Zero-valent Zinc Research Articles

Enhanced ozonation degradation of di-n-butyl phthalate by zero-valent zinc in aqueous solution: Performance and mechanism

Enhanced ozonation degradation of di-n-butyl phthalate (DBP) by zero-valent zinc (ZVZ) has been investigated using a semi-continuous reactor in aqueous solution. The results indicated that the combination of ozone (O3) and ZVZ showed an obvious synergetic effect, i.e. an improvement of 54.8% on DBP degradation was obtained by the O3/ZVZ process after 10min reaction compared to the cumulative effect of O3 alone and O2/ZVZ. The degradation efficiency of DBP increased gradually with the increase of ZVZ dosage, enhanced as solution pH increasing from 2.0 to 10.0, and more amount of DBP was degraded with the initial concentration of DBP arising from 0.5 to 2.0mgL−1. Recycling use of ZVZ resulted in the enhancement of DBP degradation, because the newly formed zinc oxide took part in the reaction. The mechanism investigation demonstrated that the enhancement effect was attributed to the introduction of ZVZ, which could promote the utilization of O3, enhance the formation of superoxide radical by reducing O2 via one-electron transfer, accelerate the production of hydrogen peroxide and the generation of hydroxyl radical. Additionally, the newly formed zinc oxide on ZVZ surface also contributed to the enhancement of DBP degradation in the recycling use of ZVZ. Most importantly, the O3/ZVZ process was also effective in enhanced ozonation degradation of DBP under the background of actual waters.

An Efficient Synthesis of 1-chloro-2,2-difluoroethylene via the Reductive Dechlorination of 1,2,2-trichloro-1,1-difluoroethane

1-Chloro-2,2-difluoroethylene was prepared from 1,2,2-trichloro-1,1-difluoroethane by reductive dechlorination in the presence of zero-valent zinc. Eleven different solvents were investigated and the best results were obtained in methanol, dimethyl formamide and ethanol at 80 °C. A large-scale experiment using ethanol as a solvent gave 1-chloro-2,2-difluoroethylene in high yield. These results provide a method for producing 1-chloro-2,2-difluoroethylen e on an industrial scale because 1,2,2-trichloro-1,1-difluoroethane is the waste material arising from 2,2-dichloro-1,1,1-trifluoroethane production. These results can also provide a method for solving the recycling problem of 1,2,2-trichloro-1,1-difluoroethane in 2,2-dichloro-1,1,1-trifluoroethane production.

Reductive dechlorination of octachlorodibenzo-p-dioxin by nanosized zero-valent zinc: Modeling of rate kinetics and congener profile

Polychlorinated dibenzo-p-dioxins (PCDDs), a group of recalcitrant toxic compounds, are ubiquitous in nature. Amongst them, octachlorodibenzo-p-dioxin (OCDD) is not only prevalent in soil and sediment due to its high lipophilicity and hydrophobicity, but also detected in ground water and surface water. The present study examined the degradation of OCDD in aqueous solutions using four different zero-valent metal nanoparticles; zero-valent aluminum (nZVAL), zero-valent zinc (nZVZ), zero-valent iron (nZVI) and zero-valent nickel (nZVN). Only nZVZ was found to efficiently degrade OCDD into lower chlorinated congeners [OCDD→1,2,3,4,6,7,9-HxCDD (63%)→1,2,3,6,8,9-HpCDD (21%)→1,2,4,7,8-PeCDD (46%)→1,2,4,7-TeCDD (19%)] under ambient conditions. Simulations were also performed to predict the OCDD dechlorination pathway using a linear free energy relationship (LFER) model. Additionally, toxic equivalent quantity (TEQ) and homologue patterns were calculated by LFER modeling. The experimentally observed congener profiles were in excellent agreement with the model-predicted results, especially considering the complexity of the OCDD dechlorination pathway (256 theoretically possible reactions). This study proposes nZVZ as a suitable candidate for OCDD dechlorination and constitutes the first report on OCDD degradation using zero-valent metal nanoparticles under ambient conditions.

Genome-Wide Assessment in Escherichia coli Reveals Time-Dependent Nanotoxicity Paradigms

The use of engineered nanomaterials (eNM) in consumer and industrial products is increasing exponentially. Our ability to rapidly assess their potential effects on human and environmental health is limited by our understanding of nanomediated toxicity. High-throughput screening (HTS) enables the investigation of nanomediated toxicity on a genome-wide level, thus uncovering their novel mechanisms and paradigms. Herein, we investigate the toxicity of zinc-containing nanomaterials (Zn-eNMs) using a time-resolved HTS methodology in an arrayed Escherichia coli genome-wide knockout (KO) library. The library was screened against nanoscale zerovalent zinc (nZn), nanoscale zinc oxide (nZnO), and zinc chloride (ZnCl(2)) salt as reference. Through sequential screening over 24 h, our method identified 173 sensitive clones from diverse biological pathways, which fell into two general groups: early and late responders. The overlap between these groups was small. Our results suggest that bacterial toxicity mechanisms change from pathways related to general metabolic function, transport, signaling, and metal ion homeostasis to membrane synthesis pathways over time. While all zinc sources shared pathways relating to membrane damage and metal ion homeostasis, Zn-eNMs and ZnCl(2) displayed differences in their sensitivity profiles. For example, ZnCl(2) and nZnO elicited unique responses in pathways related to two-component signaling and monosaccharide biosynthesis, respectively. Single isolated measurements, such as MIC or IC(50), are inadequate, and time-resolved approaches utilizing genome-wide assays are therefore needed to capture this crucial dimension and illuminate the dynamic interplay at the nano-bio interface.

Reduction of N-Nitrosodimethylamine with zero-valent zinc

N-Nitrosodimethylamine (NDMA) is known as the disinfection by-product and the pollutant in the source water. Reduction with zero-valent zinc (Zn(0)) was investigated as a potential technology to treat NDMA. The results showed that Zn(0) was effective for NDMA reduction at initial pH 7.0. There were lag period and rapid period during the process, the corresponding zero-order rate constant (kzero) was 2.968 ± 0.245 μg L−1 h−1 ([Zn(0)]0 = 10g L−1),the mass normalized pseudo-first-order rate (kM) was 0.1215 ± 0.0171 L g−1 h−1. The reactivity of zinc on NDMA removal was consistent with the zinc corrosion rate. NDMA had little effect on the corrosion of zinc. Lower solution pH benefited the reduction of NDMA with Zn(0). The consumption of the oxygen and the localized acidification should be the cause of the shift from lag to rapid reaction period in the aerobic experiments. 1,1-dimethylhydrazine (unsymmetrical dimethylhydrazine, UDMH), dimethylamine (DMA) were detected as the products of NDMA degradation. The nitrogen mass balance at 24 h was 56%, the loss can be due to the formation of ammonium, the degradation of UDMH and other unmeasured products. DMA formed during the degradation of UDMH with Zn(0), the nitrogen loss could be caused by the formation of unmeasured products. Catalytic hydrogenation is proposed to be the mechanism based on the results and the redox properties of zinc and NDMA. One reduction process is: the active hydrogen atoms initially cleave and reduce the N=O bond in NDMA, generating UDMH. Then the N–N bond in UDMH is cleaved to form DMA and ammonium.

Transformation and biodegradation of 1,2,3-trichloropropane (TCP).

Purpose1,2,3-Trichloropropane (TCP) is a persistent groundwater pollutant and a suspected human carcinogen. It is also is an industrial chemical waste that has been formed in large amounts during epichlorohydrin manufacture. In view of the spread of TCP via groundwater and its toxicity, there is a need for cheap and efficient technologies for the cleanup of TCP-contaminated sites. In situ or on-site bioremediation of TCP is an option if biodegradation can be achieved and stimulated. This paper presents an overview of methods for the remediation of TCP-contaminated water with an emphasis on the possibilities of biodegradation.ConclusionsAlthough TCP is a xenobiotic chlorinated compound of high chemical stability, a number of abiotic and biotic conversions have been demonstrated, including abiotic oxidative conversion in the presence of a strong oxidant and reductive conversion by zero-valent zinc. Biotransformations that have been observed include reductive dechlorination, monooxygenase-mediated cometabolism, and enzymatic hydrolysis. No natural organisms are known that can use TCP as a carbon source for growth under aerobic conditions, but anaerobically TCP may serve as electron acceptor. The application of biodegradation is hindered by low degradation rates and incomplete mineralization. Protein engineering and genetic modification can be used to obtain microorganisms with enhanced TCP degradation potential.

Evaluation of Zerovalent Zinc for Treatment of 1,2,3‐Trichloropropane‐Contaminated Groundwater: Laboratory and Field Assessment

AbstractThe efficacy and feasibility of using zerovalent zinc (ZVZ) to treat 1,2,3‐trichloropropane (TCP)‐contaminated groundwater was assessed in laboratory and field experiments. In the first portion of the study, the reactivity of commercially available granular ZVZ toward TCP was measured in bench‐scale batch‐reactor and column experiments. These results were used to design columns for on‐site pilot‐scale treatment of contaminated groundwater at a site in Southern California. Two of the ZVZ materials tested were found to produce relatively high rates of TCP degradation as well as predictable behavior when scaling from bench‐scale to field testing. In addition, there was little decrease in the rates of TCP degradation over the duration of field testing. Finally, no secondary impacts to water quality were identified. The results suggest that ZVZ may be an effective and feasible material for use in engineered treatment systems, perhaps including permeable reactive barriers.

2-(Dimethylamino)ethanol-κ2N,O][2-(dimethylamino)ethanolato-κ2N,O]iodidocopper(II)

The title compound, [Cu(C4H10NO)I(C4H11NO)], was obtained unintentionally as the product of an attempted synthesis of a Cu/Zn mixed-metal complex using zerovalent copper, zinc(II) oxide and ammonium iodide in pure 2-(dimethyl­amino)­ethanol, in air. The mol­ecular complex has no crystallographically imposed symmetry. The coordination geometry around the metal atom is distorted square-pyramidal. The equatorial coordination around copper involves donor atoms of the bidentate chelating 2-(dimethyl­amino)­ethanol ligand and the 2-(dimethyl­amino)­ethano­late group, which are mutually trans to each other, with four approximately equal short Cu—O/N bond distances. The axial Cu—I bond is substanti­ally elongated. Inter­molecular hydrogen-bonding inter­actions involving the –OH group of the neutral 2-(dimethyl­amino)­ethanol ligand to the O atom of the monodeprotonated 2-(dimethyl­amino)­ethano­late group of the mol­ecule related by the n-glide plane, as indicated by the O⋯O distance of 2.482 (12) Å, form chains of mol­ecules propagating along [101].

Effects of Solution Chemistry on the Dechlorination of 1,2,3-Trichloropropane by Zero-Valent Zinc

The reactivity of zerovalent zinc (ZVZ) toward 1,2,3-trichloropropane (TCP) was evaluated under a variety of solution conditions, including deionized water, groundwater, and artificial groundwater, over a pH range of about 6.5-12. In deionized water, first-order rate constants for TCP disappearance (k(obs)) exhibit a broad minimum between pH 8 and 10, with increasing k(obs) observed at lower and higher pH. The similarity between this trend and zinc oxide (ZnO) solubility behavior suggests pH related changes to the ZnO surface layer strongly influence ZVZ reactivity. Values of k(obs) measured in acidic groundwater are similar to those measured in DI water, whereas values measured in alkaline groundwater are much smaller (>1 order of magnitude at pH values >10). Characterization of the surfaces of ZVZ exposed to deionized water, acidic groundwater, and alkaline groundwater suggests that the slower rates obtained in alkaline groundwater are related to the presence of a morphologically distinct surface film that passivates the ZVZ surface. TCP degradation rates in artificial groundwater containing individual solutes present in groundwater suggest that silicate anions contribute to the formation of this passivating film.

Remediation technology of groundwater contaminated by perchloroethylene

To dechlorinate perchloroethylene (PCE), batch experiments were conducted for five groups of reactors: zero-valent iron (Fe0), zero-valent zinc (Zn0), an anaerobic microbial community (MB), a combination of iron powder and an anaerobic microbial community (FeMB), and combination of zinc powder and an anaerobic microbial community (ZnMB). After about 25 days, PCE in the FeMB degradation system has been almost completely removed; about 43% of PCE has been dechlorinated to the nontoxic hydrocarbons acetylene, ethylene or ethane. This major reaction pathway in the FeMB system was different from those in other systems. The chain pseudo-first-order kinetic reaction model was applied to predicting the complete dechlorination of PCE to non-chlorinated hydrocarbons in different systems.

살균제 Tolclofos-methyl의 화학적 처리에 의한 분해

Tolclofos-methyl is one of the most widely used organophosphorous pesticides in control of soil-borne diseases in ginseng field. In Korea, residues of tolclofos- methyl in ginseng and cultivation soil is quite often detecting. The objective of this study was to know the possibility for the accelerated degradation of tolclofos-methyl by various chemical treatment under soil slurry condition. The degradation of tolclofos-methyl was accelerated by zerovalent metals treatment in soil slurry. The degradation rate of tolclofos-methyl was found to be at higher zerovalent zinc than unannealed zerovalent and annealed zerovalent iron. The effect of different sizes of zerovalent iron on tolclofos-methyl degradation was showed that the smaller size of zerovalent iron, the greater the degradation rate. In aqueous solution of pH 4.0 below the degradation rate of tolclofos-methyl was very high. Under this experimental condition, tolclofos-methyl degradation was the greatest at 2% (w/v) of ZVI under 0.1 N of HCl in 24 hours, the degradation rate was 94.4%. By testing various chemicals, it was found that Fe2(SO4)3 as iron source showed better for degrading tolclofos-methyl in H2O2 500 mM treatment and sodium sulfite also showed the degradable possibility tolclofos-methyl in soil slurry.

Kinetics of Nitrite Reduction by Zinc Metal: Influence of Metal Shape on the Determination of Kinetic Parameters

Heterogeneous chemical reactions are complicated, especially in the case of competitive reactions. The aim of this study was to investigate the elimination of nitrite (NO2(-)) by applying a metallic reduction using zero-valent zinc (Zn0). The effect of pH, stirring, and metal shape (powder and chips) on the rate and products of nitrite reduction were studied in a batch-stirred reactor. The obtained data have been used to optimize the conditions for metallic reduction of NO2(-) and for kinetic parameters identification. It was found that the dissolution of zinc involves a pseudo-first-order reaction independent of the shape of the metal. Further, the influence of operating conditions on nitrogen (N2) and ammonium (NH4(+)) formation has been determined. It was found that a decrease in pH and in the Zn0 content enhances NH4(+) production. If kinetic parameters can be approximated easily for constant surface area, it was demonstrated that surface evolution had to be integrated for metal powder. Finally, a numerical simulation has been used to determine the kinetic parameters for NO2(-) reduction with zinc powder.

Rules of thumb for assessing reductive dechlorination pathways of PCDDs in specific systems

This paper reports a theoretical validation and proposition of the reductive dechlorination pathways for polychlorinated dibenzo- p-dioxin (PCDD) congeners. Density functional theory (DFT) calculations were carried out at the B3LYP/6-31G(d) level for all PCDDs and Mulliken atomic charges on chlorine atoms were adopted as the probe of the dechlorination reaction activity. The experimentally substantiated dechlorination pathways of 1,2,3,4-tetrachlorodibenzo- p-dioxin (1,2,3,4-TCDD) and its daughter products in the presence of zero-valent zinc were validated and the complete pathway of dechlorination of octachlorodibenzo- p-dioxin (OCDD) was proposed. The proposed pathways were found to be consistent with anaerobic biotransformation of several PCDD congeners. Four rules of thumb arrived from this study include (1) the chlorine atoms in the longitudinal (1,4,6,9) positions are removed in preference to the chlorine atoms on lateral (2,3,7,8) positions; (2) the chlorine atom that has more neighboring chlorine atoms at ortho-, meta- and para-positions is to be eliminated; (3) reductive dechlorination prefers to take place on the benzene ring having more chlorine substitutions; and (4) a chlorine atom on the side of the longitudinal symmetry axis containing more chlorine atoms is preferentially eliminated. These rules of thumb can be conveniently used for rapidly predicting the major dechlorination pathway for a given PCDD in specific systems.

Accelerated Effect of Ferric Salts on Degradation of Thiophosphate Fungicide, Tolclofos-methyl by Zerovalent Iron

In this work, the degradation of fungicide tolclofos-methyl (TM) by zerovalent metals (ZVMs) was first investigated prior to investigation of the effect of ferric salts during zerovalent iron treatment to better understand the feasibility of abiotic degradation mediated by ZVMs. Unannealed zerovalent iron (uZVI), annealed zerovalent iron (aZVI) and zerovalent zinc (ZVZn) were used as ZVMs sources. uZVI was found to be the most effective catalyst for degrading TM in aqueous solution. The TM degradation rate depends not only on the amount and sizes of uZVI used but also on the initial concentration of fungicide itself. By testing various kinds of salt, it was found that ferric sulfate and ferric chloride showed better results for degrading TM by uZVI in aqueous solution. The order of effectiveness of salts is FeCl3>Fe2(SO4)3>FeSO4>Al2(SO4)3.

Rapid transformation of 1,2,3,4-TCDD by Pd/Fe catalysts

This study investigated reductive dechlorination of 1,2,3,4-tetrachlorodibenzo- p-dioxin (1,2,3,4-TCDD) in the presence of Pd/Fe, Cu/Fe and Ni/Fe bimetallic catalysts or zero-valent iron (ZVI) under aqueous and ambient temperature conditions. It was found that the Pd/Fe catalyst can rapidly transform 1,2,3,4-TCDD to chlorine-free dibenzo- p-dioxin (DD) with >95% of conversion and half lives on the order of 5 h, whereas Cu/Fe, Ni/Fe and ZVI were not reactive over 10 d. The dechlorination of 1,2,3,4-TCDD in the Pd/Fe systems was found to be stepwise, with 1,2,3-tri (TrCDD), 1,2-di (DCDD), and mono-(MCDD) chlorodibenzo- p-dioxins as the three intermediate products at low concentrations and DD as the final and dominant product. The reaction rate of 1,2,3,4-TCDD dechlorination appeared to decrease as the coverage of palladium on iron particle surface increases from 0.0060 mol% to 0.0228 mol%. Compared to our prior study of reductive dechlorination of 1,2,3,4-TCDD with zero valent zinc (ZVZ) as the reductant, the reactions with Pd/Fe bimetallic materials were relatively slower for TCDD, but much faster for TrCDDs and DCDDs, yielding DD as the major product. The study suggested that Pd/Fe bimetallic catalyst-based reactions may be suitable for rapid decontamination of polychlorinated dibenzo- p-dioxin/dibenzofuran (PCDD/F) polluted soils and sediments.

Reduction of 2,4,6-trichlorophenol with zero-valent zinc and catalyzed zinc

Reductive dechlorination of 2,4,6-trichlorophenol (2,4,6-TCP) was conducted with Zn and Zn bimetals, Pd/Zn, Ni/Zn, Cu/Zn, Pt/Zn. Zn showed relatively low reaction rate toward 2,4,6-TCP, while Pd/Zn had dramatically increased reactivity and other bimetals had higher reaction rates than that of plain zinc. Phenol and less chlorinated phenols were found as dechlorination products. Pd/Zn produced cyclohexanone which is a product of aromatic ring reduced. Surface area normalized kinetic constants and second metal contents normalized kinetic constants were calculated and compared. Two mechanisms, mainly catalytic activation and enhanced corrosion, were proposed for the reactivity enhancement.

Cyanocobalamin Enables Activated Sludge Bacteria to Dechlorinate Hexachloro-1,3-butadiene to Nonchlorinated Gases

ABSTRACT The ability of activated sludge obtained from a local wastewater treatment plant to dechlorinate hexachloro-1,3-butadiene (HCBD) in the presence of either acetate or lactate and cyanocobalamin was investigated. Results from headspace analysis indicated complete dechlorination of HCBD by the accumulation of fully dechlorinated C4 gases (1-buten-3-yne, 1,3-butadiene, and 1,3-butadiyne). Dechlorination products were not observed in the control cultures without cyanocobalamin. Examination of control cultures revealed that the disappearance of HCBD from the headspace was partly due to adsorption into the biomass. However, the key for dechlorination was the shuttle (cyanocobalamin) rather than specific microbial enzymatic activity. The hypothesis that the bacteria reduced cyanocobalamin, which in turn reductively dechlorinated HCBD, was supported by the finding that cyanocobalamin reduced by zero-valent zinc resulted in complete dechlorination. The significance of the findings is that, in contrast to prior work where specific anaerobic bacteria (enrichments or pure cultures) were believed to be necessary for dechlorination resulting in only partly dechlorinated products, the currect data show that nonspecific aerobic activated sludge bacteria can be employed for complete HCBD dechlorination at rates sufficiently high to be considered for bioremediation projects.

Amendment of hydroxyapatite in reduction of tetrachloroethylene by zero-valent zinc: Its rate enhancing effect and removal of Zn(II)

The effects of hydroxyapatite (HAP) on dechlorination of tetrachloroethylene (PCE) by zero-valent zinc (ZVZ) were examined in batch systems. PCE was primarily transformed to trichloroethylene by ZVZ, with 1,2- trans-dichloroethylene representing a minor product. Dechlorination of PCE was accelerated by the presence of HAP, and the pseudo-first order rate constants increased with increasing amount of HAP. Zn(II), mostly generated from oxidative dissolution of ZVZ by PCE, was effectively removed from the solution by HAP. Ion substitution, coprecipitation, and adsorption are proposed as the possible mechanisms for Zn(II) removal. These reactions appeared to occur simultaneously and the contribution of each reaction to overall removal of Zn(II) was primarily dependent on HAP loading at constant ZVZ loading. The results indicate that the use of HAP in combination with conventional zero-valent metals is promising in that it can achieve both degradation of organic contaminants and stabilization of inorganic contaminants.

Kinetics of reductive dechlorination of 1,2,3,4-TCDD in the presence of zero-valent zinc

Polychlorinated dibenzo- p-dioxins (PCDDs) are toxic and widespread persistent organic pollutants (POPs). Cost-effective technologies for destroying or detoxifying PCDDs are in high demand. The overall purpose of this study was to develop a zero-valent zinc based technology for transforming toxic PCDDs to less- or non-toxic forms. We measured the dechlorination rates of 1,2,3,4-tetrachlorodibenzo- p-dioxin (1,2,3,4-TCDD) in the presence of zero-valent zinc under aqueous conditions, identified the daughter compounds of the reaction, and constructed possible pathways for the reactions. The reaction rates of daughter compounds with zero-valent zinc were also measured independently. Our results showed that the zero-valent zinc is a suitable candidate for reducing PCDDs. Reductive dechlorination of 1,2,3,4-TCDD was stepwise and complete to dibenzo- p-dioxin (DD) mainly via 1,2,4-trichlorodibenzo- p-dioxin (1,2,4-TrCDD), 1,3-dichlorodibenzo- p-dioxin (1,3-DCDD), 1-chlorodibenzo- p-dioxin (1-MCDD) to DD and via 1,2,4-TrCDD, 2,3-dichlorodibenzo- p-dioxin (2,3-DCDD), 2-chlorodibenzo- p-dioxin (2-MCDD) to DD. In each separate system, the observed half-lives of 1,2,3,4-TCDD, 1,2,3-TrCDD, 1,2,4-TrCDD, 1,2-DCDD, 1,3-DCDD, 1,4-DCDD and 2,3-DCDD are 0.56, 2.62, 5.71, 24.93, 41.53, 93.67 and 169.06 h respectively. The tendency of rate constant follows TCDD > TrCDD > DCDD. Our results suggest that zero-valent zinc is a suitable candidate for rapidly reducing highly chlorinated PCDDs to less or non-chlorinated daughter products.

Low-temperature isotopic fractionation of uranium

Uranium is the heaviest naturally occurring element and isotope fractionation between 235U and 238U is not normally considered significant given the small ∼ 1% difference in mass. It is therefore usual to assume that 238U/ 235U is constant in the terrestrial environment and equal to 137.88. We have developed experimental protocols for the precise measurement of 235U/ 238U by multiple-collector ICPMS (MC-ICPMS) and have analyzed a suite of samples formed in a range of low-temperature environments. Using a high-purity 233U– 236U double spike to internally monitor the large (percent-level) but essentially constant instrumental mass bias effects that are inherent to plasma source mass spectrometry, we are able to resolve variations in 235U/ 238U at the 0.4 epsilon level (2 σ; 1 epsilon = 1 part in 10,000) on sample sizes comprising 50 ng of uranium. Here we demonstrate sizeable (13 epsilon units) natural variability in 235U/ 238U, exceeding the analytical reproducibility by more than an order of magnitude. Compositions that are both isotopically heavier and lighter than our terrestrial standard, by 4 and 9 epsilon units respectively, are observed. The largest excursions are found in speleothem samples. Furthermore, 235U/ 238U appears broadly correlated with 234U/ 238U in samples showing the most extreme isotopic compositions. The present study investigates the role of abiotic processes in fractionating 235U from 238U. Sequential leaching experiments of U-rich minerals indicate that mineral weathering is a possible mechanism by which 235U can be fractionated from 238U in groundwaters and incorporated into speleothems. The observed variability in 235U/ 238U indicates that uranium isotopes may offer the potential to monitor new reaction pathways, such as those activated during the redox transition between the U(IV) and U(VI) oxidation states. Experiments involving the redox transition of U(VI) to U(IV) in the presence of zero-valent zinc did not produce a resolvable shift in 235U/ 238U towards anomalous values, although fractionation need not occur if the reaction is governed by a fast kinetic process. Our observations have a direct impact on the U-series and U–Th–Pb chronometers, when applied to samples formed in low-temperature environments, as these chronometers currently assume an invariant 238U/ 235U equal to 137.88.