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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