Phytoremediation of Nickel-Contaminated Soils: An Integrative Review of Plant Species, Remediation Mechanisms, and Soil Factors

Abstract

Increasing soil contamination by potentially toxic elements (PTEs) compromises environmental quality and ecosystem health, reinforcing the need for effective remediation strategies. Phytoremediation uses plants to remove, immobilize, or neutralize contaminants and represents a sustainable approach for restoring contaminated soils. Although nickel (Ni) is an essential plant micronutrient, elevated concentrations can cause phytotoxicity. This integrative review aimed to synthesize and critically analyze the scientific literature on the phytoremediation of Ni-contaminated soils published between 2000 and 2026, focusing on the plant species evaluated, their phytoremediation potential, and the main research trends and knowledge gaps. The Web of Science Core Collection search identified 230 records. Original research articles addressing the phytoremediation of Ni-contaminated soils and providing sufficient information to characterize phytoremediation potential were considered eligible, resulting in 91 included studies. Plant responses varied according to species and Ni concentration. Among 449 plant records, defined as the occurrence of a plant taxon within a study, more than 86% corresponded to herbaceous plants; Poaceae accounted for 27.2% of the records, followed by Brassicaceae (15.1%). Brassica juncea was consistently identified as a promising species for Ni phytoremediation, showing Ni tolerance, phytoextraction, and hyperaccumulation across different studies. Alyssum murale stood out among the reported hyperaccumulators for its high Ni accumulation and phytoextraction potential. Overall, the reviewed evidence supports phytoremediation as a sustainable strategy for Ni-contaminated soils. However, important knowledge gaps remain, particularly the scarcity of long-term field studies, methodological heterogeneity, limited understanding of plant–soil–microbiota interactions, underrepresentation of major Ni-producing regions, and insufficient evidence on technical and economic feasibility. Future research should address these gaps through standardized approaches and integrated field assessments under contrasting edaphoclimatic conditions to advance the effective and sustainable application of Ni phytoremediation.

1. Introduction

Environmental contamination by potentially toxic elements (PTEs) is a global concern that poses significant threats to human health, biodiversity, and ecosystem stability [1,2]. Originating from both natural and anthropogenic sources, PTEs can contaminate soils through improper disposal of industrial waste, mining residues, inappropriate use of agrochemicals, and other human activities that contribute to ongoing and accelerated environmental degradation [3,4]. PTEs include heavy metals, which, even at low concentrations, may cause adverse effects due to their bioaccumulation in living organisms [2,4]. Some metals, such as nickel (Ni), are essential micronutrients for plant growth and development; however, when present at elevated concentrations, they become phytotoxic [1].

It is estimated that up to 17% of the world’s agricultural land exceeds the intervention guideline values established for agricultural areas for at least one toxic metal [5]. However, there is significant variation in these guideline values among countries, depending on their regulatory standards for heavy metals in agricultural soils [1]. In Finland, the Ministry of the Environment has established a soil Ni threshold value of 50 mg kg⁻¹ and a lower guideline value of 100 mg kg⁻¹ for non-industrial land uses [6]. In Brazil, the National Environment Council (CONAMA) Resolution No. 420/2009 [7] establishes a prevention value of 30 mg kg⁻¹ and an agricultural investigation value of 70 mg kg⁻¹. Increases in Ni concentrations in agricultural soils, even at low to moderate levels, may result from the presence of Ni in chemical fertilizers, pesticides, and sewage, as well as from the improper use of these materials in agricultural practices [8]. Despite these established guideline values, assessing PTE concentrations in agricultural soils remains a complex challenge, particularly in understanding how different contamination levels interact with environmental processes and agricultural production. Consequently, more comprehensive studies are required to support the development of effective remediation strategies [1].

Phytoremediation, a technology that employs plants capable of removing, immobilizing, or neutralizing PTEs from the environment, has gained considerable attention within the scientific community as a promising remediation strategy [9]. Compared with conventional remediation methods, phytoremediation offers several advantages, including low implementation costs, high efficiency in areas with low to moderate levels of contamination, and environmental sustainability, making it a more socially acceptable remediation approach [9,10].

Although numerous studies on phytoremediation have been conducted and published in recent years, several important questions regarding heavy metal contamination remain unanswered: (a) Can phytoremediation be established as a viable technology for the remediation of Ni-contaminated areas? (b) Which plant species have the potential to act as barriers against Ni contamination in soils? (c) Which plant groups can be recommended for specific phytoremediation applications?

Previous reviews have addressed phytoremediation from a broad perspective; however, among the literature retrieved from the Web of Science Core Collection, no review was identified that specifically integrated evidence on Ni phytoremediation while simultaneously considering plant species, soil textural classes, Ni concentrations, and remediation mechanisms. Therefore, this integrative review aims to synthesize and critically analyze the literature on the phytoremediation of Ni-contaminated soils, focusing on the plant species evaluated, their phytoremediation potential, the influence of soil textural classes and Ni concentrations, and the main research trends and knowledge gaps.

2. Materials and Methods

2.1. Search Strategy and Study Identification

An integrative literature review was conducted to synthesize and critically analyze the available evidence on the phytoremediation of nickel (Ni)-contaminated soils. The review covered the period from 2000 to 2026, corresponding to the publication range identified by the predefined search strategy, with the earliest relevant study retrieved dating from 2000.

The literature search was conducted in the Web of Science Core Collection, which was selected because of its broad coverage of international scientific journals, standardized bibliographic records, and comprehensive citation indexes. The search included the Science Citation Index Expanded (SCIE), Social Sciences Citation Index (SSCI), and Arts & Humanities Citation Index (AHCI), thereby broadening the coverage across different fields potentially related to the multidisciplinary nature of phytoremediation. The use of this database is also consistent with previous studies addressing phytoremediation and soil contamination by potentially toxic elements [11,12].

The search strategy was defined in advance and used the terms “nickel AND phytoremediation AND soil AND contamination”, combined using Boolean operators. All records retrieved for the period from 2000 to 2026 were considered, with no restrictions on publication language. The search yielded 230 records.

2.2. Eligibility Criteria and Study Selection

The retrieved records were screened and their content assessed to determine their relevance to the objective of the review. Original research articles published between 2000 and 2026 were included when they reported data related to the phytoremediation of Ni-contaminated soils, including information on the plant species evaluated, soil Ni concentrations, and plant responses to the metal, such as uptake, accumulation, translocation, phytoextraction, hyperaccumulation, or tolerance.

The following were excluded: (i) studies that did not address phytoremediation; (ii) phytoremediation studies that did not involve Ni; (iii) studies in which Ni was not associated with soil; (iv) review articles; (v) conference abstracts; (vi) books and book chapters; (vii) editorials, notes, letters, and other documents without original data; (viii) duplicate records; and (ix) records that, after assessment, did not provide sufficient information to characterize the phytoremediation potential of the plant species.

During the study selection and eligibility assessment, 139 records were excluded, including three duplicate records and studies that did not meet the predefined eligibility criteria, primarily because they were not directly related to Ni phytoremediation or corresponded to ineligible document types. Ultimately, 91 original research articles were included in the synthesis.

2.3. Data Extraction and Organization

Data from the 91 included studies were extracted and organized into a structured database. The following variables were considered: author and year of publication, geographical location of the study, plant species and botanical family, soil textural classes, soil Ni concentration, experimental conditions, plant responses to Ni, Ni concentrations in plant tissues, phytoextraction and/or hyperaccumulation potential, and phytoremediation mechanism.

To adequately represent the diversity of plant responses reported in the literature, each occurrence of a plant taxon within an included study was considered a plant record. This approach allowed different species evaluated within the same study to be distinguished while preserving taxon-specific information on phytoremediation performance. Thus, although 91 studies were included, they yielded 449 plant records, corresponding to the different occurrences of plant taxa identified across the studies analyzed.

Accordingly, the 449 records do not represent 449 independent studies but rather occurrences of plant species within the included studies. This distinction was considered when interpreting the results to avoid treating the number of plant records as the number of independent studies.

2.4. Data Analysis

Data were analyzed descriptively to characterize the distribution of the available evidence and identify patterns in the literature on Ni phytoremediation. Absolute (n) and relative (%) frequencies were calculated for plant species and families, soil textural classes, Ni concentration ranges, phytoremediation mechanisms, and geographical distribution of the studies, and the results were presented in tables and figures.

Relative frequencies were calculated as the ratio between the number of occurrences within a given category and the total number of records considered for that variable, expressed as a percentage. To characterize the temporal development of the scientific literature, the number of studies published per year and their distribution over the 2000–2026 period were also evaluated.

Information on soil texture was extracted from the original studies as reported by the respective authors. Textural classes were grouped according to the terminology adopted in the primary studies, including sandy, sandy loam, clay loam, clayey, silty, silt loam, and medium-textured soils. No independent soil taxonomic classification was assigned when such information was not provided in the original study.

Frequency and percentage calculations and the initial organization of the database were performed using Microsoft Excel 2024. Graphical representations were generated using R^® version 4.5.1 and SigmaPlot^® version 14.5. No inferential statistical tests, meta-analysis, or effect-size models were performed because the purpose of the synthesis was to describe and integrate evidence from studies characterized by substantial experimental heterogeneity.

The overall workflow of the integrative review, including study identification and selection, data extraction and organization, and descriptive analysis, is summarized in Figure 1.

3. Effects of Nickel Toxicity

Nickel (Ni) is a mineral element classified as a heavy metal and is essential for plant growth and development because it participates in important metabolic functions [1,13]. Nickel is also essential for certain microorganisms and is involved in a wide range of cellular processes [14].

The toxic potential of Ni is associated with several factors, including its concentration and chemical speciation in the soil. Consequently, its behavior is directly influenced by soil pH, organic matter content, and the presence of metal oxides [15]. The different chemical and mineral forms of Ni in soil determine its bioavailability to plants and its mobility in the environment.

Most plant species require relatively low Ni concentrations for normal growth and development, generally ranging from 0.1 to 5.0 mg kg⁻¹ on a dry-matter basis, although this requirement may vary considerably depending on the plant species and Ni availability in the soil [16]. Nickel is primarily involved in nitrogen metabolism, iron uptake, and the activity of specific enzymes, including urease, glyoxalase-I, hydrogenase, and superoxide dismutase [13]. Therefore, in the absence of Ni, plants are unable to complete their life cycle.

However, elevated Ni concentrations can induce phytotoxicity in crops because Ni promotes the production of reactive oxygen species (ROS), such as superoxide anion (O₂•⁻), hydrogen peroxide (H₂O₂), and hydroxyl radicals (•OH). This oxidative imbalance can impair several physiological and biochemical processes, including photosynthesis, transpiration, and mineral nutrition [13,17]. Elevated Ni concentrations have been associated with leaf chlorosis, growth inhibition, nutrient imbalances, and reduced photosynthetic activity [13]. Ionic imbalances under Ni stress also delay the uptake of several cations, impair plant water relations, and reduce sodium and potassium concentrations in the shoots and roots of Cynodon dactylon and Cenchrus ciliaris [18].

Critical Ni toxicity levels are generally defined as concentrations exceeding 10 mg kg⁻¹ on a dry-matter basis in sensitive species, 50 mg kg⁻¹ in moderately tolerant species, and 1000 mg kg⁻¹ in Ni-hyperaccumulator plants [19,20]. Nickel concentrations above these thresholds induce a wide range of phytotoxic symptoms in crop species [1,21], including reduced carbon dioxide (CO₂) uptake, decreased photosynthetic rate, lower chlorophyll content, inhibition of cell division and elongation, reduced plant growth, impaired enzymatic activity, and disruption of nutrient metabolism [22,23,24]. Nevertheless, establishing a universal soil Ni concentration threshold for phytotoxicity in cultivated plants remains a complex challenge.

Numerous studies have investigated the effects of Ni on agricultural crops over the past decades. In an experiment conducted by Aguilar et al. [25] with Gossypium hirsutum L., plants were grown in soil containing 0, 15, 30, 45, 60, 75, and 90 mg dm⁻³ Ni. In adult plants, photosynthetic performance peaked at 45 mg dm⁻³, whereas higher Ni concentrations, particularly 75 and 90 mg dm⁻³, reduced photosynthetic activity and chlorophyll content. Plant growth was also reduced at 90 mg dm⁻³. These findings illustrate the concentration-dependent response of cotton to Ni, with moderate availability supporting physiological performance but excessive Ni causing phytotoxic effects.

A soil Ni concentration of 40 mg kg⁻¹ was reported to impair Solanum lycopersicum plant growth and productivity, as reflected in reduced dry biomass production [26]. In another study using cherry Lycopersicon esculentum, crop productivity remained unaffected at a low Ni concentration (40 mg kg⁻¹) but declined significantly when the soil Ni concentration reached 100 mg kg⁻¹ [27]. The activation of antioxidant enzymes, particularly ascorbate peroxidase (APX), in S. lycopersicum seedlings exposed to increasing Ni concentrations (15 and 30 mg L⁻¹) was shown to be an effective mechanism for attenuating salt stress, thereby improving plant growth and productivity [28]. Similar responses have also been reported in Oryza sativa [29], Triticum spp. [30], and Zea mays [31], all of which exhibited significant increases in APX activity when exposed to different Ni concentrations.

Toxic effects of Ni have also been reported in Hordeum vulgare when cultivated in soils containing Ni concentrations above 10 mg kg⁻¹. However, this concentration also resulted in a significant increase in crop productivity, highlighting the dual role of Ni as both an essential micronutrient and a potentially toxic element [32]. In a study evaluating Helianthus annuus L., the application of 80 mg kg⁻¹ Ni to the soil did not affect plant growth, whereas concentrations exceeding 120 mg kg⁻¹ induced phytotoxicity and plant mortality [33]. In another experiment, Z. mays plants grown under hydroponic conditions and exposed to different Ni concentrations (0, 20, and 40 mg L⁻¹) exhibited increased activities of several antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), ascorbate peroxidase (APX), and peroxidases (POX), as well as enhanced nutrient translocation from roots to shoots compared with the control after exposure to Ni [34]. Nevertheless, the findings of this study indicate that Ni toxicity severely impairs Z. mays physiology by inducing oxidative damage and disrupting nutrient uptake and translocation.

4. Phytoremediation

The use of plants for phytoremediation has emerged as a viable strategy for the remediation and restoration of contaminated soils because of its low implementation cost, ease of establishment and application, and ability to mitigate the adverse effects of heavy metals in soil [35,36]. Compared with conventional remediation technologies, phytoremediation offers several advantages, including the cost-effective remediation of large contaminated areas, the ability to remediate contaminated water, soil, and subsurface environments [37], and reduced visual disturbance to the landscape.

However, several factors should be considered when selecting plant species for soil decontamination, including the plant developmental stage, the capacity to absorb and accumulate heavy metals in plant tissues, the duration of exposure to PTEs, and the different chemical species of these elements [38,39]. Furthermore, achieving satisfactory remediation through phytoremediation may require extended periods, and there is a potential risk associated with the entry of contaminated plant biomass into the food chain. Therefore, these limitations should be carefully considered when implementing this remediation strategy.

The behavior of Ni in soil depends on its chemical forms and is strongly influenced by key soil physicochemical properties, particularly pH, texture, mineral composition, organic matter content, and cation exchange capacity (CEC), which regulate its mobility and bioavailability [40].

Phytoremediation involves either the direct action of plants or indirect processes mediated by the stimulation of rhizosphere microorganisms, thereby promoting environmental decontamination through the extraction, stabilization, volatilization, or degradation of contaminants. These processes are generally classified as phytoextraction, phytodegradation, phytovolatilization, phytostimulation, and phytostabilization [41,42].

4.1. Phytoextraction

Plants used in phytoextraction are generally referred to as hyperaccumulators because of their ability to accumulate high concentrations of heavy metals in their tissues, typically ranging from 100 to 10,000 mg kg⁻¹ on a dry-matter basis, depending on the specific metal [41,43,44,45,46]. For Ni, hyperaccumulator plants are defined as those capable of extracting and accumulating more than 1000 mg kg⁻¹ Ni in their dry biomass [46,47].

Phytoextraction is currently one of the most extensively investigated phytoremediation techniques. This approach relies on hyperaccumulator species and high-biomass-producing plants that are capable of growing in soils containing elevated concentrations of heavy metals while removing these contaminants from the affected area. However, the efficiency of phytoextraction is directly dependent on the plant’s capacity to absorb contaminants from the soil and accumulate them in its roots or aboveground tissues. Consequently, prolonged remediation periods are often required to achieve substantial reductions in soil contaminant concentrations [39,48].

4.2. Phytodegradation

Phytodegradation is primarily associated with the remediation of organic contaminants; however, it may also involve inorganic substances, such as nitrate, through their uptake and metabolism by plants. These contaminants may either be degraded or absorbed by plant tissues [49]. In experiments conducted on petroleum-contaminated soils using Medicago sativa L. and Lolium multiflorum L., with or without inoculation with bacteria of the genus Ochrobactrum, the combined use of plants and bacteria resulted in greater degradation of the target contaminants, with alfalfa showing the highest remediation efficiency. Furthermore, the ability of this species to absorb and metabolize compounds such as nitrate and ammonium highlights its relevance for the remediation of contaminated sites, reinforcing its potential for use in phytoremediation strategies [50].

4.3. Phytovolatilization

Phytovolatilization is a phytoremediation strategy applied to the removal of volatile organic contaminants and certain inorganic elements that can be metabolically converted into gaseous forms. In this process, contaminants are absorbed by the roots, translocated to plant tissues, and subsequently transformed through plant metabolic pathways, often in association with rhizosphere microorganisms, into volatile compounds that are released into the atmosphere [37]. Although this release was initially considered a potential source of secondary pollution, more recent studies indicate that, depending on the plant species, the contaminant involved, and the emission rate, phytovolatilization can substantially reduce environmental toxicity, particularly when contaminants are converted into less toxic chemical forms [51].

In addition to plant metabolism, the rhizosphere microbiota plays a fundamental role in the biochemical transformation of contaminants, thereby enhancing the efficiency of the phytovolatilization process. Nevertheless, the ecological safety of phytovolatilization requires case-specific assessments to ensure that plants effectively remove and volatilize contaminants while reducing atmospheric toxicity and minimizing the potential redistribution of volatilized compounds [37,52].

4.4. Phytostimulation

Phytostimulation, also referred to as rhizodegradation or rhizoremediation, involves the stimulation of microbial activity in the rhizosphere by plant roots, thereby enhancing the degradation of organic contaminants [53]. Plant roots release exudates composed of sugars, amino acids, enzymes, and phenolic compounds, which serve as substrates for contaminant-degrading microorganisms and promote the biodegradation of pollutants. This mechanism is particularly important in sites contaminated with persistent organic compounds, where plant–microorganism interactions can substantially improve remediation efficiency [53]. Species such as Brassica napus have demonstrated the ability to stimulate microbial communities capable of degrading contaminants such as diesel fuel, thereby promoting the functional diversity of soil microbial communities [54].

4.5. Phytostabilization

Phytostabilization involves the use of plants to immobilize pollutants through adsorption onto roots, precipitation within the rhizosphere, chemical complexation, or accumulation in roots. In addition, phytostabilizing plants contribute to the control of wind and water erosion in degraded areas [55]. Consequently, this strategy minimizes leaching, groundwater contamination, and the entry of potentially toxic metals into the food chain [56,57].

Recent studies have demonstrated that certain plant species can simultaneously operate through multiple phytoremediation mechanisms, depending on the type and concentration of the contaminant. In a pot experiment evaluating the phytoremediation potential of Conocarpus erectus in soils contaminated with up to 200 mg kg⁻¹ Ni in combination with other heavy metals, including lead (Pb), chromium (Cr), and cadmium (Cd), the species behaved as a Pb hyperaccumulator while acting as a phytostabilizer for Cr, Ni, and Cd [58]. These findings demonstrate that a single plant species may employ different phytoremediation mechanisms depending on the contaminating element.

For the remediation of Ni-contaminated sites, phytoextraction and phytostabilization are the most widely applied and extensively investigated phytoremediation strategies. Phytoextraction predominates because of the availability of hyperaccumulator species capable of absorbing high concentrations of Ni and translocating it to aboveground tissues, thereby enabling its gradual removal from the soil through biomass harvesting. In contrast, phytostabilization represents an important alternative for sites with high Ni concentrations or where complete metal removal is impractical, as it reduces Ni mobility and bioavailability, thereby minimizing leaching, environmental dispersion, and entry into the food chain.

In contrast, phytodegradation and phytostimulation (rhizodegradation) have limited applicability for Ni remediation because these mechanisms were primarily associated with the degradation of organic contaminants, such as hydrocarbons, pesticides, and industrial solvents [53]. Since Ni is a chemical element and cannot be biologically degraded, these mechanisms do not remove it from the environment but may only indirectly influence its bioavailability through microbial activity in the rhizosphere.

Phytovolatilization is the least applicable phytoremediation mechanism for Ni. This process depends on the plant’s ability to convert contaminants into volatile compounds, a characteristic that has been demonstrated for elements such as mercury (Hg), selenium (Se), and arsenic (As), but not to any significant extent for Ni [1]. Consequently, the application of phytovolatilization has very limited applicability for the remediation of Ni-contaminated soils.

Therefore, the selection of an appropriate phytoremediation strategy for Ni-contaminated soils should consider the chemical form, concentration, and bioavailability of Ni in the soil. Phytoextraction is the preferred approach when the objective is to remove Ni from contaminated sites, whereas phytostabilization is more suitable for reducing Ni mobility and mitigating environmental risks in heavily contaminated areas.

5. Evolution of Research on Nickel Phytoremediation

Using the keywords “nickel,” “phytoremediation,” “soil,” and “contamination,” the literature review identified 91 studies published between 2000 and 2026. A descriptive analysis of keyword frequencies and co-occurrence patterns in the retrieved literature was conducted to generate a keyword co-occurrence map, highlighting the most frequent and closely associated terms related to Ni phytoremediation (Figure 2A). The map was constructed using the 50 most frequently occurring keywords and their connections (i.e., clusters). The most common keywords identified in this research field were phytoremediation, accumulation, cadmium, nickel, heavy metals, plants, contamination, soil, phytoextraction, lead, and zinc (Figure 2A). The different colors in the keyword map represent distinct clusters of related terms, while the connecting lines indicate the strongest co-occurrence relationships among the keywords (Figure 2A).

Analysis of the retrieved publications indicates that scientific research on the phytoremediation of Ni-contaminated areas began in the early 2000s, when only one study (1.0%) was identified. However, the number of publications increased markedly after 2020, reaching its highest level in 2023, with 14 studies accounting for 15% of all publications included in this review (Figure 2B).

This increase coincides with the growing global interest in Ni, particularly in the context of the energy transition and its increasing use in rechargeable batteries for electric vehicles and energy storage systems. The expansion of nickel mining, processing, and recycling activities has heightened concerns regarding the environmental impacts associated with its exploitation, particularly the risk of soil and water contamination. In this context, several remediation approaches have been developed in response to the global ecological transition, with particular emphasis on phytoremediation, a strategy that employs plants to remove, immobilize, or neutralize PTEs from the environment.

Furthermore, the strengthening of environmental policies and the global agenda aimed at restoring degraded areas and mitigating pollution caused by PTEs have further stimulated research in this field. As a result, studies on Ni phytoremediation have become geographically widespread, reflecting the growing interest of researchers from different countries in developing effective strategies for the remediation of Ni-contaminated environments (Figure 2C).

Considering the total of 91 studies included in this review, 9 studies (9.9%) were published between 2000 and 2009, 33 (36.3%) between 2010 and 2020, and 49 (53.8%) between 2021 and 2026 (Figure 2B). This trend highlights the growing scientific interest in Ni phytoremediation, paralleling increasing global concerns regarding environmental conservation.

The studies published between 2000 and 2026 are broadly distributed across different regions of the world (Figure 3). Of the 91 studies included in this review, most of the research was conducted in Asia (n = 50; 54.9%), particularly in India, Iran, and China, followed by Europe (n = 30; 33.0%), where Poland, Italy, and Germany were the major contributors. This distribution reflects the strong interest among researchers in these regions in using plant species to mitigate environmental contamination. A smaller proportion of studies was conducted in Africa (n = 8; 8.8%), followed by the Americas (n = 2; 2.2%), represented by Brazil and Canada and Oceania (n = 1; 1.1%), represented by Australia. This geographical distribution demonstrates the global effort to develop sustainable strategies to mitigate environmental threats and restore contaminated ecosystems.

The world’s largest Ni reserves are primarily concentrated in Indonesia, Australia, Brazil, Russia, Canada, and New Caledonia, countries that host extensive lateritic and sulfide deposits (Figure 3). Indonesia is currently the world’s leading Ni producer due to its vast laterite reserves, whereas Australia and Canada are recognized for the exploitation of high-grade sulfide deposits [59]. In Brazil, Ni reserves are mainly distributed across the states of Goiás, Pará, and Bahia, placing the country among the world’s leading nickel producers [60].

Despite the economic importance of Ni to the steel industry and the manufacture of rechargeable batteries, Ni extraction without adequate environmental planning and management can have substantial impacts on ecosystems [61]. Among the principal environmental consequences are the generation of large volumes of mining waste and the leaching of potentially toxic metals, processes that promote the contamination of soils and water resources, thereby compromising biodiversity, water quality, and agricultural productivity [62].

In this context, it is noteworthy that a substantial proportion of the studies included in this review were conducted outside the world’s major Ni-producing regions (Figure 3). This finding reveals an important scientific gap in the very countries where mining activities are most intensive and, consequently, where the risks of environmental degradation are greatest. In major Ni-producing regions, such as Indonesia, the Philippines, and New Caledonia, nickel mining plays a strategic role in economic growth and integration into the global supply chains [63]. However, despite the importance of Ni resources in these regions, the geographical distribution of the studies reviewed here reveals a notable research gap, with limited investigation of Ni phytoremediation in areas with substantial Ni resources (Figure 3).

This limited research coverage highlights the need to expand studies on the remediation of Ni-contaminated soils in major Ni-producing regions. Given the potential environmental impacts associated with Ni mining, greater research efforts in these areas are needed to assess soil contamination, monitor environmental quality, and evaluate the effectiveness of remediation strategies under local conditions. In particular, advancing field-based phytoremediation studies could provide important evidence for the development of sustainable approaches to reducing environmental liabilities associated with Ni mining. Therefore, increasing the participation of major Ni-producing countries in phytoremediation research represents an important priority for future studies.

6. Classification of Plant Species

The 449 plant records identified in the literature indicate that a wide range of plant groups have been investigated for Ni phytoremediation. Herbaceous species were the most frequently reported, accounting for 86.20% (n = 387) of all records, followed by shrubs (n = 34; 7.6%), trees (n = 22; 4.90%), and aquatic macrophytes (n = 6; 1.34%) (Figure 4).

Herbaceous vegetation is characterized by the predominance of low-growing species, including grasses, forbs, and other ground-cover plants, which are adapted to local edaphoclimatic conditions and play a fundamental role in maintaining the ecological stability of diverse landscapes. In addition to their high colonization capacity and extensive soil cover, herbaceous plants contribute significantly to soil conservation by acting as a physical barrier against erosion, reducing both the direct impact of rainfall and the velocity of surface runoff [64,65]. The fibrous root systems of herbaceous species also improve soil structure, enhance water infiltration, and promote nutrient cycling, thereby contributing to nitrogen and phosphorus retention and improving water quality in both natural and restored ecosystems [66,67]. Recent studies have further emphasized the importance of herbaceous vegetation in maintaining functional biodiversity, enhancing carbon sequestration in rapidly regenerating ecosystems, and increasing ecological resilience to climate change [65,67].

Shrub vegetation, composed of intermediate-sized woody species, represents an important structural component for ecological connectivity and the provision of ecosystem services in natural, agricultural, and restored landscapes [68]. Shrubs form a transitional layer between the herbaceous and arboreal strata, increasing habitat heterogeneity while providing shelter, nesting sites, and food resources for a wide range of animal taxa, thereby substantially enhancing biodiversity. Shrub communities also play an important role in water conservation, as their root systems stabilize slopes, reduce soil erosion, control sediment transport, and improve water infiltration and retention within the soil profile [69]. Furthermore, recent research has demonstrated that shrub communities can contribute to climate change mitigation by storing carbon, reducing land surface temperatures, and regulating biogeochemical processes, including greenhouse gas emissions, particularly in degraded landscapes and ecological restoration sites [70,71].

Arboreal vegetation comprises tree species of varying sizes, preferably native species adapted to local environmental conditions; these species are fundamental to ecosystem stability and sustainability [72,73]. The presence of native tree species enhances ecological resilience, promotes biodiversity, and supports essential ecosystem processes, including nutrient cycling, soil conservation, and hydrological regulation [73,74].

This vegetation layer performs several strategic ecological functions by acting as a natural windbreak, reducing soil erosion, enhancing water and nutrient retention, and improving water quality through soil stabilization and a reduction in surface runoff [75,76,77]. In addition, trees provide habitat and ecological connectivity for wildlife, thereby increasing biological diversity and enhancing the ecological functionality of landscapes [78].

Arboreal vegetation also plays a key role in climate regulation and air-quality improvement because of its capacity to sequester carbon, remove atmospheric pollutants, and reduce local temperatures through shading and evapotranspiration. These processes create more stable microclimates and contribute to climate change mitigation [73,77,79].

Aquatic macrophytes have been extensively investigated as a means of remediating environments contaminated with heavy metals because of their favorable characteristics for this purpose. These plants are widely used in wastewater treatment systems for domestic sewage and industrial effluents because they can reduce the concentrations of pollutants such as nickel, iron, manganese, lead, and other PTEs. For example, Silva et al. [80] reported that the aquatic species Spirodela polyrhiza and Ricciocarpos natans were effective in reducing iron and manganese concentrations. This approach has also been widely applied for the treatment of wastewater and municipal sewage, particularly in countries where constructed wetlands have been implemented. Constructed wetlands are engineered ecosystems composed of aquatic plants and suitable substrates designed to simulate natural wetlands and incorporated into one or more stages of wastewater treatment. Their purpose is to improve effluent treatment by integrating physical processes, such as sedimentation and filtration, chemical processes, including adsorption and precipitation, and biological processes, such as plant uptake and microbial degradation and transformation [81].

The integration of different vegetation strata provides synergistic benefits by creating an effective transitional zone between cultivated areas and adjacent water bodies [82]. The diversity of plant species with phytoremediation potential varies among vegetation types, with herbaceous species being the most highly represented, particularly those belonging to the genus Brassica (Figure 4).

Plant Families and Species

Among the 91 publications analyzed, several studies evaluated plant groups belonging to more than one family and exhibiting different growth habits. For example, Alghamdi and El-Zohri [83] assessed the Ni tolerance of herbaceous species (Aerva javanica, Portulaca oleracea, Eleusine indica, Cenchrus ciliaris, Pennisetum divisum, Tetraena coccinea, and Fagonia indica) as well as subshrub species (Tephrosia nubica and Dipterygium glaucum) grown in Ni-contaminated soil.

Based on the studies reviewed, Trifolium alexandrinum exhibited the highest bioaccumulation potential [84], followed by Jatropha curcas, which maintained its phytoremediation capacity up to the highest Ni concentration evaluated (23.0 mg kg⁻¹) [85]. The grass Cymbopogon citratus also demonstrated remarkable phytoremediation performance by effectively phytostabilizing soils containing up to 699.1 mg kg⁻¹ Ni derived from battery waste [47]. Among all species evaluated, Alyssum murale var. chlorocarpum Hausskn exhibited the greatest Ni phytoextraction capacity, accumulating up to 25,500 mg kg⁻¹ Ni in its dry biomass [86]. The phytoremediation potential of A. murale for metals commonly used in lithium-ion battery components has also been investigated, revealing distinct patterns of metal translocation and sequestration, together with promising levels of Ni and cobalt (Co) accumulation, thereby confirming its exceptional hyperaccumulation potential [87].

A wide diversity of Ni-tolerant plant species has been reported, many of which are capable of extracting Ni and other heavy metals present at elevated concentrations in contaminated soils and accumulating them in their tissues. The ability of Chromolaena odorata to grow across a range of Cd, Ni, and zinc (Zn) concentrations in crude oil-contaminated soils was demonstrated by Atagana [88], who reported that this species was capable of growing and phytoaccumulating heavy metals under these conditions, thereby facilitating the remediation of crude oil-contaminated soils. The species exhibited tolerance to soil Ni concentrations of up to 2000 mg kg⁻¹. For studies in which the phytoremediation mechanism was classified as not determined (N.D), it is important to note that, although the mechanisms involved were not explicitly identified, all studies reported that the plant species listed in

Table 1. Genera and species classified in the literature as Ni phytoremediators.

Genus/Species	Family	Maximum Concentration (mg kg−1)	Phytoremediation Classification	Reference
Arundo donax	Poaceae	282	Rhizofiltration	[89]
Aloe vera	Asphodelaceae	N.D	Hyperaccumulator	[90]
Alternanthera spp.	Amaranthaceae	138.4	Ni-tolerant	[91]
Alyssum spp.	Brassicaceae	25,500	Phytoextraction/Hyperaccumulator	[86,87,92]
Amaranthus spp.	Amaranthaceae	58.9	Phytostabilization	[93]
Ambrosia spp.	Asteraceae	150	Hyperaccumulator	[94]
Arthraxon hispidus	Poaceae	89.46	Phytostabilization	[95]
Beta vulgaris L.	Amaranthaceae	384.6	N.D	[96]
Bidens pilosa	Asteraceae	14.4	Phytoextraction	[97]
Boehmeria nivea	Urticaceae	89.46	Phytostabilization	[95]
Bornmuellera baldacci	Brassicaceae	19,200	Phytoextraction	[86]
Brassica spp.	Brassicaceae	480	Ni-tolerant/Phytostabilization/Phytoextraction/Hyperaccumulator	[46,98,99,100,101,102,103]
Calotropis procera	Apocynaceae	150	Hyperaccumulator	[45]
Cenchrus ciliaris	Poaceae	60	Ni-tolerant	[18]
Centaurea intricata	Asteraceae	266.06	Phytostabilization	[104]
Ceratophyllum spp.	Ceratophyllaceae	1	N.D	[105]
Chromolaena odorata	Asteraceae	2.000	Ni-tolerant	[88]
Chrysanthemum spp.	Asteraceae	89.46	Phytostabilization	[95]
Chrysopogon spp.	Poaceae	699.1	Hyperaccumulator	[47]
Conyza canadensis	Asteraceae	89.46	Phytostabilization/Phytoextraction	[95,97]
Crotalaria micans	Fabaceae	14.4	Phytoextraction	[97]
Cymbopogon spp.	Poaceae	699.1	Ni-tolerant/Phytostabilization/Hyperaccumulator	[47,106]
Cynodon dactylon	Poaceae	60	Ni-tolerant	[18]
Eichhornia crassipes	Pontederiaceae	138.4	Ni-tolerant/Bioaccumulation	[91,107]
Elymus elongatus	Poaceae	400	Phytostabilization	[108]
Eragrostis spp.	Poaceae	75	Ni-tolerant	[109]
Euphorbia marginata	Euphorbiaceae	100	N.D	[110]
Festuca arundinacea	Poaceae	150	N.D	[111]
Halimione portulacoides	Amaranthaceae	26.2	Phytostabilization	[112]
Helianthus annuus	Asteraceae	80	Ni-tolerant/Phytoextraction	[33,99]
Hibiscus cannabinus	Malvaceae	271	Phytoextraction	[113]
Hordeum vulgare	Poaceae	75	Phytoextraction	[114]
Hypolepis muelleri	Dennstaedtiaceae	500	Phytostabilization	[115]
Jatropha curcas	Euphorbiaceae	23	Bioaccumulation	[85]
Lactuca sativa	Asteraceae	22.5	Bioaccumulation	[84]
Lavandula angustifolia	Lamiaceae	75.36	N.D	[116]
Lemna minor	Araceae	5	Hyperaccumulator	[117]
Leptadenia pyrotechnica	Apocynaceae	17,1	Phytostabilization	[118]
Lactuca sativa	Asteraceae	500	N.D	[119]
Leucaena leucocephala	Fabaceae	14.4	Phytoextraction	[97]
Medicago sativa L.	Fabaceae	N.D	N.D	[120]
Megathyrsus maximus	Poaceae	40	Ni-tolerant	[121]
Melilotus spp.	Fabaceae	500	Phytostabilization	[97,119]
Miscanthus spp.	Poaceae	600	Ni-tolerant/Phytostabilization	[95,97,122,123,124,125,126]
Nephrolepis cordifolia	Nephrolepidaceae	500	Phytostabilization	[115]
Nerium oleander L.	Apocynaceae	50	Phytoextraction	[127]
Nicotiana spp.	Solanaceae	150	N.D	[128]
Origanum vulgare	Lamiaceae	75.36	N.D	[116]
Oryza sativa	Poaceae	250	Phytostabilization	[129,130]
Parthenium spp.	Asteraceae	742	Hyperaccumulator	[131]
Phragmites australis	Poaceae	N.D	Bioaccumulation	[107]
Poa pratensis	Poaceae	150	N.D	[111]
Polygonum capitatum	Polygonaceae	89.46	Phytoextraction	[95]
Pongamia pinnata	Fabaceae	19	Bioaccumulation	[85]
Populus tremuloides	Salicaceae	88.2	Phytoextraction	[132]
Portulaca oleracea	Portulacaceae	330	Ni-tolerant	[133]
Pueraria lobata	Fabaceae	14.4	Phytoextraction	[97]
Chenopodium quinoa	Amaranthaceae	5.9	N.D	[134]
Raphanus sativus	Brassicaceae	150	Phytoextraction	[135]
Ricinus communis	Euphorbiaceae	150	Ni-tolerant	[98]
Rubus spp.	Rosaceae	89.46	Ni-tolerant/Phytostabilization	[95,136]
Sanguisorba minor	Rosaceae	23.21	Ni-tolerant	[137]
Scirpus triqueter	Cyperaceae	301	Ni-tolerant	[138]
Senecio scandens	Asteraceae	89.46	Phytostabilization	[95]
Sesuvium spp.	Aizoaceae	1.26	Bioaccumulation/Phytoextraction.	[139,140]
Sorghum spp.	Poaceae	5	Ni-tolerant/Phytostabilization	[99,141]
Spinacia oleracea L.	Amaranthaceae	2.62	Phytoextraction	[99]
Tagetes spp.	Asteraceae	10	Hyperaccumulator	[142]
Tanacetum balsamita	Asteraceae	220	Ni-tolerant	[143]
Taraxacum officinale	Asteraceae	150	Hyperaccumulator	[94]
Trifolium spp.	Fabaceae	270	Bioremediation/Bioaccumulation	[84,144]
Typha angustata	Typhaceae	138.4	Ni-tolerant	[91]
Urochloa spp.	Poaceae	138.4	Ni-tolerant	[91,121]
Urtica dioica	Urticaceae	222.84	Phytoextraction	[101]
Vernonia amygdalina	Asteraceae	5.31	N.D	[145]
Vetiveria spp.	Poaceae	200	Phytostabilization; Phytoextraction	[146,147]
Zea mays L.	Poaceae	400	Ni-tolerant/Phytostabilization	[108,148]
Zygophyllum spp.	Zygophyllaceae	16.46	Phytostabilization	[118]

N.D = Phytoremediation mechanism not determined.

were able to establish and grow in the presence of Ni, supporting their potential effectiveness in phytoremediation.

Regarding the taxonomic distribution of the species evaluated, the herbaceous plant group was dominated by the family Poaceae (n = 122; 27.2% of the 449 plant records), which comprised a diverse range of species. Within this family, Cymbopogon spp. (n = 19; 4.2%), Miscanthus spp. (n = 12; 2.7%), and Oryza sativa (n = 9; 2.0%) were the most frequently investigated taxa. Other genera and species reported within Poaceae included Festuca spp. (n = 8; 1.8%), Hordeum vulgare (n = 8; 1.8%), Sorghum spp. (n = 7; 1.6%), Urochloa spp. (n = 7; 1.6%), Z. mays (n = 6; 1.3%), Cenchrus ciliaris (n = 6; 1.3%), Megathyrsus maximus (n = 6; 1.3%), and Vetiveria spp. (n = 5; 1.1%), among others.

The family Brassicaceae accounted for 15.1% (n = 68) of the 449 plant records, with Brassica spp. (n = 39; 8.7%) being the most frequently reported taxon, followed by Alyssum spp. (n = 12; 2.7%), Raphanus sativus (n = 6; 1.3%), and Lepidium sativum (n = 5; 1.1%). Within the Asteraceae (n = 47; 10.5%), the most frequently investigated taxa were Tagetes spp. (n = 14; 3.1%), Helianthus annuus (n = 7; 1.6%), and Carthamus spp. (n = 5; 1.1%). In the Amaranthaceae (n = 22; 5.0%), Chenopodium quinoa (n = 6; 1.3%) and Beta vulgaris (n = 6; 1.3%) were the most frequently evaluated species. Several other herbaceous plant families were also represented in the literature, although with lower frequencies of occurrence.

Among shrub and subshrub species, several studies evaluated combinations of different plant families and genera. The family Euphorbiaceae was the most frequently represented (n = 8; 1.8% of the 449 plant records), with Ricinus spp. (n = 5; 1.1%) being the most commonly reported taxon. The family Araliaceae was also represented, primarily by species of the genus Hedera (n = 6; 1.3%). Among arboreal species, the family Fabaceae (n = 8; 1.8%) included studies involving Pongamia pinnata (n = 4; 0.9%), Albizia lebbeck (n = 3; 0.7%), and Leucaena leucocephala (n = 1; 0.2%). Additional tree families represented in the literature included Sapindaceae (n = 4; 0.9%), Euphorbiaceae (n = 4; 0.9%), Salicaceae (n = 1; 0.2%), Betulaceae (n = 1; 0.2%), and Moraceae (n = 1; 0.2%). Regarding aquatic macrophytes, Lemna minor (Araceae) accounted for 0.7% (n = 3) of the 449 plant records. Other macrophyte species belonged to the families Pontederiaceae, Typhaceae, and Poaceae, including Eichhornia spp. (n = 1; 0.2%), Typha spp. (n = 1; 0.2%), and Phragmites australis (n = 1; 0.2%), respectively.

Species belonging to the family Poaceae were reported in 0.2% of the studies involving aquatic macrophytes and in 25.8% of those involving herbaceous plants. These included Cymbopogon spp. [47], Miscanthus spp. [95,126], Urochloa spp. [121], O. sativa [129,130], Sorghum spp. [141,149], and Z. mays [108,148]. These grasses are frequently used to prevent soil erosion owing to their extensive root systems and high biomass production. Furthermore, Z. mays not only contributes to nutrient and contaminant retention and uptake but also has potential for bioenergy production [148,150].

The family Brassicaceae, representing 17.1% of the herbaceous species and 0.7% of the shrub species reported in the literature, includes taxa such as Alyssum spp. [87] and Brassica spp. [46,99,102], which are generally recognized as hyperaccumulators of heavy metals, particularly Ni. In a study comparing the phytoremediation efficiency of three Brassica genotypes, only Brassica juncea demonstrated promising hyperaccumulation potential in Ni-contaminated soils. In contrast, Brassica napus and Brassica campestris were adversely affected by Ni phytotoxicity, exhibiting metabolic and physiological disturbances that impaired nutrient homeostasis and consequently reduced plant growth and productivity [46].

The family Asteraceae, represented by 0.5% of the shrub species and 10.5% of the herbaceous species included in this review, comprises a wide range of plants, including Tagetes spp. [142], H. annuus L. [33,151], and Carthamus spp. [141], which have been widely investigated for use in ecological restoration programs. Their morphological, physiological, and reproductive characteristics facilitate establishment in disturbed environments, making these species highly valuable for the phytoremediation of contaminated soils and water bodies [131].

The family Fabaceae accounted for 7.1% (n = 32) of the 449 plant records, comprising herbaceous species (n = 20; 4.5%), tree species (n = 8; 1.8%), and shrub species (n = 4; 0.9%). One of the principal attributes of this family for phytoremediation is its high capacity for biological nitrogen fixation. In addition, many Fabaceae species produce large amounts of aboveground biomass and possess deep, extensive root systems that enhance phytostabilization by reducing soil erosion and improving soil physical structure, as demonstrated for L. leucocephala [97]. The family includes numerous species with recognized phytoremediation potential, such as Trifolium spp. [84,144,152], Vigna radiata [153], M. sativa L. [120,152], and Crotalaria spp., which are also widely used as green manure crops [154]. Together, these characteristics confer high tolerance to environmental stress and effective mechanisms for pollutant stabilization and, in some cases, degradation, making Fabaceae species particularly suitable for sustainable and cost-effective phytoremediation strategies.

7. Nickel Concentrations and Textural Class of Evaluated Soils

Knowledge of the soil texture class and heavy metal concentration is essential for the success of phytoremediation studies, as these factors determine contaminant bioavailability and directly influence plant establishment, growth, and survival.

According to Resolution No. 420/2009 [7] of the Brazilian National Council for the Environment (CONAMA), which establishes the Soil Quality Reference Values (QRVs) for naturally occurring inorganic substances in Brazilian soils, the prevention value for Ni is 30 mg kg⁻¹ dry weight. Concentrations above this threshold indicate the potential for adverse environmental impacts and trigger the need for further investigation. The reference values established for Ni are 70 mg kg⁻¹ for agricultural investigation, 100 mg kg⁻¹ for residential investigation, and 130 mg kg⁻¹ for industrial investigation. For groundwater, the intervention value is 20 µg L⁻¹.

A total of 230 different Ni concentrations were reported in the studies included in this review. Of these, 32.2% evaluated Ni concentrations of up to 30 mg kg⁻¹, 17.4% investigated concentrations ranging from 30 to 70 mg kg⁻¹, 11.3% evaluated concentrations between 70 and 100 mg kg⁻¹, 8.3% assessed concentrations up to 130 mg kg⁻¹, and 30.9% investigated concentrations exceeding 130 mg kg⁻¹ (Figure 5A). Because QRVs represent the natural background concentrations of a given substance in soil under conditions unaffected by anthropogenic activities, comparing measured heavy metal concentrations in soils and groundwater with the CONAMA QRVs is fundamental for environmental management in Brazil. This approach enables the differentiation of anthropogenic contamination from naturally occurring (geogenic) metal concentrations.

Regarding the substrates used in the reviewed studies, a wide diversity of textural classes was identified. Sandy soils were the most frequently investigated, accounting for 21.2% of the studies, followed by sandy loam soils (16.3%). For descriptive purposes, some substrates were grouped under the category Special Soil Conditions (14.9%), which included hydroponic experiments and studies using substrates such as sand mixed with perlite, bedrock and rock outcrops, carbonate-rich substrates, serpentine soils, and calcareous soils. Another category, Anthropogenic Soils (14.4%), comprised substrates collected from agricultural lands, landfills, mining waste deposits, industrial sites, and sewage sludge. Clay loam soils accounted for 7.2% of the studies, whereas clay soils represented 5.3%. In 13.9% of the publications analyzed, the substrate used was not reported (N.D). The remaining studies were conducted under other conditions, including natural environments (3.4%), silty soils (1.4%), medium-textured soils (1.0%), and silt loam soils (0.5%) (Figure 5B).

Sandy soils are composed predominantly of sand (>70%) and contain relatively low amounts of clay (<15%). Their coarse and loose texture results in high permeability, while they generally exhibit low fertility, low organic matter content, and acidic pH. These characteristics tend to increase the bioavailability of heavy metals, making sandy soils particularly suitable for phytoremediation studies aimed at identifying species with high phytoextraction potential. However, the principal challenges associated with these soils are maintaining adequate soil moisture and plant nutrition, whereas their major advantages include greater contaminant availability to plant roots and improved soil aeration, which favors microbial activity. In one study, B. juncea effectively phytoremediated sandy soil contaminated with up to 480 mg kg⁻¹ Ni [100]. In another study conducted under the same soil conditions, Cannabis sativa exhibited pronounced phytotoxic effects of Ni, with a significant reduction in biomass production observed even at the lowest Ni concentration evaluated (500 mg kg⁻¹) [155].

Clay soils possess unique physical and chemical properties that directly influence the efficiency of phytoremediation. Their relatively high clay content and cation exchange capacity (CEC) can favor the retention of nutrients and potentially toxic elements, thereby influencing metal mobility and bioavailability. In Ni-contaminated soils, metal uptake by plants is also affected by its chemical form and interactions with the soil matrix. Molas and Baran [114], for example, demonstrated that Ni uptake and toxicity in Hordeum vulgare varied according to the chemical form of Ni in medium and heavy clay soils. Strong metal retention may reduce Ni availability for plant uptake, potentially limiting phytoextraction while favoring strategies based on reduced contaminant mobility. In addition, plant–microorganism interactions can modify Ni acquisition and toxicity in contaminated soils [144]. In a study evaluating different C. quinoa genotypes (Q-6, Q-7, Q-50, Q-51, Q-76, and Q-82), the Q-50, Q-76, and Q-82 genotypes accumulated the highest concentrations of heavy metals in older leaves, with Q-76 exhibiting the greatest potential for the phytoextraction of Cd, Pb, Cu, and Ni [134].

PTEs released through industrial emissions, mining activities, and agricultural practices have become a major source of environmental contamination worldwide [156]. In anthropogenic soils, phytoremediation represents a promising remediation strategy because it is both sustainable and cost-effective, relying on plants to remove or immobilize heavy metals in industrially contaminated soils and other disturbed environments. Nevertheless, plant species combining rapid growth with high biomass production are generally considered the most suitable for successful phytoremediation. When evaluating the phytoremediation potential of Sesuvium portulacastrum L. for the removal of heavy metals from industrially contaminated soils, Kumawat et al. [140] reported Ni removal efficiencies exceeding 79%, demonstrating that this approach is both economically viable and environmentally sustainable. Likewise, Eragrostis curvula cultivars grown in mining-contaminated soils were shown to promote the proliferation of plant growth-promoting bacterial genera. The activity of these bacteria and their associated soil enzymes may have contributed to the ability of this species to maintain normal growth under contaminated conditions, highlighting its tolerance to soils polluted by multiple heavy metals [109].

The studies analyzed indicate that B. juncea consistently demonstrated high Ni phytoremediation efficiency across different soil textures. The species demonstrated a high capacity for phytoextraction in anthropogenic (industrial) soils [102] and sandy soils [100], behaved as a Ni hyperaccumulator in sandy loam soils [46], and exhibited Ni tolerance in another study where the soil classification was not reported [98]. In contrast, B. campestris and B. napus grown in sandy loam soils [46], as well as Brassica oleracea cultivated in clay loam soil [157], showed limited effectiveness for Ni phytoremediation. These findings indicate that, in addition to soil physicochemical properties such as pH, nutrient status, and organic matter content, the biological characteristics of the plant varieties evaluated strongly influence phytoremediation performance, highlighting the complex association between phytoremediating plants and soil textural classes (Figure 6).

Soil texture plays a decisive role in Ni mobility and, consequently, in the effectiveness of phytoremediation strategies. In sandy soils, the low adsorption capacity, resulting from reduced clay and organic matter contents and a low CEC, increases the bioavailability of Ni to plants. This condition makes sandy soils particularly suitable for phytoextraction, as Ni remains in chemical forms that are more readily absorbed by plant roots. However, the high mobility of Ni also increases the risks of leaching, phytotoxicity, and reduced biomass production, while requiring more intensive management of irrigation and soil fertility.

In contrast, clay soils exhibit a high capacity to adsorb Ni because of their higher contents of clay minerals and iron and aluminum oxides, as well as their higher CEC. Although these characteristics reduce Ni bioavailability and may limit the efficiency of phytoextraction, they also decrease Ni mobility within the soil, making phytostabilization a more appropriate remediation strategy. Therefore, the choice between phytoextraction and phytostabilization should be based on the physicochemical properties of the soil, since the greater Ni bioavailability in sandy soils favors its uptake and removal by plants, whereas the stronger retention of Ni in clay soils promotes its immobilization and reduces the risk of environmental dispersion.

Foliar application of salicylic acid to P. oleracea cultivated in sandy clay loam soil contaminated with 330 mg kg⁻¹ Ni proved effective in mitigating the effects of Ni and Pb contamination while maintaining satisfactory phytoremediation potential [133]. However, the same species, when grown in sandy soil containing 30 mg kg⁻¹ Ni, exhibited impaired growth and was therefore not considered suitable for the remediation of heavy metal-contaminated soils under the evaluated conditions [83]. These findings are consistent with those of Erkoç et al. [158], who observed similar responses in P. oleracea exposed to soil Ni concentrations of up to 80 mg kg⁻¹, although soil texture was not reported. Overall, soil properties determine contaminant bioavailability and, consequently, exert a strong influence on plant establishment, growth, and survival under contaminated conditions.

8. Future Perspectives

Advances over recent decades have demonstrated the potential of phytoremediation for the remediation of Ni-contaminated soils. Nevertheless, translating findings obtained under controlled conditions into large-scale applications remains a major challenge. Important knowledge gaps include the scarcity of long-term field studies, methodological heterogeneity, limited understanding of plant–soil–microbiota interactions, and insufficient assessment of phytoremediation performance under contrasting soil and environmental conditions. Addressing these limitations will require more integrated approaches that combine agronomic, environmental, microbiological, and technological aspects.

Promising research directions include integrating phytoremediation with plant growth-promoting microorganisms, mycorrhizal fungi, biochar, soil amendments, and other soil conditioners to enhance Ni extraction or immobilization, depending on the remediation objective. A better understanding of plant–soil–microbiota interactions may also contribute to improving plant establishment, Ni tolerance, and the efficiency of phytoextraction and phytostabilization.

The identification of new hyperaccumulator species, genetic improvement of Ni-tolerant plants, and application of omics approaches, including genomics, transcriptomics, proteomics, and metabolomics, represent additional opportunities to elucidate the mechanisms underlying Ni tolerance, uptake, and translocation. These advances may support the development and selection of more effective plant materials and management strategies for the remediation of Ni-contaminated soils.

Furthermore, the limited evidence on the technical, economic, and environmental feasibility of large-scale phytoremediation highlights the need for cost–benefit analyses, life cycle assessments, and sustainability indicators. Research should also be expanded in major Ni-producing regions, where environmental pressures associated with mining reinforce the need to evaluate phytoremediation under locally relevant conditions. Integrating these economic, environmental, and regional perspectives may contribute to more efficient and sustainable remediation strategies aligned with the principles of the circular economy and sustainable mining.

9. Conclusions

This integrative review demonstrates the potential of phytoremediation for Ni-contaminated soils across a wide range of plant species and soil conditions. Among the species evaluated, Brassica juncea was consistently identified as a promising species for Ni phytoremediation, showing Ni tolerance, phytoextraction, and hyperaccumulation across different studies, whereas Alyssum murale stood out among the reported hyperaccumulators for its high Ni accumulation and phytoextraction potential. The evidence also indicates that phytoremediation efficiency depends not only on plant characteristics but also on soil properties, particularly pH, texture, organic matter, and cation exchange capacity, which regulate Ni mobility, bioavailability, and plant uptake.

Despite these advances, important methodological, geographical, and practical knowledge gaps remain. The heterogeneity of experimental designs limits comparisons among studies and the establishment of standardized criteria for assessing phytoremediation efficiency. In addition, the scarcity of long-term field studies, limited investigation of plant–soil–microbiota interactions, underrepresentation of major Ni-producing regions, and insufficient evidence on technical and economic feasibility constrain the large-scale application of phytoremediation.

Future research should therefore prioritize standardized and comparable protocols, long-term field validation under contrasting edaphoclimatic conditions, integrated assessments of plant–soil–microbiota interactions, and studies in underrepresented Ni-producing regions. Greater emphasis on remediation efficiency, long-term stability, and technical and economic feasibility is also needed. Addressing these priorities will strengthen the scientific basis for selecting suitable plant species and management strategies, while advancing Ni phytoremediation from predominantly experimental applications toward effective, economically feasible, and sustainable remediation of contaminated soils.