Adsorptive and photocatalytic remediation of aqueous organic dyes and chromium(VI) by manganese(II) substituted magnetite nanoparticles

A series of Mn-substituted magnetite nanoparticles (MN-MNPs), MnxFe3-xO4 (0.0 <= x <= 0.63), were synthesized by coprecipitation method and their structural, magnetic, and optical behaviours as a function of Mn2+ substitution were investigated by powder XRD, FT-IR, SEM-EDX, TEM, UV-Vis DRS, and magnetic moment measurements. XRD showed single-phase spinel structure for all the samples with an average crystallite sizes in the range 12.1-14.5 nm which were in good agreement with TEM results. The values of saturation magnetization (sigma(s)) were increased with increase of Mn content up to x <= 0.205 and then decreased on further increase of Mn content. The catalytic activities of Mn-MNPs were evaluated with photo-decolourisation of different organic dyes (Methyl orange, Congo red and Rhodamine B) and photo-reduction of Cr(VI) under visible light irradiation. Among all, Mn0.205Fe2.795O4 (Mn-MNP-3) showed the best performance with >= 85% decolourisation of dyes (20 mg/L) within 4 h. Aqueous Cr(VI) was efficiently reduced by Mn-MNP-3; the rate and percentage of Cr(VI) reduction was enhanced by addition of different sacrificial agents. Photo-reduction of Cr(VI) was also improved in binary component system with a dye (Congo red) showing synergetic effect between the reduction and oxidation reactions. Simultaneous decontamination of Cr(VI) and a dye by the present catalytic system with easiness of separation may be useful in treatment of textile wastewater containing dye and heavy metals like Cr(VI).