Environ. Sci. Technol. 1996, 30, 939-944
Effect of pH and Anion Type on the Aging of Freshly Precipitated Iron(III) Hydroxide Sludges 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
The effect of pH and anion composition on the aging of freshly precipitated iron(III) hydroxide sludges has been examined. The rate of transformation of the kinetically favored iron(III) oxyhydroxide, ferrihydrite, to its crystalline analogues, hematite (R-Fe2O3) and goethite [R-FeO(OH)], was promoted with increasing pH. The influence of anion type on the transformation rate was related to the affinity of the anions for the surface of the ferrihydrite particles, with rates decreasing in the order υT nitrate > υT chloride > υT sulfate (υT ) rate of transformation). The relative composition and crystal morphology of the product species was found to be dependent on both the anion type and the pH of the system. Hematite formation was favored at pH values near the point of zero charge of ferrihydrite (pH 7-9), whereas goethite formation was favored outside of this region. A correlation between enhanced hematite formation and both the relative affinities of the anions for the ferrihydrite particles and their relative aqueous phase complex stability constants was evident. This is thought to be a manifestation of the competing formation mechanisms of hematite and goethite.
Introduction Over the past few years, the use of iron(III) salts as coagulating agents has increased dramatically. Such ironbased coagulation involves the addition of a suitable base to the iron(III) solution to produce the metastable oxyhydroxide termed ferrihydrite [Fe(OH)3.xH2O] (1). As a result of the extremely rapid kinetics of the precipitation process, large amounts of nonstructural water may be entrapped within the structure of ferrihydrite, making the sludges produced voluminous and difficult to dehydrate. Upon aging, ferrihydrite transforms into its thermodynamically favored crystalline analogues, primarily goethite [R-FeO(OH)] and hematite (R-Fe2O3). As these transformation products are highly crystalline and dehydrated, sludge volumes after transformation tend to be dramatically
0013-936X/96/0930-0939$12.00/0
1996 American Chemical Society
reduced, thus minimizing the required storage volume for the aged sludges. Furthermore, the increased thermodynamic stability of these sludges reduces the likelihood of secondary leaching of the sludge back into the environment, making them potentially useful as landfill. Consequently, it is obviously desirable for aging conditions to be optimized in such a way as to provide complete transformation in the minimum amount of time possible. In order to determine such conditions, it is important that all factors controlling the overall process be identified and optimized. Various studies have shown the transformation of ferrihydrite into goethite and hematite to be highly dependent on the aging environment (2-7). Factors such as time, temperature, pH, and electrolyte composition are all involved in not only determining the nature of the final product but also determining the rate at which transformation occurs. One of the principal parameters controlling both the rate and products of the ferrihydrite transformation process is the pH of the aging environment. Its effect is best understood by considering the mechanisms by which goethite and hematite form. The formation of goethite and hematite occur through two mutually exclusive and competitive processes (5). Hematite is produced by an internal rearrangement of the ferrihydrite structure and is favored at intermediate pH values, i.e., pH 7-8, close to the point of zero charge (pzc) of ferrihydrite. Goethite formation involves the dissolution of ferrihydrite followed by nucleation and reprecipitation as goethite. Goethite transformation is favored under pH conditions in which soluble iron hydroxy complexes predominate, i.e., pH < 4 and pH > 12. Another fundamental parameter controlling transformation is temperature. It is generally agreed that hematite syntheses are best accomplished at high temperatures (>90 °C) due to the required dehydration, whereas goethite syntheses are favored at low temperatures (500a
>500a
267 123 67 37
290 113 73 43
>1000a >500a 280 97 53
Half-conversion not observed.
Results
TABLE 2
Kinetics of Transformation. The composite effect of pH and anion type on the rate of transformation of ferrihydrite to hematite and goethite is shown in Figure 1. Increases in pH favored the rate of transformation, with transformation being typically first order in nature. At the lower pH values, where transformation was slower, the appearence of an induction period was evident. Similar results have previously been observed (3). A summary of transformation data is given as the interpolated half-conversion time (T50%) versus the pH and anion type in Table 1. Significant differences between T50% times for the nitrate, chloride, and sulfate systems were evident, with the rate of transformation increasing in the order sulfate < chloride < nitrate. Product Mineralogy. Significant color variation among samples of the same anion set with pH was observed. Colors ranged from that of pure ferrihydrite to hematite and goethite as defined elsewhere (1). Variation between anion sets was also evident, with ferrihydrite coloration dominating in the order of sulfate > chloride > nitrate. Sample compositions as characterized by X-ray powder diffraction (XRD) and oxalate extraction are given in Table
Composition of Samples for Nitrate, Chloride, and Sulfate Systems after 1 Year of Aging
940
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 3, 1996
composition of product (%) ferrihydrite
goethite
hematite
pH
NO3-
Cl-
SO42-
NO3-
Cl-
SO42-
NO3-
Cl-
SO42-
7 8 9 10 11
71 33 4 chloride > nitrate, although the degree of size variability was much greater for the sulfate system. Typical diameter ranges were 100-150, 100-200, and 100-300 nm for the nitrate, chloride, and sulfate systems, respectively. The samples prepared at pH 11 consisted entirely of goethite with no hematite visually present. This is again consistent with the XRD results. Goethite existed as mixtures of acicular and twinned (twin pieces, see ref 6) crystals for both the nitrate and chloride systems. In the sulfate system, however, the main crystal morphology observed was a star-shaped twin composed of three acicular crystals rotated at 120o about a central point. Again, similar crystal types for goethite have been observed in the past (2, 6). Based on previous work (6), given the initial iron(III) concentration employed in this study and the resulting ferrihydrite suspension concentration, twinned crystals were expected to form. However, the appearance of the star-shaped twins in the case of the sulfate system was
Upon aging, ferrihydrite may potentially form a multitude of different iron(III) oxyhydroxides and oxides under varying conditions; however, the formation of hematite and goethite predominates in the natural environment. The observation that increases in pH accelerates the rate of transformation is in agreement with previous studies (3, 4). Interaction of the common anions nitrate, chloride, and sulfate with ferrihydrite has been the focus of a number of studies in the past (11-14). Fixation of these ions onto the surface of the ferrihydrite particles occurs by various adsorption mechanisms. Adsorption of this type may be broadly classified as either specific or nonspecific in nature. Specific adsorption is characterized by the formation of chemical bonds between the adsorbent and adsorbate whereas nonspecific adsorption is purely electrostatic in origin (14). Adsorption of nitrate and chloride on the surface of ferrihydrite particles has been observed to occur largely by nonspecific mechanisms, although some specific type interaction has been noted at least for the chloride ion (13). Consequently, the adsorption of nitrate and chloride mainly occurs under pH conditions below the pzc of ferrihydrite (i.e., more acidic) where the surface has a net positive charge. In contrast, the adsorption of sulfate is highly specific with adsorption occurring at pH values far in excess of the pzc. The specific adsorption of anions has been found to dramatically reduce the rate of transformation of ferrihydrite to goethite and hematite (2). An example of this is the transformation-retarding effect of silicate, which is known to specifically adsorb on the surface of ferrihydrite and can coordinate to as many as four separate particles of ferrihydrite (2). Under such conditions, the surface of ferrihydrite may be rendered an immobile network suppressing further rearrangement or dissolution as required for the formation of goethite and hematite. As the mechanism of the adsorption of sulfate to the surface of ferrihydrite is proposed to resemble that of silicate (13, 14), the ability of sulfate to immobilize the structure of ferrihydrite in a similar manner to silicate, thus retarding its transformation, is considered likely. This mechanism is supported by the diminished rate of ferrihydrite transformation for the sulfate system relative to those of the chloride and nitrate systems. Considering that the molar ratio of SO42- to Fe3+ was lower than that for the anion to Fe3+ ratio in the Cl- and NO3- systems, this effect is even further pronounced. Also, some specific type interaction may also occur for the chloride ion, as transformation rates were slightly lower than observed for the nitrate system. The inherent ability of such specifically adsorbed anions to suppress the transformation rates may be enhanced further as transformation occurs. This may be seen as a result of changes in the position of the pzc during the
VOL. 30, NO. 3, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
941
FIGURE 3. Transmission electron micrographs of the final, aged products: (a) nitrate at pH 9, (b) nitrate at pH 11, (c) chloride at pH 9, (d) chloride at pH 11, (e) sulfate at pH 9, (f) sulfate at pH 11. All micrographs are to the same scale.
942
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 3, 1996
transformation process. The pzc of hematite and goethite occurs at higher pH values than that of ferrihydrite. Thus, during the course of the transformation the adsorption of anions may be facilitated, thereby acting to further diminish transformation rates. It should be noted that the interpretation of transformation rates based entirely on the oxalate extraction process must be done realizing the potential limitations of the method. As the oxalate dissolution process is surface controlled, any competitive reactions of the surfaceadsorbed anions may result in erroneous estimations of transformation rates. For the purpose of this investigation, any competition of the specifically adsorbed anions would essentially act to block the extraction process, resulting in an increased estimate of the observed transformation rates. Thus, the oxalate extraction process may potentially yield a somewhat conservative estimate of the effect of the specifically absorbed anions on the reduction of transformation rates. In addition to affecting the rate of transformation of ferrihydrite, the nature of the anions clearly influenced the relative proportions and crystal morphologies of hematite and goethite in the final, aged products. The degree of specificity of adsorption of the anion may thus be seen as influencing the final composition and morphology. The relative fraction of hematite tended to increase with the increasing specificity of adsorption of the anion system. Furthermore, conditions that act to favour the formation of hematite also act to increase the average hematite crystal dimensions. Similar results have been previously observed (3). This effect can be interpreted according to the competitive mechanisms of hematite and goethite formation. Goethite formation involves the dissolution of the ferrihydrite phase followed by reprecipitation as goethite. Sulfate may thus be envisaged as acting as a chemical “glue”, suppressing dissolution. The formation of hematite involving only rearrangement is thus thermodynamically favored. This situation is further promoted when the effects of aqueous phase complexation are considered. Sulfate forms relatively strong complexes with iron(III) compared to nitrate and chloride (15). Since goethite production involves hydroxy complexation as a requirement, the formation of iron(III) sulfato complexes in solution should act to reduce goethite production. No such hydroxy complexation is required for hematite production, hence hematite formation is promoted at the expense of goethite by strong complex formation. Similar results have been observed for various organic acids (5), which are also capable of complex formation with iron(III) in solution. One other important factor that influences the rate of transformation is the ionic strength of the aging environment. Increases in ionic strength have been shown to reduce the rate of transformation of ferrihydrite as well as favoring the formation of hematite over goethite (6, 16). In the current study, ionic strengths between systems were not controlled, resulting in the ionic strengths of the sulfate systems exceeding those of the chloride and nitrate systems. Increases in ionic strength have been shown to reduce the adsorption of sulfate to the surface of goethite (12). Assuming that the adsorption mechanism of sulfate on ferrihydrite resembles that of goethite, the effect of increased ionic strength in the sulfate system reducing the rate of transformation may at least be partially offset by the decreased concentration of sulfate (relative to nitrate and chloride) under the conditions of the present study.
The results observed in this investigation clearly have important ramifications for the effective aging of ferrihydrite. Optimum transformation rates for the conversion of ferrihydrite to its crystalline analogues is favored under conditions of high pH. However, the relative proportion of hematite compared to goethite diminishes at pH values greater than 9. Considering that sludge volumes increase in the order ferrihydrite > goethite > hematite, the promoted formation of goethite at the expense of hematite may be undesirable. Moreover, the ability to maintain an environment of pH > 10 may be limited as a result of potential leaching of secondary hydroxide precipitates back into the environment. The effect of anion composition also plays a significant role in sludge transformation behavior. In general, iron(III) sulfate and iron(III) chloride salts are used for water treatment. As a result, the effects of specific adsorption on the transformation rate must be considered. For sulfatebased salts, the rate of transformation of ferrihydrite may be significantly reduced; however, the preferential formation of hematite may be desirable. A similar result for chloride-based salts may be observed with the exception that the rate of transformation may not be suppressed to such an extent. The method of physical storage of ferrihydrite sludges may also play a significant role in determining the rate of transformation by the composition of products. Generally, sludges are contained in clay-lined pits or ponds. Associated with such environments are moderate concentrations of the transformation-retarding species silicate and organic acids. As a result, chemical effects other than those directly associated with the type of salt used in the coagulation process may be important.
Conclusion In order to provide for the optimum transformation of the freshly precipitated iron(III) hydroxide sludge (ferrihydrite) into its crystalline analogues (goethite and hematite), the composite effect of aging time, pH, and electrolyte composition must be considered. Transformation rates are accelerated with increasing pH. However, electrolytes that have the ability to specifically adsorb to the surface of the ferrihydrite particles can act to reduce the overall transformation rates. The nature of the electrolyte also exerts a significant influence on the composition of the aged sludge, with hematite formation being favored at the expense of goethite when the electrolyte specifically adsorbs to the structure of ferrihydrite.
Acknowledgments This work was generously supported by BHP Research (Newcastle) and an Australian Postgraduate Award (Industry) Scholarship (K.A.B.). The authors also acknowledge Mr. G. Webber of the University Microscopy Unit for recording the transmission electron micrographs and Dr. D. Todd for the X-ray powder diffraction studies.
Literature Cited (1) Schwertmann, U.; Cornell, R. M. Iron Oxides in the Laboratory; VCH: Wienheim, 1991; pp XIII-XIV, 1-12. (2) Cornell, R. M.; Giovanolli, R.; Schindler, P. W. Clays Clay Miner. 1987, 35, 21-28. (3) Schwertmann, U.; Murad E. Clays Clay Miner. 1983, 31, 277284. (4) Schwertmann, U.; Fisher, W. R. Clays Clay Miner. 1975, 23, 3337.
VOL. 30, NO. 3, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
943
(5) Cornell, R. M.; Schwertmann, U.; Clays Clay Miner. 1979, 27, 402-410. (6) Cornell, R. M.; Giovanoli, R. Clays Clay Miner. 1985, 33, 424432. (7) Cornell, R. M.; Giovanoli, R. Clays Clay Miner. 1986, 34, 557564. (8) Brady, K. S.; Bigham, J. M.; Jaynes, W. F.; Logan, T. J. Clays Clay Miner. 1986, 34, 266-274. (9) Collepardi, M.; Massidda, L.; Rossi, G. Rend. Soc. Ital. Mineral. Petrol. 1973, 29, 251-270. (10) Collepardi, M.; Massidda, L.; Rossi, G. Inst. Min. Metall. Trans., Sect. C 1973, 82, C88-C91. (11) Hingston, F. J.; Atkinson, R. J.; Posner, R. J.; Quirk, J. P. Nature 1967, 215, 1459-1461. (12) Hoins, U.; Charlet, L.; Sticher, H. Water Air Soil Pollut. 1993, 68, 241-255.
944
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 3, 1996
(13) Harrison, J. B.; Berkheiser, V. E. Clays Clay Miner. 1982, 30, 97102. (14) Parfitt, R. L.; Smart, R. S. C. Soil Sci. Soc. Am. J. 1978, 42, 39-44. (15) Smith, R. M.; Martell A. E. Critical Stability Constants Volume 4: Inorganic Complexes; Plenum Press: New York, 1976; pp 4852, 79-85, 104-112. (16) Cornell, R. M.; Giovanoli, R.; Schneider, W. J. Chem. Technol. Biotechnol. 1989, 46, 115-134.
Received for review June 12, 1995. Revised manuscript received October 25, 1995. Accepted November 1, 1995.X ES950401S X
Abstract published in Advance ACS Abstracts, January 15, 1996.
