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

Abstract

Kinetics of adsorption of p-hydroxy benzoate and phthalate on hematite–electrolyte interface were investigated at a constant ionic strength, I = 5 × 10 −4 mol dm −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 × 10 −4 mol dm −3 and at constant temperature. The adsorption isotherms for both the systems were Langmuir in nature and the maximum adsorption density ( Γ max ) of p-hydroxy benzoate is ∼1.5 times more than that of phthalate on hematite at pH 5 and 30 °C in spite of an additional carboxylic group at ortho position in phthalate. This is due to the more surface area coverage by phthalate 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 ∼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 phthalate after adsorption on hematite surfaces were recorded for obtaining the bonding properties of adsorbates. The phenolic ν C O appears at ∼ 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 ν as ( COO −) and ν s ( COO −) bands and non-dissolution of hematite suggest that the p-hydroxy benzoate and phthalate form outer-sphere surface complex with hematite surfaces in the pH range of 5–7.