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Flocculation of kaolin suspension with the adsorption of N,N-disubstituted hydrophobically modified polyacrylamide
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Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 388–393
Flocculation of kaolin suspension with the adsorption of N, N-disubstituted hydrophobically modified polyacrylamide Haijing Ren, Ye Li, Shuwu Zhang, Jun Wang, Zhaokun Luan ∗ State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China Received 30 August 2007; received in revised form 5 November 2007; accepted 5 November 2007 Available online 13 November 2007
Abstract The interaction between N,N-disubstituted hydrophobically modified polyacrylamide (HMPAM) and kaolin particle in aqueous suspension was investigated by determining the adsorption isotherms and the flocculation behaviors. HMPAMs with different structures varying in hydrophobic distribution degree, NH (the initial number of hydrophobic monomers per micelle during the preparation) 4.45, 2.55 and 1.79, were used in the experiment. Compared with polyacrylamide (PAM), the HMPAMs have very high-adsorbed amount that correspond to a thick adsorbed layer. These surface loads correlate positively with the effective flocculation of HMPAMs, which are attributed to the affinity and association behavior. Additionally, in the test, the HMPAMs with blocky hydrophobic distribution were better than the ones with short hydrophobic sequences. NaCl was used to detect the effect of electrolyte. As a result of the change of environment property with addition of NaCl, association behavior is promoted; consequently, adsorption and flocculation efficiency are enhanced. © 2007 Elsevier B.V. All rights reserved. Keywords: Flocculation; Adsorption; Hydrophobically modified polymer; Kaolin
1. Introduction The interaction between solid particles and polymers in solid/liquid dispersion has been extensively studied. Polymer adsorption from solution onto solid particles is often accompanied by particle aggregation or flocculation. In the process, macromolecule passes from solution to the uncovered surface and blocks some surface area of particle. Flocculation proceeds by biparticle collision when a surface of one particle covered by polymer interacts with the uncovered surface of another particle [1]. Water-soluble polymers modified with a small amount of hydrophobic groups have been of great interest in recent years [2–6]. Several articles reported that this kind of polymer showed a different adsorption behavior characterized by the absence of a plateau region and a continual increase of the adsorbed amount in the adsorption isotherm [7,8]. This result shows that
∗
Corresponding author. Tel.: +86 10 62849198; fax: +86 10 62849198. E-mail addresses: [email protected] (H. Ren), [email protected] (Z. Luan). 0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.11.007
the hydrophobic chain in the macromolecule play an important role in the adsorption process. Furthermore, hydrophobically modified polymer shows unusual properties in water rheological properties because there are strong associations between hydrophobic units which lead to the formation of transient networks [9]. With this self-forming behavior, hydrophobically modified polymer has strong bridging ability, which is useful in the flocculation process. There are kinds of hydrophobically modified polymer derivations that have been used in the treatment of different kinds of wastewaters [10–13]. In these investigations, the flocculation behavior is characterized as a result of polymer bridging, depletion flocculation or charge patch mechanism. The hydrophobically modified polymer used in this article is a copolymer of acrylamide and N,N-dipentylacrylamide. Similar polymer has been described by Candau [14]. In his reports, the polyacrylamide contains hydrophobic blocks of N,N-dihexylacrylamide. This disubstituted acrylamide leads to an average copolymer composition independent of the degree of reaction conversion, in contrast to what is observed with the mono-substituted acrylamide. As a result, the disubstituted copolymer favors much stronger interactions than
H. Ren et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 388–393
mono-substituted polyacrylamide [15]. With these performances, the disubstituted polymer is thought to be effective in the adsorption on the solid particle surface and in the flocculation efficiency. The study in this paper is aimed at examining the flocculation behavior of N,N-dipentyl substituted hydrophobically modified polyacrylamide in kaolin suspension, shedding light on the correlation between adsorption and flocculation in the suspension, and establishing the fundamental mechanism of the flocculation for hydrophobically modified polymer. 2. Materials and methods 2.1. Materials The N,N-disubstituted hydrophobically modified polyacrylamide was prepared in the laboratory using the method of micellar radical polymerization in water, as was described by Candau [16]. The polyacrylamide was hydrophobically modified with a low amount (0.6 mol%) of N,N-dipentylacrylamide. Sodium dodecyl sulfate (SDS) was the surfactant and potassium persulfate (K2 S2 O8 ) was the initiator. The initial number of hydrophobic monomers per micelle (NH ) during the preparation, which had strong effect on the distribution of the hydrophobic chains in the polymer molecule, was 4.45, 2.55 and 1.79, respectively. The high value of NH leads to a blocky distribution of hydrophobic chain in the macromolecule, while the low value of NH leads to short hydrophobic sequences. The productions are referred as HMPAM4.45, HMPAM2.55 and HMPAM1.79 in this article. The intrinsic viscosity ([η]) of the polymer aqueous solution was determined with an Ubbelohde viscometer at 30 ± 0.02 ◦ C in sodium chloride solution (M = 1 mol/L). The [η] of the polymers are as follows: [η]HMPAM4.45 = 350 mL/g; [η]HMPAM2.55 = 314 mL/g; [η]HMPAM1.79 = 323 mL/g. Polyacylamide (PAM) with molecular weight of 1 × 106 g/mol was supplied by Fuchen Chemical Reagent Plant (Tianjin, China), and polyaluminium chloride (PAC) was supplied by Dagang Reagent Plant (Tianjin, China). In the PAC, the aluminium (Al) hydrolysis ratio (B = [OH]/[Al]) is 2.4 and the Al2 O3 concentration is 30%. The kaolin suspension was prepared by mixing 10 g kaolin in 5 L water, which was stirred at 500 rpm for 24 h, and then was diluted 20 times with water. The characters of the suspension are as followed: turbidity = 128 NTU, pH 7.5.
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2.2.2. Measurement of Zeta potential A sample was taken using a syringe immediately after the 2 min rapid mix period in the flocculation tests. The Zeta potential was measured with Zetasizer 2000 (Malvern Instruments Ltd. Company, English). 2.2.3. Polymer adsorption experiment The adsorption isotherm of polymer onto kaolin particle was determined by the depletion method at 25 ◦ C. Polymers at different concentration were added to a series of glass flasks containing defined amounts of kaolin. The suspension was agitated by Rotating Shaker (HZQ-C, Dongming Medical Treatment Apparatus Company, China) for 24 h. After getting the equilibrium, the suspensions were centrifuged at 5000 rpm for 10 min. The polymer solution concentrations were measured by Total Organic Carbon Analyzer (Phoenix 8000, Tekmar-Dohrmann Co., USA). The adsorption amount was determined from the difference between the initial polymer concentration and the residual polymer concentration in the supernatant. 3. Results 3.1. Flocculation 3.1.1. Comparison of flocculation behavior The flocculation efficiencies of hydrophobically modified PAM, no-modified PAM and inorganic flocculant, PAC, and the Zeta potential of the kaolin suspension as a function of flocculant dosage are presented in Fig. 1. As mentioned previously in this article, flocculation can be realized through several mechanisms. The inorganic flocculant, PAC, enhances the turbidity removal through neutralization of the charge density of the kaolin particles. Prior to PAC addition, the Zeta of particles were negatively charged, but with increasing the dosage of PAC, the Zeta dramat-
2.2. Methods 2.2.1. Flocculation tests The flocculation test was carried out using a standard jar apparatus (Hubei Meiyu Instrument Co., Ltd., China). The suspension was put into six 1-L breakers and the flocculants were added in as 1 g/L aqueous solution. Immediately after the addition of the polymer, the suspension was stirred at a fast speed of 250 rpm for 2 min, followed by 100 rpm for 20 min. After the floc was settled down for 1 h, the turbidity of the supernatant liquid was measured with Digital Turbidity Meter ZD-1 (Tianjin Analytical Instrument Company, China).
Fig. 1. Turbidity removal and Zeta potential of kaolin suspension as a function of HMPAM2.55, PAM and PAC dosages at pH 7.5 without addition of NaCl.
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ically migrated to cationic charge. The turbidity removal reached the maximum that roughly corresponded to charge neutrality. However, the other two macromolecular flocculants cannot act in this way. Neutralization mechanism played a relatively little role because the Zeta potential of both systems did not get reach of the isoelectric point. For these two flocculation systems, bridge and sweep are the predominant mechanism. Their molecules adsorbed on the particle surface, bridged from the surface of one particle to another, and agglomerated the particles in suspension into big flocs. With the addition of the PAM, the Zeta potential of kaolin suspension changed insignificantly, while it increased progressively with the addition of the hydrophobically modified PAM, from −10 to −2 mV. This difference in Zeta potential proves the different adsorption ability of the polymer molecules onto the kaolin particles as the negative charge is screened by the adsorbed molecules. Compared with PAM, the hydrophobically modified PAM has stronger adsorption to kaolin particles, which is enhanced by the affinity of the hydrophobic chains to the hydrophobic kaolin particle surface. This behavior enhances the anchor of polymer chains onto kaolin particles, promotes the interaction between particles. As a result, the flocculation performance is enhanced. 3.1.2. Effect of hydrophobic distribution A comparison of different hydrophobic distributed polymers in the flocculation performance of the kaolin suspension is shown in Fig. 2. With the increase of the dosage, these three samples had similar trend on the kaolin suspension turbidity removal. The residual turbidity decreased first, and then increased. HMPAM4.45 was at a less dosage than the other two samples when the best turbidity removal was reached, and it was the most effective one in turbidity removal in the flocculation. This is attributed to the blocky distributed structure, which enhances the molecular anchor to the particle surface and promotes the association among polymer molecules. HMPAM1.79 has short hydrophobic sequences in the structure and it has the lowest flocculation efficiency. As the dosage increased, resid-
Fig. 2. Turbidity removal of kaolin suspension as a function of the concentration of different hydrophobic distributed polymers, HMPAM4.45, HMPAM2.55 and HMPAM1.79, at pH 7.5 without addition of NaCl.
Fig. 3. Turbidity removal of kaolin suspension as a function of HMPAM2.55 dosage with or without 1 mol/L NaCl at pH 7 and 9.
ual turbidity increased and restabilization took place. This is due to the repulsion between the added polymer molecules and the polymer chains that already adsorbed on the particle surface [17]. 3.1.3. Effect of NaCl To study the effect of addition of electrolyte on the flocculation performance of hydrophobically modified polymer, turbidity removal of kaolin suspension as a function of HMPAM2.55 dosage was investigated in the presence of 1 mol/L NaCl at pH 7 and 9. The results in Fig. 3 showed that, despite the effect of pH, the addition of NaCl had significant effect on the flocculation behavior of hydrophobically modified polymer. The flocculation efficiency decreased at low polymer concentration, but increased at relative high polymer concentration. Furthermore, the required polymer dosage to get the maximum flocculation efficiency increased compared with that in the system without NaCl. There are two main factors that can be suggested as an explanation of these results. First, with the addition of NaCl, the solution gets more hostile [9], which promote the intra-molecular association of polymer molecules when the polymer is at low concentration. This decreases the flocculation performance of the polymer. Oppositely, when the polymer concentration is at relatively high value, the interaction of polymer molecules is promoted and the flocculation efficiency is enhanced. Secondly, the addition of NaCl promotes the anchor of polymer to kaolin surface, flocculation is promoted but more polymer molecules are needed. We will see later in this article that adsorption parameters are taken into account to explain the flocculation behavior. In Fig. 3, the lowest turbidity at neutral pH move to higher dosage compared with the one at alkaline pH. The electrostatic attraction between polymer and solid particle was not the predominant factor in the flocculation process, and the hydrophobically modified polymer turned to get association at
H. Ren et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 388–393
Fig. 4. Adsorption isotherms of HMPAM4.45 and HMPAM2.55 on kaolin particles with or without 1 mol/L NaCl, and adsorption isotherm of PAM on kaolin without NaCl.
low concentration in alkaline pH environment [18]. This selfformed network promotes polymer catching and flocculating of the solid particles. 3.2. Adsorption 3.2.1. Adsorption isotherms Adsorption isotherms of hydrophobically modified PAMs and no-modified PAM onto the kaolin particles are shown in Fig. 4. The isotherms have high polymer type dependency behaviors. No-modified PAM shows a classical adsorption isotherm. A plateau of adsorbed amount of about 40 mg/g was reached with an equilibrium concentration of 300–400 mg/L. Differently, for the hydrophobically modified PAM, the adsorbed amount increased continuously during the increase of the equilibrium concentration and no plateau was obtained. The adsorbed amount got as high as 100 mg/g and which corresponded to a thick adsorbed layer above the particle surface. Similar phenomena were observed previously [8] and were explained that, besides the hydrophobic association [19,20], the hydrophobic chain drastically changed the adsorption behavior of polymers since these chains tried to avoid the aqueous phase by depositing themselves onto the particle surface. This kind of force is stronger than the hydrogen bonding by the amino group in PAM. Consequently, the affinity of the hydrophobically modified polymers to the kaolin particles is much stronger than that of PAM. We can find in Fig. 4 that the adsorbed amount of HMPAM4.45 is higher than HMPAM2.55. This difference of these two polymers can only be due to the macromolecules structure and the hydrophobic distribution. Polymer with a blocky hydrophobic structure leads to a strong adsorbed properties and inter-association than the macromolecules with short hydrophobic sequences. This corresponds to the flocculation result. In addition, the adsorption amount of hydrophobically modified polymer increases with the present of NaCl salt. The addition of NaCl makes the hydrophobic chains prefer to bond to the surface; meanwhile, the hydrophobic interaction of the
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Fig. 5. Langmuir equation plots of adsorption of HMPAM4.45, HMPAM2.55 and PAM onto kaolin particles.
molecules is enhanced. Consequently, flocculation efficiency is promoted. 3.2.2. Langmuir and Freundlich plot The Langmuir adsorption model and Freundlich adsorption model are applied to analyze the adsorption behavior of HMPAM and no-modified PAM onto kaolin particles. The Langmuir equation is as follows Qe =
bQm Ce 1 + bCe
(2)
where Ce is the equilibrium concentration (mg/L), Qe is the amount adsorbed under equilibrium (mg/L), Qm (mg/g) is the theoretical maximum adsorption capacity, and b (L/mg) is a Langmuir constant related to the enthalpy of adsorption. The plots of Qe as a function of Ce for HMPAM4.45, HMPAM2.55 and PAM are shown in Fig. 5. The constants were calculated and given in Table 1. On the other hand, the Freundlich isotherm model is expressed as follows Qe = Kf Cen
(3)
where Kf and n are all empirical constants and they represent the sorption capacity and the sorption intensity, respectively. Plots fitted by the Freundlich equation for the three kinds of polymers are presented in Fig. 6. The constants were given in Table 1. As mentioned previously [21], the Langmuir adsorption model is based on the assumption of monolayer adsorption Table 1 The calculated adsorption constants for the adsorption of HMPAM4.45, HMPAM2.55 and PAM onto kaolin particles at pH 7.5 without addition of NaCl Sample
HMPAM4.45 HMPAM2.55 PAM
Langmuir constants
Freundlich constants
b (L/mg)
Qm (mg/L)
R2
Kf (mg/g)
n
R2
0.00827 0.0151 0.0117
103.253 80.879 50.987
0.9449 0.8352 0.9803
14.026 17.814 9.903
0.273 0.214 0.223
0.9686 0.983 0.8028
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cally modified polymers, molecular structure has strong effect on its property. The polymer with blocky hydrophobic distribution has higher adsorption ability and flocculation performance because this kind of structure tends to get association between molecules. References
Fig. 6. Freundlich equation plots of adsorption of HMPAM4.45, HMPAM2.55 and PAM onto kaolin particles.
and constant adsorption energy, while the Freundlich equation deals with physicochemical adsorption on heterogeneous surface. In Figs. 5 and 6, we can see that the adsorption isotherm for PAM prefers to obey the Langmuir model, while that for the hydrophobically modified PAMs confirm to the Freundlich model. These could be explained that the no-modified PAM form a “classical” layer consisting of loops, tails and trains driven by hydrogen bond, as described in previous studies of PAM adsorption onto smectite clay [22]. After the particle was covered by PAM macromolecules, very little polymer molecule could bond to the surface again. Unlike PAM, the HMPAM molecules could adsorb onto the kaolin particle surface continuously by inter-molecular hydrophobic association. These behaviors lead to a high molecular density around particles. This is the reason that there are not plateau for the adsorption of hydrophobically modified PAMs. With the intra- and inter-molecular associations and the affinity onto the kaolin particles, hydrophobically modified polymer could form a network around kaolin particles. At an appropriate concentration, flocculation happens. Oppositely, when the concentration of polymer is too high, restability is obtained. 4. Conclusion In this article, the flocculation behavior of the N,Ndisubstituted hydrophobically modified polymer was investigated. For the modified polymer, charge neutralization is not a predominating effect in the flocculation process, while bridge and sweep by the hydrophobic association play an important role. The flocculation efficiency of the hydrophobically modified polymer was enhanced in most case with the increased adsorbed amount. This is attributed to the hydrophobic association of the modified polymer, which lead to a multi-layer adsorption on the kaolin particle surface and a bridging network of molecules in aqueous suspension. Addition of electrolyte will promote this kind of association behavior. Therefore, the adsorption and flocculation is enhanced with NaCl salt. For the hydrophobi-
[1] N.K. Vladimir, Polymer adsorption at solid surface, in: T. Jozsef (Ed.), Adsorption: Theory, Modeling, and Analysis, Marcel Dekker Inc., New York, 2002, pp. 743–802. [2] L.M. Landoll, Copolymers of ethylene oxide with long chain epoxides, US Patent 4,304,902 (1981). [3] S. Richard Turner, D.B. Siano, J. Bock, Acrylamide–alkylacrylamide copolymer, US Patent 4,520,182 (1985). [4] K.C. Taylor, H.A. Nasr-El-Din, Water-soluble hydrophobically associating polymers for improved oil recovery: a literature review, J. Petrol. Sci. Eng. 19 (1998) 265–280. [5] F. Candau, J. Selb, Hydrophobically modified polyacrylamides prepared by micellar polymerization, Adv. Colloid Interface 79 (1999) 149– 172. [6] G. Riess, Micellization of block copolymers, Prog. Polym. Sci. 28 (2003) 1107–1170. [7] L. Nilsson, B. Bergenstahl, Adsorption of hydrophobically modified anionic starch at oppositely charged oil/water interfaces, J. Colloid Interface Sci. 308 (2007) 508–513. [8] E. Volpert, J. Selb, F. Candau, Adsorption of hydrophobically associating polyacrylamides on clay, Langmuir 14 (1998) 1870–1879. [9] G.O. Yahaya, A.A. Ahdab, S.A. Ali, B.F. Abu-Sharkh, E.Z. Hamad, Solution behavior of hydrophobically associating water-soluble block copolymers of acrylamide and N-benzylacrylamide, Polymer 42 (2001) 3363–3372. [10] H.Z. Zhao, Z.K. Luan, B.Y. Gao, Q.Y. Yue, Synthesis and flocculation properties of poly(diallyldimethyl ammonium chloride–vinyltrimethoxysilane) and Poly(diallyldimethyl ammonium chloride–acrylamide–vinyl trimethoxysilane), J. Appl. Polym. Sci. 84 (2002) 335–342. [11] A. Ye, Y. Hemar, H. Singh, Flocculation and coalescence of droplets in oil-in-water emulsions formed with highly hydrolysed whey proteins as influenced by starch, Colloids Surf. B Biointerfaces 38 (2004) 1–9. [12] S. Bratskaya, V. Avramenko, S. Schwarz, I. Philippova, Enhanced flocculation of oil-in-water emulsions by hydrophobically modified chitosan derivatives, Colloids Surf. A: Physicochem. Eng. Aspects 275 (2006) 168–176. [13] H.J. Ren, W.N. Chen, Y.F. Zheng, Z.K. Luan, Effect of hydrophobic group on flocculation properties and dewatering efficiency of cationic acrylamide copolymers, React. Funct. Polym. 67 (2007) 601–608. [14] E. Volpert, J. Selb, F. Candau, Associating behaviour of polyacrylamides hydrophobically modified with dihexylacrylamide, Polymer 39 (1998) 1025–1033. [15] E. Volpert, J. Selb, F. Candau, Influence of the hydrophobe structure on composition, microstructure, and rheology in associating polyacrylamides prepared by micellar copolymerization, Macromolecules 29 (1996) 1452–1463. [16] A. Hill, F. Candau, J. Selb, Properties of hydrophobically associating polyacrylamide: influence of the method of synthesis, Macromolecules 26 (1993) 4521–4532. [17] S. Pal, D. Mal, R.P. Singh, Cationic starch: an effective flocculating agent, Carbohydr. Polym. 59 (2005) 417. [18] R.Y. Lochhead, C.J. Rulison, An investigation of the mechanism by which hydrophobically modified hydrophilic polymers act as primary emulsifiers for oil-in-water emulsions. 1. Poly(acrylic acids) and hydroxyethyl celluloses, Colloids Surf. A: Physicochem. Eng. Aspects 88 (1994) 27– 32. [19] J.F. Argillier, A. Audibert, J. Lecourtier, M. Moan, L. Rousseau, Solution and adsorption properties of hydrophobically associating water-soluble
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[21] J.L. Wang, H. Horan, Y. Qian, Bioadsorption of 4-chlorophenol to the activated sludge, Tsinghua Sci. Technol. 5 (2000) 241–245. [22] R.Y. Lockhead, C.M. Boykin, An investigative study of polymer adsorption to smectite clay: polyslectrolytes and sodium montmorillonite, Am. Chem. Soc. Symp. Ser. 804 (2002) 85–98.
Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 388–393
Flocculation of kaolin suspension with the adsorption of N, N-disubstituted hydrophobically modified polyacrylamide Haijing Ren, Ye Li, Shuwu Zhang, Jun Wang, Zhaokun Luan ∗ State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China Received 30 August 2007; received in revised form 5 November 2007; accepted 5 November 2007 Available online 13 November 2007
Abstract The interaction between N,N-disubstituted hydrophobically modified polyacrylamide (HMPAM) and kaolin particle in aqueous suspension was investigated by determining the adsorption isotherms and the flocculation behaviors. HMPAMs with different structures varying in hydrophobic distribution degree, NH (the initial number of hydrophobic monomers per micelle during the preparation) 4.45, 2.55 and 1.79, were used in the experiment. Compared with polyacrylamide (PAM), the HMPAMs have very high-adsorbed amount that correspond to a thick adsorbed layer. These surface loads correlate positively with the effective flocculation of HMPAMs, which are attributed to the affinity and association behavior. Additionally, in the test, the HMPAMs with blocky hydrophobic distribution were better than the ones with short hydrophobic sequences. NaCl was used to detect the effect of electrolyte. As a result of the change of environment property with addition of NaCl, association behavior is promoted; consequently, adsorption and flocculation efficiency are enhanced. © 2007 Elsevier B.V. All rights reserved. Keywords: Flocculation; Adsorption; Hydrophobically modified polymer; Kaolin
1. Introduction The interaction between solid particles and polymers in solid/liquid dispersion has been extensively studied. Polymer adsorption from solution onto solid particles is often accompanied by particle aggregation or flocculation. In the process, macromolecule passes from solution to the uncovered surface and blocks some surface area of particle. Flocculation proceeds by biparticle collision when a surface of one particle covered by polymer interacts with the uncovered surface of another particle [1]. Water-soluble polymers modified with a small amount of hydrophobic groups have been of great interest in recent years [2–6]. Several articles reported that this kind of polymer showed a different adsorption behavior characterized by the absence of a plateau region and a continual increase of the adsorbed amount in the adsorption isotherm [7,8]. This result shows that
∗
Corresponding author. Tel.: +86 10 62849198; fax: +86 10 62849198. E-mail addresses: [email protected] (H. Ren), [email protected] (Z. Luan). 0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.11.007
the hydrophobic chain in the macromolecule play an important role in the adsorption process. Furthermore, hydrophobically modified polymer shows unusual properties in water rheological properties because there are strong associations between hydrophobic units which lead to the formation of transient networks [9]. With this self-forming behavior, hydrophobically modified polymer has strong bridging ability, which is useful in the flocculation process. There are kinds of hydrophobically modified polymer derivations that have been used in the treatment of different kinds of wastewaters [10–13]. In these investigations, the flocculation behavior is characterized as a result of polymer bridging, depletion flocculation or charge patch mechanism. The hydrophobically modified polymer used in this article is a copolymer of acrylamide and N,N-dipentylacrylamide. Similar polymer has been described by Candau [14]. In his reports, the polyacrylamide contains hydrophobic blocks of N,N-dihexylacrylamide. This disubstituted acrylamide leads to an average copolymer composition independent of the degree of reaction conversion, in contrast to what is observed with the mono-substituted acrylamide. As a result, the disubstituted copolymer favors much stronger interactions than
H. Ren et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 388–393
mono-substituted polyacrylamide [15]. With these performances, the disubstituted polymer is thought to be effective in the adsorption on the solid particle surface and in the flocculation efficiency. The study in this paper is aimed at examining the flocculation behavior of N,N-dipentyl substituted hydrophobically modified polyacrylamide in kaolin suspension, shedding light on the correlation between adsorption and flocculation in the suspension, and establishing the fundamental mechanism of the flocculation for hydrophobically modified polymer. 2. Materials and methods 2.1. Materials The N,N-disubstituted hydrophobically modified polyacrylamide was prepared in the laboratory using the method of micellar radical polymerization in water, as was described by Candau [16]. The polyacrylamide was hydrophobically modified with a low amount (0.6 mol%) of N,N-dipentylacrylamide. Sodium dodecyl sulfate (SDS) was the surfactant and potassium persulfate (K2 S2 O8 ) was the initiator. The initial number of hydrophobic monomers per micelle (NH ) during the preparation, which had strong effect on the distribution of the hydrophobic chains in the polymer molecule, was 4.45, 2.55 and 1.79, respectively. The high value of NH leads to a blocky distribution of hydrophobic chain in the macromolecule, while the low value of NH leads to short hydrophobic sequences. The productions are referred as HMPAM4.45, HMPAM2.55 and HMPAM1.79 in this article. The intrinsic viscosity ([η]) of the polymer aqueous solution was determined with an Ubbelohde viscometer at 30 ± 0.02 ◦ C in sodium chloride solution (M = 1 mol/L). The [η] of the polymers are as follows: [η]HMPAM4.45 = 350 mL/g; [η]HMPAM2.55 = 314 mL/g; [η]HMPAM1.79 = 323 mL/g. Polyacylamide (PAM) with molecular weight of 1 × 106 g/mol was supplied by Fuchen Chemical Reagent Plant (Tianjin, China), and polyaluminium chloride (PAC) was supplied by Dagang Reagent Plant (Tianjin, China). In the PAC, the aluminium (Al) hydrolysis ratio (B = [OH]/[Al]) is 2.4 and the Al2 O3 concentration is 30%. The kaolin suspension was prepared by mixing 10 g kaolin in 5 L water, which was stirred at 500 rpm for 24 h, and then was diluted 20 times with water. The characters of the suspension are as followed: turbidity = 128 NTU, pH 7.5.
389
2.2.2. Measurement of Zeta potential A sample was taken using a syringe immediately after the 2 min rapid mix period in the flocculation tests. The Zeta potential was measured with Zetasizer 2000 (Malvern Instruments Ltd. Company, English). 2.2.3. Polymer adsorption experiment The adsorption isotherm of polymer onto kaolin particle was determined by the depletion method at 25 ◦ C. Polymers at different concentration were added to a series of glass flasks containing defined amounts of kaolin. The suspension was agitated by Rotating Shaker (HZQ-C, Dongming Medical Treatment Apparatus Company, China) for 24 h. After getting the equilibrium, the suspensions were centrifuged at 5000 rpm for 10 min. The polymer solution concentrations were measured by Total Organic Carbon Analyzer (Phoenix 8000, Tekmar-Dohrmann Co., USA). The adsorption amount was determined from the difference between the initial polymer concentration and the residual polymer concentration in the supernatant. 3. Results 3.1. Flocculation 3.1.1. Comparison of flocculation behavior The flocculation efficiencies of hydrophobically modified PAM, no-modified PAM and inorganic flocculant, PAC, and the Zeta potential of the kaolin suspension as a function of flocculant dosage are presented in Fig. 1. As mentioned previously in this article, flocculation can be realized through several mechanisms. The inorganic flocculant, PAC, enhances the turbidity removal through neutralization of the charge density of the kaolin particles. Prior to PAC addition, the Zeta of particles were negatively charged, but with increasing the dosage of PAC, the Zeta dramat-
2.2. Methods 2.2.1. Flocculation tests The flocculation test was carried out using a standard jar apparatus (Hubei Meiyu Instrument Co., Ltd., China). The suspension was put into six 1-L breakers and the flocculants were added in as 1 g/L aqueous solution. Immediately after the addition of the polymer, the suspension was stirred at a fast speed of 250 rpm for 2 min, followed by 100 rpm for 20 min. After the floc was settled down for 1 h, the turbidity of the supernatant liquid was measured with Digital Turbidity Meter ZD-1 (Tianjin Analytical Instrument Company, China).
Fig. 1. Turbidity removal and Zeta potential of kaolin suspension as a function of HMPAM2.55, PAM and PAC dosages at pH 7.5 without addition of NaCl.
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ically migrated to cationic charge. The turbidity removal reached the maximum that roughly corresponded to charge neutrality. However, the other two macromolecular flocculants cannot act in this way. Neutralization mechanism played a relatively little role because the Zeta potential of both systems did not get reach of the isoelectric point. For these two flocculation systems, bridge and sweep are the predominant mechanism. Their molecules adsorbed on the particle surface, bridged from the surface of one particle to another, and agglomerated the particles in suspension into big flocs. With the addition of the PAM, the Zeta potential of kaolin suspension changed insignificantly, while it increased progressively with the addition of the hydrophobically modified PAM, from −10 to −2 mV. This difference in Zeta potential proves the different adsorption ability of the polymer molecules onto the kaolin particles as the negative charge is screened by the adsorbed molecules. Compared with PAM, the hydrophobically modified PAM has stronger adsorption to kaolin particles, which is enhanced by the affinity of the hydrophobic chains to the hydrophobic kaolin particle surface. This behavior enhances the anchor of polymer chains onto kaolin particles, promotes the interaction between particles. As a result, the flocculation performance is enhanced. 3.1.2. Effect of hydrophobic distribution A comparison of different hydrophobic distributed polymers in the flocculation performance of the kaolin suspension is shown in Fig. 2. With the increase of the dosage, these three samples had similar trend on the kaolin suspension turbidity removal. The residual turbidity decreased first, and then increased. HMPAM4.45 was at a less dosage than the other two samples when the best turbidity removal was reached, and it was the most effective one in turbidity removal in the flocculation. This is attributed to the blocky distributed structure, which enhances the molecular anchor to the particle surface and promotes the association among polymer molecules. HMPAM1.79 has short hydrophobic sequences in the structure and it has the lowest flocculation efficiency. As the dosage increased, resid-
Fig. 2. Turbidity removal of kaolin suspension as a function of the concentration of different hydrophobic distributed polymers, HMPAM4.45, HMPAM2.55 and HMPAM1.79, at pH 7.5 without addition of NaCl.
Fig. 3. Turbidity removal of kaolin suspension as a function of HMPAM2.55 dosage with or without 1 mol/L NaCl at pH 7 and 9.
ual turbidity increased and restabilization took place. This is due to the repulsion between the added polymer molecules and the polymer chains that already adsorbed on the particle surface [17]. 3.1.3. Effect of NaCl To study the effect of addition of electrolyte on the flocculation performance of hydrophobically modified polymer, turbidity removal of kaolin suspension as a function of HMPAM2.55 dosage was investigated in the presence of 1 mol/L NaCl at pH 7 and 9. The results in Fig. 3 showed that, despite the effect of pH, the addition of NaCl had significant effect on the flocculation behavior of hydrophobically modified polymer. The flocculation efficiency decreased at low polymer concentration, but increased at relative high polymer concentration. Furthermore, the required polymer dosage to get the maximum flocculation efficiency increased compared with that in the system without NaCl. There are two main factors that can be suggested as an explanation of these results. First, with the addition of NaCl, the solution gets more hostile [9], which promote the intra-molecular association of polymer molecules when the polymer is at low concentration. This decreases the flocculation performance of the polymer. Oppositely, when the polymer concentration is at relatively high value, the interaction of polymer molecules is promoted and the flocculation efficiency is enhanced. Secondly, the addition of NaCl promotes the anchor of polymer to kaolin surface, flocculation is promoted but more polymer molecules are needed. We will see later in this article that adsorption parameters are taken into account to explain the flocculation behavior. In Fig. 3, the lowest turbidity at neutral pH move to higher dosage compared with the one at alkaline pH. The electrostatic attraction between polymer and solid particle was not the predominant factor in the flocculation process, and the hydrophobically modified polymer turned to get association at
H. Ren et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 388–393
Fig. 4. Adsorption isotherms of HMPAM4.45 and HMPAM2.55 on kaolin particles with or without 1 mol/L NaCl, and adsorption isotherm of PAM on kaolin without NaCl.
low concentration in alkaline pH environment [18]. This selfformed network promotes polymer catching and flocculating of the solid particles. 3.2. Adsorption 3.2.1. Adsorption isotherms Adsorption isotherms of hydrophobically modified PAMs and no-modified PAM onto the kaolin particles are shown in Fig. 4. The isotherms have high polymer type dependency behaviors. No-modified PAM shows a classical adsorption isotherm. A plateau of adsorbed amount of about 40 mg/g was reached with an equilibrium concentration of 300–400 mg/L. Differently, for the hydrophobically modified PAM, the adsorbed amount increased continuously during the increase of the equilibrium concentration and no plateau was obtained. The adsorbed amount got as high as 100 mg/g and which corresponded to a thick adsorbed layer above the particle surface. Similar phenomena were observed previously [8] and were explained that, besides the hydrophobic association [19,20], the hydrophobic chain drastically changed the adsorption behavior of polymers since these chains tried to avoid the aqueous phase by depositing themselves onto the particle surface. This kind of force is stronger than the hydrogen bonding by the amino group in PAM. Consequently, the affinity of the hydrophobically modified polymers to the kaolin particles is much stronger than that of PAM. We can find in Fig. 4 that the adsorbed amount of HMPAM4.45 is higher than HMPAM2.55. This difference of these two polymers can only be due to the macromolecules structure and the hydrophobic distribution. Polymer with a blocky hydrophobic structure leads to a strong adsorbed properties and inter-association than the macromolecules with short hydrophobic sequences. This corresponds to the flocculation result. In addition, the adsorption amount of hydrophobically modified polymer increases with the present of NaCl salt. The addition of NaCl makes the hydrophobic chains prefer to bond to the surface; meanwhile, the hydrophobic interaction of the
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Fig. 5. Langmuir equation plots of adsorption of HMPAM4.45, HMPAM2.55 and PAM onto kaolin particles.
molecules is enhanced. Consequently, flocculation efficiency is promoted. 3.2.2. Langmuir and Freundlich plot The Langmuir adsorption model and Freundlich adsorption model are applied to analyze the adsorption behavior of HMPAM and no-modified PAM onto kaolin particles. The Langmuir equation is as follows Qe =
bQm Ce 1 + bCe
(2)
where Ce is the equilibrium concentration (mg/L), Qe is the amount adsorbed under equilibrium (mg/L), Qm (mg/g) is the theoretical maximum adsorption capacity, and b (L/mg) is a Langmuir constant related to the enthalpy of adsorption. The plots of Qe as a function of Ce for HMPAM4.45, HMPAM2.55 and PAM are shown in Fig. 5. The constants were calculated and given in Table 1. On the other hand, the Freundlich isotherm model is expressed as follows Qe = Kf Cen
(3)
where Kf and n are all empirical constants and they represent the sorption capacity and the sorption intensity, respectively. Plots fitted by the Freundlich equation for the three kinds of polymers are presented in Fig. 6. The constants were given in Table 1. As mentioned previously [21], the Langmuir adsorption model is based on the assumption of monolayer adsorption Table 1 The calculated adsorption constants for the adsorption of HMPAM4.45, HMPAM2.55 and PAM onto kaolin particles at pH 7.5 without addition of NaCl Sample
HMPAM4.45 HMPAM2.55 PAM
Langmuir constants
Freundlich constants
b (L/mg)
Qm (mg/L)
R2
Kf (mg/g)
n
R2
0.00827 0.0151 0.0117
103.253 80.879 50.987
0.9449 0.8352 0.9803
14.026 17.814 9.903
0.273 0.214 0.223
0.9686 0.983 0.8028
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cally modified polymers, molecular structure has strong effect on its property. The polymer with blocky hydrophobic distribution has higher adsorption ability and flocculation performance because this kind of structure tends to get association between molecules. References
Fig. 6. Freundlich equation plots of adsorption of HMPAM4.45, HMPAM2.55 and PAM onto kaolin particles.
and constant adsorption energy, while the Freundlich equation deals with physicochemical adsorption on heterogeneous surface. In Figs. 5 and 6, we can see that the adsorption isotherm for PAM prefers to obey the Langmuir model, while that for the hydrophobically modified PAMs confirm to the Freundlich model. These could be explained that the no-modified PAM form a “classical” layer consisting of loops, tails and trains driven by hydrogen bond, as described in previous studies of PAM adsorption onto smectite clay [22]. After the particle was covered by PAM macromolecules, very little polymer molecule could bond to the surface again. Unlike PAM, the HMPAM molecules could adsorb onto the kaolin particle surface continuously by inter-molecular hydrophobic association. These behaviors lead to a high molecular density around particles. This is the reason that there are not plateau for the adsorption of hydrophobically modified PAMs. With the intra- and inter-molecular associations and the affinity onto the kaolin particles, hydrophobically modified polymer could form a network around kaolin particles. At an appropriate concentration, flocculation happens. Oppositely, when the concentration of polymer is too high, restability is obtained. 4. Conclusion In this article, the flocculation behavior of the N,Ndisubstituted hydrophobically modified polymer was investigated. For the modified polymer, charge neutralization is not a predominating effect in the flocculation process, while bridge and sweep by the hydrophobic association play an important role. The flocculation efficiency of the hydrophobically modified polymer was enhanced in most case with the increased adsorbed amount. This is attributed to the hydrophobic association of the modified polymer, which lead to a multi-layer adsorption on the kaolin particle surface and a bridging network of molecules in aqueous suspension. Addition of electrolyte will promote this kind of association behavior. Therefore, the adsorption and flocculation is enhanced with NaCl salt. For the hydrophobi-
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