Unraveling the mechanisms for persulfate-based remediation of triphenyl phosphate-contaminated soils: Complicated soil constituent effects on the formation and propagation of reactive oxygen species

Abstract

The continuous accumulation of triphenyl phosphate (TPHP) in soil poses a potential threat to human health. Persulfate-based advanced oxidation processes (SR-AOPs) are promising soil remediation technologies. Naturally occurring soil constituents could interact with persulfate (PS) during soil remediation by SR-AOPs. However, the production and transformation of reactive oxygen species (ROS) by synthetic reactions between PS and different soil constituents, as well as their roles in the pollutants degradation, are still largely unknown. This study systematically investigated the generation of ROS and their roles in degradation of TPHP in soil constituents activated PS process and in soil constituents-heat (50 °C) co-activated PS process. Comprehensive studies including ROS identification, electron paramagnetic resonance analysis, and ROS-generated model systems were carried out. An appreciable degradation rate of TPHP (~21.9%) in soil-PS system and the significant discrepancy in degradation rate (69.0–97.0%) in heat-soil-PS systems with three different soils suggested that soil constituents exhibited complex effects on the activation of PS. The Fe/Mn-minerals in soil effectively activated PS and facilitated TPHP degradation. Specially, the iron redox cycle (Fe(II) ↔ Fe(III)) plays a key role in ROS generation. Small molecular soil organic matter (SOM) rapidly consumed the produced ROS which led to their mineralization, while the remaining macromolecular SOM became more hydrophilic by interaction with ROS. SO4•−, •OH, and 1O2 played major roles directly in TPHP degradation, while O2•− could not degrade TPHP directly but might promote TPHP degradation by reducing Fe(III)- to Fe(II)-minerals or converting to other ROS. This study provides new and deeper insights into the interaction between PS and soil constituents and their roles in organic pollutant degradation in contaminated soil using SR-AOPs.