Colloid-Facilitated Transport of Strongly Sorbing Contaminants in Natural Porous Media: A Laboratory Column Study
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Colloid-Facilitated Transport ofStrongly Sorbing Contaminants inNatural Porous Media: ALaboratory Column StudyD A N I E L G R O L I M U N D ,M I C H A L B O R K O V E C , *K U R T B A R M E T T L E R , A N DH A N S S T I C H E R
Institute of Terrestrial Ecology, Swiss FederalInstitute of Technology, ETH-ITO,Grabenstrasse 3, 8952 Schlieren, Switzerland
IntroductionIn a natural porous medium, a strongly sorbing substanceis mainly associated with the solid phase, and consequently,the solution concentration of this substance in the porewater will be small. Since the solid phase is commonlyassumed to be stationary, one usually surmises that suchsubstances migrate very slowly in the subsurface and thuspose no contamination risks at distant locations from thepollutant source. Recently, however, this conjecture hasbeen challenged by the proposition that mobile, colloidalparticles could act as pollutant carriers and thus providea rapid transport pathway for strongly sorbing contaminants(1-4). The anticipated mechanism is summarized in Figure1.
The existence of various processes, which are essentialfor this mechanism to be operational, is well established.It is known that mobile colloidal particles can be releasedwithin soils or aquifers and travel in the subsurface zoneover large distances and, due to size exclusion effects, ateven higher velocities than conservative tracers (5-10).
It is also clear that an increase of the complexation powerof the mobile water phase (i.e., in the presence of organicmatter, cosolvents, or colloidal particles) leads to an increaseof the travel velocity of contaminants (3, 11-16). However,whether in-situ generated colloidal particles may signifi-cantly contribute to pollutant transport still remains unclear.One may consider the existing evidence to be either indirect(17, 18) or one might regard this transport pathway to beunimportant (1, 2, 19).
The aim of this paper is to provide unambiguousevidence that, in a natural porous material, colloid-facilitated transport can indeed represent the dominanttransport pathway for a strongly sorbing contaminant. Ourevidence is based on laboratory column experiments, wherewe use a non-calcareous soil as the stationary phase andleaching solutions containing Na+ and Ca2+ as major cationsand Pb2+ as a strongly sorbing model contaminant. Weshall demonstrate that in-situ mobilized colloidal particlesmay indeed provide the pathway for rapid transport of Pb.Upon changes in the major cation composition of theinfiltrating solutions, colloidal particles are released fromthe porous matrix and are transported upstream of a
nonretarded normality front. Provided these particles arereleased from a Pb-contaminated region, they will also carrysubstantial amounts of Pb. In the present situation, aclassical transport model, which neglects the mobilizationof colloidal particles, underestimates the total solutionconcentrations by many orders of magnitude. While weonly mimic the natural conditions in our experiments,similar processes are likely to be operational in the field.
Experimental SectionColumn Experiments. Non-calcareous soil from an (E)Bhorizon of an aquic dystric Eutrochrept with silt loam texture(6 g kg-1 organic carbon, pH 4.1 in water, cation exchangecapacity70 mol/g) was employed in this study. The soilmineralogy is dominated by vermiculite, illite, kaolinite,muscovite, and quartz with traces of chlorite and goethite.Soil aggregates (1.0-2.0 mm) were filled into glass columns10 mm in diameter and 100-120 mm in length. Thesolutions were prepared by dissolving the appropriateamounts of PbCl2, NaCl, and CaCl26 H2O (p.a. Merck) inwater from a Barnstead Nanopure apparatus. The feedsolutions were pumped by a HPLC pump through a degasserinto the column and collected with a fraction collector.Prior to the experiments, the columns were stabilized with200 pore volumes of 0.5 M CaCl2 solution, and columnparameters were determined in standard fashion by pulseexperiments with conservative tracers (20). Typical porevolumes were 6-8 mL; porosities were 0.70-0.75. Thesevalues lead to 1.25 ( 0.05 g/mL of dry mass of soil per porevolume. We have used flow rates of 0.1-0.3 mL/min, whichcorresponds to pore water velocities of 2.3-6.9 m/day. Priorto the experiments, the dispersivities were 1.5 mm andcolumn Peclet numbers (Pe) were 50-75. However, aftercolloid mobilization experiments, the latter number hasdecreased to 5-15. The feed solutions were unbuffered;at the column outlet, we have measured pH 5.5 ( 0.5. Thecollected fractions were split in two parts. One was acidifiedwith distilled HNO3 to pH < 2, sonicated, and analyzed forNa, Ca, Si, Al, and Fe by inductively coupled plasma atomicemission spectroscopy (Varian Liberty 200 ICP-AES) coupled
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FIGURE 1. Schematic representation of colloid-facilitated con-taminant transport within a pore of a water-saturated porous mediumin the subsurface. Contaminants (b) are either dissolved in the liquidphase or adsorbed to the surfaces of the solid phase. The entireparticular phase is commonly assumed to be at rest, but it is possiblethat colloidal particles disperse in the liquid phase and provide arapid transport pathway for the contaminant.
Environ. Sci. Technol. 1996, 30, 3118-3123
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- to ultrasonic nebularization (Cetac U-5000AT) and for Pbby graphite furnace atomic absorption spectroscopy (VarianSpectraAA 400). These measurements yield total concen-trations (dissolved and particulate) as verified by aciddigestion of the samples. The elements Si, Al, and Fe wereused as indicators of suspended colloidal particles; theirconcentrations were always proportional to each other.These elements were used to estimate particle concentra-tions by means of a calibration curve made with dry samplesof mobile colloidal particles (see below). The second partwas centrifuged for 2 h at 2 105 g (40 000 rpm), and thesupernatant was analyzed in a similar fashion. Thismeasurement gives the elemental concentrations of dis-solved species since these samples were effectively free ofsuspended particles (
approximately constant throughout the experiment. Fromelectron micrographs, one observes that the mobile colloidalparticles are very polydisperse with the largest particlediameter around 1 m. This value is well comparable withthe dynamic light scattering results; note that this tech-niques weights the large particles much more strongly (22).The particle size distribution of the mobile colloidal particles
can be estimated from dynamic light scattering and specificsurface area data. Assuming a log-normal mass distribu-tion, we find a mean particle diameter of 10 nm and acoefficient of variation of 1.3 (number-weighted quanti-ties). The other properties of the mobile colloidal particles(e.g., elemental content, mineral composition, specificsurface area) are rather similar to the fine clay fraction; we
FIGURE 2. Two column experiments demonstrating facilitated transport of Pb by in-situ mobilized colloids. (a) Succession of infiltratingsolutions mimicking possible contamination-remediation scenarios. Concentrations of various elements in the column effluent; totalconcentrations (dissolved and particulate, b) and concentrations of dissolved species (O). Solid lines are calculations based on the two-siteion-exchange transport model (see Table 2). (b) Pb models a strongly sorbing contaminant. (c) Al is proportional to the total concentrationof colloidal particles; major cations are shown in (d) Na and (e) Ca. The inset shows total concentrations of Si, Al, Fe, and Pb in thecolloid-facilitated transport regime. These elements are all associated with mobile colloidal particles; their concentrations are all proportionalto each other (parallel curves). Experiment I (left column) demonstrates colloid-facilitated transport near a contaminated zone. After thebreakthrough of dissolved Pb2+, the decrease in normality of the infiltrating solutions leads to two normality fronts. After the second normalityfront around 2810 pore volumes, the narrow peaks in Pb and Al concentrations reveal colloid-facilitated transport so that Pb is entirelyassociated with mobile colloidal particles. Experiment II (right column) shows that colloid-facilitated transport is also possible throughan uncontaminated zone. After the normality front around 90 pore volumes, the narrow peaks in Pb and Al concentrations reveal colloid-facilitated transport. The classical breakthrough of dissolved Pb2+ follows much later and is evidenced as the shallow peak with its maximumaround 400 pore volumes. In both experiments, colloid-facilitated transport dominates the transport of Pb for at least 50 pore volumes (seeinset).
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have no evidence for any preferential release of particularminerals. An apparent difference from the whole soil isthat the mobile colloidal particles and the fine clay fractioncontain smaller amounts of quartz.
If the porous matrix was originally contaminated, thereleased particles will be contaminated as well. In thepresent case, the solid phase is loaded with2 mg of Pb/g,and thus the released particles will carry a comparableamount of Pb. Their actual Pb content is 7 mg of Pb/g.This higher value reflects the larger specific surface area ofthe mobile colloidal particles (see Table 1). The colloidalparticles are rather effective carriers of Pb. Integration ofthe measured Pb breakthrough curve between the normalityfront and the retarded exchange front shows that the mobilecolloidal particles have transported 7% of the totaladsorbed amount of Pb out of the column. We also notethat colloid-facilitated transport can be also observed forCa, but to a much smaller extent (see Figure 2e, left).
If one would attempt to forecast the outcome of thiscontamination scenario on the basis of a classical transportmodel, one would severely underestimate the contaminantmobility and the risk of contamination. In such a classicaltransport model, one neglects the colloid-facilitated trans-port pathway. To illustrate this point, we have developeda simple transport model for the present situation. Thetransport model is based on an approximation of theconvection-dispersion equation with the mixing-cell model(stirred reactors in series). This transport model is coupledto an appropriate chemical equilibrium model, whichdescribes the liquid-solid partitioning of all importantspecies involved (26, 27). The appropriateness of suchtransport models in situations where the colloidal transportpathway is negligible has been demonstrated for varioussystems and scales (25-29).
Our chemical model for the present three-componentsituation is summarized in Table 2. The chemical modelwas mainly derived from the observed breakthrough curvesand is consistent with a limited batch data set. Theadsorption process is modeled with two different exchangesites. From all possible solution complexes, only PbCl+ isrelevant (31) and is included in the calculation. In orderto incorporate effects of mass transfer kinetics (28) andchemical heterogeneity (32), we employ a larger dispersivityparameter than measured by the conservative tracerexperiments; Pe ) 6 is used in all calculations. Thecalculated breakthrough curves are shown in Figure 2 (solid
lines). In spite of the low Peclet number used in thecalculations, the widths of the self-sharpening exchangefronts are mostly predicted as too narrow. An additionalreason could have to do with the fact that the dispersivityof the column has increased due to mobilization of colloidalparticles. Nevertheless, the model is able to reproduce themajor features of the experimental breakthrough curvesfor all dissolved ions in an approximate fashion. Clearly,this model cannot reproduce those parts of the break-through curves where colloid-facilitated transport is domi-nant. In the present case, this classical modeling approachdoes underestimate the total concentration of Pb in theeffluent by 6 (!) orders of magnitude. To a first approxima-tion, the transport of lead can be considered not to beinfluenced by the protons; the lead concentrations namelyexceed the proton concentration by more than 1 order ofmagnitude. From batch experiments, we know further that,in the concentration ranges considered in the columnexperiments, the pH dependence of the lead adsorptionisotherms is rather weak.
Colloid-Facilitated Transport Through an Uncontami-nated Zone. While the previous experiment shows thatcolloid-facilitated transport can be important within thecontaminated zone, it does not clarify to what extentcontaminated colloids are able to pass an uncontaminatedzone. To investigate this question, we have performed asimilar column experiment to the one discussed above (seeright column in Figure 2, experiment II). In this experimentthe column is infiltrated with 240 M Pb2+ solution in a 50mM NaCl background electrolyte such that the self-sharpening Pb front has penetrated about three-quartersinto the column. After this infiltration, the upper part ofthe column is contaminated with Pb at a solid concen-tration of 5.1 mg/g while the lower part is uncontami-nated. The remediation is now mimicked by infiltratingthe column with ground water, namely, a pure CaCl2solution of lower normality. Again two fronts develop,namely, a normality front, that exits the column around 90pore volumes and a retarded ion-exchange front around220 pore volumes. Between these two fronts, the conditionsare favorable for colloids release. The mobilized colloidsare contaminated with Pb and cause a peak in the total Pbconcentration 140 M at the column outlet. However,since these two fronts overtake the slowly moving Pb front,the colloids have now carried Pb through an uncontami-nated zone and reached the column outlet much soonerthan the classical breakthrough of the dissolved Pb2+
(shallow peak with its maximum 5 M around 400 porevolumes). As in the previous case, the inset in Figure 2b(right) shows that colloid-facilitated transport is dominant
Characterization of Mobile Colloidal Particles, FineClay Fraction, and Whole Soil
for more than 50 pore volumes. Because of the presenceof the uncontaminated zone in this experiment, the Pbcontent of the mobile colloidal particles is4 mg/g, a valuewhich is lower than in the previous experiment. Betweenthe normality front and retarded exchange front, 3% ofthe total sorbed amount of Pb was transported by thecolloidal particles. In contrast to the previous experiment,the smaller amount of mobilized lead could be related todesorption of Pb from mobile colloidal particles, depositionof contaminated particles, or different Pb concentration.The same...