Influence of Carbonate Speciation on Hydrated Electron Treatment Processes.

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 (H 2 CO 3 *, HCO 3 - , and CO 3 2- ) and quantified the relative importance of carbonate species to other abundant quenching agents (e.g., H 2 O, H + , HSO 3 - , 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 H 2 CO 3 *, HCO 3 - , and CO 3 2- ( k H 2 C O 3 * = 2.23 ± 0.42 × 10 9 M -1 s -1 , k H C O 3 - = 2.18 ± 0.73 × 10 6 M -1 s -1 , and k C O 3 2 - = 1.05 ± 0.61 × 10 5 M -1 s -1 ), with quenching dominated by H 2 CO 3 * (which includes both CO 2(aq) and H 2 CO 3 ) 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.