Effect of 1-1 electrolyte concentration on the adsorption/desorption of copper ion on synthetic birnessite

Abstract

Purpose Oxides are ubiquitous in nature and play an important role in scavenging metal ions from soils and sediments. At the common pH range of the natural environment the well-studied Fe and Al oxides mostly carry a positive charge and adsorbed amounts of heavy metals, and their desorption percentages decrease with increasing ionic strength. The less well studied but also important Mn oxides possess negative charges in the natural environment and this will lead to a different behavior. Therefore, it is useful to further investigate how the electrolyte concentration and type affect the metal ion adsorption/desorption by Mn oxides. Materials and methods The phyllomanganate birnessite was synthesized with hydrochloric acid and potassium permanganate, and characterized by X-ray diffraction and transmission electron microscopy. The point of zero charge and specific surface area (SSA) were determined by, respectively, the rapid potentiometric titration method and BET-N2 method. The adsorption was measured after shaking the samples in contact with Cu(NO3)2 solution for 2 h and further equilibration for 22 h at pH 4.5 and 25± 1°C. The 1-1 electrolyte concentrations were adjusted to 0, 0.001, 0.01, 0.1, 0.5, and 1 mol L −1 KNO3 or KCl. Results and discussion The PZC and SSA of synthetic birnessite were 2.5 and 75 m 2 g −1 , respectively. The birnessite consisted of small needle-like particles. A maximum amount of Cu 2+ adsorbed on birnessite of 208±8 mmol kg −1 and 2.77±0.11 µmol m −2 by using the BET area is obtained. The adsorption amount decreased gradually with increasing ionic strength. This is primarily due to screening of the electrostatic attraction. For the same reason the percentage of desorption of Cu 2+ previously adsorbed on birnessite increased with increasing electrolyte concentration. Furthermore, the decrease of Cu 2+ adsorption with increasing ionic strength was higher in KCl solution than in KNO 3 solution. This difference is most likely related to Cu 2+ –Cl − complexation.