Investigation of the Adsorption of Model and Oil Sands Process-Affected Water Naphthenic Acids on Graphite

• Author / Creator

  Moustafa, Ahmed M. A.

• Naphthenic acids (NAs) are constituents of bitumen in the oil sands deposits in northern Alberta, Canada. The extraction process of bitumen creates an enormous amount of wastewater called oil sands process affected waters (OSPW). This OSPW contains elevated concentrations of NAs, and other contaminants, that need removal in order to continue its reuse in the extraction process or eventually safe disposal into natural water streams. The research herein focuses on the understanding of NAs adsorption on the expanded graphite (EG) and highly-ordered pyrolytic graphite (HOPG). The adsorption of model NAs on EG was evaluated using Langmuir/Freundlich adsorption models and free energy thermodynamics calculations. Following the model compounds, the mechanisms for NAs adsorption of a commercial Merichem mixture and OSPW were explored. The visualization of adsorption on the surface of EG was not possible due to its irregular surface morphology. Thus, the Amplitude Modulation - Frequency Modulation Atomic Force Microscopy (AM-FM-AFM) was used to characterize the adsorption of NAs on HOPG. The adsorption of 5 model NA compounds in mono/multi-compound solutions was investigated to determine adsorption mechanisms. Overall, the NAs in both mono- and multi-compound solutions fit the Freundlich adsorption isotherms (R2 > 0.89). Thermodynamic calculations were used to assign the formation of the negatively charged assisted hydrogen bond (–CAHB) between ionized solutes and the negatively charged functional groups (FGs) on the EG as the possible adsorption mechanism. The similar pKa values of the model NAs resulted in comparable free energies for –CAHB formation (ΔG-CAHB) being less than solvation free energies (ΔGSolv). Thus, additional ΔG is supplemented by increased hydrophobicity due to proton exchange of ionized acids with water (ΔΔGHydrophobicity). Adsorption capacities and competition coefficients indicated that ΔΔGHydrophobicity values depend directly on hydrophobicity as indicated by Log Kow values. Competitive adsorption implies the occurrence of multilayer adsorption via hydrophobic bonding with CH3 ends of the self-assembled layer of NAs to the EG surface. Further study visually characterized the adsorption of decanoic acid (DA) on the surface of HOPG using AM-FM-AFM. The AM-FM-AFM images showed that DA molecules formed aggregates at the functionalized steps of HOPG and over the entire functionalized HOPG (F-HOPG). This DA adsorption to FGs in HOPG and F-HOPG confirmed the previous thermodynamics findings. The last step of this research was to understand the mechanism of NAs adsorption from complex mixtures including the Merichem NAs solution and raw OSPW. Adsorption results showed that higher Log Kow NAs have higher removal efficiency for all solutions. The calculated free energy required for the formation of –CAHB was lower than the free energy of solvation for NAs; however the –CAHB formation was still triggered by the need for additional free energy ΔΔGHydrophobicity as observed for model compounds. The presence of a large number of NAs species in both mixtures did not impact the reported mechanism.

• Subjects / Keywords

  • Naphthenci acids
  • Thermodynamics
  • Weak acids
  • HOPG
  • Adsorption
  • Graphite
  • Frequency modulation
  • Amplitude modulation
  • Oil sands process-affected water
  • Atomic force microscopy

• Graduation date

  Spring 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3Q08D

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.