Influence of Anions on the Electrokinetic and Colloidal Properties of

Article pubs.acs.org/jced

Influence of Anions on the Electrokinetic and Colloidal Properties of Palygorskite Clay via High-Pressure Homogenization Jixiang Xu, Wenbo Wang, and Aiqin Wang* Center of Xuyi Attapulgite Applied Technology, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, P.R. China Graduate University of the Chinese Academy of Science, Beijing 100049, P.R. China S Supporting Information *

ABSTRACT: A series of palygorskite samples modified with electrolytes including KCl, KBr, KI, KH2PO4, KHSO4, K2HPO4, K2SO4, and K3PO4 were prepared with the aid of high pressure homogenization. The changes of electrokinetic and rheological properties, as well as suspension stability of obtained palygorskite, were investigated through measurement of the zeta potential, flow curve, and sedimentation volume. The influences of charge, concentration, and type of anions on surface charge density of palygorskite were compared. The correlation among the surface charge, the rheological behavior, and suspension stability of palygorskite was discussed. The homogenization process could favor the migration of anions onto palygorskite. It was found that the structure maker anions (H2PO4−, HSO4−, HPO42−, SO42−, and PO43−) made the surface of palygorskite more negatively charged. The flow curve measurements demonstrated that the association capacity of rods depended on the surface potential of palygorskite. A stable suspension was obtained when palygorskite was dispersed in K2SO4 solutions and homogenized at 30 MPa.

1. INTRODUCTION Palygorskite is a naturally occurring hydrated magnesium silicate mineral. In aqueous suspension, the rods and crystal bundles of palygorskite are present in a random fashion and associate with each other to form house-of-cards structures.1 A lot of water molecules are entrapped into these clusters, resulting in an increase of the viscosity of system. For this reason, palygorskite has received considerable attention in the fields of paint, adhesives, cosmetics, fertilizer, and drilling mud.2 Controlling the colloidal properties of palygorskite suspension is necessary in the quality of the final product. Palygorskite crystals are normally needle or rod-shaped fibers. The fibers carry a negative charge on the basal surfaces, which is assumed to be due to the substitution of Al3+ for Si4+ in the tetrahedral sheet or Mg2+ for Al3+ in the octahedral sheet with a consequent imbalance of negative charge.3 The edges of fiber, where imperfections usually occurred because of bond breakage, carry a little positive charge.4 Hence, there are two kinds of interparticle interactions: Electrical attraction between positively charged edges and negatively charged surfaces, and electrical repulsion between edges and/or surfaces of like charge. In general, the magnitude and nature of the interparticle forces depend on many parameters like the pH of solution, the type of electrolyte ion, surfactant, and polymer in the medium.5−12 Among them, inorganic electrolyte ions play an important role in controlling the macroscopic behavior of colloidal suspensions.13 In particular, a prudent choice of the type and concentration of electrolyte can produce a strongly flocculated suspension in which the strength of attraction © 2013 American Chemical Society

between the particles and the viscosity of suspension are greater. Until now, many researchers have investigated the electrokinetic and rheological properties of clay suspensions in the presence of various mono- and multivalent ions.5−10 It was observed that inorganic ions with opposite charge compared with clay mineral, similar ability to change the structure of water, higher concentration, and smaller hydration radius could migrate preferentially to the particle surface. Specifically, trivalent and partial divalent (such as Cu2+, Mn2+, Ca2+, Ba2+, Ni2+, Co2+, and Pb2+) cations could change the surface charge of particle from negative to positive. Whereas, the monovalent cation was an indifferent ion for the suspension, they only changed the magnitude of surface charge. For anions, Duman and Tunç,5 Alkan et al.,10 and Dikmen et al.14 have discussed the influences of Cl−, NO3−, SO42−, CO32−, and PO43− anions on the electrokinetic properties of Na-bentonite, kaolinite, and sepiolite suspensions, respectively. It was found that mono-, di-, and trivalent anions were not potential determining ions, but the trivalent anions caused higher zeta potential than di- and monovalent anions at higher electrolyte concentration. Penner and Lagaly6 have reported the influences of sulphates and phosphates on the rheological properties of montmorillonite and kaolin suspensions. The experimental results indicated that the montmorillonite dispersions were stiffened at high Received: December 5, 2012 Accepted: January 28, 2013 Published: February 12, 2013 764

dx.doi.org/10.1021/je301280u | J. Chem. Eng. Data 2013, 58, 764−772

Journal of Chemical & Engineering Data

Article

Table 1. The Chemical Composition of Used Palygorskite (Mass Fraction)

a

componenta

Al2O3

Na2O

MgO

CaO

SiO2

K2O

Fe2O3

mass fractions

0.1047

0.0152

0.2041

0.0129

0.6431

0.0013

0.0087

XRF spectrometer (PANalytical Co.).

2.2. Preparation of Electrolyte-Modified Palygorskite. A sample of 50.0 g of palygorskite was dispersed in 500 cm−3 of KCl, KBr, KI, KH2PO4, KHSO4, K2HPO4, K2SO4, and K3PO4 solutions with given concentrations, respectively, and stirred at 800 rpm for 7200 s at ambient temperature. The obtained suspension was filtered through the sieve of 200 mesh screen to remove quartz (with mass fraction of 0.01, based on the weight of palygorskite). Subsequently, the filtered suspension was homogenized at 30 MPa using a high-pressure homogenizer (GJB 8-20, Changzhou Homogenizer Machinery Corporation, Ltd., Jiangsu, China). The homogenized suspensions were centrifuged at 5000 rpm for 600 s, and the solid products were dried at 378 K with an uncertainty of ± 0.5 K for 14400 s. Finally, the dry products were ground and passed through a 200 mesh screen. The particle size distribution, determined using a laser light scattering analyzer (SCF-106, OMEC Technology Corporation, Ltd., Zhuhai, China), of obtained palygorskite was given in Figure 1. The unhomogenized samples were prepared according to a similar procedure but without high-pressure homogenization.

concentrations of orthophosphate but not of diphosphate. In contrast, kaolin slurries were liquefied at all ortho- and oligophosphate concentrations. Somewhat surprisingly, few works have been done to study systematically the influences of anions on the electrokinetic and colloidal properties of palygorskite suspensions. In general, anions can be adsorbed at positive sites by electrostatic interactions or exchanged for structural OH− groups at the edges.15 However, natural clay with permanent negative charge would produce repulsive interaction toward anion. In a previous report, we studied the influences of mono-, di-, and trivalent cations on the surface charge, yield stress, and suspension stability of palygorskite with the aid of high-pressure homogenization technique.16 It was found that the homogenization process could favor the adsorption of cations onto palygorskite surface through the produced cavitation, shear, and turbulence forces. As a part of efforts to further discuss the type of electrolytes that influence the electrokinetic and rheological properties of the palygorskite suspension, in the present study, palygorskite modified with KCl, KBr, KI, KH2PO4, KHSO4, K2HPO4, K2SO4, and K3PO4 were obtained via mechanical agitation followed by high-pressure homogenization at 30 MPa. The influences of the homogenization process, the type, and concentration of electrolytes on the surface charge, rheological behavior, and suspension stability of obtained palygorskite were examined.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Palygorskite was supplied by the Jiuchuan Nanomaterial Tech. Co. Ltd. (Jiangsu, China). The chemical composition (mass fraction) was given in Table 1 and the purity is 0.99 by XRD patterns using an X′Pert PRO diffractometer equipped a Cu Kα radiation source (40 kV, 40 mA). KCl, KBr, KI, KH2PO4, KHSO4, K2HPO4, K2SO4, and K3PO4 were used as received without further purification, and Table 2 lists the source and mass fraction purity. Deionized water was used in all experiments.

Figure 1. Particle size a distribution of palygorskite.

2.3. Measurement of Zeta Potential. Zeta potential was measured on a Malvern Zetasizer Nano system with irradiation from a 633 nm He−Ne laser (ZEN3600) at 298 K with an uncertainty of ± 0.1 K, using a folded capillary cell. The electrophoretic mobility (UE) was measured using a combination of electrophoresis and laser Doppler velocimetry techniques. The zeta potential (ζ) was calculated from the electrophoretic mobility using the Henry equation:17

Table 2. The Source and Purity of Electrolytes Used chemical name KCl KBr KI KH2PO4 KHSO4 K2HPO4 K2SO4 K3PO4

source Shanghai Co. Shanghai Co. Shanghai Co. Shanghai Co. Shanghai Co. Shanghai Co. Shanghai Co. Shanghai Co.

initial mass fraction purity

Guoyao Chemical Reagent

0.99


0.99


0.99


0.99


0.99


0.99


0.99


0.99

ζ=

3ηUE 1 2ε f (κa)

(1)

where η is the solution viscosity, ε is the solution permittivity, κ is the Debye−Huckel parameter, and a is the radius of the charged particle. Because the amount of solid particles governed the generation of surface charge by producing the ionic species at the solid/liquid interface and then affected the zeta potential value of the suspension, the variation of the zeta potential versus the solid concentrations of palygorskite was 765



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isoelectric point.21 Thus, the changes of zeta potential of palygorskite in the presence of various electrolytes were first studied in order to discuss the effects of surface charge of clay on suspension stability. The zeta potentials of homogenized palygorskite modified with various anions (0.05 mol·kg−1 in all cases) are shown in Figure 2. The unmodified palygorskite has a zeta potential of

determined (Supporting Materials Figure S.1). It was found that the zeta potential values were not affected by the palygorskite amount and remained stable in the concentration range of 0.1 g/100 cm−3 and 0.5 g/100 cm−3. Therefore, in the subsequent zeta potential measurements, the clay to liquid ratio was kept constant at 0.5 g/100 cm−3. On the other hand, the zeta potential measurement was also influenced by the particle size. The larger the particle size is, the more scattered light it produces and hence the lower the concentration that could be used. To eliminate the effects of larger particles on the zeta potential value, a sample of 0.50 g of the obtained palygorskite was fully dispersed in 100 cm−3 of distilled water under highspeed stirring at 11000 rpm for 1200 s. The suspension was allowed to stand for 180 s to let larger particles settle. Moreover, the electrophoresis cell was carefully washed with deionized water and always conditioned with the test suspension before each measurement to prevent crosscontamination. Three parallel measurements were conducted, and the averages were reported. The estimated uncertainty for the zeta potential value obtained by this approach was ± 1 mV. 2.4. Measurement of Colloidal Properties. Steady shear measurements were made using an Anton Paar Physica MCR301 rheometer. A cone−plate with water bath was used for all measurements. The shear rate range was from 0.1 to 200 s−1. Before measurements, a sample of 5.0 g of the obtained palygorskite was dispersed in 45 cm−3 of distilled water and intensively stirred at 11000 rpm for 1200 s as in the zeta potential measurement. The structure of palygorskite suspensions is quite sensitive to its shear deformation history. To ensure reproducible results by minimizing any effects arising from structural changes following introduction of suspensions into the rheometer, the rheological measurements were started after the suspensions had been allowed to stand for 180 s. Moreover, the samples were carefully loaded in the cup and then the bob was slowly lowered until the correct measurement gap was reached. A fresh sample was used for each trial and at least three replications were made for each sample used. All tests were performed at 298 K with an uncertainty of ± 0.1 K. The uncertainty in the experimental measurements has been found to be ± 0.01 Pa. The stability of the palygorskite suspensions was evaluated using the conventional sedimentation technique in a graduated cylinder. A sample of 2.0 g of the obtained palygorskite was dispersed in 120 cm−3 of deionized water with a high-speed mixer at 11000 rpm for 600 s and then transferred to 100 cm−3 graduated cylinder, where it was allowed to stand undisturbed for some time. The sedimentation volume was read directly from the graduated cylinder at fixed time intervals. The uncertainty of sedimentation volume obtained by this approach was ± 0.2 cm−3.

Figure 2. Zeta potentials ζ of homogenized (■) and unhomogenized (□) palygorskite in the presence of KCl (b); KBr (c); KI (d); KHSO4 (e); KH2PO4 (f); K2HPO4 (g); K2SO4 (h); and K3PO4 (l) electrolytes (0.05 mol·kg−1 in all cases). a, pure palygorskite.

−23.6 mV. The negative zeta potentials decrease in the order of I− > Br− > Cl− for palygorskite dispersed in KCl, KBr, and KI solutions. The suspension based on KI-modified palygorskite displays a value of −20.6 mV. The results can be explained in the following ways: First, the sign and magnitude of the surface charge of palygorskite were governed largely by the K+ cations owing to the surface negative charge of clay which would produce repulsion toward anions. Second, the K+ ions presented in aqueous solution in the form of solvation ions, and the degree of solvation affected their adsorption onto palygorskite surfaces.7 The viscosity B coefficients, which are related to the change of solution viscosity as a function of electrolyte concentration and are a measure of the ability of each ion to structure water, of Cl−, Br−, and I− anions are −0.005, −0.033, and −0.073, respectively,22 meaning that these anions are structure breaker ions for water.23 The Cl−, Br−, and I− anions could disrupt the structure of water and enhance the hydration of the K+ counterions.6 The sequence for ability to disrupt the structure of water (also the order of decreasing the adsorption of K+ counterions) is Cl− < Br− < I−. The negative viscosity B coefficients of the I− anion is larger than that of the Cl− and Br− anions, which means that the K+ counterions (KI) could be easily hydrated and proved difficult to neutralize the negative charge of clay compared with KBr and KCl electrolytes. The surface charge became more negative when palygorskite was dispersed in KH2PO4 (−26.4 mV), KHSO4 (−25.3 mV), K2HPO4 (−36.1 mV), K2SO4 (−31.2 mV), and K3PO4 (−43.8 mV) solutions in comparison with unmodified sample. According to previous reports,5,10,14 the mono-, di-, and trivalent anions were not potential determining anions. They could not adsorb specifically onto palygorskite, but accumulate as co-ions in the electrical double layer and contribute the surface charge of the clay by increasing the thickness of double layer. Therefore, the increase of surface negative charge of

3. RESULTS AND DISCUSSION 3.1. Zeta Potential. Zeta potential is often used as an index of the magnitude of electrostatic interaction between particles and is thus a measure of the stability of the solution.18 Generally, particles with a zeta potential less than −15 mV or more than 15 mV are expected to be stable from electrostatic considerations.19 However, particles with zeta potentials between −15 and 15 mV can still be stable if they are stabilized sterically18 or the viscosity of the continuous phase is high.20 In addition, the relationship between suspension stability and zeta potential is useless at high ionic strength because the coagulation of suspension is fast even far from the 766



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Figure 3. Schematic illustration of the migration of inorganic ions into the double layer of palygorskite suspension after mechanical stirring or homogenization at 30 MPa.

Figure 4. Zeta potentials ζ of homogenized (■, ●, ▲, ▼) and unhomogenized (□, ○, Δ, ▽) palygorskite in the presence of KH2PO4 (a), K2HPO4 (b), K2SO4 (c), and K3PO4 (d) with various concentrations C.

where Z is the valency of ion and C is the ion concentration. The formula indicated that for the same cation concentration, the valency of ion contributed significantly to the thickness of double layer and consequently caused a change of the zeta potential. For example, the calculated thickness of double layer values of palygorskite suspensions were 1.34, 0.67, and 0.45 nm, respectively, in the presence of 0.05 mol·kg−1 of H2PO4−, HPO42−, and PO43− anions. The zeta potentials of the samples without homogenization were also measured to study the influences of high-pressure homogenization process on the migration of anions into the electrical double layer of suspensions (Figure 2). Unhomogenized palygorskite samples dispersed by KCl, KBr, and KI solutions have larger negative zeta potential than the corresponding homogenized samples. Whereas, the surfaces became more negatively charged for palygorskite dispersed by KH2PO4, KHSO4, K2HPO4, K2SO4, and K3PO4 solutions and

suspensions containing sulfates and phosphates was attributed to the migration of H2PO4−, HSO4−, HPO42−, SO42−, and PO43− anions into the electrical double layer of palygorskite. The special adsorption of H2PO4− onto palygorskite had also been proved by Zhang et al. using XRF.24 It can be also seen from Figure 2 that the zeta potential values of palygorskite modified with KH2PO4, K2HPO4, and K3PO4 electrolytes followed the order of PO43− > HPO42− > H2PO4−, indicating that a trivalent co-ion caused higher negative zeta potential than mono- and divalent co-ions. A similar trend was also found in sulphates-modified palygorskite. These results could be explained by the relationship between ionic strength and thickness of the electrical double layer (K−1):25

1 3 = K ZC1/2

(2) 767



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homogenized at 30 MPa. The viscosity B coefficients of H2PO4−, HSO4−, SO42−, HPO42−, and PO43− are 0.340, 0.127, 0.206, 0.382, and 0.495, respectively, which indicate that these anions are structure maker ions for water.22,23 On the other hand, the isoelectric point (iep) of used palygorskite is 4.21 (Supporting Materials Figure S.2), its surface is a structure maker surface and can promote the structure of water.26 According to the states of Dumont and co-workers,27,28 the structure maker surfaces preferentially adsorb structure maker ions and structure breaker surfaces preferentially adsorb structure breaker ions. So it is reasonable to conjecture that H2PO4−, HSO4−, SO42−, HPO42−, and PO43− anions in suspensions were more easily attracted to the inner part of electrical double layer of suspensions compared with structure breaker ions of Cl−, Br−, and I−. Furthermore, the produced high temperature and high pressure atmosphere, as well as strong cavitation, shear, and turbulence forces during the homogenization process could provide driving forces for these H2PO4−, HSO4−, SO42−, HPO42−, and PO43− anions migrated inside the electrical double layer of suspensions, and were thus more efficient in increasing the thickness of electrical double layer (Figure 3). Because the viscosity B coefficients of monovalent HSO4− and divalent SO42− are less than H2PO4− and HPO42−, respectively, the tendency and quantities of H2PO4− and HPO42− anions migrating into the electrical double layer of suspensions were stronger than HSO4− and SO42− anions, respectively, resulting in more negatively charged palygorskite. The zeta potential values shown in Figure 2 also supported this behavior. The reason for the smaller zeta potential values of homogenized palygorskite dispersed by halide solutions compared with unhomogenized systems was that more K+ ions attached onto mineral surface during the high-pressure homogenization process.16 The variations of the zeta potentials of homogenized and unhomogenized palygorskite as a function of KH2PO4, K2HPO4, K2SO4, and K3PO4 concentrations are given in Figure 4. The surface charge of palygorskite became more negative with the increase in KH 2 PO 4 and K 2 HPO 4 concentrations to 0.1 mol·kg−1, and the zeta potential values of homogenized samples were higher than that of unhomogenized samples. It indicated that more H2PO4− and HPO42− ions, respectively, were migrated onto palygorskite as the KH2PO4 and K2HPO4 concentrations increased in the solutions. Further increasing KH2PO4 and K2HPO4 concentrations to 0.25 mol·kg−1 had no obvious influence on the zeta potentials of palygorskite. The negative zeta potential gradually increased with increasing K2SO4 concentration to 0.1 mol·kg−1 and a value of −32.4 mV was obtained when palygorskite was dispersed in 0.25 mol·kg−1 K2SO4 solution and homogenized at 30 MPa. Whereas, an increase of the negative zeta potential of the unhomogenized samples was first observed as K2SO4 concentration increased from (0.05 to 0.5) mol·kg−1, and then decreased with further increasing K2SO4 concentration to 1.0 mol·kg−1. Similar trends were found for the homogenized and unhomogenized samples in the presence of K3PO4 as shown in Figure 4. According to the equation for the calculation of the ionic strength (I) of a solution (I = 1 /2ΣCiZi2), the higher the concentration and the valency of ions, the higher the ionic strength of the solution was, and then the larger was the decrease of the zeta potential. The viscosity B coefficients of monovalent HSO4− (0.127) and divalent SO42− (0.206) are less than H2PO4− (0.340) and HPO42− (0.382), respectively; the quantities of H2PO4− and HPO42− anions

migrating into the electrical double layer of suspensions were larger than HSO4− and SO42− anions, respectively, resulting in more free HSO4− and SO42− ions presented in the solutions. For the sample containing K3PO4, although the tendency and quantities of PO43− anions migrating into the electrical double layer of suspensions were stronger, the valency of PO43− anions is higher than other anions. Moreover, the K + ions concentration in the K3PO4-modified system was high in comparison with suspensions containing other electrolytes. As a result, the ionic strengths of the solution in the presence of SO42− and PO43− anions were high and could compress the thickness of the electrical double layer more largely.29 It can be concluded from the above results that the surface negative charge of palygorskite was not of unlimited increase with an increase in anions concentration, and the method to increase the surface charge of palygorskite through dispersion of clay in an electrolyte solution depended on the concentration and type of added species. 3.2. Shear Flow Curves. In pure palygorskite suspension, the rods and crystal bundles associate with each other by attractive van der Waals and repulsive electrical double layer forces to form flocculated clusters. The rheological properties of suspension depend on the association strength of the cluster, and can be varied with the surface charge of palygorskite.30 Moreover, it was found that the yield stress and stability of suspension were obviously enhanced after homogenization compared to the unhomogenized palygorskite.16 Here, the influences of anions on the rheological behavior of homogenized palygorskite samples were studied in terms of the relationship between the shear stress and the shear rate. As can be seen from Figure 5, the suspension based on K3PO4-

Figure 5. The shear stress τ vs shear rate γ̇ curves of homogenized palygorskite suspensions in the presence of ○, KCl; Δ, KBr; ▽, KI; ◊, KH2PO4; ☆, K2HPO4; □, KHSO4; ●, K2SO4; and ▲, K3PO4 electrolytes (0.05 mol·kg−1 in all cases). ■, pure palygorskite.

modified sample exhibits Bingham plastics behavior in the studied range of shear rates, but the flow becomes pseudoplastic for suspensions containing KCl, KBr, KI, KH2PO4, KHSO4, K2HPO4, and K2SO4 electrolytes.31 In addition, an improvement in shear stress of suspensions was obtained in the studied range of shear rate when 0.05 mol·kg−1 KCl, KBr, KI, KH2PO4, KHSO4, K2HPO4, and K2SO4 solutions were used to disperse the palygorskite as compared with unmodified palygorskite, but the shear stress decreased for the suspension containing K3PO4 electrolyte. The pseudoplastic flow curves of 768



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Table 3. Yield Stress τo (Pa) of Homogenized Palygorskite Suspensions in the Presence of Various Anions (0.05 mol·kg−1) as well as KH2PO4, K2HPO4, K2SO4, and K3PO4 with Various Concentrations C/mol·kg−1 type of anions

τo

CKH2PO4

τo

CK2HPO4

τo

CK2SO4

τo

CK3PO4

τo

Cl− Br− I− H2PO4− HSO4− HPO42− SO42− PO43−

63.5 66.0 69.0 35.0 52.5 31.0 63.0

0.01 0.025 0.05 0.10 0.25

42.0 44.0 35.0 27.0 24.0

0.01 0.025 0.05 0.10 0.25

61.0 52.0 31.0 25.0 18.0

0.01 0.025 0.05 0.10 0.25

77.0 68.0 63.0 60.0 42.0

0.01 0.025 0.05 0.1

54.0 42.0

Figure 6. The shear stress τ vs shear rate γ̇ curves of homogenized palygorskite suspensions in the presence of KH2PO4 (a), K2HPO4 (b), K2SO4 (c), and K3PO4 (d) with various concentrations C. □, 0 mol·kg−1; ○, 0.01 mol·kg−1; Δ, 0.025 mol·kg−1; ▽, 0.05 mol·kg−1; ◊, 0.1 mol·kg−1; and ☆, 0.25 mol·kg−1.

On the basis of the results of zeta potential shown in Figure 2, the addition of KCl, KBr, and KI to palygorskite made the surface less negatively charged. The electrostatic repulsion of interparticle was less strong, but the association capacity between particles increased due to an increase of van der Waals attraction forces.34 So the rheological properties enhanced in comparison with unmodified palygorskite. Specifically, the zeta potential values of palygorskite modified with KCl, KBr, and KI followed the order of I− > Br− > Cl−. It meant that strong electrostatic repulsive forces between rods existed in the KImodified system, which could provide a better dispersion of rods and favor the formation of cluster with larger size. It was proven that while keeping a constant solid volume fraction and shear rate imposed, the viscosity of suspension increased with the increase of the clusters size.35 Therefore, a higher yield stress of 69.0 MPa was obtained for the I−-modified palygorskite sample. Interestingly, although the surface charge

suspensions can be described by the Herschel−Bulkley model to obtain the yield stresses:32 τ = τ0 + mγ ṅ

(4)

where τ is the shear stress, τo is the yield stress, γ̇ is the shear rate, m the consistency coefficient, and n the flow behavior index. The fitting was carried out according to the method described by Coussot and Piau.33 The theoretical rheological curve obtained by this model could fit very well with the experimental data (correlation coefficients > 0.98). Table 3 lists the yield stresses of suspensions in the presence of various electrolytes (the other fitting parameters of m, n, and R2 were given in Supporting Materials Table S.1). The estimated uncertainty of yield stress is ± 0.3 Pa. It can be seen that the yield stress of the suspension was strongly dependent on the type of the added electrolyte. 769



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became more negative for palygorskite samples dispersed by KHSO4, KH2PO4, K2HPO4, and K2SO4 solutions an increase in yield stresses was observed for these corresponding suspensions. The rheological properties of the palygorskite suspension are related to the morphologies such as the dispersion and aspect ratios of the rods.36 The colloidal nature of palygorskite suspension is not apparent until the crystal bundles are separated into individual rods.37 For suspensions containing HSO4−, H2PO4−, HPO42−, and SO42− anions, the surface negative charge of rods and the electrostatic repulsion between rods increased in comparison with unmodified palygorskite, the continuity of network structure, to some extent, remain integrated at low electrolyte concentration, but the size of network structure of suspensions increased. Hence, a relatively high yield stress was generated. The zeta potential increased largely to −43.8 mV for palygorskite dispersed by 0.05 mol·kg−1 K3PO4 solution, but the shear stress of corresponding suspension decreased in the studied range of shear rate compared with unmodified palygorskite. It indicated that strongly increased negative charge density could cause the interparticle distance to increase and the strength as well as continuity of association between rods to decrease. More entrapped water was released into suspension, and the space available for the particles to move increased with a rising volume fraction of bulk water. As a result, the rheological properties of the suspension decreased. Similar findings have been reported in a number of other studies of concentrated aqueous suspensions.38,39 The shear stress of suspensions based on the phosphates-modified palygorskite decreased in the order of PO43− > HPO42− > H2PO4−, confirming that the trivalent phosphate anion at higher concentration had an obvious liquefying effect on the rheological properties of the palygorskite suspension. The variations of flow curves and yield stresses of obtained palygorskite samples as a function of KH2PO4, K2HPO4, K2SO4, and K3PO4 concentrations are shown in Figure 6 and Table 3, respectively. The yield stresses of the palygorskite increased as KH2PO4 concentration increased from 0.01 mol·kg−1 to 0.025 mol·kg−1, and then decreased with further increasing KH2PO4 concentration to 0.25 mol·kg−1. Whereas, a gradual decrease of the yield stresses occurred with increasing K2HPO4, K2SO4, and K3PO4 concentrations. The surface of palygorskite became more negative with an increase in KH2PO4, K2HPO4, K2SO4, and K3PO4 concentrations (Figure 3). The electrostatic repulsion forces between rods increased and some association of rods was broken. So the decrease of yield stresses at higher electrolyte concentrations was attributed to the collapsed network structure of suspensions. The result was in line with the observations reported in the literature.6 It can be deduced from the above discussion that the phenomenon of rheological properties of palygorskite suspension remaining constant or higher in the presence of electrolyte solutions depends on the charge density of rods and can be varied by changing the concentration and type of added electrolytes. 3.3. Suspension Stability. The effects of the types (Figure 7) and the concentrations (Figure 8) of anions on the suspension stability of palygorskite were discussed by the measurement of sedimentation volume of the suspensions after allowing it to stand for a given time. The higher the sedimentation volume, the more stable the suspension will be. The stability of suspension is due to the existence of potential energy barrier between the particles, which arises as a

Figure 7. Variation of sedimentation volumes V of homogenized palygorskite suspensions in the presence of ○, KCl; Δ, KBr; ▽, KI; ⧫, KH2PO4; ◊, K2HPO4; ●, K3PO4; ☆, K2SO4;, and □, KHSO4 electrolytes (0.05 mol·kg−1 in all cases) as a function of time t. ■, pure palygorskite.

result of interactions of the electrical double layers and the van der Waals forces.40,41 As can be seen from Figure 7, the sedimentation volumes increase in the order of I− > Br− > Cl− for homogenized samples containing monovalent anions, which follows the same trend to that of the increase in negative zeta potential with electrolytes (Figure 2). It indicated that the reductions of charge density and potential energy barrier made the rods of palygorskite easily agglomerate and settle. No obvious change in sedimentation volumes is observed in the studied range of settling time for the suspension containing KHSO4, K2HPO4, and K2SO4 electrolytes. It was likely that the higher viscosity of suspensions favored the slow sedimentation of particles.20 For the suspensions based on phosphates-modified palygorskite, the sedimentation volumes increase in the order of K2HPO4 > K3PO4 > KH2PO4, indicating that the higher were the zeta potential and suspension viscosity, the more stable the system was. The variations of sedimentation volumes of palygorskite suspensions with KH2PO4, K2HPO4, K2SO4, and K3PO4 concentrations are shown in Figure 8. An increase in sedimentation volumes occurred when KH2PO4 concentrations increased from (0.01 to 0.025) mol·kg−1, after which the sedimentation volumes of suspensions gradually decrease with further increasing KH2PO4 concentration to 0.25 mol·kg−1. It indicated that more negatively charged rods in less viscous suspension could easily settle with prolonging the incubation time. For the suspensions containing K2HPO4, the sedimentation volumes remain almost constant as K2HPO4 concentration increased from 0.01 mol·kg−1 to 0.1 mol·kg−1, but decrease with further increase in the K2HPO4 concentration. Such a decrease in suspension stability at higher K2HPO4 concentration (0.25 mol·kg−1) was attributed to the depletion attraction of single rods resulted from higher surface charge, which led to the neighboring rods sticking to each other and forming large clusters.42 The sedimentation volume of 0.01 mol·kg−1 K2SO4modified suspension is still 96.5 cm−3 after incubation for 6 days, and no obvious change in sedimentation volumes is observed in the studied range of K2SO4 concentrations. The higher viscosity of suspensions might be responsible for such a high stability of suspensions. For K3PO4-modified samples, a relatively stable suspension was obtained when palygorskite was 770



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Figure 8. Variation of sedimentation volumes V of homogenized palygorskite suspensions in the presence of KH2PO4 (a), K2HPO4 (b), K2SO4 (c), and K3PO4 (d) with various concentrations C as a function of time t. □, 0 mol·kg−1; ○, 0.01 mol·kg−1; Δ, 0.025 mol·kg−1; ▽, 0.05 mol·kg−1; ◊, 0.1 mol·kg−1; and ☆, 0.25 mol·kg−1.

dispersed by 0.01 mol·kg−1 K3PO4 solution, but a fast settlement was occurred for palygorskite dispersed by 0.025 mol·kg−1 K3PO4 solution. Further increasing K3PO4 concentration to 0.1 mol·kg−1 had a positive effect again in enhancing the stability of suspensions. In addition, a sudden decrease of sedimentation volume of the suspensions containing a higher concentration of K3PO4 (> 0.025 mol·kg−1) was found with a prolonged settling time. These results further confirmed that well dispersed palygorskite rods favored the formation of large clusters, and resulted in poor suspension stability after incubation for a long time.

(4) The mechanism by which electrolytes affected the rheological properties of suspension was through changing the surface charge of the rods and then the flocculation state of the suspension. A slight change in the surface charge of rods could increase the size of cluster and enhance the rheological properties of suspensions. On the other hand, the great increase of surface charge of rods resulted in high interparticle repulsive forces and low attractive forces, which could destroy the entanglement of network structure and decrease the rheological properties of suspension. (5) No direct relationship between suspension stability and zeta potential was ascertained. The stability of suspension was dependent on the relative contribution of zeta potential and viscosity. The higher were zeta potential and suspension viscosity, the larger the sedimentation volume of the suspension was. The suspension based on K2SO4-modified palygorskite was stable.

4. CONCLUSIONS In the current work, palygorskite samples modified with KCl, KBr, KI, KH2PO4, KHSO4, K2HPO4, K2SO4, and K3PO4 were prepared via high-pressure homogenization techniques. The electrokinetic and colloidal properties of obtained samples were discussed. The following results were obtained: (1) The migration of a given structure maker anion from an aqueous solution into the electrical double layer of palygorskite suspension became easy during high-pressure homogenization process. (2) The surface charge of palygorskite was varied by changing the concentration, the charge, and the type of anions. (3) The surface negative charge of homogenized palygorskite first increased and then remained nearly constant with increasing KH2PO4, K2HPO4, and K2SO4 concentrations, but first increased and then decreased with increasing K3PO4 concentrations.

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ASSOCIATED CONTENT

S Supporting Information *

Zeta potential of palygorskite as a function of clay concentration C, pH, the Herschel-Bulkley fitting parameters (m, n, R2) of homogenized palygorskite suspensions in the presence of various anions (0.05 mol·kg−1) as well as KH2PO4, K2HPO4, K2SO4, and K3PO4 with various concentrations C (mol·kg−1). This material is available free of charge via the Internet at http://pubs.acs.org. 771



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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 931 4968118. Fax: +86 931 8277088. E-mail: [email protected]. Funding

The authors would like to thank the Science and Technology Support Project of Jiangsu Provincial Sci. & Tech. Department (No. BY2010012) and Jiangsu Provincial Joint Innovation and Research Funding of Enterprises, Colleges and InstitutesProspective Cooperative Research Project (No. BY2011194) for financial supports. Notes

The authors declare no competing financial interest.

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