Use of H_2~(18)O_2 To Measure Absolute Rates of Dark H_2O_2 Production in Freshwater Systems

摘要

Photochemical production is usually considered to be the main source of H_2O_2 in freshwater systems; here we show that significant dark production also occurs. We used isotope-labeled H_2O_2 as a tracer to simultaneously determine H_2O_2 production and decay rates in incubations of unfiltered water samples. Our new technique for H_2~(18)O_2 analysis, requiring only small sample volumes and simple field equipment allows for preservation of samples in remote locations, followed by gas chromatography mass spectrometry (GCMS) analysis up to six days later. Dark H_2O_2 production rates of 29-122 nM/h were observed in several lakewater samples. Measured production and decay rates were consistent with pseudo steady-state, early morning [H_2O_2] measurements made in each water body. Dark H_2O_2 production is likely to be more important than photochemical production for the total H_2O_2 budget over 24 h in the freshwater systems we examined. Our results imply that processes usually assumed to be photochemically induced in freshwaters, such as metal redox cycling mediated by H_2O_2 and O_2~-, and production of strong oxidants from the reaction of H_2O_2 with Fe(II) (Fenton's reaction) could also be occurring at significant rates in the absence of light.