Effect of nitric acid modification on the lead(II) adsorption of mesoporous biochars with different mesopore size distributions

Two mesoporous biochars AC-1 and AC-2 with similar chemical properties but different mesopore size distributions were prepared to study the effect of HNO₃ modification on the lead(II) adsorption. AC-2 possesses higher mesopore volume and broader pore diameter than AC-1, while their surface area and micropore volume are similar. Adsorption experiments showed that AC-2 had far better removal efficiency, indicating the important role of mesopores played in the adsorption. HNO₃ modification enhanced the adsorption capacity of lead AC-1 and AC-2 by 15 and 27 mg g⁻¹, respectively. In particular, the removal rate of lead for AC-1 was improved from 46 to 99 % by HNO₃ treatment at a low initial lead concentration of 10 mg L⁻¹. Results of Boehm’s titration demonstrated that the amounts of oxygenic acid groups of AC-1 and AC-2 increased to 2.456 and 2.705 mmol g⁻¹ after HNO₃ treatment, respectively. Analyses of FTIR spectrum revealed that AC-2 was more likely to graft oxygen-containing acidic functional groups than AC-1, indicating that higher mesoporosity takes advantage of grafting more oxygenic functional groups, thus forming more active adsorption sites. The above results indicate that mesoporous biochars with wider pore width are more favorable to be introduced with oxygenic groups for enhanced lead removal efficiency.