The FeSe₂(cr) solubility determined by solubility experiments of Se co-existing with Fe

Figures & data

Figure 1. E_h–pH diagram for Se based on the thermodynamic data provided by Kitamura et al. [1]. The total concentrations of Se and Fe are 1 × 10⁻⁵ and 1 × 10⁻² mol dm⁻³, respectively.

Figure 2. E_h–pH diagram for the prediction of Se aqueous species based on the thermodynamic data provided by Kitamura et al. [1]. The total concentration of Se is 1 × 10⁻⁷ mol dm⁻³. The experimental data obtained from the over- and under-saturation directions are plotted as open circles and solid symbols, respectively.

Figure 3. Titration of 0.80 mol kg⁻¹ NaCl with 0.0100 mol dm⁻³ HCl using a combination glass electrode. [H⁺_{free, add}] is the moles of added free acid per liter and [H⁺_obs] = 10^−pHobs, where pH_obs is the observed pH measured with a combination glass electrode. [H⁺_obs] as a function of [H⁺_{free, add}] is shown as open circles. The solid line denotes the linear least-squares fit of data. The logarithm of the slope of this line corresponds to B = 0.03. This value is required to convert the pH_obs reading to the −log₁₀ [H⁺] value of the samples used in this study, using the following equation: −log₁₀ [H⁺] = pH_obs + B.

Figure 4. XRD pattern of the solid sample from 158-day aged suspension produced from the over-saturation direction. The pH of this suspension was 6.08.

Figure 5. XRD pattern of the solid sample from 105-day aged suspension produced from the under-saturation direction. The pH of this suspension was 6.64.

Table 1. Summary of solubility experiments of Se performed from both the over- and under-saturation directions in the presence of Fe under moderately reduced and neutral pH conditions.

	Saturation	Aging time			Se concentration	Fe concentration
Sample no.	direction	(days)	pH^a	Eh (mV vs. SHE)	(mol dm^−3)	(mol dm^−3)	log10K^○2^b
1	Over	29	7.37	−67.2	5.99 × 10^−8	1.76 × 10^−5	−17.15
2	Over	63	8.00	−124.5	6.12 × 10^−8	2.53 × 10^−6	−16.97
3	Over	92	8.76	−188.6	1.53 × 10^−7	1.30 × 10^−7	−17.02
4	Over	120	6.00	−8.5	1.12 × 10^−7	2.51 × 10^−4	−16.58
5	Over	158	6.08	−4.9	5.58 × 10^−8	1.59 × 10^−4	−17.40
6	Over	217	8.18	−175.5	5.57 × 10^−8	3.14 × 10^−7	−17.12
7	Under^c	76	6.08	−28.1	4.25×10^−8	1.53×10^−4	−16.78
8	Under^d	83	6.98	−103.0	1.54 × 10^−7	2.80 × 10^−6	−17.36
9	Under^e	105	6.64	−75.0	1.98 × 10^−7	7.50 × 10^−6	−17.21

^apH = −log₁₀ a_H⁺ = −log₁₀ γ_H⁺ + pH_obs + 0.03, where pH_obs is the pH value measured with a combination glass electrode and log₁₀ γ_H⁺ = − A(298 K) I_m^1/2/[1 + 1.5 I_m^1/2] + ϵ(H⁺, Cl⁻)m_Cl-.

^bK^○₂ $=a_{\mathrm{F}{\mathrm{e}}^{2+}}^2{a}_{{\mathrm{Se}}_4^{2-}}{a}_{{\mathrm{e}}^{-}}^2.$

^cThe resulting precipitate of sample 3 was used as the starting precipitate of sample 7.

^dThe resulting precipitate of sample 1 was used as the starting precipitate of sample 8.

^eThe resulting precipitate of sample 2 was used as the starting precipitate of sample 9.

Figure 6. E_h–pH diagram for Fe based on the thermodynamic data provided by Lemire et al. [12]. The total concentration of Fe is 1×10⁻⁷ mol dm⁻³. The experimental data obtained from the over- and under-saturation directions are plotted as open circles and solid symbols, respectively.

Figure 7. Determination of the n value for Fe_nSe which is the solubility-controlling solid and the log₁₀K^○₅ value for Fe_nSe dissolution reaction represented by (4Fe_nSe(cr) ⇌ 4nFe²⁺ + Se₄²⁻ + (8n − 2)e⁻) using the SIT model. The value of (${log}_{10}{a}_{{\mathrm{Se}}_4^{2-}}-2{log}_{10}{a}_{{\mathrm{e}}^{-}}$) is plotted against the value of (${log}_{10}{a}_{\mathrm{F}{\mathrm{e}}^{2+}}+2{log}_{10}{a}_{{\mathrm{e}}^{-}}$). Solubility data obtained from the over- and under-saturation directions are shown as open circles and solid symbols, respectively. The solid line denotes the linear least-squares fit of data. The slope and intercept of this line correspond to −4n = −2.01 ± 0.09 and log₁₀K^○₅ = −17.09 ± 0.28, respectively.

Figure 8. E_h–pH diagram for Fe based on the thermodynamic data provided by Lemire et al. [12]. The total concentrations of Fe are 1 × 10⁻⁷ mol dm⁻³ (solid line) and 1 × 10⁻⁴ mol dm⁻³ (dotted line). The experimental data obtained from the over- and under-saturation directions are plotted as open circles and solid symbols, respectively.

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