.. _example-phreeqc-manual-05:

05 - Irreversible Reactions
===========================

Pyrite oxidation: the reaction behind acid mine drainage, and a good example
of a process that cannot be modelled as an equilibrium because it does not
reverse.

Oxygen is added to a water in contact with pyrite, calcite and goethite, a
little at a time, and the model follows what each addition does.


Irreversible means titrated, not equilibrated
---------------------------------------------

.. raw:: html
   :file: study_05 - Irreversible Reactions.html

An equilibrium calculation asks where a system would settle. An irreversible
reaction asks what happens *along the way*, and is expressed by adding a
reactant in increments and solving for equilibrium after each one.

Pyrite oxidation has to be done this way. Once FeS₂ has oxidised to sulfate
and ferric iron, putting the oxygen back does not restore it, so there is no
equilibrium state of "pyrite plus oxygen" to find. What there is, is a path.


The result
----------

.. figure:: linePlot1.svg
   :alt: Millimoles of pyrite, goethite, calcite, CO2 and gypsum against oxygen added
   :align: center

   Everything plotted against oxygen added. The chain is legible in the
   curves:

   **Pyrite dissolves** as oxygen is consumed. Its iron reappears as
   **goethite**, which precipitates in step -- the oxidation does not leave
   iron in solution at this pH, it moves it from one solid to another.

   Its sulfur becomes sulfate, and the acid released by the reaction
   **dissolves calcite**. That is the buffering: as long as there is
   carbonate, the pH is held and the drainage is not acid. The **CO₂** curve
   is the carbonate leaving as gas, which is where the dissolved calcite
   goes.

   The calcium from the calcite then meets the sulfate from the pyrite, and
   **gypsum** precipitates once the product of the two is high enough. The
   ``SI Gypsum`` curve shows it reaching zero before any gypsum appears,
   which is the correct order and worth checking in any model that
   precipitates a phase.

   The whole sequence -- sulfide to sulfate, iron to oxide, acid to
   carbonate, calcium to gypsum -- follows from adding one reactant.


What it shows
-------------

That the interesting part of a reaction is often the path and not the
endpoint, and that a model with the right phases available will produce the
secondary mineralogy without being told to.

The practical reading is the carbonate: the amount of calcite present decides
whether this water ends up neutral or acid, and the model says how much
oxygen that supply is worth.


Source
------

* Parkhurst, D. L. and Appelo, C. A. J. (2013). *Description of input and
  examples for PHREEQC version 3.* U.S. Geological Survey Techniques and
  Methods, book 6, chapter A43. This is Example 5 of that manual.