Langmuir 2002, 18, 3459-3465
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Deposition/Adsorption of Colloids to Surface-Modified Granules: Effect of Surface Interactions X. Zhang and Renbi Bai* Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received October 15, 2001. In Final Form: January 24, 2002
Granular media with positive surface charges were prepared by surface modification with polypyrrole (PPy) and were used in a fixed bed column for removing colloidal substances, such as kaolin particles and humic acid. The surface-modified granules were found to possess high positive ζ potentials at pH < 10.5, whereas kaolin particles and humic acid have negative ζ potentials at pH > 2.5 and pH > 1.8, respectively. Deposition and adsorption experiments showed that the granules with positive surface charges significantly enhanced the removal of the negatively charged kaolin particles and humic acid, indicating the importance of electrostatic surface interactions in filtration and adsorption. X-ray photoelectron spectroscopy (XPS) revealed that the positive surface charges of the surface-modified granules can be attributed to the N+ present in the PPy coating, and kaolin particle deposition or humic acid adsorption reduced the amount of N+ in the PPy coating, suggesting the existence of charge neutralization during the filtration or adsorption process. Scanning electron microscope (SEM) studies visually showed that a substantial amount of kaolin particles or humic acid was deposited or adsorbed to the surface-modified granules. The study concluded that surface interactions play an important role in the removal of colloidal substances in a granular media filtration or adsorption system.
Introduction Deposition/adsorption of colloids on solid surfaces has an important effect on soil structure, contaminant translocation, and aquifer permeability in the natural environmental system.1-3 It also plays an important role in engineered systems such as water purification through a granular media bed where colloids that remain suspended in surface or groundwater are removed through attachment to the granular media surfaces.4 Although deposition/ adsorption of colloids in granular media is complex and is influenced by a large number of variables, colloidal attachment to solid surfaces is largely a matter of interaction between the media grains and the colloids. In an aqueous environment, substances, i.e., both colloids and granular media, are almost always electrically charged, resulting in the formation of an electrical double layer at the solid-water interface. The forces responsible for colloid attachment may therefore be simply given as
FT ) FL + FD
1 Hd3 12 δ2(δ + d)2
FL )
(1)
where FT is the net or total interaction force, FL, the London-van der Waals force, and FD, the electrostatic or double layer interaction force. Using the premise that the granular media grain is at least 2 orders of magnitude larger than the colloidal particle (or macromolecule), one may estimate the magnitudes of London-van der Waals force and the electric double layer force by4,5 * To whom correspondence should be addressed. Phone: (65) 874 4532. Fax: (65) 779 1936. E-mail:
[email protected]. (1) Miller, W. P.; Baharuddin, M. K. Soil Sci. 1986, 142, 235. (2) O’Melia, C. R. Colloid Surf. 1989, 39, 255. (3) Kretzschmar, R.; Sticher, H. Environ. Sci. Technol. 1997, 31, 3497. (4) Tien, C. Granular Filtration of Aerosols and Hydrosols; Butterworths: Boston, MA, 1989. (5) Bai, R. B.; Tien, C. J. Colloid Interface Sci. 1997, 186, 307.
FD ) -
2π0(ζg2 + ζp2)κe-κδ -2κδ
(1 - e
)
(
(2)
2ζgζp 2
ζg + ζ p
2
)
- e-κδ (3)
where H is the Hamaker constant, d is the diameter of colloid, δ is the separation distance, and 0 and are the permittivity in a vacuum and the relative permittivity of fluid media, respectively. ζg and ζp are the ζ potentials of the two interacting parties (i.e. granular media and colloid), respectively, and κ is the reciprocal of double layer thickness, which is defined as
κ)
x
e2Σcizi2 0kT
(4)
where k is the Boltzmann’s constant, T, the absolute temperature, and e, the elementary charge. ci and zi are the concentration and valence of the ith ion species in the solution. In general, the London-van der Waals force, FL, is always attractive (with positive value), but the double layer force, FD, can be either attractive or repulsive (with negative value), depending on whether the ζ potentials of the two interacting parties have the same or opposite sign. In most water treatment processes, both the granular media and the colloids to be removed have negative ζ potentials and FD is therefore repulsive. Thus, the net surface interaction force, FT, in eq 1 can be attractive (positive value) or repulsive (negative value), depending on the relative magnitudes of FL and FD. Attractive FT is often referred to as the favorable surface interaction, and repulsive FT the unfavorable surface interaction. This can also explains the ineffectiveness of the conventional granular media such as sand that is commonly used in
10.1021/la015632t CCC: $22.00 © 2002 American Chemical Society Published on Web 03/27/2002
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deep bed filtration for removing colloidal particles because sand usually possesses the same type of negative ζ potential as the colloids and thus results in an unfavorable surface interaction between sand grains and the colloids. This has motivated many studies examining deposition/ adsorption of colloids to granular media surfaces under unfavorable surface interactions.5-12 Particularly, the addition of polyelectrolyte (changing ci and/or zi and therefore κ in eq 4) has been studied as a way of reducing FD and therefore making FT be less unfavorable for colloidal attachment.6, 9-11 Intuitively, since most colloids in water treatment have negative ζ potentials, it can be seen from eqs 1-3 that using granular media with positive ζ potentials would enhance colloid deposition or adsorption because FT in such a case will always be attractive (FT > 0). Toward this end, several attempts were made to modify conventional granular materials (such as sand) to develop the desired positive surface charges through surface coating of metallic hydroxides, metallic peroxides, or adsorption of metallic flocs onto the media surfaces.13-15 Although improvement in colloidal removal was observed, the coated surface was subject to dissolution and the performance was hard to assess.16 In recent years, polypyrrole (PPy), because of its environmental stability to oxygen and water and its easiness of synthesis, has been considered as a highperformance material used in preparing actuators, chemical sensors and biosensors, electrodes, and electronic devices.17-24 PPy also displays strong adsorptive capabilities toward gases and macromolecules and has high conductivity at room temperature.25 The possible use of PPy in water treatment for colloid deposition and adsorption has however not been explored. In this study, granular media with positive surface charges were prepared by coating glass beads with PPy through chemical polymerization of pyrrole and were investigated for kaolin particle deposition and humic acid adsorption, respectively. The selection of kaolin particles and humic acid as model colloids was simply based on a consideration that they are common in many water resources and they usually carry negative surface charges in water.26 The mechanisms of surface interactions were examined through (6) Bai, R. B.; Tien, C. J. Colloid Interface Sci. 1999, 218, 488. (7) Huang, C.; Pan, J. R.; Huang, S. Water Res. 1999, 33, 1278. (8) Jegatheesan, V.; Vigneswaran, S. Water Res. 2000, 34, 2119. (9) Vaidyanathan, R.; Tien, C. Chem. Eng. Commmun. 1989, 81, 123 (10) Elimelech, M.; O’Melia, C. R. Langmuir 1990, 6, 1153 (11) Elimelech, M. Water Res. 1992, 26, 1 (12) Bai, R. B.; Tien, C. J. Colloid Interface Sci. 1996, 179, 631. (13) Farrah, S. R.; Preston, D. R. Appl. Environ. Microbiol. 1985, 50, 1502. (14) Edwards, E.; Benjamin, M. M. J.sWater Pollut. Control Fed. 1989, 9, 1523. (15) Lukasik, J.; Cheng, Y. F.; Lu, F.; Tramplin, M.; Farrah, S. R. Water Res. 1999, 33, 769. (16) Chen, J.; Truesdail, S.; Lu, F.; Zhan, G.; Belvin, C.; Koopman, B.; Farrah, S.; Shah, D. Water Res. 1998, 32, 2171. (17) Abel, M. L.; Camalet, J. L.; Chehimi, M. M.; Watts, J. F.; Zhdan, P. A. Synth. Met. 1996, 81, 23. (18) Deronzier, A.; Moutet, J. C. Coord. Chem. Rev. 1996, 147, 339. (19) Skotheim, T. A., Elsenbaumer, R., Reynolds, J., Eds. Handbook of conducting Polymers; Marcel Dekker: New York, 1998. (20) Lee, H. S.; Hong, J. Synth. Met. 2000, 113, 115. (21) Wang, L. X.; Li, X. G.; Yang, Y. L. React. Funct. Polym. 2001, 47, 126. (22) Perruchot, C.; Chehimi, M. M.; Delamar, M.; Lascelles, S. F.; Armes, S. P. Langmuir 1996, 12, 3245. (23) Cho G. J.; Glatzhofer, D. T.; Fung, B. M.; Yuan, W. L.; O’Rear, E. A. Langmuir 2000, 16, 4424. (24) Cho G. J.; Fung, B. M.; Glatzhofer, D. T.; Lee, J. S.; Shul, Y. G. Langmuir 2001, 17, 456. (25) Kim, D. Y.; Lee, J. Y.; Kim, C. Y.; Kang, E. T.; Tan, K. L. Synth. Met. 1995, 72, 243. (26) Jone, M. N.; Bryan, N. D. Adv. Colloid Interface 1997, 78, 1.
Zhang and Bai
ζ potential analysis, X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM) observation. (The surface interactions between kaolin particles and humic acid are out of the scope of this study.) Materials and Methods Materials. Pyrrole (99%), FeCl3‚6H2O (97%), and humic acid were purchased from the Aldrich Chemical Co. Ballotini glass beads, obtained from Jecons (Scientific) Ltd., Bedfordshire, U.K., were used as the model granular media for surface modification. Kaolin particles were obtained from Kaolin Sdn. Bhd., Tapah, Malaysia, and consisted mainly of kaolinite. HCl and NaOH were used to adjust the solution pH, and NaCl was used to vary the solution ionic concentration when necessary. Obtaining a PPy coating on the glass beads involved chemical oxidation-polymerization of pyrrole, with Fe3+ as oxidant and water as solvent. A 7 g amount of FeCl3‚6H2O was dissolved in 30 mL of deionized (DI) water with stirring at room temperature. A 100 g amount of glass beads was then added into the solution. After 5 min, pyrrole monomer (in diluted aqueous solution) was added in droplets into the mixture with vigorous stirring, and the polymerization of pyrrole was allowed to proceed for 3 h for the formation of PPy coating on the surface of glass beads. To ensure complete coverage of the surfaces, the coating process was conducted twice. The product was then washed with DI water and alcohol, air-dried for 24 h, and stored in a desiccator prior to use. The average diameters of the uncoated and coated glass beads were determined as 549.96 and 590.36 µm, respectively, through measuring 50 individual grains using an optical microscope (BX60, Olympus Optical Co. Ltd., Tokyo, Japan) that was equipped with an advanced image analysis software (AnalySIS3.0 Imaging System, GmbH) which could give sizes down to the 0.01 µm level. The thickness of the PPy coating on the glass beads was therefore around 20.2 µm. Deposition/Adsorption Experiments. Experiments were conducted in a Plexiglas column of 24.8 mm internal diameter, packed with varying depths (from 10 to 20 cm) of the PPy-coated glass beads. Kaolin suspensions or humic acid solutions were prepared in a feeding tank with stirring and passed through the test column using a metering pump at a constant approach flow velocity v ) 1.5 or 3 m/h (i.e. flow rate at 12 or 24 mL/min). The histories of the effluent concentrations were monitored by taking and analyzing samples periodically. For kaolin suspension preparation, kaolin particles were first suspended in DI water and ultrasonically bathed for 10 min and then fractionated by sedimentation for 0.5 h. The large particles were settled out and discarded, and the supernatant was used to make the test suspension by diluting it with DI water to the desired concentrations. The size distribution of the particles in the test suspensions was from submicrometer to about 10 µm, with most of them smaller than 3 µm. The turbidity of kaolin suspension samples was determined by a turbidity meter (model 800, Spectral Tecknik Pte. Ltd., Singapore). Special attention was paid to minimize the possible errors in converting turbidity to concentration. In each experiment, the samples taken at the beginning and at 10 min and then at every 0.5 h were analyzed for both turbidity and concentration (the latter was done by drying) to establish a turbidity-concentration relationship with time. With this curve, the other turbidity readings in the experiment were converted to concentration through interpolation on the curve. Humic acid solution was prepared by dissolving a certain amount of humic acid in a known volume of DI water. The solution was then mixed with a magnetic stirrer for 1 h and filtered through a Whatman membrane filter (0.45 µm) before conducting the adsorption tests. The sizes of humic acid macromolecules, as estimated from a Coulter Series 230 Particle Sizer (Coulter Counter Corp.) and a Zeta-Plus4 Instrument (Brookhaven Instruments Corp.), were in the range of from subnanometer up to 100 nm. Humic acid concentrations in the experiments were measured by an ultraviolet-visible (UV-vis) spectrometer (Hitachi UV-2000) at 400 nm. The absorbance was pH and salt dependent, and calibration lines were made for each required pH and ionic concentration level in the study. Morphology Observation. The surface morphologies of the coated and uncoated glass beads were examined using a scanning
Deposition/Adsorption of Colloids to Granules
Figure 1. ζ potentials of PPy-coated glass beads, humic acid, and kaolin particles at different solution pH values. electron microscope (SEM, JEOL JSM-6400) at 10-20 kV. To investigate the deposition of kaolin particles or adsorption of humic acid on the coated glass beads, samples from the top layer of the granular media bed after an operation run were observed by an on-site optical microscope (BX60, Olympus, Japan) or further studied by a SEM. ζ Potential Measurement. A Zeta Plus4 Instrument (Brookhaven Instruments Corp.) was used to determine the ζ potentials of kaolin particles, humic acid macromelecules, and the coated glass beads. For the case of coated glass beads, the method described by Bai and Tien6 was adopted. Coated glass beads were placed in a 100-mL vial with 50 mL of DI water, and the vial was vibrated in a sonic bath for 24 h. The liquid in the vial with small fragments from the PPy coating in it was then decanted for measurements. The ζ potentials so determined were found to be similar to those determined using PPy particles from the polymerization reaction, indicating that the fragments in the decanted liquid were essentially from the PPy coating and were completely made of PPy. XPS Study. X-ray photoelectron spectroscopy (XPS) has been used in studying surface compositions of various materials in recent years.27,28 In this study we used XPS to determine the surface composition and the chemical elemental oxidation state of the PPy coating with or without deposited kaolin particles or adsorbed humic acid. XPS signals were recorded using a VG ESCALAB MkII spectrometer with an Al KR X-ray source (1486.6 eV photons). The core-level signals were obtained at a photoelectron takeoff angle of 75° relative to the sample surface. The X-ray source was run at a reduced power of 150 W (15 kV and 10 mA). The pressure in the analysis chamber was maintained at 7.5 × 10-9 Torr or lower during each measurement. To compensate for surface charging effects, all binding energies were referenced to the C 1s neutral carbon peak at 284.6 eV.29 Surface elemental stoichiometries were determined from sensitivityfactor-corrected peak area ratios, and the software XPSpeak 4.1 was used to fit the XPS spectra peaks.
Results and Discussion ζ Potentials. Figure 1 shows the ζ potentials of PPycoated glass beads, humic acid, and kaolin particles as a function of the solution pH. For pH below 10.5, ζ zeta potentials of the coated glass beads are positive. This is in contrast with the uncoated glass beads that have negative ζ potentials for pH > 3.6 From pH ) 4 to pH ) 10, the ζ potentials of the coated glass beads are relatively constant. This may be attributed to an instant dissociation of the dopant Cl- from PPy on the coated surface (i.e. PPyCl) when pH is reduced to 10. The coated surface however appears to be very stable for pH from 4 to 10. The rapid increase of the ζ potentials at pH below 4 and the (27) Maeda, S.; Gill, M.; Armes, S. P.; Fletcher, I. W. Langmuir 1995, 11, 1899. (28) Greaves, S.; Watts, J. F.; Beadle, P. M.; Armes, S. P. Langmuir 1996, 12, 1784. (29) Kang, E. T.; Neoh, K. G.; Tan, K. L. Adv. Polym. Sci. 1993, 106, 135.
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Figure 2. Removal of kaolin particles by uncoated and PPycoated glass beads in a granular media column (bed depth ) 11 cm, flow velocity v ) 3 m/h, CInf ) 45 mg/L, pH ) 6.5).
rapid decrease of the ζ potentials at pH above 10 may be an indication of protonation and deprotonation, respectively, occurred in the PPy coating. The humic acid used in this study is found to have positive ζ potentials at pH < 1.8 and negative ζ potentials at pH > 1.8. The measured point of zero ζ potential at pH ) 1.8 in this work is close to that reported by Beck et al.30 but appears to be lower than the value of pKa ) 3 reported for the carboxylic groups in humic acid.26 Similar to humic acid, the kaolin particles in this study have positive ζ potentials at pH < 2.5 and negative ζ potentials at pH > 2.5. Kaolin particles are reported to retain a platelike structure and expose two crystallographic surfaces to aqueous environment, with face surfaces carrying negative charges and edge surfaces carrying positive charges.31,32 The results in Figure 1 indicate that, over a wide range of pH values (pH > 2.5), the overall ζ potentials of kaolin particles are dominated by SiO2 on the face surfaces. Because of the positive surface charges, the coated glass beads can be expected to give improved performance for the deposition/adsorption of negatively charged kaolin particles and humic acid in the pH range of 2.5 to 10.5. Deposition of Kaolin Particles. The typical results for kaolin particle deposition at around neutral pH are shown in Figure 2. PPy-coated glass beads were much more effective in kaolin particle deposition than uncoated glass beads. This can be attributed to the favorable surface interactions between coated glass beads and kaolin particles. Since uncoated glass beads, similar to kaolin particles, had negative ζ potentials, the surface interaction between uncoated glass beads and kaolin particles was therefore less favorable (i.e. less attractive or even repulsive). Due to the strong electrostatic attraction between PPycoated glass beads and kaolin particles, removal of deposited kaolin particles from the coated glass beads by back-washing with DI water (carried out at 50% bed expansion for 15 min in the filter column) was incomplete. When the coated glass beads after back-washing were reused, the rate of particle deposition was found to be lower, as compared to that of the fresh ones; see Figure 2. (30) Beck, A. J., Jones, K. C., Hayes, M. B. H., Mingelgrin, U., Eds. Organic Substances in Soil and Water: Natural Constituents and their Influences on Contaminant Behaviour; The Royal Society of Chemistry: Cambridge, U.K., 1993; pp 74-77. (31) Besra, L.; Sengupta, D. K.; Roy, S. K. Int. J. Miner. Process. 2000, 59, 89. (32) Williams, D. J. A.; Williams, K. P. J. Colloid Interface Sci. 1978, 65, 79.
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Figure 3. Effect of ionic concentrations on the deposition of kaolin particles in the granular media column with PPy-coated glass beads (bed depth ) 11 cm, flow velocity v ) 3 m/h, CInf ) 45 mg/L, pH ) 6.5).
To further investigate the effect of surface interaction on particle deposition, experiments were conducted with suspensions of different ionic concentrations (Figure 3). The results show that deposition of kaolin particles in the column was enhanced with increasing ionic concentrations. As suggested by eqs 4 and 3, a higher ionic concentration (i.e. a greater value of ci) would result in a smaller double layer thickness and, as a consequence, a lower electrostatic repulsion force between kaolin particles. Therefore, more kaolin particles can be expected to deposit on the surfaces of PPy-coated glass beads since the media grain-particle surface interaction was favorable and the deposited particles could stay closer on the media surface, due to the reduced electrostatic repulsion between kaolin particles in the lateral directions. The observed improvement in kaolin particle deposition at [NaCl] ) 0.001 M in Figure 3 can be attributed to this type of deposition. When ionic concentration was significantly increased, particle-particle attachment may become possible. The retailed kaolin particles on the media grain surface could act as additional collectors for the removal of other suspended kaolin particles. This would result in a reduction of CEff/CInf with time, a phenomenon that is often called as “filter ripening” in filtration.4 The results of [NaCl] ) 0.1 M in Figure 3 appear to show this type of deposition. Image study by SEM analysis indicated that kaolin particles indeed deposited directly on the surface of PPy coated glass beads (Figure 4a) but formed clusters through particle-particle attachment under high ionic concentrations (Figure 4b), which provides visual evidences on the different types of particle deposition. To obtain insight information on the surface interactions of kaolin particle deposition, XPS analyses were conducted. The N 1s peak of the PPy coating without kaolin particle deposition can be decomposed into four components (Figure 5a). The peaks of imine (-Nd) and amine (-NH-) nitrogens are centered at binding energies (BE) of 397.6 and 399.4 eV, respectively. The peaks at 401.1 and 402.6 eV are assigned to two high oxidation states of nitrogen with positive charges (NI+ and NII+) in doped polypyrrole. The proportion of positively charged nitrogen atoms in the PPy coating was about 26.9% (in terms of [N+]/[N]). It is these protonated N+ atoms that contribute to the positive surface charges of PPy-coated glass beads. However, when a spectrum of the N 1s peak of the PPy coating with kaolin particle deposition was obtained, the [N+]/[N] ratio was found to be about 18.4%; see Figure 5b. The reduction of the [N+]/[N] ratio indicates the occurrence of interaction between kaolin particles and the positively charged nitrogen atoms in PPy on the coated glass beads. The XPS spectra of Al 2p and Si 2p of the kaolin particles are shown in Figure 5c,d. Since there is only a single peak
Figure 4. SEM images showing kaolin particle deposit on the surface of PPy-coated glass beads and the cluster growth of deposit at high ionic concentration (pH ) 6.5).
in these figures, the chemical states of Al and Si in kaolin particles may be assumed to be relatively uniform. With deposited kaolin particles, the XPS spectra of Al 2p and Si 2p of the kaolin particles (see Figure 5e,f) shifted to higher BEs and may be decomposed into several peaks. Together with the N 1s results, we may infer the existence of PPy-N+‚‚‚O-Al or PPy-N+‚‚‚O-Si on the PPy coating with kaolin deposition. A portion of the positively charged nitrogen atoms in the PPy coating was therefore neutralized, resulting in a reduction of [N+]/[N] in PPy and an increase in BEs of Al 2p and Si 2p. Adsorption of Humic Acid. To further investigate the role of positive surface charges of PPy-coated glass beads, humic acid adsorption tests were conducted. Removing humic substances from water is desirable or even necessary because of their adverse effect on water quality and human health.33 As a topic of study, it has also attracted much attention in recent years.34-35 Since humic acid molecules used in this study had much smaller sizes (
