Isotopes of an atom have the same number of protons and electrons, but a variable number of neutrons. This gives each isotope a different mass. Bond energy is determined in quantum mechanics by equations in which the mass of the nucleus is a factor. As such, the difference in mass alters the physical, chemical and especially biological properties of a substance. This difference exists between the isotopes of every atom. In heavier isotopes the difference is too small to significantly effect bond energy, but the low mass of hydrogen make these differences more pronounced.
There are two stable (non-radioactive) isotopes of Hydrogen: Hydrogen-1 and Deuterium (Hydrogen-2). Both isotopes exist naturally. 99.9855% of all Hydrogen is Hydrogen-1, 0.0145% is Deuterium. Tritium (Hydrogen-3) and heavier isotopes of Hydrogen are all radioactive and decay quickly. Deuterium is commonly found in the form of heavy water, D2O. A mixture of "light" water (H2O) and "heavy water" (D2O) will rapidly exchange hydrogen ions and form "semi-heavy water" (HDO).
The melting and boiling temperatures of heavy water are higher than light water. Light water will evapourate faster, increasing the concentration of heavy water in what remains behind. Higher concentrations of heavy water are found in the leaves and juices of plants due to transpiration. Deuterium can also accumulate in biological systems with age. This is a problem because biological systems are very sensitive to small changes in the solvent properties of water. Enzymes rely on their finely tuned networks of hydrogen bonds, both in the active center with their substrates and outside the active center, to stabilize their tertiary structures. The normal metabolism of cell is disrupted in the presence of deuterium, while high concentrations of deuterium in multi-cellular organisms (plants, fungi, animals, humans) are fatal.
D2O has the following effects on biological systems:
It stops mitotic spindle formation, necessary for cell division in multi-cellular organisms. Seeds won't germinate and plants won't grow.
It increases the glomerular filtration rate and decreases the renal plasma flow of the kidneys. This decreases the overall function of the kidneys.
Blood glucose level drops and is unable to be maintained. Glucose can't be stored as liver glycogen.
The intestinal barrier functions breakdown causing dirrhea and fluid loss.
It alters the geometry and folding of DNA, which can increase replication errors. It produces genetic mutations in some bacterial DNA but not others.
It alters the pH of internal fluids.
It affects cell processes such as diffusion, nutrient transport and nerve conduction.
It accumulates in the fats of mammals. Deuterated fats accumulate around the liver, and the liver enlarges.
It decreases the production of cholesterol and fatty acids and reduces the ability to breakdown these substances.
Increasing concentrations of D2O in water produce symptoms of increasing severity:
0-15% Rats stop gaining weight. They're physically smaller.
15-20% Rats become excitable
Up to 20%, metabolic rate and body temperature increase.
20-25% Rats go into convulsions when stimulated. Skin lesions, ulcers on the paws and muzzle, and necrosis (death) of the tail begins. Males become very aggressive.
Above 20%, metabolic rate and body temperature decrease.
25% The formation of mature erythrocytes (red blood cells) stops. Rats begin dying from anemia.
25-30% Sterility caused by the inability of both gametes and zygotes to develop.
Below 30% The effects of poisoning are usually reversible by administering "light" water soon after poisoning.
30% Rats refuse to eat and fall into a coma.
30-35% Slow death occurs. Death appears to be due to dehydration even when the animal is almost bursting with water.
90% Rapid death of aquatic species occurs.
Bacteria are more resilient in heavy water, but only after slow adaptation. Once adapted they can survive in 99.8% heavy water and produce deuterated organic compounds, but they grow more slowly than in light water.
100% No known species can survive.
It's interesting to note: when an animal is starving, deuterated fats continue to be stored and not metabolised. It's only when a normal diet is resumed after fasting that deuterated fats are metabolised and the deuterium is excreted from the body as heavy water.
The most efficient way of seperating heavy water from light water is hydrolysis. The water mixture is hydrolysed into Hydrogen and Oxygen, then reformed into water. Hydrogen-1 hydrolyses around six times faster than deuterium. The reformed water has a significantly lower concentration of D2O. The reformed water is then hydrolysed again and again, each time reducing the D2O concentration until it's no longer detectable.
Deuterium should never be used when terraforming low gravity planets.
|Melting point (°C)||0.00||3.82|
|Boiling point (°C)||100.0||101.4|
|Density (Kg/L, at 20°C)||0.9982||1.1056|
|Temperature of maximum density (°C)||4.0||11.6|
|Viscosity (centipoise, at 20°C)||1.005||1.25|
|Surface tension (dyn.cm, at 25°C)||71.97||71.93|
|Heat of fusion (J/mol)||6008||6338|
|Heat of vapourisation (J/mol)||43995||45455|
|pH (at 25°C)||7.00||7.41|