Langmuir 2005, 21, 3765-3772
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Flocculation with Poly(ethylene oxide)/Tyrosine-Rich Polypeptide Complexes Chen Lu and Robert Pelton* Department of Chemical Engineering, McMaster University, 307-1710 Main Street West, Hamilton, Ontario L8S 1G6, Canada Received October 7, 2004. In Final Form: February 10, 2005 New insights into the mechanism for the flocculation of aqueous colloids by the sequential addition of a water-borne phenolic polymer, called cofactor, followed by very high molecular poly(ethylene oxide) (PEO) are presented. It is proposed that PEO/cofactor complexes form in the aqueous phase and adsorb onto the surfaces of the target colloidal particles. Flocculation will occur if PEO/cofactor complex on one particle will bind to adsorbed complex on a second particle; i.e., if the complexes are sticky. The proposed mechanism was illustrated by flocculation experiments with precipitated calcium carbonate, very high molecular weight PEO, and a polypeptide cofactor called PEY1 which was a 1:1 random copolymer of L-glycine and L-tyrosine. Independent measurements of the PEO/PEY1 complex properties, in the absence of calcium carbonate, were used to support the mechanism. In order for PEO/PEY1 complexes to be sticky, they must simultaneously have unbound PEY1 and polymer segments. With time the complexes deactivate (i.e., lose their stickiness) by a reconfiguration process which results in elimination of either unbound PEY1 or PEO segments.
Introduction High molecular weight water-soluble polymers are routinely used to flocculate aqueous colloidal suspensions in papermaking, water treatment, and other applications. Cationic copolymers of acrylamide are particularly effective if the ionic strength is low and the solution does not contain high concentrations of anionic polyelectrolytes. However, for more demanding applications it is common industrial practice to sequentially add two or more polymers1 or a polymer followed by an oppositely charged microparticle.2 These multicomponent flocculant systems tend to give more complete flocculation and more robust flocs and are generally less sensitive than singlecomponent polymers to process variations and polymer concentration. This paper deals with the mechanism of an unusual dual component polymer flocculant consisting of a polymeric cofactor followed by very high molecular weight poly(ethylene oxide) (PEO). Cofactors are low molecular weight, water-borne phenolic polymers bearing negative charges and can be based on phenolic resins, tannin, lignin, vinyl phenol copolymers, and tyrosine-rich polypeptides. The cofactor/PEO flocculation system is unusual because the adsorption of the polymers onto colloidal particles is not driven by electrostatic attraction. This is an advantage because, unlike the situation for cationic flocculants, the performance of cofactor/PEO is rather insensitive to the presence of dissolved anionic polyelectrolytes present in some types of papermaking suspensions. In the following paragraphs, the key features of the cofactor/PEO flocculation mechanism are summarized, setting the stage for this work. There have been many publications discussing the flocculation mechanism with PEO/cofactor. The older literature has been summarized in review papers,3-5 and there is general agreement that PEO forms complexes * Corresponding author: telephone, (905) 529 7070 ext. 27045; e-mail,
[email protected]. (1) Petzold, G. Dual-Addition Schemes. In Colloid-Polymer Interactions: From Fundamentals to Practice; Farinato, R. S., Dubin, P. L., Eds.; John Wiley & Sons: New York, 1999. (2) Swerin, A.; O ¨ dberg, L. Nord. Pulp Pap. Res. J. 1993, 4, 389.
with cofactor in solution and that these complexes adsorb onto colloidal particles, inducing flocculation. The early work proposed that hydrogen bonding between phenolic hydroxyl and polyether oxygens was the driving force for complex formation. By using well-defined cofactors based on poly(vinylphenol-co-styrene sulfonate), we have shown that in the complexes, PEO segments lie on top of the aromatic π electrons, a configuration not consistent with conventional hydrogen bonding.6 Thus, while more complicated than initially thought, it is clear that PEO forms complexes with cofactors in water. All commercial and most experimental cofactors are anionic polyelectrolytes with carboxyl, sulfate, or sulfonate charged groups. Since most cofactors have a high content of aromatic groups, the charged groups are required for water solubility. There has been an evolution of colloid systems used to evaluate PEO/cofactor flocculation. The earliest work involved mechanical wood pulp suspensions used to make newsprint.7 For mechanistic studies, these very complex mixtures were soon replaced by well-defined surfactantfree polystyrene latex.8-10 In retrospect this was a poor choice since these fairly bare, hydrophobic particles interact strongly with hydrophobic cofactors and the latex is very easy to flocculate. By contrast, papermaking suspensions, wastewater, and most other important commercial applications involve suspensions of very hydrophilic particles which are difficult to flocculate. (3) Smith-Palmer, Truis; Pelton, Robert Flocculation of Particles. In Encyclopedia of Surface and Colloid Science; Hubbard, A., Ed.; Dekker: New York, 2002; pp 2207-2224. (4) Pelton, Robert Polymer-colloid Interactions in Pulp and Paper Manufacture. In Colloid-Polymer Interactions; Farinato, R. S., Dubin, P. L., Eds.; John Wiley and Son: New York, 1999; pp 51-83. (5) van de Ven, T. G. M. J. Pulp Pap. Sci. 1997, 23, J447. (6) Cong, R.; Pelton, R. H.; Russo, P.; Bain, A. D.; Negulescu, I.; Zhou, Z. Colloid Polym. Sci. 2003, 281, 150. (7) Pelton, R. H.; Allen, L. H.; Nugent, H. M. Pulp Pap. Can. 1980, 81, 54. (8) Lindstrom, T.; Glad-Nordmark, G. J. Colloid Interface Sci. 1984, 97, 62. (9) Couture, L.; van de Ven, T. G. M. Colloids Surfaces 1991, 54, 245. (10) Pelton, R. H.; Xiao, H.; Brook, M.; Hamielec, A. Langmuir 1996, 12, 5756.
10.1021/la047519j CCC: $30.25 © 2005 American Chemical Society Published on Web 03/19/2005
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Our more recent work on PEO/cofactor mechanisms has used precipitated calcium carbonate (PCC) as the target colloid. PCC is a 1 µm diameter fairly uniform colloid which is used as a papermaking filler. To improve PCC as a model, dextran sulfate was used to treat PCC to simulate the presence of anionic polyelectrolytes indigenous to most applications.11 The dextran sulfate (DS) saturates the surface of the PCC particles changing them from slightly positive to negatively charged electrosterically stabilized particles which are much more difficult to flocculate than polystyrene latex.12 PEO molecular weight is a particularly fascinating variable in PEO/cofactor-induced flocculation. Since the 1970s it was known that only the very highest molecular weight PEOs are effective flocculants: 8 MDa PEO is effective, 1 MDa is marginal, and 0.1 MDa PEO, which is still a very long chain, shows essentially no flocculation.13 More recently van de Ven and co-workers have advanced our understanding of the role of molecular weight by showing that very high molecular weight PEO in freshly prepared solutions is present as multichain entangled aggregates which are effective flocculants.14 Sub-megadalton PEO tends not to form entangled structures, and if the PEO is not present as entangled structures due to time, extreme dilution, or the use of lower molecular weight PEO, the flocculation efficiency is reduced. Despite this progress, a convincing explanation for the molecular weight sensitivity has not been presented, why do we need very large entangled PEO aggregates? In this work we present a comprehensive explanation of both the molecular weight sensitivity and the observation that complete flocculation is rarely achieved with the PEO/cofactor flocculants. In recent years our approach to understanding the PEO/ cofactor flocculation mechanism has been to prepare verywell-defined cofactors and to relate the properties of PEO/ cofactor complexes, prepared in the absence of colloidal particles, to colloidal flocculation under similar conditions. Our first attempt at this involved a series poly(vinylphenolco-styrene sulfonate) cofactors in which the vinyl phenol content was varied.15 The main conclusions of this work were that the larger the PEO/cofactor complexes, the greater the initial flocculation rate. Factors such as increasing temperature or ionic strength that gave smaller complexes also gave slower initial flocculation rates. It was possible to generate large complexes by lowering the vinylphenol content in the copolymer. However, when used in flocculation, the flocs were too weak to withstand hydrodynamic forces. In summary, this work did establish that there are links between PEO/cofactor complex properties and the corresponding flocculation. However, there were some difficulties with this series of experiments. Foremost was the fact that even the most effective poly(vinylphenol-co-styrene sulfonate) were relatively poor flocculant cofactors in that the resulting colloidal flocs were weak and tended to redisperse with simple stirring. Another difficulty which we still have not resolved is that complexes formed by the very highest molecular weight PEO (8 MDa) were too big to be characterized by light scattering and electrophoresis. Therefore, as a compromise (11) Cong, R.; Smith-Palmer, T.; Pelton, R. J. Pulp Pap. Sci. 2001, 27, 379. (12) Pelton, R. H.; Allen, L. H.; Nugent, H. M. Sven Papperstidn. 1980, 83, 25. (13) Pelton, R. H.; Allen, L. H.; Nugent, H. M. Pulp Pap. Can. 1980, 81, 54. (14) Kratochvil, D.; Alince, B.; van de Ven, T. G. M. J. Pulp Pap. Sci. 1999, 25, 331. (15) Cong, R.; Pelton, R. H. J. Colloid Interface Sci. 2003, 203, 65.
Lu and Pelton
Figure 1. The schematic illustration of the interaction of PEO with PEY1.17
1 or 2 MDa PEOs were used to give smaller complexes which can be characterized. This paper is the third in a series of papers describing the flocculation properties of a novel flocculant system based on mixtures of PEO and synthetic polypeptides based on tyrosine and glutamic acid. Unlike the vinylphenol based cofactors, the peptides gave strong flocs if the tyrosine content was greater than 30%. Our first note introduced the system and defined the range of peptide compositions.16 The second paper summarized PEO/ peptide complex behavior in the absence of colloidal particles;17 the relevant conclusions are summarized below. This paper describes and explains colloidal flocculation with the PEO/peptide complexes. The peptide-based flocculant was an important discovery from a number of perspectives. First, PEO is widely researched as a protein-repellent surface treatment for implanted materials,18 so the existence of peptides which form strong complexes with PEO is significant. Second, it may be possible to replace environmentally questionable, commercial synthetic cofactors with biodegradableengineered proteins. Finally, peptides offer the advantage that circular dichroism can be used to monitor changes in peptide configuration with complex formation. Using this, we were able to show that in the presence of calcium ions, peptide conformation changed significantly upon complexation with PEO whereas without calcium there was no change in peptide configuration with PEO binding.19 Our work has focused on a specific peptide, PEY1, which is poly(L-glutamate, L-tyrosine) 1:1 with a molecular weight of 36.1 kDa. In the absence of calcium ions, many negatively charged PEY1 molecules bound to every PEO molecule causing the PEO coils to expand, presumably due to intrachain electrostatic repulsion.17 van de Ven reported similar behavior with a different cofactor.20 Without calcium ions these complexes were electrostatically stabilized and did not aggregate. In the presence of 1 mM calcium chloride the situation was very different. Figure 1 summarizes our view of the interaction of PEY1 with PEO in the presence of calcium. An important concept (16) Lu, C.; Pelton, R. H.; Valliant, J.; Bothwell, S.; Stephenson, K. Langmuir 2002, 18, 4538. (17) Lu, C.; Pelton, R. H. Langmuir 2004, 20, 3962. (18) Harris, J. M. Poly(ethylene Glycol) Chemistry: Biotechnical and Biomedical Applications; Plenum Press: New York, 1992. (19) Lu, C. Mechanisms ff Filler Flocculation with Peo/Cofactor DualComponent Flocculants. Ph.D. Thesis, McMaster University, 2003. (20) Carignan, A.; Garnier, G.; van de Ven, T. G. M. J. Pulp Pap. Sci. 1998, 24, 94.
Mechanism of Flocculation
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occurs because there are no free PEO segments to which the PEY1 can bind when two particles come into contact. In other words, the complexes are deactivated (nonsticky) because the PEO is saturated with bound cofactor. This paper summarizes the results of a set of flocculation experiments which had the following goals. First, to relate flocculation efficiencies with the properties of PEO/PEY1 complexes formed in the absence of colloidal particles which we reported previously.17 Second, to test our proposed flocculation mechanism (Figure 2), including the concept of active (sticky) versus inactive PEO/cofactor complexes. Finally, to show that the flocculation behavior of PEO/PEY1 is representative of other PEO/cofactor flocculants, giving our conclusions general applicability. Finally, only a few researchers and technologists around the world have any direct interest in PEO/cofactor flocculation. However, we propose that this work has relevance to other multicomponent flocculant systems where deactivation may be operative. Figure 2. The schematic illustration of PCC flocculation by PEO and PEY1.
portrayed in this figure is active versus deactivated complex. An activated complex is one which is sticky and will adhere to another complex upon contact. By contrast, a deactivated complex is no longer sticky and will not adhere. Adhesion occurs when a free cofactor segment on one complex can bind to a free PEO segment on the second complex. Thus, a number of processes can lead to deactivated complex. If PEO is in excess (the top path in Figure 1), the complex rapidly becomes deactivated because PEY1 completely binds to a single PEO chain. At the other extreme of excess PEY1, the complex is saturated with bound PEY1 and thus is inactive because there are no free PEO segments available to form an intermolecular complex. Only at intermediate PEO/PEY1 ratios can active complexes exist, leading to large aggregated structures. However, even at the optimum PEO/PEY1 ratio, the complex ultimately becomes inactive because bound PEY1 chains reorganize so that they are no longer available for binding. We see this very much like a reconfiguration process when a polymer spreads on a surface. However, unlike a surface, both the PEO and the PEY1 segments can move. The hypothesis behind the work in this paper was that the same factors which control the size of PEO/PEY1 complexes in the absence of colloidal particles also control the rate and extent of flocculation in the presence of colloid. In other words, complex-coated particles will only flocculate with other coated particles if the complex is active (sticky). Thus, On the basis of Figure 1 we can predict, at least qualitatively, that flocculation should behave as shown in Figure 2 when the ratio of PEO and PEY1 is varied. Figure 2 shows three cases: excess PEO (top path), optimum PEO/PEY1 concentration ratio (middle path), and excess PEY1 (bottom path). With excess PEO, complex does form and does adsorb onto the PCC. However, the external surfaces of the coated PCC particles are essentially pure PEO and two such particles will not adhere; i.e., the complex is not sticky. At the optimum PEO/PEO1 ratio, the complexes form and adsorb on the particles. Furthermore, the surfaces are sticky for a sufficient time to give particle/particle aggregation. Note that all the surfaces are shown to be coated with complex; this is in contrast to conventional bridging where adsorbed polymer on one particle adheres to a bare spot on the second particle. When the cofactor PEY1 is in excess, we propose that complex forms and adsorbs. However, no flocculation
Experimental Section Materials. PEO 309 (molecular weight (MW) ) 8 MDa), PEO 301 (MW ) 4 MDa), PEO N-60 (MW ) 2 MDa), and PEO N-12K (MW ) 1 MDa) were obtained from Union Carbide. Random l-polypeptide poly(Glu, Tyr) (1:1) (MW ) 36.1 kDa) (PEY1) was purchased from Sigma in the form of sodium salt. PEO stock solutions were prepared by dissolving polymer (0.5 g/L) in water with gentle end-to-end rotation for 24 h. PEY1 stock solutions were prepared by dissolving peptides (1 g/L) in water under gentle shaking for 10 min. Precipitated calcium carbonate (PCC) (Albacar HO, Specialty Minerals Inc.) consists of the aggregates of scalenohedral needles with a mean particle size of 1.34 µm (Brookhaven Disk Centrifuge) and a specific surface area of 12 m2/g (nitrogen adsorption). Dextran sulfate (DS) (MW ) 10 000 Da) was purchased from Sigma in the form of sodium salt with an average of 2.3 sulfate groups per glucose residue. NaCl and CaCl2 (BDH) were used without further purification. All the experiments were performed with water from a Millipore Milli-Q system fitted with one Super C carbon cartridge, two ion-exchange cartridges, and one Organex Q cartridge. Flocculation Experiments. In a typical flocculation experiment, 200 mL of 0.5 g/L PCC suspension dispersed in 0.001 M NaCl was placed in a 500 mL beaker and stirred at 475 rpm using a three-bladed propeller (55 mm diameter). The pH was adjusted to 7.8 with 0.1 M HCl. Usually, PEY1 was added to the stirred suspension and PEO was added 60 s after the PEY1. Both PEY1 and PEO were introduced in one injection using an Eppendorf micropipet. Flocculation was monitored using a photometric dispersion analyzer (PDA) (Rank Brothers, Cambridge, U.K.). The suspension was circulated through the PDA via a silicone rubber tube at a rate of 45 mL/min. V, the amplified dc output and Rrms, the ratio of root mean square (rms) voltage to V were recorded simultaneously. V was proportional to the transmitted light intensity and converted to the relative turbidity of the suspension by the following relationship
( ) ( )
τr ) ln
Vw Vw /ln Vt Vt)0
(1)
where Vw was the output voltage for water, Vt)0 was the output voltage for the PCC suspension, and Vt was the output voltage at time t during flocculation.21 The relative turbidity was used as a measure of the overall extent of flocculation. A new approach to estimating the initial flocculation rate from Rrms data is now presented. Consider a suspension initially containing N1° uniform primary particles. The evolution of aggregates was described by Overbeek and is given by the (21) Gregory, J.; Nelson, D. W. A New Optical Method for Flocculation Monitoring. In Solid-Liquid Separation; Gregory, J., Eds.; Ellis Horwood: Chichester, 1984; p 172.
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following equation22,23
dNm/dt )
[
Substituting (7) and (8) into (6) gives ∞
1 i)m-1
∑
2
ki,m-iNiNm-i -
i)1
∑k
imNiNm
i)1
]
E
(2)
∞
∞
1
∑N /dt ) - 2 k E(∑ N ) i
11
i
i)1
2
(3)
i)1
Equation 3 can be solved to obtain
1
1
-
∞
∑N
)
N1°
1
Ek11t
2
(4)
i
i)1
∞ Ni is a linear function of time with a slope of Therefore, 1/∑i)1 1/2Ek1. From eqs 2 and 4, the initial flocculation rate constant, kI, is given by
∞
∑N)
d(1/ kI )
1 2
i
i-1
k11E )
(5)
dt
It will now be shown that kI can be estimated from Rrms.21,25 Gregory derived the following expression
x
∞
L
Rrms ) C
∑N
i
i)1
A
(6)
where L is the path length of the incident light, A is the crosssection area of the incident light, and C is the average scattering cross section of the suspended particles, Furthermore, for chainlike aggregates Gregory21 argued that C is given as
C ) nNπr12Q1
(7)
where r1 is the primary particle radius, Q1 is the scattering coefficient of the primary particles, and nN is the average number primary particles per aggregate and is given by ∞
nN ) N1°/
∑N
i
x
A
where Nm is the number concentration of PCC aggregates containing m primary particles, kim is the collision rate constant for aggregates of size i interacting with aggregates of size m, and E is the flocculation efficiency which gives the fraction of collisions leading to aggregation. Since we are interested in the initial flocculation rate, E is assumed to be independent of particle size. Furthermore, for low extents of flocculation kim is assumed equal to k11, the collision rate for primary particles. Appling these simplifications to eq 2, Overbeek showed that the total concen∞ Ni, is governed by the following tration of aggregates, ∑i)1 equation22,24
d
Rrms ) πr1N1°Q1
(8)
i)1
Q1 values for the primary PCC particles used in this work fall in the Mie scattering regime. Furthermore, eq 7 is a significant assumption in this approach. C values for more compact aggregates will be lower.21 Thus eq 7 represents an upper limit. (22) Overbeek, J. Th. G. Kinetics of Flocculation. In Colloid Science, 1, Irreversible Systems; Elsevier: Rotterdam, Netherlands, 1952. (23) Lu, C.; Pelton, R. H. Langmuir 2001, 17, 7770. (24) Saffman, P. G.; Turner, J. S. J. Fluid Mech. 1956, 1, 16. (25) Gregory, J. J. Colloid Interface Sci. 1985, 105, 357.
L
(9)
∞
∑N
i
i)1
Equation 9 suggests that Rrms2 is directly proportional to ∞ Ni. Furthermore, the initial slope (kR) of Rrms2 versus time 1/∑i)1 is directly proportional to 1/2Ek1, that is
kR )
dRrms2 Ek11 ∝ kI ) dt 2
(10)
Therefore, kR values were calculated from the Rrms data and were used as estimates of the initial flocculation rate constants. Note that other authors have arbitrarily used dRrms/dt as an estimate of the initial flocculation rate constant;14 we consider eq 10 as a better analysis. Adsorption Methods. The amount of PEO adsorption onto PCC surfaces with and without DS pretreatment was measured by the tannic acid method. A linear calibration equation of the absorbency of PEO and tannic acid complex as a function of PEO concentration was obtained in the following manner. PEO was dissolved in water to give a concentration of 0.5 g/L. Various amounts of PEO solution were added into a 50 mL volumetric flask containing 5 mL of 1 mol/L NaCl solution. Thirty milliliters of water was then introduced, and the volumetric flask was shaken up and down 20 times. Five milliliters of 2 g/L tannic acid and more water were added to give a total volume of 50 mL. The volumetric flask was shaken another 20 times and allowed to stand for 30 min at room temperature. The absorbency of PEO and tannic acid complex was measured with a HP8452A UV-vis spectrophotometer (Hewlett-Packard) at a wavelength of 600 nm. In the PEO adsorption experiments, PCC suspension was added into 0.001 mol/L NaCl solution to give a concentration of 0.2 g/L. The suspension pH was adjusted to 7.8 using 0.1 M HCl. Various amounts of PEO were added, and the suspensions were allowed to stand for 4 days to reach adsorption equilibrium. The PCC particles were extracted by spinning the suspension for 30 min at 40 000 rpm at 25 °C. Fifteen milliliters of supernatant was transferred to a 50 mL volumetric flask. The PEO concentration in the supernatant was measured by the absorbency of PEO and tannic acid complex, and the adsorbed PEO amount was calculated accordingly. The amount of PEY1 equilibrium adsorption amounts on DS saturated PCC surfaces was measured using UV spectroscopy. A calibration curve of UV absorbency at 276 nm as a function of PEY1 concentration was obtained. In the PEY1 adsorption experiments, PEY1 was mixed with DS saturated PCC suspension. The suspension pH was adjusted to 7.8 using 0.1 M HCl. After being allowed to stand for 1 day, the PCC suspension was centrifuged for 30 min at 40 000 rpm at 25 °C. The PEY1 concentration in the supernatant was calculated from the UV absorbency, and the adsorbed PEY1 concentration was calculated from the material balance. Electrophoretic Mobility. The adsorption of polymer on PCC particles was monitored by the change in electrophoretic mobility of PCC particles determined using a Brookhaven ZetaPALS instrument at 25 °C. According to Ohshima’s derivation, the mobility of a particle in an electrical field is proportional to the volume charge density of the adsorbed polymer layer.26 Thus, the change in mobility can reflect the adsorption of polymer on the particle surface. The reported mobility was the average of 10 cycles, where each cycle contained 20 scans.
Results and Discussion The following sections describe the results of PCC (precipitated calcium carbonate) flocculation experiments conducted as functions of soluble calcium ion concentra(26) Ohshima, H. J. Colloid Interface Sci, 1994, 163, 474.
Mechanism of Flocculation
Figure 3. PCC electrophoretic mobility as a function of DS or PEY1 concentration. [PCC] ) 0.5 g/L, [NaCl] ) 0.001 M, pH ) 7.8 (adjusted with HCl), T ) 25 °C. The polymer concentrations are expressed in terms of the available PCC surface area assuming the specific surface of PCC was 12.0 m2/g.
tion, the ratio of PEO to PEY1 (poly(tyrosine-co-glutamate), 1:1, 36.1 kDa), PEO molecular weight, and PEO concentration. In most cases, results were obtained for both PCC suspended in simple electrolyte solutions and PCC pretreated with dextran sulfate (designated PCC+DS). Dextran sulfate, an anionic polyelectrolyte, served as a model for anionic polymers present in many commercially important suspensions.11 Most of the flocculation experiments employed either 2 MDa PEO or 8 MDa PEO. The lower molecular weight give poorer flocculation; however, it offered the advantage that the 2MDa PEO/PEY1 complexes were characterized in our previous work, whereas 8 MDa PEO complexes were more difficult to characterize. Polymer Adsorption on PCC Surfaces. Electrophoresis was used to monitor the polymer adsorption onto PCC, and the results are summarized in Figure 3. The initial mobility of PCC suspension was slightly positive under the experimental conditions. The surface charge density and thus the electrophoretic mobility of pure calcium carbonate suspensions are determined by the activity of soluble calcium ions. Figure 3 shows the mobility values expressed as a function of added PEY1 or DS concentration. The concentrations of both polymers are expressed in terms of the available PCC surface area. Both anionic polyelectrolytes (DS and PEY1) adsorbed onto the PCC reversing the sign of the mobility. If we assume high affinity binding, the polymer concentration at which the mobility becomes constant is an indication of the amount of DS (0.3 mg/m2) or PEY1 (0.4 mg/m2) required to saturate the PCC surface. The adsorption of 8 MDa PEO on the PCC surface was measured using the tannic acid method.27 There was no detectable PEO adsorption. In addition, 8 MDa PEO alone did not flocculate PCC particles. These results confirm the observation of van de Ven and co-workers that PEO does not adsorb onto aqueous PCC.5 Similarly neither PEO nor PEY1 adsorbed onto DS saturated PCC. Presumably electrostatic replusion between PEY1 and DS prevented adsorption. The top half of Table 1 shows the changes of PCC electrophoretic mobilities upon the consecutive addition of PEY1 and PEO. The suspensions remain colloidally stable under the experimental conditions because of electrostatic repulsion. The PCC mobility changed from +0.60 to -2.15 upon PEY1 adsorption, showing that the net surface charge was dominated by the carboxyl groups (27) Attia, Y. A.; Rubio, J. Br. Polym. J. 1975, 7, 135.
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on the glutamate moieties. Subsequent addition of PEO causes the mobility to decrease to -0.61 × 10-8 m2 V-1 s-1 reflecting the effect of a nonionic polymer on electrophoretic mobility.26 Finally, the second row in Table 1 confirmed the absence of PEO adsorption onto PCC. The bottom half of Table 1 shows changes in PCC+DS mobility upon the sequential addition of PEY1 and 1 MDa PEO. Neither PEO nor PEY1 influenced PCC+DS mobility when added alone. By contrast, the bottom row in Table 1 showed that PEO/PEY1 complex adsorbed, as evidenced by a decrease in the mobility. In summary, PEO did not adsorb on the PCC surface, while both PEY1 and PEO/PEY1 complexes adsorbed. Only PEO/PEY1 complexes adsorbed on the PCC+DS surface, even though neither PEO nor PEY1 adsorbed. Effect of Soluble Calcium Ions on PCC Flocculation. Figure 4 shows the effect of soluble calcium ions on the PCC flocculation induced by 8 MDa PEO and PEY1 at pH 9.8. In these experiments, the dissolved calcium ion concentration of PCC suspension was first adjusted adding either CaCl2 or EDTA. Then, 5 mg/L PEY1 and 2.5 mg/L PEO were added to the PCC suspension sequentially. Before comparison of the results in Figure 4, the interpretation of an individual flocculation curve is reviewed. The flocculation curves in Figure 4 show the time dependence of the turbidity. The first jump in relative turbidity corresponds to the addition of PCC to the stirred flocculation vessel which contained only dilute electrolyte. By definition, the relative turbidity equaled 1 with colloidally stable PCC. Once the readings were stable, PEY1 was introduced. Figure 4 shows a very slight increase in turbidity with PEY1 addition indicating perhaps a small increase in the degree of PCC dispersion with PEY1 adsorption. In our standard procedure, the PEO was added 60 s after the polypeptide cofactor, PEY1. In most cases the turbidity immediately dropped upon addition of PEO to approach a minimum turbidity. If the flocs were strong, the minimum turbidity was the steady value (e.g., curve c in Figure 4) whereas weaker flocs redispersed with time causing the turbidity to rise; curve b in Figure 4 shows a slight indication of floc rupture. This interpretation is based on our previous work in which we showed that the tendency of turbidity to increase with time correlates with floc strength directly measured by a micromechanics technique.28,29 Two parameters were used to describe the flocculation curves: flocculation rate kR, calculated as the initial slope of Rrms2 versus time, and the minimum turbidity when we use as an indication of the extent of flocculation. Figure 4 shows three flocculation experiments. Note that starting time was arbitrary so there is no significance to the displacement of the various curves along the time axis. Curve b is a standard flocculation experiment for PCC (i.e., no DS), whereas the soluble calcium ion concentration was drastically reduced for curve a and enhanced for curve c. Clearly, increasing calcium concentration gave faster flocculation, increased degree of flocculation. A possible, but incorrect, explanation for the effect of soluble calcium ion concentration was that by lowering the soluble calcium the surface charge density of PCC lowered and reversed, thus retarding the adsorption of anionic PEO/PEY1 complex.30 Table 2 shows the influence of dissolved calcium ion concentration on the electro(28) Yeung, A.; Gibbs, A.; Pelton, R. J. Colloid Interface Sci. 1997 196, 113. (29) Gibbs, A.; Pelton, R. J. Pulp Pap. Sci. 1999, 25 (7), 267. (30) Foxall, T.; Peterson, G.; Rendall, H. M.; Smith, A. L. J. Chem. Soc., Faraday Trans. 1979, 75, 1034.
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Table 1. Electrophoretic Mobilities of PCC in Different Polymer Solutionsa samples
mobility (m2 V-1 s-1 × 10-8)
500 mg/L PCC 500 mg/L PCC f 2.5 mg/L 1 MDa PEO 500 mg/L PCC f 5 mg/L PEY1 500 mg/L PCC f 5 mg/L PEY1 f 2.5 mg/L 1 MDa PEO 500 mg/L PCC f 10 mg/L DS 500 mg/L PCC f 10 mg/L DS f 5 mg/L PEY1 500 mg/L PCC f 10 mg/L DS f 2.5 mg/L 1 MDa PEO 500 mg/L PCC f 10 mg/L DS f 5 mg/L PEY1 f 2.5 mg/ L 1 MDa PEO
+0.60 ( 0.01 +0.56 ( 0.02 -2.15 ( 0.05 -0.61 ( 0.01 -2.17 ( 0.02 -2.15 ( 0.04 -2.20 ( 0.04 -0.83 ( 0.03
a [NaCl] ) 0.001 M, T ) 25 °C, pH was adjusted to 7.8 using HCl. NaCl, HCl, and PCC were premixed. Required polymer solutions were added in the order as indicated in the table. Mobility error bar is 1 standard deviation of 10 measurement cycles.
Figure 4. The influence of dissolved calcium ions on PCC flocculation by 8 MDa PEO/PEY1. [PCC] ) 0.5 g/L, [PEO] ) 2.5 mg/L, [PEY1] ) 5 mg/L, T ) 25 °C, pH was adjusted by the addition of NaOH and [Na+] was adjusted by the addition of NaCl. PCC was first premixed with the salts. PEY1 and PEO were added sequentially with an interval of 60 s. (a) [EDTA] ) 0.0004 M, [Na+] ) 0.002 M, pH ) 9.8; (b) [Na+] ) 0.002 M, pH ) 9.8; (c) [Na+] ) 0.002 M, [CaCl2] ) 0.0005 M, pH ) 9.8.
phoretic mobility of PCC at pH 9.7. As expected, lowering calcium with EDTA gave more negative particles while adding calcium chloride made the particles positive. However, in all cases subsequent addition of PEY1 made the mobility values more negative suggesting adsorption. Comparison of rows 2 and 3 in Table 2 shows that PEO subsequently adsorbed onto PEY1 coated PCC even under conditions of reduced soluble calcium ions. Thus, it seems that sensitivity of PCC flocculation to soluble calcium ion concentration is not due to the inhibition of either PEY1 or PEO/PEY1 complex adsorption onto PCC. PCC Flocculation. The following paragraphs summarize the flocculation results in which PEY1 followed by PEO were added to PCC suspensions in water containing a little added electrolyte. In our previous work it was shown that, under these solution conditions, but without dispersed PCC, multiple PEY1 molecules bind to PEO chains forming a range of structures depending upon PEO/PEY1 ratio. The interpretation of our previous results is summarized schematically in Figure 1. Figure 5 shows the effect of the PEO/PEY1 mass ratio on PCC flocculation at pH 7.8. In these experiments, 10 mg/L PEY1 and various amount of 2 MDa PEO were added sequentially. The fastest initial rate, kR, and the lowest minimum turbidity values were obtained with a PEO/PEY1 mass ratio of 0.25. Either a lower or higher ratio gave less flocculation. Our previous characterization of PEO (1MDa)/PEY1 complexes formed in 1 mM CaCl2 showed that the diameter, mass, and density of the complexes were maximum at the mass concentration ratio between 0.1 and 0.3 (see Figure 6 in ref 17). Thus, it appears that conditions which give the largest complexes in the absence of PCC also give the fastest and most extensive PCC flocculation.
Figure 6 shows the effect of PEO concentration on PCC flocculation at a fixed PEO/PEY1 ratio of 0.05. Increasing the PEY1 concentration from 10 to 50 mg/L increased the initial PCC flocculation rate by almost 3 orders of magnitude. Increasing flocculation extent with increasing polymer concentration is a general property of PEO/ cofactor flocculation which has been reported before.8,31 This behavior is in contrast to conventional bridging flocculation with a single polymer where excessive flocculant dosages give reduced flocculation. The extreme PEO molecular weight dependence of PEO/ cofactor flocculation was described in the Introduction. Figure 7 summarizes the PEO MW effect on PCC flocculation with a PEO/PEY1 mass ratio of 0.5. As expected, when PEO MW was increased from 1 MDa to 8 MDa, the initial flocculation rate increased from almost 0 to 1.4 s-1 and the minimum relative turbidity decreased from 1 to 0.24. We have shown that with increasing PEO molecular weight, over the range 100-1000 kDa, the size and mass of complexes formed increases in the absence of PCC (see ref 17). However, the properties of PEO/PEY1 complexes are dependent on time and mixing, making it difficult to produce quantitative correlations between complex size and flocculation efficiency. PCC+DS Flocculation. In the papermaking process, PCC flocculation is normally carried out in the presence of many anionic dissolved and colloidal substances, which can adsorb on the PCC surface and interfere with the flocculation. In this work, dextran sulfate (DS) was premixed with PCC to simulate anionic dissolved and colloidal substances.32 Figure 8 shows the initial flocculation rate constants of PCC+DS as a function of the PEO 2 MDa/PEY1 mass ratios. The intermediate PEO/PEY1 ratio of 0.25 gave the highest kR value of 0.027 s-1. Figure 8 also shows the initial rates of PCC flocculation experiments (from Figure 5). It is clear that the addition of DS reduced the flocculation rate at all PEO/PEY1 ratios. It was shown earlier that adsorbed DS prevents PEY1 and PEO adsorption. We suspect that DS also lowers the rate and extent of PEO/PEY1 complex adsorption, which in turn slows flocculation. Many researchers have shown that premixing PEO and cofactor gives poor colloidal flocculation.33 This is also true for PCC flocculation with PEO + PEY1. Figure 9 compares premixing with sequential addition of PEY1 followed by PEO. Premixing gave little flocculation. Furthermore, when PEO and PEY1 were premixed in the presence of 0.001 CaCl2, an aqueous gel formed and there was no PCC flocculation (data not shown). (31) Gibbs, A.; Xiao, H.; Deng, Y.; Pelton, R. H. Tappi J. 1997, 80, 163. (32) Gibbs, A.; Pelton, R. H.; Cong, R. Colloids Surf., A 1999, 159, 31. (33) Stach, K. R.; Dunn, L. A.; Roberts, N. K. Appita 1990, 43, 125.
Mechanism of Flocculation
Langmuir, Vol. 21, No. 9, 2005 3771 Table 2. The Influence of Calcium Ions on PCC Mobility at 25 °Ca samples
mobility (m2 V-1 s-1 × 10-8)
500 mg/L PCC + 0.002 M Na+ + 0.0004 M EDTA (pH)9.8) 500 mg/L PCC + 0.002 M Na+ + 0.0004 M EDTA f 5 mg/L PEY1 (pH ) 9.8) 500 mg/L PCC + 0.002 M Na+ + 0.0004 M EDTA f 5 mg/L PEY1 f 2.5 mg/L 8 MDa PEO (pH ) 9.8) 125 mg/L PCC + 0.002 M Na+ (pH ) 9.7) 125 mg/L PCC + 0.002 M Na+ + 0.0005 M CaCl2 (pH ) 9.7) 125 mg/L PCC + 0.002 M Na+ + 0.0005 M CaCl2 f 10 mg/L DS (pH ) 9.8)
-1.26 ( 0.05 -3.27 ( 0.04 -1.44 ( 0.08 -0.46 ( 0.04 +0.57 ( 0.03 -2.66 ( 0.05
a Required components were added in the order indicated in the table. The mobility error bar is 1 standard deviation of 10 measurement cycles.
Figure 5. The influence of 2 MDa PEO/PEY1 mass ratios on flocculation. [PCC] ) 0.5 g/L, [NaCl] ) 0.001 M, [PEY1] ) 10 mg/L, pH was adjusted to 7.8 by the addition of HCl, T ) 25 °C. PCC was premixed with salt before the addition of PEY1. PEO was introduced around 60 s after the addition of PEY1 to initiate the flocculation.
Figure 6. The influence of 2 MDa PEO concentration on PCC flocculation at a constant PEO/PEY1 mass ratio of 0.05. pH 7.8, T ) 25 °C, [PCC] ) 0.5 g/L.
The objective of this project was to measure PEO/PEY1 complex properties as functions of mixing ratio, PEO molecular weight, etc., in absence of a target colloidal particle and then to conduct colloidal flocculation experiments under the same conditions in an attempt to relate flocculation to complex properties. The complex properties were reported previously. This paper gives flocculation results. The flocculation results, summarized in Figures 4-9, show classic PEO/cofactor flocculation characteristics in that similar ratio, molecular weight, dosing order, and PEO concentration results can be found in the literature. However, this is the first example of an effective PEO/ cofactor pair where the flocculation results have been augmented with detailed characterization of the PEO/ cofactor complexes. We propose that PEO/cofactor complexes will induce flocculation if (a) the complexes adsorb onto the target colloidal particles and (b) the absorbed complex is active (sticky), meaning it will adhere to complex adsorbed on
Figure 7. PCC flocculation with PEO of different MWs. [PCC] ) 0.5 g/L, [NaCl] ) 0.001 M, [PEY1] ) 10 mg/L, [PEO] ) 5 mg/L, pH was adjusted to 7.8 by the addition of HCl, T ) 25 °C. PCC was premixed with salt before the addition of PEY1. PEO was introduced more than 60 s after the addition of PEY1 to initiate the flocculation.
Figure 8. The initial flocculation rates of PCC and PCC+DS as a function 2 MDa PEO/PEY1 mass ratio. For PCC+DS flocculation, PCC was premixed with DS before the additions of PEY1 and PEO. [PCC] ) 0.5 g/L, [PEY1] ) 10 mg/L, [NaCl] ) 0.001 M, pH was adjusted to 7.8 by the addition of HCl, T ) 25 °C. 2 MDa PEO was introduced more than 60 s after the addition of PEY1 to initiate the flocculation.
a second particle. The proposal that PEO/cofactor complexes are the active flocculating agent is not new; indeed, all previous mechanistic discussions assume this.8-10,34 However, the concept of complex activity (stickiness) is new and we hope to show that it is a dominant factor. Lindstro¨m was the first to propose that PEO/cofactor complexes underwent an irreversible contraction (he called it syneresis) in water to form inactive species with respect to flocculation.8 In Lindstro¨m’s work and also in our own mechanistic discussions,35 it was assumed that shrinkage produced complexes which were too small for mechanical entrapment of colloids.8 We now believe that complex stickiness, and not complex size, is the key factor. The (34) Stack, K. R.; Dunn, L. A.; Roberts, N. K. Colloids Surf., A 1993, 70, 23. (35) Xiao, H.; Pelton, R. H.; Hamielec, A. J. Pulp Pap. Sci. 1996, 22, J475.
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Figure 9. Effect of PEO/PEY1 addition order on PCC+DS flocculation at 25 °C. 0.5 g/L PCC, 0.001 M NaCl, and 10 mg/L DS were premixed in the flocculation testing vessel. pH was adjusted to 7.8 by HCl. [8 MDa PEO] ) 2.5 mg/L, [PEY1] ) 5 mg/L. (a) PEO was added 60 s after the addition of PEY1; (b) PEY1 and PEO were premixed and introduced into flocculation vessel to initiate flocculation.
requirements for flocculation (adsorption and stickiness) are now discussed in more detail. In this work we have shown that PEO/PEY1 complexes adsorb onto PCC and PCC+DS. Previous authors have shown that PEO/cofactor complexes have a strong tendency to adsorb onto latex and clay.8,35,14 Most cofactors such as PEY1 are based on hydrophobic phenolic groups and thus are barely water soluble. The corresponding PEO/ cofactor complexes are even less water soluble, particularly in the presence of calcium ions. The only surface we have seen where PEO/cofactor complex did not absorb was polystyrene latex coated with a thick corona of poly(Nisopropylacrylamide). Thus, although complex adsorption is a requirement for PEO/cofactor flocculation, we propose that adsorption is so pervasive that the rate and strength of adsorption are not critical factors in most cases. Furthermore, in classical bridging flocculation the size of the adsorbed flocculant layer is a key parameter because the adsorbed layer must expand beyond the distances of electrically double layers, the invisible bumpers preventing the contact of charged particles. All measurements of the size of PEO cofactor complexes we have made indicate that they are very large.17,36 For example, we have shown that 1 MDa PEO and PEY1 form the complexes with an average diameter of 70 nm. Complexes formed with 8 MDa PEO, the molecular weight used industrially, were too big to measure by colloidal techniques. Similarly, van de Ven and co-workers have shown that large, multi-chainentangled PEOs are the active flocculating agents.14 In summary, we propose that very large PEO/cofactor complexes adsorb onto most surfaces. Since PEO/cofactor adsorption is pervasive and thus not critical, we propose that complex to complex activity or stickiness is the critical requisite for flocculation. In the following paragraphs we will discuss the molecular mechanism by which active PEO/cofactors deactivate. Following this, we will summarize the evidence supporting the existence of the active to deactivated transition. The critical issue is whether two particles coated with PEO/cofactor complexes will adhere when they are brought together. PEO/PEY1 complexes are hydrogels with water contents typically of 95%. Thus, van der Waals forces are likely to be too weak to give adhesion under turbulent (papermaking) conditions. Instead, for adhesion to occur (a) the electrostatic repulsive forces must be sufficiently (36) Cong, R.; Pelton, R. H.; Russo, P.; Doucet, G. Macromolecules 2003, 36, 204.
Lu and Pelton
low that complex coronas on the particles come into contact and (b) cofactor segments attached to one surface must bind to bare PEO segments on the opposing surface. Thus, under optimum flocculation conditions, both unbound PEY1 segments and PEO segments coexist. With time however, even at the optimum PEO/PEY1 ratio, flocculation will cease indicating that the complex has deactivated. The only exception to this is when the total dose of PEO is high (∼50 mg/L); under such conditions a single, macroscopic floc (gel) forms. We propose that deactivation at the optimum PEO/ cofactor ratio involves the slow rearrangement of bound PEY1 chains so that most of the polypeptide segments are in close contact with PEO. This is an intramolecular process whose rate is determined by the mobility of the bound PEY1 and neighboring PEO segments. Ultrahigh molecular weight (8 MDa) entangled chains of PEO will be relatively immobile compared to isolated chains of low molecular weight PEO. Thus we propose that very high molecular weight PEO is required for PEO/cofactor flocculation because with high molecular weights the deactivation rate is slow enough for flocculation to occur. Although unproven in an absolute sense, deactivation is consistent with the following experimental observations: 1. Preformed PEO/cofactor complexes are not effective flocculants. 2. There is an optimum ratio of PEO to cofactor, whereas the greater the overall PEO concentration, the better the flocculation. As long as the PEO/cofactor ratio is near optimum, flocculation is not decreased by over-dosing. 3. Flocculation is not very sensitive to cofactor molecular weight provided it is greater than about 1 kDa, which is long enough to give simultaneous binding onto two PEO chains. 4. Flocculation is sensitive to the cofactor structures too strong an interaction with PEO gives immediate deactivation and poor flocculation. We have shown this with vinylphenol copolymers15 and with other polypeptides.16 Lindstro¨m showed that tannic acid gives colloidal complexes; however, it is a poor cofactor.8 At the other extreme, weak PEO/cofactor interactions will not survive hydrodynamic forces. 5. The conditions which lead to the largest PEO/cofactor complexes in the absence of an added colloid give the fastest flocculation in the presence of a colloid, emphasizing the importance of complex-to-complex adhesion. 6. Deactivation seems to be the only reasonable explanation for the extreme PEO molecular weight sensitivity in flocculation. In summary, we have proposed that the critical feature dictating the efficiency of PEO/cofactor flocculation is the stickiness of the PEO/cofactor complexes. Complex coated particles must be sticky to aggregate. A wide range of published flocculation results and the new results with PEO/PEY1 are consistent with the concept of initially active (sticky) PEO/cofactor complexes which deactivate with time. If deactivation is too rapid, flocculation is limited. At the molecular level, stickiness is a result of the simultaneous presence of unbound cofactor segments and available PEO segments. Acknowledgment. This work was supported by the Nalco Chemical Company and the Canadian Government NSERC CRD program. Chen Lu also thanks the Ontario Graduate Scholarship Program and Shell Canada for scholarships. LA047519J
