Preparation Of Lignin-based Micro-nanospheres And Its Application For Cr(Ⅵ) Adsorption

With the increasing severity of environmental and energy pollution,the exploitation and utilization of lignin materials with extensive sources and excellent properties have been paid increasing attention in both industrial and academic worlds.So far,there is still a lack of relevant research in the field of high-value utilization such as micro-nanotechnology,and thus,the application value of lignin micro-nanomaterials is highly worth exploring.In recent years,the removal technology of hexavalent chromium(Cr(VI))pollution in water needs to be improved urgently.Among various technologies,adsorption has unique advantages of low cost and ease of handling because no additional equipment is required.Consequently,it is important to design and create an adsorbent with economic,environmental friendly and excellent adsorption effect.In this study,three lignins were extracted from the kraft pulping black liquor by acid fractionation,and micro-nanospheres were prepared respectively.Furthermore,the optimal raw material was selected and compounded with graphene oxide(GO)to prepare lignin-graphene oxide composite micro-nanospheres(LGNs),which were further applied to the adsorption of Cr(VI).This article reports a facile self-assembly strategy for preparing super long-term stable hollow and solid micro-nanospheres based on lignin fractionation.We found that different lignins obtained at different p Hs during fractionation can form spheres with different particle sizes and structures.And the prepared micro-nanospheres have excellent stability after being stored for 90 days or under a wide range of p H 3～12.The self-assembled and formation mechanisms of the micro-nanospheres were surveyed by dynamic light scattering(DLS),elemental analysis,transmission electron microscopy(TEM),functional group contents,Fourier transform infrared spectroscopy(FT-IR)and two-dimensional heteronuclear single quantum coherence(2D HSQC)NMR.The analysis results showed that the hydrophobicity and the intermolecularπ-πinteraction play a decisive effect in the formation of micro-nanospheres.Through analyzing the mechanism,it provides theoretical support for the structure and particle size control of lignin micro-nanomaterials.However,single lignin shows unsatisfied adsorption performance,and thus,construction of lignin-based nanocomposites is worth considering.Herein,we introduced graphene oxide(GO)into lignin to form lignin/GO(LGNs)composite micro-nanospheres by self-assembly method.FT-IR and ~1H NMR spectroscopy illustrated that lignin and GO are tightly connected by hydrogen bonds.The LGNs as an environmental friendly material,also exhibit superior performance for Cr(VI)removal.The maximum sorption capacity of LGNs is 368.78 mg/g,and is better than that of other lignin-based materials.The removal process of Cr(VI)via LGNs mainly relies on electrostatic interaction,and it also involves the reduction of Cr(VI)to Cr(III).Moreover,LGNs still have high adsorption performance after repeating five times with the sorption capacity of 150.4 mg/g in 200 mg/g Cr(VI)solution.Therefore,the prepared lignin–GO composite nanospheres have great prospect as a low-cost,high-capacity and recyclable sorbent,and can be used in sewage treatment.This study can not only facilitate the advance of lignin-based micro-nanotechnologies but also provide a broad prospect for the use of black liquor.