ARTICLE pubs.acs.org/IECR
Sugar Decolorization through Selective Adsorption onto Functionalized Accurel Hydrophobic Polymeric Support Kaman Singh,* Ram Bharose, Vimalesh Kumar Singh, and Sudhir Kumar Verma Surface Science Laboratory, Department of Chemistry, University of Lucknow, Lucknow-226007, India ABSTRACT: The functionalized polypropylene-based hydrophobic Accurel products were probed for their use in sugar decolorization. The pore distribution of Accurel products falls between the macroporous and mesoporous domains. Their specific surface areas are typically in the range SBET = 2035 m2/g. Commercial decolorizing activated charcoal and ion-exchange resin (Indion 803-S) were chosen for comparison with the Accurel products. The adsorption capacities of the characterized adsorbents were found to be comparable to those of adsorbents currently used for sugar refining. Geometrical optimization was performed at the density functional theory (DFT) level to gain insight into the mode of interaction between the quaternary ammonium cation containing methyl groups attached to the nitrogen center. The alkyl chain can be assumed as the bulky surface on which the quaternary ammonium nitrogen was attached. The electrostatic potential surface of the ammonium cation indicates that methyl groups are bulky enough to resist the attack of the carboxylate anion of the phenolics on the nitrogen center. However, there are possibilities for hydrophobic interactions between the methyl functional group and phenolic acids. Urea, a hydrogen-bond breaker, reduced the decolorizing efficiency, suggesting hydrophobic adsorption. In this article, we describe a new sugar decolorization process for application in sugar refineries. It is our view that the information contained herein will be useful for designing sugar decolorization units.
’ INTRODUCTION The color in sugar juice arises from a complex mixture of organic compounds originating from their basic sources (beet and cane). The color in raw sugar comes from phenolics and flavonoids, as shown in Figure 1, amounting to approximately to 90% of the total color.1,2 Farber and Carpenter2 reported 21 phenolic compounds in the cane plant, 10 in raw sugar, and 4 in refined sugar. In the presence of phenol oxidase,1 the phenolic compounds are oxidized to quinones (red color), subsequently producing indole polymers and/or melanin. In fact, polyphenols are considered as the whole realm of color formation during sugar processing. Melanodins and caramel-type3 colors are very common at all stages of sugar processing. Color is also formed by the alkaline degradation products (ADPs) of hexoses4 at alkaline pH and higher temperature. In addition, moisture and iron5 also contribute to the color. In the presence of moisture, polyphenols present in the form of humic acids react with iron naturally present in juice or introduced from the sugar processing equipment contribute to the color by complex formation. The complex origins of color have thus made it very difficult to develop a single, widely applicable decolorization system. Objective of Sugar Refining. Over the past several decades, many industries have advanced/updated their processes to meet the challenges and opportunities of the 21st century. However, conventional processes are still the norm in the sugar industry. With increasing energy costs and pressing environmental issues related to the production of refined sugar, it is imperative that sugar technologists undertake a critical reevaluation of the old decolorization technologies with respect to their effectiveness. Specifically, these include mechanical decolorization (affination process), primary decolorization (carbonation or phosphatation), and secondary decolorization r 2011 American Chemical Society
(bone char system, granular activated carbon process, ionexchange resin system, and powdered activated carbon system). Sugar refining involves purification of sugar to meet food safety requirements and customers demand.6 There are six groups of nonsugar components: suspended matters, color, ash, turbidity, microbes, and heavy metals. Food safety requirements mandate that microbes and heavy metals in the final sugar products must meet food-grade quality. Color is an important parameter not only in terms of routine process control but also in the evaluation of the quality of the refined sugar. The quantitative value of color is an indicator of various process steps during sugar processing such as milling (raw juice, 20000 IU), clarification (clear juice, 1200015000 IU), and end product (sugar crystals, 2000 IU) in the aqueous sugar feed stream. Ion-exchange decolorization despite, being an excellent process, suffers the disadvantage of requiring regeneration, associated with high costs of chemicals, water, and wastewater treatment. The classical method of removing colorants by affination and clarification7 is insufficient to produce the fine liquor necessary for refined sugar production. Hence, it is imperative to use an additional decolorization step. The use of granular activated carbon (GAC) is popular; bone char and activated charcoal have also become customary for this purpose, but they are expensive and their regeneration processes are relatively uneconomical. Furthermore, losses of carbon8,9 (4% per cycle) and sugar (26% based on the weight of carbon) are unavoidable. Ionexchange technologies1012 are applied in sugar refineries to remove large quantities of color, enabling the production of white sugar. This technique removes not only color but also salts, so it is a demineralizing process. However, such a technology has not been routinely applied within sugar mills. Furthermore, ion exchangers are associated with high costs of chemicals, water, and wastewater treatment. We describe herein an economical method for its use in sugar refining. Mechanism of Hydrophobic Interactions. The colorants and other high-molecular-weight nonsugar components in solution could be precipitated by adding cationic surfactants7 such as quaternary ammonium salts, as shown in Figure 2. Generally, moderately charged colorants form flocs that are not visible to the naked eye. Binding of the surfactant and sorbent phase to nonionic polymers occurs through hydrophobic adsorption, and a minimum hydrophobicity is essential for the polymer to be used. Hypothesis. The efficiency of any adsorbent material to adsorb the targeted compounds (color, ash, colloids) depends on several factors13,14 such as porosity, surface area, pore size distribution, bulk density, surface chemistry, hardness, pH, particle density, particle size, amount of water-soluble minerals, and total ash content. Each of these characteristics might be of
Figure 2. Mode of interaction of the surfactant with the colorant.
special importance depending on the desired use. In the case of sugar decolorization, the decolorizing material should have a pore size distribution that is favorable for the adsorption of a mixture of polydisperse constituents from highly concentrated sugar solutions, including the pH-sensitive sugar colorants such as phenols, phenolic acids, and flavonoids. Because 90% color in sugar solution is due to these compounds, a satisfactory approach to eliminate phenolics from their aqueous solution is to employ a positively charged polyelectrolyte cationic (PEC) surface that, when it comes into contact with phenolics (which are usually anionic15,16 in nature), can initiate precipitation under suitable pH conditions. Such precipitates are water-insoluble and can easily be skimmed off. A unique substance that has been reported17 as an excellent adsorbent of all organic compounds is polypropylene. Accurel is the registered trade name given to a family of polypropylene and polyethylene products offered by Membrana GmbH (Obernberg, Germany). These products are also reported1820 to be excellent carriers. From the manufacturer’s reported porosity (73 ( 2% void volume) of the aforesaid Accurel products, we concluded that they could be very good adsorbents for sugar refineries on an industrial scale. Scanning electron micrographs of these products enhanced our confidence in the manufacturer’s claim.
’ MATERIALS AND METHODS The Accurel MP products were generous gifts by Accurel System, Membrana GmbH, Obernberg, Germany. Cetrimide (Sigma) cationic surfactant was used as a loading agent. The activated charcoal, Darco (reference carbon), was purchased from Sigma, and ion-exchange resin (Indion 830-S) was a gift from Ion Exchange India Ltd. (Mumbai, India). Preparation of Surfactant-Loaded Accurel Samples. Surfactant was loaded onto Accurel samples by a solvent evaporation technique. The surfactant dissolved in alcohol was poured into a beaker, and Accurel was added. The mixture was stirred very gently at intervals under ambient conditions until the solvent evaporated, and the sorbent was then dried. Alternately, Accurel products were wetted with ethanol (3 mL per 100 mg of Accurel), and then most of the ethanol was removed under vacuum. The supernatant liquid was added to the wetted Accurel and stirred at room temperature for 15 h and then filtered. The resulting residue was dried for 3 days over silica gel desiccators. Characterization of New Adsorbent. Fourier Transform Infrared (FT-IR) Spectroscopy. A 2-mg sample of each pure and
preloaded Accurel was mixed with 200 mg of KBr and then pelleted. FT-IR spectra of the pellets were recorded using a PerkinElmer Precisely FT-IR spectrophotometer (model Spectrum BX). 10075
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Table 1. Comparative Adsorption Characteristics of Adsorbents type
a
particle size (μm)
average pore diameter (μm)
porositya (%)
SBET (m2/g)
Accurel MP 1000
550
0.015
73 ( 2
30
Accurel MP 1001
442
0.014
73 ( 2
35
Accurel MP 1002
195
0.003
73 ( 2
21
Accurel MP powder
376
0.0028
73 ( 2
27
ion-exchange resin
0.05
20
209
charcoal
0.110
0
50809
activated carbon
0.01
38
100020009
Internal method (manufacturer’s data).
Figure 3. FT-IR spectrum of Accurel MP powder.
The FT-IR spectra of reference carbon and ion-exchange resin Indion 830-S were also obtained for comparison. Scanning Electron Microscopy (SEM). The particle morphologies of the Accurel products were studied using a scanning electron microscope (LEO 430, Cambridge, U.K.). Samples were mounted on an aluminum stab with double-sided tape. Mounted stabs were coated with gold and palladium prior to analysis using a Polaron sputter coater. N2 and H2 Adsorption. Nitrogen and hydrogen adsorption/ desorption isotherms were obtained using a Beckmann Coulter SA 3100 surface area analyzer at 77 K. Prior to measurements, all samples were degassed at 398 K for 4 h. Development of Experimental Setup. Batch-Mode Adsorption. Experiments involving the relevance of sorbent particle size were performed using cationic surfactants loaded onto Accurel supports to test their comparative sorbent capacities. Sugar solutions of 30 °Bx (degrees brix) were prepared, and the pH of each solution was maintained at pH 7.0 using 3-(N-morpholino)propanesulfonic acid (MOPS) buffer. These solutions were then taken into four flasks, Accurel (2.0 g) was added to each flask, and the solutions were shaken for 2 h. These solutions were then filtered (0.45 μm), and their absorbance spectra were recorded at 420 nm with a UVvis spectrophotometer (double-beam UVvis spectrophotometer model UV 5704SS). The absorbance data were converted into color values using the equation given elsewhere.21 Single-Column Experiments. The supports listed in Table 1 were packed into a glass column of 2.25-cm internal diameter to form a bed volume of approximately 35 mL. The surfactant for each test was loaded onto the supports by pouring 50 mL of
a 5% (w/w) aqueous solution of the surfactant at the top of the column and allowing the solution to pass through a bed of 5 g of Accurel powder. Sugar solutions of known color were passed downward through the column for each test run. Multiple-Column Experiments. Column experiments are considered more important for all practical standpoints. Therefore, decolorization trials were performed using different bed volumes and varying feed colors. Quality Assurance (QA) and Quality Control (QC). A bed column containing 250500 mg of Accurel and the surfactant was held for stabilization for 810 h. The bed was then rinsed with doubly distilled water. Color and turbidity were measured according to the officially accepted ICUMSA protocols.21 Triplicate samples were examined, and reproducibility was checked precisely. Sugar solutions of known color were run through the column/bed, and color was again determined. Turbidity is considered undesirable in direct consumption of white sugar, and thus, it is one of the most important parameters22 for determining the quality of sugar products. Regeneration. The saturated bed material can be regenerated easily using NaOH brine solutions, as in the regeneration of ionexchange columns. The sorbent bed was first flushed with 2.5 BV of water to remove the sugar from the bed. The bed was next flushed with 1.5 BV of an aqueous solution containing 5% NaCl and 0.2% NaOH. The bed was then rinsed with 2.5 BV of water.
’ RESULTS AND DISCUSSIONS Characterization of New Adsorbent. FT-IR Spectroscopy. The FT-IR spectra of preloaded and loaded Accurel products 10076
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Figure 4. SEM images of Accurel MP 1000 (
