Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 178

where
K O

= total oxidation rate constant [day −1 ]

[ RO 2 ] = concentration of oxidant [moles l −1 ]

k Oij = second-order oxidation rate constant for the polluting chemical as species

i in phase j [l mole −1 day −1 ]

The individual rate constant can be specified as independent of temperature,
with activation energy constants set to 0. If it is necessary to determine rates as a
function of temperature based on the Arrhenius function, then nonzero activation
energies, specified as constants, will invoke the following calculation for each rate
constant k :

k ( T K ) = k ( T R ) exp [1000 E ao ( T K - T R )/( RT K T R )] (4.82)

where E ao is the Arrhenius activation energy for the oxidation reaction [kcal mole −1 K −1 ].
Activation energies can be specified for each ionic species simulated.

4.2.4.6 Photolysis

The basic equation for direct photolysis is of the form

dC / dt = - k p C (4.83)

where C is the concentration of organic chemical being photolyzed and k p is the rate
constant for direct photolysis.
The first-order rate constant can be estimated directly as

k p = 2.303 J −1 Φ Σ I Λ e Λ (4.84)

where
k p = rate constant for direct photolysis [s −1 ]

J = conversion constant [6.02 × 10 20 ]

Φ = quantum yield [number of moles of chemical reacted/number of einsteins

absorbed]

I Λ = sunlight intensity at wavelength Λ [photons cm −2 s −1 ]

e Λ = molar absorbtivity or molar extinction coefficient at wavelength Λ [molarity −1

cm −1 ]

An einstein is the unit of light on a molar basis (a quantum or photon is the unit of
light on a molecular basis). The quantum yield may be thought of as the efficiency
of photoreaction. Incoming radiation is measured in units of energy per unit area
per time (e.g., cal cm −2 s −1 ). The incident light in units of einsteins cm −1 s −1 nm −1
can be converted to watts cm −2 nm −1 by multiplying with the wavelength (nm) and
3.03 × 10 39 .