Insights into ultrasound-promoted degradation of naphthenic acid compounds in oil sands process affected water. Part II: In silico quantum screening of hydroxyl radical initiated and propagated degradation of benzoic acid

Abstract

In Part I, we outlined the importance of sustainable sonochemical treatment to intensify oil sands process affected water (OSPW) treatment empirically and hypothesized degradation pathways. Herein, we elucidate the formation of intermediate products with well-defined molecular level solutions. Proposed mechanisms describe hydroxylation, decarboxylation and bond scission which drive the degradation of intermediates towards mineralization. This comprehensive first study on in silico screening of sonochemical degradation investigates quantum methods using density functional theory to explain the postulated degradation mechanisms through a theoretical radical attack approach, based on condensed Fukui reactivity indicators. A nudged elasticity band (NEB) approach is applied to find a minimum energy path (MEP), allowing the determination of intermediate products and energy barriers associated with naphthenic acid degradation. This approach provides structures and energies of the breakdown compounds formed along the reaction pathway enabling the determination of molecular reaction kinetics. In continuation of Part 1, the focus of this study is to evaluate sonochemically-induced hydroxyl radical (OH^•) reactions of benzoic acid using density functional theory. Hydroxylation and decarboxylation mechanisms of the model naphthenic acid compound and its intermediates were simulated to determine the prospective pathway to ideal mineralization. DFT was applied to calculate interaction energies, Mulliken charges, Hirshfeld population analysis, dipole moments, frontier orbitals, and polarizability. Electronic properties and frontier orbital trends were also compared to computational work by Riahi et al.[1] to confirm the transition states by Nudged Elastic Band Transition State theory (NEB-TS). In combination with Hirshfeld Population analysis, Fukui indices suggest a more linear degradation pathway narrowed down from earlier experimental work by Singla et al.[2]. The linear free energy relationship for the newly suggested computational benzoic acid degradation can be determined by lnk_TST/W=-1.677ΔG-15.41 with a R² of 0.9997 according to classic transition state theory and Wigner tunneling. This computational method can be used to explore possible degradation pathways of other NAs and bridges molecular-to-macroscale sonochemical degradation of NA's through a manifestation of molecular solutions.

Keywords: Benzoic acid; Degradation modeling; Density functional theory; In silico screening; Naphthenic acids; Quantum mechanics.