Biomass Based Polymer Functional Materials:Preparation Andadsorbing Performance Research

Water pollution has become an urgent task to be solved on the way to construct ecological civilization of China.Among various water pollutants,heavy metal has become the most prominent one.Heavy metal ions are cumulative,persistent,difficult to degrade and toxic.Moreover,it is a serious threat to the ecological environment and human health.Of the multiple water treatment technologies,adsorption technology has become one of the simplest and most efficient ways to remove water pollutants.As the key of adsorption technology,the adsorption materials are various in kind,shape,and performance at present.In practical account,adsorbents such as activated carbon,clay minerals,resins etc.have the disadvantages of not renewable,poor adsorption performance,difficult to separate the solid-liquid phase and the limited reuse performance.In addition,on account of the specificity of the heavy metals and the complexity of the environment,certain functional adsorbents are able to achieve a variety of heavy metals removal in aqueous phase.Therefore,the design and preparation of specific functional and highly effective adsorbents are of great significance,as well as the hot spots and difficulty ofresearches currently.Researches on natural biomass adsorbents for heavy metal removalhave been quite popular recently.Biomass is the most abundant material on Earth,withsignificant advantages ofvarious sources of raw materials,low cost,easy to degrade etc.In general,it is an environmental friendly material.Biomass substrate possesses a unique natural structure and chemical reactivity,and it shows a good adsorption-desorption property after being modified.Therefore,this paper aims to synthesize ahigh value-added biomass based adsorbent for the removal of metal ions and agricultural and forestry wastes in different forms of water.Based on the structure and properties of biomass raw materials,the characteristics of heavy metal pollutants,adopting the methods of photocatalytic grafting,thermal initiated chemical grafting and free radical chain transfer grafting etc.,the bio-based adsorbents with different adsorbing functions have been designed,prepared and systematically investigated and evaluated.The synthesis of bio-based adsorbents has overcome the disadvantages of low adsorption capacity of biomass raw materials,the poor reusability of powder adsorbents,and theunstable property of the acrylic polymer,and provides a universal preparation method for the functionalization of the biomass material as the environmental adsorption material.The specific work and innovative achievements of this research are as follows:?1?By adopting the method of UV photocatalytic grafting for the surface modification of biomass materials,and utilizing the three-dimensional structure of sponge and the outstanding adsorption performance of polyacrylic acid brushes,the efficient adsorbent of carboxyl loofah fiber?LF @ PAA?was obtained for the first time,and its adsorption performance of Cd???was also studied at the same time.Under the irradiation,photosensitizer BPtakes away hydrogen from the surface of the biomass substrate toform macromolecular free radicals and then lead to the grafting reaction with acrylic acid.The technology of ultraviolet photocatalytic grafting can evenly graft polyacrylic acid on the natural three-dimensional sponge-like skeleton of loofah and can open the channels between the fiber bundles to a certain degree.The optimum conditions of synthesizing LF@PAA were as follows: NaOH concentration of 6.0%,photosensitizer concentration of 0.4%,monomer concentration of 8.0%,UV lamp power of 100 W,and reaction time of 120 min.The FE-SEM picture shows that the surface of the LF @ PAA becomes rough and has a pleural morphology and is porous in the cross section.All these clearly show that there is a uniform polymer layergrafting on its surface,and there is an increase amount of Na on the surface.FT-IR results at 3337 cm^-1,2892 cm^-1,1638 cm^-1,895 cm^-1 all demonstrate the characteristic adsorption peaks of polyacrylic acid.XPS results of the LF@PAA shows characteristic peaks of Na 1s and Na KLL at 1071 eV,and its energy spectra of C 1s have also changed significantly.The adsorption capacity of modified bio-based adsorbent was greatly enhanced,the specific adsorption amount of Cd???of LF@PAA increased from 24.1 mg/g to 320.0 mg/g,and it was found that the adsorption capacity increased as the pH increased,the adsorption capacity between pH 4 to pH 7 could be more than 315.0 mg/g.Coexisting ions of Na⁺ and K⁺ have obvious effects on the adsorption of Cd???by LF@PAA,the order of their influence is: Na⁺> K⁺.Temperature has a positive effect on the adsorbing process of LF@PAA.The processes of adsorption of Cd???by LF@PAA first undergoes a membrane diffusion controlled velocity stage with a high adsorption rate,and then a relatively slow intra-particle diffusion controlled velocity phase until equilibrium.The thermodynamics shows that ?H⁰> 0,?S⁰> 0,?G⁰< 0 and ?G⁰ decreases with the increase of temperature,the adsorption of adsorbate on the adsorbent is a spontaneous process,and a higher temperature is favorable for adsorption,the adsorption process will increase the disorder degree of the system.At the same time,LF@PAA has a stable adsorption performance and good application stability.After 8 consecutive reusability experiments,it still has fine adsorption performance of Cd???.Dynamic adsorption studies have shown that the higher the filled column,the longer the penetration time and the bigger the column volume;the lower the Cd???concentration in the original water,the slower the penetration process,the bigger amount of treated water;the slower the original water run through the filled column,the better performance of the treatment.The adsorption of Cd???by LF@PAA is in accordance with Langmuir adsorption isotherm model and pseudo-second order kinetic model(the process can be regarded as a monomolecular layerof adsorbate adsorbed onto the surface of adsorbent,and the adsorption rate is proportional to the square of the driving force?qe-qt?of the adsorption process.The mechanism of LF@PAA adsorbing Cd???is presented as follows: carboxyl groupon the surface of adsorbent isnegatively charged,heavy metal cationsare electrostatically attracted and then ion exchanged withNa⁺ on carboxyl groups,form a stable coordination structure,and achieve the adsorption of heavy metal ions by LF@PAA.?2?Using the method of chemical coprecipitation and etherification of biomass,combining the advantages of easy separation of magnetic materials and the good adsorption properties of chemically modified biomass materials,the magnetic anionic loofah adsorbent has been prepared.This method can endow high efficient adsorption performance of natural biomass materials as well as achieve the good solid-liquid separation.LF-MA is prepared by loading magnetic Fe₃O₄ on the surface of the biomass fiber under the action of hydrogen bonds,hydroxyl groups and van der Waals forces.Epoxy cellulose ethers are prepared by nucleophilic substitution of the H+of hydroxyl groups on cellulose with epoxypropane,this is for the introduction of amine functional groups.The epoxy cellulose ethers is then etherified with biomass amine reagents and introduce thehighly efficient adsorbing quaternary amine groups onto the surface of biomass substrate.The optimum conditions for the preparation of LF-MA are presented as follows: biomass raw material of 0.2 g,solvent DMF of 12 m L,epichlorohydrin of 4.0 m L,ethylenediamine of 1.5 mL,temperature of etherification at 85 ?,etherification time of 60 min,crosslinking reaction temperature at 85 ?,crosslinking time of 60 min,triethylamine of 3.0 mL,grafting temperature at 85 ?,grafting time of 120 min.The amine polymer layer encapsulates the magnetic particles can effectively overcome the problem of the magnetic adsorbent'seasy falling off in magnetic fluid and its poor stability etc.The modified LF-MA fibers were uniform Ly thickened,the color became darker,the surface had obvious glass luster and obviously uniform smooth polymer bulge.The element composition of the surface changed,with 5.11% of nitrogen,1.04% of Cl and 14.53% of Fe.The FT-IR absorption peaks were significantly changed at 1328 cm^-1,1649cm^-1,1560cm^-1,605cm^-1,3402cm^-1.The XPS results of the binding energy appeared at 399.0 eVwas amine nitrogen peak of-NH,and 400.8 eV the quaternary ammonium nitrogen peak of-N⁺,all these formed a functional group similar to the commercial resin: N+?CH₂CH₃?Cl^-.The maximum adsorption capacity of LF-MA to Cr???was 344.4 mg/g,which was 17.12 times higher than that of the unmodified adsorbent.Its adsorption capacity was decreased as the pHincreased,and the adsorption effect of dichromate ion was the best under acidic condition.Adsorption is a spontaneous process,and a relatively higher temperature can benefit this process.The adsorbing process is in accordance with Langmuir adsorption isotherm model and pseudo-second order kinetic model(the process can be regarded as a monomolecular layerof adsorbate adsorbed onto the surface of adsorbent,and the adsorption rate is proportional to the square of the driving force?qe-qt?of the adsorption process.The adsorption can be divided into three steps: first,the adsorbate diffuses from the solution to the surrounding or outer surface of the adsorbent particles,this is a relatively faster process;then the adsorbate diffuses into the adsorbent and the adsorption rate becomes slower;the adsorption sites are gradually run out and the boundary layer is thickened until equilibrium is reached.In the process of adsorption,?H⁰> 0,?S⁰> 0,?G⁰< 0 and ?G⁰ decrease with the increase of temperature,the adsorption will increase the disorder degree of the system.The mechanism of LF-MA adsorbingCr???can be divided into the following three stages: First,under the electrostatic attraction,Cr???of anionic form diffuses to the adsorption site,and is ion exchanged with the Cl-on the surface of LF-MA;Secondly,Cr???immobilized on the surface of the adsorbent is reduced to Cr???under acidic conditions;Finally,Cr???and the negatively charged-OH,-COOH,-OCH₃ on biomass surface is electrostatically attracted and formed a stable complex.?3?The ATRP initiator was "anchored" on the substrate by the chemical bonding of the ATRP initiator and the natural groups on the surface of the substrate in view of the physical and chemical characteristics of the biomass and the functional polymer brush.Then,ATRP technique was used to "target grafted" methacrylic acid onto the surface of the substrate and prepared the polymer brush modified loofah adsorbent LF-g-MAA.In addition,the enrichment and recovery of rare earth ions were studied.The conditions for single factor optimization of LF-g-MAA are listed as follows:LF&Br addition amount of 0.2 g,monomer volume of 60 m L,monomer mass fraction of 40.0%,catalytic system mass addition ratio [PMDETA]:[CuBr]:[CuBr₂] of 1.5:1:0.1 at 60 ? for 12 h.The results showed polymer brush modified loofah sampleswere uniform Ly thickened,there were obviously and uniform Ly smooth polymer bulge but originalloofah fiber morphology on the surface of LF-g-MAAcould still be observed.The amount of Na on its surface was significantly increased while Br decreased.The FT-IR results demonstrated stretching vibration of C=Ogroup,stretching vibration and the symmetric stretching vibration of the-COO-grouprespectively at 1694cm^-1,1482cm^-1,1145cm^-1.The values of zeta potential of the samples wereboth negativebefore and after the modification,and the absolute value of the Zeta potential of LF-g-MAA was larger than that of LF.The grafted polymer layer can improve the thermal stability of the biomass material.The adsorption capacity of La???by LF-g-MAA increased from 73.21 mg/g to 351.38 mg/g after modification,its adsorption capacity increased with the increase of pH,and the adsorption capacity maintained at about 330.0 mg/gfrompH 5 to pH 7.The degree of influence of coexisting ions to the adsorption of La???by LF-g-MAA was Ca²⁺> Mg²⁺> K⁺> Na⁺.The order of adsorption capacity of LF-g-MAA to different rare earth ions was Dy???> Eu???> Ce???>La???>Y???,the adsorption rate was Y???>La???>Eu???.Raising the temperature will help the adsorbing of La???by LF-g-MAA,and the adsorbing process is in fine accordance with Langmuir adsorption isotherm model and pseudo-second-order kinetic model.The study of internal diffusion model shows that the adsorption process first undergoes the membrane diffusion controlled velocity stage with faster adsorbing rate,and then undergoes the sloweradsorbing rate of the intra-particlediffusion until equilibrium is achieved.Adsorption is a spontaneous process,and the disorder degree of system increases after adsorption.After 6 times of reusability,LF-g-MAA had a decreased adsorption capacity of La???,which was only 6.02% lower than that of the original maximum adsorption capacity.At the same time,the functional polymer brusheswere firmLy chemically bonded with the substrate.Of all the adsorption-desorption cycles,the desorption rate remained more than 95.6% which allowed to effectively enrich and recover rare earth ions.The adsorbed LF-g-MAA&La had good stability in phosphorus solution at different pH,and its adsorption capacity of phosphorus was above 68.00 mg/g at pH 4-9.The mechanism of LF-g-MAAadsorbing La???is mainly ion exchange.First,under the attraction of electrostatic,the rareearth ions in the solution diffuse to the surface of the adsorbent,then ion exchange with the monovalent metal ions on the carboxyl group,and finally form a monodentate or tridentate chelate structure with the carboxyl group.The removal of phosphorus by LF-g-MAA&La is achieved by the electrostatic attraction between rare earth ions La and phosphate and interaction between phosphate and Lewis acid-base.