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Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 465-468 McClure, T. A.; Scantland, D. A.; Woodford, P. 0.;Gordon, W. A.; Kresovich, S.; Arthur, M. F.; Jackson, D. R.; Llpinky, E. S. Batteile Columbus Laboratories Report BMI-2054;Vol. 11, July 1980. McClure, T. A.; Scantland, D. A.; Woodford. P. G.; Gordon, W. A.; Kresovlch, S.; Arthur, M. F.; Jackson, D. R.; Lipinky, E. S. Batteile Columbus Laboratories Report BMI-2054;Vol. 111, December 1980. Schwartz, R. D.; Keller, F. A., Jr. Appl. Envlron. Microbial. 1982, 4 3 ,
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Webb, F. C. “Biochemical Engineering”; Van Nostrand: New York, 1964.
Received for review July 30, 1984 Revised manuscript received March 18; 1985 Accepted April 12, 1985
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1dRS-lRQ3
Vaughn, R. H. I n “Industrlal Fermentations”; Underkofler, L. A.; Hickey, R. J., Eds.; Chemical Publishing Company, Inc.: New York, 1954;Vol. I. Wang. D. I. C.; Fleishchaker, R. J.; Wang, G. Y. AICh€ Symp. Ser. 1978,
74(181),101-110.
The work reported here was supported by the U.S.D~~~~~~~ of Fd3‘al Highway Administration (FHWA) and the states listed in Table IV.
Synthesis of NaX Zeolite Using Silica from Rice Husk Ash Ajay K. Dalalt and Mus11 S. Rao* Department of Chemical Engineering, Indian Institute of Technolow, Kanpur 2080 16, India
K. V. G. K. Gokhale Deparfment of Civil Engineering, Indian Institute of Technology, Kanpur 2080 16, India
Sodium X zeolite has been synthesized by using rice husk ash as a source of silica for the first time. The synthesis has been carried out for temperatures ranging from 95 to 120 OC and duration of synthesis from 1 to 24 h with Si02/A1,03, Na,O/Ai,O,, and H,O/AI,O, molar ratios ranging from 2.0 to 7.0, 2.4 to 10.0, and 96.0 to 890.0, respectively. The roles of temperature, duration of synthesis, and composition of initial mix in the formation of NaX zeolite and the stability of NaX zeolite were studied. In the present study, it has been observed that the NaX zeolite can be synthesized from an initial mix with SiO2/Ai,O3 and Na,O/SiO, molar ratios varying from 3 to 5.5 and 0.75 to 2.22, respectively, and having H,O/Na,O molar ratios from 30 to 60 at 100 O C with reaction times of 6 to 8 h. The product at each stage has been characterized by X-ray diffraction analysis. Studies on the crystallization kinetics and morphological investigations for the formation of NaX zeolite were also carried out.
Introduction The typical oxide formula of NaX zeolite is Na20.A12O3-2.5SiO2.6H20.With the most open structure (50% void volume and 7.4-A dehydrated free apertures) and with a high Si/A1 molar ratio of 1 to 1.5, the NaX zeolite has extensive industrial application as a catalyst for a variety of reactions (Cross et al., 1971; Venuto et al., 1966a-c; Jones, 1966; Schwartz, 1979; Rabo, 1976). NaX zeolite is also used as a molecular sieve (Meier and Uytterhoeven, 1973). In the earlier work on the synthesis of zeolite X, the starting materials for silica and alumina have been from pure chemical sources (Barrer et al., 1959; Breck and Flanigen, 1968; Milton, 1959). Efforts also have been made earlier to use clay minerals like metakaolin (Howell and Acara, 1964). The concentration and kind of the reacting components in the initial mix is very important in determining the end product. As experiences in India reveal, rice husk is readily available in large quantities at an extremely low cost. Rice husk, on complete burning, yields a porous, cellular, light gray ash, which on analysis has been found to contain amorphous silica up to about 90 wt % . This silica, which can be extracted from the rice husk ash by a suitable alkali, such as sodium hydroxide, has been found to be very reactive for zeolite synthesis (Bajpai Present address: Production Section, ONGC, Chand Kheda, Ahmedabad 5, India. 0196-432118511224-0465$01,50/0
et al., 1981). In developing countries such as India, the possibility of using rice husk (an agricultural waste) is attractive economically for the synthesis of sodium zeolites. Mordenite-type zeolite has been synthesized by using this source of silica (Bajpai et al., 1981).
Experimental Section Materials. In the present investigation, rice husk ash, sodium hydroxide, aluminum hydroxide, and silica gel were used as the starting materials in the initial mix for the synthesis of NaX zeolite. Pellets of sodium hydroxide (supplied by ARC Industries, Kanpur) were used in the present study. Aluminum hydroxide in the form of fine powder was obtained from APEX Chemicals, Bombay. Silica gel of 98.59% purity (BDH) was ground in a ball mill to -200-mesh size. Rice husk ash obtained from a rice mill in Kanpur was burned in a muffle furnace at lo00 “C for 10 h, and the carbon-free ash obtained was ground in a ball mill to -200 mesh. The X-ray diffraction pattern of the ash revealed the presence of silica as a-cristobalite. NaX zeolite (13X zeolite) from Linde (US.)was used for comparison. Methods. The experimental procedure for the synthesis of NaX zeolite is quite similar to the procedure reported for mordenite synthesis (Bajpai et al., 1978, 1981). NaX was characterized by X-ray diffraction analysis, 1R spectra, thermal properties, surface properties, and morphology. Quantitative estimation of NaX zeolite in the end product in each run was made by comparing the X-ray I
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A. Si 02
(a) 95"C,6hr
(b) 100 O C , 6 h r
60
' A NaX I 0 Analcime,
( d ) Composite picture
y_ 10
Figure 1. Reaction composition diagrams for 4 and 6 h.
Figure 2. Reaction composition diagrams for 8 h.
diffraction pattern of the product with that of the Linde NaX zeolite. The associated zeolites formed during synthesis were found to be analcime, P,, hydroxysodalite, and 2-21 (Duecker et al., 1971) zeolites. The lattice parameters of NaX zeolite and associated zeolites were also estimated. Results and Discussion Effect of Starting Composition. The molar percentage of the initial mix was varied as follows: Na20, 38.1-64.5; A1203, 5.7-18.5; SiO,, 29.0-53.4; H20, 92.9-98.4; H20/Na20 (molar ratio), 23.0-104.7. To represent the stability fields of NaX zeolite as a function of the composition in the system Na20-A1203Si02-H20, triangular diagrams for the synthesis at temperatures of 95 "C (for 6 and 8 h), 100 "C (for 6 and 8 h), and 120 "C (for 4 and 8 h) were plotted (Figures 1 and 2). From the plots it is evident that an SiOz/A1,03 ratio of 3 to 5 in the initial mix requires an NazO/A1203ratio of 4 to 5 for the formation of NaX zeolite at constant temperature and time. Effect of Temperature. The stability field of NaX is indicated in the triangular plots (Figures la-c and 2a-c). To bring out the trend of change with increase in temperature, these plots were superimposed (Figures Id and 2d). The NaX zeolite starts crystallizing from an initial mix containing more SiOz and less A1203at lower temperatures, while at higher temperatures the crystallization starts with less Si02 and more A1203 in the initial mix. This shift in crystallization field for the NaX zeolite with an increase in the temperature of synthesis can be explained as follows: The concentration of silicate in the liquid phase of the gel is the main controlling factor in the formation of a zeolite (Zhdanov, 1971). As the temperature increases, the solubility of silicate ions increases, causing a shift in the concentration of the liquid phase of the mix. Thus, at low temperatures, lesser amounts of SiOzwill be required to maintain the desired concentration in the liquid phase of the mix to produce NaX zeolite. With an increase in temperature, the sequence of formation of the products is in the direction amorphous/Z-21 zeolite NaX zeolite P, zeolite/HS zeolite analcime. The conversion of NaX zeolite to either P, zeolite or HS zeolite depends mainly on the concentration of NaOH in the solution. In general, with a further increase in the temperature, the ultimate product from NaX zeolite is analcime. I t is found that HS, P,, and analcime zeolites
are the dominant stable phases at higher temperatures. For any mix, as the temperature increases up to 120 "C, the crystallization of NaX zeolite increases. The crystallization starts after about 3 h and increases in its degree with reaction time up to 6 h. At the end of 8 h, it converts to analcime along with other associated zeolites depending on the concentration of silicate and aluminate ions in the solutions. The present study established that for an initial composition of reaction mixture of 7.5Naz0.A1203.5.2Si02. 315H20, the best temperature range for the synthesis of NaX zeolite with rice husk ash as a source of silica was 95-120 "C. For varying compositions of initial mix with pure chemicals, the same has been reported to be around 25-120 " C (Breck, 1974). Effect of Time. For the effect of time on the synthesis of NaX zeolite, the batch composition 7.5Na20.A1,03. 5.2Si02.315H20was chosen. The X-ray diffraction patterns of the products on synthesis at 100 OC for the above batch composition are given in Figure 3. The percent of NaX zeolite crystallized a t different periods of time was estimated from the X-ray data. The X-ray diffraction study indicated the following trend in the development of phases during synthesis: amorphous NaX zeolite HS zeolite analcime. The crystallization curve for NaX zeolite at different times for the above batch composition is indicated in Figure 4. The curve is characterized by a long induction period followed by a rapid crystallization, ultimately reaching an asymptotic value. Thus, it is sigmoidal in shape. Once the crystallization starts, the first conversion rate of the amorphous batch into NaX zeolite indicates that the rate-limiting step in the overall process is the nucleation. The crystallization curve in the present study compares well in its nature to the one reported by Breck and Flanigen (1968). Pattern of Product Crystallization The morphological characteristics for the different species obtained at different time intervals are presented in Figure 5. Scanning electron microscopy enables the recognition of the characteristic morphological patterns of the zeolite species. The products were identified by X-ray diffraction (Figure 3). A t the end of 2 h, the X-ray diffraction pattern revealed the dominant amorphous nature of the constituents.
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30
20 . IS 28(dogreer)
10 CuK,
Figure 3. X-ray diffraction patterns of products after different reaction periods at 100 'C with batch composition: 7.5Na20. AI,0s5.2SiOr315H20.
P
d
r
Figure 5. Typical product formation from a mix of composition (7.5Na20~A1,0~5.2Si02~315H,0) at 100 " C (a, top) after 2 h, indicating reaction rims (SEM, 6000X); (b. middle) after 4 h, with NaX an the main product (SEM, 3000x); (c, bottom) after 6 h, with NaX 88 the only product (SEM, 5 4 0 0 ~ ) .
0
2
L
6
8
Time,hr
Figure 4. Crystallization curve of NaX zeolite with time.
Presence of reaction rims around particles (Figure 5a) has been evidenced in the micrographs. A t the end of 4 h, the development of NaX in the product is distinctly noticed by the presence of subhedral particles of this species exhibiting tvpical octahedral crystal morphology (Figure 5b), and NaX zeolite dominates in its percent in the product in the next 6 h (Figure 5c). The approximate pore diameter of the synthesized NaX zeolite was found to be 10 A by the preferential adsorption method using mixtures of organic compounds. For two pure NaX samples synthesized in the present work, the surface area has been estimated (by BET method) as 1370 and 1378 m2/g whereas the same for a standard NaX zeolite is reported to be 1390 m2/g. The water adsorption capacity for the NaX zeolite in the present m e was found
to be 29.4 w t %. NaX zeolite synthesized by using rice husk ash as a source of silica has a lattice parameter of 25.03 A, which is in agreement with the earlier reported data for the NaX species synthesized from pure chemicals (Breck, 1974; Dubinin, 1965). Registry No. SiO,. 7631-86-9. Literature Cited A m . L. L.. Jr.; Sand. L. 8. Am. Mbwal19511. 43. 478. Bajpi. P. K.; Rao. M. S.; Gokhale. K. V. Q. K. I n d . ET. chan.Rod. Res. h v . 1978. 17, 223. Balpal. P. K.; Wo. M. S.: Whale. K. V. 0. K. Ind. Ew. chan.Rod. Res.
De". 1981, 20. 721. Baner. R. M. J . Cl".SOC. l e a s .127. BanW. R. M.: Baynham. J. W.; BuI1IMI). F. W.;
.. '"ZdlleW u l a r
M. W. M. J. 0".SOC.
40" D.*, ,OG . - - _ , . " . I ."I.
Brsck. 0. W. s(sveS"; W y : New Y a k . 1974. heck. 0. W.: F l a n w . E. M. '"MolecularSlevea"; Society of Chw+al Irc dum: LoMon. 1968. p 47. Cross. N. E.; KemhlI. C.: Lead. H. F. A&. chan.Ssr. 1971. No. 102.
389. Lhbinin. M. M. Ah.. G%WiInscl. 1965. 2.2. DUBCkw. H. C.; W&. A,: Qana. C. R. U.S. Patent 3567372. 1971. Howell. P. A.;Acara.N. A.U.S. Pmml3111680. 1964. Jones. D. 0. U.S. P a l m 3231818. 1964. Mew. W. M.: UynemoeWn. J. 0. r"Slsves": A m n Chw+al Socletv: Washington. Dc. 1973; A b . Oam. Sw. 121. Mikm. R. M. U S . Patent 2882244. 1959. R s b . J. A. "Zedlie Chemloby and CalalysW American Chemkal S x M y : Wash-. Dc. 1976: ACS W a p h 171. p 621.
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Schwartz, A. 0. U S . Patent 4 174 272, 1979. Venuto, P. 6.; tlamllton, L. A,; Landis, P. S. J. Catal. 1966a, 5, 484. Venuto, P. 6.: Hamilton, L. A.; Landis, P. S.J . Catal. I066b, 6.253. Venuto, P. H.; Hamilton, L. A,; Landis, P. S.; Wise, T. T. J. Catal. ISSSc, 4 , 81. Zhdanov, S. P. "Some Problems of Zeolite Crystallization"; American Chem-
ical Society: Washington, DC, 1971; Adv. Chem. Ser. 101.
Received for reuieu, September 5 , 1984 Revised manuseript received February 19, 1985 Accepted March 16, 1985
Removal of Colloidal Silica in Simulated Seawater by a Dynamic Multi-Short-circuited Galvanic Cell Jan-E. Osterholm, Paul W. Kramer, and Hlrotsugu K. Yasuda' Graduate Center for Materials Research, University of Missouri-Rolla,
Rolla, Missouri 6540 1
A dynamic multi-short-circuited galvanic cell (MSCGC) with R-AI electrodes has been used to study the removal
of colloidal siHca from seawater as a pretreatment step for the desalination of seawater by use of reverse osmosis. The rate of s i b removal is observed to be affected by the flow rate of the solution through the cell and the dissolved oxygen content of the solution as well as by the nature of the aluminum electrode surface and the number of contact points (short circuits) between the aluminum and platinum. The total silica content is reduced by 95 % by use of this cell configuration.
Introduction In order to avoid membrane fouling during the reverse osmosis desalination of seawater or brackish water, it is important to remove effectively the membrane-fouling substances. unlike heavy contaminants and those which float, colloidal materials cannot be easily removed by conventional mechanical methods, e.g., filtration, settling, etc. These membrane-fouling substances are (Liss, 1975): (a) inorganic suspended solids and colloids, such as silica, talc, quartz, floc of oxidized iron and manganese, calcium sulfate, and carbonate (scale-forming constituents when their solubility limits are exceeded) and (b) organic nutrients which promote biological growth on membrane surfaces (such as polysaccharides, phosphates, proteins, and lipids). Colloidal silica was selected as a suitable model for the inorganic colloids that are capable of fouling reverse osmosis membranes for the following reasons. Silicon in seawater is found in solution as silicate ions, in suspension as silicon dioxide, both free and in diatoms and other living organisms, and as clay minerals. Its amount is affected by geochemical and biological events and shows a wider range of variation than that of any other element (Armstrong, 1965). Coventional methods of treating raw water include lime softening and coagulation followed by filtration. The coagulation or flocculation process to remove colloidal materials from water is ancient, dating to the Egyptians's use of almonds, beans, and alum as coagulators to clarify muddy water (Powell, 1954). The process of coagulation involves the addition of certain soluble metallic salts to the water whereupon coagulation takes place and gelatinous substances or flocs form. These then agglomerate to form larger flocs which can be removed by sedimentation or filtration. The principal role of the coagulant is to provide highly charged ions that will neutralize the electrical charges of the colloidal material, causing it to precipitate. Many materials are capable of coagulating suspended solids and colloids; however, aluminum and iron salts are the most widely used, e.g., aluminum sulfate, sodium aluminate, 0196-43211851 1224-0468$Q7.5O/O
ferrous sulfate, ferric sulfate, ferric chloride, etc. (Powell, 1954). A large amount of work on the effect of various factors on the coagulation process in water treatment has been published and is reviewed by Packham (1962a,b). One of the important results of several works (Peterson and Bartow, 1928; Miller, 1924; Buswell and Edwards, 1922; Black et al., 1933; Marion and Thomas, 1946) is that the effectiveness of a given coagulant occurs over a limited pH range. One of the difficulties of the above-mentioned coagulation process is that anions of the coagulant materials are introduced as contarhinants. In order to avoid this, Okamot0 et al. (1950) studied the use of dc or ac electrolysis with aluminum electrodes to prepare aluminum hydroxide sol for the removal of SiOzfrom underground water. The silica content was reduced from 45 ppm to 1.5 ppm with an electric power consumption of 60 kW to treat 5 tons of water/h. It was also found that pH control and violent agitation were necessary for effective operation. A similar study (Okamoto and Morozumi, 1953a) using iron electrodes instead of aluminum gave similar results for the SiOz variation. In further studies (Okamoto and Morozumi, 1953b, Okamoto et al., 1956) it was found that a rise in the electrolytic resistance occurred during the electrolysis, which could be controlled by the addition of electrolytes and agitation, and that this rise was caused by scale formation on the cathode. A recent study (El-Nokaly et al., 1985)showed that using a bimetallic, platinum-aluminum, galvanic cell without externally applied voltage reduced the silica content in simulated seawater by 93% (from 100 ppm to 7 ppm) in about 5 min. The process also had the advantage over adding aluminum salts directly in that the aluminum/ silicate ratio was much lower. These previous studies (Okamoto et al., 1950; Okamoto and Morozumi, 1953a; Okamoto and Morozumi, 1953b; Okamoto et al., 1956; El-Nokaly et al.) and the present one depend on the corrosion of the aluminum electrode to introduce aluminum ions into the solution. These alu0 1985 American Chemical Society
