title: Abiotic reduction of contaminants by iron minerals
title: rates, redox potentials, and greenhouse gases
titleType: Subtitle
Creators: name: Robinson, Thomas Cloudman
Publisher: name: University of Iowa
Year: 2022
Subjects: Magnetite
Nitrite
Nitrous oxide
Redox potentials
public abstract
Iron

description: Fe mineral redox properties are an important characteristic of many different environmental processes such as contaminant remediation, nutrient and metal cycling, as well as biological respiration. A new framework has been developed where Fe minerals have been conceptualized as a "biogeobattery" where they store and release charge (i.e., electrons) to conduct these processes. Here, I looked at how these Fe mineral biogeobatteries reduce nitrite (NO2-) with a focus on the production of the greenhouse gas, nitrous oxide (N2O). I also characterized the redox properties of magnetite to estimate a standard redox potential (E_H^o) to determine which redox couple is relevant for interpreting magnetite potentials, as well as evaluating the influence magnetite stoichiometry has on measured redox potentials.I measured NO2- reduction rates and extents of N2O production by several Fe minerals under consistent, environmentally relevant conditions as it is unclear how chemodenitrification rates and N2O yields vary for Fe minerals under reducing conditions. Nitrite reduction rates and nitrogen recovery as N2O varied substantially among the heterogeneous Fe(II) species with half-lives (t1/2) ranging from as low as an hour to over two weeks following the trend of goethite/Fe(II) ~ hematite/Fe(II) ~ magnetites &gt; maghemite/Fe(II) &gt; sediment/Fe(II) and nitrogen mass recoveries ranging from 21% to 100%. Interestingly, I observed no clear trend between higher magnetite stoichiometry (x = Fe2+ / Fe3+) and faster NO2- reduction rates similar to other contaminants such as mercury, uranium, or nitrobenzene. I further compared chemodenitrification and biological denitrification in the absence and presence of added aqueous Fe(II) with a native Iowa sediment. I observed much slower NO2- reduction compared with heterogeneous Fe(II) species, however, I observed the greatest N2O production during chemodenitrification. In addition, my results suggest both abiotic and biological NO2- reduction was observed at my experimental conditions. My results provide compelling evidence that redox active sediments and soils could be an important source of N2O emissions and should be further evaluated and incorporated into global climate models.
description: Magnetites were one of the more reactive Fe minerals for nitrite reduction and is often suspected as one of the more important reductants of many groundwater contaminants. There is, however, little agreement among reported redox potentials for magnetite under reducing conditions which makes predicting magnetite behavior in sediments and soils difficult. I measured open-circuit redox potentials of stoichiometric magnetite suspensions over a range of pH values and aqueous Fe(II) concentrations and found that electron transfer mediators are not required to measure accurate magnetite redox potentials. I observed that the measured potential decreased with increasing pH and increasing aqueous Fe(II) concentrations, and my results followed Nernstian behavior over a pH and Fe(II) concentration range where ferrous hydroxide precipitation was limited. Surprisingly, my results differed from the expected couples of magnetite | Fe(II)aq and hematite | magnetite similar to what others have previously suggested. Rather, my estimated parameters more closely agreed with what is expected for a variety of redox couples between Fe(III) oxides such as maghemite, lepidocrocite, hematite, and goethite, and aqueous Fe(II). I estimated a formal potential (E_H^o=861±8 mV vs.SHE) with the Nernst equation which closely agreed with the expected E_H^o values for both the lepidocrocite | Fe(II)aq, and maghemite | Fe(II)aq redox couples. While previous work has suggested that lepidocrocite may be the oxidation product for Fe(II)aq in the presence of magnetite, my results and others suggest that the oxidation product could similarly be maghemite. I further observed an interesting shift in my measured potentials where the pH relationship shifted from what was expected with the Fe(III) oxides | Fe(II) to be closer to the hematite | magnetite couple at high pH conditions when no aqueous Fe(II) was added. Taken together, my measurements suggest that predicting the redox behavior of magnetite in natural soils and sediments is complex, and that the dominant redox couple will depend on the pH, aqueous Fe(II) concentrations, as well as magnetite stoichiometry.
description: I further explored the effect of magnetite stoichiometry (x = Fe(II)/Fe(III)) on magnetite redox potentials. Magnetite stoichiometry is known to substantially influence magnetite properties, such as conductivity, magnetic susceptibility, and the minerals behavior in soils and sediments (e.g., contaminant reduction and biological respiration rates). It remains unclear, however, how magnetite stoichiometry influences magnetite redox potentials and whether the relevant redox couple shifts. I measured open-circuit redox potentials of magnetite suspensions over a range of magnetite stoichiometries, pH values, and aqueous Fe(II) concentrations and I found that magnetite stoichiometry and pH influence the measured potential, but the behavior is complex and does not appear to follow Nernstian behavior. Magnetite stoichiometry, pH, and aqueous Fe(II) all influence one another, and additional experiments are needed to evaluate how to interpret our data.
description: The Sustainable Water Development (SWD) graduate program requires each student participate in a professional training experience (PTE) tailored for their career interests. I worked with Senator Joe Bolkcom to contribute weekly posts to the blog Iowa Environmental Focus to improve my writing and communication skills. I also shadowed Senator Bolkcom and learned more about how Iowa state government functions through two visits to the state house and attendance to a subcommittee meeting for a proposed senate file.
description: My work on nitrite reduction by Fe minerals and magnetite redox potentials provides an improved framework for evaluating greenhouse gas emissions from agricultural soils as well as predicting magnetite redox behavior in soils and sediments. More specifically, my work provides rates and extents of nitrite reduction to nitrous oxide gas and quantifies the standard redox potential and couple relevant to magnetite under reducing conditions. My hope is that these values will help climate scientists as well as environmental engineers better predict climate change and more effectively cleanup groundwater contamination.