Insight into the mechanism of pyrolysis temperature on the performance of biochar-based MOF materials in capacitive deionization for heavy metal removal

Removing heavy metals from electroplating wastewater and selectively capturing precious metals is a significant challenge. This work combines biochar with MOF materials to design a green CDI electrode with high adsorption capacity and selectivity. The CDI electrode realizes the efficient separation of metal ions through the synergistic action of physical and chemical adsorption. The experimental results show that it can achieve 99 % selective recovery of Ag + and 100 % selective removal of Pb 2+ in mixed metal solutions. In comparison, the removal capacity of other metal ions does not exceed 7 %, such as Al 3+ , Cu 2+ and Zn 2+ . This work also studied the regulation mechanism of the material at different temperatures. Experimental results and theoretical calculations show that with the increase of pyrolysis temperature, the metal ions in the biochar-based MOF material are gradually converted into metal oxides (300 °C), metal nitrides (600 °C), and pure metals (900 °C). The generated metal oxides and metal nitrides have a higher affinity for Pb 2+ and significantly enhance the electrode material's charge storage capacity and capacitance. The life cycle assessment results show that this strategy's impact on the environment is significantly smaller than that of the current industrial electrocoagulation technology, providing a new idea for the harmless and resource-based treatment of electroplating wastewater.