Electron-rich phenols for probing the photochemical reactivity of freshwaters

Different Swiss freshwater samples spiked with 3,4-dimethoxyphenol (DMOP) or 2,4,6-trimethylphenol (TMP) were irradiated using UV-A and visible light from a medium-pressure mercury lamp. The kinetics of depletion of both phenols at pH 8 revealed that in almost all samples the probe phenols disappeared more rapidly at 0.1 microM than 5 microM initial concentration. Pseudo-first-order rate coefficients were on average 2-3 times greater at the lower initial phenol concentration. A comparable effect was observed using buffered solutions of Suwannee River fulvic acid, which was also used as a model photosensitizer to study the influence of various parameters on such rate coefficients. Sensitizer concentration and photon fluence rate were found to be directly proportional to the rate coefficients for DMOP transformation at both initial concentrations. For both phenols, the rate coefficients increased with increasing pH in the range 4-10, but such an increase was much more pronounced at 0.1 microM than at 5 microM initial phenol concentration. The observed kinetic behavior is compatible with the assumption that electron-rich phenols are transformed by photooxidants of different lifetimes generated by photosensitization from the dissolved organic matter (DOM).