Water Conditioning & Purification Magazine

Advanced Activated Carbon Test Methods

Henry Nowicki, Wayne Schuliger, Barbara Sherman, George Nowicki, Joseph Bellisimo

Introduction
We focus here on advanced activated carbon testing methods available today to the market place. (A June WC&P article will cover standard activated carbon test methods approved by the American Standard Testing and Materials {ASTM} Committee D28 on activated carbon.) By advanced methods, we mean those that many readers may be less aware of as compared to the better known standard methods used by the majority of the market place.

Activated carbon has a long and growing list of applications, but it is most often used to improve water and air quality. Since all activated carbons are not the same, advanced testing methods are needed to select the best carbon for specific applications. After selecting the best media for the application, adsorption systems need to be designed, manufactured and certified; then clients need to monitor the field performance and change the media when it becomes exhausted. Also, advanced activated carbon testing is central to developing improved and new adsorption products.

First and best choice
The best available technology to characterize activated carbons is called the Gravimetric Rapid Pore Size Distribution (GRPD) test method, invented by Dr. Mick Greenbank. Some new advancements in GRPD applications from our laboratory have been accepted for presentations1-5 which could not have been obtained by any other method besides GRPD. It enables direct testing of all forms1 of activated carbon: powder, granular, bead, pellet, carbon block, felt and cloth. Also, composite mixtures of activated carbons have been successfully tested with GRPD: fly ash2, chemically impregnated activated carbons3, used and unused carbon comparisons, soil or river bottom sediments with activated carbons, military and civilian personal protective fabrics containing activated carbons.

Prior articles in WC&P have provided selected pieces of GRPD testing results. Table 1 contains a summary of standard benefits provided by GRPD runs and Table 2 shows additional benefits provided by GRPD beyond the standard outputs. It is an ideal test method for users and suppliers of unused sorbent media because it provides a cleaning step to the media before testing begins and the media is challenged with (effectively) seven orders of adsorbate concentrations ranging from trace to near saturation. Revealing the full distribution of binding site(s) energy is unique to GRPD; this method reveals more critical sorbent information than any other single method1.

For the GRPD test method, the autographic charts constitute the core standard report. Each individual sorbent’s characteristic specific values for its adsorption forces for the high-, mid-, and low-adsorption energies is most useful for sorbent users, suppliers and manufacturers. Revealing the full-range of adsorptive binding forces so indicated, expressed in calories per cubic centimeter (cal/cc) of adsorption spaces, provides the data pertaining to aqueous- and vapor-phase isotherms. Isotherms are the most useful information for solving a wide variety of activated carbon application problems; they provide the information for a ‘go’ or ‘no go’ decision for economic activated carbon applications. Economical activated carbon applications usually provide a high adsorbate loading, grams of adsorbate per 100 grams of specific activated carbon. The GRPD method provides an economical way to get target compound isotherms in both the aqueous and vapor phases.

Activated carbon surface area
The Brunauer, Emmett and Teller (BET) method has been widely used to provide the surface area in squared meters per gram of activated carbons. The BET surface area is obtained by dividing the weight of a monolayer of adsorbed inert gas, like nitrogen, by its molecular area, for a single nitrogen molecule. The method’s authors clearly stated this technique was designed for homogeneous sorbents and not heterogeneous sorbents like activated carbons; therefore, we need to be cautious on how we use BET surface areas for them. The GPRD method provides a lower-cost route for determining the calculated BET surface area. Compared to conventional instrumental surface area methods, the GRPD determinations have a major advantage because all of the data necessary to provide the standard and additional GRPD information (see Tables 1 and 2) is stored digitally for later use.

Thermogravimetric analysis sorbent testing
Thermogravimetric analysis (TGA) of sorbent materials provides a way to reveal information about the cleanliness of purchased materials and activated carbon samples from process units. TGA consists of heating a one-gram representative sample using a well-defined temperature program with an inert gas to sweep away the off vapors. Some TGA instrument vendors provide mass spectrometry structure elucidation of the TGA off-gas vapors. An example of a TGA of three samples is provided on Tables 3 and 4 along with the five figures to show what it can do for you.

When applied to the investigation of solid surface properties, gas solid chromatography is usually called inverse gas chromatography. The temperature at which known adsorbates come off of activated carbons depends on the vaporization temperature and the binding energy of the adsorbate to the activated carbon surface. We have run standard carbons with known adsorbates in TGA and observed multiple peaks coming off of the activated carbon from the same single loaded compound. These are due to different adsorbate binding energies to the carbon surface: the higher the binding energy, the higher the temperature of elution from the carbon in TGA analysis. In the example TGA thermograms here, toluene is the valuable adsorbate for recovery and reuse. Note on the last graph there are two peaks for toluene, 225oC and 375oC. The higher temperature toluene peak is broader and its elution temperature higher by 150oC, because it is coming from the high energy binding sites; the lower temperature peak is coming from the relatively low binding energy sites. Toluene has a boiling point of 110.6oC; as you can see, it takes much higher temperatures to release it from this activated carbon surface. Activated carbons can be used to reduce the vapor pressure of toxic materials. (Safe storage and handling chemicals on activated carbon is another important underutilized application opportunity we are exploring at our laboratory.)

For each of the three samples, we made two tables, one showing analysis of sample weight loss in the 25-100 oC, 100-250 oC, 250-450 oC and 450-980 oC ranges and another one showing weight changes from minimum to minimum on the differential temperature gravimetric (DTG) curve.

Comments for samples W-228, W-229 and W-230:

  • TGA measurements were carried out in dry nitrogen using the high-resolution option with the maximum heating rate of five °/min. TGA curves were measured up to 980oC. Each figure shows the weight-loss curve (TGA curve) and its derivative (DTG curve). Table 3 shows the weight loss changes for the strictly defined temperature ranges, while Table 4 shows the values for the temperature ranges determined on the basis of the location of the minima on the DTG curves (the temperature range for each weight change is given in the Table 4 brackets).
  • For each of the three carbons, the first figure below shows analysis of the TGA and DTG curves for 25-100 °C, 100-250 °C, 250-450 °C and 450-980 °C ranges, while the second figures shows analysis for the ranges determined by the minima on the DTG curves. In the latter case, these minima vary slightly from sample to sample.
  • The last two figures show the comparison of the TGA and DTG profiles for all three carbons studied on the same thermogram.
  • This time, the carbon samples show a large weight loss in the range up to 100oC, which may be due to water vapor adsorption.

Activated carbon tester
A simple, low-tech device to estimate the remaining service life in unused and used activated carbons (AC) was developed6; illustrated here. ASTM recently approved this device as a standard screening method7. It is based on the fact that when activated carbon adsorbs, heat is given off. The amount of adsorption heat is directly related to the types and amounts of empty adsorption spaces in the partially used or unused activated carbon test samples.

The tester consists of a 50 ml polypropylene screw-capped container graduated to 30 ml, filled with mineral oil with a 10 – 30 oC thermometer. Typically, a tablespoon of fresh unused activated carbon gives a three-to-five degree (C) rise, depending on the activated carbon quality, in about 10 minutes when immersed into the mineral oil. No sample preparation is required and all forms of activated carbon have been tested in the device. As an unused activated carbon fills its initial empty adsorption spaces with adsorbates, one will observe a decreased temperature rise in the tester. The used carbon provides the smaller heat rise value (compared to the starting activated carbon). It is important for the carbon user to save a sample of the starting carbon for use as a control in the AC Tester estimation of remaining service life. Typically, activated carbon user clients do not save some of the starting media put into the adsorbers; thus, the ideal control sample for the AC Tester is often not available. With our many years of activated carbon experiences we can reasonably estimate the heat rise for the starting carbon. When all of the adsorption spaces in the initial carbon are filled, there is no additional temperature rise in the tester. Comparing the tester temperature rise in the AC control sample with the partially used AC sample provides an estimate of remaining service. The first author of this paper provided and presented the early experimental work on the device1. There are many applications for this simple and now ASTM-approved screening device. A common question for carbon users is, how much longer can I use the activated carbon adsorber or where is the mass transfer zone (MTZ) in the working carbon system? Taking small, tablespoon-sized samples out of the working adsorption system at different depth locations is a way to find the MTZ, which is where contaminants are being removed. Above the MTZ, the carbon is completely exhausted; below the MTZ, the carbon is completely unused. ASTM is recommending ‘unused’ and ‘used’ as a replacement for the older terms ‘virgin’ and ‘spent’. Newly purchased media is easy to screen with the tester and answers can be provided in the field, but you will need a second tester to monitor ambient temperature changes to provide temperature corrections to the heat-of-immersion result for samples. By comparing the total adsorption heat upon mineral oil immersions from the initial starting control carbon with a partially used carbon, an estimate of the remaining service life is obtained.

Most, if not all, activated carbon test methods use small portions of carbon for testing as compared to the applications. It is critical that the analytical testing laboratory obtain and test a representative sample. The analytical service lab staff must educate the individuals who will collect samples for testing. In the activated carbon industry, typically the sample collectors are different than the lab testers. We could use some volunteers to write articles and provide sampling equipment on the sampling issue!

In the GRPD, TGA, the illustrated tester and most ASTM test methods, the lab tests small portions of the whole sample received. We recommend that samplers collect 200-300 grams of activated carbon, even though we only use gram-sized samples for testing. Our lab assures a representative test sample by passing the received sample through a riffler, which provides an opportunity for every particle in the received field sample to be represented in the sample tested by the lab. Simply taking a lab test sample off of the top of a large field sample can result in non-representative results. Another important error made by activated carbon users is that they typically put all of the carbon in the adsorber; they must remember to get a sample of incoming activated carbon. To compare the starting unused carbon against used requires a retained sample of the starting carbon.

ASTM adsorption isotherms
Aqueous- and vapor-phase isotherms results are important to engineers throughout the chemical process industry, from small POU devices to large 100,000 pound activated carbon adsorbers. Knowledge about target compounds adsorption capacity and kinetics is desirable in the design of process units. Adsorption isotherms conducted using ASTM test methods provide a ten-point isotherm. In the ASTM aqueous isotherm method, carbons are ground to a 325 mesh and require 95 percent passing through this sieve (smaller particles decrease the time to reach solid-liquid equilibrium but do not change the material on a nanotechnology scale size) and dosed low to high weights into separate flasks. Each flask received the same volume of challenge water, with known concentrations of contaminants. Typically carbon doses are a few milligrams to 20 grams and 100 to 1000 ml of challenge water.

In the ASTM method it is critical to reach equilibrium before separating the solid particles form the liquid, i.e. there is no further changes in the liquid-phase contaminant concentration. The time to reach equilibrium is often 24 hours or longer. A few preliminary activated carbon dosage runs before performing the 10- point isotherm is recommended. Carbon dosages need to be chosen which remove 50 ±40 percent of the contaminant. Not allowing enough time to reach solid-liquid equilibrium is the single major cause of developing poor isotherms. We have previously reported data in WC&P about a computer program to interpret the raw isotherm data and provide a Freundlich graphical isotherm presentation.

The GRPD provides standard aqueous-phase isotherms which can (additionally) be translated into vapor-phase isotherms at your chosen temperatures using the Polanyi model of 1914, perhaps the most powerful model for heterogeneous adsorbents like activated carbons. This enables additional utilization of the standard GRPD for single component isotherms to multi-component and other problem solving opportunities.

Microbiological sorbent testing
Our laboratory has reported a simple way to test for coliform and fecal coliforms on activated carbon samples8. This method consists of using sterile water to extract the activated carbon and use the extract in the US EPA-approved IDEXX diagnostic test method. Dr. Kelly Reynolds has been doing an outstanding job for WC&P to educate us on the advancements in microbiological issues; advanced test methods will be needed to deal with microorganisms.

Software programs
Software programs drive productivity and quality assurance of the information used to make decisions. We recommend developing software solutions for all technical and business management aspects of your business. Computers and internet services have completely changed our business opportunities. Take full-advantage of these modern technologies.

Accelerated sorbent mini-columns
Small sorbent columns can provide quick and accurate representations of large pilot columns. A schematic of a small mini-column is enclosed.

The use of 1-3 millimeter diameter columns to provide activated carbon performance evaluations was initiated when Professor Milton Manes noted a colleague purchased a High Performance Liquid Chromatography (HPLC) analytical instrument at Kent State University in Ohio. Dr. Manes and his students astutely put together small, low-cost equipment to mimic HPLC and provide Accelerated Column Tests for activated carbon performance evaluations.

Before the small columns came into industry use, it took a month to provide the client aqueous expected activated carbon performance. These earlier evaluations used large columns and took a lot of time because four six-inch diameter, five-foot tall columns were run in series. These relatively large columns required large volumes of challenge water to get breakthrough curves. Advanced test methods using mini-columns provide a breakthrough curve in about one day with only one to five liters of raw water (depending on the concentration of contaminants) instead of hundreds of gallons of raw water for the six-inch diameter columns. These advanced miniaturized test methods have been important for faster and more economical problem solving. Mini-columns provide bed volumes treated to client specifications and the shape of the mass transfer zone, i.e., a sharp fast or slow sigmoidal shaped contaminant breakthrough profile.

Inch-sized pilot columns
In order to determine the amount of activated carbon needed and its replacement rate for domestic and industrial applications of activated carbon, inch sized pilot columns can be used. It is not a good idea to rely solely on the mini-column performance evaluations. After many years of activated carbon experiences, we recommend providing inch diameter sized columns or larger with the specific activated carbon you plan to use in your application. Another option for meaningful pilot sized evaluation is to use commercially available five-gallon activated carbon adsorbers that are designed for relatively small clean-up jobs. They are convenient for the client at their site: the client passes raw contaminated water through the five-gallon unit until the effluent is equal to the influent quality. When the ‘in’ and ‘out’ water are equal, the media is completely exhausted. This treated volume-to-media exhaustion is useful to design engineers. Engineers like to put activated carbon adsorbers in series, to maximize the carbon utilization before disposal, gallons treated per pound of carbon. These advanced field methods are useful to demonstrate activated carbon applications and provide on-site staff education on how to operate these adsorbers.

Commercial field monitoring
Once adsorption systems are in field operation, they need to be monitored for performance: they do not last forever and will require media replacement. Advanced test methods using gas chromatography-mass spectrometry for organics, inductively coupled plasma optical emission spectroscopy for metals and ion chromatography for ionic species are available today to the market place. Often regulatory limits on contaminants determine when the sorbent media must be changed.

The major POU/POE certification organizations have laboratories with advanced analytical testing equipment to evaluate vendor performance claims. (NSF International in Ann Arbor, Mich. and WQA in Lisle, Ill.)

Education on sorbent materials
It is understandable that sorbent users do not fully know all of their options with commercial products, nor can they learn all that they need via formal education. Users should ask vendors to supply education and if they are purchasing sorbents the vendors will be willing to educate. Activated carbon users, suppliers and manufacturers need to continue their professional education to take full advantage of the opportunities offered by activated carbon. Groups like NSF International, WQA, NGWA, ACS, PACS and others provide education to cover these advanced test methods available in today’s market place.

References

  1. H. Nowicki, et. al. GRPD instrument reveals sorbent materials critical information. Pittsburgh Conference (PittCon) New analytical Instruments 230-12P, March 2, 2008.
  2. H. 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.
  3. H. Nowicki, et. al. GRPD location of positional placement of chemical impregnants into Activated Carbons. PittCon Material Sciences, 2740-5P, March 5, 2008.
  4. H. Nowicki, et. al. GRPD reveals new sorbent material for improved MTBE removal in drinking water. PittCon Process Analytical Chemistry 2770-11P March 6, 2008.
  5. Homer Yute, et. al. GRPD Polynomical Adsorption Equations for multi-component aqueous and vapor phase adsorption.” International Activated Carbon Conference (IACC-22) October 7-8, 2008.
  6. M. Manes and H. Nowicki. “A simple AC Tester device to estimate activated carbons remaining service life.” IACC-1, 1993
  7. Amos Turk, ASTM communication formal approval for AC Tester. IACC-20, 2007.
  8. G. Nowicki, et al. “Microbiological testing of activated carbon materials.” WC&P June 2007 page 38-40.

About PACS
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.

About the authors
Corresponding author Henry Nowicki, Ph.D. and M.B.A. provides the introductory course for the Activated Carbon School 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 by phone 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 Process Systems. He utilizes his 40 years of activated carbon adsorption engineering experiences to provide consulting services and directs the technical activities at PACS. He can be reached via e-mail: Waynepacs@aol.com.

Barbara Sherman, BA and MBA directs the day-to-day PACS short courses and the International Activated Carbon Conference. She can be reached at Barb@pacslabs.com.

H. George Nowicki III, BA, BS 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

Joseph Bellisimo, BS has 30 years of experience in providing isotherm testing, accelerated column testing, and building and evaluating activated carbon pilot systems, and helping clients to get commercial systems up and operational. He can be reached at joepacs@aol.com.

Authors’ note: Two standard GPRD reports are at www.pacslabs.com to provide you an example of a typical 21-25 page GRPD reports and the type and amount of information you can obtain from your sorbent materials with GRPD testing.

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