A novel diagnostic method for distinguishing between Fe(IV) and •OH by using atrazine as a probe: Clarifying the nature of reactive intermediates formed by nitrilotriacetic acid assisted Fenton-like reaction

Ultrasound enhanced destruction of tetracycline hydrochloride with peroxydisulfate oxidation over FeS/NBC catalyst: Governing factors, strengthening mechanism and degradation pathway

In this study, FeS/N-doped biochar (NBC) derived from the co-pyrolysis of birch sawdust and Mohr's salt was applied to evaluate the efficiency of catalyzed peroxydisulfate (PDS) oxidation for tetracycline (TC) degradation. It is found that the combination of ultrasonic irradiation can distinctly enhance the removal of TC. This study investigated the effects of control factors such as PDS dose, solution pH, ultrasonic power, and frequency on TC degradation. Within the applied ultrasound intensity range, TC degradation increases with increasing frequency and power. However, excessive power can lead to a reduced efficiency. Under the optimized experimental conditions, the observed reaction kinetic constant of TC degradation increased from 0.0251 to 0.0474 min-1, with an increase of 89%. The removal ratio of TC also increased from ∼85% to ∼99% and the mineralization level from 45% to 64% within 90 min. Through the decomposition testing of PDS, reaction stoichiometric efficiency calculation, and electron paramagnetic resonance experiments, it is shown that the increase in TC degradation of the ultrasound-assisted FeS/NBC-PDS system was attributed to the increase in PDS decomposition and utilization, as well as the increase in SO4•- concentration. The radical quenching experiments showed that SO4•-, •OH, and O2•- radicals were the dominant active species in TC degradation. TC degradation pathways were speculated according to intermediates from HPLC-MS analysis. The test of simulated actual samples showed that dissolved organic matter, metal ions, and anions in waters can undercut the TC degradation in FeS/NBC-PDS system, but ultrasound can significantly reduce the negative impact of these factors.

Efficient removal of recalcitrant naphthenic acids with electro-cocatalytic activation of peroxymonosulfate by Fe(III)-nitrilotriacetic acid complex under neutral initial pH condition

This work investigated the activation of peroxymonosulfate by electrochemical (EC) system assisted with Fe(III)-nitrilotriacetic acid (NTA) complex for degradation of persistent naphthenic acids (NAs) under neutral initial pH conditions. As NAs are a complicated mixture, 1-adamantanecarboxylic acid (ACA) was selected as the model NA compound for degradation experiment. The addition of NTA is to chelate with Fe(III), gaining stability under neutral pH condition to facilitate the circulation of Fe(II)/Fe(III) by the electrochemical process to activate PMS. The EC/Fe(III)-NTA/PMS system was explored with applicable pH range of 3-9 and an optimized molar ratio 1: 2 for Fe: NTA. Results of quenching and chemical probe experiment together with results of electron paramagnetic resonance (EPR) analysis revealed the main reactive species of the system, including ^•OH, SO₄^•-, ¹O₂ and possibly Fe(IV). With the addition of NTA, the yields of ^•OH, SO₄^•-, ¹O₂ were enhanced. Results of mass spectrometry analysis and DFT calculations indicated the formation of 9 degradation byproducts of ACA via three primary degradation pathways such as hydroxyl substitution, carbonyl substitution, and decarboxylation. Furthermore, the EC/Fe(III)-NTA/PMS system could achieve excellent removal efficiency of ACA with different anions such as Cl^-, HCO₃^-, NO₃^- and H₂PO₄^- in the background. The practical applicability of the system was also verified with the high removal of commercial NAs mixture standard. Overall results have indicated the EC/Fe(III)-NTA/PMS system could be utilized for efficient reclamation of authentic oil and gas industrial wastewater under natural pH conditions.

Removal of Bisphenol A via peroxymonosulfate activation over graphite carbon nitride supported NiCx nanoclusters catalyst: Synergistic oxidation of high-valent nickel-oxo species and singlet oxygen

In this work, a g-C₃N₄ supported NiCx nanoclusters catalyst (NiCx-CN) was developed, and its performance in activating peroxymonosulfate (PMS) was evaluated. Mechanism investigation stated that although singlet oxygen (¹O₂) was formed in the catalytic process, its contribution to BPA elimination was weeny. Interestingly, through the experiment with dimethyl sulfoxide as the probe, it was considered that the high-valent nickel-oxo species (Ni^&+=O), generated after the interaction of NiCx-CN and PMS, was the dominating reactive oxygen species (ROS). Theoretical calculations (DFT) implied that NiCx-CN might lose electrons to generate high-valent Ni, which was consistent with the detection of Ni³⁺ on the surface of the used NiCx-CN. Besides, the prepared NiCx-CN showed advantages in resisting the interference of inorganic anions. Meanwhile, three BPA degradation routes had been proposed based on the transformation intermediates. This study will establish a new protocol for PMS activation using heterogeneous Ni-based catalysts to efficiently degrade organic pollutants via a nonradical mechanism.

Enhancing sludge dewatering efficiency through bioleaching facilitated by increasing reactive oxygen species

Bioleaching facilitated by iron-oxidizing bacteria is regarded as a promising sludge dewatering method due to excellent dewaterability and low cost. However, a two-days bioleaching time for sludge conditioning decreased its daily treatment capacity. In fact, Fe²⁺ easily reacts with O₂ to produce reactive oxygen species (ROS) with high oxidizing activity. Can bioleaching performed in Fe²⁺-rich system generate ROS to decompose sludge extracellular polymeric substances (EPS)? Here both contribution of ROS produced in bioleaching to improve sludge dewaterability and the increase of ROS content to shorten sludge bioleaching treatment time were investigated. The introduction of H₂O₂ in sludge bioleaching treatment (BS+H₂O₂) to increase ROS could simultaneously improve sludge dewaterability and decrease bioleaching time. Specific resistance to filtration (SRF) and capillary suction time (CST) reduction ratios (90.3% and 80.9%) in BS+H₂O₂ process were much higher than those in other processes after only 30 min reaction. Mechanisms of improving sludge dewaterability in BS+H₂O₂ mainly included ROS oxidation and Fe³⁺ flocculation by analysis of the contribution factors. These findings not only provide an effectively method to promote sludge dewatering efficiency of bioleaching, but also give new sights into the design of cost-efficient processes for improving the sludge dewatering.

Degradation of tetracycline by UV/Fe3+/persulfate process: Kinetics, mechanism, DBPs yield, toxicity evaluation and bacterial community analysis

As a widely produced and used antibiotic, tetracycline (TC) has been frequently found in rivers, soil and drinking water. In this study, the degradation of TC was investigated by UV/Fe³⁺/persulfate (PS) coupled process. The degradation behavior was well fitted with pseudo-first-order model. Hydroxyl radicals (·OH), sulfate radicals (SO₄^-·) and superoxide radical (O₂^-·) were identified as the primary reactive oxygen species (ROS) in UV/Fe³⁺/PS process, the contribution to TC degradation were found to be 41.94%, 33.94% and 17.44% at pH 3.0, respectively. Fe(IV) generated from the system also played a crucial role in TC removal. The effects of process parameters (PS/Fe³⁺ dosages, pH, humic acid, Cl^-, HCO₃^-, NO₃^- and CO₃^2-) on degradation were investigated. It was found that the degradation of TC was highly pH-dependent, and the optimal performance was obtained at pH 3.0. Except for Cl^-, the presence of HA, HCO₃^-, NO₃^- and CO₃^2- inhibited TC degradation. The possible transformation pathway involving the hydroxylation, N-demethylation, hydrogenation and dehydroxylation was proposed. Furthermore, the toxicity and mutagenicity of TC and transformation products (TPs) were estimated using ECOSAR and TEST softwares, demonstrating that the toxicity level of most TPs was lower/equal to their precursors. The evaluation of DBPs showed that UV/Fe³⁺/PS process could reduce the potential of DBPs formation, especially for TCAA and TCM. Microbial community composition was analyzed by 16 S rDNA sequencing, and the relative abundance of ARG-carrying opportunistic pathogens was significantly declined after UV/Fe³⁺/PS treatment. In general, this study provides an economical, efficient and safe strategy for TC removal.

Probing the roles of pH and ionic strength on electrostatic binding of tetracycline by dissolved organic matters: Reevaluation of modified fitting model

The binding performance of dissolved organic matters (DOM) plays a critical role in the migration, diffusion and removal of various residual pollutants in the natural water environment. In the current study, four typical DOMs (including bovine serum proteins BSA (proteins), sodium alginate SAA (polysaccharides), humic acid HA and fulvic acid FA (humus)) are selected to investigate the binding roles in zwitterionic tetracycline (TET) antibiotic under various ionic strength (IS = 0.001-0.1 M) and pH (5.0-9.0). The dialysis equilibration technique was employed to determine the binding concentrations of TET, and the influence of IS and pH on binding performance was evaluated via UV-vis spectroscopy, total organic carbon (TOC), and Excitation-Emission-Matrix spectra (EEM), zeta potentials and molecule size distribution analysis. Our results suggested that carboxyl and phenolic hydroxyl were identified as the main contributors to TET binding based on the fourier transform infrared spectroscopy (FTIR) analysis, and the binding capability of four DOMs followed as HA > FA » BSA > SAA. The biggest binding concentrations of TET by 10 mg C/L HA, FA, BSA and SAA were 0.863 μM, 0.487 μM, 0.084 μM and 0.086 μM, respectively. The higher binding capability of HA and FA is mainly attributed to their richer functional groups, lower zeta potential (HA/FA = -15.92/-13.54 mV) and the bigger molecular size (HA/FA = 24668/27750 nm). IS significantly inhibits the binding interaction by compressing the molecular structure and the surface electric double layer, while pH had a weak effect. By combining the Donnan model and the multiple linear regression analysis, a modified Karickhoff model was established to effectively predict the binding performance of DOM under different IS (0.001-0.1 M) and pH (5.0-9.0) conditions, and the R² of linear fitting between experiment-measured logK_DOC and model-calculated logK_OC were 0.94 for HA and 0.91 for FA. This finding provides a theoretical basis for characterizing and predicting the binding performance of various DOMs to residual micropollutants in the natural water environment.

Insight into the role of binding interaction in the transformation of tetracycline and toxicity distribution

The transformation of free state organic micro-pollutants (MPs) has been widely studied; however, few studies have focused on mixed and bound states MPs, even though numerous ionizable organic MPs process a strong tendency to combine with dissolved organic matters in aquatic environments. This study systemically investigated the distribution and toxicity assessment of tetracycline (TET) transformation products in free, mixed and bound states during UV, UV/H₂O₂, UV/PS and CNTs/PS processes. A total of 33 major transformation products were identified by UPLC-Q-TOF-MSMS analysis, combining the double bond equivalence and aromaticity index calculations. The binding interaction would weaken the attack on the dimethylamino (-N(CH₃)₂) group and induce the direct destruction of rings A and B of TET through the analysis of 2D Kernel Density changes and density functional theory (DFT) calculations. Toxicity assessment and statistics revealed that the intermediate products with medium molecular weight (230≤ m/z ≤ 380) exhibited higher toxicity, which was closely related to the number of the rings in molecular structures (followed as 2»3 > 1≈4). A predicted toxicity accumulation model (PTAM) was established to evaluate the overall toxicity changes during various oxidation processes. This finding provides new insight into the fate of bound MPs during various oxidation processes in the natural water matrix.

Biomimetic design of graphdiyne supported hemin for enhanced peroxidase-like activity

Effective electronic interactions between molecular catalysts and supports are critical for heterogeneous enzyme mimics, yet they are frequently neglected in most catalyst designs. Taking the enzyme mimics of hemin immobilized on graphdiyne (Hemin-GDY) as an example, we explicate for the first time the underlying role of GDY as a co-catalyst. Based on the robust conjugation between GDY and hemin, the delocalized π-electrons in GDY act as a ligand for Fe ions so that the orbital interactions including electron transport from GDY → Fe can induce the formation of an electron-rich Fe center and an electron-deficient π-electron conjugated system. This mechanism was validated by electron paramagnetic resonance (EPR), Raman spectroscopy, and DFT calculations. Moreover, both EPR spetra and Lineweaver-Burk plots revealed that Hemin-GDY could efficiently catalyze the decomposition of hydrogen peroxide (H₂O₂) to produce hydroxyl radical (•OH) and superoxide anion (O₂^•-) by a ping-pong type catalytic mechanism, and particularly, the catalytic activity was increased by 2.3-fold comparing to that of hemin immobilized on graphene (Hemin-GR). In addition, Hemin-GDY with the exceptional activity and stability was demonstrated for efficient catalytic degradation of organic pollutants under acidic conditions. Collectively, this work provides a theoretical basis for the design of GDY supported catalysts and renders great promises of the GDY based enzyme mimics.

Interactions between the antibiotic tetracycline and humic acid: Examination of the binding sites, and effects of complexation on the oxidation of tetracycline

The binding between dissolved organic matter (DOM) and micro-pollutants (MPs) results in significant impacts on their migration, transformation and degradation. However, the role of the DOM/MP binding on their oxidative transformation remains poorly studied. The binding of MPs by DOM, in combination with DOM's roles as a photosensitizer and/or a competitor for free radicals, needs to be considered in the context of understanding the DOM's impacts on the oxidative degradation of MPs. This study aims to explore this aspect of DOM/MP interactions based on the quantitation of humic acid (HA) and tetracycline (TET) complexation and its role in TET removal. This study also compared the degradation of free TET versus that bound in HA-TET complexes in different oxidation processes. Fourier transform infrared (FTIR) data show that the carboxyl and phenolic hydroxyl groups in HA are the main binding sites of TET, while nuclear magnetic resonance (NMR) analysis shows the binding of TET engages its -N(CH₃)₂ groups, and two-dimensional correlation spectroscopy (2D-COS) data show that the carboxyl groups in DOM are sensitive than phenolic groups in the binding of TET. The difference between the degradation rates (Δk_obs) of the free and bound TET decreased with the increase of ionic strength using sodium nitrate, but increased with the introduction of Ca²⁺ and Mg²⁺ due to the formation of TET-Ca²⁺/Mg²⁺ complexes. Quenching experiments showed that the free radicals (•OH and •SO₄^-), PMS oxidant and UV light were the main contributors to the TET degradation in UV/PS, UV/PMS and UV/H₂O₂ processes, respectively. In-situ fluorescence time scanning and differential absorbance spectra showed that free TET was preferentially oxidized over the bound TET in all the tested treatments except UV/PS. These results provide new insights into the role of DOM/MP complexation in the degradation of MPs in natural and engineered systems.