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Environmental fate & pathways

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Degradation

The high water solubility and low vapour pressure indicate that sodium carbonate will be found predominantly in the aquatic environment. In water, sodium carbonate dissociates into sodium and carbonate and both ions will not adsorb on particulate matter or surfaces and will not accumulate in living tissues. An emission of sodium carbonate to water will result in an increase in alkalinity and a tendency to raise the pH value.

The carbonate ions will react with water, resulting in the formation of bicarbonate and hydroxide, until an equilibrium is established (McKee et al., 1963). It is obvious that both the sodium and bicarbonate ion have a wide natural occurrence (UNEP, 1995).

Sodium carbonate is an inorganic substance which cannot be oxidized or biodegraded by micro-organisms.

 

The degradation and environmental fate of sodium carbonate have been described on page 9 of the OECD SIDS dossier (2002). Please find hereafter the text: 

 

The sodium ion is ubiquitously present in the environment and it has been measured extensively in aquatic ecosystems. Sodium and chloride concentrations in water are tightly linked. They both originate from natural weathering of rock, from atmospheric transport of oceanic inputs and from a wide variety of anthropogenic sources. The sodium concentration was reported for a total number of 75 rivers in North and South America, Africa, Asia, Europe and, with a 10th-percentile of 1.5 mg/L, mean of 28 mg/l and 90th-percentile of 68 mg/L (UNEP, 1995).

 

If carbonate is dissolved in water, a re-equilibration takes place according to the following equations:

 

 HCO3-↔ CO32-+ H+             pKa = 10.33 

 

 CO2+ H2O ↔ HCO3- + H+    pKa = 6.35 

 

The carbonate will finally be incorporated into the inorganic and organic carbon cycle.

Only a small fraction of the dissolved CO2is present as H2CO3, the major part is present as CO2. The amount of CO2in water is in equilibrium with the partial pressure of CO2in the atmosphere. The CO2/ HCO3-/ CO32-equilibriums are the major buffer of the pH of freshwater throughout the world.

Based on the above equations, CO2is the predominant species at a pH smaller than 6.35, while HCO3-is the predominant species at a pH in the range of 6.35-10.33 and CO32-is the predominant species at a pH higher than 10.33.

 

The natural concentration of CO2/ HCO3-/ CO32- in freshwater is influenced by geochemical and biological processes. Many minerals are deposited as salts of the carbonate ion and for this reason the dissolution of these minerals is a continuous source of carbonate in the freshwater environment. Carbon dioxide is produced in aquatic ecosystems from microbial decay of organic matter. On the other hand, plants utilise dissolved carbon dioxide for the synthesis of biomass (photosynthesis). Because many factors influence the natural concentration of CO2/ HCO3-/ CO32-in freshwater, significant variations of the concentrations do occur.

If the pH is between 7 and 9 then the bicarbonate ion is the most important species responsible for the buffer capacity of aquatic ecosystems. UNEP (1995) reported the bicarbonate concentration for a total number of 77 rivers in North-America, South-America, Asia, Africa, Europe and the 10th-percentile, mean and 90th-percentile were 20, 106 and 195 mg/L, respectively.

 

Environmental distribution

Sodium carbonate is an inorganic substance and therefore standard computer models cannot be used to determine the transport or distribution between environmental compartments.

 

The environmental distribution of sodium carbonate has been described already on page 8, 9 and 37 of the OECD SIDS dossier (2002). Please find hereafter the text: 

  

If sodium carbonate is emitted to water it will remain in the water phase. If the pH is decreased then carbonic acid (H2CO3or CO2) can be formed. If the concentration of carbon dioxide water is above the water solubility limit, the carbon dioxide will distribute to the atmosphere. 

If sodium carbonate is emitted to soil it can escape to the atmosphere as CO2(see above), precipitate as a metal carbonate, form complexes or stay in solution.

The high water solubility and low vapour pressure indicate that sodium carbonate will be found predominantly in the aquatic environment. In water, sodium carbonate dissociates into sodium and carbonate and both ions will not adsorb on particulate matter or surfaces and will not accumulate in living tissues. An emission of sodium carbonate to water will result in an increase in alkalinity and a tendency to raise the pH value. 

The carbonate ions will react with water, resulting in the formation of bicarbonate and hydroxide, until equilibrium is established (McKee and Wolf, 1963). It is obvious that both the sodium and bicarbonate ion have a wide natural occurrence (UNEP, 1995).