Complexation and reduction of soil iron minerals by natural polyphenols enhance persulfate activation for the remediation of triphenyl phosphate (TPHP)-contaminated soil

Peroxydisulfate-based in-situ chemical oxidation (PDS-ISCO) is a promising technology for soil remediation. The weak potential of soil constituents for PDS decomposition results in the low degradation efficiency of soil con-taminants. Herein, we propose a new soil remediation strategy by accelerating the activation efficiency of PDS using chelating and reducing reagents (CRs) to complex and reduce soil Fe-minerals. Experimental results demonstrated that the introduction of tea polyphenols (TP) into soil-PDS system could effectively improve the reduction of Fe-minerals and the degradation of triphenyl phosphate (TPHP) in soil. With increasing TP con-centration from 0 to 40 mM, the degradation efficiency of TPHP increased from 10.8% to 58.6% in an upland soil (T1 soil). TP could directly complex and reduce the solid-phase Fe-minerals, leading to a significant increase of Fe(II) content, especially the insoluble Fe(II) content, which could further induce the PDS activation. Results of radical quenching experiments, electron paramagnetic resonance (EPR) analysis, and kinetic solvent isotope effect (KSIE) evaluation showed that sulfate radicals (SO4 & BULL;-, especially surface-bound SO4 & BULL;-), hydroxyl radicals (HO & BULL;, especially surface-bound HO & BULL;), and singlet oxygen (O-1(2)) played major roles in TPHP degradation, with the estimated contributions of 39.03%, 43.99%, and 16.98%, respectively. The higher content of high active Fe fractions, such as amorphous Fe, the larger yields of Fe(II) species and TPHP degradation. Increased content of soil organic matter (SOM) and pH value from 2.5 to 8.0 may reduce TPHP degradation. The effects of inorganic anions and Mn-minerals, presented in natural soil, on TPHP degradation were very limited. Simulated in-situ soil remediation using soil column confirmed that this strategy is suitable for the large-scale remediation of contaminated soil.