| Coal is the main energy of our country,and the fly ash produced after burning will bring a series of problems.Its accumulation wastes land resources,causes environmental contamination if it gets into water and atmosphere.Otherwise,the harmful substances contained in fly ash are also potentially harmful to human health.However,fly ash is porous and has a large specific surface area.The adsorption capacity of fly ash will be significantly improved especially after modification.It can be used as adsorbents to remove heavy metal ions in wastewater,which can make a rational use of wastes.In this study,the fly ash modified by microwave-assisted chemical method was used as an adsorbent for hexavalent chromium and divalent mercury.First of all,five kinds of chemical reagents were chosen together with microwave to modify the fly ash,and a batch of adsorption experiments for hexavalent chromium and divalent mercury under the same conditions were carried out.The one which performed the best adsorption capacity was selected.Secondly,through a series of single factor and orthogonal experiments,the effects of modifier concentration,microwave temperature,microwave time and microwave power on the adsorption performance for modified fly ash were studied.Then,the adsorption properties of modified fly ash on hexavalent chromium and divalent mercury were investigated under the different adsorption conditions,such as the dosage of adsorbents,the time of adsorption and the pH value.Finally,the adsorption isothermal properties and kinetic properties of modified fly ash for hexavalent chromium and divalent mercury were investigated.The experimental results showed that NaOH is the best modifier for fly ash among the five modifiers,which exhibited the best adsorption performance for hexavalent chromium and divalent mercury.In the study of modification conditions,the single factor experiments illustrated that the adsorption capacity of fly ash increased first and then decreased with the concentration of NaOH,microwave temperature,microwave time and microwave power increased.The adsorption capacity of modified fly ash for hexavalent chromium and divalent mercury was obtained through orthogonal experiments.The optimum conditions for the preparation of modified fly ash were as follows:NaOH concentration 10%,microwave power 700 W,microwave temperature 60?,microwave time 10 min.Adsorption of hexavalent chromium and divalent mercury by modified fly ash reached adsorption equilibrium in 60 min and 90 min,respectively.pH value significantly affected the adsorption of modified fly ash for hexavalent chromium and divalent mercury.Modified fly ash has excellent adsorption capacity for hexavalent chromium under acidic conditions and has the worst adsorption performance under neutral conditions.The adsorption of divalent mercury by modified fly ash slightly increased then dropped significantly with the increase of pH value,and it reached the maximum when pH=5.The adsorption decreased with dosage of modified fly ash increased.The adsorption isotherms of the modified fly ash on hexavalent chromium and divalent mercury were assessed by Freundlich adsorption isotherm and Langmuir adsorption isotherm.The results showed that the Langmuir adsorption isotherm model fitted the curve better which were 0.9930 and 0.9913,respectively.The quasi-first-order kinetic equation,quasi-second-order kinetic equation and intra-particle diffusion model were used to investigate the adsorption kinetic characteristics of modified fly ash on hexavalent chromium and divalent mercury at two different concentrations.The results show that the quasi-first-order kinetic equation can only describe the initial stage of the adsorption process,while the quasi-second-order kinetic equation can well describe the whole adsorption process.The curve of intra-particle diffusion equation does not pass through the origin,indicating that the whole adsorption process is not only affected by intra-particle diffusion but also by the joint control of other functions such as liquid film diffusion. |
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