Solid polymer desiccants based on poly(acrylic acid-co-acrylamide) and Laponite RD: Adsorption isotherm and kinetics studies

This study investigates the potential application of superporous hydrogels (SPHs) and their composites with Laponite-RD (Lap-RD) for the adsorption of water vapors from humid air. Super-hydrogel composites (SPHCs) wee synthesized via gas blowing and foaming technique. Synthesized adsorbents were characterized using different techniques such as fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermos-gravimetric analysis (TGA). The incorporation of Lap-RD within the polymer matrix increased its thermal stability. Further, P(AA-co-AM) SPHs as well as SPHCs were used to adsorb water vapors from humid air. The adsorption of water vapors in both of these materials exhibited type-III isotherm with a maximum adsorption capacity of 1.03 and 1.24 g_w/g_ads for P(AA-co-AM) SPH and SPHC at 25 °C, respectively. Higher adsorption capacity of SPHC was due to the combined effects of capillary condensation and the presence of hydrophilic Lap-RD particles within the polymer matrix. Adsorption isotherm for both materials followed Guggenheim, Anderson and Boer (GAB) model which suggested that the main driving force for water vapors adsorption was capillary condensation. The adsorption capacity of both materials decreased with increasing temperature. Furthermore, the adsorption capacity for these materials was quite low in the relative humidity range of 20–50 % which increased abruptly with increasing the relative humidity above 50 %. The isosteric heat of water vapors adsorption for P(AA-co-AM) SPHs and SPHC were found to be in the range of 59−48 and 57−49 kJ/mol, respectively. Adsorption kinetics followed linear driving force model and the diffusion of water molecules followed non-Fickian type diffusion mechanism. Finally, the synthesized adsorbents showed very good reusable efficiency and used successfully for ten cycles of adsorption.