By Henry Nowicki, Ph.D., Barbara Nowicki and Homer Yute

Summary: Selecting the best activated carbon for a specific application is an important decision. Laboratory testing methods are available to help in this choice. This article focuses on the ASTM activated carbon standardized test methods and new methods to evaluate sorbent performance for MTBE reduction from drinking water.


Activated carbon testing has been fueled by the need for information to enable decision making. During the last decade, more suppliers have emerged and users need to know that they offer comparable quality, which is determined by lab testing. Obviously, sorbent performance evaluations need representative samples; lab tests are conducted with gram quantities of sorbent. Real-world applications can use thousands to millions of pounds. Assuring a representative sorbent test sample is especially important for activated carbons. Activated carbons are a family of materials that can have significant performance variation from batch to batch or within a lot.

Standard measurements
This article focuses not so much on sorbent sampling but test methods developed by the American Society for Testing and Materials (ASTM) and American Water Works Association (AWWA) to determine the physical-chemical properties and performance of granular activated carbon (GAC) and powdered activated carbon (PAC). These methods—excluding gas phase methods in this discussion—facilitate purchasing decisions made in the potable water industry. A list of available standardized methods is shown in Table 1. An abbreviated glossary of common testing terms is provided in Table 2.

Apparent density
The test method for apparent density (AD) of activated carbon covers determination of the apparent or bulk density of GAC. For purposes of ASTM test methods, GAC is defined as a minimum of 90 percent being larger than 80 mesh (0.18 millimeter, mm).

This determination is made on free falling GAC (each particle falls independently) into an appropriate sized graduated cylinder. Dividing the mass by the volume gives the AD in grams per cubic centimeter (g/c3). The AD can be determined on the lab-received material or it can be dried using ASTM method D 2867-95. The dry bulk density (pounds per cubic foot) is obtained by multiplying this dry AD value by 62.43. This packed density is essential when designing vessels to hold GAC and for ordering materials to fill existing vessels.

Particle size distribution
In the test method for particle size distribution of GAC, a known weight of GAC is placed on the top sieve in a stack of five sieves with a catch pan. The sieves are shaken using standard conditions and the weight on each sieve and bottom pan is recorded. The data obtained may also be used to calculate mean particle diameter (MPD), effective size and uniformity coefficient. Previously we have described a software program designed to analyze and graph data from the ASTM sieve weights.1

The distribution of GAC particle sizes is important to provide the proper liquid contact times. Smaller GAC particles have a faster adsorption rate and can filter smaller particles out of water flowing through GAC beds. When GAC particles are too small, they can cause head pressure loss with decreased flow through the GAC beds, and abnormal carbon loss on backwashing to remove accumulated particulate matter.

Total ash content
This test method describes a procedure for the determination of total ash content of activated carbon. The ash is determined by heating a weighed dried sample at 650 ± 25oC for three to 16 hours until constant weight is obtained. Data have been presented demonstrating use of laboratory microwave ovens to determine percent ash in GAC.2 The microwave oven method can be conducted in one hour, but it isn’t an approved ASTM method.

The amount and composition of the ash can limit the activated carbon’s applications. For example, high acid soluble iron can cause discoloration and off taste to the GAC finished water.

Historically, the ash was thought to be responsible for the 1-3 pH unit rise when carbon was put into water. The present thought for this pH phenomena is that the carbon surface has hetero-atoms which bind protons resulting in the pH increase until these active sites are satisfied. This pH rise can last until 200-300 bed volumes have passed through the GAC. This can be a problem for bottled water manufacturers.

Moisture content
These ASTM test methods provide two procedures for determining moisture content of activated carbon. The oven-drying method is used when water is the only volatile material present and the activated carbon isn’t heat-sensitive. The second method, xylene-extraction, is used when a carbon is known or suspected to be heat sensitive or to contain non-water-miscible (mixable) organic compounds in addition to water.

Carbons can look dry and have 20 percent moisture by weight. If you’re purchasing on a weight basis, moisture content is important. The moisture level in carbons is often required to present chemical properties on a dry weight.

Ball-pan hardness
To determine the ball-pan hardness number of GAC, a screened and weighed carbon sample is placed in a hardness pan with stainless steel balls and shaken using standard test conditions. After shaking the particle size degradation is determined by screen sieving again. The higher the ASTM hardness number the better; 100 indicates no particle degradation.

A new hardness test method, based on applying 70-100 pounds per square inch of pressure on thin beds of GAC, is designed to simulate the home tap water supply units in the marketplace.3

Accounting for pH
This test method covers determination of the pH of a water extract of activated carbon. A pH change may affect the adsorption capacity of some adsorbates. Organic acids and bases are most susceptible to pH change, because the non-ionic form of these compounds are adsorbed much better than the ionic form. The relative distribution of ionic/non-ionic organic acids/bases depends on the pH.

Aqueous phase isotherm
Using the aqueous phase isotherm technique, this practice covers the determination of the adsorptive capacity of activated carbon to reduce undesirable constituents from water. It can be used to evaluate the adsorptive capacity of both unused and reactivated GAC and PAC.

It’s recommended to determine the adsorptive capacity of activated carbon for—but not limited to—the following applications: 1) reduction of color from drinking water supplies, 2) reduction of taste and/or odor constituents from potable waters, and 3) reduction of toxic agents like methyl tertiary butyl ether (MTBE) from water. Previously, we reported a software program to automate data interpretation for this ASTM isotherm method.4

This practice can be applied to all types of water, including industrial wastewaters, potable waters, sanitary wastes and effluent waters. This practice is recommended to determine the operating capacity of GAC on a weight basis for (but not limited to) reduction of the following from water: color, taste and odor, toxic agents, surface active agents and total organic carbon (TOC).

Iodine number
This test method covers the determination of the relative activation level of unused or reactivated carbons by adsorption of iodine from aqueous solution. The milligrams of iodine adsorbed by 1 g of carbon using test conditions is called the iodine number, which provides standard information about carbons porosity. It can be used as an estimate of the carbons B.E.T. surface area (see Table 2). This is probably the most often-used test method by carbon manufacturers and users.

Dusting attrition
For the purpose of this GAC test method, dusting attrition (DA) is defined as the weight (or calculated value) of dust per unit time, collected on a preweighed glass-fiber filter in a given vibrating device during a designated time per unit weight of carbon. The initial dust content of the sample may also be determined.

New problems, new test methods
Even emerging water treatment problems can be solved with new test methods. The need to reduce trace levels of MTBE from drinking water supplies has challenged the activated carbon suppliers. The consensus of opinion is that activated carbons with small pores (voids between graphitic plates) have a competitive advantage over carbons with larger pore size distributions. Most ASTM and AWWA test methods determine the carbon’s adsorption performance with high doses of adsorbates. The iodine number, perhaps the most used ASTM test method, provides high capacity information. Iodine is an extremely well adsorbed material; it adsorbs into a wide spectrum of pore sizes.

C-14 & trace capacity Thus, a test method is needed that monitors and evaluates selectively the small pore content of different activated carbons. Small pores that efficiently reduce trace levels of aqueous MTBE need to be determined to evaluate different carbons. A new method to determine the trace capacity performance for MTBE is based on using C-14 radio-labeled MTBE and determining its reduction rate when in contact with different sorbents.5 Using MTBE-C-14 allows low part-per-billion (ppb) measurements on small volumes of water with excellent analytical precision and accuracy. This new method or others designed to determine trace MTBE reduction efficiencies will be useful for selecting the best sorbent for this application in the future. Calgon Carbon Corp. scientists have presented work on another new test method called the Trace Capacity Number.6

Rapid column test (RCT)
Contaminated water is typically treated by passing water through columns filled with sorbent, with flow rate adjusted to give sufficient contact time for adsorption/absorption of the target contaminants. Pilot studies using large columns with 4-8 inch diameters take a lot of time, but small column technology has been developed to accelerate sorbent evaluations. The rapid column test consists of grinding the activated carbon to a fine mesh (to decrease the time to obtain equilibrium of the adsorbate with GAC) and using a pump to push the water through a small column. This experimental set-up is shown in Figure 1.

Influent and column effluent samples are taken for chemical analysis. Column testing is used to define the sorption characteristics of different test materials under dynamic flow conditions. Small columns are used to limit the amount of materials and time required for testing. Columns have a 1.94 millimeter (mm) inside diameter and are packed with about 0.5 g of sorbent, resulting in a bed volume of about 1 milliliter (ml). A peristaltic pump is used to transfer the previously filtered feed solution through a 0.45 µm SuporÔ filter into the base of the sorbent column. The flow rate is 1.0 ± 0.2 milliliters per minute (ml/min). Feed is introduced at the bottom of the column to maximize contact of the solution and sorbent. Column effluent samples are collected through the breakthrough of the contaminants of interest. This method is useful to compare different sorbents for MTBE reduction as well as other specific organic compound evaluations.

Conclusion
The heart of any water treatment system is the treating sorbent. Carbon is most often used in the first step to reduce organics and chlorine. Failure to reduce these can result in fouling ion-exchange media and membrane based processes. Properly using several types of carbon together can produce water containing no detectable chlorine and less than 1 ppb of TOC. Laboratory tests can be used to monitor carbon performance and select the best carbon for the specific application.

References

  1. Nowicki, H.G., and H. Yute, “Software Programs for the Activated Carbon Industry: ASTM Particle Sizing Program,” p. 38-40, WC&P May 2000.
  2. LeBlanc, G., and H.G. Nowicki, “The Rapid Determination of Total Ash Content in Activated Carbon Using a Microwave Muffle Furnace and Comparison with ASTM Method,” 8th International Activated Carbon Conference (IACC-8), Pittsburgh, September 2000.
  3. Nowicki, H.G., “New Activated Carbon Hardness Test Method for Home Water Units,” IACC-9, Pittsburgh, September 2001.
  4. Nowicki, H.G., and W. Schuliger, “Carbon Software Program: Sorbent Performance Evaluation ASTM Aqueous Phase Isotherm Program,” p.102-106, WC&P October 2000.
  5. Nowicki, H.G., “MTBE Remediation Technologies and New Test Method,” IACC-9, Pittsburgh, September 2001.
  6. Megonnell, N.E., and A.F. McClure, “Carbon Variability and the Effect on Performance for MTBE Adsorption,” IACC-8, Pittsburgh, September 2000.

About the authors
Henry Nowicki, Ph.D., directs the PACS Inc. testing and consulting services business. Dr. Nowicki has published over 100 technical articles and made more than 200 presentations. He also is a member of the WC&P Technical Review Committee.

Barbara Nowicki directs the PACS short courses and conferences business. PACS has 57 courses and four conferences for scientists and engineers. Four courses and one conference are focused on activated carbon adsorption, including the International Activated Carbon Conference held each September in Pittsburgh.

Homer Yute, a mathematician and computer programmer with PACS Inc., has developed seven software programs for the activated carbon industry.

All authors can be reached at (724) 457-6576, (724) 457-1214 (fax), email: hnpacs@aol.com or website: http://www.pacslabs.com

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