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Environmental fate & pathways

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Degradation

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

In water, sodium bicarbonate dissociates into sodium and bicarbonate. Bicarbonate re-equilibrates according to the following equations:

HCO3- ↔ CO32- + H+                        pKa = 10.33

CO2 + H2O ↔ HCO3- + H+              pKa = 6.35

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

Environmental distribution

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

Solid sodium bicarbonate has a negligible vapour pressure and for this reason it will not be distributed to the atmosphere.

If sodium bicarbonate is emitted to water, it will remain in the water phase. If the pH is decreased, then carbonic acid (H2CO3 or CO2) can be formed. If the concentration of carbon dioxide in water is above the water solubility limit, the carbon dioxide will distribute to the atmosphere.

An addition of bicarbonate to water will converge the pH to a value of 8.34. The value of 8.34 is equal to (pKa1 + pKa2)/2. In other words, if the initial pH of the receiving water is for example 7.0 then an addition of bicarbonate will increase the pH but it will never be higher than 8.34. However, if the initial pH of the receiving water is for example 9.0 then an addition of bicarbonate will decrease the pH but it will never be lower than 8.34.

If sodium bicarbonate 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 bicarbonate will be found predominantly in the aquatic environment. Sodium bicarbonate is present in the environment as sodium and bicarbonate ions, which implies that it will not adsorb on particulate matter or surfaces and will not accumulate in living tissues. It is obvious that both the sodium and bicarbonate ion have a wide natural occurrence.

[partly taken from OECD SIDS on sodium bicarbonate (2002), pg. 10 -11]

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