[Passivation of Cadmium and Arsenic in Acidified Paddy Soil by Calcium Fertilizer with Biochar-ferromanganese Composites]

Due to the aggravation of atmospheric nitrogen and sulfur deposition and the unreasonable application of fertilizer, soil acidification is becoming increasingly serious. In heavy metal-contaminated soils, acidification not only seriously affects fertility but also the effectiveness and sustainability of conventional passivation remediation materials such as biochar. The application of calcium fertilizer may improve soil acidification, alleviate the aging of biochar materials in soil, and improve its remediation ability to composite polluted soil. However, the mechanism of its effect is still unclear. Based on this, this study selected iron-manganese oxide （FM） and hickory cattails biochar （BC） to prepare biochar-ferromanganese composites （BFM） and conducted simulated acidification on it. Through characterization and an aqueous adsorption test, the changes in physicochemical properties and adsorption properties of the material after acidification were explored. Then, orthogonal tests were carried out to explore the effects of the combination of BFM and calcium fertilizer on soil pH and the availability of Cd and As under acidification conditions and to obtain the best combination scheme. The results showed that the optimal ratio of BC and FM in BFM was 7∶3 （quality ratio）, the removal rates of Cd（Ⅱ） and As（Ⅲ） were as high as 94.58% and 97.14%, respectively, and the adsorption capacities were 120.74 mg·g^-1 and 129.29 mg·g^-1 （solid-liquid ratio 1∶500）. After acidification treatment, the pore structure of BFM surface decreased, and the types and quantities of functional groups changed, resulting in the removal rates of Cd（Ⅱ） and As（Ⅲ） in aqueous solution decreasing by 73.97%-92.84% and 73.56%-93.61%, respectively. The combined application of calcium fertilizer and BFM could significantly increase soil pH, with an increase range of 3.06%-37.84%. The effect of increasing pH decreased with the increase in culture time and acidification degree. Compared with that in the blank control, the content of available Cd in soil was significantly reduced by 22.67%-97.78%. The content of available As in soil was generally stable. According to the effect curve analysis of the orthogonal test results, under the condition of weak acidification degree （pH=5.6）, the application of 2% supplemental amount with 2% silica-calcium fertilizer and 2% calcium-magnesium phosphate fertilizer had a good passivation effect on soil Cd. Under the condition of strong acidification degree （pH=4.0）, the application of 2% supplemental amount and 2% silica-calcium fertilizer had a good passivation effect on soil As. In summary, simulated acidification will affect the adsorption performance of BFM, and calcium fertilizer combined with it can increase soil pH, improve soil acidification, alleviate the aging of BFM in acidified soil, and improve its repair ability to heavy metal-polluted soil.