Study On The Preparation Of Carbon Microspheres By Hydrothermal Method And The Adsorption Properties And Mechanism For Pb²⁺ And Cu²⁺

In order to treat lead and copper wastewater,a large amount of works had been carried out.Adsorption had advantages such as no secondary pollution and high treatment efficiency which were widely applied.Carbon microspheres were prepared by hydrothermal method to treat lead and copper wastewater.Carbon microspheres were coated with magnetic chitosan by the embedding method.Surface morphology,fabric characteristics,preparation,formation mechanism,adsorption performance and mechanism of the two adsorbents were analyzed.The studies were as follows:?1?Carbon microspheres with relatively uniform particle size,good sphericity and aromatization were prepared by hydrothermal method on D-xylose concentration of 0.1 g/mL,heating time 16 h,fullness of 80%and hydrothermal temperature of190?.?2?Adsorption properties and mechanism of 190?-carbon microspheres was studied by static adsorption method.Under the reaction condition with temperature of45?,pH=5,dosage of 4 g/L and initial concentration of 8 mg/L,the maximum adsorption amount of Pb²⁺was 1.32 mg/g.Cu²⁺maximum adsorption amount was0.90 mg/g.The main force for 190?-carbon microspheres to adsorb two ions was electrostatic attraction.-OH,-COOR and Pb²⁺exist in the form of COOR-Pb-OH and dicarboxyl group combined with Pb,while COOR and Cu²⁺combined to form COOR-Cu-COOR.-OH and Cu²⁺were combined by hydrogen bonding.The adsorption process of Pb²⁺and Cu²⁺by 190?-carbon microspheres complied with the pseudo-second kinetics.Isothermal adsorption fitting results showed that the Langmuir isothermal model fitted the adsorption process of 190?-carbon microspheres on Pb²⁺and Cu²⁺.The adsorption process was a monolayer chemical adsorption.At 25?,35?and 45?,the?G of Pb²⁺adsorbed by carbon microspheres were negative and showed an increasing trend,indicating that the adsorption process spontaneously proceeded.?H=19.66 kJ·mol^-1was a positive value,indicating that the temperature increases the adsorption capacity of the carbon microspheres to Pb²⁺.?S was 56.07 J/mol/K.It mean that the temperature increase enhanced the surface activity of the material.?G of Cu²⁺adsorption by carbon microspheres were negative and increased with temperature.It indicated that the adsorption of Cu²⁺by carbon microspheres was also a spontaneous reaction,similar to the adsorption of Pb2+by carbon microspheres.?H=5.64 kJ/mol was a positive value.It indicated that the adsorption of Cu²⁺was an endothermic reaction,and the entropy change?S=9.90J/mol/K was a positive value.It indicated that the temperature increases and the adsorption capacity of carbon microspheres for Cu²⁺enhanced.The adsorption process was an endothermic reaction.Temperature enhancement led to higher solid-liquid mass transfer efficiency which reflected that randomness of adsorbent surface increased during the adsorption of two ions.Increasing temperature enhanced the adsorption capacity.?3?Carbon microspheres were coated with magnetic chitosan by embedding method to prepare magnetic chitosan/carbon microspheres?MCS/CMS?.Adsorption Compared with chitosan,magnetic chitosan,carbon microspheres and MCS/CMS were compared.Adsorption capacity of MCS/CMS was the best among the others.Based on single-factor control variables,MCS/CMS had the best adsorption capacity which synthesized by carbon microspheres:magnetic chitosan ratio=1:1,fixed crosslinking time 3 h,1.25 mol/L NaOH+0.1 mol/L sodium citrate as the crosslinking agent.Morphological comparison of cross-linked chitosan,magnetic chitosan,and MCS/CMS was performed.It could be observed that the magnetic chitosan has been attached and wrapped on the carbon microspheres to form a thin film and produce a rough textured surface.MCS/CMS had superparamagnetism with saturation magnetization of 1.17 emu/g.Chemical characterization results showed that there were several functional groups of C=C,C-O/C-N,C=O,N-H,and O-C=O on MCS/CMS.The presence of N-H and C-N indicated that chitosan was successfully coated on carbon microspheres.Zeta potential test showed that the isoelectric point HIEP of MCS/CMS was 4.27.It indicated that the surface of MCS/CMS was negatively charged and electrostatically adsorbed Pb²⁺and Cu²⁺.?4?MCS/CMS adsorption capacity of Pb²⁺was 11.06 mg/g and the adsorption capacity of Cu²⁺was 7.07 mg/g at pH=5 and initial concentration of 15 mg/L.The adsorption process of Pb²⁺and Cu²⁺by MCS/CMS complied with the pseudo-second-order kinetic model and Langmuir adsorption isotherm model.The adsorption was dominated by chemical adsorption.It can be calculated that adsorption activation energy for Pb²⁺and Cu²⁺were 10.68 kJ/mol and 5.68 kJ/mol.Activation energy was less than 40 kJ/mol.Adsorption of Pb²⁺and Cu²⁺by MCS/CMS was Ion-exchange process.At 25?,35?,and 45?,?G?-1.99 kJ/mol,-1.06 kJ/mol,-0.05 kJ/mol?of Pb²⁺adsorption by MCS/CMS and?G?-4.34 kJ/mol,-3.83 kJ/mol and-3.54 kJ/mol?of Cu²⁺adsorption were all negative values and increased with increasing temperature.It indicated that the adsorption of Pb²⁺and Cu²⁺was a spontaneous adsorption process,which was easier to perform under high temperature conditions.?H of Pb²⁺adsorption was 30.74 kJ/mol,and the?H of Cu²⁺adsorption was 16.30 kJ/mol,indicating that the adsorption process of Pb²⁺and Cu²⁺were endothermic.The?S of Pb²⁺adsorption was 96.44 J/mol/K,and the?S of Cu²⁺adsorption was 40.24 J/mol/K.The positive values of?S indicated that the adsorption of Pb²⁺and Cu²⁺by MCS/CMS was a process involving increasing entropy.According to the calculation of thermodynamic parameters,it showed that adsorption reactions were all spontaneous and endothermic.As the temperature increased,the activity of the adsorption site of the adsorbent increased.The interface disorder of the adsorption phase increased.Characterization of the adsorbed materials by Fourier Infrared Spectrometer?FT-IR?and X-ray Photoelectron Spectroscopy?XPS?revealed the adsorption characteristics of the two ions by MCS/CMS.The adsorption of metals can be attributed to the formation of N:Pb²⁺and N:Cu²⁺complexes.The carboxyl group was combined with Cu²⁺and Pb²⁺in a covalent bond manner.?5?The material reproducibility test showed that the desorption rate of HCl was the highest among HCl,NaCl,HNO3,CH3COOH,H2O.The desorption rates of HCl to Pb²⁺and Cu²⁺were 90.38%and 89.72%,respectively.After 5 cycles of utilization,MCS/CMS still showed good adsorption performance for Pb²⁺and Cu²⁺and removal rates were still 61.55%?54.16%when the initial concentration was 13 mg/L.