Preparation And Heavy Metal Adsorption Performance Study Of PH-responsive Lignin-based Hydrogel

Heavy metal pollution in water bodies is a serious ecological hazard,and there is an urgent need to research and develop green and low-cost environmentally friendly adsorbents.In this study,p H-responsive lignin hydrogels（（LS-co-AA）/MMT）were produced by graft copolymerisation of lignin and acrylic acid using sodium lignin sulfonate（LS）and montmorillonite（MMT）as the main raw materials and acrylic acid as the monomer,and then combined with montmorillonite intercalation to produce p H-responsive lignin hydrogels（（LS-co-AA）/MMT）,which were applied to the adsorption and desorption of heavy metal ions Cd（II）and Zn（II）.The p H response interval,structural changes and recycling performance of the lignin hydrogels were also analysed with a view to providing some practical value for the treatment of heavy metal ions in water by lignin hydrogels.The adsorption of two heavy metal ions,Cd（II）and Zn（II）,by lignin hydrogels was used as an indicator to study the optimal preparation process of lignin hydrogels.The results showed that the lignin-based hydrogel had the best adsorption effect on two metal ions,Cd（II）and Zn（II）,when the feeding ratio of sodium lignosulfonate and montmorillonite was 1:1,the neutralization degree of acrylic acid was 80%and the dosage of cross-linking agent was2wt%.The swelling rate of the hydrogel composites was only 18%when the p H of the aqueous solution was adjusted to 2.Later,the swelling rate of the hydrogel composites gradually increased with the gradual increase of the p H of the aqueous solution,and reached a maximum of 4400%at p H=6,indicating that the lignin-based hydrogel was responsive to the p H of the solution,and at the same time,the high swelling rate would make the hydrogel retain more metal ions.Analysis by transmission electron microscopy（TEM）,atomic force microscopy（AFM）,scanning electron microscopy（SEM）,infrared spectroscopy（FTIR）and X-ray（XRD）showed that after the graft copolymerisation reaction between sodium lignosulfonate and acrylic acid,the layered montmorillonite was uniformly dispersed in the p H-responsive hydrogel by intercalation compounding to produce p H-responsive lignin-based hydrogels.The adsorption,desorption and recycling properties of p H-responsive lignin-based hydrogels for Cd（II）ions were investigated by single-factor experiments.The experimental results showed that the maximum adsorption capacity reached 587.14 mg/g when the initial solution concentration of Cd（II）ion was 500 mg/L,p H=6.0 of Cd（II）ion solution,the adsorption temperature was 35℃and the adsorption time was 100 min.Under the conditions of the desorption solution HNO₃ concentration was 0.12 mol/L,the desorption temperature was 50℃and the time was 120 min,the best desorption rate reached 86.88%.The best desorption rate reached 86.88%.Among the four adsorption kinetic models,the quasi-secondary kinetic model had the highest linear correlation of 0.99939,indicating that the adsorption performance of the hydrogel was related to the initial concentration of the solution.in chemisorption as single molecular layer adsorption and filled adsorption.The adsorption,desorption and recycling properties of p H-responsive lignin-based hydrogels for Zn（II）ions were investigated by single-factor experiments.The experimental results showed that the maximum adsorption capacity was 285.39 mg/g when the initial solution concentration of Zn（II）ion was 600 mg/L,the p H of Zn（II）ion solution was 4.5,the adsorption temperature was 45℃and the adsorption time was 140 min.The best desorption rate was 74.39%when the desorption solution HNO₃ concentration was 0.1mol/L,the desorption temperature was 50℃and the time was 60 min.The best reached74.39%.The linear correlations of the four adsorption kinetic models were all greater than0.9,indicating that the adsorption process of Zn（II）ions by the hydrogel composites was not only related to the concentration of the solution;among the four isothermal adsorption models,the Langmuir model had the highest linear correlation,indicating that the adsorption of Zn（II）was a single molecular layer adsorption in chemisorption.