Synthesis Of HAP By Egg Shells And Its Application On Harmless Treatment Of Municipal Solid Waste Incineration Fly Ash

Egg shells is a common solid waste in our daily life,it has been widely studied because of its well mechanical properties and proper chemical composition.Hydroxyapatite has strong capacity in ion exchange and adsorption,and it is a good catalyst carrier material.In this paper,different kinds of hydroxyapatite were synthesized by egg shells,and they were used in the treatment of stabilization of heavy metals and degradation of PAHs in MSWI fly ash aimed to realize the purpose of"make waste harmless by waste"which is important to promote the sustainable development of China.(1)In this paper,six kinds of hydroxyapatite were synthesized,it was found that the purity of HAP synthesized by low-temperature-bath method and ultrasonic method is relatively low,and all the four kinds of HAP synthesized by hydrothermal methods had high purity and crystallinity which characterized by XRD and FTIR,and the higher the synthesis temperature was,the higher the crystallinity was,the crystallinity of HAP synthesized by microwave hydrothermal method is obviously higher than that by traditional hydrothermal method.TEM picture showed that the morphology of HAP obtained by microwave hydrothermal method was rod-shaped with a size about 240nm,while the morphology of HAP obtained by traditional hydrothermal method was irregular,with poor dispersion and unclear interface.Therefore,HAP synthesized by microwave hydrothermal was studied in next experiments.The effects of PH,dosages,reaction time,reaction temperature and initial concentration of Pb²⁺about adsorption experiment were further studied.The results showed that the optimal p H was 4,the optimal dosage was1g/L,and the optimal adsorption temperature was 35?.the adsorption model was conformed to the second order kinetics and Freundlich model.(2)To further study the stabilization of heavy metals in fly ash under microwave hydrothermal conditions with the addition of hydroxyapatite synthesized by egg shells,the influence of reaction temperature,reaction time,liquid-solid ratio and addition were considered and designed to orthogonal experiment,the result was evaluated by leaching rate and comprehensive toxicity index of heavy metals.It was found that the addition is the most important factor to leaching rate,concentration and the comprehensive toxicity index,meanwhile,the liquid-solid ratio was the least important factor.The best parameters in experiment are as follows:reaction temperature is 140?,reaction time is45 min,liquid-solid ratio is 8 ml/g,addition of HAP is 20 wt%.In order to further verify the best parameters of the experiment,single factor experiments for addition was designed,the result showed that the concentration of Pb decrease slightly under two different leaching standards,while the concentration of Zn,Cd under the acetate buffer solution method is much higher than sulfuric acid&nitric acid method,the reason might be the difference of the extraction solution in the two different leaching methods,the reaction products of HAP and Pb barely dissolved with the decrease of PH of extraction solution,however,the reaction products of HAP and Zn,Cd dissolved significantly.(3)The stabilization of heavy metals in fly ash with the addition of hydroxyapatite prehumors liquid(ESHAP)was studied in this paper.It was found that the two-step hydrothermal method with the addition of ESHAP could not only reduce the leaching concentration,leaching rate of heavy metals in fly ash,but also reduce the amount of heavy metals transferred into liquid phase during hydrothermal process.When 100ml ESHAP compounded 15wt%HAP was added,the leaching concentrations of Pb,Zn,Cu,Cr and Cd in the products decreased to 0.21 mg/L,0.18 mg/L,0.09 mg/L,0.27 mg/L and0.14 mg/L which could meet the standard of fly ash landfill.After BCR morphological analysis,it was found that the addition of ESHAP can significantly reduce the content of Pb and Zn in weak acid extraction state and increase the residue state,while it has little influence on the content of reducible state and oxidizable state.Combined the previous studies and the XRD analysis of hydrothermal products,it was found that the mechanism of stabilization of heavy metals in fly ash by ESHAP during hydrothermal treatment might be the synergistic effect between HAP and tobermorite and A-zeolite.(4)This paper further synthesized magnetic hydroxyapatite(MHAP)which characterized by XRD,FTIR,XRF identifying the composition and functional groups,then studied the oxidation degradation of PAHs in fly ash which catalyzed by MAHP in traditional hydrothermal conditions,it was found that the concentration degradation rate of PAHs reached 52.4%and the TEQ degradation rate of PAHs reached 59.2%without any addition due to hydrolytic reaction and carbonization reaction of low cyclic aromatic hydrocarbons under high temperature environment,the concentration degradation rate of PAHs reached 61.47%.The concentration degradation rate increased into 68.41%and the TEQ degradation rate of PAHs increased into 81.23%with the addition of H₂O₂(0.5 mol/L)because of the strong oxidizing·OH and peroxide free radical(·OOH)generated during the hydrothermal decomposition of H₂O₂.The toxicity reduction rate reached 65.87%with 10wt%HAP,since some free transition metal ions in fly ash,such as Cu,Zn,Ag,which can exchange Ca²⁺or the load on the HAP surface make HAP become catalytic active center for catalyzing the hydrolysis and carbonization of low-cyclic PAHs.when added 15wt%MHAP and H₂O₂,the concentration degradation rate of PAHs reached88.9%,the TEQ degradation rate of PAHs increased into 98.3%owing to the degradation of 4-6 ring PAHs,which proved MHAP has an excellent catalytic performance.It was also found that the addition of H₂O₂ promoted the liquid phase transfer of heavy metals,however,HAP and MHAP could significantly reduce the liquid phase transfer of heavy metals,and the stabilization ability of HAP to heavy metals was stronger than that of MHAP under 0.5 mol/L H₂O₂,the leaching concentrations of Pb,Zn,Cu,Cr and Cd in the hydrothermal products decreased into 1.63 mg/L,0.62 mg/L,0.14 mg/L,0.32 mg/L and 0.05 mg/L,respectively with 15wt%MHAP and 0.5 mol/L H₂O₂.