Environmental Engineering Reference
In-Depth Information
upper surface waters, and UV-B radiation inhibited bacterial production by
39-82 % in a high mountain lake (Carrillo et al.
2002
). Therefore, microbial
activity may be less efficient during the photoinduced degradation of DOM in
natural waters.
7 Scope of Future Challenges
Photoinduced and microbial degradation of DOM is an important research subject
in the photochemistry of the aquatic environment. Till now, a few researches have
been conducted on the degradation of bulk DOM in natural waters. Photoinduced
and microbial degradation of organic substances such as standard fulvic acid,
humic acid, fluorescent whitening agents (DAS1 and DSBP) and chlorophyll was
seldom conducted to examine their end photoproducts in the aquatic environ-
ments. For a better elucidation of the degradation processes of DOM in natural
waters, it is vital to conduct an extensive study on streams, rivers, lakes, coastal
and seawaters. The photolytically produced low molecular weight (LMW) organic
substances are microbiologically important, but in some cases they might be toxic.
Till now only a few researches have been conducted to identify the LMW organic
compounds in the aquatic environment. The effects of water temperature and pH
on the coupled photoinduced and microbial degradation of DOM and of standard
organic compounds have not been studied so far. Obviously, variation in water
temperature and pH might have a significant role on degradation processes and on
the concentration levels of the end photoproducts. Therefore, a number of impor-
tant research needs for future challenges can be distinguished as: (i) Photoinduced
and microbial degradation of various molecular size fractions of DOM for a vari-
ety of waters. (ii) An extensive study on the microbial degradation of DOM for a
variety of waters, and the development of the mechanism for microbial degrada-
tion of DOM. (iii) Effect of temperature and pH on photoinduced and microbial
degradation of DOM for a variety of waters and for standard organic substances.
(iv) Interactions between photoinduced and microbial degradation of DOM,
and its impact on microorganisms in the aquatic ecosystem. (v) Investigation on
LMW organic substances produced from photoinduced and microbial degrada-
tion of DOM in natural waters and from standard HMW organic substances.
(vi) Elucidation of the microbiological changes involving the macromolecules
(fulvic acid, humic acid and autochthonous fulvic acid) under dark incubation.
(vii) Understanding of the mechanisms of the photoinduced degradation of the
macromolecules (fulvic acid, humic acid, and autochthonous fulvic acid) by natu-
ral sunlight in aqueous media. The mechanism depicted in this chapter may pave
the way for future directions in the field. (viii) Refinement of the group contribu-
tion method (GCM) to predict HO
•
reaction rate constants. Because of the limited
data availability for the rate constants of various compounds in the aqueous phase,
the GCM still has many gaps and its implementation should be the focus for future
research (Minakata et al.
2009
).
