Comparative study of the adsorption-desorption behaviour of copper(II), zinc(II), cobalt(II) and lead(II) at the goethite solution interface

Abstract

The relative position of the adsorption curves for copper(II), zinc (II), cobalt(II) and lead(II) with respect to pH is shown to be related to the hydrolysability of the metal ion in solution. Thus the pH values at which the same molar amounts of these cations are adsorbed on the oxide surface are correlated with their respective first hydrolysis constants (pKt*) in solution. Such adsorption occurs well below the pKi* values, resulting in the release of one to two moles of protons per mole of cation specifically adsorbed. The relative effect of anions such as chloride and nitrate on the adsorption-desorption behaviour of zinc(II), cobalt(II) and lead(II) has also been investigated. Desorption trends for similar molar amounts of adsorbed cations, copper(II) zinc(II) and cobalt(II) are identical. The existence of two distinct types of adsorption sites on the oxide surface corresponding with the 'readily desorbed' and 'less readily desorbed' fractions of micronutrient cations such as copper, zinc or cobalt has been confirmed. The metal cation concentration and proton concentration play a dual role in determining the partition of heavy metal cations between the solution and surface phases. The position of the adsorption curve for lead(II) with respect to pH is close to that for copper(II), but desorption of copper(II) shows a considerable hysteresis, while lead(II) does not. Specifically adsorbed lead in the presence of either chloride or nitrate is reversible with respect to its concentration in solution with no hysteresis between adsorption and desorption isotherms. All the adsorbed lead is 'readily desorbable'.