Physical Chemical Properties of Bioavailable Contaminants

Contaminants where bioavailability is considered an important component for assessing impacts to ecosystems are typically defined as persistent, bioaccumulative, and toxic (PBT). The list of PBT contaminants continues to grow with developing technologies. New emerging contaminants, such as personal care products, pharmaceuticals, brominated flame retardants, and fluorinated surfactants are released as uncontrolled, non-point sources into the environment. Characterization of their persistence, bioavailability, and toxicity are generally not always well understood. To assess bioavailability, a compound's physical-chemical characteristics can be plugged into models, such as quantitative structure-activity relationships, QSARs, and the biotic ligand model. The breadth in physical-chemical characteristics and their influence on bioavailability are briefly described next, using examples for common organic and metal contaminants.

Millions of synthetic and naturally occurring organic compounds have been dispersed in the environment by the production of fuels, foods, personal products, and other manufactured goods. Polycyclic aromatic hydrocarbons (PAHs) are naturally occurring and anthropogenic compounds generated by the combustion of organic material such as coal, wood, garbage, and gasoline. PAHs are a class of chemical compounds that consist of many congener types having two or more fused aromatic rings. Examples of PAHs are naphthalene, pyrene, and benzo[«]pyrene. Polychlorinated biphenyls (PCBs) represent a class of 209 individual chemical congeners originating from multiple sources, such as components in industrial hydraulic oils, electrical capacitors, carbonless paper, and from forest fires. PCBs are a group of chemical compounds that consist of two fused benzene rings and containing between two and ten chlorine atoms. The PAH compounds have water solubilities that span 4 orders of magnitude, while the range of Kow for PCBs spans 3 orders of magnitude resulting in a large distribution of lipophilicity characteristics. Some, though not all, PAH and PCB congeners are persistent and toxic. As illustrated by PAHs and PCBs, even within a single chemical class, the breadth ofphysical-chemical properties is large, affecting distribution in the environment and thus bioavailability.

Inorganic contaminants, also known as elements or metals, represent nearly half of the 25 most detected hazardous substances in water. The most frequently detected metals include arsenic, beryllium, cadmium, copper, chromium, lead, mercury, selenium, silver, thallium, and zinc. Metals in the environment may occur from natural deposit redistribution due to mining activities; other sources of metals include storm water runoff, batteries, plating, wood preservation, paints, and pesticides. The chemical form, or speciation, of inorganic contaminants plays a major role in determining their bioavailability. The speciation of metals includes their valence state and complexation with natural ligands. An important property of metals is the ability to bind with organic matter, cations, and anions in waters, soils, and sediments. Examples of natural ligands include fulvic acid, carbonates, chlorides, and sulfides. The ability of metal ions to chelate, that is, bind strongly, with humic-type organic matter or colloids reduces metal bioavailability.

The distribution of a compound between environmental compartments may be estimated with the use of partition coefficients. A partition coefficient is defined as the ratio of the compound concentration in two specific phases under constant environmental conditions. For example, one of the most widely used physical-chemical characteristics of an organic compound is the »-octanol/ water partition coefficient, defined as

Kow = [cOncentratiOn] oCtanol/[concentrationl water

This partition coefficient parameter is important as it may mimic in part the biota lipid/water distribution process. Octanol is somewhat similar to biota lipids and the Kow can provide some indication about the likely distribution of organic compounds between environmental compartments and their general lipophilicity. However, a high Kow may indicate sorption to both biological membranes, indicating increased bioavailability, and also sorption to natural organic matter, resulting in reduced bioavailabil-ity. Therefore, it is important to recognize that knowledge of Kow does not always predict bioavailability. Examples of other physical-chemical contaminant properties that can influence organic and metal environmental distribution and thereby affect bioavailability include pKa, pH, oxidation-reduction potential, vapor pressure, and partition coefficients, such as with organic carbon, Koc.

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