| Abstract | The potential leachability of metals from sewage sludge amended soil is of great
importance in enhancing contamination of the metals to groundwater. Evaluation of those
potential leachability, bioavailability and accumulation following sludge applied to
agricultural soil would be focused in this study. In addition, geochemical forms of sludgeamended
soil were also subjected to fractionate using a sequential extraction procedure to
better understanding of their potential remobilization.
The potential leachability was obtained by soil column leaching test technique from
reconstructed soil profile where dewatered sewage sludge was incorporated into the 10-cm
soil depth. Two-sludge application rates; 150 and 300 kgN/ha, were applied to two typical
types of agricultural clay soil in Thailand; Rangsit soil (acid soil) and Thonburi soil
(neutral to slightly alkaline soil). These soil columns were leached with 32 liters equivalent
to 600 mm by different leachants in the range of pH 3 - 6.5. Mn leachability was the
highest in both sludge application rates from sludge amended Rangsit soil. With Thonburi
soil, Cd had the highest leachability at both rates. Results from the experiments indicated
that varied pH (3 - 6.5) of leaching solution had much less effect on potential leachability.
Leachability of elements from soil column depends on a rate of sludge applied, element
itself, linkage forms of metals associated with soil constituents and soil properties such as
organic matter, CEC, soil pH and minerals in soil, which have led to high buffering
capacity of soils. Focusing on Cu and Zn in soil column found that close to 100% of the
added metals from sludge was recovered within 10 cm depth at both application rates.
As a result of lysirneter experiment ( 40-cm soil depth), Chinese kale grown in sewage
sludge treatment at the rate of 150 kgN/ha gave a yield equivalent to 94 kgN/ha of
chemical fertilizer treatment. Soil pH is considered as a main effect responsible for metal
solubility and availability for plant uptake and growth. Yields of plant increase with
increasing soil pH. Therefore, Rangsit soil (soil pH 4.5) crop yield was lower than
Thonburi soil (soil pH 8.3) crop yield in every treatment from this experiment. Precaution
should be made that sewage sludge from Bangkok applied to acid sulfate soil for
agriculture at a rate equivalent to 150 kgN/ha (4-ton dry wt.Iha of sludge) is the awareness
rate concerning on heavy metal toxicity in soil.
Balancing Cu and Zn from lysimeter experiment data indicated that Cu and Zn from
sewage sludge were mainly accumulated in both soils over 99%. Cu was presented in
exchangeable form in Rangsit soil more than in Thonburi soil; therefore, such an easily
soluble form of Cu is phytotoxic in Rangsit soil. In addition, Zn was also toxic to plant
higher in acidic soil than neutral soil. However, Zn toxicity in any plant depends not only
on the bioavailable fractions of Zn in the soil such as exchangeable form but also the plant
itself and some other environmental fractions.
The selected sludge application rate of 150 kgN/ha, according to AIT recommendations
(1998) for sludge reuse in agricultural land, was found to be safe for application in neutral
or slightly alkaline soil, but not in acidic soil. This study showed that plants grown in
Rangsit soil assimilated more metals resulting in yields suppressed. Therefore, the soil pH
of acid soil should be raised up to at least 6.0 or above before sludge application. For tropical soils as Rangsit and Thonburi soil series with heavy texture types, there is no
doubt that the soils initially provide a strong protection against metal leachability as
leachant pH is in the range of 3 - 6.5; it is likely that the nature of the major inorganic
constitutes in soils (Fe, Al, Ca or P) as well as the native properties of the sludge applied to
the soils have a profound effect over the solubility of heavy metals. Thus, metal
remobilization becomes restricted only in the top few centimeters near the soil surface.
Since there was no lateral movement observed from the lysimeter experiments, therefore,
metal losses by runoff and erosion were not examined. However, metal transport with
water by rainfall and irrigation in lateral movement might possibly less occurred due to
small slope (less than 0.1 %) of the potential target areas. Most water will be moved down
through a soil profile as downward movement. Nevertheless, the potential for metal
movement both in the vertical and horizon directions, resulting from land application can
be minimized if good management practices are followed accordingly. Based on the results
obtained from this study, the incorporation of sewage sludge within 10 cm of soil depth at
the recommended rates should be operated before starting of rainy season to protect
leaching of metal to groundwater or transport with surface waters. Application of sewage
sludge in dry season is recommended to stabilize metals contained in sludge with soil
minerals, which metals' forms will be further transformed into non - easily water soluble
forms.
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