A comparative study of the adsorption of transition metals on kaolinite

Adsorption of Pb(II), Cu(II), Zn(II), Co(II), and Mn(II) to kaolinite was m easured at 25 degrees C in the presence of 5 mM KNO3. Adsorption edges (pH 3-10; total metal concentration 100 mu M) for Pb(II) and Cu(II) were sigmoi d, but those for Zn(II) and Co(II), and especially Mn(II), were characteriz ed by distinct steps after about 40% adsorption. Adsorption isotherms (conc entrations up to 60 mu M; fixed pH) at pH 5.50 followed the simple Langmuir equation, but those at pH 7.50 (Zn(II), Co(II), and Mn(II)) required a two -site model. More protons were released during adsorption at the higher pH. All adsorption data, and the results of potentiometric titrations of kaoli nite suspensions (alone and in the presence of metals at 100 mu M), can be fitted closely by a constant-capacitance surface complexation model that in corporates two bidentate surface complexes. One involves electrostatic attr action between transition metal ions and the permanent, negatively charged sites on the silanol faces of kaolinite. The other is an inner-sphere compl ex at the variable-charge surface hydroxyl groups situated at the crystal e dges and on the aluminol faces. (C) 1999 Academic Press.