Production of Photo-oxidants by Dissolved Organic Matter During
UV Water Treatment
Yaal Lester,
†
Charles M. Sharpless,
‡
Hadas Mamane,
§
and Karl G. Linden*
,†
†
Department of Civil, Environmental, and Architectural Engineering, University of Colorado, UCB 428, Boulder, Colorado 80309,
United States
‡
Department of Chemistry, University of Mary Washington, Fredericksburg, Virginia 22401, United States
§
School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University, Tel Aviv, 69978, Israel
* S Supporting Information
ABSTRACT: Dissolved organic matter (DOM) irradiated by sunlight
generates photo-oxidants that can accelerate organic contaminant
degradation in surface waters. However, the significance of this process
to contaminant removal during engineered UV water treatment has not
been demonstrated, partly due to a lack of suitable methods in the deep
UV range. This work expands methods previously established to detect
1
O
2
, HO•,H
2
O
2
, and DOM triplet states (
3
DOM*) at solar wavelengths
to irradiation at 254 nm, typical of UV water treatment. For transient
intermediates, the methods include a photostable probe combined with
selective scavengers. Quantum yields for
1
O
2
,
3
DOM* and H
2
O
2
were in
the same range as for solar-driven reactions but were an order of
magnitude higher for HO•, which other experiments indicate is due to
H
2
O
2
reduction. With the quantum yields, the degradation of metoxuron
was successfully predicted in a DOM solution irradiated at 254 nm. Further modeling showed that the contribution of DOM
sensitization to organic contaminant removal during UV treatment should be significant only at high UV fluence, characteristic of
advanced oxidation processes. Of the reactive species studied,
3
DOM* is predicted to have the greatest general influence on UV
degradation of contaminants.
■
INTRODUCTION
Ultraviolet irradiation (UV) is an established tool for the
treatment of drinking and wastewater, effective for inactivating
human pathogens
1
and for degrading various organic
contaminants (e.g., pharmaceuticals).
2
UV is an attractive
physical alternative to traditional chemical oxidants (i.e.,
chlorine and chloramine) that often produce disinfection
byproducts.
A contaminant may be degraded by direct UV photolysis
when it absorbs light and decomposes unimolecularly or reacts
with dissolved oxygen. However, direct photolysis is only
effective when the contaminant absorption spectrum overlaps
the emission spectrum of the UV lamp and when the direct
photolysis quantum yield is reasonably large. This is often not
the case, and in these instances indirect photolysis is typically
more important. Water constituents such as nitrate and
dissolved organic matter (DOM) can act as photosensitizers
to produce reactive species such as singlet oxygen (
1
O
2
) and
hydroxyl radical (HO•). In the case of DOM sensitization,
absorption of light leads to the formation of triplet excited
states (
3
DOM*), themselves a potent oxidant for many aquatic
contaminants
3
that react with O
2
via energy or electron
transfer mechanisms to generate reactive oxygen species such as
1
O
2
, HO•, and H
2
O
2
.
4-6
DOM photochemistry at solar wavelengths (λ > 300 nm) has
been investigated for several years,
7-11
and the photo-oxidants
are known to enhance organic contaminant transformation. For
example, Gerecke et al.
12
attributed the photochemical
elimination of numerous phenylurea herbicides from surface
water to reaction with
3
DOM*, while others have demonstrated
the involvement of
1
O
2
and HO• in the elimination of different
contaminants.
13
With UV-C radiation (200-280 nm),
commonly used in water treatment, the efficiency of photo-
oxidant generation by DOM is presently unknown. This is
primarily because available detection methods were developed
for use with longer wavelengths, and many of the probe
chemicals cannot be used (or require technical refinements)
due to their direct degradation by UV-C radiation. As a result,
the concentration of these species in deep UV systems and
their contribution to UV degradation of water contaminants has
not been previously evaluated.
The goals of this study were to validate methods for
determining the quantum yields of photo-oxidants (•OH,
1
O
2
,
3
DOM*, and H
2
O
2
) generated by DOM under UV-C radiation
Received: June 28, 2013
Revised: September 5, 2013
Accepted: September 6, 2013
Published: September 6, 2013
Article
pubs.acs.org/est
© 2013 American Chemical Society 11726 dx.doi.org/10.1021/es402879x | Environ. Sci. Technol. 2013, 47, 11726-11733