Mechanisms and influencing factors for electron transfer complex in metal-biochar nanocomposites activated...

The mechanisms and influencing factors for electron transfer complex need to be further studied by comparing radical and nonradical pathways. Herein, metal-biochar (BC) nanocomposites including CuO/BC, Fe₃O₄/BC and ZnO/BC were prepared to activate peroxydisulfate (PDS) for bisphenol A (BPA) degradation. The existence of electron transfer complex in CuO/BC-PDS system were directly demonstrated. Whereas radical pathway was dominant in Fe₃O₄/BC- and ZnO/BC-PDS systems for BPA degradation. There was a relationship between PDS adsorption and catalytic reaction. The rate-limiting step for BPA degradation in nonradical pathway was PDS adsorption, but in radical pathway was BPA degradation. Interestingly, among metal-BC, CuO/BC had the most effective performance in transformation of adsorbed PDS to electron transfer complex via out-sphere complexation. After pretreatment by PDS solutions, the separated CuO/BC achieved an efficiency of 60% in ensuing BPA degradation without re-addition of PDS. In addition, the activity of electron transfer complex in BPA degradation (k_obs > 0.0480 min^-1) was not affected by water matrix (e.g., Cl^-, HCO₃^-, natural organic matter (NOM) and actual water bodies), but affected by solution property (i.e., dissolved oxygen and conductivity) and oxidant species. Moreover, in BPA degradation process, nonradical pathway exhibited lower ecotoxicity instead of radical pathway.