Adsorption, desorption, and redox reactions at iron oxide nanoparticle surfaces

Forskningsoutput: AvhandlingDoktorsavhandling (monografi)

Abstract

Soils play a critical role in the carbon (C) cycle by regulating the atmospheric carbon dioxide (CO2) levels, and correspondingly the Earth’s climate. However, there are still countless questions of how biological and geochemical soil processes affect the C cycle. To be able to predict future implications on the Earth’s climate, we need to understand these processes. Globally, soils store more C than the terrestrial biomass and the atmosphere combined. The soil environment has an enormous impact on the soil C dynamics, whether C is captured, stored or released. Thus, depending on the soil environment, some soil organic matter (SOM) can persist for decades, while some SOM decomposes more rapidly. It has been shown that soil microbes (fungi, bacteria, archaea, etc.) play an important role in SOM decomposition into smaller molecules, ultimately, releasing CO2 to the atmosphere or playing an important role in the formation of soil aggregates, thus contributing to increased SOM stability.
Boreal and temperate forests store a large part of the terrestrial C, and in this environment ectomycorrhizal (ECM) fungi are abundant. ECM fungi form symbiotic relationships with plants i.e., plant hosts provide C as energy source for fungal growth and in return fungi transport nutrients to the plant host. A major part of soil nutrients is found in organic form, as part of SOM. Thus, to access and mobilize these nutrients, fungi are required to decompose SOM. Some ECM fungi use a degradation mechanism involving the Fenton reaction (Fe2+ + H2O2 → Fe3+ + •OH + OH-). For this mechanism to occur, the fungi need ferrous iron (Fe2+) and hydrogen peroxide (H2O2). H2O2 can be provided by fungi, however, in soils iron is primary found in oxidation state +3 and in solid state as iron oxides and other iron- containing minerals, and thereby, is not easily available. During the initial SOM decomposition, fungi produce low molecular weight organic compounds, secondary metabolites, which are not involved in sustaining fungi or the plant host growth. Some of these secondary metabolites have iron reducing capacity, which can reduce soluble ferric iron (Fe+3) salts, thus making Fe2+ more accessible. However, there is limited knowledge on how these secondary metabolites interact with and possibly reduce solid iron oxides.
In this thesis, common boreal and temperate forest soil iron oxides, ferrihydrite and goethite, were used to investigate whether Fenton reactions can be initiated by organic reductants, similar to fungal secondary metabolites or as part of dissolved organic matter (DOM), under different geochemical conditions. The aims of this
13
13
PhD project were achieved by studying adsorption, desorption, and redox reactions between the iron oxide nanoparticles and these organic reductants. This research seeks to answer the question of whether some of these reactions can promote the generation of hydroxyl radicals (•OH) via Fenton reaction. Increased understanding of these mechanisms can improve our understanding of the stability of SOM and SOM-mineral aggregates.
Results obtained at pH 4.5 and 4.0 showed that a model compound (2,6-DMHQ), which is similar to a compound secreted by brown rot wood decay fungi, was able to reductively dissolve iron oxide nanoparticles and produce H2O2 under aerobic reaction conditions. Thus, reactions between 2,6-DMHQ and iron oxides allow the formation of both reactants to initiate the Fenton reaction. In anaerobic environments, due to a lack of oxygen (O2), the formation of H2O2 was negligible from reactions between 2,6-DMHQ and iron oxides, thus suppressing the Fenton reaction. Moreover, results showed that initiation of the Fenton reaction was not only affected by O2 concentrations, but also by different geochemical factors, such as pH, redox potentials and adsorption of organic and inorganic molecules.
Soils contain a wide range of inorganic and organic molecules that can adsorb on iron oxides, thus interfering with redox reactions at iron oxide surfaces. Results showed that 2,6-DMHQ was able to compete with inorganic and organic molecules for surface iron and to initiate the Fenton reaction. Moreover, adsorption of organic and inorganic molecules in some cases promoted the Fenton reaction to occur close to iron oxide nanoparticle surfaces. These surface reactions can have a considerable impact on adsorbed SOM decomposition and the provision of nutrients to plant hosts. At the same time, more extensive SOM decomposition can result in a greater CO2 release.
At neutral pH values, various organic pollutants, with molecular structures similar to 2,6-DMHQ, are found in groundwater and agricultural soils due to pesticide and fertilizer use. Injection of H2O2 into the soil or aquatic systems is a widely-applied technique to degrade these pollutants. Results at pH 7.0 showed that adsorption of inorganic and organic molecules on iron oxides resulted in a higher yield of the Fenton reaction from the 2,6-DMHQ-iron oxide interactions. Therefore, naturally occurring processes between iron oxides and 2,6-DMHQ-like molecules can help to increase organic contaminant degradation, for example, when the commonly added plant nutrient, phosphate, is adsorbed on iron oxide surfaces.
DOM modified by the ECM fungus Paxillus involutus, which contained secondary metabolites, has a higher affinity towards iron oxide surfaces than the initial DOM. This is in agreement with the hypothesis that organic matter decomposition can contribute to SOM stability. Results showed that modified DOM reductively dissolved ferrihydrite and goethite, but in order to initiate the Fenton reaction the addition of H2O2 was required. Further, results suggested that in the absence of
14
14
H2O2, some Fe2+ was complexed to adsorbed DOM. When H2O2 was added the Fenton reaction occurred in close vicinity to the organic matter, which resulted in direct oxidation of DOM components. Moreover, experiments with DOM modified by P. involutus suggested that the produced secondary metabolites might act as antioxidants. Thus, these metabolites may inhibit the oxidation of other DOM components, thereby increasing the partial recalcitrance of DOM.
In summary, the results obtained in this PhD project suggested that in aerobic soil environments and in the presence of iron oxide nanoparticles, fungal secondary metabolites with molecule structures similar to 2,6-DMHQ might initiate the Fenton reaction and produce •OH. Yet, further studies are required to understand if and how these reactions affects the stability of the soil C. Moreover, the generation of •OH at iron oxide nanoparticles surfaces can be harmful and cause damage to organisms exposed to these nanoparticles. Thus, the reactions characterized in this study can be related to the potential toxicity of iron oxide nanoparticles.

Detaljer

Författare
  • Lelde Krumina
Enheter & grupper
Forskningsområden

Ämnesklassifikation (UKÄ) – OBLIGATORISK

  • Naturvetenskap

Nyckelord

Originalspråkengelska
KvalifikationDoktor
Tilldelande institution
Handledare/Biträdande handledare
Tilldelningsdatum2018 jan 25
UtgivningsortLund
Förlag
  • Lund University, Faculty of Science, Centre for Environmental and Climate Research (CEC), Department of Biology
Tryckta ISBN978-91-7753-503-4
Elektroniska ISBN978-91-7753-504-1
StatusPublished - 2017 dec
PublikationskategoriForskning

Nedladdningar

Ingen tillgänglig data