Probing Typical Chemical Active Organic Matters In Environments And Their Related Environmental Behaviours Using Molecular Spectroscopy

Chemically active organic matters, such as environmental electron mediators and dissolved organic matter (DOM), are widely distributed in environments, and participate in various physiochemical reactions. Probing the physiochemical characteristics of these organic matters and their related environmental behaviours is of vital importance for understanding the transformation/removal of environmental contaminants and designing new technologies for environment remediation. With molecular spectroscopic techniques and newly developed spectral data processing methods, this dissertation aims to investigate the physiochemical properties of chemically active organic matters in the environment and their related environmental behaviours. The main contents and results of this dissertation are listed below:1. Spectroelectrochemical investigation into the redox reaction mechanisms of environmental electron mediators. Electrochemical fluorescence and UV-Vis spectral methods were integrated to examine the redox behaviours of environmental electron mediators and their detection. First, by combining cyclic voltammetry, cyclic voltafluorometry, derivative cyclic voltafluorometry, and density functional theory calculations, the redox reactions of riboflavin in aprotic media were studied. The results show that the reduction contained a series of proton-coupled electron transfers dependent of potential driving force. Electron transfer-chemical reaction-electron transfer process and a disproportionation process were involved in the reduction. Second, a UV-Vis spectroelectrochemical method was developed to explore the redox behaviours of 1-hydroxyphenazine (1-OHPZ), a representative phenazine secreted by Pseudomonas aeruginosa. The redox reaction of 1-OHPZ in aqueous solution was a proton-coupled electron transfer process, with a reversible one-step 2e-/2H+ transfer reaction. Based on the spectral difference between the oxidized and reduced states of 1-OHPZ, a quantitative spectroelectrochemical detection method was developed. This spectroelectrochemical approach exhibited good linear response covering two magnitudes to 1-OHPZ with a detection limit of 0.48 μM, and was successfully applied to detect 1-OHPZ from a mixture of phenazines produced by Pseudomonas aeruginosa cultures. This method might also be applicable in exploring the redox processes of a wide range of other redox-active molecules in the environment.2. Two-dimensional correlation spectroscopic analysis on the interaction between humic acids (HA) and artificial TiO2 nanoparticles (NPs). Elucidation of the interaction between NPs and DOM can help to better understand the fates, features and environmental impacts of NPs. Two-dimensional (2D) Fourier transformation infrared (FTIR) correlation spectroscopy (COS) assisted by fluorescence excitation-emission matrix (EEM) method was used to explore the interaction mechanism of HA with TiO2 NPs at a molecular level. The results show that the C=O bonds (carboxylate, amide, quinone or ketone) and C-O bonds (phenol, aliphatic C-OH and polysaccharide) of HA played important roles in their interaction with TiO2 NPs. The adsorption process of HA onto the surface of TiO2 NPs was different from the bonding process of the two species in solution. The forms of the relevant groups in HA and their interaction with TiO2 NPs were affected by the solution pH and the surface charge of NPs. The 2D-FTIR-COS method was found to be able to construct a comprehensive picture about the NOM-TiO2 NPs interaction process.3. FTIR and synchronous fluorescence heterospectral two-dimensional correlation analyses on the binding characteristics of copper onto DOM. DOM is known to form strong complexes with heavy metals and thus governs the distribution, toxicity, bioavailability, and the ultimate fate of heavy metals in the environment. Due to the heterogenity of DOM structure, the interaction process between DOM and heavy metals remains unclear. With the integration of 2DCOS, synchronous fluorescence and infrared absorption spectroscopy, the binding process of copper to DOM was elucidated. A series of heterogeneous binding sites in HA and the subsequent subtle changes of these sites within the molecular interactions were elucidated by 2DCOS analysis. The band assignments and the correspondence between the results obtained by two spectral probes (synchronous fluorescence and infrared absorption spectra) were verified by hetero-2DCOS. The results show that the carboxyl and polysaccharide groups made the fastest responses to copper binding. Then, fluorescence quencing of humic-like moieties occurred with the vibrational change of phenolic and aryl carboxylic groups, which further induced the fluorescence quenching of fulvic-like fractions. Finally, small amounts of amide and aliphatic groups participated in the copper binding after the protein-like fraction. With these results, a comprehensive picture of structural changes of HA in the copper binding process was presented, highlighting the applicability of 2D-heterospectral correlation spectroscopy in studying complex interactions in the environment.4. Temperature-dependent conformational variation of DOM and its consequent interaction with phenanthrene. Temperature variation caused by climate change, seasonal variation and geographic locations affects the physicochemical compositions of DOM, resulting in difference in the fate of DOM-related species. Exploration into the thermal induced structural variation of DOM can help to better understand their environmental impacts. With the integration of EEM-PARAFAC analysis and synchronous fluorescence 2DCOS, an in-depth insight into the temperature-dependent conformational transitions of DOM was provided. The fluorescence components in DOM changed linearly to water temperature with different extents and temperature regions. The thermal induced transition priority in DOM was protein-likeâ†'fulvic-likeâ†'humic-like component. The transition point and the transition range varied with the origin of the DOM. The conjugation degree of the aromatic groups in fulvic- and humic-like substances, and the unfolding of the secondary structure in the protein-like substances with aromatic structure, were responsible for the conformation variation. Furthermore, the impact of thermal-induced structrual change of DOM on its hydrophobic interaction with phenanthrene was also studied. Such an integrated approach jointly enhances the characterization of conformational variation of DOM and provides a new way to elucidate the environmental behaviours of DOM.5. Exploration of membrane fouling via infrared attenuated total reflection mapping coupled with multivariate curve resolution. DOM is usually a main contributor to membrane fouling. Understanding of membrane fouling induced by DOM is of primary importance for developing effective fouling control and prevention strategies. Multivariate curve resolution-alternating least squares analysis was combined with infrared attenuated total reflection mapping to explore the fouling process of micro filtration and ultrafiltration membranes caused by two typical DOMs, HA and bovine serum albumin (BSA). The spectral contributions of different foulants and the membrane substrate were successfully discriminated, enabling the diagnosis of fouling origins. Membrane fouling caused by HA was more severe than that by BSA. Three periods, i.e., the initial adsorption stage, the equilibrium stage, and the accumulation stage, were observed during HA fouling. The integrated approach shows the spatial and temporal characterization of membrane fouling processes, and suggests a promising perspective for understanding the interaction mechanisms between foulant species and membranes at a molecular level.6. Stimulated Raman scattering (SRS) imaging of BSA/dextran induced membrane fouling. SRS imaging is an emerging imaging method with ultrafast, label-free and nondestructive characteristics. A SRS imaging platform for membrane fouling was established to study the fouling behaviours of simulated pollutants, i.e., BSA and dextran. The results show that the foulants were mainly aggregated on the membrane surface, and BSA foulants showed a larger particle size. The penetration depths for both foulants were within 10 μm, while BSA exhibited a slight deeper penetration depth. Furthermore, the mixture of BSA/dextran caused severer fouling compared with either BSA or dextran alone. The feasibility of using SRS imaging for membrane fouling study was well demonstrated. Since this vibrational imaging method is fast and label-free, and gives the chemical information of foulants and their concentration distribution and penetration depth, it offers new insights into membrane-based processes.