Contribution of Soluble Minerals in Biochar to Pb Adsorption in Aqueous Solutions

Biochar is widely used as an adsorbent to remove heavy metals from aqueous solutions. To investigate the contribution of soluble minerals (mainly anions) to Pb2+ removal in solution, wheat straw biochar was washed with deionized water to remove soluble minerals. Batch adsorption was conducted using washed biochar (WBC) and unwashed biochar (BC) to absorb Pb2+. After washing, the pH and ash content of biochar were reduced, while the specific surface area and total pore volume were increased. Adsorption kinetics of Pb2+ onto BC and WBC were well fitted to the pseudo-second-order model (R2 > 0.99). Pb sorption on BC and WBC were better fit with the Langmuir model (R2 = 0.96 to 0.97) than the Freundlich model (R2 = 0.71 to 0.87). The Langmuir maximum adsorption capacity of Pb on BC was 99.7 mg g-1, which was 4.5-fold higher than that on WBC when the initial solution pH was 5.0. The concentration of SO4, CO3, SiO3, and PO4 in the equilibrium solution was reduced by 69, 89, 97, and 41%, respectively, with the increase of initial Pb2+ concentration. The difference of Pb2+ adsorption capacity between BC and WBC proved that the soluble anions in biochar play an important role in Pb2+ sorption onto biochar.