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

Hexavalent chromium uptake in rice (Oryza sativa L.) mediated by sulfate and phosphate transporters OsSultr1;2 and OsPht1;1

Chromium (Cr) soil contamination is a critical global environmental concern, with hexavalent chromium (Cr[VI]) being especially perilous due to its high mobility, bioavailability, and phytotoxicity. This poses a significant threat to the cultivation of crops, particularly rice, where the mechanisms of Cr(VI) absorption remain largely unexplored. This study uncovered a competitive interaction between Cr(VI) and essential nutrients-sulfate and phosphate during the uptake process. Notably, deficiencies in sulfate and phosphate were associated with a marked increase in Cr(VI) accumulation in rice, reaching up to 76.5 % and 77.7 %, respectively. Employing q-PCR, this study identified significant up-regulation of the sulfate transporter gene, OsSultr1;2, and the phosphate transporter gene, OsPht1;1, in response to Cr(VI) stress. Genetic knockout studies have confirmed the crucial role of OsSultr1;2 in Cr(VI) uptake, with its deletion leading to a 36.1 % to 69.6 % decrease in Cr uptake by rice roots. Similarly, the knockout of OsPht1;1 resulted in an 18.1 % to 25.7 % decrease in root Cr accumulation. These findings highlight the key role of the sulfate transporter OsSultr1;2 in Cr(VI) uptake, with phosphate transporters also contributing significantly to the process. These insights are valuable for developing rice varieties with reduced Cr(VI) accumulation, ensuring the safety of rice grain production.

Oxidation of chromium(Ⅲ): A potential risk of using chemical oxidation processes for the remediation of 2-chlorophenol contaminated soils

Chemical oxidation processes are widely used for the remediation of organically contaminated soils, but their potential impact on variable-valence and toxic metals such as chromium (Cr) is often overlooked. In this study, we investigated the risk of Cr(Ⅲ) oxidation in soils during the remediation of 2-chlorophenol (2-CP) contaminated soils using four different processes: Potassium permanganate (KMnO4), Modified Fenton (Fe2+/H2O2), Alkali-activated persulfate (S2O82-/OH-), and Fe2+-activated persulfate (S2O82-/Fe2+). Our results indicated that the KMnO4, Fe2+/H2O2, and S2O82-/Fe2+ processes progressively oxidized Cr(III) to Cr(Ⅵ) during the 2-CP degradation. The KMnO4 process likely involved direct electron transfer, while the Fe2+/H2O2 and S2O82-/Fe2+ processes primarily relied on HO• and/or SO4•- for the Cr(III) oxidation. Notably, after 4 h of 2-CP degradation, the Cr(VI) content in the KMnO4 process surpassed China's 3.0 mg kg-1 risk screening threshold for Class I construction sites, and further exceeded the 5.7 mg kg-1 limit for Class II construction sites after 8 h. Conversely, the S2O82-/OH- process exhibited negligible oxidation of Cr(III), maintaining a low oxidation ratio of 0.13%, as highly alkaline conditions induced Cr(III) precipitation, reducing its exposure to free radicals. Cr(III) oxidation ratio was directly proportional to oxidant dosage, whereas the Fe2+/H2O2 process showed a different trend, influenced by the concentration of reductants. This study provides insights into the selection and optimization of chemical oxidation processes for soil remediation, emphasizing the imperative for thorough risk evaluation of Cr(III) oxidation before their application.

Systems biology of chromium-plant interaction: insights from omics approaches

Plants are frequently subjected to heavy metal (HM) stress that impedes their growth and productivity. One of the most common harmful trace metals and HM discovered is chromium (Cr). Its contamination continues to increase in the environment due to industrial or anthropogenic activities. Chromium is severely toxic to plant growth and development and acts as a human carcinogen that enters the body by inhaling or taking Cr-contaminated food items. Plants uptake Cr via various transporters, such as sulfate and phosphate transporters. In nature, Cr is found in various valence states, commonly Cr (III) and Cr (VI). Cr (VI) is soil's most hazardous and pervasive form. Cr elevates reactive oxygen species (ROS) activity, impeding various physiological and metabolic pathways. Plants have evolved various complex defense mechanisms to prevent or tolerate the toxic effects of Cr. These defense mechanisms include absorbing and accumulating Cr in cell organelles such as vacuoles, immobilizing them by forming complexes with organic chelates, and extracting them by using a variety of transporters and ion channels regulated by various signaling cascades and transcription factors. Several defense-related proteins including, metallothioneins, phytochelatins, and glutathione-S-transferases aid in the sequestration of Cr. Moreover, several genes and transcriptional factors, such as WRKY and AP2/ERF TF genes, play a crucial role in defense against Cr stress. To counter HM-mediated stress stimuli, OMICS approaches, including genomics, proteomics, transcriptomics, and metallomics, have facilitated our understanding to improve Cr stress tolerance in plants. This review discusses the Cr uptake, translocation, and accumulation in plants. Furthermore, it provides a model to unravel the complexities of the Cr-plant interaction utilizing system biology and integrated OMICS approach.

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.

Progress in phytoremediation of chromium from the environment

As chromium (Cr) in ecosystems affects human health through food chain exposure, phytoremediation is an environmentally friendly and efficient way to reduce chromium pollution in the environment. Here, we review the mechanism of absorption, translocation, storage, detoxification, and regulation of Cr in plants. The Cr(VI) form is more soluble, mobile, and toxic than Cr(III), reflecting how various valence states of Cr affect environmental risk characteristics, physicochemical properties, toxicity, and plant uptake. Plant root's response to Cr exposure leads to reactive oxygen species (ROS) generation and apoptosis. Cell wall immobilization, vacuole compartmentation, interaction of defense proteins and organic ligand with Cr, and removal of reactive oxygen species by antioxidants continue plant life. In addition, the combined application of microorganisms, genetic engineering, and the addition of organic acids, nanoparticles, fertilization, soil amendments, and other metals could accelerate the phytoremediation process. This review provides efficient methods to investigate and understand the complex changes of Cr metabolism in plants. Preferably, fast-growing, abundantly available biomass species should be modified to mitigate Cr pollution in the environment as these green and efficient remediation technologies are necessary for the protection of soil and water ecology.

Manganese losses induced by severe soil acidification in the extensive Lei bamboo (Phyllostachys violascens) plantation stands in Eastern China

Manganese (Mn) is a critical element in soils, essential to plant growth. Long-term and intensively managed Lei bamboo (Phyllostachys violascens) stands are usually subjected to severe soil acidification and Mn activation. However, Mn migration from topsoil to deep soil induced by severe soil acidification was poorly recognized and studied. The distribution and changes of the total and the operationally defined Mn forms in soil profiles and its potential stress and environmental effect were investigated in a chronosequence of Lei bamboo stands (0, 2, 6, 11, and 16 years of stand age). The results showed that the Mn amount was significantly decreased in topsoil and accumulated in subsoil with the long-term and intensive fertilizer application. Soil exchangeable Mn and superphosphate extractable Mn demonstrated large different variation to total Mn, whereas their sum was largely higher than and highly correlated with 8-hydroxyquinoline (HQN) extractable Mn. Soil organic carbon, pH value, exchangeable bases, and soil redox simultaneously controlled soil Mn depletion. In conclusion, long-term and intensive fertilizer application leads to soil acidification and accelerated soil Mn depletion in bamboo stand soil, promoting Mn accumulation in bamboo shoots.

The occurrence of "yellowing" phenomenon and its main driving factors after the remediation of chromium (Cr)-contaminated soils: A literature review

Chromium (Cr) is a highly toxic element, which is widely present in environment due to industrial activities. One of most applicable technique to clean up Cr pollution is chemical reduction. However, the Cr(VI) concentration in soil increases again after remediation, and meanwhile the yellow soil would appear, which is commonly called as "yellowing" phenomenon. To date, the reason behind the phenomenon has been disputed for decades. This study aimed to introduce the possible "yellowing" mechanism and the influencing factors based on the extensive literature review. In this work, the concept of "yellowing" phenomenon was explained, and the most potential reasons include the reoxidation of manganese (Mn) oxides and mass transfer were summarized. Based on the reported finding and results, the large area of "yellowing" is likely to be caused by the re-migration of Cr(VI), since it could not sufficiently contact with the reductant under the effects of the mass transfer. In addition, other driving factors also control the occurrence of "yellowing" phenomenon. This review provides valuable reference for the academic peers participating in the Cr-contaminated sites remediation.