Behavior of Fe^2+/3+ Cation and Its Interference with the Precipitation of Mg²⁺ Cation upon Mineral Carbonation of Yallourn Fly Ash Leachate under Ambient Conditions

A variety of leachates derived from the acid leaching of a unique Fe-rich fly ash, namely Yallourn from the Latrobe Valley of Australia at different temperatures, have been processed to achieve two key goals: synthesis of Fe-rich precipitate and mineral carbonation of alkaline earth metal cations for the storage and utilization of carbon dioxide (CO2). The behavior and interference of unprecipitated Fe-bearing cations on the carbonation stage was for the first time examined by us. The research findings are also applicable to the leachate derived from other industry wastes and even natural minerals, which also contain varying amounts of iron as one impure metal. To precipitate Fe out of the leachate, sodium hydroxide (NaOH) was added to adjust the pH to 4. Subsequently, the pH of the resultant supernatant was further increased to ∼13 and bubbled with CO2 to precipitate the remaining cations. As has been confirmed through the characterization of solid products by synchrotron Fe K-edge X-ray adsorption spectroscopy (XAS), quantitative X-ray diffraction (Q-XRD), and scanning electron microscopy (SEM), Fe was precipitated out as a mixture that was predominantly ferrihydrite with a nanoscale size for its primary nuclei. Its composition and size also varied largely with the leachate, i.e. the acid leaching temperature. For the unprecipitated Fe2+ that is predominant in the high-temperature leachate (i.e., ≥150 °C), it was preferentially oxidized and converted into nanoscale magnetite during the carbonation process, providing seed/nuclei for the crystallization and growth of magnesian calcite (Ca0.2Mg0.8CO3). Accordingly, the carbonation of Mg2+ was enhanced remarkably and reached completion in 20 min at room temperature. In addition, all the resulting products, Fe-precipitate (i.e., ferrihydrite), magnetite, and magnesian calcite, are relatively pure, presenting cheap and suitable precursors for a variety of value-added environmental applications

MSE middle-range ion interaction parameters in this study.

<p>MSE middle-range ion interaction parameters in this study.</p

Normalized Ni K-edge XANES spectra of nickel complexes as a function of concentration (thin lines).

<p>Baseline-removed pre-edges are shown as a function of concentration in inset for clarity. Thick lines are the calculated (convoluted) XANES spectra for Ni aqueous species using fully hydrated regular (re.) and distorted (dis., DFT optimized), as well as [NiCl(H₂O)₅]²⁺ (ADF., DFT optimized) octahedral configuration models from literature [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119805#pone.0119805.ref005" target="_blank">5</a>].</p

Distribution of Ni(II) species in NiCl₂-H₂O (a, b), and NiCl₂-MgCl₂-H₂O (c) solutions and average number of ligand in the first shell of Ni²⁺ as a function of NiCl₂ solution concentration.

<p>In (a) and (c), the distributions of species are calculated with the formation constant and MSE parameters determined in this study (all lines); in (b), the distributions of species was calculated for NiCl₂ solutions using the model and parameters from literature [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119805#pone.0119805.ref010" target="_blank">10</a>]; In (a), the calculated average numbers of ligands in the first shell of Ni²⁺ are compared with the number of ligands extracted by EXAFS (Cl ligand, empty circles with error bar; O ligand, triangles with error bar) and XRD [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119805#pone.0119805.ref021" target="_blank">21</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119805#pone.0119805.ref025" target="_blank">25</a>] (Cl ligand, filled circles) analysis are shown for comparison; and (b) the result using the model and parameters from literature [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119805#pone.0119805.ref010" target="_blank">10</a>].</p

Molar absorptivity spectra of individual Ni(II)-chloride species obtained from the analysis of two absorptive band (left, 350–550 nm and right 580–850 nm) spectroscopic data for NiCl₂-H₂O and NiCl₂-MgCl₂-H₂O systems at room temperature.

<p>The molar spectrum for Ni(ClO₄)₂ solution at room temperature is plotted as solid line for comparison.</p

Comparison experimental and predicted water activity for NiCl₂ solution at room temperature.

<p>The line is predicted one in this work and the dot line represents the prediction one without considering effect of complexes in system, corresponding to the dot line in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119805#pone.0119805.g009" target="_blank">Fig 9</a>; the circles are literature values [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119805#pone.0119805.ref063" target="_blank">63</a>].</p

The solubility product of solid phase in MSE and Pitzer model and the formation constant of NiCl⁺ and [NiCl₂]⁰ species determined in this study.

<p>The association constant of [HCl]⁰ aqueous species used in the calculation is also listed.</p><p>Uncertainty limits are given in parentheses.</p><p>* The solubility products were converted from mole fraction-scale in the literatures to molality-scale.</p><p>^£ Value was calculated by Pitzer−Simonson−Clegg (PSC) model [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119805#pone.0119805.ref046" target="_blank">46</a>].</p><p>^§ Values were calculated by HSC software (H = enthalpy, S = entropy, C = heat capacity) in literature [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119805#pone.0119805.ref048" target="_blank">48</a>].</p><p>^¶ The activity coefficient model assumed <i>γ</i>_ion = <i>γ</i>_<i>±</i>,_ion.</p><p>The solubility product of solid phase in MSE and Pitzer model and the formation constant of NiCl⁺ and [NiCl₂]⁰ species determined in this study.</p

Experimental data used for parameters estimation.

<p>* Water activity data</p><p>** Solid liquid equilibrium data</p><p>Experimental data used for parameters estimation.</p

Ni K-edge <i>k</i>³-weighted EXAFS (a), and the amplitude (b) and imaginary part (c) of their Fourier transforms of the two solid compounds and four NiCl₂ solutions.

<p>The dot line is the position of Ni-O and Ni-Cl peaks.</p

Normalized Ni K-edge XANES spectra for the two solid compounds.

<p>Normalized Ni K-edge XANES spectra for the two solid compounds.</p