Adsorption of thallium(I), lead(II), copper(II), bismuth(III) and chromium(III) by electrolytic manganese dioxide

The adsorption of thallium(I), lead(II), copper(II), bismuth(III) and chromium(III) by electrolytic manganese dioxide (EMD) has been followed between pH 2-5 [except for bismuth(III)), where a single pH of 1.5 was employed] at 20 degrees C and an ionic strength of 0.5, employing a wide range of initial metal ion concentrations from 5 x 10(-5) mol/dm(3) to 5 x 10(-2) mol/dm(3). The adsorption isotherms all fitted a simple Langmuir adsorption equation quite well. The amount of metal ion adsorbed increased with pH for lead and copper ion. However, for both thallium and chromium, a significant drop in adsorption between pH 3 and 4 was observed, associated with the dominant presence of lower-charged hydroxy species at higher pH values. At pH 4, adsorption was in the order Pb-II > Cu-II > Tl-I >> Cr-III, whereas at pH less than or equal to 2 the order was Bi-III approximate to Pb-II > Tl-I > Cr-III > Cu-II. In the low pH range, which is largely free of hydrolysis species, the adsorption of the isoelectronic bismuth, lead and thallium series appears to be dominated by charge, and charge appears a dominant factor with the transition metal pair also. The adsorption of lead (at pH 4 and 5) and bismuth (at pH 1.5) to samples of EMD subjected to prolonged heat treatment at 110, 400 and 650 degrees C were also examined. Samples dried at 110 degrees C mainly lost only surface water, and showed adsorption characteristics similar to untreated samples. Samples heated at 400 degrees C lost all their combined water but still showed significant adsorption, whereas samples heated at 600 degrees C were converted to Mn2O3, and showed very small though measurable metal ion adsorption.