Kinetic and mechanistic study for the abiotic oxidation of iron(II) catalyzed at the ferric (oxyhydr)oxide and solution interface

This research dealt with the heterogeneous oxidation of Fe(II) by some environmentally significant contaminants including nitrite, uranium, and arsenate using Hydrous Ferric Oxide (HFO) as the catalytical solid phase in anoxic soil medium and sediments. It is the first study of the Fe(II)/Fe(III)/HFO system in which mass balance has been performed for both Fe(II) and all the oxidant species using a strictly anoxic environment at circumneutral pH. Fe(II) and HFO were used as the electron donor and catalytical solid phase because Fe(II)/Fe(III) redox couple is often dominant in controlling the redox potential in anoxic groundwater system and HFO is ubiquitous in soils and sediments, and it is an important adsorbent for a wide range of chemicals. The results showed that the reaction between Fe(II) and either NO2- or U(VI) was fast in the presence of HFO. The rate for the oxidation of Fe(II) and the reduction of either NO2- or U(VI) was found to be a function of dissolved Fe(II), solid-bound Fe(II) and oxidant concentration, and can be described by Rate = - koverall • [Fe(II)diss] • [Fe(II)solid-bound] • [Oxidant]. However, the reaction rate for Fe(II)/HFO/U(VI) was an order of magnitude faster than for nitrite, possibly due to high affinity of U(VI) to the HFO surface which could provide a shorter pathway for electron-transfer. Conservation of solid-bound Fe(II) was observed for either oxidant throughout the reaction with Fe(II)/HFO, therefore solid-bound Fe(II) functioned as a catalyst for the reduction of either oxidant by Fe(II). In experiments with nitrite or no oxidant in Fe(II)/HFO, concentration of Fe(II)solid-bound first increased less than 100% and then decreased coincident with partial conversion of HFO to goethite. When U(VI) was used as an oxidant, Fe(II)solid-bound increased and then remained constant such that solid-bound Fe(II) density was increased by 3 to 5 times compared to absence of U(VI). The significant increase of solid-bound Fe(II) in Fe(II)/U(VI) reaction could be due to the newly formed sorption sites created by the sorption and reductive precipitation of U(VI) and oxidative precipitation of Fe(II). The characterization of Fe(II)/Fe(III) systems from this study is consistent with several other recent studies that have supported the hypothesis that Fe(II) is incorporated into the bulk phase of Fe(III) oxides, rather than remaining at the interface, and that this unusual behavior results in apparently unique redox reactivity. Interestingly, overall rate constants declined when Fe(II)solid-bound exceeded 0.02 mol Fe(II) per mol Fe(III), similar to results from previous studies using O2 and NO2- as oxidants.;We also investigated the effectiveness of two extractants, bicarbonate and phosphoric acid, on the quantification of U(VI) species during Fe(II)/U(VI) reaction in HFO suspension. Higher phosphate extractable U(VI) (U(VI) phosphate) than bicarbonate extractable U(VI) (U(VI) bicarbonate) was observed in all experiments. Interestingly, consumed U(VI)bicarbonate/consumed U(VI)phosphate ratios were about 2 in all the experiments. Using H3PO4 as an extracting reagent could increase U(VI) extractability from HFO suspension due to high solubility of HFO at pH 1.5. It is also possible that carbonate ligand stabilize U(V) species which was formed as a intermediate product of U(VI) reduction to U(IV), consistent with the mechanism that U(VI) reduction is an one-electron-transfer reaction followed by disproportionation of U(V) to U(VI) and U(IV) species. It was also found that when freshly prepared HFO was used as the solid phase in Fe(II)/U(VI) reaction, multiple stages reaction and a lag phase was observed. This study showed that the presence of reducing agents and dynamic mineral phases could increase the complexity of both uranium removal processes and operational procedures for quantifying uranium reduction.;Fe(II)/NO2- reaction was simulated using quantum mechanical calculation in both homogeneous and heterogeneous system to identify the favorable reaction pathway for electron transfer. Molecular orbital/density function theory (MO/DFT) calculation showed that Fe(II)/NO2 - redox reaction in aqueous phase without the presence of solid phase was thermodynamically favorable in contrast to the wet chemistry result. The absence of the spontaneous reaction could be due to the difficulty in forming inner-sphere Fe -- O -- N -- O -- Fe complexes in dilute solution in batch study. In heterogeneous system, the result showed that direct electron transfer pathway was more thermodynamically favorable than indirect electron transfer pathway for Fe(II)/NO2- reaction in the presence of Fe(III) (oxyhydr)oxide. However, due to the limitation on simulation of Fe(III) (oxyhydr)oxide cluster with Gaussian 03, density functional theory (DFT) calculation of Fe(II)/NO2- redox reaction on periodic Fe(III) (oxyhydr)oxide structure using Vienna Ab Initio Simulation Package (VASP) is under way to justify thermodynamic data of indirect pathway. NBO population analysis showed sorbed Fe(II) became more oxidized than dissolved Fe(II) in the model product in heterogeneous system indicating that sorbed Fe(II) could be the electron donor in Fe(II)/NO 2- redox reaction.;Catalytical effect of solid-bound Fe(II) on Fe(II)/As(V) reaction using HFO as the solid phase was not observed in our study. The inertness of redox reaction between As(V) and Fe(II) could be due to the requirement of two-electron transfer for As(V) reduction to As(III), whereas it is possible for O 2, nitrite, and uranyl to undergo one electron transfer with Fe(II) in the presence of HFO.