Zero-valent Zinc Research Articles

Debromination of polybrominated biphenyls (PBBs) by zero valent metals and iron-based bimetallic particles: Mechanisms, pathways and predicting descriptor

Polybrominated biphenyls (PBBs) are toxic and persistent brominated flame retardant. Yet their debromination by zero valent metal (ZVM) or iron based bimetals has received far less attention. Here we reported the kinetics and debromination pathways of PBBs in nanoscale zerovalent iron (n-ZVI), microscale zerovalent zinc (m-ZVZ), n-ZVI/Ag and n-ZVI/Pd systems. The results show that the singly occupied molecular orbital (SOMO) of PBB anion can be well correlated with the debromination pathways of PBBs by n-ZVI, which verify its ability to predict their debromination pathways through electron transfer (e-transfer) mechanism. We also found that the reaction rates of PBBs in m-ZVZ, n-ZVI/Ag and n-ZVI/Pd systems are much faster than that in n-ZVI system, and the debromination pathways of PBBs in n-ZVI, m-ZVZ and n-ZVI/Ag are exactly the same, suggesting that n-ZVI/Ag debrominate PBBs through an e-transfer mechanism. However, Pd was found to be able to utilize H2 to debrominate PBBs and the debromination pathways of PBBs in Pd-H2 system was the same to those in n-ZVI/Pd system, but was partially different from those in n-ZVI, m-ZVZ and n-ZVI/Ag systems, suggesting that n-ZVI/Pd debrominate PBBs through a H-atom transfer mechanism.

Rapid debromination of polybrominated diphenyl ethers (PBDEs) by zero valent metal and bimetals: Mechanisms and pathways assisted by density function theory calculation

Polybrominated diphenyl ethers (PBDEs) undergo debromination when they were exposed in zerovalent metal or bimetallic systems. Yet their debromination pathways and mechanisms in these systems were not well understood. Here we reported the debromination pathways of three BDE congeners (BDE-21, 25 and 29) by nano-zerovalent iron (n-ZVI). All these BDE congeners have three bromine substituents that were located in ortho-, meta- and para-positions. Results demonstrated that BDE-21, 25 and 29 preferentially debrominate meta-, ortho- and para-bromines, respectively, suggesting that bromine substituent at each position (i.e. ortho-, meta- or para-) of PBDEs can be preferentially removed. Singly occupied molecular orbitals of BDE anions are well correlated with their actual debromination pathways, which successfully explain why these BDE congeners exhibit certain debromination pathways in n-ZVI system. In addition, microscale zerovalent zinc (m-ZVZ), iron-based bimetals (Fe/Ag and Fe/Pd) were also used to debrominate PBDEs, with BDE-21 as target pollutant. We found that the debromination pathways of BDE-21 in m-ZVZ and Fe/Ag systems are the same to those in n-ZVI system, but were partially different from those in Fe/Pd systems. The debromination of BDE-21 in Pd-H2 system as well as the solvent kinetic isotope effect in single metal and bimetallic systems suggests that H atom transfer is the dominant mechanism in Fe/Pd system, while e-transfer is still the dominant mechanism in Fe/Ag system.

Efficient enhancement of ozonation performance via ZVZ immobilized g-C3N4 towards superior oxidation of micropollutants

A functional organic-metal composite material zero-valent zinc immobilized graphitic carbon nitride (ZVZ-g-C3N4) was prepared by a fast and facile two-step synthetic approach with an optimal ZVZ content of 5.4 wt%. The structure, surface morphology and chemical composition of the as-synthesized ZVZ-g-C3N4 were characterized by BET surface area, XRD, FT-IR, SEM, TEM, and XPS, respectively. ZVZ-g-C3N4 composite exhibited superior catalytic ozonation activity with an improvement of 61.2% on atrazine (ATZ) degradation efficiency in 1.5 min reaction, more than 12 times of the pseudo-first-order rate constant, and almost 16-fold of the Rct value obtained in O3/ZVZ-g-C3N4 process compared to O3 alone. Meanwhile, the ATZ degradation efficiency was gradually enhanced with increasing ZVZ-g-C3N4 dosage and initial solution pH in the range from 3.0 to 9.0, and a higher amount of ATZ was degraded when the initial concentration of ATZ rose from 1 to 10 mg L−1. The enhanced catalytic ozonation activity of ZVZ-g-C3N4 is attributed to the synergistic effects among ZVZ, ZnO and g-C3N4, as well as the improved dispersibility, increased surface area, and intensive electron-transfer ascribed to the electronic and surface properties modification. The radical scavengers experiments demonstrated that O2−, OH, and 1O2 were the dominant reactive radical species in the multifunctional processes. Moreover, an empirical kinetic model was proposed to predict ATZ degradation. The results indicated that the ZVZ-g-C3N4 composite was a highly efficient, recoverable, and durable catalyst, which would provide a promising alternative in catalytic ozonation.

Assessing the role of silica gel in the degradation of p -nitrophenol via Zn(0)-activated persulfate

Abstract In this study, silica gel (SG) was utilized as an assistant to improve the degradation efficiency of p-nitrophenol (PNP) via persulfate (PS) activated with zero-valent zinc (Zn(0)). The effects of various parameters such as the dosages of SG, PS, Zn(0), and the initial pH on PNP degradation via Zn(0)/SG/PS were investigated through batch experiments. The results demonstrated that SG exhibited a strong synergistic role in the decomposition of PNP via Zn(0)-activated PS due to the dispersion and buffering character of SG. The degradation efficiency of PNP was strongly related to the dosage of SG, PS, or Zn(0). A low pH favored of the degradation of PNP. Nevertheless, PNP could be efficiently removed in a pH range of 3 to 9. The radicals of SO4•− and •OH were verified to be responsible for the oxidative degradation of PNP. Nevertheless, the contribution of SO4•− to the degradation of PNP was much larger than that of •OH. Based on the analyses of HPLC-MS data, it could be determined that C6H7NO, C6H6O2, and C12H7N3O8 were the main intermediates generated from the degradation of PNP via Zn(0)/SG/PS. Furthermore, the possible mechanism and the degradation pathways of PNP via Zn(0)/SG/PS were proposed. PS activated with Zn(0) in the presence of SG could be concluded as a promising approach for the degradation of PNP in aqueous solution.

Debromination of polybrominated diphenyl ethers (PBDEs) by zero valent zinc: Mechanisms and predicting descriptors

Polybrominated diphenyl ethers (PBDEs) are a class of brominated flame retardants that are ubiquitous in the environment. The physical and chemical properties of PBDEs make them difficult to degrade, with the conventional remediation methods being relatively inefficient. In this study, the reactivity of zero valent zinc (ZVZ) toward 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47) was evaluated under aqueous solution. First-order rate constants (kobs) for BDE-47 disappearance increased with decreased pH, which is attributed to the dissolution of surface zinc oxides that promote the contact between the active site on zinc surface and BDE molecules. The kobs of ten investigated PBDEs in ZVZ system are positively correlated with the energy of lowest unoccupied orbitals (ELUMO) of PBDEs (R2 = 0.902). The debromination pathways of BDE-47 in ZVZ system are: BDE-47 → BDE-28 → BDE-15 → BDE-3 → DE, which is the same to the debromination pathways of BDE-47 in zero valent iron (ZVI) in previous study. In addition, the singly occupied molecular orbitals (SOMOs) of the BDE anions can well reflect the actual debromination pathways of PBDEs by comparing the size of the CBr antibonding characterized lobes. Our results suggest that the debromination of PBDEs by ZVZ is based on the electron transfer mechanism, and the SOMOs of BDE anions can be used to predict the debromination pathways of untested PBDEs.

Modeling of Malachite Green Removal from Aqueous Solutions by Nanoscale Zerovalent Zinc Using Artificial Neural Network

The commercially available nanoscale zerovalent zinc (nZVZ) was used as an adsorbent for the removal of malachite green (MG) from aqueous solutions. This material was characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The advanced experimental design tools were adopted to study the effect of process parameters (viz. initial pH, temperature, contact time and initial concentration) and to reduce number of trials and cost. Response surface methodology and rapidly developing artificial intelligence technologies, i.e., artificial neural network coupled with particle swarm optimization (ANN-PSO) and artificial neural network coupled with genetic algorithm (ANN-GA) were employed for predicting the optimum process variables and obtaining the maximum removal efficiency of MG. The results showed that the removal efficiency predicted by ANN-GA (94.12%) was compatible with the experimental value (90.72%). Furthermore, the Langmuir isotherm was found to be the best model to describe the adsorption of MG onto nZVZ, while the maximum adsorption capacity was calculated to be 1000.00 mg/g. The kinetics for adsorption of MG onto nZVZ was found to follow the pseudo-second-order kinetic model. Thermodynamic parameters (ΔG0, ΔH0 and ΔS0) were calculated from the Van’t Hoff plot of lnKc vs. 1/T in order to discuss the removal mechanism of MG.

Alternate Reductants with VB12 to Transform C8 and C6 Perfluoroalkyl Sulfonates: Limitations and Insights into Isomer-Specific Transformation Rates, Products and Pathways.

Previous studies evaluating Vitamin B12 (VB12) with Ti(III)-citrate for potential use in in situ remediation of perfluorooctanesulfonate (PFOS) found that linear (L)-PFOS was unaltered. We explored if alternate reductants could overcome this limitation with a primary focus on nanoscale zerovalent zinc (nZn0). Transformation over time with VB12-nZn0 was quantified at 22, 70, and 90 °C for PFOS, at 70 °C for perfluorohexanesulfonate (PFHxS), and VB12-nFe0 and VB12-Pd0/nFe0 at 70 °C for PFOS. Only branched (br-) isomers were transformed generating F- (no SO42-) and polyfluoroalkyl intermediates/products. The absence of L-PFOS transformation by VB12 appears to be due to the inability of L-perfluoroalkyl sulfonates to complex with VB12 and not an activation energy issue that can be overcome by stronger reductants/catalysts. At 90 °C, 95% of br-PFOS isomers were transformed within 5 days. Isomer-specific removal rates were positively correlated to the br-CF3's proximity to the terminal CF3. Br-PFHxS transformation was approximately two times slower with less defluorination than br-PFOS. C8/C7 and C6/C5 polyfluorinated sulfonates from br-PFOS and br-PFHxS, respectively, were identified as both intermediates and apparent dead-end products. Pathways included 4 F replaced by 2 H and a C═C bond, and serial F replacement by H with up to 12 F atoms removed from br-PFOS.

Study of chlordecone desorption from activated carbons and subsequent dechlorination by reduced cobalamin.

Since 1972, the French departments of Guadeloupe and Martinique have intensively used organochlorinated pesticides such as chlordecone (CLD) and hexachlorocyclohexane (HCH) isomers to prevent the proliferation of banana weevil (Cosmopolite sordidus). These molecules are stable in the environment, leading to a continuous contamination of soils, water, and food chain in the banana-producing areas. In these polluted areas, water treatment plants are equipped with activated carbon (AC) filters. In order to improve treatment of CLD-contaminated waters by AC, CLD adsorption and desorption kinetic studies are carried out using different ACs produced from sugar cane bagasse as adsorbents and subsequent CLD degradation is performed using reduced vitamin B12 (VB12). A GC-MS method for CLD quantification is as well optimized. This study shows that bagasse ACs are able to capture the pollutant, leading to a CLD concentration decrease from 1 to 73μgL-1, with an adsorption capacity of 162μgmg-1. Adsorption capacity increase with the temperature indicates an endothermic process. Polar solvents favor CLD desorption from ACs, suggesting hydrogen bonding between CLD and surface groups of ACs, the best solvent for chemical desorption being ethanol. Subsequent degradation of CLD in ethanol is performed using vitamin B12 reduced by either 1,4-dithiotreitol (DTT) or zerovalent zinc, leading to 90% of CLD removal and to the molecule cage structure opening for formation of a pentachloroindene intermediate product, characterized by GC MS/MS. A pathway for pentachloroindene formation from CLD is proposed.

Zinc-Zinc Double Bonds: A Theoretical Study.

While double bonds are known for transition metals of Groups 9 and 10 as well as for boron and p-block elements of Groups 14-16, Zn sits in a small region of the periodic table with no well-characterized double bonds. A qualitative reasoning indicates that zero-valent zinc has the potential to form Zn=Zn double bonds. A computational study in search for complexes that might showcase this new bond type is presented here.

Degradation of nitrobenzene by sodium persulfate activated with zero-valent zinc in the presence of low frequency ultrasound

In the presence of low frequency ultrasound (US) generated from an ultrasonic cleaning bath, the degradation efficiency of nitrobenzene (C6H5NO2, NB) by Zn0-activated persulfate (PS) reached 96% within 2h, which is much higher than that by PS or Zn0 alone in the presence of US or that by Zn0-activated PS in the absence of US. The synergistic effect in the US/Zn0/PS system and the optimum degradation conditions was investigated by batch experiments. The variations of pH and concentrations of Zn2+ and SO42− over time and the removal of chemical oxygen demand (COD) and total organic carbon (TOC) in different systems were analyzed, which agreed well with the degradation pattern of NB in different systems. The formation of free radicals was measured using electron paramagnetic resonance (EPR) spectroscopy to further investigate the possible mechanism of the enhanced degradation of NB by Zn0-activated PS in the presence of low frequency US.

Modeling and Optimization of Ciprofloxacin Degradation by Hybridized Potassium Persulfate / Zero Valent-Zinc/Ultrasonic Process

The presence of pharmaceutical wastewaters containing antibiotic compounds is one of the new problems that need to be addressed in environmental pollution. This study examines ciprofloxacin removal from aqueous solutions through persulfate activated with hybridized ultrasonic waves and synthesized nano zero-valent zinc (NZVZ) particles. NZVZ particles were synthetized by chemical precipitation and stabilized by starch as an eco-friendly stabilizer before application. Modeling and optimization of the process were performed by central composite design (CCD) as a response surface methodology (RSM). The effect of operational parameters, such as initial pH, NZVZ dosage, and persulfate concentration on the removal of the antibiotic have been investigated. The results indicated 55% antibiotic removal observed with initial pH of 4.5, Potassium persulfate (KPS) concentration of 1200 mg/L and NZVZ dosage of 120 mg/L.

Efficient reductive elimination of bromate in water using zero-valent zinc prepared by acid-washing treatments

Although zero valent zinc (ZVZ) is a strong reductant, studies using ZVZ for bromate reduction are rare. In this study, ZVZ is prepared by acid-washing zinc powder with HCl and used to reduce bromate. The effect of acid-washing on the morphology of zinc powder is also examined. Zinc powder inefficiently reduces bromate, but ZVZ obtained by acid-washing zinc powder eliminates bromate and converts it to bromide. A higher dose of ZVZ enhances elimination efficiency perhaps because the formation of a passivation layer of zinc oxide could be scattered on the large surface of ZVZ. Elevated temperature also substantially improves both elimination efficiency and kinetics. The effect of pH is shown to have the most significant impact on the bromate elimination; elimination efficiency and kinetics are tremendously bolstered at pH = 3, whereas the elimination of bromate is completely suppressed under alkaline conditions. ZVZ can reduce bromate to bromide even in the presence of other anions and also be reused multiple times. Thus, ZVZ can be easily prepared and used to efficiently reduce bromate to bromide. The findings presented here are essential to the design and implementation of bromate elimination in water using zero-valent metals.

Enhanced degradation of nitrobenzene by combined ultrasonic irradiation and a zero-valent zinc catalyst

Enhanced ultrasonic catalytic activity in nitrobenzene degradation was found by combining ultrasonic irradiation and a zero-valent zinc (Zn0) catalyst. In this system, the influence of the reaction parameters, reaction kinetics, intermediates, and mechanism was investigated. Under the optimum conditions (initial nitrobenzene concentration of 0.01 mmol/L, Zn0 dosage of 0.4 g/L, initial pH 7, ultrasonic intensity 4.0 W/cm2), approximately 91.2% of nitrobenzene could be removed within 30 min. The degradation process followed the pseudo-first-order kinetics model, and the reaction rate constant was 0.0670 min−1. The mechanism of the synergetic effect of ultrasound and Zn0 was proposed. Three intermediates of nitrophenol, phenylhydroxylamine, and aniline were identified by liquid chromatography–mass spectrometry (LC-MS). The degradation pathways of hydroxyl radical oxidation and Zn0 reduction were deduced. This combined system is promising for the removal of nitrobenzene pollutants from water.

Rapid degradation of aniline in aqueous solution by ozone in the presence of zero-valent zinc

The effects of Zn(0) dosage from 0.1 to 1.3gL−1, pH from 2 to 12 and temperature from 288 to 318K on the degradation of aniline in aqueous solution by ozone in the presence of Zn(0) were investigated through batch experiments. The results demonstrated that Zn(0) had a significantly synergistic role in the degradation of aniline by ozone. A complete decomposition of the initial aniline (10mgL−1) was achieved by ozone together with Zn(0) within 25min, and meanwhile nearly 70% of the total organic carbon in the solution was removed. The decomposition efficiency of aniline markedly increased with an increase of Zn(0) dosage. However, temperature exerted a slight impact on the degradation of aniline and the optimum removal efficiency of aniline was realized at 298K. Aniline was efficiently degraded at all the tested pHs except for 12. Free radicals were investigated by electron paramagnetic resonance technique and free radical scavengers. H2O2 concentration generated during the reactions was analyzed using a photometric method. Based on the results obtained in this study, it is proposed that O2− instead of OH is the dominant active species responsible for the degradation of aniline. It is concluded that ozone combined with Zn(0) is an effective and promising approach to the degradation of organic pollutants.

Rapid removal of p-chloronitrobenzene from aqueous solution by a combination of ozone with zero-valent zinc

The degradation of p-chloronitrobenzene (pCNB) in different systems including ozone alone, ozone together with zero-valent zinc (Zn(0)) and Zn(0) bubbled with air instead of ozone was investigated. The results demonstrated that the degradation of pCNB by ozone alone was weak. However, the coexistence of Zn(0) and ozone exhibited a significantly synergistic role in pCNB degradation and approximately 98% of the initial pCNB (10mg/L) was removed within 18min. The degradation efficiency of pCNB was related to Zn(0) dosage and ozone flow amount, and the optimum conditions were 0.5g/L and 4.8mg/min, respectively. pCNB could be effectively degraded in the initial pH range of 4–10, but a higher or a lower pH was not beneficial to the removal of pCNB. Accompanying anions also affected pCNB degradation. Chloride and sulfate slightly improved pCNB degradation, whereas bicarbonate and phosphate markedly suppressed the degradation of pCNB. The reuse of Zn(0) (4 cycles) for pCNB ozonation indicated that the catalytic activity of Zn(0) maintained high and stable. The experiments of free radical scavengers (tert-butyl alcohol and p-benzoquinone) corroborated the dominant active species responsible for pCNB degradation by a combination of ozone with Zn(0) was O2− rather than OH. Based on the results obtained in this study, it can be concluded that ozone activated by Zn(0) is an effective and promising approach to degrade organic contaminants.

Rapid degradation of endosulfan by zero-valent zinc in water and soil

Endosulfan has been included in the list of persistent organic pollutants (POPs) in 2011. The degradation of endosulfan by zero-valent zinc in water and soil was first investigated. The results showed that >90% endosulfan could be degraded in 180 min. The degradation was accelerated under acidic conditions with the absence of dissolved oxygen, while the nature of the soil only exhibited a negligible effect. The half-life was decreased from 130.75 min to 41.75 min with the increment of Zn(0) from 0.1 g to 1 g in soil. The use of Zn(0) was more effective than Fe(0) for the degradation of endosulfan with a half-life of 110 min and 330 min. The cationic surfactant was more effective at enhancing the degradation of endosulfan than anionic and nonionic surfactant. The degradation pathway was speculated, and four chlorine of endosulfan were proposed to be reduced. The method exhibited obvious advantages over traditional endosulfan treatments, and the research results will lay a foundation for practical application of the method.

Modeling Perchloroethylene Reduction in Zero-Valent Metal System Using Mathematical Method

The dechlorination of perchloroethylene (PCE) in aqueous solution is significant for the decontamination of water. Previous study based on batch tests was conducted on the degradation of PCE using zero-valent metal. The degradation reaction fit well with pseudo-first-order kinetic model. In this study a hypothesized reaction sequence for reduction of chlorinated ethylene and related compounds was presented to describe the reticulation degradation pathway of PCE. Mathematical model was developed to predict the subsequent degradation of PCE and intermediates; results showed that 170 days were needed to transform all of PCE to non-chlorinated hydrocarbons in degradation reactors contained zero-valent iron, while 240 days were needed for zero-valent zinc. The variation of each species probably occurred in degradation process were also predicted, and the intermediates showed the same variation tendency of increasing in the first stage, reaching to peak value, and then decreasing in later period.

Efficient Degradation of Congo Red by Sodium Persulfate Activated with Zero-Valent Zinc

Zn0-activated persulfate as a novel and potential approach to the degradation of azo dyes has hardly been reported. In this study, the effects of initial pH, persulfate concentration, Zn0 dosage, and temperature on the decomposition of Congo red (CR), an azo dye, were investigated. The results demonstrated that Zn0-activated persulfate could effectively mineralize CR. At the initial pH 5.5 and 25 °C, chemical oxygen demand (COD) and total organic carbon (TOC) in the solution with 95 mg/L CR decreased by approximately 87 and 60 %, respectively, within 3 h. The optimum dosages of persulfate and Zn0 were approximately 95 mg/L and 2 g/L, respectively. The highest decolorization efficiency of CR was realized at the initial pH 5.5. Both ·OH and SO4−· contributed to the degradation of CR, and the spectra of free radicals showed that SO4−· was gradually converted to ·OH with pH increasing from weak acidic to neutral condition.

Degradation of methyl orange by sodium persulfate activated with zero-valent zinc

Zn0-activated persulfate (PS) is a novel advanced oxidation technology for the degradation of organic pollutants in aqueous solution. The effects of the initial solution pH, the dosages of PS and Zn0, and the temperature were investigated through a series of batch experiments using methyl orange (MO), an azo dye, as a model organic pollutant. The results demonstrated that MO could be effectively degraded by Zn0-activated PS. The chemical oxygen demand and the total organic carbon decreased by approximately 85% and 58%, respectively, in the solution containing 98mg/L MO at the initial pH 5 and 25°C within 3h. The optimum dosages of PS and Zn0 were 71mg/L and 1.3g/L, respectively. The highest removal of MO was realized at an initial pH 5. Tertiary butyl alcohol, an OH-specific radical scavenger, and l-histidine, a universal radical scavenger, corroborated that both OH and SO4− contributed to MO degradation. Three stages were observed during the degradation of MO at 15 and 25°C, a rapid removal of MO in the initial stage of the reaction, followed by a very slow one and then a relatively quick degradation process.

Synthesis and application of alumina supported nano zero valent zinc as adsorbent for the removal of arsenic and nitrate

Arsenic and nitrate are ill-famed environmental pollutants that are responsible for various lethal diseases. Their removal from drinking water is very essential. In present study, newly synthesized alumina supported nano zerovalent zinc (Alumina-nZvZ) has been tested to remove arsenic and nitrate. Quantitative analyses of arsenic have been performed spectrophotometrically and while that of nitrates ions colorimetrically. After optimization of time and amount of adsorbent, Langmuir, Freundlich and D-R isotherms were applied to determine different parameters for the assessment of adsorption. Synthesized samples were characterized by scanning electron microscopy (SEM) to evaluate porosity and void size. Alumina coated with reduced ZnCl 2 showed better efficiency for removal of arsenic and nitrate ions. Kinetics of adsorption was evaluated by using pseudo first-order and pseudo second-order rate equations.