A novel two-step coprecipitation process using Fe(III) and Al(III) for the removal and immobilization of arsenate from acidic aqueous solution

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.

The role of doped-Mn on enhancing arsenic removal by MgAl-LDHs

To meet the challenges posed by global arsenic water contamination, the MgAlMn-LDHs with extraordinary efficiency of arsenate removal was developed. In order to clarify the enhancement effect of the doped-Mn on the arsenate removal performance of the LDHs, the cluster models of the MgAlMn-LDHs and MgAl-LDHs were established and calculated by using density functional theory (DFT). The results shown that the doped-Mn can significantly change the electronic structure of the LDHs and improve its chemical activity. Compared with the MgAl-LDHs that without the doped-Mn, the HOMO-LUMO gap was smaller after doping. In addition, the -OH and Al on the laminates were also activated to improve the adsorption property of the LDHs. Besides, the doped-Mn existed as a novel active site. On the other hand, the MgAlMn-LDHs with the doped-Mn, the increased of the binding energy, as well as the decreased of the ion exchange energy of interlayer Cl^-, making the ability to arsenate removal had been considerably elevated than the MgAl-LDHs. Furthermore, there is an obvious coordination covalent bond between arsenate and the laminates of the MgAlMn-LDHs that with the doped-Mn.

Molecular structures of dissolved and colloidal AsV-FeIII complexes and their roles in the mobilization of AsV under strongly acidic conditions

The effect of high concentration of iron (Fe^III) on the speciation and mobility of arsenic (As) under strongly acidic conditions remains unclear. This work studied the redistribution and speciation of As^V and Fe^III at Fe/As molar ratio of 1-14 and pH 1.5-2.0 in the dissolved, colloidal, and solid phases. Results showed that the elevated Fe^III induced the decomposition of the precipitated poorly crystalline ferric arsenate by forming dissolved (< 3 kDa) and colloidal (3 kDa-0.1 µm) As-Fe complexes. The fraction of particulate As (> 0.1 µm) decreased from 70-90% to less than 20% when the Fe/As molar ratio increased from 1 to 14. The particle size of the bulk samples decreased significantly with the increase of Fe^III concentration. The FTIR results suggested that As^V in dissolved/colloidal As-Fe complexes dominantly occurred as HAsO₄^2- species. The EXAFS results indicated that each HAsO₄^2- coordinated with approximately two Fe atoms in dissolved/colloidal As-Fe complexes at Fe/As ≥ 2. The findings suggest that high aqueous Fe^III concentration can promote the mobility of As by forming dissolved/colloidal Fe-As complexes in acidic waters, potentially accelerating As transport from source to downstream in acid mine drainage systems.

A novel method for in situ stabilization of calcium arsenic residues via yukonite formation

Stabilizing the hazardous calcium arsenic residues (CAR) and monitoring the subsequent fate of arsenic (As) are critical to reduce its risk to the environment. In this work, a novel in situ method has been proposed to stabilize CAR by adding Fe^III solution and subsequent formation of the secondary mineral (yukonite). The experiments were conducted at pH 6-9 with different Fe/As molar ratios (0.28-0.66) and the solid phases were characterized by using X-ray diffraction and scanning/transmission electron microscopy. Results showed that the stability of the CAR was significantly increased after the addition of Fe^III solution, indicating good fixation effectiveness. The dissolved As concentration in the treated CAR samples continuously decreased to <5 mg/L after 490 days of treatment at Fe/As molar ratio ≥ 0.54 and pH ≥ 8, with the leached As concentration lower than 5 mg/L (US EPA standard) for most of the treated CAR in the TCLP and HVM tests. The formation of yukonite under different experimental conditions is closely related to the enhanced stability of the treated CAR. This work provides a novel in situ method to treat CAR which might have potential for future industrial applications.

Partitioning and transformation behavior of arsenic during Fe(III)-As(III)-As(V)-SO42- coprecipitation and subsequent aging process in acidic solutions: Implication for arsenic mobility and fixation

The coprecipitation and subsequent aging of Fe(III)-As(III)-As(V)-SO₄^2- play an important role in controlling As behavior in acidic systems, such as acid mine drainage and hydrometallurgical acid waste. In this study, we investigated the redistribution and transformation of As in the Fe(III)-As(III)-As(V)-SO₄^2- system (As(III)/As(V) ≈ 1) at different Fe/As molar ratios (i.e., 0.25, 0.5, and 1) and pH (1.2 and 1.8) at 60 °C. The results showed that As(III) and SO₄^2- can be incorporated into the amorphous ferric arsenate and scorodite host phases by forming a Fe(AsO₄)_x(AsO₃)_y(SO₄)_z solid solution. As(III) contents in the freshly coprecipitated solids increased with pH and initial As(III) concentrations. During aging process, As(III) contents in the solid products with Fe/As molar ratios of 0.5 and 1 increased with aging time at pH 1.8. In contrast, As(III) was gradually expelled from aging products with aging time at pH 1.2, regardless of Fe/As molar ratio. X-ray diffraction (XRD), scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy characterization results showed that an As(III)-SO₄^2--doped scorodite was formed at Fe/As molar ratio ≤0.5 during the aging process. It was also found that As(III) had an inhibitory effect on the transformation of poorly crystalline ferric arsenate to scorodite. The present study may have important implications for understanding the geochemical cycle of As, Fe, and SO₄^2- in acidic solutions and give further understanding on the mechanisms involved in As removal and fixation in hydrometallurgical unit operations.

High-efficiency degradation of p-arsanilic acid and arsenic immobilization with iron encapsulated B/N-doped carbon nanotubes at natural solution pH

p-arsanilic acid (p-ASA) is still widely applied as feed additive in many countries. Accompanied with chemical reactions in the environment, p-ASA will release more toxic inorganic arsenic. In order to safely and efficiently treat p-ASA flow washing into the environment, iron encapsulated B/N-doped carbon nanotubes (Fe@C-NB) were fabricated and used as the catalyst for the degradation of p-ASA. The calcination temperature and the dose of the iron salt have significant effects on the structure and properties of the catalysts. We have produced a series of catalysts of the same type to facilitate the degradation of p-ASA. Under optimal conditions of material (Fe@C-NB) syntheses, both 95% degradation of p-ASA and 86% total arsenic immobilization can be obtained with oxidant (Peroxymonosulfate, PMS) and catalyst (Fe@C-NB) treatment after 60 min. The effects of oxidant types (peroxydisulfate (PDS), PMS, hydrogen peroxide (H₂O₂)), amount, initial solution pH, inorganic anion, and other reaction conditions were studied in the p-ASA removal. In this Fenton-like reaction, the Fe@C-NB exhibits high efficiency and excellent stability without complex preparation methods; besides, the advantages of short reaction time and natural reaction conditions in Fe@C-NB/PMS system will promote the practical application of Fenton-like.

A combined abiotic oxidation-precipitation process for rapid As removal from high-As(III)-Mn(II) acid mine drainage and low As-leaching solid products

A combination process of Fenton-like and catalytic Mn(II) oxidation via molecular oxygen-induced abio-oxidation of As(III)-Mn(II)-rich acid mine drainage (AMD) is developed to rapidly and efficiently remove As and obtain low As-leaching solids in this study. The effect of pH, temperature, oxygen flow rate and neutralization reagent on As removal was investigated. The results showed that pH was important to As removal efficiency, which achieved maximum in 0.25-2 h, but decreased from ∼100 % to ∼92.6 % with the increase of pH 5-9. pH, temperature and oxygen flow rate played key roles in As(III) oxidation. The increase of As(III) oxidized from 16.8 to 67.1% to 98.6-99.0 % occurred as increasing the pH 5-9, 25-95 °C and oxygen flow rate of 0-2.4 L min^-1. NaOH or Ca(OH)₂ as base was less important to As removal. The mechanism involved Fenton-like reaction between Fe(II) and O₂ for produced Fe(III) (oxy)hydroxide association with As(III + V) and Mn(II), catalytic Mn(II) oxidation for the formation of Mn(III, IV) oxides, and further As(III) oxidation by Mn(III, IV) oxides. As-bearing six-line ferrihydrite was the main solid product for low As-leaching fixation. pH 8, 95 °C and oxygen flow rate of 1.6 L min^-1 were optimal for As removal.

Long-term stability of the Fe(III)-As(V) coprecipitates: Effects of neutralization mode and the addition of Fe(II) on arsenic retention

The coprecipitation of arsenic with Fe(III) by lime neutralization is widely used in industrial practices to treat arsenic-containing waste waters generated from mineral processing operations. In this work, coprecipitation was conducted directly at pH 8 to simulate the operations in hydrometallurgical practices, which differed from the conventional laboratory operations. Moreover, although ferric is the major species of iron in arsenic-containing waste waters, the coexistence of ferrous ions cannot be ignored. Therefore, the effect of different neutralization modes, as well as the effect of ferrous ions on the removal of arsenic and the stability of the generated arsenic-bearing wastes, was systematically investigated. The result showed that arsenic was still released back into the liquid phase under alkaline conditions even for the samples formed directly at alkaline pH. It was found that the extra addition of Fe(II) may exert negative effect on the stability of the as-formed Fe(II)-Fe(III)-As(V) coprecipitates at pH 7 - 10. The concentration of ferrous ions in the liquid/solid phase decreased with increasing pH for each sample formed at different Fe(II)/Fe(tot). The results indicated that complete oxidation of the ferrous ions before coprecipitation with arsenic should be conducted to achieve optimal stability of the arsenic-bearing wastes for hydrometallurgical practice and waste disposal.

The long-term stability of calcium arsenates: Implications for phase transformation and arsenic mobilization

It is well known that calcium arsenates may not be a good choice for arsenic removal and immobilization in hydrometallurgical practices. However, they are still produced at some plants in the world due to various reasons. Furthermore, calcium arsenates can also naturally precipitate under some specific environments. However, the transformation process of poorly crystalline calcium arsenates (PCCA) and the stability of these samples under atmospheric CO₂ are not yet well understood. This work investigated the transformation process of PCCA produced by using different neutralization reagents (CaO vs. NaOH) with various Ca/As molar ratios at pH 7-12 in the presence of atmospheric CO₂. After aging at room temperature for a period of time, for samples neutralized with NaOH and precipitated at pH 10 and 12, release of arsenic back into the liquid phase occurred. In contrast, for the samples precipitated at pH 8, the aqueous concentration of arsenic was observed to decrease. XRD, Raman, and SEM results suggested that the formation of various types of crystalline calcium carbonates and/or calcium arsenates controls the arsenic behavior. Moreover, the application of lime may enhance the stability of the generated PCCA. However, no matter what neutralization reagent is used, the stability of the generated PCCA is still of concern.

Self-enhanced and efficient removal of arsenic from waste acid using magnetite as an in situ iron donator

High arsenic-containing waste acid from the heavy nonferrous metallurgical sector (Cu, Pb, Zn, Ni, Sn, etc.), one of the most dangerous arsenic hazardous wastes with extremely high arsenic concentrations, has presented enormous challenges to the environment and caused severe environmental pollution over the past few decades due to the lack of affordable and environmentally friendly disposal technologies. Here, we report a green process for the self-enhanced and efficient removal of arsenic from waste acid using magnetite as an in situ iron donator. Firstly, the room-temperature predissolution of magnetite in waste acid provides initial iron ions as a starting precipitator of arsenic, simultaneously providing a suitable pH range and an active surface that are ready for the nucleation and growth of scorodite. Afterwards, arsenic is precipitated in form the of scorodite, which is driven by a mutually improved cycle composed of arsenic precipitation and magnetite dissolution on the surface of magnetite particles. This cycle creates a low supersaturation of iron and constant pH in the waste acid, ensuring the continuous precipitation of arsenic as well-crystallized and environmentally stable scorodite by using magnetite as an in situ iron donator via the reaction of 2Fe₃O₄ + 6H₃AsO₄ + H₂O₂ = 6FeAsO₄ + 10H₂O. Under optimal conditions, including a 6-h room-temperature predissolution, a 12-h atmospheric reaction at 90 °C and a pH of 2.0 with a magnetite dosage at the Fe₃O₄/As molar ratio (the molar ratio of Fe₃O₄ in magnetite to As in waste acid) of 1.33, 99.90% of arsenic was successively removed from waste acid with an initial arsenic concentration of 10300 mg/L. In combination with the good adaptability of this process, the performed case study and prospective process show the successful removal of arsenic from waste acid as well as great potential for large-scale applications.

Application of Response Surface Methodology and Desirability Function in the Optimization of Adsorptive Remediation of Arsenic from Acid Mine Drainage Using Magnetic Nanocomposite: Equilibrium Studies and Application to Real Samples

A magnetic multi-walled carbon nanotube/zeolite nanocomposite was applied for the adsorption and removal of arsenic ions in simulated and real acid mine drainage samples. The adsorption mechanism was investigated using two-parameter (Langmuir, Freundlich, Temkin) and three-parameter (Redlich–Peterson, and Sips) isotherm models. This was done in order to determine the characteristic parameters of the adsorptive removal process. The results showed that the removal process was described by both mono- and multilayer adsorptions. Adsorption studies demonstrated that a multi-walled carbon nanotube/zeolite nanocomposite could efficiently remove arsenic in simulated samples within 35 min. Based on the Langmuir isotherm, the adsorption capacity for arsenic was found to be 28 mg g⁻¹. The nanocomposite was easily separated from the sample solution using an external magnet and the regeneration was achieved by washing the adsorbent with 0.05 mol L⁻¹ hydrochloric acid solution. Moreover, the nanoadsorbent was reusable for at least 10 cycles of adsorption-desorption with no significant decrease in the adsorption capacity. The nanoadsorbent was also used for the arsenic removal from acid mine drainage. Overall, the adsorbent displayed excellent reusability and stability; thus, they are promising nanoadsorbents for the removal of arsenic from acid mine drainage.

Enhanced arsenate removal from aqueous solution by Mn-doped MgAl-layered double hydroxides

In this study, Mn-doped MgAl-layered double hydroxides (LDHs) were successfully synthesized for efficient removal arsenate from aqueous solution. The structure and composition of Mn-doped MgAl-LDHs intercalated by different ions such as CO₃^2-, Cl^-, or NO₃^- were investigated. The characterizations of XRD, ATR FT-IR, SEM, TG-DTA, and N₂ adsorption-desorption presented that the Mn-doped MgAl-LDHs (donated as Mn-LDHs) have very similar physical morphologies and properties to the MgAl-Cl-LDHs (donated as Mg-LDHs). However, the Mn-LDHs exhibits more preferable arsenate adsorption than Mg-LDHs. The As(V) removal kinetics data of Mn-LDHs is followed pseudo-second-order expression. The adsorption capacity of As(V) on Mn-LDHs via Langmuir isotherm model was 166.94 mg g^-1. The results of XPS revealed that the enhanced removal mechanism can be attributed to surface complexation of As(V) with Mn on the surface of Mn-LDHs. These results prove that Mn-doped LDHs can be considered as a potential material for adsorption As(V) from wastewater.

The stability of Fe(III)-As(V) co-precipitate in the presence of ascorbic acid: Effect of pH and Fe/As molar ratio

The potential hazards of Fe(III)-As(V) co-precipitate under reducing conditions are incompletely known. This work investigated the effect of Fe(III) reduction by ascorbic acid (AH₂) on the stability of Fe(III)-As(V) co-precipitate at different pHs and Fe/As molar ratios. The results showed that As (14-98.9%) and Fe (27.9-99.3%) were significantly released into solution by 79.9-97.5% Fe(III) reduction of the co-precipitate (Fe/As molar ratios of 3 and 5) at pH 5-9. More As release was observed with the increase of pH (6-9) or decrease in Fe/As molar ratio (from 5 to 3). This could be attributed by oxalate, the final product of AH₂ decomposition, which strongly competed with As(V) for Fe(II) at higher pH or lower Fe/As molar ratio, inhibiting parasymplesite accumulation and then causing more As mobilization. The stability of Fe(III)-As(V) co-precipitate with AH₂ upon Fe(III) reduction was lower than that in oxic environment. Compared with produced Fe(II,III) (hydr)oxides in the presence of hydroquinone (QH₂), humboldtine was formed during the long-term reactions of Fe(III)-As(V) co-precipitate with AH₂. The findings of this study implied that parasymplesite and humboldtine as secondary solid products were environmental relevant and mainly responsible for As(V) and Fe(II) immobilization.

Arsenic associated with gypsum produced from Fe(III)-As(V) coprecipitation: Implications for the stability of industrial As-bearing waste

Gypsum is a high-volume by-product of hydrometallurgical arsenic removal using the Fe(III)-As(V) coprecipitation technique. However, the role of gypsum in As fixation during the Fe(III)-As(V) coprecipitation process and the potential risk of As-bearing gypsum are still not well known. In this study, the fixation of As by gypsum was quantitatively investigated as a function of the initial As and H₂SO₄ concentrations, Fe/As molar ratio, pH, and neutralization mode. Column leaching tests were also performed to evaluate the potential risk of Fe(III)-As(V) coprecipitates. The results showed that the gypsum isolated from Fe(III)-As(V) coprecipitates by ascorbic acid (VC) treatment could fix up to 4.5% of the total As in the system at pH 8 in fixed-pH mode. The gypsum produced in fixed-pH mode contained a significantly higher concentration of As than that produced in acid-alkaline neutralization mode. The column leaching tests showed that a faster leaching rate could enhance the release of As from Fe(III)-As(V) coprecipitates. It seems that the presence of gypsum could prevent the release of As from Fe(III)-As(V) coprecipitates. Linear combination fits of the As and Fe K-edge XANES spectra suggested that As was likely to occur as adsorbed As and yukonite in VC-isolated gypsum.

A new technique for removing strontium from seawater by coprecipitation with barite

Strontium (Sr) removal from seawater has recently attracted attention from an environmental perspective after the Fukushima Nuclear Power Plant accident, but there is a lack of effective removal techniques for removing Sr from seawater. In the present study, we looked at the removal efficiency of Sr by using barite (BaSO₄) under various experimental conditions to develop techniques for the direct removal of Sr from seawater. The effects of pH, saturation state, ionic strength, competitive ions, and [Ba²⁺]/[SO₄^2-] ratio in the initial aqueous solution were examined. Among them, Sr uptake by barite was found to be dependent on pH, saturation state, and [Ba²⁺]/[SO₄^2-] ratio in initial aqueous solution, showing that most of the aqueous Sr can be removed from the aqueous solution by adjusting these parameters. However, the effects of ionic strength and competitive ions were negligible, suggesting the effectiveness of its application to removal of Sr from seawater. Batch experiments were also conducted in a seawater system, and a rather high removal efficiency of Sr from seawater (more than 90%) was achieved. Considering its high removal and retention efficiency of Sr in seawater systems, barite is a reliable material for the removal of Sr from seawater.

Simultaneous suppression of acid mine drainage formation and arsenic release by Carrier-microencapsulation using aluminum-catecholate complexes

Pyrite (FeS₂), the most common sulfide mineral in nature, plays an important role in the formation of acid mine drainage (AMD), one of the most serious environmental problems after the closure of mines and mineral processing operations. Likewise, arsenopyrite (FeAsS) is an important sulfide mineral because its dissolution releases toxic arsenic (As) into the environment. To mitigate the serious environmental problems caused by pyrite and arsenopyrite, this study investigated carrier-microencapsulation (CME) using Al-catecholate complexes, a technique that selectively forms protective coatings on the surfaces of sulfide minerals, by electrochemical techniques and batch leaching experiments coupled with surface sensitive characterization techniques. Cyclic voltammetry (CV) of Al-catecholate complexes (mono-, bis-, tris-catecholate) suggest that these three species could be oxidatively decomposed in this order: [Al(cat)₃]^3-→[Al(cat)₂]^-→[Al(cat)]⁺→Al³⁺, and these reactions were irreversible. Among these three species, [Al(cat)]⁺ was the most effective in suppressing pyrite and arsenopyrite oxidations because it requires less steps for complete decomposition than the other two complexes. Analyses of CME treated minerals by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated that they were covered with Al-oxyhydroxide (γ-AlO(OH)), which became more extensive at higher [Al(cat)]⁺ concentrations. In addition, this coating was stable even at relatively high applied potentials that simulated surface oxidizing conditions. Based on these results, a detailed mechanism of Al-based CME is proposed: (1) adsorption of [Al(cat)]⁺ on the surface of mineral, (2) oxidative decomposition of [Al(cat)]⁺ and release of "free" Al³⁺, and (3) precipitation and formation of Al-oxyhydroxide coating.

Regulation of oxidative stress and mineral nutrient status by selenium in arsenic treated crop plant Oryza sativa

The present study was conducted to examine the impact of selenium (Se) on mineral nutrient status and oxidative stress in crop plant Oryza sativa treated with arsenic (As). Scanning electron microscopy (SEM) coupled with Energy dispersive x-ray spectroscopy (EDS) study revealed the morphological deformities in leaf veins along with granular deposition on the leaf surface. The EDS analysis exhibited loss of elements (S, Si, Cl, K, Ca, Fe and Cu) in As(III) treatment in rice roots as compared to untreated root. In the case of As(III) treated shoot, changes in elements content in term of percent atomic weight was K (1.17-0.90%), Cl (1.04-24.75%), Na (0.65-3.52%) and S (0.49-2.52%) when compared with untreated shoot. The result of EDS analysis showed that As limits the concentration of important mineral elements present in the rice root and shoot. Rice plant treated with Se (10µM) and sub lethal dose of As(III) (60µM) showed better growth responses in term of root, shoot length (11.4% and 10.71%, respectively), biomass (11.7%), reduced malonyldialdehyde content (35.14%) and stimulated antioxidant level indicating better As tolerance potential against As. Further, a selenium dependent significant reduction in As accumulation was also observed in root (14.24%) and shoot (23.78%) of rice plant when compared with plant treated with As alone. This study highlights the potential of Se to ameliorate the ecotoxicological risks associated with the As buildup in agricultural land.

Effect of iron reduction by enolic hydroxyl groups on the stability of scorodite in hydrometallurgical industries and arsenic mobilization

Scorodite (FeAsO₄·2H₂O) is an important arsenic-bearing solid waste in hydrometallurgical industries, but its stability in reducing environments is not well understood. This study investigated the effect of Fe(III) reduction by enolic hydroxyl groups on the stability of scorodite and arsenic mobilization at various pH values and ascorbic acid/scorodite molar ratios (AH₂/Sc). The results showed that 47-89% Fe(III) reduction by ascorbic acid caused approximately 10-69% (~ 37-260 mg L^-1) arsenic release and 4.5-63% (~ 13-176 mg L^-1) Fe(II) release at pH 5-8. The releases of arsenic and Fe(II) increased with increasing AH₂/Sc, whereas they decreased as pH increased. The results of the solid characterization and chemical analysis indicated that the mixture of poorly crystalline parasymplesite and probably amorphous FeHAsO₄⋅xH₂O was the new arsenic sink. The high solubility of this ferrous arsenate with the Fe(II)/As(V) molar ratio > 1 was deemed to be a major contributor to the relatively high arsenic release. This work differed from our previous finding that almost all arsenic was retained in the solid phase after similar Fe(III) reduction in scorodite with hydroquinone. Phenolic hydroxyl groups complexed with aqueous Fe(II), unlike enolic hydroxyl groups, was possibly the dominant reason for the formation of different secondary minerals, which strongly influenced arsenic redistribution between aqueous and solid phases.

Formation of tooeleite and the role of direct removal of As(III) from high-arsenic acid wastewater

In this study, an earth-mimetic method was proposed for the direct removal of As(III) by the formation of tooeleite, a ferric arsenite sulfate mineral. A series of batch experiments was used to study the relationship between the formation of tooeleite and the removal of As(III). The results indicate that As(III) removal efficiency reached up to 99% under the treatment condition of pH 1.8-4.5, initial As(III) concentration higher than 0.75g/L, and Fe/As ranged from 0.8 to 2 at room temperature. Various characterizations confirm that the precipitate obtained by this treatment was tooeleite with relatively high stability. In addition, it is assumed that ferrihydrite exists as a precursor, which is vital to the formation of tooeleite and the removal of As(III). This study suggests that tooeleite formation may be an alternative method in the direct removal of As(III) from high-arsenic acid wastewater.

MS title: Catalytic oxidation and removal of arsenite in the presence of Fe ions and zero-valent Al metals

Arsenic immobilization in acid mine drainage (AMD) is required prior to its discharge to safeguard aquatic organisms. Zero-valent aluminum (ZVAl) such as aluminum beverage cans (AlBC) was used to induce the oxidation of As(III) to As(V) and enhance the subsequent As removal from an artificially prepared AMD. While indiscernible As(III) oxidation was found in aerated ZVAl systems, the addition of 0.10-0.55mM Fe(II) or Fe(III) into the AMD significantly promoted the As(V) production. Reactions between Fe(II) and H2O2, which was produced through an oxidative reaction of ZVAl with dissolved oxygen, generated OH radicals. Such OH radicals subsequently induced the As(III) oxidation. Over the course of the Fenton like reaction, ZVAl not only directly generated the H2O2, but indirectly enhanced the OH radical production by replenishing Fe(II). Arsenite oxidation in the aerated ZVAl/Fe and AlBC/Fe systems followed zero- and first-order kinetics. Differences in the kinetic reactions of ZVAl and AlBC with respect to As(III) oxidation were attributed to higher productive efficiency of the oxidant in the AlBC systems. After the completion of As(III) oxidation, As(V) could be removed simultaneously with Al(III) and Fe(III) by increasing solution's pH to 6 to produce Al/Fe hydroxides as As(V) scavengers or to form Al/Fe/As co-precipitates.

Fenton-Like Catalysis and Oxidation/Adsorption Performances of Acetaminophen and Arsenic Pollutants in Water on a Multimetal Cu-Zn-Fe-LDH

Acetaminophen can increase the risk of arsenic-mediated hepatic oxidative damage; therefore, the decontamination of water polluted with coexisting acetaminophen and arsenic gives rise to new challenges for the purification of drinking water. In this work, a three-metal layered double hydroxide, namely, Cu-Zn-Fe-LDH, was synthesized and applied as a heterogeneous Fenton-like oxidation catalyst and adsorbent to simultaneously remove acetaminophen (Paracetamol, PR) and arsenic. The results showed that the degradation of acetaminophen was accelerated with decreasing pH or increasing H2O2 concentrations. Under the conditions of a catalyst dosage of 0.5 g·L(-1) and a H2O2 concentration of 30 mmol·L(-1), the acetaminophen in a water sample was completely degraded within 24 h by a Fenton-like reaction. The synthesized Cu-Zn-Fe-LDH also exhibited a high efficiency for arsenate removal from aqueous solutions, with a calculated maximum adsorption capacity of 126.13 mg·g(-1). In the presence of hydrogen peroxide, the more toxic arsenite can be gradually oxidized into arsenate and adsorbed at the same time by Cu-Zn-Fe-LDH. For simulated water samples with coexisting arsenic and acetaminophen pollutants, after treatment with Cu-Zn-Fe-LDH and H2O2, the residual arsenic concentration in water was less than 10 μg·L(-1), and acetaminophen was not detected in the solution. These results indicate that the obtained Cu-Zn-Fe-LDH is an efficient material for the decontamination of combined acetaminophen and arsenic pollution.

Stability of arsenate-bearing Fe(III)/Al(III) co-precipitates in the presence of sulfide as reducing agent under anoxic conditions

Currently, the co-precipitation of arsenate with ferric iron at molar ratios Fe(III)/As(V) ≥ 3 by lime neutralization produces tailings solids that are stable under oxic conditions. However not much is known about the stability of these hazardous co-precipitates under anoxic conditions. These can develop in tailings storage sites by the action of co-discharged reactive sulfides, organic reagent residuals or bacterial activity. The ferric matrix can then undergo reductive dissolution reactions, which could release arsenic into the pore water. Co-ions like aluminum could provide a redox-immune sink to scavenge any mobilized arsenic as a result of reduction of ferric. As such, in this work Fe(III)/As(V) = 4 and aluminum substituted Fe(III)/Al(III)/As(V) = 2/2/1 co-precipitates were produced in a mini continuous co-precipitation process circuit and subjected to excess sulfide addition under inert gas to evaluate their stability. It was found that the ferric-arsenate co-precipitate could retain up to 99% (30 mg/L in solution) of its arsenic content despite the high pH (10.5) and extremely reducing (Eh < -200 mV) environment. There was no significant reduction of arsenate and only 45% of ferric iron was reduced. Partial aluminum substitution was found to cut the amount of mobilized arsenic by 50% (down to 15 mg/L) hence mixed Fe(III)/Al(III)-arsenate co-precipitates may offer better resistance to reductive destabilization over the long term than all iron co-precipitates.

Rapid degradation of p-arsanilic acid with simultaneous arsenic removal from aqueous solution using Fenton process

Although banned in some developed countries, p-arsanilic acid (p-ASA) is still used widely as a feed additive for swine production in many countries. With little uptake and transformation in animal bodies, nearly all the p-ASA administered to animals is excreted chemically unchanged in animal wastes, which can subsequently release the more toxic inorganic arsenic species upon degradation in the environment. For safe disposal of the animal wastes laden with p-ASA, we proposed a method of leaching the highly water-soluble p-ASA out of the manure first, followed by treatment of the leachate using the Fenton process to achieve fast oxidation of p-ASA and removal of the inorganic arsenic species released (predominantly arsenate) from solution simultaneously. The effects of solution pH, dosages of H2O2 and Fe(2+), and the presence of dissolved organic matter (DOM) on the treatment efficiency were systematically investigated. Under the optimum treatment conditions (0.53 mmol L(-1) Fe(2+), 2.12 mmol L(-1) H2O2, and initial pH of 3.0), p-ASA (10 mg-As L(-1)) could be completely oxidized to As(V) within 30 min in pure water and 4 natural water samples, and at the final pH of 4.0, the residual arsenic levels in solution phase were as low as 1.1 and 20.1-43.4 μg L(-1) in the two types of water matrixes, respectively. The presence of humic acid significantly retarded the oxidation of p-ASA by scavenging HO, and inhibited the As(V) removal through competitive adsorption on ferric hydroxide. Due to the high contents of DOM in the swine manure leachate samples (TOC at ∼500 mg L(-1)), much higher dosages of Fe(2+) (10.0 mmol L(-1)) and H2O2 (40.0 mmol L(-1)) and a longer treatment time (120 min) were required to achieve near complete oxidation of p-ASA (98.0%), while maintaining the levels of residual arsenic in the solution at <70.0 μg L(-1). The degradation pathway of p-ASA in the Fenton process was proposed based on the major degradation products detected. Together, the results demonstrate that the Fenton process is promising as an efficient, robust, and low-cost treatment method for controlling the risk of p-ASA in the animal wastes generated at factory farms.

Pathway of zinc oxide formation by seed-assisted and controlled double-jet precipitation

ZnO can be well formed in a short time at room temperature via seed-assisted and controlled double-jet precipitation.

Yeast protective response to arsenate involves the repression of the high affinity iron uptake system

Arsenic is a double-edge sword. On the one hand it is powerful carcinogen and on the other it is used therapeutically to treat acute promyelocytic leukemia. Here we report that arsenic activates the iron responsive transcription factor, Aft1, as a consequence of a defective high-affinity iron uptake mediated by Fet3 and Ftr1, whose mRNAs are drastically decreased upon arsenic exposure. Moreover, arsenic causes the internalization and degradation of Fet3. Most importantly, fet3ftr1 mutant exhibits increased arsenic resistance and decreased arsenic accumulation over the wild-type suggesting that Fet3 plays a role in arsenic toxicity. Finally we provide data suggesting that arsenic also disrupts iron uptake in mammals and the link between Fet3, arsenic and iron, can be relevant to clinical applications.