Adsorption and Decomposition Pathways
Many APIs and their metabolites once excreted from the patient or discharged into water courses are recalcitrant to further breakdown in sewage treatment plants (STPs) and natural water courses (rivers, lakes etc.). When entering the environment via the patient, APIs may be excreted unchanged, as mixtures of unchanged API alongside one or several major metabolite(s), or completely metabolised to single or mixtures of metabolite(s).
In the environment, there may be no, partial or full chemical breakdown of the excreted compounds. As with metabolites, breakdown products can be more problematic than the parent compound – potentially more toxic and more recalcitrant.  Many APIs are not removed during typical sewage treatments, but can be adsorbed onto sewage sludge.
Therefore once released into the environment, an API or metabolite may:
- Decompose to innocuous fragments and eventually be mineralised – converted to carbon dioxide, nitrates, sulphates etc.
- Decompose to more recalcitrant fragments that are persistent.
- Be recalcitrant – i.e. does not decompose, and may or may not show undesirable biological activity.
- Bind to organic solids such as soils, river/lake sediments and sludge in STPs where it could accumulate or slowly decompose. This binding rapidly reduces the concentration of the API in solution, but may not necessarily remove it from the environment. The extent (efficiency) of binding is dependent on the nature of the solid and thus will be location dependant.
Loss to air is almost never seen for most APIs, therefore the two main environments are water (major) and land (minor).
- Recalcitrant pharmaceuticals in the aquatic environment: a comparative screening study of their occurrence, formation of phototransformation products and their in vitro toxicity, Environmental Chemistry, 2014, 11, 431-444. ,
- Is the phototransformation of pharmaceuticals a natural purification process that decreases ecological and human health risks?, Environmental Pollution, 2014, 186, 203-215. ,