By: Henry Nowicki, Ph.D., Mick Greenbank, Ph.D. and Homer Yute, M.S., PACS
The American Society for Testing Methods (ASTM) and American Water Works Association (AWWA) have standard test methods for commerce between
buyers and sellers of activated carbons (AC). These methods have been discussed
in other articles 1,2 and are listed in Table 1 by title and method number. The
methods are used for purchasing and monitoring after the AC is installed. New
challenges are emerging in the industry that require new methods and product
developments. This article discusses additional test methods for the AC
industry.
Blending GAC Lots
All ACs are not equal. It is common practice to blend
different lots of AC with different standard method iodine number values to
meet the customer specifications. Often in drinking water applications, used
granular activated carbon (GAC) from drinking water adsorbers is furnace
reactivated to provide a resource for additional use cycles. Only dedicated
furnaces for regeneration of used GAC from water filters can be used when the
GAC will be returned to drinking water filtration. Since each cycle of the
reactivation process loses abut 10 percent of the original GAC volume, unused
make-up GAC must be added to the reactivated lot to get the original volume to
fill the customers adsorber.
New test methods are needed to detect blended lots of GAC.
The ASTM iodine number is an average of the blended lot, thus 50:50 blend of
1,000 and 700 iodine numbers yields a 850 iodine number. This heterogeneous lot
(1,000:700) meets the 850 client specification but will have significantly
different performance compared to a homogenous 850. The authors are developing
new methods to detect heterogeneous GAC (granule-to-granule for intergranule
heterogeneity and intragranule, outside granule versus inside granule
heterogeneity). Currently, the AC industry does not have test methods for
heterogeneity determination; presently, average and index values are provided.
GAC granules from blended lots can be separated based on
apparent specific gravity test methods. Once the granules are separated they
can be tested with standard AWWA, ASTM or heat-of-immersion test methods under
development, commonly referred to as the AC Tester, which is the method PACS
and ASTM’s method development group committee D-19 are developing. The
external portion of the separated granules can be removed by using a gem
polisher. Once the exterior is worn off, the dust can be separated by using
standard U.S. sieves. The gem polisher also can be used to remove coatings from
other media. Again, the separated portions, exterior and interior granule
parts, can be tested with standard AWWA or ASTM methods or the AC Tester.
The AC Tester results should be used by a professional who
has knowledge about the specific adsorption systems being studied. The decision
when the carbon is exhausted and needs replaced is a site-specific decision
process. It depends on the toxicity of the adsorbates, competition of
adsorbates and possibly moisture effects on the adsorption capacity.
AC Tester—Remaining AC Service Life
Two common questions concerning GAC installations are
“How good is this GAC?” and “How long can I use this GAC
before it needs to be changed?” The first question partially is answered
using ASTM methods listed in Table 1, before purchase and installation. The
second question is more difficult because there are no standard methods for
testing used GAC to estimate remaining service. The authors have been working
on a method to answer this important question and they have come up with a
simple and low-cost test device called the AC Tester. This method is based on
the concept that as the adsorption space in GAC fills, less heat is given off
when adsorptive materials challenge it. In an initial space distribution in a
bed of GAC, 40 percent of the volume is inside the GAC granules. Thus the AC
Tester determines the heat rise of the unused GAC to compare with an equal
volume of the used GAC. Rationing the temperature rises gives an estimate of
the adsorption volume used (i.e., 4° C rise for unused and 1° C rise
for used indicates only 50 percent of the original adsorption space is
occupied.) Thus this example suggests adsorption space still remains in this
used GAC, but the operators need to proceed with caution.
EPA Toxicity Characteristic Leaching Procedure
The EPA toxicity characteristic leaching procedure (TCLP) is
a third-generation regulatory test method to determine if a waste material has
potential to contaminate groundwater. This method consists of leaching used GAC
with a simulated landfill aqueous acetate solution. The water extracted GAC
adsorbates are analyzed for a target list of organic and inorganic toxic
compounds. The 10 highest concentrations of organics and metals in the extracts
in addition to the target list are qualitatively and quantitatively determined.
Experience using the TCLP on used AC has shown the adsorbed organics have a
negligible recovery using TCLP, but metals often are recovered. These
recoveries are reasonable because the carbon adsorbed were accumulated from
water passing through carbon. The high energy binding sites in the carbon hold
on to adsorbates strongly and are not detached by buffered acetate solutions.
California has a more aggressive extraction method based on using citric acid
to evaluate a materials potential to leach and contaminate the ground water.
The authors have reported on liquid cocktails that can
recover organics from used GAC.3,4 These extracting liquids were developed for
a new commercial regeneration process, “Liquid Regeneration of Commercial
Used GAC,” and found to be very efficient.4 A version of this
kicker/chase solvent for used GAC adsorbate recovery and analysis is under
development as a stream monitor.5
Presently, there needs to be a test method capable of
detecting 10-9 to 10-12 molar organics in water. The pharmaceutical food and
specialty chemical industry have the potential of adding these concentrations
to public waters. Some of these compounds are suspected to have endocrine
disrupter and other deleterious biological effects.6 GAC can enrich water
organics 10,000–100,000 fold, which is the basis of this new stream
monitor method.
Acknowledgement
The authors thank Professor Emeritus Dr. Milton Manes for
regularly sharing his thoughts about new opportunities in the AC industry. He is
the originator of the kicker/chaser concept and many other innovations
presently used in the AC industry. Manes has provided a short course for 12
years titled, “AC Adsorption: Principles and Applications,” for
PACS and the American Chemical Society.
About The Author:
Henry G. Nowicki, Ph.D. and MBA, directs the PACS Laboratory testing and consulting services and new business developments at PACS. He has obtained three patents and published more than 100 articles about environmental issues and AC ads
