Degradation of phenol by ball-milled activated carbon (AC_BM) activated dual oxidant (persulfate/calcium peroxide) system: Effect of preadsorption and sequential injection

This study explored pre-adsorption and sequential injection of dual oxidant (DuOx) of persulfate (PS) and calcium peroxide (CP) for phenol degradation in an aqueous solution. Ball-milled activated carbon (AC_BM) was used as the catalyst in the following systems: pre-adsorption and sequential injection of PS and CP (AC_BM + PS + CP), pre-adsorption and simultaneous injection of PS and CP (AC_BM + PS/CP), simultaneous injection of AC_BM, PS, and CP (AC_BM/PS/CP), simultaneous injection of AC_BM and PS (AC_BM/PS), and simultaneous injection of AC_BM and CP (AC_BM/CP). The AC_BM had a larger specific surface area, more graphitic structures, and more defects. Moreover, it showed better phenol removal when introduced simultaneously with PS and CP. The phenol removal was most the efficient in AC_BM + PS + CP (98.8%) with a near-neutral final pH, followed by AC_BM + PS/CP, AC_BM/PS, AC_BM/PS/CP, and AC_BM/CP. This indicates that pre-adsorption and separate injection of PS and CP were the key strategy for improved performance and maintained favorable pH for the activation of PS and CP. The dual oxidant system (PS/CP) is superior to single oxidant systems (PS or CP). Scavenger experiments and the electron spin resonance spectra (ESR) demonstrated that non-radical species (¹O₂) were dominantly involved in AC_BM + PS + CP, but radical species (HO^•, SO₄^•−) also contributed. HCO₃⁻ and HPO₄²⁻ inhibited phenol degradation in AC_BM + PS + CP, whereas Cl⁻ and HA had negligible effects. The AC_BM + PS + CP showed high total organic carbon removal and AC_BM was recyclable with a slight decrease in activity. This work is important as it provides a detailed insight into the strategy of pre-adsorption and sequential injection of dual oxidants for a practical and cost-effective method of groundwater remediation.