The presence of heavy metals in the environment leads to a number of adverse impacts. Such impacts affect all spheres of the environment, that is, hydrosphere, lithosphere, biosphere and atmosphere. Until the impacts are dealt with, health and mortality problems break out, as well as the disturbance of food chains. Figure 3 summarises the health impacts of heavy metals. Impacts of heavy metals on the environment [ 13 ]. Heavy metals contamination is becoming a serious issue of concern around the world as it has gained momentum due to the increase in the use and processing of heavy metals during various activities to meet the needs of the rapidly growing population.
Soil, water and air are the major environmental compartments which are affected by heavy metals pollution. Emissions from activities and sources such as industrial activities, mine tailings, disposal of high metal wastes, leaded gasoline and paints, land application of fertilisers, animal manures, sewage sludge, pesticides, wastewater irrigation, coal combustion residues and spillage of petrochemicals lead to soil contamination by heavy metals.
Soils have been noted to be the major sinks for heavy metals released into the environment by aforementioned anthropogenic activities. Most heavy metals do not undergo microbial or chemical degradation because they are nondegradable, and consequently their total concentrations last for a long time after being released to the environment [ 5 , 14 ]. The presence of heavy metals in soils is a serious issue due to its residence in food chains, thus destroying the entire ecosystem.
As much as organic pollutants can be biodegradable, their biodegradation rate, however, is decreased by the presence of heavy metals in the environment, and this in turn doubles the environmental pollution, that is, organic pollutants and heavy metals thus present.
There are various ways through which heavy metals present risks to humans, animals, plants and ecosystems as a whole. Such ways include direct ingestion, absorption by plants, food chains, consumption of contaminated water and alteration of soil pH, porosity, colour and its natural chemistry which in turn impact on the soil quality [ 15 ]. Although there are many sources of water contamination, industrialisation and urbanisation are two of the culprits for the increased level of heavy metal water contamination.
Heavy metals are transported by runoff from industries, municipalities and urban areas. Most of these metals end up accumulating in the soil and sediments of water bodies [ 15 ]. Heavy metals can be found in traces in water sources and still be very toxic and impose serious health problems to humans and other ecosystems. This is because the toxicity level of a metal depends on factors such as the organisms which are exposed to it, its nature, its biological role and the period at which the organisms are exposed to the metal.
Food chains and food webs symbolise the relationships amongst organisms. Therefore, the contamination of water by heavy metals actually affects all organisms. Humans, an example of organisms feeding at the highest level, are more prone to serious health problems because the concentrations of heavy metals increase in the food chain [ 16 ].
Industrialisation and urbanisation, due to rapid world population growth, have recently made air pollution as a major environmental problem around the world. The air pollution was reported to have been accelerated by dust and particulate matters PMs particularly fine particles such as PM 2.
Natural processes which release particulate matters into air include dust storms, soil erosion, volcanic eruptions and rock weathering, while anthropogenic activities are more industrial and transportation related [ 17 ]. Particulate matters are important and require special attention as they can lead to serious health problems such as skin and eyes irritation, respiratory infections, premature mortality and cardiovascular diseases.
These pollutants also cause deterioration of infrastructure, corrosion, formation of acid rain, eutrophication and haze [ 9 ]. Treatment processes for acid mine water typically generate high-density sludge that is heterogeneous due to variety of metals, metalloids and anionic components, and this makes it difficult to dispose the sludge [ 19 ].
Recent researches have therefore focused on the recovery of chemical species from acid mine drainage AMD and secondary sludge. This is aimed at recovering valuable resources and also enabling easier and safer disposal of the treated sludge, hence reducing their environmental footprints.
It may also lead to soil contamination, hence affecting their productivity [ 19 ]. In order to protect the human health, plants, animals, soil and all the compartments of the environment, proper and careful attention should be given to remediation technologies of heavy metals. Most physical and chemical heavy metal remediation technologies require handling of large amounts of sludge, destroy surrounding ecosystems and are very expensive [ 19 ] Figure 4.
Mechanisms for the removal of heavy metals [ 20 ]. A variety of alkaline chemical reagents have been used over the years for neutralisation of acid mine drainage AMD in order to increase the pH and consequently precipitate and recover the metals. Some processes have recovered metals at varying pH regimes Table 1 and synthesised commercially valuable materials such as pigments and magnetite [ 22 ]. Some minerals are recovered and sold to metallurgical industries, hence off-setting the treatment costs [ 19 ].
Adsorption occurs when an adsorbate adheres to the surface of an adsorbent. Due to reversibility and desorption capabilities, adsorption is regarded the most effective and economically viable option for the removal of metals from aqueous solution.
Although efficient, adsorption is not effective with very concentrated solution as the adsorbent easily gets saturated with the adsorbate. It is only feasible for very dilute solutions, is labour intensive because it requires frequent regeneration and it is not selective in terms of metal attenuation [ 21 ]. Adsorption is therefore not applied in a large scale of metal remediation. Ion exchange is the exchange of ions between two or more electrolyte solutions.
It can also refer to exchange of ions on a solid substrate to soil solution. High cation exchange capacity clay and resins are commonly used for the uptake of metals from aqueous solutions. However, this method requires high labour and is limited to certain concentration of metals in the solution. This system also operates under specific temperature and pH. Natural and synthetic clays, zeolites and synthetic resins have been used for removal and attenuation of metals from wastewater [ 19 , 23 ].
Biosorption refers to the removal of pollutants from water systems using biological materials, and it entails the absorption, adsorption, ion exchange, surface complexation and precipitation. Biosorbents have an advantage of accessibility, efficiency and capacity.
This process is readily and easily available. Regeneration is easy, hence making it very favourable. However, when the concentration of the feed solution is very high, the process easily reaches a breakthrough, thus limiting further pollutant removal [ 24 ]. The use of membrane technologies for the recovery of acid mine drainage is very effective for water that has high concentration of pollutants.
It uses the concentration gradients phenomenon or the opposite which is reverse osmosis. There are different types of membranes that are used for mine water treatment including: ultrafiltration, nano-filtration, reverse osmosis, microfiltration and particle filtration [ 19 , 25 , 26 ]. South Africa is well endowed by mineral reserves and this has triggered its immense dependence on mineral resources for gross domestic product and economy.
However, the legacy of coal and gold mining has left in its wake serious environmental problems. The major problem is acid mine drainage. Acid mine drainage AMD is formed from the hydro-geochemical weathering of sulphide-bearing rocks pyrite, arsenopyrite and marcasite in contact with water and oxygen [ 23 , 27 ].
This reaction is also catalysed by iron Fe and sulphur-oxidising microorganisms [ 28 , 29 ]. In a nutshell, the formation of AMD can be summarised as follows [ 19 , 23 , 30 , 31 ]:. Either these reactions can occur spontaneously or can be catalysed by microorganisms sulphur- and iron-oxidising bacteria that derive energy from the oxidation reaction [ 26 ]. The ferric cations produced can also oxidise additional pyrite into ferrous ions:.
Because of the high acidity and elevated concentration of toxic and hazardous metals, AMD has been a prime issue of environmental concern that has globally raised public concern [ 33 ].
The discharge of metalliferous drainage from mining activities has rendered the environment unfit to foster life [ 22 ]. Pragmatic approaches need to be developed to counter for this mining legacy that is perpetually degrading the environment and its precious resources [ 21 ].
Researches and piloted studies have indicated that active and passive approaches can be successfully adopted to treat acid mine drainage and remove potentially toxic chemical species [ 23 , 31 ]. Metalloids of As and earth alkali metal Ca and Mg are also present in significant levels [ 33 ]. Several studies have shown the feasibility of treating acid mine drainage to acceptable levels as prescribed by different water quality guidelines, but the resultant sludge has been an issue of public concern due to its heterogeneous and complex nature loaded with metal species [ 23 , 34 ].
Based on that evidence, research studies have been firmly embedded on the recovery of valuable minerals from AMD [ 19 , 23 ]. There are several mechanisms used for the recovery of chemical components from AMD including: precipitation [ 35 ], adsorption [ 36 ], biosorption [ 24 ], ion exchange [ 19 , 25 , 26 ], desalination [ 37 ] and membrane filtration [ 38 , 39 ].
Out of those techniques, precipitation has been the promising technology due to the ability to handle large volumes of water with very little dosage [ 35 ]. Adsorption and ion exchange have a challenge of poor efficiency at elevated concentrations and quick rate of saturation. Membrane technologies have the problem of generating brine that creates another environmental liability. Desalination has a problem of producing salts that has impurities, hence making them unsuitable for utilisation. Freeze desalination has been the promising technology, but it has never been tried in a large scale [ 19 , 23 , 34 ].
This is evident from the massive number of mines found around the country. Zinc can be a danger to unborn and newborn children.
When their mothers have absorbed large concentrations of zinc the children may be exposed to it through blood or milk of their mothers.
The world's zinc production is still rising. This basically means that more and more zinc ends up in the environment. Water is polluted with zinc, due to the presence of large quantities of zinc in the wastewater of industrial plants. This wastewater is not purified satisfactory.
One of the consequences is that rivers are depositing zinc-polluted sludge on their banks. Zinc may also increase the acidity of waters. Some fish can accumulate zinc in their bodies, when they live in zinc-contaminated waterways. When zinc enters the bodies of these fish it is able to bio magnify up the food chain.
Large quantities of zinc can be found in soils. When the soils of farmland are polluted with zinc, animals will absorb concentrations that are damaging to their health. Water-soluble zinc that is located in soils can contaminate groundwater. Zinc cannot only be a threat to cattle, but also to plant species. Plants often have a zinc uptake that their systems cannot handle, due to the accumulation of zinc in soils.
On zinc-rich soils only a limited number of plants has a chance of survival. That is why there is not much plant diversity near zinc-disposing factories.
Due to the effects upon plants zinc is a serious threat to the productions of farmlands. Despite of this zinc-containing manures are still applied. Finally, zinc can interrupt the activity in soils, as it negatively influences the activity of microrganisms and earthworms.
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