Characterization of Co-doped birnessites and application for removal of lead and arsenite

Nanostructured Co-doped birnessites were successfully synthesized, and their application for the removal of Pb2+ and As(III) from aquatic systems was investigated. Powder X-ray diffraction, chemical analysis, nitrogen physical adsorption, field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the crystal structure, chemical composition, micromorphologies and surface properties of the birnessites. Doping cobalt into the layer of birnessite had little effect on its crystal structure and micromorphology. Both chemical and XPS analyses showed that the manganese average oxidation state (Mn AOS) decreased after cobalt doping. The Co dopant existed mainly in the form of Co(III)OOH in the birnessite structure. Part of the doped Co3+ substituted for Mn4+, resulting in the gain of negative charge of the layer and an increase in the content of the hydroxyl group, which accounted for the improved Pb2+ adsorption capacity. The maximum capacity of Pb2+ adsorption on HB, CoB5, CoB10 and CoB20 was 2538 mmol kg−1, 2798 mmol kg−1, 2932 mmol kg−1 and 3146 mmol kg−1, respectively. The total As(III) removal from solution was 94.30% for CoB5 and 100% for both CoB10 and CoB20, compared to 92.03% for undoped HB, by oxidation, adsorption and fixation, simultaneously.