Arsenic and Biosorption

Abstract

Arsenic, either in (III) or (V) oxidation states forms, is a hazardous element to humans; thus, its removal from aqueous environments is of the utmost priority in the countries where this problem arises. From the various separation technologies, the removal of arsenic via biosorption processing attracted an interest, because besides the removal of the element, allows the recycle materials that in many cases are considered as wastes. The present chapter reviewed the most recent proposals (2022 year) about using biosorbents to remove this toxic element.

1. Introduction

Water contamination by heavy metals is one of the most serious risks to public health and the environment [1], being this contamination due to natural or anthropogenic causes. However, even these toxic elements can be present in flavored and functional drinking waters and in mineral waters [2].

Among these toxic metals, arsenic has a preeminent position due to its influence on living organisms, particularly on humans. The ingesta of this element is associated with a series of diseases, including cancer [3]; thus, its removal from waters or liquid effluents is of the utmost importance to prevent fatal consequences on the population. Moreover, this element is also associated to murder [4].

Arsenic can be present in aqueous solution in (III) or (V) oxidation states, and with both oxidation states, existed the possibility to find inorganic and organic As-bearing compounds; generally, organoarsenic is formed from inorganic arsenic by a process called biomethylation, and the toxicity of all these compounds depends of several issues. It is of general consent that the order to hazardousness of these arsenic compounds is ranked (from the more toxic to the less one) as: (i) inorganic As(III), (ii) organic As(III), (iii) inorganic As(V), and (iv) organic As(V). A comprehensive list of naturally occurring inorganic and organic As(III) and As(V) species is reported in the literature [5].

Conventional arsenic removal strategies, from metal-contaminated waters (including liquid effluents) included: oxidation, coagulation/precipitation, membrane filtration, adsorption, ion exchange, biosorption, and solvent extraction. Several recent literature reviewed the use of some of these technologies on this important toxicological issue [6, 7, 8, 9, 10]. The use of microbes in the remediation of arsenic from the environment is of particular interest, which is claimed as an eco-friendly and economical technology. To contend with arsenic, microorganisms have specific mechanisms such as biotransformation, biosorption, and homeostasis. Also, microbial approaches are mentioned in the remediation of metals from extraterrestrial materials, that is, meteorites [11]. Also, ferrate (FeVIO₄²⁻) is now increasingly being synthesized and used as a novel adsorbent for the remediation of hazardous chemicals, including arsenic [12].

Among the above technologies, and due to the economic and relative easiness of operational modes, biosorption has a preeminent position in the removal of arsenic from contaminated waters. This chapter reviewed the literature published in 2022 about this important issue.

2. Arsenic(III)

Like in many metals involving systems, the removal of arsenic(III) from contaminated waters depends on its speciation in aqueous solutions. Figure 1 shows this speciation at various pH values [13]. Obviously, the range of predominant species versus pH is a function of the arsenic(III) concentration in the solution, but it always follows the pattern as shown in the figure.

Figure 1.

Distribution of arsenic(III) species with the aqueous pH.

Accordingly to this distribution, arsenic(III) existed as soluble anionic species at alkaline pH values, whereas at acidic, neutral, or even slight alkaline pH values, As(OH)₃ or H₃AsO₃ appeared as predominant species; however, as it can be seen later, some investigations are performed at near neutral pH values, indicating the existence of As³⁺ cationic species at this pH range.

Cotton stalks-derived biochar (CSB) is an adsorbent material that has the potential to remove As(III) from aqueous solutions [14]. This material is characterized by different techniques: Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and point of zero charges (PZC) in order to known surface moieties that facilitate As(III) uptake onto the biochar. Using experimental conditions of 1 g/L biochar dosage, pH value of 6, reaction time of 2 h and an initial As(III) in the solution of 200 μg/L, experimental results revealed that the maximum As(III) adsorption capacity is 89.90 μg/g. The percentage of As(III) removal, from the solution, decreases with increasing As concentration in this phase. The linear form of the Langmuir model (Eq. (1), explains As(III) loading onto CSB, with maximum uptake of 102.78 μg/g (r² = 0.99). The linear form of the Langmuir isotherm is represented by the next equation:

$$\frac{{\left[As\right]}_{aq,e}}{{\left[As\right]}_{ba,e}}=\frac{1}{{\left[As\right]}_{ba,m}{K}_L}+\frac{1}{{\left[As\right]}_{ba,m}}{\left[As\right]}_{aq,e}$$E1

in the above equation, [As]_aq,e and [As]_ba,e are the arsenic concentration in the aqueous phase and in the adsorbent at the equilibrium, respectively, [As]_ba,m is the maximum arsenic concentration in the bioadsorbent, and K_L is the Langmuir constant. As(III) uptake is attributed to high surface area of the bioadsorbent (103.62 m²/g) and the presence of different functional groups, i.e. ∙OH, C〓O, C∙O on the CSB surface that facilitated As(III) adsorption and removal from the solution. Note from the authors of this review: in this work, AsO₃³⁻ is the responsible for metal uptake onto the bioadsorbent; however, at the pH value at which these experiments are done (pH 6), this species does not exist (see Figure 1 of this review). Moreover, no desorption data are included in the work.

Next reference [15] isolated As-resistant lactic acid bacteria (LAB) and assessed the metal adsorption stoichiometry of LAB to validate its practical application as a bioremediation tool. In the investigation, 50 As-resistant LAB colonies from human (HS1–25) and albino mice (MM1–25) fecal samples are isolated. Considering these 50 As-resistant LAB, the HS12 isolate exhibited the highest arsenic removal efficiency (0.021 mg/h g). The metal bioremediation equilibrium investigation determined the contact time of 10 min and pH values in the 5–7 range for optimum arsenic biosorption from water. The non-linear Langmuir isotherm (r² = 0.993) fitted well with the experimental data. The arsenic bioaccumulation and scanning electron microscopy studies proved that, the mechanisms of metal sequestration of LAB HS12 is by binding of arsenic onto the cell membrane (0.000037 mg/g) as well as within the cell (0.000036 mg/g). The phylogenetic analysis of 16S rDNA amplicon (500 bp) of isolated potential HS12 LAB strains showed 97% similarity to Lactobacillus reuteri. Note from the authors of this review: no desorption data are included in the published manuscript.

A solid waste-based biosorbent derived from the Cassia fistula pod biomass is used to remove arsenic from aqueous solutions [16]. The prepared biosorbent has been characterized through different techniques, including field emission scanning electron microscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction, to investigate the physiochemical properties of the material. The experiments have been performed considering four experimental variables: pH, biosorbent dosage, initial concentration of As³⁺, and reaction time. The experimental results have been analyzed using the design-expert software for the optimization of different parameters. The maximum removal of arsenic (91%) could be achieved, whereas monolayer adsorption capacity is found to be 1.13 mg/g in 80 min at pH 6.0 and 30°C by using 60 mg dose of bioadsorbent. The arsenic adsorption behavior of the bioadsorbent has been well interpreted in terms of pseudo-first-order and Freundlich model. Note from the authors of this review: a major error occurs in this work since authors considered the existence of As³⁺ species in the aqueous phase, and this element (see Figure 1) does not form this cationic species at any aqueous pH value.

A metal oxide-based biosorbent for As(III) was designed and synthesized from eggshell biomass and characterized using FTIR, FE-SEM, EDX, XRD, and chemical analysis [17]. Raw eggshell (RES) powder is first dissolved with HCl and mixed with ZrOCl₂·8H₂O solution and then precipitated to obtain hydrated double oxide precipitate (HDOP), which is the material used for As(III) removal. Metal biosorption onto HDOP is fast and depends on pH and As(III) concentration. Pseudo-second order kinetics and Langmuir isotherm models (both in this linear form) are described As(III) uptake onto the bioadsorbent. HDOP provided exchangeable hydroxide/or chloride ligands for improved biosorption of As(III). The Langmuir model indicates that the maximum As(III) biosorption capacity onto HDOP is 40 mg/g at an optimum pH value of 10. Chloride and nitrate cause negligible interference on arsenic loading onto the adsorbent, whereas sulfate and phosphate significantly decreased the As(III) biosorption capacity of the biomaterial. HDOP completely removed arsenic from contaminated groundwater, and the remaining concentration reached values below the safe drinking water standard (10 μg/L) set by WHO. Metal desorption using three aqueous media, acid (1 M HCl), neutral (1 M NaCl), and alkaline (1 M NaOH), showed that in the alkaline medium, 94% removal rate from As-loaded bioadsorbent is reached, against 16% (NaCl) or 41% (HCl). After eight consecutive cycles of adsorption-desorption (with 2 M NaOH), results indicated that there is a continuous decrease in As(III) adsorption (99% in the first cycle) and 85% (fifth cycle), though the percentage of As(III) desorption always is greater than 95%.

A bioremediation study showed Bixa orellana as an accumulator of As(III) (and Cr(VI)); results are validated by SEM-EDX, FTIR, and other kinetic analyses [18]. Maximum percentage removal of As(III) is near 40% using an initial metal concentration of 6 mg/L. The bioadsorption data are well fitted to Freundlich and Elovich models. Three bacteria isolated from the coal mines of Rajmahal hills (India) showed As(III) resistance and bioremediation potentials (up to 150 mg/L). The 16S rRNA genotyping of these isolates is done (GenBank accession no: MK231250, MK23125, MK231251, and MK231252), which showed similarity with Stenotrophomonas maltophilia, uncultured gamma proteobacteria clone, and Bacterium E1. Further, the presence of genes involved in arsenic biotransformation like aox, acr, and ars is also confirmed in these bacterial isolates. Maximum percentage removal of As(III), from a 50 mg/L solution, by ASBBRJM16, ASBBRJM85, and ASBBRJM87 bacteria are 61%, 28%, and 10%, respectively. The isolated bacteria promoted the oxidation of As(III) to As(V) via the arsenic oxidase enzyme. Note from the authors of this review: desorption data are not presented in the manuscript.

Different biosorbents, from the weed Rumex acetosella, are used to remove metal cations in wastewater [19]. Drying, grinding, and sieving of the stems is carried out to obtain the biomass, retaining the fractions of 250–500 μm and 500–750 μm, which served to obtain the biosorbents in natura (unmodified), acidic, alkaline, and mixed. The 250 μm mixed treatment was the one that presented the highest removal percentages, mainly due to the OH, NH, ∙C∙H, COOH, and C∙O functional groups, achieving the removal of up to 33% of arsenic (also 96% for lead, 36% for zinc and 34% for cadmium). For contact times of 120 min and an optimum pH of 5.0, a loss of cellulose mass of 59% at 328°C and a change in the surface of the material are also observed, which allowed for obtaining topography with greater chelating capacity. Langmuir and pseudo-second-order models fitted the experimental data. Note from the authors of this review: this manuscript also presented the major error of considering the presence of As³⁺ in the solution. Moreover, no desorption data are included in the work.

Another type of exchanger is developed from waste biomass from watermelon rind after increasing the carboxyl functional groups by saponification [20]. Saponified watermelon rind (SWR) is further loaded with La(III) to attenuate the contamination of As(III) from water. As it is usual, different techniques are used to characterize the biosorbent. Arsenic speciation of adsorption product through X-ray photoelectron spectroscopic (XPS) analysis revealed that As(III) is partially oxidized to As(V) during the biosorption process. As(III) uptake onto La(III)-SWR is best described by Langmuir isotherm and pseudo-second-order kinetic model. Arsenic(III) removal from the solution is due to the exchange of H₂AsO₃⁻ from the solution and OH∙ groups of the bioadsorbent:

$$\text{P}\equiv La-OH+{\text{H}}_{2}As{\text{O}}_3^{-}\to \text{P}\equiv La-{\text{H}}_{2}As{\text{O}}_3+{\text{OH}}^{-}$$E2

where P represented the non-reactive part of the bioadsorbent. At a pH of 12.08, the optimum biosorption capacity is found to be 38, 49, and 63 mg/g, at temperatures 25°C, 30°C, and 35°C, respectively. The presence of chloride and nitrate in the solution has negligible interference with As(III) adsorption, whereas sulfate and phosphate significantly decrease As(III) biosorption. A thermodynamic study showed the spontaneous and endothermic nature of As(III) biosorption onto La(III)-SWR. As it is noted above, As(III) partially oxidized to As(V) due to aerial oxidation occurring during the adsorption process; in any case, H₂AsO₄⁻ species also is bioadsorbed onto La(III)-SWR bioadsorbent. Elution is carried out with NaOH solutions, increasing the percentage of arsenic removal from the bioadsorbent when the NaOH concentration increases from 0.01 M to 2 M (near 98% arsenic removal).

To investigate arsenic biosorption by Chondracanthus chamissoi and Cladophora sp. macroalgaes [21], approximately 5 kg of algae are collected from Huanchaco’s beach and Sausacocha lake (Huamachuco, Peru), La Libertad, and used in the study [21]. Arsenic biosorption is carried out in four column systems, with 2 g of algae in pellets form each, circulating arsenic solutions of 0.25 and 1.25 mg/L at 300 mL/min cm². Metals concentration is determined at 3 and 6 h of treatment. At 6 h, Chondracanthus chamissoi presented an As biosorption of 96% and 85% from the 0.25 mg/L and 1.25 mg/L solutions, respectively. At the same time, Cladophora sp. presented arsenic biosorption of 96% and 42% from the above solutions. It is concluded that Chondracanthus chamissoi achieves higher percentages of biosorption than Cladophora sp. from solutions of 1.25 mg/L As, and that there is no significant difference between the biosorption percentages of Chondracanthus chamissoi and Cladophora sp. from 0.25 mg/L solutions of the metal after 6 h of treatment. In the investigation, the algae are pretreated with HCl solutions, since this pretreatment improves the biadsorption capacity of the algae. This improvement is due to the protonated form, and there is a release of protons that favors the exchange of metals from the solution. Note from the authors of this review: this release of protons should be indicative of a cation exchange reaction; thus, do the authors consider once more that arsenic(III) is present as a cation in the solution? No desorption data are included in the work.

Magnetite nanoparticles (MNPs) are synthesized using the seaweed–Ulva prolifera, an amply found marine source in the Western coastal regions of India [22]. This biosorbent material is used as nanoadsorbent for the removal of As(III) from an aqueous solution. The optimum conditions to achieve the best arsenic(III) removal (97.5%) from the solution are: pH 9, rotation speed of 150 rpm, 90 min of reaction time, and 1.15 g/L MNPs dosage. The uptake process fitted with the non-linear forms of Langmuir isotherm and pseudo-second-order kinetic model. The highest arsenic adsorption capacity is 12 mg/g in a spontaneous and endothermic process. Note from the authors of this review: desorption experiments are absent in work.

Feather keratin-derived biosorbents using water-dispersed graphene oxide are used to create a biadsorbent for arsenic(III) [23]. Cross-linking of feather keratin with graphene oxide is investigated through X-ray photoelectrons spectroscopy (XPS), scanning and transmission electron microscopy, and Brunauer-Emmett-Teller (BET) analysis. The modifications resulted in increased surface area of the keratin proteins with substantial morphological changes, including the development of cracked and rough patches on the surface. These biosorbents exhibited excellent characteristics for the simultaneous removal (up to 99% in 24 h) of metal oxyanions, including arsenic, selenium, chromium, and cations including nickel, cobalt, lead, cadmium, and zinc from polluted synthetic water containing 600 μg/L of each metal. The insights into the biosorption mechanism revealed that the electrostatic interaction, chelation, and complexation primarily contributed to the removal of multiple heavy metal ions in a single treatment. Note from the authors of this review: desorption data are not included in the work.

Dry microalga Chlamydomonas sp. is used to remove arsenic(III) from aqueous solutions [24]. The experimentation was performed at a constant temperature of 25°C and a shaking speed of 300 rpm. Best conditions for As(III) removal are: pH 4, contact time of 60 min, temperature 25°C, and biomass concentration of 0.6 g/L. Metal uptake follows the Langmuir isotherm in its linear form, being the adsorption process of the endothermic character. Several desorbents are used (0.1 N solutions of HNO₃, H₂SO₄, HCl, NaOH, and EDTA), with the best desorption percentages yielded when EDTA is used, being this attributable to the chelating properties of this acid. Again, there is a continuous decrease in adsorption capacity after continuous use, that is, 76% removal capacity in the first cycle against 65% in the fifth cycle.

A recyclable biofilm material is prepared by loading Herminiimonas arsenicoxydans (H. arsenicoxydans) onto electrospun biomass-activated carbon nanofibers [25]. The removal of arsenic(III) is accompanied by catalytic oxidation of this oxidation state to As(V), being this oxidation attributable to a large amount of biomass accumulated and the formation of biofilms on the surface of the biomass-activated carbon nanofibers. The oxidation process responded to the first-order kinetic model. After five consecutive runs, the biofilms presented a convenient recyclability.

An adsorbent based on hydrous ferric oxide-impregnated agarose beads is used to remove As(III) from pharmaceutical wastewater [26]. The adsorption process fits well with the Langmuir model with nearly 76 mg/g of maximum arsenic load. The system is implemented in a column packed with the adsorbent beads, decreasing an initial arsenic concentration of 250–10 μg/L after five continuous cycles. Desorption is accomplished with NaOH solutions in the 0.05–3 M range, though column experiments used 0.1 M NaOH solutions as a compromise between desorption kinetics and rate of arsenic desorption. Continuous use does not affect the beads morphology. Whereas arsenic(III) is loaded onto the beads via and inner and outer sphere complexation-association, desorption is mainly governed by the removal of the outer sphere arsenic species.

Rice husk is the precursor of an eco-friendly Fe-Al bimetallic oxide/biochar adsorbent composite used in the removal of arsenic(III) from aqueous medium [27]. Maximum adsorption is reached at pH 5.5, being the experimental adapt fitted to the Freundlich isotherm and the pseudo-first-order kinetic model. Desorption is accomplished with 0.5 M NaOH solutions; however, there is a continuous decrease in the adsorption rate after continuous use, that is, 75% in the first cycle and 30% after the fifth cycle; this decrease is attributable to (i) incomplete desorption after each cycle and (ii) loss of adsorbent material in the recycling process.

Psidium guajava (guava) leaf has been investigated as effective adsorbent for As(III) [28]. Different experimental variables are considered, and results show that regardless of arsenic concentration (0.01–0.05 g/L), the rate of arsenic(III) removal from the solution reached a maximum at pH 10, and decreased from pH 10 to pH 2. Also, it is shown that the percentage of arsenic(III) removal from the solution decreased as the initial metal concentration in the solution increased from 0.01 to 0.05 g/L. Several solutions, at 0.1 N concentration, are tested to desorb arsenic loaded onto the adsorbent, and the HCl medium presented the best rate of desorption if compared with sulfuric and nitric acid, sodium hydroxide, and water.

A bionanocomposite bead UiO-66/CB was fabricated through simple gelation followed by the freeze-casting method and used to decrease the concentration of As(III) from contaminated water [29]. The biocomposite was fabricated by dispersion of UiO-66 nanoparticles in networking pores of macroporous chitosan beads. Moreover, the millimeter-sized UiO-66/CB-2.0 performed better, with respect to arsenic(III) removal, than the freely dispersed UiO-66 nanoparticles. 1 g/L UiO-66/CB-2.0 reduced As(III) concentrations from 200 μg/L in groundwater to below 10 μg/L. Arsenic(III) can be desorbed with 0.1 M NaOH solutions, though a slight reduction in the adsorption efficiency was observed after five cycles. The investigation also presented data on the use of the modified biochar in a fixed-bed column.

Dithiocarbamate (DTC)-modified cellulose adsorbents can selectively separate metal ions from water, though they can be dissolved in this medium. Trying to resolve this problem, the adsorbent was cross-linked with epoxy or complexed with iron [30]. The iron-complexed adsorbents still had solubility problems, but cross-linkage with 6.0 mol% of epoxy resulted in a material that was almost insoluble and dispersed in the solution. The optimum contact time and pH for As(III) removal were 20 min and 3.0, respectively; the adsorption responded to the Langmuir isotherm equation, being the adsorption attributed to the next reaction:

$$3DTC+As{\left(OH\right)}_3=As{\left(DTC\right)}_3+3O{H}^{-}$$E3

thus, the adsorption process was increased at acidic pH values. Note from the authors of the review: apparently, the authors of the manuscript did not interpret well the adsorption mechanism, Eq. (3), because it was doubtful that As(OH)₃ or H₃AsO₃ species predominant at these pH values (see Figure 1) lost OH⁻ groups; moreover, at these acidic pH values OH⁻ species never exists. No desorption data are included in the manuscript.

Using biochar derived from cotton stalks, the next reference [31] used a chemical treatment to modify the adsorption properties of the biomaterial. Thus, in the investigation, three biochars were used in the removal of arsenic from contaminated As(III) skipped-drinking water: CSB (cotton stalk biochar), HN-CSB (treated with nitric acid, and Na-CSB (treated with sodium hydroxide). An isotherm model showed that arsenic biosorption was best fitted to the Langmuir isotherm in all the three biochars CSB (qmax = 103 μg/g), Na-CSB (qmax = 151 μg/g), and HN-CSB (qmax = 157 μg/g). The chemical modification of the biochar produced biomaterials with the largest surface area, porous structure than the pristine material, and also promoted the presence of new functional groups on the surface of the modified biochars. Note from the authors of the review: desorption data were not given in the manuscript.

3. Arsenic(V)

Similarly to arsenic(III), arsenic(V) presented different speciation at the various pH values of the aqueous solution. This speciation is shown in Figure 2 [32], which demonstrates the sequence of neutral to anionic arsenic(V) species as the pH of the solution increased. Thus, the bioadsorption of this element presumably will be dependent, among other variables, on this speciation, and thus, of the pH of the aqueous phase.

Figure 2.

Distribution of arsenic(V) species with the aqueous pH.

The removal of As⁵⁺ in cassava wastewater, using an efficient biosorbent from chemically pretreated unshelled Moringa oleifera seeds, is investigated [33]. Several experimental conditions are used to investigate, in order to achieve efficient metal removal from the wastewater. The results of Fourier-transform infrared spectroscopy clearly suggested that additional functional groups attributed to esters are formed in the pretreated biosorbent, which is responsible for the improvement in biosorption. It was found that contact time, bioadsorbent dosage, and bioadsorbent pretreatment concentration had a significant influence on arsenic removal. Maximum percentage removal of 99.9% is reached in the synthetic solution at pH 4.0, contact time of 30 min, and dosage of 2 g for biosorbent pretreated with 1 M of chemical solution. It is shown that the bioadsorption process for untreated bioadsorbent is by ion exchange, while for treated material, the removal of the metal is attributable to a multifarious adsorption mechanism. The biosorption process was exothermic and spontaneous. Note from the authors of this review: once again, a major error occurs since the authors considered the existence of cationic As⁵⁺ species in the aqueous solution (also chromium(VI) as Cr⁶⁺), that evidently (see Figure 2) does not exist. No desorption data are included in the manuscript.

The treatment of arsenic-contaminated water with low-cost and environmental-friendly adsorbents, such as biochar, is considered a promising technique [34]. Thus, the treatment of As-contaminated water using eggshell biochar was investigated. Various parameters affecting the adsorption process, such as pH, contact time, adsorbent dosage, As(V) concentration, and the effects of anions, are studied. The results revealed that at a pH of 4.5, maximum adsorption of near 6 mg/g is reached (after 2 h), with a percentage of As(V) removal of 96%, from an initial metal concentration of 0.6 mg/L and a bioadsorbent dosage of 0.9 g/L. The SEM-EDS data illustrated that biochar consisted of a large number of active sites for As(V) adsorption, appearing the metal species on the biochar surface after metal uptake. This uptake is well represented by the Freundlich and the pseudo-second-order kinetic models. The presence of phosphates in the solution is detrimental for arsenic(V) loading onto the biochar, this being probably attributable to the fact that phosphate ions are adsorbed, on the same adsorption sites of the biochar, as arsenic(V) species. Note from the authors of this review: The manuscript does not include desorption data.

Artocarpus heterophyllus seed powder is used as a biosorbent material to remove different heavy metals [35]. The batch adsorption studies confirmed the higher removal percentage of this bioadsorbent (jackfruit) seed powder for arsenic (As⁵⁺), cadmium (Cd²⁺), and chromium (Cr⁶⁺), while lower efficiency is observed for other heavy metals like copper (Cu²⁺), zinc (Zn²⁺), and nickel (Ni²⁺). Optimization of various process parameters is carried out, and the optimum conditions are: adsorbent weight of 0.5 g for initial metal concentrations of 40 μg/L, 30 mg/L, and 30 mg/L; contact time of 10 h, 8 h, and 6 h; process temperature from 25 to 30°C; pH of 7, 7.5, and 7.5 for arsenic, cadmium, and chromium, respectively. The equilibrium data of the study are well fitted for Langmuir isotherm (arsenic, cadmium, and chromium) and Freundlich isotherm (arsenic and chromium). The kinetic and thermodynamic study confirmed that the adsorption of all three heavy metals followed the pseudo-second-order kinetics in an endothermic and spontaneous process. Several solutions are considered to desorb arsenic(V) from an As-loaded bioadsorbent. The best results are obtained when 1 M HCl solutions are used. However, after various adsorption–desorption cycles, a continuous decrease of the adsorption is observed: near 70% in the first cycle and 20% in the fifth cycle. Note from the authors of this review: again, authors considered arsenic(V) species as the non-existent As⁵⁺, and the same major error occurs in the case of chromium(VI) speciation: consideration of the existence of Cr⁶⁺ cation in the aqueous solution.

An adsorbent based in a matrix of diatomitex and chitosan as the modifier is used to investigate its properties to remove As(V) from solutions [36]. In the hybrid adsorbent, open chains of chitosan are grafted onto the matrix surface, and arsenic is loaded onto the adsorbent through ∙NH2 groups of the biomaterial. Maximum arsenic(V) removal from the solution occurs at pH 5, presenting the hybrid adsorbent with better performance, with respect to As(V) removal, than the pristine diatomite. Desorption is investigated using HCl solutions, with a slight decrease in As(V) uptake onto the adsorbent after continuous cycles, 94.2% in the first cycle and 90.4% in the fifth cycle.

Chitosan, supported onto a modified polypropylene membrane, is another bioadsorbent used to remove arsenic(V) from aqueous solutions [37]. The removal of arsenic from the solutions depended on the pH of the solution and the degree in which chitosan is grafted to the membrane. Metal adsorption, at pH 6.5, is related to a chemisorption process:

$$R-N{H}_3^{+}+H_{3}As{O}_4\to R-N{H}_3^{+}{H}_{2}As{O}_4^{-}+H^{+}$$E4

obeying the pseudo-second-order kinetic model equation. Note from the authors of this review: accordingly to the diagram of distribution (Figure 2), at this pH of 6.5, H₃AsO₄ is not the predominant As(V) species in solution. No desorption data is included in the work.

Miscanthus biochar has been used to investigate its performance on As(V) (and Cd(II) from aqueous media of different pH values [38]. Arsenic(V) is best removed from the solution at alkaline pH values; in the removal process, arsenic(V) is reduced to arsenic(III) and zero-valent arsenic. This reduction being attributable to electrons from the biochar under a physisorption and hydrophobic interactions between arsenic and the adsorbent. Note from the authors of this review: as it is noted in the manuscript, the process leads to an adsorbent loaded with arsenic(III), which represented a further hazard to humans and environment. No desorption data included in the work.

A composite of MgO nanoparticles with Itsit biochar (MgO-IBC) was used to remove arsenate from contaminated water [39]. Experimental results showed that both pH of the water and temperature influenced the overall adsorption efficiency of the biocomposite: low pH and high temperature resulted in higher arsenate bioadsorption. The removal process fitted the pseudo-second-order model and the Langmuir isotherm, through a chemisorption and spontaneous process. The arsenic-loaded adsorbent was washed with 6% HCl solution; however, the percentage of arsenic adsorption dramatically decreased from the first (96%) to the fourth cycle (32%).

Two biocomposites: iron(III)-chitosan and iron(III)-chitosan-CTAB composites, were prepared using an ionotropic gelation method [40]. Compared with the former composite, iron(III)-chitosan-CTAB material was more effective for As(V) adsorption at a wide range of pH (3.0–8.0). Again, adsorption processes fitted well with the pseudo-second-order model, though in these cases, the removal of arsenic responded to the Freundlich isotherm. In the presence of H₂PO₄^–, the adsorption of As(V) decreased. 1% w/v NaOH solution was the best desorption agent. Both bioadsorbents maintained their initial adsorption capacities after five adsorption-desorption cycles. Characterization results indicated that both electrostatic attraction and surface complexation mechanisms were responsible for arsenic(V) uptake (H₂AsO₄⁻) onto these two bioadsorbents.

4. Biosorption systems for As(III) and As(V)

A magnetic biochar is generated by pyrolysis of waste leaves of Raphanus sativus or Artocarpus heterophyllus peel pretreated with FeCl3 and used to remove As(III) and As(V) from solutions [41]. Maximum uptake onto the two adsorbent is very similar for both oxidations states, that is, 2.08 mg/g and 2.03 mg/g for As(III) and As(V), respectively. The pseudo-second-order kinetic model fitted well with the adsorption of both arsenic oxidation states and the two bioadsorbents. Note from the authors of this review: no desorption data included in the manuscript.

Calcium alginate caged graphene oxide functionalized with metformin (Alg@GMet) forming beads is used to investigate the removal from aqueous solutions, both for As(III) and (V) [42]. Best removal rate depended on the arsenic oxidations state: (i) in the (III) state maximum loading resulted in pH values of 2–6, being H₃AsO₃ the loaded arsenic species, (ii) arsenic(V) is loaded at pH values in the 2–4 range, being H2AsO4- the species removed from the solution. In both cases, the adsorption process fitted well with the linear form of the Langmuir isotherm and the pseudo-second-order kinetic equation. Also, in both cases, the adsorption process is exothermic, indicating the values of ΔG° that in both cases, the adsorption responded to a physisorption process. Desorption is accomplished with 0.1 M NaOH; however, there is a loss of adsorption capacity, that is, As(III) from 81 to 64%, As(V) from 98 to 62%, being the values from the first to the seventh cycle. The system is used in the removal of arsenic from tap water and sewerage waste; removal rates are: As(III) 69% (tap water) and 73% (sewerage waste), As(V) 62% (tap water), and 66% (sewerage waste).

Two activated biochar materials: peanut char (δ-MnO₂/A-PC) and corn char (δ-MnO₂/A-CC), were used in the treatment of a solution containing 97.5% As(III) and 2.5% As(V) [43]. Using δ-MnO₂/A-PC, 18.8% of As(III) and 35.4% of As(V) remained in the solution after 24 h of contact between both phases, indicating that (i) part of As(III) was removed from the solution by the adsorption process and (ii) that As(III) was also partially oxidized to As(V). Experimental results indicated that δ-MnO₂/A-CC was more suitable for removing waters with low As(V) concentrations, whereas δ-MnO₂/A-PC performed betters in solutions containing high As(III) concentrations. Fourier-transform infrared spectroscopy and X-ray diffraction analyses demonstrated that δ-MnO₂ was coated onto the surfaces of the biochars. Note from the authors of the review: no desorption data are included in the work.

The next investigation [44] utilized Fe²⁺ (FeCl₂) and zirconium oxychloride (ZrOCl₂) to synthesize a modified biochar (FeZrO-BC) by a co-precipitation method. The biochar was used to remove arsenite and arsenate from contaminated natural water. The addition of these two metals resulted in the formation of positively charged Zr-O and Fe-O groups. Kinetics and isothermal adsorption results indicated that both arsenic species are removed from the water via chemisorption on a monolayer, with maximum adsorption capacities of 46.7 mg/g As(III) and 47.8 mg/g As. Water was ineffective in desorbing arsenic-loaded materials, though both As(III) and As(V) can be desorbed with 0.05 M NaOH or nitric acid media. There was a loss in adsorption capacity from the first to the third cycle (Table 1).

Cycle	NaOH	HNO3
First	89.7	89.4
Second
Third	49.3	55.4

Table 1.

FeZrO-BC loss of capacity after continuous use.

Data for As(III). As(V) adsorption performed the same.

Iron(III) chloride impregnation of bagasse fly ash produced a biomaterial used in the removal of As(III) and As(V) from aqueous solutions [45]. Maximum removal of As(III) (95%) and As(V) (97%) was yielded from solutions containing less than 20 μg/dm3), whereas with solutions of 500 μg/dm3, the percentage of removal was 86% (As(III)) and 87% (As(V)) 3 g/dm3. The adsorption of both arsenic species responded to the pseudo-second-order kinetic model in an exothermic and spontaneous process. The regeneration study was carried out by different solvent and thermal methods; with HCl, the best desorption results were yielded: As(III) (83%), As(V) (74%). Thermal desorption produced a continuous decrease in the adsorption capacity, obviously due to a degradation of the adsorbent.

A magnetite-impregnated nitrogen-doped hybrid biochar (N/Fe₃O₄@BC) was used for the removal of arsenate and arsenite from aqueous environment [46]. Maximum monolayer adsorption values were 18.15 mg/g (arsenate) and 9.87 mg/g (arsenite), which were higher values than that of pristine biochar 9.89 and 8.12 mg/g, respectively. Adsorption fitted to the pseudo-first-order model, indicative of a physicochemical process. The bioadsorption of arsenic species was attributable to the presence of surface groups (OH∙, ∙NH₂, and ∙COOH), electrostatic attraction (via H- bonds), surface complexation and ion exchange followed by external mass transfer diffusion, and As(III) oxidation into As(V) by (N/Fe₃O₄@BC) reactive oxygen species. NaOH medium was used to desorb As(III) and As(V) from the loaded adsorbent, and it was observed a continuous loss in adsorption properties from the first (90% As(III), 80% As(V)) to the seventh cycle (60% As(III), 55 As(V)). Also, some degree of arsenic oxidation was observed in the desorption step.

5. Biosorption systems without mentioning the metal oxidation state

The next reference [47] investigates the combined photosynthetic activities of two green microalgal species, Tetradesmus obliquus and Tetradesmus reginae, on an integrated biophotovoltaic (BPV) platform for simultaneous wastewater treatment, toxic metal biosorption, carbon biofixation, bioelectricity generation, and biodiesel production. The wastewater used in the experimentation is collected at the Das-poort Wastewater Treatment Plant (DWWTP) located at the Tshwane Metropolitan Municipality in Central Pretoria, South Africa. The experimental setup comprised a dual-chambered BPV with copper anode surrounded by T. obliquus in BG11 media, and copper cathode with T. reginae in municipal wastewater separated by Nafion 117 membrane. The investigation reported a maximum power density of 0.344 W/m² at a cell potential of 0.415 V with external resistance of 1000 Ω and 0.3268 V maximum open-circuit voltage. The wastewater electrical conductivity and pH increased from 583 ± 22 to 2035 ± 29.31 mS/cm and 7.4–8.3, respectively, signifying increased photosynthetic and electrochemical activities. Residual nitrogen, phosphorus, chemical oxygen demand, arsenic, cadmium, chromium, and lead removal efficiencies by T. reginae are 100%, 81%, 72%, 48%, 89%, 71%, and 93%, respectively. Note from the authors of this review: no adsorption data were included in the work.

Natural waste adsorbents are used to eliminate arsenic and cadmium from aqueous solutions, and at the same time, reducing the amount of waste products [48]. The adsorbents used in the investigation are coconut husk and banana peel. Different experimental conditions are used in the work: adsorbent dosages (0.1–0.3 g), contact time (30–70 min), and temperature (25–45°C). The FTIR analysis revealed that certain heavy metals are more likely to load onto these adsorbents due to the presence of ∙OH and C〓O functional groups. The optimum removal conditions are 0.1 g dose of adsorbent, and 70 minutes of contact time at a temperature of 25°C. The results revealed that banana peel removed 0.148 mg/L of arsenic (0.948 mg/L of cadmium) from the aqueous solution, suggesting that it is a more efficient adsorbent than coconut husk. Using banana peel as bioadsorbent, the percentage of arsenic removal is in the 8–22% range (94–99% for cadmium). Note from the authors of this review: no adsorption data were included in the work.

Apple residues, banana peel, eggshell, potato peel, and sweet potato peel were tested as biosorbents to remove toxic metals (As, Cd, Hg, and Pb) from contaminated waters [49]. Adsorption experiments are performed using 0.5 g of each biosorbent in 1 L of natural tap water spiked with a mixture of the above metals at realistic concentrations (50 μg/L) under different pH values (4.5, 6.5, and 8.5) and water salinities (0, 10, and 30). The analysis by scanning electron microscopy showed differences among biosorbents, mainly in pore size and fibrous structures. Fourier-transform infrared spectroscopy identified cellulose, hemicellulose, pectin, and lignin in all biosorbents, except in eggshells, which are constituted mainly by carbonates. Results showed that the levels of all the above metals in water are considerably reduced by the biosorbents and in less than 3 h (more accentuated for apple and banana peel). However, with all the above bioadsorbent the percentage of arsenic removal never is greater than 6%, value much lower than those of mercury (up to 99%), cadmium (76%), and lead (86%). Note from the authors of this review: again, desorption data are not included in the manuscript.

6. Organoarsenics

This review manuscript [50] considered a series of organoarsenics (roxarsone, p-arsalinic acid, carbazone, triphenylarsine, phenylarsenic acid, etc.) to describe some features in their removal from aqueous solutions. Data collected included the effect of the pH, temperature, initial concentration of the organoarsenics, adsorbent dosages, contact time, and other variables on organoarsenics removal from solutions. The manuscript also insights into the mechanism of organoarsenics uptake onto various adsorbents and also the fit of experimental data on loading isotherms and kinetics models (these adsorptions generally fit better to the Langmuir isotherm and the pseudo-second-order kinetics model). Finally, some data about desorption and regeneration operations are included in the manuscript. Generally speaking, there is a progressive decline of active sites during continuous adsorption-desorption cycles that could be responsible of this decrease in adsorbent performance.

Biochar composites fabricated from polyaluminum chloride (PAC) sludge were used to investigate their adsorption properties toward dimethylarsinic acid (DMA) and also inorganic As(III) and As(V) [51]. In the case of DMA, the removal process fitted the pseudo-second-order kinetics and Freundlich isotherm models. In the adsorption process, DMA suffered a demethylation process, and As(V) was reduced to As(III) due to microorganisms present in the system. A large-scale field experiment carried out in an artificial ecological wetland showed that the addition of biochar reduced the total arsenic concentration to be immobilized in wetland sediment by 19%. Note from the authors of the review: desorption data were not included in the manuscript.

7. Conclusions

This work reviews the most recent additions to the study of arsenic removal from aqueous solutions using bioadsorbents. This technology appears to be the most used separation technology, to investigate the removal of this hazardous metal, in comparison with other recently published data about the use of other technologies in this important issue.

It is worth noting that these investigations mainly focus on scientists in Africa and Asia. An analysis of the works reviewed here, from ref. [14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31] and [33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51], 66.7% of the manuscripts are generated from Asia (India, Nepal, Pakistan, Saudi Arabia, Malaysia, China, Iran, Republic of Korea, and Taiwan), 18.5% from Africa (Egypt, Nigeria, and South Africa), 7.4% from South America (Peru) and 3.7% each from North America (Canada) and Europe (Portugal) (the classification is done in the basis of the institution in which the corresponding author is located).

Despite the great interest raised within these investigations, these reviewers found several important drawbacks in some of the published manuscripts:

1. in a number of published manuscripts, authors consider that both arsenic(V) and arsenic(III) elements are present, in the aqueous solutions, as the respective cationic species As⁵⁺ and As³⁺, which is a major chemical error since these cations never exist in aqueous solutions The above decreases the potential interest of the respective published works,

2. a number of the published manuscripts, 17 from a total of 27, which means 63%, do not consider the desorption step. This can also be considered an error, since the removal of a given element via biodesorption (or other separation technology) always consists of two steps: metal uptake onto the bioadsorbent-desorption, thus, without knowledge of how the potential biadsorbent performs in the desorption step, one has not an overall view of the whole removal process,

3. in the case in which this desorption step is considered, not a single manuscript mentioned what to do with the desorbed solution, which presumably contains a greater arsenic concentration than the original feed solution, resulting in a more toxic waste.

The potential of bioadsorbents, to remove arsenic from solutions, is here and is of current interest (both as a research tool and in a practical form), and hopefully, some of the drawbacks mentioned above will be resolved in future publications.

Besides these potential approaches to remove this contaminant from waters, governments must still apply strict laws to remediate this issue; if not, life together to the contamination problem go on.

Acknowledgments

The authors thank CSIC (Spain) for Project 202250E019.

Conflict of interest

The authors declare no conflict of interest.