Enhanced removal efficiency of Cd2+ and Pb2+ from aqueous solution by H3PO4–modified tea branch biochar: Characterization, adsorption performance and mechanism

Biochar has been widely recognized as an environmentally efficient adsorbent for removal of heavy metals in wastewater. However, inferior adsorption capacity limits its practical application. In this study, a H3PO4–modified tea branch biochar (PTBB) was firstly prepared through the one step pyrolysis of H3PO4-pretreated tea branch powder for enhancing removal efficiency of Cd2+ and Pb2+ from aqueous solutions. H3PO4 modification altered the physicochemical properties of pristine biochar. The PTBB exhibited superior adsorption performance. Its maximum adsorption capacities of Cd2+ and Pb2+ were 98.25 mg g−1 and 127.5 mg g−1 at the pH 6 and dosage of 2 g L−1, which was 1.5 and 1.3 folds of the pristine biochar, respectively. Cd2+ and Pb2+ adsorption data by PTBB were well fitted by the Langmuir isothermal and pseudo-second order kinetic models, indicating monolayer chemical adsorption controlled the adsorption process. The adsorption of Cd2+ and Pb2+ by PTBB was a spontaneous endothermic process and less affected by coexisting cations. PTBB demonstrated a greater affinity for Pb2+ as compared with Cd2+ in binary metal systems. Moreover, PTBB could maintain good stability and regeneration properties. The adsorption of Cd2+ and Pb2+ by PTBB was attributed to cation exchange, precipitation, complexation, and π electron interactions. The quantitative analysis indicated that complexation and precipitation were the primary adsorption mechanisms, which accounted for 50.52–50.56% and 36.7–42.18% to total Cd2+ and Pb2+ adsorption for PTBB, respectively. Therefore, PTBB exhibits great potential for application in the remediation of wastewater containing Cd2+ or Pb2+.