Environ. Sci. Technol. 1997, 31, 656-664
Chemical Factors Influencing Colloid-Facilitated Transport of Contaminants in Porous Media SUJOY B. ROY† AND DAVID A. DZOMBAK* Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
The effects of colloids on the transport of two strongly sorbing solutessa hydrophobic organic compound, phenanthrene, and a metal ion, Ni2+swere studied in sandpacked laboratory columns under different pH and ionic strength conditions. Two types of column experiments were performed as follows: (i) sorption/mobilization experiments where the contaminant was first sorbed in the column under conditions where no colloids were released and mobilized under conditions where colloids were released as a result of ionic strength reduction in the influent; and (ii) transport experiments where the contaminant, dissolved or sorbed on colloids, was injected into columns packed with a strongly sorbing porous medium. In the first type of experiment, contaminant mobilization was significant only when all releasable colloids were flushed from the column. In all other cases, although high colloid particle concentrations were encountered, there was no marked effect on total contaminant concentrations. In the second type of experiment, colloid deposition efficiencies were shown to control the enhancement of transport. The deposition efficiency was a function of the pH (for a high organic content sand) and of the contaminant concentration (for a charged species such as Ni2+).
Introduction Transport of particles of colloidal size has been observed in several aquifers as a result of natural groundwater conditions (1, 2), of anthropogenic changes to groundwater chemistry (3, 4), or upon injection as artificially introduced tracers (5). One of the important implications of colloid transport in aquifers is the possible enhancement of contaminant transport associated with colloidal particles. The potential of manipulating colloids in aquifers to aid remediation, by reducing permeability or enhancing contaminant mobilization, has also been discussed (6, 7). A limited number of studies have provided evidence for colloid-enhanced contaminant transport at the field scale. Colloid-associated migration of actinide metal ions (such as plutonium and americium) has been reported at several sites (8-11). In all these studies, a significant fraction of the contaminants was found to be associated with groundwater colloids, and transport distances in three of them were greater than those predicted by considering dissolved-phase transport alone. In a study observing the effect of colloids on the mobility of hydrophobic organic compounds (HOCs), Backhus et al. (4) observed a slight correlation between turbidities (a surrogate for colloidal particle concentrations) of groundwater * Corresponding author fax: 412-268-7813; e-mail: dzombak@ ce.cmu.edu. † Present address: Tetra Tech Inc., 3746 Mt. Diablo Blvd., Suite 300, Lafayette, CA 94549.
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samples and concentrations of hydrophobic organic compounds, although the evidence of enhancement of contaminant transport was not conclusive. At a site where colloidenhanced transport of radionuclides was indicated (10), Kaplan et al. (2) found no significant effect on the transport of transition metals. Given the considerably more accurate analytical techniques for detecting radioactive emissions from isotopes [e.g., Bates et al. (12) were able to measure radioactivity from individual colloid-size particles], convincing data for colloid-enhanced transport are more likely to be obtained for these chemicals than for HOCs or radioactively stable metal ions. In spite of the limited evidence for field-scale colloidenhanced transport in porous media, this mechanism continues to be of great interest because several studies have clearly demonstrated in batch tests the association of contaminants with colloidal materials similar to those found in the subsurface (e.g., refs 13-16). In addition, the enhancement of contaminant transport in porous media columns in the presence of natural organic matter or charged, mineral colloids has also been demonstrated (17-23). Our objective in this work was to conduct a systematic laboratory investigation of the potential for enhancement of contaminant transport by charged colloidal particles in response to changes in solution chemistry. Colloid effects on the transport of two chemicals [a neutral, hydrophobic organic compound, phenanthrene, and a heavy metal ion, nickel (present as Ni2+)] were studied in four different porous media. These chemicals were chosen because of their strong sorption to natural materialssphenanthrene to organic matter containing materials and Ni2+ to mineral surfacessand because these chemicals represent two broad classes of contaminants in the subsurface whose transport can be affected by the presence of colloids. The molecular-level phenomena involved in the sorption of these two chemicals are different, but their macroscopic behavior is similar in that both sorb strongly to natural particles under certain conditions and thus are candidates for colloid-enhanced transport. The four porous media studied were all sands and chosen because of their high permeabilities and related potential for rapid contaminant transport. Two types of column experiments were performed as follows: (i) sorption/ mobilization experiments where a contaminant was first sorbed in the column under conditions where no colloids were released and mobilized under conditions where colloids were released as a result of ionic strength reduction in the influent; (ii) transport experiments where a contaminant, dissolved or sorbed on colloids, was injected into columns packed with a strongly sorbing porous medium. In this paper, mobilization is defined as the process in which sorbed compounds are removed from a porous medium by desorption and/or by association with colloidal particles that are being released. The first type of experiment was designed to explore the potential for rapid mobilization of a contaminant from a porous medium by release of colloids. In previous work (24), we demonstrated the relatively rapid release of colloids in sand columns upon reduction of the ionic strength of the influent due to increase of interparticle repulsions. In the experiments described in this work, the same technique was used to release colloids from sand columns, and the effect on contaminant mobilization was studied. Such a mechanism has been proposed as a remediation option (7) but has been evaluated for only a limited set of conditions (23). The second type of experiment was designed to study the potential enhancement of transport when colloids were preequilibrated with contaminants and injected into columns.
S0013-936X(96)00064-8 CCC: $14.00
1997 American Chemical Society
TABLE 1. Properties of Sand Used sand name
approx. depth
particle sizea (µm)
sand size fractiona (0.075-4.75 mm) (%)
porosityb
focc (%)
fine fraction
Lincolnd
near surface 35-40 ft near surface
d50 ) 250 d50 ) 550 d50 ) 230 600-850i
96 99 97 100
0.35 0.31 0.32 0.37
0.037 NDg 0.5 ND
∼1.5% by mass
