Rice straw biochar and lime regulate the availability of heavy metals by managing colloid-associated- but dissolved-heavy metals

Heavy metal (HM) pollution has extensively spread in agricultural soils, posing potential threats to food safety and human health. Biochar and lime are two amendments used to remediate the soils contaminated with HMs. However, colloids have been shown to increase the mobility of HMs in paddy soils. Nevertheless, limited investigations have been made into the impact of biochar and lime on the formation of colloid-associated (colloidal) HMs in paddy soils. In this study, column and microcosm incubation experiments were conducted to examine how biochar and lime affected the availability of HMs (arsenic, cadmium, copper, iron, manganese, lead, and zinc) in different layers of paddy soils. The results revealed that biochar significantly inhibited the formation of colloidal HMs in the soil flooding phase, whereas the lime increased the colloidal HMs. These colloids containing HMs were identified as poorly dissolved metal sulfides. When the soil was drained, colloidal HMs transformed into dissolved forms, thereby improving the availability of HMs. Biochar decreased HM availability by reducing colloidal- but dissolved- HMs, whereas lime had the opposite effect. Hence, biochar demonstrated a stable and reliable remediation ability to decrease HM availability in paddy soil during flooding and drainage processes. In conclusion, this study highlighted that biochar efficiently reduced HM availability by mitigating the formation of colloidal HMs during flooding and their transformation into dissolved HMs during drainage in paddy soils.