Urban Pruning Waste Biochar Enhances Lead, Cadmium, and Zinc Sorption: A Potential Strategy for Soil Liners in Ribeira Valley (Brazil)

This study assessed the effect of incorporation of urban pruning waste biochar at 2.5% and 5.0% (w/w) on the sorption and desorption behavior of lead (Pb), cadmium (Cd), and zinc (Zn) in an Ultisol from Ribeira Valley (Brazil), regarding its potential for improving soil-based liners in waste disposal areas. Batch equilibrium sorption and desorption tests were conducted for Pb, Cd, and Zn using a soil-to-solution ratio of 1:20 (w/v), and 24-h contact time. All experiments were performed in triplicate. The materials showed more affinity for Pb compared to Cd and Zn. The maximum Pb, Cd, and Zn sorption capacities increased from 1164.5 ± 188.9 μg g−1, 1659.5 ± 112.0 μg g−1, and 549.5 ± 173.6 μg g−1, respectively, in unamended soil to 1898.6 ± 114.0 μg g−1, 1858.7 ± 126.3 μg g−1, and 790.7 ± 404.4 in the mixture with 2.5% biochar, and to 2786.8 ± 286.9 μg g−1, 2094.2 ± 101.2 μg g−1, and 1653.2 ± 104.5 μg g−1 in the mixture with 5.0% biochar. Increasing the biochar content also reduced the amount of metal desorbed in water. Such an improvement in metal retention is likely related to the high pH of biochar (10.0), the ion exchange with biochar-derived calcium (Ca), magnesium (Mg), and potassium (K), and the deprotonation of functional groups such as carboxyl, hydroxyl, and phenolic ones on the biochar particle surface. The urban pruning waste biochar has potential to improve the retention of potentially toxic metals in soil, which may contribute to both sustainable waste management and environmental protection objectives.