ISO 22190:2020

INTERNATIONAL ISO
STANDARD 22190
First edition
2020-03
Soil quality — Use of extracts for the
assessment of bioavailability of trace
elements in soils
Qualité du sol — Utilisation d'extraits pour l'évaluation de la
biodisponibilité des éléments traces dans les sols
Reference number
ISO 22190:2020(E)
©
ISO 2020

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ISO 22190:2020(E)

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ISO 22190:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Background . 2
4.1 General . 2
4.1.1 Presence of trace elements in the soil matrix . 2
4.1.2 Neutral extracts for measurement of actual availability (ISO 21268-1) . 4
4.1.3 Acid extracts for measurement of potential availability (ISO 17586) . 4
4.2 Tiered approach based on bioavailability . 5
5 General procedure using an extract . 5
6 Reporting . 6
7 Calibration . 6
7.1 Introduction . 6
7.2 Applicability of soil extracts . 7
7.3 Limits of soil extracts to estimate trace element bioavailability . 7
Annex A (informative) Calibration towards biological targets . 8
Annex B (informative) Examples of transfer functions .11
Bibliography .12
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ISO 22190:2020(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 190, Soil Quality, Subcommittee SC 7, Soil
and site assessment.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
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ISO 22190:2020(E)

Introduction
As already mentioned in ISO 17402, laboratory and field studies have demonstrated that biological
effects are not related to the total concentration of a contaminant in the soil. Instead, an organism
responds only to the fraction that is biologically available (bioavailable) for that organism. In the
conservative approach of exposure assessment as typically described in a regulatory context, it is
assumed that the total concentration of a contaminant present in a soil or soil-like material is available
for uptake by organisms, including man, which will overestimate the risks. Therefore, a risk assessment
can be optimised by using an approach that is based on estimated exposure representing the available,
effective concentration of the contaminant(s) and on (existing) intrinsic toxicity data.
In standardization of methods for assessing the bioavailability of trace elements a framework of
standards is used with the following layering of standards (see Figure 1). Starting point is ISO 17402 in
which chemical and biological methods are distinguished and where guidance for selection of relevant
methods is given. If a chemical method is to be used to establish environmental availability, there are
the following possibilities:
a) Extractions based on equilibrium. For this approach standards are available or under development.
b) Method based on non-equilibrium. For this approach standards are not yet under development.
If these standards become available they will also be included in this document (dashed line in
Figure 1).
The methods referred to in this standard are all based on extraction. Extraction can be considered as a
model to simulate the pore water concentration. The extraction methods give results that can be used
in assessment and this standard gives guidance for that use.
The method for human bioaccessibility (ISO 17924) is not presented in Figure 1. It is an extraction
method that simulates the human intestinal system and is specific for assessment of human risks.
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ISO 22190:2020(E)

Key
Red this document
Grey existing standards
Green not yet available — for future development
Figure 1 — Layering of standards for bioavailability of trace elements
(situation April 2018)
In the scientific research to bioavailability a large number of definitions and concepts are in use, which
reflect the discussion in the scientific world. However, for regulatory purposes a more clear and simple
approach is necessary. In a regulatory context, contaminants are either bioavailable or non-bioavailable.
To support decisions, both should be measurable.
As presented in Figure 2, the bioavailable fraction can be measured using the method described in this
document.
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INTERNATIONAL STANDARD ISO 22190:2020(E)
Soil quality — Use of extracts for the assessment of
bioavailability of trace elements in soils
1 Scope
This document provides guidance on the use of chemical methods establishing the bioavailability of
trace elements in soil and soil-like materials and to stimulate the use of bioavailability in assessments.
The methods themselves are not subject of this document.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 11074, Soil quality — Vocabulary
ISO 17402, Soil quality — Requirements and guidance for the selection and application of methods for the
assessment of bioavailability of contaminants in soil and soil materials
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 11074, ISO 17402 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
bioavailability
degree to which chemicals present in the soil can be absorbed or metabolised by a human or ecological
receptor or are available for interaction with biological systems
Note 1 to entry: The concept of bioavailability is further explained in ISO 17402.
Note 2 to entry: This document follows the approach of Reference [20] as illustrated in Figure 2, in which all
defined fractions are measurable as further explained in Clause 4.
Note 3 to entry: In ISO 17924 a definition specific for human uptake through ingestion is defined as the fraction
of a substance present in ingested soil that reaches the systemic circulation (blood stream).
[SOURCE: ISO 11074:2015, 5.2.2, modified — Note 2 to entry was added and the following note to entry
renumbered.]
3.2
environmental availability
fraction of contaminant physico-chemically driven by desorption processes potentially available to
organisms
[SOURCE: ISO 17402:2008, 3.3]
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ISO 22190:2020(E)

3.3
environmental bioavailability
fraction of the environmentally available compound which an organism takes up through physiologically
driven processes
[SOURCE: ISO 17402:2008, 3.5]
3.4
toxicological bioavailability
internal concentration of pollutant accumulated and/or related to a toxic effect
3.5
actual availability
concentration present in the soil pore water to which organisms are directly exposed.
Note 1 to entry: This definition refers to internal concentrations in humans, mammals and other organisms.
[SOURCE: ISO 17402:2008, 3.18]
3.6
potential availability
amount present in the soil sample (mg/kg) that can be released from the solid phase to the pore water
within a specific time frame
3.7
bioaccessibility
fraction of a substance in soil or soil-like material that is liberated in (human) gastrointestinal juices
and thus available for absorption or the amount available to cross an organism’s cellular membrane
from the environment if the organism has access to the chemical
Note 1 to entry: See ISO 10390 for more information on the chemical.
[SOURCE: ISO 17924:2018, 3.2, modified — The definition was modified by adding "or the amount
available to cross an organism’s cellular membrane from the environment if the organism has access to
the chemical" and a Note 1 to entry was added.]
4 Background
4.1 General
4.1.1 Presence of trace elements in the soil matrix
Because the total exposure of organisms depends on time, the available fraction is not a fixed fraction,
but should be divided into multiple fractions or be described as a continuum. The release of the
contaminants depends on local environmental conditions (e.g. pH). The simplest approach determines:
a) an actually available fraction or the actual dissolved amount at ambient conditions;
b) a potentially available fraction, which is the maximum amount that can be released to the soil
pore water under (predefined) worst-case conditions. This can also be expressed as the reactive
fraction;
c) a non-available fraction.
All together represent the total concentration. For environmental purposes it is generally accepted that
the amount measured using aqua regia (see ISO 11466) represents the total concentration. In Figure 2
this is called the total extractable concentration. The ‘real’ total concentration also includes the amount
within the silica matrix. To measure this amount an HF extraction should be included (see ISO 14869-3).
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ISO 22190:2020(E)

NOTE The coloured boxes at the left of the biological membrane represent the distribution of pollutant
molecules among four classes (Within the silica structure, strongly bound, potential available and actual
available) in soils and sediments. In the scheme in Figure 2, the bioavailable chemical is represented by the
potential an actual available concentrations. The chemical methods able to measure the pollutant present in each
specific fraction are given in the grey boxes. The green box to the right of the cell membrane represents the
processes that occur within the organism exposed to the pollutant. These biological processes can also serve as
the basis for standard methods used for bioavailability measurements.
Figure 2 — Measurement of bioavailability: a simplified conceptual framework
[1]
(Source: Modified from ISO 8245 )
The subdivision in terms of dissolved/actual and potential bioavailability is important, because it
broadens the role of the pore water. Bioavailable is not only the amount in the pore water, but may
include the amount that desorbs during the time an organism is in contact with the soil. Regarding
[3]
the organisms a "bio-influenced" zone could be defined . This zone comprises the pore water and
depending on the organism, parts of the soil matrix. Consequently, the available amount may have
different values. Thus, there could be numerous bioavailabilities depending on the type of target
organisms and time scale and, in turn, there could be numerous specific definitions (operational
definition).
The bioavailability of trace elements for several organisms (flora and fauna) is regulated by the
concentration and the speciation of trace elements in the water phase and the solid phase of soil
(environmental availability). From a chemical point of view, this concentration can be expressed as
(see ISO 17402):
a) The dissolved concentration and its chemical speciation at ambient conditions, which can be
characterised as
1) Free ion (activity);
2) Total concentration dissolved, including inorganic and organic complexes.
b) The amount bound to the soil solid-phase that can re-supply the dissolved concentration when the
latter is depleted during repetitive and ongoing uptake processes by organisms, for example the
maximum amount that can be released under (predefined) worst-case conditions.
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ISO 22190:2020(E)

It is noteworthy that this document suggests methods, i.e. soil extracts, based on an equilibrium
approach. Indeed, the time course of the different extraction methods is usually long enough to reach
the equilibrium between trace elements in the soil solid-phase and the water phase. Consequently, these
methods are only suitable for the assessment of the environmental bioavailability when this is driven
by soil equilibria rather than by kinetic constraints (non-equilibrium approach).
4.1.2 Neutral extracts for measurement of actual availability (ISO 21268-1)
For regulatory purposes simple and cheap methods are required and a simple extraction that simulates
the pore water quality is desirable. A neutral aqueous solution (i.e., limiting changes of the soil pH
during extraction as much as possible) can be used for this purpose. The concentration of trace element
measured in a neutral extract is assumed to reflect the concentration in the pore water [as well as ionic
strength, temperature, pH, DOC (Dissolved Organic Carbon)]. These properties may show a variation
during the year and can be influenced by external factors (e.g., rain, drought, addition of manure).
Extraction of a soil sample with demineralised water may have impact on the soil. For the purpose of
estimating the actual availability of trace elements, it is desirable to reduce the influence of external
factors and to obtain data that are more independent of the time of sampling. Extraction procedures
have been developed using aqueous solutions containing a fixed concentration of a specific salt (neutral
extract) in order to simulate the soil pore water.
The stronger the extract (high ionic strength), the higher the amount of trace element released from the
soil solid phase. On the other hand, the concentration of extracted DOC is also dependent on the choice
2+
of the neutral extract especially the concentration of divalent cations (Ca ) affects DOC. The ratio soil/
[4]
extract also affects the DOC concentration . With a higher amount of DOC extracted, a higher amount
of extracted trace elements can be expected, especially trace elements with a high affinity for binding
to DOC (e.g. Cu, Pb, Cr).
Originally 0,01 mol/l CaCl has been applied as neutral extractant. For several soils this methods reflects
2
[5][6][7][8]
the pore water concentration . A concentration of 0,01 mol/l CaCl is often higher than can be
2
measured in the pore water and consequently 0,01 mol/l CaCl can reduce DOC below concentrations
2
[9][10]
in actual pore water, thereby having an effect on the amount of trace elements dissolved . In this
document, the 0,001 mol/l CaCl extract (ISO 21268-1) is adopted as the currently most suitable soil
2
extraction method, enabling an estimation of trace element concentration in the water phase with a
result close to the actual pore water concentration. Results from this extraction can also be used in
geochemical modelling of specific bioavailable trace element species in subsequent tiers of the risk
assessment, as indicated in 4.2.
Although the 0,001 mol/l CaCl extraction is adopted in this document as a general procedure to
2
simulate the pore water concentration of trace elements, other neutral solutions have been shown to be
suitable for specific purposes (see Annex A).
[40]
NOTE The use of a high salt concentration like 1 mol/l ammonium nitrate as described in ISO 19730 and
0,01 mol/l CaCl has a positive effect on reproducibility and repeatability (see validation of ISO 19730). Results
2
have however a lower truthiness, because high salt concentrations do not simulate the pore water composition.
4.1.3 Acid extracts for measurement of potential availability (ISO 17586)
Strong acids like HNO can be used as an estimate for the potential available fraction. It will be clear
3
that the acid extraction has a very large impact on the composition of the solution. The obtained
solution has no relevance anymore to the pore water. A strong acid is a stronger solvent and will give
the amount sorbed on the CEC, but also the trace elements in acid soluble salts, which is the amount
that comes potentially available. At pH 0,5 the potential available fraction is estimated and this pH is
approached with 0,43 mol/l HNO . Non potential available trace elements included in the soil matrix
3
are not extracted at pH 0,5. These are only extracted with a method for the total concentration like
aqua regia. The difference between aqua regia and the acid extract are the non-available trace elements
and therefore aqua regia is not suitable to estimate the available trace elements.
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ISO 22190:2020(E)

4.2 Tiered approach based on bioavailability
A hierarchy in test use (tiered approach) is promoted, in which stepwise more realistic and sophisticated
tests and calculations are used for the determination of environmental availability in the framework
of impact assessment. At higher Tiers, more site specific information is required. The following
Tiered approach is advised when the bioavailability of trace elements is to be included in soil and site
assessment. In this approach, results from a previous tier can always be used in the following Tiers:
a) First Tier: Measurement of potential environmental availability by using 0,43 mol/l HNO
3
(ISO 17586). In this first tier also basic soil properties like clay, organic matter and pH are
measured, which makes it possible to make predictions of the actual environmental availability at
this initial stage.
NOTE 1 In general, a (limited) number of total concentration measurements (aqua regia) will be necessary
to test compliance with regulatory limit values. These data are not suitable for assessment of bioavailability
and are, therefore, completed with measurements of the potential availability.
b) Second Tier: Measurement of actual environmental availability using 0,001 mol/l CaCl
2
(ISO 21268-1) and if necessary, application of general biological test. If risks are more specified
it can be preferred to use 1 mol/l NH NO (see ISO 19730) to predict plant uptake, leaching tests
4 3
to predict mobility or to use specific biological tests like ISO 16198 for plant uptake. Modelling is
already part of this Tier and may ask for specific measurements. Leaching procedures, such as
ISO 12782-series and ISO 21268-series may be included within the assessment.
[11]
NOTE 2 The US EPA LEAF procedure makes use of comparable leaching procedures and tiered
approach.
c) Third Tier: Site specific measurement and site specific modelling. The measurement of human
bioaccessibility can be part of this tear.
NOTE 3 There is no strict separation between the second and third Tier. Depending on the risks, a method can
be part of the second or the third Tier.
In this approach the concept of bioavailability is already used in the first tier. Modelling is possible in
an earlier stage. However, risk assessment, requires limit values for the potential available or accessible
fractions. Having these, the step to a limit value for the actual available fraction will be small.
5 General procedure using an extract
The methods mentioned in this document are suitable for soils in contact with the atmosphere. They
are not applicable for strongly reducing soil-like materials like sediments.
NOTE 1 If the methods are applied to reducing or anaerobic soils and sediments, the procedure has effect on
the composition, for instance by oxidation of sulphide, thereby generally increasing the concentration that will
be measured.
The following steps are standardized:
— Pre-treatment: Apply extraction procedures to untreated soil (see ISO 14507). During pre-
treatment it is allowed to remove particles that are not representative. The test portion to be
prepared shall have a grain size less than or equal to 2 mm. On no account shall the material be
finely ground. If the laboratory sample cannot be crushed or sieved because of its water content, it is
allowed, in this case only, to reduce the water content until the laboratory sample can be sieved. The
drying temperature shall not exceed ambient temperature or 30 °C. Higher drying temperatures
[4]
increase the DOC and consequently the amount of several dissolved trace elements .
— Extraction procedure: A specific amount of soil and extractant are shaken during a fixed period.
— Measurement: The concentrations of elements in the extracting solution are determined by
appropriate analytical methods. Because the contaminant is often present at a low concentration,
the use of blanks is necessary.
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ISO 22190:2020(E)

An important precondition in ISO 17402 is that the method should have a mechanistic basis. Chemical
interactions are important mechanisms that influence the concentration in the water phase. Therefore,
measurement of only the trace element involved is not enough. The following parameters are also
important:
— Soil:
— pH (see ISO 10390);
— clay (see ISO 11277);
— organic matter (see ISO 10694 and ISO 12782-4);
— Fe-/Al-oxides (see ISO 12782-1, ISO 12782-2 and ISO 12782-3).
— CaCl extract:
2
— pH (see ISO 10390);
— dissolved organic matter (see ISO 8245, ISO 12782-5);
— composition of macro parameters (see specific ISO water standards);
— ionic strength;
NOTE 2 In most cases the salt in the extraction liquid determines the ionic strength. Saline soils may
influence the ionic composition and thereby the ionic strength.
— all other compounds that may form complexes with trace elements and are known to be present
in the soil sample.
The soil parameters pH, clay and organic matter are already measured in the first tier. Specification of
the organic matter and Fe-/Al-oxides are part of the second tier. The parameters in the CaCl extract
2
can only be measured if the extract is available and is therefore part of the second tier.
Having these parameters, it is possible to predict the actual availability from the potential availability
[12]
using transfer functions (see Annex B) or geochemical modelling. The application and use of different
parameters in geochemical modelling is described in Reference [13] and ISO 17402.
[15]
Results of the methods are also useful for deriving soil quality standards and soil protection
[16]
guidelines .
6 Reporting
Results can be reported as concentration measured (mg/l) or based on the original soil (mg/kg).
For neutral extracts, the total amount extracted depends on the soil/extractant ratio. Therefore, results
of neutral extracts shall be reported as concentration measured in the extract (mg/l).
The acid extract is stronger and extracts the potentially available amount. Therefore, reported results
of acid extracts shall be based on the weight of the original soil sample (mg/kg).
Necessary equations for calculation are given in the specific standards.
7 Calibration
7.1 Introduction
The chemical extractions described in this document are used to establish the bioavailability of
trace elements. The real bioavailability is characterized by uptake of trace elements reflecting in
bioaccumulation or a toxic effect. To be used for specific trace elements and specific organisms, the
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correlation between bioavailability, measured using a chemical extractant, and the effect on the specific
organism has to be shown (calibration).
7.2 Applicability of soil extracts
Examples of calibration using 0,43 mol/l HNO , 1 mol/l NH NO and CaCl are presented in Annex A.
3 4 3 2
This standard recommends the use of 0,001 mol/l CaCl (ISO 21268-1) for measuring of the actual
2
bioavailability and 0,43 mol/l HNO extraction (ISO 17586) for measuring the potential bioavailability.
3
The use of 0,001 mol/l CaCl and an acid extraction do fit in a theoretical framework as described in
2
this document. 1 mol/l NH NO (see ISO 19730) can be used as a measure for the actual environmental
4 3
availa
...