Removal of Cadmium(II) by hydrated manganese dioxide: behaviour and mechanism at different pH

ABSTRACT

Homogeneous precipitation was proposed to prepare hydrated manganese dioxide (HMO) with KMnO₄ as oxidant, NaCl as reductant and HNO₃ as reaction auxiliary. HMO was applied to remove Cd(II) and the effect of contact time, initial concentration, adsorbent dose and pH value on adsorption efficiency were investigated. The removal mechanisms at various pH values were analysed in detail. Adsorption thermodynamics parameters were calculated as ΔG < 0, ΔH > 0 and ΔS > 0, which meant that the adsorption process was endothermic. The result of adsorption kinetics indicated the adsorption process conformed to pseudo-second-order kinetics. When adsorbing Cd(II) with initial concentration equaling 100 mg·L⁻¹, the activation energy (Ea) was 62.740 kJ·mol⁻¹. The Langmuir model could describe adsorption behaviour on HMO better than the Freundlich model, indicating that the adsorption sites of HMO were homogeneous and that single-layer adsorption was a dominant way in this process. The maximum adsorption capacity of Cd(II) on MnO₂ calculated by the Langmuir model was 267 mg·g⁻¹. The adsorbent HMO could be recycled and reused for several times with a high efficiency above 70% by adding HCl. SEM, EDS, FTIR and XPS were used to analyse the mechanisms of removal of Cd(II) at pH = 3,7 and 10. The mechanisms included electrostatic attraction, ion exchange and chemical precipitation. With pH increasing, the zeta potential decreased and the surface negative charge increased, promoting Cd(II) removal through enhanced electrostatic attraction. Meanwhile, ion exchange mechanisms including inner-sphere complexation and outer-sphere complexation occurred during adsorption process at different pH.

GRAPHICAL ABSTRACT

KEYWORDS:

• Homogeneous precipitation
• pHpzc and pH effect
• removal mechanisms
• complexation
• precipitation

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data support the findings of this study are available within the article and its supplementary materials.

Additional information

Funding

This work was kindly supported by Program of Guangdong Science and Technology Department of China (B2152990).