One-pot hydrothermal preparation of manganese-doped carbon microspheres for effective deep removal of hexavalent chromium from wastewater.

ABSTRACT

Mn-doped activated carbon microspheres (MnOx/ACS) with super-high adsorption capacities and deep removal capability for hexavalent chromium (Cr(VI)) were successfully prepared via an ammonium persulfate-assisted hydrothermal method followed by potassium oxalate activation using KMnO4 and sucrose as raw materials. Their -physical and chemical properties, as well as those of Mn-doped non-activated carbon spheres (MnOx/CS), were characterized by XRD, SEM, TEM, EDS-mapping, XPS, N2 adsorption-desorption, ICP-AES, and elemental analysis. It was found that the manganese oxide (MnOx) particles were uniformly embedded within the carbon spheres via layer-by-layer capture, and the MnOx/ACS exhibited strong redox activity because of the multivalent nature of MnOx, resulting in excellent adsorption performance via reduction. In particular, MnOx/ACS-4 with a Mn content of 1.06 wt% and a specific surface area of 1405.7 m2 g-1 achieved a maximum adsorption capacity of 660.7 mg g-1; this can reduce Cr(VI) content to less than 0.05 mg L-1, which meets the corresponding Chinese drinking water quality standard when the initial concentration of Cr(VI) is less than 400 mg L-1. Furthermore, this highly efficient method can be extended to prepare V-, Mo-, or W-doped carbon microspheres with significantly enhanced adsorption performance for Cr(VI) compared to bare activated carbon sphere, indicating their good application prospect for the deep removal for heavy metal ions from wastewater.