Linker impact on the adsorption capacity of polyaspartate/montmorillonite composites towards methyl blue removal

- Preparation of Na+-Mt with poly(aspartate-co-succinimde) composite with two different linkers.
- The adsorption of methyl blue dye from aqueous solution onto the two composites was investigated.
- The equilibrium amount of adsorbed dye depends largely on the chemical composition of the linkers.
- The adsorption process is physical, endothermic, and spontaneous.

In this work, the effect of linkers such as alkyl quaternary ammonium and alkyl quaternary phosphonium salts intercalated with Na+-montmorillonite (Na+-Mt) via cation exchange followed by the reaction with poly(aspartate-co-succinimide) as biodegradable and superabsorbent polymer has been investigated. The prepared composites were spectroscopically characterized by elemental analysis, X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TAG), and Brunauer-Emmett-Teller (BET) surface analyzer. Polyaspartate-montmorillonite composite with quaternary phosphonium salt (Mt/IPS2) exhibited higher surface area and pore volume compared with polyaspartate-montmorillonite composite with quaternary ammonium (Mt/IPS1), due to its bulky structure. Mt/IPS2 composite displays a maximum adsorption capacity of 9.95 mmol/g for methyl blue (MB) dye which is larger than Mt/IPS1 composite with 8.55 mmol/g. The kinetic data of adsorption were evaluated with pseudo first-order, pseudo second-order and intra-particle diffusion models. The equilibrium time of 20 min toward MB adsorption and the pseudo-second-order was the best fit model for adsorption process. The adsorption isotherm of MB on the IPS1 and IPS2 has been investigated at 25, 30, and 35 °C and the data were in good agreement with the Langmuir isotherm model. Furthermore, the thermodynamic parameters namely; Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) were calculated and the negative ΔG° values indicate the spontaneity of the process. The adsorption mechanism was studied by FT-IR and the electrostatic interaction between cationic active sites of adsorbent surface and anionic MB dye was proposed.

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