An in situ reactive zone approach using calcium peroxide for the remediation of benzene and chlorobenzene in groundwater: A field study

There is a gap in understanding the different contributions of biodegradation and free radical oxidation using calcium peroxide (CaO₂) for the remediation of mixed contaminants of benzene and chlorobenzene in groundwater. In this study, the remedial efficiency and mechanisms of benzene and chlorobenzene co-contaminants using CaO₂ were explored by an integrated approach of field study and laboratory validation. It was found that in the field demonstration program, the radius of influence for each injection point using Geoprobe direct-push was larger than the designed value of 0.75 m in the reactive zones created by CaO₂ supplemented with a buffer solution (Area A) and CaO₂ only (Area B). Both benzene and chlorobenzene were remediated to meet the cleanup goals within 5 months. The benzene and chlorobenzene concentration rebounds observed in monitoring wells were treated effectively with sustained effect of reagents. The laboratory validation experiments verified CaO₂ with a buffer solution could maintain the pH values within the range of 6.05-7.69, and higher DO concentrations for prolonged period. The contributions of biodegradation for benzene were 43.47% and 42.02% in CaO₂ group and CaO₂ adjusted with buffer solutions group, respectively, while those for chlorobenzene were 16.87% and 19.61%. In addition, it was demonstrated in the laboratory that the application of CaO₂ supplemented with a buffer solution had the best remediation efficiency for benzene and chlorobenzene, due to the contributions from both the free radicals HO• and the biodegradation of co-contaminants by the native microbial consortium. Furthermore, the intermediate byproducts, including phenol, 2-chlorophenol and pyruvate, were detected in groundwater collected in the field, and the biodegradation and oxidative degradation pathways of benzene and chlorobenzene with the application of CaO₂ were proposed. The microbial composition analyses for groundwater samples revealed that multiple functional bacteria, which are capable of degrading benzene and chlorobenzene, were enriched. The findings of the current study take one step further for the understanding of the fundamentals of CaO₂ as a slow oxygen releasing reagent, as well as its engineering applications for the remediation of organic contaminants in soil and groundwater.