Study On Preparation Of Sodium Alginate/Polyvinyl Alcohol Double Network Hydrogel Composites And Adsorption Performance Of Heavy Metal Pollutants

In recent years,with the rapid development of modern industry,heavy metal ions frequently appear in all kinds of polluted water.Due to heavy metal ions are easy to accumulate in organisms,they pose a serious threat to human health.The adsorption method has high absorption efficiency and thorough removal for heavy metal ions.Hydrogel can be used as an ideal heavy metal adsorbent.Sodium alginate（SA）is a natural polysaccharide with good gel properties.However,the single alginate hydrogel has the disadvantages of low mechanical strength and low adsorption capacity.Polymer blending is an effective method for modification of sodium alginate.In many blending materials,polyvinyl alcohol（PVA）has attracted much attention due to its low cost,non-toxic and excellent mechanical properties.Blending SA and PVA into double crosslinked network hydrogel can improve the defects of single material,but its adsorption capacity for heavy metal ions is weak.Based on this,two kinds of double network composite gel beads,SA/PVA/CD and SA/PVA@CTS-CD,were prepared by means of SA and PVA as raw materials,Ca Cl₂ and H₃BO₃ as crosslinking agents,and cyclodextrin（CD）and chitosan（CTS）as the raw materials.,SEM-EDS,FTIR,XRD,BET and XPS were used to characterize the morphology,structure,element content,physical and chemical properties of the gel beads.The Pb²⁺and Cr（Ⅵ）were selected as the adsorption objects.The influence of the factors such as pH,dosage,ionic strength and reaction time on the adsorption properties were studied.The adsorption behavior and adsorption mechanism of gel beads were investigated by adsorption kinetics and isotherm model.The main research contents and conclusions are as follows:（1）SA/PVA/CD gel beads were prepared by ionic gel,condensation polymerization and multiple blending methods using SA,PVA and CD as raw materials and used to adsorb Pb²⁺in water.The optimum preparation conditions for SA/PVA/CD gel beads are 1%SA,3%PVA,3%CD and 3%Ca Cl₂/H₃BO₃ crosslinking agent concentration.The removal rate of 100 mg/L Pb²⁺was 95.6%when the gel beads was20 pH=5,1 g/L and 360 min.The adsorption process conforms to pseudo first-order and pseudo second-order kinetic models and Langmuir isotherm model.The adsorption process is dominated by chemical adsorption and physical adsorption,and the adsorption behavior is monolayer adsorption;At 40℃,the theoretical maximum adsorption capacity of Pb²⁺fitted by Langmuir isotherm model is 154.63 mg/g.Thermodynamic studies show that the adsorption process is spontaneous endothermic.After 5 cycles of adsorption,the removal rate of Pb²⁺from gel beads was 81.4%.The adsorption mechanisms of gel beads on Pb²⁺are mainly inclusion complexation and ion exchange.（2）SA,PVA,CTS and CD were used as raw materials to prepare SA/PVA@CTS-CD gel beads by ionic gel,condensation polymerization and embedding method,and used to adsorb Pb²⁺and Cr（Ⅵ）in water.The optimum preparation conditions for SA/PVA@CTS-CD gel beads were 1%SA,3%PVA,0.4 g CTS-CD and 3%Ca Cl₂/H₃BO₃ crosslinking agent concentration.The removal rate of 50 mg/L Pb²⁺was96.2%when the gel beads was 20 g/L,pH=5,1 g/L and adsorption time was 720 min.Under the conditions of 20℃,dosage of 1 g/L,pH=2 and adsorption time of 720 min,the removal rate of 50 mg/L Cr（Ⅵ）is 82.7%.In the composite system with initial concentration of Pb²⁺and Cr（Ⅵ）of 5-20 mg/L,the gel beads exhibited a competitive and synergistic effect on the simultaneous adsorption of Pb²⁺and Cr（Ⅵ）.The adsorption process of Pb²⁺and Cr（Ⅵ）conforms to the pseudo second-order kinetic model and Langmuir isothermal model.The adsorption process is mainly chemical adsorption,and the adsorption behavior is monolayer adsorption;At 40℃,the theoretical maximum adsorption capacities of Pb²⁺and Cr（Ⅵ）were 252.33 mg/g and97.63 mg/g respectively;Thermodynamic parameters show that the adsorption process is spontaneous endothermic;After 5 cycles of adsorption,the removal rates of gel beads to Pb²⁺and Cr（Ⅵ）were 75.6%and 50.9%respectively.The mechanism of gel beads removal of Pb²⁺in water is mainly inclusion interaction and cation-ion exchange.The adsorption mechanism of Cr（Ⅵ）is mainly inclusion complexation,reduction reaction,electrostatic adherence and ion exchange.