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I found this article in an Inorganic Chemistry textbook and thought I would share it with those who may find it interesting.
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The global nitrogen cycle is complex. It includes chemical changes and nitrogen transport between the oceans, land and atmosphere, and involves natural and man-made sources of nitrogen. The process of ammonification and assimilation refer respectively to the hydrolysis of organic nitrogen-containing compounds present in biomass. Sources of nitrates in groundwater (i.e. levels above those occurring naturally) include nitrate-based fertilizers and decaying organic material, as well as septic tanks, industrial effluent and waste from food processing factories.
Levels of [NO3]- in waste water are controlled by legislation, the Environmental Protection Agency (in the US) and the European Community. Nitrites, because of their toxicity, must also be removed. One of the principal concerns arising from nitrates and nitrites in drinking water is their association with the disease methaemoglobinemia. This is primarily suffered by infants and results in the blood having a lower than normal O2-carrying capacity. Hence, the common name for the disease is "˜blue baby syndrome'. In the body, bacteria in the digestive system convert [NO3]- to [NO2]-. Once formed, [NO2]- is able to irreversibly oxidize Fe2+ in haemoglobin to Fe3+. The product is called methaemoglobin and in this state, the iron can no longer bind to O2.
Nitrate salts are highly soluble and, therefore, their removal from aqueous solution by techniques based on precipitation is not viable. Methods of nitrate removal include anion exchange, reverse osmosis and enzymatic denitrification.
Ion-exchange involves passing the nitrate-containing water through a tank filled with resin beads on which chloride ions are adsorbed. In most conventional water-purification systems, resins bind to anions preferentially in the order [SO4]2- > [NO3]- > Cl- > [HCO3]- > [OH]-. Thus, as water containing nitrate ions passes through the resin, [NO3]- exchanges for Cl-, leaving [NO3]- ions adsorbed in the surface. However, if the water contains significant amounts of sulfate, [SO4]2- binds preferentially, lowering the capacity of the resin to remove [NO3]-. Specialized resins must therefore be used for sulfate-rich wastes. Once the ion-exchange process has exhausted the resin of Cl- ions, the system is regenerated by passing brine (aqueous NaCl) through the resin.
Removal of nitrates using enzymatic denitrification takes advantage of the fact that certain anaerobic bacteria can reduce [NO3]- and [NO2]- to N2. The process takes place in a sequence of steps, each involving a specific enzyme:
[NO3]- -> nitrate reductase -> [NO2]- -> nitrite reductase -> NO
NO -> nitric oxide reductase -> N2O -> nitrous oxide reductase -> N2.
Nitrate reduction half-reaction:
2[NO3]- + 12H+ +10e-<=> N2 + 6H20
Source:
Housecroft, Catherine E. and Sharpe, Alan G. 2008 Inorganic Chemistry. Pearson Education Limited 2001, 2008
[Adapted from: D.S. Powlson and T.M. Addiscott (2004) in Encyclopedia of Soils in the Environment, ed. D. Hillel, Elsevier, Oxford, vol. 3, p. 21.]
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The global nitrogen cycle is complex. It includes chemical changes and nitrogen transport between the oceans, land and atmosphere, and involves natural and man-made sources of nitrogen. The process of ammonification and assimilation refer respectively to the hydrolysis of organic nitrogen-containing compounds present in biomass. Sources of nitrates in groundwater (i.e. levels above those occurring naturally) include nitrate-based fertilizers and decaying organic material, as well as septic tanks, industrial effluent and waste from food processing factories.
Levels of [NO3]- in waste water are controlled by legislation, the Environmental Protection Agency (in the US) and the European Community. Nitrites, because of their toxicity, must also be removed. One of the principal concerns arising from nitrates and nitrites in drinking water is their association with the disease methaemoglobinemia. This is primarily suffered by infants and results in the blood having a lower than normal O2-carrying capacity. Hence, the common name for the disease is "˜blue baby syndrome'. In the body, bacteria in the digestive system convert [NO3]- to [NO2]-. Once formed, [NO2]- is able to irreversibly oxidize Fe2+ in haemoglobin to Fe3+. The product is called methaemoglobin and in this state, the iron can no longer bind to O2.
Nitrate salts are highly soluble and, therefore, their removal from aqueous solution by techniques based on precipitation is not viable. Methods of nitrate removal include anion exchange, reverse osmosis and enzymatic denitrification.
Ion-exchange involves passing the nitrate-containing water through a tank filled with resin beads on which chloride ions are adsorbed. In most conventional water-purification systems, resins bind to anions preferentially in the order [SO4]2- > [NO3]- > Cl- > [HCO3]- > [OH]-. Thus, as water containing nitrate ions passes through the resin, [NO3]- exchanges for Cl-, leaving [NO3]- ions adsorbed in the surface. However, if the water contains significant amounts of sulfate, [SO4]2- binds preferentially, lowering the capacity of the resin to remove [NO3]-. Specialized resins must therefore be used for sulfate-rich wastes. Once the ion-exchange process has exhausted the resin of Cl- ions, the system is regenerated by passing brine (aqueous NaCl) through the resin.
Removal of nitrates using enzymatic denitrification takes advantage of the fact that certain anaerobic bacteria can reduce [NO3]- and [NO2]- to N2. The process takes place in a sequence of steps, each involving a specific enzyme:
[NO3]- -> nitrate reductase -> [NO2]- -> nitrite reductase -> NO
NO -> nitric oxide reductase -> N2O -> nitrous oxide reductase -> N2.
Nitrate reduction half-reaction:
2[NO3]- + 12H+ +10e-<=> N2 + 6H20
Source:
Housecroft, Catherine E. and Sharpe, Alan G. 2008 Inorganic Chemistry. Pearson Education Limited 2001, 2008
[Adapted from: D.S. Powlson and T.M. Addiscott (2004) in Encyclopedia of Soils in the Environment, ed. D. Hillel, Elsevier, Oxford, vol. 3, p. 21.]
