COMPARATIVE EVALUATION OF MEDIATED ELECTROCHEMICAL REDUCTION AND CHEMICAL REDOX TITRATION FOR QUANTIFYING THE ELECTRON ACCEPTING CAPACITIES OF SOILS AND REDOX-ACTIVE SOIL CONSTITUENTS

Abstract

The production, handling and disposal of chemicals have led to contamination of soil and groundwater. Conventional remediation techniques are expensive and difficult, as a result, there is a need for cost-effective remediation strategies based on natural or enhanced contaminant transformation. Redox reactions are one of the most viable transformation pathways due to the redox characteristic of various relevant contaminants, including halogenated compounds, chromium, and munition compounds (MCs). To design an effective remediation system, it is important to first analyze the redox properties of a given soil, these properties involve the capacities to store and exchange electrons with the environment through oxidation and reduction of their redox-facile constituents. One of the most relevant parameters to determine the redox properties of soils is the electron accepting capacity (EAC), becoming a key parameter to estimate the extent to which contaminants can be degraded in situ and the frequency at which electrons need to be replenished in a contaminated site. In this thesis, two methods to measure the EAC of 8 soils were compared and analyzed, namely mediated electrochemical reduction (MER) with diquat as an electron transfer mediator and chemical redox titration (CRT) with dithionite as a reductant. All soils were air-dried, 2 mm-sieved, deoxygenated in an anoxic glove box, and redox-titrated. Results indicate a divergence of CRT from MER of 23.142 µmol e-/g soil for every 1 % of TOC present, since MER did not access all sites in organic carbon (O.C.) due to the short equilibration time (1 hr.) compared to CRT (24 hrs.). For Fe, the average accessibility in soils was about 40.8%, reflecting some slow reacting minerals, including iron clays. For O.C., the EAC estimated in soils was 4.03 mmol e-/ g O.C, this value is substantially higher than previous values reported using other techniques. In conclusion, our results suggest that CRT is a more suitable method to accurately measure the EAC of soils with high content of O.C, but also provide tools for the estimation of the EAC when Fe and O.C. content are known, this allowed to establish a linear relationship between CRT and MER methodologies. To test the transfer of electrons in soils, a munition compound (MC) was selected, 3-nitro-1,2,4-triazol-5-one (NTO). Results indicate that the kinetics of the reaction increase as the content of electrons increases, in other words, NTO reduction is faster as the EAC is filled up because of the change in reduction potential and willingness of the soil to donate electrons. A relationship between kinetics and thermodynamics is also described, and further work is required to find the adequate descriptors and coefficients that will allow for the estimation of rates of reduction of NTO and other MCs in soil and groundwater. To the best of our knowledge, this work provides the first direct comparison of MER and CRT in soil for the estimation of EAC. The results of this work will potentially increase the efficiency of remediation system aimed for the reduction of redox-active contaminants, while the findings shown here will serve as a baseline for the modeling of redox properties in soil.