Enhanced Phototransformation of Tetracycline at Smectite Clay Surfaces under Simulated Sunlight via a Lewis-Base Catalyzed Alkalization Mechanism

Insights into sunlight-driven transformation of tetracycline by iron (hydr)oxides: The dominating role of self-generated hydrogen peroxide

Iron (hydr)oxides are abundant in surface environment, and actively participate in the transformation of organic pollutants due to their large specific surface areas and redox activity. This work investigated the transformation of tetracycline (TC) in the presence of three common iron (hydr)oxides, hematite (Hem), goethite (Goe), and ferrihydrite (Fh), under simulated sunlight irradiation. These iron (hydr)oxides exhibited photoactivity and facilitated the transformation of TC with the initial phototransformation rates decreasing in the order of: Hem > Fh > Goe. The linear correlation between TC removal efficiency and the yield of HO• suggests that HO• dominated TC transformation. The HO• was produced by UV-induced decomposition of self-generated H2O2 and surface Fe2+-triggered photo-Fenton reaction. The experimental results indicate that the generation of HO• was controlled by H2O2, while surface Fe2+ was in excess. Sunlight-driven H2O2 production in the presence of the highly crystalline Hem and Goe occurred through a one-step two-electron reduction pathway, while the process was contributed by both O2-induced Fe2+ oxidation and direct reduction of O2 by electrons on the conduction band in the presence of the poorly crystalline Fh. These findings demonstrate that sunlight may significantly accelerate the degradation of organic pollutants in the presence of iron (hydr)oxides.

The impact of dissolved organic matter on the photodegradation of tetracycline in the presence of microplastics

Microplastics (MPs), an emerging class of pollutants, significantly impact the photoconversion dynamics of tetracycline (TC). But the effect of prevalent dissolved organic matter (DOM) on TC photodegradation in the presence of MPs remains a gap in current research. In this study, the photoconversion behavior and mechanism of TC under simulated sunlight conditions were systematically investigated, both in the presence of DOM and in combination with polystyrene (PS) MPs. The results demonstrated that both DOM and MPs enhanced the photodegradation of TC when compared to its direct degradation. However, DOM, particularly humic acid (HA, 10 mg/L), exhibited a more pronounced enhancing effect on TC photodegradation within 1 h reaction, regardless of the presence or absence of MPs, reaching up to 80%. In reaction systems involving TC-HA and TC-HA-PS, the primary contributors to TC degradation were direct photolysis and HA photosensitization (free radical reactions). Conversely, photosensitization effects were not significant in the presence of fulvic acid (FA). Furthermore, even under dark reaction conditions, HA exhibited a 10% degradation effect on TC. Quenching experiments and electron spin resonance (ESR) results indicate that dark reaction processes involve free radical reactions. Additionally, toxicity test results showed a reduction in the acute toxicity of TC photodegradation products, yet the long-term cumulative risks to organisms deserved attention. In general, this investigation significantly advances our understanding of the intricate photoconversion behavior of TC in the presence of coexisting chemical components.

Ball-milled layer double hydroxide as persulfate activator for efficient degradation of organic: Alkaline sites-triggered non-radical mechanism

Considerable efforts have been put into enhancing the activation performance of peroxydisulfate (PDS) by catalysts toward oxidative degradation of organic pollutants, while the oxidative selectivity is somehow overlooked. Here, we reported an enhanced non-radical oxidation pathway of PDS, activated by ball-milled Mg/Al-layered double hydroxide (BM-LDH), to reconcile the selectivity and reactivity. EPR and quenching experiments suggested that 1O2 dominated the oxidative pathway for phenol degradation without generating carcinogenic halide by-products. Multiple interfacial characterizations and density functional theory (DFT) calculations revealed that BM-LDH played dual roles in PDS activation: (1) the interlaminar BM-LDH allowed PDS intercalation to form complexed PDS, resulting in decreases in the activation barrier of PDS; (2) abundant terminal hydroxyls in the layers of BM-LDH acted as alkaline-activation sites that can efficiently activate PDS to generate 1O2 toward phenol degradation. Ball-milling treatment of LDH refined the structural hierarchy of LDH to create pore volumes, which greatly enhanced the diffusion of phenol to the intercalated PDS, resulting in more than twice the reaction rate for phenol degradation. This study provided a promising approach to simultaneously control over the reactivity and selectivity toward PDS activation that are critical for the degradation of organic pollutants particularly in drinking water treatment.

Floc aging: Crystallization and improving low molecular weight organic removal in re-coagulation

Iron coagulants have been used extensively in drinking water treatment. This typically produces substantial quantities of insoluble iron hydrolysis products which interact with natural and anthropogenic organic substances during the coagulation process. Previous studies have shown that the removal of low molecular weight (MW) organics is relatively poor by coagulation, which leads to their presence during disinfection, with the formation of halogenated byproducts, and in treated water supplies as potentially biodegradable material. Currently, there is little knowledge about the changes that occur in the nature of coagulant flocs as they age with time and how such changes affect interactions with organic matter, especially low MW organics. To improve this deficiency, this study has investigated the variation of aged flocs obtained from two commonly used iron salts and their impact on representative organic contaminants, natural organic matter (NOM) and tetracycline antibiotic (TC), in a real surface water. It was found that aging resulted in increasing crystallization of the flocs, which can play a beneficial role in activating persulfate oxidant to remove the representative organics. Furthermore, acidification was also found to further improve the removal of low MW natural organics and tetracycline. In addition, the results showed that the low MW fractions of NOM (<1 K Dalton) were substantially removed by the aging flocs. These results are in marked contrast to the poor removal of low MW organic substances by conventional coagulation, with or without added oxidants, and show that aged flocs have a high potential of reuse for re-coagulation and activation of oxidants to reduce low MW organics, and enhance drinking water quality.

Recent advances in luminescence and aptamer sensors based analytical determination, adsorptive removal, degradation of the tetracycline antibiotics, an overview and outlook

Tetracycline antibiotics (TCs), one of the important antibiotic groups, have been widely used in human and veterinary medicines. Their residues in foodstuff, soil and sewage have caused serious threats to food safety, ecological environment and human health. Here, we reviewed the potential harms of TCs residues to foodstuff, environment and human beings, discussed the luminescence and aptamer sensors based analytical determination, adsorptive removal, and degradation strategies of TCs residues from a recent 5-year period. The advantages and intrinsic limitations of these strategies have been compared and discussed, the potential challenges and opportunities in TCs residues degradation have also been deliberated and explored.

Efficient photodegradation of polystyrene microplastics integrated with hydrogen evolution: Uncovering degradation pathways

Photocatalytic microplastics (MPs) conversion into valuable products is a promising approach to alleviate MPs pollution in aquatic environments. Herein, we developed an amorphous alloy/photocatalyst composite (FeB/TiO₂) that can successfully convert polystyrene (PS) MPs to clean H₂ fuel and valuable organic compounds (92.3% particle size reduction of PS-MPs and 103.5 μmol H₂ production in 12 h). FeB effectively enhanced the light-absorption and carrier separation of TiO₂, thereby promoting more reactive oxygen species generation (especially ‧OH) and combination of photoelectrons with protons. The main products (e.g., benzaldehyde, benzoic acid, etc.) were identified. Additionally, the dominant PS-MPs photoconversion pathway was elucidated based on density functional theory calculations, by which the significant role of ‧OH was demonstrated in combination with radical quenching data. This study provides a prospective approach to mitigate MPs pollution in aquatic environments and reveals the synergistic mechanism governing the photocatalytic conversion of MPs and generation of H₂ fuel.

Photochemical degradation of perfluorooctanoic acid under UV irradiation in the presence of Fe (III)-saturated montmorillonite

Perfluorooctanoic acid (PFOA) has attracted worldwide attention owing to its widespread distribution and potential ecological risks. Developing low-cost, green-chemical and highly efficient treatment approaches is significant for treating PFOA caused environmental issues. Herein, we propose a feasible PFOA degradation strategy under UV irradiation by adding Fe (III)-saturated montmorillonite (Fe-MMT), and the Fe-MMT could be regenerated after reaction. In our system consisting of 1 g L^-1 Fe-MMT and 24 μM PFOA, nearly 90 % initial PFOA could be decomposed within 48 h. The enhanced PFOA decomposition could be explained by the ligand-to-metal charge transfer mechanism based on the generated reactive oxygen species (ROSs) and the transformation of iron species in the MMT layers. Moreover, the special PFOA degradation pathway was revealed according to the intermediate identification and the density functional theory calculation. Further experiments demonstrated that even in the presence of co-existing natural organic natter (NOM) and inorganic ions, efficient PFOA removal could still be obtained in UV/Fe-MMT system. This study offers a green-chemical strategy for PFOA removal from contaminated waters.

Autochthonous DOM had solar disinfection effect but nitrate counteracted with them

Pathogens in natural water can pose great threat to public health and challenge water quality. In sunlit surface water, dissolved organic matters (DOMs) can inactivate pathogens due to their photochemical activity. However, the photoreactivity of autochthonous DOM derived from different source and their interaction with nitrate on photo-inactivation remained limited understood. In this study, the composition and photoreactivity of DOM extracted from Microcystis (ADOM), submerged aquatic plant (PDOM) and river water (RDOM) were studied. Results revealed that lignin and tannin-like polyphenols and polymeric aromatic compounds negatively correlated with quantum yield of ³DOM*, whilst lignin like molecules positively correlated with •OH generation. ADOM had highest photoinactivation efficiency of E. coli, followed by RDOM and PDOM. Both the photogenerated •OH and low energy ³DOM* could inactivate bacteria damaging cell membrane and causing increase of intracellular reactive species. PDOM with more phenolic or polyphenols compounds not only weaken its photoreactivity, also increase regrowth potential of bacteria after photodisinfection. The presence of nitrate counteracted with autochthonous DOMs on photogeneration of •OH and photodisinfection activity, as well as increased the reactivation rate of PDOM and ADOM, which might be attributed to the increase of survival bacteria and more bioavailable fractions provided in systems.

Mechanism insight into the facet-dependent photoaging of polystyrene microplastics on hematite in freshwater

Hematite, as an extensive natural mineral with multiple crystal facets, profoundly affects the migration and transformation of pollutants in the natural environment. However, little is known about the photochemical behavior of microplastics on different facets of hematite in the aquatic environment. In this work, the photoaging of polystyrene microplastics (PS-MPs) on different crystal planes ({001}, {100}, and {012} facets) and related mechanisms were studied. Two-dimensional correlation spectroscopy analysis illustrated that the reaction pathways of PS-MPs photoaging on hematite tended to preferential chemical oxidization. The stronger performance of PS-MPs photoaging, expressed by particle size reduction and surface oxidation, was observed on the {012} crystal facet. Under irradiation, {012} facet-dominated hematite with a narrower bandgap (1.93 eV) reinforced the photogenerated charge carrier separation, and the lower activation energy barrier (1.41 eV calculated from density functional theory) led to effective •OH formation from water oxidation. These findings elucidate the underlying photoaging mechanism of MPs on hematite with different mineralogical phases.

Dissolved organic matter derived from biodegradable microplastic promotes photo-aging of coexisting microplastics and alters microbial metabolism

Dissolved organic matter (DOM) leaching from biodegradable microplastics (BMPs) and its characteristics and corresponding environmental implication are rarely investigated. In this study, the main component of DOM leachate from the two BMPs (polyadipate/butylene terephthalate (PBAT)/polycaprolactone (PCL)) was verified by using excitation-emission matrix-parallel factor analysis (EEM-PARAFAC). The PBAT-DOM (PBOM) was aromatized and terrestrial. Comparatively, PCL-DOM (PLOM) had low molecular weight. PBOM contained protein-like components while PLOM contained tryptophan and tyrosine components. Interestingly, both PBOM and PLOM could accelerate the decomposition and oxidation of coexisting polystyrene (PS) under light irradiation. Further, the difference in composition and the properties of BMPs-DOM significantly affected its photochemical activity. The high territoriality and protein-like component of PBOM significantly promoted the generation of ¹O₂ and O₂^•-, which caused faster disruptions to the backbone of PS. Simultaneously, the microbial community's richness, diversity, and metabolism were obviously improved under the combined pressure of aged PS and BMPs-DOM. This study threw light on the overlooked contribution of DOM derived from BMPs in the aging process of NMPs and their impact on the microbial community and provided a promising strategy for better understanding of combined MPs' fate and environmental risk.

Enhanced photodegradation of tylosin in the presence of natural montmorillonite: Synergistic effects of adsorption and surface hydroxyl radicals

Tylosin (TYL) is a ubiquitous macrolide antibiotic which has been frequently detected in natural aqueous environment. Montmorillonite (MMT), a major component of natural suspended particles, plays essential roles in the transportation and transformation processes of various organic contaminants. This study systematically investigated the photodegradation behavior and mechanism of TYL in MMT suspensions under simulated sunlight irradiation. In the existence of 0.1 g L^-1 Na-MMT, >80.8 % TYL was degraded after 8 h irradiation, which was significantly higher than that in the absence of MMT (42.5 %). Further mechanistic studies suggested that the synergistic effects including the formation of surface complex and the generation of surface hydroxyl radicals play essential roles in the accelerated TYL phototransformation. Meanwhile, other factors like exchangeable cations of MMTs, pH and ionic strength could also strongly influence the TYL photodegradation. The probable degradation pathways of TYL in MMT suspension was further proposed based on the detected intermediates and DFT calculations. Photobacterium phospherium T3 bioluminescent assay revealed that the photodegradation products of TYL have a lower acute toxicity than bulk TYL, especially in the presence of MMT. This study provides new insights for the photodegradation pathways of organic contaminants in aqueous environments, which is of great importance for assessing the fate and risk of emerging pollutants in natural surface water bodies.

Lewis Acid-Base Interaction Triggering Electron Delocalization to Enhance the Photodegradation of Extracellular Antibiotic Resistance Genes Adsorbed on Clay Minerals

The transformation of extracellular antibiotic resistance genes (eARGs) is largely influenced by their inevitable photodegradation in environments where they tend to be adsorbed by ubiquitous clay minerals instead of being in a free form. However, the photodegradation behaviors and mechanisms of the adsorbed eARGs may be quite different from those of the free form and still remain unclear. Herein, we found that kaolinite, a common 1:1-type clay, markedly enhanced eARG photodegradation and made eARGs undergo direct photodegradation under UVA. The decrease in the transformation efficiency of eARGs caused by photodegradation was also promoted. Spectroscopy methods combined with density functional theory calculations revealed that the Lewis acid-base interaction between P-O in eARGs and Al-OH on kaolinite delocalized electrons of eARGs, thus resulting in increased photon absorption ability of eARGs. This ultimately led to enhanced photodegradation of kaolinite-adsorbed eARGs. Additionally, divalent Ca²⁺ could reduce the Lewis acid-base interaction-mediated adsorption of eARGs by kaolinite, thereby weakening the enhanced photodegradation of eARGs caused by electron delocalization. In contrast, the 2:1-type clay montmorillonite without strong Lewis acid sites was unable to delocalize the electrons to enhance the photodegradation of eARGs. This work allowed us to better evaluate eARGs' fate and risk in real aqueous environments.

Peroxymonosulfate-based photocatalytic oxidation of tetracycline by Fe2(MoO4)3/Cd0.5Ni0.5S heterostructure; DFT simulation

The current research is meant to develop novel semiconductor photocatalysts, for the decomposition of tetracycline (TC) as a model organic contaminant in the aquatic environment. The fabrication of Fe₂(MoO₄)₃/Cd_0.5Ni_0.5S (FMO/CNS) composite has proven to be an effective method for improving the sustainability and photocatalytic activity of Cd_0.5Ni_0.5S (CNS). Under visible light irradiation, FMO/CNS nanocomposite demonstrated significant PMS activation which led to 1.36 and 1.81 times TC removal efficiency as compared to immaculate Fe₂(MoO₄)₃(FMO) and CNS. FMO/CNS composite potentially promotes the segregation of electron-hole pairs (e^--h⁺) and exemplifies amazing photocatalytic performance for TC degradation. Its significant photocatalytic activity is due to its unique structure, which includes tiny pores on the surface that confine the PMS molecule to the interface. The FMO/CNS composite has significantly greater piezocatalytic activity than pure FMO and CNS, demonstrating the synergistic effect of FMO and CNS. In the degradation of TC, holes and key reactive radicals (^•O₂^-/^•OH/SO₄^-•) played a major role. Computational studies (DFT) estimates, including the determination of intermediates, confirmed that the hydroxyl addition and C-N cleavage pathways were responsible for TC degradation. As a result, this work delivers a new approach to developing novel photocatalysts with high photocatalytic activity for the abatement of organic contaminants in water.

Dissolved Organic Matter Promotes the Aging Process of Polystyrene Microplastics under Dark and Ultraviolet Light Conditions: The Crucial Role of Reactive Oxygen Species

Microplastics (MPs) interact frequently with dissolved organic matter (DOM) commonly found in the environment, but information on the aging behavior of MPs under the participation of DOM is still lacking. Thus, the polystyrene microplastic (PSMP) aging process with DOM participation was systematically studied by electron paramagnetic resonance spectroscopy, high-performance liquid chromatography, Fourier transform infrared (FTIR) spectroscopy, and two-dimensional correlation spectroscopy analyses under dark and ultraviolet (UV) light conditions. DOM was found to promote electron transfer to generate reactive oxygen species (ROS) under dark conditions and the aging of PSMPs, while the process of DOM generating ROS under UV light was more susceptible to photoelectrons and accelerated the aging process of PSMPs. However, among the four DOM types, fulvic acid (FA) has a more significant promoting effect on the aging process of PSMPs than humic acid, which can be attributed to the stronger conversion ability of FA to semiquinone radicals. Density functional theory calculations are used to describe the difference in the aging process of different structures of plastics with the participation of DOM. This study provides a necessary theoretical basis for the study of the migration of MPs in groundwater and deep surface water.

The photodegradation processes and mechanisms of polyvinyl chloride and polyethylene terephthalate microplastic in aquatic environments: Important role of clay minerals

It is well known that microplastics (MPs) may experience weathering and aging under ultraviolet light (UV) irradiation, but it remains unclear if these processes are impacted by natural components, such as clay minerals. In this study, we systematically investigated the photodegradation behaviors of polyvinyl chloride (PVC) and poly (ethylene terephthalate) (PET), two utmost used plastics, in the presence of clay minerals (kaolinite and montmorillonite). The results demonstrated that the clay minerals, particularly kaolinite, significantly promoted the MPs photodegradation, and the aging of PET was more prominent. The photodegradation was the most distinct at pH 7.0, regardless of the presence or absence of the clay minerals. The results of electron paramagnetic resonance and inhibition experiments of reactive oxygen species indicated that the minerals, particularly kaolinite, remarkably facilitated production of •OH, which was the key species contributing to the photodegradation of MPs. Specifically, UV irradiation facilitated the photo-ionization of MPs, producing hydrated electrons and MP radical cations (MP⁺). The Lewis base sites prevalent on the clay siloxane surfaces could stabilize the MP radical cations and prevent their recombination with hydrated electrons, which promoted the generation of •OH under aerobic conditions, and facilitated the degradation of MP. Two-dimensional (2D) Fourier transformation infrared (FTIR) correlation spectroscopy (COS) analysis and ultra-high-performance liquid chromatography coupled to a Q Exactive Orbitrap HF mass spectrometer were used to identify the sequential changes of functional groups, and the degradation products of the MPs. This study improves our understanding on the aging of MPs in the complex natural environment.

Montmorillonite promoted photodegradation of amlodipine in natural water via formation of surface complexes

The photolysis of amlodipine (AML) as a ubiquitous pollutant in natural water has been extensively studied. Montmorillonite (MMT), a major component of suspended particles in surface aquifers, plays key roles in the natural transportation and transformation of organic contaminants in the environment. However, literature has scarcely focused on whether and how suspended particles affect the phototransformation of AML. This study systematically investigated the phototransformation behavior of AML in MMT suspensions under simulated sunlight. The results obtained showed that MMT significantly enhanced the photolysis of AML. The photodegradation of AML in 0.05 g/L MMT suspension reached 92.2 % after 3 h irradiation under the simulated sunlight. The photodecomposition followed the pseudo-first-order kinetic with a rate constant of 0.803 h^-1 in the presence of 0.05 g/L MMT, which is about 19 times larger than that in the absence of MMT (0.0421 h^-1). Further mechanistic investigation suggested that MMT accelerated the photolysis of AML by the formation of surface complexes between cationic amino groups of AML and the negatively charged sites on MMT surface, which greatly facilitated light absorption and electron transfer for the production of cationic radical AML^+·. Meanwhile, the hydroxyl radicals generated by irradiated MMT also played an important role in the photocatalytic degradation of AML. The probable photodegradation pathways of AML in MMT suspension further supported the proposed mechanisms. The toxicity evaluation of phototransformation products of AML with ECOSAR program indicated that photolysis could reduce its potential threats. These findings reveal an important and previously overlooked phototransformation mechanisms of AML in the presence of MMT clays, which is of importance in assessing the environmental fate of other similar organic contaminants.

Construction of SnNb2O6/MgIn2S4 heterojunction photocatalysts with enhanced visible-light-driven activity for tetracycline hydrochloride degradation and Cr(vi) reduction

A novel type II SnNb2O6/MgIn2S4 composite photocatalyst was prepared. The prepared composite photocatalyst exhibited significantly improved photocatalytic activity for Cr(vi) reduction and tetracycline hydrochloride degradation under visible light.

Photochemical Behavior of Microbial Extracellular Polymeric Substances in the Aquatic Environment

Microbially derived extracellular polymeric substances (EPSs) occupy a large portion of dissolved organic matter (DOM) in surface waters, but the understanding of the photochemical behaviors of EPS is still very limited. In this study, the photochemical characteristics of EPS from different microbial sources (Shewanella oneidensis, Escherichia coli, and sewage sludge flocs) were investigated in terms of the production of reactive species (RS), such as triplet intermediates (³EPS*), hydroxyl radicals (^•OH), and singlet oxygen (¹O₂). The steady-state concentrations of ^•OH, ³EPS*, and ¹O₂ varied in the ranges of 2.55-8.73 × 10^-17, 3.01-4.56 × 10^-15, and 2.08-2.66 × 10^-13 M, respectively, which were within the range reported for DOM from other sources. The steady-state concentrations of RS varied among different EPS isolates due to the diversity of their composition. A strong photochemical degradation of the protein-like components in EPS isolates was identified by excitation emission matrix fluorescence with parallel factor analysis, but relatively, humic-like components remained stable. Fourier-transform ion cyclotron resonance mass spectrometry further revealed that the aliphatic portion of EPS was resistant to irradiation, while other portions with lower H/C ratios and higher O/C ratios were more susceptible to photolysis, leading to the phototransformation of EPS to higher saturation and lower aromaticity. With the phototransformation of EPS, the RS derived from EPS could effectively promote the degradation of antibiotic tetracycline. The findings of this study provide new insights into the photoinduced self-evolution of EPS and the interrelated photochemical fate of contaminants in the aquatic environment.

Accelerating FeIII-Aqua Complex Reduction in an Efficient Solid-Liquid-Interfacial Fenton Reaction over the Mn-CNH Co-catalyst at Near-Neutral pH

The sluggish regeneration rate of Fe^II and low operating pH still restrict the wider application of classical Fenton process (Fe^II/H₂O₂) for practical water treatment. To overcome these challenges, we exploit the Mn-CNH co-catalyst to construct a solid-liquid interfacial Fenton reaction and accelerate the Fe^III/Fe^II redox cycle at the interface for sustainably generating ^•OH from H₂O₂ activation. The Mn-CNH co-catalyst exhibits an excellent regeneration rate of Fe^II (∼65%) and a high tetracycline removal rate (K_obs) of 0.0541 min^-1, which is 19.0 times higher than that of the Fe^II/H₂O₂ system (0.0027 min^-1) at a near-neutral pH (pH ≈ 5.8), and it also attains 100% degradation of sulfamethoxazole, rhodamine B, and methyl orange. The cyclic mechanism of Fe^III/Fe^II is further elucidated in an atomic scale by combining characterizations and density functional theory calculations, including Fe_aq^III specific adsorption and the electron-transfer process. Mn active sites can accumulate electrons from the matrix and adsorb Fe_aq^III to form Mn-Fe bonds at the solid-liquid interface, which accelerate electron transfer from Mn-CNH to Fe_aq^III and promote the regeneration of Fe^II at a wide pH range with a lower energy barrier. The regeneration rate of Fe^II in the Mn-CNH/Fe^II/H₂O₂ system outperforms the benchmark Fenton system and other typical metal nanomaterials, which has great potential to be widely applied in actual environment remediation.

Boosting the activation of molecular oxygen and the degradation of tetracycline over high loading Ag single atomic catalyst

Photocatalytic activation of molecular oxygen (O₂) is a promising way in oxidative degradation of organic pollutants. However, it suffers from low efficiency mainly due to the limited active sites for O₂ activation over traditional photocatalysts. Therefore, we established a single atomic Ag-g-C₃N₄ (SAACN) catalyst with 10 wt% loading of Ag single sites for boosting the O₂ activation during the degradation of tetracycline (TC), and 10 wt% loading of nanoparticle Ag-g-C₃N₄ (NPACN) was studied as a comparison. When using SAACN, the accumulative concentration of superoxide (•O₂^-), hydroxyl radical (•OH), singlet oxygen (¹O₂) reached up to 0.66, 0.19, 0.33 mmol L^-1h^-1, respectively, within 120 min, 11.7, 5.7 and 4.9 times compared with those using NPACN, representing 17.24% of dissolved O₂ was converted to reactive oxygen species (ROS). When additionally feeding air or O_2, the accumulative concentrations of •O₂^-, •OH, ¹O₂ were even higher (air: 4.21, 0.97, 2.02 mmol L^-1 h^-1; O₂: 17.13, 1.32, 9.00 mmol L^-1 h^-1). The rate constants (k) for degrading the TC were 0.0409 min^-1 over SAACN and 0.00880 min^-1 over NPACN, respectively (mineralization rate: 95.7% vs. 59.9% after 3 h of degradation). Moreover, the degradation ability of SAACN did not decrease in a wide range of pH value (4-10) or under low temperature (10 °C). Besides the high exposure of Ag single sites, other advances of SAACN were: 1(O₂ was more energetic favorable to adsorb on single atomic Ag sites; 2) Positive Ag single sites were easier to obtain the electrons from the surrounding N atoms, and facilitated electron transfer towards adsorbed O₂.

Comparative investigation on removal characteristics of tetracycline from water by modified wood membranes with different channel walls

The alkali-innocuous citric acid (CA) modified wood membranes (WMs) have been developed as facile, economical, and effective adsorption membranes to remove tetracycline (TC) from water. However, TC removal by modified WMs with different types of wood channel walls have rarely been compared. Therefore, in this study, modified WMs were prepared with pinewood (PW) and basswood (BW). The PW and BW WMs before and after modification were characterized by SEM, EDX, XRD, ATR-FTIR, TGA, contact angle and zeta potential. After modification, cellulose I in cellulose crystal structures of raw WM transformed to cellulose II and the contents of carboxylic groups for PW and BW were enhanced to make the hydrophilicity of WM surface increased. Compared with modified PW WMs, particles formed on the channel walls of modified BW WMs containing vessel pits to make more carboxylic groups introduced. The TC adsorption breakthrough curves showed that the 6 wt% alkali- CA modified BW had an effective filtration volume of 1968 bed volume (BV) compared with the 4 wt% alkali-CA modified PW of 1205 BV as the influent TC and breakthrough point were chosen at 2 and 0.5 mg/L, respectively. At low pH, TC complex with WM surfaces through Lewis acid-base interaction. Zwitterionic TC was shown to favor adsorption onto WM via hydrogen bonding at pH of around 5. With further increasing pH, TC adsorption efficiency decreased due to the electrostatic repulsion. The costs of modified BW and PW are about 0.0054-0.0126 US$/m³ and 0.01-0.024 US$/m³ for a low TC concentration effluent (0-0.5 mg/L), respectively. This work shed a new sight on how to develop economical and effective adsorption WMs for contaminants removal from water.

Surface catalyzed hydrolysis of chloramphenicol by montmorillonite under limited surface moisture conditions

Phyllosilicates possess high surface acidity under limited surface moisture conditions and are thus able to mediate the abiotic transformation of antibiotics. This route of abiotic transformation has long been ignored given that most of the studies carried out in aqueous phase. In this study, the catalytic performance of cation-exchanged montmorillonites (Mⁿ⁺-Mts) to the hydrolysis of chloramphenicol (CAP) was investigated under different moisture conditions. Montmorillonite exchanged with Fe³⁺ and Al³⁺ show the greatest catalytic activities. Multiple spectroscopic techniques and theoretical calculations indicate that the surface Brønsted- and Lewis-acid properties are sensitive to surface wetting. At lower moisture level (<10%, wt/wt), the strong Brønsted-acid catalysis predominates the hydrolysis of CAP. Attributing to the strong Lewis-acidities, Fe³⁺-Mt and Al³⁺-Mt could perform high catalytic activities over a wider moisture range (10- 100%, wt/wt). However, such hydrolysis reaction was almost suppressed at water content >400%. In addition, the presence of natural organic matter (NOM, 1%, wt/wt) had little impact on the catalytic activities of Fe³⁺-Mt and Al³⁺-Mt. The results of this study highlight the environmental significance of dry surface reaction by clay minerals as an effective abiotic transformation pathway to the elimination of antibiotics in natural field soil, which is commonly partly hydrated.

Enhanced phototransformation of atorvastatin by polystyrene microplastics: Critical role of aging

Reactive oxygen species (ROS) generated from light irradiation of microplastics (MPs) can potentially affect the environmental fate of organic contaminants when they coexist in the same environment. This study investigated the effect of polystyrene (PS) MPs with different aging degrees on the phototransformation of atorvastatin (ATV) under simulated sunlight. Results showed that the presence of PS MPs facilitated the phototransformation of ATV, and the degradation rate was linearly correlated with the aging degree (i.e., carbonyl index) of MPs. The enhanced effects mainly depended on the contents of oxygen-containing functional groups of PS MPs, which increased the absorption of light energy and the generation of ROS (e.g., singlet oxygen (¹O₂) and triplet-excited state PS (³PS*)). Quenching experiments indicated that ¹O₂ generated from photosensitization of PS was the major contributor to the increased phototransformation of ATV. Additionally, the role of ³PS* became more important in the photodegradation mediated by higher degree aged MPs because much more ¹O₂ was generated from the ³PS* . PS MPs also increased the types and yields of degradation products, especially for higher degree aged MPs, despite the low effect on leachate toxicity. The findings provide a novel insight into the critical role of MPs in the fate of organic contaminants in aquatic environments.

Bi2O3-Sensitized TiO2 Hollow Photocatalyst Drives the Efficient Removal of Tetracyclines under Visible Light

The complete removal of tetracycline residuals under visible light still is a challenging task because of their robust ring structure. To tackle this issue, we explore a novel Bi₂O₃-sensitized TiO₂ visible-light photocatalyst by combining p-n heterojunction with hollow structure. The hollow TiO₂/Bi₂O₃ photocatalyst manifests excellent photocatalytic performance and recyclability toward the complete degradation (100%) of antibiotics under visible light (λ > 420 nm) because of the synergistic effect of p-n heterojunction and hollow structure, successfully overcoming the challenge of the incomplete removal of antibiotics over almost all of the reported visible-light photocatalysts. Additionally, the effects of inorganic ions, pH value, water matrix, and outdoor light on the degradation of tetracyclines were investigated with many details. Notably, the degradation pathways and mechanism of tetracycline were revealed according to trapping experiments, HPLC-MS, and photoelectrochemical characterizations. Therefore, this work provides a new insight into developing visible-light photocatalysts with excellent photocatalytic performances for the complete removal of other refractory contaminants.

Singlet oxygen production abilities of oxidated aromatic compounds in natural water

Singlet oxygen (¹O₂) is well known to be formed through energy transfer from excited state organic matters to O₂, playing an important role in the transformations of contaminants. However, the contribution of small oxidated aromatic compounds (OACs) to the production of ¹O₂ in surface water is unclear. In this study, 28 OACs were selected to investigate the correlations between their photochemical production abilities of ¹O₂ and molecular structures. Our results showed that the steady-state concentrations and quantum yields of ¹O₂ (Φ_1O2) generated by OACs were in the range of 7.0 × 10^-14-1.4 × 10^-12 M and 2.2 × 10^-4-4.7 × 10^-2, respectively, indicating that the photochemical production abilities of ¹O₂ by OACs varied greatly with types and positions of functional groups on the molecule. More importantly, the observed photochemical production of ¹O₂ was most notable in cases of molecules containing -OCH₃ group and benzoquinone. A good quantitative structure-property relationship model was established between ¹O₂ producing ability, energy of the lowest unoccupied molecular orbitals (E_LUMO) and the most positive net charge of hydrogen atoms (qH⁺) of OACs. In addition, the role of ¹O₂ produced by 2, 6-dimethoxy-1, 4-benzoquinone, the OAC with the highest Φ_1O2, in the photodegradation of organic contaminants was validated by the enhanced degradation of atorvastatin under simulated sunlight, suggesting that OACs ubiquitously existed in surface water may greatly affect the fate and ecological risks of organic contaminants.

Catalytic degradation of micropollutant by peroxymonosulfate activation through Fe(III)/Fe(II) cycle confined in the nanoscale interlayer of Fe(III)-saturated montmorillonite

Low cost, green, regenerable catalyst for persulfate activation is the popularly concerned topic for the degradation of persistent organic micropollutants in drinking water. In this work, natural montmorillonite (MMT) saturated with Fe(III) ions was used to activate peroxymonosulfate (PMS) for the degradation of atrazine in raw drinking water. Results showed that the adsorption of atrazine was quickly completed within 1 min and the percentage degradation was finally increased up to 94.1% in 60 min. The d₀₀₁-spacing of MMT was enlarged to 2.91 nm at the most by Fe(III) saturation. Atrazine was adsorbed into the nanoscale interlayer of Fe(III)-saturated montmorillonite (Fe-MMT), where the Fe(III)/Fe(II) cycle was sustainably realized through the accelerated transformation of electrons between Fe(III) and PMS. Meanwhile, the in-situ generated Fe(II) accelerated the decomposition of PMS to further proceed the degradation of atrazine through the oxidation of HO• and SO₄•^- radicals. This nanoconfined effect of PMS activation by Fe(III) was further confirmed through the degradation of various micropollutants in the backgrounds of river water. The selective catalytic oxidation of micropollutants through PMS activation was attributed to the 2D mesoporous structure of Fe-MMT, inhibiting the interlayer adsorption of larger molecular backgrounds (humic acids etc.). Fe(III)-saturated montmorillonite (Fe-MMT) provided a feasible and scalable method of PMS activation by Fe(III) for the degradation of micropollutants in drinking water.

Exploiting natural chemical photosensitivity of anhydrotetracycline and tetracycline for dynamic and setpoint chemo-optogenetic control

The transcriptional inducer anhydrotetracycline (aTc) and the bacteriostatic antibiotic tetracycline (Tc) are commonly used in all fields of biology for control of transcription or translation. A drawback of these and other small molecule inducers is the difficulty of their removal from cell cultures, limiting their application for dynamic control. Here, we describe a simple method to overcome this limitation, and show that the natural photosensitivity of aTc/Tc can be exploited to turn them into highly predictable optogenetic transcriptional- and growth-regulators. This new optogenetic class uniquely features both dynamic and setpoint control which act via population-memory adjustable through opto-chemical modulation. We demonstrate this method by applying it for dynamic gene expression control and for enhancing the performance of an existing optogenetic system. We then expand the utility of the aTc system by constructing a new chemical bandpass filter that increases its aTc response range. The simplicity of our method enables scientists and biotechnologists to use their existing systems employing aTc/Tc for dynamic optogenetic experiments without genetic modification.

Formation of free radicals by direct photolysis of halogenated phenols (HPs) and effects of DOM: A case study on monobromophenols

The free radicals play an important role to understand direct/indirect transformation mechanisms of organic pollutants. However, very few efforts have been made to elucidate the radicals produced by direct photolysis. In this study, the short-lived radicals generated under simulated sunlight irradiation from representative halogenated phenols (HPs), monobromophenols, were investigated by electron paramagnetic resonance (EPR). The results showed that three radicals, carbon-centered radical (C), hydrogen radical (H) and hydroxyl radical (OH), were generated from the direct irradiation of HPs. Compared to other substitutions, halogenated atom at para-position led to the highest production of these radicals which is in accordance with the energies calculated by density functional theory. Based on the analyses of the reactive species and corresponding intermediate adducts, the possible reaction pathways for these radicals were tentatively proposed. Dissolved organic matters (DOM) could enhance the photodegradation of HPs by directly affecting the radicals' formation, mainly due to generation of excited triplet DOM (³DOM*). A positive correlation was found between the concentrations of hydrated electron and the steady state ³DOM* from different DOM. Our findings provided insights into environmental photochemical fate of HPs through their direct photolysis and will help more accurately understand their phototransformation mechanisms in the environment.

Photolysis mechanisms of tetracycline under UV irradiation in simulated aquatic environment surrounding limestone

As the most typical geological environment, limestone landforms are widespreading in the world and affect the waters that flow around them, which may also change the fate of organic contaminants in these waters. In this study, aquatic environment surrounding limestone was simulated with calcium carbonate, and the photolysis of tetracycline was evaluated under UV irradiation (30 μW/cm²). More tetracycline (up to 98%) was removed in 4 h in the presence of calcium carbonate while only 50% of tetracycline was eliminated in control experiment. The removal of tetracycline was greatly enhanced due to the major roles of alkaline pH and minor roles of Ca²⁺ and HCO₃^-/CO₃^2-. In alkaline pH, tetracycline existed as TCs^- with higher electronic density in the ring structures, which was more easily attacked by OH. Besides, it could also change the bond orbital energy to facilitate tetracycline absorbing more photon. Moreover, alkaline pH was beneficial to generate more OH and thus promote the indirect photolysis. In addition, alkaline pH also changed the degradation path of tetracycline and rapidly convert tetracycline to the byproducts with m/z 457 via hydroxylation and hydrogen abstraction. This work provides not only better understanding about the fate of tetracycline in aquatic environments but also new insights into the treatment of antibiotic-contaminated water.

Bioremediation of tetracycline antibiotics-contaminated soil by bioaugmentation

Bioaugmentation using specific microbial strains or consortia was deemed to be a useful bioremediation technology for increasing bioremediation efficiency. The present study confirmed the effectiveness and feasibility of bioaugmentation capability of the bacterium BC immobilized on sugarcane bagasse (SCB) for degradation of tetracycline antibiotics (TCAs) in soil. It was found that an inoculation dose of 15% (v/w), 28–43 °C, slightly acidic pH (4.5–6.5), and the addition of oxytetracycline (OTC, from 80 mg kg⁻¹ to 160 mg kg⁻¹) favored the bioaugmentation capability of the bacterium BC, indicating its strong tolerance to high temperature, pH, and high substrate concentrations. Moreover, SCB-immobilized bacterium BC system exhibited strong tolerance to heavy metal ions, such as Pb²⁺ and Cd²⁺, and could fit into the simulated soil environment very well. In addition, the bioaugmentation and metabolism of the co-culture with various microbes was a complicated process, and was closely related to various species of bacteria. Finally, in the dual-substrate co-biodegradation system, the presence of TC at low concentrations contributed to substantial biomass growth but simultaneously led to a decline in OTC biodegradation efficiency by the SCB-immobilized bacterium BC. As the total antibiotic concentration was increased, the OTC degradation efficiency decreased gradually, while the TC degradation efficiency still exhibited a slow rise tendency. Moreover, the TC was preferentially consumed and degraded by continuous introduction of OTC into the system during the bioremediation treatment. Therefore, we propose that the SCB-immobilized bacterium BC exhibits great potential in the bioremediation of TCAs-contaminated environments.

Bioaugmentation using specific microbial strains or consortia was deemed to be a useful bioremediation technology for increasing bioremediation efficiency.

Transformation of tetracyclines induced by Fe(III)-bearing smectite clays under anoxic dark conditions

Smectite clays are widely found in subsurface soils and waters. Although they strongly sequester tetracyclines (TCs), little is known about their reactions with these antibiotics under dark anoxic conditions. This study investigated the interactions between TCs and Fe-bearing smectite clays and the influences of environmental factors. Fe-bearing smectite clays were shown to significantly induce the transformation of TCs, including tautomerization, dechlorination, and dehydration. Moreover, the adsorbed TCs reduced the structural Fe(III) in clay particles to structural Fe(II) through electron transfer. The transformation of TCs was more readily induced by smectite clays with a higher rather than a lower Fe content. Tetrahedral Fe(III), and distorted cis- or trans-octahedral Fe(III), were more reactive as an electron acceptor than cis-octahedral Fe(III), as observed on the Mössbauer and FTIR spectra. A lower pH facilitated the adsorption of TCs through dimethyl-amino, amide, and conjugated -OH functional groups and induced a higher rate of TCs transformation. The transformation of chlortetracycline (CTC) was faster than that of oxytetracycline or tetracycline (TTC) due to -Cl substitution. The major transformation CTC products included keto-CTC, epi-CTC, iso-CTC, anhydro-CTC and TTC. Mixtures of these transformed products were found to have a higher acute toxicity than their parent compounds to Photobacterium phosphoreum T3. Our study revealed several previously overlooked interactions between TCs and clay particles that could cause these antibiotics to become unstable in the subsurface environment, with negative effects on the soil-borne microbial community.

Hydrolysis of Chloramphenicol Catalyzed by Clay Minerals under Nonaqueous Conditions

Soil contamination with antibiotics has raised great environmental concerns, while the abiotic degradation of antibiotics on drought soil particles has been largely ignored. In this study, we examined the transformation of chloramphenicol (CAP) on phyllosilicates under nonaqueous conditions. A significant hydrolysis of CAP mediated by kaolinite occurred under moderate relative humidities (RH: 33-76%) with the half-lives of 10-20 days. By contrast, incubation with montmorillonite did not result in detectable degradation of CAP. Infrared and Raman spectroscopies together with density functional theory calculations suggested that the surface-catalyzed CAP hydrolysis was mainly attributed to the basal plane hydroxyl groups of kaolinite, which formed hydrogen-bond interactions with the carbonyl of CAP such that the hydrolysis activation energy of CAP was greatly reduced. Neither the Brønsted nor the Lewis acidity was the determinant for the hydrolysis reaction. The surface moisture content played an essential role in CAP hydrolysis. Specifically, water facilitated the mass transfer of CAP over the low-RH range, whereas excessive water competed for the reactive hydroxyl sites. These results highlight an important but long-overlooked abiotic transformation pathway for antibiotics in field soil, where the soil moisture is low and the microbial activity is suppressed.