J. Ikhsan, B.B. Johnson, J.D. Wells. (1999), 'A Comparative Study of the Adsorption of Transition Metals on Kaolinite', Journal of Colloid and Interface Science, 217, p. 403-410

ABSTRACT

Adsorption of Pb(II), Cu(II), Zn(II), Co(II), and Mn(II) to kaolinite was measured at 25 oC in the presence of 5 mM KNO3. Adsorption edge (pH 3-10; total metal concentration 0.1 mM) for Pb(II) and Cu(II) were sigmoid, but those for Zn(II) and Co(II), and especially Mn(II), were characterized by distinct steps after about 40% adsorption. Adsorption isotherms (concentration up to 0.06 mM; fixed pH) at pH 5.50 followed the simple Langmuir equation, but those at pH 7.50 (Zn(II), Co(II), and Mn(II)) requires a two-site model. More protons were released during adsorption at the higher pH. All adsorption data, and the results of potentiometric titrations of kaolinite suspensions (alone and in the presence of metal at 0.1 mM), can be fitted closely by a constant-capacitance surface complexation model that incorporates two bidentate surface complexes. One involeves electrostatic attraction between metal ions and the permanent, negatively charged sites on silanol faces of kaolinite. The other is an inner-sphere complex at the variable-charge surface hydroxyl gropus situated at the crystal edges and on the aluminol faces.

Key words: adsorption; kaolinite; lead; copper; zinc; cobalt; manganese; Langmuir; surface complexation modeling.