Carboxymethyl-Β-Cyclodextrin Enhanced Persulfate/Fe2+ System for Organic Contaminant Removal in Groundwater: Performance, Influencing Factors, and Mechanisms

Abstract

The removal of contaminants by persulfate (PDS)/Fe2+ system is a promising groundwater decontamination technology, while the non-recyclability of Fe2+ and the restricted mass transfer of contaminants in water limit the decontamination efficiency. Here, we developed carboxymethyl-β-cyclodextrin (CM-β-CD) to chelate Fe2+ coupled with enhancing the solubization of contaminants in groundwater for efficient decontamination. Results showed that a 4.60-fold enhancement in the perchloroethylene (PCE) degradation rate following the addition of 1 g/L CM-β-CD to the PDS/Fe2+ system. The ternary complex formed by CM-β-CD, Fe2+ and PCE was confirmed via ultraviolet absorption spectra and nuclear magnetic resonance hydrogen spectrum. Electron paramagnetic resonance, complemented by chemical probe and radical quenching experiments, identified singlet oxygen (1O2), high-valent iron (Fe(IV)), and hydroxyl radicals (•OH) as the dominant reactive species in the PDS/Fe2+/CM-β-CD system, with relative contributions to PCE degradation quantified at 30.37%, 26.15%, and 18.08%, respectively. The system exhibited reduced PCE degradation efficiency under neutral pH conditions and at lower temperatures. Notably, the presence of HA and common inorganic ions (Cl−, SO42−, NO3−, Na+, K+, Ca2+, and Mg2+) showed negligible effects on PCE degradation efficiency. The primary degradation intermediate of PCE in the PDS/Fe2+/CM-β-CD system was identified as dichloromethane, with the degradation pathways predominantly mediated by 1O2, Fe(IV), and •OH, and PCE could undergo dechlorination and mineralization during the degradation. This study developed an environmentally sustainable remediation strategy utilizing CM-β-CD as a dual-functional agent for simultaneous Fe2+ chelation and contaminant solubilization, enabling efficient groundwater decontamination.

Keywords: Persulfate and iron (II) system, carboxymethyl-β-cyclodextrin, ternary complex, Reactive species, Influencing factors