Investigation On The Adsorption Behavior Of Cd(ii)and Pb(ii)onto Carboxylated Chitosan Fibers

Cadmium and lead are highly toxic heavy metals,and untreated wastewater containing cadmium and lead discharged into the water bodies can pose serious hazards and potential risks to water ecosystems and human life.In addition,cadmium and lead are playing an important role in industrial production such as electroplating,battery production and pigment synthesis,etc.due to their excellent physicochemical properties.Therefore,it is important to remove and recover cadmium and lead from wastewater efficiently.In this study,a cost-effective,efficient and green carboxylated chitosan（CL-CTS）fibers adsorbent was developed for the removal and recovery of Cd²⁺and Pb²⁺from wastewater,which was guided by density functional theory（DFT）calculations.The adsorption and desorption behaviors of Cd²⁺and Pb²⁺onto carboxylated chitosan were investigated by a series of performance evaluation experiments and characterization analysis methods.The DFT method was employed to build a model and optimize the geometry of the chitosan（CTS）adsorbent obtained by different functional groups,such as carboxyl group,amino group,amino thiourea,phosphoric acid group,sulfonic acid group and sulfhydryl group modification.Through the calculation and comparison of the adsorption energy of Cd²⁺and Pb²⁺onto different functional groups modified chitosan,the carboxyl group was determined as the optimal adsorption functional group for Cd²⁺and Pb²⁺.In addition,the adsorption energy of Cd²⁺and Pb²⁺onto carboxylated chitosan at different sites of chitosan molecular monomer modification was classified and compared,and it was found that the adsorption performance of the adsorbents was not significantly affected whether they were modified at the amino site or the hydroxyl site.Four carboxylated chitosan fibers adsorbents were prepared using citric acid and chloroacetic acid as modifiers.Through the adsorption performance comparison experiments on Cd²⁺and Pb²⁺,the optimal method for the carboxylation modification of chitosan fibers was finally determined to be chloroacetic acid modification.By comparing the type of cross-linker and cross-linking method,the final decision was made to use ethylene glycol diglycidyl ether（EGDE）as the cross-linker and adopt a one-step cross-linking method for the preparation of CL-CTS.By optimizing the EGDE dosage and chloroacetic acid concentration,the optimum preparation conditions were obtained as follows:The optimum preparation method of the optimized CL-CTS is to take Ethylene glycol diglycidyl ether（EGDE）as crosslinking agent and adopt one-step crosslinking method,the molar ratio of CTS monomer to EGDE was 1:1.2,and the mass ratio of CTS to chloroacetic acid was 1:14（g/g）.The adsorption amount of CL-CTS for Cd²⁺and Pb²⁺was 132 mg/g and 156 mg/g,respectively,which was increased by about 111.5%and 48.6%,respectively,compared with the unoptimized preparation method.FT-IR analysis showed that EGDE was crosslinked primarily with hydroxyl sites on chitosan,and the carboxyl functional group was successfully grafted onto chitosan.The adsorption and desorption behavior of CL-CTS for Cd²⁺and Pb²⁺showed that the adsorption of Cd²⁺by CL-CTS was described better by the Langmuir isotherm model,while the adsorption of Pb²⁺was described better by the Freundlich isotherm model.CL-CTS has stronger adsorption capabilities for the adsorption of Pb²⁺at the same adsorption temperature.The adsorption process of CL-CTS on Cd²⁺and Pb²⁺was spontaneous,endothermic and was an entropy-increasing reaction.At the same adsorption temperature,the adsorption reaction of CL-CTS on Pb²⁺was preferred to occur comparing that on Cd²⁺.The kinetic studies of the adsorption of Cd²⁺and Pb²⁺by CL-CTS are well described by the Avrami model.The rate constants obtained for the adsorption of Pb²⁺by CL-CTS fitted to the different kinetic models were all greater than those for Cd²⁺,indicating that CL-CTS adsorbs Pb²⁺more rapidly.The 0.2 M HNO₃ was chosen as the best eluent for the exhausted CL-CTS after Cd²⁺and Pb²⁺adsorption,the separation of Cd²⁺and Pb²⁺could be achieved by a stepwise desorption method using sulfuric acid followed by nitric acid desorption.Cl^-and NH₄⁺ions had less effect on the adsorption of Cd²⁺,high concentration of Ca²⁺had obvious effect on the adsorption of Cd²⁺,while the presence of EDTA would significantly inhibit the adsorption of Cd²⁺by CL-CTS.The results of DFT calculation of the adsorption energy of CL-CTS on different interfering metal ions and EDTA on Cd²⁺were consistent with the experimental results.The FT-IR and XPS characterization analyses indicated that amino and oxygen-containing functional groups were involved in the adsorption reaction of CL-CTS on Cd²⁺and Pb²⁺,and the electrostatic attraction between-COO^-and Cd²⁺and Pb²⁺and the complexation of amino on Cd²⁺and Pb²⁺were the main adsorption mechanisms,and the desorption mechanism was mainly the replacement of adsorbed Cd²⁺and Pb²⁺by high concentrations of H⁺.