INFLUENCE OF CITRIC ACID ON THE FORMATION OF SHORT-RANGE ORDERED ALUMINOSILICATES

Transcription

1 Clays and Clay Minerals, Vol. 33, No. 4, , INFLUENCE OF CITRIC ACID ON THE FORMATION OF SHORT-RANGE ORDERED ALUMINOSILICATES K. INOUE ~ AND P. M. HUANO Department of Soil Science, Saskatchewan Institute of Pedology, University of Saskatchewan Saskatoon, Saskatchewan S7N 0W0, Canada Abstract--Reactions between hydroxy-a1 ions and orthosilicic acid as influenced by citric acid were studied at an initial Si concentration of M, Si/A1 molar ratios of 0.5 and 1.0, OH/AI molar ratios of , and citric acid/a1 molar ratios of In the absence of citric acid and at OH/A1 ratios of , imogolite (>0.01 #m) was dominant in the precipitates. At citric acid/al ratios of , imogolite and/or pseudoboehmite were dominant in the precipitates at OH/A1 ratios of 1.0 and 2.0, and imogolite and/or ill-defined aluminosilicate complexes at OH/A1 ratio of 2.8. Instead of allophane or "proto-imogolite" allophane being the predominant species in the precipitates, the formation of illdefined aluminosilicate complexes at OH/A1 ratio of 3.0 was steadily promoted by increasing the solution citric acid/a1 ratios from 0 to 0.3. The freeze-dried soluble products (<0.01 /~m) ranged from silica gel to "proto-imogolite," depending upon the basicity of A1 and the level of citric acid of the parent solution. The amount of"proto-imogolite" increased with increasing citric acid/a1 ratios from 0 to 0.1 in solution. Complexing low molecular weight organic acids, such as citric acid, impeded the formation of the shortrange ordered aluminosilicates, auophanes and imogolite. Key Words--Allophane, Aluminosilicate, Citric acid, Hydroxy-A1, Imogolite, Noncrystalline, Pseudoboehmite. INTRODUCTION The short-range ordered hydrous aluminosilicates, allophane and imogolite, have been isolated from Ando soils and weathered pyroclastic deposits (Wada and Harward, 1974; Wada, 1977, 1980), basaltic saprolites (Wada et al., 1972), and spring and stream sediments (Wells et al., 1977; Inoue et al., 1980). "Proto-imogolite" (Farmer et al., 1978; Farmer and Fraser, 1979; Violante and Tait, 1979; Farmer, 1981), "proto-imogolite" allophane (Farmer et al., 1980; Parfitt and Henmi, 1980), and allophane-like constituents (Wada and Greenland, 1970; Wada and Harward, 1974; Wada, 1977, 1980) have been proposed as the analogues of imogolite and allophanes in Ando soils and Podzols. "Proto-imogolite" and "proto-imogolite" allophane have imogolite structural units (OH)3A1203SiOH on the atomic scale (Cradwick et al, 1972) but do not exhibit the tubular morphology of imogolite. "Protoimogolite" allophane exhibits a morphology similar to auophane, whereas "proto-imogolite" has no well-defined morphology. The concept of allophane-like constituents, however, defined by solubility in dithionitecitrate-bicarbonate, has been disputed by Farmer et al. (1983), who showed that the dithionite-citrate-bicarbonate treatment extracts an ill-defined fraction of the auophane-imogolite complex. Farmer et al. (1980) and Farmer (1981) proposed that a soluble aluminosilicate Permanent address: Laboratory of Soil Science, Department of Agricultural Chemistry, Faculty of Agriculture, Iwate University, Morioka, Japan 020. Copyright , The Clay Minerals Society 312 complex ("proto-imogolite") could be a mobile form of aluminum in Podzols. The finding ofallophane and imogolite in B2(B3) horizons of Podzols (Brydon and Shimoda, 1972; Tait et al, 1978; Ross and Kodama, 1979; Young et al., 1980; Farmer et al., 1980; Mc- Keague and Kodama, 1981) has important implications in pedogenic processes and necessitates a revision of many current hypotheses of podzolization (Farmer et al, 1980; Anderson et al., 1982). Imogolite has been formed from solutions containing hydroxy-a1 ions and orthosilicic acid with Si/A1 molar ratios of 0.5 and 1.0 at ph <5 by heating at 95~100"C (Farmer et al, 1977b, 1979; Farmer and Fraser, 1979; Wada et al., 1979). Wada et al. (1979) prepared allophane by heating solutions containing orthosilicic acid, A1CI3, and varying amounts of NaOH at 95"-100~ for 113 hr. The interaction of hydroxy-al ions with orthosilicic acid is influenced by the ionic environment of the soil system. Organic acids are commonly present in minute but measurable amounts in soils (Stevenson, 1967; Bruckert, 1970), and these organic compounds are constantly being introduced to soils through natural vegetation and farming. Complexing organic acids, e.g., citric acid, have strong chelating properties which disrupt the hydroxyl-bridging mechanism indispensable for the formation of crystalline Al-hydroxides (Kwong and Huang, 1975, 1977, 1979; Kodama and Schnitzer, 1980; Violante and Violante, 1980). Therefore, these acids could compete with orthosilicic acid in influencing the hydrolytic reaction of A1 and thus perturb the formation ofallophane and imogolite. The effect of low mo-

2 Vol. 33, No. 4, 1985 Influence of citric acid on short-range ordered aluminosilicates 313 lecular weight organic acids on the stability ofimogolite (Farmer, 1981) and on its formation at very low organic acid/a1 molar ratio (-<0.1) (Inoue and Huang, 1984) has been investigated; however, little systematic information is available on the influence of organic ligands on the formation of aluminosilicates with shortrange order, namely, allophane, imogolite, "proto-imogolite," and "proto-imogolite" allophane. The objective of the present study was to investigate the influence of citric acid, one of the most common compounds in the tricarboxylic acid cycle and thus common in nature, on the formation of these aluminosilicates. EXPERIMENTAL METHODS Preparation of orthosilicic acid Monomeric orthosilicic acid solution was prepared by passing Na-metasilicale through a H+-saturated Dowex 50W-X8 resin column (Inoue and Huang, 1984). The initial concentration of silicic acid was maintained below 2 x 10-3 M Si to avoid polymerization of Si. The effluent which passed through the exchange column was analyzed for Na + by flame emission to ensure complete conversion of Na-metasilicate to orthosilicic acid. Reactions between hydroxy-al ions and orthosilicic acid An AICI3 solution was mixed with solutions containing orthosilicic acid arid citric acid to give Si/A1 molar ratios of 0.5 and 1.0 and citric acid/a1 molar ratios of The terms Si/AI, OH/AI (basicity of AI), SiO2/A1203, and citric acid/al molar ratios will henceforth be stated as Si/A1, OH/A1, SiO2/ A1203, and C.A./A1 ratios for the sake of brevity. About 900 ml of these solutions was titrated with 0.1 M NaOH with continuous stirring at the rate of -0.5 ml NaOH/min to OH/ A1 ratios of 1.0, 2.0, 2.8, and 3.0. The ph of the solution was determined, and the solutions containing citric acid were adjusted to the same ph as the solution without citric acid to avoid acidity effects and then diluted to 1000 ml. The Si concentration of the resulting solution was M. About 900 ml of the parent solutions was heated at *C for 110 hr. After cooling to room temperature, the ph of the suspension was determined. The experiment was carried out in duplicate. Parent solution and fdtrate analyses The colloidal precipitates were collected through ultrafiltration using a Sartorius cellulose nitrate membrane filter of 0.01-~m pore size. The total Si in the parent solution and in the filtrate was determined by the amino sulfonic acid reduction of silicomolybdate according to the method of Weaver et al (1968). The total A1 was determined by the Aluminon method (Hsu, t963) after destruction of citric acid by digestion with nitric and sulfuric acids (Higashi and Ikeda, 1974). The amounts of Si and A1 removed from the solution were, respectively, determined by taking the difference between the Si and AI concentrations in the parent solution and those in the filtrate. Precipitate phase analysis The precipitates collected by ultrafiltration were dialyzed (molecular weight cut off = 3500) against deionized water until free of C1-. A portion of the dialyzed suspension was used for X-ray powder diffraction (XRD) and electron microscopic analyses. The remaining suspension was freeze-dried and used for infrared OR) spectroscopic and organic carbon analysis. The XRD analysis was carried out on a Philips X-ray diffractometer with Fe-filtered CoKa radiation generated at 35 kv and 16 ma using a parallel-oriented sample on a glass slide. For 1R analysis, 1 mg of freeze-dried materials was ground and mixed with 170 mg of spectroscopic grade KBr, heated at 125"C, and pressed into discs. The spectra were recorded on a Perkin-Elmer 983 instrument. For transmission electron microscope examination, a drop of diluted suspensions was deposited on a grid covered with a carbon-coated Formvar film. The transmission electron micrographs (TEM) were taken at 60 kv with a Philips EM 400 instrument. For the high magnification electron microscopy, a micro-grid method (Henmi and Wada, 1976) was used for supporting the specimen, and the electron micrographs were taken at 100 kv with a JEM 7A instrument. Allophane and imogolite samples separated from a Kitakami pumice bed (Henmi and Wada, 1976) were used as reference materials. The content of imogolite and "proto-imogolite" in the precipitates was determined by the semiquantitative procedure on the basis of absorbance at near 345 cm -~ in the IR spectra (Farmer et al, 1977a). The organic C content of the precipitates was determined by using a CHN Analyzer. Soluble phase analysis The filtrates which passed through a cellulose nitrate membrane ultra-filter of 0.01-#m pore size were freeze-dried. The nature of the freeze-dried soluble products were examined by IR spectroscopy and electron microscopy. Reactions in solution RESULTS AND DISCUSSION In the absence of citric acid, heating the parent solution to 96"-100~ for 110 hr generated dispersed colloids at OH/A1 ratios of (initial ph ) and precipitates at an OH/A1 ratio of 3.0 (initial ph ) (Table 1). As shown by Farmer et al (1979), Farmer and Fraser (1979), Wada et al. (1979), and Inoue and Huang (1984), the interaction of hydroxy- A1 ions with orthosilicic acid always caused a marked decrease in ph, ending at ph from initial values of at OH/A1 ratios of and in the absence of citric acid (Table 1). At an OH/AI ratio of 3.0, little change in ph was observed. In the presence of citric acid, the ph shift decreased with increasing C.A./AI ratio of the parent solution from 0.01 to 0.1 at OH/A1 ratios of 1.0 and 2.0. At an OH/A1 ratio of 3.0, however, the heated solution prominently decreased in ph at higher C.A./A1 ratios. The ph decrease after heating (Table 1) appears to have been predominantly induced by reaction mechanisms involving the release of hydrogen ions by the hydrolysis and polymerization of A1, the interaction of hydroxy-a1 ions with orthosilicic and citric acids, and/or the dissociation of the hydrogen from citric acid. In the absence of citric acid, only silica gel was found in the freeze-dried soluble products as shown by the absorption maxima or shoulders at 1200, 1077, 958, 797, and 464 cm -~ (Figures la, ld, and Ig). In the presence of citric acid and at a Si/A1 ratio of 0.5 and

3 314 Inoue and Huang Clays and Clay Minerals OH/AI molar ratio Table 1. ph of the systems studied. Citric Si/AI molar ratio t acid/ AI molar ratio ph~oj 2 phl~? ph~0~ ~ prim ~ citric ocld/ai molor rotio (o) Initial Si concentration of 1.6 x 10 3 M. 2 PH(0~ = ph of parent solution after addition of NaOH. 3 ph m = ph of suspension after 110 hr heating at 96 ~ 100~ Z u3 Z o:: k- OH/A1 ratios of , however, the IR spectra of the freeze-dried soluble products showed absorption maxima and shoulders at 1635, 1450, 1400, 1200, 1077, 973, 860, 800, 565, 465, 430, and 343 cm -1 (Figures lb, lc, le, and If). The absorption maxima at 973, 565, 430, and 343 cm t are attributable to "protoimogolite." On the other hand, the IR spectra of the freeze-dried soluble products at a Si/A1 ratio of 1.0, an OH/A1 ratio of 3.0, and C.A./A1 ratios of showed absorption maxima and shoulders at 1632, , 1200, 1077, 958, 797, and 464 cm -1, indicating the predominance of silica gel and citrate (Figures lh and I i). Under these conditions, "proto-imogolite" was not formed in the soluble products. The TEM of the freeze-dried soluble products formed at a Si/A1 ratio of 0.5, and OH/A1 ratio of 2.0, and a C.A./A1 ratio of 0.1 shows the presence of irregular shaped aggregates (Figure 2a) and a gel-like material (Figure 2a, see arrow; Figure 2b). The former, which was formed by freeze-drying the filtrates, are silica gels (Figure If). The latter are morphologically similar to the "proto-imogolite" identified in volcanic ash soils in Italy (Violante and Tait, 1979). Silica gels in the freeze-dried soluble products may be formed from the polymerization process of silicic acid by freeze-drying WAVENUMBER, cm "~ Figure 1. Infrared spectra of freeze-dried soluble products formed by the reaction of hydroxy-a1 ions with orthosilicic acid at C.A./A1 ratios of (a)-(c): Si/A1 ratio of 0.5 and OH/AI ratio of 1.0, (d)-(f): Si/A1 ratio of 0.5 and OH/A1 ratio of 2.0, (g)-(i): Si/A1 ratio of 1.0 and OH/A1 ratio of 3.0. Therefore, the soluble products formed in the presence of citric acid at the OH/A1 ratios of before freeze-drying are characterized by the predominance of "proto-imogolite" sol which consists of small fragments of the imogolite structure < 100 A in size (Inoue and Huang, 1984). The citrate ion may be adsorbed on the "proto-imogolite" sol rather than be present as a separate Al-citrate complex. The amount of "proto-

4 Vol. 33, No. 4, 1985 Influence of citric acid on short-range ordered aluminosilicates 315 Figure 2. Transmission electron micrographs of freeze-dried soluble products formed by the reaction of hydroxy-a1 ions with orthosilicic acid in the presence of citric acid. (a) Si/A1 ratio of 0.5, OH/A1 ratio of 2.0, and C.A./A1 ratio of 0.1; (b) high magnification of (a). Scale bar = 1 ~tm (a), 500,~ (b). imogolite" sol formed increased with increasing C.A./ A1 ratio (Figures l a - l f ). Nature of the precipitate phase Chemical properties. Generally, the percentage of Si and A1 removed from the reaction system by the ultrafiltration increased with the increase of the OH/A1 ratio of the parent solution, namely, the degree of hydrolysis of A1 (Figure 3). The precipitation of the solid phase products was invariably hindered by citric acid; however, the effect of citric acid on the interaction of hydroxy-al ions with orthosilicic acid varied with the initial Si/A1, OH/A1, and C.A./AI ratios of the system (Figure 3). In the presence of citric acid and at OH/A1 ratios of , the percentage of Si and A1 removed from the solutions, namely, the a m o u n t o f the precipitates, was drastically decreased when the C.A./A1 ratio of the parent solution was increased from 0.01 to 0.3 (Figure 3). Notably, only a little or no precipitate formed in certain systems (P-4, P-5, P- 11, P- 16, P- 17, P-27, P-28, P-32 to P-34, and P-38 to P-40) (Table 2), evidently because of the formation of soluble "proto-imogolite" sol complexed with citric acid. The percentage of Si (Y) removed from the solution was plotted against the percentage of A1 (X) removed from the solution, and the correlation was calculated according to the regression analysis as follows: Si/A1 ratio Equation 0.5 Y ~ 0.84X R ~ = 0.97*** 1.0 Y ~ 0.59X R : = 0.96*** Triple asterisks denote significance at the 0.1% level. High correlation coefficients indicate the significant coprecipitation of hydroxy-a1 ions with orthosilicic acid in the systems studied. The extent of the coprecipitation, however, decreased with increasing C.A./A1 ratio (Figure 3). Si/A1-0.5 S i / A I z,oo -~8 8o o~ 6o Legend ~ ~,o g ~ ~o ~ ~ _~,oo g d 8o,~ ~o ~,o o> ~ ~o ~ ~o oor Ol CITRIC o1~o3 ACID/AI MOLAR RATIO Figure 3. Percentage of Si and AI removed from solutions containing hydroxy-a1 ions, orthosilicic acid and citric acid at an initial Si concentration of M, Si/A1 ratios of 0.5 and 1.0, and OH/A1 ratios of and C.A./A1 ratios of

5 316 Inoue and Huang Clays and Clay Minerals << ~ m < m <<zzz<~<~<< 8~ c~ A =.= ~ ~ r~ i! 0 a ~u ==~ = <~ < < ~ ~.~ 0~ E~ T~ ~ I I ' ~ ~ I ~ r ' ~ I I ~ ~ 9 ~ ~ ~ I I ~ I ~ I I ~ ~ e.~,..~ "~ N N o.. o,r =

6 Vol. 33, No. 4, 1985 Influence of citric acid on short-range ordered aluminosilicates 317 >,- I.- m (f) Z LIJ )-- Z 2te~ : f 5 IO 5 DEGREES 219, Co KK RADIATION Figure 4. X-ray powder diffractograms of the precipitates formed from solutions containing hydroxy-a1 ions, orthosilicic acid, and citric acid at St/A1 ratio of 0.5, OH/AI ratio of 2.0, and C.AJA1 ratios of In the absence of citric acid, the SiO2/AlaO3 ratios of the precipitates (>0.01 ~tm), P-I, P-6, and P-12, were (Table 2). The relatively low SIO2/A1203 ratios are attributed to the coexistence of boehmite and/or bayerite with imogolite in the precipitates. On the contrary, SIO2/A1203 ratios of 1.00 to 1.53 of the precipitates, P-24, P-29, and P-35 (Table 2) suggest that imogolite, allophane, and small amounts of boehmite were formed. The precipitates, P- 18 and P-41, had SIO2/A1203 ratios of from 0.90 to 1.35 (Table 2), which were about the same as that found for soil allophanes (Wada, 1977). In the presence of citric acid, however, the SIO2/A1203 ratio of the precipitates was greatly influenced by the organic ligand and decreased markedly with increasing C.A./A1 ratios of the parent solution from 0.01 to 0.3 (Table 2). The organic C content, which is a measure of the amount of citrate in the precipitates, increased with increasing C.A./AI ratios of the parent solution (Table 2). The results clear- ly show that citric acid added to the reaction system complexed with hydroxy-a1 ions and aluminosilicates during their formation. X-ray powder diffraction data. The XRD pattern of the precipitates formed in the absence of citric acid indicates that imogolite (d = 21.6, 15.1, 8.7, and 6.4,~) (Farmer and Fraser, 1979; Wada et al., 1979) and small amounts ofboehmite (d = 6.1 A) and bayerite (d = 4.7 ~) (Hsu, 1977) are dominant in the products (Figure 4). Imogolite was found in the precipitates (P-l, P-6, P-12, P-24, P-29, and P-35) formed in the absence of citric acid (Table 2). The intensity of the XRD peaks of imogolite, however, were significantly reduced with increasing C.A./AI ratios (Figure 4). All of the precipitates, P-18 to P-23 and P-41 to P-46, were X-ray amorphous. IR spectra. The IR spectra of the precipitates, P-1, P-6, P-12, P-24, P-29, and P-35 (Figures 5a, 5d, and 6a; cf. Table 2 for sample numbers) resembled those obtained from natural imogolite. The characteristic absorption bands of imogolite at 995, 936, 700, 565, 504, 422, and 345 cm -1 (Wada and Harward, 1974; Wada, 1977, 1980; Farmer et al., 1977a, 1979; Farmer and Fraser, 1979) were distorted or weakened with increasing C.A./ A1 ratios of the parent solution (Figures 5a-5c, 5d-5h, 6a-6c). The IR spectra of the precipitates, P-26, P-9, and P-10 (Table 2) showed absorption maxima at 1068, 750, 630, 480, and 365 cm -1 and a shoulder at 1165 cm -1 (Figure 5c, 5g, and 5h), which are characteristic bands of pseudoboehmite (Inoue and Huang, 1984). The IR spectra of the precipitates, P-15 (Table 2, Figure 6c), showed absorption bands at 965, 563, 424, and 342 cm-1, which are very similar to those of"proto-imogolite" (Farmer et al., 1978; Farmer and Fraser, 1979; Farmer et al., 1979; Inoue and Huang, 1984). "Proto-imogolite" is, however, known to be water soluble. Therefore, these precipitates may be termed illdefined aluminosilicate complexes. This type of illdefined aluminosilicate complex was also identified in the precipitates, P-36 and P-37 (Table 2). The IR spectra of the precipitates, P-41 (Table 2) showed a broad absorption maximum at 990 cm -1 (Figure 6d), which is a feature common to allophanic clay separated from Ando soils (Wada and Harward, 1974; Wada, 1977, 1980). With increasing C.A./A1 ratio of the parent solution from 0.01 to 0.3, however, the IR spectra of the precipitates showed a gradual shift of the Si-O stretching maximum from 985 to 965 cm- (Figures 6e-6h). Especially, at C.A./A1 ratios of (Figures 6g and 6h), highly disordered products were formed, which are obviously different from allophane precipitated from the solution in the absence of citric acid. The disordered products showed a sharp absorption maximum at 965 cm -1 and other absorption bands at 576, 424, and 345 cm -1 (Figure 6g and

7 318 Inoue and Huang Clays and Clay Minerals citrlc acld/ai molar ratio I WAVENUMBER, cm -I Figure 5. IR spectra of precipitates formed from solutions containing hydroxy-a1 ions, orthosilicic acid and citric acid at C.A./A1 ratios of (a)-(c): Si/AI ratio of 1.0 and OH/ AI ratio of 1.0, (d)--(h): Si/A1 ratio of 0.5 and OH/A/ratio of WAVENUMBER, cm-~ Figure 6. Infrared spectra of precipitates formed by reaction ofhydroxyl-al ions with orthosilicic acid both in the absence and presence of citric acid. (a)-(c): Si/A1 ratio of 0.5 and OH/ A1 ratio of 2.8, (d)-(h): Si/A1 ratio of 1.0 and OH/A1 ratio of h) which are similar to those of "proto-imogolite" allophane (Farmer et al, 1980; Parfitt and Henmi, 1980). The absorption bands at 1640 and 1400 cm- a in the IR spectra of the precipitates that formed in the presence of citric acid are due to the COO--bending vibration of citrate, and those at cm -~ are due to the Si--O- and/or Si(A1)-O-stretching vibrations (Figures 5 and 6). The band at 1640 cm-~ also is due to the HOH-bending vibration of adsorbed water (Farmer, 1979). The intensity of absorption bands at 1640 and 1400 cm -~ increased with increasing C.A./ AI ratios of the parent solution. Citric acid added into the system was largely converted from un-ionized COOH groups to the ionic carboxylate form, as indicated by the appearance of strong absorption bands at 1640 (CO0-) and 1400 (CO0-) cm -~, but not at 1725 (C=O of COOH) and 1200 (CO stretch of OH-deformation ofcooh) cm- ~ (Schnitzer, 1978). The weak absorption band at 1280 cm -~ (Figures 5 and 6) was probably due to the un-ionized COOH groups of citrate complexed with aluminosilicates during their formation (Inoue and Huang, 1984). The IR spectra (Figures 5 and 6) indicate that citric acid reacted with hydroxyaluminosilicates and/or hydroxy-a1 ions to form COOions. The imogolite content markedly decreased with

8 Vol. 33, No. 4, 1985 Influence of citric acid on short-range ordered aluminosilicates 319 Figure 7. Transmission electron micrographs of precipitates formed by reaction of hydroxy-a1 ions with orthosilicic acid at C.A./AI ratios of (a) P-6, (b) P-9, (c) P-15, (d) P-29 (the high magnification), (e) P-37 (the high magnification). Sample numbers are described in Table 2. Scale bar = 1 /zm (a--e), 500/~ (d, e).

9 320 Inoue and Huang an increase of the C.A./A1 ratio (Table 2). The formation of imogolite was completely perturbed by the organic ligand at high C.A./A1 ratios. The critical C.A./ A1 ratios which inhibited the imogolite formation were in turn influenced by the OH/A1 and Si/A1 ratios of the parent solution. Electron micrographs. In the absence of citric acid, smooth and curved imogolite threads appeared to be of micrometer length with diameter of A (Figure 7a). A high magnification TEM of the precipitates (Figure 7d) shows that the threads consisted of fine tube units with inner and external diameters of about 10 and 20/~, respectively. Irregular-shape particles with different morphological characteristics and their aggregates were, however, observed in the precipitates, P-9 (Figure 7b; cf. Table 2 for sample number), which were characterized by the predominance of disordered products with low SiOz/ A1203 ratios (Table 2) and pseudoboehmite (Figure 5g). A TEM of the precipitates, P-15, shows the presence of a gel-like material (Figure 7c). The precipitates, P-37, are composed of gel-like materials, very distorted imogolite tubes, and hollow spherules (Figure 7e). Figure 8a shows a high magnification TEM of noncrystalline precipitates, P-41 (Table 2). The presence of hollow spheres with diameters of A in the TEM (Figure 8a) and the IR spectra of the products (Figure 6d) reveal that the noncrystalline aggregates observed in the TEM mainly consist ofallophane. High magnification TEM has shown that natural allophane in soils (Wada and Harward, 1974; Wada, 1977) and river sediments (Wells et al., 1977; Inoue et al., 1980), and synthetic allophane (Wada et al., 1979) consist of hollow spheres 35-50,~ in external diameter with walls 7-10,~ thick (Wada, 1979). The morphology of the hollow spheres was, however, markedly distorted by the presence of citric acid during their formation; only a few hollow spherules with diameters of A were found in the irregular aggregates (Figure 8b and 8c). GENERAL DISCUSSION In the absence of citric acid, all soluble products after freeze-drying were characterized by the predominance of silica gel. In the presence of citric acid and at a Si/ AI ratio of 0.5 and an OH/A1 ratio of , however, "proto-imogolite" was found in the soluble products which passed through the 0.01-#m pore-size membrane filter and were recovered by freeze-drying the Figure 8. Transmission electron micrographs of precipitates formed by the reaction of hydroxy-a1ions with orthosilicic acid at Si/AI ratio of 1.0 and OH/A1 ratio of 3.0 both in the absence and presence of citric acid. (a) P-41, (b) P-44, (c) P-46. Sample numbers are described in Table 2. Scale bar 500 A (a--c). Clays and Clay Minerals

10 Vol. 33, No. 4, 1985 Influence of citric acid on short-range ordered aluminosilicates 321 filtrates (Figure 1). "Proto-imogolite" has positive charges in the acidic condition (Farmer, 1981); therefore, "proto-imogolite," which is composed of < 100- /~ size particles (Inoue and Huang, 1984), presumably formed complexes with citric acid. Thus, the adsorbed citric acid impeded the conversion of "proto-imogolite" to imogolite. The IR spectra (Figures 6g and 6h) of the precipitates, P-44 and P-46 (Table 2), have many of the features of imogolite, but the band in the range of cm -1 is very similar to that of natural "protoimogolite" allophane (Parfitt and Henmi, 1980). "Proto-imogolite" allophane samples isolated from soils and pumices in New Zealand show IR spectra similar to that of "proto-imogolite," but their unit particles appear to be hollow spherules or polyhedra 35-50/~, in diameter with SiOJA1203 ratio close to 1.0 (Parfitt and Henmi, 1980). "Proto-imogolite" allophane was formed in the precipitates, P-18 (Table 2). They were composed of many hollow spherules; however, the precipitates, P44 and P46 (Figures 8b and 8c), appear to be neither tubular particles nor distinctly hollow spheres. These precipitates are also very similar to "protoimogolite" in their IR spectra (Figures 6g and 6h). They are A1 complexes of mixed ligands containing hydroxyl, citrate, and orthosilicate. The decrease of the SiO2/ A1203 ratio of the precipitates (Table 2) and the gradual shift of IR absorption maximum from 990 to 965 cm- (Figures 6g and 6h) seem to increase with the increase in the degree of complexation of hydroxy-aluminosilicates with citric ligands. This type of precipitates along with samples P-15, P-36, and P-37 (Table 2) may be tentatively classified as ill-defined aluminosilicate complexes. Such groups of aluminosilicates seem to resemble so-called"allophane-like material" (Wada and Greenland, 1970; Wada and Harward, 1974) and the ill-defined fraction of the allophane-imogolite complex (Farmer et al., 1983). A1 released from the parent materials by weathering is bound to be strongly complexed with certain low molecular weight organic ligands, such as citric acid (Kwong and Huang, 1975, 1977), and/or highly polymerized fulvic (Kodama and Schnitzer, 1980) and humic acids in the soils, leading to the perturbation of the interaction of hydroxy-a1 ions with silicic acid. Therefore, under soil conditions where organic ligands tend to accumulate, the formation of poorly to noncrystalline hydrous aluminum hydroxides, oxyhydroxides, and/or aluminosilicates with low SiOJA1203 ratios and with considerable amounts of organic ligands would be promoted. Poorly ordered aluminosilicate colloids associated with organic matter which co-exist with imogolite were isolated not only from Ando soils, but also from the B2 horizons of several Scottish Podzol and Brown Forest soils (Tait et al., 1978). The results obtained in this study indicate that the complexing low-molecular-weight organic acids, such as citric acid, merit close attention in the formation and stabilization of"proto-imogolite" and in the perturbation of the formation of the short-range ordered aluminosilicates, allophanes and imogolite. ACKNOWLEDGMENTS This study was funded by the Natural Sciences and Engineering Research Council of Canada A2383 and G1296--HUANG. The authors thank M. Yoshida, Iwate University, for help in collecting the pyroclastic samples from the Kitakami pumice bed. Thanks are also due to T. Henmi, Ehime University, for assistance in electron microscopy at the higher magnification. Saskatchewan Institute of Pedology Publication No. R427. 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