Mechanism of interaction between ascorbic acid and soil iron-containing minerals for peroxydisulfate activation and organophosphorus flame retardant degradation

Soil constituents may play an important role in peroxydisulfate (PDS)-based oxidation of organic contaminants in soil. Iron-containing minerals (Fe-minerals) have been found to promote PDS activation for organics degradation. Our study found that ascorbic acid (H2A) could enhance PDS activation by soil Fe-minerals for triphenyl phosphate (TPHP) degradation. Determination and characterization analyses of Fe fractions showed that H2A could induce the reductive dissolution of solid Fe-minerals and the increasing of oxygen vacancies/hydroxyl groups content on Fe-minerals surface. The increasing of divalent Fe (Fe(Ⅱ)) accelerated PDS activation to generate reactive oxygen species (ROS). Electron paramagnetic resonance (EPR) and quenching studies showed that sulfate radicals (SO4•−) and hydroxyl radicals (HO•) contributed significantly to TPHP degradation. The composition and content of Fe-minerals and soil organic matter (SOM) markedly influenced ROS transformations. Surface-bond and structural Fe played the main role in the production of Fe(Ⅱ) in reaction system. The high-concentration SOM could result in ROS consumption and degradation inhibition. Density functional theory (DFT) studies revealed that H2A is preferentially adsorbed at α-Fe2O3(012) surface through Fe–O–C bridges rather than hydrogen bonds. After absorption, H atoms on H2A may further is migrated to adjacent O atoms on the α-Fe2O3(012) surface. With the transformation of H atoms to the α-Fe2O3(012) surface, the Fe–O–C bridge is broken and one electron is transferred from the O to Fe atom, inducing the reduction of trivalent Fe (Fe(III)) atom. MS/MS2 analysis, HPLC analysis, and toxicity assessment demonstrated that TPHP was transformed to less toxic 4-hydroxyphenyl diphenyl phosphate (OH-TPHP), diphenyl hydrogen phosphate (DPHP), and phenyl phosphate (PHP) through phenol-cleavage and hydroxylation processes, and even be mineralized in reaction system.