Phosphate Adsorption from Aqueous Solutions onto Goethite, Bentonite, and Bentonite–Goethite System

The present paper examines the phosphate adsorption from aqueous solutions onto goethite, bentonite, and bentonite–goethite system. The properties of the materials were studied by X-ray diffraction (XRD), attenuated total reflectance (ATR), and NMR spectra and by the measurement of the specific surface area, the point of zero charge (p.z.c.) and the pore-specific volume. ATR and NMR spectra of bentonite and bentonite–goethite system show peaks which correspond to tetrahedrally and octahedrally coordinated Al. The specific surface area of the system differs according to the appropriate method used, while system’s p.z.c. is higher than bentonite and lower than goethite. The pore-specific volume of bentonite–goethite system is higher than that of bentonite or goethite. According to XRD spectrum of bentonite–goethite system, goethite coats the (001) spacing of bentonite while the coating of (010) plane of bentonite is limited. The crystallinity of the system decreases and the negative permanent charge increases. Phosphate adsorption experiments took place at different pH (3.8–9.0) and concentrations (40.3–443.5 μmol L−1) and constant capacitance model was applied to describe adsorption. A ligand exchange mechanism characterizes the model because the charge is divided among adsorbate and adsorbent. The constant capacitance model describes the adsorption mechanism in all examined pH. This model can be utilized in such systems using the surface protonation-dissociation constant of goethite and showing the exact shape of the adsorption isotherms for different pH values. Τhe produced low-cost bentonite–goethite system presents the highest adsorption of P per kilogram of goethite.