## 1 Introduction
### Main Classes of Emerging Contaminants and Their Environmental Significance  
Emerging contaminants encompass a wide variety of chemical substances with diverse sources and properties; the main classes include the following (Figure 1) [2; 3; 4; 7; 8; 9].  
* Pharmaceuticals and Personal Care Products (PPCPs):  
This category includes prescription and over-the-counter drugs, antibiotics, analgesics, hormones, and ingredients found in cosmetics, fragrances, sunscreens, and soaps. These compounds enter water systems through excretion, improper disposal and runoff from agricultural activities involving veterinary drugs [10; 11].  
* Endocrine-Disrupting Compounds (EDCs):  
EDCs are a subset of ECs that interfere with hormonal systems in animals and humans. They include natural and synthetic hormones, certain pesticides, plasticizers (like bisphenol A), and industrial chemicals. These compounds can lead to reproductive and developmental abnormalities even at very low exposure levels [12; 13].  
* Per- and Polyfluoroalkyl Substances (PFASs):  
PFASs are a group of highly persistent, synthetic, fluorinated compounds used in firefighting foams, non-stick cookware, water-repellent clothing, and food packaging. Their strong carbon-fluorine bonds make them resistant to degradation, leading to their nickname as \"forever chemicals\" [14; 15].  
* Industrial and Household Chemicals: This category encompasses solvents, flame retardants (e.g., polybrominated diphenyl ethers or PBDEs), plasticizers (e.g., phthalates), surfactants (e.g., nonylphenol), and other compounds used in manufacturing or domestic cleaning agents [16; 17].
* Pesticides and Herbicides: While many are regulated, newer formulations or breakdown products may still be unregulated and classified as emerging. These substances can leach into surface and groundwater from agricultural fields and urban landscapes [18; 19; 20].
* Nanomaterials: Engineered nanoparticles such as silver, titanium dioxide, and carbon nanotubes are increasingly used in consumer products. Their small size and unique properties raise questions about their environmental fate, transport, and potential toxicity [7; 21; 22].
* Artificial Sweeteners and Food Additives: Compounds like sucrose and acesulfame-K are widely used and excreted unchanged, making them markers of anthropogenic pollution in water bodies. Though not acutely toxic, their environmental persistence is a concern [23; 24].
* Microplastics and Nanoplastics: Fragmented plastic particles less than 5 mm (microplastics) or smaller than 100 nm (nanoplastics) have been identified in freshwater, marine, and drinking water sources. The complexity, diversity, and persistence of emerging contaminants present significant challenges for water treatment systems worldwide [25; 26].  
Figure 1: Types of ECs (schematic representation of major classes of emerging contaminants (ECs), including pharmaceuticals, endocrine-disrupting chemicals (EDCs), personal care products (PCPs), industrial chemicals, and nanomaterials). Each category is associated with potential environmental or health impacts, highlighting the complexity of EC sources and their relevance in water treatment research (reused from [PERSON] et al., 2024 [7], under the Creative Commons CC-BY-NC-ND license and permits non-commercial use of the work as published, without adaptation or alteration provided the work is fully attributed).  
These trace-level contaminants pose major challenges to conventional water treatment technologies, which often fail to ensure complete removal and may generate harmful byproducts [2; 7].