Specific ion effects have been observed to play a role not only in macromolecular biological systems such as proteins but also in inorganic systems such as colloidal nanoparticles. The relative importance of ion-water interactions versus ion-ion pairing continues to be a topic of investigation. For poorly hydrated ions such as ammonium, though, it is expected that ion-ion interactions may dominate any solution phase behavior that is governed by ion effects. Recently, when exploring systems capable of oxidizing pyrite for oilfield applications, sodium bromate and ammonium persulfate were found to react synergistically with iron sulfide as well as other inorganic substrates. Underlying the synergism is a reaction between bromate and ammonium, where oxidation at low temperature visibly produces Br2(g) and liberates H+. The approach was tailored to produce a series of strong mineral acids by reacting sodium bromate with alternative ammonium-based salts, producing in situ generated acids that are convenient for subterranean applications. A series of anions were explored including persulfate, sulfate and other polyatomic ions as well as various halides. As the ammonium salts were oxidized with sodium bromate, their products were analyzed by electrospray ionization mass spectrometry (ESI-MS), inductively coupled plasma (ICP), and elemental analysis. Real-time FT-IR and multinuclear NMR solution analyses were also performed to observe the formation of intermediate species. Each ammonium-based salt produced an oxidized nitrogen species, whereby the activation temperatures and rates were dictated by anion selection. Ammonium persulfate was found to react with sodium bromate at ~80 °C, whereas ammonium chloride required ~150 °C, and ammonium fluoride &gt;200 °C. ESI-MS confirms product formation such as the conversion of persulfate to sulfate, the apparent complete loss of nitrogen, and the presence of some Br- in solution though the majority of bromate is converted to bromine. FT-IR and NMR established the time scales over which the reactions occurred as well as the identity of intermediates. While this study elucidates the pathway for pyrite oxidation and acid generation, it further contributes to the growing body of knowledge of the role of counter ions in solution, particularly when solvent effects are minimized. Furthermore, processes involving oxidation of ammonium or ammonia have important implications for studies involving nitrification in nature.

Specific ion effect manifested in oxidation of ammonium salts and inorganic substrates

254th National Meeting (2017)

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