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
of the SOr need be converted t o particulate S042- t o account for a large fraction of the particles measured by the authors. Furthermore, the rate of decay of SOr must therefore be considerably slower than estimated in the paper. The SO2 to SOi2- conversion by catalytic oxidation in solution (Junge and Ryan, 1958; Scott and Hobbs, 1967) is likely to be greater at high than at low relative humidities. The results of the paper seem t o confirm this quite well. As a final note it is pointed out that in stack plumes as well as in many urban atmospheres, the mass of sulfur in the form of sulfur dioxide is much larger than the mass of sulfur in
SIR: With regard to the correspondence concerning our recent article, we would like t o point out that the term SO2 decaj’ was used rather than oxidation to cover several possible mechanisms of SO2 removal from the plume. It is quite possible, of course, that SO2may be absorbed or adsorbed on particulate matter, or it may go into solution in the presence of sufficient moisture, and/or it may certainly undergo heterogeneous or homogeneous oxidation processes. The question is, what is the probability that these potential SOs loss mechanisms will affect the numbers of particles larger than the size range measured and thus influence the particulate/SO? ratio? One of the more probable mechanisms of loss would be the catalyzed oxidation of SO, at one of the already established particulate surfaces. In our studies, particle counts were recorded for sizes greater than 0.3 or 0.5 p diam. There are large numbers of particles in the size range below these values which were not counted, and which would also present surfaces for SO2 reactions. Since it has been shown that further oxidation ceases at a pH of about 3, which would be achieved quite soon on a small particle surface in the absence of a neutralizing agent, we feel that this mechanism would not exert a major influence on the particulate/S02 ratio. The effect of humidity on the particle count was, of course, acknowledged in the paper. Inasmuch as there would be a distribution of SOr over all
particulate sulfate. Nonetheless, sulfate comprises a large fraction of the particle mass. Literature Cited Junge, C. E., Ryan, T. G., Quart. J. Roy. Meteorol. SOC., 84,46-56 (1958). Scott, W. D., Hobbs, P. V., J . Atmos. Sci., 24, 54-7 (1967).
James P. Friend Department of Meteorology and Oceanography New York Unicersity Unirersity Heights Bronx, N . Y . 10453
sizes of particulate matter and the reaction most probably occurs at the surface of existing particles, the probability of the formation of new particles, and more important perhaps, the formation and growth of new particles t o sizes larger than 0.3 and 0.5 p which would have been counted would be of much less significance. It should be noted that plume age was a maximum of 4 hr. It has been shown, of course, that gas-phase photochemical oxidation of SO2 may produce sulfuric acid mist, although the rate of reaction is on the order of 0.1 z,’hr. If one considers that even this rate requires sunlight and that these studies were conducted just after sunrise in plumes formed and transported at night, then it seems likely that contributions to the particle population of a size large enough t o be counted from this source would be of minor significance. As previously pointed out, the technique is certainly not definitive, however we feel that the effort was not without merit, particularly in view of the corroborative nature of the results with data obtained by widely different techniques and the contributions it may have t o future development of more precise procedures.
N. Thomas Stephens Air Pollution Research Laboratory Department of Cicil Engineering Virginia Polyteclinic Institute and State Unicersity, Blacksburg, Va.
Volume 6, Number 2, February 1972
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