Fate of toxic metals during estuarine mixing of fresh water with saline water

Abstract

Metals are among the most common environmental pollutants, and their presence in high concentration in waters and biota have devastating effects on flora, fauna, and human health. Flocculation process of metals during estuarine mixing can reduce the environmental hazards of metals and also can provide micronutrients to the aquatic system. The present investigation provides a thorough study of eliminating colloidal elements of copper, manganese, nickel, lead, and zinc during estuarine mixing of Shalmanrood River water with Caspian Sea water in Iran. The processes of flocculation were carried out in six different salinity regimes (0.45–2.4 ppt). The obtained result is indicative of non-conservative behavior of the studied metals. Higher flocculation resulted in a lower salinity regime. The obtained results indicated that most of the metals were eliminated during the initial mixing of fresh water with sea water at 0.45–0.9 ppt salinity interval. The trend of flocculation rates of elements is as follows: Zn (59.3%) > Pb (47.6%) > Mn (37.5%) > Cu (29.2%) > Ni (27%).The annual average load of copper (Cu), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn) from Shalmanrood River to Caspian Sea decreases as a result of flocculation process from 9.9, 7.7, 8.9, 5.1, and 23.2 tons per year to 7, 4.8, 6.5, 2.7, and 9.4 tons per year, respectively. According to the cluster analysis, parameters such as temperature, pH, and Eh do not have any impact on flocculation of elements expect for Pb. The only parameter that influences the flocculation of Mn is the salinity. Metal speciation studies that are carried out by Eh-pH software show that the studied metals are present as oxides (Zn, Cu, and Ni) and hydroxides (Mn and Pb).