Iron (Fe) and water
Iron and water: reaction mechanisms, environmental impact and health effects
| Seawater contains approximately 1-3 ppb of iron. The amount varies strongly, and is different in the Atlantic and the Pacific Ocean. Rivers contain approximately 0.5-1 ppm of iron, and groundwater contains 100 ppm. Drinking water may not contain more than 200 ppb of iron. Most algae contain between 20 and 200 ppm of iron, and some brown algae may accumulate up to 4000 ppm. The bio concentration factor of algae in seawater is approximately 104 - 105. Sea fish contain approximately 10-90 ppm and oyster tissue contains approximately 195 ppm of iron (all are dry mass). Dissolved iron is mainly present as Fe(OH)2+ (aq) under acidic and neutral, oxygen-rich conditions. Under oxygen-poor conditions it mainly occurs as binary iron. Iron is part of many organic and inorganic chelation complexes that are generally water soluble. In what way and in what form does iron react with water? Iron does not clearly alter in pure water or in dry air, but when both water and oxygen are present (moist air), iron corrodes. Its silvery colour changes to a reddish-brown, because hydrated oxides are formed. Dissolved electrolytes accelerate the reaction mechanism, which is as follows: 4 Fe + 3 O2 + 6 H2O -> 4 Fe3+ + 12 OH- -> 4 Fe(OH)3 or 4 FeO(OH) + 4 H2O Usually the oxide layer does not protect iron from further corrosion, but is removed so more metal oxides can be formed. Electrolytes are mostly iron (II) sulphate, which forms during corrosion by atmospheric SO2. In sea regions atmospheric salt particles may play an important role in this process. Iron (II) hydroxide often precipitates in natural waters. Solubility of iron and iron compounds Elementary iron dissolves in water under normal conditions. Many iron compounds share this characteristic. Naturally occurring iron oxide, iron hydroxide, iron carbide and iron penta carbonyl are water insoluble. The water solubility of some iron compounds increases at lower pH values. Other iron compounds may be more water soluble than the examples mentioned above. Iron carbonate has a water solubility of 60 mg/L, iron sulphide of 6 mg/L, and iron vitriol even of 295 g/L. Many iron chelation complexes are water soluble. Usually there is a difference between water soluble Fe2+ compounds and generally water insoluble Fe3+ compounds. The latter are only water soluble in strongly acidic solutions, but water solubility increases when these are reduced to Fe2+ under certain conditions. Why is iron present in water? The main naturally occurring iron minerals are magnetite, hematite, goethite and siderite. Weathering processes release the element into waters. Both mineral water and drinking water contain iron carbonate. In deep sea areas the water often contains iron fragments the size of a fist, manganese and small amounts of lime, silicon dioxide and organic compounds. Iron is applied worldwide for commercial purposes, and is produced in amounts of 500 million tons annually. Some 300 million tons are recycled. The main reason is that iron is applicable in more areas than possibly any other metal. Alloys decrease corrosivity of the metal. Steel producers add various amounts of carbon. Iron alloys are eventually processed to containers, cars, laundry machines, bridges, buildings, and even small springs. Iron compounds are applied as pigments in glass and email production, or are processed to pharmaceutics, chemicals, iron fertilizers, or pesticides. These are also applied in wood impregnation and photography. Aluminum waste products containing iron were discharged on surface water in the earlier days. Today, these are removed and applied as soil fillers. Iron compounds are applied in precipitation reactions, to remove compounds from water in water purification processes. The 59Fe isotope is applied in medical research and nuclear physics. What are the environmental effects of iron in water? Iron is a dietary requirement for most organisms, and plays an important role in natural processes in binary and tertiary form. Oxidized tertiary iron cannot be applied by organisms freely, except at very low pH values. Still, iron usually occurs in this generally water insoluble form. Adding soluble iron may rapidly increase productivity in oceanic surface layers. It might than play an important role in the carbon cycle. Iron is essential for nitrogen binding and nitrate reduction, and it may be a limiting factor for phytoplankton growth. Solubility in salt water is extremely low. The iron cycle means reduction of tertiary iron by organic ligands (a process that is photo catalysed in surface waters), and oxidation of binary iron. Iron forms chelation complexes that often play an important role in nature, such as haemoglobin, a red colouring agent in blood that binds and releases oxygen in breathing processes. Organisms take up higher amounts of binary iron than of tertiary iron, and uptake mainly depends on the degree of saturation of physical iron reserves. Iron is often a limiting factor for water organisms in surface layers. When chelation ligands are absent, water insoluble tertiary iron hydroxides precipitate. This is not thought to be hazardous for aquatic life, because not much is known about hazards of water borne iron. Mollusks have teeth of magnetite of goethite. Green plants apply iron for energy transformation processes. Plants that are applied as animal feed may contain up to 1000 ppm of iron, but this amount is much lower in plants applied for human consumption. Generally plants contain between 20 and 300 ppm iron (dry mass), but lichens may consist up to 5.5% of iron. When soils contain little iron, or little water soluble iron, plants may experience growth problems. Plant uptake capacity strongly varies, and it does not only depend on soil iron concentrations, but also upon pH values, phosphate concentrations and competition between iron and other heavy metals. Limes soils are often iron deficit, even when sufficient amounts of iron are present. This is because of the generally high pH value, which leads to iron precipitation. Iron usually occurs in soils in tertiary form, but in water saturated soils it is converted to binary iron, thereby enabling plant iron uptake. Plants may take up water insoluble iron compounds by releasing H+ ions, causing it to dissolve. Micro organisms release iron siderochrome, which can be directly taken up by plants. Iron may be harmful to plants at feed concentrations of between 5 and 200 ppm. These cannot be found in nature under normal conditions, when low amounts of soil water are present. A number of bacteria take up iron particles and convert them to magnetite, to apply this as a magnetic compass for orientation. Iron compounds may cause a much more serious environmental impact than the element itself. A number of LD50 values are known for rats (oral intake): iron (III) acetyl acetonate 1872 mg/kg, iron (II) chloride 984 mg/kg, and iron penta carbonyl 25 mg/kg. There are four naturally occurring non-radioactive iron isotopes. There are eight instable iron isotopes. What are the health effects of iron in water? The total amount of iron in the human body is approximately 4 g, of which 70% is present in red blood colouring agents. Iron is a dietary requirement for humans, just as it is for many other organisms. Men require approximately 7 mg iron on a daily basis, whereas women require 11 mg. The difference is determined by menstrual cycles. When people feed normally these amounts can be obtained rapidly. The body absorbs approximately 25% of all iron present in food. When someone is iron deficit feed iron intake may be increased by means of vitamin C tablets, because this vitamin reduces tertiary iron to binary iron. Phosphates and phytates decrease the amount of binary iron. In food iron is present as binary iron bound to haemoglobin and myoglobin, or as tertiary iron. The body may particularly absorb the binary form of iron. Iron is a central component of haemoglobin. It binds oxygen and transports it from lungs to other body parts. It than transports CO2 back to the lungs, where it can be breathed out. Oxygen storage also requires iron. Iron is a part of several essential enzymes, and is involved in DNA synthesis. Normal brain functions are iron dependent. In the body iron is strongly bound to transferrin, which enables exchange of the metal between cells. The compound is a strong antibiotic, and it prevents bacteria from growing on the vital element. When one is infected by bacteria, the body produces high amounts of transferrin. When iron exceeds the required amount, it is stored in the liver. The bone marrow contains high amounts of iron, because it produces haemoglobin. Iron deficits lead to anaemia, causing tiredness, headaches and loss of concentration. The immune system is also affected. In young children this negatively affects mental development, leads to irritability, and causes concentration disorder. Young children, pregnant women and women in their period are often treated with iron (II) salts upon iron deficits. When high concentrations of iron are absorbed, for example by haemochromatose patients, iron is stored in the pancreas, the liver, the spleen and the heart. This may damage these vital organs. Healthy people are generally not affected by iron overdose, which is also generally rare. It may occur when one drinks water with iron concentrations over 200 ppm. Iron compounds may have a more serious effect upon health than the relatively harmless element itself. Water soluble binary iron compounds such as FeCl2 and FeSO4 may cause toxic effects upon concentrations exceeding 200 mg, and are lethal for adults upon doses of 10-50 g. A number of iron chelates may be toxic, and the nerve toxin iron penta carbonyl is known for its strong toxic mechanism. Iron dust may cause lung disease. Which water purification technologies can be applied to remove iron from water? Iron removal from water is mostly carried out in drinking water preparation, because mineral water contains high amounts of iron ions. These influence water colour, odour and turbidity. Iron is present in all wastewaters. Iron removal from wastewater may be achieved by oxidation of binary iron to tertiary iron. Hydrolysis subsequently causes flake formation, and flakes can be removed by sand filtration. Oxidation may be achieved by adding oxygen or other oxidants, such as chlorine or potassium permanganate. The reaction rate depends upon pH values, and is slower under acidic than under alkalic conditions. To speed up the reaction under acidic conditions, the water may be aerated for carbon dioxide removal and pH recovery. The total reaction causes acid formation and thereby diminishes itself. Iron is often reduced together with manganese. Applying ion exchangers for iron trace removal from drinking water and process water is another option, but this is not very suitable for removing high iron concentrations. Iron compounds are applied in wastewater treatment, usually as coagulants. One example is iron sulphate application in phosphate removal. |