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Users and purchasers of activated carbons for water and wastewater treatment and other applications may not be aware of the benefits that advanced test methods can provide them in selecting the best activated carbon for their applications.

An advanced test method ?called gravimetric rapid pore size distribution (GRPD) ?can differentiate activated carbons and help in the purchaser’s selection of the best activated carbon for an application.

Twenty to thirty years ago, most water treatment applications for activated carbon involved the removal of contaminants on the order of milligrams per liter (parts per million, or ppm), but many of today’s applications are used for removal at the level of micrograms per liter (parts per billion, or ppb) in raw water to meet regulatory requirements.

A major application for the GRPD test method is determining the best activated carbon for these trace applications.

Beyond classical standards
Decisions about purchasing and specifying activated carbons typically involve use of classical American Society for Testing Materials (ASTM) and American Water Works Association (AWWA) test methods for determining these parameters:

• iodine or butane activity number
• apparent density
• moisture
• total and water-soluble ash
• hardness or abrasion number
• particle size distribution
• BET surface area
• total pore volume.

Some purchasers of activated carbon are now using the GRPD tool, and more are expected to use it in the future. The GRPD test is a supplemental test which complements the classical tests. It is not designed to replace the classical tests, but it provides additional sorbent information not obtainable from classical tests.

Finding differences
New applications for GPRD technology continue to be found, providing customers with valuable information. Here are some examples:

• GRPD compared impregnated with starting activated carbons to determine the impregnation penalty. What this means is that when the starting activated carbon is impregnated, the impregnant fills selected pores. The GRPD can reveal the number and sizes of pores filled by the impregnant.
• GRPD provided a way for a point-of-use (POU) carbon filter manufacturer to track batch-to-batch or lot-to-lot variations in activated carbon quality.
• It allowed a municipal drinking water plant to evaluate nine activated carbon suppliers to choose the best product for the plant’s needs.
• GRPD provided a POU device manufacturer with a way to evaluate the impact of a possible vendor change.

All carbons not the same
For some activated carbon applications, the historical ASTM and AWWA test methods are adequate. However, when the activated carbon purchaser’s proposed application is trace removal of substances from water or air, the historical testing methods can lead to a poor choice of activated carbon.

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.

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.

One client’s experience
For example, our company 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 carbon for their trace removal application was the one with the higher apparent density and lower ASTM iodine number, which also had the lower cost. Many purchasing decisions are based on iodine numbers, with the assumption that the higher the iodine number the better.

GRPD testing for this client revealed those activated carbons with the larger numbers of high-energy binding sites (adsorption spaces in the carbon to which contaminants are concentrated and removed from water). The client needed activated carbon with these strong adsorption spaces for their application, which was removal of small water-soluble organic compounds.

Challenged carbons
With challenging applications such as removing traces of aqueous MTBE (methyl tertiary-butyl ether, a gasoline additive), THMs (trihalomethanes, undesirable by-products of chlorination), benzene (a petrochemical component), and 2-Methylisoborneol (a bacterial by-product that can contribute to an earthy or musty odor in water) from potable waters (as well as vinyl chloride or military toxic agents from air), the ASTM iodine number or butane number is of no use for distinguishing the relative merits of different activated carbons.

GRPD test runs on client samples are available at laboratory testing services for activated carbon users and manufacturers. Each GRPD sample run is provided with a detailed report.

Some 5,000 different samples have been analyzed using GRPD technology. Most of this GRPD database has been developed 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 because it allows quick and low-cost comparisons of clients?samples with the world of activated carbons.