Preparation Of Wood Aerogel-Based Functional Films And Their Adsorption Performance On Heavy Metal Ions

In recent years,with the development of society,heavy metal wastewater poses a serious threat to the ecological environment and human health.Since the traditional methods of heavy metal wastewater treatment have disadvantages such as low efficiency,secondary pollution and slow adsorption rate,the search for new adsorbent materials has become a research direction.Nanocellulose is widely used in the preparation of adsorbents due to its advantages of large specific surface area,high mechanical strength and biodegradability.However,the preparation of nanocellulose has high energy consumption,complicated process and high production cost.Wood,as a natural renewable porous material,has the characteristics of easy modification and abundant sources,and at the same time,its multi-level layered pore structure facilitates the flow of water.Therefore,in this study,wood aerogels with rich pore structure and high specific surface area were prepared based on the formation of cellulose microfibril network structure on the cell wall of wood by using TEMPO oxidation system after delignification of wood,and the densified TEMPO oxidized wood aerogel film and acrylic acid polymerization were prepared by densification and in situ polymerization of AA initiated by UV light,respectively.TEMPO oxidized wood aerogel films,the specific results of the study are as follows:（1）Wood aerogel（TDWA-N）was prepared from natural balsa wood by removing lignin and part of hemicellulose from the wood by acidic Na Cl O₂ solution,and then using TEMPO/Na Cl O/Na Br oxidation system.The effects of time variables on the structure and chemical composition of TEMPO oxidized wood aerogels were discussed in terms of microstructure and chemical composition during the TEMPO oxidation process with TEMPO oxidation time as the variable.The results showed that the carboxyl group content of the aerogel increased with the increase of TEMPO oxidation time,reaching the maximum of 1.21 mmol/g when the oxidation time was5 h.However,when the TEMPO oxidation time exceeded 5 h,the cellular structure of the wood surface began to collapse due to the degradation of cellulose,making it difficult to form a complete aerogel.At the same time,the hydrogen bond structure was destroyed due to the enhanced effect of electrostatic repulsion between carboxyl groups,and the originally dense cell wall was decomposed into single cellulose microfibrils,forming a cellulose microfibril network structure;the chemical components of wood aerogels did not change significantly,while the crystallinity increased due to TEMPO oxidation occurring first in the amorphous region of cellulose.wood aerogels with TEMPO oxidation time of 5 h showed the largest specific surface area of 15 m² g^-1,which was twelve times higher than that of delignified wood(1.26 m² g^-1)and much higher than that of natural wood.（2）The aerogel prepared in the first part（TDWA-5）was used as the substrate,and the TEMPO oxidized 5 h aerogel was pressed into densified TEMPO oxidized wood aerogel film by hot pressing at different ratios,and the static adsorption properties of the TEMPO oxidized wood aerogel film were tested,and the microstructure,film performance parameters and dynamic adsorption performance were investigated,while three layers of TDWAM and two layers of TDWAM-15 were assembled in the filter model,which can effectively remove Cu（II）ions from water.The results are as follows:the adsorption capacity of TEMPO oxidized wood aerogel for Cu（II）ions can reach 115 mg/g,and its adsorption process is in accordance with the proposed second-order kinetic model,and the adsorbent surface is homogeneous and belongs to monomolecular layer adsorption,while the study of its adsorption isotherm shows that it is in accordance with the Langmuir adsorption isotherm model,and the adsorption process is chemisorption.With the increase of densification ratio,the densification degree of aerogel also increased,and the original aerogel structure was destroyed to some extent,and changed from the original porous structure of TDWAM to the lamellar structure of TDWAM-N.When the compression rate was 20%,the cell interstices of TDWAM-20 completely disappeared,and the pore size of the duct was obviously reduced.At the same time,its average pore size was also reduced,from 47.52?m to 26.25?m in TDWAM-20,and the flux was also reduced from 2000L h^-1 m^-2 in TDWAM to 926 L h^-1 m^-2 in TDWAM-20,with more dispersed TDWAM cell wall structure connections and the porosity was higher.After densification,the porosity decreased from 97.5%to 79.87%due to the reduction of the gap between the layers in the cell wall.Meanwhile,as the membrane thickness increased,the membrane provided more adsorption sites and took longer to reach penetration,and the penetration curve became smoother.For the 5 mm aerogel membrane,the penetration curve became flatter and the depletion time became longer as the densification degree increased from 0%to 15%,from 200 min to 350 min.The penetration curve changed significantly under different influent flow conditions.At higher flow rates,the mass transfer rate of heavy metal ions from liquid phase to solid phase is accelerated,the contact time between the solution and the membrane becomes shorter,the slope of the penetration curve increases,and the membrane penetrates more easily.When the initial concentration of Cu（II）in the feed water increases,the mass transfer driving force increases,and the high concentration difference accelerates the transfer rate of metal ions from the liquid phase to the membrane interior,leading to a decrease in the penetration time,the membrane quickly reaches adsorption saturation,and the slope of the penetration curve becomes larger and steeper.Comparing the removal efficiency of Cu²⁺by three-layer TDWAM and two-layer TDWAM-15 membranes,the removal efficiency of the filter for copper ions increased from 35%for single-layer TDWAM to100%for three-layer.The removal efficiency of the single-layer TDWAM-15 is 52%,which is greater than that of the single-layer TDWAM,while the removal efficiency of the two-layer TDWAM-15 alone can reach that of the three-layer TDWAM filter.The decrease in treatment efficiency after cycling of the two-layer TDWAM-15 filter was greatly reduced compared to the uncompacted one,indicating that the compacting treatment effectively improved the cycling stability of TDWAM-15.（3）Using TEMPO oxidized wood aerogel membranes as substrates,after introducing unsaturated double bonds using methacryloyl chloride fumigation,in situ polymerization of acrylic acid（AA）monomers inside the aerogel membranes was triggered by UV light irradiation to form polyacrylic acid（PAA）,introducing more carboxyl groups,increasing the adsorption sites while reducing the flux of the membranes and increasing the contact time between heavy metal ions and the adsorption sites.In the study,acrylic acid polymerized TEMPO oxidized wood aerogel membranes（TDWAM-N PAA）were prepared using different AA concentrations as the study variables.Meanwhile,the effects of different AA concentrations in internal polymerization on the microstructure and chemical components of the aerogel membranes were further compared,and their static and dynamic adsorption properties were investigated.The results showed that as the AA concentration increased,the flux of the membrane decreased,the membrane provided more adsorption sites,the time required to reach penetration was longer,and the penetration curve became flatter.With TDWAM-1.5 PAA as the study object,as the flow rate increases,the mass transfer rate of metal ions from the solution to the interior of the material accelerates,the contact time between metal ions and adsorption sites becomes shorter,the penetration curve becomes steeper,and the membrane penetrates more easily.When the initial concentration of Cu²⁺in the feed water increases,the mass transfer driving force increases,leading to a decrease in the penetration time,the membrane quickly reaches adsorption saturation,the slope of the penetration curve becomes larger and steeper,and the concentration difference accelerates the metal ion transfer rate from the liquid phase to the interior of the membrane.The number of membrane layers has a significant effect on t_e,which increases with the number of layers,and the rate of increase of metal ion concentration in the effluent is faster when the number of layers is smaller.