Preparation And Application Of Carboxylated Graphene Oxide Adsorbent In The Removal Of Heavy Metal Copper Ions: Adsorption Behavior And Adsorption Mechanism

Heavy metal pollutants were mainly discharged from the by-products derived from the fast development of basic industry and heavy industry,and threatening the environment and biological survival.As one member of carbon-based nanomaterials,graphene had an extensive application in pollutants adsorption.But its low adsorption capacity for pollutants and limited application area restricted its wide application.Therefore,it was urgent to explore novel preparation methods for graphene materials and to prepare graphene adsorption materials applicable to different media.This work based on the adsorption for Cu²⁺by graphene oxide(GO)sponge,took the advantage of electrophilic addition reaction under alkaline condition and put forward a thought of carboxylation modification to improve the adsorption performance of GO sponge.Carboxylated graphene oxide/chitosan(GCCS)gel beads could be formed with chitosan in an alkaline coagulating bath and achieved the Cu²⁺adsorption from both the media of water and soil.The gel balls could be mechanically strengthened by improved dopants and cross-linking agents.And the overall property of the mechanically strengthened graphene adsorption materials could meet more practical requirements.First,carboxylated graphene oxide(GO-COOH)sponge with some stackings and wrinkle structures was prepared from GO via an electrophilic addition reaction under alkaline condition.Adsorption investigation found that its adsorption towards Cu²⁺was 93.8 mg/g,3.9times that of GO sponge and for cationic dye(MB)was 1.8 times that of GO sponge.The Cu²⁺concentration after GO-COOH sponge adsorption could be reduced(0.86?g/m L)to meet the standards recommended by the Chinese government and its adsorption process could be fitted into the Langmuir model suggesting a monolayer adsorption.Its adsorption for Cu²⁺could be finished in 0.5 h and the adsorption rate was mainly controlled by the intra-particle diffusion process.Experimental foundings suggested that low temperature(303 K),high p H(p H=6)facilitated the Cu²⁺adsorption onto GO-COOH sponge,while the variation of ionic strength(0-100 m M)has ignorable effect on Cu²⁺adsorption.After 5cycles regeneration,its re-adsorption performance for Cu²⁺could still maintain 40%of its initial maximum adsorption capacity.Density functional theory(DFT)calculation suggested that the carboxyl groups in GO-COOH sponge were more prone to be 4-coordinated with Cu²⁺,the carboxyl groups on the edge(in the form of-OCH₂COOH)and on the surface(in the form of-COOH)exhibited a large coordination strength.The adsorption application of GO-COOH sponge could be further extended into cationic pollutant(MB)adsorption with an adsorption capacity of 780 mg/g.Second,by using the chitosan balls formed under alkaline condition,carboxylated graphene oxide/chitosan(GCCS)gel balls were prepared with diameters of 2.2±0.2 mm.The GCCS gel balls were obviously ball structures and there were some gaps with diameters of 30?m under its transection scanning electron microscope(SEM)characterization and some structural stackings could be observed under transmission electron microscope(TEM)characterization.The adsorption capacity of GO-COOH sponge for Cu²⁺was(77.65 mg/g)3.3 times that of GO sponge,15.3 times that of activated carbon and the adsorption of Cu²⁺onto GCCS gel balls could be fitted into the Temkin model.The adsorption of Cu²⁺onto GCCS gel balls tended to reach equilibrium after4 h,the adsorption behavior could be well fitted into the pseudo-second order model and the second step of intra-particle diffusion process inhibited the fast adsorption process.The adsorption would release H⁺and the increase of p H(reduction of H⁺)facilitated the Cu²⁺adsorption onto GCCS gel balls which was consistent with the Le Chatelier's principle.Competitive ion(Na⁺,0-100 m M)and temperature(303-333 K)have negative effect on Cu²⁺adsorption.The stable porous structure of GCCS gel balls inhibited the diffusion of soil media into the interior structure and protected the adsorptive sites on GCCS gel balls.Therefore,GCCS gel balls could be applied in Cu²⁺adsorption from the media of soil.The removal efficiency of raw soil for Cu²⁺reduced from 95.2%to68.3%with the increase of Cu²⁺concentration from 40-160?g/m L,and the addition of GCCS gel balls could maintain an excellent Cu²⁺adsorption removal efficiency(99.6%-98.5%).Even with a high Cu²⁺initial concentration(C₀)of 120?g/m L in soil media,the addition of GCCS gel balls could reduce the Cu²⁺concentration to 0.94?g/m L via adsorption.The adsorption of GCCS gel balls could evidently reduce Cu²⁺bioaccumulation in the root(59.94%)and stem(66.81%)of wheat seedling and there were negative effects on leaves.Moreover,GCCS gel beads could not intensify the influence of Cu²⁺to the biomass content and photosynthesis of wheat seedlings.Third,mechanically strengthened carboxylated graphene oxide/chitosan/cellulose(GCCSC)gel beads were prepared via the interaction with cellulose and cross-linking agent.There were some sheet-like structures of graphene component under TEM characterization and some porous structure could be observed under the SEM characterization of GCCSC gel beads.In the direction of outer pressure,GCCSC gel balls could sustain 3.2 N at an elastic limit and the force was26 N before brittle failure.The mechanically strengthened gel balls could be applied in column adsorption separation.The adsorption behavior of Cu²⁺onto GCCSC gel balls could be fitted into the Temkin model with an adsorption capacity of 22.4 mg/g.The adsorption for Cu²⁺could reach equilibrium in 6 h and the adsorption process was independent of p H(2-6),ionic strength(Na⁺or Ca²⁺:0-100 m M)and temperature(293-323K).After a column adsorption separation which was filled with GCCSC gel balls,the Cu²⁺concentration(C₀=10?g/m L)could be reduced.The collected Cu²⁺(10%C₀)filtrates were conducted into the toxicity investigation of wheat seedlings,results found that the mass of root,stem and leaves increased for about 38.1%and 8.3%to that of the control,respectively.The porous structure and stable mechanical property inhibited the direct contact between soil and graphene surface endowing it a good Cu²⁺adsorption removal efficiency from 63.1%to 91.5%with an initial Cu²⁺concentration of 200?g/m L.