2.3.4.2.6 Adsorption
The adsorption process involves transportation of the solute in the bulk liquid phase, followed by film and pore diffusion, and culminating in adsorption onto the solid phase. Adsorption involves electrostatic interactions, hydrogen bonding, charge-transfer complexation, van der Waals forces, and occasionally strong chemical bonding and electron transfer producing electrostatic interactions (Yahya et al., 2018). The efficiency of adsorption is usually determined by the surface area, pore volume, porous structure, and surface functional groups of the adsorbent. It also depends on the chemical characteristics of the pharmaceutical compounds (Patel et al., 2019). The benefits of the adsorption process for water treatment include low capital investment, low pollutant concentrations can be removed, batch and fluidized/fixed-bed reactor systems can be used, and there are possibilities of regenerating adsorbents for multiple reuse (Gisi et al., 2016; Rosales et al., 2017). Compared to other methods, adsorption removes a wide range of pharmaceuticals and produces less toxic products. Adsorption is also applicable to a wide range of aqueous systems with variable concentration of pollutants (Oliviera et al., 2017). Adsorbents, including biosorbents, activated carbons, biochar, mineral oxides, polymeric materials, and nanomaterials, have been used for the removal of pharmaceuticals in aqueous systems.
2.3.4.3 Hybrid Removal Processes
When used on their own, many treatment methods cannot effectively remove pharmaceuticals from aquatic systems. Integrating multiple methods can be beneficial in enhancing removal. For instance, because microorganisms are susceptible to toxic chemicals, pretreatment with advanced oxidation processes can reduce the death rates of microorganisms, and enhance biodegradation of pharmaceuticals as a result (Patel et al., 2019). In addition, RO processes are sensitive to organics and will benefit from a pretreatment step using carbon filtration, for instance. Adsorption methods have also been combined with photocatalysis to take advantage of the strengths of the individual techniques (Yahya et al., 2018). Overall, integrating different removal methods has great potential; however, the compatibility and synergism of different processes deserves further research.
2.4 Knowledge Gaps and Future Research Directions
Research on pharmaceuticals in aquatic systems is an emerging field replete with several knowledge gaps. Most of these knowledge gaps also apply to other emerging organic contaminants, thus can be considered as generic. A detailed discussion of the knowledge gaps is presented in an earlier review in the context of organic contaminants in Africa (Gwenzi and Chaukura, 2018). The key ones are summarized as follows.
2.4.1 Increasing Africa’s Research Footprint
The bulk of research on pharmaceuticals in aquatic systems is drawn from developing countries, while Africa remains under-represented. Thus, there is need for further research on various aspects of pharmaceuticals in aquatic systems in Africa. This is critical due to the several human exposure risk factors highlighted.
2.4.2 Hotspot Sources and Reservoirs
Studies investigating the sources and reservoirs of pharmaceuticals have largely focused on centralized wastewater systems and receiving aquatic systems. Several potential hotspot sources and reservoirs in developing countries remain under-studied. These include on-site sanitation systems such as septic tanks and pit latrines, solid waste repositories such as non-sanitary landfills or waste dumps, and cemeteries and gravesites.
2.4.3 Behaviour and Fate in Aquatic Systems
Our understanding of the behaviour and fate of pharmaceuticals in aquatic systems remain imperfect. This is because limited long-term data exists on speciation, phase partitioning among various aquatic components, and uptake and bioaccumulation by aquatic organisms including food plants and animals. Moreover, the available data is limited to a few pharmaceuticals, while the biogeochemical behaviour and fate of a myriad of pharmaceuticals are still poorly understood.
2.4.4 Ecotoxicology of Pharmaceuticals and Metabolites
Pharmaceuticals co-occur in aquatic systems with other environmental stressors such as toxic metals, nutrients, and other emerging contaminants. The ecotoxicology of single pharmaceuticals and their mixtures, and their interaction with other environmental stressors at environmentally relevant concentrations has received limited research attention. Thus, ecotoxicological findings based on individual pharmaceuticals investigated at concentrations not representative of those in the environment may yield misleading results.
2.4.5 Human Exposure Pathways
To date, information of the human exposure pathways remain largely qualitative. Few studies have quantitatively estimated the contribution of the various intake pathways to human exposure to pharmaceuticals in aquatic systems. Yet, such data is critical for quantitative human health risk assessment.
2.4.6 Human Toxicology and Epidemiology
Systematic studies investigating human toxicology and epidemiology of pharmaceuticals in aquatic systems are still lacking. Thus, there is need for comprehensive studies based on established research protocols such as case-control experiments to better understand the human toxicology and epidemiology of pharmaceuticals and their metabolites.
2.4.7 Removal Capacity of Low-Cost Water Treatment Processes
Literature investigating the removal of pharmaceuticals in aquatic systems is dominated by water treatment processes used in large-scale centralized systems common in developed countries. The capacity of several low-cost methods (e.g., biosand filtration, solar disinfection, and boiling) commonly used in developing countries to remove pharmaceuticals and their metabolites remains unknown. This calls for further research using field samples such as surface and groundwater contaminated with pharmaceuticals and their metabolites as reported in literature (e.g., Sorensen et al., 2015).
2.5 Summary, Conclusions, and Outlook
The current chapter presented an overview of the occurrence, behaviour, human health risks, and removal of pharmaceuticals in aquatic systems. The major classes of pharmaceuticals detected in aquatic systems were antibiotics, beta-blockers, analgesics, cancer therapeutics, antiinflammatory drugs, lipid regulators, endocrine disruptors, and illicit drugs. Hotspot sources include medical facilities, pharmaceutical industries, veterinary facilities, and municipal wastewater treatment systems which act as reservoirs of pharmaceuticals and their metabolites. Hydrological processes disseminated pharmaceuticals into various aquatic systems, where they undergo sorption, biochemical degradation, phase partitioning, and uptake and bioaccumulation by aquatic organisms. Ingestion of contaminated water and aquatic foods, inhalation, and dermal contact contribute to human exposure. Barring the risk for antimicrobial resistance caused by pharmaceuticals, the evidence linking pharmaceuticals to human health outcomes remain poor. However, human health risks could be significant in Africa and other developing regions due to several risk factors. These exposure risk factors include: (1) the high prevalence of consumption of raw drinking water and aquatic foods from polluted sources, (2) a putative high pharmaceutical pollution associated with intensive use of pharmaceuticals to control the high animal and human disease burden in the tropics, (3) high abuse and misuse of pharmaceuticals caused by the existence of informal markets and weak and poorly enforced environmental, public health, and medicines regulations. The capacity of conventional and advanced water treatment processes to remove pharmaceutics in aqueous systems was discussed. Finally, future research directions were highlighted to address the lack of comprehensive data on the ecotoxicology, epidemiology, and behaviour and fate of pharmaceuticals in aquatic systems especially in the tropics.
Author Contributions