Influence of Gluconic Acid on Dissolution of Si, P and Fe from Steelmaking Slag with Different Composition into Seawater

Abstract

In order to continuously supply nutrient elements such as Si, P and Fe into seawater for the multiplication of phytoplankton, steelmaking slag has always been utilized at coast. Yet Fe as the obligatory micronutrient element shows an extremely low solubility under the natural seawater condition. As one kind of the broadly existing organic ligands, gluconic acid is able to form a complex with iron in alkaline aqueous solution, by which the soluble iron will be stabilized and thus the solubility of iron will be improved. In the current research, the influence of gluconic acid on the dissolution of Si, P and Fe as well as the variation of pH was investigated by using shaking experiment. Present results show that gluconic acid has little effect on the variation of pH and the increase of the dissolved Si and P concentrations, whereas gluconic acid enhances the dissolution of Fe greatly by forming the iron-gluconate complexes. However, the photo-reduction reaction of iron-gluconate complex occurred during shaking in the day time results in a slight decline of the concentration of the soluble iron.

1. Introduction

Steelmaking slag is a byproduct of steelmaking process. In Japan approximately 110 million tonnes of steel was produced accompanied by 14.5 million tonnes of steelmaking slag in 2010. Most of the steelmaking slag was used in civil construction, cement resource and base course of road, and so on. While still about 0.8% of converter slag and 8.8% of electric furnace slag have been landfilled without any utilization.¹⁾ For the environmental protection and sustainable growth of steelmaking industry, more valuable utilization methods of steelmaking slag should be developed nowadays.

Iron as an essential micronutrient element for the multiplication of phytoplankton is believed to be able to dissolve from steelmaking slag into seawater. For restoring seaweed beds at barren ground area in coast, various pilot-scale tests by setting steelmaking slag and humus substances mixture were conducted and its effectiveness was confirmed. At the same time, a number of laboratory-scale studies convinced the effective supply of nutrient elements such as Fe, Si and P by steelmaking slag too. Hayashi et al.²⁾ conducted a field experiment by setting a mixture of steelmaking slag and dredged soil in a marine area of Kawasaki City, Japan, and seaweed beds and shoals were restored effectively, which indicated that Fe dissolved efficiently from the steelmaking slag. Yamamoto et al.^3,4) adopted steelmaking slag together with the addition of humic substances to improve the growth of seaweeds, and their results also showed that the mixture of steelmaking slag and humus materials was effective for increasing iron concentration in seawater.

Though ferrous iron (Fe²⁺) has a relatively larger solubility in aqueous solution compared to ferric iron (Fe³⁺), it is easy to be oxidized to ferric iron in the oxic seawater. The solubility of ferric iron is extremely low at the normal pH range of seawater since ferric iron readily hydrolyzes to form insoluble amorphous hydrous ferric oxides. As common sequestering agents, organic acids can chelate iron species not only to improve its solubility but also to extend its elution life time. For instance, Sugie and Taniguchi⁵⁾ convinced ethylenediaminetetraacetic acid (EDTA), as the chelating agent of Fe dissolved from steelmaking slag, having a sufficient duration of bioavailability.

Among various kinds of organic acids, gluconic acid, 2,3,4,5,6-Pentahydroxyhexanoic acid, HOCH₂[CH(OH)]₄ COOH, whose dissociation constant ( $K_{\text{d}}^{\ominus }$ ) in water is 2.5 × 10^–4, is extremely soluble in water.⁶⁾ It exists in fruits, honey, amylum and wine as one kind of sugar acids, and is used extensively as a sequestering agent for metal species in alkaline systems.⁷⁾ In the present study, the influence of gluconic acid on the dissolution of Si, P and Fe from steelmaking slag into seawater was investigated by the shaking experiment. The shaking time, the CaO/SiO₂ ratio and the Fe content of slag as well as the concentration of gluconic acid were chosen as variable parameters. The variation of pH and the dissolution behaviors and mechanisms of Si, P and Fe were discussed.

2. Experimental

2.1. Materials

Six kinds of synthesized slag containing Fe_tO, CaO, SiO₂ and P₂O₅, and two practical slags containing Fe_tO, CaO, SiO₂, P₂O₅, Al₂O₃ and MgO were adopted. The CaO/SiO₂ ratios of two practical slags, SA and SB, are 1.15 and 3.63, respectively. These slags were ground and sieved with particle diameter under 150 μm.

The synthesis processes for the six slags are identical to the previous work.⁸⁾ The mixtures of 60 g were produced from CaO and FeO, and reagent grade SiO₂ and Ca₃(PO₄)₂·xH₂O. The mixture was put into an iron crucible (outer diameter 40 mm, inner diameter 32 mm and height 115 mm) and melted at 1723 K for 50 minutes at Ar flow with a rate of 1 L/min. Then the sample was taken out quickly, poured on a stainless steel plate and quenched by blowing Ar gas. Compositions of synthesized slags are shown in Table 1. The contents of main constituents in practical slags are shown in Table 2, in which iron is contained as metallic iron, ferrous oxide and ferric oxide.

Table 1. Compositions of synthesized slags.

Slag	FeO (mass%)	CaO (mass%)	SiO2 (mass%)	P2O5 (mass%)	CaO/SiO2 (–)
S1	15.0	41.5	41.5	2.0	1.0
S2	15.0	43.5	39.5	2.0	1.1
S3	15.0	50.0	33.0	2.0	1.5
S4	25.0	36.5	36.5	2.0	1.0
S5	25.0	38.0	35.0	2.0	1.1
S6	25.0	44.0	29.0	2.0	1.5

Table 2. Compositions of main constituents in practical slags.

Slag	Total Fe (mass%)	CaO (mass%)	SiO2 (mass%)	P2O5 (mass%)	CaO/SiO2 (–)
SA	18	33	29	3.8	1.15
SB	30	35	10	1.5	3.63

Artificial seawater was prepared from raw seawater substance (Osaka Yakken, MARINE ART SF-1) dissolving into distilled water. Its pH value is around 8.20 and its composition is shown in Table 3. The concentrations of Ca and Mg contained in the artificial seawater are about 410 and 1200 mg/L, respectively.

Table 3. Chemical composition of artificial seawater.

Constituent	Concentration (mg/L)
NaCl	22100
MgCl2·6H2O	9900
Na2SO4	3900
CaCl2·2H2O	1500
KCl	610
NaHCO3	190
KBr	96
Na2B4O7·10H2O	78
SrCl2	13
NaF	3
LiCl	1
Others	0.12

The 50% gluconic acid solution (analytical reagent grade, Wako Pure Chemical Industries, Ltd., concentration of gluconic acid: from 47.0 to 53.0 mass%, density: 1.24 g/cm³) was diluted 100 times to obtain 6.2 g/L gluconic acid. One hundred milliliters of seawater containing 0.062 g/L gluconic acid was prepared by mixing 1.0 mL of 6.2 g/L gluconic acid solution and 99 mL of artificial seawater, and that containing 0.12 g/L by mixing 2.0 mL and 98 mL, respectively, in a 250 mL polyethylene bottle. On the contrary, 100 mL seawater containing 0.25 g/L gluconic acid was made by mixing 4.0 mL of 6.2 g/L gluconic acid solution, 94 mL of artificial seawater, 1.0 mL of 0.1 mol/L NaOH solution and 1.0 mL of 0.01 mol/L NaOH solution to control the pH of the solution in a 250 mL polyethylene bottle. Hereafter the prepared acid solutions are, respectively, marked as 0.06 g/L, 0.12 g/L and 0.25 g/L seawater-gluconic acid solutions.

2.2. Shaking Experiments

The shaking experiments were conducted at 298 K controlled by an air conditioner. The shaking speed was 160 cycles/min with an oscillation of 20 ± 5 mm and the shaking time was varied from 24 to 240 h. Five grams of slag were added into 100 mL seawater or seawater-gluconic acid solution for shaking. After the prescribed experimental time elapsed, the pH of the shaking solution was immediately measured by a pH meter (TOA Electronics Ltd., HM-25R), and filtrated subsequently with a 0.45 μm Millipore membrane filter. Then the adequate amount of reagent grade of dilute hydrochloric acid was added to the filtrate for keeping the dissolved species stable. The concentrations of Si, P and Fe were measured by an inductively-coupled plasma optical emission spectrometry (ICP-OES, Seiko Instruments Inc., SPS7800 Plasma Spectrometer).

2.3. Solubility Diagrams

The calculated solubility diagrams of Si, P and Fe are adopted to discuss the dissolution mechanism of them from steelmaking slag into seawater.⁸⁾ The results of the shaking experiments are plotted on the solubility diagrams, where the square symbols stand for the results without gluconic acid, and the circle symbols stand for the results with gluconic acid.

3. Results and Discussion

3.1. Variation of pH

Figure 1 shows the variation of pH with various concentrations of gluconic acid. On the whole, pH increases sharply in the first shaking day, and then decreases little with increasing the shaking time. At the same time, pH increases with increasing the CaO/SiO₂ ratio of slag. For example, the pH value of the seawater-gluconic acid solutions reaches between 12.00 and 12.40 at maximum in the case of slag SB with a CaO/SiO₂ ratio of 3.63. The pH of the solution is similar for the slags with the same CaO/SiO₂ ratio and no relationship with the FeO content of slag was observed. It is well known that the dissolution of CaO from slag will induce the increase of pH by the reaction (1).^8,9,10) Hence a slag with large CaO/SiO₂ ratio has relatively larger content of CaO, and the pH of the shaking solution becomes larger too. Simultaneously the pH changing tendency reflects the dissolution of CaO from slag into seawater. The fast increase of pH in the first shaking day indicates the rapid dissolution of Ca after immerging slag into seawater.   

$$\mathrm{C}\mathrm{a}\mathrm{O}\left(\mathrm{s},\mathrm{i}\mathrm{n}\mathrm{s}\mathrm{l}\mathrm{a}\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O}=\mathrm{C}{\mathrm{a}}^{2+}+2\mathrm{O}{\mathrm{H}}^{-}$$(1)

Fig. 1.

Variation of pH with various concentrations of gluconic acid; (a) 0 g/L, (b) 0.06 g/L, (c) 0.12 g/L and (d) 0.25 g/L.

In addition, gluconic acid decreases the initial pH of seawater but gluconic acid with different concentrations changing from 0 to 0.25 g/L has little effect on the pH during shaking, because gluconic acid is a weak acid as shown in Eq. (2),⁶⁾   

$$\begin{array}{c}\mathrm{H}\mathrm{G}{\mathrm{H}}_4\leftrightarrow {\mathrm{H}}^{+}+\mathrm{G}{\mathrm{H}}_4^{-}\\ K_{\left(2\right)}=\left[{\mathrm{H}}^{+}\right]\left[\mathrm{G}{\mathrm{H}}_4^{-}\right]/\left[\mathrm{H}\mathrm{G}{\mathrm{H}}_4^{}\right]=2.5\times {10}^{-4}\end{array}$$(2)

where HGH₄ is gluconic acid and GH₄⁻ is gluconate ion.

In the case of normal seawater, pH is 8.20 and the molar ratio of [GH₄⁻]/[HGH₄] is calculated to be 4.0 × 10⁴, which implies that the most of gluconic acid has already dissociated into gluconate ion in the seawater. When slags were immerged into the seawater-gluconic acid solution, the further dissociation of gluconic acid is negligible during the shaking experiments. Therefore, the effect of gluconic acid on the pH variation is also little.

3.2. Dissolution of Si

Dissolution behavior of Si from slag into seawater with various concentrations of gluconic acid is shown in Fig. 2. The concentration of Si has a tremendous increase in the first shaking day, and then it is maintained stable or declines slightly after 10 days shaking. The larger CaO/SiO₂ ratio of slag has an adverse effect on the dissolution of Si. For example, the shaking solution with slags S3, S6 and SB with the CaO/SiO₂ ratio of 1.5 and 3.63 has little soluble Si as shown in Fig. 2(a). Meanwhile, Si concentration in the case of slag SA is lower than those in the case of slags S2 or S5 despite the similar CaO/SiO₂ ratio among three slags. This is explained by slightly larger pH of the solution with slag SA. Therefore, it is clear that Si concentration is greatly affected by pH of the solution. At the same time, the FeO content of slag does not affect the dissolution of Si. Moreover, the concentration of soluble Si in the shaking solution with adding gluconic acid is a little larger than that without adding gluconic acid, whereas it has no relationship with the concentration of gluconic acid.

Fig. 2.

Dissolution behavior of Si from slag into seawater with various concentrations of gluconic acid; (a) 0 g/L, (b) 0.06 g/L, (c) 0.12 g/L and (d) 0.25 g/L.

Figure 3 shows the solubility diagram of Si together with the shaking results. For the large pH region with shaking slag SB, the soluble Si species is SiO₃²⁻, and its dissolution mechanism can be deduced as reaction (3):   

$$3\mathrm{C}\mathrm{a}\mathrm{O}\cdot \mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{s}\right)+2{\mathrm{H}}_2\mathrm{O}=3\mathrm{C}{\mathrm{a}}^{2+}+\mathrm{S}\mathrm{i}{\mathrm{O}}_3^{2-}+4\mathrm{O}{\mathrm{H}}^{-}$$(3)

Fig. 3.

Solubility diagram of Si together with the shaking results.

For the dissolution of Si from other slags, the soluble species is H₄SiO₄⁰ which is equilibrated with H₄SiO₄(s) phase. Due to no H₄SiO₄(s) phase contained in the slag, the dissolution mechanism of Si can be described as two steps. The first step is the reaction of SiO₂ in slag with H₂O to form H₄SiO₄(s) according to reaction (4). The second step is the dissolution of H₄SiO₄(s) into seawater according to reaction (5).   

$$3\mathrm{C}\mathrm{a}\mathrm{O}\cdot \mathrm{S}\mathrm{i}{\mathrm{O}}_2\left(\mathrm{s}\right)+5{\mathrm{H}}_2\mathrm{O}={\mathrm{H}}_4\mathrm{S}\mathrm{i}{\mathrm{O}}_4\left(\mathrm{s}\right)+3\mathrm{C}{\mathrm{a}}^{2+}+6\mathrm{O}{\mathrm{H}}^{-}$$(4)

  

$${\mathrm{H}}_4\mathrm{S}\mathrm{i}{\mathrm{O}}_4\left(\mathrm{s}\right)={\mathrm{H}}_4\mathrm{S}\mathrm{i}{\mathrm{O}}_4^0$$(5)

With adding gluconic acid, the slight larger concentration of Si is attributed to the decline of pH effect by gluconic acid, which is propitious to the hydration reaction of SiO₂ rather than the sequestering action of gluconic acid, because both Si and gluconic acid exist as anion forms under the shaking conditions.

3.3. Dissolution of P

The dissolution behavior of P from slag into seawater with various concentrations of gluconic acid is shown in Fig. 4. The concentration of P reaches a maximum value after shaking for 1 day, and then it declines gradually with increasing the shaking time as well as increasing the CaO/SiO₂ ratio of slag. At the same time, the effect of the FeO content of slag on the dissolution of P is still unknown.

Fig. 4.

Dissolution behavior of P from slag into seawater with various concentrations of gluconic acid; (a) 0 g/L, (b) 0.06 g/L, (c) 0.12 g/L and (d) 0.25 g/L.

Moreover, the concentration of soluble P with adding gluconic acid is a little larger than that without adding gluconic acid, for example, the concentration of soluble P dissolved from synthesized slags into seawater varies from 0 to 0.84 mg/L, and that dissolved from synthesized slags into seawater-gluconic acid solution varies from 0.13 to 1.39 mg/L. However, the relationship between the concentrations of gluconic acid and soluble P is obscure.

Figure 5 shows the solubility diagram of P together with the shaking results. No matter whether gluconic acid was added or not, the soluble P is located in the regions of the soluble species of HPO₄²⁻ or PO₄³⁻. The concentration of P with adding gluconic acid is slightly larger than that without adding gluconic acid. For the shaking solution with these slags having a smaller pH value, the dissolution mechanism of P can be expressed as reaction (6):   

$$2\mathrm{C}\mathrm{a}\mathrm{O}\cdot {\mathrm{P}}_2{\mathrm{O}}_5\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}=2\mathrm{H}\mathrm{P}{\mathrm{O}}_4^{2-}+2\mathrm{C}{\mathrm{a}}^{2+}$$(6)

Fig. 5.

Solubility diagram of P together with the shaking results.

This reaction proceeds backward with increasing Ca concentration in the solution, namely concentration of P decreases with increasing the shaking time.

For the shaking solution with slag SB having a larger pH value, the dissolution mechanism of P can be written as reaction (7):   

$$2\mathrm{C}\mathrm{a}\mathrm{O}\cdot {\mathrm{P}}_2{\mathrm{O}}_5\left(\mathrm{s}\right)+2\mathrm{O}{\mathrm{H}}^{-}=2\mathrm{P}{\mathrm{O}}_4^{3-}+2\mathrm{C}{\mathrm{a}}^{2+}+{\mathrm{H}}_2\mathrm{O}$$(7)

In the case of slag SB, the concentration of P is controlled by reaction (7). Since Ca concentration continuously increases with shaking time without pH increase at this large pH region,⁸⁾ this reaction also proceeds backward with time. However, this phenomenon was not clearly observed since the concentration of P in the case of slag SB is constantly small.

The improvement effect of gluconic acid on the dissolution of P is due to its decreasing effect on pH value, which indirectly affects the dissolution of P.

3.4. Dissolution of Fe

The dissolution behavior of Fe from slag into seawater with various concentrations of gluconic acid is shown in Fig. 6. The concentration of Fe shows a considerable increment in the first shaking day. It increases with the shaking time and decreases with increasing the content of FeO in the case of synthesized slags as shown in Fig. 6(a). On the contrary, the concentration of Fe increased during the initial two shaking days and then gradually decreased. This tendency was considerable in the case of slag SA. The concentration of Fe dissolved from slag SA was larger initially, while it from slag SB was larger after long shaking time. It would be due to the form of iron, namely practical slags may contain not only FeO but Fe₂O₃ or even metallic iron. In addition, several iron-containing mineralogical phases may exist because of many minor constituents or various slag cooling rate. Therefore, further investigation is required. The CaO/SiO₂ ratio of slag has no distinct effect on the dissolution of iron regardless of whether gluconic acid is added or not. The concentration of iron dissolved from synthesized or practical slags increases greatly with increasing the concentration of gluconic acid. It changes from 0.01 to 0.25 mg/L, from 0.04 to 0.38 mg/L, from 0.05 to 1.77 mg/L and from 1.39 to 11.10 mg/L with adding 0, 0.06, 0.12 and 0.25 g/L gluconic acid, respectively.

Fig. 6.

Dissolution behavior of Fe from slag into seawater with various concentrations of gluconic acid; (a) 0 g/L, (b) 0.06 g/L, (c) 0.12 g/L and (d) 0.25 g/L.

Figure 7 shows the solubility diagram of Fe together with the shaking results. In the case of Fe dissolved from slag into seawater, the concentration of Fe is mainly along the solubility line of Fe(OH)₂ or Fe(OH)₃ which are the hydrate of FeO or Fe₂O₃, respectively. In the case of Fe dissolved from slag into seawater-gluconic acid solution, the concentration of Fe is much larger than the solubility lines of Fe(OH)₂ and Fe(OH)₃. Therefore, it is assured that iron and gluconic acid form soluble complexes under the present shaking condition.

Fig. 7.

Solubility diagram of Fe together with the shaking results.

In order to clarify the relationship between the concentrations of soluble Fe and gluconic acid, the average concentration of Fe in the shaking solutions from the first to tenth day was calculated for each slag as shown in Table 4. Figure 8 shows the relationship between concentrations of Fe and gluconic acid.

Table 4. Average concentration of Fe dissolved from each slag.

Gluconic acid (g/L)	S1 (mg-Fe/L)	S2 (mg-Fe/L)	SA (mg-Fe/L)	SB (mg-Fe/L)
0.00	0.06	0.15	0.06	0.05
0.06	0.17	0.22	0.09	0.09
0.12	0.32	1.00	0.67	0.48
0.25	3.16	3.14	3.16	4.03

Fig. 8.

Relationship between concentrations of soluble iron and gluconic acid.

Ferric iron and gluconic acid form a series of complexes with the mole ratio of 1 to 1 with increasing pH as shown in reactions (8) to (11).^6,11) Generally the equilibrium constants of these complexes decrease with increasing pH.   

$$\begin{array}{c}\mathrm{F}{\mathrm{e}}^{3+}+\mathrm{G}{\mathrm{H}}_4^{-}=\mathrm{F}\mathrm{e}{\left(\mathrm{G}{\mathrm{H}}_4\right)}^{2+}\\ K_{\left(8\right)}=\left[\mathrm{F}\mathrm{e}{\left(\mathrm{G}{\mathrm{H}}_4\right)}^{2+}\right]/\left[\mathrm{F}{\mathrm{e}}^{3+}\right]\left[\mathrm{G}{\mathrm{H}}_4^{-}\right] \hspace{1em}\hspace{1em}\mathrm{l}\mathrm{o}\mathrm{g}{K}_{(8)}=17.1\end{array}$$(8)

  

$$\begin{array}{c}\mathrm{H}\mathrm{F}\mathrm{e}{\left(\mathrm{G}{\mathrm{H}}_3\right)}^{2+}=\mathrm{H}\mathrm{F}\mathrm{e}\left(\mathrm{G}\mathrm{H}\right)+2{\mathrm{H}}^{+}\\ K_{\left(9\right)}=\left[\mathrm{H}\mathrm{F}\mathrm{e}\left(\mathrm{G}\mathrm{H}\right)\right]{\left[{\mathrm{H}}^{+}\right]}^2/\left[\mathrm{H}\mathrm{F}\mathrm{e}{\left(\mathrm{G}{\mathrm{H}}_3\right)}^{2+}\right] \\ \mathrm{l}\mathrm{o}\mathrm{g}{K}_{(9)}=-4.6\end{array}$$(9)

  

$$\begin{array}{c}\mathrm{H}\mathrm{F}\mathrm{e}\left(\mathrm{G}\mathrm{H}\right)=\mathrm{F}\mathrm{e}{\left(\mathrm{G}\mathrm{H}\right)}^{-}+{\mathrm{H}}^{+}\\ K_{\left(10\right)}=\left[\mathrm{F}\mathrm{e}{\left(\mathrm{G}\mathrm{H}\right)}^{-}\right]\left[{\mathrm{H}}^{+}\right]/\left[\mathrm{H}\mathrm{F}\mathrm{e}\left(\mathrm{G}\mathrm{H}\right)\right] \\ \mathrm{l}\mathrm{o}\mathrm{g}{K}_{(10)}=-4.0\end{array}$$(10)

  

$$\begin{array}{c}\mathrm{F}\mathrm{e}{\left(\mathrm{G}\mathrm{H}\right)}^{-}+\mathrm{O}{\mathrm{H}}^{-}=\mathrm{F}\mathrm{e}\left(\mathrm{G}\mathrm{H}\right){\left(\mathrm{O}\mathrm{H}\right)}^{2-}\\ K_{\left(11\right)}=\left[\mathrm{F}\mathrm{e}\left(\mathrm{G}\mathrm{H}\right){\left(\mathrm{O}\mathrm{H}\right)}^{2-}\right]/\left[\mathrm{F}\mathrm{e}{\left(\mathrm{G}\mathrm{H}\right)}^{-}\right]\left[\mathrm{O}{\mathrm{H}}^{-}\right] \\ \mathrm{l}\mathrm{o}\mathrm{g}{K}_{(11)}=-13.3\end{array}$$(11)

The efficiency of gluconic acid forming complexes with Fe is calculated. When gluconic acid was added into seawater, it forms complexes with the major elements such as Ca²⁺ and Mg²⁺ and the minor elements such as Sr²⁺ in the normal seawater as reactions (12), (13) and (14).¹¹⁾ The concentration of Sr²⁺ (8.2 × 10⁻⁵ mol/L from Table 3) is small enough to be practically neglected.   

$$\begin{array}{c}\mathrm{M}{\mathrm{g}}^{2+}+\mathrm{G}{\mathrm{H}}_4^{-}=\mathrm{M}\mathrm{g}{\left(\mathrm{G}{\mathrm{H}}_4\right)}^{+}\\ K_{\left(12\right)}=\left[\mathrm{M}\mathrm{g}{\left(\mathrm{G}{\mathrm{H}}_4\right)}^{+}\right]/\left[\mathrm{M}{\mathrm{g}}^{2+}\right]\left[\mathrm{G}{\mathrm{H}}_4^{-}\right] \hspace{1em}\hspace{1em}\mathrm{l}\mathrm{o}\mathrm{g}{K}_{(12)}=0.70\end{array}$$(12)

  

$$\begin{array}{c}\mathrm{C}{\mathrm{a}}^{2+}+\mathrm{G}{\mathrm{H}}_4^{-}=\mathrm{C}\mathrm{a}{\left(\mathrm{G}{\mathrm{H}}_4\right)}^{+}\\ K_{\left(13\right)}=\left[\mathrm{C}\mathrm{a}{\left(\mathrm{G}{\mathrm{H}}_4\right)}^{+}\right]/\left[\mathrm{C}{\mathrm{a}}^{2+}\right]\left[\mathrm{G}{\mathrm{H}}_4^{-}\right] \hspace{1em}\hspace{1em}\mathrm{l}\mathrm{o}\mathrm{g}{K}_{(13)}=1.22\end{array}$$(13)

  

$$\begin{array}{c}\mathrm{S}{\mathrm{r}}^{2+}+\mathrm{G}{\mathrm{H}}_4^{-}=\mathrm{S}\mathrm{r}{\left(\mathrm{G}{\mathrm{H}}_4\right)}^{+}\\ K_{\left(14\right)}=\left[\mathrm{S}\mathrm{r}{\left(\mathrm{G}{\mathrm{H}}_4\right)}^{+}\right]/\left[\mathrm{S}{\mathrm{r}}^{2+}\right]\left[\mathrm{G}{\mathrm{H}}_4^{-}\right] \mathrm{l}\mathrm{o}\mathrm{g}{K}_{(14)}=1.01\end{array}$$(14)

In the case of seawater containing 0.25 g/L of gluconic acid, a part of gluconic acid was consumed to form complexes with Ca and Mg, and 9.1 × 10⁻⁴ mol/L of remaining gluconic acid is free from complexes in the aqueous solution.

The complexation reactions of gluconic acid with Fe²⁺ and Fe³⁺ are shown as reactions (15) and (16).^7,11)   

$$\begin{array}{c}\mathrm{F}{\mathrm{e}}^{2+}+\mathrm{G}{\mathrm{H}}_4^{-}=\mathrm{F}\mathrm{e}{\left(\mathrm{G}{\mathrm{H}}_4\right)}^{+}\\ K_{\left(15\right)}=\left[\mathrm{F}\mathrm{e}{\left(\mathrm{G}{\mathrm{H}}_4\right)}^{+}\right]/\left[\mathrm{F}{\mathrm{e}}^{2+}\right]\left[\mathrm{G}{\mathrm{H}}_4^{-}\right] \mathrm{l}\mathrm{o}\mathrm{g}{K}_{(15)}=1.0\end{array}$$(15)

  

$$\begin{array}{c}\mathrm{F}{\mathrm{e}}^{3+}+\mathrm{G}{\mathrm{H}}_4^{-}+4\mathrm{O}{\mathrm{H}}^{-}=\mathrm{F}\mathrm{e}{\left(\mathrm{G}\right)}^{2-}+4{\mathrm{H}}_2\mathrm{O}\\ K_{\left(16\right)}=\left[\mathrm{F}\mathrm{e}{\left(\mathrm{G}\right)}^{2-}\right]/\left[\mathrm{F}{\mathrm{e}}^{3+}\right]\left[\mathrm{G}{\mathrm{H}}_4^{-}\right]{\left[\mathrm{O}{\mathrm{H}}^{-}\right]}^4\\ \mathrm{l}\mathrm{o}\mathrm{g}{K}_{(16)}=37.2\end{array}$$(16)

Because the equlibrium constant of reaction (16) is much larger than that of reaction (15), the ferric gluconate in the aqueous solution is focused and the 9.1 × 10^–4 mol/L of remaining gluconic acid is believed to form complexes completely with ferric iron, namely there will be 9.1 × 10^–4 mol/L of ferric gluconate complex, which is equivalent to 51 mg/L of soluble Fe in seawater. However, there are only 3.16 mg/L, 3.14 mg/L, 3.16 mg/L and 4.03 mg/L of average iron concentrations for slags S1, S2, SA and SB, respectively. Therefore, the utilization efficiency of gluconic acid forming ferric gluconate complex changes from 6.2% to 7.9%, which is very low.

The concentration of soluble Fe with adding gluconic acid appears a maximum value during shaking and then decreases with increasing the shaking time. The same phenomenon was also reported by Kuma et al.¹²⁾ The dissolution mechanism of iron with gluconic acid is deduced as follows: In the initial shaking time, the complexes between the ferric iron dissolved from slag into seawater and gluconic acid is formed rapidly. In the day time, the ferric–gluconate complex is reduced by photo-reduction under light irradiation to form ferrous–gluconate complex. Since ferrous–gluconate complex has a smaller equilibrium constant, Fe²⁺ is released from the complex and the produced Fe²⁺ is readily re-oxidized to the Fe³⁺ in the seawater. On the one hand, one part of the Fe³⁺ reacts with OH⁻ to form colloidal hydrous ferric oxides. On the other hand, the other part of the Fe³⁺ reforms complexes with gluconic acid. The hydrolytic precipitation reaction and the complexation reaction occur simultaneously. However, the hydrolytic precipitation rate of re-oxidized ferric iron is a little faster than the rate of re-complexation reaction due to the rapid photo-degradation of gluconic acid.¹²⁾ Therefore, the soluble iron concentration declines in the aqueous solution.

4. Conclusions

The influence of gluconic acid on the dissolution of elements from steelmaking slag into seawater was investigated by shaking experiments. The effect of concentration of gluconic acid, the shaking time, the CaO/SiO₂ ratio and the iron content of slag on the dissolution behavior of elements were observed.

The pH value and the concentrations of Si, P and Fe increased in the first shaking day. The slags with a larger CaO/SiO₂ ratio had a larger pH value, but it was not beneficial to the dissolution of Si and P. At the same time, gluconic acid had little effect on the variation of pH and slightly enhanced the dissolutions of Si and P. On the contrary, gluconic acid improved the dissolution of Fe greatly according to the formation of iron-gluconate complex by the molar ratio of 1. The effective utilization efficiency of gluconic acid is calculated as less than 10% as a sequestrant of iron. Concentration of Fe showed a maximum value during shaking and then decreased with the shaking time. This phenomenon was attributed to the photo-reduction reaction of the soluble ferric-gluconate complex.

References

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