New insights into the dynamics of adsorption equilibria of humic matter as revealed by radiotracer studies

Abstract

The mobility of contaminants in the subsurface hydrosphere can be governed by their interaction with aquatic humic substances, which may act as carriers. For modelling migration processes, retardation of humic molecules at mineral surfaces must be considered. There is, however, a lack of clarity concerning the reversibility of adsorption of these natural polyelectrolytes. In this work, evidence was provided that a dynamic adsorption equilibrium exists. For this purpose, adsorption of humic substances (purified Aldrich humic acid and an aquatic fulvic acid) onto kaolinite was examined in tracer exchange studies by means of 14C-labelled humic material. In addition, the kinetics of adsorption and desorption were investigated in batch experiments.Attaining the equilibrium state of adsorption took considerably longer for the humic acid than for the fulvic acid (24h and 4h, respectively). In desorption experiments, initiated by diluting the supernatant at constant pH, no net release was observed for both substances within a time frame of 4weeks. However, when introducing radiolabelled humic or fulvic acid as a tracer into pre-equilibrated adsorption systems in the state of surface saturation, quantitative exchange was found to take place. This indicates that adsorption of humic matter is in fact a reversible process, albeit an exchange time of more than 4weeks was required for both humic materials. Models for humic-bound contaminant transport (presuming dynamic equilibria) are thus applicable under appropriate conditions.