Study of Al(OH)3−Polyacrylamide-Induced Pelleting

Langmuir 2008, 24, 14015-14021

14015

Study of Al(OH)3-Polyacrylamide-Induced Pelleting Flocculation by Single Molecule Force Spectroscopy Wei Sun, Jun Long,† Zhenghe Xu, and Jacob H. Masliyah* Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G6 ReceiVed August 5, 2008. ReVised Manuscript ReceiVed September 22, 2008 A nanocomposite polymer, Al(OH)3-polyacrylamide (Al-PAM) hybrid, was tested as a flocculant. This novel hybrid polymer was found to induce pellet-like floccules, leading to a more effective solid-liquid separation than common polyacrylamide (PAM)-based flocculants. To understand the mechanism of Al-PAM-induced pelleting flocculation, the molecular structure of this hybrid polymer was studied using an atomic force microscope (AFM). The interactions between Al-PAM molecules and a silica surface were measured using single molecule force spectroscopy (SMFS). The Al-PAM molecules were found to have a star-like structure with Al(OH)3 colloidal particles as cores connecting PAM chains. The SMFS results showed a strong attachment of the Al(OH)3 cores to the silica surface with an adhesion force of ∼1250 pN, in contrast to a weaker adhesion force of only ∼250 pN for PAM chains on the silica surface. The Al-PAM-induced pelleting flocculation is attributed to its star-like structure.

1. Introduction Polymer flocculants have been widely used for enhanced solid-solid or solid-liquid separation in municipal water treatment, industry process effluent clarification and dewatering of coal and mineral tailings.1-5 To enhance fine solids flocculation, attempts have been made, among which a combination of polymer flocculants and microparticles was found to be effective. The microparticles used include polymers and inorganic matters, such as cationic polymers,6 anionic bentonite,7 and cationic colloidal alumina (Al2O3).8 A strong synergy between the microparticles and polymers was found to be responsible for improved flocculation. Instead of a simple combination of microparticles and polymer flocculants, further efforts were made to synthesize microparticle-polymer hybrids for more efficient flocculation.9-12 Yang et al.,12 for example, synthesized a novel Al(OH)3-polyacrylamide hybrid flocculant (Al-PAM). In this case, ionic bonds between Al(OH)3 colloidal particles and polyacrylamide (PAM) chains were recognized. The flocculation efficiency of Al-PAM in treating kaolinite suspensions was demonstrated to be much higher than that of a commercial PAM or a blend of PAM and AlCl3. The Al-PAM-induced floccules were denser, larger, and of a spherical shape. * To whom correspondence should be addressed. E-mail: jacob.masliyah@ ualberta.ca. † Currently works at Sycrude Canada, Ltd.

(1) Pearse, M. J. Miner. Eng. 2005, 18, 139–149. (2) Selomulya, C.; Liao, J. Y. H.; Bickert, G.; Amal, R. Int. J. Miner. Process 2006, 80, 189–197. (3) Li, H. J.; Long, J.; Xu, Z. H.; Masliyah, J. H AIChE J. 2007, 53, 479–488. (4) Li, H.; Long, J.; Xu, Z.; Masliyah, J. H. Energy Fuels 2005, 19, 936–943. (5) Bolto, B.; Gregory, J. Water Res. 2007, 41, 2301–2324. (6) Xiao, H.; Liu, Z.; Wiseman, N. J. Colloid Interface Sci. 1999, 216, 409– 417. (7) Asselman, T.; Garnier, G. Colloids Surf., A: Physicochem. Eng. Aspects 2000, 170, 79–90. (8) Ovenden, C.; Xiao, H. N. Colloids Surf., A: Physicochem. Eng. Aspects 2002, 197, 225–234. (9) Liu, Y. F.; Wang, S. Z.; Hua, J. D. J. Appl. Polym. Sci. 2000, 76, 2093– 2097. (10) Qian, J. W.; Xiang, X. J.; Yang, W. Y.; Wang, M.; Zheng, B. Q. Eur. Polym. J. 2004, 40, 1699–1704. (11) Rong, J. F.; Li, H. Q.; Jing, Z. H.; Hong, X. Y.; Sheng, M. J. Appl. Polym. Sci. 2001, 82, 1829–1837. (12) Yang, W. Y.; Qian, J. W.; Shen, Z. Q. J. Colloid Interface Sci. 2004, 273, 400–405.

In a recent study, a high oil (bitumen) recovery with an improved product quality was achieved by using a combination of Al-PAM and partially hydrolyzed polyacrylamide (HPAM).13 However, the role of Al-PAM in the bitumen extraction process is not clearly understood. It was speculated that Al-PAM would flocculate the fine particles to form large and dense floccules. Thus the number of individual fine particles in the oil sands slurry would be decreased. As a result, the probability of slime coating, i.e., attachment of fine solids onto bitumen surface, was reduced, thereby improving bitumen aeration and floatation efficiency. The large and dense floccules were more difficult to be brought to the rising bitumen froth, but easy to settle in the tailings stream of gravity separators. As a result, the bitumen recovery, bitumen froth quality, and tailings settling were all improved, contributing to improved environment and reduced greenhouse gas (GHG) emissions. Understanding the mechanism of Al-PAM-induced flocculation would help validate such a speculation. The focus of the present study is on the investigation of the molecular structure of Al-PAM and its flocculation mechanism. Such an investigation would provide new insights into the critical role of Al-PAM in flocculation processes. To derive the molecular structure of Al-PAM and determine interactions between the polymer and solid surfaces, atomic force microscopy (AFM) and single molecule force spectroscopy (SMFS) were used. The SMFS has been successfully applied to the study of intermolecular and intramolecular interactions at a single molecule resolution. This technique is powerful not only for understanding the nanomechanical properties of single polymer strands, but also for exploring the conformational changes of single polymer molecules. Until now, many issues have been successfully resolved by the use of SMFS, such as protein unfolding,14 DNA unzipping,15 single molecule chain elasticity,16 binding force of (13) Li, H. H.; Long, J.; Xu, Z. H.; Masliyah, J. H. Can. J. Chem. Eng. 2008, 86, 168–176. (14) Rief, M.; Gautel, M.; Oesterhelt, F.; Fernandez, J. M.; Gaub, H. E. Science 1997, 276, 1109–1112. (15) Krautbauer, R.; Rief, M.; Gaub, H. E. Nano Lett. 2003, 3, 493–496. (16) Neuert, G.; Hugel, T.; Netz, R. R.; Gaub, H. E. Macromolecules 2006, 39, 789–797.

10.1021/la802537z CCC: $40.75  2008 American Chemical Society Published on Web 11/11/2008

14016 Langmuir, Vol. 24, No. 24, 2008

single polymer chain on a substrate,17-20 and aggregation mechanism of asphaltene molecules in solvents.21 In the present study, SMFS was used to stretch/detach single Al-PAM strands attached to a silica surface. The force to pull and extend a single Al-PAM strand as a function of the extension distance was recorded. The information retrieved from the obtained force profiles provided unique new insight into the molecular structure of the Al-PAM hybrids and the interactions between the polymer and solid surfaces. To directly observe the molecular structure, AFM was used to image the hybrid polymer molecules deposited on solid surfaces. On the basis of the information obtained from both the SMFS measurements and AFM imaging, a mechanism for Al-PAM-induced pelleting flocculation is discussed.

2. Experimental Section Materials. Potassium chloride (>99.999%, Aldrich) was used as the supporting electrolyte. Reagent-grade hydrochloric acid and sodium hydroxide (Fisher Scientific) were used as pH modifiers of the aqueous solutions. Deionized water with a resistivity of 18.2 MΩ cm, prepared with an Elix 5 followed by a Millipore-UV Plus water purification system (Millipore Inc., Canada), was used where applicable throughout this study. Sample Preparation for AFM Imaging and Force Measurement. Al-PAM was synthesized through in situ polymerization of acrylamide monomers with Al(OH)3 colloids using NaHSO3 and (NH4)2S2O8 as initiator. The details on the synthesis are described elsewhere.13 In this study, silicon wafers with an oxidized surface layer of ∼0.6 µm (NANOFAB, University of Alberta, Canada) were used as substrates for the adsorption of Al-PAM. The wafers (10 × 10 mm2) were cleaned by extensive rinsing with deionized water and ethanol, followed by blow-drying with ultrapure-grade nitrogen. They were then exposed to an ultraviolet light for more than 5 h to remove any possible organic contaminants. In order to pick up a single polymer molecule in the SMFS measurements, a very thin layer of polymer had to be deposited onto a substrate. The use of an appropriate concentration of polymer solution is critical for this type of study. A series of Al-PAM in 1 mM KCl solutions with concentrations from 20 to 300 ppm (mg/L) were tried. It was found that a concentration of 100 ppm led to the highest probability of picking up a single polymer molecule on the silica surface. The deposition procedure was as follows. A 0.05 wt % Al-PAM stock solution was prepared in 1.0 mM KCl solutions (pH ) 5.0, 8.4 and 10.0, respectively) at least 24 h prior to its use to ensure complete dissolution of the polymer. Lower concentration Al-PAM solutions were prepared by appropriate dilution using 1.0 mM KCl stock solutions of the same pHs. A total of 200 µL of diluted Al-PAM solution was placed on a pretreated silica surface and incubated for 10 min in a sealed container to minimize evaporation of the solution. The sample was then thoroughly rinsed with deionized water to remove any unattached and loosely attached polymer molecules. After blow-drying with pure nitrogen, the sample was then immediately used for AFM imaging or force measurements. AFM Imaging. The molecular structure of Al-PAM deposited on the silica surface was imaged by a MultiMode atomic force microscope (Digital Instruments, Santa Barbara, CA). Imaging was conducted in air using the standard tapping mode at room temperature. All images were acquired using silicon tips on cantilevers with a spring constant of ∼403 N/m and a resonance frequency of ∼300 kHz (Applied NanoStructures, Santa Clara, CA). A J-scanner with a maximum scan size of 20 µm × 20 µm was used. SMFS Measurements. A nanoscope E atomic force microscope with a vendor-supplied fluid cell (Digital Instruments) was used for (17) Friedsam, C.; Seitz, M.; Gaub, H. E. J. Phys.: Condens. Matter 2004, 16, S2369-S2382. (18) Long, J.; Xu, Z. H.; Masliyah, J. H. Langmuir 2006, 22, 1652–1659. (19) Zhang, Y. H.; Liu, C. J.; Shi, W. Q.; Wang, Z. Q.; Dai, L. M.; Zhang, X. Langmuir 2007, 23, 7911–7915. (20) Liu, C.; Jiang, Z.; Zhang, Y.; Wang, Z.; Zhang, X.; Feng, F.; Wang, S. Langmuir 2007, 23, 9140–9142. (21) Long, J.; Xu, Z. H.; Masliyah, J. H. Langmuir 2007, 23, 6182–6190.

Sun et al. the force measurements. Silicon nitride cantilevers (lever type 100 µm narrow) with a spring constant of ∼0.32 N/m, also from the Digital Instruments, were used to measure single molecule forces. The spring constants of the cantilevers were calibrated by measuring their thermal fluctuation before each force measurement.22 Force measurements were performed in an aqueous environment in the fluid cell. The experimental details of SMFS were described elsewhere.23,24 Briefly, a prepared sample mounted on the AFM scanner was brought into contact with the AFM tip in aqueous solutions. Because of the electrostatic interactions between an Al(OH)3 colloidal particle and the AFM tip, or nonspecific interactions between PAM chains and the AFM tip, Al(OH)3 colloidal particles or one (or more) PAM chain(s) would adhere to the AFM tip, resulting in a number of “bridges” between the tip and the sample. During the separation of the tip from the sample, the polymer chain was stretched or pulled off from the substrate. The deflection of the cantilever as a function of the substrate displacement on the piezo sample holder was recorded and then converted to a force-extension curve. A full cycle of the tip-substrate approach and retraction process was recorded during the experiments, but only the retraction force profiles are shown in the present paper. Before recording the force curves, the fluid cell was left undisturbed for 1 h to allow the system to equilibrate. All the force measurements were conducted at room temperature (21 ( 1 °C). For each condition, the measurement was performed at a number of different surface locations, and more than 1000 force curves were recorded to achieve statistical representation of the measured force profiles. Formation of Clay Floccules. Clay suspensions were prepared by dispersing kaolinite clays (∼6 µm, Wards Natural Science, Ltd., Ontario, Canada) in deionized water to a concentration of 0.6 wt % solids. The pH of the clay suspension was adjusted to 8.4. The 0.6 wt % kaolinite suspension was placed in a jar, and an appropriate dosage of Al-PAM solution was added. After gently mixing, the suspension was allowed to settle for 5 min. The floccules that had settled at the bottom of the jar were then photographed by a Nikon D2000 camera. In the flocculation study, polymer dosages were expressed with reference to the volume of kaolinite suspension.

3. Results and Discussion AFM Images of Al-PAM Molecules. AFM images of AlPAM adsorbed from 1.0 mM KCl solutions of pH 5.0, 8.4, and 10.0, respectively, are shown in Figure 1. Images obtained for samples prepared in solutions of pH 5.0 and 8.4 (Figure 1a,b) show similar star-like features. In these two images, some bright dots are seen to randomly distribute on the surface. They are attributed to Al(OH)3 colloidal particles. These particles are surrounded by PAM chains and act as cores connecting the chains. One Al(OH)3 core can have several PAM side chains as arms, exhibiting a star-like structure. Although the structure at these two pHs is similar, the surface coverage by the Al(OH)3 colloidal particles at pH 5.0 (1.25%) is much less than that at pH 8.4 (1.93%), while the mean diameter of the particles (17 ( 10 nm) at pH 5.0 is smaller than that (25 ( 10 nm) at pH 8.4. The AFM image obtained at pH 10.0 (Figure 1c) shows fewer but larger particles or aggregates. Force Profiles of Al-PAM. (a). 1.0 mM KCl Solution at pH 8.4. Interpretation of force-distance curves obtained from SMFS can be difficult. It is seldom the case that only a single chain is attached to both the AFM tip and the substrate. Usually many chains are picked up simultaneously. These chains could detach from either the substrate or the tip shortly after stretching to exceed the binding force. In our SMFS measurements, only about 10% of the force curves demonstrated characteristics of stretching (22) Butt, H. J.; Jaschke, M. Nanotechnology 1995, 6, 1–7. (23) Li, H. B.; Liu, B. B.; Zhang, X.; Gao, C. X.; Shen, J. C.; Zou, G. T. Langmuir 1999, 15, 2120–2124. (24) Marko, J. F.; Siggia, E. D. Macromolecules 1995, 28, 8759–8770.

Study of Al-PAM-Induced Pelleting Flocculation

Langmuir, Vol. 24, No. 24, 2008 14017

Figure 1. AFM images (2 µm × 2 µm) of Al-PAM deposited on a silica surface in 1.0 mM KCl solution at pH (a) 5.0; (b) 8.4; and (c) 10.0.

Figure 3. (a) Type I force profiles obtained from Al-PAM deposited on a silica surface in 1.0 mM KCl solution at pH 8.4. Inset: a proposed scheme for the detachment of a single PAM chain. (b) Histogram of the plateau height. Gaussian distribution was used to fit the experimental data.

Figure 2. A typical single molecular force profile obtained from AlPAM deposited on a silica surface in 1.0 mM KCl solution at pH 8.4 where the AFM tip picked up multiple overlapping PAM chains as shown in the inset. Gray sphere: Al(OH)3 colloidal particle; magenta line: PAM chain.

or detaching long-chain single molecules.25-27 In most cases, complex force curves were obtained as shown in Figure 2, resulting from the stretching and detachment of multiple PAM chains and/or Al(OH)3 particles from the substrate. It is difficult to distinguish individual events from the force profile in Figure 2. Therefore, no attempt was made to analyze this type of force curves in the current study. The first, strongest force peak at the closest approach of the surface is attributed to a very strong adhesion force between the tip and the uncovered regions of the silica surface.28 Since the interactions between the tip and the substrate are not the interest of the study, the first strong force peak representing this type of interactions will not be shown in the following force profiles. (25) Janshoff, A.; Neitzert, M.; Oberdorfer, Y.; Fuchs, H. Angew. Chem., Int. Ed. 2000, 39, 3213–3237. (26) Conti, M.; Bustanji, Y.; Falini, G.; Ferruti, P.; Stefoni, S.; Samori, B. ChemPhysChem 2001, 2, 610–613. (27) Zhang, W. K.; Cui, S. X.; Fu, Y.; Zhang, X. J. Phys. Chem. B 2002, 106, 12705–12708. (28) Hugel, T.; Seitz, M. Macromol. Rapid Commun. 2001, 22, 989–1016.

Five types of force profiles demonstrating single-molecule characteristics were observed. Type I force profile (Figure 3a) shows a long plateau. The measured long plateau suggests an extended constant force to detach a polymer chain from the substrate. This type of retracting force profile suggests a trainlike conformation of a polymer chain with multiple anchor points on the silica surface.26,27,29 The scheme of this detachment is shown in the inset of Figure 3a. When the AFM tip picked up such an adsorbed single PAM chain through nonspecific interactions between the tip and PAM chains, the retraction of the substrate progressively unzipped the binding sites of the molecules on the surface. Since the force used to unzip each anchoring point is similar, the obtained force curves exhibit a wiggling plateau of equal force. Two important values need to be noted here: the length of the plateau and the height of the plateau. The length of the plateau directly reflects the length of the detached PAM chain from the substrate. Because of both the polydispersity and the uncontrolled point of a polymer chain picked by the AFM tip, the measured lengths of the detached polymer chains are over a quite wide range. The height of the plateau, on the other hand, corresponds to the force required to detach a single PAM chain from the silica surface. A histogram of the plateau heights in Figure 3b shows an adhesion force of 251 ( 51 pN. The Type II force profile in Figure 4 shows two sequential plateaus of equal step height. The first plateau has a force of (29) Cui, S. X.; Liu, C. J.; Zhang, X. Nano Lett. 2003, 3, 245–248.

14018 Langmuir, Vol. 24, No. 24, 2008

Figure 4. Type II force profiles obtained from Al-PAM deposited on a silica surface in 1.0 mM KCl solution at pH 8.4. Inset: a proposed scheme for the detachment of two single PAM chains.

Sun et al.

Al(OH)3 colloidal particle as shown in Figure 5b. Because of a strong affinity between aluminum and oxygen in the form of -O-Al-O-, Al(OH)3 colloidal particles could adsorb onto the silica surface either through the affinity between aluminum and oxygen, or through electrostatic interactions between Al(OH)3 colloids and the silica surface. The single PAM chain with Al(OH)3 colloidal particles at each end adopted a loop conformation, i.e., no anchoring points along the PAM chain on the silica surface. When the tip picked an Al(OH)3 particle connected with a PAM chain to another Al(OH)3 particle anchoring on the silica surface, the PAM chain was stretched first (Region I in Figure 5). As the pulling force reached a certain value, the PAM chain was fully stretched. Further retraction eventually led to the detachment of the Al(OH)3 colloidal particle from the substrate (Region II in Figure 5). Immediately after this detachment, the force dropped to nearly zero. The observed peak force corresponds to the adhesion force of a Al(OH)3 colloidal particle to the silica surface. As the substrate continued to retract from the tip, the next cycle of stretching PAM chain and detaching the contact of Al(OH)3 with the surface occurred (Region III and IV in Figure 5). Eventually the tip disconnected from the substrate and returned to its undeflected state due to either the breakage of PAM-Al(OH)3 linkage, or the detachment of the Al(OH)3 particle from the tip (Region V in Figure 5). This process is very similar to the unfolding of proteins, in which case, sequential unfolding of individual domains of proteins results in saw-tooth types of force profiles, and each unfolding is marked by a force peak.14,30-32 When an external pulling force is applied to a macromolecule chain, the molecule will be first aligned along the direction of the external force field and then be stretched if the force is sufficiently strong. For many polymers, the relation between the elastic elongation of the polymer chain and the external force can be quantitatively described by a modified wormlike-chain (M-WLC) model,25 which treats a polymer chain of a wormlike conformation as a string of constant bending elasticity. To confirm that the rising force in Figure 5 (a) indeed originates from the stretching of a single chain, the M-WLC model was used to analyze the force profiles. By considering the enthalpic contribution, the M-WLC model is mathematically represented by the following expression:25

F(x) )

Figure 5. (a) Type III force profiles obtained from Al-PAM deposited on a silica surface in 1.0 mM KCl solution at pH 8.4 (solid black curve). The dash lines represent the M-WLC fits with lp ) 0.20 ( 0.03 nm and K ) 888 ( 25 nN for four force peaks. (b) A proposed scheme of the corresponding regions of the force profile shown in (a).

∼500 pN. The second plateau has a force of ∼250 pN. This feature indicates the detachment of two PAM chains from the silica surface in series: the two-fold height plateau of ∼500 pN corresponds to the simultaneous unzip of two PAM chains, whereas the one-fold height plateau of ∼250 pN corresponds to the unzip of the remaining single PAM chain followed by the complete detachment of this PAM train (see the inset of Figure 4). Force profiles with more than two plateaus were not observed in our force measurements. The Type III force profile in Figure 5a shows a saw-tooth pattern. In most cases, 1-4 force peaks for each force curve were observed. The characteristic saw-tooth pattern indicates sequential events of stretching a PAM chain and detaching an

kBT 1 x F · 1- × Ip 4 L K

[(

-2

)

+

x F 1 - L K 4

]

(1)

where F represents the applied force upon a single polymer chain; x is the extension of the polymer chain (in our SMFS experiments, x is the distance between the tip and substrate); L is the contour length of the polymer chain, which is the length of the linearly extended molecule without stretching its backbone; kB is the Boltzmann constant; T is the temperature; lp is the persistence length, characterizing the stiffness/flexibility of the polymer chain; and K represents the specific stiffness of the polymer chain or the elasticity of a chain segment. The lp in eq 1 is mainly determined by the lower force portion (or the relative flat portion) of the force profiles, while the K affects mainly the higher force portion of the force profiles. Therefore, the lp and K are relatively independent of each other. According to the relation between these two lengths described by Ortiz et al.,33 the contour length (30) Tskhovrebova, L.; Trinick, J.; Sleep, J. A.; Simmons, R. M. Nature 1997, 387, 308–312. (31) Kellermayer, M. S. Z.; Smith, S. B.; Granzier, H. L.; Bustamante, C. Science 1997, 276, 1112–1116. (32) Oberhauser, A. F.; Marszalek, P. E.; Erickson, H. P.; Fernandez, J. M. Nature 1998, 393, 181–185. (33) Ortiz, C.; Hadziioannou, G. Macromolecules 1999, 32, 780–787.

Study of Al-PAM-Induced Pelleting Flocculation

Figure 6. (a) Histogram of the adhesion force of Al(OH)3 colloidal particles to a silica surface. Gaussian distribution was used to fit the experimental data. (b) Histogram of the length between two successive saw-tooth peaks.

(L) in our fitting approach is set as L ≈ Lmax/0.92, where Lmax is the experimentally obtained length of the extended chain for a given peak. There are two adjustable parameters of lp and K in the fitting practice. It was found that each peak on the force profile could be fitted well by the M-WLC model with nearly identical parameters lp ) 0.20 ( 0.03 nm and K ) 888 ( 25 nN, as shown by the dash curves in Figure 5a. The good fit between the experimental data and M-WLC model with unique lp and K values for all the peaks strongly suggests that the measured rising force in Figure 5a most likely represents the stretching of PAM chains. The peak force observed in Figure 5a corresponds to the strength of adhesion between Al(OH)3 colloidal particles and the silica surface. The histogram of the adhesion force in Figure 6a is shown to be centered at 1253 ( 230 pN. This force is much larger than the force of 251 ( 51 pN holding PAM chain on silica. The results are very reasonable if we consider the tethering of PAM chains mainly through hydrogen bonding, in contrast to Al-O linkage between Al(OH)3 particles and silica surface. The histogram of the distance between the two successive sawtooth peaks in Figure 6b extends up to 420 nm. This distance is considered as the PAM chain length between the two adjacent Al(OH)3 colloidal particles. The broader distribution of the segment length is attributed to the polydispersity of PAM chains in our Al-PAM. The Type IV force profile in Figure 7 features a plateau followed by a peak (I f II in (a)) or a peak followed by a plateau (I f

Langmuir, Vol. 24, No. 24, 2008 14019

Figure 7. Type IV force profile obtained from Al-PAM deposited on a silica surface in 1.0 mM KCl solution at pH 8.4. (a) A force curve with a plateau followed by a peak (solid black curve) with the fitting results (dash lines) by the M-WLC model with lp ) 0.19 nm and K ) 900 nN. Inset: a proposed scheme of the corresponding regions of the force curve. (b) A force curve with a peak followed by a plateau (solid black curve) with the M-WLC model fitting results (red dash curves)of lp ) 0.22 nm and K ) 900 nN. Inset: a proposed scheme of the corresponding regions of the force curve.

II in (b)). The inset of Figure 7a accounts for the force profile of Figure 7a. The PAM chain with a train-like structure was picked up by the tip and unzipped during the retraction period I. The plateau of unzipping force, representing the adhesion of the PAM chain to the silica surface, was measured to be ∼250 pN. Such an unzipping process continued until a loop was encountered, leading to a sudden drop of the force to zero. Because of the presence of an anchoring point of an Al(OH)3 colloidal particle, further retracting of the substrate away from the tip resulted in the stretching of the unzipped PAM chain and loop. This stretching region can be well fitted by the M-WLC model with lp ) 0.19 nm and K ) 900 nN. The scheme in the inset of Figure 7b accounts for the force profile in Figure 7b. In this case, a PAM chain in a loop conformation anchored on the substrate surface by an Al(OH)3 particle was picked up by the tip and stretched during the retraction of the substrate from the tip. The force increased to a maximum where the Al(OH)3 particle was detached. This stretching region can be fitted well by the M-WLC model with lp ) 0.22 nm and K ) 900 nN. After the detaching of the particle, the force did not drop to zero because a PAM chain remained attached to the surface. This PAM train was then detached from the surface, leading to a plateau of ∼250 pN force, representing the adhesion force of a PAM chain to the silica surface.

14020 Langmuir, Vol. 24, No. 24, 2008

Figure 8. Floccules of 0.6 wt % kaolinite in deionized water at pH 8.4 induced by 20 ppm Al-PAM. Taken by Nikon D2000.

To our best knowledge, force profiles shown in Figure 7 were first observed in this study. Usual SMFS profiles show either plateaus or peaks, but not both. The feature shown in Figure 7 with both a plateau and a peak on one force profile could be attributed to the special structure of the hybrid Al-PAM polymer. The Al(OH)3 colloidal particles were incorporated into the polymer and acted as cores connecting PAM chains. The adhesion force of the Al(OH)3 colloidal particles on the silica surface was much larger than that of PAM chains. Therefore, Al-PAM polymer adsorbs strongly onto the silica surface through the anchor of Al with oxygen on silica. It is important to note that regardless of the type of force profiles, the measured unzipping force of a PAM chain is ∼250 pN, and the rising force regions are well fitted by the M-WLC model with very similar lp and K values in all force profiles. Such a generality supports our interpretation of the measured force profiles. (b). 1.0 mM KCl Solution at pH 5.0 and 10.0. The effect of pH on the interactions between Al-PAM and a silica surface was investigated by SMFS. Very similar results were observed for pH 5.0 and 8.4. This is attributed to similar conformation of Al-PAM adsorbed on a silica surface under these two pH values (see AFM images in Figure 1a,b). Al-PAM molecules adsorbed on a silica surface at pH 5.0 and 8.4 feature a star-like structure with the coverage and the diameter of Al(OH)3 colloidal particles being similar. At pH 10.0, the probability of picking up single polymer chains was reduced significantly. This can be explained by the AFM image in Figure 1c: few but larger particles or aggregates adsorbed on the substrate, which are more difficult to be picked by the AFM tip. Flocculation of Clays. Without flocculant addition, the clay particles were well dispersed in the suspension.34 Immediately after addition of 20 ppm Al-PAM to a 0.6 wt % suspension, small floccules with a pellet structure were clearly observed. While settling, these primary floccules underwent a growth process, leading to the formation of larger floccules with a size of 1 cm or even larger. This growth process from the primary floccules led to pellet-like floccules with a raspberry structure, as shown in Figure 8. The pellet-like floccules are dense, large in size (several millimeters) and nearly spherical in shape.35 As a result of flocculant-assisted settling, the supernatant became very clear. This finding indicates that Al-PAM is effective in flocculating particles of a wide particle size range, which was also observed by others.12 (34) Long, J.; Li, H.; Xu, Z.; Masliyah, J. H. AIChE J. 2006, 52, 371–383. (35) Yusa, M. Int. J. Miner. Process. 1977, 4, 293–305.

Sun et al.

Mechanism of Al-PAM-Induced Flocculation. As shown in Figure 8, pellet-like floccules with a raspberry structure were formed by Al-PAM flocculant. For normal flocculants such as HPAM, pelleting flocculation can only occur in a specially designed equipment under appropriate operating conditions,36-38 such as an appropriate polymer concentration range, stirring method and intensity. In general, the addition of normal flocculants leads to the formation of random, loose, and open structure floccules with entrapped dispersion solutions. In specially designed equipment, uneven shear forces are locally applied around the surface of these loose and random floccules. In regions of weak forces around the floccules, the entrapped solution in the floccules egresses, and the floccules start to shrink and densify, leading to the formation of pellet-like floccules. The process of shrinkage and densification of loose and irregular floccules with simultaneous release of dispersion solutions due to uneven mechanical forces is called mechanical syneresis.35 The decisive factor for the formation of pellet-like floccules is the application of uneven force around the surface of loose and irregular floccules. If applied uniformly, the forces will be supported by entrapped solutions in the floccule with little shrinkage and densification. On the basis of the information retrieved from AFM images, SMFS results, and the observation of the flocculation tests, the formation process of Al-PAM-induced pellet floccules appears to be different from the mechanism described above for normal polymer flocculants, which requires a specially designed equipment. The Al-PAM-induced pelleting flocculation might be described as follows. First, dense, small and nearly spherical primary floccules were formed upon Al-PAM addition. Two mechanisms would lead to the formation of these primary floccules. One is the synergism of two components in Al-PAM: Al(OH)3 colloidal particles and PAM chains. The attachment of positive Al(OH)3 colloidal particles in an Al-PAM to a negatively charged clay particle reduces electrostatic repulsion among the clay particles. Such a reduction allows clay particles to reach a much closer approximation, leading to the formation of dense floccules. PAM chains can bridge clay particles through hydrogen bonding. The other mechanism is attributed to the star-like molecular structure of Al-PAM. From a molecular structure point of view, the star-like structure is more beneficial for the bridging process compared to linear chain molecules, due to easy accessibility of PAM chains in Al-PAM to clay particles. Thus, dense, small and nearly spherical primary floccules would form. Such primary floccules were clearly observed immediately after the Al-PAM addition to the clay suspensions. While settling after gentle mixing, the formed primary floccules underwent a growth. The Al-PAM on the surface of the formed primary floccules attaches to nearby primary floccules, leading to the formation of larger floccules of a raspberry structure. Eventually, larger and denser floccules as those shown in Figure 8 were formed. It should be noted that a simple blend of PAM and AlCl3 did not induce pelleting flocculation.12 It was also observed that, when the star-like structure of Al-PAM was destroyed by hydrochloric acid, there was no pelleting flocculation at all.12 Therefore, the star-like molecular structure, i.e., Al(OH)3 colloidal particles as cores connecting PAM chains (see Figure 1), was the key for the pelleting flocculation.

4. Conclusions AFM and SMFS were used to investigate the structure of a novel polymer hybrid, Al-PAM, and its interactions with a silica (36) Higashitani, K.; Shibata, T.; Kage, H.; Matsuno, Y. J. Chem. Eng. Jpn. 1987, 20, 152–157. (37) Walaszek, W.; Ay, P. Int. J. Miner. Process. 2005, 76, 173–180.

Study of Al-PAM-Induced Pelleting Flocculation

surface. Al-PAM shows a star-like structure and enhanced flocculation performance attributed to the formation of pelletlike floccules. Several types of force profiles show complex interactions of single Al-PAM molecule with solid surfaces. There is a correlation between the force pattern and the adsorption conformation of the polymer. AFM images and information retrieved from SMFS force profiles suggest that Al-PAM molecules have a star-like structure with Al(OH)3 colloidal particles as cores connecting PAM chains. SMFS results also show that Al(OH)3 particles are strongly attached to the silica surface with an adhesion force of ∼1250 pN, in contrast to the

Langmuir, Vol. 24, No. 24, 2008 14021

adhesion force of ∼250 pN for PAM chains. The Al-PAMinduced pelleting flocculation was attributed to its strong adsorption on negatively charged clay particles and star-like structure. Acknowledgment. Financial support from NSERC Industrial Research Chair in Oil Sands Engineering (held by J.H.M.) is gratefully acknowledged. LA802537Z (38) Walaszek, W.; Ay, P. Colloids Surf., A: Physicochem. Eng. Aspects 2006, 280, 155–162.