Characterizing Optical and Chemical Properties of Dissolved Organic Matter under Changing Environments
Ruosha Zeng is a PhD student in the Department of Water Resources. (Co)Promotors are dr.ir. S. Salama, prof.dr. D. van der Wal and dr.ing. M.J.M. Penning de Vries from the Faculty ITC.
Organic matter is the largest reservoir of carbon-based compounds on Earth. It provides nutrients and energy for microbial and plant growth, determining soil fertility and aquatic productivity, thereby impacting climate and ecosystems. The transformation of organic matter between land, water, the atmosphere, and the biosphere maintains the balance of the global carbon cycle. In aquatic environments, dissolved organic matter (DOM) transforms into dissolved inorganic carbon (DIC) through photomineralization and decomposition, thereby regulating carbon dioxide levels from water to the atmosphere. The subset of DOM that absorbs solar radiation, also named chromophoric DOM (CDOM), affects the penetration of solar radiation into the water column, influencing water color, photosynthesis, and overall aquatic ecosystem health.
DOM in aquatic environments is a complex mixture of organic compounds derived from various sources, such as terrestrial vegetation, algae, and microbial activity. Further study on the behavior of DOM is needed to assess water quality, track its sources and sinks, and comprehend its role in biogeochemical cycles and the impacts of climate change on aquatic ecosystems. Understanding the mechanisms and dynamics of DOM requires the integration of chemical, physical, biological, and geological processes. Its characteristics are influenced by environmental factors like pH, temperature, salinity, oxygen concentration, light, and other dissolved substances. Additionally, the composition and properties of DOM vary across different water bodies and change over time due to factors like seasonality, hydrological events, and human activities. Among the main environmental factors, solar radiation drives various photochemical reactions affecting DOM, and the abundant presence of iron in water further influences these light-induced processes, impacting DOM. The thesis investigates the influence of visible light and ferric ions on the complex chemical degradation process of DOM and elucidates its impact on the spectrometric properties.
The characteristics and monitoring of DOM utilize various techniques such as spectroscopy, colorimetry, elemental analysis, and remote sensing over different scales. In this thesis, we opt for a multiscale interdisciplinary approach that combines chemical analyses with spectroscopic and colorimetric measurements. Through laboratory-controlled experiments and multiple chemical analyses, this thesis explores DOM’s optical properties and their correlation with its chemical structure. The results contribute to water quality monitoring, remote sensing, and citizen observations of DOM. In pursuit of investigating how initial material composition and environmental conditions influence the transformation process of organic matter and the optical properties of CDOM, we conducted laboratory-scale controlled experiments to investigate the variations in their chemical composition and optical properties of representative CDOM precursors. We analyzed the key spectrometric parameters, such as the spectral slope in the 275–295 nm range (S275–295), spectral slope ratio (SR), and absorbance ratios, and correlated these with CDOM’s chemical composition indices like the hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios. The dissertation consists of six chapters:
The first chapter introduces the research background, highlighting the importance of understanding DOM in aquatic systems. It outlines the research objectives and the challenges associated with DOM research, emphasizing the need for an interdisciplinary approach.
Chapter 2 discusses spectroscopic parameters and mathematical tools, such as Gaussian fitting and derivative analysis, to quantify CDOM’s absorption properties and for better discrimination of organic components. This chapter also explores the absorption spectra of CDOM precursors and their potential application in tracing CDOM sources.
Chapter 3 focuses on the impact of visible light on CDOM degradation. It presents results from controlled experiments that investigate the photochemical degradation process and its correlation with spectroscopic parameters. Visible light enhanced the degradation of lignin and humic acid but not cellulose. Spectroscopic analyses revealed that photodegradation produces smaller organic molecules with decreased aromaticity, particularly in lignin and humic acid, affecting their absorption properties. Despite the limitations of the setup of controlled experiments, these findings improve our understanding of carbon transformation and the spectroscopic characterization of CDOM in aquatic environments.
Chapter 4 explores the joint influence of iron ions and light on CDOM degradation and its optical properties. It discusses how iron affects the transformation processes (hydrolysis and photolysis) of CDOM. Our experiments revealed that forming Fe-CDOM complexes without light exposure generally inhibits the degradation of CDOM. When light is involved, iron plays multiple roles, i.e., as a coagulant, a catalyst, and a quenching agent. The differences observed under light and non-light conditions may primarily be due to the formation of Fe-CDOM complexes affecting the absorption properties of CDOM. Our spectroscopic analyses revealed this phenomenon. Adding ferric ions in photodegradation generally increases CDOM’s light absorption and promotes energy transfer from light to chemicals.
In Chapter 5, the focus shifts to citizen science-based water color observation. The chapter details the design of a low-cost digital colorimetry setup for water color monitoring and introduces methods to correct biases from varying light conditions and camera settings. It explores using color indices to quantify optical active constituents (OACs), especially CDOM, and their correlation with concentrations, suggesting potential applications of digital colorimetry for monitoring water quality.
The final chapter summarizes the key findings and contributions of the thesis. The findings contribute to a better understanding of biogeochemical cycles in aquatic systems within the context of climate change, particularly how solar radiation changes affect these cycles. It emphasizes the importance of understanding DOM’s role in aquatic systems, biogeochemical cycles, and climate change impacts. The chapter highlights the potential applications of DOM research and the need for further interdisciplinary efforts in this field.
