The chemical nature of precipitates formed in solutions of partially neutralized aluminum sulfate

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Wat. Res. Vol. 29, No. 6, pp. 1461-1464, 1995 Copyright © 1995 ElsevierScienceLtd Printed in Great Britain. All rights reserved 0043-1354/95 $9.50 + 0.00

THE C H E M I C A L N A T U R E OF PRECIPITATES F O R M E D IN SOLUTIONS OF P A R T I A L L Y N E U T R A L I Z E D ALUMINUM SULFATE J. R. D. B E E C R O F T , M. C. K O E T H E R and G. W. v a n L O O N * Department of Chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6 (First received April 1994; accepted in revised form October 1994)

Abstract--This study examined the solid degradation products formed in partially neutralized aluminum sulfate (PNAS) solutions. The PNAS, which has superior properties as a coagulant for treatment of some waters, suffers from instability as is evidenced by a precipitate which forms with age. The precipitate was found to contain aluminum and sulfate, but the ratio of these two species changed over time. It is suggested that the observed increase in sulfate content in the solids is consistent with the view that polymeric aluminum species are preferentially precipitated during the initial stages followed by monomeric species as degradation continues. Key words--alum, coagulant, partially neutralized aluminum sulfate, precipitate

INTRODUCTION In the wastewater and drinking water treatment industries, aluminum sulfate [or alum, AI2(SO4) 3. 16H20] is the most frequently-used coagulant (Edzwald, 1993). Increasingly, however, other aluminum compounds are finding application as replacements for alum. The replacements work well at cold temperatures and remove turbidity more efficiently. Most of these "advanced chemicals" are thought to contain polymeric species of the metal; perhaps the best known example is polyaluminum chloride (PAC) (Edzwald, 1993). Recently, Koether et al. (1993a) have developed a low-cost partially-neutralized form of alum that has been found to reduce residual turbidity and AI compared to alum for cold, low turbidity, highly coloured waters. The material, here referred to as partiallyneutralized aluminum sulfate (PNAS), also contains polymeric species of aluminum. P N A S has the potential to be an economically viable alternative to alum or PAC as a water treatment coagulant. The P N A S is synthesized in a single step by the addition of calcium carbonate to a concentrated alum solution in a molar ratio (carbonate to aluminum) of 0.75:1. The conditions of synthesis include high power input and the evolved CO2 is allowed to escape. It has been determined by Koether et al. (1993b), using 27A1 N M R , Size Exclusion Chromatography, and the Ferron Method of aluminum speciation, that the P N A S solution contains several aluminum species including *Author to whom all correspondence should be addressed.

monomers [AI(OH)] 2+, small octahedrally coordinated, polymeric A1 species and a polymer denoted as Alia ([AIO4(AII2(OH)24H20)I2] TM ). When stored in its concentrated form, the P N A S is stable for several weeks to several months depending on storage temperature. In dilute solution, the stability is somewhat reduced. In either case as P N A S ages, at some stage a precipitate appears and its ability to act as a coagulant diminishes at the same time as the concentrations of Al~3 and the other polymeric species decrease. The composition of the precipitate was the focus of this study. It was believed that it could consist of Al, SO42- , O H - , and be hydrated to some extent (i.e. the empirical formula could be presented as Al(OH)x(SO4)y'zH 20). It is also possible that the precipitate could contain some insoluble form(s) of calcium. In the present study the composition of the solid material was analyzed as a function of time in order to gain an understanding of its nature as the P N A S underwent ageing. For convenience, the study focused largely on the composition of the precipitate formed from dilute solutions stored at room temperature because of the relatively rapid rate of precipitate formation. Bench-scale studies of dilute solutions have been well documented and readily provide information on the properties of polymeric AI species (Parthasarathy and Buffle, 1985). EXPERIMENTAL Synthesis of PNAS The PNAS was produced by mixing I00 ml of commercial liquid alum (aluminum sulfate, 8.3% AlzO3), 10 ml distilled deionized water (DDW), and 16.13 g CaCO 3 (Anachemia)

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in an Osterizer Galaxie 14 blender on high speed for two minutes (Koether et al., 1993a). For "concentrated" PNAS solutions, the slurry was filtered using a 0.45 ttm filter to remove insoluble C a S O 4 which is a product of the synthesis. For "'dilute" PNAS solutions, the slurry was diluted (52.100 g/100 ml) with DDW to a final concentration of 20 g AI/I prior to filtering through the 0.45/~m filter. Samples of PNAS were stored at room temperature, 20°+ 2°C. Six replicate samples were prepared.

Analytical measurements Immediately before analysis, samples were filtered using a 0.45pm filter in order to separate any degradation precipitate that may have formed over the specified time interval from the PNAS solutions. The solid residues were then oven-dried at 80°C, digested in a minimum amount of boiling 1 M HC1, and diluted to volume using DDW. For samples and standards to be analyzed by flame absorption spectroscopy (FAAS), 0.1% in KC1 was added to control ionization interferences. The AI concentrations were determined by FAAS using a Perkin-Elmer Atomic Absorption Spectrometer Model 1100B with a CANLAB A1 hollow cathode lamp. Measurements were taken at a wavelength of 309.3 nm using a nitrous oxide-acetylene flame. Calcium concentrations were also determined by FAAS. Measurements were done at a wavelength of 422.7 nm using an air-acetylene flame. A 500mg/l Ca standard stock solution was made by dissolving 1.2491 g CaCO 3 in 50ml DDW and 10ml concentrated HCI, and then diluting to 1 1. with DDW. Calcium standards in the range 0 75 mg/l were obtained by appropriate dilution of the stock solution. Sulfate concentrations were determined by high performance liquid chromatography (HPLC). Instrumentation included a SSI 222C HPLC pump, a 200 mm Vydac Ion Chromatography anion separator column, a HewlettPackard 3390A Integrator, and a Wescan Model 215100 Conductivity Detector. Sample loop volume was 0.100 ml. The potassium hydrogen phthalate (KHP) eluent (0.003 M) was prepared in degassed DDW and the pH was adjusted to 5.05 or 4.5 with a 0.1 M KOH solution so that a well defined SO42- peak could be obtained. The flow rate was 1.8 ml/min. A standard 4.00g/1 SO42- stock solution was made by dissolving 0.7257g K2SO4 (BDH) in 100ml of DDW. Sulfate concentrations were determined using peak areas. Retention times (RT) were measured at maximum peak height. With the eluent pH at 5.05, the RT for SO4- was 5.00 min and a calibration curve of Area = 3378{conc (mg/l)} - 5 8 2 4 was obtained with R2= 0.998. In order to achieve better resolution, as the column degraded due to age, later analyses were conducted with the eluent solution adjusted to a pH of 4.5. The SO4 RT was increased to

5.76 min, and a calibration curve of Area =4964{conc (mg/l)} + 5208 was obtained with R 2 = 0.998. All analyses were done in at least triplicate. RESULTS AND DISCUSSION

p H Measurements The pH o f the concentrated P N A S solution was slightly lower than that for the dilute P N A S solution (3.31 vs 3.45) and remained relatively constant ( + 0.04 units) over the 25 day period o f observation. C o m p a r e d to the dilute one, the concentrated P N A S solution was much slower to precipitate which suggests a possible role o f the solution acidity in the development o f the precipitate. The analysis also shows that the composition o f the precipitate from the concentrated samples was slightly different from that from the dilute samples (Table 1). This may also be an effect o f pH. Both dilute and concentrated P N A S had pH values higher than that for commercial liquid alum (pH = 2.43) due to partial neutralization.

Composition o f precipitates The results o f analysis for Ca showed that there was no measurable Ca in the P N A S precipitate and in dilute P N A S solutions, Ca concentrations were less than 1 mg/l. Therefore, AI was the only significant cation present in the P N A S precipitate. The analytical results for the samples and a summary o f average sample composition is found in Table 1. The O H - concentration was estimated by balancing the charge after the A1 and SO42 concentrations were determined. A sample o f A12(SO4) 3 was also analyzed as a reference. In this, the ratio o f mol SO 2 : mol AI was found to be 1.52 + 0.02, very close to the actual value o f 1.50. The n u m b e r o f water molecules o f hydration for the sample was determined to be 16. Because the mole ratio SO~ :AI is much less than 1.52 for all the P N A S samples, the precipitate cannot be AI2(SO4h alone. It is important to note that the precipitate would not be a single well-defined A1 species, but a mixture o f c o m p o u n d s . Literature on the composition o f precipitates from partially neutralized aluminum sol-

Table 1. Composition of P N A S precipitates Sample precipitate formed (days) AI2(SO4)3 Dilute P N A S 0 to 3 0 to 7 7 to 14 14 to 21 0 to 14 0 to 21 0 to 50 Concentrated P N A S 0 to 21 33 to 94 94 to 101

CA~ ( % by wt)

o Cs°] (Vo by wt)

9.16 + 0.03

49.6 + 0.50

21.15_+0.07 23.07 _4_0.00 23.38 + 0.08 21.53 -+ 0.08 21.70+0.13 21.40+0.11 23.62 _+ 0.08 18.33-+0.08 18.82_+0.15 18.52+0.17

Composition AI(OH)'(SO4)' "z H 2 0 ~x y 0

1.52

24.11 + 0 . 7 0 30.81 + 1.40 33.68 -+ 0.50 22.68 + 0.27 37.85-+ 1.06 27.60_+ 1.57 35.52 + 1.95

2.36 2.24 2.20 2.40 2.02 2.28 2.14

0.32 0.38 0.40 0.30 0.49 0.36 0.43

41.90+0.88 42.60+0.55 4 5 . 1 2 + 1.21

1.72 1.72 1.64

0.64 0.64 0.68

Nature of PNAS precipitates utions containing A1~(SO4)3 suggest the presence of several species, and that the composition of precipitates varies with time (Bersillon e t al., 1980; Tsai and Hsu, 1984). The detailed mechanism for precipitation from those solutions remains obscure because the nature and the individual concentrations of AI species in solution cannot be determined without ambiguity (Bersillon e t al., 1980). It is believed that in partially neturalized aluminum(III) solutions, AI(OH)3 precipitates out of solution during ageing (Smith, 1971). In the PNAS solution, it is possible for SO~ to become part of the AI(OH)3 precipitate through a ligand exchange reaction. In an acidic medium, a hydroxyl is protonated to form a water ligand that could then be displaced by SO~- (Greenland and Hayes, 1981; Hingston e t al., 1972). Table 2 shows the amount of precipitate obtained as a function of time during the ageing of dilute PNAS solutions. The results for Sample 1 show that of the total amount of precipitate formed over 3 weeks, 81% by weight formed in the first 7 days, 8% from 7 to 14 days, and 11% from 14 to 21 days. For Sample 2, 88% by weight of the total precipitate formed over the 21 day observation period formed in the first 14 days and 12% in the last 7. This is in agreement with Tsai and Hsu (1984), who observed decreasing amounts of precipitate in going from the first to third weeks of separation of solutions of partially neutralized aluminum. The observed average composition of the threeday-old PNAS precipitate, AI(OH)2.36(SO4)0.32 (see Table 1) is close to the average composition of a precipitate obtained by Tsai and Hsu (1984) early in the ageing of the partially neutralized aluminum solutions. The precipitate they described had a spherical morphology and average composition of Al(OU)2.34(SO4)0.33. Mesmer and Baes (1971) determined that the molar ratio of OH/AI that best represents the initial hydrolysis products of alumi-

Table 2. Summaryof amount of precipitate (by weight) obtained from dilutePNASsolutionsof variousages.The % AI by weightstill remaining in solution is also presented. Dilute PNAS solutions initiallycontain 2 g AI/100ml Sample Mass of Mass of % AI precipitate precipitate AI in ppt remaining formed (days) (g) (g) in solution Sample No. I

0 to 7 7 to 14 14 to 21 Total

1.2924 (81%) 0.1358 (8%) 0.1746 (11%) 1.6028

0.297 0.032 0.038 0.367

85 84 82 82

1.0497 (88%) 0.1507 (12%) 1.2004

0.228 0.032 0.260

89 87 87

1.3289

0.260

87

1.0812

0.229

89

2.6267

0.620

69

1.2960

0.297

--

Sample No. 2

0 to 14 14 to 21 Total Sample No. 4

0 to 3 Sample No. 5

0 to 50 Sample No. 6

31 to 181

num was 2.43. Since Mesmer and Baes postulated that the AI species present were predominantly polymeric, this could indicate that the early precipitate of dilute PNAS is composed in part of polymeric AI species complexed with SO~ . This early precipitation of the polymeric A1 species would also account for their disappearance in PNAS solution during ageing found by Koether e t al. (1993b). The analysis of the precipitates also revealed that the average composition of precipitates from concentrated and dilute PNAS solutions differed considerably in the OH/AI and SOl /A1 ratios (see Table 1). The concentrated PNAS precipitate had less O H and more SO]- per mole of AI than dilute PNAS precipitate. As the dilute PNAS solutions aged, the composition of the precipitate changed. Inspection of the average compositions of the 0 to 3 day, 7 day, 7 to 14 day and 50 day precipitates reveals that, relative to the AI concentration, the amount of SO4 increased with ageing (0.32, 0.38, 0.40, 0.43 mol SOl per mol AI respectively) while the concentration of OH decreased (2.36, 2.24, 2.20, 2.14mol OH per mol AI respectively). The 14-21 day precipitate is anomalous with respect to this trend. A possible explanation for this observation is that in the AI species postulated to be in PNAS by Koether e t al. (1993b), the OH:AI mol ratio is greater in the polymeric than the monomeric species. Thus, the observed trend in the average composition of the samples could reflect a large percentage of polymeric AI species precipitating early in the ageing, which later gives way to predominantly monomeric species with less OH per mol of aluminum. CONCLUSIONS The average compositions for the PNAS precipitates indicate that a mixture of aluminum species is responsible for the solid, rather than single welldefined species. The average composition of the precipitate changed over time, and this may reflect the precipitation of polymeric aluminum species such as AI~3 early in the ageing, followed by predominantly monomeric species later in the ageing. Coagulants prepared by partial neutralization of aluminum solutions contain polymeric aluminum species which are responsible for enhanced efficiency in water treatment. Where such coagulants are unstable, loss of the polymers by degradative precipitation leads to a substantial drop in their coagulative properties. REFERENCES

Sample No. 3

0 to 21

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Bersillon J. L., Hsu P. H. and Fliessinger F. (1980) Characterization of hydroxy-aluminum solutions. Soil Sci. Soc. Am. J. 44, 630~634. Edzwald J. K. (1993) Coagulation in drinking water treatment: particles, organics and coagulants. Wat. Sci. Technol. 27, 21 35.

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Greenland D. J. and Hayes M. H. B. (Editors) (1981) The Chemistry of Soil Processes. Wiley, New York. Hingston F. J., Posner A. M. and Quirk J. P. (1972) Anion adsorption by geothite and gibbsite. I. The role of the proton in determining adsorption envelopes. J. Soil Sci. 23, 177-192. Jenke D. R. and Pagenkopf G. K. (1984) Models for prediction of retention in nonsuppressed ion chromatography. Analyt. Chem. 54, 8-91. Jupille T. (1987) Single ion chromatography. In Ion Chromatography (Edited by Tarter J. G.), pp. 23-81. Marcel Dekker, New York. Koether M. C., Deutschman J. E. and vanLoon G. W. (1993a) A bench-scale evaluation of the performance of a polynuclear aluminum coagulant. In Disinfection Dilemma: Microbiological Control Versus By-products (Edited by Robertson W., Tobin R. and Kjartanson K.), pp. 303-319. Am. Wat. Wks Assoc., Denver, Colorado. Koether M. C., Deutschman J. E. and vanLoon G. W. (1993b) Chemical characterization of a polynuclear

aluminum coagulant. Int. Syrup. on Chemistry and Biology of Municipal Water Treatment: Current Status and Future Directions, Burlington, ON. Mesmer R. E. and Baes C. F. Jr (1971) Acidity measurements at elevated temperatures. V. Aluminum ion hydrolysis, lnorg. Chem. 10, 2290-2296. Parthasarathy N. and Buftte J. (1985) Study of polymeric aluminum(Ill) hydroxide solutions for application in waste water treatment. Properties of the polymer and optimal conditions of preparation. War Res. 19, 25-36 Smith R. W. (1971) Relations among equilibrium and nonequilibrium aqueous species of aluminum hydroxy complexes. In Nonequilibrium Systems in Natural Water Chemistry (Edited by Hem J. D.) Advances in Chemistry Series 106, pp. 250-279. Am. Chem. Soc.: Washington, D.C. Tsai P. P. and Hsu P. H. (1984) Studies of aged OH-A1 solutions using kinetics of Al-ferron reactions and sulfate precipitation. Soil Sci. Soc. Am. J. 48, 59~5.