CORRESPONDENCE
SIR: A recent publication by Davis and Leckie ( I ) discusses the influence of adsorbed complexing ligands on trace metal adsorption by hydrous oxides. Trace metal adsorption was decreased if the metal-complexing ligands were not adsorbed onto the hydrous oxide, unaffected or decreased if the ligands were adsorbed via their major coordinating group, or enhanced if the ligands were adsorbed with a strongly coordinating group directed outward toward the solution. In conjunction with those ligands enhancing adsorption, the authors state that “metal-ligand complexes can adsorb, or alternatively, (that) trace metals can be complexed by adsorbed ligands. A t the present time there are no analytical methods capable of distinguishing between these two mechanisms” ( I ). The authors, and the geochemical-hydrogeological community in general, apparently are not aware of the concept of anion-induced adsorption: it has been concluded from electroanalytical studies a t mercury electrodes that “the adsorption of the metal-anion complex is induced by means of, not in competition with, the adsorption of free anions” (2). The purposes of this letter are to: (a) acquaint those working in this field with the concept of anion-induced adsorption; (b) point out its applicability to the results of Davis and Leckie ( I ) ; and (c) indicate its possible usefulness in guiding future adsorption studies. (a) Anson ( 3 , and references therein) has studied the adsorption of metal complexes onto mercury electrode surfaces by chronocoulometry. In a recent review ( 3 ) ,he has discussed the requirements for anion-induced adsorption and has shown that the nature of not only the anion but also that of the metal ion are important. Anions such as C - , Br-, I-, S2032-, NCO-, NCS-, and Ns(Anson’s Class 113) are strongly adsorbed onto positively charged surfaces; their adsorption decreases as the negative charge on the electrode increases, but significant adsorption may be observed in spite of substantial negative charge densities on the elecmde surface. Covalent bonding with the electrode surface appears to be quite prominent among these anions, a fact which may account for the following sequence -;
