Frederico Marques Penha

This study investigated the dissolution of niobic acid (Nb2O5·nH2O) and potassium niobate (mix of phases K4Nb6O17 and KNbO3) using the polythermal method in different concentrations of a KOH solution (0–2 mol/L). The software OLI Studio Stream Analyzer and PHREEQC were used for the thermodynamic modeling. The equilibrium time of niobic acid dissolution in an alkaline medium was investigated. Aqueous niobium concentration was measured by ICP-OES and UV–vis spectroscopy methods. The measured dissolved Nb concentrations at 30 °C show deviations from predicted values, indicating the lack of niobium solubility data. The experimental results from the polythermal method evidenced that the dissolution of niobic acid increases with temperature and decreases with KOH concentration. Nevertheless, potassium niobate exhibited a complex behavior, with different phases dissolving preferentially under the studied conditions (0–1 mol/L). Our findings contribute to the understanding of niobium chemistry in alkaline medium, crucial for the development of hydrometallurgical processing of niobium primary sources, which is directly related to the efficiency of leaching processes, crystallization control, and subsequent recovery steps of secondary sources.

Marine sediments represent a valuable source for critical minerals that prompts growing research interest in their potential for sustainable resource recovery. This study investigated the impact of bioleaching microorganisms, evaluating three different microbial inocula: sediment indigenous Fe-oxidizing bacteria, sediment indigenous Fe- and S-oxidizing bacteria, and Acidhiobacillus ferrooxidans. Key operational parameters including initial pH (2.0-7.0), energy source availability (Fe and S), and total solids content (5-10%) were also systematically varied to optimize bioleaching efficiency. The highest metal bioleaching for Al, Mg, and K was achieved using Fe- and S-oxidizing bacteria enriched from the marine sediment. The maximum concentrations in the leachate were detected as 806 mg/L (17.6%), 506 mg/L (65.7%), and 169 mg/L (22%) for Al, Mg and K, respectively. Alicyclobacillaceae was one of the key microorganisms which dominated microbial community over time. On the other hand, Sulfuristis played a key role in the early shifts in the microbial community. Additionally, Least Square Boosting (LSBoost), Random Forest Regression (RF), and Linear Regression (LR) were employed to evaluate the influence of experimental variables. All models demonstrated predictive performance for critical mineral release from the sediment, with LSBoost showing superior predictive accuracy.

This study investigates the precipitation of niobic acid (Nb ₂ O ₅ . n H ₂ O) from potassium niobate solution with addition of sulphuric acid, and its subsequent thermal conversion into niobium oxide (Nb ₂ O ₅ ). Thermodynamic modelling with PHREEQC and OLI Studio was used to predict the optimal conditions for niobic acid formation. Precipitation with different H ₂ SO ₄ concentrations produced amorphous niobic acids similar to commercial reference, with impurity removal optimized through a washing protocol. X‐ray fluorescence (XRF) analysis indicated that precipitation using 0.50 mol/L H ₂ SO ₄ and a solid‐to‐liquid ratio of 20 g/L yielded the highest niobium content (68.93 wt.%). Characterization revealed that residual potassium promoted the formation of crystalline potassium niobium oxide phases (K ₂ Nb ₈ O ₂₁ , K ₂ Nb ₁₄ O ₃₆ ) during calcination, particularly in samples with ~2 wt.% potassium, which exhibited rod‐like agglomerates (~30 μm). Precipitate washing at low solid‐to‐liquid ratios significantly reduced potassium content in the product. A decrease from 60 to 20 g/L reduced the potassium concentration by a factor of three (to 0.06 wt.%), eliminating potassium niobium oxide phases. The pure Nb ₂ O ₅ phase, after the 20 g/L‐washing, was confirmed by X‐ray diffraction (XRD) and transmission electron microscopy–energy dispersive X‐ray spectroscopy (TEM‐EDS). These results emphasize the importance of supersaturation control, calcination conditions, and purification protocols in producing high‐purity niobium oxide from secondary niobium sources.

This study explores fractional and simultaneous precipitation methods to recover metals from a synthetic solution containing the major components from lithium-ion battery recycling leachates: Co, Ni, Mn, Li, and H ₂ SO ₄ . Thermodynamic simulations analyzed the behavior of the metal-bearing solutions during hydroxide precipitation to guide process design. The fractional precipitation process was divided into three steps: pH-adjustment (D1), Co and Ni recovery (D2), and Mn recovery (D3). D2 achieved 89.7% Ni and 76.8% Co recovery; alongside Mn and Li were also removed (15% and 25% respectively). D3 showed mainly Mn recovery (68%) along with 18.7% Co and 7.3% Ni. Simultaneous precipitation resulted in over 99.7% recovery of Co, Ni, and Mn, with a small amount of Li (15%) being recovered from the solution. Na removal from the solution was observed across all experiments. X-ray diffraction analysis revealed that the phases formed were distinct from the predictions. Regardless of the presence of NH ₄ OH as a chelating agent in solution, a mixed nickel-cobalt-manganese oxide could be obtained after calcination. This approach offers a potentially less laborious method for recovering metals in products relevant to cathode precursors in a single step from recycling leachate.

This study explores the dual challenge of enhancing the properties of protein‐based foams produced from agricultural by‐products through the incorporation of red mud waste from alumina production. Foams were manufactured using an extrusion process, employing gluten and zein proteins, with raw red mud and its oxalic acid leachates serving as additives. A factorial design was utilized to assess the significance of various parameters on the mechanical properties of materials. The results indicate that red mud‐based additives do not improve foam mechanical stability in terms of stiffness (as measured by Young's modulus) and thus do not function effectively to form short crosslinking bridges. However, the results show red mud serves mainly as a plasticizer and reducing/oxidizing agent, while also potentially enhancing the formation of long crosslinking bridges. This is evidenced by a significant increase in foam strain when red mud powder is extruded with gluten, reaching 190% strain at break and densities between 500 and 1500 kg/m³. Consequently, red mud shows potential to be repurposed as an additive in protein‐based foams, suitable for applications requiring elastic deformation while keeping a stable porous structure manufactured via continuous extrusion.

Calcium oxalate (CaOx) crystallization under laminar flow conditions, relevant for kidney stone formation, was studied in a microfluidic device simulating the geometry of kidney collecting ducts. In a typical microfluidic experiment, two reactive solutions with designated concentrations of calcium (Ca) and oxalate (Ox) ions were brought into contact in a microfluidic channel to create a laminar co-current flow of the two streams. As the streams flow co-currently in the channel, diffusion takes place between the two streams across the channel width, resulting in reactive crystallization leading to CaOx nucleation and growth of CaOx crystals along the mixing front. We studied the growth of these crystals in artificial urine as a function of the fluid flow rate in the channel, the molar ratio of Ca : Ox in the medium and the presence of an organic protein, osteopontin (OPN), known to inhibit the growth of CaOx crystals. Three different flow velocities at a fixed molar ratio of Ca : Ox = 7.5 and four molar ratios of Ca : Ox at a fixed mean flow velocity of 0.035 m s⁻¹ were tested. Lastly, three additive OPN concentrations were evaluated: 2.4 × 10⁻⁸ mol m⁻³, 6 × 10⁻⁸ mol m⁻³ and 8.4 × 10⁻⁸ mol m⁻³. The mean flow velocity did not alter the crystal growth of CaOx in the studied range, whereas altering the molar ratio of Ca : Ox had a high impact on the growth rate. In addition, the type of pseudopolymorph which nucleated appears to depend strongly on the molar ratio. At a low Ca : Ox ratio, both calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD) nucleated simultaneously and the growth of the two pseudopolymorphic forms of CaOx crystals was observed. The lowest applied OPN concentration decreased the growth rate of COD, while higher concentrations of OPN slowed down the nucleation kinetics to a point that it completely inhibited the formation of any CaOx crystal in artificial urine within the investigated timeframe. COD was seen under all the conditions investigated, whilst COM was seen in experiments for Ca : Ox molar ratio values between 5 and 6. Our results were rationalized using finite element simulations supported by solution chemistry modelling.

Herein, we study the influences of the laser-exposed volume and the irradiation position on the nonphotochemical laser-induced nucleation (NPLIN) of supersaturated potassium chloride solutions in water. The effect of the exposed volume on the NPLIN probability was studied by exposing distinct milliliter-scale volumes of aqueous potassium chloride solutions stored in vials at two different supersaturations (1.034 and 1.050) and laser intensities (10 and 23 MW/cm²). Higher NPLIN probabilities were observed with increasing laser-exposed volume as well as with increasing supersaturation and laser intensity. The measured NPLIN probabilities at different exposed volumes are questioned in the context of the dielectric polarization mechanism and classical nucleation theory. No significant change in the NPLIN probability was observed when samples were irradiated at the bottom, top, or middle of the vial. However, a significant increase in the nucleation probability was observed upon irradiation through the solution meniscus. We discuss these results in terms of mechanisms proposed for NPLIN.