Effect of alkaline lignin on immobilization of cadmium and lead in soils and the associated mechanisms

Lignin is a low-cost and environmental-friendly material and could increase the solubility of phosphorus (P) in soils. Meanwhile, application of P compounds to soils decreases the bioavailability of heavy metals. However, there are few reports on whether lignin-induced P release immobilizes heavy metals in soil. This study investigated this possibility by adding alkaline lignin to forest, paddy and upland soils differing in pH and available P. The amendment of alkaline lignin increased soil P availability and enhanced the adsorption and decreased the desorption percentages of Cd in acid forest and paddy soils. The P released from the soil could immobilize Pb and Cd but the presence of Pb decreased the adsorption capacity of Cd on the acid soils. In comparison, the alkaline lignin decreased Cd adsorption and raised Cd desorption in the alkaline upland soil, due to the formation of soluble complex of hydrophilic organic matter with Cd. In addition, precipitation, complexion, and competition effect among Cd, P and lignin in different soils led to various P concentrations in the experiment. The study suggests that alkaline lignin was effective in Cd/Pb immobilization partly via enhanced P availability in acid soils, but was ineffective in Cd immobilization in alkaline soils.