Introduction:Humans have constantly been exposed to heavy metals for thousands of years through the use of medical, industrial and technological applications. Although the harmful health effects of heavy metals have been for millenniums, individuals continue to increasingly expose themselves to heavy metals.
Since exposure to heavy metals is growing in some first world countries, such as the United States and Canada, this prompts us to question how third world countries would possibly be able to cope against this kind of exposure to heavy metals. The adequate protection and restoration of ground and water ecosystems contaminated by heavy metals require remediation processes which aid in reducing health and ecological risks. What is considered a heavy metal?A heavy metal contaminant is defined as any metallic element which possesses dense properties and demonstrates the ability to be toxic and/or poisonous at minimal concentrations (Lenntech, 2017). The term heavy metal is considered very broad, but generally includes metals with an atomic number greater than 20 and excludes alkaline earth metals, alkali metals, lanthanides and actinides (Lenntech, 2016). They typically have low ionization energies, low electron affinities and a low electronegativity which is known based on the general placement of heavy metals on the periodic table. Where are heavy metals sourced from?Often, heavy metals are naturally occurring elements which are sourced from the Earth’s lithosphere in various concentrations and cannot be destroyed (Järup, 2003).
Usually, they are emitted from natural sources considered as terrestrial, marine, volcanic, forest fires, erosion and surface winds. They are also leachates from mines and landfills as well as leakage from industrial processes such as electroplating (Järup, 2003). Most heavy metals are sourced due to anthropogenic reasons. They are introduced through three different methods which include deposition of atmospheric particulates, disposal of metal enriched sewage and emissions from fossil fuel combustion, metal mining and petroleum production (Shawai, 2017).Why are heavy metals so detrimental to the environment?Mining and metallurgical practices have led to heavy metal water contamination (Shawai, 2017). The environmental risk occurs from the immobility of these metals in sediments and soils at the bottom of bodies of water (Shawai, 2017). Geochemical weathering processes and erosion which acts upon these pollutants distribute these toxic substances to follow environmental pathways and forces them to accumulate into environmental sinks (Shawai, 2017). This can damage certain environmental mechanisms which support the ecosystem equilibrium (Shawai, 2017).
Heavy metals can infiltrate a body of water due to industrial waste, or from the disintegration of soil due to acidic rain, releasing heavy metals into streams, lakes, rivers, and groundwater (Lenntech, 2016). Metals are non biodegradable and cannot be broken down into less harmful constituents in the environment (Shawai, 2017).As these metals dissociate into ions, the ionic form of a metal is more toxic due to the charges associated with them. When metals are in ionic form, they possess the ability to attract tform toxic compounds with other dissociated ions.Figure 1.0: Summary of basic information on common heavy metals found in drinking waterHeavy MetalsWater Quality Standard (Virginia Cooperative Extension, 2011)Specific SourcesArsenicZero (MCLG)30.
010 ppm (MCL)2Pesticides, wood preservatives (Shawai, 2017).Unfiltered water sources and certain raw foods (Järup, 2003). Cadmium0.005 ppm (MCL and MCLG)2,3Rechargeable batteries and cigarettes (Järup, 2003).Paints and pigments, electroplating incineration, and phosphate fertilizers (Shawai, 2017)Chromium1.3 ppm (MCL and MCLG)2,3; 1.0 ppm (SMCL)4Metal alloys and pigments for paints, cement, paper, rubber, and other materials (Järup, 2003).Results from treating animal skin to produce leather, steel industries and coal combustion (Shawai, 2017).
Mercury0.002 ppm (MCL and MCLG)2,3Result from gold or silver mining, coal combustion and medical waste (Shawai, 2017).Fish such as tuna, bass and walleye (Järup, 2003).LeadZero (MCL)20.
015 ppm (Action Level)1Aerial emission from the combustion of petrol, created as a result from the manufacturing of batteries, and herbicides (Shawai, 2017).Drinking water, food, air, soil and dust from old paint containing lead (Järup, 2003).1Action Level: A level at which, when detected in drinking water systems, triggers immediate treatment. If more than 10% of tap water samples exceed the action level, steps must be taken quick to reduce the level (Virginia Cooperative Extension, 2011).
2(MCL) Maximum Contaminant Level: The highest level of a contaminant that is allowed in drinking water (Virginia Cooperative Extension, 2011).3(MCLG) Maximum Contaminant Level Goal: The level of a contaminant in drinking water where there is no risk to health (Virginia Cooperative Extension, 2011).4(SMCL) Secondary Maximum Contaminant Level: Guidelines regulating contaminants that can cause cosmetic effects or aesthetic effects. Non-enforceable (Virginia Cooperative Extension, 2011).What are potential negative effects to aquatic ecosystems?http://www.infomine.com/library/publications/docs/Mining.
com/Apr2008c.pdf https://www.researchgate.net/publication/265849316_Effects_of_Heavy_Metals_on_Soil_Plants_Human_Health_and_Aquatic_Life http://scialert.net/fulltext/?doi=jest.2014.
1.15 Why is heavy metal contamination considered a danger to human health?Heavy metals have adverse effects on human health. Many heavy metals are toxic, enabling them to cause undesirable effects and severe problems at very low concentrations. Heavy metals can result in various health problems to living organisms, especially humans. Additionally, some can act as carcinogens depending on the heavy metal concerned, as well as taking into consideration the concentration that an individual is exposed to it. Figure 3 describes the negative impacts of some of the most significant heavy metals. How does the overall aesthetics of water change?Most heavy metals do not negatively impact the odour or taste of drinking water itself in a noticeable manner, however, there are a few particular heavy metals discussed previously that have issues related to aesthetics (Virginia Cooperative Extension, 2011). This includes problems such as leaving stains on fixtures, linens and clothes when they are washed, or altering the taste or odour of the water itself (Virginia Cooperative Extension, 2011).
Specifically, when the water contains lead ions, it has been acknowledged to leave dull gray stains on tiles, laundry, tubs and sinks (Virginia Cooperative Extension, 2011). Lead’s presence in water can also leave a metallic or abnormal taste in comparison to what is perceived as normal (Virginia Cooperative Extension, 2011). On the other hand, when there are concentrations of copper higher than 1 milligram per liter, the resulting consequences includes blue-green stains on sinks, tubs and tiles (Virginia Cooperative Extension, 2011). Also, it is reported that heavy metals often leave water to be slightly cloudy. This cloudiness is due to turbidity: the presence of fine solid particles in water (Sedifilt, 2011). These particles are inorganic minerals that do not dissolve and cause the scattering of light rays, which gives the water a cloudy appearance (Sedifilt, 2011).
Any potential remediation methods?There any many ways to approach the remediation of heavy metal contamination in water.Plants:Phytoremediation refers to the use of plants and soil microbes to reduce concentrations of contaminants in the environment. It is cost-effective, efficient and eco-friendly, as well as a solar driven remediation method (Shawai, 2017).
Plants are selected for phytoremediation based of their ability to evapotranspirate groundwater (ground evaporation and plant transpiration from the Earth’s land and ocean surface to the atmosphere), the enzymes they produce, their growth and yield rates and their ability to bioaccumulate contaminants (Shawai, 2017).Phytoextraction is the most useful phytoremediation technique for removal of heavy metals from sediments, oils and/or water. The efficiency of phytoextraction depends on factors such as the quantity of contaminant in soil and the properties of soil (Shawai, 2017).
Plants suitable must follow the following criteria: easy collection and cultivation, resistance to pathogens, high growth rate, repulsion to herbivores to avoid bioamplification, ability to tolerate the toxic effects of heavy metals and good adaptation to environmental conditions (Shawai, 2017). Phytovolatilization is described as the process in which plants use contaminants from the soil and release the contaminants in a volatile state through transpiration, which contains very minimal concentrations of the heavy metal (Shawai, 2017).Precipitating out the heavy metal: In the removal of heavy metals, several processes are used. First, metals are concentrated in an ion exchanger and once the capacity is reached, it is transferred to a wastewater treatment plant where appropriate reactants are added and the pH is adjusted (Harbauer, 2016). These treatments cause the dissolved metals precipitate out (Harbauer, 2016).
The solution undergoes a different treatment in the ion exchanger where any remaining ions are retained and captured through flocculation and precipitation (Harbauer, 2016).Specifically, precipitation is initiated by adding a treatment reagent such as a hydroxide, sulfide or carbonate to the contaminated water, which forms insoluble heavy metal hydroxide, sulfide or carbonate precipitates (Porex Filtration Group, 2017). Many hydroxides reach minimum solubility at a specific pH for each metal. This pH can be adjusted to minimum solubility and this can aid in the precipitation process. Figure 4: Theoretical Solubility of Metal Hydroxides vs. pH If heavy metals in household water is a concern, installing an acid neutralizing filter will add calcium carbonate or magnesium oxide to the water.
Pipes should also be flushed for 5 minutes which can help remove the metals leached into the water.If this does not help, distillation, which involves boiling water and collecting the steam, is best suited for small quantities. Reverse osmosis involves forcing water molecules through a semipermeable membrane, which works effectively with a large amount of water.
Activated carbon filters will gather the heavy metals in the water and create a layer. This is the most efficient method for household removal of heavy metals, but filters must be changed regularly.