You have a plume of hexavalent chromium as chromate (CrO_4^2-) that will be redu

Question

You have a plume of hexavalent chromium as chromate (CrO_4^2-) that will be reduced to insoluble Cr^3+. The molecular mass of chromate is 115.9 g/mol. The concentration is 238 ppm in water. How much zero valent iron (ZVI) will you need to add to completely transform the chromium. The ZVI will be converted to Fe(OH)_3. The molar mass of iron is 55.9 g/mol. Trichloroethylene (TCE, C_2HCl_3) can be reduced to cis-dichloroethylene (Cis-DCE, C_2H_2C1_2). If you have 34 mg/L TCE in solution how much ZVI will you need to reduce the TCE to cis-DCE? Assume the ZVI is transformed to Fe^2+. TCE molar mass is 131.5 g/mol. Will this work better in oxidizing or reducing environments? What happens in environments with oxygen (generally)?

Explanation / Answer

Basis: 1 litre of water.

Therefore,amount of Chromate ions present = 238 mg

Molar mass of Chromate = 115.9 g/mol

Moles of Chromate present = (238/1000)/115.9 = 0.00205 moles= 2.05 milli moles

Chromate reacts with ZVI as:

Fe0+ CrO42-+ 4 H2O Fe(OH)3 + Cr(OH)3 + 2 OH-

(insoluble Cr3+ is present as Cr(OH)3)

From reaction it is seen that 1 mole of Chromate will require one mole of ZVI.

Thus, 2.05 millimoles of Chromate will require = 2.05 milli moles of ZVI

Molar mass of ZVI = 55.09 g/ml

Mass of iron required = moles x molar mass = 0.00205 x 55.09 = 0.11293 gms

Amount of iron required = 0.11293 gms

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