Overlooked effects of organic solvents from sample preparation on reaction constants of micropollutants in UV-based advanced oxidation processes

Abstract

Stock solutions of micropollutants with low water solubility are commonly prepared using organic solvents in laboratory studies on degradation of these organic compounds. Dilution of the stock solution unavoidably introduces a small amount of organic solvent into the experimental working solutions. This could possibly affect the estimation of the degradation rate constants of these organic micropollutants by UV-based advanced oxidation process such as UV/H2O2. To demonstrate this problem, the effect of organic solvents on the reaction rate constant of malathion has been investigated in the UV/H2O2 process at the concentration levels that would likely be derived from stock solutions. Several organic solvents commonly used for stock-solution preparation were selected, including acetonitrile (ACN), acetone (Ac), methanol (MeOH), ethanol (EtOH), 1-propanol (PrOH), 1-butanol (BuOH) and 1-pentanol (PeOH). The results show that the reaction rate of malathion in the UV/H2O2 process could be affected by the presence of these organic solvents, even at a concentration well below that possibly introduced during the preparation of working solutions from the organic solvent stock solutions (e.g. 0.00005%, v/v). The suppressive effect on the reaction rate constant depends on the type and concentration. With the increase of organic solvent concentration, the reaction rate constant of UV/H2O2 gradually decreases to the value for photolysis alone. The organic solvents having a stronger reaction activity with OH tend to impose a greater effect on the reaction rate constant. The findings here provide a plausible explanation for the discrepancies in the rate constants reported in the literature for some organic micropollutants during the UV-based advanced oxidation processes.