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Flocculation efficiency of blends of short and long chain polyelectrolytes
Colloids and Surfaces A: Physicochem. Eng. Aspects 385 (2011) 166–170
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Flocculation efficiency of blends of short and long chain polyelectrolytes Gemma González, José C. de la Cal, José M. Asua ∗ Institute for Polymer Materials (POLYMAT) and Grupo de Ingeniería Química, Departamento de Química Aplicada, Facultad de Ciencias Químicas, University of the Basque Country, Apdo. 1072, ES-20080 Donostia-San Sebastián, Spain
a r t i c l e
i n f o
Article history: Received 22 March 2011 Received in revised form 6 June 2011 Accepted 8 June 2011 Available online 15 June 2011 Keywords: Polymeric flocculants Cationic polyelectrolytes Blends of flocculants Microemulsion polymerization
a b s t r a c t The efficiency of blends of short and long chain cationic polyelectrolytes in the flocculation of waterborne colloidal silica dispersions was investigated. The polyelectrolytes were synthesized by inverse microemulsion copolymerization of acrylamide and [2-(acryloyloxy)ethyl]-trimethylammonium chloride. It was found that short and long chains provided complementary properties in terms of clarity of the supernatant, sedimentation rate and compactness of the sediment. The optimum balance was achieved with blends containing 20–30 wt% of short chains. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polymeric flocculants are widely used in solid–liquid separation processes where colloidal particles fail to settle by themselves to improve the efficiency of sedimentation, clarification, filtration and centrifugation of small particles [1]. Applications include treatments of wastewater sludge from city sewer systems and industries [2], paper retention aids [3] and mineral processing [4]. An efficient flocculation requires the fast formation of compact sediment leaving a clear supernatant. This involves destabilization of the colloidal particles and the formation of large and compact floccules that may sediment rapidly. Because most of the colloidal particles are negatively charged [5], they can be destabilized by charge neutralization. In addition, floccule formation can be improved by bridging flocculation caused by high molecular weight water soluble polymers. Consequently, high molecular weight water soluble polymers with a certain degree of ionization in the polymer chain are efficient flocculants. The high molecular weight promotes particle bridging flocculation, whereas the ionic group provides charge neutralization [6–9]. Flocculation is also affected by the architecture of the polymer chain. Thus, branched polyelectrolytes may be advantageous [3,10–14]. On the other hand, clarification of the supernatant increases as charge neutralization is enhanced, and hence it depends on the charge content of the flocculant [13]. Short highly chargedpolyelectrolytes
∗ Corresponding author. E-mail address: [email protected] (J.M. Asua). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.06.007
are often particularly efficient for this purpose [14]. Sequential addition of polyelectrolytes of different charge has been reported to be beneficial for flocculation provided that the polyelectrolyte with charge opposite to that of the colloid is added first [15,16]. Cationic polyelectrolytes produced by the copolymerization of acrylamide and cationic acrylates are especially suitable for this purpose because most colloidal particles are negatively charged and acrylamide polymerizes forming very high molecular weight chains [6,17]. Inverse emulsion polymerization and inverse microemulsion polymerization are particularly well suited for the production of these polymers because, due to the radical compartmentalization inherent in these processes, ultra high molecular weights can be attained, which favours an efficient bridging flocculation. It has been shown that acrylamide–cationic acrylates polyelectrolytes can be conveniently produced by inverse microemulsion polymerization in continuous stirred tank reactors (CSTRs) [18,19]. The use of two CSTRs in series led to a product with the best performance as flocculant, which was attributed to the presence of short and long polymer chains. The concept is interesting because the conditions in each reactor can be adjusted in such a way that mixtures of polyelectrolytes of different characteristics can be produced. However, the adequate proportion of the different copolymers is unknown. In this work, the flocculation efficiency of physical blends composed by short chains and long-branched chains polyelectrolytes is studied and the optimum balance between both products established.
G. González et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 385 (2011) 166–170
167
Table 1 Characteristics of long and short polyelectrolytes. Product
Polymerization process
CTA (wt%)
Conversion
Cationicity (wt%)
Particle diameter (nm)
Viscosity (mPa s)
Long Short
Semi-continuous Continuous
– 0.7
0.97 ± 0.02 0.95 ± 0.02
0.80 ± 0.03 0.81 ± 0.03
163 ± 4 149 ± 3.5
123 ± 4 21 ± 1
at 27.6 g min−1 and 0.15 g min−1 (namely, 4.5 × 10−4 g min−1 of initiator), respectively. The mean residence time () for the CSTR, which defined as the ratio between the reactor volume and total flow rate, was 10 min. The chain transfer agent (CTA, 2-propanol) was included in the microemulsion feed (0.7 wt%). The reaction was carried out at 35 ◦ C. Monomer conversion and copolymer composition were calculated by determining the free monomer content of the samples by HPLC (HP 1100). Particle size was determined by dynamic light scattering using a Malvern Zetasizer Nano ZT. For the viscosity measurements, the microemulsion was inverted in water to form a solution of polymer in water (0.5 g L−1 water), and the viscosity of the solution was measured at room temperature in a Brookfield DVII using spindle 00 at 2.5 rpm. The characteristics of both reactions and products are shown in Table 1. Table 1 shows that the cationicity and particle size of the two polyelectrolytes were similar, but the viscosity of the short polymer solution was five times smaller than that of the long chain polymer. The low viscosity was an indication of the low molecular weight that was attributed to the use of a continuous polymerization process [18,19] and to the presence of a chain transfer agent in the formulation. Blends of these polymers were prepared (Table 2). The pKa of the Adamquat is unknown, but that of the tetramethylammonium chloride (pKa = 50), which may serve as a reference, indicates that the polyelectrolyte was completely protonated.
2. Materials and methods 2.1. Materials Technical monomers, acrylamide (Am; 50 wt% aqueous solution) and [2-(acryloyloxy)ethyl]-trimethylammonium chloride (Adamquat; 80 wt% aqueous solution) both supplied by SHF Floeger were used as received. Ethylenediaminetetraacetic acid tetrasodium salt hydrate (EDTA, chelating agent which suppresses the effect of the inhibitor) was supplied by Dow Chemical and used without further purification. Isopar M (an isoparaffinic fluid C12–C15, typically containing less than 1 ppm benzene and less than 1 ppm sulphur) supplied by Proquibasa was used as continuous organic phase. A mixture of Crill 43 (Croda Inc.) and Softanol 90 (supplied by Quimidroga) were used as surfactants and sodium metabisulfite (SMB) was used as initiator. Deionized water was used throughout this work.
2.2. Methods 2.2.1. Microemulsion polymerization reactions The polymerizable inverse microemulsion was prepared by mixing under agitation aqueous and the oil phases previously prepared. The aqueous phase contained the monomers, deionized water and EDTA, and was adjusted to a pH = 4 to avoid hydrolysis of acrylamide. The oil phase contained Isopar M and the surfactant system. The whole microemulsion included 31.8 wt% of Isopar M, 8.6 wt% of Am, 34.3 wt% of Adamquat, 0.8 wt% of an aqueous solution of EDTA (16 g L−1 ), 2.9 wt% of Crill 43 and 4.4 wt% of Softanol 90. The total monomer content was 42.9 wt%. The production of long/branched chains copolyelectrolyte (referred as long) was performed in semi-continuous mode. The experimental set-up included a 0.6 L jacketed reactor equipped with a combination of a six-blade turbine impeller in the lower part of the reactor and a two pitched blades (45◦ ) in the upper part, rotating at 400 rpm. The initial charge of the reactor comprised 100 g of a product previously synthesized in batch mode with the same formulation and fully converted. The temperature was set to 35 ◦ C and then the microemulsion and an aqueous solution of initiator (SMB, 0.33 g L−1 ) were fed for 60 min via separate streams at 6.7 g min−1 and 0.342 g min−1 , respectively. The reaction was carried out under nitrogen atmosphere and once the feeding was stopped, the reactor was kept at 35 ◦ C for 60 min. For the short chain flocculant (referred as short), a chain transfer agent was included in the formulation and a continuous stirred tank reactor (CSTR) was used. Polymerization was carried out in a 0.3 L jacketed reactor equipped with the same type of impeller as for the semicontinuous reactor. The agitation rate was 400 rpm. The operation started with the reactor completely filled with a previously polymerized and fully converted latex in order to reduce the time required to achieve the steady state [20]. The inverse microemulsion and the initiator (SMB, 3 g L−1 ) were fed in separate streams
2.2.2. Flocculation procedure A commercial waterborne silica dispersion (Aerodisp W7215S, Degussa, pH = 5, particle diameter dp = 210 nm, density = 1.09 g/cm3 and a solids content of 14%) was employed to test the efficiency of the flocculants. Removal of colloidal silica is not en easy task as illustrated by the work of Chuang et al. [21] where the colloidal silica content was not reduced by using poly(diallyl dimethylammonium chloride) with molecular weights in the range 200,000–350,000 g/mol. The stability of the silica dispersion was checked by means of a TURBISCAN LAbexpert equipment. In this equipment, a cylindrical glass cell containing the dispersion is scanned along the cell height with a near infrared light and the transmitted and backscattered light are recorded over time. Backscattering is best suited to the analysis of opaque samples whereas transmission is recommended for clear or translucent dispersions. The technique is suitable for the monitoring of sedimentation, coalescence and flocculation processes [22–24]. It was found that the silica dispersions were stable for at least 10 h. In order to study the flocculation behaviour of the different blends a diluted silica dispersion in water (18 mL distilled water + 0.5 mL Aerodisp W7215S) was placed in the cylindrical glass cell of the TURBISCAN LAbexpert and 1 mL of the flocculant blend solution was added for a final solids concentration of 0.38 wt% of silica in water and a dose of flocculant of about 11 ppm. Then, the
Table 2 Physical blends composition of long and short polyelectrolytes. Blends
Long
95/5
90/10
80/20
70/30
30/70
20/80
10/90
5/95
Short
Wt% of long
100
95
90
80
70
30
20
10
5
0
168
G. González et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 385 (2011) 166–170
Fig. 1. Experimental results obtained throug light scatteringh by the use of TURBISCAN LAbexpert for 80/20 blend.
cylindrical glass cell was manually shaken for 60 s and placed in the equipment for the data acquisition. The dispersion was scanned with a near infrared light and the transmitted light recorded for 40 min. An example of the evolution of the transmitted light for the blend 80/20 is shown in Fig. 1. It can be observed that the initial dispersion (colloidal silica, pink line (for interpretation of the references to color in this figure text, the reader is referred to the web version of the article)) presented a transmission value of 50%. Upon addition of the flocculant, the silica dispersion became opaque (presumably because of the formation of the floccules) and no light was transmitted. Later, sedimentation of the floccules occurred leading to the formation of two separate regions: a clear phase at the upper part (transmission about 80%), which in most cases was basically devoid of particles, and a silica rich opaque phase at the bottom (transmission = 0%). Sedimentation rate was defined as the time evolution of the sediment front between the two phases. In order to estimate the solids content of the upper clear phase, a calibration curve was prepared
by measuring the transmission for different concentrations of colloidal silica (Fig. 2). It is worth pointing out that the calibration curve is only accurate if the size of the colloidal particles remains unchanged. For a given solids content, particle agglomeration leads to a lower transmission. The silica content in the sedimentated volume, was calculated from the material balance of the silica, (hF + hS )0 = hF F + hS S
(1)
where hF and hS are the heights of the sediment and supernatant, respectively, and 0, F and S , are the solids content of the initial, sediment and supernatant, respectively. 2.2.3. Charge neutralization Zeta potential measurements were used to determine the neutralization of the silica particles by the polyelectrolyte. A 0.05 wt% dispersion of silica in deionized water was titrated with a polyelectrolyte solution in a Malvern Zetasizer Nano, and the zeta potential
100
Transmission (%)
80
60
40
20
0 0
0.1
0.2
0.3
0.4
0.5
Silica wt% Fig. 2. Effect of the silica content of the dispersion on the transmitted light.
Fig. 3. Effect of the polyelectrolyte concentration on the zeta potential of the silica (0.05 wt% of silica; short polyelectrolyte).
G. González et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 385 (2011) 166–170
169
Fig. 4. View of the dispersions 48 h after the addition of the flocculant.
100
80
80
60
60
40
40
10/90 5/95 Short
20/80
30/70
0
70/30
0
80/20
20
Long 95/5 90/10
20
Sediment height (%)
Particle removal (%)
100
Fig. 5. Particle removal and sediment height measured 40 min after the addition of the flocculant. Legend (䊉) particle removal and () sediment height.
0.01 0.008 0.006 0.004
10/90 5/95 Short
20/80
70/30
80/20
0
30/70
0.002
Long 95/5 90/10
Fig. 4 presents a picture of the different dispersions 48 h after the addition of the flocculant. It can be seen that the clarity of the supernatant increases as a higher fraction of the short flocculant was used. This indicated that short polyelectrolyte chains were efficient in removing the colloidal particles. However, the sediment height increased with the fraction of the short flocculant leading to less compact sediments. In a more quantitative way, Fig. 5 shows that the supernatant was virtually free of colloidal particles 40 min after the addition of a blend containing at least 20–30 wt% of short chains. The fraction of the particles removed from the supernatant by the long flocculant could not be accurately estimated from the transmitted light because the transmitted light was lower than that of the silica dispersion due to formation of agglomerates that remained in the supernatant. Fig. 6 shows the effect of the composition of the flocculant blend on the concentration of silica in the sediment, which is a measure of its compactness. It can be seen that the compactness of the sediment increased as the content of the long chain polyelectrolyte increased. The effect was weaker at high fractions of the long chain polyelectrolyte. The silica concentration in the sediment could not be determined in the case of 100% long chain polyelectrolyte because, as explained above, the silica content of the supernatant could not be measured. The results presented above corresponded to systems that had virtually reached the final state. However, in practice, the rate at
Silica weight fraction in the sediment
3. Results and discussion
0.012
Fig. 6. Effect of the composition of the blend of flocculants on the silica concentration in the sediment measured 40 min after the addition of the flocculant.
which this final state is reached is crucial. Therefore, the kinetics of the formation of the sediment was studied. Fig. 7 shows that the flocculation rate increased with the fraction of the long chain polyelectrolyte. The results in Figs. 4–7 indicate that the short polyelectrolyte was very efficient in removing the colloidal particles from the supernatant, but the rate of sediment formation was very low 40
Sediment height (mm)
measured after equilibration. Fig. 3 shows that the negative charge of the silica particles decreased with the addition of short polyelectrolyte. It is worth mentioning that above about 3 ppm of polyelectrolyte, flocculation of the silica was observed.
35 30 25 20 15 10 5
From botton to top Long, 90/10, 80/20, 70/30,30/70,20/80, 10/90, Short
0 5
10
15
20
25
30
35
40
Time (min) Fig. 7. Evolution of the sediment from along time for the different flocculant blends employed.
170
G. González et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 385 (2011) 166–170
and the sediment was not compact. On the other hand, the long polyelectrolyte improves the rate of sediment formation and the compactness of the sediment, but it was not effective removing the colloidal particles from the supernatant. These results suggest that the short chains were very efficient destabilizing the colloidal particles likely because, as compared with the long polyelectrolyte, their small size resulted in a higher number of chains and a higher diffusion coefficient, which allowed a better charge neutralization of most of the colloidal particles. However, the size of the floccules was small, and hence the sedimentation rate was low. In addition, sedimentation that tends to concentrate the floccules is opposed by diffusion that tends dispersed them in the system. Every system reaches a sedimentation–diffusion equilibrium, which defines the compactness of the sediment. The smaller the size of the floccule, the weaker the sedimentation and the stronger the diffusion, consequently the less compact the sediment. On the other hand, for the long chain polyelectrolyte, the smaller number of polymer chains and their relatively slow diffusion rate make difficult the efficient neutralization of the charges of the silica particles and although large floccules were formed, they sediment rapidly leaving silica particles in the supernatant. The large size of the floccules resulted in a rapid formation of a more compact sediment. A blend of short and long polyelectrolyte chains, containing about 20–30 wt% of short chains, provides the optimal balance of properties. 4. Conclusions The flocculation efficiency of blends of short and long chain cationic polyelectrolytes was investigated. The polyelectrolytes were synthesized by inverse microemulsion copolymerization of acrylamide and [2-(acryloyloxy)ethyl]-trimethylammonium chloride. A waterborne dispersion of silica particles was used as case study. It was found that the short chains were very efficient destabilizing the colloidal particles, which resulted in a clean supernatant. However, the size of the floccules formed was small, and hence the sedimentation rate and the compactness of the sediment were low. High molecular weight cationic polyelectrolytes led to a fast sedimentation rate and to a compact sediment, but plenty of colloidal particles remained in the supernatant. Blends of short and long chains containing about 20–30 wt% of short chains, provided the optimal balance of properties. Acknowledgements The financial support provided by Diputación Foral de Gipuzkoa, Gobierno Vasco (GIC 10/17-IT-373-10) and Ministerio de Educación y Ciencia (MEC CTQ 2006-03412) is gratefully acknowledged. The authors would like to thank Dr. Amaia Agirre for the electrophoretic measurements. References [1] B.M. Moudgil, S. Behl, Flocculation in solid–liquid separation processes, in: K.A. Matis (Ed.), Flotation Science and Engineering, Marcel Dekker, New York, 1995.
[2] P.A. Rey, R.G. Varsanik, Application and function of synthetic polymeric flocculants in waste-water treatment, Advances in Chemistry Series 213 (1986) 113–143. [3] A. Blanco, E. Fuente, M.C. Monte, N. Cortes, C. Negro, Polymeric branched flocculant effect on the flocculation process of pulp suspensions in the papermaking industry, Industrial and Engineering Chemistry Research 48 (10) (2009) 4826–4836. [4] P. Bimal, L. Singh, A. Besra, Ravi Prasad, Coagulation and flocculation study of iron ore fines, Separation Science and Technology 34 (5) (1999) 743– 753. [5] F.N. Kemmer, J. McCallion, The Nalco Water Handbook, McGraw-Hill, New York, 1979. [6] T. Smith-Palmer, R. Pelton, Flocculation of particles, in: P. Somasundaran (Ed.), Encyclopedia of Surface and Colloid Science, 2nd edition, CRC Press, Boca Raton, 2006. [7] J. Gregory, Flocculation by polymers and polyelectrolytes, in: Th.F. Tadros (Ed.), Solid–Liquid Dispersions, Academic Press, London, 1987, pp. 163–181. [8] F. Lafuma, The role of water-soluble polymers at the solid/liquid interface in the mechanisms of flocculation/stabilization of aqueous colloidal suspensions, Polimery 43 (1998) 104–108. [9] M.B. Hocking, K.A. Klimchuk, S. Lowen, Polymeric flocculants and flocculation, Journal of Macromolecular Science. Reviews in Macromolecular Chemistry and Physics C39 (2) (1999) 177–203. [10] P. Williams (Ed.), Handbook of Industrial Water Soluble Polymers, Blackwell Publishing, Oxford, 2007. [11] H. Chen, R. Farinato, P. Hawkins, High performance polymer flocculating agents, World Patent Application PCT/US96/17440 (1997) to Cytec Technology Corporation. [12] R.E. Neff, J.J. Pellon, R.G. Ryles, Water-soluble, branched polymers with high molecular weight as flocculating agents, European Patent Application 89120781.3 (1990) to American Cyanamid Company. [13] H. Ni, D. Hunkeler, Prediction of copolymer composition drift using artificial neural networks: copolymerization of acrylamide with quaternary ammonium cationic monomers, Polymer 38 (1997) 667–675. [14] J. Yu, D. Wang, X. Ge, M. Yan, M. Yang, Flocculation of kaolin particles by two typical polyelectrolytes: a comparative study on the kinetics and floc structures, Colloids and Surfaces A: Physicochemical Engineering Aspects 290 (2006) 288–294. [15] N. Böhn, W.M. Kulicke, Optimization of the use of polyelectrolytes for dewatering industrial sludges of various origins, Colloid and Polymer Science 275 (1997) 73–81. [16] A. Fan, N.J. Turro, P. Somasundaran, A study of dual polymer flocculation, Colloids and Surfaces A: Physicochemical Engineering Aspects 162 (2000) 141–148. [17] S.Y. Huang, D.W. Lipp, R.S. Farinato, Acrylamide Polymers. Kirk-Othmer Encyclopedia of Chemical Technology, 5th edition, John Wiley, New Jersey, 2004, pp. 304–342. [18] G. González, J.M. Ugalde, J.C. de la Cal, J.M. Asua., Synthesis of cationic polyelectrolytes by inverse microemulsion polymerization, Macromolecular Rapid Communications 30 (2009) 2036–2041. [19] G. González, J.C. de la Cal, J.M. Asua., Synthesis of cationic flocculants in continuous reactors, Chemical Engineering Journal 162 (2) (2010) 753– 759. [20] C. Abad, J.C. de la Cal, J.M. Asua, Start-up procedures in the emulsion copolymerization of vinyl esters in a continuous loop reactor, Polymer 36 (1995) 4293–4299. [21] S.H. Chuang, T.C. Chang, C.F. Ouyang, J.M. Leu., Colloidal silica removal in coagulation processes for wastewater reuse in a high-tech industrial park, Water Science and Technology 55 (2006) 187–195. [22] M. Balastre, J.F. Argillier, C. Allain, A. Foissy, Role of polyelectrolyte dispersant in the settling behavior of barium sulphate suspension, Colloids and Surfaces A: Physicochemical Engineering Aspects 211 (2002) 145– 215. [23] R. Chanamai, D.J. McClements, Creaming stability of flocculated monodisperse oil-in-water emulsions, Journal of Colloid Interface Science 225 (2000) 214–218. [24] O. Mengual, G. Meunier, I. Cayre, K. Puech, P. Snabre, TURBISCAN MA 2000 multiple light scattering measurement for concentrated emulsion and suspension instability analysis, Talanta 50 (1999) 445–456.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Flocculation efficiency of blends of short and long chain polyelectrolytes Gemma González, José C. de la Cal, José M. Asua ∗ Institute for Polymer Materials (POLYMAT) and Grupo de Ingeniería Química, Departamento de Química Aplicada, Facultad de Ciencias Químicas, University of the Basque Country, Apdo. 1072, ES-20080 Donostia-San Sebastián, Spain
a r t i c l e
i n f o
Article history: Received 22 March 2011 Received in revised form 6 June 2011 Accepted 8 June 2011 Available online 15 June 2011 Keywords: Polymeric flocculants Cationic polyelectrolytes Blends of flocculants Microemulsion polymerization
a b s t r a c t The efficiency of blends of short and long chain cationic polyelectrolytes in the flocculation of waterborne colloidal silica dispersions was investigated. The polyelectrolytes were synthesized by inverse microemulsion copolymerization of acrylamide and [2-(acryloyloxy)ethyl]-trimethylammonium chloride. It was found that short and long chains provided complementary properties in terms of clarity of the supernatant, sedimentation rate and compactness of the sediment. The optimum balance was achieved with blends containing 20–30 wt% of short chains. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polymeric flocculants are widely used in solid–liquid separation processes where colloidal particles fail to settle by themselves to improve the efficiency of sedimentation, clarification, filtration and centrifugation of small particles [1]. Applications include treatments of wastewater sludge from city sewer systems and industries [2], paper retention aids [3] and mineral processing [4]. An efficient flocculation requires the fast formation of compact sediment leaving a clear supernatant. This involves destabilization of the colloidal particles and the formation of large and compact floccules that may sediment rapidly. Because most of the colloidal particles are negatively charged [5], they can be destabilized by charge neutralization. In addition, floccule formation can be improved by bridging flocculation caused by high molecular weight water soluble polymers. Consequently, high molecular weight water soluble polymers with a certain degree of ionization in the polymer chain are efficient flocculants. The high molecular weight promotes particle bridging flocculation, whereas the ionic group provides charge neutralization [6–9]. Flocculation is also affected by the architecture of the polymer chain. Thus, branched polyelectrolytes may be advantageous [3,10–14]. On the other hand, clarification of the supernatant increases as charge neutralization is enhanced, and hence it depends on the charge content of the flocculant [13]. Short highly chargedpolyelectrolytes
∗ Corresponding author. E-mail address: [email protected] (J.M. Asua). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.06.007
are often particularly efficient for this purpose [14]. Sequential addition of polyelectrolytes of different charge has been reported to be beneficial for flocculation provided that the polyelectrolyte with charge opposite to that of the colloid is added first [15,16]. Cationic polyelectrolytes produced by the copolymerization of acrylamide and cationic acrylates are especially suitable for this purpose because most colloidal particles are negatively charged and acrylamide polymerizes forming very high molecular weight chains [6,17]. Inverse emulsion polymerization and inverse microemulsion polymerization are particularly well suited for the production of these polymers because, due to the radical compartmentalization inherent in these processes, ultra high molecular weights can be attained, which favours an efficient bridging flocculation. It has been shown that acrylamide–cationic acrylates polyelectrolytes can be conveniently produced by inverse microemulsion polymerization in continuous stirred tank reactors (CSTRs) [18,19]. The use of two CSTRs in series led to a product with the best performance as flocculant, which was attributed to the presence of short and long polymer chains. The concept is interesting because the conditions in each reactor can be adjusted in such a way that mixtures of polyelectrolytes of different characteristics can be produced. However, the adequate proportion of the different copolymers is unknown. In this work, the flocculation efficiency of physical blends composed by short chains and long-branched chains polyelectrolytes is studied and the optimum balance between both products established.
G. González et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 385 (2011) 166–170
167
Table 1 Characteristics of long and short polyelectrolytes. Product
Polymerization process
CTA (wt%)
Conversion
Cationicity (wt%)
Particle diameter (nm)
Viscosity (mPa s)
Long Short
Semi-continuous Continuous
– 0.7
0.97 ± 0.02 0.95 ± 0.02
0.80 ± 0.03 0.81 ± 0.03
163 ± 4 149 ± 3.5
123 ± 4 21 ± 1
at 27.6 g min−1 and 0.15 g min−1 (namely, 4.5 × 10−4 g min−1 of initiator), respectively. The mean residence time () for the CSTR, which defined as the ratio between the reactor volume and total flow rate, was 10 min. The chain transfer agent (CTA, 2-propanol) was included in the microemulsion feed (0.7 wt%). The reaction was carried out at 35 ◦ C. Monomer conversion and copolymer composition were calculated by determining the free monomer content of the samples by HPLC (HP 1100). Particle size was determined by dynamic light scattering using a Malvern Zetasizer Nano ZT. For the viscosity measurements, the microemulsion was inverted in water to form a solution of polymer in water (0.5 g L−1 water), and the viscosity of the solution was measured at room temperature in a Brookfield DVII using spindle 00 at 2.5 rpm. The characteristics of both reactions and products are shown in Table 1. Table 1 shows that the cationicity and particle size of the two polyelectrolytes were similar, but the viscosity of the short polymer solution was five times smaller than that of the long chain polymer. The low viscosity was an indication of the low molecular weight that was attributed to the use of a continuous polymerization process [18,19] and to the presence of a chain transfer agent in the formulation. Blends of these polymers were prepared (Table 2). The pKa of the Adamquat is unknown, but that of the tetramethylammonium chloride (pKa = 50), which may serve as a reference, indicates that the polyelectrolyte was completely protonated.
2. Materials and methods 2.1. Materials Technical monomers, acrylamide (Am; 50 wt% aqueous solution) and [2-(acryloyloxy)ethyl]-trimethylammonium chloride (Adamquat; 80 wt% aqueous solution) both supplied by SHF Floeger were used as received. Ethylenediaminetetraacetic acid tetrasodium salt hydrate (EDTA, chelating agent which suppresses the effect of the inhibitor) was supplied by Dow Chemical and used without further purification. Isopar M (an isoparaffinic fluid C12–C15, typically containing less than 1 ppm benzene and less than 1 ppm sulphur) supplied by Proquibasa was used as continuous organic phase. A mixture of Crill 43 (Croda Inc.) and Softanol 90 (supplied by Quimidroga) were used as surfactants and sodium metabisulfite (SMB) was used as initiator. Deionized water was used throughout this work.
2.2. Methods 2.2.1. Microemulsion polymerization reactions The polymerizable inverse microemulsion was prepared by mixing under agitation aqueous and the oil phases previously prepared. The aqueous phase contained the monomers, deionized water and EDTA, and was adjusted to a pH = 4 to avoid hydrolysis of acrylamide. The oil phase contained Isopar M and the surfactant system. The whole microemulsion included 31.8 wt% of Isopar M, 8.6 wt% of Am, 34.3 wt% of Adamquat, 0.8 wt% of an aqueous solution of EDTA (16 g L−1 ), 2.9 wt% of Crill 43 and 4.4 wt% of Softanol 90. The total monomer content was 42.9 wt%. The production of long/branched chains copolyelectrolyte (referred as long) was performed in semi-continuous mode. The experimental set-up included a 0.6 L jacketed reactor equipped with a combination of a six-blade turbine impeller in the lower part of the reactor and a two pitched blades (45◦ ) in the upper part, rotating at 400 rpm. The initial charge of the reactor comprised 100 g of a product previously synthesized in batch mode with the same formulation and fully converted. The temperature was set to 35 ◦ C and then the microemulsion and an aqueous solution of initiator (SMB, 0.33 g L−1 ) were fed for 60 min via separate streams at 6.7 g min−1 and 0.342 g min−1 , respectively. The reaction was carried out under nitrogen atmosphere and once the feeding was stopped, the reactor was kept at 35 ◦ C for 60 min. For the short chain flocculant (referred as short), a chain transfer agent was included in the formulation and a continuous stirred tank reactor (CSTR) was used. Polymerization was carried out in a 0.3 L jacketed reactor equipped with the same type of impeller as for the semicontinuous reactor. The agitation rate was 400 rpm. The operation started with the reactor completely filled with a previously polymerized and fully converted latex in order to reduce the time required to achieve the steady state [20]. The inverse microemulsion and the initiator (SMB, 3 g L−1 ) were fed in separate streams
2.2.2. Flocculation procedure A commercial waterborne silica dispersion (Aerodisp W7215S, Degussa, pH = 5, particle diameter dp = 210 nm, density = 1.09 g/cm3 and a solids content of 14%) was employed to test the efficiency of the flocculants. Removal of colloidal silica is not en easy task as illustrated by the work of Chuang et al. [21] where the colloidal silica content was not reduced by using poly(diallyl dimethylammonium chloride) with molecular weights in the range 200,000–350,000 g/mol. The stability of the silica dispersion was checked by means of a TURBISCAN LAbexpert equipment. In this equipment, a cylindrical glass cell containing the dispersion is scanned along the cell height with a near infrared light and the transmitted and backscattered light are recorded over time. Backscattering is best suited to the analysis of opaque samples whereas transmission is recommended for clear or translucent dispersions. The technique is suitable for the monitoring of sedimentation, coalescence and flocculation processes [22–24]. It was found that the silica dispersions were stable for at least 10 h. In order to study the flocculation behaviour of the different blends a diluted silica dispersion in water (18 mL distilled water + 0.5 mL Aerodisp W7215S) was placed in the cylindrical glass cell of the TURBISCAN LAbexpert and 1 mL of the flocculant blend solution was added for a final solids concentration of 0.38 wt% of silica in water and a dose of flocculant of about 11 ppm. Then, the
Table 2 Physical blends composition of long and short polyelectrolytes. Blends
Long
95/5
90/10
80/20
70/30
30/70
20/80
10/90
5/95
Short
Wt% of long
100
95
90
80
70
30
20
10
5
0
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Fig. 1. Experimental results obtained throug light scatteringh by the use of TURBISCAN LAbexpert for 80/20 blend.
cylindrical glass cell was manually shaken for 60 s and placed in the equipment for the data acquisition. The dispersion was scanned with a near infrared light and the transmitted light recorded for 40 min. An example of the evolution of the transmitted light for the blend 80/20 is shown in Fig. 1. It can be observed that the initial dispersion (colloidal silica, pink line (for interpretation of the references to color in this figure text, the reader is referred to the web version of the article)) presented a transmission value of 50%. Upon addition of the flocculant, the silica dispersion became opaque (presumably because of the formation of the floccules) and no light was transmitted. Later, sedimentation of the floccules occurred leading to the formation of two separate regions: a clear phase at the upper part (transmission about 80%), which in most cases was basically devoid of particles, and a silica rich opaque phase at the bottom (transmission = 0%). Sedimentation rate was defined as the time evolution of the sediment front between the two phases. In order to estimate the solids content of the upper clear phase, a calibration curve was prepared
by measuring the transmission for different concentrations of colloidal silica (Fig. 2). It is worth pointing out that the calibration curve is only accurate if the size of the colloidal particles remains unchanged. For a given solids content, particle agglomeration leads to a lower transmission. The silica content in the sedimentated volume, was calculated from the material balance of the silica, (hF + hS )0 = hF F + hS S
(1)
where hF and hS are the heights of the sediment and supernatant, respectively, and 0, F and S , are the solids content of the initial, sediment and supernatant, respectively. 2.2.3. Charge neutralization Zeta potential measurements were used to determine the neutralization of the silica particles by the polyelectrolyte. A 0.05 wt% dispersion of silica in deionized water was titrated with a polyelectrolyte solution in a Malvern Zetasizer Nano, and the zeta potential
100
Transmission (%)
80
60
40
20
0 0
0.1
0.2
0.3
0.4
0.5
Silica wt% Fig. 2. Effect of the silica content of the dispersion on the transmitted light.
Fig. 3. Effect of the polyelectrolyte concentration on the zeta potential of the silica (0.05 wt% of silica; short polyelectrolyte).
G. González et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 385 (2011) 166–170
169
Fig. 4. View of the dispersions 48 h after the addition of the flocculant.
100
80
80
60
60
40
40
10/90 5/95 Short
20/80
30/70
0
70/30
0
80/20
20
Long 95/5 90/10
20
Sediment height (%)
Particle removal (%)
100
Fig. 5. Particle removal and sediment height measured 40 min after the addition of the flocculant. Legend (䊉) particle removal and () sediment height.
0.01 0.008 0.006 0.004
10/90 5/95 Short
20/80
70/30
80/20
0
30/70
0.002
Long 95/5 90/10
Fig. 4 presents a picture of the different dispersions 48 h after the addition of the flocculant. It can be seen that the clarity of the supernatant increases as a higher fraction of the short flocculant was used. This indicated that short polyelectrolyte chains were efficient in removing the colloidal particles. However, the sediment height increased with the fraction of the short flocculant leading to less compact sediments. In a more quantitative way, Fig. 5 shows that the supernatant was virtually free of colloidal particles 40 min after the addition of a blend containing at least 20–30 wt% of short chains. The fraction of the particles removed from the supernatant by the long flocculant could not be accurately estimated from the transmitted light because the transmitted light was lower than that of the silica dispersion due to formation of agglomerates that remained in the supernatant. Fig. 6 shows the effect of the composition of the flocculant blend on the concentration of silica in the sediment, which is a measure of its compactness. It can be seen that the compactness of the sediment increased as the content of the long chain polyelectrolyte increased. The effect was weaker at high fractions of the long chain polyelectrolyte. The silica concentration in the sediment could not be determined in the case of 100% long chain polyelectrolyte because, as explained above, the silica content of the supernatant could not be measured. The results presented above corresponded to systems that had virtually reached the final state. However, in practice, the rate at
Silica weight fraction in the sediment
3. Results and discussion
0.012
Fig. 6. Effect of the composition of the blend of flocculants on the silica concentration in the sediment measured 40 min after the addition of the flocculant.
which this final state is reached is crucial. Therefore, the kinetics of the formation of the sediment was studied. Fig. 7 shows that the flocculation rate increased with the fraction of the long chain polyelectrolyte. The results in Figs. 4–7 indicate that the short polyelectrolyte was very efficient in removing the colloidal particles from the supernatant, but the rate of sediment formation was very low 40
Sediment height (mm)
measured after equilibration. Fig. 3 shows that the negative charge of the silica particles decreased with the addition of short polyelectrolyte. It is worth mentioning that above about 3 ppm of polyelectrolyte, flocculation of the silica was observed.
35 30 25 20 15 10 5
From botton to top Long, 90/10, 80/20, 70/30,30/70,20/80, 10/90, Short
0 5
10
15
20
25
30
35
40
Time (min) Fig. 7. Evolution of the sediment from along time for the different flocculant blends employed.
170
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and the sediment was not compact. On the other hand, the long polyelectrolyte improves the rate of sediment formation and the compactness of the sediment, but it was not effective removing the colloidal particles from the supernatant. These results suggest that the short chains were very efficient destabilizing the colloidal particles likely because, as compared with the long polyelectrolyte, their small size resulted in a higher number of chains and a higher diffusion coefficient, which allowed a better charge neutralization of most of the colloidal particles. However, the size of the floccules was small, and hence the sedimentation rate was low. In addition, sedimentation that tends to concentrate the floccules is opposed by diffusion that tends dispersed them in the system. Every system reaches a sedimentation–diffusion equilibrium, which defines the compactness of the sediment. The smaller the size of the floccule, the weaker the sedimentation and the stronger the diffusion, consequently the less compact the sediment. On the other hand, for the long chain polyelectrolyte, the smaller number of polymer chains and their relatively slow diffusion rate make difficult the efficient neutralization of the charges of the silica particles and although large floccules were formed, they sediment rapidly leaving silica particles in the supernatant. The large size of the floccules resulted in a rapid formation of a more compact sediment. A blend of short and long polyelectrolyte chains, containing about 20–30 wt% of short chains, provides the optimal balance of properties. 4. Conclusions The flocculation efficiency of blends of short and long chain cationic polyelectrolytes was investigated. The polyelectrolytes were synthesized by inverse microemulsion copolymerization of acrylamide and [2-(acryloyloxy)ethyl]-trimethylammonium chloride. A waterborne dispersion of silica particles was used as case study. It was found that the short chains were very efficient destabilizing the colloidal particles, which resulted in a clean supernatant. However, the size of the floccules formed was small, and hence the sedimentation rate and the compactness of the sediment were low. High molecular weight cationic polyelectrolytes led to a fast sedimentation rate and to a compact sediment, but plenty of colloidal particles remained in the supernatant. Blends of short and long chains containing about 20–30 wt% of short chains, provided the optimal balance of properties. Acknowledgements The financial support provided by Diputación Foral de Gipuzkoa, Gobierno Vasco (GIC 10/17-IT-373-10) and Ministerio de Educación y Ciencia (MEC CTQ 2006-03412) is gratefully acknowledged. The authors would like to thank Dr. Amaia Agirre for the electrophoretic measurements. References [1] B.M. Moudgil, S. Behl, Flocculation in solid–liquid separation processes, in: K.A. Matis (Ed.), Flotation Science and Engineering, Marcel Dekker, New York, 1995.
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