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.
Equilibrium batch studies were conducted to obtain solubility data of Pb and Cd in soils. The data were plotted on equilibrium solubility diagrams using pH as the master variable. In the construction of the diagram the hydroxide, carbonate, and phosphate compounds of Pb and Cd were given particular attention. Both Pb and Cd solubility decreased in the soils as pH increased. The lowest values were obtained in the calcareous soil. Under a given set of conditions, however, Cd activity in solution was always notably greater than that of Pb. In noncalcareous soils the solubility of Pb appeared to be regulated by Pb(OH)2, Pb3(PO4)2, Pb4O(PO4)2, Pb5(PO4)3OH, depending on the pH. In calcareous soils, PbCO3 also assumed importance. At higher Cd concentrations the precipitation of Cd3(PO4)2 and/or CdCO3 regulated cadmium solubility. At low Cd concentrations the equilibrium solution was undersaturated with regards to both Cd3(PO4)2 and CdCO3.
The kinetics of the phosphate interaction with calcite were studied. The results showed that the reaction did not proceed in the absence of the calcite surface. The kinetics of interaction could be described by two simultaneous reactions. The first reaction was second‐order and was ascribed to the adsorption of phosphate on the calcite surface. The second reaction was first‐order and was considered to be associated with the surface arrangement of phosphate clusters into calcium phosphate heteronuclei.Solubility criteria were used to show that at low phosphate concentrations the ultimate calcium phosphate surface mineral formed was hydroxylapatite.Desorption kinetics were studied by using an anion exchange resin technique. The desorption process could be described as two simultaneous first‐order reactions. The desorption mechanism was postulated to correspond to the dissolution of a surface nucleated calcium phosphate mineral, with the second reaction step being the desorption of phosphate from the calcite surface sites.The rate constants for adsorption and desorption were determined at four temperatures between 0C and 40C. The rate constants were used to compute the activation energies of adsorption and desorption. In addition, the thermodynamic parameters for the enthalpy of activation (ΔH†), the entropy of activation (ΔS†), and the free energy of activation (ΔG†), for both the adsorption and desorption processes were computed and discussed.
The adsorption of zinc chloride on calcite, dolomite and Casubstituted magnesite crystals from aqueous solution is treated theoretically; and a standard adsorption process is thereby rigorously defined. The thermodynamic characteristics Δ F0θ, Δ H0θ and Δ S0θ are determined for this process by means of adsorption isotherm data at various temperatures with crystals of measured surface area. It is found that about 10% of the adsorption sites probably available on calcite are occupied by zinc when the equilibrium Zn++aq concentration is 0.90 × 10‐6 M at 25.1° C. The Ca‐magnesite shows a somewhat greater affinity for zinc ion than calcite, while dolomite is intermediate. The temperature coefficients of Δ F0θ are opposite in sign for calcite and the other two minerals. The resulting endothermic heats of adsorption of Zn++aq on the dolomite and the Ca‐magnesite (Δ H0θ = 8.21 ± 2.4 and 22.7 ± 5.9 kcal/mole at about 27° C, respectively), as well as the corresponding very large positive entropies of adsorption (Δ S0θ = 56 ± 8 and 106 ± 20 cal/deg × mole at about 27° C, respectively) indicate that Zn++aq is dehydrated when adsorbed by these two minerals. The known compatability of Zn++ with the MgCO3 crystal lattice is suggested as the reason for the strong interaction of this ion with the dolomite and Ca‐magnesite, relative to calcite.
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