Environ. Sci. Technol. 2006, 40, 3794-3799
Metal-Solid Interactions Controlling the Bioavailability of Mercury from Sediments to Clams and Sipunculans HUAN ZHONG AND WEN-XIONG WANG* Atmospheric, Marine, and Coastal Environment Program and Department of Biology, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong
The bioavailability of sedimentary Hg(II) and methylmercury (MeHg) was quantified by measuring the assimilation efficiency (AE) in the clam Ruditapes philippinarum and the extraction of the gut juices from the sipunculan Sipunculus nudus. Three factors (Hg concentration in sediment, Hg sediment contact time, and organic content of sediments) were modified to examine metal-solid interactions in controlling Hg bioavailability. The Hg AEs in the clams were strongly correlated with the extraction from the sipunculan gut juices for both Hg species. The bioavailability of both Hg(II) and MeHg generally increased with increased sediment Hg concentration but decreased with sedimentmetal contact time and increasing organic content (except that MeHg was not influenced by organic content). Hg(II) speciation in sediments, quantified by sequential chemical extraction (SCE), was dependent on geochemical conditions and greatly controlled the mobility and bioavailability of Hg(II) in sediments. Most bioavailable Hg(II) originated from the strongly complexed phase (e.g., Hg bound up in Fe/Mn oxide, amorphous organosulfur, or mineral lattice), whereas Hg bound with the organocomplexed phase (Hg humic and Hg2Cl2) was not bioavailable. Hg bound with the other geochemical phases (water soluble, HgO, HgSO4, and HgS) contributed very little to the bioavailable Hg due to their low partitionings. Further, the amount of bioavailable Hg was inversely related to the particle reactivity of Hg with the sediments. Detailed analyses of metal-solid interactions provide a better understanding of how Hg in sediments can predict Hg concentration and therefore bioavailability in benthic invertebrates.
Introduction In aquatic environments, ingestion of sediments may account for up to 100% of the total amount of metal accumulated in some deposit-feeding invertebrates (1, 2). The mobility and bioavailability of sedimentary metals are thus critical in the risk assessment of contaminated sediments. Sediment geochemistry (e.g., organic carbon content and metal speciation) varies among different sediments and has a great control over the bioavailability of metals in sediments (3-7). Several studies have identified a few geochemical phases * Corresponding author phone: (852) 23587346; fax: (852) 23581559; e-mail:
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(e.g., easily exchangeable, reducible, sulfide-based) in sediments that control metal bioavailability (8-11). Therefore, understanding metal-solid interactions is a necessary first step in assessing the bioavailability of metals (12). Mercury, especially the methylated form (e.g., methylmercury or MeHg), can be toxic to aquatic organisms at very low environmental concentrations (subnanomolar levels) (12). In aquatic environments, the dominant forms of Hg are the inorganic Hg(II) and MeHg. Geochemical factors such as the organic carbon content may reduce the bioavailability of sedimentary-bound Hg to benthic organisms (13, 14). Previous studies also considered the influences of metalsolid interactions, especially partitioning among various geochemical components in sediments, on metal bioaccumulation by benthic invertebrates (15-17). However, the inherent methodological limitations in determining the solid speciation of mercury (e.g., high detection limit and defined geochemical species) have impeded our understanding of the geochemical controls on mercury bioavailability. The recently developed sequential selective extraction (or sequential chemical extraction, SCE) method provides a new tool to separate inorganic mercury in solids into biogeochemically relevant fractions and to assess the mobility of Hg(II) in sediments (18). It is thus now possible to use SCE to examine the geochemical distribution of mercury in sediments and its relationship with Hg bioavailability to different species of benthic invertebrates. Although this method is an empirical indirect one and has some ambiguities, its higher biogeochemical relevance and lower detection limits (0.1-5 ng/g) as compared with previous methods make it a good tool to study the bioavailability of different Hg solid speciations (18). In this study, we modified three factors (spiking Hg concentration, contact time between Hg and sediments, and organic content of sediments) to explore how mercury geochemical speciation controls its bioavailability. We measured Hg assimilation efficiency (AE) in the suspensionfeeding clam Ruditapes philippinarum and the gut juice extraction in the deposit-feeding sipuncula Sipunculus nudus using the radiotracer technique (203Hg). Both dietary assimilation and digestive solubilization are considered key processes in metal accumulation (12). The biomimetic approach or in vitro extraction using gut juices of deposit feeders (19) and direct assimilation efficiency measurements are two current methods for assessing the potential bioaccumulation of sediment-bound metals (20). Metal assimilation from food is associated with the solubilization process during passage through digestive systems (21, 22). We examined both Hg(II) and MeHg. Our aims were to identify the important Hg solid species that possibly affect the uptake by benthic invertebrates. We also explored whether the particle reactivity of Hg is a surrogate to indicate the bioavailability of Hg to benthic invertebrates. In this study, bioavailability is defined as the extent to which the sedimentbound Hg is transported from the sediments to the animals (12) and is quantified by both the assimilation efficiency and the extraction by gut juice as a first-order kinetic parameter.
Materials and Methods Animals and Isotopes. The oxic sediments (64%) extracted by the digestive fluids of the sipunculans were higher than those in previous measurements using the digestive fluids of the polychaete, Arenicola marina (5-40% for MeHg and 5-10% for Hg(II)) (26). Differences in extractant/sediment ratios (0.1-0.8 g/mL used in previous studies as compared to 0.025 g/mL in our study) may partially account for these differences since the solubilization process was controlled by the amount of available complexing ligands (e.g., amino acids) (26, 27). In addition, spiking may result in a higher extraction as compared to naturally contaminated sediments. Despite the use of two invertebrate species, there was a significant linear relationship between the AE in clams and the gut juice extraction in sipunculans, at least suggesting that the geochemical factors had similar influences on the bioavailability of Hg to both species. For Hg(II), both the AE and the gut juice extraction increased with increasing Hg concentration in sediments, which may be caused by the increasing binding of Hg(II) with weaker sites (which were more easily extractable) when the stronger sites became saturated (e.g., organic matter) (28, 29). Speciation analysis showed that when the sediment Hg(II) concentration increased from 6.77 to 30.4 µg/g (the highest Hg concentration tested in this study), the percentage of Hg(II) in the F3 and F4 phases decreased from 96 to 71%, whereas the percent in F2 (stomach acids, such as HgO and HgSO4) increased from 0.3 to 23%. This notable increase in the F2 fraction may explain the significant increase in AE (19-32%) and gut juice extraction (46-60%). In addition, since the metals should be solubilized within the digestive tract before assimilation (21), the binding strength of Hg(II) with particles may have strong control of the bioavailability of sediment-bound Hg. This was further demonstrated by a significant negative correlation between the Hg bioavailability and the Kd. For MeHg, a positive effect of concentration on its AE was found, but the gut juice extraction was comparable at the lower concentrations and decreased at the higher one. Such a decrease may be due to the gradual saturation of binding sites of MeHg in the gut juice, leading to a higher percentage of MeHg binding with sediments and less extraction when the concentration further increased. Meanwhile, there was no correlation between the Kd and the MeHg extraction. In general, differences in MeHg extraction among different experimental treatments were much smaller as compared to Hg(II) extraction. The lack of correlation or major difference in MeHg extraction may be explained by its weak binding with particle sites and thus a high efficiency of extraction by the gut juices. The quantified Kd of MeHg was also a few orders of magnitude lower than that of Hg(II). Contact time may reduce the assimilation of metals (e.g., Cd and Zn) as a result of changes in metal distribution in different geochemical phases (9, 10, 20). We also found that the AE and extraction of both Hg species decreased with a longer contact time between the metal and the sediments, but over a much shorter time frame than with other metals. For example, the AE of Hg(II) decreased by over 6 times (from 20 to 3%) when the contact time increased from 2 to 48 h. Speciation analysis suggested that Hg was dominated by F4, which was relatively independent of the contact time. However, the fraction of Hg in F3 increased, whereas that in F1 decreased with increasing contact time, suggesting that the migration of Hg from the weakly binding sites to the stronger binding sites may partially account for the reduced Hg bioavailability. In addition, given the very low AE (3%) when Hg(II) was spiked for 48 h, the methylation rate was likely negligible within this radiolabeling period, as demonstrated by previous studies (24). VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Generally, the bioavailability of Hg(II) decreased with an increase in LOI, whereas MeHg was much less influenced by the LOI. Speciation of Hg in the ashed sediments was clearly different from other treatments, in which 23 and 37% was in F1 and F3, respectively. Thus, Hg2Cl2 (instead of humic) in F3 was likely important in binding with Hg in the ashed sediments. In our experiments, humic acid coating did not significantly affect the AEs of Hg(II) but reduced the gut juice extraction from 55 to 44%. Gagnon and Fisher (30) reported that organic coatings favored the assimilation of MeHg (the largest change was from 27 to 87% after fulvic acid coating) from ingested particles but had the opposite effect on Hg(II). The organic content can also affect the binding strength (e.g., Kd) and thus the mobility of Hg (31). Hg Speciation. Hg(II) from F1 + F2 + F5 and F4 was extracted by the gut juices of sipunculans, suggesting that Hg(II) associated with these phases was bioavailable, but to different extents, and thus, it contributes differently to the overall gut juice extraction. At our low experimental Hg concentrations (0.69-6.77 µg/g), much less Hg was lost from F1 + F2 + F5 as compared to the loss from F4, but when the Hg concentration increased to 30.4 µg/g, the loss from F1 + F2 + F5 increased accordingly. These data indicated that the mobility of Hg(II) was dependent not only on the Hg speciation but also on its concentration. Hg from F3 was extracted only in the humic acid coating treatment (HA), possibly because of the solubilization of the newly added humic acid, which may also explain the lower MeHg Kd of humic coating sediment than that of the ashed sediment. The extreme immobility of Hg in the organocomplexed fraction may be due to the high affinity of inorganic Hg with organic matter. Overall, our data indicated that Hg in F1 + F2 + F5 contributed a very small fraction to the overall bioavailable Hg due to the low partitioning in these fractions. F3 was the refractory phase and did not contribute to the bioavailable Hg pool in the sediments. F4 may be the initially dominant binding phase for Hg when it was absorbed on the sediments, which may explain the smaller dependence on F4 with different geochemical factors. Hg subsequently migrated to other geochemical phases as the Hg concentration or contact time increased. The concentration of Hg in F4 may also affect the extraction because it can modify the ratio of Hg in sediments to complexing ligands (e.g., amino acids) in the gut. Thus, the extraction was dependent on the contact time or Hg concentration despite the Hg partitioning in F4 remaining rather constant. It is necessary to consider simultaneously the Hg speciation and its concentration in different geochemical phases in interpreting the data. Our study suggests that the key factor determining Hg availability was the Hg distribution in different geochemical phases (especially the organocomplexed and strongly complexed phases), with F4 being the main contributor to the bioavailable Hg pool in sediments and F3 being the refractory Hg pool. However, Hg geochemical distribution in sediment itself could not entirely explain the different bioavailabilities since the Hg concentrations in different geochemical phases may also affect the Hg assimilation. SCE provides a tool to study the geochemical controls of Hg bioavailability to aquatic invertebrates. In addition to these geochemical controls on Hg bioavailability, other environmental conditions such as the nutritional quality of sediments as well as the digestive physiology may also be important in controlling the Hg uptake by invertebrates, but these remain to be further determined. Furthermore, it will be relevant to further examine Hg bioavailability at lower environmental concentrations. 3798
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Acknowledgments We thank the anonymous reviewers for critical comments on this paper. This study was supported by a Competitive Earmarked Research Grant from the Hong Kong Research Grants Council (HKUST6405/05M) to W.-X.W.
Supporting Information Available Figures of Hg(II) or MeHg retention by the clams, extraction by the gut juice of sipunculans, and Hg(II) concentrations in different geochemical pools before and after gut juice extraction. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review November 21, 2005. Revised manuscript received March 21, 2006. Accepted April 13, 2006. ES0523441
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