Environ. Sci. Technol. 1994, 28, 799-804
Hydrophobic CIS Bound Organic Complexes of Chromium and Their Potential Impact on the Geochemistry of Chromium in Natural Waters Scott E. Kaczynski and Robert J. Kieber'
Department of Chemistry and Marine Science Program, University of North Carolina at Wilmington, Wilmington, North Carolina 28403-3297 The complexation of aqueous, inorganic chromium by naturally occurring, relatively hydrophobic dissolved organicmatter was investigated in a wide variety of natural waters. Levels of complexed chromium ranged from a few picomolar in open ocean organic-poor waters to several nanomolar in inland DOM-rich waters. Organic chromium concentrations were positively correlated to both dissolved organic carbon and UV absorbance. Humic substances appear to be important complexing agents in all waters studied. Photolysis experiments indicated that the organochromium species were photodegradable, with significant degradation occurring even after short-term exposure to ambient sunlight. Laboratory spike experiments with inorganic chromium standards indicated that the complexation was rapid with the majority of the metal bound within the first 2 h. The existence of organically bound Cr and its photoreactivity may explain some of the inconsistencies regarding chromium chemistry in natural waters and shed new insight into the cycling of this important trace metal in the aquatic environment.
Introduction
A good deal of attention concerning trace metal chemistry in natural waters has been focused on their organic complexation. A variety of metals including iron, zinc, nickel, manganese, magnesium, and copper are already known to exist in the aqueous environment as organic complexes (1-7). Under certain environmental conditions, these organo-metal species constitute the predominant form of the metal. The ramifications of organic complexation are significant with respect to the geochemistry and environmental behavior of these metals since the organic forms very often behave quite differently relative to their inorganic counterparts. It has been suggested in the literature that chromium may also be complexed by naturally occurring dissolved organic material (8-11). Some of the earliest research was done in the Sea of Japan, where approximately 60% of the total dissolved chromium was reported to be organically complexed (8). Several years later, organic chromium concentrations in shelf waters off the coast of Australia were found to range from 0.002 to 0.160 nM (9). A similar study of Australian coastal waters revealed that 0-90% of total dissolved chromium was organically bound (IO). Although these studies suggest organically bound chromium exists, they are limited in their scope since they are confined to a few isolated marine sites. In the present study, a more detailed investigation concerning organic chromium and a discussion of its potential impact on Cr biogeochemistry in natural waters are presented. These results provide the first detailed information on the distribution, behavior, and significance of organically bound Cr in a wide array of aquatic environments. 0013-936X/94/0~28-0799.$04.50/0
0 1994 American Chemical Society
Materials and Methods Solid-phaseextraction employing CIScartridges has been used extensively to characterize organic metal complexes isolated from natural waters (3,6,12-15).This approach is desirable because it is rapid, requires minimal sample preparation, is reproducible, and has extremely low blanks. In addition, the stationary-phase solute interactions based on van der Waals attractions are relatively weak; therefore, the possibility of chemical transformations of the organic material isolated is greatly reduced (12,16). cartridges to isolate hydrophobic organic The use Of Cr species from natural waters has certain limitations. Naturally occurring Cr organic complexes most likelycover a wide range of polarities, molecular weights, and chemical labilities. No analytical method exists that can successfully separate and quantify all fractions from natural samples. The c18 cartridge employed in the present study is capable of isolating the more hydrophobic fraction retained on the cartridge and eluted by the solvent. The organic chromium concentrations reported, therefore, should be viewed as minimum estimates of the amount of metal organicallybound. Furthermore, since no suitable analysis for determining hydrophilic organic chromium fractions exists, a mass balance on total Cr cannot be determined. A second limitation of the use of cartridges for complex natural systems is the inability to demonstrate quantitative elution of the complexed metal from the CIS columns. Approximately 80 % of the total organic carbon in aquatic systems is uncharacterized. It is impossible with organicCr complexes created from naturally occurring dissolved organic matter to test for quantitative recovery from the cartridge since no standard reference material and/or suitable alternate method exists to determine the precartridge concentrations of bound metal. The excellent precision of this analysis (9% RSD), however, indicates reproducible recovery of the analyte. Reagents and Standards. The acetonitrile (ACN) used as the eluant was HPLC grade. Deionized water was obtained from a Milli-Q water system. Humic acid sodium salts were obtained from Aldrich Chemical Co. Inc. All glassware and Teflon containers were cleaned in a 10% HCl acid bath for a period of at least 24 h at approximately 70 "C.Nuclepore filters (0.4 pm) were soaked in a Teflon petri dish containing concentrated trace metal-grade HCl for 24 h and rinsed with deionized water several times prior to use. Sample Collection and Storage. Samples were collected using a 2.5-L glass bottle with a Teflon-lined cap and then filtered using precleaned 0.4-pm filters. Plastic gloves were worn during collection to avoid sample contamination. Samples were filtered and analyzed within 5 min after collection. A map of the study sites is provided in Figure 1. Analysis of Inorganic Chromium. Dissolved chromium(V1) and chromium(II1) species were isolated using Environ. Sci. Technol., Vol. 28, No. 5, 1994 799
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so0oo' w SF Figure 1. Location of sampling sites. Slngletary Lake (SL), GreenfieldLake (GL), Masonboro Inlet (MI), Gulf of Mexico (GM), Tamiami Rlver (TR), Shark Rlver (SR), Florida Bay (FB), and Stralghts of Florida (SF). @a'@
a previously described iron hydroxide coprecipitation technique (I7,181.Two 140-mLaliquota of filtered sample were placed in separate polyethylene bottles, which were precleaned in a 1% hydrochloric bath at approximately 65 "C for at least 4 days. Dissolved Cr(II1) was collected in one aliquot by the addition of 1 mL of preformed 0.1 M Fe(OH)a(s), which selectively binds Cr(II1). The selectivityof the iron(II1)hydroxide for trivalent chromium was tested with 1and 10nM standard chromate solutions. In both cases, no Cr(V1) was adsorbed onto the Fe(OH)3(s) reagent. One milliliter of freshly prepared 0.1M ferrous hydroxide was added to a second aliquot. The iron(I1) reduces in situ Cr(V1) to Cr(III), which is subsequently scavenged by iron(II1)hydroxide,resulting in a net removal of all dissolved chromium species. The concentration of Cr(V1) is then found by difference. Both sample aliquots were filtered in the field, and the iron chromium precipitates were transported back to laboratory where they were dissolved with 1 mL of 6 M Ultrex HC1. Chromium concentrations in the lechate were determined by graphite furnace atomic absorption spectroscopy (AAS). Analysis of Organic Chromium. The isolation and determination of the hydrophobic organic chromium fraction were performed using a reverse-phaseCUcartridge (Sep-Pak, Waters). The pretreatment, washing, and elution procedure was similar to that employed successfully by several authors to isolate metal organic fractions from natural waters (3, 6, 7, 14-16). Cl8 cartridges were precleaned and conditioned by passing 5 mL of acetonitrile followed by a rinse of 10 mL of deionized water. A known volume of the filtered sample was pumped via Teflon tubing, through the cartridge at a flow rate of 6.5 mL1min. After loading the organic complexes onto the c18cartridge, 5 mL of deionized water was passed through to rinse away salts. A 1.5-mL sample of a 50/50 (vohol) mixture of acetonitrile/water was subsequently passed through the cartridge, effectivelyeluting off the analyte. Occasionally, in organic-rich waters, larger volumes of eluant (2-5 mL) were used and a second CIS cartridge was aligned in a series to compensate for any overloading of the first. 800
Envlron. Sci. Technol., Vol. 28. No. 5, 1994
Determination of Cr concentrations in the eluant were performed by flameless AAS. To determine if ACN, deionized water, or the c18 cartridges were a source of chromium contamination, 3 L of 0.4-rm filtered deionized water was passed through a precleaned C18 cartridge. The c18 cartridge was rinsed with 5 mL of deionized water followed by 3.0 mL of a 50150 mixture of ACNIdeionized water. This procedure was done twice and yielded no detectable chromium. In order to determine a seawater blank, 3 L of O.4-rm filtered Gulf Stream water containing ambient levels of the inorganicmetal were passed through another Cla cartridge. No organic chromium was observed in the eluant after passage through the Sep-Pak. This latter experiment also demonstrates that there was no artificial enrichment of the Cr signal on the cartridge. After passage of the Gulf Stream sample containing natural concentrations of the inorganic metal through a cartridge, which had a 1000fold enrichment of DOM on the cl8packing, no detectable Cr was observed in the eluant. This procedure was repeated twice with two different samples with the same result. Samples were collected for a cartridge extraction precision study from a variety of fresh and oceanic locations. These samples were analyzed multiple times (n = 10) for organic Cr with an average RSD of 9 % , which is within the reported precision of the instrument. Dissolved Organic Carbon and Absorbance Measurements. Natural water samples were collected and filtered through precleaned 0.4-pm filters within 5 min of collection. Dissolved organiccarbon (DOC)concentrations were determined with a Shimadzu carbon analyzer, Model TOC-5000. Absorbance measurements were performed on a Hewlett Packard photodiode array spectrophotometer, Model 84501A, having matched 5-cm quartz cells. Humic Chromium Spike Analysis. Organic-poor Gulf Stream water was spiked with 5 nM Cr(1II) and varying concentrations of humic acid sodium salts in 100mL volumetric flasks. The spiked samples were allowed to stand at ambient temperatures for 24 h, and the
Table 1. Concentration of Organically Bound Chromium (nM) in a Variety of Natural Waters. organic chromium total inorganic chromium mean SD range mean SD range water type
av salinity (ppt)
NDb-0.233 0.0366 0.059 0.369 0.141 0.095-0.623 Greenfield Lake, NC (n = 25) 0.009-0.322 0.106 0.101 0.635 0.409 0.302-1.811 Singletary Lake, NC (n = 12) 0.085-0.512 0.178 0.136 0.110 0.008 0.103-0.120 Masonboro Inlet, NC (n = 5) 0.215-0.242 0.230 0.069 0.007 0.004 0.003-0.011 Gulf of Mexico (n = 3) 0.162-0.168 0.165 0.031 0.443 0.207 0.130.780 Shark River (n = 9) 0.172-0.305 0.222 0.049 0.014 0.011 0.003-0.035 Florida Bay (n = 9) 0.248-0.387 0.311 0.054 0.020 0.016 0.007-0.059 Straits of Florida (n = 9) 0.270 *O 0.261-0.281 0.498 0.259 0.311-0.870 Tamiami River, FL (n = 4) * * * * ND * North Carolina Gulf Stream (n = 2) a Total inorganic chromium (nM) is provided for comparison. b ND = none detected. 0 Asterisk (*) = not determined.
concentration of organically bound chromium was subsequently determined. Cr(III)-Cr(VI)Organic Complexation Study. Natural water samples were collected, filtered through 0.4-pm precleaned filters, and placed in separate 100-mL volumetric flasks. The water sample in the first vessel was analyzed within 5 min of filtration to determine the initial organic chromium concentration (T = 0). The second vessel was spiked to a final concentration of 20 nM with Cr(II1)and the third to 20 nM Cr(V1). Eachspiked sample was allowed to stand for 24 h at ambient temperature. The samples were then analyzed for organic chromium using the cartridge extraction technique. Time Course Study. Shark River water was collected, filtered through a 0.4-pm filter, and placed in an acidcleaned, 2.5-L bottle, which was thoroughly rinsed with several portions of the sample. A spike of inorganic trivalent chromium was added to the flask to a final concentration of 20 nM, and 100-mL aliquots were withdrawn and analyzed for organic chromium a t various intervalsfor 12 h. The initial sample (T=0) was analyzed immediately following spiking. Sunlight Irradiations. Natural water samples were filtered through 0.4-pm precleaned Nuclepore filters, and initial (T = 0) organic chromium concentrations were determined. Additional samples were placed in four stoppered 500-mL quartz round-bottomed flasks and irradiated in ambient sunlight (solar noon, cloudless sky) for a period of 4-5.5 h. Two identical flasks containing a separate aliquot of the same water sample were wrapped in aluminum foil and placed alongside the photolyzed sample to act as a photochemical control.
Results Organic Chromium Concentrations in Natural Waters. C18bound organic chromium was determined at a number of different sites in the southeastern United States (Figure 1). The results are presented in Table 1. Concentrations of the complexes were less than 1 nM except in Singletary Lake, where concentrations were as high as 1.8 nM. Oligotrophic open ocean sites contained the smallest organic chromium signal relative to coastal waters, which in turn contained less than inland lakes studied. In organic-rich inland waters (Shark River, GreenfieldLake, and Singletary Lake), organic chromium concentrations exceeded total inorganic chromium concentrations by more than three times. Samples from coastal water in Masonboro Inlet, NC, which was lower in dissolved organic material, had concentrations of organic chromium that were approximately equal to total inorganic chromium concentrations. In relatively organic-poor
0.0 0.0 34.6 36.1 19.2 35.0 36.6 0.0 36.2
n.37
A A A A A A
AA
A A A A
A
A
A A
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5
IO
15
20
25
DOC (ppm) Flgure 2. Organic chromium as a function of dissolvedorganic carbon in a variety of natural waters (n = 37). The line is a regression fit to the data.
waters in the Gulf of Mexico, Florida Bay, and Straights of Florida, total inorganic chromium was the predominant form of the dissolvedmetal. The concentrationsof organic chromium reported in Table 1 for the marine locations were of the same order of magnitude as levels of organic chromium reported earlier for seawater samples (8, 9). Organic chromium existed at measurable levels in all but the organic-poor waters of the Gulf Stream off the North Carolina coast. This distribution of organicallycomplexed metal is of interest since such different water matrices were studied comprising many different physical and chemical environments. Relationship between Organic Chromium and DOC. The data presented in Table 1suggest that organicrich waters contain higher concentrations of hydrophobic, Sep-Pak-retained organic chromium relative to organicpoor waters. In order tovisualize this relationship, organic chromium concentrations are plotted as a function of dissolved organic carbon (DOC) concentrations in Figure 2. The concentration of organic chromium is positively correlated to DOC levels with a correlation coefficient of 0.83 (n = 37, p < 0.001). This relationship suggests that natural waters with higher DOC would generally contain higher concentrations of this type of organic chromium. This correlation is very interesting since the source of DOC is vastly different in these samples coming as a result of Environ. Scl. Technol.. Vol. 28. No. 5, 1994
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