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

Structural and mineralogical variation upon reoxidation of reduced Fe-bearing clay minerals during thermal activation

The hydroxyl radicals (OH) produced from Fe(II) oxidation upon reoxidation of reduced Fe-bearing clay minerals (RFC) have received increased attention and thermal activation was used to enhance Fe(II) oxidation to improve OH production. However, changes in mineral morphology and structure during thermally-activated RFC reoxidation are not yet clear. Herein, the Fe(II) oxidation extent was measured by chemical analysis during the reoxidation of model RFC (reduced nontronite (rNAu-2) at elevated temperatures. Mineralogical variation of rNAu-2 particles was observed by scanning electron microscopy (SEM), Mössbauer spectra, and X-ray photoelectron spectroscopy (XPS). The structural Fe(II) oxidation in rNAu-2 was accelerated with increasing temperature, accompanied by the transformation of structural entities and the dissolution of Fe and Si, while the overall structure of rNAu-2 minerals was relatively intact. The surface microstructure of particles showed the dissolved holes, net-shape flocs, and even large pore channels after Fe(II) oxidation by thermal activation. Moreover, the rearrangement of structural Fe(II) entities, the regeneration of edge Fe(II), and the electron transport from the interior to the edge were enhanced during rNAu-2 reoxidation by thermal activation. The increasing electron transfer at elevated temperatures could possibly be owing to the increasing number of reactive sites by increasing the internal disorder of rNAu-2. This work provides novel insights into the structural and mineralogical changes in promoting electron transfer upon RFC reoxidation.

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.

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.

Insights into the degradation process of phenol during in-situ thermal desorption: The overlooked oxidation of hydroxyl radicals from oxygenation of reduced Fe-bearing clay minerals

In-situ thermal desorption (ISTD) has attracted increasing attention owing to the efficient removal of organic contaminants from contaminated sites. However, it is poorly understood that whether and to what extent contamination degradation occurs upon oxygenation of reduced Fe-bearing clay minerals (RFC) in the subsurface during ISTD. In this study, we evaluated the mechanism of contaminant degradation upon oxygenation of reduced clay minerals during the ISTD. Reduced nontronite (rNAu-2) and montmorillonite (rSWy-3) were selected as RFC models. Results showed that thermal treatment during ISTD could significantly enhance phenol degradation, which increased from 25.8 % at 10 °C to 74.4 % at 70 °C in rNAu-2 and from 17.7 % at 10 °C to 49.8 % at 70 °C in rSWy-3. Correspondingly, the cumulative •OH at steady-state ([•OH]_ss) increased by 3.7 and 1.5 times, respectively. The acceleration of Fe(II) oxidation with increasing temperature could be mainly responsible for [•OH]_ss generation, which degrades phenol. Moreover, thermal treatment improved the fast oxidation of trioctahedral entities Fe(II)Fe(II)Fe(II) (TOF) and the slow oxidation of dioctahedral entities Fe(II)Fe(II) (DTF₁), AlFe(II) (DAF₁), and Fe(II)Fe(III) (DTF₂). Our study suggests that the overlooked degradation progress of phenol by oxygenation of RFC during ISTD, and it could be favorable for contaminant degradation during remediation.

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.

Oxytetracycline induced the redox of iron and promoted the oxidation of As(III)

Antibiotics and arsenic are two frequently detected contaminants in soils and waters, both of which have potential threats to human health. There are few studies focusing on the interaction between these two groups of contaminants in the environment. In this study, we found that the presence of oxytetracycline could significantly promote the oxidation of inorganic As(III) to As(V) with trace Fe(III) (10 μM) and H₂O₂ (100 μM) at near natural pH, and OTC was degraded simultaneously. The most possible mechanism was that OTC could complex with Fe(III) and reduce Fe(III) to Fe(II), which further induced the Fenton-like reaction. Furthermore, structural Fe(III) of α-FeOOH and adsorbed Fe(III) of montmorillonite could also induce these reactions, and the oxidation rate of As(III) was higher with Fe(III)-montmorillonite than aqueous Fe(III). Based on this study, the transformation of As(III) and OTC could occur in four natural water samples, including river water, groundwater and livestock wastewaters. The results of this study revealed the overlooked effect of residual tetracyclines antibiotics on the transformation of co-existing As(III) in natural waters and soils, which might greatly reduce the toxicity of As(III) in the environment.

Acceleration of the Dehydrogenation of d-Glucose to 2-Keto-d-gluconate in Aqueous Amino Acid via Hydrated Stacked Clay Nanosheets

The assembly of discrete active species to form periodical nanostructures is essential in realizing low-cost artificial enzymes that mimic natural enzymatic functions in extraordinary bio(chemo)selective reactions. In this study, we developed artificial bifunctional glucose/gluconic acid dehydrogenase from naturally abundant resources: l-aspartic acid (Asp) and montmorillonite (a subgroup of smectite natural clay minerals). β-d-Glucose (Glc) was dehydrogenated to 2-keto-d-gluconate (2-KGA) at 25 and 30 °C in an aqueous acidic solution (pH = 3, 4, and 5). The reaction involved sequential steps that yielded d-gluconic acid (GA) as an intermediate. The second step of the dehydrogenation (GA to 2-KGA) occurred at a higher rate than the first (Glc to GA), which is comparable to the natural process. A negatively charged carboxylate in Asp was required for the dehydrogenation, which donates an electron pair (COO:^-) to the hydroxyl group bonded to the C(1)-position of Glc. The acidic sites in clay served as coenzymatic sites (electron acceptor), promoting the Glc dehydrogenation as the Glc reduced by Asp approached the clay coenzymatic sites. The active coenzymatic structures were developed in 48 h (induction period) through the rearrangement of the adsorbed Asp and Glc molecules on montmorillonite in water (intermediate structure). The spontaneous assembling of the intermediate structures facilitated the one-pot dehydrogenation of Glc to 2-KGA via periodic "hydrated stacked layers" comprising clay nanosheets, Asp, and Glc. The facile synthetic route proposed here is inexpensive and would be beneficial without using both GDH and GADH enzymes bound to a cell membrane.

Efficient degradation of tetracycline by singlet oxygen-dominated peroxymonosulfate activation with magnetic nitrogen-doped porous carbon

Nonradical reaction driven by peroxymonosulfate (PMS) based advanced oxidation processes has drawn widespread attention in water treatment due to their inherent advantages, but the degradation behavior and mechanism of organic pollutants are still unclear. In this study, the performance, intermediates, mechanism and toxicity of tetracycline (TC) degradation were thoroughly examined in the constructed magnetic nitrogen-doped porous carbon/peroxymonosulfate (Co-N/C-PMS) system. The results showed that 85.4% of TC could be removed within 15 min when Co-N/C and PMS was simultaneously added and the degradation rate was enhanced by 3.4 and 14.7 folds compared with Co-N/C or PMS alone, respectively. Moreover, the performance of Co-N/C was superior to that of most previously reported catalysts. Many lines of evidence indicated that Co-N/C-PMS system was a singlet oxygen-dominated nonradical reaction, which was less interfered by pH and water components, and displayed high adaptability to actual water bodies. Subsequently, the degradation process was elaborated on the basis of three-dimensional excitation-emission matrix spectra and liquid chromatography-mass spectrometry. At last, the toxicity of treated TC was greatly reduced by using microalgae Coelastrella sp. as ecological indicator. This study provides a promising approach based on singlet oxygen-dominated nonradical reaction for eliminating TC in water treatment.

Hydroxyl radicals induced mineralization of organic carbon during oxygenation of ferrous mineral-organic matter associations: Adsorption versus coprecipitation

The iron (Fe) phases have been widely proposed to preserve organic carbon (OC) via adsorption or coprecipitation pathways, however, such role of Fe phases might be largely reversed under redox-fluctuation conditions, especially for Fe(II) minerals-protected OC. In this study, we synthesized the Fe(II)-OC associations via adsorption and coprecipitation using FeCO₃ and three types of low-molecular-weight organic compounds (LMWOCs) at different C/Fe molar ratios, and investigated the OC mineralization induced by hydroxyl radicals (OH) during oxygenation processes. Abundant OH can be produced upon oxygenation of FeCO₃-LMWOCs associations within 96 h, giving values of 28.49-151.36 μM in adsorption and 12.63-76.41 μM in coprecipitation treatments depended on types of LMWOCs and C/Fe molar ratios. Fe(II) species in coprecipitates with hydroquinone (HQ) mainly transformed into Goethite-like phases after oxygenation, while adsorption samples induced more formation of lower-crystalline Fe phase (e.g., ferrihydrite). The surface-Fe(II) was the primary electron donors to O₂, which further induced hydrogen peroxide (H₂O₂) formation via one- and two-electron transfer pathways. Finally, the produced OH removed 0.55-9.65 and 0.16-85.54 mg L^-1 total OC in adsorption and coprecipitation treatments after oxygenation. Collectively, this study highlights that OC associated with Fe(II) minerals might be labile due to the oxidation of formed OH, and the role of Fe phases in OC sequestration may be further re-evaluated under redox fluctuation conditions.

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.

Natural attenuation characteristics and comprehensive toxicity changes of C9 aromatics under simulated marine conditions

Microcosmic experiments were performed under a simulated marine environment to investigate the natural attenuation of C9 aromatics using nine components (propylbenzene, isopropylbenzene, 2-ethyltoluene, 3-ethyltoluene, 4-ethyltoluene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, and indene). This research aims to assess the contribution of biodegradation and abiotic activity to total attenuation of C9 aromatics and ascertain the changes in the comprehensive toxicity of seawater in the natural environment. The process of natural attenuation indicates the agreement with pseudo-first-order kinetics for all nine components in microcosmic experiments. The half-lives of the nine main compounds in C9 aromatics ranged between 0.34 day and 0.44 day under optimal conditions. The experiments showed that the natural attenuation of nine aromatic hydrocarbons mainly occurred via abiotic processes. Seawater samples significantly inhibited the luminescence of P. phosphoreum (the luminescence inhibition ratio reached 100%) at the beginning of the experiment. In addition, the toxicity declined slowly and continued for 25 days. The attenuation kinetics and changes in toxicity could be applied to explore the natural attenuation of C9 aromatics in the marine environment.

Enhanced reactivity of zero-valent aluminum/O2 by using Fe-bearing clays in 4-chlorophenol oxidation

Zero-valent aluminum (ZVAl) is a promising reductant because of its relatively low redox potential, which can efficiently activate molecular oxygen to generate reactive oxygen species. However, its long-term performance is limited by the intrinsic dense oxide layer and the passivation effect of the accumulative Al-(hydr)oxide on its surface during the reaction. In this study, four clay minerals with different compositions were mixed with ZVAl by ball milling to obtain four composites of ZVAl and clay (ZVAl-Clay), which were used to degrade a high concentration of 4-chlorophenol (4-CP) under ambient conditions. The oxidation efficiencies of different ZVAl-Clays were strongly relevant to Fe contained in the clay minerals. The Fe-free ZVAl-Clay presented poor oxidation performance, whereas the reaction efficiencies of those ZVAl composites with Fe-bearing clays exhibited varying degrees of improvement. In comparison with the original ZVAl, the highest oxidation rate increased by 23 times, the maximum increased OH production was approximately 8 times, and the corresponding mineralization efficiency improved by 38.7%. However, the levels of improved oxidation performance of various ZVAl-Clays were not positively correlated with their actual total Fe contents, and their degradation efficiencies might also be affected by other physical and/or chemical properties of different clays. The synergistic mechanism revealed by various characterizations was that electron transfer might occur from ZVAl to the structural Fe(III) of the clay through the basal plane or edge of clays triggered by ball milling. Thus, the partially produced Fe(II) on the clay surface promoted the Fenton-like reaction to decompose H₂O₂ into OH for efficient oxidation of 4-CP. In short, the ZVAl composites with Fe-bearing clays deserved further exploration as potential materials for efficient degradation of organic matters in wastewater samples.

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.

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.

Surface-bound radical control rapid organic contaminant degradation through peroxymonosulfate activation by reduced Fe-bearing smectite clays

Heterogeneously activated peroxymonosulfate (PMS)-based advanced oxidation technologies (AOTs) have received increasing attention in contaminated water remediation. However, PMS activation by reduced clay minerals (e.g., reduced Fe-bearing smectite clays) has rarely been explored. Herein, PMS decomposition by reduced Fe-bearing smectite clays was investigated, and the hydroxyl radical (OH) and sulfate radical (SO₄^-) formation mechanisms were elucidated. Reduced nontronite NAu-2 (R-NAu-2) activated PMS efficiently to induce rapid degradation of diethyl phthalate (DEP) within 30 s. Mössbauer spectroscopy, FTIR and XPS analyses substantiated that distorted trans-coordinated Fe(II)Fe(II)Fe(II)OH entities were mainly responsible for rapid electron transfer to regenerate clay surface Fe(II) for PMS activation. Chemical probe, radical quenching, and electron paramagnetic resonance (EPR) results confirmed that OH and SO₄^- were mainly bound to the clay surface rather than in bulk solution, which resulted in the rapid degradation of organic compounds such as DEP, sulfamethoxazole, phenol, chlortetracycline and benzoic acid. Anions such as Cl^- and NO₃^- had a limited effect on DEP degradation, while HCO₃^- inhibited the DEP degradation due to the increase of reaction pH. This study provides a new PMS activation strategy using reduced Fe-bearing smectite clays that will contribute to rapid degradation of organic contaminants using PMS-based AOTs.