CLUSTER FORMATION VERSUS ISOLATED-SITE ADSORPTION - A STUDY OF MN(II) AND MG(II) ADSORPTION ON BOEHMITE AND GOETHITE

The experiments measured the extent of adsorption of Mn(II) and Mg(II) on goethite left bracket alpha -FeOOH right bracket and boehmite left bracket gamma -AlOOH right bracket from pH congruent 4 to Ph congruent 11 and the resulting changes in surface charge. The electron spin resonance spectra for the Mn(II)/AlOOH system to monitor changes in the local environment of Mn(II), were also measured as adsorption occurred. The results indicate that Mg(II) adsorbs at relatively few, isolated sites on the goethite surface. When Mn(II) adsorbs on boehmite it forms magnetically ordered arrays, which are referred to as surface 'clusters'. On the basis of the negligible effect of adsorption on surface charge, it was concluded that there is a high probability of 'cluster' formation when Mn(II) adsorbs on goethite and Mg(II) adsorbs on boehmite. The mechanism of adsorption and the tendency to either form 'clusters' or adsorb at isolated sites is strongly dependent on the nature of the oxide surface.