Effect of Electrolyte on Adsorption/Desorption of Cu2+ on Nano-particle Mn Oxide

Abstract

Oxides are the active and important component in soils and sediments. They are generally in fine particles and attached to surface of soil clay as a cutan or coating. Therefore, they play an important role in controlling chemical reactions of soil interface, and thus affect bioavailability and transformation of heavy metals in soils and sediments. At the common pH range of the natural environment, Fe and Al oxides possess positive charge. The adsorption amounts of heavy metals on Fe and Al oxides increased and their desorption percentage decreased with increasing ionic strength. However, Mn oxide has a low point of zero charge (PZC) and thus displays negative charges in the natural environment. It is unclear how electrolyte affects adsorption-desorption by Mn oxides. This paper studied the influences of electrolytes (KNO3, KCl) on Cu2+ adsorption and desorption on synthetic birnessite. The PZC and specific surface area (SSA) of birnessite was 2.5 and 75 m2·g−1, respectively. The birnessite consisted of clusters or ball-like aggregates with a size of 50∼100 nm. The pH of birnessite in suspension increased slightly with increasing electrolyte concentration, but the effect of KCl solution was higher than in the effect of KNO3 at a given concentration. The amount of Cu2+ absorbed on birnessite decreased gradually with increasing ionic strength. However, the desorption percentage of Cu2+ absorbed on birnessite surface increased with increasing electrolyte concentration. Furthermore, the desorption percentage of Cu2+ was higher in KCl solution than in KNO3 solution. These results are different from the adsorption/desorption of Fe and Al oxides.