By Henry Nowicki, Wayne Schuliger, George Nowicki and Barbara Sherman
Granular activated carbons (GAC) are the best available technology (BAT) to purify and protect global drinking water supplies. GAC can be manufactured from a wide variety of natural resource materials. Coal and coconut shells are the dominant raw materials to manufacture GAC for drinking water applications1.
All activated carbons are not the same. This variable family of activated carbons have fine micro-, meso- and macro-pore structures. Micro and meso pores are most useful for accumulating water contaminants 1. These sized pores can be revealed with the Gravimetric Rapid Pore Size Distribution (GRPD) test method.
Over the last nine years, PACS Laboratories has provided many applications for GRPD 2-16. A new application for GRPD is now being used to compare the difference between unused and used GAC in drinking water applications and other applications to reveal which pores are filled with adsorbates.
A couple of definitions are offered. Unused GAC means starting GAC that has never been used before. Used GAC means unused GAC that has been through a specific application, such as drinking water or industrial wastewater applications.
What pores are used?
If you are a potential or actual activated carbon user, do you know what pores (adsorption spaces) are needed in your application? There are myriad different activated carbons and applications. Most activated carbon users do not know which pores are used in their applications.
Today it is possible for users to determine what pores are needed. Knowing what pores are filled in your application provides the opportunity to select a carbon with more of the pores you need. With more pores in your application, expect your finished water to be improved and provide longer service times.
Four client samples of granular activated carbon (GAC) were fully characterized using the GRPD method. Two samples were from the unused GAC and two were used from the unused GAC samples.
The unused GAC were Calgon F-400 (Y-334) and Norit 830 (Y-336). (The client has determined the definition of used. Virgin is an older term for unused and spent is an older term for used. Reference the American Society for Testing and Materials (ASTH International) for recommended terms17.)
All four samples were compared to each other and a commercially available PCB coconut-based carbon of approximately 1,200 iodine number. None of the samples had a weight loss greater than 2.3 percent on conditioning (heating the samples to 150º C (302º F) in argon and holding for 25 minutes).
Losses of less than eight percent indicate a well-stored sample that has been protected from moisture pick-up from ambient air during handling and storage and was also fresh and not oxidized. In terms of total adsorption pore volume, both unused samples out-performed the used version by 16.8 percent and 14.9 percent respectively.
The difference between the unused GAC themselves is only about three percent and the difference between the used GAC samples is less than one percent. However, the difference in used samples is dependent on the material adsorbed and the degree to which the carbon is saturated.
All four samples were similar in terms of the shape of their characteristic curves. But subtle differences in pore structure at different adsorption energy levels can be seen in the differential characteristic curves.
GRPD method summary
Each GRPD run measured over 300 adsorption and desorption data points covering seven orders of magnitude in relative pressure (isothermal basis) and three orders of magnitude in carbon loading. The mass adsorbed was also divided by the carbon mass to generate a weight percent loading for easier comparison.
The raw data is plotted (Figure 2) such that at 150ºC the adsorbent gas, C134a or 1,1,1,2-tetrafluoroethane (TFE), is introduced and loading increases as temperature is decreased. Mass loading was plotted against temperature but the relative pressure was also changing. There are three variables affecting performance that change from point to point: vapor pressure, partial pressure, and temperature.
To make comparisons easier, the large data file of adsorption/desorption points at different temperatures and relative pressures was simplified. First the data was interpolated to get 30 evenly spaced points covering the entire data range.
Next, adsorption and desorption results were averaged to get the equilibrium values (the difference between adsorption and desorption was minimal for this sample – no hysteresis). The y-axis is converted to pore volume measures, in cubic centimeters (cc) of liquid adsorbed or cc of pores filled per 100 grams carbon, instead of weight percent. (Table 1 series).
Performance prediction models
These curves are the only carbon-related information required to predict physical adsorption performance using the Polanyi Adsorption Potential theory. These single and multicomponent, gas- and liquid-phase computer models are used to predict carbon performance. To do performance predictions the following polynomials describes these carbon samples:
In the equation, y is the common logarithm of pore volume in cc/100g carbon and x is the e/4.6V adsorption potential in cal/cc. Characteristic curve polynomials are also noted (Appendix A.)
Characteristic curves are also translated into adsorption isotherms using the programs mentioned (Appendix A.). MTBE, a weakly adsorbed material (Figure 3), a more strongly adsorbed species like benzene (Figure 4) and a quite strongly absorbed material for phenol at pH=7 (Figure 5) are key factors.
Pore size distributions
The Kelvin equation, modified by Halsey, can be used to convert the characteristic curve data to calculated BET surface areas or pore size distributions. This is not useful in terms of performance evaluations, but some audiences are more comfortable with the concepts of pore radius and a series of capillary sizes when thinking about activated carbon.
Cumulative pore size distributions (Figure 6) were included, but have little use. The single- and multi-point BET surface area was calculated from these curves.
The amount of contaminants loaded onto GAC is directly related to influent concentrations. In drinking water applications, raw water has low concentrations that result in lightly loaded used GAC.
GAC is completely used when influent and effluent are equal. Drinking water GAC applications use three to20 percent of the original total pore volume in unused GAC. This pore volume difference between unused and used GAC can be revealed by the GRPD test method.
In contrast to drinking water applications, industrial GAC usage typically results in use of 20 to70 percent of the original total pore volume in the unused GAC. Industrial GAC applications typically have ppm concentrations in the raw water and drinking water has ppb concentrations in the influent. Concentration differences determine how much of the unused pore volume is filled in the used GAC.
- Ken Schaeffer and Robert Potwora. Coconut Shell Versus Bituminous Coal Activated Carbon. Water Conditioning and Purification, June 2008, pp. 70-74.
- “New Directions in the Activated Carbon Industry.” Henry Nowicki, Mick Greenbank, and Barb Sherman. Filtration News, Vol. 20, No. 5, Sept. 2002, pp. 33-39.
- Advanced Instruments for Sorbent Adsorbate(s) Determination and Sorbent Adsorption and Desorption Evaluation. H. Nowicki and M. Greenbank. Presented at 23rd Army Science Conference, Orlando, FL, Dec. 2-5, 2002.
- New Differential Heat-of-Adsorption Instrument. H. Nowicki and M. Greenbank. Presented at 23rd Army Science Conference, Orlando FL Dec. 2-5, 2002.
- H.G. Nowicki, et al. GRPD instrument reveals sorbent materials critical information. Pittsburgh Conference (PittCon) New Analytical Instruments, 230-12P, March 2, 2008
- H.G. Nowicki et al. GRPD determination of activated carbons in coal fired electric power plants fly ash samples. PittCon Analysis for Energy Production, 1570-8P, March 4, 2008.
- H.G. Nowicki, et al. GRPD location of positional placement of chemical impregnants into Activated Carbons. PittCon Material Sciences, 2740-5P, March 5, 2008.
- Henry Nowicki, Barbara Sherman, George Nowicki. GRPD Modern Activated Carbon Testing. Filtration News, August/September 2007, pp. 28-33.
- Henry Nowicki, Bud Carr, George Nowicki. GRPD Comparison of Two Activated Carbon Felts. Water Conditioning and Purification, June 2007, pp. 28-33.
- Henry Nowicki. Modern GRPD Test Differentiates Activated Carbons. Water Conditioning and Purification, February 2007, pp. 62-64.
- Henry Nowicki and Barbara Sherman. Activated Carbon: Advanced Test Method. Filtration News, July/August 2006, pp. 14-21.
- Henry Nowicki and Barbara Sherman. Activated Carbon: Advanced Test Method. Water Conditioning and Purification, pp. 32-36. March 2006 at https://wcponline.com/PDF/Nowickietal0306.pdf.
- Henry Nowicki and Barbara Sherman. Activated Carbon: Comparison of Classical and Advanced GRPD Test Methods. Water Conditioning and Purification, April 2006, pp. 28-35.
- H. Nowicki. Determination of Adsorption Energy Distributions in Activated Carbons and other Sorbents. 200-20P. Pittsburgh Conference. 2006.
- H. Nowicki. Selecting the Best Activated Carbon for the Process Application. Filtration News, July/August 2005, pp. 18-22
- H. Nowicki. GRPD Helps to Select the Best GAC for a Municipal Drinking Water Plant. 22nd International Activated Carbon Conference. Pittsburgh, PA, Oct 7-8, 2008.
- G. Nowicki, et al. The Importance of Activated Carbon Terminology: A Glossary of Terms. Water Conditioning & Purification, February 2008, pp. 88-94.
About PACS and the Authors
PACS is celebrating its 25th year of providing technical products and services for the activated carbon industry. PACS will host the 24th International Activated Carbon Conference and Courses program October 3-11, 2009 near Pittsburgh, PA.
Henry Nowicki, PhD, has 34 years of broad and deep activated carbon experience. Nowicki is a teacher in the Activated Carbon School and his short course is titled “AC Adsorption: Principles, Practices and Opportunities.”
Wayne Schuliger, PE, Technical Director for PACS, has 43 years of activated carbon experience. Schulinger is a teacher in the Activated Carbon School; his short course is titled “Design, Operation and Troubleshooting of AC Adsorbers.”
George Nowicki, BS, is an activated carbon chemist, having been at PACS Laboratories for 11 years. He combines business and technical skills for PACS Activated Carbon Services.
Barbara Sherman, BA, is the manager of operations, having directed day-to-day business and conferences for 25 years at PACS Testing, Consulting & Training.
All authors can be reached at 409 Meade Dr, Coraopolis PA 15108, by phone at (724) 457 – 6576 or by e-mail at www.pacslabs.com.
For a GRPD Full Characterization 20 page report e-mail Henry@pacslabs.com or call (724) 457-6576.