Making Activated Carbons More Useful
By Henry Nowicki, Ph.D., George Nowicki, Wayne Schuliger, P.E. and Murty Hari, Ph.D
The technical community that deals with water and wastewater systems is well aware of current and near-future adverse implications regarding sources of safe drinking water. Population growth, global urbanization, deforestation and increases in per capita consumption all contribute to the problem of regional water replenishment via natural rainfalls and/or aquifers. In effect, sufficient and reliable drinking water sources are becoming marginalized. Additionally, growing energy demands (which inadvertently produce water and air pollution) present new opportunities to make activated carbons (AC) more useful. The primary uses of activated carbons are to purify water and air, but the list is growing.
Emerging contaminant compounds
Recent newspaper articles have noted that over 60,000 compounds can be found in drinking water at low concentrations and that government regulations require control of only about a hundred compounds to date. Also, a high percentage of monitored wells have failed US EPA’s microbiology standard. Man-made chemicals detected in drinking water include personal care, cleaning, pharmaceutical and food-grade products.
It is becoming easier to detect emerging contaminants in drinking water that come from food-grade chemicals, rocket fuel, pharmaceuticals, household chemicals, agriculture, hydraulic fracturing, energy production and other sources. The problem, however, is that it takes years to understand the deleterious toxicological health and environmental effects. Presently, municipal wastewater plants do not utilize available AC technologies to clean wastewater before putting it back into major drinking water supplies. To reduce exposure to unintended emerging compounds in drinking water supplies with unknown toxicology, consumers can filter drinking water with activated carbons, other sorbents and treatments. The more sophisticated the filter, the more compounds can be eliminated. POU water purification with AC, which has broad and deep markets with much growth potential, is vital in many regions, such as under-developed countries that lack a commitment to safe municipal water supplies. Campers, outdoorsmen, military applications and domestic households also would benefit from a POU or POE device to provide a final filtration step for water used for human, animal and plant consumption.
Catalytic, special carbons or engineering
Chlorine disinfection has saved countless lives but has become unpopular more recently because it forms carcinogenic DBPs called trihalomethanes (THMs). Total THMs are regulated under US EPA’s maximum contaminant level (MCL), which drives potable water treatment plants to switch disinfectants from chlorine to chloramine. Chloramine is a stable, long-lasting disinfectant made by combining ammonia with chlorine, a process that has been in practice by water treatment plants for six decades. Switching to chloramine use also avoids US EPA THM rules compliance problems for water plants. This apparent performance difference has been viewed as a business opportunity by some to develop new catalytic or special activated carbons or make engineering process changes. An unintended consequence is that ordinary AC does not remove chloramine in POU/POE filters as well as it removes chlorine.
There are three options to deal with this chlorine/chloramines performance difference. Option one is to employ patented catalytic GAC to increase performance for chloramine removal. This is a manufacturing change that adds nitrogenous compounds before thermal activation and provides nitrogen insertion into the graphitic platelets, which provides a single, free electron radical to degrade chloramines to nitrogen and chloride ion. Option two is to use a different commercial AC which performs better. Option three could be to change the engineering parameters. Some are reporting success by changing the empty bed contact time (EBCT) of at least three times that of a typical chlorine application. For example, AC can scrub chlorine @ 15 gpm per square foot or less, depending on total water chemistry.
Feedstocks determine structures and uses
AC can be manufactured from any feedstock with a high carbon content, but the main raw feedstocks for manufacturing AC are different ranks of coal (lignite through anthracite but mostly meteorological grade bituminous coal-based), coconut shells and various woods. Each raw material produces a different physical adsorbent that has unique distribution of adsorption spaces with unique adsorption energies (see Figure 1). Manufacturing transit time in AC activation furnaces, after pretreatment and charring the starting raw material can be varied to produce a family of bituminous coal based AC. Densities can range from 0.80 to 0.20 g/cc. The high-density product has a smaller total pore volume than the low density, but has higher adsorption energy (AE) sites per gram. The higher density product is harder and more useful when mechanical strength is needed. Physical adsorption sorbents like AC can concentrate aqueous organics seven- to nine-fold in the gas- or liquid-phase passing through an AC bed or column. This is enhanced liquid faction in the high adsorption energy micropores.
Testing for best products
Advanced understanding of the nano-molecular- structural differences in AC can be helpful in selecting the appropriate AC for the application and develop new products to satisfy emerging markets and better service existing customers. Historically, advanced test methods have opened the door to more useful products . These advanced test methods expand and complement the classical ASTM routine test methods. Independent activated carbon testing to verify original manufacturing specifications and AC life-cycle monitoring should be standard operating procedure for AC users. AC does not last forever and must be replaced periodically. Laboratory testing is the only way to know when the AC needs to be replaced. Ideally, independent testing service providers should be knowledgeable on all aspects of AC. Unfortunately, most testing laboratories are not knowledgeable in AC applications or uses, and the inner workings and operations of the activated carbon industry.
One such testing method, the Gravimetric Adsorption Energy Distribution (GAED) full characterization for physical adsorbents test method starts out with thermal gravimetric analysis (TGA) or thermal cleaning with a stream of inert argon to carry away the adsorbates released at 240oC (464oF) in order to compare all activated carbons on an equal footing. TGA cleaning and GAED methodology eliminate the need for expensive and troublesome vacuum technology. TGA weight loss is reported on all samples to alert data users about AC cleanliness. After cleaning the sorbent sample, it is challenged with 1,1,1,2-Tetrafluoroethane (TFE). Due to fluorine’s electronegativity, and ability to inhibit electronic delocalization, TFE is difficult to adsorb at high temperature. As the temperature is cooled using an automated temperature program, the AC sample reveals its full range of adsorption energies via its characteristic curve, a plot of adsorption energy in cal/cc on the X-axis, and its corresponding pore volume in cc per 100 grams of carbon or cc per 100 ml of carbon on the Y-axis. These characteristic curves are expressed in polynomial equations, which enable development of isotherms, a plot of equilibrium concentration on the X-axis and loading in grams target compound per 100 grams carbon on the Y-axis, for any water soluble organic compounds of interest to clients. This is possible once an isotherm is available for TFE; re-mapping the pores or adsorption spaces and AE can be done using the physical and chemical properties of pores or molecules of interest. Freundlich and Langmuire isotherms are not appropriate for most AC because they are based on the assumption that the sample sorbent is homogeneous (i.e., all adsorption sites are the same and equally accessible).
Regeneration of used granular activated carbon (GAC)
Since reactivated or regenerated AC costs about half that of unused or virgin GAC, there is a large rejuvenation market.
Major Reactivation of used GAC for additional usage has not changed for over 70 years. With increased emphasis on green chemistry, however, more emphasis on new ways to regenerate used GAC to reuse GAC is expected. The present reactivation process is the same as original GAC manufacturing, converting char to AC. The water gas reaction at 1,750 oF (954oC) is applied:
C + H2O —> CO + H2
C is the elemental symbol for carbon, representing the solid and liquid adsorbates in used GAC nano-pores. The 1-5 nm- sized (10-50 angstroms) adsorption spaces, or micro-pores, provide sufficient adsorption energies to provide enhanced liquefaction and/or enhanced solidification of water-soluble organics, depending on their state when pure. Carbon monoxide and hydrogen gas, produced while mineralizing adsorbed contaminants, can be used for their heat value. This is done by introducing small amounts of oxygen or air to the gas phase and avoiding oxygen and solid interactions:
CO + H2 + O2 —> CO2 + H2O + Heat
It is critical that added oxygen does not contact solid GAC with its contaminants; it will burn up like a campfire log. Added oxygen needs to be introduced so it is only in the gaseous phase in the reactivation furnace. Modern plants use this for reducing energy costs, using the extra heat to generate electricity and dehydrate incoming wet GAC and satisfy the temperature needed for the water-gas reaction. The removed carbon needs a critical minimal temperature for the endothermic water-gas reactions. This is when carbon adsorbate is removed. The active site becomes cold and needs to be re-heated to its critical temperature for the water-gas reaction. This exothermic-endothermic exothermic cyclic reaction is the critical phenomenon to convert char to activated carbons. When removing adsorbates from the porous GAC structures, however, some of the native pore structure is changed. Some pores or adsorption spaces are widened compared to the starting GAC.
Figure 1. Molecular scale models for coconut, bituminous coal and wood-based activated carbons
GAC from water treatment plants is relatively lightly loaded with adsorbates and possibly could
may be used to treat wastewater. Getting more total gallons treated per pound of GAC is another way to make AC more useful. A recent proposal uses a new process to regenerate used GAC from drinking water plants, which eliminates present thermal process problems. This new used GAC regen process is based on competitive desorbtion of GAC adsorbates. It is well known that adsorbates can be displaced by stronger binding adsorbates. This displacement means the original adsorbates float away and the AC is now loaded with the competitive displacer. The displacer can be removed by changing the temperature or pH, depending on the physical-chemical properties of displacer. The advantages of a chemical regen process over the classical thermal process are:
- It is an aqueous-based process.
- It is relatively mild.
- Regen chemicals can be recovered and reused multiple times.
- The new regen process can be used to recover used GAC adsorbates.
- The chemical process generates much less greenhouse gas emissions
- Improved quality is provided with lower cost regenerated GAC.
There are several new technologies on the horizon that will enhance the quality of activated carbon usage in the water treatment industry. Advanced technology can unlock even more potential uses, though unintended consequences (such as toxicity for use in kidney dialysis machines) should also be carefully considered. To meet future water quality requirements around the world, an open-minded approach is needed to explore and develop possible new uses for activated carbon. Regeneration processes are being refined to meet future requirements for a host of carbon applications and further the technological impact of activated carbon in water treatment.
- Henry Nowicki, Principal Investigator, FY2013 Environmental Protection Agency, Small Business Innovative Research (SBIR) Proposal. “Removing Chloramine in Point-of-Use and Point-of-Entry Drinking Water Treatment Units,” pages 27, April 29, 2012.
- Henry Nowicki, Principal Investigator, FY2013 Environmental Protection Agency, Small Business Innovative Research (SBIR) Proposal, “Addition of Safe Antimicrobial Activity to GAC,” pages 26, April 17, 2012.
- Henry Nowicki, Principal Investigator, FY2013 Environmental Protection Agency, Small Business Innovative Research (SBIR) Proposal, “New Regeneration of Drinking Water Plants Used Granular Activated Carbons,” pages 27, April 29, 2012.
About the authors
Henry Nowicki, Ph.D., MBA, PACS President and Senior Scientist for 29 years, provides the introductory course for PACS Activated Carbon School, and serves on WC&P’s Technical Review Committee and the editorial review committee of Filtration News.
George Nowicki, BA/BA, Laboratory Manager for PACS Laboratories, has 11 years of varied activated carbon experience and advises clients on selecting activated carbon tests based on their specific applications. He helped develop the heat-of-immersion (HOI) test as a way to estimate remaining AC service time; recently this HOI test has been approved by ASTM.
Wayne Schuliger, P.E., Technical Director for PACS, has 43 years of activated carbon experience and provides PACS consulting and inspections on activated carbon adsorber operation, design and troubleshooting. He provides the PACS short course Design, Operation and Troubleshooting Aqueous- and Vapor- AC Adsorbers. Schuliger is a member of the AC Hall of Fame for his work at Calgon, for development of the technical-business model for the drinking water and industrial sectors.
Murty Hari, Ph.D. is President of Superior Adsorbents and is on the PACS Board of Directors. He advises PACS clients on the best manufacture of carbon materials and how to add value to commercial carbons.Hari is a member of the AC Hall-of-Fame, based on IR100 awards and contributions to the manufacture of carbons. Authors can be reached at (724) 457-6576 or www.pacslabs.com or e-mail Henry@pacslabs.com.
About the company
Professional Analytical and Consulting Services Inc. (PACS) is a 28-year-old incorporated firm providing independent services for industrial, environmental and activated carbon industries: activated carbon services, include routine and advanced testing, PACS short course programs, R&D, consulting, contract research, expert witness, and host the International Activated Carbon Conference (IACC) and the Activated Carbon School. PACS serves over 950+ clients and has been awarded nine government grants and contracts for R&D on activated carbon projects.