Walnut Shell As Adsorbent For The Optimal Removal Of Methylene Blue And Congo Red In Aqueous Solutions

Dye wastewater with significant damage to humans and aquatic organisms has attracted a great deal of attention.However,the way to remove dye cheaply has not been sufficiently studied.In this study,original walnut shell(WNS)was used to deal with cationic dye Methylene blue and anionic dye Congo red,two kinds of dyes which are harmful and widely used.The study demonstrated the sorption mechanism by adsorption experiment with chemical modification to different functional groups and characterizing the adsorbent.Two methods were proposed to improve the removal of Methylene blue and Congo red by WNS called optimization of adsorbent and optimization of adsorbate which can increase the removal ratio of Methylene blue and Congo red effectively.In addition,the recyclability of the used adsorbent was performed.The main conclusions are as follows:(1)The scanning electron microscopy(SEM),fourier transform infrared spectrum(FTIR)and Zeta potential analysis were used to characterize the adsorbent.The results of SEM and FTIR showed that WNS had a large number of micropores and abundant functional groups which were helpful for sorption.The Zeta potential data proved that the surface of walnut shell is negatively charged under the pH value of 2.0-12.0.It was conducive to adsorb the cationic dye methylene blue.(2)In order to clarify the sorption mechanism,chemical modification was used to determine functional groups contribution to the sorption of dyes.It was concluded that electrostatic attraction between WNS and dyes is the dominant sorption mechanism.Carboxyl and hydroxyl are beneficial to the sorption of Methylene blue but have inhibition effects on sorption of Congo red.Amino is the main functional group to adsorb Congo red by walnut shell.(3)This study also proposed two methods to improve the removal of Methylene blue and Congo red by raw walnut shell called optimization of adsorbent and optimization of adsorbate.Optimization of adsorbent used walnut shell covered with Methylene blue as adsorbent to remove Congo red.The second method mixed Methylene blue with Congo red,capture the precipitation,then use walnut shell to adsorb the dye in the water.They all can improve the removal efficiency of pollutants,increase the removal ratio of Methylene blue and Congo red effectively even up to 100%.(4)The results of desorption experiment showed that significant desorption hysteresis was observed for Methylene blue or Congo red on WNS,with desorption hysteresis coefficient were 0.40 and 0.03,respectively.This is related to the number of sorption sites and the sorption strength.From the desorption experiment of the dye,it is obvious that at initial concentration of 0.3 M HCl and 0.2 M NaOH,the desorption of Methylene blue and Congo red from adsorbent is better.Besides,it can be concluded that WNS shows excellent recycling ability up to three cycles for the treatment of cationic Methylene blue and anionic Congo red from aqueous solution.