Persistence and fate of acidic hydrocarbons in aquatic environments : naphthenic acids and resin acids

Abstract

The novel application of combination, or two stage, photochemical and microbial degradation systems for removal of resin acids from natural river water and single stage photolysis for degradation of naphthenic acids in natural river water was investigated. The organic compounds included in this project comprise naphthenic acid model compounds and mixtures as well as four resin acids. Naphthenic acids are crude oil-derived and accumulate to significant concentrations (>100 mg/L) in tailings pond water at oil sands extraction facilities. Resin acids are pulp and paper mill-derived compounds that tend to persist at low levels in receiving waters.

For each compound group, analytical methods utilizing liquid chromatography negative ion electrospray ionization mass spectrometry (LC/ESI/MS) were developed. The main hurdle to developing analytical methods for the naphthenic acids and resin acids are related to their polarity, complexity, and lack of available standards for the various individual components. As well, co-extractives, such as humic and fulvic acids, tend to interfere with the detection of naphthenic acids in aquatic samples (Headley et al., 2002a). Resin acid mixtures are not as complex as the naphthenic acids, although each group of hydrocarbon acids may include several isomeric compounds.

The application of photochemical degradation prior to biodegradation was proven to be effective here for rapid degradation of the resin acids. In general, the resin acid precursors were more susceptible to the photolysis than were the naphthenic acids. Through thermal maturation and increased complexity, the naphthenic acids seemingly become more resistant to degradation, as evidenced by their commercial use as anti-microbial agents and the observed resistance to photolysis noted in this research. The results of this research may be significant for the design of staged treatment for reduced microbial shock loading and increased bioavailability (defined here as the ability of microbial organisms to degrade the target contaminants) in both bioremediation systems and receiving waters.

Specifically, four selected pulp and paper mill-associated resin acids were exposed to several ultraviolet/visible (UV/vis) spectrum radiation sources in water collected from the River Saale in Germany. Background resin acid concentrations were observed in water collected in 2001 and 2002 from various locations along the well-forested River Saale and a manuscript detailing these results published. Analyses of water samples collected in the pulp and paper milling region of the river (in the state of Thuringia) indicated that resin acids persist through biodegradation treatment systems and for several hundred kilometres downstream. All four resin acids were degraded by facile photochemical and microbial degradation with pseudo-first-order kinetics. Half-life values were in the ranges of 18 to 200 minutes for photolysis applications, 8 to 40 hours for biodegradation applications and 3 to 25 hours for two-stage photochemical-microbial degradation processes, in which photolysis was limited to three hours. From these results, it was shown conclusively that photolysis pre-treatment is a viable and efficient method for reducing both resin acid concentrations and the associated acute toxicity.

The naphthenic acids investigated in this study were not effectively degraded via UV/vis radiation, including UV-A/UV-B radiation between 300-400 nm, near-monochromatic UV254-radiation, full spectrum artificial solar radiation and natural sunlight. The photochemical degradation potential of three model naphthenic acid compounds and three naphthenic acid mixtures (one extract from the Athabasca Oil Sands and two commercial mixtures) were examined in Athabasca River water. Photolysis at UV254 was the most successful degradation source in all instances, although most naphthenic acids were not significantly degraded by any of the radiation sources. Therefore, it was determined that photolysis is not likely to contribute significantly to environmental degradation and attenuation in the aquatic ecosystem. The results observed from the various naphthenic acids photodegradation processes, coupled with their low affinity for adsorption to soils, reveal that naphthenic acids are likely to persist in the water column. However, UV/vis radiation is capable of significantly changing the composition of mixtures in the aquatic ecosystem, but not reducing overall naphthenic acid concentrations. This may not be a beneficial as there is the potential for increased toxicity toward the lower molecular weight naphthenic acids.