A pH-sensitive composite hydrogel based on sodium alginate and medical stone: Synthesis, swelling, and heavy metal ions adsorption properties

New pH-sensitive composite hydrogels were synthesized by free-radical graft copolymerization among sodium alginate (NaAlg), sodium acrylate (NaA), and medical stone (MS). Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), tronsmission electron microscopy (TEM), and thermogravimetric analysis (TGA) analyses confirmed that NaA was grafted onto the NaAlg chains and MS participate in polymerization by its active silanol groups, and the surface morphologies and thermal stability was clearly improved after incorporating MS. The swelling capacity and rate of the hydrogel were clearly enhanced by introducing MS, and the intriguing deswelling in gelatin solution and overflowing behaviors in dimethyl sulfoxide (DMSO) and glycerin solutions were observed. In addition, the composite hydrogels exhibited excellent adsorption capacity on heavy metal ions, which enhanced the adsorption of Ni(2+), Cu(2+), Zn(2+) and Cd(2+) ions by 10.4, 8.0, 23.0, and 14.3 fold compared to active carbon (AC), and by 17.3, 16.0, 38.3 and 23.8 fold compared to MS, respectively. The biopolymer-based composite hydrogel can be used as a potential water-saving material and candidate of AC for heavy metal removal.