The Adsorption Behavior Of Heavy Metal Ion At Different Manganese Dioxides

Heavy metal pollution was a great problem for a long time. The pollutant's environmental behavior had attracted a lot of people's attention. Manganese-oxides are widespread minerals. They can adsorb many kinds of heavy metals; oxidize some heavy metals. To a certain extent, Manganese-oxides control the bioavailability and toxicity of the heavy metals.α-MnO₂,γ-MnO₂,δ-MnO₂ and non-crystal MnO₂ are four kinds of Manganese dioxides which are widespread, stable and have typical texture. In this thesis, the behavior of Pb²⁺and Cd²⁺ adsorption on these four manganese dioxide was studied, and discussed the influence of surrounding factors, then gave further investigations of one manganese dioxides which has the highest adsorption capacity.(1) Adsorbing capacity of Pb²⁺ was higher than that of Cd²⁺ on all manganese dioxide studied by this thesis, but the adsorption capacity of the same heave metal ion on different manganese dioxide was otherness. For Pb²⁺ the adsorption capacity on different manganese dioxide was:δ-MnO₂ï¹¥non-crystal MnO₂﹥α-MnO₂ andγ-MnO₂,for Cd²⁺ the adsorption capacity on different manganese dioxide was:δ-MnO₂﹥α-MnO₂﹥γ-MnO₂ï¹¥non-crystal MnO₂(2)The equilibrium data of heave metal ion on different manganese dioxide was analyzed using the Langmuir, Freundlich, Temkin isotherms. The Freundlich isotherms provided the best correlation for both Pb²⁺ and Cd²⁺ onα-MnO₂ andγ-MnO₂. Forδ-MnO₂, the adsorption equilibrium data of Pb²⁺ fitted very well to Temkin isotherm, the adsorption equilibrium data of Cu2+ fitted very well to Freundlich isotherm, the adsorption equilibrium data of Cd²⁺ fitted very well to Langmuir isotherm. For non-crystal MnO₂, the Langmuir isotherms provided the best correlation for Pb²⁺; the Temkin isotherms provided the best correlation for Cd²⁺.(3)The effects of pH to adsorption of heave metal ion on different manganese dioxide were similar. The lower pH went against the adsorption of heavy metal ion. Along with the increasing of pH, both the adsorbing capacity and adsorption rate increased. The change of adsorbing capacity caused by pH value is related to the texture of manganese dioxide. Electrolytes in the singerle-component system inhibited the adsorption of heave metal ion, and the higher concentration or higher quantivalency of the cation of the electrolye made the inhibited effect more obvious.(4)In the studied temperatures range, the saturation adsorbing capacity of Pb²⁺, Cu2+and Cd²⁺ onδ-MnO₂ was increased when the temperature increased. The values ofΔG 0 andΔH 0showed that Pb²⁺, Cu2+and Cd²⁺ adsorbed byδ-MnO₂ is a spontaneous and endothermic process.(5)The process of the kinetics of Pb²⁺and Cd²⁺ adsorption onδ-MnO₂ can be well described with the models of Elovich and double constants. For Pb²⁺the adsorption kinetics can be best described by Elovich model, while the double constants model did for Cd²⁺. The process of the kinetics of Cu2+ adsorption onδ-MnO₂ can be described only by double constants model.(6)The competitive adsorption onδ-MnO₂ showed that the uptake of each metal was considerably reduced with increasing concentrations of the other. When both Cu2+and Pb²⁺were present in the solution together, the adsorption of Cu²⁺being more strongly influenced by Pb²⁺. When both Cd²⁺and Pb²⁺ were present in the solution together, the interference of Pb²⁺ with the Cd²⁺ uptake was more pronounced than the interference of Cd²⁺ with the Pb²⁺ uptake. The adsorption affinity is Pb²⁺ï¹¥Cd²⁺. When both Cu2+and Cd²⁺were present in the solution together, the interference of Cd²⁺with the Cu2+ uptake was sub equal with interference of Cu2+ with the Cd²⁺uptake. Considering the adsorbing capacity, the adsorption affinity is Cu2+ï¹¥Cd²⁺.