Preparation Of Lignin-based Hydrogels And Their Adsorption To Heavy Metals

When the heavy metals entering the ecological environment system exceed the environmental capacity,it may cause ecological damage or environmental pollution.Biomass is introduced into the preparation of environmentally functional materials hydrogels.Lignin-based rich functional groups such as-COOH,-OH,and C=O are used to improve the adsorption and removal of heavy metal ions,which is helpful for the treatment of heavy metal pollution.In this study,lignin was extracted from waste secondary mulberry branches,and grafted intercalation was used to prepare lignin-based hydrogels（referred to as hydrogels）.Hydrogels were used to simulate the simulation of industrial wastewater and heavy metal ions in soil.The preparation of material hydrogel and its application in the treatment of heavy metal pollution provide theoretical support.In order to obtain a hydrogel with excellent performance,in this paper,the Mn²⁺adsorption amount was used as the evaluation index.The lignin quality,montmorillonite quality,and crosslinker quality were selected as the factors for consideration.The uniform formulation method was used to optimize the preparation of a suitable hydrogel;The morphology and phase structure of the prepared hydrogel were characterized by SEM and FTIR analysis.The swelling rate and water retention of the hydrogel were measured,and the hydrogel adsorption performance and its application to the adsorption of Ni2⁺in electroplating wastewater were investigated.The hydrogel grafting was speculated.Intercalation preparation mechanism.In order to find out the adsorption and action mechanism of lignin-based hydrogels on heavy metal ions in water,based on the hydrogel’s adsorption time,temperature,and solution pH value of Mn²⁺and Pb²⁺in simulated wastewater,an isotherm and thermodynamic study was carried out.Investigate the physicochemical characteristics and reaction mechanism of heavy metal ions adsorbed by hydrogels;Kinetics was used to reveal the rate controlling factors for the adsorption of heavy metals by hydrogels,and the diffusion model was used to analyze the adsorption process of Mn²⁺and Pb²⁺by hydrogels.In order to test the effect of hydrogel on the adsorption of heavy metal ions in soil,the effect of concentration,pH value and leaching time on the removal of heavy metal in soil was investigated by liquid hydrogel leaching method.The kinetic parameters of Mn,Cu and Cd were obtained,and the morphological changes of Mn,Cu and Cd in leaching process were revealed.The results are as follows:（1）Optimum composition and ratio of hydrogel obtained by uniform design method are:lignin mass is 0.7000g,montmorillonite mass is 0.1470g,crosslinker mass is 3.9730g;The fitted equation R²=0.998,F=283.82>F（4,2）=19.3,the model is available;The interaction between lignin and montmorillonite had a significant effect on Mn²⁺adsorption by hydrogel.The SEM showed the honeycomb porous structure of the hydrogel.FTIR showed that the surface of the hydrogel was rich in-OH,C=O,-NH₂and other groups.The non-conjugated C=O at 1642cm^-1 and the benzene ring skeleton at 1446cm^-1.The vibrational absorption peak was a characteristic lignin peak.The non-conjugated C-N at 1123cm^-1 coincided with Si-O at 1010cm^-1,indicating that the lignin-grafted polymer intercalates montmorillonite.（2）The hydrogel prepared under this suitable composition can absorb 60 minutes of water and the swelling ratio can reach 60%,and the water retention rate of natural air-drying for 70 hours can reach 20%.The initial mass concentration of Mn²⁺,Zn²⁺,and Pb²⁺solutions of 320mg?L^-1,160mg?L^-1,and 1000mg?L^-1 were 720min,and the adsorption amounts reached 77.09mg?g^-1,73.95mg?g^-1,and 383.90mg?g^-1,respectively.The removal rates were 48.18%,92.41%,and 38.39%respectively.The electroplating wastewater containing Ni²⁺with an initial concentration of 54.56mg?L^-1 was adsorbed.The Ni²⁺adsorption amount reached 14.64 mg?g^-1,and the removal rate reached 53.65%,these show good adsorption properties.（3）Studies on the isothermal adsorption model of Mn²⁺and Pb²⁺in wastewater prepared by hydrogels show that the adsorption of Mn²⁺by hydrogel conforms to the Langmuir isothermal adsorption model and belongs to monolayer adsorption.The maximum adsorption amounts qmax and K_L increase with increasing temperature;Hydrogel adsorption of Pb²⁺satisfies Freundlich’s isothermal adsorption model and Temkin’s isothermal adsorption model.Its adsorption is multi-molecular layer heterogeneous surface adsorption,and K_F and n values decrease with increasing temperature,indicating that effective adsorption of Pb²⁺decreases with temperature.（4）Thermodynamic analysis showed that the process of preparing hydrogel to adsorb Mn²⁺was a spontaneous endothermic reaction,and the process of adsorbing Pb²⁺was a spontaneous exothermic reaction.The quasi-second-order kinetic model showed that the adsorption of Mn²⁺and Pb²⁺by hydrogel was mainly chemical,and the reaction rate K₂ decreased as the initial mass concentrations of Mn²⁺and Pb²⁺increase.The intra-particle diffusion model showed that the adsorption of metal ions on hydrogels was divided into three stages.Coordination,ion exchange,and hydrogen bonding were the factors that affected the adsorption rate of Mn²⁺and Pb²⁺on hydrogels.（5）Hydrogel eluent for leaching of heavy metals in soil.When the volume concentration of the hydrogel eluent was 34%,the pH was 3.0,and the leaching time was 6 hours,the initial mass concentration in the soil was 18.11mg?L^-1,0.16mg?L^-1,0.067mg?L^-1,Cu,Cd removal rates can reach 64.92%,34.83%,24.51%.The removal of Mn,Cu,and Cd in the hydrogel eluent is mainly based on the iron-manganese oxide combination,and the Mn and Cu leaching kinetics meet the Elovich equation,and the Cd leaching kinetics meet the double constant equation.The morphological properties and content of heavy metals affect the removal rate of heavy metals.