Investigation
of Mono/Competitive Adsorption of Environmentally
Relevant Ionized Weak Acids on Graphite: Impact of Molecular Properties
and Thermodynamics
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Abstract
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><sub>a</sub> values of current acids resulted in comparable free energies
for −CAHB formation (Δ<i>G</i><sup>–CAHB</sup>) being less than solvation free energies (Δ<i>G</i><sub>Solv</sub>). Thus, additional Δ<i>G</i> is supplemented
by increased hydrophobicity due to proton exchange of ionized acids
with water (ΔΔ<i>G</i><sub>Hydrophobicity</sub>). Adsorption capacities and competition coefficients indicated that
ΔΔ<i>G</i><sub>Hydrophobicity</sub> values depend
on the neutral and ionized acid <i>K</i><sub>ow</sub>. Competitive
adsorption implies that multilayer adsorption may occur via hydrophobic
bonding with the CH<sub>3</sub> 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