Quantifying interactions between propranolol and dissolved organic matter (DOM) from different sources using fluorescence spectroscopy

Influence of dissolved organic matter on the anaerobic biotransformation of roxarsone accompanying microbial community response

Roxarsone (ROX), commonly employed as a livestock feed additive, largely remains unmetabolized and is subsequently excreted via feces. ROX could cause serious environmental risks due to its rapid transformation and high mobility in the anaerobic subsurface environment. Dissolved organic matter (DOM) is an important constituent of fecal organics in livestock waste and could affect the ROX biotransformation. Nonetheless, the underlying mechanisms governing the interaction between DOM and ROX biotransformation have not yet been elucidated in the anaerobic environment. In this study, the changes of ROX, metabolites, and microbial biomass in the solutions with varying DOM concentrations (0, 50, 100, 200, and 400 mg/L) under anaerobic environments were investigated during the ROX (200 mg/L) degradation. EEM-PARAFAC and metagenomic sequencing were combined to identify the dynamic shifts of DOM components and the functional microbial populations responsible for ROX degradation. Results indicated that DOM facilitated the anaerobic biotransformation of ROX and 200 mg/L ROX could be degraded completely in 28 h. The tryptophan-like within DOM functioned as a carbon source to promote the growth of microorganisms, thus accelerating the degradation of ROX. The mixed microflora involved in ROX anaerobic degrading contained genes associated with arsenic metabolism (arsR, arsC, acr3, arsA, nfnB, and arsB), and arsR, arsC, acr3 exhibited high microbial diversity. Variations in DOM concentrations significantly impacted the population dynamics of microorganisms involved in arsenic metabolism (Proteiniclasticum, Exiguobacterium, Clostridium, Proteiniphilum, Alkaliphilus, and Corynebacterium spp.), which in turn affected the transformation of ROX and its derivatives. This study reveals the mechanism of ROX degradation influenced by the varying concentrations of DOM under anaerobic environments, which is important for the prevention of arsenic contamination with elevated levels of organic matter.

Effects of clarithromycin exposure on the growth of Microcystis aeruginosa and the production of algal dissolved organic matter

Antibiotics are commonly found in the aquatic environment, which can affect microbial community compositions and activities, and even have potential adverse impacts on human and ecosystem health. The current understanding of the effects of antibiotics on microalgae growth and algal dissolved organic matter (DOM) remains indistinct. To understand the toxic effects of antibiotics on the microalgae, Microcystis aeruginosa was exposed to clarithromycin (CLA) in this study. Cell density determination, chlorophyll content determination, and organic spectrum analysis were conducted to show the effect of CLA exposure on the growth, photosynthetic activity, and organic metabolic processes of Microcystis aeruginosa. The findings revealed that the physiological status of algae could be significantly influenced by CLA exposure in aquatic environments. Specifically, exposure to 1 μg/L CLA stimulated the growth and photosynthetic activity of algal cells. Conversely, CLA above 10 μg/L led to the inhibition of algal cell growth and photosynthesis. Notably, the inhibitory effects intensified with the increasing concentration of CLA. The molecular weight of DOM produced by Microcystis aeruginosa increased when exposed to CLA. Under the exposure of 60 μg/L CLA, a large number of algal cells ruptured and died, and the intracellular organic matter was released into the algal liquid. This resulted in an increase in high molecular weight substances and soluble microbial-like products in the DOM. Exposure to 1 and 10 μg/L CLA stimulated Microcystis aeruginosa to produce more humic acid-like substances, which may be a defense mechanism against CLA. The results were useful for assessing the effects of antibiotic pollution on the stability of the microalgae population and endogenous DOM characteristics in aquatic ecosystems.

Indirect photodegradation of sulfadimidine and sulfapyridine: Influence of CDOM components and main seawater factors

This study investigated the indirect photodegradation of sulfadimidine (SM₂) and sulfapyridine (SP) in the presence of chromophoric dissolved organic matter (CDOM), and studied the influences of main marine factors (salinity, pH, NO₃^- and HCO₃^-). Reactive intermediate (RI) trapping experiments demonstrated that triplet CDOM (³CDOM*) played a major role in the photodegradation of SM₂ with a 58% photolysis contribution, and the contributions to the photolysis of SP were 32%, 34% and 34% for ³CDOM*, hydroxyl radical (HO·) and singlet oxygen (¹O₂), respectively. Among the four CDOMs, JKHA, with the highest fluorescence efficiency, exhibited the fastest rate of SM₂ and SP photolysis. The CDOMs were composed of one autochthonous humus (C1) and two allochthonous humus (C2 and C3). C3, with the strongest fluorescence intensity, had the strongest capacity to generate RIs and accounted for approximately 22%, 11%, 9% and 38% of the total fluorescence intensity of SRHA, SRFA, SRNOM and JKHA, respectively, indicating the predominance of CDOM fluorescent components in the indirect photodegradation of SM₂ and SP. These results demonstrated the photolysis mechanism: The photosensitization of CDOM occurred after its fluorescence intensity decreased, and a large number of RIs (³CDOM*, HO· and ¹O₂, etc.) were generated by energy and electron transfer, then these RIs reacted with SM₂ and SP to cause photolysis. The increase in salinity stimulated the photolysis of SM₂ and SP consecutively. The photodegradation rate of SM₂ first increased and then decreased with increasing pH, whereas the photolysis of SP was remarkably promoted by high pH but remained stable at low pH. NO₃^- and HCO₃^- had little effect on the indirect photodegradation of SM₂ and SP. This research may contribute to a better understanding of the fate of SM₂ and SP in the ocean and provide new insights into the transformation of other sulfonamides (SAs) in marine ecological environments.

Study on the cadmium and copper binding characteristics of dissolved organic matter released from human-feces-biochar (HFDOM) using parallel factor analysis (PARAFAC) and two-dimensional correlation spectroscopy (2D-COS)

The benefits of applying manure directly compared to carbonized applications are controversial, and the impact of dissolved organic matter (DOM) released from human feces-based biochar (HFDOM) on the soil environment is not yet known. Comparing the properties of the HFDOM at different pyrolysis temperatures and its binding properties to heavy metals (HMs) can provide some valuable information for the application of human manure-based biochar for soil amendment or HM remediation in soils. The result of EEM-PARAFAC shows that HFDOM contains four components. A comparison of the maximum fluorescence intensity of HFDOM bound to HMs at different pyrolysis temperatures indicates that high pyrolysis temperatures reduce the amount of HFDOM. Upon comparative analysis of 2D-SFS-COS maps, cadmium (Cd(II)) preferentially binds to humic substances and fulvic-like substances at different pyrolysis temperatures (280 °C, 380 °C), respectively, while copper (Cu(II)) preferentially complexes with fulvic-like substances in both cases. The 2D-FTIR-COS graphs with different pyrolysis temperatures (280 °C, 380 °C) are observed, and Cd(II) binds preferentially to the C-H stretching (peak 3030 cm^-1) of olefins, the C-O stretching vibration, and the O-H bending vibration (peak 1231 cm^-1) of carboxyl groups, respectively, while Cu(II) binds predominantly to the C-O stretching of alcohols, ethers, and esters (peak 1084 cm^-1), the C = O stretch of the carboxylic acid (peak 1590 cm^-1), respectively. The variability of these findings suggests that the pyrolysis temperature exerts a significant influence on the functional groups in HFDOM. Furthermore, the complexation stability constant between Cu(II) and the components decreases as the pyrolysis temperature increases, probably because the pyrolysis temperature changes the content of HFDOM and the distribution of functional groups, but the assessment of the influence still needs to be further investigated.

Molecular linkages between chemodiversity and MCPA complexation behavior of dissolved organic matter in paddy soil: Effects of land conversion

Complexation of dissolved organic matter (DOM) plays a crucial role in regulating the fate and risk of agrochemicals. Here, taking a toxic herbicide MCPA (4-chloro-2- methylphenoxyacetic acid) as the target, the effect of land conversion on complexation behavior of DOM to agrochemicals was investigated in paddy soil. Furthermore, the mechanisms were explored in a new perspective of DOM chemodiversity. Soil DOMs were selected from four long-term cropping systems, including paddy field (PF), vegetable field (VF), rice-vegetable rotation (RV) and abandoned land (AL). The results showed that the DOMs in PF and AL were rich in hydrophilic substances (e.g., carbohydrates or protein-like molecules) with low aromaticity. However, after converting PF to VF and RV, abundant aromatic macromolecules and aliphatic alkanes were observed in DOM. Due to those changes in DOM chemodiversity, the binding site and capability of DOM were highest in VF and RV, and were positively correlated with DOM aromaticity, MW, humus and polar groups (e.g., amino). This was because the complexation of "DOM-MCPA" was static binding via ligand exchange and H-bonding among polar groups and hydrophobic interaction among aromatic skeletons. The EEM-PARAFAC confirmed that microbial humic-like substances dominated the complexation of DOM rather than terrestrial humic-like and tryptophan-like matters. The 2D-COS analysis further revealed that the complexation of DOM preferentially occurred in amino, polysaccharide C-O and aliphatic C-H for PF and AL, but in aromatic C=C, amide C=N for RV and VF. In summary, these findings provide molecular insight into the effect of land conversion on DOM complexation activity, which highlight the importance of DOM chemodiversity. These results will contribute to the risk assessments of agrochemicals in paddy soil.

Inferring Ecosystem Function from Dissolved Organic Matter Optical Properties: A Critical Review

Over the last 30 years, the optical property community has shifted from conducting dissolved organic matter (DOM) measurements on new complex mixtures in natural and engineered systems to furthering ecosystem understanding in the context of past, present, and future carbon (C) cycling regimes. However, the appropriate use of optical properties to understand DOM behavior in complex biogeochemical systems is of recent debate. This critical review provides an extensive survey of DOM optical property literature across atmospheric, marine, and terrestrial biospheres using a categorical approach that probes each biosphere and its subdivisions. Using this approach, a rubric of ecosystem variables, such as productive nature, C cycling rate, C inputs, and water quality, sets the foundation for interpreting commonly used optical property DOM metrics such as fluorescence index (FI), humification index (HIX), and specific ultraviolet absorbance at 254 nm (SUVA₂₅₄). Case studies and a meta-analysis of each biosphere and subdivision found substantial overlap and characteristic distributions corresponding to ecosystem context for FI, HIX, and SUVA₂₅₄, signifying chromophores and fluorophores from different ecosystems may be more similar than originally thought. This review challenges researchers to consider ecosystem connectivity when applying optical property results rather than making traditional "if this, then that" results-style conclusions.

Comparison of the binding interactions of 4-hydroxyphenylpyruvate dioxygenase inhibitor herbicides with humic acid: Insights from multispectroscopic techniques, DFT and 2D-COS-FTIR

4-Hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor is one of the important herbicides to solve the problem of weed control. With the widespread and continued use of HPPD inhibitor (HPPDi) herbicides, it may inevitably put pressure on the environment. Humic acid (HA) can effectively interact with pesticides through sorption or covalent bond formation and promote the degradation of pesticides, which can reduce the risk of pesticides in the environment. In the present study, the interactions of four HPPDi herbicides (sulcotrione, tembotrione, topramezone and mesotrione) with HA were reported and comparative assessment of the binding using multispectral technology, density functional theory (DFT) calculation and two-dimensional correlation spectroscopy (2D-COS). Time-resolved measurements and the Stern-Volmer constant at different temperature verified that HPPDi can bind with HA through the static quenching mechanism. From the thermodynamic parameters, the interaction force between HA and sulcotrione, tembotrione, topramezone and mesotrione was provided by electrostatic force. DFT, binding constant and three-dimensional (3D) fluorescence peak variation all indicated that the order of the binding ability of the four HPPDi and HA was mesotrione > tembotrione > sulcotrione > topramezone. According to dynamic light scattering (DLS), pH 7 is most conducive to the formation of HA-HPPDi complexes. Fourier transform infrared spectroscopy (FTIR) and 2D-COS showed that HA combined with HPPDi through aromatic C-H, CO and C-X, and the first binding group to HA was almost all CO. Sulcotrione, tembotrione, topramezone and mesotrione quench the endogenous fluorescence of HA by a static quenching mechanism and bind to HA through electrostatic interaction to form a complex. These results provide important insights into the combination of environmental pollutants with HA.

New insights into the interactions between humic acid and three neonicotinoid pesticides, with multiple spectroscopy technologies, two-dimensional correlation spectroscopy analysis and density functional theory

The widespread use of neonicotinoid pesticides in agricultural production has caused pressure on the environment. In the present work, the interactions between humic acid (HA) and three neonicotinoid insecticides, dinotefuran, clothianidin and nitenpyram, were investigated by using multiple spectroscopy techniques combined with two-dimensional correlation spectroscopy analysis and density functional theory (DFT). Dinotefuran, clothianidin and nitenpyram could quench the endogenous fluorescence of HA through a static quenching process dominated by hydrogen bonds and van der Waals forces. According to the revised Stern-Volmer equation and DFT calculation, the binding abilities of the three pesticides with HA were ranked as dinotefuran < clothianidin < nitenpyram. The results of dynamic light scattering showed that neutral conditions were more conducive to the combination of HA and dinotefuran, clothianidin and nitenpyram. Through Fourier transform infrared spectroscopy (FTIR) combined with two-dimensional correlation analysis (2D-COS), the functional group with the strongest binding ability in the HA-dinotefuran, HA-clothianidin and HA-nitenpyram system was CH, CO and CO, respectively. The work will help to further understand the behavior of neonicotinoid pesticides in the environment.

Characteristic of dissolved organic matter polar fractions with variable sources by spectrum technologies: Chemical properties and interaction with phenoxy herbicide

Dissolved organic matter (DOM) is ubiquitous with high biological and chemical activity. The large intake of DOM from compost, plant residues or soil can modify the behaviors of agrochemicals. Phenoxy herbicide is the third widely used herbicide around the world with both aromaticity and polarity. However, how the diverse fractions of DOM interacting with phenoxy herbicide and the underlying mechanisms remain unknown. Thus, it is crucial to investigate the heterogeneous chemical properties of DOM fractions from variable sources and explore the interactive mechanisms. In this study, polar DOM derived from compost, rice straw and soil were fractionated, and the chemical properties of fractions were analyzed by spectrum technology and the complex interaction with phenoxy herbicide was assessed by infrared spectroscopy. Results showed that hydrophobic acid (HOA) was the largest component (49.6%) in compost DOM, while hydrophilic matter (HIM) was the main component in the polar DOM from rice straw and soil. The 4-chloro-2-methylphenoxyac etic acid (MCPA) as one representative of phenoxy herbicides was used in our study, and the results showed the interaction between different DOM fractions and MCPA was heterogeneous. HOA containing abundant fulvic-like component and polar groups resulted a greatly complex interaction with MCPA mainly via hydrophobic force, ligand exchange and hydrogen bonding. Hydrophobic neutral fraction and acid-insoluble matter showed a medium interaction with MCPA as a result of enrichment with the high aromatic humic-like molecules. Inversely, no significant interaction between HIM and MCPA was observed. Our research revealed that the aromatic framework associated with polar groups in DOM dominated the interaction with phenoxy herbicide, which might affect the bioavailability, toxicity, and mobility of phenoxy herbicide.

Investigation of eluted characteristics of fulvic acids using differential spectroscopy combined with Gaussian deconvolution and spectral indices

The characteristics of subfractions of soil fulvic acid (FA₃, FA₅, FA₇, FA₉, and FA₁₃) using stepwise elution from XAD-8 resin with pyrophosphate buffers were investigated by differential absorption spectroscopy (DAS) and differential fluorescence spectroscopy (DFS) combined with mathematical deconvolution and spectral indices. The log-transformed absorbance spectra (LTAS) exhibited three regions for both acidic-buffer-eluted subfractions (AESF) and neutral-buffer-eluted subfraction (NESF) and four regions for basic-buffer-eluted subfractions (BESF) according to the differences in spectral slopes. The DAS spectra of FA subfractions were closely fitted with seven Gaussian bands with maxima location at 199.66, 216.18 ± 1.50, 246.20 ± 3.85, 285.22 ± 7.26, 345.64 ± 5.30, 389.27, and 307.37 nm, respectively (R² > 0.993). The content of salicylic-like and carboxyl groups in FA subfractions decreased, while the phenolic chromophore increased with elution sequence. From the 11 spectral indices, AESF had greater molecular weight, condensation, polymerization, hydroxyl radical production, humification degree, and terrigenous contribution, as well as contained more conjugated aromatic structures and less N-containing groups than NESF and BESF. The humification degree and humic characters of FA subfractions were closely associated to the aromaticity, molecular condensation, and DOM-metal-bound functional groups. The proper separation of FA into subfractions is beneficial for reducing its complexity and heterogeneity, which helps us to further explore its chemical properties and interactions with various contaminants in soil environments. Graphical abstract.

Interactions between Imidacloprid and Thiamethoxam and Dissolved Organic Matter Characterized by Two-Dimensional Correlation Spectroscopy Analysis, Molecular Modeling, and Density Functional Theory Calculations

The heavy application of neonicotinoid insecticides in agricultural production has burdened the environment. In the present study, interactions of two neonicotinoid insecticides imidacloprid and thiamethoxam with dissolved organic matter (DOM) were investigated by spectroscopic techniques, molecular modeling, and density functional theory (DFT) calculations. The static mechanism of imidacloprid and thiamethoxam quenching the endogenous fluorescence of DOM was assessed through time-resolved analyses. During the binding process, a protein-like substance binds imidacloprid and thiamethoxam later than a humic-like substance, as analyzed by two-dimensional correlation spectroscopy, but more strongly than the humic-like substance, as suggested by molecular modeling and DFT calculations. The conformational changes of DOM are attributed to imidacloprid and thiamethoxam, as assessed with three-dimensional spectra. Fourier transform infrared spectroscopy indicated that DOM binds imidacloprid and thiamethoxam by hydroxyl, aliphatic C-H, amide I, and carboxyl to form stable DOM-imidacloprid and DOM-thiamethoxam complexes. Understanding the changes in the structural conformation of humic-like and protein-like substances with imidacloprid and thiamethoxam helps further understand the fate of the neonicotinoids in the environment.

Fluorescence regional integration combined with parallel factor analysis to quantify fluorescencent spectra for dissolved organic matter released from manure biochars

Dissolved Organic Matter (DOM) in biochars is important to carbon dynamics and contaminant transport in soils. Fluorescence excitation-emission matrices (EEMs) have been widely used to characterize dissolved organic matter (DOM). In this study, fluorescence regional integration (FRI) and parallel factor analysis (PARAFAC) applied to EEM allows good quantitative assessment of the composition of DOM derived from manure biochars. Manure biochars were produced using four types of manure, chicken, pig, cow, and sheep manure under various pyrolysis temperatures (300–600 °C) and holding times (0–120 min). The results from the determination of dissolved organic carbon (DOC), SUVA₂₅₄, and humification index (HIX) reflected that high pyrolysis temperature and long holding time led to a significant decrease in DOM quantity, aromaticity and humification. The FRI result showed that the pyrolysis process of DOM released from manure biochars included three changes, aromatic protein-like substance and microbial by-product-like substance generation (300–600 °C), fulvic-acid like substance decomposition (300–500 °C) and humic acid-like substance decomposition (600 °C). The PARAFAC modeling result showed that the pyrolysis process of DOM released from manure biochars contained two changes: three high molecular-weight humic-like substances decomposition and a low molecular-weight humic-like substance generation. The effect of the holding time on biochar-DOM is more significant at higher pyrolysis temperatures than lower pyrolysis temperatures. The correlation analysis result revealed that the generation of aromatic proteins, microbial by-products and fulvic acid came from the decomposition of humic-like substances including marine humic-like, UVA humic-like, and UVC + UVA humic-like substances. The results obtained in this study would be beneficial to guide the rational production and application of manure biochars.

Dissolved Organic Matter (DOM) in biochars is important to carbon dynamics and contaminant transport in soils

New insight into the applicability of spectroscopic indices for dissolved organic matter (DOM) source discrimination in aquatic systems affected by biogeochemical processes

Despite numerous studies on changes to optical proxies of dissolved organic matter (DOM) by biogeochemical processing, the applicability of commonly-used spectroscopic indices has not been explored as DOM source tracking tools under conditions where biogeochemical processes may alter them. For this study, two contrasting DOM end members, Suwannee River fulvic acid (SRFA) and algogenic DOM (ADOM), and their mixtures, were used to examine the potential changes in the conservative mixing behaviors of several well-known optical indices via end member mixing analysis under the influence of biodegradation, UV irradiation, and clay mineral (kaolin) adsorption. Most of the source tracking indices exhibited large deviations from conservative mixing behavior between the two end-members. Biodegradation tended to lower the portions of labile and ADOM in the mixtures, while the allochthonous end member (SRFA) was reduced by a greater extent after the process of UV irradiation or adsorption. The extent of the variations in biological index (BIX) and fluorescence index (FI) was smaller for more allochthonous DOM mixtures under the processes of biodegradation and UV irradiation. Overall, the process-driven variations in ratios of humic-like: protein-like fluorescence (as modeled by parallel factor analysis, PARAFAC) were greater for the SRFA versus ADOM. Evaluation criteria used in this study suggested that BIX, specific UV absorbance (SUVA), and FI each could be the reliable discrimination parameter least affected by biodegradation, UV irradiation, and adsorption, respectively. This study provided criterion information for the choice and the interpretation of the optical indices for DOM source discrimination in aquatic environments after substantial biogeochemical processing.

How do root exudates of bok choy promote dibutyl phthalate adsorption on mollisol?

This study investigates the interaction between the bok choy root exudates and dibutyl phthalate (DBP) onto mollisol during the adsorption. The result elucidated that the adsorption reached equilibrium within 12 h, the adsorption capacity of rhizosphere mollisol containing root exudates and ordinary mollisol were 243.46 mg kg^-1 and 281.95 mg kg^-1, separately. The adsorption kinetics and isotherm model followed the pseudo-second order and the Frendlish model, respectively, which hinted that the adsorption process was multi-layer heterogeneous chemisorption. We characterized the root exudates and analyzed its effects on soil physical and chemical properties and structure. The result revealed that the root exudates contained hydrocarbons, sulfur compounds and acids. Root exudates made the dissolved organic matter (DOM) dissolution from soil and the increase of organic matter, which might be one of the reasons that root exudates promote DBP adsorption on mollisol. We selected three-dimensional excitation-emission matrix (3D-EEM), synchronous fluorescence and Fourier transform infrared spectroscopy (FTIR) to analyze the interactions between root exudates and DBP, DOM and DBP, respectively. Fluorescence spectrum revealed that the main component of root exudates was protein, for DOM was humic acid, the fluorescence of root exudates and DOM gradually disappeared with the increase of DBP concentration. FTIR revealed that -COO in root exudates and -CH₂ in DOM respectively reacted with DBP. The results of this study are of great importance to reveal that the root exudates are significant in the environmental behavior of DBP adsorption on mollisol, and also provide more useful information for phytoremediation of organic pollutants in the mollisol.

Effects of inorganic ions on the photolysis of propranolol in FA solution

Photolysis of the widely used beta-blocker propranolol (PRO) was investigated in the presence of fulvic acid (FA) and inorganic ions under simulated solar irradiation. PRO undergoes direct photolysis proceeding mainly via degradation of the triplet excited state, ³PRO*. FA and inorganic ions inhibited photolysis of PRO in the order of FA > Fe³⁺ > Cl^- > Ca²⁺ > Mg²⁺ > NO₃^- > K⁺. An antagonistic effect between FA and inorganic ions toward the suppression of PRO photolysis was exhibited. The binding behaviors of PRO, FA, and inorganic ions were examined through fluorescence quenching experiments, which showed that inorganic ions affected the binding between FA and PRO through competing for the binding sites of FA or PRO. The correlation analysis demonstrated a positive correlation between the binding constant (K_OC) of FA-PRO and the inhibition rate of FA on PRO photolysis. The observed rate constants of photolysis (k_obs) have opposite correlations with the concentration of singlet oxygen (¹O₂). These findings strongly suggest that inorganic ions decrease the inhibition effect of FA on PRO photolysis via restraining the complexation of FA-PRO and production of ¹O₂.

Influence of DOM components, salinity, pH, nitrate, and bicarbonate on the indirect photodegradation of acetaminophen in simulated coastal waters

The indirect photodegradation behaviors of acetaminophen (APAP) were investigated in the presence of four kinds of dissolved organic matter (DOM) and were also assessed in the presence of seawater components and conditions such as salinity, pH, nitrate and bicarbonate. The results showed three important findings: firstly, in the indirect photolysis of APAP, the contributions of ³DOM*, ·OH and ¹O₂ were >85.0%, 2.3-9.9% and 0.8-2.6% at pH 8.0. Secondly, DOM was divided into four terrestrial humic-like components by Excitation-emission matrix spectroscopy (EEMs) combined with parallel factor analysis (PARAFAC). This study showed a good linearity between DOM fluorescence components and the indirect photodegradation of APAP (R² = 0.92) and the differences in photodegradation rates of APAP among various DOM solutions were due to the diverse compositions of DOM. Finally, salinity was an important factor influencing the removal of APAP, and the APAP photodegradation rate constants increased from (3.33 ± 0.07) × 10^-5 s^-1 to (1.25 ± 0.05) × 10^-4 s^-1 with increasing salinity. The increased pseudo-first-order rate constants for photolysis of APAP with increasing salinity, pH and nitrate were attributed to the enhanced generation of reactive intermediates (RI) and easier reactions between RI and APAP. The increased APAP removal rate constant with increasing bicarbonate was likely ascribed to the yield of ∙CO₃^-. This is the first report of the roles of DOM components and salinity on the indirect photolysis of APAP. These findings would be essential to predict the photochemical fate of APAP and would also allow for a better understanding of the environmental fate of other phenolic contaminants.

Sorption of active pharmaceutical ingredients in untreated wastewater effluent and effect of dilution in freshwater: Implications for an "impact zone" environmental risk assessment approach

Evidence of ecotoxicological effects of active pharmaceuticals ingredients (APIs) has increased research into their environmental fate. In low and low-middle income countries (LLMICs) the main source of APIs to surface waters is from discharge of untreated wastewater. Consequently, concentrations of APIs can be relatively high in the "impact zone" downstream of a discharge point. Little is known about the fate of APIs in these impact zones. In this laboratory scale investigation, the effect of successive dilution of synthetic untreated wastewater (dilution factor 1 to 10) on the distribution of APIs was studied. The sorption was consistent with the chemical properties of each compound: charge, lipophilicity, and structure. Dilution increased desorption of the basic and neutral APIs (up to 27.7%) and correlated with their lipophilicity (R²>0.980); the positive charge was of secondary importance. Anions did not significantly desorb (<10% loss). Increased concentrations of dissolved organic matter at dilutions of 8 and 10 times that of untreated wastewater coincided with lower dissolved API concentrations. The data showed a clear trend in the desorption process of APIs that may lead to higher exposure risk than anticipated. Therefore, it is suggested that these aspects should be accounted for in the development of dedicated environmental risk assessment approach for APIs in riverine impact zones of LLMICs countries.

Investigating the behavior of binding properties between dissolved organic matter (DOM) and Pb(II) during the soil sorption process using parallel factor analysis (PARAFAC) and two-dimensional correlation spectroscopy (2D-COS)

Dissolved organic matter (DOM) is the most active component in an environmental system. It can influence the chemical and structural characteristics of soil. In this work, three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy, parallel factor analysis (PARAFAC), and two-dimensional correlation spectroscopy (2D-COS) integrated with synchronous fluorescence were used to explore the interaction between soil-derived DOM and Pb(II) during the soil sorption process. According to the data of batch sorption experiments, the adsorbing capacities of soil, soil + 5 mL DOM, and soil + 10 mL DOM were 16.96, 18.29, and 19.32 mg g^-1, respectively, which indicated that DOM significantly enhanced the adsorption efficiency of Pb(II). The pseudo-second-order kinetic equation could well explain the adsorption process. The adsorbing data conformed to the isotherm of Langmuir adsorption. According to EEM-PARAFAC results, there are two major components from DOM. Protein-like substances were represented by component 1, and humic-like and fulvic-like substances were represented by component 2. Based on 3D-EEM, the results further showed that the intensities of component 1 and component 2 were obviously quenched with the increase of Pb(II) concentrations. The combined interpretations of the 2D-COS map for the DOM revealed that Pb(II) binding might occur sequentially in the order of humic-like fraction > protein-like fraction (346 > 282 nm). According to synchronous fluorescence spectra, static fluorescence quenching was the major process of quenching. Graphical abstract ᅟ.

Roxarsone binding to soil-derived dissolved organic matter: Insights from multi-spectroscopic techniques

The fate and transport of roxarsone (ROX), a widely used organoarsenic feed additive, in soil is significantly influenced by the ubiquitous presence of soil-derived dissolved organic matter (DOM). In this study, fluorescence quenching titration and two-dimensional correlation spectroscopy (2D-COS) were employed to study ROX binding to DOM. Binding mechanisms were revealed by fluorescence lifetime measurement and Fourier transform infrared spectroscopy (FTIR). Humic- and protein-like fluorophores were identified in the excitation-emission matrix and synchronous fluorescence spectra of DOM. The conditional stability constant (log KC) for ROX binding to DOM was found to be 5.06, indicating that ROX was strongly bound to DOM. The binding order of ROX to DOM fluorophores revealed by 2D-COS followed the sequence of protein-like fluorophore ≈ the longer wavelength excited humic-like (L-humic-like) fluorophore > the shorter wavelength excited humic-like (S-humic-like) fluorophore. 2D-COS resolved issues with peak overlapping and allowed further exploration of the interaction between ROX and DOM. Results of fluorescence lifetime and FTIR spectra demonstrated that ROX interacted with DOM through the hydroxyl, amide II, carboxyl, aliphatic CH, and NO2 groups, yielding stable DOM-ROX complexes. The strong interaction between ROX and DOM implies that DOM plays an important role in the environmental fate of ROX in soil.