Metals entering food chain

Standards imposed to the industrial waste streams charged in heavy metals are more and more drastic in accordance with the updated knowledges of the toxicity of mercury, cadmium, lead, chromium... when they enter the human food chain after accumulating in plants and animals (Forster Wittmann, 1983). Nowadays, the use of biosorbents (Volesky, 1990) is more and more considered to complete conventional (physical and chemical) methods of removal that have shown their limits and/or are prohibitively expensive for metal concentrations typically below 100 mg.l-i. 

Corrosion inhibitors used in offshore oil production are highly cationic. However, the use of such cationic-based corrosion inhibitors for offshore oil platforms is becoming less acceptable for environmental reasons. Cationic inhibitors are attracted to metal surfaces, thereby controlling the acid-type corrosion. When these cationic corrosion inhibitors enter seawater, they are attracted to a particular type of algae, diatomes. These algae are part of a food-chain for mussels. The cationic inhibitors inhibit the growth of these algae. Betaines and ampholytes [1067] can be used instead of cationic inhibitors or... 

The release of heavy metals into the environment presents a serious threat. Over recent decades, the annual worldwide release of heavy metals reached 22,000 T for cadmium, 939,000 T for copper, 783,000 T for lead, and 1,350,000 T for zinc.3 Because of their high solubility in the aquatic environments, heavy metals can be absorbed by living organisms and enter the food chain.6 Exposure to high levels of these metals has been linked to cytotoxic, mutagenic, and carcinogenic effects on... 

However, if the raw materials are contaminated or the composting process is incomplete, unfavorable effects must be expected. Heavy metals may be introduced into the compost with communal waste. To ensure that these do not enter the food chain, authorised limit values must be strictly adhered to. The same is true of organic contaminants (particularly polyaromatic or chlorinated hydrocarbons), the effect of which is extremely complex. If the fermentation process is not satisfactory, putrefaction will occur, the by-products of which (S02, NH3, N02, organic acids, cadaveric alkaloids, etc.) inhibit plant growth and attract pests. 

On the basis of chemical profile, Wood (38) predicted that arsenic, selenium, and tellurium will be methylated in the environment, and lead, cadmium, and zinc will not. Elemental concentration in the aquatic food chain has been reported for As (39), Hg (40), Cd (41), Pb (42), and Cu (43). The biological half-life of methylmercury in fish, for example, is one to two years (44). Pillay et al. (40) implicated heavy coal burning in the mercurial contamination of plankton and fish populations of Lake Erie. Other metals, notably cadmium, have been shown to be incorporated into the grazing grasses surrounding a coal burning source (27). Trace element contamination, therefore, can enter the food chain at various points. Disposal of solid wastes in the form of ash and slag is yet another environmental consideration (45). 

PTMs-contaminated soils pose a human health risk on the basis of the potential of the contaminant to leave the soil and enter the human bloodstream. In order to assess human health risk, several pathways of transfer of metals from soil to humans have to be taken into account. The most important metal intake takes place via the food chain in which plants or meat of animal play a key role. The direct ingestion of soil can be a major route of exposure for humans to many low mobile soil contaminants, particularly for small children through putting hands into the mouth (Gupta et al., 1996). The contribution from the inhalation of particles smaller than 10 pm and from dermal contact with soil have little meaning compared with oral ingestion and are found to be less than 1% and 0.1% of the total intake, respectively (Paustenbach, 2000). 

Heavy metals are usually available in the structure of the Lithosphere. Together with those originating from human activities they can enter living organisms through water, air and the food chain (Figure 1). 

In appraising the potential environmental hazards of metals found in petroleum, it is necessary to consider where they enter the environment, how they move in the environment, and how they might enter the food chain. 

Most metals disposed of in surface water are rapidly removed from the aqueous phase at a rate largely dictated by the amount of sediment or suspended solids and eventually settle out. Once trapped in the sediment, they are relatively immobile however, there will always be some freedom of movement of the metals between the sediment and the overlying water. Aquatic plants such as algae are excellent scavengers and concentrators of metals in surface waters. These plants are important to the lower end of the food chain, and serve as a source of metals they can be passed up the food-chain ladder to the point where they ultimately serve man as a food. Contamination of groundwater is usually not a problem unless surface water enters the groundwater supply directly, through a surface fault. 

In marine chemistry, major and minor elements in marine system are the key points studied. According to the combination of major and minor elements in sediments, we can trace their source and evolve paleoenviromnental changes inversely. However, heavy metals still draw comprehensive attention because of their accumulation, which could affect the growth and reproduction of aquatic animals and plants. Moreover, they can enter the human body through the food chain to threaten the health and development of humans. In recent years. 

Heavy metals can also accumulate in soil and eventually enter the food chain through their water-soluble compounds, which poses a potentially severe danger. 

For many years, cells with mercury cathodes were used in the chloralkali industry. Mercury is not very reactive or soluble and was thought to be harmless in the environment. Then some individuals who ate fish from mercury-contaminated waters became seriously ill. This event brought to light the fact that aquatic microorganisms convert metallic mercury to a toxic, water-soluble compound that enters the food chain (Section 15.6). 

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