Natural Attenuation of Pharmaceuticals in the Aquatic Environment and Role of Phototransformation

Abstract

Pharmaceutical compounds are categorized as contaminants of emerging concern owing to their widespread consumption, persistence in the aquatic environment, development of antibiotic resistance in microorganisms, and growing evidence of chronic and endocrine disruption effects. Therefore, pharmaceuticals have been reported to exist in numerous environmental compartments, including surface water, groundwater, and drinking water throughout the world. The principal pathway of entry of these compounds into water bodies is via treated and untreated sewage. In aquatic environments, natural attenuation of pharmaceuticals takes place through various physical, chemical, and biological processes, including dispersion and dilution, volatilization, sorption onto sediments, biotransformation, and phototransformation. Among the listed attenuation processes, both direct and indirect phototransformation play an important role in deciding the fate of pharmaceuticals. In addition to cations and anions present in the aquatic environment, dissolved organic matter (DOM) can have multiple impacts during photolytic degradation, such as photosensitization, light screening, scavenging, and oxidative inhibition. The absorption of UV rays by DOM results in the formation of a variety of photochemically produced reactive radicals, such as triplet excited states of DOM (3DOM), hydroxyl radical (HO•), and singlet oxygen (1O2). This work presents a thorough assessment of various attenuation processes, with specific attention to aqueous photochemistry of pharmaceuticals evaluated using laboratory-scale and on-site observations. This review will highlight the general extent of natural attenuation for specific classes of pharmaceuticals and focus on the governing mechanisms—underlying such attenuation processes based on studies conducted worldwide.