Adsorption-desorption behaviour of copper(II) at the goethite-solution interface

Abstract

Adsorption-desorption studies using copper(II) as the adsorbate and synthetic crystalline iron oxide, goethite (á-FeO.OH), as the adsorbent have been carried out on the acid side of the pH of isoelectric point (7.5) in the presence of a large excess of indifferent electrolyte. Specific adsorption of copper(II) on goethite is accompanied by the release of one to two moles of protons per mole of cation adsorbed. Desorption experiments have revealed two types of adsorption sites for copper(II) on the oxide surface, one of low bonding energy and the other of high bonding energy, corresponding with the 'readily desorbed' and 'less readily desorbed' fractions of copper(II) respectively. It is suggested that the two kinds of bonding are associated with the cation being coordinated to one and two surface-OH moieties respectively. The apparent hysteresis between adsorption and desorption isotherms is attributed to the existence of two types of adsorption sites of differing binding constants corresponding with the 'readily desorbed' and 'less readily desorbed' fractions of the cation respectively. The gradual interchange of some 'readily desorbed' copper(II) into a category that is not readily desorbed after an initial 'time-lag' between adsorption and desorption is attributed to a possible time-dependent reaction involving isomorphous substitution of lattice Fe3+ by Cu2+ of comparable ionic size. Some aspects of the relevance of present work to soil-plant system are indicated.