Electron-rich phenols for probing the photochemical reactivity of freshwaters

Different Swiss freshwater samples spiked with 3,4-dimethoxyphenol (DMOP) o r 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 phen ols at pH 8 revealed that in almost all samples the probe phenols disappear ed more rapidly at 0.1 muM than 5 muM initial concentration. Pseudo-first-o rder rate coefficients were on average 2-3 times greater at the lower initi al phenol concentration. A comparable effect was observed using buffered so lutions of Suwannee River fulvic a cid, which was also used as a model phot osensitizer to study the influence of various parameters on such rate coeff icients. Sensitizer concentration and photon fluence rate were found to be directly proportional to the rate coefficients far DMOP transformation at b oth initial concentrations. For both phenols, the rate coefficients increas ed with increasing pH in the range 4-10, but such an increase was much more pronounced at 0.1 muM than at 5 muM initial phenol concentration. The obse rved kinetic behavior is compatible with the assumption that electron-rich phenols are transformed by photooxidants of different lifetimes generated b y photosensitization from the dissolved organic matter (DOM).