Adsorptive removal of thiazine dyes from aqueous solutions by oil shale and its oil processing residues: Characterization, equilibrium, kinetics and modeling studies

• The oil shale and its pyrolysis residues at 200 and 500 °C were used for dye removal.
• The oil shale adsorbents were characterized using TGA/DTG, DRIFT, XRD and AFM.
• Sorption kinetics and equilibria of thiazine dyes TH, TB and MB were investigated.
• Adsorption rate decreased with increasing molecular size of the dyes.
• The byproduct at 500 °C has a great potential for remediation of water quality.

Adsorption characteristics of oil shale (OS) and its pyrolysis byproducts for cationic thiazine dyes were investigated using thionine (TH), toluidine blue (TB) and methylene blue (MB).The time dependent data for the OS adsorption were analyzed using the linear driving force (LDF) model by combining with the Langmuir isotherm. The external mass transfer (kf) and surface diffusion (Ds) coefficients were analytically calculated based on McKay equation. They decreased with increasing molecular sizes of the dyes in the following order TH > TB > MB. In contrary, desorption efficiency in 0.1 M NaCl solution increased in the same order.The thermogravimetric analysis (TGA/DTG), atomic force microscopy (AFM), diffuse reflectance infrared Fourier transforms (DRIFT) and X-ray diffraction (XRD) spectroscopy techniques were used in conjunction to characterize the OS samples. A comparison of (DRIFT) spectra of the dye loaded and unloaded adsorbents showed that the larger dye molecules are primarily adsorbed on the negatively charged surface hydroxyl groups and/or on organic functional groups with electrostatic interactions while the smaller molecules replace with structural cations in the interlayer space of the mineral matrices.Adsorption capacity of the pyrolysis residue obtained in nitrogen atmosphere at 200 °C (OS-200) for the dyes decreased with respect to the OS whereas the byproduct at 500 °C (OS-500) had considerably higher adsorption ability. All of the dyes were completely removed by the OS and OS-500 from dilute solutions (<0.5 mmol L−1). The OS-500 was also a good candidate for water remediation due to its high dye removal capacity in concentrated dye solutions.

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