Occurrence, Fate And Effects Of Medicinal Substances In The Environment

The pharmaceutical industry has expanded greatly during the last century, and this growth also means an increase in chemical waste. Organic pollutants, arising from this source, are among the major concerns to the scientific community. Approximately 3000 different pharmaceutical ingredients are reportedly used, resulting in tons of waste products being discharged in effluents each year. Owing to their diverse chemical compositions, these chemicals make one universal solution unfeasible and raises such questions as: should we enforce the 3R (reduce, reuse and recycle) strategy, develop Greener technologies or focus on wastewater remediation? Currently the latter is proving to be both the easiest to achieve and to return satisfactory results.

Despite there being as early mentions as the 1960s that dealt with the fate of pharmaceutical compounds after their intended use, the topic first started to gain recognition by a review article from Richardson and Bowron in 1985 and the work of Jørgensen and Halling-Sørensen in the 2000s. The former dealt with the question about the risk of long-term exposure to micro-concentrations of pharmaceuticals for the public. The domestic source was identified as main reason for pharmaceutical pollution in potable water supplies compared to the pharmaceutical industry, which is more controlled by guidelines like GMP. The lack of data about the biodegradation of these compounds was also addressed by Halling-Sørensen, which emphasized that the physico-chemical properties of such compounds that make them good in the clinical use, are also often responsible for bioaccumulation. It was concluded that new pharmaceuticals should be ecotoxicologically tested in sewage treatment works and further research on the persistence of medical substances and their risk on the environment needed to be performed. Major water pollutants, collectively termed as emerging contaminants (ECs), generally include such pharmaceuticals (PhACs) as antibiotics (half of total personal care product (PPCP) contamination), analgesic/anti-inflammatory drugs, hormones and cardiovascular therapeutics to name but a few.

The common pathways of the drugs' emergence in water systems include excretion and improper disposal. Excretion is not such as a minor issue as it might appear. Many pharmaceuticals are only partially transformed via metabolic processes. In sewage treatment plants (STPs) they can often be found as polar conjugates, and their removal efficiency is currently far from ideal. The main means of elimination can be attributed to biodegradation, however, low interaction tendencies with the solid surfaces favour the drugs' mobility and thus hinders their removal efficiency. The consequence is large discharges into the aquatic environment, where pharmaceuticals may not only cause individual toxicity even at trace concentrations (ng/L or mg/L), but may also exhibit mixture toxicity, as well as non-target effects.

The number of studies on the problem and thus knowledge is scarce. Introducing a sustainable strategy against the aquatic contamination remains a major challenge. The dominating primary and secondary treatments, i.e. adsorption and biodegradation respectively may be insufficient. Evidently, exposure to antibiotics and β-blockers can be toxic and /or inhibitory to the sludge bacteria. The urgency of tertiary systems is therefore growing. Research within advanced oxidation processes (AOP) including photo-Fenton, electrochemical oxidation, radiation and ozonation have reported successful results at lab scale for the elimination of a wide range of pharmaceuticals. AOP utilize highly reactive oxygen species with low selectivity such as hydroxyl radicals (HO.), H2O2, O3 and superoxide anion radicals (O2.-) to progressively oxidize organic compounds, yielding oxidized intermediates or, in the case of complete mineralization, the production of CO2, H2O and inorganic ions or acids.

There is a significant increase in the number of studies from the last eight years regarding AOP applications in tackling the removal of pharmaceuticals. It is evident that AOPs are efficient in their degradation. However, the identification of transformation products and toxicity levels are crucial, since those compounds might be more biologically active or toxic than their precursors. Moreover, the complex nature of wastewater samples makes the identification of degradation products challenging. A close examination of the quality of treated effluents, particularly the toxicity level, which could be affected due to oxidation by-products, is thus crucial. Developing an AOP degradation protocols on mixtures of pharmaceuticals, given that they do not occur individually, should also be a goal of the future research. Additionally, it is important that environmentally relevant conditions (water matrices, concentration of pharmaceuticals etc.) are accounted for when planning an experimental design for pharmaceutical degradations.

AOP using TiO2 photocatalysis is an emerging field of research where mercury-based lamps have been utilized. It has been proposed that future research should explore the usage of UV-LED instead of mercury-based lamps because of its long-life times, it is more robust and consumes less energy. Even though there is a future potential in the AOP technology, it has been considered to be limited by the expensive energy requirements. Its effectivenessbeyond the laboratory setup is also yet to be demonstrated. Therefore, it has been regarded that a more feasible alternative nowadays is adsorption onto activated carbon. This method is already in use in Switzerland where most of the WWTPs have been optimized including activated carbon adsorption processes.

Although these materials hold many benefits to them, such as low cost and large specific surface areas, there are also several deficiencies, namely large ash contents and volatile impurities. The application of innovative carbon materials of very high quality with reproducible textural and chemical properties is therefore a research area within removal of pharmaceuticals in water for the near future. A recent study (2018) utilized carbide-derived carbons which appeared as promising candidates as they proved to be highly efficient adsorbents for the removal of pharmaceuticals from water.

As with many other industries, the awareness of how waste affects us and the environment is growing. Fresh water comprises only about 2.5 % of the total Earth's water. What is more, only a fraction of it is readily accessible, leaving around a billion people with no access to this vital commodity. WHO has estimated 500 million yearly deaths, caused by consumption of contaminated water. Purification of the water waste is thus an important research area. Today our cleaning and waste control has come a long way since the awareness was raised in the 60s, but problems still arise and are being worked on.