Tetracycline-Induced Release and Oxidation of As(III) Coupled with Concomitant Ferrihydrite Transformation

Ferrihydrite/B, N co-doped biochar composites enhancing tetracycline degradation: The crucial role of boron incorporation in Fe(III) reduction and oxygen activation

Harnessing the redox potential of biochar to activate airborne O2 for contaminant removal is challenging. In this study, ferrihydrite (Fh) modified the boron (B), nitrogen (N) co-doped biochars (BCs) composites (Fh/B(n)NC) were developed for enhancing the degradation of a model pollutant, tetracycline (TC), merely by airborne O2. Fh/B(3)NC showed excellent O2 activation activity for efficient TC degradation with a apparent TC degradation rate of 5.54, 6.88, and 22.15 times that of B(3)NC, Fh, and raw BCs, respectively, where 1O2 and H2O2 were identified as the dominant ROS for TC degradation. The B incorporation into the carbon lattice of Fh/B(3)NC promoted the generation of electron donors, sp2 C and the reductive B species, hence boosting Fe(III) reduction and 1O2 generation. O2 adsorption was enhanced due to the positively charged adsorption sites (C-B+and NC+). And 1O2 was generated via Fe(II) catalyzed low-efficient successive one-electron transfer (O2 → O2·- → 1O2, H2O2), as well as biochar catalyzed high-efficient two-electron transfer (O2 → H2O2 → 1O2) that does not involve .O2- as the intermediate. Moreover, Fh/B, N co-doped biochar showed a wide pH range, remarkable anti-interference capabilities, and effective detoxification. These findings shed new light on the development of environmentally benign BCs materials capable of degradading organic pollutants.

Target recognition and selective photocatalytic degradation of trace disinfection By-products by innovative molecular imprinting strategy

Target recognition and selective degradation of trace disinfection by-products (DBPs) in water are crucial for the safety of drinking water and surface water. However, the interference of low concentrations of chlorinated DBPs (Cl-DBPs) and other factors in water remains a major challenge. This study aims to develop a novel catalyst for targeted recognition and removal of target pollutants by constructing specific imprinting cavities (IPC) on the surface of a composite photocatalyst composed of CeO2 and biochar (BC) (CeO2@BC). As an intermediate Cl-DBPs, p-chlorophenol (4-CPs) is highly toxic, prone to bioaccumulation, and difficult to remove. Therefore, 4-CP was chosen as the representative Cl-DBPs, and the 4-CP template molecule was successfully imprinted on BC through non covalent interactions between the functional monomer methyl acrylic acid and the template molecule, which was confirmed by Fourier transform infrared and X-ray photoelectron spectroscopy analysis. The imprinted CeO2@BC (MIP-CeO2@BC) showed highly selective recognition and preferential degradation of 4-CP, with a 38 % increase (from 56 % to 94 %) in the overall removal (adsorption and photodegradation) of 4-CP than the non-imprinted precursor (CeO2@BC). In the presence of competing co-solutes, enrofloxacin (ENR, completely different molecular structure from 4-CP) or 2-chlorophenol (2-CP, has the same chemical formula as 4-CP but different substituent position of Cl), MIP-CeO2@BC removed 2.94 and 2.54 times more 4-CP in the presence of ENR or 2-CP, respectively, than CeO2@BC. This feature also enhanced the ability of the material to resist interference of dissolved organic matter in complex water matrices, with a 4-CP removal of > 60 % in the presence of high concentrations of dextrose, humic acid, or trypsin proteins. Mechanism analysis revealed that molecular imprinting (MIP) can not only selectively uptake 4-CP, but also alter the main degradation pathways, among which photo-generated electrons (e-), holes (h+), and ·OH were identified as the main active substances for degrading 4-CP. Part of the photo-generated e- can be transmitted into the IPC through BC "tunnels", activating the degradation process of 4-CPs trapped in the IPC by the active substances. The combination of MIP and active metals seemed to have the potential to improve the selectivity and effectiveness of photocatalytic degradation of Cl-DBPs in water.

Unveiling the dual role of calcium peroxide in enhancing green rust-catalyzed arsenite oxidation and stabilization

Iron oxyhydroxide redox processes coupled with reactive oxygen species (ROS) generation play a critical role in the transformation of arsenic (As). However, low ROS generation limits the detoxification and stabilization of As(III). In this study, we propose a novel strategy by integrating Fe(II) rich green rust (GR) with calcium peroxide (CaO2) for the efficient transformation of As(III) under oxic conditions. The results demonstrate that the introduction of CaO2 greatly enhances the oxidation and immobilization of As(III) by GR. Quenching experiments reveal that H2O2, ·O2-, and 1O2 are the primary ROS responsible for the oxidation of As(III). Importantly, CaO2 significantly boosts the production of key ROS in the GR/CaO2 system. Additionally, the introduction of CaO2 facilitates the transformation of both non-specific and specific adsorbed As into non-extractable As, improving the stability of immobilized As. Our findings unveil that CaO2 serves a dual role in promoting GR-catalyzed As(III) oxidation and stabilization by enhancing ROS production and forming iron-arsenic-calcium complexes. Moreover, the application of GR/CaO2 in As-contaminated soil can rapidly reduce the risk of As leaching in different scenarios. Therefore, this study provides a new strategy using CaO2 coupled with iron redox process to enhance oxidation and stabilization of arsenic in water and soil.

Fe3O4 loaded biochar to enhance persulfate activation for tetracycline degradation: Performance and mechanism

Waste sodium lignosulfonate (LS) is widely converted into biochar-based catalysts due to its rich functional groups and high carbon content. However, inefficient peroxydisulfate (PDS) activation by original sodium lignosulfonate biochar (LB) limits its ability to remove organic pollutants from water bodies. Here, LS was employed as a precursor to synthesize Fe3O4-loaded biochar (Fe3O4@LB) through a one-step pyrolysis process for activating PDS to remove tetracycline (TC). Compared with the original LB, Fe3O4@LB exhibited a larger specific surface area (459.78 m2/g), which is advantageous in providing more adsorption and reaction active sites. Quenching experiment and electron paramagnetic resonance (EPR) analysis revealed that 1O2 and O2•- are the primary active species involved in TC degradation. Characterization results showed that Fe3O4 is uniformly distributed on the biochar, providing abundant Fe(II) to activate the PDS to generate reactive species (ROS). The Fe(III) generated after the reaction was reduced by electron-rich biochar to promote Fe(III)/Fe(II) cycling. The Fe3O4@LB-3/PDS system displayed excellent performance, degrading 90% of TC within 20 min, with a rate constant k of 0.092 min-1, which is about three times that of the LB (0.036 min-1). Based on the liquid chromatography-mass spectrometry (LC-MS) analysis, three possible degradation pathways were proposed. Besides, mung bean growth experiments confirmed the detoxification of TC by the Fe3O4@LB-3/PDS system. This work highlights the feasibility of waste LS usage to produce highly efficient biochar catalysts, providing a sustainable and green alternative to address water contamination by antibiotics.

Silicate impedes arsenic release and oxidation from ferrihydrite

Silicate fertilization is a common farming practice and an effective method to mitigate arsenic (As) pollution in paddy. Investigating the interaction between silicate and ferrihydrite on As retention is key to comprehensively understand the mechanism of As sequestration by silicate fertilization. Our results indicated that the transformation of ferrihydrite into goethite and hematite was inversely proportional to Si/Fe ratios. The added silicate impeded the decrease of solution pH from neutral to acidity, and imposed strong inhibitory effect on goethite formation. The aqueous As in silicate-free system was 3.43 times higher than that with Si/Fe ratio at 0.33, but similar results were not observed in those with high-level As pollution due to the inhibitory effect of As on ferrihydrite transformation. Solid characterization showed that silicate was monomerically adsorbed to ferrihydrite through Si-O-Fe bond, which impeded the reductive dissolution, Fe atom exchange, internal atomic rearrangement and dehydration of ferrihydrite. As(III) oxidation weakened in silicate-coprecipitated ferrihydrite due to the lack of Fe(II) catalysis stem from ferrihydrite dissolution. This work demonstrated that As release could be effectively impeded through the inhibitory effect of silicate on ferrihydrite transformation, thereby providing new insights into the understanding of As accumulation reduction in rice by silicate fertilization.

Synergistic release of Cd(II) and As(V) from ternary sorption systems containing ferrihydrite nanoparticles: The role of binary and ternary surface complexes

Cadmium (Cd) and arsenic (As) generally exhibit mutually beneficial co-sorption behavior on iron oxyhydroxides through multiple mechanisms, including surface precipitation, ternary surface complexes, and electrostatic interactions. However, the numerous factors that control the immobilization of Cd and As in turn complicated the processes and mechanisms involved in their co-desorption from iron minerals, which hindered the full understanding of their geochemical behaviors. Here, the simultaneous release of Cd(II) and As(V) from newly precipitated ferrihydrite nanoparticles by either Ca or P was investigated through kinetics and isothermal desorption experiments. We showed that the Cd(II) and As(V) present two-phase desorption processes (rapid desorption and slow desorption) in both binary (Fe-Cd or Fe-As alone) and ternary systems (Fe-Cd-As co-presence). Compared to their binary counterparts, Cd(II) and As(V) in the ternary systems are more prone to detachment from ferrihydrite. Further desorption of Cd(II) and As(V) at different co-presence scenarios (different initial concentrations) demonstrated mutual promotion behaviour towards their counterparts; the co-presence of Cd(II) facilitates the desorption of As(V), while the co-presence of As(V) also promotes the desorption of Cd(II). XPS and FTIR results demonstrated that either Ca or P showed minor effects on the binding environment of Cd and As. Further results from the in-situ ATR-FTIR experiment and second derivative peak fitting analysis indicate that the enhanced detachment of Cd(II) and As(V) from the ternary system may be due to the synergistic desorption of the ternary surface complexes and other surface complex species. Our results provide new insights into the prediction of the environmental behaviour of the coexistence of Cd(II) and As(V) in iron-rich geological settings. The potential environmental risks of iron-based remediation methods should be considered due to the enhanced bioavailability of Cd(II) and As(V) in co-presence circumstances.

Uptake, subcellular distribution, and fate of tetracycline in two wetland plants supplemented with microbial agents: Effect and mechanism

The use of wetland plants in the context of phytoremediation is effective in the removal of antibiotics from contaminated water. However, the effectiveness and efficiency of many of these plants in the removal of antibiotics remain undetermined. In this study, the effectiveness of two plants-Phragmites australis and Iris pseudacorus-in the removal of tetracycline (TC) in hydroponic systems was investigated. The uptake of TC at the roots of I. pseudacorus and P. australis occurred at concentrations of 588.78 and 106.70 μg/g, respectively, after 7-day exposure. The higher uptake of TC in the root of I. pseudacorus may be attributed to its higher secretion of root exudates, which facilitate conditions conducive to the reproduction of microorganisms. These rhizosphere-linked microorganisms then drove the TC uptake, which was higher than that in the roots of P. australis. By elucidating the mechanisms underlying these uptake-linked outcomes, we found that the uptake of TC for both plants was significantly suppressed by metabolic and aquaporin inhibition, suggesting uptake and transport of TC were active (energy-dependent) and passive (aquaporin-dominated) processes, respectively. The subcellular distribution patterns of I. pseudacorus and P. australis in the roots were different, as expressed by differences in organelles, cell wall concentration levels, and transport-related dynamics. Additionally, the microbe-driven enhancement of the remediation capacities of the plants was studied comprehensively via a combined microbial-phytoremediation hydroponic system. We confirmed that the microbial agents increased the secretion of root exudates, promoting the variation of TC chemical speciation and thus enhancing the active transport of TC. These results contribute toward the improved application of wetland plants in the context of antibiotic phytoremediation.

Incorporation of Shewanella oneidensis MR-1 and goethite stimulates anaerobic Sb(III) oxidation by the generation of labile Fe(III) intermediate

Dissimilatory iron-reducing bacteria (DIRB) affect the geochemical cycling of redox-sensitive pollutants in anaerobic environments by controlling the transformation of Fe morphology. The anaerobic oxidation of antimonite (Sb(III)) driven by DIRB and Fe(III) oxyhydroxides interactions has been previously reported. However, the oxidative species and mechanisms involved remain unclear. In this study, both biotic phenomenon and abiotic verification experiments were conducted to explore the formed oxidative intermediates and related processes that lead to anaerobic Sb(III) oxidation accompanied during dissimilatory iron reduction. Sb(V) up to 2.59 μmol L-1 combined with total Fe(II) increased to 188.79 μmol L-1 when both Shewanella oneidensis MR-1 and goethite were present. In contrast, no Sb(III) oxidation or Fe(III) reduction occurred in the presence of MR-1 or goethite alone. Negative open circuit potential (OCP) shifts further demonstrated the generation of interfacial electron transfer (ET) between biogenic Fe(II) and goethite. Based on spectrophotometry, electron spin resonance (ESR) test and quenching experiments, the active ET production labile Fe(III) was confirmed to oxidize 94.12% of the Sb(III), while the contribution of other radicals was elucidated. Accordingly, we proposed that labile Fe(III) was the main oxidative species during anaerobic Sb(III) oxidation in the presence of DIRB and that the toxicity of antimony (Sb) in the environment was reduced. Considering the prevalence of DIRB and Fe(III) oxyhydroxides in natural environments, our findings provide a new perspective on the transformation of redox sensitive substances and build an eco-friendly bioremediation strategy for treating toxic metalloid pollution.

Structural Fe(II)-induced generation of reactive oxygen species on magnetite surface for aqueous As(III) oxidation during oxygen activation

Magnetite is a reductive Fe(II)-bearing mineral, and its reduction property is considered important for degradation of contaminants in groundwater and anaerobic subsurface environments. However, the redox condition of subsurface environments frequently changes from anaerobic to aerobic owing to natural and anthropogenic disturbances, generating reactive oxygen species (ROS) from the interaction between Fe(II)-bearing minerals and O2. Despite this, the mechanism of ROS generation induced by magnetite under aerobic conditions is poorly understood, which may play a crucial role in As(III) oxidation. Herein, we found that magnetite could activate O2 and induce the oxidative transformation of As(III) under aerobic conditions. As(III) oxidation was attributed to the ROS generated via structural Fe(II) within the magnetite octahedra oxygenation. The electron paramagnetic resonance and quenching tests confirmed that O2•-, H2O2, and •OH were produced by magnetite. Moreover, density function theory calculations combined with experiments demonstrated that O2•- was initially formed via single electron transfer from the structural Fe(II) to the adsorbed O2; O2•- was then converted to •OH and H2O2 via a series of free radical reactions. Among them, O2•-and H2O2 were the primary ROS responsible for As(III) oxidation, accounting for approximately 52 % and 19 % of As(III) oxidation. Notably, As(III) oxidation mainly occurred on the magnetite surface, and As was immobilized further within the magnetite structure. This study provides solid evidence regarding the role of magnetite in determining the fate and transformation of As in redox-fluctuating subsurface environments.

Sub-inhibitory concentrations of ampicillin affect microbial Fe(III) oxide reduction

Antibiotics are ubiquitous in the iron-rich environments but their roles in microbial reduction of Fe(III) oxides are still unclear. Using ampicillin and Geobacter soli, this study investigated the underlying mechanism by which antibiotic regulated microbial reduction of Fe(III) oxides. Results showed that sub-minimal inhibitory concentrations (sub-MIC) of ampicillin significantly affected ferrihydrite reduction by G. soli, with a stimulatory effect at 1/64 and 1/32 MIC and an inhibitory effect at 1/8 MIC. Increasing ampicillin concentration resulted in increasing cell length and decreasing bacterial zeta potential that were beneficial for ferrihydrite reduction, and decreasing outer membrane permeability that was unfavorable for ferrihydrite reduction. The respiratory metabolism ability was enhanced by 1/64 and 1/32 MIC ampicillin and reduced by 1/8 MIC ampicillin, which was also responsible for regulation of ferrihydrite reduction by ampicillin. The ferrihydrite reduction showed a positive correlation with the redox activity of extracellular polymeric substances (EPS) which was tied to the cytochrome/polysaccharide ratio and the content of α-helices and β-sheet in EPS. These results suggested that ampicillin regulated microbial Fe(III) oxide reduction through modulating the bacterial morphology, metabolism activity and extracellular electron transfer ability. Our findings provide new insights into the environmental factors regulating biogeochemical cycling of iron.

Degradation of tetracyclines via calcium peroxide activation by ultrasonic: Roles of reactive species, oxidation mechanism and toxicity evaluation

Tetracyclines (TC) frequently detected in the aqueous environment pose threats to humans and ecosystems. The synergistic technology coupling ultrasound (US) and calcium peroxide (CaO₂) has a great potential to abate TC in wastewater. However, the degradation efficiency and detailed mechanism of TC removal in the US/CaO₂ system is unclear. This work was carried out to assess the performance and mechanism of TC removal in the US/CaO₂ system. The results demonstrated that 99.2% of TC was degraded by the combination of 15 mM CaO₂ with ultrasonic power of 400 W (20 kHz), but only about 30% and 4.5% of TC was removed by CaO₂ (15 mM) or US (400 W) alone process, respectively. Experiments using specific quenchers and electron paramagnetic resonance (EPR) analysis indicated that the generation of hydroxyl radicals (•OH), superoxide radicals (O₂^-•), and single oxygen (¹O₂) in the process, whereas •OH and ¹O₂ were mainly responsible for the degradation of TC. The removal of TC in the US/CaO₂ system has a close relationship with the ultrasonic power, the dosage of CaO₂ and TC, and the initial pH. The degradation pathway of TC in the US/CaO₂ process was proposed based on the detected oxidation products, and it mainly included N,N-dedimethylation, hydroxylation, and ring-opening reactions. The presence of 10 mM common inorganic anions including chloridion (Cl^-), nitrate ion (NO₃^-), sulfate ion (SO₄^2-), and bicarbonate ion (HCO₃^-) showed negligible influences on the removal of TC in the US/CaO₂ system. The US/CaO₂ process could efficiently remove TC in real wastewater. Overall, this work firstly demonstrated that •OH and ¹O₂ mainly contributed to the removal of pollutants in the US/CaO₂ system, which was remarkable for understanding the mechanisms of CaO₂-based oxidation process and its future application.

Highly stable and efficient Cr(VI) immobilization from water by adsorption with the La-substituted ferrihydrite as a naturally-occurring geosorbent in soils

Ferrihydrite (Fh) is a vital geosorbent in the natural environment. Here, Fh materials with lanthanum (La) substituted in varied La/La + Fe ratios were synthesized, and these La-Fh materials were investigated in-depth via adsorption kinetic and isothermal experiments to explore their adsorption performance for chromate [Cr(VI)] in soils. Material properties of La-Fh were further characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectrometer (FTIR), and X-ray photoelectron spectroscopy (XPS). The results clearly indicate that La³⁺ can be integrated into the Fh lattice, but the increase in La amount substituted into Fh is slowed down when the La/La + Fe ratio reaches to a larger value. Those La³⁺ cations that fail to become integrated may either get adsorbed or form a phase of La(OH)₃ on La-Fh surfaces. We also find that La substitution reduces the specific surface area (SSA) of La-Fh samples but raises their pH_pzc, which hampers La-Fh conversion to hematite and thus increases the chemical stability. These changes are related to the La-Fh structure and surface aspects, but they do not negatively affect the Cr(VI) adsorption efficacy, which can be promoted over a wide pH range to an alkaline pH. For instance, the maximum adsorption amount of Cr(VI) by 20%La-Fh is 30.2 mg/g at a near-neutral pH. However, the entire chromate adsorption processes are affected by H₂PO₄^- and humic acid due to their strong affinities for Cr(VI), but almost not influenced by NO₃^- and Cl^-. All the Cr(VI)-Fh reactions are well described by the fitted adsorption Freundlich model and conform to the pseudo-second-order reaction kinetic equation. The mechanisms which enhance La-Fh's adsorption ability for Cr(VI) are governed by chemical interactions, because La substitution can increase the hydroxyl density on Fh surfaces and thus improve the reactivity of La-Fh towards Cr(VI), leading to an evidently enhanced Cr(VI) immobilization onto La-Fh.

Mechanistic insights into the interfacial adsorption behaviors of Cr(VI) on ferrihydrite: Effects of pH and naturally coexisting anions in the environment

Interfacial interaction of hexavalent chromium (Cr[VI]) with ferrihydrite (Fh) plays a key role in the behavior of Cr(VI) in the environment. In this study, H2PO4-, SO42-, NO3-, Cl-, and HCO3- were chosen as coexisting anions to explore their inhibition of the capacity of Fh to adsorb Cr(VI). We employed X-ray diffraction, scanning electron microscopy, attenuated total reflection Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy to thoroughly characterize Fh reaction products before and after adsorption of Cr(VI). The results clearly revealed that pH has a marked effect on the extent of Cr(VI) adsorption onto Fh, and this process is also highly dependent on the types of anions present. H2PO4- exhibited the most evident inhibition of Cr(VI) adsorption, even at low concentrations. Similarly, the inhibition of Cr(VI) adsorption by HCO3- increased markedly with increasing pH. In contrast, SO42- only slightly competed with Cr(VI) for reactive Fh surface sites. The anions Cl- and NO3- exhibited almost no inhibitory effect on Cr(VI) adsorption. The differential order of adsorptive affinity of all six anions for Fh was as follows: H2PO4- > HCO3- > SO42- ≈ HCrO4- > NO3- ≈ Cl-. Based on these results, we further provide mechanistic insights into the complexities of Cr(VI) adsorption/desorption behaviors on Fh surfaces. Using Fh as a geosorbent, these interfacial properties could be exploited to mediate the immobilization and release of chromate from and/or into contaminated environments such as aquifers.

Phase transformation of nanosized zero-valent iron modulated by As(III) determines heavy metal passivation

Nanoscale zero-valent iron (nZVI) has been extensively used for the passivation of cadmium (Cd) or arsenic (As) from wastewaters, while the underlying mechanisms of nZVI reaction with coexisting Cd and As are largely overlooked. Herein, the interactions of Cd and As during the course of nZVI transformation and the corresponding effects on respective pollutant removal have been systematically investigated. Batch experiments results show that As(III) addition significantly promotes the passivation of Cd(II) by nZVI, and the removal capacity increases by 7.8 times compared to that of Cd(II) alone. However, the adsorption and oxidative transformation of As(III) are barely affected under a relatively low Cd(II) concentration. It is conducive to the adsorption of Cd(II) and As(III) using nZVI under neutral conditions. The transformation of nZVI to lepidocrocite dominates in the Cd(II) single system, while it mainly converts to amorphous Fe oxyhydroxide with the addition of As(III). As(III) notably reduces the surface charge of Fe oxyhydroxide intermediates and to form the ternary complexes with Cd (Fe-As-Cd), which is the predominant mechanism for the promoted Cd(II) passivation. This work provides new understanding of nZVI transformation coupled to Cd(II) and As(III) passivation, which are likely contributing to the heavy metalloids regulation in waters and subsurface environments.