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Further insight into the mechanism of formation of hydroxyaluminosilicates
Polyhedron 25 (2006) 3399–3404 www.elsevier.com/locate/poly
Further insight into the mechanism of formation of hydroxyaluminosilicates Stanislav Strekopytov 1, Eve Jarry, Christopher Exley
*
Birchall Centre for Inorganic Chemistry and Materials Science, Lennard-Jones Laboratories, Keele University, Staffordshire ST5 5BG, UK Received 23 March 2006; accepted 16 June 2006 Available online 30 June 2006
Abstract Hydroxyaluminosilicates (HAS) are formed by the reaction of silicic acid (Si(OH)4) with adjacent hydroxyl groups on an aluminium hydroxide (Al(OH)3(s)) framework or template. They are important secondary mineral phases in the biogeochemical cycling of aluminium and are extremely insoluble. Two discrete forms of HAS have been identified. HASA which is formed when [Si(OH)4] 6 [Al] and HASB which is formed when [Si(OH)4] [Al]. The formation of HASB has been suggested to involve the further reaction of Si(OH)4 with HASA and it is this contention that, in the main, we have tested herein. Applying a number of analytical and structural tools we have demonstrated the critical importance of both absolute concentrations and relative ratio’s of Si(OH)4 and Al in solution in determining which form of HAS will be precipitated from solution. In addition, by collecting HAS both almost immediately upon their precipitation from solutions (ca 0.5 h) and after ageing in solutions for up to 336 h and analysing their stoichiometries and structural configurations we have shown that the formation of HASA was the first step in the formation of HAS including in those solutions in which [Si(OH)4] [Al] and ultimately (P72 h ageing) only HASB was identified. These are the first experimental results which support the long held belief that Al(OH)3(s) is a prerequisite to the formation of HASA which, in turn, is the precursor to the formation of HASB. The insight we have gained should enable a better understanding of the role of HAS both in their control of the biological availability of aluminium and in their potential future applications in materials science. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Aluminium; Silicic acid; Hydroxyaluminosilicate; Biogeochemistry; Bioinorganic chemistry
1. Introduction Silicic acid (Si(OH)4) is the neutral, monomeric form of biologically available silicon and is characterised by an extremely limited inorganic chemistry [1]. We were surprised to find that Si(OH)4 protected against acute aluminium toxicity in fish in acid waters [2] and this finding led us to propose that the reaction of Si(OH)4 with aluminium to form hydroxyaluminosilicates (HAS) might explain this ameliorative effect [3]. Subsequent research has supported the contention that HAS were not acutely toxic to fish in *
Corresponding author. Tel.: +44 1782 584080; fax: +44 1782 712378. E-mail address: [email protected] (C. Exley). 1 Present address: Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK. 0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.06.013
acid water [4]. We have investigated the mechanism of formation of HAS [5] and established that it involves the competitive condensation of Si(OH)4 across adjacent hydroxyl groups on a framework of aluminium hydroxide (Al(OH)3(s)) [6]. Subsequently we precipitated HAS under different solution conditions and, following their structural characterisation by, for example, solid-state NMR, we were equally surprised to find two discrete forms which we imaginatively called HASA and HASB [7]. HASA was formed under conditions where [Al] P [Si(OH)4] and had an ideal Si:Al ratio of 0.5 and all of the constituent Al was octahedrally coordinated. HASB was formed under conditions when [Si(OH)4] [Al] and had an ideal Si:Al ratio of 1.0 with up to 50% of the constituent Al being in tetrahedral coordination. The existence of HASB was the major surprise, particularly as its formation appeared to
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involve a room temperature dehydroxylation of the HASA form. We proposed that further substitution of Si(OH)4 into HASA was driving the dehydroxylation [8] and it is this proposition that is tested further in the current research. We have used a combination of solution and solid-state structural techniques to provide evidence that the formation HASA is a prerequisite to the formation of HASB. 2. Experimental 2.1. Preparation of solutions All reagents used were of the highest purity available. Solutions which included a wide range of concentrations of Si(OH)4 (25–2000 lmol/L) applied across three molar ratios (0.5, 1.0, 2.0) of Si(OH)4 to Al were prepared in PTFE phials and containers (Cowie Technology Ltd., UK) using ultrapure water (conductivity < 0.067 lS/cm, Elga UK) which included 100 mmol/L KNO3 as background electrolyte and which were buffered at pH 6.0 with 25 mmol/L PIPES (piperazine-N,N 0 -bis-2-ethanesulphonic acid). Stock solutions of Si(OH)4, up to a maximum concentration of 2.0 mmol/L, were prepared by cation exchange of Na4SiO4 and either diluted as required or used in the place of ultrapure water. Al was added from a freshly prepared 1 mol/L stock of Al(NO3)3 Æ 9H2O in 1% HNO3. An established method was used for the preparation of all HAS solutions [5] and all solutions were incubated at 20 °C for the requisite time period prior to sampling. The precipitation experiments (see Fig. 1) involved the preparation of 20 mL volumes of 5 replicates of 21 different combinations of Si(OH)4 and Al. These were aged for 24 h and then filtered through 0.1 lm polyvinylidene fluoride membrane filters (Durapore). For the ageing study (see Fig. 2) 100 mL volumes of 5 replicates of 4 preparations which included 800 lmol/L Al and either 800, 1100, 1500 or 2000 lmol/L Si(OH)4 were aged for up to 500 h. These solutions were periodically sampled and filtered using syringe filters (Millipore) which incorporated the same 0.1 lm polyvinylidene fluoride membrane filters (Durapore). Analagous solutions of larger volume, 500 mL, were run in parallel and, following ageing for typically, 12, 72, 168 or 336 h, were filtered (0.1 lm membrane filtration) to collect significant quantities (>200 mg) of precipitate for subsequent solid-state analyses. For all studies, postfiltration [Si(OH)4] were measured spectrophotometrically (kmax 810 nm) as the reduced blue-molybdosilicic acid complex [9] and post-filtration [Al] by graphite furnace atomic absorption spectrometry (GFAAS) using a method developed in our laboratory [10]. 2.2. Solid-state analyses Solid phases collected by membrane filtration were dried to a constant weight at 37 °C and thereafter gently ground
Fig. 1. The precipitation of Si(OH)4 from solutions (Si(OH)4 ppt) in which the initial ratio of Si(OH)4 to Al was (a) 1:2; (b) 1:1; (c) 2:1. Mean and SD are plotted, n = 5. Solid lines indicate the theoretical precipitation of Si(OH)4 if all of the available Al were precipitated by the formation of HASA (a) and HASA or HASB (b,c).
to a fine powder using an agate mortar and pestle. The elemental composition, structural identity and thermal chemistry of each of the solid phases were determined by electron microprobe analysis, solid-state NMR and thermogravimetric analysis, respectively, using methods which have been described in detail in recent publications [8,11].
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Fig. 2. The concentration of Si(OH)4 remaining in solution (Si(OH)4soln.) following the reaction of (a) 800; (b) 1100; (c) 1500; (d) 2000 lmol/L Si(OH)4 with 800 lmol/L Al and ageing of the solutions at 20 °C for up to 500 h. Mean and SD are plotted, n = 5. The arrows indicate the times at which solid phases were collected and their relative contents of Si and Al determined by electron probe microanalysis.
3. Results
3.2. Influence of ageing on the precipitation of Si(OH)4 and the nature of the solid phase formed
3.1. Precipitation of Si(OH)4 by Al Si(OH)4 was precipitated within the 24 h period of ageing under each of the solution conditions which ranged from ca 25 lmol/L Si(OH)4/50 lmol/L Al to ca 2000 lmol/ L Si(OH)4/1000 lmol/L Al. The proportion of available Si(OH)4 precipitated in the former was ca 20% whereas this increased up to ca 100% in the latter. There was no Al postfiltration for any of the initial combinations of Si(OH)4 and Al. When these filtration data were expressed according to the theoretical composition of the solid phase formed (i.e. either HASA or HASB) it was clear that the probable composition of precipitate depended upon the initial solution concentrations of Si(OH)4 and Al and their ratio (Fig. 1). Thus for Si(OH)4:Al ratios of 0.5 and 1.0 the filtration data predicted the formation of HASA whereas for a ratio of 2.0 the data predicted the formation of HASA at lower total concentrations (<1000 lmol/L Si(OH)4) and HASB, or a mixture of HASA and HASB, at higher total concentrations (1000–2000 lmol/L Si(OH)4) (Fig. 1).
Four different solution compositions were investigated over an ageing period up to 500 h (Fig. 2). The compositions were chosen to reflect a range of Si(OH)4 to Al ratios over which the formation of either HASA or HASB was predicted [7]. 3.2.1. 800 lmol/L Si(OH)4:800 lmol/L Al The [Si(OH)4] was rapidly reduced to ca 500 lmol/L within only 0.5 h ageing and thereafter continued to fall steadily before approaching a plateau [Si(OH)4] of ca 370 lmol/L after approximately 300 h ageing (Fig. 2a). There was no Al post-filtration and so ca 800 lmol/L Al resulted in the co-precipitation of ca 400 lmol/L Si(OH)4 within 300–336 h. The Si:Al ratio of solid phases collected after 12 (Si0.44Al1.00), 72 (Si0.50Al1.00) and 336 h (Si0.57Al1.00) increased with ageing of solutions and mirrored the slow reductions in [Si(OH)4]soln.. Investigation of these three solid phases by TGA revealed a sharp exotherm at 191 °C at 12 h (Fig. 3) which was absent at 72 and 336 h. Each of
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800 lmol/L Al co-precipitated ca 400 lmol/L Si(OH)4 within 0.5 h and 600 lmol/L Si(OH)4 after 336 h. The Si:Al ratios of solid phases collected after 12 (Si0.61Al1.00), 72 (Si0.71Al1.00) and 336 h (Si0.84Al1.00) increased with ageing and reflected the continued precipitation of Si(OH)4 from the solutions. TGA of solid phases showed a small exotherm at 176 °C for the solid collected after 12 h (Fig. 3) which was not evident after 72 or 336 h. All three solid phases indicated endotherms at 115–120 °C and sharp exotherms at ca 980 °C.
Fig. 3. Representative TGA–DSC plots of solid phases collected after 12 h ageing at 20 °C from PIPES-buffered solutions containing (a) 2000 lmol/L Si(OH)4 and 800 lmol/L Al; (b) 1500 lmol/L Si(OH)4 and 800 lmol/L Al; (c) 1100 lmol/L Si(OH)4 and 800 lmol/L Al; (d) 800 lmol/L Si(OH)4 and 800 lmol/L Al; (e) 800 lmol/L Al. Heat flows are indicated by solid lines and weight losses by dashed lines.
the solid phases revealed an endotherm at 115–120 °C and a sharp exotherm at ca 980 °C. 3.2.2. 1100 lmol/L Si(OH)4:800 lmol/L Al The initial [Si(OH)4] was rapidly reduced to ca 700 lmol/L within 0.5 h and continued to fall to ca 500 lmol/L after ageing for 336 h (Fig. 2b). There was no Al post-filtration and ca 800 lmol/L Al co-precipitated ca 400 lmol/L Si(OH)4 within 0.5 h and 600 lmol/L Si(OH)4 after 336 h. The Si:Al ratios of solid phases collected after 12 (Si0.53Al1.00), 72 (Si0.61Al1.00) and 336 h (Si0.71Al1.00) increased with ageing and again mirrored the reductions in [Si(OH)4]soln.. TGA of solid phases revealed a sharp exothermic peak at 204 °C in the precipitate collected after 12 h ageing (Fig. 3) which was not present in precipitates collected after 24, 72, 168 and 336 h. Each of the solid phases revealed an endotherm at 115–120 °C and a sharp exotherm at ca 980 °C. 3.2.3. 1500 lmol/L Si(OH)4:800 lmol/L Al The initial [Si(OH)4] was rapidly reduced to ca 1100 lmol/L within 0.5 h and thereafter fell less quickly and reached a plateau [Si(OH)4] of ca 930 lmol/L after 336 h (Fig. 2c). There was no Al post-filtration and ca
3.2.4. 2000 lmol/L Si(OH)4:800 lmol/L Al The initial [Si(OH)4] was rapidly reduced to ca 1600 lmol/L within 0.5 h and continued to fall to ca 1300 lmol/L after more than 500 h ageing (Fig. 2d). There was no Al post-filtration and ca 800 lmol/L Al co-precipitated ca 400 lmol/L Si(OH)4 within 0.5 h and ca 700 lmol/ L Si(OH)4 after 500 h. These changes in [Si(OH)4]soln. were mirrored by increased Si:Al ratios of solid phases collected after 0.5 (Si0.60Al1.00), 12 (Si0.64Al1.00), 72 (Si0.77Al1.00) and 336 h (Si0.92Al1.00). TGA spectra for solid phases collected after 12, (Fig. 3) 72 and 336 h were identical and showed an endotherm at 115–120 °C and a sharp exotherm at ca 980 °C. Solid-state 27Al MAS NMR of solid phases collected after 0.5, 72 and 336 h revealed two discrete shifts at 1.25 (AlVI) and 55 (AlIV) ppm and that the proportion of AlIV to AlVI increased with the age of the solid phase (Fig. 4a). Solid-state 29Si MAS NMR (Fig. 4b) indicated a single shift, the peak of which increased from 83 to 89 ppm with increased age of the solid phase, which was generally indicative of Si in the vicinity of 1-2Al (Q3(1-2Al)). 4. Discussion The formation of HAS and their subsequent nucleation and growth towards aggregates which could be collected by 0.1 lm membrane filtration occurred in all combinations of concentrations of Si(OH)4 and Al. For some of the combinations the precipitation cascade was sufficiently rapid to enable solid phases to be collected within only 0.5 h ageing. The experiments in which different combinations of initial concentration of Si(OH)4 and ratio’s of Si(OH)4 to Al were aged for only 24 h appeared to demonstrate that within this time period the nature of the precipitate formed was finely balanced between three possible idealised forms; AlðOHÞ3ðsÞ $ HASA $ HASB In solutions in which the initial ratio of Si(OH)4 to Al was 0.5 (the stoichiometry of HASA) the amount of Si(OH)4 which was precipitated from solution was always less than would be expected if HASA was the only product (Fig. 1a). Post-filtration analyses of [Al] in each combination of initial [Si(OH)4] and Si(OH)4 to Al ratio indicated that ca 100% of the available Al was co-precipitated and thus the solid phases formed for Si(OH)4 to Al ratios of 0.5 must have included both Al(OH)3(s) and HASA. This indicated
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Fig. 4. Representative (a) 27Al and (b) 29Si NMR spectra of solid phases collected after 0.5, 72 and 336 h ageing at 20 °C from PIPES-buffered solutions containing 2000 lmol/L Si(OH)4 and 800 lmol/L Al.
that while HASA was rapidly formed something prevented all of the theoretically possible Si(OH)4 being incorporated into this phase within the 24 h period of ageing. The results of the precipitation experiments in which the initial ratio of Si(OH)4 to Al was 1.0 (the stoichiometry of HASB) suggested that the ‘something’ was the residual [Si(OH)4] which followed the initial rapid formation of HASA (Fig. 1b). In these experiments the amount of Si(OH)4 which was co-precipitated was in each case equivalent to what would have been expected from the formation of HASA (Fig. 1b). Thus HASA was the dominant solid phase formed under these conditions and the higher residual
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[Si(OH)4] which was due to the initial Si(OH)4 to Al ratio of 1.0 was critical in ensuring that all of the available Al was incorporated into HASA. For example, if we compare the efficacy with which 800 lmol/L Al precipitated Si(OH)4 in solutions in which the initial Si(OH)4 to Al ratio was either 0.5 (Fig. 1a) or 1.0 (Fig. 1b) we find that 248.4 ± 20.9 lmol/L of Si(OH)4 were precipitated in the former (ca 150 lmol/L Si(OH)4 remained in solution) whereas 374.7 ± 40.8 lmol/L Si(OH)4 were precipitated in the latter (ca 425 lmol/L Si(OH)4 remained in solution). These results have demonstrated that for initial solution conditions of 800 lmol/L Al and 400 lmol/L Si(OH)4 while HASA was the predominant solid phase formed the reaction was prevented from reaching completion (i.e. all Al(OH)3(s)/ Si(OH)4 precipitated as HASA) by a limiting residual [Si(OH)4]. However, when the initial [Si(OH)4] was 800 lmol/L the additional residual Si(OH)4 ensured that all available Al(OH)3(s) reacted with Si(OH)4 to give a solid phase consisting of ca pure HASA. By logical extension, an initial Si(OH)4 to Al ratio of 1.0 should, theoretically, have been sufficient to allow the formation of HASB and yet there was little evidence that HASB was formed in these solutions during 24 h ageing (Fig. 1b). Thus while the residual [Si(OH)4] promoted the formation of HASA it was not sufficient to support the subsequent formation of HASB through the reaction of Si(OH)4 with HASA. In these 24 h ageing experiments evidence for the formation of HASB was only obtained from those solutions in which the initial Si(OH)4 to Al ratio was 2.0 (Fig. 1c). However, it was not simply the ratio of Si(OH)4 to Al that was important since the evidence for the formation of HASB only became clear at initial [Si(OH)4] P 1200 lmol/L (Fig. 1c). The results of the 24 h precipitation experiments appeared to suggest that the formation of HAS was dependent upon the combination of both the initial [Si(OH)4] and the Si(OH)4 to Al ratio. In addition they afforded strong evidence that the formation of HASA was a prerequisite to the formation of HASB. When the formation of HAS in solutions including 800 lmol/L Al and 800, 1100, 1500 or 2000 lmol/L Si(OH)4 were investigated over time we found further evidence that HASA was a prerequisite to the formation of HASB. It was remarkable that in each of these conditions the initial [Si(OH)4] was reduced by ca 400 lmol/L within the first 0.5 h (Fig. 2a–d). This strongly suggested that regardless of the initial solution conditions the first step in the precipitation of HAS (or the co-precipitation of Si(OH)4 by Al) was the formation of HASA. Electron microprobe analyses of the stoichiometries of the solid phases confirmed that HASA was the predominant solid phase formed during the first 12 h in solutions which included 800 lmol/L Al and 800, 1100 or 1500 lmol/L Si(OH)4. Thereafter, ageing of solutions resulted in the further precipitation of Si(OH)4 and its concomitant incorporation into solid phases such that after 336 h there was clear evidence of the formation of HASB in those solutions in which Si(OH)4 had initially been present to excess
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(Fig. 2b–d). The formation of HASB appeared to be most rapid in the presence of the greatest excess of Si(OH)4 (Fig. 2d) and analyses of solid phases collected after only 0.5 h confirmed its presence in these rapidly formed precipitates. Solid-state 27Al NMR indicated the presence of AlVI and a significant component of AlIV the latter only being found in HASB [7]. In addition solid-state 29Si NMR did not indicate a significant shift at 78 ppm (Q3(3Al)) which would normally be indicative of HASA [7]. The results of the NMR did not tally well with those of filtration (only 400 lmol/L Si(OH)4 was co-precipitated at this time) or the electron microprobe as the Si:Al ratio of the solid phase collected after only 0.5 h was 0.60 and would normally be indicative of a phase composed primarily of HASA with, perhaps, some HASB. TGA of solid phases collected at 12h included exotherms in the region ca 175–200 °C for all conditions except that with the highest excess of Si(OH)4 (Fig. 3). Exotherms in this temperature region have not hitherto been observed for solid phases collected from solutions with initial Si(OH)4 to Al ratio’s of P1.0 [11]. However, the exotherms were not evident in solid phases collected from solutions after more than 12 h ageing. The initial presence and thereafter absence of exotherms in aged precipitates, including those collected from solutions which contained equimolar [Si(OH)4] and [Al] and were likely to be composed of almost exclusively HASA, strongly suggested that the exothermic peaks were not characteristic of HASA but indicative of contaminating amounts of Al(OH)3(s). The latter being absent in precipitates collected from solutions with the highest excess of Si(OH)4 and in precipitates collected from solutions after more than 12 h ageing. We have investigated the mechanism of formation of HAS and provided further evidence of equilibria between Al(OH)3(s), HASA and HASB and, in particular, the role of Si(OH)4 in determining which of these solid phases will predominate for any given solution conditions. We have shown that HASA is formed rapidly in solutions which include stoichiometric concentrations of Si(OH)4 and Al (i.e. 1:2) though the reaction does not approach completion (i.e. Si:Al ratio of 0.5 in solid phase) in the absence of an excess of Si(OH)4 in the ‘equilibrated’ solution (i.e. initial solution conditions of 1:1). We have observed that HASA was the likely initial product of the reaction of Al(OH)3(s) with Si(OH)4 even in solutions in which Si(OH)4 was present to significant excess and this supported the contention that HASA was the necessary precursor to HASB. The latter was not fully supported by the 29Si NMR of the
precipitate collected after only 0.5 h from solutions containing 2000 lmol/L Si(OH)4 and 800 lmol/L Al since we did not observe a significant HASA-characteristic shift at 78 ppm. Future research on similar freshly precipitated solid phases should help to resolve this conundrum. Finally TGA of the solid phases confirmed what we had believed to be true in our earlier study [11] that exotherms in the temperature region ca 175–300 °C were not characteristic of HASA but were actually indicative of the presence of residual Al(OH)3(s). In relation to the latter, we have investigated the thermal chemistry of Al(OH)3(s) prepared in the presence of PIPES and collected after only 12 h ageing. This solid phase was prepared as a control for the HAS phases. We observed two significant exotherms (Fig. 3e) one at ca 271 °C, similar to that seen previously [11] and a second at ca 771 °C which has not hitherto been observed. Remnants of the former were observed in several of the other solid phases collected after only 12 h (Fig. 3b–d) whereas the latter exotherm was absent from these precipitates. The significance of these exothermic peaks to the structure of Al(OH)3(s) formed in the presence of a pH buffer remain to be determined. Acknowledgements This research was funded by NERC. D.C. Apperley is thanked for all his help with the solid-state NMR which were obtained through the EPSRC solid-state NMR service at Durham. D. Plant is thanked for electron microprobe analyses which were carried out at the Electron Microprobe Facility at Manchester. References [1] C. Exley, J. Inorg. Biochem. 69 (1998) 139. [2] J.D. Birchall, C. Exley, J.S. Chappell, M.J. Phillips, Nature 338 (1989) 146. [3] C. Exley, J.D. Birchall, Polyhedron 11 (1992) 1901. [4] C. Exley, J. K Pinnegar, H. Taylor, J. Theor. Biol. 189 (1997) 133. [5] C. Exley, J.D. Birchall, Polyhedron 12 (1993) 1007. [6] C. Exley, C. Schneider, F.J. Doucet, Coord. Chem. Rev. 228 (2002) 127. [7] F.J. Doucet, C. Schneider, S.J. Bones, A. Kretchmer, I. Moss, P. Tekely, C. Exley, Geochim. Cosmochim. Acta 65 (2001) 2461. [8] S. Strekopytov, C. Exley, Polyhedron 24 (2005) 1585. [9] Department of the Environment, Silicon in Waters and Effluents, HMSO, London, 1980. [10] C. Schneider, C. Exley, J. Inorg. Biochem. 87 (2001) 45. [11] S. Strekopytov, C. Exley, Polyhedron 25 (2006) 1707.
Further insight into the mechanism of formation of hydroxyaluminosilicates Stanislav Strekopytov 1, Eve Jarry, Christopher Exley
*
Birchall Centre for Inorganic Chemistry and Materials Science, Lennard-Jones Laboratories, Keele University, Staffordshire ST5 5BG, UK Received 23 March 2006; accepted 16 June 2006 Available online 30 June 2006
Abstract Hydroxyaluminosilicates (HAS) are formed by the reaction of silicic acid (Si(OH)4) with adjacent hydroxyl groups on an aluminium hydroxide (Al(OH)3(s)) framework or template. They are important secondary mineral phases in the biogeochemical cycling of aluminium and are extremely insoluble. Two discrete forms of HAS have been identified. HASA which is formed when [Si(OH)4] 6 [Al] and HASB which is formed when [Si(OH)4] [Al]. The formation of HASB has been suggested to involve the further reaction of Si(OH)4 with HASA and it is this contention that, in the main, we have tested herein. Applying a number of analytical and structural tools we have demonstrated the critical importance of both absolute concentrations and relative ratio’s of Si(OH)4 and Al in solution in determining which form of HAS will be precipitated from solution. In addition, by collecting HAS both almost immediately upon their precipitation from solutions (ca 0.5 h) and after ageing in solutions for up to 336 h and analysing their stoichiometries and structural configurations we have shown that the formation of HASA was the first step in the formation of HAS including in those solutions in which [Si(OH)4] [Al] and ultimately (P72 h ageing) only HASB was identified. These are the first experimental results which support the long held belief that Al(OH)3(s) is a prerequisite to the formation of HASA which, in turn, is the precursor to the formation of HASB. The insight we have gained should enable a better understanding of the role of HAS both in their control of the biological availability of aluminium and in their potential future applications in materials science. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Aluminium; Silicic acid; Hydroxyaluminosilicate; Biogeochemistry; Bioinorganic chemistry
1. Introduction Silicic acid (Si(OH)4) is the neutral, monomeric form of biologically available silicon and is characterised by an extremely limited inorganic chemistry [1]. We were surprised to find that Si(OH)4 protected against acute aluminium toxicity in fish in acid waters [2] and this finding led us to propose that the reaction of Si(OH)4 with aluminium to form hydroxyaluminosilicates (HAS) might explain this ameliorative effect [3]. Subsequent research has supported the contention that HAS were not acutely toxic to fish in *
Corresponding author. Tel.: +44 1782 584080; fax: +44 1782 712378. E-mail address: [email protected] (C. Exley). 1 Present address: Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK. 0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.06.013
acid water [4]. We have investigated the mechanism of formation of HAS [5] and established that it involves the competitive condensation of Si(OH)4 across adjacent hydroxyl groups on a framework of aluminium hydroxide (Al(OH)3(s)) [6]. Subsequently we precipitated HAS under different solution conditions and, following their structural characterisation by, for example, solid-state NMR, we were equally surprised to find two discrete forms which we imaginatively called HASA and HASB [7]. HASA was formed under conditions where [Al] P [Si(OH)4] and had an ideal Si:Al ratio of 0.5 and all of the constituent Al was octahedrally coordinated. HASB was formed under conditions when [Si(OH)4] [Al] and had an ideal Si:Al ratio of 1.0 with up to 50% of the constituent Al being in tetrahedral coordination. The existence of HASB was the major surprise, particularly as its formation appeared to
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involve a room temperature dehydroxylation of the HASA form. We proposed that further substitution of Si(OH)4 into HASA was driving the dehydroxylation [8] and it is this proposition that is tested further in the current research. We have used a combination of solution and solid-state structural techniques to provide evidence that the formation HASA is a prerequisite to the formation of HASB. 2. Experimental 2.1. Preparation of solutions All reagents used were of the highest purity available. Solutions which included a wide range of concentrations of Si(OH)4 (25–2000 lmol/L) applied across three molar ratios (0.5, 1.0, 2.0) of Si(OH)4 to Al were prepared in PTFE phials and containers (Cowie Technology Ltd., UK) using ultrapure water (conductivity < 0.067 lS/cm, Elga UK) which included 100 mmol/L KNO3 as background electrolyte and which were buffered at pH 6.0 with 25 mmol/L PIPES (piperazine-N,N 0 -bis-2-ethanesulphonic acid). Stock solutions of Si(OH)4, up to a maximum concentration of 2.0 mmol/L, were prepared by cation exchange of Na4SiO4 and either diluted as required or used in the place of ultrapure water. Al was added from a freshly prepared 1 mol/L stock of Al(NO3)3 Æ 9H2O in 1% HNO3. An established method was used for the preparation of all HAS solutions [5] and all solutions were incubated at 20 °C for the requisite time period prior to sampling. The precipitation experiments (see Fig. 1) involved the preparation of 20 mL volumes of 5 replicates of 21 different combinations of Si(OH)4 and Al. These were aged for 24 h and then filtered through 0.1 lm polyvinylidene fluoride membrane filters (Durapore). For the ageing study (see Fig. 2) 100 mL volumes of 5 replicates of 4 preparations which included 800 lmol/L Al and either 800, 1100, 1500 or 2000 lmol/L Si(OH)4 were aged for up to 500 h. These solutions were periodically sampled and filtered using syringe filters (Millipore) which incorporated the same 0.1 lm polyvinylidene fluoride membrane filters (Durapore). Analagous solutions of larger volume, 500 mL, were run in parallel and, following ageing for typically, 12, 72, 168 or 336 h, were filtered (0.1 lm membrane filtration) to collect significant quantities (>200 mg) of precipitate for subsequent solid-state analyses. For all studies, postfiltration [Si(OH)4] were measured spectrophotometrically (kmax 810 nm) as the reduced blue-molybdosilicic acid complex [9] and post-filtration [Al] by graphite furnace atomic absorption spectrometry (GFAAS) using a method developed in our laboratory [10]. 2.2. Solid-state analyses Solid phases collected by membrane filtration were dried to a constant weight at 37 °C and thereafter gently ground
Fig. 1. The precipitation of Si(OH)4 from solutions (Si(OH)4 ppt) in which the initial ratio of Si(OH)4 to Al was (a) 1:2; (b) 1:1; (c) 2:1. Mean and SD are plotted, n = 5. Solid lines indicate the theoretical precipitation of Si(OH)4 if all of the available Al were precipitated by the formation of HASA (a) and HASA or HASB (b,c).
to a fine powder using an agate mortar and pestle. The elemental composition, structural identity and thermal chemistry of each of the solid phases were determined by electron microprobe analysis, solid-state NMR and thermogravimetric analysis, respectively, using methods which have been described in detail in recent publications [8,11].
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Fig. 2. The concentration of Si(OH)4 remaining in solution (Si(OH)4soln.) following the reaction of (a) 800; (b) 1100; (c) 1500; (d) 2000 lmol/L Si(OH)4 with 800 lmol/L Al and ageing of the solutions at 20 °C for up to 500 h. Mean and SD are plotted, n = 5. The arrows indicate the times at which solid phases were collected and their relative contents of Si and Al determined by electron probe microanalysis.
3. Results
3.2. Influence of ageing on the precipitation of Si(OH)4 and the nature of the solid phase formed
3.1. Precipitation of Si(OH)4 by Al Si(OH)4 was precipitated within the 24 h period of ageing under each of the solution conditions which ranged from ca 25 lmol/L Si(OH)4/50 lmol/L Al to ca 2000 lmol/ L Si(OH)4/1000 lmol/L Al. The proportion of available Si(OH)4 precipitated in the former was ca 20% whereas this increased up to ca 100% in the latter. There was no Al postfiltration for any of the initial combinations of Si(OH)4 and Al. When these filtration data were expressed according to the theoretical composition of the solid phase formed (i.e. either HASA or HASB) it was clear that the probable composition of precipitate depended upon the initial solution concentrations of Si(OH)4 and Al and their ratio (Fig. 1). Thus for Si(OH)4:Al ratios of 0.5 and 1.0 the filtration data predicted the formation of HASA whereas for a ratio of 2.0 the data predicted the formation of HASA at lower total concentrations (<1000 lmol/L Si(OH)4) and HASB, or a mixture of HASA and HASB, at higher total concentrations (1000–2000 lmol/L Si(OH)4) (Fig. 1).
Four different solution compositions were investigated over an ageing period up to 500 h (Fig. 2). The compositions were chosen to reflect a range of Si(OH)4 to Al ratios over which the formation of either HASA or HASB was predicted [7]. 3.2.1. 800 lmol/L Si(OH)4:800 lmol/L Al The [Si(OH)4] was rapidly reduced to ca 500 lmol/L within only 0.5 h ageing and thereafter continued to fall steadily before approaching a plateau [Si(OH)4] of ca 370 lmol/L after approximately 300 h ageing (Fig. 2a). There was no Al post-filtration and so ca 800 lmol/L Al resulted in the co-precipitation of ca 400 lmol/L Si(OH)4 within 300–336 h. The Si:Al ratio of solid phases collected after 12 (Si0.44Al1.00), 72 (Si0.50Al1.00) and 336 h (Si0.57Al1.00) increased with ageing of solutions and mirrored the slow reductions in [Si(OH)4]soln.. Investigation of these three solid phases by TGA revealed a sharp exotherm at 191 °C at 12 h (Fig. 3) which was absent at 72 and 336 h. Each of
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800 lmol/L Al co-precipitated ca 400 lmol/L Si(OH)4 within 0.5 h and 600 lmol/L Si(OH)4 after 336 h. The Si:Al ratios of solid phases collected after 12 (Si0.61Al1.00), 72 (Si0.71Al1.00) and 336 h (Si0.84Al1.00) increased with ageing and reflected the continued precipitation of Si(OH)4 from the solutions. TGA of solid phases showed a small exotherm at 176 °C for the solid collected after 12 h (Fig. 3) which was not evident after 72 or 336 h. All three solid phases indicated endotherms at 115–120 °C and sharp exotherms at ca 980 °C.
Fig. 3. Representative TGA–DSC plots of solid phases collected after 12 h ageing at 20 °C from PIPES-buffered solutions containing (a) 2000 lmol/L Si(OH)4 and 800 lmol/L Al; (b) 1500 lmol/L Si(OH)4 and 800 lmol/L Al; (c) 1100 lmol/L Si(OH)4 and 800 lmol/L Al; (d) 800 lmol/L Si(OH)4 and 800 lmol/L Al; (e) 800 lmol/L Al. Heat flows are indicated by solid lines and weight losses by dashed lines.
the solid phases revealed an endotherm at 115–120 °C and a sharp exotherm at ca 980 °C. 3.2.2. 1100 lmol/L Si(OH)4:800 lmol/L Al The initial [Si(OH)4] was rapidly reduced to ca 700 lmol/L within 0.5 h and continued to fall to ca 500 lmol/L after ageing for 336 h (Fig. 2b). There was no Al post-filtration and ca 800 lmol/L Al co-precipitated ca 400 lmol/L Si(OH)4 within 0.5 h and 600 lmol/L Si(OH)4 after 336 h. The Si:Al ratios of solid phases collected after 12 (Si0.53Al1.00), 72 (Si0.61Al1.00) and 336 h (Si0.71Al1.00) increased with ageing and again mirrored the reductions in [Si(OH)4]soln.. TGA of solid phases revealed a sharp exothermic peak at 204 °C in the precipitate collected after 12 h ageing (Fig. 3) which was not present in precipitates collected after 24, 72, 168 and 336 h. Each of the solid phases revealed an endotherm at 115–120 °C and a sharp exotherm at ca 980 °C. 3.2.3. 1500 lmol/L Si(OH)4:800 lmol/L Al The initial [Si(OH)4] was rapidly reduced to ca 1100 lmol/L within 0.5 h and thereafter fell less quickly and reached a plateau [Si(OH)4] of ca 930 lmol/L after 336 h (Fig. 2c). There was no Al post-filtration and ca
3.2.4. 2000 lmol/L Si(OH)4:800 lmol/L Al The initial [Si(OH)4] was rapidly reduced to ca 1600 lmol/L within 0.5 h and continued to fall to ca 1300 lmol/L after more than 500 h ageing (Fig. 2d). There was no Al post-filtration and ca 800 lmol/L Al co-precipitated ca 400 lmol/L Si(OH)4 within 0.5 h and ca 700 lmol/ L Si(OH)4 after 500 h. These changes in [Si(OH)4]soln. were mirrored by increased Si:Al ratios of solid phases collected after 0.5 (Si0.60Al1.00), 12 (Si0.64Al1.00), 72 (Si0.77Al1.00) and 336 h (Si0.92Al1.00). TGA spectra for solid phases collected after 12, (Fig. 3) 72 and 336 h were identical and showed an endotherm at 115–120 °C and a sharp exotherm at ca 980 °C. Solid-state 27Al MAS NMR of solid phases collected after 0.5, 72 and 336 h revealed two discrete shifts at 1.25 (AlVI) and 55 (AlIV) ppm and that the proportion of AlIV to AlVI increased with the age of the solid phase (Fig. 4a). Solid-state 29Si MAS NMR (Fig. 4b) indicated a single shift, the peak of which increased from 83 to 89 ppm with increased age of the solid phase, which was generally indicative of Si in the vicinity of 1-2Al (Q3(1-2Al)). 4. Discussion The formation of HAS and their subsequent nucleation and growth towards aggregates which could be collected by 0.1 lm membrane filtration occurred in all combinations of concentrations of Si(OH)4 and Al. For some of the combinations the precipitation cascade was sufficiently rapid to enable solid phases to be collected within only 0.5 h ageing. The experiments in which different combinations of initial concentration of Si(OH)4 and ratio’s of Si(OH)4 to Al were aged for only 24 h appeared to demonstrate that within this time period the nature of the precipitate formed was finely balanced between three possible idealised forms; AlðOHÞ3ðsÞ $ HASA $ HASB In solutions in which the initial ratio of Si(OH)4 to Al was 0.5 (the stoichiometry of HASA) the amount of Si(OH)4 which was precipitated from solution was always less than would be expected if HASA was the only product (Fig. 1a). Post-filtration analyses of [Al] in each combination of initial [Si(OH)4] and Si(OH)4 to Al ratio indicated that ca 100% of the available Al was co-precipitated and thus the solid phases formed for Si(OH)4 to Al ratios of 0.5 must have included both Al(OH)3(s) and HASA. This indicated
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Fig. 4. Representative (a) 27Al and (b) 29Si NMR spectra of solid phases collected after 0.5, 72 and 336 h ageing at 20 °C from PIPES-buffered solutions containing 2000 lmol/L Si(OH)4 and 800 lmol/L Al.
that while HASA was rapidly formed something prevented all of the theoretically possible Si(OH)4 being incorporated into this phase within the 24 h period of ageing. The results of the precipitation experiments in which the initial ratio of Si(OH)4 to Al was 1.0 (the stoichiometry of HASB) suggested that the ‘something’ was the residual [Si(OH)4] which followed the initial rapid formation of HASA (Fig. 1b). In these experiments the amount of Si(OH)4 which was co-precipitated was in each case equivalent to what would have been expected from the formation of HASA (Fig. 1b). Thus HASA was the dominant solid phase formed under these conditions and the higher residual
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[Si(OH)4] which was due to the initial Si(OH)4 to Al ratio of 1.0 was critical in ensuring that all of the available Al was incorporated into HASA. For example, if we compare the efficacy with which 800 lmol/L Al precipitated Si(OH)4 in solutions in which the initial Si(OH)4 to Al ratio was either 0.5 (Fig. 1a) or 1.0 (Fig. 1b) we find that 248.4 ± 20.9 lmol/L of Si(OH)4 were precipitated in the former (ca 150 lmol/L Si(OH)4 remained in solution) whereas 374.7 ± 40.8 lmol/L Si(OH)4 were precipitated in the latter (ca 425 lmol/L Si(OH)4 remained in solution). These results have demonstrated that for initial solution conditions of 800 lmol/L Al and 400 lmol/L Si(OH)4 while HASA was the predominant solid phase formed the reaction was prevented from reaching completion (i.e. all Al(OH)3(s)/ Si(OH)4 precipitated as HASA) by a limiting residual [Si(OH)4]. However, when the initial [Si(OH)4] was 800 lmol/L the additional residual Si(OH)4 ensured that all available Al(OH)3(s) reacted with Si(OH)4 to give a solid phase consisting of ca pure HASA. By logical extension, an initial Si(OH)4 to Al ratio of 1.0 should, theoretically, have been sufficient to allow the formation of HASB and yet there was little evidence that HASB was formed in these solutions during 24 h ageing (Fig. 1b). Thus while the residual [Si(OH)4] promoted the formation of HASA it was not sufficient to support the subsequent formation of HASB through the reaction of Si(OH)4 with HASA. In these 24 h ageing experiments evidence for the formation of HASB was only obtained from those solutions in which the initial Si(OH)4 to Al ratio was 2.0 (Fig. 1c). However, it was not simply the ratio of Si(OH)4 to Al that was important since the evidence for the formation of HASB only became clear at initial [Si(OH)4] P 1200 lmol/L (Fig. 1c). The results of the 24 h precipitation experiments appeared to suggest that the formation of HAS was dependent upon the combination of both the initial [Si(OH)4] and the Si(OH)4 to Al ratio. In addition they afforded strong evidence that the formation of HASA was a prerequisite to the formation of HASB. When the formation of HAS in solutions including 800 lmol/L Al and 800, 1100, 1500 or 2000 lmol/L Si(OH)4 were investigated over time we found further evidence that HASA was a prerequisite to the formation of HASB. It was remarkable that in each of these conditions the initial [Si(OH)4] was reduced by ca 400 lmol/L within the first 0.5 h (Fig. 2a–d). This strongly suggested that regardless of the initial solution conditions the first step in the precipitation of HAS (or the co-precipitation of Si(OH)4 by Al) was the formation of HASA. Electron microprobe analyses of the stoichiometries of the solid phases confirmed that HASA was the predominant solid phase formed during the first 12 h in solutions which included 800 lmol/L Al and 800, 1100 or 1500 lmol/L Si(OH)4. Thereafter, ageing of solutions resulted in the further precipitation of Si(OH)4 and its concomitant incorporation into solid phases such that after 336 h there was clear evidence of the formation of HASB in those solutions in which Si(OH)4 had initially been present to excess
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(Fig. 2b–d). The formation of HASB appeared to be most rapid in the presence of the greatest excess of Si(OH)4 (Fig. 2d) and analyses of solid phases collected after only 0.5 h confirmed its presence in these rapidly formed precipitates. Solid-state 27Al NMR indicated the presence of AlVI and a significant component of AlIV the latter only being found in HASB [7]. In addition solid-state 29Si NMR did not indicate a significant shift at 78 ppm (Q3(3Al)) which would normally be indicative of HASA [7]. The results of the NMR did not tally well with those of filtration (only 400 lmol/L Si(OH)4 was co-precipitated at this time) or the electron microprobe as the Si:Al ratio of the solid phase collected after only 0.5 h was 0.60 and would normally be indicative of a phase composed primarily of HASA with, perhaps, some HASB. TGA of solid phases collected at 12h included exotherms in the region ca 175–200 °C for all conditions except that with the highest excess of Si(OH)4 (Fig. 3). Exotherms in this temperature region have not hitherto been observed for solid phases collected from solutions with initial Si(OH)4 to Al ratio’s of P1.0 [11]. However, the exotherms were not evident in solid phases collected from solutions after more than 12 h ageing. The initial presence and thereafter absence of exotherms in aged precipitates, including those collected from solutions which contained equimolar [Si(OH)4] and [Al] and were likely to be composed of almost exclusively HASA, strongly suggested that the exothermic peaks were not characteristic of HASA but indicative of contaminating amounts of Al(OH)3(s). The latter being absent in precipitates collected from solutions with the highest excess of Si(OH)4 and in precipitates collected from solutions after more than 12 h ageing. We have investigated the mechanism of formation of HAS and provided further evidence of equilibria between Al(OH)3(s), HASA and HASB and, in particular, the role of Si(OH)4 in determining which of these solid phases will predominate for any given solution conditions. We have shown that HASA is formed rapidly in solutions which include stoichiometric concentrations of Si(OH)4 and Al (i.e. 1:2) though the reaction does not approach completion (i.e. Si:Al ratio of 0.5 in solid phase) in the absence of an excess of Si(OH)4 in the ‘equilibrated’ solution (i.e. initial solution conditions of 1:1). We have observed that HASA was the likely initial product of the reaction of Al(OH)3(s) with Si(OH)4 even in solutions in which Si(OH)4 was present to significant excess and this supported the contention that HASA was the necessary precursor to HASB. The latter was not fully supported by the 29Si NMR of the
precipitate collected after only 0.5 h from solutions containing 2000 lmol/L Si(OH)4 and 800 lmol/L Al since we did not observe a significant HASA-characteristic shift at 78 ppm. Future research on similar freshly precipitated solid phases should help to resolve this conundrum. Finally TGA of the solid phases confirmed what we had believed to be true in our earlier study [11] that exotherms in the temperature region ca 175–300 °C were not characteristic of HASA but were actually indicative of the presence of residual Al(OH)3(s). In relation to the latter, we have investigated the thermal chemistry of Al(OH)3(s) prepared in the presence of PIPES and collected after only 12 h ageing. This solid phase was prepared as a control for the HAS phases. We observed two significant exotherms (Fig. 3e) one at ca 271 °C, similar to that seen previously [11] and a second at ca 771 °C which has not hitherto been observed. Remnants of the former were observed in several of the other solid phases collected after only 12 h (Fig. 3b–d) whereas the latter exotherm was absent from these precipitates. The significance of these exothermic peaks to the structure of Al(OH)3(s) formed in the presence of a pH buffer remain to be determined. Acknowledgements This research was funded by NERC. D.C. Apperley is thanked for all his help with the solid-state NMR which were obtained through the EPSRC solid-state NMR service at Durham. D. Plant is thanked for electron microprobe analyses which were carried out at the Electron Microprobe Facility at Manchester. References [1] C. Exley, J. Inorg. Biochem. 69 (1998) 139. [2] J.D. Birchall, C. Exley, J.S. Chappell, M.J. Phillips, Nature 338 (1989) 146. [3] C. Exley, J.D. Birchall, Polyhedron 11 (1992) 1901. [4] C. Exley, J. K Pinnegar, H. Taylor, J. Theor. Biol. 189 (1997) 133. [5] C. Exley, J.D. Birchall, Polyhedron 12 (1993) 1007. [6] C. Exley, C. Schneider, F.J. Doucet, Coord. Chem. Rev. 228 (2002) 127. [7] F.J. Doucet, C. Schneider, S.J. Bones, A. Kretchmer, I. Moss, P. Tekely, C. Exley, Geochim. Cosmochim. Acta 65 (2001) 2461. [8] S. Strekopytov, C. Exley, Polyhedron 24 (2005) 1585. [9] Department of the Environment, Silicon in Waters and Effluents, HMSO, London, 1980. [10] C. Schneider, C. Exley, J. Inorg. Biochem. 87 (2001) 45. [11] S. Strekopytov, C. Exley, Polyhedron 25 (2006) 1707.