The influence of functionality on the adsorption of p-hydroxy benzoate and phthalate at the hematite-electrolyte interface

Kinetics of adsorption of p-hydroxy benzoate and plithalate on hematite-electrolyte interface were investigated at a constant ionic strength., I = 5 x 10(-4) moldm(-3), pH 5 and at three different temperatures. The state of equilibrium for the adsorption of p-hydroxy benzoate onto hematite surfaces was attained at 70 h, whereas it was 30 h for phthalate-hematite system. None of the three kinetics models (Bajpai, pseudo first order and pseudo second order) is applicable in the entire experimental time period; however, the pseudo second order kinetics model is considered to be better than the pseudo first order kinetics model in estimating the equilibrium concentration both the p-hydroxy benzoate-hematite and phthalate-hematite systems. The variation of adsorption density of p-hydroxy benzoate and phthalate onto hematite surfaces as a function of concentration of adsorbate was studied over pH range 5-9 at a constant ionic strength, I = 5 x 10(-4) moldm(-3) and at constant temperature. The adsorption isotherms for both the systems were Langmuir in nature and the maximum adsorption density (Tau(max)) of p-hydroxy benzoate is similar to 1.5 times more than that of phthalate on hematite at pH 5 and 30 degrees C in spite of an additional carboxylic group at ortho position in plithalate. This is due to the more surface area coverage by plithalate than that of p-hydroxy benzoate on hematite surface. The activation energy was calculated using Arrhenius equation and the activation energy for adsorption of p-hydroxy benzoate at hematite-electrolyte interface is similar to 1.8 times more than that of phthalate-hematite system. The negative Gibbs free energy indicates that the adsorption of p-hydroxy benzoate and phthalate on hematite surfaces is favourable. The FTIR spectra of p-hydroxy benzoate and plithalate after adsorption on hematite surfaces were recorded for obtaining the bonding properties of adsorbates. The phenolic nu(C-O) appears at similar to 1271 cm(-1) after adsorption of p-hydroxy benzoate on hematite surfaces, which shifted by 10 cm(-1) to higher frequency region. The phenolic group is not deprotonated and is not participating in the surface complexation. The shifting of the nu(as)(-COO-) and nu(s)(-COO-) bands and non-dissolution of hematite suggest that the p-hydroxy benzoate and phthalate form outer-sphere surface in the pH range of 5-7. (c) 2006 Elsevier Inc. All rights reserved.