By Ken Schaeffer and Robert Potwora
Activated carbon (AC) filter media is commonly used in point of use/point of entry (POU/POE) water applications. The type of raw materials used to produce AC has a major impact upon its characteristics. The two most common are coconut shell and bituminous coal.
Coconut shell-based AC has the most micropores. Micropores are defined as pores less than 20-angstrom units (two nm) in diameter. AC produced from bituminous coal have fewer micropores, but more mesopores and macropores. Mesopores are between 20 and 500 angstrom units (two and 50nm); macropores exceed 500 angstrom units (50nm). See Table 1.
The scanning electron micrographs (SEM) shown in Figure 1 were taken at 1,000X magnification. At this magnification, the prevalence of the micropores can be seen in the coconut shell-based activated carbon.
Coconut shell-based Bituminous coal-based
Pore volume among different pore sizes is measured by nitrogen adsorption and mercury intrusion. Nitrogen adsorption and mercury intrusion pore volume results can be plotted to obtain a pore volume distribution (Figure 2). The amount of micropores present in coconut shell-based AC is about 50 percent higher than bituminous coal-based activated carbon.
Figure 2. Pore volume distribution
The most common method used to distinguish micropore volume is the ASTM D 4607, standard Test Method for Determinaqtion of Iodine Number of Activated Carbon, ASTM International. Table 2 compares iodine numbers and other key properties between coconut shell- and bituminous coal-based AC.
Table 3. Benzene at 10 ppb
Hrbons means they have much higher capacity to adsorb small molecules, such as volatile organic chemicals (VOC). Higher capacity means coconut shell-based activated carbon will last longer before it needs to be changed out.
Volatile organic chemicals include most disinfection by-products (DBPs) such as trihalomethanes (THMs), and other VOC contaminants such as trichloroethylene (TCE). Many VOC’s have a low molecular weight, typically less than 200, and have low water solubility.
To measure the AC’s theoretical saturation capacity for VOC’s, carbon adsorption isotherms were conducted on coconut shell- and bituminous coal-based activated carbons. The results for benzene in water are shown in Figure 3.
Column 2 in Table 3 summarizes the isotherm results for 10 ppb benzene in water. The coconut shell AC had almost twice the saturation capacity compared to bituminous coal-based AC, 11 mg of benzene per gram of coconut shell carbon versus six for the coal based. Column 3 in Table 3 calculates the carbon usage (exhaustion) rate based upon this saturation capacity at an influent of 10 ppb benzene. 0.0076 pounds of coconut shell AC would be exhausted for every 1,000 gallons of water treated versus 0.014 pounds of bituminous coal AC. The cost of the carbon exhausted per every 1,000 gallons of water treated was then calculated (see Column Four, Table 3). The cost of acid-washed bituminous coal activated carbon and unwashed coconut shell activated carbon was used for this comparison. Acid-washed was selected because unwashed bituminous coal carbon has ash or impurity levels three times higher, compared to unwashed coconut shell carbon. Acid-washed bituminous coal AC has ash levels two times higher, compared to unwashed coconut shell AC. In Column 4, the treatment cost is almost three times higher, with washed bituminous AC compared to unwashed coconut shell AC, indicating a significant cost savings with coconut shell AC.
The carbon adsorption isotherm results for methyl tertiary-butyl ether (MTBE) in water are shown in Figure 4. At all MTBE concentrations in water, MTBE capacities of the coconut shell-based activated carbon were higher. Table 4 summarizes the results for 568 ppb MTBE in water.
Table 4. MTBE at 568 ppb
Again the saturation capacity of the coconut shell carbon was significantly higher. This translates into a treatment cost 2.5 times higher with washed bituminous carbon compared to unwashed coconut shell carbon.
The capacity advantage shown with coconut shell AC may not always be realized in actual full-scale use. Background organics like total organic carbon (TOC) will diminish the AC’s saturation capacity for a VOC to some extent. Also, low empty bed contact times (water residence time) in the AC filter will prevent the carbon’s saturation capacity to be reached.
Table 2 compares the ASTM hardness number of the two types of activated carbon. A higher hardness number means less particle attrition and less dust. Coconut shell AC requires less water to rinse (backwash) a new bed and generate fewer fines during operation.
Coconut shell AC is also more environmentally friendly or green. The coconut tree produces coconuts three times a year. The tree is not killed to harvest the coconut shells; therefore it’s from a renewable resource. It took millions of years for bituminous coal to be formed; hence, it’s a non-renewable resource. In addition, part of the carbon dioxide produced during manufacture of coconut shell carbon is removed by the coconut trees and produces oxygen.
Activated carbon supply
The United States has two large domestic manufacturers of bituminous coal-based activated carbon and, for the last two decades, bituminous coal-based activated carbon was readily imported from China. That has changed dramatically in the last year.
In 2007, in response to a petition filed by domestic manufacturers, the US levied new anti-dumping duty fees on most activated carbon imported from China, to the tune of 49 – 228 percent duty (average was 62 percent) on top of the standard 4.8 percent duty for such products. In addition, China’s demand for coal has been ever increasing due to hundreds of new power plants being built. At the same time, many coalmines were closed for safety and environmental reasons, resulting in shortages and price increases for Chinese coal.
Adding to the mix of forces that combined in this perfect storm, China also eliminated the 13 percent value added tax (VAT) rebate that AC exporters previously received from their government. Finally, as the dollar has weakened, Chinese carbon prices have increased due to currency exchange rates. As China’s coal AC prices increased, the domestic manufacturers followed suit.
The icing on the China coal AC cake is the impending government-ordered shutdown of most activated carbon plants in China on May 31 for approximately three months, in an effort to reduce air pollution for the Olympics. This will certainly exacerbate the coal activated carbon supply and pricing situation. In addition, the cost of coal in China has almost tripled in the past year and for the first time ever, last year China imported more coal than it exported.
We have plenty of coal in the US; domestic activated carbon producers do not have a problem getting raw material, but they are at peak production capacity. The planned new capacity is mainly for powdered activated carbon for the looming mercury emission control market (mainly power plant flue gas) in the US. As a result of anti-dumping duties and energy and labor costs and high domestic production levels, coal AC pricing has risen rapidly in the past year.
Coconut shell activated carbon is manufactured by scores of factories in a dozen countries and imported into the US by several resellers. It is suitable for many applications and for the first time ever, it is less expensive than comparable coal-base AC. Several factors (energy, labor, currency exchange rate, etc. have contributed to bituminous AC carbon price increases; coconut shell activated carbon pricing is up also, but not nearly as high.
Coconut shell activated carbon is preferred to remove VOCs from drinking water in POU/POE systems because it is more economical than bituminous coal-based activated carbon. It has higher hardness, less dust, lower ash and is more environmentally friendly. The days of low-cost activated carbon are gone and one needs to make sure they are buying and using the right AC for their application. There are many types and grades of activated carbon made from various raw materials; Discuss your application with your activated carbon supplier and get the right carbon to help reduce your costs.
About the authors
Ken Schaeffer has over 25 years experience in the activated carbon industry and is president of Carbon Resources, LLC. He has a BA in biology and an MS in environmental science from the University of Texas at Dallas, is an officer on ASTM D28 Committee on Activated Carbon, and is on the Pacific Water Quality Association Board of Directors. He can be reached at firstname.lastname@example.org or (760) 630-5724
Robert Potwora is Technical Director with Carbon Resources, LLC and has over 25 years experience in the activated carbon industry. He is current vice-chairman on ASTM D28 Committee on Activated Carbon. He may be reached at Robert@carbonresources.com or (760) 630-5724.
About the company
Carbon Resources, based in Oceanside, CA, is a quality supplier of activated carbon products and services that is backed by technical support and individualized customer service. The Carbon Resources Management Team has over 85 years of experience in the activated carbon industry and offers an unmatched line of the most diverse activated carbon products on the market. The sabre series®, Spartan series® and Guardian Adsorber series® and newly introduced Sentry series activated carbon products are widely recognized in the industry. For more information, please visit www.carbonresources.com.