Laboratory based approaches for arsenic remediation from contaminated water: recent developments

A comprehensive review on spatial and temporal variation of arsenic contamination in Ghaghara basin and its relation to probable incremental life time cancer risk in the local population

Spatial and temporal variations have been found in the levels of arsenic (As) throughout the groundwater of the Ghaghara basin. Fifteen out of twenty-five districts in this basin are reported to be affected by As, where the levels of As in groundwater and soil exceed the permissible limits set by the WHO (10 μgl-1) and FAO (20 mgkg-1) respectively. These districts include a total of four municipalities in Nepal and eighty-six blocks in India, all of which have varying degrees of As contamination. Approximately 17 million people are at risk of As poisoning, with more than two orders of magnitude higher potential lifetime incremental cancer risk, constituting over 153 thousand potential additional cases of cancer due to As-contaminated drinking water. Out of the 90 As-contaminated blocks in the Ghaghara basin, 4 blocks have about 7-fold higher potential risk of developing cancer, 49 blocks have 8-37-fold higher risk, and 37 blocks have up to 375-fold higher risk compared to the upper limit of the USEPA acceptable range, which is 1 × 10-6-1 × 10-4. High accumulation of As has been reported in the nails, hair, and urine of local inhabitants, with higher levels observed in females than males. The toxicity of As is manifested in terms of a higher occurrence of various diseases. Reproductive endpoints, such as increased incidences of preterm birth, spontaneous abortion, stillbirth, low-birth weight, and neonatal death, have also been reported in the basin. The level of As in tube wells has been found to be negatively correlated with the depth (r = -0.906), and tube wells with high levels of As (>150 μgl-1) are generally located within close proximity (<10 km) to abandoned or present meander channels in the floodplain areas of the Ghaghara river. In addition to As contamination, the water quality index (WQI) in the Ghaghara basin is poor according to the BIS standards for drinking water. Groundwater in six out of fifteen districts is unsuitable for drinking purposes, with a WQI exceeding 100. The levels of As in agricultural soil in many villages of Ballia, Bahraich, and Lakhimpur Kheri districts have exceeded the FAO limit. Water from deep tube wells has been found to be relatively safe in terms of As content, and thus can be recommended for drinking purposes. However, the use of surface water needs to be encouraged for irrigation purposes in order to preserve soil health and reduce As contamination in the food chain, thereby minimizing the risk of cancer.

Arsenic Levels and Seasonal Variation in Pasture Soil, Forage and Horse Blood Plasma in Central Punjab, Pakistan

The present study aimed to determine the accumulation levels of arsenic in pasture soil, forage and animals. An atomic absorption spectrophotometer was used to determine the concentration of metals in the samples of soil, forage and blood plasma of horses. The level of arsenic ranged between 4.26 mg/kg (summer) and 4.66 mg/kg (winter) in soil samples and 2.67 mg/kg (summer) and 2.94 mg/kg (winter) in forage samples. In blood plasma samples, the mean arsenic (As) values varied between 1.38 and 1.52 mg/L. In the blood plasma samples, the mean As values varied between 1.38 and 1.52 mg/L. No statistically significant changes were observed for arsenic concentrations in plasma samples of horses for sampling seasons (p > 0.05). A positive correlation was observed for forage and blood plasma to a certain degree for arsenic but for other media, arsenic correlations were negative and insignificant. It is therefore suggested that regular monitoring of heavy metals in soils/plants/animals is essential to prevent excessive build-up of arsenic.

Design of a Whole-Cell Biosensor Based on Bacillus subtilis Spores and the Green Fluorescent Protein To Monitor Arsenic

A green fluorescent protein (GFP)-based whole-cell biosensor (WCB-GFP) for monitoring arsenic (As) was developed in Bacillus subtilis. To this end, we designed a reporter gene fusion carrying the gfpmut3a gene under the control of the promoter/operator region of the arsenic operon (Pars::gfpmut3a) in the extrachromosomal plasmid pAD123. This construct was transformed into B. subtilis 168, and the resultant strain was used as a whole-cell biosensor (BsWCB-GFP) for the detection of As. The BsWCB-GFP was specifically activated by inorganic As(III) and As(V), but not by dimethylarsinic acid [DMA(V)], and exhibited high tolerance to the noxious effects of arsenic. Accordingly, after 12 h exposure, B. subtilis cells carrying the Pars::gfpmut3a fusion exhibited 50 and 90% lethal doses (LD50 and LD90) to As(III) of 0.89 mM and As 1.71 mM, respectively. Notably, dormant spores from the BsWCB-GFP were able to report the presence of As(III) in a concentration range from 0.1 to 1,000 μM 4 h after the onset of germination. In summary, the specificity and high sensitivity for As, as well as its ability to proliferate under concentrations of the metal that are considered toxic in water and soil, makes the B. subtilis biosensor developed here a potentially important tool for monitoring environmental samples contaminated with this pollutant. IMPORTANCE Arsenic (As) contamination of groundwater is associated with serious worldwide health risks. Detection of this pollutant at concentrations that are established as permissible for water consumption by WHO is a matter of significant interest. Here, we report the generation of a whole-cell biosensor for As detection in the Gram-positive spore former B. subtilis. This biosensor reports the presence of inorganic As, activating the expression of the green fluorescent protein (GFP) under the control of the promoter/operator of the ars operon. The biosensor can proliferate under concentrations of As(III) that are considered toxic in water and soil and detect this ion at concentrations as low as 0.1 μM. Of note, spores of the Pars-GFP biosensor exhibited the ability to detect As(III) following germination and outgrowth. Therefore, this novel tool has the potential to be directly applied to monitor As contamination in environmental samples.

Wheat PHT1;9 acts as one candidate arsenate absorption transporter for phytoremediation

Arsenate (AsV) is one of the most common forms of arsenic (As) in environment and plant high-affinity phosphate transporters (PHT1s) are the primary plant AsV transporters. However, few PHT1s involved in AsV absorption have been identified in crops. In our previous study, TaPHT1;3, TaPHT1;6 and TaPHT1;9 were identified to function in phosphate absorption. Here, their AsV absorption capacities were evaluated using several experiments. Ectopic expression in yeast mutants indicated that TaPHT1;9 had the highest AsV absorption rates, followed by TaPHT1;6, while not for TaPHT1;3. Under AsV stress, further, BSMV-VIGS-mediated TaPHT1;9-silencing wheat plants exhibited higher AsV tolerance and lower As concentrations than TaPHT1;6-silenced plants, whereas TaPHT1;3-silencing plants had similar phenotype and AsV concentrations to control. These suggested that TaPHT1;9 and TaPHT1;6 possessed AsV absorption capacity with the former showing higher activities. Under hydroponic condition, furthermore, CRISPR-edited TaPHT1;9 wheat mutants showed the enhanced tolerance to AsV with decreased As distributions and concentrations, whereas TaPHT1;9 ectopic expression transgenic rice plants had the opposite results. Also, under AsV-contaminated soil condition, TaPHT1;9 transgenic rice plants exhibited depressed AsV tolerance with increased As concentrations in roots, straws and grains. Moreover, Pi addition alleviated the AsV toxicity. These suggested that TaPHT1;9 should be a candidate target gene for AsV phytoremediation.

Arsenic Contamination in Groundwater: Geochemical Basis of Treatment Technologies

Arsenic (As) is abundant in the environment and can be found in both organic (e.g., methylated) and inorganic (e.g., arsenate and arsenite) forms. The source of As in the environment is attributed to both natural reactions and anthropogenic activities. As can also be released naturally to groundwater through As-bearing minerals including arsenopyrites, realgar, and orpiment. Similarly, agricultural and industrial activities have elevated As levels in groundwater. High levels of As in groundwater pose serious health risks and have been regulated in many developed and developing countries. In particular, the presence of inorganic forms of As in drinking water sources gained widespread attention due to their cellular and enzyme disruption activities. The research community has primarily focused on reviewing the natural occurrence and mobilization of As. Yet, As originating from anthropogenic activities, its mobility, and potential treatment techniques have not been covered. This review summarizes the origin, geochemistry, occurrence, mobilization, microbial interaction of natural and anthropogenic-As, and common remediation technologies for As removal from groundwater. In addition, As remediation methods are critically evaluated in terms of practical applicability at drinking water treatment plants, knowledge gaps, and future research needs. Finally, perspectives on As removal technologies and associated implementation limitations in developing countries and small communities are discussed.

Modified oyster shell powder with iron (II) sulfate heptahydrate to improve arsenic uptake in solution and in contaminated soils

Arsenic is a metalloid whose presence can be due to natural or anthropological causes. It is considered as a toxic chemical that puts human health at high risk. In this study, we evaluated a novel modified oyster shell (MOS) that was coated with iron (II) sulfate heptahydrate using two different proportions through batch sorption experiments in an arsenic solution and in arsenic-contaminated soils. The arsenic solution was prepared using As(III)-standard solution. The arsenic contaminated soils were extracted from a contaminated site in Cheonan, South Korea, where the average arsenic concentration of the soil was reported as 136.28 mg/kg. Different doses of oyster shell and modified oyster were used to understand the effect of the addition of iron (II) sulfate heptahydrate via sorption batch experiments in solution and sorption tests in soils. The sorption tests were conducted with 50 g of contaminated soil; then, 150 g of soils was used for the pot cultivation tests, and finally, 150 g of contaminated soils was used for column percolation test. Through the experiments, the authors observed a comparable improvement of arsenic stabilization from 10 to 60% with the addition of iron (II) sulfate heptahydrate to oyster shell.

Characteristics and mechanisms of As(III) removal by potassium ferrate coupled with Al-based coagulants: Analysis of aluminum speciation distribution and transformation

This study was carried out to investigate the enhanced removal of arsenite (As(III)) by potassium ferrate (K₂FeO₄) coupled with three Al-based coagulants, which focused innovatively on the distribution and transformation of hydrolyzed aluminum species as well as the mechanism of K₂FeO₄ interacted with different aluminum hydrolyzed polymers during As(III) removal. Results demonstrated that As(III) removal efficiency could be substantially elevated by K₂FeO₄ coupled with three Al-based coagulants treatment and the optimum As(III) removal effect was occurred at pH 6 with more than 97%. K₂FeO₄ showed a great effect on the distribution and transformation of aluminum hydrolyzed polymers and then coupled with a variety of aluminum species produced by the hydrolysis of aluminum coagulants for arsenic removal. During enhanced coagulation, arsenic removal by AlCl₃ was main through the charge neutralization of in situ Al₁₃ and the sweep flocculation of Al(OH)₃, while PACl₁ mainly depended on the charge neutralization of preformed Al₁₃ and the bridging adsorption of Al₁₃ aggregates, whereas PACl₂ mainly relied on the sweep flocculation of Al(OH)₃. This study provided a new insight into the distribution and transformation of aluminum species for the mechanism of As(III) removal by K₂FeO₄ coupled with different Al-based coagulants.

Removal of As(V) from aqueous solution using modified Fe3O4 nanoparticles

The removal of arsenic contamination from the aqueous environment is of great importance in the conservation of the Earth's water resources, and surfactants are a promising material used to modify magnetic nanoparticles to improve adsorption properties. Therefore, it is important to develop efficient and selective adsorbents for arsenic. Surface modification of Fe₃O₄ was carried out using anionic, cationic and zwitterionic surfactants to obtain composite Fe₃O₄@SDS, Fe₃O₄@CTAB, Fe₃O₄@SNC 16 and Fe₃O₄@NPC 16 (collectively referred to as Fe₃O₄@surfactants). The synthesized composite Fe₃O₄@surfactants magnetic nanoparticles were characterized by XRD, TEM and FTIR. The As(V) removal characteristics of the composite magnetic nanoparticles from the aqueous solution were evaluated by adsorption batch experiments which indicated the possibility of effective application of the surfactant-modified Fe₃O₄ magnetic nanoparticles for the removal of As(V) from aqueous solution. The adsorption equilibrium of the composites was reached in 30 min and the kinetic data followed the pseudo-second-order model. Langmuir equation could represent the adsorption isotherm data very well. Moreover, under the identical conditions, Fe₃O₄@CTAB showed maximum capacity of adsorption for As(V) (55.671 mg g^-1), with its removal efficiency being much higher than that of the other composites. In addition, the Fe₃O₄@surfactants composite magnetic nanoparticles retained 93.5% of its initial arsenic removal efficiency even after re-using it five times. The mechanism of arsenic adsorption by Fe₃O₄@surfactants composite magnetic nanoparticles was proved to be complexation via electrostatic attraction, which was mainly innersphere in nature.

Optimization of Coagulation-Flocculation Process in Efficient Arsenic Removal from Highly Contaminated Groundwater by Response Surface Methodology

Elevated arsenic (As) contamination in water, especially groundwater, has been recognized as a major problem of catastrophic proportions. This work explores As(V) removal via the coagulation-flocculation process by use of ferric chloride coagulant and polyacrylamide k16 co-coagulant as a first time. The effects of major operating variables such as coagulant dosing (50, 125 and 200 mg/L), co-coagulant dosing (5, 12.5 and 20 mg/L), pH (6, 7and 8), fast mixing time (1, 2 and 3 min), and fast mixing speed (110, 200 and 300 rpm) on As(V) removal efficiency were investigated by a Box-Behnken statistical experiment design (BBD) and response surface methodology (RSM). According to factors F values, coagulant dosing, rapid mixing speed, pH, and co-coagulant dosing showed the most effect on As(V) removal efficiency, and the rapid mixing time factor indicated the slightest effect. The proposed quadratic model was significant with a p value < 0.0001 and has satisfactorily described the experimental data with R2 and adjusted R2 values of 0.9855 and 0.9738, respectively. Predicted model optimal conditions with target of complete As(V) removal were coagulant dosing = 197.63 ppm, co-coagulant dosing = 19.55 ppm, pH = 7.37, fast mixing time = 1.43 min and fast mixing speed = 286.77 rpm. The treatment of Nazarabad well water sample with an initial As(V) concentration of 5 mg/L under the optimal conditions removed 100% As(V) with the volume of produced sludge of 10.7 mL/200 mL. Increasing coagulant dosing, co-coagulant dosing, fast mixing time and fast mixing speed operation parameters from low-level to high-level values indicated 78%, 20%, 10.52% and 9.47% increases in volume of the produced sludge, respectively. However, a reduction of 13.63% in volume of the produced sludge resulted via pH increases.

Removal of As(III) via adsorption and photocatalytic oxidation with magnetic Fe–Cu nanocomposites

Magnetic Fe-Cu nanocomposites with high adsorption capacity and photocatalytic properties were prepared via the precursor method using soluble substances isolated from urban biowaste (BBS) as carbon sources and different temperatures of the pyrolysis treatment (400, 600, and 800 °C). BBS is used as complexing agent for the Fe³⁺ and Cu²⁺ ions in the precursors. The as-prepared magnetic materials were tested in As(III) removal processes from water. Dark experiments performed with the materials obtained at 400 and 600 °C showed excellent adsorption capacities achieving a significant uptake of 911 and 840 mg g^-1 for As(III), respectively. Experiments conducted under steady-state irradiation showed a reduction of 50-71% in As(III) levels evidencing the meaningful photocatalytic capacity of Fe-Cu nanocomposites. The best photocatalytic performance was obtained for the nanocomposite synthesized at the highest pyrolysis temperature, in line with the reported trend of HO^· radicals production. Transient absorption spectroscopy experiments revealed the occurrence of an alternative oxidation pathway involving the valence band holes and yielded relevant kinetic information related to the early stages of the As(III) photooxidation. The higher absorption of the electron-hole pairs observed for the samples treated at lower temperature means that controlling the pyrolysis temperature during the synthesis of the Fe-Cu nanocomposites allows tuning the photocatalyst activity for oxidation of substrates via valence band holes, or via HO^· radicals.

Transient Glycolytic Complexation of Arsenate Enhances Resistance in the Enteropathogen Vibrio cholerae

The ubiquitous presence of toxic arsenate (As^V) in the environment has raised mechanisms of resistance in all living organisms. Generally, bacterial detoxification of As^V relies on its reduction to arsenite (As^III) by ArsC, followed by the export of As^III by ArsB. However, how pathogenic species resist this metalloid remains largely unknown. Here, we found that Vibrio cholerae, the etiologic agent of the diarrheal disease cholera, outcompetes other enteropathogens when grown on millimolar concentrations of As^V. To do so, V. cholerae uses, instead of ArsCB, the As^V-inducible vc1068-1071 operon (renamed var for vibrio arsenate resistance), which encodes the arsenate repressor ArsR, an alternative glyceraldehyde-3-phosphate dehydrogenase, a putative phosphatase, and the As^V transporter ArsJ. In addition to Var, V. cholerae induces oxidative stress-related systems to counter reactive oxygen species (ROS) production caused by intracellular As^V. Characterization of the var mutants suggested that these proteins function independently from one another and play critical roles in preventing deleterious effects on the cell membrane potential and growth derived from the accumulation As^V. Mechanistically, we demonstrate that V. cholerae complexes As^V with the glycolytic intermediate 3-phosphoglycerate into 1-arseno-3-phosphoglycerate (1As3PG). We further show that 1As3PG is not transported outside the cell; instead, it is subsequently dissociated to enable extrusion of free As^V through ArsJ. Collectively, we propose the formation of 1As3PG as a transient metabolic storage of As^V to curb the noxious effect of free As^V. This study advances our understanding of As^V resistance in bacteria and underscores new points of vulnerability that might be an attractive target for antimicrobial interventions. IMPORTANCE Even though resistance to arsenate has been extensively investigated in environmental bacteria, how enteric pathogens tolerate this toxic compound remains unknown. Here, we found that the cholera pathogen V. cholerae exhibits increased resistance to arsenate compared to closely related enteric pathogens. Such resistance is promoted not by ArsC-dependent reduction of arsenate to arsenite but by an operon encoding an arsenate transporter (ArsJ), an alternative glyceraldehyde 3-phosphate dehydrogenase (VarG), and a putative, uncharacterized phosphatase (VarH). Mechanistically, we demonstrate that V. cholerae detoxifies arsenate by complexing it with the glycolytic intermediate 3-phosphoglycerate into 1-arseno-3-phosphoglycerate (1As3PG). 1As3PG is not transported outside the cell; instead, it is subsequently dissociated by VarH to enable extrusion of free arsenate through ArsJ. Collectively, this study proposes a novel mechanism for arsenate detoxification, entirely independent of arsenate reduction and arsenite extrusion, that enhances V. cholerae resistance to this metalloid compared to other enteric pathogens.

Effects of phosphate, silicate, humic acid, and calcium on the release of As(V) co-precipitated with Fe(III) and Fe(II) during aging

The effects of phosphate (P), silicate (Si), humic acid (HA), and calcium (Ca) on the release of As(V) co-precipitated with Fe(III) and Fe(II) during aging were investigated. As(V) in synthetic groundwater could be efficiently removed by both Fe(III) and Fe(II) processes. The addition of P remarkably decreased As(V) removal efficiency while no obvious release of As(V) during aging was observed. Si and HA reduced As(V) removal to a less extent than P but caused notable As(V) release during aging. FTIR spectra and particle size of the precipitates before and after aging indicated that As(V) release in the presence of Si was due to the serious structural transformation and particle aggregation of the precipitates during aging. While for HA, As(V) release was caused by sorption of HA on the precipitates and dissolution of the precipitates by HA. The addition of Ca partially counteracted the adverse impacts of P, Si, and HA and promoted As(V) removal efficiency but had limited inhibitory effect on As(V) release as it induced more serious particle aggregation during aging. The results demonstrated that the release of As(V) caused by Si and HA should be considered when using Fe coagulation for in-situ treatment of As(V) contaminated groundwater.

In Situ Remediation of Arsenic-Contaminated Groundwater by Injecting an Iron Oxide Nanoparticle-Based Adsorption Barrier

Arsenic contamination of groundwater occurs due to both geogenic and anthropogenic processes. Conventional arsenic remediation techniques require extraction of groundwater into pump-and-treat systems, which are expensive and require long operational times. Hence, there is a need for cost-effective remediation. In this study, we assessed and validated the in situ remediation of arsenic contamination in groundwater resources using permeable reactive barriers (PRBs) made of injectable, colloidal iron oxide nanoparticles in the laboratory and in field-scale pilot tests. Sand-packed, flow-through column studies were used in order to assess the sorption behavior of the iron oxide nanoparticles using field materials (sand, groundwater) in the laboratory. The breakthrough curves were analyzed using a reactive transport model considering linear and nonlinear adsorption isotherms and were fitted best with a chemical nonequilibrium consideration. The results were used to design a pilot-scale field test. The injected 28 m3 of nanoparticles (ca. 280 kg dry weight of iron oxide) were successfully delivered to the aquifer via an injection well. No mobile iron was detected downstream, confirming that a stable in situ barrier was formed that did not move with the groundwater flow. Arsenic concentrations in groundwater were reduced to the aimed 50% of the background value, despite the relatively short contact time between arsenic and the iron oxide in the barrier, due to the high flow velocity of 1.21 m/day. We compared the results of the laboratory and field tests and concluded that the single-parameter models based on retardation factor and/or adsorption capacity fail to predict the longevity of the barrier and the evolution of arsenic breakthrough with time, most likely because they do not consider the chemical nonequilibrium effects. Therefore, we propose that upscaling the laboratory findings to field design must be carried out with care and be coupled with detailed reactive transport models.

Co-cultivation as a Strategy to Reduce Food Chain-Mediated Arsenic Contamination in Human Beings

Arsenic (As) is a highly toxic metalloid present naturally in the earth's crust. In developing countries apart from drinking water, one major reason for arsenic toxicity among human beings is through contaminated crops and vegetables. The nutritional quality of the crops and vegetables grown in the arsenic-infested area gets compromised. A major challenge is to protect the vegetables and crops from arsenic contamination. Attempts have been made through different remediation technologies. The present research is designed to reduce the arsenic load in arsenic-sensitive (non-hyperaccumulator) plants by co-cultivation with hyperaccumulator plants, thus saving food chain contamination to humans. In the present study, done in potted plants, it has been found that co-cultivated B. oleracea has 1.5 times decreased arsenic translocation compared to the control plant; on the contrary, hyperaccumulator B. juncea showed higher translocation. Plant health biomarkers like total chlorophyll and protein contents were two times higher in co-cultivated B. oleracea compared to the As-treated control which actually seconds the fact of less translocation in the co-cultivated plants. The stress marker like proline content, super oxide dimutase, and malondialdehyde content showed a decrease in co-cultivated B. oleracea compared to the control plant grown in arsenic-infested soil which again reflected less stress in co-cultivated plants. From these findings of the research, we can hypothesize that hyperaccumulator B. juncea might save B. oleracea from arsenic-induced toxicity when co-cultivated and thus can save food chain-mediated contamination to human beings.

Harnessing plant microbiome for mitigating arsenic toxicity in sustainable agriculture

Heavy metal toxicity has become an impediment to agricultural productivity, which presents major human health concerns in terms of food safety. Among them, arsenic (As) a non-essential heavy metal has gained worldwide attention because of its noxious effects on agriculture and public health. The increasing rate of global warming and anthropogenic activities have promptly exacerbated As levels in the agricultural soil, thereby causing adverse effects to crop genetic and phenotypic traits and rendering them vulnerable to other stresses. Conventional breeding and transgenic approaches have been widely adapted for producing heavy metal resilient crops; however, they are time-consuming and labor-intensive. Hence, finding new mitigation strategies for As toxicity would be a game-changer for sustainable agriculture. One such promising approach is harnessing plant microbiome in the era of 'omics' which is gaining prominence in recent years. The use of plant microbiome and their cocktails to combat As metal toxicity has gained widespread attention, because of their ability to metabolize toxic elements and offer an array of perquisites to host plants such as increased nutrient availability, stress resilience, soil fertility, and yield. A comprehensive understanding of below-ground plant-microbiome interactions and their underlying molecular mechanisms in exhibiting resilience towards As toxicity will help in identifying elite microbial communities for As mitigation. In this review, we have discussed the effect of As, their accumulation, transportation, signaling, and detoxification in plants. We have also discussed the role of the plant microbiome in mitigating As toxicity which has become an intriguing research frontier in phytoremediation. This review also provides insights on the advancements in constructing the beneficial synthetic microbial communities (SynComs) using microbiome engineering that will facilitate the development of the most advanced As remedial tool kit in sustainable agriculture.

Removal of As(V) from wastewaters using magnetic iron oxides formed by zero-valent iron electrocoagulation

Electrocoagulation of zero-valent iron has been widely applied to the removal of dissolved arsenic, but the solid-liquid separation of arsenic-containing precipitates remains technically challenging. In this work, zero-valent iron was electrochemically oxidized to magnetic iron oxides for the removal of As(Ⅴ) from simulated and actual mining wastewaters. The results indicated that lepidocrocite was formed when zero-valent iron was oxidized by dissolved oxygen, but ferrihydrite and green rust were first formed and then transformed to magnetic iron oxides (mainly magnetite and maghemite) in the electrochemical oxidation from 0 to 0.9 V (vs. SCE), which facilitates the adsorption of As(V) and subsequent solid-liquid separation under a magnetic field. In simulated As(V)-containing solution with initial pH 7.0, zero-valent iron was electrochemically oxidized to magnetite and maghemite at 0.6 V (vs. SCE) for 2 h. The As(V) concentration first decreased from 5127.5 to 26.8 μg L^-1 with a removal ratio of 99.5%. In actual mining wastewaters, zero-valent iron was electrochemically oxidized to maghemite at 0.6 V (vs. SCE) for 24 h, and the As(V) concentration decreased from 5486.4 to 3.6 μg L^-1 with a removal ratio of 99.9%. The removal ratio of As(V) increased slightly with increasing potential, and increased first and then decreased with increasing initial pH. Compared with that of SO₄^2- and NO₃^-, the presence of Cl^- significantly enhanced the removal of As(V). This work provides a highly efficient, facile and low-cost technique for the treatment of arsenic-containing wastewaters.

Novel design of amine and metal hydroxide functional group modified onto sludge biochar for arsenic removal

This study involved novel-designed sludge biochar (SB) adsorbed for arsenic removal with lower operating costs and higher adsorption efficiency properties. Generally, biochar only relies on micropores for pollutant adsorption, but physical adsorption is not highly efficient for arsenic removal. Therefore, in order to improve the removal efficiency of arsenic by SB, diethylenetriamine (DETA) and FeCl₃ were used in this study to modify the surface of SB by an immersion method. The objectives of this research are to obtain optimum operation conditions by assessing the effect of different Fe content, pH and initial concentration on adsorbing arsenic. This study is the first to use Density Functional Theory (DFT) to simulate and verify the adsorption mechanism of arsenic by SB. Results showed the presence of amine/iron oxyhydroxides functional groups greatly promoted SB surface activity and its arsenic adsorption potential. The surface area, pore volume and pore size of the SB were estimated to be 525 m² g^-1, 0.35 cm³ g^-1 and 8.71 nm, respectively. The DFT model result is the same as the result of arsenic adsorption performance with high adsorption energy (-246.3 kJmol^-1) and shorter bond distances (1.42 Å), indicating strong chemical adsorption between arsenic and material. The reaction mechanism is divided into four pathways, including oxidation-reduction, complexation, electrostatic adsorption and pore adsorption.

The Utilization of Algae and Seaweed Biomass for Bioremediation of Heavy Metal-Contaminated Wastewater

The presence of heavy metals in water bodies is linked to the increasing number of industries and populations. This has serious consequences for the quality of human health and the environment. In accordance with this issue, water and wastewater treatment technologies including ion exchange, chemical extraction, and hydrolysis should be conducted as a first water purification stage. However, the sequestration of these toxic substances tends to be expensive, especially for large scale treatment methods that require tedious control and have limited efficiency. Therefore, adsorption methods using adsorbents derived from biomass represent a promising alternative due to their great efficiency and abundance. Algal and seaweed biomass has appeared as a sustainable solution for environmentally friendly adsorbent production. This review further discusses recent developments in the use of algal and seaweed biomass as potential sorbent for heavy metal bioremediation. In addition, relevant aspects like metal toxicity, adsorption mechanism, and parameters affecting the completion of adsorption process are also highlighted. Overall, the critical conclusion drawn is that algae and seaweed biomass can be used to sustainably eliminate heavy metals from wastewater.

Effect of Stevia rebaudiana Bertoni residue on the arsenic phytoextraction efficiency of Pteris vittata L

Soil contamination by arsenic (As) presents a high risk to public health, necessitating urgent remediation. This study sought to develop an efficient strategy for the phytoremediation of As-contaminated soil. The effects of Stevia rebaudiana Bertoni residue (SR) on the available As (A-As) concentration of soil and As extraction from the soil by Pteris vittata L. were studied by soil simulation, pot, and field experiments. The A-As concentration in the soil simulation experiment increased significantly by 84.20% after 20 days. The biomass, As concentration, and total extracted As of SR-treated P. vittata L. in the pot experiment increased significantly by 50.66%, 120.2%, and 171.2%, respectively, compared to the untreated control. The SR-treated rhizosphere soil in the pot experiment displayed a significant 21.72% decrease in total As concentration. In the one-year field experiment, treatment with SR resulted in a significant 191.1% increase in As extraction by P. vittata L. and a significant 10.26% reduction in rhizosphere soil As concentration compared to the control. This study proposes a potential mechanism for SR-mediated enhancement of P. vittata L. As extraction ability and provides a new, economic, and environmentally friendly method for As-contaminated soil remediation.

Activated Carbons for Arsenic Removal from Natural Waters and Wastewaters: A Review

The arsenic pollution of waters and wastewaters is concerning many countries across the world, and because of the effects of arsenic on human health, its removal from waters is of great importance. Adsorption using functionalized activated carbons as a technique for the removal of arsenic from water streams has gained great attention. In the present review, we summarize synthesis technologies, the characterization of materials and arsenic removal capacity, and we clarify the parameters which play a critical role in the removal of arsenic, such as the pH value of the water, the active group in the functionalization and temperature. The review article concludes that most of the experimental data fit both Langmuir and Freundlich isotherms. In this review, the recyclability and reuse of the materials are also reported, and the findings show that for both arsenite and arsenate, even after several adsorption cycles, the material can be further used as an efficient adsorbent for arsenic removal.

A review on sources, identification and treatment strategies for the removal of toxic Arsenic from water system

Arsenic liberation and accumulation in the groundwater environment are both affected by the presence of primary ions and soluble organic matter. The most important influencing role in the co-occurrence is caused by human activity, which includes logging, agricultural runoff stream, food, tobacco, and fertilizers. Furthermore, it covers a wide range of developed and emerging technologies for removing arsenic impurities from the ecosystem, including adsorption, ion exchangers, bio sorption, coagulation and flocculation, membrane technology and electrochemical methods. This review thoroughly explores various arsenic toxicity to the atmosphere and the removal methods involved with them. To begin, the analysis focuses on the general context of arsenic outbreaks in the area, health risks associated with arsenic, and measuring techniques. The utilization of innovative functional substances such as graphite oxides, metal organic structures, carbon nanotubes, and other emerging types of composite materials, as well as the ease, reduced price, and simple operating method of the adsorbent material, are better potential alternatives for arsenic removal. The aim of this article is to examine the origins of arsenic, as well as identification and treatment methods. It also addressed recent advancements in Arsenic removal using graphite oxides, carbon nanotubes, metal organic structures, magnetic nano composites, and other novel types of usable materials. Under ideal conditions for the above methods, the arsenic removal will achieve nearly 99% in lab scale.

Simultaneous removal of arsenic, fluoride, and manganese from synthetic wastewater by Vetiveria zizanioides

Main aim of the present research is to explore the potential use of Vetiveria zizanioides L. for phytoremediation of arsenic, fluoride, and manganese simultaneously from synthetic wastewater in a batch scale floating platform unit. Half strength Hoagland's nutrient solution spiked with arsenic, fluoride, and manganese concentrations of 1, 20, and 10 mg/L, respectively has been used. The effects of pH and treatment time on simultaneous removal of arsenic, fluoride, and manganese have been performed. V. zizanioides has exhibited optimum growth at pH 8 and the removal of arsenic and fluoride is observed to be 59.6 and 38.1%, respectively. This plant has successfully removed all of the manganese (99.3%). The uptake of manganese is found to be faster than the arsenic and fluoride. The trend of arsenic, fluoride, and manganese accumulation in various parts of V. zizanioides is found as roots > stems > leaves. Result showed that the use of V. zizanioides would be appropriate to treat arsenic, fluoride, and manganese contaminated wastewater.

Abiotic oxidation of arsenite in natural and engineered systems: Mechanisms and related controversies over the last two decades (1999-2020)

Abiotic oxidation of toxic As(III) to As(V) is being deemed as a necessary step for the overall arsenic decontamination in both natural and engineered systems. Direct oxidation of As(III) by chemical oxidants, such as ozone, permanganate, ferrate, chlorine and chloramine, or naturally occurring minerals like Mn, Fe oxides, seems straightforward. Both O₂ and H₂O₂ are ineffective for arsenite oxidation, but they can be activated by reducing substances like Fe²⁺, Fe⁰ to increase the oxidation rates. Photo-induced oxidation of As(III) has been demonstrated effective in Fe complexes or minerals, NO₃^-/NO₂^-, dissolved organic matter (DOM), peroxygens and TiO₂ systems. Although a variety of oxidation methods have been developed over the past two decades, there remain many scientific and technical challenges that must be overcome before the rapid progress in basic knowledge can be translated into environmental benefits. To better understand the trends in the existing data and to identify the knowledge gaps, this review describes in detail the complicated mechanisms for As(III) oxidation by various methods and emphasizes on the conflicting data and explanation. Some prevailing concerns and challenges in the sphere of As(III) oxidation are also pointed out so as to appeal to researchers for further investigations.

Oxygen-rich poly-bisvanillonitrile embedded amorphous zirconium oxide nanoparticles as reusable and porous adsorbent for removal of arsenic species from water

A new oxygen-rich porous polymer based on bisvanillonitrile was synthesized and characterized. This polymer was employed as support for the anchoring of 14.5 w% amorphous zirconium oxide nanoparticles. The formation of homogeneously dispersed nanoparticles in the poly-bisvanillonitrile (PBVN) host material was confirmed using N₂-sorption, XRPD, XPS and electron microscopy. The combination of zirconium oxide nanoparticles having active adsorption sites with the porous supporting material showed excellent adsorption of arsenic species. The resulting adsorption capacities of the hybrid material extend to 245 mg g^-1 for arsenite (As^III) and 115 mg g^-1 for arsenate (As^V). Moreover, adsorption kinetics showed a fast removal of both arsenic species with initial adsorption rate h of 0.0646 mg g^-1 min^-1 for arsenite and 0.0746 mg g^-1 min^-1 for arsenate. The immobilization was not interfered by the presence of other compounds in solution, indicating the applicability in real working environments. The material could be regenerated in a continuous mode using a 0.1 mol L^-1 sodium hydroxide solution at 70 °C to desorb arsenic.

A powerful arsenite-oxidizing biofilm bioreactor derived from a single chemoautotrophic bacterial strain: Bioreactor construction, long-term operations and kinetic analysis

Microbial oxidation of As(III) by biofilm bioreactors followed by adsorption is a promising and environment friendly approach to remediate As(III) contaminated groundwater; however, poor activity, stability and expandability of the bioreactors hampered their industrious applications. To resolve this issue, we constructed a new biofilm bioreactor using a powerful chemoautotrophic As(III)-oxidizing bacterium Rhizobium sp. A219. This strain has strong ability to form biofilms and possesses very high As(III)-oxidizing activities in both planktonic and biofilm forms. Perlites were used as the biofilm carriers. Long-term operations suggest that the bioreactor has very high efficiency, stability and scalability under different geochemical conditions, and it is cheap and easy to construct and operate. During the operations, it is only required to supply air, nothing else. All the common contaminants in groundwater slightly affected the bioreactor As(III)-oxidizing activity. The common contaminants in groundwater can be largely removed through assimilation by the bacterial cells as nutrition. The bioreactor completely oxidize 1.0, 5.0, 10.0, 20.0 and 30.0 mg/L As(III) in 12, 18, 20, 25 and 30 min, respectively. Approximately 18, 18, 12, 12 and 21 min were needed to oxidize 1.1 mg/L As(III) at 20, 25, 30, 35 and 40 °C, respectively. The bioreactor works well under the pH values of 5-8, and the most optimal was 7.0. The data suggest that this bioreactor possesses the highest efficiency and stability, and thus has the great potential for industrial applications among all the described As(III)-oxidizing bioreactors derived from a single bacterium.

Artificial Floating Island with Vetiver for Treatment of Arsenic-Contaminated Water: A Real Scale Study in High-Andean Reservoir

Arsenic found in agriculture water reservoirs represents a threat to water security and safe agricultural products in developing countries. Small farms do not implement traditional water treatments due to the high cost; hence, a nature-based solution is an alternative to tackling this challenge. This paper investigated the potential of artificial floating island with Vetiver (AFIV) for the geogenic arsenic removal present in the reservoir of the Ilinizas páramo in Ecuador. We constructed two AFIV systems using PVC pipes in a reservoir batch type with a 3.6 m3 treatment capacity. Arsenic and iron were analyzed in duplicated every 30 days at the affluent and effluent through 120 days. The average remediation of arsenic was recorded as 97% in water and 84% in sediment, while the average remediation of iron was 87% in sediment. The survival rate of macrophytes was 92%; they accumulated arsenic in its roots that acted as a barrier against the translocation. The research demonstrated that the use of AFIV has the potential to rehabilitate reservoirs contaminated with arsenic under adverse climatic conditions such as the páramo ecosystem.

Treatment of aqueous arsenic - A review of biosorbent preparation methods

Arsenic (As) is a worldwide human health issue with the major exposure route being the consumption of As-contaminated drinking water. Sorption is considered to be an efficient treatment method, among other technologies, for As removal from various water and wastewater matrices. There are common commercially available sorbents, however, the use of locally or regionally available biomasses have recently been of interest as potentially cost-effective and environmentally friendly alternatives. Despite these benefits, untreated biomasses often show low sorption capacity, can be too fragile, and can lead to coloration of waters when used in treatment processes. Treatment methods of biomasses can include chemical processes using acid or alkaline solutions, developing of biomass composite by deposition of activating agents, and preparation of biochars. This review includes an overview of 53 recent studies that assess a variety of biomass modification methods meant to overcome these issues such as activation with acids or bases and biomass-based composites. Furthermore, future perspectives have been provided to assist in the further optimization of methods for biomass modifications to enhance their As sorption capacities.

Chemical Treatment of Highly Toxic Acid Mine Drainage at A Gold Mining Site in Southwestern Siberia, Russia

The critical environmental situation in the region of southwestern Siberia (Komsomolsk settlement, Kemerovo region) is the result of the intentional displacement of mine tailings with high sulfide concentrations. During storage, ponds of acidic water with incredibly high arsenic (up to 4 g/L) and metals formed on the tailings. The application of chemical methods to treat these extremely toxic waters is implemented: milk of lime Ca(OH)2, sodium sulfide Na2S, and sodium hydroxide NaOH. Field experiments were carried out by sequential adding pre-weighed reagents to the solutions with control of the physicochemical parameters and element concentrations for each solution/reagent ratio. In the experiment with Ca(OH)2, the pH increased to neutral values most slowly, which is contrary to the results from the experiment with NaOH. When neutralizing solutions with NaOH, arsenic-containing phases are formed most actively, arsenate chalcophyllite Cu18Al2(AsO4)4(SO4)3(OH)24·36H2O, a hydrated iron arsenate scorodite, kaatialaite FeAs3O9·8H2O and Mg(H2AsO4)2. A common specificity of the neutralization processes is the rapid precipitation of Fe hydroxides and gypsum, then the reverse release of pollutants under alkaline conditions. The chemistry of the processes is described using thermodynamic modeling. The main species of arsenic in the solutions are iron-arsenate complexes; at the end of the experiments with Ca(OH)2, Na2S, and NaOH, the main species of arsenic is CaAsO4−, the most toxic acid H3AsO3 and AsO43−, respectively. It is recommended that full-scale experiments should use NaOH in the first stages and then Ca(OH)2 for the subsequent neutralization.

Arsenic Toxicity: Molecular Targets and Therapeutic Agents

High arsenic (As) levels in food and drinking water, or under some occupational conditions, can precipitate chronic toxicity and in some cases cancer. Millions of people are exposed to unacceptable amounts of As through drinking water and food. Highly exposed individuals may develop acute, subacute, or chronic signs of poisoning, characterized by skin lesions, cardiovascular symptoms, and in some cases, multi-organ failure. Inorganic arsenite(III) and organic arsenicals with the general formula R-As²⁺ are bound tightly to thiol groups, particularly to vicinal dithiols such as dihydrolipoic acid (DHLA), which together with some seleno-enzymes constitute vulnerable targets for the toxic action of As. In addition, R-As²⁺-compounds have even higher affinity to selenol groups, e.g., in thioredoxin reductase that also possesses a thiol group vicinal to the selenol. Inhibition of this and other ROS scavenging seleno-enzymes explain the oxidative stress associated with arsenic poisoning. The development of chelating agents, such as the dithiols BAL (dimercaptopropanol), DMPS (dimercapto-propanesulfonate) and DMSA (dimercaptosuccinic acid), took advantage of the fact that As had high affinity towards vicinal dithiols. Primary prevention by reducing exposure of the millions of people exposed to unacceptable As levels should be the prioritized strategy. However, in acute and subacute and even some cases with chronic As poisonings chelation treatment with therapeutic dithiols, in particular DMPS appears promising as regards alleviation of symptoms. In acute cases, initial treatment with BAL combined with DMPS should be considered.

Investigating the protective actions of D-pinitol against arsenic-induced toxicity in PC12 cells and the underlying mechanism

Arsenic is awfully toxic metalloid responsible for many human diseases all over the world. Contrastingly, D-pinitol is a naturally occurring bioactive dietary compound has antioxidant properties. The objective of this study is to elucidate the protective actions of D-pinitol on arsenic-induced cytotoxicity and explore its controlling role in biomolecular mechanisms in PC12 cells. Obtained results demonstrated that co-exposure of D-pinitol with arsenic increases cell viability, decreases DNA damage and protects PC12 cells from arsenic-induced cytotoxicity by increasing glutathione (GSH) level and glutathione reductase (GR). Protein expression of western blot analysis showed that co-exposure of D-pinitol and arsenic significantly inhibited arsenic-induced autophagy which further suppressed apoptosis through up-regulation of survival factors; mTOR, p-mTOR, Akt, p-Akt, NF-кB, Nrf2, ERK1, GR, Bcl-x and down-regulation of death factors; p53, Bax, cytochrome c, LC3, although arsenic regulated those factors negatively. These results of this study suggested that D-pinitol protects PC12 cells from arsenic-induced cytotoxicity.

Expression of New Pteris vittata Phosphate Transporter PvPht1;4 Reduces Arsenic Translocation from the Roots to Shoots in Tobacco Plants

Arsenic-hyperaccumulator Pteris vittata is efficient in As uptake, probably through phosphate transporters (Pht). Here, for the first time, we cloned a new PvPht1;4 gene from P. vittata and investigated its role in arsenate (AsV) uptake and transport in yeast and transgenic tobacco plants. On the basis of quantitative real-time polymerase chain reaction (qRT-PCR), PvPht1;4 was abundantly expressed in P. vittata fronds and roots, with its transcripts in the roots being induced by both P deficiency and As exposure. PvPht1;4 was localized to the plasma membrane, which complemented a yeast-mutant defective in P uptake and showed higher P transport affinity than PvPht1;3. Under AsV exposure, PvPht1;4 yeast transformants showed comparable tolerance as PvPht1;3, but higher As accumulation than PvPht1;2 transformants, indicating that PvPht1;4 had considerable AsV and P transport activity. However, in soil and hydroponic experiments, PvPht1;4 expressing tobacco lines accumulated 26-44 and 37-55% lower As in the shoots than wild type plants, with lower root-to-shoot As translocation. In the roots of PvPht1;4 lines, higher glutathione (GSH) contents and expression levels of GSH synthetase gene NtGSH2 were observed. In addition, the transcripts of AsIII-GSH transporter NtABCC1 in PvPht1;4 lines were upregulated. The data suggested that PvPht1;4 lines probably detoxified As by reducing AsV to AsIII, which was then complexed with GSH and stored in the root vacuoles, thereby reducing As translocation in transgenic tobacco. Given its strong AsV transport capacity, expression of PvPht1;4 provides a new molecular approach to reduce As accumulation in plant shoots.

Waste sludge derived adsorbents for arsenate removal from water

Aqueous arsenate [As(V)] was removed using an aluminum-based adsorbent (ABA) and coal mine drainage sludge coated polyurethane (CMDS-PU) prepared using alum and coal mine sludge, respectively. Their As(V) removal efficiencies were compared with each other and granular ferric hydroxide (GFH). The mineralogy and surface chemistry of materials were determined using wavelength dispersive X-ray fluorescence (WD XRF) and Fourier transform infrared spectroscopy (FTIR), respectively. The angle-resolved X-ray photoelectron spectroscopy (AR-XPS) studies confirmed As(V) retention on the adsorbent surfaces. The adsorption kinetics data were fitted to pseudo second-order rate equation. The faster As(V) uptake kinetics of GFH and ABA (GFH > ABA > CMDS-PU) were attributed to their large pore volume and mesoporous nature. Langmuir adsorption capacities of 22, 31 and 10 mg/g, were achieved for GFH, ABA and CMDS-PU, respectively. As(V) adsorption on GFH, ABA and CMDS-PU was endothermic. GFH and ABA were efficient over a wide pH range (3-10). In column studies, GFH, ABA, and CMDS-PU successfully treated 23625, 842, and 158 bed volumes (BVs) and 2094, 6400, and 17 BVs of As(V)-contaminated water with 9.5 and 27 EBCT, respectively (at pH = 6.0, As_i = 600 μg/L). The GFH and ABA have a potential to be used at large-scale aqueous phase As(V) remediation.

Emerging technologies for arsenic removal from drinking water in rural and peri-urban areas: Methods, experience from, and options for Latin America

Providing drinking water with safe arsenic levels in Latin American (LA) countries (a total of 22 countries) is a major current challenge. Arsenic's presence in water has been neglected for many decades since it was first reported ~100 years ago in Argentina. The major arsenic source in this region is geogenic. So far, arsenic has been reported in 15 LA countries. Arsenic concentrations in drinking water have been reported up to >200 fold (2000 μg/L) the WHO limit of 10 μg/L. About 14 million people in the arsenic affected LA countries depend on contaminated water characterized by >10 μg/L of arsenic. Low-cost, easy to use, efficient, and sustainable solutions are needed to supply arsenic safe water to the rural and peri-urban population in the affected areas. In the present study, >250 research articles published on various emerging technologies used for arsenic remediation in rural and peri-urban areas of LA countries are critically reviewed. Special attention has been given to arsenic adsorption methods. The manuscript focuses on providing insights into low cost emergent adsorbents with an implementation potential in Latin America. Natural, modified and synthetic adsorbents used for arsenic decontamination were reviewed and compared. Advantages and disadvantages of treatment methods are summarized. Adsorbent selection criteria are developed. Recommendations concerning emerging adsorbents for aqueous arsenic removal in LA countries have also been made.

Heavy metal bioremediation of coal-fired flue gas using microalgae under different CO2 concentrations

Sustainability assessments have revealed that integration of CO₂ from coal-fired flue gas with microalgae cultivation systems could reduce greenhouse gas emissions. The technical goal of this integration is to utilize exhaust from coal power plants to enhance microalgae cultivation processes by capturing and recycling of carbon dioxide from a more toxic to a less toxic form. However, heavy metals are also introduced along with CO2 to the cultivation system which could contaminate biomass and have deleterious effects on products derived from such systems. The present study aimed at shedding some light on capability of microalgae to sustain their diversity and propagate them under different CO₂ concentrations from coal-fired flue gas. Mixed microalgal culture was grown in nutrient rich medium and heavy metals (Al, Cu, Fe, Mn and Zn) are expected to be introduced from flue gas. Three concentrations (1%, 3% and 5.5%) of CO₂ were evaluated (reference concentrations from flue gas). Comparative studies were carried out by flue gas and control systems in photobioreactors. Under the 3% CO₂ (30% flue gas), the highest fraction of B, Mn and Zn were found to be internalized by the cells (46.8 ±9.45 gL-1, 253.66 ± 40.62 gL-1 and 355.5 ±50.69 gL-1 respectively) during their cultivation period into biomass. Hence, microalgae may offer solution to two major challenges: providing potential biofuel feedstock for energy security and reducing heavy metal pollution to the air.

Use of heavy metals resistant bacteria-a strategy for arsenic bioremediation

A large number of industries release their untreated wastes in the environment causing an increase in the concentration of toxic pollutants including heavy metal ions in ground and drinking water which is above the WHO limit. The presence of toxic pollutants in the industrial wastes pollutes our environment. Arsenic (As) is a ubiquitous toxic metalloid. Its amount varies in different parts on the earth, and its concentration is increasing in our environment day by day both by natural and anthropogenic activities. It is found in two forms; one is arsenate (As⁵⁺) and other is arsenite (As³⁺) and the latter is more toxic due to high mobility across the cell membrane. The long-term use of arsenic-containing water causes arsenicosis. High arsenic consumption, revealed by skin harms, color change, and spots on hands and feet, may cause skin cancer and affect lungs and kidneys. Hypertension, a state of high blood pressure, and lack of insulin which causes diabetes and many other disorders which relate to reproduction are the consequences of arsenic contamination. Several methods have been employed to decontaminate arsenic pollution, but the bioremediation by using biomass of bacteria, algae, fungi, and yeasts is the most compromising approach and has gained much attention from researchers in the last few decades. The microbial detoxification of arsenic can be achieved by reduction, oxidation, and methylation. High bioremediation potential and feasibility of the process make bacteria an impending foundation for green chemistry to exterminate arsenic in the environment.

Simplex-Centroid mixture design applied to arsenic (V) removal from waters using synthetic minerals

Arsenic (As) is a toxic and carcinogenic element. Therefore, it is necessary to carry out research on As-contaminated water management in order to achieve the World Health Organization (WHO) standard for drinking water (0.010 mg L^-1). A Simplex-Centroid mixture design (SCMD) was used to determine the best mineral composition for both maximum adsorption capacity of As(V) (MAC-As) and residual concentration of As(V) (RC-As), using synthetic poorly crystallized aluminum hydroxide (pAlHyd), calcined layered double hydroxide (cLDH), and two-line ferrihydrite (2ℓFh). The analysis of variance results and the predicted values of models showed a good agreement with the experimental data, indicating that SCMD is a reliable method to optimize As removal through determination of the best mineral composition. The ability of pure synthetic minerals to remove As from water was different among those mixtures thereof, which indicate that the mineral components interacted with each other. Results showed that cLDH was the best As adsorbent. However, it showed a RC-As higher than the WHO standard. The pAlHyd and 2ℓFh exhibited smaller MAC-As, but they lowered RC-As to below 0.010 mg L^-1, showing no direct relationship between high MAC-As and low RC-As. Therefore, mineral compositions which combine high adsorption capacity with low residual concentration should work better for removing As from drinking water, ensuring it meets the WHO potability standard. Ternary diagrams for MAC-As and RC-As showed that the best combination for maximizing MAC-As and reducing RC-As should be a mixture of 75-90% of cLDH, 10-20% of pAlHyd, and 0-5% of 2ℓFh.

Redox synergistic Mn-Al-Fe and Cu-Al-Fe ternary metal oxide nano adsorbents for arsenic remediation with environmentally stable As(0) formation

Arsenic mitigation behavior in aqueous systems is being evaluated for Mn-Al-Fe, Cu-Al-Fe nano adsorbents. Morphological, and vibrational spectroscopy analysis are observed with As-OH, and As-O surface complexes. XPS study of individual As(3d) spectra at different parameters is observed with multiplet peak behavior attributed to redox behavior of Mn-Al-Fe, Cu-Al-Fe. Significant proportions of As(0) signal (∼25 at.% in pH 7, ∼78 at.% in pH 2, ∼58 at.% in pH 12) implicate an environmentally stable behavior of these adsorbents to address the arsenic leaching issue. Adsorption kinetics are observed with Pseudo Second Order (PSO) model, and Freundlich model supported multilayer adsorption behavior is observed for adsorption isotherms. Trace metal voltammetry studies are observed with 75-90 % of As(III) mitigation in aliquot samples. Detailed study of Mn(2p), Cu(2p), Fe(2p), and O(1 s) spectra explains redox active, and surface ligand exchange synergism in arsenic adsorption. Low equilibrium concentrations (Ce < 10 ppb) in As(V) systems (C_i ∼ 100 and 500 ppb) indicate the drinking water application of these systems. Cyclic-voltammetry (CV) studies implicate the mitigation and immobilization of arsenic species onto adsorbent by both reduction, and sorption phenomenon.