Particle-Size Distribution of Chromium: Total and

Particle-Size Distribution of Chromium: Total and Hexavalent Chromium in Inspirable, Thoracic, and Respirable Soil Particles from Contaminated Sites in New Jersey Vasiliki Kitsa* and Paul J. Lioy The Environmental and Health Sciences Institute, University of Medicine and Dentistry of New Jersey (UMDNJ)-Robert Wood Johnson Medical School, 681 Frelinghuysen Road, Piscataway, NJ 08854-5635

Judith C. Chow and John G. Watson Downloaded At: 21:36 13 October 2008

Desert Research Institute, Reno, NV 89506

Saul Shupack and Timothy Howell Chemzstiy Department, Yillanoua University, Philadelphia, PA 19085

Paul Sanders New Jersey Department of Enuironrnental Protection and Energy, Trenton, NJ 08625

Chromate slag is found mixed with soil at several sites in New Jersey. Previous analyses of the contaminated soil were limited to particle sizes > 30 pm. This study focuses on the inspirable, thoracic, and respirable particles that would enter the human respiratory tract, should the contaminated soil become airborne. This article discusses the soil-sampling procedure used at a slag-contaminated site and a site with visible hexavalent chromium crystals. In addition, the drying and sieving procedures and the techniques needed to resuspend the contaminated dust in a sealed chamber are described for aerodynamic size fractionation (30, 10,

" T o whom correspondence should be addressed; affiliated with Joint Graduate Teaching Program in Exposure Assessment, Department of Environmental Sciences, Rutgcrs University and UMDNJ-Robert Wood Johnson Medical School. Aerosol Sciencc and Technology 17:213-229 (1992) O 1992 Elsevicr Sciencc Publ~shingCo., Inc.

and 2.5 pm). The resuspended dust was collected on filters and analyzed for 38 elements by x-ray fluorescence (XRF). Wet chemistry techniques were used to measure total extractable and hexavalent chromium. It was found that 1.6% of the slag had a particle aerodynamic diameter (at 50% cut-point) d,, = 30 pm, of that 1.1% was d,, = 10 p m and 0.26% was d,, = 2.5 pm. Total chromium and lead concentrations above 1000 ppm were found in all particle sizes. Comparison of the total chromium results derived for two analytical techniques showed that the total chromium, in the contaminated soil exists in soluble and nonsoluble 30% forms, and that the nitric acid method extracts of the total chromium present in the slag measured by XRF. Hexavalent chromium concentrations > 1000 ppm were found in the contaminated soil that contained visible chromium blooms, and were 1800, 1700, and 1200 ppm for the inhalable, thoracic, and respirable size ranges, respectively.

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INTRODUCTION

From 1905 to 1976, Hudson County was a major center for chromate and bichromate production. Chromate slag waste from the refining industries was used as fill and diking material at several sites in New Jersey. Later these sites were developed as residential, commercial, industrial, and public areas and recent investigations detected chromium contamination above 1000 ppm in many locations (ESE, 1988; Sheehan et al., 1991). Chromium in the wastes has a number of oxidation states, ranging from Cr(0) to Cr(VI), but only the trivalent and hexavalent forms are of biological significance. In the natural environment chromium is present almost exclusively in the trivalent form. There is no evidence that trivalent chromium is converted to hexavalent forms in biological systems. Hexavalent chromium, however, readily crosses cell membranes and is reduced intracellularly to trivalent chromium (Goyer, 1986). The known harmful effects of chromium in humans have been attributed to the hexavalent form. Hexavalent chromium is very irritative and corrosive and can cause ulcers from dermal irritations, and respiratory cancer by the inhalation route (USEPA, 1984a, b, c; Sittig, 1985). Trivalent chromium compounds are considerably less toxic. Studies on the concentration of chromium in soil have focused on analysis of the total mass or of sieved fractions, where the smallest particle size obtained is 38 p m (ESE, 1988; Sheehan et al. 1991). Information on the particle-size distribution of chromium for particles < 38 p m does not exist. Considering the possibility that soil contaminated with chromium can become airborne, it is critical to approach the problem from the point of view of the human inhalation. Such an approach requires measurements of the chromium concentration in smaller particle sizes. In-

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spirable, thoracic, and respirable particles are a focus of these experiments. Inspirable particles are those with a diameter < 100 p m and can deposit anywhere in the respiratory tract. Thoracic are the particles < 10 p m in diameter, which deposit anywhere within the lung airways, whereas respirable particles deposit in the gas exchange region and are < 3.5 p m (in our case approximated as 5 2 . 5 pm). In order to obtain data in these size fractions, chromium contaminated soil was sieved and particles < 38 p m were resuspended in a sealed chamber under controlled conditions and size-selectively analyzed for mass, total, extractable, and hexavalent chromium and selected trace elements. Total chromium is defined as the values measured by x-ray fluorescence (XRF) or after hydrofluoric acid digestion, and extractable chromium is defined as the values measured after nitric acid digestion.

METHODOLOGY

Surface Soil Sampling Surface soil was collected from a chromium contaminated site (40 x 48 m) in the State Liberty Park, New Jersey, during the spring of 1990 (Figure 1). The sampling was repeated in 1991. Only the top 2 cm of the surface soil was collected because only the top layer of the soil can be readily disturbed by local traffic or wind and can become airborne. A systematic sampling design was used to spread the sampling points uniformly over the study area (Wilding, 1984; Sabbe et al., 1987). Such a design is more efficiently used in the field than a random sampling design (Cochran, 1977). The starting point for sample collection was selected using a randomized process to avoid selection bias (USEPA, 1983) and the individual sampling points were deter-

Particle-Size Distribution of Chromium

6 4240ppm

9

14

19

0

0

0

0

2527ppm

383 1 ppm

6 138ppm

5080ppm

1556pprn

FIGURE 1. Soil-sampling site: site 15 Liberty State Park. 48 X 40 m (8 X 8 m grid).


VEGETATION

mined using an 8 x 8-m2 grid. The number of samples is estimated according to the formula: where n = number of samples; CV = coefficient of variation in percent; p = the percentage of error that will be allowed; and t is the two-tailed t value obtained from t tables for the significance level at ( n - 1) degrees of freedom. Assuming a CV = 65% and a margin of error p = 20%, it was estimated that 63 samples were required to obtain a significance level of -98% (USEPA, 1983). Samples were collected in a regular pattern. At each of 21 equally spaced points located at the grid intersections, three Zcm-deep samples labeled A, B, and C were taken with d = 5.1 cm (63 total) using a plastic scraper, after rernoving the coarse top layer of the soil (e.g., rocks and slag clinkers). Samples were kept separately in plastic containers and were stored at 4°C (USEPA, 1984d). A portion of each soil sample was ana-

lyzed for extractable total chromium. The technique is described in the Analytical Techniques subsection. The same technique was used for the analysis of the nylon filters. The remaining portions of the 63 samples were combined to form a composite sample, which was used in the resuspension study. As mentioned above, surface soil samples were also collected in 1991. This time only 21 soil samples were collected from the same location in Liberty State Park. The same collection procedures as described above were followed. In 1991 soil was also collected from another site in Kearny, New Jersey, where yellow hexavalent chromium crystals (blooms) were visible on the surface of the site. All the 1991 soil samples were analyzed for total extractable and hexavalent chromium. the technique is described in the Analytical Techniques subsection. Three uncontaminated soil samples from Highland Park, New Jersey, were also collected. These samples formed the background composite sample, which was

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analyzed together with the contaminated soil samples. Chamber Resuspension The components of the resuspension experiments included drying, sieving, and resuspending the soil in a sealed chamber. The 1990 composite sample was dried by spreading the soil on a clean surface in a hood operating at 150 ft3/min at ambient temperature and low humidity. To ensure that drying was complete the soil remained in the hood for a period of 3 days. During a second experiment using the 1991 soil, a portion of the soil collected at the Liberty State Park was air-dried as described above and the rest was freezedried using Lyph-Lock 4.5-L freeze dry system (Labconco Corp., Kansas City, MO), in an effort to determine whether the method of drying affects the measured total chromium concentrations. The soil containing the hexavalent chromium blooms was freeze-dried. For all the composite samples dry sieving was qsed as a simple method for differentiating particles by size fraction, down to 38 pm. The sieve sizes used were 400, 250, 175, 75, 45, and 38 pm. Brass sieves were employed instead of steel to avoid possible analysis interferences caused by chromium contained in the steel. The material retained on each sieve was stored separately (Head, 1980). The dried and sieved soil was resuspended in a sealed chamber to determine the size distribution for particles < 38 pm. The size fractionation of the resuspended soil was completed with a series of impactors. The resuspension chamber used in our experiments (Figure 2) was developed by the Desert Research Institute (DRI) and it consists of the parallel impactor particulate sampler (PIPS), which is contained in a cellulose fiber, static-free enclosure, 72 x 51 x 107 cm ( -0.4 m3 volume); see Houck et al. (1989)

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and Chow et al. (1991). A 20 X 13-cm quartz fiber filter is mounted on top of the cellulose enclosure to provide filtered makeup air during sampling of the resuspended soil. Approximately 0.1 g of the sieved soil is transferred to a 250-mL side-arm vacuum flask, and a rubber stopper containing a hollow glass tube is placed on the flask. The flask is positioned with its side arm at the inlet of the resuspension chamber. An air stream at a flow rate of 45-50 L/min is moved through the glass, via a metal bellows pump. The aerosolized sample is carried into the resuspension chamber where it is mixed with the filtered air. Samples are introduced into the chamber for 5 s every 10 min until the filters obtained an optimum loading of 0.5-3 mg for XRF analysis (Chow et al., 1991). When the PIPS is in operation, the air with the aerosolized particles is pulled through the dust cap or through the cyclone. The larger particles ( > 10 pm) are collected on an impactor plate, which is coated with a thin layer of Apiezon grease (Apiezon Products Ltd., London, U.K.) to eliminate particle bounce and reentrainment. The sample gas then moves through the length of a 32 X 5-cm tubular aluminum diffuser where the flow becomes uniform and particles passing an impactor are collected on a filter. Subsequently, the gas passes through a critical orifice which is used for flow control. Each of the eight ports has a critical orifice allowing a nominal flow rate of 10 L/min (and is individually calibrated with a mass flow meter. Vacuum gauges on both sides of the critical orifice simultaneously measure pressure drop to ensure proper critical orifice operation (Chow et al., 1991). By using the above approach the PIPS simultaneously collects particles with a cut size of 1, 2.5, and 10 p m (Figure 2). To obtain values for the total suspended particulate mass, which in the current study is particles < 30 pm, dust is also col-



RESUSPENSION TABLE

' 1pUMp MODULES

FIGURE 2. Overview of Desert Research Institutc resuspension sampling system (Chow et al., 1991). Key: (1) vacuum gauge, (2) critical orifice, (3) filter holder, (4) dust cap, (5) cyclone, (6) impactor, (7) cyclonc/manifold assembly, (8) tubular aluminum diffuser, (9) cellulose chamber, (10) dust injection inlet, (11) opening for makeup air, (12) main vacuum gauge, (13) elapsed time meter, (14) pump switch, (15) exhaust, (16) power inlet, (17) resuspension platform.


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lected on a filter without an impactor (Houck et al., 1989). The dust resuspended in the chamber has a slightly higher d,, of 38 pm. However, particles > 30 p m generally have a short half-life in the atmosphere (Houck et al., 1989). For the resuspension experiments the following impactor cut sizes were used: 2.5 pm, respirable fraction (approximately); 10 pm, inhalable fraction. Particles with diameter < 30 p m (Total Suspended Particulates, TSP) were collected without an impactor. A cyclone separator with a cut size of 4 p m was attached at the front of the 2.5-pm impactor sampling tube to prevent overloading of the impactor plate. The resuspended dust was collected on teflon and nylon filters for the 1990 samples and on teflon and quartz filters for the 1991 samples. Nylon filters were not used in 1991 mainly because of the static problems we faced during filter weighing. The optimum filter loading was 1-3 mg. The 30- and 10-pm d,, size fractions reached optimum loadings first. The 2.5p m size fraction loaded much more slowly. In cases where a filter reached optimal loading during a run, it was removed and replaced by a teflon-coated quartz filter. Fifty-seven teflon, 34 nylon, and 4 replacement filters were collected for the 1990 samples. Twelve teflon and 12 quartz filters were collected for the 1991 samples. The filters were weighed and the mass was determined for each size fraction.

Analytical Techniques The technique for extractable total chromium analysis of the contaminated soil consisted of nitric acid digestion and detection by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) (Leeman Labs, Lowell, MA). The analytical procedure was the same as that used on the resuspended soil collected on ny-

lon filters: 1. Place soil or filter in beaker and add 10 mL of nitric acid solution ( 5 mL of concentrated HNO,, 5 mL of distilled deionized water); cover with a watchglass and heat to just under boiling. 2. Reflux for 2 h, remove watchglass, and reduce the volume to 5 mL. 3. Add 1 mL of 30% H 2 0 2 and heat until effervescence stops. 4. Filter after cooling into a 50-mL volumetric flask, dilute to volume with 10% HNO,. 5. Determine Cr by ICP-AES.

The detection limit is 12 ppb and the error is < 5%. The teflon filters from the resuspension studies were analyzed for total elemental chromium and 37 other elements at DRI using XRF (Model 0700/8000, Kevex, Foster City, CA). Five separate excitation conditions were used to analyze each filter. The interference-free minimum detection limit for Cr is 2 ng/cm2. The XRF method measures the total chromium in a sample while the ICP analyses measure that fraction of the total and hexavalent chromium which is extractable in a weak acid. The quartz filters were analyzed for both extractable total and hexavalent chromium. The technique used pyridine dicarboxylic acid (PDCA), which complexes with the Cr(II1) whereas it does not react with Cr(V1). The technique was as follows: Place sample in acid washed beaker Add 5 mL of PDCA stock solution and water to cover the sample (5 mL approximately) Heat to boiling for 10 min. Filter and analyze filtrate. Analysis: Direct injection into ICP, to determine total Cr concentration.



2. For speciation, a Dionex column (CS-5, Dionex Corp., Sunnyvale, CA) and the sample was sent into the ICP for analysis. The detection limit is 0.09 ppm and the error < 10%.

QUALITY CONTROLQUALITY ASSURANCE

Soil Sampling Disposable sampling tools, plastic scoops, and plastic sample containers were used to avoid cross contamination of the samples. Tags were prepared for each sample and attached to the container at the time of sampling. All sampling information and site data were included on the tag. The field conditions encountered, equipment used, procedures followed, and investigators involved with sample collection were recorded on each container. Photographs were taken of each sampling point in an effort to document physical characteristics of the soil (Houck et al., 1989). Field blanks were taken to the site. A container with uncontaminated soil which remained uncovered during sampling was analyzed to detect accidental contamination (Houck et al., 1989; Lippmann, 1990). Triplicate samples were taken at three of the 21 sampling points at the site to provide blind replicates for the analytical laboratories. The triplicate samples were taken next to the original sample. At 13A sampling point, for example, three samples were taken: 13A, 13A (right), and 13A (left). Sample 13A was combined with the rest to form the composite, whereas samples 13A (right) and 13A (left) were analyzed for total extractable chromium but were not composited with the rest of the samples. Standards with known Cr concentration were analyzed with the samples and showed -99% recovery.

Resuspension Sampling The teflon filters used in the resuspension study were provided by the DRI. They were inspected for imperfections and equilibrated in a controlled environment at 24°C and 23% relative humidity (RH). The initial weights were recorded subsequent to equilibration. Each filter was equilibrated for 24 h after sampling before determining the final weight. The quartz filters were heated for at least 3 h at 700-800°C. This procedure is followed by DRI in order to lower the quantifiable limits for organic and elemental carbon measurements (Houck et al., 1989). The filters were stored at 4°C. Nylon filters were prewashed in 10% nitric acid and were rinsed five times with distilled water. They were allowed to dry and equilibrate at 23°C and 30% RH. The initial weight was recorded. Each filter was placed in a cassette and all were taken to DRI. After sampling the resuspended dust, the filters were returned to UMDNJ and equilibrated for 24 h at the initial conditions before measuring the final weights. Initial and final weights for each type of filters were recorded under the same temperature and relative humidity conditions. Standard operating procedures for the resuspension chamber were followed during the resuspension sampling (Houck et al., 1989). The sampling system was disassembled and cleaned to avoid cross contamination between samples. The impactors, table surface, and sampling columns were cleaned after every fifth run.

XRF Analysis

Peak overlaps occur in XRF analysis because of the number and spacing of the characteristic x-ray lines relative to detector resolution. Peak overlap coefficients were used to successively subtract inter-


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ferences from peak integrals, in order to determine net elemental peak areas (Houck et al., 1989). The ability of an x-ray to penetrate matter depends on the energy of the x-ray and the composition and thickness of the material (Houck et al., 1989). In that attenuation factors (absorption of x-rays within the particle) for particles with diameter > 2.5 p m and up to 30 p m could be significant for the lightest elements, concentration corrections calculated by Dzubay and Nelson (1975) were used in this study. Attenuation factors for fine particles with diameter < 2.5 p m are negligible even for the lightest elements (Dzubay and Nelson, 1975). A standard (elemental thin film standard from pMatter, Deer Harbor, WA) is analyzed routinely with each XRF sample run as a check of instrument calibration and performance. The pMatter standards are accurate to f5% as stated by the manufacturer. National Institute of Standards and Technology (NIST) certified thin film standards, Standard Reference Material (SRM) 1833 and SRM 1832, are analyzed every 3 months to assure the instrument calibration.

RESULTS

Bulk Soil Sample The total chromium concentrations in the samples collected at each grid intersection in the spring of 1990 are reported in Figure 1. Total extractable chromium concentrations of the unsieved soil were determined by the nitric acid digestion technique and ranged from 1360 to 6140 ppm with an average concentration of 3430 ppm, a = 1360, and a CV of 38.7%. This analysis of the bulk soil samples indicates high variability in the chromium concentration at sampling locations throughout the site. Changes of several thousand parts

Kitsa et al.

per million occur within distances of 15 or 20 m. This can be attributed mainly to the types of chromium waste slag deposited because it is known that the contaminated sites accepted wastes from several local chromate-producing companies. The individual wastes contained different amounts of chromium slag (ESE, 1988; Sheehan et al., 1991). The individual samples were mixed together as a composite sample. Sieving of the composite sample found that -5.8% of the total soil is in the resuspendable form ( d < 75 pm). The sieved soil was analyzed after digestion by the nitric acid technique (Table 1). The extractable chromium concentrations of the sieved fractions ranged from 1920 to 3420 ppm with a mean of 2600 ppm, a u = 556, and a CV of 21.3%. In 1991 some of the Liberty State Park soil and the bloom soil were analyzed for hexavalent chromium. The results are reported in Table 2. The soil with the visible chromium crystals (bloom soil) had -60 times higher hexavalent chromium concentrations than the contaminated soil without the chromium crystals. Resuspended Sieved Soil Studies Twelve resuspension experiments were completed with the 1990 Liberty State Park samples and three with uncontamiTABLE 1. Total Extractable Chromium in Sieved Fractions of Composite Soil Sample (1990) Nitric Acid Digeslion Sample diameter ( pm), Concentration LSPAD" ( p g Cr/g soil) d > 500 250 < d < 500 150 < d < 250

2480 1920 2080

7 5 < d < 150 38