Impact of Biochar on Release Kinetics of Pb (II) and Zn (II) in a Calcareous Soil Polluted with Mining Activities

Abstract

Release kinetics governs the detachment and migration of potentially toxic trace elements (PTEs) over time between the solid phase and soil solutions. The assessment of PTEs’ release behavior through time supports more accurate prediction models of toxicity in highly polluted soils. Recently, biochar, a charcoal black carbon ameliorant, was extensively applied for decreasing the release of Zn and Pb to the soil solution. This study was arranged during a short-term (45 days) incubation experiment to estimate the possible effects of walnut leaves (WL) and their biochars produced at three temperatures (200 (B200), 400 (B400), and 600 (B600) °C) on the kinetics of Zn and Pb release in a naturally calcareous and highly polluted soil. Results showed that the biochars could reduce the release of Zn and Pb to the soil solution. The rate of Zn release (“b” in power function) at different biochar levels was 0.35 (2% B600)–0.38 (control) (mg kg−1)−1, respectively; also, the release of Pb was 0.23 (2% B600)–0.33 (control) (mg kg−1)−1, respectively. The results illustrated that a lower concentration of these metals was associated with exchangeable and oxides of Fe-Mn, whereas the higher content of residual fractions of Zn and Pb reduced the release of Zn and Pb in soils treated with biochar. Overall, the more the level and temperature of biochar increased, stabilizing PTEs through changing the distribution of metals in the soil, the more biochars succeeded significantly in lessening the release of PTEs, which ultimately could retard the further toxicity in the soil solution.