Emerging Contaminants in Natural and Engineered Water Environments: Environmental Behavior, Ecological Effects and Control Strategies

1. Introduction to the Special Issue

The acceleration of industrialization and urbanization has rendered water pollution one of the most pressing environmental issues worldwide [1]. Historically, the focus of water pollution research has primarily been traditional pollutants, such as heavy metals, pesticides, and organic compounds [2]. However, in recent years, the emergence of a new class of pollutants—including antibiotics, personal care products (such as shampoos, soaps, and cosmetic ingredients), pharmaceutical residues, industrial chemicals, and their metabolites—has garnered widespread attention [3]. These pollutants are widespread in aquatic environments, have various sources (including daily consumption and industrial production), and pose significant risks to both humans and ecosystems [4].

In natural water bodies, such as rivers and lakes, the sources of emerging contaminants are diverse and complex [5]. They include industrial wastewater discharge, agricultural runoff, and domestic waste and pharmaceutical residues [6]. The nature of these sources is often elusive, with varying discharge methods, making tracing the pollution sources and controlling these pollutants extremely challenging [7]. Furthermore, the behavior of emerging contaminants in water bodies is highly complex, involving processes such as adsorption, dissolution, degradation, and migration, all of which are influenced by a combination of physical, chemical, and biological factors [8]. The varying states and behaviors of pollutants complicate the prediction of their migration pathways and transformation mechanisms, thereby increasing the uncertainty of the effectiveness of pollution control measures [9]. For example, antibiotics in water can disrupt the physiological functions of aquatic organisms, leading to immune suppression, developmental abnormalities, and decreased reproductive success [10]. Some antibiotics persist in the aquatic environment for extended periods and can accumulate through the food chain, potentially affecting higher trophic levels [11]. Research has also shown that antibiotic residues in water may facilitate the spread of antibiotic resistance genes, which can alter microbial community structures and impair water’s self-purification capacity. Although emerging contaminants typically exist in low concentrations, their bioaccumulation potential and long residence times in the environment make them highly toxic [12]. These pollutants can exert organ toxicity, neurotoxicity, reproductive and developmental toxicity, immune toxicity, endocrine-disrupting effects, carcinogenicity, and teratogenicity, leading to persistent ecological harm and even species extinction or a loss of ecosystem functions, threatening the stability of aquatic ecosystems [13]. Emerging contaminants’ cumulative effects and diverse toxicity mechanisms complicate their detection and impact assessment using traditional monitoring methods [14]. Many pollutants are present in low concentrations, making them difficult to detect with standard water-quality-monitoring techniques. They can have potential biological effects on aquatic organisms and, through cumulative exposure, result in long-term ecological damage. As such, the ubiquity and potential hazards of these pollutants necessitate the study of their characteristics, ecological impacts, and control strategies. However, there is currently a lack of in-depth research on the mechanisms of emerging contaminants, presenting significant challenges regarding their management in water environments.

The aim of this Special Issue was to compile the latest research on the identification, characterization, ecological effects, and management of emerging contaminants. We invited contributions from researchers and practitioners in fields such as environmental chemistry, toxicology, engineering, and environmental policy. This Special Issue focuses on the mechanisms of risk formation and strategies for managing emerging contaminants, covering the following topics (among others): (1) contaminant source analysis; (2) environmental behavior; (3) ecological effects and toxicology; (4) risk assessment; and (5) control and mitigation strategies. This Special Issue will serve as a valuable platform for exchanging knowledge, fostering collaboration, and exploring new research avenues in the field of water pollution management.

2. The Main Contributions of This Special Issue

Since the call for papers in November 2023, five manuscripts have been accepted for publication after a thorough review process. Below is a summary of the key highlights from the published papers.

Leung et al. (Contribution 1) investigated the ecological risks parabens, specifically methylparaben and propylparaben, pose to daphnids, an indicator species for aquatic ecosystems. They assessed the toxic effects of these parabens, both individually and in combination, by focusing on phenotypic endpoints such as mortality, feeding behavior, and enzymatic activity. Controlled exposure experiments revealed that propylparaben and its mixture with methylparaben were more toxic than methylparaben alone, significantly altering these endpoints. The results indicated dose-dependent effects, suggesting that even low levels of these contaminants could disrupt key physiological processes in daphnids. Moreover, their study highlighted the synergistic toxic effects of the mixtures, emphasizing the need to consider pollutant mixtures rather than individual chemicals. While this study provides valuable insights into the toxicity mechanisms of parabens, the authors acknowledged a gap in the existing literature: the lack of long-term data on the chronic effects of these contaminants. They identified a need for further research on transgenerational or chronic exposure in order to better reveal the long-term ecological implications of parabens, as such work could provide a more realistic perspective on the persistence and impacts of these pollutants in natural environments. In conclusion, Leung et al. underscored the importance of developing stronger mitigation strategies and monitoring systems to address the growing ecological risks posed by emerging contaminants such as parabens.

Gu et al. (Contribution 2) explored the distribution and component characteristics of dissolved organic matter (DOM) in the Yongding River, focusing on how DOM responds to natural and anthropogenic influences. Using techniques such as the three-dimensional fluorescence excitation–emission matrix (EEM) technique, fluorescence regional integration (FRI), and parallel factor analysis (PARAFAC), they identified three major fluorescent components: C1 (microbial humic-like), C2 (terrestrial humic-like), and C3 (protein-like). The results showed that the DOM in the river was influenced by both microbial and terrestrial sources, with the upper reaches displaying strong autochthonous characteristics and high humification, while the lower reaches were most impacted by human activities. They also established that fluorescence indices such as P_I,n/P_II,n and (C1 + C2)/C3 were useful for evaluating the humification degree of DOM in the river. Gu et al. concluded that their findings provide a novel approach to understanding the spatial distribution and environmental behavior of fluorescent DOM in inland water systems, emphasizing the critical role of DOM in the carbon cycle. Their study highlighted the need for an extensive molecular-level DOM database to further explore DOM’s geochemical behavior.

Zhang et al. (Contribution 3) investigated nitrogen removal efficiency in horizontal subsurface flow constructed wetlands (HSSFCWs) using different treatments. They compared four treatments: aeration with an external carbon source (CW_CA), external carbon source alone (CW_C), aeration alone (CW_A), and a control treatment (CW_CK). The results showed that nitrogen removal rates were significantly higher in the enhanced treatments (CW_CA, CW_A, and CW_C) compared to the control, with CW_CA achieving the highest removal rates for chemical oxygen demand (COD), ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃⁻-N), and total nitrogen (TN). Zhang et al. also observed significant changes in bacterial community structure, particularly in the CW_CA and CW_C treatments, enhancing the abundance of nitrogen-converting bacteria. They concluded that CW_CA significantly improved nitrogen removal by promoting nitrification and denitrification processes. Their study identified the optimal conditions for nitrogen removal, including an aeration rate of 0.80 ± 0.02 L·min⁻¹ and a COD/TN ratio of 5. These findings could improve the efficiency of wastewater treatment by optimizing the composition of functional bacteria.

Lyu et al. (Contribution 4) investigated the impact of acetochlor (ACE) on the soil microbial nitrogen cycle in riparian zones, specifically how pesticide accumulation affects nitrogen transformation. Their 60-day exposure experiment showed that ACE inhibited soil nitrogen transformation, including in terms of a decline in nitrification and denitrification rates. The expression of functional genes involved in nitrate reduction was most significantly reduced at higher ACE doses. This study emphasizes the dose-dependent nature of ACE’s impact, with higher doses causing more significant disruption. Lyu et al. concluded that pesticide accumulation could potentially disrupt microbial nitrogen cycles in riparian soils, impairing nitrogen removal processes. However, this study, based on short-term laboratory conditions, recommended conducting further long-term field studies to assess the ongoing impacts of pesticides in natural environments.

Liang et al. (Contribution 5) explored the use of environmental DNA (eDNA) metabarcoding technology for monitoring biodiversity and assessing the health of aquatic ecosystems. They highlighted eDNA’s advantages over traditional biomonitoring techniques, including timeliness, accuracy, and cost-effectiveness. By combining eDNA sampling with high-throughput sequencing, this technology serves as a sensitive and efficient method for detecting biodiversity changes, particularly in freshwater environments. However, Liang et al. noted the current limitations of this technology, including the need for further optimization. They suggested that integrating eDNA metabarcoding with drone sampling and automatic sequencing could streamline the monitoring process, reducing human error and improving data accuracy. They concluded that eDNA metabarcoding technology holds promise for environmental ecological assessments, offering new insights into aquatic biodiversity monitoring and ecosystem health. They also suggested that future advancements could enhance its application in aquatic ecosystem protection and water quality monitoring.

3. Conclusions and Future Prospects

This Special Issue addresses the growing concern over emerging contaminants, particularly those linked to pharmaceuticals, personal care products, and other anthropogenic pollutants, which increasingly threaten the water environment. The contributions highlight the urgent need for better identification and characterization and a better understanding of the ecological impacts and management strategies associated with these pollutants. The research presented underscores the wide range of ecological effects caused by emerging contaminants. For example, the studies examine the toxicity of parabens with respect to aquatic organisms (Leung et al.), the impact of dissolved organic matter (DOM) in river systems (Gu et al.), and the influence of pesticides like acetochlor on microbial nitrogen cycles in riparian zones (Lyu et al.). Other studies explore nitrogen removal in constructed wetlands (Zhang et al.) and the use of environmental DNA (eDNA) for monitoring biodiversity and ecosystem health (Liang et al.). These studies demonstrate the interdisciplinary nature of environmental pollution research and reveal that while some contaminants have direct toxicological effects, others—such as those affecting microbial processes—may result in long-term, ecosystem-wide consequences. Moreover, these contributions highlight the growing need to develop integrated monitoring tools, such as eDNA metabarcoding technology, to assess biodiversity and pollution levels in aquatic environments with greater sensitivity and efficiency. As our understanding of pollutants’ behaviors in complex environmental systems deepens, opportunities will arise to develop more effective management strategies. However, as the studies included herein note, further long-term research and field studies are required in order to fully understand the persistence and cumulative effects of emerging contaminants.

The advancement of technological tools, including automated eDNA analysis and machine learning algorithms, will play a crucial role in enhancing monitoring capabilities and improving risk assessment processes. To address the increasing complexity of the pollution landscape, future efforts must focus on closing knowledge gaps, improving pollutant detection techniques, and optimizing environmental management strategies to mitigate the negative impacts on aquatic ecosystems and human health. Collaborative research across disciplines—i.e., environmental chemistry, toxicology, engineering, and policy—will be essential for tackling these emerging environmental challenges.