Cadmium removal with thiosulfate/permanganate (TS/Mn(VII)) system: MnO2 adsorption and/or CdS formation

Abstract

Removal of Cd(II) from aqueous solution with thiosulfate/permanganate (TS/Mn(VII)) system was systematically investigated at different TS/Mn(VII) molar ratios (0.375, 0.5, 1, 2, 3) and various pH (5–9). The nano-MnO2 in situ formed in TS/Mn(VII) system showed excellent adsorption capacity for Cd(II) (235 mg/g at pH 7) under stoichiometric ratio (TS/Mn(VII) = 0.375), where higher pH would promote the removal of Cd(II). Cd(II) removal efficiency decreased from 98.1% to 60.2% as the TS/Mn(VII) molar ratio increased from 0.375 to 3 at neutral pH, mainly due to the reduction of MnO2 by excess Na2S2O3 as well as the competitive adsorption of Mn(II) generated therein. At pH 9, TS/Mn(VII) ratio had a negligible influence on the removal efficiency of Cd(II) (about 90%), resulted from the slow redox reaction between MnO2 and excessive Na2S2O3 at alkaline conditions. Unexpectedly, at pH 5, the removal efficiency of Cd(II) remained at a high level (85.2% and 96.0%) at high TS/Mn(VII) molar ratios (2 and 3) where negligible nano-MnO2 existed. This result could be rationalized by the formation of solid CdS, which was confirmed by Scanning Electron Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). This work demonstrated the important role of TS/Mn(VII) molar ratio in TS/Mn(VII) system for Cd(II) removal and identified the formation of solid CdS in TS/Mn(VII) system, which provides a promising engineering perspective for Cd(II) removal by MnO2 adsorption and/or CdS formation.