inorganic chemistry

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responsible for their higher solubility in water at all pH values.
Humic acids, being more aromatic, become insoluble when the
carboxylate groups are protonated at low pH values. This struc-
ture allows the humic materials to function as chelating ligands
with the ability to bind various metal centers( 4 ). The effective
distribution of metal ion affinities for binding to a HS measured
under typical freshwater conditions showed that the three
groups of cations could be distinguished: (a) Al, H, Pb, Hg, and
Cr, which are preferentially bound to the phenolic sites of the
fulvic ligand; (b) Ca, Mg, Cd, Fe(II), and Mn, which display a
greater effective affinity for carboxylic sites; and (c) Fe(III), Cu,
Zn, and Ni, for which phenolic and carboxylic distributions
overlap ( 5 ).
Complexation in combination with detergent and colloidal pro-
perties, make humic and fulvic materials, effective agents to
transport both inorganic and organic contaminants in the envi-
ronment. Organic contaminants that are associated with HS
are essentially unavailable for uptake by biota, but these natural
organic acids can enhance or retard the photochemical decompo-
sition of waste or toxic organics. Consequently, dissolved organic
matter (DOM) can either enhance or inhibit the rate of
photodegradation of organic contaminants in water basins and
thus the effect of HS on photodegradation of organic con-
taminants on soil surfaces is broadly investigated( 4 ).
The HS found in most natural waters absorb solar light in the
range 300–500 nm, and upon laser (355 nm) excitation three pri-
mary transient species were detected: triplet state (^3 HS),
hydrated electron (ehydr), and radical cation (HSþ):


HS•+ + e−hydr

(^1) HS
(^3) HS
hn *HS ð 6 Þ
The results of steady-state irradiation suggested, however, that
hydrated electrons are mostly trapped by molecular oxygen and
their role in sunlight-irradiated natural surface waters is likely
to be minor. Thus, the main species responsible for the photoin-
duced degradation of aquatic pollutants are triplet excited states
of HS. The triplet state lifetime was estimated as longer than
2 ms with an energy of 170 kJ mol^1 ( 6 ). These parameters are
adequate to generate the excited singlet O 2 state (^1 Dgor^1 Sg),
first of which is relatively low lying (94 kJ mol^1 ) and thus can
be easily populated by the energy transfer from the triplet states
METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 297

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