Influences Of Alkali Metals On Heavy Metals Migration And Competition In Sewage Sludge

The sewage sludge incineration, which is the efficient solutions to reducing quantity, hazard-free treatment and reclamation for sludge have been paid much more attention. In this paper, the migration characteristic of heavy metals （Pb, Zn, Cr, Cd, Ni, Cu）and the effects of alkali metals （K, Na） on the migration of heavy metals during sludge thermal treatment were researched in detail based on the combination theoretical and experimental method.The thermodynamic equilibrium calculations by HSC-Chemistry 5.0 software were used to predict heavy metal speciation transformation and influence of alkali metals on their migration during thermal treatment. The impact of K, Na on the heavy metal volatility characteristics and speciation transformation was researched by constant temperature experiments with high-temperature tubular furnace based on five-steps sequential extraction.Thermodynamic analysis results showed that Pb, Zn, Cd were mainly in silicate condensate at low temperature.When the temperature is higher than 800℃（Cr is above 400℃）, the heavy metals would transfer from condensed phase into gaseous phase. Ni,Cu exist in the form of oxide in condensed phase. There will be a few of elementary gases at high temperature. Cr existed mainly as K₂CrO₄ and Na₂CrO₄ in the condensed phase. The addition of K could promote the volatilization of Pb/Zn at 850℃, while at 1100℃it could promote the volatilization of Pb, Zn, Ni. The increase of Na content has no effect on the migration of heavy metals.It indicated that K could promote the the volatilization of Pb, Zn, Cd, Cu, Ni,the volatility increased significantly when K is 4.5%wt according to the constant temperature experiment and speciation analysis.Na contribute to the volatilization of Pb, Zn, Cd, Cu, Ni.While Cr volatilization rates decreased gradually with the addition of K, Na.The addition of K（When the content of K is lower than 4.5%wt）could promote the state of Pb, Zn, Cd, Ni to trsnsfer to unstable state,the content of unstable state would decrease with the continuous increasing of K. The content of unstable state which include ion exchange form and copper bonded to carbonates of Pb, Zn, Cd, Ni with the addition of Na.While Cr would transfer to stable state with the addition of K, Na. Toxicity analysis showed that adding K and Na can reduce the potential ecological risk of combustion residues during sludge incineration.