Hydrogel of HEMA, NVP, and Morpholine-Derivative Copolymer for Sulfate Ion Adsorption: Behaviors and Mechanisms

SO(4)(2−)-containing compounds are widely present in wastewater generated from various industries and mining industries, such as slag leachate, pulp and paper wastewater, modified starch wastewater, etc. When the concentration of SO(4)(2−) is too high, it will not only be corrosive to metal equipment but also accumulate in the environmental media. Based on this, a novel cationic hydrogel HNM was synthesized in this study by introducing morpholine groups into the conventional hydrogel HEMA–NVP system for the adsorption of SO(4)(2−) in aqueous solutions. Characterizations by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) indicated that morpholine groups had been introduced into the as-synthesizedhydrogels. The scanning electron microscope (SEM) characterization results show that the introduction of morpholine groups changed the surface of the hydrogel from micron-scale wrinkles to nanoscale gaps, increasing the contact area with the solution. The results of static water contact angle (WCA), equilibrium water content (EWC), and SO(4)(2−) adsorption capacity show that the introduction of morpholine groups not only further improved the equilibrium water content and hydrophilicity of the hydrogel but also greatly improved the SO(4)(2−) adsorption capacity of the hydrogel, with the maximum SO(4)(2−) adsorption amount of 21.59 mg/g, which was much higher than that of the hydrogel without morpholine groups of 5.15 mg/g. Further studies found that the adsorption of SO(4)(2−) on the hydrogel HNM was pH-dependent, and acidic conditions were favorable for the adsorption. Therefore, the introduction of morpholine groups greatly enhanced the ability of conventional HEMA–NVP hydrogels to remove SO(4)(2−) from aqueous solutions.