Activated carbon is an effective material for the removal organic micropollutants from water. Many legacy pollutants are neutral organic compounds, but many emerging contaminants, such as pharmaceutically active compounds (PhACs) and per- and polyfluoroalkyl substances (PFASs) are ionizable. Many PhACs are weak acids or bases while many PFASs are strong acids. Apart from non-specific interactions between the adsorbate and the adsorbent, the adsorption behavior of ionizable organic contaminants involves Coulombic interactions that are affected by solution pH and ionic strength. Here, pH dependent adsorption isotherm data will be discussed for the following probe compounds: (1) the weak organic acid sulfamethoxazole (SMX), (2) the weak organic base trimethoprim (TMP), and (3) the strong organic acid perfluoropentanoic acid (PFPeA). For SMX and TMP, adsorption isotherm data were collected at pH values that were at least 2 pH units above and below the pKa value such that the adsorbate was either predominantly in the neutral or ionic form. When the equilibrium aqueous phase concentration was normalized against the pH-dependent aqueous solubility, adsorption isotherm data overlapped when pH was pKa +/- 2. This result suggests that the adsorptive removal of ionizable organic compounds is largely controlled by adsorption of the neutral species, even at pH values at which the ionic form begins to dominate. Adsorption of ions and Coulombic interactions became important at pH values farther from the pKa. For PFPeA, adsorption capacity increased with decreasing solution pH and ionic strength. These results were primarily a result of (1) enhanced Coulombic interactions between the PFPeA anion and an activated carbon surface with an increasing concentration of positively charged sites and (2) decreased screening of attractive Coulombic interactions at lower ionic strengths. However, ionic strength and pH effects were subtle in ultrapure water and disappeared once background organic matter was present in the system.

Factors controlling the adsorption of ionizable compounds to activated carbon

255th National Meeting (2018)

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