Manganese chloride nanoparticles: A practical adsorbent for the sequestration of Hg(II) ions from aqueous solution

Removal of mercury from aqueous solution has been investigated at different pHs, adsorbent dosage, contact time, temperature and initial concentrations of Hg(II) using an immobilized Mn nanoparticles (MnNPs) on mixed-oxides support. The as-prepared nanoadsorbent was characterized by elemental analysis, Fourier transform infrared (FTIR), UV–visible diffuse reflectance spectra (UV–vis DRS), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), Brunauer–Emmett–Teller (BET), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). EPR, CV and EIS of Mn ions evidenced that most of the covalently bond active sites of the nano-adsorbent are in the form of Mn(III) ions at the surface. The heterogeneous Mn(III)Cl NPs (5–30 nm) were found to be effective adsorbent for the removal of Hg(II) from solution. The adsorption of Hg(II) has been studied in terms of pseudo-first-order and pseudo-second-order kinetics, and the Freundlich, Langmuir and Langmuir–Freundlich isotherm models have also been applied to the equilibrium adsorption data. The adsorption process was spontaneous and endothermic in nature and followed pseudo-second-order kinetic model. A high removal ability of 289.5 mg of mercury per gram of Si/Al-PAEA = SA@MnNP was achieved at pH 6.0. The nanoadsorbent showed high reusability due to its high adsorption capacity after 7th adsorption–desorption cycles. The mechanism of interaction of MnNPs with Hg(II) was studied using various analytical techniques such as DR UV–vis, CV and EIS at the surface of multi-walled carbon nanotubes. The results confirm that the surface of MnCl was occupied by Hg(II), that is, suppresses electron transfer rate of the reaction related to Mn(III)/Mn(IV) redox couple. The ease of preparation of the nanomaterial by sol–gel method, high density of reactive sites on the surface and affordable cost will make it possible to use this nanomaterial in field applications, especially for the treatment of Hg(II) from wastewater.