Composite Adjusting And Structure Controlling Of Chitosan/graphene Oxide Based Adsorbents For Fluoride Removal

Dissolution of fluoride-containing mineral and improper discharge of industrial wastewater lead to increasing concentration of fluoride in groundwater,which pose a risk to drinking-water safety.Adsorption is considered to be one of the most promising technology with low cost and easy operation for fluoride removal.However,low adsorption capacity and slow adsorption kinetics are the main problems of the developed fluoride adsorbent up till now.In this study,chitosan?CTS?was employed as supporting materials,and then composite was adjusted by changing the species and existing forms of active components,also graphene oxide?GO?was used to control the structure of the adsorbent to improve the adsorption capacity and accelerate the adsorption rate.This study would provide basic theory to remediation of groundwater fluoride pollution by adsorption technology.First,a granular Fe-impregnated chitosan?Fe-CTS?adsorbent was synthesized via chelation of Fe ions to two–OH and two–NH₂ groups of CTS,also,the Fe linked to a water molecule and a chlorine,and fluoride removal by Fe-CTS was due to Fe–F chelation occurred via ion exchange between fluoride and chloride,which was supported by EXAFS?Extended X-ray absorption fine structure?and other analysis.In addition,Fe-CTS exhibited high selectivity for fluoride,and the maximum adsorption capacity was estimated by Langmuir isotherm model to be 20.75 mg/g,but,the adsorption would reach equilibrium in 6 h.A 150 mg/L Fe³⁺solution showed great regeneration efficiency.As a result,a fluoride adsorption capacity of regenerated adsorbent?FeCl₃-Fe-CTS?of 14.44 mg/g was obtained,which approximated the adsorption capacity of Fe-CTS?14.99 mg/g?.The observation obtained from SEM?Scanning electron microscopy?images and the EDS?Energy dispersive spectroscopy?indicated that the regeneration using FeCl₃accelerated the internal diffusion of fluoride by increasing and broadening the mesopores.After regeneration,FeCl₃-Fe-CTS was positively charged,which enhanced the electrostatic attraction with fluoride.After seven regeneration/adsorption cycles,FeCl₃-Fe-CTS did not show significant loss in the adsorption ability.A zirconium-chitosan/graphene oxide membrane?Zr-CTS/GO?was synthesized by the immobilization of Zr?IV?on CTS and GO,and the active epoxy and carboxyl groups in graphene reacted with the amino groups in CTS.Stacked GO sheets were retained in the Zr-CTS/GO,and the thermal stability and mechanical strength of Zr-CTS/GO were enhanced by introducing GO.The ion-exchange mechanism was involved in fluoride adsorption on Zr-CTS/GO via fluoride replaced chloride and-OH from the surface of Zr-CTS/GO.Compared with granular Fe-CTS,Zr-CTS/GO membrane exposed much more adsorption sites to fluoride,resulting in higher adsorption capacity of 29.06 mg/g and much more rapid adsorption kinetic of 1 h to reach equilibrium.ZrO?OH?_1.33Cl_0.66-rGO hybrid adsorbent was synthesized through the controlled hydrolysis of ZrOCl₂·8H₂O in GO aqueous solution by thermal treatment,and the Zr ions was enriched on rGO sheets with the assist of cation-?interaction,then,ZrO?OH?_1.33Cl_0.66-rGO membrane was obtained by vacuum-assisted self-assembly.ZrO?OH?_1.33Cl_0.66-rGO showed high fluoride adsorption capacity of 110.75 mg/g.The fluoride adsorption was achieved by ion-exchange between fluoride and chloride,and it can reach equilibrium in 5 min.This study revealed the both mechanisms of adsorbents synthesis and fluoride removal,provided basic theory for design of adsorbent with high adsorption capacity and fast adsorption kinetics,also,provided support to remediation of groundwater fluoride pollution by adsorption technology.