Environmental Engineering Reference
In-Depth Information
The soil was enriched with Cd applied as nitrate. The artificially polluted
soil was moistened to field capacity and allowed to dry in cycles of
approximately 15 days within a 3-month period in order to reach equilibrium
with soil colloids (Martínez, Motto, 2000). At the end of the stabilization
period, total Cd concentration was determined by atomic absorption
spectrometry after digestion of the samples with aqua regia according to ISO
11466 (1995). Blanks were measured in parallel.
Biosolids
Biosolids from Buenos Aires City was provided by the local water
operator Agua y Saneamientos Argentinos S.A. (AySA). The aerobically
stabilized biosolids used in these experiments was previously dried in holding
pools in the biosolids treatment plant. Biosolids was thoroughly mixed with
pine sawdust (1:1, V:V) and composted in piles. Composite samples were
taken at three different stages of the composting process: i) a day after
composting started (compost 1); ii) at the end of the thermophilic stage, 10
days after the process started (compost 2); iii) mature compost, after 90 days
composting started (compost 3).
Incubation Experiments
Artificially soil contaminated samples were amended at proportions
equivalent to a field application rate of i) 50 Mg ha
-1
of compost 1 (C1); ii) 50
Mg ha
-1
of compost 2 (C2); iii) 50 Mg ha
-1
of compost 3 (C3); iv) 50 Mg ha
-1
of compost 3 + 20 Mg ha
-1
lime (LC3); v) 20 Mg ha
-1
lime (L). A control
(artificially contaminated soil CS) with no amendment was also included.
Deionized water was regularly added to keep amended and control soils to
moisture contents of 60% to ensure minimum denitrification (Rowell, 1994).
Soils were incubated in glass flasks (360 ml) in darkness in a temperature-
controlled incubator at 24º C to achieve stable soil conditions. Sampling was
managed by removing three pots per treatment at 0, 14, 28 and 42 days.
The sequential extraction procedure of McGrath and Cegarra (1992) was
used to investigate the mechanism of action of the amendments. Briefly, soil
was sequentially extracted with 0.1 M CaCl
2
for 16 h (exchangeable fraction,
EXCH-Cd); 0.5 M NaOH for 16 h (organic matter bound fraction, OM-Cd );
0.05 M Na
2
EDTA for 1 h (inorganic precipitate fraction, INOR-Cd) at a soil:
solution ratio of 1:10. The concentration of Cd in each fraction was
determined by Flame Atomic Absorption Spectroscopy (FAAS).
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