Preparation, characterization and sorption properties for phosphates of hematite, bentonite and bentonite–hematite systems

• Published in: Advances in colloid and interface science Vol. 97; no. 1; pp. 37 - 61
• Main Authors: Dimirkou, A., Ioannou, A., Doula, M.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 29.03.2002
• Subjects: Adsorption; Bentonite; Bentonite–hematite system; Constant capacitance model; Hematite; Phosphate; Hematite; Bentonite; Constant capacitance model; Adsorption; Phosphate; Bentonite–hematite system
• ISSN: 0001-8686; 1873-3727
• DOI: 10.1016/S0001-8686(01)00046-X

Hematite was prepared using Fe(NO 3) 3·9H 2O. Bentonite was first saturated with sodium and then was transformed to the hydrogen form. The bentonite–hematite (b–h) system was prepared by the mixing of hematite and bentonite at pH 6.0, and a hematite-coated bentonite surface was created. The physical and chemical properties of the above solids were studied from the information given by X-ray, IR and NMR spectra and by measurement of specific surface and point of zero charge. The results of the above study show that the bentonite–hematite system is not a physical mixture of hematite and bentonite. Hematite moved into bentonite's interlayer space, coating these planes throughout. Consequently, Al-substitution and structural H 2O/OH displacement of hematite took place. The system has a specific surface, less than bentonite and greater than hematite, while its p.z.c. is higher than bentonite and lower than hematite. Adsorption experiments of phosphates at different concentrations and pH were carried out and the constant capacitance model was used to describe phosphate adsorption by hematite, bentonite and the bentonite–hematite system. The model is characterized by a ligand exchange mechanism based on the assumption that the charge is attributed both on adsorbate and adsorbent, and is proven to successfully describe the adsorption of phosphate on these materials along with the effect of varying pH values. Ma-Za 1 and Ma-Za 2 (two computer programs) were used for this purpose. The model successfully gave a description (both quantitative and qualitative) of the adsorption of phosphate anions across the pH range (3.8–9.0). Furthermore, it can be successfully applied in systems as the bentonite–hematite system by utilizing the surface protonation–dissociation constant of hematite. The model, when applied in such systems, procreates the exact shape of the adsorption isotherms for the entire pH range of 3.8–9.0. The constant capacitance model was able to describe phosphorous adsorption on hematite and the bentonite–hematite system. The accurate fit of the model to phosphorous adsorption on hematite and the bentonite–hematite system suggests that inner sphere surface complexation is the appropriate adsorption mechanism for these materials.