“…The copper ligands must be well-defined structurally and chemically, and also numerous because NOM has a high sorption capacity and selectivity for Cu(II) over a large concentration range (Gao et al, 1997;Covelo et al, 2004). Since carboxylate moieties comprise the large majority of reactive sites below pH 7 in NOM, and can form a vast number of synthetic coordination complexes with Cu(II) (Melnik et al, 1998a(Melnik et al, ,b, 1999, the strongest bonds are expected to be with carboxyl ligands (Sposito et al, 1979;Boyd et al, 1981). The predominance of oxygen ligands does not exclude other electron donors from being involved in Cu bonding, such as nitrogen as suggested by electron spin resonance (ESR) spectroscopy (Boyd et al, 1983;Senesi and Sposito, 1984;Senesi et al, 1985;Luster et al, 1996).…”