EFFECT OF PH, TEMPERATURE, AND CONCENTRATION ON THE ADSORPTION OF CADMIUM ON GOETHITE

Adsorption of cadmium onto goethite has been investigated as a function of both solution concentration and temperature. The pH range over which adsorption occurred was shifted to higher pH values as the initial cadmium concentration was increased and as the solution temperature was decreased. Over the temperature range studied (10–70 °C), the results of adsorption experiments carried out at pH 6.5 and 7.0 could be fitted by using a one-site Langmuir adsorption isotherm. However, at pH 7.5 a two-site Langmuir model was needed to fit the data. The integral heat of adsorption found from the Langmuir constants was ~ 13 kJ mol−1 at pH 6.5 and 7.0, while the two-site types at pH 7.5 yielded heats of adsorption of 20 and 6 kJ mol−1, respectively. The isosteric heat of adsorption was also determined and found to decrease with increasing surface coverage. © 1990, American Chemical Society. All rights reserved.