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silica co-precipitates
Journal of Colloid and Interface Science 348 (2010) 65–70
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Regular Article
Synthesis and characterisation of ferrihydrite/silica co-precipitates Laurence Dyer a, Phillip D. Fawell b, O.M.G. Newman c, William R. Richmond a,* a
Nanochemistry Research Institute, Department of Chemistry and Parker Centre for Integrated Hydrometallurgy Solutions, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia b CSIRO Minerals and Parker Centre for Integrated Hydrometallurgy Solutions, PO Box 90, Bentley, Western Australia 6982, Australia c Nyrstar Metals Ltd., PO Box 377, Hobart, Tasmania 7009, Australia
a r t i c l e
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Article history: Received 12 January 2010 Accepted 26 March 2010 Available online 14 April 2010 Keywords: Ferrihydrite Silica Continuous crystallisation Composite materials
a b s t r a c t The effect of the presence of soluble silicates on ferrihydrite precipitation and some properties of the products formed in co-precipitation of ferrihydrite and silica have been investigated. The co-precipitates were formed using a continuous crystallisation process in which a combined iron/silicon feed solution was reacted with sodium hydroxide at a constant rate, while maintaining pH at 2.65 and temperature at 85 °C. The products of co-precipitation and the supernatant solutions were characterised using a variety of analytical techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM) and surface charge measurements. The addition of silicates was shown to have a significant impact on the crystallinity and surface charge of the precipitates formed. For products collected after five residence times in the continuous crystalliser, co-precipitates formed from ferric sulfate solution were found to contain considerably less silica than those formed from ferric nitrate. We conclude that adsorption of silicate species on ferrihydrite surfaces speeds up the polymerisation process, and that sulfate ion competes with silicate for surface adsorption sites. Thus, the precipitation of silica proceeds much more rapidly in ferric nitrate media, than in ferric sulfate. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Ferrihydrite (5Fe2O39H2O) is a poorly-crystalline iron oxyhydroxide that is common in the environment, precipitating in iron-laden springs, drainage lines, stagnant water soils and river sediments [1]. It is also a common form of biogenic iron, formed as a precursor to more stable iron oxides such as magnetite and hematite, which occur as components of limpet teeth, for example [1,2]. Fe(II)-oxidising bacteria can induce ferrihydrite formation in both intra-cellular and extra-cellular pathways. Ferrihydrite precipitation occurs under a wide variety of conditions, some of which produce ferrihydrite only, while others produce a mixture of iron oxide or iron oxyhydroxide phases. Controlled precipitation of iron from solution is important in many industrial processes, and it is essential in some hydrometallurgical processes – such as zinc and nickel recovery for example. In many situations where ferrihydrite occurs, dissolved silicates are also present. Examples where the concentrations of iron are reasonably high include industrial process liquors, thermal springs, mine waste residues and acid mine drainage sites. The stability and adsorptive properties of ferrihydrite derived from mine wastes or mineral processing residues is a critical issue in determining their
* Corresponding author. Fax: +61 4 (08) 9266 4699. E-mail address: [email protected] (W.R. Richmond). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.03.056
handling and disposal. These residues often contain associated adsorbed species such as As, Sb, Cu and Cd, so it is important that they remain stable in the environment in order for these contaminants to be contained. Ferrihydrite’s low crystallinity and nanoscale crystallite size have presented a challenge to those trying to determine its structure, and it is only in recent years that a detailed crystal structure of ferrihydrite has been reported [3]. Ferrihydrite has been classified according to the number of reflections in its XRD pattern: 6-line ferrihydrite and 2-line ferrihydrite being two extremes in a continuum of phases with XRD patterns displaying a decrease in long-range order which can be explained by a decrease in the size of coherent scattering domains in ferrihydrite – the average size of such domains is about 6 nm in 6-line ferrihydrite and 2 nm in 2-line ferrihydrite [7]. Soluble silicates have been shown to adsorb readily onto ferrihydrite surfaces, and to have a significant effect on the structural characteristics of ferrihydrite precipitates [4]. The presence of silicates is also known to inhibit the transformation of ferrihydrite to more stable iron oxide phases such as goethite and hematite [5,6]. The generally accepted mechanism for silica precipitation involves the polymerisation of monosilicic acid to form silicate oligomers, and subsequent dehydration to form silica. Polymerisation of silicic acid occurs spontaneously at concentrations exceeding 100–200 ppm [7]. Conditions that govern silicate polymerisation
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are the silicate concentration, pH, temperature and the concentration of other ions [8]. At low pH, polymerisation occurs relatively slowly, and particle aggregation dominates over growth [9]. This is a consequence of the low point of zero charge for silica (PZC – pH 2); at higher pH the particles are negatively charged and do not aggregate as quickly. Aggregation is a significant factor when silica co-precipitates during the acid hydrolysis of iron, since at moderately low pH, ferrihydrite particles are positively charged and silica particles are negatively charged. Previous studies of ferrihydrite/silica co-precipitates are limited to very few examples although some examination of the properties of mixed oxides of iron and silicon has been reported, generally describing sol–gel precipitate composites. However, little work has been reported on continuous co-precipitation processes or the products thereof. There is a void in current knowledge as to how these species are produced and how they behave when precipitated simultaneously. Some mechanistic aspects have been proposed – such as formation of a silica layer on the surface of ferrihydrite particles [10] – but in general there is very little structural information about ferrihydrite/silica co-precipitates. Some precipitate properties have been analysed, including crystallinity, structure and surface charge. The presence of dissolved silica during iron oxide/oxyhydroxide precipitation has been shown to decrease the crystallinity of ferrihydrite [11–13]. In experiments where silica was precipitated onto iron particles it was shown that silicic acid monomers bond to the iron oxide phase that inevitably forms on the metal surface through Si–O–Fe bonds [14–16], and the formation of such a chemical linkage was also implicated in the case of iron oxide coated silica particles [17]. When concentrations are sufficiently high, soluble silicates polymerise, bonding to the Si species adsorbed on an iron oxide surface through silanol bridges (Si–O–Si) [7,10,16]. A study of goethite precipitation in the presence of silicates has shown uptake of Si on the particle surfaces, but none throughout single crystal domains. In this case, non-surface associated Si was thought to be present in layers between crystal boundaries in aggregates [18]. Many studies have demonstrated that the presence of silica in iron oxyhydroxides increases their stability with respect to transformation into other iron oxide phases. Pure 2-line ferrihydrite transforms to hematite at 340 °C, while in silica-associated 2-line ferrihydrite the transformation temperature has been shown to increase to 740 °C at a Si/(Si + Fe) ratio of 0.27 [19]. In aqueous suspension, the transformation time for ferrihydrite to goethite increases from less than 24 h to 1–2 weeks with the addition of Si at high pH [20]. The PZC of ferrihydrite is approximately 8, but when the Si/Fe ratio reaches 0.35 it becomes closer to 4, although some literature surface charge values have been shown to be flawed [21]. In studies where iron oxide particles have been coated with silica, the PZC is dominated by the silica as would be expected [22]. However, the surface charge of both ferrihydrite and silica particles has been shown to vary significantly depending on the type and concentration of ions in solution [23,24]. This paper examines the structure and properties of the precipitates formed in iron silica co-precipitation reactions as well as the effect of altering the proportion of silica present on the reaction itself and on the material produced. In addition, we describe the use of energy filtered TEM to examine the distribution of silica and iron within aggregates in the co-precipitates; a level of structural detail that has not been described previously. The samples have been produced in a continuous crystallisation set-up, with constant reaction conditions operating at steady state so as to give the best approximation for systems occurring both naturally and industrially. Temperature and pH conditions were chosen to broadly approximate those found in some hydrometallurgical iron-removal stages, and thus we anticipate that the results may be a useful aid to understanding the behaviour of such systems.
2. Materials and methods Precipitates were produced in a continuous crystallisation apparatus comprising a 110 mL jacketed glass crystallisation vessel and three medical dosing pumps. The reactor has an internal diameter and height of 50 mm and 75 mm respectively. An agitator with four angled blades rotated at 570 rpm was positioned approximately 5 mm from the bottom of the reactor. Feed solutions (iron(III)/silicon and NaOH) were delivered into the agitated, temperature-controlled crystallisation vessel at constant rates using two medical dosing pumps: one to deliver the Fe/Si solution and the other to deliver the NaOH solution. A third pump was used for product removal. In a range of co-precipitation experiments, both ferric nitrate and ferric sulfate were used as the source of iron(III) and these feed solutions were dosed with various amounts of silica, which was added as sodium metasilicate. All Fe/Si feed solutions had a total Fe + Si concentration of 0.1128 mol L 1 while Si:Fe ratios were varied between 0 and 0.43. The sodium hydroxide concentration was 8 mol L 1 and the feed rate of the NaOH delivery pump was adjusted to control the pH during the precipitation experiments. The reaction mixtures were maintained at 85 °C and pH 2.65 and flow rates were set to achieve a mean residence time of 45 min. In a typical experiment the reactor was operated over a period of at least five residence times to achieve steady state conditions. Product samples were collected at regular intervals and XRD patterns examined to ensure that steady state was achieved. These conditions were found to produce 6-line ferrihydrite, among more crystalline iron oxide/oxyhydroxide phases, at steady state in silica-free control runs. Samples for X-ray diffraction were prepared by filtering through 0.45 lm membranes and drying the solid in an oven at 60 °C. X-ray powder diffraction was carried out at 25 °C using Cu Ka radiation (40 kV, 30 mA) on a Seimens D500 powder diffractometer, or on a Phillips X’pert powder diffractometer, with a cobalt long fine focus tube. Line profile analysis, and deconvolution of multiple peaks in the XRD patterns was carried out using the Xfit program.1 This analysis provided full width at half maximum (FWHM) values and peak positions for selected reflections in the ferrihydrite patterns. Peak positions were determined relative to a CaF2 internal standard. Slurry samples were filtered twice through 0.45 lm syringe filters to remove solids in preparation for ICP-AES. Analyses were conducted using both an ICP-AES Vista and 735-ES instruments. For surface charge measurements, dilute suspensions of precipitates were analysed on a Malvern Zetanano DLS particle sizer. For these measurements, samples were dialysed with pure water (MilliQÒ), then diluted and the pH adjusted to the desired value using nitric acid or sulfuric acid as appropriate. Specimens for transmission electron microscopy (TEM) were prepared by dialysing a product slurry with pure water to remove surface-adsorbed species and then diluting further. A droplet of this diluted suspension was then placed onto a conventional holey-carbon grid and allowed to dry in air. The grids were examined using a JEOL 2011 TEM for basic imaging, and a JEOL 3000F or JEOL 2100 TEM for high resolution or energy filtered TEM (EFTEM) imaging.
3. Results and discussion Powder X-ray diffraction patterns for co-precipitates prepared using a ferric nitrate feed solution reveal subtle changes in the crystallinity of the ferrihydrite/silica products as the silica content 1 Cheary R.W., Coelho, A.A., Programs Xfit and FOURYA. CCP14 Powder diffraction Library–Daresbury Laboratory, Warrington UK; 1996.
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of the feed increases. The control sample displayed the characteristic pattern of 6-line ferrihydrite [1] along with a few sharper peaks indicating the presence of small amounts of poorly-crystalline goethite and hematite. As the Si concentration was increased, the peaks due to these more crystalline phases broadened and at the highest Si concentrations were not present at all. The ferrihydrite peaks appeared to broaden a little as the Si concentration was increased. In the patterns of samples with high Si levels, a very broad, low intensity background peak could be seen in the patterns, centered near 25–30° 2h, which we attributed to the presence of amorphous silica. It has been documented that ferrihydrite decreases in crystallinity when precipitated in the presence of soluble silicates [25]; however, there is little information as to why. Michel et al. [26] presented pair distribution function (PDF) analysis of X-ray scattering data which suggested that the only difference between 6line ferrihydrite and its less crystalline phases is the size of the coherent scattering domains. Thus if the crystallinity is governed by the particle size, the observed decrease in crystallinity would suggest that adsorbed silicates are restricting the growth of the individual ferrihydrite particles. Interestingly, the sulfate systems behaved slightly differently – in this case the control sample provided a pattern with considerably less evidence of goethite formation than was the case in the nitrate system. In addition there was very little, if any, indication of a change in crystallinity with increasing silicate levels XRD. A slight peak-broadening was observed as the Si content increased but the diffraction pattern of the product obtained from the 30% Si system still contained all six characteristic peaks of 6-line ferrihydrite. In an attempt to quantify the observed change in crystallinity, line profile analysis was conducted using the Xfit program to obtain peak widths and positions. Peak positions were determined in samples containing CaF2 as an internal standard. The full width at half maximum (FWHM) values obtained from profile analysis of the peaks that were least influenced by overlap with other reflections reveal a trend associated with increasing silica content. For co-precipitates prepared in the ferric nitrate systems, the Si-containing samples displayed a slight broadening of the peaks as the Si concentration was increased. Those from the sulfate solutions, on the other hand, displayed no significant trend in relation to silica content. We also found that there was no significant shift in peak positions associated with increasing silicon content. The subtle peak-broadening with increasing silica content in the co-precipitates formed from nitrate solution, may be attributed to a greater degree of silicate adsorption on the surface of the ferrihydrite crystallites, relative to the sulfate system. A significant degree of silicate adsorption in the early stages of ferrihydrite precipitation might be expected to restrict crystallite development
and lead to a ferrihydrite precipitate with smaller ordered crystal domains. This interpretation is consistent with the structural interpretation of ferrihydrite proposed by Michel [6,7]. The presence of more stable ferric oxide/oxyhydroxides in the silica-free precipitates is an indication that the ferrihydrite precipitates are undergoing transformation within the timescale of the mean residence time. The silica-free precipitates contain goethite and hematite when prepared in nitrate solutions, but only goethite in the sulfate systems. This is not surprising as the presence of sulfate is known to favour goethite formation over hematite [1]. It is noteworthy however that the presence of only small amounts of added silicate is sufficient to prevent transformation of ferrihydrite to these more crystalline phases. This observation is consistent previous work in which the presence of silica has been shown to retard the formation of hematite and goethite by transformation of ferrihydrite in suspension [19]. Supernatant solutions were sampled and analysed to determine the amounts of unprecipitated iron and silicon after the continuous crystalliser had been running for five residence times. The results of these analyses were then used to calculate the proportion of the total iron and silicate in the feed solution that was ultimately removed from solution in the co-precipitate. These results are presented in Table 1. It can be seen that while iron precipitation is essentially complete in both nitrate and sulfate media, the extent of silica precipitation is quite different in the two different electrolytes. In the sulfate systems, only 25–30% of the Si precipitated, but in the nitrate systems the proportion was much greater, with roughly 80–90% being removed from the solution. The polymerisation of silicates and the subsequent precipitation of silica is a much slower process than the precipitation of iron as ferrihydrite, and these results indicate that the nature of the electrolyte from which the silica precipitates can have a significant influence on the rate of precipitation. It has been demonstrated that the equilibrium solubility of silica is somewhat lower in the presence of nitrate ions than in the presence of sulfate [27], and our analyses showed that in sulfate systems, after five residence times, the residual silicon concentrations remained above the expected solubility limit – i.e. the solutions remain supersaturated with respect to silica. It was also clear that the measured residual silicon levels were progressively greater in samples with higher added silicate in the feed solution, demonstrating that silica precipitation did not reach equilibrium during the timeframe of the experiments. In the nitrate systems however, the residual silica levels were found to be below the expected solubility limit, and the level of residual silicon was independent of the silicate concentration in the feed. So we conclude that differences in the amount of silica precipitated from the different electrolyte media result from differences in the rate of silicate polymerisation in the different electrolytes.
Table 1 Residual iron and silicon concentrations, and proportions of iron and silicon precipitated in the continuous crystallisation experiments after five residence times. These values were determined by analysis of residual concentration of Fe and Si in the reactor supernatant liquor after completion of the experiments. Mol% added silicon
[Fe] in liquor (mg/L)
Ferric sulfate 0% Si 5% Si 10% Si 20% Si 30% Si
179 61.5 59.1 88.6 99.7
Ferric nitrate 0% Si 5% Si 10% Si 20% Si 30% Si
0.38 8.14 <0.2 0.81 0.69
% Fe precipitated 97.16 98.97 98.96 98.24 97.74 99.99 99.86 100.00 99.98 99.98
[Si] in liquor (mg/L)
% Si precipitated
– 119 212 433 696
– 24.95 33.32 31.73 26.86
71.7 19.3 87.2 89.6 71.7
– 54.81 93.92 86.25 90.58
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Table 2 Proportions of iron and silica precipitated in the continuous crystallisation experiments after five residence times, showing the effect of varying the iron concentration in the feed solution. The feed was iron sulfate solution, and the silicon concentration was 0.95 g L 1 Si in all cases. Fe concentration 1.58 g L 3.15 g L 12.6 g L
1 1 1
Fe (1=4 standard) Fe (½ standard) Fe (2 standard)
% Fe precipitated
% Si precipitated
97.03 98.34 99.92
25.66 33.58 63.39
We propose that the major difference between the ferrihydrite phases precipitated from nitrate and sulfate solutions is that in sulfate electrolytes, the ferrihydrite surface will have a much greater amount of adsorbed anion than in nitrate solution, and consequently will have considerably more surface sites available for silicate adsorption. So the observed difference in the amount of silica precipitated in each of the two electrolytes suggests that surface adsorption of silicate on ferrihydrite plays a significant role in speeding up the rate of silicate polymerisation. To investigate this possibility in more detail, we carried out a series of continuous crystallisation experiments using a ferric sulfate feed solution in which the added silicate concentration was kept constant, while the total iron concentration was varied. Analysis of iron and silicon levels in the supernatant solutions provided the data that is presented in Table 2, which clearly shows that an increase in the iron concentration in the feed, and hence an increase in the total amount of ferrihydrite precipitated, leads to a corresponding increase in the amount of silica precipitated. High resolution TEM was employed in an attempt to distinguish between areas of nanocrystalline ferrihydrite in which lattice
planes would be visible and those of amorphous silica. Initial micrographs confirmed that all precipitates had a morphology typical of ferrihydrite, in that they were comprised of aggregates formed from roughly spherical particles of less than 20 nm [28]. Viewing lattice planes in ferrihydrite samples of varying crystallinity has been conducted successfully in several studies [26,28]. Fig. 1 provides a comparison of the high resolution images obtained from the silica-free samples, which were the most crystalline samples observed, and those observed in a co-precipitate formed from solutions with a Si:Fe molar ratio of 3:7. As can be seen in Fig. 1, lattice fringes were not apparent in the silica/ferrihydrite co-precipitate. It is possible that a surface layer of amorphous silica or a silica/ferrihydrite mixture obscures lattice information that might otherwise be visible. However, although these images provide a qualitative illustration of the decrease in overall crystallinity of the co-precipitates as silica content increases, no specific structural information could be obtained from this analysis, and since the nature of the particles appeared to be uniform throughout the sample, we could not conclusively identify discrete regions of silica or ferrihydrite in the aggregates. Elemental maps across aggregates of co-precipitate particles were acquired using EFTEM. Several series of images were collected to enable us to compare the unfiltered image, iron map, silicon map and thickness maps of aggregates. Generally, this analysis revealed differing behaviour between iron and silicon in terms of the distribution of the elements through the aggregates. As expected, the iron signal was the most intense, and its intensity corresponds to the signal obtained in the thickness maps (highest intensity in thickest areas). The silicon distribution, on the other hand, appears to be more topographically related, with its regions
Fig. 1. (a and b) High resolution TEM images of ferrihydrite precipitated from ferric nitrate in the absence of silica. (c and d) High resolution TEM images of ferrihydrite/silica co-precipitates prepared from a ferric nitrate feed solution containing 30% added silica.
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of highest intensity unrelated to thickness, but rather tending to show some localisation of Si around the aggregate surface. To illustrate this difference more clearly, a series of elemental maps is presented in Fig. 2. The aggregate shown in these maps contains a region of high thickness in the left side of the image. As expected, the iron map shows a peak in intensity at this point. However, there is no corresponding increase in the silicon map intensity for this region, and the silicon signal intensity appears to fluctuate over areas of constant thickness, suggesting there is little or no thickness relationship. This indicates that the co-precipitate contains a higher proportion of silica around the surface of ferrihydrite aggregates rather than having a uniform distribution of silica throughout the ferrihydrite aggregates. This does not rule out the possibility that silicate species may also be adsorbed on ferrihydrite particles within the aggregate, but clearly the amount of silica within the aggregate is significantly lower than that appearing on the surface. In Fig. 3, the relationship between zeta potential and pH is plotted for silica-free and 30% silica samples obtained from both the nitrate and sulfate systems. Literature sources indicate that the generally accepted PZC for ferrihydrite is approximately 7.8–8 [1,29] and the silica-free samples shown in Fig. 3 display PZCs in this range. In co-precipitates from both the nitrate and sulfate media, the effect of silicate addition is to produce a significant shift in the zeta potential plot, moving the PZC to a lower pH. The PZC for silica is approximately pH = 2 [7] (though literature values range from 0 to 4 [21]) so the shifts observed in Fig. 3 are as expected given the presence of silica in the co-precipitates. Although the zeta potentials are generally more negative in the silica–ferrihydrite co-precipitates, and the PZC has moved to lower pH, the surface charge curves are still not representative of a pure silica surface. This suggests that the surfaces are a mixture of both ferrihydrite and silica. However, it should be noted that this effect arises in samples for which the amount of Si in the initial feed solution is only 30 mol% in comparison to iron. Consequently, even at these levels silica has a significant effect on the surface properties, which will in turn affect the adsorptive, aggregation, settling and filtration properties of the precipitate. When comparing the zeta potential-pH curves for similar coprecipitates prepared from the nitrate and sulfate solutions, it can be seen that with the sample prepared in ferric sulfate, the PZC occurs at lower pH, and at pH values below the PZC, the magnitude of the surface charge is lower than for the co-precipitate prepared in ferric nitrate. These features of the samples prepared from sulfate solution may be due to the presence of chemisorbed
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Fig. 3. Zeta potential measurements from samples of the precipitate formed in the nitrate control, nitrate 30% Si, sulphate control and sulphate 30% Si systems.
sulfate ions on the particle surfaces thus altering the surface charge to a more negative value. The presence of soluble silicates in the ferrihydrite precipitation process produces ferrihydrite/silica co-precipitates, the properties of which are dependent on the amount of silica present, and also on the nature of the electrolyte solution in which the co-precipitates are formed. Iron hydrolysis, and consequently the precipitation of ferrihydrite, is considerably more rapid than the rate of silicate hydrolysis and it appears that this difference in the rate of hydrolysis has a significant effect on the nature of the co-precipitates.
Fig. 2. A series of EFTEM elemental maps of a section of an aggregate formed in the co-precipitation of ferrihydriteand silica with an Fe:Si ratio of 7:3. (a) Thickness map, (b) iron map and (c) silicon map.
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Analysis of our powder XRD data indicates that when silica and iron co-precipitate from a ferric nitrate solution, the crystallinity of the ferrihydrite/silica product decreases with increasing silica content, although this effect is considerably more marked for co-precipitates formed from ferric nitrate solution than in ferric sulfate media. This difference is likely to arise from the fact that sulfate anions will have a much greater tendency to adsorb on ferrihydrite surfaces than nitrate, and so reduce the relative amount of silicate species being surface-adsorbed. Further evidence of this effect is provided by the analysis of residual silicate in supernatant solutions following co-precipitation reactions. These data show that in nitrate media, 80–90% of the dissolved silica is precipitated during the course of the experiments, while in sulfate media, little more than 30% of the silica precipitates. This seems to indicate that surface adsorption of silicates on ferrihydrite is a key factor in speeding up the polymerisation and hydrolysis of silicate species. This interpretation is further supported by our observation that the amount of silica precipitated in the continuous crystallisation process can be controlled by varying the concentration of iron in the feed solution. An increase in the total surface area of ferrihydrite precipitated provides a greater surface area for silicate adsorption and thus a more rapid silicate polymerisation to form silica. On the basis of the results of our co-precipitation experiments, we believe that the following mechanism best outlines the sequence of steps in ferrihydrite/silica co-precipitation in an acidic continuous crystallisation process. 1. Rapid hydrolysis of Fe3+ results in precipitation of ferrihydrite and a rapid drop in the concentration of free iron in the solution. 2. Silicate anions adsorb on the ferrihydrite surfaces preventing further crystal development and inhibiting transformation of ferrihydrite to more crystalline phases such as goethite. 3. Surface-adsorbed silicate species polymerise and dehydrate to form silica. The evidence provided by transmission electron micrographs and elemental mapping provides further indication that the surface adsorption of silicates is an important step in the precipitation of silica, since all silicon observed in the elemental maps is closely associated with iron. Elemental mapping of aggregates using EELS showed that aggregates typically had high levels of iron at their center, and that most of the silicon was associated with the periphery of the aggregates. We could not identify any aggregates that were composed of silica alone, nor any that were composed of ferrihydrite alone. The co-precipitates can thus be described as an intimate mixture of ferrihydrite crystallites and amorphous silica, although the nature of the distribution of silica within the aggregates is dependent to some extent on the nature of electrolyte medium in which the co-precipitate is formed. Acknowledgments The work described here has been supported in part by the ARC Linkage project LP 0560753. L. Dyer wishes to acknowledge
the Australian Research Council and the Parker Cooperative Research Centre for Integrated Hydrometallurgy Solutions for financial support in the form a of a PhD stipend. We acknowledge the Centre for Microscopy, Characterisation and Analysis at the University of Western Australia for the use of the JEOL 2100 and JEOL 3000 transmission electron microscopes, and the kind assistance of Dr. Martin Saunders with EFTEM imaging and elemental mapping.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Regular Article
Synthesis and characterisation of ferrihydrite/silica co-precipitates Laurence Dyer a, Phillip D. Fawell b, O.M.G. Newman c, William R. Richmond a,* a
Nanochemistry Research Institute, Department of Chemistry and Parker Centre for Integrated Hydrometallurgy Solutions, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia b CSIRO Minerals and Parker Centre for Integrated Hydrometallurgy Solutions, PO Box 90, Bentley, Western Australia 6982, Australia c Nyrstar Metals Ltd., PO Box 377, Hobart, Tasmania 7009, Australia
a r t i c l e
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Article history: Received 12 January 2010 Accepted 26 March 2010 Available online 14 April 2010 Keywords: Ferrihydrite Silica Continuous crystallisation Composite materials
a b s t r a c t The effect of the presence of soluble silicates on ferrihydrite precipitation and some properties of the products formed in co-precipitation of ferrihydrite and silica have been investigated. The co-precipitates were formed using a continuous crystallisation process in which a combined iron/silicon feed solution was reacted with sodium hydroxide at a constant rate, while maintaining pH at 2.65 and temperature at 85 °C. The products of co-precipitation and the supernatant solutions were characterised using a variety of analytical techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM) and surface charge measurements. The addition of silicates was shown to have a significant impact on the crystallinity and surface charge of the precipitates formed. For products collected after five residence times in the continuous crystalliser, co-precipitates formed from ferric sulfate solution were found to contain considerably less silica than those formed from ferric nitrate. We conclude that adsorption of silicate species on ferrihydrite surfaces speeds up the polymerisation process, and that sulfate ion competes with silicate for surface adsorption sites. Thus, the precipitation of silica proceeds much more rapidly in ferric nitrate media, than in ferric sulfate. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Ferrihydrite (5Fe2O39H2O) is a poorly-crystalline iron oxyhydroxide that is common in the environment, precipitating in iron-laden springs, drainage lines, stagnant water soils and river sediments [1]. It is also a common form of biogenic iron, formed as a precursor to more stable iron oxides such as magnetite and hematite, which occur as components of limpet teeth, for example [1,2]. Fe(II)-oxidising bacteria can induce ferrihydrite formation in both intra-cellular and extra-cellular pathways. Ferrihydrite precipitation occurs under a wide variety of conditions, some of which produce ferrihydrite only, while others produce a mixture of iron oxide or iron oxyhydroxide phases. Controlled precipitation of iron from solution is important in many industrial processes, and it is essential in some hydrometallurgical processes – such as zinc and nickel recovery for example. In many situations where ferrihydrite occurs, dissolved silicates are also present. Examples where the concentrations of iron are reasonably high include industrial process liquors, thermal springs, mine waste residues and acid mine drainage sites. The stability and adsorptive properties of ferrihydrite derived from mine wastes or mineral processing residues is a critical issue in determining their
* Corresponding author. Fax: +61 4 (08) 9266 4699. E-mail address: [email protected] (W.R. Richmond). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.03.056
handling and disposal. These residues often contain associated adsorbed species such as As, Sb, Cu and Cd, so it is important that they remain stable in the environment in order for these contaminants to be contained. Ferrihydrite’s low crystallinity and nanoscale crystallite size have presented a challenge to those trying to determine its structure, and it is only in recent years that a detailed crystal structure of ferrihydrite has been reported [3]. Ferrihydrite has been classified according to the number of reflections in its XRD pattern: 6-line ferrihydrite and 2-line ferrihydrite being two extremes in a continuum of phases with XRD patterns displaying a decrease in long-range order which can be explained by a decrease in the size of coherent scattering domains in ferrihydrite – the average size of such domains is about 6 nm in 6-line ferrihydrite and 2 nm in 2-line ferrihydrite [7]. Soluble silicates have been shown to adsorb readily onto ferrihydrite surfaces, and to have a significant effect on the structural characteristics of ferrihydrite precipitates [4]. The presence of silicates is also known to inhibit the transformation of ferrihydrite to more stable iron oxide phases such as goethite and hematite [5,6]. The generally accepted mechanism for silica precipitation involves the polymerisation of monosilicic acid to form silicate oligomers, and subsequent dehydration to form silica. Polymerisation of silicic acid occurs spontaneously at concentrations exceeding 100–200 ppm [7]. Conditions that govern silicate polymerisation
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are the silicate concentration, pH, temperature and the concentration of other ions [8]. At low pH, polymerisation occurs relatively slowly, and particle aggregation dominates over growth [9]. This is a consequence of the low point of zero charge for silica (PZC – pH 2); at higher pH the particles are negatively charged and do not aggregate as quickly. Aggregation is a significant factor when silica co-precipitates during the acid hydrolysis of iron, since at moderately low pH, ferrihydrite particles are positively charged and silica particles are negatively charged. Previous studies of ferrihydrite/silica co-precipitates are limited to very few examples although some examination of the properties of mixed oxides of iron and silicon has been reported, generally describing sol–gel precipitate composites. However, little work has been reported on continuous co-precipitation processes or the products thereof. There is a void in current knowledge as to how these species are produced and how they behave when precipitated simultaneously. Some mechanistic aspects have been proposed – such as formation of a silica layer on the surface of ferrihydrite particles [10] – but in general there is very little structural information about ferrihydrite/silica co-precipitates. Some precipitate properties have been analysed, including crystallinity, structure and surface charge. The presence of dissolved silica during iron oxide/oxyhydroxide precipitation has been shown to decrease the crystallinity of ferrihydrite [11–13]. In experiments where silica was precipitated onto iron particles it was shown that silicic acid monomers bond to the iron oxide phase that inevitably forms on the metal surface through Si–O–Fe bonds [14–16], and the formation of such a chemical linkage was also implicated in the case of iron oxide coated silica particles [17]. When concentrations are sufficiently high, soluble silicates polymerise, bonding to the Si species adsorbed on an iron oxide surface through silanol bridges (Si–O–Si) [7,10,16]. A study of goethite precipitation in the presence of silicates has shown uptake of Si on the particle surfaces, but none throughout single crystal domains. In this case, non-surface associated Si was thought to be present in layers between crystal boundaries in aggregates [18]. Many studies have demonstrated that the presence of silica in iron oxyhydroxides increases their stability with respect to transformation into other iron oxide phases. Pure 2-line ferrihydrite transforms to hematite at 340 °C, while in silica-associated 2-line ferrihydrite the transformation temperature has been shown to increase to 740 °C at a Si/(Si + Fe) ratio of 0.27 [19]. In aqueous suspension, the transformation time for ferrihydrite to goethite increases from less than 24 h to 1–2 weeks with the addition of Si at high pH [20]. The PZC of ferrihydrite is approximately 8, but when the Si/Fe ratio reaches 0.35 it becomes closer to 4, although some literature surface charge values have been shown to be flawed [21]. In studies where iron oxide particles have been coated with silica, the PZC is dominated by the silica as would be expected [22]. However, the surface charge of both ferrihydrite and silica particles has been shown to vary significantly depending on the type and concentration of ions in solution [23,24]. This paper examines the structure and properties of the precipitates formed in iron silica co-precipitation reactions as well as the effect of altering the proportion of silica present on the reaction itself and on the material produced. In addition, we describe the use of energy filtered TEM to examine the distribution of silica and iron within aggregates in the co-precipitates; a level of structural detail that has not been described previously. The samples have been produced in a continuous crystallisation set-up, with constant reaction conditions operating at steady state so as to give the best approximation for systems occurring both naturally and industrially. Temperature and pH conditions were chosen to broadly approximate those found in some hydrometallurgical iron-removal stages, and thus we anticipate that the results may be a useful aid to understanding the behaviour of such systems.
2. Materials and methods Precipitates were produced in a continuous crystallisation apparatus comprising a 110 mL jacketed glass crystallisation vessel and three medical dosing pumps. The reactor has an internal diameter and height of 50 mm and 75 mm respectively. An agitator with four angled blades rotated at 570 rpm was positioned approximately 5 mm from the bottom of the reactor. Feed solutions (iron(III)/silicon and NaOH) were delivered into the agitated, temperature-controlled crystallisation vessel at constant rates using two medical dosing pumps: one to deliver the Fe/Si solution and the other to deliver the NaOH solution. A third pump was used for product removal. In a range of co-precipitation experiments, both ferric nitrate and ferric sulfate were used as the source of iron(III) and these feed solutions were dosed with various amounts of silica, which was added as sodium metasilicate. All Fe/Si feed solutions had a total Fe + Si concentration of 0.1128 mol L 1 while Si:Fe ratios were varied between 0 and 0.43. The sodium hydroxide concentration was 8 mol L 1 and the feed rate of the NaOH delivery pump was adjusted to control the pH during the precipitation experiments. The reaction mixtures were maintained at 85 °C and pH 2.65 and flow rates were set to achieve a mean residence time of 45 min. In a typical experiment the reactor was operated over a period of at least five residence times to achieve steady state conditions. Product samples were collected at regular intervals and XRD patterns examined to ensure that steady state was achieved. These conditions were found to produce 6-line ferrihydrite, among more crystalline iron oxide/oxyhydroxide phases, at steady state in silica-free control runs. Samples for X-ray diffraction were prepared by filtering through 0.45 lm membranes and drying the solid in an oven at 60 °C. X-ray powder diffraction was carried out at 25 °C using Cu Ka radiation (40 kV, 30 mA) on a Seimens D500 powder diffractometer, or on a Phillips X’pert powder diffractometer, with a cobalt long fine focus tube. Line profile analysis, and deconvolution of multiple peaks in the XRD patterns was carried out using the Xfit program.1 This analysis provided full width at half maximum (FWHM) values and peak positions for selected reflections in the ferrihydrite patterns. Peak positions were determined relative to a CaF2 internal standard. Slurry samples were filtered twice through 0.45 lm syringe filters to remove solids in preparation for ICP-AES. Analyses were conducted using both an ICP-AES Vista and 735-ES instruments. For surface charge measurements, dilute suspensions of precipitates were analysed on a Malvern Zetanano DLS particle sizer. For these measurements, samples were dialysed with pure water (MilliQÒ), then diluted and the pH adjusted to the desired value using nitric acid or sulfuric acid as appropriate. Specimens for transmission electron microscopy (TEM) were prepared by dialysing a product slurry with pure water to remove surface-adsorbed species and then diluting further. A droplet of this diluted suspension was then placed onto a conventional holey-carbon grid and allowed to dry in air. The grids were examined using a JEOL 2011 TEM for basic imaging, and a JEOL 3000F or JEOL 2100 TEM for high resolution or energy filtered TEM (EFTEM) imaging.
3. Results and discussion Powder X-ray diffraction patterns for co-precipitates prepared using a ferric nitrate feed solution reveal subtle changes in the crystallinity of the ferrihydrite/silica products as the silica content 1 Cheary R.W., Coelho, A.A., Programs Xfit and FOURYA. CCP14 Powder diffraction Library–Daresbury Laboratory, Warrington UK; 1996.
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of the feed increases. The control sample displayed the characteristic pattern of 6-line ferrihydrite [1] along with a few sharper peaks indicating the presence of small amounts of poorly-crystalline goethite and hematite. As the Si concentration was increased, the peaks due to these more crystalline phases broadened and at the highest Si concentrations were not present at all. The ferrihydrite peaks appeared to broaden a little as the Si concentration was increased. In the patterns of samples with high Si levels, a very broad, low intensity background peak could be seen in the patterns, centered near 25–30° 2h, which we attributed to the presence of amorphous silica. It has been documented that ferrihydrite decreases in crystallinity when precipitated in the presence of soluble silicates [25]; however, there is little information as to why. Michel et al. [26] presented pair distribution function (PDF) analysis of X-ray scattering data which suggested that the only difference between 6line ferrihydrite and its less crystalline phases is the size of the coherent scattering domains. Thus if the crystallinity is governed by the particle size, the observed decrease in crystallinity would suggest that adsorbed silicates are restricting the growth of the individual ferrihydrite particles. Interestingly, the sulfate systems behaved slightly differently – in this case the control sample provided a pattern with considerably less evidence of goethite formation than was the case in the nitrate system. In addition there was very little, if any, indication of a change in crystallinity with increasing silicate levels XRD. A slight peak-broadening was observed as the Si content increased but the diffraction pattern of the product obtained from the 30% Si system still contained all six characteristic peaks of 6-line ferrihydrite. In an attempt to quantify the observed change in crystallinity, line profile analysis was conducted using the Xfit program to obtain peak widths and positions. Peak positions were determined in samples containing CaF2 as an internal standard. The full width at half maximum (FWHM) values obtained from profile analysis of the peaks that were least influenced by overlap with other reflections reveal a trend associated with increasing silica content. For co-precipitates prepared in the ferric nitrate systems, the Si-containing samples displayed a slight broadening of the peaks as the Si concentration was increased. Those from the sulfate solutions, on the other hand, displayed no significant trend in relation to silica content. We also found that there was no significant shift in peak positions associated with increasing silicon content. The subtle peak-broadening with increasing silica content in the co-precipitates formed from nitrate solution, may be attributed to a greater degree of silicate adsorption on the surface of the ferrihydrite crystallites, relative to the sulfate system. A significant degree of silicate adsorption in the early stages of ferrihydrite precipitation might be expected to restrict crystallite development
and lead to a ferrihydrite precipitate with smaller ordered crystal domains. This interpretation is consistent with the structural interpretation of ferrihydrite proposed by Michel [6,7]. The presence of more stable ferric oxide/oxyhydroxides in the silica-free precipitates is an indication that the ferrihydrite precipitates are undergoing transformation within the timescale of the mean residence time. The silica-free precipitates contain goethite and hematite when prepared in nitrate solutions, but only goethite in the sulfate systems. This is not surprising as the presence of sulfate is known to favour goethite formation over hematite [1]. It is noteworthy however that the presence of only small amounts of added silicate is sufficient to prevent transformation of ferrihydrite to these more crystalline phases. This observation is consistent previous work in which the presence of silica has been shown to retard the formation of hematite and goethite by transformation of ferrihydrite in suspension [19]. Supernatant solutions were sampled and analysed to determine the amounts of unprecipitated iron and silicon after the continuous crystalliser had been running for five residence times. The results of these analyses were then used to calculate the proportion of the total iron and silicate in the feed solution that was ultimately removed from solution in the co-precipitate. These results are presented in Table 1. It can be seen that while iron precipitation is essentially complete in both nitrate and sulfate media, the extent of silica precipitation is quite different in the two different electrolytes. In the sulfate systems, only 25–30% of the Si precipitated, but in the nitrate systems the proportion was much greater, with roughly 80–90% being removed from the solution. The polymerisation of silicates and the subsequent precipitation of silica is a much slower process than the precipitation of iron as ferrihydrite, and these results indicate that the nature of the electrolyte from which the silica precipitates can have a significant influence on the rate of precipitation. It has been demonstrated that the equilibrium solubility of silica is somewhat lower in the presence of nitrate ions than in the presence of sulfate [27], and our analyses showed that in sulfate systems, after five residence times, the residual silicon concentrations remained above the expected solubility limit – i.e. the solutions remain supersaturated with respect to silica. It was also clear that the measured residual silicon levels were progressively greater in samples with higher added silicate in the feed solution, demonstrating that silica precipitation did not reach equilibrium during the timeframe of the experiments. In the nitrate systems however, the residual silica levels were found to be below the expected solubility limit, and the level of residual silicon was independent of the silicate concentration in the feed. So we conclude that differences in the amount of silica precipitated from the different electrolyte media result from differences in the rate of silicate polymerisation in the different electrolytes.
Table 1 Residual iron and silicon concentrations, and proportions of iron and silicon precipitated in the continuous crystallisation experiments after five residence times. These values were determined by analysis of residual concentration of Fe and Si in the reactor supernatant liquor after completion of the experiments. Mol% added silicon
[Fe] in liquor (mg/L)
Ferric sulfate 0% Si 5% Si 10% Si 20% Si 30% Si
179 61.5 59.1 88.6 99.7
Ferric nitrate 0% Si 5% Si 10% Si 20% Si 30% Si
0.38 8.14 <0.2 0.81 0.69
% Fe precipitated 97.16 98.97 98.96 98.24 97.74 99.99 99.86 100.00 99.98 99.98
[Si] in liquor (mg/L)
% Si precipitated
– 119 212 433 696
– 24.95 33.32 31.73 26.86
71.7 19.3 87.2 89.6 71.7
– 54.81 93.92 86.25 90.58
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Table 2 Proportions of iron and silica precipitated in the continuous crystallisation experiments after five residence times, showing the effect of varying the iron concentration in the feed solution. The feed was iron sulfate solution, and the silicon concentration was 0.95 g L 1 Si in all cases. Fe concentration 1.58 g L 3.15 g L 12.6 g L
1 1 1
Fe (1=4 standard) Fe (½ standard) Fe (2 standard)
% Fe precipitated
% Si precipitated
97.03 98.34 99.92
25.66 33.58 63.39
We propose that the major difference between the ferrihydrite phases precipitated from nitrate and sulfate solutions is that in sulfate electrolytes, the ferrihydrite surface will have a much greater amount of adsorbed anion than in nitrate solution, and consequently will have considerably more surface sites available for silicate adsorption. So the observed difference in the amount of silica precipitated in each of the two electrolytes suggests that surface adsorption of silicate on ferrihydrite plays a significant role in speeding up the rate of silicate polymerisation. To investigate this possibility in more detail, we carried out a series of continuous crystallisation experiments using a ferric sulfate feed solution in which the added silicate concentration was kept constant, while the total iron concentration was varied. Analysis of iron and silicon levels in the supernatant solutions provided the data that is presented in Table 2, which clearly shows that an increase in the iron concentration in the feed, and hence an increase in the total amount of ferrihydrite precipitated, leads to a corresponding increase in the amount of silica precipitated. High resolution TEM was employed in an attempt to distinguish between areas of nanocrystalline ferrihydrite in which lattice
planes would be visible and those of amorphous silica. Initial micrographs confirmed that all precipitates had a morphology typical of ferrihydrite, in that they were comprised of aggregates formed from roughly spherical particles of less than 20 nm [28]. Viewing lattice planes in ferrihydrite samples of varying crystallinity has been conducted successfully in several studies [26,28]. Fig. 1 provides a comparison of the high resolution images obtained from the silica-free samples, which were the most crystalline samples observed, and those observed in a co-precipitate formed from solutions with a Si:Fe molar ratio of 3:7. As can be seen in Fig. 1, lattice fringes were not apparent in the silica/ferrihydrite co-precipitate. It is possible that a surface layer of amorphous silica or a silica/ferrihydrite mixture obscures lattice information that might otherwise be visible. However, although these images provide a qualitative illustration of the decrease in overall crystallinity of the co-precipitates as silica content increases, no specific structural information could be obtained from this analysis, and since the nature of the particles appeared to be uniform throughout the sample, we could not conclusively identify discrete regions of silica or ferrihydrite in the aggregates. Elemental maps across aggregates of co-precipitate particles were acquired using EFTEM. Several series of images were collected to enable us to compare the unfiltered image, iron map, silicon map and thickness maps of aggregates. Generally, this analysis revealed differing behaviour between iron and silicon in terms of the distribution of the elements through the aggregates. As expected, the iron signal was the most intense, and its intensity corresponds to the signal obtained in the thickness maps (highest intensity in thickest areas). The silicon distribution, on the other hand, appears to be more topographically related, with its regions
Fig. 1. (a and b) High resolution TEM images of ferrihydrite precipitated from ferric nitrate in the absence of silica. (c and d) High resolution TEM images of ferrihydrite/silica co-precipitates prepared from a ferric nitrate feed solution containing 30% added silica.
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of highest intensity unrelated to thickness, but rather tending to show some localisation of Si around the aggregate surface. To illustrate this difference more clearly, a series of elemental maps is presented in Fig. 2. The aggregate shown in these maps contains a region of high thickness in the left side of the image. As expected, the iron map shows a peak in intensity at this point. However, there is no corresponding increase in the silicon map intensity for this region, and the silicon signal intensity appears to fluctuate over areas of constant thickness, suggesting there is little or no thickness relationship. This indicates that the co-precipitate contains a higher proportion of silica around the surface of ferrihydrite aggregates rather than having a uniform distribution of silica throughout the ferrihydrite aggregates. This does not rule out the possibility that silicate species may also be adsorbed on ferrihydrite particles within the aggregate, but clearly the amount of silica within the aggregate is significantly lower than that appearing on the surface. In Fig. 3, the relationship between zeta potential and pH is plotted for silica-free and 30% silica samples obtained from both the nitrate and sulfate systems. Literature sources indicate that the generally accepted PZC for ferrihydrite is approximately 7.8–8 [1,29] and the silica-free samples shown in Fig. 3 display PZCs in this range. In co-precipitates from both the nitrate and sulfate media, the effect of silicate addition is to produce a significant shift in the zeta potential plot, moving the PZC to a lower pH. The PZC for silica is approximately pH = 2 [7] (though literature values range from 0 to 4 [21]) so the shifts observed in Fig. 3 are as expected given the presence of silica in the co-precipitates. Although the zeta potentials are generally more negative in the silica–ferrihydrite co-precipitates, and the PZC has moved to lower pH, the surface charge curves are still not representative of a pure silica surface. This suggests that the surfaces are a mixture of both ferrihydrite and silica. However, it should be noted that this effect arises in samples for which the amount of Si in the initial feed solution is only 30 mol% in comparison to iron. Consequently, even at these levels silica has a significant effect on the surface properties, which will in turn affect the adsorptive, aggregation, settling and filtration properties of the precipitate. When comparing the zeta potential-pH curves for similar coprecipitates prepared from the nitrate and sulfate solutions, it can be seen that with the sample prepared in ferric sulfate, the PZC occurs at lower pH, and at pH values below the PZC, the magnitude of the surface charge is lower than for the co-precipitate prepared in ferric nitrate. These features of the samples prepared from sulfate solution may be due to the presence of chemisorbed
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Fig. 3. Zeta potential measurements from samples of the precipitate formed in the nitrate control, nitrate 30% Si, sulphate control and sulphate 30% Si systems.
sulfate ions on the particle surfaces thus altering the surface charge to a more negative value. The presence of soluble silicates in the ferrihydrite precipitation process produces ferrihydrite/silica co-precipitates, the properties of which are dependent on the amount of silica present, and also on the nature of the electrolyte solution in which the co-precipitates are formed. Iron hydrolysis, and consequently the precipitation of ferrihydrite, is considerably more rapid than the rate of silicate hydrolysis and it appears that this difference in the rate of hydrolysis has a significant effect on the nature of the co-precipitates.
Fig. 2. A series of EFTEM elemental maps of a section of an aggregate formed in the co-precipitation of ferrihydriteand silica with an Fe:Si ratio of 7:3. (a) Thickness map, (b) iron map and (c) silicon map.
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Analysis of our powder XRD data indicates that when silica and iron co-precipitate from a ferric nitrate solution, the crystallinity of the ferrihydrite/silica product decreases with increasing silica content, although this effect is considerably more marked for co-precipitates formed from ferric nitrate solution than in ferric sulfate media. This difference is likely to arise from the fact that sulfate anions will have a much greater tendency to adsorb on ferrihydrite surfaces than nitrate, and so reduce the relative amount of silicate species being surface-adsorbed. Further evidence of this effect is provided by the analysis of residual silicate in supernatant solutions following co-precipitation reactions. These data show that in nitrate media, 80–90% of the dissolved silica is precipitated during the course of the experiments, while in sulfate media, little more than 30% of the silica precipitates. This seems to indicate that surface adsorption of silicates on ferrihydrite is a key factor in speeding up the polymerisation and hydrolysis of silicate species. This interpretation is further supported by our observation that the amount of silica precipitated in the continuous crystallisation process can be controlled by varying the concentration of iron in the feed solution. An increase in the total surface area of ferrihydrite precipitated provides a greater surface area for silicate adsorption and thus a more rapid silicate polymerisation to form silica. On the basis of the results of our co-precipitation experiments, we believe that the following mechanism best outlines the sequence of steps in ferrihydrite/silica co-precipitation in an acidic continuous crystallisation process. 1. Rapid hydrolysis of Fe3+ results in precipitation of ferrihydrite and a rapid drop in the concentration of free iron in the solution. 2. Silicate anions adsorb on the ferrihydrite surfaces preventing further crystal development and inhibiting transformation of ferrihydrite to more crystalline phases such as goethite. 3. Surface-adsorbed silicate species polymerise and dehydrate to form silica. The evidence provided by transmission electron micrographs and elemental mapping provides further indication that the surface adsorption of silicates is an important step in the precipitation of silica, since all silicon observed in the elemental maps is closely associated with iron. Elemental mapping of aggregates using EELS showed that aggregates typically had high levels of iron at their center, and that most of the silicon was associated with the periphery of the aggregates. We could not identify any aggregates that were composed of silica alone, nor any that were composed of ferrihydrite alone. The co-precipitates can thus be described as an intimate mixture of ferrihydrite crystallites and amorphous silica, although the nature of the distribution of silica within the aggregates is dependent to some extent on the nature of electrolyte medium in which the co-precipitate is formed. Acknowledgments The work described here has been supported in part by the ARC Linkage project LP 0560753. L. Dyer wishes to acknowledge
the Australian Research Council and the Parker Cooperative Research Centre for Integrated Hydrometallurgy Solutions for financial support in the form a of a PhD stipend. We acknowledge the Centre for Microscopy, Characterisation and Analysis at the University of Western Australia for the use of the JEOL 2100 and JEOL 3000 transmission electron microscopes, and the kind assistance of Dr. Martin Saunders with EFTEM imaging and elemental mapping.
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