Recitation 2

Kinetics

 

Problem:

Arsenic-rich water discharges from a spring at El Tatio geyser field at a rate of 9.8 m3 min-1 and is used as irrigation water for several villages in the Rio Loa valley. Aqueous arsenic can exist in two redox states, the reduced As+3 species (arsenite) and  the oxidized As+5 species (arsenate). Microorganisms living in the spring water oxidize the arsenite to arsenate which happens to be 100 times less toxic to humans.

                                                    As(III)   g As(V)

Assume: An outflow stream cross-section of 0.5 m2; constant temperature of 85 C; homogenous system; constant flow.

(a) What is the reaction order?

(b) What is the rate constant (k) of the microbial oxidation of arsenic.

(c) What is the half-life (t1/2) of arsenite?

(d) How far downstream from the spring will you need to go to find most of the arsenite converted to arsenate (such that the concentration of As(III) is less than about 100 ppb)? Should a farmer living 1 km away from the spring have any arsenite in his/her irrigation waters?

Be sure to include the following 3 plots and to explain why they might be important:

            * reactants vs time

            * semi-log plot of reactants vs time

            * inverse of reactants (1/[As]) vs time

Arsenic speciation data:

Sample

Distance along flow path (m)

total As  (ppm)

As(III)  (ppm)

TAT05-001

1

 36.54

35

TAT05-002

103

 36.27

17.68

TAT05-003

197

 35.81

8.79

TAT05-004

398

 36.56

2.37

TAT05-005

612

 37.01

0.54

TAT05-006

722

 37.13

0.27

 

(d) Actually the temperature is not a constant 85C, the water cools rapidly. What is the general relationship between temperature and reaction rate? Is it a linear relationship? And what about microbial enzymatic reactions, how might the relationship between reaction rate and temperature look?

(e) What is the relationship between products (in this case As(V)) and time? What order with respect to the production of As(V) is the reaction?   If you need a hint, peruse Snoeyink and Jenkins.

 

Solutions can be turned in at the end of class or emailed to me here jlandrum@mail.utexas.edu remember to include Recitation 2 and your name in the subject line.