Preparation Of Magnetic Carbon Nano-adsorbent And Its Performance In Removing Hexavalent Chromium Ions In Water

Heavy metal chromium ions were widely distributed in surface water and groundwater due to their wide application in electroplating,printing,dyes and other industries,and were listed as one of the most toxic pollutants in wastewater.Chromium ions were mainly present in the forms of Cr(VI)and Cr(III)in water,and Cr(VI)is higher than 100 times more toxic than Cr(III).Therefore,reducing Cr(VI)to Cr(III)in wastewater or immobilizing is a very important pollution control strategy.The adsorption method has the advantages of simple operation,low cost and high efficiency;therefore it has a broader prospect in the purification and removal of Cr(VI)ions.In this thesis,nitrogen-doped magnetic carbon nano-adsorbents(MCNs)were prepared by one-step carbonization method.Their physicochemical properties were analyzed by various characterization techniques.Degradation adsorption experiments of hexavalent chromium ions were carried out.The effects of initial ion concentration,solution pH and reaction contact time on adsorption efficiency were explored.Adsorption kinetics model and adsorption isotherm model were used to investigate the adsorption mechanism of MCNs.The main results obtained in the thesis are as follows.(1)The adsorption efficiencies of Cr(VI)ions of MCNs prepared at different carbonization temperatures were compared.The results showed that the material prepared by carbonization temperature of 700℃(MCN700)had the best adsorption effect.When pH is 2,the initial concentration of Cr(VI)ions was 50 mg/L and the amount of adsorbent was 50 mg,after the adsorption of Cr(VI)ions by MCN700 was balanced at the contact time of 2 h,the adsorption rate and adsorption capacity can reach 96.90%and 56.28 mg/g,respectively.Through the characterization of MCNs,it was found that MCN700 had the irregular morphology of tubular carbon and flake carbon,and it had a large specific surface area(817.0 m~2/g).The magnetization and crystallinity of the adsorbent was very beneficial to the improvement of adsorption performance and recycling of adsorbents.(2)The adsorption mechanism of MCN700 was explored by adsorption kinetics model,adsorption isotherm model,X-ray Photoelectron Spectroscopy and Vibrating Sample Magnetometer characterization.The results showed that the pseudo second-order kinetic equation and Langmuir equation could well fit the adsorption process of Cr(VI)ions by MCN700,indicating that the adsorption process was mainly dominated by monolayer adsorption,and chemical adsorption was the main mechanism affecting the adsorption process,but not mass transfer within the particle.The adsorption and reduction processes in the adsorption reaction existed simultaneously.After the Cr(VI)ions were adsorbed on the surface of adsorbents,some of them were reduced to Cr(III)ions by ZVI on the surface of the adsorbent.After 6 adsorption-desorption cycles of the adsorbent MCN700,the adsorption rate only decreased by 11.61%,indicating that the regeneration performance of MCN700 was very good.