Development of magnetic Sr₅(PO₄)₃(OH)/Fe₃O₄ nanorod for adsorption of Congo red from solution

Magnetic Sr₅(PO₄)₃(OH)/Fe₃O₄ (SPF) nanorod with rough surfaces was prepared and characterized using different techniques, such as SEM, EDS, TEM, SAED, HRTEM, XRD, XPS, PPMS and FT-IR. The adsorption behavior of Congo red (CR) from aqueous solution onto the as-prepared SPF nanorod was systematically investigated. Adsorption isotherm data indicated that the adsorption of CR onto SPF was monolayer adsorption which followed well the Langmuir isotherm model. The maximal CR uptake by SPF was 396 mg/g in the test. Kinetics parameters were in accord with pseudo-second-order equation, and the maximum sorption capacity calculated was 294 mg/g, which was close to the experimental value. For thermodynamic studies, parameters such as the Gibbs free energy, the enthalpy and the entropy indicated that the adsorption process was spontaneous and exothermic in nature. The removal of CR was attributed to the hydrogen bond and ionic interactions. Moreover, SPF can be easily magnetic separated and reused. The adsorption capacity was approximately 89% of the initial saturation adsorption capacity after being used five times.