Abstract
The adsorption of salicylic acid, acetaminophen, and methylparaben (pharmaceutical products derived from phenol) on carbons activated with different surface chemistries was carried out. We evaluated the effect of the physicochemical properties of the adsorbent and adsorbates on the adsorption capacity. A study of the adsorbate–adsorbent interactions via immersion calorimetry in the analytes solutions at different concentrations was included, in addition to the equilibrium data analysis. The results show that the pharmaceutical compounds (2.28–0.71 mmol g−1) have lower adsorption capacities in the activated carbon with the highest content of oxygenated groups (acids), while the activated carbons with amphoteric characteristics increase the capacities of adsorption (2.60–1.38 mmol g−1). This behavior may be associated with the increased affinity between the adsorbent and solvent due to the presence of polar groups, which was corroborated by the high immersion enthalpy value in water (ΔHimmH2O = −66.6 J g−1). The equilibrium data, adjusted to the Freundlich adsorption model, indicated that the heterogeneous adsorption processes involve immersion enthalpy values between −9.42 and −24.3 J g−1.
Highlights
The most common use of activated carbons is in adsoption processes, because the adsorbent presents the necessary physicochemical characteristics that allow it to capture substances that are desired to be removed from systems in gaseous or liquid phases [1,2].Due to anthropogenic activities, water sources are contaminated with substances of different natures: heavy metals, surfactants, products of oil refineries, and chemical agents
The effect of the physicochemical changes of the adsorbent in the adsorption capacity of the compounds was determined from the equilibrium data, and the adsorbate–adsorbent affinity was evaluated by immersion enthalpy in the studied compound solutions
Adsorption tests were carried out using solutions of phenol, salicylic acid, acetaminophen, and methylparaben in concentrations between 0.07 and 10.6 mmol L−1, since at these concentrations there is a sufficient amount of adsorbate molecules to evaluate the adsorbate-adsorbent interactions without the interference of the activated carbon–solvent interactions
Summary
The most common use of activated carbons is in adsoption processes, because the adsorbent presents the necessary physicochemical characteristics that allow it to capture substances that are desired to be removed from systems in gaseous or liquid phases [1,2]. Adsorption can be favored on adsorbents with low surface polarity, so an option to increase the adsorption capacity is to modify the surface chemistry through the reduction of chemical groups present on the activated carbon. This process can be carried out by increasing the adsorbent temperature above the thermal stability of the surface chemical groups on the activated carbon. In this way, the oxygenated groups are converted to CO2 and CO. The effect of the physicochemical changes of the adsorbent in the adsorption capacity of the compounds was determined from the equilibrium data (adsorption isotherms), and the adsorbate–adsorbent affinity was evaluated by immersion enthalpy in the studied compound solutions
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