The effect of rhizosphere pH on removal of naphthenic acid fraction compounds from oil sands process-affected water in a willow hydroponic system

The extraction and processing of bitumen from the oil sands in northern Alberta, Canada generates large volumes of oil sands process-affected water (OSPW). OSPW contains a complex mixture of inorganic and organic compounds, including naphthenic acid fraction compounds (NAFCs) that are of particular concern due to their toxicity to aquatic organisms. Phytoremediation is a cost-effective, scalable approach that has the potential to remove NAFCs from OSPW and reduce OSPW toxicity. Environmental pH influences the chemical form and bioavailability of NAFCs. However, little is known about the influence of pH on the uptake of NAFCs in plant systems. This study sought to elucidate the impact of rhizosphere pH on the uptake of NAFCs using a sandbar willow (Salix interior) hydroponic system. To mimic and maintain the naturally low pH conditions of the root, OSPW solutions in these systems were adjusted to a low pH level (pH 5.0) and their NAFC uptake from solution was compared to that of OSPW at native pH (pH 8.0). Our findings revealed that the lower pH hydroponic systems demonstrated enhanced NAFC removal from solution as determined by LC-MS analysis, where up to 26% of NAFCs were removed from OSPW over 72 h at pH 5.0 compared to 8% removed at pH 8.0. Similarly, analysis of spike-in 13C-labeled NAs demonstrated that the OSPW hydroponic system rapidly removed a relatively labile NA (13C-cyclohexane carboxylic acid) from solution at both pH levels, whereas near complete removal of a recalcitrant NA (13C-1-adamantane carboxylic acid) was observed in pH 5.0 solutions only. These results provide insight into the importance of rhizosphere pH on efficient NAFC uptake by plant root systems. Further research will determine whether OSPW phytoremediation efficiency can be enhanced using field treatment conditions that promote low rhizosphere pH levels. [Display omitted] •Low pH (pH 5.0) hydroponic solutions enhanced the uptake of NAFC by willow roots.•Classical O2-NAs are more rapidly depleted than O3- and O4-NAs in pH 5.0 conditions.•A recalcitrant model NA is efficiently taken up by root systems in low pH solutions.•Recurring exposure of OSPW to roots demonstrates continual, efficient NAFC uptake.