I.2.3. METALLOIDS AND ORGANOMETALLIC COMPOUNDS
Mihaela Carmen CHEREGI
The most significant water pollutant metalloid is arsenic, a toxic element that has been the chemical villain of more than a few murder plots. Acute arsenic poising can result from the ingestion of more than about 100 mg of it. Excessive exposure to this element can lead to health effects on the digestive and central nervous system, heart and kidneys and some of its compounds may cause cancer and genetic damage. Arsenic is also toxic for aquatic live. However, the impact on human health and the environment depend on the form and bioavailability of this metalloid.
Arsenic has been a politically charged issue the last three years. Unlike many other contaminants that are anthropogenic, arsenic contamination of waters generally comes from natural sources, through the erosion of rocks, minerals and soils. The arsenic concentration in the Earth’s crust is at a level of 2–5 ppm. The combustion of fossil fuels (e.g. coal) introduces large quantities of arsenic into the environment, much of it reaching natural waters. Arsenic occurs with phosphate minerals and enters into the environmental along with some phosphorous compounds.
It is used in the manufacture of wood preservatives, glass and non-ferrous alloys. The use in agricultural products (pesticides) is banned in almost all-western countries. Arsenic is also used in bronzing and pyrotechnics. Another source of arsenic is mine tailing. Arsenic produced as a by-product of copper, gold, and lead refining exceeds the commercial demand for arsenic, and it accumulates as waste material.
The most important compounds are white arsenic, the sulfide Paris green (copper arsenate), calcium arsenate and lead arsenate, the last three being used as agricultural insecticides.
Like mercury, arsenic may be converted to more mobile and toxic methyl derivatives by bacteria, following the reactions:
H3AsO4 + 2H+ + 2e- H3AsO3 + H2O
(CH3)2AsO(OH) + 4H+ + 4e- (CH3)2AsH + 2H2O
International agencies of environmental protection have conducted researches on arsenic (occurrence, health effects, bioavailability) and indented to lower the existing standard in drinking water of 50 g/L to a level that would better protect human health. In January 2001, the U.S. EPA proposed to lower the standard to 10 g/L, this rule became effective on February 2002 and drinking water systems must comply with this new standard by January 2006.
On the non-political front, arsenic research issues that have become important are determining individual species of this metalloid and their occurrence in water, food, and biological sample. Different arsenic species have different toxicity and chemical behavior in aquatic systems, therefore, it is important to be able to identify and quantify them.
I.2.3.2. Organically Bound Metals and Metalloids
In aquatic systems, two major types of metal-organic interactions are considered:
a. Complexation, usually chelation when organic ligand is involved. A definition of complexation in natural water or wastewater is: a process in which a species that is present reversibly dissociates to a metal ion and an organic ligand as a function of pH:
ML + 2H+ M2+ + H2L
where M2+ is a metal ion and H2L is the protonated form of the ligand L-2.
b. Metals bonded to organic entities by way of carbon atom that are non-dissociated at lower pH or large dilution. The organic component and the particular oxidation state of the metal involved may not be stable apart from the organometallic compounds.
A simple way to classify these species from their toxicology point of view may be:
With alkyl group such as ethyl in tetraethyl lead, Pb(C2H5)4;
Carbonyls, some of them are quite volatile and toxic, having carbon monoxide bonded to metals :C O:
With an organic group donating electron such as ethylene or benzene.
The most prominent of the compounds outlined above are the arene carbonyl species in which the metal ion is bonded to both an aryl entity such as benzene and to several carbon monoxide molecules.
A large number of compounds exist that have at least one bond between the metal and a C atom on an organic group, as well as other covalent ionic bonds between the metal and atoms other than carbon. Because they have at least one metal – carbon bond, as well as properties, uses, and toxicological effects typical of organometallic compounds, it is useful to consider such compound along with organometallic compounds. Examples are monomethylmercury chloride in which the organometallic CH3Hg+ ion is ionically bonded to the chloride anion; phenyldichloroarsine, C6H5AsCl2, in which a phenyl group is covalently bonded to arsenic through an As–C bond, and two Cl- anions are also covalently bonded to As.
Another type consists of organic groups bonded to a metal atom through atoms other than carbon. These compounds do not meet the strict definition; such compounds can be classified as organometallics for discussion of their toxicology and aspects of chemistry. An example is isopropyl titanate (or titanium isopropylate), Ti( i–C3H7)4.
The interaction of trace metals with organic compounds in natural waters is too vast, it may be noted that metal–organic interactions may involve organic species of both pollutants (such as EDTA) and natural (such as fulvic acid) origin. These interactions are influenced by redox equilibrium, formation–dissolution of precipitates; colloid formation and stability; acid–base equilibrium; and microorganisms–mediated reaction in water. Metal–organic interactions may increase or decrease the toxicity of metals in aquatic ecosystems, and they have a strong influence on the growth of algae in waters.
Organotin compounds are generally man-made chemicals with a global production on the order of 40,000 tones/year. Of all the metals, tin has the greatest number of organometallic compounds in commercial use.
In the past, one of the main sources of release into the marine environment was triphenyltins and tributyltins, which had been used in paints for the ships and boats. Other sources of organotin release included the industries that they are used in, particularly the chemical industry, and through their applications in fungicides, acarides, disinfectants, antifouling paint, stabilizers to lessen the effects of heat and light in PVC plastics, catalysts, precursors for formation of films of SnO2 on glass, and wood preservative products. Tributyltin chloride compounds (TBT) have bactericidal, fungicidal, and insecticidal properties and have particular environmental significance because of their use as industrial biocides. Tributyltin hydroxide, naphthenate, bis (tributyltin) oxide, and tris (tributylstannyl) phosphate are also used as biocides. Antifungal TBT compounds have been used as slimicides in cooling tower water.
The toxicity of organotins generally follows the order: trialkyl > dialkyl > monoalkyl but the dialkyl form is much more neurotoxic, with an effect in brain cells from as low as 30 ppb. European countries have proposed banning the use of PVC pipe to transport potable water, due to the leaching of organotin from PVC plastic products (dibutyltin is used as a heat stabilizer in PVC pipe).
In addition to synthetic organotins compounds, methylated tin species can be produced biologically in the environment.
Excessive exposure to some organic tin compounds may cause adverse health effects on brain, eye, immune system, lung, skin and the unborn child, and may cause cancer. Most local environmental concerns arise from organotin pollution in marine waters. TBT is very toxic for algae, mollusks, crustaceans and fish. It has been identified as an endocrine disrupting substance with observable effects in gastropod mollusks and suggested effects in marine mammals. Also, it impairs the immune system of organisms and lead to shellfish developing shell malformations. Triphenyltin may have similar effects.
Because of such concerns, several countries, including U.S., England and France, prohibited TBT application on vessel smaller than 25 meters in length during the 1980s. In 1998 the International Marine Organization agreed to ban organotin antifouling paints on all ships by 2003.