Abstract

Owing to the increasing industrial revolution in recent times, the pollution of aquatic and soil environments by heavy metals has been on the increase. The heavy metals generated from industrial activities can endanger humans as well as other living organisms. These heavy metals are hazardous and persist in the environment posing a great threat to public health. In order to address this serious environmental problem, gamut of remedial strategies are currently being applied with each having its merits and demerits. Interestingly, the physicochemical remediation strategies can be considered as one of the best and viable techniques in rehabilitating heavy metal contaminated sites, especially water and soil. The main advantages of the physicochemical remediation strategies stem from its high removal efficiency, speedy recovery of contaminants, ease of implementation and operational convenience. Hence, the scope of this review encompasses a brief account of heavy metal pollution, its sources, toxic effects and a critical review of the current physicochemical remedial approaches utilised in the removal of heavy metals, which ultimately restores polluted environmental media back to their original status. This study is significant, because it sensitises the general public on the dangers of heavy metal pollution and at the same time provides invaluable information to environmental scientists, public health workers and engineers who work to restore degraded ecosystems as well as sustainable ways to protect human and ecosystem health.

1. Introduction

The growing human population with its concomitant anthropogenic activities has resulted in significant metal accumulation in soil and water ecosystems. In recent decades, heavy metal contamination has become a severe danger to ecosystems and human health ^{1, 2}. Industrialization, technological progress, rising human population and resource exploitation, as well as agricultural and household waste run-off, continue to pollute the environment with a wide spectrum of heavy metals. Metals and metalloids with a specific density of more than 5 g cm³ or greater than water fall into this category ³. Zinc (Zn), nickel (Ni), lead (Pb), silver (Ag), cobalt (Co), chromium (Cr), arsenic (As), iron (Fe), cadmium (Cd) and the elements in platinum group are examples of heavy metals. Some heavy metals, such as chromium, zinc, nickel and copper are paramount for organism survival (a shortage or excess can cause sickness), while others, like cadmium and lead, are deadly even in minute amounts ^{4, 5}. Owing to their food chain contamination potentials, non-degradability, tendency to accumulate in organism, persistent nature and toxicity, heavy metals are one of the most harmful forms of contaminants. Heavy metals having unfavourable health effects in human metabolism (arsenic, mercury, cadmium, lead) represent substantial concerns due to their persistence in the environment and demonstrated potential for large health ramifications ⁶. These heavy metals come from a variety of sources, including metalliferous mining and smelting, agricultural chemical use, metal discharge and waste disposal ^{3, 7}. Weathering produces these metals, which are found naturally in the rocks. The presence and levels of heavy metals are determined by the nature of rock as well as the weathering conditions that began the process. Pb, Cr, Hg, Zn, Ni, Cd, Co, Zn, Sn and Mn are predominant in sedimentary rocks ⁸. Only a few of these operations which metal mining and smelting, fossil fuel combustion, pesticide use in agriculture, battery as well as other metal product manufacturing in industries, sewage sludge, and municipal trash disposal release heavy metals to the environment ⁹.

Therefore, the goal of this review is to present a comprehensive account of environmental heavy metal pollution, evaluate the physicochemical remedial techniques for restoring damaged ecosystems, and assess the pros and cons of each technique that may influence the successful rehabilitation of contaminated media. The issues raised in the work are very important, topical and closely related to engineering and environmental protection and will probably serve as useful information for environmental scientists and engineers whose work involves the rehabilitation of contaminated sites. However, the biological and other remedial strategies are outside the scope of this review paper.

2. Heavy Metal Pollution and Environmental Health

According to Hou et al., ¹⁰ “the soil is a non-renewable and precious resource, generated at a rate of a few centimetres per thousand years”. To buttress this point, Cassidy et al., ¹¹ and Jansson and Hofmockel ¹² stated soil benefits the ecosystems and human society by providing habitat for the majority of the world's species and producing agricultural output. However, anthropogenic activities have resulted in widespread soil contamination and degradation ^{13, 14}. According to studies, the EU has 2.8 million polluting sites, while 19% of arable lands in China have toxic substances at levels that exceed environmental quality standards ^{15, 16}. Lopez et al., ¹⁷ believe that “heavy metals can affect soil microbial community biomass, diversity, composition, structure, and function, as well as nitrogen, phosphorus, and potassium solubility, which affects plant uptake and growth.” According to research on biosolids with engineered nanomaterials used for agricultural production in several regions, the symbiosis between nitrogen-fixing bacteria (i.e., the key supplier of nitrogen in agro-ecosystem rejuvenation) and legumes is destroyed in the presence of ZnO/TiO₂ mixtures and their transformation products ¹⁸. Furthermore, Chen et al., ¹⁹ reported that “heavy metals could trigger a decrease in bacterial species population and a relative increase in soil actinomycetes, as well as losses in both the biomass and diversity of bacterial communities in contaminated soils”. Also, Karaca et al., ²⁰ submitted that “various metals influence enzyme activity in different ways according to the chemical affinities of the enzymes in the soil system.” Another study discovered that when Pb levels are high, Sporosarcina pasteurii can't generate enough exopolysaccharides, and the number of live cells drops, lowering “Pb” precipitation capability through carbonate precipitation induced by microbes ^{21, 22}. In addition, heavy metal contamination is building up in the soil as a result of ongoing Pb/Zn smelting ²³, posing a long-term hazard to human health and ecological security, as well as threatening the sites' long-term sustainability ^{24, 25}. “The high levels of heavy metals in the environment cause a number of health risks for living and non-living organisms,” according to a paper by Shimod et al., ²⁶. Contaminated soil contaminants can induce major neurological problems and life-threatening cancers when they reach the human body through contaminated crops, contaminated soil dust inhalation, or unintentional ingestion of contaminated soil ^{10, 27}.

Heavy metal pollution alters the composition, size and microbial population activities, as well as negatively impacting a number of plant quality and yield indices ²⁸. Abdu et al., ²⁹ are of the view that “Zinc (Zn), iron (Fe), copper (Cu), molybdenum (Mo), and manganese (Mn) are all required in plant and animal nutrition in small levels”. Excessive levels of these components, on the other hand, can be harmful to organisms. Heavy metal buildup in plants is determined by plant species, whereas plant absorption or metal transfer factors from soil to plant determine the efficacy of various plants in absorbing metals ³⁰. Stunted foliage, dark green leaves, brown short roots and wilting of older leaves are all toxic indicators. Too much lead in the soil can diminish soil production, while too little lead can inhibit essential plant activities like water absorption, mitosis and photosynthesis, resulting in stunted foliage, fading older leaves, dark green leaves and brown short roots ³¹. Heavy metals are phytotoxic by nature, which means that even at low levels, they have a significant negative impact on plant growth ^{32, 33}. This shows that when heavy metal concentrations are high, plant development is severely hampered. Heavy metal concentrations in the soil have a substantial impact on agriculture, slowing crop growth, inducing oxidative stress as well as affecting metabolic processes ^{34, 35, 36}. “Chlorophylls (Chl) and carotenoids are significant photosynthetic pigments that play a key role in converting solar energy to chemical energy during photosynthesis,” according to the study by Rai et al., ³⁷. They went on to say that “heavy metals, in particular, have an impact on photosynthetic pigment biosynthesis.” Chlorosis and plant growth retardation occur in metal-polluted environments, indicating an impairment of production pathways for photosynthetic pigment, which affects photosynthetic performance, plastid development and total metabolism. Plant structure has long been known to be affected by heavy metals, and elevated metal concentrations in plants, according to Bini et al., ³⁸, have a considerable impact on plant morphology. Plants exposed to extreme concentrations of metal have structural alterations such as mitochondrial structural changes, reduced leaf thickness and palisade loss ³⁸. Heavy metals have an impact on plant structure and have been linked to toxicological effects ³³. Depending on the metal's deadly potential, heavy metal toxicity has diverse impacts on cellular structure and function ³⁹.

Heavy metal ingestion by humans through the food chain has been documented in a number of nations, and the subject is garnering public and governmental attention, particularly in developing countries. They are dangerous because they cannot be destroyed or degraded in any way, and bio-accumulate in the bodies of living organisms. When compared to its concentration in the environment, bio-accumulation is described as an increase in a chemical's dose in a living organism over time. When plants absorb heavy metals from contaminated soil, they build up in plant tissue ⁴⁰. Heavy metals enter the human system through ingestion as humans are only exposed to them through the in-take of contaminated plants. They become harmful when they are not absorbed and accumulate in the soft tissues ⁴¹. According to a report, Sankhla et al., ⁴² “heavy metals enter human systems mostly through air, food, and water”. Humans are adversely affected by chronic exposure to hazardous metals, and the effects do not become apparent for several years ³⁰. Heavy metals can harm humans in trace amounts after acute or chronic exposure, wreaking havoc on major organs in adults such the brain, kidney, central nervous system, heart, reproductive, and digestive systems, as well as in children's neurological system, blood circulation, kidney and the brain ^{43, 44}. In people and animals, non-essential heavy metals can induce chronic or acute poisoning, resulting in symptoms such as motor neuron dysfunction, nausea, irregular bowel movements, brain and heart damage, vomiting, hypertension, visual and hearing impairment, and more ^{45, 46, 47}. Nerve cell and DNA structure disruption, as well as a delay in mitochondrial metabolic tasks including ATP synthesis and oxidative photophosphorylation, can all be caused by cellular and internal toxicity ^{48, 49, 50}. Heavy metals and toxic metalloids' bioavailability in suspended particulate matter of air was recently revealed, potentially providing a hidden threat to human health through inhalation of atmospheric air ⁵¹. Arsenic was also discovered in the air and urine of people who lived near copper smelters ⁵². As a result, these major challenges, when combined with repeated exposures to the food product or environmental pollution with hazardous heavy metals, could lead to global death. Concentrations of heavy metals above a specific level may pose major health risks. Heavy metal toxicity can injure or diminish brain and central nervous system functions, as well as the lungs, liver, kidneys, blood components, and other vital organs ⁵³. Long-term exposure to dangerous heavy metals has been related to cancer, Alzheimer's disease, muscular dystrophy, and multiple sclerosis. Heavy metals enter people through three primary routes: oral consumption, inhalation, and skin contact ^{54, 55}.

Water is essential for the survival of all living things on the planet. Water is both a source of life and a necessity for many people around the world, but many do not have access to new and safe drinking water. While the world's population has quadrupled, anthropogenic water consumption has increased sevenfold in the previous century ⁵⁶. The number of people who have access to improved drinking water sources has climbed from 2.6 billion in 1990 to 663 million in 2015, according to a World Health Organization (WHO) report titled “Progress on Sanitation and Drinking Water-2015 Update and MDG Assessment.” The rapid pace of industrialisation, population explosion, and spontaneous urbanisation, on the other hand, has resulted in significant water contamination. Hazardous metals, pesticides, medicines, dyes, surfactants, and other dangerous substances have contaminated water supplies and are harmful to humans and animals on an ecological level ^{57, 58, 59}. Heavy metals are generally associated to particulate debris in aquatic systems, which settles and becomes absorbed into sediments over time. High amounts of micropollutants, especially metals, can affect diatom community structure, according to Morin et al., ⁶⁰ and Jong et al. ⁶¹. By breaking down rocks and releasing heavy metals into surface waters and groundwater and the formed acid rain can contribute to heavy metal poisoning of water systems ⁶². Heavy metals accumulate in numerous organs of marine species over time due to their toxicity, resulting in metal-related illnesses in the long term, putting the aquatic biota and other organisms at risk. Heavy metals can be passed up the food chain after being acquired by an aquatic organism. Although some heavy metals, such as enzyme cofactors, micronutrients, osmotic pressure regulators, and molecule normalisation, are important in living organisms' physiological, biochemical, and metabolic processes, the vast majority of them have no known biological function and are toxic when present in high concentrations ⁶³. Bioavailability of heavy metals, as well as the amount consumed, determine this ⁶⁴. Heavy metals' risks to living beings are exacerbated by their widespread. When the medium becomes acidic and nutrient-deficient, as well as when the soil structure is weak, as it is in mining sites, toxicity rises ⁶⁵.

3. Heavy Metal Remediation Strategies

The World Health Organization (WHO) and the Environmental Protection Agency (EPA) have both said that heavy metal contamination is a severe concern to human health, and as a response, they have proposed a number of methods for restricting heavy metals' spread in the environment ⁶⁶. The widespread consequences and challenges associated with heavy metal-related disorders have prompted further study and trials in the field of heavy metal contamination remediation. More so, the long term consequences of heavy metals have prompted academics and researchers to work on remediation technologies, although the majority of the work, according to research, is channeled on contaminated soil and water.

In practical terms, remediation is the process of returning soil, water, or air to their pre-contamination state. This is accomplished by degrading and transforming pollutants into less dangerous, if not completely harmless forms. According to Mu'azu et al., ⁶⁷, “certain sustainable strategies for the treatment of heavy metal contaminated soils and water are available in research articles,” with each possessing its benefits and drawbacks. However, the initial concentration of metal ions, physical and chemical features of the contaminated sites determine how each procedure is applied ⁶⁸. In addition, Gomes et al., ⁶⁹ are of the opinion that “the several remediation strategies available depends on the media (e.g. air, water, or soil) and the type of pollutant (e.g. heavy metals, PCBs).” The techniques accomplish one of two functions: a) removing pollutants, or b) transforming metals into harmless forms ^{70, 71}. As a result, it is critical to employ a strategy (or a combination of approaches) that is appropriate and sustainable for the contaminated site. It is worth noting also that several variables such as efficiency, time, cost, reliability as well as the degree of toxicity and the need for the employed method's post-remediation treatment, must all be taken into account when choosing the appropriate approaches. Either in-situ or ex-situ, the biological and physicochemical approaches could be used ^{72, 73}. For the most effective clean-up of some heavy metal-contaminated sites, a combination of treatments may be required. As a result, physicochemical, and biological technologies can be utilised in tandem to reduce pollution to a level that is safe and acceptable. However, this review presents only the account of the physicochemical approaches. In order to understand all that the physicochemical strategy entails, its different unit processes have been shown in Figure 1, and comprehensively described as follows, with the some pros and cons presented in Table 1.

4. Physicochemical Approaches

The well-known physicochemical approaches include; soil replacement/Excavation and landfilling, thermal desorption, vitrification technology, soil washing, chemical immobilisation, chemical precipitation, electrokinetic remediation, ion-exchange, chemical coagulation and flocculation, membrane filtration, adsorption ^{53, 74, 75}. The physicochemical approach of restoring a degraded ecosystem is cheap and effective. However, the procedure is time-consuming, costly, only ideal for small contaminated sites, and environmentally unfriendly because harmful compounds are released into adjacent ecological systems during the remediation process ^{72, 76, 77}.

Soil replacement, otherwise known as soil change entails replacing polluted soil with clean and uncontaminated soil in order to reduce pollutant concentrations, improve soil absorption capacity, and achieve soil rehabilitation ^{78, 79}. It could be a total or partial replacement of the soil. The original polluted soil is replaced with new, fertile soil, which decreases the harmful effects of pollutant on the environment while also improving soil durability ^{74, 80}. Although expensive and only suited for highly polluted small areas, the soil replacement approach efficiently isolates polluted soil and habitats, minimising their effects on the ecosystem ⁸¹. Similar to soil replacement is the excavation and landfilling. During the excavation process, polluted soil is removed and moved to landfills. Landfill liners, such as clay, are widely used to prevent contaminants from migrating. Landfills take up a lot of space and are potentially hazardous to people. Excavation and disposal are more costly. There are concerns regarding safety, odour emission, and the danger of groundwater contamination from runoff ⁸².

This process, also known as thermal remediation or incineration, involves heating contaminated soil matrix in a chamber at a high temperature, typically between the range of 90-320 and 320-560°C for low and high desorption, respectively, where organic pollutants and some metals can be vaporised ⁷⁶. Using a thermal incinerator, the excavated soil is subjected to high temperatures. The high temperature breaks down the pollutants, and the liberated volatiles are subsequently combusted or recovered with solvents in the afterburner. Fixed bed tubular reactor, electrically-heated furnace, heating systems with rotating kiln, quartz tube and other apparatus have been used in the laboratory experiments ^{83, 84, 85}. This method is “simple, less expensive, easy to reuse remediated soil, and eco-friendly,” according to Narendrula-Kotha et al., ⁷¹. However, its major drawbacks are the high cost of equipment and the extended desorption time. Furthermore, this procedure is not equally effective for all soil types, and it is not applicable to all metals ⁷⁶.

This is a heavy metal polluted soil treatment process in which the soil is exposed to high temperatures and pressure for a length of time, then cooled to create a vitreous substance. After treatment, the molten solidified product is discarded. The process of vitrification necessitates an increase in paste intensity between 1400 and 2000 degrees Celsius, resulting in the natural breakdown or solubilisation of hazardous substances. The pyrolysis products are derived from the fumed gas produced by the treatment framework, which is manufactured by this technique ⁸⁶. According to Wuana and Okieimen ¹ “high temperature treatment reduces the mobility of metals, resulting in the production of vitreous materials, usually a solid oxide”. It is important to note that the vitrification technology is high-performing, can dispose of heavy metals, and can be used on contaminated water and soil. Regardless, this approach is complicated and requires more combined resources, increasing the cost with minimal under applications ⁸⁷.

This process more often referred to as chemical leaching is a process of removing pollutants from contaminated soil by leaching it with clean water and often reagents, surfactants, or other liquids ^{88, 89}. In the washing process, surfactants, chelating agents, acids, bases, water and other solvents can be used ^{90, 91, 92}. Heavy metals are leached out of the soil using chemicals and reagents such as Ethylenediamine tetraacetic acid (EDTA), which immobilises hazardous metals in a less accessible form. The contaminated soil is dug up and treated with the proper extractants during this process ⁹³. The leachate is subsequently collected and treated or disposed of downstream of the site using a collection system. Furthermore, it is a low-cost, easy technique that may be applied to field remediation ⁹⁴. The desorbability/release of the contaminant from the soil, as well as the leaching rate of pollutants through soil, are all influenced by the hydraulic conductivity, texture, porosity, and mineralogy of the soil ⁸². Furthermore, the choice of extractant is influenced by the soil and the heavy metal to be removed, and it is crucial for the soil washing process. Because of its ability to totally remove contaminants and its efficiency, soil washing is a popular method of heavy metal cleanup ⁹³. Inorganic and organic acids, as well as chelating chemicals are often used as washing solutions in the rehabilitation of heavy metal polluted soil. Acids, both organic and inorganic, have been found to remove much more heavy metals from polluted soils ^{95, 96}. However, they are inefficient in locations contaminated with both Arsenic due to variations in their characteristics. The chemical speciation's redox potential has an impact on the remediation process ⁹⁷. Soil washing has not been successful in removing As., which is tightly linked to Fe oxides in the majority of cases. Although, the majority of research have compared the impact of experimental parameters (such as pH, soil/liquid (S/L) ratio, and other factors) on heavy metal removal effectiveness using various washing agents, only a few have looked at the influence of soil parameters on soil washing ^{98, 99, 100, 101}. Additional study is needed to develop an effective soil washing method, as the detrimental impacts of the washing treatment process on soil properties must be reduced ⁹⁵. Kim et al., ¹⁰¹ submitted that “soil washing combined with physical separation, for example, can concentrate metals into smaller soil volumes, resulting in effective heavy metal removal and a lower amount of contaminated soil”.

It entails mixing an additive into excavated, polluted soil to encapsulate the contaminants. After that, the mixture is dumped on the ground. For inorganic contaminants, this approach is employed. Chemical immobilisation has no effect on removing or extracting pollutants from the soil. Instead, the mobility of heavy metals in soil or water is much reduced, decreasing their potential for transfer to plants, microbes and water ¹⁰². The solidification and stabilisation procedures are the two components of this approach. The solidification process, which uses chemical agents to control heavy metal solubility and maintain species in the soil, removes heavy metals from soil systems ¹⁰³. Solidification can transform liquid wastewater, semi-solid sludge, or powder into a granular material, making treatment and transportation easier. The binder employed in this method, on the other hand, depletes soil fertility and decreases the value of agricultural land ¹⁰⁴. Solidification is a type of soil remediation that can be used in and out of the field. An auger is used to mix a binding agent, such as fly ash, clay, asphalt and cement into the contaminated soil, resulting in a solid block ¹⁰⁵. A crane with an injector head and a large mixer may inject binding slurries and mix them with the waste, if the contaminants are deep in the soil. Because the solid block is water impermeable, the contaminants trapped inside are not leachable ¹⁰². However, if natural weathering and unmanaged mechanical disturbances undermine the solid block's structure, the trapped contaminants may become mobile again in the long run ¹⁰⁶. In addition, future uses of the stabilised site could be constrained. In-situ solidification has been used to treat 60 heavy metal-contaminated sites in the United States since the 1980s ^{105, 107}. A solidification project takes an average of 1.1 months to complete, which is much less time than alternative treatment options. The pollutants, however, are neither destroyed nor removed by solidification. The cemented portions could prevent a more thorough rehabilitation in the future. As a result, solidification should only be used as a last resort for soil rehabilitation if all other options have failed. Because solidification necessitates a large volume of binding agent, its usefulness is mostly controlled by its availability and transportation costs. The total cost of solidification varies by location and can be as high as $1500 m³ (averaged $520 m³ in 2012 in the United States), including material, drilling, and mixing costs ¹⁰⁶. Stabilization, on the other hand, is the technique of chemically converting hazardous contaminants into less dangerous or transportable forms ¹⁰⁸. Stabilization (also known as “in-situ fixing”) immobilises pollutants while leaving the soil intact. Chemical reactions between pollutants and adhesives are used in this procedure. Tajudin et al., ¹⁰² stated that “in addition to binding agents, precipitation reagents/stabilizing chemicals are injected into polluted soil to create physiochemical interactions between the stabilising reagents and heavy metals, decreasing their mobility”.

Electroremediation, also known as electrokinetic remediation or electrokinetic soil flushing, is embedding electrodes in contaminated soil or groundwater and creating an electric field with a low current ¹⁰⁸. The contaminated matrix is exposed to an electric field between anode and cathode, allowing pollutants in the soil to be removed or transferred into the flushing fluid, which can then be treated or mobilised using electro-osmosis, electromigration, and electrophoresis ¹⁰⁹. In other words, this is a unique remediation technology in which voltage is applied at both ends of the soil to create an electric field gradient. As pollutants migrate in the direction of the electric field, electro-precipitation, deposition, and co-deposition will enrich the pollutants that eventually reach the electrode area ¹¹⁰. Heavy metal is transferred to the cathode or anode via electroosmosis and electromigration of the electric field. Thereafter, precipitation on the stick indicates the reduction or elimination of metal contamination in soil ¹¹¹. The three main electrokinetic processes in this method are electrostatic, electromigration, and electrophoresis ^{80, 112}. Interestingly, strong metals such as lead, arsenic, and mercury can be extracted from soil using extraction or electromigration methods ⁸⁰. Organic and inorganic contaminants containing metals like lead, chromium, cadmium, and uranium are treated by electroremediation. Electroremediation is advantageous in low-permeability soils because poor permeability has no effect on the transport rate created by the electrical field. Depending on the circumstances at the site, it can be employed on-site, off-site, or in-situ. Another advantage of electro remediation is that it can be used to treat contamination that is deep or difficult to reach. This method is suitable for low permeable soils and has the advantages of being simple to install and operate, as well as being cost-effective and not degrading the natural environment, allowing for environmental remediation and the preservation of the original ecotope [75,113-119]. This is a new form of physical repair technology that has a fast cycle time and great efficiency. In recent years, it has sparked widespread concern and research. The pH, permeability, and electrolyte components of the soil, on the other hand, have an impact on its removal efficiency. This approach is also limited by its high cost and significant energy usage ⁷⁴.

This technology has been widely employed in the removal of heavy metals from wastewater due to its high removal rate, higher treatment capacity, and efficient kinetics [119-123]. This method is affected by the pH of the aqueous phase ¹²⁴. To remove heavy metal ions from a solution, a specialised ion exchanger, either naturally occurring inorganic zeolites or synthesised organic ion exchange resins containing cations or anions, is utilised ¹²⁵. Even after electroplating eliminates cyanide, copper, nickel, and cadmium from a water solution, the remaining solution contains cyanide, copper, nickel, and cadmium, all of which are hazardous to one's health. The ions (cations or anions) in the solution are replaced with ions (cations or anions) on the insoluble material known as ion-exchange resin in this technique. Heavy metal wastewater is fed into one end of the ion-exchange column, then passed through the bed, which removes the heavy metals. If the column becomes saturated with heavy metals, it is cleaned and dried before being regenerated to remove the heavy metals that have been deposited ^{126, 127, 128, 129}. Heavy metals must be removed from wastewater using ion-exchange resin, which might be natural or synthetic. Synthetic resins are chosen because they are more effective in removing heavy metals from wastewater than various natural and synthetic resin materials ¹³⁰. Strong acidic resins with sulfonic acid groups and weak acidic resins with carboxylic acid groups are the most common cation exchangers. The hydrogen ions in the resins' acidic groups can act as an exchangeable ion with the cationic metal ions in the effluent. Natural zeolites and natural silicate minerals, as well as synthetic resins, have been widely used to remove heavy metals from wastewater due to their widespread availability and affordability ^{131, 132, 133}. Despite the fact that the ion-exchange method is widely used, the chemicals needed to regenerate ion-exchange resins produce significant secondary pollutants. Secondary pollutants must be treated properly. Because a large amount of resin is required to treat a large volume of wastewater with low metal ion concentrations, the ion-exchange technique is expensive. An ion exchanger's mesh bed should be rinsed using a simple and cost-effective counter-current technique.

This procedure entails adding chemicals to polluted water, which react with heavy metals and result in the development of insoluble precipitates or compounds. These insoluble complexes or precipitates will separate from their parent liquids by sedimentation and filtration. Carbonates (soda ash and sodium bicarbonate), lime, caustic soda, and sulphides (sodium sulphide, sodium hydrosulphide) are some of the most commonly used inorganic precipitating agents for heavy metal precipitation and removal ^{134, 135, 136}. Because the solubility of metals is highly affected by pH, the process exhibits a variety of precipitation characteristics at different pH levels. When the pH is low, heavy metals are more readily available, but as the pH rises, they precipitate. Heavy metals can form insoluble hydroxide, sulphide, carbonate, and other insoluble compounds. The solubility of products is improved by lowering the pH of wastewater with an acid ¹³⁷. Chemical precipitation is also influenced by the size, density surface charge, and type of pollutant, as well as the dose of precipitants that are continuously evaluated during the precipitation process, according to relevant studies ^{104, 112, 138}. Hydroxide and sulfide precipitation are two of the most common chemical precipitation reactions. In the pH range of 8.0 - 11.0, the solubility of several hydroxides of metals is limited. Flocculation followed by sedimentation operations can be used to separate the produced metal hydroxides. Because of their ease of handling and cost-effectiveness, various hydroxides have been employed to precipitate heavy metals from wastewater ¹³⁹. In most industrial wastewater treatment processes, lime has been used as a major hydroxide precipitating agent ¹⁴⁰. Heavy metals such as Cu and Cr were removed from wastewater using calcium hydroxides and sodium hydroxides as precipitating agents ¹⁴¹. On the other hand, sulphide precipitation is an effective method for removing hazardous heavy metals ¹⁴². The solubility of metal sulphide precipitates is substantially lower than that of hydroxide precipitates, and sulphide precipitates are not amphoteric, which is one of the most critical considerations when using sulphides. In comparison to hydroxide precipitation methods, the sulphide precipitation method is capable of achieving a significant amount of metal removal over a wide pH range ¹⁴³. In comparison to metal hydroxide sludges, metal sulphide sludges thicken and dewater better. In any case, the use of the sulphide precipitation technique has the potential to be hazardous ¹⁴⁴. In addition, heavy metal ions are frequently in acidic environments, and sulphide precipitants in acidic environments can result in harmful hydrogen sulphide vapour formation. This precipitation procedure must be carried out in basic or neutral conditions. Furthermore, metal sulphide precipitation produces colloidal precipitates, which can cause separation problems in filtration and sedimentation operations. In a nutshell, the chemical precipitation method is suitable due to its cost-effectiveness, ease of operation and process control ^{145, 146, 147}, and applicability across a wide temperature range. However, to reduce the metals to an acceptable level for release into the environment, the process necessitates a large amount of chemicals ¹⁴⁸. Furthermore, the enormous volume of sludge produced, rising sludge disposal costs, slower metal precipitation, poor settling, and long-term environmental effects of sludge disposal are some of the process' limitations ¹⁴⁹.

Heavy metals are removed from wastewater or solutions using this chemical method ^{150, 151}. The electrostatic repulsion mechanism decreases the net surface charge of colloidal particles. It's when particles' charges are balanced. This method has successfully treated drinking water and decontaminated industrial effluents ^{152, 153}. Coagulation is the transformation of coagulants or dissolved organic materials into bigger forms. Magnesium chloride (MgCl₂), Alum, aluminium hydroxide, polyaluminium chloride (PACL), polyethyleneimine (PEI), and other coagulants are employed in this process ¹⁵⁴. Coagulation is the process of applying a chemical coagulant to destabilise colloidal particles, which results in sedimentation. To enhance the particle size, the unstable particles are flocculated into huge floccules after coagulation ¹⁵³. Through successive collisions and interactions with inorganic and dissolved organic polymers, flocculation gradually mixes the destabilised particles, increasing particle size. Filtration, flotation, or straining can easily separate smaller particles from bigger particles once they have been agglomerated ¹⁵⁵. Finally, in the sedimentation tank, the huge floccules will settle. Alum, ferric chloride, ferrous sulphate, and other coagulants for the removal of contaminants from wastewater are widely used in traditional wastewater treatment systems ¹⁵⁶. Macromolecule flocculants have also been successfully used in the removal of heavy metals from wastewater ^{157, 158, 159, 160}. According to the US Environmental Protection Agency ¹⁶¹, report “the pH range of 8.5-11.3, using lime softening and coagulation (ferric sulphate or alum) was successful in eradicating lead, chromium, and cadmium up to 98 percent. Cadmium removal improves as pH rises; ferric sulphate and alum coagulation were combined to remove 97 percent of lead from river water with 0.15 mg/l lead. At a pH range of 6.5-9.3, ferric sulphate coagulation removed 98 percent of the chromium ¹⁶². At an ideal pH of 6.2-7.8 for alum, 8.7-10.9 for MgCl₂, and 8-9.3 for PACL, 99 percent of the lead was eliminated ¹⁵⁴. In the study conducted by Johnson et al., ¹⁶³, FeCl₃ was utilised as a coagulant to improve metal (Ni, Zn, Pb and Cu) removal from synthetic wastewater. Heavy metals cannot be completely removed by coagulation/flocculation treatment units alone, however they can be completely removed when combined with other treatments ^{164, 165, 166, 167, 168}. Coagulation/flocculation offers advantages, but it also has drawbacks, such as high operational expenses due to extensive chemical use.

Membrane technology is a physical treatment method that filters pollutants out through a membrane based on their size and qualities ¹⁶⁹. The hydrostatic pressure around the membrane is the fundamental driving force for filtration through the membrane ¹⁷⁰. The presence of functional groups, surface charge and degree of hydrophilicity, pore size, pore distribution and solution flow are the most important elements in the membrane process ¹⁷¹. These factors are crucial in determining the total water production rate and heavy metal removal effectiveness of the membrane process ¹⁷¹. The flux rate and selectivity of the membrane determine its overall performance. Heavy metals were removed from wastewater using membrane filtering treatment technologies, which yielded outstanding results ¹⁷². Water permeability and heavy metal ion rejection are influenced by the chemical and physical properties of the membrane ^{173, 174}. Membrane filtration is well-known for its ability to remove organic chemicals and suspended solids from inorganic effluents rich in heavy metals ¹⁷⁵. Because of its cost-effectiveness and great efficiency, this approach has gained more popularity and has become a popular method for the removal of heavy metal from contaminated medium ¹⁷⁶. For the removal of hazardous metal ions from wastewater, various membrane technologies such as nanofiltration, reverse osmosis, ultrafiltration and electrodialysis have been effectively used. A semi-permeable membrane in the reverse osmosis system allows water to pass through while rejecting dangerous heavy metals from wastewater. This is one of the most effective methods for eliminating dissolved metals from water and waste water. Mohsen-Nia et al., ¹⁷⁷, Zhang et al., ¹⁷⁸, You et al., ¹⁷⁹ and Vital et al., ¹⁸⁰ employed reverse osmosis to extract several hazardous metals from wastewater. The high power costs, membrane fouling and handling of rejection are all major disadvantages of this approach. The ultrafiltration technique uses low transmembrane pressures to extract dissolved and colloidal particles ¹⁸¹. Dissolved solids pass through the ultrafiltration membrane rather quickly since the pore diameters are larger than the dissolved particles. Nano-filtration is a membrane-based method for removing contaminants with particle sizes that fall in between ultra-filtration and reverse osmosis. Commercially available nanofiltration membranes are made up of synthetic polymers with charged groups. For eliminating a specific group of charged entities, the setup must be designed specifically ¹³⁴. The elimination of hazardous heavy metals from wastewater is achieved with this treatment process ^{182, 183, 184, 185}. High efficiency, relatively low energy consumption, reliability, and ease of operation are only a few of the benefits of this treatment procedure ¹⁸⁶. Electrodialysis is a membrane operation that uses charged ion exchange membranes and an electric field to separate ions from one solution to another. This treatment approach has been effectively used to treat industrial wastewater, generate drinking and process water from seawater, recover usable elements from industrial effluents, and create salts ¹⁸⁷. Schlichter et al., ¹⁸⁸, Nataraj et al., ¹⁸⁹, Nemati et al., ¹⁹⁰ and Jiang et al., ¹⁹¹ employed this treatment strategy to eliminate dangerous heavy metals from industrial wastewater. This method has the advantage of being more efficient in terms of removal, requiring less space, and being easier to use ¹⁸¹. Membrane treatment technologies can remove harmful heavy metals from wastewater, but they have drawbacks such as process complexity, higher costs, membrane fouling, and decreased permeate flux ⁵³.

5. Adsorption

According to Abdel-Raouf and Abdul-Raheim ¹⁹² “Adsorption is a mass transfer process in which impurities are attached to a solid surface by physical or chemical interactions, most commonly van der Waals forces of attraction or hydrogen bonding”. The size of the pollutants, their concentration, temperature, molecular mass, and other chemical parameters influence adsorption efficiency ¹⁶⁹. For the removal of heavy metals from wastewater, adsorption process is now believed to be the most cost-effective, efficient, and selective treatment and decontamination approach [137,193-198]. This technique offers flexibility in operation and design for 100% recovery of heavy metals from wastewater. It occurs when species are drawn to the solid surface, or when one or more species in the liquid interact with the solid, and it can also happen between the solid and gaseous phases. The two types of adsorption are physical and chemical adsorption ¹⁹⁹. Physical adsorption is induced by weak Van der Waals forces of attraction, whereas chemical adsorption is caused by a strong covalent bond between the adsorbent and the adsorbate. Surface modified magnetic adsorbents, modified biopolymers, biomass-based activated charcoal, and other adsorbents of varied sorts and cost-effectiveness have all been employed to extract heavy metals from metal-contaminated soil and water ^{80, 200}. With the correct desorption techniques, heavy metal adsorption can sometimes be reversed, allowing the adsorbent to be regenerated. Activated carbon (AC) is one of the most widely utilised and popular adsorbents for heavy metal removal from wastewater because of its higher surface area and affinity for heavy metals. The advantages of this technology with reference to Yang et al., ²⁰¹ are “high removal capacity, low operating costs, ease of installation, and simple treatment by regenerating adsorbed heavy metal ions”. Because heavy metal can exist with a variety of other organic and inorganic contaminants, which might obstruct heavy metal removal, a suitable adsorbent should be selected as part of the multi-component system. The use of coal-fired fly ash (CFFA) to remove, copper, nickel and cadmium from single and multi-contaminated systems has been investigated ²⁰². According to various research, heavy metals can be eliminated using sawdust. Yu et al., ²⁰³ found that “maple sawdust had a clearance effectiveness of more than 80% for Cr (VI)”. Cadmium, nickel, zinc and copper were removed from beech sawdust by Božić et al., ²⁰⁴, while manganese, copper, nickel, zinc and cadmium were removed from poplar and linden tree sawdust by the authors in another study. Li et al., ²⁰⁵ used poplar tree sawdust to demonstrate Cr, Cu, and Pb adsorption. Adsorption has a number of advantages over other traditional approaches, including the reduction of biological and chemical sludge, as well as being cost-effective, environmentally friendly, and incredibly efficient. Adsorbent regeneration and metal recovery are also possible with this approach ²⁰⁶.

6. Conclusion and Recommendation

Heavy metal pollution is a global issue with public health implications. To reduce the toxic effects of these metals and restore ecosystem functioning at contaminated sites, a variety of remediation procedures have been devised, each with its own set of benefits and drawbacks. Physicochemical strategies for removing heavy metals from ecological systems were surveyed and summarised in this review. Chemical precipitation, coagulation, and flocculation are the most simple and inexpensive techniques, although, they have problems complying with higher environmental standards. Other strategies, such as thermal desorption, soil washing and flushing, membrane technology, vitrification technology, electroremediation, chemical immobilisation, adsorption, ion-exchange, and others, can be more effective in overcoming the drawbacks of large-scale sludge production, but commercial application is still difficult in many cases due to economic viability. Hence, heavy metal concentration, environmental impact, ease of operation and financial cost should all be considered when selecting a method for a contaminated medium. The application of a soil remediation technique, on the other hand, is determined through a multitude of factors such as public acceptability, remediation objectives, site and pollutant properties, remediation efficacy, financial intensiveness and duration. Also, treatability studies should be conducted prior to deciding on the appropriate remedial treatments and in initiating full-scale treatment.

Acknowledgments

Authors sincerely thank the faculty members of their respective universities for the immeasurable support that they received in the course of the preparation of this scientific review. In a very special way, Anyiam Ngozi Donald expresses his unalloyed appreciation to the Association of Commonwealth Universities (ACU) for funding his Master’s programme via the Queen Elizabeth Commonwealth Scholarship scheme.

References