Environ. Sci. Technol. 1985, 19, 345-352
Determination of Chromium Speciation in Environmental Particles. Multitechnique Study of Ferrochrome Smelter Dust X. B. Cox, 111, and Richard W. Linton’ Department of Chemistry, Venable and Kenan Laboratories of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514
Frank E. Butler U S . Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1
The chemical speciation of particulate Cr has a si nificant influence on its environmental impact, with Cr compounds of greatest current concern. A novel analytical regimen was evaluated for the determination of the con centration, speciation, and potential bioavailability of environmental Cr by using the example of ferrochrome smelter dust. Various wet chemical extractions were compared for possible use in the routine environmental monitoring of “bioavailable”Cr and CrW. Complementary nondestructive instrumental methods included bulk elemental analysis via instrumental neutron activation (INAA), microscopic studies via scanning electron microscopy-energy dispersive X-ray microanalysis (SEMEDX), and surface chemical analysis via X-ray photoelectron spectroscopy (XPS). Roughly half of the total Cr in the primary smelter dust was extractable by routine acid/base leaching (bioavailable),of which about 40% was Cr6+. The Cr6+exists as Cr,0,2- or Cr02- and predominates in submicron particles probably formed during smelting. The remainder of the Cr is primarily insoluble Cr,03 which is located in large particles similar to the original chromite ore.
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Introduction As a current focus on the environmental importance of Cr, two comprehensive surveys for Cr were completed in 1983 (1,2) summarizing potential sources and hazards. These studies indicate the importance of the determination of specific chemical states of environmental Cr. Although Cr in inorganic compounds may occur in valence states ranging from -2 to +6, the Cr3+and Cr6+species are of primary environmental interest (3,4).Compounds containing Cr6+,present largely from anthropogenic processes (I),are believed to be responsible for most of the health problems associated with all Cr compounds. The toxicity and carcinogenicity of Cr6+compounds also are reported to be much higer than that for Cr3+compounds (5, 6). Therefore, a more complete understanding of the health effects of Cr source emissions requires information on Cr concentration, speciation, and bioavailability. There are a number of pollutant Cr sources, including kilns, smelting furnaces, boilers, and leaching and plating tanks. The burning of oil, coal, and wood and the incineration of municipal refuse and sewage sludge also contribute to atmospheric Cr. Cement production, asbestos mining, and the routine wear of Cr-containing products such as vehicular brake linings are additional contributors to the environmental mobilization of Cr. The source of Cr chosen for this initial investigation is a ferrochrome smelter that processes mixtures of Cr-containing ores and lime (CaC03)in an electric arc furnace. Both Cr3+and Crs+ species are present in the dust captured by pollution control devices, in this case a baghouse. As baghouse dusts are disposed of in landfills, where 0013-936X/85/09 19-0345%01.50/0
leaching mechanisms can extract species into the environment, the dust provides a useful analytical sample to determine the amount and chemical state of Cr potentially available for biological uptake (operationally designated as ”bioavailable”Cr). The main goal of this study is to use nondestructive instrumental techniques in concert with conventional wet chemical analysis (6-10)to establish the fraction of bioavailable Cr present as Cr6+. An additional objective is the evaluation of various wet chemical techniques to help establish a reliable, routine approach to environmental source monitoring of particulate Cr species, ultimately to include ambient particle samples. Supplemental information on inter- and intraparticle Cr distribution is provided by specialized surface and microprobe analytical techniques. Wet chemical techniques, as applied to routine environmental monitoring, have many potential advantages including high sample throughput, low cost, low detection limits, and the ability to determine both elemental concentration and chemical state. The latter aspects are complicated in that a particulate sample is physically altered (extracted, dissolved, etc.) with the possibility of induced chemical changes, matrix effects, and differential solubility artifacts. Particularly well suited to complement the wet chemical techniques used in this study is X-ray photoelectron spectroscopy (XPS). The X P S experiment (11)allows for essentially nondestructive analysis of Cr on the surface of the smelter dust. Surface specificity is important in that it is the particle surface which comes in direct contact with the environment. Changes in chemical speciation and/or concentration in the surface region may cause an element to have an environmental importance far greater than its bulk concentration would indicate (12). Furthermore, particle surfaces may show a high concentration of water-soluble compounds (12)which can be accessed by the environmental leaching actions of atmospheric water or groundwater or by biological fluids following particle ingestion or inhalation. Chemical state information also is provided in XPS via shifts in binding energies of photoelectrons, enabling a comparison of the relative proportion of surface region Cr3+vs. Cr6+in the specific instance of the smelter dust sample. In addition to the intraparticle (surface vs. bulk) Cr chemistry, it is of interest to establish the interparticle distribution of Cr; Le., is the Cr localized in Cr-rich particles? Scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX) provides both morphological information (secondary or backscattered electron imaging) and X-ray maps of the distribution of elements among particles (13).Correlating the characteristic X-ray emission with the position of the electron beam allows for spatially resolved elemental analysis with a resolution on the order of 1 pm. Because of the inherent X-ray line width and the energy resolution of the EDX detector, little
0 1985 American Chemical Society
Environ. Sci. Technol., Vol. 19, No. 4. 1985
345
Table I. Wet Chemical Methods Employed in the Analysis of Extractable Cret and Total Extractable Cr in Ferrochrome Smelter Dusts ( 1 4 ) O description (1) borate fusionb (ASTM D3682) (2) mild acidb (method 3020) (3) mild acid' (method 3020) (4) acid digestion' (method 3020, method 3020 with HzOz, or method 3050.) (5) EP toxicityb (method 1310) (6) EP toxicity (method 1310) (7) EP toxicity (method 1310 or method 1310 with H,O,) (8)strong acid, sonication' (ASTM D3683) (9) alkaline digestion' (method 3060 or method 3060 with
resultse 45.6-48.1 mg/g of total Cr; spike recovery 18-55% 0.009-0.01 mg/g of CrSt; spike recovery
