Two transformation pathways of Acetaminophen with Fe³⁺ saturated clay particles in dark or light

Acetaminophen (AP) has been frequently detected in different environments due to its wide usage as a common analgesic and antipyretic pharmaceutical. Excess residual of AP in the environment may cause biological risk. However, information about its environmental behaviors was limited, especially the interactions with clay minerals. In this study, AP transformation mediated by Fe³⁺ saturated clay particles was systematically investigated. The results showed 47.6 ± 1.1% or 78.9 ± 0.5% of AP was removed in the presence of Fe³⁺-montmorillonite respectively in dark or under simulated sunlight irradiation after 10 h. The hypothesized mechanism was that exchangeable ferric ions can either obtain electron from AP to form AP radical, or produce ^•OH under light, which can further react with AP. In dark condition, AP radicals could cross-couple with each other to form dimers, while oxidation products were also detected under light irradiation due to ^•OH attacking. Moreover, higher concentration of dissolved oxygen (DO) facilitated Fe³⁺ regeneration on clay surfaces and more reactive Fe species distributed in lower pH, which could significantly enhance the removal of AP both in dark and light. Results of this study revealed that clay minerals played important roles in the abiotic transformation of AP either in dark or under light irradiation, and oligomerization other than mineralization were the dominant processes.