EDTA-Chitosan/Graphene Oxide Composite Adsorbent:Preparation,Characterization And Application In Simulated Electroplating Wastewater

With the rapid development of China’s economy,the problem of heavy metal pollution caused by industrial wastewater discharge has become increasingly serious.Copper plating and nickel plating are common processes in the electroplating industry,which produce a large amount of wastewater that are highly harmful to humans.Adsorption method shows many advantages such as low cost,high efficiency,and simple operation in the treatment of heavy metal pollution.However,the composition of electroplating wastewater is very complicated,which contains various inorganic cations,anions or organic ligands,which will seriously interfere with the removal of heavy metal ions by the adsorbents.Therefore,the treatment of electroplating wastewater with ordinary adsorbents usually does not meet the national discharge standards.In response to this problem,we have developed a new and efficient chelating adsorbent to remove copper and nickel heavy metal ions from electroplating wastewater.In this paper,EDTA-modified chitosan/graphene oxide composite adsorbent（referred to as EC@GO）was prepared by"one-pot method",in which EDTA-disodium salt,chitosan（CS）and graphene oxide（GO）were used as raw materials,and water-soluble carbodiimide（EDAC）was used as the condensing agent.In addition,the synthesis conditions for EC@GO were optimized,and the chemical compositions,functional groups,specific surface area and morphology of EC@GO were characterized by FTIR,XRD,SEM and BET.The effects of various factors such as adsorbent dosage,solution pH,adsorption time,and concentration on the adsorption of copper（II）and nickel（II）ions by EC@GO were also investigated.Moreover,the adsorption mechanism was explored,and EC@GO was further applied to deal with simulated copper or nickel-containing electroplating wastewater.The detailed results are presented as follows:1)The synthesis conditions for EC@GO was investigated.In order to dissolve chitosan with EDTA-disodium salt solution,the molar ratio of the amino groups of chitosan to EDTA-disodium salt should be 1:3,and the concentration of chitosan was about 1.5%.In addition,when the composite ratio of GO to chitosan（w/w）was 1:9 and the molar ratio of EDAC to EDTA-disodium salt was 2:1,the synthesized EC@GO material had better adsorption performance for heavy metal ions.2)The performance of EC@GO for absorbing Cu²⁺and Ni²⁺was little affected by the pH of the solution.And EC@GO exhibited high removal efficiency for heavy metal ions even in strongly acidic solutions.The isothermal adsorption process of EC@GO followed the Sip model,and its maximum adsorption capacity for Cu²⁺in aqueous solution of pH=2 was123.9 mg g^-1.The maximum adsorption capacities for Ni²⁺in aqueous solutions of pH=2and pH=5 were 113.2 and 115.3 mg g^-1,respectively.EC@GO had a fast adsorption rate for Cu²⁺and Ni²⁺,which could reach 99%of the maximum adsorption capacity within 1～5 h.The kinetic process was in accordance with the Elovich model.3)The adsorption mechanism of EC@GO for heavy metal ions was chemical adsorption,and the EDTA-chelating group was the main adsorption site.EC@GO showed good reusability for Cu²⁺and Ni²⁺in the aqueous solutions.After repeated use for six times,the removal percentage of Cu²⁺and Ni²⁺was still more than 63%.4)After the low-concentration and high-concentration simulated copper-containing electroplating wastewaters（pH=3）were treated by EC@GO,the residual copper concentrations in the electroplating wastewaters were both lower than 0.1 mg L^-1.The concentration of nickel ions in the simulated nickel-containing electroplating wastewater was96.75 mg L^-1,and the total concentration of organic and inorganic ligands in the wastewater was 10 times that.The residual nickel ions in electroplating wastewater treated by EC@GO could also be less than 0.1 mg L^-1.The national discharge standards for copper and nickel ions in electroplating wastewater are 0.5 mg L^-1.5)A fixed bed column with a length of 15 cm was prepared by loading the mixture of EC@GO and silica gel into a glass column with an inner diameter of 0.6 cm,which was used to treat simulated copper-containing electroplating wastewater.The concentration of the wastewater was 50 mg L^-1,and the pH of the wastewater was 3.In addition,2 mol L^-1 of nitric acid was used as the eluent for the column.The fixed bed column showed high efficiency in removing copper ions from wastewater.The residual concentration of copper ions in the first 160 m L of wastewater treated by the column was almost undetectable.After three adsorption and desorption cycles,the adsorption performance of EC@GO for removing copper ions was basically unchanged,indicating that the EC@GO adsorption column was regenerated well and had good reusability.