Unraveling the photochemical behavior of dissolved organic matter derived from hydrothermal carbonization process water: Insights from molecular transformation and photoactive species

Hydrothermal carbonization process water (HTPW) has been utilized as a substitute for chemical fertilizers in agricultural applications. However, the input of HTPW into paddy water, particularly the significant proportion of dissolved organic matter (DOM) in HTPW (DOM-HTPW), directly engages in photochemical transformations, a phenomenon often overlooked. This study observed a consistent decrease in humification (SUVA280, 7.7-53.9%) and aromaticity (SUVA254, 6.1-40.0%) of DOM-HTPW after irradiation. The primary active photobleaching components of DOM-HTPW varied depending on the feedstock, such as protein for chicken manure DOM-HTPW and lignin for rice straw DOM-HTPW. The photochemical activity of DOM-HTPW was augmented by its lower molecular weight and higher hydrophilic composition, particularly evident in chicken manure DOM-HTPW, which exhibited higher generation rates for 1O2 (35.1-37.1%), 3DOM* (32.8-43.9%), and O2•- (28.6-48.8%) as measured by molecular probes. DOM-HTPW effectively facilitated the phototransformation of tetracycline, with the contribution of O2•- being more significant than 3DOM* and 1O2. These findings shed new light on the understanding the photochemical processes of DOM-HTPW as exogenous DOM and the interconnected fate of contaminants in aquatic environments.

Photochemical behavior of dissolved organic matter derived from Alternanthera philoxeroides hydrochar: Insights from molecular transformation and photochemically reactive intermediates

Hydrochar-derived dissolved organic matter (HDOM) enters aquatic ecosystems through soil leaching and surface runoff following the application of hydrochar. However, the photochemical behavior of HDOM remains unclear. The photo-transformation of HDOM was analyzed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), multiple spectroscopy methods, high-performance liquid chromatography, and combining synchronous fluorescence and Fourier-transform infrared spectroscopy with two-dimensional correlation spectroscopy. The results showed that with the increase of carbonization temperature, amide II in protein-like substances were observed to be preferentially photolyzed, and the protein-like substances were more sensitive to low irradiation time, while the duration time of the photochemical behavior of amide II and aliphatic C-H were more persistent. FT-ICR MS results showed that N and S-containing molecules, including lignins and lipids were more sensitive to ultraviolet irradiation. Furthermore, the photo-transformation of HDOMs was accompanied by the generation of triple excited state dissolved organic matter and singlet oxygen. Our findings will be beneficial for understanding the mechanisms of photo-transformation of HDOM and for predicting the possible behaviors of hydrochar produced at different temperatures before large-scale application.

DFT/TDDFT insights into effects of dissociation and metal complexation on photochemical behavior of enrofloxacin in water

Elucidation of the mechanisms underlying the effects of different dissociated forms and metal ion complexation on the photochemical behavior of antibiotics in aqueous media is a key problem and requires further research. We examined the mechanism of the direct photolysis of enrofloxacin (ENRO) in different dissociated forms in water and the impact of metal ions (Mg²⁺) on the photolysis of ENRO using density functional theory and time-dependent density functional theory. The results showed that different dissociated forms of ENRO exhibited diverse maximum electronic absorbance wavelengths (ENRO³⁺ (264 nm) < ENRO^- (278 nm) < ENRO⁰ (280 nm) < ENRO²⁺ (282 nm) < ENRO⁺ (306 nm)). The calculations of the reaction pathways and activation energies (E_a) in the photolysis of ENRO⁰/ENRO⁺/ENRO^- showed that defluorination was the main reaction pathway. The removal of cyclopropane was the main reaction pathway for the direct photolysis of ENRO²⁺/ENRO³⁺. Furthermore, the presence of Mg²⁺ was observed to change the order of the maximum electronic absorbance wavelengths and increases the intensities of the ENRO absorbance peaks. Calculations of the photolysis reaction pathways showed that the presence of Mg²⁺ increased the E_a for the most direct photolysis pathways of ENRO, while its presence decreased the E_a for several partial direct photolysis pathways such as the pathway in which the piperazine ring moiety of ENRO⁰/ENRO³⁺ is damaged and the pathway in which cyclopropane is released from ENRO³⁺. The findings on the photolysis behavior of ENRO in water system have provided useful information on the risk assessment of antibiotics.

Photochemical behavior of carbon nanotubes in natural waters: reactive oxygen species production and effects on •OH generation by Suwannee River fulvic acid, nitrate, and Fe (III)

The photochemical activities of three kinds of carbon nanotubes (CNTs) were investigated in the present study. Efficient procedures of dispersing the three kinds of carbon nanotubes in water were established, and the quantitative analysis methods were also developed by TOC-absorbance method. High pH value or low ionic strength of the colloidal solutions facilitated the dispersion of CNTs. The suspensions of three kinds of CNTs could generate singlet oxygen ((1)O2) and hydroxyl radical (•OH) under irradiation of simulated sunlight, while superoxide radical (O2 (•-)) was not detected. The steady-state concentrations of (1)O2 and •OH generated by these CNTs were also determined. The presence of CNTs in natural waters can affect the photochemical behavior of water constituents, such as nitrate, dissolved organic matter, and Fe(3+). Specifically, in nitrate solution, the presence of CNTs could inhibit the generation of •OH by nitrate through light screening effect, while the quenching effect of hydroxyl radicals by CNTs was not observed. Besides light screening effect, the three kinds of CNTs used in the experiments also have a strong inhibiting effect on the ability of DOM to produce •OH by binding to the active sites. Moreover, the adsorption of Fe(3+) on MWCNT-OH and MWCNT-COOH could lead to its inactivation of formation of •OH in acidic conditions. However, the presence of the three kinds of CNTs did not affect the ligand-to-metal charge transfer (LMCT) reaction of DOM-Fe (III) complex.