Titanium-pillared montmorillonite composite as an efficient catalyst for 2-nitrophenol reductive transformation by Fe(II): The effects of aqueous chemistry and mechanistic insights

Abstract

The abiotic reduction of 2-nitrophenol (2-NP) to 2-aminophenol is the major remediation strategy used to remove 2-NP from contaminated aquifers and groundwaters. In the present study, both the aqueous chemistry and surface nature of a titanium-pillared montmorillonite (TPMnt) catalyst were investigated in-depth, to obtain comprehensive insight into the reductive transformation mechanisms of 2-NP in a Fe(II)/TPMnt reaction system. The results show that almost all the 2-NP disappeared within 0.5 h in the Fe(II)/TPMnt3 reduction system in stark contrast to just 26% of 2-NP removal within 4.0 h in a Fe(II) solution, and 85% of 2-NP removal within 4.0 h in a Mnt system. These results demonstrate that the specially adsorbed Fe(II) species bound to the mineral surface is one key species crucial for achieving the 2-NP transformation. Among the aqueous chemistry, i.e., the ratio of [Ti]/Mnt, the solution pH, the initial Fe(II) concentration, the dose of TPMnt, the solution pH, and the initial Fe(II) concentration significantly influenced the 2-NP transformation. High effectiveness was achieved under these conditions: TPMnt3 with a content of 20.0 mmol/g TiO2, pH of 7, an initial Fe(II) concentration of 10.0 mmol/L, and a dose of TPMnt at 2 g/L. There was a positive correlation between the constant k values of the reduction rate and the amounts of adsorbed Fe(II) species on the TPMnt surface. Further, the TPMnt was shown to possess good recyclability, and the catalytic activity of TPMnt3 for the 2-NP reduction remained mostly unchanged when reused four times. Our results for cyclic voltammograms and electrochemical impedances confirmed that the enhanced performance of the 2-NP reduction can be attributed to a negative shift of the oxidation potentials of Fe(II) along with decreased impedance values (RCT) of the Fe(II)/Fe(III) coupling on the GC electrodes. The intercalated TiO2 in TPMnt functionalized as an ideal promoter for the efficient transformation of 2-NP by Fe(II). Taken together, these results suggest the TPMnt surface-mediated 2-NP reduction by Fe(II) is a promising application for the remediation of 2-NP-contaminated aquifers and groundwaters.