Effect of the soil biochar aging on the sorption and desorption of Pb²⁺ under competition of Zn²⁺ in a sandy calcareous soil

Applying biochar in contaminated soils can reduce the availability of the heavy metals (HMs). However, there is no sufficient evidence regarding the effects of biochar on adsorption and desorption of lead (Pb²⁺) in the calcareous soils. Thus, the present study aimed to investigate the effect of walnut leaf biochars (1% w/w) produced at different temperatures (200 (B200), 400 (B400), and 600 (B600) °C) on the adsorption and desorption properties of Pb²⁺ in individual (Pb) and competitive (Pb + Zn) systems at two incubation times (30 and 90 days). The results showed that the maximum adsorption capacity of Pb²⁺ (qm) in the soils treated with B400 and B600 was higher than the soils treated with feedstock and B200. In the presence of Zn²⁺, the qm value decreased. Strength of adsorption (K_L) increased with an increment in the pyrolysis temperature, and was lower in competitive system than the individual system in all treatments. Also, the KL was lower in competitive system than the individual system in all treatments. Strength of Pb²⁺ adsorption in soils treated with B400 and B600 enhanced at 90 days compared to 30 days of incubation. Lead desorbed in 10 mM CaCl₂ (< 1% of adsorbed Pb²⁺) and DTPA–TEA solutions decreased with an increase in the pyrolysis temperature. The results revealed that walnut leaf biochars produced at high temperatures had a higher capacity and strength of Pb²⁺ adsorption, while lowering Pb²⁺ desorption compared with other biochars. The results of this study revealed that walnut leaf biochars produced at high temperatures could adsorb and immobilize Pb²⁺ in sandy calcareous soils at either individual or competitive systems.