Biomass based hydrogel as an adsorbent for the fast removal of heavy metal ions from aqueous solutions

In order to achieve the reutilization of waste biomass soybean dregs, a low-cost hydrogel, soybean dregs-poly(acrylic acid) (SESD-PAA) was prepared through a one-step reaction. The synthesized polymer was verified by Fourier transform infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy. As an adsorbent, SESD-PAA was used to remove Zn(ii), Fe(iii), Cu(ii), and Cr(iii) from aqueous solutions. The effect of pH, initial concentration, and contact time on metal ion adsorption was examined. It was found that SESD-PAA showed high and fast adsorption capacities for Zn(ii), Fe(iii), Cu(ii), and Cr(iii), which were 121.2, 78.5, 75.4, and 41.7 mg g-1, respectively. Moreover, before and after the adsorption of these ions, the hydrogels were analyzed by X-ray photoelectron spectroscopy, which confirmed that the metal ions were adsorbed by the polymer by interactions of carboxyl, hydroxyl, and amine groups with metal ions. Additionally, the adsorption process followed a pseudo-second-order kinetics model, the Fickian diffusion model, and the Freundlich adsorption model. Furthermore, the competitive adsorption of metal ions in mixtures and the desorption/regeneration capacity of SESD-PAA were investigated. As an efficient, low-cost, easily obtained, and environmentally friendly adsorbent, SESD-PAA was expected to be used for the processing of large amounts of sewage.