Methylene blue and methyl orange removal using green rust as a low-cost, sustainable adsorbent and photocatalyst

Abstract

Green rust (GR), a mixed-valent iron mineral from the layered double hydroxides family, has gained attention for its potential in environmental and energy applications. Despite its widespread presence and possible role in life's evolution, GR's susceptibility to oxidation has limited its practical use. In this study, we prepared a stabilized GR, synthesized through a one-pot solvothermal method using iron(III) chloride and glycerol. X-ray diffraction, field emission scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis characterized the prepared green rust. The adsorption of the anionic dye methyl orange (MO) and its mixture with the cationic dye methylene blue (MB) onto GR was explored for the first time. Various parameters, such as pH (5–11), adsorbent dose (0.015–0.030 g), and initial dye concentration (10–200 mg L⁻¹), were studied to determine the efficiency of GR for removing MO from water. To get insights into the adsorption mechanism and kinetics, different isotherms and kinetic models were applied to fit the data of adsorption obtained at pH 7. The obtained data revealed that the adsorption process obeys pseudo-first-order, pseudo-second-order, mixed 1,2-order, and Avrami models. The isotherm data showed strong agreement with the Langmuir–Freundlich, Sips, and Baudu models. At 25 °C, the maximum adsorption capacity for MO was 93.18 mg g⁻¹. Monte Carlo and molecular dynamics simulations were used to identify specific binding sites, quantify adsorption energies, and elucidate the mechanisms driving the removal process.