inorganic chemistry

FeIIðÞedtaðÞH 2 O

hi 2 


þNO^ !½ŠFe edtaðÞðÞNO^2 
þH 2 O ð 14 Þ

is characterized by the extremely fast NO-uptake (rate constant
of 2.4 108 M
^1 s
^1 and high stability constant of 2.1 106 M
^1
at 25C(60,81– 87 )). The FeII(edta) complex also reacts fast with
dioxygen:

2 FeIIðÞedtaðÞH 2 O

hi 2 


þO 2 þH 2 O!2 FeIIIðÞedtaðÞH 2 O

hi


þH 2 O 2

ð 15 Þ

but the reaction is slower than that with NO (k¼320 M
^1 s
^1 at
25 C and pH 6 (88,89)). Moreover, its oxidized form, the
[FeIII(edta)(H 2 O)]
complex, is inactive toward NO. This means
that to ensure effective NO-uptake under atmospheric
conditions, the Fe(III) product should be rapidly reduced:

FeIIIðÞedtaðÞH 2 O

hi


þe
! FeIIðÞedtaðÞH 2 O

hi 2 


ð 16 Þ

and different chemical and biological methods for the reduction
of Fe(III) were proposed( 90 – 92 ).
To avoid any additional pollution by the oxidation products
and to ensure the low cost of the process, the BiodeNOx proce-
dure was proposed, which is a novel technique for NOxremoval
from industrial flue gases. In principle, BioDeNOx is based on
NO absorption into an aqueous [FeII(edta)(H 2 O)]^2
solution
(Eq. 14) followed by NO reduction and [FeII(edta)]^2
regeneration
by the denitrifying bacteria:

6 FeIIðÞedtaðÞNO

hi 2 


þC 2 H 5 OH!6 FeIIðÞedta

hi 2 

þ3N 2 þ2CO 2 þ3H 2 O

ð 17 Þ

The technical and economical feasibility of the BioDeNOx con-
cept is strongly determined by high rate biological regeneration
of the scrubber liquid and by edta degradation. Detailed studies
showed, however, a contradiction between the optimal redox
state of the aqueous [FeII(edta)]^2
solution for NO absorption
and the biological regeneration. [FeIII(edta)(H 2 O)]
reduction
was found to be too slow to keep all [Fe(edta)] in the reduced
state. Moreover, since flue gases generally contain oxygen, part
of the [FeII(edta)]^2
is oxidized to [FeIII(edta)]
(Eq. 15). There-
fore, not NO reduction but [FeIII(edta)]
reduction was found to
be the rate limiting factor of BioDeNOx reactors treating flue
gas that contains more than 1% oxygen (77,92).

METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 309