Mechanochemical synthesis of kaolin-KH2PO4 and kaolin-NH4H2PO4 complexes for application as slow release fertilizer

A milling process to reduce kaolin to amorphous phase in the presence of KH₂PO₄ or NH₄H₂PO₄ and allow mechanochemical (MC) reaction for incorporation of KH₂PO₄ and NH₄H₂PO₄ into the kaolin structure was investigated in this work. Mixtures of kaolin and KH₂PO₄ and NH₄H₂PO₄ in separate systems were prepared by milling in a planetary ball mill. Tests with kaolin contents ranging from 25 to 75 wt.% and mill rotational speeds from 200 to 700 rpm were performed to evaluate incorporation of KH₂PO₄ and NH₄H₂PO₄ and release of K⁺, NH₄⁺ and PO₄³⁻ ions into solution. Analyses by XRD, DTA and ion chromatography indicated that the MC process was successfully applied to incorporate both KH₂PO₄ and NH₄H₂PO₄ into the amorphous kaolin structure. Release of K⁺ and PO₄³⁻ ions from the system (kaolin-KH₂PO₄) when dispersed in water for 24 h reached only up to 10%. Under similar conditions for the system (kaolin-NH₄H₂PO₄), release of NH₄⁺ and PO₄³⁻ ions reached between 25 and 40%. These results indicated that the MC process can be developed to allow amorphous kaolin to act as a carrier of K⁺, NH₄⁺ and PO₄³⁻ nutrients to be released slowly for use as fertilizer.     

Modification of titanium oxide layer by calcium and phosphorus

The aim of the study was to investigate the possibility of calcium and phosphorus ion implantation into an oxide film applied onto titanium during anodic passivation. The corrosion resistance of modified titanium in Tyrode's physiological solution has been identified. Anodic oxidation was carried out in two solutions. The first contained 20 g dm⁻³ NaH₂PO₂ in 4.3 M H₃PO₄ (K1), whereas the other, 20 g dm⁻³ Ca(H₂PO₂)₂ in 4.3 M H₃PO₄ (K2). Voltage of 100 and 150 V was applied. It has been found out that it is possible to incorporate Ca and P into the emerging passive layer. The application of the voltage of 150 V makes it very porous. It has been also demonstrated that titanium so modified presents higher resistance to corrosion in the investigated environment than titanium not modified in Tyrode's solution.     

Continuous salt precipitation and separation from supercritical water. Part 1: Type 1 salts

The precipitation and separation performance of various binary type 1 salt-water mixtures was systematically studied for the first time in a continuously operated laboratory plant. The aim was to find a field of operation for the salt separator where salts can be separated with high efficiency. Experiments with aqueous solutions of the salts NaNO₃, KNO₃, Ca(NO₃)₂, K₂CO₃, KHCO₃, (NH₄)₂CO₃, K₃PO₄, K₂HPO₄, KH₂PO₄, NaCl, KCl, NH₄Cl and (NH₄)₂SO₄ were carried out at 30 ± 0.5 MPa varying the salt separator temperature from sub-critical to supercritical. For most of these salts separation efficiencies ranging from 80 to 97% were obtained. For the nitrates the separation efficiency increased in the order NaNO₃ < KNO₃ < Ca(NO₃)₂, whereas for potassium salts the separation efficiency of the phosphates was significantly higher than that of KNO₃. Considerable hydrolysis of the phosphate and the hydrogen phosphate salts in supercritical water was found, although this had no negative influence on the phosphate separation efficiency. It was found that the ammonium salts decompose in supercritical water, probably to ammonia and the corresponding mineral acids, leading to reduced separation of the ammonia due to its high solubility in supercritical water.     

Effects of phosphate species on localised corrosion of steel in NaHCO3 + NaCl electrolytes

Pitting corrosion of carbon steel electrodes in 0.1 M NaHCO₃ + 0.02 M NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E_b with the phosphate concentration was investigated by linear voltammetry. E_b increased from −15 ± 5 mV/SCE for [HPO₄²⁻] = 0 to 180 ± 40 mV/SCE for [HPO₄²⁻] = 0.02 mol L⁻¹. During anodic polarisation (E = 50 mV/SCE), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via the scanning vibrating electrode technique (SVET). The addition of a Na₂HPO₄ solution after the beginning of the polarisation did not lead to the repassivation of pre-existing well-grown pits. The corrosion products forming in the pits were identified in situ by micro-Raman spectroscopy. They depended on the phosphate concentration. For [HPO₄²⁻] = 0.004 mol L⁻¹, siderite FeCO₃ was detected first. It was oxidised later into carbonated green rust GR(CO₃²⁻) by dissolved O₂. The beginning of the process is therefore similar to that observed in the absence of phosphate. Finally, GR(CO₃²⁻) was oxidised into ferrihydrite, the most poorly ordered form of Fe(III) oxides and oxyhydroxides. Phosphate species, adsorbing on the nuclei of FeOOH, inhibited their growth and crystallisation. For [HPO₄²⁻] = 0.02 mol L⁻¹, siderite was accompanied by an amorphous precursor of vivianite, Fe₂(PO₄)₃·8H₂O. This shows that, in any case, phosphate species interact strongly with the iron species produced by the dissolution of steel.     

Preparation of hydroxyapatite coating on smooth implant surface by electrodeposition

A novel process for the deposition of a hydroxyapatite (HA) coating on a smooth implant surface has been developed. Specimens were firstly subjected to electrodeposition at −1.8 V (versus Ag/AgCl) in a mixed solution of 0.042 M Ca(NO₃)₂·4H₂O and 0.025 M NH₄H₂PO₄ at 85 °C for 5 s, and then post-treated in 1 M NaOH solution for 30 min. The experimental results showed the specimens prepared by the designed process to have better adhesion properties than those prepared by the traditional electrodeposition process.     

Electrochemical performance of the carbon coated Li3V2(PO4)3 cathode material synthesized by a sol-gel method

A carbon coated Li₃V₂(PO₄)₃ cathode material for lithium ion batteries was synthesized by a sol-gel method using V₂O₅, H₂O₂, NH₄H₂PO₄, LiOH and citric acid as starting materials, and its physicochemical properties were investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDAX), transmission electron microscope (TEM), and electrochemical methods. The sample prepared displays a monoclinic structure with a space group of P2₁/n, and its surface is covered with a rough and porous carbon layer. In the voltage range of 3.0-4.3 V, the Li₃V₂(PO₄)₃ electrode displays a large reversible capacity, good rate capability and excellent cyclic stability at both 25 and 55 °C. The largest reversible capacity of 130 mAh g⁻¹ was obtained at 0.1C and 55 °C, nearly equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹). It was found that the increase in total carbon content can improve the discharge performance of the Li₃V₂(PO₄)₃ electrode. In the voltage range of 3.0-4.8 V, the extraction and reinsertion of the third lithium ion in the carbon coated Li₃V₂(PO₄)₃ host are almost reversible, exhibiting a reversible capacity of 177 mAh g⁻¹ and good cyclic performance. The reasons for the excellent electrochemical performance of the carbon coated Li₃V₂(PO₄)₃ cathode material were also discussed.     

Comportement electrochimique de l'uranium,du fer et du vanadium,dans l'acide orthophosphorique et dans le melange H3 PO4 5M—HF 1M. Etude cinetique de la reduction de l'uranium (VI) par le fer (II)

The electrochemical behaviour of uranium, iron and vanadium in 0,1 M–10 M phosphoric acid solutions, has been investigated by polarography, cyclic voltammetry and controlled-potential electrolyses. In concentrated phosphoric acid solutions (|H₃ PO₄| > 3 M), uranium (VI) can be reduced by iron (II). Kinetic measurements of the reduction in 5 M H₃PO₄ and 5 M H₃PO₄—1 M HF mixtures have been carried out.     

Electrochemical hydrogen reactions on tungsten carbide

The kinetics of hydrogen ionisation and electrochemical evolution on a smooth rotating tungsten carbide electrode have been investigated. Polarisation curves in 1 N H₂SO₄, H₃PO₄ and HCl solutions, in the temperature range of 20?80°C, have been obtained. For hydrogen ionisation at ?_r=0·3 V the apparent activation energies (Kcal/moles) were 8·1 in H₂SO₄, 7·4 in H₃PO₄ and 12·8 in HCl. At potentials of ?_r>0·4V a decrease in the hydrogen ionisation rate was observed, explained as a decrease in the number of active surface sites due to surface oxidation and adsorption of anions. However, after a moderate anodic polarisation not leading to the formation of a thick tungsten carbide oxides layer, reiterated recording of the polarisation curve yields an increase in electrode activity.In all acids the first order of reaction with respect to molecular hydrogen was found. The hydrogen ionisation rate was independent of the pH-value of the solution. These results showed adsorption, accompanied by molecular hydrogen dissociation, to be the rate-determining step.Modifying the concentration of sulphuric acid in the presence of excess normal salt or without it, as well as passing from acidic to alkaline solutions caused no change in the course of the polarisation curves. This suggests that recombination of adsorbed hydrogen atoms into molecules on tungsten carbide is the rate determinating step, ie, the same step as in the reverse process of hydrogen ionisation.     

An EQCM study of the deposition and doping/dedoping behavior of polypyrrole from phosphoric acid solutions

The potentiostatic deposition of polypyrrole (PPy) from 0.1 M aqueous phosphoric acid solution and the doping/dedoping behavior of the resulting films have been investigated by the electrochemical quartz crystal microbalance (EQCM) technique. The change of the complex shear modulus during the film growth and during the doping/dedoping were calculated using the acoustic impedance method. It was found that the films contain only ca. 1 wt% water in the oxidized state which is in accordance with their relatively high storage modulus of 38 MPa and low loss tangent of 0.05. Comparing H₃PO₄ (pH 1) and NaH₂PO₄ (pH 8) it was found that the doping/dedoping in the latter is accompanied by a strong increase of the surface roughness which is a direct result of cation exchange present at pH 8.     

A comparison of crystal growth kinetics by use of chemical potential difference and concentration difference

A Spherical Void Electrodynanic Levitator Trap (SVELT) was used to measure the water activity in the supersaturated aqueous solutions of six materials, NaCl, (NH₄)₂SO₄, KH₂PO₄, NH₄H₂PO₄, KA1(SO₄)₂ · 12H₂O and glycine. The relationship between chemical potential and concentration was obtained using a fifth order polynomial fit. A comparison of the order of the crystal growth rate obtained from the use of chemical potential difference and concentration difference was made. The order of crystal growth rate calculated from concentration difference was found to be close to that obtained from chemical potential difference at low supersaturation, while at higher supersaturation a deviation was noted.     

Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions

The electrochemical reduction of oxygen on thin-film platinum electrodes in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions has been investigated using the rotating disk electrode (RDE) method. Thin films of Pt (0.25-20 nm thick) were prepared by vacuum evaporation onto glassy carbon substrate. The surface morphology of Pt films was examined by transmission electron microscopy (TEM). The specific activity of O₂ reduction was higher in HClO₄ and decreased with decreasing film thickness. In H₂SO₄, the specific activity was lower and appeared to be independent of the Pt loading. The values of Tafel slopes close to −120 mV dec⁻¹ in high current density range and −60 mV dec⁻¹ in low current density range were obtained for all electrodes in both solutions, indicating that the mechanism of O₂ reduction is the same for thin-film electrodes as for bulk Pt. The number of electrons transferred per O₂ molecule was close to four for all thin Pt films studied.     

Studies on permeation of uranium (VI) from phosphoric acid medium through supported liquid membrane comprising a binary mixture of PC88A and Cyanex 923 in n-dodecane as carrier

Present studies deal with the application of supported liquid membrane (SLM) technique for the separation of uranium (VI) from phosphoric acid medium using a binary mixture of 2-ethyl hexyl phosphoric acid-mono-2-ethyl hexyl ester (PC88A) and neutral donor which is a mixture of four tri-alkyl phosphine oxide better known as Cyanex 923 in n-dodecane as a carrier and (NH₄)₂CO₃ as a receiving phase. Various parameters like feed acidity, nature of strippant, carrier concentration, membrane pore size, membrane thickness etc. which affect the transport of U(VI) have been studied in detail. Experiments have also been carried out to see the transport behaviour of different fission products from a diluted High Level Waste (HLW) solution. Stability of the membrane against the leaching of the extractant and stability of the membrane support have also been investigated. We have tried to model the physicochemical transport of U(VI) in SLM as well as establishing the mechanism (Diffusion controlled) of transport. More than 95% uranium (VI) is recovered in 360 min using a binary mixture of 0.60 M PC88A and 0.15 M Cyanex 923 in n-dodecane as carrier and 0.5 M (NH₄)₂CO₃ as stripping phase from the 0.5 M H₃PO₄ feed. Lower concentration of H₃PO₄ (0.5 M) and optimum carrier concentration (0.60 M PC88A + 0.15 M Cyanex 923) in the mole ratio of 4:1 is found to be the most suitable condition for maximum transport of uranium (VI). The optimum conditions obtained from this study was also applied to recover uranium from analytical waste in phosphoric acid medium generated in the laboratory.     

Characterization and performance of hydrous manganese oxide prepared by electrochemical method and its application for supercapacitors

Hydrous manganese oxide was deposited on graphite substrates at anodic potentials of 0.5-0.95 V versus saturated calomel electrode (SCE) in 0.25 M Mn(CH₃COO)₂ solution at 25 °C. Morphology of manganese oxide prepared was examined by scanning electron microscopy (SEM). Manganese oxide deposited at various anodic potentials was evaluated by cyclic voltammetry with various potential scan rates in different electrolytes. Results indicated that the pseudocapacitive behaviors of manganese oxide were excellent both in 2 M KCl and 2 M (NH₄)₂SO₄ solutions at room temperature. Manganese oxide deposited at 0.5 V versus SCE showed better capacitive behaviors, the specific capacitances were 275 F/g in 2 M KCl solution and 310 F/g in 2 M (NH₄)₂SO₄ solution, respectively. Besides, better electrochemical reversibility could be obtained in 2 M KCl solution.     

New mechanism of gas transport at the interface solid/liquid

It was recently shown that an abnormally fast transport of CO molecules takes place at the electrode/electrolyte interface of Pt and PtRu electrodes in H₂SO₄ and HClO₄ solutions. In the present paper, this phenomenon is tested for other gases, such as hydrogen and oxygen. The fast transport is also observed at the solid/electrolyte solution interface of other electrode materials and at the glass/electrolyte interface. Several experiments are shown, demonstrating that mass transfer takes place at a velocity, which is more than one order of magnitude higher than expected for usual diffusion conditions.Assuming radial mass transfer at the interface of a Pt disc, the activation energy, E_a = 23 kJ mol⁻¹, was calculated from Arrhenius plots. The same value was measured in H₂SO₄ and HClO₄ as supporting electrolytes. The mass transport parameter, Y, at 298 K was 4.8 × 10⁻³ cm² s⁻¹ and 2.9 × 10⁻³ cm² s⁻¹ in 0.5 M H₂SO₄ and 1 M HClO₄ respectively.     

Corrosion inhibition of mild steel by natural product compound

Corrosion inhibition of mild steel in H₃PO₄ containing chloride or sulphate ions have been studied using different electrochemical techniques. The corrosion and hydrogen evolution of mild steel alloy in 2 M H₃PO₄ acid containing 0.5 M NaCl can be effectively inhibited by addition of natural product compound, Thymol (IPMP), of different concentrations. However, in 2 M H₃PO₄ containing 0.5 M Na₂SO₄ corrosion cannot be effectively inhibited. The results of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements confirm the synergistic effects which describe the increase in the effectiveness of a corrosion inhibitor in the presence of Cl⁻ ions in the corrosive medium. At any temperature, an increase in it leads to an increase of the corrosion rate and hydrogen evolution on mild steel. Polarization and EIS results are in good agreement with each other. The obtained results were confirmed by surface examination using scanning electron microscope.     

Dissolution of serpentine using recyclable ammonium salts for CO2 mineral carbonation

Low efficiency of mineral dissolution and unrecyclable use of additives are two barriers for the development of CO₂ mineral carbonation. A new pH-swing CO₂ mineralisation process using recyclable ammonium salts is proposed to overcome these barriers. This paper presents the studies of mineral dissolution with ammonium salts. In this study, dissolution of a serpentine sample was performed using a series of aqueous solutions, which include (NH₄)₂SO₄, NH₄Cl, NH₄HSO₄ and H₂SO₄. NH₄HSO₄ is the most efficient in terms of extracting Mg extraction from the serpentine sample. At 100 °C 1.4 M NH₄HSO₄ extracted 100% of Mg from serpentine in 3 h, as well as 98% of Fe and 17.6% of Si. The rate limiting mechanism of serpentine dissolution with NH₄HSO₄ is a chemical reaction with product layer diffusion control and the activation energy of this dissolution was 40.9 kJ mol⁻¹.     

Synthesis and electrochemical performance of LiNi0.5Mn1.5O4 spinel compound

Spinel compound LiNi_0.5Mn_1.5O₄ was synthesized by a chemical wet method. Mn(NO₃)₂, Ni(NO₃)₂·6H₂O, NH₄HCO₃ and LiOH·H₂O were used as the starting materials. At first, Mn(NO₃)₂ and Ni(NO₃)₂·6H₂O reacted with NH₄HCO₃ to produce a precursor, then the precursor reacted with LiOH·H₂O to synthesize product LiNi_0.5Mn_1.5O₄. The product showed a single spinel phase under appropriate calcination conditions, and exhibited a high voltage plateau at about 4.6-4.8 V in the charge/discharge process. The LiNi_0.5Mn_1.5O₄ had a discharge specific capacity of 118 mAh/g at about 4.6 V and 126 mAh/g in total in the first cycle at a discharge current density of 2 mA/cm². After 50 cycles, the total discharge capacity was above 118 mAh/g.     

Corrosion resistance of plasma-anodized AZ91D magnesium alloy by electrochemical methods

Anodic coatings formed on magnesium alloys by plasma anodization process are mainly used as protective coatings against corrosion. The effects of KOH concentration, anodization time and current density on properties of anodic layers formed on AZ91D magnesium alloy were investigated to obtain coatings with improved corrosion behaviour. The coatings were characterized by scanning electron microscopy (SEM), electron dispersion X-ray spectroscopy (EDX), X-ray diffraction (XRD) and micro-Raman spectroscopy. The film is porous and cracked, mainly composed of magnesium oxide (MgO), but contains all the elements present in the electrolyte and alloy. The corrosion behaviour of anodized Mg alloy was examined by using stationary and dynamic electrochemical techniques in corrosive water. The best corrosion resistance measured by electrochemical methods is obtained in the more concentrated electrolyte 3 M KOH + 0.5 M KF + 0.25 M Na₃PO₄·12 H₂O, with a long anodization time and a low current density. A double electrochemical effects of the anodized layer on the magnesium corrosion is observed: a large inhibition of the cathodic process and a stabilization of a large passivation plateau.     

Electrochemical behavior of AZ91D magnesium alloy in phosphate medium—part I. Effect of pH

The influence of pH on the corrosion behavior of Mg-based AZ91D alloy was investigated in a constant composition phosphate medium using various electrochemical techniques, complemented with surface analysis data. The studied solutions were 0.1 M H₃PO₄, NaH₂PO₄, Na₂HPO₄ and Na₃PO₄ having pH values of 1.8, 4.5, 9.1 and 11.8, respectively. Spontaneous passivation was substantiated from monitoring the continuous positive shift of the open circuit corrosion potential with both immersion time and solution pH. The impedance data indicated more improvement in the insulating properties of the corrosion products formed on the alloy surface with increase in pH. The electrolyte pH plays a determinant influence on surface film properties, as films formed in phosphate solutions with higher pH values are thicker, thus affording better protection for the alloy than those formed in acidic solutions. Good agreement was observed between the results obtained from electrochemical techniques and those from EDX and XRD examinations. The alloy is more susceptible to corrosion in acidic phosphate solutions than in the alkaline ones. Crystalline magnesium (Mg), magnesium hydride (MgH₂) and magnesium oxide (MgO) were found to be the main constituents of the surface film after holding for 2 h in the acidic phosphate medium.     

Mass transportation in diethylmethylammonium trifluoromethanesulfonate for fuel cell applications

To use the protonic mesothermal fuel cell without humidification, mass transportation in diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), trifluoromethanesulfuric acid (TfOH)-added [dema][TfO], and phosphoric acid (H₃PO₄)-added [dema][TfO] was investigated by electrochemical measurements. The diffusion coefficient and the solubility of oxygen were ca. 10⁻⁵ cm² s⁻¹ and ca. 10⁻³ M (=mol dm⁻³), respectively. Those of hydrogen were a factor of 10 and one-tenth compared to oxygen, respectively. The permeability, which is a product of the diffusion coefficient and solubility, of oxygen and hydrogen were almost the same for the perfluoroethylenesulfuric acid membrane and the sulfuric acid solution; therefore, these values are suitable for fuel cell applications. On the other hand, a diffusion limiting current was observed for the hydrogen evolution reaction. The current corresponded to ca. 10⁻¹⁰ mol cm⁻¹ s⁻¹ of the permeability, and the diffusion limiting species was the hydrogen carrier species. The TfOH addition enhanced the diffusion limiting current of [dema][TfO], and the H₃PO₄ addition eliminated the diffusion limit. The hydrogen bonds of H₃PO₄ or water-added H₃PO₄ might significantly enhance the transport of the hydrogen carrier species. Therefore, [dema][TfO] based materials are candidates for non-humidified mesothermal fuel cell electrolytes.