Optimizing decolorization of methyl blue solution by two magnetic hydroxyapatite nanorods

• Two magnetic nanorods were compared and used for MB adsorption.
• High adsorption capacity and good reusability of these nanorods were observed.
• The adsorption process was spontaneous and exothermic.
• Hydrogen bonding and associated interactions exist in SPF and SBPF, respectively.

In this study, the application of two magnetic hydroxyapatite nanorods Sr5(PO4)3(OH)/Fe3O4 and Sr5xBa3x(PO4)3(OH)/Fe3O4, called SPF and SBPF for short, was investigated as effective adsorbents for removal of methyl blue (MB) from aqueous solution. Effects of three influence parameters (i.e., pH, temperature, and adsorbent dosage) were studied to optimize the adsorption processes. Kinetic data for both materials fitted well with pseudo-second-order kinetic model. The isotherm data agreed well with Langmuir model for SPF and Freundlich model for SBPF. The maximum adsorption capacity of SBPF nanorods (702 mg/g) is higher than that of SPF (531 mg/g), suggesting higher potential for MB removal. Thermodynamic studies indicated the adsorption process by these nanorods was spontaneous and exothermic. Moreover, these magnetic adsorbents can be easily recovered, and the capacity was maintained over 86% of the initial capacity after being used for five cycles. Mechanistic studies indicate that there are hydrogen bonding for MB-adsorbed SPF; whereas both hydrogen bonding and ionic interaction for MB-adsorbed SBPF. Interestingly, these magnetic nanorods show not only high adsorption capacity for MB but also facile regeneration from aqueous solution.

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