Facile synthesis and application of cellulosic coagulant from banana peels in cadmium‐spiked water

Abstract

Cellulosic coagulant with low crystallinity and surface charge of −19.2 mV were extracted from wet banana peels (WBE) using kitchen‐blending method. Functionalization with ferric chloride and aluminium chloride yielded higher surface charge of −23.8 mV (mWBE). Both WBE and mWBE coagulants were used to target cadmium ions from aqueous solution. Coagulants and the floccules (WBEA and mWBEA) were characterized by XRD, FT‐IR, zeta sizer nano series, and SEM/EDs. The amount of cadmium ion coagulated was determined using ICP‐OES. The FTIR analysis revealed the functional groups involved in the coordination and subsequent removal of the metals ions around 1634 cm⁻¹, ascribed to the C = O vibrational band of carbonyl group. Microscopic analysis revealed that the mWBE is porous and exhibited microfibers with rod‐like morphology. The effects of parameters such as the initial concentration, coagulant dosage and solution pH were investigated. Coagulation results showed that 10 mg of WBE and mWBE could remove about 80% and 90% of the Cd²⁺ ions respectively. However; the difference in the performance of both materials does not justify the essence of surface modification. Therefore, WBE is considered more efficient and environmentally friendly. Notwithstanding, the performance of these coagulants in real environmental samples will confirm their robustness.

1 Introduction

The prevention of toxic metals from reaching downstream and the receiving waters has become a centre of research interest for decades. One of the toxic metals that pose major public health to society through water contamination is cadmium, especially in South Africa. High levels of the cadmium have been found in South African soil and water by some researchers, and this is due to the excessive use of fertiliser contaminated with this trace element [1, 2]. The result also confirms the higher concentration of Cd beyond the allowable limit, therefore total health risk for human and animals was evaluated and it was found to be highly disastrous. Cd is a common component in many electronic devices, which includes batteries, solar cells, paints and pigments, however, it can also emanate to water sources through industrial waste and run‐offs [3]. Cadmium is a confirmed carcinogen, mutagen, and teratogen [4, 5] targeting liver, kidney, lungs or gastrointestinal tract upon accumulation. Therefore, efficient water treatment processes are required to be adopted to ensure that adverse effects on the environment are eradicated by reducing Cd to the minimum allowable discharge limit.

Water treatment methods such as physical, chemical and biological techniques used to alleviate the negative environmental impact of toxic metals and pollutants are in existence. However, the major limitation of these techniques depends on the choice of inappropriate water treatment media [6, 7, 8]. For example, coagulation/flocculation is a century old and proven technology [9]; however, the choice of coagulant usually determines the success of this technique. Some of the available commercial and chemically synthesised coagulants, which include aluminium sulphate and lime, are highly efficient especially in the coagulation of organic matter, turbidity reduction and even in the removal of metal ions; still, the challenge faced by leaching of chemicals into the processed water requires further studies.

Fibrous forestry industry wastes such as sugarcane bagasse, pineapple peels, coconut coir, apple and banana peels [10, 11] reported as potential media for the removal of several contaminants from wastewater [12, 13]. This is possible due to the presence of some organic groups such as carboxylic, phenolic groups, citric and cellulose, which are considered to be highly efficient in the water treatment. Therefore, the use of naturally synthesised coagulants such as extracted cellulose from biomass could be a good replacement to chemically synthesised products. Furthermore, it has been reported that the higher the percentage of cellulose in plant waste, the higher the efficiency of the material for metal coordination [14].

Cellulose is a glucose polymer bounded in the β ‐1, 4‐linkage configuration and it is the most abundant organic polymer on earth [15]. The cellulose content in banana peels is higher than other components present in this agricultural resource. As such, the major structural components of banana peels are cellulose (75%), nanofibres (5.1%) and carboxylic compounds (6%) [16]. Previous studies confirmed the efficiency of banana peels adsorbents in toxic metals removal [17, 18, 19]. The presence of the carboxylic compound in banana peels can also enhance the coagulation process [20]. Therefore, the synthesis of coagulant from this agricultural waste is worth exploring.

The objective of this present study was to reveal the potential of banana peels extract as a green‐synthesised coagulant for cadmium removal from polluted water. Consequently, a slight modification of this material with Al and Fe was also explored to improve its coagulation capacity. The characterisation of the unmodified, modified coagulants and the flocs obtained during the experiment were investigated.

2 Material and methods

2.1 Materials

2.1.1 Preparation of cadmium synthetic water

The preparation of cadmium synthetic water involves decomposition of cadmium sulphate in deionised water. Using this method, 1.79 g of cadmium sulphate was added to 200 ml of distilled water and the mixtures were shaken for 10 min. Afterwards, the Cd concentrate obtained was diluted with 800 ml of distilled water in 1000 ml volumetric flask. The pH of the solution was then adjusted with 0.1 M NaOH or 0.1 M HCl to the desired pH values. Other chemicals used in this study such as AlCl₃, FeCl₃ and sodium bicarbonate were purchased from Merck Chemicals Ltd. South Africa. All chemicals used were of analytical grade.

2.2 Methods

2.2.1 Preparation and modification of cellulosic coagulants

Banana peels (Musa paradisiaca L) were gently removed from its fruit; it was double washed with deionised water to remove the adhering dirt and then cut into smaller pieces. The wet banana peels were blended for 20 min at 400 rpm using deionised water as a process control agent. The peel particles were separated from cellulose using <65 µm sieve and then stored in the refrigerator [wet banana peel extract (WBE)]. Cellulose extracted was then modified with Al and Fe in 1:1 using sodium bicarbonate to speed up the reaction [modified WBE (mWBE)]. The different molar ratio of Al and Fe was investigated to determine the optimum modification regime. The similar method of modification using Al and Fe was reported elsewhere [21].

2.2.2 Characterisation of cellulosic coagulants

Functional groups, mineralogical properties, charges and elemental composition of the coagulation materials were determined using Fourier transform infrared (FT‐IR) spectroscopy, scanning electron microscopy (SEM), zeta potentiometer, X‐ray diffraction (XRD) and energy‐dispersive X‐ray spectroscopy (EDS) analyses. The composition of these cellulosic coagulants was analysed before and after modification to ascertain the changes in chemical stability. The FT‐IR analysis was done using a Perkin Elmer Spectrum 100 spectrometer with the spectra recorded in the range of 500–4000 cm⁻¹ at a resolution of 4 cm⁻¹. The coagulants zeta potential analysis was done using zeta sizer nano series (Malvern instruments). The XRD results were recorded on a Bruker‐AXS D8 advance diffractometer, using a quartz sample holder, with Cu Kα radiation (λ  = 0.15406 nm; 40 kV, 40 mA and an increment of 0.01°). The morphology and composition of the sorption media were characterised by SEM/EDS using a JEOL JSM‐7600F field emission SEM, running at an accelerating voltage of 2 kV. To avoid charging effects during observation; an ion‐sputtering device was used for fine gold coating of the samples. The samples used for all the material characterisation analyses were air‐dried for 2 days before the commencement of these analyses.

2.2.3 Coagulation experiment

Jar tester in a six‐gang stirrer conducted coagulation experiments. Certain amounts of coagulant were added to six different reactors containing 500 ml of Cd synthetic wastewater. The test was conducted by rapid mixing of the solution for 3 min at 200 rpm followed by 20 min of slow mixing at 60 rpm at room temperature. After the mixed samples had settled without any agitation for 30 min, samples were taken from 2.0 cm below the surface of the processed water for analysis. The coagulation experimental procedure depends on the matrix of the water to be treated; however, the method adopted in this study was reported elsewhere based on the similarity in the polluted water matrix [22]. Each coagulation test was run in triplicate and the result reported is the arithmetic average result of the three. The flocs were freeze‐dried for further characterisation analyses.

3 Results and discussion

3.1 Characterisation of cellulosic coagulants

3.1.1 FT‐IR spectroscopy

The FT‐IR spectra of unmodified cellulosic coagulant (WBE), modified coagulant (mWBE), unmodified after‐coagulation form (WBEA) and modified after‐coagulation form (mWBEA) are shown in Fig. 1. The bands at 3286–3386 cm⁻¹ are due to O–H stretching while the C–H stretching is identified at 2113 cm⁻¹. The band at 1634 cm⁻¹ is attributed to the C = O bond of carboxylic acid. The wavelength of this band becomes lower in (WBEA and mWBEA) suggesting that this functional group might be responsible for the removal of metal ion from water [23]. The band at 1416 cm⁻¹ corresponds to the stretching vibration of CH₂ and it is known as crystallinity band in cellulose due to its deformation from crystalline fractions of cellulose [24]. The wavelength of this band disappears in the spectrum of modified cellulosic coagulant suggesting the reduction in crystallinity of modified coagulant (mWBE) due to the interaction between modifier (Al/Fe) and CH₂.

Fig. 1.

FT‐IR spectra of unmodified coagulant (WBE), modified coagulant (mWBE), unmodified after‐use form (WBEA) and modified after‐use form (mWBEA)

3.1.2 Scanning electron microscopy/Transmission electron microscopy

The morphological properties of green‐synthesised cellulosic coagulants and flocs obtained after coagulation composite were investigated by SEM analysis and the results are presented in Figs. 2 a–d. The micrographic scan of WBE in Fig. 2 a shows that the particles were clustered with no significant appearance of fibres, but an onset appearance of fibres could be noticed on the surface of WBEA with the irregularity of shape. The modification of WBE to mWEB has a substantial effect on the morphologies of this material, although the particles on mWEB are also clustered but brighter and denser due to the presence of Al/Fe on the surface of this material. The porosity of this material was observed on mWBEA form and the pores are uniformly distributed across the samples with a homogeneous rough surface. The disappearance and reappearance of fibres on WBE and WBEA might be due to cohesive and adhesive forces between the fibres and water. Cohesive forces between the fibres keep them intact before coagulation while adhesive forces between the fibres and water keep the fibre apart in the flocs obtained after coagulation; hence, they are visible [17, 18, 19, 20, 21, 22, 23, 24]. TEM analysis displayed pseudo‐spherical appearance of mWBE compared with WBE which was dot‐shaped in Figs. 3 a and b. Unmodified cellulosic coagulants are monodispersed shape with an average particle size of 4.60 nm.

Fig. 2.

SEM images of

(a) Unmodified coagulant (WBE), (b) Unmodified after‐use coagulant (WBEA), (c) Modified coagulant (mWBE), (d) Modified after‐use form (mWBEA)

Fig. 3.

TEM images of

(a) Unmodified coagulant (WBE), (b) Modified coagulant

3.1.3 Energy dispersive spectroscopy

Elemental composition analysis conducted displayed major elements in WBE, WBEA, mWBE and mWBEA (Table 1). These elements include C, O, N, H, Si, Al, Fe, Ca, Cl and K. Additional minerals such as Al, Fe and Cl that were introduced as modifiers can be noticed in mWBE, which improve the performance of this material. The chemical interaction between these coagulants and Cd²⁺ was noticed in WBEA and mWBEA elemental compositions due to the appearance of this metal ion on the surface of flocs obtained after the coagulation process. These coagulants are rich in carbon, oxygen and nitrogen with higher percentages compared to other elements, therefore, could be regarded as organic materials.

Table 1.

EDS analytical data of cellulosic coagulants materials

Element, wt%	WBE	WBEA	mWBE	mWBEA
C	47.76	59.07	44.82	53.11
O	25.62	15.72	14.92	27.06
N	10.49	2.53	10.22	—
H	5.92	—	—	—
Al	1.95	1.30	5.42	0.60
Si	1.77	2.72	1.91	—
K	0.22	1.14	8.46	1.96
S	1.40	1.49	1.45	0.69
Ca	4.44	2.81	4.10	0.39
Cu	0.43	1.78	0.49	—
Cd	—	10.9	—	14.73
Fe	—	—	6.98	0.97
Cl	—	—	1.23	0.50
Pb	—	−0.40	—	—
total	100	100	100	100

3.1.4 XRD analysis

XRD patterns of WBE, WBEA, mWBE and mWBEA can be seen in Fig. 4. It can be observed that these materials displayed an amorphous character with a certain degree of crystallinity, which can be seen as a bump at 2θ angle between 10 and 30 most especially in the after‐coagulation form (WBEA and mWBEA). This is indicating that the material becomes purely amorphous due to the increase in the particle size formed (flocs) during the coagulation process [25]. As it was reported in our previous study that reduction of banana peels particle size increases its crystallinity, it can be confirmed that the size of coagulant particle (<65 µm) is responsible for the obtained amorphous properties [17]. The reduction or disappearance of phase (C₈₄ H₇₂ N₁₆ O₁₆) was also noticed on the XRD scan of mWBE, obviously due to the surface modification with Al/Fe. The disappearances of Ca₂ and C₆ Ca₆ O₁₈ was also noticed on the patterns of after‐coagulation materials (WBEA and mWBEA), suggesting the participation of this phase in the removal of cadmium ion from simulated water.

Fig. 4.

XRD pattern of unmodified coagulant (WBE), modified coagulant (mWBE), unmodified after‐use form (WBEA) and modified after‐use form (mWBEA)

3.1.5 Zeta potential analysis

Zeta potential describes the stability and the surface potential of the material. The particles are adjudged stable when their zeta values are more positive than +30 mV or more negative than −30 mV [25, 26, 27]. In the two synthesised coagulants (WBE and mWBE), the zeta potential values are −19.2 and −23.8 mV, respectively, as shown in Table 2. These values reflect less stability of the coagulants but the negativity of the values showed the coagulants were extracted by the plant biomolecules and also stabilised, although not very efficiently. Moreover, the type of plant used in the coagulant synthesis also affects the values of the zeta potential. The knowledge of the degree of stability of nanoparticles aids in their application thus, highly electronegative material can attract contaminant of positive charge and vice versa.

Table 2.

Summary of unmodified and modified coagulants zeta potential parameters

Sample name			Zeta potential, mV	Zeta deviation, mV	Conductivity, mS/cm	Standard deviation, mV
(a)	WBE	peak 1	−19.2	9.47	0.711	6.43
		peak 2	0.00	0.00	0.00	3.07
		peak 3	0.00	0.00	0.00	0.00
(b)	mWBE	peak 1	−23.8	6.03	0.347	6.03
		peak 2	0.00	0.00	0.00	0.00
		peak 3	0.00	0.00	0.00	0.00

3.2 Coagulation results

3.2.1 Effects of Al/Fe molar ratio on coagulant performance

Green‐synthesised coagulant was modified with aluminium and iron as described above. The influence of modifier molar ratio was investigated to ascertain the best modification regime to improve the coordination of metal ions (Cd²⁺) from aqueous solution. As such, three different ratios of Fe/Al (1.1, 2:1 and 1:2) was investigated in Fig. 5.

Fig. 5.

Effects of Al/Fe molar ratio on Cd removal from aqueous solution (pH, 7.24; initial conc., 20 mg/l; volume, 500 ml)

The higher performance was observed with the ratio of Fe/Al (1:1) and a very low removal of Cd²⁺ was obtained using Fe/Al (1:2 and 2:1), indicating that the performance of this material shares an indirect relationship with Al and Fe.

3.2.2 Effect of cadmium solution pH

Solution pH control is very necessary for any coagulation because the rapid mixing and the increase in hydroxyl ions can induce precipitation of metal ion, which can remove or reduce metal ions in the solution without adding the coagulant. In this case, Cd²⁺ concentration of 20 mg/l was reduced to 18.7 at pH 7.1, therefore little or no removal was observed in the pH control experiment. The increase in Cd removal was noticed with an increase in pH (Fig. 6). WBE and mWBE removed very little Cd²⁺ at pH2, a gradual increase of Cd²⁺ removal was observed with an increase in pH up to 7.10 indicative of the interaction of the cationic Cd²⁺ species with the hydrophilic surface of these coagulants. It could be seen that the removal of this metal ion is directly proportional to the increase in pH therefore; neutral pH (7.10) was considered as the optimum for efficient reuse of process water. Adsorption and charge neutralisation of Cd²⁺ on to the hydrophilic surface of WBE and mWBE could be the mechanisms involved in the coagulation process [28, 29].

Fig. 6.

Effects of solution pH on unmodified (WBE) and modified (mWBE) cellulosic coagulants on Cd removal from aqueous solution (coagulant dosage, 20 mg; initial conc., 20 mg/l; volume, 500 ml)

3.2.3 Effect of dosage

Fig. 7 shows the effect of coagulant dosage using both unmodified and modified cellulosic coagulants. It could be observed that the coagulation performance of modified cellulosic coagulant mWBE is better than that of unmodified WBE at the same dosage, which demonstrated that the modified coagulant is more efficient compared to unmodified. The enhanced coagulation performance in modified mWBE might be due to the strong interaction between the cadmium and the modifier, which resulted in modifier–pollutant complexes that could be retained in the flocs [28, 29, 30]. A dosage of 20 mg WBE and mWBE was responsible for 90 and 80% removal of Cd²⁺, respectively; the increase in the performance of mWBE is negligible, therefore it can be considered unnecessary. Charge neutralisation might be a dominant mechanism other than absorption bridging and sweeping coagulation.

Fig. 7.

Effects of modified (WBE) and unmodified (mWBE) cellulosic coagulants dosage on Cd removal from aqueous solution (initial conc., 20 mg/l; pH, 7.10; volume, 500 ml)

3.2.4 Effect of initial concentration

The cadmium initial concentration was examined by varying concentration from 5 to 100 mg/l. It was observed that as the initial concentration increases, the removal efficiencies decrease in both cases (Fig. 8). For instance, the removal efficiency of Cd onto mWBE decreases from 93 to 74% as the initial concentration increases from 5 to 100 mg/l. This is because the driving force was very fast at higher concentration due to the excess availability of active sites, but later, a constant removal capacity was observed, which means that active sites are limited. Similar outcome was reported earlier [31].

Fig. 8.

Effects of Cd initial concentration using WBE and mWBE cellulosic coagulants (dosage, 20 mg; pH, 7.10; volume, 500 ml)

4 Conclusion

Overall, it can be concluded that the naturally synthesised WBE is more eco‐friendly, efficient, facile and can even be used to purify domestic water. Although modification of this coagulant has a substantial effect on the density and morphologies of this material; the increase in the Cd²⁺ removal is negligible, therefore, it can be considered unnecessary to inflict more damage to processed water by adding more chemicals (Al/Fe) to the media. For instance, 20 mg dosage of WBE and mWBE was responsible for 80 and 91% removal of Cd²⁺ from an initial concentration of 20 mg/l at pH 7.10. However, more studies may still be necessary using domestic and environmental water containing toxic metals from different sources to assess the robustness of WBE as a coagulant.