Colloid-Enhanced Ultrafiltration of Chlorophenols in Wastewater: Part II. Apparent Acid Dissociation Constants of Chlorophenols in Colloid Solutions at Different Ionic Strengths and Effect of pH on Solubilization of Phenolic Compounds

ABSTRACT

The effects of pH, ionic strength, and type of colloid on the efficiency of removal of an organic solute in colloid-enhanced ultrafiltration, as indicated by the tendency of the solute to solubilize into micelles and surfactant–polymer complexes, are investigated in this paper. The apparent acid dissociation constants (K_aapp) of 2-monochlorophenol (MCP), 2,4-dichlorophenol (DCP), and 2,4,6-trichlorophenol (TCP) are determined in surfactant solutions and surfactant–polymer mixtures at different salinities using a spectrophotometric titration technique. The distribution coefficients of charged species and neutral species of MCP and the solubilization of TCP into micelles and into surfactant–polymer aggregates are measured using dialysis methods. Cetylpyridinium chloride (CPC) is the cationic surfactant and sodium polystyrenesulfonate (PSS) is the anionic polymer used. The apparent values of pK_a(pK_a,app) of the solutes in micellar solution are less than the corresponding values in aqueous solution (pK_a,aq), whereas the values of pK_a,app for the solutes in the surfactant–polymer mixtures are slightly higher than those in the aqueous and micellar solutions. The pK_a,app values increase as salt concentration increases in the micellar solution while remaining almost unchanged in the surfactant–polymer mixtures. In the micellar solutions, the distribution coefficient into the surfactant aggregate of the anionic species of MCP is greater than that of the neutral species, by factors of approximately 30 to 1.5 when salt concentration increases from zero to 0.10 M. However, in the surfactant–polymer mixtures, the distribution coefficient of the neutral species is higher than that of the charged species by factors of 5 to 8 when salt concentration increases from zero to 0.05 M. In the micellar solutions, the distribution coefficients of the neutral species are less dependent on salinity than those of the charged species. The solubilization and solute rejection are influenced by pH, primarily for solutes with low hydrophobicity.

Key Words::

• Distribution coefficient
• Solubilization constant
• Chlorophenols
• Surfactant–polymer interaction