Toxicity biological availability

Silver compounds having anions that are inherently toxic, eg, silver arsenate and silver cyanide, can cause adverse health effects. The reported rat oral LD values for silver nitrate, silver arsenate [13510-44-6] and silver cyanide are 500—800 (29), 200—400 (29), and 123 mg/kg (30), respectively. Silver compounds or complexes ia which the silver ion is not biologically available, eg, silver sulfide and silver thiosulfate complexes, are considered to be without adverse health effects and essentially nontoxic. 

One of the questions that arises in soil analysis is whether a determination of the total amount of a component in soil is desired or if just the biologically available amount is more relevant. Related to this is the question of which species of the component is present. In some cases, speciation is of utmost importance. For instance, chromium can be present as Cr(III) or Cr(VI). Cr(VI) is more toxic and thus of greater concern [15], This concern is also related to the biological availability of a specific species. In this case, while knowing the total chromium content (i.e., the sum of Cr(III) and Cr(IV)... 

It is often assumed that if something is in the soil, it will be in plants. This is incorrect. Plants do not take up all of the elements or molecules present in their immediate environment. However, there are some plants, called hyperaccumulators, that accumulate higher than normal levels of some toxic elements. These plants still do not take up all the elements in their environment and they are often small, so the total amount of toxic elements removed from soil is limited. In addition, not all species of an element are toxic and some are not biologically available and thus do not enter biological systems. For example, chromium as Cr(VI) is more toxic and more biologically available than is Cr(IH) [1]. 

Biological extractions are carried out to determine if biologically important elements are at levels that are sufficient, yet not toxic, for plant needs. Acid soil extraction to determine the biologically available plant nutrients is the most common type of extraction of soil carried out. The objective is to extract a portion, not all, of a particular nutrient or metal that is correlated to the amount available to plants. The plants of primary interest are crop plants such... 

In some cases, resins have been used to try to determine only the plant or more generally the biological availability of an ionic species. Resins placed in soil have also been used to study ion speciation, soil microbiology, various phosphorus measurements, soil nutrient supply rate, nutrient transformations and movement, and micronutrient and metal toxicity [22-25],... 

Biologically available organic contaminants may find their way into the food chain and be toxic, but they are also more easily decomposed and are thus removed from the environment. Slowly decomposed biologically available organic compounds, such as pesticides, may have long lifetimes in soil and thus pose a hazard to animals and humans. Biologically unavailable compounds, such as tars, produce undesirable characteristics in soil, such as water... 

Complexation by organic matter causes a wide variety of effects on the bioavailability of metals. As noted earlier, the toxicity of dissolved copper can be reduced by complexation with organic matter. In this case, complexed copper is not biologically available to phytoplankton. The other extreme is exhibited by iron, in which the metal s bioavailability is enhanced by complexation with organic matter. There are two likely... 

Metals cannot be created or destroyed, but can change form, altering their biological availability and toxicity. Metallic mercury evaporates and is redistributed... 

Toxicity. Cadmium pigments are cadmium compounds with a low solubility, however, small quantities of cadmium dissolve in dilute hydrochloric acid (concentration equivalent to stomach acid), and in cases of long-term oral intake of cadmium pigments, they can accumulate in the human body. On inhalation of subchronic amounts of cadmium pigments, a small proportion of cadmium is biologically available [3.115], [3.116]. 

Several elements, particularly zinc and copper, could play a role as trace nutrients for phytoplankton. They are known to be important for growth of terrestrial plants, but neither the requirement for these nutrients nor the elemental distributions in seawater are well known. The biological availability of both zinc and copper is controlled by their complexation with organic material. Analytical methods that have the distinction of being able to discriminate chemical forms of the metal are needed. These measurements reflect the chemical reactivity and biological availability or toxicity of the metal more accurately. 

Appraising the toxic potential of biologically available contaminants in sediment should include three compartments the whole sediment (with standardized direct contact assays when these are available), the porewater, and the elutriate (aqueous extract). Additional hazard information can also be obtained from toxicity testing conducted on organic extracts using methanol or acetone. 

Extrapolations between different media are common in risk assessment. Such extrapolations are based on physical and chemical interactions between components of the matrix and the toxic substance that may enhance or reduce the biological availability of the substance, thus affecting its apparent toxicity because of the change in exposure. In this chapter, the word medium is reserved to indicate the major environmental compartments air, water, sediment, and soil. The word matrix is associated with the physicochemical properties of the media. The problems associated with extrapolating between one medium or type of matrix to another are intricate and are generally due to the varying chemical, physical, biological, and spatial characteristics associated with the different media. 

It is generally accepted that the distribution, mobility, biological availability and toxicity of PTMs in soil depend not simply on their total concentrations but, critically, on their forms (Morgan and Stumm, 1995). These may be soluble, readily... 

The section on general toxicological and biological parameters reviews and summarizes chemical data and basic toxicity information available on the agent of interest and reviews data on absorption, distribution, metabolism, and excretion in humans and experimental animals. 

Chemical models of metal speciation have been used to assess the biological availability of different solute metal forms. Pagenkopf and Andrew (l ) used equilibrium models to suggest that the availability of Cu to fishes was controlled by the concentration of the free Cu ion. Equilibrium models were also used to show that the toxicity of Cu to phytoplankton followed the activity of metals rather than total metal concentrations l8) and that the concentration of free Zn ion plus additional factors (e.g. competition from Ca and Mg) may affect the availability of solute Zn to fishes (19>20). 

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