pubs.acs.org/Langmuir © 2010 American Chemical Society
Effect of Cationic Polyacrylamide Adsorption Kinetics and Ionic Strength on Precipitated Calcium Carbonate Flocculation Ping Peng* and Gil Garnier* BioPRIA, Australian Pulp & Paper Institute, Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia Received August 26, 2010. Revised Manuscript Received September 16, 2010 The effect of polymer adsorption kinetics and ionic strength on the dynamics of particle flocculation was quantified using a model system consisting of precipitated calcium carbonate (PCC) and cationic polyacrylamide (CPAM) at a low shear rate. All early flocculations detectable by a photodispersion analyzer (PDA) happened in nonequilibrium polymer adsorption regimes. We observed discrepancies in flocculation rates with the surface coverage theory, which is based on a simple monolayer adsorption model, in both early and late flocculation stages. For instance, the same amount of adsorbed CPAM reached at different polymer doses demonstrated different flocculating capabilities. This highlighted the importance of polymer adsorption kinetics upon flocculation. The transient conformation of the adsorbed CPAM during the kinetic process sometimes even superceded the adsorbed amount in the determination of PCC flocculation. Both antagonistic and synergetic effects of increased ionic strength on the CPAM-induced PCC aggregation were observed during early flocculation. However, late-stage PCC flocculation shared some similarities, irrespective of polymer dose and ionic strength. Despite the decreased amount of adsorbed polymer from the increased ionic strength, the combination of CPAM and salt, at certain concentrations, demonstrated a synergy to promote PCC aggregation more efficiently than the same amount of the respective components.
1. Introduction Aggregation processes play a critical role in many industrial and environmental strategies. A better understanding and control of particle aggregation can significantly enhance solid-liquid separation processes by sedimentation, filtration, flotation, and centrifugation and can thus serve a wide range of traditional industries such as water treatment, mineral recovery, and paper manufacturing. In addition, controlled particle aggregation allows for better control of the surface property of particles and thus renders an improved interface between the particles and their matrix. This has led to its wide application in biotechnology and nanotechnology. As a typical flocculant, polyelectrolytes are widely used to turn off colloidal stability and thus initiate flocculation. During polyelectrolyte-induced flocculation, polymer adsorption onto colloid surfaces is vital.1 On the basis of the polymer adsorption theory under equilibrium conditions, many flocculation mechanisms have been proposed, such as bridging, patching, and charge neutralization mechanisms.2 According to these theories, the amount of adsorbed polymer at equilibrium, or the surface coverage of particles by polymer, is the governing variable.1,3 Experiments and theory correlate well when the time required for polymer adsorption to reach equilibrium is far less than the timescale of particle aggregation. However, when the corresponding timescales become comparable, the effect of polymer adsorption on flocculation becomes more complicated.3 Although the *Corresponding authors. E-mail:
[email protected] and gil.garnier@ monash.edu. (1) Zhou, Y.; Franks, G. V. Langmuir 2006, 22, 6775–6786. (2) Hubbe, M. A.; Nanko, H.; McNeal, M. R. Bioresources 2009, 4, 850–906. (3) Borkovec, M.; Papastavrou, G. Curr. Opin. Colloid Interface Sci. 2008, 13, 429–437. (4) Cohen Stuart, M. A.; Mieke Kleijn, J. In Physical Chemistry of Polyelectrolytes; Radeva, T., Ed.; Surfactant Science Series; Marcel Dekker: New York, 2001; Vol. 99, p 281.
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kinetics of polymer adsorption has received some attention, the process remains obscure and is much less understood than the static ones.3-6 In addition, most adsorption kinetics studies rely on a difference in refractive index between the polymer and substrate. This normally requires either a macroscopic planar substrate or a carefully selected monodisperse particle substrate such as model latex or silica spheres.3,6 It is rare to find studies on polymeric flocculant adsorption kinetics based on an imperfect particle of industrial significance that has a broad size range during aggregation, from colloidal to macroscopic. The imperfect particle can invalidate the principle for quantifying adsorbed polymer on colloids such as that based on the hydrodynamic thickness measured by dynamic light scattering. Also, the characterization of such particle flocculation across a broad size range is challenging itself, owing to the limitation of an instrument when a single technique is normally used. However, such imperfect systems prevail in industry. In addition, when the polymer adsorption kinetics study is based on measuring the polyelectrolyte charge, any instability, such as the hydrolysis of the widely used cationic polyacrylamide (CPAM) under alkaline conditions,7,8 adds further complexity to the flocculation mechanisms. Polyelectrolyte-induced flocculation is further complicated by an increase in the ionic strength of the colloidal suspension, which usually leads to a significant loss of polymeric flocculant efficiency and a change in floc properties.9 For polyelectroytes of high molecular weight, the increased ionic strength may shield the repulsion between charged groups along a polymer chain and (5) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1998. (6) Samoshina, Y.; Nylander, T.; Shubin, V.; Bauer, R.; Eskilsson, K. Langmuir 2005, 21, 5872–5881. (7) Aksberg, R.; Wagberg, L. J. Appl. Polym. Sci. 1989, 38, 297–304. (8) John, G. Particles in Water: Properties and Processes; CRC Press: Boca Raton, FL, 2006. (9) Monagle, D. J. Flocculation and Settling of Inorganic Particles in a Salt Solution. U.S. Patent 3,617,572 (A), 1973.
Published on Web 10/07/2010
DOI: 10.1021/la103410j
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enable the macromolecule to coil up into a tighter, less extended conformation that in turn reduces the bridging capability of the polymer. However, the increased ionic strength also reduces the electrical double-layer thickness of a particle and thus enables two particles to approach one another more closely, which decreases the bridging distance a polymer must span in order to induce flocculation. In this way, the increased ionic strength can favor the polymer-induced flocculation, so the effect of ionic strength on polyelectrolyte-induced colloidal flocculation is not straightforward and the net effect depends on the force balance of all interactions. Although progress has been made to understand the influence of ionic strength through theoretical modeling,6 most insights are from empirical studies of a specific colloid and polymeric flocculant system.1,10 Specific systems are studied because of flocculation complexity and instrumental limitation in floc characterization.1 Calcium carbonate (CaCO3) is widely used as a filler in many industrial applications such as papermaking, paints, plastics, and rubber. Owing to its osteoconductivity, calcium carbonate is also a good filling material in orthopedics and dentistry. Recently, there has been increased interest in the application of calcium carbonate for microcapsule fabrication, pharmaceutics,11-13 stem cell tissue regeneration,14 and bioactive paper. Among all applications, calcium carbonate aggregation has a great influence on its interaction with its matrix/host. The control and predictability of calcium carbonate flocculation is then critical to the performance of many of these novel materials. It is the objective of this study to quantify the effect of cationic polyacryamide adsorption kinetics and ionic strength on the flocculation kinetics of polydisperse calcium carbonate. Complementary techniques of photodispersion analysis (PDA) and focused beam reflectance measurements (FBRM) were used for the continuous monitoring of PCC flocculation kinetics from a colloidal state to a macroscopic particle. The instability of CPAM was quantified, and a novel derivatized polyelectrolyte titration technique was developed to measure CPAM adsorption kinetics on PCC particles and aggregates. The correlation between flocculation and polymer adsorption kinetics was analyzed under various ionic strengths.
2. Materials and Methods a. Flocculation. Precipitated calcium carbonate (PCC) was provided by Schaefer Kalk (Malaysia) SDN.BHD (Schaefer Precarb 100). The specific surface area (BET) is 8 ( 2 m2/g, and the average particle size d50 (Sedigraph) is 1.0 ( 0.2 μm according to the manufacturer. One gram of PCC in 0.5 L of deionized water was magnetically stirred for 10 min, followed by dilution of the suspension to 1 L. Prior to each experiment, a 500 mL suspension of PCC at a fixed concentration of 200 ppm was made from the stock solution, which was then dispersed ultrasonically for 10 min. When salt was added, it was dissolved in the 200 ppm PCC slurry before ultrasonication. The pH of the suspension was around 9.7 and remained constant during the study. The PCC suspension was pumped from the outlet of a 1 L beaker by a peristaltic pump through 3-mm-diameter tubing into the photocell of the photometric dispersion analyzer (PDA 2000, Rank Brothers) and then back to the beaker via a recirculation (10) William, E. S. Principles of Wet End Chemistry; TAPPI Press: Atlanta, GA, 1996; p 122. (11) Joshi, A. B.; Srivastava, R. Adv. Sci. Lett. 2009, 2, 329–336. (12) Shan, D.; Zhu, M.; Xue, H.; Cosnier, S. Biosens. Bioelectron. 2007, 22, 1612–1617. (13) Shan, D.; Zhu, M.; Han, E.; Xue, H.; Cosnier, S. Biosens. Bioelectron. 2007, 23, 648–654. (14) Champa Jayasuriya, A.; Bhat, A. J. Tissue Eng. Regener. Med. 2010, 4, 340-348.
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loop. The reaction beaker was equipped with a flat paddle stirrer operating at 100 rpm. The flow was maintained constant at 30 mL/min, which resulted in a stable PDA signal over time for the 200 ppm PCC suspension. The maximum shear rate was less than 150 s-1.15 The delay between the suspension leaving the beaker and entering the PDA was 2 s. The change in the PCC aggregation state in deionized water was monitored continuously by measuring the light transmitted through the 200 ppm PCC suspensions flowing through the PDA. The root-mean-square (rms) value of the fluctuating light intensity relative to the average transmitted light intensity (dc signal) was recorded. Then the ratio (R) of rms to dc signal was derived; R is sensitive to changes in the degree of flocculation, especially in the earlier stage. With the light beam wavelength at 820 nm, the minimum detectable size was about 0.5 μm and the maximum was at about 100-200 μm.15 A cationic dimethylamino-ethyl-methacrylate polyacrylamide (CPAM) of high molecular weight (13 MDa) and a charge density of 40 wt % (F1, SnowFlake Cationics, AQUAþTECH, Switzerland) was used as received. A stock solution of 1 mg/mL CPAM in deionized water was prepared on the day of the experiment. The effect of CPAM on PCC aggregation was tested at different polymer concentrations for a 200 ppm PCC suspension under varying ionic strengths of 0, 0.01, 0.05, and 0.1 made by adding NaCl. The dimensionless ionic strength (I) was calculated as follows16 I ¼
1 mi 2 Zi Σ 2 m0
ð1Þ
where mi is the ionic concentration in units of molality or mol/kg, m0 = 1 mol/kg, and Zi is the number of charges on the ion. The 1 M solution is assumed to be equivalent to 1 mol/kg. Thorough cleaning of the beaker and flowing system was achieved by circulating a diluted hydrochloric acid solution followed by a copious amount of water between each measurement. The PDA ratio (R) of the suspension was measured before adding the polymer to the suspension. After 5 min of equilibrium, the predetermined amounts of CPAM (2-15 mg of CPAM/g of PCC) were added quickly while the ratio was continuously measured every second until reaching the plateau or maximum. The stability ratios (W), or flocculation efficiency of the polymer (1/W), for a particular flocculation was calculated as follows W ¼
KmaxðfastÞ ðdR=dtÞmaxðfastÞ = KmaxðiÞ ðdR=dtÞmaxðiÞ
ð2Þ
where Kmax(i) is the steepest slope of curve i and Kmax(fast) is the fastest maximum growth rate of flocculation described by ln W = 0 among all conditions studied; ln W = ¥ represents the stable suspension, and dR/dt is the slope of the R versus time curve. The focused beam reflectance measurement (FBRM) was also used to follow the evolution of floc size. During the measurement, the FBRM provides a focused laser beam that spins at a controlled constant high speed and propagates into the particle system through a sapphire window.17 Particles located in the scanning-focused spot backscatter distinct pulses of reflected light, which are translated into chord lengths (CLs) by multiplying the scan speed (velocity) by the pulse width (time). CL is defined as the shortest distance from one edge of a particle to another edge. Thousands of individual CLs are measured each second to produce the CL distribution, which is used to detect particle dimensions and count in real time. The CL counts grouped by channels (for example, with a restrictive CL range such as 50150 μm) are the primary data from which a variety of statistical (15) Rank Brothers Ltd Photometric Dispersion Analyser. http://www.rankbrothers.co.uk/prod5.htm, accessed Feb 20, 2010. (16) Solomon, T. J. Chem. Educ. 2001, 78, 1691–1692. (17) Yu, Z. Q.; Chow, P. S.; Tan, R. B. H. Org. Process Res. Dev. 2008, 12, 646–654.
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presentations are obtained from the control interface. In theory, the total number of CL counts is an indication of the number of particles of a given size represented by its respective channel. Experiments were carried out with a Lasentec D600L FBRM system (Mettler-Toledo) equipped with a 1 L beaker and an electric-motor-driven stirrer at 100 rpm. A 500 mL suspension of 200 ppm PCC was pretreated in the same way as for the PDA experiments. After 5 min of stabilization, the predetermined amounts of CPAM (2-15 mg of CPAM/g of PCC) were then added quickly while the chord length (CL) ranging from 1 to 1000 μm was continuously measured every 10 s. Prior to performing the FBRM test, the FBRM probe window was cleaned. The repeatability of the FBRM probe was confirmed by performing FBRM tests on three PCC suspensions under the same conditions. b. Particle and Colloid Charge. The zeta potential measurements were performed with a Zetasizer Nano ZS (Malvern Instruments) in a folded capillary cell (DTS1060) at 25 C. The zeta potential was calculated with the supplied software by determining the electrophoretic mobility from an electrophoresis experiment using laser Doppler velocimetry and applying the Henry equation. The streaming potential measurements were performed with the particle charge detector (M€ utek PCD-03, BTG Instruments GmbH). The instrument was calibrated with both deionized water and the standard of 0.001 N poly(diallyldimethylammonium chloride) (poly-DADMAC) and sodium poly(ethylene sulfate) (PES-Na). During the measurements, a 200 ppm PCC suspension in water was dispensed ultrasonically for 10 min before measurements. For CPAM, a concentration of 0.0004-0.003 mg/mL in either deionized water or PCC-saturated water solution was used. c. Polymeric Flocculant Adsorption. The amount of polymer adsorbed onto PCC was determined by subtracting the concentration of CPAM in the supernatant after centrifugation from that of the dosed polymer. The adsorption kinetics was measured in time-lapse mode by sampling the PCC suspension at different times after injecting CPAM, followed by immediate centrifugation, twice, for 2 min each at 2000 rpm before decantation. The adsorption isotherm was measured from the supernatant after stirring the PCC and CPAM suspension for 2 h. The quantification of CPAM without salt was made through polyelectrolyte titration, which used the streaming potential of the particle charge detector to identify the point of zero charge. An automatic titrator continuously added an oppositely charged polyelectrolyte (poly-DADMAC or PES-Na) of known concentration (0.0005 N) to the 10.0 mL aqueous sample until the point of zero charge was reached. Titrant consumption in milliliters was measured and then transformed into CPAM concentration through a standard curve of known concentration of CPAM under the same conditions. The quantification of CPAM under salt conditions was made through the analysis of the total nitrogen content of CPAM. Alkaline persulphate digestion was used to convert the N-containing compounds into nitrate. The total nitrogen was then measured by a colorimetric flow injection analysis method (Lachat QuickChem 8500) at 520 nm.18 The CPAM concentration was obtained through a standard curve of known concentration of CPAM under the same conditions. The detection limit is below 0.1 μg (CPAM)/mL.
3. Results a. Flocculation Measured by PDA. The CPAM-induced PCC flocculation in the early stage was monitored by PDA. Figure 1 presents the PDA ratio signal (R) of a flowing PCC suspension as a function of time for different doses of CPAM (2, 5, and 15 mg of CPAM/g of PCC) injected at 5 min. In the (18) APHA-AWWA-WEF Standard Methods for the Examination of Water and Wastewater, 21 ed.; American Public Health Association: Washington, DC, 2005; Vol. Method 4500-PJ.
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Figure 1. PDA ratio signal (R) of PCC suspensions as a function of time at different CPAM dosages of (a) 15, (b) 5, and (c) 2 mg of CPAM/g of PCC, respectively. CPAM was injected at 5 min.
absence of salt, a 200 ppm sheared PCC slurry forms a stable colloid suspension as indicated by the flat lines within the first 5 min before CPAM addition (Figure 1). Upon addition of CPAM, the lags of PCC flocculation were observed at the lower polymer dosage (2 mg of CPAM/g of PCC) as indicated by the flat R value (5-10 min, Figure 1). However, the growth rate of PCC flocculation gradually increased after an induction period and remained steady (13-15 min) before approaching a flocculation plateau (Figure 1). In contrast, a high polymer dose (15 mg of CPAM/g of PCC) instantly induced PCC flocculation, which proceeded at an almost constant flocculation growth rate. Nevertheless, the largest maximum flocculation ratio growth rate (steeper curve) was observed at the medium polymer dose (5 mg of CPAM/g of PCC), where the PCC flocculation was also instantly induced but proceeded at an increasing flocculation growth rate with time. Despite the different polymer dosages, the maximum ratio achieved was almost the same, beyond which the PDA loses its detection ability as the PCC flocs become larger than the detection range limit. Other floc analytical techniques are needed thereafter. The maximum growth rate of the PDA ratio was used to calculate the system stability (ln W) in this study because it is a clear indication of the particle aggregation at a given time. This is in agreement with the method recommended by the supplier.15 The maximum growth rate of the ratio is a valuable criterion for quantifying the system instability or flocculant efficiency in a dynamic process. The PCC stability (ln W) as a function of CPAM dosage is shown in Figure 2. The maximum aggregation rate was at 5 mg of CPAM/g of PCC without salt. Both the insufficient and excess addition of CPAM led to comparatively stable particle systems as expected from the electrostatic charge theory. Adding salt to a PCC suspension shifted the larger flocculation rates to ranges of higher CPAM concentration. The maximum aggregation rate was measured at 15 mg of CPAM/g of PCC under 0.01 M NaCl. Salt addition also induced PCC flocculation with the critical coagulation concentration (CCC) of the PCC suspension for NaCl at 0.01 M (Figure 3). The PCC stability value (ln W ) in the sole presence of salt at any concentration (Figure 3) was larger than those measured with polymer (Figure 2). b. Flocculation Measured by FBRM. The particle numbers of sheared PCC suspensions, represented by the CL counts, within the respective ranges of particle size were determined as a function of time by a focused beam reflectance measurement (FBRM). The evolution of particle size distribution with time and various distribution statistics were then derived. The unweighted median cord length (CL) is very sensitive to changes in the fine portion of the size distribution,19 which was almost unchanged for DOI: 10.1021/la103410j
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Figure 2. PCC stability ratio (ln W) of PCC suspensions as a function of CPAM dosage under different ionic strengths of (O) 0.00, (() 0.01, (2) 0.05, and (b) 0.10.
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Figure 5. (a) PDA ratio and the number of counts evolution for PCC suspensions in 0.1 M NaCl in particle ranges of (b) 50-150 and (c) 150-300 μm under a CPAM dosage of 5 mg/g of PCC injected at 5 min.
Figure 3. PCC stability ratio (ln W) as a function of salt concentration (NaCl).
Figure 4. Number of counts evolution for PCC suspensions in a particle range of either (a-c) 50-150 μm or (d-f) 150-300 μm under CPAM dosages of (a, d) 2, (b, e) 5, and (c, f) 15 mg of CPAM/g of PCC, respectively. CPAM was injected at 5 min; no salt was present.
the PCC suspensions within 45 min in the absence of salt and polymer. However, another statistical presentation consisting of the number of counts for particles smaller than 10 μm gradually increased with time for PCC suspensions. The evolution of the CL distribution with time also showed an increased number of particles with time, especially in the size range of around 3 μm, but no detectable increase was recorded beyond 30 μm (data not shown). Salt-free PCC particles flocculated more vigorously to form larger particles (beyond 50 μm) upon injection of polymer (Figure 4). At a low dose of 2 mg of CPAM/g of PCC, the increasing number of medium-sized aggregates (50-150 μm) toward a (19) Mettler-Toledo AutoChem, I. FBRM Control Interface Users’ Manual (ver. 6.7.0). Columbia, MD, 2005.
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Figure 6. Zeta potential of CPAM in saturated PCC solution (10-4 mg CPAM/mL) as a function of mixing time.
plateau was accompanied by a gentle increase in the number of large aggregates (150-300 μm). At polymer dosages of 5 and 15 mg of CPAM/g of PCC, however, the PCC flocculation behaviors in the late stage were rather similar to each other. In both of the higher polymer dosages, the medium-sized aggregates (50-150 μm) continued to flocculate into large aggregates (150-300 μm). The addition of salt (0.1 M NaCl) to PCC suspensions also promoted PCC aggregation, but the particle size was limited to a maximum of 50 μm (data not shown). Under such a high ionic strength, the polymer dose showed little effect on the continued flocculation of PCC in the late stage as characterized by FBRM; typical results are presented in Figure 5. The number of PCC aggregates with size ranging from 50 to 150 μm was also quite similar to that without salt for up to 20 min, irrespective of the ionic strength. However, the number of PCC aggregates continued to grow beyond 20 min and proceeded to around 30 min. Nevertheless, the number of PCC aggregates of 150-300 μm induced by CPAM (Figure 5) was much smaller than without salt at polymer doses of 5 and 15 mg of CPAM/g of PCC (Figure 4). c. Adsorption of CPAM on PCC Particles. The CPAM remaining in the supernatant of the PCC suspension gradually lost its positive charge and even underwent charge reversal as indicated by the particle charge detector; this was confirmed by the zeta potential measurement (Figure 6). As a result, the end point of the polyelectrolyte titration was blurred. This made it difficult to determine the CPAM concentration in the supernatant by using PES-Na as a titrant. Immediate pH adjustment of the CPAM supernatant to 2.5 by the addition of hydrochloric acid also failed to deliver reproducible measurements. Therefore, the derivation of CPAM through complete hydrolyzation was performed by adding sodium hydroxide to CPAM Langmuir 2010, 26(22), 16949–16957
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Figure 7. (A) Kinetics of CPAM adsorption on PCC at different polymer dosages of (*) 15, (0) 5, and (O) 2 mg/g of PCC and (B) isotherm of CPAM adsorption on PCC. CPAM was injected at 0 min. No salt was present.
standards and supernatant at 0.02 N and then holding at 55 C for at least 16 h. The hydrolyzed CPAM was negatively charged and stable as detected by the Zetasizer; it was then titrated by polyDADMAC. Good polymer concentration measurement and reproducibility were thus achieved with a limit of detectability of 0.1 μg of CPAM/mL. The CPAM adsorption kinetics as a function of polymer concentration were measured (Figure 7A). There was a lag time of up to 6 min before adsorption was detectable for the low polymer dose of 2 mg of CPAM/g of PCC. Afterwards, the adsorption kinetics demonstrated similar trends to those at the higher CPAM dosages: an initial linear increase up to around 10 min followed by a plateau. CPAM adsorption plateaus of 1.7, 4.6, and 13.6 mg of CPAM/g of PCC were measured at increasing CPAM dosages of 2, 5, and 15 mg of CPAM/g of PCC, respectively. The equilibrium adsorption isotherm of CPAM on salt-free PCC is shown in Figure 7B. The isotherm reached an adsorption plateau at a polymer dose of around 15 mg of CPAM/g of PCC. The quantification of CPAM polymer through the titration of hydrolyzed CPAM was disrupted by the presence of salt. Therefore, an alternative CPAM quantification method was developed on the basis of the analysis of the total nitrogen content of CPAM. For CPAM solution in the absence of salt, polymer quantification by both methods was consistent. The CPAM adsorption kinetics as a function of polymer dose under two salt concentrations (0.01 and 0.1 M NaCl) is shown in Figure 8. At a low ionic strength of 0.01 and a small polymer dose (2 mg of CPAM/g of PCC), there was an initial lag time for polymer adsorption. However, the adsorption soon resembled the adsorption kinetics at polymer doses of 5 and 15 mg of CPAM/g of PCC, increasing exponentially and then approaching an adsorption plateau (Figure 8A). With salt, the adsorbed amount of CPAM polymer at the plateau Langmuir 2010, 26(22), 16949–16957
Figure 8. Kinetics of CPAM adsorption on PCC at different polymer dosages of (*) 15, (0) 5, and (O) 2 mg/g of PCC under different ionic strengths of (A) 0.01 and (B) 0.1. CPAM was injected at 0 min.
was lower than that without salt. Under high ionic strength (0.1 M NaCl), there was no detectable adsorbed CPAM at the low polymer dosage (2 mg of CPAM/g of PCC). For polymer doses of 5 and 15 mg of CPAM/g of PCC, both sets of adsorption kinetics displayed a lag time up to 6 min after an injection of polymer. The subsequent adsorption displayed a common initial exponential increase with time, followed by a plateau (Figure 8B). Again, at high salt concentration (0.1 M) the amount of adsorbed CPAM polymer at the plateau was lower than at low salt concentration (0.01 M).
4. Discussion a. PCC Flocculation Characterized by PDA. Photometric dispersion analysis (PDA) is a well-suited technique for the early detection of small flocs (as low as 0.5 μm) but is restricted for large flocs (e.g., above 150 or 200 μm). PDA analysis indicated a stable PCC suspension in the absence of salt and polymer, which came from the repulsion of the negatively charged double layers of PCC, having a zeta potential of -1.65 ( 0.17 mV at pH 9.7 ( 0.3. Adding salt beyond the CCC level induced PCC flocculation because of the decreased activation energy for aggregation from the decreased double-layer thickness of PCC. This is in agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for colloid stability. Regardless of the ionic strength of the PCC suspension, CPAM injection always induced PCC aggregation more vigorously than that induced by salt only. This is consistent with the previous report by Gregory.20 However, the early PCC (20) Gregory, J. J. Colloid Interface Sci. 1973, 42, 448–456.
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Figure 9. Schematic illustration of the effect of salt on (A) the CPAM conformation and (B) the PCC double-layer thickness.
flocculation, characterized by PCC stability (ln W), varied depending on the ionic strength of the PCC suspension and polymer dose. Without salt, a classical behavior was observed: an initial decrease in the stability of the PCC suspension with the polymer dosage was followed by its increase and the transition to stabilization. Nevertheless, such regularity was not observed with salt in the suspension, indicating more complex flocculation mechanisms with the polyelectrolyte in the presence of salt. At a medium polymer dose of 5 mg of CPAM/g of PCC, the maximum aggregation rate was obtained for the salt-free PCC suspension, which is normally considered to be optimum when fast flocculation is required. At 5 mg of CPAM/g of PCC, the efficiency of CPAM was systematically reduced with increasing ionic strength of the PCC suspension (Figure 2). This is a typical challenge faced by industries such as papermaking under high water-recycling conditions. The antagonistic effect of ionic strength on PCC flocculation induced by CPAM may arise from the dominance of the charge screening of the polymer by the salt ions as illustrated in Figure 9A. Superimposed with the effect of polymer concentration, the resulting compact polymer conformation under the medium dose reduced the bridging capability. At low and high doses of CPAM, however, the addition of salt significantly promoted PCC flocculation (Figure 2). Nevertheless, the effect of ionic strength on PCC flocculation induced by CPAM was not monotonic. The least amount of NaCl at the CCC of the PCC suspension led to the largest increase in the PCC flocculation rate. The synergy between the salt and CPAM concentrations on the dynamic flocculation behavior of PCC may arise from the dominance of the PCC particle double-layer compression as illustrated in Figure 9B. The decreased particle double-layer thickness favors PCC flocculation. In addition, adding salt to a PCC suspension shifted the larger aggregation rates to higher CPAM dose ranges. This suggests that the optimized polymer concentration for fast flocculation is a nonlinear function of the ionic strength of particle suspensions. Both antagonistic and synergetic effects of increased ionic strength on the polyelectrolyte-induced particle aggregation were previously reported for flocculation systems involving different chemistries.1,20 However, both effects observed in the flocculation system with the same chemistry system is novel. There is a potential to optimize flocculation conditions by manipulating only the engineering process while keeping the system chemistry constant. For instance, the fast PCC flocculation can be achieved by choosing the right combination of CPAM and salt concentrations instead of changing the type of polymeric flocculant. b. PCC Flocculation Characterized by FBRM. The stable PCC suspension indicated by PDA also demonstrated the constant 16954 DOI: 10.1021/la103410j
unweighted median CL in FBRM analysis. However, another statistical method;the number of counts for particles smaller than 10 μm and the evolution of the CL distribution with time;implied the progressive aggregation of PCC suspensions for up to 30 μm. The increased counts for particles of a few micrometers are hypothesized to arise from the flocculation of the small particles below 1 μm that were initially not detectable by FBRM. The size measurement of the PCC suspensions by the Zetasizer confirmed the hypothesis (Figure 10). More than 50% of the particles indeed had a size below 1 μm, which was also confirmed by scanning electron microscopy (data not shown). Similar phenomena were previously reported for FBRM analysis.21 For CPAM-induced PCC flocculation, not all counts in the restrictive CL ranges and the distribution statistics obtained from FBRM could be used to interpret the particle size population evolution. The complex data analysis may arise from the nonspherical shape of PCC particles or a possible change in their surface properties and relative refractive indices during aggregation.17 Nevertheless, the counts for CLs between 50 and 150 μm indicated that the injection of CPAM into a salt-free PCC suspension promoted PCC flocculation. The formation of particles beyond 50 μm in the late stage was observed at all polymer doses studied. In addition, the abrupt count increases for CLs between 50 and 150 μm (Figure 4) coincided with the PDA ratio maximum or plateau (Figure 1). This implies that the aggregates immeasurable by PDA were successfully monitored by FBRM. The large flocs formed during late PCC flocculation as characterized by FBRM were complementary to PDA data, which enabled the continuous characterization of PCC flocculation across a wide particle size range as schematically illustrated in Figure 11. In the absence of salt, the polymer dose had a mild effect on PCC flocculation in the late stage. At a low polymer dose of 2 mg of CPAM/g of PCC, there was no significant increase in large aggregates (150 to 300 μm), but the medium aggregates (50 to 150 μm) continued to grow with time. At polymer dosages of 5 and 15 mg of CPAM/g of PCC, the starting point for the formation of large aggregates (150 to 300 μm) almost coincided with the sharp decrease in the medium aggregates (50 to 150 μm) observed for PCC suspensions (Figure 4). This coincidence, as schematically illustrated in Figure 11, indicates a progressive particle aggregation process where invisible primary particles become visible aggregates that continue to grow into medium and large aggregates. The flocculation mechanisms might be dependent on the polymer surface coverage on fillers in both (21) Blanco, A.; De La Fuente, E.; Negro, C.; Monte, M. C.; Tijero, J. Tappi J. 2002, 1, 14–20.
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Figure 10. Size distribution of PCC particles after 10 min of ultrasonication.
Figure 11. Schematics of PCC flocculation kinetics in the (A) early (by PDA) and (B, C) later (50-150 μm and 150-300 μm by FBRM, respectively) stages in relation to (D) the CPAM adsorption kinetics on PCC particles without salt. The data was extracted from Figures 1, 4, and 7A.
early and late stages. Under all conditions investigated, the aggregation rate was higher than the rate of floc breakup because of the low stirring and flow rate investigated. Under higher ionic strength (0.1 M NaCl), the polymer dose showed little influence on the medium and large PCC aggregates developed in the late flocculation stage. Similar to the case without salt, the abrupt count increases for the CLs between 50 to 150 μm coincided with the PDA ratio maximum or plateau (Figure 5). The extended growth of medium aggregates with time up to 30 min resulted from an increase in the detectable particles and inhibited the aggregation toward large particles. The inhibited growth patterns of large PCC aggregates under a high ionic strength of 0.1 resembled the respective pattern at a low polymer dosage and under salt-free conditions. This implies that the charge screening of polymer by a high salt concentration is equivalent to the reduction of the effective polymer concentration and thus the polymer flocculant efficiency. The inhibited growth may also arise from the performation of PCC aggregates up to 50 μm as induced by salt. c. CPAM Adsorption and Its Correlation to PCC Flocculation: No Salt. The instability of CPAM at the natural pH of the PCC suspension (pH 9.7) caused by its hydrolysis into a negatively charged polymer is in agreement with previous studies.7,22 Under alkaline conditions, the hydrolysis rate of the ester groups of the cationic quaternized dimethylamino-ethylacrylate is high with an estimated half-life of less than 15 min.7 The polymer hydrolysis reduces the positive charge density of CPAM and leads to the formation of anionic acrylate groups, which can form an internal salt with the cationic trimethylaminoethyl groups to disrupt the identification of the end point of titration. However, (22) Kamiti, M.; van de Ven, T. G. M. J. Pulp Pap. Sci. 1994, 20, J199–205.
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titration of the completely hydrolyzed CPAM with poly-DADMAC enabled the determination of the CPAM concentration in the salt-free supernatant. The time required to reach the CPAM adsorption plateau (Figure 7A) being similar to the time required to reach the maximum flocculation ratio (R) (Figure 1) was a coincidence. This implies that the early flocculation detected by PDA proceeded under a nonequilibrium polymer adsorption regime (Figure 10). At low polymer concentration (2 mg of CPAM/g of PCC), the collision probability between the PCC particles (200 ppm) and the CPAM molecules was low. This resulted in a lack of polymer adsorption for up to 6 min (Figure 7A) and inhibited PCC flocculation (5-10 min, Figure 1). After the induction periods, the CPAM adsorption increased linearly up to 10 min where the same climbing trends are displayed by the corresponding PCC flocculation growth rate. The adsorption plateau (after 10 min, Figure 7A) also coincided with the PCC flocculation plateau in PDA measurements (13-15 min, Figure 1). The maximum quantity of adsorbed polymer (1.7 mg of CPAM/g of PCC) was less than the amount of polymer required for half surface coverage, assuming monolayer adsorption (around 3.9 mg of CPAM/g of PCC).23 The early PCC flocculation detected by PDA at the lower polymer concentration well obeyed the polymer bridging theory with more adsorbed polymer leading to the increased flocculation rate. However, the PCC flocculation continued in the late stage, as shown in Figure 4, despite the amount of adsorbed CPAM having reached equilibrium. This suggests that the adsorbed CPAM at equilibrium consisted of long loops and tails, which retained good flocculating ability. At the medium polymer concentration (5 mg of CPAM/g of PCC), the amount of adsorbed polymer increased with time, followed by a plateau at 4.6 mg of CPAM/g of PCC after 10 min. The corresponding early flocculation growth rate (slope of R, in Figure 1) initially increased exponentially with time and then a reached steady rate between 10 and 12.5 min, before approaching a flocculation plateau at 14.5 min. The corresponding adsorbed amount of polymer at the steady flocculation rates was between 2.8 and 3.5 mg of CPAM/g of PCC, which corresponds to 72-90% monolayer surface coverage. This is slightly higher than the half surface coverage required for the fast aggregation rate predicted from the bridging theory. Such a discrepancy implies a different phenomenon than the simple polymer bridging mechanism. In addition, the transient conformation of the adsorbed CPAM could contribute significantly to the nonequilibrium flocculation. Similar to the low polymer dose, the PCC flocculation continued in the late stage (Figure 4) despite the CPAM adsorption having reached equilibrium. The fastest adsorption rate was observed at the high polymer dose (15 mg of CPAM/g of PCC), which arose from the increased collision probability between the numerous polymer coils and PCC particles. Within 3.5 min after polymer addition, the PCC particles reached full surface coverage. However, the polymer adsorption continued until reaching a plateau at 13.6 mg of CPAM/g of PCC in 7 min. During the nonequilibrium polymer adsorption period, the PCC flocculation proceeded at a constant rate, regardless of the amount of adsorbed CPAM. Despite different amounts of adsorbed polymer, the PCC flocculation in the late stage detected by FBRM was similar for 5 and 15 mg of CPAM/g of PCC. This indicates the importance of polymer adsorption kinetics with respect to particle flocculation. (23) Mabire, F.; Audebert, R.; Quivoron, C. Polymer 1984, 25, 1317–1322.
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Figure 12. Schematic mechanism for multilayer adsorption of (C) CPAM on PCC particles as a result of the hydrolysis of (A) CPAM into (B) locally negatively charged hydrolyzed CPAM in PCC suspensions as a result of the increased pH.
Correlating polymer adsorption with PCC flocculation reveals that the same amount of adsorbed polymer (2.2-4.6 mg of CPAM/g of PCC) reached at different polymer additions (5 and 15 mg of CPAM/g of PCC) (Figure 7A) demonstrated different flocculating activities, which is indicated by the R slopes at 7-9 min in Figure 1a,b, respectively. These discrepancies are evidence of the strong effect of polymer concentration on the adsorbed polymer conformation, in addition to the maximum quantity of adsorbed polymer at equilibrium. A previous study by quartz crystal microgravimetry also indicated that the adsorbed polymer configuration is changed by the polymer concentration.24 The transitional conformation of CPAM adsorbed on PCC before reaching its maximum adsorption capacity overrode the adsorbed amount in the determination of the polymer flocculating activity. The effect of the reconformation/relaxation of adsorbed polymer on its flocculating efficiency is inconsistent with previous studies based on adsorption equilibrium.1,25 These discrepancies highlighted the importance of polymer adsorption kinetics and conformation on flocculation and the shortcomings of the flocculation rate theory based on the simple monolayer adsorption model. The polymer adsorption isotherms (Figure 7B) were found not to obey the Langmuir equation, confirming a more complex flocculation mechanism. The maximum amount of polymer adsorbed on PCC was more than 3 times that expected from the simple flat monolayer adsorption of polymer coils (Figure 7B). The excess amount of adsorbed polymer may arise from the extended conformation with many loops and tails upon adsorption, which left more spaces to adsorb more macromolecules. In addition, the multilayer polymer adsorption from the local charge reversal of CPAM molecules upon hydrolysis (Figure 6) may also have contributed to the excess amount of adsorbed polymer. This is because the local charge reversal of the CPAM molecules could happen much faster when in contact with PCC suspensions, although it took a few hours for the net charge reversal of CPAM molecules to be detected. Such local-charge-reversed CPAM molecules could have a random mosaic distribution of positively and negatively charged regions. The negatively charged segments can attract the oppositely charged segments of CPAM, thus leading to multilayer adsorption. This is illustrated in Figure 12. In the equilibrium adsorption regime, the conformations of adsorbed CPAM directly in contact with the PCC suspension could be rather similar to each other as long as the particles were completely covered with polymer, regardless of the total adsorbed amount of polymer. Such similarity led to similar PCC flocculation behavior in the late stage for polymer dosages of 5 and 15 mg of CPAM/g of PCC, as detected by FBRM. d. CPAM Adsorption and Its Correlation to PCC Flocculation: With Salt. The effect of ionic strength on CPAM
adsorption kinetics on PCC was significant and monotonical. Although the trends in the CPAM adsorption kinetics on PCC particles under a low ionic strength of 0.01 (Figure 7A) were similar to those that were salt-free, the respective adsorbed amount was reduced by more than 50%. Under a high ionic strength of 0.1, the adsorbed amount was less than 25% of the respective amount without salt. At a low polymer dose of 2 mg of CPAM/g of PCC, the polymer adsorption under an ionic strength of 0.1 was almost eliminated by the charge screening of CPAM from the small ions and fewer collisions between the PCC particles and the CPAM molecules at a low polymer concentration. For the other two polymer doses, a lag time of up to 6 min was required for the CPAM molecules to compete with the small cations for the adsorption sites and to replace the previously adsorbed small cations (Figure 8B). The comparison of Figures 7A and 8 demonstrated that the addition of salt consistently led to a decreased amount of adsorbed polymer for the same dose of CPAM. The higher the concentration of salt, the lower the concentration of adsorbed polymer at a given polymer concentration. This indicates that the attraction between CPAM and PCC is mainly electrostatic in nature such that salt screens the attraction and reduces polyelectrolyte adsorption. In the presence of salt, the maximum amount of adsorbed polymer (Figure 8) was comparable to or well below the amount of polymer required for monolayer adsorption in a salt-free PCC suspension.23 It is well known that the hydrodynamic diameter of a cationic polymer decreases with increasing ionic strength.1,26 Therefore, the actual surface coverage of PCC by CPAM under salt conditions should be even lower than predicted from the adsorption kinetics and salt-free monolayer adsorption. The decreased adsorption could also arise from the screening of the attraction between the locally oppositely charged segments of the hydrolyzed CPAM (Figure 12), thus preventing multilayer adsorption. The salt within the PCC suspension also causes the flocculation of PCC particles prior to injection of the CPAM. As a result, part of the PCC particle surfaces become inaccessible and thus the effective PCC surface area on which to adsorb the polymer is reduced. Other conjecture on reduced adsorption is that the conformation of adsorbed polymer under a high ionic strength tends to be liquidlike instead of glasslike, which could enhance the mobility of adsorbed CPAM.27 During the centrifugation process, the adsorbed polymer of a liquidlike conformation could easily detach from the particles and remain in the supernatant, thus resulting in an apparent reduced amount of adsorbed polymer. However, no detectable adsorption difference was identified for centrifugation conducted between 5400 and 2000 rpm. The minimum time and speed required for the sufficient separation of PCC aggregates from the supernatant was thus used in this study. The reduced polymer adsorption with
(24) Enarsson, L.-E.; Wagberg, L. Langmuir 2008, 24, 7329–37. (25) Falk, M.; Odberg, L.; Wagberg, L.; Risinger, G. Colloids Surf. 1989, 40, 115–124.
(26) Aoki, K.; Adachi, Y. J. Colloid Interface Sci. 2006, 300, 69–77. (27) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Cohen Stuart, M. A. Langmuir 2002, 18, 5607–5612.
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increasing ionic strength in the dynamic process resembled the adsorption isotherm of CPAM on silica28 and montmorillonite29 in equilibrium. Flocculation with salt also operated within the nonequilibrium polymer adsorption regimes as represented in Figure 5. The period required to reach the PCC flocculation maximum/plateau (PDA) was always shorter than that needed for the polymer adsorption plateau with one exception: the low CPAM dosage (2 mg of CPAM/g of PCC) at 0.01 M NaCl. There was no monotonic relationship between the amount of adsorbed polymer and PCC flocculation behavior when a salt was present. Under a high ionic strength (0.1), all PCC flocculations detected by PDA corresponded to the nondetectable polymer adsorption regimes. Because the detection limit of CPAM is 0.1 μg/mL, the nondetectable polymer adsorption implied that the estimated CPAM adsorption was lower than 11 individual polymer molecules per square meter of PCC particles. Nevertheless, such nondetectable polymer adsorption indeed enhanced the PCC flocculations in comparison to that for salt alone. Similarly, there was no detectable adsorbed CPAM at the low polymer dosage (2 mg of CPAM/g of PCC) under a higher ionic strength. However, the synergy between such a negligible amount of adsorbed CPAM and the high salt concentration promoted the PCC aggregation more efficiently than the same amounts of the respective components applied alone. This implied that the polymer conformation may override the polymer quantity (mg of CPAM/g of PCC) in determining the flocculant efficiency. The mobility of the highmolecular-weight CPAM molecules that were adsorbed could be enhanced by the presence of salt.27 This enhanced mobility, or ability to diffuse through the polymer network, together with the polymer conformation change, could enable some adsorbed CPAM molecules to possess a higher flocculant efficiency, although the total amount of adsorbed CPAM consistently decreased with increasing ionic strength. The Hamaker equation30 predicts that the van der Waals attraction between PCC particles/aggregates increases under high ionic strength (0.1) because of the decreased double-layer thickness (∼1 nm) and the increased particle dimensions caused by the salt prior to injection of the polymer. The increased van der Waals attraction could significantly contribute to the PCC flocculation detected by PDA in addition to the electrostatic interaction under salt conditions where only a few active CPAM molecules with tails were sufficient to serve as a bridge for the progressive aggregation of preflocculated PCC. This could explain the similarity of the PCC flocculation at a high ionic strength under different polymer dosages in the late stage, regardless of the amounts of adsorbed polymer.
happened in the nonequilibrium polymer adsorption regime, regardless of the polymer dose or ionic strength of the PCC suspension. There were discrepancies between the amount of adsorbed CPAM and the surface coverage theory for the flocculation rate based on the simple monolayer adsorption model in both early and late PCC flocculation. This highlights the importance of polymer adsorption kinetics on flocculation. The transient conformation of the adsorbed CPAM significantly affected the flocculation kinetics and even overrode the effect of the amount of adsorbed polymer. The significant effect of the ionic strength of the PCC suspension on CPAM adsorption was monotonic, which contrasts with its effect on PCC flocculation. In the absence of salt, the maximum amount of adsorbed CPAM on PCC was more than 3 times that expected from the simple flat monolayer adsorption of polymer coils. This, may arise from the extended polymer conformation and multilayer formation from the partial local charge reversal of CPAM molecules upon hydrolysis. The polymer adsorption isotherms in the absence of salt did not obey the Langmuir equation. The presence of salt screened the polymer charge and reduced the accessible surface area of PCC owing to the salt-induced flocculation, which significantly reduced the amount of adsorbed polymer. Unexpected synergetic PCC aggregation was discovered by combining CPAM and salt. Full PCC flocculation dynamics, from colloids to large aggregates (100 μm), was characterized by two complementary techniques, PDA and FBRM, and was related to the kinetics of polymer adsorption. A progressive polymer-induced particle aggregation process was revealed where the PCC particles, invisible initially, became visible aggregates that grew into medium and large aggregates. Classical behavior for the effect of polymer dosage on flocculation was observed for PCC suspension in the absence of salt: an initial decrease in the stability of the PCC suspension, followed by its increase and the transition to system stabilization. However, such regularity was not observed when salt was present in PCC suspensions because the highest flocculation rates were shifted to higher CPAM dose ranges. The optimized polymer dose for fast flocculation is a nonlinear function of the ionic strength of the particle suspension. This study has two main industrial implications. First, the adsorbed polymer conformation on colloids governs their flocculation rate. This means that the polymer dissolution history, polymer addition point (contact time), and shear are critical variables. Second, high-molecular-weight polyelectrolytes are still effective flocculants at very high ionic strength. The synergetic conditions observed challenge our current fundamental understanding.
5. Conclusions The effects of cationic polyacrylamide adsorption kinetics and ionic strength on the dynamics of precipitated calcium carbonate flocculation were quantified. The early stages of flocculation
Acknowledgment. An Australian Research Council Linkage grant is gratefully acknowledged. We thank David Hunkeler at SnowFlake Cationics, AQUAþTECH, Kee Eu-Leong at Schaefer Kalk (Malaysia), and Nafty Vanderhoek and Michael Wedding at CSIRO for invaluable support in supplying materials and instruments and for advice. We also thank Dr. E. Perkins, Department of Chemical Engineering, Monash University, for proof reading.
(28) Tong, K. W.; Audebert, R. J. Colloid Interface Sci. 1988, 121, 32–41. (29) Durandpiana, G.; Lafuma, F.; Audebert, R. J. Colloid Interface Sci. 1987, 119, 474–480. (30) Hamaker, H. C. Physica 1937, 4, 1058–1072.
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