Overcoming Barriers to the Remediation of Carbon Tetrachloride Through Manipulation of Competing Reaction Mechanisms

Quantify the kinetics of all competing product-formation pathways, over a range of conditions relevant to groundwater remediation, using well-mixed batch reactors and analysis primarily by chromatography. At OGI, batch experiments were conducted on Fe(0) systems (both Fisher Electrolytic and Nano-sized iron). The experiments were done with and without buffer. The buffered experiments tried to contrast two buffers: an organic buffer (EPPS, presumably a H atom donor), and the inorganic borate. In the buffered experiments, the pH was varied (7.3 and 8.4). For the pre-exposure treatment, after trying a variety of methods, like shaking and not shaking for varied amounts of time, it was decided to stick with not shaking and have a pre-exposure of 24 hours. The unbuffered data did not show any marked trend with increasing mass of Felc. However, 3.5 g of Fe showed about 100% conversion to CHCl3, and 1g of Fe showed 50% conversion. At pHs 8.4 and 7.3, there was no trend observed for branching ratios between EPPS and Borate buffer. kCT (disappearance rate constant of carbon tetrachloride) values were found to be different from CT and CF fits. Experiments with nano-iron (unbuffered, buffered with both buffers at pH 8.3), did not show any trend with respect to Fisher Iron, except for the unbuffered experiments, where the CF ''yield'' was less in the nano iron case. Future experiments involve testing for chloride, formate and CO, and performing experiments over a wider range of pH and buffers. Batch experiments were conducted at PNNL to compare the efficiency and product distribution of representative Fe(II) and Fe(0) systems applied to dechlorination of CCl4. These experiments involved (1) a smectite clay with Fe(III) in its structure that had been reduced to Fe(II) by dithionite treatment, (2) the same clay to which Fe(II) was added as an exchangeable cation, (3) electrolytic Fe(0) from Fisher, and (4) a mixture of the reduced clay and Fe(0). Experiments were conducted in headspace vials at pH 7 in either bicarbonate or bis-tris propane buffers. Reactant and product concentrations were determined by headspace analysis using GC/MS. Results from the first run of this experiment showed relatively little dechlorination by the Fe(II) system, and from 50-80% dechlorination by the Fe(0) system after 48 h. Essentially no CHCl3 was seen in the Fe(II) system, whereas as 30% of the original CCl4 was converted to CHCl3 in the Fe(0) system. Very low amounts of CH2Cl2 were seen in all treatments. Plans are underway to repeat this experiment and use a cryo-GC capability that will allow simultaneous determination of CO as well as the chlorinated methanes. This is important because CO represents the end product of the second hypothetical dechlorination pathway that may compete with the CHCl3 pathway and will aid in mass balance calculations. An additional experiment to evaluate the Henry's Law constant for CCl4 in aqueous solutions in the presence and absence of clay showed no significant difference due to the presence of clay, although slightly higher gas-phase concentrations were seen when clay was present

Accessible versions of figures from Tratnyek et al. (2017) "In silico environmental chemical science: Properties and processes from statistical and computational modelling"

<p>Accessible versions of selected figures from Tratnyek et al. (2017) "In silico environmental chemical science: Properties and processes from statistical and computational modelling" Environ. Sci. Processes Impacts 19(3): 188-202. DOI: 10.1039/C7EM00053G.</p> <p>The Abstract Art figure shows a classification of variables for predictive/diagnostic models used in silico environmental chemical science, in terms of system scales and variable types. Figure 3 shows a continuum of system scales encompassing the whole scope of predictive/diagnostic modelling for in silico environmental chemical sciences, juxtaposing earth and biological scales. The published version of Figure 3 is tall, for two-column page-layouts, but a wide version of Figure 3 is provided for landscape oriented formats.</p> <p>This work is from the perspectives/review paper at the beginning of a themed issue on "Quantitative Structure-Activity Relationships (QSARs) and Computational Chemistry Methods in the Environmental Chemical Sciences", published in the March 2017 issue of the Royal Society of Chemistry journal Environmental Sciences: Process and Impacts. The whole collection of papers can be accessed at rsc.li/qsars.</p

Overcoming Barriers to the Remediation of Carbon Tetrachloride through Manipulation of Competing Reaction Mechanisms--Final Technical Report

The premise of this project was that if we understood the fundamental chemistry that controls the branching among product formation pathways for the degradation of CCl4, we could design remediation strategies that minimize the formation of CHCl3 and thereby provide badly needed alternatives for remediation of the large plumes of CCl4 that contaminate several DOE sites. To this end, we performed a series of coordinated batch, spectroscopic, and modeling experiments, to study the effect of a variety of factors on the yield of CHCl3 from CCl4 during reduction with zero-valent iron (Fe0). The factors studied include those with direct implications for field performance (e.g., the concentration of CCl4 relative to the amount of iron surface area) and others chosen for diagnosis of the reaction mechanism (e.g., incorporation of deuterium into CCl4 reduction products in the presence of D2O). The key mechanistic findings of this study are (i) that CCl3• probably is not an intermediate in the formation of CF, but CCl3− probably is, (ii) the high reductive capacity of the Fe0 core favors the concerted 2e− reduction, and (iii) magnetite on Fe0 favors the benign product formation pathway. The latter conclusion is based on the observation that one type of nano-sized Fe0 that is coated with magnetite shell produces low yields of chloroform (0-40%), whereas others produce the higher yields of chloroform (60-100%) that are typical of most methods for reducing CCl4 (including biodegradation). Since nano-Fe0 can, in principle, be introduced into the deep subsurface by injection, our results would suggest that the right type of nano-Fe0 introduced in the right way might be highly effective at dechlorinating CCl4 with minimal formation of CHCl3 or other undesirable by-products. This conclusion may offer a breakthrough in the search for remediation technologies that are suitable for the deep CCl4-contamination at DOE sites such as the 200-W area of Hanford

Effects of Solution Chemistry on the Dechlorination of 1,2,3-Trichloropropane by Zero-Valent Zinc

The reactivity of zerovalent zinc (ZVZ) toward 1,2,3-trichloropropane (TCP) was evaluated under a variety of solution conditions, including deionized water, groundwater, and artificial groundwater, over a pH range of about 6.5–12. In deionized water, first-order rate constants for TCP disappearance (<i>k</i>_obs) exhibit a broad minimum between pH 8 and 10, with increasing <i>k</i>_obs observed at lower and higher pH. The similarity between this trend and zinc oxide (ZnO) solubility behavior suggests pH related changes to the ZnO surface layer strongly influence ZVZ reactivity. Values of <i>k</i>_obs measured in acidic groundwater are similar to those measured in DI water, whereas values measured in alkaline groundwater are much smaller (>1 order of magnitude at pH values >10). Characterization of the surfaces of ZVZ exposed to deionized water, acidic groundwater, and alkaline groundwater suggests that the slower rates obtained in alkaline groundwater are related to the presence of a morphologically distinct surface film that passivates the ZVZ surface. TCP degradation rates in artificial groundwater containing individual solutes present in groundwater suggest that silicate anions contribute to the formation of this passivating film