Study On Optimization Of Preparation Technology For Modified Fly Ash And Its Adsorption Properties For Pb(?) And Cd(?)

In recent years,with the development of industry,an increasing amount of heavy metals were emitted into environment,which lead to a serious environmental heavy metal pollution.Among them,Cd???and Pb???have been considered as highly toxic metal ions.Heavy metal could not be biodegraded after entering the environment and could be accumulated and transferred in organisms through the food chain.Thus its possessing threats to the ecosystem and human health.Fly ash?FA?was a large amount of available and low-cost solid industrial waste.FA has become a potential adsorbent for removing heavy metals in water due to its large specific surface area and porous structure.The adsorption capacity of fly ash was improved through chemical modification.Therefore,adopting a reasonable fly ash modification technology is great significant to improve the adsorption efficiency of fly ash,reduce the material preparation cost as well as energy consumption.In this study,fly ash was used as feedstock and Na OH used as activator to prepare high efficient mesoporous materials adsorbent through a simple one-step low-temperature alkali roasting method.This study was aim to determine the optimum preparation technology,reduce the cost and energy consumption in the modification process,at the same time increase its specific surface area and improve the adsorption capacity of Pb???and Cd???in aqueous solution.Optimum modification condition for preparing adsorbents was investigated through comparing the adsorption capacity of modified fly ash obtained by different preparation processes.The physical and chemical properties of fly ash before and after modification were analyzed by phase characterization.The static adsorption test was used to determine the optimal adsorption conditions of modified fly ash for heavy metal.The adsorption kinetics and adsorption isotherm were used to fit the heavy metal adsorption process,and reveal the adsorption mechanism of modified fly ash for Pb???and Cd???in single and binary metal solution.The main findings are as follow:?1?The effect of different mass ratio?FA:Na OH?,roasting temperature and time on the adsorption performance of modified fly ash and optimum preparation technology were investigated.The adsorption capacity of Cd???was high when the content of Na OH was large and the adsorption capacity of heavy metal by modified fly ash which were prepared at different mass ratio follows 5:8>5:7>5:6>5:5.Low temperature could improved the adsorption capacity of material,however,the higher modification temperature was not conducive to the removal of heavy metal.When the temperature lower than350?,the adsorption capacities of heavy metal by modified fly ash were increased with the increasing of calcination temperature.However,there was a dramatically decrease in the sorption amounts of heavy metal when the calcinations temperature was higher than 350?.Interestingly,long roasting time was not conducive to improve the adsorption performance of modified fly ash,the adsorption capacity of heavy metal by modified fly ash which were prepared at different roasting time followed 3 h>1 h>5 h.It was found that the modified fly ash prepared at the conditions of the FA and Na OH mass ratio was 5:8,calcination temperature of 300?and reaction time was 3 h showed the better adsorption capacities.?2?The physical and chemical properties of FA before and after modification were analyzed.The surface micromorphology,specific surface area,surface functional groups and mineral composition of FA and MFA were analyzed by Scanning Electron Microscope?SEM?,Energy Dispersive Spectrometer?EDS?,X-ray Diffraction?XRD?,Surface Area Analyze?BET?,Fourier Transform Infrared Spectroscopy?FTIR?and X-ray photoelectron spectroscopy?XPS?techniques.The results showed that quartz and mullite diffraction peak in fly ash disappeared after modification and formed an amorphous structure.The BET of modified fly ash was enlarged by a factor of 21.77.?3?The adsorption performance of modified fly ash on Pb???and Cd???in single and binary systems was revealed.The single adsorption results indicated that the adsorption equilibrium basically reached when the MFA dosage was 2 g L^-1while the solution p H was about 6,the concentration of Pb???and Cd???were 200 mg L^-1and 100 mg L^-1,respectively,the adsorption time was 30 min for Pb???and 120 min for Cd???.Under 308 K,the maximum adsorption amounts of Pb???and Cd???in a single solution were126.55 and 56.31 mg g^-1,respectively.Competitive adsorption results showed that under the same adsorption conditions,the maximum adsorption amounts of Pb???and Cd???were reduced to 120.48 and36.10 mg g^-1,respectively.Modified fly ash has higher adsorption selectivity to Pb???,the coexisting cations(K⁺,Na⁺,Mg²⁺and Ca²⁺)may inhibited the heavy metals adsorption.In addition,desorption experiments showed that the adsorbed saturated modified fly ash was desorbed 6 times with 0.1 M HNO₃and still had the ability to adsorb Pb???and Cd???.?4?The adsorption mechanism of modified fly ash on Pb???and Cd???in single and binary metal solution was investigated.The results of adsorption isotherm studies showed that the Langmuir could better fit the adsorption process of Pb???and Cd???,which indicated that the adsorption of Pb???and Cd???by modified fly ash is an endothermic process and chemisorption.The study of adsorption kinetics showed that the pseudo-second-order kinetics model could better fit the adsorption process of modified fly ash for Pb???and Cd???,this also indicated that the adsorption process was a chemical adsorption based on ion exchange.In summary,low temperature alkali roasting method is an economical and feasible method for modifying fly ash.Low temperature Na OH modified fly ash can be used as an effective adsorbent for removing heavy metals in water.It not only enriches the modification technology of fly ash,but also provides new ideas for the treatment of wastewater pollution by heavy metals.