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Aquatic Chemistry

Problem 1. Carbon sequestration technologies are being actively explored by researchers as methods to remove CO2 from the atmosphere, thereby diminishing the greenhouse effect. The most popular repository for CO2 is considered to be the deep ocean, where the CO2 would theoretically be isolated from the atmosphere for thousands of years.

Incorporation of CO2 into phytoplankton is one approach that has been considered carefully. In some such scenarios, certain iron-poor regions of the ocean would be fertilized with iron, inducing phytoplankton to grow and take up CO2 for conversion into biomass. When the microbes were to die, their shells would ideally sink to the floor. Diatoms, planktonic microbes with calcium carbonate (CaCO3) shells, or "tests", are particularly attractive targets for these efforts because they consume so much more CO2 during growth than non-shelled microbes. You may be familiar with "diatomaceous earth", a white powder made of the shells of these microbes. The White Cliffs of Dover also owe their whiteness to the shells of these microbes.

The composition of a particular tropical ocean surface is as follows:

pH = 8.5 T = 25° C [Mg2+] = 0.06 M

[Na+] = 0.46 M PCO2 = 10-3.5 atm [Cl-] = 0.54 M

[Ca2+] = 0.01 M [HCO3-] = 2.3 mM [SO42-] = 0.03 M

Write a balanced equation for the reaction of atmospheric CO2 with free calcium ions to form calcite at the ocean surface.

Find the equilibrium constant for this reaction. Is the system described at equilibrium with respect to this reaction? If not, in which direction is it proceeding?

In light of your answer to (b), do the diatoms need to do anything to facilitate the formation of calcite? If so, what? Describe in words only.

Calcium carbonate may form in crystals of either calcite or aragonite. Which of these two is the thermodynamically most stable form, that therefore would be the ultimate form of calcium carbonate to accumulate on the ocean floor? Show all calculations.