Environ. Sci. Technol. 1997, 31, 3010-3016
Strongly Complexed Cu and Ni in Wastewater Effluents and Surface Runoff DAVID L. SEDLAK,* JONATHAN T. PHINNEY, AND WILLIAM W. BEDSWORTH Department of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, California 94720-1710
Although a variety of analytical techniques have been developed to characterize metal speciation, few have been used successfully in the complicated matrices encountered in wastewater effluents and surface runoff. In this study, competitive ligand equilibrium-cathodic stripping voltammetry (CLE-CSV) and chelating resin column partitioninggraphite furnace atomic absorption spectroscopy (CRCPGFAAS) are used to determine the speciation of Cu and Ni in point and non-point pollutant sources discharging into South San Francisco Bay. As expected, most of the dissolved Cu and Ni in wastewater effluents and surface runoff are complexed. Moderately strong metal-complexing ligands, which likely consist of activated sludge biopolymers and humic substances, are responsible for the complexation of only about 20% of the Ni and 5-50% of the Cu. The remaining Cu and Ni is complexed by ligands with apparent stability constants comparable to those of synthetic chelating agents. Strongly complexed Cu is present at concentrations below 40 nM and accounts for 5-60% of the Cu discharged by these sources. Strongly complexed Ni is present at concentrations ranging from approximately 10 to 100 nM and accounts for >75% of the Ni discharged by wastewater treatment plants and approximately 25% of the Ni in surface runoff. Strong Ni complexes, which are not removed during wastewater treatment, are extremely stable in seawater. The existence of strong metal-complexing ligands in wastewater effluent and, to a lesser degree, in surface runoff must be accounted for when evaluating metal treatability and biogeochemistry. In response to concerns about the effects of pollutant metals on aquatic ecosystems, most municipalities dedicate considerable resources to the monitoring of point and non-point pollutant sources and to the implementation of metal source control programs (1). The environmental regulations that are the impetus for these activities are intended to maintain total metal concentrations below levels known to be toxic to sensitive aquatic organisms. This approach has been criticized (2, 3) because it fails to consider the decrease in toxicity that occurs when metals are adsorbed onto particles or are complexed by dissolved ligands. As the resources dedicated to controlling pollutant metals continues to increase, it is likely that these concentration-based regulations will be reevaluated. For example, the U. S. Environmental Protection Agency (U.S. EPA) has recently begun to consider partitioning * Corresponding author. E-mail:
[email protected]. Telephone: (510) 643-0256. FAX: (510) 642-7483.
3010
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 10, 1997
of metals onto particles when issuing discharge permits for wastewater treatment plants (4). Future water quality regulations and policies may distinguish between the species of dissolved metals discharged by different pollutant sources. The development of such regulations requires a better understanding of the speciation of pollutant metals in wastewater effluents and surface runoff. Over the past 20 years, numerous researchers have characterized the metal-complexing ligands in surface waters (5, 6). Because it is difficult to identify ligands in the aquatic environment, metal species often are classified according to the nature of the metal-organo complexes that they form (7-14). For the purposes of predicting the fate and effects of metals in aquatic ecosystems, operational measurements are often made to distinguish labile metal species (i.e., metal cations, inorganic complexes, and weak metal-organo complexes) from moderately strong metal-organo complexes (e.g., metal complexes with humic substances) and strongly complexed metals complexed (e.g., metals complexed by polydentate ligands). After metal-containing waters are discharged, speciation changes in response to ligands and metals in the receiving water. Under conditions in which they are thermodynamically unstable, weak and moderately strong metal complexes dissociate in minutes (15, 16). In contrast, dissociation of strongly complexed metals is kinetically limited, requiring weeks or months, if it occurs at all (16-18). To predict the fate and effects of metals discharged to aquatic systems, it is therefore important to identify sources of different metal species. A variety of analytical techniques has been used to measure metal speciation, but few have been applied successfully in the complicated matrices encountered in surface runoff or wastewater effluents. Anodic stripping voltammetry (ASV) and ion-selective electrodes are of limited use because they lack the necessary sensitivity and are subject to interference from surface-active compounds (19-21). Competitive ligand equilibrium-cathodic stripping voltammetry (CLE-CSV) and chelating resin column partitioning-graphite furnace atomic absorption spectroscopy (CRCP-GFAAS) have been used successfully in studies of coastal and estuarine waters (2226). CLE-CSV has never been used in wastewater effluents, surface runoff, or highly polluted waters. CRCP-GFAAS has been employed on a limited number of samples collected from polluted waters (27-30) but has never been used in conjunction with a technique, such as CLE-CSV, that is capable of discriminating between strong and moderately strong metal-organo complexes. The objective of this study was to determine the speciation of Cu and Ni in wastewater effluents and surface runoff discharging into South San Francisco Bay. Cu and Ni were chosen because their concentrations in South San Francisco Bay (31, 32) often exceed water quality objectives (i.e., 80 and 140 nM, respectively). Using techniques that distinguish between moderately strong and strong metal complexes, it is demonstrated that significant concentrations of strongly complexed metals are present in wastewater effluents and surface runoff. With knowledge of metal speciation in wastes from different sources, municipalities can target their control efforts to protect aquatic ecosystems from adverse effects of pollutant metals.
Materials and Methods Chemicals. Unless otherwise specified, all chemicals were purchased from Fisher Scientific. Most solutions were prepared by dissolving chemicals of the highest available purity in Milli-Q water. Stock solutions of 0.1 M dimethylglyoxime (DMG) and 0.01 M 8-hydroxyquinoline (8-HQ)
S0013-936X(97)00271-X CCC: $14.00
1997 American Chemical Society
FIGURE 1. Map of South San Francisco Bay indicating locations where samples were collected. Modified from ref 33 with permission from AAAS. were prepared in methanol (HPLC grade). A 0.1 M solution of tropolone (Arcos Chemical Co.) was prepared in 0.035 M NH4OH (trace metal grade). All bottles, filters, and tubing used for sample collection or storage were made of Teflon, polypropylene, or polyethylene. All equipment was cleaned in Micro detergent (Baxter Scientific), rinsed in methanol (HPLC grade), and soaked in 2 N HCl (reagent grade) for 1 week. Sample manipulation was conducted under a laminar flow hood. Study Site and Sample Collection. Samples were collected from the three water pollution control plants (WPCPs) and the two largest surface waters that discharge into San Francisco Bay south of the Dumbarton Bridge (Figure 1). This area is of particular concern because the dilution of pollutants is limited by the slow exchange of water south of the Dumbarton Bridge. Descriptions of the hydrology and ecology of South San Francisco Bay are available elsewhere (33). All three WPCPs are equipped with primary, secondary, and tertiary treatment systems. The San Jose/Santa Clara plant (SJSC WPCP) is the largest WPCP. It discharges approximately 450 × 106 L day-1 (34, 35) of treated effluent. The Palo Alto and Sunnyvale WPCPs discharge approximately 71 × 106 and 65 × 106 L day-1 of treated wastewater, respectively (35). Wastewater effluent was collected using trace metal clean procedures (31). A peristaltic pump equipped with Bev-line and Masterflex tubing (Cole-Parmer) was used to draw samples through an in-line 0.45 µm polypropylene filter cartridge (MSI Inc.). Samples were collected in acid-cleaned double-bagged bottles and stored at 4 °C until analysis. Surface runoff samples were collected from the Guadalupe River and Coyote Creek, approximately 5 km from where they
discharge to San Francisco Bay. Both sampling sites, which are located proximate to USGS monitoring stations, receive runoff from urban and agricultural areas. Average rainy season (i.e., December through May) discharges from the Guadalupe River and Coyote Creek are approximately 400 × 106 and 200 × 106 L day-1 , respectively (36). Dry weather discharges average 30 × 106 and 130 × 106 L day-1, respectively (36). At each site, water samples were collected prior to and approximately 1 week after the first significant rain of the season. At the Guadalupe River site, an additional sample was collected near the middle of the rainy season (February 4, 1997). Grab samples were collected by submerging sample bottles approximately 10 cm below the water surface. In addition, 2 h composites were collected during the first significant rain of the season (October 29, 1996) using an automated sampler (ISCO) with polypropylene sampling line. All samples were filtered through acid-cleaned 0.45 µm polypropylene filters within 24 h of collection. Total suspended solids (TSS) were measured in selected surface runoff samples (37). Samples from San Francisco Bay were obtained from the R. V. David Johnston at 1 m depth during August, 1996 using trace metal clean sampling techniques (31, 32). Total and Dissolved Cu and Ni. [Cu] and [Ni] were determined in filtered and unfiltered samples by graphite furnace atomic absorption spectroscopy (GFAAS) (PerkinElmer 3300 spectrometer; HGA-600 furnace) using standard conditions (37). Samples were analyzed in duplicate by standard additions. Quality assurance samples (NIST SRM 1643d) and method blanks were analyzed with each set of samples.
VOL. 31, NO. 10, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3011
[Nidiss] in some samples could not be determined with sufficient precision by GFAAS because concentrations were near or below the method detection limit. In these samples, [Nidiss] was determined using CLE-CSV (discussed in the following section) after UV-oxidation of the organic ligands. Samples were placed in 8 mL stoppered quartz tubes and irradiated with a 500 W Hg lamp (Ace Glass) for 7 h. UVoxidation followed by CLE-CSV has yielded similar results to GFAAS in previous studies (26) and in samples that we analyzed using both methods. Colloid-associated Cu and Ni in wastewater effluents were measured by dialysis. Dialysis tubing (Spectrum Medical; 3000 MW cutoff; 80% of the Cu discharged by the SJSC and Palo Alto WPCPs and 30% of the Cu discharged by the Sunnyvale WPCP. It also accounted for approximately 60% of Cutot in surface runoff. [Nitot] in wastewater effluents and in surface runoff ranged from 95% of the Nitot in all but two of the wastewater effluent samples. Dissolved Ni accounted for approximately 45% of the Nitot in the surface runoff samples. In the dialysis experiment, more than 95% of the dissolved Cu and Ni in effluent from the SJSC WPCP passed through the dialysis membrane within 2 days.
FIGURE 3. Typical results for CLE-CSV titrations in surface runoff and wastewater effluents. (A) Cu speciation (using 8-HQ) in wastewater effluent from the Palo Alto WPCP. (B) Ni speciation in wastewater effluent from the SJSC WPCP (3/18/96).
FIGURE 4. Typical results for CRCP-GFAAS titrations in surface runoff and wastewater effluents. (A) Cu from the Palo Alto WPCP. (B) Ni from the SJSC WPCP (3/18/96). Error bars are included for all measurements, but in many cases are smaller than the symbols.
Speciation of Cu and Ni by CLE-CSV. All samples exhibited a linear relationship between the height of the reduction peak (ip) and [Cudiss] at concentrations less than approximately 300 nM (Figure 3A). The sensitivity of the analysis, as measured by the slope of the lines, was significantly higher (p > 0.95) in artificial wastewater effluent than in wastewater effluents or surface runoff. Decreases in the sensitivity of CLE-CSV are consistent with the presence of surface-active compounds (19-21). In effluent from the Sunnyvale WPCP and two surface water samples, the sensitivity of CLE-CSV using tropolone was too low for quantitative speciation measurements. For these samples, acceptable results were obtained using 8-HQ. When tropolone and 8-HQ were both used on different aliquots of an effluent sample collected from the Palo Alto WPCP, higher concentrations of strongly complexed Cu were detected for tropolone (39 ( 8 nM) than for HQ (8 ( 2 nM). These differences were expected because 8-HQ forms stronger complexes with Cu2+ than tropolone. The value of ip in samples to which Cu was not added prior to analysis provides information on the concentration of Cu species that form complexes with the competing ligand (i.e., HQ or tropolone). We refer to these species as Cu*CSV. [Cu*CSV] is calculated by dividing ip, measured in samples to which Cu had not been added, by the slope obtained in the Cu titration. [Cu*CSV] can be used to calculate [CuLCu 1 ], the strongly complexed Cu remaining after the competing ligand is added:
proximately 75% of Nidiss in wastewater effluents, whereas an average of 25% of Nidiss consisted of NiLNi 1 in surface runoff (Figure 2b). Speciation of Cu and Ni by CRCP-GFAAS. Titration of surface runoff and wastewater effluents with Cu or Ni prior to CRCP-GFAAS (Figure 4) yielded qualitatively similar results. At Cu or Ni concentrations greater than approximately 300-600 nM, the slopes of the titration curves were approximately equal to unity, indicating that the chelating capacity of the sample had been exceeded. In all samples to which Cu or Ni was not added prior to analysis, a small percentage of the Cu or Ni present in the sample was retained on the chelating resin column (i.e., mean ) 24% for Cu and 21% for Ni). This value, referred to as Me′CRCP, provides an estimate of species that are present as metal cations, inorganic complexes, and CRCP-labile metalorgano complexes. Moderately strong metal complexes (i.e., [MeL2]) were calculated from [MeL1], [Mediss], and [Me′CRCP]:
[CuLCu 1 ] ) [Cudiss] - [Cu* CSV]
(1)
CuLCu 1 was present at concentrations below 40 nM in all samples. The percentage of [Cudiss] that could be accounted for by CuLCu 1 varied from 5-25% in wastewater effluent from the Palo Alto WPCP to more than 80% in samples from the Sunnyvale WPCP and in the mid-winter sample from the Guadalupe River (Figure 2A). All samples tested exhibited linear relationships between [Nidiss] and ip at [Nidiss] below approximately 1000 nM (Figure 3B). The sensitivity of the analysis was similar for each matrix (p > 0.36), indicating insignificant interference from surfaceactive compounds. A relationship analogous to eq 1 can be used to calculate Ni [NiLNi 1 ] from [Ni* CSV]. In contrast to the Cu results, NiL1 exhibited distinctive trends in wastewater effluent and surface runoff samples: NiLNi 1 accounted for an average of ap-
[MeL2] ) [Mediss] - [MeL1] - [Me′CRCP]
(2)
accounted for more than 50% of [Cudiss] in the CuLCu 2 Guadalupe River (10/15/96) and the Palo Alto WPCP and
