Research Hydrogen Peroxide Cycling in Surface Geothermal Waters of Yellowstone National Park CINDY L. WILSON,† N A N C Y W . H I N M A N , * ,† WILLIAM J. COOPER,‡ AND C H R I S T O P H E R F . B R O W N †,§ Department of Geology, University of Montana, Missoula, Montana 59812, and Department of Chemistry, University of North Carolina at Wilmington, Wilmington, North Carolina 28403
Hydrogen peroxide (H2O2), iron, and sulfide ion were measured every 4 h over 40-h periods at four hot springs in Yellowstone National Park, WY: an alkaline spring (Black Sand Pool); the sulfur-rich, near-neutral Roadside Spring near Nymph Lake; and two iron-rich, acidic springs (Chocolate Pots and the iron-rich Roadside Spring near Nymph Lake). Hydrogen peroxide concentrations reached 200-600 nM by late afternoon and decreased, in most cases, to less than 50 nM during the night. Diel changes in H2O2 concentrations suggest that photochemically mediated processes were responsible for its formation. Photochemical reactions with DOC are likely the primary pathway responsible for H2O2 formation in geothermal waters. Although microbially mediated processes are important in limiting the buildup of H2O2, the inverse relationship between H2O2 and sulfide ion suggests that H2O2 decay may also occur via chemically mediated processes in the sulfur-rich waters.
Introduction Photochemical processes in natural waters have implications in metal redox cycling (1-4) and biological activity (5). Of the possible reactive photochemical products, hydrogen peroxide (H2O2) is one of the more stable species (4). Whereas numerous studies report H2O2 variability in atmospheric waters (6 and references therein) and surface waters (7-11), relatively little is known about H2O2 variability in metal-rich systems (10), and no studies have been conducted in geothermal waters as they surface in springs. In surface waters, H2O2 concentrations are controlled primarily by in situ formation and decay (11) involving photochemical/ chemical reactions and biological activity. The main pathway for in situ photochemical formation of H2O2 is believed to be the interaction of ultraviolet (UV) radiation (280-400 nm) with the fraction of dissolved organic carbon (DOC) that absorbs solar radiation (10, 11). When photoactive DOC absorbs light, an electron is transferred from the ground state to an excited state and then electron transfer occurs, by an as yet undefined process, to O2, forming the superoxide radical ion (O2•-) (10): * Corresponding author e-mail:
[email protected]; phone: (406)243-5277; fax: (406)243-4028. † University of Montana. ‡ University of North Carolina at Wilmington. § Present address: Battelle PNNL, Richland, WA 99352. 10.1021/es9906397 CCC: $19.00 Published on Web 05/23/2000
2000 American Chemical Society
DOC + hv f DOC*
(1a)
DOC* + O2 f DOC•+ + O2•-
(1b)
The superoxide radical ion and its conjugate hydroperoxyl radical (HO2•; pKa ) 4.8) disproportionate to form H2O2 (12):
O2•- + H+ f HO2•
(2a)
HO2• + HO2• f H2O2 + O2
(2b)
Several studies have reported biological H2O2 formation by marine and freshwater organisms (13-16). In these studies, H2O2 was found to form as a byproduct of microbial processes, including nutrient acquisition (eq 3) (15), incomplete aerobic respiration (eq 4), and photosynthesis (eq 5) (13, 16). However, in most environments this appears to be a relatively minor source of H2O2: L-amino
acid + O2 f keto acid + H2O2 + NH4+ (3) O2 + 2e- + 2H+ f H2O2
(4)
2H2O f H2O2 + 2e- + 2H+
(5)
Metal redox cycling is another pathway for H2O2 formation (11, 17-19). Reduced metals (e.g., Fe, Cu, Mn) may react with oxygen, forming O2•-/HO2• (11), which in turn forms H2O2 either through disproportionation (eq 2) or through reactions with reduced metals (17):
M(n)+ + O2 (+ H+) f M(n+1)+ + O2•-(HO2•)
(6)
M(n)+ + O2•-/HO2• f M(n+1)+ + H2O2
(7)
H2O2 formation rates via these pathways depend on pH, dissolved metal concentrations, and oxygen concentrations (17-19):
H+ + HO2• + Fe(II) f H2O2 + Fe(III)
pH < 4.8, k ) 1.2 × 106 M-1 s-1 (8)
HO2• + Fe(III) f O2 + Fe(II) + H+
pH < 4.8, k < 104 M-1 s-1 (9)
2H+ + O2•- + Fe(II) f H2O2 + Fe(III)
pH > 4.8, k ) 1.0 × 107 M-1 s-1 (10)
O2•- + Fe(III) f O2 + Fe(II)
pH > 4.8, k ) 1.5 × 108 M-1 s-1 (11)
Surface water H2O2 concentrations are controlled by H2O2 decay. Moffett and Zafiriou (14) determined that photolysis removes insignificant quantities of H2O2. Instead, biological activity of small algae and bacteria (0.2-1.0 µm) is responsible for the majority of H2O2 removal in surface waters (11, 14, 20). Small algae and bacteria produce catalases and/or peroxidases that destroy H2O2 (14, 20). Chemical decay of H2O2 can also occur in aqueous systems when H2O2 reacts with dissolved metals (e.g., Fe, Cu, Cr, S). Hydrogen peroxide can react with either reduced metals via Fenton reactions (17-19, 21, 22): VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2655
TABLE 1. Hot Spring Water Chemistry of Select Analytesa Black Sand Pool pool July
Chocolate Pots
channel Aug
temp (°C) 88.8 81.4 pH 8.39 8.45 H2O2 (nM) 159 219 DOC (mg L-1) 1.60
