Measurement and Photochemical Dynamics of Reactive Oxygen Species and Chromophoric Dissolved Organic Matter in Japanese Rivers

概要

Doctoral Thesis

Measurement and Photochemical Dynamics of Reactive
Oxygen Species and Chromophoric Dissolved Organic
Matter in Japanese Rivers

(Summary)

Taiwo Tolulope Ayeni
Graduate School of Biosphere Science
Hiroshima University
September 2021

Chapter one introduces this study in a concise manner. Just as their name depicts, reactive
oxygen species are very reactive in whatever environment they are found, whether in the
human body, plant tissues, or natural environments ranging from soils to natural waters like
rivers, lakes, coastal waters, seas, and oceans. The atmosphere is not left out as the impacts of
reactive oxygen species could result in the greenhouse effect which affects the whole world.
The present study is into natural water and river to be specific. Three reactive oxygen species
(ROS) comprising hydroxyl radical (•OH), nitric oxide radical (NO•), and singlet oxygen (1O2)
across rivers in Japan were investigated. In addition, the optical properties of the rivers
including chromophoric dissolved organic matter (CDOM) and fluorescent dissolved organic
matter (FDOM) were elucidated.
Chapter two discusses the concurrent measurement of the photoformation rates, steady-state
concentrations, and scavenging rates of the ROS across rivers ranging from small to large
rivers and relatively clean to polluted rivers. Nine rivers comprising of sixty-five stations
located along the west to the east axis of Japan were investigated. The absorption coefficient
at 300 nm (a300, m

–1

), which ranged from 2.44 to 36.2 m

–1

,

was used to investigate the

chromophoric dissolved organic matter (CDOM) properties of the rivers. The photoformation
rate ranges were (13.9–944) × 10 –12 M s –1 for •OH, (2.76–2610) × 10 –12 M s –1 for NO•, and
(9.48–133) × 10 –9 M s –1 for 1O2. The steady-state concentration ranges were (1.53–16) × 10
–16

M for •OH, (10.2–1520) × 10 –12 M for NO•, and (3.79–53.4) × 10 –14 M for 1O2. The results

showed that nitrite (NO2–) was a major source for both •OH and NO•, and CDOM was a major
source for 1O2 across all the rivers. According to significant relationships with these sources,
models were generated to predict the formation rates of the ROS (in M s

i

–1

) from known

concentrations of source compounds using the equations R•OH (10 –12) = 19.2 [NO2–]_µM + 36.9,
RNO• (10 –12) = 41.4 [NO2–]_µM + 44, and R1 O (10 –9) = 3.52 (a300)_m–1 + 1.61. Dissolved organic
2

matter (DOM), escape to the atmosphere, and water molecules were the major sinks for river
•OH, NO•, and 1O2, respectively. A general scavenging rate constant of •OH as a function of
the dissolved organic carbon (DOC) concentration was obtained [kC, OH = [(7.5 ± 6.8) × 108 L
(mol C)

–1

s –1]. These models would allow for easy prediction of ROS concentrations on a

large scale.
In Chapter Three, the optical properties of DOM in five rivers, including Ohta, Kurose, Yodo,
Yamato, and Kokubu, were investigated. In addition to this, their contributions to the
photogeneration of ROS were elucidated. The average DOC concentrations across the rivers
were the lowest (0.95 mg C L –1) in the Ohta, and the highest (2.85 mg C L –1) in the Yamato
and were correlated with some optical parameters. Absorption ratios such as the E2:E3, the
A280/A350, and the spectra slope, S275–295 showed that the Yodo and the Kokubu rivers had the
lowest and highest DOM molecular weights, respectively. PARAFAC modelling combined
with the excitation-emission matrices of the DOM in the rivers was used to characterize their
sources and fates. The PARAFAC analysis showed that the major components of DOM across
the rivers were the terrestrial humic-like (TH-L) and tryptophan-like (TP-L) substances.
Conversely, the Kokubu River contained other components like the fluorescent whitening
agent, autochthonous humic-like, and extracellular polymeric substances (EPS). The
significant relationships between the DOC versus the TH-L, TP-L components, and EPS
suggested that they were significant contributors to the total DOM in the rivers. On
investigating the ROS-generating abilities of these fluorescent components, TH-L and TP-L
components contributed significantly to ROS photoformation, whereas the Kokubu River

ii

components seemingly had no contribution, probably due to the few observations of such
components as reported in this study. The CDOM was observed to play a dual role in both
generating and scavenging the ROS. Comprehensive models for estimating the photoformation
rates of each ROS (in M s –1) were deduced by integrating contributions from their respective
major and minor sources. They include: R•OH (10 –12) = 21.0 [NO2–]_µM + 0.460 [TH-L]_QSU +
10.9; RNO• (10 –12) = 67.9 [NO2–]_µM + 35.2 [a300]_m–1 – 2.51 [TH-L]_QSU – 0.765 [TP-L]_QSU
– 8.14; and R1 O (10 –9 ) = 3.81 [a300]_m–1 – 0.101 [TP-L]_QSU + 11.1.
2

Finally, Chapter four discusses the major findings, conclusions, and possible
recommendations for future studies.

iii