Investigation of Tetrachloro-Aurate(III) Adsorption in the Presence of Dichromate(VI) Species on Mg/Al Layered Double Hydroxides

Abstract

We investigated the adsorption behaviour of tetrachloro-aurate(III) (AuCl₄^-) under a single solution and in the presence of dichromate(VI) (Cr₂O₇^2-) ion on Mg/Al LDH. The most effective adsorption of AuCl₄^- ion occurred at a pH of 3; the optimum time was 600 minutes. The kinetics studies demonstrated that the adsorption of AuCl₄^- ion on Mg/Al LDH adhered to a pseudo-second-order kinetic model. The rate constant (k) was 2.877 x 10^-3 and 13.696 x 10^-3 g. mg^-1. min^-1 for the single and binary conditions, respectively. The isotherm model was evaluated by employing many models, including Langmuir, Freundlich, Temkin, Dubinin-Radushkevich, Flory-Huggins, Fowler-Guggenheim, Hill-de Boer, Jovanovic, Harkins-Jura, and Halsey. The R² analysis indicates that the isotherm accurately fits the adsorption data of AuCl₄^- onto Mg/Al LDH. The X-ray diffraction (XRD) patterns of the Mg/Al LDH exhibit noticeable variations in the d₀₀₃, d₀₀₆, d₀₀₉, and d₀₀₉. In the peaks, a slight shift towards higher 2θ values was observed, resulting in a minimal reduction in the interlayer spacing. The IR spectra detected a specific absorption peak at 3456.44, 1381.03, 678.94, 447.48, and 563.21 cm⁻¹. These peaks correspond to the stretching vibrations of the O–H, O–N–O, N=O, Mg–O, and Al–O bonds in the Mg/Al LDH. The initial specific surface area of Mg/Al LDH was 20.49 m² g^-1. However, after AuCl₄^- adsorption in both conditions, the surface area reduced to 13.81 and 3.74 m² g^-1. The Mg/Al LDH had a coarse and irregular shape, with the adsorbent surface predominantly composed of smaller particles.