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fluorescens exhibited a much stronger affinity for the Na + ion compared to other
EPS that originated from S. putrefaciens and Clostridium sp. This finding allows
the hypothesis that the deprotonated carboxylic sites of EPS from P. fluorescens
are mostly bound to Na + in solution at pH 4 (Harper et al. 2008 ). The mechanism
behind the lower binding capacity at high ionic strength is probably that compl-
exation at one site will decrease the tendency of a neighboring functional group to
form an electrostatic complex with another metal ion. The increasing counter ion
condensation in the diffuse double layer of the macromolecule will also weaken
the affinity of the binding site (Stevenson et al. 1993 ; Bryan et al. 2002 ).
5.5 Effect of Photoinduced Processes
Photoinduced processes have a significant role in metal-DOM complexation in
natural waters (Wu et al. 2004a ; Bergquist and Blum 2007 ; Brooks et al. 2007 ;
Yang and Sturgeon 2009 ; Sanchez-Marin et al. 2010 ; Vidali et al. 2010 ; Yin et al.
2010 ; Zheng and Hintelmann 2009 ). The protective effect of DOM on Cu and Pb
toxicity greatly disappears when the samples are irradiated with high intensity
UV-light (Sanchez-Marin et al. 2010 ). After UV irradiation, the bulk DOC has
been found to decrease by between 60 and 75 %, whereas the decreases in fluo-
rescence and absorbance of CDOM range from 85 to 99 % (Sanchez-Marin et al.
2010 ). The capacity of humic acid to bind copper appears significantly reduced
for irradiated humic acid solutions in the pH range from 3 to 6 (Vidali et al. 2010 ).
The observed apparent convergence of the percentage of copper bound to humic
acid for photolytically unaltered and irradiated humic acid in the pH range from
6 to 7 is due to the precipitation of copper-soluble species and to the binding on
available ionized binding sites (Vidali et al. 2010 ).
The photoinduced reduction of ionic Hg in natural water can result in the pro-
duction of elemental Hg in presence of DOM, which is strongly affected by the
Hg-DOM interaction. The subsequent reoxidation of elemental mercury to ionic
mercury can occur in the presence of DOM in natural waters (Bergquist and Blum
2007 ; Zheng and Hintelmann 2009 ; Xiao et al. 1995 ; Costa and Liss 1999 ; Zhang
and Lindberg 2001 ; Ravichandran 2004 ). Photochemistry can thus affect loss by
volatilization and bioavailability of mercury to organisms (Ravichandran 2004 ).
Photoinduced degradation processes can significantly decrease the absorb-
ance (approximately 8-100 %) and fluorescence intensity of humic substances
(fulvic and humic acids) (up to 84 %), tryptophan (up to 88 %) and tyrosine
(0-100 %) upon irradiation for hours to months in natural waters (Brooks et al.
2007 ; Mostofa et al. 2011 ; Stedmon et al. 2007 ; Moran et al. 2000 ; Mostofa et al.
2007 , 2010 ; Skoog et al. 1996 ; Winter et al. 2007 ; Abboudi et al. 2008 ; Osburn et
al. 2009 ; Zhang et al. 2009 );Norman et al. 2011 Photoinduced degradation is able
to sequentially decompose the functional groups of DOM, particularly in macro-
molecular fulvic and humic acids. It is induced the formation of low molecular
weight photo-products, with simultaneous mineralization of dissolved organic
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