Das Manash R, Mahiuddin Sekh
Material Science Division, Regional Research Laboratory, Jorhat 785006, Assam, India.
J Colloid Interface Sci. 2007 Feb 15;306(2):205-15. doi: 10.1016/j.jcis.2006.10.049. Epub 2006 Oct 26.
Kinetics of adsorption of p-hydroxy benzoate and phthalate on hematite-electrolyte interface were investigated at a constant ionic strength, I = 5 x 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 x 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 (Gamma(max)) of p-hydroxy benzoate is approximately 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 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 approximately 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 nu(CO) appears at approximately 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 complex with hematite surfaces in the pH range of 5-7.
在恒定离子强度（I = 5×10⁻⁴ mol dm⁻³）、pH值为5以及三个不同温度下，研究了对羟基苯甲酸盐和邻苯二甲酸盐在赤铁矿 - 电解质界面上的吸附动力学。对羟基苯甲酸盐在赤铁矿表面的吸附在70小时达到平衡状态，而邻苯二甲酸盐 - 赤铁矿体系则为30小时。三种动力学模型（Bajpai模型、准一级动力学模型和准二级动力学模型）在整个实验时间段内均不适用；然而，在估计对羟基苯甲酸盐 - 赤铁矿和邻苯二甲酸盐 - 赤铁矿体系的平衡浓度时，准二级动力学模型被认为比准一级动力学模型更好。在恒定离子强度（I = 5×10⁻⁴ mol dm⁻³）和恒定温度下，研究了对羟基苯甲酸盐和邻苯二甲酸盐在赤铁矿表面的吸附密度随吸附质浓度的变化，pH范围为5 - 9。两个体系的吸附等温线均为朗缪尔型，在pH值为5和30℃时，对羟基苯甲酸盐在赤铁矿上的最大吸附密度（Γ(max)）比邻苯二甲酸盐大约高1.5倍，尽管邻苯二甲酸盐在邻位有一个额外的羧基。这是因为邻苯二甲酸盐在赤铁矿表面的表面积覆盖比对羟基苯甲酸盐更大。使用阿伦尼乌斯方程计算了活化能，对羟基苯甲酸盐在赤铁矿 - 电解质界面的吸附活化能比邻苯二甲酸盐 - 赤铁矿体系大约高1.8倍。吉布斯自由能为负表明对羟基苯甲酸盐和邻苯二甲酸盐在赤铁矿表面的吸附是有利的。记录了对羟基苯甲酸盐和邻苯二甲酸盐吸附在赤铁矿表面后的傅里叶变换红外光谱，以获得吸附质的键合性质。对羟基苯甲酸盐吸附在赤铁矿表面后，酚类的ν(CO)出现在约1271 cm⁻¹处，向高频区域移动了10 cm⁻¹。酚基未去质子化，不参与表面络合。ν(as)(COO⁻)和ν(s)(COO⁻)谱带的移动以及赤铁矿的不溶解表明，在pH值为5 - 7的范围内，对羟基苯甲酸盐和邻苯二甲酸盐与赤铁矿表面形成了外层表面络合物。
