We investigated simultaneous arsenic and fluoride removal from ground water by electrocoagulation (EC) using aluminum as the sacrificial anode in a continuous filter-press reactor. The groundwater ...was collected at a depth of 320 m in the Bajío region in Guanajuato Mexico (arsenic 43 µg L−1, fluoride 2.5 mg L−1, sulfate 89.6 mg L−1, phosphate 1.8 mg L−1, hydrated silica 112.4 mg L−1, hardness 9.8 mg L−1, alkalinity 31.3 mg L−1, pH 7.6 and conductivity 993 µS cm−1). EC was performed after arsenite was oxidized to arsenate by addition of 1 mg L−1 hypochlorite. The EC tests revealed that at current densities of 4, 5 and 6 mA cm−2 and flow velocities of 0.91 and 1.82 cm s−1, arsenate was abated and residual fluoride concentration satisfies the WHO standard (CF < 1.5 mg L−1). Spectrometric analyses performed on aluminum flocs indicated that these are mainly composed of aluminum-silicates of calcium and magnesium. Arsenate removal by EC involves adsorption on aluminum flocs, while fluoride replaces a hydroxyl group from aluminum aggregates. The best EC was obtained at 4 mA cm−2 and 1.82 cm s−1 with electrolytic energy consumption of 0.34 KWh m−3.
•Arsenic and fluoride removal from real groundwater by electrocoagulation.•Aluminum as sacrificial anodes.•Electrolyzes at different flow velocities and current densities.•Electrocoagulation led to 100% arsenic removal (43 µg L−1) with 0.34 KWh m−3.•Fluoride depletion from 2.5 to 0.15 mg L−1 meet the WHO standard (<1.5 mg L−1).
The simultaneous removal of hydrated silica, fluoride and arsenic from deep well water (hydrated silica 72 mgL−1, fluoride 4.4 mgL−1, arsenic 106.2 μgL−1, sulfate 50 mgL−1, phosphate 0.99 mgL−1, ...pH = 8.2 and conductivity 659 μScm−1) by electrocoagulation (EC) was investigated. The EC was performed in a continuous electrochemical reactor using aluminum plates as sacrificial anodes coupled directly to a jar test device. The effect of current density (4 ≤ j ≤ 8 mA cm−2) and mean linear flow rates in the EC reactor (0.057 ≤ u ≤ 0.57 cm s−1) on the hydrated silica, fluoride, and arsenic removal efficiencies was analyzed. The abatement of hydrated silica was obtained at 8 mA cm−2 and 0.057 cm s−1, while the residual concentrations of F− and As after the same electrolysis were 0.19 mg L−1 and 9.8 μg L−1, satisfying the WHO guidelines for F− (≤1.5 mg L−1) and As (≤10 μg L−1). Spectroscopic analyses on aluminum flocs revealed that they are predominantly composed of aluminum silicates. Arsenates adsorb on aluminum flocs and fluoride replaces a hydroxyl group from aluminum aggregates.
•Hydrated silica abatement from real groundwater by electrocoagulation.•Aluminum electrodes in an up-flow reactor with twelve cell stack.•Hydrated silica reacts with large dose of coagulant to form aluminum silicates.•Fluoride replaces a hydroxyl group from flocs, and arsenate adsorbs on aggregates.•Fluoride and arsenic concentrations after EC fulfill the WHO guidelines.
Information on the spectral shape of prompt emission in gamma-ray bursts (GRB) is mostly available only at energies ≳10 keV, where the main instruments for GRB detection are sensitive. The origin of ...this emission is still very uncertain because of the apparent inconsistency with synchrotron radiation, which is the most obvious candidate, and the resulting need for considering less straightforward scenarios. The inclusion of data down to soft X-rays (∼0.5 keV), which are available only in a small fraction of GRBs, has firmly established the common presence of a spectral break in the low-energy part of prompt spectra, and even more importantly, the consistency of the overall spectral shape with synchrotron radiation in the moderately fast-cooling regime, the low-energy break being identified with the cooling frequency. In this work we further extend the range of investigation down to the optical band. In particular, we test the synchrotron interpretation by directly fitting a theoretically derived synchrotron spectrum and making use of optical to gamma-ray data. Secondly, we test an alternative model that considers the presence of a black-body component at ∼keV energies, in addition to a non-thermal component that is responsible for the emission at the spectral peak (100 keV–1 MeV). We find that synchrotron radiation provides a good description of the broadband data, while models composed of a thermal and a non-thermal component require the introduction of a low-energy break in the non-thermal component in order to be consistent with optical observations. Motivated by the good quality of the synchrotron fits, we explore the physical parameter space of the emitting region. In a basic prompt emission scenario we find quite contrived solutions for the magnetic field strength (5 G < B′< 40 G) and for the location of the region where the radiation is produced (Rγ > 1016 cm). We discuss which assumptions of the basic model would need to be relaxed in order to achieve a more natural parameter space.
•Comparison of Pt, IrO2-based, RuO2-based and BDD anodes for naproxen degradation.•Growing oxidation power: EO-H2O2<EF<PEF, regardless of the anode.•Quicker TOC removal: IrO2-based anode better in ...EO-H2O2 and EF, BDD better in PEF.•Increasing decay kinetics of naproxen: RuO2-based<Pt<BDD<IrO2-based.•Identification of 6 aromatic products along with oxalic and maleic acids.
This study focuses on the role of the anode material for the electrochemical degradation of the top-selling anti-inflammatory drug naproxen (NPX). Aqueous solutions containing 40mgL−1 NPX sodium in 0.050M NaClO4 at pH 3.0 were comparatively treated by electrochemical advanced oxidation processes (EAOPs) like electro-oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF). The experiments were performed in a 2.5L flow plant equipped with an annular glass photoreactor coupled to a cell with a Pt, IrO2-based (DSA-O2), RuO2-based (DSA-Cl2) or boron-doped diamond (BDD) anode and an air-diffusion cathode to electrogenerate H2O2. In EF and PEF, 0.50mM Fe2+ was added as catalyst. At 50mAcm−2, the oxidation power of EAOPs rose in the order: EO-H2O2<EF<PEF, regardless of the anode used. The IrO2-based anode led to greater mineralization in EO-H2O2 and EF. In contrast, the BDD anode allowed an almost total mineralization in PEF, being superior to 85% attained with the other three materials. DSA, a significantly cheap anode compared to Pt and BDD, can then be a suitable candidate for treating NPX solutions by EAOPs. For each process, the mineralization current efficiency and specific energy consumption were determined. The NPX concentration decay always followed a pseudo-first-order kinetics and, in PEF, it was enhanced in the sequence: RuO2-based<Pt<BDD<IrO2-based. GC–MS analysis of treated solutions allowed detecting six aromatic products, whereas maleic and oxalic acids were identified by ion-exclusion HPLC. A reaction sequence for the degradation of NPX by EAOPs is finally proposed.
•Use of a 10dm3 solar plant with a FM01-LC cell and a CPC photoreactor.•Predominant H2O2 production at the graphite-felt cathode and photolysis at the CPC.•Close agreement was obtained between ...theoretical and experimental data.•Four heterocyclic, one aromatic, one tertiary amine and five carboxylic acids detected.•Release of nitrate ions and, to a lesser extent, ammonium ion.
A solar photoelectro-Fenton (SPEF) plant containing a filter-press FM01-LC flow reactor in series with a compound parabolic collector (CPC) as photoreactor, operating in batch recirculation mode, was simulated using a parametric model. The degradation of 10dm3 of solutions of the heterocyclic antibiotic erythromycin (ERY) in 0.050moldm−3 Na2SO4 at pH 3.0 was used for validation. The filter-press reactor contained a platinized titanium plate anode and a graphite-felt cathode that produced H2O2 from the reduction of dissolved oxygen (0.24mmoldm−3). Trials were performed under potentiostatic and galvanostatic conditions with predominance of H2O2 production, minimizing H2 evolution reaction. The effect of initial catalyst (Fe2+) concentration, current density (j), initial antibiotic concentration as dissolved organic carbon (DOC) and volumetric flow rate on the ERY mineralization was studied. Good agreement between simulations and experimental DOC decays was obtained. Mineralization current efficiencies and specific energy consumptions were also determined. The best performance under galvanostatic conditions was found for 0.225mmoldm−3 ERY (100mgdm−3 DOC), 0.50mmoldm−3 Fe2+, volumetric flow rate of 3.0dm3min−1 and jcath=-0.16mAcm−2, reaching 69% mineralization with current efficiency of 75% and specific energy consumption of 0.059kWh (g DOC)−1. Six organic by-products were identified by gas chromatography-mass spectrometry, whereas final short-chain carboxylic acids like formic and oxalic acid were detected by ion-exclusion high-performance liquid chromatography. The initial N atom of ERY was predominantly converted into NO3− ion, although NH4+ ion was formed as well.
Detection of prompt emission by Swift-XRT provides a unique tool to study how the prompt spectrum of gamma-ray bursts (GRBs) extends down to the soft X-ray band. This energy band is particularly ...important for prompt emission studies, since it is towards low energies that the observed spectral shape is in disagreement with the synchrotron predictions. Unfortunately, the number of cases where XRT started observing the GRB location during the prompt phase is very limited. In this work, we collect a sample of 34 GRBs and perform joint XRT+BAT spectral analysis of prompt radiation, extending a previous study focused on the 14 brightest cases. Fermi-GBM observations are included in the analysis when available (11 cases), allowing the characterization of prompt spectra from soft X-rays to MeV energies. In 62% of the spectra, the XRT data reveal a hardening of the spectrum, well described by introducing an additional, low-energy power-law segment (with index α1) into the empirical fitting function. The break energy below which the spectrum hardens has values between 3 keV and 22 keV. A second power-law (α2) describes the spectrum between the break energy and the peak energy. The mean values of the photon indices are 〈α1〉 = −0.51 (σ = 0.24) and 〈α2〉 = −1.56 (σ = 0.26). These are consistent, within one σ, with the synchrotron values in fast cooling regime. As a test, if we exclude XRT data from the fits we find typical results: the spectrum below the peak energy is described by a power law with 〈α〉 = −1.15. This shows the relevance of soft X-ray data in revealing prompt emission spectra consistent with synchrotron spectra. Finally, we do not find any correlation between the presence of the X-ray break energy and the flux, fluence, or duration of the prompt emission.
Supernova remnants are believed to be the main sources of galactic cosmic rays (CR). Within this framework, particles are accelerated at supernova remnant shocks and then released in the interstellar ...medium. The mechanism through which CRs are released and the way in which they propagate still remain open issues. The main difficulty is the high non-linearity of the problem: CRs themselves excite the magnetic turbulence that confines them close to their sources. We solve numerically the coupled differential equations describing the evolution in space and time of the escaping particles and of the waves generated through the CR streaming instability. The warm ionized and warm neutral phases of the interstellar medium are considered. These phases occupy the largest fraction of the disc volume, where most supernovae explode, and are characterized by the significant presence of neutral particles. The friction between those neutrals and ions results in a very effective wave damping mechanism. It is found that streaming instability affects the propagation of CRs even in the presence of ion-neutral friction. The diffusion coefficient can be suppressed by more than a factor of ∼2 over a region of few tens of pc around the remnant. The suppression increases for smaller distances. The propagation of ≈10 GeV particles is affected for several tens of kiloyears after escape, while ≈1 TeV particles are affected for few kiloyears. This might have a great impact on the interpretation of gamma-ray observations of molecular clouds located in the vicinity of supernova remnants.
The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow ...relativistic jet or an isotropic outflow. High-spatial-resolution measurements of the source size and displacement can discriminate between these scenarios. We present very-long-baseline interferometry observations, performed 207.4 days after the merger by using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milli-arc seconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.
We report our observation of the short gamma-ray burst (GRB) GRB 170817A, associated to the binary neutron star merger gravitational wave (GW) event GW 170817, performed in the X-ray band with ...XMM-Newton 135 d after the event (on 29 December, 2017). We find evidence for a flattening of the X-ray light curve with respect to the previously observed brightening. This is also supported by a nearly simultaneous optical Hubble Space Telescope observation and successive X-ray Chandra and low-frequency radio observations recently reported in the literature. Since the optical-to-X-ray spectral slope did not change with respect to previous observations, we exclude that the change in the temporal evolution of the light curve is due to the passage of the cooling frequency: its origin must be geometric or dynamical. We interpret all the existing afterglow data with two models: i) a structured jet and ii) a jet-less isotropic fireball with some stratification in its radial velocity structure. Both models fit the data and predict that the radio flux must decrease simultaneously with the optical and X-ray emission, making it difficult to distinguish between them at the present stage. Polarimetric measurements and the rate of short GRB-GW associations in future LIGO/Virgo runs will be key to disentangle these two geometrically different scenarios.
Knowledge of the bulk Lorentz factor Γ0 of gamma-ray bursts (GRBs) allows us to compute their comoving frame properties shedding light on their physics. Upon collisions with the circumburst matter, ...the fireball of a GRB starts to decelerate, producing a peak or a break (depending on the circumburst density profile) in the light curve of the afterglow. Considering all bursts with known redshift and with an early coverage of their emission, we find 67 GRBs (including one short event) with a peak in their optical or GeV light curves at a time tp. For another 106 GRBs we set an upper limit tpUL. The measure of tp provides the bulk Lorentz factor Γ0 of the fireball before deceleration. We show that tp is due to the dynamics of the fireball deceleration and not to the passage of a characteristic frequency of the synchrotron spectrum across the optical band. Considering the tp of 66 long GRBs and the 85 most constraining upper limits, we estimate Γ0 or a lower limit Γ0LL. Using censored data analysis methods, we reconstruct the most likely distribution of tp. All tp are larger than the time Tp,γ when the prompt γ-ray emission peaks, and are much larger than the time Tph when the fireball becomes transparent, that is, tp>Tp,γ>Tph. The reconstructed distribution of Γ0 has median value ~300 (150) for a uniform (wind) circumburst density profile. In the comoving frame, long GRBs have typical isotropic energy, luminosity, and peak energy ⟨ Eiso ⟩ = 3(8) × 1050 erg, ⟨ Liso ⟩ = 3(15) × 1047 erg s-1, and ⟨ Epeak ⟩ = 1(2) keV in the homogeneous (wind) case. We confirm that the significant correlations between Γ0 and the rest frame isotropic energy (Eiso), luminosity (Liso), and peak energy (Ep) are not due to selection effects. When combined, they lead to the observed Ep−Eiso and Ep−Liso correlations. Finally, assuming a typical opening angle of 5 degrees, we derive the distribution of the jet baryon loading which is centered around a few 10-6M⊙.
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