Design Of A Corn Stalk Cellulose-based Adsorbent For Adsorption Of Cd(?)and Ni(?):Adsorption Behaviors And Mechanisms

In recent years,nickel-cadmium(Ni-Cd)and nickel-metal hydride(Ni-MH)batteries have occupied a high proportion in the market due to their mature production process and high electrical capacity.In the process of battery production and recovery,Cd(?)and Ni(?)are always discharged with wastewater.The Cd(?)and Ni(?)enriched in the water will pose a huge threat to human health and ecological environment,so it is urgent to develop an economic and efficient method to deeply remove Cd(?)and Ni(?)in the battery wastewater.Adsorption technology has attracted much attention in the field of heavy metal wastewater treatment due to its simple operation,stable adsorption products and applicability to large-scale wastewater treatment.Corn straw is one of the common agricultural wastes,its main component is cellulose.Due to its excellent hydrophilicity and a large number of hydroxyl groups,cellulose is suitable to be used as a matrix material to prepare a heavy metal ion adsorbent with good adsorption performance by introducing appropriate functional groups.In this paper,the microcrystalline cellulose extracted from corn straw was used as the matrix,and the chemical structure of microcrystalline cellulose was modified by grafting and crosslinking to synthesize the grafted and cross-linked product(Cell-g-GMA-c-MBA).On this basis,the grafted and cross-linked product was further reacted with aminating agent,and finally a high adsorption capacity amination modified product(Cell-g-GMA-c-MBA-DETA)with various amino groups was obtained.SEM,FTIR,XRD and other technical means were used to analyze and characterize the two materials,so as to clarify the influence of each step of modification on the surface structure and functional group types and content of corn straw cellulose.Taking Cd(?)and Ni(?)as the study object,the effects of the dosage of adsorbent,initial pH value,adsorption time,adsorption temperature and other factors on the adsorption performance of modified cellulose were studied.Simultaneously,the results of FTIR and XPS and related adsorption experiment could infer the adsorption mechanism of aminated modified adsorbent for two heavy metal ions,and the reliability of the relevant mechanism was verified by the ion competitive adsorption experiment.The main results were as follows:(1)Firstly,Glycidyl methacrylate(GMA),potassium persulfate(KSP),N,N-methylenebisacrylamide(MBA)and diethylenetriamine(DETA)was used as graft monomer,initiator,crosslinking agent and aminating agent,respectively.Then,epoxy groups and amino groups were successfully introduced into the cellulose structure,and the grafted and cross-linked product(Cell-g-GMA-c-MBA)and the amination modified product(Cell-g-GMA-c-MBA-DETA)were prepared.The modification conditions of each step were optimized by single factor variable experiment.Under the optimum conditions,namely the mass ratio of GMA:Cell=3:1,20 mmol/L KSP,5.0 wt%of MBA to Cell and 60? for 2 h,the maximum GE%and E could reach 93.69%and 4.96 mmol/g,respectively.The content of N-containing group in Cell-g-GMA-c-MBA-DETA can reach 7.20 mmol/g under the condition of 80 g DETA,50?and 8 h.(2)The results of SEM analysis showed that the surface of Cell-g-GMA-c_MBA became rough and porous after grafting and crosslinking and there were many granular polymers,and after amination,the surface morphology of Cell-g-GMA-c-MBA-DETA had more uniform pores and raised lamellae.The results of FTIR analysis showed that both GMA and MBA had been successfully introduced to the cellulose skeleton,and DETA was successfully introduced to the Cell-g-GMA-c-MBA surface by the ring opening reaction of epoxy group.The reluts of XRD analysis showed that the crystal structure of cellulose would be destroyed by the two-step modification.(3)The results showed that it was more favorable for the adsorption of Cd(?)and Ni(?)under acid condition.The optimal pH values of the two materials were 4 and 4.5,respectively.According to the thermodynamic and kinetic studies of aminated cellulose,the adsorption process of Cell-g-GMA-c-MBA-DETA for Cd(?)and Ni(?)was consistent with Langmuir model and it was endothermic.The maximum adsorption capacity of Cell-g-GMA-c-MBA-DETA for Cd(?)and Ni(?)can reach 369.9 mg/g and 321.2 mg/g at 45? and pH=4.5,respectively.In addition,the adsorption of Cd(?)and Ni(?)by Cell-g-GMA-c-MBA-DETA was in accordance with the quasi second-order kinetic model,which indicated that the rate determining step of adsorption process should be chemical adsorption.The results of regeneration experiments showed that after six adsorption-desorption cycles,the regeneration efficiency of Cell-g-GMA-c-MBA-DETA for Cd(?)and Ni(?)can still be maintained at 80%and 82%,indicating that the adsorption material has good repeatability and stability.(4)By comparing the changes of FTIR and XPS before and after the adsorption of Cd(?)and Ni(?)of two step modified material,it could obtain that the primary,secondary and tertiary amino groups introduced after amination were the most important active sites of Cd(?)/Ni(?)adsorption in Cell-g-GMA-c-MBA-DETA.It could infer tthat 1 mmol Cd(?)was inclined to be adsorbed by forming chelation complexes with 2-3 mmol N atoms in N-containing groups,while the adsorption of 1 mmol Ni(?)would require at least of 1 mmol N atom from the amino groups to form coordinate bonds.The different adsorption forms of the two ions at the same adsorption site resulted in the difference of the maximum adsorption capacity of the aminated modified corn straw cellulose to the two heavy metal ions.The above adsorption mechanism can be used to reasonably explain the results of competitive adsorption experiments and further prove the reliability of the adsorption mechanism.