Adsorption of As(V), As(III), and Se(IV) anions from municipal landfill leachate solutions by the clay minerals kaolinite and montmorillonite is strongly dependent upon solution pH.The H2AsO4‐ and HSeO‐3 ions are the apparent primary As(V) and Se(IV) species being adsorbed by the clay minerals. Activation of the anion adsorption sites on the clay minerals by H+ ions is important in determining the amount of HSeO3‐ ions adsorbed by the clay minerals, but it is apparently not important in determining the amount of H2AsO4‐ ions adsorbed.Montmorillonite adsorbed more As(V), As(III), and Se(IV) anions than kaolinite. Although no definite proof is given, the higher adsorption by montmorillonite is attributed to a higher edge surface area.The results of the study suggest that disposal of As and Se wastes in municipal landfills, especially under alkaline conditions, could produce ground water contamination by As and Se.
Adsorption isotherms of phosphate on calcite were shown to be described by a two‐region Langmuir isotherm equation, i.e., the plot showed two distinct linear portions. The break in the slope of the Langmuir plot was found, on a solubility diagram of the calcium phosphates, to correspond closely to the division between octocalcium phosphate and hydroxylapatite.The adsorption data were also found to be described by the BET equation. The monolayer capacity computed from the BET equation was found to correspond closely with the adsorption maximum computed from the initial solpes of the Langmuir plots. Comparison of monolayer capacities with the total surface area of the calcite indicated that only approximately 5% of the total surface was covered with phosphate ions.The differential isosteric heat of adsorption, was computed from the adsorption data. A plot of as a function of phosphate sorbed revealed a discontinuity at the region 1 boundary of the adsorption data.Kinetics of phosphate reaction with calcite were found to be consistent with heterogeneous nucleation theory and are discussed in terms of the previous adsorption data.It was concluded that the interaction of phosphate with the calcite surface could be described as a heterogeneous nucleation process.
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