Mechanism and efficiency of contaminant reduction by hydrated electron in the sulfite/iodide/UV process

Advanced reduction by the extremely strong reducing species, hydrated electron (e_aq^-), is a promising and viable approach to eliminate a wide variety of persistent and toxic contaminants. In this study, we proposed a sulfite/iodide/UV process, which offered efficient production of e_aq^- for contaminant reduction. Using monochloroacetic acid (MCAA) as a simple e_aq^- probe, the availability of e_aq^- was assessed, and the mechanism involving the roles of S(IV) and iodide in the process was elucidated. A pronounced synergistic effect of S(IV) and iodide was observed in MCAA reductive dechlorination. The efficiency was much more dependent on the iodide concentration due to its higher absorptivity and quantum yield of e_aq^-. S(IV) played a dual role by producing e_aq^- via photoionization of SO₃^2- and by reducing the reactive iodine species formed to avoid their scavenging of e_aq^-. When S(IV) was available, cycling of iodide occurred, favoring the constant e_aq^- production. The formation and transformation kinetics of sulfite radical were studied to verify the roles of S(IV) and iodide in the process. A kinetic model of MCAA dechlorination was also developed to quantify the e_aq^--initiated reduction efficiency, highlighting the effects of S(IV), iodide, and pH. High pH favored the reduction, and the process was still effective in field surface water. This study underscores the importance of producing e_aq^- efficiently and of minimizing the e_aq^- scavenging of intermediates inherently formed and accumulated, and highlights the potential of the sulfite/iodide/UV process to efficiently eliminate recalcitrant contaminants.