HR: 1340h
AN: B33C-1041    [Abstracts]
TI: Modeling the Kinetics of Silica Polymerization and Precipitation in Aqueous Solutions as a Function of pH and Ionic Strength
AU: * Conrad, C F
EM: cfc11@psu.edu
AF: The Pennsylvania State University, Center for Environmental Kinetics Analysis 2217 EES Building, University Park, PA 16802 United States
AU: Heaney, P J
EM: pjh14@psu.edu
AF: The Pennsylvania State University, Center for Environmental Kinetics Analysis 2217 EES Building, University Park, PA 16802 United States
AU: Bandstra, J
EM: jxb88@psu.edu
AF: The Pennsylvania State University, Center for Environmental Kinetics Analysis 2217 EES Building, University Park, PA 16802 United States
AU: Icopini, G
EM: gicopini@lanl.gov
AF: Los Alamos National Laboratory, Chemistry Division , Los Alamos, NM 87545 United States
AU: Yasuhara, H
EM: huy103@psu.edu
AF: The Pennsylvania State University, Center for Environmental Kinetics Analysis 2217 EES Building, University Park, PA 16802 United States
AU: Brantley, S L
EM: brantley@essc.psu.edu
AF: The Pennsylvania State University, Center for Environmental Kinetics Analysis 2217 EES Building, University Park, PA 16802 United States
AB: Understanding the nucleation, growth and crystallization of colloids from solution is important in both industrial and environmental processes. The growth and nucleation of silica colloids is especially interesting as they are ubiquitous in nature and are one of the most important industrial materials. In this study, we examine the changes in the kinetics of the oligomerization rate of silica as a function of pH, ionic strength and silica concentration. Previous work has investigated the kinetics of the disappearance of monomeric SiO2 in solutions from pH 3 to 7 in a low ionic strength solution (0.01 molal) and geothermal brine (0.24 molal). Results from this work showed the reaction kinetics to be fourth order. This work extends these results to the precipitation of nanocolloidal SiO2 under identical experimental conditions. Results show that the reaction is first order with respect to nanocolloidal concentrations. The rate of precipitation is fastest at near neutral pH and in high ionic strength solutions, reflecting the decreased stability of the colloids under these conditions. Analysis of the variability of the rates of reaction with ionic strength and initial SiO2 concentration shows a linear dependence of the reaction constants with initial SiO2 concentration, but no such dependence with ionic strength, suggesting that the extent of supersaturation in solution is the overriding control on the rate of precipitation. As was seen with the monomeric SiO2 results, the reaction rate constants for the conversion of nanocolloid to precipitated SiO2 vary linearly with pH under each initial concentration and ionic strength condition. Additionally, it was found that for each SiO2 concentration, the reaction rate constants for high and low ionic strength conditions could be fit to a single function over the pH range studied in this work.
DE: 0412 Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912)
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
SC: Biogeosciences [B]
MN: Fall Meeting 2005