MOBILITY AND SPECIATION OF SELENIUM IN THE PRESENCE OF NATURAL ORGANIC MATTER

Natural organic matter (NOM) is known to form aqueous and surface complexes with a wide variety of metals, including both cationic and oxyanionic metals, and potentially affects the mobilities of such metals in natural systems. Selenium (Se) is an essential nutrient, but several forms are toxic above certain concentrations. For this reason, Se mobility is a subject of wide interest. Se mobility is influenced strongly by its speciation, with Se(VI) as selenate usually found to be more mobile than Se(IV) as selenite. Elemental selenium, Se⁰, is typically found as a red solid with limited mobility as well as low toxicity. One goal of this ongoing study is to examine the influence of NOM on the mobility of the Se oxyanions, selenate and selenite. A second goal is to compare this influence to the influence of NOM on the mobility and aqueous speciation of arsenic, studied previously. Toward this goal, we have undertaken a set of experiments to examine the ability of natural, unfractionated NOM samples to form aqueous complexes with selenate and selenite. In addition, we have developed a new method to distinguish NOM-complexed Se from inorganic Se, while simultaneously distinguishing inorganic Se(VI) and Se(IV), using ion-exchange chromatography. NOM was obtained from a mine drainage in Leadville, CO, the Suwannee River in Georgia, and the Black River in the Upper Peninsula of Michigan. In batch experiments, Se oxyanions were combined with NOM at environmentally realistic concentrations, incubated in darkness until a complexation steady state was reached, and analyzed. Total Se was quantified using graphite furnace atomic absorption spectrometry, and total organic matter was quantified by total organic carbon analysis. Mass balances were obtained for all experiments. Preliminary results and details of the speciation method will be presented.