Sequestration of naphthenic acids from aqueous solution using β-cyclodextrin-based polyurethanes

Abstract

The sorption characteristics of naphthenic acids (NAs) in their anion form with β-cyclodextrin (β-CD) based polyurethanes, as sorbents, from aqueous solutions that simulate the conditions of oil sands process water (OSPW) are presented. The copolymer sorbents were synthesized at various β-CD ∶ diisocyanate monomer mole ratios (e.g., 1 ∶ 1, 1 ∶ 2, and 1 ∶ 3) with diisocyanates of variable molecular size and degree of unsaturation. The equilibrium sorption properties of the copolymer sorbents were characterized using sorption isotherms in aqueous solution at pH 9.00 with electrospray ionization mass spectrometry to monitor the equilibrium unbound fraction of anionic NAs in the aqueous phase. The copolymer sorbents were characterized in the solid state using ¹³C CP-MAS NMR spectroscopy, IR spectroscopy and elemental analysis. The sorption results of the copolymer sorbents with anion forms of NAs in solution were compared with a commercially available carbonaceous standard: granular activated carbon (GAC). The monolayer sorption capacities of the sorbents (Q_m) were obtained from either the Langmuir or the Sips isotherm model used to characterize the sorption characteristics of each copolymer sorbent. The estimated sorption capacity for GAC was 142 mg NAs per g sorbent whereas the polymeric materials ranged from 0–75 mg NAs per g sorbent over the experimental conditions investigated. In general, significant differences in the sorption capacities between GAC and the copolymer sorbents were related to the differences in the accessible surface areas and pore structure characteristics of the sorbents. The Sips parameter (K_eq) for GAC and the copolymer materials reveal differences in the relative binding affinity of NAs to the sorbent framework in accordance with the synthetic ratios and the value of Q_m. The diisocyanate linker plays a secondary role in the sorption mechanism, whereas the β-CD macrocycle in the copolymer framework is the main sorption site for NAs because of the formation of inclusion complexes with β-CD.