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Cubic phase of MgO nanoparticles were prepared by microwave-assisted combustion synthesis and investigated for the removal of toxic dyes like Congo Red (CR) and Trypan blue (TB). The crystallite size of the MgO nanoparticle was calculated to be 18 nm from XRD pattern. The sample was further characterized by FTIR, TGA and FESEM techniques. The dyes were subjected to prototypical batch adsorption process, including investigation of different parameters like MgO dosage, dye concentration, solution pH, agitation speed and temperature. It was found that, 0.2 g of MgO NPs showed maximum removal efficiency for both the dyes (more than 98%), having 25 ppm of dye concentration at an acidic pH (3–4). The maximum loading capacity of MgO NPs was obtained to be 136 mg/g and 132 mg/g for CR and TB, respectively. Different thermodynamic parameters like ΔG0, ΔH0 and ΔS0 were measured. The negative ΔH0 and the positive ΔS0 values for both the dyes correspond to an exothermic process and an increase in randomness of the adsorbent and dye. The isotherm analysis exhibited that the Freundlich model fits better to the experimental equilibrium data, suggesting heterogeneous surface of the nanoparticles. Whereas, the kinetic data revealed a pseudo 2nd order rate for adsorption process.

Green synthesis of manganese oxide nanoparticles (NPs) was carried out by sol-gel method using Acacia Concinna fruit extract for removal of reactive dye. The effect of calcination temperature on its morphology was investigated. α-MnO

Green synthesis of manganese oxide nanoparticles (NPs) was carried out by sol-gel method using Acacia Concinna fruit extract for removal of reactive dye. The effect of calcination temperature on its morphology was investigated. α-MnO2 and Mn3O4 NPs were synthesized at 400 °C and 900 °C respectively and were characterized by PXRD, SEM, TEM, FTIR, BET, Raman and TGA. As-synthesized MnO2 NPs were investigated for the adsorption of Reactive Blue 21 (RB-21) dye. The effect of pH, adsorbent dose, agitation speed, initial dye concentration and temperature on dye removal was explored. pHpzc was calculated from zeta potential study showing positive surface charge below pH 3.18 resulting in electrostatic force of attraction between adsorbate and adsorbent. Both linear and non-linear regression approaches were utilised for the fitting of kinetic models and adsorption isotherms. Adsorption data follows a pseudo second order kinetics and fits well with the Freundlich isotherm model. Thermodynamic parameters such as ΔHo, ΔSo and ΔGo were determined. The dye removal efficiency, in case of MnO2 NPs at pH 3.0 was obtained to be 98% whereas for Mn3O4, no such dye adsorption was observed. The mechanism of adsorption was studied theoretically confirming π-π interaction and H-bonding between the MnO2 and RB dye molecules.