Phytoremediation Potential of Silicon-Treated Brassica juncea L. in Mining-Affected Water and Soil Composites in South Africa: A Review

Abstract

Heavy metal pollution due to mining activities poses a significant threat to agricultural production, ecosystem health, and food security in South Africa. This review integrates current knowledge on the use of mustard spinach (Brassica juncea (L.) Czern.) for the bioremediation of polluted water and soil, focusing on enhancing phytoremediation efficiency through the use of silicon-based biostimulant treatments. Mustard spinach is known for its capacity to accumulate and tolerate high levels of toxic metals, such as Pb, Cd, and Hg, owing to its strong physiological and biochemical defense mechanisms, including metal chelation, antioxidant activity, and osmotic adjustment. However, phytoremediation potential is often constrained by the negative impact of heavy metal stress on plant growth. Recent studies have shown that silicon-based biostimulants can alleviate metal toxicity by reducing metal bioavailability, increasing metal immobilization, and improving the antioxidative capacity and growth of plants. Combining silicon amendments with mustard spinach cultivation is a promising, eco-friendly approach to the remediation of mining-impacted soils and waters, potentially restoring agricultural productivity and reducing health risks to the resident populations. This review elucidates the multifaceted mechanisms by which silicon-enhanced phytoremediation operates, including soil chemistry modification, metal sequestration, antioxidant defense, and physiological resilience, while highlighting the practical, field-applicable benefits of this combined approach. Furthermore, it identifies urgent research priorities, such as field validation and the optimization of silicon application methods.

1. Introduction

Globally, mining has long served as a major economic growth contributor, significantly contributing to GDP and employment, and particularly benefiting rural and economically disadvantaged communities [1,2,3]. However, mining activities disrupt ecological balance, alter landscapes, and generate hazardous waste, often resulting in soil contamination by heavy metals, which pose threats to agricultural yields and public health [3,4,5,6,7]. Heavy metals, such as chromium (Cr), manganese (Mn), cadmium (Cd), and lead (Pb), persist in environmental matrices at concentrations exceeding ecotoxicological safety levels, and they are easily absorbed by plants through phytoaccumulation. This contributes to their introduction into the food chain, thereby affecting food security and ecosystem integrity [8,9,10]. Extensive research has demonstrated that heavy metal pollution adversely affects agricultural productivity. For example, mercury (Hg) pollution inhibits root growth and shoot biomass in Indian mustard (Brassica juncea (L.) Czern.), whereas zinc (Zn) pollution decreases root dry weight, shoot length, and leaf area in Mathiola flavida (M. flavida) [11,12]. In addition, Pb and Cd cause decreases in seed germination and biomass accumulation in other crops [13,14], posing a significant threat to crop productivity and the health of nearby communities.

Despite these detrimental effects of heavy metals on plants, it is also important to recognize that certain heavy metals, such as Mn, Zn, copper (Cu), iron (Fe), molybdenum (Mo), and nickel (Ni), are essential micronutrients for plants at trace levels. They serve as cofactors and activators for enzymes, participate in redox reactions, and support vital metabolic processes [15].

Globally, incidents such as tailings dam failures and toxic spills have heightened concerns regarding the environmental hazards of mining, resulting in severe ecological degradation, reduced agricultural productivity, and threats to water resources, livestock, and human health [16,17,18,19,20].

In South Africa, the 2022 Jagersfontein tailing dam collapse released large quantities of mining waste, contaminating soils and waterways, destroying crops, and displacing communities [16]. Similarly, the toxic spill at the Okhokho coal mine contaminated local water resources and compromised ecosystem health [17]. These two incidents illustrate the immediate and far-reaching impact of mining mismanagement in the region. These two incidents are presented as representative examples of the broader environmental threat posed by mining, which underscores the urgent need for effective remediation strategies to address these issues.

Given these challenges and the persistence of heavy metals in the environment, phytoremediation has emerged as a promising, cost-effective, and eco-friendly strategy for remediating contaminated soil and water. B. juncea offers a promising solution to soil and water pollution. B. juncea from the Brassicaceae family is recognized for its remarkable capacity to absorb heavy metals through its roots, thereby aiding in the rehabilitation of polluted land [21,22]. This is supported by field trials by Nepal et al. [23], where B. juncea varieties demonstrated hyperaccumulation of Cd and Mo with bioaccumulation factors (BAF) ranging from 1.31 to 2.22 for Cd and from 1.97 to 3.42 for Mo, significantly exceeding soil concentrations. These BAF values >1 indicate the plant’s capacity to concentrate metals in shoot tissues above soil levels, enabling repeated phytoextraction cycles that progressively reduce soil metal burdens. Similarly, this outcome was observed in studies conducted by Ali et al. [24] (Ni, Pb, Cr, and Cd) and Ojha et al. [25] (Pb) on B. juncea cultivated in contaminated soils, where their BAF was greater than 1.

However, their effectiveness is constrained by the limited growth rate and low biomass production of plants, both of which are adversely affected by heavy metal toxicity [26,27]. Wijekoon et al. [28] noted that phytoremediation is a naturally slow process that requires considerable time to reduce contamination levels significantly. Nonetheless, with a steady approach, it may be feasible to grow plants for consumption in the affected areas in the future, particularly with the support of biostimulants.

Several studies have emphasized the potential of silicon (Si) biostimulants to increase the effectiveness of phytoremediation in different crops [29,30]. Si, as the second most found element in soil after oxygen, is part of soil chemistry and plays a very important role in various necessary processes regarding plant physiology [31]. These observations indicate that Si-based biostimulants increase nutrient uptake, improve tolerance to environmental stress, and facilitate plant development, even under adverse conditions [32,33,34]. Taking advantage of the natural ability of B. juncea to sequester heavy metals, along with Si-based biostimulants for plant development, there is great potential for developing more effective and sustainable methods for cleaning soils contaminated with heavy metals from mining activities. This review makes a significant contribution to the field of environmental remediation by introducing a framework for silicon-enhanced phytoremediation using B. juncea in soils and waters affected by mining. Rather than focusing on single-target methods, this comprehensive approach examines the combined impact of applying silicon and plant-based remediation techniques. Current findings indicate that silicon may enhance the uptake of heavy metals, bolster plant stress resilience, and boost remediation effectiveness, potentially through mechanisms like pH alteration, interactions between metals and silicates, and improved antioxidative responses, although these pathways need further investigation. By connecting laboratory results with real-world applications, this study aids in developing practical and sustainable strategies for restoring environments impacted by mining and for enhancing food security.

2. Methodology

This qualitative review was developed through a comprehensive analysis of the existing literature on the role of biostimulants in mitigating plant stress in B. juncea, with a particular focus on plant growth, agricultural productivity, and crop quality under conditions of heavy metal contamination. The review is contextualized within both general and site-specific South African environmental conditions.

2.1. Literature Search Strategy

To achieve this, the authors conducted a desktop-based scoping review, following the methodological framework outlined by Fusar-Poli et al. [35]. The review emphasized various bioremediation strategies applicable to near-mine water and soil environments. The literature was primarily sourced with Google Scholar (https://scholar.google.com/) and ResearchGate (https://www.researchgate.net/), using the following targeted search terms related to the study’s thematic focus:

“Silicon-based biostimulant on bioremediation or Si-based biostimulant on bioremediation”; “Brassica juncea on heavy metals bioremediation or mustard spinach on heavy metals or Mineiation”; “Brassica juncea as phytoremediation on mine soils or mustard spinach as phytoremediation on mine soils”; “Brassica juncea as phytoremediation on mine water or mustard spinach as phytoremediation on mine water”; “Silicon-based biostimulant and Brassica juncea as phytoremediation strategy on mine water and soil composites or Si-based biostimulant and mustard spinach as phytoremediation strategy on mine water and soil composites”; “Environmental pollution remediation using silicon biostimulant and Brassica juncea or Environmental pollution remediation using Si biostimulant and mustard spinach”; “Mine soil and water pollution in South Africa or Mine soil and water pollution in Free State or Mine soil and water pollution in KwaZulu Natal”; and “Mercury, cadmium, lead, zinc, chromium and iron as heavy metals in mine soil and water”.

2.2. Inclusion and Exclusion Criteria, and the Selection Process

The authors applied a twenty-year publication window as the primary inclusion criterion, encompassing a range of sources such as peer-reviewed journal articles, review papers, book chapters, academic theses, short research communications, and industrial briefs.

2.3. Limitation of the Methodological Framework

No formal quality appraisal was conducted beyond the temporal exclusion criterion; therefore, this review does not claim to offer a comprehensive synthesis of all available evidence [35]. Instead, the selected materials were analyzed and discussed in alignment with this study’s central objective. The aim of the selected framework is based on the subjective bias associated with the areas and industry in the selected areas of South Africa, and the potential challenges associated with reliability and validity in the industry and selected areas of the study.

3. Findings on Heavy Metal Contamination: Sources, Environmental Pathways, and Impact on the Ecosystem and Agriculture

The main sources of heavy metal contamination include mining, smelting, industrial production, and other anthropogenic activities such as using pesticides in agriculture and sewage sludge for land irrigation. Mining has been identified as a major contributor to environmental pollution, owing to the large amount of waste generated during mineral processing [36,37]. The waste generated includes mine tailings and waste rocks, both of which have long-term effects on ecosystems.

Waste rocks are coarse geological materials produced during mining that have no economic value and require disposal. During weathering and decomposition, these rocks allow for the leaching of harmful substances into neighboring bodies of water and land [38]. Acid mine drainage (AMD) is one of the most critical causes of water and soil pollution [39,40,41]. Due to air and water, minerals such as sulfides in waste rocks oxidize and give rise to acidic solutions containing high levels of harmful metals such as Pb, Cd, and Cu [42]. Such acidic effluents degrade groundwater quality, cause considerable damage to ecosystems, and negatively affect fertile land by introducing harmful substances into the soil [43,44]. Such contaminants may cause serious physiological harm to the exposed plants, resulting in disruptions to normal metabolic processes and impairing the uptake of essential nutrients [45].

Similarly, the generation of metal-contaminated mine tailings with low water retention capacities as byproducts of mining activities poses severe ecological risks to the surrounding environment. These substances, typically stored in tailing ponds, have low water retention capacity, low organic matter content, and high metal concentrations, and thus act as a perpetual source of pollution. If left untreated and uncontrolled, tailings can leak or disperse via wind and water, thereby expanding the scope of contaminants far beyond the confines of mining activities [46]. Tailings can have pH values ranging from highly acidic to alkaline, depending on the immediate environment, and thus have serious ecological implications [42]. This pH variability makes tailings difficult to manage, as they pose risks to both terrestrial soils and aquatic environments, eventually leading to detrimental effects on local agriculture.

In addition, when animals living in polluted areas feed on or consume polluted plants, serious health risks arise. Long-term exposure to these environmental toxins has been linked to immune suppression, oxidative stress, and reproductive problems in various livestock species [47,48,49]. This poses a risk not only to the health of livestock, but also to consumers through the food chain.

Additionally, farming practices in polluted areas pose significant health risks to consumers. It is widely known that crops can take up essential nutrients, such as nitrogen, phosphorus, and potassium, from the soil and accumulate dangerous heavy metals, such as zero-valent Pb and Cd. Heavy metals do not undergo biological degradation and are, therefore, likely to bioaccumulate in human organs after the ingestion of polluted crops, resulting in cumulative toxicity in humans. This toxicity may cause adverse health effects in the form of cirrhosis of the liver, kidney injury, mental impairments, degeneration of the central nervous system, cardiac conditions, sterility, basal ganglia deterioration in the brain, and, in severe cases, death [50,51]. Therefore, agricultural production in these areas will be considerably undermined, as farmers will be unable to utilize their polluted lands, and people in these areas will be discouraged from planting gardens in their homes because of the health risks associated with consuming these crops.

Another major contributing factor to this situation is the prevalence of untreated mining wastewater, which discharges harmful contaminants into land and water, further intensifying pollution levels [52]. In addition, polluted water sources are used for irrigation in agriculture, driven by increasing water scarcity [53]. These factors cumulatively reduce agricultural production in the affected regions.

Heavy metal buildup in the tissues of plants disrupts physiological and metabolic processes, resulting in various harmful effects [54]. Although different heavy metals impart different biochemical effects on plants, they universally display toxicity, which negatively affects plant growth and development [54]. Several studies have shown that the occurrence of heavy metal pollutants is harmful, inhibits growth, causes tissue necrosis, and reduces seed germination, resulting in changes at molecular and structural levels in plant tissues [55,56,57,58]. As shown in

Table 1. The effect of the heavy metals on Brassica species and other vegetables.

Crop	Heavy Metal	Mode of Application	Concentrations	Regions	Results	Citation
Indian mustard (B. juncea)	Hg	Hydroponics/nutrient solution	0, 25, 50, 100, 200, 400 µM (HgCl2)	India	Inhibited growth: plant height ↓3–54%, fresh/dry weight ↓4–65%, yield attributes ↓20–33%, seed germination ↓4–76% (significant at ≥100 µM)	Ansari et al. [11]
Indian Mustard	Zn	Artificially contaminated soil	20, 40, 80, and 160 mg/L (ZnSO4)	Pakistan	Reduced root length, shoot length, root biomass, and shoot biomass (significant at ≥100 mg/L)	Chaudhry et al. [12]
Spinach (Spinacia oleracea)	Arsenic (As)	Hydroponics/nutrient solution	0, 50, 100 µM (Na2HAsO4·7H2O)	Pakistan	Dry root weight ↓21.1–58%, stem length ↓18.7%, leaf area ↓22.1–61%; photosynthetic pigments and gaseous exchange parameters ↓29–66% (significant at 50 and 100 µM)	Saleem et al. [59]
Rapeseed (Brassica napas) Indian Mustard (B. juncea) B. campretris	Pb	Hydroponics/nutrient solution	0, 50, 100, 150 mM (as Pb(NO3)2)	Pakistan	Plant growth ↓, biomass ↓, photosynthetic pigments ↓, shoot length ↓(up to 34%), root length ↓, number of leaves ↓, plant biomass ↓(significant at ≥100 and 150 mM)	Shehzad et al. [14]
Basil (Ocimum basilicum)	Cd	Irrigation	0, 50, 100, 200 mg/L	Iran	Reduced seed germination (↓up to 50% at 200 mg/L), reduced shoot/root length, altered oil composition	Fettahi et al. [13]
Rucola (Eruca sativa)	Cd	Irrigation	30 µmol/L Cd	Not specified	Reduced shoot length ↓19–44%, and root length ↓36–52%, reduced photosynthetic pigment (Chl-a: ↓25–53%; Chl-b: ↓18–63%), increased oxidative stress (↑53–96%)	Waheed et al. [60]
Sweet potato (Ipomoea batatas)	Fe	Hydroponics/nutrient solution	0.45, 0.9, 4.5, 9.0 mmol/L Fe (as Fe-EDTA)	Not specified	Reduced stomatal density, mitochondrial impairment in radicle cells, decreased nutrient uptake (Significant at 4.5 and 9.0 mmol/L)	Adamski et al. [61]
Indian Mustard	Cr	Artificially contaminated soil	0, 20, 40, 60, 80 ppm (K2Cr2O7)	India	Reduced plant height (↓56% at seedling and 3% harvest), reduced biomass, impaired root/shoot growth, number of leaves ↓51%, germination percentage ↓36% (Significant at ≥40 and ppm)	Kumar et al. [62]
Indian mustard	Cd	hydroponic/solution exposure	0, 50, 100, 200 µM (as CdCl2)	Kenya	Reduced plant length, shoot/root length ↓, dry biomass ↓(significant at higher concentrations ≥ 100 µM)	Chowardhara et al. [63]
Indian mustard	Cd	Artificially contaminated soil	0, 25, 50 or 100 mg Cd/kg	India	Reduced photosynthesis, nutrient uptake, photolysis of water, sugar accumulation, and stunted growth	Siddiqui et al. [64]
Solanum nigrum L.	Cu, Zn, Pb, Cd, Mn and Hg	Mine Tailings	-	China	Shoot length ↓72.7% Root length ↓69.1% Shoot fresh weight ↓74.5% Root fresh weight ↓67.6%	Li et al. [65]
Brassica juncea	As, Fe, and Zn	Mine Tailings	-	South Africa	Reduced growth Poor leaf development	Arthur et al. [66]

, a wide body of research has confirmed the harmful implications of polluted water and land on the growth, production, and quality of crops grown in agricultural environments.

Analysis of the compiled data (Table 1) reveals a consistent pattern of growth inhibition and physiological disruption across multiple crops exposed to heavy metals, with the effects being strongly dependent on both the metal type and concentration. For instance, B. juncea exposed to Hg in a hydroponics system showed a marked reduction in plant height (3–54%), biomass (4–65%), yield (20–33%), and seed germination (4–76%) at concentrations greater than 100 µM. Similarly, exposure to Zn in soil led to significant reductions in root and shoot length and biomass at ≥100 mg/L. Spinach subjected to As displayed reductions in root weight (21.1–58%), stem length (18.7%), leaf area (22.1–61%), and photosynthetic parameters (29–66%) at concentrations of 50 and 100 µM.

Pb exposure of the rapeseed, B. juncea, and B. campestris via hydroponics resulted in decreased growth, biomass, and chlorophyll content, with the shoot length declining up to 34% at ≥100 mM. Cd toxicity was evident in basil, rucola, and B. juncea, manifesting as reduced shoot and root length, chlorophyll content, and increased oxidative stress, with effects intensifying at higher concentrations. Fe excess in sweet potato led to physiological impairment, such as reduced stomatal density and mitochondrial dysfunction at ≥4.5 mmol/L. Cr exposure in B. juncea caused substantial reductions in plant height, biomass, number of leaves, and germination, particularly at ≥40 ppm. In a multi-metal context, Solanum nigromum grown on mine tailings exhibited severe reductions in shoot and root length and biomass, highlighting the compounded impact of mixed heavy metal stress.

These findings underscore that heavy metal toxicity disrupts key physiological and developmental processes in crops, including photosynthesis, nutrient uptake, and cellular integrity, ultimately leading to stunted growth and reduced yields. The adverse effects are generally dependent and become significant at higher concentrations. Such outcomes not only threaten crop productivity, but also raise concerns about food safety and human health due to potential heavy metal accumulation in edible plant parts. This highlights the urgent need for effective remediation strategies and careful monitoring of heavy metals in agricultural systems to ensure sustainable crop production and food security.

3.1. Impact of Heavy Metal Contamination on Soil Microbial Communities

Soil serves as a critical microbial reservoir on Earth and hosts a vast array of microorganisms that are essential for maintaining soil health. These microorganisms play a fundamental role in microbial-mediated processes that enhance soil quality, including the bioremediation of contaminants and biocontrol of soil-borne phytopathogens, both of which are considered eco-friendly strategies for sustaining soil health [67].

However, under severe water and soil contamination conditions, microbial populations are significantly affected. Chu [68] highlighted that the structure and diversity of soil microbial communities are affected by the presence of heavy metals. This is further supported by Naz et al. [69], who demonstrated that increasing concentrations of heavy metals in soil induce structural changes and disrupt normal microbial community function. Heavy metals affect various functional genes, altering the ecological roles of soil microorganisms. These disruptions, which target functionally significant microbial groups, ultimately lead to reduced microbial diversity and shifts in community structure [70].

Zhao et al. [71] indicated that bioavailable heavy metals significantly influence bacterial community diversity. Similarly, Zhang et al. [72] found that the stochastic processes that govern prokaryotic community assembly decline as heavy metal concentrations increase. This suggests that prokaryotic microbial communities are particularly sensitive to environmental disturbances under heavy metal stress, making them more vulnerable to the disruption of microbial ecosystem stability. A study conducted by Diaconu et al. [73] showed that soil microorganisms can only tolerate low heavy metal concentrations in the soil and that Cd and Cr contamination affect microbial growth (biomass development and enzymatic activity). Moreover, Abbas et al. [74] showed that the presence of Cr in soil has a detrimental impact on soil fertility and microbial activity, leading to reduced crop yields.

3.2. Agroecological Importance and Genetic Origin of B. juncea

B. juncea, also known as Indian mustard, is a cruciferous plant cultivated extensively in subtropical and temperate regions. It originated through interspecific hybridization between B. rapa (AA genome, 2n = 20) and B. nigra (BB genome, 2n = 16), resulting in a stable amphidiploid genome (AABB, 2n = 36), which confers a high degree of genetic variability [75,76]. This hybrid vigor makes it well suited for diverse agroecological environments, including areas prone to abiotic stress. The species is widely used for food, oilseed, and phytoremediation applications across Asia, Africa, and North America [77,78]. Genetic studies have classified B. juncea into four cultivar groups based on morphological traits: integrifolia, juncea, napiformis, and tsatsai. This intraspecies diversity is particularly valuable for environmental remediation, enabling the selection or breeding of cultivars with enhanced metal uptake and tolerance traits. In South Africa, B. juncea is primarily grown for its edible foliage, rich in vitamins and minerals [79,80]

3.3. Tolerance Mechanisms of B. juncea Under Heavy Metal Stress

B. juncea exhibits significant tolerance to heavy metal stress, which accounts for its extensive use as an effective phytoremediator in contaminated environments. This species can accumulate substantial concentrations of toxic metals, including Cd, Pb, Zn, Cu, and As, often at levels detrimental to most other plants, in both its roots and shoots. Its ability to survive under metal stress is linked to several tolerance mechanisms, including metal sequestration in vacuoles, the production of metal-binding peptides like phytochelatins, and the activation of antioxidant enzymes that mitigate oxidative stress [81,82,83]. These physiological and biochemical adaptations allow for B. juncea to maintain its growth and metabolic functions in contaminated soils, making it more resilient than may other species. The remarkable metal tolerance and accumulation capacity of B. juncea are attributed to a suite of well-coordinated physiological and biochemical mechanisms (Figure 1). Understanding these tolerance mechanisms is essential for optimizing the use of B. juncea in phytoremediation strategies and for the development of cultivars with improved stress resilience.

B. juncea’s tolerance to heavy metals is partially facilitated through internal chelation, wherein metal ions are bound to organic molecules to mitigate their reactivity and toxicity. Two critical chelation processes in plants involve phytochelatins, their precursor glutathione, and organic acids.

A primary mechanism by which B. juncea detoxifies metals is through phytochelatins (PCs), which are glutathione-derived compounds that play a significant role in heavy metal binding and detoxification. The levels of PCs in B. juncea increase in a dose- and time-dependent manner when exposed to metal toxicity, particularly under Cd stress [82]. Phytochelatin synthase, the enzyme responsible for the synthesis of PCs, is activated in the presence of heavy metals.

Glutathione (GSH), a precursor of phytochelatins, also serves a dual role as an antioxidant and substrate in the phytochelatin biosynthetic pathway [83]. Under Pb stress, B. juncea exhibits increased GSH levels at low Pb concentrations and decreased levels at higher concentrations of Pb. This response indicates the initial stimulation of the GSH biosynthetic pathway, with decreased GSH levels resulting from its utilization in phytochelatin formation. Further research has demonstrated that transcripts encoding γ-glutamylcysteine synthetase (γ-ECS), a key enzyme in GSH biosynthesis, increase under copper stress in B. juncea [83]. Transgenic plant studies further corroborate the significance of the glutathione metabolic pathway, as B. juncea samples overexpressing genes involved in GSH synthesis exhibit enhanced tolerance to heavy metals. A group of enzymes known as glutathione-S-transferases (GST) is also crucial for metal tolerance, as they conjugate GSH with metals and facilitate their sequestration [84].

Moreover, organic acids represent a crucial category of chelating agents in the defense mechanism of B. juncea against heavy metal toxicity. Maurato et al. [82] documented an elevation in citrate levels in response to heavy metal stress. Citrates have been identified as playing a pivotal role in both xylem and sap shoots, suggesting that citrate facilitates the translocation of Pb from the roots to the shoots, thereby enabling the plant to sequester the metal away from the sensitive regions of the root tissue [82].

The exposure of B. juncea to heavy metals results in the production of reactive oxygen species (ROS), which induce oxidative stress. In response, B. juncea activates a comprehensive enzymatic antioxidant defense mechanism [83]. The primary enzymes involved in this process include superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), and peroxidase (POD).

SOD serves as a critical defense mechanism by catalyzing the conversion of superoxide radicals (O^2-) into hydrogen peroxide (H₂O₂). This reaction is essential due to the highly reactive nature of superoxide radicals, which can disrupt cellular signaling pathways and potentially lead to significant cellular damage or cell death if not adequately controlled [83]. Research by Malecka et al. [83] demonstrated that B. juncea exhibits increased SOD activity when exposed to various metals, including lead (Pb), copper (Cu), and zinc (Zn). SOD exhibits both metal-specific and organ-specific responses in plants, with essential metals such as copper and zinc inducing a higher production of reactive oxygen species (ROS) compared to non-essential metals like cadmium (Cd) and lead (Pb).

In addition to enzymatic antioxidants, B. juncea utilizes a variety of non-enzymatic antioxidants to alleviate oxidative stress. Ascorbate (vitamin C) serves as a potent reactive oxygen species (ROS) scavenger and acts as a substrate for ascorbate peroxidase (APX). Sharma et al. [84] reported an increase in ascorbate content in B. juncea under heavy metal stress, particularly stress induced by copper (Cu) exposure. Ascorbate also contributes to the recycling of other antioxidants and the maintenance of membrane integrity [85].

Beyond phytochelatin biosynthesis, glutathione plays a direct antioxidant role through ROS scavenging and participation in the ascorbate–glutathione cycle, which is essential for sustaining redox balance during stress. B. juncea plants subjected to heavy metal treatment exhibit alterations in glutathione metabolism, characterized by early accumulation and potential reduction at elevated doses or prolonged exposure times [77].

Phenolic compounds and flavonoids further enhance the non-enzymatic antioxidant defense in B. juncea. These compounds function as both metal-chelating agents and free radical scavengers. Under cadmium (Cd) stress, phenolic acid levels in B. juncea increase, indicating their role in metal detoxification. Additionally, anthocyanins have been observed to protect the photosynthetic apparatus of B. juncea from metal ions by chelating metals and sequestering them in vacuoles [77].

Proline accumulation constitutes a critical adaptive mechanism in B. juncea when confronted with heavy metal stress. Proline functions as an osmoregulator, metal chelator, and antioxidant [86,87]. Empirical evidence suggests that proline accumulates in B. juncea in response to stress induced by heavy metals such as Cd, Pb, and Cu. This accumulation aids in regulating cellular water balance (osmotic adjustment) and prevents protein denaturation under heavy metal stress conditions [86]. The protective role of proline extends beyond osmotic adjustment, as it actively scavenges reactive oxygen species (ROS), stabilizes subcellular structures, and maintains protein integrity. Additionally, proline possesses the ability to form complexes with heavy metals, thereby facilitating detoxification processes [86]. In general, the extent of proline accumulation is correlated with the severity of metal stress [87].

3.4. Use of Si-Based Biostimulants as a Bioremediation Strategy for Contaminated Mine Soil and Water

Silicon-based biostimulants offer a practical approach for remediating contaminated mine and water. Although silicon (Si) is not among the essential nutrients, Si is known for its beneficial function in conferring protection against a range of abiotic stresses, such as heavy metal toxicity. Silicon can be added to the environment either in soil amendment (such as silicate minerals or Si-supplied biochar), hydroponics nutrient solutions, or foliar sprays. Numerous researchers have proven the usefulness of Si in mitigating stress triggered by Pb, As, and Hg in various plant species and growing substrates [88,89,90].

3.4.1. Mechanisms of Si-Mediated Heavy Metal Toxicity Alleviation

The mechanisms outlined in Figure 2 depict the processes through which silicate mitigates heavy metal toxicity in plants. Silicate amendments increase soil pH, thus decreasing the bioavailability of heavy metals. This enhances avoidance strategies, such as the chelation of metals by root exudates and the regulation of the activity of metal transporters. Silicate further improves tolerance strategies, such as the limitation of metal translocation from roots to shoots, the evenly distributed sequestration of metals in leaf tissues, chelation by endogenous ligands, compartmentation into vacuoles or cell walls, the activation of antioxidant defense mechanisms, and anatomical changes that work together to enhance plant tolerance under heavy metal stress.

In soil systems, the application of Si amendments significantly mitigates the bioavailability of heavy metals by modifying the soil chemistry, primarily through the elevation of rhizosphere pH and the formation of insoluble metal compounds. Many Si rich amendments, such as calcium silicate and Si-rich biochar, exhibit alkaline properties and release silicate ions into the soil solution. This process increases soil pH, particularly under acidic or neutral conditions, facilitating the adsorption of toxic metal cations (e.g., Pb²⁺ and Hg²) onto soil particles, thereby reducing their solubility and uptake by plants [92].

Rachappanavar et al. [92] emphasized that exogenous Si applications enhance heavy metal tolerance by increasing soil pH and forming metal-Si precipitates, which decrease metal phytoavailability. Supporting this, a pot experiment conducted by Naz et al. [89] on Pb-contaminated soils demonstrated that Si supplementation (100–200 mg/kg) increased soil pH and immobilized Pb, resulting in a 31–62% reduction in shoot Pb content. Furthermore, the application of 125 mg/kg nano-silica led to an up to 84% reduction in shoot Pb levels compared to untreated plants. These findings confirm that Si induces Pb immobilization within the soil and root zone, thereby limiting translocation to aerial tissues.

Si has been demonstrated to alleviate As toxicity, facilitating its precipitation or adsorption in soil. Satter et al. [93] reported that the combined application of biochar and Si to As-contaminated soil (12 mg/kg) increased soil pH and decreased extractable As levels, thereby reducing As accumulation in maize. The addition of alkaline biochar further amplified this effect by enhancing both pH and Si availability [93]. These findings highlight that one primary mechanism that Si mediates detoxification is indirect, subsequently altering the soil environment to decrease metal solubility and bioavailability.

Beyond pH modulation, Si can directly interact with metal ions to form stable complexes or coprecipitates in the soil solution. Monosilicic acid (H₄SiO₄), the soluble form of Si, can bind with metal ions such as Pb, Cu, and As, forming less soluble metal silicate complexes, particularly under pH conditions between 5.5 and 7.5 [94]. Cai et al. [95] found that soluble Si added to an aqueous metal solution generated particulate metal silicate complexes, thereby significantly reducing free metal ion concentrations.

Within plants, Si plays a crucial role in the detoxification of heavy metals through the mechanisms of coprecipitation and compartmentalization. Si aids in the sequestration of toxic metals within the root tissues by forming metal inert metal silicate aggregates in the cell walls and vacuoles, thereby preventing their entry into the cytoplasm [88,96].

Naz et al. [89] reported that, in Pb-stressed lentil, the application of Si resulted in increased Pb retention within the root cell wall and in silicate-bound form, with limited translocation to the shoots. This root-bound sequestration was similarly observed for Cd and Pb in rice, where Si enhanced the retention of metals in silicified root cell structures. Similarly, David et al. [90] demonstrated that the application of nanoparticles to As-contaminated soil led to the immobilization of As in maize roots. The As were predominantly bound within the root cell wall and vacuole, consistent with silica deposition, resulting in significant reduced translocation to the shoots.

Silicon (Si) also modifies the speciation of metals and metalloids in both soil and plant tissues. For example, in the context of arsenic, arsenites are more toxic and mobile compared to arsenates. Si has been demonstrated to interfere with arsenic uptake and speciation by competing for adsorption sites and transport pathways. In rice, Si and arsenite share uptake routes via Lsi1 and Lsi2 transporters, and an abundant supply of Si can competitively inhibit arsenite absorption [88,97]. A similar pattern was observed in tomatoes, where Si nanoparticles reduced both arsenic uptake and its translocation to shoots by maintaining arsenic in less mobile forms within the roots [98].

In addition to its impact on metal transporters, Si affects the biochemical speciation of heavy metals. David et al. [90] reported that, in arsenic-stressed maize, Si application enhanced phytochelatin production, leading to the formation of As (III)–phytochelatin complexes sequestered in vacuoles, thereby effectively reducing arsenic toxicity.

A similar mechanism is applicable to mercury (Hg). In soil, Hg predominantly exists as Hg²⁺, a highly toxic form that can be transformed into methylmercury. Li et al. [97] demonstrated that the application of nano-silica amendments immobilized Hg²⁺, potentially converting it into less toxic species. This treatment markedly decreased the availability and translocation of Hg to soybean plants, indicating the formation of inert Hg-silicate compounds.

3.4.2. Physiological and Biochemical Plant Responses to Si

In addition to abiotic and chemical interactions, Si induces a series of physiological and biochemical responses in plants to adapt to heavy metal stress. One of the first strategies adopted was the reduction in oxidative stress levels. Lead, As, and Hg generate ROS within plant cells, leading to lipid peroxidation, oxidative modification of proteins, and deoxyribonucleic acid (DNA) degradation. Si-treated plants under metal stress exhibit enhanced antioxidant defense systems [94,99]. Si treatment enhances enzymatic antioxidant activity, such as that of SOD, POD, and CAT, as well as non-enzymatic antioxidant activity, such as that of glutathione and ascorbate [94]. The enhancement of antioxidant potential allows for the detoxification of ROS generated as an outcome of metal toxicity, protecting cellular structure and function.

The addition of Si nanoparticles to Pb-stressed lentils enhanced the activity of SOD, CAT, and POD. This enhanced antioxidant defense led to the lower accumulation of hydrogen peroxide in the plant tissues compared to plants exposed to Pb alone, an indication that Si nanoparticles effectively reduced oxidative stress in Pb-stressed lentils [89]. In mercury-stressed garlic, the addition of Si increased catalase activity in conjunction with glutathione reductase activity in the leaves, whereas the malondialdehyde content, an indicator of lipid peroxidation, decreased compared to the control treatments [100]. The authors concluded that Si protects against mercury toxicity by strengthening antioxidant defense mechanisms [100]. Increased antioxidative protection is a ubiquitous response to Si treatment and is prevalent in several species exposed to heavy metal stress [94].

Si is important for maintaining photosynthetic efficiency and growth under heavy metal stress conditions. Many studies have reported that Si-treated plants have higher chlorophyll levels and photosynthetic rates than untreated plants in heavy-metal-enriched environments [94,99]. Si appears to strengthen chloroplast membranes and mitigate the breakdown of chlorophyll induced by heavy metal stress. For instance, Naz et al. [89] found that lentil plants subjected to Pb stress and treated with Si had a greener phenotype, with total chlorophyll content increasing by up to 68% (for bulk Si) and 130% (for nano-Si) compared to control plants, thereby boosting photosynthetic efficiency. The beneficial effect of Si on photosynthesis was due to both indirect pathways and improvements in the leaf ultrastructure of treated plants. Si is often deposited below the cuticle, which encourages leaf erectness and light absorption while simultaneously reducing water loss via transpiration, thereby maintaining turgor and facilitating gas exchange.

Several studies have confirmed that Si enhances plant tolerance to abiotic stresses, particularly heavy metal toxicity (

Table 2. The impact of silicon application under heavy metals on Brassica species and other plants.

Crop	Heavy Metal	Si Concentrations Used	Regions	Mode of Application	Results	Citation
Spinach (Giant Forkhook), Kale (Holland variety), White amaranth (Dubia Giant variety)	Cd and Pb	-	Kenya	Soil amendment	In the greenhouse, Si reduced root Cd uptake by 33% (spinach) and 45% (kale). Si reduced shoot uptake by 32% in spinach (greenhouse) In the field, Si reduced root uptake by 42% (spinach), 30% (kale), and 24% (amaranths).	Ngugi et al. [29]
Cabbage (B. oleraceae)	Ni, Cd and As	1 mM, 2 mM, 3 mM Si	Pakistan	Foliar application	Increased photosynthetic rate by 71% (Ni), 59% (As), and (Cd) 75%. Increased chlorophyll by 78% (Ni), 82% (As), and 103% (Cd). SOD: Increased by 79% (Ni), 81% (As), 112% (Cd) with Si at 2 mM. POD: Increased by 43% (Ni), 41% (As), 58% (Cd). CAT: Increased by 94% (Ni), 135% (As), 115% (Cd). APX: Increased by 47% (Ni), 69% (As), 85% (Cd).	Zubair et al. [101]
Lettuce (Lettuca sativa), spinach (S. oleraceae) and cabbage	Cd and Pb	Brassinosteroid 3.5 µM + Silicon 2 mM; Brassinosteroid 3.5 µM + Silicon 3 mM	Pakistan	Foliar application	↑Root length, root fresh weight, root dry weight, shoot fresh weight, shoot dry weight, and leaf area. ↑SOD activity, POD activity, CAT activity and APX activity. ↓Cadmium (Cd) accumulation in edible parts. ↓Lead (Pb) accumulation in edible parts.	Balal et al. [102]
Sedum alfredii	Cd	0, 0.5, 1, 1.5, and 2 mM (Na2SiO3·9H2O)	China	Nutrient solution	Shoot fresh weight ↑22.67–52%. Shoot dry weight ↑21.74–52.17%. Root fresh weight ↑10.71–28.57%. Root dry weight ↑40–80%. Plant height ↑10.75–23.26%. Root length ↑11.90–31.44%. (Significant at 2 mM Si concentration)	Hu et al. [103]
S. alfredii	Cd	0.15, 1, and 2 mM Si	China	Nutrient solution	Shoot biomass ↑33.1–63.6%. Root biomass ↑28.3–55.1%. Enhanced Cd accumulation ↑31.9–96.6%. Improved Cd translocation 1.38 times higher than control. Chlorophyll a and b: Increased by up to 17.1% and 22.7%, respectively. Antioxidant enzymes: Activities of SOD ↑2.47, CAT ↑2.69, and POD ↑2.57 times. Upregulated genes for catalase (CAT), superoxide dismutase (CSD), and peroxidases (PER) were upregulated, boosting ROS detoxification.	Yang et al. [104]
Indian mustard (B. juncea)	As	1.5 mM Si	Not specified		Reduced root length inhibition 27%. Root tissue density ↑64.7%. Root fineness ↑40.1%. Increased As accumulation. Reduced H2O2 ↓8%. SOD ↓30%, CAT ↓32%, and APX ↓19%.	Pendey et al. [105]
B. juncea	As	1.5 mM	India	hydroponic/solution exposure	Plant growth ↑109%, fresh weight ↑35%, and dry weight ↑50%. Chlorophyll ↑55% and carotenoid content ↑76%. As concentration in shoots ↓30%. Enzymatic antioxidants: SOD ↑14% and CAT ↑15%.	Praven et al. [30]

), through both internal and external mechanisms. Si mitigates heavy metal toxicity by reducing metal adsorption, altering metal speciation through Si compound formation, and limiting metal bioavailability. Internally, it alleviates toxicity by stimulating antioxidant activity, facilitating metal complexation and compartmentalization, and modifying cell wall structures to regulate transport [31].

Several studies (Table 2) have underscored the substantial efficacy of silicon (Si) application in mitigating heavy metal stress in various crop species. Research conducted on spinach, kale, and white amaranth in Kenya demonstrated that soil supplementation with Si significantly decreased cadmium (Cd) and lead (Pb) uptake by 33–45% at the root level and 24–42% at the shoot level, both in controlled greenhouse conditions and field settings. This reduction is crucial for enhancing the safety of edible plant tissues, thereby ensuring the production of relatively safe food.

In Pakistan, the foliar application of silicon (Si) at concentrations of 1–3 mM in cabbage plants exposed to stressors such as nickel (Ni), arsenic (As), and cadmium (Cd) resulted in notable physiological improvements. Photosynthetic rates increased by up to 75%, and chlorophyll content increased by 103%. Furthermore, the activities of antioxidant enzymes, specifically superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), were significantly enhanced. These improvements highlight the pivotal role of Si in bolstering the plant’s antioxidative defense system, thereby augmenting tolerance to heavy-metal-induced toxicity in plants.

Similar enhancements were observed in lettuce, spinach, and cabbage treated with a combination of brassinosteroid (3.5 µM) and Si (2–3 mM), where root and shoot growth and antioxidant enzyme activities were markedly improved. Importantly, these treatments also resulted in reduced Cd and Pb accumulation in edible plant parts, reinforcing the contribution of Si to both crop productivity and food safety.

For the hyperaccumulator Sedum alfredii, Si supplementation (up to 2 mM) in nutrient solutions significantly increased the biomass, plant height, and root length of the plants. Si also facilitated Cd accumulation and translocation, enhanced chlorophyll content, and upregulated genes associated with antioxidant defense, thereby improving the plant capacity for reactive oxygen species (ROS) detoxification. These findings are particularly pertinent to phytoremediation, as they demonstrate the ability of Si to aid in the removal and immobilization of heavy metals from contaminated environments.

In arsenic-exposed Indian mustard, Si supplementation at 1.5 mM effectively alleviated root growth inhibition by 27%, improved root tissue fineness and density, and reduced oxidative stress markers. Although silicon increased As uptake, it concurrently reduced phytotoxicity, underscoring the dual role of silicon in both enhancing metal uptake and safeguarding plant health, which is advantageous for agricultural and environmental remediation strategies.

Collectively, these results provide compelling evidence that Si supplementation is a highly effective strategy for managing heavy metal toxicity in crops. Si not only promotes plant growth and fortifies antioxidative defenses, but also enables tailored outcomes, either reducing heavy metal content in food crops or increasing it in hyperaccumulators for remediation, depending on the species and application context. These findings strongly advocate for the integration of silicon-based amendments into agricultural management and phytoremediation programs to enhance crop safety and achieve sustainable practices in metal-contaminated areas.

4. Conclusions and Future Perspectives

Pollution from mining and industrial operations remains a major threat to environmental health, agricultural productivity, and the global food supply. This review underscores the potential of B. juncea for phytoremediation as an effective and eco-friendly strategy due to its ability to accumulate heavy metals and tolerate them through mechanisms like chelation, antioxidative defense, and osmotic adjustment.

However, the success of phytoremediation using B. juncea is often hindered by stress from heavy metals, which can limit plant growth and biomass production. Research suggests that Si biostimulants might help overcome these obstacles through the following:

• Decreasing heavy metal bioavailability through changes in soil chemistry and metal immobilization;
• Boosting the antioxidative capacity and overall health of plants;
• Promoting better plant growth and yield even in contaminated environments.

This study enhances the current understanding of silicon’s role in improving phytoremediation and illustrates its practical significance when used with B. juncea. The approach of combining silicon amendment with B. juncea has proven to be a scalable and promising method for cleaning up mining-affected soils and waters, restoring agricultural productivity, and reducing health risks in affected communities. The use of silicon-based biostimulants for phytoremediation presents a viable and cost-effective solution for environmental restoration and safer crop production in polluted areas. This method shows particular promise for boosting local food production efforts, such as food gardens, and supporting food security.

Future Directions

To advance the field and address the remaining research gaps, future work should focus on the following:

• Field validation of Si-enhanced phytoremediation under real-world conditions.
• Optimization of Si application methods and rates for different soil types and contamination levels.
• Economic assessments, which are required to determine the commercial viability of this approach.
• Exploration of synergistic effects with other biostimulants or soil amendments.