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A Review of Methods to Measure Heavy Metal Bioavailability
for Plants from Soils
Rog-Young Kim, Jeong-Ki Yoon, Ji In Kim, Gyoung-Hun Park, Sung Mi Yun, Jong Mo Kim, and
Tae-Seung Kim
National Institute of Environmental Research, Soil and Groundwater Research Division, 42 Hwanggyong-ro,
Seo-gu, Incheon 404-708, Republic of Korea
INTRODUCTION
Bioavailability is the key issue in risk assessment and remediation decision of
contaminated sites. Bioavailability of heavy metals in soils, however, should not be
considered as a fixed quantity, but as a complex dynamic process depending on the type
of metal, soil, organism, and exposure. But until now, there have been no standardized
methods for the measurement of heavy metal bioavailability to be applied as a tool for
risk assessment. Therefore, firstly we reviewed the metal-specific characteristics which
determine soil availability of heavy metals, to provide an insight into how to develop a
measurement tool. And then we reviewed the most frequently used methods for the
measurement of heavy metal bioavailability, in particular in relation to phytoavailability.
AVAILABILITY OF HEAVY METALS IN SOILS
Definition of heavy metal availability in soils
Soil availability of metals can be determined by an available amount of the total content
in soils including the actual available fraction (dissolved in the pore water) and the
potential available fraction (adsorbed to the soil matrix; ISO 17402, 2008). To measure
the soil availability, it is important to know both the concentrations and chemical forms
in the pore water. They are controlled by adsorption/desorption, complexation/
dissociation, precipitation/dissolution or very slow diffusion into the interior of clay
minerals and oxides. These processes are strongly metal-specific because of the
different adsorption affinities of metals on clay mineral/oxide binding sites, the different
propensities to form stable complexes with organic/inorganic ligands (McBride, 1994).
Concept of specific and nonspecific adsorption
In a moderately acidic soil; up to a slightly alkaline soil pH, heavy metals exist mainly
specifically adsorbed at hydroxyl surfaces of oxides or clay minerals. The specific
adsorption of metals increased with the increasing hydroxide constant of the metal ions
as follows: Cd2+ < Ni2+ < Zn2+ << Cu2+ = Pb2+ << Cr3+, indicating a decrease in metal
mobility with the increasing hydroxide constant (Herms and Bruemmer, 1984). In
contrast, at low pH, heavy metals are principally non-specifically adsorbed at the
binding sites of cation exchange of clay minerals and oxides via electrostatic bonds,
which are considered to be easily available.
Concept of stability of metal-organic complexes
Organic matter can also modify the adsorption or solubility of metal ions depending on
the soil pH and the type of organic matter. In many cases, the stability constants of
metal-organic complexes increase with increasing pH and are higher with humic acids
than with fulvic acids. In general, for a typical soil pH range, the stability of metalorganic complexes can be summarized as follows: Cd, Zn, and Ni being of low stability
and Pb, Cu, and Cr being high (Smith, 2009).
Concept of mobility
Based on these results, we categorized heavy metals into two groups: Cd, Ni, and Zn of
relative high mobility and Cu, Cr, and Pb of relative low mobility. It can be useful to
design a more robust measurement tool.
METHODS TO MEASURE BIOAVAILABILITY FOR PLANTS FROM SOILS
The methods can be classified depending on (1) the type of tools: chemical extraction or
mechanistic calculation, (2) strength of extraction media: weak or strong, (3)
mechanism of extraction: exchange, chelation, diffusion or equilibrium sampling, or (4)
models: FIAM or surface complexing model (Brand et al., 2009).
Chemical extraction
The extraction methods can be divided into two groups: (1) those which measure the
actual available concentrations in the pore water, either the total dissolved
concentrations extracted by neutral salts such as 0.01 M CaCl2, 0.1 M NaNO3, 1.0 M
NH4NO3 or the free metal ion concentrations extracted by DGT and DMT and (2) those
which measure the potential available contents in the soil solid phase, extracted either
by exchange with strong acids such as 0.1 M HCl and 0.43 M HNO3 or by chelation
with strong complexing agents such as 0.05 M EDTA and 0.5 M DTPA.
Geochemical models
The geochemical models, which are developed to calculate the dissolved free metal ion
concentration in the pore water and the actual available metal fraction, can be grouped
into several types: (1) models which calculate the free metal ion activity: MINTEQ3.0,
PHREEQ3, and ORCHESTRA, (2) models which calculate metal ion binding to geocolloids such as humic and fulvic acid: NICA-Donnan model and WHAM/Model VI,
and (3) models which additionally predict the metal interaction at the site of toxic action
(i.e. biotic ligands) and can calculate the toxic effect levels: BLM (Biotic Ligand
Model) and TBLM (Terrestrial Biotic Ligand Model).
CONCLUSIONS
For relatively highly mobile metals (Cd, Ni, and Zn), a neutral salt solution such as 0.01
M CaCl2 or 1 M NH4NO3 was recommended for routine analysis, whereas a strong acid
or chelating solution such as 0.43 M HNO3 or 0.05 M DTPA was recommended for
strongly soil adsorbed and less mobile metals (Cu, Cr, and Pb). While methods which
assessed the free metal ion activity in the pore water are advantageous for providing a
more direct measure of bioavailability, few of these models have to date been properly
validated.
REFERENCES
1. Brand, E., Peijnenburg, W., Goenenberg, B., Vink, J., Lijzen, J., Ten Hulscher, D., Jonker, C.,
Romkens, P. and Roex, E. (2009). “Towards implementation of bioavailability measurements in the
Dutch regulatory framework”, RIVM Report 711701084/2009.
2. Herms, U. and Bruemmer, G. W. (1984). “Einflussgoressen der Schwermetallloeslichkeit und bindung in Boeden”, J. Plant Nutr. Soil Sci., Vol.147, p.400-424.
3. ISO 17402 (2008). “Soil quality - Requirements and guidance for the selection and application of
methods for the assessment of bioavailability of contaminants in soil and soil materials”, Geneva,
Switzerland.
4. McBride, M. B. (1994). “Environmental chemistry of soils”, New York, Oxford University Press.
5. Smith, S.R. (2009). “A critical review of the bioavailability and impacts of heavy metals in municipal
solid waste composts compared to sewage sludge”, Eviron. Int., Vol.35, p.142-156.