Environ. Sci. Technol. 1986, 20, 891-894
Kinetics of Aluminum Hydrolysis: Measurement and Characterization of Reaction Products Bill Batchelor,* J. Brock McEwen,+and Roberta Perrys Civil Engineering Department, Texas A&M University, College Station, Texas 77843
A timed colorimetric analysis procedure is described to characterize products formed during aluminum hydrolysis. The procedure is based on measuring the rate and extent of reaction of the hydrolysis products with ferron and can provide information on the amounts and relative sizes of the hydrolysis products. The procedure was applied to partially neutralized solutions of aluminum sulfate at dilute concentrations (0.1 mM AI). No particles of a size similar to the AllSpolymer were found in these solutions. Marked differences in the characteristics of hydrolysis products formed in batch and continuous flow reactors were observed, Four types of aluminum species were measured in effluents from continuous flow reactors. Two of them were characterized with respect to their dissolution rates in a timed colorimetric analysis procedure.
Introduction The hydrolysis reactions of aluminum are of interest in the field of water treatment where solutions of aluminum sulfate are often used as coagulants. Despite their importance, little work has been done on the kinetics of these reactions. A critical part of a kinetic study is development of an analysis procedure that can measure the concentrations of various hydrolysis products. Measurement Techniques. Short reviews of various methods for analysis of the aluminum hydroxide species produced by hydrolysis reactions are available (1-3). Most of the techniques discussed in these reviews, and elsewhere in the literature (4-16), are not suitable for analysis of short-lived reaction products, because they require too much time for sample prepartion or analysis. However, the timed colorimetric procedure can be started within 30-60 s of sampling and so is suitable for analysis of reaction products that are stable for a time period of minutes. The timed colorimetric procedures are based on the observation that different types of aluminum hydroxide species dissolve and then react with a colorimetric reagent at different rates. Different dissolution rates can arise from differences in the strength of the bonds between the aluminum and hydroxide ions or from differences in the size of the species. If the bonds are similar, the dissolution rates should be proportional to the surface area of the aluminum hydroxide species. Objectives. This paper will describe development of a timed colorimetric analysis technique for characterizing the aluminum hydroxides formed under conditions similar to those found in water treatment, and the application of this technique to solutions neutralized in batch and continuous flow reactors. Experimental Methods Batch Reactor. Experiments were conducted in batch and continuous flow reactors to produce aluminum hy'Present address: CHSM-Hill, 5995 S. Syracuse, Denver, CO 80231.
Present address: CH2M-Hill, 310 W. Wisconsin Avenue, Miwaukee, WI 53202. 0013-936X/86/0920-0891$01.50/0
drolysis products for evaluation by the manual timed colorimetric procedure. The effects of pH and reaction time on formation of aluminum hydrolysis products were evaluated in filtered and unfiltered solutions of aluminum sulfate (0.1 and 0.2 mM Al). The effect of reaction time on the ability of unfiltered alum solutions to coagulate clay was also studied. The batch reactors were rectangular, acrylic containers that held 1L of sample. They were stirred at 100 rpm by a paddle stirrer such as used in coagulation jar tests (Phipps and Bird Inc., Richmond, VA). A mean velocity gradient of approximately 100 s-l has been reported for a similar system (17). The standard procedure was to prepare the aluminum salt solution of the desired concentration and then neutralize with sodium bicarbonate or sodium carbonate to the desired pH. In some cases, the pH of a distilled water solution was adjusted to the desired value, and then a concentrated aluminum salt solution was added with a syringe to achieve the desired total aluminum concentration. Some samples from the batch reactors were filtered before manual colorimetric analysis to allow direct characterization by particle size. Cellulose acetate membrane filters (Millipore Corp., Bedford, MA) with pore sizes of 450 and 10 nm were used along with an ultrafilter (XM100A, Amicon Corp., Lexington, MA) with a nominal molecular weight cutoff of 100000 (an approximate pore size of 6 nm). The time for sampling, filtration, and addition of reagents was from 1 to 2 min for the 450-nm filters. The time increased to about 5 min when the 10-nm filters were used. Analysis of sequential 5 mL volumes of filtrate did not indicate that the amount of aluminum passing the filters was decreasing. Continuous Flow Reactor. Aluminum solutions neutralized in a continuous flow reactor were used to develop an automated procedure for the timed colorimetric test. A 2.7-L glass reaction kettle with a temperature controller (Versa-Therm Model 2156, Cole-Parmer Instrument Co., Chicago, IL) and pH controller was used in the continuous flow studies. The pH controller consisted of a pH meter (Chemcadet Model 5652, Cole-Parmer Instrument Co., Chicago, IL) and a data acquisition interface (ISAAC Model 91A, Cyborg Corp., Newton, MA) connected to a microcomputer (Apple Computer Inc., Cupertino, CA). The feed solution contained 0.01 M NaNO,, 0.5 mM NaCO,, and varying amounts of nitric acid depending on the desired reactor pH. Nitric acid was also used by the Apple/ISAAC controller to maintain pH. A concentrated aluminum sulfate solution (1M Al) was fed to the reactor through a Teflon tube (0.30 mm i.d.), and the reactor was mixed with a heavy-duty magnetic stirrer (Model 4815, Cole-Parmer Instrument Co., Chicago, IL). The mean velocity gradient for this reactor can be estimated as 2200 s-' by assuming that 10% of the rated power of the mixer is transmitted to the water. An impulse response test showed that the system behaved as a completely mixed reactor. Adding aluminum to a buffered solution avoids formation of local areas of excess hydroxide ions that have been associated with artifacts in titration experiments (18).
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 9, 1986 891
I
Table I. Summary of Experimental Conditions for Timed Colorimetric Analysis Applied to Dilute (0.1 mM Al) Solutions of Aluminum Sulfate in Batch Reactors
TOTAL AL
pH
MODERATELY DISSOLVINQ AL
5.5 5.5
RAPIDLY DISSOLVING AL
TIME Flgure 1. Differentiation of aluminum hydroxide species in timed colorimetric analysis procedure.
Timed Colorimetric Test. The timed colorimetric procedure was based on the aluminum analysis technique of Rainwater (19). Absorbances were measured manually for samples from batch reactors while the automatic data acquisition system obtained absorbance measurements for samples from the continuous flow reactor. Slight modifications to the referenced procedure were made to facilitate automatic analysis of samples from the continuous flow reactor. For these samples, the colorimetric reagent was prepared with 5 volumes of saturated ferron, 5 volumes of distilled water, 4 volumes of acidified hydroxylamine (100 g of hydroxylamine plus 40 mL of concentrated hydrochloric acid), and 4 volumes of 4.3 M sodium acetate. A 2 mL volume of this reagent was mixed with a 5 mL volume of sample, and its increase in absorbance was monitored at 370 nm in 1-cm cuvettes. The Apple/ISAAC data acquisition system was standardized with 0.1 mM aluminum standards at the start and the end of the monitoring period. Measured absorbance values taken at 4-min intevals could then be easily corrected for any drift in the spectrophotometer. Values for absorbance as a function of time, whether measured manually or automatically, were used to calculate first-order dissolution rate constants by nonlinear regression. Equation 1 was used to fit the absorbance data when one aluminum hydroxide species was believed to be dissolving, while eq 2 was used when two species were believed to be dissolving: A, = At, t (A, - Ato)(l - exp(-kt)) (1) At = At0
+ (A,,,
- At,)(1 exp(-k,t)) + (A,
- A,,J(l
- exp(-k,t))
(2)
where At, A,, and A, = absorbances measured at any time, at infinite time, and at time equal to zero, k = first-order rate constant (T-l), t = time (T), A,,, = absorbance that would be measured at infinite time if only the rapidly dissolving species were present, and k, and k, = first-order rate constants for the rapidly and moderately dissolving species. After values of the coefficients (A,, A,,,, A,, k,, k,) were determined, the concentrations of the various species (instantaneously, rapidly, moderately, and slowly dissolving aluminum) were calculated from the differences among the critical absorbances (Ab, A,,,, A,) as indicated by Figure 1. Coagulation Test. Coagulation tests were performed by transferring 1 L of the neutralized aluminum salt solution to rectangular acrylic containers. A concentrated solution of kaolin clay was then added to achieve a final concentration of 150 mg/L. The solution was mixed 892
Environ. Sci. Technol., Vol. 20, No. 9, 1986
6.0 6.0 6.0'rd 6.0 6.0 6.0 6.0 6.0C,d 6.0'~~ 6.0' 6.0 6.0 6.0 6.0Ctd
reaction time, min 30 30
0.5 0.5 5 10.5 10.5 20.5 20.5 30 30 30 30 30.5 30.5 320
450
filter pore size,!' nm 220 10 6*
+ -
-
-
-
-
-
$.
-
-
-
-
-
-
-
a (+) filtrate contained aluminum hydroxide species that dissolved during timed colorimetric test; (-1 filtrate did not contain species that dissolved. *M,lo6. 'Total aluminum = 0.2 mM. dpH achieved in solution before aluminum salt injected.
rapidly (100 rpm) for 1 min and then slowly (20 rpm) for up to 30 min. During the slow mixing period, samples were taken to measure the decrease in turbidity. These samples were allowed to settle for 30 min before their absorbance was measured at 650 nm, Results and Discussion Batch Reactors. Table I summarizes the experimental conditions used to evaluate the timed colorimetric analysis procedure in dilute solutions (0.1 mM Al) of alum hydrolyzed in the batch reactor. None of the alum solutions filtered through filters with pore sizes less than 220 nm gave a positive response, and only one filtrate of the 450nm pore-size filter was found to be positive. These results indicate that small, complex aluminum hydroxide species are not generally formed in solutions of alum with total aluminum concentrations on the order of those used in water treatment (0.01-0.2 mM Al). Documentation of the existence of the small (1.5 nm) All, polymer by NMR has been limited to solutions with aluminum concentrations greater than 1 mM (2,3,20-23). Hence, the importance of this polymer in alum coagulation at aluminum concentrations near 0.1 mM has not been documented. The results presented here do not rule out the existence of the All, species under these conditions. It may exist in loosely bound agglomerates which can be removed by membrane filtration. These results indicate that characterization of aluminum hydroxide species in dilute alum solutions is not improved by analysis of filtrates. An analysis procedure for characterization of aluminum species that are important to coagulation should be applied to unfiltered solutions. Figure 2 shows the effect of batch reaction time on the progress of the timed colorimetric test conducted with unfiltered samples from a dilute solution of alum (0.1 mM Al) at pH 6.0. The reduced rate of dissolution of the aluminum hydroxide species indicates that they are probably growing larger with time. The potential usefulness of the timed colorimetric test for characterizing aluminum hydrolysis products can be seen in Figure 3. This figure shows the effect of batch reaction time on the flocculation efficiency of an unfiltered solution of alum (0.1 mM Al) at pH 6.0. Preneutralization
.60
I
I
40
80
120 160 TIME (MINI
240
200
Figure 2. Timed colorimetric analysis of dilute (0.1 mM AI) aluminum sulfate solutions allowed to react at pH 6.0 for various times in batch reactor.
.ool 0
10
20
40
30
50
TIME(MIN.1 reaction time
= 20 min.
0
n
v v
n
0
0
Figure 4. Timed colorimetric analysis of dilute (0.1 mM AI) aluminum sulfate solutions formed in continuous flow reactor (pH 6.0, 9.6-min hydraulic retention time, 25 "C).
10 min.
.60
v
1
.40 .50:
-
v - : : 5
10
I
I
15
20
I
25
r
S
-
BATCH CONTINUOUS FLOW
30
TIME (MIN.)
v)
m 4
Figure 3. Flocculation by dilute (0.1 mM AI) aluminum sulfate solutions allowed to react at pH 6.0 for various times in batch reactor.
significantly decreases the ability of the aluminum hydroxide hydrolysis products to flocculate clay. After 20 min of reaction time, the aluminum hydroxides are nearly completely ineffective. The results shown in Figures 2 and 3 indicate that the aluminum hydroxides that form during batch neutralization grow with time and that the larger hydrolysis products are less effective coagulants. Continuous Flow Reactor. Close analysis of results of applying the automated timed colorimetric procedure to solutions neutralized in continuous flow reactors indicates the existence of two types of hydrolysis products. Figure 4 shows early data points obtained from analysis of a sample from a continuous flow reactor. The rapid dissolution rate observed during the first 20 min indicates that two types of aluminum hydroxide species are dissolving with two different dissolution rates. These species are called rapidly dissolving and moderately dissolving aluminum (Figure 1). Figure 5 shows the results of the timed colorimetric procedure applied to 0.1 mM A1 solutions from aluminum sulfate that were neutralized to pH 6.0 in batch and continuous flow reactors. The hydraulic retention time in the continuous flow reactor was 9.6 min which is similar to the batch reaction time of 10.5 min. Figure 5 shows that very different aluminum hydroxide products are formed in batch and continuous flow reactors. The differences could be explained in terms of precipitation kinetics. The aluminum in the batch reactor becomes highly oversaturated when the solution is first neutralized. This leads to formation of a large number of nuclei. These
4
e
12
16
TIME (HR)
Figure 5. Timed colorimetric analysis of dilute (0.1 mM AI) aluminum sulfate solutions allowed to react at pH 6.0 in batch and continuous flow reactors.
nuclei will grow, but their ultimate size will be limited by the total concentation of aluminum. The degree of oversaturation in the continuous flow reactor is much lower, so fewer nuclei are formed. Since both systems have similar total aluminum concentrations, the hydrolysis products in the batch system will be larger in number but smaller in size than those in the continuous flow system. The smaller particles will dissolve more rapidly in the timed colorimetric procedure. The results shown in Figure 5 may have practical importance in water treatment where batch jar tests are often conducted to determine optimal coagulant dosages. If alum is being introduced to the water in a completely mixed rapid mix basin in the full-scale plant, the results of a laboratory batch jar test may not accurately model the processes occurring in the treatment plant. However, it should be noted that all of the conditions used to obtain the data in Figure 5 are not the same as would be found Environ. Sci. Technol., Vol. 20, No. 9, 1986
893
in water treatment plants. The reaction times are significantly longer, and the concentrations of particles are much lower in these experiments than would be observed at a typical water treatment plant.
Conclusions This paper describes the development of a timed colorimetric procedure for characterizing the aluminum hydroxide hydrolysis products formed during alum coagulation. On the basis of results of applying this test procedure to dilute solutions of aluminum sulfate in batch and continuous flow reactors, the following conclusions can be drawn. (1) Small hydrolysis products were not generally observed during batch neutralization of dilute (0.1 mM Al) solutions of aluminum sulfate. Therefore, it is unlikely that the 13 aluminum polymer is an important coagulant in typical water treatment practice, unless it is present in the form of larger agglomerates. Furthermore, filtration is not a valuable tool in characterizing products of aluminum hydrolysis in this system. (2) The aluminum hydroxide hydrolysis products produced in batch reactors increase in size and decrease in ability to coagulate clay as hydrolysis reaction time increases. (3) Nonlinear regressions of data from timed colorimetric experiments on solutions from continuous flow reactors can be used to calculate concentrations of four aluminum species (instantaneously, rapidly, moderately, and slowly dissolving aluminum). These regressions can also be used to determine the dissolution rate constants and initial dissolution rates for two of these species (rapidly and moderately dissolving aluminum). (4) Aluminum hydroxide hydrolysis products produced in batch reactors dissolve more rapidly than those produced in continuous flow reactors. Registry No. HzO,7732-18-5; AI, 7429-90-5. Literature Cited (1) Baes, C. F.; Mesmer, R. E. T h e Hydrolysis of Cations; Wiley: New York, 1976. (2) Buffle, J.; Parthasarathy, N.; Haerdi, W. Water Res. 1985, 19, 7-24.
894
Environ. Sci. Technol., Vol. 20, No. 9, 1986
(3) Parthasarathy, N.; Buffle, J. Water Res. 1985,19, 25-36. (4) Bersillon, J. L.; Brown, D. W., Fiessinger, F.; Hem, J. D. J . Res. U.S. Geol. Surv. 1978, 6, 325-337. ( 5 ) Okura, T.; Goto, K.; Murai, M.; M e n . Fac. Eng. Hokkaido Univ. 1960, 11, 25-39. (6) Turner, R. C. Can. J. Chem. 1969, 47, 2521-2527. (7) Turner, R. C.; Ross, G. J. Can. J. Chem. 1970,48,723-729. (8) Turner, R. C. Can. J. Chem. 1976,54, 1910-1915. (9) Bersillon, J. L.; Hsu, P. H.; Fiessinger, F. Soil Sci. SOC.Am. J. 1980,44, 63-634. (10) Johansson, G. Acta Chem. Scand. 1962,16,403-420. (11) Nayayama, M.; Goto, K.; Yotsuyanagi, T. K e n k y u Hokoku-Asahi Garasu Kogyo Gijutsu Shoreikai 1966,12, 403-415; Chem. Abstr. 1968,69, 46477. (12) Schonherrr, S.; Frey, H. P. 2. Anorg. Allg. Chem. 1979,452, 167-175. (13) Fripiat, J. J. J. Phys. Chem. 1965, 69, 2458-2461. (14) DeHek, H.; Stol, R. J.; DeBruyn, P. L. J. Colloid Interface Sci. 1978, 64, 72-89. (15) Saito, Y.; Shinata, Y.; Wakaki, J.; Yamada, M. Therm. Anal., [Proc. I n t . Conf.], 1977, 79-82. (16) Waters, D. N.; Henty, M. S. J.Chem. SOC.,Dalton Trans. 1977 1977, 243-245. (17) Hudson, H. H.; Wagner, E. G. J. Am. Water Works Assoc. 1981, 73, 218-222. (18) Vermeulen, A. C.; Geus, J. W.; Stol, R. J.; deBruyn, P. L. J. Colloid Interface Sei. 1975, 51, 449-458. (19) Rainwater, F. H.; Thatcher, L. L. U.S. Geol. Surv. Water S u p p l y Pap. 1960, No. 1454. (20) Akitt, J. W.; Mann, B. E. J. Magn. Res. 1981,44,584-589. (21) Akitt, J. W.; Farthing, A. J. Chem. SOC., Dalton Trans. 1981, 1981, 1606-1608. (22) Akitt, J. W.; Farthing, A.; Howarth, 0. W. J. Chem. SOC., Dalton Trans. 1981, 1981, 1609-1614. (23) Bottero, J. Y.; Poirer, J. E.; Fiessinger, F. Water Sci. Technol. 1981, 23, 601-612.
Received for review September 5, 1985. Revised manuscript received April 14,1986. Accepted April 30,1986. Although the research described i n this article has been funded wholly or i n part by the US.Environmental Protection Agency under Assistance Agreement R809774-01-0 to the Texas A&M Research Foundation, it has not been subjected to the Agency’s required peer and administrative review and, therefore, does not necessarily reflect the view of the Agency, and no official endorsement should be inferred. Portions of this paper were presented before the Division of Environmental Chemistry, at the National Meeting of the American Chemical Society, Washington, DC, September 1983.
