Characterization of mesoporous SiO2 particles for the characterization of non-ideal sorption behavior
EMSL Project ID
11090
Abstract
Contamination of soils with volatile organic compounds (VOC) is a common problem on Department of Energy Reservations. As these solvents are transported through the subsurface to groundwater, they become an increased risk to human health and welfare as well as the environment. Understanding the fundamental controlling mechanisms of transport has been challenging due to the complexities inherent to the subsurface environment. Factors that typically control subsurface transport include physical and chemical soil attributes, moisture content, percent organic matter, and contaminant characteristics. Recent interest has been focused on often-observed discrepancies between measured and predicted partition coefficients. For example, models often under predict the time required to achieve target residual contaminant concentrations during soil vapor extraction operations. These under predictions can be the result of models not accounting for desorption kinetics which are significantly different (slower) than adsorption kinetics. One hypothesis for the inability to achieve accurate model predictions is that current equilibrium sorption equations do not account for contaminant-soil interactions that occur in near-molecular size intraparticle pore spaces. Due to the heterogeneous and complex nature of soils, however, it is difficult to isolate all the factors that can impact sorption in order to distinguish the significance or role of each potential factor.The recent advances in nano materials development may help provide a solution. Silicate-based mesoporous molecular sieves (MMS) have been developed in our laboratory to evaluate the effects of pore size on VOC adsorption and desorption rates. MMS with narrow pore size distribution and well defined surface chemistry were synthesized in an acidic medium using nonionic surfactants as structure directing agents and tetraethoxy silane as the silicon source. Porosimetry results indicate that these ideal sorbents have a narrow pore size distribution (PSD) in the mesoporous range (2-50 nm). Secondary Ion Mass Spectrometry (SIMS) indicates that the surface functional groups are predominately silica oxyhydroxides. However, to correlate the sorption phenomena with respect to the characteristics of these nano-materials, they must be more fully characterized.
The proposed research will combine the previous findings with the results obtained at the EMSL user facility to more fully characterize the MMS. The proposed analytical methods to be carried out at EMSL will be as follows: 1) low angle XRD which will be used to verify the pore size, wall thickness and pore shape; 2) high resolution TEM imaging which will be used to demonstrate that the particles are of uniform pore size distribution and will verify the porosimetry interpretation method; and 3) Solid state NMR spectroscopy will be used to determine the molecular structure of solids.
Project Details
Project type
Exploratory Research
Start Date
2004-09-13
End Date
2007-03-22
Status
Closed
Released Data Link
Team
Principal Investigator