Effect of pH on photo-oxidation of dissolved organic carbon by hydroxyl radicals in a coloured, softwater stream

Previous work has shown that the photo-oxidation rate of dissolved organic carbon (DOC) exposed to solar radiation was significantly enhanced in acidic stream waters and that the average photo-oxidative loss of DOC in the UVA region was more than 60%. This study examined the effect of pH on photobleaching and the photo-oxidative loss of DOC and the degree of photo-oxidative loss by hydroxyl radicals ((OH)-O-•) over the pH range 4-9 in a coloured, softwater stream. Binding iron with NaF lowered the photo-oxidation rate constant by about 50% at pH 4 and by 10-20% at pH 4.6 and 4.9. Hence, iron plays an important role in the photo-oxidative loss of DOC at pH 4 with declining importance as the pH increases. The importance of (OH)-O-• in photo-oxidative loss of stream DOC was evaluated by adding KI to quench (OH)-O-•. After 69 hours of artificial irradiation without KI, DOC loss decreased as pH increased from pH 4 to 9 whereas the minimum loss of absorbance at 300 nm occurred at pH 7. KI significantly reduced loss of DOC at pH 4, 5 and 7 but not at pH 9 with the fraction of DOC lost by non-(OH)-O-• mechanisms gradually increasing from 58% to 75% between pH 4 and 7, and 100% at pH 9. Since many northern freshwater lakes on the Precambrian Shield receive large fluxes of UV-labile humic matter from peatlands and have pH < 7.5, photo-oxidative loss of DOC by both non-(OH)-O-• and (OH)-O-• mechanisms is important in C dynamics in northern freshwaters but with photo-oxidative loss dominated by non-(OH)-O-• mechanisms likely operating in the UVA region.