Heterogeneous Chemistry of Iron at the Liquid/Solid and Air/Solid Interfaces

Hematite is one of the common and stable phases of iron oxide that is ubiquitous in nature. It is involved in many heterogeneous reactions through liquid-solid and gas-solid interfacial interactions in aquatic geochemical systems. Moreover, the interfacial chemistry of metal-oxide and organic matter plays a significant role in the mobility and bioavailability of iron and other components such as arsenic in the soil and aquatic systems. The interactions of organic matter with metal oxide surfaces occur through several mechanisms in aquatic environment. Chapter 2 of this thesis describes the interactions of low molecular weight model organics that include citric acid (CA), oxalic acid (OA) and pyrocatechol (PC) on hematite nanoparticles that have been investigated and characterized by in situ attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR) measurements. H2O/D2O (H/D) exchange experiments were performed to observe the effect of hydration. It was found that strong hydration influenced the spectra for both CA and OA whereas less in the case of PC. Chapter 3 illustrates the initial binding kinetics of arsenicals such as dimethyl arsinate (DMA) onto hematite nanoparticles pre-exposed to low molecular weight organics such as OA and PC. These kinetic experiments were conducted using ATR-FTIR with an emphasis on the role that electrolytes (KCl, NaCl, and KBr) play in the adsorption process. It was concluded that the rate of adsorption for the arsenical onto pre-exposed model organic-hematite surface was found greater in the presence of electrolytes, based on the initial kinetic rate of adsorption for arsenical. Chapter 4 focuses on the investigation of the hygroscopic properties of organic and organometallic polymeric particles, namely polycatechol, polyguaiacol, Fe-polyfumarte, and Fe-polymuconate. These particles are efficiently formed in iron-catalyzed reactions with aromatic and aliphatic dicarboxylic acid compounds detected in field-collected Secondary Organic Aerosol (SOA). The structure of surface water was studied using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and the uptake of gas water was quantified using quartz crystal microbalance (QCM) as a function of relative humidity. Spectroscopic data show that water bonding with organic functional groups acting as hydrogen bond acceptors causes shifts in their vibrational modes. Analysis of the hydroxyl group stretching region revealed weak and strong hydrogen bonding networks that suggest cluster formation reflecting water-water and water-organics interactions, respectively. A modified Type II multilayer Brunauer-Emmett-Teller (BET) adsorption model described the adsorption isotherm on the nonporous materials, polycatechol, polyguaiacol, and Fe-polymuconate. However, water adsorption on porous Fe-polyfumarate was best described using a Type V adsorption model, namely the Langmuir-Sips model that accounts for condensation in pores. It was found that the organic polymers are more hydrophobic than the Fe-containing organometallic polymers. In Chapter 5, the efficiency of iron-containing materials such as Fe-BTC (BTC = 1,3,5-benzenetricarboxylate) metal-organic frameworks (MOFs) and CoFe2O4 nanomaterials was examined to explore their performance in reducing NOx in NH3-SCR (Selective Catalytic Reduction) by DRIFTS. Urea was used as an in situ production of NH3(g) as a reductant agent for NO(g). It appeared that the rate of conversion of NO(g) in the presence of CoFe2O4 nanomaterials (2.3±0.03 ppm·meter-2·min-1) is better than that of Fe-BTC MOFs (0.22±0.04 ppm·meter-2·min-1)