Influence of Citric and Tannic Acids on Hydroxy-a1 Interlayering in Montmorillonite

-The formation ofhydroxy-Al-interlayered montmorillonite was affected by complexing organic acids. Montmorillonite (<2.0 t~m) was aged for three months at an initial pH of 5.0 or 6.0 in A1C13 solutions containing citric or tannic acid at organic acid/A1 molar ratios from 0 to 1.0. The A1/clay ratio in the system was 900 meq AP+/100 g of montmorillonite. Ion-exchange experiments revealed that organically complexed A1 ions have both positive and negative charges. Evidence from X-ray powder diffraction, electron microscopic examination, measurements of specific surface, cation-exchange capacity, organic carbon, and the nature of sorbed A1 indicates that citric and tannic acids influence differently the hydroxy-A1 interlayer formation in montmorillonite. Hydroxy-Al-citrate can be adsorbed as interlayers in montmorillonite, but hydroxy-Al-tannate exists principally as a separate phase binding the clay particles. The differences observed between the influence of citric and tannic acids on AI interlayering are probably due to their differences in molecular weight (size) and structure. Key Words--A1 polymers, Hydroxy-A1 interlayer, Ion-exchange resin, Montmorillonite, Organic acid, Specific surface. I N T R O D U C T I O N Aluminum interlayering o f expandable 2:1 clay minerals is a well-known natural process (Pearson and Ensminger, 1949; Sawhney, 1958; Dixon and Jackson, 1960; Rich, 1968) and has been successfully reproduced in laboratory studies (Shen and Rich, 1962; Turner and Brydon, 1965; Violante and Violante, 1978). The formation of Al-interlayered clay is accompanied by changes in many properties of the original clay minerals, such as a reduction in the cation-exchange capacity (CEC) (Jackson, 1963; Rich, 1968; Hsu, 1968), the development o fpH-dependen t layer charge (de Villiers and Jackson, 1967), the appearance o f a ti tratable third-buffer-range acidity (Schwertmann and Jackson, 1963), stable 14-~ X-ray powder diffraction spacings, modification to the retention and movement o f certain anions, and changes in the physical and engineering properties of the clays (Barnhisel, 1977). The formation of A1 interlayers in montmori l loni te occurs readily in moderately acid condit ions and is affected by the presence of inorganic cations and anions in the systems (Rich, 1968; Turner and Brydon, 1967; Hsu, 1968). Organic acids are part of natural environments (FSrsther, 1981; Stevenson, 1982); they are introduced to the terrestrial and aquatic systems through natural vegetation and/or farming, and range from simple to complex molecules. Kwong and Huang (1981) demonstra ted that differences in the properties of the hydrolytic products o r A l formed in the presence of low-molecular-weight orgarlic acids (e.g., citric acid) and high-molecular-weight polyphenolic organic acids (e.g., tannic acid) are att r ibutable to the differences in the chemical as well as the physical behavior of the organic acids. Goh and Huang (1984) reported that the citric acid/Al molar ratio significantly influenced the degree to which citric acid perturbed the format ion of hydroxy-Al-montmoril lonite complexes at the A1/clay ratio of 600 meq AP+/100 g. They further noted that a ci trate/Al molar ratio of 0.5 was apparent ly critical in perturbing the formation of the hydroxy-Al-montmori l loni te complexes. The differences in the mechanism by which citric and tannic acids would influence the format ion ofhydroxy-Al-montmor i l lon i te was, however, not discussed. A study of the reactions of citric and tannic acids with hydroxy-A1 and montmori l loni te would therefore facilitate our understanding o f the complex reactions of hydroxy-Al with montmori l loni te in the presence of small and large chelating organic ligands. The objective o f the present study was to compare the influence of citric and tannic acids on the format ion of hydroxy-Al-montmori l loni te complexes in the pH range 5.00 and 6.00 which is most conducive to A1 interlayering. MATERIALS AND METHODS Retention of Al-organic acid complexes by exchange resins The resins used were Rexyn 101, H+-exchange resin (CEC = 460 meq/100 g, 16-50 mesh average spherical diameter), Dowex 50W-XB, Na+-exchange resin (CEC = 510 meq/100 g, 20-40 mesh average spherical diameter), and Dowex 1-XB, Cl--exchange resin (anion exchange capacity = 420 meq/100 g, 20-50 mesh average spherical diameter). The two cationexchange resins were chosen to test whether or not the difCopyright 9 1986, The Clay Minerals Society 37 38 Goh and Huang Clays and Clay Minerals ference in ionic forms, H + vs. Na +, significantly affected the exchange properties of the Al-organic acid complexes. The Al-organic acid complexes were prepared by pipeting 20 ml of 2.308 x 10 -2 M A1C13 solution into 80 ml ofdeionized distilled water to which citric or tannic acid had been added so as to give organic acid/A1 molar ratios of 0, 0.1, 0.5, and 1.0. The solutions which contained the Al-organic acid complexes were stirred for 20 rain and the pH was recorded. The solution was then added dropwise to buret columns packed with 20 g of the appropriate exchange resin. The columns were 34 cm in length and 1 cm in diameter. The rate of addition and of elutriation was 2 ml/min. The elutriate was collected in a beaker and then passed through the column four more times to ensure that the exchange reaction was complete. At the end of the elutriation, the elutriate was analyzed for A1 by the Aluminon method (Hsu, 1963) after destruction of organic acids by digestion with nitric and sulfuric acids (Weaver et al., 1968). The percentage of A1 exchanged was calculated by the difference between the concentration of A1 in the initial Al-organic acid complexes in solution and the final concentration of A1 in the elutriate. Preparation and aging of mineral suspensions Three liters each of 5 x 10 -3 M A1C13 containing citric or tannic acid at the required organic acid/A1 molar ratios of 0, 0.1, 0.3, 0.5, 0.7, and 1.0 were prepared using reagent grade chemicals. Five grams of montmorillonite (from Upton, Wyoming, previously saturated with Na by treating it with 1 M NaC1 and then washing it free of C1-) were added from a 2% stock suspension to the solutions containing A1C13 and citric or tannic acid. The control treatment (sample F1) contained clay in water. The suspensions were then brought to a final volume of 4 liters or were titrated against 0.2 M NaOH (1 ml/min) before bringing to the final volume with deionized distilled water. The amount of 0.2 M NaOH added was such that the initial pH of the suspensions after dilution to the final volume was 5.00 or 6.00. The AI concentration in the solution after dilution was 3.75 x 10 -3 M. The suspensions were aged at 250 + 0.5"C for 3 months in polypropylene bottles and were agitated daily. The initial and final pH of the suspensions were recorded. After 3 months of aging, each suspension was separated into its filtrate and solid phase by ultrafiltration through a Millipore MF filter of 0.025-#m pore size. The solid phase of each sample was then dialyzed in a SpectraPor 2 dialysis membrane tube (molecular weight cutoff= 12,00014,000) against deionized distilled water, with constant renewal of water, for 72 hr until a negative test for C1was obtained. Examination of filtrate and solid-phase products The total AI in the filtrate was determined by the Aluminon method (Hsu, 1963) after the organic acids were destroyed by digestion with nitric and sulfuric acids (Weaver et aL, 1968). The surface reactivities of the solid-phase reaction products were analyzed with respect to their CEC, extractable A1, exchangeable AI, and specific surface. The CEC was determined according to the procedure outlined by Alexiades and Jackson (1965). Exchangeable A1 was determined by washing 100 mg of the sample five times with 1 M KC1 (10 ml each) and determining A1 in the combined supernatant after centrifuging for 7 rain at 1200 g each time. Acid-extractable AI was similarly determined in separate samples by five washings each of 7 rain with 10 ml of 0.2 M HC1 (Shen and Rich, 1962). The percentage of the adsorbed A1 that was exchanged by 1 M KCI was calculated as follows: % KCl-exchangeable AI Amount of KCl-exchangeable A1 = x 100 Total A1 removed from solution The percentage of the fixed A1 that was extracted by 0.2 M HC1 was estimated as follows: % fixed A1 extracted by HC1 Fixed A1 extractable by HCI x 100 Total fixed-A1 (AI extracted by HC1) (KCl-exchangeable A1) x 100 (Total A1 sorbed) (KCl-exchangeable A1) The carbon content of selected freeze-dried samples was determined by ignition and gas chromatography using a Hewlett Packard Carbon, Hydrogen, Nitrogen Analyzer. The specific surface of the solid phase products was evaluated gravimetrically by the retention of ethylene glycol monoethylether (EGME) as proposed by Carter et al. (1965) and modified by Eltantawy and Arnold (1973) using a theoretical value of 3.71 x 10 -4 g of EGME for a complete monomolecular layer coverage of 1 m 2 of surface. X-ray powder diffraction (XRD) analysis of oriented specimens was undertaken using Ni-filtered CuKa radiation. For infrared (IR) analysis, the KBr pellet method (1.5% w/w) was used on a Perkin Elmer 621 IR instrument. Dilute (0.1%) suspensions in deionized distilled water of selected samples were deposited on carbon-coated Formvar film on copper grids and dried at 35~ before examination with a Philips 400 transmission electron microscope. R E S U L T S A N D D I S C U S S I O N Retention o f aluminum-organic acid complex by exchange resins T h e resul t s o f the r e t e n t i o n o f AI by the ca t ion a n d a n i o n exchange res ins are p r e s e n t e d in T a b l e 1. T h e fact t h a t all t he AI was r e t a i n e d by H +a n d Na+-ex change res ins d e m o n s t r a t e s t h a t the A1 or A l -o rgan ic ac id c o m p l e x in so lu t ion possessed pos i t ive ly cha rged sites. T h e p H o f the so lu t ion also ind ica tes t h a t the hydro lys i s a n d p o l y m e r i z a t i o n o f A1 was ins ign i f i can t (Kwong a n d Huang , 1979). A t the s a m e t ime , the c o m plexes m u s t also h a v e possessed nega t ive ly cha rged sites. In the absence o f organic acid, the p H o f the so lu t i on (Tab le 1) was 3.90, a n d n o A1 was r e t a i n e d by the C1-exchange resin. T h e a d d i t i o n o f o rgan ic ac id to the A1 so lu t i on a t c i t ra te /A1 ra t ios o f 0.1, 0.5, a n d 1.0 success ive ly inc reased the to ta l A1 r e t a i n e d b y C I exchange res in to 8.2%, 13.3%, a n d 17.0%, respect ively. T h e c o m p l e x a t i o n r eac t i on b e t w e e n ci t r ic ac id a n d A1 p roduces 1:1 a n d also 2:1 c o m p l e x e s ( K w o n g a n d Huang , 1979) w h i c h can fu r t he r d i ssoc ia te p r o t o n s ( P a t t n a i k a n d Pan i , 1961). T h e nega t ive l o g a r i t h m o f the first d i s soc i a t i on c o n s t a n t o f t he Al -c i t r a te (pK) is 3.49 ( P a t t n a i k a n d Pani , 1961). T h e nega t ive ly cha rged sites the re fo re m u s t h a v e a r i sen f r o m the d i s soc i a t i on o f p r o t o n s f r o m the Al -c i t r a te complexes . T h e d i s soc i a t i on c o n s t a n t o f A l t a n n i c ac id c o m plexes ha s n o t b e e n r epo r t ed in the l i te ra ture . S o m e A1, howeve r , was also r e t a i n e d b y the C1-exchange Vol. 34, No. 1, 1986 Influence of citric and tannic acids on Al-interlayering 39 Table 1. Retention of Al-organic acid complexes by cation and anion exchange resins. Organic Organic acid/Al molar Solution acid ratio pH % A l exchanged H§ Na+-resin Cl--resin Citric 0 3.90 100 100 0 0.1 3.06 100 100 8.2 _+ 1.9 0.5 2.65 100 100 13.3 -+ 1.6 1.0 2.49 100 100 17.0 _+ 2.1 Tannic 0.1 3.09 100 100 0 0.5 2.87 100 100 0 1.0 2.78 100 100 2.6 _+ 0.02 res in a t a t anna te /A1 ra t io o f 1.0, a l t h o u g h to a lesser degree t h a n t h a t o b s e r v e d for Al-c i t r ic ac id c o m p l e x e s (Tab le 1). Acco rd ing to H s u a n d R i c h (1960), t he poros i ty o f the res ins d e t e r m i n e s the size o f molecu les t h a t will diffuse in to the resin. T h e r e t e n t i o n o f A1c i t r ic ac id complexes b y the C1---exchange res in m a y h a v e b e e n eas ier t h a n the r e t e n t i o n o f A l t a n n i c ac id complexes because the la t ter ha s a larger size a n d m a y no t h a v e diffused to the exchange si tes as easily. pH W h e n ci tr ic ac id was p r e sen t in the sys tem, sma l l e r p H s were r e c o r d e d u p to a c i t ra te /A1 ra t io o f 0.5; a b o v e a ci t ra te/A1 ra t io o f 0.5, the p H o f the sys tems ac tua l ly inc reased f r o m the in i t ia l va lues af te r 3 m o n t h s o f aging (Tab le 2). T h e smal l e r decreases in p H as the c i t r a t e / A1 ra t io was inc reased to 0.5, as c o m p a r e d to the p H d r o p in the con t ro l s ( samples F3 a n d F9), were p r o b ab ly due to the buffer ing ac t ion o f the c i t ra te a n i o n a n d the c o n s t r a i n t t h a t the a n i o n h a d o n the hydro lys i s o f A1 (Kwong a n d Huang , 1977). A t the c i t ra te /A1 ra t ios o f 0.7 ( sample F7) a n d 1.0 ( samples F8 a n d F14) , t he increases in p H af ter 3 m o n t h s o f aging were p r o b a b l y due to par t i a l b r e a k d o w n o f the hydroxy-A1 p o l y m e r s i n to smal l e r un i t s u n d e r the in f luence o f h igh c i t r a t e / A1 m o l a r rat ios. T h e final p H s o f the sys tems aged w i th t a n n i c ac id (Tab le 2) i nd i ca t e t h a t m o r e p r o t o n s were p r o d u c e d by the hydro lys i s o f A1 a n d by the c o m p l e x a t i o n r e a c t i o n b e t w e e n A1 a n d t a n n i c ac id t h a n cou ld b e buf fe red b y the ca rboxy la te g roups o n t a n n i c acid. T h e p H dec rease was par t ia l ly due to the c o m p l e x a t i o n b e t w e e n A1 a n d the pheno l i c g roups ( M c H a r d y et aL, 1974). T h e larger n u m b e r o f func t iona l g roups pe r m o l e o f t a n n i c ac id Table 2. Surface properties of the solid phase reaction products of montmoril lonite with hydroxy-A1 ions and organic acids. Retained AP Fixed A14 CEC ~ before CEC after Organic pH A1 removed 2 exchanged by extracted by Specific extraction by extraction by acid/A1 molar from solution 1 M KCI 0.2 M HCI surface 0.2 M HCI 0.2 M HCI Sample ~ ratio Initial Final (rag/g) (%) (%) (m2/g) (meq/100 g) (meq/100 g) F1 in water 7.35 7.37 0 0 > 100 794 96 94 F2 in A1C13 3.80 3.70 5.94 44.9 > 100 811 82 79 F3 0 5.00 4.45 79.89 4.8 15.3 322 17 34 F4 0.1 5.00 4.83 70.80 4.6 10.4 346 18 23 F5 0.3 5.00 4.85 41.91 14.1 29.6 498 34 51 F6 0.5 5.00 4.98 19.69 42.6 94.3 771 56 63 F7 0.7 5.00 5.25 2.71 100 >100 779 80 79 F8 1.0 5.00 5.54 0.79 100 > 100 777 85 86 F9 0 6.00 4.93 80.95 4.9 13.2 326 18 42 F10 0.1 6.00 5.26 69.55 5.0 10.1 348 20 29 F11 0.3 6.00 5.49 43.69 10.5 18.9 490 37 43 F12 0.5 6.00 5.72 21.55 39.3 43.2 692 51 63 F13 0.7 6.00 6.00 4.99 60.1 78.9 780 79 67 F14 1.0 6.00 6.24 1.02 98.0 > 100 771 83 94 F15 0.1 5.00 4.78 80.95 1.2 66.6 733 60 87 F16 0.3 5.00 4.89 80.95 4.6 46.6 785 94 34 F17 0.5 5.00 4.75 80.95 4.8 37.1 998 92 30 F18 0.7 5.00 4.87 73.23 4.5 33.9 1286 94 39 F19 1.0 5.00 4.76 66.72 4.4 21.2 1144 86 33 F20 0.1 6.00 5.72 80.95 0.8 57.7 733 54 71 F21 0.3 6.00 5.57 80.95 0.2 48.5 944 99 38 F22 0.5 6.00 4.93 80.95 3.7 37.8 1000 99 36 F23 0.7 6.00 4.65 80.95 6.2 35.3 1269 90 40 F24 1.0 6.00 4.62 75.06 4.6 29.4 1180 74 42 1 Samples F4-F8 and F10-F14 were aged in presence of citric acid. Samples F15 to F24 were aged in presence of tannic acid. Samples F3 and F9 were aged in presence of hydroxy-A1 ions without organic acids. 2 Mean error of duplicates = _+0.43 mg A1/g clay. 3 The average error is 4.7% of the values indicated. 4 The average error is 3.5% of the values indicated. 5 Mean error of duplicates = _+3 meq/100 g. 40 Goh and Huang Clays and Clay Minerals in comparison to citric acid also accounts for more proton generating sites in the former. Exchangeability and extractability of Al removed from solution The removal of A1 from solution was influenced by the type of organic acid, the organic acid/A1 molar ratio that was initially present in solution, and the p H of the medium (Table 2). In the absence of organic acid and NaOH (sample F2), only 5.94 mg of A13§ was adsorbed per gram of montmori l loni te . The aging of this sample at an initial pH of 3.80 for three months could also have caused some dissolution of the clay inasmuch as more than 100% of the fixed A1 was removed by the five combined washes with 0.2 M HCI (Table 2). The small amount (0.79 mg/g) of Al extracted from the clay that was aged in water by the 0.2 M HC1 was likely due to the dissolution of structural A1 from montmorillonite. Increasing the citrate/A1 ratio reduced the amount of A1 that was removed from solution, because the citrate competed with the montmori l loni te and formed soluble complexes with AI. The perturbat ion of polymerizat ion reactions of A1 by citrate is shown in Table 2 by the results of the KC1exchangeable AI (samples F4 -F8 and F I0 -F14) . According to Hsu (1968), stable hydroxy-A1 polymers in montmori l loni te have an OH/A1 ratio of 2.5-2.7. In suppressing the hydrolysis of the A1 a+, the citrate ligands reduced the growth o f the hydroxy-A1 polymers and, hence, their net posit ive charge. Furthermore, the structural distort ion in the interlayer sheet (see Goh and Huang, 1984) reduced the tenacity with which the hydroxy-Al-ci trate polymers were held. Thus, the proport ion of the adsorbed A1 that was exchanged by 1 M KC1 increased as the citrate/A1 ratio increased to 1.0. At both initial pHs o f 5.00 and 6.00, a smaller proport ion of A1 was extracted by 0.2 M HCI at a citrate/ A1 ratio of 0.1 (samples F4 and F10, Table 2) as compared to samples where citric acid was absent (samples F3 and F9). Aluminum was increasingly more extractable, however, when the citrate/Al ratio was gradually increased, as was found by Goh and Huang (1984). Increasing the tannate/A1 ratio did not always reduce the amount of A1 removed from solution (Table 2). The removal of A1 from solution was probably due to adsorption by the clay and to precipitat ion as discrete oxides. The complexat ion reaction between AI and tannic acid generated protons, as shown below: E" At-OOC -~ complex] j . : ; ~ Jc~ -] " / hydrogen'bonding xr~/OH /--OH oH : .~O-A ,~