Study On The Removal Of Heavy Metal Ions From Aqueous Solution By Zeolite NaA Synthesized From Fly Ash

With the development of the economy and industry in our country,the amount of industrial wastewaters discharged are growing rapidly,which may contain heavy metal pollutants and brings a great harm to human and ecology.Fly ash,the major solid wastes generated during the combustion of coal for energy production,can be used to synthesize zeolite,which not only diminished the harm caused by fly ash,but also provided an effective adsorbent for the removal of heavy metal ions from wastewater.In this study,zeolite NaA was synthesized from fly ash by the alkali fusion-ultrasound assisted hydrothermal method.The factors such as alkali fusion,ultrasound assist,the ratio of silicon to aluminum,alkalinity,crystal temperature and crystal time were investigated to get the optimum conditions.The optimum conditions were as follows: the ratio of silicon to aluminum in raw material is 1.0,alkalinity of NaOH 2.0 mol/L,crystal temperature 100 °C and crystal time was 24 h.Zeolite that synthesized in this study was also tested by scanning electron microscopy?SEM?,X-ray powder diffraction?XRD?,Differential thermal analysis and nitrogen adsorption-desorption.The result of SEM method demonstrated that Zeolite NaA was regular shaped,uniform cubic particle and good dispersity;the XRD analysis shows that the synthesized product are zeolite NaA without the presence of other phases;the analysis of TG-DSC indicated that the synthesized zeolite has good thermal stability.The zeolite synthesized in this study were used for the removal of heavy metal ions,(Pb²⁺,Cu²⁺,Cd²⁺,Ni²⁺,Mn²⁺)from aqueous solution.Bath method was employed to study the influential parameters such as adsorption dosage,pH and coexisting cations.Adsorption isotherms and kinetics studies were carried out in single-heavy metal system and multi-heavy metal systems,respectively.The Langmuir isotherm model was fitted to the equilibrium data better than the Freundlich model.The maximum equilibrium adsorption capacity?qm?for Pb²⁺,Cu²⁺,Cd²⁺,Ni²⁺ and Mn²⁺ ions was 65.75,56.06,52.12,34.40 and 30.89 mg/g respectively in the single system,which exhibited the same sequence and decreased to 45.28,32.86,26.93 and 16.25 mg/g respectively in the multi-heavy metal system.The kinetics of the adsorption were well described by the pseudo-second-order model,except for Cd²⁺ and Ni²⁺ ions which were fitted for pseudo-first-order model in the multi-heavy metal system.It can be seen that most adsorption took place at the first 60 min and 240 min was enough for the adsorption to reach equilibrium.Distribution coefficients?Kd?was used to evaluate the selectivity sequence of the heavy metals in this study.The result indicates the selectivity sequence of metal ions onto zeolite is Pb²⁺> Cu²⁺> Cd²⁺>Ni²⁺>Mn²⁺.Based on the results of the heavy metal removal efficiency from aqueous solutions,tests were conducted to evaluate these results using industrial wastewaters,including desulfurization wastewater from wet limestone-gypsum system and metal plating wastewater.The results demonstrated that zeolite could be used as an effective adsorbent for the removal of heavy metals from industrial wastewater.