| Abstract: Pure goethite (Goe) and Al-doped goethite (Goe-Al0.1, Goe-Al0.2 and Goe-Al0.4) 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-Al0.1 < Goe-Al0.2 < Goe-Al0.4, and the pore size distributions of the four samples show the opposite order. The points of zero charge (PZC) of Goe, Goe-Al0.1, Goe-Al0.2 and Goe-Al0.4 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 (qmax) of Goe, Goe-Al0.1, Goe-Al0.2 and Goe-Al0.4 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. |
