Research Communications Transformation of Chloropicrin in the Presence of Iron-Bearing Clay Minerals JAVIERA CERVINI-SILVA,† JUN WU, RICHARD A. LARSON,* AND JOSEPH W. STUCKI Department of Natural Resources and Environmental Sciences, University of Illinois at UrbanasChampaign, 1102 South Goodwin Avenue, Urbana, Illinois 61801
Chloropicrin (trichloronitromethane, CP) reacted with reduced ferruginous smectite (SWa-1) in aqueous suspension. SWa-1 promoted CP-dechlorination to produce dichloro(DCNM) and chloronitromethane (CNM) with yields of up to ∼80% within 30 min. The simultaneous formation of DCNM and CNM suggests that CP and SWa-1 surface participate in two electron-transfer pathways. A series of CPadsorption and dechlorination rate constants in the presence of SWa-1 showed a strong correlation with the Fe(II) content of SWa-1 (r 2 ) 0.96 and 0.98, respectively), {Fe(II)}. This study indicates that the oxidation state of structural Fe greatly alters surface chemistry and has a large impact on electron transfer during clay-organic interactions.
Introduction Chloropicrin (CCl3NO2, CP), once a war agent (1), is today a major component of commercial fumigants (e.g., VAPAM, BUSAM 1020, VORLEX) and is applied together with methyl bromide to strawberry, broccoli, and potato fields. In addition to direct application, chloropicrin may be accidentally introduced to the environment by anthropogenic activities such as freshwater disinfection (chlorination/ozonation) (2, 3). Chloropicrin is transformed in the environment by abiotic (e.g. photohydrolysis (4)) and biotic (e.g. metabolic reduction (5)) processes. The transformation pathway of CP in the presence of naturally occurring clays has received no attention. Nitroaromatic compounds (NACs) have been reported to adsorb to clay surfaces (kaolinite, montmorillonite (6)), whereas chlorinated ethanes (e.g. pentachloroethane, 1,1,2,2-tetrachloroethane (7)) adsorb nonreversibly to smectites (montmorillonite, ferruginous smectite, and nontronite) and undergo R,β-elimination (dehydrochlorination). Recent studies indicate that the transformation rates of organic compounds are greatly accelerated by reduced Fe clay minerals (7, 8). Because most clay minerals contain some Fe which is susceptible to redox cycling in their crystal structures, CP could be transformed by interacting with oxidized and reduced clay minerals. * Corresponding author phone: (217)333-7269; fax: (217)333-8046; e-mail:
[email protected]. † Present address: University of California at Berkeley, Berkeley, CA. 10.1021/es990900j CCC: $19.00 Published on Web 01/28/2000
2000 American Chemical Society
The objective of this study was to characterize the reaction of CP with iron-bearing clay minerals and to determine the extent of structural Fe participation and the effect of Fe oxidation state. The outcome of this study will be of particular importance in predicting the fate of CP in natural soils and sediments during burial, submersion, wetting, drying, and other events.
Methods A 0.1 mM solution of chloropicrin (98% Mallinckrodt) was prepared in 9:1 water-ethanol under an argon atmosphere. The concentration of CP was confirmed by gas chromatography with a standard curve. The initial pH in solution was 8. A sample of the
