Gravimetric Adsorption Energy Distribution
PACS Advanced GAED Testing
State-of-the-Art Tests for Activated Carbons
Whereas the American Society for Testing Materials (ASTM) iodine number is the most popular figure of merit for activated carbons, the present state-of-the-art is more powerful by at least an order of maginitude. One can now rapidly determine what amounts to a calibrating isotherm (“correlation curve”) on a carbon, and then compute, for that carbon, the adsorption isotherms of a wide variety of gases/vapors or solutes from water solution, from traces to saturation, using software available from Professional Analytical and Consulting Serivces Inc. (d.b.a. PACS).
Whereas there are a number of methods for calibrating carbons from adsorption data in the gas or liquid phase, by far the most reliable, rapid and convenient is a gravimetric method invented by Dr. Mick Greenbank, that has been named the “Gravimetric Adsorption Energy Distribution (GAED) Method.” The computation of adsorption isotherms from the results of this method is readily carried out by PACS software that uses algorithms published by Dr. M. Manes and coworkers, and an extensive database compiled by Dr. Yaws. By contrast, the ASTM iodine number is of no use for distinguishing the relative merits of different carbons for challenging (i.e., low capacity) applications such as removing traces of aqueous MTBE, THM, methylphosphonic acid, ethanolamine, or trace taste and odor components (Geosmin and 2-Methylisoborneol) from potable waters or vinyl chloride from air. The GAED method, for which PACS is the only commercially available source, can also be used to determine the location (coarse or fine pores) of impregnants on the carbon, to follow the process of activation or reactivation, to select the best carbon for a particular application, to benchmark new activated carbons, against existing commercial products, or to evaluate other adsorbents. PACS has experience with all of these GAED applications.
In the Polanyi model, which was used by both Manes and Greenbank, the calibrating isotherms do not determine pore dimentional measurements (indeed, Polanyi did not mention pore measurements), but rather the adsorption energies in various locations. Although they correlate roughly with pore size (which cannot be characterized by a single number), they are much better adapted for the calculation of isotherms. For a gravitational analogy, imagine the process of pouring some liquid into a vessel of undetermined shape, where one can determine only the total volume of the liquid and the height of its surface from the bottom of the vessel. Having determined the relation of volume to height for one liquid, one would find the same relation for almost any other liquid. With knowledge of the density of such liquid one would know the relation, for that vessel, of cumulative mass to liquid height.
PACS Testing, Consulting, and Training Services plans to work toward obtaining American Society for Testing Materials (ASTM) approval as a standard method for this robust GAED methodology. We encourage you to use this test method.
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409 Meade Drive
Coraopolis, PA 15108
PACS provides testing, courses and consulting at the client’s time and place, and twice a year carbon conferences.
PACS Positions Available:
- Laboratory chemist for activated carbon group. Will train!
PACS has positions available for professional scientific service providers and has provided these services for over 38 years. PACS will accept proposals for short courses, consultants, activated carbon conference directors for focused conference subjects, and other needed services. New ideas are welcomed.
409 Meade Drive
Coraopolis, PA 15108