Advances in Phytoremediation-Based Strategies for Co-Contaminated Riparian Soils: A Review

Abstract

Riparian soils co-contaminated with heavy metals and organic pollutants present a formidable environmental challenge; conventional single-target remediation strategies are frequently insufficient due to the synergistic interactions between contaminant classes. This review offers a systematic synthesis of phytoremediation as an integrative and ecologically sustainable paradigm for addressing these complex multi-pollutant scenarios. Through a critical examination of underlying mechanisms—namely phytoextraction, rhizodegradation, phytostabilization, and phytovolatilization—we evaluate the efficacy of selected hyperaccumulator and pollution-tolerant species in simultaneously mitigating inorganic (e.g., Pb, Cd, As) and organic (e.g., PAHs, pesticides) contaminants. Furthermore, the discussion highlights emerging strategic integrations, including genetic engineering for enhanced metal accumulation, the application of engineered nanomaterials to modulate pollutant bioavailability and plant stress tolerance, rhizosphere amendment with low-molecular-weight organic acids, and biochar-mediated immobilization coupled with microbial stimulation. The analysis posits that phytoremediation, particularly when augmented by these advanced synergies, constitutes a viable, multifunctional, and environmentally benign strategy for the holistic restoration of riparian ecosystems. Future inquiries should prioritize the mechanistic elucidation of combined technologies, the development of predictive performance models, and rigorous long-term field validation to guarantee operational efficacy and environmental safety.

1. Introduction

Soil serves as the fundamental basis for human survival and development, underpinning critical ecosystem functions such as agricultural productivity and ecological regulation [1]. Distinct from agricultural systems, the riparian zone constitutes a vital ecotone bridging terrestrial and aquatic environments, playing a pivotal role in water quality protection, biodiversity conservation, and microclimate regulation [2]. Consequently, the contamination of these zones poses severe threats to both environmental integrity and human well-being. The accelerating global industrialization and urbanization have imposed unparalleled stress on urban riparian soils, with combined contamination emerging as a particularly acute bottleneck restricting the ecological restoration and safe utilization of these vital soil resources [3]. It is reported that approximately 10 million soil sites have been polluted worldwide [4]. Most of these sites are co-contaminated with inorganic and organic pollutants rather than individual pollutants, including heavy metals (HMs), polycyclic aromatic hydrocarbons (PAHs), pesticides, and antibiotics [5]. The co-existence of HMs and organic contaminants can induce negative effects on soil function and quality, subsequently threatening ecology safety and even human health [6]. Furthermore, soil clay minerals and organic matter exhibit a high affinity for heavy metals (HMs) and organic pollutants, which enables the long-term retention of these co-contaminants in soils [7]—with this phenomenon being particularly prominent in riparian soils.

Currently, in contrast to phytoremediation, conventional remediation technologies—including physical, chemical, and microbial-based remediation approaches—exhibit distinct inherent advantages and limitations, respectively, in the remediation of soil co-contamination.

Physical remediation technologies aim to restore soil integrity by employing specific techniques to separate and remove contaminants from the soil matrix. Commonly utilized methods include thermal desorption, soil replacement, isolation/containment, vitrification, and electrokinetic remediation. These technologies are recognized for their rapid treatment efficiency [8], broad applicability, precise process controllability, and low potential for secondary pollution. However, they are frequently associated with substantial energy consumption and high operational costs. Additionally, their efficacy can be limited when addressing deep or heterogeneous contaminated soil profiles.

Chemical remediation involves the application of chemical reagents or reactions to treat polluted media, thereby reducing, removing, or transforming contaminant toxicity to meet environmental safety standards. Prevalent methods include chemical immobilization/stabilization, soil washing, and chemical reduction. While chemical remediation offers relatively high treatment efficiency, operational simplicity, and applicability for both in situ and ex situ scenarios, a significant drawback is its lack of selectivity. This frequently leads to secondary contamination, as chemical agents may degrade beneficial soil constituents or react with non-target contaminants. This not only compromises removal efficiency [9] but also potentially generates more hazardous by-products.

Microbial remediation leverages the metabolic activities of specific microorganisms to degrade pollutants and restore contaminated environments. Its core principle involves modulating microbial communities to achieve efficient removal of organic matter, nutrients (e.g., nitrogen and phosphorus), heavy metals, and oils, while simultaneously improving soil and water quality. Microorganisms can degrade organic pollutants, utilizing them as carbon and energy sources or through co-metabolism. Organic contaminants can be transformed by microbes or form organo-complexes, altering their water solubility and reducing their toxicity [10]. Heavy metal ions can also be detoxified via enzymatic valence state transformations within microbial cells [11]. These processes are associated with minimal environmental disturbance [12]. However, microbial metabolic rates are highly dependent on and sensitive to ambient conditions (e.g., pH, temperature, nutrient availability), making remediation outcomes difficult to predict consistently.

For all three aforementioned traditional methods, a critical challenge remains the precise selection of initial reactants or agents. Elucidating the underlying mechanisms is paramount to preventing the formation of undesirable by-products [13].

In contrast, phytoremediation—characterized by its cost-effectiveness and environmental compatibility—has demonstrated considerable potential for managing soil co-contamination and has increasingly become a focal point of research. To assess research trends, a bibliometric analysis was conducted using VOSviewer software (version 1.6.20) to visualize approximately three hundred recent publications (from the past three years) retrieved from the CNKI database using the keyword “soil remediation technology” (Figure 1). The resulting network map revealed that “phytoremediation technology” constitutes a significant cluster within the literature. This observation underscores that, amidst rapid scientific advancement and growing global emphasis on environmental issues, phytoremediation holds substantial promise for the ongoing research and development of soil remediation technologies.

This review systematically synthesizes developments in bioremediation over the past decade, with a specific emphasis on soil co-contamination. It initially outlines the sources of soil co-contaminants and introduces conventional remediation techniques. The core discussion centers on phytoremediation, detailing its methodologies, applicable plant species, key influencing factors, and future research trajectories. The overarching objective is to enhance the understanding of co-contamination in riparian zone soils and promote the advancement of targeted remediation technologies. This includes a critical evaluation of the effectiveness of these approaches in addressing soil pollution, while concurrently considering their environmental, social, and economic implications to facilitate the development of sustainable, long-term solutions.

Using “soil remediation technology” as the keyword, 300 documents published in the past three years were retrieved from the CNKI database, and the VOSviewer software was used for visual analysis. (This network visualization illustrates the co-occurrence relationships of keywords in the literature. Lines between nodes represent the frequency of co-occurrence (thicker lines indicate stronger associations), while overlapping/aggregated nodes reflect clustered research themes.)

2. Status of Soil Pollution in Riparian Zones

China is characterized by an extensive river network, comprising over 50,000 rivers with drainage areas exceeding 100 km² and more than 1500 rivers with drainage areas greater than 1000 km². These watercourses are classified as either exorheic, discharging into seas or oceans, or endorheic, flowing into inland lakes or evaporating within deserts and salt flats. Exorheic river basins encompass approximately 64% of the national land area, while endorheic basins constitute the remaining 36%. As transitional ecotones between terrestrial and aquatic ecosystems, riparian zones play a critical role in preserving water quality, maintaining biodiversity, and regulating microclimates [14]. However, rapid urbanization and industrialization over recent decades have severely degraded the environmental quality and ecological functions of urban riparian zones due to anthropogenic activities, including non-point source runoff pollution, infrastructure construction, industrial discharges, and traffic emissions [3]. For instance, an assessment of heavy metal concentrations in the riparian soils of the Yellow River Delta revealed elevated levels, with arsenic (As) and cadmium (Cd) identified as posing moderate to severe pollution risks [15].

To systematically evaluate the national soil environment, the former Ministry of Environmental Protection (MEP) and the Ministry of Land and Resources (MLR) conducted a comprehensive nationwide survey from 2005 to 2013. The joint report, the National Soil Pollution Status Survey Bulletin, indicated a concerning overall status: 16.1% of all sampled sites exceeded the environmental quality standards established by the MEP. Soil pollution was particularly acute in specific regions, where heavy metals and metalloids were the primary contaminants, accounting for 82.8% of contaminated sites. These contaminants included manganese (Mn), mercury (Hg), arsenic (As), lead (Pb), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn), with chromium (Cr) being the predominant element. While comprising a smaller proportion, organic pollutants also contributed significantly to soil contamination. Riparian zones encompass various land-use types, including cropland, forest, grassland, and unused land. The soil environmental quality across these land-use types is concerning; notably, cropland soils exhibited a high exceedance rate of 19.4%, underscoring the severity of pollution in riparian zones.

From a global perspective, a recent survey investigated the presence of mesoplastics and macroplastics in surface water and soil at the most heavily contaminated site of the Langueyú Stream in the Pampas region of Argentina [16]. The plastic waste extracted from the surface occupied 19.4% of the total area, of which 70.7% corresponded to single-use plastics, predominantly bags. A total of 331 plastic items were extracted from the soil, 93.7% of which were macroplastics, consisting primarily of threads, wrappers, bags, and other categories. The most abundant mesoplastic categories were bags, followed by threads and coarse plastics. Approximately 73.6% of the plastics extracted from the surface and the macroplastics extracted from the soil were white or transparent; these items occupied a total area of 8090 cm², with a higher concentration of items measuring less than 100 cm². The mesoplastic-to-macroplastic ratio was 0.07 on the surface and 14.76 in the soil. Surface macroplastics likely originate primarily from stream flooding and inappropriately deposited waste. Wind action also likely plays a role, transporting macroplastics from vegetation entrapment into the water or away from the stream banks. Furthermore, a significant process in this context is the burial of plastics within the stream banks, followed by their degradation and fragmentation into mesoplastics.

3. Sources of Compound Pollution in Soil

3.1. Sources of Heavy Metals

This figure (Figure 2) roughly reflects the types and sources of soil pollutants in the riparian zone.

The origins of heavy metals (HMs) in riparian zones stem from two primary factors: natural and anthropogenic sources [17,18].

3.1.1. Natural Factors

Rocks within the surrounding riparian regions contain inherent concentrations of heavy metals. Over prolonged weathering processes, these metals are gradually released into the soil and adjacent water bodies [19]. Furthermore, riparian soils previously contaminated with HMs are susceptible to erosion by water flow. This process facilitates the transport of sediment-bound HMs into river channels [19,20]. Concurrently, erosion can expose HMs from deeper soil layers, thereby increasing the overall HM burden within the riparian zone.

3.1.2. Anthropogenic Factors

Over the past decades and even centuries, numerous human activities have been major contributors to soil HM pollution [21,22].

Industrial Activities: Mining operations generate waste residues and wastewater containing high concentrations of HMs, such as lead, zinc, and mercury. These contaminants enter rivers and infiltrate soils via atmospheric deposition, rainfall runoff, and direct wastewater discharge, thereby polluting riparian areas. Additionally, wastewater, exhaust gases, and solid wastes from manufacturing industries (e.g., electroplating, chemical, electronics) contain significant amounts of highly toxic and bioaccumulative HM particles, including cadmium, mercury, lead, arsenic, and chromium [23], often causing irreversible impacts on riparian soil.

Agricultural Practices: With the advancement of agricultural mechanization and supply chains, the prolonged application of fertilizers, pesticides, and plastic films containing HMs (e.g., cadmium) has resulted in their gradual accumulation in soils [24]. These metals subsequently enter river systems via surface runoff and leaching, further exacerbating the contamination of riparian soils.

Power and Energy Sector: Operations including coal mining, thermal power plants, cement factories, and metal processing plants contribute significantly to HM pollution. For instance, coal combustion releases 1.25 to 13.1 mg of Cd per ton via flue gas [25], which also contains substantial amounts of arsenic and lead [26].

Urbanization: Urbanization leads to high population density, intensifying the challenge of managing ever-increasing production waste [27]. Landfill leachate constitutes a primary vector for pollutants [28], characterized by a complex composition including high concentrations of organic matter, inorganic ions, microorganisms, and HMs [29]. Unlike many organic pollutants, toxic HMs are non-degradable and resistant to chemical or biological breakdown [30]. Furthermore, HMs from municipal landfills migrate into surrounding water bodies and soils via leachate discharge. They are assimilated by biota and subsequently impact the riparian environment [30]. Through biogeochemical processes, these trace metals continuously accumulate in riparian soils, sediments, and plants, eventually exceeding regional background concentrations and leading to HM pollution in the riparian zone [18,31].

3.2. Sources of Organic Pollutants

In contrast to natural inputs, organic pollutants in riparian soils predominantly originate from anthropogenic activities.

Industrial Sources: Industrial operations in the petroleum, chemical, and metallurgical sectors generate organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) [32], per- and polyfluoroalkyl substances (PFAS) [33], microplastics [34], and antibiotics [35,36].

Agricultural and Aquacultural Sources: The excessive application of pesticides (e.g., DDT, organophosphates) and herbicides (e.g., glyphosate), coupled with the rapid expansion of aquaculture (leading to the discharge of antibiotics, hormones, and ammonia-derived organic matter from animal waste), contributes to organic pollutants that enter river systems and ultimately persist in soils.

Research utilizing oysters (Saccostrea mordax) from coastal Okinawa to assess the status and geographical distribution of persistent organic pollutants (POPs) revealed widespread contamination not only by organochlorine pesticides (OCs) but also by various brominated flame retardants, including polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecanes (HBCDs), across the coastal region [37]. These pollutants, characterized by environmental persistence and bioaccumulation potential, are typically discharged into rivers and lakes via wastewater and exhaust gases. They contaminate water and infiltrate riparian soils, exerting adverse effects on various flora and fauna [38].

3.3. Difficulties in Dealing with Complex Pollution in Riparian Zones

3.3.1. Synergistic Effects of Pollutants

Within soil environments, organic pollutants and heavy metal ions engage in interactions through mechanisms such as physical adsorption, chemical precipitation, and complexation [39]. These interactions create complex contamination scenarios that pose significant difficulties for synergistic remediation technologies. For instance, microplastics (MPs), owing to their hydrophobicity and large specific surface area, often act as effective carriers for other pollutants [40]. Through physical adsorption [41], chemical co-precipitation [41], and complexation with hydrous oxides [42], MPs can directly adsorb heavy metals (HMs) and subsequently alter their distribution within the soil matrix [43]. Furthermore, MPs can impact soil microbial communities and physicochemical properties (e.g., pH, organic matter content, porosity). These indirect effects may subsequently modify the chemical speciation of heavy metals, thereby influencing their bioavailability [44]. This phenomenon serves as a representative example of pollutant synergy.

Moreover, interactions between different heavy metals within co-contaminated systems significantly increase the difficulty of soil remediation. A pertinent example is the competitive adsorption between copper (Cu) and cadmium (Cd). Studies have demonstrated that the individual adsorption and retention of Cu²⁺ in soil are primarily influenced by pH, effective cation exchange capacity (CECe), dissolved organic carbon (DOC), and the contents of vermiculite, chlorite, and total clay. Conversely, the individual and competitive adsorption/retention of Cd²⁺ are mainly governed by pH, CEC, and the contents of manganese oxides, vermiculite, chlorite, and total clay [45]. Notably, the influencing factors for both metals are largely overlapping. Experimental results from such studies indicate that most soils exhibit a significantly higher affinity for Cu²⁺ than for Cd²⁺, a direct consequence of competitive adsorption between the two ions [45]. Given the differential affinities of adsorbents for different heavy metals, identifying a single adsorbent capable of efficiently and simultaneously sequestering multiple heavy metal species is challenging. This adds considerable complexity to the selection and application of adsorbents during the remediation process.

Simultaneously, co-contamination remediation is constrained by limitations in materials and technologies. In practical applications, there is often a lack of materials and processes that can efficiently remove multiple classes of pollutants concurrently. Technologies targeting a single pollutant class may exert adverse effects on others. For example, when ozone is employed to degrade organic pollutants in water, it may oxidize As(III) to As(V) [46], which is more mobile in the environment, thereby complicating the water treatment process. Furthermore, after organic and heavy metal pollutants interact, originally effective treatment methods may become significantly less efficient or even yield unpredictable adverse effects.

3.3.2. Complexity of Contaminant Speciation

Heavy metals and organic pollutants can coexist in various states, including dissolved, adsorbed, precipitated, and complexed forms. Moreover, the concentration of organic pollutants can influence the speciation of heavy metal contaminants. The respective concentrations and distribution of these species collectively determine their interactions with the surrounding environment [47], rendering single-treatment technologies often inadequate for simultaneous removal. Studies have further revealed that the presence of coexisting anions significantly alters the affinity between dissolved organic matter (DOM) and heavy metal particles, while competitive cations can markedly reduce the adsorption rate of heavy metals in soils [48]. These findings highlight the considerable challenges that contaminant speciation complexity poses to remediation processes.

Furthermore, fluctuations in riparian zone environmental conditions can induce speciation transformations among pollutants of different valences and types. Consequently, their interaction mechanisms are anticipated to be highly complex. This underscores the necessity of employing advanced analytical tools to gain a more profound understanding of the underlying principles of co-contamination [47]. Such complexity necessitates real-time adjustment during the remediation process, which simultaneously increases costs and operational difficulty. A pertinent example is the remediation of sites co-contaminated with Cr(VI) and polycyclic aromatic hydrocarbons (PAHs). While reducing agents can transform toxic Cr(VI) into less toxic Cr(III) precipitates, intermediate products generated during PAH degradation may subsequently re-mobilize the precipitated Cr(III) [49], thereby complicating the overall treatment strategy.

3.3.3. Influence of the Self-Characteristics of Riparian Zones

Riparian soils, acting as the core interface of the terrestrial–aquatic ecotone, develop unique characteristics characterized by strong physicochemical heterogeneity, high biological activity, and complex interfacial processes under the combined influence of hydrological fluctuations, vegetation succession, sediment deposition, and biological activities. Concurrently, they perform vital ecological functions such as pollution buffering, water conservation, and bank stabilization [50]. Studies indicate that the pollutant retention and removal efficiency of riparian zones can be influenced by factors including season, vegetation type, plant species, and riparian width [51,52], with flood regimes being the most significant driver.

Flood regimes primarily alter the soil environment (e.g., oxygen levels, nutrient availability, pollutant speciation) through variations in inundation duration, frequency, and intervals, thereby exerting a bidirectional effect on plant survival, root function, and remediation efficiency. Moderate flooding, characterized by short-duration inundation and appropriate intervals, can enhance root aeration, promote the dissolution and migration of pollutants (heavy metals/organics), and facilitate plant uptake and accumulation, leading to increased remediation efficiency. Conversely, prolonged or excessively frequent flooding induces soil anoxia, resulting in root decay, plant mortality, and suppressed microbial activity, which can cause a sharp decline in remediation efficiency and potentially trigger secondary pollutant release. Similarly, overly short or infrequent flooding leads to insufficient soil moisture, restricted plant growth, low pollutant bioavailability, and suboptimal remediation performance.

For instance, researchers conducting greenhouse experiments in Xalapa, Veracruz, Mexico, to investigate flood frequency, found that under conditions of intermittent flooding (14 h drainage/10 h inundation) coupled with nutrient addition, the removal rate of total petroleum hydrocarbons (TPHs) by mangroves was significantly enhanced, reaching up to 47%. In contrast, permanent inundation resulted in extremely low remediation efficiency of only 2.99% [53]. Similarly, another study examining the effects of different water regimes on the growth and Cd accumulation capacity of the hyperaccumulators Rorippa sylvestris (L.) Besser and Rorippa amphibia Besser in Cd-contaminated soil demonstrated that the translocation factor (TF) for Cd was significantly higher under periodic flooding (alternating wet and dry conditions) compared to either permanent inundation or drought [54]. These cases collectively underscore the substantial influence of flood regimes on riparian plant growth and remediation processes. Therefore, when selecting plants for field-scale phytoremediation, it is crucial to rigorously consider the impact of the local riparian environment and choose species adapted to the specific hydrological regime. When implementing a contaminated soil remediation project at a specific site, it is essential not only to systematically assess the types and composite characteristics of pollutants but also to comprehensively consider multiple factors, including site-specific water-level gradients, plant tolerance to extreme climatic conditions, and the response mechanisms of plants to water-level fluctuations. It is recommended to adhere to the principles of “gradient-based configuration, native-species prioritization, water-level regulation, and periodic harvesting” in plant selection and remediation system design. This approach aims to synergistically enhance remediation efficiency, system stability, and long-term ecological security, ultimately achieving the dual objectives of maximizing remediation benefits while minimizing environmental disturbance.

4. Phytoremediation Technology

Phytoremediation is an ecologically sustainable technology that employs specific plant species and their associated microbial communities to remove, degrade, stabilize, or transform contaminants via natural physiological and metabolic processes. This approach principally operates through five distinct mechanisms: phytoextraction, phytostabilization, rhizofiltration, phytovolatilization, and phytodegradation [55]. Although phytoremediation presents certain limitations—including extended remediation timeframes and potential phytotoxicity under elevated contaminant loads—it has garnered significant attention within the field due to its inherent ecological compatibility and sustainability. Furthermore, relative to conventional remediation techniques, phytoremediation offers distinct advantages: it minimizes secondary soil disturbance and pollution, ameliorates soil structure, augments microbial diversity, and facilitates the direct restoration of remediated sites for use as green spaces or agricultural land. The cost-effectiveness and operational simplicity of this approach further bolster the economic viability of rehabilitating contaminated land. A schematic diagram illustrating the specific mechanisms of phytoremediation is presented in Figure 3.

4.1. Phytoaccumulation

Phytoaccumulation refers to the process by which plants assimilate contaminants from the environment into their tissues via mechanisms such as root uptake and foliar adsorption. Certain heavy metals, including Fe, Mo, Cu, Mn, Si, Se, Co, and Zn, function as essential micronutrients for plant metabolism. Plant species capable of absorbing and accumulating significantly elevated concentrations of these essential elements, as well as non-essential heavy metals, are designated as hyperaccumulators. Typically, these plants demonstrate metal concentrations in their shoots that are at least two orders of magnitude greater than those observed in non-accumulator plants, although exact thresholds may vary depending on environmental conditions [56].

Studies have demonstrated that typical hyperaccumulators, such as the fast-growing tree species Casuarina, can effectively remove heavy metal ions (Cd, Pb, Ni, and Zn) from secondary-treated municipal wastewater (SWW), achieving removal efficiencies of 92%, 77%, 83%, and 73%, respectively. Furthermore, the accumulation of these heavy metals was found to be higher in the roots compared to the shoots [57]. This indicates that, in contrast to the classical hyperaccumulation profile where metals are translocated to aerial tissues, this species primarily sequesters heavy metal ions within root cells. Although hyperaccumulators possess a robust capacity for metal uptake, their practical application is constrained by numerous challenges, including limited extraction rates, low biomass, potential ecological risks, susceptibility to heavy metal toxicity, and extended remediation timelines. Nevertheless, these limitations can be mitigated by integrating phytoaccumulation with complementary technologies to enhance overall heavy metal removal efficiency [58].

4.2. Phytostabilization

Phytostabilization is extensively employed for the remediation of soils in abandoned mining areas contaminated with metals such as Zn, Pb, Cd, Mn, Cu, Cr, Fe, As, and Ni [59,60,61]. Root exudates facilitate the exchange of materials, energy, and information between plant roots and the rhizosphere [62]. This technology capitalizes on the chelation, adsorption, precipitation, and degradation capabilities of root exudates—comprising organic acids, amino acids, polysaccharides, enzymes, and analogous compounds—to immobilize heavy metal ions. Subsequently, these ions are sequestered through processes such as intracellular deposition and transformation, thereby reducing the mobility and bioavailability of heavy metal contaminants within the soil matrix.

Additionally, root exudates modulate soil pH and redox potentials [62], fostering the growth and metabolic activity of plants and rhizosphere microorganisms. This, in turn, indirectly influences the speciation and stability of heavy metals [63] and enhances overall phytoremediation efficiency. The composition and quantity of root exudates are governed by variables including plant species, developmental stage, soil texture, pH, temperature, nutrient availability, pollutant bioavailability, and the rhizosphere microbial load [64]. Consequently, research indicates that soil amendments significantly enhance the efficacy of phytostabilization. For instance, the application of compost effectively immobilizes heavy metals and stimulates plant biomass production [65]. Furthermore, the combination of biochar and compost promotes the establishment of plant cover and improves phytostabilization efficiency in soils co-contaminated with Cd, Cu, Ni, Pb, and Zn [66].

Nevertheless, despite these interventions, phytostabilization primarily serves to convert heavy metals into less toxic or inert forms, immobilizing them within the soil or plant tissues to mitigate migration rather than reducing the total contaminant concentration. A persistent risk of pollutant re-mobilization exists subsequent to plant senescence and decomposition, or if soil physicochemical conditions shift (e.g., a decrease in pH precipitating metal dissolution).

When employing hyperaccumulator plants for phytoremediation, intercropping with companion species—such as Lolium perenne L. (ryegrass) and Astragalus sinicus L. (Chinese milk vetch), characterized by slow decomposition rates and high adsorptive capacities—presents a viable mitigation strategy [67]. The residues of these companion plants can induce a “physical encapsulation” effect on hyperaccumulator residues, retarding the decomposition rate of the latter and attenuating the peak release of heavy metals. Furthermore, the prompt removal of all plant residues from the remediated site is imperative to prevent in situ decomposition, thereby effectively eliminating the risk of secondary contaminant release into the soil or aquatic environment.

4.3. Rhizofiltration

Rhizofiltration harnesses the adsorptive, absorptive, and degradative capacities of plants and their associated symbiotic microorganisms to sequester pollutants—including heavy metals, organic compounds, and nitrogen and phosphorus nutrients—from aquatic environments and concentrate them within plant tissues. Consequently, plant species exhibiting hypoxia tolerance, metal resistance, and extensive root surface areas are considered optimal candidates for rhizofiltration [68]. For instance, numerous studies have investigated the deployment of aquatic macrophytes for the remediation of industrial and domestic wastewater [69,70]. In macrophyte-based wastewater treatment systems, high removal efficiencies have been documented for biochemical oxygen demand (BOD), nitrogen, suspended solids, and coliform bacteria [71,72].

Concurrently, a novel approach involves constructed wetlands featuring floating emergent plants, in which vegetation grows hydroponically on rafts rather than being rooted in sediments. Such systems offer the advantages of passive operation, low maintenance requirements, and operational simplicity [73]. However, these systems are not without limitations. Studies utilizing the comet assay on fish exposed to effluent from floating emergent plant wetlands have demonstrated increased DNA damage, suggesting that such treatment systems may inadvertently augment the genotoxicity of the wastewater [74]. Crucially, once the adsorptive capacity of the plant roots reaches saturation, the timely harvesting and appropriate disposal of the biomass are imperative to prevent secondary pollution [75,76,77].

4.4. Phytovolatilization

Phytovolatilization refers to the process by which plants absorb contaminants and subsequently release volatile compounds or their metabolites into the atmosphere through transpiration. This technology has been widely applied to remove highly volatile metals such as mercury, arsenic, and selenium. Research indicates that Arabidopsis thaliana can convert Hg²⁺ to Hg⁰, which increases mercury’s volatility, allowing it to diffuse freely into extracellular spaces or enter the xylem, eventually escaping from plant tissues through stomatal transpiration [78]. However, the number of natural plant species capable of converting metals into volatile forms is very limited. Therefore, phytovolatilization technology often employs transgenic plants to enhance the ability of plants to volatilize metals [78]. Factors influencing phytovolatilization include plant species, growth stage, and soil conditions, which collectively determine the efficiency of the process. Nevertheless, when implementing phytovolatilization, it is essential to monitor the migration of pollutants in the atmosphere promptly, as the process primarily transfers contaminants from soil to the atmosphere. Over time, heavy metal pollutants may be re-deposited into the soil through processes like dry and wet deposition, potentially leading to secondary pollution.

4.5. Phytodegradation

Phytodegradation encompasses the metabolic transformation of organic pollutants—such as pesticides, petroleum hydrocarbons, and polycyclic aromatic hydrocarbons (PAHs)—into less toxic or inert substances. This process occurs either through direct plant metabolism or synergistically with rhizosphere microorganisms. Degradation proceeds via two primary pathways: first, through direct plant-mediated enzymatic catalysis involving oxidoreductases, hydrolases, and transferases [79]; and second, through the stimulation of rhizosphere microbial communities. Root exudates supply labile carbon and nitrogen sources to the rhizosphere, potentially augmenting microbial metabolic activity by 10- to 100-fold, thereby accelerating the degradation of organic contaminants [75]. However, rhizosphere microbial activity is contingent upon specific environmental parameters, such as pH and oxygen availability. Furthermore, the generation of degradation intermediates possessing elevated toxicity relative to the parent compounds (e.g., the conversion of DDT to DDE) presents a significant challenge. Consequently, these factors introduce inherent complexities and uncertainties into the remediation process.

5. Types of Remediation Plants

5.1. Plants for Remediation of Heavy Metal Contamination

As previously discussed, hyperaccumulators are plants whose leaf metal content is at least 2–3 orders of magnitude higher than that of plants growing in normal soils and at least one order of magnitude higher than that of plants growing in metalliferous soils, commonly referred to as super accumulators [80,81]. We have counted several common heavy metal hyperaccumulator plants as shown in

Table 1. Heavy metal hyperaccumulator plants.

Plant Name	Accumulated Heavy Metals	Effects	Citations
Sedum alfredii	Cd, Zn	Cadmium content in leaves and stems can reach up to 9000 mg/kg (dry weight, DW) and 6500 mg/kg (DW), respectively; zinc content in roots can reach approximately 14,000 mg/kg DW.	[82,83,84]
Brassica juncea (Indian Mustard)	Zn, Ni	Zinc concentrations ranged from 147.2 ± 13.9 to 155.5 ± 6.3 mg kg−1 (stems) and 134.7 ± 7.7 to 171.9 ± 3.9 mg kg−1 (leaves). Nickel concentration can reach up to 2687 mg kg−1 (though chlorophyll is inhibited at this level).	[85,86]
Dicranopteris pedata (Gleicheniaceae fern)	Rare Earth Elements	Concentration of rare earth elements can reach up to 3358 µg/g.	[87,88]
Pityrogramma calomelanos	As	Arsenic concentration accumulated in leaves can reach up to 4260 mg kg−1.	[89]
Berkheya coddii	Ni, Co	Accumulates > 30,000 µg g−1 Ni in dry leaves while co-accumulating up to 600 µg g−1 Co.	[90,91]
Solanum nigrum L. (Black Nightshade)	Cd, Pb	Citric and polyglutamic acids in the plant enhance the translocation of Cd and Pb from roots to shoots and increase available Cd and Pb concentrations in rhizosphere soil, improving uptake efficiency.	[92,93]
Pistia stratiotes L. (Water Lettuce)	Fe, Al, Zn	Fe content in roots can reach 19,726 mg kg−1; Al content, 15,128 mg kg−1.	[94]
Silybum marianum(L.) Gaertn. (Milk Thistle)	Cu, Pb, Cd, Zn	-	[95,96]
Leersia hexandra Swartz	Cd	Under high-concentration Cd culture solution, the average Cr concentration in its leaves was 2932 mg/kg.	[97]
Celosia argentea L.	Cd, Mn	Maximum Cd and Mn concentrations in leaves were 276 and 29,000 mg/kg, respectively.	[98,99]
Boehmeria nivea (L.) Gaudich. (Ramie)	Cd	Highest Cd content reached 146 mg/kg in roots and 102 mg/kg in leaves.	[100]
Chrysopogon zizanioides (L.) Roberty (Vetiver)	Zn	Maximum total zinc accumulation reached (8068 ± 407) mg/kg.	[101,102,103]
Pteris vittata L. (Chinese Brake Fern)	As	Exhibits strong As uptake, efficient As translocation, and high cellular As tolerance; leaf As concentration can reach up to 3210 mg/kg.	[89,93,104]

, and recorded their respective enrichment effects on heavy metals.

To this end, we calculated the translocation factors of several hyperaccumulator plants for metal Zn, as shown in the figure below (

Table 2. Translocation coefficient of heavy metal Zn.

Plant Names	Translocation Factor (TF)	Citations
Sedum alfredii	1.2~3.5	[105,106]
Viola baoshanensis	2.3~5.1	[107]
Brassica juncea	1.1~2.7	[108,109]
Arabidopsis halleri	1.3~3.2	[110]

).

It was found that the translocation factors of several hyperaccumulator plants were similar. However, relatively speaking, Viola baoshanensis had a better ability to accumulate Zn than the other plants, which also provided inspiration for the future remediation of Zn-contaminated riparian zone soils.

Phytoremediation of Organic Contaminants

Concurrent with rapid economic and industrial development, the intensity of petroleum extraction has escalated significantly. Operational failures during extraction and production (e.g., well blowouts), leakage during transportation and processing, accidental spills, and the indiscriminate discharge of petroleum by-products and oily wastewater have collectively contributed to the environmental release of petroleum hydrocarbons [95,96]. These contaminants pose a substantial threat to ecological systems, representing a particularly acute risk to riparian zones. We have counted several common plants that absorb organic pollutants, as shown in

Table 3. Organic-tolerant plants.

Plant Name	Accumulated/Degraded Organic Compound(s)	Effects/Performance	Citations
Brassica juncea (Indian Mustard)	Ciprofloxacin; Bisphenol A and Ibuprofen	Showed phytotoxicity symptoms at higher ciprofloxacin concentrations (200 mg kg−1 and above) but demonstrated 65.71% remediation potential at 100 mg kg−1. Under high pollutant (BPA and Ibuprofen) concentrations, soil enzyme activities including dehydrogenase activity (DHA) were reduced by 45%.	[111,112]
Salix rubens and Salix triandra (Willow species)	Petroleum Hydrocarbons	Initial concentrations of pyrene and chrysene (23.06 μg kg−1 and 126.27 μg kg−1, respectively) decreased to undetectable levels in most cases. Concentrations of benzo[k]fluoranthene and benzo[a]pyrene also decreased from 28.44 and 3.82 μg kg−1 to undetectable levels.	[113]
Chrysopogon zizanioides (L.) Roberty (Vetiver)	Nitrogen- and Phosphorus-containing organics (in wastewater)	The highest overall removal efficiency was achieved at a high planting density (60 plants/m2), with removal rates for BOD, COD, nitrogen, and phosphate reaching 89.7%, 80.6%, 60.5%, and 40.8%, respectively.	[114]
Hybrid Poplar (Populus spp.)	Trichloroethylene (TCE)	Takes up TCE into above-ground tissues and degrades it into metabolites trichloroacetic acid (TCAA), dichloroacetic acid, and trichloroethanol, while the parent chlorinated solvent volatilizes into the atmosphere.	[115,116]
Taraxacum ohwianum K., Potentilla discolor B., and Chelidonium majus L.	Benzo[a]pyrene (BaP)	Shoot enrichment factors were 1.01, 4.98, and 38.24, respectively, indicating a strong ability to transport BaP from roots to shoots.	[117]
Medicago sativa L. (Alfalfa)	Petroleum Hydrocarbon Pollutants	Stimulates the activity of soil dehydrogenase and peroxidase, thereby promoting the oxidative decomposition of pollutants.	[118,119,120,121]

, and recorded their respective enrichment effects on pollutants.

To date, few hyperaccumulator plants for organic pollutants have been identified, and there is no widely accepted list.

5.2. Summary

Given the vast diversity of plant species utilized for remediation, the systematic screening of candidates is a critical determinant of phytoremediation success. Currently, however, there is no unified standard or methodology for such screening. Common approaches typically involve field surveys and the analysis of plant samples collected from contaminated riparian zones. Laboratory experiments, such as greenhouse hydroponic systems or soil pot simulations, are also widely documented in the literature. While these controlled environments facilitate an in-depth exploration of remediation mechanisms, they often fail to account for the myriad environmental variables inherent in natural settings—including soil composition, climatic conditions, water availability, nutrient competition, biological pests, and light exposure [122]. For instance, researchers evaluating the cadmium remediation potential of marigolds observed that maximum Cd content in branches reached 19.48 mg·kg⁻¹ under greenhouse conditions. In contrast, field experiments revealed significantly lower accumulation, with stem and root contents reaching only 6.66 and 7.29 mg·kg⁻¹, respectively [123]. While these findings affirm the potential of marigolds for remediating cadmium-contaminated soil, the marked divergence between laboratory and field data warrants critical attention. This limitation is particularly pronounced in riparian zones, where soil composition and environmental variability are highly complex. Consequently, studies relying on controlled simulations often struggle to reflect actual remediation efficacy, frequently leading to suboptimal results in practical applications [124].

Accordingly, effective plant screening must integrate a holistic set of criteria, including the specific phytoremediation technology, pollutant characteristics, plant biological traits, cultivation requirements, management practices, and biodiversity conservation objectives [125].

6. Factors Influencing Phytoremediation Efficiency

The efficacy of phytoremediation is governed by three primary categories of variables: plant characteristics, pollutant properties, and environmental conditions.

6.1. Plant Characteristics

As the principal biological agents in the remediation process, the inherent traits of plants directly determine their remediation potential. As previously discussed, hyperaccumulator species—capable of sequestering exceptionally high contaminant levels in their tissues without exhibiting phytotoxic effects—represent promising candidates for phytoremediation [126]. Given the extensive diversity of candidate flora, species selection is paramount for the successful deployment of phytoremediation technologies. Nevertheless, universally standardized screening criteria and methodologies remain lacking.

6.2. Pollutant Properties

Environmental pollutants are generally categorized as either inorganic metals or organic compounds based on their chemical nature. Regarding inorganic pollutants, such as heavy metals, plants primarily facilitate accumulation through root adsorption and active uptake. Certain hyperaccumulators further sequester heavy metals within vacuoles via chelation, thereby mitigating cellular toxicity. In contrast, the remediation of organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs) and organic pesticides, involves complex pathways including uptake, degradation, and volatilization. Plant enzymatic systems facilitate the breakdown of these contaminants into less toxic or non-toxic, low-molecular-weight substances.

For heavy metals, three key indices are utilized to assess phytoremediation efficacy: the Translocation Factor (TF = [concentration in aboveground parts]/[concentration in roots]); the Bioaccumulation Factor (BF = [concentration in plant]/[concentration in soil]); and the Phytoremediation Factor (PF = ([aboveground concentration] × [aboveground biomass])/[soil concentration]) [127]. For organic pollutants, the primary evaluation metric is the degradation efficiency of the target compounds.

Furthermore, in scenarios of combined contamination involving both heavy metals and organics, the synergistic interactions between metal ions and organic molecules, along with the resultant complexity of contaminant interactions, constitute a major determinant of phytoremediation efficiency.

Contaminant concentration directly dictates plant health and remediation performance. Low concentrations typically induce insignificant phytotoxicity, allowing for normal plant growth and contaminant accumulation. Conversely, excessively high concentrations can inhibit physiological processes such as photosynthesis and respiration, leading to growth stunting, reduced biomass, or plant mortality. Additionally, the chemical speciation of the pollutant is of paramount importance. Pollutants existing in dissolved or ionic forms are more readily absorbed by plant roots, whereas those in precipitated or strongly bound states exhibit reduced bioavailability and may require environmental conditioning to convert them into bioaccessible forms.

For organic pollutants, the primary evaluation metric is the degradation efficiency of the target compounds.

6.3. Environmental Conditions

Phytoremediation is a complex process occurring within natural environments, where external conditions indirectly regulate efficiency by modulating both plant growth and pollutant activity.

6.3.1. Soil Physicochemical Properties

As the medium supporting nearly the entire plant life cycle from germination to fruiting, soil physicochemical properties are fundamental to plant development. These properties encompass soil texture, pH, organic matter content, nutrient status, and aeration. Among these, soil pH exerts a particularly pronounced influence on phytoremediation efficiency. For instance, research indicates that sulfur amendment in alkaline soil can lower pH, increase bioavailable cadmium content, and thereby enhance cadmium uptake by chicory (Cichorium intybus), improving remediation outcomes [128]. Specifically regarding riparian soils, heterogeneity in these properties stems from the combined effects of natural processes (e.g., hydrology, sedimentation-erosion, vegetation) and anthropogenic activities, further compounded by sampling and analytical methodologies. Effective ecological remediation designs must rigorously account for this variability by employing monitoring strategies that utilize multiple sampling points, depths, and seasonal timings, combined with statistical methods for quantitative analysis.

6.3.2. Pollutant Bioavailability

Pollutant bioavailability refers to the extent to which a contaminant can be absorbed, transformed, and exert ecotoxicological effects on an organism. Studies have demonstrated that Cd toxicity can significantly inhibit plant growth, particularly at high concentrations, by increasing the translocation factor and root cell wall thickness, and disrupting the ultrastructure of leaf chloroplasts [129]. Consequently, this mechanism inhibits the plant’s capacity to absorb and transform cadmium, thereby affecting remediation efficiency. Furthermore, heavy metal bioavailability is influenced by several parameters, including soil pH, redox potential (Eh), and the quantities of organic matter, clay, and oxide minerals. Elevated soil pH can reduce the concentration of metals in the soil solution, often due to metal complexation by functional groups associated with organic matter and oxides [130,131].

6.3.3. Plant–Microbe Interactions

Rhizosphere microorganisms are indispensable components of the phytoremediation system. The symbiotic relationships established with plants can significantly enhance remediation efficiency. Microbes not only degrade high-molecular-weight organic pollutants that are recalcitrant to plant assimilation but also secrete phytohormones that promote plant growth and increase contaminant tolerance [132,133]. Reciprocally, plant root exudates supply carbon sources and energy to microbial communities, stimulating their proliferation and metabolic activity. Consequently, combined plant–microbe remediation is regarded as a promising strategy for co-contaminated soils.

One study investigated the phytoremediation potential of Catharanthus roseus in copper (Cu)- and lead (Pb)-contaminated soil via inoculation with Pseudomonas fluorescens RB4 and Bacillus subtilis 189. These bacterial strains significantly increased the availability of water-soluble Cu and Pb. Compared to non-inoculated controls, plants inoculated with P. fluorescens RB4 exhibited fresh weight increases of 102%, 48%, and 45% in Cu-, Pb-, and Cu + Pb-contaminated soils, respectively. Similarly, inoculation with B. subtilis 189 resulted in fresh weight increases of 108%, 43%, and 114% under the same conditions. Co-inoculation with both bacteria yielded even greater improvements, with fresh weight increases of 121%, 102%, and 177% in Cu-, Pb-, and Cu + Pb-contaminated soils, respectively. Furthermore, co-inoculated plants demonstrated higher accumulation of Cu and Pb in shoots, alongside improved translocation and bioaccumulation factors [134]. These results highlight the enhanced phytoremediation efficiency achieved through inoculation with P. fluorescens RB4 and B. subtilis 189.

7. Development Directions for Phytoremediation

Functioning as an ecotone between terrestrial and aquatic ecosystems, the riparian zone supports a diverse array of flora. Comprising predominantly shrubs and herbaceous species, these plants facilitate the accumulation of metals and organic pollutants from the soil surface [135]. While a portion of these contaminants may be naturally degraded by environmental microorganisms, this process is often protracted [136,137]. Consequently, pollutants inevitably migrate to adjacent riparian soils and groundwater, where they may persist for centuries, imposing a significant ecological burden and posing health threats to humans and other organisms [138]. Therefore, the development of more efficient and comprehensive methods for decontaminating riparian soils is imperative. As previously discussed, while conventional remediation technologies possess distinct merits, they are frequently constrained by environmental variables [139,140]. In contrast, phytoremediation is characterized by its ecological compatibility and sustainability. Thus, the integration of phytoremediation with complementary technologies represents an essential strategy for optimizing remedial efficacy while minimizing environmental disruption.

7.1. Plant-Gene Editing Combined Remediation Technology

Genetic engineering exhibits substantial promise for mitigating environmental toxins. The strategic introduction of genetic modifications can endow plants with enhanced metabolic pathways, enabling them to target and remediate pollutants more effectively than their wild-type counterparts [141]. This technological advancement not only augments the efficiency of phytoremediation but also broadens the spectrum of addressable contaminants, offering scalable interventions for some of the most refractory environmental problems [142,143]. Consequently, integrated plant-gene editing remediation remains a viable strategy for addressing the complex environmental conditions and co-contamination scenarios typical of riparian zones.

Based on the phytoremediation process, three primary categories of transgenic technologies can facilitate efficient remediation: (1) modifying plants with genes encoding metal-binding proteins, (2) modifying plants with genes encoding metal transporters, and (3) modifying plants with genes encoding enzymes that detoxify metals and metalloids.

7.1.1. Modifying Plants with Genes Encoding Metal-Binding Proteins

The metal sequestration capacity of plants can be effectively augmented by the introduction of genes encoding metal-binding proteins [144]. Research indicates that plants engineered with genes for metal-binding proteins, or enzymes capable of degrading organic pollutants, can more efficiently target specific contaminants. Advances in genetic technology progressively address traditional limitations—such as low uptake rates or sensitivity to environmental stressors—thereby facilitating the deployment of phytoremediation in relatively harsh and complex contamination scenarios. This underscores the transformative potential of genetic engineering in enhancing phytoremediation. Relative to non-transgenic plants, those engineered with metal-binding protein genes demonstrate the ability to absorb and sequester substantial quantities of heavy metals. For instance, transgenic tobacco and Arabidopsis thaliana exhibit increased uptake rates of zinc and cadmium, suggesting their applicability in the remediation of contaminated soils [145]. Furthermore, genetic modification of Brassica napus has resulted in a significant increase in cadmium resistance, with engineered plants displaying a 16-fold increase in tolerance compared to unmodified variants [146]. These advancements not only improve the efficiency of phytoremediation—expediting the remediation timeline—but also enhance adaptability to diverse environmental and contamination contexts.

7.1.2. Genes Encoding Metal Transporters

Metal transporters are integral to metal uptake, translocation, and compartmentalization—key processes underlying metal tolerance and accumulation. Consequently, the modulation of metal transporter gene expression is critical following the initial absorption of heavy metal ions. However, the manipulation of transporters responsible for metal uptake and translocation can disrupt metal homeostasis; therefore, such strategies require careful optimization to avoid rendering the resulting transgenic plants unsuitable for phytoremediation purposes [147]. For example, plasma membrane phosphate transporters have been investigated as a strategy to increase arsenic uptake, given the chemical analogy between arsenate (As(V)) and inorganic phosphate. The overexpression of the AtPHT1 and AtPHT7 genes (expressed in vegetative and reproductive tissues of Arabidopsis, respectively) in tobacco induced As(V) hypersensitivity due to increased As uptake and accumulation. However, when co-introduced with the YCF1 gene (encoding a yeast vacuolar cadmium transporter from Saccharomyces cerevisiae), the stacked-transgenic lines exhibited superior As(V) tolerance and accumulation compared to wild-type plants and single-gene transformants [148]. Moreover, the effects of overexpressing the same metal transporter can vary across different plant species. Overexpression of AtHMA4 in Arabidopsis increased tolerance to Cd, Zn, and Co and led to greater accumulation of Zn and Cd in shoots [149]; conversely, expression of the same gene in tobacco did not significantly affect tolerance to or accumulation of cadmium or zinc [150].

7.1.3. Genes Encoding Enzymes That Detoxify Metals and Metalloids

Following the uptake and translocation of heavy metals, plants must convert these elements into less toxic or non-toxic forms for storage, utilization, or excretion. This process is mediated by enzymes that catalyze their chemical transformation. The majority of examples involving the genetic detoxification of metals/metalloids focus on selenium metabolism, achieved by converting inorganic selenium into less toxic volatile forms [147]. Central to this pathway are ATP sulfurylase (APS), which catalyzes the rate-limiting step in selenate reduction, and selenocysteine methyltransferase (SMT), which methylates selenocysteine to prevent its misincorporation into proteins. Additionally, enzymes that scavenge oxidative damage play a pivotal role. For instance, transgenic tobacco lines overexpressing LmSAP exhibited reduced oxidative damage upon exposure to Cd, Cu, Zn, and Mn. This response was attributed to increased activity of antioxidant enzymes (SOD, CAT, and POD), suggesting that the enhanced metal stress tolerance observed in LmSAP transgenic plants is associated with attenuated ROS accumulation and the maintenance of redox homeostasis via elevated antioxidant enzyme activity [151].

7.2. Plant–Nanoparticle Combined Remediation Technology

Research indicates that nano-phytoremediation, which leverages the characteristics of both nanotechnology and phytoremediation, represents a promising approach for remediating contaminated soils [152]. Incorporating nanomaterials into plants shows remarkable potential for multiple benefits, including enhanced purification, detoxification, and elimination of toxic pollutants. Applied nanoparticles assist the phytoremediation process through three distinct aspects: direct remediation by nanoparticles, enhancement of pollutant phytoavailability, and promotion of plant health and growth under adverse conditions [153]. The mechanism of the specific plant–nanoparticle combined remediation technology is illustrated in Figure 4. The figure reflects the basic principle of plant–nanoparticle combined remediation.

7.2.1. Direct Remediation by Nanoparticles

Most nanoparticles employed in nano-phytoremediation are capable of directly eliminating pollutants at contaminated sites during the remediation process. This capability not only reduces pollutant toxicity but also alleviates the phytotoxic burden on plants. The primary mechanisms governing direct pollutant removal by nanoparticles are redox reactions and adsorption [153,154]. For instance, composites of iron oxide (FeOOH) and nano zero-valent iron (nZVI) can remove or detoxify pollutants via redox processes. nZVI functions as a potent electron donor for the reductive transformation or degradation of pollutants and is widely utilized in the treatment of metal contaminants such as Cr(VI), Hg(II), Cu(II), Cd(II), and Ni(II) through adsorption and reduction mechanisms. The FeOOH shell of nZVI acts as an adsorbent, providing reactive sites and facilitating electrostatic interactions with pollutants, while the Fe(0) core confers reductive capacity. nZVI catalyzes reduction reactions, converting highly toxic Cr(VI) into less toxic Cr(III), which subsequently precipitates as iron chromate. Furthermore, nZVI has demonstrated efficacy in the purification of chlorinated pollutants, such as organochlorine pesticides and PCBs, through reductive dechlorination processes [155]. Its high specific surface area and robust reducing capacity are instrumental in the remediation of soil contaminants.

7.2.2. Enhancing Pollutant Phytoavailability

Pollutant phytoavailability is a pivotal determinant in the phytoremediation process, particularly regarding phytoaccumulation. Plants can exclusively accumulate pollutants present in bioavailable forms. Phytoavailability is governed by a multitude of factors, including soil physicochemical properties, pollutant distribution within the soil, and chemical speciation [156]. Regarding metals, soluble forms exhibit the highest phytoavailability, whereas complexed and crystalline forms display the lowest. The degree of pollutant phytoavailability directly dictates remediation efficiency; low phytoavailability can significantly impede the process. Consequently, nanoparticles offer a potential strategy for modulating pollutant phytoavailability. Nanoparticles can function as carriers for pollutants, increasing their accessibility to plants [157]; conversely, they may form complexes with pollutants, preventing cellular uptake and thereby reducing phytoavailability [158]. Studies indicate that nanoparticles such as TiO₂ NPs, Fe₃O₄ NPs, quantum dots, carbon nanotubes, and silica NPs can be directly accumulated by plants [159]. Furthermore, evidence suggests that phytoavailable fullerenes can form complexes with trichloroethylene (TCE), enhancing its phytoaccumulation in eastern cottonwood (Populus deltoides W.) due to the co-transport of TCE with fullerene nanoparticles [160]. These findings underscore the potential of nanoparticles to enhance pollutant phytoavailability in future phytoremediation applications.

7.2.3. Promoting Plant Growth

Plant growth rate and uptake capacity are among the most critical determinants of effective phytoremediation. Investigations have demonstrated that nano-silica can significantly improve the absorption of lead (Pb) by plant roots [161]. Further mechanistic studies revealed that silicon dioxide nanoparticles ameliorate plant growth under lead stress by enhancing the activity of antioxidant enzymes in lead-affected plants [162]. Additionally, nanoparticles can be engineered to facilitate the targeted delivery of nutrients and chemicals to plants, thereby augmenting growth and pollutant removal capacity while minimizing adverse effects on the plants and the surrounding ecosystem [163]. Concurrently, nanomaterials contribute to soil remediation and increase plant protein levels, ultimately promoting the proliferation of beneficial microorganisms in the soil and rhizosphere. This, in turn, enhances the plant’s capacity to absorb pollutants and improves overall soil fertility [164]. These findings highlight the potential of integrating nanomaterials with plants to address environmental pollutants with greater efficacy.

7.3. Plant–Low-Molecular-Weight Organic Acid Combined Remediation Technology

Low-Molecular-Weight Organic Acids (LMWOAs) are organic compounds characterized by short carbon chains and the presence of at least one acidic functional group (–COOH). As weak acids exhibiting distinct dissociation behaviors, organic acids (OAs) at the soil–plant interface carry one or more negative charges. OAs are ubiquitous in living organisms, playing essential roles not only as intermediates in the tricarboxylic acid cycle for energy production but also in numerous cellular metabolic pathways [165]. Owing to their acidity, OAs function as ligands, binding metals to form organometallic complexes. OAs can form complexes of varying stoichiometries and structures with metals; those possessing multiple electron-donating groups, such as oxalic acid and citric acid, can form one or more chelate rings upon complexation. Under specific conditions, LMWOAs function as chelating agents, a property that significantly enhances bioavailability and accelerates phytoextraction rates. Plant uptake of metals is more efficient when metals are in soluble forms, maximizing root surface contact and facilitating dissolution in the transpiration stream for transport into the plant [166]. LMWOAs possess considerable potential for improving pollutant bioavailability, suggesting their utility in enhancing phytoremediation processes. The application of LMWOAs as phytoremediation amendments can mitigate the excessive mobilization and leaching risks associated with synthetic chelating agents while simultaneously promoting higher plant biomass. The principal challenge for LMWOA-enhanced phytoremediation lies in identifying an optimal amendment that is plant-tolerant and capable of maintaining metal solubility to facilitate phytoextraction without persisting excessively in the soil profile. Nonetheless, LMWOAs hold significant promise for the remediation of riparian soils characterized by diverse pollution sources and complex compositions.

7.4. Phytoremediation–Microbial Synergistic Technology

The significance of riparian zone soil derives primarily from its function as a critical nexus between terrestrial and aquatic resources, necessitating the restriction of pollutant mobility and dispersion within this zone. Traditional microbial remediation is often constrained by environmental variables and technical limitations, typically demonstrating efficacy against only specific pollutants. The overall process, from microbial cultivation to the remediation phase, can be protracted, potentially requiring several weeks or months. Furthermore, when introduced microbial consortia are applied to contaminated sites, they often face competition and exclusion by indigenous microbial populations, which can compromise the final remediation outcome. Consequently, integrating microorganisms with phytoremediation—a technology characterized by environmental friendliness, ecological compatibility, and cost-effectiveness—optimizes these ecological interactions, overcomes the limitations of single-method bioremediation, and exhibits substantial potential for the remediation of contaminated environments.

Phytoremediation–microbial synergistic remediation is a green remediation technology that employs the cooperative interactions between plants and functional microorganisms, such as plant growth-promoting rhizobacteria and mycorrhizal fungi, to efficiently remove or immobilize pollutants including heavy metals and organic compounds in soil or water. Its core principle leverages the complementary advantages of plant uptake/accumulation and microbial transformation/degradation, thereby addressing the limitations of single-approach remediation, such as prolonged duration, low efficiency, and poor stability [167].

7.4.1. Enhancement of Phytoremediation via Plant Growth Promotion

The rhizosphere microbiome plays a crucial role in promoting plant growth and health. Microorganisms such as plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) secrete bioactive substances including indole-3-acetic acid (IAA) and cytokinins, which stimulate root development and nutrient uptake, thereby enhancing plant stress tolerance. For instance, one study identified a core bacterial operational taxonomic unit, ASV245 (affiliated with the genus Pseudomonas), which consistently enriched across different maize genotypes. The corresponding strain WY16, isolated from maize roots, significantly promoted shoot and root growth by activating phytohormone signaling pathways, thereby enhancing the plant’s capacity for pollutant uptake [168]. Another study demonstrated the remediation potential of legume–rhizobia symbionts in soil contaminated with chlorinated organic compounds [169,170]. Experiments revealed that nitrogen fixation not only promotes plant growth and increases legume yield but also enhances the plant’s dechlorination capacity in contaminated soil.

Moreover, microorganisms can optimize the rhizosphere microenvironment to further stimulate plant growth and pollutant uptake. For example, a composite amendment of biochar and palygorskite was shown to enhance the complexity and stability of the rhizobacterial co-occurrence network, improve rhizosphere conditions, reduce the bioavailability of Cd and Zn, and regulate nitrogen and phosphorus cycling, thereby significantly improving the remediation efficiency of woody plants. These findings were validated through both field trials and controlled laboratory experiments, collectively underscoring the considerable potential of microorganisms in promoting plant growth and enhancing pollutant assimilation.

7.4.2. Activation and Transformation of Pollutants

Microorganisms enhance the bioavailability of pollutants to plants by secreting organic acids, chelators, extracellular enzymes, and other metabolites that alter the physicochemical state of contaminants.

Activation Mechanism: The core of activation lies in converting pollutants from stable, bound forms into soluble or exchangeable forms, thereby increasing their environmental mobility and uptake efficiency by plants. For example, studies have shown that the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1 can induce the Kirkendall effect in aged nano zero-valent iron (nZVI), leading to its bioregeneration. The regenerated nZVI exhibited an approximately 50-fold increase in the rate constant for Cr(VI) reduction compared to the aged material, significantly enhancing the reductive transformation of heavy metal contaminants [171]. It should be noted, however, that the activation of heavy metals must be carefully controlled. Excessive activation may lead to the uncontrolled migration of heavy metal ions in the environment, posing risks of secondary pollution. Therefore, in practical remediation applications, it is essential to select highly efficient yet controllable microbial strains and integrate them with appropriate plant species to achieve a synergistic balance between pollutant activation and uptake.

Transformation Mechanism: The toxicity of certain heavy metals varies dramatically with their valence states. For instance, hexavalent chromium (Cr(VI)) is over 100 times more toxic and significantly more carcinogenic than trivalent chromium (Cr(III)). Similarly, inorganic mercury is more toxic than most organic mercury species, with methylmercury being particularly hazardous. Microorganisms can alter the valence state or chemical form of heavy metals through biochemical processes such as oxidation-reduction and methylation/demethylation, thereby reducing their toxicity and lowering their environmental mobility. A similar transformation logic applies to organic pollutants. For example, Pseudomonas myxofaciens has been shown not only to degrade specific organic compounds (e.g., M, C, and F) but also to function as a plant growth-promoting rhizobacterium [172]. It produces indole-3-acetic acid (IAA), siderophores, and can solubilize insoluble phosphates. Consequently, P. myxofaciens VITVJ1 enhances both plant root–shoot development and the degradation of pesticides into forms more readily assimilated by plants, thereby concurrently promoting plant growth and facilitating soil remediation.

7.5. Plant–Chemical–Microbial Combined Remediation Technology

Biochar is a carbon-rich material produced via the pyrolysis of biomass under oxygen-limited or anaerobic conditions at elevated temperatures. The application of biochar for soil remediation involves multiple mechanisms, including physical adsorption, electrostatic adsorption, co-precipitation, complexation, and redox reactions. The synergistic interplay of these mechanisms dictates the efficacy of biochar in immobilizing pollutants [173]. In recent years, biochar application has garnered increasing attention due to its versatile properties in environmental remediation. When integrated with phytoremediation strategies, biochar can enhance pollutant removal efficiency by improving plant performance and soil conditions [174]. The high specific surface area and porous structure of biochar facilitate the adsorption of pollutants, thereby reducing their bioavailability and mobility within the soil matrix [175] and lowering the risk of leaching into groundwater. While adsorbing toxic compounds and exerting a protective effect, biochar also mitigates plant stress, supports healthier growth, and ultimately improves phytoremediation outcomes. Furthermore, beyond the capacity of native soil humus to promote plant growth and drive microbial dehalogenation [176], biochar application ameliorates overall soil health by enhancing nutrient retention, water-holding capacity, and the maintenance of microbial activity in contaminated soils. The preservation of microbial diversity not only facilitates microbial secretion of phytohormones, which promote plant growth and enhance pollutant tolerance [132,133], but also enables the production of biosurfactants that lower surface tension and augment the bioavailability of hydrophobic pollutants, thereby stimulating microbial degradation [177].

The mechanism of this combined remediation technology is illustrated in Figure 5. The figure reflects the mechanism of action of biochar in phytoremediation.

7.6. Feasibility of Phytoremediation

The utilization of phytoremediation as an environmentally benign strategy for environmental restoration is anticipated to confer significant societal advantages. This sustainable approach facilitates land regeneration while concurrently enhancing human quality of life and well-being. Numerous studies have elucidated the social benefits derived from the reclamation of urban brownfields utilizing phytoremediation, including the promotion of social justice and cohesion, esthetic enhancement, and improvements in physical and psychological health [173]. The engagement of community members and stakeholders in activities such as tree planting, watering, and weeding fosters teamwork, mutual learning, and knowledge exchange, thereby bolstering public acceptance of ecological restoration initiatives. Research investigating human responses to diverse vegetation types has revealed that while floral colors may enhance short-term esthetic experiences, trees and shrubs confer more substantial long-term psychological benefits [174]. Specific phytoremediation species can be incorporated into urban hedgerows, thereby improving the visual appeal of reclaimed sites [178]. Interaction with nature, particularly within urban green spaces, yields comprehensive positive impacts on human well-being, including notable improvements in physical and mental health. In the context of mixed-contamination urban brownfields, the application of diverse phytoremediation species can ensure high biodiversity while optimizing the psychological benefits conferred by green spaces [179].

Concurrently, phytoremediation strategies present tangible economic benefits. For instance, a phytoremediation project implemented on agricultural soils with mild cadmium contamination employed a multi-crop intercropping pattern. Results indicated that the intercropping of two species represents a promising approach that integrates remediation with agricultural production. Moreover, the cultivation of marketable cash crops can provide substantial economic incentives for local farmers throughout the remediation process [180]. These findings validate the economic feasibility of applying phytoremediation to lightly contaminated soils. Future strategies to further enhance the cost-effectiveness of phytoremediation may include advancing mechanization and reducing the costs associated with hyperaccumulator seedlings.

8. Conclusions

Co-contamination of riparian zone soils, stemming from diverse pollutant sources and complex chemical mixtures, represents a significant global environmental challenge. This review synthesizes current understanding regarding the interactions between heavy metals and organic pollutants in riparian zones, emphasizing the distinct obstacles these interactions pose to conventional remediation strategies. Our analysis demonstrates that phytoremediation possesses distinct and promising potential for addressing this complexity, capitalizing on its inherent ecological compatibility and sustainability. Functioning via multiple mechanisms, it facilitates the simultaneous attenuation of both inorganic and organic contaminants. While hyperaccumulators such as Pteris vittata and Solanum nigrum demonstrate efficacy in metal extraction, species like alfalfa and poplars exhibit considerable promise for organic pollutant remediation. Furthermore, plant–microbe synergies can significantly augment remediation efficiency.

Despite the advantages of cost-effectiveness and environmental compatibility, limitations such as extended treatment timelines and phytotoxicity under high-concentration stress persist. Future research must prioritize elucidating the mechanisms underlying integrated, multi-technological approaches. The application of genetic engineering and nanotechnology offers strategic avenues to enhance remedial efficiency and adaptability. Critically, the implementation of a comprehensive pre-application toxicity risk assessment is imperative, alongside the long-term monitoring of remediation efficacy to ensure sustainable outcomes.

In summary, phytoremediation provides a viable and sustainable strategy for managing complex soil pollution within riparian systems. Through continuous technological refinement and strengthened interdisciplinary collaboration, it is poised to assume an increasingly pivotal role in environmental restoration, ultimately fostering the recovery and sustainable stewardship of riparian ecosystems.