Adsorption Characteristics,Mechanism And Application Of Functionalized Graphene Oxide Composites For Hg(?) In Water

Mercury(Hg)is one of the most toxic heavy metals,with the features of persistence,bioaccumulation,and migration.In the production of polyvinyl chloride(PVC)by the calcium carbide method,Hg is added in large quantities as a catalyst,resulting in the high concentration of Hg in the chlor-alkali wastewater,which is difficult to remove and recycle.The adsorption has the advantages of simplicity,low cost,high efficiency,reproducibility,etc.,which is widely used in the treatment of mercury-containing wastewater.As a new type of material,graphene oxide(GO)contains masses of oxygen-containing functional groups with simple structure and good environmental stability.In the study,L-cysteine modified graphene oxide(L-Cysteine-GO)and poly-cyanoguanidine functionalized graphene oxide(DCDA-GO)were prepared.Through batch adsorption experiments,it was found that the adsorption performance of L-Cysteine-GO significantly depended on pH,the highest removal rate was achieved at pH 7,while DCDA-GO was not limited by pH,the removal rate was higher than 90% when pH ranged at 1-13.When the dosage of the materials increased,the removal rate increased,while the adsorption capacity decreases,hence the most suitable dosage was 1 g/L.When the initial concentration was set as 200 mg/L and 600 mg/L,the maximum adsorption capacity of L-Cysteine-GO and DCDA-GO could be achieved at 160.71 mg/g and 341.35 mg/g,respectively.The modified material kept good performance on removing Hg(?)in wastewater even at high ionic strength and high chloride ion matrix.It can enrich Hg(?)with high selectivity in the water containing a variety of heavy metals.The packed column was filled with the proposed materials to filter Hg(?)solution(20 mg/L)at 15-20 min,the concentration of effluent was lower than 4 (?)g/L,realizing the short-time processing of high-concentration mercury solution.The relevant instrument was utilized to characterize the morphology and microstructure,it was discovered that the industrial-grade GO was at sheet structure,the crystallinity of the functionalized material improved after modified,additionally,the density declined,and the hydrophilicity and dispersibility improved.Infrared and Raman spectra confirmed that L-cysteine and polycyanoguanidine were successfully loaded on the surface of GO.Some wrinkles and internal gullies on the surface of L-Cysteine-GO increased significantly;the surface of DCDA-GO showed a flower-like layering.The adsorption mechanism of the two functionalized materials was revealed with the comprehensive analyzing the results from the batch experiment and microscopic characterization.The dynamic and thermodynamic behavior can be accurately revealed by the Quasi second-order model and the Freundlich model.Adsorption belonged to multi-layer chemical adsorption;the adsorption reactions of L-Cysteine-GO and DCDA-GO were proved to be exothermic,and endothermic spontaneous processes,respectively.The capacity of L-Cysteine-GO was dominantly ascribed to electrostatic attraction,the binding of sulfhydryl,amino,carboxyl with Hg(?).The adsorption capacity of DCDA-GO is mainly due to the chelating effect of cyano and imine groups on Hg(?)and the binding of carboxyl groups.Moreover,The adsorption capacity of amino-modified materials was better than that sulfhydryl-modified materials,while sulfhydryl-modified materials had a better affinity for Hg(?),of which the mechanism was relatively more complicated.In the treatment of low concentration simulated wastewater from PVC produce process(200 (?)g/L),the Hg(?)removal rates of GO,L-Cysteine-GO and DCDA-GO were 50.4%,95.68%,and 97.02%,respectively.In the treatment of simulated high-concentration(20 mg/L)chlor-alkali wastewater,their maximum removal rates were 17.45%,82.08%,and 64.61%,respectively.These showed that the modification of L-cysteine and cyanoguanidine can significantly improve the adsorption effect of GO in practical applications.The results of this study provide a reference for the modification of graphene oxide composite materials,and a certain idea for the treatment of mercury industrial wastewater with high ionic strength and high salinity.