Insight into adsorbate–adsorbent interactions between aromatic pharmaceutical compounds and activated carbon: equilibrium isotherms and thermodynamic analysis
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The adsorption of phenol, salicylic acid and methylparaben on activated carbon is carried out using solvents with different pH, the adsorption and calorimetric data are analyzed in order to determine the effect of the substituent on the adsorption capacity. The adsorption isotherms were adjusted to the Langmuir model, which allows to assume the formation of specific adsorbate–adsorbent interactions between groups present in the adsorbate molecules, the substituents of the aromatic ring, and chemical groups on the activated carbon. According to the Lagmuir model the formation of specific interactions generates the monolayer adsorption so it is possible to correlate the adsorption capacity with changes in the interactions present in the system. It was determined that the adsorption process is disadvantaged at extremes pH values. From the Langmuir model it was calculated that the maximum adsorbed capacity of phenol and methylparaben in activated carbon granular activated carbon using water as solvent was 3.11 and 1.58 mmol g−1 respectively. The adsorption process of salicylic acid does not adjust to the Langmuir model due to the presence of different interactions that includes repulsion forces. From thermodynamic calculations and calorimetric data, it was determined that the immersion enthalpies vary between − 8.33 and − 59.3 J g−1, while the change in the enthalpy associated with interactions substituents-activated carbon is between − 15.1 and 6.40 J g−1 for the carboxylic acid and between − 0.50 and 20.0 J g−1 for the ester group.
KeywordsAdsorption activated carbon Gibbs energy Immersion enthalpy Interactions Methylparaben Phenol and salicylic acid Salicylic acid
The authors thank the Framework Agreement between the Universidad de Los Andes and the National University of Colombia and the act of agreement established between the Chemistry Departments of the two universities. The authors also appreciate the grant for the funding of research programs for Associate Professors, Full Professors, and Emeritus Professors announced by the Faculty of Sciences of the University of the Andes, 20-12-2019-2020, 2019, according to the project “Enthalpy, free energy and adsorption energy of the activated carbon interaction and solutions of emerging organic compounds”.
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