George Nowicki, Henry Nowicki, Wayne Schuliger, and Barbara Sherman
In the first article in this series we covered some advanced activated carbon (AC) test methods (1). The highlight of (1) that article, and the take home lesson, was the advanced test method called Gravimetric Rapid Pore-Size Distribution (GRPD) invented by Dr. Mick Greenbank. The GRPD is delivering information about commercial activated carbons and other new research sorbent products and composites that no other testing method provides. The GRPD test method is important to the future activated carbon industry. It is worth your time to understand the GRPD benefits.
This second article in the series is designed to focus on standard testing methods for solving problems for the activated carbon industry. Although the GRPD method can provide information not attainable from standard methods the industry often relies on them to conduct business. The standard methods presented in this article and later articles are not intended for you to be a laboratory technician. They are presented in a way to show how the standard methods can be applied by activated carbon users to better manage their purchasing and trouble shooting working activated carbon adsorption systems. Activated carbon adsorbers range from less than a gram in cigarette filters to tons at municipal drinking water plants. The standard testing methods are relevant to all sizes of activated carbon adsorption units.
Activated carbon misunderstandings
Misunderstandings are a part of life. A consensus on word usage is a way to help decrease misunderstandings (2). Five important misunderstandings in the activated carbon industry are shared with you below. You are invited to send additional industry misunderstandings to the authors for later publications.
Often in the print media and at trade conferences authors state that chlorine taste and odor removal by activated carbon is a physical adsorption phenomenon. This is a misunderstanding because it is a chemical reaction between this disinfectant and activated carbon. You can easily demonstrate this by taking a few particles of activated carbon and treating them with five milliliters of hypochlorous acid, like grocery store Clorox® bleach or a generic version of the product. If you do this treatment you will see disintegration of the particles and their degradation to a dark brown solution. It is true that pure chlorine gas will physically adsorb and desorb on activated carbon when there is absolutely no water present. When traces of water are present on the carbon chlorine chemically reacts with the carbon and the chlorine is converted to chloride. Chlorine is an oxidizing agent and carbon is a reducing agent, similar to copper metal. At low concentrations chloride adds no taste to your drinking water. Standard test methods are available to provide (AC) capacity for chlorine taste and odor reduction in unused and used activated carbons.
Usage of the terms “virgin” and “spent or exhausted” as related to activated carbon status can be misleading and needs to be replaced by the terms “unused” for virgin and “used” for spent as recommended by ASTM International.
The Phenol Number test method has been removed from the list of accepted test methods because it gives erratic results, although it is still quoted by some scientists as a useful test method. It is bad because the test result has unacceptable variance due to the amount of oxygen in the test solution. Most of the oxygen comes from the test carbon samples. The AC micro-pores concentrate atmospheric oxygen two to three times above the ambient atmospheric pressure. Phenol is oxidatively polymerized on the carbon surface in the presence of oxygen. Thus you have a chemical reaction in addition to the physical adsorption to explain the removal of Phenol from solution. The original Phenol Number test method (assumed physical adsorption but it was later learned the chemical reaction was dominant) was based on a five-point isotherm. This test should not be used in your purchasing decisions. A good rule is to avoid the use an unstable chemical as the basis of a test method, if it can be avoided.
Activated carbons that have been stored for a long time can dramatically change their performance. Atmospheric oxygen and contaminants can negatively impact the carbon performance. Organic air contaminants can fill adsorption spaces in the carbon. Oxygen can slowly oxidize (add oxygen functional groups) AC: organic acids, peroxy acids, ketones, aldehydes, epoxides, anhydrides, lactones, esters, hydroxyl groups, and other oxygen containing groups. Treating activated carbons with ozone and other strong oxidizing agents are used to provide surface modifications. These surface modifier chemicals accelerate formation of oxygen functional groups in the starting activated carbon. These oxygen containing organic functional groups are added to the periphery of the graphitic platelets. Adding these groups increases the ion exchange and aqueous ionic metal attractions and can degrade selected physical organic adsorptions.
Another misunderstanding is that lower apparent densities, grams per cubic centimeter of material, are better than higher apparent densities because they adsorb more iodine or butane in standard ASTM activity test methods. Lower apparent densities (AD) are do to the fact that manufacturers increase the residence time in the activation furnace and pluck off more carbon atoms to create larger adsorption spaces. Manufacturers can make a family of activated carbons with densities between 0.4 to 0.8 grams per cubic centimeter by controlling the time thru the activation furnace. These lower density products have larger adsorption spaces, do to slower transit thru the furnace on average lower densities mean more weakly adsorbate binding adsorption sites, loss of some high energy binding sites, and decreased mechanical strength. Activity numbers and mechanical strength are inversely related for a given type of activated carbon. Higher iodine and butane activity can be attained at the cost of lower high energy
binding site pore volume, and decreased mechanical strength and bulk AD. In todays regulatory market it is often the higher density that is preferred for applications that involve the adsorption of small molecules that are difficult to remove such as dicloromethane and trihalomethanes because they need high energy adsorption sites to attract and hold them to the carbon surface. The GRPD test is used to determine the number of high- and low- energy sites. Iodine and butane activity numbers provide total binding sites and do not provide the distribution of binding sites like the GRPD does. Theoretically these higher apparent density carbons should be lower cost because they go thru the activation furnace faster, and all other manufacturing costs are independent of apparent density.
ASTM standard test methods
The single-body of professionals who have contributed the most to the activated carbon industry development are the volunteers to the ASTM International activated carbon standards committee. These methods are the most widely used between buyers and sellers of these materials. A table is provided listing the latest version of ASTM test method numbers and titles for activated carbon.
Apparent bulk density test method
The bulk received and dry apparent densities (AD) are very useful ASTM test methods. These test methods are easy to do and require low cost equipment with low maintenance. Laboratories should do these tests on all incoming samples and users should request these tests on their projects. As the name received AD implies it is the material received at the testing laboratory. Labs need to assure that all parts of the received sample have an equal opportunity to be in the test sample which is a 100 milliliter volume. The test result is obtained by a controlled free fall of the particles into a 100 milliliter graduated cylinder as defined in the ASTM test method. If the particles in the process application get reduced in size or gain weight by adsorbing organics the AD goes up. Dry apparent density is after heating the sample for three hours at 150̊C. Oven-drying of unused carbons provides a moisture estimate. Both received and dry apparent density tests are reported in grams per milliliter. Grams per cubic centimeter can be easily converted to pounds per cubic foot by multiplying by 62.43; engineers work in pounds per cubic foot.
Some sources of AD measurement error and solutions are provided. Representative samples: lab does riffling and the sampler gets multiple samples of the incoming load for AD. In laboratory testing slow free fall of irregularly shaped AC particles and not shaking the 100 milliliter graduated cylinder are critical. Sources of error due to improper filling techniques or shaking the graduated cylinder while filling can greatly affect the AD results. As a part of the training program for new analysts have them invert the graduated cylinder after they complete the AD test. The analyst will see a 5-15% volume increase.
Determining the moisture content on an activated carbon provides useful information. There are two approved ASTM test methods. The oven drying moisture determination test method uses the received and dry apparent densities to determine the percent moisture on activated carbon. Oven-drying should only be used when water is the only volatile matter present and when the carbon is not heat sensitive. Some carbonaceous materials change size and shape upon heating in air. The Dean-Stark moisture determination test, which uses the xylene-extraction method, is used when the sample is suspected to contain nonwater-miscible volatile organic compounds in addition to water, or if it is suspected that the activated carbon sample may be heat sensitive. The Dean Stark test apparatus is shown schematically.
If you would open your home point-of-use (POU) device and pass the water contents thru a coffee filter you would see the carbon particles agglomerated together. After a few hours of air drying you would see the particles become free flowing. However, the particles are still filled 18-25 percent on a weight-weight basis with water. The intra-particle water can be removed by oven-drying and organic solvent extraction. A typical specification between buyer and seller is 1-2% moisture.
Iodine activity test method
All activated carbons are not the same, although this ASTM test method often indicates that vastly different carbons are the same. This observation is due to the fact that the ASTM Iodine Number and Butane Activity Number are both approaching the total pore volume in samples. The iodine number indicates the amount of iodine adsorbed (in milligrams per one gram of activated carbon) under controlled testing conditions.
Often AC users apply the iodine test to help make decisions when to replace used carbon. Carbon does not last forever and needs to be replaced with unused AC periodically to maintain specified process control. For example the installed iodine number is 1000 and the user periodically takes samples out of the working AC beds for iodine number tests. When the iodine number hits 400, it is replaced with unused AC and the replacement cycle begins again.
The Gravimetric Rapid Pore-Size Distribution (GRPD) test method invented by Dr. Greenbank is the most practical and cost effective test to get the cumulative and differential pore volumes as a function of adsorbate(s) binding strength (in units of calories per cubic centimeter) for the specific carbon that will be used in your application. We previously tested and reported ten samples of activated carbon for ASTM iodine number and BET surface areas and all the samples had similar iodine numbers and BET surface areas, but when the same ten samples were tested using the GRPD test it showed that some of the ten samples were quite different in their high energy binding sites. The ASTM iodine, butane, and carbon tetrachloride only measures the total pore volume; they do not provide information that the GRPD reveals about the relative proportions of micro-, meso-, and macro-pores, which is the required information if you want to measure the carbon’s ability to adsorb compounds that are in your application(s). Macro-pores sizes are best characterized by mercury intrusion, because mercury does not spread on the carbon surface because its cohesive forces are stronger than its AC adsorptive forces.
Butane activity test method
The ASTM butane activity number and its predecessor carbon tetrachloride (CTC) tests are ratios (in percent) of the butane or CTC weight adsorbed by an activated carbon to the weight of the activated carbon sample. Iodine number, butane activity, and carbon tetrachloride testing are used to determine the relative activation level and the general total adsorption capacities of unused or used activated carbons by measuring the carbons initial or remaining total pore volume. These tests are a relative indicator of the porosity and surface area of an activated carbon sample but these test results should not be generalized. Making a purchasing decision solely on an iodine or butane number from different vendors is often not good judgement. However, it is common for purchasing agents who do not understand the fundamentals of activated carbon to use this approach. Singly comparing the iodine or butane number from different vendors and cost per unit volume or pound is still done today. These popular ASTM activity tests do not differentiate the total pore volume into its incremental adsorption binding energies and corresponding partial pore volumes with different binding energies like the GRPD provides.
The GRPD test should be used to compliment the ASTM butane and iodine activity tests. Large numbers of samples are run for iodine and butane activity to drive the carbon industry. For example AC manufacturers and reactivators take samples off their furnaces and do AD, iodine number, and butane activity to control their process, also AC users make replacement decisions based on these activity lab numbers.
Standard activated carbons for quality assurance
It is good laboratory practices (GLP) to have standard known valued activated carbon materials for the test methods being provided available. Standards are used to have the analyst(s) running them periodically to check the testing performance, on a regular schedule. Since our mission statement at PACS Inc. is to provide the global activated carbon industry (users, manufacturers, and suppliers of activated carbon products) with services and product which are needed: we provide standard activated carbons for quality assurance programs. Standards include materials for the Iodine Number, Butane Activity, Butane Working Capacity, Carbon Tetrachloride Activity Number, Received and Dry Apparent Densities, and both high and low Molasses Number decolorization standard activated carbons. Without representative test samples the value of test results are less valuable.
Attaining a representative sample is quite possibly the most important part of performing laboratory testing yet the laboratory technicians performing the laboratory testing are typically not directly involved in collecting the samples. The people who collect the sample should be collecting samples from the top, middle, and bottom of the adsorbers or supersacks. When the sample of activated carbon arrives at the laboratory for testing it is necessary to riffle the sample down to the required test size sample for the specific test that is to be performed. By riffling the sample it gives all the particles an equal chance of making it into the test sample. Grinding particles to a powder is another way to get representative samples for testing. Grinding is most important when the test users a small amount of material and the test results depend on attaining equilibrium between solid and liquid phases.
When the laboratory management uses commercial known standards, or other sources of know materials, it is an easy way to assure the laboratory is in “statistical control” which means there is evidence that known materials are run on a defined schedule and the test results for each parameter fall within an acceptable range around the true value for that test parameter. Every test method has an acceptable precision. Known standards are run as blind and double-blinds. A blind standard means the analyst knows they are testing an unknown standard and a double-blind standard is when the analyst thinks they are running another regular sample but it is a known standard. For example iodine numbers are reported in milligrams of iodine adsorbed per gram of dry powdered activated carbon, butane and carbon tetrachloride are reported as grams adsorbed per 100 grams of as received sample activated carbon, butane working capacity is reported as the difference between the butane activity and the amount blown off with a define air stream thru the butane saturated carbon in grams per 100 grams carbon. The molasses number has no units, as it is a ratio of the molasses decolorization optical density of the standard divided by the samples optical density after standard contact with the same standard molasses solution, times 100.
The National Institute of Science and Technology (NIST), previously called NBS, supplies a wide variety of standard materials for many industries and academic markets but presently does not supply standards for the activated carbon industry. It is possible that NIST will supply these kinds of standards in the future. Since there were no commercial supplies of these important activated carbon standards we have been supplying them to the market place until, and we will continue to do so, someone takes over this important basic GLP tool of running standards along with samples.
Purchasing ASTM standard testing methods
The ASTM International test methods for testing activated carbon were developed under the jurisdiction of ASTM Committee D-28. Individual test methods can be purchased from ASTM for roughly $30 per test method, or a book containing all of the activated carbon test methods can be purchased. If you purchase a book ask for the small book containing only activated carbon test methods. The table listing the latest ASTM test methods titles and numbers are provided in the small book. Individuals who order laboratory tests typically use the D- method numbers which shows the latest revision. The test methods for testing activated carbon begin with the letter D- and are followed by the test method number, a hyphen, followed by the year that the test method was originally developed, followed by the year that the test was last updated. The ASTM test methods are updated from time-to-time and new test methods are slowly added to the editions of the ASTM Standards.
ASTM provides testing methods for many different types of materials to facilitate the buyer-seller specifications for a wide variety of commercial materials. ASTM standards test methods for testing activated carbon are used by activated carbon manufactures, users, and academia in characterizing and buying activated carbon. The ASTM methods cover the proper way to perform the test, the required equipment, the repeatability and the precision of the tests, and how to interpret the data.
The ASTM test methods provide good instructions for laboratory technicians to follow however they are lacking in some critical areas. They do not fully describe the possible dangers and personal safety equipment that would be required to ensure employee safety, the required steps to get a representative sample are not defined with the method, and an easy way to interpret the data generated by performing the tests with computer programs should be required.
The specific safety issues involved in performing ASTM tests are not adequately described; only general statements are given about safety issues. Interpreting the data that the ASTM test methods generate is were most errors are made. The ASTM test methods do not explain how to interpret the data in a way that a laboratory technician without a Ph.D. could easily interpret the data. Recognizing that there was a need for software to help interpret the data that ASTM test methods generates, PACS Inc. has developed software for sale that can aid laboratory technicians in interpreting the user data. (3) The granular activated carbon (GAC) ASTM particle sizing test method for example has many parameters that must be met for the test to be performed properly, and the mathematics involved in interpreting the particle sizing data to get the required effective size, uniformity coefficient, and mean particle diameter are quite complex. The PACS GAC particle sizing software (3) helps laboratory technicians choose the correct screens, and alerts them if any of the parameters set by the ASTM are not met, and it calculates the effective size, uniformity coefficient, and mean particle diameter after all the required parameters have been met and generates a two page report, which can be put into the laboratory archives and given to the client. A copy of a typical particle PACS sizing software report is enclosed.
About PACS: Testing, R&D, Consulting, Training and Conferences
Professional Analytical and Consulting Services, Inc. (PACS) is in its third decade of providing activated carbon services and other services to engineers and scientists: laboratory testing, R&D, consulting, training and conferences. PACS hosts the International Activated Carbon Conference and Courses programs in Pittsburgh, PA every October. Information about the firm’s services are available on its website www.pacslabs.com or by phone at (724)-457-6576.
About the authors
H. George Nowicki III, B.S., B.A. is a laboratory technician and new business developer for PACS. He can be reached at e-mail: Georgepacs@aol.com or by calling 724. 457. 6576
Henry Nowicki, Ph.D. and M.B.A. provides the introductory course for the Activated Carbon School titled “Activated Carbon Adsorption: Principles, Practices, Applications and Opportunities”. Dr. Nowicki directs the day-to-day Testing and Consulting services for PACS. He can be reached at 724. 457. 6576 or e-mail: Henry@pacslabs.com and web site: www.pacslabs.com.
Wayne Schuliger, P.E. provides the PACS short course titled “Design, Operation and Trouble Shooting Activated Carbon Adsorption Systems”. He utilizes his 40 years of activated carbon adsorption engineering experiences to provide consulting services and directs the technical activities at PACS engineering services. He can be reached at e-mail: Waynepacs@aol.com.
Barbara Sherman, B.A. and M.B.A. directs the day-to-day operations of sixty different PACS short courses and the bi-annual International Activated Carbon Conference and Courses program. In 2009 the IACC conferences are in Johannesburg, South Africa in May, and Pittsburgh, PA in October in 2008 and 2009. She can be reached at Barb@pacslabs.com.
- Henry Nowicki et. al., Advanced Activated Carbon Test Methods. Water Conditioning and Purification, March 2008, p. 80-86
- George Nowicki et. al., Activated Carbon Glossary. Water Conditioning and Purification, February 2008, p. 88-94
- Henry Nowicki et. al., Software Programs For The Activated Carbon Industry. Filtration News, July 2008