Adsorptive features of polyacrylamide--aluminosilicate composites for methylene blue

Polyacrylamide--montmorillonite (PAAm-Mt) and polyacrylamide--clinoptilolite (PAAm-Z) composites were prepared by cross-linked polymerization of acrylamide in dispersions of Mt and Z. The composites were characterized by FT-IR, SEM, XRD, CEC, and PZC analysis. The adsorptive features of PAAm-Mt and PAAm-Z for methylene blue (MB) were investigated by their relevance to initial dye concentration, temperature, contact time, PZC, and pH. The monolayer adsorption capacities were 0.14 and 0.08 mol kg⁻¹ for PAAm-Z and PAAm-Mt, respectively. The changes in pH of MB solution had an insignificant influence on their adsorption capacities. The values of enthalpy and entropy changes were positive (ΔHPAAm-Mt = 7.6 and ΔHPAAm-Z = 17.6 kJ mol⁻¹, ΔSPAAm-Mt = 55.7, and ΔSPAAm-Z = 107.2 J mol⁻¹ K⁻¹). The negative free enthalpy change was proof of the spontaneity of adsorption (ΔGPAAm-Mt = --9.0, ΔGPAAm-Z = --14.4 kJ mol⁻¹). The adsorption kinetics obeyed a pseudo-second order model (kPAAm-Mt = 11.5 and kPAAm-Z = 1.18 mol⁻¹ kg min⁻¹, R2\,> 0.99). The adsorption mechanism was the combination of chemical and physical adsorption governed by Coulombic forces (liquid film diffusion) on which the contribution of intraparticle diffusion was not significant. This was consistent with a low value of the adsorption activation energy (Eₐ = 9.4 kJ mol⁻¹). The composites had amphoteric features buffering the pH of the studied adsorption environment to approximately neutral levels.