A comparative assessment of humic acid and biochar altering cadmium and arsenic fractions in a paddy soil

Purpose Contamination of agricultural soils by multiple heavy metal(loid)s poses a threat to environmental quality and food safety. Humic acid (HA) and biochar (BC) are common amendments for soil remediation. This study systematically investigated the effects of HA and BC on the redistribution of Cd and As fractions in contaminated soil and assessed the side effects of HA and BC in soil remediation. Materials and methods The experiments were designed to probe the dynamic behavior of soil pH, bioavailable Cd and As, and the five fractions of Cd and As in the presence of HA or BC during the 180-day incubation. A leonardite-derived HA (L-HA) and bamboo willow-derived BC (BWB) were added as amendments at 1% and 3% doses to a paddy soil (pH 5.88) contaminated by multiple heavy metal(loid)s. The treatments were denoted as L-HA1, L-HA3, BWB1, and BWB3, respectively, with cadmium (soil Cd, 4.47 mg/kg) and arsenic (soil As, 97.05 mg/kg) as the target contaminants. Results The soil pH reduced to 5.69 at L-HA1 and 5.33 at L-HA3 but increased to 6.27 at BWB1 and 6.75 at BWB3. L-HA1 and L-HA3 lowered bioavailable Cd by 10.79% and 11.27% and bioavailable As by 21.16% and 23.58%, respectively. While, BWB1 and BWB3 increased bioavailable Cd and increased by 18.13% and 18.94% bioavailable Cd and no apparent effect on bioavailable As in comparison with control. In terms of the fractions of heavy metal(loid)s during the 180-day incubation, L-HA and BWB dynamically altered Cd and As redistributions. L-HA1 and L-HA3 reduced exchangeable Cd by 17.60% and 48.75%, respectively, but increased residual Cd by 40.17% and 46.93% and residual As by 18.97% and 20.14%, respectively. BWB1 and BWB3 decreased exchangeable Cd by 11.65% and 37.16% but increased residual Cd by 33.84% and 37.02%, respectively. In contrast, BWB1 and BWB3 increased exchangeable As by 4.07% and 46.89% but lowered residual As by 5.31% and 27.06%, respectively. Conclusion The L-HA and BWB can alter soil properties and heavy metal(loid)s fractions, and L-HA is better in reducing the potential risk of Cd and As in contaminated soil. A higher dose of HA (L-HA3) is better in immobilizing Cd and As than L-HA1, and the BWB showed a similar effect on Cd but an opposite effect on As. Therefore, HA and BC are applicable amendments for the remediation of metal(loid)-contaminated soil. Still, their side effects should be assessed based on specific soil and amendment properties.