Poly(acrylic acid) as a rheology modifier for dense alumina dispersions in high ionic strength environments

Colloids and Surfaces A: Physicochem. Eng. Aspects 362 (2010) 71–76

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Poly(acrylic acid) as a rheology modifier for dense alumina dispersions in high ionic strength environments Prasad S. Bhosale, John C. Berg ∗ Department of Chemical Engineering, University of Washington, Seattle, WA 98195, USA

a r t i c l e

i n f o

Article history: Received 7 December 2009 Received in revised form 20 March 2010 Accepted 26 March 2010 Available online 3 April 2010 Keywords: High ionic strength Dense alumina dispersions Poly(acrylic acid) as a rheology modifier

a b s t r a c t Rheological modifiers are currently being investigated for effective transport of dense radioactive nuclear waste slurries in the vitrification process developed for more permanent radioactive waste disposal. Recently, poly(acrylic acid) (PAA) was found to be a promising candidate for reducing the yield stress and viscosity of these waste slurries, of which aluminum hydroxides are a major component. This work focuses on dense alumina dispersions stored in an environment similar to that of the nuclear waste slurries, viz., strong basic conditions (0.5N NaOH) with high salt concentrations (0.1N–1N KNO3 ). It was found that the correct choice of molecular weight of the PAA was especially important to its effectiveness as a rheology modifier: too low a value provides insufficient steric stabilization, while too high a value induces bridging. Experiments are conducted to determine the effective molecular weight range for minimization of yield stress and viscosity and its dependence on ionic strength. An observed shift in the effective molecular weight range with salt concentration was attributed to changes in PAA conformation with ionic strength. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Dense colloidal systems are often rheologically complex since the particulates that compose them can aggregate, forming microstructures of various types that can induce solid-like behavior [1–3]. One such system is that of the radioactive nuclear waste slurries stored at the US Department of Energy sites at Hanford, WA and Savanna, GA. As the storage tanks are beginning to fail, vitrification facilities are currently being developed for more permanent disposal [4]. One of the challenges of this strategy is the need for effective and efficient transport of the dense nuclear waste slurries to and through the vitrification processes [4,5]. The slurries are multi-component, dense (25–35 wt%) colloidal dispersions of particulates in the size range of 0.1–12 ␮m [5,6]. The major solid components are aluminum hydroxides, zirconium hydroxides, ferrous hydroxides and silicon oxides residing in a strong basic environment (pH 12–13) with very high salt concentration (1.5–2 N) [5,6]. Recent studies by Chun et al. [5] with a nuclear waste slurry simulant have shown that poly(acrylic acid) (PAA) and citric acid (CA) are promising rheology modifiers for these slurries. The focus of the present study is PAA addition to dense alumina suspensions for purposes of rheology modification. PAA is known

∗ Corresponding author at: Department of Chemical Engineering, University of Washington, Box 351750, Seattle, WA 98195, USA. Tel.: +1 206 543 2029; fax: +1 206 543 3778. E-mail address: [email protected] (J.C. Berg). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.03.043

as a stabilizing agent for dense alumina dispersions in the ceramic industry under low salt and moderately basic conditions [7,8]. Low molecular weight PAA (MW 2000–10,000 g/mol) has been used as a dispersant, allowing the handling of high solids slurries for gel casting, freeze casting and slip casting processes at pH 9–10 and low ionic strength: 0.001–0.01 N [7–11]. Aggregation is then induced in these processes by addition of salt to collapse the PAA adlayers and thereby destabilize the colloids [12]. On the other hand, high molecular weight PAA (1 × 103 kg/mol and above) is commonly used as a flocculent in wastewater treatment processes [13–17]. In particular, high molecular weight PAA is used as a flocculent in the high salt and strong basic environment of the Bayer process for concentrating waste slurries [18,19]. The use of PAA as either a dispersant [7–12] or a flocculent [13–16] with alumina and other metal oxides has been extensively studied with respect to the dependence of the desired performance on molecular weight and concentration under low salt conditions. For example, Cesarano et al. [7] studied the PAA molecular weight range 1.8–50 kg/mol for its use as a dispersant for dense alumina dispersions, and Das et al. [16,17] studied a range of PAA molecular weight (MW 2–4 × 103 kg/mol) and concentration (1 ␮g/m2 to 5 mg/m2 ), at pH 3.5 and 0.03N NaCl. This study demonstrated patch aggregation at ultra low concentration with low MW PAA (5–50 kg/mol), steric stabilization at high concentrations of low MW PAA (2–50 kg/mol), and flocculation by bridging at low concentrations of high MW PAA (200 kg/mol and above). Studies with dense alumina dispersions suggest that optimum PAA concentrations for dispersing alumina corresponded to the inception of the knee in the adsorption isotherm [7,10], the location of

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which depended on salt concentration. In low salt media with PAA MW’s of 5–50 kg/mol, this corresponded to 0.5–0.8 mg/m2 . The stability and rheology studies above were limited to low salt concentration (0.001–0.10N) systems. To the best of our knowledge, PAA has not been studied as a dispersant in media beyond 0.1N in ionic strength, and in particular, the important variable of molecular weight has not been investigated under these conditions. Increasing the ionic strength screens the electrostatic forces between COO− groups, allowing PAA chain collapse [20], and it is likely that this collapse is not complete even at 0.1N ionic strength. Furthermore, studies involving other colloids in high salt media have revealed a number of unusual effects [21]. Dense alumina suspensions show shear thinning behavior with a yield stress,  y . They may be envisioned as weakly aggregated or percolated particulate structures. As the yield stress is reached, inter-particle bonds in the aggregates are broken, and a sudden drop in viscosity is noted, followed by Newtonian or visco-plastic flow [1]. If the suspension is subsequently unperturbed, aggregates can re-form, and the suspension can regain its yield stress and high apparent viscosity. In dense colloids, the yield stress is a direct manifestation of the inter-particle force F, and the number of particle–particle contacts per unit area. Using the model suggested by Russel et al. [3] the latter should scale as 2 /a2 , where  is the volume fraction of the particles and a is the particle radius, i.e.: y ∝

2 F. a2

(1)

Most rheological modifiers are designed to alter the interparticle interaction force, which is in general composed of van der Waals (FvdW ), electrostatic (Felec ), steric (Fsteric ), bridging (Fbridge ), depletion (Fdepletion ) and other components (Fother ), such as those of solvent structure or hydrophobic effects [1–3]. Thus: F = FvdW + Felec + Fsteric + Fbridge + Fdepletion + Fother .

(2)

Van der Waals forces are always present and strongly attractive at close enough proximity, but electrostatic forces are effectively screened out and negligible in high enough ionic strength environments such as those under investigation [10]. Steric repulsion results when adsorbed polymer layers on approaching particles overlap producing osmotic pressure differences that seek to redilute the overlap regions [1]. Bridging occurs when dissolved polymers present in the system adsorb onto multiple particles. Attractive depletion forces result from the presence of unadsorbed polymer. In the systems under study, PAA is present in sufficient amount (0.717 mg PAA/m2 of alumina surface) to cover the alumina particle surfaces [13,16] producing full coverage, with a minimum of free polymer remaining in the solution, so that depletion effects may be neglected. Finally, neither solvent structuring nor hydrophobic effects are expected to play a significant role, so that to good approximation for the systems under study: F = FvdW + Fsteric + Fbridge .

Fig. 1. Schematic of the different interaction mechanisms observed with change in the PAA molecular weight and ionic strength.

extension depends primarily on its molecular weight, it is also influenced by its solvency, which depends on the salt concentration. PAA molecules are known to undergo significant conformation changes over the range of monovalent salt concentrations of 0.01–2 N, and the adsorption of PAA onto metal oxide surfaces is also increased at high ionic strengths due to the decrease in solvency [13]. In the vitrification process, nuclear waste slurries go through dilution and concentration cycles [4]. Thus in order to evaluate PAA as a dispersant in slurries relevant to the radioactive tank wastes cited above, the present study examines the stability and rheological behavior of dense alumina slurries over a wide PAA molecular weight range at a concentration of 0.717 mg/m2 (0.3 wt% of alumina) in media of high salt concentration (0.1–1N KNO3 ) and pH corresponding to the presence of 0.5N NaOH. 2. Materials and methods The dispersoid used was APA 0.5 (491 nm), a high purity alumina obtained from Sasol Corp., Tucson, AZ. The particle size distributions of the alumina powder in water and in 0.5N NaOH with 0.1–1N KNO3 were measured by dynamic light scattering (DLS) using a Brookhaven Zeta-PALS (Brookhaven Instruments Corp., Holtsville, NY) instrument. Results are shown in Fig. 2. Fig. 3 shows an electron micrograph of alumina particles obtained using a JSM 7000F scanning electron microscope (JEOL Ltd., Tokyo, Japan). The BET surface area was measured to be 4.18 m2 /g by nitrogen adsorption using a Quantachrome NOVA 4200e apparatus (Quantachrome Corp., Boynton Beach, FL).

(3)

The decisive factor in determining the net inter-particle attraction is usually just the relative magnitude van der Waals attraction and steric repulsion, which depends on the inter-particle spacing, S0 , which corresponds roughly to twice the polymer adlayer thickness, i.e., 2ı. When this is large enough, the net attractive forces are zero, and the colloid is stable against aggregation. The adlayer thickness depends on the extension of the adsorbed polymer molecules from the particle surface, which in turn depends on the polymer molecular weight and its degree of solvency in the medium. For purposes of yield stress reduction it would thus appear that the higher MW polymer the better, but when the chain extension becomes too great, polymer bridging can induce aggregation and thus increase the yield stress [20,21]. Fig. 1 represents the effect of changing the polymer molecular weight. While the degree of polymer chain

Fig. 2. Smoothed particle size distribution of alumina particles in water and in 0.5N NaOH, 0.5N KNO3 solution measured using DLS.

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Fig. 3. Electron micrograph of alumina particles (scale bar 1 ␮m).

The desired concentration (66.67 wt% or 34.65 vol%) of particles was mixed in a 0.5N NaOH solutions containing the PAA and 0.1–1N KNO3 . The concentration of PAA was maintained at 0.3 wt% of alumina (to produce 0.717 mg/m2 ) for all the experiments. Mechanical agitation was applied for 4 min to obtain a consistent slurry in each case. The slurry obtained was then left unperturbed for 40 min in a Physica MCR 300 rheometer (Anton Paar, Ashland, VA) before testing. Viscosity vs. shear–stress plots were generated using the cup-and-bob tool as the shear–stress was increased from 0.4 Pa until the yield stress point was crossed, and Newtonian behavior was observed. The wait time between successive data points was 5 s. In dense colloidal dispersions, the yield stresses measured are often a function of the approach rate. Here we have maintained a constant ramp rate for all the experiments to compare yield stresses obtained in different chemical environments. Shear rates measured at very low shear–stresses were beyond the instrument capability, indicating the possibility of slip of the solid material on the wall. However, we expect the slip to be low at high shear rates were yield stress point was observed. The instrument setup used should be able to represent, at least qualitatively, the change in yield stress due to different chemical environments. To confirm this, trials were conducted using a vane tool, and similar changes in the yield stress were observed. A range of poly(acrylic acid) molecular weights: 1.8, 50, 90, 200, 450, 1 × 103 and 4 × 103 kg/mol, obtained form Polyscience Corp. (Warrington, PA), were tested. All other chemicals used were purchased from Fisher Scientific. The change in conformation of the poly (acrylic acid) with ionic strength was investigated using DLS. For these studies, dilute solutions of PAA were prepared in 0.5N NaOH with different KNO3 concentrations (0.01N, 0.1N, 0.5N and 1N). The solutions were filtered through 0.45 ␮m (for PAA 1 × 103 and 450 kg/mol) and 0.2 ␮m (for PAA 200 and 90 kg/mol) nylon filters obtained form Whatman Corp. (Piscataway, NJ) before analysis. The change in conformation was tracked by measuring the hydrodynamic radius of the PAA. PAA adsorption measurements were carried out in the concentration range of 10–150 ppm at different KNO3 concentrations (0.1N, 0.5N and 1N) in 0.5N NaOH solution. Alumina powder (4 mg) was mixed with 10 mL of PAA solution at the desired PAA concentration and salt conditions. The solution was shaken for 3 h, followed by centrifugation to separate the alumina particles. One mL of supernatant was then removed for further analysis. The amount of PAA adsorbed was calculated by measuring the difference in the bulk PAA concentration before and after adsorption equilibrium was achieved. For that purpose, turbidity changes caused by the PAA reaction with hyamine [13,14] was measured

Fig. 4. Rheology of 34.65 vol% alumina slurries in a 0.5N NaOH solution at different KNO3 concentrations. (a) Shear–stress vs. shear rate plot and (b) viscosity () vs. shear–stress () plot.

using a Model Evolution 300 spectrophotometer (Thermo Scientific, Asheville, NC) at 500 nm. 3. Results and discussion 3.1. Alumina dispersions in the absence of added polymer The mean particle diameter of the alumina particles in a dilute dispersion in DI water was found to be 491 nm, while in a dilute dispersion of 0.5N NaOH and 0.1–1N KNO3 it was measured at 1.5 ␮m, as shown in Fig. 2, indicating aggregation under the high electrolyte conditions. Similar particle size distributions and sediment heights were observed in the 0.5N NaOH solution over the range of KNO3 concentrations investigated. Fig. 4a shows the shear–stress vs. shear rate behavior observed for concentrated (34.65 vol%) alumina slurries at different ionic strengths. All the slurries tested showed shear thinning behavior. In a shear–stress vs. shear rate plot, the yield stress point is the point where the strain rate in the rheometer increased rapidly as the shear–stress is increased, followed by a constant shear rate. However, it is difficult to pinpoint the yield stress through shear–stress vs. shear rate graphs. Barnes [22] therefore argued that to obtain the yield stress point, one can plot the slope of the curve against shear–stress, as shown in Fig. 4b. The slope is the effective viscosity of the dispersion, and the yield stress point is the shear–stress at which a sudden drop in the viscosity is observed. Herein we have presented all the rheological data in the form of viscosity vs. shear–stress plots which are easy to interpret in terms of yield stress. The yield stresses of 33 + 4 and 31 + 5 Pa were measured at 0.1N and 0.5N KNO3 , respectively, and a significantly lower value (19 + 3 Pa) at 1N KNO3 . Since there was no difference in particle size distribution or sediment heights between the three cases, the decrease in the yield stress at 1N KNO3 was evidently not caused

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Fig. 5. Adsorption isotherms for PAA 90 kg/mol on alumina at 0.1N, 0.5N, and 1N KNO3 concentration in 0.5N NaOH solution.

by different microstructure formation. The decrease in yield stress at 1N KNO3 was consistent with colloidal probe AFM studies by Yilmaza et al. [23], who showed a large decrease in the friction force between the alumina surfaces under high salt (1N and above) conditions. The lower value of the yield stress may also be the result of a lower rate of particle sintering under high salt conditions. Sintering of inter-particle bonds within an aggregate is a well known phenomenon [24–27], and the contact strength has been found to vary with time [27], salt concentration [23,26] and the type of salt [24,25]. Similar colloidal probe AFM studies by Vakarelski et al. [26] have shown, for example, that the adhesion force between silica surfaces rapidly increases with time at 0.1N K+ concentrations, but very slowly at 1N K+ . The aging time of 40 min before testing in the present study may not have been sufficient for full sintering in the 1N K+ slurries. 3.2. PAA adsorption isotherms Fig. 5 shows the PAA 90 kg/mol-alumina adsorption isotherms measured at different KNO3 concentrations (0.1N, 0.5N and 1N) in 0.5N NaOH solution. The results are consistent with adsorption studies conducted by Chibowski et al. [13,14], who showed an increase in adsorption with an increase in the ionic strength. The optimum PAA concentrations for dispersing alumina corresponded to the inception of the knee in the adsorption isotherm [10,7]. In the systems under study, this occurred approximately at a value of 0.5 mg/m2 (Fig. 5). Therefore, the PAA concentration selected was a sufficient amount (0.717 mg/m2 of alumina surface) to cover the alumina particle surfaces [13,16] to produce full coverage, but with a minimum of free polymer remaining in the solution, so that depletion effects could be neglected [28].

Fig. 6. Viscosity () vs. shear–stress () for 34.65 vol% alumina slurries in a 0.5 N NaOH, 0.5 N KNO3 solution with PAA molecular weights of 50, 90, 200 and 450 kg/mol (50 K, 90 K, 200 K and 450 K, respectively). PAA concentration was maintained at 3 mg/g of alumina for all PAA molecular weights.

the highest molecular weight polymers tested produced bridging [16,17]. At the added electrolyte KNO3 concentration of 0.5N, the optimum molecular weight polymer among the polymers tested (200 kg/mol) dropped the yield stress by almost half. 3.4. Effect of ionic strength PAA adsorption on metal oxide surfaces is known to be significantly influenced by the ionic strength of the solution [14,15,20]. In contrast to Fig. 6, which shows the rheological behavior of the alumina-PAA system at 0.5N KNO3 , Fig. 8 shows the viscosity vs. shear–stress behavior at the lower electrolyte concentration of 0.1N KNO3 . The yield stress values with PAA MW 90 and 50 kg/mol dropped to 19 + 2 Pa and 23 + 3 Pa, respectively from 33 + 4 Pa measured for alumina without PAA. For PAA 200 kg/mol and PAA 450 kg/mol, the yield stress increased to 40 + 2 and 63 Pa, respectively, and further increases in molecular weight increased the yield stress to still higher values. These experimental results suggest that the optimum molecular weight (corresponding to the lowest  y ) decreased from 200 to 90 kg/mol (among the polymers tested) when the ionic strength was decreased from 0.5 to 0.1N KNO3 . Fig. 9 shows results for the highest electrolyte concentration studied (1N KNO3 ). In this case, the yield stress value for the alumina dispersion (without PAA) dropped to 19 + 3 Pa. PAA molecular weights of 50, 90 and 200 kg/mol showed yield stresses of 15 + 5, 15 + 6 and 17 + 4 Pa, respectively, an insignificant change from the values for the alumina dispersion in the absence of PAA, while for a molec-

3.3. Effect of PAA addition at 0.5N KNO3 Fig. 6 shows viscosity vs. shear–stress plots obtained for different PAA molecular weights, with the KNO3 concentration maintained at 0.5N (1N total ionic strength, 0.5N K+ ions and 0.5N Na+ ions). The addition of PAA reduced the viscosity by an order of magnitude for all the molecular weights studied. The yield stress, however, varied with the molecular weight. The addition of 1.8, 5, 50 and 90 kg/mol MW PAA all produced  y values roughly comparable to those of the pure alumina dispersion, while a molecular weight of 200 kg/mol produced a significant decrease in the yield stress. In contrast, a PAA molecular weight of 450 kg/mol more than doubled the yield stress. For PAA molecular weights of 1 × 103 and 4 × 103 kg/mol, the yield stresses were even higher, as shown in Fig. 7. The lower molecular weight polymer adlayers (<200 kg/mol) evidently provided only an inadequate barrier to aggregation, while

Fig. 7. Viscosity () vs. shear–stress () for 34.65 vol% alumina slurries in 0.5N NaOH, 0.5N KNO3 , with PAA molecular weights of 450, 1 × 103 and 4 × 103 kg/mol (450 K, 1 M and 4 M, respectively). PAA concentration maintained at 3 mg/g of alumina for all PAA molecular weights.

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Fig. 8. Viscosity () vs. shear–stress () of a 34.65 vol% alumina slurries in 0.5N NaOH, 0.1N KNO3 for different PAA molecular weights. PAA concentration maintained at 3 mg/g of alumina for all the molecular weights.

ular weight of 450 kg/mol, the value jumped to 60 Pa, indicating polymer bridging. Therefore in this case, one can expect the optimum molecular weight to be higher than 200 kg/mol but less than 450 kg/mol. From Figs. 6, 8 and 9 it is clear that the optimum molecular weight for yield stress reduction increased with increasing the ionic strength. This change was due to conformational changes in the PAA molecules. PAA in basic environments with moderate ionic strength (0.1N) is in a partially stretched conformation due to intra-molecular electrostatic repulsive forces between carboxylate (–COO− ) groups [20]. Increases in ionic strength screen these forces [20] and decrease the PAA solubility [13], both of which allow the PAA chains to fold upon themselves and collapse, decreasing adlayer thickness. To corroborate the expected changes in PAA conformation with ionic strength, the hydrodynamic radii of the PAA molecules in 0.5N NaOH, KNO3 solutions of varying KNO3 concentration were studied using DLS. Fig. 10(a) shows the expected sharp decreases in the hydrodynamic radius of PAA 1 × 103 and 450 kg/mol with ionic strength and similar reductions in the hydrodynamic dimensions of the PAA 90 and 200 kg/mol molecules are shown in Fig. 10(b). Fig. 11 shows the effect of PAA conformational changes on the rheology of the alumina slurries in terms of relative yield stress vs. ionic strength for PAA molecular weights of 90 and 200 kg/mol. The relative yield stress is the ratio of  y in the presence of polymer to that observed in its absence at the same KNO3 concentration level. For PAA 90 kg/mol at 0.1N KNO3 , the PAA chains were stretched enough to form thick enough adlayers to produce sufficient steric repulsion to reduce the yield stress (relative ␶y ∼0.57), but as the

Fig. 9. Viscosity () vs. shear–stress () of 34.65 vol% alumina slurries in 0.5N NaOH, 1N KNO3 solutions with different PAA molecular weights. PAA concentration maintained at 3 mg/g of alumina for all PAA molecular weights.

Fig. 10. Change in hydrodynamic radius of free PAA molecules vs. KNO3 concentration in 0.5N NaOH, as measured by dynamic light scattering. (a) MW 1 × 103 and 450 kg/mol (1 M and 450 K). Inset: schematic of adlayer change with increasing ionic strength. (b) MW 200 and 90 kg/mol (PAA 200 K and 90 K).

ionic strength increased to 0.5N, the PAA chains collapsed, decreasing the adlayer thickness so that the yield stress increased back to nearly that of the alumina dispersion in the absence of polymer (relative  y ∼1 and 0.87). At PAA 200 kg/mol an effective polymer adlayer was formed at 0.5N (relative  y ∼0.6), but increasing the KNO3 concentration to 1N collapsed the PAA chain, and the yield stress returned to the pure alumina value (relative  y ∼0.9). At 0.1N KNO3 the PAA 200 kg/mol molecules were more stretched, producing aggregation by polymer bridging, increasing the yield stress to a value higher than that of alumina dispersions (relative  y ∼1.2). In summary, the most effective PAA molecular weights, producing approximately 40% reduction in yield stress, were found to be 90 and 200 kg/mol at 0.1N and 0.5N KNO3 , respectively. At 1N KNO3 , one might anticipate an effective PAA molecular weight to be within the 200–450 kg/mol range.

Fig. 11. Change in relative yield stress for alumina-PAA system (90 and 200 kg/mol) at different KNO3 concentrations.

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4. Conclusions Modification of the rheology of dense alumina dispersions in highly basic media (0.5N NaOH) using poly(acrylic acid) (PAA) has been investigated as a function of polymer molecular weight and solution ionic strength (0.1–1N KNO3 ). Significant reductions in yield stress and viscosity could be achieved if the PAA molecular weight was high enough to produce robust steric repulsion, but not so high that bridging flocculation resulted. The particular molecular weight that produced the greatest reductions in slurry yield stress depended on the ionic strength of the medium. Among the systems and conditions investigated, optimum yield strength reductions of approximately 40% (compared with comparable slurries in the absence of polymer) were achieved with a PAA molecular weight of 90 kg/mol in 0.1N KNO3 media, but 200 kg/mol in 0.5N KNO3 . For 1N KNO3, the apparent optimum would be between 200 and 450 kg/mol. Acknowledgments This work was supported by the U.S. Department of Energy’s Office of Technology Innovation and Development (EM-30) through the Pacific Northwest National Laboratory (PNNL), Richland, WA. The authors gratefully acknowledge helpful discussions with Dr. Jaehun Chun and Dr. Paul Bredt. References [1] J.C. Berg, An Introduction to Interfaces and Colloids: The Bridge to Nanoscience, World Scientific Publishers, Singapore, 2010 (Chapters 7 and 8).

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