Sorption and desorption kinetics of chlorophenols in hexadecyltrimethyl ammonium-montmorillonites and their model analysis

Sorption and desorption kinetics of chlorophenols, 2-chlorophenol (2-ChP), 2,4-dichlorophenol (2,4-DChP) and 2,4,5-trichlorophenol (2,4,5-TChP), in montmorillonite modified with hexadecyltrimethyl ammonium cations (HDTMA-mont) were investigated by using laboratory batch adsorbers. To investigate the effect of chemical concentration and sorbent weight on the sorption or desorption rate constants, the initial chemical concentration and sorbent weight were varied from 50 to 150 mg/L and from 0.2 to 1.0 g, respectively. A one-site mass transfer model (OSMTM) and two compartment first-order kinetic model (TCFOKM) were used to analyze kinetics. The OSMTM applicable to desorption rate analysis was newly derived. As expected from the number of model parameters involved, the three-parameter TCFOKM was better than the two-parameter OSMTM in describing sorption and desorption kinetics of chlorophenols in HDTMA-mont. The mass transfer coefficient for sorption (k_s) in OSMTM generally increased as K_ow value increases, except for 2,4,5-TChP, while the mass transfer coefficient for desorption (k_d) consistently increases as K_ow value decreases, due to the weaker hydrophobic interaction between the solute and the organoclay. Since most sorption and desorption complete in an hour and half an hour, respectively, k_d values were found to be greater than k_s values for all three solutes studied. The fraction of the fast sorption (or desorption) and the first-order sorption (or desorption) rate constants for the fast and slow compartments in TCFOKM were determined by fitting experimental data to the TCFOKM. The results of kinetics reveal that the fraction of the fast sorption or desorption and the sorption rate constants in the fast and slow compartments were in the order 2,4,5-TChP>2,4-DChP>2-ChP, which agrees with the magnitude of the octanol-water partition coefficient, K_ow. The first-order sorption rate constants in the fast and slow compartments were found to vary 10 ¹-10² hr^-1 and 10^-3-10^-2 hr^-1, respectively. However, the desorption rate constants in the fast and slow compartments were not correlated well with K_ow. The first-order desorption rate constants in the fast compartment (10 ¹-10² hr^-1) were found to be much larger than those in the slow compartment (10^-3-10^-4 hr^-1). Sorption affinity and desorption resistance of each chlorophenol in 50% HDTMA-mont were found to show the same tendency: the weakly-sorbed chlorophenol (i.e., 2-ChP) was easily desorbed, while the strongly-sorbed chlorophenol (i.e., 2,4,5-TChP) was rather resistant to desorption.