Synthesis And Properties Of Functionalized Calcium Alginate-based Hydrogel Adsorbent

With the rapid development of human industrialization,large amounts of wastewater containing heavy metal ions has been generated.Unlike organic pollutants,heavy metal ions are non-degradable and easy to accumulate in organisms.They can enter the human body through the food chain,which severely endangers human health and even causes regional diseases.Therefore,how to quickly and effectively remove heavy metal ions in wastewater has attracted wide attention from researchers.At present,the methods commonly used to remove heavy metal ions in wastewater mainly include chemical precipitation,ion-exchange,electrochemical treatment,membrane technology and adsorption,among which adsorption is widely used to remove heavy metal ions in wastewater.Compared with other methods,the adsorption method has obvious advantages such as simple operation,high efficiency,and low energy requirement,and the adsorbent can be reused after elution treatment.However,the current adsorption materials have evident disadvantages such as small adsorption capacity,long adsorption time,and high cost of the adsorbents.Therefore,the development of low-cost adsorbents with large specific surface area,high density active sites,and high efficiency has become a research hotspot.Sodium alginate（SA）has the advantages of rich resources,hydrophilicity,non-toxicity,and biodegradability and has been widely used in food production.In its structure,there exist a large number of carboxyl and hydroxyl functional groups,which have good binding ability with heavy metal ions.Consequently,SA has attracted great attention in the field of wastewater treatment containing heavy metal ions.In the reported literatures,alginate adsorption materials have the following problems: 1.The preparation of calcium alginate（CA）adsorbents with three-dimensional（3D）network structure need to use divalent cations as a crosslinker or foreign objects as a skeleton;2.The sizes of CA adsorbents reported in the literatures are mostly at the micron and millimeter levels and their functions are relatively single,only using for adsorption.Therefore,preparing multifunctional CA hydrogel adsorbents with 3D network nanostructure is the key to improve their practicality and broaden their using range.In this work,SA is used as raw material to prepare CA adsorbent with 3D network nanostructure by simple and green route.In order to study its adsorption performance and adsorption mechanism,heavy metal ions Cd²⁺ and Cu²⁺ are selected as inorganic pollutant models in aqueous solution.At the same time,on the basis of such 3D network nanostructure,CA hydrogel is functionalized by adding different inorganic nanoparticles（NPs）: 1.by adding Fe₃O₄ and Ag NPs,CA hydrogel is endowed with magnetic separation function and antibacterial function;2.through coating a layer of Si O2 shell on the surface of Fe₃O₄ NPs,the magnetic separation function is reserved;at the same time,the existence of Si O2 shell can protect the stability of Fe₃O₄ NPs during cyclic regeneration,while the hydroxyl group on the surface of Si O2 can combine with heavy metal cations to improve the adsorption performance.The specific work is as follows:（1）By a simple and green synthesis route,3D network nanostructured sodium alginate（NNSA）hydrogel adsorbent is prepared by using SA as raw material,without the introduction of divalent cations and other skeleton supports.With the aid of cross-linking of Ca²⁺,calcium alginate（NNCA）hydrogel adsorbent（32.343 m2 g-1）with the same morphology is obtained,and its stability in aqueous solution is evidently improved.The effects of p H value,contact time,initial concentration,adsorbent dosage and temperature on the adsorption properties of Cd²⁺ and Cu²⁺ are investigated.The results show that NNCA hydrogel adsorbent has faster adsorption rate and small adsorbent dosage.Compared with adsorbents of the same kind,NNCA hydrogel adsorbent show high removal efficiency.The adsorption behavior of Cd²⁺ and Cu²⁺ onto NNCA hydrogel matches well with Freundlich isothermal adsorption model and Pseudo-second-order kinetic model.Comparative studies show that the adsorption performance of the synthesized NNCA hydrogel adsorbent is superior to that of commercial SA.（2）Functionalization of NNCA hydrogel: Firstly,Fe₃O₄ NPs is synthesized by co-precipitation method,and then dispersed in the formed NNCA hydrogel.Finally,Ag NPs are synthesized through in-situ reduction method.Subsequently,adsorption performance,adsorption mechanism,cyclic performance,antibacterial performance and magnetic property of the synthesized Fe₃O₄/Ag-NNCA hydrogel adsorbent are studied.The results show that Fe₃O₄/Ag-NNCA hydrogel adsorbent（65.564 m2 g-1）with large specific surface area has the high removal efficiencies > 95 % for Cd²⁺ and > 89 % for Cu²⁺,and can be quickly separated from the solution by external magnetic field.The study on regeneration performance shows that after five cycles,the removal efficiency of Cd²⁺ and Cu²⁺ is still over 53 % and 68 %,respectively.The antibacterial properties of Fe₃O₄/Ag-NNCA hydrogel are tested by Escherichia coli（E.coli）and Staphylococcus aureus（S.aureus）.The results display that Fe₃O₄/Ag-NNCA hydrogel adsorbent has good inhibitory effects on two common bacteria.（3）Based on NNCA and Fe₃O₄/Ag-NNCA,a layer of Si O2 shell is coated on the surface of Fe₃O₄ NPs,thus obtaining Fe₃O₄@Si O2-NNCA hydrogel adsorbent functionalized by the core-shell NPs.Subsequently,the corresponding adsorption property,adsorption mechanism,magnetic property and cyclic performance for Fe₃O₄@Si O2-NNCA hydrogel adsorbent are studied.The results show that Fe₃O₄@Si O2-NNCA hydrogel adsorbent has higher removal efficiency for Cd²⁺ and Cu²⁺,which can reach 97 %.The study on the cyclic performance reveals that after ten cycles,the removal efficiencies of the regenerated Fe₃O₄/Ag-NNCA for Cd²⁺ and Cu²⁺ still remain above 86 %,showing a good regeneration ability.