Superfund Research Program
Functional Nanostructures of Groundwater Remediation
Project Leader: Thomas J. Pinnavaia
Grant Number: P42ES004911
Funding Period: 2000 - 2006
- Project Summary
Final Progress Reports
Dr. Pinnavaia’s work on the direct assembly of organofunctional silica mesostructures from low cost sodium silicate has been completed and published. Up to 50% of the framework silica centers in these materials have been functionalized with mercaptopropyl groups and other organic functionalities. On-going studies indicate that these compounds have unprecedented properties for the efficient removal of divalent mercury from ground water. Also, recent experiments show that arsenic in both As(III) (arsenite) and As(V) (arsenate) forms can be efficiently removed from water under near – neutral pH conditions at ambient temperatures. This is the first example of the remediation of arsenite by a mesoporous material.
Also, the team’s X-ray absorption spectroscopy studies of the mechanism of mercury(II) cation binding to mercaptopropyl-functionalized mesostructures has been completed and published. These materials have the anhydrous formula (SiO2)1-x(LSiO1.5)x, where L is the mercaptopropyl group. Monolayer -S-Hg-OH and double layer -S-Hg-O-Hg-OH binding modes provide the best fits to the absorption data, depending on the mercury loading.
Nanoscale FeS has been tested for the removal of the oxyacid forms of arsenic. FeS effectively removes As(III) as As2S3 at low pH and as a mixed Fe-As sulfide at intermediate pH. To extend the application of FeS to permeable reactive barriers (PRB) field application, methods to coat FeS onto quartz particles have been on-going in order to create reactive FeS surfaces that can be placed in PRB walls without reducing hydraulic conductivity. The FeS coated quartz particles are being tested for uptake of arsenic in comparison to nanoscale FeS.
XAS characterization of Cd and Hg sorption by nanoscale FeS has also continued during the current year. Characterization of nanoscale FeS by a variety of surface area measurement techniques has been undertaken in the current year. The team’s sorption studies suggest that FeS is made up of extremely small particles with a high surface area. Surface area estimates by BET gas adsorption SEM, TEM, and X-ray diffraction vary significantly. This variation is due, in part, to particle aggregation during the freeze-drying of FeS prior to surface area measurement or microscopic characterization. To overcome the particle aggregation effects, Ethylene Glycol Monoethyl Ether (EGME) method and Photon Correlation Spectroscopy (PCS) have been used to measure the FeS surface area and particle size. Based on PCS the researchers have confirmed that FeS nanoscale particles form initially with particle size on the order of 4 to 15 nm but grow up to 180 nm aggregates under the experimental conditions. The total surface area of FeS as measured by EGME has been found to be 1,086 m2/g with the interlayer area estimated to be 880 m2/g and the external area of 206 m2/g based on crystallographic data for mackinawite. These results indicate that the high reactivity of FeS is in part related to its extremely high surface area.
The presence of metals may impact the reductive dechlorination of chlorinated hydrocarbons by reduced iron sulfide. Fe(II), Co(II), Ni(II), and Hg(II) were found to have a significant impact on hexachloroethylene (HCA) dechlorination. Chlorinated ethylenes are of much greater concern and importance at mixed waste sites. The researchers have been investigating the ability of FeS nanoparticles to reductively dechlorinate trichloroethylene (TCE) and tetrachloroethylene (PCE) in the presence of transition metals (Cd, Hg, Ni, and Co). Similar to the reductive dechlorination of HCA, the rate increase of PCE dechlorination in systems containing Co(II) and Hg(II) and TCE dechlorination containing Co(II) can be attributed to formation of a more reactive mixed metal sulfide phases. Other metals resulted in decreased rates from the formation of non reactive mixed metal sulfide phases or inactive iron oxide coatings on FeS. The implications of these results are that when metal concentrations are sufficiently high, reductive dechlorination in FeS systems is usually impeded, the exception being when Co is present, in which case the reactivity can increase dramatically. The unusual reactivity of mixed CoS-FeS systems is under further investigation.