561 Journal of Oceanography, Vol. 61, pp. 561 to 568, 2005 Keywords: ⋅ Fe(II), ⋅ hydrogen peroxide, ⋅ red soil pollution, ⋅ oxidizing power, ⋅ photochemistry, ⋅ hydroxyl radical, ⋅ Okinawa. * Corresponding author. E-mail: arakakit@sci.u-ryukyu.ac.jp † Present address: Graduate School of Biosphere Sciences, Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8521, Japan. Copyright © The Oceanographic Society of Japan. Simultaneous Measurement of Hydrogen Peroxide and Fe Species (Fe(II) and Fe (tot) ) in Okinawa Island Seawater: Impacts of Red Soil Pollution TAKEMITSU ARAKAKI 1 *, HIROYUKI FUJIMURA 1 , ASHA MANSOUR HAMDUN 1 , KOUICHIROU OKADA 1 , HIROAKI KONDO 1† , TAMOTSU OOMORI 1 , AKIRA TANAHARA 2 and HATSUO TAIRA 1 1 Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, Senbaru, Nishihara-cho, Okinawa 903-0213, Japan 2 Instrument Research Center, University of the Ryukyus, Senbaru, Nishihara-cho, Okinawa 903-0213, Japan (Received 12 September 2003; in revised form 8 April 2004; accepted 15 April 2004) The northern part of Okinawa Island suffers from red soil pollution—runoff of red soil into coastal seawater—which damages coastal ecosystems and scenery. To eluci- date the impacts of red soil pollution on the oxidizing power of seawater, hydrogen peroxide (HOOH) and iron species including Fe(II) and total iron (Fe (tot) , defined as the sum of Fe(II) and Fe(III)) were measured simultaneously in seawater from Taira Bay (red-soil-polluted sea) and Sesoko Island (unpolluted sea), off the northern part of Okinawa Island, Japan. We performed simultaneous measurements of HOOH and Fe(II) because the reaction between HOOH and Fe(II) forms hydroxyl radical (•OH), the most potent environmental oxidant. Gas-phase HOOH concentrations were also measured to better understand the sources of HOOH in seawater. Both HOOH and Fe(II) in seawater showed a clear diurnal variation, i.e. higher in the daytime and lower at night, while Fe (tot) concentrations were relatively constant throughout the sampling period. Fe(II) and Fe (tot) concentrations were approximately 58% and 19% higher in red-soil-polluted seawater than in unpolluted seawater. Gas-phase HOOH and seawater HOOH concentrations were comparable at both sampling sites, rang- ing from 1.4 to 5.4 ppbv in air and 30 to 160 nM in seawater. Since Fe(II) concentra- tions were higher in red-soil-polluted seawater while concentrations of HOOH were similar, •OH would form faster in red-soil-polluted seawater than in unpolluted seawater. Since the major scavenger of •OH, Br – , is expected to have similar concen- trations at both sites, red-soil-polluted seawater is expected to have higher steady- state •OH concentrations. Hydroxyl radical can react with many organic compounds at almost diffusion-limited rates, 10 9 –10 10 M –1 sec –1 (Buxton et al., 1988). The •OH can be formed from the photolysis of various constituents such as nitrate, nitrite, dissolved organic compounds, and Fe(OH) 2+ in seawater (Zhuang et al ., 1992; Faust, 1994; Arakaki and Faust, 1998; Finlayson-Pitts and Pitts, 2000). The •OH can also be formed from the reaction between photo-reduced Fe(II) and HOOH, i.e. the photo-Fenton reaction (Zepp et al., 1992; Arakaki and Faust, 1998). Fe(II) + HOOH → •OH + OH – + Fe(III). (R-1) In some cases, the photo-Fenton reaction is the domi- nant pathway of •OH formation in the aqueous phase 1. Introduction Hydrogen peroxide (HOOH) and hydroxyl radical (•OH) are powerful, naturally occurring oxidants in the environment (Sakugawa et al., 1990; Thompson, 1992; Stumm and Morgan, 1996). HOOH in seawater can be formed from the combination of photochemical reactions of dissolved chemical species (e.g. organic compounds) and gas-to-seawater partitioning of HOOH (Warneck, 1988; Faust et al., 1993; Finlayson-Pitts and Pitts, 2000).