Adsorption-desorption of copper(II) by temperature-sensitive nano-biochar@PNIPAM/alginate double-network composite hydrogel: Enhanced mechanisms and application potentials

Novel temperature-sensitive double-network composite hydrogels (denoted as nBC@PA) were prepared by copolymerization-crosslinking poly (N-isopropyl acrylamide), ion-crosslinking alginate-M2+ (M = Ca, Mg and Fe) and adding nano-biochar (nBC) as functional additive, which was further applied for treatment of simulated Cu(II)-containing water and remediation of Cu-contaminated sediments. Main factors including pre-swelling treatment, solution pH and temperature on adsorption kinetics while that of desorption agents, temperature (pretreating and simultaneous heating) on desorption kinetics were both explored in batch mode. Besides, the adsorption isotherms, salt effect and potential application of in-situ sediments remediation were further probed to evaluate the comprehensive performance of nBC@PA. Basic physicochemical properties, swelling features, SEM-EDS, XRD, FTIR, and XPS, etc. were applied for characterization analysis. The adsorption behavior of nBC@PA hydrogels to remove Cu(II) ions from water was evaluated by adsorption isotherms and kinetics. With the incorporation of nBC and increasing its contents, it could help to promote mechanical strength and adsorption performance to a certain extent. Tough the nBC@PA was not achieved enhanced adsorption from temperature responsive property, faster and more efficient desorption could be achieved. Preswelling and temperature pretreating could be an important supplementary method to enhance adsorption and desorption process, respectively. Under in-situ capping of nBC@PA crosslinked with Mg(II) rather than Ca(II) and Fe(II), Cu in sediment could be immobilized and reduced substantially. The temperature-sensitive nature of nBC@PA hydrogels determines their ability to perform different roles at specific temperatures, while the combination of high versatility, easy treatment and separation of hydrogels endows them considerable potential for industrial applications.