Dual Carbon and Hydrogen Compound-Specific Isotope Analysis to Assess the Performance of Chemical Oxidation of Hydrocarbon Compounds using Persulfate

Felipe Marques Solano, Ramon Aravena, Massimo Marchesi, Neil Thomson

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Contaminated Sites and Remediation II

ABSTRACT: In situ chemical oxidation (ISCO) has emerged as an effective, rapid, and low cost active remediation method at hydrocarbon-impacted sites. The interpretation of contaminant concentration data alone is often insufficient to quantify remediation efficiency. The use of stable isotope analysis to complement these concentration data can be very helpful in distinguishing between chemical oxidation and other processes, and therefore in appropriately quantifying the efficiency of chemical oxidation treatment. The aim of this study was to evaluate the potential application of compound specific isotope analysis (CSIA) as a tool to monitor the performance of ISCO for benzene, toluene, ethylbenzene, m-xylene (BTEX), and trimethylbenzenes (TMBs) using persulfate (S2O82-). Our methodology involved two components: (1) batch experiments run at different persulfate/contaminant ratios to determine the carbon and hydrogen isotope enrichment factors, and (2) field tests (e.g., push-pull tests) performed at a former Manufactured Gas Plant (MGP) site located in Clearwater, Florida, USA (Figure 1). The field tests included a baseline component run to evaluate dissolution from impact sediments without the presence of persulfate. Laboratory results showed that as the concentration of BTEX and TMBs decreased 13C and 2H values steadily increased following an almost exponential trend. Similar enrichment factors were obtained for all the compounds explored with different oxidant/contaminant ratios suggesting that the isotope enrichment factor is not influenced by the oxidant/contaminant ratio. Enrichment factors from near-zero to -2.0 › for 13C, and from near-zero to -30› for 2H were determined. The higher hydrogen isotope fractionation factors were obtained during oxidation of the higher molecular weight compounds which suggest that 2H can be used as a tracer to evaluate the treatment performance of these compounds by persulfate. Furthermore, 2H could be used in the field to differentiate between the role of biodegradation and chemical oxidation since for some of the compounds (e.g., see benzene in Figure 2) no hydrogen isotope fractionation was observed during chemical oxidation whereas hydrogen isotope fractionation has been reported for aerobic and anaerobic biodegradation. The results from this study indicate that CSIA can be a complementary tool to evaluate the performance of in situ chemical oxidation of BTEX and TMBs by persulfate. Figure 1: Plan-view of Former MGP site. Figure 2: Benzene D 13C and D 2H by S2O82- oxidation.

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