Release of chromium from Cr(III)- and Ni(II)-substituted goethite in presence of organic acids: Role of pH in the formation of colloids and complexes.

High levels of Cr(III) are hosted in Fe (oxyhydr)oxides in soils derived on (ultra)mafic rocks, which can pose potential risks to the environment. Organic acids can cause the solubilization of Fe (oxyhydr)oxides and the release of Cr(III). However, the release behaviors of Cr(III) from Fe (oxyhydr)oxides by organic acids and its main factors remain unclear. This study investigates the speciation of Cr released from Cr(III)-substituted goethite in the presence of citrate and oxalate and the effects of pH (3-7). Batch experiments showed that Fe(III) and Cr(III) dissolution were significantly enhanced by citrate and oxalate, and the extent of dissolution was negatively correlated with pH. When at relatively high pH (5-7), AF4-ICP-MS results revealed that large proportions of dissolved Fe (>58 %) and Cr (18 %-73 %) were presented in the form of Cr(III)-citrate colloids in the sizes of 1-125 nm and 125-350 nm. Further, FTIR and cryogenic XPS characterization demonstrated that the formation of·Cr(III)-citrate colloids was attributed to the adsorption and complexation of citrate on the substituted goethite surface. However, Cr was mainly released as soluble Cr(III)-organic complexes when presented at pH 3. While low pH inhibited the formation of Cr(III)-organic colloids, it promoted the release of Cr by facilitating the dissociation of surface Cr(III)-organic complexes. In addition, the incorporation of Ni(II) in Cr(III)-substituted goethite weakened the adsorption of organic acid by shortening the crystal size of goethite, thus significantly inhibiting the formation of Cr(III)-organic complexes and colloids. This study confirms the formation of Cr(III)-organic acid colloids and highlights the importance of pH on Cr release behavior, which is essential for evaluating Cr transport and fate in soils with high background values.

Increasing soil Mn abundance promotes the dissolution and oxidation of Cr(III) and increases the accumulation of Cr in rice grains.

Hexavalent chromium (Cr(VI)) is more readily taken up by plants than trivalent chromium (Cr(III)) due to its similar chemical structure to phosphate and sulfate. In paddy soils, Cr(VI) of natural origin are mainly produced from Cr(III) oxidized by O2 and Mn(III/IV) oxides, which are affected by rice radial oxygen loss (ROL) and Mn(II)-oxidizing microorganisms (MOM). However, little is known about the effect of ROL and Mn abundance on rice Cr uptake. Here, we investigated the effects on Cr(VI) generation and the subsequent Cr uptake and accumulation with the involvement of two rice cultivars with distinct ROL capacities by increasing soil Mn abundance. Results showed that Mn(II) addition to the soil led to more Cr(III) being released into the pore water, and the dissolved Cr(III) was oxidized to Cr(VI) by ROL and biogenic Mn(III/IV) oxides. The concentration of Cr(VI) in soil and pore water increased linearly with the addition of Mn(II) doses. Mn(II) addition promoted the root-to-shoot translocation and grain accumulation of Cr derived mainly from newly generated Cr(VI) in the soil. These results emphasize that rice ROL and MOM promote the oxidative dissolution of Cr(III) at a high level of soil Mn, resulting in more Cr accumulation in rice grains and increasing dietary Cr exposure risks.

Water Management Impacts on Chromium Behavior and Uptake by Rice in Paddy Soil with High Geological Background Values.

Chromium (Cr) is an expression toxic metal and is seriously released into the soil environment due to its extensive use and mining. Basalt is an important Cr reservoir in the terrestrial environment. Cr in paddy soil can be enriched by chemical weathering. Therefore, basalt-derived paddy soils contain extremely high concentrations of Cr and can enter the human body through the food chain. However, the water management conditions' effect on the transformation of Cr in basalt-derived paddy soil with high geological background values was less recognized. In this study, a pot experiment was conducted to investigate the effects of different water management treatments on the migration and transformation of Cr in a soil-rice system at different rice growth stages. Two water management treatments of continuous flooding (CF) and alternative wet and dry (AWD) and four different rice growth stages were set up. The results showed that AWD treatment significantly reduced the biomass of rice and promoted the absorption of Cr in rice plants. During the four growth periods, the root, stem and leaf of rice increased from 11.24-16.11 mg kg-1, 0.66-1.56 mg kg-1 and 0.48-2.29 mg kg-1 to 12.43-22.60 mg kg-1, 0.98-3.31 mg kg-1 and 0.58-2.86 mg kg-1, respectively. The Cr concentration in roots, stems and leaves of AWD treatment was 40%, 89% and 25% higher than CF treatment in the filling stage, respectively. The AWD treatment also facilitated the potential bioactive fractions conversion to the bioavailable fraction, compared with the CF treatment. In addition, the enrichment of iron-reducing bacteria and sulfate-reducing bacteria with AWD treatment also provided electron iron for the mobilization of Cr, thus affecting the migration and transformation of Cr in the soil. We speculated that the reason for this phenomenon may be the bioavailability of Cr was affected by the biogeochemical cycle of iron under the influence of alternating redox. This indicates that AWD treatment may bring certain environmental risks in contaminated paddy soil with high geological background, and it is necessary to be aware of this risk when using water-saving irrigation to plant rice.

Nano zero-valent iron enhances the absorption and transport of chromium in rice (Oryza sativa L.): Implication for Cr risks management in paddy fields.

Chromium (Cr) accumulating in soil caused serious pollution to cultivated land. At present, nano zero-valent iron (nZVI) is considered to be a promising remediation material for Cr-contaminated soil. However, the nZVI impact on the behavior of Cr in the soil-rice system under high natural geological background value remains unknown. We studied the effects of nZVI on the migration and transformation of Cr in paddy soil-rice by pot experiment. Three different doses of nZVI (0, 0.001 % and 0.1 % (w/w)) treatments and one dose of 0.1 % (w/w) nZVI treatment without plant rice were set up. Under continuous flooding conditions, nZVI significantly increased rice biomass compared with the control. At the same time, nZVI significantly promoted the reduction of Fe in the soil, increased the concentration of oxalate Fe and bioavailable Cr, then facilitated the absorption of Cr in rice roots and the transportation to the aboveground part. In addition, the enrichment of Fe(III)-reducing bacteria and sulfate-reducing bacteria in soil provided electron donors for Cr oxidation, which helps to form bioavailable Cr that is easily absorbed by plants. The results of this study can provide scientific basis and technical support for the remediation of Cr -polluted paddy soil with high geological background.

Natural source of Cr(VI) in soil: The anoxic oxidation of Cr(III) by Mn oxides.

Cr(VI) from oxidation of geogenic Cr(III) minerals is gradually becoming the primary source of Cr(VI) in soils and groundwater instead of direct emissions. Thermodynamically, natural oxidants of Cr(III) are limited to O2 and Mn oxides. The oxidation of Cr(III) occurs commonly in oxic soils but the difference in the oxidative dissolution of Cr(III) by Mn oxides in different redox soils (especially under anoxic conditions) is not fully understood and field evidence is lacking. Here, the relationship between Cr(VI) and Mn oxides in basalt-origin soil profiles under three different redox conditions (anoxic, suboxic and oxic) was studied. The oxidative dissolution of chromite was validated by synthesising δ-MnO2 that was close to biogenic Mn oxides under anoxic and oxic conditions. In anoxic soils, high levels of Cr(VI) were detected in the same horizons as those where Cr(III)-minerals co-existed with Mn(III/IV) oxides, suggesting an exclusive pathway for Cr(VI) generation through oxidation by Mn oxides where there was a deficiency of other oxidants, such as O2. In oxic soils, the highly abundant Fe oxides combined with Cr(III) to form Cr(III)-Fe(III) oxyhydroxides and Cr(VI) was generated mainly via slow oxidation by O2. The chromite oxidation experiment results also indicated that a high abundance of Mn oxides could promote chromite oxidative dissolution to generate Cr(VI), even under anoxic conditions. Additionally, the form of Cr and the reactivity and abundance of Mn oxides and reducing agents controlled the net content of Cr(VI) in the soil. This study showed that, even under reducing conditions, Cr(III) is readily oxidised by Mn oxides to generate Cr(VI) in reductant-deficient and Mn-rich soils, which may lead to the continuous introduction of Cr(VI) into groundwater and agricultural soils.

Enrichment and speciation of chromium during basalt weathering: Insights from variably weathered profiles in the Leizhou Peninsula, South China.

Basalt-derived soils are widespread worldwide. Such soils contain high levels of heavy metals like chromium (Cr), which is a serious environmental concern. However, little is known regarding the enrichment and speciation of Cr during the basalt weathering process. Therefore, two basalt-derived soil profiles (Nitisol and Ferralsol) in the Leizhou Peninsula, south tropical China, were investigated to explore the redistribution and transformation of Cr during basalt weathering. All profiles could be divided into three layers: rocks, saprolites, and soils. The Nitisol and Ferralsol profiles exhibited strong (kaolinization) and extreme (laterization) degrees of weathering, respectively. Results showed that Cr concentrations in the saprolites (234 to 315 mg·kg-1) were higher than those in basalt rocks (139 to 159 mg·kg-1), indicating that Cr was enriched with the continuous loss of Si and other mobile macro-elements. While high levels of Cr were also enriched in the soils (178 to 430 mg·kg-1) accompanied with Fe. However, in the upper soils of the Ferralsol profile, the acidity and organic matter could promote the leaching of Cr. Geochemical fractions and EPMA mapping showed that chromite and olivine were the main Cr-bearing minerals in basalt, but Fe-oxides (e.g., goethite and hematite) contained the highest portion of Cr in weathered saprolites and soils. The availability of Cr in the soil was extremely low due to the high stability of Cr bound to Fe-oxides. However, the decreasing contents of Cr bound to Fe-oxides in the upper soils of the Ferralsol profile indicated that Cr could also be released during Fe leaching. In conclusion, the weathering of basalt can lead to the enrichment of Cr in Fe-(hydro)oxides, which are the main controlling minerals for Cr mobility in basalt-derived soils. Further research is needed to evaluate the effect of Fe-(hydro)oxide formation and dissolution on the release of soil Cr.

Chromium biogeochemical behaviour in soil-plant systems and remediation strategies: A critical review.

Chromium (Cr) is a toxic heavy metal that is heavily discharged into the soil environment due to its widespread use and mining. High Cr levels may pose toxic hazards to plants, animals and humans, and thus have attracted global attention. Recently, much progress has been made in elucidating the mechanisms of Cr uptake, transport and accumulation in soil-plant systems, aiming to reduce the toxicity and ecological risk of Cr in soil; however, these topics have not been critically reviewed and summarised to date. Accordingly, based on available data-especially from the last five years (2017-2021)-this review traces a plausible link among Cr sources, levels, chemical forms, and phytoavailability in soil; Cr accumulation and translocation in plants; and Cr phytotoxicity and detoxification in plants. Additionally, given the toxicity and hazard posed by Cr(VI) in soils and the application of reductant materials to reduce Cr(VI) to Cr(III) for the remediation of Cr(VI)-contaminated soils, the reduction and immobilisation mechanisms by organic and inorganic reductants are summarised. Finally, some priority research challenges concerning the biogeochemical behaviour of Cr in soil-plant systems are highlighted, as well as the environmental impacts resulting from the application of reductive materials and potential research prospects.

[Key processes and progress in phytomining of nickel contaminated soils: a review].

Phytomining technology cultivates hyperaccumulator plants on heavy metal contaminated soils, followed by biomass harvesting and incineration to recover valuable metals, offering an opportunity for resource recycling and soil remediation. Large areas of ultramafic soils, naturally rich in nickel (Ni), are present in numerous places around the world. As an environmentally friendly and cost-effective soil remediation technology, phytomining has a broad application prospect in such areas and thus has attracted great attention from global researchers. The key processes of phytomining include: (1) high-selectivity response of hyperaccumulator plants to Ni the underlying mechanisms involved in the rhizosphere; (2) underlying mechanisms of high-efficiency uptake and translocation of Ni in hyperaccumulators; and (3) resource recycling of high-added value Ni products from the Ni-rich bio-ore of hyperaccumulators. In recent 30 years, phytomining practices have successfully carried out in United States, Albania and Malaysia. However, the research and application of this technology in China are still in the fledging stage. This paper reviews the key processes and research progress of phytomining, and points out the bottleneck, to provide theoretical basis and technical guidance for phytomining.

Direct laser writing of pyramidal plasmonic structures with apertures and asymmetric gratings towards efficient subwavelength light focusing.

Efficient confining of photons into subwavelength scale is of great importance in both fundamental researches and engineering applications, of which one major challenge lies in the lack of effective and reliable on-chip nanofabrication techniques. Here we demonstrate the efficient subwavelength light focusing with carefully engineered pyramidal structures fabricated by direct laser writing and surface metallization. The important effects of the geometry and symmetry are investigated. Apertures with various sizes are flexibly introduced at the apex of the pyramids, the focusing spot size and center-to-sidelobe ratio of which could be improved a factor of ~4 and ~3, respectively, compared with the conical counterparts of identical size. Moreover, two pairs of asymmetric through-nanogratings are conceptually introduced onto the top end of the pyramids, showing significantly improved focusing characteristics. The studies provide a novel methodology for the design and realization of 3D plasmonic focusing with low-noise background and high energy transfer.