I.2.4 ANIONIC INORGANIC SPECIES
In this subchapter the most important inorganic anionic species, which have an impact on the environment, are presented. These species are chosen since they are either direct toxic or they have an indirect effect in the area where they are released. Besides the environmental effect, information about the most common use and sources are also included.
Chloride is the ionic form of elemental chlorine. It occurs naturally in a wide range of concentrations in waters. Upland reservoirs of water generally contain low concentrations of chlorides whereas the concentration increases in rivers and groundwater. The highest concentration is found in the sea due to the partial evaporation of water.
Chloride is spread into the environment from several different sources. In the industry hydrochloride is an important chemical and it is used in many different areas such as cleaning of metal surfaces in order to remove the oxide layer and in the production of different organic chemicals. After use, the chlorides are disposed of in the wastewater. In areas with cold climate, sodium chloride is used for preventing a slippery road surface. During its use, the chloride compound is spread on the road and from there it is transported into the environment with rainfall and snowfall. Another chloride salt, calcium chloride, is used during the summer to prevent gravel roads from giving off dust. Chloride is also spread into the environment with wastewater from households. This chloride originates from the food that we consume and is excreted from the human body in particularly the urine.
Reasonable concentrations of chloride are not harmful to humans. If there is a limit for chlorides it is usually set by the salty character it gives to the water at a concentration of 250 mg/L. The type of flora and fauna that develops in natural waters is depending on the composition of the water. If this is changed from low ionic to high ionic due to the discharge of high strength wastewater the composition of organisms and plants will also change.
Fluoride is the most common halogen and large amounts can be found in the minerals apatite (Ca5(F, OH)(PO4)3 (the ions F and OH can substitute each other)) and fluorspar (CaF2). The exact composition of the type of mineral in the ground and its distribution influences the natural background concentration of fluoride in the groundwater. Normally this range is between 0 and 0.2 ppm. Fluoride can also be found in the air but this is usually restricted to areas affected by human activities. Unaffected air normally does not contain measurable concentrations of fluoride.
Fluoride is an important component in the production of aluminum. The raw material is aluminum oxide (Al2O3), which is dissolved in cryolite (Na3AlF6) to form an electrolyte. Aluminum is then formed by the reduction of aluminum ions. During this process fluorides escape from the molten to the gas phase and can be spread in the immediate area around the aluminum production plant. Introduction of gas cleaning by electrostatic filters or wet scrubbers have reduced the amount of fluorides released into the air. However, the cleaning of the gas does not solve the problem but instead transfers it into a water problem instead. Many of the industries that discharge fluorides in the air also release fluorides by water pollution. The largest sources for fluoride pollution are phosphate fertilizer producer and metallurgic industry like aluminum producers. Another major source of fluorides is the domestic sewage. The reason for this is the positive effect that fluorides have on the strength of the dental enamel and its ability to prevent the formation of caries. Therefore, in some areas fluoride is added to the drinking water in order to improve the dental status. It is however, important not to be exposed to too much fluoride. It was found that in areas were the groundwater contained more than 1 mg/L of fluoride people suffered from mottled enamel or dental fluorosis.
What effect enhanced concentrations of fluorides could have on freshwater and marine organisms is not completely examined. It is however, reasonable to conclude that marine organisms, which normally exist in an environment with a small concentration of fluoride (around 0.6 ppm in the sea), would not be affected by a modest release of fluoride. The situation for freshwater organisms is a little bit different. They live in an environment almost total fluoride free and it is possible that these organisms could be more affected by fluoride containing effluent. More research is required in this area since little is known about the impact from fluoride on both marine and freshwater organisms.
The situation for fluorides in the air is a little bit different. One conclusion from the US Department of Agriculture is that fluoride has done more damage to livestock than any other pollutant. Fluoride released to the air from industries is spread in the nearest surroundings and is accumulated in the plants. This fluoride enriched forage can be seriously toxic when cattle ingest it. Little is known about the long-term effects of fluoride exposure and it is important to gather more information in this area especially since fluoride accumulates in the food chain.
I.2.4.3. Nitrate and Nitrite
These are very important ions since they contain nitrogen, which is vital for all organisms both plants and animals. Both nitrate and nitrite are also part of the natural nitrogen cycle (Figure I.2.1.), which explains the transformations between different nitrogen compounds.
Nitrogen is reduced to ammonia by nitrogen-fixing bacteria. Ammonia is also produced during thunderstorms by the lightning. The ammonia is taken up by plants and is used for the production of vital molecules. Ammonia could also be consumed by nitrifying bacteria, which in two steps transform ammonia to nitrate via nitrite. Plants can also use nitrate as a source of nitrogen in order to produce new plant material. Animals and humans utilize the nitrogen in the plants as material for, among other things, protein synthesis. When oxygen is absent some bacteria have the ability to use nitrate as an electron acceptor, which then will be reduced to nitrogen. Microorganisms decompose dead animals and plants and during this process the bound nitrogen is released as ammonia.
Figure I.2.1. Nitrogen cycle.
Both nitrite and nitrate could contribute to the eutrophication if nitrogen is a limiting factor. This would lead to production of organic matter, which when it is decomposed would consume oxygen. If the concentration of organic matter is high, oxygen free areas in the water system could develop severe effects on the water living organisms. Nitrite itself is also toxic towards fish.
When hemoglobin is exposed to nitrite it is oxidized to methemoglobin. This change leads to a loss of the protein ability to bind to oxygen, which is very severe. Such a situation could occur when there is nitrate in the drinking water and it is very dangerous for infants and herbivores especially ruminants. All of these have a high number of nitrate reducing bacteria in their digestive system.
I.2.4.4. Sulfate and Sulfide
The sulfate ion is the second most common anionic ion in natural water after chloride. This ion has a laxative effect and usually there is a sulfate limit for drinking water. In a similar manner as the nitrogen, sulfate can be transformed into different oxidations states according to the sulfur cycle, which is a little bit more complex (Figure I.2.2.). Most of these conversions involve microorganisms or higher organisms in some way although the oxidation of hydrogen sulfide occurs rapidly at neutral and aerobic conditions. Sulfate, under anaerobic conditions, can be reduced to sulfide ion by microorganisms, which in the form of hydrogen sulfide has a very unpleasant smell of rotten eggs. It is a toxic gas and just a few breaths of hydrogen sulfide in a high concentration are sufficient to cause death. It could be produced in the sewer system were anaerobic conditions prevail in the water phase. Some of the hydrogen sulfide is released to the gas phase were it is dissolved in moisture on the sewer walls. Another type of organisms oxidizes the sulfide under the production of sulfuric acid, which causes corrosion of the sewer system. Sulfur is also needed for growth of plants and animals since there are e.g. proteins that contain sulfur. Plants normally fulfill this need by taking up sulfate.
Figure I.2.2. Sulfur cycle
The cyanide ion has an electrical charge of minus one and is made up of one carbon atom and one nitrogen atom. It exists naturally in e.g. cassava and in bitter almond. The cyanide is very toxic. An excess concentration of 200 micrograms per liter is toxic to most species of fish. If cyanide enters the human body it binds to hemoglobin and prevents it from taking up oxygen. When this happens the skin develops a blue color and the condition is called cyanosis. Prolonged exposure to low concentrations of cyanide may lead to breathing difficulties and enlargement of the thyroid gland.
Cyanide forms easily a cyanic-complex with some metal ions and this property is used in concentration of gold. In this process the ore is leached by an aqueous solution of cyanide with a high pH to prevent formation of hydrogen cyanide. The gold is dissolved into the solution and further processing of the liquid results in solid gold. Cyanide is also used in other types of industries like metallurgy, chemical production, plastic production and electroplating.
There have occurred accidents with release of leaching water with high concentration of cyanide into the environment. In January 2000 a dam filled with tailings ruptured in Northwest Romania. It was estimated that around 100 tons of cyanide were released into the Sãsar river, which then were further transported into Somes/Szamos, Tisa/Tisza - Danube river system. This accident led to serious damages to the ecosystem in several of the East European countries.
The term phosphate is used for the different salts of phosphoric acid that contain the ions PO43-, HPO42- and H2PO4-. Phosphate is essential to all living organisms and is used as a component in several cell components as the DNA and coenzymes. It is also an important molecule for storage and transportation of energy in the cell. The growth of microorganisms in aquatic system is often limited by some element and it is usually either phosphate and/or nitrogen. The release of large quantities of phosphate could lead to eutrophication of these waters. Most of the world's production of phosphate is used as fertilizer in the agriculture. Another large field of application for polyphosphates is as a water softener in household's detergents and industrial boiler water. The polyphosphate then forms a complex with cations such as calcium and magnesium, which otherwise would interact with the detergent and increased the consumption of the detergent. It also forms a complex with the same cations in the water boiler in order to prevent them from producing scaling with carbonates. Phosphate is also used as feed phosphates and is added as phosphoric acid to soft drinks.