Review of kinetics of chemical and photocatalytical oxidation of Arsenic(III) as influenced by pH

Artificial neural network modeling for the oxidation kinetics of divalent manganese ions during chlorination and the role of arsenite ions in the binary/ternary systems

This study investigated the coexistence and contamination of manganese (Mn(II)) and arsenite (As(III)) in groundwater and examined their oxidation behavior under different equilibrating parameters, including varying pH, bicarbonate (HCO3-) concentrations, and sodium hypochlorite (NaClO) oxidant concentrations. Results showed that if the molar ratio of NaClO: As(III) was >1, the oxidation of As(III) could be achieved within a minute with an extremely high oxidation rate of 99.7 %. In the binary system, the removal of As(III) prevailed over Mn(II). The As(III) oxidation efficiency increased from 59.8 ± 0.6 % to 70.8 ± 1.9 % when pH rose from 5.7 to 8.0. The oxidation reaction between As(III) and NaClO releases H+ ions, decreasing the pH from 6.77 to 6.19 and reducing the removal efficiency of Mn(II). The presence of HCO3- reduced the oxidation rate of Mn(II) from 63.2 % to 13.9 % within four hours. Instead, the final oxidation rate of Mn(II) increased from 68.1 % to 87.7 %. This increase can be attributed to HCO3- ions competing with the free Mn(II) for the adsorption sites on the sediments, inhibiting the formation of H+. Moreover, kinetic studies revealed that the oxidation reaction between Mn(II) and NaClO followed first-order kinetics based on their R2 values. The significant factors affecting the Mn(II) oxidation efficiency were the initial concentration of NaClO and pH. Applying an artificial neural network (ANN) model for data analysis proved to be an effective tool for predicting Mn(II) oxidation rates under different experimental conditions. The actual Mn(II) oxidation data and the predicted values obtained from the ANN model showed significant consistency. The training and validation data sets yielded R2 values of 0.995 and 0.992, respectively. Moreover, the ANN model highlights the importance of pH and NaClO concentrations in influencing the oxidation rate of Mn(II).

Bioaccessibility and human health risks of arsenic from geological origin in lateritic red soil on construction land

Current human health risk assessments of soil arsenic (As) contamination rarely consider bioaccessibility (IVBA), which may overestimate the health risks of soil As. The IVBA of As (As-IVBA) may differ among various soil types. This investigation of As-IVBA focused As from geological origin in a typical subtropical soil, lateritic red soil, and its risk control values. The study used the SBRC gastric phase in vitro digestion method and As speciation sequential extraction based upon phosphorus speciation extraction method. Two construction land sites (CH and HD sites) in the Pearl River Delta region were surveyed. The results revealed a high content of residual As (including scorodite, mansfieldite, orpiment, realgar, and aluminum arsenite) in the lateritic red soils at both sites (CH: 84.9%, HD: 91.7%). The content of adsorbed aluminum arsenate (CH: 3.24%, HD: 0.228%), adsorbed ferrum arsenate (CH: 8.55%, HD: 5.01%), and calcium arsenate (CH: 7.33%, HD: 3.01%) were found to be low. The bioaccessible As content was significantly positively correlated with the As content in adsorbed aluminum arsenate, adsorbed ferrum arsenate, and calcium arsenate. A small portion of these sequential extractable As speciation could be absorbed by the human body (CH: 14.9%, HD: 3.16%), posing a certain health risk. Adsorbed aluminum arsenate had the highest IVBA, followed by calcium arsenate, and adsorbed ferrum arsenate had the lowest IVBA. The aforementioned speciation characteristics of As from geological origin in lateritic red soil contributed to its lower IVBA compared to other soils. The oxidation state of As did not significantly affect As-IVBA. Based on As-IVBA, the carcinogenic and non-carcinogenic risks of soil As in the CH and HD sites decreased greatly in human health risk assessment. The results suggest that As-IVBA in lateritic red soil should be considered when assessing human health risks on construction land.

Magnesium ferrite-nitrogen-doped graphene oxide nanocomposite: effective adsorptive removal of lead(II) and arsenic(III)

Magnetic nanocomposites have received immense interest as adsorbents for water decontamination. This paper presents adsorptive properties of nitrogen-doped graphene oxide (N-GO) with magnesium ferrite (MgFe₂O₄) magnetic nanocomposite for removing lead(II) (Pb(II)and arsenite As(III) ions. Transmission electron microscope (TEM) image of synthesized nanocomposite revealed the wrinkled sheets of N-GO containing MgFe₂O₄ nanoparticles (NPs) with particle size of 5-15 nm distributed over its surface. This nanocomposite displayed higher BET surface area (72.2 m²g^-1) than that of pristine MgFe₂O₄ NPs (38.4 m²g^-1). Adsorption on the nanocomposite could be described by the Langmuir isotherm with the maximum adsorption capacities were 930 mg/g, and 64.1 mg/g for Pb(II) and As(III), respectively. Whereas, maximum removal efficiencies were observed to be 99.7 [Formula: see text] 0.2% and 93.5 [Formula: see text] 0.1% for Pb(II) and As(III), respectively. The study on the effect of coexisting anions on the adsorption of metal ions showed that the phosphate ions were potential competitors of Pb(II) and As(III) ions to adsorb on the nanocomposite. Significantly, the investigation on adsorption of metal ion in the presence of coexisting heavy metal ions indicated the preferential adsorption of Pb(II) ions as compared to Cd(II), Zn(II) and Ni(II) ions. The effectiveness of the nanocomposite to remove the metal ions in electroplating wastewater was demonstrated.

Importance of the ligand-to-metal charge transfer (LMCT) pathway in the photocatalytic oxidation of arsenite by TiO2

Photooxidation of As(III) by TiO₂ is a complicated process in which the oxidation mechanisms are always controversial. In this study, the enhanced photooxidation rates of As(III) with increasing pH values from 8.0 to 11.0 indicate the high photocatalytic reactivity of TiO₂ under alkaline conditions. Moreover, As(III) improves the production of H₂O₂, indicating H-abstraction from As(III) (soluble or adsorbed) for H₂O₂ production. Although O₂˙^-, h⁺, ˙OH and -OOH are always regarded as the reactive oxygen species in the UV-TiO₂ system, the superoxo and peroxo species formed on the surface of TiO₂ also contribute to As(III) oxidation. The As(III)-O-Ti(IV) surface complexes formed on TiO₂, as well as the decreased bandgaps of TiO₂ with increasing concentrations of As(III) indicate that the ligand-to-metal charge transfer (LMCT) pathway also contributes to the oxidation of As(III) under alkaline conditions. Electrochemical analyses further reveal that As(III) enhances the electron density on the surface of TiO₂, thereby improving the catalytic reactivity of TiO₂. We therefore suggest that H-abstraction from As(III) or H₂O to the formed superoxo and peroxo species results in the formation of H₂O₂, accompanied by the oxidation of As(III). This enriches our knowledge on the oxidation of As(III), as well as other contaminants rich in -OH groups during the photocatalytic oxidation processes.

Arsenic: A Global Environmental Challenge

Arsenic is a naturally occurring metalloid and one of the few metals that can be metabolized inside the human body. The pervasive presence of arsenic in nature and anthropogenic sources from agricultural and medical use have perpetuated human exposure to this toxic and carcinogenic element. Highly exposed individuals are susceptible to various illnesses, including skin disorders; cognitive impairment; and cancers of the lung, liver, and kidneys. In fact, across the globe, approximately 200 million people are exposed to potentially toxic levels of arsenic, which has prompted substantial research and mitigation efforts to combat this extensive public health issue. This review provides an up-to-date look at arsenic-related challenges facing the global community, including current sources of arsenic, global disease burden, arsenic resistance, and shortcomings of ongoing mitigation measures, and discusses potential next steps.

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.

Nanofiltration for Arsenic Removal: Challenges, Recent Developments, and Perspectives

Arsenic (As) removal is of major significance because inorganic arsenic is highly toxic to all life forms, is a confirmed carcinogen, and is of significant environmental concern. As contamination in drinking water alone threatens more than 150 million people all over the world. Therefore, several conventional methods such as oxidation, coagulation, adsorption, etc., have been implemented for As removal, but due to their cost-maintenance limitations; there is a drive for advanced, low cost nanofiltration membrane-based technology. Thus, in order to address the increasing demand of fresh and drinking water, this review focuses on advanced nanofiltration (NF) strategy for As removal to safeguard water security. The review concentrates on different types of NF membranes, membrane fabrication processes, and their mechanism and efficiency of performance for removing As from contaminated water. The article provides an overview of the current status of polymer-, polymer composite-, and polymer nanocomposite-based NF membranes, to assess the status of nanomaterial-facilitated NF membranes and to incite progress in this area. Finally, future perspectives and future trends are highlighted.

Thallium(I) Oxidation by Permanganate and Chlorine: Kinetics and Manganese Dioxide Catalysis

The oxidation of the toxic heavy metal thallium(I) (Tl(I)) is an efficient way to enhance Tl removal from water and wastewater. However, few studies have focused on the kinetics of Tl(I) oxidation in water, especially at environmentally relevant pH values. Therefore, the kinetics and mechanisms of Tl(I) oxidation by the common agents KMnO₄ and HOCl under environmentally relevant pH condition were explored in the present study. The results indicated that the pH-dependent oxidation of Tl(I) by KMnO₄ exhibited second-order kinetics under alkaline conditions (pH 8-10) with the main active species being TlOH, while the reaction could be characterized by autocatalysis at pH 4-6, and Mn(III) might also play an essential role in the MnO₂ catalysis. Furthermore, a two-electron transfer mechanism under alkaline conditions was preliminarily proposed by using linear free energy relationships and X-ray photoelectron spectroscopy (XPS) analysis. Distinctively, the reaction rate of Tl(I) oxidation by HOCl decreased with increasing pH, and protonated chlorine might be the main active species. Moreover, the Tl(I)-HOCl reaction could be regarded as first order with respect to Tl(I), but the order with respect to HOCl was variable. Significant catalysis by MnO₂ could also be observed in the oxidation of Tl(I) by HOCl, mainly due to the vacancies on MnO₂ as active sites for sorbing Tl. This study elucidates the oxidation characteristics of thallium and establishes a theoretical foundation for the oxidation processes in thallium removal.

Treatment of As-rich mine effluents and produced residues stability: Current knowledge and research priorities for gold mining

Refractory ores, in which gold is often embedded within As-bearing and acid-generating sulfide minerals, are becoming the main gold source worldwide. These ores require an oxidizing pre-treatment, prior to cyanidation, to efficiently breakdown the sulfides and enhance gold liberation. As a result, large volumes of As-rich effluents (> 500 mg/L) are produced through the pre-oxidation of refractory gold ores and/or the exposure of As-bearing tailings upon exposure to air and water. Limited information is available on performant treatment of these effluents, especially of pre-oxidation effluents characterized by a complex chemistry, extremely acidic or alkaline pH and high concentrations of arsenic. The treatment of As-rich effluents is mainly based on precipitation (using Al or Fe salts and/or Ca-based compounds) and (electro)-chemical or biological oxidation processes. A performant treatment process must maximize As removal from contaminated mine water and allow for the production of residues that are geochemically stable over the long term. An extensive literature review showed that Fe(III)-As(V) precipitates, especially bioscorodite and (nano)scorodite, appear to be the most appropriate forms to immobilize As due to their low solubility and high stability, especially when encapsulated within an inert material such as hydroxyl gels. Research is still required to assess the long-term stability of these As-bearing residues under mine-site conditions for the sustainable exploitation of refractory gold deposits.

Introducing Frontal Chromatography-Inductively Coupled Plasma-Mass Spectrometry as a Fast Method for Speciation Analysis: The Case of Inorganic Arsenic

A frontal chromatography-ICP-MS method (FC-ICP-MS) is proposed as an innovative approach for fast elemental speciation analysis: inorganic arsenic speciation was selected as the first case study to prove the feasibility of the technique and to explore its potentialities and limits. The principal benefits of the FC-ICP-MS approach are the short analysis time and the very simple instrumental setup. As(III) and As(V) front separation is performed over a strong anion exchanger at pH 7.5. After the optimization of the instrumental setup and the frontal chromatographic parameters, As(III) and As(V) concentrations up to 240 μg/kg can be determined within 120-140 s using different univariate and multivariate calibration approaches. Best results in terms of accuracy in prediction were obtained using the partial least squares (PLS) calibration achieving limits of detection of 0.18 and 0.21 μg/kg for As(III) and As(V), respectively. This approach was also used to establish the figures of merit of the method. The proved feasibility and good performances (in terms of analysis time and accuracy) of this technique lay the groundwork for future applications of FC-ICP-MS for the speciation of other elements.

An Adverse Outcome Pathway Linking Organohalogen Exposure to Mitochondrial Disease

Adverse outcome pathways (AOPs) are pragmatic tools in human health hazard characterization and risk assessment. As such, one of the main goals of AOP development is to provide a clear, progressive, and linear mechanistic representation of pertinent toxicological key events (KEs) occurring along the different levels of biological organization. Here, we present an AOP framework that depicts how exposure to organohalogens can lead to mitochondrial disease. Organohalogens are disinfectant by-products (DBPs) found in our drinking water. Chloroform, trichloroacetic acid, and trichlorophenol were selected to represent specific types of organohalogens for the development of this AOP. Although each of these compounds contains chlorine atoms, they differ in aromaticity and solubility, which have a significant impact on their potency. This AOP consists of two main pathways, both of which are triggered by the molecular initiating event (MIE) of excessive reactive oxygen species generation. Pathway 1 details the downstream consequences of oxidative stress, which include mitochondrial DNA damage, protein aggregation, and depolarization of the mitochondrial membrane. Pathway 2 shows the KEs that result from inadequate supply of glutathione, including calcium dysregulation and ATP depletion. Pathways 1 and 2 converge at a common KE: opening of the mitochondrial membrane transition pore (mPTP). This leads to the release of cytochrome c, caspase activation, apoptosis, and mitochondrial disease. This AOP was developed according to the Organisation for Economic Co-operation and Development guidance, including critical consideration of the Bradford Hill criteria for Weight of Evidence assessment and key questions for evaluating confidence. The presented AOP is expected to serve as the basis for designing new toxicological tests as well as the characterization of novel biomarkers for disinfectant by-product exposure and adverse health effects.

An overview of main arsenic removal technologies

Arsenic (As) is metalloid, naturally present in the environment but also introduced by human activities. It is toxic and carcinogenic and its exposure to low or high concentrations can be fatal to human health. Arsenic contamination in drinking water threatens more than 150 million peoples all over the world. Therefore, treatment of As contaminated water is of unquestionable importance. The present review begins with an overview of As chemistry, distribution and toxicity, which are relevant aspects to understand and develop remediation techniques. The most common As removal processes (chemical precipitation, adsorption, ion exchange, membrane filtration, phytoremediation and electrocoagulation) are presented with discussion of their advantages, drawbacks and the main recent achievements.

The influence of irrigation-induced water table fluctuation on iron redistribution and arsenic immobilization within the unsaturation zone

Given the long-term potential risk of arsenic (As)-contaminated agricultural soil to public health, the redistribution of iron (Fe) and immobilization of As within the unsaturation zone during irrigation and consequent water table fluctuations were studied via a column experiment and corresponding geochemical modeling. Experimental results show that As and Fe accumulated significantly at the top of the column during irrigation. A tremendous increase in As and Fe accumulation rates exists after water table recovery. It was deduced that Fe(II) and As(III) were oxidized directly by O₂ at the period of low water table. But the production of hydroxyl radical (OH) was promoted at the period of high water table due to the oxidation of adsorbed Fe(II). The generated OH further accelerate the oxidation of Fe(II) and As(III). Moreover, the combination of As and Fe is more stronger at the top of the column due to the transformation of combined states of As from surface complexation into surface precipitation with the growth of Fe(III) minerals. This study details the processes and mechanisms of As and Fe immobilization within the unsaturation zone during different irrigation periods and accordingly provides some insights to mitigate As accumulation in topsoil.

Removal of Cu(II) in water by polymer enhanced ultrafiltration: Influence of polymer nature and pH

This study presents an efficient removal of Cu(II) in water using the polymer enhanced ultrafiltration (PEUF) method. Polymer of different molecular weight (MW) (polyethyleneimine (PEI), sodium lignosulfonates (SLS) and dextrans) were investigated to evaluate efficiency in removal of Cu(II) in water by the PEUF method. The decomposition of Cu(II)-polymer complex was also evaluated in order to reuse polymers. Cu(II) complexation depends on the MW of chelating polymer and the pH of feed solution. It was found that the Cu(II) rejection increased with the polymer dosage with high removal of Cu(II) when using PEI and SLS at a 10:20 (mg/mg) ratio ([Cu(II)]:[polymer]). It was found that the maximum chelating capacity was 15 mg of Cu(II) per 20 mg of PEI. The Cu(II)-PEI complex could be decomposed by acid addition and the polymer could be efficiently reused with multiple complexation-decomplexation cycles. A conceptual flow chart of the integrated process of efficient removal of Cu(II) by PEUF method is suggested.

Efficiently Visible-Light Driven Photoelectrocatalytic Oxidation of As(III) at Low Positive Biasing Using Pt/TiO2 Nanotube Electrode

A constant current deposition method was selected to load highly dispersed Pt nanoparticles on TiO2 nanotubes in this paper, to extend the excited spectrum range of TiO2-based photocatalysts to visible light. The morphology, elemental composition, and light absorption capability of as-obtained Pt/TiO2 nanotubes electrodes were characterized by FE-SEM, energy dispersive spectrometer (EDS), X-ray photoelectron spectrometer (XPS), and UV-vis spectrometer. The photocatalytic and photoelectrocatalytic oxidation of As(III) using a Pt/TiO2 nanotube arrays electrode under visible light (λ > 420 nm) irradiation were investigated in a divided anode/cathode electrolytic tank. Compared with pure TiO2 which had no As(III) oxidation capacity under visible light, Pt/TiO2 nanotubes exhibited excellent visible-light photocatalytic performance toward As(III), even at dark condition. In anodic cell, As(III) could be oxidized with high efficiency by photoelectrochemical process with only 1.2 V positive biasing. Experimental results showed that photoelectrocatalytic oxidation process of As(III) could be well described by pseudo-first-order kinetic model. Rate constants depended on initial concentration of As(III), applied bias potential and solution pH. At the same time, it was interesting to find that in cathode cell, As(III) was also continuously oxidized to As(V). Furthermore, high-arsenic groundwater sample (25 m underground) with 0.32 mg/L As(III) and 0.35 mg/L As(V), which was collected from Daying Village, Datong basin, Northern China, could totally transform to As(V) after 200 min under visible light in this system.

Oxidation of arsenite to arsenate on birnessite in the presence of light

The effect of simulated solar radiation on the oxidation of arsenite [As(III)] to arsenate [As(V)] on the layered manganese oxide, birnessite, was investigated. Experiments were conducted where birnessite suspensions, under both anoxic and oxic conditions, were irradiated with simulated solar radiation in the presence of As(III) at pH 5, 7, and 9. X-ray absorption spectroscopy (XAS) was used to determine the nature of the adsorbed product on the surface of the birnessite. The oxidation of As(III) in the presence of birnessite under simulated solar light irradiation occurred at a rate that was faster than in the absence of light at pH 5. At pH 7 and 9, As(V) production was significantly less than at pH 5 and the amount of As(V) production for a given reaction time was the same under dark and light conditions. The first order rate constant (k_obs) for As(III) oxidation in the presence of light and in the dark at pH 5 were determined to be 0.07 and 0.04 h^-1, respectively. The As(V) product was released into solution along with Mn(II), with the latter product resulting from the reduction of Mn(IV) and/or Mn(III) during the As(III) oxidation process. Post-reaction XAS analysis of As(III) exposed birnessite showed that arsenic was present on the surface as As(V). Experimental results also showed no evidence that reactive oxygen species played a role in the As(III) oxidation process.

Mesoporous Magnesium Oxide Hollow Spheres as Superior Arsenite Adsorbent: Synthesis and Adsorption Behavior

Arsenic contamination in natural water has posed a significant threat to global health due to its toxicity and carcinogenity. Adsorption technology is an easy and flexible method for arsenic removal with high efficiency. In this Article, we demonstrated the synthesis of mesoporous MgO hollow spheres (MgO-HS) and their application as high performance arsenite (As(III)) adsorbent. MgO-HS with uniform particle size (∼180 nm), high specific surface area (175 m(2) g(-1)), and distinguished mesopores (9.5 nm in size) have been prepared by hard-templating approach using mesoporous hollow carbon spheres as templates. An ultrahigh maximum As(III) adsorption capacity (Qmax) of 892 mg g(-1) was achieved in batch As(III) removal study. Adsorption kinetic study demonstrated that MgO-HS could enable As(III) adsorption 6 times faster as a commercial MgO adsorbent. The ultrahigh adsorption capacity and faster adsorption kinetics were attributed to the unique structure and morphology of MgO-HS that enabled fast transformation into a flower-like porous structure composed of ultrathin Mg(OH)2 nanosheets. This in situ formed structure provided abundant and highly accessible hydroxyl groups, which enhanced the adsorption performance toward As(III). The outstanding As(III) removal capability of MgO-HS showed their great promise as highly efficient adsorbents for As(III) sequestration from contaminated water.

Magnetic graphene-carbon nanotube iron nanocomposites as adsorbents and antibacterial agents for water purification

One of the biggest challenges of the 21st century is to provide clean and affordable water through protecting source and purifying polluted waters. This review presents advances made in the synthesis of carbon- and iron-based nanomaterials, graphene-carbon nanotubes-iron oxides, which can remove pollutants and inactivate virus and bacteria efficiently in water. The three-dimensional graphene and graphene oxide based nanostructures exhibit large surface area and sorption sites that provide higher adsorption capacity to remove pollutants than two-dimensional graphene-based adsorbents and other conventional adsorbents. Examples are presented to demonstrate removal of metals (e.g., Cu, Pb, Cr(VI), and As) and organics (e.g., dyes and oil) by grapheme-based nanostructures. Inactivation of Gram-positive and Gram-negative bacterial species (e.g., Escherichia coli and Staphylococcus aureus) is also shown. A mechanism involving the interaction of adsorbents and pollutants is briefly discussed. Magnetic graphene-based nanomaterials can easily be separated from the treated water using an external magnet; however, there are challenges in implementing the graphene-based nanotechnology in treating real water.

Arsenic contamination, consequences and remediation techniques: a review

The exposure to low or high concentrations of arsenic (As), either due to the direct consumption of As contaminated drinking water, or indirectly through daily intake of As contaminated food may be fatal to the human health. Arsenic contamination in drinking water threatens more than 150 millions peoples all over the world. Around 110 millions of those peoples live in 10 countries in South and South-East Asia: Bangladesh, Cambodia, China, India, Laos, Myanmar, Nepal, Pakistan, Taiwan and Vietnam. Therefore, treatment of As contaminated water and soil could be the only effective option to minimize the health hazard. Therefore, keeping in view the above facts, an attempt has been made in this paper to review As contamination, its effect on human health and various conventional and advance technologies which are being used for the removal of As from soil and water.

Arsenic contamination of groundwater: a review of sources, prevalence, health risks, and strategies for mitigation

Arsenic contamination of groundwater in different parts of the world is an outcome of natural and/or anthropogenic sources, leading to adverse effects on human health and ecosystem. Millions of people from different countries are heavily dependent on groundwater containing elevated level of As for drinking purposes. As contamination of groundwater, poses a serious risk to human health. Excessive and prolonged exposure of inorganic As with drinking water is causing arsenicosis, a deteriorating and disabling disease characterized by skin lesions and pigmentation of the skin, patches on palm of the hands and soles of the feet. Arsenic poisoning culminates into potentially fatal diseases like skin and internal cancers. This paper reviews sources, speciation, and mobility of As and global overview of groundwater As contamination. The paper also critically reviews the As led human health risks, its uptake, metabolism, and toxicity mechanisms. The paper provides an overview of the state-of-the-art knowledge on the alternative As free drinking water and various technologies (oxidation, coagulation flocculation, adsorption, and microbial) for mitigation of the problem of As contamination of groundwater.

Improved arsenic(III) adsorption by Al2O3 nanoparticles and H2O2: evidence of oxidation to arsenic(V) from X-ray absorption spectroscopy

We have investigated the oxidation of inorganic As(III) with H2O2 catalysed by Al2O3, using X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy. The effects of different reaction conditions (pH, time and initial H2O2 concentration) were also studied as were the kinetics of the oxidation reaction. We demonstrated that As(III) was oxidized to As(V) in the presence of H2O2 and Al2O3. Furthermore, all arsenic species found on the Al2O3 surface were in the As(V) state. The presence of both Al2O3 and H2O2 was necessary for oxidation of As(III) to take place within the period of time studied. The oxidation kinetics indicate a mechanism where reversible As(III) binding to the alumina surface is followed by irreversible oxidation by H2O2 leading to strongly bound As(V). Results from this study indicate that there is a surface-catalysed oxidation of As(III) on Al2O3 by H2O2, a reaction that can take place in nature and can be of help in the development of novel treatment systems for As(III) removal.

Engineering aspects of ferrate in water and wastewater treatment - a review

There is renewed interest in the tetra-oxy compound of +6 oxidation states of iron, ferrate(VI) (Fe(VI)O4(2-)), commonly called ferrate. Ferrate has the potential in cleaner ("greener") technologies for water treatment and remediation processes, as it produces potentially less toxic byproducts than other treatment chemicals (e.g., chlorine). Ferrate has strong potential to oxidize a number of contaminants, including sulfur- and nitrogen-containing compounds, estrogens, and antibiotics. This oxidation capability of ferrate combines with its efficient disinfection and coagulation properties as a multi-purpose treatment chemical in a single dose. This review focuses on the engineering aspects of ferrate use at the pilot scale to remove contaminants in and enhance physical treatment of water and wastewater. In most of the pilot-scale studies, in-line and on-line electrochemical ferrate syntheses have been applied. In this ferrate synthesis, ferrate was directly prepared in solution from an iron anode, followed by direct addition to the contaminant stream. Some older studies applied ferrate as a solid. This review presents examples of removing a range of contaminants by adding ferrate solution to the stream. Results showed that ferrate alone and in combination with additional coagulants can reduce total suspended solids (TSS), chemical oxygen demand (COD), biological oxygen demand (BOD), and organic matter. Ferrate also oxidizes cyanide, sulfide, arsenic, phenols, anilines, and dyes and disinfects a variety of viruses and bacteria. Limitations and drawbacks of the application of ferrate in treating contaminated water on the pilot scale are also presented.

Formation and toxicity of brominated disinfection byproducts during chlorination and chloramination of water: a review

Disinfection byproducts (DBPs) in drinking water exhibit considerable adverse health effects; recent focus is on the brominated disinfection byproducts (Br-DBPs). The chlorination and chloramination of bromide ion containing water produce reactive bromo species, which subsequently react with natural organic matter (NOM) to yield Br-DBPs. The possible reactions involved in generating DBPs are presented. Identified Br-DBPs include bromomethanes, bromoacetic acid, bromoacetamides, bromoacetonitriles, and bromophenols. Mixed chloro- and bromo-species have also been identified. Pathways of the formation of Br-DBPs have been described. The concentration of Br- ion, pH, reaction time, and the presence of Cu(II) influence the yield of DBPs. The effects of water conditions on the production of Br-DBPs are presented. The epidemiological studies to understand the potential toxic effects of DBPs including Br-DBPs are summarized. Brominated DBPs may have higher health risks than their corresponding chlorinated DBPs. A potential role of an emerging alternate disinfectant, ferrate (FeV)O(2-)4), in minimizing DBPs is briefly discussed.

Oxidation of Ni(II)-cyano and Co(III)-cyano complexes by Ferrate(VI): effect of pH

Free cyanide (CN(-)) and metal-cyanide complexes (tetracyanonickelate(II)), Ni(CN)4(2-) and hexacyanocobaltate(III)), Co(CN)6(3-) are common constituents of effluents of mining, coal gasification, and petroleum refining. This article presents the degradation of Ni(CN)4(2-) and Co(CN)6(3-) by ferrate(VI) (Fe(VI)O4(2-), Fe(VI)) in alkaline media. The effect of pH (9.0-11.0) and reactant molar ratios on the degradation of the cyanide complexes was investigated. The removal of Ni(CN)4(2-) ion in 200 min was found to be > 90% at pH 9.0; forming cyanate (NCO(-)) ions as the stoichiometric products ([Fe(VI)]:[Total CN(-)] = [Fe(VI)]:[NCO(-)] ≈ 1.0). The degradation efficiency decreased with an increase in pH from 9.0 to 11.0. Comparatively, the Co(CN)6(3-) ion could be degraded only up to 10% in 200 min at pH 9.0 and the final oxidized products were nitrite and nitrate ions. The oxidation efficiency of removing Co(CN)6(3-) did not vary significantly with pH. Fe(VI) consumptions as a result of the oxidation of free cyanide and metal-cyanides and their products are compared and discussed.

Remediation of inorganic arsenic in groundwater for safe water supply: a critical assessment of technological solutions

Arsenic contaminations of groundwater in several parts of the world are the results of natural and/or anthropogenic sources, and have a large impact on human health. Millions of people from different countries rely on groundwater containing As for drinking purposes. This paper reviews removal technologies (oxidation, coagulation flocculation, adsorption, ion exchange and membrane processes) with attention for the drawbacks and limitations of these applied technologies. The technologies suggested and applied for treatment of As rich water have various problems, including the need for further treatment of As containing secondary waste generated from these water treatment processes. More efficient technologies, with a lower tendency to generate waste include the removal of As by membrane distillation or forward osmosis, instead of using pressure driven membrane processes and subsequently reducing soluble As to commercially valuable metallic As are surveyed. An integrated approach of two or more techniques is suggested to be more beneficial than a single process. Advanced technologies such as membrane distillation, forward osmosis as well as some hybrid integrated techniques and their potentials are also discussed in this review. Membrane processes combined with other process (especially iron based technologies) are thought to be most sustainable for the removal of arsenic and further research allowing scale up of these technologies is suggested.

Concentration-dependent photoredox conversion of As(III)/As(V) on illuminated titanium dioxide electrodes

The photoconversion of As(III) (arsenite) and As(V) (arsenate) over a mesoporous TiO(2) electrode was investigated in a photoelectrochemical (PEC) cell for a wide range of concentrations (μM-mM), under nonbiased (open-circuit potential measurements) and biased (short-circuit current measurements) conditions. Not only As(III) can be oxidized, but also As(V) can be reduced in the anoxic condition under UV irradiation. However, the reversible nature of As(III)/As(V) photoconversion was not observed in the normal air-equilibrated condition because the dissolved O(2) is far more efficient as an electron acceptor than As(V). Although As(III) should be oxidized by holes, its presence did not increase the photooxidation current in a monotonous way: the photocurrent was reduced by the presence of As(III) in the micromolar range but enhanced in the millimolar range. This abnormal concentration-dependent behavior is related with the fate of the intermediate As(IV) species which can be either oxidized or reduced depending on the experimental conditions, combined with surface deactivation for the water photooxidation process. The lowering of the photooxidation current in the presence of micromolar As(III) is ascribed to the role of As(IV) as a charge recombination center. Being an electron acceptor, the addition of As(V) consistently lowers the photocurrent in the entire concentration range. A global concentration-dependent mechanism is proposed accounting for all the PEC results and its relation with the photocatalytic oxidation mechanism is discussed.

Application of titanium dioxide in arsenic removal from water: A review

Natural arsenic pollution is a global phenomenon and various technologies have been developed to remove arsenic from drinking water. The application of TiO(2) and TiO(2)-based materials in removing inorganic and organic arsenic was summarized. TiO(2)-based arsenic removal methods developed to date have been focused on the photocatalytic oxidation (PCO) of arsenite/organic arsenic to arsenate and adsorption of inorganic and organic arsenic. Many efforts have been taken to improve the performance of TiO(2) by either combing TiO(2) with adsorbents with good adsorption property in one system or developing bifunctional adsorbents with both great photocatalytic ability and high adsorption capacity. Attempts have also been made to immobilize fine TiO(2) particles on supporting materials like chitosan beads or granulate it to facilitate its separation from water. Among the anions commonly exist in groundwater, humic acid and bicarbonate have significant influence on TiO(2) photocatalyzed oxidation of As(III)/organic arsenic while phosphate, silicate, fluoride, and humic acid affect arsenic adsorption by TiO(2)-based materials. There has been a controversy over the TiO(2) PCO mechanisms of arsenite for the past 10 years but the adsorption mechanisms of inorganic and organic arsenic onto TiO(2)-based materials are relatively well established. Future needs in TiO(2)-based arsenic removal technology are proposed.

Iron and aluminium based adsorption strategies for removing arsenic from water

Arsenic is a commonly occurring toxic metal in natural systems and is the root cause of many diseases and disorders. Occurrence of arsenic contaminated water is reported from several countries all over the world. A great deal of research over recent decades has been motivated by the requirement to lower the concentration of arsenic in drinking water and the need to develop low cost techniques which can be widely applied for arsenic removal from contaminated water. This review briefly presents iron and aluminium based adsorbents for arsenic removal. Studies carried out on oxidation of arsenic(III) to arsenic(V) employing various oxidising agents to facilitate arsenic removal are briefly mentioned. Effects of competing ions, As:Fe ratios, arsenic(V) vs. arsenic(III) removal using ferrihydrite as the adsorbent have been discussed. Recent efforts made for investigating arsenic adsorption on iron hydroxides/oxyhydroxides/oxides such as granular ferric hydroxide, goethite, akaganeite, magnetite and haematite have been reviewed. The adsorption behaviours of activated alumina, gibbsite, bauxite, activated bauxite, layered double hydroxides are discussed. Point-of-use adsorptive remediation methods indicate that Sono Arsenic filter and Kanchan™ Arsenic filter are in operation at various locations of Bangladesh and Nepal. The relative merits and demerits of such filters have been discussed. Evaluation of kits used for at-site arsenic estimation by various researchers also forms a part of this review.

Oxidation of inorganic contaminants by ferrates (VI, V, and IV)--kinetics and mechanisms: a review

Inorganic contaminants are found in water, wastewaters, and industrial effluents and their oxidation using iron based oxidants is of great interest because such oxidants possess multi-functional properties and are environmentally benign. This review makes a critical assessment of the kinetics and mechanisms of oxidation reactions by ferrates (Fe(VI)O(4)(2-), Fe(V)O(4)(3-), and Fe(IV)). The rate constants (k, M(-1) s(-1)) for a series of inorganic compounds by ferrates are correlated with thermodynamic oxidation potentials. Correlations agree with the mechanisms of oxidation involving both one-electron and two-electron transfer processes to yield intermediates and products of the reactions. Case studies are presented which demonstrate that inorganic contaminants can be degraded in seconds to minutes by ferrate(VI) with the formation of non-toxic products.

Oxidation of X-ray compound ditrizoic acid by ferrate(VI)

Iodinated X-ray contrast media (ICM) such as diatrizoic acid (DTZA) is used in large amounts in hospitals to enhance imaging of organs and blood vessels during radiography. Due to its persistence and non-biodegradability, it is found in treated water, sewage effluent, surface waters, and aquatic environments. This paper presents the kinetics of the oxidation of DTZA by ferrate(VI) (Fe(VI)O4(2-), Fe(VI)) as a function of pH (7.1-9.6) at 25 degrees C in order to determine the effectiveness of Fe(VI) to remove DTZA from water. The reaction was determined to be first-order with respect to concentrations of Fe(VI) and DTZA. The rate of the reaction was found to be pH dependent and the rate decreased nonlinearly as the pH increase from 7.1 to 9.6. The speciation of Fe(VI) (HFeO4(-) and FeO4(2-)) was used to explain the rate dependence on pH. The calculated rate constant of Fe(VI) with DTZA at pH 7.0 was compared with nitrogen-containing pollutants and is briefly discussed.

Methodologies for the analytical determination of ferrate(VI): a review

This paper reviewed different analytical techniques to determine concentrations of the environmentally-friendly oxidant ferrate(VI) (Fe(VI)O(2-)(4), Fe(VI)) anion in solid and solution samples. The techniques included volumetric (chromite and arsenite), electrochemical (cyclic voltammetry and potentiometry), and spectrophotometric methods (Fourier transform infrared, Mossbauer, UV-Visible, 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonate (ABTS), and fluorescence). The importance of the techniques were briefly described. This paper also presented a newly developed method for the determination of low concentrations (0.25 μM) of Fe(VI)) using the reaction between Fe(VI) and iodide (I(-)), which formed the yellowish color of I(-)(3) that could be measured spectrophotometrically at 351 nm. The increase in I(-)(3) was linear with respect to Fe(VI) added (0.25-76 μM) in buffered solutions at pH 5.5-9.3. The linearity was not affected by the ions present in tap water.

Oxidation of nitrogen-containing pollutants by novel ferrate(VI) technology: a review

Nitrogen-containing pollutants have been found in surface waters and industrial wastewaters due to their presence in pesticides, dyes, proteins, and humic substances. Treatment of these compounds by conventional oxidants produces disinfection by-products (DBP). Ferrate(VI) (Fe(VI)O(4)(2-), Fe(VI)) is a strong oxidizing agent and produces a non-toxic by-product Fe(III), which acts as a coagulant. Ferrate(VI) is also an efficient disinfectant and can inactivate chlorine resistant microorganisms. A novel ferrate(VI) technology can thus treat a wide range of pollutants and microorganisms in water and wastewater. The aim of this paper is to review the kinetics and products of the oxidation of nitrogen-containing inorganic (ammonia, hydroxylamine, hydrazine, and azide) and organic (amines, amino acids, anilines, sulfonamides, macrolides, and dyes) compounds by ferrate(VI) in order to demonstrate the feasibility of ferrate(VI) treatment of polluted waters of various origins. Several of the compounds can degraded in seconds to minutes by ferrate(VI) with the formation of non-hazardous products. The mechanism of oxidation involves either one-electron or two-electrons processes to yield oxidation products. Future research directions critical for the implementation of the ferrate(VI)-based technology for wastewater and industrial effluents treatment are recommended.

Simultaneous photocatalytic oxidation of As(III) and humic acid in aqueous TiO2 suspensions

The simultaneous photocatalytic oxidation of As(III) and humic acid (HA) in aqueous Degussa P25 TiO(2) suspensions was investigated. Preliminary photocatalytic studies of the binary As(III)/TiO(2) and HA/TiO(2) systems showed that As(III) was oxidized more rapidly than HA and the extent of photocatalytic oxidation of each individual component (i.e. As(III) or HA) increased with decreasing its initial concentration and/or increasing catalyst loading. The simultaneous photocatalytic oxidation of As(III) and HA in the ternary As(III)/HA/TiO(2) system showed that both As(III) and HA oxidation was reduced in the ternary system compared to the corresponding binary systems. The effect of operating conditions in the ternary system, such as initial As(III), HA and TiO(2) concentrations (in the range 3-20mg/L, 10-100mg/L and 50-250 mg/L respectively), initial solution pH (3.6-6.7) and reaction time (10-30 min), on photocatalytic As(III) and HA oxidation was assessed implementing a two-level factorial experimental design methodology. Seven and ten factors were found statistically important in the case of photocatalytic As(III) and HA oxidation respectively. Based on these statistically significant factors, a first order polynomial model describing As(III) and HA photocatalytic oxidation was constructed and a very good agreement was obtained between the experimental values and those predicted by the model, while the observed differences may be readily explained as random noise.

Removal of arsenite by Fe(VI), Fe(VI)/Fe(III), and Fe(VI)/Al(III) salts: effect of pH and anions

The removal of arsenate and arsenite from drinking water poses challenges, especially when arsenite is present in a significant amount. The removal of arsenite by K(2)FeO(4), K(2)FeO(4)/FeCl(3), and K(2)FeO(4)/AlCl(3) salts was studied at pH 6.5 and at an initial As concentration of 500 microg As(III)L(-1). The arsenite removal in Fe(VI)/Fe(III) and Fe(VI)/Al(III) systems was also examined as a function of pH (6.0-8.0). Arsenite was first oxidized by Fe(VI) to arsenate, which was subsequently removed through adsorption by Fe(III) or mixed Fe(III)-Al(III) oxy/hydroxide phases. Fe(VI)/Al(III) salts had higher removal efficiency of arsenite than Fe(VI) and Fe(VI)/Fe(III) salts. A molar ratio of 6(3/3):1 for Fe(VI)/Al(III) to As(III) decreased arsenite concentration from 500 to 1.4 microg L(-1) at pH 6.5. Arsenite removal increased with a decrease in pH from 8.0 to 6.0 and exhibited less pH dependence in the Fe(VI)/Al(III) system than in the Fe(VI)/Fe(III) system. Aluminum chloride salts performed better than FeCl(3) and FeCl(3)/AlCl(3) salts (Fe:Al=1:1) in removing As(V) from water. Effect of anions (phosphate, silicate, bicarbonate, nitrate, and sulfate) on the arsenite removal by Fe(VI)/Al(III) salts at pH 6.5 was examined. Phosphate, silicate, and bicarbonate ions interfered with the removal of arsenite in water. Nitrate and sulfate had none to minimal effect on arsenite removal. Fe(VI)/Al(III) salts showed a potential for removing arsenite below the current drinking water standard (10 microg L(-1)).