- Application of sewage sludge to soils increases the trace metal
content and the organic content of soils. The complexation of indigenous and added metals with organic constituents must be studied to
evaluate metal availability to plants and animals.
The water soluble organic fractions (WSOF) from an agricultural
soil (W), a soil treated with sludge for 6 years (WS₆), a sludge-soil
mixture incubated for 1 week (WS₁), and sewage sludge (SS) were
extracted and purified. The WSOF samples were assayed for C, H, N, S,
0, P, total acidity, C00H, and phenolic OH, and by UV-visible, infrared,
and proton nuclear magnetic resonance spectroscopy. The Cu(II) binding
mechanisms in the WSOF samples were studied by electron spin resonance
(ESE.) spectroscopy and potentiometric titrations. Conditional ratios
of complexed metal (Mc) to free metal ions (M) at 296 K were calculated.
The SS contained higher organic H, N, and P contents, a higher H/C,
and a lower C/N ratio than W. Total acidity, carboxyl, and phenolic
hydroxyl group contents were generally highest in SS, intermediate in
WS₆ and WS₁, and lowest in W. Overall aromatic character and aromatic carboxyl group contents were highest in W, and lowest in SS while the
aliphatic proton and carboxyl group contents were highest in SS, and
lowest in W. Protein decomposition products were most prevalent in SS,
and decreased in concentration as the sludge component in the mixture
At 77 K, the Cu(II)-W complex showed an anisotropic ESR spectrum
while the Cu(II)-SS complex showed an isotropic spectrum. These spectral
results suggest that the oxygen donor ligands of W may form relatively
strong bonds with Cu(II) due to extensive equatorial chelation while
ligands of SS may form little or no chelate bonds with Cu(II).
The ratio of complexed metal to free metal ions in WSOF samples
increased as sewage sludge was added to the soil. A maximum Cu(II)
binding capacity for W and W₆ was attained near the value of 0.1 mol Cu(II)/kg WSOF while it was not reached for WS₁ or SS.
Although the bond strength between Cu(II) and W is stronger than
the bond between Cu(II) and SS due to chelate bond in the former, the
Cu(II) complexing capacity of a soil appears to increase with sludge