A good question for every manufacturer and user of physical adsorbents is what sized adsorption spaces and adsorption energies are used in specific real-world applications? This two-part article covers the answer to this important question for a drinking water plant using granular activated carbon (GAC) to polish Lake Ontario water.

All activated carbons are not the same — one size does not fit all applications. They have different ranges of size and shape adsorption spaces. Smaller nanometer sized and shaped adsorption spaces hold adsorbate molecules stronger than larger adsorption spaces. Most purchasers buy activated carbons (AC) because it provides physical adsorption of unwanted molecules from water or air streams passing through various forms of AC. AC media comes in powder, granular, pellet, block, fabric, composites and chemical impregnations of these forms. A wide variety of chemical impregnated ACs are available to provide chemical and catalytic reactions on the graphitic AC surfaces. Gravimetric Adsorption Energy Distribution or GAED has been used on all forms of AC for both aqueous- and vapor-phase applications. AC has an amazing list of market needed applications.

Using GAED tests to answer tough questions

GAED is the best available test method for you to answer these important questions: What is the best technical and cost effective AC for specific applications and when should I change out the used AC with fresh? GAED test method has its roots in the early 1900s largely due to Dr. Michael Polanyi. He developed fundamental concepts for heterogeneous physical adsorption on solid media, which are utilized in the present day GAED instrument and software package. GAED provides full characterizations for AC physical adsorption performance and answers your questions for both aqueous- and vapor-phase applications.

All you need to do to determine what adsorption energies are consumed in your AC applications is do comparative GAED runs on the starting unused sample and its used sample. The starting test sample needs to be representative of what you used in your application. Comparison of the initial and final GAED sample runs reveals the adsorption spaces consumed in your application, because these spaces are filled with adsorbate(s) in the used AC. This information, knowing what adsorption energies you need, is useful in improving your next AC purchase, and when to change out the partially used AC and what your options are for disposal of the used AC. This case study reported here is for two different virgin or unused GAC samples and their corresponding spent or used samples.

How GAED works and strategies for answering questions

Mick Greenbank capitalized on Polanyi's potential adsorption energy model to design and build instruments to provide full physical adsorption characterizations of activated carbons and other sorbent materials. This robust and rugged instrument allows the activated carbon industry to solve prior refractory problems for clients. Also, GAED solves modern and classical problems and guides new research and development product developments more cost effectively, compared to existing classical test methods.

An archival record of how GAED works and some applications is available online. The most recent publication points out GAED's use to reveal markets available for a particular AC or new material. The AC industry is in the process of transforming from an art to a science and GAED and other testing methods help to facilitate this important advancement.

GAED problem-solving strategy is to use a standardized routine set of test parameters on client field samples and known commercial benchmark AC samples or appropriate standards for the specific client sample, to get the best marketplace comparison. GAED measurements, along with American Society Test Methods or ASTM and other available needed laboratory methods, can satisfy the lab part. Professional knowledge and experiences about different AC products and how to use them in different applications coupled with GAED and ASTM lab results is a useful flexible problem-solving strategy.

Characteristic curves and uses

It is important to clean the test sample before the GAED test is performed. When comparing a group of different vendor samples we want to provide dry samples to the GAED instrumental analysis. Some field samples in the testing group may have more moisture and volatile matter. The GAED report provides these moisture/VM values as a measure of cleanliness. You need to know the cleanliness of purchased AC. Do not pay for excess water on the carbon and/or VM, which can indicate a contaminated AC. AC can become contaminated because they adsorb organics and moisture from the ambient air they are stored or transported thru.

After samples are thermally cleaned they are challenged with 1, 1, 1, 2-Tetrafluoroethane (TFE) or R134a gas at 240° C. This gas was chosen because it is difficult to be adsorbed by AC (at 240° C because only the highest adsorption energy sites are able to capture TFE from the mobile challenge gas stream), its low cost, availability worldwide, non-toxic and friendly to the environment. After the highest adsorption energy pore volume or adsorption space is determined at 240° C the automatic temperature program lowers the sample temperature hot to cold slowly reaching a quasi-equilibrium state before starting the next incremental temperature lowering. Physical adsorption is a function of temperature, the lower the temperature the better the adsorption capacity, weight of TFE picked up. Lowering the temperature no lower than -20° C avoids TFE condensation on the sample, which would be false positive for adsorption.

This resulting hot to cold curve (temperature on the X-axis and weight gained on the Y-axis) is called the adsorption Characteristic Curve; it is characteristic of the sample when it is run through the standard GAED test parameters. At the lowest temperature, the temperature program is reversed, cold to starting hot, to obtain the desorption Characteristic Curve. AC typically do not demonstrate any hysterias, i.e. the adsorption and desorption curves are essentially the same. Other physical adsorbents, such as organoclays and zeolites, do show hysterias.

From the graphical Characteristic Curves in Figure 1 polynomial equations are derived, see Table 5. These equations allow you to plug in a desired minimum adsorption energy, needed to capture a target molecule, and calculate its corresponding available pore volume for that sample run material.

Using the Polanyi heterogeneous adsorption model concepts allows the Characteristic Curves to be transformed into isotherms, see Figure 4. GAED reported example isotherms covers a weakly, moderately and very well adsorbed molecule from water or air, i.e. Methyl-tertiarybutyl-ether or MTBE, Benzene and Phenol respectively. An advanced feature of GAED analysis is that specific client molecules of interest can be transformed into isotherms. Additionally, GAED could provide simultaneously multiple adsorbate(s) determinations and reveal the molecular mixture in the sample.

An important feature of this transformation from the Characteristic Curve to isotherm is that you can get isotherms at any temperature you want. Determining classical empirical ASTM isotherms have limitations for the temperature and also cost. Cost is many orders more than GAED derived isotherms at any temperature the client wants.

Unused and used GAC case study

Figure 1 contains the Volume based Carbon Characteristic Curves - Cumulative. Note that PACS sample identifications are: AA-686 is unused GAC and AA-687 is its used version and AA-688 is a different unused starting GAC and AA-689 is its corresponding used counterpart material. These curves provide on the Y-axis adsorption space volumes, classically called pore volumes as a function of Adsorption Energies on the X-axis in cal/cc. Providing characteristic curves in volume presentation eliminates the problem with the wide range of adsorbates densities. For example, liquid butane has a density of 0.600 g per cc and solid iodine density is 4.98. When presented on an optional mass bases, presentations are grams adsorbed per 100 g carbon on the Y-axis. In a drinking water plant and many other applications volume based presentations are better than mass because the end user and service provider has a finite pore volume to fill with adsorbates which can be used to provide estimates on how long the AC will perform for the application.

The last of five samples presented in Figure 1 and shown on Table 1 is a benchmark bituminous coal based liquid phase standard GAC. The standard functions as a quality control and assurance check to validate the GAED instrument is completely operational and its output agrees with prior runs. The GAED instrument was within its statistical control limits during sample analysis. This instrument has been used continuously the last 15 years with no malfunctions. This has created a large database of GAED runs on a wide variety of sorbents. It literally runs itself. Interpretation of the raw data and report writing consumes most of the lab time for GAED analysis. We are working on ways to reduce this time.

Figure 1 graphical presentations are summarized in Table 1 as numerical tabular values. Table 4 uses these data values from Table 1 to let us know what adsorption spaces were filled with adsorbates in these two starting unused AC drinking water adsorbers. Both used GAC filters are near their time to be replaced with fresh unused AC. AC does not last forever. AC is also expensive and should not be replaced prematurely. When the water going in and out of the GAC adsorber is the same, GAC is exhausted and no longer removing adsorbates. In drinking water plants used AC is still removing suspended particulate matter and reducing finished product water turbidity during the automated backwashing cycles. You can do suspended solids determinations on backwash water samples to prove this benefit.

In next month's article, we'll continue with ASTM unused and used tests and draw our conclusions. For more information, please contact PACS Testing, R&D, Consulting, Expert Witness Services, Short Courses and Conferences. Phone: 724-457-6576; email: Henry@pacslabs.com; or visit www.pacslabs.com.

Henry Nowicki is president and senior research scientist for Professional Analytical and Consulting Services Inc.(PACS) and Activated Carbon Services. His is also chairman for the International Activated Carbon Conference (IACC) and Activated Carbon School.

Wayne Schuliger, P.E. is technical director for PACS and provides the short course, Activated Carbon Adsorbers Design, Operation and Troubleshooting.

George Nowicki is the PACS laboratories director and software expert.

Barbara Sherman is manager of operations and contact person for the International Activated Carbon Conference and Activated Carbon Courses. Contact Barb@pacslabs.com or call (724) 457-6576.