Interlayer expansion of dimethyl ditallowylammonium montmorillonite as a function of 2-chloroaniline adsorption

The expansion behaviour of an organically modified montmorillonite during the adsorption of increasing amounts of an organic pollutant: 2-chloroaniline (2-CA). The organophilic montmorillonite, a commercial product, was obtained exchanging the inorganic cation with dimethyl ditallowylammonium ions (DMDTA). 2-CA was added to the organoclay, starting from 193.9 ppm up to 23,531.7 ppm going through twenty-two steps of growing quantities of 2-CA. For each of these twenty-two steps – corresponding to a range of adsorbed 2-CA (Cs) between 0.029 mol/kg (grams of 2-CA per kg of organoclay) and 2.763 mol/kg – the basal spacings were determined. The adsorption isotherm was performed according to ASTM D 4646-87 Standard and, by comparing the experimental Cs data with the corresponding d001 values, the basal expansion of the polluted organoclay is characterized by a steep rise for the first steps of adsorption of 2-CA (concentration in water at equilibrium Cwb0.00373 mol/L), then the d001 remains almost constant up to the higher amounts of pollutant adsorbed. The 001 reflections are sharper, their intensity higher, and up to four orders of 00l reflections are observed as the uptake of 2-CA increased. Since in aqueous solution the staking of organoclay platelets is poorly ordered, the observed behaviour may be partially explained by the 2-CA sorbed on the external surface of the clay mineral particles, which brings the silicate layers together. Finally, a tentative interpretation of the adsorption phenomena was carried out by fitting the experimental data according to the most common theoretical models: Freundlich, Langmuir, Dual Mode and Dual Langmuir Model.