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 p<i>K</i>_a values of current acids resulted in comparable free energies for −CAHB formation (Δ<i>G</i>^–CAHB) being less than solvation free energies (Δ<i>G</i>_Solv). Thus, additional Δ<i>G</i> is supplemented by increased hydrophobicity due to proton exchange of ionized acids with water (ΔΔ<i>G</i>_{Hydrophobicity}). Adsorption capacities and competition coefficients indicated that ΔΔ<i>G</i>_{Hydrophobicity} values depend on the neutral and ionized acid <i>K</i>_ow. 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