Determination of photoformation rates and scavenging rate constants of hydroxyl radicals in natural waters using an automatic light irradiation and injection system Original Research Article

Photoformation rates and scavenging rate constants of hydroxyl radicals (radical dotOH) in natural water samples were determined by an automatic determination system. After addition of benzene as a chemical probe to a water sample in a reaction cell, light irradiation and injection of irradiated water samples into an HPLC as a function of time were performed automatically. Phenol produced by the reaction between radical dotOH and the benzene added to the water sample was determined to quantify the radical dotOH formation rate. The rate constants of radical dotOH formation from the photolysis of nitrate ions, nitrite ions and hydrogen peroxide were comparable with those obtained in previous studies. The percent of expected radical dotOH photoformation rate from added nitrate ion were high in drinking water (97.4%) and river water (99.3%). On the other hand, the low percent (65.0%) was observed in seawater due to the reaction of radical dotOH with the high concentrations of chloride and bromide ions. For the automatic system, the coefficient of variance for the determination of the radical dotOH formation rate was less than 5.0%, which is smaller than that in the previous report. When the complete time sequence of analytical cycle was 40 min for one sample, the detection limit of the photoformation rate and the sample throughput were 8 × 10−13 M s−1 and 20 samples per day, respectively. The automatic system successfully determined the photoformation rates and scavenging rate constants of radical dotOH in commercial drinking water and the major source and sink of radical dotOH were identified as nitrate and bicarbonate ions, respectively.