Preparation Of Microwave Aminated And Porous Lignin Microspheres And Study On Their Adsorption Performance For Pb²⁺

Water pollution caused by heavy metal ions is highly toxic and carcinogenic,extremely damaging the natural environment and endangering human health.Lignin is a natural amorphous polymer that has great application potential in the field of heavy metal wastewater pollution treatment.Due to its unique polyphenol structure,physicochemical properties,and abundance,a wide variety of adsorption materials can be potentially obtained from lignin.The research on the development of high-efficiency lignin-based adsorption materials is of great significance for realizing the high-value application of industrial lignin,protecting the environment,and maintaining the sustainable development of the ecosystem.At present,lignin-based adsorbents are mostly used as powders and fine particles,which are inconvenient to operate and difficult to recycle.Spherical adsorbent has a hydrophilic network,good hydraulic performance,and it is suitable for bed adsorption and seldom causes secondary pollution.Spherical lignin-based adsorbents are divided into gel type and porous type.Gel-type lignin microspheres have a small adsorption capacity and need to be chemically modified to increase the adsorption capacity,but they are basically broken after chemical modification due to weak swelling of the lignin microspheres.However,it is difficult to prepare porous lignin microspheres directly from water-soluble lignin,and it is easy to collapse during the pore-making process.In view of these two problems,this paper proposes two solutions to prepare lignin gel-type microspheres with both strength and high adsorption properties and high-strength,porous lignin microspheres for efficient adsorption of Pb²⁺.First,using industrial-grade sodium lignosulfonate as raw material,formaldehyde as cross-linking agent,and PEG-400 as reinforcing agent,the high-strength PLM microspheres were prepared by inverse suspension polymerization.Further,gel-type lignin microspheres A-PLM with good sphere strength,swelling performance and large adsorption capacity,was obtained by gentle and rapid microwave modification to introduce amine group.The introduction of PEG-400 can increase the strength of the lignin microspheres by 5 times,so as to withstand microwave modification.Adding 27 wt%anhydrous ethylenediamine solution to each gram of PLM microspheres and reacting at 70°C for 20 min in a microwave,the prepared A-PLM microspheres have the best adsorption performance with the nitrogen content is 1.07%.A-PLM microspheres have good spherical morphology,good strength,excellent thermal stability and acid and alkali resistance.The volume of A-PLM microspheres in water can swell 3.3 times that of PLM.A-PLM can remain intact spherical after 5 cycles of column adsorption.The adsorption performance and mechanism of Pb²⁺on A-PLM were systematically studied.The results show that the saturated adsorption capacity of A-PLM for Pb²⁺is 218.66mg/g,which is about 2.3 times that of PLM microspheres.The adsorption process of Pb²⁺by A-PLM is a monolayer adsorption,which conforms to pseudo-secondary adsorption kinetics and is controlled by chemical adsorption.On the basis of XPS characterizations,the mechanism for the Pb²⁺adsorption of A-PLM involves electrostatic attraction,ion exchange,and complexation.The dynamic column adsorption experiment proved that A-PLM microspheres can reduce the concentration of 50 mg/L Pb²⁺wastewater treatment to 0.01 mg/L,reaching the national drinking water standard.The microspheres are easy to desorb,and the adsorption performance does not decrease after 5 cycles.Secondly,using sodium lignosulfonate as the raw material,epichlorohydrin as the crosslinking agent,and PEA-230 as the chain extender,the inverse suspension polymerization method combined with freeze-drying technology was used to prepare a porous amine-based spherical lignin-based adsorbent（PALM）.After process optimization,60 wt%of sodium lignosulfonate concentration,5 m L of epichlorohydrin,10 wt%of polyetheramine,1 m L of ethanol,and 1 m L of 50 wt%Na OH solution were used to react at 90℃.The PALM microspheres with the best strength,high yield and N content of 1.67%can be prepared in 2 h.PALM microspheres have good spherical morphology,excellent acid and alkali resistance and thermal stability.The strength of PALM is twice that of PLM,and there is no obvious swelling in water,which is beneficial to storage and transportation.PALM microspheres are rich in super-large pore structure,with porosity and total pore area of 61.12%and 18.32 m²/g,respectively,which ensures that it has good adsorption capacity and recycling performance.The adsorption performance of porous PALM microspheres for Pb²⁺was systematically studied.The results show that the maximum adsorption capacity of porous PALM microspheres for Pb²⁺is 156.82 mg/g,which exceeds the unmodified PLM gel microspheres and many adsorption materials.The adsorption process of Pb²⁺by A-PLM microspheres is a monolayer adsorption,which conforms to pseudo-secondary adsorption kinetics and is controlled by chemical adsorption.On the basis of XPS characterizations,the mechanism for the Pb²⁺adsorption of PALM microspheres involves electrostatic attraction,ion exchange,and complexation.The dynamic column adsorption experiment proved that PALM microspheres can reduce the Pb²⁺wastewater treatment with a concentration of 50 mg/L to 0.98 mg/L,reaching the national industrial wastewater discharge standard.