Synthesis of MnO2 Hollow Nanospheres through Selective Etching Method as an Effective Absorbent to Remove Methyl Orange from Aqueous Solution

The discharge of dye wastewater has become an unavoidable problem for human health and the environment. Developing an economical and rapid method to prepare effective adsorbents for selective removal of dyes is extremely urgent. In this work, MnO2 hollow nanospheres (MHNSs) were prepared through the selective etching method with the MnCO3 as the sacrificial template. The effect of the pH value, contact time, and initial concentration on the adsorption of methyl orange (MO) onto the MHNSs was systematically investigated. The unique mesoporous hollow structure and large BET surface area (43.74 m(2)/g) of MHNSs lead to an excellent adsorption capacity (1677.14 mg/g) at the optimal condition. Furthermore, the prepared MHNSs also showed great stability (90% removal rate after four cycles). The adsorption kinetics data fitted well with the pseudo-second-order kinetic model (R2>0.9997). The overall process was jointly controlled by external mass transfer and intraparticle diffusion, and intraparticle diffusion was the dominant factor. The adsorption isotherm results showed that the Freundlich model was more accurate to describe the experimental data than the Langmuir model. The thermodynamic analysis showed that the adsorption of MO on MHNSs was spontaneous and exothermic. Moreover, the calculated Delta G degrees and the XPS spectra showed that the process was mainly a physical process. It is expected that MHNS has a potential application for purifying dye wastewater due to its great adsorption performance and excellent stability.