Filter Method for the Measurement of Atmospheric Hydrogen Sulfide Toshiichi Okita, James P. Lodge, Jr,,l and Herman D. Axelrod National Center for Atmospheric Research, Boulder, Colo. 80302
m A dry Pb(OAc)s-impregnated membrane filter ( 0 . 8 ~pore diameter) was prepared by soaking the filter in 10 % Pb(OAc)t, 5 % HOAc for 5 min, drying in a desiccator, and storing in darkness. Sampled air was passed through the filter at up to 20 liters/min, and the H2S present produced a brown stain (PbS). The filter was returned to the desiccator and kept in the dark. Analysis was done by dissolving the filter in a mixed organic solvent (5.6 ml CHBCOOCH3,1.4 ml CH,OH, and 0.5 ml HOAc). The resulting brown suspension was measured with a spectrophotometer at 350 nm. The H2Samount can be obtained from a standardization curve. The reproducibility was = t l 5 with very little humidity effect. At all flow rates, the filter will react with atmospheric H2S concentrations by volume. Below this volume, none of greater than 2 X the H2Sis apparently captured. There were small interference problems with high levels of SO?, NO?, and O s , but these were overcome with selective prescrubbers; hence, the filters have wide application and are easy to use.
H
ydrogen sulfide in substantial concentrations in the ambient atmosDhere constitutes a serious air ~ o l lutant because of its malodor and toxicity. Somewhat lower concentrations still have adverse effects upon lead-based paints and upon a number of metals, including silver and copper. Even the unpolluted atmosphere is not entirely free from hydrogen sulfide. Analytical methods for hydrogen sulfide have been reviewed extensively by Jacobs (1960) and by Smith et al. (1961). Two popular classes of methods are those using liquid collection media and those using more or less dry impregnated paper or other suitable substrates. In the liquid sampling techniques, hydrogen sulfide is normally brought into contact with an alkaline solution of a suitable metal ion, and the dissolved gas precipitates the metal sulfide. Usually the hydrogen sulfide is regenerated and made to react with N,Ndimethyl-p-phenylenediamine in the presence of a mild oxidizing agent to form methylene blue, which is measured colorimetrically. The dry methods usually involve either passage of air through a chemically impregnated filter or the simple suspension of the filter or other substrates in an airstream, followed by estimation of the stain of metal sulfide either by eye or by means of a reflectance measurement. A dry filter system has numerous advantages, many of which were pointed out by Lodge et al. (1963); an additional advantage is the ease with which the collection method can be scaled up so that significant quantities of H& can be collected from mixtures as dilute as the unpolluted atmosphere. Combining the techniques of dry filter sampling and a sensi-
To whom reprint requests should be sent. 532 Environmental Science & Technology
tive liquid analysis procedure, we have developed a hydrogen sulfide analysis method. Hydrogen sulfide is passed through a lead acetate-impregnated membrane filter t o form lead sulfide. The filter is then dissolved in a mixed organic solvent, and the attenuation of light by the lead sulfide suspension is used to determine atmospheric hydrogen sulfide levels. Experimental
Preparation of Filters. Lead acetate-impregnated filters were prepared by soaking plain, white 47 mm AA (0.8~),ss (3.0~), and SM (5.0~)Millipore membrane filters in 10% Pb(OAc)2, 5 HOAc solution, After 5 min of immersion in the solution, the filter was removed and dried in a desiccator. The filters were then stored in a desiccator and protected from light until use. Reagents. All chemicals were of analytical reagent grade, and deionized water was used throughout. Preparation of Diluted Gas Streams. Diluted H2S in air was prepared by filling Mylar bags with the appropriate ratio of air and H2S; the bag concentrations were determined by the method of Jacobs et a/. (1957). The bag air was then drawn through the filters under various experimental conditions. In an alternate procedure, the filter was exposed to a diluted HzS airstream produced from the apparatus of Axelrod et a/. (1970). Procedure. For atmospheric HzS, remove a Pb(0Ac)n filter from its container in the desiccator. Place the filter in the filter holder and draw air through at a fixed flow rate for a fixed length of time. (The use of an inert prefilter to remove airborne particles is suggested.) When sampling is completed, return the filter prior to analysis to its container and store in a desiccator without exposure to light. Once back in the laboratory, dissolve the filter in 7.5 ml of a mixed organic solvent made from 5.6 ml CH3COOCH3, 1.4 ml C H 3 0 H , and 0.5 ml HOAc. (Prepare this mixture at least 24 hours in advance.) The filter rapidly dissolves, and the suspension is allowed to stand with frequent agitation for 30 min. Place the solution in a spectrophotometer, and measure the apparent absorption from the suspended PbS at 350 nm. The “absorption” is converted to concentration from a standardization curve. Results and Discussion
Standardization of Filters. The filters were standardized by exposing them to known dilutions of H2S (previously described apparatus) and analyzed according to the described procedure. The wavelength of choice, 350 nm, has the greatest sensitivity (Figure 1) and exhibits a linear relationship with concentration. After the standardization is completed, similarly prepared membrane filters should not need further standardizations. Reproducibility. The reproducibility of the method was determined for several concentrations of H2S. Table I shows
1.0r
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IT
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-
0.7 0.6
-
0.5
-
4 0.4
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0.3 0.2
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C 600
400 50 0 WAVE L E N G T H nm
300
Figure 1. Light scatter or effective absorbance for PbS sol as a function of wavelength The numbers on the curve are the ratio of sample absorbance to blank absorbance. A, 18 p g HzS; B, 3 pg HzS; C, filter blank
Table I. Reproducibility of the Method Sfandard HzS on deviation, pg filtera, pg k0.07 zt0.15 ~t0.14
0.82 1.6 2.1 a
Four samples at each amount.
Table 11. Effect of Added Sulfur Dioxide on Hydrogen Sulfide Determination Absorbance at 350 nm Sampling Concentration, ppm with without Ratio time, HzS SO2 SOz(A) SOs(B) A/B min 0.65
0.46
0.18
0.033
1.45
0.98
0.57
0.30
1.01 0.93 0.98 0.95
0.97 0.80 0.93 0.89
1.04 1.16 1.05 1.07
10 10 10 10
0.80 0.75 0.69 0.91
0.64 0.64 0.73 0.69
1.25 1.17 0.95 1.32
10 10 10 10
0.47 0.45 0.40 0.40
0.38 0.38 0.33 0.42
1.24 1.18 1.21 0.95
15 15 15 15
0.147
0.104
1.41
20
the results of this experiment and indicates that the degree of reproducibility (coefficient of variation) is less than 10% a t the concentrations examined. However, parallel sampling typically shows a =t 15 variation. Interferences. Smith et al. (1961) indicated that SOz usually interferes with the determination of H2S. Our studies also found this to be a problem. The brownish PbS stain on the impregnated filter became yellow in the presence of SOs, the yellow color presumably from the formation of elemental sulfur. This exposure caused a n increase in the absorbance values (Table 11). The following methods for the selective removal of SOL were examined: 1. scrubbing through 5 aqueous KC103 solution (Junge, 1959); 2. scrubbing through a solution of 10 grams of Ba(OH), in 100 ml of 30% acetic acid (Smith et al., 1961); 3. passage through a glass-fiber filter (Gelman Type A) impregnated with the above Ba(0Ac)J solution; 4. passage through Millipore-type AA filter impregnated with the Ba(OAc)? solution; 5. passage through glass-fiber filter impregnated with the above solutions with 10% glycerol added; 6. passage through glass-fiber filters impregnated with 5 % aqueous KHCO, (Pate et al., 1963). Of these methods, the last three appeared t o work best. Table 111 shows a more detailed study of these last three methods as a function of concentrations of H?S and SO?. Because of the simplicity of the KHCO, prefilter, it is recommended for areas of high SO?levels. Ozone also strongly faded the PbS stain, resulting in lower absorbances. However, this problem was overcome with the use of packed tubes containing A1 wool or granulated Na2S03as prefilters. The N a S 0 , tubes were also moderately effective in removing NO? as an interferent. The NO1 again lowered the absorbance. Experiments indicate, however, that NO2 does not interfere at levels below 0.2 ppm. Collection Efficiency. The collection efficiency of the Pb(OAc)z-impregnated filters was found to vary with flow rate, relative humidity, and H2S concentration. The capture efficiency was greater than 90% for flow rates of 20 liters/min and below. Relative humidity has a minimal effect a t flow rates below 20 litersjmin except that high humidities (>80%) tend t o give higher analytical results, but they are still within the experimental variations of + 15 %. The efficiency also changes with HIS concentration and with the larger filter pore sizes (Figure 2). F r o m these data, it appears that good results can be obtained with AA filters at flow rates under 20 liters/min.
Table 111. Efficiencies of Removal of SO2 or HSImpregnated Glass-Fiber Filter Eff. of
Concentration, ppm elimination, 2 ~~
Substance
SOz
HzS
SO2
HzS
KC103
1.00
0.38
KC103, glycerol added
0.86 0.86
1.18 0.21
... ...
4 2
Ba(OAc)z
1.00
0.38
99.5
0
Ba(OAc)?, glycerol added
0.86 0.86
1.18 0.21
... ...
0 0
KHCOB
1.00 0.86 0.86
0.38 1.18 0.21
100
6 2 0
100
...
...
12
Volume 5, Number 6, June 1971 533
100
other phenomena, and, therefore, this method is useful only for H2S concentrations of 2 ppb and above. Despite the rather large coefficient of variation for this H2S measurement method, the convenience of using the Pb(OAc)n filters for field sampling is certainly attractive and accurate enough for most applications.
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Acknowledgment
FLOW RI 17.5
Assistance and consultation in various portions of the experiment were given by Alexander Goetz, John B. Pate, and Humbert Bravo A. T. Okita was a n NCAR visiting scientist from the Institute of Public Health, Tokyo, Japan. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
+ 0
i 80 0 0
70
IO-’
10-1
10-1 CONCENTRATION OF H,S
I
10
(p.p.m.l
Figure 2. Collection efficiency as a function of the H2S concentration in the airstream
Lower Limit of Detection. The lower limit of detection is about OSlg using the described analytical procedure. Obviously, the use of high flow rates for low H S concentrations can result in the capture of measurable amounts of H2S. Further experiments have shown, however, that even with high volumes of sampled air, H2S will not be collected if the H2S concentration is below approximately 2 ppb. It is not clear whether this is a matter of reaction kinetics or some
Literature Cited Axelrod, H. D., Pate, J. B., Barchet, W. R., Lodge, J. P., Jr., Atmos. Enuiron. 4, 209 (1970). Jacobs, M. B., “The Chemical Analysis of Air Pollutants,” Interscience Publ., Inc., New York, N. Y . , 1960, p 185. Jacobs, M. B., Braverman, M. M., Hochheiser, S., Anal. Chem. 29, 1349 (1957). Junge, C. E., in “Atmospheric Chemistry of Chlorine and Sulfur Compounds,” J. P. Lodge, Jr., Ed., American Geophysical Union, 1959, p 34. Lodge, J. P., Jr., Pate, J. B., Huitt, H. A., Amer. Ind. Hyg. Ass. J . 24, 380 (1963). Pate, J. B.. Lodge. J. P.., Jr.., Nearv. - ,M. P., Anal. Chim. Acta 28; 341 (1963).- ’ Smith, A. F., Jenkins, D. G., Cunningworth, D. E., J . Appl. Clzem. 11, 317 (1961). Receiced ior reciew May 18,1970. Accepted February 18,1971.
Effects of Carbonate and Magnesium on Calcium Phosphate Precipitation John F. Ferguson’ and Perry L. McCarty Department of Civil Engineering, Stanford University, Stanford, Calif. 94305
B The effect of magnesium and carbonate on calcium phosphate Precipitation was investigated at concentration conditions resembling those in the anaerobic digestion process and at concentration ratios typical of waste waters. Chemically defined systems were allowed to precipitate at constant temperatures (20 O t o 30 “C). The precipitation time was usually 24 hr, but in some tests varied from 1 min to several weeks. After precipitation, p H was determined, solids were separated, and the soluble components measured. Chemical analyses and x-ray diffraction of solids from some tests were made. With magnesium present, the phosphate residual decreased gradually; low values were found at pH above 9. Without magnesium, local minimum and maximum in the residual phosphate were found, respectively, at p H 8 and 9.5. Calculated activity products and solids analyses supported a hypothesized effect of magnesium on the precipitated solids. Higher carbonate concentrations increased the phosphate residual at p H 8 to 11. The effect was reduced in systems with high magnesium concentrations.
534 Environmental Science & Technology
vv
ater quality problems concerning excess algal productivity or accelerated eutrophication are common and often associated with the introduition of nutrients with waste discharges. Nutrients that stimulate excess growth include carbon, nitrogen, phosphorus, trace metals, vitamins and growth factors, chelating agents, and other organic nutrients. The techniques for assessing the relative importance of the various components in a waste discharge are being developed, with only tentative conclusions available (Gerloff, 1969). Phosphorus has often been considered the principal stimulant of algal growth, and much effort has been devoted to the development of methods for its removal from waste discharges (Nesbitt, 1969). This study was directed to phosphorus removal, and, more specifically, to the effects of magnesium and high concentrations of inorganic carbon on precipitation of calcium phosphates. Present address: Department of Geography and Environmental Engineering, Johns Hopkins University, Baltimore, Md. 21218.
