Adsorption of dyes from aqueous solutions onto sand and their kinetic behavior

Three organic dyes namely, Coomassie Blue, Malachite Green and Safranin Orange were removed from solution by adsorption on sand at 298 K. Characteristics of local sand sample used as an adsorbent in this work were initially found from the low-temperature adsorption of nitrogen on sand samples at 77 K. Conditions for maximum adsorption of these dyes on sand sample were then optimized. It was seen that under these conditions, 65–70% of dye could be removed from solution onto the sand surface. Adsorption data was fitted to Freundlich equation for the calculation of various parameters and it was found that for all the three dyes 1/n < 1, which indicates that adsorption was favorable. The adsorption kinetics followed the pseudo second order equation for all the three dyes investigated in this work with the k values lying in the region of 6.2×10^5 to 3.0×10^6. The intra-particle diffusion rate coefficient values obtained by using the Morris–Weber equation showed that Malachite Green reached equilibrium faster than Coomassie Blue and Safranin Orange. The diffusion coefficient values calculated for the three dyes were in the range of 7.8×10^−8 to 9.3×10^−8 cm2/s and are compatible with other studies reported in the literature. The adsorption behavior of the dyes was also investigated in terms of added cations and anions and it was found that adsorption of Commassie Blue and Safranin Orange decreased substantially in the presence of sulphate, thiosulphate, acetate, potassium, nickel and zinc ions.