Low-temperature densification of Mg-doped hydroxyapatite fine powders under hydrothermal hot processing conditions

Densification of calcium hydroxyapatite fine powders doped with different concentrations of Mg (2, 4 and 6 mol % Mg, MgHA) was successfully achieved for the first time in a nearly fully dense state using the hydrothermal hot pressing (HHP) technique at low temperatures. Consolidation of MgHA powders was studied under different temperatures (150–240 °C), reaction times (1–6 h), and powder particle size (20 nm–1.5 μm). X-Ray diffraction analyses indicated that the particle densification under HHP conditions proceeded without any variation in the crystalline structure and regardless of the Mg content. The results from this work showed that an increase in temperature accelerates the reaction between MgHA particles and water (solvent) mixed during the hydrothermal treatment. Particle packing associated with bulk densification was achieved through a massive dissolution-recrystallisation mechanism, which induced the formation of small particles that rapidly crystallised on the surface of the partially dissolved original MgHA particles. The optimum conditions to obtain pellets with a high apparent density of 3.0758 ± 0.001 g/cm3 and tensile strength value of 12.6 ± 0.6 MPa were 10 wt% of water at a temperature of 240 °C with a 6 h reaction time and 6 mol % of Mg (MgHA3). The use of the HHP technique coupled with the fine particle size and reactivity of the MgHA precursor powders with water allowed us to produce disks that were compacted to a nearly full dense state with a low content of open porosity of 2.0%

Low-temperature densification of Mg-doped hydroxyapatite fine powders under hydrothermal hot processing conditions

Densification of calcium hydroxyapatite fine powders doped with different concentrations of Mg (2, 4 and 6 mol % Mg, MgHA) was successfully achieved for the first time in a nearly fully dense state using the hydrothermal hot pressing (HHP) technique at low temperatures. Consolidation of MgHA powders was studied under different temperatures (150–240 °C), reaction times (1–6 h), and powder particle size (20 nm–1.5 μm). X-Ray diffraction analyses indicated that the particle densification under HHP conditions proceeded without any variation in the crystalline structure and regardless of the Mg content. The results from this work showed that an increase in temperature accelerates the reaction between MgHA particles and water (solvent) mixed during the hydrothermal treatment. Particle packing associated with bulk densification was achieved through a massive dissolution-recrystallisation mechanism, which induced the formation of small particles that rapidly crystallised on the surface of the partially dissolved original MgHA particles. The optimum conditions to obtain pellets with a high apparent density of 3.0758 ± 0.001 g/cm3 and tensile strength value of 12.6 ± 0.6 MPa were 10 wt% of water at a temperature of 240 °C with a 6 h reaction time and 6 mol % of Mg (MgHA3). The use of the HHP technique coupled with the fine particle size and reactivity of the MgHA precursor powders with water allowed us to produce disks that were compacted to a nearly full dense state with a low content of open porosity of 2.0%

Iron Calcium Carbonate Instability: Structural Modification of Siderite Corrosion Films

Corrosion research related to CO2-containing environments has focused over the past few decades on siderite formation (FeCO3) as a main corrosion product on carbon steel, yet the influence of Ca and other ions on its chemical and structural characteristics is not fully understood. Metal-localized corrosion is the biggest industrial challenge because of the unknown and unpredictable character of this phenomenon that frequently leads to failure. We report here the role of Ca and formation of iron-calcium carbonate (FexCayCO3) through a spiral growth model as in the calcite system and quantify the replacement of Fe2+ by Ca2+ ions in the structure of FeCO3 to form FexCayCO3. The incorporation of Ca2+ inhibits the completion of spiral segments on the growth of the rhombohedral crystals of FeCO3, promoting an enlargement of its structure along the c-axis. This leads to distortions in the chemical structure and morphology affecting the chemical and mechanical properties. Under flow conditions over time in an undersaturated environment, Ca is leached out from the expanded structure of FexCayCO3 increasing the solubility of the crystals, weakening the mechanical properties of the resulting corrosion films and stimulating localized corrosion

Study of a Local Structure at the Interface between Corrosion Films and Carbon Steel Surface in Undersaturated CO₂ Environments

Industries transporting CO2 gas-saturated fluids have infrastructures made of carbon steel. This is a good material with great mechanical properties but prone to corrosion and potential failure. Corrosion in sweet environments involves the formation of FeCO3 as a corrosion film, which is recognized to play a protective role under certain conditions. This work on the dissolution of corrosion films in sweet environments, under acidic and undersaturated conditions, demonstrates that the effects on the integrity of steel are far more significant than the damage observed on the surface of the corrosion film. Our results prove that dissolution of FeCO3 involved the presence of an amorphous phase, the intermediate formation of FeCl2 or FeCl+, and the presence of a phase with short distance atom–atom correlations. The amorphous phase was identified as a mixture of retained γ-Fe and Fe3C. Partially broken α-Fe and Fe3C structures were identified to prove the damage on the material, confirming the interface zone without evident damage on the corrosion film. Dissolution affected both the α-Fe and FeCO3, with the lattice [102̅] from the FeCO3 crystalline structure being the fastest to dissolve. The damage of steel at the molecular scale was evident at the macroscale with pit depths of up to 250 μm. The impact on the integrity of steel can be, therefore, more drastic than frequently reported in industrial operations of CO2 transport industries that use cleaning procedures (e.g., acid treatment, pigging) as part of their operational activities

An Effective Surrogate Tracer Technique for S. aureus Bioaerosols in a Mechanically Ventilated Hospital Room Replica Using Dilute Aqueous Lithium Chloride

Finding a non-pathogenic surrogate aerosol that represents the deposition of typical bioaerosols in healthcare settings is beneficial from the perspective of hospital facility testing, general infection control and outbreak analysis. This study considers aerosolization of dilute aqueous lithium chloride (LiCl) and sodium chloride (NaCl) solutions as surrogate tracers capable of representing Staphylococcus aureus bioaerosol deposition on surfaces in mechanically ventilated rooms. Tests were conducted in a biological test chamber set up as a replica hospital single patient room. Petri dishes on surfaces were used to collect the Li, Na and S. aureus aerosols separately after release. Biological samples were analyzed using cultivation techniques on solid media, and flame atomic absorption spectroscopy was used to measure Li and Na atom concentrations. Spatial deposition distribution of Li tracer correlated well with S. aureus aerosols (96% of pairs within a 95% confidence interval). In the patient hospital room replica, results show that the most contaminated areas were on surfaces 2 m away from the source. This indicates that the room’s airflow patterns play a significant role in bioaerosol transport. NaCl proved not to be sensitive to spatial deposition patterns. LiCl as a surrogate tracer for bioaerosol deposition was most reliable as it was robust to outliers, sensitive to spatial heterogeneity and found to require less replicates than the S. aureus counterpart to be in good spatial agreement with biological results

Influence of pH and Temperature on Struvite Purity and Recovery from Anaerobic Digestate

The precipitation of struvite (MgNH4PO4.6H2O) from wastewater streams simultaneosuly recovers nitrogen (N) and phosphorus (P) for reuse as fertilisers. Struvite crystallisation is controlled by pH, saturation index, temperature and other ions in the solution (e.g., Ca2+, Mg2+ and CO32−). This work studies the effect of pH and temperature on phosphorus and nitrogen removal via struvite precipitation and the quality of the resulting precipitate product (i.e., crystal size, morphology and purity). Struvite was precipitated in batch reactors from the supernatant produced during anaerobic sludge dewatering at a wastewater treatment works, under controlled pH (8, 9 and 10) and temperature (25, 33 and 40 °C) conditions. The optimal P removal as struvite, reduction of the co-precipitation with Ca and the increase in particle size of the struvite precipitates were determined. The results showed that temperatures of 33 °C and 40 °C are not recommended for struvite precipitation—i.e., at 33 °C the purity is lower, and at 40 °C the ammonia losses are induced by volatilisation. At all pH-tests, the P removal efficiency was >93%, but the highest phosphate content and purity as struvite were obtained at a pH of 9.0. The optimum pH and temperature for the formation of large crystals (84 µm) and a high purity (>70%) of the struvite precipitates were 9 and 25 °C, respectively

Nanoparticle Assembly Leads to Mackinawite Formation

Iron sufides are important mineral phases in natural environments where they control global elemental cycles. Fe–S phases have been suggested to form through the transformation of several possible precursors to finally reach stable crystalline structures. Mackinawite is a metastable intermediate, of which a full chemical and structural characteristization of various possible intermediate stages in its formation pathways, or the chemical conditions that affect the transformations to the metastable mackinawite, are well understood. Here we report, the various steps of mackinawite formation via oriented aggregation (OA) from a nanoparticulate precursor. During OA, the formation of aggregates is a crucial stage for self-assembly of primary particles to reach stable structures. The formation occurs in five steps: (1) homogeneous nucleation of primary FeSnano particles; (2 and 3) formation of mass fractal-like aggregates from the FeSnano as precursor toward the transformation to mackinawite; (4) oriented alignment and self-assembly of these mackinawite-like aggregates; and (5) transformation to a still metastable but typical layered mackinawite structure

What controls selenium release during shale weathering?

This study demonstrates that only a combination of a chromous chloride reduction with dual sequential extraction schemes can clearly separate the proportions of Se present in the sulphide versus the organic pools in shales. The data reveals that even small amounts of pyrite outcompete the organic matter for the available Se and pyrite oxidation will control the release of selenium during shale weathering

Mechanistic Insights of Dissolution and Mechanical Breakdown of FeCO3 Corrosion Films

Carbon steel is a universally used material in various transportation and construction industries. Research related to CO2 corrosion environments agrees on the occurrence of siderite (FeCO3) as a main product conforming corrosion films, suggested to impart protection to carbon steel. Identifying and understanding the presence of all corrosion products under certain conditions is of greatest importance to elucidate the behavior of corrosion films under operation conditions (e.g., flow, pH, temperature), but information regarding the nature and formation of other Fe corrosion products apart from FeCO3 is lacking. Corrosion products in CO2 environments typically consist of common Fe minerals that in nature have been demonstrated to undergo transformations, forming other Fe phases. This fact of nature has not been yet explored in the corrosion science field, which can help us to describe mechanisms associated with industrial processes. In this work, we present a multiscale and multidisciplinary approach to understand the mechanisms occurring on corrosion films under the key factors of flow and pH through the combination of molecular techniques with imaging. We report that certainly siderite (FeCO3, cylindrical with trigonal-pyramidal caps) is the main product identified under the conditions used (representative of brine transport at 80 °C), but wustite (FeO) and magnetite (Fe3O4) minerals also form, likely from the de-carbonation of FeCO3 → FeO → Fe3O4, depending on pH under the action of flow. These minerals exist across the corrosion films evidencing a more complex nature of the three-dimensional layer not currently accounted for in the mechanistic models. A relatively low flow velocity (1 m/s), as recognized for industrial operations, is enough to produce chemo-mechanical damage to the FeCO3 crystals, causing breakage at low pH where dissolution of FeCO3 occurs with a rapid crystal size reduction of the cylindrical FeCO3 geometry of ∼80% in just 8 h, changing also the local chemical structure of Fe3C under the film. Similarly, a flow velocity of 1 m/s is capable of inducing crystal removal at neutral pH, promoting further degradation of the steel, compromising the protectiveness assumption of FeCO3 corrosion films. The chemo-mechanical damage and Fe phase transformations will affect the critical localized corrosion, and therefore, they need to be accounted for in mechanistic models aiming to find new avenues for control and mitigation of carbon steel corrosion

The Irish kelp, Fucus vesiculosus, a highly potential green bio sorbent for Cd (II) removal: Mechanism, quantitative and qualitative approaches

Looking for new green and environmentally friendly bio sorbents for metal removal from polluted wastewater, the present study investigates the potential new bio sorbent for Cd(II) removal from wastewater namely, the mechanism and uptake capacity of Cd(II) by brown algae, Fucus vesiculosus from the Irish Sea. This work takes a comprehensive approach involving the combination of qualitative and quantitative information collected from macro to atomistic scale, in a direct and non-destructive manner. Our results demonstrate that Cd(II) is adsorbed on the algal surface based on carboxylic of alginate groups. Effective Cd(II) adsorption is achieved at pH conditions between 5 and 7, at which the uptake occurs rapidly (∼2 h), with increasing Cd(II) concentration. Cd maximum uptake capacity (i.e., 1.203 mmol Cd g−1 dried algae) in first adsorption cycle show superior uptake as opposed to other species. Quantitatively the bio sorbent has an increasing uptake capacity (more than two folds) in the second cycle, after metal elution and biomass surface sites functioning. Desorption of Cd(II) and the regeneration of the biomass is effectively achieved with HCl (10 mM) and EDTA (1 mM), but they can only be used for two cycles, before the efficiency decreases. Microprecipitation occurs at high pH (>9) when using NaOH as an eluent. Results from this work shed new light on understanding Cd(II) binding mechanisms on Fucus v., providing crucial information for further process optimization, pilot testing, scaling up and implementation as a clean, environmentally friendly biotechnology applied to wastewater treatments