Application of Iron-Bimetal Biochar for As and Cd Reduction and Soil Organic Carbon Preservation Under Varying Moisture

The contamination of paddy soils with arsenic (As) and cadmium (Cd), coupled with the depletion of soil organic carbon (SOC), poses significant threats to rice yields and quality. There is an urgent need to identify a suitable soil additive capable of achieving simultaneous heavy metal remediation and promotion of organic matter enrichment. The current study introduced two novel iron (Fe)/magnesium (Mg)-based bimetal-oxide-modified rice straw biochar (RSB), namely RSB-Fe/Mn and RSB-Fe/Mg. It evaluated their effectiveness in As/Cd immobilization and SOC preservation. An 8-week cultivation experiment was carried out in sequential drying–flooding moisture fluctuation conditions, with the soil pore water As/Cd (PWAs/Cd) and SOC fractions monitored. The mechanisms of As/Cd immobilization were investigated using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS) characterizations. Results revealed that PWAs and PWCd were reduced by up to 67.1% and 80.2% during the drying period and by 27.0% and 76.5% during the flooding period, respectively. Additionally, SOC content increased by 16.3% and 33.9% with RSB-Fe/Mn addition during the drying and flooding period, respectively, with an increase in the mineral-associated organic carbon (MAOC) fraction. The study proves that RSB-Fe/Mn and RSB-Fe/Mg are effective for soil As/Cd passivation and SOC stabilization, offering a promising solution to mitigate As and Cd pollution in paddy soils while maintaining soil quality.