Optimization of compound-specific chlorine stable isotope analysis of chloroform using the Taguchi design of experiments.

RATIONALE: Chloroform, a probable human carcinogen, is commonly detected in various concentration levels in many surface water and groundwater sources. Compound-specific chlorine stable isotope analysis (Cl-CSIA) is significant in investigating the fate of chlorinated contaminants in the environment. Analytical conditions should, however, be thoroughly examined for any isotopic fractionation. In this study, we simultaneously optimize three analytical parameters for a robust online Cl-CSIA of chloroform using the Taguchi design of experiments. METHODS: For Cl-CSIA, a purge-and-trap autosampler coupled to a gas chromatograph in tandem with a quadrupole mass spectrometer, with electron ionization in selected ion monitoring (SIM) mode, was used. Using the Taguchi method, the dominant parameter affecting the results of Cl-CSIA for chloroform was identified through concurrent investigation of the signal-to-noise ratios (S/N) of three parameters, each at three levels: purging time (5, 10, 15 min), transfer time (80, 120, 160 s), and dwell time (20, 60, 100 ms). Moreover, the optimum combination of the levels was identified. RESULTS: The purging time, with a maximum S/N, resulted in the highest influence on the isotope ratios determined. It was further refined through additional experiments to sufficiently extract chloroform from the aqueous phase. Accordingly, 8 min of purging time, 120 s transfer time and 100 ms dwell time were the optimum conditions for Cl-CSIA of chloroform. Post-optimization, a precision of ±0.28 ‰ was achieved for 8.4 nmol of chloroform (equivalent to 0.89 µg or approx. 25 nmol Cl-mass on column). CONCLUSIONS: A simple online method for Cl-CSIA of chloroform was optimized with the Taguchi design of experiments. The Taguchi method was very useful for the optimization of the analytical conditions. However, the purging conditions should be fine-tuned and selected so that sufficient extraction of a target compound is confirmed to acquire a stable and higher precision of the method.

Modelling of geochemical and isotopic changes in a column experiment for degradation of TCE by zero-valent iron.

Zero-valent iron (ZVI) permeable-reactive barriers have become an increasingly used remediation option for the in situ removal of various organic and inorganic chemicals from contaminated groundwater. In the present study a process-based numerical model for the transport and reactions of chlorinated hydrocarbon in the presence of ZVI has been developed and applied to analyse a comprehensive data set from laboratory-scale flow-through experiments. The model formulation includes a reaction network for the individual sequential and/or parallel transformation of chlorinated hydrocarbons by ZVI, for the resulting geochemical changes such as mineral precipitation, and for the carbon isotope fractionation that occurs during each of the transformation reactions of the organic compounds. The isotopic fractionation was modelled by formulating separate reaction networks for lighter ((12)C) and heavier ((13)C) isotopes. The simulation of a column experiment involving the parallel degradation of TCE by hydrogenolysis and beta-elimination can conclusively reproduce the observed concentration profiles of all collected organic and inorganic data as well as the observed carbon isotope ratios of TCE and its daughter products.

Carbon isotope fractionation during reductive dechlorination of TCE in batch experiments with iron samples from reactive barriers.

Reductive dechlorination of trichloroethene (TCE) by zero-valent iron produces a systematic enrichment of 13C in the remaining substrate that can be described using a Rayleigh model. In this study, fractionation factors for TCE dechlorination with iron samples from two permeable reactive barriers (PRBs) were established in batch experiments. Samples included original unused iron as well as material from a barrier in Belfast after almost 4 years of operation. Despite the variety of samples, carbon isotope fractionations of TCE were remarkably similar and seemed to be independent of iron origin, reaction rate, and formation of precipitates on the iron surfaces. The average enrichment factor for all experiments was -10.1 per thousand (+/- 0.4 per thousand). These results indicate that the enrichment factor provides a powerful tool to monitor the reaction progress, and thus the performance, of an iron-reactive barrier over time. The strong fractionation observed may also serve as a tool to distinguish between insufficient residence time in the wall and a possible bypassing of the wall by the plume, which should result in an unchanged isotopic signature of the TCE. Although further work is necessary to apply this stable isotope method in the field, it has potential to serve as a unique monitoring tool for PRBs based on zero-valent iron.