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The term heavy metals refer to any metallic chemical element characterized by their relatively high density and potential to be toxic at low concentrations1. Some heavy metals include mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), and lead (Pb).
Other heavy metals, such as copper (Cu), selenium (Se), and zinc (Zn), are referred to as trace elements, which are necessary contributors to various metabolic pathways within the human body, making small amounts of these chemical elements essential for survival.
Heavy metals possess certain useful physical and chemical properties, which have been utilized throughout history for various consumer, industrial, and even medical products.
The Bioaccumulation of Heavy Metals
As natural components of the Earth’s crust, heavy metals cannot be degraded or destroyed, often allowing these metals to bioaccumulate within the environment. Bioaccumulation is an increase in the concentration of a chemical within a biological organism over time, compared to the chemical’s natural concentration within the environment1.
The bioaccumulation of heavy metals in solid waste is particularly concerning in developing countries, where electronics composed of heavy metals are improperly disposed of.
Due to their resistance to natural degradation processes that organic chemicals undergo, heavy metal concentrations exceed acceptable amounts within soil.
As municipal solid waste is often applied to agricultural soils as manure, the presence of these toxic metals within the environment becomes more concerning as they enter human and animal consumption pathways.
Preventing the Effects of Heavy Metals
In an effort to prevent the inevitable acute and chronic effects upon exposure to heavy metals within the environment and food supply, risk analysis investigators often conduct assessments of the concentrations within landfills, dumpsites, and other areas containing municipal solid waste. Researchers use a variety of ways to collect, extract, preserve and store samples collected from municipal waste sites.
Following collection, the United States Environmental Protection Agency (EPA) has over 200 accepted preparative and analytical techniques available for scientific use, each of which are specific to the metal(s) of interest in an extraction.
For example, total mercury within solid waste can be determined by USEPA SW-846 Method 7471A, which involves acid digestion by sulfuric acid and nitric acid to 0.5-0.6 g of the untreated sample. 5 mL of a saturated permanganate solution is added to the sample, followed by a 15-minute period of autoclaving. The cooled, diluted solution is mixed with 100 mL of water and 6 mL of sodium chloride-hydroxylamine sulfate solution in order to reduce excess permanganate2.
Using AAS and ICP-MS to Measure Metal Concentrations
While sample preparation for each chemical extraction varies significantly from each other, atomic absorption spectrophotometry (AAS) and inductively coupled plasma-mass spectroscopy (ICP-MS) are often utilized in order to determine the final metal concentrations within solid waste. AAS depends on a process known as atomic emission, where a solution containing metal ions is placed in a flame, producing a color that is characteristic of the metal ion of interest.
For example, a sodium solution produces a yellow color, while a potassium solution produces a violet color3. Within the flame, atomization occurs, converting metal ions into atoms in either an excited or ground state. AAS utilizes a light beam to excite found state atoms within the flame, measuring the light absorption as the metal’s concentration within the sample.
ICP-MS is a similar analytical technique used for elemental determinations, where a high temperature ICP source is combined with a mass spectrometer for increased detection capability. In contrast to the principle of an AAS, the ICP source within an ICP-MS converts atoms within the sample to ions, which are later separated and detected by the mass spectrometer4.
Measurements obtained from AAS and ICP-MS procedures are often used to confirm elevated metals present within a municipal solid waste site and other areas of concern. Despite the prevalence of available regulatory measures, there is a clear need for continued long-term monitoring of heavy metal concentrations within landfills and other municipal solid waste sites.
The Threat of Landfill Leachates
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Landfill leachates, an aqueous solution that forms as waste decomposes, is of increasing concern as well, as this runoff solution can contain harmful metals and other waste products from within landfills5. Due to this formation of leachates, it is also important to investigate how the solubility of metals and relating metal compounds within the leachate can pose hazardous health effects upon exposure. As production of electronic devices and other products containing heavy metals continues to rise, it may be beneficial for federal regulatory agencies to reconsider acceptable values for heavy metals within the environment, particularly within solid waste.
References and Further Reading
- Heavy Metals
- Method 7471B - Mercury in Solid or Semisolid Waste
- AAS Theory
- What Is ICP-MS?
- The Fate of Heavy Metals in Landfills: A Review