Oxidative decolorization of methylene blue by leached sea-nodule residues generated by the reductionroasting ammoniacal leaching process

The leached residue, generated after selective extraction of Cu, Ni and Co by reductive-roasting ammoniacal leaching of sea nodules, was characterized by various physicochemical methods. The finely divided residue, containing mainly manganese carbonate/silicates and manganese(III,IV) (hydr)oxides along with iron oxides, showed a lower surface area (66.3 m2g− 1)than that of the parent sea nodule (130 m2g− 1). The catalytic efficiency of water-washed sea nodule residue (WSNR) was evaluated taking oxidative decolorization of methylene blue (MB) as the test reaction. The extent of decolorization was decreased with increase in pH but increased in the presence of H2O2 or NaCl. Decolorization of MB occurred in two consecutive steps; the rate constant of the first step was ∼ 10 times higher than that of the second step. The formation of a surface precursor complex between WSNR and MB at a rate-limiting step, followed by electron transfer from MB to the active metal centre of WSNR and release of product(s), was proposed as the ecolorization process