Improved monitoring of the reductive dechlorination of PCE in polluted soils by using geophysical and electrochemical measurements carried out in columns
Résumé
Today, reductive dehalogenation (RDH), using strong reducers, is one of the most important emerging remediation techniques for halogenated hydrocarbons (HHC). The use of nanomaterials, all alone or coupled with strong reducers, for RDH is emerging too as a good alternative and promising technological approach to clean up polluted environments by HHC. The purpose of the present study was to set up an enhanced monitoring of the reductive dechlorination (RDC) of Perchlorethylene (PCE) in soils, based on geophysical and electrochemical measurements and practiced, at this stage, only in columns. This study is accomplished within the framework of DECHLORED, a project that aims to clean up aquifers by using strong reducers with or not the combined effect of Nano-iron particles (NIP), reaching the RDC of chlorinated hydrocarbons (CHC) and illuminating the field under RDC process. For this purpose, columns (height 50cm; diameter 20cm) are designed and made in Kynar® and equipped with sophisticated systems for non-destructive monitoring of time changing physical parameters. This monitoring was complemented by physicochemical probes (temperature, pH, redox, conductivity, oxygen and chloride) and Gas Chromatography for PCE and its degradation products (DCE, ethene and ethane) and Ion Chromatography for Cl- and SO42- in water at the column output to correlate RDC, NP migration and electrical properties. The entire system was placed in air-conditioned cupboard. Columns are implemented with two rings of Hasteloy® as electrodes for alternative sinusoidal current (AC) injection and six ports equipped with Ag-AgCl electrodes for potential measurement. Two methods are used: i) frequency-domain induced polarization (IP) measurements (which consist of imposing an AC current at a given frequency and measuring the resulting electrical potential difference between two other non-polarizing electrodes), and ii) galvanostatic electrochemical impedance spectroscopy (GEIS). Both by measuring complex electrical conductivity in the mHz to kHz range, are sensitive to electrolytic conductivity, grain surface conductivity and metallic conductivity. Under time varying electric field, pore fluid/solid polarization effects could be measured from surface using IP impedance analyzer or time-domain resistivimeter. Indeed, IP can map real and complex resistivities of the media at any time and particularly, before, during and after the RDC process application on the column. Two different soils, polluted or not by PCE, different solutions/suspensions of reactants (reducers or NIPs separately or reducers and NIPs together) at two different temperatures (12 and 25°C) are tried. Experiments in columns were carried out to test reactants behavior, in the absence and in the presence of PCE, and allow us to: * estimate how reactants and PCE chemical degradation products transform electrical properties * assess reactant dispersion, distribution, transport and reactivity on PCE degradation, inside the columns (over all its length) and as a function of time and for different soils * Improve understanding of reactant efficacy, their transformation and persistence, and also their potential negative effects * model these flow-through systems (1D or 2D column set up) by modeling softwares (PHREEQC® and MARTHE® reactive transport simulations). Our results claim for combining geophysical with electroanalytical measurements to monitor RDC processes and will allow us to give an advantageous method for monitoring and checking in situ remediation.