Question

You are the lead engineer at a bioremediation field site that is contaminated with 330 mg/L PCE, which has a molecular formula of C2Cl4. The other available electron acceptors are: oxygen at 8 mg/L, nitrate (NO3-) at 600 mg/L, Fe(OH)3 at 600 mmol/L, and sulfate (SO42-) at 250 mg/L. When these are reduced they become: water, NH4+, Fe2+, and S2-, respectively. PCE must be reduced completely to ethene (C2H4) or the local community group will protest. However, all of the other electron acceptors must be completely reduced for this to happen (i.e. everything needs to be completely reduced.) You are going to use ethanol (C2H6O) as the electron donor for all of these reactions, and it is oxidized to bicarbonate (HCO3-). What is the minimum concentration of ethanol you need to get the job done in mg/L ethanol?

Answer 1

Redox reactions involved-

Oxidation-half rxn

C2H6O +5H2O=2HCO3- + 14H+ + 12e

REDUCTION RXNS

1)C2Cl4 +2H2 +4e=C2H4 + 4Cl-

2)NO3- +10H+ +8e=NH4+ +3H2O

3)O2 +4H+ +4e=2H2O

4)Fe3+ +e=Fe2+

5)SO42- +10H+ +8e=H2S+4H2O

[PCE]=330 mg/L=0.330 g/L=0.330 g/(165.83g/mol)=0.00199 mol/L

Moles of electron =4*0.00199 mol/L=0.00796 mol/L

[NO3-]=600mg/L=0.6 g/L=0.6g/(62g/mol)=0.00968 mol/L

Moles of electron=8*0.00968 mol/L=0.07744 mol/L

[O2]=8mg/L=0.008 g/(32 g/mol)=0.00025 mol/L

Moles of electron=4*0.00025 mol/L=0.001 mol/L

[Fe3+]=600 mmol/L=0.6 mol/L

Moles of electron=0.6 mol/L

[SO42-]=250mg/L=0.250 g/(96 g/mol)=0.0026 mol/L

Moles of electron=8*0.0026 mol/L=0.0208 mol/L

Total moles of electron=(adding up)=0.7072 mol/L

So total moles of electrons required for reduction=0.7072 mol/L

1 mol ethanol produces 12 moles electrons ,so moles of ethanol required to produce 0.7072 mol of electrons=1/12*0.7072=0.05893 mol of ethanol per litre=0.05893 mol * 46 g/mol( molar mass of ethanol)=2.710 g/L =2.710 *1000=2710.78 mg/L of ethanol

Answer=2710.78 mg/L