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Evidence of ecotoxicological effects of active pharmaceuticals ingredients (APIs) has increased research into their environmental fate. In low and low-middle income countries (LLMICs) the main source of APIs to surface waters is from discharge of untreated wastewater. Consequently, concentrations of APIs can be relatively high in the "impact zone" downstream of a discharge point. Little is known about the fate of APIs in these impact zones. In this laboratory scale investigation, the effect of successive dilution of synthetic untreated wastewater (dilution factor 1 to 10) on the distribution of APIs was studied. The sorption was consistent with the chemical properties of each compound: charge, lipophilicity, and structure. Dilution increased desorption of the basic and neutral APIs (up to 27.7%) and correlated with their lipophilicity (R>0.980); the positive charge was of secondary importance. Anions did not significantly desorb (<10% loss). Increased concentrations of dissolved organic matter at dilutions of 8 and 10 times that of untreated wastewater coincided with lower dissolved API concentrations. The data showed a clear trend in the desorption process of APIs that may lead to higher exposure risk than anticipated. Therefore, it is suggested that these aspects should be accounted for in the development of dedicated environmental risk assessment approach for APIs in riverine impact zones of LLMICs countries.

The global consumption and production of pharmaceuticals is increasing concomitantly with concern regarding their environmental fate and effects. Active pharmaceutical ingredients are mainly released into the aquatic environment through wastewater effluent discharge. Once in the environment, pharmaceuticals can undergo processes of natural attenuation, i.e. dilution, sorption, transformation, depending on physico-chemical properties of the compound, such as water solubility, lipophilicity, vapour pressure, and environmental conditions, such as pH, temperature and ionic strength. A major natural attenuation process is the sorption on dissolved organic matter, colloids, suspended solids and sediments, which in turn control pharmaceuticals distribution, residence time and persistence in aquatic systems. Here we review studies of sorption capacity of natural sorbents to pharmaceuticals. These report on the importance of several environmental and sorbent-specific properties, such as the composition, quality, and amount of the sorbent, and the environmental pH, which determines the speciation of both the sorbent and compound. The importance of accounting for distribution processes on freshwater sorbents for any determination of environmental concentrations of pharmaceuticals is apparent, while the reliability of surrogate standards for measuring dissolved organic matter (DOM) distribution is evaluated in the context of the need for robust environmental risk assessment protocols.