Photocatalytic activity of α‐MnO2 nanorod synthesized through a low temperature (90 °C) single step precipitation route in the absence of surfactant and template is reported. Dependence of precipitation time on morphology of the synthesized α‐MnO2 has been investigated and the photocatalyst has been tested for the degradation of organic cationic and anionic dyes. Detailed study on the degradation of Rhodamine B (RhB) has been carried out. The lower precipitation time (of 6 h) is found to be ideal for the synthesis of the photocatalyst. The mechanism of RhB photodegradation under visible light using α‐MnO2 nanorods has been established through mass spectra analysis. The intermediate products during degradation exhibits de‐ethylation and mineralisation steps. Experimental results suggest that both super oxide and hydroxyl radicals are the main active species in the process. Total organic carbon (TOC) analysis of treated RhB reveals complete mineralisation. The photodegradation efficiency of α‐MnO2 for cationic and anionic dyes are found to be 95–100 % under visible light irradiation. The excellent photocatalytic activity of α‐MnO2 can be correlated with its 1‐D morphology of high aspect ratio and low photoluminescence intensity. The complete dye discoloration within 10 min and total mineralisation of RhB in 25 min is quite significant especially under visible light irradiation and has never been reported earlier.

Systematic experimental investigation of MnO2–BSA complexes in terms of the structure and stability of the protein as well as the aggregation of the nanoparticle.

Herein, we report a highly efficient template or surfactant free hydrothermal method to fabricate a graphene based single-crystal tetragonal needle-and nanorod-like α-MnO 2 composite, a kind of semiconductor photocatalyst for the reduction of hexavalent chromium [Cr(VI)] under visible irradiation. Hydrothermal reaction time and in situ introduction of graphene played a important role in tuning the morphology of α-MnO 2 . The as-synthesized reduced graphene oxide (RGO)/α-MnO 2 nanorod composite exhibited outstanding photoreduction ability as compared to RGO/α-MnO 2 needle composite. The higher activity of RGO/α-MnO 2 nanorod composite was successfully derived from photoluminescence (PL) and photocurrent measurements. The low PL intensity and high photocurrent density of RGO/α-MnO 2 nanorod composite concludes that change in aspect ratio as well as the presence of RGO favors intimate strong interaction of a two-dimensional layer of graphene and one-dimensional α-MnO 2 nanorod which facilitates for enhanced photoexcited charge (e − /h + ) separation and simultaneously increases its photoreduction ability under visible light irradiation.

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Polymetallic sea nodules are the potential resources of copper, cobalt, manganese, and nickel. The exploration and exploitation of manganese nodules are necessary to meet the future demands of Mn in the world. This work describes the valuable extraction of Mn through an innovative approach of using sucrose as a reductant. The metal extraction from the nodule leaching was studied in ambient temperature (27 °C) and at an elevated temperature 90 °C with and without agitation. In both cases, 99.9% of Mn was extracted while the extraction time appears to be significantly low (2 h) at 90 °C but it requires a longer time of 24 h at 27 °C. The optimum sucrose concentration selected was 7% (w/w) of nodule for maximum metal extraction. With use of 10% (v/v) H 2 SO 4 , 7% (w/w) sucrose at 90 °C, and solid to liquid (S/L) ratio of 1:10 the achieved metal recovery figures were the following: Mn >99%, Ni 98%, Cu 87%, Co 83% in 2 h. The oxidation pathway of sucrose is outlined in this study with the aid of mass spectrometry during reductive leaching of ocean bed nodule. Sucrose in acidic environment generates 1-or 6-monoacid of sucrose with ions of m/z (mass-to-charge ratio) 355 and 127, 5-hydroxymethylfurfural (5HMF). A dehydrated glucose complex (m/z 325) was generated at elevated temperature. These released organics act as reductant for the leaching of Mn 2+ from MnO 2 . Glucaric acid (m/z 211) is generated as the end product of the sucrose oxidation in the solution.

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