By Henry Nowicki, Ph.D./M.B.A.

Many purchasers of activated carbons for water and waste water remediations and vapor-phase applications are not aware of the benefits of having advanced test methods provided as a part of the purchasing process.

New test method’s advantages
Purchasers need to be aware that there is an advanced test method called – Gravimetric Rapid Pore Size Distribution (GRPD) which can differentiate activated carbons and help them to select the best for their applications. Typically, purchasing decisions between buyers and sellers of activated carbons use classical American Society for Testing Materials (ASTM) and American Water Works Association (AWWA) test methods: iodine or butane activity number, apparent density, moisture, total and water soluble ash, hardness or abrasion number, particle size distribution, BET (definition) surface area, total pore volume and (of course) cost to make purchasing decisions. Some purchasers are now using the GRPD tool and more are expected to adopt it in the future. The GRPD test is supplemental and compliments the classical tests; it is not designed to replace them, but to provide additional sorbent information not obtainable from the classical tests.

New Uses
New applications for the GPRD technology continue to arise, providing customers with valuable information. Some of the more interesting uses of GRPD to date have been:

  • A comparison of impregnated with starting activated carbons to determine the impregnation penalty. Should this be defined?
  • Providing a way for a POU carbon filter manufacturer to track batch-to-batch or lot-to-lot variations in activated carbon quality.
  • Allowed a municipal drinking water plant to evaluate nine activated carbon suppliers to choose the best product for their needs.
  • Provided a POU device manufacturer a way to evaluate the impact of a possible vendor change.

GRPD data files are now mined to provide clients with a calculated BET surface area and Trace Capacity Number. Even though the GRPD provides clients with useful isotherms, some still have a comfort zone with BET surface areas.

More information for specific uses
Most water applications for activated carbon 20-30 years ago removed milligrams-per-liter, but many of today’s applications are used to remove micrograms-per-liter in the raw water, which needs to be finished to meet regulatory requirements. A major use for the GRPD test method is determining the best activated carbon for these trace applications.

For some activated carbon applications, the historical ASTM and AWWA test methods are adequate. However, when the activated carbon purchaser’s application is a trace removal from water or air, the historical testing methods can lead to a poor choice of activated carbon. All activated carbons are not the same. Classical test methods cannot differentiate activated carbons with the same BET surface area or ASTM iodine or butane activity number.

For example, we ran GRPD tests and classical tests on nine activated carbon supplier samples under consideration for purchase by a major municipal drinking water plant. If the decision maker only had classical test data they would not have been aware that the better activated carbons for their needs was the higher density and lower ASTM iodine number activated carbon which also had the lowest cost. Many purchasers today use the higher iodine number to make decisions. GRPD testing for this client revealed the activated carbons with the larger numbers of high energy binding sites. These binding sites with strong adsorption forces were used to differentiate the activated carbons for the client. The client needed activated carbon with these strong adsorption sites for their application, removal of small water-soluble organic compounds.

The GRPD advanced test method is required for trace removal applications and it is useful in many other scenarios where the classical test methods are inadequate.

Isotherm computation
Whereas the ASTM iodine number is the most popular figure of merit for activated carbons, the present state-of-the-art GRPD test method is more powerful by at least an order of magnitude. One can now rapidly determine what amounts to a calibrating isotherm for any activated carbon and then compute, using software, for that carbon, the adsorption isotherms of a wide variety of gases/vapors or solutes from water solution from traces to saturation. Whereas there are a number of methods for calibrating activated 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, which has been named the AGravimetric Rapid Pore Size Distribution (GRPD) method. The computation of water-phase or gas-phase adsorption isotherms from the results of this GRPD method is readily carried out by PACS software that uses algorithms published by Dr. M. Manes and coworkers, and an extensive database of physical and chemical properties of molecules compiled by Dr. K. Yaws. These isotherms derived from the GRPD method are very useful to activated carbon users. Isotherms give the equilibrium capacity (grams of solute adsorbed per gram of carbon) of solute loading on carbon from the water solution. This capacity information is used to design activated carbon adsorption systems. The GRPD data provides a calculated BET surface area and trace capacity number.

The ASTM iodine number or butane number is of no use for distinguishing the relative merits of different activated carbons for challenging applications such as removing traces of aqueous MTBE, THM, methylphosphonic and 2-Methylisoborneol from potable waters or vinyl chloride or military toxic agents from air. GRPD runs on client samples are available at PACS as well as many other services for activated carbon users and manufacturers. Each GRPD sample run is provided with a detailed report of at least 15 pages. Some 5,000 different samples have been analyzed using GRPD technology. Most of this GRPD database has been performed on activated carbon samples but many other materials have also been analyzed. This database of compiled GRPD runs on a wide variety of sorbents is very useful for clients because it allows quick and low cost comparisons of their samples with the world of activated carbons. Users and manufacturers of activated carbon are encouraged to use the GRPD technology.

About the author
Henry Nowicki, Ph.D./M.B.A. is President of PACS Testing, Consulting, and Training. Dr. Nowicki is the chairperson for the 2007 19th and 20th International Activated Carbon Conferences and Short Course program which is now accepting one-page abstracts for presentations in Sydney, Australia in July and Pittsburgh, PA in October (2007). Dr. Nowicki presents a two-day course on the activated carbon subject and has arranged seven other instructors and courses on carbon. Contact him by email at, telephone: (724)-457-6576; fax: (724)-457-1214 or visit the

For a complimentary sample
Anyone who wants a typical gravimetric rapid pore size distribution (GRPD) laboratory report along with a list of GRPD applications should visit and use the ‘contact us’ button on the website’s homepage or e-mail Nowicki will send you a sample report free of charge, entitled “Have Your Tried a GRPD?”


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