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Langmuir 1997, 13, 6260-6266
Adsorption of Dextrin and Guar Gum onto Talc. A Comparative Study Rajendra Kumar Rath,† S. Subramanian,*,† and J. S. Laskowski‡ Department of Metallurgy, Indian Institute of Science, Bangalore, 560012, India, and Department of Mining and Mineral Process Engineering, The University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 Received May 19, 1997. In Final Form: September 2, 1997X The interaction of guar gum and dextrin with talc has been investigated through adsorption, flotation, and electrokinetic measurements. The adsorption densities are independent of pH and the isotherms exhibit Langmuirian behavior. Higher adsorption density of guar gum onto talc is observed vis-a`-vis that of dextrin. Pretreatment of talc with a complexing agent such as ethylenediaminetetraacetate results in a decrease in the adsorption density, highlighting the importance of metallic magnesium sites for the adsorption process. The adsorption density of guar gum is decreased when the talc sample is subjected to leaching and/or calcination attesting to the contribution of metallic sites and brucite structure in enhancing the adsorption process. An increase in the surface face-to-edge ratio leads to increase in adsorption density. The flotation recoveries are independent of pH, with the guar gum exhibiting better depressant ability compared to dextrin, complementing the adsorption results. However, polymer depressant ability is reduced in the case of leached or calcined talc sample in comparison to that on talc. Electrokinetic measurements portray conformational rearrangements of macromolecules with the loading, resulting in the shift of the shear plane, further away from the interface. Coprecipitation, conductance, and viscosity tests confirm polymer-magnesium ion interaction in the bulk solution. The adsorption process is governed by hydrogen bonding as well as chemical interaction between the polysaccharides and the surface metal hydroxide groups of talc. The better depressant activity of guar gum may be attributed to its more favorable cisconfiguration of the hydroxyl groups as opposed to the trans-hydroxyl groups of dextrin, apart from its higher molecular weight.
Introduction Depressants play an important role in effecting selective separation of minerals from ores by flotation. Polysaccharides and their derivatives are effective depressants for talc and other magnesia-bearing minerals. There are numerous reports on the application of carboxymethyl cellulose as a depressant for talc and other magnesiabearing minerals present as impurities in various sulfide ores.1-5 The mechanisms of depressant action and application of various dispersing and depressing agents have been critically examined by Laskowski and Pugh.6 The important factors governing the effectiveness of the polymeric depressants from a chemical viewpoint have been highlighted by Lin and Burdick.7 The constitution of talc has been extensively investigated by Pask and Warner.8 Some aspects of the structure and surface properties of talc have been the subject of discussion of a number of papers.9-11 The role of hydrolyzed cations in * To whom correspondence should be addressed. † Indian Institute of Science. ‡ The University of British Columbia. X Abstract published in Advance ACS Abstracts, October 15, 1997. (1) Rhodes, M. K. In Proceedings of the 13th International Mineral Processing Congress; Laskowski, J., Ed.; Elsevier: Amsterdam, 1981; Vol. 2A, p 346. (2) Edwards, C. R.; Kipkie, W. B.; Agar, G. E. Int. J. Miner. Process. 1980, 7, 33. (3) Barker, C. W.; Featherstone, S. F.; Storey, M. J. Trans. Inst. Min. Metall. 1982, 91, C135. (4) Bakinov, K. G.; Vaneev, I. I.; Gorlovsky, S. I.; Eropkin, J. I.; Zashilhin, N. V.; Koney, A. S. 7th Int. Min. Process. Congress, New York, 1964. (5) Oliveira, J. F.; Gomes, L. M. B. In Processing of Hydrophobic Minerals and Fine Coal; Laskowski, J. S., Poling, G. W., Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, 1995; p 341. (6) Laskowski, J. S.; Pugh, R. J. In Colloid Chemistry in Mineral Processing; Laskowski, J. S., Ralston, J., Eds.; Elsevier: Amsterdam, 1981; p 346. (7) Lin, K. F.; Burdick, C. L. In Reagents in Mineral Technology; Moudgil, B. M., Somasundaran, P., Eds.; Surfactant Science Series 27; Dekker: New York, 1988; p 471. (8) Pask, J. F.; Warner, M. F. J. Am. Ceram. Soc. 1954, 37, 118.
S0743-7463(97)00518-0 CCC: $14.00
the natural hydrophobicity of talc has been examined by Fuerstenau et al.12 The adsorption and depressant action of poly(oxyethylene) alkyl ether on talc has been studied by Pugh and Tjus.13 Dextrin has also been reported as a depressant for talc.14 In an earlier communication, we discussed the adsorption of guar gum onto talc.15 In the present investigation, two natural polysaccharides having different configurations of hydroxyl groups, namely, guar gum and dextrin, have been compared with respect to their adsorption, electrokinetic, and flotation behaviors onto talc. Viscosity, conductivity, and coprecipitation tests have also been carried out to ascertain the interaction of the polymeric reagents with the magnesium ions in the bulk solution. Possible mechanisms of adsorption are discussed. Experimental Section Materials. A pure mineral sample of talc was obtained from Alminrock Indscer Fabriks, Bangalore, India. Mineralogical and X-ray powder diffraction data confirmed that the talc sample was of high purity with a trace amount of quartz. The sample was dry ground using a porcelain ball mill at a speed 60 rpm. The ground sample was then dry screened through 150, 75, 63, 53, and 37 µm BSS sieves. All the different size fractions were collected and kept separately for various studies. The -37 µm size fraction was used for the electrokinetic studies and the (-150 + 75) µm fraction was used for the flotation tests whereas (-150 + 75), (-63 + 53), and -37 µm fractions were used for the adsorption studies. The BET nitrogen specific surface areas of (9) Pauling, L. Proc. Nat. Acad. Sci. 1930, 16, 123. (10) McHardy, J.; Salman, T. Trans. Inst. Min. Metall. 1974, 83, C25. (11) Malhammar, G. Colloids Surf. 1990, 44, 61. (12) Fuerstenau, M . C.; Lopez-Valdieso, A.; Fuerstenau, D. W. Int. J Miner. Process. 1988, 23, 161. (13) Pugh, R. J.; Tjus, K. Colloids Surf. 1990, 47, 179. (14) Makarinsky, F. M. Can. Mining J. 1975, 96, 26. (15) Rath, R. K.; Subramanian, S.; Laskowski, J. S. In Processing of Hydrophobic Minerals and Fine Coal; Laskowski, J. S., Poling, G. W., Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, Quebec, Canada, 1995; p 105.
© 1997 American Chemical Society
Adsorption of Dextrin and Guar Gum onto Talc the three fractions of talc (-150 + 75), (-63 + 53), and -37 µm were found to be 0.69, 2.07, and 3.87 m2/g, respectively. The sample of guar gum used in the study was obtained from s.d.Fine-Chem Ltd., Bombay, India, whereas the white dextrin sample was a J.T. Baker product, prepared by the thermal degradation of starch under acidic conditions.16 The molecular weights of guar gum and white dextrin were determined by highperformance liquid chromatography (HPLC) to be 4.22 × 106 and 3980, respectively. The polysaccharide solutions were prepared by dispersing a known weight in cold distilled water and then dissolving it with boiling distilled water. The solutions were prepared fresh each day. Potassium nitrate was used to maintain the ionic strength and nitric acid and potassium hydroxide were used as pH modifiers. All the reagents used in this study were of analytical grade. Deionized double distilled water with a final conductivity of 1.5 µΩ-1 was used for all tests. Methods. Adsorption Studies. Preliminary kinetic experiments indicated that within 15 min, adsorption equilibrium was attained. Hence in all adsorption tests, the time of equilibration was fixed at 30 min. For the adsorption tests 0.5 g of talc powder was taken and made up to 50 mL after addition of desired amounts of 10-2 M KNO3, polymer solution of known concentration and pH in 250 mL Erlenmeyer flasks. The suspensions were then agitated for half an hour using a Remi Orbital Shaking Incubator at 250 rpm at 25 °C. After equilibration the solution pH was again recorded. The solution was then centrifuged for 10 min at 5000 rpm using a Remi Laboratory Centrifuge R8C. The supernatant liquid was then clarified using Whatman 42 filter paper and analyzed for equilibrium concentration of polymer using a Shimadzu UV-260 spectrophotometer as per the method suggested by Dubois et al.17 Flotation Tests. Flotation tests were carried out using a modified Hallimond tube.18 For the flotation tests, 1 g of (-150 + 75) µm talc sample was conditioned with 200 mL of double distilled water at a particular pH for 10 min using a magnetic stirrer. The sample was then transferred to the Hallimond tube and nitrogen gas was passed at the rate of 100 mL per min. After flotation for 3 min, the concentrate and the tailing were separately filtered, dried, and weighed. The recoveries are expressed on weight basis. In experiments where polymers were used as depressants, the sample was conditioned for a total period of 30 min at a given pH with the polymer prior to flotation. Electrokinetic Measurements. Electrophoretic mobility measurements on talc with and without polymers were carried out using a Zeta meter 3.0 (Zeta meter Inc, USA) with bright molybdenum anode and platinum cathode electrodes. Potassium nitrate was used to maintain the ionic strength at 10-3 M. A suspension of talc at 0.2 g/L was conditioned at a particular pH and equilibrated for 1 h at room temperature (27 °C). Measurements were made on the talc alone and in the presence of polymers such as guar gum and dextrin at different concentrations. Coprecipitation Tests. Coprecipitation tests were carried out between dextrin and magnesium ions in aqueous solution to elucidate the interaction mechanism. A known amount of magnesium nitrate solution was mixed with polymer solution in such a way that the final concentration of magnesium nitrate was 7 × 10-4 M and that of dextrin solution was 100 ppm in 100 mL of solution. The desired pH was adjusted by adding either nitric acid or potassium hydroxide solution. After 30 min, the solution was centrifuged at 5000 rpm for 10 min in a Remi Laboratory Centrifuge R8C. The supernatant was then analyzed for magnesium ions by using a Thermo Jarrell Ash atomic absorption spectrophotometer. Polymer concentration was determined using the method of Dubois et al.17 Similar tests were also carried out in 100 mL solutions of Mg2+ ions or polymer as control experiments. Conductance Tests. The specific conductances of 400 ppm dextrin solutions were measured as a function of pH in the absence and presence of Mg2+ ions using a Systronics direct reading digital conductivity meter type 304. (16) Product Literature of J. T. Baker Chemical Co., Phillipsberg, NJ. (17) Dubois, M.; Gilles, K. A.; Hamilton, J. K.; Rebers, P. A. Smith, F. Anal. Chem. 1956, 28, 350. (18) Fuerstenau, D. W.; Metzger, P. H.; Seele, G. D. Eng. Min. J. 1957, 158, 93.
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Figure 1. Adsorption isotherms of dextrin and guar gum onto talc. Viscosity Measurements. The relative viscosities of 400 ppm dextrin and guar gum were measured at room temperature (27 °C) as a function of pH and Mg2+ ion concentration using a Cannon-Fenske capillary viscometer immersed in a constant temperature bath.
Results and Discussion Adsorption Studies. Detailed adsorption experiments have been carried out to study the effect of pH, particle size, sample weight, temperature, and calcination on the adsorption densities of guar gum and dextrin onto talc. Adsorption Isotherms. The adsorption isotherms of dextrin and guar gum onto talc are depicted in Figure 1. It is readily evident that the adsorption density of dextrin onto talc is much lower as compared to that of guar gum. It is well-known19,20 that the two neighboring hydroxyl groups in galactomannan such as guar gum have cisconfiguration while those in dextrin have a trans-configuration. The higher affinity of guar gum toward talc may be attributed to its higher molecular weight in addition to the more favorable conformation of the cishydroxyl groups of guar gum to associate with the mineral sites vis-a`-vis the trans-hydroxyl groups of dextrin.7 The isotherms are found to comply with the Langmuir equation by plotting the reciprocal of the adsorption density verses the reciprocal of the residual concentration (Figure 2). The isotherms may be categorized to follow Langmuirian behavior according to the Giles classification.21 Effect of pH. The effects of pH on the adsorption densities of guar gum and dextrin onto talc are portrayed in Figure 3. The figure also shows the adsorption density of guar gum onto talc sample pretreated with disodium ethylenediaminetetraacetate (EDTA). The amount of guar gum and dextrin adsorbed onto talc is independent of pH in the entire range investigated. However, the magnitude of adsorption density of guar gum onto talc is about one and one-half times that of dextrin adsorption. The fact that the adsorption densities are not affected by pH indicates that the process is not sensitive to the changes in the magnitude of the surface charge characterstics of talc. The adsorption of dextrin onto other hydrophobic solids such as molybdenite and graphite was also found to be independent of pH.22,23 (19) Caesar, G. V. In Starch and its Derivatives; Radley, J. A., Ed.; 1953, 1, 291. (20) Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Wiley: New York, 1980; Vol. 12, p 58. (21) Giles, C. H.; MacEwan, T. H.; Nakhwa, S. N.; Smith, D. J. Chem. Soc. 1960, 3973. (22) Wie, J. M.; Fuerstenau, D. W. Int. J. Miner. Process. 1974, 1, 17.
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Figure 2. Plot of reciprocal of adsorption density versus reciprocal of equilibrium concentration.
Figure 4. Effect of pH on the adsorption densities of dextrin and guar gum for three different particle sizes of talc.
Figure 3. Effect of pH on adsorption densities of dextrin and guar gum onto talc. Table 1. EDAX Analysis of Talc and EDTA-Treated Talc element (%)
size (µm)
talc (1)
Mg Si Al Mg Si Al
-37 -37 -37 -150 + 75 -150 + 75 -150 + 75
28.42 71.37 0.21 30.44 59.18 0.38
talc (2)
EDTAtreated talc (1)
EDTAtreated talc (2)
29.58 70.1 0.32 29.73 69.91 0.36
20.64 78.87 0.49 25.75 73.91 0.34
23.55 74.69 1.76 25.35 74.25 0.41
In order to ascertain whether the lattice magnesium ions present in the talc matrix are involved in the interaction process with guar gum, a few experiments were carried out on talc samples pretreated with the complexing agent EDTA. It is interesting to note that there is a decrease in the adsorption density of guar gum onto EDTA-treated talc in the entire pH range investigated. The importance of metallic magnesium sites in enhancing the adsorption density is clearly borne out. These results are further corroborated by the decrease in the magnesium content observed in the case of a EDTAtreated sample as confirmed by energy dispersive analysis of X-rays (EDAX) experiments. The EDAX results obtained by using EDAX attachment supplied by Oxford Instruments, U.K., are presented in Table 1. Effect of Particle Size. In considering the effect of particle size on adsorption, it is pertinent to recall the structure of talc. Talc crystals are comprised of twodimensional sheet structures, consisting of two layers of (23) Subramanian, S.; Laskowski, J. S. Langmuir 1993, 9, 1330.
silica tetrahedra held together with brucite (Mg(OH)2). The atoms within the layers are held together by ionic bonds, while oxygen-oxygen interlayer atoms are held only by weak residual forces.24 Consequently, when talc particles are broken, two different surfaces are formed, one resulting from the easy cleavage of the layers, called “faces” and the other arising from the rupture of the ionic bonds within the layers, termed “edges”. The adsorption densities of guar gum and dextrin onto talc as a function of pH for three different particle sizes, namely, (-150 + 75), (-63 + 53), and -37 µm are shown in Figure 4. In all cases, as expected, the adsorption densities of guar gum are higher compared to that of dextrin. The pH behavior follows a similar trend as that shown in Figure 3. It is noteworthy that the adsorption density follows an inverse trend with respect to particle size, i.e., the coarser size has a greater adsorption density compared to the finer sizes, when the adsorption densities are expressed on a surface area basis. Since in the coarser size, the face-to-edge ratio is higher compared to the finer sizes, it can be inferred that the adsorption takes place to a greater extent on the “faces”. These are made up of fully compensated oxygen atoms, present a very low electrical charge, and are nonpolar in water. On the other hand the edges composed of hydroxyl ions, silicon, oxygen, and magnesium ions which easily undergo hydrolysis, present a relatively high electrical charge and are polar in water.25 Effect of Sample Weight. The adsorption densities of guar gum and dextrin as a function of pH for different sample weights ranging from 0.1-1 g are depicted in Figure 5. Here again, the adsorption densities of guar gum are higher compared to that of dextrin for all the sample weights studied. It is apparent that the adsorption density increases with decrease in weight of sample taken for both guar gum and dextrin. In other words, as the solid-to-liquid ratio decreases, the adsorption density increases. Effect of Temperature. The effects of temperature as well as of pH on the adsorption densities of dextrin and (24) Aplan, F. F.; Fuerstenau, D. W. In Froth Flotation 50th Anniversary Volume; Fuerstenau, D. W., Ed.; AIME: New York, 1962; p 170. (25) Chander, S.; Wie, J. M.; Fuerstenau, D. W. AIChE Symp. Ser. No.150; AIChE: New York, 1975; Vol. 71, p 176.
Adsorption of Dextrin and Guar Gum onto Talc
Figure 5. Effect of pH on the adsorption densities of dextrin and guar gum for different amounts of talc.
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Figure 7. Effect of pH and particle size on the adsorption density of guar gum onto original and calcined talc sample. Table 2. Effect of Pretreatment of Talc on the Adsorption of Guar Gum (pH 3.1) amount adsorbed (mg/m2)
Figure 6. Effect of pH on the adsorption densities of dextrin and guar gum onto talc at two different temperatures.
guar gum onto talc are shown in Figure 6. The temperatures were controlled at 25 and 55 °C in an incubator shaker. In both cases, there is an increase in the adsorption density with temperature in the entire pH range investigated, suggesting the possibility of chemical interaction of dextrin and guar gum with talc. Earlier studies on the adsorption of octylhydroxamate on ferric oxide and manganese dioxide revealed increased adsorption densities with rise in temperature.26,27 Similar enhanced adsorption with the increase in temperature have been observed for the adsorption of starch on hematite.28,29 Chemisorption is favored at higher temperatures due to faster reaction kinetics. Additionally, the diffusion rate of polymeric molecules such as guar gum and dextrin would be enhanced at higher temperatures leading to higher adsorption densities. Further, dissolution and aging processes are expected at higher temperatures. Effect of Calcination and Leaching. The effect of pH on the adsorption density of guar gum for calcined talc samples is shown in Figure 7. In these tests, talc samples of two different sizes, -37 and (-150 + 75) µm were calcined at 1000 °C in a Barnstead/Thermolyne furnace for 3 h. As a result of calcination, dehydroxylation of the (26) Raghavan, S.; Fuerstenau, D. W. J Colloid Interface Sci. 1975, 50, 319. (27) Natarajan, R.; Fuerstenau, D. W. Int. J. Miner. Process. 1983, 11, 139. (28) Khosla, N. K.; Bhagat, R. P.; Gandhi, K. S.; Biswas, A. K. Colloids Surf. 1984, 8, 321. (29) Subramanian, S.; Natarajan, K. A. Miner. Eng. 1988, 1, 241.
size (µm)
talc
leached talc
calcined talc
leached calcined talc
-37 -37 (-150 + 75) (-150 + 75)
0.97 0.98 4.5 4.52
0.93 0.95 3.96 4.03
0.83 0.82 3.6 3.68
0.76 0.79 3.5 3.45
brucite structure takes place with the loss of structural water of talc. As anticipated, a concomitant decrease in the adsorption density of guar gum for the calcined sample is observed as shown in Figure 7. It is evident from the figure that the finer size fraction shows a lower adsorption density in both the original and calcined talc samples. It is worthy to mention that the magnitude of decrease in the adsorption density is greater in the case of (-150 + 75) µm calcined sample compared to the -37 µm fraction. These results attest to the contribution of hydroxylated (brucite) talc surface in enhancing the adsorption process. The results of the dissolution tests indicated that greater amounts of magnesium ions are released in the acidic pH range 2-4 (Figure 12b). It thus becomes of interest to investigate the effect of leaching, particularly at acidic pH on the adsorption characteristics of guar gum onto talc. In these experiments, the pH was maintained at 3.1, the initial guar gum concentration was 100 ppm, and two different particle sizes of talc, i.e., -37 µm and (-150 + 75) µm, were studied. The results are summarized in Table 2. It is evident from the table that the adsorption density is decreased marginally for both the fractions, namely, -37 µm and (-150 + 75) µm leached talc. The importance of magnesium sites is further reinforced. It is pertinent to recall that the adsorption density of guar gum onto talc is decreased after calcination for both the size fractions studied (Figure 7). For the sake of comparison, the adsorption data obtained at pH 3.1 are also incorporated in Table 2. In order to assess the combined effects of leaching and calcination, a few adsorption experiments were conducted on talc sample initially leached at pH 3.1 followed by calcination at 1000 °C. It is interesting to note from Table 2 that adsorption density is further decreased in the case of leached calcined sample for both size fractions studied. These results further demonstrate the contribution of hydroxylated talc surface in addition to the magnesium sites in enhancing the adsorption process.
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Figure 8. Effect of pH on the flotation recovery of talc in the absence and presence of polymer.
Figure 9. Effect of pH on the flotation recovery of leached or calcined talc in the absence and presence of polymer.
Flotation Tests. The effect of pH on the floatability of talc in the absence and presence of dextrin/guar gum is shown in Figure 8. The results of the flotation studies carried out on EDTA-treated talc using guar gum as a depressant are also shown in the same figure. It is evident that in all cases, the flotation recovery of talc is independent of pH. Additionally, it is clear that guar gum depresses talc to a greater extent compared to dextrin. It is interesting to note that the depressant ability of guar gum on EDTA-treated talc is less compared to that on talc. The complexation of magnesium ions with EDTA decreases the availability of magnesium sites for guar gum interaction. This leads to an increase in the flotation recovery, consequent to lesser adsorption of guar gum. The trend observed with respect to flotation of the two different talc samples in the presence of guar gum complements the adsorption data. The pH behavior observed in the flotation tests is in good agreement with the adsorption results. Similar trends have been observed in the case of molybdenite22 and graphite.23,30 Additionally, contact angle measurements on graphite,30 molybdenite,22 and paraffin wax30 showed a close parallelism to the floatabilities and both were found to be independent of pH. The effect of pH on the flotation recovery of talc that has been either calcined or leached is portrayed in Figure 9. It is apparent that there is a marginal decrease in the floatability in the case of leached talc, whereas, for the calcined talc, the flotation recovery is further reduced. It can be inferred from the same figure that the floatability (30) Arbiter, N.; Fujii, Y.; Hansen, B; Raja, A. AIChE Symposium Series No. 150; AIChE: New York, 1975; Vol. 71, p 183.
Rath et al.
Figure 10. Effect of polymer concentration on the flotation recovery of talc.
of the calcined sample is not depressed in the presence of either dextrin or guar gum due to loss of brucite structure of the talc, consequent to calcination. Additionally, it has been reported that the natural hydrophobicity of the talc is decreased after calcination due to the desorption of nitrogen and nitrogen species from the talc surface.31 However, in the case of leached talc sample, there is a significant decrease in the natural floatability in the presence of guar gum. A comparison of the depressant ability of guar gum with respect to unleached talc (Figure 8) and leached talc (Figure 9) indicates that the unleached talc is depressed to a greater extent vis-a`-vis the leached sample. These results are in good agreement with the adsorption behavior. The effect of dextrin and guar gum concentration on talc flotation is depicted in Figure 10. The better depressant action of guar gum compared to dextrin is readily observed. In both the cases, there is a steep decrease in the flotation recovery up to about 10 ppm, and thereafter it remains constant. Electrokinetic Studies. The electrophoretic mobility of talc particles as a function of pH in the absence and presence of different concentrations of dextrin is shown in Figure 11. The measurements have been conducted using 10-3 M KNO3 as an indifferent electrolyte. In the absence of dextrin, the electrophoretic mobility of talc is negative in the entire pH range investigated. The electronegative character increases steadily with increase of pH up to about 6 and remains more or less constant in the alkaline range. A similar behavior for talc has been reported by other researchers.25,32 Addition of different concentrations of dextrin in the range of 1-50 ppm, correspondingly reduces the negative electrophoretic mobility values in proportion to the dextrin concentration, presumably without any shift in the isoelectric point (iep) value. Such a trend is akin to the effect of an indifferent electrolyte. Since the polymer appears merely to reduce the electrophoretic mobility in absolute magnitude under all the pH conditions studied, the primary effect of the large macromolecule seems to be to shift the slipping plane further away from the interface. These results suggest that some conformational rearrangements of the macromolecules take place with increasing extension of the looping chain, when the dextrin concentration is further increased. These observations are in consonance with the results of the effect of the adsorption of large nonionic (31) Michot, L.; Yvon, J.; Cases, J. M. In Advances in Measurement and Control of Colloidal Processes; Williams, R. A., de Jaeger, N. C., Eds.; Butterworth, Heinemann: Woburn, MA, 1991; p 233. (32) Brien, F. B.; Kar, G. The Trend in Engineering; University of Washington Press: Seattle, WA, 1968; Vol. 20, p 8.
Adsorption of Dextrin and Guar Gum onto Talc
Figure 11. Electrophoretic mobility of talc as a function of pH in the absence and presence of different concentrations of dextrin.
molecules on the zeta potentials reported by many researchers.23,33-35 From Figure 11, it is evident that the electrophoretic mobility of the leached talc as a function of pH is more or less the same as that of the talc. On the other hand, there is a significant change in the electrophoretic mobility values of the talc after calcination in the entire pH range investigated. The electrophoretic mobilities become more negative compared to talc and this may be attributed to the loss of structural water from talc and the transformation of brucite to magnesium oxide.36 Electrophoretic mobility measurements on talc particles in the absence and presence of different concentrations of guar gum showed a similar behavior and the results have been reported in an earlier communication.15 Polymeric depressants such as guar gum and dextrin show a great capacity for hydrogen bonding with hydrated minerals in a colloidal system. Thus adsorption of such polymers at the talc surface brings about a reduction of the negative potential rendering the talc hydrophilic and, hence, depressed. This is confirmed by the flotation tests as shown in Figures 8 and 10. Dissolution and Coprecipitation Tests. In order to ascertain the dissolution of magnesium from talc, leaching tests were performed as a function of time and pH, and the results are shown in parts a and b of Figure 12, respectively. It is apparent that the dissolution of magnesium increases as a function of time (Figure 12a). It is quite plausible that chemical interaction between the magnesium ions so leached from talc could take place with the polymer particularly at acidic pH values. There are several reports confirming the formation of chemical complexes between polysaccharides and metal ions.37-39 (33) Brooks, D. E. J. Colloid Interface Sci. 1973, 43, 687. (34) Lyklema, J. A. J. Pure Appl. Chem. 1976, 46, 149. (35) Garvey, M. J.; Tadros, Th. F.; Vincent, B. J. Colloid Interface Sci. 1976, 55, 440. (36) Blazek, A. Thermal Analysis; Van Nostrand, Reinhold: New York, 1972; p 176. (37) BeMiller, J. N. In Starch Chemistry and Technology; Whistler, R. L., Paschall, E. F., Eds.; Academic Press: New York, 1965. (38) Zaidi, S. A. H.; Mahdihassan, S. Pak. J. Sci. Ind. Res. 1963, 6, 103. (39) Williams, R. M.; Atalla, R. H. In Solution Properties of Polysaccharides; Brant, D. A., Ed.; ACS Symposium Series, 150; American Chemical Society: Washington, DC, 1981.
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Figure 12. (a) Dissolution of magnesium from talc as a function of time. (b) Dissolution of magnesium from talc as a function of pH.
Figure 13. Effect of pH on the residual concentration of dextrin and magnesium.
From Figure 12b, it is evident that the dissolution of magnesium decreases with increase in pH. The tendency for precipitation of magnesium with the increase of pH, especially in the alkaline range, leads to lower magnesium content in solution. The dissolution of magnesium from calcined talc as a function of pH is also shown in Figure 12b. It is evident that a lesser amount of magnesium is solubilized from the calcined sample in the pH range investigated. The structural changes brought about by calcination, namely, transformation of brucite to magnesium oxide, possibly contribute to the observed effect. The interaction of the polymer with talc at the solid/ solution interface may be correlated with the metal ionpolymer interaction in the bulk solution. With this in view, coprecipitation tests were conducted with dextrin and magnesium ions as a function of pH and the results are shown in Figure 13. It is evident that the maximum pH for coprecipitation for the magnesium-dextrin system is close to 12. An identical behavior is observed in the variation of magnesium content as a function of pH, in the case of the magnesium-dextrin system and magnesium ions present alone. Thus the pH range for coprecipitation of dextrin and magnesium coincides with the range where magnesium hydroxide appears in the solu-
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Table 3. Discrepancies from Additive Values of Conductivity for the Mg2+-Dextrin System dextrin concn (ppm)
Mg2+ concn (M)
pH
discrepancy from the additive values (%)
400 400 400 400
10-3 10-3 10-2 10-2
4.8 9.9 4.9 9.9
-10.7 -25.7 -1.4 -6.2
Table 4. Relative Viscosities of Guar Gum and Dextrin as a Function of pH and Mg2+ Ion Concentration polymer concn (400 ppm)
Mg2+ concn (M)
pH
relative viscosity
guar gum guar gum guar gum guar gum dextrin dextrin dextrin dextrin
10-3 10-3 10-2 10-2 10-3 10-3 10-2 10-2
2.1 9.9 2.0 9.6 2.3 9.8 2.3 9.5
1.50 1.58 1.52 1.59 1.29 1.34 1.32 1.38
tion. It is interesting to observe that while there is no change in dextrin concentration as a function of pH, there is a slight decrease in dextrin concentration in the magnesium-dextrin system up to about pH 10 and a sharp decrease in the dextrin concentration is observed thereafter. This further testifies to the magnesium-dextrin complex formation and is in close agreement with the results of interaction of various metal ions with dextrin.40,41 Similar results were obtained for the coprecipitation tests between guar gum and magnesium ions as a function of pH, as reported earlier.15 Conductivity Measurements. The results of conductivity measurements carried out with dextrin containing varying concentrations of magnesium at different pH are illustrated in Table 3. In all the cases, negative deviations from the additive values of the conductances are observed. Davidson has suggested that in alkaline solution, the hydroxyl group in polysaccharide could ionize; when complexation with metal ions occurs, there is an associated change in the conductance of the solution.42 Earlier studies carried out on the guar gum-magnesium system also showed deviations in the additive values of the conductances.15 Viscosity Tests. The relative viscosities of guar gum and dextrin as a function of pH and magnesium concentration are summarized in Table 4. It is evident that they increase with increase in pH from 2 to 9 as well as magnesium ion concentration from 10-3 to 10-2 M, presumably due to magnesium-polymer interaction. Adsorption Mechanisms. Based on the studies carried out at the talc-solution interface and in the bulk solution, the possible mechanisms of adsorption of dextrin and guar gum onto talc can be explained as follows: (a) The dissolution of talc releases magnesium ions, particularly in acidic solutions, which could form neutral or charged magnesium hydroxo complexes. The hydroxo complexes could react with the polymer both at the hydroxylated talc surface as well as in the bulk solution by hydrogen bonding and chemical interaction. (b) Polymer could directly interact with the hydroxylated talc surface by hydrogen bonding, especially at alkaline pH values. At alkaline pH values, the metal hydroxide (40) Liu, Q.; Laskowski, J. S. J. Colloid Interface Sci. 1989, 130, 101. (41) Liu, Q.; Laskowski, J. S.; Li, Y.; Wang, D. Int. J. Miner. Process. 1994, 42, 251. (42) Davidson, E. A., Carbohydrate Research; Rinhert, H., Ed.; Winston: New York, 1967.
precipitate on the talc faces could interact with water molecules through hydrogen bonding. (c) The adsorption process could also be brought about by chemical interaction with the magnesium ions released, particularly at acidic pH. In this study, the two polymers chosen, namely, dextrin and guar gum, have shown different adsorbabilities onto talc and consequently exhibit differences in the magnitude of depressant action. The higher adsorption density and better depressant action of guar gum may be attributed to the more favorable cis-configuration of the hydroxyl groups, as opposed to the trans-configuration in dextrin, apart from the differences in their molecular weights. The various adsorption tests with different particle sizes have significantly revealed that greater adsorption takes place on the hydrophobic faces vis-a`-vis the edges with metallic magnesium sites contributing to enhanced adsorption. These findings are in line with the results of adsorption of dextrin on lead-coated methylated quartz.43 Thus it is not the hydrophobicity of the matrix per se but the metallic sites that contribute to enhanced adsorption. Conclusions From the results of this investigation, the following conclusions can be drawn: 1. Adsorption measurements indicate that the adsorption densities of guar gum and dextrin onto talc are independent of pH and the adsorption isotherms follow Langmuirian behavior. Higher adsorption densities onto talc are exhibited by guar gum compared to dextrin, due to the more favorable cis-configuration of hydroxyl groups in guar gum, in addition to its higher molecular weight. 2. Talc samples treated with EDTA to complex the magnesium ions show decreased adsorption density, highlighting the importance of magnesium sites for the interaction with the polymer. 3. The samples subjected to calcination exhibit decreased adsorption density attesting to the contribution of the brucite structure of talc surface in enhancing the adsorption process. A marginal decrease in adsorption density is observed at acidic pH in the case of leached talc sample due to loss of magnesium sites. 4. Talc sample initially subjected to leaching at acidic pH, followed by calcination, showed a pronounced decrease in the adsorption density of guar gum reinforcing the importance of magnesium sites and brucite structure in accentuating the adsorption process. 5. Adsorption densities decrease with decrease in particle size, due to decrease in the surface face-to-edge ratio in the finer fractions. 6. Floatability of talc is depressed to a greater extent by guar gum compared to dextrin and is independent of pH, complementing the adsorption results. 7. Electrokinetic experiments show that the macromolecules become fully extended with the loading at the talc surface driving the shear plane further away from the interface. 8. Greater dissolution of magnesium ions takes place from talc at acidic pH, while complexation in solution is observed at alkaline pH values. 9. Coprecipitation, conductivity, and viscosity studies confirm polymer-magnesium complex formation in the bulk solution. 10. The adsorption mechanisms are postulated to be governed by hydrogen bonding and chemical interaction. LA970518P (43) Liu, Q.; Laskowski, J. S. Int. J. Miner. Process. 1989, 26, 297.
