Children's exposure to environmental lead: A review of potential sources, blood levels, and methods used to reduce exposure.

Lead has been used for thousands of years in different anthropogenic activities thanks to its unique properties that allow for many applications such as the manufacturing of drinking water pipes and its use as additives to gasoline and paint. However, knowledge of the adverse impacts of lead on human health has led to its banning from several of its applications, with the main goal of reducing environmental pollution and protecting human health. Human exposure to lead has been linked to different sources of contamination, resulting in high blood lead levels (BLLs) and adverse health implications, primarily in exposed children. Here, we present a summary of a literature review on potential lead sources affecting blood levels and on the different approaches used to reduce human exposure. The findings show a combination of different research approaches, which include the use of inspectors to identify problematic areas in homes, collection and analysis of environmental samples, different lead detection methods (e.g. smart phone applications to identify the presence of lead and mass spectrometry techniques). Although not always the most effective way to predict BLLs in children, linear and non-linear regression models have been used to link BLLs and environmental lead. However, multiple regressions and complex modelling systems would be ideal, especially when seeking results in support of decision-making processes. Overall, lead remains a pollutant of concern and many children are still exposed to it through environmental and drinking water sources. To reduce exposure to lead through source apportionment methods, recent technological advances using high-precision lead stable isotope ratios measured on multi-collector induced coupled plasma mass spectrometry (MC-ICP-MS) instruments have created a new direction for identifying and then eliminating prevalent lead sources associated with high BLLs.

Material- and Site-Specific Partition Coefficients for Beneficial Use Assessments.

Partition coefficient (Kd) values available in the literature are often used in fate and transport modeling conducted as part of beneficial use risk assessments for industrial byproducts. Because element partitioning depends on soil properties as well as characteristics of the byproduct leachate, site-specific Kd values may lead to more accurate risk assessment. In this study, contamination risk to groundwater of beneficially reused byproducts was assessed using batch leaching tests on waste to energy bottom ash and coal combustion fly ash. Leachates were equilibrated with eight different soils to obtain the waste-soil-specific Kd,exp values for the metals of interest. The Kd,exp values were used as inputs in the Industrial Waste Management Evaluation Model to demonstrate the degree to which Kd estimates affect risk assessment outcomes. Measured Kd,exp values for the most part fell within the large range of Kd values reported in the literature, but IWEM results using default Kd values for some types of soils resulted in overestimated risk compared to those derived from Kd,exp values. Modeled concentration at the receptor location was much lower for some elements for those soils with high concentrations of iron and aluminum.

Strongly Bound Sodium Dodecyl Sulfate Surrounding Single-Wall Carbon Nanotubes.

NMR techniques have been widely used to infer molecular structure, including surfactant aggregation. A combination of optical spectroscopy, proton NMR spectroscopy, and pulsed field gradient NMR (PFG NMR) is used to study the adsorption number for sodium dodecyl sulfate (SDS) with single-wall carbon nanotubes (SWCNTs). Distinct transitions in the NMR chemical shift of SDS are observed in the presence of SWCNTs. These transitions demonstrate that micelle formation is delayed by SWCNTs due to the adsorption of SDS on the nanotube surface. Once the nanotube surface is saturated, the free SDS concentration increases until micelle formation is observed. Therefore, the adsorption number of SDS on SWCNTs can be determined by the changes to the apparent critical micelle concentration (CMC). PFG NMR found that SDS remains strongly bound onto the nanotube. Quantitative analysis of the diffusivity of SDS allowed calculation of the adsorption number of strongly bound SDS on SWCNTs. The adsorption numbers from these techniques give the same values within experimental error, indicating that a significant fraction of the SDS interacting with nanotubes remains strongly bound for as long as 0.5 s, which is the maximum diffusion time used in the PFG NMR measurements.

Unique toxicological behavior from single-wall carbon nanotubes separated via selective adsorption on hydrogels.

Over the past decade, extensive research has been completed on the potential threats of single-wall carbon nanotubes (SWCNTs) to living organisms upon release to aquatic systems. However, these studies have focused primarily on the link between adverse biological effects in exposed test organisms on the length, diameter, and metallic impurity content of SWCNTs. In contrast, few studies have focused on the bioeffects of the different SWCNTs in the as-produced mixture, which contain both metallic (m-SWCNT) and semiconducting (s-SWCNT) species. Using selective adsorption onto hydrogels, high purity m-SWCNT and s-SWCNT fractions were produced and their biological impacts determined in dose-response studies with Pseudokirchneriella subcapitata as test organism. The results show significant differences in the biological responses of P. subcapitata exposed to high purity m- and s-SWCNT fractions. Contrary to the biological response observed using SWCNTs separated by density gradient ultracentrifugation, it is found that the high-pressure CO conversion (HiPco) s-SWCNT fraction separated by selective adsorption causes increased biological impact. These findings suggest that s-SWCNTs are the primary factor driving the adverse biological responses observed from P. subcapitata cells exposed to our as-produced suspensions. Finally, the toxicity of the s-SWCNT fraction is mitigated by increasing the concentration of biocompatible surfactant in the suspensions, likely altering the nature of surfactant coverage along SWCNT sidewalls, thereby reducing potential physical interaction with algal cells. These findings highlight the need to couple sample processing and toxicity response studies.

An XAS study of Hg(II) sorption to Al-based drinking water treatment residuals.

Drinking water treatment residuals (DWTRs) are produced from the coagulation and flocculation processes in conventional drinking water treatment. The abundant metal oxide content of these materials resulting from the use of coagulants, like alum and ferric chloride, has driven strong research interest into the reuse of DWTRs as sorptive materials. Using a suite of aluminum-based DWTRs, we provide new insights into Hg(II) sorption mechanisms. Experiments performed at circum-neutral pH show that sorption capacities are related to the amount of organic carbon/matter present in DWTRs. We found that carbon rich samples can scavenge about 9000 mg/kg of Hg, in contrast to 2000 mg/kg for lime based DWTRs. X-ray absorption spectroscopy (XAS) at the Hg L3 edge further characterizes mercury coordination. X-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) results point to a partial association of mercury with sulfur at low mass loadings, transitioning to a full association with oxygen/carbon at higher concentrations of sorbed Hg(II) and in DWTRs with limited sulfur content. These results suggest that sorption of Hg(II) is primarily controlled by the carbon/organic matter fraction of DWTRs, but not by the coagulants.

Interactive forces between sodium dodecyl sulfate-suspended single-walled carbon nanotubes and agarose gels.

Selective adsorption onto agarose gels has become a powerful method to separate single-walled carbon nanotubes (SWCNTs). A better understanding of the nature of the interactive forces and specific sites responsible for adsorption should lead to significant improvements in the selectivity and yield of these separations. A combination of nonequilibrium and equilibrium studies are conducted to explore the potential role that van der Waals, ionic, hydrophobic, π-π, and ion-dipole interactions have on the selective adsorption between agarose and SWCNTs suspended with sodium dodecyl sulfate (SDS). The results demonstrate that any modification to the agarose gel surface and, consequently, the permanent dipole moments of agarose drastically reduces the retention of SWCNTs. Because these permanent dipoles are critical to retention and the fact that SDS-SWCNTs function as macro-ions, it is proposed that ion-dipole forces are the primary interaction responsible for adsorption. The selectivity of adsorption may be attributed to variations in polarizability between nanotube types, which create differences in both the structure and mobility of surfactant. These differences affect the enthalpy and entropy of adsorption, and both play an integral part in the selectivity of adsorption. The overall adsorption process shows a complex behavior that is not well represented by the Langmuir model; therefore, calorimetric data should be used to extract thermodynamic information.

High-precision Pb isotopes of drinking water lead pipes: Implications for human exposure to industrial Pb in the United States.

Millions of lead (Pb) pipes are still used in the drinking water distribution systems in many regions in the world. Human exposure to Pb from contaminated drinking water continues to be of concern in the United States (U.S.), as illustrated by the widely publicized "Flint Water Crisis" in 2015. The Pb isotopic composition of Pb-pipes potentially can be useful to identify human exposure to Pb from lead service lines (LSLs). In addition, as the LSLs were likely manufactured from similar industrial Pb sources as other Pb objects and materials in the USA, the Pb-pipes isotope data can provide information about the overall isotopic composition of the U.S. industrial Pb. In this work we present high-precision Pb isotope data from Pb-pipes excavated from different U.S. municipalities. The Pb-pipes show an extremely wide range of Pb isotopic compositions, with 206Pb/204Pb ranging from 17.004 to 22.010, 207Pb/204Pb from 15.460 to 15.921, and 208Pb/204Pb from 36.687 to 41.120. The wide isotope range is observed even in a single town, suggesting that no regional Pb isotope patterns can be expected within the continental USA. However, the high-precision MC-ICP-MS Pb data form a clear linear trend that, depending on the context, can be used to identify human Pb exposure. Furthermore, as the linear trend is a result of utilization of Pb ores from different domestic and international sources and secondary recycling of metallic Pb, it is likely representative of the overall isotopic composition of the U.S. industrial Pb pool. Therefore, the identified trend is the most accurate isotope representation of the U.S. anthropogenic Pb at present and can be used as first-order evaluation to determine if a person with elevated blood Pb levels was exposed to U.S. industrial Pb sources.

Acute and Chronic Toxicity Testing of Drinking Water Treatment Residuals in Freshwater Systems.

The beneficial use of drinking water treatment residuals (DWTRs) faces barriers due primarily to uncertainties and concerns about their potential environmental impacts. We used total and water leachable toxic metal concentrations and 2 benthic organism-based bioassays to identify suitable DWTR substrates for introduction to freshwater systems. Using total metal contents and the consensus probable effect concentration concept, 3 DWTRs were selected and used in elutriate and toxicity studies. The concentrations of water leachable Ag, As, Cd, Cu, Cr, Ni, Pb, and Zn were below the US Environmental Protection Agency's ambient water quality criteria. Using the long-term 65-d life cycle Chironomus tentans test and 4 different endpoints (survival, adult emergence, egg case production, and number of eggs produced per female), no statistical differences were found between the DWTR treatments and the controls. Similarly, results obtained using the 10-d Hyalella azteca test showed no toxicity. However, although both survival and growth were recorded in all bioassays, the results of the 10-d C. tentans and the 28-d H. azteca tests were ambiguous. For C. tentans, 2 of the 3 DWTRs resulted in significantly lower survival rates compared to the controls. For H. azteca, no significant growth differences were observed between controls and DWTR treatments, but 2 of the 3 DWTRs resulted in significantly lower survival rates than the controls. Overall, these results suggest that certain DWTR substrates could be suitable for introduction to aquatic systems. Environ Toxicol Chem 2021;40:2005-2014. © 2021 SETAC.

Removal of type-A, type-B, and borderline metals from contaminated soils using zero valent iron and magnetic separation technology: A predictive approach for metal resources recovery.

Despite the limitations reported on the efficiency of metals used as sorbents, recent advances in chemical and material sciences make it possible to use remediation technologies based on zero valent iron (ZVI) to restore the ecosystem services of metal-contaminated soils. In addition, recent studies showed that remediation by in situ immobilization could be avoided by taking advantage of the strong magnetic characteristics of ZVI. We combined these well-established concepts and conducted laboratory experiments to predict the removal efficiency of metals from contaminated soils based on their chemical classification into type-A, type-B and borderline metals. The Nieboer-Richardson separation of metal ions based on covalent and ionic indexes was used, and beryllium (Be2+), mercury (Hg2+) and lead (Pb2+) were selected as representative of type-A, type-B and borderline, respectively. The results showed a significant decrease in total metal concentrations of treated soils, with a removal efficiency of about 80% for Be, 90% for Pb and 97% for Hg. This ranking followed the increasing order of the covalent indexes, which are 1.11, 3.36, and 3.92 for Be, Pb and Hg, respectively. Therefore, the ability to form strong covalent bonds with oxygen atoms in maghemite (Fe2O3, γ-Fe2O3) identified on ZVI surfaces seems to drive metal recovery. Validation studies conducted on soil samples collected from sites contaminated with either Pb or Hg, confirmed the above trend. Overall, the results suggest that borderline and type-B metals can be successfully recovered from contaminated soils with rates ≥90%, while the performance would be much lower for type-A metals.

A Screening Approach for the Selection of Drinking Water Treatment Residuals for Their Introduction to Marine Systems.

Drinking water treatment residuals (DWTRs) produced in large quantities worldwide show strong sorption capacities for several contaminants including metals. These by-products of the water-treatment process are primarily discharged as wastes, to either natural or engineered systems, based on the regulations in place in the country where they are produced. To assess how DWTRs can be repurposed to limit the mobility of metals in aquatic systems, we tested their propensity to release toxic metals and their potential ecotoxicity. To account for the wide variability in their physicochemical characteristics, DWTR samples were obtained from 15 water-treatment plants across the United States. A screening procedure based on a combination of 1) the toxicity characteristics leaching procedure (TCLP), 2) total metal contents and sediment quality guidelines, and 3) acute 10-d Americamysis bahia and chronic 28-d Neanthes arenaceodentata survival and growth bioassays was used. All tested samples were found to be nonhazardous based on TCLP results. However, the concentrations of As, Cu, and Ni exceeded the sediment quality guidelines in some samples, resulting in the exclusion of 7 DWTR samples. All of the DWTRs evaluated for toxicity were nontoxic to the tested organisms. The results of the present study suggest that certain DWTRs can be introduced safely into the marine environment and, therefore, used as potential amendments or capping materials to control the mobility of certain sediment contaminants. Environ Toxicol Chem 2021;40:1194-1203. © 2020 SETAC.

Nanowastes and the environment: using mercury as an example pollutant to assess the environmental fate of chemicals adsorbed onto manufactured nanomaterials.

Emerging nanotechnologies hold great promise for creating new means of detecting pollutants, cleaning polluted waste streams, and recovering materials before they become wastes, thereby protecting environmental quality. Studies focusing on the different advantages of nanoscience and nanotechnology abound in the literature, but less research effort seems to be directed toward studying the fate and potential impacts of wastes that will be generated by this technology. Using a combination of biogeochemical and toxicological methods, we conducted a preliminary investigation of the potential environmental fate of Hg as an example pollutant bound to nanomaterials used in treatment of gas effluents. Methylation of Hg sorbed onto SiO(2)-TiO(2) nanocomposites was used as a proxy for Hg bioavailability to sedimentary microorganisms, and the FluoroMetPLATE assay was used to assess the toxicity of both virgin and Hg-loaded SiO(2)-TiO(2) nanocomposites. Our results show that the bioavailability of Hg sorbed onto SiO(2)-TiO(2) nanocomposites to sedimentary microorganisms is pH dependent, with decreasing reaction rates as the pH increases from 4 to 6. Toxicity tests conducted using liquid extracts obtained by leaching of Hg-loaded SiO(2)-TiO(2) nanocomposites with the synthetic precipitation leaching procedure solution showed an average inhibition of 84% (vs 57% for virgin SiO(2)-TiO(2) nanocomposites). These results suggest that Hg sorbed onto engineered nanoparticles could become bioavailable and toxic if introduced into natural systems. Accordingly, studies focusing on the environmental implications of nanomaterials should include determination of the fate and impacts of pollutants that enter the environment bound to engineered nanomaterials.

Toxicity of functionalized fullerene and fullerene synthesis chemicals.

Fullerene is one of the most studied carbon-based nanoparticles due to its unique structure and potential for diverse applications. This study focuses on toxicological effects of two fullerene nanomaterials, contributing to ecological as well as human risk assessment strategies. The biological responses from two basic fullerene materials, aqueous-nanoC60 and alkaline-synthesized fullerenol, were examined using four model organisms. Bioassays were conducted on bacteria (Pseudomonas aeruginosa and Staphylococcus aureus) to determine population impacts and to assess mechanisms of cellular effects for both Gram-negative and Gram-positive species. LC50 of aqu-nC60 stirred for 28 days for P. aeruginosa was estimated to be 1336 mg/L; however, toxicity of the same aqu-nC60 preparation for S. aureus was insignificant. Freshwater green algae Raphidocelus subcapitata and invertebrate Ceriodaphnia dubia were exposed to 28-day stirred aqu-nC60 with no significant toxicological impact. Aqu-nC60 stirred for 14 days bore no toxicity within two orders of magnitude greater than the highest concentration administered. LC50 for organisms exposed to alkaline-synthesized fullerenol prepared in the laboratory was 2409 mg/L for P. aeruginosa with no determinable toxicity to S. aureus, and 1462 mg/L and 45.2 mg/L for R. subcapitata and C. dubia, respectively. Toxicity thresholds for commercially-prepared fullerenol were lower for all species, an impact attributed to the presence of impurities. Mechanistic analysis of membrane damage on bacteria by laboratory-prepared fullerenol indicated necrotic and apoptotic responses with and without photoactivation. Toxicological responses from fullerenol synthesis by-products were only determinable for C. dubia with effects attributable to impurities.

Removal of phyto-accessible copper from contaminated soils using zero valent iron amendment and magnetic separation methods: Assessment of residual toxicity using plant and MetPLATE™ studies.

Zero valent iron (ZVI) has been widely tested and used in remediation of both contaminated soils and groundwater, and in general, the in situ amendment of the contaminated media is used as remediation approach. However, concerns remain as to the potential detrimental effects of both the immobilized ZVI and the adsorbed pollutants as the treated system could undergo transformations over time. Accordingly, plans for soil remediation by in situ immobilization of sorbents should include a long-term monitoring of the treated systems. Here, we report on a comparative study in which artificially Cu-contaminated sandy and organic soils characterized by different metal binding capacities were treated by either (i) in situ immobilization of ZVI in the soils, or (ii) by a ZVI amendment followed by magnetic retrieval of formed ZVI-Cu complexes prior to plant growth studies. The latter relies on the combination of the high metal adsorption capacity and magnetism of ZVI. Two plant species, Lactuca sativa (lettuce) and Brassica juncea (Indian mustard) were used to assess the efficiency of the two treatment methods in eliminating the bioavailable fraction of Cu. Overall, the results showed that, if soil remediation by in situ immobilization reduces the bio-accessible fraction of Cu, treatment using ZVI amendment followed by magnetic separation performs better. The latter resulted in less Cu accumulated in the shoots and roots of plants. In parallel to the plant growth study, we used MetPLATE™, a short-term bioassay based on the inhibition of the ß-galactosidase enzyme by the bioavailable fraction of heavy metal cations, to predict the efficiency of the two treatment methods with regard to the elimination of Cu phyto-toxicity. The results of the bioassay confirmed the trends of phyto-toxicity results, suggesting that MetPLATE™ could be an adequate alternative to the more expensive, labor intensive, and time consuming plant growth studies.

Linking landscape development intensity within watersheds to methyl-mercury accumulation in river sediments.

An indicator of the disturbance of natural systems, the landscape development intensity (LDI) index, was used to assess the potential for land-use within watersheds to influence the production/accumulation of methyl-mercury (MeHg) in river sediments. Sediment samples were collected from locations impacted by well-identified land-use types within the Mobile-Alabama River Basin in Southeastern USA. The samples were analyzed for total-Hg (THg) and MeHg concentrations and the obtained values correlated to the calculated LDI indexes of the sampled watersheds to assess the impact of prevalent land use/land cover on MeHg accumulation in sediments. The results show that unlike THg, levels of MeHg found in sediments are impacted by the LDI indexes. Overall, certain combinations of land-use types within a given watershed appear to be more conducive to MeHg accumulation than others, therefore, pointing to the possibility of targeting land-use practices as potential means for reducing MeHg accumulation in sediments, and ultimately, fish contamination.

Insights into the mechanisms of mercury sorption onto aluminum based drinking water treatment residuals.

Several studies have demonstrated the ability of drinking water treatment residuals (WTRs) to efficiently sorb metal cations from aqueous solutions. Reported results have stimulated interest on the potential use of WTRs as sorbent for metal removal from contaminated aqueous effluents as well as in metal immobilization in contaminated soils. However, knowledge on mechanisms of metal sorption by WTRs remains very limited and data on the long-term stability of formed metal-WTR complexes as a function of changing key environmental parameters are lacking. In this study, chemical selective sequential extraction (SSE), scanning electron microscopy combined with X-ray energy dispersive spectrometer (SEM-EDS), and X-ray photoelectron spectroscopy (XPS) were used to gain insight into the different mechanisms of mercury (Hg) binding to aluminum based WTR (Al-WTRs). Results from sorption studies show that a significant portion of Hg becomes incorporated in the operationally defined residual fraction of Al-WTRs, and therefore, not prone to dissolution and mobility. The results of solid phase analyses suggested that Hg immobilization by Al-WTR occurs largely through its binding to oxygen donor atoms of mineral ligands driven by a combination of electrostatic forces and covalent bonding.

Selective desorption of high-purity (6,5) SWCNTs from hydrogels through surfactant modulation.

Selective desorption of (6,5) single-wall carbon nanotubes from hydrogels only occurs at specific co-surfactant ratios. High-purity fractions are obtained at this ratio even with long elution times and different total co-surfactant concentrations. These results suggest that each (n,m) type forms a thermodynamically-stable surfactant structure in the co-surfactant solution, enabling high-fidelity separations in a single column.

Effect of watershed parameters on mercury distribution in different environmental compartments in the Mobile Alabama River Basin, USA.

Total mercury (THg) and mono-methylmercury (MeHg) levels in water, sediment, and largemouth bass (LMB) (Micropterus salmoides) were investigated at 52 sites draining contrasting land use/land cover and habitat types within the Mobile Alabama River Basin (MARB). Aqueous THg was positively associated with iron-rich suspended particles and highest in catchments impacted by agriculture. Sediment THg was positively associated with sediment organic mater and iron content, with the highest levels observed in smaller catchments influenced by wetlands, followed by those impacted by agriculture or mixed forest, agriculture, and wetlands. The lowest sediment THg levels were observed in main river channels, except for reaches impacted by coal mining. Sediment MeHg levels were a positive function of sediment THg and organic matter and aqueous nutrient levels. The highest levels occurred in agricultural catchments and those impacted by elevated sulfate levels associated with coal mining. Aqueous MeHg concentrations in main river channels were as high as those in smaller catchments impacted by agriculture or wetlands, suggesting these areas were sources to rivers. Elevated Hg levels in some LMB were observed across all types of land use and land cover, but factors such as shallow water depth, larger wetland catchment surface area, low aqueous potassium levels, and higher Chl a concentrations were associated with higher Hg burdens, particularly in the Coastal Plain province. It is suggested that the observed large variability in LMB Hg burdens is linked to fish displacement by anglers, differences in food web structure, and sediment biogeochemistry, with surficial sediment iron oxides buffering the flux of MeHg from sediments to deeper water pelagic food webs.

Increasing UV-B radiation at the earth's surface and potential effects on aqueous mercury cycling and toxicity.

In the past two decades, a great deal of attention has been paid to the environmental fate of mercury (Hg), and this is exemplified by the growing number of international conferences devoted uniquely to Hg cycling and its impacts on ecosystem functions and life. This interest in the biogeochemistry of Hg has resulted in a significant improvement of our understanding of its impact on the environment and human health. However, both past and current research, have been primarily oriented toward the study of direct impact of anthropogenic activities on Hg cycling. Besides a few indirect effects such as the increase in Hg methylation observed in acid-rain impacted aquatic systems or the reported enhanced Hg bioaccumulation in newly flooded water reservoirs; changes in Hg transformations/fluxes that may be related to global change have received little attention. A case in point is the depletion of stratospheric ozone and the resulting increase in solar UV-radiation reaching the Earth. This review and critical discussion suggest that increasing UV-B radiation at earth's surface could have a significant and complex impact on Hg cycling including effects on Hg volatilization (photo-reduction), solubilization (photo-oxidation), methyl-Hg demethylation, and Hg methylation. Therefore, this paper is written to provoke discussions, and more importantly, to stimulate research on potential impacts of incoming solar UV-radiation on global Hg fluxes and any toxicity aspects of Hg that may become exacerbated by UV-radiation.

Impact of land use and physicochemical settings on aqueous methylmercury levels in the Mobile-Alabama River System.

Mercury (Hg) concentrations above levels that could pose health risks have been measured recently in predatory fish from many aquatic systems in the southeastern region of the United States. Based on hypotheses derived from published experimental data on the aqueous geochemistry of Hg, we investigated the effect of certain natural and human-imposed conditions on in situ levels of methylmercury (MeHg) in the Mobile-Alabama River System (MARS). Water samples were collected from different types of environments, hypothesized to have contrasting levels of MeHg in the aqueous phase, and were analyzed for total-Hg (THg) and MeHg concentrations, as well as some key geochemical parameters. The results showed the following. i) Overall, total Hg concentrations in waters of the MARS are quite uniformly distributed and vary from 0.2 to 6 ng L(-1), suggesting that besides geological sources, atmospheric deposition is certainly the main source of Hg inputs in the studied system. ii) In locations with comparable THg levels, the Hg fraction present as MeHg was consistently higher in samples collected from the Coastal Plain portion of the MARS as compared to those from other geological provinces. iii) Our in situ observations confirmed conclusions derived from laboratory experiments, in that, MeHg abundance in aquatic systems correlates with sulfate (but only within a narrow range of concentrations); decreasing pH; and has no direct relationships with either nitrate or phosphate. iv) The investigation of Hg accumulation in biota at a single site showed that an aquatic system with low THg concentrations but a high MeHg:THg ratios, could have organisms with Hg content above safe levels. Therefore, potential health risks to fish eating populations can exist even when the aqueous phase does not show signs of significant Hg enrichment.

Volatilization and sorption of dissolved mercury by metallic iron of different particle sizes: implications for treatment of mercury contaminated water effluents.

Batch experiments were conducted to investigate the interactions between metallic iron particles and mercury (Hg) dissolved in aqueous solutions. The effect of bulk zero valent iron (ZVI) particles was tested by use of (i) granular iron and (ii) iron particles with diameters in the nano-size range and referred to herein as nZVI. The results show that the interactions between Hg(n+) and Fe(0) are dominated by Hg volatilization and Hg adsorption; with Hg adsorption being the main pathway for Hg removal from solution. Hg adsorption kinetic studies using ZVI and nZVI resulted in higher rate constants (k) for nZVI when k values were expressed as a function of mass of iron used (day(-1)g(-1)). In contrast, ZVI showed higher rates of Hg removal from solution when k values were expressed as a function iron particles' specific surface area (gm(-2)day(-1)). Overall, nZVI particles had a higher maximum sorption capacity for Hg than ZVI, and appeared to be an efficient adsorbent for Hg dissolved in aqueous solutions.