Modeling studies of malachite green sorption onto macroporous hydrogel pillared-clay

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A series of macroporous hydrogel pillared-clay (MPHPC) solids was developed using iron-pillared clay (Fe-PC). These solids were encapsulated in sodium alginate hydrogel either with or without polyvinyl alcohol (PVA) and calcium carbonate. The macroporous network of the MPHPC materials was created through an acidification step using an HCl solution. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), and X-ray fluorescence (XRF) were employed to characterize the encapsulated MPHPC composites. In addition, a series of experiments using Malachite Green (MG) was conducted to assess their sorption performance. XRD results confirmed that the two polymer molecules did not intercalate into the interlamellar spaces of the clay. FTIR results confirmed the presence of alginate (–OH and –COO− groups) and the silicate chain (–Si–O bond) in the solid. SEM images revealed that the MPHPC sorbents exhibited a highly porous structure. Comparisons based on the EDS and XRF results before and after the sorption of MG onto beads (S 2 and S 3 samples) show that both analyses established the presence of chlorine on the surface of the samples after MG sorption. A comparative study of kinetic models, including the Elovich model, the Weber–Morris (W–M) intraparticle diffusion model, the pseudo-first-order (PFO) model, and the pseudo-second-order (PSO) model, was performed. The kinetic results indicated that the MG sorption on the MPHPC sorbent was well described by the PSO model with sorption occurring through both surface sorption and intraparticle diffusion. Fitting the experimental MG sorption data to the Langmuir, Freundlich, Dubinin–Radushkevich (D–R), and Temkin isotherm models revealed that the Langmuir and Temkin models provided the best fit for the MPHPC matrix prepared with PVA. At the optimal pH of 8, the Langmuir isotherm results indicated maximum MG sorption capacities of approximately 130 and 182 mg g −1 for samples corresponding to MPHPC prepared with and without PVA, respectively. Studies on the reusability of MPHPC sorbents using 90% ethanol as a desorbing agent showed that they can be used for three cycles with an insignificant drop in removal efficiency. • Synthesis of new porous spherical hydrogel based on pillared-clay with macropore network. • Evaluation of the isotherm and kinetic parameters for the sorption process. • High MG sorption capacity around 182 mg g −1 . • MG sorption on MPHPC is modelled by pseudo-second-order kinetics and Langmuir or Temkin isotherms. • Under optimized conditions, 91% of MG is removed in aqueous solution.

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