MECHANISM OF ZINC ADSORPTION BY IRON AND ALUMINUM OXIDES

Abstract

Adsorption of Zn from aqueous solutions of ZnCl2 by hydrated Al2O3 and Fe2O3 was highly pH-dependent and increased markedly with increasing pH. Fe2O3 had a higher Zn adsorption capacity, at equivalent pH values, than Al2O3. Hydrogen ions were released upon adsorption of Zn2+ on the surface of Al2O3 and Fe2O3, suggesting that surface aquo (-OH2) and hydroxo (-OH) groups were involved in Zn adsorption. The molar ratio of H+ released to Zn2+ adsorbed onto Fe2O3 increased from 1.53 to 1.96 as suspension pH increased from 5.93 to 6.96, whereas the molar ratio of adsorbed Cl- to adsorbed Zn2+. Similiar results were obtained for Al1O3. It was postulated that two types of adsorption, specific and nonspecific, depending upon reversibility (exchangeeability with Ba2+, occured. Specific adsorption involved adsorption of Zn2+ and release of two H+ions for each mole of Zn2+ adsorbed and accounted for 60 to 90 percent of total Zn adsorption by Fe2O3. An olation bridge structure between Zn2+ and two surface -OH2 groups was postulated. Nonspecfic adsorption of Zn accounted for 40 to 10 percent of the total Zn adsorption, and was thought to involve adsorption of ZnCl+ or Zn2+ plus Cl- and the release of one H+ion for each mole ov Zn2+ adsorbed. A single-bond structure in which ZnCl+ replaced one H+ from surface -OH2 groups was postulated. The raio of specific to nonspecific adsorption increased with increasing pH. It was concluded that specific adsorption of Zn by Al2O3 and Fe2O3 may be at least partially responsible for the frequently responsible for the frequently reported fixation and unavailability of added Zn to soils.