Abstract
The environmental problem stemming from toxic and recalcitrant naphthenic acids (NAs) present in effluents from the oil industry is well characterized. However, despite the numerous technologies evaluated for their destruction, their up-scaling potential remains low due to high implementation and running costs. Catalysts can help cutting costs by achieving more efficient reactions with shorter operating times and lower reagent requirements. Therefore, we have performed a laboratory investigation to assess iron-TAML (tetra-amido macrocyclic ligand) activators to catalyze the oxidation of NAs by activating hydrogen peroxide — considered environmentally friendly because it releases only water as by-product — under ultra-dilute conditions. We tested Fe-TAML/H2O2 systems on (i) model NAs and (ii) a complex mixture of NAs in oil refining wastewater (RWW) obtained from a refining site in Colombia. Given the need for cost-effective solutions, this preliminary study explores sub-stoichiometric H2O2 concentrations for NA mineralization in batch mode and, remarkably, delivers substantial removal of the starting NAs. Additionally, a 72-h semi-batch process in which Fe-TAML activators and hydrogen peroxide were added every 8 h achieved 90–95% removal when applied to model NAs (50 mg L−1) and a 4-fold reduction in toxicity towards Aliivibrio fischeri when applied to RWW. Chemical characterization of treated RWW showed that Fe-TAML/H2O2 treatment (i) reduced the concentration of the highly toxic O2 NAs, (ii) decreased cyclized constituents in the mixture, and (iii) preferentially degraded higher molecular weight species that are typically resistant to biodegradation. The experimental findings, together with the recent development of new TAML catalysts that are far more effective than the TAML catalysts deployed herein, constitute a foundation for cost-effective treatment of NA-contaminated wastewater.
Highlights
Naphthenic acids (NAs, Fig. 1) are natural constituents of crude oil and bitumen, with the general formula CnH2n−z O2, where n = number of C atoms and Z = hydrogen deficiency (Table S1)
Preliminary reactions with model NAs (50 mg L−1) were conducted with TAML catalysts ranging from 40 to 100 μg L−1 (1a = 74.4–186 nM, 1b = 82.4–206 nM) and H2O2 from 20 to 100 mg L−1 (588–2940 μM) in order to optimize the subsequent stage for removal of NAs in refining wastewater (RWW)
Decomposition was observed at 100 μg L−1 of 1 (1a = 186 nM, 1b = 206 nM) in combination with 40 mg L−1 of H2O2 (1176 μM), these conditions were used for the degradation reactions conducted subsequently
Summary
While 1b is ten times less reactive than 1a (Tang et al, 2016; Popescu et al, 2008), the fact that it is fluorine-free and relatively inexpensive are attractive features that lead us to evaluate its properties in TAML/H2O2 degradation of NAs. Previous studies with Fe-TAML/H2O2-based treatment processes have reported successful removal of a wide variety of organics in wastewater, including estrogenic compounds (Chen et al, 2012; Mills et al., 2015; Shappell et al, 2008), dibenzothiophene derivatives (Mondal et al, 2006), pharmaceuticals (Shen et al, 2011; Somasundar et al, 2018), a molluscicide (Tang et al, 2016; Tang et al, 2017), halogenated phenols (Wang et al, 2017; Gupta et al, 2002), nitrophenols (Kundu et al, 2015), and bisphenol A (Onundi et al, 2017). A. fischeri comes in the form of a standardized lyophilized reagent providing a low coefficient of variation
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