MESOPOROUS MOLECULAR SIEVES FOR THE DESIGN OF SELECTIVE HEAVY METAL ION ADSORBENTS
|L. Mercier and T.J. Pinnavala, Michigan State University, Department of Chemistry, 435 Chemistry Building, East Lansing, MI 48824||
The contamination of water systems by heavy metal ions such as Hg and Pb represents an important threat to human health. The improvement of adsorbent materials that can remove toxic metal ions from aqueous media is a continuing objective of environmental remediation efforts. Recently the immobilization of ionspecific ligands on supporting substrates, such as oxides and polymers, has resulted in promising new technologies for the treatment of metal-contaminated effluent streams.
There remains, however, a need to enhance the adsorption properties of such materials, particularly in increasing pollutant-loading levels and in achieving greater adsorbate specificity. Materials advances in these areas can be expected to lead to more efficient remediation of polluted effluents by fixed bed adsorption technologies. In this paper, we describe the preparation of heavy metal ion absorbents with vastly improved binding properties based on the chemical functionalization of surfactant-templated mesoporous molecular sieves.
Owing to their large surface areas and size exclusion properties, open framework solids with pore dimensions in the mesopore range (diameter from 20 to 100 A) are ideal supports for the design of selective pollutant adsorbents. We have achieved the synthesis of highly effective heavy metal adsorbents by the covalent grafting of ligating sulfhydryl functionalities to the framework walls of MCM-41 and HMS mesoporous silica molecular sieves. These materials possess exceptionally high metal ion loading capacities (up to 1.5 mmol/g and exhibit selective binding of heavy metal ions (such as Hg and Pb) from ionic solutions.
Moreover, the large, well-defined pore channels of the mesoporous molecular sieves allow total access of the adsorbate pollutant to all of the binding sites in the adsorbents. In contrast, significant pore blockage occurs for grafted oxides with amorphous pore channels, such as silica gel, resulting in only a fraction of the total ligating sites being accessible for metal lon binding. The effect of the adsorbent pore channel structures on metal ion adsorption, as well as other properties pertaining to the practical applicability of these adsorbents for environmental remediation, will be presented.
Key words: metal adsorbents, molecular sieves, mercury, lead
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