Bioavailability, in environmental and

soil science Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to ...

s, represents the amount of an element or compound that is accessible to an organism for uptake or

adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which ...

across its

cellular membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (th ...

.American Society for Testing and Materials (ASTM). 1998. Standard guide for conducting laboratory soil toxicity or bioaccumulation test with the lumbricid earthworm ''Eisenia foetida''. E 1676-97. Philadelphia, PA: ASTM.Casarett, & Doull. 2001. Toxicology. ''The Basic Science of Poisons'', Sixth Edition. C. D. Klaassen (ed.)Semple, K. T., Doick, K. J., Jones, K. C., Burauel, P., Craven, A., & Harms, H. 2004. Peer reviewed: defining bioavailability and bioaccessibility of contaminated soil and sediment is complicated. ''Environmental Science & Technology'', 38(12), 228A-231A. New York: McGraw-Hill. In environmental and agricultural applications, bioavailability most often refers to availability of

contaminant Contamination is the presence of a constituent, impurity, or some other undesirable element that spoils, corrupts, infects, makes unfit, or makes inferior a material, physical body, natural environment, workplace, etc. Types of contamination Wi ...

s, such as organic pollutants or heavy metals, in soil systems and is also used frequently in determining potential risk of land application of

sewage sludge Sewage sludge is the residual, semi-solid material that is produced as a by-product during sewage treatment of industrial or municipal wastewater. The term " septage" also refers to sludge from simple wastewater treatment but is connected to si ...

or other inorganic/organic waste materials. Almost exclusively,

plant root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...

s and

soil organism Soil biology is the study of microbial and faunal activity and ecology in soil. Soil life, soil biota, soil fauna, or edaphon is a collective term that encompasses all organisms that spend a significant portion of their life cycle within a soil ...

s uptake contaminants that are dissolved in water. Therefore, the bioavailable fraction is often likened to the dissolved (aqueous) fraction in these systems. Depending on its chemical properties, a contaminant may or may not be found in the aqueous phase. Organic contaminants may become sorbed or sequestered in organic matter through weak

Van der Waals interactions A van is a type of road vehicle used for transporting goods or people. Depending on the type of van, it can be bigger or smaller than a pickup truck and SUV, and bigger than a common car. There is some varying in the scope of the word across th ...

or through

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

- or

covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

bonding.National Research Council (US). 2003. Committee on Bioavailability of Contaminants in Soils and Sediments. ''Bioavailability of contaminants in soils and sediments: Processes, tools, and applications''. Washington, DC: National Academies Press.Semple, K. T., Morris, A. W. J., & Paton, G. I. 2003. Bioavailability of hydrophobic organic contaminants in soils: fundamental concepts and techniques for analysis. ''European journal of soil science'', 54(4), 809-818. Ionic compounds, such as heavy metals, can be

precipitated In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...

into the solid phase.Traina, S. J., & Laperche, V. 1999. Contaminant bioavailability in soils, sediments, and aquatic environments. ''Proceedings of the National Academy of Sciences'' 96 (7): 3365-3371. Volatile compounds can be lost as

aerosol An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of ant ...

s to the soil atmosphere. In these forms, contaminants are relatively inaccessible to microbial or plant uptake and must dissociate and re-dissolve into the soil solution to become biological available.

Factors influencing soil bioavailability

Bioavailability is a function of soil properties, time, environmental conditions, and plant and microbial characteristics Naidu, R. (ed). 2011. ''Chemical Bioavailability in Terrestrial Environments'' (Vol. 32). Elsevier. * Soil properties, such as pH,

ion exchange Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, ...

capacity,

soil organic matter Soil organic matter (SOM) is the organic matter component of soil, consisting of plant and animal detritus at various stages of decomposition, cells and tissues of soil microbes, and substances that soil microbes synthesize. SOM provides numerous ...

content,

texture Texture may refer to: Science and technology * Surface texture, the texture means smoothness, roughness, or bumpiness of the surface of an object * Texture (roads), road surface characteristics with waves shorter than road roughness * Texture ...

and

porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...

influence bioavailability. Because soils with higher ion exchange and organic matter content offer more opportunities for

, typically they exhibit lower bioavailability. * As the contact time between the contaminant and soil increases, a decrease in bioavailability is observed, termed “ageing”, due to diffusion and sorption processes with mineral and organic fractions of soil. * Environmental conditions influence bioavailability. Drought conditions result in lower soil water content. This can reduce the access of plants and organisms to dissolved contaminants but also can enhance precipitation of salts from solution.

Measuring bioavailability in soil ecosystems

Site-specific characteristics have a major influence on contaminant bioavailability and no standardized tests have been developed. However, there are a number of chemical and biological tests used to estimate bioavailability including a direct measurement of contaminant

bioaccumulation Bioaccumulation is the gradual accumulation of substances, such as pesticides or other chemicals, in an organism. Bioaccumulation occurs when an organism absorbs a substance at a rate faster than that at which the substance is lost or eliminated ...

in earthworms (''

Eisenia fetida ''Eisenia fetida'', known under various common names such as manure worm, redworm, brandling worm, panfish worm, trout worm, tiger worm, red wiggler worm, etc., is a species of earthworm adapted to decaying organic material. These worms thrive ...

''). Estimates of bioavailability can also be obtained from chemical solid-phase soil extractions.

Fugacity In chemical thermodynamics, the fugacity of a real gas is an effective partial pressure which replaces the mechanical partial pressure in an accurate computation of the chemical equilibrium constant. It is equal to the pressure of an ideal gas ...

modelling of bioavailability is based on the

solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solub ...

and partitioning of compounds into aqueous and non-aqueous phases.Mackay, D. and Fraser, A., 2000. Bioaccumulation of persistent organic chemicals: mechanisms and models. ''Environmental pollution'', 110(3), pp.375-391. This model describes the tendency for contaminants to be dissolved in the soil solution.

References

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