3. Chemical precipitation can be an effective means of removing toxic metals fro

Question

3. Chemical precipitation can be an effective means of removing toxic metals from groundwater. In this process, the groundwater is pumped into a tank and a soluble compound is added to form a precipitate of the toxic ion. The precipitate is allowed to settle. It is then separated from the water and disposed of properly, leaving the water essentially free of the contaminant Opponents of fracking (hydraulic fracturing) claim that it contaminates groundwater with metals such as barium. The EPA has published safe drinking water levels for barium of 2 x 10 g/L. In a particular well, the barium concentration was found to be 3 x 103 g/L. You are tasked with removing the barium from the well water. You have your choice of the following salts for chemical precipitation. Which salt would be the most effective salt to use considering cost and the introduction of other ions into the solution? Justify your response. Salt Molar mass Cost (Sper Precipitate sp 1000 kg) $12-16 $4-6 $ 8-10 (g/mol) 40.00 NaOH Na2CO3 105.99 Na2SO4 142.04 Ba(OH)2 BaCO3 BaSO4 5.0 x 10-3 1.6 x 109 1.5 x 109

Explanation / Answer

The three salts given for the precipitation of Ba are NaOH, Na2CO3, Na2SO4

From the data given

For NaOH,

When NaOH is added to the solution Ba(OH)2 precipitate is formed which has a solubility product of 5*10-3. This solubility factor is higher than the other cases. Moreover the cost per 1000kg is 12-16$ which is also higher compared to other two salts. Therefore NaOH cannot be used for the precipitation of Barium.

Form the solubility prodcut data

Concentration of Ba = cube root((5*10-3)/4) = 1.07* 10-1 M (higher than the permiscible value of the Barium concentration)

For Na2CO3 and Na2SO4

Among these two salts,Na2CO3 is preferred over the Na2SO4 because of the two reasons mentioned below.

1) The cost per 1000kg of Na2CO3 salt ($4-6) is less compared to cost of 1000kg salt of Na2SO4 ($8-10)

2) Moreover from the solubility product data,

Ba concentration in BaCO3 = 4*10-5M

Ba concentration in BaSO4 = 3.8*10-5M

There is a negligible difference between the two concentrations. Hence the cost factor dominates and Na2CO3 is considered to be the best salt for removing the Barium from the solution.

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