Heavy metal contamination in water

Heavy metals are a group of chemical elements whose atomic weights are between 63.55 (copper) and 200.59 g/mol (mercury), and their densities range between 4 and 7 g/cm³. The more used and known heavy metals For their environmental problems they are:

• Lead (PB).
• El Mercurio (HG).
• The zinc (zn).
• Cadmium (CD).
• Copper (Cu).
• The Molybdenum (MO).
• The manganese (mn).
• The nickel (Ni), among others.

Other lighter toxic elements such as:

• Aluminum (al).
• The beryllium (be).
• The arsenic (AS).

Heavy metals come from a wide variety of sourcesboth natural and artificial:

• The Natural sources They correspond to metal -rich minerals from the mother or volcanoes.
• The Anthropic sources o Artificial correspond to mining deposits, industries and energy sources and everything related to their transport.

Several heavy metals are essential for the proper functioning of the ecosystem. Metals such as iron, copper, zinc, and molybdenum are necessary for plants and animals since they are part of enzymes and other proteins. For example, hemocyanine, blood plasma protein in charge of gaseous exchange in crustaceans, molluscs and arachnids, contains copper. Especially in the sea, but also in fresh water, iron acts as a limitation of primary production, while molybdenum acts as a limiting in the nitrogen fixing rate. These metals must be present in nature at low concentrations But if they are in higher concentrations, even if they are slightly, they can be toxic. Other heavy metals such as arsenic, cadmium, lead and mercury are simply toxic and are not required in biological activities.

Environmental pollution with heavy metals appears When the extraction and use of these intensifies. Urban development has also contributed to the entry of heavy metals in the middle, because to urbanize it is necessary to transform the land and the mother rock underneath. In addition, without treated wastewater, leachate of landfills or the abandonment of waste in the environment are also a source of heavy metal contamination.

The Industrial and Minera Activity He is responsible for liberation to the environment of lead, mercury, cadmium, arsenic and chromium, harmful to both human health and other living beings.

Much of the lead is released from Recycling of batteries and industrial remains as welds, metals, cable coatings, etc. Lead contaminates water by means of water soluble salts that are mostly generated in the industry of the Painting and pyrotechnicsin the manufacture of pottery with enamel, in photothermography and glass coloration techniques, in the production of chemicals such as lead tetraetil (antidetonist in gasoline) and in the mining industry among others.

Mercury has the special characteristic that in environmental state it is in a liquid state. However, this is not as toxic as its vapors and derivatives. Some mercury compounds come from Polyvinyl chloride factories (PVC) and other chlorinated compounds, paintings and fungicidal pesticides, explosive and plastic detonators, by mining activities such as in the extraction of cinnabar (mercury sulphide ore), gold and silver and silver and by oil refineries.

A small part of pollution From the waters with Mercury, it comes from the Biological activity. Some anaerobic bacteria that live in lake funds are capable of transforming mercury and other inorganic derivatives into organic mercury compounds by methylation processes (addition of groups -CH3).

Another especially toxic metal is cadmium, which tends to form aqueous compounds. The most used cadmium compounds in the industry are haluros complexes, cyanide and amine. Cadmium contaminates water especially by Wastewater spills Without trying industries such as metal finish, electronics, iron alloys and iron and zinc production, pigments manufacturing (paintings and dyes), batteries (cadmium, nickel), plastic stabilizers, fungicides, treatments such as electrodeposition and use in nuclear reactors.

Some cadmium derivatives are used as catalysts and their organic acid salts (laurerate, stearate or cadmium benzoate) are used as Light and temperature stabilizers in plastics. These stabilizers can contaminate food if stored in plastics that contain them.

Cyanides, from the galvanic industry, of the refineries and the cleaning of metals are poured into wastewater, contaminating aquatic ecosystems. Other metals such as arsenic, copper and chromium are widely used as wooden preservatives and coal ashes They contain traces of many heavy metals.

In general, heavy metals, except arsenic, molybdenum and selenium, are little soluble in alkaline waters (pH> 7) and can join organic particles. In this way, metals can appear in very high toxic concentrations in waters that are apparently pure, pristine and clear, such as the oligotrophic waters of a river on the mountain. Heavy metal concentrations can be especially high in soft water that flow through areas with sulphide minerals or mining residues.

Heavy metals are powerful agents that have significant effects on sweetacuícolas ecosystems. These metals are typically found in low concentrations, in parts per million (ppm) or parts per billion (PPB), so they are difficult and expensive to detect.

There are some bioindicators such as aquatic macroinvertebrates that can accumulate heavy metals in their exoskeleton, even for weeks and months. For example, Plecopter larvae are good Bioindicators of the average metal concentration such as cadmium and molybdenum in mountain streams affected by mining wastewater releases.

Although metals are in low concentrations, they have a series of consequences on ecosystemswhich we will explain below.

Lethal and subletal effects on organisms

The effects of heavy metals can classify in acute or chronic. Acute effects occur in a short period of time and normally end up killing or severely damaging organisms. Typical uprising effects are the decrease in the growth rate and changes in behavior or development.

On the other hand, chronic effects are those that occur in the long term. Many metals are:

• Carcinogenic: They produce cancer.
• Teratogenic: They have negative effects on development.
• Mutagenic: They damage DNA.
• Neurotoxic: negatively affect neuronal and cognitive functions.
• Endocrine disruptors: They act as or interfere with hormones.
• They can even damage the immune system of organisms.

Bioacumulation

Heavy metals also have indirect effects on living beings because bioacumulate in organisms and move through trophic networks. Many of the metals, such as organic mercury compounds, are lipophilic, that is, they are more soluble in fats than in waterand therefore tend to accumulate in the fatty tissue of the animal. Although many metals are found in low concentrations, of the order of parts per trillion (ppt), they may or may not be toxic directly for living beings, however, due to bioaccumulation, these can accumulate at very toxic levels in organisms.

Biomagnification

Biomagnification is the process by which lipophilic metals amount to the trophic chain, from the prey to the predator. For example, phytoplankton when filtering water accumulates in its body lipophilic metals, and therefore the concentration of metals will become greater in the body than in the water. When the zooplankton consumes the phytoplankton, a portion of these metals is transferred to the fats of the zooplankton, in turn increasing its concentration with respect to that of the phytoplankton. This process continues towards the top of the trophic chain. A very common rule says that Each trophic level is capable of accumulating up to ten times more toxicity than the previous trophic level.

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