Special Issue “Treatment and Remediation of Organic and Inorganic Pollutants: Advances, Challenges and Future Directions”

1. Introduction

Increasing industrial activity [1], population growth [2], and unsustainable land and water management practices [3] have intensified the release of various organic and inorganic pollutants into the environment. These pollutants, ranging from pharmaceuticals and heavy metals to dyes and emerging pollutants such as microplastics and antibiotics, significantly affect ecosystems and human health, contributing substantially to the increase in human diseases, affecting climate change as well as public and individual health, and leading to increased rates of illness and mortality [4,5]. Thus, environmental pollution caused by pollutants is one of the central challenges of the 21st century. In response, there has been a growing interest in the development and optimization of effective treatment and remediation strategies [6,7,8]. This Special Issue of Process, entitled “Treatment and Remediation of Organic and Inorganic Pollutants”, is designed to present the latest advances in this field, provide insights into new methodologies, and provide a platform for interdisciplinary collaboration. The nine published articles in this Special Issue [9,10,11,12,13,14,15,16,17], two of which are reviews, represent a diverse and innovative set of contributions that reflect both the complexity of environmental pollution and the ingenuity of modern remediation approaches.

2. Overview of the Contributions

The articles included in this Special Issue cover a broad spectrum of topics related to pollution treatment, ranging from adsorption and advanced oxidation to electrochemical, hydrothermal, and biological processes, as well as digital and policy-oriented perspectives. Below is an overview of key thematic areas.

2.1. Adsorption Processes

One of the most promising and widely applied approaches for pollutant removal is adsorption. Thus, the development of novel adsorbent materials—including highly porous structures, multifunctional hybrid materials, agricultural and industrial byproducts or waste-derived materials, natural substances and their modified forms, nanomaterials, and COF/MOF systems—has attracted significant attention from the scientific community [18,19,20,21]. In this Special Issue, the study by Obradović et al. [8] explores the use of surfactant-modified bentonites for the removal of non-steroidal anti-inflammatory drugs, specifically ibuprofen and diclofenac sodium, from aqueous media. The findings indicate that both the type of surfactant and its arrangement within the interlayer space and on the adsorbent surface play a significant role in the adsorption process. Thus, the modification of natural clays with surfactants significantly enhances their affinity for target pollutants, demonstrating the potential of engineered, low-cost adsorbents in the purification of contaminated waters.

2.2. Advanced Oxidation Processes (AOPs)

Advanced oxidation processes (AOPs) utilize highly reactive radical species, primarily hydroxyl radicals (·OH), but also other radical species, to degrade or remove harmful substances from water and wastewater. These processes involve the in situ generation of radicals, which then degrade the pollutants into less harmful substances. Thus, AOP technologies are essentially a process intensification through the use of hybrid methods for wastewater treatment. They are highly effective for treating a wide range of organic and inorganic pollutants, including those that are resistant to conventional treatment methods [22,23]. The three papers featured in this Special Issue highlight the promising application of advanced oxidation processes (AOPs) in wastewater treatment, with a particular focus on how thermal activation, catalytic enhancement, and reactor configuration contribute to improved pollutant removal. Hayet Amichi et al. [9] examined the solar-driven degradation of Rhodamine B (RhB) using hypochlorite, supported by comprehensive kinetic modeling. Their study sheds light on the respective roles of reactive oxygen species (ROS) and reactive chlorine species (RCS) in the degradation mechanism. They demonstrated that solar-activated hypochlorite effectively degrades RhB, and by applying a free-radical kinetic model in COPASI^®, they successfully established kinetic profiles for ROS and RCS, emphasizing their dependence on operational parameters such as temperature and hypochlorite dosage. Kovačić et al. [11] focused on the removal of polyethylene terephthalate microplastics (PET MPs) through the Fenton process followed by oxidative coagulation, revealing distinct behavior between unweathered and UV-weathered particles. Their findings provide valuable input for developing targeted remediation approaches, underlining the necessity of incorporating environmental aging effects into treatment strategies. In another contribution, Talbi et al. [12] explored the use of a tubular microreactor (2–6 m in length, 1 mm diameter), immersed in a temperature-controlled bath, for the activation of persulfate by thermal and Fe(II) catalysis—both individually and as a combined hybrid method—to degrade Safranin O (SO). The thermo-catalytic approach resulted in more than 50% total organic carbon (TOC) reduction, indicating significant mineralization of organic contaminants. The study emphasizes the advantages of microreactor systems, such as their compact form and high surface-to-volume ratio, which favor laminar flow, enhance mass transfer, and shorten diffusion paths compared to conventional reactor designs [24,25].

2.3. Combined Hybrid Approaches

Hybrid wastewater treatment combines different treatment technologies to increase overall efficiency, minimize operating costs, and reduce environmental impact. These systems essentially integrate two or more physical, chemical, or biological methods, implemented simultaneously or sequentially (one after the other), exploiting their complementary advantages to achieve improved pollutant removal and resource consumption/recovery [26]. In this Special Issue, Svilović et al. [13] presented a novel hybrid treatment system that integrates electrocoagulation, zeolite-assisted ion exchange and adsorption, and ultrasound to treat composting wastewater highly loaded with organic matter. Although the combined use of electrocoagulation, zeolite, and ultrasound generally did not lead to notable improvements in wastewater quality with respect to COD and turbidity, the inclusion of ultrasound contributed to reduced voltage requirements and lower electrode consumption. This, in turn, can lower overall process costs. Consequently, this area remains promising and warrants further investigation.

2.4. Hydrothermal Treatment

Hydrothermal treatment uses hot, compressed water to treat materials, often biomass or waste, under high temperature and pressure conditions. This method can be used for various purposes, including waste management [27], biomass conversion [28], and material synthesis [29]. Švarc-Gajić et al. [14] explored the use of subcritical water oxidation, a novel hydrothermal method, to break down various organic contaminants such as pesticides (tembotrione, clomazone), pharmaceuticals (ciprofloxacin, 17α-ethinyl estradiol), and mycotoxins (zearalenone, deoxynivalenol, fumonisin B1). Their study demonstrated complete degradation of tembotrione and all tested mycotoxins at 200 °C without catalysts. Other pollutants were also effectively degraded at this temperature, with removal rates ranging from 89.5% (clomazone) to 98.7% (17α-ethinyl estradiol). These findings highlight a promising, cost-effective, and environmentally friendly approach for treating food, feed, and environmental samples. Nevertheless, further investigation is necessary to fully understand the green breakdown of specific compounds and to identify and assess the safety of intermediate products.

2.5. Biochar and Photocatalytic Materials

Biochar, derived from pyrolyzed biomass, has unique properties to remove pollutants from soil, water, and gas [30,31]. These materials offer a unique advantage over conventional photocatalysts by enabling the simultaneous valorization of waste biomass and efficient elimination of priority pollutants from the environment. A review by Bratović and Tomašić [15] explored the potential of biochar-based composites for the removal of antibiotics and synthetic dyes from wastewater. The article emphasizes the importance of material modification—including biochar sources, preparation techniques, and the physicochemical properties of biochar-based photocatalytic composites—as key factors influencing performance. Biochar stands out due to its low cost, non-toxicity, and abundance of surface functional groups, making it a versatile and sustainable platform for developing multifunctional adsorbents and catalysts. Special attention is given to the role of biochar in enhancing the photocatalytic activity of photoactive species and in influencing the reaction mechanisms of organic pollutant degradation. These findings support the classification of biochar-based composites as a promising, environmentally friendly solution to water pollution. However, further research is necessary to enable large-scale production and real-world application.

2.6. Digital Tools and Artificial Intelligence

Ramos et al. [16] made a significant contribution by developing a machine learning framework capable of classifying and tracking plastic waste based on image data. This cutting-edge approach reflects the growing trend toward digital transformation in environmental monitoring, presenting scalable opportunities for real-time waste identification and sorting. Despite these advancements, key obstacles persist. The lack of consistency in methodologies, datasets, and evaluation metrics across different studies limits comparability and hinders the development of standardized benchmarks. Moreover, although laboratory experiments have demonstrated encouraging results, a considerable gap remains between experimental accuracy and the resilience needed for deployment in complex, real-world waste management systems.

2.7. Air Pollution and Urban Policy

Extending research beyond water remediation, Pastor-Fernández et al. [17] conducted an evaluation of NO₂ concentrations within low-emission zones (LEZs) in Seville, highlighting the significant impact of traffic-related pollution on urban air quality. Their results highlight the crucial role of urban mobility policies in enhancing air quality and provide empirical evidence in support of traffic regulation measures. However, to determine whether the effectiveness of LEZ implementation is sustained or enhanced over time, further long-term studies—particularly those encompassing extended enforcement periods and the issuance of fines—are essential.

3. Identified Knowledge Gaps and Challenges

The collected contributions highlight several enduring challenges within the field of environmental treatment and remediation. One of the most pressing issues is the presence of multicomponent pollutant mixtures in real wastewater streams, which limits the effectiveness of technologies designed to target individual contaminants and underscores the need for integrated, multi-step treatment systems. Additionally, while many solutions demonstrate promising results in controlled laboratory settings, their scalability and long-term performance in real-world applications remain uncertain. There is a noticeable lack of comprehensive studies addressing the durability of materials, regeneration potential, and economic feasibility over extended periods. Furthermore, although digital technologies such as machine learning are advancing rapidly, their practical integration into treatment infrastructure is still in its infancy. Finally, regulatory and societal factors pose significant barriers; aligning technological innovation with policy frameworks, fostering public acceptance, and ensuring economic viability are essential yet complex tasks that continue to challenge the widespread implementation of advanced remediation strategies.

4. Impact of the Special Issue

This Special Issue has achieved significant dissemination within the scientific and professional community. The published articles have collectively attracted thousands of views and downloads, reflecting a high level of interest in the topic. The thematic diversity and international authorship exemplify the growing global effort to address environmental pollution through multidisciplinary innovation. Notably, the inclusion of digital technologies, smart materials, and urban policy insights enhances the relevance and applicability of the findings.

5. Future Directions

Based on the presented findings, several promising directions for future development can be identified. One key avenue is the creation of hybrid and synergistic treatment systems that integrate physical, chemical, and biological processes, thereby enhancing efficiency and adaptability across diverse wastewater matrices. Equally important is the adoption of green and circular approaches, with a focus on environmentally benign materials and processes that enable resource recovery and reduce ecological impact. The integration of real-time monitoring technologies, including sensors, AI-driven diagnostics, and automation, holds great potential to improve the reliability, responsiveness, and overall performance of treatment systems. Additionally, stronger collaboration between scientists, industry stakeholders, and policymakers is crucial for translating research innovations into practical applications and ensuring alignment with broader sustainability goals. Finally, increasing public awareness and investing in environmental education are essential to foster societal engagement, support the transition toward cleaner technologies, and promote more responsible approaches to waste management.