Debromination Mechanism Research Articles

Catalytic Pyrolysis of Waste-Printed Circuit Boards Using a Cu/Fe Bimetal Synergistic Effect to Enhance Debromination

Deep and efficient debromination is a critical step in achieving environmentally friendly recycling and ensuring the sustainability of waste-printed circuit boards (WPCBs) because of their high toxicity and carcinogenicity. To this end, this study used a copper–iron (Cu/Fe) bimetal as a debromination agent to remove bromides from WPCBs using in situ catalytic pyrolysis technology. The results show that the maximum debromination efficiency was 97.14% under the following conditions: a Cu mole ratio of 0.20 (Cu/Fe-0.20), a Cu/Fe-0.20 dosage of 0.4, a pyrolysis temperature of 600 °C, and a retention time of 10 min. The main bromine species in pyrolysis oil and gas were bromophenol, bromomethane, HBr, and Br2. The conversion of bromine species and the debromination of the Cu/Fe-0.20 bimetal were analyzed in real time using a thermogravimetry-coupled Fourier transform infrared and mass spectrometer (TG-FTIR-MS). Using the Cu/Fe bimetal synergistic effect, we determined that the debromination mechanism could be used for bromide conversion and fixing. The Cu in the Cu/Fe-0.20 transformed the organic Br (bromophenol and bromomethane) into inorganic Br (HBr and Br2) by providing empty orbitals for lone pairs of electrons. Then, the generated HBr and Br2 reacted with Fe in the Cu/Fe-0.20 and were fixed in pyrolysis residue. This study provides theoretical support and a practical method for WPCB deep debromination and recycling.

Green, solventless debromination of C70Br10 to C70 with CsI

A smooth, solventless and solid state debromination of C70Br10 can be achieved in a CsI matrix with the restoration of the pristine C70 in quantitative yields. Such debromination reaction was studied with FT-IR spectroscopy and its kinetics was fitted by a pseudofirst order law, with a rate constant in the range of 3.6-5.2 x 10−5 s−1. The thermal properties of the compound C70Br10 were studied with TGA and DSC analysis focusing on the thermal decomposition processes. From the DSC data both the activation energy for the thermal decomposition of C70Br10 as well as its enthalpy of formation were determined. Some considerations on the relative weakness of C-Br bonds in brominated fullerenes compounds were made. Accordingly, the debromination mechanism is discussed in terms of unimolecular elimination reaction (E1) in a concerted mechanism which leads to the restoration of the pristine C70 fullerene.

Green, solventless debromination of C70Br10 To C70 with CsI

A smooth, solventless and solid state debromination of C70Br10 can be achieved in a CsI matrix with quantitative restoration of the pristine C70 in quantitative yields. The debromination kinetics was studied with FT-IR spectroscopy and according to the pseudofirst order kinetics law, the rate constant was found in the range 3.6–5.2 × 10−5 s−1. The compound C70Br10 was studied with TGA and its thermal decomposition by DSC analysis. From the DSC data both the activation energy for the thermal decomposition of C70Br10 was determined as well as its enthalpy of formation, which has led to some considerations on the relative weakness of C–Br bonds in brominated fullerenes compounds. The debromination mechanism is discussed in terms of unimolecular elimination reaction (E1) in a concerted mechanism which leads to the restoration of the pristine C70 fullerene.

Quantum chemical study on the catalytic debromination mechanism of brominated epoxy resins

The study employed Density Functional Theory (DFT) to investigate the catalytic debromination mechanism of brominated epoxy resins (BERs) by iron (Fe) and copper (Cu) catalysts. By introducing electric field (EF), intramolecular electron transfer and polarization effects on BERs debromination were explored and experimentally validated. Results indicated that the bond dissociation energy (BDE) of the C-Br bond was 312.27 kJ/mol without catalysis, while with Fe, Cu, and EF, it was 114.47 kJ/mol, 94.85 kJ/mol, and 292.59 kJ/mol, respectively, enhancing reactivity. EF parallel to the C-Br bond and oriented toward the C atom, altered electrostatic potential and dipole moment around C-Br bond, leading to 68.60% and 50.19% increment in electronic contribution difference and molecule polarity, respectively, thereby reducing the C-Br BDE. Fe and Cu facilitated electron transfers with BERs, inducing reactions between their negative electrostatic potentials and Br's positive potential, changing electron sharing, resulting in 19.87% and 12.11% increase in polarity, respectively, and further BDE reduction. Structural modifications by the EF and catalysts also intensified van der Waals forces with bromine atoms and decreased spatial hindrance, collectively making C-Br bond breakage easier. Experiments revealed the EF enhanced BERs' debromination efficiency but hindered Fe/Cu's catalysis at lower temperatures.

Solid state debromination of C60Br24 back to C60 with cesium iodide: kinetics, thermodynamics and mechanism

The bromofullerene C60Br24 undergoes a complete and quantitative debromination to C60 when it is mixed and ground in the solid state with cesium iodide (CsI). The kinetics of this unique solid state debromination reaction was studied with FT-IR spectroscopy on C60Br24 embedded in CsI pellet. The debromination rate constant was measured k = 8.4 x 10−4 s−1 and found independent from the C60Br24 concentration. Chemical thermodynamics calculations show that the C60Br24 debromination in CsI matrix is characterized by a largely favorable free energy of reaction (ΔGr) and the reaction is exothermal. A debromination mechanism is discussed in terms of concerted elimination of the bromide anions and formation of carbocations on the fullerene cage in agreement with the E1 type elimination reaction mechanism. The carbocations sites are reduced to radicals and stabilized as trivinylmethyl radicals (the reduction occurs by the action of iodide ions of CsI which are oxidized to molecular iodine) and then a rearrangement of the double bonds leads back to C60 quantitatively.

Investigating the debrominations of a subset of brominated flame retardants by biogenic reactive sulfur species

Brominated flame retardants (BFRs) are persistent organic pollutants. Many bacteria are able to debrominate BFRs, but the underlying mechanism is unclear. Herein, we discovered that reactive sulfur species (RSS), which have strong reductive activity and are commonly present in bacteria, might be one of the reasons leading to such ability. Experiments performed with RSS (H2S and HSSH) and BFRs indicated that RSS can debrominate BFRs via two different mechanisms simultaneously: the substitutive debromination that generates thiol-BFRs and the reductive debromination that generates hydrogenated BFRs. Debromination reactions rapidly happened under neutral pH and ambient temperature, and the debromination degree was around 30% − 55% in one hour. Two Pseudomonas strains, Pseudomonas sp. C27 and Pseudomonas putida B6-2 both produced extracellular RSS and showed debromination activity. C27 debrominated HBCD, TBECH, and TBP by 5.4%, 17.7%, and 15.9% in two days. Whereas, B6-2 debrominated the three BFRs by 0.4%, 0.6%, and 0.3% in two days. The two bacteria produced different amounts and species of RSS, which were likely responsible for the contrasted degrees of the debromination. Our finding unveiled a novel, non-enzymatic debromination mechanism that many bacteria may possess. RSS producing bacteria have potentials to contribute to bioremediation of BFRs-polluted environments.

Highly efficient debromination of 4,4'-dibrominated diphenyl ether by organic palygorskite-supported Pd/Fe nanoparticles.

Organic palygorskite (OP)-supported Pd/Fe nanoparticles composite (OP-Pd/Fe) was prepared by stepwise reduction method. The removal capacity of 4,4'-dibrominated diphenyl ether (BDE15) by OP-Pd/Fe was compared with other various materials. For better understanding the possible mechanism, the synthesized and reacted OP-Pd/Fe materials were characterized by TEM, SEM, XRD, and XPS, respectively. The effects of major influencing parameters on the degradation of BDE15 were also studied. Benefit from the synergistic effect of the carrier and bimetallic nanoparticles, BDE15 could be completely debrominated into diphenyl ether (DE) under suitable conditions. A two-stage adsorption/debromination removal mechanism was proposed. The degradation of BDE15 with OP-Pd/Fe was mainly stepwise debromination reaction, and hydrogen transfer mode was assumed as the dominated debromination mechanism. The removal process fitted well to the pseudo first-order kinetic equation. The observed rate constants increased with increasing Pd loading and OP-Pd/Fe dosage while decreased with increasing initial BDE15 concentration, the tetrahydrofuran/water ratio, and the initial pH of the solution. The work provides a new approach for the treatment of PBDEs pollution.

New insights into the debromination mechanism of non-metallic fractions of waste printed circuit boards via alkaline-enhanced subcritical water route

The debromination of brominated epoxy resins (BERs) from waste printed circuit boards (WPCBs) is of great challenge due to its high structural stability. An enhanced subcritical water (SCW) process with alkaline additive is proposed for efficient debromination of BERs, and the mechanism of alkaline additive on bromine removal has been investigated. More than 95.11% of bromine is removed under the optimal conditions. Both enhanced fiberglass destruction and BERs degradation contribute to the efficient debromination process. Structure of Ca−Al, Si−O, and Al−O bonds in fiberglass is destroyed by alkaline corrosion, releasing BERs into the solution and facilitating the debromination process. Meanwhile, phenol, 2-bromophenol and catechol are the main products of BERs degradation. Nucleophilic substitution reaction by the bimolecular process dominantly contribute to the debromination. The energy barrier of the bimolecular process is only 15.9 kcal/mol, which is much lower than that of the direct dissociation energy of C−Br bond (168.3 kcal/mol). Ion association of KOH is weaker than that of NaOH in the alkaline-enhanced SCW process due to the difference of cation radius, proved by ab initio molecular dynamics simulation. An efficient approach for WPCBs debromination under mild conditions has been proposed.

In-situ debromination mechanism based on self-activation and catalysis of Ca(OH)2 during pyrolysis of waste printed circuit boards

Completely and deeply removed bromide from waste printed circuit boards (WPCBs) is necessary due to their toxicity and carcinogenicity. To achieve this purpose, calcium hydroxide (Ca(OH)2) as a debromination agent was added during pyrolysis process of WPCBs. The results showed that hydrogen bromide (HBr), 4-bromophenol, 2-bromophenol and 2,4-dibromophenol were the main bromide species in pyrolysis products. The Ca(OH)2 plays a significant role for removing HBr and organic bromide, but not affects products yield. Optimal removal efficiency for 4-bromophenol, 2-bromophenol and 2,4-dibromophenol reached 87.5 %, 74.6 % and 54.5 %, respectively. And debromination efficiency was related to the steric hindrance caused by bromide atoms. The Ca(OH)2 can be activated by captured HBr and its thermal decomposition. And the newly-generated calcium bromide and calcium oxide significantly facilitate debromination due to their high surface energy and reactivity. The debromination mechanism was clarified by experiments coupled with computational chemistry: the coordination of bromide and calcium to form [Ph-Br···Ca2+] or [Ph-Br···Caatom]. Then, electrons were delivered form bromide atom to Ca2+ or Caatom, which resulted in the stretch and weaken the C-Br bond. Hence, the C-Br bond was more easily to break. This work can provide support for designing novel and efficient debromination agents applied for high-temperature system.

Rapid Evaluation of the Debromination Mechanism of Eosin in Oil Paint by Direct Analysis in Real Time and Direct Infusion-Electrospray Ionization Mass Spectrometry.

Eosin is a synthetic organic colorant prone to fading under the influence of light. On the basis of the growing interest in the understanding of the discoloration mechanism of eosin-based lakes, this study compares the ability of two ultrafast and ultrasensitive mass spectrometry techniques to detect eosin derivatives in complex matrices, such as oil media without the use of conventional separation columns or additional sample preparation protocols. Direct analysis in real time mass spectrometry (DART-MS) and direct infusion electrospray ionization mass spectrometry (DI-ESI-MS) were used to characterize the degradation pathway of eosin in oil media. The analysis protocols developed in this study are applied to discern the degradation mechanism of the lake pigment eosin (comprising the molecule per se complexed to an inorganic substrate) dispersed in linseed oil to create an oil paint. The analysis of oil paints by high resolution MS without an extraction methodology that modifies the system chemistry allowed us to identify the degradation forms without causing any additional fragmentation. Both techniques revealed the primary photodegradation pathway of eosin in linseed oil, and DI-ESI-MS provided additional information on the native conformation of the lake.

Improved photoelectrocatalytic degradation of tetrabromobisphenol A with silver and reduced graphene oxide-modified TiO2 nanotube arrays under simulated sunlight

In present study, reductive graphene oxide and silver nanoparticles co-comodified TiO2 nanotube arrays were prepared, and which was investigated to degrade tetrabromobisphenol A. The arrays co-modified with silver nanoparticles and reductive graphene oxide prepared by electrodeposition method exhibited good photoelectrocatalytic degradative activity for tetrabromobisphenol A, and the degradation efficiency reached 99.6% within 80 min. The synergistic effect of high photoresponse of Ag nanoparticles with their high capture ability for photogenerated electrons and the extended wavelength absorption range of reductive graphene oxide resulted in the highest degradation efficiencies. Degradation is postulated to follow a stepwise reductive debromination mechanism.

Heterogeneous Sono-Fenton treatment of decabromodiphenyl ether (BDE-209): Debromination mechanism and transformation pathways

Polybrominated diphenyl ethers, used as flame retardants are a cause of concern over past several years, due to their toxicity, bioaccumulation and widespread distribution. In this work, the combination of ultrasound and advanced Fenton process was investigated for the degradation of Decabromodiphenyl ether (BDE-209). The effect of pollutant concentration, solution pH, ultrasonic power density and loading of Fenton’s reagent on the extent of BDE-209 degradation were studied. The results indicated an enhancement in the degradation of BDE-209 during Sono-Fenton treatment, leading to a complete degradation within 80 min of treatment, whereas only 40% and 25% degradation were achieved during ultrasound and Fenton treatment alone. To understand the degradation mechanism, analytical techniques including High performance liquid chromatography and Gas chromatography mass spectroscopy were employed to monitor the concentrations of parent compound along with degradation products. The degradation mechanism was proposed based on the identified degradation products. Total organic carbon (TOC) analysis demonstrated 60% TOC removal within 80 min of treatment. The obtained results suggest that the initiation of BDE-209 degradation during Sono-Fenton treatment followed thermal decomposition at bubble-vapor interface along with reductive debromination at iron surface, accompanied by oxidative degradation of lower brominated congeners at liquid bulk. This work illustrated an effective method for the removal of BDE-209 and hence providing an efficient wastewater treatment solution for recalcitrant pollutants.

Enhanced debromination of decabrominated diphenyl ether in aqueous solution by attapulgite supported Fe/Ni bimetallic nanoparticles: kinetics and pathways

In this study, Fe/Ni bimetallic nanoparticles were supported on the attapulgite (A-Fe/Ni) to enhance the degradation reactivity of decabrominated diphenyl ether (BDE209) in aqueous solution. The Fe/Ni nanoparticles were well distributed on the attapulgite surface with an average diameter of 20–40 nm. The removal percentage of BDE209 by A-Fe/Ni was 1.59 times higher than Fe/Ni nanoparticles alone because attapulgite could act as supporting material to disperse Fe/Ni nanoparticles and prevent Fe/Ni nanoparticles from aggregation. The degradation kinetics for BDE209 debromination by A-Fe/Ni could be well described by a pseudo-first-order model, and the debromination rate constant of BDE209 increased with increasing the dosage of A-Fe/Ni, water/THF ratio, and decreasing the initial BDE209 concentration and solution pH. The degradation products were identified using a third-order polynomial regression equation between the experimental and reference gas chromatography relative retention times. Stepwise debromination from n-bromo-DE to (n − 1)-bromo-DE was a possible pathway with bromines being substituted sequentially by hydrogen. The preferred elimination of bromines of BDE209 by A-Fe/Ni followed the debromination preference of para-Br > meta-Br > ortho-Br. The results provide evidences for understanding the debromination mechanism of polybrominated diphenyl ether by clay-supported Fe/Ni nanoparticles.

Rapid photocatalytic debromination on TiO2 with in-situ formed copper co-catalyst: Enhanced adsorption and visible light activity

The cheap metal Cu particles in-situ loaded on TiO2 surface is able to co-catalyze the photocatalytic debromination of polybrominated pollutants. Over 15 times of enhancement for the reductive debromination of deca-brominated diphenyl ether (BDE209) over Cu/TiO2 is achieved, relative to that on the bare TiO2. Moreover, the presence of metal copper markedly changes the distribution of the formed intermediates, indicating the change in the debromination mechanism because of the co-catalysis of metal Cu. The detailed ex-situ and in-situ characterizations of the Cu/TiO2 photocatalyst indicate that the copper remains its Cu0 valence state in photocatalytic turnover. The mechanistic investigation suggests that the presence of metal copper nanoparticles greatly enhance the adsorption of hydrophobic BDE209 on the photocatalyst, which facilitates the electron transfer from the conduction band of TiO2 to the pollutants. Our experimental results also implies that the interaction of BDE209 with metal copper co-catalyst would position-specifically activate the C-Br bonds during the dissociative electron transfer. We further show that the surface plasmon resonance of metal copper can contribute to the enhancement of photocatalytic activity by utilizing the visible-light part of the irradiation. The present study provides a simple and economic method for elimination of organobromide pollutants, and would be promising in reductive dehalogenation by other techniques.

Debromination of 1,2,4,5-Tetrabrominated Benzene in Alcohol Solution with Nanoscale Pd/C Particles: Factors, Pathways, and Mechanism

In this study, the catalytic debromination of 1,2,4,5-tetrabrominated benzene (1245TBB) was performed in a solution of NaOH in alcohol (Visopropanol:Vmethanol = 99:1) in the presence of a nanoscale catalyst Pd/C at a temperature of 50°C. The final degradation product of 1,2,4,5-tetrabrominated benzene is benzene. The debromination efficiency of 1245TBB and its degraded intermediate products (1,2,4-tribromobenzene, 1,2,-dibromobenzene, and bromobenzene) were highly dependent on the Pd/C loading and NaOH concentration. The catalyst life was evaluated, which showed that the deactivated catalyst could be reactivated about 30% after washing with 100 mmol L-1 NaOH solution in isopropanol. The possible 1245TBB debromination mechanism and pathways are also proposed and discussed in this study.

Photodecomposition of bromophenols

Photodecomposition of bromophenols (BPs) represents a potent channel of debromination and elimination of these species in the environment. From this perspective, the present contribution reports geometrical parameters, electronic absorption spectra and excited states of the complete series of BPs in their ground state (S0), as well as their first singlet exited state (S1). We calculate excitation energies for S0 → S1 transition within the framework of the time-dependent density functional theory (TDDFT). We estimate and discuss charges on bromine atoms and HOMO-LUMO energy gaps (EH−L) as molecular descriptors for the photoreactivity of BPs and photo-induced debromination mechanism of BPs. Spectral patterns reveal that, as the degree of bromination increases, peaks of absorption spectra red-shift toward wavelengths near 300 nm, for the pentabrominated phenol. Based on the analysis of optimised geometries and Hirshfeld's atomic charges, photodebromination of BPs commences via the loss of an ortho Br atom. The excitation energies decrease linearly with increasing number of bromine atoms. This indicates that, higher brominated congeners of BPs photodecompose more readily than lower brominated congeners.

Which Oxidation State Initiates Dehalogenation in the B12-Dependent Enzyme NpRdhA: CoII, CoI, or Co0?

The quantum chemical cluster approach was used to elucidate the reaction mechanism of debromination catalyzed by the B12-dependent reductive dehalogenase NpRdhA. Various pathways, involving different oxidation states of the cobalt ion and different protonation states of the model, have been analyzed in order to find the most favorable one. We find that the reductive C–Br cleavage takes place exclusively at the CoI state via a heterolytic pathway in the singlet state. Importantly, the C–H bond formation and the C–Br bond cleavage proceeds via a concerted transition state, as opposed to the stepwise pathway suggested before. C–Br cleavage at the CoII state has a very high barrier, and the reduction of CoI to Co0 is associated with a very negative potential; thus, reductive dehalogenation at CoII and Co0 can be safely ruled out. Examination of substrates with different halogen substitutions (F, Cl, Br, I) shows that the dehalogenation reactivity follows the order C–I > C–Br > C–Cl > C–F, and the barrier for ...

Theoretical Investigation on Debromination Model of Deca- BromoDiphenyl Ester (BDE209)

The debromination of deca-bromodiphenyl ester (BDE209) is one of the most important processes in its removal from the environment. Despite numerous experimental studies improved the efficiency of poly brominated diphenyl ethers (PBDEs’) debromination, few studies focused on the mechanism. This work provides a preliminary investigation on studying the debromination mechanism of BDE209 with density functional theory (DFT). Experimental results suggest a stepwise process catalyzed by Ni⁄Fe bimetallic nano particles. So, a potential debromination mechanism of BDE209 via a four-membered ring transition state has been proposed. The B3LYP calculations indicate lower activation energies catalyzed by Ni relative to Fe, which agrees with the experimental result that a much higher debromination efficiency with Ni⁄Fe bimetallic nano particles than nano zero-valent iron. Also water has small solvent effect on the activation energies of debromination. The efficiency of debromination improves greatly in solvent mixture with higher ratio of water in experiment probably because of more protons provided by more water for the corrosion and debromination. Finally, the meta-bromine atoms are the most susceptible to be debrominated. Keywords: Deca-bromodiphenyl ester (BDE209), Debromination mechanism, Fe⁄Ni bimetallic nano particles, Density functional theory (DFT), Catalytic capacity.

Visible-light-driven photocatalytic and photoelectrocatalytic debromination of BDE-47 on a macroporous silicon/graphene heterostructure

Macroporous silicon/graphene (MPSi/Gr) heterostructure was fabricated by electrophoresis depositing Gr sheets on the surface of MPSi and then annealing the samples to improve adhesion and electrochemical property. In the photoelectrochemical measurements, MPSi/Gr heterostructure displayed stable and high photocurrent compared with MPSi. The stability profits from protection of Gr layer, and the enhancement of photocurrent is ascribed to effective photogenerated charge separation of MPSi/Gr heterostructure. The results of photocatalytic and photoelectrocatalytic debromination of BDE-47 showed MPSi/Gr possesses excellent photocatalytic capability, and this capability can be enhanced by applying a bias potential. The kinetic constant of MPSi/Gr (1.13h−1) heterostructure in photoelectrocatalytic process was five times as much as that of MPSi/carbon film heterostructure (0.22h−1), indicating Gr, as a charge collector which can promote separation and transfer of photogenerated carries, is superior to other carbon materials for building heterostructures. The majority (87.1%) of debromination was induced by visible light which was confirmed by comparing reaction efficiency under xenon lamp irradiation to that under the same lamp irradiation via a 422nm filter. According to the results of photocatalytic debromination experiments with O2 or N2 bubbling, debromination mechanism of BDE-47 was proposed as a reductive debromination by photogenerated electrons.

Debromination of polybrominated diphenyl ethers by Ni/Fe bimetallic nanoparticles: Influencing factors, kinetics, and mechanism

Polybrominated diphenyl ethers have been identified as a new class of organic pollutants with ecological risk due to their toxicity, bioaccumulation, and global distribution. Proper remediation technologies are needed to remove them from the environment. In this paper, Ni/Fe bimetallic nanoparticles were synthesized by chemical deposition and used to degrade decabromodiphenyl ether (BDE209). The characteristics of Ni/Fe nanoparticles were analyzed by transmission electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, and Brunnaer–Emmett–Teller surface area analysis. Ni/Fe bimetallic nanoparticles with diameters in the order of 20–50 nm could effectively degrade BDE209 in the solvent (tetrahydrofuran/water). Influence factors, such as Ni/Fe nanoparticle dosage, initial BDE209 concentration, and Ni loading, on the removal of BDE209 were studied. The results indicated that the degradation of BDE209 followed pseudo-first-order kinetics, and the degradation rate of BDE209 increased with increasing the amount of nano Ni/Fe particles, Ni/Fe ratio, and decreasing the initial concentration of BDE209. Through analyzed the mass balance of the BDE209 removal, degradation was the main process of BDE209 removal. The mechanism of debromination was deduced by analyzing the reaction products using gas chromatography–mass spectrometry, the bromide ion in the solution and varying the solvent conditions. Stepwise hydrogen reduction is the main process of debromination, and the hydrion play an important role in the reaction. Moreover, the experiment of long term performance and leaching of Ni were also carried out to test the stability and durability of Ni/Fe nanoparticles in BDE209 degradation.