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 pHdependent 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 CO 2 3 * = 2.23 ± 0.42 × 10 9 M −1 s −1 , k HCO 3 = 2.18 ± 0.73 × 10 6 M −1 s −1 , and k CO 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.

Deeper Defluorination and Mineralization of a Novel PFECA (C7 HFPO-TA) in Vacuum UV/Sulfite: Unique Mechanism of H/OCF3 Exchange

“…The absorption coefficient of OH – at pH 10.01 and 12 was 0.3218 and 32.18 cm –1 , much higher than 0.2732 cm –1 of C7 HFPO-TA (0.1 mM). In addition, HO • may dissociate to O • – at higher solution pH, lowering its redox potential. , At a solution pH of 3.99, we also observed the degradation decline of C7 HFPO-TA in the VUV process, which may be ascribed to the light screening of H 2 O 2 (ε 185 nm = 341 M –1 cm –1 ) (Table S11), and e aq – scavenging by H + /H • /H 2 O 2 or H 2 CO 3 */HCO 3 – (eq –). , In consideration of the ineffectiveness of HO • , the e aq – or direct photolysis was the main force for the decay of C7 HFPO-TA in the VUV process. normale aq − + normalH • → normalH 2 + OH − goodbreak0em1em⁣ italick = 2.5 × 10 10 .25em normalM − 1 .25em normals − 1 normale aq − + normalH + → normalH • goodbreak0em1em⁣ italick = 2.3 × 10 10 .25em normalM − 1 .25em normals − 1 normale aq ...…”

“…In addition, both CO 3 2– and HCO 3 – could compete 185 nm photons . Obviously, the decreasing trend of k was also observed with the decreasing solution pH from 12 to 10, which was mainly caused by the generated HCO 3 – with a faster e aq – quenching rate of 2.18 ± 0.73 × 10 6 M –1 s –1 . By contrast, the maximum decay/defluorination (99.9%/70.3%) of C7 HFPO-TA in the VUV process occurred at pH 5.98 (Figure S7).…”

“…31 Obviously, the decreasing trend of k was also observed with the decreasing solution pH from 12 to 10, which was mainly caused by the generated HCO 3 − with a faster e aq − quenching rate of 2.18 ± 0.73 × 10 6 M −1 s −1 . 30 By contrast, the maximum decay/ defluorination (99.9%/70.3%) of C7 HFPO-TA in the VUV process occurred at pH 5.98 (Figure S7). The neutral conditions benefited the degradation in the VUV process, which was consistent with the previous study.…”

“…The enhanced effect of C7 HFPO-TA at pH 10.02 to 12 was ascribed to the highly efficient generation and utilization of e aq − and slower quenching by dissolved carbonate species. 16,30 Figure S6 displayed the effect of dissolved carbonate species on the degradation of C7 HFPO-TA. All k values of the VUV/SF process were higher than that of the UV/SF process under the same reaction condition, showing its excellent resistance to dissolved carbonate species.…”

“…The main carbonate species at solution pH 12 were CO 3 2− , which could quench e aq − at the rate of 1.05 ± 0.61 × 10 5 M −1 s −1 . 30 In addition, both CO 3 2− and HCO 3 − could compete 185 nm photons. 31 Obviously, the decreasing trend of k was also observed with the decreasing solution pH from 12 to 10, which was mainly caused by the generated HCO 3 − with a faster e aq − quenching rate of 2.18 ± 0.73 × 10 6 M −1 s −1 .…”