Environ. Sci. Technol. 1984, 18, 319-326
Correlative Surface Analysis Studies of Environmental Particles Martha E. Farmer? and Richard W. Linton”
Department of Chemistry, The University of North Carolina, Chapel Hill, North Carolina 27514 Various surface analysis techniques [scanning electron microscopy/energy dispersive X-ray microanalysis (SEM EDX), electron spectroscopy for chemical analysis (ESC ), and secondary ion mass spectrometry (SIMS)] were evaluated in a correlative regimen for the chemical characterization of particulate pollutants. Analytical capabilities were demonstrated by using particles derived from steel blast furnaces. The ESCA studies of “bottom ashes” suggest highly water-soluble species enriched in sulfates on particles consisting primarily of iron oxides. Comparisons of SEM/EDX and SIMS data for selected metals (e.g., Cr, Cu, Fe, Mn, Mo, Pb, and Zn) in unleached and water-leached particles were used to estimate the extent of enrichment and aqueous solubility of surface species. Accessibility to the environment (via washout, rainout, groundwater leaching, lung fluids, etc.) is governed by both metal surface accessibility (extent of surface enrichment) and metal surface solubility (surface speciation). The unique combination of direct surface analysis and time-resolved solvent leaching experiments enables the relative importance of the two factors to be estimated for individual elements of environmental interest.
d
Introduction The role of the particle surfaces in mediating the chemical behavior of pollutants is an issue of environmental interest (1-7). Examples of the areas of potential atmospheric impact are briefly summarized as follows. Studies of airborne particles from several pollution sources (1-3) have shown that various elements are enriched in concentration on particle surfaces and may be transported in the atmosphere in this manner. Consequently, the enhanced environmental accessibility of toxic species present on particle surfaces may increase their biological availability to physiologic fluids and tissues. Particle surface chemistry also influences the performance of pollution control technologies. For example, the surface enrichment of alkali metals, which helps to determine the resistivity of fly ash particles, will enhance the ability of the particle surface to hold a charge and in turn improve the efficiency of electrostatic precipitation (8, 9). The focus of this research is the evaluation and development of correlative analytical techniques for the determination of the speciation, availability, and surface association of elements present in pollutant particles (Figure 1). An integral aspect of all experiments is the combination of time-resolved solvent leaching with bulk and surface analysis measurements, both to extend and substantiate surface analysis information and to address the origin and potential bioavailability of toxic species in the surface layer (10). The surface analysis techniques of principal interest in this investigation are electron spectroscopy for chemical analysis (ESCA) and secondary ion mass spectrometry (SIMS). Basic reviews of technique fundamentals (11) and the previous applications of surface spectroscopic analysis to environmental particle characterization may be found elsewhere (2, 3). Present address: Research Staff, Ford Motor Co., Dearborn, MI 48226. 0013-936X/84/09 18-0319$01.50/0
In conjunction with ion sputtering, ESCA and SIMS provide a measure of relative elemental concentrations as a function of depth. The ESCA technique provides semiquantitative surface analysis data using empirical sensitivity factors and chemical speciation information via electron binding energy shifts. The SIMS technique described herein is used only for depth profiling studies (concentrationsvs, depth) of metals detectable by scanning electron microscopy/energy dispersive X-ray microanalysis (SEM/EDX).
Methods Sample Preparation. Precipitator dusts (PD) from several open hearth blast furnaces were obtained as suitable samples for analytical technique evaluation. Steel furnaces represent a major source of particulate pollutants (12)and offer the prospect of environmental mobilization of toxic trace metals via volatilization in the high-temperature zone followed by condensation on the surfaces of more refractory particles (primarily iron oxides) in the lower temperature exhaust regions. The particles available for this study were primarily “bottom ashes”. Hence, the extent of condensation of volatiles could be somewhat diminished in comparison to “instack” samples collected at closer to ambient temperatures. The bulk composition of PD generally reflects the elemental content of steel products, with substantial contributions from many of the first-row transition metal oxides (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn). Although relative toxicity may be highly dose and speciaion dependent, many of these metals are known to be somewhat toxic and/or mutagenic when inhaled in particulate form (13). Samples of PD were sprinkled onto In foil and pressed in a vise between sheets of A1 foil. For ESCA, complete surface coverage of the In substrate was desired to maximize sensitivity. With this mounting procedure, no problems were encountered with electrical charging of more than a few electron volts in the ESCA spectra. For SIMS studies less dense coverages were required to minimize more severe electrical charging encountered during ion irradiation of the insulating particles. No significant problems were encountered with the “cover-up”of particles by In during ion sputtering as has been suggested by Hock et al. for sample preparations involving widely dispersed single particles (14). The above mounting procedures also have the advantages that In is electrically conducting, presents few spectral interferences, is soft and malleable allowing embedding of particles without physical alteration, and has a low-vapor pressure for ultrahigh vacuum compatibility (15). A Mo mask was placed over the specimen for ESCA examination. Surface and Depth Profiling Analysis. The ESCA studies were conducted on a Perkin-Elmer Physical Electronics Model 548 electron spectrometer equipped with a Mg anode and double-pass cylindrical mirror analyzer. Sputter depth profiling was achieved by using a 3M Minibeam I rastered ion gun with a 2.5-keV Ar+ primary beam produced by using a 15-mA filament current and backfilling the vacuum chamber with 5 X torr of Ar. Residual vacuum level was typically torr. Charge correction was accomplished by referencing all peaks to
0 1984 American Chemical Society
Envlron. Sci. Technol., Vol. 18, No. 5, 1984 319
SOLVENT EXTRACTION
TIME-RESOLVED LEACHING
LEACHATE RESIWE
LEACHED PARTICLES
UNLEACHED PARTICLES
Figure 2. Time-resolved leaching apparatus. (a) Nuclepore filter; (b) magnetic stirrer; (c) peristaltic pump; (d) stirrer fllter cell. AA (SPECIATION)
xps
1
SIMS (TRACE ANALYSIS)
7 BULK ANALYSIS
SEM/EDX (METAL ANALYSIS)
Figure 1. Experimental protocol for the determination of inorganic species present on particle surfaces.
the adventitious hydrocarbon (C Is, 284.6 eV) on the particle surfaces. Estimates of relative surface concentrations were obtained by using tabulated relative atomic sensitivity factors (16) and normalizing to the Fe atomic concentration. The SIMS elemental depth profiles were performed on a CAMECA IMS-3f ion microscope (17). An 8-keV posprimary beam of 1-pA itive oxygen (predominantly 02+) current and 50-pm diameter was rastered over an area of 400 pm by 400 wm containing various dust particles. Apertures in the secondary ion extraction optics limited secondary ion detection to a circular area of 150-pm diameter, thereby ensuring relatively uniform primary beam current density over the analysis area. Because of the highly oxidized nature of the PD particles, the effects of primary beam oxygen implantation on secondary ion yields is considered to be small (1-3, 10). Because changes in relatiue concentrations following removal of surface layers via water leaching or ion sputtering were the important observables, it was not necessary to quantify absolute concentrations via use of empirical standards such as NBS standard glasses (18). Use of a generalized set of SIMS atomic sensitivity factors analogous to those used in the ESCA studies also was not desired since matrix effects are much more significant in influencing SIMS quantitation m 3 , 11). Microscopic Analysis. Scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX) were performed on an ETEC scanning electron microscope equipped with a Kevex 5100 energy dispersive X-ray analyzer with a 30-mm2Si(Li) detector. Beam current was approximately lo-@A; accelerating voltages were 30 keV for EDX and 20 keV for SEM. Samples were C-coated to minimize charging, while still permitting X-ray microanalysis. Bulk Analysis. The bulk metal content of the PD particles was determined by dissolution of particles and atomic absorption spectroscopy (AAS) using an air/ acetylene flame (Perkin Elmer Model 560). Dissolution of the refractory metal oxides in the PD was accomplished by borax-carbonate fusion as per the method of Cobb and Harrison (19). This method also reduces atomic absorption signal suppression caused by flame oxidation and chemical interference by silicates. The alkali content of the sodium carbonate-sodium tetraborate flux serves as ionization buffer; lanthanum chloride (LaC1,) serves as the releasing agent. 320 Environ. Sci. Technol., Vol. 18, No. 5, 1984
Soxhlet Leaching. A several gram sample of PD was placed in a Soxhlet thimble elevated by prongs above the siphon level to prevent precipitation of dissolved metals onto the residual PD. Fitted Teflon sleeves were used on all joints. Leaching was continued for 83 h at approximately 75 "C. An empty thimble was concurrently leached to serve as a blank. The leachate solutions were rinsed from the flasks, filtered through Whatman No. 42 filter paper, and diluted to 250 mL, with acidification to pH C3 with HN03. Metals were determined by AAS. Time-Resolved Leaching (TRL). A 1.7-g sample was placed in a Nuclepore stirred filter cell (Figure 2) equipped with a 0.2-pm pore size Nuclepore polycarbonate membrane filter. An initial 10 mL of distilled, deionized water was added, with immediate initiation of stirring and solvent flow. Constant flow was maintained by a peristaltic pump at an initial rate of -900 mL/h. After the fifth fraction, the flow rate was slowed to 60 mL/h to increase the metal concentration in later fractions facilitating analyte detectability. The pH of the eluting water was not adjusted or buffered. Successive 6-mL fractions were collected, diluted 20X, and acidified to pH 5 3 with reagent A.C.S. grade 70% HN03, in order to prevent adsorption or hydroxide precipitation of metals during storage (20). Metals chosen for study by AAS (Cr, Cu, Fe, K, Mn, and Zn) correspond to most of those metals which had been characterized by SEM/EDX and/or SIMS. Although Pb characterizationby flame AAS was attempted, it was below the estimated detection limit of 0.4 pg/mL for all leachate solutions. No attempt was made to characterize Si or Mo with AAS techniques. Sulfate was determined by Ba precipitation and nephelometry, as per the method of Toennies and Bakay (21), which is adaptable to sulfate concentrations ranging from 0.01 to 100 ppm. The stated precision of the method is 1% , with a detection limit of 3 pg of S042-. Ions such as NH4+,Nat, Kt, Ca2+,Fe3t, CY, NO
