Influence of Carbonate Speciation on Hydrated Electron Treatment Processes

Abstract

Advanced reduction processes (ARPs) that generate hydrated electrons (e_aq^-; e.g., UV-sulfite) have emerged as a promising remediation technology for recalcitrant water contaminants, including per- and polyfluoroalkyl substances (PFASs). The effectiveness of ARPs in different natural water matrices is determined, in large part, by the presence of non-target water constituents that act to quench e_aq^- or shield incoming UV photons from the applied photosensitizer. This study examined the pH-dependent quenching of e_aq^- by ubiquitous dissolved carbonate species (H₂CO₃*, HCO₃^-, and CO₃^2-) and quantified the relative importance of carbonate species to other abundant quenching agents (e.g., H₂O, H⁺, HSO₃^-, and O_2(aq)) during ARP applications. Analysis of laser flash photolysis kinetic data in relation to pH-dependent carbonate acid-base speciation yields species-specific bimolecular rate constants for e_aq^- quenching by H₂CO₃*, HCO₃^-, and CO₃^2- ($k_{H_2{CO}_3^{*}}$ = 2.23 ± 0.42 × 10⁹ M^-1 s^-1, $k_{{HCO}_3^{-}}$ = 2.18 ± 0.73 × 10⁶ M^-1 s^-1, and $k_{{CO}_3^{2-}}$ = 1.05 ± 0.61 × 10⁵ M^-1 s^-1), with quenching dominated by H₂CO₃* (which includes both CO_2(aq) and H₂CO₃) at moderately alkaline pH conditions despite it being the minor species. Attempts to apply previously reported rate constants for e_aq^- quenching by CO_2(aq), measured in acidic solutions equilibrated with CO_2(g), overpredict quenching observed in this study at higher pH conditions typical of ARP applications. Moreover, kinetic simulations reveal that pH-dependent trends reported for UV-sulfite ARPs that have often been attributed to e_aq^- quenching by varying [H⁺] can instead be ascribed to variable acid-base speciation of dissolved carbonate and the sulfite sensitizer.

Keywords: PFAS; acid−base speciation; advanced reduction process; hydrated electron; laser flash photolysis.