Relationship between Pb/Cd adsorption and metal oxides on surface coatings at different depths in Lake Jingyuetan

Adsorption of heavy metals to metal oxides on surface coatings of sediments in aquatic environments is one of the most important factors governing their toxicity. Many previous studies were carried out to examine relationships between characteristics of surface coatings and heavy metal adsorption. However, a number of uncertainties existed in the related mechanisms. Especially, the effect of depth, at which surface coatings are developed, was not seriously considered. In this study, surface coatings were collected on glass slides at different depths in the Jingyuetan Lake, which is located in the northeast of China, and the related chemical characteristics (Fe and Mn oxides in the surface coatings) were analyzed. Pb and Cd adsorption onto the surface coatings was measured under controlled laboratory conditions. Nonlinear regression analyses and Langmuir adsorption isotherms were used to estimate contributions of Fe and Mn oxides developed at different depths. The results indicated that a strong linear relationship existed between the depth of water and the contents of iron/manganese oxides. The depth of water can also influence the Langmuir parameters (Γ,max) of Pb and Cd adsorbed onto the surface coatings, by reducing values of Γmax from water surface to the bottom and reaching the lowest level when approaching the sediments. For surface coatings at different depths in the lake, analyses of correlation between the Γmax and the coating constituents indicated that adsorption of Pb and Cd by Mn and Fe oxides are statistically significant. Based on results of this study, the role of water depth in governing processes of Pb and Cd adsorption to metal oxides in homogeneous surface coatings was verified.