Enhancing naphthenic acid attenuation in mesocosm wetlands: The role of temperature, plant species, and microbial communities.

Abstract

Oil sands process-affected water (OSPW) is a by-product of bitumen extraction from oil sands surface mining in Alberta, Canada. A major group of organics in OSPW known as naphthenic acid fraction compounds (NAFCs) are of concern due to their persistence and toxicity. Constructed wetland treatment systems have emerged as potential biological treatment approaches for reducing NAFC concentrations within OSPW. In this study, greenhouse-scale mesocosms simulating a constructed wetland consisting of coarse sand tailings (CST) and OSPW were used to evaluate the ability of Scirpus microcarpus, Triglochin maritima, and unplanted controls to attenuate NAFCs under spring/fall and summer temperatures (10°C/5°C and 20°C/10°C day/night). Overall, in this mesocosm system, NAFC attenuation was similar regardless of different design parameters such as plant type, plant presence, and temperature. By the end of the study, NAFCs attenuation was 30% to 50% lower than the initial OSPW depending on plant species, plant presence, and temperature. The relative abundance of the acutely toxic O₂-NAFCs decreased over time, with an increase in the less toxic O₃, O₄, and SO₃ classes. Various hydrocarbon-degrading microbial families such as Comamonadaceae and Xanthobacteraceae were found to be dominant in OSPW, while cyanobacteria (Trichormus) were enriched in the CST. Principal component analysis indicated that only time led to distinct clusters for NAFC composition, while plant type, temperature, and time influenced the microbial communities. Shifts in microbial communities over time corresponded to shifts in NAFCs, possibly due to a decrease in toxicity with increased oxidation of NAFCs and/or an increase in available nutrients from a decrease in plant fitness in the planted mesocosms. PRACTITIONER POINTS: Constructed wetland mesocosms for NAFC attenuation from OSPW comparing three planted/unplanted conditions under two temperatures. Mesocosms had 30%-50% removal of total NAFCs, with a decrease in O₂-NAFCs and increase in O₃, O₄, and SO₃ classes. NAFC composition only shifted with time, while microbial communities were influenced by plant type, temperature, and time. Lack of difference in NAFC attenuation between treatments could indicate a high level of functional redundancy between the microbial communities.