Anion-SAMMS™
We have synthesized and demonstrated the use of metal-chelated ligands immobilized on mesoporous silica as a novel anion-binding materials for toxic anions such as chromate, arsenate, pertechnetate, and selenite. Novel chemical interfaces with chelate-SAMMS™ have shown selective removal of toxic metal oxoanions by a ligand exchange mechanism. This approach allows the construction of binding sites that satisfy the stereoelectronic requirements of tetrahedral anions. This SAMMS™ form can remove chromate and arsenate to low levels, even with competing sulfate ion present.
Nearly complete removal of arsenate and chromate is achieved in the presence of interfering anions for solutions containing up to 100 ppm toxic metal anions under a variety of conditions. The material remains effective for even higher concentration solutions (in excess of 1000-ppm anions). Anion loading of more than 130 mg/g (1.12 mmol/g) of SAMMS™ and distribution coefficients of more than 100,000 have been observed. These properties are comparable to the performance of thiol-SAMMS™.
Our anion removal tests were performed in water containing 1-, 10-, and 100-ppm arsenate and chromate with a solution-to-silica (SAMMS™) ratio of 100. In all tests, virtually all of the chromate was removed in a single treatment. The addition of 150-ppm-sulfate-competing anions had little effect on the adsorbing behavior. At the same solution-to-silica ratio (100 mL/g), chromate concentrations higher than 1000 ppm began to produce saturation of the binding sites. The maximum adsorbing capacity is about 130 mg/g or 1.12 mmol/g. For a much higher solution-to-silica ratio (500 mL/g), virtually complete removal of the chromate is observed for chromate concentrations up to 100 ppm. Higher concentration of chromate under these conditions results in saturation of the binding sites.
Similar results were also obtained for arsenate removal. The maximum loading capacity is 140 mg/g or 1.00 mmol/g. Under the same conditions, the residual concentrations of arsenate are all slightly higher than chromate at low anion concentrations. This suggests the binding chemistry has higher affinity for chromate than arsenate.
The test results can be summarized as follows:
- The SAMMS™ anion materials are very efficient oxoanion binding materials. Efficient anion removal can be achieved over a wide concentration range.
- Oxometallate anions (chromate and arsenate) are preferred over sulfates.
- Low concentrations of chromate and arsenate can be removed from high concentrations of sulfates.
- The binding chemistry is more sensitive for chromate than arsenate at low concentrations.
Supporting Publications
"Design and Synthesis of Selective Mesoporous Anion Traps", Glen E. Fryxell, Jun Liu, Meiling Gong, Teresa A. Hauser, Zimin Nie, Richard T. Hallen, Mauxu Qian, and Kim F. Ferris, Chemistry of Materials, 1999, 11, 2148-2154.
"An X-Ray Absorption Fine Structure Spectroscopy Study of the Interactions Between Contaminant Tetrahedral Anions to Self-Assembled Monolayers on Mesoporous Supports", Shelly Kelly, Ken Kemner, Glen E. Fryxell, Jun Liu, Shas V. Mattigod, K. F. Ferris J. Phys. Chem. B 2001, 105, 6337-6346.