Binary Systems of Kaolinite and Hematite: Preparation, Characterization and Adsorption Characteristics for Fluoride

In soils and sediments kaolinite (Kao) and hematite (Hem) are often cemented together as binary systems, which have a significant influence on the mobility of nutrients and pollutants in natural environments. In this study, the binary systems of kaolinite-hematite complex (KHC) and kaolinite-hematite mixture (KHM) were prepared, and the surface properties and fluoride adsorption of the samples were investigated. Compared to KHM, the XRD peaks for both kaolinite and hematite in KHC were lower. The crystallinity of hematite in KHM and KHC was 94.13% and 80.99% respectively. This indicates that the surface of kaolinite in KHC was largely coated by poorly crystalline hematite, and the coating in KHM was relatively weak. The total pore volume of the samples followed the sequence of Hem > KHC > KHM > Kao, and the specific surface area (SSA) decreased in the order of KHC > Hem > KHM > Kao. The isoelectric point (IEP) of Kao, Hem, KHM and KHC was 3.3, 7.6, 5.9 and 6.5, and the zeta potential at pH 5.5 was −18.6, 25.3, 2.8 and 10.3 mV, respectively. The fluoride adsorption data of the samples were fitted using one-site Langmuir, two-site Langmuir and Freundlich models, and results indicated that the binary systems possessed the surface with various adsorption sites for fluoride. At pH 5.5, the adsorption capacity (qmax) of Kao, Hem, KHM and KHC was 1.92, 13.79, 7.48 and 10.46 mg·g−1, respectively. Compared to the average of Kao and Hem, the qmax of KHC increased significantly (P < 0.05), whereas that of KHM had no significant change (P > 0.05).