Environ. Sci. Technol. 2002, 36, 5321-5327
Chromium Speciation in Rainwater: Temporal Variability and Atmospheric Deposition ROBERT J. KIEBER,* JOAN D. WILLEY, AND SUZANNE D. ZVALAREN Department of Chemistry, University of North Carolina at Wilmington, Wilmington, North Carolina 28403-3297
The concentrations of the various chromium species were determined in 89 rainwater samples collected in Wilmington, NC, from October 1, 1999, to December 31, 2001. Volume-weighted annual average concentrations of Crtotal, particulate Cr, Cr(III)(aq), and Cr(VI)(aq) were 4.6, 2.2, 0.8, and 1.2 nM, respectively. There was distinct seasonal and diurnal variability in the concentrations of the various chromium species. Chromium emissions to the global atmosphere by both natural and anthropogenic sources are estimated to be 2.2 × 109 mol/yr. Using our rainwater concentration data along with other published rainwater Cr concentrations and an estimate for total global annual rain, the total global flux of chromium removed from the atmosphere via wet deposition is 2.1 × 109 mol/yr. This represents complete removal of Cr and indicates that essentially all chromium released into the global atmosphere is removed via rain. About half this chromium is dissolved with roughly equal concentrations of toxic Cr(VI) and relatively harmless Cr(III) species.
Introduction Chromium is released into the atmosphere by a variety of anthropogenic activities, which include steel manufacturing, leather tanning, wood preservation, and fossil fuel combustion. Approximately 75 000 ton (1.3 × 109 mol) of chromium is put into the atmosphere annually by these sources, of which one-third occurs as toxic Cr(VI) species (1). Additionally, 54 000 ton (9.4 × 108 mol) of chromium is added to the environment by natural sources. These sources include weathering of rocks and soils, dry fallout due to volcanic ash, lithogenic inputs from dust, and runoff from terrestrial sources (1, 2). The atmosphere has become a major pathway for long-range chromium transport, and rain may be an important removal mechanism for atmospheric chromium. To assess the role of rainwater removal of the 129 000 ton (2.2 × 109 mol) per year of Cr emitted to the global atmosphere, it is essential to quantify the total rainwater chromium concentration on a global scale. Earlier studies have focused on measuring total Cr concentrations and indicate that there is a significant quantity present in wet deposition, even at locations far removed from anthropogenic sources (Table 1). Cr has been implicated in many important atmospheric redox reactions, all of which depend on the oxidation state of the metal. Cr is a central reactant or catalyst in several important redox processes * Corresponding author e-mail:
[email protected]; telephone: (910)962-3865; fax: (910)962-3013. 10.1021/es020777n CCC: $22.00 Published on Web 11/13/2002
2002 American Chemical Society
occurring in the atmosphere including transformations of S(IV), hydrogen peroxide, free radicals (OH and HO2/O2-), organic acids, and other trace metals. Seigneur and Constantinou (3) developed a detailed kinetic speciation model for atmospheric chromium. The reactions they considered include many rainwater components, including Fe(II), S(IV), Mn(IV), sulfate, chloride, hydrogen ion, formic acid, oxalic acid, nitrite, and hydrogen peroxide. The reactions of the various Cr species with these other analytes occurred over time periods of minutes to days, a time frame relevant for many atmospheric processes. Their kinetic model predicts that Cr(III) should be predominant in rain. They stress, however, the importance of actual analyses of precipitation to test their theoretical results. In addition to its important role in controlling many significant atmospheric redox reactions, the valence state of Cr directly impacts its human and ecotoxicological effects and physicochemical properties in the troposphere both before and during deposition. Cr(III) and Cr(VI) have vastly different environmental behaviors and impacts on human health. Trivalent chromium has low solubility and readily adsorbs onto particles. Chromium(III) forms organic complexes and is considered to be an essential nutrient for organisms. Chromium(III) helps maintain normal metabolism of glucose, cholesterol, and fat and aids in the synthesis of nucleic and amino acids (4, 5). In contrast, hexavalent chromium is highly soluble as chromate, does not readily adsorb onto particles, and has toxic effects on organisms even at low concentrations (1, 5). Determining the speciation of chromium, in addition to its total concentration, is therefore essential in order to assess the atmospheric reactivity and environmental fate of Cr in the troposphere.
Experimental Section Sample Collection. Rain filtration, standard and sample preparation, and reagent storage were all conducted in a class 100 laminar flow clean bench to minimize sample contamination. Rainwater samples were collected on an event basis at the University of North Carolina at Wilmington collection site from October 1, 1999, to December 31, 2001. The UNCW rainwater collection site is a large open area, approximately 1 ha, within a turkey oak, long leaf pine, and wiregrass community typical of inland coastal areas in southeastern North Carolina. This rainwater site (34°13.9′ N, 77°52.7′ W) is on the UNCW campus, approximately 8.5 km from the Atlantic Ocean. Because of the close proximity of the collection site to the laboratory, analysis can be completed quickly, which reduces the possibility of compositional changes between time of collection and analysis. Event rain samples were collected using four AerochemMetrics (ACM) model 301 automatic sensing wet/dry precipitation collectors. One of the collectors contained a 2-L muffled Pyrex glass beaker from which samples for analysis of pH, inorganic ions, dissolved organic carbon, hydrogen peroxide, and organic acids were collected. The trace metal sample collectors consist of a HDPE funnel connected by Tygon tubing to a 2-L Teflon bottle extensively cleaned using trace metal clean procedures and protocols (6-8). Rainwater analyzed for Cr and Fe was taken from these trace metal collectors. Meteorological data including rain amounts, rain duration, time of day, surface temperature, wind speed, wind direction, and storm origin were also recorded. Real time precipitation maps were used to define the end of specific rain events, which initiated the sampling process. Analytical. Concentrations of total Cr (not filtered), containing both particulate and dissolved species, were VOL. 36, NO. 24, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Chromium Concentrations in Atmospheric Watersa sample type
location
n
wet + dry atmos dep. weekly samples cloudwater snow
Quillayte, WA Ontario, Canada Lenox Mountain, MA Sweden (urban) Sweden (rural) Lake Superior Lake Erie Lake Michigan Wilmington, NC Netherlands Ankara, Turkey Wales, U.K. Whiteface Mountain, NY Warren, MI
19 202 12 150 82 52 52 52 44 34 76 14 yr 59 39
rainwater
total chromium (nM)
dissolved chromium (nM)
