Junqing Chen,Zede Yi,Shiyu Fu

State Key Laboratory of Pulp and Paper Engineering,South China University of Technology,Guangzhou,Guangdong Province,510640,China

Abstract:The fabrication of directionally driven oil-water separation materials has great significance for the removal of oil spills and organic pollutants.In this study,an oil-water separation aerogel capable of directionally adsorbing oil was designed using an anisotropic wood aerogel with a layered structure and a top-down fabrication strategy.Specifically,a magnetic wood-based superhydrophobic aerogel(methyltrimethoxysilane(MTMS)/Fe3O4 wood aerogel)was developed through the in situ coprecipitation of Fe3O4 nanoparticles and chemical vapor deposition.Owing to its highly porous structure,lipophilicity,hydrophobicity(water contact angle of 160°),and high compressibility,the MTMS/Fe3O4 wood aerogel exhibits excellent oil-water separation performance and compression cycle stability.Additionally,the Fe3O4 endows the material with excellent magnetic and photothermal conversion capabilities.These excellent properties make MTMS/Fe3O4 wood aerogel a promising recyclable and sustainable oil-water separation material.

Keywords:wood aerogel; superhydrophobic; magnetic; photothermal conversion;oil-water separation

1 Introduction

With the extensive development and utilization of petroleum resources by human beings,oil spills,organic liquid spills,industrial chemical spills,and discharges of oily domestic sewage seriously threaten the sustainable development of human life,local economies,and natural ecosystems[1-4].Therefore,the treatment of oil spill pollution and organic wastewater has attracted increasing attention.Various methods have been proposed to solve this problem,such as adsorption,microbial degradation,and catalysis[5].Adsorption is recognized as an ideal method owing to its simple operation,high efficiency,and low cost[6].Adsorbents can generally be classified into natural,inorganic,commercial,and synthetic adsorbents.Among them,natural adsorbents and inorganic adsorbents have poor adsorption effects on oil,whereas commercial adsorbents and synthetic adsorbents have poor environmental friendliness and are not easily degraded[5].Owing to their unique framework structures,high porosity,large specific surface areas,and hydrophobic and lipophilic surface properties,hydrophobic and lipophilic aerogels exhibit excellent oil absorption capacities[7].Carbon-based aerogels such as carbon nanotube sponges and carbon nanofibers have been widely used to adsorb various oils and organic solvents from wastewater[8].Carbon-based aerogels have desirable mechanical properties and a good adsorption effect,but their complex preparation processes and high costs obstruct their application as adsorbents[7].In addition,there are polydimethylsiloxane sponges,polyurethane sponges,etc.,but their mechanical properties are poor,and cannot meet the growing demands for sustainable green development[9-10]. Therefore, developing efficient and sustainable biomass-based oil-water separation materials is of great significance.

A large number of studies have been conducted to prepare aerogels from biomasses such as nanocellulose[11-13],bacterial cellulose[14],and pectin[15]for oil-water separation, but single-component materials such as cellulose usually require complex extraction processes.As a renewable resource,wood has a natural 3D porous structure,and can serve as an important structural basis for green functional materials.After lignin and hemicellulose are removed from natural wood,an anisotropic porous structure composed almost entirely of cellulose is obtained with a large specific surface area,high porosity,and high compressibility[16].At present,materials based on this top-down modification process have been widely used in oil-water separation[17],electromagnetic shielding[18],seawater desalination[19], and other fields[20-22].However,continuous separation is very important for oil-water separation materials.Fe3O4has a good photothermal conversion capability and excellent magnetic property[23].The introduction of Fe3O4into a material can endow the material with an excellent photothermal conversion capability and magnetic recyclability.The photothermal effect can accelerate the volatilization of oil,especially low-boiling-point oil, thereby achieving continuous oil-water separation.

In this study,we prepared a directional oil-absorbing aerogel based on the excellent porous structure of natural wood using a top-down strategy,and by loading Fe3O4nanoparticles.The detailed preparation process is shown in Fig.1.First,a mixture of sodium chlorite and glacial acetic acid was used to remove lignin and part of the hemicellulose from the wood.Further treatment with NaOH was performed to remove residual lignin and hemicellulose,so as to obtain a wood aerogel composed almost entirely of cellulose. A methyltrimethoxysilane(MTMS)/Fe3O4wood aerogel was then obtained byin situco-precipitation of the Fe3O4nanoparticles and chemical vapor deposition(CVD).The MTMS/Fe3O4wood aerogel retains the 3D porous structure of the natural wood well,and possesses high mechanical compressibility and a good elastic recovery rate.In addition,the aerogel also exhibits an excellent magnetic property,a photothermal conversion capability, superhydrophobicity, and lipophilicity.Under an external magnetic field,the MTMS/Fe3O4wood aerogel can be easily controlled by the magnetic field to move towards pollutants.The excellent magnetic property also makes the aerogel easy to recycle.Lastly,as a benefit from its excellent photothermal conversion capability,the MTMS/Fe3O4wood aerogel has the potential to provide sustained oil-water separation.This work further broadens the application of wood in the field of oil-water separation.

Fig.1 Schematic illustration of the preparation of MTMS/Fe3O4 wood aerogel

2 Experimental

2.1 Materials

Balsa wood was obtained from Guangzhou Qigao Light Wood Trading Co.,Ltd.The FeCl3·6H2O(≥99%)and FeSO4·7H2O(101%-102.8%)were supplied by the Guangzhou Chemical Reagent Factory.The sodium chlorite(80%)and MTMS (98%)were purchased from Aladdin Reagent Co.,Ltd.,China.NaOH (≥96%)was purchased from Guangzhou Dongzheng Chemical Glass Instrument Co.,Ltd.The n-Hexane(C6H14,≥99%)was supplied by Shanghai Runjie Chemical Reagent Co., Ltd., and the trichloromethane(CHCl3,≥99%)was purchased from Chengdu Kelon Chemical Co.,Ltd.The Sudan I was supplied by Tianjin Zhiyuan Chemical Reagent Co.,Ltd.The indigotin(C16H10O2N2,98%)was purchased from Shanghai Macklin Biochemical Co.,Ltd.All of the experimental water was deionized water.All chemicals were used without purification.

2.2 Preparation of MTMS/Fe3O4 wood aerogel

A previously reported method was used[19].Specifically,3 cm×3 cm×2 cm of balsa wood was delignified at 105℃for 9 h with a mixed solution of sodium chlorite and glacial acetic acid at pH value=4.6.Then,the residual hemicellulose in the samples was removed by treatment with 8%NaOH at 85℃for 4 h.After careful cleaning,the sample was pre-frozen and lyophilized in a freeze dryer for 48 h to obtain the wood aerogel.The wood aerogel was immersed in a mixed solution of FeCl3·6H2O and FeSO4·7H2O(molar ratio of Fe3+:Fe2+=1:1),and then was immersed in a 1 mmol/L NaOH solution.Next,the sample was washed with deionized water until reaching a neutral pH value,and was then lyophilized in a freeze dryer for 48 h to obtain the Fe3O4wood aerogel.Finally,the CVD of the Fe3O4wood aerogel was conducted using the MTMS.

2.3 Characterization

The morphologies and structures of the samples were characterized by a scanning electron microscope(SEM;Zeiss Merlin,Germany),and the distributions of MTMS and Fe3O4in the aerogel were characterized using an energy dispersive spectrometer(EDS).The Fourier transform infrared(FT-IR)spectroscopy was performed using an IR Thermo Fisher Scientific Nicolet IS50-Nicolet Continuum spectrometer.The Xray photoelectron spectroscopy(XPS)of each sample was carried out using the Thermo Fisher ESCALAB 250Xi.The room temperature magnetic properties of the samples were examined on a vibrating sample magnetometer(VSM;LakeShore7404,USA)under a maximum magnetic field of 30 kOe.The compression tests were performed on a universal material testing machine(INSTRON 5565,USA)at room temperature.The crystal structures of the samples were measured using an X-ray diffractometer(XRD;X'pert Powder,Germany).The water contact angle was measured with a contact angle meter(DCAT 21,Germany).The thermal stability of the samples was measured using a thermogravimetric analyzer(TG209F3,Germany).The sample was irradiated with an 808-nm near-infrared laser,and the temperature change on the surface of the sample was recorded by a Fluke thermal infrared imager(Ti400)to reflect its photothermal conversion capacity.The selective absorptions of floating oil and sinking oil by the MTMS/Fe3O4wood aerogel were conducted in a beaker.The reusability of the MTMS/Fe3O4wood aerogel was tested using a simple finger squeeze method.

2.4 Oil absorption experiments

The n-hexane,vacuum pump oil,and trichloromethane were chosen for evaluating the oil adsorption capability of the MTMS/Fe3O4wood aerogel at room temperature by using a method in a previous report[24].The mass absorption capacity was calculated as follows:

whereQWis the mass absorption capacity;m0andm1are the masses of the adsorbent before and after adsorption,respectively.The oil absorption tests were repeated three times.

3 Results and discussion

3.1 Characterization of MTMS/Fe3O4 wood aerogel

Cellulose,hemicellulose,and lignin are the main constituents of wood[17].The removal of lignin and hemicellulose through a top-down strategy results in an anisotropic porous cellulose skeleton,which is favorable for the infiltration of chemicals for valueadded applications of wood.FT-IR was used to characterize the compositional changes in the process.As can be seen in Fig.2(a),the removal of lignin can be confirmed by the disappearance of the characteristic peaks at 1493,1504,and 1595 cm-1[25].Notably,the characteristic peaks of hemicellulose at 1251 and 1737 cm-1are still present in the delignified wood[25].After further alkali treatment,the characteristic peaks of the hemicellulose disappeared,resulting in a wood aerogel composed almost entirely of cellulose.As shown in Fig.2(b),the results from XRD indicated that the cellulose and its arrangement did not change during the process.After loading the Fe3O4and a silanization modification,new characteristic peaks appeared at 2θ=30.3°,35.6°,43.3°,53.6°,57.4°,and 63.2°.They correspond to the(220),(311),(400),(422),(511),and(440)planes of Fe3O4,respectively[22].In the XPS analysis of the MTMS/Fe3O4wood aerogel(Fig.2(c)),the presence of Si 2p and Si 2s confirmed the success of the silanization modification[22].The appearance of the Fe 2p peak confirmed that the Fe3O4nanoparticles were successfully combined with the wood aerogel.According to Fig.2(d),the obtained MTMS/Fe3O4wood aerogel has a good magnetic property.With the applied magnetic field ranging from-30 to+30 kOe at room temperature,the sample exhibits a saturation magnetization(M)of 25.27 emu/g.The excellent magnetic property makes the MTMS/Fe3O4wood aerogel easily attracted by magnets.

Fig.2 (a)FT-IR spectra of different wood samples;(b)XRD patterns of different wood samples;(c)XPS survey spectrum of MTMS/Fe3O4 wood aerogel;(d)VSM curve of MTMS/Fe3O4 wood aerogel

The structure of the MTMS/Fe3O4wood aerogel was characterized by SEM,as shown in Fig.3.It can be seen from Fig.3(a)that the MTMS/Fe3O4wood aerogel inherits the honeycomb porous structure of natural wood.The Fe3O4nanoparticles were grownin situ,and attached to the cell wall and pores of the wood.Meanwhile,the fiber arrangement in the same direction as the wood growth can be clearly seen from Fig.3(b),indicating that the cell wall arrangement did not change during the entire chemical treatment process.It can be seen from the local enlargement of Fig.3(b)that the Fe3O4nanoparticles are uniformly attached to the fiber surface.Furthermore,Fig.3(c)and Fig.3(d)show the EDS elemental maps of a cross-section and longitudinal section of the MTMS/Fe3O4wood aerogel.The distribution of the Si and Fe elements further proves the successful introduction and uniform distribution of the Fe3O4nanoparticles and MTMS,thereby endowing the wood aerogel with excellent hydrophobicity and magnetic properties.

Fig.3 SEM images of MTMS/Fe3O4 wood aerogel:(a)cross-section;(b)longitudinal section.EDS elemental maps of MTMS/Fe3O4 wood aerogel:(c)cross-section;(d)longitudinal section

3.2 Mechanical and thermodynamic properties of MTMS/Fe3O4 wood aerogel

The material was designed for oil-water separation,so it was necessary to characterize its compressionresilience properties.As benefiting from the excellent porous structure of natural wood,the MTMS/Fe3O4wood aerogel shows an excellent elasticity perpendicular to the direction of the wood growth.We recorded the cyclic compression curves of a single MTMS/Fe3O4wood aerogel at a 40%strain and the stress-strain curves at different strains using a universal testing machine.The compression direction is shown in Fig.4(a).As shown in Fig.4(b),the compression curves of the MTMS/Fe3O4wood aerogel almost overlap after the tenth cycle,and the maximum pressure does not change significantly.In addition,the MTMS/Fe3O4wood aerogel has good stability without evident deformation during the whole process.Fig.4(c)shows the stress-strain curves of the MTMS/Fe3O4wood aerogel at strain values of 40%,60%,and 80%.Notably,even under 80%strain,the MTMS/Fe3O4wood aerogel can still recover without collapse.These results indicate that the MTMS/Fe3O4wood aerogel has excellent compression resilience and stability,which is beneficial to the removal of adsorbed oil during oilwater separation.

Fig.4 Mechanical properties of MTMS/Fe3O4 wood aerogel:(a)compression diagram;(b)stress-strain curves of MTMS/Fe3O4 wood aerogel under cyclic compression at a maximum strain of 40%;(c)stress-strain curves of MTMS/Fe3O4 wood aerogel at various strains

The thermodynamic properties of a wood aerogel and MTMS/Fe3O4wood aerogel are shown in Fig.5.In the TG analysis(TGA)curves of Fig.5(a),both samples have a period of mass loss at 100℃;this can be attributed to the adsorbed water in the samples.The wood aerogel is pyrolyzed at 230℃,and the mass loss is approximately 70%.The pyrolysis temperature of the MTMS/Fe3O4wood aerogel is approximately 260℃,and the thermal stability is slightly improved compared with that of the wood aerogel.It can be seen from the derivative thermogravimetric(DTG)curves in Fig.5(b)that the pyrolysis rate of the MTMS/Fe3O4wood aerogel is significantly lower than that of the wood aerogel.Moreover,Fig.5(c)shows the temperature change of the MTMS/Fe3O4wood aerogel under 808-nm near-infrared light irradiation. The Fe3O4nanoparticles endow the MTMS/Fe3O4wood aerogel with an excellent photothermal conversion capability,which can rapidly heat up and reach equilibrium in a short time.This characteristic is conducive to the volatilization of oil(especially low-boiling-point oil)and prevents the adsorption saturation of the MTMS/Fe3O4wood aerogel,which is expected to achieve continuous oil-water separation.

Fig.5 (a)TGA curves of wood aerogel and MTMS/Fe3O4 wood aerogel;(b)DTG curves of wood aerogel and MTMS/Fe3O4 wood aerogel;(c)thermal infrared imaging images of MTMS/Fe3O4 wood aerogel under 808-nm near-infrared light irradiation

3.3 Hydrophobic and lipophilic properties of MTMS/Fe3O4 wood aerogel

It is well-known that cellulose is amphiphilic,which greatly limits its application in the field of oil-water separation[24].The hydrophobicity and lipophilicity of a material are the keys to its application in oil-water separation[26].Fig.6(e)shows that the MTMS/Fe3O4wood aerogel has a water contact angle of 160°,indicating that it is superhydrophobic.As shown in Fig.6(a),the superhydrophobic MTMS/Fe3O4wood aerogel floats on the water surface,whereas the unmodified wood aerogel sinks underwater.As shown in Fig.6(b),the MTMS/Fe3O4wood aerogel immersed in water exhibits a mirrorlike surface,further illustrating its superhydrophobicity.As shown Fig.6(c),when the MTMS/Fe3O4wood aerogel was put into n-hexane,it quickly sank below the liquid surface.This was owing to the lack of surface tension in the n-hexane.When dyed alkaline solution(left),pure water(middle),and acid solution(right)were dropped on the surface of the MTMS/Fe3O4wood aerogel,the droplets stood on the aerogel,with spherical shapes(Fig.6(d)).This shows that the modified wood aerogel has excellent hydrophobicity under different pH conditions.Moreover,an MTMS/Fe3O4wood aerogel sample filled with n-hexane solution(stained with Sudan I)was squeezed by fingers,as shown in Fig.6(f).The excellent elasticity of the MTMS/Fe3O4wood aerogel allows for the n-hexane to be easily extruded,indicating that the material has good reusability.

Fig.6 (a)Photo of wood aerogel and MTMS/Fe3O4 wood aerogel in water;(b)photo of MTMS/Fe3O4 wood aerogel immersed in water;(c)photograph of MTMS/Fe3O4 wood aerogel in n-hexane;(d)photograph of dyed alkaline solution(left),pure water(middle),and acid solution(right)on the surface of MTMS/Fe3O4 wood aerogel;(e)water contact angle of MTMS/Fe3O4 wood aerogel;(f)picture of squeezing MTMS/Fe3O4 wood aerogel filled with n-hexane

3.4 Oil adsorption and oil-water separation

Benefiting from its highly porous structure,excellent mechanical properties,and superhydrophobicity,the MTMS/Fe3O4wood aerogel demonstrates great potential for the field of oil-water separation.As shown in Fig.7(a),when placed in a mixture of n-hexane(stained with Sudan I)and water,the MTMS/Fe3O4wood aerogel quickly absorbs the n-hexane completely.The same is true for heavy oil.As shown in Fig.7(b),the MTMS/Fe3O4wood aerogel can also absorb trichloromethane(stained with Sudan I)from the bottom.The excellent magnetism of Fe3O4endows the MTMS/Fe3O4wood aerogel with magnetism.Under an external magnetic field,the MTMS/Fe3O4wood aerogel is controlled by the magnetic field to move to the polluted area to remove pollutants(Fig.7(c)).In addition,the MTMS/Fe3O4wood aerogel with the adsorbed pollutants can be easily recycled after being attracted to a magnet.

Fig.7 Oil absorption performance of MTMS/Fe3O4 wood aerogel:(a)n-hexane;(b)trichloromethane;(c)MTMS/Fe3O4 wood aerogel removes n-hexane under magnetic field

The MTMS/Fe3O4wood aerogel was immobilized in a homemade separator to process the oil-water mixture,as shown in Fig.8(a).It can be seen that the water stained with the indigotin is filtered out,whereas the nhexane(stained with Sudan I)is adsorbed by the MTMS/Fe3O4wood aerogel.There is no n-hexane residue in the filtered water,indicating that the MTMS/Fe3O4wood aerogel has a good oil-water separation effect.In addition,the high porosity of the MTMS/Fe3O4wood aerogel provides enough space for it to adsorb the liquid.Fig.8(b)shows the mass adsorption capacity of the MTMS/Fe3O4wood aerogel for nhexane,tri-chloroform,and pump oil.The results show that the MTMS/Fe3O4wood aerogel has good mass adsorption capacity;the mass adsorption capacity can reach 6.2-12 times its own weight.In general,the MTMS/Fe3O4wood aerogel has good adsorption to oil,and has good application prospects for the field of oilwater separation.

4 Conclusions

In conclusion,we successfully constructed a magnetic superhydrophobic wood aerogel(methyltrimethoxysilane(MTMS)/Fe3O4wood aerogel)with an excellent oil-water separation performance using a top-down strategy.The obtained MTMS/Fe3O4aerogel inherits the excellent 3D network structure of natural wood with excellent compressibility and elasticity,and can be repeatedly extruded without structural damage.In view of the excellent mechanical properties as combined with its superhydrophobic and lipophilic properties,the MTMS/Fe3O4wood aerogel has an excellent oil absorption capability(in a range of 6.2-12 times its weight),and can be recycled using a simple mechanical extrusion.In addition,the Fe3O4provides the material with excellent magnetic and photothermal conversion capabilities.The MTMS/Fe3O4wood aerogel can be controlled to move towards pollutants using an applied magnetic field,and the photothermal effect provides the potential for continuous oil-water separation.These excellent properties make the MTMS/Fe3O4wood aerogel a good adsorbent for removing oil and organic pollutants.This strategy provides a pathway to constructing low-cost and sustainable materials for oilwater separation.

Acknowledgments

This work was supported by the National Natural Science Foundation of China(22078114),Natural Science Foundation of Guangdong Province(2021A151 5010360),and International Cooperation Project of National Key Research Program(2021YFE0104500).