Geoscience Reference
In-Depth Information
The increase in DOM fluorescence observed as pH increases can be partially attributed
to alkaline hydrolysis. Kumke et al. (
2001
) investigated the changes to a brown water Lake
Hochsee and to wastewater effluent upon hydrolysis with NaOH. An increase in fluores-
cence intensity monitored at 260 and at 330 nm excitation produced nearly a doubling
of DOM fluorescence emission intensity near 450 nm. This increase in fluorescence was
coincident with a decrease in molecular size. The smaller compounds produced by alkaline
hydrolysis had stronger ultraviolet (UV) absorption and thus excitation, but the authors
argue that hydrolysis may also disrupt the quenching activity.
More recent studies have focused on natural organic matter (NOM) rather than extracted
humic substances. Using sequential tangential ultrafiltration (STUF), NOM from the
Amazon basin was separated into particulate, colloidal, and dissolved fractions (Patel-
Sorrentino et al.,
2002
). A and C peak fluorescence intensity both increased with increas-
ing pH. The effect was similar for both “black water” and clear water Amazon basin rivers.
As noted previously, phenolic moiety deprotonation, tertiary structural changes (coiling
and uncoiling), and ionization were suggested as possible mechanisms for pH-induced
variations.
Although no concrete picture for how spectral properties vary with pH, applicable to
all environments and DOM substrates seems to be coming to light, it is clear that many
properties do vary such that caution should be applied when comparing samples - particu-
larly if pH ranges significantly. In natural waters, for example, an acidic river end member
could potentially deliver DOM whose fluorescent properties could significantly change
on mixing with the buffered seawater end member during estuarine mixing. Changes in
pH along with buffering and divalent metal-organic matter interactions (cf. Willey,
1984
)
offer the potential to introduce nonconservative spectral signals during estuarine mixing.
Several conceptual processes have been offered to explain DOM spectral changes with pH
changes at the molecular and colloidal level (see earlier). In addition, humic substances
could be a source for sorbing other fluorescent organic compounds (e.g., polycyclic aro-
matic hydrocarbons) in catchment areas with relatively low pH. This might allow estuarine
transport via molecular protection, particularly if aggregate particles are formed. Then, as
pH increases in estuaries, disaggregation could occur - releasing compounds and poten-
tially impacting estuarine bulk DOM optical properties. Although this may be a concen-
tration-dependent process (Avena and Wilkinson,
2002
), such pH-transitional regions are
most likely to exhibit the optical property changes described here.
7.6 Effect of Metals
Natural organic matter (NOM) interacts with metals in the environment primarily through
complexation with organic ligands. For example, metals such as Al and Fe released via
weathering reactions can be complexed to NOM and thus mobilized in surface waters,
increasing their dispersion from the continental to the marine environment. Toxic met-
als (e.g., Cu, Cd, Hg, Pb) may also serve as complexing constituents for NOM in solu-
tion, though metal toxicity may also be mitigated by ligand complexation (Boyd et al.,
