Desorption Kinetics of Cadmium ²⁺ and Lead ²⁺ from Goethite

Abstract

It is generally accepted that trace metal concentrations in soil solution are primarily controlled by sorption and desorption reactions at the particle‐water interface. While numerous studies have been conducted to understand adsorption of these metals to soil minerals, less is known about long‐term adsorption‐desorption processes. The objective of this study was to examine the influence of residence time and organic acids on the desorption of Pb 2+ and Cd 2+ from goethite. Adsorption experiments were conducted at pH 6.0 for 1 wk (short‐term) and 20 wk (long‐term). Lead adsorption was nearly complete after 4 h, with very little additional sorption occurring during a 20‐wk period. In contrast, Cd showed a continuous slight increase in the remaining adsorption. Desorption experiments were conducted at pH 4.5 and desorption kinetics for Pb 2+ and Cd 2+ were slow compared with the sorption reaction. Trace metal removal from the goethite surface was not completely reversible during an 8‐h desorption period for all of the experiments, except for short‐term Cd 2+ in the presence of salicylate. For all experiments except long‐term Pb 2+ desorption, the quantity of metal desorbed from goethite followed the order salicylate > NaNO 3 > oxalate. It is postulated that the greater effectiveness of salicylate compared with oxalate was related to the ability of oxalate to form bridging or ternary complexes between the metal and the goethite surface. For all experiments except Pb 2+ sorption in the presence of oxalate a greater quantity of metal was desorbed for the short‐term compared with the long‐term experiment. However, these results were only statistically significant for Pb 2+ in the presence of salicylate. These results suggest that residence time effects observed by many researchers are much less prevalent at low pH values, and hence natural or anthropogenic reduction in soil pH may reduce the ability of the soil to naturally sequester trace metal cations over time.