Effects of biochar derived from sewage sludge and sewage sludge/cotton stalks on the immobilization and phytoavailability of Pb, Cu, and Zn in sandy loam soil

Highlights: • Biochar was prepared via pyrolysis of sewage sludge and sludge plus cotton stalks. • Heavy metal contaminated mining soils were amended with the biochars. • Biochars effectively immobilized heavy metals (Pb, Cu, Zn) in the soil. • Biochars reduced metal bioaccumulation by ryegrasses and promoted plant growth. • Co-pyrolysis of sewage sludge and cotton stalks shows promise for soil remediation. Co-pyrolysis of sewage sludge and straws has been used to improve the pore structure and reduce the ecological risks of heavy metals in sewage sludge-derived biochars. However, to date, no study has focused on the effects of biochar derived from sewage sludge/straws on the immobilization and phytoavailability of heavy metals in soil. Here, we studied the effects of biochar derived from sewage sludge/cotton stalks (SCB) and that derived from sewage sludge alone (SSB) on the remediation of sandy loam soil contaminated by Pb, Cu, and Zn. SCB amendment decreased the bioavailable forms of Pb, Cu, and Zn in the soil by 19.0%, 34.9%, and 18.2%, respectively, and reduced their accumulation in ryegrass by 28.6%, 50.1%, and 30.0%, respectively, compared with those by SSB amendment. Furthermore, SCB amendment transformed more metals from the acid-soluble fraction to the oxidizable fraction than SSB amendment, indicating that complexation played a more critical role in SCB amendment than in SSB amendment. Both biochar amendments effectively improved soil water holding capacity, increased the supply of available P, N, and K, and promoted ryegrass growth. The findings of this study show the benefits of SCB over SSB for the remediation of heavy metal-contaminated soil.