Walnut Shell Biomass Triggered Formation of Fe3C-Biochar Composite for Removal of Diclofenac by Activating Percarbonate

Percarbonate (SPC) as a promising substitute for liquid H2O2 has many advantages in the application of in situ chemical oxidation (ISCO). Developing efficient, cost effective and environmentally friendly catalysts for SPC activation plays the key role in promoting the development of SPC-based ISCO. Herein, the walnut shell biomass was combined with ferric nitrate for the catalytic synthesis of Fe3C@biochar composite (Fe3C@WSB), which demonstrated high efficiency in activating SPC for the removal of diclofenac (DCF). The Fe3C showed average crystallite size of 32.6 nm and the composite Fe3C@WSB demonstrated strong adsorptivity. The prepared Fe3C@WSB could activate both SPC and H2O2 with high efficiency at ca. pH 3 with extremely low leaching of iron, while in a weak acidic condition, higher efficiency of DCF removal was obtained in the Fe3C@WSB/SPC process than in the Fe3C@WSB/H2O2 process. Moreover, the Fe3C@WSB/SPC and Fe3C@WSB/H2O2 processes did not show significant differences when supplied with varying amounts of catalyst or oxidant, but the Fe3C@WSB/SPC process exhibited stronger capability in dealing with relatively highly concentrated DCF solution. Based on quenching experiments and electron spin resonance (ESR) analysis, heterogeneous activation of SPC was assumed as the dominant route for DCF degradation, and both the oxidation by radicals, including •OH, •O2− and CO3•−, combined with electron transfer pathway contributed to DCF degradation in the Fe3C@WSB/SPC process. The cycling experiment results also revealed the stability of Fe3C@WSB. This work may cast some light on the development of efficient catalysts for the activation of SPC.