Photodegradation of 17β-estradiol on silica gel and natural soil by UV treatment

Cross-Coupling of 1,2,3,4-Tetrachlororodibenzo-p-dioxin with Six Coexisting Polycyclic Aromatic Hydrocarbons during Photodegradation on a Fly Ash Surface

The adverse conditions of the garbage incineration process can lead to the generation of dioxins and polycyclic aromatic hydrocarbons (PAHs). This study aimed to investigate the removal efficiency and possible cross-coupling effect of 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TCDD) and six coexisting low-molecular-weight PAHs during photodegradation on the fly ash surface. Due to their higher photoreactivity and light-shielding effect, the six PAHs exhibited inhibitory effects on the photodegradation of 1,2,3,4-TCDD, causing a reduction of 4.1%-21.2% in the removal efficiency. Common degradation products of 1,2,3,4-TCDD and PAHs were identified by LC-MS and GC-MS, and the formation of primary products was verified by theoretical calculations of bond dissociate energies, excitation energy, frontier electron densities, and transition states. In addition, high-molecular-weight coupling products of 1,2,3,4-TCDD and its interaction products with PAHs were observed in the mixed irradiation samples, and two coupling elimination mechanisms were proposed to illustrate their formation through C-O-C bonding and -COO- bonding, respectively. According to toxicity prediction analysis, the developmental toxicity and mutagenicity of most interaction products were higher than 1,2,3,4-TCDD. This study provided some new insights into the transformation, interaction, and related ecological risks of dioxins and PAHs coexisting on the surface of fly ash during the waste incineration process.

Photodegradation of 2-chlorodibenzo-p-dioxin (2-CDD) on the surface of municipal solid waste incineration fly ash: Kinetics and product analysis

Considering that waste incineration fly ash is the main carrier of dioxins and can migrate over long distances in the atmosphere, it is of great significance to study the photochemical transformation behavior of dioxins on the surface of fly ash. In this work, 2-chlorodibenzo-p-dioxin (2-CDD) was selected to conduct a systematic photochemical study. The influence of various factors on the photodegradation of 2-CDD were first explored, and the results showed that small particle size of fly ash, low concentration of 2-CDD and appropriate level of humidity were more conducive to photodegradation, with the highest degradation percentage reaching 76%-84%. The components of fly ash (Zn (Ⅱ), Al (Ⅲ), Cu (Ⅱ) and SiO2) also had a certain promoting effect on the degradation of 2-CDD, which increases the degradation efficiency by 10%-20%, because they could act as effective photocatalysts to produce free radicals for reaction. With a higher total light exposure intensity, natural light environments led to a more complete degradation of 2-CDD than laboratory Xe lamp irradiation (90% degradation Vs. 79% degradation). Based on chemical probe and radical quenching experiment, hydroxyl radical also contributed to 2-CDD photodegradation on fly ash. A total of 16 intermediate products were detected by mass spectrometry analysis, and four initial reaction pathways of 2-CDD were speculated in the process, including dechlorination, ether bond cleavage, hydroxyl substitution, and hydroxyl addition. According to the results of density functional theory calculation, the reaction channels of ether bond cleavage and •OH attack were determined. The toxicity assessment software tool (TEST) was used to assess the toxicity and bioconcentration coefficient of reaction products, and it was found that the overall toxicity of the photodegradation products was reduced. This study would provide new insights into the environmental fate of dioxins during long-range atmospheric migration process.

An inevitable but underestimated photoaging behavior of plastic waste in the aquatic environment: Critical role of nitrate

Photoaging is an important reaction for waste plastics in the aquatic environment and plays a key role in the lifetime of plastics. Nevertheless, when natural photosensitive substances such as nitrate participate in this process, the physiochemical changes in plastics and the corresponding reaction mechanisms are not well-understood. In this work, the photochemical behavior of polyethylene terephthalate (PET) bottles in deionized water and nitrate solution was systematically investigated under ultraviolet (UV) irradiation. The analyses of the surface physicochemical properties of the photoaged PET bottles indicated that, after 20 days of photo-irradiation, the presence of nitrate reduced the contact angle from 69.8 ± 0.9° to 60.0 ± 0.3°, and increased the O/C ratio from 0.23 to 0.32, respectively. The leaching rate of dissolved organic carbon (DOC), which was 0.0193 mg g^-1·day^-1 in nitrate solution, was twice that of 0.00941 mg g^-1·day^-1 in deionized water. Furthermore, fluorescence spectroscopy revealed that the increasing DOC had aromatic rings with hydroxyl on the side-chain formed after UV irradiation. The positive effect of nitrate on the degradation of PET bottles was mainly through the generation of hydroxyl radicals that were produced through the photolysis of nitrate. In addition, two-dimensional correlation spectroscopy analysis showed that the chain scission of PET plastics could be initiated by nitrate-induced ·OH attacking the carbon-oxygen bonds instead of forming peroxides with oxygen. This work elucidates the mechanism of photodegradation of plastics that was induced by nitrate and highlights the important role of natural photosensitive substances in the photoaging process of plastics.

Biological responses of Eisenia fetida towards the exposure and metabolism of tris (2-butoxyethyl) phosphate

The toxicity of various organophosphorus flame retardants (OPFRs) is of increasing concern. However, there is still a lack of research on the toxicity of OPFRs to terrestrial invertebrates and its metabolism in vivo. Herein, earthworms (Eisenia fetida) were exposed to soil spiked with 0, 0.05, 0.5, and 5 mg/kg tris(2-butoxyethyl) phosphate (TBOEP, a typical alkyl OPFRs) for 28 d to study the biological responses to the exposure and metabolism of TBOEP. TBOEP exposure inhibited the activity of acetyl-cholinesterase (64.4-68.6% of that in the control group), increased the energy consumption level, and affected calcium-dependent pathways of E. fetida, which caused a 3.6-12.4% reduction in the weight gain rate (developmental toxicity), a 10.6-69.4% reduction in the number of juveniles (reproduction toxicity), and neurotoxicity to E. fetida. The 5 mg/kg TBOEP exposure caused a significant accumulation of malondialdehyde (1.68 times higher than that in the control group) in E. fetida, which indicated that the balance of oxidation and anti-oxidation of E. fetida was broken. Meanwhile, E. fetida maintained the absorption and metabolic abilities to TBOEP under the environmental condition. The removal rate of soil TBOEP was increased by 25.1-35.5% by the presence of E. fetida. Importantly, TBOEP could accumulate in E. fetida (0.09-76.0 μg/kg) and the activation of cytochrome P450 and glutathione detoxification pathway promoted the metabolism of TBOEP in E. fetida. These findings link the biological responses and metabolic behavior of earthworms under pollution stress and provide fundamental data for the environmental risk assessment and pollution removal of OPFRs in soil.

Persistent Free Radicals from Low-Molecular-Weight Organic Compounds Enhance Cross-Coupling Reactions and Toxicity of Anthracene on Amorphous Silica Surfaces under Light

Polycyclic aromatic hydrocarbon (PAH) contamination has raised great environmental concerns, while the effects of low-molecular-weight organic compounds (LMWOCs) on PAH photodegradation at amorphous silica (AS)/air interfaces have been largely ignored. In this study, the phototransformation of anthracene (ANT) at amorphous silica (AS)/air interfaces was investigated with the addition of LMWOCs. ANT removal was attributed to ^•OH attacking and the energy transfer process via ³ANT*. Light irradiation induced the fractured ≡SiO^• or ≡Si^• generation on AS surfaces, which could react with absorbed H₂O and O₂ to generate ^•OH and further yield a series of hydroxylated products of ANT. The presence of citric acid and oxalic acid improved ^•OH generation and enhanced ANT removal by 1.0- and 2.2-fold, respectively. For comparison, the presence of catechol and hydroquinone significantly decreased ANT removal and produced coupling products. The results of density functional theory calculations suggest that persistent free radicals (PFRs) on AS surfaces from catechol or hydroquinone after ^•OH attacking prefer to cross-couple with ANT via C-C bonding rather than self-couple. Dianthrone and cross-coupling products might possess higher ecotoxicity, while hydroxylated products were less ecotoxic than their parent compounds based on Ecological Structure Activity Relationships (ECOSAR) estimation. The results of this study revealed the potential ecotoxicity of PAH-adsorbed particulates coexisting with LMWOCs and also provided a new insight into PAH transformation through PFR pathways.

Adsorption-desorption behavior of the endocrine-disrupting chemical quinestrol in soils

Quinestrol (QUN), a synthetic estrogen used as an oral contraceptive or emergency contraceptive component, has been shown to be an endocrine-disrupting chemical. To assess the environmental risk of QUN, batch equilibration experiments were conducted to investigate the adsorption–desorption of QUN in five contrasting soils from different areas of China. The leaching properties were also calculated based on the adsorption and degradation data from our previous study with the same soils. The Freundlich and Langmuir models were applied to the sorption–desorption data to examine the affinity towards QUN of the soils, which had varying physical and chemical properties. The K_f and K_f^des values of QUN in the tested soils ranged from 3.72 to 20.47 mg¹⁻ⁿ Lⁿ kg⁻¹ and from 1.26 to 7.8 mg¹⁻ⁿ Lⁿ kg⁻¹, respectively, and Q_m ranged from 28.25 to 126.58 mg/kg. The desorption data showed that hysteresis occurred. The K_f and K_f^des values of QUN were positively correlated with the soil total organic carbon (OC) and cation exchange capacity (CEC), and it may be due to the content of TOC and CEC exhibited a positive correlation. A low mobility potential of QUN in soils was predicted and verified the adsorption results by the groundwater ubiquity score (GUS) and retardation factor (R_f).

Formation of hydroxylated derivatives and coupling products from the photochemical transformation of polyfluorinated dibenzo-p-dioxins (PFDDs) on silica surfaces

Polyfluorinated dibenzo-p-dioxins (PFDDs) are dioxin compounds that have been detected in industrial fluoroaromatic chemicals and can cause adverse effects to organisms. In this work, the photochemical behaviors of PFDDs congeners on silica was systematically investigated. The pseudo-first-order rate constants (k, h^-1) of surface photolysis changed with the substitution number and position of fluorine atoms, and the tetra-fluorinated PFDDs tended to degrade more efficiently. Octafluorinated dibenzo-p-dioxin (OFDD) was selected as a representative to explore the reaction mechanisms. Product analysis showed that OFDD was decomposed into hydroxylated PFDDs (OH-PFDDs) and hydroxylated polyfluorinated diphenyl ethers (OH-PFDEs) via hydroxyl substitution and (OH radical mediated or direct) C-O bond cleavage. Coupling elimination reaction was also observed, resulting in the formation of three-membered and four-membered ring compounds. According to the extracted peak areas in mass spectra and the energy barrier in potential energy surface, direct homolysis of C-O bond occurs as the dominant reaction pathway. This work could provide some new insights into the environmental fate of dioxin compounds.

Degradation Kinetics and Transformation Products of Levonorgestrel and Quinestrol in Soils

Levonorgestrel (LNG) and quinestrol (QUN) are typical endocrine disruptors that enter the soil via sewage irrigation and sludge return. However, the fates of both compounds in soil are not well-understood. Laboratory microcosm studies were conducted to fill the gap of understanding of LNG and QUN behavior in soils. High values of goodness-of-fit indices (GFIs) were obtained using the double-first-order in parallel (DFOP) model and the single-first-order (SFO) model to fit the degradation kinetics of LNG and QUN in soils, respectively. The end-points (DT₅₀ and DT₉₀) of LNG and QUN were positively correlated with soil total organic carbon (TOC). Soil water content and temperature were observed to be critical factors in degradation of LNG and QUN. The degradation rates of LNG and QUN were very slow under sterile and flooded conditions, indicating that the aerobic microbial degradation was dominant in the degradation of LNG and QUN. Moreover, major transformation products were identified, and biodegradation pathways of LNG and QUN were proposed. The present study is expected to provide basic information for ecological risk assessment of LNG and QUN in the soil compartment.

Occurrence, sorption, and transformation of free and conjugated natural steroid estrogens in the environment

Natural steroid estrogens (NSEs), including free estrogens (FEs) and conjugated estrogens (CEs), are of emerging concern globally among public and scientific community due to their recognized adverse effects on human and wildlife endocrine systems in recent years. In this review, the properties, occurrence, sorption process, and transformation pathways of NSEs are clarified in the environment. The work comprehensively summarizes the occurrence of both free and conjugated estrogens in different natural and built environments (e.g., river, WWTPs, CAFOs, soil, and sediment). The sorption process of NSEs can be impacted by organic compounds, colloids, composition of clay minerals, specific surface area (SSA), cation exchange capacity (CEC), and pH value. The degradation and transformation of free and conjugated estrogens in the environment primarily involves oxidation, reduction, deconjugation, and esterification reactions. Elaboration about the major, subordinate, and minor transformation pathways of both biotic and abiotic processes among NSEs is highlighted. The moiety types and binding sites also would affect deconjugation degree and preferential transformation pathways of CEs. Notably, some intermediate products of NSEs still remain estrogenic potency during transformation process; the elimination of total estrogenic activity needs to be addressed in further studies. The in-depth researches regarding the behavior of both free and conjugated estrogens are further required to tackle their contamination problem in the ecosystem. Graphical abstract ᅟ.