Electron transfer-based peroxydisulfate activation by waste herb residue biochar: Adsorption versus surface oxidation

Waste herb residue biochar (WBC) was synthesized to activate peroxydisulfate (PDS) for the removal of different micropollutants. WBC prepared at 700 C (WBC700) exhibits discrepant adsorption and PDS catalytic perfor-mance to three pharmaceuticals and four phenolic compounds. At 60 min reaction time, the adsorptive removal by 0.5 g/L WBC700 varied from 31.3% (clofibric acid) to 97.6% (4-chlorophenol), while the apparent removal ranged from 36.1% (clofibric acid) to 99.5% (4-chlorophenol) when 0.5 g/L WBC700 and 1.0 mM PDS were applied. The oxidation of the micropollutants involves electron transfer mechanism via the surface-confined metastable reactive complexes (WBC-PDS*), which was verified by electrochemical tests, quenching experiments, electron paramagnetic resonance, Fourier transform infrared and in-situ Raman spectroscopy. The observed pseudo first-order kinetic constant (k(obs)) of organic removal did not exhibit a good correlation with its electrochemical redox descriptor (half-wave potential, phi(1/2), for example) as reported in the previous studies. Instead, the surface oxidation rate constant (k(oxid)), determined from a dynamic model considering liquid-solid mass transfer of micropollutant and the sequent surface oxidation by WBC-PDS* complexes, was highly related to phi(1/2). In parallel, the liquid-solid mass transfer coefficient (K(L)a) of micropollutant was obtained from a similar model excluding the surface oxidation, and k(obs) showed a more significant association with K(L)a than with adsorption capacity (Q(e)). This study provides a promising approach to understand the role of adsorption and surface oxidation in an electron transfer-dominated persulfate activation process.