1439
Langmuir 1995,11, 1439-1442
Magic Angle Spinning NMR Study of Adsorbate Reactions on Activated Charcoal George W. Wagner" Geo-Centers, Inc., Gunpowder Branch, P.O. Box 68, Aberdeen Proving Ground, Maryland 21010-0068
Brian K. MacIver and Yu-Chu Yang Research and Technology Directorate, U.S.Army ERDEC, Aberdeen Proving Ground, Maryland 21010-5423 Received December 5, 1994. I n Final Form: March 22, 1995@ Magic angle spinning (MAS)NMR can distinguish between molecules adsorbed in monolayers within the micropores (> 20 A) of activated charcoal and those present in multilayers or capillary-condensedliquid in the mesopores (20-500 A). The differentiation is possible due to the NMR shifts to low frequency induced by the large, diamagnetic susceptibility of the graphitic surface of the micropores (upon which primary adsorption occurs) in combination with slow molecular exchange between the micropores and mesopores. This study uses MAS NMR to characterize the adsorption and oxidation of 2-chloroethyl phenyl sulfide (CEPS) on activated charcoal. Micropore-filling by solvents and solvent displacement of micropore-adsorbed CEPS are also demonstrated for charcoal using this technique.
Introduction Activated charcoal is widely used as a filter material for air and water purification and as an adsorbent for remediating chemical spills due to its ability to strongly adsorb and remove contaminants from the environment. 1,2 However, a t some point the contaminated charcoal in spent filters and adsorbents must be disposed of in a safe manner. Perhaps a n extreme problem of this nature is the safe disposal of charcoal impregnated with about 40% by weight of mustard (bis(2-chloroethyl)sulfide, S(CH2CHzC1)z)) and other vesicant chemical warfare agent^.^ These materials were packaged in individual vials by the military as a particular variety of the chemical agent identification sets known as "sniff sets". The sets are slated for chemical detoxification prior to disposal by incineration. This study demonstrates the utility of solid-state MAS NMR to examine reactions of adsorbates on activated charcoal with applied decontamination solutions. Moreover, the technique is perhaps unique in its ability to allow spectroscopic discGmination between adsorbates in the micropores (I'='~ " ' ~ ' ' ~ ~ I " " ' " " 1 ~ I O Bb 6b 40 20 PPH
Figure 5. 13C MAS NMR spectra obtained for oxidation of 41.2% CEPS*lO/charcoal: (a) before reaction; (b) after 0.5 h of reaction with PSNPOlchloroform-d;( c ) chloroform-d added to sample after 11days.
36.6 ppm) and micropores (broad peaks a t 38.1 and 31.3 ppm) of charcoal (Figure 5a). Like CPBA, PSNPO reacts quickly with micropore-adsorbed CEPS. After a 0.5-h of reaction time (Figure 5b), no CEPS* is detected ( ~ 2 % the initial amount ofCEPS* remains unreacted), and only CEPSO* and/or CEPS02" are evident in the mesopores (sharp peaks a t 60.1 and 37.3 ppm) and micropores (broad
Conclusions MAS NMR spectroscopy is useful for studying the reactions of adsorbates within the micropores of activated charcoal. In addition, MAS NMR allows the discrimination of micropore-adsorbed (monolayer) vs capillarycondensed (multilayer) adsorbates, thus permitting the study of micropore filling and other adsorption phenom~ ~ ena. Acknowledgment. The authors thank Mr. Christopher Kanvacki, ERDEC, for many helpful discussions about adsorption on activated charcoal. We further acknowledge Professor Franklin A. Davis, Drexel University, for supplying the PSNPO used in this study and Professor Douglass F. Taber, University of Delaware, for furnishing CEPS*. LA940963+ (12) 13CNMR analysis ofthe solution phase from this reaction found 97% CEPSO* (60.0 and 37.3 ppm) and 3% CEPS02* (58.7 and 36.2 ppm) but no CEPS*.
