Study On The Fraction Distribution And Leachabilitiy Of Heavy Metals In Municipal Sludges

The major restrictive factor for agronomic application of sewage sludge is heavymetals concentrated in sewage sludges. The environmental behavior of heavy metalsdepends strongly on their specific chemical fractions or binding ways in sludges. Totalcontents and fractions of heavy metals in sludges are studied, which were collectedfrom five typical municipal wastewater treatment plants of China. Leachabilities ofheavy metals release from sewage sludge by simulated acid rain are also studied.(1) Cr, Cu, Mo, Ni, and Pb mainly distributed in oxidizeable fraction and residualin sludges, indicating that these metals were relatively liable and lower potential riskin environment. Cd, Mn and Zn showed higher portion of exchangeable fractions andreducible fractions, suggesting their higher potential mobolities and risks inenvironment. The results of the distribution of heavy metal fractions in sludgesshowed the decreasing order of potential environmental risk for these studied metalswas Zn＞Mn＞Cd＞Cu＞Ni＞Mo＞Cr＞Pb.(2) The potential behavior of heavy metals in sludges depended not only on theirown metallic characteristics, but also on the compositions and physicochemicalproperties of the sludges. Further more, the effects of physicochemical properties ofthe sludges on the potential behavior of heavy metals differed from the difference ofmetals. The significance of the pH values of sludges for these metals could be ordereddecreasingly by Cd＞Mn＞Zn＞Mo＞Cu＞Pb＞Cr. And the same order for LOI andEC were Cd＞Zn＞Mn＞Cu＞Pb＞Mo＞Cr, and Cd＞Cu＞Zn＞Mn＞Pb＞Mo≈Cr, respectively.(3) The pH of leachates of sludges raised fleetly and then reaching anequilibrium status finally after 24 hours. Similar variation trends of EC, TN, TP andCOD_Cr could be found in leachates of sludges. Power function could be used todescribe the variation trends.(4) Similar variation trends of metal concentrations in leachates of sludges couldbe found for all studied metals in sludges, descending gradually with the time andreaching an equilibrium status finally. The releasing process of metals from sludgeswas simulated by the mathmetics equations in the paper. It was evidents from the thecomparation of these correlation coefficients of these equations that the double constant velocity equation(amended Freundlich equation) was more suitable for thedescription of Cr, Cu, Ni, Pb, and Mn, the parabola equation was more suitable for Cdand Mo, and the most suitable description for Zn was the rectified Elovich equation.(5) The sequence of the rates of removing for these metals from sludges is Mn＞Mo＞Cd＞Ni＞Pb＞Cr＞Cu＞Zn. Except Cd and Zn, the sequence was basically fitin with the sequence based on fractions of heavy metals in sludges.