Insights Into the Mn(VII) and Cr(VI) Adsorption Mechanisms on Purified Diatomite/MCM-41 Composite: Experimental Study and Statistical Physics Analysis.

Inas A Ahmed,Adrián Bonilla-Petriciolet,Michael Badawi,Moaaz K Seliem,Mohamed Mobarak,Eder C Lima

doi:10.3389/fchem.2021.814431

Abstract

In this study, a purified diatomite (PD) with a concentration of diatom frustules more than 92% SiO2 was utilized to synthesize a composite of MCM-41 silica under hydrothermal conditions. The as-synthesized PD/MCM-41 composite was characterized and tested as an adsorbent for the removal of Cr(VI) and Mn(VII) ions from aqueous solution. Results of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier-transform infrared spectroscopy (FTIR) revealed that the diatom frustules of the PD were coated with MCM-41 mesoporous silica. Experimental isotherms of Cr(VI) and Mn(VII) adsorption were fitted to classical and advanced statistical physics models at 25°C–55°C and pH 3. The Langmuir model estimated monolayer adsorption capacities ranging from 144.1 to 162.2 mg/g for Cr(VI) and 166.2 to 177.0 mg/g for Mn(VII), which improved with increasing the solution temperature. Steric and energetic parameters obtained from a monolayer adsorption model with one adsorption site was utilized to explain the adsorption mechanism at a microscopic level. The number of Cr(VI) and Mn(VII) ions adsorbed on PD/MCM-41 active site (n) were 1.25–1.27 for Cr(VI) and 1.27–1.32 for Mn(VII), thus suggesting multi-interaction mechanisms. The density of PD/MCM-41 active sites (D M) was a key parameter to explain the adsorption of these heavy metals. The adsorbed quantities were maximum at 55°C, thus obtaining 102.8 and 110.7 mg/g for Cr(VI) and Mn(VII), respectively. Cr(VI) and Mn(VII) adsorption energies ranged from 18.48 to 26.70 kJ/mol and corresponded to an endothermic adsorption with physical forces. Entropy, free enthalpy, and internal energy associated to the adsorption of Cr(VI) and Mn(VII) ions were calculated, thus indicating that the removal of these pollutants was spontaneous. Overall, this article offers new interpretations for the Cr(VI) and Mn(VII) adsorption mechanisms on PD/MCM-41 composite, which are relevant to contribute to the development of effective water treatment processes.

Highlights

During the last few decades, a significant amount of industrial effluents and solid has been discharged into the environment, especially the aquatic systems, due to industrialization and anthropogenic activities
Characterization of the composite (PD/MCM-41) via X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier-transform infrared spectroscopy (FTIR) techniques was considered in a previous study (Selim et al, 2018)
The adsorption equilibrium of Cr(VI) and Mn(VII) on purified diatomite (PD)/MCM-41 was evaluated via experimental investigation and statistical physics analysis at three temperatures and pH 3.0

Summary

Introduction

During the last few decades, a significant amount of industrial effluents and solid has been discharged into the environment, especially the aquatic systems, due to industrialization and anthropogenic activities. The high-efficiency and simple design, wide accessibility of natural materials, and low operating cost are important factors associated with the application of adsorption strategy in water remediation (Meena et al, 2005; Seliem and Komarneni, 2016; Selim et al, 2018; Sellaoui et al, 2021a). In this direction, the modeling of the adsorption equilibrium is mandatory to characterize and understand the physicochemical factors that could govern the removal of water pollutants in adsorption systems (Li et al, 2019; Seliem and Mobarak, 2019; Mohamed et al, 2020; Ramadan et al, 2021). SPMs can be used to outline the interactions at the micro- and macroscopic levels, and they can offer deep and novel insights into the adsorption mechanism (Li et al, 2020; Abu Sharib et al, 2021)

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