Preparation Of Gradient Polyelectrolyte Hydrogel And Its Performance And Mechanism Of Self-driven Removal Of Heavy Metal

In recent years,a wide variety of nanomaterials based on adsorption or advanced oxidation techniques have been widely used for the removal of heavy metal contaminants from water bodies.However,small-sized nanomaterials generally suffer from problems such as easy agglomeration and difficult recovery,and they are usually encapsulated in materials such as polymeric membranes,microspheres or hydrogel matrices.However,most of the matrix materials usually have problems such as ionic impermeability,partial shielding of active sites of nanomaterials and slow diffusion rate of contaminants.In this paper,a gradient chitosan(CS)hydrogel and a highly salt-tolerant gradient P(APTC-BZA)hydrogel were designed as efficient substrates for nanomaterials with the removal of pollutants such as Sb(V),As(V)and Cr(VI),respectively,under the induction of an applied electric field and unilateral UV light to address the above problems,with the counter ions on the gradient polyelectrolyte chain from the dense side of the gel to the sparse side and diffusion in the solution,electrostatic fields were formed at the interface and inside of the gradient hydrogel,respectively,which in turn promoted the mass transfer rate and removal efficiency of heavy metal ions.The main studies are as follows:(1)Natural polycationic CS and acrylamide(AM)were used as raw materials to prepare gradient CS/PAM hydrogels with gradient distribution of network chains by thermal polymerization under the induction of an applied electric field.The hydrogel crosssections were characterized by SEM,XPS and CLSM with gradient structure distribution.The gradient hydrogels were prepared at a CS concentration of 2% and a thickness of 2 mm with the best internal potential.As an example,thanks to the dual electrostatic field of the gradient PCS/AM hydrogel,it exhibited a high removal rate of Sb(V)at different CS mass fractions and Sb(V)initial concentrations.The gradient and homogeneous polyelectrolyte hydrogels and nonpolyelectrolyte hydrogels were used as efficient carriers of nanomaterials,respectively.Compared with the homogeneous polyelectrolyte hydrogels,the gradient polyelectrolyte hydrogels could reach adsorption equilibrium at 40 min,which was three times faster than the homogeneous polyelectrolyte hydrogels,and maintained the removal rate of 75% after five cycles.(2)To further solve the electrostatic shielding problem of traditional polyelectrolyte hydrogel in high ionic strength solution.A highly salt-tolerant gradient polyelectrolyte hydrogel was constructed based on the cation-π interaction.The salt-tolerant gradient P(APTC-BZA)hydrogel was prepared by copolymerization of benzyl acrylate(BZA)and3-(acrylamide propyl)trimethyl ammonium chloride(APTC)at an equimolar ratio under the induction of single-side UV light.It has the best internal potential when the polymerization time is 6 h,the gel thickness is 2 mm and the content of UV absorber is 3%,and it has stable Zeta potential and internal potential in high ionic strength solution.The gradient P(APTC-BZA)composite hydrogel loaded with the Ui O-66 MOFs showed 80%removal of Sb(V)in 500 m M Na Cl solution,which was more than five times higher than that of the P(APTC)composite hydrogel.The gradient P(APTC-BZA)composite hydrogel loaded with MIL-68 MOFs can still maintain good catalytic performance for Cr(VI)under high ionic strength,and can completely catalytic reduction of Cr(VI)within 40 min of light,and the catalytic efficiency is still above 90% after 5 cycles of experiments.