Oxidative ageing of biochar and hydrochar alleviating competitive sorption of Cd(II) and Cu(II)

Biochar and hydrochar have been served as attractive adsorbents for remediation of polluted water and soil, but it is lack of the long-term ageing effects on competitive adsorption of co-existing heavy metals by these carbonized materials. By this, corn stalk was used as carbon precursor to prepare biochar (500 °C) and hydrochar (200 °C). The single-metal and binary-metal Cd(II)/Cu(II) sorption were conducted on biochar and hydrochar before and after ageing using artificial accelerated ageing of 5% H₂O₂ treatment. The elemental analysis, BET, SEM, FTIR, XRD and Zeta potential were used to characterize the physicochemical properties of carbonized material samples. The results showed that oxidative ageing could increase O content and O-containing functional groups but decrease C content, metal content and aromaticity degree. Ageing hardly affected the SSA and crystallographic structures of biochar and hydrochar. The reduction of metal content in Aged-BC caused a decline of sorption capacity, indicating that cation exchange would be the predominant factor involved in biochar sorption for Cd(II) and Cu(II). As for hydrochar with more O-containing functional groups than biochar, the dominated sorption mechanism would be surface complexation, due to higher sorption capacity of Aged-HC with richer O-containing functional groups. In binary-metal system, the competitive sorption of Cd(II) and Cu(II) on biochar was observed obviously but that on hydrochar was limited. Ageing could increase the sorption capacity of Cd(II) in binary-metal system, resulting in alleviating competitive adsorption. The total sorption amount of Cd(II) and Cu(II) by biochar was markedly greater than that of hydrochar before or after ageing, suggesting that biochar can be still more capable than hydrochar for handling Cd(II) and Cu(II) in single-metal or binary-metal. These findings suggest us to consider the long-term effect on immobilization of co-existing heavy metals and alleviating competitive adsorption of carbonized materials as alternative amendment for contaminated sites.