Competitive sorption of Pb, Cd, and Zn on soil-biochar mixtures: Kinetic studies and post-sorption analysis

This study analyzed the sorption kinetics of Pb, Zn, and Cd in mixtures of a Brazilian Ultisol with 2.5% and 5.0% biochar from urban pruning waste and the chemical speciation of these metals after sorption, toward their use as liners in mining waste disposal areas. Sorption kinetics experiments, desorption in water, and post-sorption sequential chemical extractions were conducted. The pseudo-first-order model provided the best fit for Zn sorption kinetics in the studied materials compared to Cd and Pb. The soil and soil-biochar mixtures displayed greater affinity for Pb than for Cd and Zn. Although biochar incorporation reduced metal desorption in water, sequential extraction revealed most retained metals remained in soluble and exchangeable fractions, posing a potential environmental risk. Among the studied metals, Cd exhibited the highest mobility, indicated by its lower sorption, higher desorption, and greater proportion in the soluble/exchangeable phase (up to 89.4%). Overall, biochar derived from urban pruning waste effectively enhanced Pb, Cd, and Zn retention in the Brazilian Ultisol; however, potential remobilization remains a concern, requiring further investigations.