Chitosan Sponge Reinforced By Citric Acid Modified ?-Cyclodextrin For Highly Efficient Removal Of Dyes And Heavy Metals From Dyeing Effluents

The rapid development of industry and agriculture resulted in discharging of wastewater rich in heavy metals and persistent organic pollutants causing ecological and environmental problems.Adsorption is one of the most widely used methods for treating industrial wastewater.However,traditional adsorbents,such as activated carbon,natural mineral,solid waste,and synthetic resins are often limited by their cost-efficiency and poor renewability.Therefore,it is very important to develop a low-cost,stable,highly efficient and environmentally friendly adsorbent.Chitosan-based adsorbents have been explored for wastewater treatment for being renewable,inexpensive,and readily modifiable with tunable adsorption properties.However,chitosan is susceptible to acid hydrolysis,which limits its application in treating acidic effluents.In the current study,citric acid modified cyclodextrin?CA-CD?was used enhance chitosan via chemical cross-linking.The resulting chitosan-based porous adsorbent showed excellent stability and favorable efficiency in removal of reactive dyes and heavy metal ions from dyeing wastewater.Specifically,CA-CD rich-in carboxyl functionalities were prepared via esterification of beta cyclodextrin with citric acid using sodium hypophosphite as the catalyst.Adsorbents containing varying amount of CA-CD were then prepared by coupling of chitosan and CA-CD under the catalysis of 1-ethyl-3-?3-two methyl amino propyl?two carbonyl imine hydrochloride?EDC?and N-hydroxysuccinimide?NHS?using a sol-gel method.The resulting CA-CD/CS adsorbent was finally freeze-dried and characterized by Fourier Transformed Infrared Spectroscopy?FTIR?,Scanning Electron Microscopy?SEM?,Thermogravimetric Analysis?TGA?and mechanical compression test.Reactive Blue 49 in its hydrolyzed form?HRB49?was used to study the adsorption properties of CA-CD/CS on reactive dyes.The time for reaching the adsorption equilibrium was determined and the effects of CA-CD contents on adsorption capacity was investigated,using a static adsorption approach.It was found that the adsorption of HRB49 reached equilibrium in 24 h at room temperature.A maximum adsorption capacity of 485 mg/g was achieved with the sample containing20 wt%CA-CD at pH=5.5,room temperature.The adsorption of HRB49 by CA-CD/CS fits with the pseudo 2^nd order kinetic adsorption model with a correlation coefficient R² of 0.996,indicating the domination of chemical adsorption.in the adsorption isotherm was determined and found to conform to the Langmuir adsorption model with aR² of 0.998,showing that it was monolayer adsorption.Considering the fact that hydrolyzed dyes coexist with large amount of salts and surfactants in the actual spent dyebath,the effect of ionic strength,pH,and surfactant on the adsorption process were also investigated.The presence of NaCl had a negative effect on adsorption of HRB49 possibly via competitive interaction with the charged binding sites.At[NaCl]of 2 M the adsorption capacity was reduced by 9.5%.The adsorption was also compromised by soap and peregal O.The anionic soap had a more pronounced effect in that the adsorption was brought down by 20%at a concentration of 0.4 M soap.The reusability of the CA-CD/CS adsorbent was demonstrated by a six-cycle adsorption-desorption experiment.The removal rate for HRB49 remained unchanged after4 cycles and was still up to 80%of the original value at the end of 6 cycles.The CA-CD/CS adsorbent was also found to be generally applicable to hydrolyzed reactive dyes of different size and structure.The adsorption of common heavy metal pollutants in dyeing wastewater(Pb²⁺,Cu²⁺and Cd²⁺)by CA-CD/CS was also studied.The time for reaching the adsorption equilibrium was determined and the effects of CA-CD contents on adsorption capacity was investigated,using a static adsorption approach.The results showed that the adsorption of Pb²⁺,Cu²⁺and Cd²⁺on CA-CD/CS reached equilibrium in 120 min after the initial contact.The equilibrium adsorption capacity of Pb²⁺,Cu²⁺and Cd²⁺on CA-CD/CS reached the maximum at pH 6Vand was determined to be 85.487 mg/g,71.23 mg/g,and 22.158 mg/g respectively.In conclusion,the CA-CD/CS adsorbent prepared in the current work had been demonstrated as a promising adsorbent for removal of dyes and metal ions from dyeing effluents,which is stabl,renewable,and applicable in relatively large pH range.