Effects of natural organic matter on the photolysis of tetracycline in aquatic environment: Kinetics and mechanism

Photo-assisted conversion of tetracycline in regulated persulfate system: Multiple roles of natural dissolved organic matter

Advanced oxidation processes (AOPs) using persulfate system can effectively remove organic pollutants. However, dissolved organic matter (DOM) has multiple effects on AOPs efficiency, and the influence of DOMs from natural sources on AOPs is still unclear. In this study, we explored the effects of soil DOM (SDOM) and fertilizer DOM (FDOM) on tetracycline (TC) removal by persulfate systems. DOMs introduction decreased light transmittance, slightly increased the pH of the systems, and destroyed original adsorption-desorption equilibrium. SDOM promoted most reactive species generation in the initial stage, thus improving the initial TC degradation rate. However, introduction of SDOM and FDOM increased the final TC residual rate. FDOM produced more obvious inhibitory effects on TC degradation. The final TC residual rates in systems containing 7.5 and 15 mg L-1 FDOM (F7.5-TC-PS and F15-TC-PS, respectively) were 25.85 % and 25.52 %, respectively. The inhibitory effects of FDOM on TC degradation were related to the combination between TC and FDOM, with humic acid-like component in FDOM being the main contributor. Besides, the main components in DOMs underwent transformation in the persulfate systems. This study sought to provide insights into the regulatory effects of DOM on TC photo-assisted conversion by AOPs.

Photo-transformation of isoproturon under UV-A irradiation: The synergy of nitrite and natural organic matter

Isoproturon (IPU), a widely utilized phenylurea herbicide, is recognized as an emerging contaminant. Previous studies have predominantly attributed the degradation of IPU in natural waters to indirect photolysis by natural organic matter (NOM). Here, we demonstrate that nitrite (NO2-) also serves as an important photosensitizer that induces the photo-degradation of IPU. Through radical quenching tests, we identify hydroxyl radicals (•OH) and nitrogen dioxide radicals (NO2•) originating from NO2- photolysis as key players in IPU degradation, resulting in the generation of a series of hydroxylated and nitrated byproducts. Moreover, we demonstrate a synergistic effect on the photo-transformation of IPU when both NOM and NO2- are present in the reaction mixture. The observed rate constant (kobs) for IPU removal increases to 0.0179 ± 0.0002 min-1 in the co-presence of NO2- (50 μM) and NOM (2.5 mgC/L), surpassing the sum of those in the presence of each alone (0.0135 ± 0.0004 min-1). NOM exhibits multifaceted roles in the indirect photolysis of IPU. It can be excited by UV and transformed to excited triplet states (3NOM*) which oxidize IPU to IPU•+ that undergoes further degradation. Simultaneously, NOM can mitigate the reaction by reducing the IPU•+ intermediate back to the parent IPU. However, the presence of NO2- alters this dynamic, as IPU•+ rapidly couples with NO2•, accelerating IPU degradation and augmenting the formation of mono-nitrated IPU. These findings provide in-depth understandings on the photochemical transformation of environmental contaminants, especially phenylurea herbicides, in natural waters where NOM and NO2- coexist.

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.

Aqueous fate of furaltadone: Kinetics, high-resolution mass spectrometry - based elucidation and toxicity assessment of photoproducts

Furaltadone (FTD) is an antibiotic belonging to the nitrofurans group. It has been broadly used in livestock and aquaculture for therapeutic purposes, as well as for stimulating promotion. Although the European Union has imposed restrictions on the use of FTD since 1995 due to concerns regarding its toxicity, in many cases FTD has been excessively and/or illegally applied in productive animals in developing countries, because of its high efficacy and low-cost. Unlike other nitrofuran compounds, the hydrolytic and photolytic behavior of FTD in natural aquatic systems has not been thoroughly investigated. To this end, hydrolysis in different pH values and photolysis in aquatic environment, including lake, river and sea water have been both examined. Hydrolysis was found to have an insignificant impact on degradation of FTD in the aquatic environment relevant pH values, whereas indirect photolysis proved to be the main route of its elimination. The identification of tentative photoproducts (PPs) was performed using ultra high performance liquid chromatography coupled to hybrid LTQ/Orbitrap high resolution mass spectrometry. A possible pathway for photolytic transformation of FTD was proposed. Additionally, in silico simulations were used to evaluate the toxicity such as the mutagenicity of FTD and PPs. Complementary to the low-cost and time-limited simulations, an in vitro method (Vibrio Fischeri bioluminescence) was also used to assess ecotoxicity.

Comparing the photodegradation of typical antibiotics in ice and in water: Degradation kinetics, mechanisms, and effects of dissolved substances

New antibiotic contaminants have been detected in both surface waters and natural ice across cold regions. However, few studies have revealed distinctions between their ice and aqueous photochemistry. In this study, the photodegradation and effects of the main dissolved substances on the photolytic kinetics were investigated for sulfonamides (SAs) and fluoroquinolones (FQs) in ice/water under simulated sunlight. The results showed that the photolysis of sulfamethizole (SMT), sulfachloropyridazine (SCP), enrofloxacin (ENR) and difloxacin (DIF) in ice/water followed the pseudo-first-order kinetics with their quantum yields ranging from 4.93 × 10-3 to 11.15 × 10-2. The individual antibiotics experienced disparate photodegradation rates in ice and in water. This divergence was attributed to the concentration-enhancing effect and the solvent cage effect that occurred in the freezing process. Moreover, the main constituents (Cl-, HASS, NO3- and Fe(III)) exhibited varying degrees of promotion or inhibition on the photodegradation of SAs and FQs in the two phases (p < 0.05), and these effects were dependent on the individual antibiotics and the matrix. Extrapolation of the laboratory data to the field conditions provided a reasonable estimate of environmental photolytic half-lives (t1/2,E) during midsummer and midwinter in cold regions. The estimated t1/2,E values ranged from 0.02 h for ENR to 14 h for SCP, which depended on the reaction phases, latitudes and seasons. These results revealed the similarities and differences between the ice and aqueous photochemistry of antibiotics, which is important for the accurate assessment of the fate and risk of these new pollutants in cold environments.

Elucidating the inhibitory effects of natural organic matter on the photodegradation of organic micropollutants: Atrazine as a probe compound

Natural organic matter (NOM) is a complex mixture of heterogeneous compounds with varying functional groups and molecular sizes. Understanding the impact of NOM on the generation of photochemically produced reactive intermediates (PPRIs) and their potential inhibitory effects on photolysis has remained challenging due to the variations in the reactivities and concentrations of these functional groups. To address this gap, tannic acid (TA), gallic acid (GA), catechin (CAT), and tryptophan (Trp), were chosen as potential substitutes for NOM. Their effects on the photochemical transformation process were evaluated and compared with the widely used Suwannee River NOM (SRNOM). Atrazine (ATZ) was selected as a probe organic micropollutant (OMP). In this investigation, a significantly higher concentration of HO• was observed compared to O21, and the triplet excited state ( NOM*3). The findings suggest that the substituted phenols, particularly those with carboxylate-substitutions, played a substantial role in HO• formation, while electron-rich moieties acted as antioxidants, consuming NOM*3. Hydroxyl, carboxylic, and amino acid were the active groups for O21 formation. However, the inhibitory effects induced by the NOM surrogates were significant and mainly attributed to the direct photolysis inhibition caused by the inner filter effect. The scope of this work was further extended to include SRNOM, where similar trends with less pronounced formation of PPRIs and inner filter effects were observed. Therefore, this study sheds some light on the role of the functional groups in NOM during photochemical transformations of OMPs, thereby deepening our understanding of their fate in aqueous systems.

Photo-Reactivity of dissolved black carbon unveiled by combination of optical spectroscopy and FT-ICR MS analysis: Effects of pyrolysis temperature

Dissolved black carbon (DBC) has high photoactivity, which plays an important role in contaminants photodegradation. However, it is unclear how pyrolysis temperatures would affect the composition and photo-reactivity of DBC at the molecular level. Herein, we combined complementary techniques to study the characteristics of DBC pyrolyzed at 200 - 500 ℃, as well as the photoproduction of reactive species and the photodegradation of tetracycline (TC). Bulk composition characterization found that condensed aromatic carbonyl compounds (ConAC) with narrow molecular weights in DBC experienced an increase from 200 to 500 °C, which enhanced the photoproduction of 3DBC*,1O2, and ·OH. Molecular-level data suggested that 3DBC* and 1O2 were both related to the same DBC compounds. Comparatively, the patterns for ·OH were less pronounced, implying its precursor was not 3DBC* and had more complexity. Plentiful CHOx species of ConAC in DBC400 and DBC500 (DBCT, where T = pyrolysis temperature) accelerated the generation of 3DBC* and 1O2, enhancing the photodegradation of TC, and mainly triplet states of quinones reacted with TC. In contrast, DBC200 and DBC300 exhibited inhibition since massive CHOx species in lignin-like reduced 3TC* to TC. Our data revealed the diverse photochemical behavior mechanisms of DBC pyrolyzed at 200 - 500 ℃ at the molecular level and the implications for aquatic contaminants photochemistry.

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.

Enhanced photolysis of tetracycline by Zn(II): Role of complexation

Zn(II) is a necessary additive during antibiotic production and aquaculture, leading to the coexistence of Zn(II) and antibiotics in aquatic environment, especially in receiving waters of pharmaceutical and aquaculture wastewater. However, the roles of Zn(II) in the photochemical behavior of antibiotics are still not clear, which limits the understanding of the fate of antibiotic in nature. In this study, tetracycline (TC) was selected as typical antibiotic to evaluate the effect of Zn(II) on antibiotic photolysis. The removal of TC was accelerated by 22.75 % with TC:Zn(II) molar ratio at 1:5. The mechanism of Zn(II)-induced TC photolysis was explored via reactive oxygen species (ROS) analysis and density functional theory (DFT) calculation for the first time. Zn(II) could enhance the formation of TC excited states and further produce more singlet oxygen (12.54 % higher than control group) to promote indirect photolysis. Besides, Zn(II) could react with TC via complexation, and the complex was more vulnerable to attack by reactive oxygen species due to more active sites. Furthermore, the structure and toxicity of intermediates were identified with mass spectrometer, T.E.S.T. and ECOSAR software. Zn(II) hardly changed the degradation path of TC, and TC was mainly degraded via ring opening, demethylation, deamidation, and hydrogen abstraction with more toxic intermediates than the parent molecule. This work is significant to better understand the environmental fate of antibiotics, and also provides new insight into wastewater treatment in the pharmaceutical and aquaculture industry.

Ecotoxicological Study of Tannic Acid on Soil and Water Non-Target Indicators and Its Impact on Fluvial and Edaphic Communities

Tannic acid (TA) is a key tannin extensively used in the leather industry, contributing to around 90% of global leather production. This practice leads to the generation of highly polluting effluents, causing environmental harm to aquatic ecosystems. Additionally, tannins like TA degrade slowly under natural conditions. Despite efforts to reduce pollutant effluents, limited attention has been devoted to the direct environmental impact of tannins. Moreover, TA has garnered increased attention mainly due to its applications as an antibacterial agent and anti-carcinogenic compound. However, our understanding of its ecotoxicological effects remains incomplete. This study addresses this knowledge gap by assessing the ecotoxicity of TA on non-target indicator organisms in both water (Vibrio fischeri, Daphnia magna) and soil environments (Eisenia foetida, Allium cepa), as well as natural fluvial and edaphic communities, including periphyton. Our findings offer valuable insights into TA's ecotoxicological impact across various trophic levels, underscoring the need for more comprehensive investigations in complex ecosystems. Our results demonstrate that TA exhibits ecotoxicity towards specific non-target aquatic organisms, particularly V. fischeri and D. magna, and phytotoxicity on A. cepa. The severity of these effects varies, with V. fischeri being the most sensitive, followed by D. magna and A. cepa. However, the soil-dwelling invertebrate E. foetida shows resistance to the tested TA concentrations. Furthermore, our research reveals that substantial TA concentrations are required to reduce the growth of river microbial communities. Metabolic changes, particularly in amino acid and amine metabolism, are observed at lower concentrations. Notably, the photosynthetic yield of river periphyton remains unaffected, even at higher concentrations. In contrast, soil microbial communities exhibit greater sensitivity, with significant alterations in population growth and metabolic profiles at a very low concentration of 0.2 mg/L for all metabolites. In summary, this study offers valuable insights into the ecotoxicological effects of TA on both aquatic and terrestrial environments. It underscores the importance of considering a variety of non-target organisms and complex communities when assessing the environmental implications of this compound.

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.

Pervasive Selection for Clinically Relevant Resistance and Media Adaptive Mutations at Very Low Antibiotic Concentrations

Experimental evolution studies have shown that weak antibiotic selective pressures (i.e., when the antibiotic concentrations are far below the minimum inhibitory concentration, MIC) can select resistant mutants, raising several unanswered questions. First, what are the lowest antibiotic concentrations at which selection for de novo resistance mutations can occur? Second, with weak antibiotic selections, which other types of adaptive mutations unrelated to the antibiotic selective pressure are concurrently enriched? Third, are the mutations selected under laboratory settings at subMIC also observed in clinical isolates? We addressed these questions using Escherichia coli populations evolving at subMICs in the presence of either of four clinically used antibiotics: fosfomycin, nitrofurantoin, tetracycline, and ciprofloxacin. Antibiotic resistance evolution was investigated at concentrations ranging from 1/4th to 1/2000th of the MIC of the susceptible strain (MICsusceptible). Our results show that evolution was rapid across all the antibiotics tested, and selection for fosfomycin- and nitrofurantoin-resistant mutants was observed at a concentration as low as 1/2000th of MICsusceptible. Several of the evolved resistant mutants showed increased growth yield and exponential growth rates, and outcompeted the susceptible ancestral strain in the absence of antibiotics as well, suggesting that adaptation to the growth environment occurred in parallel with the selection for resistance. Genomic analysis of the resistant mutants showed that several of the mutations selected under these conditions are also found in clinical isolates, demonstrating that experimental evolution at very low antibiotic levels can help in identifying novel mutations that contribute to bacterial adaptation during subMIC exposure in real-life settings.

Natural detoxification of antibiotics in the environment: A one health perspective

The extended concept of one health integrates biological, geological, and chemical (bio-geo-chemical) components. Anthropogenic antibiotics are constantly and increasingly released into the soil and water environments. The fate of these drugs in the thin Earth space ("critical zone") where the biosphere is placed determines the effect of antimicrobial agents on the microbiosphere, which can potentially alter the composition of the ecosystem and lead to the selection of antibiotic-resistant microorganisms including animal and human pathogens. However, soil and water environments are highly heterogeneous in their local composition; thus the permanence and activity of antibiotics. This is a case of "molecular ecology": antibiotic molecules are adsorbed and eventually inactivated by interacting with biotic and abiotic molecules that are present at different concentrations in different places. There are poorly explored aspects of the pharmacodynamics (PD, biological action) and pharmacokinetics (PK, rates of decay) of antibiotics in water and soil environments. In this review, we explore the various biotic and abiotic factors contributing to antibiotic detoxification in the environment. These factors range from spontaneous degradation to the detoxifying effects produced by clay minerals (forming geochemical platforms with degradative reactions influenced by light, metals, or pH), charcoal, natural organic matter (including cellulose and chitin), biodegradation by bacterial populations and complex bacterial consortia (including "bacterial subsistence"; in other words, microbes taking antibiotics as nutrients), by planktonic microalgae, fungi, plant removal and degradation, or sequestration by living and dead cells (necrobiome detoxification). Many of these processes occur in particulated material where bacteria from various origins (microbiota coalescence) might also attach (microbiotic particles), thereby determining the antibiotic environmental PK/PD and influencing the local selection of antibiotic resistant bacteria. The exploration of this complex field requires a multidisciplinary effort in developing the molecular ecology of antibiotics, but could result in a much more precise determination of the one health hazards of antibiotic production and release.

Double Z-scheme Co3O4/Bi4O7/Bi2O3 composite activated peroxymonosulfate to efficiently degrade tetracycline under visible light

Co₃O₄/Bi₄O₇/Bi₂O₃ (CBB) composites were prepared, in which Co₃O₄ was synthesized from Co-MOF as precursor. The peroxymonosulfate (PMS) activated by CBB catalyst under visible light was used to degrade tetracycline (TC). Owing to the synergistic effect of photocatalysis and PMS activation, 98.4% of TC was removed within 60 min. The optimal loading of Co₃O₄ was determined, and the influence of PMS dosage, initial pH, and disturbing anions on the degradation effect were investigated. The "CBB + Vis + PMS" system showed good reusability, and the degradation was only reduced by 1.7% after 5 cycles. This system also had a good degradation of other five pollutants. The quenching experiment showed that holes (h⁺), superoxide radicals (·O₂^-), and singlet oxygen (¹O₂) were the main active species. The degradation products of TC were determined by liquid chromatography-mass spectrometry, and the degradation pathway was proposed. Finally, a double Z-scheme degradation mechanism was proposed in the "CBB + Vis + PMS" system. The peroxymonosulfate activated by CBB under visible light to degrade organic pollutants has widespread application prospects in environmental remediation.

Roles of microplastic-derived dissolved organic matter on the photodegradation of organic micropollutants

Microplastic-derived dissolved organic matter (MP-DOM) is ubiquitous in water environment and exhibits photosensitivity. However, little is known about the effects of MP-DOM on the photodegradation of organic micropollutants in natural water. In this study, we investigated the effect of MP-DOM derived from two typical plastics, i.e., polystyrene (PS), and polyethylene (PE), on the photodegradation of a typical organic micropollutants sulfamethoxazole (SMX) in a simulative natural water system. MP-DOM exerted a significant inhibition on the SMX photodegradation, mainly attributed to the direct photolysis inhibition of SMX caused by the inner filter effect and the complexation effect. Despite the enhanced reactive oxygen species (ROS) generation with the increase of their steady-state concentration by 41.1 - 160.7 %, PS-DOM exhibited high oxidation resistance, causing an inhibition on the photodegradation of SMX probably through transferring electrons to the SMX intermediates. This study helps to deepen the understanding of microplastic photochemical behavior in natural water.

How Nitrogen and Sulfur Doping Modified Material Structure, Transformed Oxidation Pathways, and Improved Degradation Performance in Peroxymonosulfate Activation

Current research has widely applied heteroatom doping for the promotion of catalyst activity in peroxymonosulfate (PMS) systems; however, the relationship between heteroatom doping and stimulated activation mechanism transformation is not fully understood. Herein, we introduce nitrogen and sulfur doping into a Co@rGO material for PMS activation to degrade tetracycline (TC) and systematically investigate how heteroatom doping transformed the activation mechanism of the original Co@rGO/PMS system. N was homogeneously inserted into the reduced graphene oxide (rGO) matrix of Co@rGO, inducing a significant increase in the degradation efficiency without affecting the activation mechanism transformation. Additionally, S doping converted Co₃O₄ to Co₄S₃ in Co@rGO and transformed the cooperative oxidation pathway into a single non-radical pathway with stronger intensity, which led to a higher stability against environmental interferences. Notably, based on density functional theory (DFT) calculations, we demonstrated that Co₄S₃ had a higher energy barrier for PMS adsorption and cleavage than Co₃O₄, and therefore, the radical pathway was not easily stimulated by Co₄S₃. Overall, this study not only illustrated the improvement due to the heteroatom doping of Co@rGO for TC degradation in a PMS system but also bridged the knowledge gap between the catalyst structure and degradation performance through activation mechanism transformation drawn from theoretical and experimental analyses.

Why the cooperation of radical and non-radical pathways in PMS system leads to a higher efficiency than a single pathway in tetracycline degradation

Current research focused on developing multiple active species in peroxymonosulfate (PMS) system to degrade contaminants, but deepening concern lacks over why cooperation of those active species facilitated a faster degradation. Here, we employed Co₃O₄, rGO and Co₃O₄@rGO composite to activate PMS for tetracycline (TC) degradation, and detected crucial factors toward highest performance of Co₃O₄@rGO/PMS system. Batch experiments exhibited a satisfactory TC degradation efficiency under Co₃O₄@rGO/PMS, complete degraded 50 mg/L TC within 20 min. Analytical tests discovered that radical active species generated by Co₃O₄/PMS and non-radical species by rGO/PMS were successfully co-existed in Co₃O₄@rGO/PMS system, significantly improving the performance of TC removal. Subsequently, a combination of density functional theory (DFT) calculation and intermediates analysis revealed that, in Co₃O₄@rGO/PMS system, the cooperation rather than independent effect of radical and non-radical active species expanded TC degradation pathways, enhancing the degradation performance. Furthermore, decent adaptability, stability, and recyclability toward affecting factors variation of Co₃O₄@rGO/PMS demonstrated it as a potent and economical system to degrade TC. Overall, this study developed a novel Co₃O₄@rGO/PMS system with a cooperative oxidation pathway for highly efficient TC removal, and managed to clarify why this oxidation pathway achieved high efficiency through a combination of theoretical and experimental method.

Enhanced degradation mechanism of tetracycline by MnO2 with the presence of organic acids

In this study, we constructed MnO₂/organic acid (OA) systems using MnO₂ colloid, the most reactive phase of Mn(IV), and two kinds of OA (oxalic acid and l-tartaric acid). We investigated the effect of OA on tetracycline (TC) degradation by MnO₂. The results show that both OA obviously accelerate TC degradation by MnO₂. Mn(III) formed during the reaction lead to the acceleration. Mn(III)-oxalate complex formed in oxalic acid system resulted in the lower degradation efficiency than that in l-tartaric acid system. The acceleration of oxalic acid was decreased when the concentration was more than 75 μM, and even completely disappeared with the concentration of 500 μM, owning to the fact that excess oxalic acid decreased the pH and some MnO₂ was fast reduced to Mn²⁺ by oxalic acid and unable to react with TC. The impact of pH on TC degradation resulted from the influences of H⁺ on MnO₂ redox potentials and TC deprotonation. And acidic conditions accelerated TC degradation. The addition of Mg²⁺, Ca²⁺, Fe³⁺ and Zn²⁺ exhibited an inhibitory effect in both systems for their occupying reactive sites on MnO₂ surface and blocking the access of TC to MnO₂. Similar intermediates in the two systems were detected, indicating a similar TC degradation mechanism including a series of reactions like dehydration, hydroxylation and oxidation. The MnO₂/OA system provides an efficient treatment of TC in wastewater. And it is also noticeable that MnO₂/OA system should also have an important effect on the fate of pollutants in environment, from our results.

UV-Irradiation Facilitating Pb Release from Recycled PVC Microplastics

Microplastics (MPs) are ubiquitously in ecosystem and have evoked wide attention. The potential risk of MPs to the ecosystems is associated with MPs and the additives such as Pb, which serves as a traditional stabilizer. However, the release of Pb from MPs remains largely unknown. In this study, we evaluated the release of Pb from recycled polyvinyl chloride (PVC) under UV-irradiation. The release process was dominated by two processes: H⁺ facilitated dissolution of Pb, and light-induced hydroxyl radical (·OH) caused C-H bond cleavage from PVC with the generation of alkyl radical. The effects of pH and coexisting low molecular weight organic acids (LMWOAs) were also evaluated. Lower pH speeds up the Pb release from MPs. The LMWOAs act as a filter of UV to restrain the Pb release. Overall, this study shows the release of Pb from recycled PVC MPs and indicates the potential risk of Pb to the environment.