Oxidative coupling of acetaminophen mediated by Fe³⁺-saturated montmorillonite

The wide usage of acetaminophen as human medicine has resulted in its ubiquitous occurrence in various environmental compartments. However, the information for the transformation of acetaminophen in soil is still limited. In this study, oxidative coupling of acetaminophen in bulk solution mediated by Fe³⁺-saturated montmorillonite was observed under different environmental conditions. In the absence of natural phenolic acids, acetaminophen could be fully eliminated from the solution within 72h at pH3.5, acetaminophen dimer was identified as the major reaction product. Reduction of montmorillonite associated Fe³⁺ coupled with the oxidation of acetaminophen was considered as the main mechanism for acetaminophen transformation on Fe³⁺-saturated montmorillonite. The clay associated Fe³⁺ showed higher reactivity than Fe³⁺ in solution due to the strong complexation between surface Fe³⁺ and acetaminophen. The cross-coupling reaction between acetaminophen and phenolic acids was also observed when phenolic acids were present in the system. While with the increase of phenolic acid concentration, the competition for the reactive sites between acetaminophen and phenolic acids significantly suppressed acetaminophen removal. These results demonstrated the importance of transition metal saturated clay minerals for the abiotic transformation of anthropogenic micropollutants.