The role of copper in aquatic photochemistry of dissolved organic matter

Dissolved organic matter (DOM) is a complex and heterogeneous mixture of organic compounds ubiquitous in the aquatic environment. The aquatic photochemistry of DOM is an important mechanism for driving biogeochemical element cycling and for the attenuation of organic contaminants in natural sunlit waters. However, the impact of trace metals on the photochemical processes of DOM remains poorly understood. Trace amount of copper (Cu) (i.e. nanomolar concentrations), a prevalent redox active metal on earth, has been shown to significantly inhibit the DOM-photosensitized degradation of various electron-rich contaminants (e.g., phenols and amines) in our previous studies. The inhibition is not attributed to the quenching of photochemically produced reactive intermediates by Cu and the impact of Cu on the optical properties of DOM; rather, it is due to the reduction of oxidation intermediates of contaminants by the photo-produced Cu(I), resulting in the reconstruction of their parent compounds. This thesis further pursued this line of research and extended the Cu inhibition effect to the photodegradation of contaminants in seawater systems as well as the photooxidation of DOM itself. Also, Cu was found to act as a novel quencher to investigate the fundamental mechanism for the photobleaching and photooxidation of DOM from diverse sources in this thesis. In Chapter 2, the effect of nanomolar concentrations of Cu on 4-carboxybenzophenone (CBBP) and Suwannee River natural organic matter (SRNOM)-photosensitized degradation of seven contaminants including phenols and amines was investigated under conditions typical of estuarine and coastal systems. The results showed that strong Cu inhibition of the photosensitized degradation of all contaminants occurred even in high chloride solutions. The inhibitory effect of Cu was found to decrease with the increase in Cl‒ concentration due to the change in the Cu(I) speciation from reactive Cu(I)-DOM complexes to less reactive Cu(I)‒Cl complexes. In Chapter 3, nanomolar concentrations of Cu was also shown to remarkably reduce the photobleaching rates and the photo-induced loss of electron-donating moieties of three selected DOM isolates (including both terrestrial and microbially derived DOM) under simulated sunlight irradiation. By using ultra-high resolution mass spectrometry, the results indicated that Cu selectively inhibited the photooxidation of lignin- and tannin-like phenolic moieties present in DOM by reducing the phenoxyl radicals (i.e., DOMD•+) back to phenols. Based on the maximum inhibitory effect of Cu on the DOM photobleaching rate, the contribution of DOMD•+ to DOM photobleaching was calculated to be at least 29−55% in the wavelengths of 220−460 nm. In Chapter 4, the interaction between DOMD•+ and phenolic antioxidant moieties in DOM photooxidation was evaluated by using Cu as a competing quencher for DOMD•+ under varying pH (5.2‒10.0) conditions. A significant decrease in inhibitory effect of Cu on the DOM photobleaching and the loss of electron-donating moieties of DOM was observed with increase in pH, suggesting more pronounced competition for DOMD•+ quenching from antioxidant phenolic moieties within DOM. Employing the optimized conditions for Cu quenching method, the contribution of DOMD•+ to the photobleaching and photooxidation of DOM from a broad range of origins, such as soil, terrestrial aquatic and wastewater effluent, was determined. The results presented in this thesis have important implications for understanding the fate and transformation of organic contaminants and DOM in Cu-rich natural sunlit waters. Moreover, this is the first study in which a novel quencher (i.e. Cu) has been developed to determine the importance of DOMD•+ in DOM photobleaching and photooxidation. The insights gained from use of Cu as a competing quencher for DOMD•+ advance our understanding of the fundamental chemistry underlying this process.

Grantor dc:publisher

UNSW, Sydney

Year dc:date

2025

Author dc:creator

• Pan, Yanheng

• Aquatic photochemistry
• Dissolved organic matter
• Copper redox cycle
• Carbon cycle
• Photobleaching
• Long-distance charge transfer
• Phenolic moieties
• anzsrc-for: 4011 Environmental engineering
• anzsrc-for: 3703 Geochemistry
• anzsrc-for: 41 ENVIRONMENTAL SCIENCES

Statement dc:rights

• open access
• CC BY 4.0
• free_to_read

Licence

• https://purl.org/coar/access_right/c_abf2
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

en

Identifier

https://doi.org/10.26190/unsworks/31031

OAI identifier oai:identifier

oai:unsworks.library.unsw.edu.au:1959.4/104636