Brönsted Catalyzed Hydrolysis of Microcystin-LR by Siderite

Water Vapor Condensation Triggers Simultaneous Oxidation and Hydrolysis of Organic Pollutants on Iron Mineral Surfaces

Increasing worldwide contamination with organic chemical compounds is a paramount environmental challenge facing humanity. Once they enter nature, pollutants undergo transformative processes that critically shape their environmental impacts and associated risks. This research unveils previously overlooked yet widespread pathways for the transformations of organic pollutants triggered by water vapor condensation, leading to spontaneous oxidation and hydrolysis of organic pollutants. These transformations exhibit variability through either sequential or parallel hydrolysis and oxidation, contingent upon the functional groups within the organic pollutants. For instance, acetylsalicylic acid on the goethite surface underwent sequential hydrolysis and oxidation that first hydrolyzed to salicylic acid followed by hydroxylation oxidation of the benzene moiety driven by the hydroxyl radical (•OH). In contrast, chloramphenicol underwent parallel oxidation and hydrolysis, forming hydroxylated chloramphenicol and 2-amino-1-(4-nitrophenyl)-1,3-propanediol, respectively. The spontaneous oxidation and hydrolysis occurred consistently on three naturally abundant iron minerals with the key factors being •OH production capacity and surface binding strength. Given the widespread presence of iron minerals on Earth's surface, these spontaneous transformation paths could play a role in the fate and risks of organic pollutants of health concerns.

Dry-to-wet fluctuation of moisture contents enhanced the mineralization of chloramphenicol antibiotic

Due to livestock wastewater irrigation, soil is becoming one of the major sinks of antibiotics in the environment. Recently, it is getting recognized that a variety of minerals under low moisture conditions can induce strong catalytic hydrolysis to antibiotics. However, the relative importance and implication of soil water content (WC) for natural attenuation of soil residual antibiotics has not been well recognized. In order to explore the optimal moisture levels and the key soil properties dominating for the high catalytic hydrolysis activities of soils, this study collected 16 representative soil samples across China, and assessed their performances to degrade chloramphenicol (CAP) under different moisture levels. The results showed that the soils with low organic matter contents (< 20 g/kg) and high amounts of crystalline Fe/Al were particularly effective in catalyzing CAP hydrolysis when exposed to low WC (< 6%, wt/wt), leading to CAP hydrolysis half-lives of <40 d Higher WC greatly suppressed the catalytic activity of the soil. By utilizing this process, it is possible to integrate abiotic and biotic degradation to enhance the mineralization of CAP, attributing to that the hydrolytic products are more available for soil microorganisms. As expected, the soils experienced periodic dry-to-wet moisture conditions (i.e., the WC shifting from 1 to 5% to 20-35%, wt/wt) exhibited higher degradation and mineralization of ¹⁴C-CAP, in comparison with the constant wet treatment. Meanwhile, the bacterial community composition and the specific genera showed that the dry-to-wet fluctuation of soil WC relieved the antimicrobial stress to bacterial community. Our study verifies the critical role of soil WC in mediating the natural attenuation of antibiotics, and guides to remove antibiotics from both wastewater and soil.

Fluorine-doped BiVO4 photocatalyst: Preferential cleavage of C-N bond for green degradation of glyphosate

With increasing concerns on the environment and human health, the degradation of glyphosate through the formation of less toxic intermediates is of great importance. Among the developed methods for the degradation of glyphosate, photodegradation is a clean and efficient strategy. In this work, we report a new photocatalyst by doping F ion on BiVO₄ that can efficiently degrade glyphosate and reduce the toxic emissions of aminomethylphosphonic acid (AMPA) through the selective (P)-C-N cleavage in comparison of BiVO₄ catalyst. The results demonstrate that the best suppression of AMPA formation was achieved by the catalyst of 0.3F@BiVO₄ at pH = 9 (AMPA formation below 10%). In situ attenuated total reflectance Fourier transforms infrared (ATR-FTIR) spectroscopy indicates that the adsorption sites of glyphosate on BiVO₄ and 0.3F@BiVO₄ are altered due to the difference in electrostatic interactions. Such an absorption alteration leads to the preferential cleavage of the C-N bond on the N-C-P skeleton, thereby inhibiting the formation of toxic AMPA. These results improve our understanding of the photodegradation process of glyphosate catalyzed by BiVO₄-based catalysts and pave a safe way for abiotic degradation of glyphosate.

Production of reactive oxygen species from oxygenation of Fe(II)-carbonate complexes: The critical roles of carbonate

Hydroxyl radicals (•OH) production upon the oxygenation of reduced iron minerals at the oxic/anoxic interface has been well recognized. However, little is known in the influencing environmental factors and the involved mechanisms. In this study, much more •OH could be efficiently produced from oxygenation of Fe(II) with 20-200 mM carbonate. Both carbonate concentration and anoxic reaction time play a critical role in •OH production. High carbonate facilitates the formation of Fe(II)_{high reactivity}, i.e., surface-adsorbed and structural Fe(II) with low crystalline that is reactive toward O₂ reaction for •OH production, while long anoxic reaction time enables the transfer from Fe(II)_{high reactivity} to Fe(II)_{low reactivity}, i.e., Fe(II) at interior sites with high crystalline, that is hardly oxidized by O₂. Furthermore, the degradation pathway of p-nitrophenol (PNP) is highly dependent on the carbonate concentration that low carbonate facilitates •OH oxidation of PNP (80.2%) while high carbonate enhanced O₂^•- reduction of PNP (48.7%). Besides, carbonate also influences the structural evolution of Fe mineral during oxygenation by retarding its hydrolysis and following transformation. Our finding sheds new light on understanding the important role of oxyanions such as carbonate in iron redox cycles and directing contaminant attenuation in subsurface environment.

In-situ production of iron flocculation and reactive oxygen species by electrochemically decomposing siderite: An innovative Fe-EC route to remove trivalent arsenic

The removal of trivalent arsenic (As (III)) from water has received extensive attention from researchers. Iron electrocoagulation (Fe-EC) is an efficient technology for arsenic removal. However, electrode passivation hinders the development and application of Fe-EC. In this work, an innovative Fe-EC route was developed to remove As (III) through an electrochemical-siderite packed column (ESC). Ferrous ions were produced from siderite near the anode, and hydroxide was generated near the cathode during the electrochemical decomposition of siderite. As a result, an effect of Fe-EC-like was obtained. The results showed that an excellent removal performance of As (III) (>99%) was obtained by adjusting the parameters (As (III) concentration at 10 mg/L, pH at 7, Na₂SO₄ at 10 mM and the hydraulic retention time at 30 min) and the oxidation rate of As (III) reached 84.12%. The mechanism analysis indicated that As (III) was oxidized to As (Ⅴ) by the produced active oxide species and electrode, and then was removed by capturing on the iron oxide precipitates. As (III) was likely to be oxidized in two ways, one by the reactive oxygen species (possibly •OH, Fe(IV) and •O₂^- species), and another directly by the anode. The long-term effectiveness of arsenic removal demonstrated that ESC process based on the electrochemical-siderite packed column was an appropriate candidate for treating As (III) pollution.

A comprehensive review on the photocatalytic inactivation of Microcystis aeruginosa: Performance, development, and mechanisms

Harmful algae blooms (HABs), caused by severe eutrophication and extreme weather, have spread all over the world, posing adverse effects on eco-environment and human health. Microcystis aeruginosa is the dominant harmful cyanobacterial species when HABs occur, and the toxic metabolites produced by it, microcystins, are even fatal to humans. Photocatalytic technology has received wide attention from researchers for its clean and energy-efficient features, while the basic mechanisms and modification methods of photocatalysts have also been widely reported. In recent years, photocatalytic technology has shown great promise in the inhibition of HABs. In this article, we systematically reviewed the progress in photocatalytic performance and algae removal efficiency, discuss the damage mechanisms of photocatalysts for algae removal, including physical damage and various oxidative stresses, and also explore the degradation rates and possible pathways of microcystins. It can be concluded that during the photocatalytic process, the cytoarchitectural integrity of algae cells was damaged, a variety of important protein and enzyme systems were disrupted, and the antioxidant systems collapsed due to the continuous attack of ROS, which adversely affected the normal physiological activities and growth, resulting in the inactivation of algae cells. Moreover, photocatalysts have a degrading effect on microcystins, thus reducing the adverse effects of HAB. Finally, a brief summary of future research priorities regarding the photocatalytic degradation of algae cells is presented. This study helps to enhance the understanding of the destruction mechanism of Microcystis aeruginosa during the photocatalytic process, and provides a reference for the photodegradation of HAB in water bodies.

Structure-dependent surface catalytic degradation of cephalosporin antibiotics on the aged polyvinyl chloride microplastics

Microplastics (MPs) have been recognized as a global concern due to their potential health effect, as MPs could adsorb and carry various pollutants in aquatic environment. In the present study, a new environmental behavior related to polyvinyl chloride microplastics (PVC-MPs) and the underlying mechanism were described. Our results showed that the photo-aged PVC-MPs could affect the transformation of cephalosporin antibiotics. For instance, the presence of altered PVC-MPs significantly accelerated the hydrolysis of cefazolin (CFZ), but exhibited negligible effect on the degradation of cephalexin (CFX). As indicated by in situ Fourier transform infrared spectra and theoretical calculations, hydrogen bonds could be formed between β-lactam carbonyl of CFZ and the oxygen-containing moieties on the aged PVC-MP surfaces. The hydrogen-bonding was able to significantly increase the positive atomic Mulliken charge on the β-lactam carbonyl carbon, thus narrowing the energy gap of CFZ hydrolysis and subsequently enhancing the disruption of β-lactam ring. While for CFX, instead of the β-lactam carbonyl, the amide amino group was involved in the hydrogen-bonding due to the structural difference. Therefore, in addition to increasing the adsorption capacity, the aged PVC-MPs could act as the catalyst to mediate the transformation of antibiotics. Our study would help improve the understanding for interactions between contaminants and MPs in natural environments.

Iron Minerals Mediated Interfacial Hydrolysis of Chloramphenicol Antibiotic under Limited Moisture Conditions

Iron minerals are important soil components; however, little information is available for the transformation of antibiotics on iron mineral surfaces, especially under limited moisture conditions. In this study, we investigated the catalytic performance of four iron minerals (maghemite, hematite, goethite, and siderite) for the hydrolysis of chloramphenicol (CAP) antibiotic at different moisture conditions. All the iron oxides could efficiently catalyze CAP hydrolysis with the half-lives <6 days when the surface water content was limited, which was controlled by the atmospheric relative humidity of 33-76%. Different minerals exhibited distinctive catalytic processes, depending on the surface properties. H-bonding or Lewis acid catalysis was proposed for surface hydrolytic reaction on iron oxides, which however was almost completely inhibited when the surface water content was >10 wt % due to the competition of water molecules for surface reactive sites. For siderite, the CAP hydrolysis was resistant to excessive surface water. A bidentate H-bonding interaction mechanism would account for CAP hydrolysis on siderite. The results of this study highlight the importance of surface moisture on the catalytic performance of iron minerals. The current study also reveals a potential degradation pathway for antibiotics in natural soil, which has been neglected before.

Selective removal of common cyanotoxins: a review

The development of cyanobacterial blooms can have adverse effects on water bodies and may produce cyanotoxins. Several physical and chemical methods have been applied to remove cyanotoxins, but they have been significantly challenged due to extensive energy footprint and over-used chemicals, which limits practical application on a large scale. Selective removal has been regarded as the most promising approach recently for the elimination of prevalent and major bloom-forming cyanotoxins (e.g., microcystins and cylindrospermopsin) as natural organic matters and radical scavengers are ineluctably present in real scenarios. This paper reviews current advancements in research on selective oxidation and adsorption of cyanotoxins. Its goal is to provide comprehensive information on the treatment mechanism and the process feasibility involved in the cyanotoxin removal from real-world waters. Moreover, perspectives of cyanotoxin control and in situ selective elimination approaches are also reviewed. It is expected that the information gathered and discussed in this review can provide a useful and novel reference and direction for future pilot-scale applications.

Photodegradation of microcystin-LR by pyridyl iron porphyrin immobilized on NaY zeolite

A novel photocatalyst, FeTPyPY, was prepared by immobilizing water-soluble tetra(4-pyridyl)phenyl iron-porphyrin (FeTPyP) on NaY zeolite to degrade microcystin-LR (MC-LR), one of the most toxic microcystins (MCs). UV-Vis analysis, UV-Vis diffuse reflectance spectroscopy, infrared spectroscopy, cyclic voltammetry and transmission electron microscopy were employed to characterize immobilized FeTPyPY. Under visible light (λ ≥ 420 nm), MC-LR was degraded utilizing immobilized FeTPyPY by activating molecular oxygen. The results showed that 85% of MC-LR was efficiently degraded by FeTPyPY with loading amount 100:1 (m_NaY:m_FeTPyP) after 300 min of visible light illumination. Moreover, FeTPyPY was stable in the degradation system with pH 7.0. The degradation mechanism was evaluated using electron spin resonance, and the results demonstrated that highly reactive oxygen species (^•OH radical) were generated in the system to degrade MC-LR. Therefore, immobilized FeTPyPY was available to break down the toxic groups within MC-LR by utilizing environmental ^•OH radical under circumneutral condition.

Noble-metal-free TiO2 photocatalysis for selective CC reduction of α,β-enones by CF3SO3H modification

The highly selective CC reduction of α,β-enones was realized by CF₃SO₃H-modifying noble-metal-free TiO₂ photocatalysis.

Structural basis of microcystinase activity for biodegrading microcystin-LR

Microcystinase (MlrA) catalyzes the first and most important biodegradation step of hepatotoxic microcystin-LR (MC-LR) produced and released from cyanobacterial cells, and the underlying catalytic mechanism is not completely understood yet. MlrA was postulated previously to be a metalloprotease with an active site of H²⁶⁰AIH²⁶³NE²⁶⁵, a variant of the common metal-binding motif of HEXXH. Through comparison with representative modes in HEXXH-containing metalloproteases, molecular dynamics simulation, homology modeling, and docking, the active sites of MlrA involved in the MC-LR biodegradation by Sphingomonas sp. USTB-05 were predicted. Site-directed mutants of MlrA were constructed for verification then. The results show that MlrA is likely not a metalloprotease, but a glutamate protease belonging to type II CAAX prenyl endopeptidases. Combined with the biodegradation of MC-LR by MlrA and its mutants, a complete enzymatic mechanism for MC-LR biodegradation by MlrA is proposed: Glu¹⁷² and His²⁰⁵ activate a water molecule facilitating a nucleophilic attack on the Adda-Arg peptide bond of MC-LR; Trp¹⁷⁶ and Trp²⁰¹ contact the carboxylate side chain of Glu¹⁷²and, by raising its pKa potentially, accelerate the reaction rates; His²⁶⁰ and Asn²⁶⁴ (located in the previous postulated active center of H²⁶⁰AIH²⁶³NE²⁶⁵) function as an oxyanion hole to stabilize the transition states. This study reveals the enzymatic mechanism of MlrA for catalyzing MC-LR in both the representative modes and the experiments of site-directed mutagenesis.

Hydrolysis of Chloramphenicol Catalyzed by Clay Minerals under Nonaqueous Conditions

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

Rapid Hydrolysis of Penicillin Antibiotics Mediated by Adsorbed Zinc on Goethite Surfaces

The soil environment is an important sink for penicillin antibiotics released from animal manure and wastewater, but the mineral-catalyzed transformation of penicillins in soil has not been well studied. To simulate this environmental process, we systematically investigated the behavior of penicillin G and amoxicillin, the two most widely-used penicillin antibiotics, in the presence of goethite and metal ions. The results demonstrated that Zn ions significantly promoted the hydrolysis of penicillins in goethite suspensions, as evidenced by the degradation rate nearly 3 orders of magnitude higher than that of the non-Zn-containing control. The spectroscopic analysis indicated that the specific complexation between penicillins, adsorbed Zn, and goethite was responsible for the enhanced degradation. Metastable interactions, involving hydrogen bonds between carbonyl groups in the β-lactam ring and the double/triple hydroxyl groups on goethite surface, and coordination bonding between carboxyl groups and surface irons were proposed to stabilize the ternary reaction intermediates. Moreover, the surface zinc-hydroxide might act as powerful nucleophile to rapidly rupture the β-lactam ring in penicillins. This study is among the first to identify the synergic roles of Zn ion and goethite in facilitating penicillin degradation and provides insights into β-lactam antibiotics to assess their environmental risk in soil.

Detoxification of Cylindrospermopsin by Pyrite in Water

Cylindrospermopsin (CYN) is a cyanobacterial toxin released from eutrophic water. It persistently remains in the environment because its degradation under solar light is extremely low. In this study, pyrite, an abundant mineral, was investigated as a catalyst for decomposing and detoxifying CYN in water. A detailed examination of intermediates provided insights into the degradation pathway. Electron spin resonance spectra revealed that H2O2 and hydroxyl radicals (•OH) were generated at the pyrite surface while promoting the recycling of Fe(III) into Fe(II) during the degradation process. This degradation system could be uniquely efficient in the presence of relatively high levels of natural organic matter because the structure of the uracil ring is decomposed to detoxify CYN. This work confirms a new approach to selectively and effectively detoxifying CYN in water using an inexpensive, environmentally friendly, and bio-compatible mineral.

α-Ferrous oxalate dihydrate: an Fe-based one-dimensional metal organic framework with extraordinary photocatalytic and Fenton activities

An Fe(ii)-based metal organic framework, α-ferrous oxalate dihydrate (α-FOD), with remarkable n-type semiconductor characteristics has been successfully prepared for use as a bifunctional catalyst in both photocatalysis and Fenton oxidation.