Aged biochar for simultaneous removal of Pb and Cd from aqueous solutions: Method and mechanism

In this study, we prepared maize stover biochar and used artificial simulated aging to prepare three types of aged biochar. The effects of aging on the adsorption capacity of Pb and Cd of biochar were studied. It was found that the pH, ash, and aromaticity of the aged biochar decreased compared to fresh maize straw biochar. On the contrary, the polarity, oxygenated functional groups, specific surface area, and average pore size of the aged biochar increased. The sorption behavior of Pb2+ and Cd2+ on biochar can be well explained by quasi-second-order kinetics and the Langmuir equation. The maximum adsorption of Pb2+ in the single metal system was CBC-N (74.401 mg g−1)>CBC (72.959 mg g−1)>CBC-H (69.421 mg g−1)>CBC-F (66.460 mg g−1), the maximum adsorption of Cd2+ was CBC-N (52.983 mg g−1)> CBC-H (52.728 mg g−1) > CBC-F (50.692 mg g−1) > CBC (45.966 mg g−1). The adsorption of both Pb2+ and Cd2+ by biochar is lower in the binary metal system than in the single metal system, implying a competitive relationship and that biochar is more selective for Pb2+ than Cd2+. The mechanisms for removing Pb2+ and Cd2+ from solution by aged biochar consist mainly of pore filling, electrostatic adsorption, ion exchange, mineral precipitation, π−π interaction, and surface complexation. This paper demonstrates that aged biochar is an ideal adsorbent but that environmental changes and pollutant types should be considered in practical applications.