METAL LEACHABILITY FROM SEWAGE SLUDGE-AMENDED THAI SOILS

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METAL LEACHABILITY FROM SEWAGE SLUDGE-AMENDEDTHAI SOILSPreeda Parkpian a , Katerachada Klankrong b , Ronald DeLaune c & Aroon Jugsujinda ca School of Environmental Resource and Development (SERD) , Asian Institute of Technology(AIT) , Urban Environmental Engineering and Management Program, P.O. Box 4, Klong Luang,Pathumthani, 12120, Thailandb Water Quality Management Division , Department of Drainage and Sewerage , BangkokMetropolitan Administration , Dingdaeng, Bangkok, 10320, Thailandc Wetland Biogeochemistry Institute , Louisiana State University , Baton Rouge, Louisiana,70803, U.S.A.Published online: 06 Feb 2007.

To cite this article: Preeda Parkpian , Katerachada Klankrong , Ronald DeLaune & Aroon Jugsujinda (2002) METALLEACHABILITY FROM SEWAGE SLUDGE-AMENDED THAI SOILS, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 37:5, 765-791, DOI: 10.1081/ESE-120003588

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METAL LEACHABILITY FROM SEWAGE

SLUDGE-AMENDED THAI SOILS

Preeda Parkpian,1 Katerachada Klankrong,2

Ronald DeLaune,3,* and Aroon Jugsujinda3

1Urban Environmental Engineering and Management Program,School of Environmental Resource and Development (SERD),

Asian Institute of Technology (AIT), P.O. Box 4,Klong Luang, Pathumthani 12120, Thailand

2Water Quality Management Division, Department of Drainageand Sewerage, Bangkok Metropolitan Administration,

Dingdaeng, Bangkok 10320, Thailand3Wetland Biogeochemistry Institute, Louisiana State University,

Baton Rouge, Louisiana 70803, USA

ABSTRACT

Determining mobility and availability of metals in sewage sludgeamended soil is an environmental concern. Potential leachability andbioavailability of metals following sludge applied to agricultural soilwas evaluated. Geochemical forms of metal occurring in sludge-amendedsoil were subjected to fractionation for understanding heavy metal trans-formation and remobilization in sludge-amended soil. Metal leachabilitywas determined using reconstructed soil profiles where dewatered sewagesludge was incorporated into the surface 0–10 cm of soil. Two-sludgeapplication rates; 150 and 300 kgN/ha, equivalent to sludge at 4 and 8ton/ha, were applied to soil columns representing typical agriculturalclay soils of Thailand (Rangsit acid sulfate soil). The soil columnswere leached with 32 l equivalent to 600mm of surface water using dif-ferent leachants (distilled water pH 6, distilled water adjusted to pH 3

765

Copyright # 2002 by Marcel Dekker, Inc. www.dekker.com

*Corresponding author. E-mail: [email protected]

J. ENVIRON. SCI. HEALTH, A37(5), 765–791 (2002)

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and rainwater pH 5). Among metals measurement, results showed Mnleachability from sludge-amended Rangsit soil were high at both sludgeapplication rates (18–29% of total Mn applied). The leachability of othermetals was less than 2.5 and 7.2% following application of 150 and300 kgN/ha of sludge, respectively. Results from the experiments indi-cated that the leachant at pH 3 had the most effect on potential leach-ability of Cu, Zn, Cd, and Ni, except Fe and Mn, at low sludgeapplication rate. Whereas, only the leachability of two metals was influ-enced by the lowest pH (pH 3) when sludge applied was increased.Besides pH of leachant, it appeared that leachability of elements fromthe soil column depended on rate of sludge applied, the particular metal,and metal form or fraction. The soil studied had organic matter, CEC,pH, clay content, etc., that resulted in high buffering capacity, whichfavors metal retention. Less than 0.5 and 1.8% of the added Cu andZn applied at the 150 and 300 kgN/ha application rates, respectively,were detected in the leachate. Approximately 100% of the added Cuand Zn in the sludge remained in the surface 10 cm at each sludge appli-cation rate. Sequential extraction of sludge-amended soil following leach-ing (at the high sludge application rate) revealed that most of the Curemained in the surface sludge-amended soil layer (0–2 cm depth) in theform of organic and residual Cu fractions that are not easily mobilized.An exchangeable fraction of Zn increased, approximately representing60% total Zn applied in sludge–soil layer as compared with its nativesoil Zn fractions. These results demonstrate that Zn availability in the soilwould increase as a result of sludge application. However, the total Zn inthe leachate is safe for agricultural use, because it represents an amountof less than 2% of total Zn applied.

Key Words: Heavy metal; Leaching; Sewage sludge; Linkage form;Soil column

INTRODUCTION

Like many countries in Southeast Asia, Thailand is regarded as anagricultural country, and the majority of national income comes from agri-cultural exports. Most of Thai agricultural soils have been used extensivelyfor crop production for extended periods of time and are usually low in soilorganic matter (SOM) and essential plant nutrients. Added nutrients (chemi-cal fertilizer) are needed for maintaining and increasing crop production.As a result, the fertilizer applications have increased substantially.Chemical fertilizers are most commonly imported, being paid for in foreigncurrency equivalent to several thousand millions Baht. To reduce such expen-ditures for chemical fertilizers, sewage sludge (which is considered an organicfertilizer) is available from wastewater treatment plants in Bangkok for use asan alternative source of plant nutrients for crops. Agricultural application of

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sewage sludge has great potential as a fertilizer source and as a meansfor waste management of sludge material.

Bangkok sewage sludge contains a large quantity of organic matter,nutrients (N, P, and K), and major and minor nutrients (Cu, Zn, Fe, etc.)that can be of value for maintaining crop production. Not only can sludge beused as a fertilizer source to maintain crop productivity, but sludge can alsoimprove physical properties of soil (1), e.g., by increasing soil porosity andthe stability of soil aggregation (2). Sludge application can increase the com-plexation and adsorption capacity of soil, enhancing the ability of the soil toretain metals (3).

Sewage sludge can be beneficial, but it can also contain potentiallyhazardous heavy metals. Sewage sludge, upon mineralization by microorgan-isms, could possibly release solubilized heavy metals into the soil solution andmove into groundwater (4). Likewise, the accumulation of sludge-borneheavy metals in soil following repeated sludge application can cause phyto-toxic concentration. Heavy metals may persist in the soil environment andbe taken up by agronomic plants, entering the edible parts of vegetabletissues, and thus, representing a direct pathway for incorporation of heavymetals into the human food chain.

Thailand has a wide range of soil types. Acid sulfate soils represent asignificant portion of the soil types used for agricultural production. Acidsulfate soil is estimated to cover a total of 40% of Thailand’s agriculturalland area and is used for many of the major cropping systems (5). Acidsulfate soils support poor agricultural productivity because of acidity. LowpH of soil greatly influences the release of metals (6).

At present, the feasibility of using sludge as a soil amendment or ferti-lizer in Thailand is a concern for consumers and the environment. The con-sequences of sludge application to agricultural soil, especially acid sulfatesoil, with regard to the long-term bioavailability and movement of heavymetals into the environment are incompletely understood. The determinationof potential hazards of heavy metals released from sewage sludge applied tosoil requires a knowledge of metal remobilization and bioavailability char-acteristics that are dependent on chemical and physical forms of the metals insludge and reactions with soil (7,8). To provide answers to these concerns, aseries of laboratory experiments (using packed soil columns and leachingtechniques adapted from various standard column-leaching tests) (9) wereconducted to simulate the long-term effect of leaching behavior of heavymetals from sludge-amended soil. Many researchers applied leaching testtechniques to estimate the potential and actual leachabilities of heavymetals (4,10–12).

Tropical weather conditions (rainfall and temperature effects) influencemetal transformation in soil differently than in temperate zones, in terms ofturnover rate and immobilization in various geochemical soil fractions. Tobetter understand heavy metal mobility down to subsurface soil layers

METAL LEACHABILITY FROM SEWAGE SLUDGE 767

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following surface application of sewage sludge, selective and sequentialextraction techniques have been employed (13–15). Modifications of theprocedure, such as strength and pH of leachants and use of rainwater,have been utilized in evaluating metal movement. Such modifications havebeen designed to simulate the actual tropical field conditions utilizing varioussludge liquid ratios equal to rainfall scenarios and sludge application rates.

The objectives of this study were to (i) evaluate the potential leach-ability, availability, and accumulation of heavy metals in Thai soil followingsludge application at several rates (150 and 300 kgN/ha); (ii) identify thedevelopment and distribution of different geochemical forms of metal-sludge-amended soil using the sequential extraction procedure; and (iii) for-mulate recommendations for sludge application in soil that will avoid excessmetal mobility and phytotoxicity to agricultural systems.

MATERIALS AND METHODS

Material Sampling and Characterization

Plough (topsoil) layer (0–15 cm) with no previous history of sludgeapplication was utilized in this study. The soil obtained from Khlong 5,Rangsit, Pathum Thani is classified as acid sulfate soil (Rangsit soil series).The field had been abandoned for 2 years prior to sampling. Average annualrainfall for the site is 1040mm (600mm for the rainy season) (MeteorologicalDepartment, Thailand 1991–1999). Soil was collected at 0–15 cm depth sur-face after plant residue or debris was removed. At the experimental site, soilwas air-dried under shade at ambient temperature (25–30�C) and crushed bymortar and by hand and sieved through 2mm stainless steel screen. Sievedsoil was kept in plastic bags prior to experiments and analyses. Variouscharacteristics of soil, both physical and chemical properties, which influenceleaching, adsorption, and decomposition of metals in soil, were analyzed.Dewatered sewage sludge was obtained from Sri Phraya WastewaterTreatment Plant and stored at 5�C. Sewage sludge was also chemically andphysically characterized before sludge application.

Soil Column Leaching Experiments

Leaching experiments were conducted at the Asian Institute ofTechnology (AIT). Seven soil columns constructed from PVC pipe (60 cmlength and 25.4 cm diameter) were maintained in ambient laboratoryconditions. In each column, as shown in Figure 1, a porous acrylic platewas installed inside PVC pipe at the height of 20 cm from the bottom, anda plastic funnel was attached under that plate. The inside PVC pipe wall wascoated with resin to protect it from corrosion. Above the porous acrylic plate,

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the column wall was sanded to increase roughness of the wall for minimizingany edge effect. To facilitate drainage and soil retainage, 3 cm depth of acid-washed gravel was put over the porous plate. A geotextile was then placed onthe top of the gravel to prevent soil loss during leaching. Washed sand wasplaced onto the geotextile to inhibit soil clogging in the geotextile. The soilwas packed into the column and initially rewetted by distilled water to avoidentrapment of air in the soil pore, until bulk density and water contentreached field conditions at 1.04 g/cm3 and 47%. Soil thickness after soilsaturation was 10 cm. The quantity of sewage sludge applied to the soilcolumns was determined according to AIT recommendation (16) for sludgeapplication rates for agricultural soils. Dewatered sewage sludge under wetcondition was applied and mixed with topsoil (0–2 cm depth) at an appli-cation rate of 150 and 300 kgN/ha as a dry weight basis. The columns wererested to stabilize for 1 week, while water content was maintained at 70%field capacity during stabilization. The soil columns were leached using lea-chants shown in Table 1.

Eight liters of a leachant, which was equivalent to 150mm of simulatedrainfall on a monthly basis, was continuously passed downward through thesoil columns. A second leaching was conducted after gravitational flowceased 3 days afterward. A total of four leachings (32 l as being equivalentto 600mm, representing average annual rainfall statistics in Pathum Thaniprovinces during the rainy season for 4 months) was sprayed onto topsoil ineach soil column. Leachate following each leaching episode was collected inplastic bottles by gravity flow. Volume, pH, and dissolved organic carbon(DOC) were immediately measured and recorded. One liter of aliquot wasfiltered through Whatman no. 5 filter paper and stored in a refrigerator until


Figure 1. A schematic of a soil column.

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analyzed. The columns, following a 1-week rest after the last leaching, weredismantled. The soil core was subsequently removed from the column cutinto five portions: 0–2, 2–4, 4–6, 6–8, and 8–10 cm.

Sample Analysis

All samples were characterized for selected parameters utilizing a spe-cific standard method (Table 2) prior to the leaching experiments. In addi-tion, geochemical forms or metal speciation was conducted from the sludge-amended soil samples after leaching soil columns were completed.

RESULTS AND DISCUSSION

Characteristics of Soil

Physical and chemical properties of Rangsit soil were analyzed to eval-uate metal availability, leachability, and distribution in the soil column of theprofile. The soil properties are presented in Table 3. Soil pH was relativelylow with a value of 4.58. Any soil with pH below 6.5 could increase metalsolubility, resulting in phytotoxicity to plants. Rangsit soil contained 53%clay, which is classified as a clay soil (17). Total nitrogen content of the soilwas 0.05%, which is considered low for plant growth. Nitrogen applicationis necessary for supplying sufficient nutrients for plant growth in this soil.Phosphorous content of Rangsit soil was low, because soil from the field sitehad not been amended with P fertilizer for the last 3 years. Exchangeablecations in Rangsit soil were adequate and were derived mainly from a

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Table 1. Summary of Soil Column Leaching Experiments

Treatment Leachants

At 150 kgN/ha sludge applied1. Rangsit soil (Control) Distilled water (pH 6)2. Rangsit soilþ sewage sludge Distilled water (pH 6)3. Rangsit soilþ sewage sludge Distilled water adjusted

to pH 3 with H2SO44. Rangsit soilþ sewage sludge Rainwater* (pH 5)

At 300 kgN/ha sludge applied5. Rangsit soilþ sewage sludge Distilled water (pH 6)

6. Rangsit soilþ sewage sludge Distilled water adjustedto pH 3 with H2SO4

7. Rangsit soilþ sewage sludge Rainwater* (pH 5)

*Rainwater was collected after 15min rainfall during the rainyseason.

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previous application of lime and fertilizer. Soils with high clay content, suchas Rangsit soil, have a high cation exchange capacity (35meq/100 g). Thewater-soluble fraction of heavy metals was low in this soil. Total heavy metalcontent of the soil was also relatively low due to no previous history ofreceiving metals in the form of sludge application.

Characteristics of Sewage Sludge

Sewage sludge from Bangkok Wastewater Treatment Plant was alsoanalyzed for selected parameters, which are shown in Table 4. Sludge pH(6.94) was near neutral using a 1 : 5 sludge-to-water ratio. Total nitrogenin sewage sludge was approximately 3.60%, with available nitrogen beingapproximately 755mg/kg. Such a nitrogen level is sufficient for maintain-ing plant growth. The sewage sludge used in the study is an excellentsource of many plant nutrients. High amounts of calcium in sludgeresulted from liming in a dewatering process. The Ca could increase theadsorption of metals by exchanging with cations and coprecipitation as


Table 2. Sample Analysis Method

Material Parameter Method

Sludge and soil Particle sizedistribution

Hydrometer method described by Sheldrickand Wang (47)

pH Electrometric method described byHendershot, Lalande, and Duquette (47)

Total nitrogen Semi-micro kjeldahl method described byMcGill and Figueiredo (47)

Total phosphorous HC1O4 acid digestion method described by

O’Halloran (47)Available phosphorous Bray II method described by Olsen and

Sommer (48)

Organic carbon Walkley-Black method described byRhoades (48)

Cation exchange capacity Ammonia saturation method described by

Rhoades (48)Exchangeable cations Ammonium Acetate Method described by

Hendershot, Lalande, and Duquette (48)Water soluble trace metal Extract by 0.01M CaCl2Total trace metalconcentrations

AAs after using CEM MDS-2000 microwavefor digestion

Metal speciation Sequential extraction procedure described by

Tessier et al. (13)

Leachate pHDissolved organic carbon

pH meterShimadzu 5000A TOC analyzer

Soluble trace metals Shimadzu AA-6701F atomic absorption

spectrometer

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carbonates and oxides/hydroxides-bound metal. The amount of basicexchangeable cations present in sludge would help serve as a soil condi-tioner, by neutralizing soil pH and increasing the buffering capacity of soil(18). Copper had the highest solubility among the metals measured insewage sludge. Because organic matter in dewatered sewage sludge isactively degraded, humic substances are present. Those dissolved acidichumic products could complex with Cu and other metals, resulting inhigh solubility of metals. A comparison of the concentrations of heavymetal in sludge and USEPA standards with High Quality MetalConcentrations Limits for Land Application (19) showed that all of themetals were below allowable concentrations, except for Cu concentration(Table 4). Results suggest Cu should be carefully monitored if this sludgematerial is applied to agricultural soils.

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Table 3. Characteristics of Originating Rangsit Soil

Parameter Analyzed Value

Bulk density (g/cm3)* 1.04Soil texture (%)Sand 16Silt 31

Clay 53

Soil pH (1 : 5 sludge : water) 4.58Total N (%) 0.05Total P (mg/kg) 360

Available P (mg/kg) 7.75Cation exchange capacity (meq/100 g) 35Exchangeable cations (mg/kg)

K 109Ca 1419Mg 266

Water-soluble trace metal (mg/kg)

Cd 0.23Cu 0.51Ni 2.99

Zn 1.03Fe 2.52Mn 0.29

Total trace metal content (mg/kg)Cd 0.72

Cu 21Ni 13Zn 23

Fe 24 831Mn 15

*From Sirisukhodom (1).

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Potential Leachability in Sludge-Amended Soil Column

In this study, the leachability of selected metals (Fe, Cd, Cu, Mn, Zn,and Ni) was also investigated in soil column experiments following two rates(150 and 300 kgN/ha) of sludge application to determine the amount of metalreleased within these treatments. Leachability of metals from soil columnexperiments was based on dry material weight of metal applied, as shownin Table 5.


Table 4. Characteristics of Sewage Sludge

Parameter Analyzed Value

Sludge pH (1 : 5 sludge : water) 6.94Moisture content (%) 85Total N (%) 3.60Available N (mg/kg)* 755

Total P (mg/kg) 13 536Available P (mg/kg) 476Organic matter (%) 46

Alkalinity (g/cm3)* 750Exchangeable cations (mg/kg)K 1307

Ca 5387Mg 355

Water-soluble trace metal (mg/kg)Cd 0.19

Cu 20.58Ni 14.16Zn 3.94Fe 2.29

Mn 12.45

Total trace metal content (mg/kg)Cd 1.89Cu 1437

Ni 107Zn 1607Fe 23 852

Mn 1841

Total metal in sludge standardsfor land application (mg/kg)**Cd 39

Cu 1500Ni 420Zn 2800

Fe —Mn —

*Potential leachability of toxic heavy metals in Bangkok sewage sludge (45).

**High-quality metal concentrations limits for land application (19).

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Table

5.MetalLeachabilityat150and300kgN/haofSludgeApplied(BasedonDryMetalWeight)

InputofMetals(mg)*

TheTotalAmountof

MetalsinLeachate(mg)

LeachabilityofIndividualMetal

(mg/kgofIndividualMetalApplied)

TheSequenceof

Leachability

DOCin

Leachate

(mg/kg)

Treatment

Cu

ZnCdNi

Fe

Mn

Cu

Zn

Cd

Ni

Fe

Mn

Cu

Zn

Cd

Ni

Fe

Mn

At150kgN/ha

1.Rangsitsoil

(nosludge)

leachedby

DW(pH�6)

1201304.2

72139000

810.340.130.05Trace

3.1623.76

2835

100211905

—23291892Mn�Cd>Cu>Zn>Fe>Ni

3438

2.Rangsit

soilþsludge

leachedby

DW(pH�6)

1501634.3

751396001200.570.390.10

0.18

4.3624.03

3800

239323256

240031200250

Mn�Cd>Cu>Ni�Zn>Fe

3598

3.Rangsit

soilþsludge

leachedby

DWpH3

1501634.3

751396001200.620.750.11

0.40

3.6322.11

4133

460125581

533326184250Mn�Cd>Ni>Zn>Cu>Fe

4012

4.Rangsit

soilþsludge

leachedby

rainwater(pH�5)

1501634.4

731396001200.550.190.09

0.25

2.5826.28

3667

116620455

342518219000Mn�Cd>Cu>Ni>Zn>Fe

3851

At300kgN/ha

5.Rangsit

soilþsludge

leachedby

DW(pH�6)

1801974.3

771400601591.693.180.31

2.21

9.1035.18

938916142720932870165221258Mn�Cd�Ni>Zn>Cu>Fe

5303

6.Rangsit

soilþsludge

leachedby

DWpH3

1801974.3

771400601591.943.590.26

2.37

8.8034.361077818223604653077963216101Mn�Cd�Ni>Zn>Cu>Fe

8572

7.Rangsit

soilþsludge

leachedbyrainwater

(pH�5)

1801974.4

751400601591.772.590.30

2.37

4.4731.42

983313147681823160032197610Mn�Cd�Ni>Zn>Cu>Fe

5275

*Inputofmetals¼metalsfromsludgeþmetalsfromsoilþmetalsfromdistilledwaterorrainwater.

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Leachability of Selected Metals at 150 kgN/ha Sludge Applied

Manganese and Iron

The sequence of metal leachability in the 150 kgN/ha sludge applica-tion rates in Rangsit soil indicated that Mn had the highest potentialleachability in each treatment studied. Generally, Mn is present in soilas oxides and hydroxides in coating layers of soil particles, and its com-pounds and fractions can be dissolved easily by combined factors such assoil pH, complexation by organic ligands, and microbial effects (20).However, microbiological activity in soil is also known to be responsiblefor the oxidation and reduction of Mn compounds (21). Therefore,released Mn ions from microbial activity are further complexed by sulfateand become more soluble. In general, oxidation of sulfur by oxygen isvery low, but may be catalyzed by autotrophic bacteria, which seem to beubiquitous in acid sulfate soil (22); therefore, sulfate could be produced inhigh amounts in the Rangsit soil.

Soil pH levels below 6 favor reduction of Mn to the more mobileand plant available forms, resulting in increased levels of dissolved andexchangeable Mn (23). From Table 5, Mn leachability in soil columnsleached by DW pH 3 would be expected to be high as compared to otherleachate, but the result did not follow such hypothesis. This could be attrib-uted to Mn–Fe antagonism, which has been previously reported in acidic soilthat contains large amounts of solubilized Mn (24). Higher levels of plantMn were found at high Mn/Fe ratios in acid sulfate soil in Thailand (25). It issuggested that high levels of Fe in the soil solution might exert a negativeeffect on the uptake of Mn. In a study under acid sulfate soil, little water-soluble Fe was detected at 250mV and pH 4.5, around 39mg/kg (25),whereas 18–31mg/kg of the added Fe was found in the leachate in thisstudy. It is possible that the conversion of soluble Fe into insoluble ferricforms under oxidizing conditions occurred in the wet–dry cycle of soil duringthe experiment. The content of Fe in the leachate was, therefore, low incomparison with the other trace metals as well as with total Fe content (6).The pH variation of leachant in this study apparently had a minimal effect onMn and Fe leachability.

Cadmium and Zinc

Generally, Cd mobility at soil pH below 5 is high (26). Resultsobtained in this study indicated that Cd was the most mobile element inthe soil columns leached with DW pH 3, followed by soil columns leachedwith DW and rainwater, respectively. Under acidic conditions in theRangsit soil, portions of sulfide and oxide of trace metals might have


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been transformed to water-soluble forms, which combined with Cd,resulting in dissolved Cd (27). In general, Cd exhibits higher mobilitythan Zn under acidic soil systems. When compared to total Zn applied,the leachability of Zn was restricted and accounted for less than 1%(0.1–0.45%). Soil column leached by DW pH 3 also had the highest leach-ability of Zn.

Copper

Copper is a metal that has a high affinity for binding with electron-donating groups, such as carboxylate and amide groups, in dissolvedorganic carbon (DOC) (3,28). DOC is an important component of solu-tions in terrestrial and aquatic ecosystems through its influence on acidity,nutrient availability, toxicity, and transport of metals and contaminants.Generally, DOC is a major product of the decomposition process, mole-cular structures ranging from simple to complex fulvic acids (FA) (29).Fulvic acids, due to high contents of oxygen-containing functional groups,are the most active components of organic matter in complexingmetals (30). Fulvic acids contained in sludge-amended soil in the columnshould, thus, serve as a good available carrier of metal mobility in soilsolution.

The stability of the metal fulvic acid complexes decreased in theorder Cu>Fe>Ni>Pb>Co>Ca>Zn>Mn>Mg was reported (31).Therefore, Cu could form stable complexes with fulvic acids. Becausesewage sludge from Sri Phraya WWTP had not stabilized, microorganismswere still actively degrading and producing DOC that was detected inleachate from the soil column (Table 5). It should be noticed here thatorganic matter in sewage sludge could be decomposed within 24 h (32).This is attributed to the high water content and initially relative oxidizednature of the sludge, resulting in a faster rate of microbial degradation(33). Based upon this phenomenon, a large portion of the DOC would beremoved with Cu complexes and transported further down the soil columnor profile. Ion-selective electrode measurement was the method that con-firmed that a large fraction of soluble Cu was organically complexed,existing in mobile forms (34). The high concentration of DOC observedin the leachate seemed to indicate that bacterial activity still functionedactively in this experiment. Results from this experiment indicated that Cucombined with DOC in columns leached by DW pH 3 had the highestleachability. This was supported with high total DOC concentration(4012mg/kg), which was detected in the leachate (Table 5). Leachabilityof DOC from sludge-amended soil was a function of pH (9). At low pH,DOC was released in soil solution, because it was replaced by H ions foradsorption sites.

776 PARKPIAN ET AL.

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Nickel

In addition to Cu, Ni is another transition metal that tends to formcovalent bonds with organic ligands (35), but the bond is not as strong as Cu.In general, Ni is controlled with pH-dependent solubility and complexationwith soluble carbon (28). However, under acid sulfate soil such as Rangsitsoil, the form of Ni is mainly in Ni-sulfate in soil solution. As pH of leachantwas increased, Ni leachability was decreased. This was because H ionsin low pH leachant are strongly attracted to the soil surface (negativecharges), and they have the power to replace Ni. The sequence of metalleachability in soil column leached with DW pH 3 showed that Ni mobilitywas different from the other sludge-amended soil leachates. This was becauseNi solubility was affected by the low pH (pH 3). Low pH metal bindingcompeted with H ions, resulting in fewer sites available for binding (30).Therefore, under acid soil Ni, with water solubility (2.99mg/kg) higherthan Cu (0.51mg/kg), had more leachability than Cu in the soil columnleached by DW pH 3.

Leachability of Selected Metals at 300 kgN/ha Sludge Applied

Most metals, except Mn, showed increasing leachability in accordancewith a high sludge application rate (300 kgN/ha) (Table 5). This indicatedthat the sludge application rate affected metal leachability. The sequences ofmetal leachability in soil columns at a high sludge application rate were in thesame sequence or order, except Cu and Ni were altered. Cu leachability wasfar below Ni leachability. This was possibly because Ni possessed intermedi-ate chemical behavior, being influenced both by pH dependency and bythe ability to form organic complexes with DOC (28). Another factor thatresulted in less Cu leaching out from soil than Ni was that Cu fractionsdeveloped and were present in the soil after the sludge was applied. Table8 indicated that more than 50% of the least soluble fraction (residual frac-tion) predominated after a high sludge application rate, which was not easilysoluble. Zn was greater than Cu at the higher rate of application. Thisdiffered from that of Zn at a low sludge application rate, where Cu wasmore easily solubilized than Zn. Results from sequential extraction procedure(Table 8) showed that Zn’s forms were transformed to an exchangeablefraction (approximately 60% of total Zn applied in topsoil), which wasexpected to become available after the sludge-applied rate was increased.Therefore, under acid soil, Zn (1.03mg/kg), which has a higher solubilitythan Cu (0.51mg/kg), would be found in more soluble forms in Rangsit soil.

Soil pH in the leachate at both sludge application rates increased from 5to 5.5 after the first leaching and remained constant at pH around 7 after thethird leaching (Figure 2). This was possibly due to the presence of exchange-


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able cations, especially calcium associated with previous lime application,and the Ca leached increased the pH of the leachate. Because of the nearneutral pH in the leachate, precipitation of metals in the leachate would haveoccurred. Therefore, metals became less solubilized instead of remainingdissolved in the leachate. However, leachate pH at a high sludge applicationrate (300 kgN/ha) was lower than that at a low sludge application rate, likelyas a result of the production of organic acids from degradable organic mate-rials in sludge (soluble forms) that could lower pH of the leachate.

Recovery Leachability of Selected Metals from Sewage Sludge

Under the same conditions, the amount of metals leached from sludge-amended soil column by DW was subtracted from the amount of leachablemetals in the control column in order to quantify metals leached as a result of

778 PARKPIAN ET AL.

Figure 2. pH in the leachate of sludge-amended soil after receiving different amounts of

leachant: (a) pH in the leachate at sludge rate 150 kgN/ha; (b) pH in the leachate at sludgerate 300 kgN/ha.

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sewage sludge addition or metal recovery determination. The results shown inTable 6 indicated that Cd was most readily leached from sewage sludge fol-lowed by Ni, Zn, Cu, Fe, and Mn. Apparently, Mn, which became moresoluble in sludge-amended soil, was not from the sewage sludge but fromMn content in the native soil material. Cd and Ni were the first two metalsthat were released from sludge into solution. However, their concentrations inthe sludge were below EPA standard levels, reported to not be harmful to theenvironment. Cu and Zn leachability from sludge were not as high as Cd andNi. Nevertheless, Cu and Zn have been selected to identify geochemical formsin this study. This was likely because Cu concentration in sewage sludge(1437mg/kg) was near EPA regulatory standards (1500mg/kg) allowing forsludge application to agricultural lands (19). The Zn forms in the sewagesludge were mainly transformed into an exchangeable form, which was readilyavailable, following sludge application (Table 8). As a result, these two metalsshould be closely monitored if sludge is applied to Thai soil.

Potential Environmental Pollution from Sludge-Amended Soil

The concentrations of metals released into the leachate from soil columnleached with rainwater (natural condition) at low and high sludge applicationrates were compared to the groundwater quality standard of Thailandreported in Table 7. Metal concentrations in the leachate, except Cd concen-tration (at high application rate) and Mn concentration (at both applicationrates), were below the maximum permissible levels for groundwater. Theseallowable concentrations for the metals were saved, despite some Cd andMn that moved below 10-cm depth in the soil surface profile. However, Cdand Mn concentrations as a result of the metal re-adsorption process wouldnot expect to move down the soil profile and reach the water table depths(20–30m depth).


Table 6. Metal Leachability of Sewage Sludge

Leachability of Individual Metal (mg/kg)

Treatment Cu Zn Cd Ni Fe Mn

At 150 kgN/ha

1. No sludgeþ leached by DW 2835 1002 11 905 Trace 23 291 8922. Sludgeþ leached by DW 3800 2393 23 256 2400 31 200 2503. Leachability of

sewage sludge (3)¼ (2)� (1)

955 1391 11 351 2400 8 —

At 300 kgN/ha4. Sludgeþ leached by DW 9389 16 142 72 093 28 701 65 221 2585. Leachability of

sewage sludge (5)¼ (4)� (1)

6554 15 140 60 188 28 701 42 —

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Under acid sulfate soil (clay soil) in these leachate studies, Mn showedthe highest concentration in the leachate. Whereas in other studies, Zn con-centration from sewage sludge applied to sandy loam soil and silty clay loamsoil was the highest concentration in the leachate (36,37). The difference ofthose metal concentrations or movement in the soil profile was associatedwith difference in soil type and background metal contents in sludge.

Copper and Zinc Mobility in Sludge-Amended Soil

The concentrations of Cu and Zn remain in a sludge-amended soilprofile after leaching soil columns were analyzed to quantify depth metalspenetrated following sludge application. Mobility of Cu and Zn withinsoil depth is present in Figures 3 and 4. At the 150 kgN/ha application rateto the Rangsit soil, there was no evidence of Cu movement from sludge–soillayer (0–2 cm depth) with depth into the soil column. As mentioned pre-viously, microbial oxidation of organic components in sewage sludge is animportant aspect governing the movement of sludge in soil. When organicmaterial is degraded, both fulvic (soluble) and humic (insoluble) acidsare produced. Insoluble organic material effectively inhibits uptake ofmetal cations such as Cu, which binds strongly with organic material, pre-venting plant uptake. Conversely, soluble organics increased the carryingcapacity of soil solution for Cu (38). Therefore, DOC containing fulvicacids resulting from the origin sludge application could significantly influencethe mobility of Cu. However, Cu movement was restricted to surface 4 cmbelow sludge–soil layer. Beyond 4-cm depth, Cu concentration was still inline with native Rangsit soil Cu (21mg/kg) concentration. When sludgeapplication was doubled (300 kgN/ha), two soil columns leached with DW

780 PARKPIAN ET AL.

Table 7. Metal Concentrations in the Leachate from Soil Column Leached by Rainwater

Metal Concentrations (mg/L) Maximum LevelGroundwater

Quality Standard3Element Control1

1501

3002

Cu 0.012 0.017 0.056 1.5Zn 0.005 0.006 0.083 15.0

Cd 0.002 0.008 0.029 0.01Ni Trace 0.008 0.076 —Fe 0.114 0.081 0.143 1.0Mn 0.861 0.821 1.004 0.5

1Soil with no sludge applied.2Sludge applied at 150 kgN/ha.3Sludge applied at 300 kgN/ha.4Groundwater for consumption by pollution control department, Thailand (46).

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19.64

20.81

22.87

0 20 40 6 00 8

0 8

8-10

4-6

0-2

So

il D

epth

(cm

)

18.16

17.75

43.94

19.72

18.52

68.59

0 20 40 6 0

8-10

4-6

0-2

So

il D

epth

(cm

)

Cu Conc (mg/kg)

Sludge Applied at 150 kgN/ha Sludge Applied at 300 kgN/ha

18.71

20.78

45.98

20.12

23.8

58.28

0 10 20 30 40 50 6 00 7

8-10

4-6

0-2

So

il D

epth

(cm

)

Cu Conc (mg/kg)


20.94

19.76

39.14

21.42

21.44

71.69

0 20 40 6 00 8

8-10

4-6

0-2

So

il D

epth

(cm

)

Cu Conc (mg/kg)


(a)

(b)

(c)

(d)

Cu Conc (mg/kg)

Figure 3. Cu mobility in Rangsit soil as influenced by different leachants: (a) No sludgeamended soilþ leached by DW pH 6; (b) Sludge amended soilþ leached by DW pH 6; (c)Sludge amended soilþ leached by DW pH 3; (d) Sludge amended soilþ leached by rainwater

pH 5. Background total Cu in soil¼ 21mg/kg.

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782 PARKPIAN ET AL.

21.86

21.57

22.78

0 20 40 6 00

00

8

0 8

8-10

4-6

0-2

So

il D

epth

(cm

)

Zn Conc (mg/kg)

15.55

24.89

46.22

27.05

35.92

67.78

0 20 40 6 0

8-10

4-6

0-2

So

il D

epth

(cm

)

Zn Conc (mg/kg)


16.84

21.53

49.04

29

25.54

68.94

0 20 40 6 8

00 8

8-10

4-6

0-2

So

il D

epth

(cm

)


15.31

24.52

52.71

29.84

32.45

68.42

0 20 40 6

8-10

4-6

0-2

So

il D

epth

(cm

)

Zn Conc (mg/kg)


(a)

(b)

(c)

(d)

Zn Conc (mg/kg)

Figure 4. Zn mobility in Rangsit soil as influenced by different leachants: (a) No sludge

amended soilþ leached by DW pH 6; (b) Sludge amended soilþ leached by DW pH 6; (c)Sludge amended soilþ leached by DW pH 3; d) Sludge amended soilþ leached by rainwaterpH 5. Background total Zn in soil¼ 23mg/kg.

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and rainwater showed that Cu still accumulated only in such layer (0–4 cm),the same as the 150 kgN/ha application rate. However, soil columns leachedwith DW pH 3, which had the highest amount of DOC (4012mg/kg) Cureached down to the 6-cm depth of a 10 cm depth packed soil column.

Mobility of Zn in Rangsit (Figure 4) after receiving sludge at a low rate(150 kgN/ha) seemed to be restricted to the surface 0–8 cm layer. Beyond8-cm depth in the soil column, Zn concentration of sludge-treated soil wassimilar to values found for background Zn concentration (23mg/kg) in soil.When sludge application rate was increased (300 kgN/ha), Zn concentrationbelow 8-cm depth in all soil columns increased above background levels.

Increase in the sludge application rate influenced Zn mobility more thanCu mobility under acid sulfate soil. This was due mainly to the linkage formof Zn that is favorable to remobilization, whereas Cu was mainly accumu-lated in the topsoil as predominated by less mobilizable forms. However, Cuand Zn concentrations were recovered close to 100% within 10-cm soil depthbecause re-adsorption and precipitation of metals occurred during theirmovement from the sludge–soil layer.

Other reported research observed similar results, which showed that themajority of sludge applied accumulated in the soil surface. As a result, themovement of metals from the application site was typically minimal.Following are a few examples of such observations; no sludge-borne metalswere detected below the 15-cm depth in the soil columns, even on thePlainfield sand with a pH of 4.9 (39). Over 90% of the deposited heavymetals (Cd, Cr, Ni, Pb, and Zn) were found in the 0–15 cm soil depth,where sludge was incorporated, and no statistically significant increase inheavy metal content of the soil was detected below the surface 30 cm of thesoil profile after 6 years of continued annual sludge application in coarse andloamy soil (40). Similar to the work in California that found that the metalsinclude Cd, Cu, Zn, and Hg, which had concentrations in sludge that weremarkedly greater than those in the untreated soil that had moved in smallamounts into the 25–30 cm depth over the 9-year period (41). Metals trans-ported from soil surface in grassland soil treated with sewage sludge also didnot go below 10 cm. Most of the 7 metals (Cd, Cr, Cu, Mo, Ni, Pb, and Zn)(60–100%, mean 87%) remained in the upper 5 cm of soil (42). These earlierstudies showed soil below the topsoil layer (0–15 cm depth) had no significantenrichment in heavy metals following sludge application. Less than 1% of themetal applied had moved below the topsoil layer.

Soil pH at 0–2 cm depth in sludge-amended Rangsit soil increased overbackground topsoil pH (Figure 5). Generally, sewage sludge is treated withlime to control odor at wastewater treatment plants during sludge processing.Corresponding organic biodegradation under the acidic condition takesplace, and no acidic products are produced to further decrease soil pH.The increased soil pH resulted from calcium content of sludge, as such,would alleviate metal leaching in the soil profile, helping to neutralize soil


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784 PARKPIAN ET AL.

4.59

4.51

4.48

4.4 4.5 4.6 4.7 4.8 4.9 5

8-10

4-6

0-2

So

il D

epth

(cm

)

Soil pH

4.57

4.57

4.61

4.64

4.78

4.97

4.4 4.5 4.6 4.7 4.8 4.9 5

8-10

4-6

0-2

So

il D

epth

(cm

)

Soil pH

Sludge Applied at 150 kN/ha Sludge Applied at 300 kgN/ha

4.54

4.5

4.48

4.52

4.52

4.48

4.4 4.5 4.6 4.7 4.8 4.9 5

8-10

4-6

0-2

So

il D

epth

(cm

)

Soil pH


4.56

4.68

4.72

4.67

4.69

4.74

4.4 4.5 4.6 4.7 4.8 4.9 5

8-10

4-6

0-2

So

il D

epth

(cm

)

Soil pH


(a)

(b)

(c)

(d)

Figure 5. Soil pH distribution as influenced by different leachants: (a) No sludge amended

soilþ leached by DW pH 6; (b) Sludge amended soilþ leached by DW pH 6; (c) Sludgeamended soilþ leached by DW pH 3; and (d) Sludge amended soilþ leached by rainwaterpH 5.

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pH. Except for pH of soil column leached with DW pH 3, pH did notincrease over background soil pH (4.58). Soil pH at a high sludge applicationrate was slightly greater than that at the low rate, because the greater decom-position rates produced higher NHþ

4 levels and organic acids.

Copper and Zinc Speciation in Sludge-Amended Soil

The speciation or metal form in soil following the application of sewagesludge to agricultural land is likely to be more important than the totalconcentration of metals, because metal forms determine the availability ofmetals for plant uptake and the potential for further contamination ofgroundwater. The observed influence of total metal concentrations andsludge characteristics on metal speciation emphasized the necessity for con-sideration of all five fractions, including exchangeable, carbonates, Fe andMn oxides, organic bound, and residues. This investigation using sequentialextraction procedure developed (13) to identify and quantify the differentchemical forms in which Cu and Zn were present in sewage sludge and


Table 8. Sequential Extraction of Sludge-Amended Soil on the Distribution of Cu and Znat Sludge High Application Rate (300 kgN/ha)

Material TreatmentSoil Depth(cm) Exch Carb Ox Org Res

(a) Copper Copper (% of Total Cu Applied)

Sewage

sludge

— — 11.98 10.40 12.18 59.35 6.09

Rangsit soil No sludgeþ leached 0–2 10.77 13.48 9.49 12.49 53.77by DW (pH 6) (control) 8–10 16.74 11.09 10.37 9.04 52.75With sludgeþ leached 0–2 10.14 7.73 9.97 14.67 57.49

by DW (pH 6) 8–10 16.13 10.08 9.84 11.3 52.64With sludgeþ leached 0–2 14.28 7.60 11.83 15.72 50.57by DW3 8–10 14.60 11.24 13.53 15.46 45.17

With sludgeþ leached 0–2 12.18 7.47 15.16 12.78 52.40by rainwater (pH 5) 8–10 11.20 8.14 12.60 13.37 54.69

(b) Zinc Zinc (% of Total Zn Applied)

Sewagesludge

— — 11.71 24.95 52.98 6.12 4.25

Rangsit soil No sludgeþ leached 0–2 7.94 4.14 8.10 7.00 72.82

by DW (pH 6) (control) 8–10 13.17 6.05 11.08 10.40 59.30With sludgeþ leached 0–2 62.36 7.76 11.60 6.16 12.12by DW (pH 6) 8–10 20.16 3.91 12.01 6.23 57.68

With sludgeþ leached 0–2 59.63 4.69 15.45 4.88 15.35by DW3 8–10 19.89 2.54 10.68 9.75 57.14With sludgeþ leached 0–2 54.99 4.61 20.83 4.71 14.86

by rainwater (pH 5) 8–10 13.99 4.61 13.99 4.28 63.12

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sludge-amended soil. From Table 8, the major forms of Cu extracted insewage sludge were in the order: organic bound�oxides>exchangeable>carbonates>residue. The order of Zn speciation was oxides>carbonates>exchangeable>organic bound>residue. An organic complex of Cu was pre-dominated in sewage sludge, because most sludge was abundant with organicmaterial (bacteria cells) and the binding capacity of organic matter wasgreatest for Cu (3), whereas the most adsorption of Zn in sewage sludgewas oxide fraction.

Following sludge application at 300 kgN/ha and being leached withvarious leachants, control soil columns (no sludge) and soil columns lea-ched with DW, DW pH 3, and rainwater were fractionated to determine Cuand Zn speciation. Comparing Rangsit soil column leached with DW withcontrol columns showed that organic bound and residual fractions of Cu insludge–soil layer (0–2 cm depth) increased slightly from 12.49 and 53.77%to 14.67% and 57.49%, respectively. Only the carbonate fraction decreased.The other fractions remained the same as the fractions in the control. Atthe lowest section of the soil column (8–10 cm), no significant changeoccurred in all Cu fractions, suggesting that most Cu in sewage sludgeaccumulated and remained at the origin of level of application to the sur-face soil.

More than 70% of Zn was found in the residual forms as surface soillayer (0–2 cm depth) of Rangsit soil, however, the exchangeable fraction ofZn (62.36%) significantly increased in soil treated with sludge that wasleached with DW. The explanation for such increase of exchangeable fractionwas that under acid sulfate soil, H ions replaced organic and inorganicligands easily from Zn, and thus, Zn formed soluble zinc sulfate. When theZn concentration in soil solution increased, Zn was attracted by the negativecharge of exchangeable sites (43). Such an effect suggested that the applica-tion of sewage sludge at 300 kgN/ha to Rangsit soil provided more labile Znthat might potentially become bioavailable than in nonamended soil (44).At 8–10 cm depth of soil column, an exchangeable fraction of Zn (20.16%)previously leached by DW was also increased higher than its control soil ofthe same depth (13.17%), as a result, Zn became more easily mobilized thanCu.

In the surface soil layer (0–2 cm) of soil column leached with DW pH 3,Cu occurred in organic fraction slightly higher than in a column leached withDW and rainwater. This is because organic matter decomposed wasrestricted due to extremely low pH of soil. At 8–10 cm depth section, Cu inresidual fraction (45.17%) of column leached with DW pH 3 became dis-solved and re-adsorbed by other fractions including carbonates (11.24%),oxides (13.53%), and organics (15.46%). Comparing soil columns leachedwith DW, DW pH 3, and rainwater, Zn in exchangeable fraction of thosecolumns increased more than 55% (55–62%) in the surface soil (0–2 cmdepth) after sludge was applied. However, regardless of pH of the leachants,

786 PARKPIAN ET AL.

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Zn was predominated in the exchangeable soil fraction. Based on this finding,leaching of sludge-amended soil induced Zn remobilization as indicated bythe higher level of its exchangeable fraction.

Recommendations for Sludge Reuse in Agriculture

Sewage sludge application to Thai soils should be further treated fol-lowing the dewatering process to remove organic matter. Humic substancessuch as humic and fulvic acids could be produced if sludge has not stabi-lized. Those acid products could complex with metals, especially Cu,increasing solubility. Although almost 100% of metals were restricted tothe surface 10-cm soil depth in this study, plants could possibly assimilatethose soluble metals and translocate them to plant parts and into the foodchain. Metals in the sludge could be transformed into exchangeable forms,which were easily bioavailable. This study found that most metals appliedfrom sewage sludge under acid sulfate soil remained in the topsoil (0–10 cmdepth), with the major concentration at the 0–2 cm surface layer. This wasattributed to the fact that the soil had higher CEC due as a result of limeapplication. This phenomenon helps alleviate metal mobility. Further studyshowed that when the sludge application rate was increased, potentialleachability also increased. This could be of concern in no lime-amendedsoil with low soil pH. Therefore, caution should be taken in not applyingmore than the recommended rate of 150 kgN/ha recommended by AIT (16).Metal leachability should not have any significant impact on groundwatercontamination, but because of potential increases in bioavailability frac-tions in the surface, caution should be taken regarding plant uptake andtoxicity. Therefore, phytotoxicity might be expected in plant species sensi-tive to Zn toxicity.

CONCLUSIONS

The leachability of metal from the soil profile depended on pH of lea-chant, rate of sludge applied, chemical nature of metal and metal species, andfraction. Properties such as organic matter, CEC, soil pH, and minerals insoil increased the buffering capacity of soils, favoring higher metal retentionin the surface layer. Results of this study showed that for Cu and Zn, lessthan 0.5% of the two metals applied at 150 kgN/ha and less than 1.8% ofeach metal applied at 300 kgN/ha were detected in the leachate. Whereas,close to 100% of the added metals from sludge were recovered within 10 cmof the soil column from both sludge application rates. Sequential extractionof sludge-amended soil after leaching at a high application rate revealed thatCu was associated with organic and residual fractions, which are not easily


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remobilized. Exchangeable fraction of Zn increased predominately (around60% of total Zn applied) in the Rangsit soil, as compared to original frac-tions found in soil receiving no sludge. This could lead to the conclusion thatavailability of Zn could be expected from sludge application. However, thetotal Zn in the leachate detected was far less than that in the regulatoryguideline and, thus, is safe for agricultural use. Because soil used in thisstudy was clay texture with high CEC and organic matter content, theamount of metal binding sites in the soil was, therefore, far higher thansoils from other areas. In terms of potential leachability, results obtainedfrom this study also confirmed that this soil had a high capacity for retainingmetals following sludge application.

ACKNOWLEDGMENTS

The author would like to thank Mr. Somchai Sornwanee, Ms. JariratSrinatpat, and Ms. Noppawan Jantavee for their excellent technical supportand assistance on sample analyses throughout the experiment at the Depart-ment of Drainage and Sewerage, Bangkok Metropolitan Administration(BMA). The Royal Thai Government provided the financial support forthis research.

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Documents

METAL LEACHABILITY FROM SEWAGE SLUDGE-AMENDED THAI SOILS