Sorption of acetaminophen and nalidixic acid onto polar and nonpolar adsorbents

Abstract

Pharmaceuticals play a vital role in human and animal daily life. Apparently, pharmaceutical compounds show up in groundwater and surface water in many continents. Although these compounds are detected at low concentrations in the environment, a long term effect of such exposures has not yet known. The available information on their fate and transport are still limited and fragmented. The understanding of the sorption of these molecules is greatly important not only in the removal of those compounds from drinking water to support the idea of the water reuse but also in the migration of these compounds in the environment. The sorption of two pharmaceuticals predominantly in the neutral form, acetaminophen and nalidixic acid, have been investigated in the laboratory using batch experiments. The nonionic polymeric resins (Amberlite XAD2, XAD7, and XAD761) and activated carbon were used as polar and nonpolar adsorbents, respectively. The results indicated that nalidixic acid shows greater sorption capability than acetaminophen onto all adsorbents as acetaminophen has higher water solubility with less K[subscript ow]. The sorption kinetics studies showed that the sorption equilibrium of acetaminophen and nalidixic acid are achieved within 24 hours for all adsorbents. Sorption of acetaminophen and nalidixic acid onto polymeric resins are well fitted with the Langmuir isotherm. The sorption capacity in an area basis corresponds to the polarity of the polymeric adsorbents for both pharmaceuticals. For sorption of acetaminophen and nalidixic acid onto activated carbon, both sorptions follow the Freundlich isotherm with the equation of q = 131.3107 (mg/g activated carbon) (L/g) [superscript N] x C [subscript e]. [superscript 0.1243] for acetaminophen and q = 78.5778 (mg/g activated carbon) (L/g) [superscript N] x C [subscript e]. [superscript 0.3273] for nalidixic acid. In the sorption of pharmaceuticals onto polymeric resins, the interaction caused by the affinity between polar moieties of pharmaceutical compounds and adsorbents plays an important role. Alternatively, the hydrophobic interaction controls the sorption of pharmaceuticals onto activated carbon as the hydrophobic part of both acetaminophen and nalidixic acid partition onto the nonpolar surface of activated carbon. The activated carbon shows the greater sorption capacity than the polymeric resins for the sorption of these two pharmaceuticals. The results thus imply that the pharmaceuticals can truly adsorb on both polar and nonpolar surfaces due to their amphiphilic characteristic and the sorption behavior strongly depends on the molecular structure of pharmaceuticals and the surface properties of adsorbents.