Adsorption dynamics, diffusion and isotherm models of poly(NIPAm/LMSH) nanocomposite hydrogels for the removal of anionic dye Amaranth from an aqueous solution

• NPX hydrogel with less clay content is beneficial adsorption in Amaranth solution.
• Pseudo second-order model is applicable for the studied adsorption process.
• Film diffusion controls the adsorption rate, but not the only factor.
• D-R model reveals chemical adsorption type between NPX hydrogel and Amaranth.
• The prepared hydrogel is suitable for super-adsorbent to dispose real industrial waste.

To investigate diffusion model and adsorption mechanism of hydrogel for the removal of anionic dye Amaranth from aqueous solution, poly(N-isopropylacrylamide)(abbreviated as PNIPAm)/clay lithium magnesium silicate hydrate (abbreviated as LMSH) nanocomposite hydrogels with various clay percentage of LMSH/NIPAm (named as NPX hydrogel) were prepared. Adsorption kinetics of NPX samples were investigated under different experimental conditions (including solution pH, initial concentration of Amaranth dye solution and clay percentage of LMSH/NIPAm). Based on the adsorption experiments, adsorption dynamics, diffusion mechanisms and adsorption isotherms of NPX samples in Amaranth solution were analyzed through pseudo first-order model, pseudo second-order model, film diffusion model, intra-particle diffusion model and different isotherm models. The adsorption kinetics showed that in acidic (pH = 2) Amaranth solution NPX nanocomposite hydrogel was more effective for the removal of Amaranth dye. With increasing clay percentage of LMSH/NIPAm from 5 to 40 wt.%, adsorption capacity of Amaranth dye onto NPX samples decreased due to shrunk pore effect of NPX nanocomposite hydrogels and enhanced electrostatic repulsive interactions between NPX samples and Amaranth molecules. Adsorption dynamics indicated pseudo second-order model was more suitable for the investigated adsorption process. The adsorption rate-controlling step mainly came from film diffusion, but not the only factor. Adsorption isotherm indicated the adsorption process from monolayer adsorption to multilayer adsorption. According to Dubinin-Radushkevich isotherm model (abbreviated as D-R model), the calculated mean free energy implied chemisorption nature between NPX nanocomposite hydrogel and Amaranth. Based on adsorption mechanism of NPX nanocomposite hydrogel on Amaranth established, the conclusion confirms the prospect of NPX nanocomposite hydrogel as effective adsorbent to dispose real dyeing wastewater.

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