Preparation Of Mercaptopropionyl Corn Stalk And Its Adsorption Performance For Cd(Ⅱ) In Water

As a natural biomass material,crop straw can be used as an adsorbent to treat heavy metal wastewater by using its own porous structure,but the removal of heavy metal is poor by the physical adsorption mechanism of crop straw.Since the surface of straw has more active hydroxyl,carboxyl and amine groups,the adsorption performance can be improved by chemical modification for straw.In this thesis,the adsorbent for removing Cd（II）,called mercaptopropionyl corn stalk（MPCS）,was prepared by introducing sulfhydryl groups（-SH）into CS through a chemical reaction using corn stalk（CS）,levocysteine（L-Cy SH）,sodium hydroxide（Na OH）,carbodiimide hydrochloride（EDC·HCl）and N-hydroxysuccinimide（NHS）as raw materials.The removal rate of Cd（II）in water sample by MPCS was investigated,and the influencing factors in the preparation of MPCS were studied by using the single-factor experimental method.The better preparation conditions of MPCS were determined.Furthermore,the preparation conditions of MPCS were optimized by Plackett-Burman experiments,steepest climb experiments,and response surface experiments.The functional groups,morphological structures and energy spectrum changes of CS,activated CS,MPCS and MPCS-Cd were analyzed by infrared spectroscopy（FTIR）,field emission scanning electron microscopy（SEM）and energy spectrum analyzer（EDS）,respectively.These could reveal the preparation mechanism of MPCS.The adsorption performance of MPCS for Cd（II）in water sample was investigated under the conditions of different dosages,oscillation rate,adsorption temperature,adsorption time,initial p H and initial concentration of Cd（II）.The adsorption kinetics,adsorption thermodynamics and adsorption isotherm model were used to explore the adsorption mechanism of Cd（II）by MPCS.The main experimental results are as follows:（1）The better preparation conditions of MPCS determined by the single-factor experimental method were particle size of 40 mesh（0.45 mm）,activator of 0.5 mol·L^-1 Na OH solution,n（EDC·HCl）:n（Cy SH）of 0.025:1,m（NHS）:m（EDC·HCl）of 0.75:1,ratio of reactants m（CS）:m（Cy SH）of 1:5,p H 10.0 of the reaction medium,reaction temperature of55℃,and reaction time of 3 h.The highest removal rate of Cd（II）by MPCS prepared under these conditions was 98.96%.（2）The Plackett-Burman experiment screened the significant factors affecting the preparation conditions of MPCS as activator,p H of reaction medium,and reaction temperature.The steepest climbing experiment determined the horizontal center points of activator,p H of reaction medium,and reaction temperature as 0.05 mol·L^-1 Na OH solution,10.5,and 65℃,respectively.（3）The optimal preparation conditions of MPCS were obtained after optimization of response surface experiment:the particle size of CS was 40 mesh（0.45 mm）,the activator was 0.05 mol·L^-1 Na OH solution,n（EDC·HCl）:n（Cy SH）was 0.025:1,m（NHS）:m（EDC·HCl）was 1:1,m（CS）:m（Cy SH）was 1:2,the p H of the reaction medium was 9.73,the reaction temperature was 68～69℃,and the reaction time was 3 h.The highest removal rate of Cd（II）by MPCS prepared under these conditions was 95.25%.（4）Infrared analysis（FTIR）,scanning electron microscopy（SEM）,and energy spectrum analysis（EDS）showed that sulfhydryl groups were successfully introduced into the CS molecular chain in the preparation of MPCS,and a large amount of Cd（II）was adsorbed onto the surface of MPCS after adsorption.（5）The optimal adsorption conditions of MPCS for Cd（II）in water sample were that the oscillation rate was 100 r·min^-1,the adsorption temperature was 40℃,and the initial p H value was 6.0.The highest removal rate of Cd（II）in water sample with an initial concentration of 50 mg·L^-1 was 96.45%by MPCS under the adsorption conditions.（6）The adsorption isotherms showed that the Langmuir isothermal adsorption model was a good fit for the adsorption of Cd（II）by MPCS,which was mainly based on monomolecular layer adsorption.The adsorption kinetics showed that the R² value of the quasi-secondary kinetic model were greater than 0.99,which was mainly based on chemisorptions for the adsorption of Cd（II）by MPCS.The adsorption thermodynamics showed that the adsorption process was endothermic absorption and the solid-liquid interface confusion increased.