Influence of biochar on the partitioning of iron and arsenic from paddy soil contaminated by acid mine drainage.

Paddy fields contaminated by arsenic-containing acid mine drainage (AMD) may also have rich iron in soil. Whether this iron can be loaded onto biochar to passivate the dissolved arsenic is worth further exploration. Soil was mixed with biochar prepared at 400, 550, and 700 °C and incubated under alternating anaerobic and aerobic conditions. Soil, soil solution and biochar samples were analysed using ICP-MS, FTIR, SEM, XPS, etc. The results showed that biochar prepared at lower pyrolysis temperatures contained a higher number of functional groups. Under the combined action of microorganisms, primarily from the Firmicutes phylum, biochar promoted the dissolution of arsenic-containing iron oxides in soil, with the residual arsenic also undergoing transformation. The biochar rapidly loaded dissolved iron onto its surface, likely in the form of Fe3O4 and FeOOH, and adsorbed arsenic primarily as As(III). Although the iron oxides detached over time, they were more stable on the biochar prepared at 400 °C compared to those prepared at higher pyrolysis temperatures. Meanwhile, the arsenic content on the biochar increased, raising the As/Fe molar ratio to above that of the soil. This study lays the foundation for further exploring the long-term use of biochar in AMD-contaminated paddy fields.