Rate of Cd²⁺ Release from Dissolved Fulvic Acid and Natural Dissolved Organic Carbon as a Function of UV_B Dose

Environmental Context. Atmospheric ozone depletion results in an increase of UV_B radiation impinging on the surface waters of aquatic ecosystems. Radiative absorption by dissolved humic substances results in bleaching and photochemical decomposition to smaller molecular weight dissolved components. With respect to the lake biota, this can reduce the effectiveness of a natural absorptive protective UV screen, as well as enrich the surface waters with microbial substrates and previously bound biologically unavailable trace metals. In controlled experiments using low-level Cd-contaminated dissolved fulvic acids and natural lakewater dissolved organic carbon, we examine the relationship between increasing UV dose and cadmium free-ion (Cd²⁺) concentrations.

Abstract. Using controlled UV_B exposures in a laboratory incubator, the photolytic release of bound cadmium from cadmium-contaminated dissolved fulvic acid and cadmium-amended natural lakewater dissolved organic carbon was examined using an ion-exchange technique, developed to measure the cadmium free-ion concentration (Cd²⁺). In the fulvic acid experiments, with increasing UV_B dose, the increasing cadmium free-ion concentration followed an exponential saturation function, whereas the decrease in dissolved organic carbon was linear. Experiments using natural lakewaters did not reveal any increase in Cd²⁺, even at high UV_B exposures. Given the much greater dissolved iron concentration in humic natural lakewaters, relative to the fulvic acid medium, iron photoreduction and reoxidation produces fresh amorphous iron oxide surfaces. We hypothesize that these bind the cadmium free-ion, thus reducing its aqueous concentration. Depending on Cd²⁺ affinity to biological surfaces, this mechanism might thus competitively further protect the biota from trace metal toxicity.