Mercury in Aquatic Systems of the Gulf Islands National Seashore, Southeastern USA

This study reports on levels and speciation of mercury (Hg) in different environmental compartments of selected park units in the Gulf Islands National Seashore (USA), and on potential rates of methyl-Hg (MMHg) formation and degradation in sediments. In the aqueous phase, total (THg) and MMHg concentrations ranged from 0.19 to 14.26 ng/L (n=32) an

Effects of simulated acid rain and carbon-rich water on mercury mobilization in soils amended with aluminum-based drinking water treatment residuals

Mercury (Hg) contamination in soils is of concern because of its known adverse effects on ecosystem functions and human health. Research on how to reduce Hg contamination in soil is still needed, mainly because of the difficulties in remediating Hg-contaminated soils while minimizing adverse effects on treated systems. We investigated the potential of a waste substrate, aluminum (Al)-based drinking water treatment residuals (Al-WTRs), as a low-cost sorbent for immobilizing the mobile fraction of Hg in contaminated soils using column leaching studies. Because of the known role of acidic deposition and dissolved organic matter on the environmental cycling of Hg, columns packed with Hg-contaminated alluvial soils collected from the watershed of Poplar Creek in Tennessee of USA were leached using either the synthetic precipitation leaching procedure (SPLP) to simulate the effects of acid rain or low pH dissolved organic carbon (DOC) rich river water (Suwannee River water, pH 4.20) to mimic soil flooding events with DOC-rich waters. The results show that, for soils with very high mobile Hg fractions, control columns without Al-WTR leached with the SPLP solution retained only 51% of total-Hg, which was significantly less (p ​< ​0.05) than in the Al-WTR treated soil columns (up to 80%). Leaching with DOC-rich river water (53.3 ​mg ​C/L) decreased the sorption capacity of Al-WTR. Using waters with increasing DOC concentrations (from 5.33 to 40 ​mg ​C/L) resulted in the removal of 63% of the initial mass of Hg in the control columns compared to 22–29% in the columns amended with 2 and 5% Al-WTR. Overall, Al-WTR can immobilize Hg under extreme leachability conditions and should be considered as a potential sorbent for in situ remediation of Hg-contaminated soils. However, further studies are needed on the fate of Al-WTR-immobilized Hg

Removal of elemental mercury from simulated coal-combustion flue gas using a SiO2–TiO2 nanocomposite; 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

Development of silica/vanadia/titania catalysts for removal of elemental mercury from coal-combustion flue gas

SiO2/V2O5/TiO2 catalysts were synthesized for removing elemental mercury (Hg0) from simulated coal-combustion flue gas. Experiments were carried out in fixed-bed reactors using both pellet and powder catalysts. In contrast to the SiO2-TiO2 composites developed in previous studies, the V2O5 based catalysts do not need ultraviolet light activation and have higher Hg0 oxidation efficiencies. For Hg0 removal by SiO2-V2O5 catalysts, the optimal V2O5 loading was found between 5 and 8%, which may correspond to a maximum coverage of polymeric vanadates on the catalyst surface. Hg0 oxidation follows an Eley-Rideal mechanism where HCI, NO, and NO2 are first adsorbed on the V2O5 active sites and then react with gas-phase Hg0. HCI, NO, and NO2 promote Hg oxidation, while SO2 has an insignificant effect and water vapor inhibits Hgo oxidation. The SiO2-TiO2-V2O5 catalysts exhibit greater Hg0 oxidation efficiencies than SiO2-V2O5, may be because the V-O-Ti bonds are more active than the V-O- Si bonds. This superior oxidation capability is advantageous to power plants equipped with wet-scrubbers where oxidized Hg can be easily captured. The findings in this work revealed the importance of optimizing the composition and microstructures of SCR (selective catalytic reduction) catalysts for Hg0 oxidation in coal-combustion flue gas

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

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 <i>Pseudokirchneriella subcapitata</i> as test organism. The results show significant differences in the biological responses of <i>P. subcapitata</i> 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 <i>P. subcapitata</i> 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

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