Comparative assessment of biochars from multiple sources based on persulfate activation capability: Role of iron component in enhancing thermal treatment effect on carbocatalytic performance

In this study, we explored the correlation between the catalytic activity of UK Biochar Research Center biochars (BCs) and their physicochemical properties, suggesting a potential role of endogenous iron in promoting their persulfate activation capability by heat treatment. A steady improvement in the persulfate activation efficiency with increasing annealing temperature was observed exclusively for iron-containing BCs (e.g., sewage sludge (SS) BCs). Along with increasing the Brunauer–Emmett–Teller surface area, thermal annealing formed carbon-wrapped iron particles on the SS BC surfaces, produced crystalline elemental iron and iron carbides, and caused an sp³-to-sp² conversion in the carbon phase. Comparing the annealed SS BCs in terms of the open circuit potential shift, electrical conductivity, and thermodynamic binding parameters showed that the heat-initiated modification enhanced the electron transfer-mediating capacity and surface affinity toward persulfate, which led to the beneficial effect of annealing on the carbocatalytic activity of iron-containing BCs for non-radical persulfate activation.