Water Conditioning & Purification Magazine

GAED Reveals Pore Sizes in Carbon Blocks Causing Aqueous Dechlorination Failure

By Henry Nowicki, H. George Nowicki, Wayne Schuliger and Barbara Sherman

All activated carbons (AC) are not the same. The main feed-stock sources and types are By Henry Nowicki, H. George Nowicki, Wayne Schuliger and Barbara Sherman performance sample is the fifth run in Table 1. It validates that the measurement process is working and is within commercial bituminous coal-, coconut shell- and wood-based AC. Activated carbons can be made from many raw materials, but these three dominate; they are readily available at low cost. There are many forms of AC: powder, granular, pellet, tablets, blocks, fiber, cloth and composites. Sorbent product developers have many types and forms of materials to work with. The Gravimetric Adsorption Energy Distribution (GAED) test method has been applied to many types and forms of activated carbon problems. An important use of activated carbon filters is to reduce taste and odor (T&O) for consumers; the most common source is from chlorine disinfection. It can be quite variable, depending on municipal plant operations. Activated carbon removes chlorine T&O by a chemical reaction, not physical adsorption. Activated carbon is a reducing agent (not quite as strong as copper metal) and chlorine is an oxidizing agent. Activated carbon is consumed by hypochlorous acid and aqueous chlorine. Placing some granular activated carbon (GAC) in a glass container and adding a small amount of five-fold diluted Clorox bleach yields black GAC particles dissolved to a brown solution.

This case study compared two batches of POU block activated carbons to help define why the recent batch was inferior for dechlorination of drinking water. GAED advanced test method was used to provide a full comparative characterization of these good and bad block activated carbons.

Test results

Pore sizes, adsorption spaces, and access to C-H reaction sites at the periphery of graphitic platelets are important to the dechlorination performance of activated carbons. Larger pores provide better water diffusion inside GAC and improved access to C-H reaction sites. Table 1 contains GAED-summarized adsorption data for 1,1,1,2-Tetrafloroethane (TFE) for two prior superior dechlorination carbon blocks with sample identifications Z-261 and Z-262, and two recent block samples AA- 11 and AA-12, with inferior dechlorination. A quality-control instrument performance sample is the fifth run in Table 1. It validates that the measurement process is working and is within its statistical control limits.

Figure 1 provides a graphical presentation for Table 1 summarized tabular data, called characteristic curves. Third-degree polynomials they yield are shown in Figure 1a. In the equation, y is the common logarithm of pore volume in cc/100 g carbon, and x is the e/4.6V adsorption potential in cal/cc.

These equations are all that is needed to obtain a wealth of information from the test method.

Performance prediction models

These polynomials are all that is needed to provide aqueous-phase, and gas-phase performances for AC and other sorbents. The equations provide the tested materials’ pore size distributions (PSD), cumulative adsorption energy versus pore volume distribution, differential adsorption energy versus pore volume, isotherms for client compounds of interest, BET surface area, mid-capacity and trace-capacity determinations.

Although these four runs on carbon blocks were similar, they revealed significant pore size differences between the superior and inferior dechlorination activities. The lower panel in Figure 1b is displayed with an expanded y-axis to better illustrate the differences in pore sizes in angstroms (Å). Note that the top of these graphs show pore sizes in Å, while the lower x-axis is adsorption energy in calories per cc.

Classical pore structures are defined as: one to five nano- meters as micropores, five to 50 as mesopores, and 50 to 100 as macropores. Greenbank simplified this classification into ad- sorption pores one to five nanometers and transport five to 100 nanometers.2 His classification divides the pores into those that provide physical adsorption and those that function to provide transportation to the physical adsorption sites.2 Increased pore sizes in the superior dechlorination carbon blocks provide easier access to oxidation-reduction C-H sites to chemically reduce hypochlorous acid, hypochlorite ions and chlorine to chloride (which has no taste and odor) at low concentration. Munici- palities typically supply drinking water with one to three ppm (milligram per liter) chlorine residuals. These hypochlorous acid and chlorine concentrations contribute significant T&O that most consumers find objectionable. During reduction of chlorine to chloride ions, oxygen functional groups (hydroxyl, aldehyde, carboxylic, ketone and eposide) are added to the periphery (chemically active sites are C-H) of the graphitic platelets. There are a limited number of these C-H functional groups, which are related to the dechlorination capacity. The interior of graphitic platelets has only C-C single and double bonds and is stable to these oxidizing agents (graphitic platelets are among the most stable compounds known; more stable than carbon in a diamond ring). Greenbank presents a good analogy to iron rusting, with oxidizing the graphitic platelet.1 In both cases, oxygen is added to the substrate via chemical reactions, iron oxide and oxygen functional groups added to graphite platelets.

Larger pores in the good block AC provide quicker water wetting and faster chemical kinetic diffusion and thus, more efficient dechlorination reactions with interior block AC graphitic platelets.2 The single most significant reason why AC users claim AC is not working is that they simply do not extract the compressed air from AC adsorption spaces before starting their water application. Smaller pores compress the atmospheric air pressure five-to-seven fold. It takes 72 hours of water soaking to remove this compressed air. Smaller pores (adsorption spaces) resist water flux through these one- to five-nanometer slits and crevices and thus produce stagnant flow. Larger pores (five to 100 nanometers) have lower resistance to water flow and thus better chemical diffusion, which increases the rate and capacity of oxidative chemical reactions at exterior graphitic platelet C-H sites. There are two major views on how activated carbons work in physical adsorption: pore sizes and adsorption energies. Actually, both camps approach the same conclusions on how AC works.

Pore size distributions

The Kelvin equation (modified by Halsey) can be used to convert the characteristic curve data to calculated BET surface areas and pore size distributions. This is not useful in terms of performance evaluations using adsorption isotherms, but some audiences are more comfortable with the concepts of pore radius and a series of capillary sizes when thinking about activated carbon. Figure 1b shows the differential pore size distributions; it clearly shows that loose coconut GAC has much more pore volume at each pore diameter. Manufactured carbon blocks consume significant amounts of carbon’s pore volume. The binder holding the GAC particles together in the block penetrates into some of the GAC pore volume. This loss of pore volume is a penalty that block AC manufacturers pay to obtain its benefits.

The multi-point BET surface area was calculated from these characteristic curves and is presented in the summary Table 2 with total pore volumes and high-energy adsorption pore volumes. BET surface area is the first adsorption layer on the activated carbon surface. It represents the high-energy binding sites and not the total adsorption pore volume.

Results and discussion

This project did not include GAED full characterization of the starting granular activated carbon used to manufacture these blocks. Thus, we cannot discern unequivocally if the differences in pore diameters are due to the starting carbon or the manufacturing process used to make blocks. The pore structures for these two batches are different.

Table 2 and Figure 1 reveal an average 58.6-percent difference in highest adsorption energy sites (25 to 29 cal/cc cumulative) and only a seven-percent difference in the total pore volume 16.00- versus 14.87-cc per 100 g C average differences between prior good and recent bad dechlorination carbon blocks.

Other problems solved with pore sizes

GAED has been used to provide forensic information on AC cases3; pores occupied by the binder used to manufacture activated carbon blocks; sources of raw material used to manufacture AC; unused or used AC questions; reactivated or unused profile pore size changes available in working AC adsorbers; changes in pore structure with time in furnace; changes in pore structure when chemicals are added to carbons; selection of the best AC for a client’s aqueous-phase and gas-phase applications; identification of AC chemical additives and what pores they occupy and their quantification. Table 3 summarizes pore volumes from test runs of unknown and known AC samples. The ratios of high-adsorption energy pore volume (25 to 29 cal/cc) to total pore volume (zero to 29 cal/cc) data indicate the carbon is a coal-based material.

In Table 3, the major types of AC were compared to an unknown AC to help determine its source by using ratios of high-adsorption energy pore volume to their respective total pore volume. These values were obtained from characteristic curves and Table 1 data using known and unknown AC commercial materials (note that the unknown ratio best matches the coal based AC).

Confirming AC source with PIXE

GAED and PIXE (Photon Induced X-ray Emission) are useful for providing AC source determinations. PIXE provides sodium through uranium elemental composition analysis. It is a way to confirm GAED claims for AC sources. Different sources of AC have unique elemental fingerprints. PIXE only determines the surface elements, as it has low penetration into the material being analyzed. The characteristic PIXE test determines differences in elements for different AC types. These are:

  • Coal: Si, Al, Fe, Mg, Ca, Sb
  • Wood: P, Zn
  • Coconut: Na, P, Cl

PIXE analysis also strongly indicates a coal-based activated carbon, based on elements analysis above. It is always good to obtain confirming data to strengthen your decisions. PIXE and GAED are based on completely different physical phenomena.

Conclusion

ASTM, AWWA standard methods and GAED advanced testing methods are recommended to solve sorbent problems. This newer testing method provides adsorption information for many POU suppliers and batch-to-batch GAC variations. It also provides relative product life and performance information, as well as being capable of determining DBP performance.

Since the client with a well-defined dechlorination problem had prior GAED test runs on good carbon blocks, a baseline for comparison was available when subsequent batches had poor dechlorination performance. It is important for GAED users to build up a database to understand and monitor their activated carbons beds and incoming material. This type of testing can help with deciding when to replace the used media with unused GAC4 and provide life-cycle monitoring. GAED example reports are available at www.pacslabs.com and from the first author listed below. Note that these reports provide more information than presented in this article, due to space limitations.

Acknowledgment for GAED user

Henry Nowicki thanks Dennis Brown from Aquamira Technologies Inc. for providing the testing samples used in this case study, and approval to publish their case study project on GAED relevant data. To read more about the testing method, several previous articles can be found at www.wcponline.com using Nowicki as the search keyword.

References

  1. Greenbank, M. “Using a Gas-Phase Test to Predict Liquid-Phase Activated Carbon Performance at Trace Concentrations”. 26th International Activated Carbon Conference. Pittsburgh PA. October 12-13, 2010.
  2. Schuliger W., Nowicki G., Nowicki H., Sherman B., “Importance of Proper Wetting GAC.” Filtration News. June 2010, pp. 27-29.
  3. Nowicki, H., et al. “Many Applications for GAED Full Sorbent Characterizations”. 27th International Activated Carbon Confer- ence. Seattle, WA. April 25-26, 2011 and Pittsburgh Conference, Atlanta, GA. March 8-12, 2011.
  4. Nowicki, H., et al. “GAED Test method: Optimum Time for GAC Replacement in Potable Water Treatment Plants.” WC&P. June 2010, pp. 24-28.

About the authors

Henry Nowicki, Ph.D., MBA, President of PACS, provides routine and advanced laboratory testing and R&D. He has won nine SBIR and other government contracts on AC new product developments, published over 100 papers and provided numerous PACS Short Courses and presentations. Nowicki provides the introductory course for the PACS Activated Carbon School monthly and at the client’s time and place. Henry@pacslabs.com.

H. George Nowicki, B.S. is Laboratory Manager and business devel- oper for PACS. He has 12 years of varied activated carbon experiences and advises clients on selecting activated carbon tests based on their specific applications. George@pacslabs.com

Wayne Schuliger, P.E., Technical Director for PACS, has 43 years of activated carbon experience and provides consulting on AC adsorber operation and design. He provides a short course titled “Design, Opera- tion and Trouble Shooting Activated Carbon Adsorption Systems” in the PACS Activated Carbon School. In addition to courses, Schuliger provides independent reviews and recommendations for operating and proposed adsorption process equipment. Wayne@pacslabs.com

Barbara Sherman, MS, Manager of Operations for PACS for 27 years, manages the company’s day-to-day business activities and practices. She is the Registrar for PACS Short Courses and the bi-annual International Activated Carbon Conferences. Barb@pacslabs.com

All authors can be reached by e-mail, by phone at (724) 457-6576 or at www.pacslabs.com.

About the company

Professional Analytical and Consulting Services Inc. (PACS) is celebrating its 27th year of incorporated business with a center of excellence in activated carbon adsorption. To view laboratory testing, R&D, consulting, short-course descriptions with the public schedule and activated carbon conference and other services, please refer to the website www.pacslabs.com for information.

About the testing

GAED provides the most precise, accurate and comprehensive test method to evaluate sorbents, including: characteristic curve, differential curve, pore size distribution, temperature versus loading capacities, BET surface area, trace- and mid-capacity data, isotherms for a wide range of organics of interest to clients, and a preliminary market survey for new sorbent developers relative to existing commercial sorbents. This test method is well suited to help select the best AC for the application, monitor its life-cycle performance, and determine the optimum time for replacement of used AC.

2011 International Activated Carbon Conferences (IACC)

PACS hosts the bi-annual International Activated Carbon Conference (IACC) and Activated Carbon School. The conference provides techni- cal presentations, commercial products and services providers’ mar- keting sheets, Hall-of-Fame awardees and leadership speakers. PACS is now accepting one-page abstracts for oral, poster and abstract- only technical presentations and exhibitor registrations. Conference proceedings are available after each conference is completed. The 27th International Activated Carbon Conference (IACC-27) will be held April 27, 2011 in Seattle, WA; IACC-28 will be held near Pittsburgh, PA, October 11-12, 2011. This conference is held every October near Pittsburgh, PA and at a variable site each spring.

Activated Carbon School

PACS provides The Activated Carbon School—see pacslabs.com for course descriptions. A highly qualified staff of practicing profession- als provides one- to two-day short courses and consulting services on your issues. There are some 14 courses on activated carbon in the curriculum because the education covers a wide variety of needed subjects. PACS provides advice to interested parties for the best courses for individual needs. Courses are provided monthly in major cities and at client’s time and place. See www.pacslabs.com for registration forms and more information.

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