3. A laboratory studying the reductive dechlorination of dissolved aqueous phase

Question

3. A laboratory studying the reductive dechlorination of dissolved aqueous phase PCE (tetrachloroethene, see figure below) reports anaerobic degradation rate constants of 0.021/day and 0.017/day for the first and second reactions in the series, respectively. Assuming negligible trans-1,2-DCE formation, an initial PCE concentration of 0.112 mg/L, and an experimental volume of 1L, answer the following questions: How long did it take for the PCE concentration to fall below its MCL of 0.005 mg/L? What was the maximum concentration of TCE during the experiment? How long did TCE remain above its MCL of O.005 mg/L? After how many days were both PCE and TCE below their respective MCLs? (Hint: It might help if you plot the concentrations of PCE, TCE, and cis-1,2-DCE with time) (a) (b) (c) trans-DCE cI CI CI PCE TCE CI CI cis-DCE The sequential reductive dechlorination of PCE under anaerobic conditions

Explanation / Answer

In Given question

PCE---K1-->TCE ---K2---->cis-DCE

This reaction is a sequential reaction, first PCE changes into TCE having rate constant (K1) 0.021/day with first order kinetics as indicated by unit of rate constant, K1 .

a) K1 =(2.303/t ) *log (initial conc./final conc.)

0.021 =(2.303/t)log(0.112/0.005)

t=(2.303/0.021)log22.4

t=109.6*1.35=148days

After this TCE changes in to DCE with first order kinetics having K2=0.017/day

b) max. Conc. Of TCE =initial conc. Of PCE*(K1/K2)^x , where x=K2/(K2-K1)

max. Conc. Of TCE=0.112*(0.021/0.017)x

here x=0.017/(0.017-0.021)

x=-4.25

Using this ,we get,

max. Conc. Of TCE=0.045mg/l

Now , we have to calculate time

t=( 2.303/K2 )log 0.045/0.005

=(2.303/0.017)*0.96

=130days

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