United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/123 Jan. 1986
&EPA Project Summary
Soil Physicochemical
Parameters Affecting
Metal Availability in
Sludge-Amended Soils
R. P. Gambrell, Susan Landes, C. N. Reddy, and W. H. Patrick, Jr.
Laboratory studies were conducted
to examine the effects of pH and
oxidation-reduction status (redox po-
tential) on soil levels of trace and toxic
metals in selected chemical forms.
Studies were also conducted to see
how metal availability to plants was af-
fected by soil redox conditions. The ele-
ments studied included Cu, Zn, Cd, Pb,
Cr, Ni, and As.
Chemical availability and plant up-
take studies under controlled pH and
redox potential indicated that various
organic phases predominate in retain-
ing Cu under reducing conditions (i.e.,
low redox potential). As redox potential
is increased, however, a very marked
transformation occurs so that poten-
tially available Cu becomes strongly as-
sociated with the reducible (hydrous
iron oxide) phase under well oxidized
conditions. Soluble, exchangeable, and
chelate-extractable Cu also increased
with increasing redox potential. Results
with rice were mixed, but increasing
oxidation conditions tended to increase
Cu in corn.
Dissolved and exchangeable Zn in-
creased with decreasing soil pH and in-
creased to a lesser degree with increas-
ing redox potential. A very large
increase in plant Zn levels occurred
with increasing redox potential.
Compared with most other trace and
toxic metals studied, a much larger pro-
portion of the total Cd extracted was
recovered in the most readily available
chemical forms (dissolved and ex-
changeable). Cd appears to be less
strongly associated (immobilized) with
high-molecular-weight organics and is
weakly bound to hydrous iron oxides.
This factor contributes to the potential
for ready accumulation of Cd by plants
grown in Cd-contaminated soils. In
most of the experimental combinations
of sludge-soil mixtures and pH used in
the laboratory microcosms, plant Cd
concentrations increased greatly as the
redox potential increased.
Except for exchangeable lead being
greater at pH 5.0 than at higher pH lev-
els, changing pH or redox potential had
little effect on Pb levels in other chemi-
cal forms.
Unlike results for Cd and Zn, oxida-
tion status had little, if any, effect on
plant levels of Cr, Ni, and As. Predomi-
nant chemical forms in the sludge-
amended soils and oxidation effects on
levels in these chemical forms are dis-
cussed in the report.
This research demonstrates the im-
portant effects of soil redox potential in
regulating the chemical mobility and
availability to plants of Zn and Cd—two
key contaminants in sludge materials
that may be applied to soils. Where al-
ternatives exist for disposal of contami-
nated sludge, wet or poorly oxidized
soils would be more effective for immo-
bilizing Zn and Cd and would thus re-
sult in lower accumulations of these
metals in crop plants or native plant
populations on uncultivated land.
This Project Summary was devel-
oped by EPA's Water Engineering Re-
search Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
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separate report of the same title {see
Project Report ordering information at
back).
Introduction
The Federal Water Pollution Control
Act of 1972 (PL 92-500) has stimulated
increasing interest in the use of terres-
trial ecosystems to achieve water qual-
ity standards. Section 201 (b) requires
that technology development consider
advanced waste treatment techniques
and include the reclaiming and recy-
cling of water and the confined disposal
of pollutants to prevent other environ-
mental pollution. The application of mu-
nicipal sewage sludge to soil-crop sys-
tems may be the most environmentally
and economically feasible method of
meeting these requirements in many in-
stances. The benefits of applying mu-
nicipal sludge to land include
(1) reducing the pollution load of river
and coastal waters that have tradition-
ally received sewage wastes,
(2) recycling water and nutrients
through crops to recover these valuable
resources, (3) more rapid inactivation or
destruction of potentially serious micro-
bial pathogenic organisms than some
conventional wastewater treatment
methods, and (4) possible improvement
of marginal soils to develop land for
parks and other urban uses.
The most serious limitation of apply-
ing municipal sludge to agricultural
soils concerns the fate of pathogens and
toxic substances that may be associated
with municipal wastes. Toxic heavy
metals in sludge-amended soils and the
crops grown on them have been studied
extensively because of the tendency for
these metals to accumulate in soils and
to be assimilated by plants. Crop uptake
of toxic metals may increase the dietary
intake of livestock and humans who
consume these plants.
Extensive agricultural research has
shown that a number of interacting
physical, chemical, and biological fac-
tors affect the availability of nutrients
and toxic materials in soil-plant sys-
tems. For example, the kind and amount
of clay minerals and soil organic matter
and the presence of additional chemi-
cally reactive or adsorptive soil compo-
nents such as sulfides (if present) and
hydrous oxides play an important role
in regulating bioavailability. Plant spe-
cies and management of soils, crops,
and waste applications are also impor-
tant factors. Fortunately, most soils—
even those that are highly contami-
nated— have a large capacity to
immobilize toxic metals so that only a
small proportion of the metal applied is
available to plants. However, all of the
metals applied with sludge applications
must be considered potentially avail-
able for plant uptake. Because of the
probable long-term accumulation of po-
tentially available toxic metals in
sludge-amended soils and the dynamic
nature of soil-plant systems, it is imper-
ative that we understand all of the sig-
nificant factors and processes affecting
their availability.
Soil pH and oxidation-reduction (re-
dox) conditions are two physicochemi-
cal parameters that affect metal mobi-
lization and immobilization.
Considerable research has been con-
ducted on the effects of soil pH on the
chemical availability and uptake of trace
and toxic metals by crop plants grown
on soils treated with municipal sludge.
As presented in the literature review,
these studies have conclusively demon-
strated that an increase in soil acidity
usually enhances the availability of
trace and toxic metals to plants. In con-
trast, very little is known about the ef-
fects of soil redox conditions on toxic
metal availability, but they are probably
as important as soil pH in regulating the
availability of some metals.
Very limited published information is
available on the effects of soil redox
conditions on trace and toxic metal up-
take by plants. The early work focused
on trace elements, primarily Fe and Mn.
In recent years, a few papers have been
published on chemical mobility and
plant uptake of trace and potentially
toxic metals (e.g., Zn) and toxic ele-
ments such as Cd in wetland soils. How-
ever, little attention has been paid to the
processes regulating trace and toxic
metal availability urder different oxida-
tion conditions and the effects of soil
oxidation on the availability of metals to
plants in sludge-amended soils. Most
soils currently receiving municipal
sludge are upland soils classified as
moderately well to well drained. Such
soils are generally aerobic or oxidized.
Municipal sludge may also be applied
to wetland soils characterized by poor
internal drainage or water saturation for
much of the time. These soils are likely
to be anaerobic and chemically re-
duced. Because of increased restric-
tions on dumping pollutants in coastal
water and the chronic shortage of con-
veniently located land suitable for dis-
posal, pressures will probably increase
to use poorly drained agricultural and
wetland soils for waste disposal.
In addition to the various oxidation
conditions of different soils, adding
large quantities of sludge (frequently as
a water slurry) with high levels of or-
ganic matter is likely to contribute to in-
termittent, strongly reduced conditions
in all or part of the rhizosphere of an
upland soil that is normally considered
well drained and oxidized. Thus a wide
range of redox conditions may be en-
countered in sludge-amended soils, re-
sulting in varying degrees of toxic metal
availability to plants.
The primary objective of this research
was to study the effects of soil pH and,
especially, redox conditions on the soil
chemical form and the availability of
trace and toxic metals to plants grown
on agricultural soils receiving municipal
sludge applications. The elements in-
cluded were Cu, Zn, Cd, Pb, Cr, Ni, As,
Fe, Mn, Ca, Mg, Mo, Si, P, and Al, but the
report evaluated only the first seven of
these in detail.
A secondary objective of the research
was to focus on the effects of pH and
redox conditions on trace metal com-
plex formation with soluble and insolu-
ble organic matter. This important, if
not predominant, regulatory process af-
fects the availability of metals in sludge-
amended soils.
Research Approach and
Findings
Sludge Ratio Study
A simple study was conducted in
which mixtures of Chicago reference
sludge and Drummer soil were incu-
bated as suspensions under conditions
of continuous air purging or oxygen-
free nitrogen purging. The sludge: soil
ratios used were 0:100, 5:95, 25:75, and
100:0. At the end of the incubation, se-
quential extractions were made on the
suspensions to measure exchangeable
and diethylenetriaminepentaacetic-
acid-(DTPA-) extractable levels of se-
lected elements to determine the effect
of sludge:soil ratios and oxidation con-
ditions on the chemical availability of
these elements.
At ratios of 75 and 100 percent sludge,
the data suggested that oxidizing condi-
tions were probably not achieved; thus
the evaluation of the results was limited
to the 0-, 5-, and 25-percent sludge ra-
tios. Except for Mn, there was generally
an increase in exchangeable and DTPA-
extractable levels of the elements stud-
ied with an increase in the sludge ratio.
Also, except for Mn and Mo, the largest
proportion of the elements was in the
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DTPA fraction. This result was espe-
cially apparent as the sludge ratio in-
creased, indicating the important role of
high-molecular-weight organics in
binding these elements in soils and
sludge-amended soils. Levels of ex-
changeable and DTPA-extractable Cu,
Zn, Cd, and Pb were generally much
greater in oxidized sludge-soil suspen-
sions than in reduced suspensions. Ex-
changeable and DTPA-extractable lev-
els of Fe, Mn, and P responded to redox
potential conditions in the opposite
manner. This result was anticipated for
Fe and Mn, since these metals are sub-
ject to redox-mediated valence state
changes, which make them much more
chemically mobile under reducing con-
ditions as ferrous Fe and manganous
Mn. This preliminary experiment indi-
cated that (1)the addition of metals
through sludge increases metal levels
in several chemical forms, but the
organic-bound pool may be especially
important, and (2) soil redox potential
affects the levels of metals in various
chemical forms. One implication is that
soil redox conditions may affect the
availability of trace and toxic metals to
plants as well.
Effect of Sludge Digestion
Method on Plant Uptake of Se-
lected Elements
The ojective of this study was to de-
termine whether sludge derived from
aerobic or anaerobic digestion of
sewage influenced the chemical
availability or plant uptake of trace and
toxic elements in sludge-amended
soils. Soybean and corn seedlings were
grown for 3 weeks in the Drummer soil
material amended with 5 percent of aer-
obically or anaerobically digested Baton
Rouge sludge. Only the plant tissue
samples were analyzed for elemental
content. Each experimental combina-
tion was replicated five times.
Soybeans grown for 3 weeks in the
5-percent-amended Drummer soil
showed no indication that the addition
of sludge or the digestion method af-
fected the above-ground plant tissue
content of Cu, Zn, Cd, Pb, Cr, Ni, or As.
Corn plants grown in the soil amended
with 5 percent anaerobically digested
sludge contained significantly more Cu,
Cd, Pb, Cr, and As than did the control
plants. Regarding the method of diges-
tion, significant differences were ob-
tained only for Cu and Pb: levels were
higher for both elements from the
sludge that was derived from an anero-
bic digestion process.
Trace and Toxic Elements in
Humic and Fulvic Acids Ex-
tracted from Sludge and
Sludge-Soil Mixtures
Levels of selected elements in humic,
fulvic, and precipitate forms were deter-
mined in Baton Rouge sludge materials
that had been prepared by either an
anaerobic or an aerobic digestion proc-
ess. In another study, levels of selected
elements in the three humic material
fractions were determined in extracts of
many of the sludge-soil suspensions in-
cubated under controlled pH and redox
potential.
Following the incubations of sludge-
soil mixtures under controlled pH and
redox potential, aliquots of the suspen-
sions were shaken to extract humic and
fulvic acids and a precipitate fraction.
Fulvic acid was apparently recovered in
the greatest amount, though a substan-
tial part of this fraction is composed of
extracting reagents (primarily sodium).
Cu, Zn, and Ni were most strongly asso-
ciated with the fulvic acid fraction than
with the other two forms. However, the
much greater quantity of fulvic acid ex-
tracted would result in a greater quan-
tity of most elements of interest in this
fraction. Copper was also present at
about equal levels in the humic acid
fraction. The concentration of all other
elements in the humic acid fraction was
either about the same as or less than
levels in the fulvic acid and precipitate
forms. No consistent, unidirectional
changes occurred in the concentration
of any metal in any of the three fractions
with increasing pH and redox potential.
However, levels of Cu were generally
lower in all three fractions under strong
reducing conditions (-150 mv) than in
better oxidized treatments at pH levels
of 5.0 and 6.5. At pH 5.0, Zn levels
tended to decrease with increasing
redox potential. Cadmium levels in the
humic and precipitate fractions were
low (undetectable). Cadmium levels in
the fulvic acid fraction generally in-
creased with increasing redox potential.
Effect of pH and Oxidation
Conditions on Chemical Form
and Plant Availability of Metals
in Sludge-Amended Soils
A 10-percent mixture of Chicago ref-
erence sludge and Drummer (or Loring)
soil material was incubated as an
aqueous slurry at different pH levels
(5.0, 6.5, and 8.0) and at different redox
potential levels (-150 mv, 50 mv, 250
mv, and 500 mv) representing redox
conditions ranging from strongly re-
duced to well oxidized. After a 2-week
period, duplicate suspension aliquots
were taken and extracted for dissolved,
exchangeable (sodium-acetate-ex-
tractable), reducible (ammonium-
oxalate-/oxalic-acid-extractable), DTPA-
extractable, and residual organic-bound
metals. At this time, plant seedlings
were introduced into the microcosms
so that the roots were suspended in the
slurry being maintained under con-
trolled pH and redox potential. After 2
weeks of growth in the microcosms, the
above-ground tissue was harvested and
analyzed for trace and toxic metals.
The chemical availability and plant
uptake studies under controlled pH and
redox potential provided considerable
information on the forms and transfor-
mations of trace and toxic metals in
sludge-amended soils.
Various organic phases predominate
in retaining Cu under reducing condi-
tions, but as redox potential is in-
creased, a very marked transformation
occurs in the reducible (hydrous iron
oxide) phase, with which potentially
available Cu is most strongly associated
under well oxidized conditions. The
trend was for dissolved, exchangeable,
and chelate-extractable Cu to increase
with increasing redox potential. Results
with rice were mixed, but increasing ox-
idation conditions tended to increase
Cu in the young corn plants grown in
the laboratory microcosms.
In the studies using Chicago sludge
and Drummer soil, there was a weak
trend for readily available (dissolved
and exchangeable) Zn to increase with
increasing redox potential, and the
highest levels were at pH 5.0, as ex-
pected. A very large increase in plant Zn
levels occurred with increasing redox
potential. For example, in the young
corn plants, Zn levels increased from
around 100 jxg/g under strong reducing
conditions to greater than 900 and 500
(xg/g in well oxidized sludge-soil mix-
tures at pH levels of 6.5 and 8.0, respec-
tively. Most of the potentially available
Zn was found in the reducible fraction
under oxidized and reduced conditions.
Compared with most other trace and
toxic metals studied, a much larger pro-
portion of the total Cd extracted was re-
covered in the most readily available
chemical forms (dissolved and ex-
changeable). Cd appears to be less
strongly associated (immobilized) with
high-molecular-weight organics than
most toxic metals and, in particular, is
weakly bound to hydrous iron oxides.
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This factor contributes to the potential
for ready accumulation of Cd by plants
grown in Cd-contaminated soils. In
most of the experimental combinations
of sludge-soil mixtures and pH used in
the laboratory microcosms, plant Cd
concentrations increased greatly as the
redox potential increased. For example,
a 20-fold or greater increase in the Cd
content of rice plants occurred between
-150 mv and 500 mv at pH 6.5 in the
suspensions of Baton Rouge sludge and
Loring soil and in the combination of pH
8.0 Chicago sludge and Drummer soil. A
50-fold increase in plant content oc-
curred between the redox potential ex-
tremes in the Chicago sludge and Drum-
mer soil incubated at pH 6.5. Thus this
research supports other studies indicat-
ing that Cd contamination of soils and
sediments represents a greater environ-
mental risk than contamination with
many other toxic metals at equivalent
levels because of Cd's relatively high
mobility and availability to plants. This
study also demonstrates the impor-
tance of soil oxidation conditions on Cd
mobility and availability to plants.
Pb was most strongly associated with
the residual organic phase. Exchange-
able Pb was very pH-dependent, and the
greatest levels were found at pH 5.0.
Changing redox and/or pH had little ef-
fect on levels of Pb in other chemical
forms.
Unlike results with Zn and Cd, soil ox-
idation status had little, if any, effect on
plant levels of Cr, Ni, and As. Predomi-
nant chemical forms in the sludge-
amended soils and oxidation effects on
levels in these chemical forms are dis-
cussed in the full report.
Conclusions
This research demonstrates the im-
portant effects of soil redox potential in
regulating the chemical mobility and
availability to plants of Zn and Cd—two
key contaminants in sludge that may be
applied to soils. Obviously, most crop
species are not grown on soils that
range from strongly reduced to well ox-
idized. Most crops are grown on well
drained (oxidized) soils. Rice is an im-
portant exception in that it is normally
grown on flooded (reduced) soils. How-
ever, rice may be grown in upland, oxi-
dized soils as well, and in some regions
of the country, substantial acreage is
devoted to such dry-land rice. Sugar-
cane is grown on soils that are some-
what low and wet. The redox potentials
of these soils would probably be inter-
mediate but subject to frequent and
wide fluctuations. Nonagricultural soils
may also be used to dispose of sewage
sludge, and the oxidation status en-**
countered in soils of various natural
habitats or other uncultivated soils may
vary considerably.
In considering the potential problems
associated with the disposal or produc-
tive use of contaminated sewage
sludge, many factors and options must
be considered in developing acceptable
management plans. This research indi-
cates that the redox status of soils being
considered for receiving municipal
sludge is also an important consider-
ation, particularly for Zn and Cd. Where
feasible alternatives exist, wet or poorly
oxidized soils would be more effective
for immobilizing Zn and Cd. Sludge ap-
plications to these soils would therefore
be less likelv to result in accumulations^
by plants. ~
The full report was submitted in fulfill-
ment of Cooperative Agreement R-
805789 by Louisiana State University
under the sponsorship of the U.S. Envi-
ronmental Protection Agency.
R. P. Gambrell, S. Landes, C. N. Reddy, and W. H. Patrick, Jr., are with Louisiana
State University, Baton Rouge, LA 70803.
J. A. Ryan is the EPA Project Officer (see below).
The complete report, entitled "Soil Physicochemical Parameters Affecting Metal
Availability in Sludge-Amended Soils," (Order No. PB 86-116 621 /AS; Cost:
$16.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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