Environ. Sci. Technol. 1996, 30, 3477-3486
Copper Speciation and Binding by Organic Matter in Copper-Contaminated Streamwater ROBERT F. BREAULT AND JOHN A. COLMAN* U.S. Geological Survey, 28 Lord Road, No. 280, Marlborough, Massachusetts 01752
GEORGE R. AIKEN AND DIANE MCKNIGHT U.S. Geological Survey, 3215 Marine Street, Boulder, Colorado 80303
Fulvic acid binding sites (1.3-70 µM) and EDTA (0.0017-0.18 µM) accounted for organically bound Cu in seven stream samples measured by potentiometric titration. Cu was 84-99% organically bound in filtrates with 200 nM total Cu. Binding of Cu by EDTA was limited by competition from other trace metals. Water hardness was inversely related to properties of dissolved organic carbon (DOC) that enhance fulvic acid binding: DOC concentration, percentage of DOC that is fulvic acid, and binding sites per fulvic acid carbon. Dissolved trace metals, stabilized by organic binding, occurred at increased concentration in soft water as compared to hard water.
Introduction Binding of trace metal ions by dissolved organic matter in aquatic environments is important in controlling the chemical speciation (1-3) and bioavailability and toxicity (4-6) of trace metals. Gardner (1) determined that most divalent trace metals would be complexed by ethylenediaminetetraacetic acid (EDTA) at concentrations of EDTA and metal commonly measured in streams. Mantoura and Riley (2) concluded from investigation of fulvic acid isolated from streamwater that fulvic acid complexes of Cu were likely to be the major chemical species for Cu. Tipping (3) has come to similar conclusions from modeling investigations. McKnight (4) showed that fulvic acid complexation could account for control of Cu toxicity to algae during the treatment of a reservoir with copper sulfate. Despite findings of past investigations, it is not generally known how well predictions of metal binding by dissolved organic matter account for actual metal speciation in streamwater, which organic materials cause the binding, what the effect of competition among metals for binding sites might be, or whether metal toxicity is likely to be controlled by organic matter. Metal toxicity masking effects of dissolved organic matter are not included in the U.S. Environmental Protection Agency (USEPA) water-quality criteria (7), although the toxicity masking effects of hardness are included. In S0013-936X(96)00130-7 This article not subject to U.S. Copyright. Published 1996 by the American Chemical Society.
order to further assess toxicity and geochemistry of trace metals in streams, the correspondence between binding potential of dissolved organic constituents and observed metal speciation in streamwater samples should be determined as functions of streamwater type. Cu is commonly selected to study metal binding by organic compounds because of its known affinity for carboxylate and other organic ligands, its importance in toxicity in streams, and its role in control of algae. Investigators have studied Cu-organic binding by experimenting with and modeling fractions of organic material isolated from soils or natural waters (2, 3, 8, 9) and by measuring concentrations of synthetic chelating materials, principally EDTA and nitrilotriacetic acid (NTA), in natural waters (1, 10, 11). Complementary to the fractionation approach, copper binding capacity of whole waters has been measured (4, 12), and many measurements of freeion Cu concentrations have been made in whole water (12, 13). In this investigation, we combine the fractionation and whole water approaches by measuring the fulvic acid fraction of dissolved organic carbon and a synthetic organic compound and by comparing Cu binding predicted by these materials to binding measured in the corresponding stream sample. The combined approach is applied to samples from Massachusetts streams that have a range of hardness (sum of dissolved Ca and Mg concentration) and of effluent contributions from point sources. In addition, constituents involved in binding are characterized according to streamwater type to assess their geochemical and toxicological significance. Agreement between summed constituent binding properties and measurements of binding in river samples would confirm that the constituents investigated do in fact account for the amount of Cu binding measured. This test for closure has not been included in previous investigations of metal binding by organic constituents. The investigation focused on speciation for total dissolved copper concentrations greater than 100 nM in order to address the relation of Cu binding to Cu toxicity. Detailed speciation is worked out for total dissolved copper of 200 nM. This range is reflective of conditions in Cu-contaminated streams that approach regulatory limits for toxicity criteria (7).
Methods Sampling and Sample Processing. Samples were collected December 1993-August 1994 from seven stream sampling stations in Massachusetts (Table 1). Wadeable streams were sampled using a 3-L, all-Teflon DH-81 sampler (14) at the centroid of flow. Deeper streams were sampled from bridges using a D-77 sampler (14) with a 3-L Teflon bottle and Teflon nozzle. Teflon sampling equipment and bottles were precleaned with dilute nitric acid. Trace element, clean-method protocol was used during sample collection and processing. Samples analyzed for major ions and trace elements were filtered through a 0.45-µm Gelman capsule filter (polyether sulfone), bottled in Teflon, and acidified with Fisher Optima concentrated nitric acid to 0.4%. One liter of water was pumped through the filters before the sample was collected. Samples analyzed for dissolved organic carbon (DOC) were filtered through 0.45-µm silver
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TABLE 1
Site Descriptions and Sampling Dates stream name USGS station no.
sampling date
NPDESa sites
Blackstone River 01112000
12/28/93 8/24/94
9
Bungey River 01109369
6/30/94
1b
Marsh Brook 01197300
7/21/94
0
Nashua River 01096500
1/3/94 8/25/94
2
Ten Mile River 01109382
12/29/93
14
unnamed tributary 010974573
8/01/94
0
Williams River 01197802
8/03/94
0
stream description stream draining the Blackstone River basin of south-central Massachusetts to Narragansett Bay; drainage basin consists of forested terrain and the city of Worcester; the basin is underlain by granitic and metamorphic rocks small stream draining to the Ten Mile River of southeastern Massachusetts; drainage area consists of sparsely settled woodlands and wetlands; the basin is underlain primarily by sedimentary and igneous rocks and some coal small stream draining to the Housatonic River. Drainage area consists of sparsely settled woodlands and marsh; the basin is underlain by limestone, dolomite, marble, quartzite, schist, and gneiss stream draining the Nashua River basin of north-centralMassachusetts; drainage basin consists mainly of forested terrain; the basin is underlain by several types of crystalline rocks small stream draining the Ten Mile River basin of southeastern Massachusetts; drainage area consists of sparsely settled woodlands, wetlands, agricultural lands, and some brushlands; the basin is underlain primarily by sedimentary and igneous rocks and some coal small stream draining to the Sudbury River of north-eastern Massachusetts; drainage area consists of wetlands; the basin is underlain by a variety of crystalline rocks small stream draining to the Housatonic River; drainage area consists of sparsely settled woodlands and wetlands; the basin is underlain by limestone, dolomite, marble, quartzite, schist, and gneiss.
a Number of National Pollution Discharge Elimination System permit sites upstream of sampling site (46). from fish hatchery).
membrane filters with a pressure stainless steel filtration unit, collected in 125-mL precombusted amber glass bottles, and stored on ice. All samples for inorganic and DOC analyses were sent to the USGS laboratory in Arvada, CO. Samples for copper titrations were filtered through 0.45µm Gelman capsule filters and collected in amber glass bottles. Large samples (5-10 L) for fulvic acid isolation and EDTA analysis were collected in precleaned stainless steel milk cans and filtered in the field serially through 1and 0.3-µm glass filters using glass and stainless Balston filtration units. The filtered water was collected in 1-L, amber prebaked bottles and sent, on ice, to the USGS laboratory in Boulder, CO, for isolation and analysis. Quality Assurance. Measurement of dissolved trace metals in river water is subject to contamination during collection and processing of samples (15, 16). This potential problem was assessed in our samples, by checking sources of systemic contamination (leaching from inappropriate or unclean sampling materials) with equipment blanks and by checking sources of random contamination (as from airborne particles) with duplicate samplesssampling twice within 10 min at a site. Chemical Analysis. Specific conductance, pH, dissolved oxygen, temperature, and alkalinity were determined in the field using standard USGS procedures (17). Analysis of DOC was by wet oxidation with carbon dioxide detection by infrared spectrometry (18). Trace metal analyses for Ag, Al, Ba, Be, Cd, Co, Cr, Cu, Mn, Mo, Ni, Sb, and Zn were by inductively coupled plasma-mass spectrometry (19). Methods for other dissolved inorganic constituents are described by Fishman and Friedman (17): Major-ion analyses for Ca, Fe, K, Mg, and Na were done by atomic absorption spectrometry; analyses of F- were by ion-selective electrode; analyses of Cl- and SO42- were by ion-exchange chromatography; analyses of ammonia and ammonia plus organic nitrogen were by a salicylate-hypochloride colorimetric method; analyses of nitrite and nitrate were by a diazotization and colorimetric method, after cadmium reduction
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b
Non-NPDES point source (discharge
of nitrate to nitrite; and analyses of dissolved P and dissolved ortho-P were by a phosphomolybdate colorimetric method. Analysis for EDTA used a derivitization-gas chromatography procedure described by Schaffner (20). The method is highly specific for EDTA, even in a complex matrix and has a detection limit of about 1.0 nM. Isolation of Fulvic Acids. Isolation of fulvic acids used fractionation procedures described by Aiken et al. (21). Filtered samples were acidified with concentrated HCl to a pH of 2.0 and passed through an XAD-8 resin column, where fulvic acids are retained. The column was rinsed with distilled water to remove chloride ion until specific conductance of the rinse decreased to 750 µS/cm at 25 °C. The XAD-8 resin column in series with a cation-exchange resin column (AG-MP 50) in the hydrogenated form was back-eluted with 0.1 M NaOH. Back-elution was complete when monitored specific conductance of the eluent increased. The hydrogen saturated eluent was freeze-dried, weighed, and stored in amber glass vials. Cu Titrations. Cu titrations, with monitoring of cupric ion activity, were performed on filtered water samples and on solutions of fulvic acids made from freeze-dried material, using the methods of McKnight et al. (9) with slight modifications. Titrations used 0.1 M KNO3 background electrolyte, pH regulation at 6.25, and temperature at 25.0 °C. Cupric ion activity and pH were measured against Orion Model 90-02 double-junction reference electrodes. The pH of the sample was measured and recorded with a Ag/ AgCl glass pH electrode. Cupric ion activity was measured with an Orion Model 94-29 cupric ion-selective electrode. After use in high Cu concentration solutions, the sensing element was rinsed with deionized organic-free water and immersed in 100 mL of 0.025 M H2SO4 solution for 5 min to remove absorbed Cu ions (23). Electrode drift was minimized by polishing before each calibration-titration sequence with Orion polishing strips, and photo-induced reaction at the probe surface was reduced by covering the titration cell with black tape (24). Reaction progress after
TABLE 2
General Water Chemistry Data for Sample Filtrates from Each Sampling Sitea sample Blackstone River 12/28/93 8/24/94 Bungey River Marsh Brook Nashua River 1/3/94 8/25/94 Ten Mile River unnamed tributary Williams River a
pH
Ca (mM)
Mg (mM)
DOC (mg/L)
EDTAb (nM)
Al (nM)
Cu (nM)
Fe (nM)
Mn (nM)
Ni (nM)
Zn (nM)
6.2 6.9 6.8 7.9
0.27 0.25 0.25 0.90
0.08 0.07 0.10 0.35
4.2 4.9 3.8 6.7
158 62 1.9
2 076 1 112 408 148
94 173
