Effects of dissolved organic matter on adsorbed Fe(II) reactivity for the reduction of 2-nitrophenol in TiO2 suspensions

• Adsorbed Fe(II) reactivity affected by dissolved organic matter (DOM).
• DOM affect adsorbed Fe(II) density and peak oxidation potential of active species.
• High molecular weight DOM fractions have stronger electron transfer capacities.
• A lower peak oxidation potential indicates a higher reduction rate.
• A higher electron transfer capacities value indicates a higher reduction rate.

Dissolved organic matter (DOM) is widespread in aquatic and terrestrial environments. Iron is the most abundant transition metal in the Earth’s crust. The biogeochemistry of iron and the strength of Fe(II) as a reducing agent while adsorbed on minerals are affected by DOM. This study investigated the effects of Fe(II)/DOM interactions on the reduction of 2-nitrophenol (2-NP) in TiO2 suspensions. Kinetic measurements demonstrated that rates (k) of 2-NP reduction by adsorbed Fe(II) species are affected by adding DOM (denoted O-DOM), and the obtained k values under the impact of the Fe(II)/DOM interaction with different molecular weight DOM fractions [including MW < 3500 Da (L-DOM), 3500 < MW < 14 000 Da (M-DOM), and MW > 14 000 Da (H-DOM)] showed significant differences. The enhanced rates of 2-NP reduction contributed to increases in the amount of adsorbed Fe(II) species and negative shifts in peak oxidation potential values (EP) in CV tests. For different molecular weight DOM fractions, increases in k (O-DOM < L-DOM < M-DOM < H-DOM) are consistent with increases in molar electron transfer capacities (ETC) based on k values at a fixed DOM concentration (O-DOM < L-DOM < M-DOM < H-DOM). This study attributed the impact of DOM on the enhanced reductive reactivity of Fe(II) to the higher level of adsorbed Fe(II) and the lower EP values. In addition, the ETC values were slightly higher in the TiO2 suspension containing the H-DOM fraction as compared the other two DOM fractions, which would further enhance the reduction rate of 2-NP. These findings promote a general understanding of Fe(II)/DOM interactions and their impact on the fate of contaminants in actual subsurface environments.

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