The Research On The Behavior Of Heavy Metals During The Pyrolysis Of Sewage Sludge

The present study focuses on the total content and speciation of Cu, Zn, Pb, Cd in the sewage sludge and the in the pyrolysis residues obtained at different temperatures. The influence that the pyrolysis temperature exerts on the speciation distribution and the leachability of these metals in the residues was evaluated. At first, Atomic Adsorption Spectrometry was (AAS) was employed to analyze the total concentration of the heavy metals in the sludge and the residues. The concentration of Zn (2104.8mg/kg) in the sludge was beyond the permitted value which is stipulated in Control standards for pollutants in sludges from agricultural use. The concentration of Cd (5.2mg/kg) and Cu (312.3mg/kg) were beyond the permitted value when the sludge was applied to the soil with pH<6.5. The concentration of Zn in the sludge was lower than the permitted values. When it came to the residues, the same conclusion could be attained except Cd in the residue produced during the pyrolysis at the temperature of 700℃.Secondly, BCR (Community Bureau of Reference) was used to extract the speciation of these metals and AAS was used to determine the concentration of these speciation. Cu, Cd, Pd in the sludge have great potential environment risk because the sum of the first three speciation accounts for 80%. Their bio-availability and toxicity can not be neglected. The sum of the first two speciation of Zn in the sludge is high. However some of exchangeable speciation can be fixed in the inorganic crystal lattice and pose no toxicity to the environment, so its bioavailability may be overestimated. All the absolute value of correlation coefficients between pyrolysis temperatures and the fractions of Cd, Cu, Pb, Zn in the residues are far less than 0.8 which is the minimum of high correlation. More than half of these correlation coefficients were below 0.5 and some were even below 0.3. The pyrolysis temperatures didn't effectively contribute to the distribution of metal speciation in the residues when the temperature was below 700℃, because the energy was not high enough to break the bonds.At last, modified Toxicity Characteristic Leaching Procedure was applied to determine the leachability of these metals in the sludge and residues. TCLP concentrations of Cd, Cu, Pb, Zn maximized at 0.74mg/L,15.69mg/L,10.25mg/L, 85.63mg/L at 400℃,350℃,500℃,550℃respectively. When the temperatures exceeded those points, the TCLP concentrations declined sharply. There are three reasons:Firstly, the heavy metals exist in organic chelate, with the increase of the temperature, a lot of chelate bonds were easily to break, and the metal ions can be released in the test. However, when the temperature came to some points, these metals can be fixed in the ash crystal lattice and hardly to release in the test. Secondly, the pH rose in the residues with the increasing of the temperature. So the pH buffering capacity of the residues is the other reason for the change of TCLP concentration of the metals. Thirdly, the BET surface of the residues increased when the temperature increased except 650℃(it rose to 33m3/g when the temperature got to 700℃)。It means that the residue adsorption absorbability to these metals could be improved.