A Phytoremediation Approach for the Restoration of Coal Fly Ash Polluted Sites: A Review

1. Introduction

Globally, South Africa has been classified as the sixth largest coal producer and exporter, and the mining industry contributes almost 27% of the mineral resource sales, and therefore contributes significantly to the economy of this country (Webb, 2015; Hassan, 2023). Many countries including South Africa mainly depend on coal for power generation, largely due to the fact that it is the most available fossil fuel and affordable means of supplying electricity to consumers (Zou et al., 2016; Xu et al., 2020). Due to the enormous economic contributions made by coal mining over the past few decades, it has gained prominence among the world economies. In 2019, coal generated almost 37% of the world’s electricity, up from 27% in 2007. This supported the prediction that between 2007 and 2020, the world’s coal consumption would increase by an average of 1.1% per year and by an average of 2.2% annually for the period 2020-2035 (IEO, 2020). Coal remains the main non-renewable source of generating electricity in South Africa and Eskom (national electricity supply utility) generates almost 90 per cent of usable electricity in the country from coal (Hancox, & Götz, 2014; DoE, 2016). Depending on the coal combustion methods and emission controls, large quantities of solid waste that include coal fly ash (CFA), boiler slag, bottom ash and fluidized-bed combustion are generated (Ahmedi, & Kusari, 2012).

Coal combustion waste accounts for 5-20 wt.% of burned coal and includes coarse bottom ash (BA) (5-20 wt.%) and fine CFA represents 80-95 wt.% of the ash produced (Yao et al., 2015; Bhatt et al., 2019; Vilakazi et al., 2022). Approximately 600 million tons of CFA is produced per year, dominated by China, India, USA, Germany, and South Africa contributing more than 300 million tons and it is estimated that CFA production will increase yearly by 5% (Dwivedi, & Jain, 2014; Bhatt et al., 2019). Of the CFA produced globally, more than 200 million tonnes remain unused and is stored in ash dumps, thereby presenting serious environmental pollution problems. In South Africa, approximately 28 million tons of CFA is produced annually with utilization rate of less than 10% (ESKOM, 2018; Reynolds-Clausen, & Singh 2022; Vilakazi et al., 2022). Therefore, approximately 26 million tons of CFA are stored in disposal facilities, this presenting serious risks to the health of humans, animals and the environment. Many power stations are currently running out of ash disposal sites and there is a need to expand the disposal facilities (SACCA, 2021), this suggesting that more CFA will be produced and the issues of pollution will continue to be a burden. CFA storage in disposal facilities results to air pollution in windy seasons, groundwater and surface contamination, and affecting soil quality and human health (He et al., 2012). The tiny spherical particles of CFA ranging between 1 and 100 µm in size suspend in air for longer periods and sometimes reducing atmospheric visibility (Karun, & Sridhar, 2016; Maiti, & Prasad, 2016). Human mortality and morbidity, different chronic obstructive pulmonary diseases, hypertension, kidney disease, cardiopulmonary disease, and cancers were reported in people who live in the vicinity of coal-fired power plants (C-FPPs) due to the inhalation of coal nanoparticles, micro-particles and its by-products (Saikia et al., 2020; Gasparotto, & Martinello, 2021). All fossil fuels when they are burnt can release carbon dioxide and other greenhouse gases leading to global warming. The CFA is disposed of as slurry which is stored in dumping areas and can be released directly into nearby surface water body (Muriithi, 2009). Due to the fact that its reusability rate is lower than its production, its disposal still remains an issue to many countries where they reach to the point all space near the C-FPPs has been used and this suggesting increase in the number of degraded lands (Reynolds-Clausen, & Singh, 2019). Long-term storage can result to leaching of toxic heavy metals and organic contaminants from CFA, ultimately contaminating groundwater and the underlying soils (Muriithi, 2009; Vilakazi et al., 2022). Although CFA is predominantly composed of carbon, it also contains nitrogen, Sulphur, organometallic compounds, toxic metals such as mercury, chromium, cadmium, arsenic, and lead and organic pollutants that are harmful to the whole biosphere and can lead to serious health issues including the nervous system and cancer impacts such as developmental delays, cognitive deficits and behavioral problems (Gasparotto, & Martinello, 2021). Furthermore, they can also cause respiratory distress, gastrointestinal illness, heart damage, kidney and lung diseases, reproductive problems, impaired bone growth, and birth defects in children (USEPA, 2010; Speight, 2021). Because of all these problems, CFA is classified as a health hazard in South Africa. The management of CFA is an important environmental challenge which requires sustainable strategies to reduce air contamination by suspended particulate matters and prevent water and soil pollution through curbing movement of inorganic and organic pollutants into aquatic systems and restore degraded lands (Karun, & Sridhar, 2016; Maiti, & Prasad, 2016).

Among conventional and sustainable remediation methods of polluted areas, phytoremediation is a strategy that applies various plant species to transfer, stabilize, remove, and lower availability of pollutants in the soil, groundwater and other waste products (Tripathi et al., 2019; Golubev, 2011; Nedjimi, 2021). It is the most eco-friendly and cost-effective strategy as the plants can colonize the ash dumps, by establishing a self-sustaining vegetative layer for recovery of the ecosystem health (Gajic´ et al., 2018). The polluted area is restored gradually over time, and this results to reduction of pollutants in the environment. Although, there are many reports on phytoremediation of polluted sites, investigations regarding the potential use of phytoremediation strategies for restoration of ash dams is rather sparse. This review aims to discuss the pollution problems related to CFA, production and physicochemical properties of CFA, conventional clean-up methods, CFA disposal methods, various means of CFA utilization and their limitations and we explore the use of plants for addressing pollution problems due to CFA disposal and the economic opportunities that restoration of CFA disposal sites may present. Commercialization is relevant for developing countries, as this will not only solve pollution problems, but will also address societal challenges. Guidance is provided for future studies in selecting suitable species for phytoremediation strategies. In an attempt to suggest suitable species, many studies focus on species that grow in the vicinity of ash disposal sites. However, these plants grow in soils polluted with CFA and therefore may not adapt to 100% CFA media, hence there is sparse vegetation on the CFA disposal sites. To support plant growth, the physicochemical properties of CFA must be altered. In this review, we highlight the advantages offered by the legumes, aromatic plants and grasses due to their capabilities in altering the properties of CFA, making it favourable for plant growth.

2. Production of CFA

Ash is produced inside the boiler furnace from the unburnt carbon and unburnt inorganic components of coal (Wang et al., 2015; Speight, 2021). The ash produced differ based on how they are formed and exit from the boiler. The bottom ash has a gritty and sandy texture fraction that falls into the grates below the boilers where it is mixed with water and pumped to ponds. The CFA is composed of very fine spherical glassy nanoparticles that are formed due to incomplete combustion of coal (Ghassemi, 2004). Powdery CFA is captured before reaching the atmosphere by highly efficient electrostatic precipitators or by a variety of other devices such as baghouses or wet scrubbers (Vilakazi et al., 2022; Molchanov et al., 2023). CFA is the solid by-product produced from the burning of inorganic materials mainly composed of iron and alumina oxides, silica with low levels of magnesium oxide, calcium oxide, unburned carbon, sulphur oxides, and other substances (Fu, 2010). The production of ash rich in minerals is linked to the type of coal used (Kgabi et al., 2009). The ash yields of coal samples collected from different coal mines in South Africa range between 20.0% and 49.6% (Hancox, & Götz, 2014; Matjie et al., 2016). According to Yang (2019), a reasonably clean coal should contain an ash content less than 12.5%, this indicating that much of the coal used is of poor quality, hence high production of ash.

Coal combustion for generation of energy is expected to continue to grow in South Africa due to consumer demands and therefore the generation of CFA production will certainly increase. The coal reserves remaining are estimated for 116 years and therefore will remain a valuable resource for the economy of South Africa. Coal is the dominant source of energy for South Africa, comprising more than 80 percent of the country’s system load. Generating electricity through coal combustion results to the production of solid residues with environmental problems. More than 119 million tons of coal has been consumed between 2014 and 2015, resulting to approximately 35 million tons of CFA (Reynolds-Clausen, & Singh, 2019). Currently, the worldwide generation of ash is estimated at approximately 750 million tons (Szwalec et al., 2022), suggesting that more than 500 million tons is CFA. Though several alternative energy sources have been proposed and implemented, the rate at which the coal is being used as the main source of energy cannot be counterbalanced (Basu et al., 2009; Verma et al., 2014).

3. Characterisation of CFA

CFA can be defined as a fine powder material mostly composed of silica and almost all particles are spherical. The particles result from the burning of pulverised coal at high temperatures that range between 1400 and 1700 °C (Bayat, 1998; Nyale et al., 2013).

It is a heterogeneous material that poses health problems due to its chemical and physical composition. Its composition depends on combustion and cooling procedures, the type of coal (bituminous, subbituminous, lignite and pozzolanic) used, geographical origin of coal, emission control measures, ash collection methods and other factors which impact to a considerable extent, its suitability of applications and remediation processes (van der Merwe et al., 2014; Kelechi et al., 2022). The broad variety of the CFA composition makes it a complex material to be characterized (Alterary, & Marei, 2021). CFA is classified into two classes C and F, as highlighted by American Society for Testing and Materials (ASTM) (Alterary, & Marei, 2021; Saljnikov et al., 2022). Class C is derived from subbituminous and lignite coals and is used as the replacement material in cement. Class C is mainly composed of calcium, alumina and silica and its calcium oxide content is 20% or more and has cementitious properties (Bentz et al., 2011; Yousaf et al., 2020). Class F has lesser content of calcium oxide as compared to Class C and is mainly pozzolanic in nature with no value in cementing. The total amount of ferric oxide, silicon dioxide and aluminium oxide is greater than 70% for class F, while Class C is characterised by contents between 50-70% of these minerals (Bhatt et al., 2019). Classification, crystallinity, shapes, mineralogy, morphology, and chemical composition reveal the properties of this material and these provide guideline for its reusability and potential as a source of valuable metals (Alegbe et al., 2018).

Physical properties of CFA include particle size distribution, specific surface area, permeability, angle of friction, thermal stability, colour, optimum moisture, specific gravity, and bulk density (Pandian et al., 1998; Mishra, & Das, 2010; Bhatt et al., 2019). CFA consists of fine grey powder of which the particles are principally sphere-shaped of aluminosilicate glass, some hollow and others solid, and mostly amorphous in nature while carbonaceous fraction consists of angular particles (Ghosal, & Self, 1995; Ahmaruzzaman, 2010). The hollow spheres in CFA, commonly known as cenospheres range between 45 and 150 μm (Ngu et al., 2007). CFA particles have a smooth outer surface, which is due to the occurrence of the aluminosilicate glass phases (Inada et al., 2005; Nyale et al., 2013). The particle surface area and the size distribution are important in determining the activity of CFA. For example, particle size and distribution are important factors for geo-polymerization process. Mishra and Das (2010) conducted a study to determine the particle size, shape, chemical composition and mineralogical phase of CFA. They found that CFA is spherical in shape and the bulk ash is porous in nature, mineralogical phase was composed of mullite, quartz and hematite while CFA was predominantly composed of alumina, silica, iron oxide and a little amount of calcium oxide. The South African CFA sample is classified as class F and is mainly composed of crystalline phases quartz (6.1%) and mullite (31.8%) and an amorphous silica-alumina glass phase (62.1%) and has been confirmed to be a good substrate for silica nanoparticle and zeolite syntheses (Musyoka, 2009; Imoisili, Nwanna, & Jen, 2022). Besides the above physical properties, optimal moisture content value ​​for CFA varies between 11% and 53%, with maximum dry density values ​​between 1 and 1.78 g/cm³. CFA is not plastic and it does not swell when used as a base material for buildings (Cokca, & Yilmaz, 2004). CFA has a high specific surface area and low bulk density (Bhatt et al., 2019). The amount of iron and unburned carbon (UBC) impacts the colour of CFA, which varies from orange to deep red or brown and white to yellow. The grey colour of CFA is usually explained by the percentage of unburnt carbon. In small amounts, the presence of ferrous ion-containing compounds can cause a brown coloration, which changes the colour from bluish-grey to grey (Kang et al., 2013).

The chemical properties of CFA highly depend on the origin of coal and the power plant mode of operation. CFA is composed of three main groups including inorganic compounds constituted of crystalline and amorphous phases, organic materials and fluid materials occurring in both inorganic and organic compounds (Alterary, & Marei, 2021). CFA contains minerals and elements in various amounts. The most abundant materials in CFA are inorganic compounds, formerly present in the raw coal, which remain after the coal burning process with a small amount of unburnt carbon that remains from incomplete combustion (Bayat et al., 1998; Nyale et al., 2013; Ingelsson et al., 2020). The most abundant elements found include Si, O₂, Ca, Fe, Al, C, K, Na, Ti, P, Mg, and Ba (El-Mogazi et al., 1988; Styszko-Grochowiak et al., 2004; Vassilev, & Vassileva, 2007; Akinyemi et al., 2012). The major oxide constituents in CFA include CaO, quartz (SiO₂), Al₂O₃, and hematite (Fe₂O₃) (Mainganye et al., 2013). The calcium-bearing minerals include anorthite, akermanite, gehlenite and various calcium aluminates and silicates identical to those found in Portland cements (Snellings, 2012). CFA particles are dominated by Ca-rich plagioclase feldspars, Mg-rich phases, complex Ca-Mg-Al-Si-Ti-Fe grains, trace elements, base metals and trace amounts of rare earth elements (Alegbe et al., 2018; Hower et al., 2022). The CFA with a range of calcium oxide content extending above 15% indicates its self-cementing properties. Other available minerals are sodium oxide, potassium oxide and magnesium oxide which are very important for plant development. The pH values of CFA differ depending on the sulphur/calcium ratio. Depending on pH, CFA is classified as acidic ash (pH 1.2-7), mild alkaline ash (pH 8-9) and strongly alkaline ash (pH 11-13) (Kolbe et al., 2011; Bhatt et al., 2019; Saljnikov et al., 2022). The amount of mercury is usually in the range of 0.01-1.00 mg/L for bituminous coal. The concentrations of other trace elements vary as well according to the type of coal combusted to generate it (Speight, 2017). The constituents of CFA include also a small number of radioisotopes such as ²³⁵U, ²²⁶Ra, ²²²Rn, ²³⁸U, ²³²Th, and ⁴⁰K, which present radiation risk depending on distribution and concentrations (El-Magazi et al., 1998). Acceptable radioactivity level is a key environmental factor to consider for safe utilization of solid wastes (Zielinski, & Finkelman, 1997; Barescut et al., 2011). The toxicity of CFA is normally indicated by the occurrence of toxic heavy metals such as Cd, Ni, As, Ba, and Pb among others, alongside with organic substances (Anjum, 2017). Higher amount of As, Cr, Pb and Hg were reported in CFA than in raw coal, highlighting serious health risks due to CFA disposal (Altıkulaç et al., 2022). The organic matter found in CFA include unburned carbon and various organic compounds. During the coal combustion and pyrolysis processes, chemical and physical changes occur resulting to the formation and emission of various compounds including polycyclic aromatic hydrocarbons (PAHs) which are known carcinogens (Liu et al., 2008). Relatively higher amounts of PAHs are found in CFA than that in the raw coal (Liu et al., 2000). Low molecular weight-PAHs are the ones that predominate in CFA samples (Sahu et al, 2009). Research also found that polychlorinated biphenyls (PCBs), aliphatic hydrocarbons and small number of dioxins are also present in CFA (Shibayama et al., 2005; Nomura et al., 2009; Alterary, & Marei, 2021). These organic compounds adsorbed on CFA particles can be dispersed into the environment and therefore posing a risk of contamination and can result to serious health challenges (Ribeiro et al., 2014).

4. Coal Fly Ash Disposal

The disposal of high volume of CFA from C-FPPs consumes too much water and land area. There are two main CFA disposal mechanisms that are used by several C-FPPs worldwide, namely the wet and dry method (Mbugua, 2012). In case of dry disposal system, CFA is directly disposed of in basins and landfills covering several areas of valuable land near the power plants, while in the wet system the CFA slurry is allowed to settle in ash dams, ponds and lagoons and the water is recovered through channelling (Qadir et al., 2019). Disposal of CFA requires huge amount of water burdening the already stressed water systems (Paliwal, 2013). Although wet ash handling systems are closed circuits allowing for the ash water to be recycled, the recirculating is lost through seepage, evaporation and absorption on the ash (Eskom, 2018). The ashes can be dispersed due to wind in the surroundings which affect respiratory system and other human parts leading to cancer, bronchitis, anemia and asthma (USEPA, 2007). To reduce the spread of CFA dust, suppression systems or water sprinklers are installed but these is not a sustainable strategy due to high water consumption. Furthermore, even covers are placed onto the damping site to prevent the carrying away to tiny CFA dust. Watertight covering precautions have also been approved to protect the discarded CFA from the rainwater (Jain, & Tembhurkar, 2022). These common disposal practices are regarded as unsightly, converting lands to non-productive sites, environmentally unsafe and are not feasible financially due to long term conservation (Menghistu, 2010; Mainganye, 2013). CFA contains a high concentration of heavy metals and metalloids that leach out into the environment, which further cause soil or water contamination in the surroundings of power stations (Pandey et al., 2011). It also contains various persistent organic pollutants (POPs) that may lead to the contamination of soils, air and water (Jala, & Goyal, 2006). Large volumes of land are used for CFA disposal and this has become a global concern.

5. Coal Fly Ash Utilization

Alternative to CFA disposal, many countries are reusing it and this has been beneficial for the environment and contributes to economic growth. The biggest producers of CFA include China, India and USA contributing up to approximately 390 million tons of CFA per year as shown in Table 1 and the production of CFA increases by about 5% yearly. In China, CFA utilization is reported to account 45% and mainly used in production of building materials, paving, mine backfilling and low-end building (Luo et al., 2021). More than 70% of produced CFA is recycled in the USA and is mainly used to produce cement products (Lu et al., 2023). According to Yousef (2020), India produced 217 million tonnes between 2018-2019 and utilized 168.40 million tonnes of CFA for cement production, construction of dams and road, suggesting a utilization of more than 70%. The EU has reported utilization rate over 90%, which can be attributed to small volumes produced and the involvement in projects aimed at turning waste into valuable materials. Countries including Netherlands, Denmark and Italy use up to 100% of the produced CFA followed by Japan which utilizes 96.4% (Kelechi et al., 2022).

CFA is considered as a value-added material in construction and building industry (concrete and cement) including grout, block and brick development, geopolymers, environmental management such as backfilling and mine drainage treatment, land reclamation and soil amelioration, road construction, agriculture, raw material for glass-ceramics production, and recovery of valuable minerals (Dwivedi, & Jain, 2014; Wang, & Chen, 2014). The cement and concrete industries contribute greatly to the recycling of CFA accounting to 44.19% of its utilization, followed by the road construction (15.25%) and 12.49% is reported for remediation of landfills (Loya, & Rawani, 2014). The use of 40% CFA in replacing cement resulted to cost reduction in the process of producing concrete production (Kelechi et al., 2022). Supplementing with CFA results to hardened concrete through pozzolanic and hydraulic activity. An even higher amount of CFA up to 60% can be used in structural uses (Scott, & Thomas, 2007). CFA is also used to synthesize zeolite that can be used in agricultural activities and water treatment processes. CFA-based zeolites have the ability to retain water and cation exchange capacity and therefore widely used for the controlled release of fertilizers in agriculture (Harja, 2016). Zeolites are minerals principally composed of ferro-aluminosilicate elements containing both crystalline and amorphous phases and have stable structures. The characteristics of zeolite include noteworthy ion-exchange capacity, high surface area and unique pore characteristics and therefore have been applied for treatment of ionic and heavy metals from industrial sludges, acid mine drainage and wastewater activities (Zhang et al., 2021; Parra-Huertas et al., 2023). Zeolites have high sorption capacity due to the smaller particle size that increases the specific surface area ranging between 2500 and 4000 cm²/g (Alonso, & Wesche, 1991). Consequently, widely used as a sorbent to clean wastewater containing toxic metals, pigments, dyes, and flue gas of toluene vapors, NO_x and SO_x (El-Mogazi et al., 1988). Moreover, CFA has been identified as a suitable material for photocatalysis and adsorption of p-nitrophenol because it contains metal oxides such as TiO₂, SiO₂, Fe₃O₄ and Al₂O₃, which are effective in these processes (Park, & Bae, 2019). For example, synthesised zeolite doped with silver nanoparticles was used for mercury (II) removal and about 90% removal efficiency was achieved (Tauanov et al., 2018). In another study, CFA coated with carbon hybrid nanocomposite was synthesised from CFA for the effective removal of cadmium ion from wastewater and the spent adsorbent was reused in the photocatalytic degradation of methylene blue (Umejuru et al., 2020). Furthermore, geopolymeric materials synthesized from CFA can offer valuable solutions to emerging environmental as they can immobilize toxic heavy or radioactive metals through process such as physical encapsulation and chemical stabilization (Adewuyi, 2021). CFA can be applied as a geopolymer concrete binder material due to its wide availability, greater potential and low cost for preparing geopolymers (Pavithra et al., 2016; Feng et al., 2019; Wulandari et al., 2021). However, CFA-based geopolymer materials may be affected by many parameters which are considerably related to the primary materials and their physical and chemical properties such as chemical composition, fineness and glassy phase content (Bouaissi et al., 2018; Feng et al., 2019). The FA/kaolinite ratio, the water content and metal silicates utilized significantly affect the final geopolymer product (Ahmed et al., 2021). CFA is considered as an alternative source of various valuable resources. While natural resources are decreasing with time, CFA contains a variety of beneficial metals and mineral substances whose demand is increasing in industries (Sahoo et al., 2016). CFA is a potential alternative source for rare earth elements (REEs) and carbon nanoparticles, which are important components in various applications including high-technology (micro- and nano-technology) products and a variety of consumer goods such as cell phones, computers, fluorescent lighting, catalysis, permanent magnets, advanced defense technology, and medical devices (Dai et al., 2016; Silva et al., 2020; Wu et al., 2022).

CFA has demonstrated potential in agriculture due to its efficiency in modification of soil health and crop performance. It is valuable for soil amendment, thereby providing plant and animal nutrients and improving its chemical and physical characteristics. CFA can serve as a fertilizer for agricultural activities as it may contain essential elements such as Ca, P, K, S, Na, Mg, Zn, Fe, Cu, Co, Mo, and B that are beneficial for crop development (Mupambwa et al., 2015; Belviso, 2017). Unfortunately, although CFA is characterized by high total concentration of nutrients, its application in agriculture is limited by low bioavailability of these nutrients (Mupambwa, 2016). Furthermore, this product is characterised by toxic metals that may be taken-up by plants. Chemical and physical properties of CFA affect their options for re-usability. For example, 75% of the CFA must have carbon content, measured by the loss on ignition (LOI) of less than 4% and have a fineness of 45 μm or less. Some countries such as India and China have made significant progress with CFA utilization, but the amount remaining is still high and this remains an environmental risk. In South Africa, approximately 10% of the produced CFA is recycled and therefore the amount stocked is increasing daily due to much dependency on coal for generation of electricity. Million tons of stocked CFA is a potential hazard to the environment, hence it is paramount to find alternative means to mitigate environmental health challenges associated with its disposal. In South Africa, Eskom has embarked on CFA utilization project aimed at identifying markets that can require high usage of CFA and have proposed four applications including mine drainage treatment and backfilling, soil amelioration and land reclamation, road construction, and brick and cement development (Reynolds-Clausen, & Singh, 2019). The building and construction industry use CFA as a cement extender (blended cements) or as a supplementary cementation material. Substituting with CFA has significant economic benefit and it also enhances the concrete performance, allowing the use of less water (SACCA, 2021). Other sectors utilizing CFA in South Africa are the polymer and rubber industry and agriculture (Krueger, & Krueger, 2005). Converting CFA into reusable materials is an excellent strategy of reducing high volumes that need to be dumped into landfills, and consequently reducing the need for more storage space that is costly (Umejuru et al., 2020). However, presently far less CFA is recycled and these low utilization rate in South Africa and many other countries highlight the need for effective strategies to prevent further degradation of the ecosystem.

6. Treatment Methods of Contaminated Sites

Several remediation strategies are being applied for clean-up of contaminated soils and ash disposal sites but most of them are ineffective and expensive (Koul, & Taak, 2018; Sharma et al., 2018). Methods for the treatment of contaminated sites include biological, physical and chemical methods which have advantages and disadvantages as well. Remediation methods are dependent on the contaminant and environmental factors, therefore there is no single or combined treatment methods that can be effective for remediation of all pollutants under same environmental conditions (Yeung, 2010). In addition to the use of landfills, the major treatment and disposal strategies of CFA include stabilization and solidification, chemical treatment and heat treatment (Sharma et al., 2018; Liu, & Ren, 2021). The chemical and thermal techniques are technically difficult and not suitable for practical applications, because are often associated with high cost, poor efficiency and are dependent upon the pollutants in the substrate (Negri et al., 1996).

6.1. Chemical Remediation

This technology aims to alter the chemical characteristics of pollutants to decrease their hazardous properties. The most commonly known chemical remediation methods include chemical oxidation and extraction, nano-remediation, ion-exchange, soil amendment, precipitation, and chemical leaching (Sharma et al., 2018). Chemical fixation fixes toxic heavy metals in the contaminated site through adsorption resulting to less availability of them. It is not a permanent treatment as heavy metals can get released into the environment under conducive conditions of weathering. It also consists of addition of reagents into the polluted sites to form slightly insoluble materials (Yang, 2005; Dhaliwal et al., 2020). Chemical leaching is based on dissolving heavy metal ions into the acids such as H₂SO₄, HCl or HNO₃ and extracting them out (Dermont et al., 2008). The limitation of this method is the need of large quantities of acids, which later require pre-treatment and therefore too costly. In the soil amendment technique which also called chemical fixation, the mobility and solubility of toxic heavy metals are reduced using chemical reagents such as FA (Sharma et al., 2018). Nano-remediation is suitable for clean-up of hazardous elements in industrial waste. The technology involves the complex formation of heavy metals with modified nanoparticles, followed by co-precipitation, and ultimate removal of toxicants from the contaminated media (Li et al., 2018; Sharma et al., 2018). The chemical oxidation is a remedial technique applied ex-situ and in-situ effectively used for treatment of organic pollutants (Oprea et al., 2009; Ranc et al., 2016; Liao et al., 2019). These methods require the use of oxidizing agents such as ozone, potassium permanganate, chlorine dioxide, hypochlorite, and hydrogen peroxide. The limitations of these methods include the production of secondary pollutants or by-products that need further processing and the need to use chemical reagents (Nam et al., 2008; Rada et al., 2019). Furthermore, the use of these treatments is associated with high costs (Istrate, 2009).

6.2. Physical Remediation

The treatment of contaminated sites via physical remediations include soil replacement, excavation and off-site disposal technology, vitrification and heat treatment (Zhou et al., 2004; Yeung, 2010; Sharma et al., 2018). Soil replacement technique is based on the reduction of contaminant levels through completely or partially replacing the polluted soils. Physical technique for migrating contaminated land and disposal in landfills is labour-intensive, quite expensive, time consuming and not commercially sustainable (Dhaliwalet al., 2020). Vitrifying technology consists of removing heavy metals through weathering process of organic matter by maintaining high temperature in the soils. This is a complicated technique which requires more energy, making it expensive and it has limited application Thermal desorption is suitable for treatment of most semi-volatile and volatile pollutants such as PCBs and PAHs. In this method, these organic pollutants in contaminated media are heated first to an appropriate temperature by indirect or direct heating under vacuum or into carrier gas to separate target pollutants from the substrate. The advantages of this technique include possibility of recycling soil and contaminants, high safety, high efficiency, short treatment period, and capability to treat different types of pollutants. However, this method produces off gases which can result in secondary contamination. This treatment method requires soil excavation and transportation that can create new contamination sources such as rainwater contamination, dust and noise (Kristanti et al., 2022). In addition, the other limitation is the high energy consumption and the resulting costs. In the excavation and off-site disposal technology, polluted soils are physically excavated and transported to disposal sites and treatment facilities, but this technology is not cost-effective (Yeung, 2010). Incineration method is a complex system representing an integrated system of components for waste preparation, feeding, combustion, and emission control. Thermal technology such as incineration method is a complex system that require utilizing electricity and therefore not a viable option for South Africa which is currently experiencing energy crisis. Remediation of polluted sites by biological methods can be advantageous to other methods as it does not require chemicals, cost-effective and no possibility of producing secondary pollutants. Bioremediation techniques involve the use of several animals, microorganisms and plants for remediation and has advantages over the traditional methods and therefore should be explored.