Iron monosulfide as a scavenger for dissolved hexavalent chromium and cadmium

Grinding siderite with ferric sulfate to generate an active ferrous source for Cr(VI) reduction

Activated siderite, endowed with excellent properties, was simply prepared by co-grinding with Fe sulfate to enhance its high reducing ability for Cr(VI). Batch experiments were conducted to investigate the main affecting parameters, such as material ratio, pH, temperature, etc. The removal of Cr(VI) by activated siderite was completed within 4 h of the reaction. The activated siderite maintained a high removal effect of Cr(VI) within a wide pH range (3-9). Various analytical methods, including XRD, SEM/EDS, XPS, etc., were employed to characterize the samples and discover variations before and after the reaction. The Fe (Ⅱ) in activated siderite becomes highly active, and it can even be released from the solid phase in the mildly acidic liquid phase to efficiently reduce Cr(VI) and mitigate its toxicity. These findings introduce an innovative approach for activating various minerals widely distributed in nature to promote the recovery of the ecological system.

Chromium speciation and mobility in contaminated coastal urban soils affected by water salinity and redox conditions

Chromium (Cr) is a redox-sensitive element in contaminated coastal urban soils. Sea level rise (SLR) with subsequent soil inundation may facilitate Cr transformation and mobilization through alterations in local redox conditions and porewater ion composition. We investigated the impact of water salinity and redox conditions on Cr chemistry in these environments. Synchrotron-based X-ray spectroscopy and wet chemical analyses revealed that the soils contained very high levels of Cr (up to 4320 mg kg-1) and that chromite (∼52%) and Fe-Cr hydroxide coprecipitates (∼44%) were the predominant Cr species. The abundance of these two components resulted in low Cr mobility under non-flooded conditions. Chromium(II) was identified in the soils, potentially derived from the waste parent material. Seawater and anoxic conditions resulted in lower Cr release compared to freshwater and aerobic conditions. Up to three to eight times more Cr was released under aerobic conditions versus anaerobic conditions in the freshwater versus saltwater, respectively, with total dissolved Cr values remaining below 0.02 mg L-1. The decrease in Cr release was likely due to Cr reduction by Fe(II) and sulfide. This work provides important information on how salinity and redox fluctuations impact Cr cycling which is likely to occur during SLR.

Physicochemical Effects of Sulfur Precursors on Sulfidated Amorphous Zero-Valent Iron and Its Enhanced Mechanisms for Cr(VI) Removal

Amorphous zerovalent iron (AZVI) has gained considerable attention due to its remarkable reactivity, but there is limited research on sulfidated amorphous zerovalent iron (SAZVI) and the influence of different sulfur precursors on its reactivity remains unclear. In this study, SAZVI materials with an amorphous structure were synthesized using various sulfur precursors, resulting in significantly increased specific surface area and hydrophobicity compared to AZVI. The Cr(VI) removal efficiency of SAZVI-Na₂S, which exhibited the most negative free corrosion potential (-0.82 V) and strongest electron transfer ability, was up to 8.5 times higher than that of AZVI. Correlation analysis revealed that the water contact angle (r = 0.87), free corrosion potential (r = -0.92), and surface Fe(II) proportion (r = 0.98) of the SAZVI samples played crucial roles in Cr(VI) removal. Furthermore, the enhanced elimination ability of SAZVI-Na₂S was analyzed, primarily attributed to the adsorption of Cr(VI) by the FeS_x shell, followed by the rapid release of internal electrons to reduce Cr(VI) to Cr(III). This process ultimately led to the precipitation of FeCr₂O₄ and Cr₂S₃ on the surface of SAZVI-Na₂S, resulting in their removal from the water. This study provides insights into the influence of sulfur precursors on the reactivity of SAZVI and offers a new strategy for designing highly active AZVI for efficient Cr(VI) removal.

Enhanced Cr (VI) removal with Pb (II) presence by Fe2+-activated persulfate and zero-valent iron system

Combined heavy metals such as chromium (Cr (VI)) and lead (Pb (II)) in natural water have globally posed severe environmental and public health risk. Here the removal of Cr (VI) and Pb (II) mixed pollutants using Fe²⁺-activated persulfate (PS) with extra zero-valent iron (ZVI), which was not only a supplementary Fe²⁺ source, but also a high-efficiency absorbent, was investigated. During removal, pivotal factors of initial pollutant concentration, dosages of ZVI and PS, initial pH and temperatures were examined. Interestingly, generating a lot of H⁺ in the process of Fe (II) activating persulfate were helpful to the corrosion of ZVI over a large range of pH (1-9). Under the optimum condition, removal efficiency of Pb (II) and Cr (VI) have reached 100% and 94.26% respectively. The removal mechanism was suggested as a three-step reaction that the Pb (II) boosted the removal of Cr (VI) by co-precipitated in wastewater, and the Pb (II) and Cr (VI) were adsorbed and subsequently reduced to Pb⁰ and Cr³⁺ as Cr(OH)₃ or Cr³⁺-Fe³⁺ hydroxides on ZVI surface. Cr (VI) and Pb (II) adsorption kinetics agreed with the pseudo-second-order reaction rate expression. In addition, we were surprised to found that the contribution effect of chromium and lead co-precipitation for their removal by Fe (II) - PS-ZVI has strong dependence on initial pH and concentration ratio of Cr (VI) and Pb (II). The result indicated that Fe (II)-PS-ZVI system should be a favourable removal technology for Cr (VI) and Pb (II).

The pH-sensitve oxygenation of FeS: Mineral transformation and immobilization of Cr(VI)

Iron sulfide (FeS) has been widely used to reduce toxic Cr(VI) into Cr(III) in anoxic aquatic environments, where pH could strongly influence Cr(VI) removal. However, it remains unclear how pH regulates the fate and transformation of FeS under oxic conditions and the immobilization of Cr(VI). The results of this study showed that typical pH conditions of natural aquatic environment significantly affected the mineral transformation of FeS. Under acidic conditions, FeS was principally transformed to goethite, amarantite, and elemental sulfur with minor lepidocrocite through proton-promoted dissolution and oxidation. Instead, under basic conditions, the main products were lepidocrocite and elemental sulfur via surface-mediated oxidation. In typical acidic or basic aquatic environment, the pronounced pathway for the oxygenation of FeS solids may alter their ability to remove Cr(VI). Longer oxygenation impeded Cr(VI) removal at acidic pH, and a decreasing ability to reduce Cr(VI) caused a drop in Cr(VI) removal performance. Cr(VI) removal decreased from 733.16 to 36.82 mg g^-1 with the duration of FeS oxygenation increasing to 5760 min at pH 5.0. In contrast, newly generated pyrite from brief oxygenation of FeS improved Cr(VI) reduction at basic pH, followed by a drop in Cr(VI) removal performance due to the impaired reduction capacity with increasing to the complete oxygenation. Cr(VI) removal increased from 669.58 to 804.83 mg g^-1 with increasing oxygenation time to 5 min and then decreased to 26.27 mg g^-1 after the full oxygenation for 5760 min at pH 9.0. These findings provide insight into the dynamic transformation of FeS in oxic aquatic environments with various pHs and the impact on Cr(VI) immobilization.

Efficient removal of aqueous Cr(VI) with ferrous sulfide/N-doped biochar composites: Facile, in-situ preparation and Cr(VI) uptake performance and mechanism

FeS nanoparticles loaded on nitrogen-doped biochar (FeS/BNC) were fabricated by pyrolyzing coffee husks pretreated with Mohr's salt. The nitrogen doping and FeS loading of biochar are simultaneously achieved in one-pot pyrolysis. The elemental analysis, SEM, TEM, XRD, XPS, Raman, FTIR and N₂ adsorption-desorption technologies were used to characterize the composition and structure of FeS/NBC. The appraisement for removing aqueous Cr(VI) testified that FeS/NBC offered a synergistic scavenging effect of Cr(VI) by FeS and NBC. The effect of crucial experimental conditions (FeS/NBC dosage, foreign ions, initial pH and concentration of Cr(VI) solution) were investigated. The Cr(VI) removal capacity was as high as 211.3 ± 26 mg g^-1 under the optimized condition. The practicability of FeS/NBC was examined by using simulated actual samples from tap water and lake water. The mechanism examination showed that surface adsorption/reduction and solution reduction were implicated in the removal of Cr(VI). The current work introduces a novel FeS/NBC composite prepared by an in situ pyrolysis method with excellent potential for chromium pollution remediation.

Effect of FeS on mercury behavior in mercury-contaminated stream sediment: A case study of Pohang Gumu Creek in South Korea

This study investigated mercury contamination with respect to the sediment characteristics in Gumu Creek near the Pohang Industrial Complex, South Korea. The contaminated sediment had high levels of Hg, exceeding 250 mg Hg/kg sediment at the sampling position, and high concentrations of iron, sulfur, and organic carbon under extreme anaerobic conditions. The anoxic condition of the sediment produced large amounts of FeS. Hg L₃-edge EXAFS analysis revealed that FeS controlled the Hg species in the sediment mainly as β-HgS like precipitation or Hg-S complexation. We also speculated that the presence of FeS induced the abiotic reduction of Hg(II) to Hg(0) and consequently suppressed the formation of highly toxic methylated mercury species. The results obtained in this study are mostly consistent with those reported in previous studies of geochemical reactions of FeS in controlling Hg(II) under pure FeS mineral systems under laboratory scenarios. This study demonstrates that the laboratory controlled reaction scenarios can explain the field behavior of Hg in the contaminated anoxic sediment of the Gumu Creek site.

In-situ remediation of hexavalent chromium contaminated groundwater and saturated soil using stabilized iron sulfide nanoparticles

Hexavalent chromium (Cr(VI)) is one of prevalent toxic and mobile heavy metal contaminants in the environment. In this study, synthetic iron sulfide nanoparticles (FeS NPs) stabilized with carboxymethyl cellulose (CMC) were applied to remediate Cr(VI) contaminated groundwater and saturated soil. The batch test results showed that aqueous Cr(VI) was removed with a capacity as high as 1046.1 mg Cr(VI) per gram of FeS NPs. The removal of aqueous Cr(VI) mainly involves adsorption, reduction and co-precipitation. Aqueous Cr(VI) removal by using FeS NPs was a strong pH-dependent process. Dissolved oxygen (DO) would compete with Cr(VI) for Fe(II) and S(-II) and inhibit the process of Cr(VI) reduction at pH 5.6. For ionic strength and natural organic matter (NOM), there were no significant influences on the aqueous Cr(VI) removal. Column tests demonstrated that the concentrations of Cr(VI) in the effluent were lower than 0.005 mg L^-1 after an elution of 45 pore volumes (PVs) of stabilized FeS NPs suspension. The leached Cr(VI) decreased from 4.58 mg L^-1 of raw Cr(VI)-contaminated soil to 46.8-80.7 μg L^-1 from the surface to bottom treated soil in column through Toxicity Characteristic Leaching Procedure (TCLP). Therefore, the synthesized FeS NPs hold high potential for the in-situ remediation of Cr(VI)-contaminated groundwater and saturated soil.

Modulation of hexavalent chromium toxicity on Οriganum vulgare in an acidic soil amended with peat, lime, and zeolite

Dynamics of chromate (Cr(VI)) in contaminated soils may be modulated by decreasing its phytoavailability via the addition of organic matter-rich amendments, which might accelerate Cr(VI) reduction to inert chromite (Cr(III)) or high-cation exchange capacity amendments. We studied Cr(VI) phytoavailability of oregano in a Cr(VI)-spiked acidic soil non-treated (S) and treated with peat (SP), lime (SL), and zeolite (SZ). The addition of Cr(VI) increased the concentrations of Cr(VI) and Cr(III) in soils and plants, especially in the lime-amended soil. The plant biomass decreased in the lime-amended soil compared to the un-spiked soil (control) due to decreased plant phosphorus concentrations and high Cr(VI) concentrations in root at that treatment. Oregano in the peat-amended soil exhibited significantly less toxic effects, due to the role of organic matter in reducing toxic Cr(VI) to Cr(III) and boosted plant vigour in this treatment. In the lime-amended soil, the parameters of soil Cr(VI), soil Cr(III), and root Cr(III) increased significantly compared to the non-amended soil, indicating that Cr(VI) reduction to Cr(III) was accelerated at high pH. Added zeolite failed to decreased Cr(VI) level to soil and plant. Oregano achieved a total uptake of Cr(III) and Cr(VI) of 0.275 mg in plant kg^-1 soil in a pot in the non-amended soil. We conclude that peat as soil amendment might be considered as a suitable option for decreasing Cr(VI) toxicity in soil and plant, and that oregano as tolerant plant species has a certain potential to be used as a Cr accumulator.

Application of iron sulfide particles for groundwater and soil remediation: A review

Rapid industrialization and urbanization have resulted in elevated concentrations of hazardous inorganic and organic contaminants in groundwater and soil, which has become a paramount concern to the environment and the public health. In recent years, iron sulfide (FeS), a major constituent of acid-volatile sulfides, has elicited extensive interests in environmental remediation due to its ubiquitous presence and high treatment efficiency in anoxic environment. This paper provides a comprehensive review on recent advances in: (1) synthesis of FeS particles (including nanoscale FeS); and (2) reactivity of FeS towards a variety of common environmental contaminants in groundwater and soil over extended periods of time, namely, heavy metals (Hg(II), Cu(II), Pb(II), and Cr(VI)), oxyanions (arsenite, arsenate, selenite, and selenate), radionuclides (e.g., uranium (U) and neptunium (Np)), chlorinated organic compounds (e.g., trichloroethane, trichloroethylene, and p-chloroaniline), nitroaromatic compounds, and polychlorinated biphenyls. Different physiochemical and biological methods for preparing FeS with desired particle size, structure, and surface properties are discussed. Reaction principles and removal effectiveness/constraints are discussed in details. Special attention is placed to the application of nanoscale FeS particles because of their unique properties, such as small particle size, large specific surface area, high surface reactivity, and soil deliverability in the subsurface. Moreover, current knowledge gaps and further research needs are identified.