Influence of Carbonate Speciation on Hydrated Electron Treatment Processes

Advanced reduction processes (ARPs) that generate hydrated electrons (eaq–; 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 eaq– or shield incoming UV photons from the applied photosensitizer. This study examined the pH-dependent quenching of eaq– by ubiquitous dissolved carbonate species (H2CO3*, HCO3–, and CO32–) and quantified the relative importance of carbonate species to other abundant quenching agents (e.g., H2O, H+, HSO3–, and O2(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 eaq– quenching by H2CO3*, HCO3–, and CO32– (kH2CO3* = 2.23 ± 0.42 × 109 M–1 s–1, kHCO3− = 2.18 ± 0.73 × 106 M–1 s–1, and kCO32− = 1.05 ± 0.61 × 105 M–1 s–1), with quenching dominated by H2CO3* (which includes both CO2(aq) and H2CO3) at moderately alkaline pH conditions despite it being the minor species. Attempts to apply previously reported rate constants for eaq– quenching by CO2(aq), measured in acidic solutions equilibrated with CO2(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 eaq– quenching by varying [H+] can instead be ascribed to variable acid–base speciation of dissolved carbonate and the sulfite sensitizer.