United States
Environmental Protection
Agency
OSWER Directive # 9355.4-26FS
June 2015
Office of Superfund Remediation and
Technology Innovation
Phosphate Amendment Fact Sheet
Glossary of Terms and Acronyms
Amendment: Agents that are added
to soil to cause lead to be less soluble
or less mobile in soil
Bioaccessibility (oral): Soluble
fraction of lead in gastrointestinal
fluids; used to estimate RBA
Bioavailability (oral): Fraction of an
ingested dose of lead that is absorbed
from the gastrointestinal tract
Contaminant: Harmful or hazardous
matter introduced into the
environment
Co-contaminant: Contaminants
commonly found together with lead
Dissolved: made soluble
In vivo: Measured in a living organism
(human or animal model)
In vitro: Measured in a test tube
Pb: Lead in any form
Phosphate: A phosphorus-oxygen
chemical having the structure PO4B"
Phosphate Amendment: Any one of a
group of soil amending agents that
contain phosphate
Pyromorphite: Any one of a group of
highly insoluble lead-phosphate
minerals (e.g., chloropyromorphite,
Pb5[P04]3CI)
Relative Bioavailability (oral): Ratio
of oral bioavailability of lead in soil of
that of a highly water soluble form of
lead (typically lead acetate)
RBA: Relative bioavailability
What happens when soil containinR lead (Pb)
is inRested?
Lead (Pb) is a ubiquitous environmental
constituent and children are particularly
sensitive to the effects of lead. Exposure of
children to Pb can cause adverse health
outcomes, such as neurocognitive impairment.
Ingested soil and surface dust can be important
contributors to elevated blood Pb levels in
children exposed to Pb contaminated
environments.
Following ingestion of soil contaminated with
Pb, Pb is released from soil and dissolved in
gastrointestinal fluids. Once the Pb is dissolved,
it is absorbed from the gastrointestinal tract
into the body. Oral bioavailability measures
how much of the ingested Pb is absorbed
following ingestion.
The oral bioavailability of Pb is strongly
influenced by its solubility in fluids of the
gastrointestinal tract and the form of Pb that is
ingested.
How is the oral bioavailability of Pb in soil
measured?
The oral bioavailability of Pb from soil is
measured using in vivo bioassays. This type of
study is typically performed using juvenile
swine. In the swine assay, the oral relative
bioavailability (RBA) is used to estimate the
bioavailability of Pb in soil. Animal studies to
assess the bioavailability of Pb in soil are
expensive and time consuming.
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United States
Environmental Protection
Agency
OSWER Directive # 9355.4-26FS
June 2015
Office of Superfund Remediation and
Technology Innovation
Phosphate Amendment Fact Sheet
The oral relative bioavailability of Pb can also
be estimated using an in vitro method which
measures Pb bioaccessibility. This is the most
common and cost-effective method used to
determine the oral relative bioavailability of Pb
in soil.
How are soils that are contaminated with Pb
treated to reduce (mitiRate) risk?
Typically, strategies to mitigate human health
risk from exposure to Pb contaminated soil
have focused on excavation and removal of the
contaminated soil, capping with clean soil, or
covering with vegetation.
Soil amendments, agents that are added to soil
to cause Pb to be less soluble or less mobile in
soil, used in combination with other methods
(e.g., removal or containment) have been used
to mitigate exposure to soil Pb.
Phosphate amendments are agents that
contain phosphate, such as triple super
phosphate, rock phosphate, phosphoric acid,
and hydroxyapatite (e.g., fish bones).
Phosphate amendments have been studied as a
means to mitigate risks from exposure to Pb in
soils.
How can phosphate amendments affect the
oral bioavailability of Pb?
The rationale for amending soils with
phosphate is that phosphate will promote
formation of highly insoluble Pb species (e.g.,
pyromorphite minerals) in soil, which will
remain insoluble after ingestion and, therefore,
decrease oral bioavailability.
Results of in vitro and in vivo studies provide
evidence that amending soils with phosphate
can reduce the bioaccessibility and
bioavailability of Pb from soil within a certain
concentration range of Pb.
How is the effectiveness of phosphate
amendments measured?
Laboratory studies have shown that
pyromorphite has very low solubility and forms
rapidly in soils. However, direct measurements
of the bioavailability of pyromorphite in the
mammalian gastrointestinal tract have not
been reported. Therefore, it is not possible to
predict the maximum effectiveness of any
phosphate amendment treatment.
Animal bioassays can be used to determine the
RBA of Pb from soils treated with soil
amendments. Currently, this is the method
recommended by EPA.
At this time, use of in vitro, bioaccessibility
methods to predict the relative bioavailability
of Pb from phosphate-amended soils is not
recommended. Although a large number of in
vitro studies have evaluated the effect of
phosphate soil amendments on soil Pb
bioaccessibility, the relationship between
bioaccessibility and RBA has not been
rigorously established for soils amended with
phosphate. The U.S. EPA is currently conducting
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United States
Environmental Protection
Agency
OSWER Directive # 9355.4-26FS
June 2015
Office of Superfund Remediation and
Technology Innovation
Phosphate Amendment Fact Sheet
research to develop in vitro methods that could
be used to estimate the RBA of Pb in phosphate
amended soils.
What factors may influence the effectiveness
of phosphate amendments?
If other metals, such as iron (Fe), aluminum
(Al), and manganese (Mn), are present in soil,
they may react with phosphate amendments.
This may decrease the amount of phosphate
available to react with Pb to form pyromorphite
The pH level of soil may influence the chemical
form of Pb in soil. Certain forms of Pb do not
easily react with phosphate to form
pyromorphite.
Water in soil is necessary to transport
phosphate amendments through the soil and
sustain the formation of pyromorphite. If
phosphate amendments are applied to soils
that have low water content, pyromorphite
formation may be reduced.
There is very little information about long-term
stability of pyromorphite or the environmental
conditions that could cause it to break down
and release soluble Pb into soil.
What happens to other metals when
phosphate amendments are applied to soils?
In many instances, Pb-contaminated soils also
contain other co-contaminants of concern, such
as antimony (Sb), arsenic (As), cadmium (Cd),
vanadium (V), and zinc (Zn).
Investigations of effects of phosphate
amendments on co-contaminated soils are
limited and studies have not examined the
bioavailability of co-contaminants.
Studies have shown that phosphate
amendments may cause co-contaminants, such
as As, to be released from soil and to enhance
mobility of these contaminants within soil.
Enhanced mobility may cause co-contaminants
to migrate to ground or surface water or be
more available for uptake into plants.
High soil content of organic matter can reduce
formation of pyromorphite.
It is unknown if increased mobility of co-
contaminant mobility results in an increase in
co-contaminant bioavailability.
Are there other concerns about usinR
phosphate amendments?
Phosphate amendments may migrate to and
contaminate areas off the application area.
If applied in excess, phosphate amendments
may run off the application area and
contaminate ground or surface water.
How should phosphate amendments be used
for soil remediation?
Formation of pyromorphite in soil from the site
should be demonstrated.
Results of in vitro and in vivo studies show that
amending soils with phosphate reduced
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United States
Environmental Protection
Agency
OSWER Directive # 9355.4-26FS
June 2015
Office of Superfund Remediation and
Technology Innovation
Phosphate Amendment Fact Sheet
bioaccessibility and bioavailability of Pb from
soil. However, these studies cannot be used to
predict how well phosphate amendments will
work at a specific site. Therefore, plans to
amend soils with phosphate need to include
assessment of site-specific efficacy to reduce
Pb bioavailability.
Phosphate amendments should be used in
combination with other methods, such as re-
vegetation, raised garden beds, or gravel.
The long-term effectiveness of the phosphate
amendment should be established to
determine if repeated applications are
necessary to maintain reduced bioavailability.
What are research needs for use of phosphate
amendments?
Research needs for assessment of efficacy of
phosphate amendments on bioavailability
include the following:
• Measurement of the bioavailability of
pyromorphite in a suitable animal
model and/or human clinical study;
• Development of an in vitro
bioaccessibility assay that reliably
predicts in vivo bioavailability of Pb in
phosphate-amended soils;
• Additional studies of efficacy of specific
amendments in a variety of settings
where Pb contamination is an issue and
cannot be feasibly remediated by
removal (e.g., urban gardens);
• Studies to determine effects of
phosphate amendments on the mobility
and bioavailability of important co-
contaminants (e.g., As); and
• Studies to determine duration of
efficacy and requirements for repeated
amendments.
Reference:
Scheckel, K.G., Diamond, G., Maddaloni, M., Partridge, C., Serda, S., Miller, B.W., Klotzbach, J. and Burgess,
M. 2013. Amending soils with phosphate as means to mitigate soil lead hazard: A critical review of the state
of the science. 7. Toxicol. Environ. Health B 16(6): 337-380.
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