Environ. Sci. Technol. 1982, 76, 62-66
NOTES Stabilization and Analysis of Sulfur( IV) Aerosols in Environmental Samples Christopher R. Fortune and Barry Delllnger"
Northrop Services, Inc., P.O. Box 12313,Research Triangle Park, North Carolina 27709 A method is proposed for sampling plume and ambient sulfur(1V)aerosols on formalin-impregnated paper filters. Analysis of filters spiked with sulfite (SO?-) yielded SO3,recovery efficiencies of >96% , even on filters loaded with oxidation-promoting cement dust. The effect of sulfur dioxide (SO,) concentration, relative humidity, and aerosol composition on SO?- artifact formation due to conversion of SOz was evaluated and found to be small under the conditions tested. Introduction The emission of sulfur oxides from stationary sources has been one of the most intensely studied sources of air pollution. Primary sulfur dioxide emissions and subsequent photochemical oxidation to sulfuric acid have long been considered the primary source and mechanism for the formation of acid rain (1). The liquid-phase oxidation of sulfur dioxide is also an important mechanism to consider with respect to the air-pollution problems attributed to the gas (2). More recently, there has been increased concern over the impact on air quality of primary sulfuric acid and sulfate emissions by power plants burning sulfur-containing fuels (3). Environmental research has concerned itself with measuring sulfur dioxide and sulfate concentrations both at the source of the emission and in the atmosphere. Of equal concern has been determination of the mechanisms for sulfuric acid and sulfate formation from sulfur dioxide. Apparently successful methods have been developed for the speciation and accurate analysis of sulfur dioxide, sulfuric acid, and sulfates at both source (4, 5 ) and atmospheric (6) concentrations. Although an integral role for sulfur(1V) species (sulfite and bisulfite) as necessary intermediates in the liquid-phase oxidation of sulfur dioxide has long been accepted (7), sampling and analysis of sulfur(1V) aerosols have largely been ignored. This is most likely due to the difficulty of stabilizing these species for analysis, rather than to any concrete evidence that they either are nonexistent or exist at such low levels that consideration is unwarranted. Previously introduced wet chemical methods designed for sulfur dioxide and sulfate sampling and analysis are not capable of detecting sulfur(1V) aerosols, since the methods are predicated on oxidizing all sulfur oxides to sulfate for analysis. In this note, we propose a method for the collection and analysis of sulfur(1V) aerosol using formalin-impregnated filters. Analysis is by ion chromatography. The purpose of our preliminary study was threefold: to determine the effectiveness of the proposed sampling method in preventing oxidation of sulfur(1V) aerosol to sulfate, to develop a method for recovery and analysis that allows continued stabilization, and to evaluate the level of sulfite artifact formation resulting from conversion of sulfur di62
Environ. Sci. Technol., Voi. 16, No. 1, 3982
oxide and its effects on measurements under various conditions of relative humidity and sulfur dioxide concentration. The results of these experiments and a brief discussion of possible sources of sulfur(1V) aerosols are presented. Experimental Section Filter Preparation. The formalin-impregnated filter substrates used throughout this study were prepared by using Whatman 41 filter paper. In the initial experiments, which involved testing the stabilizing effect of the filters, 47-mm-diameter disks were used, while in the later experiments, which involved testing for artifact formation, 37-mm disks were used. A Buchner funnel and a vacuum pump were used to individually wash the filters with 50 mL of 1.0 M nitric acid and rinse them with 100 mL of deionized water. The filters were then transferred to a 2.0-L beaker filled with deionized water and allowed to soak for 10 min, to ensure removal by dilution of any traces of nitric acid not removed by the rinsing procedure. The filters were then placed in a Pyrex baking dish and dried at 100 "C for 15 min. The filters were impregnated with formalin by soaking the cooled, acid-washed filters for 10 min in a beaker containing a 10% (v/v) aqueous formalin solution prepared from reagent-grade formalin (37% w/w; Fisher Scientific Co.). With tweezers, the excess solution was scraped off the filters onto the edge of the beaker, and the filters were placed in a Pyrex baking dish and dried at 100 "C for 15 min. Triethanolamine (TEA)-impregnated filters were prepared as described above, except that the filters were soaked in a 10% (w/v) aqueous TEA solution prepared by dissolving 100 g of reagent-grade triethanolamine (Fisher Scientific Co.) in 1.0 L of deionized water. Three treated filters from each set of twelve were placed in marked plastic petri dishes for analysis as blanks. Treated filters and blanks were stored in a silica gel desiccator at all times before and after use, prior to extraction for analysis. Sample Recovery, Following experimental sampling runs, the filters were extracted by using an ultrasonic bath. The filters were transferred from the polystyrene petri dishes to 150-mLPyrex beakers containing 25 mL of 0.1% (v/v) formalin solution. A 5-mL rinse of the petri dish bottoms was also added to the beakers. The samples were sonicated for 10 min and then filtered through prewashed Whatman 541 filter paper into 50-mL volumetric flasks by using 0.1% formalin from a squeeze bottle. The samples were diluted to the mark and transferred to labeled 60-mL polypropylene bottles. Sample Analysis, All sulfite/sulfate analyses were performed with a Dionex Model 10 ion chromatograph interfaced with a Hewlett-Packard Model 3385A automation system. The majority of the samples were analyzed by manual injection into the instrument; however, the
0013-936X182/0916-0062$01.25/0
0 1981 American Chemical Society
samples generated in the SOz artifact study were analyzed automatically with a Gilson Autosampler system. The 47-mm-filter extracts generated in the sulfite stabilization tests were analyzed by using a column arrangement consisting of a 3 X 150 mm precolumn, a 3 X 250 mm anion separator column, and a 6 X 250 mm anion suppressor column, A 100-pL sample injection loop was used, and the eluent consisted of 0.003 M Na2C03pumped at a flow rate of 160 mL/h. Samples were analyzed on the 1.0 pmholcm full-scale range setting on the conductivity meter. External standards used for calibration contained 5.0 pg/mL sulfite ion (S032-and 1.00 pg/mL sulfate ion (S04z- in 0.1% (v/v)) formalin solution, with retention times of 5.2 and 8.6 min for SO?- and SO:-, respectively, under the above conditions. Control standards prepared at the same concentrations as the calibration standards were routinely analyzed after every four sample analyses. Typically, 25 samples (including 5 control standards) were analyzed in an 8-h day. The precision of the method, as indicated by the relative standard deviation of the five control standards, was typically 1.7% for S032-and1.1% for SO-.: The detection limit for sulfite ion under these conditions was 50 ng/mL. Ion-chromatographic analysis of sulfite ion in sample matrices containing significant concentrations of nitrate (NO3-)ion required the use of an alternate analysis scheme. Nitrate ion was only approximately 50% resolved from sulfite ion when the scheme described above was used, resulting in a marked decrease in the precision and reproducibility of sulfite ion determinations. The 37-mmfilter extracts were analyzed to determine S03z-, NO3-, and SO:- ions in one sample run, using a column arrangement consisting of a 3 X 50 mm guard column (anion concentrator column) followed in sequence by a 3 X 250-mm anion separator column, a 3 X 500 mm anion separator column, and a 6 X 250 mm anion suppressor column. A 1000-pL sample injection loop was used, and the eluent consisted of 0.0005 NaHC03/0.0035 M Na2C03pumped at a flow rate of 160 mL/h. Samples were analyzed on the 3.0 pmho/cm full-scale range setting on the conductivity meter. External standards used for calibration and as controls contained 0.5 pg/mL SO3%,1.0 pg/mL NO3-, and in 0.1% (v/v) formalin, with retention 1.0 pg/mL Sod2times of 8.2, 10.5, and 14.4 min for S032-,NO3-,and S04zions, respectively, under the above conditions. Samples were routinely analyzed automatically by using the Gilson Autosampler system. The precision of the method, determined as previously outlined, was 6.7%, 2.3%, and LO% for SO:-, NO3-, and S04z-ions, respectively, for the above concentration levels. The detection limit for sulfite ion under these conditions was 20 ng/mL. Preparation of Analytical Standards. On the basis of data obtained in this study and in earlier work, it was possible to prepare stable ion-chromatography sulfite ion standards by dissolving sodium sulfite in 0.1 % (v/v) formalin solution. When stock 100 pg/mL standards stored in polypropylene storage containers for 12 months and longer were checked for stability by analysis for sulfate ion against freshly prepared standards, it was found that less than 1% of the sulfite ion was converted to sulfate. For comparison, additional sulfite standards were prepared in 0.1% TEA (w/v) and 10% 2-propanol (v/v). Sulfate and nitrate standards were prepared by dissolving potassium sulfate and sodium nitrate in deionized water. All stock standard solutions were prepared from certified analytical-reagent-grade chemicals (Fisher Scientific Co.) dried at 105-110 "C for 2 h and corrected for purity. Stock solutions and solvent solutions were routinely analyzed for
GASMIXING
CHAMBER iTEFLOYl
FILTER HOLDER (TEFLON1
SILICAGEL DRYINGTUBE IOPTIONALl
ZERO A I R ROTAMETERS
SO2 ROTAMETERS CYLINDER
I
CYLINDER
/
R2
R3 R-4 R S
R 6
D 1000 0 la00 D 60 0 18 000 0 18 000
Figure 1. Diagram of the S02/moisture generation system and sulfuroxide sampling train used for most of the test runs. The impinger train configuration and the composition of the implnger solutions were varied to meet the needs of each specific experimental design.
NO3-and ,9042- impurities, and the small but detectable amounts determined were used to correct the nominal concentrations of these ions in low-levelworking standards. A typical working standard containing 1.0 pg/mL NO3in 0.1% formalin would be corrected to 1.016 and S042pg/mL NOy and 1.034 pg/mL SO:- because of impurities in the reagents or deionized water. S02/Moisture Generation System. The sulfur dioxide and moisture generation system and combined Greenburg-Smith impinger sampling train used for the majority of the experiments in this study are shown schematically in Figure 1. The filter holder preceded the first impinger of the sampling train. A silica gel drying tube was placed directly behind an empty filter holder in initial test runs, to verify the relationship of percent relative humidity to percent of total airflow through the deionized-water-filled modified Greenburg-Smith impinger. Gravimetric determinations of the moisture generated under various conditions verified that percent relative humidity was equal to percent of total airflow through the deionized-water impinger. The drying tube was not used for actual test runs. The relative humidity was varied from 50% to 100% by controlling the airflow through the deionized-water impinger using calibrated rotameters (R-5 and R-6). Sampling was conducted at a nominal flow rate of 18.0 L/min for 30 min. Sulfur dioxide concentrations in the range 0.1-105 ppm (v/v) were generated by using analyzed cylinders of sulfur dioxide in air (either 101 or 1050 ppm) with the flow controlled through one of four calibrated rotameters (R-1, R-2, R-3, or R-4). Dry air, moisture, and sulfur dioxide were mixed in a Teflon mixing chamber that preceded the filter holder. The Greenburg-Smith impingers in the sampling train generally contained 200 mL of 3% H202for removal of sulfur dioxide. Preparation of Loaded Filters. Filters loaded with fly ash or specially prepared fly ash/reagent mixtures were prepared by using a specially designed puffing apparatus used for the preparation of standards for X-ray fluorescence analysis. A small amount of the material to be loaded was placed in the bottom section of the glass device and, with a vacuum, the filter was held in place on a wire mesh screen in the top section of the device. The two sections were then coupled and a vacuum allowed to form Envlron. Sci. Technol., Vol. 18, No. 1, 1982 83
Table I. Sulfite Aerosol Stabilization (High-Humidity Case) filter no. 1
2 3
pretreatment
initial SO,2-, pmol
elapsed time, day
none none none
0 0 0
6 6 6
1 0 % formalin 10% formalin 10% formalin
2.667 2.667 2.667
6 6 6
10% TEA 10% TEA
10% TEA
2.667 2.667 2.667
6 6 6
none none none
2.667 2.667 2.667
1 1 1
SO,2- found, pmol
-
mean 4 5 6
mean 10 11 12
Below detection limit (3 x
2
39
pmol).
inside the closed container. With a stopcock, the vacuum was then broken, causing air to rush through four nozzles directed at the material to be loaded on the filter. The material was thereby dispersed, and the vacuum applied to the filter caused the material to be deposited thereon. The two sections of the puffing apparatus were then separated, the vacuum was shut off, and the loaded filter was stored in a plastic petri dish. A second set of filters used in this study was obtained by operating two dichotomous samplers (Sierra Model 244) side by side to obtain 20-h samples of the ambient air near our laboratory. The fine-fraction (0-2.5-pm) filters from each pair of runs were retained for testing, while the coarse-fraction (2.5-15-pm) filters were discarded. Preparation of Fly Ash/Reagent Mixtures. It was determined that nitrate ion would make an excellent internal standard from which fdpr loadings using the puffmg technique could be determined. With the above technique, a mixture of fine (2.5-pm mean diameter) coal fly ash containing 2% (by weight) nitrate ion was prepared by using finely ground, dried, reagent-grade sodium nitrate (NaN03). This mixture was used in lieu of pure fly ash in the loaded-filter SO2 artifact studies. A mixture of dried cement kiln ash and sodium sulfite was prepared for loading on treated filters by finely grinding dried, reagent-grade sodium sulfite with a mortar and pestle. The appropriate amounts of each material were weighed and transferred to a mixing vial for shaking on a mechanical mixer, to ensure a homogeneous mixture. The amounts of ash and Na2S03used in the mixture were such that 1000 pg of the mix contained 333 pg of sulfite ion.
Results Initial experiments were concerned with the ability of formalin and TEA to stabilize sulfite once it had been collected on the filter. It was decided to simulate the two extreme cases: extremely high humidity, where one might expect the moisture of the sample airstream to dissolve collected sulfite or sulfate and allow permeation into the filter material; and low humidity, where dry particles might simply impact on the filter surface with little exposure to moisture. To determine the effectiveness of the treated filters under high-humidity conditions, we spiked both treated and untreated filters with sodium sulfite by pipetting 0.25 mL of a 1000 ppm stock solution onto each filter in the bottom of a glass petri dish and storing the filters in a 84
