pubs.acs.org/Langmuir © 2010 American Chemical Society
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Retention of Latex Colloids on Calcite as a Function of Surface Roughness and Topography )
Gopala Krishna Darbha,† Thorsten Sch€afer,‡,§ Frank Heberling,‡ Andreas L€uttge, and Cornelius Fischer*,†,
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† Georg-August-Universit€ at, Goldschmidtstr. 3, D-37077 G€ ottingen, Germany, ‡Institut f€ ur Nukleare Entsorgung (INE), Karlsruher Institut f€ ur Technologie (KIT), D-76021 Karlsruhe, Germany, §Institute of Geological Sciences, Department of Earth Sciences, Freie Universit€ at Berlin, Berlin, Germany, and Rice University, 6100 Main Street, Houston, Texas 77005
Received September 7, 2009. Revised Manuscript Received February 6, 2010 Adhesion of colloidal particles to mineral and rock surfaces is important for environmental and technological processes. Surface topography variations of mineral and rock surfaces at the submicrometer scale may play a significant role in colloid retention in the environment. Here, we present colloid deposition data on calcite as a function of submicrometer surface roughness based on surface data over a field of view of several square millimeters, sufficient to trace the pattern of common inhomogeneities on mineral surfaces. A freshly cleaved calcite crystal was reacted to produce a well-defined etch pit density of ∼3.4 ( 1.2 to 8.3 ( 1.6 [10-3 μm-2] and etch pit depth ranging from ∼4 to 50 nm. This surface was exposed at the point of zero charge (PZC) of calcite to a colloidal suspension. We used a bimodal particle size distribution of nonfunctionalized polystyrene latex spheres with average diameters of 499 and 903 nm. Vertical scanning interferometry (VSI) was applied to quantify calcite surface topography variations as well as the retention of latex colloids. For both particle sizes, the experiments showed a positive correlation between the surface roughness (Rq) and the number of adsorbed particles. Etch pits were preferred sites for colloidal deposition in contrast to surface steps. The majority of adsorbed particles were trapped at etch pit walls compared to etch pit bottoms. Increasing pit density (D) and depth (d) resulted in an increase of colloidal retention. Deposition of smaller particles exceeded that of the larger-sized fraction of the bimodal system investigated here. Our results show that colloidal deposition at rough mineral and rock surfaces is an important geochemical process. The results about surface roughness dependent particle adsorption will foster the understanding and predictability of colloidal retention for a multitude of natural and technical processes.
Introduction In recent years, colloid-aided mobilization of contaminants in the subsurface environment has gained widespread acceptance.1-5 Sorption of contaminants to colloids in the aquifer leads to a dramatic enhancement of mobility and poses a potent threat to pollute nearby bodies of water.6,7 Numerous studies have been *Corresponding author. Email:
[email protected]. (1) McCarthy, J. F.; Zachara, J. M., Subsurface transport of contaminants mobile colloids in the subsurface environment may alter the transport of contaminants. Environ. Sci. Technol. 1989, 23, (5), 496-502. (2) Pelley, A. J.; Tufenkji, N., Effect of particle size and natural organic matter on the migration of nano- and microscale latex particles in saturated porous media. J. Colloid Interface Sci. 2008, 321, (1), 74-83. (3) Geckeis, H.; Sch€afer, T.; Hauser, W.; Rabung, T.; Missana, T.; Degueldre, C.; M€ori, A.; Eikenberg, J.; Fierz, T.; Alexander, W. R., Results of the colloid and radionuclide retention experiment (CRR) at the Grimsel Test Site (GTS), Switzerland Impact of reaction kinetics and speciation on radionuclide migration-. Radiochim. Acta 2004, 92, (9-11), 765-774. (4) M€ori, A.; Alexander, W. R.; Geckeis, H.; Hauser, W.; Sch€afer, T.; Eikenberg, J.; Fierz, T.; Degueldre, C.; Missana, T., The colloid and radionuclide retardation experiment at the Grimsel Test Site: influence of bentonite colloids on radionuclide migration in a fractured rock. Colloids Surf., A 2003, 217, (1-3), 33-47. (5) Sen, T. K.; Khilar, K. C., Review on subsurface colloids and colloidassociated contaminant transport in saturated porous media. Adv. Colloid Interface Sci. 2006, 119, (2-3), 71-96. (6) Baumann, T.; Fruhstorfer, P.; Klein, T.; Niessner, R., Colloid and heavy metal transport at landfill sites in direct contact with groundwater. Water Res. 2006, 40, (14), 2776-2786. (7) Ryan, J. N.; Elimelech, M., Colloid mobilization and transport in groundwater. Colloids Surf., A 1996, 107, 1-56. (8) Elimelech, M.; Omelia, C. R., Kinetics of deposition of colloidal particles in porous-media. Environ. Sci. Technol. 1990, 24, (10), 1528-1536. (9) Torkzaban, S.; Bradford, S. A.; Walker, S. L., Resolving the coupled effects of hydrodynamics and DLVO forces on colloid attachment in porous media. Langmuir 2007, 23, (19), 9652-9660.
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conducted on colloidal retention in both the unsaturated porous2,8-10 (vadose zone) and saturated porous11,12 media to determine the influence of various factors such as zeta potentials of colloids and interfaces,13,14 pH,15 surface charge heterogeneity,16 ionic strength of the solution,17-19 and colloidal size,20,21 (10) Delos, A.; Walther, C.; Sch€afer, T.; B€uchner, S., Size dispersion and colloid mediated radionuclide transport in a synthetic porous media. J. Colloid Interface Sci. 2008, 324, (1-2), 212-215. (11) Crist, J. T.; McCarthy, J. F.; Zevi, Y.; Baveye, P.; Throop, J. A.; Steenhuis, T. S., Pore-scale visualization of colloid transport and retention in partly saturated porous media. Vadose Zone Journal 2004, 3, (2), 444-450. (12) Bradford, S. A.; Torkzaban, S., Colloid transport and retention in unsaturated porous media: A review of interface-, collector-, and pore-scale processes and models. Vadose Zone Journal 2008, 7, (2), 667-681. (13) Adamczyk, Z.; Siwek, B.; Musial, E., Kinetics of colloid particle adsorption at heterogeneous surfaces. Langmuir 2001, 17, (15), 4529-4533. (14) Adamczyk, Z.; Jaszczolt, K.; Siwek, B.; Weronski, P., Irreversible adsorption of particles at random-site surfaces. J. Chem. Phys. 2004, 120, (23), 11155-11162. (15) Filby, A.; Plaschke, M.; Geckeis, H.; Fanghanel, T., Interaction of latex colloids with mineral surfaces and Grimsel granodiorite. J. Contamin. Hydrol. 2008, 102, (3-4), 273-284. (16) Chen, J. Y.; Klemic, J. F.; Elimelech, M., Micropatterning microscopic charge heterogeneity on flat surfaces for studying the interaction between colloidal particles and heterogeneously charged surfaces. Nano Lett. 2002, 2, (4), 393-396. (17) Adamczyk, Z., Particle adsorption and deposition: role of electrostatic interactions. Adv. Colloid Interface Sci. 2003, 100, 267-347. (18) Litton, G. M.; Olson, T. M., The Influence of Particle-Size on Latex Colloid Deposition Kinetics. Abstr. Pap. Am. Chem. Soc. 1995, 209, 140-Envr. (19) Litton, G. M.; Olson, T. M., Particle size effects on colloid deposition kinetics: Evidence of secondary minimum deposition. Colloids Surf., A 1996, 107, 273-283. (20) Alonso, U.; Missana, T.; Patelli, A.; Ceccato, D.; Albarran, N.; GarciaGutierrez, M.; Lopez-Torrubia, T.; Rigato, V., Quantification of Au nanoparticles retention on a heterogeneous rock surface. Colloids Surf., A 2009, 347, (1-3), 230-238.
Published on Web 03/05/2010
DOI: 10.1021/la9033595
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including the non-DLVO forces such as hydrophobic19,22 and capillary forces.23 In most of these studies, the interaction between artificial substrates and polystyrene particles were investigated. Although theoretical24-30 and experimental31-34 investigations suggest an enhanced colloid deposition with increasing surface roughness, it is often disregarded. For example, recently field experiments were conducted at Grimsel Test Site, Switzerland to counter the effects of bentonite colloid mediated transport of radionuclides (Am(III), Pu(IV)) in the fractured rock.3,4 While the analysis clearly reveals the size dispersion effects during the migration of natural submicrometer bentonite colloids,10 only 80-90% of total colloids were recovered under unfavorable (in terms of electrostatic forces) attachment conditions, and recovery strongly depended on fracture residence time.35 As another example, batch sorption experiments were performed to study the interaction of gold nanoparticles with granite.20,21 The experimental comparison of favorable vs unfavorable electrostatic conditions showed non-negligible colloidal retention in the crystalline rock under unfavorable electrostatic conditions. The main mechanisms responsible are discussed as mostly related to higher porosity of certain minerals or to physical defects of the granite surface, e.g., mineral surface roughness or surface inhomogeneities at grain boundaries.20,21 Results of experiments performed by Das and coauthors demonstrated that surface roughness provides a large enough restraining torque in predicting the hydrodynamic detachment of particles.36 In the experiments carried out for colloid straining and filtration in saturated porous media, the surface roughness of the grains caused an increased (21) Alonso, U.; Missana, T.; Patelli, A.; Rigato, V.; Ravagnan, J., Colloid diffusion in crystalline rock: An experimental methodology to measure diffusion coefficients and evaluate colloid size dependence. Earth Planet. Sci. Lett. 2007, 259, (3-4), 372-383. (22) Prescott, S. W.; Fellows, C. M.; Considine, R. F.; Drummond, C. J.; Gilbert, R. G., The interactions of amphiphilic latexes with surfaces: the effect of surface modifications and ionic strength. Polymer 2002, 43, (11), 3191-3198. (23) Lazouskaya, V.; Jin, Y.; Or, D., Interfacial interactions and colloid retention under steady flows in a capillary channel. J. Colloid Interface Sci. 2006, 303, (1), 171-184. (24) Bhattacharjee, S.; Ko, C. H.; Elimelech, M., DLVO interaction between rough surfaces. Langmuir 1998, 14, (12), 3365-3375. (25) Hoek, E. M. V.; Bhattacharjee, S.; Elimelech, M., Effect of membrane surface roughness on colloid-membrane DLVO interactions. Langmuir 2003, 19, (11), 4836-4847. (26) Hoek, E. M. V.; Agarwal, G. K., Extended DLVO interactions between spherical particles and rough surfaces. J. Colloid Interface Sci. 2006, 298, (1), 50-58. (27) Martines, E.; Csaderova, L.; Morgan, H.; Curtis, A. S. G.; Riehle, M. O., DLVO interaction energy between a sphere and a nano-patterned plate. Colloids Surf., A 2008, 318, (1-3), 45-52. (28) Jaiswal, R. P.; Kumar, G.; Kilroy, C. M.; Beaudoin, S. P., Modeling and validation of the van der Waals force during the adhesion of nanoscale objects to rough surfaces: a detailed description. Langmuir 2009, 25, (18), 10612-10623. (29) Rabinovich, Y. I.; Adler, J. J.; Ata, A.; Singh, R. K.; Moudgil, B. M., Adhesion between nanoscale rough surfaces - I. Role of asperity geometry. J. Colloid Interface Sci. 2000, 232, (1), 10-16. (30) Rabinovich, Y. I.; Adler, J. J.; Ata, A.; Singh, R. K.; Moudgil, B. M., Adhesion between nanoscale rough surfaces - II. Measurement and comparison with theory. J. Colloid Interface Sci. 2000, 232, (1), 17-24. (31) Bowen, W. R.; Doneva, T. A., Atomic force microscopy studies of membranes: Effect of surface roughness on double-layer interactions and particle adhesion. J. Colloid Interface Sci. 2000, 229, (2), 544-549. (32) Cooper, K.; Ohler, N.; Gupta, A.; Beaudoin, S., Analysis of contact interactions between a rough deformable colloid and a smooth substrate. J. Colloid Interface Sci. 2000, 222, (1), 63-74. (33) Cooper, K.; Gupta, A.; Beaudoin, S., Substrate morphology and particle adhesion in reacting systems. J. Colloid Interface Sci. 2000, 228, (2), 213-219. (34) Zhao, K.; Mason, T. G., Directing colloidal self-assembly through roughness-controlled depletion attractions. Phys. Rev. Lett. 2007, 99, (26), -. (35) Sch€afer, T.; Geckeis, H.; Bouby, M.; Fangh€anel, T., U, Th, Eu and colloid mobility in a granite fracture under near-natural flow conditions. Radiochim. Acta 2004, 92, 731-737. (36) Das, S. K.; Schechter, R. S.; Sharma, M. M., The role of surface-roughness and contact deformation on the hydrodynamic detachment of particles from surfaces. J. Colloid Interface Sci. 1994, 164, (1), 63-77.
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colloidal retention due to enhanced collision efficiency by a factor of 2 to 3.37 Deposition of colloids on surfaces is predominantly caused by factors such as electrostatic forces of attraction, chemical heterogeneity of surfaces, impurities, and so forth.15 However, close to the zero point of charge (PZC) of the mineral surface where minimal or no net electrostatic effects prevail, as well as under conditions of repulsive forces between particles and substrate, the importance of roughness for deposition of colloids is increased.20 Calculations of DLVO interaction energy between a sphere and simulated membrane surfaces predict significant reduction in the energy barrier with increasing surface roughness.25 Furthermore, experimental results show that the van der Waals (vdW) contact interactions of a polystyrene latex sphere and rough silicon substrate reduced the interaction energy by 90% because of the effectively larger separation distance between them.33 In light of these observations and calculations, deeper insight into the interaction between particles and rough surfaces is required. To understand and address potential colloidal adhesion pathways in the environment, it is essential to consider natural mineral surfaces and their potential surface topography variations for deposition experiments.38,39 The question of how mineral surface roughness variations can influence the colloidal deposition is still unanswered. Are there preferred crystallographic sites for particle adsorption related to mineral surface roughness variations? With the study presented here, we investigate and quantify the deposition of polystyrene particles at calcite crystal surfaces as a function of surface roughness. Although the retention of monodisperse latex colloids on several mineral surfaces has been studied by many groups under varying conditions, little work has been reported considering well-defined and reproducible surface topography variations on natural mineral and rock surfaces.15,21,40,41 For our experimental approach, we used a simple bimodal particle size distribution of polystyrene latex spheres (average diameter 903 and 499 nm) to represent important modes of natural grain-size distribution ranges. In the environment, aggregates of clay minerals and/or hydrated oxides and hydroxides, mainly of Si, Fe, and Al, are major components of suspensions due to weathering processes, soil formation, or diagenesis.42,43 An important example of natural iron oxides (formed during oxidative weathering) is ferrihydrite, often found to occur as spherical aggregates with diameters of approximately 500 nm.44 Furthermore, clay minerals in weathering profiles and soils often show a broad grain size distribution. Important
(37) Auset, M.; Keller, A. A., Pore-scale visualization of colloid straining and filtration in saturated porous media using micromodels. Water Resour. Res. 2006, 42, W12S02. (38) Fischer, C.; Karius, V.; L€uttge, A., Correlation between sub-micron surface roughness of iron oxide encrustations and trace element concentrations Sci. Total Environ. 2009, 407, 4703-4710. (39) Fischer, C.; Karius, V.; Weidler, P. G.; Luttge, A., Relationship between micrometer to submicrometer surface roughness and topography variations of natural iron oxides and trace element concentrations. Langmuir 2008, 24, (7), 3250-3266. (40) Patelli, A.; Alonso, U.; Rigato, V.; Missana, T.; Restello, S., Validation of the RBS analysis for colloid migration through a rough granite surface. Nucl. Instrum. Methods Phys. Res. Sect. B 2006, 249, 575-578. (41) Stumpf, S.; Stumpf, T.; Lutzenkirchen, J.; Walther, C.; Fanghanel, T., Immobilization of trivalent actinides by sorption onto quartz and incorporation into siliceous bulk: Investigations by TRLFS. J. Colloid Interface Sci. 2008, 318, (1), 5-14. (42) Cornell, R. M.; Schwertmann, U. The Iron Oxides; Wiley-VCH: Weinheim, 2003. (43) Jasmund, K.; Lagaly, G. Tonminerale und Tone; Steinkopff: Darmstadt, 1993; p 490. (44) Schwertmann, U.; Taylor, R. M. Iron Oxides. In Minerals in Soil Environments, Dixon, J. B., Weed, S. B., Eds.; Soil Science Society of America: Madison, WI, 1989; Vol. 1, pp 379-438.
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examples are aggregates of illite or kaolinite crystals with particle sizes of approximately 0.5-2 μm.45-47 Vertical scanning interferometry (VSI) was applied to characterize the calcite surface topography as well as to quantify the colloidal retention. The application of VSI enables to distinguish and quantify the more or less reactive sections of mineral surfaces within a large field of view, i.e., within several square millimeter, sufficient to trace the pattern of common inhomogeneities on mineral surfaces.39,48-51 In this study, we present quantitative results about the interaction of colloids with a common rock-forming mineral (calcite) surface as a function of surface roughness and topography parameters. The initial experimental approach involves the comparison of colloidal deposition rate between a flat (unreacted) to rough (etched) calcite at, above, and below the PZC. Later, the deposition behavior was compared at several surface sections bearing varying topographical parameters to assess the influence of roughness on interaction energy.
Materials and Methods Colloidal Particles and Calcite Surface Preparation. The nonfunctionalized polystyrene latex colloids were purchased from Duke Scientific (Thermo Fischer Scientific). Zeta potentials and photon correlation spectroscopy (PCS) measurements of the colloids were obtained with a ZetaPlus (Zeta Potential Analyzer, Brookhaven Instruments Corporation, USA). Calcite crystals (Icelandic spar from WARD’S Natural Science Establishment, LLC, Ontario, USA) were freshly cleaved with a clean razor blade along the (1014) plane. Typical dimensions of the crystal are 10 mm 7 mm 3 mm. All the experiments were performed at room temperature (22 C). The specific conductivity of the water was 0.054 μS/cm. The chemicals (NaHCO3, NaOH, and NaCl) used throughout this study are of research grade and were purchased from Merck and used without any further purification steps. Before the calcite surface is exposed to colloidal solution, it was etched by dipping the surface for 15 min in stirring water (200 rpm). The ζ-potential of calcite was determined by a streaming potential analyzer (Anton Paar Surpass Electrokinetic Analyzer) with plane-parallel channel cell method at ionic strength of 10-2 M NaCl solution and PZC was found to be at pH ∼8.9 (see Appendix, comparable to literature values).52 For each size class, the particle concentration was adjusted to 24 106 particles/mL in 10-2 M NaCl solution and pH was adjusted using 0.01 M NaOH. A systematic study on the dissolution of calcite by Lea et al.53 showed that the kinetics of dissolution is significantly retarded by an increased carbonate concentration (>900 μM) in the solution. Under these conditions, the rate of retreat of (45) Fanning, D. S.; Keramidas, V. Z.; El-Desoky, M. A. Micas. In Minerals in Soil Environments, Dixon, J. B., Weed, S. B., Eds.; Soil Science Society of America: Madison, Wisconsin, 1989; Vol. 1. (46) Meunier, A.; Velde, B. Illite; Springer: Berlin, 2004; p 300. (47) Velde, B. Origin and Mineralogy of Clays - Clays and the Environment; Springer: Berlin, 1995. (48) Luttge, A.; Winkler, U.; Lasaga, A. C., Interferometric study of the dolomite dissolution: A new conceptual model for mineral dissolution. Geochim. Cosmochim. Acta 2003, 67, (6), 1099-1116. (49) Luttge, A., Etch pit coalescence, surface area, and overall mineral dissolution rates. Am. Mineral. 2005, 90, (11-12), 1776-1783. (50) Fischer, C.; Luttge, A., Converged surface roughness parameters - A new tool to quantify rock surface morphology and reactivity alteration. Am. J. Sci. 2007, 307, (7), 955-973. (51) Adi, S.; Adi, H.; Chan, H. K.; Young, P. M.; Traini, D.; Yang, R. Y.; Yu, A. B., Scanning white-light interferometry as a novel technique to quantify the surface roughness of micron-sized particles for inhalation. Langmuir 2008, 24, (19), 11307-11312. (52) Somasundaran, P.; Agar, G. E., Zero point of charge of calcite. J. Colloid Interface Sci. 1967, 24, (4), 433-440. (53) Lea, A. S.; Amonette, J. E.; Baer, D. R.; Liang, Y.; Colton, N. G., Microscopic effects of carbonate, manganese, and strontium ions on calcite dissolution. Geochim. Cosmochim. Acta 2001, 65, (3), 369-379.
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Figure 1. Schematic diagram of the flow cell. Table 1. VSI Data Set Parameters Mirau objective magnification
field of view [μm μm]
virtual pixel length [nm]
20 1.6 100 1.6
467 467 93 93
228 45
structurally more opened obtuse steps [441]þ and confined (acute) [441]- steps slowed to a much greater extent with a decrease in step velocity (from Vss = 4.0 nm/s at 900 μm2 area were considered for deposition analysis.
Quantification of Colloid Deposition onto Calcite Surface. The individual number of colloids adsorbed was counted manually based on their size from the processed images. The kinetics of the colloidal deposition was determined by calculating the dimensionless Sherwood number (Sh)57 Jap Sh ¼ C0 D¥
ð1Þ
where ap is the radius of the particle, C0 is the bulk colloidal concentration, D¥ is the bulk diffusion coefficient, and J is the deposition flux and was found by determining the initial slope of the number of deposited colloids versus time and the value was normalized to the area under consideration. Here, for the bimodal system, Shtotal is the sum of the individual Sh values of 499 nm (Sh499) and 903 nm (Sh903). Previous studies usually reported higher Sherwood numbers because of experiments where particle deposition is due to strong electrostatic forces acting between oppositely charged surfaces.57 In the present case, since the number of colloids adsorbed is relatively small (at PZC of the substrate), in eq 1 ap governs the high or low values of Sh. Hence, for an equal number of small and big adsorbed particles, the ratio of Sh903 to Sh499 is 3.26.
(56) Frank, M.; Anderson, D.; Weeks, E. R.; Morris, J. F., Particle migration in pressure-driven flow of a Brownian suspension. J. Fluid Mech. 2003, 493, 363-378. (57) Kline, T. R.; Chen, G. X.; Walker, S. L., Colloidal deposition on remotely controlled charged micropatterned surfaces in a parallel-plate flow chamber. Langmuir 2008, 24, (17), 9381-9385.
4746 DOI: 10.1021/la9033595
-1 N -1 X X 1 M ½zðxk , yl Þ MN k ¼0 l ¼0
ð2Þ
The parameter F expresses the ratio between the measured surface area, Fm, and the area of the flat xy plane, which is the sampling area, F0. For a totally flat surface, the total and geometrical surface area and the sampling area are the same (F = 1). Fm F ¼ ð3Þ F0 In our case, Rq is sensitive for the number and density of surface steps as well as etch pits, e.g., higher amounts of Rq were observed for surfaces dominated by surface steps as well as for surfaces dominated by a high density of etch pits.
Determination of Parameters: Step Height (s), Etch Pit Depth (d), and Etch Pit Density (D). The average s and d for overall surface-section were determined by using the frequencyheight profile tool from SPIP. This is a convenient tool to identify the fraction of etch pits or steps with characteristic height difference. D is calculated as follows: D ¼
number of etch pits total area under considerationðμm2 Þ
Results and Discussion Characterization of Calcite Surface Topography. An etched calcite surface is characterized by rhombic etch pits and surface steps of varying height.53,58-62 An example of an etched calcite surface subsection and related height histogram is presented in Figure 3. A surface map, the respective histogram graph, as well as height range maps (1-5) show the occurrence of a distinct surface step and three depth maxima of etch pits. The observed calcite surface topography variations were characterized by quantified ranges of the parameters step height (s), etch pit depth (d), and etch pit density (D). The sample surface is characterized by flat etch pits (d = 4 to 18 nm), etch pits (58) Arvidson, R. S.; Ertan, I. E.; Amonette, J. E.; Luttge, A., Variation in calcite dissolution rates: A fundamental problem? Geochim. Cosmochim. Acta 2003, 67, (9), 1623-1634. (59) Compton, R. G.; Pritchard, K. L.; Unwin, P. R., The direct measurement of dissolution kinetics at the calcite water interface. J. Chem. Soc., Chem. Commun. 1989, (4), 249-251. (60) Eriksson, R.; Merta, J.; Rosenholm, J. B., The calcite/water interface I. Surface charge in indifferent electrolyte media and the influence of low-molecularweight polyelectrolyte. J. Colloid Interface Sci. 2007, 313, (1), 184-193. (61) Liang, Y.; Baer, D. R.; McCoy, J. M.; Amonette, J. E.; LaFemina, J. P., Dissolution kinetics at the calcite-water interface. Geochim. Cosmochim. Acta 1996, 60, (23), 4883-4887. (62) Pokrovsky, O. S.; Golubev, S. V.; Schott, J., Dissolution kinetics of calcite, dolomite and magnesite at 25 degrees C and 0 to 50 atm pCO(2). Chem. Geol. 2005, 217, (3-4), 239-255.
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Figure 3. Height histogram (upper left side) of an etched calcite surface (upper right side). The dark-brown to yellow shaded region in the frequency-height profile shows the frequency of surface increments that are below the reference (unreacted) surface and corresponds to the height map series (lower section of the image) were yellowish-brown colors indicate the height variation according to height range numbers 1-5, respectively. Parameter s is the height of surface steps; d is etch pit depth.
Figure 4. Zeta potential measurements for 499 and 903 nm colloids at different pH.
of medium depth (d = 20 to 28 nm), deep etch pits (d = 39 to 60 nm), and an unreacted surface portion (yellow portion). The surface data show that the distribution of etch pit depths is heterogeneous on scales of tens of micrometers. Some areas of the surface show only isolated shallow pits (d ∼ 3 nm). Similarly, the lateral dimensions (L1 and L2 along the diagonals of rhombic pit) show ranges between L1 = 5.6 ( 2 μm to 13.6 ( 4 μm and L2 = 6.6 ( 2 μm to 14 ( 3 μm. It was observed that there was no significant change in the topography of the surface in the experimental time frame of deposition studies because of inhibition of further dissolution of calcite by addition of NaHCO3. Calcite Surface and Colloid Stability Measurements. Under the implemented experimental conditions over a duration of 24 h, the individual PCS measurements show that the mean diameters of the colloids were 499 ( 12 nm and 903 ( 13 nm. The zeta potential values of the colloids at various pH conditions are shown in Figure 4. The Smoluchowski equation was applied to Langmuir 2010, 26(7), 4743–4752
convert electrophoretic mobility measurements of the colloids to zeta potentials.57 The colloids possess enough repulsive potential values to maintain the stability of the bimodel system from homo or heterocoagulation. Even at 100 ppm of Ca2þ (2.5 mM/L CaCl2) the colloidal suspension showed a stable behavior for 24 h. This confirms that the colloids are highly stable over the period of experiment (60 min). This is in agreement with results from Elimelech et al.8 about the stability of latex particles at 70 nm). For an equivalent Rq, a surface dominated by a high density of medium-sized etch pits are rather productive in colloidal deposition compared to a surface with isolated etch pit of high lateral (L1 and L2) or vertical dimension (d). It can be concluded that the Sherwood number is not a function of one of the parameters d, D, or s. Sh is rather controlled by a combination of all of these surface topography features. Hence, in determining the colloidal adhesion rate, it is essential to consider the total surface topography in terms of roughness parameters (Rq and F) that explains the overall surface distributions rather individual surface functions (d, D, or s). Previous reports based on theoretical or experimental approaches about membranes mostly predict the interaction energy between spherical particles and a positive or negative asperity range that is similar or smaller than the size of the particle.1,25,33 Those studies did not regard particle dimensions considerably smaller than the asperity size range. Unlike the discussed models from literature,26 the investigated substrate in this study lacks any protruding asperities and has only negative asperities (etch pits) over a flat area in conjunction with surface steps that were originated from cleaved mineral surface. The interaction is considered between colloids (499 and 903 nm) and a negative asperity whose average lateral dimension (∼7 μm) is larger (∼14 times for 499 and ∼7 times for 903 nm) and vertical dimension (∼30 nm) is smaller (∼16 times for 499 and ∼30 times for 903 nm) than the colloidal particle. In aqueous media, when a particle under flow approaches the rough calcite surface, it is attracted toward the etch pit. The preferential deposition of colloids at etch pits could be due to a reduced interaction energy barrier at a close vicinity of the pit. Etch pits are the low-energy pockets where the particle gets trapped into. Most of the particles were observed to attach to the pit walls far from corners. Only minor numbers of particles attached at pit corners. A particle approaching the etch pit is 4750 DOI: 10.1021/la9033595
Figure 10. Sh values of individual 499 and 903 nm colloids for three surface types at PZC of calcite.
restricted by the lateral movement due to the interaction with the edges of the pit that act as asperities (walls) and probably forced to cross over the energy barrier and into the primary minimum. This behavior of the particle interface is an outcome of the interplay between the DLVO and the steric interactions that lead to complex particle deposition phenomena at rough surfaces. The so-called “Shadow effect” which is a result of hydrodynamic interactions, has not been observed in the current case, as the Peclet number for both size particles is ,1.67,68 As illustrated in Figure 9, particles were not detected at the deepest part of the pit. Always the particles tend to attach at the pit wall far away from the bottom of the pit. This can be explained from theoretical considerations26 about the ascending order of magnitude of the interaction energy according to the regime: sphere-positive asperity (protrusion) < sphere-smooth surface < sphere-negative asperity (valley). When the particle penetrates into the deep region of the pit where a significant volume of particle is embedded, the interaction energy will be up to five times larger than the sphere-smooth plate interaction energy profile (etch pit bottom). Study of Specific Deposition of 499 and 903 nm Colloids. From our experiments, it was observed that, at pH 8.9, although the difference is small in number, a higher number of 499 nm colloids adsorbed per unit area compared to 903 nm colloids. The rate of deposition of 903 nm colloids was affected greatly by (67) Ko, C. H.; Elimelech, M., The “shadow effect” in colloid transport and deposition dynamics in granular porous media: Measurements and mechanisms. Environ. Sci. Technol. 2000, 34, (17), 3681-3689. (68) Lopez, P.; Omari, A.; Chauveteau, G., Simulation of surface deposition of colloidal spheres under flow. Colloids Surf., A 2004, 240, (1-3), 1-8.
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Figure 11. Collision frequency as a function of particle diameter. Flow velocity = 9.3 10-6 m/s, viscosity = 8.9 10-4 Pa s, colloid
density = 1.05 g/cm3, fluid density = 1 g/cm3.
variation in etch pit depth and density, while 499 nm colloids did not show any site-specific deposition. The number of 903 nm colloids adsorbed on type 1 is high compared to the remaining parts (Figure 10). This is due to the high density of pits as well as the occurrence of deep pits (Figure 8). The decrease in d and D from type 1 to type 2 (by a factor of ∼2 and ∼1.5) and type 2 to type 3 (by a factor of ∼2.1 and ∼1.75) correlates well with the number of adsorbed 903 nm colloids. A general interpretation is that particles tend to adsorb rather at walls of the etch pit. Even though a pit of few nanometers in depth has shown influence on the colloidal deposition (type 2 and type 3), high pit depth causes more effect as seen in type 1. Therefore, Sh903 nm is high for type 1 samples where higher pit depths occur. The smaller colloid type shows a gradual decrement in their Sh499 from type 1 to type 3 that follows a regular trend with respective etch pit frequency, etch pit depth, and step height values. Etch pit wall asperities are formed during dissolution due to the retreat of crystal layers in the nanometer range.69 For a constant etch pit volume, the effective contact area between etch pit walls and colloids varies because of the contact area differences along the pit wall. An increase in surface roughness results in the decrease of the contact area with a colloidal particle. That implies the reduction in particle-substrate interactions. This results in making attractive interactions less attractive and repulsive interactions less repulsive. Moreover, the deposition process can be augmented by the number of protruding asperities (contributing to roughness) exposed to a particle that decreases electrostatic and van der Waals forces between particle and substrate. Hence, it is expected that the bigger colloid should have more tendency to adsorb onto surfaces, which have high effective contact area with the interacting substrate compared to the smaller colloid. However, in contrast, it was noticed that the colloidal density of 499 nm colloids fraction is higher compared to 903 nm colloids. Though it is commonly assumed that the surface potentials of particles and substrate are normally distributed, most mineral surfaces in aqueous media are heterogeneously charged at both the molecular and macroscopic levels.70 Even though the chemical (69) Jordan, G.; Rammensee, W., Dissolution rates of calcite (10(1)over-bar4) obtained by scanning force microscopy: Microtopography-based dissolution kinetics on surfaces with anisotropic step velocities. Geochim. Cosmochim. Acta 1998, 62, (6), 941-947. (70) Elimelech, M.; Gregory, J.; Jia, X.; Williams, R. A., Particle deposition & aggregation; Butterworth-Heinemann: Woburn, 1995.
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conditions indicate the PZC of calcite at pH 8.9, it is realistic to observe the large distributions of surface potentials that differentiate local surface properties of the substrate (as discussed above). At PZC of the mineral surface, particle deposition follows different deposition mechanism not only due to inherent physical and electrochemical inhomogeneities of substrate but also due to the difference in surface potentials of bimodal colloids. For further examination of size-dependent deposition patterns, we applied filtration theory to analyze the particle size with respect to different transport mechanisms: diffusion, interception, and gravitational sedimentation.71 On the basis of the flow velocity in the cell, the calculated results (Figure 11) show that because of high collision frequency the larger particles (903 nm) encounter the surface much more frequently than the smaller particles (499 nm). Therefore, deposition at roughness type 1 sites (Figure 10, left section) can be explained by filtration theory (collision frequency is roughly two times higher for bigger particles: 1.5 10-6 for 499 nm colloids vs 3.2 10-6 for 903 nm colloids), but the difference in roughness types 2 and 3 deposition is not expected on the basis of filtration theory and can therefore be interpreted as an effect of increasing surface roughness.
Summary and Conclusion An experimental approach was applied to test whether or not topography variations of a crystal surface in the micrometer and submicrometer scale are able to influence the number and frequency of adsorbed particles close to PZC. For surface topography deviations 900 μm2. Smaller areas show (i) a broad variation (0.0063 to 0.0081 μm-2) of deposition density (N/A) and (ii) a deviation from the correlation between roughness parameter Rq and deposition density. The results show the significance of mineral surface roughness and topography variations in the micrometer to submicrometer range for colloidal adsorption. Colloidal adsorption at rough mineral and rock surfaces is an important geochemical process. We therefore conclude that resulting applications will foster the predictability and quantification of colloidal retention at interfaces in the environment, as well as for technical processes such as contaminant transport and filtration, drinking-water quality control, mining and nuclear waste management, and biofouling of transplants.
Appendix Classification Based on Roughness Parameters. To provide the insight into how the topographical roughness parameters controlled the colloidal deposition behavior, the surfaces were classified based to their etch pit depth values into three types (as demonstrated in Figure 3) and each type is studied in detail (partitions shown in Figure 6). The 3D data sets of the three example surfaces exhibit a range of surface morphologies: Surface step height s, pit depth d, pit density D, lateral dimensions L1 and L2 of the etch pit diagonals. An inverse correlation between d and L1, 2 as well as between d and D was found. This explains the topography variations responsible for enhanced colloidal adhesion of type 1 compared to type 2 or type 3 surfaces. Zeta Potential Measurements
Note Added after ASAP Publication. This article was published ASAP on March 5, 2010. Due to a production error, reference 52 was omitted. The correct version was published on March 12, 2010. Acknowledgment. The authors thank Andre Filby and Johannes L€utzenkirchen (Institut f. Nukleare Entsorgung (INE), Karlsruher Institut f€ ur Technologie, Germany) for fruitful discussions. We thank Volker Karius and Alexander Michler (Georg-August-Universit€at G€ottingen) for discussions and analytical assistance. We thank an anonymous reviewer for helpful comments and suggestions that significantly improved this manuscript. This is publication # GEOTECH-1293 of the GEOTECHNOLOGIEN R&D program funded by the German Ministry of Education and Research (BMBF) and German Research Foundation (DFG), grant # 03G0719A. KIT-INE work was partly funded by the Federal Ministry of Economics and Technology (BMWi) under the research project “KOLLORADO-2”.
4752 DOI: 10.1021/la9033595
Figure A-2. Zeta potential measurements at calcite as a function of pH. pHPZC of calcite is ∼8.9.
Langmuir 2010, 26(7), 4743–4752
