Study of Photochemical Formation of Hydroxyl Radical in Natural Waters

This dissertation mainly focuses on the sources of the hydroxyl radical (•OH) from photochemical reactions in natural waters, in particular from reactions involving dissolved organic matter (DOM). Firstly, an accurate method for estimating •OH formation rate during long-term irradiation was developed. It was observed that previous methods for measuring •OH formation rates in the natural waters, which were based upon sequentially determined cumulative concentrations of probe photoproducts, significantly underestimated actual •OH formation rates. It was found that the underestimation was mainly due to the degradation of the probe photoproducts and that only ‘instantaneous’ formation rates were appropriate for accurately estimating •OH photochemical formation rates. ‘Instantaneous’ •OH formation rates were obtained by adding probes to a sub-sample at each time point during the long-term irradiation and irradiating the sub-sample for a short time. By employing this approach, •OH formation rates were measured during a photoflocculation study in natural waters. In addition to •OH formation, hydrogen peroxide concentration, dissolved organic carbon, total dissolved nitrogen, water optical properties, and iron speciation were measured. The results showed that in iron- and DOM-rich water samples •OH appears to be mainly produced from the Fenton reaction initially, but subsequently from other sources, in particular DOM photoreactions. In order to elucidate possible photoreaction sites and mechanisms of •OH photoformation from DOM, phenolic compounds were used as model DOM chromophores. •OH quantum yields (Фs) at 280~340 nm were measured by •OH trapping reaction with benzene. It was found that many phenolic acids are capable of producing •OH, especially those with para hydroxyl and carboxyl groups (especially 2,4-dihydroxybenzoate and 4-hydrodroxybenzoate), which have markedly high •OH Фs. By conducting methane trapping and competition kinetics experiments, it was confirmed that free •OH was produced from these compounds. The results suggest that hydroxybenzoic acid moieties within DOM play an important role in the photoproduction of •OH. Finally, it was hypothesized that a quinoid enol tautomer present as a water cluster was responsible for •OH production from phenolic compounds with para hydroxyl and carboxyl groups. Phenolic acids with para hydroxyl and carbonyl groups are common components of lignin, which is a major source of DOM in freshwaters. In order to examine the lignin phenolic composition of natural samples, a simplified method using alkaline copper oxide oxidation coupled with solid-phase extraction and high performance liquid chromatography (HPLC) was developed. In this study, an interlaboratory comparison of the simplified HPLC approach with the conventional, but much more complex and expensive, high pressure reaction vessel GC-MS method was also conducted. The agreement between the two different methods was generally very good. A major benefit of this simplified HPLC method is that it allows any investigator with standard HPLC equipment to analyze lignin components, whereas previously only a small number of specialized labs were able to perform these analyses