Adsorption of Congo red dye from aqueous solution by using siderite waste accompanying phosphate mines: Performance and mechanism

Abstract

Natural siderite (NS) collected from Abu Tartur area, Egypt was used as an adsorbent of Congo red from an aqueous solution. Activated siderite (AS) was prepared by calcination at 350°C for 2 hrs. The AS adsorbent was characterized by various physicochemical techniques such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Surface area measurement (BET), XRF analyses, and texture analysis. The impact of various adsorption parameters (adsorbent dosage, Congo red concentration, pH, contact time, and temperature) was studied in a batch system in order to optimize the maximum dye sorption. The optimum parameters were found to be pH. 2, initial dye concentration 150 mg/L, contact time 30 minutes, and adsorbent dose 0.2 g. Adsorption kinetics and equilibrium isotherm of the ANS adsorbent were studied using pseudo-second-order, intra-particle diffusion, and Elovich models. Moreover, Langmuir and Freundlich isotherms were applied for the interpretation of the obtained equilibrium data. The obtained data were found to best fit the Langmuir model, while the adsorption kinetic followed the pseudo-second-order equation. The intra-particle diffusion investigations indicate that the diffusion process is not the rate-controlling step. The analysis of our data was extended to estimate the kinetic and thereafter the thermodynamic functions. Accordingly, the functions such as free energy change (ΔG°), enthalpy change (ΔH°), the activation energy of adsorption (Ea), and entropy change (ΔS°) were estimated and found to be -226.42, -2.443, 33.71 kJ/mol and 0.751 kJ/mol.K, respectively. Overall these findings let us infer that the use of ANS as an adsorbent has the potential to be an effective and cheap adsorbent for removing dyes from industrial wastewater.