Construction Of Multifunctional Nanocellulose/Exfoliated Bentonite-Based Aerogels And Their Mechanism Towards Efficient Oil-Water Separation

The discharge of oil-contained wastewater from industrial production and human daily life,as well as the frequent incidences of crude oil spills have caused great harm to water resources and posed a serious threat to ecological environment and human health.Among the treatment methods for oil pollution,adsorption is currently recognized as one of the most effective methods for treating oily wastewater.Therefore,the development of high-performance adsorbent materials for the rapid removal and recovery of oil spills is a simple and efficient method.Aerogels are a new type of porous and lightweight material,possessing high porosity,low density and large specific surface area,and have demonstrated great potential in the field of environmental remediation.Among them,nanocellulose aerogels,being biomass materials,present the characteristics of renewability and environmental friendliness.In addition,nanocellulose aerogels are inexpensive adsorbent materials,which could provide a broad application prospect in the field of oily wastewater adsorption.However,the currently reported nanocellulose aerogels shown demerits of having single function,poor mechanical properties,poor recycling performance and poor adsorption performance,which limit their further application for oil-water separation.Therefore,the functionalization of nanocellulose aerogels through chemical modification to make them more conducive to the practical treatment of oily wastewater is also a hot topic in current research.Aiming at the shortcomings of carboxylated cellulose nanofibers（CNF-C）based aerogel performance,the introduction of network structures like magnetic exfoliated bentonite/polyethyleneimine MBTex/PEI,coupling of novel exfoliated bentonite/titanium carbide（BTex/Ti₃C₂）functionally enhanced materials and synergism of the smart materials of poly（N-isopropylacrylamide）and BTex/Ti₃C₂ were adopted to improve the oil-water separation performance of CNF-C-based aerogel.Moreover,CNF-C/BTex-based aerogels with multiple functionalities,hydrophobicity,ultra-lightweight,good mechanical properties and good cycling performance were successfully obtained by combining chemical vapor deposition/solution immersion and freeze-drying techniques to achieve efficient and rapid clean-up of oily wastewater.The structural properties,oil-water separation performance,recycling and stability of CNF-C/BTex-based aerogels were thoroughly investigated systematically the research work,and the synthesis mechanism of CNF-C/BTex-based aerogels was clarified.Meanwhile,the effects of various extreme environmental factors on the wettability,oil-water separation performance and recycling performance of CNF-C/BTex-based aerogels were investigated,and the adsorption mechanism of CNF-C/BTex-based aerogels for oily wastewater were proposed.The detailed results of the investigation are as follows:（1）Magnetic exfoliated bentonite（MBTex）has good magnetic response properties,anti-mold behavior and extrusion resistance.Simultaneously,CNF-C could be cross-linked with polyethyleneimine（PEI）to form a porous network structure to prevent corrosion of magnetic media.A novel carboxycellulose nanofibre/polyethyleneimine/magnetic bentonite（CNF-C/MBTex/PEI）magnetically driven aerogel was synthesized via a simple hydrothermal and chemical vapor deposition（CVD）method based on the properties of MBTex and PEI.The characterization results confirmed that CNF-C and PEI were inserted into the surface of MBTex with a network structure,and CNF-C/PEI acted as the support,which successfully inhibited the oxidation and agglomeration of Fe₃O₄.CNF-C/MBTex/PEI possessed superhydrophilicity（contact angle up to 151°）and excellent mechanical properties（full resilience at80%pressure）.At the same time,BTex and Fe₃O₄ provided CNF-C/MBTex/PEI with good anti-mold properties（anti-mold efficiency exceeding 90%）and magnetic response,respectively.CNF-C/MBTex/PEI exhibited high adsorption performance for various oils with low-viscosity（adsorption capacity up to24.6～77.8 times of its original weight）.Additionally,CNF-C/MBTex/PEI showed good recycling performance and stability,and the adsorbed oil could be recovered by simple mechanical squeezing（adsorption ability remained more than 90%after 20 cycles）.Based on the XPS,XRD and FT-IR characterization results,oil-water separation process and recycling performance,the proposed path of CNF-C/MBTex/PEI aerogel interacting with oil was proposed for explaining the adsorption mechanism.（2）Compared with low-viscosity oils,high-viscosity oily wastewater with poor flowability could cause serious environmental pollution.Therefore,the rapid and effective removal and recovery of highly viscous oil remains a global problem.Presently,two-dimensional（2D）Ti₃C₂-MXene materials with high photothermal properties are attracting widespread attention in photothermally assisted crude oil wastewater treatment.Nevertheless,the poor stability of Ti₃C₂severely restricted its exploitation and application in the development of photothermal aerogels.In this study,cost-effective BTex/Ti₃C₂ co-adsorption components was constructed,which were synergistically introduced into the CNF-C solution.The process of CNF-C/BTex/Ti₃C₂ photothermal aerogel construction was accomplished by adopting a simple in-situ co-precipitation method coupled with microwave hydrothermal and solution impregnation processes.The results revealed that the introduction of BTex could effectively inhibit the oxidation of Ti₃C₂ during the preparation process,and ensure the formation of the aerogel,which synergistically improved the structural stability,mechanical properties and adsorption properties of the aerogel.At the same time,CNF-C/BTex/Ti₃C₂ displayed excellent structural stability,thermal stability,hydrophobicity（water contact angle of 149°）and mechanical properties（full recovery at 60%strain）,which facilitated the removal of oils with low-viscosity（the adsorption capacity reached as high as 46.1 to 90.2 times than its original weight）.Furthermore,the visible light absorption property of CNF-C/BTex/Ti₃C₂ was improved（99%）,and it exhibited excellent photothermal conversion efficiency（the surface temperature of CNF-C/BTex/Ti₃C₂ reached up to 75.2°C under 1 solar intensity irradiation）and possessed the thermal conductivity(104 mW·m^–1·K^–1).This enabled CNF-C/BTex/Ti₃C₂ aerogel to adsorb highly viscous crude oil(adsorption capacity up to 48.2 g·g^–1)more than48.2 times than its original weight within 25 s at 1 solar intensity radiation(1kW·m^–2).In addition,the enhanced adsorption mechanisms of CNF-C/BTex/Ti₃C₂ photothermal aerogel for low-viscosity oils and high-viscosity crude oils were further proposed.（3）Despite the significant effect of aerogel for oil-water separation,mechanical squeezing could cause damage to the pore size structure of regenerated aerogel.Therefore,the regeneration of aerogels after oil-water separation was a critical issue that limited the practical application of aerogels.In this work,a fast temperature-responsive carboxylated cellulose nanofibre/exfoliated bentonite/Ti₃C₂-poly（N-isopropylacrylamide）（CNF-C/BTex/Ti₃C₂-PNI）aerogel with switchable wettability was developed by synergistically introducing BTex,Ti₃C₂-Mxene,and thermo-responsive poly（N-isopropylacrylamide）（PNIPAAm）into CNF-C solution through a one-step hydrothermal and in-situ co-precipitation method.The smart response aerogel of CNF-C/BTex/Ti₃C₂-PNI was able to reversibly transform between hydrophobic/lipophilic and hydrophilic/lipophobic when the ambient temperature was above or below the lower critical solution temperature（LCST was 32°C）of PNIPAAm.The results confirmed that CNF-C/BTex/Ti₃C₂-PNI could adsorb oil within 5 s at the temperature of 45°C,and its adsorption capacity reached 54.3～98.4 g·g^–1.When the temperature was 25°C,CNF-C/BTex/Ti₃C₂-PNI was able to release the adsorbed oil within 8 min,and the recovery efficiency was greater than 78%.Moreover,the addition of BTex effectively prevented the oxidation of Ti₃C₂ under a high-temperature hydrothermal environment.Simultaneously,the mechanical properties（completely recoverable deformation at 60%strain）and thermal stability of CNF-C/BTex/Ti₃C₂-PNI smart response aerogel was synergistically improved by BTex/Ti₃C₂.The reduction in adsorption and desorption performance of CNF-C/BTex/Ti₃C₂-PNI for various oils was under 15%after 20 adsorption–desorption cycles.Finally,the synthesis and fast-thermo-responsive mechanism of CNF-C/BTex/Ti₃C₂-PNI aerogel was proposed.The multifunctional carrier loading,smart-responsive material construction and novel co-sorbent coupling strategy provided in this work can provide new ideas and theoretical guidance for the development of highly efficient and stable CNF-C/BTex-based aerogels,which could provide a theoretical basis and technological references for the rapid and efficient treatment of oily wastewater.