Colloidal association of selenite and selenate in water containing dissolved organic matter and ferric iron: implication for selenium mobility in the aquatic environments

Abstract

Dissolved organic matter (DOM)-modified organic and inorganic colloids are known to enhance the mobilization and transport of different trace elements in aquatic environments. Occurrences of selenium (Se) in natural waters often positively correlate with DOM under diverse environmental conditions, suggesting that DOM may enhance Se mobilization at the soil/sediment–water interface. Despite its importance, however, the extent and mechanism of the association of Se species with DOM-modified colloids in water is poorly understood. Therefore, the size distribution of selenite and selenate in water containing Suwannee River aquatic DOM (SRNOM) was determined in the absence and presence of Fe3+ under different environmentally relevant conditions by batch incubation followed by filtration (0.2 µm) and ultrafiltration (50 and 3 kDa). The mechanism of the association of Se species with SRNOM-modified colloids was determined by Fe and Se K-edge X-ray absorption spectroscopy (XAS). Results suggest that Se species can be considerably associated with DOM-modified colloids in water, and therefore, their mobilization and transport can be enhanced in aquatic environments. The colloidal association of selenite occurred predominantly due to its inner-sphere complexation with the Fe(O,OH)6 octahedra (1E and 2C complexation), and therefore, the extent of association was dependent on the Fe3+ concentration in solution. At low Fe3+ concentrations (<200 µmol/L, C/(C + Fe) molar ratio > 0.91), selenite was complexed with organically bound monomeric Fe(O,OH)6 octahedra, resulting in Fe(III)-bridged ternary complexation with DOM. At higher Fe3+ concentrations, it was complexed with polymeric Fe(O,OH)6 octahedra, resulting in its sorption onto the neo-formed Fe oxyhydroxides. In addition, selenite could also be directly complexed with N-containing functional groups (e.g., –NH2) of the SRNOM. Selenate formed the colloidal association predominantly due to its direct binary complexation with O-containing functional groups (e.g., –COOH and –OH) of the SRNOM. In addition, Fe(III)-bridged ternary complexation (2C complexation), binary complexation with S-containing functional groups (e.g., –SH) of the SRNOM, and outer-sphere complexation could contribute to its colloidal association with DOM. Therefore, the extent of the colloidal association of selenate was not strongly dependent on the Fe3+ concentration in the solution. Selenate generally had a greater tendency to form colloids with SRNOM than selenite. Only when polymeric Fe(O,OH)6 octahedra started to form in solution (at Fe3+ concentration ≥ 200 µmol/L) did the colloidal association of selenite exceed that of selenate initially and then decreased again when the cluster size of the polymeric Fe(O,OH)6 octahedra was big enough to precipitate Fe oxyhydroxides from solution (at Fe3+ concentration 500 µmol/L), associating selenite with particulates. Changes in solution pH did not impact the colloidal association of any Se species. Similarly, changes in the ionic strength of the solution did not affect the colloidal association of selenite; however, it impacted the association of selenate considerably, making the association more extensive at low ionic strength. This study implies that the formation of Se-carrying colloids should be accounted for in determining the mobility and transport of Se in DOM-rich natural waters.