Effect of Fe(III)/Citrate Concentrations and Ratio on the Photoproduction of Hydroxyl Radicals: Application on the Degradation of Diphenhydramine

Fe­(III)-citrate complexes are photoreactive and ubiquitous in natural waters. In this study, the effect of Fe­(III)/citrate concentrations and ratio on the photoproduction of ^•OH as a function of pH (3–9) was systematically investigated. The ^•OH formation mechanism was elucidated according to the pH-dependent formation rate of ^•OH and the speciation distribution analysis of Fe­(III) species. At high Fe­(III)-to-citrate ratio (10:50), the ^•OH photoproduction increased with decreasing pH. In contrast, the ^•OH photoproduction increased in the order of pH 9.0 < 3.0 < 7.0 < 6.0 < 5.0 at low Fe­(III)-to-citrate ratios (10:100–10:300). At identical Fe­(III)-to-citrate ratio (1:10), high concentration of Fe­(III)-citrate complexes rendered a downward trend for ^•OH production with increasing pH. FeOHcit^– is the predominant reactive species responsible for the ^•OH formation at high pH. The optimal pH for ^•OH production was governed by the amount of O₂^• – and the stability of Fe­(II) species in the Fe­(III)-cit solution. The Fe­(III)-cit-induced photodegradation of diphenhydramine verified the pH-dependent trend for ^•OH production. By GC-MS and LC-ESI-MS analyses, the photoproducts of diphenhydramine were identified and the degradation pathway was proposed