Mechanisms insights into Cd passivation in soil by lignin biochar: Transition from flooding to natural air-drying

Biochar shows great potential in soil cadmium pollution treatment, however, the effect and mechanisms of biochar on cadmium passivation (CP) during the long-term process of soil from flooding to natural air-drying are not clear. In this study, a 300-day experiment was conducted to keep the flooded water level constant for the first 100 days and then dried naturally. Mechanisms of CP by lignin biochar (LBC) were analyzed through chemical analysis, FTIR-2D-COS, EEMs-PARAFAC, ultraviolet spectroscopy characterizations, and microbial community distribution of soil. Results showed that application of LBC results in rapid CP ratio in soil within 35 days, mainly in the residual and Fe-Mn bound states (total 72.80%). CP ratio further increased to 90.89% with water evaporation. The CP mechanisms include precipitation, electrostatic effect, humus complexation, and microbial remediation by promoting the propagation of fungi such as Penicillium and Trichoderma. Evaporation of water promoted the colonization of aerobic microorganisms and then increased the degree of soil humification and aromatization, thereby enhancing the cadmium passivation. Simultaneously, the biochar could reduce the relative abundance of plant pathogens in soil from 1.8% to 0.03% and the freshness index (β/α) from 0.64 to 0.16, favoring crop growth and promoting carbon sequestration and emission reduction.