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 / cm ³ . The heavy metals most used and known for their environmental problems are:

• Lead (Pb).
• Mercury (Hg).
• El zinc (Zn).
• Cadmium (Cd).
• Copper (Cu).
• Molybdenum (Mo).
• El manganeso (Mn).
• Nickel (Ni), among others.

Other lighter toxic elements are also included such as:

• Aluminum (Al).
• Beryllium (Be).
• Arsenic (As).

Heavy metals come from a wide variety of sources , both natural and man-made:

• The natural sources correspond minerals rich in metals from bedrock or volcanoes.
• The anthropogenic sources or artificial correspond to mining sites, industries and energy sources and everything related to transportation.

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, hemocyanin, a blood plasma protein responsible for gas exchange in crustaceans, mollusks and arachnids, contains copper. Especially in the sea, but also in fresh water, iron acts as a limiting factor in primary production, while molybdenum acts as a limiting factor in the rate of nitrogen fixation. These metals must be present in nature at low concentrationsbut if they are in higher concentrations, even slightly, they can be toxic. Other heavy metals like arsenic, cadmium, lead and mercury are simply toxic and are not required in biological activities.

Environmental contamination with heavy metals appears when the extraction and use of these intensifies . Urban development has also contributed to the entry of heavy metals into the environment, since to urbanize it is necessary to transform the soil and the bedrock underneath. In addition, untreated sewage, landfill leachate or waste dumping in the environment are also a source of heavy metal contamination.

The industrial and mining activities is responsible for the release to the media of lead, mercury, cadmium, arsenic and chromium, harmful to human health and other living beings both.

Much of the lead is released from battery recycling and industrial waste such as solder, metal, cable coatings, etc. Lead pollutes water through water-soluble salts that are mainly generated in the paint and pyrotechnics industry , in the manufacture of glazed pottery, in photothermography and glass coloring techniques, in the production of chemicals such as tetraethyl lead (antiknock in gasoline) and in the mining industry among others.

Mercury has the special characteristic that in an environmental state it is in a liquid state. However, it is not as toxic as its vapors and derivatives. Some mercury compounds come from the factories of polyvinyl chloride ( PVC ) and other chlorinated compounds, fungicidal paints and pesticides, explosives and plastics detonators, due to mining activities such as the extraction of cinnabar (mercury sulfide mineral), gold and silver and by oil refineries.

A small part of water pollution with mercury comes from biological activity . Some anaerobic bacteria that live on the bottom of lakes are capable of transforming mercury and other inorganic derivatives into organic mercury compounds by methylation processes (addition of -CH3 groups).

Another especially toxic metal is cadmium, which tends to form aqueous compounds. The most widely used cadmium compounds in industry are halide complexes, cyanide and amine. Cadmium pollutes water primarily from untreated wastewater discharges from industries such as metal finishing, electronics, iron alloys and iron and zinc production, pigment manufacturing (paints and colorants), etc. batteries (cadmium, nickel), plastic stabilizers, fungicides, treatments such as electroplating and their use in nuclear reactors.

Some derivatives of cadmium are used as catalysts and their organic acid salts (laurate, 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, refineries and metal cleaning are discharged into wastewater, contaminating aquatic ecosystems. Other metals such as arsenic, copper, and chromium are widely used as wood preservatives, and coal ash contains traces of many heavy metals.

In general, heavy metals, except arsenic, molybdenum and selenium, are poorly soluble in alkaline waters (pH> 7) and can bind to 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 mountain river. Heavy metal concentrations can be especially high in soft waters that flow through areas with sulfur minerals or mining residues.

Heavy metals are powerful agents that have significant effects on freshwater ecosystems. These metals are typically found in low concentrations, in parts per million (ppm) or parts per billion (ppb), making them 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, sandstone larvae are good bioindicators of the mean concentration of metals such as cadmium and molybdenum in mountain streams affected by releases of mining sewage.

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

Lethal and sublethal effects on organisms

The effects of heavy metals can be classified as acute or chronic . Acute effects occur in a short period of time and usually end up killing or severely damaging the organisms. Typical sublethal effects are decreased 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 cause cancer.
• Teratogenic : have negative effects on development.
• Mutagenic : they damage DNA.
• Neurotoxic : negatively affect neuronal and cognitive functions.
• Endocrine disruptors : act like or interfere with hormones.
• They can even damage the immune system of organisms.

Bioaccumulation

Heavy metals also have indirect effects on living beings as they bioaccumulate 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 fat than in water , and therefore tend to accumulate in the fatty tissue of the animal. Although many metals are in low concentrations, on the order of parts per trillion (ppt), they may or may not be directly toxic to living things, however, due to bioaccumulation, they can accumulate at very toxic levels in organisms.

Biomagnificación

Biomagnification is the process by which lipophilic metals move up the food chain, from prey to predator. For example, phytoplankton when filtering water accumulates lipophilic metals in your body, and therefore the concentration of metals will become higher in the body than in water. When the phytoplankton are consumed by zooplankton, a portion of these metals is transferred to the fats of the zooplankton, in turn increasing their concentration relative to that of the phytoplankton. This process continues towards the top of the food 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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