Considering ionic state in modelling sorption of pharmaceuticals to sewage sludge

Information on the partitioning of chemicals between particulate matter and water in sewage treatment plants (STPs) can be used to predict their subsequent environmental fate. However, this information can be challenging to acquire, especially for pharmaceuticals that are frequently present in ionised forms. This study investigated the relationship between the ionisation state of active pharmaceutical ingredients (APIs) and their partitioning between water and sludge in STPs. In addition, the authors investigated the underlying mechanisms of sludge sorption by using chemical descriptors based on ionized structures, and evaluated the usefulness of these descriptors in quantitative structure-property relationship (QSPR) modelling. KD values were collected for 110 APIs, which were classified as neutral, positive, or negative at pH 7. The models with the highest performance had the R(2)Y and Q(2) values of above 0.75 and 0.65, respectively. The results showed that the dominant intermolecular forces governing the interactions of neutral and positively charged APIs with sludge are hydrophobic, pi-pi, and dipole-dipole interactions, whereas the interactions of negatively charged APIs with sludge were mainly governed by covalent bonding as well as ion-ion, ion-dipole, and dipole-dipole interactions; hydrophobicity-driven interactions were rather unimportant. Including charge-related descriptors improved the models’ performance by 5-10%, underlining the importance of electrostatic interactions. The use of descriptors calculated for ionised structures did not improve the model statistics for positive and negative APIs, but slightly increased model performance for neutral APIs. The authors attribute this to a better description of neutral zwitterions.

Authors: Rybacka A, Andersson PL. ;Full Source: Chemosphere. 2016 Sep 19; 165:284-293. doi: 10.1016/j.chemosphere.2016.09.014. [Epub ahead of print] ;