Hydrogen peroxide formation and decay in iron-rich geothermal waters: The relative roles of abiotic and biotic mechanisms

Hydrogen peroxide (H2O2) is widely distributed in surface waters where the primary photochemical formation pathway involves the interaction between di ssolved organic carbon (DOC) and ultraviolet radiation (UVR). In laboratory studies using iron-rich water from Yellowstone's Chocolate Pots spring, H2 O2 formation depended on sample treatment (unfiltered, <0.2 pm filtered, au toclaved) prior to irradiation, suggesting several formation pathways. Simi lar H2O2 formation in filtered and unfiltered water indicates that it is pr imarily soluble material that is responsible for H2O2 formation. H2O2 forma tion with soluble material probably includes only photochemical reactions w ith DOC and/or metals. Greater H2O2 formation in unfiltered and filtered wa ter than in autoclaved water suggests that the agent(s) involved in H2O2 fo rmation is (are) not stable at high temperatures and pressures and degrade to nonphotoreactive species. Such unstable agents may include DOC and/or di ssolved complexes of iron or other metals. UVR absorbance occurs across the UV spectrum and, though slightly greater in the UVA range (320-400 nm), is similar to that of other surface waters. Increased UVR absorbance after au toclaving suggested degradation or alteration of some components, which in turn affected H2O2 formation. The spectral region used for irradiation affe cted net formation and yield. H2O2 formation in water irradiated with UVA r adiation was 2.5-3 times that formed in water irradiated with UVB radiation (280-320 nm) in experiments using artificial light sources. Apparent quant um yields comparable to those reported by others could not be calculated be cause the instrumental designs are not the same. However, approximate quant um yields were calculated for these experiments but should be viewed with c aution. Quantum yields were higher in these experiments (0.0040 mol H2O2 pe r mol photon at 310 nm and 0.0012 mol H2O2 per mol photon at 350 mm) than v alues reported by other researchers (<0.0007 mol H2O2 per mol photon at 300 nm and <0.0005 H2O2 per mol photon at 340 nm; [Scully, N. M., D. R. S. Lea n, D. J. McQueen and W. J. Cooper (1996) Limnol. Oceanogr. 41, 540-548]). I n natural solar source experiments, N2O2 formation was greater in experimen ts with UVA and photosynthetically active radiation (PAR; 400-700 nm) than with PAR alone or with UVB, UVA and PAR. However, H2O2 capacity (nM H2O2 W- 1 h(-1) m(2)) was greatest with UVB radiation and lowest with PAR radiation , Source regions could not be studied separately. Dark decay of H2O2 occurr ed via two mechanisms. The main mechanism responsible for H2O2 decay involv ed particulate matter (probably microorganisms), whereas a secondary mechan ism involved soluble matter (ie. DOG, metal ions and other dissolved specie s involved in Fenton reactions).