Removal of nickel ions from aqueous solutions by manganese dioxide derived from groundwater treatment sludge

In this study, manganese dioxide (MnO2) derived from groundwater treatment sludge was used in the adsorption of nickel (Ni(II)) ions from aqueous solutions. The synthetic MnO2 was prepared via permanganate reduction using manganese extracted from the sludge by reductive acid leaching and hydroxide precipitation. Scanning electron microscopy images showed aggregated micron-sized MnO2 particles. Fourier transform infrared analysis of MnO2 revealed functional groups at 3,396 cm(-1), 1,630 cm(-1), 1,427 cm(-1) and 468 cm(-1) bands. Zeta potential measurements at the pH range of 2-8 confirmed the net negative surface charge of MnO2 particles. Moreover, Ni(II) adsorption by MnO2 was best described by the Langmuir isotherm model, as indicated by the high values for the coefficients of determination (R-2 > 0.9703). The separation factor (RL) for the range of pH values and initial Ni(II) concentrations considered in this study indicated that Ni(II) adsorption by MnO2 was favorable. The kinetic data of Ni(II) adsorption by MnO2 at pH 6.5 and initial Ni(II) concentrations from 10 to 200 mg L-1 conformed to the pseudo-second order adsorption kinetic model, with R-2 > 0.9997. Chemisorption occurred through the complexation of Ni(II) ions with available MnO2 functional groups. The thermodynamic study at temperatures of 298.15, 308.15 and 318.15 K revealed that Ni(ll) adsorption by MnO2 was spontaneous and thermodynamically favorable for initial Ni(II) concentrations ranging from 50 to 200 mg L-1. Overall, Ni(II) adsorption by MnO2 was endothermic, as indicated by the positive LW values. In addition, MnO2 had good affinity towards the Ni(II) ions, as shown by the positive AS values at all Ni(II) concentrations. Simple cost analysis revealed that the MnO2 production from groundwater sludge was economically viable and may be scaled up for commercial applications. (C) 2018 Elsevier Ltd. All rights reserved.