Leaching Behavior of Edta in a Silica Sol-Gel Matrix
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KRIS S. OKA AND J.D. MACKENZIE University of California, Los Angeles, Department of Materials Science and Engineering, 5732 Boelter Hall, 405 Hilgard Avenue, Los Angeles, CA 90024 ABSTRACT
Ethylene diamine tetra-acetic acid (EDTA) doped silica gels were made by the sol-gel process for the potential application as a filter for heavy metal ions in wastewater. The behavior of the organic molecule in the matrix was studied by investigating the percentage of EDTA leached out with a variation in the timing of addition of the EDTA molecule into the starting silica gel solution. Leach tests using water as the medium were performed for 2, 12 and 24 hours in order to determine the amount of EDTA trapped in the pores or in the matrix. A minimum amount of EDTA leached from the gels was detected in the samples to which EDTA was added 4 hours after initial hydrolysis of TEOS. This result was correlated to having a greater percentage of pore volume in the range of 15-20 A, which was further substantiated with density measurements. The physical changes in the silica matrix altering the pore volume distribution were attributed to the addition of the water into which the EDTA molecule was initially dissolved. 1. INTRODUCTION Organic-inorganic materials made by the sol-gel process have become a very active area for research from sensors and filters for environmental applications to solid state lasers. 1,2 In spite of being attractive materials, little is known about the fundamental aspects of the interaction between organic dyes and the inorganic host matrix. For analysis of the structure and properties of the doped sol-gel matrices, organic dyes can be useful probes. In this research, a filter for heavy metal ions is being developed by incorporating ethylene diamine tetra-acetic acid (EDTA), a well known chelating agent for heavy metal ions, 3 into a porous silica gel matrix. The efficiency of this filter, however, depends heavily on the location of the organic molecule within the gel structure and the interaction between the organic molecule and the inorganic matrix. For example, the EDTA molecule needs to be located where it can interact with the toxicant while still maintaining its ability to form a complex with it. There are several types of locations and interactions to consider. The location of the molecule can be divided into two sites. One is on the surface of an open pore, and the other is sterically hindered within a pore. The interaction between the molecule and the matrix can be divided into three types of bonding, Van der Waals forces, hydrogen bonding and direct bonding, such as in acetate polymerization. In an attempt to determine the location and interaction of the EDTA molecule in a silica sol-gel matrix, leaching tests were performed and discussed in terms of these locations and bonding types using density measurements and surface area analysis. 2. EXPERIMENTAL PROCEDURE
Silica gels were made by sol-gel process. The inorganic precursor, tetra-ethoxy silane (98%), was purchased from Aldrich Chemicals. The acid cat
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