Geoscience Reference
In-Depth Information
Table 8.11 Range of selected trace element concentrations in dry, digested sewage sludge, and
in comparison with element concentrations in median sludges and in soils (Chaney
1989
)
Element
Reported range (mg/kg)
Typical median sludge
Typical soil
Minimum
Maximum
As
1.1
230
10
Cd
1
3,410
10
0.1
Cu
84
17,000
800
15
Cr
10
99,000
500
25
F
80
33,500
260
200
Hg
0.6
56
6
Ni
2
5,300
80
25
Pb
13
26,000
250
15
Se
1.7
17.2
5
Zn
101
49,000
1,700
50
relatively hydrophobic and lipophilic HgCl
2
complex, which is more prone to
salting out than other, more hydrophilic, complexes. Based on this study, Turner
et al. (
2001
) consider that neutral methylated forms of Hg(II) behave in a similar
fashion because they have a strong affinity for particulate OM and are known to be
salted out from aqueous solution. Extending this finding to other trace metals,
Turner et al. (
2002
) suggest that such contaminants are complexed by and sub-
sequently neutralize organic ligands and the resulting neutral assemblages are
salted out, possibly by electrostriction.
8.4.2 Nonadsorptive Retention
Nonadsorptive retention of contaminants may occur when chemicals reach the
subsurface as a separate liquid phase or are adsorbed on suspended particles or
organic residues. Contaminated suspended particles originating from sludge dis-
posal or polluted runoff, for example, can represent a substantial hazard to the
subsurface environment.
Table
8.11
shows the concentration range of potential toxic trace elements in
US sewage sludges, as summarized by Chaney (
1989
). In this table, data on
maximum concentration of toxic trace elements in dry, digested sewage sludges
are compared to concentrations of the elements in median sludges and in soils. The
subsurface contamination that may result from uncontrolled disposal on land
surfaces is evident. In a laboratory study carried out by Vinten et al. (
1983
), the
distribution of suspended particles from a sewage effluent on various types of soils
was studied. If, for example, heavy metals in the concentration range of the values
shown in Table
8.11
are adsorbed on transported sludge, a highly concentrated
aggregate of polluted solid material is formed. Vinten et al. (
1983
) examined the
effect of subsurface properties on the vertical redistribution of deposited solids and
