Soluble Trace Metals in Alkaline Stabilized Sludge Products
The application of alkaline sludge products as liming agents to agricultural soils is now commonplace. Laboratory investigations of water extracts from two alkali-stabilized sludge products used in New York State reveals that 40 to 50% of the total Cu, 15 to 25% of the total Ni, and 11 to 13% of the Mo dissolved out of the sludge product into water, whereas much smaller fractions of the Cd, Zn, Pb, Ag, and Hg were immediately water soluble. Speciation techniques, including differential pulse anodic stripping voltammetry (DPASV), electron spin resonance (ESR), and computer speciation calculations (MINEQL+), indicated that the high solubility of Cu and Ni was a result of complexation with dissolved amines or amino acids in the alkaline (pH12) water extracts. Because of the high affinity of both Cu2+ and Ni2+ for N-ligands in amines, these metals were mobilized much more than Pb, Zn, or Cd. The low solubilities of Ag and Hg may indicate that they were present in the sludge product as the insoluble metallic or sulfide forms. Prolonged exposure of the alkaline extracts to air caused CO2 to gradually lower the solution pH to 8.1 forming a CaCO3 precipitate. However, the dissolved Cu and Ni concentrations did not diminish with pH, indicating that the metal-organic complexes were sufficiently stable at pH 8.1 to inhibit metal precipitation with the CaCO3. The results suggest potential for the transport of certain heavy metals from land-applied alkaline sludges into surface waters and shallow groundwater in some situations.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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