Complexation of Dissolved Organic Matter with Trace Metal Ions in Natural Waters - Photobiogeochemistry of Organic Matter - page 768

the fluorescence quenching upon complexation between various DOM compo-
nents and metal ions.
The M-DOM interaction is generally estimated by determining the condi-
tional stability constant, which significantly depends on several important factors
in aqueous solution such as contents, nature and molecular size of DOM (Nair
and Chander 1983 ; Kim et al. 1990 ; Wu et al. 2004b ; Sonke and Salters 2006 ;
Shcherbina et al. 2007 ; Konstantinou et al. 2009 ; Vidali et al. 2009 ); effect of
pH (Zhang et al. 2009 , 2010 ; Ryan and Weber 1982a ; Sonke and Salters 2006 ;
Shcherbina et al. 2007 ; Christoforidis et al. 2010 ; Iskrenova-Tchoukova et al.
2010 ; Liu and Cai 2010 ); effect of ions (cations and anions) and ionic strength
(Sonke and Salters 2006 ; Christoforidis et al. 2010 ; Iskrenova-Tchoukova et
al. 2010 ; Smith and Martell 1987 ; Cabaniss and Shuman 1988 ; Fu et al. 2007 );
effect of photoinduced processes (Zhang et al. 2009 ; 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 ); effect of microbial processes
(Bergquist and Blum 2007 ; Yin et al. 2010 ; Tabak et al. 2005 ; Francis and Dodge
2008 ; Fletcher et al. 2010 ), and finally effects of freshwater and seawater (Wu
et al. 2004a , c ; Fu et al. 2007 ; Harper et al. 2008 ). M-DOM complexation plays
a vital role in the toxicity and bioavailability of heavy metals in the environment
(Christoforidis et al. 2010 ; Winner 1985 ; Stackhouse and Benson 1988 ; Jiang
et al. 2009 ). Optically darker DOM (or deep lake and marine DOM) has higher
metal binding capability and typically tends to decrease metal toxicity to fish
and algae in natural waters (Schwartz et al. 2004 ; Winch et al. 2002 ; Luider et al.
2004 ). Ternary complexes between organic ligands and trace metal ions and their
relative stability constants may pave the way to find out high variations in condi-
tional stability constants of binary complexes of DOM in aqueous solution (Martin
and Prados 1974 ; Khalil and Radalla 1998 ; Khalil and Attia 1999 , 2000 ; Khalil
2000a , b ; Khalil and Taha 2004 ; Khalil and Fazary 2004 ; Radalla 2010 ; Rosas et
al. 2010 ). A review by Hays et al. ( 2004 ) on the determination of conditional sta-
bility constants and ligand concentrations of fulvic acid with metal ions covers the
Stern-Volmer equation for non-linear relationship, the differences between linear
and non-linear relationship of formed complex and fluorescence quenching, and
finally the Modified Multisite Stern-Volmer Equation. A review by Mostofa et al.
( 2011 ) summarizes the studies into metal ion complexation with various organic
substances, and discusses about the possible mechanisms behind the quenching or
enhancement in metal-DOM complexation in waters.
This chapter will give a general overview on the complexation of metal ions (M)
with DOM (M-DOM), fluorescence characteristics of M-DOM complexes, theories for
binary and ternary complexes as well as their conditional stability constants in aqueous
solution, and finally the binding sites (or functional groups or flurophores or chromo-
phores) in various DOM components. A mechanism will be provided on M-DOM com-
plexation, with use of tryptophan, a well known molecularly characterized fluorescent
DOM component, as a model to understand the complexing properties. This review will
discuss the key factors affecting the M-DOM complexation and the importance of the
M-DOM complexation in waters.