Environ. Sci. Technol. 2005, 39, 807-813
Aggregation Kinetics of Kaolinite-Fulvic Acid Colloids as Affected by the Sorption of Cu and Pb ILONA HEIDMANN, ISO CHRISTL, AND RUBEN KRETZSCHMAR* Institute of Terrestrial Ecology, Swiss Federal Institute of Technology, Grabenstrasse 3, CH-8952 Schlieren, Switzerland
Aggregation kinetics of kaolinite-fulvic acid colloids, as influenced by two strongly sorbing trace metal cations, Cu(II) and Pb(II), was investigated by time-resolved dynamic laser light scattering experiments. The effects of Cu and Pb on the aggregation rate and electrophoretic mobility were compared with that of Ca, another major divalent metal cation which is less strongly adsorbed. Kaolinite-fulvic acid suspensions (in 0.01 M NaNO3 at pH 4 and pH 6) were spiked with solutions containing Cu, Pb, or Ca to give total divalent cation concentrations between 10-5 and 8 × 10-3 M. The concentration of kaolinite was varied between 25 and 200 mg L-1, while the concentration of fulvic acid ranged from 0.15 to 1.2 mg L-1. The mass ratio of kaolinite to fulvic acid was kept constant at 500:3 in all experiments. Relative aggregation rates, expressed as attachment efficiency R, were determined from linear increases in average hydrodynamic radius with time during the first 5-8 min of the aggregation experiments, always starting with a well-dispersed suspension at time zero. The corresponding slope for fast aggregation (R ) 1) was measured for pure kaolinite suspended in 0.01 M NaNO3 at pH 4. Addition of fulvic acid to the suspensions completely inhibited kaolinite aggregation at pH 4 and pH 6. Additions of Cu, Pb, and Ca resulted in strongly increased aggregation rates of the kaolinite-fulvic acid particles. The potential of the three cations to enhance aggregation of the kaolinitefulvic acid colloids increased in the order Ca < Cu e Pb. At pH 4, the relationship between particle electrophoretic mobility and aggregation rate was the same for all three divalent metal cations. In the presence of Ca, an increase in pH from 4 to 6 resulted in decreased aggregation rates. However, in the presence of Cu or Pb, the opposite trend was observed and the relationship between electrophoretic mobility and aggregation rate was different than at pH 4. The effects of Cu, Pb, and Ca on the aggregation rates of kaolinite-fulvic acid colloids are explained by the different sorption behavior of the three divalent metal cations.
Introduction Mobile colloids in soils and groundwater aquifers can facilitate the transport of strongly sorbing contaminants, such as trace metals, radionuclides, and hydrophobic organic * Corresponding author phone: +41-1-6336003; fax: +41-16331118; e-mail:
[email protected]. 10.1021/es049387m CCC: $30.25 Published on Web 12/30/2004
2005 American Chemical Society
compounds (1, 2). The mobility of colloids strongly depends on dispersion and aggregation of the particles (1-3). Dispersion leads to a release of particles from the solid matrix into solution, thereby generating mobile colloids (3). If the colloids are unstable, meaning that they aggregate, they will be immobilized in porous media because of filtration or sedimentation of larger aggregates. The aggregation rate of colloidal particles is strongly influenced by the diffuse layer charge and the resulting electrostatic forces acting between particles as a function of separation distance. Similarly charged particles must overcome a repulsive energy barrier to aggregate, leading to slow or reaction-limited aggregation kinetics. In this aggregation regime, the aggregation rate strongly depends on the diffuse layer charge, electrolyte type, and electrolyte concentration in solution. Increasing electrolyte concentrations lead to screening of surface charge and therefore increased aggregation rates. If the particles are nearly uncharged or oppositely charged, or if the salt concentration in solution is high enough to diminish the repulsive energy barrier, the aggregation kinetics is termed fast or transport-limited. In this regime, every particle collision results in aggregation and the rate is solely determined by the physical collision rate. The aggregation rate relative to the fast rate is often expressed in terms of the attachment efficiency R, which is the fraction of collisions leading to particle attachment. The minimum electrolyte concentration required to induce fast aggregation is called the critical coagulation concentration, ccc (4, 5). Clay mineral colloids in the environment are frequently coated with adsorbed natural organic matter, such as humic substances. Humic substances contain a large number of acidic functional groups, including carboxylic, phenolic, alcoholic, and carbonylic type groups (6, 7). Humic substances are negatively charged in the entire pH range between pH 3 and 10 (8). Therefore, adsorbed humic substances alter the colloidal stability of mineral particles significantly by changing the net particle charge and diffuse layer potential (9-15). In general, clay minerals coated with natural organic matter exhibit a much higher colloidal stability than the corresponding reference clay minerals (10, 14-17). The stabilization of clay colloids by adsorbed humic substances can be due to increased diffuse layer charge (electrostatic stabilization) and, additionally, by steric stabilization caused by adsorbed organic macromolecules (18). Adsorbed humic substances also add high-affinity sorption sites for trace metal cations to the clay colloid surface (19-21). Therefore, clay-humic colloids may not only exhibit a higher colloidal stability and mobility in the environment, but also a higher sorption affinity for trace metal contaminants. It is therefore important to understand the influence of strongly sorbing trace metal cations on the electrophoretic mobility and aggregation kinetics of clay-humic colloids. To date, the aggregation of clay-humic colloids has been studied in the presence of Na and Ca electrolytes, but the influence of strongly sorbing trace metals such as Cu and Pb on colloid aggregation kinetics has not been investigated. Therefore, the objectives of this study were to (i) investigate the effects of Cu(II) and Pb(II) on the aggregation kinetics of kaolinite colloids in the presence of adsorbed fulvic acid and (ii) to relate the observed aggregation rates to the electrophoretic mobility of the kaolinite particles. Kaolinite was chosen because it is a dominating clay mineral in many acidic soils of humid regions. Previous studies have shown that kaolinitic clays from surface soil horizons can exhibit high colloidal stability and mobility because of adsorbed humic substances (16, 22). In contrast, pure kaolinite VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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aggregates rapidly under acidic conditions, even at low ionic strengths (10).
Materials and Methods Kaolinite and Fulvic Acid. The size fraction
