Environ. Sci. Technol. 2005, 39, 756-763
Influence of Amine Buffers on Carbon Tetrachloride Reductive Dechlorination by the Iron Oxide Magnetite KARLIN M. DANIELSEN,† J O H N L . G L A N D , ‡ A N D K I M F . H A Y E S * ,† Department of Civil and Environmental Engineering and Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48105-2125
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 × 10-5 ( 6.3 × 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 × 10-4 ( 6.5 × 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 × 10-5 ( 8.1 × 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 × 10-5 ( 4.8 × 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 (•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.
Introduction The pH dependence of reductive dechlorination by zerovalent iron or iron oxide surfaces is well-documented (1-6). These * Corresponding author phone: (734)763-9661; fax: (734)763-2275; e-mail:
[email protected]. † Department of Civil and Environmental Engineering. ‡ Department of Chemistry. 756
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reactions may generate or consume protons, so buffers are usually added to stabilize the pH. Good et al. (7) introduced a series of amine buffers for use in biological or biogeochemical research that have been widely applied in environmental chemistry systems due to their high solubility and buffer capacity between pH 6 and pH 8. The Good’s buffers are frequently assumed to be inert based on reports of “negligible” complexation of Mg2+, Ca2+, Mn2+, and Cu2+ (7). However, more recent reports of metal ion complexation by Good’s buffers (8-10) or impurities (10) in those buffers have been reported. If buffers do complex with iron oxide surfaces as well as metal ions, they can influence reductive dechlorination reactions by altering the number of available reaction sites, their accessibility, or electron-transfer characteristics. Yu et al. (11) ascribed the interactions between Good’s buffers and hydrated metal ions to the formation of bidentate chelate rings between the hydrated metal and the buffer via one peripheral alcoholic oxygen atom and the nearest tertiary nitrogen. TRIS (tris(hydroxymethyl)aminomethane) is one Good’s buffer that forms complexes with manganese, iron, cobalt, nickel, copper, and zinc (12). X-ray analysis of the precipitated metal-buffer solids demonstrated composite three-dimensional structures (9). The presence of amine functionalities and peripheral alcohol groups on other Good’s buffers suggests that similar complexes could be formed by several of these buffers (11). Yu et al. (11) sought to eliminate metal buffer interactions by taking advantage of substituent steric effects. In earlier work, they observed a decrease in metal complexation by monoamines and diamines with increasing number or bulkiness of the substituted alkyl groups (13, 14). On the basis of this observation, Yu et al. (11) introduced a series of eight tertiary amines with the ability to buffer solutions between pH 3 and pH 11. Tertiary amine buffers offer the advantage of minimizing metal complexation, but it remains to be demonstrated whether sorption of these buffers to iron oxides is also minimized. If sorption does take place, organic buffers may influence reductive dechlorination reactions through interaction with surface-bound reaction intermediates. Dechlorination of CCl4 by magnetite can proceed via two highly reactive intermediate species; the trichloromethyl radical (•CCl3) and the trichlorocarbanion (-:CCl3) (15) (Figure 1). Several authors have examined CCl4 reductive dechlorination by reduced iron metal or iron oxides in the presence of organic compounds such as 2-propanol (16-18). The toxic product CHCl3 was produced in each case and was attributed to hydrogen abstraction by •CCl3 from 2-propanol. Elsner et al. (18) documented that 2-propanol enhanced CHCl3 production relative to controls that did not contain 2-propanol. This result implies that reductive dechlorination reactions in the presence of other organic compounds such as amine buffers with readily abstractable hydrogen atoms could likewise increase CHCl3 yields. The reaction intermediate -:CCl3 has also been hypothesized to form CHCl3 (4, 15, 18). Stromeyer et al. (19) tracked CCl4 reductive dechlorination by Acetobacterium woodii in deuterated water and found CDCl3 as a reaction product. Proton abstraction from water by -:CCl3 led to chloroform production in this experimental system. Since -:CCl3 is a very strong base (20, 21), it can abstract a proton from any species with a lower pKa including any amine with buffer capacity in the neutral pH range. Given that both hydrogen abstraction and proton abstraction have been identified in reductive dechlorination reactions, the actual mechanism that dominates CHCl3 10.1021/es049635e CCC: $30.25
2005 American Chemical Society Published on Web 12/21/2004
FIGURE 1. Proposed pathways for reductive dechlorination of carbon tetrachloride: pathways a and b yield the hydrogenolysis product chloroform while pathway c shows r-elimination and carbene hydrolysis to form carbon monoxide. Branches d and e show the pH-dependent branching between carbon monoxide and formate from free dichlorocarbene while branches f and g show the implicit pathways from surface stabilized carbene species. Boxes indicate species identified in this study. formation must be highly dependent on the experimental system. Reductive dechlorination of CCl4 by magnetite was selected for this study. The original EPA Priority Pollutant List (22) included CCl4, and it is still considered to be one of seven compounds that present the greatest threat to U.S. public groundwater supplies (23). Magnetite (FeIIFeIII2O4) forms as a corrosion product on the surfaces of zerovalent iron in permeable-reactive barriers (24), is produced by dissimilatory iron-reducing bacteria (25), and is known to reduce CCl4 (6, 15, 26). Past work by our research group and others have identified the major products and pathways (6, 15, 26), making this system an ideal one for studying buffer effects. The mechanisms examined here are not limited to magnetite systems but are broadly applicable anywhere reductive dechlorination is facilitated by reduced iron species in the presence of amine buffers. Buffers may influence CCl4 reductive dechlorination reactions through complexing or solubilizing surface iron
FIGURE 2. Chemical structures and pKa values of the amine buffers (A) TRIS (tris(hydroxymethyl)aminomethane), (B) TEA (triethylammonium), and (C) TEEN (N,N,N′,N′-tetraethylethylenediamine). The pKa values shown are for these compounds in the nondeterated form. sites, by providing a source of hydrogen that radical reaction intermediates can readily abstract, or by providing a readily available source of protons. This paper examines the influence of a series of organic buffers on reaction rate constants and products during CCl4 dechlorination by magnetite. Deuterated compounds were utilized to track the source of hydrogen in CHCl3 and isolate hydrogen abstraction or proton abstraction as the primary reaction mechanism. In particular, one of the Good’s buffers, TRIS (tris(hydroxymethyl) aminomethane), was selected to represent buffers with the ability to complex strongly to solid surfaces. The Yu’s buffer TEEN (N,N,N′,N′-tetraethylethylenediamine) was selected to represent buffers that contain abstractable hydrogen atoms but lack peripheral alcoholic oxygen functionalities needed to form bidentate chelate rings and strong sorption complexes. TEA (triethylammonium) was selected as a structural analogue for TEEN in the isotope tracking studies since TEEN is not commercially available in the deuterated form. Understanding or eliminating buffer interactions is essential for establishing reaction pathways and predictive rate constants for reductive dechlorination reactions if such results are to be extended to natural or contaminated aquatic systems.
Experimental Details Materials. Fully deuterated forms of TRIS and TEA (referred to as TRISd and TEAd in this paper) were commercially available (Sigma-Aldrich, St. Louis, MO) and prepared in H2O. TEEN was also purchased from Sigma-Aldrich but was not available in the deuterated form. All compounds were used as received. Figure 2 gives the chemical structures and pKa values for the TRISd, TEAd and TEEN buffers. Magnetite was precipitated chemically in highly basic solutions of ferric chloride based on the method described by Schwertmann and Cornell (27) and washed and characterized according to the procedures described in ref 6. The precipitate was confirmed to be magnetite by X-ray diffraction. The specific surface area measured by BET analysis and N2 adsorption was 18.01 ( 0.04 m2/g. Transformation Studies. All sample preparation for dechlorination experiments was performed in an anaerobic chamber (Coy Labs, Grass Lake, MI). Procedures for maintaining anaerobic conditions are described elsewhere (6). Solutions containing 0.01 M buffer were prepared and adjusted to 0.01 M ionic strength by NaCl to compensate for the fraction of buffer species in the uncharged amine form. A volume of 60 mL of D2O or buffer solution was added to VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Effect of the amine buffers TRISd, TEEN, and TEAd on the rate of 20 µM CCl4 reduction by 25 g/L magnetite at pH 8.9. Error bars represent 95% confidence intervals. FIGURE 3. Raw data showing the dechlorination of CCl4 ([) to CHCl3 (*) and CO (4) by magnetite in the presence of 0.01 M TRISd buffer (pH 8.9, solid concentration 25 g/L, CCl4 concentration 20 µM) as compared to the magnetite free controls (O) (0.01 M TRIS buffer). The solid line represents the data fit using the pseudo-first-order rate model and dashed lines were added to guide the eye. each reactor, and the buffer solutions were titrated to pH 8.9. Dried magnetite solids were then added at a concentration of 25 g/L and allowed to equilibrate overnight. The buffer free magnetite suspension was prepared in D2O with a background electrolyte of 0.01 M NaCl and stabilized at pH 8.9 without pH adjustment. Controls containing 0.01 M buffer solutions but no magnetite were prepared in H2O and run in parallel. Sample and control reactors were spiked with saturated aqueous solutions of CCl4 (in D2O or H2O) to a hypothetical (assuming no partitioning to the headspace) aqueous concentration of 20 µM (3 mg/L), submerged in a 25 °C rotating water bath (170 rpm), and shielded from light. Standards were prepared from 2-propanol stock solutions containing CCl4, CHCl3, and CDCl3 or from gas-phase injections of a 10:90 CO:N2 gas mix (Scotty Specialty Gases, Troy, MI). All solutions were allowed to equilibrate for 1 h before sampling to facilitate headspace partitioning. Dechlorination and product generation were monitored for at least 2 half-lives. The species CCl4 and CHCl3 were quantified periodically by headspace analysis using a HP 6890 gas chromatograph (GC) and a 30 m × 0.45 mm × 1.27 µm DB5 column. Flow after the column was split to simultaneously monitor CCl4 by a flame ionization detector and CHCl3 by an electron capture detector. The GC was operated isothermally at 30 °C. CO was measured on a RGA3 reduced gas analyzer (Trace Analytical, Sparks, MD) run isothermally at 40 °C. The source of the proton or deuterium ion in CHCl3 was inferred by GC/MS. Headspace samples were analyzed for the relative abundance of the mass fragments 83 (CHCl2+) and 84 (CDCl2+) to determine the concentrations of CHCl3 and CDCl3 respectively. Aqueous- and gas-phase samples were also analyzed by GC/MS for unidentified reaction products. Iron Analysis. Samples were also analyzed for aqueous total iron (Fe(II) + Fe(III)). The total iron analysis was performed on 5-mL aliquots filtered through a 0.22-µm filter and acidified to 0.1 M H+ with HCl. The total iron concentration was assayed by ICP-AES (Perkin-Elmer, Boston, MA) analysis at 238.2 nm. Data Analysis. A detailed description of the data analysis is presented in reference (6). Briefly, the apparent pseudofirst-order rate constants were determined by correcting the CCl4 concentration for headspace partitioning using the Henry’s constant and fitting the data with the pseudo-firstorder rate law. The concentration profiles were well predicted by the pseudo-first-order rate law (Figure 3). The apparent rate constants pertain to volatile compounds partitioning rapidly between the aqueous and the gas phase but reacting only in the aqueous phase. The apparent rate constants are rate constants that would have been observed in a hypothetical headspace-free system based on the method de758
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scribed by Burris et al. (28). The following values of dimensionless Henry’s constants measured at 25 °C were utilized in this analysis: CCl4, 1.244; CHCl3, 0.15 (29); and CO, 42.8 (30). All of the reported rate constants have been normalized to the magnetite surface area. The product distribution fractions were calculated from ratios of the moles of product evolved relative to the moles of CCl4 consumed over the time frame of the reaction using the method described in ref 6.
Results and Discussion Dechlorination Reaction Rate. The extent that buffer conditions influence CCl4 reduction rates was examined in magnetite suspensions. Preliminary experiments showed CCl4 dechlorination to be primarily surface mediated. These experiments were conducted at pH 6 and compared the CCl4 dechlorination rate constants in a magnetite suspension with an equilibrium Fe2+ concentration of 0.44 mM to the rate constants obtained from a sample containing the same aqueous Fe2+ concentration but no magnetite. The rate constant in the magnetite suspension was measured at 1.4 × 10-3 ( 3.6 × 10-5 h-1 and was 14 times faster than the rate constants measured in the aqueous Fe2+ solutions (1.0 × 10-4 ( 1.5 × 10-4 h-1) without magnetite. The Fe2+ concentration in the buffer-free magnetite suspension was below the detection limit of ∼700 nM at pH 8.9. Thus, the component of the dechlorination reactions promoted by aqueous Fe2+ in magnetite suspensions is expected to be negligible. These results indicate that the reaction in bufferfree samples is dominated by reduction at surface iron sites. The rates of reductive dechlorination were found to depend on the presence and type of pH buffer (Figure 4). A baseline value for rate constants is provided by the pH 8.9 magnetite-only system (no organic buffer) with a pseudofirst-order rate constant of 7.1 × 10-5 ( 6.3 × 10-6 L m-2 h-1. Adding TEAd or TEEN buffer caused modest but statistically significant rate enhancements with rate constants of 8.6 × 10-5 ( 8.1 × 10-6 and 8.7 × 10-5 ( 4.8 × 10-6 L m-2 h-1, respectively. The largest rate enhancement was observed in the TRISd-buffered system where the rate constant of 2.1 × 10-4 ( 6.5 × 10-6 L m-2 h-1 was nearly triple the value measured in the buffer-free systems. The rate limiting step in abiotic reductive dechlorination reactions is commonly agreed to be the transfer of the first electron (18, 31-36) followed by C-Cl bond cleavage. Changes in the overall CCl4 transformation rate constant clearly demonstrate buffer interaction during this step. The factors most likely to alter the electron-transfer rate include changing the number of reaction sites, their accessibility, or their electron-transfer characteristics. TRIS buffer has been proven to form complexes with transition metal salts (9, 12). Therefore, it is possible that TRISd might enhance dechlorination rates by dissolving Fe(III) from the magnetite surface and subsequently exposing “fresh” Fe(II) sites that were previously blocked to CCl4 access. Evidence for this effect would be increasing amounts of dissolved iron in the presence of TRISd. However, the concentrations of total iron in the 0.01 M TRIS system were below the detection limit of ∼700
nM. Increasing the TRIS concentration by a factor of 10 to 0.1 M did not raise the total iron concentration above the detection limit. Total iron concentrations in the remaining amine buffer systems and in the buffer-free system were also below the detection limits. Therefore, the possibility of rate augmentation by an effective enhancement of the iron solubility was ruled out. Nonetheless, increasing the TRISd concentration (at pH 8) from 0.001 to 0.01 enhanced the rate constant by a factor of 1.5 and further increasing the TRISd concentration by an additional order of magnitude to 0.1M enhanced the rate constants by an additional factor of 2.1 (data not shown) supporting the concept that TRISd either improves CCl4 accessibility to the reactive surface sites or enhances electron transfer through a TRISd-surface interaction. Particle Aggregation Hypothesis. Many organic compounds are known to disperse mineral suspensions (37, 38), so it is possible that the organic buffers enhanced reaction rates by inhibiting particle aggregation. Preliminary experiments examined the settling behavior of magnetite suspensions and found settling to be visibly influenced by the presence of amine buffers. Follow-up work was conducted to systematically investigate the relationship between ionic strength, particle aggregation, and reaction kinetics (39). Decreasing the background electrolyte concentration from 0.1 to 0.007 M NaCl significantly reduced the particle aggregation but increased the reaction rate constants by no more than 13%. This result demonstrated that particle aggregation could alter the reaction kinetics but only to a limited extent. Modest but statistically significant enhancement in the reaction rate constants was observed in the TEAd and TEEN systems relative to the magnetite-buffered system. Tadros and Lyklema (40) examined the surface charge of silica particles in the presence of various cations. Their work established that ions such as Na+ caused more pronounced changes in surface charge than did tetraethylammonium. Particle aggregation is dependent on surface charge, so lower surface charge and less aggregation by TEAd and TEEN relative to Na+ could explain the slightly faster rate constants in these systems relative to the buffer-free system. Particle aggregation, however, cannot explain the rate enhancement in the TRISd buffered system. Magnetite particles in the presence of TRISd settled at a faster rate than was observed in the buffer-free system, but the rate constant was enhanced by almost a factor of 3 nonetheless. The rate enhancement by TRISd is more likely related to TRISd complexation of surface sites. These complexes could enhance the reaction rate constants by improving the electron donating capacity of the surface, or if dense enough, could disrupt the electrical double layer, and could facilitate CCl4 access to the solid. The rate enhancement for TRISd could be generated by either process. Dechlorination Product Distribution. The distribution of products from CCl4 reductive dechlorination by magnetite were previously examined by McCormick and Adriaens (15). Radical trapping experiments isolated the highly reactive intermediates •CCl3 and :CCl2. The carbanion -:CCl3 was also hypothesized to be present. Further insight into the reaction pathway can be inferred from an investigation of CCl4 reductive dechlorination by Fe(II) sorbed to goethite. The first step in the CCl4 dechlorination in the Fe(II)/goethite system involves transfer of a single electron from the solid and cleavage of one C-Cl bond (18) to produce •CCl3 (Figure 1). The trichloromethyl radical may be further reduced to form a trichlorocarbanion (-:CCl3) (4, 15, 18) that can yield CHCl3 by deprotonating an organic buffer, water molecule or surface site (pathway b). Further reduction of •CCl3 followed by R-elimination can yield dichlorocarbene (:CCl2) and a chloride ion. Pathway c shows that :CCl2 may hydrolyze to
FIGURE 5. Product yields as a function of buffer condition. The products include CO (white bars), CHCl3 or CDCl3 (black bars), and other undetected compounds; (striped bars). Error bars represent 95% confidence intervals. formyl chloride (18), which rapidly decomposes to carbon monoxide (CO) (41, 42) or, under very basic conditions, formate (42). An alternate pathway to CO and formate could occur if :CCl2 is instead stabilized on the surface as an organometallic carbenoid species (15) that reacts through a series of unidentified surface species (15, 18) (pathways f and g). The products of CCl4 dechlorination were monitored in this study to examine how extensively amine buffers influenced the degradation pathway and product distribution (Figure 5), irrespective of whether overall dechlorination rates were influenced. In the magnetite system with no organic buffer, CO was the dominant reaction product at 82%, while 5.2% of the original mass underwent hydrogenolysis to CHCl3. Other minor products include methane ( 1° (ref 43 and references cited therein). Rapid oxidation of TRISd would not be expected since it is a primary amine. Hegetschweiler and Saltman (48) monitored TRIS interaction with the (batho)2CuII system and observed that TRIS buffer was not oxidized after “several days”. Therefore, TRISd interaction with the surface most likely does not occur via amine oxidation. Isotope Tracer Studies. Deuterium exchange experiments were performed to establish the source of hydrogen leading to the formation of chloroform (Table 1). There are limitations when using deuterated buffers due to the possible exchange of D for H on certain functional groups when they are placed in water. Protons in N-H and O-H functional groups are exchangeable with the protons in water while C-H groups are nonexchanging (49, 50) in time frames relevant to the present experiments. This has practical implications on the interpretation of product distributions in the presence of deuterated buffers. Only the D-H groups would remain deuterated and function as meaningful markers for tracking abstraction from the TRISd and TEAd buffers. Isotopic exchange did not significantly influence the purity of water in the present study since the concentration of D2O never exceeded 0.01%. Hydrogen Abstraction Interpretation. One possible interpretation of the results presented in Table 1 is based on a hydrogen abstraction mechanism. Based on thermodynamics, •CCl3 should be able to abstract a hydrogen atom from atomic configurations with lower bond dissociation energy (BDE) values than CHCl3. Abstraction from N-H or N-D functional groups by •CCl3 may be feasible since N-H BDE values (51) overlap with those of CHCl3 (52, 53). However, 760
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TABLE 1. Deuterated System Descriptions and Results system
form of buffer
solution
% CHCl3
% CDCl3
Fe3O4 TRISd TEAd
naa D D
D2O H2O H2O
16 96 96
84 4b 4b
a
na, not applicable.
b
Values not statistically significant.
R(C-H) bonds, which have BDE estimates (54-56) less than or equal to CHCl3, are expected to be the most readily extracted. All relevant BDE values used in this study to identify possible hydrogen abstraction functional groups on amine buffers can be found in Appendix 1 in the Supporting Information. Table 1 shows the components in each deuterated system and the fraction of chloroform that incorporated hydrogen or deuterium. The simplest sample matrix consisted of magnetite in deuterated water. Here the •CCl3 abstracted predominantly deuterium atoms. The most likely source of deuterium in this system is from deuterated surface sites and not from solution given that the BDE for CHCl3 is above that of FeOH (57) but well below the value for water (58) (Appendix 1 in the Supporting Information). Waters of hydration remaining on the solid surface after magnetite synthesis and drying were in the form of H2O but partially exchanged with solution when placed in D2O. The rate of exchange of weakly bound protons on a magnetite surface was estimated by Astumian et al. (59) at 1.4 × 105/d, so after 24 h of equilibration time, virtually all of the weakly bound FeOH sites would have exchanged with the solution to form FeOD sites. Therefore, the presence of deuterium in CDCl3 can be logically interpreted as deuterium abstraction from exchangeable surface FeOD sites. The remaining 16% of chloroform was in the form of CHCl3 and could be attributed to hydrogen abstracted from less exchangeable hydroxyl groups on the magnetite surface that retained their original FeOH isotopic signature. Surface hydroxyl groups are known to have different reactivities depending on the coordination environment (60). The exact coordination of hydroxyl groups on magnetite is unknown, but the presence of multiple types of sites with different proton exchange reactivities would be consistent with simultaneous occurrence of FeOH and FeOD. Formation of both CHCl3 and CDCl3 may indicate hydrogen and deuterium abstraction from both types of surface sites. Formation of the hydrogenolysis product chloroform from deuterated TRISd and TEAd was tracked in H2O. Hydrogenolysis in the TRISd buffer system produced 96% CHCl3 and 4% CDCl3. The 4% measurement is within the range of experimental error and is not statistically significant. The absence of CDCl3 rules out R(C-D) groups as the abstraction location. This is significant since the TRISd molecule contains six deuterium atoms located on R-carbons. While the exact BDE values for deuterium atoms on the TRISd molecule are not available, they can be approximated by the R(C-H) BDEs of methanol and ethanol (54-56). These values are generally well below those of CHCl3 and suggest that R(C-H) groups would be abstractable by CCl3 (Appendix 1 in the Supporting Information). The lack of CDCl3 suggests that hydrogen abstraction from R(C-D) groups on TRISd buffer is not an important mechanism. Although hydrogen abstraction from FeOH sites cannot be ruled out by these data, this scenario is inconsistent with the evidence supporting a strong TRISd/surface interaction. The TRISd/surface complex would have to enhance electron transfer to CCl4 but then be completely shielded from •CCl3 attack despite the fact that TRISd contains abstractable R(C-H) groups. It is far more likely that hydrogen abstraction is not an important reaction pathway to CHCl3 production in the TRISd system or that abstraction is occurring from the
N-H groups. Preferential abstraction at N-H groups was deemed unlikely given the relative energetic favorability for R(C-H) abstraction. Hydrogenolysis products in the presence of TEAd were 96% CHCl3 and 4% CDCl3. Again the 4% yield for CDCl3 was not statistically significant. The BDE of R(C-H) groups on triethylamine is also below (51) that of chloroform so deuterium abstraction from these groups is energetically favorable (Appendix 1 in the Supporting Information). The fact that deuterium atoms were not abstracted from the TEAd molecule combined with the lack of reaction rate influence and only minor changes in product distribution in the presence of TEAd provide strong evidence that TEAd does not associate with the solid surface where the trichloromethyl radical would be generated. Proton Abstraction Interpretation. Many of the conclusions noted above for hydrogen abstraction would be the same if proton abstraction were the dominant reaction pathway (step b, Figure 1). Since -:CCl3 is a strong base with pKa estimates g13.6 (20, 21) the carbanion would be able to remove H+ or D+ (hereafter referred to a protons) from any species with a lower pKa value. Figure 2 shows that the pKa values for TEEN, TRISd, and TEAd are each well below those estimated for -:CCl3. Likewise, the pKa values for dissociation of the magnetite surface species FeOH2 and FeOH have been estimated at 5.6 (61) and 8.5 (62), respectively. Therefore, the -:CCl3 could readily extract a proton from any of the buffer molecules or surface sites and would be at a minimum an effective competitor for the protons in water. The presence of both CHCl3 and CDCl3 in the magnetite/D2O system can be explained by -:CCl3 protonation from a mix of FeOH, FeOD surface sites or from D2O. Since -:CCl3 is generated at the solid surface it would be logical to expect the carbanion to encounter adjacent surface sites. The relative abundance of CDCl3 compared to CHCl3 may then be reflective of the abundance of FeOD compared to FeOH sites. Recall that no statistically significant concentrations of CDCl3 were observed in the TRISd buffered system. However the TRISd buffer contains structural configurations that have been shown to complex transition metals (9, 12), and the presence of this buffer clearly augmented the reaction rate constants. Further evidence for TRISd/surface interactions was observed in preliminary experiments using nondeuterated TRIS. Duplicate vials at pH 8.9 and containing TRIS concentrations of 0.01 M yielded 9% CHCl3 in both vials, but increasing the TRIS concentration to 0.1 M TRIS increased the CHCl3 yield to 19 and 20%. These observations are consistent with both TRISd forming surface complexes and with a -:CCl3 intermediate species. Based on pKa values, the most likely group on the TRISd molecule to be deprotonated by a -:CCl3 species would be the N-D group. However, the N-D signal would have been lost by isotopic exchange and proton extraction from this group would produce CHCl3. Even if the buffer were directly involved in chloroform production, this involvement would not be observed. Thus, the presence of the -:CCl3 is supported by the fact that only CHCl3 was observed in the TRISd system. The absence of statistically significant concentrations of CDCl3 in the TEAd system is also consistent with a proton abstraction mechanism of chloroform formation. The TEAd molecule was added as triethylamine and titrated to pH 8.9, so all of the nitrogen atoms would have been protonated instead of deuterated. Proton abstraction from this molecule would have occurred at the N-H group so once again protonation of -:CCl3 would have formed CHCl3. However, the lack of influence on the product yield is more likely due to a general lack of involvement of TEAd with the surface and therefore the reaction. Protonation of -:CCl3 from FeOH sites best explains hydrogenolysis in this system.
Reviewing the isotope tracking data collectively can provide insight into the dominant reaction pathway. It is possible to explain the CCl4 hydrogenolysis data in the magnetite system with no organic buffer by either (i) a trichloromethyl radical intermediate and hydrogen abstraction or (ii) a trichlorocarbanion intermediate and proton abstraction. However, data obtained from the more complex system involving TEAd or TRISd is more consistent with the presence of a -:CCl3 intermediate and proton abstraction. Therefore, proton abstraction by -:CCl3 is the one mechanism capable of explaining the entire isotope tracking data set. The presence of the -:CCl3 as a short-lived intermediate was also essential to explain the pH dependent product distribution of CCl4 reductive dechlorination by magnetite (6). CHCl3 production increased from
