Modification Of Pre-oxidized Polyacrylonitrile Fibers And Its Adsorption Properties For Metal Ions

Heavy metal is nonbiodegradable and tends to accumulate in humans and other living organisms.Heavy metal is harmful to the environment and human body.Sorption technology is an effective method to remove heavy metal ions from water.As a surpport of adsorbent for heavy metal,fiber has effective surface structure.In order to utilize the internal structure of the fiber,modified fiber was prepared by treating pre-oxidized polyacrylonitrile fiber with sulfuric acid.The chemical structures of the pre-oxidized fiber and the sulfuric acid treated fibers were studied by comparison.Modified fiber was then used as an adsorbent for Cu(Ⅱ)and Pb(Ⅱ)ions from aqueous solutions.The effects of initial pH,temperature,initial concentration of the soiution and contact time on Cu(Ⅱ)and Pb(Ⅱ)ions adsorption were evaluated.The adsorption equilibrium was analyzed by adsorption kinetics,adsorption isotherms and adsorption thermodynamics.Moreover,the reusability and competitive adsorption characteristics of modified fibers were studied.The relationship between the adsorption of heavy metal ions and th modified PAN pre-oxidized fiber was also established.The main results of this research can be summarized as follows:1.Pre-oxidation temperature can regulate cross-linking structure and amount of unreacted nitrile groups of the fiber.After pre-oxidization of the fiber,the amount of nitrile and methylene groups in the fiber were significantly decreased,and conjugate structures were formed simutaneously.In the process,the linear structure of PAN was converted to ladder structure in of PAN pre-oxidized fiber.More unreacted nitrile groups and higher extent of pre-oxidation were obtained when the heat treatment was at low temperature.With the increasing of the temperature of heat treatment,the pre-oxidized fibers showed an increment in initial temperature and peak temperature of the exothermic reaction,and a decrease in released heat of the reaction.2.Sulfuric acid treatment can regulate the chemical environment inside of PAN pre-oxidized fiber.The nitrile groups in the fibers were transformed into amide groups after treatment of sulfuric acid,while cyclic structures were not affected.The fiber can hold its shape after acid treatment.The existence of amide groups and cyclic structure could provide active sites for adsorption of heavy metal ions.The adsorption characteristics of copper ions on the fibers was preliminarily explored,the modified fiber prepared at 250 ~oC was found to have the highest adsorption capacity,which showed a high affinity to Cu(Ⅱ),Pb(Ⅱ)and other ions.3.The adsorption experiments of Cu(Ⅱ)and Pb(Ⅱ)ions by modified PAN fibers indicated that the adsorption capacities was significantly affected by increasing initial concentration of metal ions,reaction time,and temperature.An optimal pH values for adsorption Cu(Ⅱ)and Pb(Ⅱ)ions of solution were obtained.The adsorption equilibriums of the two metal ions were both fitted well with Langmuir isotherm,which indicated that the adsorption of metal ions took place by the formation of monolayer.Dynamics analysis demonstrated that the adsorption of the two metal ions on the modified PAN pre-oxidized fiber were both fitted well with pseudo-second-order,and the reaction process was mainly controlled by chemical adsorption.4.The modified fiber has good regenerative characteristic for adsorption Cu(Ⅱ)and ions.In competitive adsorption experiments,the adsorption of copper and lead ions can be promoted by each other,while nickel and zinc had almost no effect on copper adsorption,which greatly hindered the adsorption of nickel and zinc.Microchannels in the PAN pre-oxidized fibers were produced by the treatment of sulfuric acid,through which metal ions can diffuse from the surface to the inner of the fiber.The adsorption of metal ion can take place not only on the surface but also in the fiber with conjugated structure and acidic functional groups produced by hydrolysis of the modified fibers.