Effect of salinity and organic matter content of sediments on the sediment-water exchange of arsenic

The effect of salinity (NazCl) and the organic matter (OM) content of sediments on the mobility of arsenic as As(III) was studied in a sediment-water system. Salinity ranging from <50 to 1700 mg/L NaCl and OM content of sediments from trace level to 50% were used in a series of batch sorption, sediment-water microcosm and flume experiments. Results from batch sorption experiments indicated that NaCl and particulate OM reduced As sorption to suspended sediments (SS). Salinity up to 830 mg/L NaCl in sediment-water microcosm reduced the rate and amount of dissolved As removal from water by sorption to SS followed by deposition, and water-to-sediment diffusion. Salinity increased SS and particle-sorbed As concentrations in water by the NaCl-induced reduction in interparticle attraction followed by sediment-to-water particle diffusion. Benthic activity enhanced the rate of dissolved and particle-sorbed As and SS removal by promoting SS flocculation and deposition and As diffusion. It is hypothesized that Cl reduces As removal by competition for positively charged surface sites on particulate metal oxyhydroxides and NaCl acts by destabilizing particles, thereby promoting the release of particle-sorbed arsenic from the sediment bed and reducing particle aggregation and deposition from the water. OM also decreases As sorption through the competition for positively charged surface oxides binding sites. A transient mass balance model was developed to explain the effects of salinity and benthic activity on the sediment-water exchange of any nonvolatile, non-degrading chemical. The model estimated dissolved and particle-sorbed chemical concentrations that change due to bidirectional chemical, NaCl and particle diffusion, and particle deposition. Results from batch sorption and microcosm experiments were used to parameterize the model for three NaCl concentrations, with and without benthic activity. The model was useful in explaining the dominant process responsible for As movement in the microcosms and the effect of salinity and benthic activity on these processes. Results from flume experiments indicated that saline water passing over the sediment bed had a negligible effect on sediment resuspension rate due to the minimal contact time between water and sediment bed which did not allow the salt to diffuse into the sediment bed. However, saline pore water increased the resuspension rates due to increasing the negative surface charge density of particles, and thus reducing interparticle attraction forces. Reducing the particulate OM content (from 50% to <5%) caused the sediments to behave as uncohesive material with a uniform resuspension rate as a function of shear stress rather than increasing exponentially with shear stress as with cohesive sediments. Overall, the results from this research suggest that it is important to examine secondary factors such as salinity and OM as they affect chemical dynamics. This research demonstrated that increasing salinity will result in greater As mobility by reducing sorption and sediment cohesion. OM has two opposing effects: high OM content can increase As mobility by decreasing sorption and it can reduce and increase sediment resuspension rates at low and high shear stresses, respectively.