Photodegradation of gallic acid under UV irradiation: Insights regarding the pH effect on direct photolysis and the ROS oxidation-sensitized process of DOM

Abstract

In this study, the degradation of gallic acid (GA), a model compound for dissolved organic matter (DOM) in controlled UV/N2, UV/air, UV/Fe3+/N2, and UV/Fe3+/air systems was investigated to elucidate the contribution of direct photolysis and reactive oxygen species (ROS) oxidation to GA degradation at various pH values. In general, the order of the degradation rate of GA in these four systems was as follows: UV/Fe3+/air>UV/air>UV/Fe3+/N2≈UV/N2. In the UV/N2 system, GA underwent slow direct photolysis, the rate of which decreased with decreasing pH. In the UV/Fe3+/air system, the most rapid GA degradation was achieved at pH 5. ROS are mainly derived from two sources. The first source is attributed to the role of DO and the other is attributed to the interaction of Fe3+ and DO. The contribution of ROS to GA oxidation is much greater (>71%) than that of direct photolysis (<29%) at each pH value and is most obvious at pH 5. H2O2 formation was detected during GA degradation in the UV/air and UV/Fe3+/air systems. Using ROS scavengers, it was found that oxidation by OH was the main mechanism of GA degradation in the UV/Fe3+/air system. Based on the experimental results, a mechanism for GA degradation and ROS formation involving the effect of pH was proposed. This study furthers our understanding of changes in DOM degradation mechanisms due to global acidification.