Study On The Effect And Mechanism Of Heavy Metals Stabilizationin MSWI Fly Ash With Phosphoric Acid

Landfill, composting and incineration are the main disposal methods of municipal solid wastes?MSW? in China. Disposing of Municipal Solid Wastes?MSW? by incineration has the remarkable advantages of harmLess, utilization and volume reduction. In recent years, the incineration capacity of municipal solid waste in China has increased rapidly. By 2014, 188 MSW incinerators have been with a incineration capacity of 5330 million tons per year, accounting for 32% of the total safe dispose capacity.The incineration capacity of municipal solid waste in China will further growth during “much starker choices-and graver consequences-in”, it's estimated that the MSW with the increase of incineration rate will reach 50% by 2020. A large amount of fly ash will be generated, fly ash gathers a lot of heavy metals, which may damage the safety of ecological environment without proper treatment.The MSWI fly ash used in this was collected from a large incineration plant located in Chongqing. The basic properties of the MSWI fly ash are analyzed, including compositional analysis and mineralogical analysis. In addition, the leaching characteristics of the target heavy metals?Pb, Zn, Cu, Ni, Cd, and Cr? were also evaluated using different leaching tests?TCLP 1311, HJ/T299-2007, HJ/T300-2007, and HJ557-2009?. The analysis results revealed that heavy metal Pb, Ni and Cd exceeded the Chinese land-filling standard?GB 16889-2008?, and heavy metal Pb exceeded the hazardous waste identification standard?GB 5085.3-2007?. The high leaching toxicity of heavy metals shows that the fly ash should subject to stabilization treatment before final disposal or utilization.Addition of phosphate has been commonly adopted for stabilization of heavy metals in contaminated systems?soils, water etc.?. This study used phosphoric acid to immobilization heavy metals in fly ash. The factors that may influence the immobilization process of heavy metals were evaluated, namely addition amount of phosphoric acid, treatment temperatures and liquid/solid ratio. What's more, the concentration of the major ions(Na⁺, K⁺, Mg²⁺, Cl^-, and SO₄^2-) in the fly ash and phosphate reaction system was also analyzed. The results showed that when the added content of phosphoric acid is the quality of 4% of fly ash, the temperature is 40? and the liquid-solid ratio is 3:1, the stabilization effect is the best. Moreover, in addition to Ca²⁺, the concentration of the other major ions in the filtrate also decreased afterstabilization treatment, indicating involvement of these ions during phosphorylation stabilization which may influence the stabilization effect of heavy metals.In effort to reveal the mechanisms controlling the immobilization of heavy metals when adding phosphate acid, a series of experiments were conducted. First, the optimal condition for systhesis of hydroxyapatite?HAP? was studied, and the results showed that Ca/P of 1.67-1.8, temperature of 40? and pH of 11.5-12.5 are suitable conditions for synthesis of HAP. A comparison experiment was conducted to evaluate the effect of heavy metals stabilization effect by hydroxyapatite and metal phosphate. The results indicated the excellent performance of HAP over metal phosphate on heavy metal stabilization. The stabilization effect of heavy metal Pb by phosphate was studied by using the optimal synthesis conditions for HAP. The result showed that when Ca/P is1.67 and temperature is 40 ?, the stabilization efficiency of heavy metal Pb reached over 99.9%. pH dependence leaching test of the Pb-contained HAP indicated its resistance to acid and alkaline corrosion. However, existence of other ions likes Na⁺, Cl-,etc., showed influence on stabilization of heavy metals.In conclusion, when the adding amount of phosphoric acid is 4% of the fly ash, the temperature is 40? and the liquid-solid ratio is 3:1, the stabilization effect of the heavy metals in fly ash is the best. However, existence of other ions may pose inevitable influence on formation of HAP, thus affect the immobilization of heavy metals in the reaction systems.