DECAY of Radioactive or Biodegradable (although usually more complex than this model reflects) materials

The first-order irreversible rate reaction term represents the mass loss of both the dissolved phase and the sorbed phase.



The rate constants are

= ln(2) / half-life in dissolved phase

= ln(2) / half-life in sorbed phase

For radioactive decay, the reaction generally occurs at the same rate in both phases.

For biodegradation some reactions occur only in the dissolved phase

The sorbed concentration is determined by the sorption isotherm







The decay term is solved in Eulerian step, so the stability criterion is: