Environ. Sei. Technoi. 1993, 27, 2807-2813
Transport of Humic Matter-Coated Hematite in Packed Beds Aria Amlrbahman and Terese M. Olson'
Department of Civil and Environmental Engineering, Unlverslty of California, Irvine, California 927 17 Column filtration experiments, using quartz grains as the bed media, were conducted to study the deposition kinetics of hematite particles in the presence of three wellcharacterized humic substances. Two pH conditions, 5.2 and 7.4, were selected such that the humic matter was adsorbed to an originally positive and negative hematite surface, respectively. Background NaCl concentrations ranged from 0.015 to 0.1 M. The particle deposition rates, expressed in terms of initial experimental attachment depended on the magnitude of both efficiencies (aexp), electrostatic and steric repulsive interaction energies. For exhibited a strong a given solution chemistry, values of aexp dependence on the type of the humic matter, while electrophoretic mobility measurements were independent of the type of surface coating and were solely a function of the pH and NaCl concentration. The lowest values of aerpwere obtained for particles coated with the largest molecular size humic material at all solution conditions. Their low attachment rates were attributed to greater steric interaction forces. Attachment efficiencies of hematite particles coated with two different fulvic acids were determined. It is proposed that differences in the attachment rates are related to differences in the surface conformations of the fulvic acids. Introduction
Suspended inorganic colloids in natural aqueous systems are commonly coated with natural organic matter (NOM) and as a result carry a net negative charge, irrespective of the original surface charge of the core particle (1-3). The adsorbed NOM greatly alters the physicochemical character of these particles and has a major impact on their reactivity, mobility, and stability. The chelating carboxylic and phenolic functional groups associated with NOM, for example, greatly increase the solubility of trace metals and radionuclides. Solubilities of hydrophobic organic pollutants are also enhanced in the presence of NOM primarily by hydrophobic interactions. The stability and transport of NOM-coated colloids are promoted at all solution conditions that are encountered in natural systems by the enhancement of electrostatic and steric repulsive interaction energies. Coagulation studies involving NOM-coated inorganic colloids have demonstrated the stabilizing role of the adsorbed NOM (4-7). Colloidal particle transport, in environmentally relevant systems, has been studied under well-controlled and in field environments (10laboratory conditions (8,9) 14). Although NOM has been detected in several field experiments and in laboratory columns packed with aquifer material (13-161, its role in particle transport has not been studied systematically. Liang et al. (14) attributed the increase in turbidity of their injection water to the presence of NOM-stabilized colloids. Ryan and Gschwend (11)described the role of NOM in promoting ~~~~
* To whom correspondence should be addressed. 0013-936X/93/0927-2807$04.00/0
0 1993 American Chemical Society
the dissolution of Fe(II1) oxide coatings and the mobilization of kaolinite. The exact mechanisms by which NOM influences colloid transport are not yet understood. We have conducted packed-bed column experiments to study the deposition kinetics of submicron-sized hematite (a-Fe2O3) particles in the presence of three well-characterized humic substances of different molecular sizes and acidities. Filtration experiments have been performed using clean quartz grains as the bed media at different pH and NaCl concentrations. In this paper, a mechanistic analysis of our observations regarding the transport of the humic matter-coated particles in packed beds is presented. Experimental Procedures and Approach
Materials Preparation and Characterization. Hematite particles were synthesized by the method of forced hydrolysis (17)using Fe(C104)3salt with some modifications developed by Liang (18). X-ray diffraction patterns confirmed that the solid particles were indeed hematite. At acidic pH and in the absence of electrolytes, the synthesized hematite particles had a mean diameter of 170nm as determined by photon correlation spectroscopy (PCS, Coulter Model NIMD). Scanning electron micrographs of 85 particles, however, indicated a mean diameter of 136 nm. This discrepancy may largely be due to the presence of doublets in the hematite suspension. Potentiometric titration and electrophoretic measurements indicated that the point of zero charge (pH,,,) and isoelectric point (pHi,,) were both approximately 6.5 for clean hematite, respectively. Electrophoretic mobilities (EM) were measured using a Rank Brothers Mark I1 microelectrophoresis apparatus. Both round and flat cells were used. Measurements were conducted at 25 "C by timing at least 10 particles in each direction at both stationary points. The difference between the mobilities measured at each stationary point was less than 5 % . Surfactant-free latex particles (Interfacial Dynamics Corp., Portland, OR) with sulfate functional groups and a mean diameter of 156 nm were also used for filtration experiments. Quartz grains were utilized as the collector media (Unimin, New Canaan, CT). Size fractionation and cleaning procedures were as described in an earlier study (9). A wet-sedimentation sizingmethod was used to obtain a mean grain diameter of 275 pm. The grains were subsequently roasted at 810 "C for a minimum of 8 h. After cooling, the surface was rehydrated by boiling in deionized water for at least 1 h. Filtration experiments with quartz media cleaned by other techniques, developed by Pashley and Kitchener (19) for quartz plates, such as treatment with hot 10N NaOH or fuming concentrated "03, failed to render reproducible results. To measure EM values, quartz grains were crushed to colloidal size (approximately 1.3 pm) using an agate mortar and pestle and processed according to the procedure developed by Li and de Bruyn (20) and modified by Litton and Olson (9). Environ. Sci. Technol., Vol. 27,
No. 13, 1993 2807
Three different humic substances were used. Wellcharacterized reference peat humic acid (PHA) and peat fulvic acid (PFA) were obtained from the International Humic Substances Society (IHSS) in a freeze-dried form and were dissolved in deionized water. The fulvic acid sample dissolved readily. PHA, however, due to its incomplete dissolution in deionized water (apparently because of the abundance of hydrophobic moieties) was dissolved in a pH 10 solution and immediately passed through a column of Amberlite IR-l2O(plus) ion-exchange resin in H+ form (Aldrich Chemicals, Milwaukee, WI). The fulvic portion of a Georgetown, SC, surface water (GFA)was isolated using an XAD-8 resin (Rohm and Haas, Philadelphia, PA). The resin purification procedure and the isolation technique were conducted according to the method developed by Thurman and Malcom (21). Potentiometric titrations of the humic substances were conducted at an ionic strength of 0.05 M under an Nz atmosphere at 25 "C to evaluate their overall acidities. Elemental analyses were performed on an ash-free and moisture-free basis by Huffman Laboratories (Golden, CO). Molecular size (MS) distributions of the humic samples were determined by an ultrafiltration technique. These experiments were performed using filter membranes ranging from 500 to 100 000 Da (Amicon,Beverly, MA) at 50 psi. The solutions contained 15 mg of C (L of humic material)-' and 0.05 M NaC1, and the pH was 5.2. Adsorption isotherms at pH 5.2 and pH 7.4 and an ionic strength of 0.05 M NaCl were developed for humichematite systems at room temperature. By choosing these pH conditions, NOM adsorption to positively and negatively charged hematite was studied. The solids concentration was 200 mgL-l. The adsorption batches were prepared by diluting aliquots of humics and salt in deionized water, or a solution buffered with NaHC03, and then adjusting the pH. Particles were then added, and the pH was again adjusted to the desired value. The dispersions were shaken for 24 h and passed through 0.050pm polycarbonate filters. Adsorption of humic matter to the filter membranes was generally lower than 5%. The concentration of humic substances was determined using a double-beam spectrophotometer (Hitachi U-2000) at 254 nm. The filtrate had the same absorbance as the original humic solution for a given organic carbon concentration; that is, the adsorption was not wavelength-specific. Filtration Experiments. A general scheme of the filtration system used here is described by Litton and Olson (9). Particle dispersions were prepared at least 15 h before the experiment. Quartz grains packed by the tap-and-fill method were equilibrated with a minimum of 10pore vols of solution adjusted with the background pH and electrolyte (NaC1) to ensure uniform surface and solution chemistry throughout the column. The column was then equilibrated with a recirculating solution of background salt and humic matter for a minimum of 15 h prior to the run. This equilibration procedure was found to be absolutely essential in producing consistent results, despite the fact that the loss of NOM from solution due to adsorption to the quartz surface was not detectable. The importance of preadsorption of polymers to colloids in initial coagulation rate studies is also discussed by de Witt and van de Ven (22). Influent and effluent particle concentrations were measured with a spectrophotometer using a 10-cm flowthrough cell at wavelengths of 430 and 234 nm for hematite 2808
Envlron. Scl. Technol., Vol. 27, No. 13, 1993
(4 X lolo particles L-l) and latex (7 X 1O1O particles L-l) dispersions, respectively. These concentrations were chosen based upon the sensitivity of the spectrophotometer and minimization of the particle blocking effect. The humic matter concentration in all the filtration experiments was 1 mg of C L-l. Average velocities of 1.8 and 0.25 cm min-1 were used. Colloid Deposition Rates. Theories of colloid transport in packed beds have been extensively reviewed in the literature from both macroscale and microscale perspectives (23). Particle deposition rates are often expressed in terms of a dimensionless single collector efficiency ( v ) , which is the ratio of the rate of particle attachment to the rate of particle approach to the collector surface. The attachment efficiency (aex,,)reflects the effect of surface and solution chemistry on deposition kinetics and is defined as rate of particles attaching to the surface rate of particles reaching the surface The relationship between aexpand 7 is as follows: cyexp = vivo where 'lois the collection efficiency. The collection efficiencies in this study were determined experimentally by conducting column experiments in the absence of humic matter at favorable filtration conditions (24),at a pH of 3.8 and a salt concentration of 0.01 M. This solution chemistry was chosen such that the enhanced deposition rate, due to the long range of the attractive double layers, would be minimized (25). The experimental values for vo were slightly greater than theoretical predictions (24)over the range of flow velocities used here. For average velocities of 1.8 and 0.25 cm min-', qo,Jqothevalueswere 2.04 and 1.47, respectively. Breakthrough curves obtained from the column filtration experiments at unfavorable conditions were used to determine aexpvalues. The reaction rate constants were obtained by fitting the nondispersive portion of the effluent breakthrough curves to the solution of the linear chromatography model developed by Rajagopalan and Chu (26). Our analysis, using the solution of the advectivedispersive colloid transport equation ( 2 3 , indicates that the dispersion term in the transport equation can be neglected for the range of dispersion Peclet numbers (Pe) encountered in this work (Pe I 15). Dispersion must be considered when Pe 5 4. Peclet numbers were obtained by conducting column experiments using conservative tracers, The retention time in the 10-cm flow-through cell was also considered. The initial attachment efficiency was calculated by setting the reverse reaction rate constant that accounts for blocking effects (26) equal to zero. "exp
=
Results and Discussion
The results of potentiometric titrations for our humic samples expressed as charge densities for pH 5.2 and pH 7.4 are shown in Table I. These results indicate that PFA has the most and that PHA has the least acidic character. Different size fractions of humic substances may possess variable amounts of charge, Green et al. (28) conducted fluorescence quenching on fractionated IHSS Suwannee River humic acid and determined that higher MS fractions have higher apparent surface potentials than fractions with lower MS. Jekel(6), however,found that the larger fraction (MS > 3000) of the Ruhr River (Germany) organic matter,
Table 111. Adsorption Plateau for Humic Substances on Hematite
Table I. Charge Densities of Humic Substances humic substances
PH
charge density (mmol/g of C)
IHSS peat humic acid
5.2 7.4 5.2 7.4 5.2 7.4
-6.74 -8.58 -11.99 -14.80 -10.27 -12.29
IHSS peat fulvic acid Georgetown fulvic acid
humic substances
PH
adsorbed amount (mg of C/g of solid)
IHSS peat humic acid
5.2 7.4 5.2 7.4 5.2 7.4
14.52 9.04 7.56 4.68 9.79 6.54
IHSS peat fulvic acid Georgetown fulvic acid
Table 11. Elemental Composition of Humic Substances (in W )by Weight humic substances
C
0
H
atomic ash O/C
N
IHSS peat humic acids,* 56.82 4.06 34.91 3.74 1.92 IHSS peat fulvic acidasb 51.54 3.51 42.58 2.31 1.59 Georgetown fulvic acida 52.75 4.30 39.90 0.90 0.89
0.46 0.62 0.57
a Elemental analyses were performed by Huffman Laboratories (Golden,CO) on a dried sample basis. b Data were provided by Dr. R. L. Malcom (USGS, Denver, CO). 100 0 IHSS P e a t Fulvic Acid v Georgetown Fuivic Acid v IHSS P e a t H u m i c Acid
c 2 E-.
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,, ,
io5
Apparent Molecular Size Figure 1. Molecular size frequency distrlbutlon diagram for humic substances obtained using ultraflltratlon.
isolated using an anion-exchange resin, contained less total acidity than smaller size fractions. Potentiometric titrations of the size-fractionated humic matter were not performed in this study. Elemental composition data for the humic samples are shown in Table 11. These data suggest that PFA has the highest total oxygen to carbon ratio (O/C) followed by GFA and then PHA. The molecular size frequency distribution diagram determined from the ultrafiltration experiments is shown in Figure 1. Results obtained using this technique are highly dependent upon the conformation of the macromolecules in solution, which is in turn a function of pH, ionic strength, and organic concentration. Previous studies, using an ultrafiltration technique, have indicated that negatively charged macromolecules are retained more than neutral ones partly due to the increased hydration of charged species and orientation of the solvating water molecules (29, 30). Humic substances, because of their polyanionic nature, may behave similarly. Figure 1shows that PHA has a higher apparent MS and is more polydisperse than the two fulvic acid samples used. More than 70% of PFA and GFA are smaller than 3000 Da. PFA has a slightly higher average molecular size than GFA. In part, the size difference between the two fulvic acid samples may be explained by the higher hydration
energy of PFA due to its higher total acidity (Table I) and higher O/C (Table 11). The adsorption isotherms are of the high-affinity type. Coverages of the humic materials at saturation are shown in Table 111. For the three humic materials studied here, the extent of adsorption is inversely proportional to the solution pH, the O/C ratio, and the total acidities of the organic matter. Due to the weak polyacidic and hydrophobic nature as well as the structural, size, and surface charge variability of different moieties in a given humic material, a detailed understanding of the adsorption mechanisms as well as the conformations of the adsorbed humic substances is impractical. Ligand exchange is usually the dominant mechanism for adsorption of humic matter onto metal oxides (31), although hydrophobic effects, hydrogen bonding, and various types of bridging cannot be ruled out (32). Our isotherm data indicate an inverse relationship between the O/C and the acidity with the extent of adsorption. Similar correlations between O/C ratio have been observed by others (31, 33). In general, O/C ratios and the acidity are proportional to the energy of hydration. Higher acidity also results in greater intrachain repulsion. For a given solution chemistry, macromoleculeswith lower degrees of intrachain repulsion and solvency assume more compact conformations in the solution. Adsorption and the conformational behavior of humic matter may resemble those of the weak acidic polyelectrolytes with hydrophobic moieties. Conformations of model hydrophobic polyacids depend upon the solution chemistry as well as the size of the hydrophobic side chains (34, 35). At low degrees of ionization and high salt concentrations, these macromolecules take up coiled conformations due to intramolecular hydrophobic interactions and adsorb to a larger extent with more compact conformations. The adsorption behavior of our humic samples is consistent with these theories. PHA, which is the most hydrophobic macromolecule, has the highest adsorption plateau and, due to its larger molecular size, may possess the largest layer thickness compared to our fulvic acid samples. Despite its slightly smaller molecular size, GFA adsorbs to a higher extent and possibly forms more compact conformations than the more acidic and polar PFA. Several factors may account for the pH dependence of our isotherm plateaus. At pH 1.4,fewer favorable sites for the adsorption of humic materials exist on the negatively charged hematite. Since the humic molecules are also more ionized at this pH, the additional intra- and interchain repulsion would favor more open conformations and sterically-limited packing at the hematite surface (36). Due to their small specific surface area, adsorption experiments for quartz grains were not performed. Davis Environ. Scl. Technol., Vol. 27, No. 13, 1993 2809
3 0
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l o g [NaCl]
Flgure 2. Electrophoretic mobillty of uncoated and humic mattercoated hematite partlcles at (a) pH 5.2 and (b) pH 7.4 as a functlon of [NaCI]. Solid concentratlon was 4 X IOio particles L-I.
Flgure 3. Electrophoretic mobility of uncoated and humic mattercoated crushed quartz grains at (a) pH 5.2 and (b) pH 7.4 as a function of [NaCI].
(2) observed measurable amounts of adsorbed NOM on colloidal silica only at pH values less than 3. Results of our electrophoresis experiments, however, qualitatively suggest some adsorption of humic substances to crushed quartz surfaces (see below). The results of our electrophoresis experiments for humic-coated hematite and quartz are shown in Figures 2 and 3 as a function of ionic strength. The total humic concentration for all experiments was 1 mg of C L-l. Our adsorption isotherms indicate that nearly maximum surface coverage of humic matter is established at this concentration. Electrophoretic mobilities of the coated particles also remained constant at higher dosages of humic material. As shown in Figure 2, panels a and b, EM values for coated hematite particles were similar at the two pH conditions. Figure 3, panels a and b, however, indicate that EM values for humic-coated quartz particles are a function of solution pH, especially at lower ionic strengths. A t pH 5.2, quartz particles coated with organic matter are more electrophoretically mobile than uncoated quartz (Figure 3a). At pH 7.4, however, this trend is less apparent as the EM values for quartz are less dependent on the surface coverage (Figure 3b). The extent of NOM adsorption to quartz is apparently less at pH 7.4, therefore, than at pH 5.2. Figures 2a, 2b, and 3a show that the magnitude of the electrophoretic mobilities of hematite and quartz coated with humic matter is higher than those of uncoated colloids. Charge balance calculations for hematite-polyelectrolyte systems suggest that particles with adsorbed organic coatings have smaller surface charge densities than un-
coated particles, even if all functional groups of the adsorbed organic matter are ionized (7). The charge density of the adsorbed polyelectrolytes is, however, less than that of the polyelectrolytes in solution (37). As more adsorption takes place, for example, Tipping (1) has demonstrated that the negatively charged functional groups of the adsorbate become increasingly protonated. To explain the higher magnitude of the electrophoretic mobilities of coated particles relative to the uncoated particles, it is proposed that the adsorbed polyelectrolyte conformations further expand the diffuse double layer (38). Figures 2 and 3 also indicate that the EM values for our coated hematite and quartz particles at saturation coverage are nearly independent of the type of humic substances used. A direct surface charge determination of the humic matter-coated particles using potentiometric titrations requires major assumptions regarding the adsorption mechanisms and the extent of surface coverage for each humic sample. Due to the largely unknown nature of these mechanisms and their controlling effect on the distribution of surface functional groups, such assumptions are unwarranted. If according to the Gouy-Chapman theory, however, { potentials can be correlated to the charge density at the {-plane (for { < -50 mV); the various humic substances studied here confer comparable amounts of charge on the particle at saturation coverage (1, 37).
2810
Envlron. Scl. Technol., Vol. 27, No. 13, 1993
Filtration Experiments Results of the column filtration experiments for coated hematite particles, expressed as log aexp vs log [NaClI, are
0.0
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-16
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Figure 4. Plot of log aeqas a function of [NaCI] for humic mattercoated hematite particles. Solid and dotted lines are polynomial fits indicating asq values at pH 5.2 and pH 7.4, respectively.
shown in Figure 4. As these results suggest, deposition rates are highly dependent on the ionic strength. In systems where both particle and collector surfaces have a similar surface charge, the total interaction energy barrier increases with decreasing ionic strength. As this barrier increases, lower colloid attachment efficiencies are observed. An outward shift of the {-plane due to the presence of the adsorbed humic matter would enhance such barriers. From Figure 4 it is evident that, at both pH values, deposition rates are strongly dependent on the type of the humic material used. This is especially noteworthy since, in our systems, EM is only a function of the solution chemistry and not of the humic matter type. For a given salt concentration and pH, hematite particles in the than the two presence of PHA exhibit lower values of aexp fulvic acids. GFA-coated hematite colloids have the highest aexp. This dependence may partially be explained by molecular size differencesof the humic substances, given with MS distribution (Figure the limited correlation of aexp 1). Such direct correlations of colloid deposition rates with the total acidities (Table I) and the O K ratio of humic matter in solution (Table 11), however, do not exist. To investigate the effect of molecular size of the humic matter on the deposition kinetics, column experiments were performed using hematite coated with three specific size fractions of PHA at pH 5.2 and an ionic strength of 0.05 M. The humic matter concentration was again 1mg of C L-l. The aexp values for hematite particles coated with PHA fractions with molecular sizes less than 30 000, 10 000 and 3000 Da were 0.009,0.015, and 0.024, respectively. Comparison of these values with the data in Figure 4 reveal3 that the attachmentefficiencyof hematite coated with the PHA fraction smaller than 30 000 Da is almost halfway between the corresponding values for the unfractionated PHA-coated and the PFA-coated systems a t for the same solution conditions. The values of aeXp systems in the presence of the PHA fraction smaller than 10 000 Da and the unfractionated PFA nearly coincide. The measured EM for the coated particles were, however, independent of the size of PHA fractions and were nearly identical to the corresponding values for the unfractionated humic substances. Jekel(6) observed the same behavior in his shear-induced coagulation studies of silica and clay suspensions in the presence of fractionated NOM and divalent cations; particles coated with larger organic fractions were more stable.
Comparing just the colloids coated with the two fulvic acids and given that higher deposition rates were obtained with GFA-coated hematite, it does not seem reasonable to attribute their differences in attachment efficiency to (1) electrostatic repulsion-their electrophoretic mobilities are similar, (2) adsorption density differences-the adsorption density of GFA was actually greater than PFA, or (3) molecular size differences-the average molecular size of PFA is only slightly larger than GFA. A complex repulsion mechanism that depends on the adsorbed conformation differences, therefore, is required to explain the transport behavior of the two fulvic acid-coated particles. Factors that could perhaps explain these results are the effects of solvency differences on surface conformations (PFA is more acidic and has a higher O/C ratio than GFA) or structural rigidity differences. Sterically-induced stability has been reported for aqueous dispersions of hematite, silica, and clay particles in the presence of humic substances and divalent cations (4, 6). Unfortunately, it is not yet possible to ascertain the exact nature of the steric interaction between humic-coated surfaces. Two components, entropic and enthalpic energies, can contribute to the overall steric interaction energy. Entropic or elastic effects are induced when the adsorbed organic layers interpenetrate and compress upon the approach of two coated surfaces. Enthalpic or mixing energies are governedby the interactions between adsorbed segments and solvent molecules. For strongly hydrated chains, repulsion may result even before the interpenetration of these chains (39). Steric stabilization is induced if the resulting free steric energy of interaction is greater than the van der Waals dispersion energy between the approaching surfaces (40,41).The relative contributions of either component cannot be deduced from our data alone, since both depend on the conformation of adsorbed polyelectrolyte. Comparisons of the humic and fulvic acid-coated hematite attachment efficiencies provide qualitative evidence that steric interactions are more important in the presence of the larger humic acid coatings. Observations that support this conclusion include the following: (1) Universally lower attachment efficiencies of PHA-coated hematite were obtained relative to PFA-coated hematite, despite their similar electrophoretic mobilities. (2) Progressively lower attachment efficiencies were determined with colloids coated with increasingly larger molecular size fractions of the peat humic acid. Since many properties of humic acids co-varywith molecular size, the mechanism explaining the enhanced steric repulsion may not be simply attributed to this variable alone. The shifting of the {-plane is a very important phenomenon in explaining the enhanced stability of coated particles compared to uncoated particles. A purely electrostatic repulsion mechanism in this study, however, cannot explain the observed differences among the aexp values for various coatings, primarily due to the similarities in electrophoretic mobilities for various humic matter coatings. The litmus test for steric interactions is generally to conduct colloid stability measurements under conditions of negligible electrostatic repulsion (e.g., high ionic strength). A t pH 5.2, the aexpvalues in Figure 4 appear to tend toward unity at sufficiently high ionic strength. At pH 7.4, however, this trend is less apparent. Due to excessive particle coagulation at NaCl concentrations above 0.1 M, column experiments with NOM-coated Envlron. Scl. Technol., Vol. 27, No. 13, 1993 2811
0 0
-n
E.
1
v G e o r g e t o w n Fulvic T
1
I
Acid
IHSS P e a t Humic Acid
Literature Cited
.
,
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-1 8
pH= 7 4
T O C = 1 ppm I
-1 6
-1 4
-1 2
-10
-08
-06
-04
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log [NaCl]
Figure 5. Log cyew a s a function of [NaCi] for humic matter-coated latex colloids at pH 7.4. Latex particles have sulfate functional groups and a mean diameter of 156 nm. Solid lines are polynomlai flts to the data.
hematite were not performed at these salt concentrations. To suppress the electrostatic effects at sufficiently high NaCl concentrations, additional filtration experiments were conducted using coated latex colloids of approximately the same diameter as the hematite particles. Humic-coated latex suspensions remained relatively monodisperse even after 24 h at ionic strengths well above 0.1 M, as determined by PCS. Results of these experiments are shown in Figure 5. At these high salt concentrations, cyerp values were substantially less than unity. PHA-coated particles exhibited more stability than GFA-coated particles at all salt concentrations. Therefore, for humiccoated latex particles at high salt concentrations, steric repulsion appears to be the dominant repulsion mechanism and is sufficient to produce stability. The behavior of humic-coated hematite systems, however, may be indicative of electrosteric effects, where the stability is brought about by the combination of the two electrostatic and steric forces (42). One possible explanation for this behavior may be that the adsorbed conformations on the surface of hematite, as opposed to latex colloids, are simply not thick and hydrated enough at higher salt concentrations to overcome the dispersive forces without the presence of an electrostatic barrier. To obtain an improved mechanistic understanding of the influence of humic substances on particle stability and transport, additional studies with well-characterized surrogate polyelectrolytes are probably necessary. This is a difficult task, since no single polyelectrolyte can simultaneously reflect the polydispersity, weak acidic nature, hydrophobicity, and structural features of humic substances. Our findings underscore the need to further define the role of surface conformations of adsorbed humic material in controlling colloid stability and transport. Direct surface probe techniques that can help to verify the often indirect evidence provided by stability and filtration experiments are also needed. Acknowledgments
We acknowledge Dr. F. J. Wobber of the Office of Health and Environmental Research, Ecological Research Divi 2812
+ion, US. Department of Energy, for financially supporting this investigation under Contract DE-FG03893'1360849. The authors also thank Dr. J. F. McCarthy, of Oak Ridge National Laboratory, for providing the Georgetown surface water for this study. A scholarship from the Irvine Ranch Water District is gratefully acknowledged.
Environ. Scl. Technol., Vol. 27, No. 13, 1993
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