Environmental Engineering Reference
In-Depth Information
4.7.1 Kinetics of the Photo-Fenton Reaction
The kinetics of the photo-Fenton reaction can be determined as a function of
pH, based on the yield of HO
formed per Fe(II) oxidized by H 2 O 2 , and con-
sidering the photoreactions of aqueous organic substrates (Zepp et al. 1992 ;
Hoigné et al. 1988 ). Under illumination with constant irradiance of a diluted
probe compound (P) that reacts with HO
(Eq. 4.36 ), the hydroxyl radical would
rapidly reach a steady-state concentration. In the presence of P and of other HO
scavengers (S), the decay of HO
can be expressed as follows (Eqs. 4.36 , 4.37 )
(Zepp et al. 1992 ):
K P , [ P ]
−→
(4.36)
HO + P
rection products
K S , [ S ]
−−−−−→
HO + S
scavenging products
(4.37)
where k p is the second-order rate constant (M -1 s -1 ) for the reaction of HO
with
the probe P, and ∑ k s [S] is the pseudo-first order rate constant (s -1 ) for HO
scav-
enging by all the components present in the reaction medium, except the probe
compound.
The scavenging rate of HO
can be expressed as (Eq. 4.38 ) (Zepp et al. 1992 ):
S] } [ HO
(4.38)
R S ={ K P [P] +
K S
If the concentration of P or the reaction rate for the P is sufficiently low (i.e.,
k s [S] » k P [P]), it is r S = (∑ k s [S])[HO
]. Under the steady-state condition the rate
of generation of HO
is r OH = r S , from which the hydroxyl radical concentration
becomes (Eq. 4..39 ) (Zepp et al. 1992 ):
HO
(4.39)
ss = r HO /
k s [S]
The oxidation rate (Ms -1 ) of the probe compound in an irradiated system (con-
version per unit time) is described as (Eq. 4.40 ) (Zepp et al. 1992 ):
d [P] / dt = k P
HO
ss [P] = k [P]
(4.40)
If the concentrations of the photoactive Fe(III) species, H 2 O 2 , and the scaven-
gers show a negligible variation as compared to [P], both r OH and ∑ k s [S] (and
[HO
] ss as a consequence) would be about constant. That would give a pseudo-
first order reaction with rate constant k . If the second-order rate constant, k P and
the scavenging rate constant, ∑ k s [S] are known, then r OH can be determined from
k by the following equation (Eq. 4.41 ) (Zepp et al. 1992 ):
r HO = k
k s [S]
/( k P )
(4.41)
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