Absorption and adsorption studies of polyacrylamide/sodium alginate hydrogels

The aim of this study is to investigate the use of hydrogels as adsorbent in the removal of methylene blue (MB), a cationic dyestuff commonly used in industry, by adsorption from waste water. Therefore, sodium alginate-modified polyacrylamide-sodium alginate (PAAm/SA) hydrogels with the different concentration of sodium alginate were synthesized by free radical solution polymerization under normal atmospheric conditions. The absorbents were characterized using Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The absorption and adsorption experiments were performed and the effects of several experimental parameters including pH and contact time upon the adsorption process were evaluated. As a result of swelling tests, PAAm/SA hydrogels swelled in the range of 12.72–25.52 gwater gdry gel−1 in water, while PAAm hydrogel swelled to 8.5 gwater gdry gel−1. Methylene blue adsorption, which showed a rapid increase with increasing pH in the range of pH 2.0–6.0, reached the highest value at pH 7.0. Adsorption equilibrium was investigated by using Langmuir, Freundlich, and Temkin isotherm models. Two kinetic models, (i) pseudo-first-order and (ii) pseudo-second-order kinetic models, were applied to test the experimental data. Pseudo-second-order kinetic model well fitted the kinetic results, suggesting chemisorption as the most probable adsorption mechanism. Adsorption of methylene blue with hydrogels was well fitted with the Freundlich isotherm model, indicating the multilayer adsorption process on heterogeneous surfaces. The prepared hydrogels can be potential candidates for use as absorbents to treat wastewater.