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 (eaq-), 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 eaq- for contaminant reduction. Using monochloroacetic acid (MCAA) as a simple eaq- probe, the availability of eaq- 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 eaq-. S(IV) played a dual role by producing eaq- via photoionization of SO32- and by reducing the reactive iodine species formed to avoid their scavenging of eaq-. When S(IV) was available, cycling of iodide occurred, favoring the constant eaq- 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 eaq--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 eaq- efficiently and of minimizing the eaq- scavenging of intermediates inherently formed and accumulated, and highlights the potential of the sulfite/iodide/UV process to efficiently eliminate recalcitrant contaminants.

Synergistic photosensitized removal of Cr(VI) and Rhodamine B dye on amorphous TiO2 under visible light irradiation.

Amorphous TiO(2) (Am-TiO(2)) was prepared at room temperature by hydrolysis of Ti(OBu)(4) in water without addition of strong acids or organic solvents. Results from XRD and TEM revealed that the as-prepared Am-TiO(2) was composed of amorphous structure. For the simultaneous photosensitized removal of Cr(VI) and zwitterionic Rhodamine B (RhB) dye, Am-TiO(2) exhibited more significant synergistic effect than commercial P25-TiO(2). The removal efficiencies for RhB and Cr(VI) after 100 min visible light irradiation were 97.8% and 53.5% on Am-TiO(2), respectively. While 88.2% RhB and 42.1% Cr(VI) were removed on P25-TiO(2). Decreased synergistic activities as well as smaller surface areas were observed when Am-TiO(2) was pretreated at high temperatures (200-700°C). Thus, it was the larger specific surface area rather than better crystallinity dominated the synergistic degradation dynamics under visible light irradiation with lower pH (2), greater catalyst loading amount (2g/L), proper RhB/Cr(VI) ratios (1:8) and higher light intensity (500 W). Better synergistic performance was also obtained on Am-TiO(2) than P25-TiO(2) when Cr(VI) coexists with cationic dyes, while negligible difference was observed in the presence of anionic dyes. Superior stability and simplicity of Am-TiO(2) was also exhibited in the cyclic runs.