Environ. Sci. Technol. 2002, 36, 5164-5171
Geochemistry of Cd, Cr, and Zn in Highly Contaminated Sediments and Its Influences on Assimilation by Marine Bivalves W E N H O N G F A N , †,‡ W E N - X I O N G W A N G , * ,† A N D JINGSHENG CHEN‡ Department of Biology, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, PR China, and Department of Urban and Environmental Sciences, Beijing University, Beijing 100871, PR China
We tested the controls of metal geochemistry in sediments collected from an extremely contaminated Chinese bay on metal assimilation by marine mussels and clams. Metal speciation in the contaminated sediments, quantified by the Tessier operational extraction method, was significantly dependent on metal concentrations in the sediments. The fractions of Cd in the easily exchangeable and carbonate phases increased, while the reducible and residue phases decreased with increasing Cd concentration. The majority (72-91%) of Cr was associated with the residue component, with the remainder of Cr in the organic matter and reducible phases. Zn in carbonate phase increased, whereas in the organic matter and residue phases it decreased with increasing Zn concentration. The bioavailability of Cd, Cr, and Zn to marine green mussels (Perna viridis) and clams (Ruditapes philippinarum) was quantified using radiotracer spiked technique with concurrent measurements of speciation of spiked metals. There was a significant correlation between the Cd assimilation efficiency (AE) by both mussels and clams and Cd partitioning in the easily exchangeable and reducible phases. In contrast to previous studies, a negative correlation was found between the Cd AE and its total concentration in sediment, likely caused by the saturation of Cd binding sites in the gut or by its antagonistic interaction with a very high Zn concentration in these collected sediments. In contrast, there was no significant correlation between the AEs of Cr or Zn and any of their geochemical phases or their concentrations. The metal AEs were further quantified by experimentally manipulating different concentrations and ratios of acid volatile sulfide (AVS) and simultaneously extractable metals (SEM). There was no statistically significant relationship between the AEs of the three metals and the concentrations of AVS and SEM or [SEM-AVS]. Geochemical controls on metal assimilation from contaminated sediment are therefore only relatively apparent for Cd. The influences of metal speciation on metal bioavailability can be confounded * Corresponding author phone: (852)2358 7346; fax: (852)2358 1559; e-mail:
[email protected]. † The Hong Kong University of Science and Technology (HKUST). ‡ Beijing University. 5164
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 23, 2002
by the degree to which sediments are contaminated with metals.
Introduction Sediments are often the sinks of metal contaminants in aquatic systems (1). Contamination of sediments by metals and organic compounds is now widespread and well documented in numerous Chinese rivers and coastal waters (2, 3). Risk assessment of these contaminated sediments requires knowledge of contaminant bioavailability to sediment-ingesting aquatic animals. The bioavailability of metal contaminants associated with sediment has therefore been studied very extensively over the past two decades (4-8). Important processes in sediment geochemistry and animal physiology (e.g., feeding) in controlling metal bioavailability have been identified in numerous studies (5, 9-12). Metals are bound with different geochemical fractions in sediment, including the operationally defined easily exchangeable, carbonate, reducible, organic matter, and residue phases. It is well-known that metal bindings with different geochemical phases fluctuate greatly among different metals, locations and seasons, adding further complexity to the prediction of metal bioavailability (4, 13-15). Feeding processes controlling the metal uptake from sediments include the time at which the sediment and metal are processed within the animal’s gut, the selectivity of particles, and the ingestion rate of the animals (11, 16). Assimilation efficiency is an important physiological parameter used to quantify metal bioavailability from sediment and has been measured extensively over the past 10 years in diverse aquatic animals, with an aim to delineate the geochemical and physiological controls on metal bioavailability (17). In earlier experimental studies, metal bioavailability from sediment was generally interpreted in the context of metal bioaccumulation and tissue body burden (6). Understanding of geochemical controls on metal bioavailability is often based on concurrent field measurements of metal concentrations in benthic deposit-feeding invertebrates (such as polychaetes and bivalves) and metal concentrations in sediments collected across a wide spatial scale and quantified by various acid extraction and digestion methods (18-22). These experimental studies have generally shown that (1) labile metals (such as those extracted easily by HCl) are more bioavailable to benthic animals; (2) metals bound with the reducible phase (e.g., Fe oxide) are accumulated less by benthic animals, presumably due to the competition of Fe with binding sites in the animal gut tissues; (3) metals bound with the sulfide species are less or not bioavailable to the benthic animals (23, 24); and (4) the influence of organic coating on metal assimilation from sediment is inconsistent (9, 10, 25, 26). However, these relationships appear to be highly specific for different metals, organisms and sites, further highlighting the complexity of understanding metal bioavailability in contaminated sediments. Several previous studies have quantified metal assimilation by marine bivalves from ingested sediments (9, 25-28). In an earlier study, we used pure inorganic particles or artificially prepared sediment to demonstrate that several sediment geochemical pools, including metals in the easily exchangeable, reducible, and sulfide species can considerably affect their assimilation by a marine clam Ruditapes philippinarum (29). In this study, we further tested our previous experimental findings using natural sediments collected from a severely contaminated Chinese bay. Metal contamination 10.1021/es020122m CCC: $22.00
2002 American Chemical Society Published on Web 11/01/2002
FIGURE 1. The sediment sampling stations in Jinzhou Bay. in this bay is probably among the most severe in the world. We quantified the speciation of Cd, Cr, and Zn in the sediments collected on 3 occasions from the bay. The assimilation of these metals by both mussels and clams was quantified using the radiotracer pulse-chase feeding technique, with concurrent measurements of radiotracer speciation in these sediments. Finally, we tested the influences of simultaneously extractable metals and acid volatile sulfide in the contaminated sediment on metal assimilation by the mussels.
Materials and Methods Sediment and Bivalve Collections. Sediments were collected from different locations across a spatial scale of 25 km from Jinzhou Bay in Northern China on three occasions between 2000 and 2001 (Figure 1). A major zinc smelting factory and numerous small factories are located at the inner part of the Bay. Both surface oxic sediment and anoxic sediment were collected. The anoxic sediments were collected from depths below 5 cm and were immediately sealed in bags. After being collected from the field, the sediments were stored at -80 °C. They were finally dried at 50 °C for 3 days and sieved through a 63 µm mesh before the chemical measurements were taken as described below. The two species of bivalves (clam Ruditapes philippinarum, shell length of 2-3 cm, and green mussel Perna viridis, shell length of 2-3 cm) were collected from Tolo Harbor, Hong Kong, during low tide. Both species of bivalves were acclimated in a recirculating seawater aquarium (30 psu) at a room temperature of 23 °C for 1-2 weeks prior to the experiments. The clams and mussels were fed continuously with the diatom Thalassiosira pseudonana (clone 3H) during the acclimation period. One day before the assimilation efficiency measurements, the bivalves were fed with a mixture of sediments and diatoms such that the bivalve digestive enzymes could become acclimated to the sediments. Three metals, Cd, Cr, and Zn, were considered in this study. The assimilation of these metals was quantified by the radiotracer technique, using 109Cd, 51Cr(III), and65Zn as tracers of their respective stable metals. The radioisotopes were obtained from New England Nuclear (Boston, USA). Radioactivity was measured with a Wallac 1480 NaI gamma detector (Turku, Finland). The gamma emission of 109Cd was determined at 88 keV, of 75Se at 264 keV, and of 65Zn at 1115 keV. Sediment Metal Concentration and Speciation Measurements. Total metal concentration in the sediment was measured using established methods (30). Briefly, 0.250 g of dried sediments was digested in 4 mL of concentrated ultrapure HNO3 (70%) and 1 mL of concentrated HClO4 (60%). The glass tubes were placed in an aluminum block using the following sequential heating procedures: 50 °C for 3 h, 70 °C for 0.5 h, 100 °C for 0.5 h, 150 °C for 3 h, and finally 190
°C until complete dryness. After cooling, 10 mL of 2% HNO3 was added into the tubes, which were then placed in the heating block at 70 °C for 1 h. The digested metals were centrifuged at 800 g for 30 min, and the metal concentrations were measured using ICP-AES (Perkin-Elmer 3300 DV). Standard sediment (SRM 1646A, estuarine sediment) was used in the concurrent sediment digestion and measurements. In general, the recovery efficiency was >80% for all metals using acid extraction when compared with the total digestion of standard reference materials using HF. Sediment total organic carbon content (
