Application of surface complexation modeling to the reactivity of iron(II) with nitroaromatic and oxime carbamate contaminants in aqueous TiO₂ suspensions

This study reports on the application of surface complexation modeling to interpret observed kinetic trends for Fe^II redox reactions with model nitroaromatic (4-chloronitrobenzene) and oxime carbamate (oxamyl) contaminants in aqueous TiO_2(s) suspensions. Pseudo-first-order rate constants for reduction of the two probe contaminants (k_red, s^-1) vary by several orders of magnitude with changing conditions (100-500 μM Fe^II, 0-15 g L^-1 TiO_2(s), pH 2-9), but the relationship between reaction rates and Fe^II speciation differs considerably for the two contaminants. For oxamyl, k_red measurements are most strongly correlated with the volumetric total adsorbed Fe^II concentration (moles Fe^II adsorbed per liter of TiO_2(s) suspension), whereas k_red measurements for 4-chloronitrobenzene are proportional to the concentration of the hydrolyzed Fe^II surface complex ({triple bond, long}TiOFe^IIOH⁰). The differing trends demonstrate that Fe^II redox reactivity at the aqueous/TiO_2(s) interface is influenced, in part, by specific molecular interactions with the target oxidant. Results are also geochemically relevant in that they demonstrate unambiguously that mononuclear Fe^II-metal (hydr)oxide surface complexes are sufficiently reactive species to reduce nitroaromatic contaminants, an issue that remained open following earlier studies in Fe^III (hydr)oxide suspensions because structural Fe^II species are simultaneously present in such systems because of interfacial Fe^II-to-Fe^III electron transfer processes that occur on Fe^II adsorption.