Effects of sludge‐based biochar application on heavy metals bioavailability and migration–transformation in coastal saline soil

Abstract The objective of this study is to evaluate the environmental risks posed by heavy metals during the remediation of coastal saline soil using sewage sludge and sludge‐based biochar, with a view to providing a theoretical basis for the sustainable use of coastal saline soil and sludge recycling. Pot and soil column leaching experiments were conducted to investigate the effects of sewage sludge, sludge‐based biochar, and their combinations on maize growth, the bioavailability, migration, and transformation of heavy metals in soil. The results indicated that sewage sludge and sludge‐based biochar application promoted maize growth and increased heavy metal accumulation as well as the content of available heavy metals in soil. In comparison with sewage sludge, the application of sludge‐based biochar, either alone or in combination with sludge, reduced the accumulation of heavy metals in maize plants. The maximum reductions in the concentrations of Cd, Cr, Cu, and Zn in the aboveground parts and root of maize plants reached 37.4%, 72.4%, 76.2%, 37.3%, and 22.2%, 27.5%, 31.6%, and 27.9%, respectively. Sludge‐based biochar application significantly reduced soil available metal concentration, as well as the proportion of exchangeable and reducible metal fractions. Furthermore, sludge‐based biochar application significantly reduced the transport capacity of heavy metals in soil. The maximum reductions in cumulative leaching of Cd and Cu reached 34.8% and 24.0%, respectively. Overall, sludge‐based biochar application not only promoted maize growth but also significantly reduced the bioavailability of heavy metals in coastal saline soil, thereby enhancing heavy metal stabilization and mitigating their environmental risks.