Study On The Preparation Of Biomass Based Double Network Hydrogel And Their Adsorption Behavior For Heavy Metal Ions

Water pollution is one of the causes of water shortages.Among them,the pollution caused by heavy metal ions poses great danger to human health and ecosystem sustainability due to its bioaccumulation and non-degradability.So far,various techniques have been developed for the removal of heavy metal ions in wastewater,such as chemical precipitation,ion exchange,membrane separation and adsorption.Adsorption is generally considered as a cost-effective technology for wastewater treatment,and its economic benefits can be enhanced by the use of low cost,high performance and readily recyclable adsorbent.Biomass polymers(i.e.,chitosan,cellulose,lignin,starch,and humic substance)are a broad range of low-cost polymer resources that can be extracted from biomass waste.In addition,the biomass polymer is structurally stable and contains a variety of reactive functional groups,and can be used an ideal adsorbent.The double network hydrogel has the advantages of high water content,adjustable swelling property,good mechanical properties and easy modification.The main functions of buried water in the double network hydrogel are:(1)serving as supporting media and maintaining a certain shape,(2)providing transport nanochannels and facilitating the free diffusion of water soluble small molecules,and(3)providing hydrogen bonding and offering additional active sites for water soluble small molecules.Consequently,the double network hydrogel can be used as an ideal heavy metal adsorbent.In order to develop low-cost and high-efficiency adsorbent for heavy metal ions removal,a series of biomass based double network hydrogels were successfully prepared by selecting biomass polymer as a network and synthetic polymer as another network.The main work is as follows:(1)Chitosan(CTS)was selected as the biomass polymer network,and polyacrylamide(PAM)as the synthetic polymer network to prepare a chitosan/polyacrylamide double-network hydrogel(CTS/PAM)through one-step radical polymerization method.Ethylenediaminetetraacetic acid(EDTA)was used as the modifier and crosslinker of CTS,and contributed to the improvement of adsorption performance and mechanical strength of CTS/PAM gel.The results of the determination of carboxyl functional groups and the zero charge point as well as the Fourier infrared spectroscopy characterization indicated that the CTS/PAM gel was successfully modified by EDTA.EDTA-CTS/PAM gel exhibited high water content,strong mechanical properties,good solution and thermal stability.The results of adsorption kinetics and isotherm experiments showed that the adsorption kinetics equilibrium could reach in about 170 min,and the theoretical maximum adsorption capacities of Pb(?),Cu(?)and Cd(?)by EDTA-CTS/PAM gel at 298 K were 138.41,99.44 and 86.00 mg/g,respectively.Additionally,EDTA-CTS/PAM gel performed excellent reusability as well as good adsorption performance of various heavy metal ions in actual wastewater.(2)Cellulose derived from waste cotton fabrics(WCFs)was selected as the biomass polymer network,and polyacrylamide(PAM)as the synthetic polymer network to prepare a cellulose/polyacrylamide double-network hydrogel(Cellulose/PAM gel)through one-step radical polymerization method.To investigate the underpinned relation between the crosslinking agent amount of cellulose and structure as well as adsorption property of the obtained materials,a series of Cellulose/PAM gels were prepared by varying the amount of crosslinking agent.Scanning electron microscopy of Cellulose/PAM gel showed a three-dimensional porous structure,and the degree of sheet-like stacking of the cellulose network and the pore structure were related to the amount of crosslinking agent.The static adsorption experiment results showed that the adsorption performance of Cellulose/PAM gel decreased with the increased amount of crosslinking agent.The theoretical maximum adsorption capacity of Cellulose/PAM gel-1 for Pb(?),Cu(?)and Cd(?)were 382.80,138.90 and 198.48 mg/g,respectively,and the adsorption kinetics equilibrium could reach in 5 min.The dynamic experimental results of simulated industrial wastewater showed that the volume of wastewater could be concentrated from 6900 mL to 46 mL with a single fixed bed column equipped with Cellulose/PAM gel-3,and which could be concentrated from 13800 mL to 92 mL with tandem two fixed bed columns.Furthermore,the tandem two fixed bed columns could effectively treat the actual industrial wastewater,and the change in the adsorption amount was negligible after three adsorption-desorption cycles.(3)Humic substance(HS)was selected as the biomass polymer network,and polyacrylic acid(PAA)as the synthetic polymer network to prepare a polyacrylic acid/humic substance double network hydrogel(PAA/HS gel)through one-step radical polymerization method.PAA/HS gel had good stability in aqueous solution,and the humic substance dissolved from the network only when the pH was greater than 9.Moreover,the morphology and structure characterization showed the advantages of PAA/HS gel,such as three-dimensional macroporous network structure,high water content,large swelling property,high thermal stability and good mechanical properties,which was beneficial to the adsorption of heavy metal ions.The results of heavy metal ion adsorption in individual and multicomponent systems indicated that the theoretical maximum adsorption capacity of Pb(?),Cu(?)and Cd(?)in individual system by PAA/HS gel was 360.50,151.00 and 412.76 mg/g,respectively,while the theoretical adsorption capacity of each ion in multicomponent systems was lower than that in individual system.Noticeably,both interference and promotion of adsorption behaviors were identified in adsorption kinetic of multicomponent systems.The experimental data analysis and material characterization results showed that PAA/HS gel exhibited a heterogeneous surface and various types of active sites,and the interactions between functional groups in humic substance and heavy metal ions were different.(4)Lignin derived from rice husk(RH)was modified with chitosan(RH-CTS)and selected as the biomass polymer network,and polyacrylamide(PAM)as the synthetic polymer network to prepare a rice husk-chitosan/polyacrylamide double network hydrogel(RH-CTS/PAM gel)through one-step radical polymerization method.Fourier infrared spectroscopy characterization showed that lignin was successfully extracted from rice husk,and modified with chitosan to form a complex.The morphology and structure characterization showed that RH-CTS/PAM gel had the advantages of threedimensional macroporous network structure,high water content,large swelling property,good mechanical properties,high solution and thermal stability.The results of heavy metal ion adsorption in individual and multicomponent systems indicated that the theoretical maximum adsorption capacity of Pb(?),Cu(?)and Cd(?)in individual system by RH-CTS/PAM gel was 374.32,196.68 and 268.98 mg/g,respectively,while the adsorption capacity in multi-component systems was lower than that in individual system.RH-CTS/PAM gel exhibited ultra-fast adsorption rate in individual system,and the adsorption kinetic equilibrium of Pb(?),Cu(?)and Cd(?)at initial concentration of 200 mg/L could achieve within about 10 min.