Preparation And Adsorption Performance Of Mercaptopropionyl Wheat Straw As Heavy Metal Adsorbent

Heavy metals have the characteristics of high toxicity,easy enrichment,strong mobility and non-degradability.Recently,a large amount of wastewater containing heavy metal has been produced in the production process of industry and agriculture.If it is not disposed properly,it will cause immeasurable damage to the ecological environment and human health.Therefore,it is necessary to seek an efficient and economic method for the treatment of heavy metal wastewater.At present,the common methods for the treatment of heavy metal mainly include ion exchange,chemical precipitation,membrane separation,and adsorption,etc.The adsorption method has the advantages of wide source of raw materials,simple operation,good removal effect,no sludge,recyclable,renewable and environmental friendly,etc.Moreover,common adsorbents,such as activated carbon,carbon nanomaterials and carbon aerogels,have good removal performance for heavy metals,but their high preparation costs are not conducive to their wide application in practical wastewater treatment.Wheat straw is cheap,easy to obtain and rich in reserves,and its structure is rich in active functional groups,such as hydroxyl group（—OH）,carboxyl group（—COOH）,and amine group（—NH₂）,which has the function of capturing heavy metals.If wheat straw is used as an adsorbent to remove heavy metals from wastewater,it can not only reduce the treatment cost of heavy metal wastewater,but also alleviate the environmental pollution caused by the placement or incineration for wheat straw.However,there are a large number of hydrogen bonds in the molecule and waxy film on the surface of wheat straw,which leads to a low adsorption capacity for heavy metals.If the waxy film on the surface and the hydrogen bond inside the molecule are effectively removed by acid-base chemical pretreatment,the grafting of mercaptopropionyl group（—SH）,amino group（—NH₂）,and other functional groups will significantly improve the chemical characteristics of the straw surface and significantly enhance its adsorption performance for heavy metals.In this paper,a novel adsorbent,named mercaptopropionyl wheat straw（MPWS）,was prepared by the chemically modifying with chemical pretreatment of WS（CWS）and L-cysteine（L-Cys）.The influence of several factors on the preparation of MPWS was studied by single factor method.The preparation conditions of MPWS were optimized by combining Placket-Burman experiment,steepest climbing experiment and response surface experiment,and the MPWS with the best adsorption performance for Cd（Ⅱ）and Cu（Ⅱ）were obtained respectively.FTIR,SEM,EDS,BET and element analysis were used to explore the preparation mechanism of MPWS.Taking Cd（Ⅱ）,Cu（Ⅱ）and mixed Cu（Ⅱ）and Cd（Ⅱ）as research objects,the adsorption characteristics of MPWS were studied by single factor experiment.The adsorption mechanisms of MPWS for Cd（Ⅱ）and Cu（Ⅱ）were investigated by combining adsorption kinetics,adsorption isotherm and adsorption thermodynamics experiments,and various characterization methods.The desorption mechanisms of MPWS-Cd and MPWS-Cu were discussed by means of desorption kinetics and desorption thermodynamics experiments,and the regeneration performance of desorbed MPWS was also investigated.Furthermore,the effects of coexisting inorganic matter,coexisting organic matter and turbidity on the adsorption of mixed Cu（Ⅱ）and Cd（Ⅱ）by MPWS were investigated.The main results are presented as follows:（1）According to the single-factor experiment,when the particle size of WS was 0.60mm（30 mesh）,the concentration of Na OH solution as chemical pretreatment was 0.1 mol/L,the n（EDC·HCl）:n（L-Cys）was 0.05:1,the m（NHS）:m（EDC·HCl）was 1:1,the m（CWS）:m（L-Cys）was 1:5,the p H value of reaction medium was 10,the reaction temperature was 35℃,and the reaction time was 3 h,the prepared MPWS had the best adsorption performance for Cd（II）,and the maximum removal rate of Cd（II）could reach94.20%.The order of adsorption performance for Cd（Ⅱ）was MPWS>CWS>WS.This indicates that the adsorption performance of wheat straw for Cd（Ⅱ）was effectively improved after chemical pretreatment,and the adsorption performance was further improved after modification by mercaptopropionyl groups.（2）The main influencing factors and center point of levels in the preparation of MPWS were determined through Plackett Burman and steepest climbing experiment experiments that experiment that the concentration of Na OH solution was 0.1 mol/L,the n（EDC·HCl）:n（L-Cys）was 0.075:1,the m（L-Cys）:m（CWS）was 3:1 and the reaction time was 2 h.Through the central composite design（CCD）experiment,the optimum preparation conditions of MPWS corresponding to the adsorption of Cd（Ⅱ）were determined as follows:the particle size of WS was 30 mesh（0.60 mm）,the concentration of Na OH solution was 0.07 mol/L,the n（EDC·HCl）:n（L-Cys）was 0.099:1,the m（NHS）:m（EDC·HCl）was 0.5:1,the m（L-Cys）:m（CWS）was 4:1,the p H value of the reaction medium was 11,the reaction temperature was 40℃,and the reaction time was 2.39 h.Under this preparation condition,when the dosage of MPWS was 20 g/L,the maximum removal rate of Cd（Ⅱ）could reach95.56%.The optimum preparation conditions of MPWS corresponding to the adsorption of Cu（Ⅱ）were determined as follows:the particle size of WS was 30 mesh（0.60 mm）,the concentration of Na OH solution was 0.07 mol/L,the n（EDC·HCl）:n（L-Cys）was 0.088:1,the m（NHS）:m（EDC·HCl）was 0.5:1,the（L-Cys）:m（CWS）was 4:1,the p H value of reaction medium was 11,the reaction temperature was 40℃,and the reaction time was 2.5 h.Under this preparation condition,when the dosage of MPWS was 6 g/L,the maximum removal rate of Cu（Ⅱ）could reach 92.52%.（3）For the preparation mechanism of MPWS,Na OH solution removed a certain amount of hemicellulose,lignin and fatty waxy film from wheat straw.After the esterification and amidation reaction between—OH and—NH₂in mercaptopropionyl groups and—COOH,—NH₂and—OH in wheat straw,mercaptopropionyl groups were successfully grafted into the molecular chain of wheat straw.The prepared MPWS mainly removes heavy metals by chemical adsorption.（4）When the oscillation rate was 150 r/min,the adsorption temperature was 30℃,the adsorption time was 2 h,and the initial p H value of the water sample was 6.0,the MPWS had the best adsorption effect on Cd（Ⅱ）and Cu（Ⅱ）.The maximum removal rate of Cd（Ⅱ）in the water sample with the initial concentration of Cd（Ⅱ）of 100 mg/L was 97.21%,and the maximum removal rate of Cu（Ⅱ）in the water sample with the initial concentration of Cu（Ⅱ）of 100 mg/L was 97.28%.The adsorption process of Cd（Ⅱ）,Cu（Ⅱ）and mixed Cu（Ⅱ）and Cd（Ⅱ）by MPWS could conform to the pseudo-second-order kinetic mode.The adsorption behavior of MPWS for Cd（Ⅱ）,mixed Cu（Ⅱ）and Cd（Ⅱ）accorded with the Langmuir isotherm model,while the adsorption behavior for Cu（Ⅱ）accorded with the Freundlich isotherm model.The adsorption of Cd（Ⅱ）and Cu（Ⅱ）by MPWS was a spontaneous endothermic reaction.In the adsorption of mixed heavy metals,the adsorption of Cu（Ⅱ）by MPWS was a spontaneous exothermic process,while the adsorption of Cd（Ⅱ）by MPWS was a non-spontaneous endothermic process at low dosage,and at high dosage,the adsorption of Cu（Ⅱ）by MPWS was spontaneous at lower temperature and non-spontaneous at higher temperature.The adsorption thermodynamic behavior of Cd（Ⅱ）by MPWS was opposite to that of Cu（Ⅱ）.The adsorption mechanism of MPWS for Cd（Ⅱ）and Cu（Ⅱ）mainly includes coordination,ion exchange,electrostatic adsorption and physical adsorption,while the adsorption mechanism of MPWS for mixed Cu（Ⅱ）and Cd（Ⅱ）mainly includes coordination,ion exchange,electrostatic adsorption,physical adsorption and competitive adsorption.（5）HCl solution and EDTA solution had good desorption performance for MPWS-Cd and MPWS-Cu.The desorption process of HCl solution and EDTA solution for MPWS-Cd and MPWS-Cu could conform to the pseudo-second-order kinetic model,and the process was spontaneous and endothermic.The regeneration performance of MPWS desorbed by EDTA solution was better,with the regeneration rate of 91.20%for Cd（Ⅱ）and 83.96%for Cu（Ⅱ）;The desorption mechanisms of HCl solution to MPWS-Cd and MPWS-Cu are mainly protonation and electrostatic repulsion.The desorption mechanism of EDTA solution to MPWS-Cd is mainly coordination competition.（6）Coexisting inorganic substances mainly inhibited the adsorption of Cu（Ⅱ）in mixed heavy metals by MPWS at low concentration,and promoted it at high concentration;while they mainly showed inhibition for Cd（Ⅱ）,and the inhibition increased with the increase of inorganic salt concentration.Coexisting organics had a great influence on the adsorption performance of MPWS for Cu（Ⅱ）and Cd（Ⅱ）.EDTA,sodium citrate and sodium pyrophosphate had a strong inhibitory effect on the adsorption of Cu（Ⅱ）by MPWS.EDTA had a strong inhibitory effect on Cd（Ⅱ）in mixed water samples.Sodium citrate and sodium pyrophosphate showed a promotion at low dosage and a inhibition at high dosage.The effect order of different organic substances on the adsorption of Cu（Ⅱ）and Cd（Ⅱ）by MPWS showed EDTA>sodium citrate>sodium pyrophosphate.Turbidity could promote the adsorption of Cu（Ⅱ）by MPWS,while it could promote the adsorption of Cd（Ⅱ）at high concentration and inhibit the adsorption of Cd（Ⅱ）at low concentration.The noval heavy metal adsorbent MPWS has simple preparation process,high efficiency and low cost.MPWS has good adsorption effect on different concentrations of Cd（Ⅱ）,Cu（Ⅱ）and mixed Cu（Ⅱ）and Cd（Ⅱ）.The good desorption and regeneration performance for MPWS with Cu（Ⅱ）and Cd（Ⅱ）.Therefore,MPWS has potential value in the application of actual heavy metal wastewater.