Liming effects on trace metal plant availability of a
coarse-textured sewage sludge soil.
by
Sara Brallier
a thesis submitted in partial fulfillment
of the requirements for the degree of
Masters of Science
University of Washington
1992
Approved by
(Chairperson of Supervisory Committee)
Program Authorized
to Offer Degree
Date
Master's Thesis
In presenting this thesis in partial fulfillment of the requirements for a
Master's degree at the University of Washington, I agree that the library shall
make its copies freely available for inspection. I further agree that extensive
copying of this thesis is allowable only for scholarly purposes, consistent with
"fair use" as prescribed in the U.S. Copyright Law. Any other reproduction for
any purposes or by any means shall not be allowed without my written
permission.
Signature
Date
University of Washington
Abstract
Liming effects on trace metal plant availability of a
coarse-textured sewage sludge soil.
by Sara Brallier
Chairperson of the Supervisory Committee:
Assistant Professor Robert B. Harrison
College of Forest Resources
A field study was conducted to determine the plant availability of trace
metals from soils amended with 500 Mg ha-1 of municipal sewage sludge 16
years previously. The sludge-amended soil was adjusted with lime to four
different pH levels ranging from an untreated pH of 4.6 to pH levels of
approximately 5.8, 6.5 and 6.9. Crops selected for planting represented food
crops important in the human diet and of differing metal uptake rates,
including (i) bush beans (Phaseolus vulgaris L. cv. Seafarer), (ii) cabbage
(Brasica oleracea L. v. capitata L. cv. Copenhagen market), (iii) carrots
(Daucus carota L. cv. Toudo), (iv) corn (Zea mays L. cv. FR37), (v) lettuce
(Lactuca sativa L. cv. Parris Island), (vi) potatoes (Solanum tuberosum L. cv.
Kenebec), and (vii) tomatoes (Lycoperisicon esculentum L. cv. Burpee VF).
With the exception of corn, crop yields were significantly reduced in the
unlimed sludge-amended soils (pH 4.6). The edible portion of each crop was
analyzed for Cd, Cu, Ni and Zn. Cabbage and lettuce tissue had Cd and Zn
concentrations above suggested mean plant tolerance levels in the unlimed
sludge-amended soil. All other crops and metals had concentrations below
suggested tolerance levels. With the liming treatments, Cd and Zn
concentrations were significantly reduced in the cabbage and lettuce to below
the tolerance level. Liming also reduced total Ni in cabbage, lettuce and
tomato fruit. Copper concentrations did not significantly change after liming.
A sequential extraction method was used to determine relative solubility and
availability of metals in the soils used for plant growth experiments. The
relative solubilities of Cd, Ni and Zn were signifcantly reduced after liming the
sludge-amended soil. The majority of the soil Cu was found in the insoluble
and unavailable soil fractions before liming the sludge-amended soil and this
did not change after liming. The results indicate Cd, Ni and Zn from the
heavy sludge amendment remains extractable and plant available in acidic
soils but is rapidly made unavailable when soil acidity is reduced even 16
years after sludge application. This study shows the benefit of pH adjustment
in reducing metal uptake when sludge is applied at rates high enough to
result in excessive nitrification and leaching with subsequent acidification of
the soil.
4
TABLE OF CONTENTS
List of Figures .................................................................................................... iii
List of Tables ..................................................................................................... iv
Acknowledgments ............................................................................................. v
1. INTRODUCTION .......................................................................................... 6
2. LITERATURE REVIEW ................................................................................ 8
2.1. Soil Factors Affecting Trace Metal Availability ................................ 8
2.1.1 Soil pH................................................................................ 9
2.1.2. Soil Organic Matter ........................................................... 10
2.1.3. Soil Cation Exchange Capacity ......................................... 11
2.1.4. Hydroxides of Iron, Manganese, and Aluminum................ 12
2.1.5. Amount and Type of Clay .................................................. 12
2.2. Metal Speciation Procedures .......................................................... 13
2.3. Plant Capacity for Trace Elements.................................................. 14
2.3.1. Food Chain Implications .................................................... 14
2.3.2. Plant Species .................................................................... 14
2.3.3. Plant Cultivars ................................................................... 14
2.3.4. Plant Parts and Age .......................................................... 15
2.4. Long Term Plant Availability of Sludge-borne Metals...................... 15
2.5. Effect of Sludge on Metal Concentration in Soil .............................. 16
2.6. Effect of Sludge Application on Metal Uptake by Plants ................. 17
2.7. Effect of Soil pH on Metal Activity in Sludge Amended Soils .......... 19
3. HYPOTHESIS AND STUDY OBJECTIVE .................................................... 26
4. METHODS AND MATERIALS ...................................................................... 27
4.1. Site Description ............................................................................... 27
4.1.1. Climate .............................................................................. 27
4.1.2. Soils .................................................................................. 27
4.2. Study Design .................................................................................. 27
4.2.1. Plant Analysis Procedure .................................................. 28
4.2.2. Soil Analysis Procedure .................................................... 29
4.3. Statistical Analysis .......................................................................... 30
5. RESULTS AND DISCUSSION ..................................................................... 32
5.1. Liming Effects on Metal Speciation and Soil Chemical
Properties ............................................................................................... 32
5.1.1. Effects on Metal Speciation ............................................... 32
5.1.2. Effects on Soil Chemical Properties .................................. 33
5.2. Liming Effects on Plant Metal Uptake ............................................. 34
6. CONCLUSIONS ........................................................................................... 46
7. LITERATURE CITED ................................................................................... 48
APPENDIX 1: Plant Yield Data ......................................................................... 56
APPENDIX 2: Complete plant data analysis ..................................................... 59
ii
LIST OF FIGURES
Number
Page
1. Forms of metals. ........................................................................................... 21
2. Stability of the ionic species of several trace metals as functions of
soil pH.. ........................................................................................................ 22
3. The generalized effects of metal concentrations in nutrient solution
on yield and metal content of plants. ............................................................ 23
4. Flowchart of the sequential extraction procedure used to determine
the speciation of each metal in the soil samples. ......................................... 31
5. Soil metal speciation, in percent of total metal, by sequential
extraction for the background (pH 5.9) and sludge-amended soils
treated with lime to four different pH levels; pH 4.6, 5.8, 6.5 and 6.9. .......... 37
6. Trace metal concentrations in plant tissue grown in sludgeamended soil treated with lime to four different pH levels; pH 4.6,
5.8, 6.5 and 6.9. ........................................................................................... 38
iii
LIST OF TABLES
Number
Page
1. Essentiality and effects of trace elements on plant and animal
nutrition in terrestrial environment. ............................................................... 7
2. Sequential extraction techniques used to fractionate trace metals in
soils and sludge-amended soils. .................................................................. 24
3. Approximate concentrations of trace elements in mature leaf tissue
generalized for various species (ppm, dry wt.). ............................................ 25
4. Sequential extraction of soil metals, in percent of total metals, found
at the surface soils (0-15 cm) in the back-ground (no sludge/no
lime) and sludge-amended soils treated with lime to four different
pH levels; pH 4.6, 5.8, 6.5 and 6.9. .............................................................. 40
5. Mean (±S.D.) soil elemental analysis of the background (no
sludge/no lime) and sludge-amended soils treated with lime to four
different pH levels; pH 4.6, 5.8, 6.5 and 6.9. ................................................ 42
6. Mean (±S.D.) metal concentration (dry wt.) in plant tissue grown in
background (no sludge/no lime) and sludge-amended soils treated
with lime to four different pH levels; pH 4.6, 5.8, 6.5 and 6.9. ...................... 43
iv
ACKNOWLEDGMENTS
I wish to express sincere appreciation to Dr. Robert B. Harrison, my
supervisor, for his guidance in the course of my study and research. Special
thanks are equally due to the other members of my committee; Dr. Charles L.
Henry, Dr. Dale Cole, and Charles Treser, for their interest and assistance
during the completion of this thesis. I also wish to express my appreciation to
my fellow students whose criticism and suggestions were often invaluable to
me. In addition, special thanks to my parents for their continuous love and
support.
This work was supported by the Regional W-170 Biosolids Research
Group and funded by Northwest Regional Biosolids Committee. A special
thanks to Dr. Rufus L. Chaney, USDA-ARS, and Dr. Terry J. Logan, Ohio
State University, for their guidance and support in this research project.
v
1. INTRODUCTION
Composition of parent material and pedogenic processes in soil formation
are usually the main factors determining metal concentrations in soil.
Anthropogenic sources of trace metals are superimposed on these
background levels. Sewage sludge, a byproduct of primary and secondary
wastewater treatment processes, is one anthropogenic source. Sewage
sludge typically contains organic matter and plant and animal nutrients
primarily of domestic source, but may also contain trace metals from
stormwater and industrial sources that enter the sewage system (Rappaport
et al., 1988). The primary positive aspects of sewage sludge application to
agricultural and forest land are generally considered to be the recycling of
organic matter and nutrients and the increased plant and animal productivity
associated with the addition of nutrients found in sewage sludge.
Much research has focused the possible effects of trace metals and other
persistent chemicals that may be present in sewage sludge from industrial
sources. Logan & Chaney (1983) identified several elements as being of
particular concern because elevated levels of these metals sometimes limit
biotic productivity in nature. Those trace metals of concern include Cd, Cr,
Cu, Pb, Hg, As, Mn, Ni, Se and Zn. Several of these metals are also
necessary plant or animal nutrients, meaning their lack is detrimental to the
health of associated plants or animals. Necessary trace elements include As,
B, Co, Cr, Cu, F, Mn, Mo, Ni, Se, Sn, V and Zn (Table 1).
Of the constituents most often found in sewage sludge, Cd, Cu, Ni and Zn
may occur at concentrations, which, when applied to soils in excessive
amounts, may depress plant yields, degrade the quality of food produced, or
pose a threat to animal and human health through accumulation in plant
tissues or direct ingestion of sludge-amended soil.
7
Table 1: Essentiality and effects of trace elements on plant and animal
nutrition in the terrestrial environment (Source: Kabata-Pendias & Pendias,
1984).
Essential or
beneficial to__
Potential toxicity to
Element
Plants
Animals
Plants
Animals
As
No
Yes
Yes
Yes
B
Yes
No
Yes
Co
Yes
Yes
Yes
Cr
No
Yes
Yes
Speciation important, Cr6+
very toxic; otherwise relatively
nontoxic; carcinogenic
Cu
Yes
Yes
Yes
Easily complexed in soils;
narrow margin for plants
F
No
Yes
Yes
Accumulative toxicity for
plants and animals
Mn
Yes
Yes
<pH 5
Mo
Yes
Yes
Ni
No
Yes
Yes
Yes
Se
Yes
Yes
Yes
4 ppm
Narrow margin for animals;
interacts with other trace
metals
Sn
No
Yes
Yes
Relatively nontoxic; very low
uptake by plants
V
Yes
Yes
Yes
Narrow margin and highly
toxic in animals; carcinogenic
7
Phytotoxic before animal
toxicity; may be carcinogenic
Narrow margin, especially in
plants
Yes
Relatively nontoxic;
carcinogenic
Wide margin; toxic in acid
soils; among the least toxic
5-20 ppm
Yes
Comments
Narrow margin for animals
Very mobile in plants;
relatively nontoxic;
carcinogenic
8
Zn
Yes
Yes
Wide margin; easily
complexed in soils; maybe
lacking in some diets;
relatively nontoxic
8
2. LITERATURE REVIEW
2.1. Soil Factors Affecting Trace Metal Availability
Plant availability of trace metals depends upon the physical and chemical
properties of the soil-sludge layer. Factors affecting availability include the
sludge metal chemical characteristics and loading rate, soil pH, CEC, redox
potential, texture, and organic matter content (Verloo & Eeckhout, 1990;
Logan & Chaney, 1983; Williams et al., 1980; Haghiri, 1974; Lagerwerff,
1972). Geochemically, a trace metal introduced into the soil may end up in
one or more of the following forms (Figure 1):
(1) Water Soluble: Exists in soil solutions as either free ions or soluble
complexes with inorganic anions or organic ligands.
(2) Exchangeable: Adsorption by electrostatic forces to negatively
charged sites (for cations) or negatively charged sites (for anions) on clays,
organic matter, hydrous oxides, or on amorphous materials. Exchangeable
cations may be displaced by base cations commonly present in soil or sludge
solutions.
(3) Organic: Complexation with the organic fraction; chelated and/or
organic bound. This category includes trace metal cations immobilized on or
into living or recently dead biological material. The complexes may vary in
stability from immediately mobile, easily decomposable, and moderately
resistant to decomposition. Some of the organic material will be insoluble;
some will have been flocculated or precipitated by complexing cations,
notably iron or aluminum, but also by base cations like calcium or other trace
metals.
(4) Hydrous-oxide: Adsorption or coprecipitation with oxides,
hydroxides, and hydrous oxides of Fe, Mn, and Al present as coating on clay
minerals or as discrete particles. These oxides are rarely pure; they usually
contain cations from each other, and probably trace metal cations as well.
10
(5) Carbonate: Carbonate precipitation in soils high in free CaCO3,
bicarbonate, and alkaline in reaction. If precipitates do form, they usually
contain more than one trace metal, and often form mixed crystals or mixtures
of crystallites with the corresponding salts of major elements, usually calcium
or iron.
(6) Residual: Fixed or occluded within the crystalline lattices of soil
minerals.
Of those listed above, the first two forms are relatively mobile and plant
available; while the last four are generally immobile but sometimes become
mobile and plant available with time. Metal activity in the soil solution is
generally considered to be the result of chemical equilibrium between the
solution phase and solid phase among clay minerals, organic matter, hydrous
oxides of Fe, Mn and Al, and soluble chelators (Adriano, 1986). The kinetics
governing the chemical equilibrium of each metal species are complicated
and not well understood.
2.1.1 Soil pH
Of all the soil variables which have been reported to affect the solubility
and availability of sludge-borne metals (organic matter content, cation
exchange capacity, soil texture, pH, etc.) only pH has been shown to have
consistent significant effect (Page et al., 1987; Logan & Chaney, 1983). The
diversity of ionic species of trace metals and their various affinities to complex
with inorganic and organic ligands make possible the dissolution of each
element over a relatively wide range of pH. Figure 2 illustrates this change in
ionic speciation for two trace metals (Zn and Cu) as a function of soil pH.
Trace metals are generally more soluble and available under acid
conditions due to the hydrolysis of hydroxy groups (OH) and the dissolution of
solid phase minerals such as carbonates and phosphates (Lindsay, 1979;
Ellis & Knezek, 1972). The pH dependent OH groups are primarily found on
the edges and surfaces of the inorganic and organic colloids. The OH groups
are attached to Fe and/or Al in the inorganic colloids (e.g., -Al-OH) and to the
10
11
CO groups in organic colloids (e.g., -CO-OH) Under moderately acid
conditions, there is little or no charge on these particles, but as the pH
increases, the hydrogen dissociates from the colloid OH group, and negative
charges result (Brady, 1990). As the pH increases, more OH- ions are
availabe to force the reaction to the right, and the negative charge on the
particle surface increases.
-Al-OH
+
-CO-OH +
No charge
OHOHalkaline
soil solution

-Al-O -CO-ONegative
charge
+ H20
+ H20
acid
soil solution
If the pH is lowered, OH- ions concentrations are reduced, the reaction goes
back to the left, and the negativity is reduced. The relative mobility of some
trace metals in soils as influenced by soil pH can be summarized as follows
(Fuller & Alesii, 1979):
(1) In acidic soil (pH 4.2 to 6.6), Cd, Hg, Ni and Zn are relatively mobile;
As, Be and Cr are moderately mobile; and Cu, Pb and Se are slowly mobile.
(2) In neutral to alkaline soils (pH 6.7 to 7.8), As and Cr are relatively
mobile; Be, Cd, Hg and Zn are moderately mobile, and Cu, Pb and Ni are
slowly mobile.
2.1.2. Soil Organic Matter
Soil organic matter is often regarded as a major factor in the sorption of
metals (Ellis & Knezek, 1972). This is because of both its cation exchange
property and chelating ability. These complexes result from the binding of the
metals to partially dissociated enolic ( -OH ), carboxyl ( -COOH ) and phenolic
(
) functional groups in the organic matter (Brady, 1990). Under very
acid conditions, the negative charge is not very high because H+ is adsorbed
on the surface of the functional groups. With a rise in pH, the H+ ions
dissociate from first the carboxyl groups and then the enolic and phenolic
groups. This leaves a greatly increased negative charge on the surface and
11
12
the H+ is replaced as shown below by other cations such as calcium and
magnesium:
Somewhat stable soluble and insoluble complexes between the metals
and soil organic matter may form. Experimental results obtained for organic
matter indicated that it has a scavenging action for trace metals that is far out
of proportion to its own concentration (Tessler et al., 1979). However, organic
matter has often shown a higher affinity for Cu and Pb than Cd, Ni and Zn
with the stability of the metallo-organic complex increasing as pH increased
from 3 to 7 (Kabata-Pendias & Pendias, 1984).
2.1.3. Soil Cation Exchange Capacity
The CEC of soil is largely dependent on the amount and type of clay,
organic matter, and Fe, Mn and Al oxides. These soil components usually
have different cation exchange properties. Trace metal cations bind by
electrostatic forces to the predominant negative charges on clays, organic
matter and oxides. In general, the higher the CEC of soil, the greater the
amount of metal a soil can accept without potential hazards (Korcak &
Fanning, 1985). Numerous evidences indicate that CEC can best be viewed
as a general, but imperfect, indicator of the soil components (i.e., clay,
organic matter, and Fe, Mn and Al oxides) that limit the solubility of metals
12
13
instead of a specific factor in the availability of these metals (King, 1988;
Latterell et al., 1976).
2.1.4. Hydroxides of Iron, Manganese, and Aluminum
Considerable evidence indicates that hydroxides (Fe, Mn and Al) play a
major role in the adsorption of trace metals in mineral soils (Keeney &
Wildung, 1977). Metal oxides may occur as discrete crystalline minerals or as
coatings on other soil minerals. Their capacity to adsorb or release trace
metal cations from solution is controlled by pH and their crystalline structure.
The adsorption of metals to oxides is related to the fact that most trace metals
readily form hydroxyl species, while the more prevalent cations, Ca 2+ and
Mg2+, do not. In general, the surfaces of hydrous Fe and Mn oxides are
strong scavenging agents for trace metals. This is because the adsorption
selectivity of trace metals is on the order of 103 to 106 over ions like Ca2+
and Mg2+. This permits selective adsorption of trace metals to the oxygen
and hydroxyl surface groups of Fe, Mn and Al oxides.
2.1.5. Amount and Type of Clay
The amount of clay in relation to the amount of silt and sand determines
the soil texture, which in turn influences the CEC of soils. In general, the
higher the clay content, the higher the CEC. The 2:1 type clay minerals
usually have higher CEC than 1:1 types minerals. For example,
montmorillonite (2:1 type) may have 80 to 100 meq/100 g compared to only 3
to 15 meq/100 g for kaolinite (1:1 type) (Brady, 1990). Korte et al. (1976)
found that soil capacity for elements in the cationic form is best correlated
with the surface area (amount of clay), while those present in anionic form are
more strongly correlated with the free iron oxides in the soil. Of the trace
metals present in cationic form, Cu and Pb are highly immobile in most soils,
while Hg is relatively mobile. Of the metals in anionic form, soil capacity is
decreased in the order Se, V, As and Cr.
13
14
2.2. Metal Speciation Procedures
Several sequential chemical fractionation procedures designed to
fractionate soil metals according to relative solubility have been proposed as
a means to identify chemical forms of trace metals in soils. Some studies
have also attempted to correlate these forms with mobility and plant
availability (Lake et al., 1984). Sequential extraction methods have been
used to partition metals into fractons defined as soluble, exchangeable or
sorbed, organically complexed, precipitated, oxide-bound, occluded and
residual, though the actual chemical forms by each extraction procedure are
likely more complicated (Emmerich et al., 1982; Iyengar et al., 1981;
Rappaport et al., 1988; Silviera & Sommers, 1977; Shuman, 1986; Sims,
1986). To estimate these fractions, different extracting solutions have been
used. Table 2 lists some of the extractants used to fractionate metal species
From Table 2, it can be noted that there are a wide diversity of extractants
used to fractionate metal species, although experimental results are often
contradictory and inconsistent. This makes comparison or prediction of
results difficult. Experimental conditions must, therefore, be accurately
defined: namely, the nature of the counter cation, the nature of the anion,
ionic strength, and the pH and soil solution ratio values (Calvet & Bourgeois,
1990; Lake et al., 1984). The use of soil extractants to predict plant
availability of metals has been a subject of a great deal of attempts to
correlate with plant uptake studies, but many problems have been seen with
these studies (Giordano et al., 1979). Such simple extraction's may be useful
under certain conditions; however, such as comparing similar soils,
comparing treatment effects on the same soil, etc.
Today, there are additional analytical techniques used for speciation of
metals to help alleviate the inconsistencies found with sequential extraction
procedures. They are divided into two groups: (i) Chromatographic
techniques; (ii) Multipurpose computer programs; and (iii) kinetic chemical
methods (i.e., 8-hyroxyquinoline extraction) Chromatographic techniques
include ion-exchange, adsorbent resin, gel permeation, and high performance
liquid chromatography (HPLC). Multipurpose computer programs include
14
15
programs such as GEOCHEM. Computer programs are used for speciation
computations, predicting the metal species likely to occur in the solution
phase, on the basis of known chemical equilibria and properties of the soils
concerned. Analytical results, in combination with computer modeling, can
give fairly accurate predictions of the different metal forms. Computer
modeling, however, is useful only for setting limits on speciation and needs to
be run in conjunction with analytical procedures to validate their results
(Verloo & Eeckhout, 1990; Lake et al., 1984).
2.3. Plant Capacity for Trace Elements
2.3.1. Food Chain Implications
One of the basic environmental problems relates to the quantities of
accumulated metals in plant parts used as food. The generalized effects of
metal concentrations in nutrient solution on yield and metal content of plants
are shown in Figure 3. The approimate concentrations of metal toxicity or
deficiency found in various plant species are listed in Table 3.
In addition to the various soil factors previously discussed that can
influence plant availability of trace metals, the following factors can also affect
the ability of plants to accumulate trace metals: plant species, plant cultivars,
and plant parts and age.
2.3.2. Plant Species
Crops differ widely in their sensitivity to excess trace metals. In general,
leafy vegetables are the greatest accumulators of soil trace elements. Crops
differ considerably in their sensitivity to individual trace metals. Generally, at
about pH 5.5 to 6.5, Cu could be twice as toxic as Zinc, and Ni four times as
toxic as Zn at equilvalent concentrations (Adriano, 1986).
2.3.3. Plant Cultivars
Differences in uptake of trace metals among cultivars have long been
established. Investigators recently found differential uptake and translocation
15
16
of Cd and Zn among lettuce and corn cultivars (Giordano et al., 1979; Hinesly
et al., 1979); and differential Cd uptake among soybean cultivars (Boggess et
al., 1978). This evidence indicates there may be a genetic basis of differential
translocation of metals within plants.
2.3.4. Plant Parts and Age
It is common observation that trace elements are not uniformly distributed
among plant tissues. In general, the seeds (or grain) contain lower
concentrations of most trace elements than do the vegetative tissues. Thus,
grain crops are very likely to contribute smaller amounts of trace elements to
the diet than do leafy vegetables. Garland et al. (1981) suggested that the
distribution of elements in various vegetative tissues (exclusive of stems) is a
characteristic of xylem transport and that the ultimate concentration of an
element in a specific tissue is related to the flux of transpirational water lost
through evapotranspiration and the duration of this process for specific
tissues. Age also has an effect on trace element concentration in plants.
Data indicate a tendency for trace element concentrations (Cd, Cr, Cu, Mn,
Pb and Zn) in fescue leaves to decrease during the growing season (Boswell,
1975).
2.4. Long Term Plant Availability of Sludge-borne Metals
It has been suggested some metals (Zn, Ni and Cd) might increase in
solubility during sludge decomposition in the soil (Dudley et al., 1986; Behel
et al., 1983; Chang et al., 1983; Holtzclaw et al., 1978), however, a lowering
of metal solubility and uptake has typically been demonstrated in the results
of research projects (Emmerich et al., 1982). It has been observed from long
term sludge studies that plant availability of sludge-borne metals is highest
during the first year after sludge is applied (Bidwell & Dowdy, 1987; Chang et
al., 1987; Hinesly & Hansen, 1984). This is in contrast to the long-held belief
that once the sludge applied organic matter is oxidized complexed metal will
be released and plant uptake will increase (Beckett et al., 1979). Additionally,
16
17
this belief is not supported by studies of sludge chemistry which indicate that
digested sludge in addition to being 50% organic matter is 50% inert inorganic
mineral forms (including Fe and Al oxides, silicates, phosphates, and
carbonates) that are reactive with the metals and environmentally stable
(Essington & Mattigod, 1991; McCalla et al., 1977).
2.5. Effect of Sludge on Metal Concentration in Soil
The concentration of Zn, Cd, Cu, Ni, Mn, Ca, Mg, Na, K, PO43--P, SO42--S
and Cl in soil solution typically increases with sludge application rates,
particularly with multiple sludge applications at high rates in fine-textured
soils. In general, soil solution concentrations of Cd, Cu, Ni, Pb and Zn are
low at all sludge application rates due to the retention of metals in mineral and
organic complexes. However, soluble Cu and Ni concentrations in soil
treated with sludge at rates of 800 Mg ha-1 were markedly higher than those
receiving lesser amounts of sludge (Behel et al., 1983). Application of 200
Mg ha-1 of sludge at one time resulted in higher levels of soluble Zn, Cd, Cu,
PO43--P and Cl and higher pH than a similar total amount of sludge applied in
a consecutive annual addition of 50 Mg ha-1 yr-1.
Rappaport et al. (1988) found that DTPA extractable (a sparingly soluble
form) of Cd, Cu, Ni and Zn increased with rate of sludge application. The
order of DTPA extractable metals concentration in the soil was Cu > Zn > Ni >
Cd. Extractions with DTPA have been shown to be somewhat correlated with
plant uptake (Singh & Narwal, 1984; Street et al., 1977; Bingham et al.,
1975). However, in some cases, plant uptake did not correlate with DTPA
extraction (Giordano et al., 1979). It was further reported that the
extractability of metals from the sludge-amended soil were controlled more by
the properties of the sludge rather than the soil properties. Giordano et al.
(1979) reported that the concentration of DTPA extractable Cd, Zn, Cu, Pb
and Ni in sludge-amended soil increased apparently because of progressive
decomposition of the organic matrix, though the water solubility decreased
with time.
17
18
Liming can also have an effect on the metal solubility in soil. This is
because there is a greater complexation of metals by soluble organic carbon
with liming (Dudley et al., 1986), and for most metals, decreased solubility
through inorganic processes as well. Campbell & Evans (1987) reported that
both H+ and humic acid concentration influence the percentage of soluble Pb
and Cd. Lead is largely bound by humic acids and to a lesser extent by
carbonate but if the concentration of Ca increases in the system, more Pb will
bind with the humic acids. Cadmium binding to humic acids is much lower
than that of Pb, which will result in a higher availability of Cd than that of Pb in
sludge-amended soils, though Cd is strongly retained in many circumstances.
Over time, it appeared that Cd, Ni and Zn shifted toward less-soluble form,
while Cu remains in a relatively insoluble organic form (Emmerich et al., 1982.
2.6. Effect of Sludge Application on Metal Uptake by Plants
Various studies have reported that plants absorb substantial amounts of
additional metals from sludge-amended soils. In a recent pot study, a linear
relationship was observed between the total Cd concentration of 17
anaerobically digested sludges and Cd concentration of Sudax when the
sludges were applied at a constant Cd rate (Jung & Logan, 1992). Metal
accumulation by plants depends upon plant species, sludge and soil physical
and chemical properties as well as the metal concentrations and forms. Metal
contents of vegetative tissues, such as leaves and stems, have generally
been reported to be higher than those of the fruiting, root and tuber tissues
(Vlamis et al., 1985; Bingham et al., 1975; Dowdy & Larson, 1975).
It has been observed that sludge with low metal concentrations have lower
plant uptake at the same metal loading than do sludges with higher metal
concentrations (Logan, 1989; Corey et al., 1987). In order to see substantial
plant uptake of metals where the source of those metals is sludge, extremely
high application rates are typically necessary, usually >100 Mg ha -1 of sludge
with high metal levels. Lower rates of sludge or the application of sludge with
low metal content results in no additional metal uptake by most plants, with
18
19
the exception of certain metal accumulators such as lettuce or swiss chard.
Dowdy & Larson (1975) observed that in most edible tissue, trace metal
accumulations did not increase more than two or three fold as a result of
amending the soil with 450 Mg ha-1 of sludge. This would result in the
average soil plow layer consisting of about 25% sludge. Reductions in metal
uptake were probably the result of the binding of free metals in soil by sludge
organic matter.
These observations have led some researchers to suggest that the uptake
of trace metals by plants is a function of soil organic matter content, cation
exchange capacity, pH, soil texture, and Fe, Mn and Al oxide (King, 1988;
Korcak & Fanning, 1985; Singh & Narwal, 1984; Chang et al., 1983; King &
Dunlop, 1982; Haghiri, 1974). In general, it has been observed that the
uptake of most metals, particularly Cd, increases with decreasing soil pH
(Gerritse et al., 1982; Dijkshoorn et al., 1981; Giordano et al., 1979; Bingham
et al., 1975). However, the competition of the H+ ion with the Cd2+ ion for
plant uptake at lower pH levels may explain why the uptake is not as great as
the increase in solubility would indicate (Hatch et al., 1988), and why in some
cases, no decrease or even an increase in uptake was noted with liming
(Pepper et al., 1983; Hemphill et al., 1982; King & Dunlop, 1982). It is
speculated that in some soils, particularly those with high amounts of organic
matter, the mechanisms of metal retention may be quite different than in soils
where inorganic retention mechanisms dominate.
The presence of nitrogen in sludge can also enhance the uptake of Zn,
Cd, Cr, Pb and Ni by plants in sludge-amended soils either because of the
increased growth or due to mobilization of metals from acidification due to
nitrification (Chang et al., 1983; Giordano & Mortvedt, 1976). The
mineralization of organic matter to nitrate results in the production of 2 H+,
which can greatly acidify soil as follows:
19
20
It is important to note that most of the available studies with sewage
sludge were conducted with sludges containing metals at levels higher than
the median concentration in current U.S. sludges (EPA, 1990). The cited
studies were either deliberately conducted with high metal sludges (e.g., the
Chicago sludge of the 1970's which had a Cd concentration of approximately
200 mg/kg) or simply reflect the higher metal concentrations that were
present prior to industrial pretreatment programs. As not all studies give the
metal concentration of the sludges used, it is not possible to determine sludge
metal concentration for all studies.
2.7. Effect of Soil pH on Metal Activity in Sludge Amended Soils
Some researchers found that pH reduction in the sludge-soil layer
increased the solubility of metals and increased their potential for movement
in the soil profile. Boswell (1975) applied sewage sludge of pH 5.6 to a soil
that had been limed to a pH of 6.2. Hinesly et al. (1972) started with a soil pH
of 5.6, which at the end of two years had dropped to 4.9. Fuller & Alesii
(1979) observed that at lower soil pH levels and higher concentration of
metals in the sludge, there was more rapid movement of metals through the
soil. The amount of Pb, Ni, Cd, Cd and Zn retained by soil depended upon
the pH of the soil, nature of the clay and the chemical nature of organic
compounds present (Harter, 1983; Hatton & Pickering, 1980).
King (1988) observed that the relative retention of metals by most soils
was in the order of Pb > Cu > Cr > Zn > Ni > Co > Cd. Sorbed and
nonexchangeable Cd, Co, Cu, Ni and Zn were better related to pH than to
any of the other soil properties. Sorbed and nonexchangeable Pb, Cr and Sb
20
21
were more related to sand, clay or Fe oxide content. Additionally, retention in
organic soils were superior to retention in nearly all of the mineral soils.
Lowered pH of the soil profile beneath the sludge soil layer was attributed
to mineralization and nitrification of sludge organic nitrogen (Chang et al.,
1987; Emmerich et al., 1982). With time intense nitrification and the lowering
of pH may be conducive to the movement of metals in the soil profile.
However, no direct evidence of this has been actually observed, and there is
the potential for increased retention due to a buildup of organic matter in the
soil from the additional nitrogen and increased microbial and plant activity as
well.
21
22
Figure 1: Forms of trace metals in soil solutions.
22
23
Figure 2: The hydrolysis species of Zn2+ and Cu2+ as functions of soil pH in
equilibrium with soil Zn or Cu (Source: Kabata-Pendias & Pendias, 1984).
23
24
Figure 3: The generalized effects of metal concentrations in nutrient solution
on yield and metal content of plants (Source: Kabata-Pendias & Pendias,
1984).
24
25
Table 2: Sequential extraction techniques used to fractionate trace metals in
soils and sludge-amended soils (Source: Lake et al., 1984).
Metal
Studied
Silviera & Alloway et
Sommers al. (1979)
(1977)
Cd,Cu,
Cd
Pb, Zn
Metal
Form:
Extractant
Used:
Soluble
H2 O
Source
Exchangeable
KNO3
Petruzzelli Emmerich
Schalscha
et al. (1981) et al. (1982) et al. (1980;
1982)
Cd, Cu, Ni, Cd, Cu, Ni, Cr, Cu, Mn,
Pb, Zn
Zn
Ni, Zn
Soon &
Bates
(1982)
Cd, Ni,
Zn
H2 O
H2 O
CH3CO2- KNO3
NH4
CaCl2
KNO3
KNO3
Adsorbed
KNO3
Deionized
H2 0
NaOH
Na4P2O7
(CH3
CO2)2Cu
NaOH
Carbonate
HNO3
Precipitated
Na2-EDTA
EDTA
HNO3
Na2EDTA
Sulfide
Precipitated
HNO3
HNO3
Available
K4 P 2 O7
Cd,Cu,Ni
Pb, Zn
NaF
Ion
Exchange
Organically
Bound
Sposito et
al. (1982)
DTPA
DTPA
Occluded
HONH2HCL
Residual
HNO3**
HNO3 **
* Cu-Ox consists of oxalic acid (CO2H)2 and oxalate (CO2NH4)2
** Residual forms determined by subtracting sum of extracted forms from total metal
concentration.
25
HNO3
26
Table 3: Approximate concentrations of trace elements in mature leaf tissue
generalized for various species (ppm, dry wt.) (Source: Kabata-Pendias &
Pendias, 1984).
Deficient, if less than the
stated amounts of
Sufficient or
Excessive or
Element
essential elements
normal
toxic
Ag
As
-------------
0.5
1 - 1.7
5 - 10
5 - 20
B
Ba
Be
Cd
Co
Cr
Cu
F
Hg
5 - 30
------------------------------2-5
-------------
10 - 200
------<1 - 7
0.05 - 0.2
0.02 - 1
0.1 - 0.5
5 - 30
5 - 30
-------
50 - 200
500
10 - 50
5 - 30
15 - 50
5 - 30
20 - 100
50 - 500
1-3
Li
Mn
Mo
Ni
Pb
Se
Sn
Sb
Ti
------15 - 25
0.1 - 0.3
-------------------------------------
3
20 - 300
0.2 - 1
0.1 - 5
5 - 10
0.01 - 2
------7 - 50
-------
5 - 50
300 - 500
10 - 50
10 - 100
30 - 300
5 - 30
60
150
50 - 200
Tl
V
Zn
Zr
------------10 - 20
-------
------0.2 - 1.5
27 - 150
-------
20
5 - 10
100 - 400
15
Note: Values are not given for very sensitive or highly tolerant plant species.
26
3. HYPOTHESIS AND STUDY OBJECTIVE
The objective of this study was to determine if pH has an effect on the
extractability and plant availability of Cd, Cu, Ni and Zn in a soil amended with
a single high application of sewage sludge sixteen years earlier. The
following two hypothesis were developed to test this theory:
Hypothesis 1: Extractability of the Cd, Cu, Ni and Zn is a function of its free
ion concentration (water soluble and exchangeable form).
This will be done by measuring the solubility of Cd, Cu, Ni and Zn is a
function of pH in a sludge-amended soil treated treated with lime to four
different pH levels. By changing the soil pH, it can be observed if pH can still
affect the free ion concentration of Cd, Cu, Ni and Zn in a soil amended with
sewage sludge sixteen years earlier.
Hypothesis 2: Plant availability of Cd, Cu, Ni and Zn is a function of its free
ion concentration.
This will be done by measuring the plant availability of Cd, Cu, Ni and Zn
primarily by changing the relative solubility of metal fractions and measuring
the concentrations of metals in plant tissue grown in a sludge-amended soil
treated with lime to four different pH levels and comparing uptake to the
concentration of metal fractions. By changing the soil pH, it can be observed
if the availability of soil Cd, Cu, Ni and Zn can still be affected sixteen years
after sludge application.
4. METHODS AND MATERIALS
4.1. Site Description
This field study was conducted at the University of Washington's Charles
Lathrop Pack Forest Research Station, located approximately 100 km south
of Seattle near Eatonville, Washington.
4.1.1. Climate
The maritime climate at Pack Forest is mild and wet with a mean annual
temperature of 9° C. Mean annual precipitation is 114 cm with over 75% of
the precipitation falling between October and April. Winter temperatures are
mild enough that nearly all of the precipitation falls as rain.
4.1.2. Soils
The soil, prior to the application of sewage sludge, is classified as a
Barneston series by the U.S.D.A Soil Conservation Service (1979).
Barneston soil is a somewhat excessively drained gravelly sandy loam (mesic
Dystric xerochrepts) that formed in glacial outwash under conifers along the
foothills of the Cascades. In a typical undisturbed profile a mat of extremely
acid (pH 4.2), undecomposed needles and wood overlies a 5-inch, very dark
brown and very dark grayish brown, very strongly acid (pH 4.4) gravelly sandy
loam surface layer. The subsoil, to a depth of 13 inches, is a dark yellowish
brown, strongly acid (pH 4.8-5.2) coarse gravelly sandy loam. The
substratum, to a depth of more than 60 inches, is a brown, medium acid (pH
5.8) very gravelly sand.
4.2. Study Design
The site was amended in 1975 with a single high application (500 Mg ha 1) of anaerobically digested dewatered sewage sludge from the Municipality
of Metropolitan Seattle (METRO) wastewater treatment plants. In the
29
summer of 1991, the sludge-amended soil was divided into four 3 x 11-m
plots and planted with vegetable crops. One 3 x 11-m sludge-amended plot
was not treated with lime to serve as a control with an initial pH level of
approximately 4.6. The remaining three sludge-amended plots were treated
with hydrated lime, Ca(OH)2, at rates of 8, 15, and 22 Mg ha-1 resulting in a
soil pH of approximately 5.8, 6.5, and 6.9, respectively. To quantify
backgound soil and plant metal concentrations for comparison purposes only,
four untreated 3 x 11-m plots (unsludged and unlimed) were used with soil pH
levels of approximately 5.9. These four untreated plots were located
approximately 30-m from the treated sludge-amended plots.
Vegetable crops selected for planting represented foods important in the
human diet and showing differing metal uptake rates: (i) bush beans
(Phaseolus vulgaris L. cv. Seafarer), (ii) cabbage (Brasica oleracea L. v.
capitata L. cv. Copenhagen market), (iii) carrots (Daucus carota L. cv.
Toudo), (iv) corn (Zea mays L. cv. FR37), (v) lettuce (Lactuca sativa L. cv.
Parris Island), (vi) potatoes (Solanum tuberosum L. cv. Kenebec), and (vii)
tomatoes (Lycoperisicon esculentum L. cv. Burpee VF).
4.2.1. Plant Analysis Procedure
During the growing season, all plots were fertilized with optimum amounts
of commercial NPK fertilizer (10:10:10; without micronutrients) to ensure
nutrient availability was not limiting. Plant height at maximum vegetative
growth stage, and total above ground biomass (wet weight basis) were
measured at time of harvesting for the cabbage, corn and lettuce. Total
harvestable biomass (total wet weight of heads of lettuce and cabbage,
tubers of potato, fruit of tomato, bean pods, and corn stalk at tasseling above
the 2nd leaf) was measured at the time the edible tissue was sampled for
analysis. The edible portion of the plant tissues were dried at 70 °C to
constant weight, then ground to <2-mm in a stainless steel Wiley mill for
analysis. Total Cd, Cu, Ni and Zn in plant tissue samples were determined
using an HNO3-H2O2-HCL acid digestion (EPA Standard Method 3050) and
29
30
inductively coupled argon plasma spectroscopy (ICP; Thermo Jarrel Ash
ICAP 61E, Thermo Jarrel Ash, Franklin, MA).
4.2.2. Soil Analysis Procedure
Three soil core samples were collected from the Ap horizon (0-15 cm) in
each plot. The soil samples were analyzed for pH, total Cd, Cu, Ni and Zn,
trace metal speciation (Henry & Harrison, 1992), CEC, total N and organic C.
Samples were air dried and screened to < 2-mm prior to physical and
chemical analysis. Soil pH was determined in a 1:2 soil-to-deionized water
suspension after equilibration for 1 hour (Corning pH Analyzer 250, Corning,
NY). Sequential extractions were used to fractionate the trace metals in the
soil samples (Henry & Harrison, 1992). The extractions were done in
sequence on the same soil sample using the following extraction technique
(Figure 4):
(1) deionized water extraction for soluble metals (Msoluble);
(2) 1.0M MgCl2 extraction for exchangeable forms (Mexchangeable);
(3) 0.1M Na4P2O7 extraction for organically bound forms (Morganic); and
(4) HNO3-H2O2-HCL acid digestion of the residual fraction (Mresidual).
Metal concentrations in solutions were measured on an ICP. Results for each
fraction were reported as the percent of the total soil metal.
Total metals were determined using the HNO3-H2O2-HCL acid digestion
(EPA Standard Method 3050). Cation exchange capacity was determined
using the following steps: (1) soil was saturatedwith 1.0 M NH4Cl, (2) soil
pore water was removed with two ethanol leachings to remove soluble NH4+,
(3) NH4+ was displaced with 1.0 M KCl, and (4) NH4+ was analyzed on a
Technicon Autoanalyzer (Technicon Autoanalyzer II, Tarrytown, NY). Total N
was determined by digesting soil samples using a modified Li2SO4-H2SO4
digestion method (Parkinson & Allen, 1975) and analyzed on an
30
31
Autoanalyzer. Total organic C content was determined by dry combustion
(LECO model 761-100 C Determination: LECO Corp., Joseph, MI).
31
32
4.3. Statistical Analysis
The experiment was treated as a 4 x 3 factorial with a minimum of three
replicates per cell (four treatment levels; three cells within each treatment
level, and three replicates within each cell). An analysis of variance (ANOVA)
was used to test the differences of means in plant metal concentrations at
each pH level in the sludge-amended soil (SYSTAT, 1990). The unsludged
and unlimed plots were used to quantify background concentrations for
comparison purposes only.
32
33
Figure 4: Flowchart of the sequential extraction procedure used to determine
the speciation of each metal in the soil samples.
33
5. RESULTS AND DISCUSSION
5.1. Liming Effects on Metal Speciation and Soil Chemical Properties
5.1.1. Effects on Metal Speciation
Figure 5 shows the percentage of metals in the four fractions of the
sludge-amended soil at each pH level. Background soil fractions by
sequential extraction are included for comparison purposes. Based on the
observation that the water soluble and exchangeable metal fractions are often
plant available (Brady, 1990), the water soluble + exchangeable metal frations
(Msol+exch) were used to indicate relative solubility of metal in soils. Liming
the sludge amended soil significantly decreased Cdsol+exch, Nisol+exch and
Znsol+exch. Reductions were 45, 90 and 95%, respectively. Liming resulted
in only minor changes in Cusol+exch (<2%). For example, Cdsol+exch,
Nisol+exch and Znsol+exch fractions in the unlimed, sludge-amended soil were
high (28%, 27% and 30% respectively), while the limed sludge-amended soil
Cdsol+exch, Nisol+exch and Znsol+exch were much lower (15%, 2% and trace,
respectively). The Cusol+exch in the unlimed sludge-amended soil was only
2% of total, while the limed, sludge-amended soil had only trace
concentrations. Similar studies of the effects of liming showed Zn exchangeable
was decreased by an average of 60% to 95%, depending on the soil type,
when the soil pH was increased from 4.8 to 7.5 (Sims, 1986; Shuman, 1986).
Alloway et al. (1979) found the distribution of Cd in a sludge-amended
acid soil (pH 5.9) for the water soluble and exchangeable fractions totaled
22%, organic bound 45%, hydrous-oxide bound 20%, and the rest of the
metal was in the residual form. Street et al. (1978) observed that watersoluble Cd decreased as much as 50% as pH was increased from 5.0 to 7.8.
This study also found a similar distribution of Cd in the acidic unlimed sludgeamended soil (Table 4 and Figure 5). As the pH was increased, the
proportion of Cd in the water soluble plus exchangable fraction decreased
from 29% to 15% while the residual fraction increased from 35% to 55%.
35
This decreased solubility of Cd in soils is associated with the formation of
CdCO3 and Cd3(PO4)2 with increasing pH (Street et al., 1977). Elevated pH
can also change the nature of the exchange sites by hydrolyzing or
precipitating Al3+ ions that occupy the exchange sites, thus creating more
exchange sites (Adriano, 1986).
With the possible exception of Pb, Cu has been found to be the most
strongly adsorbed than Cd, Ni and Zn on organic matter, Fe and Al oxides
and oxyhydroxides (McBride, 1989). The stability of the Pb and Cu metalloorganic complex often increasing as pH increased from 3 to 7 (KabataPendias & Pendias, 1984). In a study conducted by McLaren & Crawford
(1973), from 20 to 50% of the Cu in 24 soils of diverse types occured in
organically bound forms.
In soil sorption studies, the amount of Ni retained was also dependent
upon the pH of the soil, with retention increasing with increasing pH (Harter,
1983; Gerritse et al., 1982). Berrow & Burridge (1990) found increasing the
soil pH from 4.5 to 6.5 decreased the plant available fraction of Ni, thus
decreasing the Ni content of oats grain by a factor of about 8.
Similar studies of the effects of liming showed exchangeable Zn was
decreased by an average of 60% to 95%, depending on the soil type, when
pH was increased from 4.8 to 7.5 (Sims, 1986; Shuman, 1986). Zinc
adsoprtion by carbonates, precipitation of Zn hydroxide or carbonates, or
formation of insoluble calcium zincate are believed partly responsible for Zn
unavailability in alkaline soils (Farrah & Pickering, 1977; Shuman, 1979;
Bingham et al., 1964).
5.1.2. Effects on Soil Chemical Properties
Soil pH was reduced in the sludge-amended soil, probably due to the
significant nitrification that followed the sludge application (Table 5). For
instance, the unamended soil had an average pH level of 5.9, while the
sludge-amended, unlimed soil had a pH level of 4.6.
Total N and organic C were increased in the 0-15 cm soil horizon as a
result of the addition of sewage sludge. For instance, total C was 67 vs 184
35
36
mg g-1 and total N was 3.4 vs. 12.9 mg g-1 in the unamended and sludgeamended soils, respectively. Total N and C concentrations were not
significantly affected by liming (Table 5). Results also indicate CEC was
significantly higher in the unamended soil compared to the unlimed sludgeamended soil (18.1 vs 11.6 cmolc kg-1, respectively). Though the large
amendment of organic matter would normally increase CEC, the effective
CEC of this soil was apparently greatly reduced by the reduction in soil pH.
The liming treatments increased CEC to levels similar to background soils
(Table 5).
5.2. Liming Effects on Plant Metal Uptake
The most consistent trends in plant metal concentrations observed in this
study occurred with Cd, Ni and Zn (Table 6). In general, metal concentrations
in plant tissue grown in the sludge-amended soil were in the order (Table 6 ,
Figure 6 and Appendix 2):
bush bean foliage > corn stalk > cabbage > lettuce > potato peels >
peeled potato tuber = tomato fruit
Tissue Cd, Ni and Zn concentrations in cabbage and lettuce tissue were
significantly reduced (p<0.01) in the limed sludge-amended soils to below
suggested mean plant tolerance levels (in foliage) of 3, 50 and 300 mg kg-1
(Figure 6). Copper concentration; however, was not reduced by liming. In
the peeled potato tuber and tomato fruit, tissue Cd, Ni and Zn concentrations
were significantly reduced by treatment with lime (p<0.01). While Cu
concentration generally did not change after liming in the peeled potato tuber,
concentration in the tomato fruit did appear to reach deficiency levels at the
highest liming rate (pH 6.9). Ni concentration was significantly reduced
(p<0.01) in the tomato fruit but was not reduced in the peeled potato tuber.
Cadmium, Ni and Zn concentrations in the potato peels were all significantly
36
37
reduced (p<0.01) by treatment with lime, while Cu concentration was not
reduced.
In general, metal concentrations in each of the different plant tissues had
the following trends (Table 6): Cadmium concentration in the plant tissue after
liming was in the order: bush bean foliage > corn stalk > lettuce > cabbage,
potato peel > peeled potato tuber, tomato fruit; Copper concentration was in
the order: bush bean foliage > potato peel > lettuce > cabbage, corn stalk >
peeled potato tuber, tomato fruit; Nickel concentrations was in the order:
bush bean foliage, cabbage > corn stalk > lettuce > peeled potato tuber,
tomato fruit; and Zinc concentration was in the order: bush bean foliage >
corn stalk > lettuce > cabbage > potato peel > peeled potato tuber > tomato
fruit.
The above-ground yield for the cabbage and lettuce was significantly
reduced in the unlimed sludge-amended soil (Appendix 1). The aboveground yield for corn was not reduced in the unlimed sludge-amended soil
and was unaffected by liming. Total above ground yield was not measured
for the bush beans. The bush beans also did not produce any legumes in the
unlimed sludge-amended soil, and consequently, I was unable to compare
the effect of liming on the uptake of metals in legumes. Total above ground
yields were not measured for the potato and tomato crops; however, total fruit
and tuber yield was increased after liming.
These results agree with studies suggesting that Cd, Ni and Zn in soils
may be made unavailable to plants by the application of hydrated lime and,
consequently, phytotoxicity prevented (Bingham et al., 1979; MacLean &
Dekker, 1978: Chaney et al., 1977). Soil pH appears to be the most
important soil property that determines plant availability for Cd, Ni and Zn.
The higher solubility of these three metals in the unlimed sludge-amended
soil was in agreement with recognized increasing solubility of the metals with
increasing soil acidity (Logan & Chaney, 1973). Cadium uptake by plants
was found to almost always to decrease with decreasing pH (Adriano et al.,
1982; Bingham et al., 1979; Mahler et al., 1978). In studies of Zn uptake at
different pH levels, uptake declined sharply when soils were limed from pH
37
38
4.3 to above 5.6 (Friesen et al., 1980; Gupta et al., 1971). Liming is effective
because these metals probably form an insoluble precipitate with hydroxides,
carbonates and phosphates and thus becomes unavailable to plants (Reddy
& Patrick, 1977).
Copper concentrations remained more stable and generally were
unaffected by changes in pH. This was in agreement with the general affinity
of Cu to form stable complexes with organic matter (Harter, 1983; Ellis &
Knezek, 1972). A number of studies have shown that Cu in soil solutions,
especially at higher pH, exists primarily in a form complexed with organic
matter which is relatively unavailable to plants (Hodgson, 1963). In fact, King
& Dunlop (1982) concluded that high organic matter content in soils can
substitute for high pH in immobilizing metals and thus sewage sludge can be
applied to organic soils that have pH values lower than the currently
suggested value of pH 6.5 for metal application from municipal sewage
sludges.
38
39
Figure 5: Soil metal speciation, in percent of total metal, by sequential
extraction for the background (pH 5.9) and sludge-amended soils treated with
lime to four different pH levels; pH 4.6, 5.8, 6.5 and 6.9.
39
40
Figure 6: Trace metal concentrations (mg kg-1, dry wt.) in plant tissue grown
in sludge-amended soil with treated with lime to four different pH levels; pH
4.6, 5.8, 6.5 and 6.9.
40
41
Figure 6 (con't): Trace metal concentrations (mg kg-1, dry wt.) in plant tissue
grown in sludge-amended soil with treated with lime to four different pH
levels; pH 4.6, 5.8, 6.5 and 6.9.
41
42
Table 4: Sequential extraction of soil metals, in percent of total metals, found
at the surface soils (0-15 cm) in the back-ground (no sludge/no lime) and
sludge-amended soils treated with lime to four different pH levels; pH 4.6, 5.8,
6.5 and 6.9.
CADMIUM
Water
Amendment
Soluble Exchangeable Organic Residual
----------------------------------%----------------------------No Sludge/No Lime - pH 5.9 ND *
31.6
31.6
36.7
Sludge/No Lime - pH 4.6 1.31
27.7
36.3
34.6
Sludge/Lime - pH 5.8 1.28
24.8
35.3
38.6
Sludge/Lime - pH 6.5
ND
18.0
30.3
51.7
Sludge/Lime - pH 6.9
ND
15.3
29.5
55.3
COPPER
Water
Amendment
Soluble Exchangeable Organic Residual
----------------------------------%----------------------------No Sludge/No Lime - pH 5.9 8.58
ND
64.1
27.3
Sludge/No Lime - pH 4.6
ND
1.88
57.8
40.2
Sludge/Lime - pH 5.8
ND
ND
58.1
41.4
Sludge/Lime - pH 6.5
ND
TR **
45.4
53.9
Sludge/Lime - pH 6.9
TR
TR
43.4
55.7
NICKEL
Water
Soluble Exchangeable Organic Residual
----------------------------------%----------------------------No Sludge/No Lime - pH 5.9 3.96
3.96
ND
92.1
Sludge/No Lime - pH 4.6 1.21
26.9
14.9
57.0
Sludge/Lime - pH 5.8 1.08
16.7
18.3
64.0
Sludge/Lime - pH 6.5 1.33
5.57
17.2
75.9
Sludge/Lime - pH 6.9 1.37
1.87
17.6
79.1
Amendment
42
43
Table 4 (con't): Sequential extraction of soil metals, in percent of total metals,
found at the surface soils (0-15 cm) in the background (no sludge/no lime)
and sludge-amended soils treated with lime to four different pH levels; pH 4.6,
5.8, 6.5 and 6.9.
ZINC
Water
Amendment
Soluble Exchangeable Organic Residual
----------------------------------%----------------------------No Sludge/No Lime - pH 5.9
ND
10.1
11.9
78.0
Sludge/No Lime - pH 4.6 1.30
29.6
47.7
21.5
Sludge/Lime - pH 5.8
TR
14.2
58.0
27.5
Sludge/Lime - pH 6.5
ND
3.35
49.4
47.2
Sludge/Lime - pH 6.9
ND
TR
50.0
49.7
* ND = nondetected
** TR = trace
43
44
Table 5: Mean (±S.D.) soil elemental analysis of the background (no
sludge/no lime) and sludge-amended soils treated with limed to four different
pH levels; pH 4.6, 5.8, 6.5 and 6.9. All soil samples were collected from a
depth of 0 to 15 cm.
Amendment
pH
%N
%C
CEC
(cmolc kg-1)
No Sludge/No Lime
5.9 (0.1)
0.34 (0.04)
6.74 (1.1)
18.1 (2.9)
Sludge-No Lime
4.6 (0.1)
1.29 (0.18)
18.4 (2.6)
11.6 (1.0)
Sludge-Limed
5.8 (0.1)
1.18 (0.27)
16.9 (3.9)
19.3 (2.1)
Sludge-Limed
6.5 (0.3)
1.19 (0.11)
17.0 (1.5)
19.6 (2.5)
Sludge-Limed
6.9 (0.2)
1.17 (0.17)
16.8 (2.5)
23.0 (2.3)
44
45
Table 6: Mean (±S.D.) metal concentration (mg kg-1, dry wt.) in plant tissue
grown in background (no sludge/no lime) and sludge-amended soils treated
with lime to four different pH levels; pH 4.6, 5.8, 6.5 and 6.9.
BEAN LEAF
Amendment
Cd
Cu
Ni
Zn
-------------------------- mg/kg -----------------------No Sludge/No Lime - pH 5.9
ND †
6.9 (1.8)
TR †
32 (6.4)
Sludge/No Lime - pH 4.6 12 (4.3)
36 (4.9)
81 (17) 1700 (360)
Sludge/Lime - pH 5.8 7.6 (1.6)
31 (8.1)
20 (6.5) 630 (180)
Sludge/Lime - pH 6.5 7.4 (1.6)
21 (3.3)
13 (5.4) 680 (130)
Sludge/Lime - pH 6.9 5.8 (2.2)
23 (5.3) 9.7 (3.3) 570 (140)
Amendment
No Sludge/No Lime - pH 5.9
Sludge/No Lime - pH 4.6
Sludge/Lime - pH 5.8
Sludge/Lime - pH 6.5
Sludge/Lime - pH 6.9
Amendment
No Sludge/No Lime - pH 5.9
Sludge/No Lime - pH 4.6
Sludge/Lime - pH 5.8
Sludge/Lime - pH 6.5
Sludge/Lime - pH 6.9
CABBAGE
Cd
Cu
Ni
Zn
-------------------------- mg/kg -----------------------ND
2.1 (0.7) 2.7 (5.6) 20 (2.6)
8.2 (2.4)
9.0 (2.2)
34 (13) 900 (270)
1.5 (0.7)
8.6 (2.6)
27 (11)
200 (59)
1.2 (0.5)
6.9 (2.0) 19 (7.9) 160 (50)
1.6 (0.5)
9.5 (2.6) 22 (8.8) 190 (41)
CORN STALK
Cd
Cu
Ni
Zn
-------------------------- mg/kg -----------------------ND
1.7 (0.3)
ND
18 (3.0)
4.5 (4.2)
4.1 (2.1) 3.6 (2.9) 230 (63)
7.9 (5.6)
9.6 (2.1)
37 (25)
480 (83)
3.7 (1.8)
7.5 (2.9) 9.4 (5.4) 440 (130)
4.3 (4.2)
6.7 (2.9)
22 (19) 430 (200)
45
46
Table 6 (con't): Mean (±S.D.) metal concentration (mg kg-1, dry wt.) in plant
tissue grown in background (no sludge/no lime) and sludge-amended soils
treated with lime to four different pH levels; pH 4.6, 5.8, 6.5 and 6.9.
LETTUCE
Amendment
Cd
Cu
Ni
Zn
-------------------------- mg/kg -----------------------No Sludge/No Lime - pH 5.9
ND
10 (2.3) 5.9 (5.8) 41 (6.3)
Sludge/No Lime - pH 4.6 3.2 (0.8)
9.5 (1.8) 13 (5.3) 360 (81)
Sludge/Lime - pH 5.8 6.4 (2.9)
12 (2.6) 7.3 (2.4) 300 (85)
Sludge/Lime - pH 6.5 2.9 (1.7)
11 (1.6) 3.8 (1.1) 170 (66)
Sludge/Lime - pH 6.9 3.1 (1.1)
12 (2.6) 139 (33) 140 (33)
Amendment
No Sludge/No Lime - pH 5.9
Sludge/No Lime - pH 4.6
Sludge/Lime - pH 5.8
Sludge/Lime - pH 6.5
Sludge/Lime - pH 6.9
Amendment
No Sludge/No Lime - pH 5.9
Sludge/No Lime - pH 4.6
Sludge/Lime - pH 5.8
Sludge/Lime - pH 6.5
Sludge/Lime - pH 6.9
TOMATO FRUIT
Cd
Cu
Ni
Zn
-------------------------- mg/kg -----------------------ND
7.4 (1.3)
19 (17)
20 (1.9)
0.9 (0.3)
8.7 (4.2)
19 (13)
49 (9.4)
1.0 (0.7)
10 (2.4) 9.5 (3.8)
60 (12)
TR
1.3 (3.3) 1.4 (3.7) 8.2 (21)
TR
TR
TR
TR
POTATO TUBER
PEELED
Cd
Cu
Ni
Zn
-------------------------- mg/kg -----------------------ND
8.2 (2.2) 2.3 (5.1) 22 (4.1)
1.3 (0.3)
12 (3.5)
27 (32)
91 (30)
TR
9.5 (1.7) 8.0 (5.0)
45 (17)
TR
9.9 (1.7)
13 (22)
45 (8.4)
TR
12 (1.6)
29 (43)
49 (6.8)
46
47
Table 6 (con't): Mean (±S.D.) metal concentration (mg kg-1, dry wt.) in plant
tissue grown in background (no sludge/no lime) and sludge-amended soils
treated with lime to four different pH levels; pH 4.6, 5.8, 6.5 and 6.9.
POTATO PEEL
Amendment
Cd
Cu
Ni
Zn
-------------------------- mg/kg -----------------------No Sludge/No Lime - pH 5.9
TR
10 (1.7) 1.3 (0.3) 18 (2.8)
Sludge/No Lime - pH 4.6 3.7 (1.2)
24 (9.0)
22 (8.3) 400 (170)
Sludge/Lime - pH 5.8 2.3 (0.6)
20 (3.9)
10 (4.4) 130 (75)
Sludge/Lime - pH 6.5 1.6 (0.3)
16 (3.3) 4.3 (2.3)
44 (11)
Sludge/Lime - pH 6.9 1.4 (0.2)
20 (9.4) 3.6 (1.1) 42 (6.5)
† ND and TR Values
Cd
Cu
Ni
Zn
-------------------------- mg/kg -----------------------Nondetected (ND)
≤0.2
≤0.3
≤0.2
≤0.2
Trace (TR)
≤0.5
≤1.0
≤0.8
≤0.5
47
6. CONCLUSIONS
Hypothesis 1: Extractability of the Cd, Cu, Ni, and Zn is a function of its free
ion concentration (water soluble and exchangeable form).
The most soluble and plant available soil fractions of Cd, Ni and Zn were
the most significantly reduced by the liming treatment. Copper extractability
was very low and soil Cu was present almost exclusively in the unavailable
organically complexed or residual fraction even at the lowest soil pH level.
The data for Cd, Ni and Zn appear to support the practice of liming soil to 6.0
or above when sludge is applied at a rate high enough to result in
mineralization of excess nitrogen, nitrification, nitrate leaching and
acidification of the soil.
This study indicates trace metals in sludge-amended soils may remain
plant available in acidic soils for several years after application. Soil pH will
markedly alter the distribution of Cd, Ni and Zn among the plant available
pools and should be considered carefully when assessing the fate of natural
or applied trace metals. Using a sequential extraction technique to fractionate
soil metals is critical when considering the fate of trace metals. This study
shows the irrelavence of using only total metal concentrations to assess the
relative mobility and plant availability of trace metals.
Hypothesis 2: Plant availability of Cd, Cu, Ni, and Zn is a function of its free
ion concentration.
Cadmium, Cu, Ni and Zn remained plant available in a soil amended with
a single high application of sewage sludge sixteen years earlier. Zinc was the
most plant available sludge-borne metal. The ability for the absorbed Cu and
Ni to move from vegetative parts into the fruit and tuber is substantially less
than for Cd and Zn. This study also agrees with reported studies in which it
has been shown that leafy green plant tissue have a higher trace metal
49
uptake rate than root or fruit tissue. General trends in the uptake of individual
metals by the different crop tissue in this study (e.g., cabbage, lettuce and
peeled potato tuber) suggest that crop uptake differences must be accounted
for in food-chain risk assessment from land application of sewage sludge. In
addition, sludge-borne metals generally do not cause serious toxicity to the
majority of crops, even if large amounts of metal are added to the soil.
49
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58
APPENDIX 1: PLANT YIELD DATA
SAMPLE
Total Above Ground
Wet Wt (kg)
Edible Wet Wt.*
(kg)
Plant Ht
(cm)
LETTUCE
Mean
SD
Mean
SD
Mean
SD
Background-pH 5.9
0.13
0.08
0.13
0.08
19.1
5.8
Sludge/No lime-pH 4.6
0.07
0.04
0.03
0.04
8.7
4.0
Sludge/Lime-pH 5.8
0.39
0.11
0.28
0.09
25.8
1.7
Sludge/Lime-pH 6.5
0.48
0.14
0.35
0.11
25.1
2.1
Sludge/Lime-pH 6.9
0.40
0.13
0.27
0.11
24.3
2.9
* Weight minus outer wrapper leaves
SAMPLE
Total Above Ground
Wet Wt (kg)
Edible Wet Wt.*
(kg)
Plant Ht
(cm)
CABBAGE
Mean
SD
Mean
SD
Mean
SD
Background-pH 5.9
0.51
0.21
0.13
0.08
21.8
3.5
Sludge/No lime-pH 4.6
0.02
0.02
0.02
0.02
11.5
3.8
Sludge/Lime-pH 5.8
0.84
0.40
0.23
0.13
27.4
2.2
Sludge/Lime-pH 6.5
0.88
0.33
0.24
0.14
25.6
2.6
Sludge/Lime-pH 6.9
0.56
0.21
0.13
0.07
22.6
2.2
* 1/2 of head minus outer wrapper
leaves
SAMPLE
Total Above Ground
Wet Wt (g)
Plant Ht
(cm)
BUSH BEAN
Mean
SD
Mean
SD
Background-pH 5.9
28.00
7.13
---
---
Sludge/No lime-pH 4.6
0.74
0.15
5.1
0.3
Sludge/Lime-pH 5.8
25.83
11.47
28.8
6.6
Sludge/Lime-pH 6.5
23.35
6.13
26.2
4.1
Sludge/Lime-pH 6.9
25.28
11.63
27.6
3.8
60
60
61
APPENDIX 1: Plant Yield Data (con't)
SAMPLE
Individual Tuber Wt.*
Unpeeled Tuber Wet Wt (g)
POTATO TUBER
Mean
SD
Background-pH 5.9
82.78
34.48
Sludge/No lime-pH 4.6
43.70
37.12
Sludge/Lime-pH 5.8
146.89
73.84
Sludge/Lime-pH 6.5
154.66
57.03
Sludge/Lime-pH 6.9
165.81
64.58
*Total above ground plant wet wt. was not measured at time of harvest.
SAMPLE
Individual Tuber Wt.
Unpeeled Tuber Wet Wt (g)
Tuber Length
(cm)
CARROT TUBER
Mean
SD
Mean
SD
Background-pH 5.9
27.99
10.41
28
12
Sludge/No lime-pH 4.6
Non
Non producing
producing
Sludge/Lime-pH 5.8
25.78
16.55
23
9
Sludge/Lime-pH 6.5
20.79
10.37
23
9
Sludge/Lime-pH 6.9
19.09
6.43
23
9
SAMPLE
Total Above Ground
Wet Wt (kg) above 2nd leaf
Plant Ht
(cm)
CORN STALK
Mean
SD
Mean
SD
Background-pH 5.9
0.25
0.04
113.5
9.7
Sludge/No lime-pH 4.6
0.31
0.08
103.3
13.9
Sludge/Lime-pH 5.8
0.56
0.09
129.8
20.7
Sludge/Lime-pH 6.5
0.41
0.13
113.3
8.4
Sludge/Lime-pH 6.9
0.43
0.12
122.5
21.8
61
62
APPENDIX 1: Plant Yield Data (con't)
SAMPLE
TOMATO
Total Above Ground
Total Fruit Yield
Wet Wt (g)
Wet Wt
(g)
Plant Ht
(cm)
Mean
SD
Mean
SD
Mean
SD
Background-pH 5.9
745
184
345
133
110
12
Sludge/No lime-pH 4.6
138
85
57
44
56
10
Sludge/Lime-pH 5.8
2171
1839
718
752
85
15
Sludge/Lime-pH 6.5
1470
541
900
341
115
12
Sludge/Lime-pH 6.9
1864
576
1143
256
119
11
62
63
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS
* Description of Type coding. An entry of 9999 signifies no sample:
BL = Bean Leaf
CBL = Background pH 5.9 (no sludge/no lime) Bean Leaf
S5.0BL = Sludge/No lime pH 4.6 Bean Leaf
S5.5BL = Sludge/Lime pH 5.8 Bean Leaf
S6.0BL = Sludge/Lime pH 6.5 Bean Leaf
S6.5BL = Sludge/Lime pH 6.9 Bean Leaf
CB = Cabbage
CCB = Background pH 5.9 (no sludge/no lime) Cabbage
S5.0CB = Sludge/No lime pH 4.6 Cabbage
S5.5CB = Sludge/Lime pH 5.8 Cabbage
S6.0CB = Sludge/Lime pH 6.5 Cabbage
S6.5CB = Sludge/Lime pH 6.9 Cabbage
CP = Carrot Peel
CCP = Background pH 5.9 (no sludge/no lime) Carrot Peel
S5.0CP = Sludge/No lime pH 4.6 Carrot Peel
S5.5CP= Sludge/Lime pH 5.8 Carrot Peel
S6.0CP = Sludge/Lime pH 6.5 Carrot Peel
S6.5CP = Sludge/Lime pH 6.9 Carrot Peel
CT = Peeled Carrot Tuber
CCT = Background pH 5.9 (no sludge/no lime) Peeled Carrot Tuber
S5.0CT = Sludge/No lime pH 4.6 Peeled Carrot Tuber
S5.5CT= Sludge/Lime pH 5.8 Peeled Carrot Tuber
S6.0CT = Sludge/Lime pH 6.5 Peeled Carrot Tuber
S6.5CT = Sludge/Lime pH 6.9 Peeled Carrot Tuber
63
64
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
PP = Potato Peel
CPP = Background pH 5.9 (no sludge/no lime) Potato Peel
S5.0PP = Sludge/No lime pH 4.6 Potato Peel
S5.5PP= Sludge/Lime pH 5.8 Potato Peel
S6.0PP = Sludge/Lime pH 6.5 Potato Peel
S6.5PP = Sludge/Lime pH 6.9 Potato Peel
PT = Peeled Potato Tuber
CPT = Background pH 5.9 (no sludge/no lime) Peeled Potato Tuber
S5.0PT = Sludge/No lime pH 4.6 Peeled Potato Tuber
S5.5PT= Sludge/Lime pH 5.8 Peeled Potato Tuber
S6.0PT = Sludge/Lime pH 6.5 Peeled Potato Tuber
S6.5PT = Sludge/Lime pH 6.9 Peeled Potato Tuber
T = Tomato Fruit
CT = Background pH 5.9 (no sludge/no lime) Tomato Fruit
S5.0T = Sludge/No lime pH 4.6 Tomato Fruit
S5.5T= Sludge/Lime pH 5.8 Tomato Fruit
S6.0T = Sludge/Lime pH 6.5 Tomato Fruit
S6.5T = Sludge/Lime pH 6.9 Tomato Fruit
L = Lettuce
CL = Background pH 5.9 (no sludge/no lime) Lettuce
S5.0L = Sludge/No lime pH 4.6 Lettuce
S5.5L = Sludge/Lime pH 5.8 Lettuce
S6.0L = Sludge/Lime pH 6.5 Lettuce
S6.5L = Sludge/Lime pH 6.9 Lettuce
64
65
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
65
66
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
66
67
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
67
68
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
68
69
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
69
70
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
70
71
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
71
72
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
72
73
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
73
74
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
74
75
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
75
76
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
76
77
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
77
78
APPENDIX 2: COMPLETE PLANT DATA ANALYSIS (CON'T)
78