The influence of biologically produced sulfide-containing solutions on nickel and cobalt precipitation reactions and particle settling properties

Hydrometallurgy 189 (2019) 105142

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The influence of biologically produced sulfide-containing solutions on nickel and cobalt precipitation reactions and particle settling properties

T

Yun Liua, , Antonio Serranoa, James Vaughanb, Gordon Southamc, Ling Zhaoa, Denys Villa-Gomeza ⁎

a

School of Civil Engineering, The University of Queensland, 4072 QLD, Australia School of Chemical Engineering, The University of Queensland, 4072 QLD, Australia c School of Earth and Environmental Sciences, The University of Queensland, 4072 QLD, Australia b

ARTICLE INFO

ABSTRACT

Keywords: Aggregation Biological sulfate reduction Metal recovery Mining and metallurgical wastewater Particle settling Sulfide precipitation

Wastewater from mining and metallurgical activities provides a potential profit source for recovering leftover but valuable metals such as nickel (Ni) and cobalt (Co). For this, sulfide precipitation is an attractive option, although the cost of providing chemical sulfide and the poor settling of metal sulfides discourage its application. As an alternative, sulfide-containing solution (i.e. biogenic sulfide) can be produced on site by sulfate reducing bacteria that can utilize sulfate present in the wastewater, thus reducing reagent and transportation costs. Additionally, the various components in biogenic ‘sulfide’, i.e., bacterial cells, cell by-products and exopolymer may enhance the settling properties of Ni and Co sulfide, thus improving metal recovery. In this project, the effect of using chemical sulfide and four biogenic sulfide-containing solutions for Ni and Co precipitation and settling was evaluated. The biogenic sulfide-containing solutions were obtained from four bioreactors fed with volatile fatty acids (VFA), leachate from fermentation of municipal solid waste, ethanol and lactate, respectively. The results showed that VFA-fed biogenic sulfide enhances the Ni precipitation reaction as compared with chemical sulfide, whereas Co was effectively precipitated from solution (> 98%) regardless of the sulfide source and metal concentration. Settling extent of both metal sulfides reached up to 80–90% in lactate-fed biogenic sulfide, while < 20% of the precipitates settled by reaction with chemical sulfide. Phosphate was the main component affecting particle settling whereas acetate and sulfate exhibited a smaller influence. This research highlights the importance of the different components from the biogenic sulfide-containing solutions in the precipitation and settling of Ni and Co precipitates, demonstrating enhanced prospects of metal recovery through biological sulfate reduction process.

1. Introduction In recent years, world demand and prices of nickel (Ni) and cobalt (Co) have increased considerably (Anonymous, 2017; Elshkaki et al., 2017), thus making the extraction of the metals from secondary streams attractive to the metal refinery industry (Simonnot et al., 2018). Recovering Ni and Co can also create revenue for remediating the associated waste streams that already pose a great risk to the environment due to the toxicity and non-degradability of these metals. Ni and Co are both present in waste streams from the mining and processing of Ni ores, and their concentrations may vary according to the ore type and process control (Crundwell et al., 2011; Mudd, 2009). The recovery of both metals from solution can be achieved by sulfide precipitation, which has advantages compared to the traditional hydroxide

⁎

precipitation. For instance, sulfide precipitation can be effective even with low metal concentrations and low pH solutions, as the metal sulfides are highly insoluble. Besides, sulfide precipitation is more selective for Ni and Co than hydroxide precipitation over Mn, Mg, Ca and Al, and the sludge formed has better thickening properties, which can be easily reprocessed (Villa-Gomez and Lens, 2017; Lewis, 2010). In conventional sulfide precipitation, either chemical H2S gas or NaHS are used, although the challenges of providing chemical sulfide requiring high energy input and transportation cost can limit its application (Yamada and Tamura, 1990). Moreover, the recovery of the metal sulfides can be difficult due to the small particle size of the precipitates, which leads to poor settling properties (Lewis, 2010; Villa-Gomez et al., 2014). The formation of fine particles is a result of the high degree of supersaturation, which causes excessive nucleation of fine particles rather than growth of larger particles (Al-Tarazi

Corresponding author. E-mail address: [email protected] (Y. Liu).

https://doi.org/10.1016/j.hydromet.2019.105142 Received 8 March 2019; Received in revised form 1 August 2019; Accepted 25 August 2019 Available online 26 August 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.

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Table 1 Biogenic sulfide sources. Sulfate reducing bioreactors

Working volume (L)

Substrate

Effluent label

Ref

Continuously stirred-tank reactor Continuously stirred-tank reactor Up-flow anaerobic sludge blanket reactor Continuously stirred-tank reactor

1.4 1.4 1.4 3.0

VFA mixture Leachate from the fermentation of MSW Ethanol Lactate

VFA-S Leachate-S Ethanol-S Lactate-S

(Patel, 2018) (Patel, 2018) (Villa-Gomez et al., 2017) (Liu et al., 2019)

Table 3 Batch experimental design.

et al., 2004; Mokone et al., 2010). Indeed, conventional precipitation of mixed NieCo sulfides associated with pressure acid leaching of Ni laterites is carried out at elevated temperatures and pressures and requires significant reaction seeding to achieve the required reaction rate, extent and a precipitate that can be readily settled and filtered (Roy, 1961). An alternative to the addition of chemical sulfide is the use of biological sulfate reduction (BSR) that allows the production of sulfidecontaining solution on site, using the sulfate present in the wastewater (Villa-Gomez and Lens, 2017; Kaksonen and Puhakka, 2007). In this process, sulfate-reducing bacteria (SRB) transform the sulfate to sulfide by using an electron donor, i.e., H2 (autotrophic) or a low molecular weight organic compound (heterotrophic) (Papirio et al., 2013). The biologically produced sulfide-containing solution, also namely biogenic sulfide, is distinguished from chemical sulfide via the by-products and leftover reagents from microbial activity, including volatile fatty acids (VFA), phosphate and sulfate, depending on the substrates used. Some of these components were reported to affect the particle size of the metal sulfide precipitates (Esposito et al., 2006; Mokone et al., 2012) that might result in a better settling properties. Previous research assessing the effect of biogenic sulfide on metal precipitation and settling mainly used a sulfide to metal molar ratio ranging from 0.5 to 2 (Reis et al., 2013; Mokone et al., 2012). However, when the sulfide to metal molar ratio is high (e.g. 3–16) as a result of the general high sulfate concentrations in metal-containing wastewaters and difficulty to separate the sulfate reduction and the metal sulfide precipitation process, there is still a lack of knowledge regarding the settling properties of metal sulfides and the role of the main components within the biogenic sulfide in the precipitation processes. Therefore, the aim of this study was to evaluate Ni and Co precipitation and settling properties for assessing the possibility of metal recovery in a single stage reactor. Studies on the differences between Ni and Co particle settling is also highly relevant when metals separation is a challenge due to similar chemical behavior of Ni and Co (Flett, 2004).

Experiment (Exp.)

Sulfide source

Metals

Initial metal concentration (mg/L) a

Sulfide to metal molar ratio

Compounds

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Na2S Na2S VFA-S VFA-S Na2S Na2S VFA-S VFA-S Na2S Na2S Na2S Na2S Na2S Na2S VFA-S VFA-S

Ni Co Ni Co Ni Co Ni Co Ni Co Ni Co Ni Co Ni + Co Ni + Co

100 100 100 100 500 500 500 500 500 500 500 500 500 500 100/100 500/500

16.3 16.3 16.3 16.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 8.2 1.6

– – – – – – – – Sulfate Sulfate Acetate Acetate Phosphate Phosphate – –

a

Initial metal concentration refers to the total Ni/Co concentration in the serum bottles after metal injection.

obtaining biogenic sulfide-containing solutions with different matrix compositions (Table 1). The four bioreactors were operated for > 100 days at different organic loading rates while maintaining a chemical oxygen demand – sulfate ratio (COD/SO4) of 0.67. The effluent was collected and stored in serum bottles at 4 °C, with N2 in the headspace. Characteristics of the different sulfide matrices are shown in Table 2. 2.2. Experimental design 2.2.1. Batch experiments Sulfide precipitation batch experiments were carried out in 200 mL serum bottles with a working volume of 160 mL to compare Ni and Co precipitation reaction extent and particle size distribution (PSD) between chemical sulfide and VFA-S. The VFA-S was chosen due to the high sulfide concentration, allowing for the sufficient metal precipitation (Table 2). The pH of the chemical sulfide was adjusted to 7.8, as in the VFA-S, by using 20 wt% HCl. NiCl2 (5000 mg/L Ni) and/or CoCl2 (5000 mg/L Co) stock solutions were injected into the serum bottles containing 160 mL of sulfide-containing solution (chemical or biogenic) and N2 in the headspace. The effect of sulfide sources, metal concentration, and relevant compounds (sulfate, phosphate and acetate) were investigated (Table 3). Sulfate, phosphate and acetate were chosen as these were found to be significant components in the biogenic

2. Material and methods 2.1. Source of sulfide Chemical sulfide and four biogenic sulfide effluents from different lab-scale sulfate-reducing bioreactors were used in this study. The chemical sulfide was prepared by dissolving analytical grade Na2S·9H2O into anoxic water which was prepared by boiling distilled water and cooling it under N2 to remove the dissolved oxygen. The biogenic sulfide-containing effluents were collected from bioreactors fed with different substrates, i.e., a VFA mixture, leachate from the fermentation of municipal solid waste (MSW), ethanol and lactate, Table 2 Characteristics of sulfide-containing solutions from different sources. Sulfide source

pH

VFA-S Leachate-S Ethanol-S Lactate-S

7.8 7.9 6.4 7.4

Sulfide (mg/L) ± ± ± ±

0.1 0.1 0.1 0.2

886 816 134 320

± ± ± ±

Sulfate (mg/L) 6 20 2 3

1230 1230 2083 2218

± ± ± ±

77 47 70 49

Phosphate (mg/L)

Ammonia (mg/L)

Nitrate (mg/L)

Acetate (mg COD/L)

Propionate (mg COD/L)

Butyrate (mg COD/L)

Valerate (mg COD/L)

816 ± 20 110 ± 2 77 ± 1 609 ± 54

131 ± 3 175 ± 4 65 ± 1 95 ± 6

3 ± 0.1 5 ± 0.1 0 5±4

366 ± 9 63 ± 20 1061 ± 25 1278 ± 8

198 ± 31 263 ± 59 0.5 ± 0.5 39 ± 27

10 ± 1 61 ± 20 29 ± 1 7±1

17 ± 2 119 ± 21 0.5 ± 0.4 6±1

2

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VFA-S. The effect of these relevant compounds was studied by adding them into the chemical sulfide, at the same concentrations encountered in the VFA-S. After metal injection, solutions in the bottles were magnetically stirred for 10 min. After this, liquid samples were extracted from the bottles and all the metals passed through 0.22 μm filters were referred to as soluble metals. Samples were also collected for PSD analysis. The experiments were done in triplicate, and standard deviations were provided. All experiments were conducted at room temperature (25 ± 2 °C), and all the reagents used were analytical grade (Merck).

sampling port (H), at given time (i) (Eq. (1)). Samples taken from the top port were regarded as the surface samples. H2 H1

p(H2 i) + p(H1 i) 2

p(Hi)dh

(H2

H1)

(1)

The overall settling extent of metal precipitates above a depth at any sampling time, R(H, i), can be calculated through Eq. (2):

R(Hi)

1

H2 H1

p(Hi)dh × 100%

(2)

The equivalent overflow rate is equal to the apparent settling velocity (v) calculated from the depth of the sampling port divided by the sampling time (Eq. (3)).

2.2.2. Column experiments Subsequently, column experiments were conducted to characterize the overall precipitation and settling rate of Ni and Co sulfide precipitates. A 1.5 L column with a diameter of 9 cm and three sampling ports at 3, 10 and 17 cm from the base was used (Fig. 1). All four biogenic sulfide-containing solutions (Table 2) as well as the chemical sulfide (896 mg/L sulfide) were studied in sequence. Each column experiment was carried out by injecting Ni and Co stock solutions to the column with one of the sulfide-containing solutions inside. An oxygen-free environment was maintained throughout the whole experiment by flushing N2 and sealing the column. Ni and Co concentrations were 100 mg/L and 10 mg/L, respectively, within the concentration range of a typical hydrometallurgical process. The pH of chemical sulfide mixture after metal addition was adjusted to ~7 with 20 wt% aqueous HCl. Magnetic stirring of the experiments was maintained for 10 min. Subsequently, samples in triplicate were taken from the three sampling ports at different time intervals, over 7 h, to measure total and soluble metal concentrations. All experiments were conducted at room temperature (25 ± 2 °C).

Overflow rate = v =

H i

(3)

2.4. Analytical methods Total and soluble Ni and Co concentrations were measured with an atomic absorption spectrometer (AAS) (AAnalyst 400, Perkin Elmer). For total Ni and Co measurements, samples were digested with HNO3 and heated at 120 °C for two hours to dissolve all the metal precipitates. For soluble metal determination, samples were filtered (0.22 μm) first and then diluted in 2% HNO3. It needs to be noted that both dissolved metals and metallic particles smaller than 0.22 μm are accounted as soluble metals. The PSD of Ni and Co sulfide precipitates were analyzed by a PSS Nicomp Accusizer 780 AD, with 0.5 mL of the samples diluted in 80 mL of membrane filtered high purity “Milli-Q” water to minimize particles in the dilution solution. Dissolved sulfide and sulfate were determined using a UV-VIS-NIR spectrophotometer (Cord-Ruwisch, 1985) and Dionex™ ICS-1100 Ion Chromatography System (IC), respectively. Soluble COD was analyzed using Spectroquant cells and a SQ 118 Photometer (Merck) after stripping H2S with N2. VFA composition was measured through an Agilent 7890A gas chromatograph with mass spectrometer (GC–MS). Nitrogen and phosphorus species were analyzed through Lachat QuicChem8500 flow injection analyzer (FIA).

2.3. Settling velocity calculation The Overcamp method was used for the data settling analysis during the column experiments (Overcamp, 2006), in which the obtained metal precipitates concentration (= total metal - soluble metal) at each sampling port and timing were integrated to determine the percentage (p) of the precipitates remaining in the column above the depth of the

2.5. Statistical analysis Visual Minteq® v3.1 was used to predict the possible Ni and Co speciation in the four biogenic sulfide-containing solution. The concentrations of the components shown in Table 2 were input into the model. Temperature, Ni and Co concentrations were maintained same as in the column experiments. The Pearson product moment correlation was applied using SigmaPlot® (version 14.0) to relate main components in different sulfide matrices (i.e., VFA-S, Leachate-S, Lactate-S, chemical sulfide) with Ni and Co settling extent. The pair(s) of variables with positive correlation coefficients and P values below 0.05 tend to increase together. For the pairs with negative correlation coefficients, one variable tends to decrease while the other increases. 3. Results 3.1. Ni/Co sulfide precipitation in the batch experiments Table 4 shows that with an initial metal concentration of 100 mg/L, Ni precipitation extent in chemical sulfide was 62.1 ± 2.9%, in comparison to 77.5 ± 2.2% when biogenic sulfide (i.e., VFA-S) was used. The experiments with 500 mg/L Ni resulted in a higher precipitation with the two studied sulfide sources, i.e., 93.0 ± 5.1% and 99.2 ± 0.2% (Table 4). The PSD analysis showed that the percentage of larger Ni particles increased significantly in VFA-S (8.1 ± 0.3 μm of mean size) in comparison to chemical sulfide (1.0 ± 0.1 μm of mean size) at 100 mg/L Ni (Fig. 2a and b). Higher Ni concentration (500 mg/

Fig. 1. Schematic of the column experiments. Note that the working volume decreased with the sample extraction. 3

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Table 4 Ni and Co precipitation extent in different sulfide sources. Initial metal concentration (mg/L)

Chemical sulfide (Exp. 1–2, 5–6) Biogenic VFA-S (Exp. 3–4, 7–8) Sulfate-containing chemical sulfide (Exp. 9–10) Acetate-containing chemical sulfide (Exp. 11–12) Phosphate-containing chemical sulfide (Exp. 13–14) Biogenic VFA-S (bi-metal, Exp. 15–16) a

100

500

Ni (%)

Co (%)

Ni (%)

62.1 ± 2.9 77.5 ± 2.2 – – – 97.7 ± 0.4

99.0 ± 0.7 98.3 ± 0.3 – – – 99.5a

93.0 99.2 90.6 98.4 99.0 99.3

± ± ± ± ± ±

Co (%) 5.1 0.2 1.3 0.6 0.4 0.1

100.0a 99.9a 99.9a 100.0a 100.0a 99.9a

Standard deviations of these values are < 0.1.

Fig. 2. PSD of Ni and Co precipitates formed in chemical sulfide or biogenic VFA-S, with initial Ni or Co concentration of 100 mg/L (a, b) or 500 mg/L (c, d).

L Ni) also increased the mean size of the Ni sulfide precipitates to a final size of 2.5 ± 0.2 μm in chemical sulfide and 22.3 ± 0.1 μm in VFA-S (Fig. 2c and d). A measurement artifact caused a decrease in percentage at ~22 μm. By contrast, the batch experiments with Co maintained a higher reaction extent (larger than 98%) and a small change in PSD regardless of the sulfide source and metal concentration throughout all the experiments (Table 4 and Fig. 2). The batch experiments with chemical sulfide along with acetate and phosphate showed an increase in Ni precipitation from 93.0 ± 5.1% to nearly 99%, while in the experiments along with sulfate, it decreased to 90.6 ± 1.3% (Table 4). The addition of any of the three compounds increased the proportion of larger Ni sulfide particles as compared with the experiments with only chemical sulfide (Fig. 3a). Contrary, the

experiments with Co showed little change in precipitation and PSD with the addition of sulfate, acetate or phosphate (Table 4 and Fig. 3b). The experiments with the joint addition of 100 mg/L of both Ni and Co showed an increase in metal precipitation as compared to the experiments with individual addition of metal: from 77.5 ± 2.2% to 97.7 ± 0.4% for Ni and from 98.3 ± 0.3% to 99.5 ± 0.1% for Co (Table 4). Contrary, no difference was observed with the experiments at 500 mg/L of Ni and Co (Table 4). 3.2. Ni/Co sulfide precipitation and settling in the column experiments The column experiments showed that in the chemical sulfide, VFAS, Lechate-S and Lactate-S, around 98% of the Ni and Co precipitated 4

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Fig. 3. PSD of (a) Ni, (b) Co sulfide precipitates formed in chemical sulfide with addition of sulfate, acetate and phosphate, with initial Ni or Co concentration of 500 mg/L.

with sulfide. According to the Visual Minteq modelling (Table A1), > 99% of the Ni and Co should be in form of insoluble Ni and Co sulfide. In contrast, the column experiments using Ethanol-S resulted in < 10% Ni and Co precipitated. This low precipitation could be related to the low amount of phosphate and high concentration of VFA (Table 2) that resulted in an acidic pH condition that affected the solubility of Ni and Co (Fig. 4). This solubilization is significantly different from the Ni and Co sulfide speciation predicted by the Visual Minteq modelling, i.e. 70% and 54%, respectively (Table A1). The difference between the experimental precipitation results and the predicted formation of Ni and Co sulfide on Visual Minteq denoted the limitation of using a speciation model to predict the real behavior of the evaluated precipitation process. Some processes affecting precipitation not considered in the model used may include complexation with extracellular polymeric substances or soluble microbial products (Zou et al., 2015). On the other hand, the precipitates produced in the biogenic sulfide experiments displayed higher settling rates than the experiments with chemical sulfide regardless the sampling port (Fig. 5 and Fig. A.1). Concretely, Lactate-S and VFA-S presented better Ni and Co settling (up

to 80–90%) than Leachate-S (up to 40–50%) and chemical sulfide (< 20%) at the same overflow rate. Although Ni and Co settling extents were similar, Co precipitates settled slightly faster than Ni in all cases, and their difference increased with lower overflow rate (Fig. 5). Therefore, better separation between Ni and Co was achieved at lower overflow rate and higher retention time. Table 5 shows the results of the Pearson product moment correlation and significance (P) of the main components in biogenic sulfide matrices with regard to the Ni and Co precipitates settling extent. A hierarchy in the significance of the examined components in relation to the Ni and Co precipitates settling follows phosphate > sulfate > acetate > sulfide. However, the P values were higher than the significance limit (P = .05), which may be due to the number of samples used. 4. Discussion 4.1. Effect of sulfide source on Ni and Co precipitation This study shows that Ni precipitation was more effective with biogenic sulfide (i.e., VFA-S) than with chemical sulfide, whereas Co sulfide precipitation was over 98% regardless of the sulfide source. Due to the high sulfide to Ni molar ratio (Table 3), Ni sulfide fines were generated in chemical sulfide and some of these fines passed through the filter during metal analysis (Lewis, 2010; Villa-Gomez et al., 2011), resulting in the lowest recovery by precipitation (Table 4). Veeken et al. (2003) also reported that when sodium sulfide was used, Cu and Ni showed lower precipitation levels compared with Cd, Pb and Zn, and the presence of Cu sulfide particles was observed in the filtrate. The formation of these fine particles can be attributed to the low solubility products and high supersaturation levels of these metal sulfides (Lewis and van Hille, 2006). On the other hand, biogenic VFA-S reduced the number of fine particles (Fig. 2) reaching a higher Ni precipitation (77.5 ± 2.2%) at 100 mg/L Ni, which may be attributed to the residual compounds after the SRB metabolism, such as the chelating agents, dissolved organic matter, macro- and micro-nutrients (Mokone et al., 2012; Moreau et al., 2004). These substances could delay the metal sulfide precipitation reaction through complexation with metals, allowing for the crystal growth and aggregation. This study shows that acetate and phosphate were responsible for higher Ni precipitation and differences in the particle size, while sulfate had less effect on Ni precipitation (Table 4 and

Fig. 4. Percentage of soluble Ni/Co and pH after metals sulfide formation in different sulfide matrices. 5

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Fig. 5. Settling extent of (a) Ni precipitates and (b) Co precipitates as a function of overflow rate in various sulfide matrices at middle sampling port (H = 10 cm).

could reveal other means to separate Co from Ni based on the possible interactions of Ni with other components.

Table 5 Pearson product moment correlation of Ni/Co precipitates settling extent against main components in different sulfide matrices. Ni

Sulfide Sulfate Phosphate Acetate

Co

Correlation coefficient

P value

Correlation coefficient

P value

−0.466 0.899 0.913 0.766

0.534 0.101 0.087 0.234

−0.460 0.892 0.920 0.764

0.540 0.108 0.080 0.236

4.2. Effect of sulfide source on the settling properties of Ni and Co sulfide precipitates The settling properties of the Ni and Co precipitates in the column experiments strongly differed between the chemical sulfide and the biogenic sulfide sources. Moreover, the differences in the components concentrations within the biogenic sulfide matrices also resulted in different settling rates. The settling rate of Ni and Co in the biogenic VFA-S and Lactate-S was higher than the obtained for Leachate-S and Ethanol-S (Fig. 5). Such differences can be attributed to particle aggregation (Al-Tarazi et al., 2004; Villa-Gomez et al., 2012) that is influenced by the substances present in the sulfide matrix (Table 2). Phosphate was found to be the most significant component affecting Ni and Co settling compared with sulfide, sulfate and acetate (Table 5). It is worth to note that the formation of aqueous Ni and Co sulfate could occur at low pH, as it was predicted by the Visual Minteq model in the column experiments with Ethanol-S (Table A1). However, at neutral pH almost all the initial sulfate should remain in free form (Table A1). In VFA-S and Lactate-S the phosphate concentration was 5 times higher than for Leachate-S (Table 2), relating to settling extents of up to 80–90% of Ni and Co precipitates. Phosphate can be found widely in many waste streams containing organics such as municipal wastewater or phosphogypsum waste leachate (Salo et al., 2018; Sanchez-Andrea et al., 2012; Yeoman et al., 1988). Therefore, the use of these waste streams as an electron donor for BSR can provide phosphate, and ultimately, enhancing metal recovery (Reis et al., 2013; Salo et al., 2018; Villa-Gomez et al., 2012). Similar to phosphate, acetate enhanced metal precipitation and PSD in the batch experiments (Table 4 and Fig. 3). This VFA is usually present in biogenic sulfide as it is generated due to the incomplete oxidation of electron donors by SRB (Sanchez-Andrea et al., 2014). The remaining sulfate, not consumed by SRB, could also increase the particle size of these fines (Fig. 3), although its effect on metal precipitation was limited (Table 4). Sulfide showed a negative correlation coefficient in both Ni and Co (Table 5), as it controls the level of supersaturation (Sampaio et al., 2010) and thus, the size of the precipitates. In contrast, in the biogenic Ethanol-S experiments, the extremely low Ni and Co precipitation (< 10%) and settling could be attributed to the low sulfide to metal molar ratio where other substances may compete with sulfide for the metal (Fig. 4) (Esposito et al., 2006). The

Fig. 3a). Acetate is one of the chelating agents enabling to complex with metals (Esposito et al., 2006), while phosphate was found to form apatite (Ca5(PO4)3(OH)) in the presence of CaCl2.2H2O, a typical component in the mineral medium for bacterial growth, which can adsorb metals to precipitate (Villa-Gomez et al., 2012). However, other research has reported that in the precipitation of Zn with chemical sulfide, higher Zn precipitation values and larger particle size were achieved in comparison with biogenic sulfide (Esposito et al., 2006). This contradiction could be explained by the difference in sulfide to metal molar ratio in the two studies (> 1 and < 1, respectively). At low sulfide to metal molar ratios, various compounds in biogenic sulfide (e.g., chelating agents, dissolved organic matter, macro- and micronutrients) could compete with sulfide to precipitate metals and inhibit nucleation (Smiljanić et al., 2011). In contrast, when sulfide was higher than the stoichiometric value, these substances tend to improve the particle size distribution of the fines and enhance aggregation rather than metal complexation (Deonarine et al., 2011; Moreau et al., 2004; Villa-Gomez et al., 2012). Despite of the similar solubility product (Ks) of Ni and Co sulfide, e.g. log Ks (beta NiS) = −11.10, log Ks (beta CoS) = −11.07 at 25 °C (Stumm and Morgan, 1995), this study shows that the Ni and Co precipitation and particle size distribution were different. Unlike Ni, the Co results remained consistent regardless of the sulfide source (Table 4, Fig. 2 and Fig. 3). This can be related to the faster water exchange kinetics of Co as compared with Ni (Williams et al., 2013). In contrast, this indicates that Ni had more time to interact with other components from the biogenic matrix, allowing for the formation of larger particles (Fig. 2 and Fig. 3). These additional compounds limited the formation of ultrafine particles, allowing better precipitation of Ni. Unfortunately, the joint addition of Ni and Co into biogenic VFA-S led to similar metal precipitation (Table 4) due to co-precipitation of both metals (Demopoulos, 2009; Lewis and Swartbooi, 2006). Still, further research 6

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low pH (4.9) in Ethanol-S after metal injection resulted in an even lower amount of sulfide. Karbanee et al. (2008) reported that at pH of 4.24, sulfide species in the liquid mainly existed in the form of non-reacting H2S affecting Ni removal. Therefore, these results highlight the importance of the different components from the biogenic sulfide, especially the phosphate, in the precipitation and settling of Ni and Co. This is a promising achievement for the selective recovery of both metals, given the different responses to the complex components from the biogenic sulfide matrix.

Co was effectively precipitated regardless of the sulfide source and metal concentration. The components of the matrices of the biogenic sulfide-containing solutions had a strong influence on the settling properties of both Ni and Co sulfide precipitates. Importantly, residual phosphate, and to a lesser extent, sulfate and acetate existing in biogenic sulfide, were the main factors from the biogenic sulfide matrix that influenced the metal sulfide precipitation extent and their settling properties. Acknowledgements

5. Conclusions

The research was financially supported by Queensland Government (Australia) through 'Women's Research Assistance Program (WRAP)'. The authors would like to thank the laboratory staff of the Australian Microscopy & Microanalysis Research Facility, Hydrometallurgy Group and Solid Waste Management at UQ for the analytical support.

This research shows that Ni was precipitated more effectively with the VFA-fed biogenic sulfide than chemical sulfide because the additional compounds prevented the formation of ultrafine particles capable of passing through a membrane filter and that do not settle. Meanwhile, Appendix A

Fig. A1. Settling extent of (a) Ni precipitates and (b) Co precipitates as a function of overflow rate in various sulfide matrices at bottom sampling port (H = 17 cm).

Table A1

Percentage distribution of the principal species according to the analytical characterization of the biogenic sulfides obtained through the Visual Minteq® speciation software. Component Cobalt

Nickel

Sulfide

Sulfate

Species name +2

Co Co-Acetate+ CoHS+ CoSO4 (aq) CoHPO4 (aq) Ni+2 NiHS+ NiSO4 (aq) NiH2PO4+ NiHPO4 (aq) Ni-Acetate+ Ni-(Acetate)2 (aq) HS− H2S (aq) NiHS+ CoHS+ SO4−2 HSO4− NiSO4 (aq) CoSO4 (aq) NH4SO4−

VFA-S

Lechate-S

Ethanol-S

Lactate-S

0.064

0.064

99.799 0.029 0.101 0.033 99.904 0.015

99.882 0.033 0.016 0.033 99.936 0.017

22.836 3.017 54.906 19.178 0.048 14.933 70.006 12.54 0.045 0.025 2.265 0.168 0.808 68.431 28.532 2.229 97.279 0.052 0.985 0.15 1.532

0.252 0.053 99.281 0.185 0.226 0.13 99.642 0.095 0.092 0.031

0.041 82.324 10.905 6.158 0.613 97.013

83.641 9.005 6.689 0.666 95.788

2.984

4.21

7

61.257 20.05 17.006 1.688 97.966

2.026

(continued on next page)

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Table A1 (continued) Component Phosphate

Ammonia Nitrate Acetate

Propionate

Butyrate Valerate

Species name

VFA-S

Lechate-S

Ethanol-S

Lactate-S

−2

88.278 11.705

89.935 10.045

1.139 98.592 0.112 0.095 0.052

75.743 24.224

HPO4 H2PO4− H3PO4 NiH2PO4+ NiHPO4 (aq) CoHPO4 (aq) NH4+ NH4SO4− NH3 (aq) NO3− NiNO3+ Acetate− H-Acetate (aq) Co-Acetate+ Ni-Acetate+ Ni-(Acetate)2 (aq) Propionate− H-Propionate (aq) Ni-Propionate+ Co-Propionate+ Butyrate− H-Butyrate (aq) Ni-Butyrate+ Valerate− H-Valerate (aq)

92.093 5.262 2.644 100

91.075 5.556 3.369 100

99.927 0.072

99.94 0.059

99.905 0.095

99.923 0.077

99.917 0.083

99.932 0.068

99.912 0.088

99.928 0.072

90.776 9.22 99.969 0.029 68.196 31.529 0.028 0.215 0.032 62.216 37.659 0.11 0.015 65.245 34.714 0.037 63.956 36.044

0.025 90.105 8.881 1.013 100 99.818 0.178

99.765 0.233 99.794 0.205 99.783 0.217

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