.. _example-phreeqc-manual-15:

15 - Transport Kinetic Biodegradation
=====================================

Cobalt and NTA moving through a column while bacteria degrade the NTA, grow
on it, and sorb -- transport, kinetics, microbiology and sorption together.

NTA (nitrilotriacetate) is a chelating agent. It binds cobalt into the
complex CoNta⁻, which does not sorb, so while the NTA survives the cobalt
travels freely. The bacteria eat the NTA. The question is what happens to the
cobalt when its ligand is destroyed.


What is coupled
---------------

.. raw:: html
   :file: study_15.html

* **Transport** along the column.
* **Kinetic biodegradation** of NTA, with the rate depending on the amount
  of biomass.
* **Microbial growth**: the biomass increases as it consumes the substrate,
  so the rate accelerates as the reaction proceeds.
* **Sorption** of cobalt, which depends on its speciation.

The growth term is what makes this more than a decay model. The rate is not a
constant: it feeds back on itself, so degradation is slow until the
population establishes and then fast.


The results
-----------

.. figure:: Dissolvedspecies.svg
   :alt: Dissolved cobalt, CoNta and HNta with pH against time, at two grid resolutions
   :align: center

   Dissolved species against time. **CoNta⁻** is the mobile form, and it
   falls as the NTA is degraded. Free **Co²⁺** appears as its ligand is
   destroyed, and free cobalt sorbs -- so biodegradation of the chelator
   immobilises the metal.

   That is the result, and it is counter-intuitive enough to be worth
   stating plainly: degrading the organic contaminant is what stops the metal
   moving.

.. figure:: Sorbedspecies.svg
   :alt: Sorbed cobalt, sorbed CoNta and biomass against time, at two grid resolutions
   :align: center

   Sorbed species and biomass. Sorbed cobalt rises as the dissolved free
   cobalt appears, and the **biomass** curve shows the growth that drove the
   whole sequence.

Both figures are drawn twice, at **10 cells and 20 cells**. That is the
convergence check of :ref:`example-phreeqc-manual-12`, applied to a problem
with no analytical solution to compare against -- which is the usual
situation. Where the two resolutions agree, the result is a property of the
model; where they differ, it is a property of the grid.

With a growing biomass and a sharp degradation front, this check is not
optional. A reaction rate that depends on its own product is exactly the kind
of non-linearity that a coarse grid resolves badly and silently.


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 15 of that manual.
* The Co-NTA system and its degradation kinetics follow Tebes-Stevens, C.,
  Valocchi, A. J., VanBriesen, J. M. and Rittmann, B. E. (1998),
  *Multicomponent transport with coupled geochemical and microbiological
  reactions*, Journal of Hydrology 209, 8-26.