Persistent and Emerging Organic Contaminants in Natural Environments

1. Introduction to the Special Issue

In recent decades, the world has experienced the detrimental effects of the unchecked growth of various human activities, including industrialization, transportation, agriculture, and urbanization [1]. Improvements in living standards and the increasing demand for consumer goods have intensified air, water, and soil pollution, particularly through the release of persistent organic pollutants (POPs) and emerging contaminants (ECs) [2]. Air pollution, for example, is primarily driven by anthropogenic sources such as combustion processes related to industrial and civil activities, energy production, and transportation—including both the vehicular and maritime sectors. These activities release harmful substances such as POPs, greenhouse gasses, and particulate matter [1,3]. Similarly, water pollution is closely linked to human activities in urban, agricultural, and industrial contexts, which lead to the release of pollutants such as chemicals, nutrients, leachates, and oil spills [4]. Wastewater treatment plants (WWTPs), while essential for managing municipal, hospital, industrial, and agricultural discharges, have limited effectiveness in removing certain classes of ECs, highlighting a critical gap in their capacity to mitigate these contaminants [1,5]. Soil contamination arises from hazardous waste disposal, pesticide application, sludge use, and the accumulation of non-biodegradable materials, and is exacerbated by insufficient waste management infrastructures [1].

Given the interconnected nature of environmental compartments, contaminants can transfer between media depending on their physicochemical properties, the environmental conditions, and the characteristics of the receiving medium [6]. This transfer process shapes the distribution of contaminants within natural ecosystems, influencing their potential for bioaccumulation, biomagnification, and adverse effects on organism health [2,6]. Furthermore, contaminants can be transported over long distances via oceanic and atmospheric currents or through migratory animals, reaching regions far from their original emission source, including remote polar areas [7]. This phenomenon is well-documented in certain groups of pollutants, such as POPs, but represents an emerging challenge for other classes of contaminants, including emerging pollutants. This issue warrants particular attention in the context of climate change, which significantly influences the transport, persistence and emission dynamics of contaminants.

POPs can be either man-made or naturally occurring compounds, characterized by their intrinsic resistance to natural degradation processes, allowing them to persist in the environment for extended periods [8]. These pollutants are regulated under the Stockholm Convention, an international treaty adopted in 2001, which aims to eliminate or restrict the production, use, and release of POPs (Stockholm Convention, 2001). The Convention facilitated the phasing-out of many harmful chemicals and promotes the safe disposal of existing stockpiles. Initially addressing 12 POPs, known as the ‘dirty dozen’, the treaty has progressively expanded to include additional substances. Despite these regulations, POPs continue to persist in the environment, including in remote polar regions, due to their past production, ongoing use, and environmental persistence [9]. These contaminants are widely distributed through both natural and human-induced processes, moving across soil, water, and especially air. POPs are particularly prone to long-range transport via atmospheric and oceanic currents. Their lipophilic nature and persistence enable POPs to bioaccumulate in organisms, including humans, and biomagnify through food chains, resulting in higher concentrations at higher trophic levels. These substances pose significant risks to human health and environmental integrity [7,8,9]. POPs have been linked to cancer, allergies, hypersensitivity, and damage to the central and peripheral nervous systems. They also adversely affect the reproductive and immune systems of exposed organisms. Furthermore, some POPs are classified as endocrine disruptors, meaning that they interfere with hormonal systems, reducing the fitness of organisms and affecting ecosystem functioning [8]. Examples of POPs include dioxins, insecticides, polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs).

ECs are unregulated substances—both known and unknown—(Norman network), and knowledge regarding their persistence, environmental fate, behavior, and (eco)toxicological effects is limited or incomplete. For certain compounds, these aspects remain entirely unknown [10]. These human-made chemicals include pesticides, cosmetics, pharmaceuticals, personal care products (PPCPs), illicit drugs, hormones, endocrine-disrupting compounds, and micro- and nano-plastics. Although these substances play a significant role in modern society, they pose increasing environmental risks due to their widespread use and potential ecological impacts [10].

The global production of synthetic ECs has risen dramatically, from 1 million tons per year in 1930 to 400 million tons by 2000 [1]. EUROSTAT data from 2013 revealed that over 50% of chemical production between 2002 and 2011 involved substances harmful to the environment, with more than 70% of these chemicals exerting significant ecological impacts. It is further estimated that the global production of such pollutants has increased to approximately 500 million tons annually [10,11].

Scientists are increasingly concerned about the impact of PPCPs on ecosystems and human health, in alignment with the One Health concept [12]. PPCPs, which are designed to elicit specific biological responses in target organisms, can also induce similar effects in non-target species, even when present at trace levels over extended periods [10,13]. Between 5% and 95% of ingested PPCPs are excreted in their active form through feces and urine, ultimately entering the environment and contaminating soils, water, and plants [14,15]. Certain endocrine disruptors (EDs) interfere with hormonal systems, with adverse effects on reproduction, neurological function, development, and immunity in humans and animals [10]. Additionally, some EPs, such as non-steroidal anti-inflammatory drugs (NSAIDs) like acetylsalicylic acid, paracetamol, diclofenac, ibuprofen, and naproxen, can induce sub-lethal effects in non-target aquatic organisms, such as freshwater invertebrates, even at low concentrations [16]. Musk fragrances and plasticizers, although widely detected in human tissues, including adipose tissue, breast milk, and blood, remain poorly understood in terms of their potential risks to human health and the environment [13].

Particular attention has been paid to antibiotics, which are extensively used in both veterinary and human medicine. Their widespread use, their improper disposal, and the low removal efficiency of WWTPs contribute significantly to the contamination of natural ecosystems. For instance, antibiotics can enter the environment through the application of manure or sludge as fertilizers on agricultural land or via the use of wastewater for crop irrigation [17,18]. Once in the environment, antibiotic residues can adversely affect biota across various trophic levels and pose risks to human health. This occurs through the consumption of contaminated food and water and through fostering the development of antibiotic-resistant bacteria. The resulting selective pressure promotes maintenance and spread resistance across different environmental compartments [18].

Consequently, monitoring these pollutants and understanding their transformation and degradation pathways in various environmental media have become critical areas of focus. This is particularly pressing given the significant rise in the use of PPCPs over the past century, a trend further accelerated by the COVID-19 pandemic [12].

Enhancing the efficiency of wastewater treatment systems is therefore essential to mitigate the escalating issue of environmental contamination and its associated risks.

2. A Summary of the Special Issue

This Special Issue comprises five research papers and one review, addressing various topics related to contamination, including biodegradation, advancements in pollutant removal technologies, bioindicators of ecotoxicological effects, and bioremediation. These studies focus on contaminants such as PPCPs, including diatrizoate, iohexol, iodipamide, flufenamic acid, diclofenac, sulfamethoxazole, paracetamol, fluoxetine, and 17 α-ethinylestradiol, as well as petroleum organic pollutants, such as hydrocarbons.

Konopka et al. [19] investigated the removal efficiency of three iodinated contrast media (ICM)—diatrizoate, iohexol, and iodipamide—from synthetic hospital wastewater using anaerobic membrane bioreactors (MBRs) with varying sludge ages (40, 70, and 100 days). Their results demonstrated that MBR performance improved significantly with an increase in sludge age. However, no clear relationship was observed between sludge age and the removal rates of iohexol and iodipamide. The authors emphasized that the sludge age is a critical parameter to optimize for effective ICM removal in wastewater treatment systems.

Yang et al. [20] examined groundwater contamination resulting from petroleum spills in an oil field. Their study demonstrated that compound-specific markers and biochemical parameters are effective tools for understanding the fate and transport of petroleum contaminants, thereby facilitating cost-effective remediation strategies. The authors identified a petroleum source, noting a decreasing trend in fresh petroleum input and an increase in biodegradation potential farther from the source. Biodegradation, primarily driven by microbial activity, emerged as the main process for hydrocarbon removal, with low-weight hydrocarbons degrading more rapidly, especially under higher temperature conditions.

Kamenická et al. [21] contributed to improving the reuse of exhausted adsorbents in water treatment by enhancing the adsorption capacity of activated carbon for flufenamic acid and diclofenac using benzalkonium chloride (BAC). The improvement in adsorption was attributed to the formation of less polar ion pairs, which exhibit a stronger affinity for the non-polar activated carbon surface.

Kalka and Drzymała [22] assessed the use of Vicia faba as a bioindicator to evaluate the toxicity of wastewater containing diclofenac and sulfamethoxazole. They measured the activity of the antioxidant enzymes catalase and superoxide dismutase in plant leaves. The study showed that Vicia faba effectively detects genotoxicity in wastewater, providing a simple and rapid method for identifying a broad range of toxic effects, although it does not reveal their mutagenic impacts.

Palma et al. [23] evaluated the bioremediation potential of Micrococcus yunnanensis TJPT4 for the removal of paracetamol, fluoxetine, and 17α-ethinylestradiol in aquatic environments. Notably, this study is the first to report the presence of Micrococcus yunnanensis in Filograna implexa. The microorganism exhibited high drug removal efficiency, making it a promising candidate for bioremediation, particularly in bioaugmentation processes in environments impacted by saline intrusions.

The review by Liberti et al. [24], proposed the use of microalgae as sustainable solution for water purification, capitalizing on their natural capacity to absorb and transform pollutants, including nutrients and heavy metals. The authors highlighted a significant scientific gap regarding the utilization of the biomass generated during the treatment process. They emphasized the need for a comprehensive approach that addresses the entire lifecycle of microalgae, from wastewater treatment to innovative biomass applications, while considering both the environmental and economic aspects of this process.