Efficacy of nitrate and biochar@birnessite composite microspheres for simultaneous suppression of As(III) mobilization and greenhouse gas emissions in flooded paddy soils.

Elevated As(III) pollution and greenhouse gas (GHG) emissions are two primary environmental concerns associated with flooded paddy soils. Herein, a novel biochar@birnessite composite microsphere was engineered using a biochar, birnessite and sodium alginate formulation. The microspheres were applied along with nitrate to examine their efficacy in suppressing As(III) mobilization and GHG emissions in an As-contaminated flooded paddy soil. After a 10-day incubation period, the combined nitrate + microsphere treatment achieved desirable remediation effects versus a nitrate-alone treatment, with mobile As(III) (initially 0.1 mM in flooded layer) completely immobilized and N2O, CH4 and CO2 emissions declining by 89 %, 73 % and 31 %, respectively. As(III) immobilization was ascribed to oxidation/adsorption/coprecipitation by FeOx/MnOx regenerated from successive cycles of Feammox/Mnammox and nitrate-reduction coupled with Fe(II) oxidation (NRFO)/nitrate-reduction coupled with Mn(II) oxidation (NRMO). Moreover, NRFO/NRMO-derived full denitrification displayed high thermodynamic feasibility, leading to full denitrification with the generation of N2 rather than N2O. The co-occurrence of anaerobic oxidation of methane (AOM) driven by biochar-shuttling and coupled reduction of nitrate/FeOx/MnOx fostered anaerobic oxidation of CH4 to CO2. A portion of the resulting CO2 was incorporated into poorly-soluble carbonate minerals leading to lower CO2 emission and soil carbon sequestration. Metagenomic sequencing revealed that the nitrate + microsphere treatment enriched the abundances of key microorganisms linked to As/Fe/Mn oxidation and GHG mitigation (e.g., Geobacter, Streptomyces, Cupriavidus and Chloroflexus). Our findings document the efficacy of nitrate + biochar@birnessite microsphere treatment as an effective remediation strategy to simultaneously mitigate As(III) pollution and GHG emissions in flooded paddy soils.