Bosio 2007 - Integrated Bacterial Process For The Treatment Of A Spent Nickel Catalyst

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Integrated bacterial process for the treatment of a spent nickel catalyst V. Bosio, M. Viera ∗ , E. Donati Centro de Investigaci´on y Desarrollo de Fermentaciones Industriales (CINDEFI), Departamento de Qu´ımica, Universidad Nacional de La Plata – Consejo Nacional de Investigaciones Cient´ıficas y Tecnol´ogicas (CONICET), 50 y 115, 1900 La Plata, Argentina Received 9 April 2007; received in revised form 20 September 2007; accepted 29 October 2007

Abstract Integrated biological processes involving the dissolution and subsequent precipitation have been used for the treatment of the spent material from the hydrogenation of vegetable oil containing a high-level of nickel. Our results show that nickel was successfully leached using Acidithiobacillus thiooxidans. The percentages of nickel leached using A. thiooxidans were higher than those obtained with dilute sulphuric acid solutions. Due to the physical characteristics of the residue, the best results were obtained when the leaching process was carried out using sulphuric acid biogenerated by an A. thiooxidans biofilm. The recovery of nickel from the leachates was performed at room temperature by precipitating with sulphide generated by Desulfovibrio cells. Indirect precipitation using sulphide generated in Desulfovibrio sp. cultures allowed the recovery of nickel as the very insoluble nickel sulphide. © 2007 Elsevier B.V. All rights reserved. Keywords: Nickel; Recovery; Spent catalyst; Desulfovibrio; Acidithiobacillus thiooxidans

1. Introduction Several chemical industries use large quantities of solid catalysts containing valuable heavy metals such as Pt, Pd, V, Mo, and Ni. Due to the gradual build up of impurities which eventually plugs the pores and deactivates the catalysts, they often require replacement after 2 or 3 years of operation [1]. Spent catalysts are managed through: (i) chemical recovery and recycling of valuable metals for different applications, (ii) regeneration for reuse and (iii) landfilling. The regeneration of spent catalysts is only possible in a limited number of catalytic systems and can only be carried out a few times. Recycling of spent catalysts lowers the catalyst cost as well as the environmental pollution caused by catalyst waste. Landfilling for the disposal of spent catalysts is becoming increasingly difficult due to both the decreasing availability of landfill space and the concern for pollution arising from possible leaching of heavy metals.

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Corresponding author. Tel.: +54 221 4833794; fax: +54 221 4833794. E-mail address: [email protected] (M. Viera).

Suitable catalysts for hydrogenation processes can be economically produced using metallic nickel and other nickel compounds. According to the Environmental, Health, and Safety (EHS) Guidelines for Vegetable Oil Processing published by the International Finance Corporation [2], nickel catalysts from hydrogenation should be either: (i) recycled and recovered for reuse as a nickel catalyst, nickel metal, nickel salt, etc. or (ii) stored and disposed of according to the waste management guidance presented in the General EHS Guidelines. In most cases the spent catalysts are mixed with filtering aids (diatomaceous earth) and these mixtures should be treated in the same way as proposed for nickel catalysts. Approximately 6–9 kg of this mixture is generated per tonne of oil produced. In our country, the spent nickel catalyst used in the hydrogenation of vegetable oil is mixed with diatoms but no further treatment is performed. Thus, this solid residue accumulates in heaps, representing a potential hazard to human health and to the environment, and also a waste of a valuable non-renewable resource. Traditionally, metal recovery from spent catalysts has been carried out by leaching with inorganic acids or smelting [3]. These processes entail the use of acids in large-scale operations, which generate large volumes of potentially hazardous waste

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and gaseous emissions. Bioleaching processes are based on the ability of microorganisms to transform solid compounds into soluble and extractable elements that can subsequently be recovered. Bioleaching processes have been successfully applied on an industrial scale for the recovery of copper metal from natural ores and for the biooxidation pretreatment of refractory gold ores [4]. This approach can be considered as a ‘clean technology’ and this is associated with lower cost and energy requirements in comparison to non-biological processes [5]. Bacteria from the genus Acidithiobacillus are the most important microorganisms applied to metal solubilisation, particularly because they are autotrophic and tolerate high concentrations of heavy metals. Cells derive the energy required for their metabolism from the catalysis of the aerobic oxidation of reduced sulphur compounds including sulphides, elemental sulphur and thiosulphate. Furthermore, Acidithiobacillus ferrooxidans can use Fe(II) as an energy source [6]. The usefulness of these microorganisms in metal solubilisation – not only from ores and concentrates but also from solid wastes – is mainly related to their ability to generate acidic media (sulphuric acid is the last product during the oxidation of sulphur compounds) and oxidising media [Fe(III) is produced during the growth of A. ferrooxidans on Fe(II)]. After metals are released from the solid wastes, they should be recovered from the liquid phase. Precipitation with lime is the conventional technique applied for the immobilisation of metals. However, this treatment has some significant limitations in terms of applications and effectiveness as it usually results in production of mixture and unstable metal hydroxides [7]. Microbial processes are especially suitable techniques for the recovery of elements present in relatively low concentrations in solution where conventional techniques are usually less effective [5,8]. Sulphate reducing bacteria (SRB) are heterotrophic bacteria that grow under anaerobic conditions and reduce sulphate to sulphide. Most heavy metal sulphides have a very low-solubility product so that even a moderate output of sulphide can remove metals to levels permitted in the environment [9]. Several laboratory experiments have been carried out to study the possibility of using SRB to precipitate heavy metals from wastewater both in situ and ex situ [10–12]. There are a few examples of processes that utilise SRB in the treatment of contaminated water on an industrial scale. The best known is the groundwater treatment process installed at the Budelco Zinc Refinery (The Netherlands) since 1992 [13]. Integrated processes that involve dissolution and subsequent precipitation can be used for metal recovery. These processes are based on the ability of Acidithiobacillus to leach metals from soils or solid wastes. The metals ions, which are released as soluble ions in a highly acidic solution, are later removed by precipitation with sulphide produced by the activity of SRB [14–16]. In this paper we present the results of an integrated process aimed at the treatment of the spent nickel catalyst generated in the hydrogenation of vegetable oil process. Nickel was leached from the solid residue using A. thiooxidans cultures. The effects of several factors (pulp density, type of reactor, pH) were analysed. Abiotic leaching experiments with sulphuric acid were also performed. The recovery of nickel from the acidic solution

was carried out using the sulphide biogenerated by Desulfovibrio sp. 2. Materials and methods 2.1. Spent catalyst The spent catalyst used throughout this work had been employed in the hydrogenation of vegetable oil. The waste material was an amorphous black mass consisting of the spent catalyst, organic material adsorbed onto its surface (vegetable oil, products from incomplete hydrogenation, polyhydric alcohols, and fatty acids) mixed with a filtering aid (diatomaceous earth). The organic material adsorbed on the catalyst did not allow adequate contact between the waste material and the solution. As a result, one part of the spent catalyst was treated with two parts of xylene at 60 ◦ C during 1 h prior to use. The residue was washed with ethanol and dried at room temperature. This procedure was repeated at least twice. The nickel content was determined as follows. Two samples (0.1 g each) of the spent catalyst were treated with xylene and then digested with HNO3 and diluted to 50 ml with distilled water. The resulting samples were each filtered through a 0.45 ␮m membrane and the nickel concentrations in the final solutions were determined. The nickel concentration in the residue was 41 ± 2 mg of nickel per gram. 2.2. Microorganisms The strain of A. thiooxidans (DSM 11478) used throughout this work was routinely maintained in iron-free 9K culture medium [17] with 1% (w/v) powdered elemental sulphur, as an energy source. Sulphate reducing bacterium Desulfovibrio sp. (ATCC 49975) was used in the precipitation experiments. The strain was routinely maintained in Postgate’s B medium [18] at 4 ◦ C in 10 ml bottles sealed with a rubber stopper and aluminium cap. 2.3. Leaching experiments 2.3.1. Chemical leaching experiments Chemical leaching tests were carried out with solutions of sulphuric acid at different pH values (1.0, 2.0, 3.0 and 5.0). Solutions were prepared by adding concentrated sulphuric acid to distilled water. The leaching tests were conducted using 100 ml of the acidic solution in a 250 ml Erlenmeyer flask with 1% (w/v) pulp density of spent catalyst at 30 ◦ C and 150 rpm. All experiments were conducted in duplicate. Samples were collected, filtered and analysed for soluble nickel and pH. Samples were taken the following days: 1st, 2nd, 8th, 15th and 22nd. 2.3.2. Bioleaching experiments Bioleaching was performed in 250 ml Erlenmeyer flasks with 100 ml of iron-free 9K medium (pH 5.0), the spent catalyst at various pulp densities (0.2, 1 and 2%, w/v) and 1% powdered elemental sulphur. Flasks were inoculated with 10% (v/v)

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cells, harvested by filtering the grown culture with blue ribbon paper followed by centrifugation (20 min, 2000 × g) and then suspended in fresh medium. The centrifugation–suspension procedure was repeated until the pH was ca 5 (washed cells). In order to overcome the inhibition in the bacterial growth due to the alkalinity of the spent catalyst, two other sets of experiments were designed. Firstly, bacterial cells from a grown culture (unwashed cells) were directly inoculated to the flask (prepared as indicated above). In this way, the acidity of the inoculum avoided the rapid increase in pH and the consequent inhibition of bacterial. In the second set of experiments, flasks were prepared as described above (with the culture medium, powdered sulphur and the spent catalyst at different pulp densities), but the inoculation (with washed cells) was made after 24 h, prior to a new adjustment of the pH through the addition of sterile 1/10 sulphuric acid (for that purpose, about 0.7–1 ml was needed). To evaluate if A. thiooxidans was able to leach the nickel from the spent catalysts without adding sulphur, a flask with 1% pulp density was inoculated with unwashed cells. Control experiments were conducted using sterile culture medium. Cultures were incubated at 30 ◦ C in a rotary shaker at 150 rpm. All experiments were carried out in duplicate. Samples were withdrawn at regular intervals for the analysis of pH, nickel concentration and bacterial population. Samples were taken the following days: 1st, 2nd, 8th, 15th and 22nd. Evaporation was compensated prior to sampling by the addition of sterile distilled water. A bioleaching experiment in a 0.85 l stirred tank reactor was also carried out. The reactor contained 0.75 l of iron-free 9K medium, 7.5 g of powdered elemental sulphur, 10 g of spent catalyst and 75 ml of inoculum (taken directly from a grown culture), the mixture was magnetically stirred and the temperature maintained at 30 ◦ C. 2.3.3. Bioleaching using sulphuric acid generated by an A. thiooxidans biofilm Experiments were carried out using two percolator columns. In the first percolator column (C1), A. thiooxidans was immobilised on elemental sulphur particles to produce sulphuric acid. The column contained 145 g of sulphur (particle size 2–4 mm), 230 ml of culture medium and 20 ml of inoculum. The immobilisation was achieved by recirculating the medium in the column until the pH was around 0.8. At that moment, the exhausted medium was replaced by fresh medium (without inoculation) and the procedure was repeated three times to ensure that the acid was produced by the attached cells. After the third replacement, once the pH of the culture reached a value of approximately 0.8, the acidic medium was collected and fresh medium was added to C1. The collected medium was pumped at a rate of 113 ml/h into the second column (C2), which contained the spent catalyst. It was observed that the simple percolation of the leaching medium through the spent catalyst did not produce a significant solubilisation of nickel, because of this, the medium was forced to recirculate in C2. The amount of spent catalyst as well as the recirculation rate in C2 were changed in different runs. The conditions employed in each run are summarized in Table 1.

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Table 1 Conditions used in the experiments of bioleaching with sulphuric acid generated by an A. thiooxidans biofilm

#1 #2 #3 #4

Volume of leaching solution (ml)

pH

Recirculation rate (ml/min)

Spent catalyst (g)

143 200 150 160

0.66 0.52 0.62 0.8

5 5 10 13

10 15 15 15

The volume and pH of leaching solution (generated in C1 by the A. thiooxidans biofilm) pumped to the C2 at a constant rate of 113 ml/min; the recirculation rate of the leaching solution in C2 and the mass of spent catalyst contained in C2 for runs #1–4.

2.4. Precipitation experiments Cultures of Desulfovibrio sp. were obtained by filtering a grown culture (2 × 108 bacteria/ml) in Postgate’s B with blue ribbon paper and adding this to 10 ml of Postgate’s C medium (10%, v/v) [18]. Anaerobic conditions were achieved by adding thioglycolate and ascorbic acid (0.1 g/l each) to the samples. Different volumes of the nickel leachates from the column C2 (see Section 2.3.3) were added to the cultures to give final nickel concentrations in the range of 18–180 ppm. Samples were withdrawn after 1 week for analysis of the nickel and sulphate concentrations. Indirect precipitation experiments using sulphide biogenerated by Desulfovibrio cells were also performed. In this case, batch cultures of Desulfovibrio sp. in Postgate’s C medium were prepared in 250-ml Erlenmeyer flasks completely filled with liquid and sealed with a rubber stopper. Cultures were run in batch mode until further reduction of sulphate was not detected. Afterwards, the exhausted medium containing the biogenerated sulphide was pumped at different rates into a column containing the leachates. The volume of leachate and the inlet rate were changed in the different runs. The concentration of sulphide in the inlet solution and the final concentration of soluble nickel were measured. 2.5. Analytical methods Dissolved sulphate was determined by a turbidimetric method [19]. Sulphate forms an insoluble precipitate with barium (BaCl2 ) under acidic conditions. The absorbance of the sample was measured at 450 nm using a Beckman DU 640 Spectrophotometer (Fullerton, CA, USA). Samples were centrifuged for 2 min at 14,000 rpm and diluted before performing the analysis. Dissolved sulphide generated in Desulfovibrio sp. cultures was measured by titration using standard iodine and sodium thiosulphate solutions and starch as indicator (iodometric method) [19]. Dissolved nickel in the leachates and in the digested catalyst was determined by atomic absorption spectrophotometry (Shimadzu AA6650, Shimadzu Corporation, Kyoto, Japan). A standard solution of 1000 mg/l of nickel (Carlo Erba, Milan, Italy) was used to prepare the calibration standards. Samples were diluted with 0.14 M HNO3 and filtered through a 0.45 ␮m membrane. The sulphuric acid produced by

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bacterial oxidation of sulphur was analysed by titration with 0.01 M NaOH. Free bacterial population was determined by counting in a Petroff Huaser chamber in conjunction with an optical microscope with phase contrast (Nikon Labophot, Kyoto, Japan). 2.6. Statistical analysis All experiments were conducted in duplicate. Nickel concentration was measured three times on each duplicate sample. Data points in figures represent means with error bars shown (± S.D.). Both one-way and two-way analysis of variance were performed where appropriate (with 95% level of confidence, α = 0.05), using the statistical package on Microsoft® Office Excel Version 2003. Also, t-test was used to compare leaching efficiencies. 3. Results and discussion 3.1. Leaching experiments 3.1.1. Chemical leaching experiments in Erlenmeyer flasks Treatment of 1 g of the spent catalyst with diluted sulphuric acid solutions gave, after 8 days, good leaching efficiencies in systems with initial pH values of 1 or 2 (100 and 73%, respectively, S.D. < 1%). Significant dissolution of nickel was not observed in samples with pH values higher than 3 (32 and 10% for pH values 3 and 5, respectively; S.D. < 1%). In all cases an increase in the pH was observed. Analysis of variance considering one factor (the pH of the solution) indicated that there are significant differences between the leaching efficiencies of the different solutions for a level of confidence of 95%. Typically, spent nickel catalysts contain metallic nickel and nickel oxide, although nickel sulphides may occasionally be present along with mixtures of coke, hydrocarbons or fat [20]. Thermodynamic data (Pourbaix Diagram) shows that Ni0 can be spontaneously oxidised to Ni2+ at pH values lower than 4.3–4.4 according to the following equation: Ni(s) 0 + 2H+ (aq) → Ni2+ + 2H2 (g)

increase in pH and this inhibited the bacterial growth, especially in the cases of the higher pulp densities. It was observed that after 1 week the pH rose from 5.6 to 7.1 (in the case of 1% pulp density) and 7.2 (in the case of 2% pulp density) and the number of bacteria decreased from the initial level of 5 × 107 to 2 × 107 bacteria/ml. Negligible nickel dissolution was observed in these cultures. Two different strategies were investigated in an effort to reduce the inhibition resulting from the pH increase. Firstly, cells were inoculated from a grown culture without washing (unwashed cells); in this case, the acidity of the inoculum avoided the rapid increase in pH and the consequent inhibition of bacterial growth. However, only in the lowest pulp density was there a decrease in pH, an observation that suggests adequate bacterial growth. The second strategy involved postponing the inoculation (with washed cells) for 24 h prior to a new adjustment of the pH through the addition of sterile 1/10 sulphuric acid (about 0.7–1 ml was needed). The percentages of nickel leached using these last two strategies for the different pulp densities are shown in Fig. 1. It can be seen from the figure that systems inoculated with washed bacteria were less efficient than those inoculated with unwashed cells for the highest pulp densities. Two-way ANOVA, performed considering the pulp density and the inoculation mode as the two independent factors, indicated that there was a significant difference between the leaching percentages obtained. t-Test performed at each pulp density also showed that there were significant differences in the leaching efficiencies obtained with each inoculation procedure. It was found that the higher the pulp density the lower percentage of dissolved nickel. This result is unexpected because the amount of sulphur employed would produce sufficient sulphuric to react with all of the nickel present in the spent catalyst. Similar

(1)

while the dissolution of nickel hydroxide can be achieved at pH values below 6.5 [21]. Both processes – the oxidation of Ni0 and the dissolution of nickel hydroxides/oxides – led to an increase in the pH of the solution. The low-level of nickel dissolution in the chemical leaching at pH 5 suggests that there is a small amount of oxidised nickel present in the spent catalyst. A similar trend in nickel dissolution behaviour with pH was observed by Cerruti et al. [22] in a study of spent nickel–cadmium batteries as opposed to spent catalyst. Al-Mansi and Abel Monem [23] reported similar results for the recovery of nickel from a spent catalyst, albeit with a much higher acid concentration. 3.1.2. Bioleaching experiments When the bioleaching experiments were carried out using washed bacteria, the alkalinity of the spent catalyst produced an

Fig. 1. Percentages of leached nickel in the bioleaching experiments in Erlenmeyer flasks using different pulp densities. Data represent the average of two independent samples analysed in triplicate, bars represent the standard deviation. Two-way ANOVA test with two factors showed that all the results have significant differences (α = 0.05). According to the t-test performed at each pulp density, the results have significant differences (α = 0.05).

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behaviour was observed by Cerruti et al. [22] on using A. ferrooxidans for the leaching of Ni–Cd batteries. They concluded that there is competition between the two surfaces for the attachment of bacteria on increasing the pulp density; thus, at the higher pulp density a large number of bacteria are attached to the spent catalyst and, consequently, they are unable to grow (initial bacterial attachment to the sulphur is required for its oxidation) and a smaller amount of sulphuric acid is produced. A similar effect of pulp density was observed in the leaching of nickel, molybdenum and aluminium from a spent refinery processing catalyst using adapted Aspergillus niger cultures [24]. According to the authors, the decrease in the level of extraction of the three metals was due to the toxicity of heavy metals. However, acidophilic microorganisms are able to inhabit metal-rich environments (such as acid mine drainage, for instance). These microorganisms are very tolerant to a wide range of metal ions. The degree of metal resistance varies according to the strain and the culture conditions, but in general it is at least one order of magnitude higher than the tolerance exhibit by neutrophilic microorganisms [25]. It has been reported that A. ferrooxidans was able to growth in the presence of 30.0 g Ni(II)/l [26] while adapted strains have been isolated that can grow in the presence of up to 58.7 g/l [27]. Unfortunately, there are very few reports concerning the toxicity of heavy metals to A. thiooxidans. Although it has been reported that sulphur oxidation by A. thiooxidans was strongly inhibited by 0.30 g Ni(II)/l [28], preliminary experiments performed in our laboratory with the strain of A. thiooxidans used throughout the present work showed there was not growth inhibition (measured as the decrease of pH culture) in the presence of soluble nickel up to a concentration of 2.0 g/l (data not shown). That concentration is much higher than the maximum achieved in our experiments. The percentage of nickel leached by the sterile control (medium 0K, pH 5) was higher than that obtained in the sulphuric acid solution of similar pH. The presence of NH4 + in the culture medium at that pH value could enhance the dissolution of nickel through the formation of the complex Ni(NH3 )6 2+ , which is more stable than Ni(H2 O)6 2+ [29]. In order to analyse whether the bacteria could grow in the presence of the spent catalyst without the addition of sulphur, an experiment was performed in which 1% pulp density (and no sulphur) was added to the culture medium. In this case, there was an initial dissolution of nickel due to the acidity of the inoculum but this did not increase even after 30 days. The possibility of scaling up the process was investigated by performing bioleaching experiments in a 0.85 l stirred tank reactor. The reactor contained 0.75 l of iron-free 9K medium, 7.5 g of sulphur, 10 g of spent catalyst and 75 ml of inoculum (taken directly from a grown culture) and the mixture was magnetically stirred with the temperature maintained at 30 ◦ C. Approximately 340 mg of nickel were leached from the waste in 48 h, representing 80% of the total nickel in the reactor. Although the recovery of nickel was significant, the system was not particularly good from a practical point of view. The physical characteristics of the spent catalyst (a very fine powder) made it adhere to the reactor wall, resulting in an inhomogeneous system with a lower efficiency.

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Fig. 2. Percentages of leached nickel obtained in the different runs using the acidic medium generated by the A. thiooxidans biofilm (see conditions of each run in Table 1). Data represent the average of three replicates, bars represent the standard deviation.

3.1.3. Bioleaching using sulphuric acid generated by an A. thiooxidans biofilm Sulphuric acid generated by A. thiooxidans growing on elemental sulphur particles in column C1 was used as leaching medium. The culture medium pH ranging from 0.52 to 0.8 was pumped at rate of 113 ml/h to C2. The volumes pumped varied between 140 and 200 ml. The volumes and the pH of each leaching medium used are shown in Table 1. As it was indicated in Section 2.3.3, the simple percolation of the leaching medium through the spent catalyst did not produce a significant solubilisation of nickel. The leaching of nickel increased when the leaching solution was forced to recirculate through the spent catalyst. The pulp density was increased from an initial value of 10–15 g which was the maximum possible without flooding. In these conditions, an increase in the recovery of nickel was found. Increasing the recirculation rate produced a decrease in the percentage of leached nickel. The percentages of nickel leached for each run after 24 h of recirculation are shown in Fig. 2. It can be seen from the figure that the recirculation rate was the most important factor affecting the leaching efficiency. When the mass of the residue was increased by 50% (runs 1 and 2), while the inlet rate of the leaching medium and the recirculation rate were constant, the percentage of leached nickel increased from 71 to 92%. When 15 g of spent catalyst were leached at the same inlet rate but with higher recirculation rates (runs 3 and 4), the efficiency of the leaching process diminished (54 and 49%, respectively)—almost certainly due to incomplete contact. 3.2. Precipitation experiments Experiments were carried out in sealed flasks by adding different volumes of the leaching nickel solution to Desulfovibrio sp. cultures. This did not allow the development of the bacteria or, therefore, the production of sulphide. Sulphate reducing bacteria are sensitive to the presence of heavy metals. Experiments involving nickel precipitation performed in our laboratory showed that Desulfovibrio sp. could grow in the presence of up to 25 ppm of dissolved nickel, but the results were highly depen-

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Table 2 Conditions employed and results obtained in the precipitation experiments using biogenerated sulphide

A B C D

Q (ml/min)

m(S2− ) (mg)

m(Ni)i (mg)

m(Ni)f (mg)

Percentage of Ni precipitated

5 5 5 10

22.74 27.01 33.47 29.29

68.37 136.7 205.1 136.7

14.3 96.6 201.2 113.3

79 29 2 17

Q: inlet flow rate of the solution containing biogenerated sulphide; m(S2− ): mass of dissolved sulphide generated; m(Ni)i : mass of nickel initially present in the precipitation column; m(Ni)f : mass of soluble nickel after the contact with the sulphide solution; percentage of Ni precipitated: (m(Ni)i − m(Ni)f ) × 100/m(Ni)i .

dent of the inoculum conditions. According to data reported in the literature, nickel concentrations between 10 and 20 ppm are toxic for pure and mixed cultures of SRB [30]. To avoid the inhibition produce by the presence of nickel on the bacterial growth, the precipitation was carried out indirectly. Thus, the recovery of nickel from the leaching solution was performed using sulphide generated in a Desulfovibrio sp. spent culture. The results obtained in each experiment are summarized in Table 2. It can be seen that the highest precipitation efficiency (79%) is achieved for the lowest inlet rate and the lowest mass of nickel. That percentage of nickel precipitation represents the maximum possible value according to the amount of sulphide introduced into the system. When the mass of soluble nickel was increased (experiment B) only 29% of the nickel was precipitated and this corresponds to 81% of the sulphide in the inlet. Thus, 19% of the sulphide entering the system did not react with the dissolved nickel, probably because a proportion of the sulphide was lost as H2 S produced during contact with the acidic leachate. A process for the indirect precipitation of heavy metals by sulphate reducing bacteria has been previously studied [12]. This method involves the production of H2 S by SRB in a fixed-bed column reactor. A gas mixture of H2 and CO2 was injected into the column while the SH2 was removed from the outlet gas flow by N2 and it was bubbled into a reactor containing a heavy metal solution. Although the system allowed copper and zinc to be selectively recovered, a preliminary estimation of costs indicated that, in that particular case the treatment would not be economically viable. A simple and low-cost system for the treatment of acid mine drainage has been designed. The system consisted in a biofilm reactor inoculated with acidophilic SRB immobilised on porous glass beads [31]. Although these SRB populations were able to efficiently reduced sulphate to sulphide, it is necessary to consider that this type of reactor, where a heavy metal solution percolates the biofilm is subject to clogging because of the accumulation of insoluble metal sulphides. The generation of sulphide in a simple reactor as a batch reactor can be used in the precipitation of heavy metals without the design of more complex reactors or the needing of gas carriers, which increase the operational costs. Although the use of SRB to precipitate heavy metals has been widely studied, this is no the only possibility for metal precipitation. Precipitates of phosphates of divalent cations can be obtained in cultures of Citrobacter sp., when glycerol 2-phosphate is present [32]. Metals like Zn, Cu, Ca, Mn and Sr can be immobilised as oxalates by oxalic acid produced by fungi such as Aspergillus and Penicillum [33]. When comparing these bioprecipitation processes, the precipitation with SRB seems to be advantageous because metallic

sulphides are more insoluble than phosphates and oxalates; in addition the culture of SRB does not require complex organic compounds as Citrobacter does [9]. Nickel could have been recovered from the leaching solution as solid NiSO4 by evaporation for 5 h at 100 ◦ C as proposed by several authors [8,23,34]. However, the solubility of NiS is much lower than that of the NiSO4 (0.00036 and 29.3 g per 100 ml for NiS and NiSO4 , respectively), thus allowing the recovery of nickel as a solid salt at room temperature. Beside this, the low-solubility of nickel sulphide would allow a safer final disposal. 4. Conclusions The results of our experiments show that nickel present in the spent material from the hydrogenation of vegetable oil can be successfully leached using A. thiooxidans. The percentages of nickel leached using A. thiooxidans where higher than those obtained with dilute sulphuric acid solutions. Batch experiments showed a decrease in the leaching efficiency on increasing the different pulp densities—probably due to a competition between the spent catalyst and the sulphur for bacterial attachment. The physical characteristics of the residue meant that the best results were obtained when the leaching process was carried out using sulphuric acid biogenerated by an A. thiooxidans biofilm. The recovery of nickel from the leachates was performed at room temperature by precipitating with sulphide generated by Desulfovibrio cells. Due to the toxic effects of nickel ions on the growth of Desulfovibrio, direct precipitation was not possible. Indirect precipitation using sulphide generated in Desulfovibrio sp. cultures allowed the recovery of nickel as the very insoluble nickel sulphide. In order to improve the efficiency of this process, the possibility of sulphide volatilisation should be minimised. Acknowledgements Dr. Edgardo Donati and Dr. Marisa Viera are research members of CONICET. This research was supported in part by ANPCyT (PICT 25300) and CONICET (PIP 5147). References [1] E. Furimsky, Spent refinery catalysts: environment, safety and utilization, Catal. Today 30 (1996) 223–286. [2] International Finance Corporation, 2006. Environmental, Health, and Safety Guidelines for Vegetable Oil Processing. Vegetable Oil Processing, Draft Document, August 2006. Available on line at: http://www.ifc.org/ ifcext/enviro.nsf/Content/EnvironmentalGuidelines.

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Please cite this article in press as: V. Bosio, et al., Integrated bacterial process for the treatment of a spent nickel catalyst, J. Hazard. Mater. (2007), doi:10.1016/j.jhazmat.2007.10.095