Advantages of aeration in arsenic removal and arsenite oxidation by structural Fe(II) hydroxides in aqueous solution

Arsenite oxidation and adsorptive arsenic removal from contaminated water: a review

Arsenic poisoning of groundwater is one of the most critical environmental hazards on Earth. Therefore, the practical and proper treatment of arsenic in water requires more attention to ensure safe drinking water. The World Health Organization (WHO) sets guidelines for 10 μg/L of arsenic in drinking water, and direct long-term exposure to arsenic in drinking water beyond this value causes severe health hazards to individuals. Numerous studies have confirmed the adverse effects of arsenic after long-term consumption of arsenic-contaminated water. Here, technologies for the remediation of arsenic from water are highlighted for the purpose of understanding the need for a single-point solution for the treatment of As(III)-contaminated water. As(III) species are neutral at neutral pH; the solution requires transformation technology for its complete removal. In this critical review, emphasis was placed on single-step technologies with multiple functions to remediate arsenic from water.

Definitive Review of Nanobiochar

Nanobiochar is an advanced nanosized biochar with enhanced properties and wide applicability for a variety of modern-day applications. Nanobiochar can be developed easily from bulk biochar through top-down approaches including ball-milling, centrifugation, sonication, and hydrothermal synthesis. Nanobiochar can also be modified or engineered to obtain "engineered nanobiochar" or biochar nanocomposites with enhanced properties and applications. Nanobiochar provides many fold enhancements in surface area (0.4-97-times), pore size (0.1-5.3-times), total pore volume (0.5-48.5-times), and surface functionalities over bulk biochars. These enhancements have given increased contaminant sorption in both aqueous and soil media. Further, nanobiochar has also shown catalytic properties and applications in sensors, additive/fillers, targeted drug delivery, enzyme immobilization, polymer production, etc. The advantages and disadvantages of nanobiochar over bulk biochar are summarized herein, in detail. The processes and mechanisms involved in nanobiochar synthesis and contaminants sorption over nanobiochar are summarized. Finally, future directions and recommendations are suggested.

Determination of bupropion by off-line coupling Fe3O4@CuO&GO nanocomposite and ion mobility spectrometry with application to biological samples

In this study, an off-line coupling of dispersive solid-phase extraction (DSPE) and ion mobility spectrometry (IMS) was introduced to extract and determine bupropion (BUP). A magnetic nanocomposite adsorbent (Fe3O4@CuO&GO) was fabricated by combining graphene oxide (GO) sheets with Fe3O4 and CuO through coprecipitation method. The synthesized adsorbent was characterized and analyzed using the analytical techniques. The effect of extraction parameters including desorption solvent (type and volume), pH, adsorbent amount, contact time, temperature, and the volume of analyte solution on the extraction efficiency was investigated and optimized. The operational parameters of IMS method were also investigated. Under the optimum conditions (DSPE-IMS), the proposed method provided a linear range 4.0-24.0 ng for BUP with a determination coefficient R2 ≥ 0.98. LOD and LOQ values were 0.7 and 2.2 ng for BUP. The repeatability of proposed method was evaluated and reported as relative standard deviation (RSD% ≤ 5.5). The developed method was applied to determine BUP in different biological samples, in which satisfactory results were obtained (93.0-98.0%).

A chrysotile-based Fe/Ti nanoreactor enables efficient arsenic capture for sustainable environmental remediation

Iron-based materials for arsenic (As) immobilization in practical groundwater and soil remediation suffer from a low removal capacity and an insufficient long-term stability. Herein, a unique chrysotile-based nanoreactor has been developed by incorporating iron/titanium oxides into the cylindrical cavity of chrysotile (TiFe-Chy), providing sufficient internal reaction sites for As immobilization. Results reveal that the adsorption capacities of TiFe-Chy for As(III) and As(V) are considerably higher than the commonly used amendments, i.e., layered double hydroxide (LDH) and Phoslock®, respectively. More importantly, TiFe-Chy exhibits a strong anti-interference capability of As immobilization in soils compared to those commercial products due to this unique incorporation approach. Fixed-bed leaching experiments indciate that this TiFe-Chy nanoreactor can efficiently decrase the As(III) and As(V) concentrations by 81.8-87.3% within a period of ten years, significantly improving the long-term stability of As immobilization in soils. Life cycle assessment analysis reveals that TiFe-Chy can reduce negative environmental impacts (such as carbon emissions), resulting in a low cost for soils and groundwater remediation. The findings of this work open a new avenue for sustainable heavy metal(loid)s remediation in groundwater and soils.

How does arsenic speciation (arsenite and arsenate) in groundwater affect the performance of an aerated electrocoagulation reactor and human health risk?

Arsenic (As) occurrence in water resources has become one of the most critical environmental problems worldwide. The detrimental health impacts on humans have been reported due to the consumption of As-contaminated groundwater resources. Consumption of As-containing water over the long term can cause arsenicosis and chronic effects on human health due to its toxicity. Several treatment processes are available for As removals such as coagulation, ion exchange, adsorption, and membrane technologies but they have various major drawbacks. In the present work, therefore, an aerated electrocoagulation (EC) system with aluminum anodes was operated for simultaneous arsenate (As(V)) and arsenite (As(III)) removal to overcome the disadvantages of other processes such as, sludge formation, difficulties in operation, high operating costs, high energy consumption, and the requirement of pre-treatment process and to enhance the conventional EC process. The combined effects of the applied current (0.075-0.3 A), aeration rate (0-6 L/min), pH (6.5-8.5), and As speciation (As(V)-As(III)) were studied on As removal efficiency. The findings revealed that As removal mostly depended on the airflow rate and the applied current in the EC system. The highest As removal efficiency (99.1%) was obtained at an airflow rate of 6 L/min, a pH of 6.5, an initial As (V) concentration of 200 μg/L, and a current of 0.3 A, with an energy consumption of 2.85 kWh/m³ and an operating cost of 0.66 $/m³. The human health risk assessment of treated water was also examined to understand the performance of the EC system. At most of the experimental runs, the chronic toxic risk (CTR) and carcinogenic risk (CR) of As were within the permissible limits except for an airflow rate of 0-2 L/min, an initial pH of 8.5, and a current of 0.075-0.15 A for high initial As (III) concentrations. Overall, the As removal performance and groundwater risk assessment show that the EC process is a promising option for industrial applications.

Arsenic (III) removal by mechanochemically sulfidated microscale zero valent iron under anoxic and oxic conditions

The interaction of As(III) with micron-sized, mechanochemically sulfidated zero-valent iron (S-mZVI^bm) has been studied under both anoxic and oxic conditions. The As(III) removal capacity varied with the increase of S/Fe molar ratio under anoxic conditions, while it continuously decreased under oxic conditions. A series of sequential extractions, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) spectroscopy analyses were used to investigate As(III) removal mechanisms. In the absence of oxygen, As(III) was removed from solution primarily through the formation of As₄S₄ with less than half of the removal resulting from the adsorption of As(III)/As(V) and FeAsS precipitation. Under oxic conditions, adsorption onto iron (oxyhydr)oxides was the dominant mechanism of As(III) removal. Increasing sulfidation decreased particle Fe(0) content, which resulted in less production of iron (oxyhydr)oxides and therefore lower As(III) removal capacities. Column experiments showed that less than 2 wt% of S-mZVI^bm in sand was able to rapidly reduce the As(III) concentration in a real groundwater from 300 to 10 µg/L, the Chinese drinking water standard, for up to 750 BV with an EBCT of 2.54 min. This study demonstrates that S-mZVI^bm is an efficient and cost-effective material in treating As-contaminated water to ensure water safety.

As(III) removal from wastewater and direct stabilization by in-situ formation of Zn-Fe layered double hydroxides

In order to remove and stabilize As(III) simultaneously from wastewater, a novel and effective method based on the in-situ formation of As(III)-containing Zn-Fe layered double hydroxides (ZnFe-As-LDHs) was developed. The influence of pH, Zn/Fe, Fe/As and adding rate on the formation of ZnFe-As-LDHs were investigated. Under the optimal conditions, the concentration of As(III) decreased from 100 to 0.13 mg/L and As leaching concentration of the ultimate sludge was 1.87 mg/L, which could meet the arsenic leaching criteria (5 mg/L) regulated by US EPA. Compared with the "ex-situ" sludge obtained by As(III) adsorbed on the pre-formed ZnFe-LDHs, the As(III) removal efficiency increased by 21.6 % and the stability of the sludge increased by 94.2 % on the in-situ formation of LDHs, which mainly attributed to 55.06 % oxidation of As(III) and co-precipitation of As with Zn and Fe. Additionally, a possible in-situ formation pathway for ZnFe-As-LDHs was illustrated. At the beginning of the process, non-crystalline ferric arsenate formed and then transformed to amorphous ferrihydrite as precursors, followed by the formation of LDHs. This work demonstrated that co-precipitating As with Zn and Fe in the wastewater to in-situ form LDHs exhibited excellent potential for removal and direct stabilization of As(III).

Green rust functionalized geopolymer of composite cementitious materials and its application on treating chromate in a holistic system

Green rust functionalized geopolymer of composite cementitious materials (GR-CCM) was synthesized to improve the adsorption and subsequent stabilization/solidification of chromate in a holistic operating system. The initial pH in solution exhibited the most significant effect on the chromate removal by GR-CCM among three adsorption factors. The maximum monolayer adsorption capacity and theoretical saturation capacity of GR-CCM for Cr(VI) in the acidic condition were 55.01 mg/g and 41.70 mg/g, respectively. Amorphousness brought by loading GR weakened the crystallinity of composite cementitious materials (CCM), which enhanced the adsorption capacity of CCM and boosted the solidification process. The mixed-valent iron species in the GR-CCM not only directly engaged in the adsorption and reduction of chromate also positively strengthened the solidification of Cr species during the whole treatment. This study facilitates the application of GRs on the geopolymer materials and demonstrates the combination of adsorption and immobilization for the treatment of other potential heavy metal contamination.

Sustainable Low-Concentration Arsenite [As(III)] Removal in Single and Multicomponent Systems Using Hybrid Iron Oxide-Biochar Nanocomposite Adsorbents-A Mechanistic Study

Rice and wheat husks were converted to biochars by slow pyrolysis (1 h) at 600 °C. Iron oxide rice husk hybrid biochar (RHIOB) and wheat husk hybrid biochar (WHIOB) were synthesized by copyrolysis of FeCl₃-impregnated rice or wheat husks at 600 °C. These hybrid sorbents were characterized using X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy (SEM), SEM-energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, physical parameter measurement system, and Brunauer-Emmett-Teller (BET) surface area techniques. Fe₃O₄ was the predominant iron oxide present with some Fe₂O₃. RHIOB and WHIOB rapidly chemisorbed As(III) from water (∼24% removal in first half an hour reaching up to ∼100% removal in 24 h) at surface Fe-OH functions forming monodentate ≡Fe-OAs(OH)₂ and bidentate (≡Fe-O)₂AsOH complexes. Optimum removal occurred in the pH 7.5-8.5 range for both RHIOB and WHIOB, but excellent removal occurred from pH 3 to 10. Batch kinetic studies at various initial adsorbate-adsorbent concentrations, temperatures, and contact times gave excellent pseudo-second-order model fits. Equilibrium data were fitted to different sorption isotherm models. Fits to isotherm models (based on R ² and χ²) on RHIOB and WHIOB followed the order: Redlich-Peterson > Toth > Sips = Koble-Corrigan > Langmuir > Freundlich = Radke-Prausnitz > Temkin and Sips = Koble-Corrigan > Toth > Redlich-Peterson > Langmuir > Temkin > Freundlich = Radke-Prausnitz, respectively. Maximum adsorption capacities, Q _RHIOB ⁰ = 96 μg/g and Q _WHIOB ⁰ = 111 μg/g, were obtained. No As(III) oxidation to As(V) was detected. Arsenic adsorption was endothermic. Particle diffusion was a rate-determining step at low (≤50 μg/L) concentrations, but film diffusion controls the rate at ≥100-200 μg/L. Binding interactions with RHIOB and WHIOB were established, and the mechanism was carefully discussed. RHIOB and WHIOB can successfully be used for As(III) removal in single and multicomponent systems with no significant decrease in adsorption capacity in the presence of interfering ions mainly Cl^-, HCO₃ ^-, NO₃ ^-, SO₄ ^2-, PO₄ ^3-, K⁺, Na⁺, Ca²⁺. Simultaneous As(III) desorption and regeneration of RHIOB and WHIOB was successfully achieved. A very nominal decrease in As(III) removal capacity in four consecutive cycles demonstrates the reusability of RHIOB and WHIOB. Furthermore, these sustainable composites had good sorption efficiencies and may be removed magnetically to avoid slow filtration.

Co-oxidation of As(III) and Fe(II) by oxygen through complexation between As(III) and Fe(II)/Fe(III) species

Previous studies have shown that the oxidation of Fe(II) by molecular oxygen can lead to the co-oxidation of As(III) at neutral pH. However, the mechanism of As(III) oxidation in the presence of Fe(II) with respect to the interaction between As(III) and Fe(II) is still unclear. In this work, we examined the oxidation of As(III) in the presence of Fe(II) in air-saturated water, which is affected by pH, co-existing phosphate, and scavengers (tert-butanol, superoxide dismutase, catalase and dimethylsulfoxide). Results confirm that the formation of the Fe(II)-As(III) complex (formation constant, K_{Fe(II)-As(III)} = 10^3.86 M^-1) plays an extremely important role in the initial stage of As(III) oxidation at pH 7.25. The oxidation of Fe(II)-As(III) promotes the production of H₂O₂ and colloidal ferric hydroxide. H₂O₂ reacts with the Fe(II)-As(III) complex through oxidizing Fe(II) to Fe(IV), which then causes the partial oxidation (ca. 50%) of As(III). The other part of As(III) oxidation by H₂O₂ occurs, in the form of Fe(III)-As(III) complex, through direct electron transfer from As(III) to H₂O₂ but not through Fe(IV). This work provides new mechanistic insight into arsenic and iron redox chemistry in the environment and furthers our understanding of Fenton reactions at neutral pH.

Tradeoff between groundwater arsenite removal efficiency and current production in the self-powered air cathode electrocoagulation with different oxygen reduction pathways

Naturally occurring arsenic enrichment in aquifers posts a huge threat to drinking water safety. To achieve energy-efficient arsenite [As(III)] removal, a self-powered iron electrocoagulation was developed that coupled iron corrosion anode with oxygen reduction air cathode for simultaneous As(III) oxidation and removal. Activated carbon (AC), which favored the four-electron oxygen reduction reaction (ORR, O₂+4e^-+4H⁺→2H₂O, E^0' = 0.816 V), and carbon black (CB), which favored two-electron ORR (O₂+2e^-+2H⁺→H₂O₂, E^0' = 0.283 V), were evaluated for As(III) removal efficiency and current production performance. The comparison showed a tradeoff between higher current (i.e., higher iron corrosion rate) attributed to the higher reduction potential with four-electron ORR, and higher H₂O₂ production for improved As(III) oxidation with two-electron ORR yet the lower reduction potential The CB cathode that favored H₂O₂ production had the best As(III) removal of 99.2 ± 0.4% and the lowest maximum power density of 60 ± 0.3 mW m^-2, while the AC cathode showed the opposite trend. These results suggested that cathode catalysts need to be carefully evaluated for the balance of As(III) removal and current production to provide a sustainable and effective solution for groundwater As(III) removal.

Improved adsorption of Congo red by nanostructured flower-like Fe(II)-Fe(III) hydroxy complex

Amorphous Fe(II)-Fe(III) hydroxy complex with flower-like nanostructure was synthesized by ferric reduction using a microwave-assisted ethylene glycol approach. Here we investigated the correlation between its chemical composition and the removal rate for Congo red (CR) dye. The results showed that the amorphous complex had similar reduction and anion exchange capacities to the green rust. Due to the synergistic effect of attractive electrostatic interaction, anion exchange, ferrous redox and hydrogen bonding, the Fe(II)-Fe(III) hydroxy complex exhibited strong adsorption of CR with an estimated adsorption capacity up to 513 mg g^-1. In contrast, the Fe(III) hydroxy complex had an adsorption capacity of 296 mg g^-1 because of the predominant mechanism based on the electrostatic interaction. The present study provides a facile synthesis of nanostructured iron hydroxy complex, with superior performance in adsorbing CR.

High catalytic oxidation of As(III) by molecular oxygen over Fe-loaded silicon carbide with MW activation

The catalytic oxidation of arsenite (As(III)) to arsenate (As(V)) and the removal of arsenic (As) in an iron loaded silicon carbide (Fe/SiC) system under microwave (MW) irradiation were studied. Fe/SiC was synthesized by electro-deposition and its capability of activating molecular oxygen was also characterized. Highly efficient As(III) removal in a wide pH range of 2.5-9.5 was achieved, involving oxidation by reactive oxidation species (OH and O₂^-) induced by MW irradiation and adsorption by the generated Fe (hrdro)oxide precipitates. Significant enhancement of As(III) oxidation was achieved at acidic pH, where sequential fresh Fe(0) exposure with MW irradiation could improve the Fenton-like reactions and oxidation efficiency for As(III). As(III) removal was accelerated via adsorption at alkaline conditions, where the adsorbed Fe(II) on Fe/SiC showed significant catalytic activity for molecular oxygen and high pH further favored the formation of (hydro)oxides and the As sequestration by adsorption. Fe/SiC showed superior performance for the treatment of As in water with MW irradiation.

Opposite effects of dissolved oxygen on the removal of As(III) and As(V) by carbonate structural Fe(II)

Freshly prepared carbonate structural Fe(II) (CSF) was used to immobilize As(III) and As(V) in wastewater under oxic and anoxic conditions. Dissolved oxygen was found to exert opposite effects on these two arsenic species. The sorption density of As(III) was higher under oxic conditions, whereas that of As(V) was higher under anoxic conditions. X-ray diffraction and infrared spectroscopic analyses indicated that crystalline parasymplesite (Fe(II)₃(AsO₄)₂·8H₂O) was formed when As(V) was removed under anoxic conditions, while an amorphous Fe-As-containing precipitate was formed when As(III) was removed under oxic conditions. The distribution of arsenic and iron between the solution and sediments suggested that the oxidation of structural Fe(II) promoted coprecipitation process and inhibited surface complexation. X-ray photoelectron spectroscopic analyses revealed that more As(III) was oxidized under oxic condition, which contributed to a higher sorption capacity for As(III). The formation of parasymplesite through surface complexation/precipitation was proposed to be more effective for the removal of As(V) by CSF, while As(III) was more efficiently removed through coprecipitation. Together, the results suggest that CSF may be an effective material for sequestering both As(III) and As(V). In addition, attention should be paid to the dissolved oxygen content when remediating different arsenic species.