Tea polyphenol reinforced polyacrylamide hydrogel as a regenerative adsorbent for low-concentration methylene blue

To remove the low-concentration methylene blue (MB) residues from liquid waste, we attempt to fabricate a hydrogel with good reusability and good removal efficiency. In this work, polyacrylamide (PAAm) hydrogel initially synthesized through polymerization and subsequently soaked in tea polyphenol (TP) aqueous solution to yield PAAm/TP hydrogels. The hydrogels were detected by Fourier transform infrared spectrometer, scanning electronic microscope, zeta electric potential instrument, full-automation surface area and porosity instrument, swelling degree, and universal testing machine. The results confirmed the successful preparation of PAAm/TP hydrogels with mesoporous structure. Compared to pure PAAm hydrogel, TP-reinforced PAAm hydrogel exhibited good mechanical properties and low swelling degree. Furthermore, PAAm/TP hydrogels were employed as an adsorbent for low-concentration MB owing to low specific surface area and mesoporosity. The effects of adsorption time, TP concentration, hydrogel level, starting MB level, pH value, and inorganic salts on MB adsorption were researched. PAAm/TP hydrogels had higher adsorption efficiency for low-concentration MB than pure PAAm hydrogel. The kinetic data of the PAAm/TP hydrogel coincided with the quasi-second-order model (R2 = 0.9997), and the adsorption rate was mainly governed by internal diffusion. The adsorption process highly followed the Freundlich isotherm model (R2 = 0.9755). Furthermore, the hydrogel demonstrated stability and excellent recyclability. Notably, the removal efficiency remained above 50 % even after five consecutive adsorption-desorption cycles. The mechanism analysis manifested that the MB adsorption was stimulated by electrostatic attraction, hydrogen bonding, and 7C-7C stacking interreactions. Hence, the prepared PAAm/TP hydrogels brought a green and effective method for removing low-concentration MB from wastewater.