Study On Carboxylation Modification Of Cotton Fabric And Its Adsorption For Heavy Metal Ions

It is crucial to effectively treat the water sources polluted by heavy metal ions for the safety of environment and human health.Among the various treatment methods,the adsorption method is widely used due to its simple operation,low cost,and good performance.In recent years,developing low-cost and green adsorption materials has become the study focus.Cellulose is a kind of widely distributed,inexpensive and renewable natural bio-based material.It contains a large number of hydroxyl groups,which enables it to adsorb heavy metal ions.However,when cellulose is directly used for adsorption,the performance is usually poor,therefore it is of great importance to modify cellulose.1,2,3,4-butanetetracarboxylic acid（BTCA）is a good modification agent for cellulose,while there is no report on the reaction sites of cellulose and BTCA,which restricts the controllable modification of cellulose and development of the cellulose-based efficient adsorbents.In this thesis,the reaction sites of cellulose and BTCA was first investigated by combining Fourier transform infrared spectroscopy（FTIR）with two-dimensional correlation spectroscopy（2Dcos）analysis.Results showed the changing sequence of related hydroxyl groups as follows（“→”means earlier than or prior to）:v（O（3）-H（3）…O（5））（intramolecular hydrogen bond）→v（O-H）（weak hydrogen bond）→v（O-H）（free）→v（OP（6）-H（6）…O’（3））（intermolecular hydrogen bond）→v（O（2）-H（2）…O（6））（intramolecular hydrogen bond）.Therefore,the 6-position（O（6）-H（6））hydroxyl group of cellulose is more active than the 2-position（O（2）-H（2））to participate in the esterification reaction.The changing sequence of related carboxyl groups was:v（C=O）（BTCA carboxyl carbonyl,hydrogen bond）→v（C=O）（BTCA carboxyl carbonyl,hydrogen bond）v（C=O）（BTCA carboxyl carbonyl,hydrogen bond）→v（C=O）（BTCA carboxyl carbonyl,free）→v（C=O）（BTCA anhydride）→v（C=O）（ester carbonyl）→v（C=O）（ester carbonyl）.Therefore,the adjacent carboxyl groups of BTCA first dehydrated to form anhydride intermediate,and then the formed anhydride reacted with cellulose hydroxyl to form ester bond.Experimental results and Gaussian calculation results of the reaction between cellobiose（cellulose model chemical）and BTCA also proved the above conclusion,and indicated that C14-and C11-carboxyl groups on BTCA molecule were more likely to preferentially participate in the reaction.For esterification reaction of cellulose（cellobiose as model）and BTCA,we proposed the following possible reaction route:BTCA→C14015 ester→C31C34 anhydride→C14015C330 ester.In addition,the difference of BTCA-esterification rates between raw cotton fabric（CF）and the selective-oxidized cotton fabric also showed that the reaction activity of cellulose O（6）-H（6）was higher than that of O（2）-H（2）.According to the above research results of reaction site mechanism between BTCA and cellulose,we adopted sodium periodate-sodium chlorite（NaIO4-NaClO2）system to selectively oxidize O（2）-H（2）and O（3）-H（3）of cellulose into carboxyl groups,and the retained active O（6）H（6）can be further modified with BTCA.Further,the BTCA-modified cotton was used to adsorb Pb2+.Results showed that the best oxidation time of NaIO4 was selected as（sample was denoted as POCF2）.And the optimum conditions for BTCA modification of cellulose were obtained as follows:concentration of BTCA was 50 g/L,curing temperature was 160℃,and curing time was 1 min（sample was denoted as B50-POCF2（160+1））.The influence of adsorption conditions on the adsorption performance of B50-POCF2（160+1）was investigated in detail.When the pH of adsorption solution was 4,B50-POCF2（160+1）showed an equilibrium adsorption capacity（Qe）of 123.92 mg/g for Pb2+.With the increase of the initial concentration of Pb2+,the Qe of B50POCF2（160+1）for Pb2+ increased gradually,and tended to be constant when the initial concentration of Pb2+ exceeded 300 mg/L.When the temperature increased from 303K to 323K,the Qe of B50POCF2（160+1）for Pb2+ increased from 127.30 mg/g to 146.38 mg/g.The adsorption process of B50POCF2（160+1）for Pb2+ was more in line with the pseudo-second-order kinetic adsorption model,and followed the Langmuir adsorption isotherm model.The Gibbs free energy change of the adsorption process was-18.92 kJ/mol at the experimental temperature（303K）and was accompanied by endothermy and entropy increase.The adsorption capacity of B50-POCF2（160+1）for Pb2+remained 73.86%after five cycles with intact morphology,reflecting a good recycling potential.It can also adsorb different heavy metal ions including Pb2+,Cu2+,Cr3+,and Cr6+,and showed a good selective adsorption for Pb2+.