The Presence of and Leaching Processes Concerning Heavy Metal Contamination in Our Environment

       In our day to day lives, we are often exposed to heavy metals, such as mercury (Hg), cadmium (Cd), lead (Pb), arsenic (As) and copper, without even knowing it. Cadmium is present in rechargeable household batteries. Mercury lies within the dental amalgam of your teeth, as well as the fish that you eat. Lead can be found in many building materials, paints and household pipes. If you're a research scientist in a lab, involved in a mining project, work in a hazardous waste facility or are working for a chemical company, you yourself probably interact with these elements frequently. But what does their presence mean for us, both environmentally and health wise as a society? Several studies, including a review by the WHO, have found that when these heavy metals leach into the ground, for example via run off or leaks during storage and transportation, they can contaminate groundwater supplies and pose serious risks to human health.

Illustration 1: Example of how heavy metals can leach
 into our water and soil.

Often, once these heavy metals have become trapped in the soil, it becomes very difficult and expensive to remove them. These removal processes involve chemical and biochemical leaching, also known as “extraction”. Traditionally, acid treatments were an important component in leaching. Now, however, there are many modern techniques that are used in removing heavy metals from the soil and groundwater supplies. For example, in a 2001 study concerning the remediation of heavy metals in the environment, around 300 gallons (20 g/minute) of calcium polysulfide (CaSn2) was dispensed into a groundwater location through 114 injection ports, at a depth of up to 20 feet. Researchers saw that, after this had been done, the chromium concentration in the groundwater almost instantly declined. Other effective techniques include electro-kinetic leaching, which utilizes electric currents, and chemical leaching, which involves organic and inorganic acids.

     In chemical leaching, substances such as sulfuric and hydrochloric acid are added to contaminated areas in order to solubilize the heavy metals and further extract them from the soil. It was noted also in a study that iron chloride (FeCl3) was a cost effective and efficient compound to use for chemical leaching. It was also noted by a researcher in the same study that as, after 18 hours of leaching, they were able to extract about 78% of copper (Cu) using inorganic acids such as nitric (HNO3) acid.

Illustration 2: Example of heavy metal contamination in Australia.

The electro-kinetic leaching process is especially useful in diluting high concentrations of heavy metals, particularly lead, in substances. In a sample that was used in one study, there was an over 70% decrease in Pb concentrations. In electro-kinetic leaching, there is a two step method: first, there is water washing of the contaminated soil in order to remove soluble salts (such as KCl, NaCl, SiCl4, and CaClOH). Second, an electric current is used in order to balance the pH and stabilize the heavy metals into less reactive complexes.

Overall, there is a large amount of controversy surrounding heavy metals. They are utilized in almost every aspect of our modern lives, in one way or another, and yet they can be extremely dangerous if not handled properly. One way to handle this issue would be to ensure that people are very well aware of the dangerous nature of these metals, and know how to properly store and transport them. Another important aspect of this issue is to make sure that people involved in the chemical industry, whether they are scientists in the lab or working out in toxic waste landfills, are aware of safe and effective methods such as chemical and electro-kinetic leaching that can be used to properly detoxify and stabilize potentially life threatening heavy metals that are leaching into our soil.

References:

Jarup, Lars. "Hazards of Heavy Metal Contamination." Hazards of Heavy Metal Contamination. British Medical Bulletin, 2003. Web. 13 Feb. 2014.

Ukiwe, Lugard, Allinor J.I, Ejele A.E, Anyadiegwu C.I.C, & Ibeneme S.I. "Chemical and Biological Leaching Methods to Remove Heavy Metals from Sewage Sludge: A Review."JOURNAL OF ADVANCES IN CHEMISTRY. 4.3 (4): 509-517. Web. 13 Feb. 2014

Wing-Ping Liao1, Renbo Yang, Zheng-Xian Zhou1, ZJui-Yuan Huang. “Electrokinetic stabilization of heavy metals in MSWI fly ash after water washing.” Wiley Online Library, 9 Jan 2014. Web. 13 Feb. 2014.

Thomasser, R. and Rouse, J.V., 2001. “In-Situ Remediation of Chromium Contamination of Soil and

Groundwater”. United States Environmental Protection Agency (EPA), 1981, US EPA Chart Comparing Sulfide and Hydroxide Solubilities; EPA publication, EPA-600/2-82-OIIC.

Images courtesy of: treehugger.com (http://media.treehugger.com/assets/images/2011/10/environmental-heavy-metal-contamination.jpg) and the Australian Department of the Environment (http://www.environment.gov.au/archive/atmosphere/airquality/publications/report3/chapter2.html).