Evaluating The Effects Of Salinity On Flocculation And Purification Mechanism Of Heavy Metals In Estuaries

Estuaries are incredibly dynamic systems with their physical, chemical, and biological structure characterized by high spatial and temporal variability. During estuarine mixing, the dissolved metal concentration in the surface waters can be significantly decreased by flocculation and adsorption processes. The present study explores the effect of salinity, pH, dissolved organic carbon and nitrate on the flocculation, adsorption and desorption processes of dissolved metals (Cu, Mn, Ni, Zn, and Pb). The experiment was conducted on a series of mixtures with salinities ranging from 1.1 parts per thousand to 7.1 parts per thousand during the mixing of fresh water of the Navrood River water with brackish water from the Caspian Sea. The flocculation trend of Cu (74.44%) > Pb (73.92%) > Zn (54.37%) > Ni (52.33%) > Mn (35.36%) at different salinity regimes (1.1 parts per thousand to 7.1 parts per thousand) indicates that Cu and Pb have a non-conservative behavior and Zn, Ni, and Mn have a relatively conservative behavior. Cluster analysis (CA) indicates that Cu and Zn are principally governed by salinity. According to the experiments, Cu, Ni, and Pb are desorbed from suspended particulate matters during estuarine mixing. The outcomes of the three-step chemical partitioning of the suspended particulate matters indicated that Mn and Pb are desorbed from the suspended particulate matters physically, while Ni is absorbed into the suspended particulate matters chemically. CA indicates Ni, Zn, and Cu are mainly governed by salinity and DOC. According to the mean annual discharge of the Navrood River (166 x 10(6) m(3)/y), the annual discharge of dissolved metals (Cu, Mn, Ni, Zn, and Pb) into the Caspian Sea would reduce respectively from 7.01, 5.59, 20.87, 27 and 3.28 to 1.79, 3.61, 9.95, 12.32, and 0.85 tons/y. The results of this study help decision makers in the local and regional levels to make realistic estimations about the pollution load of heavy metals in the Navrood River specifically and the Caspian Sea generally.