Template-free synthesis of nanoparticle-built MgO and Zn-doped MgO hollow microspheres with superior performance for Congo red adsorption from water.

Nanoparticle-built MgO hollow microspheres were synthesized through a template-free hydrothermal route using citrate as a structural director. Zn was introduced into MgO to improve the surface charge. Pure MgO and Zn-doped MgO samples were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectrometry (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and a zeta-potential analyzer. The as-prepared microspheres showed outstanding performance for the removal of Congo red (CR, anionic dye) from solutions. The maximum adsorption capacities of pure MgO and Zn-doped MgO samples were 3022.02 and 2953.39 mg g-1, respectively. The Zn-loaded sample only required 45 min to reach equilibrium, which was much shorter than that of pure MgO sample (120 min) and most previously reported adsorbents. The high adsorption capacity and efficiency for CR removal resulted from the samples' unique porous structures and positive surface charges at pH 7. The adsorption process followed Langmuir isotherm and pseudo-second-order model. Regeneration assessment was conducted by a method of calcining for four times, and the observed steady adsorption efficiency indicated a bright prospect for the two samples in wastewater treatment.