Environ. Sci. Technol. 1994,28, 730-736
Chromium Speciation and Distribution in the Great Lakes Stan Beaubien,t Jerome Nrlagu'v* Dave BIowe8,t and Greg Lawson*
Department of Earth Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1,Canada, and National Water Research Institute, Box 5050, 867 Lakeshore Road, Burlington, Ontario L7R 4A6, Canada
A technique is described for selectively measuring the forms of chromium in freshwater samples, with detection limits of 21 ng L-I for Cr(III), 4 ng L-l for Cr(VI), and 8 ng L-I for colloidal/organic Cr. Recovery of the Cr(V1) and Cr(II1) spikes were consistently >go%, but only about 70% for the added organic Cr. When the method was applied to samples from Lake Ontario, it was found that Cr(V1) made up 75-85 % of the dissolved Cr and that Cr(111)was consistently below the analytical detection limit. About 105% of the dissolved Cr was in the colloidal/organic form. Average concentrations of total dissolved Cr in the epilimnetic waters were found to be 69 ng L-l in Lake Superior, 136 ng L-I in Lake Erie, and 351 ng L-l in Lake Ontario. The increasing concentration of Cr down the Great Lakes watershed reflects the conservative geochemical behavior of Cr(V1) and the increasing input of Cr from industrial and municipal sources. Introduction Little is currently known about the chemistry of chromium in lacustrine environments. In comparison to oceans, lakes tend to have lower dissolved solid and higher particulate loads, higher biological activity, greater ratio of sediment to water surface areas, and very transient transport and mixing features and may be influenced more by inputs from rivers and industrial sources (see refs 1 and 2). These features can lead to the behavior of trace metals in lakes being quite different from that in oceans. The evaluation of the biogeochemistryof chromium species in freshwaters based on current knowledge in the marine literature cannot be made with any confidence. This report presents the first systematic investigation of the distribution and speciation of chromium in the Great Lakes using the ultraclean laboratory method. In view of the marked differences in the chemical behavior and toxicity of chromium in its different oxidation states,numerous methods have been tried in the continuing effort to quantify the chromium species in natural waters. Coprecipitation of an individual specieshas been examined with various dithiocarbamates (DTC), such as lead pyrrolidine DTC ( 3 ) , sodium dibenzyl DTC ( 4 ) , cobalt pyrrolidine DTC ( 5 , 6 ) ,and manganese diethyl DTC (9, as well as with various inorganic mineral phases including Pbsalts ( 8 ) ,Fe(OH)Zand/or Fe(0H)s (9-11), andBi(0H)S (12). The process of chelation/solvent extraction has been applied to Cr using ammonium pyrrolidine dithiocarbamate (APDC) back-extracted into methyl isobutyl ketone (13-15) or dichlorobenzene (16), and diethyl dithiocarbamate (DDTC) back-extracted into chloroform (17) or carbon tetrachloride (18). One of the simplest, but
least sensitive methods, is colorimetry. There are numerous examples of reagents that can form a colored compound with Cr (VI), including diphenylcarbazide (19221, (o-nitropheny1)fluorone (23),or Rhodamine B (24). An approach using diphenylcarbazide colorimetry in tandem with the exchange resin Dowex 50W-X4 has also been proposed (25,261. Other less common approaches include isotope dilution ( 2 9 ,adsorptive differential-pulse voltammetry (281, luminol chemiluminescence (29),and cathodic stripping voltammetry (30). The discrepancies in the published results suggest that complete separation of Cr species is not always achieved using these various methods. The selectivity, simplicity, rapid processing and low cost of exchange resins has long been recognized, and many resins have been used for the separation of the chromium species in water samples (Table 1). Some of these methods involve the separation (and sometimes preconcentration) of one inorganic form and the calculation of the concentration of the other form by the difference between total dissolved chromium and the measured species (36, 38). Other methods separate and measure directly the two inorganic forms (32,34,35,39).These approaches assume that no other species contribute significantly to the observed total concentration, i.e., that phases such as colloidal/organic Cr (a term used in this paper to designate the Cr complexed or adsorbed to various organic ligands and/or colloids) do not exist. In a significant departure from previous practices, Johnson (33) measured the Cr(111),Cr(VI), and total dissolved Cr concentrations and calculated the colloidal/organic Cr fraction by the difference. Recently Hiraide and Mizuike (31) described a method that can be used to actually isolate and measure the three dissolved forms of chromium. This technique has been optimized in this study for processing large volume samples in the field to avoid preservation problems, such as Cr(V1)reduction under acid conditions (40)or the release of Cr(II1) from the complexed/adsorbed fraction (41).
and Industrial Health, School of Public Health, University of Michigan, Ann Arbor, MI 48109, + University of Waterloo. National Water Research Institute.
Materials and Methods Reagents. Ultrapure water used in the experiments was prepared by the reprocessingof the laboratory-distilled water in a quartz still (Corning AG-3) to produce doubledistilled water (DDW), which was subsequently passed through a Milli-Q Plus water system (Millipore Corp., Bedford, MA) to produce the Milli-Q water (MQW). Double-distilled Seastar nitric and hydrochloric acids (Seastar ChemicalsInc., Sydney, BC) were used as received for sample acidification and resin cleaning/charging. Seastar ammonia solution was used as received. Acetate buffer (7.5 M) was prepared using Ultrex glacial acetic acid (J. T. Baker Chemical Co., Phillipsburg, NJ) and Seastar ammonia solution; this solution did not require further purification as blanks contained less than 20 ng L-1 of Cr. Hydroxylamine hydrochloride (HAH) was obtained from Aldrich Chemicals (Milwaukee, WI) and mixed with MQW to make a 5% (w/v) solution.
730 Envlron. Scl. Technol., Vol. 28, No. 4, 1994
0013-936X/94/0928-0730$04.50/0
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* Corresponding author, now at the Department of Environmental
@ 1994 American Chemical Soclety
Table I. Information on Various Ion-Exchange Media Used in Study of Chromium Speciation in Natural Aqueous Samples. Cr(V1) exchangemedia DEAESephadexA-25 Chelex-100 Bio-Rad 1-X4 Dionex CG5 DionexHPIC-AS’I/CSZ C18 (Adogen464) C18 (TributyltinC1) C18 (NaDDTC) alumina
detector GFAAS GFAAS GFAAS CL DCPAES GFAAS GFAAS GFAAS ICPAES
MT p/m p/m p/m s/m p/m p/m p/m p/m p/m
DL ND ND 0.02 0.5
