Kinetics and adsorption of benzoate and salicylate at the natural hematite–water interface

Kinetics of adsorption of benzoate and salicylate onto the natural hematite surfaces have been carried out at pH 5 and at a fixed ionic strength (I = 1 × 10−4 mol dm−3). The state of equilibrium was attained at 144 and 70 h for benzoate and salicylate, respectively. Adsorption behaviour of benzoate and salicylate onto the natural hematite was studied over a wide range of pH at a fixed I = 5 × 10−4 mol dm−3 NaCl and at different temperatures. The adsorption isotherms for both the adsorbates were Langmuir type up to pH 8. The Γmax for benzoate is ≈2.7–13.7 times more than that of salicylate depending on the pH of the suspension. Unlike benzoate, salicylate is not only adsorbed at one surface site but also covers around three or more surface sites of hematite surface. The rate constant for adsorption, K1 for salicylate onto the natural hematite is greater than that of benzoate and increases with the increase in temperature and ≈4 times greater at a fixed temperature. The activation energy for the adsorption of salicylate onto the natural hematite is 2.9 times more than that of benzoate on the same adsorbent resulting in lower Γmax.