Fate Of Cr Research Articles

Bioremediation mechanisms of chlorophenol-Cr(Ⅵ): The role of amines, immobilization, and DEGs in Pseudomonas sp. PC

The combined pollution of p-Chlorophenol (4-CP) and hexavalent chromium [Cr(VI)] is widespread in the environment. However, little information is available on the relationship between the fate of Cr(VI) and its amines during dechlorination, which are dominant in pH value changes. Here, Pseudomonas sp. PC can degrade 4-CP and convert Cr(VI). The removal efficiency of Cr(VI) accords with the first-order reaction kinetics. Notably, the pH value (7.95) of the chlorophenol-Cr(VI) systems increased compared to the 4-CP treatment due to the significant accumulation of amines, especially ethambutol, which was 1.06 folds higher than 4-CP treatment. The pH value of the 4-CP treatment decreased to 5.89. Thus, Cr(III) was more likely to immobilize on the surface of the cell in the form of Cr(OH)3. Interestingly, Catechol 1,2-dioxygenase (catA) was 0.98 folds higher than Catechol 2,3-dioxygenase (dmpB) in chlorophenol-Cr(Ⅵ) systems, indicating that the presence of Cr(Ⅵ) changed the metabolic pathway of Pseudomonas sp. PC for 4-CP from the dmpB meta-cleavage pathway to the catA ortho-cleavage pathway. These conclusions deepen the cognizance of the relationship between the degradation pathway of chlorophenols, the fixation of Cr(VI) and the amines, providing a better basis for the bioremediation application of chlorophenol-Cr(VI) sites.

Disclosing the influence mechanism of facet-dependent pyrite photo-activation and photo-dissolution processes on the reduction of Cr(VI)

The photo-activation and photo-dissolution processes of pyrite (FeS2) can affect the environmental behavior of the co-existing hexavalent chromium (Cr(VI)). But the photochemical performance of FeS2 is intimately dependent on its exposed facets. Herein, FeS2 nanosheets (FeS2 NS) and FeS2 nanocubes (FeS2 NC) with the dominant exposed facets of (001) and (210)/(100) respectively are prepared. The more Fe3+, Fe2+, and SO42− are released in the FeS2 NS system than the other system due to its more excellent generation ability of photogenerated electrons and reactive oxygen species. The higher surface energy on (001) facet leads to the faster dissolution rate of FeS2 NS. Due to the optimal production ability of photogenerated electrons and Fe2+ of (001) facet, the much higher Cr(VI) elimination efficiency in the FeS2 NS system is observed than that in the FeS2 NC (72.8%) system within 120 min. This work could help to unveil the influence of FeS2 on the fate of Cr(VI) in surface environment, and offer a theoretical support to clarify the influence of heavy metal ions on the iron sulfide minerals.

Adsorption of Cr(VI) and Cr(III) on layered pipe scales and the effects of disinfectants in drinking water distribution systems

Pipe scales in drinking water distribution systems (DWDS) potentially adsorb chromium (Cr). Meanwhile, the fate of Cr in pipe scales and water could be influenced by the disinfectants used in DWDS since they might influence the valence state of Cr. Therefore, the adsorption of Cr (Cr(VI) and Cr(III)) on pipe scales, the transformation between different valence states, and the effects of disinfectants present in DWDS are important research topics for improving tap water quality but have not yet been sufficiently investigated. This study investigated the properties of layered pipe scales and conducted adsorption kinetic experiments in single and binary Cr(VI) and Cr(III) systems, as well as experiments related to the oxidation and adsorption of Cr(III) under the influence of decaying disinfectants. According to the results, pipe scales exhibited distinct layered structures with varying mechanisms for the adsorption of Cr(VI) and Cr(III). Cr(VI) was adsorbed through surface complexation on the surface and porous core layers, while redox reactions predominantly occurred on the shell-like layer. Furthermore, Cr(III) was adsorbed via surface precipitation on the three-layer pipe scales. Importantly, disinfectants promoted the transformation of Cr(III) to the less readily released Cr(VI) in pipe scales, reducing the Cr exposure risk from the pipe scale phase. Pipe scales also decreased the Cr(VI) concentration in water (almost 0 mg/L), enhancing the safety of DWDS. This study provides theoretical guidance on the safe operation of DWDS.

Reduction of Chromate via Biotic and Abiotic Pathways in the Presence of Three Co-contaminating Electron Acceptors.

Bioreduction of Cr(VI) to Cr(III) is a promising technology for removing Cr(VI), but Cr(VI) reduction alone cannot support microbial growth. This study investigated the reduction of Cr(VI) in the presence of three electron acceptors that typically coexist with Cr(VI): NO3-, SO42-, and Fe(III). All three systems could reduce Cr(VI) to Cr(III), but the fate of Cr, its impacts on reduction of the other acceptors, and its impact on the microbial community differed. Although Cr(VI) was continuously removed in the NO3--reduction systems, batch tests showed that denitrification was inhibited primarily through impeding nitrite reduction. The SO42- and Fe(III) reduction systems reduced Cr(VI) using a combination of biotic and abiotic processes. Across all three systems, the abundance of genera capable of reducing Cr(VI) increased following the introduction of Cr(VI). Conversely, the abundance of genera that cannot reduce or resist Cr(VI) decreased, leading to restructuring of the microbial community. Furthermore, the abundance of sulfide oxidizers and Fe(II) oxidizers substantially increased after the introduction of chromate. This study provides fundamental knowledge about how Cr(VI) bioreduction interacts with bioreductions of three other co-contaminating electron acceptors.

Enhanced Cr(VI) stabilization by terrestrial-derived soil protein: Photoelectrochemical properties and reduction mechanisms

Glomalin-related soil protein (GRSP) is a stable iron-organic carbon mixture that can enhance heavy metal sequestration in soils. However, the roles of GRSP in the transformation and fate of Cr(VI) have been rarely reported. Herein, we investigated the electrochemical and photocatalytic properties of GRSP and its mechanisms in Cr(VI) adsorption and reduction. Results showed that GRSP had a stronger ability for Cr(VI) adsorption and reduction than other biomaterials, with the highest adsorption amount of up to 0.126 mmol/g. The removal efficiency of Cr(VI) by GRSP was enhanced (4–7%) by ultraviolet irradiation due to the hydrated electrons produced by GRSP. Fe(II) ions, persistent free radicals, and oxygen-containing functional groups on the GRSP surface as electron donors participated in the reduction of Cr(VI) under dark condition. Moreover, Cr(III) was mainly adsorbed on the –COOH groups of GRSP via electrostatic interactions. Based on 2D correlation spectroscopy, the preferential adsorption occurred on the GRSP surface for Cr(VI) in the sequential order of CO → COO– → O–H → C–O. This work provides new insights into the Cr(VI) adsorption and reduction mechanism by GRSP. Overall, GRSP can serve as a natural iron-organic carbon for the photo-reduction of Cr(VI) pollution in environments.

A multi-scale assessment of the impact of salinity on the desorption of chromate from hematite: Sea level rise implications

The solubility and transport of Cr(VI) is primarily controlled by adsorption-desorption reactions at the surfaces of soil minerals such as iron oxides. Environmental properties such as pH, ionic strength, and ion competition are expected to affect the mobility and fate of Cr(VI). Sea level rise (SLR), and consequent seawater intrusion, is creating a new biogeochemical soil environment at coastal margins, potentially impacting Cr(VI) retention at contaminated sites. We employed in-situ ATR-FTIR spectroscopy and DFT calculations to investigate at the molecular level the adsorption of Cr(VI) on the hematite surface and its desorption by sulfate, as a function of pH and ionic strength. We further used a batch experiment to assess Cr(VI) desorption at varying artificial seawater (ASW) concentrations. IR results demonstrate the complexity of Cr(VI) adsorption, showing a combination of monodentate inner-sphere complexation at high pH and dichromate outer-sphere (∼75%) at low pH. The Cr(VI)-complexes exhibited desorption induced by increasing pH values (58% of desorption) and sulfate competition (∼40% desorption). ASW desorbed ∼20% more Cr(VI), even at just 1% concentration. Our findings provide insight into Cr(VI)-adsorption complexation that controls the retention and remobilization of Cr(VI) on Fe-oxide minerals. The results point to an elevated risk of Cr(VI) mobilization in contaminated soils affected by SLR.

The effect of FeS on the fate of Cr(VI) in the presence of organic matters under dynamic anoxic/oxic conditions.

The reduction of Cr(VI) by FeS under anoxic conditions has been studied extensively. However, when the redox environments alternate from anoxic to oxic, the effect of FeS on the fate of Cr(VI) in the presence of organic matters still remains unknown. Therefore, this study investigated the effect of FeS coupled with humic acids (HA) and alga on the transformation of Cr(VI) under dynamic anoxic/oxic conditions. HA could promote the reduction of Cr(VI) from 86.6% to 100% under anoxic conditions due to the enhancement of the dissolution and dispersibility of FeS particles by HA. However, the strong complexing and oxidizing properties of alga inhibited the reduction of FeS. Reactive oxygen species (ROS) generated from FeS oxidation under oxic conditions could oxidize 3.80μM of Cr(III) to aqueous Cr(VI) at pH 5.0, while aqueous Cr(VI) reached to 4.83μM in the presence of HA, which was ascribed to the increasing amount of free radicals. In addition, acidic conditions and excess FeS would increase strong reducing Fe(II) and S(-II) species, and improve the efficiency of Fenton reaction. The findings provided new insights into the fate of Cr(VI) in aquatic systems containing FeS and organic matters under dynamic anoxic/oxic conditions.

The mobility and fate of Cr during aging of ferrihydrite and ferrihydrite organominerals

The fixation of environmentally hazardous Cr(VI) in soils is largely attributed to its retention by iron (Fe) (oxyhydr)oxides in the soil environment, which are prevailingly associated with organic carbon (OC). The effect of OC with different binding strengths on Cr(VI) adsorption and the mobility and fate of Cr(VI) during aging of Fe (oxyhydr)oxide however, are unknown. Here we investigate how OC binding strength influences Cr(VI) adsorption to ferrihydrite (Fh) organominerals, the aging of Fh organominerals and the subsequent retention or release of Cr(VI) as aging proceeds. We conduct Cr(VI) adsorption experiments onto Fh organominerals, apply surface complexation modelling to study the relative binding strengths between OC and Cr(VI), and then conduct aging experiments to track the redistribution of Cr(VI). We show based on the relative binding strength between OC and Cr(VI), that weakly-bound OC at different C loadings has almost no discernable influence on Cr(VI) adsorption, while strongly-bound OC significantly suppresses Cr(VI) adsorption via surface site blocking and/or electrostatic repulsion. The mobility and fate of Cr(VI) with Fh organominerals during the aging process is strongly influenced by the presence of OC. Weakly-bound carboxyl-poor OC not only near doubles the rate of Cr(VI) retained (i.e., Cr(VI) redistribution from a weakly-bound to a strongly-bound fraction) in neoformed Fe minerals at pH 5.0, but also increases the final proportion of Cr(VI)strongly-bound, which we attribute to the loose and porous structure of Fh organominerals and easier replacement of weakly-bound OC by Cr(VI). More strongly-bound carboxyl-rich OC and higher pH however, reduces the influence of OC, such that the rate of Cr(VI) retained is similar to or lower than that for the pure Fh system, which we attribute to the fact that the strongly-bound OC is less easily replaced by Cr(VI) and hence Cr(VI) is less able to access surface sites or surface pores and thus to be retained inside neoformed minerals. Overall we suggest that OC with higher carboxyl-richness will further suppress the rate of Cr(VI) retained and overall proportion of Cr(VI)strongly-bound, and thus that the mobility and fate of Cr(VI) in soils is strongly controlled by the carboxyl-richness of OC in Fe organominerals.

Thermal treatment of municipal solid waste incineration fly ash and nanofiltration membrane concentrate co-processing: Speciation of chromium and its leachability

The thermal treatment of municipal solid waste incineration fly ash (MSWI FA) has raised serious concerns due to the increasing leaching concentration of Cr, and Cl severely limits the reuse and utilization of MSWI FA. A nanofiltration membrane concentrate (NF-MC) is utilized in this study to leach the MSWI FA to remove Cl and the fate of Cr in the leaching MSWI FA by NF-MC (L-MSWI FA) is investigated. The results showed that Cl in the MSWI FA was effectively removed at a removal rate greater than 95%. After the thermal treatment of the proportion of the MSWI FA based on the L-MSWI FA (P-MSWI FA), it was revealed that Cr primarily as Cr2O3 hexagonally shaped flake-like crystallites, was oxidated first to Cr(VI), and the leaching concentration reached the maximum (1.17±0.05 mg/L) at 800°C. Cr(VI) was then reduced, and the chemical valence of Cr between Cr(III) and Cr(VI) confirmed that the leachability of the Cr-containing materials rose after the thermal treatment. In addition, it was believed that Cr(VI) was reduced when the temperature was greater than 1000°C. The results of this study provide theoretical guidance for controlling the enhancement of Cr leachability during the thermal treatment process.

The sensitive mobility of Cr in ashes studied by SiO2–Al2O3–Fe2O3–CaO system

Since incineration is a feasible method for stabilization/solidification of chromium (Cr)-enriched wastes, the species, distribution, and mobility of Cr in ashes deserve more studies, especially as the function of ash composition. Synthetic Cr-bearing ashes (SAs) were synthesized by SiO2–Al2O3–Fe2O3–CaO systems to investigate Cr mobility under 1100 °C. A study from simplicity to complexity. The Cr in SiO2–CaO is of high mobility with CrO42− formation, in contrary to the moderate mobility in SiO2–Al2O3 and poor mobility in SiO2–Fe2O3. However, species and mobility of Cr are affected by the values of CaO/SiO2, Al2O3/SiO2, and Fe2O3/SiO2 ratios. When other oxides are added to the two-phase systems above, the fate of Cr is affected more considerably. With the SiO2 content of 70%, adding a slight amount of CaO (<10%) strengthens the stabilization/solidification of Cr, due to the favorable solid integration under Ca2+ fusion. However, the Cr mobility is higher with increasing the CaO content further. The minimum content of CaO is ∼20% to sufficiently decrease the proportion of residual Cr (QCr-S5) in SiO2–Al2O3–CaO, much lower than in SiO2–Fe2O3–CaO, which confirms the easier release of Cr immobilized in Si–Al matrixes. Considering the opposite effects of Fe2O3 and CaO on Cr mobility, increasing Fe2O3/CaO ratios >3/2 can limit the effect of CaO, leading to the efficient stabilization/solidification of Cr waste. Additionally, the QCr-S5 is 83% with the Fe2O3 content of 15% in SiO2–Al2O3–Fe2O3, higher than in SiO2–Al2O3 and SiO2–Fe2O3. This suggests the intense stabilization/solidification of Cr, probably due to the formation of amorphous Fe-rich glass. Based on these above, an equation is developed to describe the relationship between ash compositions and QCr-S5 (QCr-S5 = −39.37X1 + 24.96X2 + 5.34X3 – 2.51X4 + 54.29).

Effect of TOC Concentration of Humic Substances as an Electron Shuttle on Redox Functional Groups Stimulating Microbial Cr(VI) Reduction.

Humic substances as an electron shuttle play an essential role in the biogeochemistry processes. However, the influence of total organic carbon (TOC) concentrations of humic substances on microbial Cr(VI) reduction remains unclear. In this study, the rates and extents of Cr(VI) reduction by Shewanella oneidensis MR-1 in the presence of Leonardite humic acids (LHA) and Pahokee peat humic acids (PPHA) with different TOC concentrations were evaluated. We found that the enhanced reduction in Cr(VI) was associated with TOC concentrations of 2.5–50 mg C/L of HA samples. The result shows that HA as an electron shuttle impacted both rates and extents of microbial Cr (VI) reduction, which delivered differently in terms of low TOC concentration range of 2.5 to 15 mg C/L and high concentration range of 15–50 mg C/L. The rates of Cr(VI) reduction significantly enhanced in the low TOC concentration range of HA compared to a high concentration range. The highest acceleration rate of Cr(VI) reduction was achieved at 15 mg C/L of HA. The quinone-like fluorophore was responsible for the main redox-active functional groups of HA by the three-dimensional excitation-emission spectroscopy. The fluorescence intensity of quinone-like fluorophore of HA in the low TOC concentration range was positively correlated with its acceleration coefficient, corresponding to the highest microbial Cr(VI) reduction rate obtained in 15 mg C/L of HA. These findings highlighted the effect of the TOC concentration of HA on microbial Cr(VI) reduction processes. It emphasized that the low TOC concentration of HA contributed to the high rates of Cr(VI) reduction, which is critical for better understanding the fate of Cr(VI) and evaluating the effectiveness of Cr(VI) restoration strategies in the future.

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.

Identification of the Chromate Sorption Mechanism Conversions in a Quartz–Montmorillonite–Ferrihydrite System

The fate of Cr(VI) in soils/sediments largely depends on its sorption by iron oxides/hydroxides. But, their combination with surrounding minerals (e.g., quartz (Qu), clays) evidently changes some properties. Since ferrihydrite (Fh) is the precursor for most iron oxides, the 10 days sorption experiments studying Cr(VI) mobility were performed using the reference Qu–montmorillonite (Mt)–Fh system. From the results of batch experiment, ζ potential, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy, chromium(VI) sorption mechanisms are sensitive in Qu–Mt–Fh mixtures and closely relate to pHs. At acidic conditions, the deposition of Fh on Qu/Mt surfaces decreases the sorption sites for Cr(VI) while the formation of outer-sphere complexation driven by electrostatic attraction increases Cr(VI) mobility in contrast to inner-sphere complexation. At alkaline conditions, the situations are different in Qu–Fh and Mt–Fh systems, even though both increase sorption compared to that with only Fh. Sorption is subject to free Fh in Qu–Fh mixtures, while considering the high loading of the Mt–Fh surface due to the strong association of Si–OH with Fe–O(H), CrO42– captures are favorable by hydrogen bonding with water as outer-sphere complexation in Mt–Fh mixtures, resulting in high mobility especially at Mt/Fh ratios of 2/3.

Mn-incorporated ferrihydrite for Cr(VI) immobilization: Adsorption behavior and the fate of Cr(VI) during aging

Chromium(VI) (Cr(VI)) is an environmental priority pollutant, and its mobility in natural environment is strongly controlled by ferrihydrite. Ferrihydrite always contains various ions, which may change the properties of ferrihydrite, thereby affecting the behavior of pollutants. This study aims to investigate the adsorption of Cr(VI) by Mn-incorporated ferrihydrite and the mobility behavior of Cr(VI) during aging. Results showed that the incorporation of Mn enhanced the adsorption of Cr(VI) on ferrihydrite, and the adsorption performance increased with the increase of Mn content. The maximum adsorption capacity for Cr(VI) reached to 48.5 mg/g with molar ratio of Mn/Fe 5%, while it was 36.1 mg/g for pure ferrihydrite. After aging for 7 days, ferrihydrite transformed into goethite and hematite. The adsorbed Cr(VI) on the surface of ferrihydrite was released into the solution during aging. The incorporation of Mn retarded the transformation of ferrihydrite, which inhibited the migration of adsorbed Cr(VI). Nevertheless, the incorporation of Mn resulted in the transformation of adsorbed Cr(VI) to non-desorbed Cr(VI), thereby enhancing the retention of Cr(VI). Our results suggest that the incorporation of Mn into ferrihydrite has an important role on the mobility of Cr(VI), which enhances our understanding of the behavior of Cr(VI) in the environment.

Photoreductive dissolution of schwertmannite loaded with Cr(VI) induced by tartaric acid

Schwertmannite (SCH) as an adsorbent for Cr(VI) removal has been widely investigated. However, there are limited reports on photoreduction driven dissolution of SCH loaded with Cr(VI) (SCH–Cr(VI)) and the fate of Cr(VI) in the presence of dissolved organic matter (DOM). In this study, the effect of tartaric acid (TA) on the stability of SCH–Cr(VI) exposed to simulated solar radiation was examined. The results demonstrated that TA could greatly enhance the release of the dissolved total Fe (TFe) from SCH–Cr(VI). Conversely, the dissolved total Cr (TCr) obviously declined. Low pH promoted the liberation of TFe and TCr. The presence of ions including Al3+, Ca2+, K+ and CO32− exerted different impact on the photoreductive dissolution of SCH–Cr(VI) induced by TA. On the basis of the species distribution of iron and chromium and the characterization of the solid samples, the underlying mechanism is proposed for the transformation and the fate of Cr(VI). Cr(VI) was reduced to Cr(III) by Fe(II) generated from Fe(III)-TAn via ligand to metal charge transfer. The produced Cr(III) was adsorbed by SCH or co-precipitates with Fe(III). Thus, this study helps us to gain an insight into the mobility and fate of Cr(VI) in acid mining drainage containing DOM, and will help design remediation strategies for Cr contamination.

Serpentinization in the Archean and Early Phanerozoic – Insights from chromium isotope and REY systematics of the Mg[sbnd]Cr hydroxycarbonate stichtite and associated host serpentinites

Chromium isotopes have emerged as powerful redox proxies for past ocean-atmosphere systems as shifts in Cr isotope ratios towards heavier isotope compositions are indicative of Cr redox cycling on Earth. Knowledge of the Cr cycle in the critical zone and the fate of Cr during serpentinization and other hydrothermal processes is therefore important to assess the geologic interpretation of Cr isotopes in the rock record. An investigation of the trace element geochemistry of stichtite may provide such insights by shedding new light on the conditions of past serpentinizing environments.Stichtite, a magnesium‑chromium-bearing hydroxycarbonate, is a rare mineral that forms during serpentinization by conversion of chromite and other Cr-rich spinels. Its occurrence is restricted to Archean and Early Phanerozoic serpentinite bodies on ancient cratons and to mine waste. In this contribution, Cr isotope and rare earth elements and Y (REY) systematics in stichtites and their respective serpentinite hosts were investigated. This comparative study of Archean and Early Phanerozoic stichtites from different localities in South Africa, Western Australia, and Tasmania revealed variable Cr isotope fractionation with δ53Cr ranging from −0.145 ± 0.052‰ up to +0.24 ± 0.04‰ in the stichtites. The serpentinites are less fractionated with δ53Cr ranging from −0.13 ± 0.04‰ to +0.05 ± 0.02‰ and show a fractionation similar to the terrestrial mantle δ53Cr inventory. Whereas δ53Cr of all stichtites, except of one, are within the chromite array, the deviations from host serpentinites indicate that Cr was subject to Cr cycling during serpentinization. The variable Cr isotope ratios of stichtite reflect a complex interplay of several physicochemical processes. These are difficult to identify solely from isotope compositions as the resulting Cr isotope fractionation represents the net result of all processes that affect the complex Cr cycling in the critical zone.The very first REY concentration data for stichtite further corroborate the notion that stichtite formation is indeed not an isochemical process and that its formation involves intense metal mobility during associated metasomatism. Rare earth elements and yttrium are depleted in the stichtites relative to the serpentinites, but the overall similar REYCN patterns indicate that the REY in both, the serpentinites and stichtites, were sourced from a chemically similar fluid that was dominated by interaction with the serpentinized rocks. Seawater and marine carbonates can be excluded as potential contributors towards REY inventory of stichtite. The presence of superchondritic Y/Ho ratios in stichtite further emphasizes intense water-rock interaction at high CO32− and HCO3− activities. Positive EuCN anomalies in some stichtites and serpentinites point to reducing conditions with regard to the Eu3+/Eu2+ redox couple and temperatures in excess of 250 °C at the site of REY mobilization, prior to stichtite formation. Lack of Ce anomalies in stichtite further confirm reducing conditions at the site of stichtite formation.

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.

The critical utilization of active heterotrophic microalgae for bioremoval of Cr(VI) in organics co-contaminated wastewater

Considering the importance of removing toxic Cr(VI) from practical wastewater containing complex pollutants, this study presented for the first time the utilization of the live heterotrophic microalgae (Botryocossuss sp. NJD-1) to achieve a concurrent abatement of Cr(VI), TOC, NO3-N and PO4-P, through a comprehensive biochemical process. The experimental results showed that the Cr(VI) removal efficiencies in the culture with different types of organic descended in the order of sodium acetate, ethanol and methanol. The highest removal efficiencies were achieved as 94.2%, 98.2%, 66.9% and 99.2% for Cr(VI), TOC, NO3-N and PO4-P, respectively, in the culture with 5 mg L−1 Cr(VI) and sodium acetate of equivalent to 2.92 g C L−1. Through mass balance calculation and characterization, the fate of Cr(VI) and Cr(III) was tracked and quantified in the culture system, which indicates that 87.17% of initial Cr(VI) were reduced to Cr(III) and then adsorbed in algal biomass for the optimal removal case. Consequently, the mechanism demonstrating the correlation among the removal process of Cr(VI), the biological activity of microalgae and the effect of organic compounds was proposed.

Insights into Anammox activity inhibition under trivalent and hexavalent chromium stresses

The implementation of Anammox process in wastewater has been a growing concern, but chromium toxicity, created by the coexistence of chromium and ammonium, is a challenge, In this study, the acute toxicity of two typical chromium ions (i.e., Cr(III) and Cr(VI)) on Anammox process as well as the kinetics under Cr(VI) stress and the fate of Cr(VI) were assessed. The results showed that Cr(III) had little influence on Anammox activity since the inhibition percentages were only 1.47% at 32 mg L−1. However, Cr(VI) imposed a more pronounced inhibition with a half inhibition concentration of 10.91 mg L-1. Kinetics results indicated that Cr(VI) exerted a non-competitive inhibition on both of ammonium oxidation and nitrite conversion. Moreover, the fate of Cr(VI) was determined by the distribution and species variation of Cr(VI). The results demonstrated that Cr(VI) immigrated into sludge and then reduced to Cr(III). Cr(III) accounted for 92.5% of the total chromium in the sludge. Our results provided deeper insights into chromium acute toxicity on Anammox activity, which will aid in implementing Anammox process into wastewater treatment.

Transport and fate of hexavalent chromium in slag–soil system

The transport and fate of hexavalent chromium Cr(VI) in slag–soil system, which remain poorly understood, are of great importance for environmental risk assessment and pollution control of Cr(VI) in soil and groundwater. Based on the migration behavior of Cr(VI) in the chromium-containing slag under the effects of rainfall, the transport and fate of Cr(VI) in soil were investigated. Batch and column experiments were undertaken to clarify the Cr(VI) adsorption and transport behavior of aqueous Cr(VI) in soil, respectively. Findings from a combination of experimental and model results clearly suggest that the adsorption of Cr(VI) onto soil is a spontaneous chemical adsorption process. For chromium-soil transport system, the constructed HYDRUS-1D dynamics model is a good simulation for Cr(VI) migration in soil, as the modeling yielded adequate fit for simulated and measured values of Cr(VI) concentration in soil (correlation coefficient R2 = 0.99). The transport modeling then was utilized to assess the transport of Cr(VI) in slag–soil system. Up to the year of 2018, the concentration levels of Cr(VI) in the soil-leaching solution attained 905 mg/L. Due to the thin layer of soil and the high mobility of Cr(VI) in soil, it is no doubt that the Cr(VI) leaching will engender serious pollution of soil and groundwater in the presence of precipitation. The applications and limitations of the HYDRUS-1D were also discussed, such as bacteria transport, more elaborate biogeochemical processes, coupling of vadose zone processes with existing larger scale groundwater flow models.