Douglas Evans

Molecular size distribution characteristics of the metal–DOM complexes in stream waters by high-performance size-exclusion chromatography (HPSEC) and high-resolution inductively coupled plasma mass spectrometry (ICP-MS)

In this work, the complexation of the metals Fe, V, Ce, Th, U, Mo, Cu, Ni, Co, Cr, Zn, Pb and Cd with different molecular size (MS) fractions of dissolved organic matter (DOM) in natural waters was investigated. In order to assess the MS distribution of the metal–DOM complexes, DOM samples from stream waters were concentrated by reverse osmosis, and were then analyzed by high-performance size-exclusion chromatography (HPSEC) coupled with on-line UV-Vis absorbance and high-resolution inductively coupled plasma mass spectrometry (ICP-MS). The MS distribution of overall DOM and its metal-bound complexes was evaluated. The results indicate the following order of decreasing number-averaged molecular weight: Cu > Ni > (Co, Zn, Cr) > Pb > Cd for the DOM-bound complexes of transitional metals, which is consistent with Irving-Williams series, and (Fe, V, Ce) > Th > U > Mo for the DOM-bound complexes of the other metals. The results suggest that the metal distribution among the different MS fractions was closely related to metal binding strength; metals with high binding strength were distributed more in the larger MS fractions, and metals with low strength were distributed more in the smaller MS fractions. Possible mechanisms for these observations were discussed. This study is significant to the understanding of metal complexation with DOM in natural waters.

Hyphenation of flow injection on-line preconcentration and ICP-MS for the rapid determination of 226Ra in natural waters

A novel, sensitive, and rapid method is developed for the preconcentration/separation of 226Ra in natural waters using a flow injection (FI) design based on a sequential on-line ion exchange preconcentration coupled with ICP-MS. 226Ra is extracted using an Ln resin (HDEHP) from the sample at pH 10.6 in the presence of ethylenediamine tetraacetic acid (EDTA) diammonium salt, whereas Ca and Mg, which form stable complexes with EDTA, are not retained on the column. Quantitative elution of 226Ra from the Ln resin is achieved using 5 M HNO3. The separation of Ra from Ba and Sr is subsequently performed by the selective retention of Sr and Ba on an Sr*Spec resin, placed between the Ln resin and the Apex-Q system. In addition to being accurate and rapid for 226Ra determination, this method is effective in eliminating spectral (88Sr138Ba) and non-spectral (Ca, Mg) interferences. Using a 20 mL sample, a detection limit (3σ) of 457 fg L−1 (16.92 mBq L−1) was obtained with a sample throughput of 3.5 samples h−1. The precision for 6 replicate measurements of 5 pg L−1 (185.18 mBq L−1) 226Ra was better than 4%. The method was applied to the analysis of natural waters, where the concentration of 226Ra was found to be in the range 1.2 to 2.4 pg L−1 (44.4 to 88.8 mBq L−1), lower than the maximum acceptable concentration (MAC) for 226Ra of 16.2 pg L−1 (600 mBq L−1), recommended in the Guideline for Canadian Drinking Water Quality.

Method intercomparison for the analysis of 239/240Pu in human urine

Following a radiological or nuclear emergency, medical intervention requires rapid assessment of the exposure of people usually through determination of internal dose. For the plutonium urine bioassay, besides thermal ionization mass spectrometry (TIMS) and alpha spectrometry methods, inductively coupled plasma mass spectrometry (ICP-MS) methods have been recently developed, which can provide much higher sample throughput. In this work, three ICP-MS methods were compared with one TIMS method and two alpha spectrometry methods for the measurement of 239Pu and 240Pu in human urine samples spiked at different concentration levels. The sample throughputs for all three ICP-MS methods are similar: each instrument measures about 80 samples in the first 24 hours and 200 samples in the first 48 hours following the emergency event, if the samples arrive at the laboratory 8 hours after the event occurs. Method accuracy and precision were determined using ANSI N13.30. Method detection limits and minimum detectable amounts (MDA) were determined to evaluate method sensitivities. The sensitivities of the three ICP-MS methods were also compared with the derived urine action level (24 h urine, 500 mSv committed effective dose equivalent, inhalation exposure, maximum dose conversion factor) to evaluate their applicability to exposure situations.

Uranium bioaccumulation in a freshwater ecosystem: impact of feeding ecology.

The objectives of our study were: (1) to determine if there was significant uranium (U) bioaccumulation in a lake that had been historically affected by a U mine and (2) to use a combined approach of gut content examination and stable nitrogen and carbon isotope analysis to determine if U bioaccumulation in fish was linked to foodweb ecology. We collected three species of fish: smallmouth bass (Micropterus dolomieu), yellow perch (Perca flavescens) and bluegill (Lepomis macrochirus), in addition to several invertebrate species including freshwater bivalves (family: Sphaeriidae), dragonfly nymphs (order: Odonata), snails (class: Gastropoda) and zooplankton (family: Daphniidae). Results showed significant U bioaccumulation in the lake impacted by historical mining activities. Uranium accumulation was 2-3 orders of magnitude higher in invertebrates than in the fish species. Within fish, U was measured in operculum (bone), liver and muscle tissue and accumulation followed the order: operculum>liver>muscle. There was a negative relationship between stable nitrogen ratios ((15)N/(14)N) and U bioaccumulation, suggesting U biodilution in the foodweb. Uranium bioaccumulation in all three tissues (bone, liver, muscle) varied among fish species in a consistent manner and followed the order: bluegill>yellow perch>smallmouth bass. Collectively, gut content and stable isotope analysis suggests that invertebrate-consuming fish species (i.e. bluegill) have the highest U levels, while fish species that were mainly piscivores (i.e. smallmouth bass) have the lowest U levels. Our study highlights the importance of understanding the feeding ecology of fish when trying to predict U accumulation.

Influence of body size on Cu bioaccumulation in zebra mussels Dreissena polymorpha exposed to different sources of particle-associated Cu

Size of organisms is critical in controlling metal bioavailability and bioaccumulation, while mechanisms of size-related metal bioaccumulation are not fully understood. To investigate the influences of different sources of particle-associated Cu on body size-related Cu bioavailability and bioaccumulation, zebra mussels (Dreissena polymorpha) of different sizes were exposed to stable Cu isotope ((65)Cu) spiked algae (Chlorella vulgaris) or sediments in the laboratory and the Cu tissue concentration-size relationships were compared with that in unexposed mussels. Copper tissue concentrations decreased with mussel size (tissue or shell dry weight) in both unexposed and algal-exposed mussels with similar decreasing patterns, but were independent of size in sediment-exposed mussels. Furthermore, the relative contribution of Cu uptake from algae (65-91%) to Cu bioaccumulation is always higher than that from sediments (9-35%), possibly due to the higher bioavailability of algal-Cu. Therefore, the size-related ingestion of algae could be more important in influencing the size-related variations in Cu bioaccumulation. However, the relative contribution of sediment-Cu to Cu bioaccumulation increased with body size and thus sediment ingestion may also affect the size-related Cu variations in larger mussels (tissue weight >7.5mg). This study highlights the importance of considering exposure pathways in normalization of metal concentration variation when using bivalves as biomonitors.

Uptake of Dissolved Organic Carbon-Complexed 65Cu by the Green Mussel Perna viridis

Stable Cu isotope ((65)Cu) was complexed with various representative dissolved organic carbon (DOC) types, including coastal seawater DOC, fulvic acid (FA), cyanobacteria spirulina (SP) DOC, histidine (His), cysteine (Cys), and lipophilic diethyl dithiocarbamate (DDC) at different concentrations. The uptake of these dissolved Cu species by the coastal green mussel Perna viridis was quantified for the first time. Copper complexed with different DOC types were taken up in some measure by mussels, depending on the DOC types. However, complexation generally reduced Cu uptake as compared to that of inorganic Cu species, and DOC type-specific negative relationships were found between DOC levels and Cu uptake. Strong Cu binding sites (including His and organic sulfur functional groups) within DOC appeared to control the inhibitory effects of DOC on Cu uptake, possibly due to the competitive binding of Cu between the dissolved phase and biological membranes. Therefore, differences in strong Cu binding site levels may explain the differences in bioavailability of Cu complexed with different types of DOC. At the same time, the variations in Cu-DOC uptake may also be partly attributed to the absorption of Cu-DOC complexes, especially for the small Cu-DOC complexes (e.g., Cu-Cys, Cu-His, or Cu-DDC). Our study highlights the importance of considering the specificity of Cu-DOC complexes when assessing biological exposure to dissolved Cu in natural waters, especially during events, such as phytoplankton bloom periods, that could modify DOC composition and concentrations.

Effects of aging on the digestive solubilization of Cu from sediments.

Solubilization of particulate Cu by different solutions, mimicking digestive fluids of deposit-feeders, was quantified in stable isotope (65)Cu-spiked sediments (with 3 days-2 months Cu-sediment contact time or aging). Copper solubilization generally decreased with prolonged aging. However, such decrease became less evident after 1 month and equilibrium of Cu in sediments could be reached after 2 months. Aging effects on Cu solubilization can be explained by the changes in Cu geochemical fractionation with aging: Cu generally transferred from more mobile phases (carbonate and Fe-Mn associated) to more refractory phases (organic associated and residual phase). Besides Cu geochemical fractionation, digestive fluid composition and different Cu solubilization pathways involved, as well as sedimentary organic content, could all affect the digestive solubilization of Cu and its change with aging. Our results emphasize the necessity of considering Cu aging in laboratory sediment toxicity experiments, and in risk assessment of Cu contaminated sediments.

Influence of contact time and sediment composition on the bioavailability of Cd in sediments.

Stable isotope (111)Cd was spiked into sediments of different organic content levels for 3 days to 2 months. Bioavailability of spiked Cd to deposit-feeders, assessed by in vitro Cd solubilization, generally decreased with contact time but became comparable with that of background Cd after 2 months. This could be explained by the gradual transfer of Cd from the more mobile geochemical phase (carbonate associated phase) to more refractory phases (Fe-Mn oxide associated phase, and organic associated phase) within 2 months. The sedimentary organic content had a weak effect on Cd solubilization, while the distribution of Cd in carbonate or Fe-Mn oxide associated phase could have a larger influence on the solubilization of sedimentary Cd and its change with contact time. The observations in this study emphasize the need to consider Cd sequestration over time in sediments of various compositions, which would be useful in risk assessment of contaminated sediments.

The impacts of ontogenetic dietary shifts in yellow perch (Perca flavescens) on Zn and Hg accumulation.

Yellow perch (Perca flavescens) undergo several ontogenetic dietary shifts, and consequently these fish feed at different trophic levels and rely on different carbon sources over their lifetime. Stable isotope ratios of nitrogen (δ(15)N) and carbon (δ(13)C) are powerful ecological tools that are used to provide a temporally integrated description of the feeding ecology of aquatic animals such as fish. The main objective of this study was to use stable isotopes of nitrogen and carbon to determine if dietary changes affected mercury (Hg) and zinc (Zn) accumulation in yellow perch ranging in size from approximately 5 cm to 27 cm. Results showed that Hg bioaccumulation generally increased with increasing trophic level in fish feeding at higher trophic levels, however, the relationship between Hg levels and δ(15)N was non-linear showing no relationship in small fish (less than 15 cm). In contrast, there was a negative, linear relationship between δ(15)N and Zn, suggesting that as perch fed at progressively higher trophic levels, less of Zn bioaccumulated. No relationship was observed between δ(13)C and metal levels in perch. Collectively, these results demonstrate a contrast in the behavior of Zn and Hg bioaccumulation in yellow perch as a function of trophic status.

Temporal and spatial variation in Hg accumulation in zebra mussels (Dreissena polymorpha): Possible influences of DOC and diet

Zebra mussels (Dreissena polymorpha) are filter feeders located near the base of the foodweb and these animals are able to utilize a variety of carbon sources that may also vary seasonally. We conducted both a spatial and a temporal study in order to test the hypotheses: (1) dissolved organic carbon (DOC) concentrations influence Hg accumulation in zebra mussels sampled from a series of lakes and (2) seasonal variations in diet influence Hg accumulation. In the spatial study, we found a significant negative relationship between Hg concentrations and DOC concentrations, suggesting an influence of DOC on Hg bioaccumulation. In the temporal study, we used stable isotope ratios of nitrogen (δ(15)N) and carbon (δ(13)C) as ecological tools to provide a temporally integrated description of the feeding ecology of zebra mussels. Both δ(15)N and δ(13)C varied seasonally in a similar manner: more depleted values occurred in the summer and more enriched values occurred in the fall. Mercury concentrations also varied significantly over the year, with highest concentrations occurring in the summer, followed by a progressive decrease in concentrations into the fall. The C/N ratio of zebra mussels also varied significantly over the year with the lowest values occurring mid-summer and then values increased in the fall and winter, suggesting that there was significant variation in lipid stores. These results indicate that in addition to any effect of seasonal dietary changes, seasonal variation in energy stores also appeared to be related to Hg levels in the zebra mussels. Collectively results from this study suggest that DOC concentrations, seasonal variation in diet and seasonal depletion of energy stores are all important variables to consider when understanding Hg accumulation in zebra mussels.