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This article was downloaded by: [University of Alberta] On: 26 November 2014, At: 18:40 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Communications in Soil Science and Plant Analysis Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lcss20 MetalAssociated Forms and Speciation in BiosolidAmended Oxisols M. L. Silveira a , A. C. Chang b , L. R. F. Alleoni c , G. A. O'Connor d & R. Berton e a Range Cattle Research and Education Center, University of Florida , Ona, Florida, USA b Environmental Sciences Department , University of California , Riverside, California, USA c Department of Soils and Plant Nutrition , University of Sao Paulo , Piracicaba, Brazil d Soil and Water Science Department , University of Florida , Gainesville, Florida, USA e Campinas Agronomic Institute , Campinas, Brazil Published online: 25 Apr 2007. To cite this article: M. L. Silveira , A. C. Chang , L. R. F. Alleoni , G. A. O'Connor & R. Berton (2007) MetalAssociated Forms and Speciation in BiosolidAmended Oxisols, Communications in Soil Science and Plant Analysis, 38:7-8, 851-869, DOI: 10.1080/00103620701263700 To link to this article: http://dx.doi.org/10.1080/00103620701263700 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

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Page 1: Metal‐Associated Forms and Speciation in Biosolid‐Amended Oxisols

This article was downloaded by: [University of Alberta]On: 26 November 2014, At: 18:40Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office:Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Communications in Soil Science and PlantAnalysisPublication details, including instructions for authors and subscriptioninformation:http://www.tandfonline.com/loi/lcss20

Metal‐Associated Forms and Speciation inBiosolid‐Amended OxisolsM. L. Silveira a , A. C. Chang b , L. R. F. Alleoni c , G. A. O'Connor d & R.Berton ea Range Cattle Research and Education Center, University of Florida , Ona,Florida, USAb Environmental Sciences Department , University of California , Riverside,California, USAc Department of Soils and Plant Nutrition , University of Sao Paulo ,Piracicaba, Brazild Soil and Water Science Department , University of Florida , Gainesville,Florida, USAe Campinas Agronomic Institute , Campinas, BrazilPublished online: 25 Apr 2007.

To cite this article: M. L. Silveira , A. C. Chang , L. R. F. Alleoni , G. A. O'Connor & R. Berton (2007)Metal‐Associated Forms and Speciation in Biosolid‐Amended Oxisols, Communications in Soil Science and PlantAnalysis, 38:7-8, 851-869, DOI: 10.1080/00103620701263700

To link to this article: http://dx.doi.org/10.1080/00103620701263700

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”)contained in the publications on our platform. However, Taylor & Francis, our agents, and ourlicensors make no representations or warranties whatsoever as to the accuracy, completeness, orsuitability for any purpose of the Content. Any opinions and views expressed in this publicationare the opinions and views of the authors, and are not the views of or endorsed by Taylor &Francis. The accuracy of the Content should not be relied upon and should be independentlyverified with primary sources of information. Taylor and Francis shall not be liable for anylosses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilitieswhatsoever or howsoever caused arising directly or indirectly in connection with, in relation to orarising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantialor systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, ordistribution in any form to anyone is expressly forbidden. Terms & Conditions of access and usecan be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: Metal‐Associated Forms and Speciation in Biosolid‐Amended Oxisols

Metal-Associated Forms and Speciationin Biosolid-Amended Oxisols

M. L. Silveira

Range Cattle Research and Education Center, University of Florida,

Ona, Florida, USA

A. C. Chang

Environmental Sciences Department, University of California,

Riverside, California, USA

L. R. F. Alleoni

Department of Soils and Plant Nutrition, University of Sao Paulo,

Piracicaba, Brazil

G. A. O’Connor

Soil and Water Science Department, University of Florida,

Gainesville, Florida, USA

R. Berton

Campinas Agronomic Institute, Campinas, Brazil

Abstract: The objective of this study was to determine the effects of pH and ionic

strength on the distribution and speciation of zinc (Zn), copper (Cu), and cadmium

(Cd) in surface soil samples from two Brazilian Oxisols amended with biosolids.

Soils and biosolids were equilibrated in an experimental dual-chamber diffusion

apparatus that permits the soils and biosolids to react through a solution phase via

diffusion across a membrane. After equilibrium was reached, soil and biosolids

samples were sequentially fractionated to identify various solid forms of Zn, Cu, and

Cd. Metal concentrations in the solution phase were determined and mass balance

Received 9 June 2005, Accepted 16 March 2006

Address correspondence to M. L. Silveira, UF/IFAS Range Cattle Research and

Education Center, 3401 Experiment Station, Ona, Florida 33865, USA. E-mail:

[email protected]

Communications in Soil Science and Plant Analysis, 38: 851–869, 2007

Copyright # Taylor & Francis Group, LLC

ISSN 0010-3624 print/1532-2416 online

DOI: 10.1080/00103620701263700

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calculated. Equilibrating pH had no major effect on Cu solubility from biosolids and, at

pH range from 4 to 7, most Cu remained in the biosolids. Soluble Zn and Cd concen-

tration increased with decreasing pH because of the increased solubility of the

biosolids. Copper and Zn were primarily associated with the residual fraction and Fe

oxides in one soil, but were primarily associated with chemically unstable fractions,

or adsorbed to the surface of oxides, in the other soil. In both soils, Cd was

primarily associated with readily bioavailable fractions. The effect of pH on the

metal distribution was more evident than the ionic strength effect. Free ions were the

predominant metal species in solution, especially at lower pH values.

Keywords: Biosolids, cadmium, chemical speciation, copper, heavy metals, oxisols,

sequential fractionation, zinc

INTRODUCTION

Biosolids are residues generated during primary, secondary, or advanced

treatment of domestic sanitary sewage through one or more controlled

processes that reduce pathogens and attractiveness to vectors. The term

biosolids is related to the definition of sewage sludge found in Part 31,

Water Resources Protection of the Natural Resources and Environmental Pro-

tection Act, 1994 PA 451, as amended; however, biosolids are only that

portion of sewage sludge that undergoes adequate treatment (pathogen

reduction) to permit application to land (USEPA 1994).

Land-applied biosolids can improve physical–chemical soil properties,

such as pH, cation-exchange capacity, and aggregate stability (Tsadilas

et al. 1995), and serve as organic sources of nitrogen (N), phosphorus (P),

sulfur (S), and micronutrients for plant nutrition. However, biosolids can

also contain high concentrations of heavy metals that can accumulate to

problematic concentrations in agricultural soils receiving biosolids for long

periods.

Metals present in the biosolids can be solubilized for reaction with soils,

and the kinetics of metal dissolution determine the rate at which equilibrium is

reached (Gerritse et al. 1983). The chemical reactivity of metals is determined

by the chemical equilibrium between metals in solution and in solid phases.

Although metal solubility can be initially reduced by soil sorption reactions,

long-term solubility is controlled by chemical forms in the solid phases

(Martinez and McBride 1998). Therefore, knowledge of metal-associated

forms in the soil and the biosolids and chemical speciation in solution

is essential for understanding soil-metal chemistry in the environment

(Mattigod and Page 1983).

Metals present in soils or biosolids can be associated with different com-

ponents. Sequential extraction techniques have been widely used to assess

heavy-metal distributions in the solid phase (Shuman 1985, 1991). This

procedure is especially useful to determine risks of soil contamination, to

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predict the potential soil’s ability to release metals into solution (Krishnamurti

and Naidu 2002), and to determine the long-term behavior of metals intro-

duced into the soil and their redistribution with time (Candelaria and Chang

1997). The soluble and exchangeable fractions have received special

attention because they are considered readily available to plants.

Heavy-metal mobility and retention are correlated with the chemical and

mineralogical soil characteristics (Matos et al. 2001; Appel and Ma 2002;

Covelo, Couce, and Vega 2004). Soils contain a variety of surface functional

groups, such as hydrous oxide minerals, organic matter, and alumosilicates,

responsible for metal sorption reactions. Crystalline and poorly crystallized

iron oxides are by far the most active components, from a chemical standpoint,

in the geochemical cycling of trace elements in soils (Martinez and McBride

1998). Thus, the impact of contaminants in tropical soils with high Fe

hydroxide concentrations, such as Oxisols, can be particularly distinctive,

necessitating a better understanding of the processes controlling heavy-metal

interactions in these soils. Although many studies have evaluated heavy-

metal behavior in temperate soils (Shuman 1985, 1986; Candelaria and

Chang 1997), relatively few experiments have been conducted on tropical

soils (Naidu 1997; Matos et al. 2001; Silveira, Alleoni, and Chang 2006).

In natural aqueous systems, many different organic and inorganic ligands

are present in solution (Sposito 1983). The soil solution is influenced by

reactions that take place at the solid–solution interface, and the concentrations

of metals in the aqueous phase can be used as potential indicators of metal

uptake by plants. Metals can exist as free ions (hydrated) or interact with

other ions or molecules forming outer-sphere complexes (ion pairs) or

inner-sphere complexes. Free ions are considered the most readily available

chemical specie of metal in the soil solution. On the other hand, the soil

solution can contain 100–200 different soluble complexes (Sposito 1994),

and the presence of ligands can improve the complexation and solubility of

metals. The stability of complexes is markedly affected by environmental con-

ditions, and pH has been identified as the most important factor regulating

metal chemical speciation in biosolid-amended soils (Obrador et al. 1997).

The main objectives of this study were 1) to determine the solid-phase

fractionation and the chemical speciation in solution of (Zn), (Cu), and (Cd)

in biosolid-amended Oxisols and 2) to evaluate the effects of pH and ionic

strength on metal-associated forms in the soil and the biosolids. An exper-

imental chamber system allowed the solutions and separate solid phases

(soils and biosolids) to be characterized independently after the system

reached the equilibrium.

MATERIALS AND METHODS

Surface samples (0–0.2 m) of a Typic Eutrorthox (TE) and a Typic

Haplorthox (TH) soil were collected from intensively cultivated sugarcane

Forms and Speciation in Biosolid-Amended Oxisols 853

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fields in the state of Sao Paulo, Brazil. Selected chemical and physical

properties of the soils are given in Table 1.

Biosolids, anaerobically digested and air dried, were collected in 1974

from Metropolitan Sanitary District of Greater Chicago, IL (MSDGC).

After, collection, dried biosolids were stored in a cold room at 48C until

used. This biosolid, sample was chosen because it was contaminated by

heavy metals, particularly Cd, due to industrial-waste discharges (Table 2).

Although Zn and Cu concentrations in this biosolid do not exceed the ceiling

limits (7500 mg kg21 and 4300 mg kg21 for Zn and Cu, respectively)

(USEPA 1994), Cd concentration was nearly twice (152 mg kg21) the

ceiling concentration for this metal (85 mg kg21). Accordingly, this material

could not be land applied. This biosolid sample represents a distinctive

material and does not reflect the characteristics of the biosolids currently

produced in the United States. Limiting the discharge of industrial wastes

has significantly decreased heavy-metal concentrations in biosolids produced

by the MSDGC. For instance, biosolid samples collected in 2001 from

Table 1. Selected chemical and physical soil attributes

Soil TE TH

pH, H2O 5.0 4.0

C (g kg21) 20 8.3

Ca (mmolc dm23) 40 0.4

Mg (mmolc dm23) 17 0.1

K (mmolc dm23) 5.8 0.02

Na (mmolc dm23) 0.1 0

Al (mmolc dm23) 0 1.43

CEC (mmolc dm23) 62.9 2.0

Sand (g kg21) 170 830

Silt (g kg21) 230 50

Clay (g kg21) 600 120

Fet (g kg21) 220 9

Note: TE, Typic Eutrorthox; TH, Typic Haplorthox;

Fet, total Fe concentration determined after sulfuric

acid digestion.

Table 2. Soil and biosolids total Zn, Cu, and Cd concentrations

Soil/biosolids Zn (mg kg21) Cu (mg kg21) Cd (mg kg21)

TE soil 184 257 0.22

TH soil 13 13 0.15

Biosolids 4319 1263 152

Note: TE, Typic Eutrorthox; TH, Typic Haplorthox.

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the MSDGC exhibited 74, 56, and 96% less Zn, Cu, and Cd, respectively, than

in the samples collected in 1974. Despite its uniqueness, the heavily contami-

nated biosolids was used herein to simplify estimates of the rates at which Zn,

Cu, and Cd are released from the biosolids and reacted with the soil and

solution phases.

For total elemental analyses, subsamples of both the biosolids and soil

samples were oven dried, grounded in an agate mortar, and passed through

a 100-mesh sieve. Approximately 0.250 g of soil and 0.100 g of biosolids

then were digested in triplicate according to USEPA 3052 protocol (USEPA

1996). For the biosolids samples, 1 mL of H2O2 30% was added to increase

the solubilization of the organic fraction. Certified soil (NIST 2709, San

Joaquin soil) and biosolid (NIST 2781, domestic sludge) samples were

digested using the same protocol for quality assurance of the analyses. Recov-

eries ranged from 90 to 110% (data not shown). Stock solutions of all reagents

(analytical grade) were prepared using deionized, distilled water. The

glassware was soaked overnight in 2 M nitric acid (HNO3) and rinsed with

deionized, distilled water prior to use.

Experiment Design

Soil samples were equilibrated with biosolids in a dual-chamber diffusion

apparatus (DCDA), modified from De Pinto (1982). The DCDA was

initially used to characterize P release from municipal wastewater particulates

(DePinto et al. 1981, Young et al. 1982) and from Great Lakes tributary

suspended sediments (DePinto, Young, and Martin 1981) and to investigate

the solution-phase speciation and solid-phase distribution of heavy metals in

biosolid-amended soils (Candelaria and Chang 1997; Berton, Chang, and

Page 1999). The apparatus consists of two polycarbonate chambers of

300 cm3 of volume, separated by a 0.45-mm membrane (Figure 1). The

design of the DCDA allows the two solid phases (soil and biosolids) to

react with each other only through the solution phase via diffusion across

Figure 1. Schematic representation of the dual-chamber diffusion apparatus

(DCDA).

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the membrane. In this way, the solid phases remain separated in each side of

the chamber, and after the equilibrium is reached, the chemical changes in one

solid phase can be easily characterized without disturbance or contamination

by the other solid phase (DePinto 1982). To maximize the diffusion rate across

the membrane, the magnets originally used by DePinto (1982) were replaced

by a stirring apparatus, which allowed more uniform mixing of the solutions.

The kinetic processes that take place in the soil and biosolid phases when

incubated in the DCDA can be functionally divided in three phases (Figure 2):

rate 1) the rate of soluble metals released from the biosolids, rate 2) the rate of

diffusion across the membrane, and rate 3) the rate of adsorption of metals by

the soil. According to Candelaria and Chang (1997), the heavy metal sorption

step (rate 3) is relatively fast, so the reaction rate in the DCDA should be

limited by either the diffusion of metals across the membrane (rate 2) or the

desorption of metals from the solid phase (rate 1). Available metal released

by the biosolids in the DCDA is expected to diffuse across the membrane

and become rapidly immobilized by the soil. By performing a mass balance

on the system, heavy metal distribution and dynamics can be examined.

Two grams of soil or biosolids (dry wt. eq.) were placed in each side of the

chamber. The large biosolids–soil ratio (1:1) was chosen primarily for

simplicity to characterize overall changes in heavy-metal distribution and

was not intended to mimic recommended biosolids rates for field application.

The DCDA was filled with 500 mL of a background solution: calcium nitrate

[Ca(NO3)2; 0.005 M]þ calcium sulfate (CaSO4; 0.003 M), which approxi-

mated the ionic strength and composition of saturation water extracts of

both Oxisols (data not shown). The ionic strength was calculated based on

the electrical conductivity of the solution according to the Debey–Huckel

equation (Lindsay 1979). Calcium, nitrate, and sulfate were chosen because

these ions dominated the soil solutions of the control soils and because of

the lower tendency of nitrate and sulfate to form inner-sphere complexes

with colloid surfaces (specific adsorption) (Sposito 1989).

The pH of the background solution was adjusted to range from 4 to 7

using either 0.15 M HNO3 or calcium carbonate (CaCO3). Additional sets

Figure 2. Kinetic rates that determine the changes in metal distribution in the solid/solution phases.

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Page 8: Metal‐Associated Forms and Speciation in Biosolid‐Amended Oxisols

of samples were equilibrated in DCDA and contained a background solution

with an ionic strength ten times greater than the control soil solution. One

DCDA served as a control and contained only the background solution and

2 g of soil. One drop of toluene was added to each chamber to control

microbial growth. The suspension was mixed constantly, and pH and electrical

conductivity (EC) were measured daily until equilibrium was established (�5

to 7 d). Equilibrium was judged to have been obtained when solutions in both

sides of the chambers exhibited the same pH and EC. Additionally, after 5 to 7

days of reaction, a subsample of the solution from each side of the chambers

(soil and biosolids) was analyzed for Zn, Cu, and Cd. At the end of the

experiment, both sides of the chamber exhibited the same heavy-metal

concentrations.

Following equilibration, the suspensions were centrifuged, and the super-

natant was filtered through a 0.45-mmmembrane filter. The solid phases (soils

and biosolids) were recovered and freeze dried before analysis. An aliquot of

the filtered solution phase was acidified with concentrated HNO3 and stored in

a cold room at 48C until analysis. The concentrations of iron, manganese, zinc,

calcium, Cu2þ, potassium, magnesium, sodium, nickel, cadmium, lead, and

aluminum in solutions were quantified by atomic absorption spectropho-

tometer or by coupled plasma atomic emission spectrometry (ICP/AES).The concentration of anions [sulfate (SO4

22), chloride (Cl2), nitrate (NO32),

and phosphate (PO432)] were measured in non-acidified solutions by a

Dionex AS-11 ion chromatograph (Sunnyvale, CA). Dissolved organic

carbon was determined using a Shimadzu TOC analyzer (TOC 5050,

Kyoto, Japan). Certified water standards (NIST 1640, trace elements in

natural water) were used to ensure the quality assurance of the analyses.

Recoveries ranged from 90 to 110% (data not shown).

Chemical speciation of Zn, Cu, and Cd in solution was calculated using

the PC-GEOCHEM model (Parker, Norvell, and Chaney 1995). The contri-

bution of organic ligands was estimated based on the DOC concentration

using the mixture model (Mattigod and Sposito 1979; Sposito et al. 1982).

In this model, the organic carbon concentration is used to estimate the

approximate quantitative distribution of various organic acids. One

advantage of the mixture model is that organic acids that complex metals

can be characterized over a wide range of pH and ionic strength (Mattigod

and Sposito 1979).

Soils and biosolids were analyzed for total Zn, Cu, and Cd (EPA 3052)

(USEPA 1996). Mass balance of metals in solid and solution phases was cal-

culated, and the recoveries ranged from 95 to 109% (data not shown). Heavy-

metal sequential fractionation was assessed using the method proposed by

Silveira et al., (2006), adapted for soils with high Fe oxide concentrations

(Table 3). Metals extracted by this procedure were operationally defined as

exchangeable (Ex), carbonate and surface adsorbed (Surf. Ox.), organic

bound (OM), Mn oxide associated (Mn Ox.), amorphous (Fe Ox.) and crystal-

line Fe (Fe Cryst.) oxides sorbed, and residual fraction (Res.).

Forms and Speciation in Biosolid-Amended Oxisols 857

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RESULTS AND DISCUSSION

Zn, Cu, and Cu Distributions between the Solid and the Solution

Phases

There was good agreement between the total Zn, Cu, and Cd concentrations of

the soils and biosolids and the sum of fractions assessed by the sequential frac-

tionation protocol (mass balance�90–110%, data not shown). The agreement

suggests that the scheme efficiently removed the metals associated with the

operationally defined fractions.

The majority of Zn, Cu, and Cd remained in the biosolids after equili-

brium with both soils at pH � 5 (Figure 3). The solution concentrations of

the metals increased with decreasing pH, attributed to greater metal solid-

phase dissolution rates of the biosolids at lower pH values.

There was no variation in the biosolids metal solubility when pH was

reduced from 7 to 6; however, the amount of Cu, Zn, and Cd released from

the biosolids increased when pH was decreased to 5 and 4. The percentage

of metals bound to the biosolids decreased as pH was reduced from 5 to 4:

from 87% to 72% for Cu, from 57 to 22% for Zn, and from 65 to 30% for

Cd (Figure 3). However, the amount of metal adsorbed by the soil did not

Table 3. Heavy-metal sequential extraction (1 g of soil or biosolids) used

Fraction Abbreviation Reagent

Extraction time/temperature

Exchangeable Ex 15 mL 0.1 M Sr(NO3)2 2 h, 258CSurface oxides/carbonate

Surf Ox 15 mL 1 M NaOAc (pH 5) 5 h, 258C

Organic matter OM 5 mL 0.71 M (pH 8.5) 30 min, 908CMn oxides Mn Ox 50 mL NH2OH . HCl

0.05 M (pH 2)

30 min, 258C

Poor-crystalline

Fe oxides

Fe Ox 40 mL 0.2 M ammonium

oxalateþ 0.2 M oxalic

acid (pH 3)

2 h, Extraction in

the dark

Crystalline Fe

oxides

Fe Cryst 250 mL 6 M HCl 24 h, 258C

Residual Res HNO3þHFþH2O Microwave

digestion (EPA

3052)

Notes: Samples were centrifuged at 1225 G for 10 min, and the suspension was

filtered through Whatmann No. 42. Between each step, samples were washed with

5 mL of 0.1 M NaCl, centrifuged, and filtered to avoid readsorption of metals by the

solid phase. All extracts were acidified, using concentrated HNO3, except the Fe Ox

fraction, in which 1 drop of toluene was added. Blanks were used to assess possible

contamination.

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consistently increase as metal concentrations increased in the soil solution.

Overall, the metal sorption of the TE soil was greater than the TH soil,

possibly due to the greater clay and Fe- and Mn-oxide contents of the TE

soil (Table 1).

The pH effect on metal solubility varied with the studied metal species.

Biosolid-Cu extractability was less affected by pH than biosolids-Zn or -Cd

extractabilities. At pH 4, the quantity of metal bound to the biosolids at

Figure 3. Percentage of total Zn, Cu, and Cd in the biosolids after the equilibrium

with soils in the DCDA. Bars indicate one standard error (n ¼ 4) (IS ¼ pH 6, but 10

times ionic strength).

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equilibrium varied from 63 to 81% for Cu to only 22% and 30% for Zn and Cd,

respectively. Clearly, biosolids exhibited greater binding affinity for Cu than

for Zn and Cd.

Increasing the ionic strength of the background solution ten-fold (IS

treatment) had no effect on metal distributions between the solid and liquid

phases. Thus, increasing the ionic strength (at pH 6) did not change the metal

release from the biosolids or the metal concentrations in the soil solution.

Solid-Phase Fractionation of Zn

In the control TE soil samples, approximately 93% of the total Zn was associ-

ated with the Fe-cryst and Res fractions (Figure 4). In the TH soil, Zn was not

detected in these fractions; rather, the Fe-ox and Mn-ox fractions were the most

important for Zn retention (Figure 4). This differential pattern of Zn distribution

can be attributed to differences in the soils mineralogy and clay contents

Figure 4. Distribution of Zn in soil and biosolids samples (TE ¼ Typic Eutrorthox,

TH ¼ Typic Haplorthox, IS ¼ ionic strength).

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(Kalbasi and Racz 1978). Similar results were reported by Abd-Elfattah and

Wada (1981) and Shuman (1979), who found Zn to be dominantly associated

with clay minerals and Fe and Al hydroxides in temperate soils.

Although the majority of Zn was found in the Fe-cryst and Res fractions,

the Surf. Ox. and Mn-Ox fractions contained more Zn after the soils were

equilibrated with biosolids than in the control soil samples. The increase in

the Zn-Surf. Ox. in both soils was likely due to metal associations with the

carbonate fraction, which became more important at higher pH values.

Decreases in pH increased Zn association with the exchangeable fraction.

The percent of exchangeable Zn found in the TE soil samples agree with

the results obtained by Sims (1986), who found 1 to 53% of the total Zn in

the exchangeable form, in the pH range from 4.1 to 7.5. Exchangeable Zn

was greater in the TH soil samples, ranging from 3 to 82% of the total Zn.

The effect of pH reduction on the increase of exchangeable forms was more

evident in the TH soil than in the TE soil samples. Possibly, this reflects the

major contribution of Fe oxides and presence of kaolinite in the TE soil and

the greater affinity of Zn for less labile chemical forms.

Overall, Zn distribution among the various fractions for IS treatment was

similar to that found in the soil samples incubated at pH 6.0; however, the

absolute amount of Zn adsorbed by both soils was reduced at greater ionic

strength. The increase in Ca concentration, due to the increase in the

solution ionic strength, most likely resulted in competition with Zn for adsorp-

tion sites in the soil. Shuman (1986) found that increasing the ionic strength of

the solution decreased Zn adsorption. Adsorption isotherms had similar shapes

at ionic strengths ranging from 0.005 to 0.1 mol L21, but Zn adsorption

increased approximately tenfold at corresponding lower ionic strength

treatment isotherm (0.005). In the IS treatment, Zn content associated with

the TH Surf. Ox. and Mn-ox fractions slightly increased. Exchangeable Zn

decreased in both soils in the IS treatment, compared to the pH 6 treatment.

In the TH soil, the percent exchangeable Zn decreased from 26% in the pH

6 treatment to 3% in the IS treatment.

For the biosolids, reducing pH decreased the amount of Zn associated

with the Surf. Ox. fraction. This was likely due to the dissolution of carbonates

as pH decreased and subsequent sorption in the exchangeable-Zn fraction. No

exchangeable-Zn was observed at pH 7, whereas at pH 4, the values varied

between 18 and 24% of total Zn.

Solid-Phase Fractionation of Cu

Copper in the control soils was dominantly present in nonsoluble forms

(Figure 5). Although soil Cu concentrations increased after the biosolids

incubation, the metal distributions among the fractions were similar to

those in the control soil samples. The pH effect on the amount of Cu in

the soil solution and/or on soil adsorption and distribution was less

marked than observed for Zn. Jeffery and Uren (1983) found no marked

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dependence of soil-available Cu on soil pH, except for Cu-contaminated

areas. Similar results were reported by Sims (1986), who found no signifi-

cant change in soil Cu distribution in the pH range from 4.1 to 7.5. This

could be explained by the fact that Cu is differentially adsorbed by the

soil in relation to Zn and Cd. McBride and Blasiak (1979) suggested that

soil Fe and Al hydroxides first adsorbed Cu and second Zn because of the

difference in the pH50 (the pH at which 50% of maximum metal adsorption

occurs). Thus, Cu is typically more strongly associated with Fe and Al

hydroxides than Zn and Cd.

On average, 79% of the total Cu was associated with the Res fraction in

the TE soil samples, whereas most Cu was associated with the amorphous Fe

and Mn oxides and OM fractions in the TH soil (Figure 5). McLaren and

Figure 5. Distribution of Cu in soil and biosolids samples (TE ¼ Typic Eutrorthox,

TH ¼ Typic Haplorthox, IS ¼ ionic strength).

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Crawford (1973), studying surface samples of 24 soils with contrasting attri-

butes, found most (�77%) of the total Cu bound to clay minerals and 35 %

associated with the Fe-ox fraction. Abd-Elfattah and Wada (1981) also

reported that amorphous Fe oxides were the most selective components in

the Cu adsorption.

In both biosolid-amended and control samples, about 1 to 3% of the total

Cu in the TE soil and 12 to 20% in the TH soil was associated with the OM

fraction. These percentages are less than those reported by Sims (1986),

who found 6 to 73% of the total Cu in the OM fraction. However, soils

studied by Sims (1986) had greater OM concentrations than the Oxisols

studied here. Further, the sequential extraction method used by Sims (1986)

did not account for the surface oxide fraction, which could have led to over-

estimation of the quantity of Cu associated with the OM fraction. Contrary to

the results obtained by Sposito, Lund, and Chang (1982), no increase in the

OM fraction Cu concentration was observed in the biosolid-treated soil

samples compared to the control soil samples. However, differences in the

sequential fractionation procedures used make comparison of the results

difficult. Sposito, Lund, and Chang (1982) extracted the OM fraction immedi-

ately after the exchangeable/sorbed fraction and subsequently the carbonate

fraction. In this study, metals associated with the carbonate fraction were

assessed before the organic fraction. The IS treatment did not affect Cu distri-

bution in soils and biosolids.

Manganese and amorphous Fe oxides, and OM, were the main fractions

responsible for the Cu retention in biosolids. As pH decreased, the percentages

of metal associated to the exchangeable fraction increased slightly (from 0 to

3%), and the percentage in the Surf. Ox. fraction decreased (from 12 to 6%).

Petruzzelli et al. (1994) reported similar percentages of exchangeable Cu.

According to these authors, although exchangeable fraction may represent a

lower percentage of total Cu than the other fractions, small changes in pH

can affect Cu bioavailability.

Solid-Phase Fractionation of Cd

Cadmium distribution in soils was greatly affected by the biosolids incubation.

Compared to control samples, Cd concentration was 52-fold greater in the TE

soil and 27-fold greater in the TH soil after the biosolids treatment (Figure 6).

Compared to Cu and Zn, Cd in the biosolids was more readily available and

easily dissolved in the solution. Possibly, the mechanisms of Cd retention in the

biosolid matrix were more susceptible to the effect of pH changes (Figure 6).

The majority of Cd (�76% of total Cd) was associated with the Surf. Ox.

and OM fractions. Because of the biosolids high pH, the Surf. Ox. fraction

may contain carbonates, which contribute to Cd retention in the solid phase

(Sposito 1983). As pH decreases, the carbonates are solubilized, and metals

associated with this fraction are either redistributed into other solid phases or

Forms and Speciation in Biosolid-Amended Oxisols 863

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solubilized to the solution. At pH 4, 19 to 26% of total Cd was associated with

the exchangeable fraction compared to 2.6 to 3.4% at pH 6.

The increased Cd dissolution from biosolids was not followed by a pro-

portional increase in Cd soil adsorption. In the TH soil, for all pH values,

about 2% of the total Cd was adsorbed, independent of the metal concentration

in solution. In the TE soil, this value ranged from 5 to 8% of total Cd. Possibly

the ability of soils to retain Cd was limited, because this metal has less

tendency to be adsorbed compared to Zn and Cu (Alloway 1990). Increasing

solution ionic strength did not affect Cd dissolution from biosolids; however,

the distribution of this metal among the solid phases shifted. Cadmium associ-

ated with OM was increased, followed by a decrease in Ex-Cd for both soils.

In both control soils, Cd was found mainly in the Surf. Ox. and OM

fractions. When pH was decreased, Cd was found primarily associated with

exchangeable fraction, which supports the results obtained by Kuo,

Heilman, and Baker (1983).

Figure 6. Distribution of Cd in soil and biosolids samples (TE ¼ Typic Eutrorthox,

TH ¼ Typic Haplorthox, IS ¼ ionic strength).

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Table 4. Total concentration and chemical speciation of Zn, Cu, and Cd in solution

Treatment

Total Zn

(mg L21)

Free Zn

(% of total)

Total Cu

(mg L21)

Free Cu

(% of total)

Total Cd

(mg L21)

Free Cd

(% of total)

TEþ Biosolids

pH 7 0.06 80.5 nd 0 0.01 88.0

pH 6 0.5 92.2 0.01 82.7 0.03 90.2

IS 1.0 87.9 0.01 80.8 0.07 86.3

pH 5 3.7 96.7 0.1 95.2 0.3 95.5

pH 4 13.4 98.3 1.1 97.8 0.5 97.8

THþ Biosolids

pH 7 0.1 81.5 nd 2.3 0.02 88.1

pH 6 0.8 92.6 0.05 26.6 0.06 90.8

IS 1.3 87.4 0.02 81.0 0.08 85.4

pH 5 8.0 97.6 0.2 96.5 0.3 96.9

pH 4 12.7 98 0.9 97.4 0.5 97.4

Note: TE, Typic Eutrorthox; TH, Typic Haplorthox; nd, not detected (below the detection limit).

Form

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The Oxisols in the present study exhibited contrasting attributes, but Cd

distribution was similar in both soils. In control samples of TH and TE, Cd

was associated with the Surf. Ox. and OM fractions (82% of total Cd)

(Figure 6), both of which are potentially bioavailable fractions. The Surf. Ox

and OM fractions, and consequently the soil Cd distribution, can be modified

with soil acidification and/or OM mineralization. Thus, the mechanisms of

Cd retention in these soils are chemically unstable, and Cd can be eventually

released to the solution (Kuo, Heilman, and Baker 1983). Biosolid addition to

the soil increased Ex-Cd. The exchangeable fraction was 4% of total Cd in

the control soil samples but increased to 11% and 80% of total Cd, at pH 7

and pH 4, respectively, after incubation with biosolids.

Metal Chemical Speciation in the Solution Phase

The distribution of soluble species, according to GEOCHEM-PC, showed that

between 81 to 98% of Zn, Cu, and Cd in solution were presented as free ions

(Table 4). There was a strong correlation between pH and the amount of ionic

species for both soils. Free-ion species percentages increased with increasing

acidity. Under high pH conditions, soil metal adsorption ability and the avail-

ability of complexing agents are increased (Salam and Helmke 1998),

favoring the complexes’ formation. There was a higher percentage of free-

Cd and -Zn ions in solution than Cu ions at the same pH values. The lower

free-Cu concentrations in solution compared to the other metals was due to

both the strong adsorption of Cu by the soil colloid surfaces (Cavallaro and

McBride 1980) as well as its higher affinity with complexing agents present

in the solution. Generally, increasing solution ionic strength favored the

formation of SO422 complexes with Zn, Cu, and Cd and decreasing concen-

trations of the free ionic species.

CONCLUSIONS

Chemical forms and total Zn, Cu, and Cd concentrations in the biosolids had a

marked effect on subsequent metal distribution in the soils. The pH markedly

affected Zn, Cd and, to a lesser extent, Cu distribution in soils and biosolids.

Increasing the solution ionic strength had minimal effects on the metal distri-

bution in soils or biosolids. Soluble-metal concentrations increased with

decreasing soil pH, due to the higher solubilization of the various metal

solid phases in the biosolids. At low pH values, the majority of the metals

in the solid phase were associated with exchangeable fractions, and free

ions were the predominant chemical specie in solution. Although Oxisols

exhibit distinctive chemical characteristics, Zn, Cu, and Cd partitioning was

similar to that in temperate soils amended with biosolids. The solubilities/exchangeabilities of the heavy metals studied were primarily dictated by the

characteristics of the biosolids and the pH of the soil solution.

M. L. Silveira et al.866

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ACKNOWLEDGMENT

Maria L. Silveira thanks the Brazilian Federal Agency CAPES for granting a

scholarship and supporting this study. The authors thank the staff of the

Environmental Sciences Department, University of California, Riverside,

for assistance with chemical analyses.

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