Determination of mercury methylation rates using A 203-HG radiotracer technique

A radiotracer method for the determination of mercury (Hg) methylation rates in bulk water and water overlying intact sediment cores has been developed. A sediment core with overlying water is collected in a core tube, the overlying water is spiked with high specific activity 203-Hg radiotracer, and the core is incubated at ambient temperature. Aliquots of the overlying water are removed, the Hg is extracted from the sample, and the activity in the extract is measured. A 10–25 fold sample preconcentration is achieved using a dithizone-chloroform extraction technique and a sodium nitrite back extraction step to separate inorganic Hg(II) from monomethylmercury (MMHg). The use of this technique, in conjunction with high specific activity 203-Hg, has allowed for spiking concentrations in the overlying water of approximately 1 ng Hg/L. This spiking level is about the same concentration as the ambient water overlying the core, thus not significantly perturbing the system. Our technique is a significant improvement over previous methodologies which used 203-Hg spike additions of 1 μg Hg/L or higher. The technique was used to measure Hg methylation rates at the Experimental Lakes Area (ELA) in Ontario, Canada during August of 1993 and at an extensively studied estuarine site in Gulf Breeze, Florida, USA during September, 1993 and June, 1994. Multiple cores were collected and spiked with a range of 1 to 11,800 ng Hg (as 203-Hg) into the overlying water. MMHg production at the ELA site indicated rates of 0.25 to 3.7 pg/cm²/day (0.08 to 2.5 % methylation/day). Results from Gulf Breeze were significantly higher at 1.5 to 425 pg/cm²/day or 0.06 to 18 % methylation/day. These rates are one to three orders of magnitude greater than previously measured “specific rates” in bulk water samples and sediments. A direct comparison of rates with previous sediment methylation assay techniques is not possible, however, because of the significant differences between our methodology and previous assay protocols.     

Stores from old reservoirs: Sediment Hg and Hg methylation in Ontario hydroelectric developments

We sampled several Ontario Hydro reservoirs to deteimine the changes in mercury (Hg) and organic profiles of sediment cores with reservoir development We also examined Hg methylation among reservoirs of different age and water chemistry. In sediment cores from four Ontario reservoirs, reservoir (watershed) specific differences appeared to supersede general trends, with the differences between headpond and run-of-river reservoirs particularly important In general, the rate of Hg and organic accumulation appeared to increase with reservoir impoundment, but we were unable to discern consistent changes in concentrations of Hg or loss on ignition (LOI) with reservoir impoundment We also observed significant positive correlations of sediment Hg with concentrations of chlorophyll derivatives and bacterial photopigments. Our results are in agreement with earlier studies which indicated that an increase in Hg supply caused by erosion and release from flooded soils is central to the changes in Hg dynamics within reservoirs following impoundment Although Hg methylation activity of sediments, from a series of reservoirs on the Mississagi River was positively correlated with organic content of the sediment, the observed rates were sufficiently variable that the potential effects of reservoir age or preparation methods could not be resolved. Similarly, in our measurements of net methylmercury (MeHg) flux in two older reservoirs, the large range of variation in net MeHg flux observed among replicate samples obscured the potential effects of differences in geology and water chemistry. Future studies should focus on resolving the underlying causes of this variability and in consolidating the ‘microscale’ measurements obtained using sediment core incubation techniques and the ‘macroscale’ values obtained by whole lake mass balance techniques.     

Denitrification potentials: Measurement of seasonal variation using a short-term anaerobic incubation technique

A short-term anaerobic incubation technique using the C₂H₂ inhibition of N₂O-reductase for comparing denitrification potentials of soils is described. Twenty grams of soil with added NO^?1₃ are incubated in the presence of He and 0.1 atm C₂H₂ at 25°C and 0 soil matric potential for 8 h. N₂O evolution is linear within 60 to 120 min. The denitrification potential of soils stored at 4°C decreased markedly over 21 days of storage in accordance with changes in the available C. Denitrification under an anaerobic atmosphere was observed at 4 C. Denitrification potentials were independent of NO^?3₃ concentrations above 25 μg NO^?₃-N g^?1 soil. Biphasic linear rates of N₂O evolution were observed in one soil. Incubation of this soil with chloramphenicol suggested the first linear phase is attributable to the in situ enzyme activity at the time of sampling. The second linear phase is indicative of the dentrification potential and is attributed to the full induction of denitrifying enzymes. The denitrification potential of a soil was maintained at or close to the maximum for 8 months of the year. During midsummer months the denitrification potential decreased markedly and the soil demonstrated a biphasic rate of denitrification suggesting an in situ denitrification activity less than the maximum potential. Results indicate that the maximum denitrification potential of this soil may often be limited not by NO^?₃ but by available C.     

Influence of incubation time on phosphorus sorption dynamics in lake sediments

Purpose  

Estimating phosphorus (P) sorption is generally achieved by measuring a decrease in P concentration over a defined period. However, information within the literature has not been able to be widely applied, probably due to the variability in incubation periods and initial P concentrations in reported experiments. The objective of this study was to investigate whether variations in incubation time and initial P concentration might result in differences in measured levels of P sorption in sediments.     

Microbial methylation of mercury in upper Wisconsin river sediments

The microbial methylation of Hg was studied in water and sediments from the Upper Wisconsin River by quantifying the methylation of radioactive Hg(II) (²⁰³Hg[NO₃]₂). Methylation activity was near detection limits in the water, highest in surface sediments (0 to 4 cm), and decreased with increasing sediment depth. Methylation had a temperature optimum of 35 °C. Organically enriched sediments exhibited higher methylation activity than less eutrophic sediments. Methylation activity in sediments was stimulated by the addition of peptone but not by glucose or starch. Oxygenation of sediments inhibited methylation activity. A summertime peak in methylation activity, observed in water, floc, and sediments, was related to factors other than temperature. More than 98% of the added ²⁰³Hg(II) was bound to sediments within 4 hr of inoculation, while more than 3% was methylated during a 10-day incubation. As much as 7% of the added ²⁰³Hg(II) was methylated in other experiments, suggesting that bound Hg is available for methylation. These data suggest that organic-rich surficial sediments in the Upper Wisconsin River have the potential to produce significant amounts of toxic methylmercury during late summer months.     

Effects of plant rhizosphere on mercury methylation in sediments

Purpose  

This research investigates the effects of different plant species on Hg methylation rates, examines the difference in mercury (Hg) methylation rates in the plant rhizospheres, and shows the trend in methylmercury (MeHg) content of sediment in the Second Songhua River (SSR) of China, thereby providing the basis for estimating the ecological risk of this area.     

Translocation of 203Hg labelled HgCl2 and CH3HgCl in an iron-humus podzol studied by radio-analytical techniques

Since increased Hg-concentrations in fish in lakes and rivers in northern Europe, northern parts of the U.S.A. and Canada were found, environmental Hg research has focused intensively on the factors determining leaching of mercury from soil into water systems. This article presents the results of a leaching experiment with undisturbed soil columns treated with HgCI₂ and CH₃HgCl using radio-analytical techniques. The columns were irrigated with rain of different acidity, rain volumes and irrigation intensities. The leaching of mercury was traced by detecting the vertical distribution of ²⁰³Hg in the soil profiles. Advantages and disadvantages of radioanalytical scanning techniques are discussed. The results of Hg leaching in the soil columns indicate a considerably stronger leaching of monomethyl mercury compared to inorganic mercury. Leaching of the two Hg-species is ruled by competition of H⁺ induced soil-Hg desorption with DOM-Hg complex formation; both being affected by rain acidity. Rain intensity had no visible effect on leaching of Hg²⁺ and CH₃Hg⁺. An extended rain duration increased the leaching of CH₃Hg⁺.     

Sulfur and nitrogen mineralization in soils compared using two incubation techniques

Net mineralization of sulfur and nitrogen was studied in three Canadian Prairie soils using two commonly used incubation methods. In the open system technique, where the soils were leached periodically II.3–11.8 μ g SO^2?₄ -S g^?1 soil was mineralized in 17 weeks. Little mineralization or a net immobilization of sulfur (from 1.4 to 1.3 μ g SO^2?₄-S g^?1 soil) was observed in a closed system where the soils were left undisturbed throughout incubation. Changes in the specific activity of ³⁵S-labelled soil solution sulfate during the closed incubation indicated that mineralization-immobilization processes were occurring simultaneously resulting in minimal net changes in CaCl₂-extractable SO_2?⁴ concentrations. The amounts of mineralized nitrogen (32.6–57.8 μg N g_?1 soil) were found to be independent of the incubation method employed.     

Mercury methylation by sulphate-reducing bacteria from sediments of an acid stressed lake

The role of freshwater sulphate-reducing bacteria in McHg production was examined by adding specific microbial inhibitors to anoxic lake sediments spiked with ²⁰³HgCl₂ and measuring net methylation. The effect of increased sulphate (such as would arise from acid deposition in the area) on the activity of sulphate-reducing bacteria both in terms of sulphate reduction rate and methylation of Hg was examined by adding sulphate to ²⁰³HgCl₂ spiked sediments. Sodium molybdate (10 mM), a specific inhibitor for sulphate-reducing bacteria, reduced the amount of MeHg produced from anoxic lake sediments by 75% compared with controls over a 7 d period. In contrast, 2-bromoethane sulfonic acid (15 mM), a specific inhibitor for methanogenic bacteria, did not alter the amount of McHg produced. Additions of sulphate that were “realistic” in terms of the normal range of the area (5 to 30 mg.L^?), increased the sulphate reduction rate of sediment slurries. However, in the experimental system, these additions did not stimulate McHg production. In our study, methylation of Hg in sediments was primarily due to the activity of sulphate-reducing bacteria. However, the methylation rate does not appear to be sensitive to the concentration of sulphate over the range typical of softwater, Precambrian Shield lakes. This could be due to reduced availability of Hg due to the formation of insoluble HgS or to the fact that the overall activity of sulphate reducers not being stimulated even if sulphate reduction rate is, or both.     

Development of a Mercury Vapour Air Analyser for Measurement of Hg in Solution

The Tekran 2537A mercury vapour analyser, designed to measure Hg in air by cold vapour atomic fluorescence spectrometry, has been modified to determine Hg in solution. The new ‘front-end’, required to generate Hg° vapour from acidified waters or acid leachates, is described. Using 1% NaBH4 as reducing agent, a 12 mL water sample can be analysed, at a rate of 1 every 6 min, for Hg to a detection limit of 0.8 ppt (ng L-1). Instrumental precision is typically 1% relative standard deviation (RSD) at levels of Hg from 10 to 200 ppt. Results for 10 analyses of the international water standard, NIST 1642b, are 1530±20 ppt Hg, agreeing well with the certified value of 1480±130 ppt. Nineteen geological standard reference materials (soils, sediments and tills) were used to assess accuracy. Results for these samples, digested in aqua regia in triplicate, showed good agreement with recommended values for all but two, SO-3 and TILL-1. However, results by this method for these two standards were confirmed by an independent method, direct atomic absorption spectrometry. Average method precision was shown to be 5% RSD over the range 10 ng g- 1 to 35 μg g-1 Hg.     

基于超声多重反射法测量高衰减流体媒质声阻抗及浓度

利用超声波在固(钢)液界面处声阻抗差异导致的多重反射回波,该文建立了测量被测流体媒质声阻抗及浓度的测量系统.用该系统测量了甲醇、乙醇、蒸馏水和丙三醇4种媒质的声阻抗,与标准值相比,发现测量的误差随被测媒质阻抗增加而迅速减小;同时用系统测量了CaCl2溶液的声阻抗,结果表明溶液阻抗与浓度呈显著线性关系,最大拟合误差小于2.7 g/L.检测中,由于无需声波穿透被测媒质,此法尤其适合于工业中测定管道、容器内高衰减媒质的阻抗及浓度.     

A simplified incubation method using chip‐trays as incubation vessels to identify sulphidic materials in acid sulphate soils

Acid sulphate soils (ASS) can pose a significant hazard to natural ecosystems and developed areas situated within ASS landscapes. Management techniques used to minimize these hazards rely on methods that can classify ASS materials. These methods have traditionally required complex and time‐consuming techniques. A new simplified incubation method, modified from existing ones, was therefore developed to classify ASS materials in a timely manner. The simplified incubation method was found to be a viable alternative where samples cannot be incubated to a stable pH because of logistical or time constraints, but where there is still a need to classify slowly acidifying sulphidic materials. The use of chip‐trays as incubation vessels was also examined. Chip‐trays offer many advantages in terms of transport, storage and analysis of soil samples compared with soil‐slabs. This study establishes an acceptable level of precision ( ± 0.1 pH units with 95% confidence) for pH measurements in the incubation method and confirms that an acceptable level of precision is obtainable when using chip‐trays in the pH incubation method.     

基于连续小波变换和随机森林的芦苇叶片汞含量反演

植物重金属污染是当今世界面临的重大生态环境问题之一,高光谱技术为快速、大面积监测植被重金属含量提供了可能性。本研究以重金属汞(Hg)和湿地植物芦苇为研究对象,采用连续小波变换(CWT)和随机森林(RF)算法相结合的方法建立芦苇叶片总汞含量反演模型,以期寻求一种较为精准的植物汞污染反演模型,未来可通过高光谱技术建立模型来无损、快速估测湿地植物重金属汞污染情况,为湿地生态系统的监测提供方法支持。结果表明:芦苇叶片总汞含量敏感波段主要分布在可见光波段419~522 nm、664~695 nm和724~876nm以及近红外波段1 450~1 558 nm和1 972~2 500 nm;经CWT变换后,小波系数与叶片总汞含量的相关系数绝对值提高0.04~0.18,所构建的预测反演模型拟合效果R~2提高0.107~0.177,模型精度RMSE提高0.008~0.013,其中利用经小波变换的去包络线光谱(CR-CWT)数据建立的RF模型对芦苇叶片总汞含量的反演精度和拟合效果最优(R~2=0.713,RMSE=0.127);同时在土壤总汞含量约为20 mg?kg~(-1)时,采用CR-CWT数据构建RF模型的方法来反演芦苇叶片总汞含量更为准确和可靠(R~2=0.825,RMSE=0.051)。因此,利用RF算法进行植被重金属含量的反演具有一定的现实可行性,而结合CWT后所构建的反演模型对指导植被重金属含量监测更具参考价值,应用前景广阔。     

Preliminary results on the role of rivers in total Hg concentrations in marine sediments and benthic organisms of a coastal area of Italy

The paper reports the results of mercury (Hg) concentrations in the <20 μm grain-size fraction of shallow sediments of the northern Tyrrhenian sea, collected near the mouths of rivers flowing down from the Monte Amiata area (central Italy), which is characterized by cinnabar mineralization which was developped as mercury mine in the past. A few species of benthic marine organisms collected in the sediment sampling area were also analyzed. From the results, it emerged that the rivers contribute to the Hg concentration in the marine sediments which already contain high natural Hg background levels. The benthic organisms, which reflect sediment contamination, showed high Hg concentrations as well.     

Soil core incubation system for the field measurement of denitrification using acetylene-inhibition

Two systems for determining rates of denitrification, both based on the acetylene-inhibition technique, have been compared. One system involved in situ treatment of soil with C₂H₂ using an enclosure (500 × 150mm) placed over the soil surface followed by measurement of nitrous oxide (N₂O) emission. The other involved incubation, in the field, of soil cores with 5% (v/v) C₂H₂ in modified fruit preserving jars. Agreement between the two systems of measurement was close for well-drained soils over a wide range of rates of denitrification (0.005–1.27 kg N ha⁻¹ day⁻¹). Results obtained with poorly-drained soils having low air-filled porosities indicated that the incubation system with soil cores overcame the problems associated with applying the enclosure technique to soils of this type. Denitrification in the incubation system could be terminated by the addition of chloroform after an appropriate period (usually 24 h). The N₂O concentration in the air space of the jars then remained essentially constant for 14 days. This provided a second advantage in allowing the system to be used at sites remote from analytical facilities.     

Land reclamation using reservoir sediments in Tigray,northern Ethiopia

Accelerated soil erosion leads to sedimentation in reservoirs and a decline in their life span. As many reservoirs in northern Ethiopia are dry at the end of the dry season, we were able to evaluate the potential of using reservoir sediments for land reclamation. Stripped land from which construction material for the reservoirs had been excavated was covered with 0, 15 and 30 cm of sediment and planted with a local garlic cultivar (Allium sativum). The applied reservoir sediments had low to medium organic C and total N contents and were high in available P and exchangeable cations. The yield of garlic increased with additional available water and the application of sediments. The results show that total biomass and bulb yield were three times higher on the reclaimed plots than on the control ones (11.7 t/ha vs. 3.6 t/ha for the biomass; 7.7 t/ha vs. 2.0 t/ha for the yield). When sediment transport and labour costs were taken into account, plots with 15 cm of sediments had in the first cropping season a cost‐benefit ratio of 3, whilst those with 30 cm had a cost‐benefit ratio of 0.9. This study demonstrates that the use of relatively small quantities of reservoir sediments is an economically viable strategy for land reclamation. The result can be improvement in income for resource‐poor farmers by as much as 76%, and the life expectancy of the reservoirs is also increased.     

Modelling post-depositional transport of PAHs in aquatic bed sediments using CoReTranS

Purpose

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous and persistent contaminants in aquatic bed sediments. A better understanding of their in-bed fate and transport is therefore key in minimising the risk to the environment over time through various remediation and monitoring strategies. Since ecological effects and risks are related to contaminant concentrations, this study developed CoReTranS, a predictive model that simulates one-dimensional organic contaminant reaction and transport in bed sediments.

Materials and methods

CoReTranS was benchmarked against analytical solutions of simplified reactive transport models and validated using a published study of marsh sediments contaminated with petroleum-derived hydrocarbons from Wild Harbour, West Falmouth, MA, USA.

Results and discussion

The CoReTranS model effectively predicted the vertical distribution of PAHs in the Wild Harbour sediments as confirmed by the modelling results from the published study. The CoReTranS model was also used to interpret results from a published study of PAH-contaminated fjord sediments from Kitimat Arm in British Columbia, Canada. Specific insights into the post-depositional fate and transport of selected PAHs in the Kitimat fjord sediments were obtained by comparing the measured concentration-depth profiles with the numerical results from the CoReTranS model. Key parameters such as effective diffusivity of contaminants and burial velocities of sediment particles were shown to possibly account for the predicted concentrations-depth profiles in the Kitimat fjord sediments.

Conclusions

As demonstrated, CoReTranS can simulate reactive transport models in order to predict PAH concentration profiles in porewater under site-specific conditions. The information derived from the use of the CoReTranS model highlighted practical application of such information by engineers to site-specific risk assessment and remediation.     

Simultaneous removal of Cu and PAHs from dredged sediments using flotation

Purpose

This study addresses the feasibility of a flotation technique, using a lab-scale flotation cell, to simultaneously remove both metals and polyaromatic hydrocarbons (PAHs) from fine sediment fractions (<250 μm) that are potentially contaminated with copper (Cu).

Materials and methods

A multiple flotation process with three consecutive flotation stages was performed on three sediments (13S, 14B, and 24A) with different particle size distributions, Cu and PAH concentrations, and organic matter contents.

Results and discussion

Flotations performed under selected conditions allowed for significant removal of both Cu (61–70 %) and PAHs (75–83 %) with acceptable froth recoveries of approximately 23–29 %. Removal rates for arsenic, lead, and zinc were 48–61, 40–48, and 32–36 %, respectively. Flotation selectivity of Cu was greatly influenced by the contents of fine particles and organic matter of the sediments. The maximum flotation selectivity was obtained for the 53–125-μm size fraction. The high flotation selectivity of Cu (2.5–3.2) and PAHs (3.0–3.6) demonstrated the feasibility of flotation to treat soils or sediments containing both organic and inorganic pollutants.

Conclusions

Overall, the flotation results showed a high selectivity for both Cu and PAHs and demonstrated the feasibility of flotation to treat media contaminated with organic and inorganic contaminants.     

Predicting bioaccessibility of contaminants of emerging concern in marine sediments using chemical methods

Journal of Soils and Sediments - A comparison was made between three chemical methods to predict bioaccessibility of triclosan (TCS), bisphenol A (BPA), and 17α-ethynylestradiol (EE2) in...     

A characterization of the soils and sediments in contaminated sites and rivers using petroleum biomarker compounds

Journal of Soils and Sediments - The objective of this study is to reveal the presence of petroleum biomarkers in sediments of contaminated sites and to prove the existence of these compounds in...