Effect of Ca2+, Mg2+, and Anion Type on the Aging of Iron(III

Effect of Ca2+, Mg2+, and Anion Type on the Aging of Iron(III) Hydroxide Precipitates .... Journal of Dispersion Science and Technology 2017 57, 1-10 ...
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Environ. Sci. Technol. 1997, 31, 1024-1032

Effect of Ca2+, Mg2+, and Anion Type on the Aging of Iron(III) Hydroxide Precipitates KARLIS A. BALTPURVINS, ROBERT C. BURNS,* AND GEOFFREY A. LAWRANCE Department of Chemistry, The University of Newcastle, Callaghan, Australia 2308 ALAN D. STUART BHP Research (Newcastle Laboratories), P.O. Box 188, Wallsend, Australia 2287

Lime and magnesia are commonly used to treat metal (e.g., iron)-laden waters and wastewaters, which can generate large volumes of sludge with their attendent handling and landfill problems. In order to address possible reductions in generated sludge and subsequent landfill volumes, the effects of pH and concentration of Ca2+ and Mg2+ on the initial precipitation and subsequent phase transformations of iron(III) hydroxide sludges have been investigated. In both Cl- and SO42- media, ferrihydrite (5Fe2O3‚9H2O) was found to be the kinetically preferred phase independent of pH and Ca2+ concentration, with residual Ca2+ solubility being controlled by adsorptive processes at pH values more alkaline than the point of zero charge of ferrihydrite. In contrast, ferrihydrite was the only phase present at pH values goethite (20.6) > a-pyroaurite (∼15.0) > hematite (7.8).

Introduction The use of iron(III) salts as coagulating agents in domestic water treatment has increased dramatically in recent times. This has been largely a result of the possible links between Alzheimer’s disease and soluble aluminium concentrations derived from, for example, alum (1). Such treatment involves the rapid hydrolysis of an Fe(III) solution to form the kinetically favored, metastable hydroxide, ferrihydrite (5Fe2O3‚ 9H2O). With time, ferrihydrite transforms into its thermodynamically preferred crystalline counterparts, goethite [R-FeO(OH)] and hematite (R-Fe2O3) (2). The rate at which this transformation occurs and the composition of the aged * Corresponding author fax: +61-49-21-6912; e-mail: csrb@ paracelsus.newcastle.edu.au.

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product(s) are of great environmental significance as the precipitated sludge volume can be altered significantly as a result of the transformation process. Consequently, optimization of the transformation conditions may potentially reduce sludge handling as well as landfill volumes required for the disposal of the generated sludges. The transformation of ferrihydrite occurs through two competing pathways: (a) dissolution and re-precipitation to form goethite and (b) an internal aggregation and rearrangement to produce hematite (3, 4). The chemical and physical conditions at which aging occurs can exert a significant effect on the rate, composition, and morphological state of the transformation products. To date, the influence of temperature, pH, various organic acids, silicate, and common anion type (NO3-, Cl-, and SO42-) and the presence of Al(III), Mn(II)/(III), Cu(II), Zn(II), Ni(II), Co(II), and Ti(IV) have been investigated (2, 5-17). One early study also investigated the effects of some common cation types, including Ca2+, and Mg2+, although in a limited study (18). The most commonly used bases for hydrolysis of Fe(III) in water treatment as well as in an industrial wastewater context are lime (CaO) and, to a lesser extent, magnesia (MgO). Thus, during the precipitation of ferrihydrite, large quantities of the cation associated with the precipitating agent may be introduced into the system, which may influence the nature of the transformation reaction. This paper presents the results of an investigation into the composite effect of pH, Ca2+, and Mg2+ concentration, and common anion type on the rate, composition, and morphology of the products for the ferrihydrite transformation process. The study is directed toward the reduction of sludge volumes in water treatment plants and the minimization of landfill volumes required for the storage of iron(III) hydroxide sludges.

Experimental Section Precipitation Studies. In order to investigate the behavior of Ca2+ and Mg2+ during the initial precipitation of Fe(III), a series of controlled precipitation experiments were performed. Precipitation profiles (residual metal ion concentration versus pH) were determined experimentally using a system termed a chemostat, which allows for the strict control of precipitation conditions (i.e., pH, temperature, reaction time, and rate and dosage of precipitant). The operation of the chemostat has been described previously (19). All systems were investigated at 25 °C in this study. Model solutions were prepared with compositions for Mn+(mol dm-3):Fe3+(mol dm-3) (where Mn+ ) Ca2+ or Mg2+) of 0.001:0.000 (a control system containing no Fe3+), 0.001:0.001, 0.001:0.010, and 0.001:0.100. Analytical reagent grade CaCl2‚H2O, MgCl2‚6H2O, and FeCl3‚6H2O were used in all preparations. The residual solubilities of Ca2+ and Mg2+ were investigated over the pH interval range 2-11. An equilibration period of 45 min was allowed between sampling events. All systems were left open to the atmosphere to simulate industrial practices. Samples were centrifuged at 2500 rpm for 15 min and then filtered through a 0.45-µm cellulose nitrate membrane filter. The Ca2+ and Mg2+ concentrations in the filtrates were determined using a Perkin Elmer Model 3110 atomic absorption spectrophotometer under standard conditions. The phases that formed were characterized by X-ray powder diffraction (XRD) analysis. Solids were isolated at pH values of 7, 9, and 11 in order to observe any pH-dependent phase modifications. The resulting solids were filtered, washed, and dried at 50 °C for 48 h (16). XRD studies were performed on a Phillips PW1700 Automated Powder Diffractometer System 1, employing graphite-monochromated Cu-KR radiation.

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Effect of pH and Cation Type on the Transformation of Ferrihydrite. In order to assess the composite effect of pH and cation type on the transformation process of ferrihydrite at ambient temperatures, a series of long-term (1-yr) transformations at 20 °C were performed. Suspensions of ferrihydrite were prepared by precipitation from solutions of 0.1 mol dm-3 FeCl3‚6H2O and either CaCl2‚H2O or MgCl2‚6H2O with 5 mol dm-3 KOH such that the mole ratio Mn+(mol): Fe3+(mol) (where Mn+ ) Ca2+ or Mg2+) was 0.5:1. The pH values of the suspensions ranged from 7 to 11 and are considered to be typical of industrially treated samples. The ferrihydrite suspensions (0.1 mol dm-3) were aged in closed polyethylene bottles at 20 °C for a period of 1 yr. Sample homogenization (by thorough shaking and stirring) and pH adjustment were made at weekly intervals for the first 2 months and monthly thereafter. No kinetic examination of the transformation reactions was made in view of the potential interferences of Ca2+ and Mg2+ on oxalate extraction (see below). After 1 yr, the solid phases were filtered, washed, and dried at 50 °C for 48 h. XRD studies were performed as described above. For the systems containing mixtures of hematite and goethite, quantitative determination of the proportions of each were achieved by comparison with a series of standards made by mixing known amounts of synthetic goethite and hematite (20) in a roller mixer for several minutes. The relative intensities of the 110 and 111 peaks of goethite and the 102 peak of hematite were used for comparison. Transmission electron micrographs (TEM) of the solid phases were recorded using a Jeol 1200 EXII transmission electron microscope. Samples were prepared by dispersing the crystals in Millipore Milli-Q water and evaporating to dryness on a carbon support film. Infrared spectra were recorded as KBr disks using a Bio-Rad FTS-7 Fourier transform spectrophotometer. Concentration Effects and Anion Type on the Transformation of Ferrihydrite. An investigation into the composite effect of anion type (Cl- and SO42-) and Ca2+ or Mg2+ concentration on ferrihydrite was performed at an accelerated rate by increasing the aging temperature of the systems to 70 °C. Ferrihydrite suspensions were prepared as described above using Cl- salts, while the mole fraction x, where x ) Mn+/(Mn+ + Fe3+) and Mn+ ) Ca2+ or Mg2+, was varied in the range from 0.00 to 0.50. All transformations were performed at pH 10.5, as this is known to produce a mixture of goethite and hematite from pure ferrihydrite (10). To compare the effect of anion type, transformation reactions were also performed on SO42--based systems. Compositions were as previously described with the substitution of the SO42--based salts, Fe2(SO4)3 and MgSO4 for Cl--based salts, and both Ca(NO3)2‚4H2O and Na2SO4‚10H2O for CaCl2‚H2O. The kinetics of the transformation reactions were followed by taking subsamples during the reactions and extracting the unconverted ferrihydrite (or other oxalate-soluble phase) with a 2-h acid/oxalate extraction (pH 3) in the absence of light (20). The extent of reaction was expressed as Fe(o)/Fe(t), where Fe(o) ) oxalate extractable fraction and Fe(t) ) total Fe(III) in the system. The latter was determined by digestion in 5 M HCl for 48 h at 70 °C. For the systems where the mole fraction of Ca2+ exceeded 0.05, 18-crown-6 ether was included as a complexing agent to prevent calcium oxalate precipitation. Inclusion of the 18-crown-6 ether did not aid in the dissolution of the hematite or goethite phases. The compositions of the final crystalline transformation products were characterized by XRD and TEM, using the procedures described in the previous section. Infrared spectra were recorded as described above. Preparation of a Synthetic Pyroaurite (a-Pyroaurite). Samples of a synthetic pyroaurite based on the known formula of the mineral pyroaurite [Mg6Fe2(OH)16CO3‚4H2O] were prepared by combining solutions of Mg(NO3)2 and Fe(NO3)3 at pH 2 such that the mole ratio for Mg2+:Fe3+ was 3:1, then

FIGURE 1. Precipitation profiles (residual metal ion vs pH) for the precipitation of Ca2+ (0.001 mol dm-3) with Fe(III) in Cl- media at Fe3+ concentrations of 0.000, 0.001, 0.010, and 0.100 mol dm-3. adding a solution of Na2CO3 until the mole ratio for Fe3+: CO32- was 1:10, so that there would be an excess of CO32-, and finally adding KOH until the pH reached 11. The resulting pale brown slurry was then divided in two in order to investigate the effects of temperature on possible aging of the freshly precipitated synthetic pyroaurite. One sample was isolated by filtration after 1 h, while the second sample was heated to 70 °C for 48 h, cooled to room temperature, and subsequently isolated by filtration. XRD data, TEM examination, and the infrared spectra of both samples were recorded as described above. Chemical analysis of the sample heated to 70 °C for 48 h gave 22.6% Mg and 19.8% Fe as compared with calculated values of 22.0% and 16.8%, respectively, based on the mineral composition given above. The Mg:Fe ratio is only 2.62, instead of the expected 3.00, which suggests that the compound does not have the exact mineral composition and/or that some ferrihydrite was also precipitated with the synthetic pyroaurite. Investigations into the formation of synthetic pyroaurite are continuing. Sludge Volume Index. Sludge volumes for the various solids that were characterized in this study were determined according to the standard Sludge Volume Index (SVI). Sample aliquots (10 cm3) of the hydrated solids were removed from systems that gave representative examples (such as when a-pyroaurite was formed in the presence of ferrihydrite) or found to produce single phases. Each aliquot was placed in a graduated cylinder and allowed to settle for a period of 1 h, after which time the observed sludge volume was recorded. The solid was then quantitatively filtered, dried at 50 °C for 48 h, and weighed. The SVI is recorded in units of cm3/g.

Results Precipitation Studies; Nature of the Solid Phases. The precipitation profiles (defined for the purposes of this study as the residual metal ion concentration versus pH) for Ca2+ and Mg2+ on the initial precipitation of Fe(III) in Cl- media are shown in Figures 1 and 2, respectively. Over this same pH range, Fe(III) is known to precipitate as ferrihydrite from pH ∼3 to ∼5 (19). The profile for Ca2+ (Figure 1) showed significant removal of Ca2+ only at pH values more alkaline than the point of zero charge (pzc) of ferrihydrite (pH ∼8), with no evidence for bulk Ca2+ precipitation occurring over the pH range examined [i.e., no formation of an insoluble ternary or quaternary species, with Ca(OH)2 being soluble under the conditions of this studyssee Figure 1, the profile with no added Fe(III)]. For Mg2+ (Figure 2) at a high Mg2+ to Fe3+ ratio (1:1), bulk precipitation occurred only at high pHs (∼11), with the residual Mg2+ concentration in solution dropping markedly from the formation of brucite, Mg(OH)2. In contrast, for lower Mg2+ to Fe3+ ratios (1:10 and 1:100), the observed Mg2+ concentration decreased during the initial Fe(III) precipitation over the pH range 3-6, which may be reasonably associated with direct co-precipitation into the ferrihydrite structure

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FIGURE 2. Precipitation profiles (residual metal ion vs pH) for the precipitation of Mg2+ (0.001 mol dm-3) with Fe(III) in Cl- media at Fe3+ concentrations of 0.000, 0.001, 0.010, and 0.100 mol dm-3. rather than adsorption because of the concurrence of both of these processes. At the lowest Mg2+ to Fe3+ ratio (1:100), this co-precipitation amounted to a reduction of ∼42% in the Mg2+ concentration, with a mole fraction of 0.004 for incorporation of Mg2+ into the ferrihydrite structure based on the resulting Mg2+ concentration, while at the higher initial Mg2+ to Fe3+ ratio of 1:10, the mole fraction for incorporation of Mg2+ was 0.012. In each case, almost no further decrease in Mg2+ concentration was then observed until the pH was more alkaline than the pzc of ferrihydrite, after which possible absorption followed by the eventual bulk precipitation of a Mg2+-containing phase occurred based on the reduction in residual Mg2+ in solution. Characterization of the solids in both systems indicated that ferrihydrite acts to control the Ca2+ concentration independently of pH and the Ca2+ to Fe3+ ratio as no other phase other than ferrihydrite was formed under these conditions. For the Mg2+ system, however, ferrihydrite was only observed independently of pH for low Mg2+ to Fe3+ ratios. When the Mg2+ to Fe3+ ratio exceeded 1:10, ferrihydrite was the sole phase present at pH values hematite (7.8). When a-pyroaurite was formed in the presence of ferrihydrite, the measured sludge volumes were a-pyroaurite/ferrihydrite (SO42- medium, 27.5) > a-pyroaurite/ ferrihydrite (Cl- medium, 17.6; aged for 1 yr at pH 11 and 20 °C).

Discussion The fate of secondary metal ions on precipitation of ferrihydrite has been found to be governed by a variety of mechanisms that depend on the pH of the system and the ion itself (28). These include specific and non-specific adsorption, surface-enhanced precipitation, and bulk precipitation mechanisms. Some argument exists as to whether these can be considered as separate modes of retention rather than a continuum of mechanistic modes (29). The observation that Ca2+ concentration is not significantly reduced until the pH is made more alkaline than the pzc of ferrihydrite indicates that nonspecific adsorption mechanisms dominate. As Ca2+ has a much larger ionic radius (crystal radius, 6-coordination, 1.14 Å) than Fe3+ (crystal radius, 6-coordination, 0.785 Å, high spin), no inclusion (co-precipitation) of Ca2+ in the ferrihydrite structure would be expected. Thus, no reduction in the Ca2+ concentration was observed during the initial precipitation of ferrihydrite prior to pH ∼8. The absence of any dramatic reduction in Ca2+ concentration at the upper end of the pH range examined (pH ∼11) suggests that no surface-enhanced precipitation or that bulk precipi-

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tation [i.e., Ca(OH)2] occurred [see the profile with no added Fe(III)]. This is in agreement with a previous investigation (30). The retarding effect of Ca2+ on the transformation to goethite is likely attributable to Ca2+ adsorption, which can provide an effective barrier to the dissolution of ferrihydrite, a prerequisite for goethite formation. As a result, both the rate of transformation and the formation of goethite are suppressed. This results in the preferential formation of hematite, which only requires internal aggregation and rearrangement (3, 4). Although the presence of Ca2+ does not prevent the nucleation of goethite, it is uncertain as to whether the presence of Ca2+ also affects the growth of goethite. This is considered likely as high levels of soluble Ca2+ are available for adsorption onto the newly formed goethite nuclei, which may suppress crystal growth. Moreover, adsorption of Ca2+ is considered likely as the pzc of goethite is only ∼6.7 (31), somewhat less than that of ferrihydrite (pH ∼8). Indeed, the TEMs of goethite crystals formed under these conditions were more highly elongated along the crystallographic c direction, as well as exhibiting etch pits, indicating conditions that are unfavorable for crystal growth (2, 7, 12). Thus, goethite formation would also be suppressed at pH values less than the pzc of ferrihydrite. Such transformation-retarding effects have been observed for other foreign cations like Cu2+ and Al3+ (13, 16). As for ferrihydrite, only a very low degree of Ca2+ incorporation into the bulk goethite and hematite phases may be anticipated as a result of the different ionic radii of Ca2+ and Fe3+, as well as their preferred coordination modes (high spin Fe3+ prefers 6-coordination, while Ca2+ is found in a range of coordination environments, commonly 8-coordination as in perovskite, CaTiO3). This is particularly significant for goethite as crystal growth is selective and governed by dissolution. Thus, Ca2+ incorporation would be highly unfavorable. If any significant incorporation of Ca2+ had occurred in either goethite or hematite, an increase in the XRD d-spacings would have been anticipated. However, this was not observed in the present study. It may be noted that previous investigations into the aging of freshly precipitated Al3+ gels have observed increases in the concentration of Ca2+ with time, which indicates that crystallization processes can facilitate desorption of Ca2+ as a result of structural limitations (28). Notably, the ionic radius of Al3+ (crystal radius, 6-coordination 0.675 Å) is even smaller than that of Fe3+, making inclusion of Ca2+ into Al3+ gels even less favorable than into Fe3+ gels (i.e., ferrihydrite). In contrast to Ca2+, it appears as though a full array of solubility-governing mechanisms are observed for the interaction of Mg2+ with Fe3+. In the initial precipitation studies of ferrihydrite with low Mg2+ to Fe3+ ratios, it was observed that the Mg2+ concentration was reduced during the actual precipitation of ferrihydrite, indicating that co-precipitation of Fe3+ and Mg2+ occurred. The limited inclusion of Mg2+ together with Fe3+ in a co-precipitated phase is not unexpected considering the similarities in their ionic radii (Mg2+, crystal radius, 6-coordination 0.860 Å, as compared to Fe3+, 0.785 Å). Indeed, Mg2+ has been observed to be accommodated deeply into the structure of other oxides, in contrast to Ca2+ (30). Any inclusion of Mg2+ into ferrihydrite would likely also require the replacement of some O2- ions by OH- ions to retain a charge balance. However, the observation that the Mg2+-Fe3+ phase, a-pyroaurite, only forms above a critical Mg2+ to Fe3+ ratio and at a critical pH, which approaches that required for the bulk precipitation of Mg2+ as Mg(OH)2 (brucite) is consistent with the fact that the pyroaurite structure is entirely different from that of ferrihydrite and not simply a Mg2+-substituted ferrihydrite phase. Indeed, pyroaurite is related to brucite. As noted above, the fact that pyroaurite has a structure in which Mg2+ and Fe3+ ions are randomly distributed in (positively charged) brucite-like layers

is significant and again related to the similarities in their ionic radii. The occurrence of Fe3+ in the brucite-like layers is counterbalanced by the negatively charged interlayer, thereby maintaining electrical neutrality. The formation of the pyroaurite-related materials in this study should be regarded as being somewhat different to the formation of the substituted magnetite-type [Fe3O4 or Fe3+(Fe2+Fe3+)O4, an inverse spinel] materials, which are formed by the co-precipitation of ions such as Cu2+ and Mn2+ with Fe3+ on addition of base (12,16). Thus, for example, up to 9 mol% of Cu2+ can be incorporated into hematite after co-precipitation and aging (at pH 12.2 and 70 °C), but above this level copper(II) magnetite (CuFe2O4) is formed in addition to copper(II) hematite. The range of continuous variability up to 9 mol % in hematite and by inference in the precursor ferrihydrite is not available to Mg2+, as shown by the initial coprecipitation studies, where only some 1 mol % of Mg2+ was incorporated into ferrihydrite. Thus in the case of the Mg2+-Fe3+ system only some Mg2+ actually co-precipitates during the initial precipitation of ferrihydrite, suggesting that when the pH exceeds that required for the precipitation of a-pyroaurite (pH ∼9), provided that the Mg2+ to Fe3+ ratio is high enough (x is g 0.10), the ferrihydrite dissolves, probably through attack by Mg2+, and the Fe3+ is re-precipitated as a-pyroaurite. It should be noted that the magnesium(II) magnetite compound MgFe2O4 (largely an inverse spinel) may be formed not, however, by direct precipitation from aqueous solution, but only by using a high-temperature procedure that requires firing of well-ground mixtures of MgO and Fe2O3 in air at temperatures from 1200 to 1400 °C (27, file 36-398). The pH dependence of the transformation reaction in the presence of Mg2+ may be seen as a reflection of the differences in the interaction of Mg2+ and Fe3+ with pH. The observation that the pyroaurite-related phase exists as the sole transformation product at pH values g9 indicates that this material is thermodynamically as well as kinetically preferred when the pH exceeds that required for direct precipitation. Thus the pyroaurite-related phase does not act as a precursor to the formation of either hematite or goethite. Indeed, neither goethite nor hematite were found to form in the presence of a-pyroaurite in this study. This is in agreement with the Mg2+ concentration study (70 °C, pH 10.5), as a critical Mg2+ concentration (x g 0.10) was required to produce this phase. For the systems in which the pH and Mg2+ concentration were below the critical values required for pyroaurite-related material production, the transformation of ferrihydrite into hematite was preferred, with some co-precipitation of Mg2+ into ferrihydrite as found from the initial co-precipitation studies. No discernible change in the XRD d values was observable with increasing Mg2+ incorporation, resulting from the similarities in their respective ionic radii, as noted above. The near absence of goethite as a transformation product for the systems where ferrihydrite was initially present suggests that the presence of co-precipitated Mg2+ acts to inhibit ferrihydrite dissolution. Minimal inhibition of goethite nucleation is considered likely, as under the conditions of the accelerated transformation studies at pH 10.5, the Mg2+ concentration was found to be quite low [i.e., 6.37 × 10-7 mol dm-3, calculated using MINTEQA2 (31)]. The reduction in the observed rate of transformation of ferrihydrite with increased Mg2+ concentration (for x < 0.1) may be seen as a reflection of the inhibition of the ferrihydrite dissolution process as a result of incorporated Mg2+. It has been suggested that incorporation of foreign ions into the crystal structure of a host phase can result in alterations to the thermodynamic stability of that phase (32). Thus, the inhibition of goethite production by Mg2+ (x < 0.1) may be considered a result of Mg2+ incorporation reducing the solubility of the ferrihydrite phase and thereby indirectly favoring hematite. The nature of the initially precipitated material and the final transformation product(s) significantly affect the result-

ing sludge volume. When precipitation conditions favor the initial immobilization of Fe(III) as ferrihydrite, transformation into highly crystalline hematite and goethite acts to reduce the sludge volume. Conditions that promote the formation of hematite over goethite [i.e., pH 0.05) reduces the initial sludge volume by producing the pyroaurite-type phases. However, no further sludge volume reductions occur with time, resulting from the thermodynamic stability of such phases and the lack of transformation of the excess ferrihydrite to its crystalline analogues in the presence of these phases. The observed sludge volumes of the solids formed may be seen as a function of their composition, degree of hydration, and morphology. Ferrihydrite exists as a highly hydrated, relatively non-crystalline solid and therefore results in a high sludge volume. Goethite, a-pyroaurite, and hematite are more crystalline and give lower sludge volumes. Thus goethite, although dehydrated in nature, generally exists as mixtures of acicular and twinned crystals. These crystals, although of similar size to hematite, apparently pack in an irregular fashion and consequently lead to moderate sludge volumes. Although a-pyroaurite is hydrated (see the formula for the mineral pyroaurite, given above), the crystals are small and pack in a compact manner, leading to a smaller sludge volume than for goethite. In contrast, hematite is both highly dehydrated and exhibits typical hexagonally-shaped plate crystals, which apparently pack in an ordered array and hence lead to a lower sludge volume than the other phases. The order observed for the sludge volumes of the mixed a-pyroaurite/ferrihydrite solids formed in Cl- and SO42- media suggests that the latter contains much more ferrihydrite than does the former, as evidenced by the considerably higher sludge volume. Moreover, XRD and TEM studies showed that the crystals were less crystalline when formed in SO42- media. Thus it would seem that the presence of SO42- both retards the formation and lowers the crystallinity of a-pyroaurite. It is evident that the presence of either Ca2+ or Mg2+ during the precipitation process acts to retard the rate of transformation, promoting the formation of hematite at the expense of goethite, and hence to minimize resultant, long-term sludge volumes. The presence of Mg2+ above a critical concentration results in the formation of a pyroaurite-like phase, leading to an immediate reduction in sludge volume. However, this phase does not act as a precursor to the crystallization of hematite or goethite with time. At Mg2+ concentrations below that required for the formation of a-pyroaurite, transformation to hematite occurs with goethite formation being effectively suppressed. The preferential formation of hematite results in an approximately 4-fold reduction in sludge volume relative to that of ferrihydrite. However, while the formation of hematite may be preferred in terms of final sludge volumes for landfill requirements, time constraints may dictate the immediate disposal of Fe(III)-containing sludges. The formation of a pyroaurite-type phase, which can be obtained immediately, may therefore serve as a useful alternative, although it results only in a 2-fold reduction in sludge volume relative to ferrihydrite.

Acknowledgments This work was generously supported by BHP Research (Newcastle) and an Australian Postgraduate Award (Industry) Scholarship (K.A.B.). The authors also thank Mr. G. Weber of the University Microscopy Unit for recording the transmission electron micrographs and Ms. Xiaomei Lin for the analytical data on the synthetic a-pyroaurite.

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Literature Cited (1) Marquis, J. K. Environmental Chemistry and Toxicity of Aluminium; Lewis Publishers, Chelsea, MI, 1989; 289-298. (2) Schwertmann, U.; Murad, E. Clays Clay Miner. 1983, 31, 277284. (3) Feitknecht, W.; Michaelis, W. Helv. Chim. Acta 1962, 26, 402410. (4) Fisher, W. R.; Schwertmann, U. Clays Clay Miner. 1975, 23, 3337. (5) Cornell, R. M.; Schwertmann, U. Clays Clay Miner. 1979, 27, 402-410. (6) Cornell, R. M. Clays Clay Miner. 1985, 33, 219-227. (7) Cornell, R. M.; Giovanoli, R. Clays Clay Miner. 1985, 33, 424432. (8) Brady, K. S.; Bigham, J. M.; Jaynes, W. F.; Logan, T. J. Clays Clay Miner. 1986, 34, 266-274. (9) Cornell, R. M.; Giovanoli, R.; Schindler, P. W. Clays Clay Miner. 1987, 35, 21-28. (10) Baltpurvins, K. A.; Burns, R. C.; Lawrance, G. A.; Stuart, A. D. Environ. Sci. Technol. 1996, 30, 939-944. (11) Stiers, W.; Schwertmann, U. Geochim. Cosmochim. Acta 1985, 49, 1909-1911. (12) Cornell, R. M.; Giovanoli, R. Clays Clay Miner. 1987, 35, 11-20. (13) Lewis, D. G.; Schwertmann, U. Clays Clay Miner. 1979, 27, 195200. (14) Schulze, D. G. Clays Clay Miner. 1984, 32, 36-44. (15) Beaufort, D. Clays Clay Miner. 1984, 32, 157-158. (16) Cornell, R. M.; Giovanoli, R. Polyhedron 1988, 7, 385-391. (17) Cornell, R. M. Clay Miner. 1988, 23, 329-332. (18) Schellmann, W. Chem. Erde 1959, 20, 104-135. (19) Baltpurvins, K. A.; Burns, R. C.; Lawrance, G. A.; Stuart, A. D. Environ. Sci. Technol. 1996, 30, 1493-1499. (20) Schwertmann, U.; Cornell, R. M. Iron Oxides in the Laboratory; VCH: Weinheim, Germany, 1991.

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(21) Ingram, L.; Taylor, H. F. W. Mineral Mag. J. Mineral. Soc. 1967, 36, 465-479. (22) Allman, R. Acta Crystallogr. 1968, B24, 972-977. (23) Feitknecht, W. Fortschr. Chem. Forsch. 1953, 2, 670-757. (24) There is a second crystalline form of [Mg6Fe2(OH)16CO3‚4H2O], termed sjo¨grenite. Pyroaurite has a rhombohedral unit cell, while sjo¨grenite is hexagonal, with the difference related to the stacking of the OH- layers. Pyroaurite appears to be the low-temperature form and sjo¨grenite is the high-temperature form (see ref 22). Those hydroxides and hydroxy salts that precipitate from solution and are related to these structures have the low-temperature pyroaurite form (see ref 22). (25) Allman, R.; Donnay, J. D. H. Am. Mineral. 1969, 54, 296-299. (26) Kohls, D. W.; Rodda, J. L. Am. Mineral. 1967, 52, 1261-1271. (27) JCPDS. Powder Diffraction File, Inorganic Phases; International Centre for Diffraction Data: Swarthmore, 1989; File 25-521. (28) Benjamin, M. M. Environ. Sci. Technol. 1983, 17, 686-692. (29) Kathikeyan, K. G.; Elliot, H. A.; Cannon, F. S. Proc. Purdue Ind. Waste Conf. 1996, 50th, 259-267. (30) Kinniburgh, D. G.; Jackson, M. L.; Syers, J. K. Soil Sci. Soc. Am. J. 1976, 40, 796-799. (31) Parks, G. A. Chem. Rev. 1965, 65, 177-198. (32) Allison, J. D.; Brown, D. S.; Novo-Gradac, K. J. MINTEQA2PRODEFA2, A Geochemical Assessment Model for Environmental Systems: Version 3.0; U.S. EPA: Athens, GA, 1991. (33) Van Nortwich, J. W.; Haas, C. N. Proc. Purdue Ind. Waste Conf. 1992, 46th, 455-464.

Received for review June 11, 1996. Revised manuscript received November 15, 1996. Accepted November 21, 1996.X ES960498Y X

Abstract published in Advance ACS Abstracts, February 1, 1997.