Immobilization of cadmium and lead using phosphorus-rich animal-derived and iron-modified plant-derived biochars under dynamic redox conditions in a paddy soil

Abstract

Functionalized biochar has gained extensive interests as a sustainable amendment for an effective remediation of paddy soils contaminated with heavy metals (HMs). We examined the efficiency of pig carcass-derived biochar (P-rich biochar, total P=8.3%) and pristine (raw biochar, total Fe=0.76%) and Fe-modified (Fe-rich biochar, total Fe=5.5%) green waste-derived biochars for the immobilization of cadmium (Cd) and lead (Pb) in a paddy soil under pre-defined redox conditions (Eh, from -400 to+300mV). Average concentrations (μg L-1) of dissolved Cd increased under reducing conditions up to 10.9 in the control soil, and decreased under oxidizing conditions to below the detection limit (LDL=2.7) in the raw and Fe-rich biochar treated soils. Application of the raw biochar decreased the concentrations of dissolved Cd by 43-59% under Eh≤-100mV, compared to the non-treated control, which was more effective than the Fe-rich biochar (31-59%) and the P-rich biochar (8-19%). The immobilization of Cd under low Eh might be due to its precipitation with sulfide (S2-), whereas its immobilization under high Eh might be due to the associated increase of pH. Concentrations (μg L-1) of Pb ranged from 29.4 to 198.2 under reducing conditions, and decreased to LDL (12.5) under oxidizing conditions. The P-rich biochar was more effective in immobilizing Pb than the raw and Fe-rich biochars, particularly under Eh≤0mV (55-82%), which might be due to the retention of Pb by phosphates. The raw and Fe-rich biochars immobilized Pb under low Eh (≤ -300mV), but both biochars, particularly the Fe-rich biochar mobilized Pb under Eh higher than -200mV, especially at+100mV, due to the decrease of pH at this point (pH=6.0 to 6.5). These results improved our understanding of using P-rich and Fe-rich functionalized biochars for the immobilization of Cd and Pb in a paddy soil under stepwise redox changes. The amendment of P-rich pig carcass-derived biochar to paddy soils could be a promising approach for mitigating the risk of Pb for human health and the environment. The raw and Fe-rich green waste-derived biochars can be used for immobilizing Cd and mitigating its risk in paddy soils under both reducing and oxidizing conditions.