Applicability of the Langmuir equation to copper sorption by loess with high carbonate content

Abstract

The Langmuir equation has been widely used for description of the adsorption characteristics in different solid–liquid systems, although it was originally advanced for adsorption of gas molecules on a planar surface. In this paper, the applicability of the Langmuir equation was discussed when it is modified or extended to describe sorption on heterogeneous surfaces such as soils or soil components. The sorption of Cu(II) by loess was investigated experimentally with sediment concentration ranging from 10 to 100 kg/m³, and copper concentration from 64.80 to 7499 mg/l. The measured results were found well fitted with the nonlinear Langmuir isotherm in form, but for the loess with high carbonate content, the Cu precipitation becomes the primary mechanism instead of the original adsorption in the ordinary cases with low carbonate content. Thus, the parameters identified from the same Langmuir form would be essentially different in their physical meanings comparing with those in the conventional Langmuir equation. Using the MINTEQA model, copper precipitation was calculated and the implications of parameters in Langmuir equation were interpreted. Both the `sorption coefficient' and the `sorption maximum' in the Langmuir equation for copper precipitation are very close to those for copper sorption, but the `sorption coefficient' for copper precipitation is greater than that for copper sorption and oppositely for the `sorption maximum', which explains why precipitation dominates the sorption process.