Effect of Molecular Structure on the Relative Reactivity of Naphthenic Acids in the UV/H₂O₂ Advanced Oxidation Process

Abstract

The large volume of oil sands process-affected water (OSPW) produced by the oil sands industry in Northern Alberta, Canada, is an environmental concern. The toxicity of OSPW has been attributed to a complex mixture of naturally occurring acids, including naphthenic acids (NAs). Highly cyclic or branched NAs are highly biopersistent in tailings ponds, thus understanding structure-reactivity relationship for NAs is very important for OSPW reclamation. In this study, we hypothesized that large, branched and cyclic NAs may be better oxidized in the UV/H(2)O(2) process than small, linear and acyclic NAs. Relative rate measurements using binary mixtures of model NA compounds confirmed that reactivity favored compounds with more carbons, and also favored NAs with one saturated ring, relative to the corresponding linear NA. However, for model compound with three rings, no increased reactivity was observed relative to monocyclic NA. UV/H(2)O(2) treatment of OSPW confirmed our findings with model compounds, indicating that the compounds with more carbons are favored for degradation. However, increasing the number of rings (or double bond equivalents) in OSPW NAs did not show any clear structure-reactivity. Microbial degradation studies of the UV/H(2)O(2) treated OSPW should be conducted to examine the overall benefit of this treatment for the real applications.

Water treatment technologies for the remediation of naphthenic acids in oil sands process-affected water

“…The radicals then went on to abstract hydrogen from the NAs, instigating their oxidative degradation. Structure selective degradation, similar to the findings of Perez-Estrada et al [44] and Afzal et al [46], was also observed for this process. The authors reported an NA removal rate of 80 ± 2% at pH $ 8.3.…”

“…The authors observed rapid degradation for both systems, with half-lives of 3.32 and 3.61 h for the commercial and native NAs, respectively. As reported in other works discussed here [44,46], selective degradation of higher molecular weight NAs was observed. Interestingly, the authors reported that in some cases the presence of TiO 2 did not result in significant half-life reduction of the NAs.…”

“…Using this ozonation process, up to 61% removal was achieved. In 2012, the same group investigated the relative reactivity of several model NA compounds using a UV/H 2 O 2 advanced oxidation process, wherein UV irradiation (253.7 nm) was used to stimulate the decomposition of H 2 O 2 into hydroxyl radicals [46]. Direct photolysis by UV irradiation was considered to be negligible compared to the indirect oxidation processes.…”

“…In the case of OSPW treatment, this could pose a significant hurdle, as it may result in products of greater toxicity than the initial NAs [46]. Moreover, advanced oxidation processes are often resource intensive, resulting in high operating costs [55].…”

“…energy or chemical reactants, depending on the oxidation method) [56]. Afzal et al [46] suggested that advanced oxidation processes be implemented in combination with other techniques, such as biodegradation and flocculation, to provide a more comprehensive system of remediation processes for OSPW.…”