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More than 50% of pharmaceuticals in current use are chiral compounds. Enantiomers of the same pharmaceutical have identical physicochemical properties, but may exhibit differences in pharmacokinetics, pharmacodynamics and toxicity. The advancement in separation and detection methods has made it possible to analyze trace amounts of chiral compounds in environmental media. As a result, interest on chiral analysis and evaluation of stereoselectivity in environmental occurrence, phase distribution and degradation of chiral pharmaceuticals has grown substantially in recent years. Here we review recent studies on the analysis, occurrence, and fate of chiral pharmaceuticals in engineered and natural environments. Monitoring studies have shown ubiquitous presence of chiral pharmaceuticals in wastewater, surface waters, sediments, and sludge, particularly β-receptor antagonists, analgesics, antifungals, and antidepressants. Selective sorption and microbial degradation have been demonstrated to result in enrichment of one enantiomer over the other. The changes in enantiomer composition may also be caused by biologically catalyzed chiral inversion. However, accurate evaluation of chiral pharmaceuticals as trace environmental pollutants is often hampered by the lack of identification of the stereoconfiguration of enantiomers. Furthermore, a systematic approach including occurrence, fate and transport in various environmental matrices is needed to minimize uncertainties in risk assessment of chiral pharmaceuticals as emerging environmental contaminants.

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Biosolids are extensively used in agriculture as fertilizers while offering a practical solution for waste disposal. Many pharmaceutical and personal care products (PPCPs), such as triclosan and triclocarban, are enriched in biosolids. Biosolid amendment changes soil physicochemical properties, which may in turn alter the persistence of PPCPs and hence the risk for secondary contamination such as plant uptake. To delineate the effect of biosolids on PPCPs persistence, triclosan and triclocarban were used as model compounds in this study and their sorption (Kd) and persistence (t1/2) were determined in different soils before and after biosolid amendment. Biosolids consistently increased sorption of triclosan and triclocarban in soil. The Kd of triclosan increased by 3.9-21 times following amendment of a sandy loam soil with biosolids at 2-10%. The persistence of both compounds was prolonged, with t1/2 of triclosan increasing from 10 d in the unamended soil to 63 d after biosolid amendment at 10%. The relationship between t1/2 and Kd was further examined through a meta-analysis using data from this study and all relevant published studies. A significant linear relationship between t1/2 and Kd was observed for triclosan (r(2) = 0.69, p < 0.01) and triclocarban (r(2) = 0.38, p < 0.05) in biosolid-amended soils. On the average, when biosolid amendment increased by 1%, t1/2 of triclosan was prolonged by 7.5 d, while t1/2 of triclocarban was extended by 4.7 d. Therefore, biosolid amendment greatly enhances persistence of triclosan and triclocarban, likely due to enhanced sorption or decreased chemical bioavailability. This finding highlights the importance to consider the effect of biosolids when evaluating the environmental risks of these and other biosolid-borne PPCPs.

Research interest in chiral pesticides has increased probably because enantiomers often exhibit different environmental fate and toxicity. An investigation into the enantiomer-specific bioactivity of chiral triticonazole enantiomers in agricultural systems revealed intriguing experimental and theoretical evidence. For nine of the phytopathogens studied ( Rhizoctonia solani, Fusarium verticillioide, Botrytis cinerea (strawberry and tomato), Rhizoctonia cereali, Alternaria solani, Gibberella zeae, Sclerotinia sclerotiorum, and Pyricularia grisea), the fungicidal activity data showed ( R)-triticonazole was 3.11-82.89 times more potent than the ( S) enantiomer. Furthermore, ( R)-triticonazole inhibited ergosterol biosynthesis and cell membrane synthesis 1.80-7.34 times higher than its antipode. Homology modeling and molecular docking studies suggested the distinct bioactivities of the enantiomers of triticonazole were probably due to their different binding modes and affinities to CYP51b. However, field studies demonstrated that ( S)-triticonazole was more persistent than ( R)-triticonazole in fruits and vegetables. The results showed that application of pure ( R)-triticonazole, with its high bioactivity and relatively low resistance risk, instead of the racemate in agricultural management would reduce the application dosage required to eliminate carcinogenic mycotoxins and any environmental risks associated with this fungicide, yielding benefits in food safety and environmental protection.

Recurrent cholera outbreaks in sub-Saharan Africa (SSA) attracted a lot of research interest, raising questions about the effectiveness of current prevention and control methods. However, research on cholera and other water-borne diseases in Africa is dominated by epidemiological studies, while investigations on the environmental drivers and reservoirs of cholera remain scarce. The current discourse relating cholera to the environment in SSA is often limited to the rudimentary statement that, “cholera is caused by the consumption of contaminated water and food”. Yet, beyond this simplistic view, literature elsewhere shows that cholera outbreaks are controlled by its complex interactions with environmental drivers and reservoirs. This brings to question whether cholera can be eradicated in SSA without understanding these complex interactions. The current review seeks to (1) highlight the nature and dynamics of recent cholera outbreaks in SSA, (2) discuss the importance of environmental reservoirs of Vibrio cholerae, and anthropogenic and hydroclimatic drivers in controlling the dynamics of cholera outbreaks, and (3) highlight key knowledge gaps and future research directions, and the need to harness emerging research tools such as modeling, machine learning, data mining, and genomics techniques to better understand the cholera dynamics. By bringing to fore these often-overlooked issues in cholera research, we seek to stimulate discussion, and promote a shift toward cross-disciplinary research on cholera and other water-borne diseases in SSA and beyond.

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