Phosphoric acid-activated biochars from orange and potato peels as adsorption barriers to prevent picloram and imidacloprid leaching in contaminated soils

Abstract

This study investigated the potential of H₃PO₄-activated biochar from agricultural waste as an adsorption barrier to mitigate groundwater contamination by picloram and imidacloprid. The biochars exhibited high surface areas, mesoporous structures, and high porosity. The maximum adsorption capacity and influence of experimental variables (adsorbent mass, initial concentration, pH, contact time, and temperature), along with their interactions, were assessed using response surface methodology. Predicted maximum capacities were 51 µg mg⁻¹ for picloram and 66 µg mg⁻¹ for imidacloprid. The most significant effects were the adsorbent mass and initial concentration, whereas temperature was non-significant. Although pH significantly affected the adsorption of picloram, adsorption remained high under environmentally relevant pH conditions. Kinetic and equilibrium data were best described by the pseudo-second order and Freundlich models, respectively, indicating rapid uptake and heterogeneous multilayer adsorption. Furthermore, 96% of imidacloprid and 65% of picloram remained adsorbed after three desorption cycles. Matrix effects, evaluated in soils with negligible adsorption capacity, were more pronounced for picloram. Nevertheless, amending with a low dose of biochar (0.5% w/w) increased K_d values by 8 (picloram) and 120-fold (imidacloprid). In soil column experiments (100 g), the addition of a low-dose biochar barrier (20 mg for imidacloprid and 100 mg for picloram, mixed with 1 g of soil) effectively limited leaching to 12–24% and 25–30%, respectively, depending on the soil type. Overall, these results demonstrate that biochar derived from agricultural waste is an effective and sustainable amendment for pesticides immobilization, providing a promising strategy for mitigating soil and groundwater pollution.