Effects of Modified Biochar Application on Cd Speciation Transformation in Soil

School of Life and Environmental Sciences, Shaoxing University, Shaoxing, 312000, China

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Abstract

This study investigated the preparation of iron/manganese-modified rice husk biochar (Fe/Mn@BC) and its application for cadmium (Cd) immobilization in contaminated acidic soils. The biochar was synthesized via pyrolysis at 450°C, followed by chemical modification with FeCh-6H2O and MnCh'4H2O. SEM and BET analyses confirmed that modification significantly increased specific surface area and introduced abundant active adsorption sites. The modified biochar was applied to acidic paddy soils (pH 4.94-5.31) at 1% and 5% (w/w) dosage rates. BCR sequential extraction results demonstrated that 5% Fe/Mn@BC effectively reduced acid-soluble Cd from 55.1% to 13.2-17.4% and increased residual Cd from 17.5% to 48.3-66.3% under anaerobic conditions. Notably, during aerobic incubation, 5% Fe/Mn@BC maintained stable Cd immobilization, whereas unmodified biochar showed significant Cd re-mobilization. The enhanced performance is attributed to Fe/Mn oxides acting as electron shuttles, facilitating iron transformation, elevating soil pH, and promoting Cd precipitation and surface complexation. These findings indicate that Fe/Mn modification effectively overcomes the limitations of pristine biochar in acidified soils, offering a sustainable and cost-effective strategy for remediating Cd-contaminated agricultural land.