Acid and salt in the Vasa; The sulfur cycle

The transformations of sulfur compounds can be illustrated schematically as follows (Sandström 2003). In the first step sulfate-reducing bacteria, metabolising organic matter under anaerobic conditions, transform sulfate ions in seawater to dissolved hydrogen sulfide. When this water penetrates waterlogged wood, the hydrogen sulfide reacts and forms solid sulfur compounds. If iron ions are present iron sulfides (e.g. pyrite FeS₂) form, otherwise e.g. thiols (-SH groups) and other reduced sulfur compounds such as elemental sulfur, S₈(s), may accumulate in the wood.

Sulfate-reducing bacteria produce hydrogen sulfide, H₂S(aq), by stripping the oxygen atoms from the tetrahedral sulfate ions, SO₄^2-, in the seawater when feeding on organic material in polluted anoxic water. The H₂S molecules look like water molecules, with sulfur instead of an oxygen atom. When dissolved hydrogen sulfide penetrates waterlogged wood, it can react to elemental sulfur, S(s), which consists of S₈ crowns, and also to thiols with -SH groups. Dimethylsulfide is the major gasform (instead of hydrogen sulfide) evaporating from seawater in the natural sulfur cycle.

When oxygen in the air gets into contact with the humid wood in the salvaged shipwreck, sulfate ions regenerate when reduced sulfur compounds, e.g. iron sulfides, oxidise to sulfuric acid. The end result is acid and sulfate salts, which reveal that acid has formed in the wood:

S(s) + ³/₂O₂ + H₂O → 2H⁺(aq) + SO₄^2- (Sulfuric acid) → Sulfate salts + acid

FeS₂(s) + ⁷/₂O₂ + H₂O → FeSO₄(aq) + 2H⁺(aq) + SO₄^2- (Sulfuric acid)


After oxidation of reduced sulfur to sulfuric acid, the tetrahedral SO₄^2- ions (with the sulfur atom in the centre and oxygen atoms at the corners) may combine with positive ions and precipitate as a sulfate salt. The crystal structures of some sulfate salts found on the Vasa, gypsum CaSO₄.2H₂O, natrojarosite NaFe₃(SO₄)₂(OH)₆ and melanterite, FeSO₄.7H₂O.

Bicarbonate makes the Vasa less acidic

Starting in 2001 the hull was examined, and in 2004, more than 1700 areas and objects with acidic salt precipitates (pH = 3 or less) had been registered with information about location, date, pH-development and treatment. The pH-values measured at the salt precipitates can occasionally be as low as 1. Inside the hull, e.g. in the hold, orlop deck, the gun decks, the cabin floor, the poop deck, and on pinewood, many precipitates display a yellowish-greenish colour typical of natrojarosite, and those are usually the most acidic ones.

pH-variations in wood surfaces following treatment with bicarbonate/soda solution. After an initial increase, the pH values often revert to levels below 4 within a few months.

All acidic areas inside the Vasa have repeatedly been treated with an alkaline solution, while more efficient long-term treatments are being developed. Poultices soaked in 5 weight% bicarbonate/soda solution in mass ratio 7:3 of the compounds Na₂CO₃·10H₂O and NaHCO₃, were placed on horizontal surfaces. This limits the pH value to at most 9 in the buffer range for the acid-base pair HCO₃^-/CO₃^2-, because a strongly alkaline solution is also harmful to the wood. In contact with acidic wood the harmless products carbon dioxide, CO₂, and sodium sulfate, Na₂SO₄(aq), are formed. Vertical surfaces were hand sprayed and covered by plastic film.

Diagram for the carbonate system in aqueous solution, showing the logarithmic concentration (M = mol/l) of species in solution vs. pH. In the buffer ranges at about pH = 6±1 and 10±1, there are comparable amounts of the species in the acid/base pairs, i.e. CO₂(aq) / HCO₃^-and HCO₃^- / CO₃^2-, respectively. Note that in the acidic range the hydrated carbon dioxide molecule CO₂(aq) dominates over the acidic form H₂CO₃ with the ratio about 600:1. The diagram also shows that goethite, i.e. solid iron(III) oxyhydroxide FeOOH(c), precipitates already at low pH-values and low iron concentration.