Sorption studies of cobalt(II) on colloidal hematite using potentiometry and radioactive tracer technique

The sorption of Co(II) on colloidal hematite was studied as a function of p H, ionic strength, and Co(II) concentration. Two different techniques were used, yielding two different sets of information: (i) potentiometric titrat ions that provide information on the number of protons released as a functi on of pH owing to the sorption of Co(II) and (ii) measurement of the amount of cobalt sorbed on the surface as a function of pH using a radioactive tr acer, Co-60. At low Co(II) concentrations (10(-8) M), the sorption was foun d to be independent of ionic strength but there seems to be a weak ionic st rength dependence at higher Co(II) concentrations (10(-4) M). The adsorptio n edge moved to higher pH with increasing Co(II) concentration. For the hig h Co(II) concentration, the number of protons released per cobalt sorbed in creased from zero to approximately 1.5. The basic charging properties of he matite were modeled with four different surface complexation models. The I- pg Basic Stern Model (BSM), with binding of electrolyte ions to the Stern p lane, seems to be the most reasonable model if the ambition is to describe experimental data at different ionic strengths. The sorption of cobalt was modeled with the 1-pK BSM. By introducing a low concentration of high affin ity surface sites for cobalt sorption it was possible to model the sorption in very wide cobalt concentrations, ranging from 10(-8) M to 10(-4) M. (C) 2000 Academic Press.