PVA hydrogel was crosslinked via trihydroxy melamine and epichlorohydrin, and β-cyclodextrin with strong adsorption capacity was added to remove the heavy metal ions. Results showed that the addition of trihydroxy melamine with 8%, the compressive strength of the hydrogel was increased by approximately 20%. The Langmuir isotherm model showed that the adsorption capacity of the hydrogel for Pb(II) and Ni(II) reached 505.9 mg/g and 286.7 mg/g, respectively, and the efficiency of removing the low-concentration heavy metal ions in water more than 99%.
The hydrogels were synthesized as follows: PVA was stirred and dissolved in deionized water at 90 °C to obtain a 90 g/L PVA solution. Then, 2 g of β-cyclodextrin, 2 mL of acrylic acide, and 2 g of acrylamide were added to 5 mL of deionized water, and the mixture was stirred to dissolve the compounds. The resulting solution was then added into 20 mL of the PVA solution. Methylbisacrylamide and potassium peroxodisulfate were added, and the mixture was stirred fully and evenly. Nitrogen was introduced into the mixing process to protect the mixed solution, and the hydrogel was obtained after 2 h at 60 ℃. Finally, with 20 mL of DMSO as the solvent, 4 mL of ECH was dissolved in DMSO, and then 2 g NaOH was added. Then, the hydrogel was immersed in the mixed solution and kept for 2 h at 60 °C to obtain hydrogels.
Inrteraction studies by XPS showed that a strong interaction exists between ‒COOH and ‒NH2 and the heavy metal ions. The binding ion exchange between these ions plays a dominant role, while electrostatic adsorption and hydrogen bonding play a secondary role. The adsorption mode of the amino group is mainly coordination adsorption, which can also explain the high adsorption capacity under low pH value. Even cyclodextrin has a role: β-cyclodextrin increased the maximum adsorption amount of the hydrogel more significantly, and the improvement compared to the hydrogel without cyclodextrin is 21.3% (Pb(Ⅱ)) and 27.3% (Ni(Ⅱ)) respectively
Wang, Z., Li, T.-T., Peng, H.-K., (…), Lou, C.-W., Lin, J.-H. (2021) Low-cost hydrogel adsorbent enhanced by trihydroxy melamine and β-cyclodextrin for the removal of Pb(II) and Ni(II) in water. Journal of Hazardous Materials 411,125029. https://doi.org/10.1016/j.jhazmat.2020.125029