Fractionation of oil sands-process affected water using pH-dependent extractions: A study of dissociation constants for naphthenic acids species

• The dissociation constants for naphthenic acids species were estimated.
• Sum of O2-, O3- and O4-NAs species accounted for 33.6% of extracted organic matter.
• Accumulative masses at different pHs revealed every O atom added increases the pKa.
• Electron-withdrawing groups (double bonds and aromatic groups) may lead to lower pKa.
• Model pKa values were 3.5 for O2-NAs, 4.8 for O3-NAs, and 6.8 for O4-NAs.

The fractionation of oil sands process-affected water (OSPW) via pH-dependent extractions was performed to quantitatively investigate naphthenic acids (NAs, CnH2n+ZO2) and oxidized NAs (Ox-NAs) species (CnH2n+ZO3 and CnH2n+ZO4) using ultra-performance liquid chromatography time-of-flight mass spectrometry (UPLC-TOFMS). A mathematical model was also developed to estimate the dissociation constant pKa for NAs species, considering the liquid–liquid extraction process and the aqueous layer acid-base equilibrium. This model provides estimated dissociation constants for compounds in water samples based on fractionation extraction and relative quantification. Overall, the sum of O2-, O3-, and O4-NAs species accounted for 33.6% of total extracted organic matter. Accumulative extracted masses at different pHs revealed that every oxygen atom added to NAs increases the pKa (i.e., O2-NAs < O3-NAs < O4-NAs), indicating that the additional O atoms exist as –OH in O3- and O4-NAs. Molecule electron-withdrawing groups such as double bonds and aromatic groups, as indicated by higher carbon and −Z number, may be responsible for the lower pKa of O2-, O3-, and O4-NAs. The model obtained estimated pKa values of 3.5 for O2-NAs, 4.8 for O3-NAs, and 6.8 for O4-NAs via nonlinear regression curve fittings. These pKa values are valuable physicochemical parameters for environmental engineering applications targeting OSPW NAs treatment.