► The review focuses on the Cr speciation, uses, toxicity and remediation aspects. ► Assessed amount of hexavalent Cr in soils/metallurgical/other wastes and sites. ► Chemical and bioremediation ...processes for Cr(VI) are summarised in detail. ► Strategy for bio-remediation of Cr(VI) in different solid wastes is suggested. ► Capability of microbes for reducing Cr(VI) contamination described. ► Mechanism of microbial reduction of Cr(VI) to Cr(III) is included.
Chromium is a highly toxic non-essential metal for microorganisms and plants, and its occurrence is rare in nature. Lower to higher chromium containing effluents and solid wastes released by activities such as mining, metal plating, wood preservation, ink manufacture, dyes, pigments, glass and ceramics, tanning and textile industries, and corrosion inhibitors in cooling water, induce pollution and may cause major health hazards. Besides, natural processes (weathering and biochemical) also contribute to the mobility of chromium which enters in to the soil affecting the plant growth and metabolic functions of the living species. Generally, chemical processes are used for Cr- remediation. However, with the inference derived from the diverse Cr-resistance mechanism displayed by microorganisms and the plants including biosorption, diminished accumulation, precipitation, reduction of Cr(VI) to Cr(III), and chromate efflux, bioremediation is emerging as a potential tool to address the problem of Cr(VI) pollution. This review focuses on the chemistry of chromium, its use, and toxicity and mobility in soil, while assessing its concentration in effluents/wastes which becomes the source of pollution. In order to conserve the environment and resources, the chemical/biological remediation processes for Cr(VI) and their efficiency have been summarised in some detail. The interaction of chromium with various microbial/bacterial strains isolated and their reduction capacity towards Cr(VI) are also discussed.
In this comprehensive review resources of lithium and status of different processes/technologies in vogue or being developed for extraction of lithium and associated metals from both primary and ...secondary resources are summarized. Lithium extraction from primary resources such as ores/minerals (spodumene, petalite and lepidolite) by acid, alkaline and chlorination processes and from brines by adsorption, precipitation and ion exchange processes, is critically examined. Problems associated with the exploitation of other resources such as bitterns and seawater are highlighted. As regards the secondary resources, the industrial processes followed and the newer developments aiming at the recovery of lithium from lithium ion batteries (LIBs) are described in detail. In particular pre-treatment of the spent LIBs, leaching of metals from the cathode material in different acids and separation of lithium and other metals from the leach liquors, are discussed. Although spent LIBs are currently processed to recover cobalt and other base metals but not lithium, there is a good prospect for the recovery of lithium in the coming years. Varying compositions of batteries for different applications require development of a suitable recycling process to recover metals from all types of LIBs.
•Review gives reserves, technologies & R&D on Li extraction from different sources.•Practice and problems of extraction of Li from primary ores/minerals are described.•Covered merits/demerits of Li recovery from brine, a major source of production.•Critically examined industrial processes & R&D efforts to recover metals from LIBs.•Highlighted recycling of spent LIBs for recovery of Li & other base metals.
•Investigated H2SO4 leaching of all metals from cathode active material of LIBs.•Optimized leaching conditions: 1M H2SO4, 368K, 50g/L pulp density and 240min.•Achieved recovery of 93.4% Li, 66.2% Co, ...96.3% Ni and 50.2% Mn.•Low recovery of Co and Mn is explained through the Eh–pH diagrams.•Leaching kinetics follows logarithmic rate law with surface diffusion control mechanism.
This work is focussed on the processing of cathodic active material of spent lithium ion batteries (LIBs) to ensure resource recovery and minimize environmental degradation. The sulfuric acid leaching of metals was carried out for the recovery of all the valuable metals including nickel and manganese along with the frequently targeted metals like lithium and cobalt. The process parameters such as acid concentration, pulp density, time and temperature for the leaching of metals from the cathode powder containing 35.8% Co, 6.5% Li, 11.6% Mn and 10.06% Ni, were optimized. Results show the optimized leach recovery of 93.4% Li, 66.2% Co, 96.3% Ni and 50.2% Mn when the material was leached in 1M H2SO4 at 368K and 50g/L pulp density for 240min. The need of a reductant for improved recovery of cobalt and manganese has been explained by the thermodynamic analysis (Eh–pH diagram) for these metals. Leaching of the valuable metals was found to follow the logarithmic rate law controlled by surface layer diffusion of the lixiviant reacting with the particles. The mode of leaching of the metals from the spent LIBs was further examined by chemical analysis of the samples at various stage of processing which was further corroborated by characterizing the untreated sample and the leach residues by XRD phase identification and the SEM-EDS studies.
The growing need for strategic and rare earth elements (REE) led to an enormous interest in the development of new technologies for the recycling and recovery of these metals from secondary resources ...especially batteries. The primary reserves, production scenario and possible secondary resources of cobalt, lead, lithium, manganese, nickel, REE, and zinc, with respect of metal content, quantity, the source of generation is discussed. The purpose of this review is to depict the status of the recycling technologies for the extraction of metals inherent in primary and secondary batteries from secondary resources like process wastes, catalysts, effluents, metal and alloy scrap, e-wastes, mine tailings, and metallurgical slags and spent batteries, depicting a summary of those under-utilized wastes. Also, the problems and the prospect of the studies of their recycling technologies especially focused on the hydrometallurgical and pyrometallurgical processes have been put forward. The future avenues in spent battery recycling are also discussed with respect to the directions of research needed for their sustainable utilization and environmental management.
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•Resource availability, concentration of Co, Li, Mn, Ni, Pb, REEs, Zn in minerals.•Metals vis-à-vis secondary resources depicts vistas for recycling.•Recycling batteries hold significance in environmental and materials sustainability.•Complete recovery of inherent valuables from batteries stand challenging.•Metal separation to high purity salts by hybrid methodologies is imperative.
ABSTRACT
The dynamics of the partially ionized solar atmosphere is controlled by the frequent collisions and charge exchange between the predominant neutral hydrogen atoms and charged ions. At signal ...frequencies below or of the order of either of the collision or charge exchange frequencies, the magnetic stress is felt by both the charged and neutral particles simultaneously. The resulting neutral-mass loading of the ions leads to the rescaling of the effective ion-cyclotron frequency (it becomes the Hall frequency), and the resultant effective Larmor radius becomes of the order of few kms. Thus, the finite Larmor radius effect that manifests as the ion and neutral pressure stress tensors operates over macroscopic scales. Whereas parallel and perpendicular (with respect to the magnetic field) viscous momentum transport competes with the Ohm and Hall diffusion of the magnetic field in the photosphere–chromosphere, the gyroviscous effect becomes important only in the transition region between the chromosphere and corona, where it competes with the ambipolar diffusion. The wave propagation in the gyroviscous effect-dominated medium depends on the plasma β (a ratio of the thermal and magnetic energies). The abundance of free energy makes gyro waves unstable with the onset condition exactly opposite of the Hall instability. However, the maximum growth rate is identical to the Hall instability. For a flow gradient of ${\sim} 0.1 \, \mbox{s}^{-1}$, the instability growth time is 1 min. Thus, the transition region may become subject to this fast-growing gyroviscous instability.
Abstract
In the Hall magnetohydrodynamics, the onset condition of the Kelvin–Helmholtz instability is solely determined by the Hall effect and is independent of the nature of shear flows. In ...addition, the physical mechanism behind the super- and sub-Alfvénic flows becoming unstable is quite different: the high-frequency right circularly polarized whistler becomes unstable in the super-Alfvénic flows whereas low-frequency, left circularly polarized ion-cyclotron wave becomes unstable in the presence of sub-Alfvénic shear flows. The growth rate of the Kelvin–Helmholtz instability in the super-Alfvénic case is higher than the corresponding ideal magnetohydrodynamic rate. In the sub-Alfvénic case, the Hall effect opens up a new, hitherto inaccessible (to the magnetohydrodynamics) channel through which the partially or fully ionized fluid can become Kelvin–Helmholtz unstable. The instability growth rate in this case is smaller than the super-Alfvénic case owing to the smaller free shear energy content of the flow. When the Hall term is somewhat smaller than the advection term in the induction equation, the Hall effect is also responsible for the appearance of a new overstable mode whose growth rate is smaller than the purely growing Kelvin–Helmholtz mode. On the other hand, when the Hall diffusion dominates the advection term, the growth rate of the instability depends only on the Alfvén –Mach number and is independent of the Hall diffusion coefficient. Further, the growth rate in this case linearly increases with the Alfvén frequency with smaller slope for sub-Alfvénic flows.
Gamma-ray bursts (GRBs) are flashes of high-energy radiation arising from energetic cosmic explosions. Bursts of long (greater than two seconds) duration are produced by the core-collapse of massive ...stars
, and those of short (less than two seconds) duration by the merger of compact objects, such as two neutron stars
. A third class of events with hybrid high-energy properties was identified
, but never conclusively linked to a stellar progenitor. The lack of bright supernovae rules out typical core-collapse explosions
, but their distance scales prevent sensitive searches for direct signatures of a progenitor system. Only tentative evidence for a kilonova has been presented
. Here we report observations of the exceptionally bright GRB 211211A, which classify it as a hybrid event and constrain its distance scale to only 346 megaparsecs. Our measurements indicate that its lower-energy (from ultraviolet to near-infrared) counterpart is powered by a luminous (approximately 10
erg per second) kilonova possibly formed in the ejecta of a compact object merger.
The Hall effect arises in a plasma when electrons are able to drift with the magnetic field but ions cannot. In a fully ionized plasma this occurs for frequencies between the ion and electron ...cyclotron frequencies because of the larger ion inertia. Typically this frequency range lies well above the frequencies of interest (such as the dynamical frequency of the system under consideration) and can be ignored. In a weakly ionized medium, however, the Hall effect arises through a different mechanism – neutral collisions preferentially decouple ions from the magnetic field. This typically occurs at much lower frequencies and the Hall effect may play an important role in the dynamics of weakly ionized systems such as the Earth's ionosphere and protoplanetary discs. To clarify the relationship between these mechanisms we develop an approximate single-fluid description of a partially ionized plasma that becomes exact in the fully ionized and weakly ionized limits. Our treatment includes the effects of ohmic, ambipolar and Hall diffusion. We show that the Hall effect is relevant to the dynamics of a partially ionized medium when the dynamical frequency exceeds the ratio of ion to bulk mass density times the ion-cyclotron frequency, i.e. the Hall frequency. The corresponding length-scale is inversely proportional to the ion to bulk mass density ratio as well as to the ion-Hall beta parameter. In a weakly ionized medium, the critical frequency becomes small enough that Hall magnetohydrodynamics (MHD) is an accurate representation of the dynamics. More generally, ohmic and ambipolar diffusion may also be important. We show that both ambipolar and Hall diffusion depend upon the fractional ionization of the medium. However, unlike ambipolar diffusion, Hall diffusion may also be important in the high fractional ionization limit. The wave properties of a partially ionized medium are investigated in the ambipolar and Hall limits. We show that in the ambipolar regime wave damping is dependent on both fractional ionization and ion–neutral collision frequencies. In the Hall regime, since the frequency of a whistler wave is inversely proportional to the fractional ionization, and bounded by the ion–neutral collision frequency it will play an important role in the Earth's ionosphere, solar photosphere and astrophysical discs.
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•Investigated H2SO4 leaching of rare earth metals (REMs) from spent Ni–MH batteries.•Optimized leaching conditions: 2M H2SO4, 348K, 120min and 100g/L pulp density.•Achieved recovery ...of 98.1% Nd, 98.4% Sm, 95.5% Pr and 89.4% Ce.•Established chemical control kinetic model for leaching REMs by XRD and SEM studies.•Recovered REMs as mixed sulfate salt by the pH adjustment to <1.8.
Nickel–metal hydride batteries (Ni–MH) contain not only the base metals, but valuable rare earth metals (REMs) viz. La, Sm, Nd, Pr and Ce as well. In view of the importance of resource recycling and assured supply of the contained metals in such wastes, the present study has focussed on the leaching of the rare earth metals from the spent Ni–MH batteries. The conditions for the leaching of REMs from the spent batteries were optimized as: 2M H2SO4, 348K temperature and 120min of time at a pulp density (PD) of 100g/L. Under this condition, the leaching of 98.1% Nd, 98.4% Sm, 95.5% Pr and 89.4% Ce was achieved. Besides the rare earth metals, more than 90% of base metals (Ni, Co, Mn and Zn) were also leached out in this condition. Kinetic data for the dissolution of all the rare earth metals showed the best fit to the chemical control shrinking core model. The leaching of metals followed the mechanism involving the chemical reaction proceeding on the surface of particles by the lixiviant, which was corroborated by the XRD phase analysis and SEM–EDS studies. The activation energy of 7.6, 6.3, 11.3 and 13.5kJ/mol was acquired for the leaching of neodymium, samarium, praseodymium and cerium, respectively in the temperature range 305–348K. From the leach liquor, the mixed rare earth metals were precipitated at pH∼1.8 and the precipitated REMs was analyzed by XRD and SEM studies to determine the phases and the morphological features.
Graphitic carbon nitride (g-C3N4) has manifested itself as an effective counterpart of titanium di-oxide (TiO2) to extend its photoactivity in the visible range. In this work, g-C3N4 functionalized ...TiO2 nanotubes were synthesized by electrodeposition of g-C3N4 in bulk on TiO2 nanotubes from a continuously rotating organic suspension at a moderate voltage (∼90 V). Additionally, samples with varying time durations were also synthesized to examine the effect of electrodeposition time on the performance of g-C3N4 functionalized TiO2 nanotubes. Photo-electrochemical measurements demonstrated strong light induced activity, with a photocurrent density of 0.14 mA/cm2 at 1 V vs Ag/AgCl in 1 M Na2SO4 electrolyte. Maximum photoresponse for g-C3N4 functionalized TiO2 nanotubes was upto 4 factors higher than bare TiO2 nanotubes. The crystallinity, morphology and chemical compositions were investigated by XRD, FESEM and XPS respectively. Further, the optical absorption edge, chemical bonds and defects states were evaluated by UV–visible, Raman, FTIR, PL and EPR spectroscopy. The properties of g-C3N4/TiO2 system were correlated with the photocurrent behaviour and a mechanism of the probable role of vacancies and defects in their photo induced activity has been proposed. The study has established a simple and scalable scheme for the functionalization of TiO2 nanotube with g-C3N4 to achieve enhanced photo-electrochemical properties.
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•Simple and scalable functionalization of TiO2 nanotube with g-C3N4 was reported.•A rotating suspension of bulk g-C3N4 electrodeposited at 90 V on TiO2 nanotubes.•Study of g-C3N4 electrodeposition time on PEC performance of g-C3N4/TiO2 nanotubes.•Four-fold increase in photocurrent density for g-C3N4/ TiO2 nanotubes was achieved.•Possible role of vacancies and defects in the photo induced activity was discussed.
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