Kinetics of Humic Acid Adsorption at Solid-Water Interfaces MARCELO J. AVENA † AND LUUK K. KOOPAL* Laboratory for Physical Chemistry & Colloid Science, Wageningen Agricultural University, P.O. Box 8038, 6700 EK Wageningen, The Netherlands The adsorption kinetics of purified Aldrich humic acid (PAHA) onto hydrophilic (Fe 2 O 3 and Al 2 O 3 ) and hydrophobic (polystyrene and silanized SiO 2 ) surfaces are studied by reflectometry. The initial rate of adsorption depends on the rate of transport and the rate of attachment. Attachment on hydrophilic surfaces is relatively fast at low pH where surface and HA attract each other electrostatically. Moreover, carboxylic and phenolic groups are exposed to the outside of the HA molecules, and these groups form complexes with surface hydroxyl groups. Due to the high attachment rate the process is transport-limited. At high pH, where surface and HA repel each other electrostatically, attachment is slow, and the adsorption rate is attachment- limited. At hydrophobic surfaces attachment of HA takes place through hydrophobic attraction. Hydrophobic groups are hidden in the inner part of HA molecules, and structural rearrangements are required before attachment can occur. The slow attachment leads to an attachment- limited rate. Increasing the pH increases the number of charged groups at the outside of the molecules, and the rate of attachment becomes even slower. Adsorption rate variations with electrolyte concentration also reveal the dynamics and flexibility of the HA molecules and the sensitivity of attachment for electrostatic effects. Introduction Humic substances such as humic acids (HA)and fulvic acids (FA) are natural weak polyelectrolytes that are very active in binding ions and organic molecules (1, 2). These properties make HA and FA important in regulating the speciation, mobility,and transport ofions,nutrients,and contaminants in soils and aquifers (3, 4). Humic substances have also a relatively strong affinity for oxide surfaces and tend to be attached to particulate matter (5-8). Thus, the reactive properties ofHAand FAcan be significantlymodified bythe presence of oxide or clay particles and vice versa. Therefore, the mechanisms ofbinding ofhumics to solid surfaces have to be known in order to improve the understanding of the role that these substances play in the environment. The adsorption of HAand FAon oxide or clay surfaces has been experimentally studied as a function of different variablessuch aspH,electrolyte concentration,type ofhumic, etc. (7-12). Most of the studies were performed by “equi- librium” adsorption experiments in which a mixture of adsorbate and adsorbent is “equilibrated” at the desired conditions, and the adsorbed amount or the remaining nonadsorbed amount is then measured. This kind of experi- ments allows studying the effects of the different variables on the affinity of the humics for different surfaces. Together with adsorption experiments, desorption studies can be performed in order to test the reversibilityofthe process and to gain insights on the HA-solid surface interaction.Although veryinterestingaspects ofthis interaction have been revealed bydesorption studies,onlya fewarticleshave been published so far (5, 13), and much work has still to be done in this field. Detailed studies about the kinetics of the adsorption or desorption processes have not appeared in the literature yet. Most of the reports point out that the adsorption is fast but do not present a detailed kinetic analysis of the process (6, 11). Due to the fact that the initial steps of adsorption take place quickly, there are not many techniques that are able to follow the attachment of molecules in a short time scale. This scarcity of techniques may be the main reason kinetics has received little attention. Pinheiro et al. (14) successfully applied alternating current voltammetry to study the ad- sorption of HAon the Hg/aqueous solution interface. They have shown that the kinetics is affected not onlybytransport processes but also by reactions at the surface as well as by electrostatic interactions between adsorbing molecules and previously adsorbed ones. Unfortunately, voltammetry can- not be easily applied to study the mentioned processes on metaloxidesorclays.Recently,Avena and Koopal(13)showed that reflectometry can be used to follow the kinetics of HA adsorption and desorption on an iron oxide surface. It was shown that the rate ofadsorption levels offstronglyafter the first 100 s. In 0.1 M salt concentration and for a HA concentration of 50 mg/L, about 1.5 mg/m 2 of material is adsorbed in the first 100 s at pH 3.25. Much slower processes thatcan continueformanyhours(8)followthisquickprocess. The slow adsorption processes can be attributed to the presence of long-range electrostatic repulsion and/or rear- rangements in the adsorbed layer either by conformation changesor byexchange ofadsorbed HAmoleculeswith newly arrivingones oflarger molecular mass.Polydispersityeffects on the adsorption have been studied both experimentally (byreplacement ofpreadsorbed FAbyHA)and theoretically by Vermeer and Koopal (8). The aim of this article is to analyze the fast initial adsorption step in more detailbyinvestigatingthe influence of supporting electrolyte concentration, pH and HA con- centration on the adsorption kinetics of a HA sample on varioussurfaces.The processesare followed byreflectometry in a stagnation point flow cell, and the results are analyzed on the basis of a simple model for the adsorption kinetics. The slow processes following the initial adsorption step will not be considered. Materials and Methods Chemicals were p.a.quality,and high puritywater was used. All experiments were performed at 25 °C at a supporting KNO3 concentration ranging between 0.003 and 0.8 M and in the pH range 3.5-10.0. Humic Acid. Apurified Aldrich humic acid (PAHA) was used. The method of purification of this sample and some physical and chemical properties have been reported previ- ously (7, 15, 16). The elemental analysis of PAHA is (wt) C, 55.8%; O, 38.9%; H, 4.6%; and N, 0.6%, and its total acidity is about 5 mmol/g. According to Vermeer (15) the weight average molecular mass, Mw, of PAHA obtained by size exclusion chromatographyusingproteinsasstandardsequals *Corresponding author phone: +31 317 482629; fax: +31 317 483777; e-mail: koopal@fenk.wau.nl. † On leave of absence from INFIQC, Departamento de Fisico- quı ´mica, Facultad de Ciencias Quı ´micas, Universidad Nacional de Co ´rdoba, Co ´rdoba, Argentina. Environ. Sci. Technol. 1999, 33, 2739-2744 10.1021/es981236u CCC: $18.00 © 1999 American Chemical Society VOL. 33, NO. 16, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2739 Published on Web 07/14/1999