Influence of Amine Buffers on Carbon Tetrachloride Reductive Dechlorination by the Iron Oxide Magnetite

Abstract

The influence of amine buffers on carbon tetrachloride (CCl4) reductive dechlorination by the iron oxide magnetite (FeIIFeIII2O4) was examined in batch reactors. A baseline was provided by monitoring the reaction in a magnetite suspension containing NaCl as a background electrolyte at pH 8.9. The baseline reaction rate constant was measured at 7.1 x 10(-5)+/-6.3 x 10(-6) L m(-2) h(-1). Carbon monoxide (CO) was the dominant reaction product at 82% followed by chloroform (CHCl3) at 5.2%. In the presence of 0.01 M tris-(deuteroxymethyl)aminomethane (TRISd), the reaction rate constant nearly tripled to 2.1 x 10(-4)+/-6.5 x 10(-6) L m(-2) h(-1) but only increased the CHCl3 yield to 11% and did not cause any statistically significant changes to the CO yield. Reactions in the presence of triethylammonium (TEAd) (0.01 M) increased the rate constant by 17% to 8.6 x 10(-5)+/-8.1 x 10(-6) L m(-2) h(-1) but only increased the CHCl3 yield to 8.8% while leaving the CO yield unchanged. The same concentration of N,N,N',N'-tetraethylethylenediamine (TEEN) increased the reaction rate constant by 18% to 8.7 x 10(-5)+/-4.8 x 10(-6) L m(-2) h(-1) but enhanced the CHCl3 yield to 34% at the expense of the CO yield that dropped to 35%. Previous work has shown that CHCl3 can be generated either through hydrogen abstraction by a trichloromethyl radical (radical CCl3), or through proton abstraction by the trichlorocarbanion (-:CCl3). These two possible hydrogenolysis pathways were examined in the presence of deuterated buffers. Deuterium tracking experiments revealed that proton abstraction by the trichlorocarbanion was the dominant hydrogenolysis mechanism in the magnetite-buffered TRISd and TEAd systems. The only buffer that had minimal influence on both the reaction rate and product distribution was TEAd. These results indicate that buffers should be prescreened and demonstrated to have minimal impact on reaction rates and product distributions prior to use. Alternatively, it may be preferable, to utilize the buffer capacity of the solids to avoid organic buffer interactions entirely.