Editorial for the Special Issue “Advances in Remediation of Contaminated Sites: 2nd Edition”

With the acceleration of industrialization and urbanization, anthropogenic activities such as mineral exploitation, industrial production, and agricultural cultivation have led to increasingly severe contamination of soil, groundwater, and sediment environments [1,2,3]. Contaminated sites not only pose threats to ecological system stability—such as disrupting soil microbial communities and inhibiting plant growth—but also endanger human health through pathways such as food chain accumulation and groundwater ingestion [4,5,6]. As non-renewable land resources become increasingly scarce, developing efficient, eco-friendly, and cost-effective contaminated site remediation technologies has become a core task to achieve global Sustainable Development Goals [7,8,9].

Remediation of contaminated sites is a complex interdisciplinary field integrating environmental chemistry, soil science, microbiology, and ecological engineering [10,11,12]. Over the past decade, significant progress has been made in remediation technologies, from traditional physical–chemical methods to emerging biological and intelligent remediation strategies [13,14]. However, challenges remain, including low remediation efficiency for complex combined pollution, high costs of large-scale engineering applications, and unclear long-term ecological risks of remediation technologies [15,16]. Addressing these bottlenecks requires continuous innovation in theoretical research and technological development.

The following Special Issue “Advances in Remediation of Contaminated Sites: 2nd Edition” focuses on the aforementioned core issues and includes 24 high-quality research works, systematically structured around four dimensions: pollution characterization and risk assessment, environmental fate of pollutants, remediation technology, and review articles and bibliometric analysis. It forms a complete research chain encompassing “baseline characterization–mechanism analysis–technological innovation–trend guidance”. In terms of research proportion, remediation technology (46%) focuses on breakthroughs in materials and processes; environmental fate of pollutants (21%) analyzes the behavioral laws of pollution; pollution characterization and risk assessment (21%) lays the foundation for remediation decision-making; and review articles and bibliometric analysis (12%) guide the development direction of the field.

1. Pollution Characterization and Risk Assessment

Pollution characterization and risk assessment serve as the prerequisite for remediation decision-making. Through three-dimensional analysis of “concentration distribution–source identification–risk classification”, they provide accurate baselines for technology selection [17,18]. This Special Issue covers typical scenarios such as industrial areas, agricultural lands, drinking water sources, and tailings ponds, integrating traditional sampling and emerging detection technologies to construct a multi-dimensional investigation system.

Targeting the pollution characteristics of different sites, research teams have accurately characterized pollutant distribution and sources. In industrial areas, Chen et al. focused on Baotou, a major industrial city in China. By utilizing the Geo-accumulation Index (Igeo) and Potential Ecological Risk Index (PERI), they found that the concentrations of Hg, Pb, Zn, and Cu in the soil of Kundulun District (dominated by iron and steel and power industries) exceeded the standard significantly, with a PERI of 386—2.3 times that of Qingshan District (dominated by machinery manufacturing). This finding confirms the decisive role of industrial structure in pollution distribution. In agricultural areas, Wang et al. focused on the “soil–rice” system in paddy fields of Guangxi province in China. The results of field experiments revealed that single lime treatment could reduce Cd content in rice by 73.15%; however, flooding would increase As accumulation by 28%. In addition, the synergistic treatment of “flooding + foliar silicon fertilizer” could simultaneously control pollution, reducing Cd content in rice to 0.18 mg/kg (below the national standard of 0.2 mg/kg), providing a promotable scheme for safe agricultural production in paddy fields. In drinking water source scenarios, Wei et al. focused on groundwater in Zhanjiang city in China and confirmed through δD–δ¹⁸O isotope analysis that atmospheric precipitation represents the main recharge source. Additionally, the Fe and Mn concentrations in the middle-deep confined aquifer exceeded Class III standards due to the presence of manganese–iron ore deposits, providing a scientific basis for the hierarchical protection of drinking water sources.

For special scenarios such as tailings ponds and riparian environments, research teams have assessed pollution release risks under extreme conditions. Shu et al. focused on low-sulfur lead–zinc tailings in the karst area of northern Guangxi province in China and challenged the traditional notion that “high acid-neutralizing capacity equals low risk” through batch leaching experiments. Although the acid-neutralizing capacity of the tailings reached 166.57 kg H₂SO₄/t, under acid rain conditions (pH = 2–3), the leaching concentrations of Pb, Zn, and Cd were 5.2 times higher than those under neutral conditions. Moreover, due to the well-developed pore structure of coarse-grained tailings, their leaching amount was 18% higher than that of fine-grained tailings, serving as a warning for the prevention and control of acid rain in tailings ponds.

Emerging detection technologies have significantly improved the efficiency of pollution investigation. Zhang et al. focused on a metallurgical slag site in Gejiu, Yunnan province, China, integrating visible–near-infrared (Vis-NIR) and X-ray fluorescence (XRF) spectroscopy and using Competitive Adaptive Reweighted Sampling (CARS) to screen characteristic variables. The constructed quantitative model for soil Cd content achieved a coefficient of determination (R²) of 0.9505, with calculation time shortened by 68% compared to single XRF technology, enabling on-site rapid and non-destructive monitoring. Additionally, Li et al. provided a supplementary basis for the quantitative analysis of pollution concentration through the study of the adsorption mechanisms of modified biochar on soil Cd.

2. Environmental Fate of Pollutants

The environmental fate of pollutants is crucial for understanding pollution diffusion paths and optimizing remediation strategies [19,20]. This Special Issue explores the behavioral mechanisms of pollutants in the “soil–plant–groundwater” system from three dimensions: heavy metal migration, organic pollutant degradation, and the interaction effects of compound pollution.

The speciation of heavy metals directly determines their bioavailability, and research teams have deeply analyzed the key factors affecting speciation transformation. Qin et al. compared Miscanthus floridulus from the Dabaoshan mining area in Guangdong province in China and the non-mining area in Boluo County, Huizhou city, China, and found that under 100 mg/L Cu stress, the chlorophyll reduction rate of the mining ecotype (18%) was only 43% of that of the non-mining ecotype (42%), and the ascorbic acid content was 2.1 times that of the latter. This finding confirms that plants in mining areas develop Cu tolerance by enhancing their antioxidant systems. The authors further studied the effects of Pb stress on the photosynthetic system and found that under high Pb concentration (240 mg/L), the maximum photochemical efficiency (Fv/Fm) of Miscanthus floridulus from the mining area remained 0.78 (compared to only 0.62 for the non-mining area), revealing the protection mechanism of its photosynthetic apparatus. At the mineral interface level, Li et al. found that the adsorption capacity of rod-like lepidocrocite (exposing the (001) crystal plane) for Cr(VI) (28.6 mg/g) was 1.8 times that of flake-like lepidocrocite (exposing the (010) crystal plane), by using in situ ATR-FTIR and two-dimensional correlation spectroscopy (2D-COS). Additionally, L-aspartic acid exhibited a significantly higher inhibition rate (45%) on adsorption by the (001) crystal plane than by the (010) crystal plane (22%), providing a molecular mechanism for the targeted design of Cr(VI) adsorption materials.

For organic pollutants such as microplastics and persistent organic pollutants, research teams have analyzed their degradation and aging laws. Wang et al. compared the changes in polylactic acid (PLA) and polyethylene (PE) under UV–water coupling. Due to the hydrolysis and cleavage of ester bonds, PLA formed 5–10 μm pores on its surface, and its particle size decreased from 1 mm to 0.3 mm; in comparison, PE only underwent surface oxidation, with the carbonyl index (CI) increasing from 0.02 to 0.06 and no significant change in particle size. Through Py-GC-MS, the characteristic degradation products of the two materials were identified (lactic acid and acetic acid for PLA and 2-hexanone for PE), providing a basis for the environmental risk assessment of microplastics.

For scenarios with coexisting multiple pollutants, research teams have analyzed the impact of interaction effects on pollution fate. Li et al. focused on in situ uranium leaching, addressing the issue whereby natural groundwater inflow dilutes leachate and reduces mining efficiency. They establish a “groundwater flow ratio” quantitative model, taking Inner Mongolia’s C1 and C2 uranium mines as study objects. By analyzing how injection volume and filter parameters affect the flow ratio, the authors propose adjusting filter depth (to confined aquifers) and injection mode (intermittent injection) to reduce natural groundwater inflow by 30–40%, providing a quantitative basis for leaching process optimization.

3. Remediation Technology

Remediation technology is the core to solving pollution problems [16,21]. This Special Issue has achieved breakthroughs in four directions: functional materials, bioremediation, chemical and electrochemical remediation, and intelligent optimization. It emphasizes the characteristics of “greenization–accuracy–low cost” and covers the remediation needs of heavy metals, organic pollutants, and radioactive pollution.

Research teams have focused on functional carriers such as biochar, micro-electrolytic materials, and nanocomposites to improve pollution removal efficiency. In the field of heavy metal remediation, Li et al. used tobacco straw as raw material and prepared modified biochar (HAP-TSB550) through co-pyrolysis with hydroxyapatite (HAP). The specific surface area increased from 3.63 m²/g to 18.36 m²/g, and the maximum adsorption capacity for Cd(II) reached 14.50 mg/g (9.24 times that of unmodified biochar). Through XPS analysis, the synergistic mechanism of “Ca²⁺ exchange (42%) + P-O complexation (38%) + electrostatic adsorption (20%)” was confirmed. Zhang et al. explored the resource utilization of agricultural waste and found that citric acid-modified spent Agaricus bisporus (SCAB) achieved a Zn(II) removal rate of 75.9% (twice that of raw waste) and maintained 58% selectivity even in the presence of coexisting Cu²⁺ and Pb²⁺, providing a scheme for low-cost heavy metal wastewater treatment. In the field of radioactive pollution remediation, Gong et al. prepared nanoflower-like MgAl-LDO/konjac gum (LDO-KGM) through freeze-drying and calcination. The maximum adsorption capacity for U(VI) reached 3019.56 mg/g (2.3 times that of pure MgAl-LDO), and the efficiency remained over 84.5% after five cycles, providing a green material for nuclear wastewater treatment. In the field of organic pollution remediation, Dai et al. prepared spherical Fe-C micro-electrolytic materials, achieving a removal capacity of 105.48 mg/g for crystal violet through the synergistic effect of “adsorption (35%) + micro-electrolytic degradation (65%)”.

Bioremediation technology has attracted considerable attention due to its environmental friendliness and low cost. This issue focuses on technology optimization and pilot-scale verification. In the field of phytoremediation, Wang et al. targeted polymetallic mining areas in Yunnan province in China and screened native accumulator plants such as Juncus effusus (BCF = 1.8) and Phragmites capitata (BCF = 1.5) through the Bioaccumulation Factor (BCF), providing a basis for plant selection in mining area remediation. In the field of microbial remediation, Chen et al. proposed zero-valent iron (ZVI)-enhanced microbial technology for the combined ammonia nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃⁻-N) pollution in acidic groundwater (pH = 3.8). ZVI increased the pH to 6.2 by slowly releasing Fe²⁺, shortening the denitrification cycle from 15 days to 10 days. Pilot-scale experiments further confirmed that the combined process of alkaline reagent injection–intermittent aeration–ZVI–microbial agent addition achieved over 80% removal rates for both NH₄⁺-N and NO₃⁻-N, breaking the application limitation of microbial remediation in acidic environments. In the field of constructed wetlands, Song et al. monitored the 4–5 year operation data of the Hongze constructed wetland in Jiangsu Province, China, and found that the COD removal rate increased from 7.6% to 15.14%, while the NH₄⁺-N removal rate decreased from 78.33% to 46.04%. Additionally, the abundance of ammonia-oxidizing bacteria (AOB) in the aeration tank was 5.3 times that of anaerobic ammonia-oxidizing bacteria, confirming that nitrification dominates NH₄⁺-N removal. It is recommended to replace wetland plants every 3 years to maintain efficiency.

For refractory pollutants such as persistent organic pollutants (POPs) and heavy metals, research teams have developed chemical and electrochemical remediation technologies. Liang et al. prepared Pd/metal foam electrodes for the degradation of 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47) and found that the 15.16% Pd/Ni electrode achieved a debromination efficiency of 94.3%. Through in situ Raman spectroscopy, the mechanism of “stepwise debromination dominated by hydrogen atom transfer” was confirmed, providing a direction for the selection of electrochemical remediation materials. Tang et al. provided theoretical support for advanced oxidation technology, using a Support Vector Machine (SVM) combined with Morgan molecular fingerprints to predict the reaction rate constants of water pollutants with sulfate radicals, achieving an R² of 0.89 and a low MAE of 0.12 for a test set. Through SHAP analysis, 10 key features were identified, such as “number of hydroxyl groups” and “aromatic ring structure”, providing a tool for parameter optimization of advanced oxidation processes (SR-AOPs).

Machine learning and numerical simulation have promoted the transformation of remediation decision-making from “empirical” to “data-driven”. Wang et al. constructed a computational framework integrating process models and random forest regression. The authors simulated 120 remediation scenarios in an arsenic-contaminated site, trained the model to establish the correlation between “excavation depth–chemical dosage–cost-removal rate”, and finally screened out the optimal strategy of “1.5 m partial excavation + 2% biochar addition + natural attenuation”, reducing costs by 35% compared to the traditional full excavation scheme. Zhang et al. further applied this method to an arsenic-polycyclic aromatic hydrocarbon compound-contaminated site of a retired shipyard in Guangzhou, verifying its applicability in complex pollution scenarios and providing technical support for the accurate optimization of remediation schemes.

4. Review Articles and Bibliometric Analysis

Review articles and bibliometric analysis provide trend guidance for the development of the field through systematic sorting and data mining. This Special Issue includes two types of studies: technical reviews and bibliometric analysis.

Alasmary et al. systematically reviewed the remediation technologies for Pb pollution in military shooting ranges, pointing out that bullet weathering is the main pollution source. When soil pH < 5.5, the proportion of exchangeable Pb increases to 15%, significantly enhancing its bioavailability. Among existing technologies, chemical stabilization (phosphate addition) has the lowest cost (200 RMB/mu); however, its long-term stability requires verification. It is recommended to develop “stabilization + Leymus chinensis-rhizobium combined bioremediation” composite technology in the future. Liu et al. focused on forest ecosystems and reviewed the mechanisms of microbial degradation of organic pollutants: acidic soil (pH = 4.5–5.5) is conducive to PAH degradation, and fungi (such as white-rot fungi) contribute 60% of the degradation amount. Genetically engineered Pseudomonas can improve the degradation efficiency of petroleum hydrocarbons by 40%; however, attention should be paid to the impact of exogenous bacteria on the native ecology, providing a technical pathway for pollution remediation in forest ecosystems.

Meng et al. analyzed the research trends of soil minerals and carbon stability using 1834 articles from the Web of Science (2013–2023) with tools such as CiteSpace (version 6.3.R1) and VOSviewer (version 1.6.20). The number of publications in this field increased at an average annual rate of 12.3%, with China (32%) and the United States (25%) leading the field. However, Sino–US cooperative papers only accounted for 8%. Research hotspots focused on “clay mineral–organic carbon adsorption” and “iron oxide catalytic transformation”, with Nature Communications as the core journal. It is recommended to strengthen the research on the coupling mechanism of “climate change–minerals–carbon cycle” in the future to promote the synergy between carbon sequestration and pollution remediation.

5. Conclusions and Future Prospects

The Second Issue of “Advances in Remediation of Contaminated Sites” showcases cutting-edge research across pollution characterization, environmental fate, and remediation technology, reflecting the interdisciplinary and practical nature of the field. Key achievements include (1) the application of multi-source spectral fusion and machine learning for rapid, accurate pollution detection; (2) the revelation of adaptive mechanisms of plants and microbes in contaminated environments; (3) the development of high-performance functional materials (e.g., modified biochar and nanoflower-like oxides) for pollution control; and (4) the integration of intelligent optimization into remediation strategy design, improving cost-effectiveness.

However, several challenges remain: (1) remediation of complex combined pollution (e.g., heavy metal–organic pollutant co-contamination) requires more efficient integrated technologies; (2) long-term ecological risks of remediation technologies (e.g., secondary pollution from nanomaterials) require further evaluation; (3) scaling up lab-scale technologies to engineering applications requires addressing cost and stability issues. Future research should focus on (1) developing synergistic remediation technologies for combined pollution; (2) applying omics and synthetic biology to enhance biological remediation efficiency; (3) establishing long-term monitoring systems for remediation sites; and (4) promoting the integration of intelligent technologies (e.g., AI and the IoT) into site management.

We would like to express our sincere gratitude to all authors for their valuable contributions, the reviewers for their rigorous comments, and the editorial team of Processes for their professional support. It is our hope that this issue will inspire more innovative research and promote the application of remediation technologies, contributing to the sustainable management of contaminated sites worldwide.