Selective adsorption of copper by amidoxime modified low-temperature biochar: Performance and mechanism

Biochars prepared at 300-700 °C were functionalized with amidoxime groups to evaluate their selective adsorptive removal capabilities towards Cu(II), Cd(II), and Pb(II). The results show that the amidoxime modification significantly enhanced the the Cu(II) adsorption capacity of the biochar prepared at 300 °C (AOBC300) by 1.6 times, reaching 0.61 mmol/g. In binary and ternary heavy metal solutions, AOBC300 exhibited preferential adsorption of Cu(II), followed by Pb(II) and Cd(II). High salinity, alkaline earth metal ions, humic substances, and other metal cations had minimal interference on the adsorption of heavy metals by AOBC300. Sample characterization revealed that amidoxime modification reduced the zeta potential and increased the hydrophilicity of the biochar. XPS analysis demonstrated that both N and O atoms of the amidoxime group are involved in the adsorption process, contributing to AOBC300's strong affinity for heavy metal ions. DFT calculations further confirmed the adsorption preference of different heavy metals on AOBC300. This study demonstrates that amidoxime grafting is an effective protocol for refining low-temperature biochar, aimed at efficiently eliminating heavy metals from waste streams.