No abstract

a b s t r a c tThe effect of temperature on nickel sulfide bioleaching was studied in the presence of mesophile (Acidithiobacillus ferrooxidans) and moderate thermophile (Sulfobacillus thermosulfidooxidans) strains and the results were discussed in terms of sulfide dissolution thermodynamics (Eh-pH diagrams) and kinetics (cyclic voltammetry). It was observed that in the pH range 1.8-2.0 the highest nickel dissolution was achieved which reached 50% for mesophiles and over 80% for moderate thermophiles. External ferrous iron addition had no effect on the metal dissolution at 34°C, but adversely affected nickel leaching at higher temperatures. The best outcomes were accomplished with low FeSO 4 additions (2.5 g/L) at 50°C. Pyrrhotite dissolution avoided the need for external iron addition, providing Fe 2+ concentrations as high as 7 g/L during bioleaching, which supports bacterial growth. Eh-pH diagrams for pentlandite and pyrrhotite show a negligible effect of temperature on the stability field of each sulfide whilst cyclic voltammetry indicated that temperature has the strongest influence on pyrrhotite oxidation. The latter along with a rapid increase in solution potential (Eh) explains the higher and faster extraction observed with S. thermosulfidooxidans.

The kinetics of the thermal decomposition of a synthetic ettringite sample was studied between 298 and 820 K in an inert atmosphere for the present work. The ettringite and its thermal decomposition products were characterized using X-ray diffraction, infrared spectroscopy, and scanning electron microscopy. Four endothermic events were observed with thermogravimetry curves, the maxima of which occurred at 366, 397, 537, and 641 K. All events were associated with the loss of water molecules with different degrees of interaction within the ettringite structure. Chemical equations for each decomposition step were proposed based on the percentages of mass loss observed. In addition, for the first time, the activation energies of each ettringite decomposition events were determined by the isoconversional methods of Ozawa-Flynn-Wall, Friedman, and Kissinger-AkahiraSunose. The modeling revealed that the activation energy varied from *50 kJ mol -1 , characteristic of mass transfer control steps, to *150 kJ mol -1 , which is typical of chemical control, as the temperature increased and the ettringite structure lost water. A total of 32 mol of water was released equivalent to 43.1 % of the initial sample mass.

No abstract

Acid mine drainage is a natural process occurring when sulfide minerals such as pyrite are exposed to water and oxygen. The bacterially catalyzed oxidation of pyrite is particularly common in coal mining operations and usually results in a low-pH water polluted with toxic metals and sulfate. Although high sulfate concentrations can be reduced by gypsum precipitation, removing lower concentrations (below 1200 mg/L) remains a challenge. Therefore, this work sought to investigate the application of ion exchange resins for sulfate sorption. The macroporous type 1 strong base IX resin Purolite A500 was selected for bath and fixed-bed sorption experiments using synthetic sulfate solutions. Equilibrium experiments showed that sulfate loading on the resin can be described by the Langmuir isotherm with a maximum uptake of 59 mg mL-resin(-1). The enthalpy of sorption was determined as +2.83 kJ mol(-1), implying an endothermic physisorption process that occurred with decreasing entropy (-15.5 J mol(-1).K(-1)). Fixed-bed experiments were performed at different bed depths, flow rates, and initial sulfate concentrations. The Miura and Hashimoto model predicted a maximum bed loading of 25-30 g L-bed(-1) and indicated that both film diffusion (3.2 × 10(-3) cm s(-1) to 22.6 × 10(-3) cm s(-1)) and surface diffusion (1.46 × 10(-7) cm(2) s(-1) to 5.64 × 10(-7) cm(2) s(-1)) resistances control the sorption process. It was shown that IX resins are an alternative for the removal of sulfate from mine waters; they ensure very low residual concentrations, particularly in effluents where the sulfate concentration is below the gypsum solubility threshold.

In a recent paper, novel polyester nanocomposites reinforced with up to 3 wt% of cellulose nanocrystals (CNCs) extracted from conifer fiber were characterized for their crystallinity index, water absorption, and flexural and thermal resistance. The use of this novel class of nanocomposites as a possible substitute for conventional glass fiber composites (fiberglass) was then suggested, especially for the 1 and 2 wt% CNC composites due to promising bending, density, and water absorption results. However, for effective engineering applications requiring impact and tensile performance, the corresponding properties need to be evaluated. Therefore, this extension of the previous work presents additional results on Izod and tensile tests of 1 and 2 wt% CNC-reinforced polyester composites, together with a comparative cost analysis with fiberglass. The chemical effect caused by incorporation of CNCs into polyester was also investigated by FTIR. In comparison to the neat polyester, the Izod impact energy increased 50% and 16% for the 1 and 2 wt% composites, respectively. On the other hand, the tensile strength and Young’s modulus remained constant within the ANOVA statistical analysis. FTIR analysis failed to reveal any chemical modification caused by up to 2 wt% CNC incorporation. The present impact and tensile results corroborate the promising substitution of a polyester composite reinforced with very low amount of CNCs for common fiberglass in engineering application.

No abstract

No abstract