Christina E. Cowan

Adsorption Of Cadmium And Copper By Manganese Oxide

Cadmium and copper adsorption by δ-MnO2 was investigated by potentiometric titration over a range of pH. Adsorption increased with increasing pH because of hydrolysis of metal cations and/or variable charge sites on δ-MnO2 δ-MnO2. The surface acidity constant for δ-MnO2 was determined using the Triple Layer Model to be pKint,a2 = 5.34. The data from titrations with pH as master variable were analyzed to determine the Triple Layer Model intrinsic stability constants, p*Kint,cd2+ = 0.81, p*Kint.cdoH+ = 6.89, p*Kint,cd2+ = 1.66, and p*K+int,CuOH+ = 3.79 for cadmium and copper adsorption onto δ-MnO2. Titrations with metal ion as the master variable were performed at constant pH to estimate the adsorption capacities. The data conformed to a Langmuir isotherm and could be modeled with the Triple Layer Model constants. For cadmium, at pH 5.5, 7.0, and 8.0, the adsorption capacity is 0.434, 1.08, and 1.92 mmol/g, respectively. The affinity of δ-MnO2 for Cu, 1.54 mmol/g at pH 5.5, is greater than that for cadmium. The results show δ-MnO2 has high adsorption capacities and high adsorption affinities for cadmium and copper even in acidic conditions.

Solution ion effects on the surface exchange of selenite on calcite

The adsorption of SeO{sub 3} on CaCO{sub 3}(s) was evaluated in equilibrium CaCO{sub 3}(aq) suspensions over a range in pH and SeO{sub 3} concentration in the absence and presence of the cosolutes, Mg, SO{sub 4}, and PO{sub 4}. Selenite was adsorbed by CaCO{sub 3}(s) in a manner consistent with its exchange on anionic specific surface sites saturated with CO{sup 2{minus}}{sub 3}/HCO{sup {minus}}{sub 3} that were quantified by H{sup 14}CO{sub 3} isotopic exchange. This exchange process was described very well using the computer program FITEQL and half-reactions for the surface exchange of HCO{sup {minus}}{sub 3}, CO{sup 2{minus}}{sub 3}, HSeO{sup {minus}}{sub 3}. The cosolutes SO{sub 4} and PO{sub 4} decreased SeO{sub 3} sorption by direct competition or site blockage, and these effects were also predicted using the surface exchange model. By contrast, the sorption of SeO{sub 3} was not influenced by Mg because the two ions sorbed over different pH regions and exchanged with different surface sites that appeared to be distinct and noninteractive. Evaluation of SeO{sub 3} sorption over a wide concentration range suggested that the surface of CaCO{sub 3}(s) was heterogeneous and that SeO{sub 3} had access to less than 5% of the total anion specific surface sites.