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Cadmium (Cd) removal from paddy soil to reduce Cd accumulation in brown rice is essential for agroecology, food safety, and human health. In this study, we demonstrate that ferric chloride (FeCl

The utilization of recycled biochar combined with chemical leaching is an appropriate method to remove cadmium (Cd) from paddy soil. Some Cd-rich soil clay particulates (particulate Cd) are reported to be removed via biochar adsorption and the potential impact of biochar on soil properties need further study. The removal efficiencies and mechanisms of Cd from soil by using floatable hydroxyapatite modified biochar (HBC) combined with CaCl

Engineered black carbon (biochar) can be introduced into groundwater through its extensive engineered applications (e.g., in-situ remediation of groundwater/soils), which can participate in geochemical processes that may alter the fate of trace contaminants such as arsenic (As(III)). Here we examined the impacts of the undissolved and dissolved fractions of reduced biochar (hereafter denoted as rUBC and rDBC, respectively) on the As(III) immobilization in the absence/presence of Ca

Biochars have demonstrated great potential for water decontamination and soil remediation; however, their redox reactivity toward trace contaminants and the corresponding redox-active moieties (RAMs, i.e., phenolic -OH, semiquinone-type persistent free radicals (PFRs), and quinoid C═O) remain poorly understood. Here we investigated the roles of the RAMs on biochar in oxidation of As(III) under varying pH and O2 conditions. The results showed that the promoted oxidation of As(III) by the RAMs is strongly pH dependent. Under acidic and neutral conditions, only the oxidation of As(III) by •OH and H2O2 produced from activation of O2 by phenolic -OH and semiquinone-type PFRs occurred. In contrast, the oxidation by semiquinone-type PFRs, quinoid C═O, and H2O2 (if O2 was introduced) appeared under alkaline conditions. This pH-dependent oxidation behavior was attributed to the varying redox activities of RAMs, as confirmed by multiple characterization and validation experiments using biochar with tuned RAMs compositions, as well as thermodynamics evaluation. Our findings provide new insights into the roles of the RAMs on biochar in the promoted oxidation of trace As(III) over a broader pH range under both anoxic and oxic conditions. This study also paves a promising way to oxidize As(III) with biochar.

Elevated As(III) and Fe(II) in shallow reducing groundwater can be frequently re-oxidized by introducing O2 due to natural/anthropogenic processes, thus leading to oxidative precipitation of As as well as Fe. Nevertheless, the geochemical process may be impacted by co-existing engineered black carbon due to its considerable applications, which remains poorly understood. Taking rice husk-derived biochar prepared at 500 °C as an example, we explored its impact on the process particularly for the As(III) oxidation and (im)mobilization during the oxygenation. The presence of the biochar had a negligible effect on the As(III) oxidation and immobilization extents within 1 d, while accelerating their rates. However, the immobilized As(III) was significantly liberated from the formed Fe(III) minerals afterward within 21 d, which was 2.2-fold higher than that in the absence of the biochar. The enhanced As(III) liberation was attributed to the presence of the surface silicon-carbon structure, consisting of the outer silicon and inner carbon layers, of the rice husk-derived biochar. The outer silicon components, particularly for the dissolved silicate primarily promoted the As(III) release via ligand exchange, while significantly impeding the transformation of ferrihydrite to lepidocrocite and goethite still resulted secondarily in the As(III) release. Our findings reveal the possible impact of biochar on the environmental behavior and fate of As(III) in the Fe(II)-rich groundwater during the oxygenation. This work highlights that biochar, particularly for its structural features should be a concern in re-mobilizing As in such scenarios when the oxygenation time reaches several days or weeks.

Magnetic biochar (MBC) has been used to remove hexavalent chromium (Cr(VI)) from water, but the roles of Fe