Microplastics as hazardous pollutants: Occurrence, effects, removal and mitigation by using plastic waste as adsorbents and supports for photocatalysts

Most sectors in our society use plastic-based materials from different polymer sources due to their different properties and wide range of applications. Combined with population growth, this has caused a vertiginous increase in plastic waste. In the last eight decades, plastic production has resulted in over 8.3 billion metric tons, and unfortunately, around 80% of these plastics have been discarded into the environment, causing harmful effects. The lack of management and control of plastic waste results in the presence of microplastics (MPs) in the environment. This affects food chains and causes problems for biodiversity, the environment and human health. In this review, the classification, environmental occurrence, hazardous effects and management of MPs in wastewater treatment plants are discussed in detail. Other issues associated with the presence of MPs are also addressed; for example, the role played by MPs as vehicles for a range of hazardous chemical contaminants, such as pharmaceuticals, hormones and pesticides, “accidentally” contaminating aquatic organisms that feed on these materials. This review also highlights the technologies that have had satisfactory results in recent years in the removal and/or degradation of MPs, such as heterogeneous photocatalysis, adsorption, and electrochemical processes. We also point out that, in order to mitigate the problem, plastic waste can be transformed into high value-added products, such as adsorbents (adsorption capacity: 0.2–1753 mg g-1) and/or supports for photocatalytic materials used in heterogeneous photocatalysis for the degradation of organic contaminants, a subject that is still little discussed in the literature. Finally, during the review, the challenges of this line of investigation are also presented. These insights will offer possibilities and strategies for the development of multifunctional materials based on plastic waste, applied in processes such as adsorption and heterogeneous photocatalysis, as well as for understanding the importance of monitoring MPs in aqueous media, due to the various problems associated with their presence as waste, as vehicles for emerging contaminants, and as possible releasers of organic contaminants arising from additives or their photoinduced weathering.