Potassium phosphate/magnesium oxide modified biochars: Interfacial chemical behaviours and Pb binding performance

Removal of lead (Pb) from aqueous solutions by biochar is a promising method. In this study, wheat straw biochar (WBC) was modified by phosphate/magnesium via pre-treatment of biomass and post-treatment of biochar, noting as WBC_PMA and WBC_PMB, respectively. Based on Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses, phosphate/magnesium chemically bound to the structures of biochar surface, increasing the contents of polar groups (i.e., -COOH and -OH) and phosphorus-containing compounds, mainly Mg₃(PO₄)₂ and Mg₂P₂O₇. Owing to pyrolysis process enhancing loading ability of phosphate/magnesium, WBC_PMA possessed more active functional groups than WBC_PMB. Results showed that maximum sorption capacity of Pb was improved by modifications, following the sequence of WBC_PMA (470.09 mg/g) > WBC_PMB (308.39 mg/g) > WBC (59.93 mg/g). Pseudo-second-order kinetics and thermodynamics study indicated that chemisorption was involved in sorption process. Precipitation, complexation and cation exchange dominated Pb sorption and the corresponding contributions accounted for 17.89-32.73%, 28.84-46.22%, and 21.05-53.27%, respectively. Additionally, desorption characteristics of Pb illustrated that WBC_PMA owned more prominent stabilization ability than that of WBC and WBC_PMB. The findings of this study suggested that pre-modification method increased the contents of active groups in biochar and strengthened the removal efficiency of Pb ultimately. Due to the complexity of the actual Pb-containing wastewater environment, it was necessary to evaluate the effects of various factors on the stabilization performance of the pre-modified biochar in further.