Sorption of acridine to unbonded silica and the influence of pH and counterion concentration

Abstract

Aqueous batch experiments were performed at four different pH's (2, 4, 6, 8) and at three different phosphate concentrations (0.001 M, 0.01 M, 0.1 M) to investigate the effects these variables have on acridine (pKₐ = 5.6) solubility. Results indicate that acridine solubility depends on both pH and counterion concentration. Twelve laboratory column experiments were conducted to investigate the sorption of acridine onto unbonded silica. Major variables included pH, counterion type, solute concentration, and pore water velocity. These experiments provided estimates of water-solid partition coefficient (range: 33 ± 5 cm³/g at pH 7 to 44 ± 5 cm³/g at pH 4), dispersion coefficient, and mass-transfer coefficient. It is hypothesized that sorption of the acridinium ion (pH 4) exceeded that of the neutral species due to the greater free energy change resulting from electrostatic interactions over solvophobic partitioning. The average time scales for desorption were 31 seconds at pH 7 and 43 seconds at pH 4. It is suggested that acridine sorption to unbonded silica is controlled primarily by the pH of the aqueous phase.