| In recent years,heavy metal pollution caused by industrial wastewater discharge has become a prominent environmental problem of concerned to the whole society,which needs to be effectively controlled.At present,adsorption is the most common and effective method for heavy metal water pollution treatment,which is simple and flexible in operation and has a good effect on wastewater treatment.Further improvement of adsorption performance,environmental friendliness and multiple recycling of adsorbent materials is the main directions of adsorbent performance optimization.Because of their three-dimensional network structure,large specific surface area,and robust adsorption abilities,hydrogels are regarded as novel materials with considerable potential for the treatment of water-soluble contaminants.However,most of the existing hydrogels are constructed by synthetic polymers,or chemical crosslinking agents are introduced in the preparation process,which are not ideal in terms of green and environmental friendliness.In addition,if the hydrogels cannot be recycled after adsorption,secondary pollution will be caused to the environment.Therefore,it is of great significance to develop green,efficient,environmentally friendly and recyclable hydrogel adsorbents for the treatment of water pollution.The sodium alginate-based hydrogel,which has a high potential for use in the treatment of water pollution,has been shown to have good adsorption properties for pollutants like heavy metals or dyes.Its raw materials are inexpensive and simple to obtain,and its physical crosslinking is straightforward and easy to cary out.However,sodium alginate hydrogel has disadvantages such as low porosity.small specific surface area,high rigidity,brittleness and unsatisfactory mechanical properties.Combining sodium alginate with other green materials to improve the adsorption performance and mechanical stability of hydrogel is an effective way to solve the above bottleneck problems.Enteromorpha is a kind of large green algae.In recent years,the adverse effect of green tide caused by Enteromorpha on ecological environment has aroused wide attention.The growth control and utilization of Enteromorpha have become a research hotspot.Enteromorpha contains a large number of heteropolysaccharides with abundant sulfate and carboxylic acid groups and other active functional groups,which have good application prospects in the treatment of heavy metal pollution.However,the molecular structure of Enteromorpha polysaccharide also makes it difficult to cross-link and the stability of the prepared hydrogel is poor,its use in the remediation of environmental contamination is restricted.If the functionalized exploitation of Enteromorpha polysaccharide is realized,it is a significant technique to transform trash into treasure.In response to the issues mentioned above,this thesis prepared algal polysaccharide-based composite hydrogel beads with good mechancal properties by compounding sodium alginate(SA)and Enteromorplia polysaccharide(EP),using the molecular structural properties of both to complement each other and adopting the calcium ion external cross-linking method,and prepared magnetic composite hydrogel beads by introducing Fe3O4 nanopaiticles,optimized the preparation conditions and explored the mechanism of gel synthesis.The morphological structure,mechanical properties and thermal stability of the hydrogel beads were characterized,as wl1ile as the adsorption performance of Pb2+,Cu2+ and Cd2+ as representative heavy metal ions was tested and the mechanism was investigated.The results showed that the seaweed polysaccharide based composite hydrogel beads have excellent heavy metal adsorption properties and mechanical stability.Moreover,they can be recycled for multiple times and have good application prospects in heavy metal pollution treatment.This thesis is divided into two parts:(1)Preparation of SA/EP composite hydrogel bead and its adsorption propery for heavy metal ions.EP was extracted and combined with SA by microwave-assisted water extraction and alcohol precipitation.SA/EP composite hydrogel beads were prepared by external crosslinking of Ca2+.The Na+ on the G unit of SA molecular structure was replaced by Ca2+ and used as a cross-linking node to link the SA polysaccharide chains to form the backbone of the hydrogel,and EP was doped in the network to properly reduce the strength of the SA hydrogel.It not only improves the defects of SA hydrogels that are too dense and have low porosity,but also further optimizes the pore structure of SA hydrogels.At the same time,EP is rich in active functional groups such as sulfuric acid group and carboxyl group,which introduces more adsorption sites and further improves the adsorption performance of hydrogels for heavy metal ions.The preparation conditions were optimized.Under the conditions that the total concentration of SA and EP polysaccharides was 2 wt%,the ratio of them was 6:1,and the concentration of CaCl2 solution was 2 wt%,the prepared hydrogel beads had continuous and uniform porous structure good mechanical properties and efficient adsorption capacity of heavy metal ions.The equilibrium adsorption capacity of SA/EP composite hydrogel beads for Pb2+,Cu2+ and Cd2+ can reach 89.4 mg/g,68.9 mg/g and 79.6 mg/g.The prepared hydrogel beads of different sizes were filled into Fixed-bed hydrogel columns to verify the efficient treatment ability of SA/EP composite hydrogel beads for wastewater containing heavy metal ions.(2)Preparation of SA/EP@Fe3O4 magnetic seaweed polysaccharide composite hydrogel beads and their adsorption properties for heavy metal ions.By using the coprecipitation approach,Fe3O4 magnetic nanoparticles were created.The particle size was about 20 nm and the dispersion was uniform.Subsequently.SA/EP@Fe3O4 magnetic hydrogel beads were prepared by external crosslinking of Ca2+ with the aid of ultrasound under the above-mentioned optimized preparation conditions.The prepared hydrogel beads have superparamagnetic properties and can be separated and recovered under the action of external magnetic field.The results showed that the magnetic gel beads prepared under the condition of mass ratio of total polysaccharide to Fe3O4 nanoparticles was 6:1 maintained excellent adsorption capacity,and the equilibrium adsorption capacities of Pb2+,Cu2+ and Cd2+ reached 84.7 mg/g.66.6mg/g and 70.6 mg/g.In addition,it was found that the nanoparticles could disperse some of the stresses,which improved the mechanical properties of the composite hydrogel and maintained the structural integrity after three cycles of compression.The introduction of magnetic nanoparticles to construct magnetic composite hydrogel beads provides a new way to solve the problem of separation and recovery of hydrogel adsorbents.For SA/EP composite hydiogel beads and SA/EP@Fe3O4 magnetic hydrogel beads,the adsorption isotherms of Pb2+,Cu2+and Cd2+were measured,and the effects of pH value of solution,concentration of heavy metal ion solution,adsorption time and other factors on adsorption properties were explored.The adsorption kinetics mechanism was studied and the adsorption mechanism of heavy metal ions was revealed.Four cycles of adsorption experiments were carried out to determine the two kinds of hydrogel beads,and the results showed that the high efficiency of adsorption was still maintained after repeated use.In conclusion,green algal polysaccharide-based composite hydrogel beads with good mechanical properties and heavy metal adsorption were prepared in this paper by compounding two water-soluble heteropolysaccharides of algal origin,sodium alginate and Enteromorpha polysaccharide.This research work not only provides a new way for the functional utilization of seaweed polysaccharides,especially Enteromorpha polysaccharide,but also expands the research of cheap and efficient biomass adsorbent.The structure and adsorption mechanism of hydrogel beads were systematically discussed.The research results have both theoretical significance and application value. |
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