Occurrence, Risk Assessment, and Removal of Emerging Contaminants in Aquatic Environment

1. Introduction

The pervasive presence of emerging contaminants (ECs) in aquatic environments represents a significant and growing challenge for environmental science and public health in the 21st century [1,2,3]. Industrialization, agricultural intensification, and expanding urbanization have led to an explosive increase in the diversity and quantity of synthetic chemicals entering water systems [1,4]. These ECs encompass a wide range of substances, including pharmaceuticals and personal care products (PPCPs) [5,6], novel persistent organic pollutants (POPs) like per- and polyfluoroalkyl substances (PFASs) [7,8], endocrine-disrupting chemicals (EDCs) [9], disinfection by-products (DBPs) [10,11], engineered nanomaterials [12], microplastics [13], and flame retardants [14]. While often detected at trace concentrations (ng/L to µg/L levels), their continuous input into aquatic ecosystems, coupled with their potential persistence, bioaccumulation, and inherent biological activity, raises substantial concerns regarding long-term ecological impacts [15] and risks to human health [16].

Recent advancements in analytical chemistry have significantly improved our ability to detect and quantify these micropollutants in various environmental compartments, including wastewater [17,18], surface water [19], groundwater [20], and even drinking water [21]. However, significant knowledge gaps remain. The complex environmental fate, transport pathways, transformation products, and potential synergistic effects of EC mixtures are often poorly understood. Furthermore, conventional wastewater treatment plants (WWTPs) were not originally designed to remove these refractory compounds, often exhibiting limited and variable removal efficiencies, sometimes even acting as conduits for ECs and associated risks like antibiotic resistance genes (ARGs) into receiving waters. Consequently, there is an urgent need for research focusing on the comprehensive assessment of EC occurrence and risks, alongside the development and optimization of effective removal technologies and sustainable management strategies.

This Special Issue, “Occurrence, Risk Assessment, and Removal of Emerging Contaminants in Aquatic Environments”, compiles ten original research articles and reviews that address these critical needs. The contributions herein explore the detection, fate, transport, and risk assessment of various ECs, evaluate conventional and advanced removal processes, and investigate novel management approaches, collectively advancing our understanding and offering potential solutions for safeguarding water quality.

2. An Overview of Published Articles

This Special Issue compiles 10 insightful contributions covering a diverse range of topics central to the theme of emerging contaminants in aquatic systems.

Contributions 1, 2, and 5 delve into the biological treatment of specific recalcitrant organic pollutants, focusing on co-metabolism strategies. Wang et al. (Contribution 1) present a feasibility study utilizing excess sludge fermentation broth (FB) as an alternative, internal carbon source for the co-metabolic degradation of 2,4,6-trichlorophenol (2,4,6-TCP) in a sequencing batch reactor (SBR) system. Their findings demonstrate that this coupled system achieved stable and efficient 2,4,6-TCP removal (up to 240.13 mg/L) without external carbon sources, with the fermentation process yielding high concentrations of volatile fatty acids (VFAs), suggesting a cost-effective and sustainable approach. Wang and Li (Contribution 2) extended this work by exploring the degradation characteristics and underlying microbial mechanisms of using sludge FB. Through batch experiments and metagenomic analysis, they identified an optimal FB concentration range, pinpointed the inhibitory effects of specific VFAs (propionic acid), and revealed the enrichment of key chlorophenol degradation genes (e.g., PcpA, chqB, or fadA) and the associated bacterial genera like Ralstonia within the adapted sludge, providing deeper insights into the process’s dynamics and genetic basis. Wang et al. (Contribution 5) further investigated the role of the carbon source by systematically comparing the effects of four common, easily degradable substrates (methanol, ethanol, sodium acetate, and sodium propionate) on 2,4,6-TCP degradation. Their results indicate that sodium acetate promoted the highest 2,4,6-TCP removal efficiency and stimulated the production of protective extracellular polymeric substances (EPSs), while different carbon sources led to distinct shifts in the microbial community structure and the abundance of functional metabolic genes, highlighting the significant influence of co-substrate choice on treatment performance and microbial ecology.

Contribution 3 explores advanced oxidation processes, specifically photocatalysis, for antibiotic removal. Lin et al. present the development and evaluation of novel composite photocatalytic membranes. Utilizing electrospinning, they successfully immobilized TiO₂ and TiO₂-reduced graphene oxide (rGO) onto polyacrylonitrile (PAN) nanofibers. Characterization confirmed the successful integration of the photocatalysts. Under simulated solar light, these PAN-TiO₂ and PAN-rGTi membranes exhibited significantly enhanced degradation rates for sulfamethoxazole (SMX) and enrofloxacin (ENR) compared to unmodified PAN membranes, addressing the challenge of catalyst recovery associated with powder forms. Importantly, the membranes demonstrated good stability and recyclability over five operational cycles, suggesting their potential for practical water purification applications.

Wastewater and sludge management strategies relevant to contaminant control are addressed in Contributions 4 and 7. Yang et al. (Contribution 4) report on a comprehensive full-chain approach to reprocessing historically landfilled municipal sludge in Shanghai, a significant challenge in megacities. Their study details the process from sludge extraction under geomembrane cover, through chemical conditioning (using PFSS/PEA or PAS/PEA) and subsequent dewatering to ~60% moisture content, to the physio-chemical treatment of the high-strength leachate generated and the evaluation of resource utilization pathways via co-incineration in power plants and solidification/modification for use as construction backfill. This work provides valuable insights into managing legacy waste streams that can act as reservoirs for various pollutants and reclaiming valuable urban land. Lu et al. (Contribution 7) focused on optimizing a critical WWTP process—chemical phosphorus removal. They developed an intelligent chemical dosing system for polyaluminum chloride (PAC) based on a feedforward prediction model combined with an adaptive fuzzy neural network P feedback controller. Pilot-scale experiments validated the optimal dosage, and subsequent implementation in a full-scale plant resulted in precise dosing control, achieving 100% effluent TP compliance and a 67% improvement in stability. Such process optimization enhances overall plant performance, potentially benefiting downstream EC removal, and reduces chemical consumption and sludge production, aligning with sustainability and low-carbon goals.

The occurrence, fate, and risks of specific EC classes are examined in Contributions 6 and 10. Liu et al. (Contribution 6) investigated 12 common organophosphate flame retardants (OPFRs) in a municipal WWTP in Hunan Province, China. Their analysis revealed that OPFRs were predominantly in the dissolved phase, with Tributoxyethyl phosphate (TBOEP) and tris(2-chloroethyl) phosphate (TCEP) being the most abundant. The overall removal efficiency was low (mean 39.1%), particularly for halogenated OPFRs which showed negative removal, indicating potential formation or release within the plant. Mass balance analysis indicated that a significant fraction (60.9%) was discharged via effluent, while adsorption to sludge (11.2%) and losses during treatment (biodegradation/biotransformation) accounted for the rest. The study highlights the inefficiency of conventional WWTPs in removing OPFRs and the associated ecological risks. Hernández et al. (Contribution 10) address disinfection by-products (DBPs) by determining the occurrence of chloroform (CHCl₃), a major trihalomethane (THM), in the drinking water supply of Cuenca, Ecuador. Sampling across three water treatment systems and their distribution networks over five months, they measured physicochemical parameters and CHCl₃ levels. While free chlorine residuals decreased along the distribution networks, CHCl₃ concentrations (ranging from 11.75 to 21.88 µg/L) remained below the Ecuadorian regulatory limit. No consistent correlation was found between CHCl₃ and the measured parameters, suggesting complex formation dynamics influenced by various factors, reinforcing the need for DBP monitoring in drinking water systems.

Finally, Contributions 8 and 9 provide valuable reviews synthesizing current knowledge. Wu et al. (Contribution 8) offer a comprehensive review of surfactant-enhanced remediation (SER) for non-aqueous phase liquid (NAPL)-contaminated soil and groundwater. The review details the mechanisms of surfactant action (mobilization, solubilization, and emulsification) and discusses the synergistic application of SER with various remediation techniques, including chemical oxidation, biodegradation, soil vapor extraction, electrokinetics, and thermal desorption. It also critically assesses the associated risks, such as residual surfactant toxicity, and explores the potential of more environmentally friendly biosurfactants. Zhu et al. (Contribution 9) present a broad review specifically focused on antibiotics within urban wastewater treatment systems. This review synthesizes information on the sources, occurrence, and potential ecological and human health risks of antibiotics and associated ARGs emanating from WWTPs. It covers the spectrum of detection methods, from enrichment techniques and immunoassays to advanced instrumental analysis and sensor technologies, and categorizes removal strategies including physical (membrane separation, adsorption), chemical (AOPs), and biological (activated sludge, MBRs) approaches, highlighting current limitations and challenges in effective antibiotic management in wastewater.

3. Conclusions and Future Directions

This Special Issue offers a multifaceted perspective on the pressing challenges posed by emerging contaminants in aquatic environments. The curated collection of ten articles spans a significant breadth of research, encompassing the investigation of specific contaminant classes such as chlorophenols, antibiotics, flame retardants, and disinfection by-products, alongside explorations into innovative remediation technologies and management strategies. Methodologically, the contributions showcase a diverse range of approaches, from detailed laboratory and pilot-scale experiments and field monitoring campaigns to advanced analytical techniques like metagenomics, comprehensive mass balance analyses, the development of intelligent control systems, and critical literature reviews.

Collectively, the research presented herein significantly advances our understanding of ECs. Studies like those by Wang et al. (Contributions 1 and 2) provide valuable insights into optimizing biological degradation pathways, demonstrating the potential of utilizing waste streams like sludge fermentate as sustainable co-substrates, and elucidating the complex interplay between carbon sources and microbial functional gene abundance. The work by Lin et al. (Contribution 3) highlights the promise of advanced materials, showing that electrospun photocatalytic membranes can effectively degrade antibiotics while overcoming the practical limitations of catalyst recovery. Investigations by Liu et al. (Contribution 6) and Hernández et al. (Contribution 10) underscore the limitations of existing treatment infrastructure in removing persistent compounds like OPFRs and managing DBP formation, respectively, providing crucial occurrence data and risk context. Furthermore, the studies by Yang et al. (Contribution 4) and Lu et al. (Contribution 7) demonstrate innovative approaches to managing legacy pollution (landfilled sludge) and optimizing existing treatment processes in ways that indirectly benefit overall contaminant control and resource management within WWTPs. The comprehensive reviews by Wu et al. (Contribution 8) and Zhu et al. (Contribution 9) synthesize the state-of-the-art in SER technologies and antibiotic management in wastewater systems, respectively, identifying key challenges and future research needs.

Despite the progress highlighted in this Special Issue, the field of emerging contaminants continues to face substantial hurdles, necessitating focused future research efforts. A critical area remains the development and scaling-up of efficient and cost-effective removal technologies. While novel materials like the photocatalytic membranes studied by Lin et al. (Contribution 3) show promise, transitioning these from lab-scale success to robust, full-scale application requires overcoming challenges related to long-term stability, fouling resistance, and economic viability. Further research into advanced oxidation processes, selective adsorbents, and optimizing biological processes, perhaps by harnessing specific microbial consortia or enzymes, is warranted. The synergistic combination of different treatment modalities, such as integrating physical pre-treatment with AOPs and biological polishing, deserves greater exploration.

Understanding the long-term ecological and health risks associated with chronic, low-level exposure to complex mixtures of ECs and their transformation products remains a paramount challenge. The work by Liu et al. (Contribution 6) on OPFR risk assessment exemplifies the need for such studies across a broader range of contaminants. Particular attention should be paid to the fate and transfer of antibiotic resistance genes through wastewater systems, a risk highlighted by Zhu et al. (Contribution 9), and the potential endocrine-disrupting effects of various ECs.

Continuous improvement in analytical methodologies is essential for accurate monitoring and risk assessment. Detecting ultra-trace concentrations of diverse ECs and their metabolites in complex matrices like wastewater and sludge requires ongoing innovation in sample preparation (enrichment/extraction) and instrumental analysis, possibly through the advanced sensor technologies discussed by Zhu et al.(Contribution 9) Developing rapid, cost-effective, and field-deployable monitoring tools would greatly enhance regulatory efforts and plant operational control.

Finally, bridging the gap between scientific understanding and effective management requires a holistic approach. This includes stronger source control measures, as emphasized by Zhu et al. (Contribution 9), optimizing existing infrastructure through approaches like the intelligent control systems developed by Lu et al. (Contribution 7), exploring sustainable solutions like biosurfactants for remediation (Contribution 8), and developing integrated assessment frameworks that consider the entire lifecycle of contaminants and treatment processes. The management of waste streams like landfilled sludge, addressed by Yang et al. (Contribution 4), is also a crucial component of preventing long-term environmental liabilities.

The research presented in this Special Issue provides valuable contributions to addressing the multifaceted challenge of emerging contaminants in aquatic environments. It is hoped that these articles will stimulate further investigation and innovation, ultimately leading to more effective strategies for protecting water resources and public health.