An extensive review on chromium (vi) removal using natural and agricultural wastes materials as alternative biosorbents

Abstract

Several conventional techniques for heavy metals decontamination for instance ion exchange, evaporation, precipitation and electroplating have been utilized in preceding years. Though these techniques have some drawbacks, adsorption using low-cost biosorbents is environmentally friendly. In this study, the potential of several natural and agricultural wastes as economical biosorbents for the reduction of Cr(VI) ions from polluted water has been reviewed. The application of adsorption models, as well as the impact of adsorption factors on heavy metals eradication, has been considered in this review. The study revealed that efficient reduction of Cr(VI) from water and wastewaters is highly dependent on the pH of the solution, shaking time, adsorbent type, initial concentration and temperature. The review of the relevant literature indicates that the maximum removal efficiency of Cr(VI) using the various low-cost adsorbents ranged from 50.0–100.0% with optimum pH and contact time ranging from 2.0–6.0 and 30.0–180.0 min, respectively at room temperature (25.0 °C). Furthermore, considering all the studies reviewed, the pseudo-second-kinetics and Langmuir isotherm are the dominant models that best described the Cr(VI) equilibrium data. The thermodynamic parameters suggested that the biosorption of Cr(VI) on the biosorbents was spontaneous, realistic and endothermic at the temperature range of 30.0–45.0 °C. It is found that the natural and agricultural wastes as cheap biosorbents are feasible replacements to commercial activated carbons for metal-contaminated water treatment. However, gaps have been identified to improve applicability, regeneration, reuse and safe discarding of the laden adsorbents, optimization and commercialization of suitable agricultural adsorbents.

Introduction

Contamination of water instigated by artificial activities has been acknowledged as a worldwide problem. Approximately, 80% of ailments in the universe are a result of water pollution [1]. The United Nations organization reports indicate that about 1.1 billion people have no access to potable water, especially in developing countries [1]. The persistent contaminants of the natural environment include effluents, sewage and agro-based wastes. Thousands of poisonous substances are released knowingly or innocently into aquatic bodies without proper treatment to remove these toxic contaminants which are usually heavy metals [1]. All heavy metals of higher concentration in wastewater are found to be toxic and carcinogenic and pose health risks to living species [2, 3]. Heavy metals are risky and unsafe though may be present in small amounts. This is attributable to the fact that they bioaccumulate in living tissues through ingestion [4].

Chromium(VI) is an ample toxic metal found in the environment and different industrial activities comprising canning, steel fabrication and paints industries are the common sources of Cr(VI) pollutants in the aqueous milieu [2, 5, 6]. High Cr(VI) concentration could lead to liver and kidney damage, stomach cancer, chronic bronchitis and irritation of the skin [7, 8].

To successfully remove toxic ions before their disposal into the environment, workable solutions must be applied. Recently, the purification of water containing toxins is an interesting area that is gaining research endowments. Conventional methods in water management systems are mostly unproductive or costly, particularly when the system contained 1.0–100.0 mg/L metal ions concentrations [4, 7]. The elimination of toxic metals by way of biosorption is reassuring as many natural substances could be employed as low-priced adsorbents [2, 7].

The present paper reviews many procedures being followed and research efforts being made with a special emphasis on biosorption with discounted materials as biosorbents for decontamination of Cr(VI) from aquatic environments.

Pollution of the aquatic environment with heavy metals

Mostly, heavy metals flow into the aquatic milieu over atmospheric deposition, erosion of geological environment or owing to man activities caused by domestic sewage, industrial sewage and mining operations [9, 10]. However, natural trace metals get to water bodies through soil leaching and chemical weathering of minerals [11]. The metal ions in aqueous media are present in suspended particulates and sediments [12, 13]. Anthropogenic activities such as metallurgy and mining, agriculture, sewage, manufacturing, and industrial wastewater discharges introduce toxic ions into water bodies [14]. Toxic metal ions assimilation by living species could be attributable to their high solubility in the aqueous environment [15]. Once they form part of the food chain, these toxic metals may be ingested beyond the permitted concentrations and may accumulate in the human system triggering severe health problems [16, 17]. The aquatic milieu with its water quality is the central pivot governing the state of fitness and ailment in both animals and humans. Water has been recognized as a “trouble spot” of the universe as countless noxious waste are waterborne and act as a solvent of most substances and as a channel in the cycle; air → soil → plants → animals. Heavy metals are recognized as the major sources of water contamination [18].

Recently, the increasing use of agricultural drainage systems and chemicals characterize the most hazardous form of chemical contamination, especially toxic metal ions. Farmers usually spray dangerous insecticides such as organochlorine and organophosphates up to five or more consecutive times when perhaps two or three applications may be adequate in a cropping season. Runoff of insecticides, fungicides, and pesticides from agricultural activities may pollute water media by spreading them within water bodies. Subsequently, the potability of such water bodies progressively declines to a level that may threaten their usage as drinking water by humans and livestock [19]. In Ghana, pollution of surface and groundwater bodies is well-known in gold mining areas [20, 21]. The exploitation of gold and other minerals puts much pressure on soil, water, vegetation and causes human health disorders. Gold mining in recent times is regarded as the common source of metal ions pollution and has since become undesirable in the environment. It has led to activities such as tailings and mines wastewaters disposal, smelting, exploitation of minerals, refining and transportation of ores [20, 22, 23]. In mining areas, the huge deposits of wastes from mines, rocks and ore stockpiles lead to the formation of a heap around the plants and weathering of such heaped materials leads to the release of toxic ions into water media. In Ghana, heavy metal contamination in mining areas has been investigated and that dangerous metals are usually discharged from tailings of mines including cadmium, arsenic, mercury, lead and others into the natural environment [20, 24]. In Ghana, the most reported environmental threats connected with mining activities are the pollution and contamination of surface water media with toxic metal ions and other chemical substances [25, 26].

Methods for the treatment of toxic heavy metals

Several conventional methods have been examined in water and wastewaters purification. These methods include precipitation [27], ultrafiltration [28], coagulation/flotation [29], electrochemical method [30], reverse osmosis [31], solvent extraction [32] and ion-exchange methods [33]. Most of these methods produce productive outcomes, but they have limitations in terms of high energy requirements, incomplete disposal, high operating costs, complex operations and the formation of toxic sludge [2] and are unable to treat low concentration levels of metal ions in water [34]. The adsorption technique is the most recognized method that is considered as a promising alternative for environmental pollution control [4, 5, 35] and most recommended as an operative technique and more economical for eliminating various contaminants from wastewaters. Besides, the adsorbent can undergo regeneration by a proper desorption procedure [35].

Among all the several treatment techniques, adsorption is reliable and perceived as the well-known method and globally acknowledged for the removal of trace elements from wastewaters [16, 36–40]. It is flexible in design and its operating processes produce potable water [41].

Some of the strengths and weaknesses of some conventional methods for removing heavy metals from aqueous media as cited by Athar et al. [42] and Ahmaruzzaman, [43] are presented in Table 1.

Table 1.

Benefits and shortcomings of various techniques adopted in removing heavy metals

Purification method	Benefits	Shortcomings
Ion–exchange	• Selects metals • Intense regeneration of materials	• Expensive • Removes fewer metals
Membrane process and ultrafiltration	• Creation of reduced solid waste • Consumption few chemicals • Proficient	• High maintenance cost • Reduced flow rates • Reduced remote performance over time • Production of concentrated sludge
Chemical coagulation	• Sludge settlement • Dewatering	• Very expensive • Consumes a large number of chemicals
Electrochemical methods	• Pick out metals • Do not consume chemicals • Pure metals extraction is possible	• High operational and startup cost
Chemical precipitation	• Cheap • Removal of most metals	• Mass production of sludge • Disposal problems
Oxidation	• A rapid removal of toxic pollutants	• High energy requirement • By-product formation
Biological Treatment	• Volume reduction • Active in removing individual metals	• Need sophisticated technology
Photochemical	• Do not produce sludge	• Formation of by-products

Elimination of Cr(VI) ions using low-priced biosorbents

Bayuo et al. [16] investigated the optimization of Cr(VI) biosorption using response surface methodology on Arachis hypogea husk. The central composite design was applied to obtain optimal values of factors impacting the decontamination of Cr(VI) from synthetic water. More so, three-parameter isotherm and kinetic studies were examined. The ANOVA results suggested that the independent factors had a significant influence on the quantity of Cr(VI) adsorbed on the Arachis hypogea husk. The optimal amount of Cr(VI) adsorbed was 2.4 mg/g at 120.0 min, 8.0 and 50.0 mg/L as maximized removal time, pH and initial concentration. The Redlich-Peterson and pseudo-second-order were the best-fitted models.

Adane et al. [44] carried out a study to maximized Cr(VI) elimination from an aquatic environment with Teff husk-derived activated carbon. The influence of interaction factors was optimized by the central composite design component of the Design-Expert software (version 7.0). At an equilibrium contact time (124.2 min), pH (1.9), biosorbent dosage (20.2 g/L) and initial Cr(VI) concentration (87.8 mg/L), the maximum percent removal of Cr(VI) by the Teff husk was 95.6%. The experimental data obtained on Cr(VI) by the Teff husk agreed well with the Langmuir and pseudo-second-order models.

Belachew and Hinsene, [45] utilized kaolin modified with cetyl trimethyl ammonium bromide (CTAB) to eliminate Cr(VI) from aqueous media. The impact of some biosorption factors on Cr(VI) depollution was studied. At the following equilibrium conditions, 180.0 min removal time, 0.1 g CTAB–kaolin dosage and 10.0 mg/L Cr(VI), Ninety-nine percent (99.0%) of Cr(VI) was removed by CTAB–kaolin. The Langmuir and Freundlich isotherms were applied to explore the biosorption of Cr(VI) onto kaolin–CTAB composites. The isotherm data gave a good representation to the Langmuir and pseudo-first-order models suggesting monolayer and chemisorption, respectively.

Dawodu et al. [46] examined the biosorption removal of Cr(VI) from polluted effluent onto Heinsia crinita seed coat (HCSC). The consequence of several operating factors encompassing biosorbent dose, temperature, contact time, pH and initial Cr(VI) concentration on the process was studied. The study uncovered that the HCSC is an appropriate biosorbent for Cr(VI) depollution from the effluent. The maximum amount of Cr(VI) adsorbed was attained at 0.3 g dosage, pH of 2.0 and 30.0 min contact time. The experimental data showed a better representation with the pseudo-second-order and Freundlich models. The thermodynamic studies suggested endothermic, physical and spontaneous processes.

Biosorbents-derived from bagasse was employed by Kumar et al. [47] for adsorptive elimination of Cr(VI) from wastewater. The consequences of diverse variables such as contact time, pH and biosorbent dose on Cr(VI) decontamination were considered. At 25.0 °C, a pH 4.0, the optimum removal efficiencies of 94.6% and 98.4% were obtained for synthetic bagasse and bagasse bio-polymeric gel beads respectively. However, at 25.0 °C and pH 6.0, the maximum percent removal for activated carbon was 64.8%.

Samaraweera et al. [48] examined the biosorption of Cr(VI) and Cr(III) ions onto NaOH-modified peel obtained from Artocarpus nobilis fruit. A batch tryouts carried out at a pH range establishes that the optimal pH for Cr(VI) and Cr(III) biosorption removal was achieved at a pH 2.0 and 5.0 correspondingly. The maximum amount of Cr(VI) and Cr(III) adsorbed on the peel of Artocarpus nobilis fruit at pH 5.0 and 2.0 was 4.9 mg/g for both ions. The kinetic data followed the pseudo-first-order model at a room temperature of 27.5 °C.

The biosorptive decontamination of Cr(VI) from aqueous media was explored by Mondal et al. [49] onto waste mosambi peel dust by applying response surface methodology as an optimization tool. The optimization operating conditions for the elimination of Cr(VI) were as follows: 5.0 mg/L initial Cr(VI) concentration, a pH of 2.0, 0.5 g dosage, 30.0 min contact time and 150.0 rpm agitation speed. The isotherm and kinetic data were best fitted to the D-R isotherm and pseudo-second-order model while the thermodynamic elements proved that Cr(VI) biosorption is spontaneous and endothermic.

The biosorption characteristics of activated carbons to eliminate Cr(VI) from wastewater was studied by Puszkarewicz, [50]. Different varieties of carbons obtained using WD-Ekstra (WDA), WD-Ekstra modified by salt acid WD (HCl) and nitrogen acid WD (HNO₃) were applied. The study revealed that the biosorption removal of Cr(VI) was improved with an increase in solution temperature. The elimination of Cr(VI) by the activated carbons was found to be successful at pH 2.0 while the equilibrium time for WD (HCl) and WDA were 150.0 and 270.0 min, respectively. The Freundlich isotherm showed good fitness to the equilibrium data.

A study was conducted by Manzoor et al. [51] to eradicate Cr(VI) and Cr(III) from wastewater using immobilized biomass from corn cob. The biosorption system factors (dosage, contact time, pH and initial ion concentration were maximized. The study indicated that the amounts of Cr(VI) and Cr(III) adsorbed on the corn cob biosorbent were 277.6 and 208.6 mg/g, respectively. The equilibrium data of both ions were found to be well represented with the Langmuir and pseudo-second-order kinetic models.

Daneshvar et al. [7] examined the decontamination of Cr(VI) from water by different microalga-based substances. Considering all the materials utilized, the microalgal biochar exhibited about 100.0% removal of Cr(VI). The experimental data displayed good agreement with the pseudo-second-order kinetic and Langmuir isotherm models.

Parlayici and Pehlivan, [52] prepared green adsorbents from cranberry (Cornus mas) kernel shell (CKS), rosehip (Rosa canina) seed shell (RSS and banana (Musa cavendishii) peel (BP). The impacts of many factors comprising adsorbent dosage, pH, contact time, temperature and initial Cr(VI) concentration on the biosorption process were verified. The results from the batch mode suggested that Cr(VI) biosorption is dependent on these process factors. The biosorption kinetic data obeyed the pseudo-second-order while the Langmuir isotherm showed compliance to the equilibrium results with Cr(VI) maximum sorption capacities as 15.2, 10.4, and 6.8 mg/g for RSS, BP and CKS, respectively.

Özsin et al. [53] investigated Cr(VI), Pb(II) and Cu(II) biosorption removal on chemical activated biosorbent produced from chickpea (Cicer arietinum) husks. The study indicated that the biosorbent was effective and economical and Cr(VI), Pb(II) and Cu(II) maximum uptake capacities were obtained as 59.6, 135.8 and 56.2 mg/g, respectively. The Langmuir isotherm and pseudo-second-order models showed the best compliance to the equilibrium results. Also, the results of the thermodynamic studies suggested that the elimination of Cr(VI), Pb(II) and Cu(II) by the chickpea husk-derived activated carbon was discovered to be spontaneous and endothermic.

Yusuff, [54] conducted a study to eliminate Cr(VI) from aquatic media utilizing Leucaena leucocephala seed shell. The biosorbent was chemically modified and independent factors (initial adsorbate concentration, pH, adsorbent dosage and temperature) influencing the system were optimized at a constant contact time (60 min). The study indicated that at an optimum aqueous pH (4.2), biosorbent dose (0.6 g), initial Cr(VI) concentration (71.5 mg/L) and temperature (26.2 °C), the highest percent removal of Cr(VI) was attained as 95.6%. The isotherm and kinetic data conformed well to Freundlich and pseudo-second-order models.

Kashif and Mukhtar, [55] applied the Box-Behnken design of response surface methodology to optimize the depollution of Cr(VI) from the synthetic aqueous environment onto Litchi chinensis. The three experimental factors considered during the optimization studies were biosorbent dose, pH and temperature. The optimum biosorption of Cr(VI) removal was accomplished at an initial pH of 2.0. The experimental and predicted results of Cr(VI) on L. chinensis were minimal suggesting the effectiveness of the adsorbent in binding Cr(VI) ions.

Alam et al. [56] conducted a study to remove Cr(VI) from tannery wastewater on spent tea leaves. The biosorptive behavior and the impacts of independent variables including contact time, pH and adsorbent dose on Cr(VI) biosorption were determined using the batch procedure. At a pH of 10.0 and an adsorbent dose of 14.0 g/L, the study revealed that maximum decontamination of Cr(VI) from the wastewater utilizing spent tea leaves was found to be 95.4% with a corresponding sorption capacity of 10.6 mg/g.

Jinhui et al. [57] utilized an activated carbon prepared from cassava sludge to decontaminate Cr(VI) from an aqueous solution. Batch tests were performed to determine the influence of pH, shaking time, biosorbent dose and initial Cr(VI) concentration. The equilibrium time was found to be in the range of 40.0–60.0 min and the biosorption ability of the biosorbent was enhanced at lower pH. The single-factor experimental results revealed that Cr(VI) decontamination upsurges with initial Cr(VI) concentration while the amount of Cr(VI) adsorbed on the biosorbent decreases. Also, increasing the biosorbent dose increased the rate of removal with a corresponding decrease in the sorption capacity.

Microporous activated carbon from almond shell powder was produced by activating it with H₃PO₄ and explored by Rai et al. [58] to eliminate Cr(VI) from wastewater. Batch trials were conducted to assess the impacts of pH of the solution, contact time, adsorbent dose, initial Cr(VI) concentration and temperature on the biosorption of Cr(VI). The study reported that at an initial pH of 2.0, a hundred percent (100.0%) of Cr(VI) removal was achieved. The equilibrium studies indicated that the Langmuir isotherm and pseudo-second-order models fitted better to the equilibrium data. The thermodynamic parameters revealed the spontaneous, endothermic and increased randomness of the biosorption system.

Adsorption of chromium ions (VI) and (III) in aqueous environments were investigated by Ba et al. [59] utilizing olive wastes-derived carbon. The influences of biosorption factors (adsorbent dosage, pH, contact time and initial solutes concentration) on chromium ions decontamination were performed by batch mode. The experimental results showed that the sorption capacity of the prepared carbon was significantly dependent on initial pH. The best initial pH for Cr(VI) biosorption was pH of 2.0 and basic pH for that of Cr(III) biosorption removal. The equilibrium studies revealed that the pseudo-second-order and Langmuir isotherm models agreed with the data. The maximum sorption capacities evaluated from the Langmuir model were 74.9 mg/g at pH 2.0 and 14.3 mg/g at pH 9.0 for Cr(VI) and Cr(III), respectively. The temperature effect was determined using the thermodynamic parameters. Negative values of ΔH⁰ and ΔG⁰ proved the feasibility of the biosorption system with its spontaneous and exothermic natures.

Gorzin, [60] prepared a low-effective activated carbon using paper mill sludge to eradicate Cr(VI) from aqueous media. The impacts of contact time, dosage, pH, metal Cr(VI) concentration and temperature on the adsorbent biosorption ability were studied. The optimum amount of Cr(VI) adsorbed on the adsorbent was determined as 23.2 mg/g at a best pH 4.0, 180.0 min contact time and temperature of 45.0 °C. The Langmuir and pseudo-second-order were the well-fitted isotherm and kinetic models. The thermodynamic studies pointed out that the Cr(VI) biosorption on the biosorbent was spontaneous, feasible and endothermic.

Hexavalent chromium [Cr(VI)] was successfully removed by Labied, [61] using waste lignocellulosic-derived activated carbon obtained from Ziziphus jujuba cores. The study revealed that the optimal operative conditions for maximum Cr(VI) biosorption removal were pH 2.0, 1.0 g/L biosorbent dose and 100.0 mg/L Cr(VI) concentration and the removal efficiency augmented from 27.2–62.1%. The Toth and Elovich models showed conformity to the isotherm data while the kinetic data was best described by a pseudo-second-order. The thermodynamic parameters revealed the biosorption of Cr(VI) on the biosorbent was spontaneous, realistic and endothermic at the temperature range of 20.0–40.0 °C.

Ankur Gupta and Balomajumder, [62] investigated a concurrent biosorption removal of Cr(VI) and phenol on Fe-treated tea waste biomass from binary solution. The consequence of biosorption factors including pH, adsorbent dose, initial Cr(VI) concentration and initial phenol concentration was maximized. The ANOVA analysis of the quadratic model establishes that the experimental data are in good agreement with the predicted results. At optimum conditions of 55.0 mg/L initial concentration of Cr(VI), 27.5 mg/L of phenol, pH of 2.0, 15.0 g/L of adsorbent dose 99.9% biosorption removal of both Cr(VI) and phenol was attained.

Pakade et al. [63] explored the biosorption of Cr(VI) from aquatic media on macadamia nutshell biosorbents physically and chemically modified using two activating agents [raw macadamia nutshell powder (RMN), acid-modified macadamia nutshell (ATMN) and base-modified macadamia nutshell (BTMN)]. The optimal values for the biosorption factors for which maximum biosorption was achieved are 2.0, 600.0 min, 0.2 g and 100.0 mg/L for pH, contact time, adsorbent dose and initial Cr(VI) concentration, respectively. The equilibrium data were best described by the Langmuir and pseudo-second-order models.

Almond green hull powder (AGHP) was utilized in removing Cr(VI) from wastewater by Nasseh et al. [64]. The effects of pH, dosage, initial Cr(VI) concentration, temperature and contact time on the process were examined. The study indicated that Cr(VI) decontamination by AGHP was dependent on the solute [Cr(VI)], solid (AGIP) and temperature in an acidic medium. The highest eradication of Cr(VI) by the biosorbent was found to >99.0% and the Freundlich model best fitted the equilibrium results.

Numerous biosorbents including natural zeolite clinoptilolite, modified zeolite, grape and olive wastes have been explored by Kučić et al. [65] to remove Cr(VI) from aqueous media. They discovered that natural zeolite, modified zeolite, grape and olive wastes were capable of removing about 5.0%, 13.0%, 73.0% and 62.0% of Cr(VI), respectively. The Langmuir isotherm and pseudo-second-order models conformed to the experimental data better in comparison to other biosorption models. At higher temperatures, the biosorption of Cr(VI) rises with free energy change (∆G°) being negative suggesting the possibility and spontaneous nature of the biosorption system.

Song et al. [66] examined the elimination of toxic Cr(VI) from synthetic water onto wheat straw and Eupatorium adenophorum. The influences of several factors comprising contact time, pH, biosorbent dose and temperature were examined. The study disclosed that at a pH of 1.0, about 99.9% of Cr(VI) was removed with an optimum sorption capacity of 89.2 mg/g at 35 °C. The amount of Cr(VI) adsorbed on the biosorbent was observed to upsurge with increasing temperature suggesting the endothermic nature of the system. From the equilibrium studies, the Langmuir and pseudo-second-order models indicated good compliance to the experimental data.

Biochar particles obtained from Onopordom heteracanthom (weed) were evaluated by Ghorbani-Khosrowshahi, [67] to eliminate Cr(VI) from aqueous milieus. The impacts of biosorption factors encompassing adsorbent dose, initial Cr(VI) concentration and pH on Cr(VI) biosorption were explored. The best biosorption pH was found to be a pH of 1.0. Among the biosorption models tested, the Langmuir and pseudo-second-order models exhibited good fitness to the equilibrium data.

Velumani et al. [68] examined the biosorption of Cr(VI) utilizing a novel biosorbent prepared from Passiflora foetida plant seed. The influences of contact time, pH, initial Cr(VI) concentration and temperature on Cr(VI) elimination were studied. The study showed that the pseudo-second-order and Freundlich models correlated well with the equilibrium data. From the thermodynamic studies, the negative values of ∆G°, ∆H° and ∆S° implied that the biosorption system was feasible and spontaneous, exothermic and lessened randomness at the solid-solution interface.

Regmi et al. [69] carried out a study using concentrated sulphuric acid-modified adsorbent obtained from Phragmities stem to eradicate Cr(VI), Al(III) and Fe(II) from water media. From the study, the maximum Cr(VI), Al(III) and Fe(II) uptake capacity of the prepared biomaterial was higher in comparison to the previous reports. Therefore, the study indicated that Phragmities waste could be utilized for Cr(VI), Al(III) and Fe(II) elimination from wastewater. At a pH 1.0 and 2.0, the optimum uptake capacity of Cr(VI) onto sulphuric modified Phragmities waste was 200.0 mg/g. However, Al(III) uptake capacity was 90.9 mg/g, 148.0 mg/g for charred and phosphorylated charred Phragmities wastes, respectively at pH 6.0 while at the pH 2.7, Fe(II) uptake capacity was 166.7 mg/g for charred Phragmities waste, and 200.0 mg/g for phosphorylated charred Phragmities waste. The well-fitted isotherm and kinetic models were Freundlich and pseudo-second-order.

Sathish et al. [70] examined mangrove leaf powder (MLP) ability for the depollution of Cr(VI) from synthetic wastewater. The impacts of biosorption factors comprising pH, biosorbent size, initial Cr(VI) concentration and biosorbent dose on Cr(VI) decontamination using MLP were considered. The optimum biosorption of Cr(VI) was attained at a biosorbent particle size of 0.5 mm and a pH of 2.0. The kinetic and equilibrium data obeyed the pseudo-second-order and Langmuir models at 30.0 min of contact time. The thermodynamic studies suggested that the adsorption of Cr(VI) on MLP was found to be spontaneous and endothermic.

Mekonnen et al. [71] utilized avocado kernel seeds (AKS), Juniperus procera sawdust (JPS) and papaya peels (PP) as low-priced biosorbents for the elimination of Cr(VI) from wastewater. The consequences of some factors including contact time, pH, adsorbent dose, initial Cr(VI) concentration and temperature on the biosorption system were considered. The maximum operative factors required to adsorbed 5.0 mg/L Cr(VI) were obtained at a pH of 1.0, adsorbent dose of 0.5 g, 160.0 min contact time and temperature of 40.0 °C. The equilibrium data were found to be well correlated with the Freundlich and pseudo-second-order models. The thermodynamic results indicated that the biosorption process was feasible with a negative free energy change value while the positive entropy change value implied improved randomness at the solid-liquid interfaces.

Duranoglu and Beker, [72] studied the removal of Cr(VI) by peach stone-derived activated carbon. The impacts of independent factors such as Cr(VI) concentration, temperature, pH and adsorbent amount on Cr(VI) removal were scrutinized. The study revealed that Cr(VI) biosorption was pH-dependent and the highest decontamination was accomplished at lower pH values. The Freundlich and pseudo-second-order models showed a good representation of the equilibrium data.

Dula et al. [73] examined the application of activated carbon produced from bamboo (Oxytenanthera abyssinica) waste in removing Cr(VI) from water media. Batch tests were conducted as a function of initial Cr(VI) concentration, pH, contact time, adsorbent dose and temperature. The removal efficiency and the amount of Cr(VI) adsorbed at optimum pH 2.0 and 27.0 °C were 98.3% and 59.2 mg/g, respectively. The equilibrium results were found to show a suitable correlation with the pseudo-second-order and Freundlich models. The biosorption thermodynamics suggested that the decontamination of Cr(VI) by bamboo waste-derived carbon was spontaneous, feasible and exothermic at the temperature range of 25–45 °C.

Ju and Okoli, [74] carried out a study to remove a toxic metal, Cr(VI) from industrial wastes before disposing it safely into the environment. Two natural adsorbents obtained from the fiber of palm kernel and coconut husk were examined for removing Cr(VI) ion from synthesized wastewater. It was found that the efficiencies of activated coconut husks and activated palm kernel fiber of Cr(VI) ion decontamination were 91.1 and 84.9%, respectively. Furthermore, the impact of removal time, Cr(VI) concentration as well as sorption isotherms and kinetics were examined on the adsorption efficiency. The removal efficacy displayed improvement with the increase of removal time and decreasing metal ion concentration. More so, the pseudo-second-order best explained the kinetic data and the Freundlich model showed the best fit to the equilibrium data.

Sunil et al. [75] used five biosorbents including groundnut shell powder, wheat bran, activated neem leaf powder, activated charcoal and modified activated charcoal in eliminating Cr(VI) present in the effluent. The study indicated that these adsorbents have the capability of removing Cr(VI) contained in synthetic water. The effect of initial Cr(VI) concentration indicated that the percentage elimination of Cr(VI) from the aqueous media unto the groundnut shell ranged from 51.7–69.0%, 79.4–87.3% and 81.9–95.7% for neem powder and modified neem leaf powder, respectively. However, for modified activated charcoal, the percentage of removal ranged from 89.0–97.6% and 78.8–83.4% for that of wheat bran. Among the isotherms, both the Langmuir and Freundlich models correlated better to the experimental data.

The removal of Cr(VI) on coffee polyphenol-formaldehyde/acetaldehyde resins as biosorbents were investigated by Mulani et al. [76]. At a pH of 2.0, optimum elimination of Cr(VI) by the biosorbents occurred. The percent decontamination of Cr(VI) was found to be reduced with increasing the pH of the solution. However, percent elimination of Cr(VI) rises and the amount of Cr(VI) adsorbed on the biosorbents declines with increasing biosorbent dose. The isotherm data were best demonstrated by the Freundlich isotherm model while the kinetic studies suggested that Cr(VI) biosorption was rapid and occurred within 150.0 min.

Kaur et al. [77] utilized agricultural residues such as Syzygium cumini leaves powder (SCLP) and Populus deltoides leaves powder (PDLP) for the biosorption removal of Cr(VI), Ni(II) and Cu(II) from water media. The influences of adsorbent dose, pH, initial ion concentration and contact time were studied. The study discovered that the elimination efficiency of Cr(VI), Ni(II) and Cu(II) onto SCLP and PDLP declined with increasing initial ion concentration at a pH of 2.0, 6.0 and 4.0 for Cr(VI), Ni(II) and Cu(II), respectively. Also, increasing biosorbent dose resulted in a decrease of the amount of Cr(VI) adsorbed on the biosorbent but with an upsurge in the percent removal. The equilibrium results were best fitted to the Langmuir isotherm and pseudo-second-order models. The thermodynamic parameters described the biosorption process as being endothermic and spontaneous.

Gandhi et al. [78] investigated Cr(VI) removal onto inexpensive adsorbents. The batch mode tests conducted proved that the biosorbents can adsorb Cr(VI). For all the ten biosorbents tested, Multhani mitti and Mango bark dust were found to be potent in removing Cr(VI). The study showed that the decontamination of Cr(VI) by the biosorbents increases with increasing contact time and biosorbent dose.

Photocatalytic reduction of hexavalent chromium, Cr(VI) in aqueous solutions with zinc oxide nanoparticles and hydrogen peroxide was investigated by Assadi et al. [79]. The effect of pH, catalyst dose, Cr(VI) initial concentration and hydrogen peroxide concentration on photocatalytic reduction of Cr(VI) was investigated. The study reveals that the adsorption of Cr(VI) was more promising in acidic solutions. More so, the process of photoreduction of Cr(VI) approximately followed first-order kinetics.

Idris, [80] conducted a study to eliminate Cr(VI) and Ni(II) from the effluent of a dye by groundnut shell. The study showed that at 120.0 and 150.0 min of contact time, optimal Cr(VI) and Ni(II) elimination were achieved. The kinetic results were best fitted to the pseudo-second-order kinetic model.

Mahajan and Sud, [81] examined Dalbergia sissoo pods in different ways as a low-priced biosorbent for Cr(VI) depollution from synthetic wastewater using batch experiments. It was found that Dalbergia sissoo pods could eliminate Cr(VI) from artificial polluted water and the uptake efficiency exhibited dependence on the initial pH of the solution, nature of adsorbent, adsorbent load and initial metal ion concentration. At the pH of 2.0, the maximum Cr(VI) elimination was achieved. The Langmuir and Freundlich isotherm models showed a correlation to the isotherm data with good coefficients of determination while the best correlated kinetic model was pseudo-second-order.

Saritha and Dash, [82] examined Cr(VI) removal using groundnut shells by batch adsorption experiment and concluded that the groundnut shells have a moderate potential and a substantial biosorption ability for removing Cr(VI) from effluent. The uptake ability of the biomaterial for removing Cr(VI) was found as 5.3 mg/g. Furthermore, the percent removal of Cr(VI) showed dependence on time, pH, biosorbent load and initial Cr(VI) concentration. Hence, at an initial 50.0 mg/L Cr(VI) concentration, the maximum adsorbent loading for maximum removal was 0.5 g/L. About 70.2% elimination of Cr(VI) was attained at 50.0 min and 50.0 mg/L. However, it was observed that the removal efficiency declined to 30.8% when Cr(VI) initial concentration was raised to 70.0 mg/L.

Attia et al. [83] produced a chemically activated carbon from olive stones to remove Cr(VI) from aqueous media at an initial Cr(VI) concentration range of 4.0–50.0 mg/L. The impacts of pH, contact time, initial Cr(VI) concentration and dosage on the removal efficiency of Cr(VI) were performed. At a pH of 1.5, the maximum removal efficiency Cr(VI) was reached. The equilibrium data showed a correlation with the Langmuir and pseudo-first-order models.

The biosorption removal of Cr(VI) and Zn(II) from aquatic media onto the skin of orange peel (Citrus sinensis) was probed by Ekpete et al. [84]. The biosorption factors that were examined include contact time, initial ions concentration, adsorbent dose, temperature and pH. The study discovered that the white inner skin of the orange eliminates more of Cr(VI) than that of Zn(II) ions in all the batch experiments. At a temperature of 30.0 °C and pH of 3.0, maximum depollution of Cr(VI) and Zn(II) ions took place. The Langmuir isotherm showed good representation with the equilibrium data with maximum sorption capacities of 8.1 mg/g and 1.1 mg/g for Cr(VI) and Zn(II) ions respectively.

Giri, [85] conducted a continuous fixed-bed column study to eradicate Cr(VI) from an aqueous environment by different particle sizes of sawdust. The study showed the appropriateness of the adsorbent in eradicating Cr(VI) ion from water media in continuous mode. The adsorbent selected was found to be a suitable biomaterial bearing in mind its economic prospects in water treatments. It was found in their study that the amount of Cr(VI) adsorbed decreased; however, the percent removal increased with decreasing initial Cr(VI) concentration in the aqueous media. The biosorption capacity of the sawdust at the different mesh sizes for Cr(VI) removal was found to follow: (−30.0 + 10.0) > (−50.0 + 30.0) > (−70.0 + 50.0).

Agarwal et al. [86] examined the binding capability of five low-cost biosorbents and indicated that tamarind seed biosorbent is more cost-effective and suitable than other biosorbents explored. The removal efficiency of Cr(VI) ions from wastewater unto tamarind seed were reduced considerably when pH was increased, slightly declined with increasing ionic strength and increased as the temperature rises.

Pinto et al. [87] investigated the potentiality of an adsorbent derived from coconut shells to remove three heavy metals. The removal efficiency of the coconut shell for Cr(VI), Cd(II) and As(III) as studied at pH values (2.0–9.0), mesh sizes (0.04–0.28 mm) and initial ion concentration (17.7–884.7 mg/L) using batch experiment. The equilibrium data obtained for the metals were investigated and fitted well to the Langmuir and Freundlich models whereas the experimental data fitted both pseudo-first-order and pseudo-second-order models.

Neem leaf powder was applied in eliminating Cr(VI) from aqueous media [88]. The batch tests were conducted at diverse initial Cr(VI) concentration. The effect of the initial pH, contact time, adsorbent dosage and temperature were explored. The equilibrium data were analyzed by the various isotherm and kinetic models. The Langmuir model revealed that the uptake efficiency of the biosorbent was 0.1 mg/g. It was discovered that 87.0% of Cr(VI) ions from the aqueous media (6.3 mg/L) could be adsorbed by a small dosage of neem leaf powder (1.6 g) for a contact time of 300.0 min at 27.0 °C. In this study, the best pH was ranged from 4.5–7.5. Similarly, Babu and Gupta, [89] investigated activated Azadirachta indica as a less expensive adsorbent. The study indicated that through the physical modification of the neem leaf using the activation procedure, the biosorption capacity of the neem leaf was remarkably enhanced. The removal of Cr(VI) using some other low-cost adsorbents are summarized in Table 2.

Table 2.

Adsorption potential of some other agricultural adsorbents for the decontamination of Cr(VI) ions from aqueous media

Biomaterial	Removal efficiency (%)	Reference
Oat waste	> 80.0	Gardea et al. [90]
Modified sawdust	62.0–86.0	Garg et al. [91]
sawdust of beech	100.0	Acar and Malkoc, [92]
Modified rice husk Bagasse	88.9	Bishnoi et al. [93]
Wheat Bran	> 82.0	Farajzadeh and Monji, [94]
Coconut Husk	> 80.0	Mohan et al. [95]
Neem leaf powder	> 96.0	Venkateswarlu et al. [96]
Rubberwood sawdust	60.0–70.0	Karthikeyan et al. [97]
Raw rice bran	40.0–50.0	Montanher et al. [98]
Fly ash of bagasse	96.0–98.0	Gupta and Ali, [99]

Factors affecting biosorption of heavy metals

Alfarra et al. [1] conducted a review on the removal of heavy metals by natural adsorbent and summarized some of the factors affecting biosorption as presented in Table 3.

Table 3.

Factors affecting biosorption of heavy metals

Factor	Effects
pH of solution	It improves the biosorption of positively-charged metals but then decreases that of negatively-charged metals or acidic dyes.
Adsorbent particle size	Small adsorbent size increases biosorption in the batch mode because of the higher surface area of the biosorbent. However, not good for column adsorption owing to its low mechanical strength and column clogging.
Biosorbent dosage	It reduces the amount of absorbed pollutants per unit mass of biosorbent then upsurges its percent removal.
Agitation speed	It increases the rate of biosorption of pollutants by reducing its mass transfer resistance; however, could destroy the biosorbent physical structure.
Initial pollutant concentration	It upsurges the amount of absorbed pollutants per unit mass of the biosorbent but declines its percent removal.
Temperature	It increases the biosorption of pollutants by rising the surface activity and adsorbate kinetic energy; however, could destroy the biosorbent physical structure.
Ionic strength	It reduces the biosorption removal of pollutants by competing with the solute for vacant binding locations of the biosorbent.
Other contaminant concentration	If coexisting pollutant competes with targeted contaminant for binding sites or forms any complex with it, a higher amount of other toxins will decrease biosorption elimination of the targeted contaminant.

Adsorption kinetic studies

The adsorption kinetics helps to find the dependency of the rate of adsorption on the biosorbent characteristics and the conditions of the adsorption system itself. Therefore, adsorption kinetics depends on the kind of adsorbent, modification procedure and the type of adsorbate being adsorbed [50]. Fundamentally, the attained adsorption equilibrium is expressed as the change in the amount of the adsorbate adsorbed over a given time.

In most of the studies reviewed, the kinetic parameters were investigated by applying pseudo-first-order, pseudo-second-order, Elovich and intraparticle diffusion models. These kinetic models are as follow:

Pseudo-first-order model expression

A kinetic model on pseudo-first-order was formulated by Lagergren [58]. The linearized-integral form of the pseudo-first-order model is represented in Eq. (1)

$$\frac{\mathrm{d}{\mathrm{q}}_{\mathrm{t}}}{\mathrm{dt}}={\mathrm{k}}_{{\mathrm{p}}_1}\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)$$ 1

where ${\mathrm{k}}_{{\mathrm{p}}_1}$ (min⁻¹) is the rate constant of pseudo-first-order, q_e (mg/g) and q_t (mg/g) represent equilibrium adsorption capacities and time t (min) respectively. Integrating Eq. (1) by applying the following conditions: q_t = 0; t = 0; q_t = q_t and t = t produces Eq. (2):

$$log\left(\frac{{\mathrm{q}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}}\right)={\mathrm{k}}_{{\mathrm{p}}_1}\mathrm{t}$$ 2

Then Eq. (2) can be reorganized as:

$$log\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)=log{\mathrm{q}}_{\mathrm{e}}-\frac{{\mathrm{k}}_{{\mathrm{p}}_1}}{2.303}\mathrm{t}$$ 3

The adsorption capacity (q_e) at equilibrium, and the rate constant (k_p1) could be evaluated experimentally from the gradient and intercept by plotting log(q_e − q_t) against t from Eq. (3).

Pseudo-second-order model expression

The integrated formula of the pseudo-second-order model is given by Bayuo et al. [5]:

$$\frac{\mathrm{d}{\mathrm{q}}_{\mathrm{t}}}{\mathrm{dt}}={\mathrm{k}}_{{\mathrm{p}}_2}{\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)}^2$$ 4

Then Eq. (4) could be written as:

$$\frac{\mathrm{d}{\mathrm{q}}_{\mathrm{t}}}{{\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)}^2}={\mathrm{k}}_{{\mathrm{p}}_2}\mathrm{dt}$$ 5

By integrating Eq. (5) with the conditions: q_t = 0; t = 0 and q_t = q_t and t = t produces;

$$\frac{1}{\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)}=\frac{1}{{\mathrm{q}}_{\mathrm{e}}}+{\mathrm{k}}_{{\mathrm{p}}_2}\mathrm{t}$$ 6

Rearranging Eq. (6) yields:

$$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{k}}_{{\mathrm{p}}_2}{\mathrm{q}}_{{\mathrm{e}}^2}}+\frac{1}{{\mathrm{q}}_{\mathrm{e}}}\mathrm{t}$$ 7

The rate constant (${\mathrm{k}}_{{\mathrm{p}}_2}$) of the pseudo-second-order model in addition to the equilibrium sorption capacity (q_e) could be obtained from the gradient of the plot and intercept by plotting $\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}$ against t from Eq. (7).

Elovich’s model equation

Elovich model expression is a kinetic expression of the chemical reaction between the adsorbent surface functional groups and adsorbate in the liquid phase. The non-linearized and linearized Elovich equations are expressed in Eqns. (8) and (9), respectively [61]:

$$\frac{\mathrm{d}{\mathrm{q}}_{\mathrm{t}}}{\mathrm{dt}}=\text{αexp}\left(-\mathrm{\beta }{\mathrm{q}}_{\mathrm{t}}\right)$$ 8

$${\mathrm{q}}_{\mathrm{t}}=\frac{1}{\mathrm{\beta }}ln\mathrm{\alpha \beta }+\frac{1}{\mathrm{\beta }}ln\mathrm{t}$$ 9

where α represents an initial adsorption rate (mg/g. min) and β denotes the constant of desorption (g/mg) for an investigation.

By plotting q_t against lnt in Eq. (9) produces a rectilinear expression having a slope of (1/β) and intercept, (1/β) ln (αβ) from which, other constants can be evaluated.

Weber-Morris intraparticle diffusion model

Weber and Morris, [100] expressed the intra-particle model as in Eq. (10);

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{id}}{\mathrm{t}}^{\mathrm{½}}+\mathrm{C}$$ 10

where q_t represents the amount of the ions adsorbed at time t, k_id (mg/gmin) denotes the constant of the intra-particle diffusion model and C signifies the intercept.

A straight-line graph of q_t against t^½ going through the origin indicates that the Weber-Morris kinetic model is the rate-limiting step [101].

Adsorption isotherm studies

The main aim of adsorption isotherms is to establish a link between the amount of the adsorbate in the bulk solution and the quantity adsorbed at the interface of the adsorbent [4, 5, 102]. An isotherm explains how an ion can be spread within the liquid and solid phases at different concentrations at equilibrium.

Several adsorption isotherms models have been applied to describe equilibrium data. However, among these, the Langmuir, Freundlich, Dubinin-Radushkevich (D-R) and Temkin isotherms are the common models used by several studies.

Langmuir adsorption isotherm model

According to the Langmuir isotherm, the adsorption of the adsorbate takes place at homogeneous sites and forms a monolayer.

The non-linear and linear expressions of Langmuir isotherm are given in Eqns. (11) and (12), respectively [45, 103].

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{q}}_{\mathrm{m}}{\mathrm{K}}_{\mathrm{L}}\frac{{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}$$ 11

$$\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{q}}_{\mathrm{m}}{\mathrm{K}}_{\mathrm{L}}}+\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{m}}}$$ 12

where q_e denotes equilibrium uptake capacity (mg/g), C_e is the concentration of the solute at equilibrium (mg/L); K_L (L/mg) and q_m (mg/g) are constants that are linked to the net enthalpy of adsorption and uptake capacity, correspondingly.

From Eq. (12) the constants K_L and q_m that can be obtained from a slope of $\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{m}}}$ and intercept $\frac{1}{{\mathrm{q}}_{\mathrm{m}}{\mathrm{K}}_{\mathrm{L}}}$ by plotting $\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}$ against C_e.

The important parameter of the Langmuir isotherm that offers a clue about the shape of the isotherm [104] is expressed in Eq. (13).

$$R_L=\frac{1}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_0}$$ 13

where R_L stands for separation factor, C₀ is the initial concentration of the metal ion and K_L represents Langmuir’s constant (L/mg) relating energy of adsorption through the Arrhenius equation.

The value of R_L provides evidence as to whether the adsorption is irreversible (R_L = 0), favorable (0 < R_L < 1), linear (R_L = 1) or unfavorable (R_L > 1).

Freundlich adsorption isotherm model

The Freundlich model establishes a link between equilibrium adsorbate and biosorbent capacity based on the multilayer adsorption characteristics involving the heterogeneous surface of the adsorbent [105].

The Freundlich expression is an exponential equation and the non-linearized and linearized expressions are presented as in Eqns. (14) and (15), respectively [71].

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}{{\mathrm{C}}_{\mathrm{e}}}^{\frac{1}{\mathrm{n}}}$$ 14

$$\log q_e=\log {\mathrm{K}}_{\mathrm{F}}+\frac{1}{\mathrm{n}}\log {\mathrm{C}}_{\mathrm{e}}$$ 15

where q_e represents uptake capacity (mg/g) at equilibrium, C_e stands for the amount of solute in the bulk solution at equilibrium (mg/L); K_F is a constant representing the comparative uptake ability of the solid (mg/g) and, n is the intensity of adsorption.

From the plot of logq_e against logC_e, the constants K_F could be obtained from the intercept of logK_F and n from the slope of $\frac{1}{\mathrm{n}}$ in Eq. (15). The adsorption intensity or surface heterogeneity is measured from the slope ranging between 0 and 1; the surface is extra heterogeneous as the value becomes close to zero. While the value of $\frac{1}{n}$ less than unity shows chemical adsorption, value $\frac{1}{n}$ above one implies cooperative adsorption [46].

Dubinin-Radushkevich (D-R) adsorption isotherm model

The Dubinin-Radushkevich (D-R) model is an analog of the Langmuir model but it is more universal than that of the Langmuir model as it discards constant sorption potential or the homogeneous surface. Generally, it is applicable to explain the occurrence of the adsorption phenomenon unto both homogeneous and heterogeneous surfaces of the adsorbent [52].

The non-linearized and linearized expressions of the Dubinin-Radushkevich model are given by Eqns. (16) and (17), respectively [46].

$${\mathrm{q}}_{\mathrm{e}=}{\mathrm{q}}_{\mathrm{s}}\exp (-{\mathrm{K}}_{\mathrm{D}\mathrm{R}}{\mathcal{E}}^2)$$ 16

$$\ln q_e=\ln q_s-K_{DR}{\mathcal{E}}^2$$ 17

$$\text{Where}\mathcal{E}=\text{RTln}\left(1+\frac{1}{{\mathrm{C}}_{\mathrm{e}}}\right)\text{is an expression of Polanyi Potential}.$$ 18

where q_e is the equilibrium uptake capacity (mg/g), C_e represents the amount of solute (mg/L) in the aqueous media at equilibrium, q_s (mg/g) is a constant linked to the uptake capacity, K_DR (mol²/kJ²) denotes the adsorption mean free energy, R (J/mol. K) signifies gas constant and T (K), absolute temperature.

The constants K_DR and q_s can be obtained by plotting ln q_e against ${\mathcal{E}}^2$ with the slope, −K_DR and intercept, lnq_s. The mean adsorption energy, E (kJ/mol) is evaluated from the D–R model as in Eq. (19).

$$E=\frac{1}{\sqrt{2{\mathrm{K}}_{\mathrm{DR}}}}$$ 19

The value of mean sorption energy (E) gives information about chemisorption and physisorption. When the value of E ranged between 1.0 to 8.0 kJ/mol, it indicates physisorption and between 8.0 to 16.0 kJ/mol implies chemisorption [46, 106].

Temkin adsorption isotherm model

This model was formulated based on the assumption that the adsorption heat declines linearly through increasing adsorbent coverage [61]. The Temkin model postulates that the decrease in the adsorption heat is straight rather than being logarithmic [58, 107].

The Temkin model is expressed in the following expressions [46, 108].

$${\mathrm{q}}_{\mathrm{e}}=\frac{\mathrm{RT}}{\mathrm{b}}ln\mathrm{A}{\mathrm{C}}_{\mathrm{e}}$$ 20

The linear form is illustrated as in Eq. (21),

$${\mathrm{q}}_{\mathrm{e}}=\frac{\mathrm{RT}}{\mathrm{b}}ln\mathrm{A}+\frac{\mathrm{RT}}{\mathrm{b}}ln{\mathrm{C}}_{\mathrm{e}}$$ 21

Then Eq. (21) can also be expressed as in Eq. (23)

$${\mathrm{q}}_{\mathrm{e}}=\mathrm{BlnA}+\mathrm{Bln}{\mathrm{C}}_{\mathrm{e}}$$ 22

$$\text{where}B=\frac{\mathrm{RT}}{\mathrm{b}}$$ 23

C_e stands for the amount of solute found in bulk solution at equilibrium (mg/L), q_e (mg/g) is denotes the equilibrium uptake capacity, T signifies temperature (K), R indicates gas constant (8.314 J/mol. K), b and A connote Temkin’s constants.

Plotting q_e against lnC_e in Eq. (22), b can be evaluated from $\frac{\mathrm{RT}}{\mathrm{b}}$ (slope) and A from $\frac{\mathrm{RT}}{\mathrm{b}}ln\mathrm{A}$ (intercept).

Adsorption thermodynamic studies

The thermodynamic parameters that aid in understanding the nature of the biosorption of Cr(VI) ions on biosorbents are the standard changes in Gibbs free energy (ΔG°), entropy (ΔS°) and enthalpy (ΔH°).

The equilibrium constant found in the Langmuir model at a given temperature can be used to investigate the thermodynamic factors such as enthalpy change (ΔH⁰), entropy change (ΔS⁰) and free energy change (∆G⁰) [109, 110].

The Gibbs free energy of adsorption (∆G⁰) can be expressed with respect to Langmuir adsorption constant by the following Eqns:

$$∆{\mathrm{G}}^0=-\text{RTln}{\mathrm{K}}_{\mathrm{L}}$$ 24

The Gibbs energy change (ΔG⁰) is linked to the change in enthalpy (ΔH⁰) and entropy (ΔS⁰) under constant temperature as expressed in Eq. (25).

$$∆{\mathrm{G}}^0={\mathrm{\Delta H}}^0-{\mathrm{T\Delta S}}^0$$ 25

Combining Eqns. (24) and (25), we get:

$$ln{\mathrm{K}}_{\mathrm{L}}=\frac{{\mathrm{\Delta S}}^0}{\mathrm{R}}-\frac{{\mathrm{\Delta H}}^0}{\mathrm{RT}}$$ 26

where R denotes the ideal gas constant (8.314 J/mol. K), K_L represents the distribution coefficient for the adsorption (mL/g) and, T signifies the absolute temperature of the aqueous solution.

The values of ΔH⁰and ΔS⁰ are obtained from $\frac{–\text{∆}\mathrm{H}°}{\mathrm{R}}$ (slope) and $\frac{\text{∆}S°}{R}$ (intercept) by plotting lnK_L against $\frac{1}{\mathrm{T}}$. After obtaining ΔH⁰ and ΔS⁰values of the adsorption, ∆G⁰ of each temperature is calculated by the well- known Eq. (25).

Conclusion

The review of the adsorption processes and biosorbents showed that the adsorption method has countless advantages for the decontamination of Cr(VI) ions from water and wastewaters. It can be concluded that adsorption using biosorbents is a reliable, safe, efficient and cost-effective method for the removal of Cr(VI) from water, wastewater and industrial effluents. Hence, natural and agricultural waste materials should constantly be utilized to curtail cost and maximize heavy metals biosorption capabilities. The study revealed that efficient reduction of Cr(VI) from water and wastewaters is highly dependent on the pH of the solution, shaking time, adsorbent type, initial concentration and temperature. From the review of the relevant literature, the maximum removal efficiency of Cr(VI) using the various low-cost adsorbents ranged from 50.0 to 100.0% with optimum pH and contact time ranging from 2.0–6.0 and 30.0–180.0 min at room temperature (25.0 °C).

Furthermore, considering all the studies reviewed, the pseudo-second-kinetics and Langmuir isotherm are the dominant models that best described the Cr(VI) equilibrium data. The thermodynamic parameters suggested that the biosorption of Cr(VI) on the biosorbents was spontaneous, realistic and endothermic at the temperature range of 30.0–45.0 °C.

Nevertheless, further studies should be carried out to improve the biosorbents applicability, regeneration, reusability and efficient management of the loaded adsorbents, optimization and commercialization of suitable agricultural adsorbents.

Declarations

Conflict of interest

The author declares that there is no conflict of interest regarding the publication of this review paper.