Using the dual-mode model to describe adsorption of organic pollutants onto an organoclay.

Adsorption of 2,4,5-trichlorophenol (TCP) and picric acid (PA) by montmorillonite preloaded with crystal violet (CV) up to 80, 100, and 125% of the cation exchange capacity was studied. The adsorption isotherms indicated a strong interaction of TCP and PA with the organoclay. While the adsorption isotherms of both pollutants can be described by the Langmuir model with relatively good fit, the adsorption of TCP is more adequately described by the dual-mode model (DMM), which combines the Langmuir equation with a partition mechanism. Competitive adsorption experiments and adsorption experiments in water:ethanol mixed medium as a solvent confirmed the assumption that the removal of TCP from water by the organoclay follows two different mechanisms: site-specific adsorption occurs in the low concentration range, whereas a partition mechanism dominates the high concentration range. This double mechanism in organoclays exhibits both complete removal at low pollutant concentrations and high sorption capacity in the high concentration range, and therefore may be effective in water treatment for a wide range of pollution levels.