1,1,1-Trichloroethane (TCA, C 2 H 3 Cl 3 , MW = 133.4 g/mol) is chlorinated solv

Question

1,1,1-Trichloroethane (TCA, C 2 H 3 Cl 3 , MW = 133.4 g/mol) is chlorinated solvent that was widely used throughout the United States. The maximum contaminant level (MCL) in drinking water is set by the USEPA at 0.20 mg/L. A municipal water treatment plant has an intermittent problem with an excessive level of TCA with their water supply. They propose to treat the water using powdered activated carbon (PAC). The TCA level in the untreated water is 1.2 mg/L. The treatment process is designed to reduce the TCA level to 0.15 mg/L (to provide a margin of safety below the MCL). The sorption of TCA on PAC follows a Freundlich isotherm: q = 2.5 C 0.34 where C is the equilibrium dissolved TCA concentration in water (mg/L) and q is the equilibrium mass ratio of TCA sorbed on the PAC (mg/g). What dose of PAC is required to meet the treatment goal? Express your answer in g of PAC per L of water.

Explanation / Answer

For TCE removal, C0 = 1 mg/L................(i)

Maximum Contaminant level (MCL) = 0.2 mg/L..............(ii)

Say X (g) of PAC is used,

Removal required = (1-0.2) mg/L = 0.8 mg/L................(iii)

Adsorption capacity of PAC = (0.8 mg/L)*(1 liter)/[X (g)] = 0.8 mg/X (this is Qeq)............(iv)

Ceq is 0.2 mg/L........................................................(v)

(0.8/X) = 129 (0.2)0.73 ...............................................................(vi)

129 (0.2)0.73 = 27.87...............................................................(vii)

Therefore X(g) = 0.8/27.87 = 0.0287(g) ...................................(viii)

So, 0.0287 g/L PAC is required to treat one liter volume of water

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