Heavy metals are one of the main problems in aquatic systems as they affect the ecological balance of the water body. Biological availability of a metal is dependent on its chemical form. Speciation of metals in aquatic systems is of high importance and can provide a proper framework for managers to determine the probable risks resulted from metals. Therefore, this study was performed in Anzali Wetland to investigate the speciation of heavy metals and metalloids, and determine the factors controlling them. For this purpose, six stations were sampled and the relations among water quality parameters were identified by cluster analysis. Water chemistry software (HSC Chemistry) was used to draw Eh/pH diagrams. According to the results, oxidation-reduction potential (Eh) values ranged from - 280 to + 135 mV. The metals including arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), mercury (Hg), cerium (Ce), copper (Cu), aluminum (Al) and iron (Fe) were affected by Eh and pH. Salinity played a significant role in controlling nickel (Ni), magnesium (Mg), calcium (Ca) and strontium (Sr) in the water. The HSC Chemistry diagrams showed that all the studied metals were within the two lines of water stability range. As, Cd, Co, Cr, Ni, lead (Pb), Al, Ca, Ce, Fe, manganese (Mn), Mg, silicon (Si) and Sr were present in a bioavailable species in the wetland water. Some metals such as As, Cd, Cr, and Fe existed in very toxic forms. It was found that changes in the Eh-pH values as a result of anthropogenic and different activities in the wetland can alter the speciation of metals. Eh/pH diagrams also showed that the elements can exceed the water stability range and become unstable in three states (pH < 5, Eh < - 380 mV or Eh > 780 mV).
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