Geochemistry and Removal of Heavy Metals and Other Pollutants in Water

1. Introduction to the Special Issue

Global water ecosystems are confronted with increasing pressure, characterized by sustained deterioration of water quality and diminishing availability of water resources, which have emerged as critical constraints on regional sustainable development [1,2]. Rapid industrialization and urbanization have drastically reconfigured watershed land- use patterns and disrupted hydrological–biogeochemical cycling, ultimately reshaping the dynamics of runoff generation, the magnitude and composition of pollutant loading, and the inherent self-purification potential of aquatic ecosystems [3,4]. Such changes present unprecedented challenges not only to the stability of aquatic ecosystems but also to the effectiveness of water environmental management strategies [2,5].

Significantly, contemporary water environmental challenges exhibit intrinsic multidimensionality and cross-scale interactions. At the watershed scale, anthropogenic perturbations govern the trajectories of water quality evolution by altering hydrological processes and the biogeochemical cycling of material fluxes [4,6]. At finer spatial scales, the co-occurrence of emerging contaminants and traditional pollutants further amplifies complexity, as the spatiotemporal dynamics of their coupled transport, transformation, and cross-pollutant interactions remain incompletely elucidated [7,8,9]. Consequently, three core scientific challenges have emerged for the shift from empirically driven to mechanism-oriented water environmental management: identifying pollutant sources and speciation, elucidating key transport and transformation pathways and their controlling factors, and developing efficient, sustainable remediation technologies.

Against this backdrop, this Special Issue focuses on the geochemical characteristics and removal pathways of representative aquatic contaminants, including heavy metals, microplastics, and nutrients. The five contributions, comprising four research articles and one review, systematically address the following research priorities: pollutant transport dynamics under heterogeneous hydrological and geological conditions, mechanistic regulation of bioavailability variations, quantitative source apportionment of groundwater nitrate pollution, and efficacy assessment of synergistic remediation technologies for heavy metal-contaminated river–reservoir systems. By synergistically bridging mechanistic insights with engineering applications, this Special Issue endeavors to establish a systematic and integrative research framework encompassing pollutant identification, process mechanistic elucidation, and targeted technological intervention, thereby offering robust scientific underpinnings and strategic decision support for pollution mitigation and sustainable aquatic environment management in complex scenarios.

2. Main Contributions of This Special Issue

This Special Issue comprises four research articles and one review paper, which synergistically address the geochemical behaviors and mitigation strategies of heavy metals and other key pollutants in aquatic systems. These contributions systematically investigate pollutant source identification, transport and transformation processes, ecological effects, and mitigation pathways across diverse water environments, forming a closed-loop research framework spanning problem diagnosis, mechanistic analysis, and management practice. Collectively, they provide theoretical underpinnings and methodological guidance for the scientific assessment and targeted remediation of complex water pollution scenarios.

With respect to single contamination, the included studies focus on both ecological responses and geochemical stability. One study based on benthic macroinvertebrate community analysis demonstrates that pollution gradients across different water function zones significantly influence species diversity and ecological network structure, with conservation and preservation zones exhibiting substantially better ecological conditions than development-oriented zones (Contribution 1). These findings highlight the critical role of functional zoning and spatial management in maintaining aquatic ecosystem stability. Complementarily, an investigation on mercury (Hg) speciation in sediments reveals that Hg predominantly occurs in stable sulfide-bound forms, which markedly reduces its bioavailability (Contribution 2). This work provides a robust geochemical basis for improving heavy metal risk assessment frameworks.

Regarding complex pollution, one contribution elucidates the heterogeneous interfacial interactions between microplastics and heavy metals. Using Fe(III) as a representative metal, experimental results show that microplastic aging, pH conditions, and particle size significantly regulate adsorption capacity. Furthermore, machine learning approaches were successfully applied to predict the environmental behavior of metal–microplastic interactions (Contribution 3). These findings confirm the role of microplastics as effective vectors for heavy metal transport in aquatic environments, highlighting the necessity of considering synergistic mechanisms involving emerging composite pollutants.

Nutrient pollution is addressed through a study focusing on nitrate contamination in groundwater within typical agricultural regions. By integrating hydrochemical indicators with positive matrix factorization modeling, the study identifies agricultural fertilization and domestic wastewater leakage as the dominant nitrate sources (Contribution 4). Spatial analysis further delineates peri-urban areas, irrigated farmlands, and livestock-intensive zones as priority control regions, providing a scientific basis for differentiated groundwater management and targeted mitigation strategies.

In terms of remediation pathways, the review paper systematically synthesizes and evaluates mitigation strategies for heavy metal pollution control in river–reservoir systems. It summarizes the current understanding of post-dam metal transport, deposition, and remobilization processes, and compares in situ stabilization, phytoremediation, and sediment capping techniques. Specifically, it highlights solidification/stabilization as a particularly promising approach in deep-water reservoirs attributed to its superior engineering feasibility and ecological compatibility (Contribution 5). These insights offer valuable guidance for integrated watershed-scale pollution control and coordinated management of hydropower development and environmental protection.

Overall, the studies presented in this Special Issue collectively advance a systematic and coherent research framework linking pollutant identification, mechanistic elucidation, and remediation technologies. They establish a solid theoretical foundation and technical support for addressing complex, multi-pollutant water contamination, and inform future environmental policy-making and management decisions. Through the continuous integration of fundamental research and engineering practice, aquatic ecosystems are expected to play an increasingly prominent role in pollution prevention and environmental security, facilitating the realization of harmonious human–water relationships and long-term sustainable development.