简介概要

Mechanism of aluminum complexation in oxidative activity of leaching bacteria in a fluoride-containing bioleaching system

来源期刊：Rare Metals2019年第1期

论文作者：Xiang Li Jian-Kang Wen Xiao-Lan Mo Wu Biao Dian-Zuo Wang Hong-Ying Yang

文章页码：87 - 94

摘要：Accumulation of toxic ions in leachate is one factor limiting bioleaching applications. The effect of fluoride ions on the growth of bioleaching microorganisms has been extensively emphasized. In this study, HF is found to be the toxic form of fluoride that affects the bacterial activity under acidic conditions. The added aluminum could compete with H⁺ to complex with F-, thus significantly decrease the concentration of HF and finally reduce the toxicity of fluoride to bacteria. When F^-/Al³⁺concentration ratio is 0.5:1.0, Fe²⁺ oxidation rate could reach 0.167 g·L^-1·h^-1, close to that of the biotic control group(0.195 g·L^-1·h^-1). The competitive complexation mechanism of fluoride by AlF_n^3-n results in stability constants of AlF_n^3-n complex（7.00） that are larger than those of HF（3.18）. The F^-/Al³⁺ concentration ratio in the medium could affect the speciation of AlF_n^3-n complex.With the decrease in F^-/Al³⁺ concentration ratio, the coordination numbers of AlF_n^3-n decrease. Finally, the feasibility of fluoride detoxification by aluminum ion is verified. This work has meaningful implications for fluoride-containing bacterial bioleaching systems.

稀有金属(英文版) 2019,38(01),87-94

Mechanism of aluminum complexation in oxidative activity of leaching bacteria in a fluoride-containing bioleaching system

Xiang Li Jian-Kang Wen Xiao-Lan Mo Wu Biao Dian-Zuo Wang Hong-Ying Yang

National Engineering Laboratory of Biohydrometallurgy,General Research Institute for Nonferrous Metals

School of Metallurgy,Northeastern University

作者简介：*Jian-Kang Wen e-mail:kang3412@126.com;

收稿日期：7 November 2017

基金：financially supported by the National Natural Science Foundation of China (Nos. 51404031 and U1608254);

Mechanism of aluminum complexation in oxidative activity of leaching bacteria in a fluoride-containing bioleaching system

Xiang Li Jian-Kang Wen Xiao-Lan Mo Wu Biao Dian-Zuo Wang Hong-Ying Yang

National Engineering Laboratory of Biohydrometallurgy,General Research Institute for Nonferrous Metals

School of Metallurgy,Northeastern University

Abstract：

Accumulation of toxic ions in leachate is one factor limiting bioleaching applications. The effect of fluoride ions on the growth of bioleaching microorganisms has been extensively emphasized. In this study, HF is found to be the toxic form of fluoride that affects the bacterial activity under acidic conditions. The added aluminum could compete with H⁺ to complex with F-, thus significantly decrease the concentration of HF and finally reduce the toxicity of fluoride to bacteria. When F^-/Al³⁺concentration ratio is 0.5:1.0, Fe²⁺ oxidation rate could reach 0.167 g·L^-1·h^-1, close to that of the biotic control group(0.195 g·L^-1·h^-1). The competitive complexation mechanism of fluoride by AlF_n^3-n results in stability constants of AlF_n^3-n complex(7.00) that are larger than those of HF(3.18). The F^-/Al³⁺ concentration ratio in the medium could affect the speciation of AlF_n^3-n complex.With the decrease in F^-/Al³⁺ concentration ratio, the coordination numbers of AlF_n^3-n decrease. Finally, the feasibility of fluoride detoxification by aluminum ion is verified. This work has meaningful implications for fluoride-containing bacterial bioleaching systems.

Keyword：

Competitive complexation; Detoxification; Fluoride; Aluminum; Bioleaching;

Received： 7 November 2017

1 Introduction

China is not rich in uranium resources.Its known reserves of uranium rank 14th in the world;hence,it cannot adapt to the long-term needs of the development of nuclear power [ 1] .Since the beginning of this century,China’s uranium industry has faced some problems,raw material prices have increased,mines have aged,and conventional hydrometallurgy processes have suffered economic losses [ 2, 3] .Bioleaching plays an important role in the leaching of lowgrade uranium ore because of certain advantages.However,applying bioleaching technology to high-fluorinecontent ores which account for a large proportion of China s uranium resources (>60%) is difficult [ 4, 5] .In a bioleaching system,fluoride ions can affect the activity of bacteria,inhibit the growth and multiplication of leaching bacteria and hinder acid production of leaching bacteria,thus reducing the efficiency of bioleaching [ 6, 7] .Many failures of industry applications have been reported [ 8, 9] .Fluoride ions have a strong inhibitory effect on various metallurgical engineering bacteria,thus severely restricting the promotion and application of uranium bioleaching technology [ 10, 11] .Frequent replacement or dilution of the leaching solution is necessary because of the presence of fluoride ions,which directly lead to a decline in labor efficiency and an increase in cost.

There are two ways to fundamentally solve the above problems.(1) One way is to improve the fluoride tolerance of the leaching bacteria.Many researchers have performed tests on domesticated and cultured the bacteria,but this method has a problem that the stability of fluoride resistance of the bacterial cannot be guaranteed [ 5, 12, 13, 14, 15, 16, 17] .(2)Another alternative way is to develop a solution chemistry method to reduce the toxicity of fluorine.Fluorine in solution is mainly affected by three basic equilibria:acidbase balance,precipitation equilibrium and complex equilibrium [ 18, 19, 20] .In the bioleaching system,bacteria have low tolerance to fluoride ions.However,a variety of metal ions such as Mg²⁺,Mn²⁺,Al³⁺and Fe³⁺are released from the dissolved associated minerals [ 21] .By exploiting the solution chemistry of fluorine,the competitive complexation of such cations can be carried out to complex the free F^-and thus to reduce their toxicity [ 22, 23] .

A method for reducing the concentration of free fluoride ions by metal cations has been reported [ 21, 24, 25] .However,the detoxification of fluoride by aluminum and the mechanism of fluoride and aluminum complexation have not yet been quantified.Therefore,the present study was undertaken to assess the effect of aluminum on the oxidative activity of leaching bacteria in a fluoride-containing bioleaching system.

2 Experimental

2.1 Bacteria and media

A mixture of Acidithiobacillus ferrooxidans and Leptospirillum ferriphilum was obtained from the National Engineering Laboratory of Biohydrometallurgy,General Research Institute for Nonferrous Metals,Beijing,China [ 15] .The composition of the 9-K medium (pH2.00±0.02) was as follows:3 g.L^-1 (NH₄)2SO₄,0.5 g·L^-1 MgSO₄·7H₂O,0.5 g·L^-1 K₂HPO₄,0.1 g·L^-1KCl,0.01 g·L^-1 Ca(NO₃)₂ and 44.2 g-L^-1 FeSO₄·7H₂O.

2.2 Standard curve for fluoride ion detection

The fluoride-selective electrode only responds to free fluoride ions.The free fluorine concentration,however,is affected by pH,ionic strength and cation concentration.In order to quantify the change in free fluorine concentration in solution,establishing a method for the determination of free fluorine is needed.Therefore,in this study the free fluorine response was determined by plotting the free fluorine standard curve for different fluoride concentrations during the complexation process.

2.3 Toxicity of fluorine at different pH values

In order to explore the toxicity of fluorine at different pH levels to bacteria,the leaching bacteria were inoculated into 9-K medium with different initial concentrations of F^-and pH values to a concentration of 10%inoculum.The culture conditions were pH 1.5,c(F^-)=10 mg·L^-1;pH2.0,c(F^-)=10 mg-L^-1;pH 2.0,c(F^-)=20 mg-L^-1;pH3.0,c(F^-)=20 mg-L^-1;pH 3.0,c(F^-)=40 mg-L^-1;pH4.0,c(F^-)=40 mg·L^-1.The shake flasks were agitated at33℃and 160 r·min^-1 in an incubator.

The total amounts of fluoride added were the same at different solution pH levels.The free fluoride ions in the standard samples with no buffer solution and those in undiluted samples at different pH levels were directly determined through the fluoride-selective electrode method.The concentration of complexed fluoride was the difference between the total fluoride concentration and the concentration of free fluoride.A series of standard samples with the same concentrations of F^-and different pH values were prepared to explore the feasibility of the determination of free fluorine ion at different pH levels.The fluoride ion concentration was 1.9 g·L^-1.Values of the solution pH were varied from 1.0 to 7.0.

2.4 Mechanism of detoxification by aluminum-fluoride complex

Throughout the experiments,the bacteria were maintained at 33℃and 160 r·min^-1 in a bacteriological incubator.The bio-oxidation experiments were performed in a conical flask with 100 ml of suspension that contained 10 vol%of the bacterial inoculum (not previously adopted with either fluoride or aluminum).

In this study,the concentration of fluoride was set at0.5 g·L^-1 to test the detoxification of fluoride by the aluminum/fluoride complex.In the shake-flask experiments,the pH value,reference electrode potential (E_h),ferrous ion oxidation rate and bacterial growth were assessed in experiments in which fluoride ions were added.Aluminum concentrations were varied from 0 to 3.0 g·L^-1,in the presence and absence of fluoride so that the following F^-/Al³⁺concentration ratios were achieved (g·L^-1):0:0,0.5:0,0.5:0.5,0.5:1.0,0.5:2.0 and 0.5:3.0.In the experiment,soluble NaF and Al₂(SO₄)3·18H₂O (both analytical grade) were selected as F^-source and Al³⁺source,respectively.

2.5 Complexation mechanism experiment

The concentration of fluoride in solution stayed at 1 g·L^-1,and the concentration ratios of F^-to Al³⁺were from 5:1 to1:10.Dilute sulfuric acid solution (H₂SO₄ to H₂O volume ratio of 1:1) was added to adjust the solution pH to 2.0,and the volume was maintained at 100 ml.The coordination number was calculated through Bjerrum’s function.The Bjerrum’s function could be defined as

where c(F)_T is the total amount of substance concentration of fluorine;c(Al)_T is the total amount of substance concentration of aluminum;c(F^-) is the amount of substance concentration of free fluoride anions; is the stability constant of coordination reaction at different levels;n is a function of c(F^-). could be found in Lange’s Handbook [ 26] .

3 Results and discussion

3.1 Fluoride concentration standard curve

E_h and the fluoride concentration standard curve are shown in Fig.1.The logarithm of fluoride ion concentration shows a good linear correlation with E_h (R²=0.997).

3.2 Acid dependence of fluoride toxicity to bacteria

9-K media with different concentrations of fluorine and pH were used to investigate the effect of fluorine toxicity on bacterial growth,as illustrated in Fig.2.When pH is 1.5,no bacterial growth is observed within 160 h during Fe²⁺bio-oxidation experiments performed with 10 mg·L^-1 total fluoride concentration.When pH increases to 4.0,the concentration of fluoride increases to 40 mg·L^-1,and the bacteria could grow slowly.With the increase in initial pH,the resistance of bacteria to fluoride increases,while the ferrous ion-oxidizing activity and the concentration of bacteria in the solution present a downward trend (Fig.2c).The results show that the mechanism of fluoride toxicity to leaching bacteria is mainly affected by pH of the fluoridecontaining solution.This phenomenon may be explained by the fluoride chemistry.The transformation of fluorine in the solution can be calculated according to Eqs.(2)-(4).Hydrofluoric acid is a weak acid with an ionization constant (pK_a) of 3.18:

Fig.1 Standard curve for correlation between logarithm of F^-and fluoride ion-selective E_h

Thus,the concentration of HF can be deduced from the concentration of F^-.The ionization constant of HF is given by:

The variable forms of fluoride at different pH values are shown in Fig.3.In this part,only fluoride ions and hydrogen ions were added to the solution,so the effect of other ions that are present is excluded.When pH value is less than 5.0,the concentration of free fluoride increases with the rise in pH.When pH value is more than 5.0,more than 90%of the fluoride is in the form of F^-.In the experiment,more than95%of the fluoride is in the form of HF at pH 1.0 and 2.0.The pH value of the solution therefore has a significant effect on the form of fluoride ions.

The study shows the detrimental effects of fluoride on bacterial growth,which are ascribed to the predominance of the HF form at the pH of bioleaching.At low-pH levels of the bioleaching system,fluoride ions are converted into HF.Unlike H⁺and F^-(the permeability coefficient of HF is 1×10⁵-1×10⁷,larger than those of H⁺and F^-),HF can penetrate the cell membrane and dissociate into H⁺and F^- [ 27] .Although the leaching bacteria adapt to a low-pH environment,the internal cell pH is neutral.H⁺then lowers the internal cell pH,and the fluoride inhibits acid production,thereby disrupting the cell membrane structure and combining with some enzymes [ 28, 29] .It can be speculated that the released form of HF is highly toxic.The result is consistent with the work of Suzuki et al. [ 30] who also indicated that fluoride exists mostly as HF at pH 2.3,which is able to penetrate the microbe and dissociate to H⁺and F^-.

3.3 Mechanism of competitive complexation of fluoride by aluminum

The detrimental effect of fluoride on bacterial growth can be overcome by the addition of aluminum to the system(Fig.4).In the acidic system,the introduced aluminum ions can compete with hydrogen ions,producing complex and reducing the concentration of HF,as has been verified as the effective form of fluoride that affects the bacterial activity.

No lag phase was evident in the experiment performed with the biotic control group (Fig.4b).However,no bacterial growth is observed within 60 h during Fe²⁺biooxidation experiments performed with 0.5 g·L^-1 fluoride,and the oxidation-reduction potential stays only at 260 mV throughout the experiment.When the aluminum concentration increases to 0.5 g·L^-1,the growth is detected and a lag phase is observed.These point to the detoxification effect of aluminum on fluoride toxicity [ 21, 24] .When the aluminum concentration increases to 1.0 g·L^-1,the lag phase disappears.Nevertheless,the lag phase is longer than the former when the aluminum concentration increases to2.0 g·L^-1.The variation of ferrous iron concentration more intuitively reveals the effect of different F^-/Al³⁺concentration ratios on the utilization of ferrous iron by bacteria(Fig.4c).The ferrous concentration in the biotic control group decreases at the same rate with the group containing0.5 g·L^-1 fluoride and 1.0 g·L^-1 aluminum.

Fig.2 Fluoride toxicity to bacteria at different pH levels:a E_h curve,b ferrous oxidation curve and c bacterial growth curve

Fig.3 Formation of fluoride at different pH values

The variation of cell density is well in accordance with the variation of ferrous concentration.The growth of cells reaches maximum density at 50 h in the control group,and it changed little in the groups containing 0.5 g·L^-1 fluoride and 0.5 g·L^-1 aluminum (Fig.4d),reaching maximum density at 70 h.When the aluminum concentration increases to 1.0 g·L^-1,the negative effects of fluoride ions on Fe²⁺oxidation rate and bacterial specific growth rate are completely overcome by Al³⁺(Fig.5).

The Fe²⁺oxidation rate of the biotic control group is0.195 g·L^-1·h^-1,and the bacterial specific growth rate is0.084 h^-1.The presence of fluoride ions also reduces the specific growth rate and iron oxidation rate of bacteria.When the concentration of F^-is 0.5 g·L^-1,the specific growth rate is-0.066 h^-1,and the bacterial concentration decreases.The Fe²⁺oxidation rate is less than0.013 g.L^-1·h^-1,showing lower bacterial oxidation activity.When Al³⁺concentration is 0.5 g·L^-1,the bacterial specific growth rate could reach 0.045 h^-1,and Fe²⁺oxidation rate could be increased to 0.131 g·L^-1·h^-1.When the aluminum concentration increases to 1.0 g·L^-1,a significant increase in the bacterial specific growth rate could be achieved at 0.07 h^-1,and the Fe²⁺oxidation rate could be increased to 0.167 g·L^-1·h^-1,close to those of the biotic control group.With the increase in the concentration of Al³⁺,the specific growth rate and the Fe²⁺oxidation rate show a downward trend.The effect of different F^-/Al³⁺concentration ratios on the growth of bacteria varies when F^-and Al³⁺concentrations reach a certain proportion;in this case,the bacteria could tolerate a high F^-concentration in solution.

3.4 Thermodynamic analysis of competitive complexation of aluminum ions

The main competitive complex reactions between aluminum and hydrogen ions when aluminum ions are introduced in solution are as follows:

Fig.4 Effects of bacterial growth at different F^-/Al³⁺concentration ratios:a pH variation curves,b E_h curves,c ferrous oxidation curves and d bacterial growth curves

Fig.5 Effects of Fe²⁺oxidation rate and specific growth rate at different F^-/Al³⁺concentration ratios

The activity of all ions in this study is considered to be1.The Gibbs free energy data were derived from HSC 6.0thermodynamics software.These standard Gibbs free energies (G^θ) were fitted with a specific temperature(T) using Origin to obtain the G^θ(T) function of the ions,as shown in Table 1.The coefficient of correlation between the curve fitted using Origin and the true value is greater than 0.99;therefore,the fitting functions are reliable in thermodynamic calculation.

The thermodynamic reactions of competition complexation are shown in Fig.6.Reactions (8)-(10) could not take place spontaneously in the acidic system under standard conditions.The change of standard Gibbs free energy,ΔG^θ(T),of Reactions (5)-(7),(11) and (12) is both less than 0.These reactions could therefore be taken place spontaneously.According to Reactions (5)-(12),if there is no aluminum in the solution,Reactions (11) and (12) are the main reactions.When aluminum ions are introduced,aluminum would react with HF to generate .This destroys the complexing structure of HF and reduces the concentration of HF,protecting the growth of bacteria.The reactions are mainly Reactions (5)-(7),and the order ofΔG^θ(T) values according to reactions is (7)>(6)>(5).Therefore,the difficulty order of competitive complexation reaction of aluminum follows the sequence(10)>(9)>(8)>(7)>(6)>(5).Combined withΔG^θ(T),aluminum ions in solution can also compete with HF at three levels of competitive complexation.In the presence of aluminum ions,fluoride ions are most likely to appear in the form of but not in the form of HF and

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Table 1 G^θ(T) function of ions from HSC 6.0 thermodynamics software (273.15-373.15 K)

Fig.6 Correlation betweenΔG^θ(T) and temperature of Reactions(5)-(12)

3.5 Modeling study of interactions between Al and F

The effect of soluble complexes on the complexation of F^-and Al³⁺was investigated using the PHREEQC chemical equilibrium model.SOLUTION＿SPREAD module is an efficient numerical framework for the solution of different elements.The model parameters,solution reactions and constants are listed in Tables 2 and 3.

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Table 2 Parameters used for calculations by PHREEQC simulation software in this work

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Table 3 Model of reactions used for calculations by PHREEQC simulation software

^a Being value of

The composition of the bio-oxidation solution (aluminum,ferrous and magnesium) was used in a thermodynamic study to estimate the distribution of soluble and insoluble fluoride complexes species in the system.This analysis covers the beginning of the experiments when Fe²⁺and Mg²⁺are also the main iron species,and the stability constants of Fe²⁺/F^-and Mg²⁺/F^-complexes are small.Hence,the effect of interfering ions on the total fluoride in the solution could be ignored.

Under acidic conditions,the fluoride is mainly present in the form of AlF₂⁺,AlF²⁺,AlF₃,AlF₄^-,HF and F^-.AlF₅^2-and AlF₆^3-are not present,in accordance with the law of thermodynamics.Without Al³⁺,more than 85%fluoride is in the form of HF (Fig.7).The presence of Al³⁺evidently alters this situation obviously.When the concentration of Al³⁺is 0.5 g·L^-1 (i.e.,when F^-/Al³⁺concentration ratio is1:1),the concentration of HF is less than 3 mg·L^-1(0.578%of total fluoride).When the concentration of Al³⁺increases to 1.0 g·L^-1,the ratio of F^-/A1³⁺is 1:2,and the concentration of HF is less than 1 mg·L^-1 (0.2%of total fluoride),which is similar to the normal state.Therefore,the bacteria present higher fluoride tolerance when Al³⁺was used with the substrate.

Fig.7 Constitutional diagram of fluoride-aluminum complex in culture medium

The acid dissociation coefficient of HF is 3.18,which is quite lower than the first-step complexation con-stant of complex,which is 7.10(Table 3).This result means that complex is more stable than HF.Therefore,F^-in solution can be roughly considered as coming from the complex of in the solutions containing Al³⁺.For F^-/Al³⁺concentration ratios studied,the complex depends on the concentrations of F^-and Al³⁺in solution.The simulation indicates that when the concentration of Al³⁺is low,AlF₂⁺is the predominant complex,followed by AlF²⁺,and a small amount of AlF₃.The calculations indicate that when the concentration of Al³⁺increases,AlF²⁺is the predominant complex,followed by AIF₂⁺,and other species are almost nonexistent.With the increase in the concentration of aluminum ions in the soIution,the coordination number of species in the solution becomes smaller.No consensus has been reached regarding the main aluminum-fluoride complexes formed during bioleaching of fluoride-containing ores.Whereas Dopson et al. [ 31, 32] suggested AlF₂⁺as the main complex,Brierley and Kuhn [ 8] proposed AlF²⁺as the predominant species.Furthermore, complexes were the main complex in the system when the aluminum was added in the solution.The HF concentration decreases rapidly to below 1×10^-4 mol·L^-1,which seems to be the threshold for bacterial growth inhibition.The toxicity of aqueous fluoride therefore seems to be strongly dependent on fluoride speciation and the presence of complexing ligands such as aluminum.

3.6 Complexation mechanism ofin acidic system

After 60-min complexation reaction,F^-concentration was measured (Fig.8).When F^-/Al³⁺concentration ratio increases from 1:1 to 5:1,the concentration of free F^-in the solution increases.When F^-/Al³⁺concentration ratio is5:1,the concentration of free F^-is about 3.65×10^-2g.L^-1,which is much lower than the initial concentration(1 g·L^-1),indicating that Al³⁺strongly complexes F^-.With the decrease in F^-/Al³⁺concentration ratio,the concentration of free F^-decreases markedly,as low as1×10^-5 g·L^-1,indicating that F^-is completely complexed with Al³⁺.

The average coordination number in the solution can be calculated by the definition of the Bjerrum’s function.The measured concentration of free F^-was substituted into the Bjerrum’s function,as shown in Fig.9.In the acidic system,F^-and Al³⁺could form complex complexes with coordination numbers of 1-4.With the decrease in F^-/Al³⁺concentration ratio, complex moves toward the low coordination numbers.The speciation of complex could be controlled by adjusting F^-/Al³⁺concentration ratio in the medium;thus,the bacteria could tolerate the high-fluoride environment.

Fig.8 Relationship between F^-/Al³⁺concentration ratio and free fluoride concentration

Fig.9 Relationship between F^-/Al³⁺concentration ratio and coor-dination number

4 Conclusion

In this study,the effect of aluminum on the oxidative activity of leaching bacteria in a fluoride-containing bioleaching system was assessed.Experimental results indicate that at the typically bioleaching pH levels,the predominant of the formation of fluoride is HF,as verified to be the form of fluoride that affects the bacterial activity.Under acidic conditions,Al³⁺can compete with H⁺to complex with F^-,which is in accordance with the laws of thermodynamics.The addition of aluminum could significantly decrease the concentration of HF and finally reduce the toxicity of fluoride to bacteria.Simulation and experimental data both indicate that the speciation of complex can be controlled by adjusting F^-/Al³⁺concentration ratio in the medium.