Surveying the elimination of hazardous heavy metal from the multi-component systems using various sorbents: a review

Abstract

In this review, several adsorbents were studied for the elimination of heavy metal ions from multi-component wastewaters. These utilized sorbents are mineral materials, microbes, waste materials, and polymers. It was attempted to probe the structure and chemistry characteristics such as surface morphology, main functional groups, participated elements, surface area, and the adsorbent charges by SEM, FTIR, EDX, and BET tests. The uptake efficiency for metal ions, reusability studies, isotherm models, and kinetic relations for recognizing the adsorbent potentials. Besides, the influential factors such as acidity, initial concentration, time, and heat degree were investigated for selecting the optimum operating conditions in each of the adsorbents. According to the results, polymers especially chitosan, have displayed a higher adsorption capacity relative to the other common adsorbents owing to the excellent surface area and more functional groups such as amine, hydroxyl, and carboxyl species. The high surface area generates the possible active sites for trapping the particles, and the more effective functional groups can complex more metal ions from the polluted water. Also, it was observed that the uptake capacity of each metal ion in the multi-component solutions was different because the ionic radii of each metal ion were different, which influence the competition of metal ions for filling the active sites. Finally, the reusability of the polymers was suitable, because they can use several cycles which proves the economic aspect of the polymers as the adsorbent.

Introduction

Water has been recognized as an essential material [80], which is important for the life of all types of organisms [186]. Our planet is contained a trace amount of drinking water, which is crucial for the usage of people. Hence, water pollution by different substances should be avoided to preserve its properties [187]. The pollution has been described as "a harmful matter has the state of solid, liquid, and gas which generates serious damages to the nature". The important origins of water contamination are sewage, dumping, oil contamination, universal warming, acid rains, eutrophication, and the wastes of the factories, which the third case is more crucial, and the rest factors are the part of the third one [88, 198]. As an example of water sources pollution, the Karoon river in Iran has been contaminated by the petrochemical and oil factories [63], the Ottawa river in Canada has been contaminated by the pulp and paper factories [168], and a large number of water sources in India has contaminated by textile factories [157]. The wastes of the factories consist of many hazardous metallic particles in comparison to the other wastes [203]. The metallic particles are artificial inorganic chemical contaminations [89]. The molecular weight of the metallic particles is defined in the range of 60.5 and 200.6 with a specific gravity of higher than 5. The release of effluents from factories into the water sources causes to the fishes pollute with the metallic particles which are diffused into the organs of people by the use of these contaminated fishes (Table 1) [129]. The other parameter is irrigation of the farming lands [131] by the wastes of the factories consisting of metallic particles which are not decomposed (Standards), which collect in the organisms and eventually transfer to the food chain [197].

Table 1.

The allowable concentration of metallic particles [45]

Metal ions	Health hazards	Permissible limit (mg/L) (WHO)
Cadmium (Cd)	Kidney damage, renal disorder, human carcinogen, emphysema	0.003
Mercury (Hg)	Neurological damage, paralysis, blindness, rheumatoid arthritis and anorexia	0.001
Arsenic (As)	Skin, lung, bladder and kidney cancer, neurological disorder, muscular weakness and nausea	0.010
Lead (Pb)	Brain damage, anaemia, anorexia, vomiting, disease of circulatory and nervous systems	0.050
Chromium (Cr)	Headache, diarrhoea, nausea, vomiting, carcinogenic, lung tumour	0.050
Cobalt (Co)	Asthma like allergy, damage to the heart, damage to the thyroid and liver, carcinogenic	0.100
Copper (Cu)	Liver damage, Wilson disease, Insomnia	2.500
Zinc (Zn)	Depression, lethargy, neurological signs, dehydration anaemia and increased thirst	5.000
Manganese (Mn)	Syndrome of motor dysfunction, memory loss resembling, Parkinson disease	0.500
Iron (Fe)	Headache, Brittle nails, Depression, Constipation, Tinnitus, Gastrointestinal complains	0.300
Nickel (Ni)	Dermatitis, nausea, chronic asthma, coughing and cancer of the lung	2.00

Metal poisoning extensively impacts the growth mechanism, including preventing the essential biological functional groups of molecules, displacing the necessary metallic particles in biomolecules, and varying the effective parts of biomolecules [110, 161]. For example, it has been confirmed that nickel and lead particles are listed among the toxic metallic particles owe to their high solubility in the solutions [29]. The admissible value of nickel and lead particles in solutions are 0.02 mg/L [204] and 0.006 mg/L [24], respectively. The origins of heavy metal ion generations are electroplating, metal surface treatment process, conversion-coating, anodizing-cleaning, milling, etching industries, electrolysis depositions, petroleum refining, and battery industries [91, 166, 220]. The metallic particles have acute and chronic influences on the people’s organisms [249]. Also, metal ions have adverse effects on the immunity, respiratory systems, and infants [65, 201], and they generate damages in the brain, circularity systems, kidney, and heart, which lead to creating severe diseases like insomnia, encephalopathy, nephropathy, and cardiovascular issues [2, 10]. In advanced countries, strict laws are explained to release wastes of factories into the nature; thus, the effluents of factories must be rectified before discharging into the nature [86, 149, 159]. The separation methods divide into four categories such as physical [55, 89], chemical [99], biological [191], and integrated [250] procedures, in which the chemical methods are broadly applied for the removal of metallic particles [243]. Sedimentation [60], floatation [89], filtration [259], and membrane separation [151] has recognized as the physical procedure. Reduction [136], precipitation [169], chemical coagulation [212], ion exchange [144], and adsorption [156, 185, 230] are the chemical group. Biological methods are sludge process [255], aerobic [137], anaerobic digestions [178], aerated lagoons process [17], and oxidation ponds [41]. Besides, the integrated procedure is a mixture of physical, chemical, and biological techniques [223]. Each of the separation methods has a number of drawbacks [23]. Adsorption is the most important method compared to the other common procedures [25, 163]. Adsorption is the transfer of mass from the aqueous phase to the surface of the adsorbent. In this technique, the metallic particle is impacted by the van-der-walls bonds in the physical state or chemical bonds like hydrogen bonds in chemical sorption [114]. This procedure has a large number of advantages including easy design, lowest secondary pollution, economical, and no sensitivity to acidity [188]. In this technique, choosing the low-cost and high-efficiency adsorbent is a crucial case. The aim of this article is to evaluate the uptake capacity and structure of various adsorbents for finding the appropriate adsorbent among them. The results are written in Table 2. Moreover, the novelty and strength of this present literature are studying the various types of adsorbents in terms of experimental and modeling cases. The modeling aspects are isothermal, kinetic, and thermodynamic for determining the behavior of each adsorbent. Also, the other novelty of this review is investigating the structural properties of each adsorbent by analyzing the different tests including SEM, XRD and etc. The weakness of this present review is the lack of studying the computational programs such as design-expert or molecular dynamics which were done for the adsorption of metallic ions using the different adsorbents.

Table 2.

Comparison of the adsorption capacity in different sorbents

Researchers	Sorbents	Metal ion	q (mg/g)						Ref
Mihajlovic et al	Zeolite	Pb, Cd, Zn	NZ					ZFe	[153]
			0.05, 0.008, 0.03					0.052, 0.048, 0.046
Qiu & Zheng	Cancrinite-type zeolite	Pb, Cu, Ni, Co, Zn	524.22, 132.24, 89.91, 73.19, 75.45						[181]
Taha et al	Na-activated bentonite	Pb, Cd, Ni	5.43, 3.14, 2.77						[224]
Bourliva et al	Natural bentonite	Cd, Cu, Ni, Pb	31.25, 32.26, 26.32, 85.47						[18]
Vhahangwele & Mugera	South African bentonite	Co, Cu, Ni, Pb, Zn	9.50, 9.90, 8.80, 9.50, 9.60						[236]
Sdiri et al	Montmorillonitic and Calcareous Clays	Pb, Cd,	RS	RY		TS		TY	[208]
			48.04, 1.15	34.61, 3.61		24.87, 1.57		35.20, 4.45
Ortega et al	Natural clays	Ni, Zn, Cd	bentonite			sepiolite			[172]
			Cd	Ni		Cd		Zn
			0.6	0.53		0.32		0.26
Ivanets et al	Hydroxyapatite	Cd, Co, Cu, Fe, Ni, Pb, Zn	I-0			M-1		P-1	[106]
			0.337,0.0058, 2.859,4.467, 1.174,22.79,0.065			0.416,0.0117 3.050, 5.025, 1.467,14.504, 0.068		0.472,0.0176, 3.367,5.472, 1.786, 12.432, 0.072
Hajahmadi et al	Treated Aspergillus Niger biomass	Zn, Co, Cd	Zn			Co		Cd	[94]
			8.4			8		8.8
Cepoi et al	Spirulina platensis	Cu, Fe, Zn, Ni	Cu	Fe		Zn		Ni	[44]
			6.1	7.2		1.5		0.5
Roy et al	Nostoc muscorum	Pb, Cu, Cd, Zn	Pb	Cu		Cd		Zn	[192]
			1.30	1.00		0.80		0.55
Khodja et al	Penicillium sp.	Pb, Cd	Pb			Cd			[117]
			0.49			0.46
Terry & Stone	Scenedesmus abundans	Cd, Cu	Cd			Cu			[227]
			6.80			7.50
Aksu et al	Chlorella vulgaris	Cu, Cr	Cu			Cr			[6]
			34.86			15.77
Vijayaraghavan et al	brown marine alga	La, Ce, Eu, Yb	La	Ce		Eu		Yb	[237]
			34.75	39.20		47.12		38.06
Monroy-Figueroa et al	Modified Byrsonima crassifolia	Cd, Ni	Cd			Ni			[158]
			3			2.5
Bind et al	Macrophytes	Cu, As, Cr	Cu			As		Cr	[36]
			0.60			0.53		0.39
Chiban et al	Carpobrotus edulis	Pb, Cd	Pb			Cd			[53]
			120			24
Bouhamed et al	Stones	Cu, Ni, Zn	Cu			Ni		Zn	[39]
			18.68			16.12		12.19
Ojedokun et al	Cow dung	Pb	37.35						[166]
El‑Tabey et al	Waste styrofoam	Ni, Cd	Ni			Cd			[74]
			21.73			20.45
Hui et al	Fly ash	Co, Cr, Cu, Zn, Ni	Co	Cr		Cu	Zn	Ni	[103]
			9.34	38.69		40.01	26.58	7.59
Reynel-Avila et al	Chicken feathers	Cd, Pb, Ni	Cd			Pb		Ni	[190]
			0.562			2.735		0.146
Liu et al	Waste sludge	Cr, Cu, Zn	Cr			Cu		Zn	[133]
			0.5			0.32		0.11
Guijarro-Aldaco et al	Modified egg shell	Cd, Ni, Zn	Cd			Ni		Zn	[90]
			5.57			9.96		10.48
Bucatariu et al	Silica/Polyethyleneimine Composite	Cu, Ni, Co, Cd	Cu	Ni		Co		Cd	[40]
			174.75	23.48		18.86		38.22
Tian et al	Chitosan	Ni ions in multi-component wastewater	200.58						[229]
Usman et al	Cyclodextrine	Hg	178.30						[234]
Sharifi et al	Fe3O4@activated carbon	Metronidazole	56.180						[213]
Amiri et al	LECA	As(III), As(V)	As(III)		As(V)				[15]
			0.035		0.058
Jaafarzadeh et al	Modified Solid Waste Vegetable Oil	As(III), As(V)	As(III)		As(V)				[107, 109]
			0.062		0.053
Teymouri et al	Ceratophyllum demersum	Cr, Cd	Cr(in the presence of H3PO4)		Cd(in the presence of NaOH)				[228]
			45		63
Jaafarzadeh et al	Modified Volcanic ash	As(III), As(V)	As(III)		As(V)				[108]
			39 μg/mg		41 μg/mg

Material and method

In this article, old and new reviews and research articles from 1983 to 2022 have been surveyed to investigate different adsorbents in the removal of metal ions. Regarding the articles, the common adsorbents such as biosorbents, microbes, waste materials, and polymers were selected for study in this review article. Also, the main criteria in various papers such as isothermal, kinetic, and thermodynamic models to investigate the adsorbent behavior are given in this review article. In addition to this criterion, the morphology and structural characteristics of the adsorbent are other important criteria that have been mentioned in various articles. Accordingly, the search strategies in this paper include conventional adsorbents, isothermal models, kinetic models, thermodynamic models, and structural properties of adsorbents.

Biosorbents

Mineral materials

Zeolite

Natural zeolites are the common low-cost sorbent with a high exchange [175] in removing metallic particles due to their exclusive features, such as low Si/Al ratio [47, 244]. Zeolite has a crystallinity network (Fig. 1a). The particles in the zeolite network have spherical and uniform shapes (Fig. 1b and c). Moreover, the XRD pattern in Fig. 1d indicated that the carbonate cancrinite was the dominant element in the zeolite structure, and the nitrogen adsorption–desorption in Fig. 1e proved that the mesopores had occupied the adsorbent surface. Also, Fig. 1d shows that the pore sizes were observed at 200 Å, while the size of metallic particles was 5 Å, which implied that the access of toxic particles into the sorbent pores had occurred quickly [181]. The ability of natural zeolites in the separation processes relies on many parameters [154], such as characteristics of particles (particle radius, hydration energy, ion electronegativity), features of zeolite, and properties of the system (solution acidity, heat degree, metal concentration, the existence of other ions). Mihajlovic et al. compared the performance of crude zeolite (NZ) and modified zeolite (ZFe) by the catalyst for the elimination of lead, cadmium, and zinc ions in multicomponent solutions [153]. Qiu and Zheng investigated the separation of lead, copper, nickel, cobalt, and zinc in the polluted water using zeolite [181]. Visa studied the elimination of cadmium, nickel, copper, zinc, and lead ions from the wastewater using zeolite [239]. Sucharda et al. removed zinc and cadmium ions from the multi-component systems by applying natural and modified zeolite [124]. The studies showed that the natural zeolites have a low surface area and uptake capacity, while modification of zeolite progresses the surface area and adsorption capability. According to Fig. 1f, the sorption of metallic particles on ZFe was so fast (30 min for Pb²⁺ and Zn²⁺, 60 min for Cd²⁺) in 0.3 mmol/L metal concentration. The whole of the metallic particles in the contaminated phase had no contest because many adequate sites were found at the surface of ZFe.

Fig. 1.

a Crystallinity network of zeolite. b Morphology of zeolite with magnitude 6 μm. (c) Morphology of zeolite with magnitude 50 μm. d XRD pattern of zeolite. e N₂ adsorption–desorption of zeolite. f The influence of equilibrium time on the separation of Pb²⁺, Cd²⁺, and Zn.²⁺ onto NZ (black symbols) and ZFe (open symbols)

The zeolite has a different affinity to the different metallic particles. For example, the sorbent affinity for lead ions is higher than zinc and cadmium ions. The slight adsorption trend for zinc and cadmium ions may be attributed to the transfer of these two metallic particles to the inner parts of the sorbent hollows. Generally, the transfer of the heavy metal ions in the adsorbent pores occurred slower than the movement of the ions at the exterior adsorbent surface (Fig. 1f) [239].

The kinetic models proved that the separation procedure obeyed the pseudo-second-order relation, which indicated that the chemical adsorption controlled the process, and the adsorption was monolayer [153].

Scientists recommended the adsorption of metallic cations owing to the plane interaction with hydroxyl species on zeolites and the blend of positive charges of cations and negative charges of metallic ions on zeolite planes [42, 174, 252]. The considered reaction onto the zeolite surface is written below:

$$nSi-OH+M^{n+}⟺{(Si-O)}_n-M+n{H}^{+}$$ 1

Equation (1) refers to the H⁺ production and pH reduction during the chelating process, while the OH⁻ groups have been created in turn, which leads to pH growth. It is essential that adjust the pH value to a certain value for preventing heavy metal precipitation and collapse of the zeolite network.

Bentonite

Materials based on clay such as bentonite were preferred over the other low-cost sorbents like activated carbon, owing to the excellent surface area, chemical resistance, mechanical stability, and good cation exchangeability. Bentonite has a high uptake capacity, and good thermostability [251]. Moreover, the adsorption kinetic using bentonite is fast (Fig. 2a) which was short and economical for using this adsorbent on larger scales [224]. The particle sizes in bentonite were measured at less than 63 nm. The pH_PZC of bentonite is 7.4 which is due to aluminum and iron oxide or hydroxides in the adsorbent network. The most content of bentonite is smectite (Table 3). According to Fig. 2b, it was noticed that bentonite has contained quartz, montmorillonite, calcite, muscovite, and dolomite. The same conclusions have been conducted by Djukic, et al. [67]. Besides, aluminum and silicon oxides are the primary particles of bentonite clay (Fig. 2c). Magnesium, calcium, sodium, and potassium are known as exchangeable elements [125]. These cations have shown excellent cation exchange strength in bentonite. The surface textural of the adsorbent is illustrated in Fig. 2d. The textural image of the crude clay can be observed in Fig. 2d, implying the rough network of sorbent. The micrographs prove the heterogeneous characteristics of the adsorbent texture of the adsorbent, which also displays a superiority of smectite crystals. The crystals have a foliated state. According to the morphology of the adsorbent particle, it has concluded crystals of a tiny extent return to the hexagonal crystalline network. These crystals are owing to the quartz accessory minerals [102].

Fig. 2.

a The fast kinetic adsorption of bentonite in removing the hazardous metal ions. b Mineralogical composition of bentonite. c Elemental composition of bentonite. d The morphology of bentonite. e The changes of ${\text{q}}_{\text{m,i}}^{\text{mix}}/{\text{q}}_{\text{m,i}}^{\text{o}}$ with the initial concentration in various metallic particles. f The schematic of penetration of metallic particles inside the layers of the Na-bentonite

Table 3.

Compositions of bentonite [181]

Chemical composition				Mineralogical composition
Element	Trace (ppm)	Element	Trace (ppm)	Material	content
Mo	0.40	Cd	< 0.10	Smectite	94%
Cu	10.90	Sb	0.10	Illite	1%
Pb	8.10	Bi	< 0.10	Calcite	4%
Zn	23.00	Hg	1.00	Quartze	1%
Ni	7.90	Se	0.10
As	10.30	Tl	0.10

Bentonite is available in a large quantity in Greece. They have attracted scientists’ interests for a long time, because of their low price and abundance [21]. Using bentonite to eliminate metallic particles in multi-component solutions has been rarely studied [93]. Taha et al. surveyed the elimination of metallic particles from the river of Alexandria in Egypt using natural bentonite collected in the western desert of Egypt with 90% montmorillonite and low impurities [224]. Bourliva et al. studied the natural bentonite performance in Greece to remove cadmium, copper, nickel, and lead ions from plating factory wastewater [18]. Vhahangwele & Mugera applied South African bentonite to eliminate toxic metallic particles from acidic contaminated water [236].

The authors have evaluated the metal ions competition. According to Fig. 2e, the values of metal ions were ${\text{q}}_{\text{m,i}}^{\text{mix}}/{\text{q}}_{\text{m,i}}^{\text{o}}<1$ which expressed the antagonistic state was occurred. Thus, metallic particles contest for the sorption channels of bentonite with other metallic particles, and adsorption is subjected to the existence of the rest metallic particles. The adsorption kinetics of metal ions onto the bentonite showed that pseudo-second-order kinetics better fitted, and metal separations were governed by chemisorption. Long-range intra-particle diffusion was also performed to consider the penetration of the solute to bentonite clay interlayers. Also, the separation process obeyed the Freundlich isotherm equation, and it indicates that the adsorption was multi-layer [18]. Also, the ion exchange technique was first initiated inside the layers of bentonite. For example, Bama & Sundrarajan utilized the Na-bentonite for the elimination of metallic particles, and they exhibited that the adsorption has occurred inside the layer (Fig. 2f) [125]. According to the studies, the activation energy for the bentonite is less than the 42 kJ/mol which expresses that the separation is diffusion-controlled more than chemically controlled, and the Van der Waals forces or electrostatic interactions are created between the bentonite and the metal ions [9].

Bourliva et al. applied bentonite in real effluent of plating units, and the removal efficiency of each metal ion has been written in Table 4; it was seen that the bentonite has a low adsorption efficiency for nickel and zinc ions. Furthermore, it has removed the iron completely at a pH of 4.47 [18].

Table 4.

Separation of metallic particles from plating effluent by bentonite [181]

Samples	Heavy metal ions	pH	Removal (%)
Wastwater I	Fe, Zn, Ni, Cu	2.46	57.58, 36.61, 38.71, 60.54
Wastewater II	Fe, Zn, Ni, Cu	4.47	100.00, 73.78, 49.01, 4.85

Clay

Clay is a natural adsorbent that is extensively used to separate toxic metal ions by researchers because of its low price and good efficiency. Figure 3e indicates the clay surface is rough and has a large number of pores. The most common clays are kaolinite, Celtek clay, bentonite, sepiolite, and montmorillonite. Sdiri et al. inspected the Montmorillonitic and Calcareous Clays in Tunisia to remove lead and cadmium ions in single and multi-component solutions. The authors believed that the understanding of the adsorption mechanism depends on both clay and metal ions, and the interaction of the mixed cations in the wastewater. In their research, two different natural clays were collected from Gabes (Y sample) and Gafsa (S sample).

Fig. 3.

The removal of a lead and b cadmium ions in binary systems. Binary adsorption isotherms of Cd(II)–Ni(II) on bentonite. The surfaces are predicted with the MRPMI model. c Cd(II) uptake and d Ni(II) uptake at pH of 7 and temperature of 25 °C. e The SEM image of clay

For improving the clay performance, both carbonate minerals and organic contents should be removed. In order to eliminate the carbonate from the clay, it was soaked in 1 mol/L acetic acid and heated using a water bath at 80 ℃. The organic matters were removed by 30% H₂O₂. Then, raw and modified clays were introduced into the water and 1 mol/L NaCl three times under stirring, and dried at 105 ℃ overnight. Also, the RS and TS, RY, and TY mention the raw and modified S and Y clays, respectively. The high SiO₂, Al₂O₃, and Fe₂O₃ compositions in RY and TY propose excellent yields of metallic particles owing to the existence of aluminum (Al–O) and silanol (Si–O) groups at the surface of both RY and TY clay samples. The separation in single-element systems in pH of 6 and mixing rate of 200 rpm. The initial concentration of lead and cadmium ions were 60 and 10 mg/L, respectively. Figure 3a and b display the uptake capacity of lead and cadmium ions by the raw and modified clay samples at pH 6 for entire metal blends in multi-component solutions. The observations demonstrate that lead ions have the most yield. In the lead–cadmium solutions, more than 24.87 mg/g of lead were eliminated, implying that the existence of cadmium has a low inhibitory impact on lead separation onto raw and modified clays. The elimination content of lead in the existence of cadmium ions was 48.04, 34.61, 24.87, and 35.2 mg/g for RS, RY, TS, and TY samples, respectively. The carbonaceous clay (RS) eliminated 48.04 mg/g of lead ions but extensively reduced its carbonate-free form (TS). Therefore, the existence of carbonates in RS sorbent has an increased lead elimination due to the sedimentation of PbCO₃ [208].

Ortega et al. investigated the elimination of metallic particles using bentonite and sepiolite. The binary adsorption results of cadmium–nickel on bentonite and the forecasting of the MPRMI model are drawn in Fig. 3c and d. The impact of nickel on the capacity of cadmium ions can be observed in Fig. 3c, and in the concentration of nickel ranging from 0 to 2 meq/L, the capacity of cadmium ions is reduced drastically as raising the number of nickel ions at equilibrium. Then, the capacity of cadmium ions was reduced gradually for the number of nickel ions more than 2 meq/L. For example, at the amount of cadmium ions at the equilibrium of 5 meq/L, the capacity of cadmium sorbed was 0.60, 0.41, 0.33, and 0.30 meq/g, while the number of nickel ions at saturation was 0, 2, 4, and 5 meq/L, respectively. The adsorption isotherm surface of nickel ions on bentonite is displayed in Fig. 3d, and the reliance of the capacity of nickel on the content of cadmium ions at the saturation state is plotted in this figure. The capacity of nickel ions reduced gradually owing to the competition of the cadmium ions, and the decrease of the nickel capacity was almost proportional to the increase of the cadmium concentration. At a nickel concentration at the equilibrium of 5 meq/L, the capacity of nickel sorbed was 0.53, 0.39, 0.32, and 0.29 meq/g for the content of cadmium at the equilibrium of 0, 2, 4, and 5 meq/L, respectively [172].

Hydroxyapatite

The ability of calcium and calcium-magnesium phosphates are excellent in eliminating the metallic particles, without producing the toxic materials, and are harmless to the ecosystem [93, 119]. Many investigations are dedicated to the survey of fabricated [71] and raw apatites [75], consisting of mineral and biological nature, derived from degradable raw substances. Researches show the consequences of observations about the rectification of contaminated real water [155] and soil [93], which proved excellent yield in metallic particles elimination by bone meal and hydroxyapatite (HA). The suitable properties of calcium phosphates are varying the physical and chemical properties, such as the porosity and amount of chemical particles, by varying the procedure and fabrication cases. HA is constituted of platelet-shaped nanoparticles with mean dimensions up to about 50 × 20 nm (Fig. 4a). HA is fabricated via chemical precipitation from the mixture of Ca(NO₃)₂·4H₂O and H₃PO₄ in the pH of 10 by introducing NH₄OH at the normal temperature. The fabrication of HA is done by gradual mixing of a phosphorous suspension with a calcium suspension at a certain agitation speed [106]. The adsorption properties rely on different factors such as degree of crystallinity, and solubility. For example, Fig. 4b expressed that enhancing the surface area returns the diminish the extent of HA crystallinity and the magnitude of crystallites which promoted the potential of the adsorbent in the removal of the metallic particles [57]. Typically, the solubility of HA is a criterion for evaluating its ability the removal of metallic particles; in other words, the high solubility of HA improves the uptake capacity even in the existence of salt-containing materials [209]. Comparative investigations are displayed that polyacrylic and citric acid are the inhibitors that decrease the fabrication of HA crystals to the most and least amount, respectively [16]. Phosphonic acids have a strong inhibitory potential, which is characterized by the amount of CaPO(OH)₂ [265]. The phosphonic and diphosphonic acids are extensively suggested for the functionalization of HA, due to improving its crystallinity which impacts the porosity of the network. Also, adding these acids develop the uptake capacity of metal particles [76, 171]. Chemical improvement of the calcium phosphates surface is conducted via sorption of phosphonates per HA from the solution; thus, these modified adsorbents are applied for the selective separation of metallic particles [58]. The phosphonates such as phenyl phosphonic acid as chelating agents including hydroxyl and phosphonic groups (Fig. 4c), thus the phosphonates create excellent chelation with metallic particles, such as Ca²⁺ and Mg²⁺ particles [128].

Fig. 4.

a The particle size of the HA. b The surface area of HA in different crystallinity degree. c The phenylphosphonic acid structure. d XRD pattern of HA with the different inhibitor. e Uptake capability of HA adsorbents at 15 min

Ivanets et al. studied the organic (HEDP, sample P-1) and inorganic (Mg²⁺ ions, sample M-1) inhibitors for the removal of metallic particles. According to Table 5, the addition of 1% magnesium particles increases the surface area of the produced HA because of the diminish in the size of crystallites. Also, adding the HEDP improved the specific surface area because it chelated with the Ca²⁺ ions, prohibit the crystallization of HA, and produce elaborate complexes. It was observed that the introduction of different inhibitors even a small amount of them change the XRD pattern which proved the changes in the chemical composition, crystal, and textural features (Fig. 4d). The good and rapid uptake efficiency of the metallic particles in the first 60 min explained the benefits of the adsorbent P-1 relative to the other sorbents. By comparison of the results of metallic particle elimination, the order of the sorbent ability is compared following: P-1 > M-1 > I-0 (Fig. 4e) [106].

Table 5.

The content of participant element, and structural properties of resins [172]

Sample	Unit	I-0	M-1	P-1
Concentration (Ca)	mmol·g−1	9.40	9.29	8.92
Concentration (P)	mmol·g−1	5.69	5.71	5.76
Asp	m2·g−1	72.00	123.00	75.00
ABET	g2·g−1	75.00	127.00	79.00
Vsp.ad	cm3·g−1	0.233	0.375	0.516
Vsp.des	cm3·g−1	0.242	0.367	0.735
Dsp.ads	nm	13.00	12.00	28.00
Dsp.des	nm	13.00	12.00	29.00
τ	nm	8.86	8.14	‒

Microbes

Aspergillus niger

Microorganisms such as bacteria, algae, fungi, and yeast have known for their practical application in effectively eliminating the metallic particles [87]. Among the essential types, fungi groups are suggested due to the benefits, including containing a vast content of cell wall which expresses suitable chelation of metallic particles. Aspergillus niger, a filamentary fungus, has not simply been used in the large-scale generation of citric acid and enzymes, and it is also recognized for its potential in trapping the metallic particles [69, 238]. In nature, it is found in soil and litter, in compost, and on decaying plant material [207]. Based on the color of the conidiospores, the genus of Aspergillus niger is determined. The color of Aspergilli has dark brown spores which consist of the A. niger species (Fig. 5a) [189]. Aspergillus niger is typically recognized as a non-pathogenic microbe. Only in a few cases has Aspergillus niger been able to colonize the human body as an opportunistic invader [207]. The microscopic image of Aspergillus niger is shown in Fig. 5b. Aspergillus niger has grown in clean ambient with the required feed at a low temperature. The feed of this microbe consisted of these matters (g/l): Sugar, 50; (NH₄)₂SO₄, 2; KH₂PO₄, 0.15; MgSO₄, 0.15, and the pH of the ambient for microbe growth was regulated to 5.5 [13].

Fig. 5.

a The macroscopic image of Aspergillus niger b The microscopic image of Aspergillus niger. The impact of c equilibrium time, d acidity, and e sorbent amount on the elimination efficiency

Some investigations have occurred in the removal of hazardous metal ions using Aspergillus niger. Hajahmadi et al. in 2015 investigated the elimination of zinc, cobalt, and cadmium particles from the ternary systems by applying Aspergillus niger [94]. Munir et al. utilized Aspergillus niger for the removal of nickel and chromium ions [160]. Akar and Tunali investigated the elimination of lead and copper particles using Aspergillus flavus [4]. Li et al. studied the elimination of uranium using modified Aspergillus niger [130]. According to these studies, the ambient situations can have a considerable impact on the uptake capacity of microorganisms in eliminating metal particles.

In studying the impact of equilibrium time on the uptake capacity, the observations explained that the greatest amount of metal separation was observed the first time. For example, in Fig. 5c, the rapid adsorption of lead and copper ions is observed in 30 and 15 min, respectively [4]. This trend suggests that the separation removal is possibly owing to the contact of the metallic particles and the effective chemical species of the cells that exist on the adsorbent surface in the initial fast step, and penetration of metallic particles in the next step [138]. This fast stage can possibly be owing to the numerous accessibility of effective locations on the adsorbent, and also transferring the metallic particles in the pores is occurred slowly in the second step which causes to less significant this step [205].

The acidity content is considered a necessary parameter in the biosorption of metallic particles using Aspergillus niger. The system acidity influences the chemical features of the particles such as the ability of dissolution, the performance of effective chemical species, and the rivalry of particles in occupying the empty places. The difference in the acidity value is the contest between the hydrogen atoms and the positive parts of metals for filling the vacant pores on the adsorbent [142]. The influence of the acidity on the adsorption efficiency at a specific time has illustrated in Fig. 5d. At the high acidity amount of the system (2–3), the uptake of the participant metallic particles using Aspergillus niger was relatively slight, since many H₃O⁺ particles are extensively involved with metallic cations for occupying the active places, which avoids the adsorption of metallic particles resulting of the repulsive force [14]. At high pH, the amount of hydrogen ions is low; thus, many positive charges are accumulated on the adsorption places, in which the metallic cations and H₃O⁺ particles are competed for filling the pores, leading to improving the removal efficiency. The removal yield enhanced sharply as acidity enhanced from 2 to 4, and the raising in removal efficiency gradually decreases from the pH of 5. At acidity amounts higher than 7.0, zinc, cobalt, and cadmium ions sediment owe to a large number of hydroxide particles in the biosorption ambient.

The biomass dosage is the primary factor as it shows the yield of the sorbent for a particular initial extent [200]. According to Fig. 5e, when the biosorbent dosage enhances the content of available active places or surface area for the metallic particles, because of increasing the active sites [232].

Temkin isotherm model is the better model for fitting with the empirical results in comparison with the rest isotherm equations. Also, the high amount of K_F in the Freundlich model implies the high feasibility of metallic ion adsorption. The typical binding energy has obtained 8–16 kJ/mol [98].

Nostoc muscorum

Vast attention has been paid to cyanobacteria as they have been investigated to efficiently disappear metal particles due to their wide surface area [33]. Their higher mucilage extent as well facilitates more effective chemical species with excellent metal-chelation potential [193]. Moreover, cyanobacteria consume a low amount of nutrients, and it does not generate any toxic material in comparison with the other microbe types. One of the most popular cyanobacteria is Nostoc muscorum (Fig. 6a and b). Nostoc muscorum has a filamentous form that grows in ponds, and other pools of water and a gelatinous matter has covered the surface of this microorganism. Also, they can expand in the alkaline ambient which can avoid pollution by other microorganisms [81]. The functional groups of the Nostoc muscorum are carboxylic acid, hydroxyl, amine, and amido (Fig. 6c). The results of the FTIR test is listed in Table 6. It consists of 15.09% carbon, 2.23% nitrogen, and 5.26% sulphur [92]. Additionally, it was observed that a live Nostoc muscorum shows a better performance than a dead species because of the appropriate membrane binding and metabolic energy-dependent intracellular uptake [68]. The Nostoc muscorum has paid much attention to eliminating toxic metal ions. Roy et al. in 2015 studied the elimination of the lead, copper, zinc, and cadmium particles utilizing Nostoc muscorum as adsorbent, and analyzed the factors using the Plackett–Burman design [192]. Dixit & Singh utilized Nostoc muscorum for the elimination of cadmium and lead ions [66]. Sheekh et al. investigated the potential of Nostoc muscorum for the elimination of copper, cobalt, lead, and manganese from industrial wastewater [73]. Raghavan et al. investigated the separation of cadmium ions utilizing Nostoc muscorum [183]. Arun et al. used Nostoc muscorum as an adsorbent for the adsorption of copper particles from the effluent [20]. For preparing the Nostoc muscorum as the adsorbent, the cyanobacterium Nostoc muscorum is firstly separated from a heavy metal-polluted place in Meghalaya, India [95]. The strain was promoted in liquid-blue green BG110 ambient utilizing glass (Fig. 6d and e) with alternate light and dark periods of 16 and 8 h, respectively. Heat degree and light power for the cyanobacteria development have been kept at 25–30 °C and 3000–3500 lx (cool white light), respectively. Components of the BG110 ambient was (g/L): K₂HPO₄·3H₂O, 40; MgSO₄·7H₂O, 75; CaCl₂, 36; citric acid, 6; ferric ammonium citrate, 6; EDTA, 1; and Na₂CO₃, 2. The ambient has improved with 1 ml/L little metal system consisting (g/L) H₃BO₃, 2.86; MnCl₂·4H₂O, 1.81; Na₂MoO₄·2H₂O, 0.390; ZnSO₄·7H₂O, 0.222; CuSO₄·5H₂O, 0.079; and Co(NO₃)₂·6H₂O, 0.0494. The culture was continuously kept by subculturing and rinsing with BG110 medium every 20 days. The schematic of the bioreactor is displayed in Fig. 6f.

Fig. 6.

The SEM of Nostoc muscorum in a 10 μm, and b 1 μm. c The chemistry structure of Nostoc muscorum. d The erlenmyer containing the mixture and Nostoc muscorum. e The macroscopic image of Nostoc muscorum. f The schematic of bioreactor. g The plot of lnK vs. T⁻¹. h The reusability test of Nostoc muscorum

Table 6.

The FTIR results for the Nostoc muscorum [92]

Wave length (cm−1)	Component
3403	Carboxylic/OH
2925	Phenolic/carboxylic
1622	C = O stretch
1429	OH bonds
1125	= C–C =
464	C–N–S scissoring

According to Fig. 6g, the amount of lnK has enhanced from 4.199 to 4.208 to 4.216 by increasing the temperature from 298 to 303 to 308 K implying the enhancement affinity of Nostoc muscorum to the temperature enhancement for adsorption of the zinc ions. Figure 6g proved the positive value of enthalpy (1.294 kJ/mol) which expresses that the separation process is endothermic. Besides, the sign of the Gibbs free energy values (ΔG^o) was negative in all temperatures implying the spontaneous nature of the adsorption (Table 7) [64].

Table 7.

The value of ΔG^o in the three different temperatures [64]

Temperature (K)	ΔGo (kJ/mol)
298	-10.404
303	-10.599
308	-10.796

The rates of metallic particles obeyed the pseudo second-order kinetic relation implying that the initial metallic adsorption by Nostoc muscorum is reaction controlled, containing chemisorption, i.e., chelating of metal particles to the effective chemical species on Nostoc muscorum [176]. Furthermore, the reusability of the Nostoc muscorum has been examined by EDTA, and the results are illustrated in Fig. 6h. Figure 6h explains that the adsorption capacity of the lead ions has not significantly changed after the 6 steps. These results demonstrated that the Nostoc muscorum could be continuously utilized for metal elimination without significant loss in adsorption efficiency [66].

Penicillium

Penicillium belongs to the fungi groups. Fungi groups like penicillium under stress progress a number of behaviors in order to tolerate inappropriate situations [165]. The removal of toxic metal particles via penicillium divides into three steps which are the following: 1) Adsorption of metallic particles on the plane of the penicillium, 2) intracellular removal of metallic particles, and 3) Chemical binding between the metallic particles and penicillium [216]. Several species of penicillium microorganisms are applied for the removal of metallic ions including Penicillium notatum, Penicillium chysogenum, Penicillium simplicissimum, and Penicillium austurianum [141]. SEM analysis in Fig. 7a shows that even though the Penicillium sp. hyphal is packed tightly, there is still sufficient microchannel and void volume for free transferring of particles, hence enhancing the accessibility of metallic particles to the active sites on the fungal biomass [132]. Generally, the cell walls of penicillium play an important in adsorbing the heavy metal ions. Figure 7b implies the cell wall of penicillium consists of polysaccharides and glycoproteins such as glucans, chitin, mannans, and phospho-mannans. These polymeric materials have a large number of sources of metal chelating [79]. Sintuprapa et al. showed the uptake of zinc particles into the cell wall of living penicillium and also chelation of zinc ions with the phosphorous group of the penicillium (Fig. 7c and d) [218]. Several investigations are accomplished to eliminate metallic particles from polluted water. Khodja et al. evaluated the sorption ability of Penicillium sp. separated from the soil to remove cadmium and lead in single and binary solutions [117]. Li et al. used Penicillium simplicissimum for the separation of Pb(II) and Cu(II) particles [132]. Ahmad et al. studied the elimination of nickel, chromium, and cadmium ions using Penicillium sp. [1]. Fan et al. investigated the separation of Cd(II), Zn(II), and Pb(II) particles using Penicillium simplicissimum [78].

Fig. 7.

a SEM analysis of penicillinium. The building of the Penicillinium: P = polyphosphate; CW = cell wall; CM = cell membrane; CY = cytoplasm b 0.2 μm, and c 0.05 μm. d Schematic representation of the outer fungal cell layers. e Macroscopic observations of Penicillium sp. f The impact of acidity on the uptake capacity of metallic particles. g The impact of the contact time on the uptake capacity

For preparing the Penicillium as the sorbent, 25 g of a soil part was poured into the 225 mL of tryptone salt broth. Petri dishes consisting of Potato Dextrose Agar (PDA) ambient were mixed with 1 ml of the suspension. The glasses stayed in the incubator at 25 °C for 5 days. Pieces of individual colonies were poured into Oxytetracycline Glucose Yeast Extract Agar (OGYA) ambient. The fabricated culture was introduced into distilled water. The strain was determined at the genus part after promotion by inspecting its macroscopic (color, texture) characteristic in Fig. 7e [253]. The removal efficiency of metallic particles relies on the metal type, and empirical situations like equilibrium time, sorbent extent, temperature, chemical additive, and pH [82, 139]. For example, Liu et al. modified the penicillium resin with 0.025% saponins because of lacking an inappropriate impact on the environment and promoting the uptake capacity via enhancing the negative charge of the resin. This negative charge returns to the enhancement of the sulfur, chlorine, and potassium amount (Table 8). The solution’s initial pH affects the system chemistry of the metals, the ability of effective chemical species on the cell wall, and the movement of the metallic particles for filling the effective places of the sorbent. Figure 7f reveals the impact of the pH on the uptake capacity of the cadmium, zinc, and lead ions, and the optimum pH has been calculated at 4, 6, and 5 for cadmium, zinc, and lead ions, respectively [218]. With respect to the figure, the sorption capability of the metallic particles is very low, because many hydrogen ions with a positive charge contest with the metallic particles for occupying the vacant places of the resin. When the acidity enhancing, a high extent of negatively charged cell plane becomes available hence facilitating greater metal sorption [3]. Also, the metallic particles precipitated at a pH of more than 7 which avoided the joining of cations to the effective chemical species of the cell wall in the penicillium. Figure 7g exhibits the impact of the equilibrium time on the uptake capability of the cadmium, lead, mercury, and arsenic ions. According to Fig. 7g, the value of the adsorption capacity was high at the beginning of the process, and the adsorption capacity has not changed after 4 h [206]. The participant’s chemical compositions in the cell wall of the Penicillium determine the intensity of the chelation [206].

Table 8.

The extent of elements in the raw and modified resin [140]

Biosorbent species	Element weight percents (%)
	N	S	Cl	K
P. simplicissimum	2.3	0.44	1.19	1.68
P. simplicissimum + 0.025% saponins	7.49	0.85	3.11	2.01

Waste materials

Stone

Applying the stone as the adsorbent has many advantages such as simplicity, low price, eco-friendly, producing no toxic sludge, renewable, and excellent uptake capacity even at a slight concentration [54]. Various stones have been applied the removal the poisoning metallic particles including peach stones, apricot stones, olive stones, cherry stones, grape stones, and date seeds [219]. The stone has a great extent of oxygen-containing functional species such as carboxylic, phenolic, and lactone which can adsorb the polar pollutants easily [217]. The SEM photograph of the stone is displayed in Fig. 8a. This figure indicates the irregular and porous structure of the stone is owing to its large surface area [120]. Figure 8b is the XPS diagram which shows the composition of the stone. According to this figure, the stone contains 90.93% carbon, 5.28% oxygen, and 3.79% bore [37]. The pH_pzc of the stone is 3.40 which confirms the acidic nature of this adsorbent.

Fig. 8.

a The SEM image of stone. b The XPS image of stone. c The effect of pH on the uptake capacity. d The effect of time on the uptake capacity. e The effect of temperature on the uptake capacity

A few studies have been conducted on the adsorption of metallic particles from multi-component systems. The work of Bouhamed et al. aimed to study sorption features of the provided activated carbon as interacts with a mixture of cations (copper, nickel, and zinc) [39]. In their research, activated carbon has fabricated from the stones in Tunisian. Kobya et al. applied apricot stone to eliminate nickel, cobalt, cadmium, copper, lead, and chromium in the polluted water [120]. Lara et al. investigated the elimination of chromium, copper, and nickel ions from real electroplating effluent [147]. Jeon utilized the medicinal stones to eliminate the copper ions [111]. Tsibranska et al. investigated the elimination of the lead, cadmium, copper, and zinc ions using apricot stones [233].

The different conditions impact the uptake efficiency of the cations such as equilibrium time, acidity, concentration, chemical reagents, and heat degree. The acidity of the system is a significant factor in the separation. The chelating of cations by functional groups in the adsorbent is extremely reliant on pH. Figure 8c reveals the impact of pH on the removal capacity. With respect to this figure, the maximum uptake capability for copper ions is obtained at pH of a 6, and the uptake capability is reduced after the pH of 7. Under the pH of 4, the removal efficiency is slight owing to the competition of metal ions and H⁺ [111]. Also, Fig. 8d explains the impact of time on the uptake capacity of copper and nickel ions in the single and multi-component systems. During Cu(II)–Ni(II) separation procedure, Cu(II) removal exhibits an enhancement as time increases until obtaining the saturation of almost 120 min, which was two folds faster than single rate sorption (240 min). It observes that copper with high dependency on the stone displaced a part of sorbed nickel with slighter dependency and participation. The sorbed content of Ni(II) was protected more or less fixed since the initial saturation times and determined 0.015 mmol/g, 85% slighter than the sorbed extent of Ni(II) in a single solution. The empirical sorbed content of Cu(II) from the multi-component solution was almost 0.076 mmol/g, 15% slighter than that in a single solution [37].

To explain empirical data, the Langmuir and Freundlich relations have been used for studying the impact of the existence of other metallic particles on the isotherm factors. The contest sorptions of Cu(II), Ni(II), and Zn(II) on the sorbent followed the Langmuir isotherm relation very suitable with the extent of R² (0.98–0.99) (Table 9), and they also fit the Freundlich isotherm relation with the values of R² (0.95–0.97), but not more appropriate than the Langmuir isotherm relation in general. The ratio of q_e’/q_e will indicate whether the impact of mixing metallic particles in a system is synergistic (q_e’/q_e > 1), no net interaction (q_e’/q_e = 1), or antagonistic (q_e’/q_e < 1). The qe’/qe ratios of copper, nickel, and zinc in the multi-component solutions have been calculated at 0.59, 0.66, and 0.57, respectively. The ratios have observed lower than unity, explaining that the separation of metallic particles was influenced by the existence of the rest metallic particles in the solution. The uptake efficiency obeyed the arrangement of copper > nickel > cadmium in single-component solutions, and the competitive uptake efficiency fell in the declining arrangement of copper > nickel > cadmium in multi-component solutions. The empirical data explain that the total uptake efficiency of metallic particles in multi-component solutions enhances, but individual metallic particle uptake will reduce owe to cation interaction for active places [39].

Table 9.

The isotherm parameters for the multi-component solution [52]

Heavy metal ion	The Langmuir constants			The Freundlich constants
	q0 (mg/g	b (l/mg)	R2	Kf	1/n	R2
Copper	18.68	0.018	0.99	0.6	0.69	0.96
Nickel	16.12	0.016	0.98	0.46	0.7	0.98
Zinc	12.19	0.018	0.98	0.32	0.74	0.95

Ceren & Eral studied the thermodynamic aspect of uranium and thorium using olive stones. Figure 8e illustrates the changes in uptake capacity of uranium and thorium with the temperature. According to this figure, the uptake capability of the metallic particles slowly enhances with enhancing the heat degree. The positive sign of heat changes shows the endothermic feature of the separation process. Also, the free Gibbs energy decreases by increasing the temperature which demonstrates the adsorption capacity is favored at the high temperatures (Table 10) [127].

Table 10.

Thermodynamic constants for the separation of uranium, and thorium ions on olive stones [127]

Metal ions	ΔHo (kJ/mol)	ΔSo (kJ/mol.K)	ΔGo (kJ/mol)
			288 K	303 K	313 K	323 K
Uranium	16.59	0.13	-20.76	-22.71	-24	-25.30
Thorium	7.42	0.08	-15.73	-16.94	-17.74	-18.54

Cow dung

Cow dung ash is an eco-friendly and low-price sorbent. It is bioorganic sewage that comprises 12.48% calcium oxide, 0.9% magnesium oxide, 0.312% calcium sulfate, 20% aluminium oxide, 20% iron oxide, and 61% silica [199, 235]. The existence of maximum removal of silica causes it to show a significant dependence on metallic particles. The benefit of applying cow dung is easily available. Releasing the cow dung is easy because it can be sun-dried and can be utilized as a landfill [30, 72]. It has several major features which have been in applying many times. It is mixed with soil bedding and urine, which is utilized as manure for agricultural aims. It is also employed in the generation of biogas which is utilized to produce power and energy [123]. The most popular and effective functional groups in cow dung are carboxyl, phenols, quinols, and amide which returns to the component in the cow dung such as fats, proteins, bile pigments, and aliphatic–aromatic species. Besides, some dead microorganisms such as Acinetobacter sp. and Pseudomonas sp. are observed in the cow dung which promotes the uptake capacity of the toxic metal particles [5]. The cow dung powder should be dried carefully before utilizing or preventing its oxidation by acid. According to the XRF diagram in Fig. 9a, carbon, nitrogen, hydrogen, and sulfur have comprised the structure of the cow dung [77]. The SEM image of cow dung is illustrated in Fig. 9b, indicating the existence of a great extent of the pores and small opening on the adsorbent texture [30].

Fig. 9.

a XRF and b SEM diagram of cow dung. The effect of c temperature and d acidity on the uptake capability of metallic particles

Several investigations were conducted about the adsorption of metallic particles using cow dung. Ojedokun et al. [166] investigated the important factors in the adsorption of metal ions using cow dung. Das et al. used modified cow dung for the removal of chromium [59]. Barot & Bagls investigated the elimination of chromium and cadmium ions [30]. Usman et al. utilized cow dung for adsorption of the lead ions [77]. These investigations implied that the experimental conditions influence the removal efficiency. The effective factors are described following. Figure 9c exhibits the impact of temperature on the uptake efficiency of the chromium ions and indicates that temperature enhancement promotes the uptake capacity, which is owing to the enhancement in chemical interaction between chromium ions and cow dung [59]. Moreover, Fig. 9d shows the impact of acidity on the removal yield. It can be proved that the appropriate pH for maximum removal efficiency of chromium ions is one, and as pH increases there is a significant reduction in removal efficiency, because a large number of hydroxyl ions attribute great hindrance to the diffusion of chromium ions, resulting in weakening of electrostatic force of attraction between cow dung and chromium ions which causes in reducing removal efficiency [30]. The observations prove in the pseudo-second-order is the most appropriate kinetic model in fitting with the empirical results which confirms the chemical interaction between the solute and the cow dung. Also, the Langmuir isotherm equation had the appropriate correlation with the empirical result, which explains the single-layer of the cow dung. Table 11 demonstrates the influence of the amount of sorbent on the separation evaluation occurred by Elaigwu et al. [77]. Generally, when the amount of the sorbent enhances, the eliminated metallic particles should promote, according to Okeimen et al. [167]. It is observed that an enhancement in the amount of the adsorbate causes an enhancement in the competition of adsorbate particles for few available active places on the surface of the resin, thus, an enhancement in the number of metallic particles eliminated. This behavior could also propose that enhance in adsorbate amount resulted in enhance in the number of available particles per binding place of the resin, hence, bringing about a higher possibility of binding of particles to the resin. The sorbent generates active places for the trapping of metallic particles. Its extent hence firmly impacts the separation of metallic particles from the system [126].

Table 11.

Impact of adsorbate content on adsorption of Pb²⁺ [166]

Concentration of adsorbate (mg/l)	Initial concentration of Pb2+ (mg/l)	Lead concentration (mg/l) at equilibrium	Amount Adsorbed (mg/g)
20	12.51	0.42	12.09
30	18.76	0.44	18.32
40	25.02	0.51	24.51
50	31.27	0.16	31.11
60	37.52	0.17	37.35

Waste styrofoam

By growing the various packages, especially food packages, the waste plastics are generated because of easy to use, comfortable, and cheap [83, 85]. Waste plastics harm the environment because of their biodegradability properties, and they comprise polystyrene [31, 118]. Moreover, they damage human health such as irritation of the skin, and eyes, and make disorders in the respiratory system, and kidney and blood diseases [226]. The problem of abundant waste plastics cannot be solved by incineration or landfilling due to being expensive and the production of toxic gases in the air, thus, recyclability is the most appropriate choice for solving this global issue [31]. For the aim of reducing the waste plastics and removing the hazardous metal ions, the waste plastics are applied as the adsorbent [248]. The waste plastic or polystyrene with the commercial name styrofoam has a long hydrocarbon chain with a phenyl ring joined to each carbon atom (Fig. 10a). Polystyrene has been fabricated via free radical vinyl polymerization of its monomer [32, 146]. The EDX test in Fig. 10b implies that the waste polystyrene contains of 13.77% oxygen and 86.23% carbon [31]. Also, Fig. 10c shows the SEM photograph has the spherical and coarse particles [257]. The peaks in FTIR test (Fig. 10d) are written following: OH (at 3,400–3,200 Cm^–1), unsaturated aromatic C–H stretching vibrations (at 3,025 Cm^–1), CH₂ bending vibration (at 2,921–2,846 Cm^–1), aromatic ring (at 1,492 Cm^–1), CH₂ (at 1,451 Cm^–1), various substitution of benzene ring between 900 Cm^–1 and 770 Cm^–1 (at 905; 837; 754 Cm^–1). The characteristic infrared absorbances of sulfonated polystyrene have been studied [148].

Fig. 10.

a The structure of polystyrene. b The EDX analysis of waste polystyrene. c The SEM image of waste polystyrene. d The FTIR graph of waste polystyrene. e Comparison the removal efficiency of lead and cadmium ions in different metal concentration. f Comparison the Langmuir and Freundlich isotherm models in the different temperature

The investigations for separation of metallic particles are written following. El‑Tabey et al. [39] have found a method to transform Styrofoam or waste plastics into a more valuable functional polymer (WPS-g MA), by grafting it with maleic acid (MA) for removing heavy metal ions (Ni(II) and Cd(II)) from effluent. Mahmoud et al. used waste Styrofoam to eliminate cadmium, lead, and mercury particles in the polluted water [146]. Ruziwa et al. studied sulphonated Styrofoam in the elimination of Pb(II) and Zn(II) particles [195]. Bekri-Abbes et al. utilized the waste polystyrene to eliminate the lead and cadmium ions [31]. Tran et al. inspected the waste of Styrofoam to eliminate cadmium, copper, and zinc particles from the wastewater [231]. Jia et al. applied the waste Styrofoam for the removal of cadmium ions [112].

Similar to the other adsorbents, many factors affect the removal efficiency including temperature, metal concentration, pH, etc. For example, Fig. 10e reveals the impact of metal extent on the separation percentage. According to this figure, the adsorption percentage decreases by increasing the lead and cadmium concentration [31]. Two adsorption isotherm equations, Langmuir and Freundlich, have been applied to fit the adsorption isotherm results at all three various temperatures (Fig. 10f). The obtained data revealed that the Freundlich equation was appropriate at 25 and 35℃, also the Langmuir equation was fitted suitably with the empirical data at 45℃. Moreover, Fig. 10f indicated that the temperature enhancement had a positive influence on the adsorption capacity which proves the endothermic feature of the sorbent [112].

Chicken feathers

Chicken feathers are produced in farming, and disposed of via burning [221]. USA and China are the biggest producers of chicken feathers [242]. Chicken feathers comprise 80 or 90% β-keratin which is a fibrous protein. Because of these hydrophilic groups including amino acids, cysteine, carboxylic, hydroxyl, and thiol groups in the structure of the protein, the toxic metal ions chelate into these groups [19, 177]. The schematic interaction between the metal ions and the chicken feather is illustrated in Fig. 11g. According to this figure, the adsorption mechanism divides into four diverse categories which are implied following: 1) electrostatic interaction 2) pore filling 3) precipitation on the adsorbent surface, and 4) connection of cations with the functional groups of the chicken feather. This natural sorbent has shown desired removal efficiency in the adsorption of organic and inorganic materials [8]. Also, some investigations have revealed that the uptake efficiency of the chicken feather was more than the other commercial sorbents like activated carbons [115, 190]. The morphology photograph of a chicken feather in Fig. 11b reveals the rough and porous surface of this adsorbent, which helps the interaction between the toxic metallic particles and the chicken feather. Chicken feathers exhibited a low acidic feature with a pH_pzc = 6.25 ± 0.1(Fig. 11c). For preparing the chicken feathers in the adsorption process, initially, they are dipped into the ethanol for 12 h for cleaning the rachis and barbs completely (Fig. 11a) from organic residues, then they are washed with the distilled water and dried [46]. The XRD pattern in Fig. 11d has exhibited the peaks of α-helix and β-sheet in the keratin architecture at the 2ө of 9 and 20°, respectively, and it demonstrates the semi-crystalline nature of this protein [62].

Fig. 11.

The a macroscopic and b microscopic image of the chicken feather. c The pH isoelectric of chicken feather. d The XRD image of chicken feather. e The reusability test of the chicken feather. f The effect of pH on the adsorption. g Schematic of interaction between the metal ions and the functional groups in the chicken feather. h The impact of time on the uptake capacity

Several investigations were done to eliminate metallic particles utilizing chicken feathers. Reynel-Avila et al. investigated the simultaneous elimination of cadmium (Cd²⁺), nickel (Ni²⁺), and lead (Pb²⁺) ions from ternary aqueous systems applying chicken feathers as resin [190]. Rahmani-Sani et al. used chicken feathers to eliminate lead, cadmium, copper, zinc, and nickel particles in the polluted water [184]. Kong et al. investigated the removal of nickel, chromium, and lead ions utilizing chicken feathers [122]. Chakraborty et al. utilized chicken feathers as the adsorbent to eliminate the cobalt, copper, iron, and nickel ions [46]. Al-Asheh et al. studied the elimination of copper, nickel, and zinc ions using chicken feathers [7]. Chen et al. used chicken feathers in eliminating cadmium and lead particles [50]. Dhaouadi et al. has removed cadmium, nickel, and lead ions using chicken feather [62]. In each of the above studies, the researchers examined the different variables like pH, heat degree, and time for finding the optimum experimental condition in which the adsorbent has the higher adsorption capacity. Figure 11f and h explain the impact of pH and equilibrium time on the separation capacity of a chicken feather. With respect to Fig. 11f, the maximum removal efficiency of all metallic particles is observed pH of 5.5. At this pH, the elimination yield of lead, cadmium, copper, zinc, and nickel ions have been calculated 96.1, 93.8, 83.3, 69.6, and 50.0%, respectively [184]. The system acidity exerts its impact via the alteration of the condition of functional species existing on the plane of the resin [246]. Besides, Fig. 11h exhibits that the equilibrium contact time for copper, iron, nickel, and cobalt are determined at 60, 40, 24, and 12 min, respectively. Thus, the separation of metallic particles by the chicken feather is rapid [46].

The reusability of the adsorbents is essential in the adsorption process due to the economic aspects [150]. Figure 11e displays the regeneration ability of the chicken feather using 0.1 M NaOH for the chromium, nickel, and lead ions. With respect to this figure, uptake capacities for Ni²⁺, Cr⁶⁺, and Pb²⁺ of 86.52, 54.29, and 37.17 mg/g were obtained after the five steps. This proved that the regenerated chicken feather exhibited the excellent adsorption of Ni²⁺, Cr⁶⁺, and Pb²⁺ [122].

Fenton Modified Volcanic Ash (FMVA)

Volcanic ash is the other natural adsorbent that can be used for the removal of heavy metal ions. Jafarzadeh et al. employed the modified volcanic ash for the elimination of As(III) and As(V). The modification agent was Fenton which is ferrous iron and hydrogen peroxide. Also, three parameters have been investigated on the adsorption potential of the volcanic ash such as pH, contact time, and Fe²⁺/H₂O₂ ratio. According to their work, the Fe²⁺/H₂O₂ ratio possessed a strong influence on the adsorption of both As(III) and As(V). The optimum experimental conditions for As(V) have been acquired at pH of 5, Fe²⁺/H₂O₂ of 0.06, and contact time of 30 min. Besides, the optimum values for removal of As(III) have been obtained at pH of 2, Fe²⁺/H₂O₂ ratio of 0.06, and contact time of 30 min. Under these mentioned optimum conditions, the uptake capacity of As(III) and As(V) were found 39 μg and 41 μg per mg of the adsorbent, respectively. The reason for the modification of the volcanic ash with the iron is owing to the remarkable affinity of this element toward the arsenic moieties because generates desired adsorption channels for the arsenic via the different adsorption mechanisms such as mono- and bidentate surface complex formation, ligand exchange, and electrostatic attraction. Regarding their work, the uptake capability of the arsenic has improved by the Fenton modification. In fact, the oxygen atoms were separated in the Fenton reaction and the hydroxyl radicals with a strong affinity were produced that decomposed the organic materials which reduced the impurity of the adsorbent. The other benefits of this chemical process are low reaction time, the use of non-toxic and low-cost reactants, and facile handling. The SEM images of the volcanic ash are displayed in Fig. 12. With respect to Fig. 12a, the structure of the volcanic ash has contained many compressed low-size moieties, which makes the surface porous and rough. The morphology of the modified volcanic ash has no considerable change from the raw one. The X-ray analysis has proved the presence of SiO₂ as the major particle and Al₂O₃ and Fe₂O₃ as the minor moieties. Also, the mass fraction of aluminum and iron enhanced from %16.57 to %20.32 and from %7.10 to %17.90, respectively. The mass fraction of CaO and SiO₂ decreased after the Fenton treatment. Moreover, the Fenton reaction has reduced the organic components of the adsorbent. It was observed from the ANOVA results, that pH and Fe²⁺/H₂O₂ had a significant impact on the uptake capacity because the p-value was lower than 0.05. Figure 12b,c indicated that enhancing the Fe²⁺/H₂O₂ ratio enhanced the removal of As(III) and As(V). Figure 12 implied that time had not a significant effect on the adsorption of arsenic. Also, Fig. 12b displayed that the adsorption of As(III) enhanced at a pH of 4 but reduced at a pH of 5. Figure 12c illustrates that the adsorption of As(V) has increased as pH enhanced from 3 to 5[108]. Fenton is not only employed for modifying the adsorbents for the removal of metallic particles, while Fenton can be used as the modification reagent for eliminating the drugs such as metronidazole. For instance, Sharifi et al. used Fenton as the modification agent for the removal of metronidazole from the real wastewater [213].

Fig. 12.

The a SEM image of volcanic ash. The diagrams of MINITAB software for b As(III) and c As(V) [108]

Ceratophyllum demersum

Plants are used as the low-cost adsorbent for the removal of metallic particles and are typically modified with diverse treatment processes such as heating pretreatment, acid pretreatment, alkali pretreatment, and organic solvent pretreatment for increasing the uptake capability of the metallic ions. For instance, Teymouri et al. employed Ceratophyllum demersum for separating Cr(VI) and Cd(II) ions from the aqueous solution. They also examined 20 various pretreatment procedures. Ceratophyllum demersum is frequently observed in ponds, lakes, streams, and ditches and this plant has been put in the aquatic macrophyte which possesses a great number of nutrients. According to their work, acid and alkali pretreatments were selected as the most effective modification procedures for the removal of Cr(VI) and Cd(II) ions, respectively. The acidic and alkali agents were H₃PO₄ and NaOH, respectively. In acidic conditions, acidic agents clean the cell wall and the ionic particles are replaced with hydrogen moieties and other functional moieties which generate active places for the diffusion of HCrO₄⁻ and Cr₂O₄⁻. Furthermore, the chelation of oxygen atoms with metallic ions can increase the porosity and surface area of the biomass. Thus, because of this mentioned reason, acidic pretreatment is appropriate for the adsorption of Cr(VI) ions. Treatment with the basic solutions can dissociate the parts of the sorbent surface and also introduce many functional moieties via eliminating the particles that occupy the pores including proteins, lipids, and polysaccharides, and for this reason, alkali pretreatment was good for the sorption of cadmium ions. Figure 13a, b and c display the morphology of the Ceratophyllum demersum and modified biomass, respectively. According to Fig. 13a, the surface of the raw biomass consists of many pores with irregular shapes. Also, the acidic biomass has a smoother surface because H₃PO₄ destroyed the cell walls of the Ceratophyllum demersum (Fig. 13b). The pretreatment of the biomass with the NaOH made the surface rougher (Fig. 13c). The FTIR graph of the raw biomass has peaks at the wavelength of 3420, 2926, 1655, and 1070 Cm⁻¹ which correspond to the hydroxyl, amine, carbonyl and amide groups, respectively. After modifying the surface with the H₃PO₄, the intensity of the wavelength was significantly altered especially peaks of hydroxyl, amine, carbonyl, and amide. With respect to the elemental tests, the number of hydrogen ions enhanced after modifying the biomass with the acid. Because of the acidic state of the adsorbent, HCrO₄⁻ and Cr₂O₇⁻ are the common forms of chromium at the pH of 2. Thus, these anionic forms are attracted by the positive charges. Additionally, the FTIR results showed that the (N)-OH, -C = O, and (N)C-O bands have changed after treating with the NaOH [228].

Fig. 13.

The SEM images of a raw biomass, b modified biomass with the H3PO4, and (c) modified biomass with NaOH [228]

Modified solid waste vegetable oil

Jaafarzadeh et al. eliminated As(III) and As(V) using modified solid waste vegetable oil (SWVOI.). They examined multiple parameters including Ph, contact time, Fe²⁺/H₂O₂ ratio, initial arsenic ion amount, and dosage of the sorbent. The highest removal efficiency has been acquired % at 84 and %78 for the As(III) and As(V), respectively. Also, the most suitable pH value for As(III) and As(V) was obtained at 7 and 4, respectively. The advantage of using Fenton in the adsorbent was the short reaction time, being economical and non-toxic reactants. Figure 14a shows the morphology of SWVOI. With respect to this figure, the architecture of this adsorbent is comprised of aggregated tiny moieties with irregular forms. The elemental analysis also indicated the mass fraction of CaO, Al₂O₃, and SiO₂ after treating with the Fenton. Also, SWVOI possessed low solubility in water and acids, thus, no composition of this adsorbent leached into the water and acid. Figure 14b displays the effect of time on the adsorption of As(III) and As(V). According to Fig. 14b, the speed of the adsorption was very high for the first 30 min. Also, the saturation time has been found at 60 min. The fast sorption at the first of 30 min was owing to a large number of available sites. Figure 14c reveals the influence of the sorbent dosage on the removal efficiency. According to Fig. 14c, the removal efficiency has enhanced from %33%90 for As(III) and from %30%80 for As(V) by increasing from 0.4 to 6 g/L. Therefore, 1 g/L of this adsorbent can provide a remarkable yield which proves that the SWVOI is the best candidate from an economical point of view. Figure 14d also illustrates the impact of pH on the yield of the arsenic ions at pH values of 2 to 9. Regarding this figure, the yield of the arsenic ions was reduced by elevating pH amounts up to 7. Figure 14c also implies that the highest yield of As(III) and As(V) has acquired %84 at pH of 7 and %78 at pH of 4, respectively. In the basic media, hydroxide ions enhanced and competed with the arsenate ions for filling the vacant sites causing the reduction of the adsorption of As(V). Furthermore, the positive charge at the interface of the oxide/solution reduces by enhancing the pH amount. As the result, the columbic attraction diminishes between the arsenate particles and the surface of the adsorbent. The R_L values for As(III) and As(V) ions were written in Table 12. According to this Table, the values of R_L for both As(III) and As(V) are in the range of 0 and 1 at the entire initial concentrations [107, 109].

Fig. 14.

The a SEM image of SWVOI. The effect of b time, c adsorbent dosage, and (c) pH [109]

Table 12.

The R_L values of As(III) and As(V) [109]

Initial Concentration (ppm)	RL of As(III)	RL of As(V)
10	0.83	0.86
50	0.50	0.56
150	0.25	0.29
300	0.14	0.17

Light Expanded Clay Aggregate (LECA)

Amiri et al. modified LECA with the Fenton for the elimination of As(III) and As(V). They investigated pH, contact time, and Fe²⁺/H₂O₂. The highest yields have been determined at %96 and %99 at the initial concentration of 150 μg/L, adsorbent dosage of one gram, time of 60 min, and pH of 2 and 4 for As(III), and As(V), respectively. LECA is comprised of tiny and low-weight moieties. Also, the charge of this composite is positive at low pH and negative at high pH. They have also examined both batch and dynamic methods. In the dynamic method, the glass column with a diameter of 4.3 Cm and length of 30 Cm. Figure 15a shows the morphology of LECA. This figure contained several small particles that caused the surface coarse. Also, SiO₂ and Al₂O₃ are the main compounds and Fe₂O₃ is the minor component. After the modification, the pH altered from 8.4 to 3.5 and its surface becomes more positive. LECA has also low solubility in water and acids, thus, no compositions or particles of LECA leach into the water and acids. The iodine number was elevated from 528 mg/g to 532 mg/g implying the high surface area of LECA. The optimum conditions have been detected at Fe²⁺/H₂O₂ ratio of 0.06, contact time of 30 min, pH of 2 for As(III), and pH of 5 for As(V). The removal efficiency of As(III) and As(V) reached to %92 and %95, respectively. The effect of time on the removal efficiency has been evaluated in the duration of 30 to 180 min and drawn in Fig. 15b. According to this figure, the yield increased by increasing time and reached the saturation state after 60 min. The rate of the process was rapid for the first 30 min. Also, Fig. 15c displays the effect of the adsorbent dosage on the yield at the initial concentration of 150 μg/L. This figure describes that the yield of As(III) and As(V) was enhanced from %33 to %97, and %40 to %100, by increasing the adsorbent dosage from 0.4 to 6 g/L, respectively, which proves that 1 g/L of this sorbent acquires a high yield which is economical for the process. In order to investigate the effect of pH on the adsorption potential, the pH was adjusted between 2 and 9 (Fig. 15d). Figure 15d shows that the uptake capacity and removal efficiency decreased with enhancing pH up to 6. The highest yields have determined %96 at pH of 2 for As(III) and %99 at pH of 4 for As(V), respectively. In the case of thermodynamic relations, the ΔG of LECA was negative for both As(III) and As(V) which implies the spontaneous and desired nature of LECA. According to the Freundlich model, the k_F of the arsenate was more than arsenite that demonstrating the higher affinity of arsenate toward the sorbent. Also, the value of n in the Freundlich model was in the range of 0 and 1 indicating that the adsorption was favored. Figure 15e is the breakthrough curve of As(III) and As(V) which reveals that no arsenic was found in the effluent up to about 10 and 18 bed volumes of As(III) and As(V) [15].

Fig. 15.

a The morphology of LECA. Effect of b time, c sorbent dosage, and d pH on the removal efficiency of As(III) and As(V). e The breakthrough curve [15]

Polymers

Polymer hydrogel

Hydrogels are implied as a cross-linked, insoluble network of polymeric architectures containing homo- or copolymers with the ability to trap a high amount of water [28]. The schematic of the hydrogel structure is displayed in Fig. 16a. Figure 16a explains the formation of the 3-D cross-linking structure, in which the attapulgite (ATP) C = C nanorod had been connected with each other by the copolymer to generate the cross-linking network (blue chains), while other copolymers were only linked onto a single ATP nanorods as the polymer brushes (green chains) [134]. Hydrogels can be natural such as gelatin, alginate, starch, and cellulose-based, or acrylamide 2-hydroxyethyl methacrylate, acrylic acid, 2-acrylamide 2-methylpropane sulfonic acid-based synthetic polymers [214, 260]. Generally, the nitrogen atoms in the amide groups and OH in the carboxylic groups are the major functional groups that chelate with the metallic particles (Fig. 16h) [170]. According to Fig. 16b and c, the morphology of the hydrogel is a coarse, irregular, and porous structure that creates many adsorption places for toxic metal ions and promotes separation performance. Also, the morphology is almost rough after the adsorption of copper ions (Fig. 16b) [258]. The thermal stability of the hydrogels such as starch-based/AcA hydrogel (St-AcAH) are good and tolerate the temperature till 450℃ (Fig. 16e) [196]. Hydrogels have several benefits over common procedures of heavy metal elimination, such as having a hydrophilic network, being lighter than water, appropriateness of monomer functional species for direct fabrication, further diffusion into contaminates owe to their three-dimensional architectures, synthesizability at controllable dimensions, alter the potential of functional species, environmentally benign due to facile biologic precipitation, and the probability of regeneration owe to desorption [28].

Fig. 16.

a Structural model of the ATP/P(AA-co-AM) nanocomposite hydrogels. The SEM image of hydrogel b before & c after the adsorption. d The reusability of the hydrogel. e TGA diagram of the hydrogel. f Effect of contact time on the adsorption of heavy metal ions. g Effect of pH on the adsorption of heavy metal ions. h Chelation mechanism of metal ions with the hydrogels

The reusability of Chitosan-g-poly (acrylic acid) hydrogel is studied by Zheng et al. and the observations are illustrated in Fig. 16d, and it was concluded that the hydrogel can be used even up to 6 cycles without the significant variations in the sorption capacity of nickel particles [262]. These hydrogels utilized are typically provided from free-radical solution polymerization and the resulting product is mainly in the gel form [261]. Many investigations are done for the removal of metallic particles using hydrogel which is brought following. Sezgin et al. studied the polyacrylic acid hydrogel for the elimination of copper, nickel, zinc, and chromium from industrial effluent [210]. Saberi et al. used starch-poly (acrylic acid) nanocomposite hydrogel to eliminate copper and lead particles from the polluted water [196]. Zhao et al. studied the uptake capacity of acrylamide/acrylic acid cellulose hydrogel in eliminating the copper, lead, and cadmium particles [258]. Zheng et al. utilized chitosan-g-poly(acrylic acid) hydrogel to eliminate nickel particles in the polluted water [262].

The equilibrium time of the hydrogels is almost long. For example, Sezgin et al. have plotted the changes in adsorption capacity vs. the contact time (Fig. 16f), and it was derived that the separation was conducted fast in the first of 3 h and then reached the equilibrium after 4 h [210]. Moreover, the stability of the hydrogel is suitable at a low pH, and typically the metal hydroxides are produced at a pH of higher than 6 (Fig. 16g). In the study of kinetic models, it is observed that the pseudo-second-order rate equation is fitted well with the empirical data, which proves the chemical reaction controlled the rate of the adsorption [43].

Polyethyleneimine

Polyethylenimine (PEI), which is recognized for forming the complex with the toxic metal ions owing to the existence of a great extent of amine species per molecule, is usually applied to alter the adsorbent plane to promote the uptake efficiency of certain resin [48, 194]. It is a type of novel chelation reagent for metallic particles even if it is under the situation of the large extent of alkaline-earth metallic ions. The property of PEI has attracted the vast attention of researchers, and its utilization in adsorption separation fields of metallic ions is progressing. PEI has the disadvantage of high aqueous solubility, so it cannot be directly used as adsorbent material [116]. For example, scientists cover PEI on the planes of ion exchange adsorbent or silica gel particles to eliminate heavy-metal metallic particles from polluted water [12]. Insoluble polymers [70], biomass [61], and cellulose [164] have been applied to crosslink PEI to avoid its leaching during the sorption process. In this section, it is attempted to collect the information about the adsorbents in which the PEI is the additive to improve the removal efficiency. Bertagnolli et al. have added the PEI to the alginate for the elimination of cadmium, copper, and zinc particles. The morphology and elemental changes are displayed in Fig. 17a and b [34]. The SEM indicated that the PEI/Alginate has a more homogenous distribution. The chemical mechanism (reduction) is typically conducted between the PEI and the other materials like graphene oxide (GO). For a better interpretation, the mechanism is illustrated in Fig. 17c, in which the GO has the carboxylic and hydroxyl species, and the PEI are chemically joined to the oxygen-containing functional species of GO like carbonyl, epoxy, etc. via the reduction process in the alginate matrix, in which enhance the chelation of metal ions (Fig. 17c). The benefit of the PEI is its rapid adsorption rate which reduces energy consumption and cost. According to Fig. 17d, the saturation time for the elimination of copper, zinc, and cadmium is found at 10 min and implying the high affinity of the metallic particles toward the PEI [173]. The reusability property of PEI has been observed in favor. For example, Hu et al. prepared a diagram to examine the regeneration of PEI in the adsorption (Fig. 13e). They reported that the rate of desorption has decreased after the three steps, but the extent of reduction is not significant [100]. Moreover, the introduction of PEI to some polymers can increase the adsorption capacity significantly. Zeng et al. modified carboxymethyl chitosan with the PEI to eliminate mercury, copper, cadmium, and lead ions. It was detected that the uptake capability of the metallic particles especially mercury ions is high (Fig. 17f) [256]. The other consequence that can be derived from this figure is the selectivity of PEI is different for the different metal ions [256]. Besides, pH is an important factor in the adsorption capability of copper ions. It was derived that the adsorption capability decreased by increasing the pH values at a pH lower than 6, and the maximum uptake capability is found at pH of 6 (Fig. 17g) [84]. In acidic media, most of the N atoms of the amino species in the macromolecular chains of PEI are in a protonation state, the protonation degree of N atoms of amino groups reduces with the decline of acidity, and the coordination potential of N atoms of amino species towards metallic particles powers, so that the uptake capability of copper enhances with the promoting of acidity amount [11].

Fig. 17.

The SEM images of a Alginate & b PEI/Alginate. c The chelation mechanism between the PEI and metal ions. d Impact of time on the adsorption efficiency. e Reusability of PEI. f Uptake capacity of PEI on the different metal ions. g Effect of pH

Chitosan

Chitosan (Fig. 18a), (1 → 4)-2-amino-2- acet- amido-2-deoxy-β-D-glucan is produced from partial N-deacetylation of chitin with a strong alkali solution such as sodium hydroxide. Chitosan is considered to be a copolymer of N-acetyl-D-glucosamine and D-glucosamine [240]. It has a unidirectional porous structure which helps with the diffusion of the metallic particles, and the pore size is about 200 μm in length and 50–100 μm in width [97]. Chitosan is a unique polysaccharide with an alkaline basis in nature and it was obtained from chitin by deacetylation reaction, which was generally applied for elimination of metallic ions [56] because of some benefits including being economic, extensive-sourced, and easy degradation. Especially, amino and hydroxyl species (Fig. 18b) are the most common functional groups in the chitosan structure which is complex with the metallic particles [254]. Nevertheless, the uptake capacity of chitosan has not attained an adequate value and its extensive utilization was vigorously limited by its low water solubility, and vast acid-solubility [38]. Therefore, some investigators have exploited physical and chemical modification for improving the uptake capacity of metallic particles such as chromium [135], lead [264], arsenic [241], copper [113], mercury [245], and etc.

Fig. 18.

a Structure of chitosan. b The connection of cations with the hydroxyl and amine groups of chitosan. c The SEM image of chitosan. The effect of d time, e pH, and f dose on the removal efficiency

Same as the other adsorbents, the experimental conditions have influenced the chitosan performances such as equilibrium time, acidity, sorbent amount, additives, etc., some of these factors are explained following.

The sorption procedure needs various steps like solute transfer from the liquid to the adsorbent, binding on the plane of the sorbent, and penetration within the empty hollows (Fig. 18c) [152]. Figure 18d shows the impact of the equilibrium time on the removal efficiency of the modified chitosan. It is derived that the removal yield of lead, copper, nickel, zinc, and cadmium ions reached the equilibrium state after 45 min, and their removal efficiency is high in the almost short contact time which is economic [105]. Also, the rapid sorption at the first step may be owing to the sufficient number of accessible places at the plane of the sorbent [104]. Figure 18e exhibits the change of the removal efficiency with the acidity values. According to Fig. 18e, as pH increases the elimination yield of copper and nickel particles enhances from 42 to 92% and 13% to 58%, respectively. At a low acidity amount, different functional groups in chitosan were protonated, and hence electrostatic repulsion was created between chitosan chains and the metallic particles, causing a poor bonding [121]. The other important factor is an adsorbent dosage which is illustrated in Fig. 18f. Figure 18f shows that the rate of separation exhibited a sharp enhancement as the extent of the modified chitosan varied from 0.02 to 0.2 g/L. In this concentration range of modified chitosan, the number of active places was increased, and the lead ions were rapidly adsorbed at the binding sites. When the modified amount enhanced from 0.2 to 0.5 g/L, a slight change in the removal efficiency was found; indeed, when the majority of lead ions were adsorbed by the sorbent, the number of unoccupied active binding sites, which would no longer contribute to the removal percentage, was increased. Besides, a high dosage could cause adsorbent aggregation and a further reduction in the removal efficiency of sorbent [211, 263].

Cyclodextrin (CD)

β-CD is a cyclic oligosaccharide that many researchers have focused on this polymer because of its exclusive features such as being inexpensive [215]. Its unique network of hydrophilic exterior and hydrophobic interior returns to selectively connect different size-matched compounds via Van der Waals forces and hydrogen bonds to form host–guest inclusion complexes [51]. Meanwhile, β-CD has many primary hydroxyl species on its plane and secondary hydroxyl species in the pores (Fig. 19a) which are easily altered by different functional groups. Moreover, the derivatives of CD have special functional groups such as carboxyl for chelating the metal ions[26].β-CD has the nontoxicity, is easily accessible and resistant in the chemical network which causes it becomes appropriate for its nature and the biocompatible option to synthesis sorbents for eliminating pollutants from effluent [182]. However, its use is limited in formulations due to low aqueous solubility and toxicity [222]. For solving this problem, hydrophilic derivatives of β-cyclodextrin like hydroxypropyl-β-cyclodextrin (HP-β-CD), sulfobutylether-β-cyclodextrin (SBE-β-CD), and randomly methylated-β-cyclodextrin (RM-β-CD) have been investigated [143, 202]. According to the micrograph of CD, it is concluded that it contains many nanorods (Fig. 19b) with an average size of 45 nm. Also, the BET analysis exhibits that its surface area is calculated at 185 m²/g. Hence, the network properties facilitate the solvent transferring through the adsorbent and prepare desired access of contaminations which promotes the removal yield [225]. The zeta potential in Fig. 19c shows that the pH_zpc for CD is 4.7. On the other hand, at pH lower than 4.7, the adsorbent surface protonates and becomes more positive which is unfavorable for adsorbing the metal ions [145]. To study the separation potential at various times, the adsorption kinetics of lead ions on β-CD polymer were investigated and displayed in Fig. 19d. The separation process occurred fast and reached a saturation state in 5 min, and it is derived from the economic aspect of the CD as the adsorbent [96]. The other effective and crucial parameter is an adsorbent dosage which indicating in Fig. 19e. According to this figure, the uptake capacity of cadmium, lead, and copper particles slowly reduce from 228.91–49.03 mg/g, 505.60–63.29 mg/g, and 188.25–49.86 mg/g, respectively. A probable response to this trend is explained that the enhancement in the uptake amount produces excessive effective groups beyond the capacity to be accepted by the metallic particles. Furthermore, the competition for active places on the sorbent between water and the metallic particles causes an enhancement in water uptake and a reduction in the elimination speed of the metallic particles [101, 162]. As reported in more manuscripts, examination of the reusability of the adsorbents is essential in practical applications. Qin et al. evaluated the reusability of cross-linked CD in the mixture of metallic particles and dyes, and the obtained empirical data are depicted in Fig. 19f. The regeneration reagent for the dye and metal ions were ethanol and sulfuric acid, respectively. After five steps, the uptake capacities of dye and Cd(II) decreased to 91.1% and 89.9% of the original values respectively (Fig. 19f), indicating that the sorbent could be recycled without a significant reduction in potential [180]. Finally, Fig. 19g exhibits the removal efficiency and contest of metallic particles in the single, binary, and ternary solutions. It is proved that the elimination yield of metal particles has decreased by increasing the metal ion types in the solution. The reduction in uptake efficiency of the same sorbent in a multi-metal system than that of a single metal ion may be attributed to the less accessibility of chelation places. In the case of a multi-metal solution, when metals contest for the same sorption locations of a sorbent, metals with a higher dependency could substitute others with weaker affinity [27, 179].

Fig. 19.

a The structure of CD in (I) general structure, and (II) the arrangement of OH and CH groups on the trauncated cone (left) and the number of atoms in glucopyranoside ring (right). b SEM graph of CD. c The zeta potential of CD. d The effect of contact time. e The effect of the adsorbent dosage. f The reusability of CD. g Comparison of the removal efficiency in the single, binary, and ternary systems

Adsorption Mechanisms

Mineral materials

Typically, three various mechanisms are involved in the adsorption of metallic particles toward the mineral materials including a) ion exchange, b) presence of pores, and c) precipitation. The ion exchange mechanism is the exchange of the oxygen and silicon of the functional groups with the metallic ions. In the case of the ion exchange process, the work of Chen et al. confirmed the participation of silicon and oxygen in the adsorption process by the XPS graph (Fig. 20). Figure 20 reveals peaks at 531, 24, 438, 346, 152, and 102 eV which are ascribed to the O1s, O2s, Ca2s, Ca2p, Si2s, and Si2p, respectively. Also, the bands at 38.2 eV and 143.4 eV corresponds to the Pb 4f 7/2 and Pb 4f 5/2, respectively, and the peaks at 406.3 eV and 411.6 eV which are corresponded to the Cd 3d 5/2 and Cd 3d 3/2, respectively. One of the mechanisms of the mineral materials returns to the existence of several pores in their skeletons which were produced owing to the hydrolysis reactions. Regarding Eqs. (2) and (3), the silanol and aluminol sites were hydrolyzed and the hydroxyl groups were released and more active sites were produced. Also, precipitation of metallic ions in neutral and alkaline conditions is the other mechanism that is written in Eqs. (4) and (5) [49].

$${\text{Ca}}_2{\text{Al}}_2{\text{SiO}}_7+{\text{5H}}_2\text{O}\to {\text{SiO}}_2+{\text{2Al(OH)}}_3+{\text{2Ca}}^{2+}+{\text{4OH}}^{-}$$ 2

$$\text{2CaO}{\text{.SiO}}_2+{\text{2H}}_2\text{O}\to {\text{2Ca}}^{2+}+{\text{H}}_4{\text{SiO}}_4+{\text{4OH}}^{-}$$ 3

$$\text{Si}-\text{OH}...\text{H}-\text{O}-{\text{H[Me(OH}}_2{\text{)}}_3{\text{]}}^{2+}\to \text{Si}-\text{O}-\text{Me}+{\text{H}}_3{\text{O}}^{+}$$ 4

$${\mathrm{Me}}^{2+}+{\text{OH}}^{-}\to \text{Me}{(\text{OH})}^{+}+{\text{OH}}^{-}\to \text{Me}({\text{OH}}_2)$$ 5

Fig. 20.

The XPS graph of SMA (synthetic mineral adsorbent) [49]

Low-cost materials

General speaking, the adsorption mechanisms of metallic particles onto the low-cost adsorbents are physical adsorption, chemisorption, electrostatic interactions, hydrogen bonding, pi-pi interactions, and precipitation (Fig. 21a). Figure 21a also explains that the waste substances have diverse functional moieties that present on their planes which have typically negative charges such as amine, carbonyl, carboxyl, and hydroxyl groups. In fact, in the case of functional groups, the contaminants connect to the plane of the adsorbents via the electrostatic attractions between the positive charges of the metallic ions and the negative charges of the functional groups. For instance, the adsorption mechanism of chromium and copper ions onto the soybean meal (SBM) waste was the adhesion of metallic ions on its functional moieties. Figure 21d is also showed that the hydroxyl and amine groups were the chelation agents. Additionally, Van der Waal and hydrogen bindings have been determined as the other attractive forces which are involved in the adsorption of metallic pollutants onto the surface of the low-cost adsorbents (Fig. 21b and c). The other important mechanism in the adsorption of the metallic ions toward the low-cost materials is the complexation in which the metal bounded by ligands occupied the central position, called mononuclear complexes. Thus, it is concluded that the electrostatic reactions were the main mechanism for the low-cost materials [35].

Fig. 21.

a The different types of the adsorption mechanisms for the low-cost adsorbents. b Adsorption mechanism of biochar on heavy metal ions in water. c Schematic diagram of six adsorption mechanisms of bio-sorbents, and d Proposed sorption mechanism of Cu(II), Ni(II), Co(II), and Fe(II) onto modified chicken feathers [35]

Microbes

In nutshell, two factors impress on the mechanism of the microbes the kind of the cell and the shapes of the cell. Moreover, the mechanism of microbes in trapping the metallic ions divides into two distinct groups: a) metabolism-independent, and b) metabolism-dependent. The first above case refers to the adhesion of metallic particles to the cell wall which typically is an immediate step. The latter case is intracellular adsorption. Some examples were explained following about the metabolism-independent. For instance, the metabolism-independent in the bacteria with the species of gram-positive is their walls which act as the chelator such as Bacillus Subtilis in which the carboxyl groups of the glutamic acid of their peptidoglycan was the main position for adsorbing the metallic ions. Also, the teichoic and teichuronic acid was the active sites of the Bacillus Licheniformis. In the gram-negative bacteria classes, two different membrane bilayers compress a thin peptidoglycan film between them in the periplasmic space. Escherichia Coli was put in the groups of the gram-negative in which the chelation occurred at the polar head group regions of constituent membranes. The metabolism-independent in the algae is the cell walls and extracellular reagents which are recognized as the appropriate adsorption sites. Generally, the ionic moieties adhere to the proteins and polysaccharides of the algae. Ultimately, the metabolism-independent in the fungi and yeast returns to the chemical reagents in their skeleton. For instance, R. Arrhizus has chitin in which the amine nitrogen of it acts the chelation. Also, the phosphate and carboxyl moieties in the cell walls of the yeast serve as the chelation agent. In the case of the metabolism-dependent, the membrane performance of the cells plays an important role in trapping the metallic particles such as electrophoretic mono- and divalent cation transport in fungi in the presence of other materials e.g. potassium ions [247].

Polymers

For surveying the adsorption mechanism of the polymers, the work of Azad et al. has been studied. They investigated the performance of the PVB-PVA blend for the removal of heavy metal ions. They indicated that the physical and chemical interactions were involved in the adsorption mechanism (Fig. 22). Figure 22 verifies the physicochemical mechanism for the trapping of copper, cadmium, and lead ions on the plane of the composite which was ascribed to the surface complexation and Lewis acid–base contact between the metallic particles and hydroxyl moieties. The other example of physical adsorption was the electrostatic interactions between the carboxyl moieties and metallic particles. Moreover, the porous tunnel shape architecture leads to create plenty of vacant sites in which the metallic ions transport to them via the diffusion process. Regarding the above cases, Azad et al. resulted that the surface complexation and the electrostatic interactions were the governed mechanisms in the polymers [22].

Fig. 22.

The adsorption mechanism in the polymers [22]

Conclusion

In this review article, various types of adsorbents in removing heavy metal ions from the wastewater were studied. It was concluded that the polymers, especially chitosan, have shown the highest uptake capacity for the elimination of each kind of metallic particle among the other sorbents. Moreover, the reusability of the polymers was suitable, which appears to be the economic aspect of the polymers for application on a commercial scale. The reasons for the excellent adsorption capacity return to the excellent surface area of the sorbent, and more effective functional groups. For example, chitosan has amine and hydroxyl which can be complex with metal ions, the polymer hydrogels have many amide and carboxyl groups, and cyclodextrin polymers have a large number of hydroxyl groups and some carboxyl moieties. It is noteworthy that each of these mentioned polymers has some superiority relative to others. In the view of reaching the equilibrium state, this order was observed: CD > PEI > Chitosan > Hydrogel. In the regeneration aspect, polymer hydrogels were the best because they can be used in up to 6 steps. Among these polymers, CDs have the lowest price. In view of thermal stability, polymer hydrogels were the best candidate because they can retain their features even at 450℃. With respect to the uptake capacity of diverse types of polymers, the following arrangement was proposed: Chitosan > CD > Polyethyleneimine > Hydrogel, because it was observed that the value of uptake capacity of chitosan, CD, polyethyleneimine, and polymer hydrogel was up to 200, 178, 174, and 14 mg/g, respectively. It was also concluded that each of these polymers has some drawbacks. For example, high saturation time, high solubility, and vast-acid solubility are the main drawbacks of polymer hydrogel, polyethyleneimine, CD, and chitosan, respectively, which can be solved with the aid of researchers in the future. Also, modifying or crosslinking procedures can dramatically enhance the uptake capacity of the polymers due to their appropriate chemistry structures. In contrast, the adsorption capacity of the other adsorbents such as the mineral materials, microbes, and plants could not be considered changed, because they have a low surface area and lower influential functional groups relative to the polymers, despite being economical and eco-friendly. Besides, the isotherm models described that the adsorbents obeyed the Freundlich model which proves the adsorbents are multi-layer and heterogeneous in nature. The kinetic relations expressed that most of the adsorbents followed the pseudo-second-order model, which explained that the dominant behavior in most adsorption processes was chemical separation. According to the results, the waste materials attained the equilibrium state, rapidly. Also, with respect to the derived results, the different metallic particles had different uptake capacities due to their various ionic radius. Finally, it is proposed the modified waste polymeric adsorbents for two reasons; first, they can remove most of the heavy metal ions, and second, enhancing the accumulation of the waste materials from the ecosystem.

Authors' contributions

Hadiseh Masoumi: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing—original draft, Writing—review & editing, Supervision Visualization. Ahad Ghaemi: Supervision, Funding acquisition, Software, Validation, Formal analysis, Investigation, Resources, Writing—review & editing, Visualization. Gannadzadeh Gilani: Methodology, Conceptualization, Supervision, Conceived and designed the experiments, Validation, Formal analysis, Investigation, Resources, Writing—review & editing, Visualization.

Data availability

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Declarations

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Competing interests

The authors declare that they have no competing interests.