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

Size-dependent uptake of chromium colloids in rice (Oryza sativa L.)

Chromium (Cr) contamination in agricultural soils poses a significant threat to food safety. In paddy soil, Cr tends to exist as colloidal Cr, instead of soluble Cr. However, little is known regarding the absorption capability of colloidal Cr in rice. Therefore, this study explores the differential absorption of soluble and colloidal Cr in rice through hydroponic experiments, further investigating size-dependent uptake of Cr colloids, focusing on the role of endocytosis and the effects of adding endocytosis inhibitors. The results demonstrate the accumulation of Cr in rice roots for treatments with 100 μM soluble Cr (Cr-EDTA), 28 nm Cr colloid, and 62 nm Cr colloid were 40.0, 797, and 2033 mg/kg, respectively. The reason for this phenomenon is that colloidal Cr is more likely to be adsorbed onto the cell wall, which increases the potential for rice to absorb colloidal Cr, particularly the larger Cr colloids. When endocytosis in rice was inhibited by wortmannin, the absorption of Cr colloid with small size was reduced by 7.30%-26.6%, thereby highlighting the important role of endocytosis in the uptake of 28 nm particles in rice. In contrast, the uptake of soluble Cr and larger colloids were less affected by endocytosis inhibition. This study underscores the importance of considering colloidal Cr and its particle size when assessing the risks associated with heavy metal pollution in agriculture. The findings have significant implications for managing soil contamination and ensuring food safety.

Phases partitioning and occurrence forms of arsenic, chromium, and vanadium in a tidal reach of the Pearl river estuary, South China

Migration characteristics and occurrence forms of redox-sensitive metal(loid)s such as arsenic (As), chromium (Cr), and vanadium (V) remained unclear in dynamic estuarine waters. In this work, size fractionation and chemical speciation of As, Cr, and V in the Jiaomen Waterway (JMW), a tidal river of the Pearl River estuary, were explored based on (ultra)filtration, the diffusive gradients in thin films (DGT) techniques and a thermodynamic chemical equilibrium model. The results showed that As was present mainly in soluble forms in the river water, and the suspended particulate matter (SPM) was identified the major carrier for Cr. The hosting phase of V converted from solid to liquid fractions during the transition from rainy to dry seasons. Decreasing Log Kd of Cr > V > As indicated particulate As presented greater potentials to be desorbed from the SPM and transferred into the liquid phase. Coagulation and flocculation of colloidal As was observed in rainy season, while changes in its partitioning behaviors between colloids and truly dissolved fractions were relatively weak in dry season. In contrast, Cr and V behaved a transfer from colloid to truly dissolved fraction along the JMW because of degradation of organic matter. During the migration to the estuary, AsO43- and CrO42- were transformed into H3AsO3 and Cr(III)-organic complexes, respectively, due to reduction of As(V) and Cr (VI) species. Meanwhile, proportions of V species decreased in order of HVO42- > VO2+>V(IV)-organic complexes without obvious redox reactions of V(V) being taken place. The results were anticipated to provide a further supplement for geochemistry of redox-sensitive metal(loid)s in estuarine regions.

Different calcium sources affect the products and sites of mineralized Cr(VI) by microbially induced carbonate precipitation

Microbially induced carbonate precipitation (MICP) is a common biomineralization method, which is often used for remediation of heavy metal pollution such as hexavalent chromium (Cr(VI)) in recent years. Calcium sources are essential for the MICP process. This study investigated the potential of MICP technology for Cr(VI) remediation under the influence of three calcium sources (CaCl2, Ca(CH3COO)2, Ca(C6H11O7)2). The results indicated that CaCl2 was the most efficient in the mineralization of Cr(VI), and Ca(C6H11O7)2 could significantly promote Cr(VI) reduction. The addition of different calcium sources all promoted the urease activity of Sporosarcina saromensis W5, in which the CaCl2 group showed higher urease activity at the same Ca2+ concentration. Besides, with CaCl2, Ca(CH3COO)2 and Ca(C6H11O7)2 treatments, the final fraction of Cr species (Cr(VI), reduced Cr(III) and organic Cr(III)-complexes) were mainly converted to the carbonate-bound, cytoplasm and cell membrane state, respectively. Furthermore, the characterization results revealed that three calcium sources could co-precipitate with Cr species to produce Ca10Cr6O24(CO3), and calcite and vaterite were present in the CaCl2 and Ca(CH3COO)2 groups, while only calcite was present in the Ca(C6H11O7)2 group. Overall, this study contributes to the optimization of MICP-mediated remediation of heavy metal contaminated soil. CaCl2 was the more suitable calcium source than the other two for the application of MICP technology in the Cr(VI) reduction and mineralization.

Enhanced Cr(VI) removal and stabilization from bioleached wastewater by zero-valent iron coupled with hetero and autotrophic bacteria

Zero-valent iron (Fe0) usually suffers from organic acid complexation and ferrochrome layer passivation in Cr(VI) removal from bioleached wastewater of Cr slag. In this work, a synergetic system combined Fe0 and mixed hetero/autotrophic bacteria was established to reduce and stabilize Cr(VI) from bioleached wastewater. Due to bacterial consumption of organic acid and hydrogen, severe iron corrosion and structured-Fe(II) mineral generation (e.g., magnetite and green rust) occurred on biotic Fe0 surface in terms of solid-phase characterization, which was crucial for Cr(VI) adsorption and reduction. Therefore, compared with the abiotic Fe0 system, this integrated system exhibited a 6.1-fold increase in Cr(VI) removal, with heterotrophic reduction contributing 3.4-fold and abiotic part promoted by hydrogen-autotrophic bacteria enhancing 2.7-fold. After reaction, the Cr valence distribution and X-ray photoelectron spectroscopy indicated that most Cr(VI) was converted into immobilized products such as FexCr1-x(OH)3, Cr2O3, and FeCr2O4 by biotic Fe0. Reoxidation experiment revealed that these products exhibited superior stability to the immobilized products generated by Fe0 or bacteria. Additionally, organic acid concentration and Fe0 dosage showed significantly positive correlation with Cr(VI) removal within the range of biological adaptation, which emphasized that heterotrophic and autotrophic bacteria acted essential roles in Cr(VI) removal. This work highlighted the enhanced effect of heterotrophic and autotrophic activities on Cr(VI) reduction and stabilization by Fe0 and offered a promising approach for bioleached wastewater treatment.

Partitioning and Mobility of Chromium in Iron-Rich Laterites from an Optimized Sequential Extraction Procedure

Chromium (Cr) leached from iron (Fe) (oxyhydr)oxide-rich tropical laterites can substantially impact downstream groundwater, ecosystems, and human health. However, its partitioning into mineral hosts, its binding, oxidation state, and potential release are poorly defined. This is in part due to the current lack of well-designed and validated Cr-specific sequential extraction procedures (SEPs) for laterites. To fill this gap, we have (i) first optimized a Cr SEP for Fe (oxyhydr)oxide-rich laterites using synthetic and natural Cr-bearing minerals and laterite references, (ii) used a complementary suite of techniques and critically evaluated existing non-laterite and non-Cr-optimized SEPs, compared to our optimized SEP, and (iii) confirmed the efficiency of our new SEP through analyses of laterites from the Philippines. Our results show that other SEPs inadequately leach Cr host phases and underestimate the Cr fractions. Our SEP recovered up to seven times higher Cr contents because it (a) more efficiently dissolves metal-substituted Fe phases, (b) quantitatively extracts adsorbed Cr, and (c) prevents overestimation of organic Cr in laterites. With this new SEP, we can estimate the mineral-specific Cr fractionation in Fe-rich tropical soils more quantitatively and thus improve our knowledge of the potential environmental impacts of Cr from lateritic areas.

Interaction between chromite and Mn(II/IV) under anoxic, oxic and anoxic-oxic conditions: Dissolution, oxidation and pH dependence

Chromite oxidative dissolution has been recognized as an important process leading to elevated Cr(VI) in soil and groundwater. Under natural conditions, direct oxidation of Cr(III) by O2 is very unfavorable, and a critical determinant of Cr(VI) generation in soil and groundwater is the interaction between chromite and Mn(II) or Mn(III/IV) oxides. Here, the effects of Mn(II) or Mn(IV) on the oxidative dissolution of chromite were investigated at pH values of 5, 7 and 9 during anoxic, oxic and anoxic-oxic processes. The results showed that the direct oxidation of Cr(III) by O2 was slow in aqueous-phase system, while the Mn oxides in chromite could oxidize dissolved Cr(III). The added Mn(II) can be catalytically oxidized to MnOOH on the chromite surface only under alkaline oxidation conditions, and the catalytic efficiency is slow, which has less effect on chromite oxidative dissolution. Compared with the direct oxidation of O2 and catalytic oxidation of Mn(II), the synthesized biogenic Mn oxides drove the oxidative dissolution of chromite to release more Cr(VI) and were the main threat to the long-term stability of chromite in the environment. Overall, both acidic and alkaline environments are favorable to the catalytic oxidation of chromite by O2, Mn(II) and δ-MnO2, while neutral conditions are favorable to the long-term stability of chromite. These above processes may occur in soils and sediments with redox fluctuations (e.g., rice paddies, river floodplains, wetlands, and peatlands), and the presence of Mn(II) and Mn(III/IV) may play an important role in the oxidation and mobilization of Cr(III), leading to elevated Cr(VI) levels in soils and groundwater.