Physical environmental electrochemistry: Electrochemical properties of natural organic matter and iron powders.

摘要

Electron transfer is ubiquitous in nature. A thorough understanding of the rates and mechanisms of electron transfer is vital to the scientific study of natural systems. It is also important for the successful design and implementation of engineered systems that rely on electron transfer (i.e., permeable reactive barriers filled with Fe0). Since electrochemistry is the science that deals with the relationship between electricity and chemical changes, it is well suited for the study of systems in which electron transfer is important. This dissertation details the electrochemical study of two important, and different environmental systems.;The first part of this dissertation deals with the redox properties of natural organic matter (NOM), fractions of NOM, and model biogeochemical electron shuttles. Several electrochemical techniques and experimental designs were adapted in order to obtain the best resolution of voltammograms of NOM. The peak potentials for various NOM fractions were collected and related to various electron donors and electron acceptors, given as a redox ladder. A significant result of this study was the observation that the voltammogram of an unfractionated NOM sample gave several peaks, indicating that several redox active species or moieties are present in the sample. This led us to hypothesize that there may be a continuum of potentials associated with redox active groups in NOM.;The second environmental system that was studied in this dissertation was that of the Fe0-H2O-contaminant system. Much work has been done on characterizing the rates and mechanisms of how Fe 0 reduces various contaminants. Most of these studies use high grade iron or polished Fe0 disk/coupon/wire electrodes. While these studies have provided a great deal of understanding of this system, the rates and mechanisms of iron mediated reduction by particles used in the field may be different. In order to get at important aspects of this system that are not represented in batch or column experiments, we designed, fabricated, and validated a powder disk electrode (PDE) that is able to hold various sizes of iron particulate (micro to nano).;In order to determine if our PDE would give electrochemically interpretable results, we started by packing the PDE with electrolytic Fe0(Felc). We found that by varying scan rate, rotation rate, and cavity volume: (i) the resulting voltammogram was due to the iron powder, not the underlying disk material, (ii) the cathodic reaction (2H+ + 2e - → H2(g)) is kinetically limited, and (iii) iron dissolution is affected by the mass transport of solutes (probably Fe 2+) out of the cavity pore space.;For the study of nano-iron powders (FeH2, FeBH ), we combined batch, spectroscopic and electrochemical approaches to characterize the properties of these nano iron samples. FeH2 is a two-phase material consisting of 40 nm α-Fe0 (made up of crystals approximately the size of the particles) and Fe3O 4 particles of similar size or larger containing reduced sulfur; whereas FeBH is mostly 20-80 nm metallic Fe particles (aggregates of <1.5 nm grains) with an oxide shell/coating that is high in oxidized boron. The FeBH particles further aggregate into chains. We found that both nano iron samples with Fe0 cores gave more cathodic corrosion potentials (Ecorr's) than either the polished iron disk or the PDE packed with Fisher electrolytic iron.