Exothermic adsorption of chromate by goethite
Exothermic adsorption of chromate by goethite
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DOI：
10.1016/j.apgeochem.2020.104785
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发表时间：
2020-12-01
影响因子：
3.4
通讯作者：
Kersten, Michael
中科院分区：
文献类型：
--
作者：
Dabizha, Anna;Kersten, Michael
针铁矿是土壤颗粒表面常见的铁羟基氧化物，在酸性条件下对Cr（VI）具有很强的吸附能力。使用离子强度为0.1、0.05和0.01 M，pH值为3-11的溶液，在10 - 75 ℃的四个温度下，进行铬酸盐浓度为0.1、0.2和0.3 mM的分批平衡吸附实验。这些实验的结果表明，铬酸盐的吸附量随着pH值向针铁矿的零点表面电荷（pHPZC 9.1）的增加而减少，这是典型的阴离子。铬酸盐吸附效率还取决于溶液的离子强度。铬酸根的吸附量随离子强度的增加而减少，表明外球表面络合作用影响吸附过程。使用电荷分布多位点表面络合（CD-MUSIC）模型框架的数据进行解释。配合常数随温度的变化进行了拟合，使用两项范特霍夫方程给出偏摩尔焓和熵常数。双折射率为负值，表明铬酸盐吸附是放热的，由此发现外球结合的放热性（Δ（r）G（OS，298）-62.19 kJ mol（-1））小于内球结合（Δ（r）G（IS，298）-112.25 kJ mol（-1））。因此，吸附强度随着温度的升高而降低，这是因为表面络合的放热性质。
Goethite is a common Fe oxyhydroxide coating soil particle surfaces, which has a high Cr(VI) adsorption capacity under acidic pH conditions. Batch equilibrium adsorption experiments with chromate concentrations of 0.1, 0.2, and 0.3 mM were performed using solutions with ionic strengths of 0.1, 0.05, and 0.01 M and pH values of 3-11 and at four temperatures between 10 and 75 degrees C. The results of these experiments show that the amount of chromate adsorbed decreases as the pH increases towards the zero-point-of-surface-charge of goethite (pHPZC 9.1), which is typical for anions. The chromate adsorption efficiency also depends on the ionic strength of the solution. The amount of chromate adsorbed decreases as the ionic strength increases, indicating that outer-sphere surface complexation affects the adsorption process. The data were interpreted using the charge distribution multisite surface complexation (CD-MUSIC) model framework. Variations in the complexation constants with temperature were fitted using the two-term van't Hoff equation to give partial molar enthalpy and entropy constants. The enthalpies were negative, indicating that chromate adsorption is exothermic, whereby outer sphere binding was found to be less exothermic (Delta(r)G(OS,298) -62.19 kJ mol(-1)) than inner-sphere binding (Delta(r)G(IS,298) -112.25 kJ mol(-1)). The adsorption strength, therefore, decreases as the temperature increases because of the exothermic nature of surface complexation.