Perchlorate contamination of ground and surface water has occurred at a number of sites, principally in the western United States. The contamination is a result of historical activities such as previously acceptable disposal procedures and the testing of solid rocket motors. The health effects of low exposure levels to perchlorate are not well known, but state action levels for perchlorate in drinking water are as low as 1 part per billion. The responsibility for perchlorate remediation at many of these sites falls to the Department of Defense, and the development of innovative remedial approaches has been an active area of research. The most widely used approach for perchlorate remediation is ion exchange using strong-base anion exchange resins and traditional pump-and-treat technology. One drawback is the relatively low perchlorate selectivity of commercial resins, which renders a large fraction of their anion exchange capacity unusable. Highly selective anion exchange resins have been prepared, but they are much more expensive. Bioremediation by perchlorate-reducing bacteria is an attractive approach for the in-situ degradation of perchlorate in groundwater; however, it is ineffective if other electron acceptors, especially oxygen and nitrate, are present.
The primary research objective was to modify activated carbon by creating additional quaternary ammonium sites and measure the capacity and selectivity of these activated carbons for perchlorate adsorption. The secondary research objective was to determine whether granular modified activated carbon (GMAC) anion exchange materials could be prepared using only gaseous reagents under conditions similar to those used to prepare conventional gas-activated carbon.
The GMACs were prepared by first increasing the nitrogen content of granular activated carbon (GAC) through treatment with ammonia at elevated temperatures, which has been shown to produce either pyridinic sites or quaternary ammonium sites, depending on the temperature. In a procedure analogous to the alkylation of poly-(4-vinylpyrinide) resins, the pyridinic sites were alkylated to produce strong-base anion exchange sites. The project attempted to prepare GMACs in a manner analogous to the production of conventional gas-activated carbon.
The main hypothesis/assumption inherent in this work was that the pyridinic nitrogen sites in activated carbon can by alkylated to produce cationic sites that are similar to those found in certain strong-base anion exchange resins. Unfortunately, the main hypothesis that the nitrogenous sites in activated carbon can be alkylated appears to be false. Instead, alkylation appears to increase the net positive charge of the activated carbons by alkylating the anionic oxygen surface sites and converting them to neutral species as well as converting some neutral oxygen sites into cationic sites. While alkylation greatly increases the selectivity of the cationic sites for perchlorate, the fact that this approach does not seem capable of increasing the number of cationic surface sites strongly suggests that modified activated carbons are not promising materials for perchlorate remediation since perchlorate selective ion exchange resins can perform the same task more economically. (SEED Project Completed – 2007)