2474
Ind. Eng. Chem. Res. 1999, 38, 2474-2481
Separation of Hydroxycitric Acid Lactone from Fruit Pectins and Polyhydroxyphenols on Polybenzimidazole Weak-Base Resin M. Chanda† and G. L. Rempel* Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Polybenzimidazole (PBI) free-base resin has been used for selective sorption and recovery of hydroxycitric acid lactone (HCAL) from aqueous solutions containing also significant proportions of polyhydroxyphenols and fruit pectins, because the study has relevance to the problem of separation and recovery of HCAL, a potent antiobesity substance, from aqueous extracts of Garcinia cambogia fruits, grown largely in coastal areas of South India. PBI resin has the saturation sorption capacity of 315 mg/g dry resin for HCAL, compared with 131, 138, and 293 for catechol, pyrogallol, and pectin, respectively, in individual sorptions from aqueous solutions. The resin selectivity for HCAL over catechol, pyrogallol, and pectin in binary sorptions varies with pH, the separation factor of HCAL being maximum over catechol and pyrogallol at a pH of 1.7-1.8 and infinite over pectin at pH < 1.8. Under vigorous agitation the initial uptake of HCAL is very fast with 30% of the equilibrium sorption taking place in 10 s, followed by a significantly lower rate, leading to an overall 75% attainment of equilibrium sorption in 30 min. In continuous column operations with PBI resin and influent containing HCAL, polyhydroxyphenols, and fruit pectins, a proper combination of relatively low flow rate, a relatively low substrate pH (1.7-1.8), and “dead-end” stripping with alkali, which involves use of less than the theoretical amount of stripping agent necessary for complete stripping, produces an excellent separation and good yield of HCAL from the mixed influent. Introduction Hoffman-La Roche, an international pharmaceutical company, and Brandais University researchers began investigating hydroxycitric acid (I) in the early 1970s. They found that the (-)-form of the acid was a potent and competitive inhibitor of citrate lyase, an enzyme found primarily in the liver which is vital in regulating fat metabolism.1 (-)-Hydroxycitric acid (HCA) inhibits lipogenesis, lowers the production of glycogen in the liver, suppresses appetite, and increases the body’s production of heat by activating the process of thermogenesis. In 1991, Citrin was introduced in the health food industry as the first all-natural plant extract standardized for (-)-HCA. This unique extract comes from premium hand-harvested Garcinia cambogia,2,3 popularly known as “Malabar Tamarind” and grown in the coastal regions of southern India. It has been described as a “weightloss wonder.”3
(-)-HCA was first isolated from the G. cambogia extract by Lewis and Neelakantan.4 An aqueous extract obtained by autoclaving the dried fruit rind with water was concentrated to a small volume and treated with alcohol to remove pectin. Neutralization of the filtrate with alkali and repeated treatment with absolute alcohol yielded the alkali salt of HCA as a pale yellow † On leave from Indian Institute of Science, Bangalore, India.
hygroscopic powder. Aqueous solutions of the alkali salt were passed through a cation-exchange resin bed for the recovery of the acid. However, the acid itself could not be crystallized because evaporation resulted in the formation of lactone (II). Hydroxycitric acid lactone (HCAL) was obtained in pure form and characterized by the determination of its physical properties.4 The principal components in the aqueous extracts of G. cambogia rind are HCA (as free acid and lactone), pectins, and polyphenols.5 Because all these components are acidic, with HCA having the lowest pKa value among them, a separation should be possible by the application of a weakly basic resin. Moffett et al.6 claimed such a process for the preparation of HCA concentrate from G. cambogia fruit rind containing 23-54 wt % free HCA, 6-20 wt % lactone of HCA, 0.001-8 wt % citric acid, and 32-70 wt % water, wherein free HCA, the lactone, and the citric acid constitute 94-99 wt % of total solutes dissolved in the water. The process uses conventional weak-base resins for concentrating HCA in sodium salt form and strong acid resin to regenerate the HCA as free acid. However, no elaborate studies have been made to compare the capacities of various weak-base resins for this separation and to evaluate the relevant kinetic parameters. Our earlier studies7,8 on the relatively new and unconventional weak-base resins, polybenzimidazole and poly(4-vinyl pyridine), have shown that they are capable of sorption of both carboxylic acids and simple phenols from aqueous solutions, the sorption, however, being strongly influenced by pH of the substrate and exhibiting some degree of preferences. In view of the considerable interest created recently in the use of HCA as an effective antiobesity drug, we undertook a systematic investigation of the possible use of such weakbase resins for the separation of HCA lactone from fruit
10.1021/ie980458y CCC: $18.00 © 1999 American Chemical Society Published on Web 04/21/1999
Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2475
pectins and polyhydroxyphenols in aqueous solutions so that the study may lead to a possible economic method of extracting HCA and its lactone directly from the aqueous extracts of G. cambogia fruits and rinds. The present article gives the results of such a study made with the commercial polybenzimidazole resin Aurorez of Hoechst-Celanese. Experimental Section Sorbent. The poly(2,2′-m-phenylene-5,5′-dibenzimidazole) sample polybenzimidazole (PBI) supplied by Hoechst-Celanese was Aurorez, a microporous spherical polymer having the following physical properties in the dry state9: bulk density, 0.2 g/mL; specific surface area, 20-35 m2/g; and swelling in water, 5%. The material was supplied as a wet resin having spherical beads of approximately 250-500 µm with 76% (w/w) moisture content. Because PBI, as noted above, has low bulk density and low degree of swelling in water, the high moisture content of the resin may be attributed to the water present in pores and voids.
Figure 1. Visible spectrum of vanadic acid-acetic acid solution (containing 0.2 g sodium metavanadate, 2.0 mL glacial acetic acid, and 8 mg HACL per 100 mL) with the reagent solution used as reference and the absorbance balanced to zero at 650 nm.
The PBI repeating unit (III) has two secondary nitrogens (>NH) and two tertiary nitrogens (dNs). The electron pair of the secondary nitrogen, being a part of the aromatic sextet, as in pyrrole, does not accept a proton readily. However, the electron pair of the tertiary nitrogen is readily available to an attacking proton. Therefore, considering only two nitrogens per repeating unit capable of accepting protons, the theoretical capacity is calculated to be 6.5 mequiv/g dry weight. However, the commercial PBI has the elemental composition C 74.1%, H 4.1%, N 16.4%, and O 5.6%. Assuming that all the nitrogen belongs to PBI, the theoretical capacity is calculated to be 5.9 mequiv/g dry weight. In comparison, the available capacity of the commercial resin, as determined by measuring its equilibrium uptake of protons in a medium of 0.1 N HCl, is 4.5 mequiv/g dry weight showing that about 24% of the active sites are not accessible for reaction. Sorbates. Because pure HCA was not available and is difficult to obtain, the extracts of G. cambogia fruit rinds commercially available as calcium salts were used to obtain HCA as lactone. The calcium salt supplied by Natural Remedies India, Bangalore, was dissolved in 2 N HCl and neutralized with calculated quantity of Na2CO3 to remove the calcium as carbonate, which also carried with it some insoluble resinous material. The filtrate, which had the dissolved sodium salts of Garcinia extracts, was treated with sulfonic acid resin Dowex 50W-X8(H+) to liberate free acids. The reddish brown filtrate was treated with activated charcoal and concentrated to a thick light brown syrup on a water bath. It was seeded with a few crystals of the lactone of HCA and left overnight in a desiccator. A light brown, crystalline material was obtained, and the yield was about 18% based on the starting calcium salt. For further purification the material was repeatedly extracted with ether, and the combined extracts were dried over anhydrous sodium sulfate. A considerable proportion of the color was removed in this way because it was ether insoluble. Ether was removed by distilla-
tion, and the syrupy mass thus obtained was heated as a thin layer on a water bath to remove traces of ether. Seeding with HCA lactone resulted in needle-shaped crystalline solid with a yield of about 12% based on the starting calcium salt of G. cambogia extracts. The crystalline solid obtained by the above-mentioned procedure was found to have an equivalent weight of 95 from alkali titrations, an elemental composition C 38.2%, H 3.1%, O 58.2%, in good agreement with C6H6O7 (C 37.9%, H 3.2%, O 58.9%) for HCAL, and a melting point of 175 °C in fair agreement with the reported value of 178 °C for the lactone.4 Being a arge γ-hydroxy acid, HCA is easily converted to the lactone on evaporation of its solution to recover the solid. Two polyhydroxyphenols used in the work, viz., catechol and pyrogallol, were >99% purity, obtained from Aldrich Chemical Co., Milwaukee. Pectin from natural fruit peel, available as Certo brand product of Kraft Canada, Don Mills, Ontario, was used. Its acid content from alkali titration was determined to be 1.26 mequiv/g dry weight. Analysis. A colorimetric method was used for measuring the concentration of HCAL in aqueous solution. To 1 mL of glacial acetic acid in a 50-mL volumetric flask was added 4 mL of 2.5% (w/v) clear aqueous sodium metavanadate solution. A measured volume of solution containing 4-30 mg HCAL was then added and the volume made up with water. (The sodium metavanadate was conveniently dissolved in water at 5060 °C, made up nearly to volume, allowed to cool, and then filtered.) A scan of the visible range spectrum (Varian Spectrophotometer Model Cary 219) after color development for 4-5 h at 25 °C showed absorption maxima at 500 and 570 nm (Figure 1). Absorbance was measured at any of these wavelengths after color development for 4 h at 25 °C, and a calibration curve was prepared using known concentrations of HCAL. For the estimation of pectin the anthrone method of Jermyn10 and the carbazole method of Dische11 were adopted with slight modifications. Both methods gave concurrent results. The anthrone and carbazole reagents were made by dissolving 100 mg of the respective material in 500 mL of 80% (v/v) sulfuric acid. An amount of pectin solution was added to the reagent such that the pectin concentration in the final solution was in the
2476 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999
range of 1-5 mg/100 mL. The absorbance was measured at 630 nm in the anthrone method and 530 nm in the carbazole method, after color development for 4 h at 25 °C. For estimation of catechol and pyrogallol, a colorimetric method based on color development with hydroxylamine - Fe(III) in aqueous solution was used. The reagent was made by dissolving 10 g of NH2OH‚HCl and 10 g FeCl3‚6H2O in 1 L of water and filtering. No adjustment of pH was made. An amount of phenolic solution was added to a definite volume (20 mL) of the reagent such that the concentration of phenol was 100500 µg in the final volume (25 mL) made up in a volumetric flask. Absorbance was measured at 630 nm for catechol and 460 nm for pyrogallol after color development for 4 h at 25 °C. Calibration curves were prepared using known concentrations of catechol and pyrogallol. Sorption Experiments. Sorption measurements were first made using HCAL, catechol, pyrogallol, and pectin individually and then with binary mixtures of HCAL with the other three sorbates. For equilibrium sorption measurements a small-scale dynamic contact between the sorbent and sorbate solution of specified composition was effected in tightly stoppered flasks at 25 °C on a gyratory shaker for 12 h. The extent of sorption was calculated from the residual concentration of the sorbate in the solution. A range of concentrations of the sorbates were used. The sorption was also measured at different pH values of the aqueous medium for the sorbates individually and in binary mixtures of HCAL and the other three sorbates. For determination of sorption kinetics wet sieved resin beads of a narrow size range were used. A rectangular basket made of polypropylene screen (0.45 mm opening) was used to hold the resin beads. The basket was fixed to the shaft of a rotor and rotated while the sorbate solution was brought into contact for a specified period. In this way, the sorbent would be instantly separated from the sorbate solution at any specified time. Dynamic contacts between the sorbent and the solution were effected at different stirring speeds using a low solution concentration of HCAL (2 mmol/L) to determine the minimum speed above which the kinetic values are not influenced by the degree of agitation and hence are not controlled by film diffusion. Sorption rates were always measured at stirring speeds much above this minimum. Column runs were conducted to study the performance of the PBI free-base resin in column operation for the separation and recovery of HCAL from mixtures containing polyhydroxyphenols and pectin besides HCAL. The study was aimed at determining the breakthrough capacity of the resin for HCAL sorption in column and the enrichment that can be achieved by the process. Results and Discussion Effect of pH. The effect of pH of the substrate on the equilibrium sorption capacity of PBI weak-base resin is shown graphically in Figure 2. The sorption falls at lower pH values in all cases. For phenols, this can be explained by the hydrogen-bonding mechanism (eq 1) of phenolic sorption on weak-base resin,7 and for carboxylic acids by the acid-base interaction of the basic resin with the acid, forming a complex with carboxylate anion as the associated counterion (eq 2):
Figure 2. Sorption isotherms of HCAL, catechol, pyrogallol, and pectin on PBI free-base resin in mildly acidic media: HCAL (pH 2.2-2.5); catechol (pH 4.2-4.6); pyrogallol (pH 3.7-4.3); pectin (pH 2.3-2.6). Resin loading 40 g (wet)/L. Temperature, 25 °C.
PhOH + N h E h PhOH‚‚‚N h E
(1)
where the overbar indicates the resin phase.
RCOOH + N h E h RCOO-‚‚‚HN+ E
(2)
Although a decrease in pH would increase the concentration of the undissociated HCAL, thus favoring the forward reaction in eqs 1 and 2, a decrease in pH in the acidic range would also reduce the amount of free-base form of PBI and thus a reduction in sorption by the mechanism above. The sorption is relatively unaffected by decreasing pH (Figure 2) when these two effects balance each other. The fact that the sorption decreases at still lower pH values may thus be attributed to the predominance of the second effect over the first at relatively low pH. Although the sorption generally decreases at lower pH, the sorption of HCAL remains significantly high even at pH 1.5 where the sorptions of phenols are reduced by about 75% and that of pectin to 0. Thus an effective separation of HCAL from the other sorbates would be achieved by appropriately controlling the pH of the substrate. Sorption Isotherm. The equilibrium data for the sorption of HCAL, catechol, pyrogallol, and pectin on PBI free-base resin are plotted against equilibrium sorption concentration in Figure 3. The results show a wide variation in the sorption magnitude of the chosen sorbents, the least acidic catechol (pK1 ) 9.45, pK2 ) 12.8) having also the smallest sorption on the weakly basic resin, PBI (pK1 ) 7.5 of benzimidazole), and the most acidic HCAL (pK1 ) 3.14, pK2 ) 4.77 for citric acid) having the highest sorption. Pectin, which is a group of polysaccharides, mainly partially methylated polygalacturonic acids, has significantly higher sorption than the less acidic pyrogallol (pK1 ) 9.0, pK2 ) 11.2, pK3 ) 14.0). At low concentrations, HCAL has nearly an order of magnitude higher sorption than the other sorbates indicating that a good separation in column operation should be possible. The Langmuir isotherm equation fitted well to the sorption data of all the four sorbates. The parameters
Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2477
Figure 3. Effect of pH on the sorption of HCAL, catechol, pyrogallol, and pectin on PBI free-base resin. Initial concentrations: HCAL, catechol, and pyrogallol, 50 mM; pectin, 10 g/L. Resin loading, 80 g (wet)/L. Temperature 25, °C. Table 1. Langmuir Isotherm Parameters for Sorption of HCAL, Catechol, Pyrogallol, and Fruit Pectin on PBI Free-Base Resin Langmuir isotherm, eq 1 sorbate
As, mg/g of dry resin
Kb, L/g
r
HCAL catechol pyrogallol pectin
315 131 138 293
2.12 0.32 0.53 0.23
0.999 0.992 0.996 0.999
of the Langmuir isotherm
C* 1 C* ) + x* KbAs As
(3)
where x* is the equilibrium sorption (mg/g dry weight), C* is the equilibrium sorbate concentration (g/L), As is the saturation sorption capacity (mg/g dry weight), and Kb is the binding constant (L/g). The values of As and Kb were determined by least-squares fit. These are presented in Table 1. HCAL has 4 to 9 times higher binding constant than the other sorbates. This is reflected in high sorptions of HCAL on the resin even at very low concentrations. Pectin is seen to have a relatively high saturation sorption (As ) 293 mg/g dry resin), compared with HCAL (As ) 315 mg/g dry resin) despite having a very low acid value of 1.26 mequiv/g dry weight, compared with 10.5 mequiv/g dry weight for HCAL. This would be expected because of the higher molecular weight of pectin. Selectivity for HCAL. The selectivity of PBI freebase resin for HCAL over catechol, pyrogallol, and pectin in acidic media was determined by calculating the separation factor RAB, defined by
RAB )
x/A × C/B x/B × C/A
(4)
where x* and C* represent the equilibrium sorption (mg/g dry resin) and equilibrium concentration (g/L),
Figure 4. Selectivity of sorption of HCAL (A) relative to catechol, pyrogallol, and pectin (denoted as B) in binary mixtures at different pH levels of the substrate. Concentration of each component in mixture, 4 g/L. Resin loading, 40 g (wet)/L. Temperature, 25 °C.
respectively; A represents HCAL and B any of the other three sorbents, catechol, pyrogallol, or pectin. The equilibrium sorption of HCAL on PBI was measured at different pH levels of the substrate, using binary mixtures of HCAL and one of the other three sorbates, each having an initial concentration of 4 g/L. The separation factor values calculated from eq 4 are plotted in Figure 4 as a function of pH. All values are significantly greater than 1, which indicates that good separation of HCAL from the other three sorbates can be achieved at all pH levels. The HCAL/catechol selectivity is nearly independent of pH until about 1.8, below which the selectivity drops relatively rapidly, whereas HCAL/pyrogallol selectivity is maximum at pH 1.7. The HCAL/pectin selectivity increases very rapidly below pH 1.9, becoming nearly infinite at pH 1.8. This agrees with the effect of pH on pectin sorption presented in Figure 2, which shows no sorption of pectin below pH 1.8. A substrate pH level of 1.7-1.8 would thus appear to be suitable for recovery of HCAL free of pectin. Sorption Rate Behavior. For comparing sorption rates of the four sorbates chosen for this study, the sorption was measured as a function of time under similar conditions. Sorption experiments were performed at stirring speeds (200-300 rpm) much greater than the experimentally determined minimum for elimination of film diffusional resistance. The results plotted in Figure 5 show that, although HCAL has the highest sorption rate, a common feature for all the four sorbates is the rapid occurrence of a significant percentage (3035%) of the respective equilibrium sorptions within 10 s, which is followed by a slow and declining rate of sorption. The very rapid initial rate may be attributed to the characteristic morphology of PBI microporous beads which, as noted before, have a rigid matrix and about 75% pores and void volume, filled with water. Although all sorption experiments were performed at stirring speeds much greater than the minimum for elimination of film diffusional resistance, the so-called “interruption test”, which is described as the best technique for distinguishing between particle and film diffusion control,12 was used with HCAL as the sorbate. The basket reactor used was especially suitable for such
2478 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999
fractional attainment of equilibrium sorption. Such concentration independence is not in accordance with the shell-core or ash-layer diffusion model, but is consistent with predictions from the ordinary pdc model.12 An ordinary pdc model was used therefore to fit the kinetic data of HCAL sorption on PBI (Figure 6). With particle diffusion rate controlling, the fractional attainment of equilibrium X as a function of time t can be described by Barrer’s integration13 of Fick’s flux equation in an “infinite solution volume (ISV)” case:
X(t) ) 1 -
Figure 5. Rate of sorption of HCAL, catechol, pyrogallol, and pectin on sieved PBI resin of bead size 0.46-0.58 mm with similar initial sorbate concentrations (HCAL, catechol, pyrogallol, 20 mM; pectin, 4 g/L) and resin loading, 20 g (wet)/L in mildly acidic media (HCAL, pH 2.4; catechol, pH 4.6; pyrogallol, pH 4.3; pectin, pH 2.6) at 25 °C.
6
∞
∑n
π2n)1
-2
( )
exp -
π2n2Dt
(5)
r20
The terms r0, D, and n in eq 5 represent the average radius of resin bead, resin diffusivity, and integer numbers, respectively. In terms of the “equivalent ratio”, w, defined as the ratio of the total sorption capacity of the resin to the total sorbate content of the external solution, finite solution volume experiments can be used to approximate ISV conditions13 for w , 1 mequivr/mequiv. Because the kinetic data of Figure 6 were obtained with experimental conditions such that w < 0.3, ISV conditions would be reasonably well approximated, especially in relation to low conversion ranges. To a good approximation, eq 5 can be simplified to
[ ( )]
X(t) = 1 - exp -
π2Dt
1/2
r20
(6)
from which the particle diffusion coefficient D is approximately obtained as
D ) -0.233r20t-1 log[1 - X2]
Figure 6. Rate of sorption of HCAL on sieved PBI resin of bead size (diameter) 0.46-0.58 mm in HCAL solutions of different concentrations, Co, at pH 2.0. Resin loading, 20 g (wet)/L; temperature, 25 °C with vigorous agitation.
tests. The sorbent basket was removed from the sorbate solution for a brief time (5 min) and then reimmersed. The interruption caused a change in the momentary sorption rate indicating particle diffusion control (pdc) of sorption rate under the experimental conditions used. The effect of external concentration of HCAL on its sorption rate is shown in Figure 6. About 30% of the equilibrium sorption in each case takes place within 10 s, whereas t1/2 is about 3 min. This suggests that the initial rapid uptake may be caused by migration of the sorbate, under vigorous agitation, into the pores which make up as much as 75% of the total volume of the resin bead. The initial rapid uptake is, however, followed by a slow and decreasing rate of uptake with 75% of the equilibrium sorption being attained in 30 min. Figure 6 also shows that the external solution concentration of HCAL has practically no effect on the rate of
(7)
With use of the data of Figure 6, eq 7 yielded values of 1.0-2.0 × 10-7 cm2/s in the conversion range 3060%. The data below 30% conversion were not used because, as discussed earlier, the initial uptake was believed to involve a significant extent of bulk diffusion into the pores constituting about three-fourths volume of a PBI resin bead. The diffusivity decreased significantly at higher conversions, decreasing to a value of 3.7 × 10-8 at 75% conversion of the resin bead. This may be attributed to Donnan exclusion effect which becomes more important at higher resin conversions because of the sorption of HCAL by an acid-base interaction mechanism (eq 2). Stripping Behavior. The sorbates, HCAL, polyhydroxyphenols, and pectins are easily and rapidly sripped by alkali. However, the relatively large difference in their acidity, especially between HCAL and phenols, offers an interesting possibility of differential or selective stripping of the sorbates from the same sorbent. This is visibly demonstrated when a PBI resin sample that has been preloaded with HCAL and catechol or a mixture of PBI resins, which have been separately preloaded with HCAL and catechol, is treated with an insufficient amount of NaOH. As soon as the resin is added to the alkali solution, the solution turns green because of the instantaneous stripping of catechol, which would be expected in view of the phenolic species having a significantly lower binding constant than
Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2479
Figure 7. “Dead-end” stripping. Percentage of stripping of cosorbed HCAL and catechol plotted against percentage of stoichiometric quantity (based on HCAL) of the stripping agent used.
HCAL (Table 1). However, if the amount of alkali is less than what is required for complete stripping of HCAL, the green color soon disappears and the pH of the solution drops, indicating that catechol has been readsorbed and HCAL has been stripped from the resin. We have termed this type of selective stripping, performed with a limited or insufficient quantity of strippant to effect separation of sorbed species differing in binding to the sorbent, as “dead-end” stripping. To test the principle of “dead-end” stripping, several stripping experiments have been conducted using a mixture of preloaded PBI resins so that the mixture has 1:1 mole ratio of sorbed HCAL and catechol. After stripping the mixed resins with different quantities of NaOH, under equilibrium, the extent of stripping is measured for each of the sorbed species. From the results plotted in Figure 7, it is evident that by stripping under equilibrium with less than the stoichiometric amount of the stripping agent (NaOH), HCAL can be separated from the cosorbed catechol. However, although the HCAL stripping closely follows the theoretical line, the extent of catechol stripping is much less than the theoretical. This may be attributed to the oxidative degradation of catechol which is clearly evident from the pronounced color changes (green turning to very dark brown) in alkaline media in the presence of air. The colored oxidized product binds more strongly to the resin. Column Operation. The performance of the PBI free-base resin in continuous operation was studied by conducting column runs. Figures 8 and 9 show typical breakthrough curves for HCAL, catechol, and pectin with a 43 cm3 resin column and an aqueous effluent containing HCAL, catechol, and pectin in 2 g/L concentration each. The runs were performed with different flow rates and different pH values of the influent. When a relatively low flow rate is used (Figure 8), the boundary between the used (yellowish) and unused (grey) portions of the resin bed appears quite sharp as it moves along the column with progressive exhaustion of the bed and touches the bottom line, coinciding closely with the appearance of the breakthrough. Figures 8 and 9 show that distinctly separate breakthroughs are obtained for HCAL, catechol, and pectin,
Figure 8. Breakthrough curves of PBI column with relatively high influent pH and relatively low flow rate. Volume of resin bed (wet), 43 mL; resin weight, 32 g (wet); column inside diameter (i.d.), 1 cm; resin bed height, 54 cm.
Figure 9. Breakthrough curves of PBI column with relatively low influent pH and relatively high flow rate. Volume of resin bed (wet), 43 mL; resin weight, 32 g (wet); column i.d., 1 cm; resin bed height, 54 cm.
the latter two appearing much earlier than HCAL, which indicate a significantly high level of sorption of HCAL, compared with phenolic species and pectin. A lower flow rate increases the distance of the breakthrough point of HCAL from those of catechol and pectin signifying a better separation of HCAL. Decreasing pH below 1.8, as seen from a comparison of Figures 8 and 9, prevents sorption of pectin and also reduces the sorption of the phenolic species, thus ensuring lesser contamination of the resin bed with these sorbates. The HCAL sorption capacity at an influent rate of 4 bed volumes (BV) per hour and an influent pH level of 2.8 (Figure 8) was determined to be 46 mg HCAL/g wet sorbent, compared with the equilibrium sorption capacity of 58 mg HCAL/g wet sorbent. At a high influent rate of 8 BV/h and an influent pH level of 1.7 (Figure 9), however, the breakthrough capacity was found to be 32 mg HCAL/g wet sorbent.
2480 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999
Figure 10. Stripping of PBI column of Figure 8 according to “dead-end” stripping method.
The stripping performance of the PBI resin bed used in the column run of Figure 8 is shown in Figure 10. In accordance with the “dead-end” stripping principle, the concentration of NaOH in the stripping solution is fixed at less than the theoretical value calculated on the basis of the stoichiometric amount of NaOH for the sorbed HCAL and 1 BV. As can be seen from Figure 10, HCAL displays a sharper elution than the phenolic component (pectin in the effluent was not determined because of low concentration), with practically all the sorbed HCAL being eluted in 1.5 BV. The stripping produces a good separation of the two sorbates. The peak concentration of HCAL in the stripping effluent is about 16 times the influent concentration, whereas the average concentration is about 8 times that of the influent.
HCAL separation factors RHCAL catechol and Rpyrogallol being maximum at a pH of about 1.8, whereas RHCAL pectin becomes infinite at pH < 1.8 as pectin sorption drops to zero. A significant feature of the sorption rate behavior of PBI for all the sorbates is the very fast initial uptake amounting to 30-35% of the equilibrium sorption in each case. This is attributed to the characteristic morphology of the PBI resin bead which has nearly three-fourths of its volume as pores and voids filled with water that facilitates rapid physical transport of the sorbate in a vigorously stirred medium. The sorption rate after the initial fast uptake is, however, slow with 75% of the equilibrium sorption of HCAL, for example, taking place in 30 min. The sorption rate of HCAL on PBI is pdc and the sorption data fit well to the ordinary pdc model applicable under “infinite solution volume” conditions, yielding diffusion coefficient values of 1.0-2.0 × 10-7 cm2/s in the particle conversion range 30-60%. The diffusivity, however, decreases significantly at higher conversions, falling to a value of 3.7 × 10-8 cm2/s at 75% conversion of the resin bead. In column operation, the PBI free-base resin displays a large gap between the HCAL breakthrough and those of polyhydroxyphenol and pectin, both of the latter occurring very early in the run. This results in a good separation and selective loading of HCAL in a continuous column run. The considerable difference in acidity between HCAL and phenols also affords selective recovery of the sorbed HCAL by stripping with a limited volume of sodium hydroxide solution of relatively low concentration determined for “dead-end” stripping. In summary, a combination of relatively low influent flow rate, a relatively low pH (∼1.7) of the influent, and stripping with a limited amount of alkali, as determined according to the quantity of HCAL sorbed in the column, results in a high degree of separation and good recovery of HCAL from mixtures containing significant proportions of polyhydroxyphenols and pectin.
Conclusions
Acknowledgment
The possibility of using PBI weak-base resins for separation of HCAL from mixtures with polyhydroxyphenols and fruit pectins has been explored because it is relevant to the problem of recovery of the valued antiobesity substance, hydroxycitric acid, from the extract of G. cambogia fruit rind. Catechol and pyrogallol have been used as representative polyhydroxyphenols. The free-base resin has the saturation sorption capacity of 315 mg/g dry resin for HCAL, compared with 131, 138, and 293 for catechol, pyrogallol, and pectin, respectively. The differences in sorption are more pronounced at low concentrations (
