A multifactor exploration of the photobleaching of Suwannee River dissolved organic matter across the freshwater/saltwater interface

A four factor central composite experimental design was applied to explore the photobleaching of Suwannee River dissolved organic matter (SRDOM) at 350 nm as a function of the tetravariate system of [SRDOM], total [Fe(III)], [NO3-], and salinity. The ranges of each factor were setto cover their likely concentrations atthe freshwater/ saltwater interface, to encompass the possible conditions encountered during the transition from the terrestrial to marine environment. Each experiment was carried out using a minimum of 25 different initial conditions, with 3-6 replicates/condition. The resulting data set mapped out the effects of multiple photoactive components on the rate of photobleaching. Under the conditions tested (nominally total [Fe(III)] 0.00-4.00 microM; [NO3-] 0.00-60.00 microM; SRDOM 0.00-30.00 mg/L; salinity 0.00-35.00 ppt, polychromatic illumination, pH 8.2) all samples photobleached at all wavelengths measured, and the absorption at 350 nm bleached the most rapidly. The most important factor for predicting the rate of photobleaching at 350 nm was the initial loading of SRDOM; the effect of all other factors on photobleaching was not significant at the 95% level of confidence. Varied salinity, Fe(III), or added D2O had no effect on the rate of photobleaching, indicating that hydroxyl radical, singlet oxygen, and superoxide did not contribute significantly to the loss of the chromophore at 350 nm. The addition of hydroquinone or thiosulfate inhibited photobleaching, suggesting photobleaching may depend on a weaker oxidant such as the excited-state acceptors (derived from SRDOM directly) produced during photoinitiated charge-transfer processes. The advantages of multifactor experimental techniques for exploring SRDOM photochemistry are discussed.