Sorption of Chlorophenols to Wood Pulp

Batch sorption experiments for the retention of 2,4-dichlorophenol and 2,4,5-trichlorophenol by pulped wood fibers indicate that sorption is governed by the interaction of the acid form of these compounds with lignin. Measurements of fiber-water distribution coefficients over a pH range of 2−12 demonstrate that chlorophenolate ions do not sorb to the solid phase, and the interaction between the neutral form of the compound and lignin is primarily responsible for the retention within this pH region. This was confirmed by experiments using lignin and cellulose model particles and pulp fibers of various lignin content. The results have led to the development of an equation that relates the overall fiber-water distribution coefficient to the ionization of the phenol, the lignin mass fraction of the fiber, and a lignin−water distribution coefficient that can be estimated with a linear free-energy relationship. Modeling the fiber-water transfer of chlorophenols based on the dominance of the hydrophobic interactions was successful in the treatment of concentration, temperature, and inorganic salt effects. Sorption isotherms were shown to be linear and to have no dependency on phenol cosolutes at concentration levels common in bleaching. Distribution coefficients demonstrated only a slight dependency on temperature and inorganic salt concentrations, which could be predicted from the solute's heat of aqueous dissolution and Setschenow constants, respectively. Measurements also indicate that sorption is unaffected by fiber surface area and concentration, but they do show dependency on the concentration of colloidal materials, which appears to be the result of third-phase sorption.