Adsorption Of Cu²⁺?Pb²⁺?Cd²⁺in Water By Polyethyleneimine Functionalized Straw Biochar

Biochar is a new type of environmental functional material,which plays an important role in improving soil physical and chemical properties,reducing greenhouse gas emissions,and generating renewable resources and so on.The performance of biochar in the process of using is easy to be limited by its own conditions.In order to further improve the physical and chemical properties of biochar and reveal the adsorption mechanism of biochar for heavy metal ions in water,Polyethyleneimine?PEI?was used to modified biochar to study its adsorption effect on Cu²⁺,Pb²⁺,Cd²⁺in water.Using corn straw biochar as substrate,PEI as ammoniator and glutaraldehyde as cross-linking agent after potassium hydroxide treatment,PEI functional biochar?PBC?with good adsorption performance was prepared.The specific surface area and porosity analysis,Zeta potential analysis,fourier transform infrared spectroscopy?FTIR?,scanning electron microscopy?SEM?and energy dispersive spectroscopy?EDS?were used to examine the structural and morphological characteristics of PBC.The results showed that the surface morphology of biochar changed obviously before and after modification.The surface of PBC changed from rough state to smooth state,and the surface porosity increased obviously.The results of EDS showed that the N content on the surface of biochar increased significantly,and FTIR showed that there was a new absorption peak of C-N stretching vibration in the wavelength of 1362 cm^-1,which indicated that PEI was successfully loaded on the surface of straw biochar.The results of Zeta potential analysis showed that the higher pH condition was beneficial to the adsorption of heavy metal ions by PBC.Environmental factors such as pH,reaction time,temperature and others were used to study the adsorption effect and adsorption characteristics of PBC on heavy metal ions.The adsorption capacity of PBC for three heavy metal ions increased with the increase of solution pH.When the solution pH is 9.0,the adsorption capacity of PBC to Cu²⁺,Pb²⁺,and Cd²⁺was91.36 mg/g,94.51 mg/g and 82.26 mg/g respectively.The adsorption capacity of PBC for three ions increased with the increase of reaction time and solution temperature.The adsorption of Cu²⁺and Cd²⁺by PBC reached equilibrium at about 80 min and Pb²⁺at about 160 min.The 1mol/L nitric acid was used to desorb the biochar that has completed the adsorption process.The experimental results showed that after six consecutive cycles of adsorption-desorption experiments,PBC still had a high adsorption capacity for heavy metal ions,and this indicates that PBC can be reused and has high economic value.The kinetic model and isothermal adsorption model were used to fit the adsorption results.The fitting results of Cu²⁺and Pb²⁺,using the first-order kinetic equations fit better,and R²values are 0.960 and 0.943 respectively.The fitting results of the second-order kinetic equation for Cd²⁺were better,with R² reaching 0.996,indicating that the adsorption of Cd²⁺by PBC was mainly chemical.The Langmuir equation fitted the adsorption results better,which indicated that the adsorption reaction was mainly carried out by single monolayer adsorption.The maximum adsorption amounts of Cu²⁺,Pb²⁺,and Cd²⁺by PBC were 513.31 mg/g,612.55 mg/g,and 301.04 mg/g,respectively.The results of adsorption thermodynamics analysis showed that the Gibbs free energy change?G<0 and the entropy change?S>0 of the three ions,and this indicated that the adsorption reaction could proceed spontaneously.The enthalpy change?H>0 of the reaction indicated that the reaction was an endothermic reaction.The results of competitive adsorption experiments showed that the selective adsorption order of PBC for three heavy metal ions was Pb²⁺>Cu²⁺>Cd²⁺,which was consistent with their adsorption quantity order on PBC,indicating that Pb²⁺has a strong adsorption capacity and is relatively easy to remove.In summary,PEI-modified biochar has good adsorption properties for Cu²⁺,Pb²⁺,and Cd²⁺,and it can be reused by using nitric acid desorption.In the future work,the comparative study of different kinds of straw needs be strengthened to reduce the preparation cost and improve the utilization rate.