H2SO4 System Research Articles

Low-acid leaching of lithium-ion battery active materials in Fe-catalyzed Cu-H2SO4 system

Leaching of active cathode materials of Li-ion batteries (LIB) is a hotly contested topic. In the published literature, the best processes utilize concentrated acid (e.g. 2–3 M H2SO4) and elevated temperatures for waste LIB leaching, along with unstable reduction reagents such as H2O2. In this study, we demonstrate the dissolution of LiCoO2 (LCO) in a low-acid leaching system that utilizes typical battery elements which can be found in impure, recycled black masses; Fe2+ as a reducing agent towards LCO, and Cu as a reducing agent towards Fe3+. We show for the first time that the Cu-Fe2+-H2SO4 system can provide an excellent performance in dissolving LCO materials at low acid environment and near-room temperature (T = 30 °C), even to the point where the acidity of the solution decreases to pH = 1.89 while reaching Co extraction of 92%. To the best of our knowledge, such high leaching efficiency has not been previously reported under such mild conditions. Nowadays, recyclability of the process waters may also be important, and herein we highlighted the influence of Na2SO4 on leaching of LCO active materials as well in this system. Minimization of the lixiviant concentration and temperature is beneficial in allowing decrease in chemical and energy consumption. High pH operation also can support further downstream processing, helping to avoid the problem of sodium accumulation towards the end-stage where lithium is recovered.

A novel approach to extract Nb, Y and Ce from a niobium-bearing ore of low grade by roasting KHSO4–H2SO4 system

In this work, a novel approach of leaching valuable metals, such as niobium, yttrium and cerium, from a niobium-bearing ore of low grade by roasting KHSO4–H2SO4 system was experimentally studied. Through the work, the effects of roasting temperature, roasting time, mass ratio of agents to ore and the types of leaching agents on the extraction of Nb, Y and Ce were investigated, and the roasting KHSO4–H2SO4–ore system was thought as an effective method to extract these valuable metals. It was found that the recovery of Nb, Y and Ce can, respectively, reach 90.20%, 97.22% and 98.41% in condition of roasting temperature of 300 °C, roasting time of 2 h, mass ratio of KHSO4-to-H2SO4 to ore of 0.5:1:1, particle size of − 74 μm (~ 95%), leaching solution of oxalic acid (30 g·L−1), leaching temperature of 90 °C and leaching liquid-to-solid ratio of 5:1. The leaching kinetics of the calcined Nb-bearing ore in oxalic acid solution was successfully modeled by chemical reaction model, and the apparent activity energy was obtained to be 14.16 kJ·mol−1. The results generated from this work will provide the fundamentals for extracting Nb, Y and Ce from a niobium-bearing ore of low grade and have potential applications value in future.

Leaching kinetic study of sulfuric acid roasted mixed-type rare earth concentrate for reducing the solid-waste production and chemical consumption

The Baotou bastnaesite and monazite mixed rare earth ore is one of the most significant rare earth ore in the world and is mainly processed by the sulfuric acid roasting method. However, the high content of Fe and P in the leaching solution contributes to a large amount of radioactive precipitate waste produced in the purification process, and that causes the primary loss of rare-earth elements (REEs) and the environmental issue and safety concern. The goal of this paper is to reduce the solid-waste by improving the selective leaching efficiency of REEs based on a leaching kinetics study of REEs, Fe and P in the H2SO4 system. The experiments were carried out focusing on the effect of acidity, roasted concentrate particle size, temperature, and liquid-solid ratio on the leaching kinetics. The results indicate that the equilibrium leaching time of REEs (<15 min) and P (<10 min) is far less than that of the Fe (60 min to more than 180 min) which is highly depended on temperature. The leaching kinetics model of REEs, Fe and P follows the shrinking core model but is governed by different limiting step: both REEs and P are controlled by diffusion, while Fe is controlled by chemical reaction. A possible leaching mechanism is proposed. Based on the kinetic study, the dissolution of Fe is inhibited and its content in the leaching liquid is reduced by controlling the leaching time. Meanwhile, the result of neutralization for purifying the leaching solution shows that the amount of solid waste generated in the neutralization process is reduced by 19% with improving 3% REEs recovery and reducing 15% MgO consumption. This study shows a good prospect for improving the sulfuric acid roasting method to be a cleaner and lower chemical consumption process in dealing with the bastnaesite and monazite mixed rare earth ore.

Highly boosting the supercapacitor performance by polydopamine-induced surface modification of carbon materials and use of hydroquinone as an electrolyte additive

In this work, we use polydopamine (PDA) to modify the surface of carbon materials, and then adopt hydroquinone (HQ) as a redox additive to jointly improve the performance of supercapacitors. This non-covalent bond PDA coating produce functional groups such as amino and hydroxyl, which can interact with HQ in H2SO4 system by hydrogen bonding. The carbon materials solely coated with PDA (without any HQ) deliver favorable capacitance of 218 F g−1 and energy density of 7.59 Wh kg−1; whereas the one not only coated with PDA but also using HQ as redox additive, higher capacitance of 557 F g−1 and energy density of 19.36 Wh kg−1 are achieved, realizing the energy density increases by 2.55 times. In addition, for carbon materials not covered with PDA, HQ kinetics is diffusion controlled; for carbon materials covered with PDA, HQ kinetics is surface controlled. Obviously, this work provides a synergistic approach to collectively boosting the performance of supercapacitors.

Investigation of persulfate production on BDD anode by understanding the impact of water concentration

The formation of persulfate (S2O82−) in aqueous solution has been frequently considered as a direct oxidation of SO42− ions on boron-doped diamond (BDD) surface. Conversely, some authors have also suggested that slower reactions between OH radicals and SO42− ions are feasible. However, no attempts have confirmed this electrochemical pathway because the studies have been frequently performed at diluted sulfuric acid solutions. Thus, the purpose of this work was to study the effect of the ratio H2O/H2SO4 on the electrochemical behavior at BDD surface in order to verify the possible participation of hydroxyl radical on the production of persulfate. Different electrochemical techniques (cyclic voltammetry, polarizations curves and impedance) were employed to understand the electrochemical behavior when different concentrations of H2O were added to concentrated H2SO4. Results clearly suggest that, intermediate species are necessary to promote the formation of S2O82−. The predominance of HSO4− ions at concentrated H2SO4 system with lowest water content (2%) suggests that sulfate radical specie (SO4−) may be produced via the reaction of HSO4− with OH radicals. Then, S2O82− production depends principally on the particular formation of OH on BDD surface or BDD(SO4−) sites rather than on the direct anodic oxidation of SO42− at BDD surface. These results are described and discussed in the light of current literature.

DFT modelling of tantalum pentafluoride extraction with phosphorus-based extractants – A molecular dynamics study

Solvent extraction (SX) is a powerful method for both the separation and extraction of metals, resulting in high purity metals. The transition metals tantalum (Ta) and niobium (Nb) are of considerable significance, for example in the nuclear energy sector, where Nb is used as a cladding material and Ta in high-temperature, oxidation-resistant alloys, capacitors, and specialised materials. For these applications, the metals are needed in high purity. In recent studies, various Ta species that could be present during SX have been identified, including TaF5, TaF5·H2O, TaF4OH, TaF4·HSO4, and TaF3OH·HSO4.In an attempt to understand how extraction occurs, molecular dynamics simulations have been utilised, whereby each species was simulated in a 3D periodic box. The stoichiometry of each system was determined from previous experimental (SX) conditions, and each species was investigated in 4 m and 10 m H2SO4. Simulations started at a perfectly mixed point, and the total simulation time was between 2 and 10 ns. Small-scale system results showed that TaF5·H2O formed at low H2SO4 concentrations and could be extracted with D2EHPA in both 4 m and 10 m H2SO4. If the solution was left to age, TaF4OH could form, which could not be extracted with D2EHPA at either concentration. In H2SO4-based media, TaF4·HSO4 will form, which could be extracted with D2EHPA from both 4 m and 10 m H2SO4. Further ageing of this solution resulted in the formation of TaF3OH·HSO4, which could not be extracted at either H2SO4 concentration. Furthermore, it was seen that in the 4 m H2SO4 system, the aqueous phase tends to form a droplet within an organic bulk solution, and when the H2SO4 concentration was increased, both phases showed droplet properties with break-away between them.

Using recurrence plots for the analysis of the nonlinear dynamical response of iron passivation-corrosion processes.

Recurrence plots (RPs) and recurrence quantification analysis (RQA) are used in this work to study different nonlinear dynamical regimes emerging in an electrochemical system, namely, the electrodissolution-passivation of iron in chloride-containing sulfuric acid solutions. Current oscillations at different applied potentials and chloride concentrations exhibit bifurcations from periodic to complex (bursting) periodic and aperiodic or chaotic behaviors, associated with different dissolution states of iron. The clarification of these transitions is essential to understand the type of corrosion (uniform or localized) taking place as well as the underlying mechanisms governing the stability of the metal. The RQA reveals that the predictability of the chloride-perturbed Fe|0.75M H2SO4 system strongly depends on the chloride concentration and the applied potential. At relatively low chloride concentrations, RQA measures, based on vertical and diagonal structures in RPs, display a decrease upon the breakdown of the passivity on iron and the initiation of localized corrosion (pitting). Phases of pitting corrosion (propagation/growth and unstable pitting) that followed pit initiation are discerned by keen changes of complexity measures upon varying the applied potential. At higher chloride concentrations, the evolution of RQA measures with the potential signifies a transition from the passive-active state dissolution to the polishing state dissolution of iron inside pits. The increase of the applied potential at late stages of pitting corrosion increases the nonlinear correlations and thus the complexity of the system decreases, which corroborates the RQA.

Recovery of scandium from sulfuric acid solution with a macro porous TRPO/SiO2-P adsorbent

A macro porous silica-polymer based TRPO/SiO2-P adsorbent was prepared for recovery of scandium from sulfuric acid solution. The as-prepared TRPO/SiO2-P was characterized by SEM-EDS, TG-DSC, BET, IR. It was found the adsorption of TRPO/SiO2-P towards Sc reached equilibrium in 120 min with the saturated adsorption amount about 13.3 mg/g. It needed about six molecules of TRPO/SiO2-P to uptake 2 molecules of Sc(III), meanwhile three molecules of SO42− were needed for charge balance. Furthermore, TRPO/SiO2-P exhibited high adsorption selectivity towards Sc and almost no adsorption towards the other RE and Al, Fe and little adsorption towards Ti which could be effectively suppressed by H2O2. And column experiments showed clear separation of Sc from Al, Ti, Fe, Y, La and Ce in the effluent, indicating TRPO/SiO2-P could successfully separate Sc from the other metal ions. In a word, TRPO/SiO2-P is a very promising adsorbent for Sc separation from the H2SO4 system.

Synthesis of carbon quantum dot-doped NiCoP and enhanced electrocatalytic hydrogen evolution ability and mechanism

Carbon quantum dots/bimetal phosphide (CQDs@NiCoP) catalysts for electrocatalytic hydrogen evolution reaction were synthesized via a facile annealing method. In a 0.5 M H2SO4 system, the 30 mg/L CQDs@NiCoP catalyst performed well with high electrocatalytic activity, lower initial overpotential than pure NiCoP (108 vs. 182 mV) and lower Tafel slope of 80 mV/decade. The enhancement of hydrogen evolution reaction activity was probably because the addition of CQDs enhanced the catalyst conductivity and significantly accelerated charge transfer.

Local structures of titanium-ion complexes in redox flow battery electrolytes as revealed by X-ray scattering with difference analysis

We report an investigation into the local structure of Ti4+ ions in sulfuric acid solutions as electrolytes for Ti-based redox flow batteries using X-ray total scattering with difference analysis. In the X-ray scattering experiments, we analyzed two concentrated aqueous solutions, one containing 1.5M TiOSO4 and 2.0M H2SO4 (termed the Ti-SO4 system) and the other containing 3.5M H2SO4 (termed the H2SO4 system), which have the same SO42− concentration, and then subtracted the observed X-ray interference function (i(Q)) for the H2SO4 system from that of the Ti-SO4 system to obtain the difference interference function (Δi(Q)). Using Fourier transformation of Δi(Q), we successfully obtained the difference distribution function (Δg(r)), which provides information on the local structure in the first solvation sphere of the Ti ions. The obtained Δg(r) function was interpreted using two possible structural models, i.e., mononuclear and multinuclear Ti-ion complexes. Consequently, we concluded that Ti ions are six-coordinated by O atoms (SO4− and/or H2O) to form octahedral complexes and that mononuclear and multinuclear complexes coexist in the Ti-SO4 system.

Effect of hydrogen bonding on the liquid crystalline behavior of Poly(p-phenylene terephthamide) in sulfuric acid

Many factors affect the formation of lyotropic liquid crystals (LLCs) in the poly(p-phenylene terephthamide) (PPTA)/H2SO4 system. In this systematic work, we prove that the hydrogen bonding between PPTA and H2SO4 plays a very important role for the LLC formation by using a combination of several characterization methods. Texture features and transition temperatures of different phases owing to hydrogen bonding in the PPTA/H2SO4 system were obtained by using polarized light microscopy and differential scanning calorimetry. 13C NMR and FTIR spectroscopy were applied to investigate the chemical structures in different phases, especially the hydrogen bonding between PPTA and H2SO4. The effect of molecular weight on phase transition was also examined. Moreover, guest compounds such as H2O, CH3SO3H, and carbon nanotube added will destroy the hydrogen-bonding network in the PPTA/H2SO4 system, resulting in different phase behaviors. All these results demonstrate that hydrogen bonding between PPTA and H2SO4 is the key factor for the highly ordered arrangement of polymer chains in the LLC state that is a nematic phase.

Catalytic oxidation of cellulose to formic acid in V(V)-Fe(III)-H2SO4 aqueous solution with O2

Carboxylic acids are considered as an important group of chemicals. Direct conversion of cellulose to formic acid (FA) by vanadium(V(V))-contained catalyst is of significant conceptual and practical interest. A second metal (from Mg(II), Ca(II), Al(III), Cr(III), Mo(VI), Mn(II), Cu(II), or Fe(III)) incorporated into V(V)-contained catalyst system was screened to further improve the oxidation of cellulose by O2 to yield FA. Fe(III) exhibited the best performance and effects of reaction parameters on the conversion of cellulose in NaVO3-FeCl3-H2SO4 aqueous solution were investigated. Cellulose could be rapidly converted, and the yield of FA (66.8%) was higher than that obtained by the single use of FeCl3 (48.4%) or NaVO3 (57.6%). The conversion of cellulose in another V(V)-contained catalyst system (H5PV2Mo10O40) was explored to verify the universal enhancement of FeCl3. Influences of the second metal Fe(III) on the V(V)-contained catalyst system and mechanism of cellulose oxidation in the V(V)-Fe(III)-H2SO4 system were studied. Compared with single use of V(V), the employment of Fe(III) alone is not active enough in the oxidative CC bond cleavage. V(V) other than Fe(III) is the main active species for selective oxidation of substrates. The incorporation of Fe(III) and V(V) shows both synergistic and competitive relationships. Both the synergistic and competitive effects are beneficial to increase FA yield, as they promote the oxidation of V(IV) to V(V), increase oxidative ability, and inhibit the overoxidation to by-product CO2. The catalyst system was reused four times without decline in activity.

Analysis of the temporal self-organizational phenomena observed during the electrodissolution of Zn in sulfuric acid solutions

This work aims to analyze the high-frequency current oscillations observed during the electrodissolution of zinc in sulfuric acid solutions. The current oscillations are emerged within a fixed potential region, ΔΕosc in the characteristic polarization curve, I = f(E) of the Zn | x M H2SO4 system at proper concentrations of the H2SO4. The ΔΕosc appears in the limiting current region under precipitation/dissolution conditions of the ZnSO4.yH2O across the interface Zn | x M H2SO4, during the electrodissolution of Zn. The oscillation characteristics (frequency, amplitude, waveform, induction time needed for their emergence) are changing by varying certain parameters, such as the sulfuric acid concentration, the angular rotational speed of the Zn disc-electrode and a variable external resistance added to the solution resistance. The time series, I = f(t) are recorded under potentiostatic conditions within the ΔΕosc and, using techniques for the analysis of non-linear dynamical systems, are characterized as chaotic. We have proposed construction of recurrence plots and the recurrence quantification analysis as appropriate tools for characterizing current oscillations far from bifurcation points and unraveling dynamical transitions of the electrochemical system as the applied potential is varied.

Oscillations of pH at the Fe│H2SO4 interface during anodic dissolution

An unmodified Pt microelectrode and a Pt microelectrode coated with polyaniline were used in conjunction with a scanning electrochemical microscope (SECM) to study anodic dissolution in the Fe│H2SO4 system. The concentrations of Fe2+ (cFe2+) measured with the unmodified microelectrode and the pH values measured with the polyaniline-modified microelectrode were recorded in situ during current oscillations in the Fe│H2SO4 system and were found to change periodically at the Fe│H2SO4 interface. The changes in cFe2+ may be caused by the periodic formation and dissolution of surface film(s), which could be salt films and/or oxide films. If a salt film is formed, it is unlikely to affect the pH. Since the pH changes periodically during the current oscillations, it can be deduced that the surface film is mainly composed of oxide, and that the formation and dissolution of the oxide film play a key role in the current oscillations of the system.

Supercapacitive vertical graphene nanosheets in aqueous electrolytes

Vertical graphene nanosheets (VGN) is a potential material for high-performance supercapacitors due to its high surface area, open interconnected network and excellent electrical conductivity. Selection of proper aqueous electrolyte is crucial for its utilization as supercapacitor electrode. The present study reports supercapacitive performance of vertical graphene nanosheets in neutral (Na2SO4), alkaline (KOH), and acidic (H2SO4) aqueous electrolytes. Factors like ionic mobility and electrolyte wettability are found to influence supercapacitor performance. The highest capacitance observed for VGN in KOH (197 μF/cm2) is attributed to the higher wettability, whereas VGN /H2SO4 system with specific areal capacitance of 188 μF/cm2 is found to have highest capacitance retention (96.8%), shortest relaxation time (50 ms), low charge transfer resistance (72 Ω cm2) and minimal change in graphitic structure. The later observation is correlated to cationic mobility. Detailed supercapacitive investigation is presented including equivalent circuit simulation for deduction of electrode/electrolyte interface behavior.

Kinetics and mechanism of 1-phenyl-5-methyltetrazole nitration in HNO3–H2SO4 system

At nitration of 1-phenyl-5-methyltetrazole with nitric acid in aqueous solutions of sulfuric acid of 84–96% concentration a mixture forms of 1-(4-nitrophenyl)- and 1-(3-nitrophenyl)-5-methyltetrazoles in a ratio 60: 40. According to kinetic data, the tetrazole ring in the substrate is present in a protonated form. The protonation constants of 1-(4-carboxyphenyl)-5-methyltetrazole at the tetrazole ring and the carboxy group were determined. The Hammet’s electronic constant of the protonated tetrazole ring was calculated.

Synthesis of hydroxy derivatives from adamantanecarboxylic acids in the system MnO2–H2SO4

A convenient procedure has been developed for the synthesis of mono- and dihydric cage alcohols from adamantanecarboxylic acids and their esters using the MnO2–H2SO4 system. The reaction at elevated temperature involved both decarboxylation and decarbonylation of the initial acid or ester.

Synthesis and Cytotoxicity of bis(pyrazol-1-yl)-Alkane Derivatives with Polymethylene Linkers and Related Mono- and Dipyrazolium Salts

Reactions of α,ω-dibromoalkanes with pyrazole and 3,5-dimethylpyrazole in superbasic medium of dimethyl sulfoxide – potassium hydroxide were used for the synthesis of bidentate ligands: bis(pyrazol-1-yl)alkanes and bis(3,5-dimethylpyrazol-1-yl)alkanes linked by alkyl chains containing between four and twelve methylene groups. Oxidative iodination of the obtained compounds with I2–HIO3–H2SO4 system in acetic acid provided diiodo derivatives: bis(4-iodopyrazol-1-yl)alkanes and bis(4-iodo-3,5-dimethylpyrazol-1-yl)alkanes. The obtained diiodo derivatives of pyrazole were further converted to mono- and dipyrazolium salts by alkylation of nitrogen atom at position 2 of the pyrazole rings with iodomethane, as well as with methyl triflate. These products are of interest as precursors to mesoionic N-heterocyclic carbene complexes. The cytotoxicity of the obtained compounds was studied against THP-1 monocytic leukemia cells.

A temporal semi-stochastic model for pitting corrosion

We have proposed a mechanism that could possibly mimic the well known phenomenon of the halide ions induced oscillations in Me|nM H2SO4 system. It is assumed that close to the Flade potential region or anodic side far from the Flade potential are the scenarios wherein the applied potential is greater than the pitting potential. Consequently, the pitting initiation process occurs with unit probability and hence the stochastic pitting growth process becomes independent from the instantaneous pit depth. This pitting growth phenomenon is captured by time dependent stochastic functions (double exponential) along with the deterministic component. The consequences of mimicking the spatial features of oxide film breakdown by using the time dependent stochastic functions, are also discussed. Furthermore, the additional results have been provided to validate the proposed temporal semi-stochastic form of the model.

Dynamic Electrodeposition of Manganese Dioxide: Temporal Variation in the Electrodeposition Mechanism

The time resolved mechanism of electrodeposition and the effect of this changing mechanism on the nucleation and growth of solid manganese dioxide has been investigated in both acidic and neutral electrolytes on the rotating ring disc electrode (RRDE). The fate of the Mn3+ intermediate is a key feature of this electrodeposition mechanism, the formation of which is dependent on the substrate, which in this case is either platinum, MnO2 or MnOOH. On the platinum surface, for all electrolytes, soluble Mn3+ is produced initially. The stability of this soluble Mn3+ species determines the initial morphology, and rate of change of mechanism for the process. In a neutral electrolyte, nucleation and growth of MnO2 occurs primarily through the precipitation of a 2D film of MnOOH on the platinum, which rapidly covers the surface. Nucleation in an acidic H2SO4 system occurs primarily via a disproportionation route which forms 3D MnO2 hemispheroids that cover the substrate slowly. Subsequent growth of MnO2 in both electrolytes then proceeds via formation of a MnOOH film, which is subsequently oxidized in the solid state to form MnO2. MnOOH oxidation to MnO2 appears kinetically limited, which is overall a limiting factor in the electrodeposition process.