Photochemical Formation of Hydroxyl Radicals in Red Soil-Polluted Seawater on the North of Okinawa Island, Japan

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 (aEuro cent OH), the most potent oxidant in the environment, in red soil-polluted waters using a 313-nm monochromatic light. aEuro cent OH was photochemically formed in the red soil-polluted water samples, and the formation rates of aEuro cent OH decreased as salinity increased, i.e., as red soil-polluted river water gets mixed with seawater. The photo-formation rates of aEuro cent OH showed good correlations with dissolved Fe concentrations (R (2) = 0.96) and [NO(2) (-)] + [NO(3) (-)] 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 NO(3) (-), Fe(OH)(2+), and hydrogen peroxide all together accounted for less than 10% of the observed aEuro cent 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 aEuro cent OH, and the photolysis of NO(2) (-) could account for at most 78% of the observed aEuro cent OH formation rates. We found that the Fenton's reaction (a reaction between Fe(II) and H(2)O(2)) could possibly account for the observed formation of aEuro cent OH in the red soil-polluted waters.