Performance and mechanism of simultaneous removal of chromium and arsenate by Fe(II) from contaminated groundwater

(Fe, Cr)(OH)3 Coprecipitation in Solution and on Soil: Roles of Surface Functional Groups and Solution pH

The simultaneous precipitation of (Fe, Cr)(OH)₃ nanoparticles in solution (homogeneous) and on soil surfaces (heterogeneous), which controls Cr transport in soil and aquatic systems, was quantified for the first time in the presence of model surfaces, i.e., bare and natural organic matter (NOM)-coated SiO₂ and Al₂O₃. Various characterization techniques were combined to explore the surface-ion-precipitate interactions and the controlling mechanisms. (Fe, Cr)(OH)₃ accumulation on negatively charged SiO₂ was mainly governed by electrostatic interactions between hydrolyzed ion species or homogeneous (Fe, Cr)(OH)₃ and surfaces. The elevated pH through protonation of Al₂O₃ surface hydroxyls resulted in higher Cr/Fe ratios in both homogeneous and heterogeneous coprecipitates. Due to ignorable NOM adsorption onto SiO₂, the amounts of (Fe, Cr)(OH)₃ precipitates on bare/NOM-SiO₂ were similar; contrarily, attributed to favored NOM adsorption onto Al₂O₃ and consequently carboxyl association with metal ions or (Fe, Cr)(OH)₃ nanoparticles, remarkably more heterogeneous precipitates harvested on NOM-Al₂O₃ than bare-Al₂O₃. With the same solution supersaturation, the total amounts of homogeneous and heterogeneous precipitates were similar irrespective of the substrate type. With lower pH, decreased electrostatic forces between substrates and precipitates shifted (Fe, Cr)(OH)₃ distribution from heterogeneous to homogeneous phases. The quantitative knowledge of (Fe, Cr)(OH)₃ distribution and the controlling mechanisms can assist in better Cr sequestration in natural and engineered settings.

Cross-redox and simultaneous removal of Cr(VI) and As(III): Influences of Fe(II), Fe(III), oxalic acid, and dissolved organic carbon

Hexavalent chromium [Cr(VI)] and arsenite [As(III)] are hazardous to both human and ecosystem. While their cross-redox reaction decreases both their toxicities, the interferences from ubiquitous substances like Fe (Fe(II) and Fe(III)) and organic compounds (oxalic acid and soil-extracted dissolved organic carbon (DOC)) on such interaction are rarely reported; thence, inspires the investigation in this study. Results showed that the cross-redox, in the absence of interfering substances, only occurred at pH≤2.0, with reaction orders of 0.676 and 0.783 in respect to the concentration of Cr(VI) and As(III). The pseudo-reaction constant, k', of such reaction was recorded at 0.087 m^1.377/(mmol^0.459 min). With the addition of Fe(II), the rate of Cr(VI) reduction is promoted in conjunction with suppressed As(III) oxidation. Upon neutralizing to pH 6.0, such reduced Cr can be entirely removed via Fe(II)-assisted adsorption and/or co-precipitation. Meanwhile, the elimination of aqueous As is relatively inferior (36 %), attributed to the largely preserved As(III), which is less susceptible to adsorptive/co-precipitative removal. Unlike Fe(II), Fe(III) did not alter Cr(VI)-As(III) cross-redox path, but triggered high adsorptive and/or co-precipitative removals of Cr and As (90 %). In contrast, both organically-altered systems exhibits plummeted As(III) oxidation, under distinctive mechanisms: oxalic acid competes with As(III) in the redox interactions while DOC reduces As(V) into As(III). Also, DOC would undergo complexion with metals and/or blocked the adsorption or co-precipitation sites, leading to even lower Cr and As precipitation. This study unravelled the interference from ubiquitous species in the co-removal of Cr(VI) and As(III), which provides insightful remediation for heavy metal contaminations.

Emerging technologies for PFOS/PFOA degradation and removal: A review

Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are highly recalcitrant anthropogenic chemicals that are ubiquitously present in the environment and are harmful to humans. Typical water and wastewater treatment processes (coagulation, flocculation, sedimentation, and filtration) are proven to be largely ineffective, while adsorption with granular activated carbon (GAC) has been the chief option to capture them from aqueous sources followed by incineration. However, this process is time-consuming, and produces additional solid waste and air pollution. Treatment methods for PFOS and PFOA generally follow two routes: (1) removal from source and reduce the risk; (2) degradation. Emerging technologies focusing on degradation are critically reviewed in this contribution. Various processes such as bioremediation, electrocoagulation, foam fractionation, sonolysis, photocatalysis, mechanochemical, electrochemical degradation, beams of electron and plasma have been developed and studied in the past decade to address PFAS crisis. The underlying mechanisms of these PFAS degradation methods have been categorized. Two main challenges have been identified, namely complexity in large scale operation and the release of toxic byproducts. Based on the literature survey, we have provided a strength-weakness-opportunity-threat (SWOT) analysis and quantitative rating on their efficiency, environmental impact and technology readiness.

Bifunctional Ionic Covalent Organic Networks for Enhanced Simultaneous Removal of Chromium(VI) and Arsenic(V) Oxoanions via Synergetic Ion Exchange and Redox Process

Chromium (VI) and arsenic (V) oxoanions are major toxic heavy metal pollutants in water threatening both human health and environmental safety. Herein, the development is reported of a bifunctional ionic covalent organic network (iCON) with integrated guanidinium and phenol units to simultaneously sequester chromate and arsenate in water via a synergistic ion-exchange-redox process. The guanidinium groups facilitate the ion-exchange-based adsorption of chromate and arsenate at neutral pH with fast kinetics and high uptake capacity, whereas the integrated phenol motifs mediate the Cr(VI)/Cr(III) redox process that immobilizes chromate and promotes the adsorption of arsenate via the formation of Cr(III)-As(V) cluster/complex. The synergistic ion-exchange-redox approach not only pushes high adsorption efficiency for both chromate and arsenate but also upholds a balanced Cr/As uptake ratio regardless of the change in concentration and the presence of interfering oxoanions.

Potential hazards of novel waste-derived sorbents for efficient removal of mercury from coal combustion flue gas

Novel waste-derived sorbents synthesized through one-step co-pyrolysis of wood and PVC (or brominated flame retarded plastic) were demonstrated as cost-effective sorbents for mercury (Hg) removal in our previous studies. To introduce magnetism and improve porosity, Fe species were further doped into such waste-derived sorbents. The ultimate fate of Hg-laden sorbents after their service is mainly disposed in landfill. Therefore, the stability of Hg/halogens on the spent sorbents is an important topic. In this work, the leachability of Hg/Cl/Br from four waste-derived sorbents was evaluated using toxicity characteristic leaching procedure (TCLP). Three traditional sorbents (Cl-impregnated activated carbon, Br-impregnated activated carbon and commercial activated carbon) were also tested for comparison. Experimental results suggested that the stability of Hg/Cl/Br on four waste-derived sorbents was far higher than that prepared by chemical impregnation. For four waste-derived sorbents, little Hg was leached out whereas certain amounts of Cl/Br escaped into the leachate. Interestingly, Fe-doping effectively improved the stability of Hg/Cl/Br on the waste-derived sorbents. Kinetic analysis revealed that diffusion process and surface chemical reaction were respectively the rate-limiting step for waste-derived sorbents before and after Fe-doping. Water-washing pretreatment could remove loosely-bonded Cl/Br from the waste-derived sorbents, while the Cl/Br essential for Hg removal was retained.

Simultaneous Removal of Arsenate and Chromate from Ground- and Surface- Waters by Iron-Based Redox Assisted Coagulation

Arsenic (As) and chromate (Cr(VI)) contamination of ground and surface waters is a major problem worldwide. Given that a new drinking water limit is anticipated for Cr(VI) and that the limit of arsenic in drinking water is quite low (10 μg/L), there is an urgent need for evaluating technologies that could be efficient for removal of both contaminants simultaneously. In this work, the use of Fe(II) redox assisted coagulation was investigated to simultaneously remove the contaminants of interest. The basic principle of this technology is that Fe(II) could react with Cr(VI) and form Fe(III)-hydroxides and insoluble Cr(III) species, while the freshly formed Fe(III) hydroxides are very efficient adsorbents for As(V). The effect of pH, the water matrix composition, Fe(II) dose, initial contaminant concentrations, NOM presence and phosphate concentration were the examined parameters. The results revealed that with a dose of 2 mg/L Fe(II), residual As(V) and Cr(VI) concentrations were both below 10 μg/L, from initial concentrations of 50 μg/L. Though, this is effective only at circumneutral pH values. This is however not a big obstacle, since most natural waters, especially groundwaters, have near neutral pH values. At these pH values, residual iron concentration was far below 200 μg/L. The presence of phosphate anions inhibited As(V) removal but had no effect on Cr(VI) removal. Increasing Fe(II) concentrations eliminated the effect of phosphate and provided simultaneous phosphate removal. Therefore, Fe(II) coagulation can be applied, with secured results, for simultaneous As(V), Cr(VI) and phosphate removal from waters.

Using porous iron composite (PIC) material to immobilize rhenium as an analogue for technetium

Technetium (⁹⁹Tc), a uranium-235 (²³⁵U) and plutonium-239 (²³⁹Pu) fission product, is a primary risk driver in low level radioactive liquid waste at U.S. Department of Energy sites. Previous studies have shown success in using Zero Valent Iron (ZVI) to chemically reduce and immobilize redox sensitive groundwater contaminants. Batch and column experiments were performed to assess the ability of a novel porous iron composite material (PIC) to immobilize Tc(VII) in comparison with two commercial Fe oxide sorbents and reagent grade ZVI in the presence and absence of NO₃^-, a competing oxidized species that is often found in high concentrations in liquid nuclear waste. Perrhenate (ReO₄^-) was used as a non-radioactive chemical analogue for pertechnetate (TcO₄^-) under both oxic and anoxic test conditions. The PIC powder was the most effective at immobilizing Re(VII) under all batch test conditions. The presence of nitrate (NO₃^-) slowed the removal of ReO₄^- from solution, presumably through chemical reduction and precipitation. Even so, the PIC and ZVI were effective at removing both Re(VII) and NO₃^- completely from solution. Nitrate was reduced to NH₃ with very little nitrite (NO₂^-) buildup during equilibration. Significant Re immobilization was observed in the column tests containing PIC sorbent, even though inlet solutions were in equilibrium with O₂. The presence of NO₃^- hastened Re breakthrough, while NO₃^- reduction to NH₃ was observed. The results suggest that PIC and ZVI would be the most effective at the removal of TcO₄^- from contaminated groundwater sites.

Hydroxyl radical dominated degradation of aquatic sulfamethoxazole by Fe0/bisulfite/O2: Kinetics, mechanisms, and pathways

In this study, batch experiments were carried out to investigate the key factors on sulfamethoxazole (SMX) removal kinetics in a new AOPs based on the combination of zero valent iron (Fe⁰) and bisulfite (S(IV)). With the increase of Fe⁰ from 0.25 mM to 5 mM, the removal rate of SMX was linearly increased in the Fe⁰/S(IV)/O₂ system by accelerating the activation of S(IV) and Fe⁰ corrosion to accelerate. In the first 10 min of reaction, the increasing concentration of S(IV) inhibited SMX removal after since the high S(IV) concentration quenched reactive oxidative species (ROS). Then SMX removal rate was accelerated with the increase of S(IV) concentration after S(IV) were consumed up. The optimal ratio of S(IV) concentrations to Fe⁰ concentration for SMX removal in the Fe⁰/S(IV)/O₂ system was 1:1. With SMX concentrations increasing from 1 to 50 μM, SMX removal rate was inhibited for the limitation of ROS yields. Although the presence of SO₄^- and OH was confirmed by electron paramagnetic resonance (EPR) spectrum, OH was identified as the dominant ROS in the Fe⁰/S(IV)/O₂ system by chemical quenching experiments. Besides, strong inhibitive effects of 1,10-phenanthroline on SMX degradation kinetics by Fe⁰/S(IV)/O₂ proved that the generation of ROS was rely on the release of Fe(II) and Fe(III). The generation of SO₄^- was ascribed to the activation of S(IV) by Fe(II)/Fe(III) recycling and the activation of HSO₅^- by Fe(II). And OH was simultaneously transformed from SO₄^- and generated by Fe⁰/O₂. Density functional theory (DFT) calculation was conducted to reveal special reactive sites on SMX for radicals attacking and predicted intermediates. Finally, four possible SMX degradation pathways were accordingly proposed in the Fe⁰/S(IV)/O₂ system based on experimental methods and DFT calculation.

Influence of fulvic acid on the colloidal stability and reactivity of nanoscale zero-valent iron

This study investigated the effect of fulvic acid (FA) on the colloidal stability and reactivity of nano zero-valent iron (nZVI) at pH 5, 7 and 9. The sedimentation behavior of nZVI differed at different pH. A biphasic model was used to describe the two time-dependent settling processes (i.e., a rapid settling followed by a slower settling) and the settling rates were calculated. Generally, the settling of nZVI was more significant at the point of zero charge (pHpzc), which could be varied in the presence of FA due to the adsorption of FA on the nZVI surface. More FA was adsorbed on the nZVI surface at pH 5-7 than pH 9, resulting in the varying sedimentation behavior of nZVI via influencing the electrostatic repulsion among particles. Moreover, it was found that there was a tradeoff between the stabilization and the reactivity of nZVI as affected by the presence of FA. When FA concentration was at a low level, the adsorption of FA on the nZVI surface could enhance the particle stabilization, and thus facilitating the Cr(VI) reduction by providing more available surface sites. However, when the FA concentrations were too high to occupy the active surface sites of nZVI, the Cr(VI) reduction could be decreased even though the FA enhanced the dispersion of nZVI particles. At pH 9, the FA improved the Cr(VI) reduction by nZVI. Given the adsorption of FA on the nZVI surface was insignificant and its effect on the settling behavior of nZVI particles was minimal, it was proposed that the FA formed soluble complexes with the produced Fe(III)/Cr(III) ions, and thus reducing the degree of passivation on the nZVI surface and facilitating the Cr(VI) reduction.

Chromate removal by surface-modified nanoscale zero-valent iron: Effect of different surface coatings and water chemistry

This study investigated the correlation between the colloidal stability and reactivity of surface-modified nano zero-valent iron (SM-nZVI) as affected by the surface coating (i.e., polyacrylic acid [PAA] and starch) under various geochemical conditions. Generally, the colloidal stability of nZVI was enhanced with increasing loading of surface coating, while there is an optimum loading for the most efficient Cr(VI) removal by SM-nZVI. At lower loadings than the optimum loading, the surface coating could enhance the particle stabilization, facilitating the Cr(VI) reduction by providing more available surface sites. However, the over-loaded surface coating on the surface of nZVI particles decreased the Cr(VI) reduction due to the occupation of the reactive sites and the inhibition of the mass transfer of Cr(VI) ions from water to the particle surface by providing the electrostatic or steric repulsion. The effects of Ca(2+) ions or humic acid (HA) on the colloidal stability and reactivity of PAA-modified nZVI (P-nZVI) and starch-modified nZVI (S-nZVI) were examined. Differing stability behavior and reactivity were observed for different SM-nZVI. It was found that the presence of Ca(2+) or HA altered surface chemistry of SM-nZVI, the particle-particle interaction and the particle-contaminant interaction, and hence influencing the stability behavior and reactivity of the particles.

Role of dissolved oxygen in metal(loid) removal by zerovalent iron at different pH: its dependence on the removal mechanisms

The role of oxygen in metal(loid)s removal by zerovalent iron (ZVI) is strongly dependent on the removal mechanisms of metal(loid)s at different pH.

The limitations of applying zero-valent iron technology in contaminants sequestration and the corresponding countermeasures: the development in zero-valent iron technology in the last two decades (1994-2014)

Over the past 20 years, zero-valent iron (ZVI) has been extensively applied for the remediation/treatment of groundwater and wastewater contaminated with various organic and inorganic pollutants. Based on the intrinsic properties of ZVI and the reactions that occur in the process of contaminants sequestration by ZVI, this review summarizes the limitations of ZVI technology and the countermeasures developed in the past two decades (1994-2014). The major limitations of ZVI include low reactivity due to its intrinsic passive layer, narrow working pH, reactivity loss with time due to the precipitation of metal hydroxides and metal carbonates, low selectivity for the target contaminant especially under oxic conditions, limited efficacy for treatment of some refractory contaminants and passivity of ZVI arising from certain contaminants. The countermeasures can be divided into seven categories: pretreatment of pristine ZVI to remove passive layer, fabrication of nano-sized ZVI to increase the surface area, synthesis of ZVI-based bimetals taking advantage of the catalytic ability of the noble metal, employing physical methods to enhance the performance of ZVI, coupling ZVI with other adsorptive materials and chemically enhanced ZVI technology, as well as methods to recover the reactivity of aged ZVI. The key to improving the rate of contaminants removal by ZVI and broadening the applicable pH range is to enhance ZVI corrosion and to enhance the mass transfer of the reactants including oxygen and H(+) to the ZVI surface. The characteristics of the ideal technology are proposed and the future research needs for ZVI technology are suggested accordingly.

Enhancing the effect of bisulfite on sequestration of selenite by zerovalent iron

The enhancing effect of HSO₃⁻ on Se(iv) sequestration varied with the headspace volume, HSO₃⁻ concentration and initial pH, respectively.

Kinetics and mechanisms of pH-dependent selenite removal by zero valent iron

The kinetics of Se(IV) removal by zero valent iron (ZVI) open to the air as a function of pH and the involved mechanisms were investigated in this study. The specific rate constants of Se(IV) removal by ZVI decreased from 92.87 to 6.87 L h(-1) m(-2) as pH increased from 4.0 to 7.0. The positive correlation between the removal rate of Se(IV) and the generation rate of Fe(II) and the depression of Se(IV) removal in the presence of 1,10-phenanthroline indicated that both ZVI and adsorbed Fe(II) on ZVI surface contributed to the reductive removal of Se(IV). The soft X-ray STXM measurement confirmed the adsorption of Fe(II) on the surface of ZVI and freshly formed ferric (hydr)oxides. Se(IV) was removed by adsorption followed by reduction to Se(0) on ZVI surface at pH 4.0-7.0, as revealed by XANES spectra. A core-shell structure was observed when ZVI reacted with Se(IV)-containing solution for 3 h at pH 6.0. Se(IV) was reduced to Se(0) and co-precipitated with the freshly formed Fe(III), forming the shell surrounding the iron core. After reaction for 24 h, the generated Se(0) was surrounded by multiple layers of Fe(III) oxides/hydroxides. SEM images and XRD patterns revealed that the corrosion products of ZVI at pH 6.0 transformed from amorphous iron hydroxides to lepidocrocite (γ-FeOOH) as reaction proceeded. The final corrosion products of ZVI contained both lepidocrocite and goethite at pH 5.0 while they were X-ray amorphous at pH 4.0 and 7.0.

A novel application of H2O2-Fe(II) process for arsenate removal from synthetic acid mine drainage (AMD) water

Batch experiments were carried out to investigate the influences of H(2)O(2)/Fe(II) molar ratio, pH, sequence of pH adjustment, initial As(V) concentration, and interfering ions on As(V) removal in H(2)O(2)-Fe(II) process from synthetic acid mine drainage (AMD). The optimum H(2)O(2)/Fe(II) molar ratio was one for arsenate removal over the pH range of 4-7. Arsenate removal at pH 3 was poor even at high Fe(II) dosage due to the high solubility of Fe(III) formed in situ. With the increase of Fe(II) dosage, arsenate removal increased progressively before a plateau was reached at pH 5 as arsenate concentration varied from 0.05 to 2.0mgL(-1). However, arsenate removal was negligible at Fe/As molar ratio <3 and then experienced a striking increase before a plateau was reached at pH 7 and arsenate concentration ≥1.0mgL(-1). The co-occurring ions exerted no significant effect on arsenate removal at pH 5. The experimental results with synthetic AMD revealed that this method is highly selective for arsenate removal and the co-occurring ions either improved arsenate removal or slightly depressed arsenate removal at pH 5-7. The extended X-ray absorption fine structure (EXAFS) derived As-Fe length, 3.27-3.30Å, indicated that arsenate was removed by forming bidentate-binuclear complexes with FeO(OH) octahydra. The economic analysis revealed that the cost of the H(2)O(2)-Fe(II) process was only 17-32% of that of conventional Fe(III) coagulation process to achieve arsenate concentration below 10μgL(-1) in treated solution. The results suggested that the H(2)O(2)-Fe(II) process is an efficient, economical, selective and practical method for arsenate removal from AMD.