MnO Flowery Nanocomposites for Efficient and Fast Removal of Mercury(ii) from Aqueous Solution: a Facile Strategy and Mechanistic Interpretation

We report the synthesis of MnO flowery nanocomposites consisting of MnO nanoflowers grown over the surface of clay nanomaterials using an easy and green approach. The MnO nanocomposites were explored as a cost-effective nanoadsorbent for mercury removal from aqueous solution and they demonstrated excellent efficiency towards mercury uptake. Monolayer molecular adsorption of Hg(ii) was attained over the surface of the MnO nanocomposites and the experimental data acquired in the kinetic study demonstrated that the Hg(ii) adsorption kinetics proceeded via a pseudo-second-order kinetic model. pH dependent adsorption study revealed that their sorption capacity increases until pH 7.0 and then gradually decreases with increasing pH. Apart from the experimental study, we have provided a mechanistic interpretation to illustrate the mechanism of kinetics and thermodynamics during Hg(ii) adsorption. Theoretical understanding along with experimental results indicates a spontaneous and highly favorable Hg(ii) uptake up to 50 °C, representing endothermicity of the adsorption process and then exothermicity above 50 °C, resulting in reduced sorption capacity. The exceptional adsorption performance of the MnO nanocomposites may be attributed to their negative surfaces, which facilitated the binding of positively charged Hg(ii) ions through electrostatic interaction. Hence, MnO nanocomposites proved to be an effective and inexpensive nanoadsorbent for the removal of Hg(ii) from aqueous solution and may hold a promise for wastewater treatment.