| The discharge of heavy metal sewage has become an environmental problem that is generally concerned in the world today.Copper and cadmium are two common heavy metals in industrial wastewater,which have bioaccumulation.In the meantime,Compared with free heavy metals,chelated heavy metals are more toxic and more stable,which will pose a serious threat to human health and the ecological environment.Because of its simple operation and good adsorption effect,adsorption method is one of the most widely used and potential heavy metal removal technologies today.A large amount of agricultural and forestry solid waste is generated every year in my country.Proper modification of these solid wastes and the treatment of heavy metal wastewater is conducive to the realization of waste treatment with waste,and has certain economic and environmental benefits.As a natural polysaccharide biopolymer material with wide sources,low cost and easy biodegradation,poplar sawdust is mainly composed of cellulose and lignin.Through chemical modification,specific functional groups can be introduced to improve its physicochemical properties,thereby enhancing its adsorption capacity and adsorption efficiency for heavy metals.In this paper,ethylenediaminetetraacetic dianhydride(EDTAD)was used to esterify poplar powder to obtain EDTAD modified poplar powder(EPSD),and the adsorption properties of copper,cadmium and two complex states of copper and cadmium were also studied.The effects of EDTAD dosage,modification reaction temperature and modification time on the adsorption performance of copper and cadmium were studied,and the EPSD preparation process was optimized by response surface method,and the best preparation conditions were obtained by fitting,and the EPSD samples were characterized and analyzed.Further through static adsorption experiments,the effects of adsorption time,p H value,initial volume and temperature on the adsorption of copper,cadmium and two complex states copper and cadmium by EPSD,as well as the adsorption kinetics,adsorption isotherm model and thermodynamic characteristics.Finally,the effect of the influent flow rate on the adsorption of free copper,cadmium and two complexed copper and cadmium by EPSD was investigated by dynamic experiments,and the dynamic adsorption process was modeled.Characterization of EPSD by scanning electron microscopy(SEM),elemental analysis(EDS),Fourier transform infrared spectroscopy(FT-IR),and specific surface area and pore size distribution(BET)showed that,the EDTAD modification did not significantly change the pore structure inside the poplar sawdust,mainly due to the esterification reaction with the hydroxyl groups on the surface of the poplar powder,which introduced a carboxyl functional group to the surface of the poplar powder,which was conducive to the chelation reaction with heavy metals.The surface of the modified EPSD became obviously rough,not only increased in bending and folds,but also some were porous.The specific surface area of EPSD increased significantly,which could provide good conditions for subsequent heavy metal adsorption.The preparation conditions of EPSD are mainly affected by factors such as the dosage of EDTAD,modification reaction temperature and modification time.The results showed that when the dosage of EDTAD was 3g,the temperature was 70℃,and the time was 4h,it was the best preparation conditions for EPSD.Meanwhile,the response surface design and optimization of EPSD preparation were carried out according to these three main factors,and the effect of EPSD on the adsorption of copper and cadmium under different preparation conditions was used as the measure of the modified preparation effect.The results of the response surface method show that the response surface optimization model of EPSD adsorption of copper and cadmium conforms to the objective reality and can be used for design and optimization experiments.The optimal preparation conditions for model fitting are that the dosage of EDTAD is 3.1 g,the temperature is 74.3 °C,and the time is 4.8 h,and the estimated maximum copper and cadmium adsorption capacities are 22.9 mg/g and 20.9 mg/g,respectively,and the deviations from the actual experimental values(22.9mg/g,20.6mg/g)are only 0.2% and 1.4%,respectively.The adsorption performance of EPSD for free copper and cadmium is greatly affected by the adsorption time,initial concentration,p H value,initial volume,temperature and other factors.Neutral conditions and higher temperature are more conducive to the adsorption of copper and cadmium by EPSD.The adsorption kinetic model of EPSD for copper and cadmium conformed to the pseudo-second-order kinetic model,and the adsorption behavior conformed to the pseudo-second-order kinetic equation.The Langmuir model is also more suitable for describing the adsorption isotherms of the two,and the fitted maximum adsorption capacities are 30.0 mg/g and 59.2 mg/g,respectively.Also,the adsorption of free copper and cadmium on EPSD is a spontaneous endothermic process,and the higher the temperature,the better the adsorption performance of copper and cadmium.The adsorption process of copper and cadmium by EPSD is an endothermic process,and the process of entropy increase also shows that it has a good adsorption capacity to copper and cadmium.The performance of EPSD in the adsorption of complex copper and cadmium is slightly lower than that of free copper and cadmium.Among them,the adsorption kinetic models of copper citrate,cadmium citrate,copper tartrate and cadmium tartrate on EPSD all conformed to the pseudo-second-order kinetic model.At the same time,the adsorption isotherms of chelated heavy metals are also more suitable to be described by the Langmuir model,and the fitted maximum adsorption capacities were 24.3 mg/g,103.1 mg/g,29.2 mg/g,and 56.8 mg/g,respectively.Within the scope of the experimental study,the adsorption thermodynamics showed that the adsorption process of complex copper and cadmium by EPSD was a spontaneous endothermic process,which also had good attraction to complex copper and cadmium.The dynamic adsorption experiment showed that with the continuous increase of the influent flow rate,the concentration of several heavy metals in the effluent decreased continuously at the same time,and the breakthrough curve gradually shifted to the right and the slope also increased.At the same time,the time that EPSD dynamic adsorption reaches the breakthrough point and saturation point is gradually advancing,and the adsorption saturation of EPSD for several heavy metals begins to decrease gradually.The study found that the Thomas model fits the dynamic experimental data of copper,cadmium and two complex states of copper and cadmium well,and is most suitable to describe the dynamic adsorption of free heavy metals copper,cadmium and two complex states of copper and cadmium by EPSD.EPSD has good regeneration ability.After three repeated adsorption and desorption experiments,the adsorption of copper and cadmium decreased by 24% and 28%,respectively,and the adsorption capacity of copper citrate,cadmium citrate,copper tartrate,and cadmium tartrate decreased by 31%,32%,32%,and 36%,respectively,indicating that EDTA has a good desorption effect on heavy metals,and EPSD can also be recycled in the removal of copper,cadmium and two complex copper and cadmium.EPSD showed good adsorption performance for copper,cadmium and two complexed copper and cadmium,and proved to be an efficient and renewable adsorbent.This study uses natural agricultural and forestry solid waste as raw materials,and provides new ideas and reference meaning for theoretical research and practical engineering application of complex heavy metal treatment in actual industrial wastewater. |
