Browsing by Author "Okada, Kouichirou"

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    Photochemical Formation of Hydroxyl Radicals in Red Soil-Polluted Seawater on the North of Okinawa Island, Japan
    (2010-05) Arakaki, T.; Hamdun, Asha; Uehara, Masaya; Okada, Kouichirou
    Land development has caused runoff of red soil into the ocean on the north side of Okinawa Island, Japan. In an attempt to clarify the impacts of this “red soil pollution” on the oxidizing power of seawater, we studied the formation of hydroxyl radical (•OH), the most potent oxidant in the environment, in red soil-polluted waters using a 313-nm monochromatic light. •OH was photochemically formed in the red soil-polluted water samples, and the formation rates of •OH decreased as salinity increased, i.e., as red soil-polluted river water gets mixed with seawater. The photo-formation rates of •OH showed good correlations with dissolved Fe concentrations (R 2 = 0.96) and [NO2−] + [NO3−] concentrations (R 2 = 0.87), while a negative and weak correlation was found with dissolved organic carbon concentrations (R = −0.78). Theoretical calculation showed that direct photolysis of NO3−, Fe(OH)2+, and hydrogen peroxide all together accounted for less than 10% of the observed •OH formation in the red soil-polluted waters. Comparison between filtered and unfiltered samples showed that red soil particles were not the main sources of •OH, and the photolysis of NO2− could account for at most 78% of the observed •OH formation rates. We found that the Fenton’s reaction (a reaction between Fe(II) and H2O2) could possibly account for the observed formation of •OH in the red soil-polluted waters.
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    Simultaneous Measurement of Hydrogen Peroxide and Fe Species (Fe(II) and Fe(tot)) in Okinawa Island Seawater: Impacts of Red Soil Pollution
    (2005-05) Arakaki, T.; Fujimura, Hiroyuki; Hamdun, Asha; Okada, Kouichirou; Kondo, Hiroaki; Oomori, Tamotsu; Tanahara, Akira; Taira, Hatsuo
    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 elucidate 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, ranging from 1.4 to 5.4 ppbv in air and 30 to 160 nM in seawater. Since Fe(II) concentrations 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 concentrations at both sites, red-soil-polluted seawater is expected to have higher steady-state •OH concentrations.