Contribution of Soluble Minerals in Biochar to Pb2+ 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 SO42-, CO32-, SiO32-, and PO43- 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.