Research On Preparation And Adsorption/Desorption Properties Of Polyaspartic Acid/Lignocellulose Hydrogels

In recent years,high adsorbing hydrogels has attracted a great deal of attention in heavy metal wastewater treatment due to their high adsorption capacity,fast adsorption rate for heavy metal and good regeneration properties.The lignocellulose,polyaspartic acid were used as raw materials to prepare polyaspartic acid/lignocellulose（PASP/LNC）hydrogels through aqueous polymerization.The effect of the pretreatment time,the pretreatment temperature,the KMnO₄ concentration,the glutaraldehyde amount,the polyaspartic acid amount,the reaction time and the reaction temperature on swelling properties and adsorption properties for Pb²⁺,Cd²⁺ of the hydrogels were all evaluated,the optimum preparation conditions of the hydrogels were determined.The microstructure of the hydrogels were inspected and analyzed by means of scanning electron microscope（SEM）,X-ray diffraction（XRD）,Brunner-Emmet-Teller（BET）,thermogravimetric analysis（TGA）,infrared spectroscope（FTIR）and X-ray Photoelectron Spectroscopy（XPS）.The swelling properties of the hydrogels were investigated by changing the ionic strength,swelling time and swelling temperature,the adsorption characteristics of the hydrogels were investigated by changing the initial concentrations of solution,pH value,adsorption time and adsorption temperature.Further more,the adsorption kinetics and adsorption isotherm models of the hydrogels were determined and the adsorption mechanism of the hydrogels for Pb²⁺,Cd²⁺ was investigated.Subsequently,the desorption properties of the hydrogels were studied based on the best adsorption conditions,the best desorption conditions of the hydrogels were determined by changing the HNO₃ concentration,desorption temperature and desorption time.The main experimental results were as follows:1.Synthesis of PASP/LNC hydrogels and the swelling properties,adsorption properties of PASP/LNCPreparation results showed that,the swelling ratio of PASP/LNC was achieved 11.3;the adsorption capacity of PASP/LNC for Pb²⁺,Cd²⁺ was achieved 977.21mg/g、812.21mg/g,under the optimum preparation conditions,with the pretreatment time is 15min,the pretreatment temperature is 50℃,the KMnO₄ concentration is 0.06mol/L,the glutaraldehyde amount is 1g,the the polyaspartic acid amount is 11g,the reaction time is 3.5h and the reaction temperature is 70℃,respectively.The characterization results showed that polyaspartic acid has been copolymerized with lignocelluloses,obtained loose and porous PASP/LNC hydrogels,which is advantage to the swelling properties and the adsorption properties for Pb²⁺,Cd²⁺,the crystallinity of PASP/LNC has declined significantly,the thermal stability has improved significantly compared with lignocellulose.Swelling results showed that,the ionic strength is 0,the swelling time is 120min,the swelling temperature is 20℃,pH value of the solution is 11,the swelling ratio of PASP/LNC could achieved 11.9 under this condition.2.The adsorption/desorption properties of PASP/LNC for Pb²⁺,Cd²⁺Adsorption results showed that,the adsorption capacity of PASP/LNC hydrogels for Pb²⁺ could achieved 972.35mg/g after using initial concentration of solution 0.04mol/L,pH of the value is 5.5,being performed for 120min at 30℃;the adsorption capacity of PASP/LNC hydrogels for Cd2 could achieved 811.71mg/g after using initial concentration of solution 0.05mol/L,pH of the value is 5.5,being performed for 60min at 30℃.The adsorption processes of PASP/LNC for Pb²⁺,Cd²⁺ were better fitted by pseudo-second-order equation,the Langmuir equation.The desorption results showed that,the HNO₃ concentration is 0.04mol/L,desorption temperature is 30℃,the desorption time is 60min,the desorption capacity of Pb²⁺,Cd²⁺on PASP/LNC could achieved 907.68mg/g,741.22mg/g,the desorption efficiency could achieved 93.29%、91.36%,respectively.The adsorption/desorption experiment indicates that,the adsorption capacity of PASP/LNC hydrogels for Pb²⁺,Cd²⁺ could achieved 705.24mg/g,530.75mg/g for four cycles,respectively.The adsorption mechanism analysis results showed that,the surface of the PASP/LNC filled with granular attachments after adsorption,the mainly adsorption of active sites are O—H、-COOH of hydrogels for Pb²⁺,Cd²⁺,complexing adsorption and exchanging adsorption both exsit in adsorption process.3.The adsorption/desorption properties of PASP/LNC for mixed Pb²⁺,Cd²⁺The adsorption results showed that,the best adsorption capacities of PASP/LNC hydrogels for Pb²⁺,Cd²⁺could be reached 974.21mg/g,803.21mg/g in single metal ion solution;the best adsorption capacities of PASP/LNC hydrogels for Pb²⁺,Cd²⁺ could be reached 782.02mg/g、571.41mg/g,the adsorption capacities of PASP/LNC hydrogels for mixed metal ions are decreasing compared with the single metal ion.The equilibrium adsorption time of Pb²⁺ same as the single metal ion and the equibrium adsorption time of Cd²⁺ extends 30min compared with the single metal ion in mixed metal ion solution.The mutual inhibitory actions exist between Pb²⁺ and Cd²⁺ in mixed metal ion solution.The adsorption processes for mixed Pb²⁺,Cd²⁺ of PASP/LNC hydrogels were better fitted by the Langmuir equation,pseudo-second-order equation.The desorption results showed that,the HNO₃ concentration is 0.04mol/L,desorption temperature is 30℃,the desorption time is 60min,the desorption capality of mixed Pb²⁺,Cd²⁺ on PASP/LNC could reached 600.38mg/g、460.7mg/g,the desorption efficiency could achieved 76.77%、80.62%,respectively.The adsorption mechanism and adsorption selectivity analysis results showed that,the crystal texture of PASP/LNC is changed,the mainly adsorption of active sites are O—H、-COOH of hydrogels for mixed Pb²⁺,Cd²⁺,complexing adsorption and exchanging adsorption both exsit in adsorption process,PASP/LNC hydrogels show the excellent adsorption selectivity for Pb²⁺ compared with Cd²⁺.