Increasing
chemical pollution of aquatic environments is a growing
concern with global relevance. A large number of organic chemicals
are termed as “micropollutants” due to their low concentrations,
and long-term exposure to micropollutants may pose considerable risks
to aquatic organisms and human health. In recent decades, numerous
treatment methods and technologies have been proposed to remove micropollutants
in water, and typically several micropollutants were chosen as target
pollutants to evaluate removal efficiencies. However, it is often
unclear whether their toxicity and occurrence levels and frequencies
enable them to contribute significantly to the overall chemical pollution
in global aquatic environments. This review intends to answer an important
lingering question: Which micropollutants or class of micropollutants
deserve more attention globally and should be removed with higher
priority? Different risk-based prioritization approaches were used
to address this question. The risk quotient (RQ) method was found
to be a feasible approach to prioritize micropollutants in a large
scale due to its relatively simple assessment procedure and extensive
use. A total of 83 prioritization case studies using the RQ method
in the past decade were compiled, and 473 compounds that were selected
by screening 3466 compounds of three broad classes (pharmaceuticals
and personal care products (PPCPs), pesticides, and industrial chemicals)
were found to have risks (RQ > 0.01). To determine the micropollutants
of global importance, we propose an overall risk surrogate, that is,
the weighted average risk quotient (WARQ). The WARQ integrates the
risk intensity and frequency of micropollutants in global aquatic
environments to achieve a more comprehensive priority determination.
Through metadata analysis, we recommend a ranked list of 53 micropollutants,
including 36 PPCPs (e.g., sulfamethoxazole and ibuprofen), seven pesticides
(e.g., heptachlor and diazinon), and 10 industrial chemicals (e.g.,
perfluorooctanesulfonic acid and 4-nonylphenol) for risk management
and remediation efforts. One caveat is that the ranked list of global
importance does not consider transformation products of micropollutants
(including disinfection byproducts) and new forms of pollutants (including
antibiotic resistance genes and microplastics), and this list of global
importance may not be directly applicable to a specific region or
country. Also, it needs mentioning that there might be no best answer
toward this question, and hopefully this review can act as a small
step toward a better answer.
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