•Cu polymers probably exist on birnessite surfaces at high loading and low pH.•Cu(II) innersphere complexes on birnessite are less stable than Ni(II) complexes.•The first CuO shell in these Cu-doped birnessites is possible octahedron.Geochemical behaviors of heavy metal contaminants, such as Cu2 +, are strongly controlled by natural birnessite-like minerals in both marine and terrestrial environments. However, the mechanisms of the interaction of Cu2 + with birnessite are not fully understood yet. In the present study, Cu2 + was coprecipitated with Mn2 + to produce hexagonal turbostratic birnessite, which is analogous to natural birnessite. The obtained Cu-doped birnessite was characterized by powder X-ray diffraction, field-emission scanning electron microscopy, and X-ray absorption spectroscopy (XANES + EXAFS). The stability of Cu(II) in the birnessite structure was investigated by acid treatment. Increasing the dopant content reduces the mineral crystallinity in the [001] direction and the unit cell parameter b from the hexagonal layers. It also shortens the bond length of MnO in the [MnO6] unit and the edge-sharing MnMn distance in the layers, and increases the average oxidation state (AOS) of Mn and the specific surface area. Analysis of Cu K-edge XANES and EXAFS data indicates that, only a small part of Cu(II) is inserted into the birnessite layers, while most of it is adsorbed on the vacancies. When the Cu/Mn molar ratio is increased from 0.08 to 0.23, an increasing part of Cu(II) is present as polynuclear clusters on the birnessite edge sites in the pH range of ~ 3.3–5.3. Reaction with H2SO4 solution is found to easily dissolve the polynuclear Cu clusters and the highly distorted Cu octahedra in innersphere complexes on the birnessite-water interface, with ~ 53% of the Cu2 + released into the solution. On the other hand, the reaction with HCl solution leads to reductive dissolution of the mineral matrix, the release of Mn2 + into solutions, the decrease in the first MnO and edge-sharing MnMn distances and Mn AOS, in addition to the release of Cu2 +. The release rate of Cu2 + is much faster than that of Ni2 + in Ni-doped birnessites, owing to the lower stability of distorted [CuO6] octahedron upon proton attack. These results indicate the formation of multinuclear Cu complexes on birnessite surfaces under the investigated conditions. The results also suggest the lower stability of Cu2 + in these minerals and thus higher potential toxicity in acidic conditions, in comparison with other metal pollutants, such as Ni2 +. This study provides new insights into the interaction mechanisms between Cu2 + and birnessite-like minerals, and help to clarify the structural stability and geochemical behaviors of Cu2 + associated with birnessite-like minerals in natural environments.Download high-res image (124KB)Download full-size image

•Co and Ni were co-doped into the structures of cryptomelane for the first time.•The co-doped cryptomelanes had remarkable increase in optical absorption property.•Co and Ni co-doping improved the complete degradation of phenol by cryptomelane.Cryptomelane exhibits excellent photocatalytic activity for the degradation of organic pollutants. Incorporation of transition metals (TMs) into the structure of Mn oxides will cause changes to their substructure and physicochemical properties. However, the synergetic effects of incorporation of two kinds of metals have yet to be investigated. Here, cobalt and nickel co-doped cryptomelanes were synthesized with different molar ratios of Co/Ni, and characterized by powder X-ray diffraction, elemental analysis, N2 physical adsorption and UV–Visible diffuse reflectance spectroscopy, and their photocatalytic performance for the degradation of phenol was investigated. These studies demonstrate that, doping of Co and Ni did not change the crystal structure of cryptomelane, but resulted in decreased crystallinity, and generally increased the specific surface area. Co(III) and Ni(II) were incorporated into cryptomelane and substituted for Mn(IV) and Mn(III), respectively, leading to a change in Mn average oxidation state. Compared with non-doped and singly doped samples, Co and Ni co-doped cryptomelanes have large increase in optical absorption properties and increase the rate of phenol degradation, i.e. the TOC removal rate was increased by 28–38%. Co and Ni co-doped cryptomelane has potential applications in the remediation of natural waters contaminated by organic pollutants.

Cryptomelane is a reactive Mn oxide and has been used in removal of heavy metal from wastewaters. Co-doped cryptomelane was synthesized by refluxing at ambient pressure and characterized by powder X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and extended X-ray absorption fine structure spectroscopy, and its performances for removal of Pb2 + and Cr3 + from aqueous solutions were investigated. Co doping has a negligible effect on the structure and morphology of cryptomelane but increases the specific surface area and Mn average oxidation state. Mn and Co K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) analysis shows that Co barely affects the atomic coordination environments of Mn, and distances of edge- and corner-sharing Co–Me (MeCo, Mn) pairs are shorter than those of the corresponding Mn–Me pairs, implying the replacement of framework Mn(III) by Co(III). These Co-doped cryptomelanes can quickly oxidize Cr3 + to be HCrO4− and remove 45%–66% of the total Cr in the reaction systems by adsorption and fixation, and they have enhanced Pb2 + adsorption capacities. Thus these materials are promising adsorbents for heavy metal remediation. The results demonstrate the design and modification of environmental friendly Mn oxide materials and can help us understand the interaction mechanisms of transition metals with Mn oxides.Download full-size image