Question

Benzo[a] pyrene, or B[a]p, a polycyclic aromatic hydrocarbon, is measured in a wastewater lagoon 2.5 hr after a release at a concentration of 3ppb.

Half life of B[a]p is 1hr via direct sunlight exposure.

a.Based on its structure, explain why light exposure can affect B[a]p

b.If only direct photodegradation is the only degradation process occurring, what is the initial concentration of B[a]p in the lagoon? Assume a first order kinetics

Answer 1

a) Benzo[a]pyrene is a crystalline, aromatic hydrocarbon consisting of five fused benzene. Its molecular formula isC₂0H₁₂ its structure shows 10 double bonds.

(C1=CC=C2C3=C4C (=CC2=C1)C=CC5=C4C(=CC=C5)C=C3)

Light exposure can affect B[a]p, because the double bonds in the aromatic rings can absorb light and direct photo degradation of B[a]p take place.

b) Calculate k

t1/2 is independent of concentration for a first--order process.

t 1/2 = 0.693/k

k = 0.693/t1/2

k = 0.693/1 hr = 0.693/hr

Calculate the initial concentration of B[a]p in the lagoon-

t = 2.5 hr

C₀ = Ct e^-kt

C_{0 =} initial concentration

C_t = concentration after time t =3ppb.

Co = Ct e^ kt

C₀ = (3ppb) e^( 0.693/h )(2.5 )

      = 3ppb. x e^1.725

      = 3ppb. x 5.61 = 16.837 ppb

      = 17 ppb.

The initial concentration of B[a]p in the lagoon is 17 ppb.