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Jeg blev af Gemini foreslået at opslemme 10g frisk sortblå sediment (transporteret lufttæt i køkkenfolie) i 50 ml. destilleret vand og gøre en pH måling så hurtig så mulig. Som jeg husker kan opslemmede partikler skade pH prober, som i dag heldigvis kun koster en brøkdel af hvad de gjorde tidligere. To målinger med en kalibreret probe viste pH på hhv. 2.38 og 2.33, d.v.s sur som stærk eddike. Gemini foreslog fornyet pH måling efter en time som (forbløffende) gav en pH på 2.17. Konklusjonen er at sedimentet er en kemisk tidskapsel - virkelig syret :). Målestokken er en meter lang. Jeg overlader ordet til Gemini:
What's happening:
pyrite oxidation (the "acid sulfate" reaction)
The black-blue color
of your sediment is a dead giveaway — it's rich in reduced
iron-sulfur minerals formed under anoxic conditions 55 million years
ago. The key culprit is pyrite
(FeS₂, fool's gold) and its metastable precursor mackinawite (FeS),
both of which are stable in oxygen-free environments but highly
reactive once exposed to air and water.
The reaction cascade:
1. Initial oxidation — when you mix the sediment with aerated water, pyrite reacts with oxygen:
2FeS₂ + 7O₂ + 2H₂O → 2Fe²⁺ + 4SO₄²⁻ + 4H⁺
Each pyrite molecule releases two protons (H⁺) — direct acid production.
2. Iron(II) oxidation — the ferrous iron further oxidizes:
4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O
3. Ferric iron attack — Fe³⁺ is an even more aggressive oxidizer of pyrite than oxygen:
FeS₂ + 14Fe³⁺ + 8H₂O → 15Fe²⁺ + 2SO₄²⁻ + 16H⁺
This is autocatalytic — once Fe³⁺ builds up, the reaction accelerates, producing even more acid.
Why the pH is so low (2.33)
• Your sediment is fresh from the cliff — the pyrite inside has been preserved unoxidized for 55 million years. Once you add water with dissolved oxygen, the oxidation proceeds rapidly on the enormous surface area of fine-grained sediment.
• The sulfate (SO₄²⁻) produced is the conjugate base of sulfuric acid (H₂SO₄), a strong diprotic acid — this drives the pH way down.
• 10 g sediment in 50 mL water is a highly concentrated suspension, so the local acid load is significant....
The 41% increase in
proton concentration after two hours without adding any fresh
oxidant - here's the feedback
loop in detail
Round 1 (fast —
your initial reading):
2FeS₂ + 7O₂ +
2H₂O → 2Fe²⁺ + 4SO₄²⁻ + 4H⁺ ---- direct O₂ attack
This jump-starts the
system. Iron(II) and protons are now in solution.
Interlude
(rate-limiting step — abiotic):
4Fe²⁺ + O₂ +
4H⁺ → 4Fe³⁺ + 2H₂O
This is slow at low
pH. In nature, Acidithiobacillus ferrooxidans catalyzes it,
but abiotically it takes time — hence the two-hour window you
observed.
Round 2 (faster —
the autocatalytic punch):
FeS₂ + 14Fe³⁺ +
8H₂O → 15Fe²⁺ + 2SO₄²⁻ + 16H⁺ --- Fe³⁺ is a MUCH
stronger oxidizer
Notice: each Fe³⁺
that oxidizes pyrite produces 15 Fe²⁺ — which then re-oxidize to
Fe³⁺ (step interlude) and attack more pyrite. The number of
oxidizers multiplies with each cycle. And each cycle pumps 4× more
protons into solution per FeS₂ unit than the oxygen route did.
This is why pH 2.18
is not the floor for your sample — leave it aerated for days and
you might see it drift toward 1.5–1.8 until the pyrite surface is
exhausted or the pH becomes so low that the Fe²⁺→Fe³⁺
oxidation stalls (iron-oxidizing bacteria tolerate down to ~1.0).