Radiotracer studies on the kinetics and equilibrium characteristics of adsorption of humic matter

Introduction
Humic substances are ubiquitous in near-surface natural waters, and they are known to act as carriers for organic and inorganic contaminants [1, 2]. In order to assess the impact of such humic-bound mobilization, transport models are developed (see [3] for a review). As a prerequisite, reaction rates for adsorption and desorption are commonly assumed to be high enough to ensure a steady local equilibrium under flow conditions. For humic matter as a polydisperse system of highly charged colloids, however, it is unclear whether a dynamic adsorption equilibrium (i.e., a permanent run of adsorption and desorption at equal rates) actually exists. Low recoveries in column experiments with geological materials suggest a limited reversibility.

Experimental
Using kaolinite as an adsorbent, the kinetics of adsorption and desorption were studied for a humic acid (Aldrich) and a fulvic acid (isolated from bog water). Their radiolabeling with 14C (accomplished by azo coupling with [14C]aniline) allowed sensitive detection and enabled tracer exchange experiments at surface saturation, providing direct insight into the dynamics of the adsorption equilibria for the first time. In these studies, a negligible amount of radiolabeled humic or fulvic acid was contacted with equilibrated systems of kaolinite and non-labeled humic material at different durations ranging from 6 hours to 4 weeks.

Results
The equilibrium state of adsorption was attained within few hours for the fulvic acid, whereas the process took considerably longer for the humic acid (~ 2 days), possibly as a consequence of competition effects within the polydisperse system [4]. In desorption experiments, initiated by diluting the supernatant, not any release was observed within a time frame of 4 weeks, neither for the humic acid nor for the fulvic acid. In view of transport modeling, this finding is rather disturbing since the basic assumptions do not hold if adsorption is irreversible. Our tracer exchange experiments, however, revealed that labeled humic material is adsorbed even though it is confronted with a saturated surface. Consequently, an exchange must take place, indicating a reversible process, albeit an exchange time of ~ 4 weeks was required for both materials until the adsorption equilibrium was quantitatively represented by the tracer. Apparently, the competitive situation in its presence is a stronger driving force for desorption than is a concentration gradient. Models for humic-bound transport are thus applicable under comparable conditions.

[1] MacKay & Gschwend (2001) ES&T 35, 1320-1328. [2] Dearlove et al. (1991) Radiochim. Acta 52/53, 83-89. [3] Lippold & Lippmann-Pipke (2009) J. Contam. Hydrol. 109, 40-48. [4] Van de Weerd et al. (1999) ES&T 33, 1675-1681.