Synthesis of polyacrylamide-layered silicate nanocomposite hydrogels and their use in heavy metal removal

Hidrogels are a class of crosslinked polymer networks that are swollen in water without dissolving. These materials are of great importance since they can be used for large amounts of adsorption applications. One of these applicatons is their usage for water purification (trapping heavy metals or organotoxic materials). In this project, our objective was to enhance metal adsorption and swelling properties of polyacrylamide (PAAm) nanocomposite hydrogels by the immobilization of Spirulina microalg onto the clay to obtain an organophilic clay (Sp-MMT). Polyacrylamide (PAAm) nanocomposite hydrogels having Na-montmorillonite (NaMMT) and Spirulina modified MMT (Sp-MMT) clays in different loading degrees were prepared by free radical polymerization of acrylamide (AAm) in the presence of N,N-methylene bis-acrylamide (BAAm), as a crosslinker. The resultant nanocomposite hydrogels were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effect of increased nanoclay loading in thermal and morphological properties were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), respectively. Chromium metal adsorption capacity was determined with UV-VIS spectroscopy. All the Sp-MMT and NaMMT reinforced nanocomposite hydrogels showed higher swelling and metal adsorption capacity compared to neat PAAm hydrogel. PAAm nanocomposite hydrogel having 1wt% Sp-MMT clay was found to have desired exfoliated structure, maximum swelling, and improved thermal stability. This nanocomposite also showed maximum metal adsorption capacity which is about 312 % higher than that of neat PAAm hydrogel at the highest metal concentration. The presence of Spirulina's open network structure observed via SEM where all potential binding sites are under receptive position was found to be responsible for the dramatic increase in the metal adsorption and water absorption. In the literature it was reported that Spirulina and MMT clay by themselves can remove only about 4.80 % and 0.36 % Cr+6 ion, respectively, with respect to their weights. In this study, this removal was found to increase up to about 10000 % Cr6+ ion with respect to Spirulina's weight in H-1SpM nanocomposite hydrogel.