Local Structure and Crystallization Transformation of Hydrous Ferric Arsenate in Acidic H2O-Fe(III)-As(V)-SO42- Systems: Implications for Acid Mine Drainage and Arsenic Geochemical Cycling

Hydrous ferric arsenate (HFA) is a common thermodynamically metastable phase in acid mine drainage (AMD). However, little is known regarding the structural forms and transformation mechanism of HFA. We investigated the local atomic structures and the crystallization transformation of HFA at various Fe(III)/As(V) ratios (2, 1, 0.5, 0.33, and 0.25) in acidic solutions (pH 1.2 and 1.8). The results show that the Fe(III)/As(V) in HFA decreases with decreasing initial Fe(III)/As(V) at acidic pHs. The degree of protonation of As(V) in HFA increases with increasing As(V) concentrations. The Fe K-edge extended X-ray absorption fine structure and X-ray absorption near-edge structure results reveal that each FeO6 is linked to more than two AsO4 in HFA precipitated at Fe(III)/As(V) < 1. Furthermore, the formation of scorodite (FeAsO4·2H2O) is greatly accelerated by decreasing the initial Fe(III)/As(V). The release of As(V) from HFA is observed during its crystallization transformation process to scorodite at Fe(III)/As(V) < 1, which is different from that at Fe(III)/As(V) ≥ 1. Scanning electron microscopy results show that Oswald ripening is responsible for the coarsening of scorodite regardless of the initial Fe(III)/As(V) or pH. Moreover, the formation of crystalline ferric dihydrogen arsenate as an intermediate phase at Fe(III)/As(V) < 1 is responsible for the enhanced transformation rate from HFA to scorodite. This work provides new insights into the local atomic structure of HFA and its crystallization transformation that may occur in AMD and has important implications for arsenic geochemical cycling.

Intrinsic Molecular Mechanisms of Transformation between Isomeric Intermediates Formed at Different Stages of Cysteine-Xylose Maillard Reaction Model through Dehydration

2-Threityl-thiazolidine-4-carboxylic acid (TTCA) and Amadori rearrangement product (ARP), the isomeric intermediates derived from the cysteine-xylose (Cys-Xyl) Maillard reaction model, possessed the ability to produce similar flavor profile during the thermal process, but the flavor formation or browning rate of heated TTCA was significantly lower than that of ARP. Macroscopically, the yield of TTCA reached the maximum when the moisture content of the reaction system just dropped to nearly 0% during the thermal reaction-vacuum dehydration process. During the subsequent dynamic intramolecular dehydration process, the reaction remained at an early stage of the Maillard reaction, and TTCA was the main intermediate. Thereinto, the water activity of the samples decreased with the increased dehydration time. From a molecular perspective, the dissipation of free water promoted the conversion of combined water to immobilized water and free water, increasing the intramolecular dehydration. Instantaneous high-temperature dehydration during the spray drying process revealed a higher efficiency than the thermal reaction-vacuum dehydration process, which facilitated the specific conversion of substrates to intermediates (TTCA, ARP). The loss of free water and immobilized water was a key driving force for the direct formation of TTCA/ARP, regulating the formation stages of MRIs. The increase of the inlet air temperature could alter the ratio of TTCA and ARP at the equilibrium state.

Theoretical Study of the Hydroxyl-Radical-Initiated Degradation Mechanism, Kinetics, and Subsequent Evolution of Methyl and Ethyl Iodides in the Atmosphere

The degradation and transformation of iodinated alkanes are crucial in the iodine chemical cycle in the marine boundary layer. In this study, MP2 and CCSD(T) methods were adopted to study the atmospheric transformation mechanism and degradation kinetic properties of CH₃ I and CH₃ CH₂ I mediated by ⋅OH radical. The results show that there are three reaction mechanisms including H-abstraction, I-substitution and I-abstraction. The H-abstraction channel producing ⋅CH₂ I and CH₃ C ⋅ HI radicals are the main degradation pathways of CH₃ I and CH₃ CH₂ I, respectively. By means of the variational transition state theory and small curvature tunnel correction method, the rate constants and branching ratios of each reaction are calculated in the temperature range of 200-600 K. The results show that the tunneling effect contributes more to the reaction at low temperatures. Theoretical reaction rate constants of CH₃ I and CH₃ CH₂ I with ⋅OH are calculated to be 1.42×10^-13 and 4.44×10^-13  cm³  molecule^-1  s^-1 at T=298 K, respectively, which are in good agreement with the experimental values. The atmospheric lifetimes of CH₃ I and CH₃ CH₂ I are evaluated to be 81.51 and 26.07 day, respectively. The subsequent evolution mechanism of ⋅CH₂ I and CH₃ C ⋅ HI in the presence of O₂ , NO and HO₂ indicates that HCHO, CH₃ CHO, and I-atom are the main transformation end-products. This study provides a theoretical basis for insight into the diurnal conversion and environmental implications of iodinated alkanes.

An Exploration of the Transformation of the 8-Oxo-7,8-Dihydroguanine Radical Cation to Protonated 2-Amino-5-Hydroxy-7,9-Dihydropurine-6,8-Dione in a Base Pair

A theoretical investigation was performed to disclose the transformation mechanism of 8-oxo-7,8-dihydroguanine radical cation (8-oxoG⋅⁺ ) to protonated 2-amino-5-hydroxy-7,9-dihydropurine-6,8-dione (5-OH-8-oxoG) in base pair. The energy profiles for three possible pathways of the events were mapped. It is shown that direct loss of H7 from base paired 8-oxoG⋅⁺ is the only energetically favorable pathway to generate neutral radical, 8-oxoG(-H7)⋅. Further oxidation of 8-oxoG(-H7)⋅ : C to 8-oxoG(-H7)⁺  : C is exothermic. However, the 8-oxoG(-H7)⁺  : C deprotonation from all possible active sites is infeasible, indicating the inaccessible second proton loss and the lack of essential intermediate 2-amino-7,9-dihydropurine-6,8-dione (8-oxoG^OX ). This makes 8-oxoG(-H7)⁺ act as the precursor of hydration leading to the generation of protonated 5-HO-8-oxoG by stepwise fashion in base pair, which would initiate the step down guanidinohydantoin (Gh) pathway. These results clearly specify the structure-dependent transformation for 8-oxoG⋅⁺ and verify the emergence of protonated 5-HO-8-oxoG in base pair.

MgAl Saponite as a Transition-Metal-Free Anode Material for Lithium-Ion Batteries

Transition-metal compounds (oxides, sulfides, hydroxides, etc.) as lithium-ion battery (LIB) anodes usually show extraordinary capacity larger than the theoretical value due to the transformation of LiOH into Li₂O/LiH. However, there has rarely been a report relaying the transformation of LiOH into Li₂O/LiH as the main reaction for LIBs, due to the strong alkalinity of LiOH leading to battery deterioration. In this work, layered silicate MgAl saponite (MA-SAP) is applied as a -OH donor to generate LiOH as the anode material of LIBs for the first time. The MA-SAP maintains a layered structure during the (dis)charging process and has zero-strain characteristic on the (001) crystal plane. In the discharging process, Mg, Al, and Si in the saponite sheets become electron-rich, while the active hydroxyl groups escape from the sheets and combine with lithium ions to form LiOH in the "caves" on sheets, and the LiOH continues to decompose into Li₂O and LiH. Consequently, the MA-SAP delivers a maximum capacity of 536 mA h·g^-1 at 200 mA·g^-1 with a good high-current discharging ability of 155 mA h·g^-1 after 1000 cycles under 1 A·g^-1. Considering its extremely low cost and completely nontoxic characteristics, MA-SAP has great application prospects in energy storage. In addition, this work has an enlightening effect on the development of new anodes based on extraordinary mechanisms.

[Emission Characteristics, Transformation Mechanism, and Reduction Potential of Ammonia Emissions from a Crop Rotation System in Yangtze River Delta]

To study the characteristics and reduction potential of the ammonia emissions of a crop rotation system in the Yangtze River Delta, we monitored and compared the ammonia fluxes from two rotation systems:a conventional rice/winter wheat rotation system and a rice-shrimp cultivation/Chinese milk vetch rotation system. This study was conducted through closing chamber methods to investigate the influencing factors and transformation mechanism of ammonium emissions between the two studied cultivation patterns. Additionally, we established the temporal-spatial emission inventory by sorting out the local ammonia emission factors of farmland in the Yangtze River Delta in the last ten years. The emission reduction effects under different ammonia emission reduction paths were also obtained. The results showed that, the cumulative amount of ammonia emissions throughout the whole monitoring year for the conventional rice/winter wheat rotation system (CR-W) and the rice-shrimp cultivation/Chinese milk vetch rotation system (RS-C) were 65.95 and 20.31 kg·hm^-2, respectively, whereas the ammonia loss rates of CR-W and RS-C were 10.86% and 9.20%, respectively. Field surface water NH₄⁺-N, field surface water pH, and topsoil NH₄⁺-N were the major internal factors of ammonia emissions from paddy fields, whereas topsoil NH₄⁺-N and atmospheric temperature had an important impact on ammonia emissions in the wheat season. The ammonia flux/field NH₄⁺-N ratio (ARN) of field surface water under the CR and RS modes in the rice season reached 0.35±0.27 and 0.14±0.19, respectively, which was 10-25 times that of topsoil in the wheat season, such that the ammonia emission flux in the rice season was significantly higher than that in the wheat season. Under the conditions of high field water pH (8.0-9.0), atmospheric temperature (>28℃), and wind speed (>5.0 m·s^-1), the ammonia flux/field NH₄⁺-N ratios (ARN) were around 1.6-4.6 times that under low pH, temperature, and wind speed conditions, indicating that those three factors were the main factors affecting the conversion of NH₄⁺-N from farmland to atmospheric NH₃. Fertilization types also had significant effects on ARN; under different conditions, the ARN of urea was 1.5-5.5 times that of organic fertilizer. In 2019, the ammonia emission flux of rice and wheat under a conventional planting pattern in the Yangtze River Delta were (49.2±17.6) kg·hm^-2 and (16.0±13.5) kg·hm^-2, respectively, whereas the ammonia loss rates of rice and wheat were (20.1±5.7)% and (5.9±3.6)%, respectively. The ammonia emission loss rate of the former was about three times that of the latter. The ammonia emission inventory built by local factors shows that the total ammonia emissions of the farmland rotation system in the Yangtze River Delta reached (400.3±206.4) kt in 2019, which was mainly concentrated in the central and northern regions of Anhui province and Jiangsu province, and the ammonia emission intensity reached (1.33±1.39) t·km^-2. The selection of different emission factors had a relatively large impact on the change range of the inventory results, reaching the standard of -51.6%~51.6%. Through combing and analyzing the six main paths of ammonia emission reduction in farmland, it was found that nitrogen fertilizer synergism was the best way to reduce ammonia emissions, with the efficiency of (30.9±51.4)%; however, the grain yield increase rate was (-4.2±17.4)%, with great uncertainty. The ammonia emission reduction effect of adding soil additives was relatively poor (-5.4±45.1)%; however, the grain yield increase rate was the highest among those of the six emission reduction paths, reaching (6.8±23.9)%. The ammonia emission reduction effect and grain yield increase rate of the ecological planting and breeding mode were (22.3±15.1)% and (5.6±3.8)%, respectively, which had the advantages of reducing ammonia emissions and increasing crop yield.

Transformation of Tetracycline by Manganese Peroxidase from Phanerochaete chrysosporium

The negative impacts on the ecosystem of antibiotic residues in the environment have become a global concern. However, little is known about the transformation mechanism of antibiotics by manganese peroxidase (MnP) from microorganisms. This work investigated the transformation characteristics, the antibacterial activity of byproducts, and the degradation mechanism of tetracycline (TC) by purified MnP from Phanerochaete chrysosporium. The results show that nitrogen-limited and high level of Mn²⁺ medium could obtain favorable MnP activity and inhibit the expression of lignin peroxidase by Phanerochaete chrysosporium. The purified MnP could transform 80% tetracycline in 3 h, and the threshold of reaction activator (H₂O₂) was about 0.045 mmol L^-1. After the 3rd cyclic run, the transformation rate was almost identical at the low initial concentration of TC (77.05-88.47%), while it decreased when the initial concentration was higher (49.36-60.00%). The antimicrobial potency of the TC transformation products by MnP decreased throughout reaction time. We identified seven possible degradation products and then proposed a potential TC transformation pathway, which included demethylation, oxidation of the dimethyl amino, decarbonylation, hydroxylation, and oxidative dehydrogenation. These findings provide a novel comprehension of the role of MnP on the fate of antibiotics in nature and may develop a potential technology for tetracycline removal.

A Practical Protocol for a Comprehensive Evaluation of Sulfur Fumigation of Trichosanthis Radix Based on Both Non-Targeted and Widely Targeted Metabolomics

Trichosanthis Radix (TR) is one of the most severely sulfur-fumigated herbs in the market, whose transformation mechanism of chemical compositions and sulfur-fumigation markers of TR have not been clarified. To excavate characteristic sulfur-fumigation markers of TR samples, this study brings up a practical protocol using both ultra-performance liquid chromatography/quadrupole time-of-flight-mass spectrum (UPLC-ESI-QTOF-MS/MS)-based non-targeted metabolomics and ultra-performance liquid chromatography/electrospray ionization/quadrupole multiple-stage linear ion-trap mass spectrum (UPLC-ESI-QTRAP-MS/MS)-based widely targeted metabolomics. The results of study demonstrated that five characteristic markers are sulfur-containing components, which were identified as p-Hydroxybenzyl hydrogen sulfite, cucurbitacin D sulfite I, cucurbitacin D sulfite II, cucurbitacin B sulfite I, and cucurbitacin B sulfite II, respectively. Additionally, cucurbitacin B and D were also filtered and identified as the characteristic sulfur-fumigation markers. Meanwhile, the different sulfur-fumigation extent of TR samples was tested by chemical transformations analysis and sulfur dioxide residues test. Further, 58.16% (139 of 239) of the differential metabolites content significantly reduced in sulfur-fumigated TR samples. Besides, 20 kinds of non-sulfur marker metabolites were tested to evaluate the quality of TR samples before and after sulfur fumigation, predominantly including phenolic acids, amino acids, lipids and nucleotides. Taking TR as an example, this work provides a comprehensive practical protocol for the quality supervision of sulfur-fumigation herbs.

The Potential Transformation Mechanisms of the Marker Components of Schizonepetae Spica and Its Charred Product

Schizonepetae Spica (SS) is commonly used for treating colds, fevers, bloody stool and metrorrhagia in China. To treat colds and fevers, traditional Chinese medicine doctors often use raw SS, while to treat bloody stool and metrorrhagia, they usually use Schizonepetae Spica Carbonisata (SSC; raw SS processed by stir-frying until carbonization). However, there have been limited investigations designed to uncover the mechanism of stir-fry processing. In the present study, a method combining gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC) was developed for the comprehensive analysis of the chemical profiles of SS and SSC samples. Principal component analysis of the GC-MS data demonstrated that there were 16 significant differences in volatile compounds between the SS and SSC samples. The simultaneous quantification of six nonvolatile compounds was also established based on HPLC, and remarkable differences were found between the two products. These changes were probably responsible for the various pharmacological effects of SS and SSC as well as the observed hepatotoxicity. Finally, the mechanisms could be rationalized by deducing possible reactions involved in the transformation of these marker components. This work reports a new strategy to reveal the chemical transformation of SS during stir-fry processing.

Constituent transformation mechanism of concentrated leachate after incineration at different temperatures

Spraying concentrated leachate into an incineration furnace and burning is encouraged by the Chinese government as a harmless method for leachate treatment. In this research, the constituent transformation mechanism was studied, by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectrometry (EDS), of residues after burning the concentrated leachate in a muffle furnace at different temperatures (750 °C, 850 °C, 900 °C, 1000 °C, and 1100 °C). XRD results showed that the main components of the residues were metal chlorides and calcium salt crystals and that the peak position of most of these crystals changed little because their crystal structure was stable at high temperatures. SEM results illustrated that the higher the burning temperature, the smaller the solid particles, and the looser the structures of the residues. EDS analysis showed that S atoms in the concentrated leachate were usually transformed into gaseous compounds during incineration, whereas most of the Cl atoms could be fastened onto solid residues if an appropriate temperature was maintained. This study concluded that 900 °C was the best burning temperature for spraying concentrated leachate into the furnace. In addition, this study suggested that material selection for the nozzle and flue gas pipelines must pay more attention to corrosion caused by gaseous sulfur compounds. Similarly, material selection for the inner wall of the incinerator furnace should pay more attention to corrosion caused by Cl atoms. This means that the municipal solid waste (MSW) incineration power plants should incorporate various and appropriate corrosion-resistant materials according to the different regions of the incinerator.

[Influencing Factors and Transformation Mechanism of Venlafaxine Degradation by UV]

As a member of antidepressants, venlafaxine degradation by UV was investigated in this study. The influence of initial concentration of venlafaxine, pH value, and water quality on venlafaxine removal were studied. The results showed that venlafaxine degradation fitted the pseudo-first-order kinetics model. The reaction rate increased with the decrease of venlafaxine concentration. It was found that the highest removal rate of venlafaxine occurred at pH 10. The degradation of venlafaxine involves direct photolysis and self-sensitized photolysis caused by hydroxyl radical (·OH) and singlet oxygen (¹O₂). Moreover, the direct photolysis played a dominant role in the venlafaxine removal. Water matrix also affected venlafaxine degradation. The results indicated that the degradation rate of venlafaxine in the secondary effluent was lower than that in the ultrapure water. Inorganic anions (Br^-, Cl^-, NO₃^-, and NO₂^-) impacted venlafaxine removal, and the inhibition of NO₂^- was stronger than others'. Both humic acid and fulvic acid also inhibited venlafaxine decomposition. UPLC-MS/MS was used to perform full scan to identify intermediates of venlafaxine degradation. Five degradation pathways were proposed:①demethylation; ②deamination; ③sequential hydroxylation of aromatic rings; ④ring opening of cyclohexane; ⑤low-molecular-weight acid transformation. In addition, 87% of the N element in the molecules were transformed into inorganic ions (ammonium ions and nitrate ions).

Study on the evolution and transformation of chlorine during co-processing of hazardous waste incineration residue in a cement kiln

The co-processing of hazardous waste in a cement kiln can eliminate a large quantity of hazardous wastes, but the excessive existence of chlorine will affect not only the operation of a cement kiln but also the quality of cement products. In this study, the mixtures of hazardous waste incineration residue and raw meal were incinerated in a high temperature tubular furnace. The distribution ratio of chlorine in flue gas, clinker and fly ash under different experiment conditions was obtained and the influence of the co-processing conditions on chlorine evolution and transformation was studied. The results showed that chlorine mainly existed in flue gas and clinker, and only less than 1% of chlorine existed in fly ash. The incineration temperature had a significant influence on the distribution of chlorine. The higher the incinerating temperature, the greater the distribution ratio of chlorine in flue gas and fly ash. The proportion of chlorine in all parts remained basically unchanged while the temperature was higher than 1300°C. With the increase of the retention time, the proportion of chlorine released into the flue gas increased. The distribution ratio of chlorine in each part remained unchanged after about 30 minutes. There were four stages of the rate of chlorine release. In addition, the chlorine content of the sample had little effect on the partition of chlorine. Some suggestions on the co-processing of hazardous waste in a cement kiln are put forward based on these experimental results.