Preparation Of Enteromorpha Polysaccharide-based Hydrogel And Its Application In The Treatment Of Heavy Metal Water Pollution

In recent years,with the rapid development of industry,the problem of water pollution,especially heavy metal pollution,needs to be repaired urgently.Adsorption is one of the most commonly used methods of treating heavy metal pollution due to its low cost,ease of operation,high efficiency and low secondary pollution.Hydrogels which has three-dimensional porous network,large surface area,abundant functional groups and good adaptability to water environment has great application potential in the treatment of heavy metal water pollution.Among them,hydrogels prepared using macromolecules of natural origin have important application prospects due to their green material origin and high environmental safety.Enteromorpha prolifera is a large green algae that has caused concern in recent years with outbreaks of growth on the Yellow Sea and other coasts.Due to the lack of effective ways to utilise it,a large amount of Enteromorpha prolifera is discarded as waste and decays,seriously affecting the marine ecosystem.It has been found that the polysaccharide content of Enteromorpha prolifera is over 60%and contains a large number of hydroxyl,sulphate and carboxyl groups,which are easy to modify and can interact with organic molecules and inorganic ions in a variety of ways.Therefore,the extraction of polysaccharides from Enteromorpha prolifera and their preparation into hydrogels for water pollution treatment not only exploits their natural advantages of abundant functional groups,but also has important environmental and economic significance as the material is green,low cost and more easily degradable.However,there are a large number of gaps in the research and application of Enteromorpha polysaccharides.The method and process of extraction of Enteromorpha polysaccharide is immature and its structure and composition are unclear;as a natural polysaccharide molecule,the preparation of hydrogel using it has the problem of poor mechanical stability,which cannot meet the demand of practical application.Based on the above research background and problems,this thesis uses Enteromorpha prolifera as raw material to extract the water-soluble Enteromorpha polysaccharide by an improved water extraction and alcoholic precipitation method,and analyses its structure and composition.The extracted Enteromorpha polysaccharide was then used to prepare a double network hydrogel,which is a low cost,environmentally friendly and has strong adsorption performance on heavy metal ions,as well as excellent mechanical stability,and has outstanding advantages and potential application prospects in the remediation of heavy metal pollution.Details are as follows.The first part is the extraction,process conditions optimization and characterization of Enteromorpha polysaccharides.The improved water extraction and alcohol precipitation method was used to extract Enteromorpha polysaccharides with good water solubility.The extraction conditions were optimized by response surface methodology(RSM)and the optimal extraction process conditions for Enteromorpha polysaccharides were determined.When the material-to-liquid ratio is 1:75,the extraction temperature is 105℃,the extraction time is 6 h and the concentration of H2O2is2 wt%,the Enteromorpha polysaccharide extraction rate can reach up to 49.6%.The characterisation and content analysis showed that the Enteromorpha polysaccharide is a heteropolysaccharide linked by β-glycosidic bonds and contains a large number of hydroxyl groups,sulphate groups and glyoxylates,with the sulphate and glyoxylate content being around 15.3%and 8.1%respectively.The second part is the preparation and performance characterization of Enteromorpha polysaccharide-based double network hydrogel.The first step was to cross-link the epichlorohydrin(ECH)with the Enteromorpha polysaccharide under alkaline conditions,followed by interpolation of the acrylic acid(AA)to obtain a Enteromorpha polysaccharide-based double network hydrogel.Hydrogels with different polysaccharide contents were prepared by adjusting the ratio of polysaccharide to acrylic acid to 1:1,2:1,3:1 and 4:1,respectively.The microstructure,composition and mechanical properties of the hydrogels were further analysed to investigate the cross-linking mechanism.Characterisation by FT-IR demonstrated that the hydrogels were cross-linked with ECH by ring opening of epoxy bonds and breaking of C-Cl bonds,and graft copolymerisation of AA by the formation of ether bonds.As the polysaccharide content increases,the cross-link density of the hydrogel increases and the pore structure becomes more continuous and homogeneous,which facilitates the diffusion of contaminant molecules in it;and as the polysaccharide content increases,the mechanical properties of the hydrogel are enhanced,with a maximum deformation rate of 95%.The third part is the exploration of the adsorption performance of Enteromorpha polysaccharide-based hydrogel and the discussion of the adsorption mechanism.Taking Pb2+,Cd2+,Cu2+as examples,the effects of pH and polysaccharide content on the adsorption performance of hydrogels were explored.The adsorption of heavy metal ions by the hydrogel gradually increased with the pH from low to high and then tended to equilibrium;the adsorption performance of the hydrogel increased with the increase of polysaccharide concentration,in which the adsorption performance of three metal ions:Pb2+>Cd2+>Cu2+,the maximum adsorption amounts were 60.6 mg/g,41.7 mg/g and 35.1 mg/g respectively.Fitting the experimental data using adsorption kinetic model,adsorption isotherm model and pore diffusion model shows that the adsorption of metal ions is mainly chemisorption and influenced by both membrane diffusion and pore diffusion processes.By XPS analysis,it was further demonstrated that the adsorption of heavy metal ions by the hydrogel relies mainly on its surface and inside active functional groups,through interactions such as electrostatic,chelation and ion exchange.In addition,the hydrogel has good recycling properties.In summary,this thesis used an improved water extraction and alcoholic precipitation method to extract the water-soluble polysaccharides from Enteromorpha prolifera and further used them to prepare a double network hydrogel.The hydrogel which has excellent mechanical properties and good adsorption performance,is low cost and easy to be degraded and has potential application prospects in the treatment of heavy metal pollution.