Effects of dissolved oxygen, salinity, nitrogen and phosphorus on the release of heavy metals from coastal sediments

Great amounts of nutrients discharged into the urbanized coastal areas, which are continuously subject to violently anthropogenic metal contamination, will result in eutrophication and hypoxic episode. In order to study the effects of dissolved oxygen (DO), salinity, nitrogen and phosphorus on the release of six metals including Zn, Pb, Cd, Cu, As and Cr from coastal sediments, a series of 60-days microcosm experiments consisting of sediments and seawater were conducted. Severe hypoxia could result in the enhanced peak values of Pb, Cd, Cu and Cr concentrations in the overlying water. A higher level of water salinity could elevate the peak value of As concentration in water column, and a higher level of nitrogen could increase the peak value of Zn concentration in water. The exchange fluxes demonstrated that the diffusion from the sediments was a dominant process during the first 10 days, However, a relative equilibrium of adsorption and precipitation in the sediment-water interface reached during the later periods. In addition, the bioavailability of the studied metals in sediments was elevated under severe hypoxia, or a high level of water salinity, or high levels of nitrogen and phosphorus. The results of linear regression analysis suggested that higher metal bioavailability in sediments could facilitate the metal release, but the process could be restrained by the higher aqueous phosphorus due to the precipitation of metal phosphates.