Degradation of phenol using Fe(II)-activated CaO₂: effect of ball-milled activated carbon (AC_BM) addition

In-situ chemical oxidation (ISCO) based on peroxide activation is one of the most promising technologies for removing organic contaminants from natural groundwater (NGW). However, use of the most common form of hydrogen peroxide (H₂O₂) is limited owing to its significantly rapid reaction rate and heat generation. Therefore, in the present study, the activation of calcium peroxide (CaO₂), a slow H₂O₂ releasing agent, by Fe(II) was proposed (CaO₂/Fe(II)), and the phenol degradation mechanisms and feasibility of NGW remediation were investigated. The optimum molar ratio of [phenol]/[CaO₂]/[Fe(II)] (phenol = 0.5 mM) was 1/10/10, resulting in 87.0-92.5% phenol removal within 120 min under a broad initial pH range of 3-9. HCO₃^-, PO₄^3-, and humic acid significantly inhibited degradation, whereas the effects of Cl^-, NO₃^-, and SO₄^2- were negligible. Reactive oxygen species (ROS) were identified based on the results of phenol degradation in the presence of scavengers and electron spin resonance (ESR) spectroscopy, which demonstrated that ¹O₂ played the dominant role, supported by ^•OH, in CaO₂/Fe(II). Phenol removal in NGW (67.81%) was less than that in distilled and deionized water (DIW, 92.5%) at a [phenol]/[CaO₂]/[Fe(II)] ratio of 1/10/10. However, phenol removal was significantly improved (∼100%) by increasing the CaO₂ and Fe(II) doses to 1/20/20-40. Furthermore, when 125-250 mg L^-1 of ball-milled activated carbon (AC_BM) was added (CaO₂/Fe(II)-AC_BM), phenol removal was enhanced from 67.81% to 90.94-100% in the NGW. CaO₂/Fe(II)-AC_BM exhibited higher total organic carbon (TOC) removal than CaO₂/Fe(II). In addition, no notable by-products were detected using CaO₂/Fe(II)-AC_BM, whereas the polymerisation products of hydroxylated and/or ring-cleaved compounds, that is, aconitic acid, gallocatechin, and 10-hydroxyaloin, were found in the reaction with CaO₂/Fe(II). These results strongly suggest that CaO₂/Fe(II)-AC_BM is highly promising for groundwater remediation, minimizing degradation byproducts and the adverse effects caused by the NGW components.