I?. The value of kBrdepends to a large extent on the choice for F(R,0. Models in which loss is determined by the rate of diffusion to the surface of reactants-e.g., 0 3 , NOz-in the atmosphere surrounding the particle or by reactions at the particle surface lead to functional forms of F(R,t) which are qualitatively similar (Robbins, 1970). Thus, additional experiments including measurement of the particle size distribution, especially for particles in the 0.01- to 0.1-p diam range, need to be made to distinguish among alternative models. Conclusions
Preliminary experiments indicate that bromine and chlorine are lost relative to lead in particulate matter present in fresh automobile exhaust from combustion of leaded gasoline. The loss rate for bromine is much greater than for chlorine during the first half hour, and the rate is apparently not affected by the presence or absence of sunlight during this time. The measured loss rates may be accounted for quantitatively by assuming outward diffusion of halide ions to particle surfaces where they are volatilized. With additional information, especially simultaneous particle size distribution measurements alternative models could be examined using techniques developed in this work.
Literature Cited Allen, H. E., Matson, W. R., Mancy, K. H., J. Water Pollut. Control Fed., 42, 573-81 (1970). Andersen, A. A., Amer. Ind. Hyg. Ass. J., 27, 160-5 (1966). Dams. R., Robbins. J. A., Rahn, K. A., Winchester. J. W.. Anal. Chem., 42, 861-7 (1970). Flesch. J. P.. J . Colloid Interface Sci.. 29. 502-9 (1969). General Motors Research Laboratories,’Search,‘ 1962. Habibi, K., Jacobs, E. S., Kunz, W. G., Pustell, D. L., “Characterization and Control of Gaseous and Particulate Exhaust Emissions from Vehicles,” Presentation at the Air Pollution Control Association Fifth Technical Meeting, San Francisco, Calif., October 8-9, 1970. Jost, W., “Diffusion in Solids, Liquids and Gases,” Academic Press, New York, N.Y., 1952, pp 45-6. Kittel, C., “Introduction to Solid State Physics,” 3rd ed., Wiley, New York, N.Y., 1966, pp 512-21. Korth, M., U S . Public Health Service Publication No. 999-AP-20, 1966. Lindeken, C. L., Morgin, R. L., Petrock, K. F., Health Phys., 9, 305-8 (1963). Lininger, R . L., Duce, R. A., Winchester, J. W., Matson, W. R., J . Geophys. Res., 71,2455-63 (1966). Loucks, R. H., Winchester, J. W., “Trace Substances in Environmental Health 111,” Conference Proceedings, University of Missouri, June 1969, pp 233-50. Middleton, J. T., Anier. Sci., 59 (2), 188-94 (1971). Pierrard, J. M., ENVIRON. Scr. TECHNOL., 3, 48-51 (1969a). Pierrard, J. M., PhD Thesis, “Photolysis of Lead Halides,” DeDt. of Chemistry. University of Washington. Seattle. Wish., (1969b). Robbins, J. A., “Precipitation Scavenging (1970),” A.E.C. Conference Proceedings. Richland. Wash.. June 1970. pp 325-51. Sunyal, A. K., Dhar, N. R., 2. Anorg. Chem., 128, 212-17 (1923). Winchester, J. W., Zoller, W. H., Duce, R. A., Benson, C. S., Attnos. Enciron., 1, 105-19 (1967). _
Acknowledgment
The authors are particularly indebted to J. W. Winchester for his support and thoughtful criticism of this work. We wish to thank L. F. Gilbert and C. F. Gordon of the Ethyl Corporation for the donation of the ethyl fluid and Sherman Funk of the Willow Run Laboratory, Motor Vehicle Compliance Section for use of the exhaust coupling equipment. Acknowledgment is also due Ronald Lininger and William Pierson for their expressions of interest, and Lawrence Hecker for his assistance in the lead measurements.
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Receiced,fiir reciew April 30, 1971. Accepted August 20, 1971. This work was supported in part bj, the U S . Atomic Energy Coiiiniission under contract AT(II-l)-I705 und by the U S . Public Health Sercice under grunt AP-00585.
COM M UN ICATION
Filtration of Ferric Hydroxide Gordon A. Lewandowski’ and Henry B. Linford Department of Chemical Engineering, Columbia University, New York, N.Y. 10027
Optimum conditions for ferric hydroxide filtration were sought by measuring the surface charge of the particles as well as by obtaining direct filtration data. Streaming potential measurements were used as a means of indicating the relative surface charge, and this data correlated well with filtration rates. That is, maximum rates occurred at the same pH as zero surface charge. Optimum conditions of filtration were pH 6.6, high iron concentration, and the use of NaOH or KOH rather than calcium hydroxide as the precipitating base. Agitation and rate of base addition did not appear to significantly affect filterability within the range of the experimental conditions.
I
ron compounds are present in many industrial waste streams, and the removal of these compounds by a hydroxide precipitation would be most efl‘ective provided this material could be separated out of suspension by a reasonable filtration process. It was the object, therefore, of this investigation to improve the filtration rate of ferric hydroxide, and in this connection, five different variables were studied: final pH, type of base used, base addition rate, rate of agitation, and concentration of iron. In order that a better understanding of the chemical and physical properties of the hydroxide precipitate might be Present address: FMC Corp.. 500 Roosevelt Ave., Carteret, N.J. 07008. To whom correspondence should be addressed. Volume 6, Number 2, February 1972 169
4 rkp
r=($)x(,> A
where E, = streaming potential; AP = pressure drop across the bed; h = conductivity of the filtrate; p = viscosity of the filtrate; D = dielectric constant of the filtrate; and k = proportionality constant, 9 X lo9 n.m2/coulomb2. This expression was used to correlate the streaming potential data. Apparatus
Apparatus used was according to Lewandowski, 1970; Linford et al., 1968. Detailed diagrams of the streaming cell and equipment are given in Figures 1 and 2. Silver-silver chloride electrodes were situated above and below the bed and then connected to a Keithley electrometer (Keithley Instruments, Cleveland, Ohio) with an impedance of 1014ohms. With measured voltages of the order of V, the streaming current was approximately 10-17 A. Therefore, polarization of the electrodes is negligible, and the measured potential difference is a true streaming potential. Furthermore, since the supporting electrolyte is more than 10-aN in KCl, surface conduction is not a problem in this study. Figure 3 describes the equipment used to study filtration time.
Figure 1. Streaming potential cell A, Starrett micrometer depth gauge: B, aluminum flanges: C, rubber gaskets; D, silver rod; E, nylon connector: F, silver-silver chloride electrodes; G , silver pin: H, fritted glass disk; I, filter support; J, deposited bed; and K, O-rings
obtained, the formation of this precipitate was also studied from the point of view of surface chemistry. Because of the complex shape, wide size distribution, and high particle concentration, reliable electrophoretic mobility measurements are difficult, if not impossible, to obtain with ferric hydroxide suspensions. However, reliable streaming potential measurements can be obtained in the apparatus described here. A well-known expression (MacInnes, 1961) for the zeta potential, in terms of the streaming potential, is given as:
~~
Procedure Under agitation (135, 260, and 490 rpm), 500 cc of ferric chloride (0.02M and 0.2M) were placed in a 1000-cc graduated cylinder and 500 cc of KOH of sufficient concentration added to bring the pH of the filtrate to the desired level. The KOH was usually poured from a flask in about 15 sec (2000 cc/min); however, data is also presented for a KOH addition rate of 4 cc/min. Agitation was normally continued for 5 min after base addition to ensure complete homogeneity. Then, under 60-cm Hg vacuum, 116 cc of the suspension were transferred to the Millipore holder (Millipore Filter Corp., Bedford, Mass,), a clock was started, and the time recorded at each 2-cc interval of collected filtrate (measured in the vacuum breaker) until the liquid level fell below the surface of the bed, The filter paper was #934AH (Reeve Angel, Clifton, N.J.) For the streaming potential measurements, the beds were formed on a disk of filter paper placed over the fritted disk
nJF& TRANSDUCERS
Figure 2. Streaming potential apparatus A, D, 1-liter flasks; B, vacuum release valve; C , streaming cell; E, vacuum trap; and F, manometer
VACUUM
ELECTROMETER---
170 Environmental Science & Technology
:
FLAT -
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PADDLE, G L A S S STIRRER
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1000 c c C Y L I N D E R
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Figure 3. Filtration apparatus
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Figure 4. Filtration time
surrounding the lower electrode (Figure l), and then the upper and lower electrodes were joined in the streaming apparatus (Figure 2 ) . A vacuum was drawn on flask A and then slowly released. The streaming potential was then recorded as a function of the pressure drop, and the zeta potential related to the slope by the above equation. Pressure transducers were used to measure the pressure drop, and E$ vs. A P was recorded automatically on a n EAI Variplotter (Electronic Associates, Inc., Long Branch, N.J.). The suspension preparation was altered for calcium hydroxide addition because of its partial solubility. In this case, a 500-cc suspension of calcium hydroxide was agitated for 5 min prior t o addition of 500 cc of 0.02M ferric chloride. In all but two cases, the method of addition was simply to pour the reagent from a flask (approximately a 15-sec total addition time). However, the two exceptions were notable. In one, the ferric chloride was added dropwise a t 11 cc/min (about a 45-min total addition time). This apparently allowed more time for reaction t o take place and ensured complete solubility of the calcium hydroxide particles prior to filtration. The filtration time was decreased by about 18%. In the other case, 50 cc of 0.2Mferric chloride was poured into the calcium hydroxide suspension, and this mixture was agitated for 30 min (instead of the usual 5 min). Thereafter, the suspension was diluted up to 1000 cc and agitated for a n additional 20 min prior to filtration. This reduced the filtration time about 68 % down to a level commensurate with the KOH addition (Figure 4). The temperature for all runs was 25.5' i 1.0"C. Results and Discussion The filtration results are summarized in Figure 4. Filtration times are for 100 cc of collected filtrate. The two sets of three curves each represent the mean and 90% confidence limits
0 KOH added at 2000 cc/min to O.02M FeCh @ 260 rpm; 0 @ 490 rpm; H @ 135 rpm; A NaOH @ 260 rpm; V KOH at 4 ccimin and 260 rpm; 0 KOH added at 2000 cc/min to 0.2iM FeCh @ 260 rpm, solids content 10 X that in the other cases; A 0.02M FeC13 added at 2000 cc/min to Ca(OH)2 @ 260 rpm; 0.2M FeCh; and 0 0.02M FeC13 at 11 cc/min
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Figure 5. Zeta potential Volume 6, Number 2, February 1972 171
when KOH was used as the precipitating base at low and high concentrations of iron. The filtration results also show that, within the range of each variable, the rate of agitation and the rate of base addition have a negligible effect on the filtration time (except when calcium hydroxide is used), while control of the final pH of the filtrate is of the utmost importance, having an optimum in the range pH 6.5-7.5. Outside of this range, the filtration times (and therefore pumping costs or filter area requirements) increase quite rapidly, particularly on the acid side. Increasing the iron concentration also appears to be beneficial from the point of view of filtration time per unit weight of filtered solids. This time improved threefold in going from 0.02M to 0.2M ferric chloride solutions (although the absolute filtration time was slower). However, any potential savings in filter area requirements (per unit weight of filtered solids) would have to be weighed against the additional capital or operating expenditure which may be required for concentrating the iron. Finally, while use of sodium and potassium hydroxide exhibit similar filtration properties, use of calcium hydroxide results in a much slower filtration time (fourfold increase), unless proper precautions are taken with regard to iron concentration and agitation time to allow the calcium hydroxide particles to react with the ferric chloride and reach equilibrium. This implies increased holding time (under agitation) of the suspension and possibly a concentration of the iron as described above, and the added costs of such modifications would have to be weighed against the additional operating cost incurred by using KOH or NaOH as the precipitant. The zeta-potential results are reported in Figure 5, which
describes a particle of ferric hydroxide that goes from a positive to a negative surface charge, with a zero point at pH 6.6. Furthermore, the shape of the zeta-potential curve indicates an adsorption phenomena at the particle surface, possibly involving several ferrihydroxyl complexes (Lewandowski, 1970; Parks and de Bruyn, 1962). The conditions of precipitation of the suspensions used in the zeta-potential study were: 260-rpm agitation, the KOH solution added to 0.02M ferric chloride by pouring the base from a flask (15-sec total addition time), and a temperature of 25.5”C. These conditions correspond approximately to ferric hydroxide suspensions in 0.03M KC1. Filtration data, under the same conditions, are plotted on the same graph as the zeta-potential curve. This indicates that a zero zeta potential corresponds to an optimum filtration rate, a result which is expected from coagulation theory (Kruyt, 1952). Literature Cited Kruyt, H. R., Ed., “Colloid Science, Vol 1: Irreversible Systems,” Chap. VI, pp 245-77, Elsevier, Amsterdam, Holland, 1952. Lewandowski. G.. PhD thesis, Columbia University, _ . New York, N.Y.’, 19iO. Linford, H., Fyfe, R., Hilborn, W., Lausangngam, S., “Cationic Polvmers as Flocculation Aids in Water Purification,” Dept.-of Interior, Federal Water Pollution Control Administration, Terminal Progress Report WP 00240-06, 1968. MacInnes, D. A., “The Principles of Electrochemistry,” p 439, Dover, New York, N.Y., 1961. Parks, G. A., de Bruyn, P. L., J . Phys. Chem., 66, 967-73 (1962). Received for review October 20, 1970. Accepted September 3, 1971.
CORRESPONDENCE
Attenuation of Power Station Plumes as Determined by Instrumented Aircraft SIR: Stephens and McCaldin (1971), in their article entitled “Attenuation of Power Station Plumes as Determined by Instrumental Aircraft,” [ES&T, 5 (7), 615 (1971)l present some interesting data on SOz and particle concentrations in plumes. They assume that the emitted particles with d > 0.3 pm remain in the plume thus constituting a “conservative” or inert tracer so that SOs decay in the plume may be estimated. They then estimate:
a. For relative humidities (RH) below 40z, there is little or no SO2 decay. b. For R H in the range 40-55z, the SOZdecay corresponds to a half-life of 140 min c. For 78-80z RH, the SOs decayed with a 70-min halflife. In their discussion, the authors properly point out that their particle measurements might have been affected by the 172 Environmental Science & Technology
humidity so that the findings are not confirmed. They hoped that they might solve the problem by using impactors or heating the particle sampler inlet directly. I would like to point out an error in their assumption which is even more grave than the above problem, and, I believe, invalidates their findings with regard to SO2 decay in power station plumes. It is well-known that atmospheric SO2 is readily oxidized to sulfate in particulate form. I contend that enough SO2 is converted to particulate matter in the plume so that the assumption of the particles as a conservative tracer is grossly in error, Consider an SO? concentration of 0.5 ppm or 1425 pg/m3 STP. The conversion of 2 z of this to sulfate particles with a density of 2 g/cm3will result in 110 particles/cm3 with diameter of 0.4 pm. (This of course neglects dilution by atmospheric dispersion.) This particle concentration is higher than any reported by the authors in Figure 6. What this calculation illustrates is that only a small fraction
