ACTIVATED CARBON
FROM PECAN WASTE

by Hyman Rudoff, Ph.D.

Carbon has been used as a water-purifying agent for centuries. The Bible (Numbers 19:9) refers to the use of the ashes from a burnt offering as a purifier for water that must be kept ritually clean. Even the use of carbon derived from pecan shells has been referred to -- in the 1970s, but the systematic exploration of the material is current.

Dr. David Rockstraw of New Mexico State University's Department of Chemical Engineering describes the beginning of this exploration thus: He and a graduate student, Reyad Shawabkeh, were sitting in a park, watching the children picking up fallen pecans from the ground. Rockstraw and Shawaekeh discussed possible topics that might be researched by the student, working toward his Ph.D.

"I challenged him to find a good use for pecan shells", says Dr. Rockstraw. Pecan shells are produced in tonnage quantities [one estimate is 40 tons/day] as waste products from the preparation for market of the very popular pecan nut, and have been disposed of in landfills, at substantial cost. Obviously if a use could be found for them, the liability would become an asset, and the environmental burden would virtually disappear.

A project was proposed to meet this challenge; it was accepted, and financial support was established by a seed grant from the Department of Energy's Waste Management Education and Research Consortium.

Activated carbon (AC) is another name for the kind of charcoal that can be used to remove trace impurities from water. It is used in tonnage quantities by water treatment plants and many industries. The commonest types come from coal and coconut shells. The differences between these two types of carbon and other less effective types lie in the intimate details of physical structure and surface chemistry.

Historically, AC has been known to work best in aqueous solutions. That produced from pecan shells is probably no different from those now in use, but AC's use in non-aqueous solutions has not yet been studied.

The focus of the current work is on the adsorption of high molecular weight dyes -- i.e.. large organic molecules -- and of radioactive materials [presumably metal ions].

However, "... we are putting very polar materials on the surface [of the AC] the way we are derivatizing them", and ... "we are making them three different ways". according to Rockstraw.

One method of preparation heavily covers the surfaces of the carbon with sulfate groups; the product adsorbs ions from the water solution, and, what is very interesting, ... "we can remove the [adsorbed] material by changing the pH. [pH is a measure of acidity]. This means that the carbon acts like an ion-exchange resin", a very useful attribute.

Ecotopics interviewed Dr. Rockstraw.

E: That means that from the standpoint of regeneration you have it almost licked in a certain sense.

Dr. R.: "Yes, we have regenerated one sample of AC five times now with less than 80% loss in activity".

E: That could mean that if you were forced to make this form of AC in relatively small quantities, and that therefore the unit costs were high, perhaps the capacity for multiple regeneration would offset the cost to a degree?

Dr. R.: "That would be the hope. We find that we don't have to go to extremely high temperatures to regenerate."

E: That's very interesting. What are the typical temperatures required?

Dr. R.: "AC from coal requires about 1000° C. AC from coconut shells needs 600°; pecan shell [AC] takes about 180°."

E: That very low regeneration temperature means that not only will the cost for heating fuel be less, but any pollution caused by its combustion will be minimal, far less than for the other ACs.

Dr. Rockstraw: "Right! All sorts of benefits!"

E: The study of pecan-shell AC is proceeding well; should the work be extended to other materials? The answer to that question is that for the moment the focus is on the pecans. However, the answers to many detailed questions might come sooner if the group were expanded to cover them. In any case, Dr. Rockstraw himself has plans to work at Los Alamos on the application of pecan-shell AC to the adsorption of plutonium.

David A. Rockstraw, Ph.D.
Assistant Professor, Chemical Engineering
New Mexico State University
office: (505) 646-7705
Chemical Engineering, Jett Hall 286
fax (505) 646-7706
e-mail: drockstr@nmsu.edu

Born in Gary, Indiana, Rockstraw received his BS at Purdue and his Ph.D. at the University of Oklahoma in Norman -- both in chemical engineering. His writings, research and working participation in his field show great versatility. Dr. Rockstraw has returned to NMSU from Los Alamos and with graduate student, Reyad Shawabkeh, plans to continue the work on pecans. Personal interests include racquetball, mountain biking and hiking, plus a green belt in Karate.

Author, Hyman Rudoff, Ph.D. is a retired industrial chemist and sailor with some 25 years' experience on the water. He writes on political, marine, and ecological topics, largely for the Shore Journal and Ecotopics International News Service.