(7) Duggan, M. J.; Williams, S. Sci. Total Enuiron. 1977, 7, 91. (8) Olson, K. W.; Skogerboe, R. K. Enuiron. Sci. Technol. 1975,9, 227. (9) Biggins, P. D. E.; Harrison, R. M. Enuiron. Sci. Technol. 1980, 14,336. (10) Forstner, U.; Wittmann, G. “Metal Pollution in the Aquatic Environment”; Springer-Verlag; West Berlin, 1979. (11) Salomons, W.; Forstner, U. Enuiron. Technol. Lett. 1980, 1 , 506. (12) McLaren, R. G.; Crawford, D. V. J . Soil Sci. 1973,24,172. (13) Stevenson, F.J. Soil Biol. Biochem. 1979,11,493. (14) Rendell, P. S.; Batley, G. E.; Cameron, A. J. Enuiron. Sci. Technol. 1980,14,314. (15) Luoma, S . N.; Bryan, G. W. ACS Symp. Ser. 1979, No. 93, 577. (16) Tessier, A.; Campbell, P. G. C.; Bisson, M. Anal. Chem. 1979, 51,844. (17) Luoma, S. N.; Bryan, G. W. Sci. Total Enuiron., in press. (18) Perhac, R. M. Tennessee University Water Resources Research Centre Research Report No. 32, NT15, Springfield, Va., PB232427, 1974,44 pp. (19) Gupta, S. K.; Chen, K. Y. Enuiron. Lett. 1975,10, 129.
(20) Hildebrand, E. E.; Blum, W. E. Z. Pflanzenernachr. Bodenkd. 1975,3,279. (21) Zimdahl, R. L.; Skogerboe, R. K. Enuiron. Sci. Technol. 1977, 21,1202. (22) Jenne, E. A,; Luoma, S.N. In “Biological Implications of Metals in the Environment”; Wildung, R. E., Drucker, H., Eds. Conf. 750929 NTIS: Springfield VA, 1977; p 110. (23) Agemian, H.; Chau, A. S. Y. Analyst (London) 1976,101,761. (24) Shuman, M. S.; Haynie, C. L.; Smock, L. A. Enuiron. Sci. Technol. 1978,12,1066. (25) McIntosh, A. W.; Shephard, B. K.; Mayes, R. A.; Atchison, G. J.; Nelson, D, W. J . Enuiron. Qual. 1978, 7,301. (26) Khalid, R. A.; Gambrell, R. P.; Patrick, W. H., J r . In “Environmental Chemistry and Cycling Processes”; Adriano, D. C., Brisbin, I. L., Jr., Eds., U.S. Department of Energy Symposium Series No. 45,1978, p 417. (27) Laxen, D. P. H. MSc. Thesis, University of Lancaster, Lancaster, England, 1975. (28) Grieve, D.; Fletcher, K. Estuarine Coastal Mar. Sci. 1977,5, 415.
Received for review February 25, 1981. Accepted July 13,1981.
NOTES
Effect of Reagent Mixing Rate on Adsorption Process for am-Fe203*H20 Alan R. Appleton, Jr., and James 0. Leckie” Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, California 94305
The effect of mixing on the physicochemical surface characteristics of precipitated am-Fe2OrH20 was investigated by using a well-mixed system with mean residence times ranging from 5 to 30 s. Using the equilibrium adsorption behavior of Cd2+as a diagnostic tool, we observed no significant variation in the surface characteristics of the freshly precipitated solid. In addition, no significant variations were observed after the am-Fe203.HzO had been aged for 1h. For the range studied in this investigation, mixing conditions do not affect the surface characteristics of am-FezO3.HzO. These results are useful in designing a coagulation/flocculation process in a water or wastewater treatment scheme. Iron(II1) salts are commonly used as coagulants and precipitants in both water and wastewater applications. Besides removing particulate pollutants through the coagulation process and phosphates through the precipitation/coprecipitation process, amorphous iron oxyhydroxide (am-Fe203. H2O) formed in these applications also removes dissolved constituents through adsorption onto the solid surface. Recent work has been directed toward an adsorption process utilizing am-Fe203.HzO for removal of heavy metals from wastewaters (1-4). In this type of process the solid is typically precipitated in the presence of the heavy metals to be removed. In typical treatment schemes a flash mixer is used to disperse the iron feed solution and the influent wastewater to be treated. Subsequent unit processes of flocculation and sedimentation allow for solid/solution contact times on the order of 1-2 h. From an engineering point-of-view, it is important to know whether the mode of application of the iron feed solution, i.e., mixing conditions, has any practical influence on the ability of the solid formed t o remove the adsorbates. At a more detailed level one could ask whether the mixing process affects the physical/chemical characteristics of the precipitated 0013-936X/81/0915-1383$01.25/0 @ 1981 American Chemical Society
arn-FezO3.HzO and, hence, the adsorption process. The work reported here addressed the question of whether variations in the mixing time of the Fe(II1) reagents influenced the adsorption behavior of cadmium a t the solid/solution interface. Adsorption of Cd(I1) was used as a diagnostic tool to evaluate indirectly changes in the physical/chemical characteristics of the amorphous iron oxyhydroxide formed under different mixing regimes. Presumably any significant changes in adsorptive behavior would reflect changes in either specific surface area (particle-size distribution) and/or chemical characteristics of the solidlsolution interface. The study was conducted in two parts: (1)evaluation of the mixing-chamber characteristics and selection of specific mixing conditions for further study and (2) evaluation of the adsorption characteristics of the amorphous iron oxyhydroxide formed under the specified mixing conditions.
Experimental Section Residence-time-distribution (RTD) studies were conducted by using a Plexiglass tank with a six-blade turbine impeller. The tank assembly (Figure 1)had a volume of 109.4 mL. The distribution of residence times in the mixing vessel was determined by the impulse method (5,6) using a pulse of 0.25 M KC1 solution as a tracer. Changes in conductivity of the effluent were monitored. In the second phase of the experimental work, the Plexiglass tank was used as a mixing chamber to form amorphous iron oxyhydroxide. The flows of a base-electrolyte-buffer solution, which was 8 X M NaOH, 0.1 M NaN03, and 5 X lop3M NH4N03, and a 6.3 X l o w 2 M solution of Fe(N03)3 were combined in an inlet manifold connected to the mixing chamber. Flow rates were partitioned such that the final concentration of Fe(II1) was 1 X M with an ionic strength, I , of 0.1 M and a p H close to 7 . Volume 15, Number 11, November 1981 1383
shaft rotation
(c w 1
a
one of three boffles(width 0 2")
cutlet , 114'' I D
I
'I
1 5"
k0.667'-
outlet, 1/4" I
D
counter with a NaI(T1) crystal was used for radioactive counting. Fractional adsorption was computed by comparing the background-corrected activity of the 4-mL supernatant sample with that of a 4-mL sample of the mixed slurry. A plot of Cd2+adsorption as a function of solution pH is termed an adsorption edge. T o investigate the effect of solid aging on the adsorption characteristics of am-Fe203-H20, we generated a second set of adsorption edges by utilizing freshly precipitated solid. In these experiments the adsorbate was added as the effluent was collected from the mixing vessel. All other procedures are as described above.
Results and Discussion Comparisons between residence-time-distribution (RTD) curves for ideal mixing conditions and the experimental cases studied were used as a measure of the performance of the mixing vessel under experimental conditions (5). In the ideal case of a continuously stirred reactor, the pulse of tracer instantaneously forms a homogeneous solution of concentration CO.The concentration of tracer in the outflow, C, changes with time and is a function of the mean residence time, t. This relationship is expressed as (6, 7)
C = Co exp(-t/t) where COand C are initial and instantaneous concentrations of tracer, and t and t are real time and mean residence time, respectively. The mean residence time, t,is defined as the ratio of the vessel volume, V ,to the fluid flow rate, Q
t
Figure 1. Mixing-tank diagram.
The kinetics of Cd2+adsorption were evaluated in this investigation a t each of the two mixing conditions. The results showed that pseudoequilibrium was reached within 15 min in both cases and agreed well with the results from previous studies ( I , 2). Thus, for solid/solution contact times of 1 h, typical of common water and wastewater treatment process trains, the adsorption of Cd2+onto am-FezOs-H2Owill be a t a pseudoequilibrium condition. Four milliliters of a 2.5 X lop4 M lo9Cd2+solution was mixed with sufficient effluent, containing the freshly precipitated iron oxyhydroxide, to yield 500 mL of slurry with a Cd2+ concentration of 2 X loT6M. The am-FepO3.HzO slurry with lo9Cd2+was then transferred to a 600-mL double-walled reaction vessel and maintained a t 20 OC. A magnetic Teflon stirring bar kept the slurry in suspension. Cd2+adsorption as a function of solution pH was measured in two types of experiments. In one set of experiments, the preformed iron oxyhydroxide was aged for 1h in the doublewalled reaction flask before the lo9Cdwas added. The p H of the solution was increased from ca. 6.3 to 8.3 with microliter additions of NaOH as eight 10-mL aliquots were removed from the slurry. The aliquots were placed on an end-over-end roller for 1h a t ambient temperature. The equilibrium pH of the magnetically stirred slurry was measured by using a combination pH electrode. The aliquots were then centrifuged for 3-5 min to achieve solid/solution separation. A 4-mL sample of the clear supernatant was pipetted into a glass counting vial. In addition, two 4-mL samples of the mixed slurry were pipetted into glass counting vials to determine total activity of lo9Cd. A Baird Atomic Model 810c well 1384
Environmental Science & Technology
= V/Q
A dimensionless measure of the change in tracer concentration with time can be obtained by dividing both sides of eq 1by COand defining the time ratio 0 = t/t.A plot of C/COvs. 0 is known as a C curve. C curves are valuable in analyzing and comparing RTD data for several different residence times. Differences among peak heights, shapes of curves, and other characteristics can easily be compared. Figure 2A shows the C curve for the ideal mixing case, when the concentration of the effluent equals COat t = 0, C curves are show! in Figure 2b-e for mixing conditions o f t = 5 s aFd 500 rpm, t = 5 s and 3000 rpm, t = 30 s and 500 rpm, and t = 30 s and 3000 rpm, respectiyely. An inspection of peak heights in Figure 2 shows that, at t = 5 s and 500 rpm, C/COexceeds 1.0 over a short time interval, which can be interpreted as short-circuiting through the mixing vessel. According to eq 1, C/COdecreases to e-l or 0.368 a t d = 1 under ideal well-mixed condition_. Thus, a comparison of the mean hydraulic residence time, t , and the elapsed time, t , a t C/C, = 0.368 for a range of mixing rates will indicate deviations from ideal mixing conditions. Figure 3 presents a plot o f t vs. t at C/CO = 0.368 for a range of mixing rates. A straight line drawn through the data points is indicative of good mixing when the slope is equal to 1. The cases for t I20 s a t all rpm values are very close to the ideal case; however, there is a d i g i t decrease in elapsed time compared to residence time for a t of 30 s. Major deviations occurred for an rpm of zero. Two sets of mixing conditions producing near-ideal mixing in the reaction vessel were chosen on the basis of the data presented in Figures 2 and 3. The t_wo mixing conditions chosen were t = 5 s with 3000 rpm and t = 30 s with 3000 rpm. Thus, fluid flow rates differed by a factor of 6, with the slower rate, corresponding to t = 30 s, indicative of a large-scale flash-mixing operations commonly found in water and wastewater treatment schemes. Adsorption experiments with cadmium provided a direct measure of variations in the removal characteristics of the iron
II\
i = 5 sec RPM=3000
(c)
2
3
4
ec+/ii
Figure 2. Mixing-characteristic curves for ideal case and four experimental systems.
0
5
,
I
IO
15
1
20
25
30
t (ELAPSED TIME) sec Figure 3. Elapsed time vs. residence time at constant CICo(0.367) for various experimental mixing rates.
oxyhydroxide formed under the two mixing regimens. Several mechanisms are a t work in this removal process. Hydrolysis, polymerization, and precipitation of the solid precede the adsorption of the metal a t the solid/solution interface. Thus, a t a more detailed level, any changes in either the kinetic or equilibrium adsorption behavior of cadmium can be assumed to reflect changes in the physical/chemical nature of the iron oxyhydroxide solid. Results of the equilibrium studies on solids formed under different mixing conditions are given in Figures 4 and 5. Figure 4 gives equilibrium adsorption data for removal of cadmium under the two mixing conditions, where lo9Cd2+was added immediately after mixing. In both cases, 50% removal of cadmium occurs a t p H 6.8. T o evaluate the characteristics of the solid after a short aging period, all other conditions being the same, we aged the amorphous iron oxyhydroxide for 1h before the cadmium was added. Figure 5 shows the results of this experiment for the two mixing conditions. Again, no perceptible differences in adsorption behavior are noted between the two mixing conditions. In addition, after comparing Figures 4 and 5 , one can find no differences between freshly precipitated and aged (1 Volume 15, Number 11, November 1981 1385
I 0.1 M NaN03 o i = 30 sec
A
o
-
1
FRESHLY PRECIPITATED SOLID
I
I 0I M i 5 sec i = 30sec
’ SOLIDS AGED
‘6
NaN03-
Ihr
8
(6.8) 7
PH
Figure 4. Adsorption edges for cadmium adsorption onto freshly precipitated am-Fe2O3.H20under different mixing regimes.
Figure 5. Adsorption edges for cadmium adsorption onto aged (1 h) am-Fez03-Hz0under different mixing regimes.
h) solid for a given set of mixing conditions. Thus,_for reasonable mixing operations of complete mixing with t I30 s, the resulting solids show no differences in adsorption characteristics as measured by the adsorption of cadmium.
ditions are achievable with currently available commercial flash-mixing devices.
Conclusions We have shown that, for solid/solution contact times of 1 h, which are typical of common water and wastewater treatment schemes, a variation in residence times in the mixing vessel does not alter adsorption characteristics of cadmium on amorphous iron oxyhydroxide. Thus, presumably the physical/chemical characteristics of the solid are substantially the same in all cases. The two residence times used in this study differ by a factor of 6 but produce a well-mixed system under the chosen conditions. Equilibrium data for adsorption of cadmium onto freshly precipitated iron oxyhydroxide and iron oxyhydroxide aged 1 h indicate no differences in solid surface characteristics. As long as complete dispersion of the Fe(II1) feed solution in the mixing vessel occurs within 30 s, mixing conditions appear to be of minor importance in the removal of trace metals through adsorption onto the Fe(II1) flocs. These con-
Acknowledgment We thank Tony Naylor for his technical advice and assistance. Literature Cited (1) Leckie, James 0.;Benjamin, Mark M.; Hayes, Kim; Kaufman, Gary; Altman, Scott. “Adsorption/Coprecipitation of Trace Elements from Water with Iron Oxyhydroxide”, Electric Power Research Institute Final Report RP-910, Sept 1980. (2) Davis, James A.; Leckie, James Q. J . Colloid Interface Sci. 1978,
67,90-107. ( 3 ) Davis, James A.; Leckie, James 0.J . Colloid Interface Sci. 1980, 74,32-43. (4) Benjamin, Mark M. Ph.D. Thesis, Stanford University, Stanford, CA, 1978. (5) Gray, J. B. In “Mixing: Theory and Practice”; Academic Press: New York 1966; Vol. 1. (6) Levenspiel, Q.; Bischoff, K. B. Adv. Chem. Eng. 1963,4. (7) Holland, F. A,; Chapman, F. S. “Liquid Mixing and Processing in Stirred Tanks”; Reinhold: New York, 1966; pp 95-192.
Received for review October 21,1980. Revised Manuscript Received April 14,1981. Accepted July 6,1981. This uork was partially supported by EPRI Contract EPRI-910.
Oxidation of Phenolic Antioxidants in a River System Viorica Lopez-Avila Midwest Research Institute, 425 Volker Boulevard, Kansas City, Missouri 641 10
Ronald A. Hites” School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
The phenolic antioxidants are very important commercial, organic chemicals. The two most well-known are 2,g-di-tertbutyl-4-methylphenol (also known as butylated hydroxytoluene or BHT) and 2- and 3-tert-butyl-4-methoxyphenol (also known as butylated hydroxyanisole or BHA). Both are widely used antioxidants in the polymer and food industries ( I ) . The environmental fate of phenolic antioxidants is of significance because of their abundance and their usage patterns. Thus, we have studied the fate of three such antioxidants which were being emitted in the wastewater of a facility which manufactures them. The compounds that we studied are 2,g-di-tert -butylphenol and the methyl and octadecyl esters of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid. These 1386
Environmental Science & Technology
antioxidants are produced by a small specialty chemicals plant located in Rhode Island ( 2 ) .The wastewater from this plant is biologically treated and released into the Pawtuxet River; we have previously reported that this plant was emitting (in 1976) -3 kglday of 2,6-di-tert-butylphenol, -25 kglday of the methyl ester, and -15 kg/day of the CISester (3).The purpose of this paper is to report on the fates of these compounds in the Pawtuxet River. Experimental Section The details of the sampling, isolation, and analytical procedures have been given elsewhere ( 2 , 3 ) .
0013-936X/81/0915-1386$01,25/0 @ 1981 American Chemical Society
