Water Treatment Technology for Emerging Contaminants

1. Introduction

The rapid advancement of industrialization and urbanization has brought about significant challenges to water quality and environmental health. Emerging pollutants, such as microplastics, pharmaceutical residues, endocrine disruptors, and heavy metals, have become increasingly prevalent, posing long-term threats to both human health and ecosystem stability. These pollutants, often present in low concentrations, are persistent and difficult to degrade, leading to their accumulation in the environment. The consequences of this pollution are far-reaching, affecting not only aquatic ecosystems but also human populations that rely on these water sources for drinking, agriculture, and industrial purposes. In response to these challenges, the scientific community has been actively developing and refining water treatment and environmental remediation technologies. This special issue brings together ten cutting-edge research articles that explore innovative approaches to addressing these pressing environmental issues. The studies presented here span a wide range of topics, from the adsorption of emerging pollutants to the development of novel disinfection methods, and from the optimization of biodegradation processes to the application of advanced oxidation techniques.

Water is a fundamental resource for life, yet its quality is increasingly compromised by human activities. Industrial discharges, agricultural runoff, and urban wastewater are major sources of pollution, introducing a wide array of contaminants into water bodies. Among these, pharmaceuticals, personal care products, and microplastics have garnered significant attention due to their persistence and potential to cause harm even at low concentrations. Traditional water treatment methods, such as coagulation, sedimentation, and chlorination, are often ineffective against these pollutants, necessitating the development of more advanced and targeted treatment technologies.

The papers in this special issue address these challenges by exploring a variety of innovative approaches. From the use of graphene oxide for pollutant adsorption to the application of black body-inspired nanostructures for solar desalination, these studies highlight the potential of new materials and technologies to improve water quality. Additionally, the integration of biological, chemical, and physical methods offers a holistic approach to tackling complex pollution problems. By combining these methods, researchers can achieve higher removal efficiencies and reduce the environmental impact of treatment processes.

2. Review of New Advances

2.1. Graphene Oxide for Adsorption of Emerging Pollutants

The first paper in this issue (contribution 1), by Tsubouchi et al. [1] investigates the use of graphene oxide (GO) for the adsorption of butylparaben (BP), an emerging pollutant commonly found in wastewater. Butylparaben is a member of the paraben family, widely used as a preservative in cosmetics, pharmaceuticals, and food products. Due to its widespread use, BP has been detected in various water bodies, raising concerns about its potential ecological and health impacts. The study highlights the effectiveness of GO in removing BP, with a removal efficiency of 84.3% under optimal conditions. The authors employed a comprehensive characterization of GO, including TGA/DTA, XRD, and SEM, to understand its physicochemical properties. The adsorption process was found to follow pseudo-first-order kinetics, and the Langmuir isotherm provided the best fit for the experimental data. The study also demonstrated that the adsorption process is spontaneous and that the GO adsorbent can be regenerated efficiently, making it a promising candidate for large-scale wastewater treatment applications.

The use of GO for pollutant adsorption is particularly promising due to its high surface area, tunable surface chemistry, and ability to form stable dispersions in water. These properties make GO an excellent adsorbent for a wide range of pollutants, including heavy metals, organic dyes, and emerging contaminants. Furthermore, the ability to regenerate GO through simple processes such as thermal treatment or chemical washing enhances its practicality for industrial applications. The findings of this study contribute to the growing body of research on the use of graphene-based materials for environmental remediation, highlighting their potential to address some of the most pressing water pollution challenges.

2.2. Solar Desalination Using Black Body-Inspired Nanostructures

In the second paper (contribution 2), Kaviti et al. [2] explored the application of black body-inspired nanostructures for solar desalination. Desalination is a critical technology for providing fresh water in arid regions, but traditional methods, such as reverse osmosis and thermal distillation, are energy-intensive and costly. The authors developed non-contact nanostructured single-slope solar stills (NCNSSSs) with varying perforation diameters and demonstrated their effectiveness in converting solar energy into infrared radiation for water evaporation. The study found that the NCNSSS with a 3.8 mm perforation diameter achieved the highest productivity, with a 9.89% and 13.47% increase in efficiency compared to smaller perforation sizes. The desalinated water produced met WHO standards for potability, highlighting the potential of this technology for providing clean drinking water in arid regions.

The use of black body-inspired nanostructures for solar desalination represents a significant advancement in the field of renewable energy-driven water treatment. By optimizing the design of solar stills, the authors were able to achieve higher evaporation rates and improve the overall efficiency of the desalination process. This approach not only reduces the energy requirements for desalination but also minimizes the environmental impact associated with traditional methods. The study underscores the importance of innovative design and material selection in enhancing the performance of solar-driven water treatment technologies.

2.3. Biodegradation of Paper Sludge

The third paper (contribution 3), by Samešová et al. [3] focuses on the biodegradation of paper sludge, a significant environmental concern due to its high global production. Paper sludge is a byproduct of the paper manufacturing process, consisting of a mixture of cellulose fibers, fillers, and chemicals. Disposal of paper sludge poses significant environmental challenges, as it can lead to soil and water pollution if not properly managed. The study employed aerobic and anaerobic biodegradability tests to optimize the decomposition process. The authors found that aerobic decomposition reached approximately 80% after 28 days, while anaerobic decomposition produced biogas with a yield of 554 m³/tVS. The study also highlighted the importance of pH and initial concentration in optimizing the biodegradation process. The findings suggest that paper sludge is suitable for both aerobic and anaerobic biodegradation, offering a sustainable solution for waste management in the paper industry.

The biodegradation of paper sludge not only addresses the environmental challenges associated with its disposal but also provides an opportunity for resource recovery. The production of biogas through anaerobic digestion offers a renewable energy source, while the residual sludge can be used as a soil amendment or fertilizer. The study highlights the potential of biological treatment methods to convert waste materials into valuable resources, contributing to the circular economy and reducing the environmental footprint of industrial processes.

2.4. Arsenic Removal Using Novel Bacteria

In the fourth paper (contribution 4), Dey et al. [4] report the isolation and identification of novel arsenic-resistant bacteria from an arsenic-contaminated region. Arsenic contamination of groundwater is a major public health concern, particularly in regions such as South Asia, where millions of people are exposed to unsafe levels of arsenic in drinking water. The study identified two Gram-positive bacteria, Bacillus sp. and Bacillus cereus, capable of withstanding high concentrations of arsenic and effectively converting toxic arsenite (As³⁺) to less toxic arsenate (As⁵⁺). The bacteria demonstrated a 50% removal efficiency for both As³⁺ and As⁵⁺, making them promising candidates for bioremediation of arsenic-contaminated water. This study underscores the potential of using natural microorganisms for the detoxification of heavy metals in water sources.

The use of bacteria for arsenic removal offers several advantages over traditional chemical and physical methods. Biological treatment is often more cost-effective and environmentally friendly, as it does not require the use of hazardous chemicals or generate secondary waste. Furthermore, the ability of bacteria to oxidize arsenite to arsenate reduces the toxicity of arsenic, making it easier to remove through subsequent treatment processes. The findings of this study contribute to the growing body of research on the use of microorganisms for environmental remediation, highlighting their potential to address some of the most challenging water pollution problems.

2.5. Disinfection of Water Using UV-C and Solar Irradiation

The fifth paper (contribution 5), by Choi et al. [5] compares the disinfection efficiency of UV-C and solar irradiation for the inactivation of bacteria and cyanobacteria. Disinfection is a critical step in water treatment, as it ensures the removal of pathogenic microorganisms that can cause waterborne diseases. The study found that UV-C irradiation achieved over 6 log removal values (LRV) for Escherichia coli and Bacillus subtilis within 1 min, while solar irradiation required 20 min for similar results. The addition of hydrogen peroxide (H₂O₂) enhanced the disinfection process, particularly for cyanobacteria. The study highlights the potential of solar irradiation as a cost-effective and environmentally friendly alternative to UV-C for water disinfection, especially in resource-limited settings.

The use of solar irradiation for water disinfection is particularly promising in regions with limited access to electricity and advanced water treatment infrastructure. Solar disinfection (SODIS) is a simple and low-cost method that can be implemented at the household level, providing an effective means of improving water quality in developing countries. The findings of this study contribute to the growing body of research on solar-driven water treatment technologies, highlighting their potential to address global water challenges.

2.6. Degradation of Antibiotics Using Fe-Loaded Biochar

In the sixth paper (contribution 6), Zhang et al. [6] investigate the degradation of the antibiotic metronidazole (MNZ) using Fe-loaded biochar (Fe-BC) synthesized from rape straw. Antibiotics are a major class of emerging contaminants, with their widespread use in human and veterinary medicine leading to their presence in water bodies. The persistence of antibiotics in the environment can lead to the development of antibiotic-resistant bacteria, posing a significant threat to public health. The study demonstrated that Fe-BC could activate peroxymonosulfate (PMS) to achieve a 95.3% degradation efficiency of MNZ within 60 min. The degradation process was driven by various reactive oxygen species, with sulfate radicals (SO₄•−) playing a dominant role. The study also highlighted the reusability and stability of the Fe-BC catalyst, making it a promising material for the removal of antibiotic pollutants from wastewater.

The use of biochar for pollutant degradation offers several advantages, including its low cost, abundance, and ability to be derived from waste materials. The incorporation of iron into the biochar enhances its catalytic properties, enabling the activation of PMS for the generation of reactive oxygen species. The findings of this study contribute to the growing body of research on the use of biochar-based materials for environmental remediation, highlighting their potential to address some of the most challenging water pollution problems.

2.7. Comparative Analysis of Disinfection Efficiency

The seventh paper (contribution 7), by Romanovski et al. [7] compares the disinfection efficiency of ozone and sodium hypochlorite solutions on steel and polymer surfaces. Disinfection of surfaces is a critical measure for the inactivation of microorganisms and viruses, particularly in healthcare settings and public spaces. The study found that ozonated water was 100–230 times more effective than sodium hypochlorite solutions, depending on the type of microorganism. The efficiency of inactivation varied with the substrate material, particularly for Gram-negative bacteria. The study suggests that ozone could be a more effective and environmentally friendly alternative to chlorine-based disinfectants for surface disinfection.

The use of ozone for disinfection offers several advantages over traditional chlorine-based methods, including its higher oxidation potential and ability to inactivate a wide range of microorganisms. Furthermore, ozone decomposes into oxygen, leaving no harmful residues and minimizing the environmental impact of disinfection processes. The findings of this study contribute to the growing body of research on the use of ozone for environmental and public health applications, highlighting its potential to improve the safety and efficacy of disinfection practices.

2.8. Enhanced Adsorption of Methylene Blue Dye

In the eighth paper (contribution 8), Hamri et al. [8] explore the enhanced adsorption capacity of methylene blue (MB) dye onto acid-treated kaolin. Dyes are a major class of pollutants in textile wastewater, with their presence leading to aesthetic and environmental concerns. The study found that sulfuric acid treatment increased the surface area and pore volume of the kaolin, resulting in a higher adsorption capacity for MB. The adsorption process followed pseudo-second-order kinetics and was best described by the Langmuir isotherm. The study also demonstrated the effectiveness of the treated kaolin in reducing chemical oxygen demand (COD) and biological oxygen demand (BOD) in real textile wastewater, highlighting its potential for industrial applications.

The use of kaolin for dye adsorption offers several advantages, including its low cost, abundance, and ability to be modified to enhance its adsorption properties. The findings of this study contribute to the growing body of research on the use of clay minerals for environmental remediation, highlighting their potential to address some of the most challenging water pollution problems.

2.9. Pollution in Huixian Wetland

The ninth paper (contribution 9), by Gao et al. [9] provides a comprehensive review of the pollution status in the Huixian Wetland, focusing on nutrients, heavy metals, emerging pollutants, and microplastics. Wetlands are critical ecosystems that provide a wide range of ecological services, including water purification, flood control, and habitat for biodiversity. However, wetlands are increasingly threatened by pollution from industrial, agricultural, and urban sources. The study highlights the seasonal variations in pollutant concentrations and the ecological risks posed by antibiotics and organochlorine pesticides (OCPs). The authors also discuss the carbon sequestration potential of the wetland and the role of anaerobic ammonia-oxidizing bacteria in wastewater treatment. The study underscores the need for further research to protect and restore the ecological balance of the wetland.

The findings of this study contribute to the growing body of research on wetland pollution and management, highlighting the importance of understanding the sources and impacts of pollutants in these critical ecosystems. The study also underscores the potential of wetlands to contribute to climate change mitigation through carbon sequestration, highlighting the need for integrated approaches to wetland management that balance ecological, economic, and social objectives.

2.10. Water Treatment Technologies for Emerging Contaminants

The final paper (contribution 10), by Wang et al. [10] reviews the current state of water treatment technologies for emerging contaminants (ECs), including microplastics, pharmaceutical residues, and endocrine disruptors. The authors discuss the advantages and limitations of physical, chemical, and biological treatment methods and highlight the need for interdisciplinary collaboration to develop efficient, economical, and environmentally friendly technologies. The study emphasizes the importance of addressing the long-term cumulative effects of ECs on human health and ecosystem security.

The findings of this study contribute to the growing body of research on emerging contaminants and their treatment, highlighting the need for innovative and integrated approaches to water treatment. The study also underscores the importance of interdisciplinary collaboration in addressing complex environmental challenges, highlighting the potential of combining different treatment methods to achieve higher removal efficiencies and reduce the environmental impact of water treatment processes.

3. Conclusions

The ten papers in this special issue collectively advance our understanding of water treatment and environmental remediation technologies. From the adsorption of emerging pollutants to the development of novel disinfection methods, these studies offer innovative solutions to some of the most pressing environmental challenges of our time. The findings presented here not only contribute to the scientific literature but also provide practical insights for the development of sustainable water treatment technologies. As we move forward, it is crucial to continue fostering interdisciplinary collaboration and innovation to ensure the protection of our water resources and the health of our ecosystems.