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Predicting Precipitates

Demonstrates how solubility constants can be used to predict the formation of precipitates.

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Practice Predicting Precipitates

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Water Purification with Seashells

Water Purification with Seashells

Credit: Colored Pencil Magazine
Source: http://www.flickr.com/photos/coloredpencilmag/8805118262/

Natural sources of fresh water will generally contain a small amount of heavy metal cations, which have toxic effects on living systems. Some of this is due to leeching from natural deposits in the Earth’s crust, but contamination is significantly higher in urban areas where industrial wastewater is released back into the environment, especially in places where industrial regulation is low. Most affluent countries use high-tech treatment systems to remove these contaminants from drinking water, but these techniques are relatively expensive to implement. A low-cost alternative involves treating the water with pulverized seashells, causing the heavy metals to precipitate as solids that can be removed by a simple filtration.

News You Can Use

• Read about how seashells are being used to treat wastewater in the following article: http://www.nbcnews.com/id/30435640/#.UoFaKuKwW-M
• Toxic metals like mercury and lead will precipitate from these water-seashell mixtures as their carbonate salts.
• Although other metals like zinc and iron are necessary nutrients, they are toxic at high concentrations. These ions are also precipitated as carbonate salts.
• Large water treatment plants require heavy machinery and large facilities that only large cities can afford.
• Credit: Ron Cogswell
Source: http://www.flickr.com/photos/22711505@N05/6250411467/

A seaside water treatment plant in Boston, MA [Figure2]

• See how other precipitation reactions are used in more advanced water treatment facilities in the following video:

Show What You Know

With the links below, learn more about the solubility of various ionic compounds in water and how these values are related to the solubility product constant (Ksp). Then answer the following questions.

1. In order to bond with the heavy metal ions, some of the carbonate ions from the calcium carbonate (CaCO3) in the seashells must dissolve into the water. However, according to our solubility rules, calcium carbonate is an insoluble salt. Explain this apparent discrepancy.
2. The pKsp values for various salts can be found in one of the above links. The relationship between pKsp and Ksp is as follows:\begin{align*}pk_{sp} = -\log(k_{sp})\end{align*} Calculate Ksp values for CaCO3, HgCO3, and PbCO3.
3. Calculate the solubility of CaCO3, HgCO3, and PbCO3 in milligrams per liter of water. Explain why the insoluble salt CaCO3 can be used to precipitate Hg2+ and Pb2+ ions.
4. According to the EPA, the maximum allowable concentration of mercury in drinking water is 2 ppb (about \begin{align*}1 \times 10^{-8} \text{M}\end{align*}). What concentration of carbonate would be required to ensure that the concentration of Hg2+ is below this standard? Can this carbonate concentration be obtained from the dissolution of calcium carbonate?

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