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Content and Fractionation of Heavy Metals in Water Treatment Sludges

Dep. of Agric. Eng., Pennsylvania State Univ., University Park, PA 16802;
Dep. of Civil Eng., Pennsylvania State Univ., University Park, PA 16802.

^* Corresponding author.

ABSTRACT

The composition and distribution of Cd, Cu, Cr, Ni, Pb, and Zn in eight alum and FeCl₃ coagulation sludges were investigated using a five-step chemical fractionation procedure to assess potential impacts following soil incorporation. The mean total metal levels (mg kg^–1 dry wt.) for these sludges were Cd (<2), Cu (234), Cr (187), Ni (102), Pb (230), and Zn (557), which are well below maximum allowable levels for land-applied wastes. Two individual FeCl₃ sludges had Ni levels slightly above maximum allowable concentrations. Mean total Cr and Ni concentrations in the FeCl₃ sludges were, respectively, 5.4 and 3.1 times the corresponding alum sludge levels, reflecting elevated concentrations in the FeCl₃ coagulant. The Cu, Cr, Ni, Pb, and Zn in sludges were predominantly bound within the oxide or silicate components. A small portion of the sludge Cd was in a readily exchangeable form, whereas the dilute acid extractable and organically bound fractions contained the majority of non-residual Cd. Fractionation results suggest sludge Cd would potentially be mobile under acidic soil conditions, but the total Cd levels were very low (1–2 mg kg^–1 compared with an allowable maximum of 25 mg kg^–1). Although fractionation data must be interpreted cautiously, the results suggest most metals in water treatment sludges occur predominantly in weakly mobile, nonbioavailable forms.

Paper no. 8168 in the Journal Series of the Pennsylvania Agric. Exp. Stn.

Received for publication April 26, 1989.

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