铝掺杂针铁矿的制备、表征及吸附氟的特性

水热条件下制备了针铁矿(Goe)和几种铝掺杂针铁矿(Goe-Al_(0.1),Goe-Al_(0.2)和Goe-Al_(0.4)),用X射线衍射(XRD)、扫描电镜(SEM)、氮气物理性吸附、酸碱滴定等手段对样品进行了表征,并研究了它们对氟离子的吸附特性。结果表明,随着铝掺杂量的增加,铝掺杂针铁矿的结晶度不断减弱、颗粒的长度不断减小。4种样品的微孔表面积、孔体积和表面分形度都表现为GoeGoeAl0.1Goe-Al_(0.2)Goe-Al_(0.4),而孔径分布表现为相反的顺序。Goe、Goe-Al_(0.1)、Goe-Al_(0.2)和Goe-Al_(0.4)的电荷零点(PZC)分别为8.2、8.3、8.5和8.7,pH=5.0时它们的表面电荷量分别为0.66、0.83、1.03和1.19 mmol·g~(-1)。准二级动力学模型适合描述4种样品对氟的吸附动力学过程,表明化学吸附是主要作用机制。一位Langmuir模型可较好的拟合等温吸附数据(R2为0.967~0.981),二位Langmuir模型对等温吸附数据的拟合度更高(R2为0.982~0.995),而Freundlich模型的拟合度较低(R2为0.877~0.912)。初始pH=5.0时,Goe、Goe-Al_(0.1)、Goe-Al_(0.2)和Goe-Al_(0.4)对氟的最大吸附容量分别为8.83、10.24、11.72和12.86 mg·g~(-1)。可见,铝掺杂针铁矿对土-水环境中氟的吸附容量高于纯针铁矿。

Preparation, Characterization and Fluoride Adsorption Characteristics of Goethite and Al-Doped Goethite

Pure goethite (Goe) and Al-doped goethite (Goe-Al_0.1, Goe-Al_0.2 and Goe-Al_0.4) were prepared under hydrothermal conditions, and the surface properties and fluoride adsorption characteristics of the samples were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen physical adsorption, potentiometric titrations and batch adsorption experiments. The results show that both the crystallinity of the samples and the length of rod nanoparticles decrease with increasing the Al contents. The micropore surface areas, pore volumes and surface fractal dimension D values of the samples follow the order of Goe < Goe-Al_0.1 < Goe-Al_0.2 < Goe-Al_0.4, and the pore size distributions of the four samples show the opposite order. The points of zero charge (PZC) of Goe, Goe-Al_0.1, Goe-Al_0.2 and Goe-Al_0.4 appear around 8.2, 8.3, 8.5 and 8.7, respectively. At pH=5.0, the surface charges of the four samples are 0.66, 0.83, 1.03 and 1.19 mmol·g^-1, respectively. The adsorption kinetic data for fluoride by the samples were well fitted using the pseudo-second-order kinetic model, suggesting that chemisorption is important in the adsorption process. One-site Langmuir model is suitable to describe the isotherm adsorption data (R²=0.967~0.981). Compared to the correlation coefficients (R²) fitted using one-site Langmuir model, the R² fitted using two-site Langmuir model are higher (R²=0.982~0.995), but the R² fitted using Freundlich model are lower (R²=0.877~0.912). At pH=5.0, Langmuir adsorption capacities (q_max) of Goe, Goe-Al_0.1, Goe-Al_0.2 and Goe-Al_0.4 are 8.83, 10.24, 11.72 and 12.86 mg·g^-1, respectively. This indicates that the adsorption capacity for fluoride onto Al-doped goethite is higher than that onto Goe.