viernes, 9 de octubre de 2015

Titanic, from iceberg to bacteria

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[Press PLAY to listen "Hymn to the sea" while you read the post]

Sometimes things occur around the world of such intensity that resonates over time, producing a kind of echo. The story of the Titanic causes a particular effect on who listen it, almost as if it were a Bible story: that huge ship, all those people in the ocean at night, the iceberg...

The Royal Mail Steamer Titanic was the product of an intense competition between British shipping companies in the first half of the twentieth century. White Star Line was in a battle for supremacy of steamship with Cunard Line, famous for the construction of two of the most sophisticated and luxurious ships of that time. On the one hand, Mauretania had set a speed record in transatlantic crossing. On the other hand, Lusitania, whose tragic final came in the annals of history when a torpedo launched by a German submarine sank it, precipitating the entry of the United States in the Great War.

The response of White Star Line was the construction of the Olympic Class: three ships including the RMS Titanic. The biggest steamship passengers ever built caused a stir when he departed from Southampton (England) on 10 April 1912 in its maiden voyage. After stops in Cherbourg (France) and Queenstown (now Cork, Ireland), the ship sailed to New York with 2,240 people aboard, including passengers and crewmembers.

RMS Titanic leaving Southampton.
Since this was the first transatlantic crossing of the most famous ship in the world, many of the passengers were high rank officials, wealthy industrialists, dignitaries and celebrities. In the second class academics, tourists and journalists travelled. But by far the largest group of passengers was in the third class, with more than 700, higher than the other two levels combined.

The fifth day, 600 km down the coast of Newfoundland (Canada), on a calm sea under a clear sky, the Titanic began to turn into a legend after meeting with an element of nature. The collision with the iceberg occurred at 23:30 on April 14th in 1912 and in less than three hours 50,000 tons of iron were submerged under the sea.

It just has been thirty years since oceanographer Robert Ballard discovered the Titanic in the depths of the Atlantic Ocean (1985) and was surprised by how the ship remained, predicting that it would not deteriorate too in a short term. But since then it has been observed that month by month, year by year, the ship is disappearing and that is happening faster than many experts expected.

Mainmast, captain's cabin and stern deck, where passengers gathered as they sank, have also collapsed. The look-out from Iceberg! was shouted it has disappeared. Huge holes in their decks have opened, metal walls have fallen and strange structures like brown stalactites have multiplied, as a result of the iron corrosion. These formations have been called rusticles.

Titanic hull is made of steel, which is an alloy of iron with small amounts of carbon. Iron is an "active" metal, which means that when it corrodes the by-products not form a protective layer on the metal to prevent further corrosion. In fact, when iron oxidize, iron oxide (Fe2O3) forms a porous crust that is permeable to water, allowing more water contact with iron under the oxide layer, even without corroding, causing that corrosion continues.

Greater degree of oxidation occurs when iron is under mechanical stress in places such as combinations, curves and rivets. When this occurs, Fe atoms are more weakly bound, so they are more easily oxidized by oxygen dissolved in water. In the following reaction, electrons are transferred from the iron to oxygen, oxidizing iron and reducing oxygen.

Fe (s) → Fe2+ (aq) + 2 e-
Iron oxidation half-reaction.

O2 (g) + 2 H2O (l) + 4 e- → 4 OH- (aq)
Oxygen reduction half-reaction.

2 Fe (s) + O2 (g) + 2 H2O (l) → 2 Fe2+ (aq) + 4 OH- (aq)
Redox reaction..

2 Fe2+ (aq) + 4 OH- (aq) → Fe(OH)2 (s)
Synthesis reaction.

4 Fe(OH)2 (s) + O2 (g) → 2 Fe2O3(s) + 4 H2O (l)
Second product of the oxidation of iron (rusticles).

Such oxidation occurs more easily at warm temperatures, where oxygen, water, salts or impurities are. But the ocean temperature decreases with depth, while pressure increases. Likewise gas solubility increases and salt solubility decreases. 4 km below the ocean surface, with water temperatures near freezing and low oxygen or salts dissolved in water, any chemical reaction would be blocked, and the metal should rot very slowly.

Canadian and Spanish researchers have isolated twenty seven bacterial strains from a rusticle of Titanic. Among them, they have identified a new species ever found in such deep waters. After this discovery, in 2010, they characterized and designated as Halomonas titanicae, taking into account the place of isolation. This type of bacteria is anaerobic, lives at very low temperatures (2-3 °C) and uses iron and minerals of boat hull to get energy. Researchers showed that bacteria are fixed to the steel surface and generate corrosion products that contribute, along with other microorganisms, to deterioration of the Titanic.

Halomonas titanicae was isolated and characterized by researchers at the University of Sevilla.

In seawater, sulfate ions are abundant and are converted to hydrogen sulfide (H2S) by these "eat-iron" bacteria:

SO42- + 10 H+ + 8 e- → H2S + 4 H2O

In seawater, pH is typically 8, but with the depth increases the solubility of carbon dioxide (CO2), making the water more acidic, as it is shown in the following reaction:

H2O + CO2 → H2CO3

Iron corrosion occurs more rapidly in acidic water, which balances the effect of depth and low temperature of the ocean delaying corrosion of the shipwreck. Some areas of the water surrounding the Titanic have a pH as low as 4. Moreover, the higher concentration of hydrogen ions, available by hydrogen sulfide dissolved in the water, contributes to the corrosion of iron as it is shown in the following reaction:

Fe (s) + 2 H+ (aq) → Fe2+ (aq) + H2 (g)

Bacteria are also able to convert hydrogen gas (H2) into hydrogen ions (H+), which then react to reduce the sulfate ions in water and, consequently, more iron is ionized. Furthermore, hydrogen sulfide (H2S) produced dissociates into ions in seawater and sulfide ions (S2-) form a precipitate with the iron ions (Fe2+) in water:

H2S (aq) ↔ 2 H+ + 2S2-
Dissociation of hydrogen sulphide ions.

Fe (s) + 2 H+ → Fe2+ + H2 (g)
Iron oxidation.

Fe2+ + S2- → FeS (s)
Synthesis of iron and sulfur.

Resulting iron sulfide (FeS) form insoluble deposits similar to rusticles, although they are black and less porous. Precipitation of FeS isolates more H+ ions, making them available for more iron oxidation. This in turn increases the speed at which the hull of Titanic deteriorates.

These "titanic" bacteria are being thoroughly investigated in order to save other structures beneath the ocean, which are also made of iron. Although they are the main responsible for the corrosion of the emblematic boat, you can learn a lot from their discovery. Scientists hope to obtain information that will be useful in creating new protective paints and covers for ships in the future. While the Titanic is deteriorating faster than was suspected at first, it is important to note that every different ecosystem that exists in the sea floor will decompose all shipwrecks over time.

While bacteria are the most significant factor, they are not the only contributing to accelerate Titanic corrosion. Natural oxidation in presence of oxygen and salts dissolved in seawater still play an important role. Another factor, relatively less significant, is due to organic material on board the ship. Due to it was a luxury ship, its interior was largely equipped with wooden panels. Since cellulose of wood decomposes, oxygen released contributes more to the iron corrosion of hull and also feeds the aerobic bacteria that produce by-products reducing pH of the water which surrounds the Titanic.

At the current rate, scientists have predicted that the wreck will be completely oxidized in the next 75-90 years. The discovery of anaerobic bacteria has allowed scientists to adjust their methods of predicting the corrosion rate of shipwrecks depending on environmental conditions.

Prediction of deterioration of RMS Titanic.

What is clear is that the Titanic is sentenced to disappear. Since disaster, it has inspired countless books and films and has not ceased to leave headlines. Meanwhile, his story has entered the public consciousness as a powerful warning about the dangers of human arrogance, as it happened with the Tower of Babel, who wanted to get to heaven and God didn’t allow it... but always at the expense of the same people.

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