Investigation of mono/competitive adsorption of environmentally relevant ionized weak acids on graphite: impact of molecular properties and thermodynamics.
The thermodynamics of adsorption and competitive interactions of five weak acids on a graphite surface was assessed in alkaline solutions. Adsorption of the acids in mono- and multicompound solutions followed their Freundlich isotherms which suggest a diversity of graphite adsorption sites as confirmed by the presence of carboxylic and phenolic groups observed on graphite surfaces. Thermodynamic calculations assigned the formation of the negatively charged assisted hydrogen bond (-CAHB) between ionized solutes and adsorbent surface groups as the possible adsorption mechanism. However, the similar pKa values of current acids 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 (ΔΔG Hydrophobicity). Adsorption capacities and competition coefficients indicated that ΔΔG Hydrophobicity values depend on the neutral and ionized acid Kow. Competitive adsorption implies that multilayer adsorption may occur via hydrophobic bonding with the CH3 ends of the self-assembled layer which affects the acid adsorption capacities in mixtures as compared to monocompound solutions. The determination of adsorption mechanisms will assist in understanding of the fate and bioavailability of emerging and classical weak acids released into natural waters.
Department of Civil and Environmental Engineering, University of Alberta , Edmonton, Alberta T6G 2W2, Canada.
Ahmed M A
AM
Moustafa
Kerry N
KN
McPhedran
Jesús
J
Moreira
Mohamed
M
Gamal El-Din
D000327:Adsorption; D002264:Carboxylic Acids / Q000737:chemistry*; D003652:Decanoic Acids / Q000737:chemistry*; D006108:Graphite / Q000737:chemistry*; D006538:Heptanoic Acids / Q000737:chemistry*; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D015394:Molecular Structure; D008970:Molecular Weight; D061329:Oil and Gas Fields; D013816:Thermodynamics
D002264:Carboxylic Acids; D003652:Decanoic Acids; D006538:Heptanoic Acids; C031071:decanoic acid; D006108:Graphite