Diffusive transport of organic colloids from sediment beds

Organic colloids resulting from sediment diagenesis in porewaters are transported across the sediment-water interface via Brownian diffusion. The sediment selected for this study was from the University Lake, Baton Rouge, La. The effects of electrolytes on the sediment-to-water flux of colloids varied depending on the type and concentration of the electrolyte. An ionic strength of 0.5 M reduced the flux of colloids; the effect being greater for KCl than for NaCl at the same ionic strength. The effects of different electrolytes (viz., NaCl, KCl, and CaCl₂) on the porewater colloid concentrations were studied in batch experiments. Below the ionic strength of 0.5 M, the electrolytes were effective in increasing the binding of the colloids to sediment surfaces, and hence decreased their pore-water concentrations. Above the ionic strength of 0.5 M, the amount of colloids released to the porewater was greatest in the presence of CaCl₂. The diffusive flux of colloids from sediment to water was modeled using an effective diffusivity as the single adjustable parameter. The effective diffusivity (D_eff) was obtained using two complementary techniques. The first method utilized the sediment-to-water flux of colloids, which was fit to a model to extract D_eff. For the second method, postmortem cores of the sediment were obtained after 82 d, the porewater colloids were extracted and analyzed. The porewater colloid profile as a function of sediment depth was used to obtain D_eff. The effects of sediment diagenesis on the porewater colloids and the sediment total organic carbon (TOC) in a natural environment are illustrated using data from New Bedford Harbor (NBH), Mass. For this specific case only 0.43% of the organic matter delivered to the sediment was found to reappear in the water column as "dissolved" species.