Three-dimensional transfer of Cr(VI) co-precipitated with ferrihydrite containing silicate and its redistribution and retention during aging

Degradation of enoxacin with different dissociated species during the transformation of ferrihydrite-antibiotic coprecipitates

Ferrihydrite acts as a natural reservoir for nutrient elements, organic matter, and coexisting pollutants through adsorption and coprecipitation. However, the degradation of emerging fluoroquinolone antibiotics during the transformation of ferrihydrite coprecipitates, especially those with various dissociated species, remains insufficiently explored. In this study, Enoxacin (ENO), employed as a model antibiotic, was introduced to prepare ferrihydrite-ENO coprecipitates. The influence of coprecipitated ENO on the transformation of the ferrihydrite-ENO coprecipitate was investigated across different pH conditions. The results revealed that ferrihydrite-ENO coprecipitates thermodynamically transformed into more stable goethite and/or hematite under all pH conditions. In neutral and alkaline conditions, ENO promoted the transformation of coprecipitates into goethite while hindering hematite formation. Conversely, under acidic conditions, ENO directly obstructed the transformation of coprecipitates into hematite. Different dissociated species of ENO displayed distinct degradation pathways. The cationic form of ENO exhibited a greater tendency for hydroxylation and defluorination, while the zwitterion form leaned toward piperazine ring oxidation, with limited preference for quinolone ring oxidation. The anionic form of ENO exhibited the fastest degradation rate. It is essential to emphasize that the toxicity of the degradation products was intricately connected to the specific reaction sites and the functional groups they acquired post-oxidation. These findings offer fresh insights into the role of antibiotics in coprecipitation, the transformation of ferrihydrite coprecipitates, and the fate of coexisting antibiotics.

Interaction of polystyrene nanoplastics with impurity-bearing ferrihydrite and implication on complex particle sedimentation

Nanoplastics (NPs) usually coexist with impurity-bearing ferrihydrite (ImFh), and their interaction is related to their environmental fate. In this study, the aggregation between ImFh (impurities: Al, Mn and Si) and polystyrene nanoplastics (PSNPs), as well as the sedimentation of ImFh-PSNP complex particles in the aqueous phase were investigated systematically with particle concentrations of 100 mg/L ImFh and 10 mg/L PSNPs. Our results revealed that the PSNP suspension was dispersive and stable under various pH values and low ion strength. After coexisting with ImFh, PSNPs aggregated with the positively charged ImFh to form ImFh-PSNP complex particles, which destroyed the stability of PSNPs. The increase in pH and Na+ concentration could inhibit their aggregation, but high Na+ concentration (>20 mM) caused the homoaggregation of PSNPs. The aggregation capacity of PSNPs with ImFh was in the order of Al-bearing Fh > Fh > Mn-bearing Fh > Si-bearing Fh. Zeta potential and Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations indicated that Al-bearing Fh showed higher positive potential than pure Fh, which caused stronger electrostatic interactions with PSNPs. However, Mn and Si in ImFh decreased the positive potential and inhibited the electrostatic interaction with PSNPs, and the effect of Si was greater than that of Mn. The aggregation between ImFh and PSNPs inhibited the sedimentation of their complex particles, and the higher aggregation capacity appeared to have a greater inhibition degree. Due to the "electrostatic patches" effect of PSNPs, the energy barrier of the ImFh-PSNPs particles was higher than that of the ImFh particles. Our findings clarified the influence of impurities on the interaction between ImFh and PSNPs and provided insight regarding their fate in the environment.

Permeable reactive composite approaching cathode enhanced Cr removal in soil using the byproduct of electrokinetic technology: emphasized energy utilization efficiency

Cost-effective techniques with significant removal rates and low energy consumption are urgently required for in-situ Cr-contaminated soil remediation to reduce potential environmental toxicity to the ecosystem and human bodies. Electrokinetic technology is a valuable and promising soil remediation technology; however, the acidic and alkaline fronts evolution induced by the electrokinetic byproducts (H+, OH-) has significant hindering characteristics for ion removal. To effectively utilize the byproducts for enhancing Cr elimination, this paper proposed the permeable reactive composite approaching cathode with rhamnolipid-modified biochar as reactive material. Power utilization efficiency (η) was presented to comprehensively evaluate the target species elimination effect, considering removal rate and energy consumption. Results suggested that biosurfactant rhamnolipid stimulated Cr removal in acid and base fronts. Acid front induced rhamnolipid protonation reducing anolyte Cr(VI) to Cr(III), and base front induced rhamnolipid deprotonation complexing with Cr(III) and expediting Cr(VI) dissolution by electrostatic repulsion. Permeable reactive composite approaching cathode induced the maximum removal rate of Cr(VI) and Cr(III) in each section by impelling the alkaline front. Approaching cathode caused increased resistance and energy consumption in the near-anode regions, ultimately decreasing energy utilization efficiency. Optimized moving frequency and applied potential magnitude could adjust power consumption distribution in a single soil layer to obtain better electrokinetic removal performance of contaminates. This work provided essential scientific and practical importance for in-situ electrokinetic remediation of Cr(VI) and Cr(III), considering elimination efficiency and energy consumption in the future.

One-pot synthesis of siliceous ferrihydrite - coated halloysite nanorods in alkaline medium: Structure, properties and cadmium adsorption performance

The application of amorphous ferrihydrite (Fh) for Cd(II) removal is restricted by its unstable and easily transformable nature. Although doping with silicates stabilized ferrihydrite, its product siliceous ferrihydrite (SiFh) again suffered from the disadvantage of spontaneous agglomeration. Herein, ferrihydrite was hybridized with halloysite nanotubes (HNTs) to prepare a novel siliceous ferrihydrite - coated halloysite nanorods (SiFh@HNTs) in alkaline medium, to break through the current barriers. The characterization results showed that SiFh@HNTs could simultaneously overcome the defects of easy phase transformation of ferrihydrite and easy aggregation of SiFh nanoparticles (NPs). Meanwhile, the optimal SiFh@HNT₄₀ with halloysite content of 40 % formed a well-developed mesoporous structure and exhibited the desired surface properties: a high specific surface area of 303.4 m²/g, an isoelectric point as low as pH_iep = 4.5, and rich functional Fe - OH groups. The formation mechanism of such excellent sturcture-properties of SiFh@HNT₄₀ were mainly attributed to two factors: the generation of smaller (∼5 nm) SiFh NPs induced by the integration of halloysite-derived SiO₄^4- into ferrihydrite, and the dispersion of SiFh NPs on clay nanotubes. Furthermore, the adsorption capacity of SiFh@HNT₄₀ for Cd(II) was up to 137.8 mg/g at 30 °C and pH 6, which was much higher than that of aggregated ferrihydrite (11.2 mg/g), halloysite (18.8 mg/g) and goethite (49.4 mg/g). The adsorption thermodynamics study revealed the adsorption of Cd(II) on SiFh@HNT₄₀ was clearly chemisorption with a (ΔH_ads)_q of 43.3 kJ/mol. Characterization results of XPS and FTIR confirmed that the rich Fe - OH groups on SiFh@HNT₄₀ was the main adsorption sites, and Cd(II) was specifically adsorbed by inner-sphere surface complexation. In addition, SiFh@HNT₄₀ had application potential in the mixed-metal wastewaters treatment. Cyclic regeneration experiments showed that SiFh@HNT₄₀ had good regeneration performance and could be reused many times.

Improvement of the Stability of IO3--, SeO32--, and SeO42--Coprecipitated Barite after Treatment with Phosphate

Coprecipitation of radionuclides with barite has been studied to remove radionuclides from radioactive liquid waste because of its excellent removal efficiency; however, little information exists concerning the stability of the ions coprecipitated with barite. This study systematically investigated the stability of iodate, selenite, and selenate coprecipitated with barite via leaching tests. These oxyanions were gradually leached from the oxyanion-bearing barite into ultrapure water over time. Leaching of the oxyanions significantly increased in leaching solutions containing NaCl (pH 5.3), NaNO₃ (pH 5.9), and Na₂SO₄ (pH 5.7). Conversely, leaching of the oxyanions was suppressed in KH₂PO₄ solution (pH 8.5), indicating that phosphate stabilized the oxyanion-bearing barite. The effect of phosphate treatment on oxyanion-bearing barite was further investigated. The results showed that the barite surface was modified with phosphate, and a thin surface layer of a barium phosphate-like structure was formed. The amount of oxyanions leached from the phosphate-treated samples into leaching solutions containing NaCl or NaNO₃ was much lower than the amounts leached from the untreated barite samples into ultrapure water. The barite coprecipitation combined with subsequent phosphate treatment may be a promising method to efficiently remove iodate, selenite, and selenate from wastewater and stabilize them as barite coprecipitates.

Aging factor and its prediction models of chromium ecotoxicity in soils with various properties

Aging of pollutants determines bioavailability and toxicity thresholds of environmental pollutants in soil. However, the ecotoxicity of chromium (Cr) rarely considers the effect of aging as well as soil properties. In order to explore the aging characteristics and establish their quantitative relationship with different soil properties, this study selected 7 soils with different properties through exogenous addition of Cr and determined its toxicity on barley root elongation. From 14d to 540d, EC₁₀ and EC₅₀ of barley root elongation ranged from 21.40 to 312.52 (mg·kg^-1) and 50.15 to 883.88 (mg·kg^-1) respectively. The hormesis appeared in the dose-response curve of acid soil as relative barley root elongation reached >110 % compared with the control. Extended aging time of Cr from 14d to 540d was associated with the attenuation of the toxicity of Cr, as the aging factor increased from 1.26 to 6.09 for EC₅₀, from 0.88 to 4.98 for EC₁₀. The prediction model of AF_EC50 and soil properties is lg (AF_360d) = 0.306lg Clay+0.026lg CEC + 0.240 (R² = 0.872, P < 0.01). The results demonstrated that with the extension of aging time, the toxicity of Cr decreased at 360d and reached a slow reaction stage, after that soil OC, Clay and CEC could well explain the aging procedure of Cr (VI). These results are beneficial for risk assessment of Cr contaminated soils and establishment of a soil environmental quality criteria for Cr.

Enhanced remediation of surface-bound hexavalent chromium in soils using the acidic and alkaline fronts of electrokinetic technology

In the subsurface environment, highly toxic hexavalent chromium (Cr(VI)) control and remediation are essential to avoid further ecological impacts and reduce environmental risks. This paper investigated the enhanced Cr(VI) electrokinetic removal in the soil through the approaching cathode method. Besides, a novel four-step sequential fractionation method was used to reflect the strength of Cr(VI) binding to the soil. The approaching cathode enhanced the electrokinetic delivery of surface-bound Cr(VI) by advancing the alkaline front for Cr(VI) desorption and improving the electric potential flattening of the soil layers. Desorption of Cr(VI) by the alkaline front involved converting the inner-sphere complexes form of Cr(VI) to a weakly adsorbed form susceptible to ionic strength. In addition, the acidic front provided a favorable environment for the photochemical reduction of Cr(VI) by soil species or the added citrate as the electron donors. Improving the potential distribution could regulate the energy consumption of individual soil layers and efficiently operate the electrokinetic transfer of pollutants. The work results have significant scientific and practical significance for applying the in-situ electrokinetic technique in subsurface pollution control.

Enhanced immobilization of phosphate by ferrihydrite during the photoreductive dissolution process

The surface interactions of iron (hydr)oxides with various environmental chemicals play a vital role in controlling their environmental transport and fate. As a bioessential element, phosphorus and its biogeochemical cycling are usually controlled by its adsorption on iron (hydr)oxides. Photoreductive dissolution of iron (hydr)oxides can change their surface structure and properties, but its influence on the adsorption of phosphate remains unknown. Here, an enhanced removal of phosphate during the photoreductive dissolution of ferrihydrite (Fh) was investigated. The K_d value of phosphate adsorption on Fh under light irradiation is evidently larger than that in the dark (21 vs 13 L/g). Zeta potential determination in combination with X-ray photoelectron spectroscopy analysis suggested that the released Fe²⁺ from Fh surface during photoreductive dissolution can be oxidized to Fe³⁺, which then co-adsorb with phosphate back to Fh surface, enhancing the immobilization of phosphate on Fh. In situ ATR-FTIR results disclosed that light irradiation could further facilitate the formation of ternary complexes and surface precipitation on Fh, even after the increment of phosphate adsorption becoming negligible in the dark, and the relative content of surface precipitation increased evidently. The desorption ratio of phosphate from the irradiated Fh sample was reduced, which should be attributed to a high content of surface precipitation that can tightly bind phosphate on Fh. The findings of this study highlight an important yet previously unappreciated pathway that light irradiation can enhance the immobilization of phosphate on iron (hydr)oxides.

Rapid Cr(VI) reduction structure in chromium contaminated soil: The UV-assisted electrokinetic circulation of background iron

Substantially decreasing the severe hazards connected with the toxic Cr(VI), developing effective reduction remediation strategies may be crucial under favorable economic conditions for the contaminated soil containing Cr(VI) to protect human health. Several typical enhancers (phosphate, fulvic acid, citric acid) were used to test electrokinetic remediation (EKR) coupled with UV radiation-induced photochemical reduction for contaminated soil containing Cr(VI). The added citrate, while improving the Cr(VI) electromigration, worked as the ultimate sacrificial electron donors, with the dissolved soil background Fe(III) as electron shuttle, to Cr(VI) rapid reduction. The dissolved soil background Fe(III) convert into Fe(II) ions through the UV radiation-induced ligand-metal charge transfers reaction, which constituted a novel electrokinetic circulation reduction pathway for the elimination of surface-bound/dissolved Cr(VI) (difficult to electromigration) in the near-anodic soil layers. More than 80% dissolved and surface-bound Cr(VI) was eliminated from the soil. In particular, the dissolved and surface-bound Cr(VI) was enhanced by more than 62.37% removal in near-anodic soil layers compared to conventional citric acid-enhanced EKR and provided no extra cost other than UV radiation. This configuration may be a cost-effective and feasible remediation design in the future for the in-situ Cr(VI) reduction of contaminated sites.

Assessing environmental fate of hexavalent chromium as influenced by fractionation of ferrihydrite with dissolved organic matter

The dynamic interactions among iron (Fe) oxides, dissolved organic matter (DOM) and toxic trace metals play crucial roles in risk assessment and environmental remediation. Although the inhibitory effects of DOM on the iron oxides transformation process have been studied previously, there is still a lack of mechanistic and quantitative understanding on the kinetics of Cr(VI) and ferrihydrite transformation in the present of DOM. In this study, we investigated the fractionation process of DOM on ferrihydrite and its influence on the fate of Cr(VI) and transformation of ferrihydrite. The result of three-dimension excitation emission matrix (3D-EEM), Q-Exactive LC-MS/MS, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) indicated that fulvic acid-like compounds of DOM were the mainly fractionated compounds on the surface of ferrihydrite, which further inhibited the transformation of ferrihydrite. Besides, bracewellite (CrO(OH)) generated as an accompanied mineral during the transformation of ferrihydrite in the present of Cr(VI). Based on the DFT theoretical calculation, we concluded that Cr(VI) mainly in the form of HCr O₄^- was more inclined to be adsorbed on iron-oxide tetrahedron by inner-sphere monodentate mononuclear configurations. The findings on the dynamic coupling among Fe oxide transformation and Cr(VI) sequestration under the effect of DOM provided the basis for accurately predicting the fate of trace elements and iron mineral.

Direct and Indirect Electron Transfer Routes of Chromium(VI) Reduction with Different Crystalline Ferric Oxyhydroxides in the Presence of Pyrogenic Carbon

Electron transfer mediated by iron minerals is considered as a critical redox step for the dynamics of pollutants in soil. Herein, we explored the reduction process of Cr(VI) with different crystalline ferric oxyhydroxides in the presence of pyrogenic carbon (biochar). Both low- and high-crystallinity ferric oxyhydroxides induced Cr(VI) immobilization mainly via the sorption process, with a limited reduction process. However, the Cr(VI) reduction immobilization was inspired by the copresence of biochar. Low-crystallinity ferric oxyhydroxide had an intense chemical combination with biochar and strong sorption for Cr(VI) via inner-sphere complexation, leading to the indirect electron transfer route for Cr(VI) reduction, that is, the electron first transferred from biochar to iron mineral through C-O-Fe binding and then to Cr(VI) with Fe(III)/Fe(II) transformation on ferric oxyhydroxides. With increasing crystallinity of ferric oxyhydroxides, the direct electron transfer between biochar and Cr(VI) became the main electron transfer avenue for Cr(VI) reduction. The indirect electron transfer was suppressed in the high-crystallinity ferric oxyhydroxides due to less sorption of Cr(VI), limited combination with biochar, and higher iron stability. This study demonstrates that electron transfer mechanisms involving iron minerals change with the mineral crystallization process, which would affect the geochemical process of contaminants with pyrogenic carbon.

Aging shapes Cr(VI) speciation in five different soils

To make sound decisions regarding management of heavy metal contamination in soils, it is necessary to understand contaminant transformations over extended periods. In this study, sequential extraction methods were applied to quantify the changes of Cr fractions [available Cr(VI), immobile Cr(VI) and immobile Cr(III)] in five contrasting soils spiked with Cr(VI) over a 240-day incubation. Results showed that available Cr(VI) in soils continually decreased during aging, with a sharp decline occurring in the first 30 days. The best fit of available Cr(VI) data was obtained using an Elovich model for Brunisol and Anthrosol-1, a fractional power model for Anthrosol-2, and a pseudo first-order kinetic model for Luvisol-1 and Luvisol-2. After aging for 240 days, immobile Cr(VI) increased by 4.5-31% and immobile Cr(III) increased by 68-95% of total spiked Cr(VI) in Brunisol, Anthrosol-1 and Anthrosol-2. The two Luvisol soils had relatively high reduction rates with no Cr(VI) immobilized. A multireaction model was developed in MATLAB Simulink toolbox to describe transformation flow rates among soluble Cr(VI), adsorbed Cr(VI), immobilized Cr(VI) and immobilized Cr(III) in soils with aging. We conclude that (i) Cr(VI) reduction and immobilization were occurring concurrently in soils and competing for available Cr(VI) species; (ii) Cr(VI) reduction is favored by low soil pH and high organic carbon, while Cr(VI) immobilization occurs with cations (such as Ca²⁺) and Fe oxides.

Interaction between Se(IV) and fulvic acid and its impact on Se(IV) immobility in ferrihydrite-Se(IV) coprecipitates during aging

Selenium (Se) is regarded as a trace element for humans, but it is toxic in excess. In natural environments, the mobility of Se is dominantly controlled by the Se oxyanions with high solubility such as selenite (Se(IV)). Se(IV) is often associated with the omnipresent ferrihydrite and coexisting organic matter. However, there is little information on the dynamic interactions among Se(IV), fulvic acid, and ferrihydrite. This study investigated the influence of fulvic acid on ferrihydrite-Se(IV) coprecipitates (Fh-Se) transformation for 8 days and the subsequent behavior of Se(IV) at varied pH (5.0, 7.5, and 10.0). Results showed that fulvic acid had different effects on Fh-Se transformation at varied pH values. Fh-Se transformation was promoted by fulvic acid at initial pH 5.0 whereas it was inhibited at initial pH 10.0. Interestingly, at initial pH 7.5, Fh-Se transformation was promoted at a low C/Fe ratio while it was suppressed at a high C/Fe ratio. Besides, fulvic acid induced the generation of more extractable Se(IV) at initial pH 5.0 and more coprecipitated Se(IV) at initial pH 7.5 and blocked the release of Se(IV) at initial pH 10.0. Fulvic acid possibly interacted with Se(IV) via carboxyl complexation and weakened the inhibition of Se(IV) on Fh-Se transformation. Thus, fulvic acid increased the transformation rate of Fh-Se. These findings help to uncover the environmental behavior of Se(IV) and organic matter during ferrihydrite transformation.

Application of UV radiation for in-situ Cr(VI) reduction from contaminated soil with electrokinetic remediation

Restoring hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)) from contaminated soil is a cost-effective alternative for attenuating Cr(VI) toxicity to the ecosystem. A new electrokinetic remediation (EKR) system with UV light was explored to overcome an energy barrier to catalyze Cr(VI) reduction from the surface soil near the anodic reservoir. Natural organic matters and minerals from the contaminated soil acted as electron donors and catalysts for Cr(VI) photo-reduction and no additional chemical reagent. There was almost no residual Cr(VI) in anolyte after UV/EKR compared with the conventional EKR. The reduction improved the efficiency of EKR in the soil near the anodic reservoir by dropped the Cr(VI) negative mass flux caused by electroosmosis advection and concentration diffusion. The pathways of Cr(VI) photo-reduction are possibly dominated by ligand-to-metal charge transfer, i.e., photocatalytic cyclic reduction by Fe(III)/Fe(II) complexes on the surface of the minerals and in soil pore fluid and the photo-induced decomposition of chromate ester. It is concluded that UV/EKR is a clean, efficient, and low-cost method for remediation of Cr(VI)-contaminated soil.

Migration behavior of Cr(VI) during the transformation of ferrihydrite-Cr(VI) co-precipitates: The interaction between surfactants and co-precipitates

Redistribution of Cr(VI) in ferrihydrite-Cr(VI) co-precipitates (Fh-Cr) was affected by co-precipitates transformation and coexisting substances. These effects were crucial for predicting the migration path of Cr(VI) in ferrihydrite-Cr(VI) co-precipitates. This work investigated the effects of the extensively used surfactants of anionic surfactant sodium dodecylbenzene sulfonate (SDBS) and cationic surfactant cetyltrimethylammonium bromide (CTAB) on the Fh-Cr transformation and redistribution of Cr(VI) for 10 days at different pH values (5.0, 7.5 and 9.0) and concentration of surfactants (0.5, 2.0 and 5.0 mM). The results showed that SDBS hindered the transformation of Fh-Cr to hematite and tended to transform into goethite. SDBS inhibited hematite formation by inhibiting the aggregation of Fh-Cr particles, and it enhanced the dissolution of Fh-Cr to facilitate the formation of goethite. Affected by the inhibition of Fh-Cr transformation, the process of Cr(VI) redistribution was delayed. CTAB did not affect the transformation of Fh-Cr, but allowed more Cr(VI) to enter the interior of iron minerals. When the surfactants were adsorbed on the Fh-Cr, SDBS decreased the adsorption of Cr(VI) by Fh-Cr, while CTAB increased the Cr(VI) adsorption. The findings of this study contribute to understand the effects of surfactants on the transformation of Fh-Cr and the behaviors of Cr(VI) during this process.

Influence of Al(III) and Sb(V) on the transformation of ferrihydrite nanoparticles: Interaction among ferrihydrite, coprecipitated Al(III) and Sb(V)

Ferrihydrite is ubiquitous in natural environments and is usually co-precipitated with impure ions and toxic contaminants like Al(III) and Sb(V) during the neutralization process of acid mine drainage. However, little is known about the dynamic interactions among ferrihydrite, Al(III) and Sb(V). In this study, the influence of coprecipitated Al(III) and Sb(V) on the transformation of ferrihydrite was investigated. The samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy before and after aging for 10 days at 70 °C. Results indicated that the Al(III) enhanced the immobilization of Sb(V) under neutral and alkaline conditions, and the presence of Sb(V) induced more production of extractable Al(III). XRD patterns revealed that the transformation rate of coprecipitated Al(III) and Sb(V) ferrihydrite was higher than Al-coprecipitated ferrihydrite. It is speculated that the presence of Sb(V) weakened the inhibition of Al(III) under experimental conditions. Competitive reaction of Al(III) and Sb(V) for substitution on the lattice Fe of ferrihydrite, likely decreased Al(III) substitution on ferrihydrite, and thus increased the observed transformation rate of ferrihydrite. These results have significant environmental implications for predicting the role of impurities and contaminants on ferrihydrite transformation processes.

Effects of oxalate and citrate on the behavior and redistribution of Cr(VI) during ferrihydrite-Cr(VI) co-precipitates transformation

Understanding the influence of organic matters on the fate of Cr(VI) during ferrihydrite-Cr(VI) (Fh-Cr) co-precipitates transformation helps to study the retention of Cr(VI) by iron oxides in the environment. In this paper, Fh-Cr was prepared by co-precipitation and the redistribution of Cr(VI) in the oxalate or citrate system during the transformation of Fh-Cr was studied. X-ray diffraction, Fourier transform infrared spectroscopy, Transmission electron microscopy, and X-ray photoelectron spectroscopy were used to characterize Fh-Cr for aging 7 days at 70 °C. Results showed that both oxalate and citrate could hinder the release of Cr(VI) from Fh-Cr and abate the harm of Cr(VI). Oxalate improved the transformation from Fh-Cr to hematite and promoted Cr(VI) to be enfolded into the secondary minerals to further immobilize Cr at initial pH of 5.0 and 7.0, while citrate evidently reduced the release of Cr(VI) through stabilizing Fh-Cr at initial pH of 9.0. Besides, reduction of Cr(VI) by oxalate and citrate was through forming the surface complexes that promoted electron transfer from oxalate or citrate to Cr(VI), which can effectively abate the harm of Cr(VI). The findings of this study can promote understanding of the influences of organic matters on Cr(VI) immobilization during transformation of iron oxides in nature.

Formation and Immobilization of Cr(VI) Species in Long-Term Tannery Waste Contaminated Soils

Chromium speciation in naturally contaminated soils appears more complex than spiked studies have shown. This study characterized Cr speciation (oxidation states; availability; molecular geometry) intended to highlight the genesis of immobile Cr(VI) species in long-term tannery waste-contaminated soils. In a series of samples obtained from Shuitou in China, chemical extraction methods showed that Cr(III) was dominant(>96.7% of total Cr), with Cr(VI) concentration up to 144 mg kg^-1. Of the total Cr(VI) present, immobile Cr(VI) represents >90%. Synchrotron-based X-ray near-edge structure spectroscopy (XANES) showed the occurrence of Cr(VI), which was not removed by phosphate buffer extraction, confirming a significant amount of immobile Cr(VI) fractions in soils. X-ray fluorescence maps exhibited the heterogeneous distribution of Cr in soils associated with both Mn and Fe. Such a distribution suggests Cr(III) oxidation to Cr(VI) by Mn oxides and a possible immobilization of both Cr(III) and Cr(VI) onto Fe (hydr)oxides. Linear combination fitting of XANES spectra revealed that fractional weights (%) in samples were CrFeO₃ (49.3-53.6), CrOOH (22.3-30.8), and CaCrO₄ (13.2-25.3). Our results demonstrate that (i) Cr(VI) is immobilized in soils and (ii) mechanisms of Cr(VI) immobilization are CaCrO₄ precipitation and recrystallization with Fe (hydr)oxides.