Selective biosorption and recovery of Ruthenium from ... - Springer Link

Appl Microbiol Biotechnol (2012) 95:381–387 DOI 10.1007/s00253-012-4053-9

BIOTECHNOLOGICAL PRODUCTS AND PROCESS ENGINEERING

Selective biosorption and recovery of Ruthenium from industrial effluents with Rhodopseudomonas palustris strains Giovanni Colica & Stefania Caparrotta & Roberto De Philippis

Received: 27 January 2012 / Revised: 22 March 2012 / Accepted: 23 March 2012 / Published online: 3 May 2012 # Springer-Verlag 2012

Abstract This study demonstrated for the first time the possibility to remove and partially recover the Ruthenium contained in industrial effluents by using purple non sulfur bacteria (PNSB) as microbial biosorbents. Up to date, the biosorption was only claimed as possible tool for the removal of the platinum-group metals (PGM) but the biosorption of Ru was never experimentally investigated. The PNSBs tested have adsorbed around 40 mg g (dry biomass)−1 of the Ru contained in the real industrial effluents. At the end of the bioremoval experiments, the amount of Ru recovered from the biomass ranged from 42 % to 72 % of that adsorbed by PNSB, depending by the characteristics of the Ru effluent used. In any case, the use of microbial sorbents such as PNSB for the biosorption and recovery of Ru can be considered a way to reduce both the costs and the impact on the environment of the mining activities needed to obtain the increasing amounts of this rare and precious metal requested by the industrial activities related to its use. Keywords Ruthenium . Biosorption . Metal recovery . Rhodopseudomonas palustris . Electroplating . Industrial effluents

Introduction The process of metal biosorption has been widely investigated in the last 30 years using a large number of biomaterials for the removal of metals from water solutions and G. Colica : S. Caparrotta : R. De Philippis (*) Department of Agricultural Biotechnology, University of Florence, Piazzale delle Cascine 24, 50144 Firenze, Italy e-mail: [email protected]

wastewaters (De Philippis et al. 2011; Chojnacka 2010; Gadd 2009), especially to disclose the biosorption mechanisms and/or to develop new technologies for the removal of heavy metals from the industrial effluents. However, in spite of the large number of studies so far available, those aimed at studying the possible use of biosorption for the recovery of precious metals from water solutions are proportionally few (Das 2010; Cui and Zhang 2008; Mack et al. 2007). In particular, the biosorption and recovery of Ruthenium from water solutions by using microorganisms was never experimentally investigated but it was only claimed as possible tool for the removal of the platinum-group metals (PGM), i.e., Ru, Rh, Pa, Os, Ir, and Pt (Dobson and Burgess 2007), in spite of the high economical value of this metal. PGM are extremely scarce on the market, compared to other precious metals (Au, Ag), due to their low abundance in the Earth and to the difficulty in extracting them from ores, where they are usually present in concentrations less than 0.0008 % (w/w) (Bernardis et al. 2005). On the other side, their request increased exponentially over the last century (Marinho et al. 2011; Bernardis et al. 2005) due to their high catalytic activity, which is exploited in many industrial chemical processes. For instance, the catalytic properties of PGM are presently exerting a positive impact on the quality of the environment through the development of catalysts to be used in the control of automotive emissions. Moreover, the excellent mechanical properties and the high resistance to corrosion and oxidation of PGM are currently exploited for processing extremely corrosive molten glass, nozzles for spinning textiles, and coatings for the turbine blades of jet engines (Marinho et al. 2011; Bernardis et al. 2005). Ru is used by metal-plating industries for the superficial treatment of a number of luxury products, but the electrodeposition of Ru is negatively affected by the increasing

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presence of other metals (usually Cu, Zn, and Ni) released by the treated articles in the galvanic bath during the plating process, even at low concentrations. The presence of contaminant metals reduces the usability of Ru solutions, leading to their early disposal even when they are still rich in Ru, thus causing a significant increase in the costs of this process. A possible strategy for extending the life of Ru-containing galvanic baths could be the selective removal of contaminant metals or of the Ru still contained in the solution. The selectivity in the metal removal is one of the main features of many microbial biosorbents (De Philippis et al. 2011; Gadd 2009), thus suggesting the possibility to find out specific microorganisms for selectively removing the contaminant metals or for recovering pure Ru from Ru-containing galvanic baths. Among the microorganisms used for metal biosorption, purple non-sulfur bacteria (PNSB) were studied by several authors, who showed their good sorption properties towards Cd, Pb, and Au (Seki et al. 1998; Bai et al. 2008; Feng et al. 2008). However, even if PNSB have been widely investigated for their capability to produce hydrogen (Eroglu and Melis 2011), their possible exploitation for the removal of precious metals other than Au was never investigated. This research was aimed at assessing the capability of five Rhodopseudomonas palustris strains to adsorb Ru, Cu, Ni, or Zn from pure water solutions and at testing their capability to selectively remove these metals from two different kinds of industrial effluents deriving from the process of Ru deposition. The possibility to recover the Ru adsorbed by bacterial biomass was also assessed.

Materials and methods Microorganisms utilized The experiments were carried out with five PNSB, Rp. palustris strains 42OL, AV33, AV32a, SC0, and CGA009. All the strains, with the exception for CGA009, are deposited at the CSMA Culture Collection (registered at WDCM with number 147) with the following collection numbers: CSMA73/42 (strain 42OL), CSMA09/33 (strain AV33), CSMA09/32a (strain AV32a), and CSMA80/52 (strainSC0). CGA009 was a kind gift of Dr. Caroline S. Harwood, Washington State University, Seattle (USA). The cultures were carried out in RPN medium containing (g l−1): DL-malic acid, 2.0; NH4Cl, 0.5; K2HPO4, 0.5; KH2PO4, 0.3; MgSO4·7H2O, 0.4; NaCl, 0.4; CaCl2·2H2O, 0.075; ferric citrate, 0.005; and yeast extract, 0.4. Trace elements were provided by adding 10 ml per liter of a solution containi ng (mg l − 1 ): ZnSO 4 ·7H 2 O, 10; MnCl2·4H2O, 3; H3BO3, 30; CoCl2·6H2O, 20; CuCl2·2H2O, 1; NiCl2·6H2O, 2; Na2MoO4.2H2O, 30. The pH of the

Appl Microbiol Biotechnol (2012) 95:381–387

medium was adjusted at 6.8 with NaOH before autoclaving (Bianchi et al. 2010). Metal solutions In a first set of experiments, single metal solutions of Ru, Cu, Ni, and Zn (20 mg l−1; pH 4), obtained by diluting 1,000 mg l−1 standard solutions (Sigma-Aldrich Co.), were used. In a second set of experiments, two real effluents were used, deriving from the Ru plating processes carried out under acid or alkaline conditions. The metal composition of the acid effluent (named RuA; pH 1.8) was the following: Ru 3,250 mg l−1, Cu 29 mg l−1, Zn 88 mg l−1, Ni 980 mg l−1, and Fe 688 mg l−1. The metal composition of the alkaline effluent (named RuB; pH 8.7) was the following: Ru 1,800 mg l−1, Cu 9.5 mg l−1, Zn 30 mg l−1, Ni 29 mg l−1, and Fe 32 mg l−1. Biosorption tests with single metal solutions In the first set of experiments, the capability of each strain to adsorb Ru or Cu, Ni, and Zn, the metals usually present in solution as contaminants, was verified using single metal solutions. If not differently reported, 15 ml of pure cultures of PNSB, in stationary phase, were centrifuged at 2,700×g; after 40 min, 15 ml of fresh culture was added at the pellet and centrifuged again at 2,700×g for 40 min. The pellet obtained after the second centrifugation was suspended in 2 ml of RPN medium (Bianchi et al. 2010) and 13 ml of the metal solution was added. The final biomass concentration in the test tubes was of 1±0.1 g (dry weight) l−1. Blanks prepared by adding 2 ml of culture medium devoid of cells to 13 ml of metal solutions were used. After 72 h, the samples were centrifuged at 3,500×g for 20 min in order to remove the biomass from the solution and to analyze the concentration of the metals remained in solution. The biomass was utilized with or without acid pretreatment. For the acid pretreatment, an aliquot of the bacterial culture was confined in a dialysis tubing (Ø28.6 mm, cut-off 16,000 Da) and dipped in a 0.03 % HCl solution for 6 h in order to remove the metal ions possibly bound to the negatively charged groups of the cell envelopes. Subsequently, the confined biomass was dialyzed against deionized water for 24 h to remove the excess of acid. For each test were recorded the pH value and the dry weight of the biomass. Biosorption tests on Ru-containing industrial effluents The five Rp. palustris strains were tested, following the above described procedure, with the two industrial Rucontaining effluents RuA and RuB after a 1:10 dilution with


distilled water. The bacterial cultures were utilized with and without acid pretreatment (see above). In the tests, 50 ml of culture or of culture medium devoid of cells (blank) were added to 200 ml of Ru solutions, diluted 1:10, for 64 h. All the tests were repeated three times while the blanks were repeated five times. The tests were carried out in a thermostat at 25 °C and at 65 °C, the latter being the temperature of the bath during the process of Ru electrodeposition. The two strains (Rp. palustris AV33 and SC0) that showed the best results in these tests were lyophilized and rehydrated with deionized water until a concentration of 40 g (cell dry weight) l−1. Fifty milliliters of this concentrated cell suspension was added to 200 ml of undiluted Ru solution and maintained at 65 °C for 64 h in a thermostatic orbital incubator. At the end of the test, the biomass was separated from Ru solutions by centrifugation, and both the pellet and the supernatant were collected and analyzed. The pellet was washed two times with distilled water and lyophilized. Sixty milligrams of each lyophilized biomass was mineralized at 150 °C for 72 h in a solution composed by 20 ml of HNO3 65 % (v/v) and 20 ml of H2O2. At the end of the mineralization process, the volume was adjusted to 50 ml with deionized water. The metal content of these solutions was determined by atomic absorption spectrometry. Analytical methods The culture dry weight (g l−1) was determined by vacuum filtration of 10 ml of the dialyzed cultures, followed by drying the filter at 50 °C until a constant weight was reached. The amount of metal removed from the aqueous solution or from the Ru-containing industrial wastewaters was calculated as the difference in the metal concentration before and after the contact with the cultures, using a blank obtained as above reported. Specific metal removal (q), expressed as milligrams of metal removed per gram of dry weight, was calculated as q (mg g−1)0V (Ci −Cf) m−1, where V is the sample volume (l), Ci and Cf are the initial and final metal concentrations (mg l−1), respectively, and m is the amount (g) of dry biomass (Volesky and May-Philips 1995).

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Following the kinetics of metal removal, it was observed that the equilibrium in the concentration of the metal remaining in water solution, corresponding to the saturation of the metal removal capability of the biomass, was always achieved within the first 12–14 h of the experimental time (data not shown). Thus, the values of final metal concentration utilized for calculating the specific metal removal (q), expressed as milligrams of metal removed per gram of cell dry weight, were always those measured after 24 h. When the bacterial biomass was utilized without any acid pretreatment (Fig. 1), it was observed a specific metal removal ranging between 86.27±14.12 mg of Ru removed per gram of cell dry weight, obtained with strain CGA009, and 0.57±8.81 mg g−1, showed by strain AV33. On the other side, the removal of Cu was always higher that the removal of Ru, ranging from 145.95±27.83 mg g−1 obtained with strain SC0 to 78.39±14.57 mg g−1 obtained with strain AV33. Ni and Zn always showed q values lower than 10 mg g−1. When the bacterial biomass was utilized after an acid pretreatment (Fig. 2), a completely different situation was observed. In this case, values of specific Ru removal ranging between 207.20±12.86 mg g−1, obtained with strain CGA009, and 130.88±7.92 mg g−1, obtained with strain SC0, were found. On the other side, the q values obtained for all the other metals tested never exceeded the value of 10 mg g−1, pointing out a very high and selective affinity of the strains for Ru. Biosorption tests on diluted galvanic effluents at 25 °C When the five Rp. palustris strains were tested at 25 °C, without any acid pretreatment, on the diluted effluent deriving from the acid Ru-plating process (RuA), a very low Ru

Results Preliminary tests with single metal solutions The first set of experiments was aimed at investigating the metal removal capability of the five Rp. palustris strains towards Ru and the metals most commonly contaminating Ru solutions in the electro-plating industrial process, namely Cu, Ni, and Zn.

Fig. 1 Metal biosorption from single metal solutions with untreated biomass of Rp. palustris strains CGA009, SC0, 42OL, AV32a, and AV33. Data are mean values of at least three independent experiments and bars represent the standard deviation

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Fig. 2 Metal biosorption from single metal solutions with acidpretreated biomass of Rp. palustris strains CGA009, SC0, 42OL, AV32a, and AV33. Data are mean values of at least three independent experiments and bars represent the standard deviation

biosorption was observed, the q values found never exceeding 4 mg g−1 (Fig. 3). Rp. palustris strain 42OL showed comparable q values towards Ru, Ni, and Zn, with q ranging between 2 and 3 mg g−1, while the other strains showed very low q values for Ni and Zn. None of the five strains showed the capability to remove Cu from RuA effluent. The same experiments done with the diluted effluent deriving from the basic Ru-plating process (RuB) at 25 °C showed an even lower metal removal capability of the five strains, with the only exception of strain 42OL, which maintained a comparable q value towards Ru in RuA and in RuB effluents (Fig. 4). In the experiments carried out with RuA and RuB effluents using HCl-pretreated biomass, the q values obtained were two to three times higher than those obtained with the untreated biomass. In the tests with pretreated biomass and

Fig. 3 Metal biosorption from the diluted Ru acid effluent (RuA) with untreated biomass of Rp. palustris strains CGA009, SC0, 42OL, AV32a, and AV33. Data are mean values of at least three independent experiments and bars represent the standard deviation


Fig. 4 Metal biosorption from the diluted Ru basic effluent (RuB) with untreated biomass of Rp. palustris strains CGA009, SC0, 42OL, AV32a, and AV33. Data are mean values of at least three independent experiments and bars represent the standard deviation

RuA solution (Fig. 5), CGA009 showed the best results, selectively removing Ru with a q value higher than 7 mg g−1, with a concomitant Ni uptake of only 0.2 mg g−1. Rp. palustris strain 42OL removed Ru 5 mg g−1 without removing any other metal present in solution. Strains AV32a and AV33, under the same conditions, showed very poor metal removal performances. In the tests with pretreated biomass and RuB solution (Fig. 6), the five strains showed a completely different behavior in comparison with the tests carried out with RuA. Indeed, strains CGA009 and SC0 did not remove any amount of Ru, while strains AV32a and AV33 drastically increased their q values towards Ru. Only strain 42OL showed a less significant change in the q value, which decreased from 5.23±1.55 to 3.01±2.11. In all the tests done with RuB solution, only very small amounts of the

Fig. 5 Metal biosorption from the diluted Ru acid effluent (RuA) with acid-pretreated biomass of Rp. palustris strains CGA009, SC0, 42OL, AV32a, and AV33. Data are mean values of at least three independent experiments and bars represent the standard deviation


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Fig. 6 Metal biosorption from the diluted Ru basic effluent (RuB) with acid-pretreated biomass of Rp. palustris strains CGA009, SC0, 42OL, AV32a, and AV33. Data are mean values of at least three independent experiments and bars represent the standard deviation

other metals were removed, being the Cu uptake activity the highest observed, with q values well below 1 mg g−1. Biosorption tests on diluted galvanic effluents at 65 °C When the metal uptake of the five Rp. palustris strains was tested using RuA and RuB effluents at 65 °C, a common working temperature of the baths in the Ruplating processes (Wang et al. 2011), the q values of the biomass drastically changed, showing a very high increase for all the strains. In the tests carried out with RuA effluent and with the five Rp. palustris strains utilized without any acid pretreatment, the highest q value towards Ru was obtained with strain SC0, which showed a value of over 132.57±10.04 mg g-1 (Fig. 7), followed by strain CGA009, with a value of 1,097.97± 14.65 mg g−1; the other three strains showed q values around 60 mg g−1. When the tests were carried out under the same conditions but with the acid-pretreated biomass (Fig. 7), a decrease in the q values towards Ru was observed for strains CGA009 (q036.18±16.27 mg g−1), SC0 (q 076.04 ± 13.68 mg g−1), and AV32a (q 047.09 ± 4.63 mg g−1), while in the two other cases, the q values did not significantly change. In the tests carried out with RuB effluent and with the five Rp. palustris strains utilized without any acid pretreatment, the q values towards Ru were around 20 mg g−1 for all the strains. However, when the tests were carried out with the acid-pretreated biomass under the same conditions (Fig. 7), a very significant increase in the q values was observed, with the best results obtained with strain AV33, which showed a q value towards Ru higher than 95 mg g−1, followed by the strain SC0 (q083.32±3.15 mg g−1).

Fig. 7 Metal biosorption at 65 °C from a undiluted Ru acid effluent (RuA), b undiluted basic effluent (RuB) with untreated (black bars), and acid-pretreated (white bars) biomass of Rp. palustris strains CGA009, SC0, 42OL, AV32a, and AV33. Data are mean values of at least three independent experiments and bars represent the standard deviation

In all the tests carried out with RuA and RuB effluents at 65 °C, the removal of the contaminant metals (i.e., Cu, Ni, and Zn) was always very low, with q values never exceeding 1 mg g−1 (data not shown). Biosorption tests on undiluted real effluents of Ru-plating process and Ru recovery These tests, aimed at investigating the possibility to recover Ru from the effluents of a Ru-plating industrial process, were carried out at 65 °C with undiluted RuA and RuB effluents by using the two strains, AV33 and SC0, which have respectively shown the best q values in the above reported experiments on RuA and RuB. In these experiments, the concentration of Rp. palustris cells used was also increased in order to have enough biomass for the recovery of the Ru sorbed at the end of the biosorption process.

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Using the undiluted RuA and RuB effluents, a drastic decrease in the specific metal biosorption was observed (Fig. 8), being the q values obtained three to four times smaller than those previously obtained with diluted effluents. Indeed, Rp. palustris strain SC0 showed a q value of 31.85±2.14 mg g−1 in the RuA effluent, while strain AV33 showed a q value of 38.08±3.18 mg g−1 in the RuB effluent. The contaminant metals Cu, Ni, and Zn were adsorbed at q values lower than 0.15 mg g−1. At the end of the biosorption process, the bacterial biomass was centrifuged, lyophilized, and mineralized to recover the sorbed metals. The amount of Ru recovered from 1 g of biomass dry weight was 22.95±1.98 mg for strain SC0 and 15.74±3.77 mg for strain AV33 (Fig. 8).

Discussion As stated in the “Introduction”, no previous experimental studies are available in literature regarding Ru biosorption using microorganisms in spite of the high economical value of this metal. A preliminary screening carried out at the beginning of this research with several PNSB, cyanobacteria, and yeasts (data not shown) showed that only PNSB were capable to remove Ru from pure metal solutions at a high specific metal uptake. A direct comparison of the above reported q values for Ru with previously published results cannot be done, owing to the lack of previous studies in the field. However, it is possible to compare the sorbing performances of PNSB with those obtained with other microorganisms and metals in terms of total amount of metal ions removed per unit of biomass (De Philippis and Micheletti 2009). The five PNSB, tested against pure Ru

Fig. 8 Amount of Ru adsorbed from RuA effluent, by Rp. palustris strain SC0, and from RuB effluent, by AV33 (light gray bars) and amount of Ru recovered from the respective biomass (dark gray bars). Data are mean values of at least three independent experiments and bars represent the standard deviation


solutions after the acid pretreatment of the biomass, showed q values ranging from 2.03 to 1.34 mmol of Ru removed per gram of cell dry weight, which are among the highest so far found with other microbial biosorbents towards other metals. In fact, most of the q values reported in literature are lower than 1 mmol g−1 (De Philippis et al. 2011; Lesmana et al. 2009). In pure metals solutions, the acid pretreatment of the biomass increased the biosorption performances towards Ru, in accordance with previous observations done with microbial biosorbents towards other metals (Lesmana et al. 2009; Kavita et al. 2011; Colica et al. 2010), but at the same time caused a drastic decrease in the removal of Cu. This behavior might be due to a modification of the binding sites on the cell wall of the PNSB strains used increasing their affinity to Ru, as it was observed with other metals (Kavita et al. 2011; Colica et al. 2010; Micheletti et al. 2008), and it was confirmed using a real wastewater containing Cu, Zn, and Ni in addition to Ru. It is worth stressing that the selectivity in a biosorbent is considered a very positive feature because it gives the possibility for use in the removal of the metal of interest from wastewaters containing a number of other contaminant metals. As a consequence, the abovementioned feature increases the interest in the use of PNSB for Ru bioremoval. However, the use of diluted real wastewaters caused a drastic decrease in the Ru uptake performances of the PNSB strains tested, both in the untreated and in acid-pretreated biomass, reducing their q values to 3–4 % of those found in pure metal solutions. More than to the presence of other metals, as it can be inferred from the selectivity shown by the PNSB towards Ru, this decrease was probably due to the presence of other, unknown chelating agents that are usually added to Ru solutions in order to enhance the performances of the industrial plating process. An example of the complexity of a bath for electrodeposition of Ru films can be found in Wang et al. (2011). A significant enhancement in the Ru sorbing capacity of the PNSB tested was observed when the strains were used on diluted wastewaters at the temperature of 65 °C, which is the operating temperature of the industrial process, instead of at 25 °C. Under these conditions, two main phenomena were observed: (1) the q values increased up to 20-fold in comparison with those obtained at 25 °C, still maintaining a high selectivity towards the removal of Ru; (2) the acid pretreatment of the biomass induced different changes in the metal sorbing properties of the PSNB strains when they were used in the wastewaters deriving from the industrial process carried out at pH 1.8 (RuA) or at pH 8.7 (RuB). In the case of RuA, the acid pretreatment reduced the performances of the bacterial biomass in comparison with the untreated biomass, while the same pretreatment enhanced the Ru sorbing properties of PNSB against RuB wastewaters. An explanation of this difference could reside in the very low


pH of RuA, which may have affected the integrity of the cells whose external structure have been already partially dismantled by the acid pretreatment (Kavita et al. 2011). The enhancement of the sorbing properties of biomasses observed by increasing the operating temperature, a wellknown phenomenon observed with many other biosorbents (Volesky 2003), suggests the direct use of PNSB on the Rucontaining wastewaters coming out from the metal plating process that, as stated above, is carried out at 65 °C. When the biomass of PNSB was used with undiluted wastewaters, a partial decrease in the q values was observed, most probably owing to increased amounts of the unknown chelating agents already mentioned. In any case, the almost negligible removal of the contaminant metals was confirmed also on the undiluted wastewaters, further pointing out the selectivity in the removal of Ru. In spite of this decrease, the Ru sorbing performances of PNSB remained quite interesting, being around 40 mg g−1 with both wastewaters. At the end of the bioremoval experiments, the recovery of 72 % of the Ru adsorbed from RuA solution by the biomass of strain SC0 can be considered rather satisfactory, while the recovery of only the 42 % from strain AV33 that was used with RuB wastewaters is too low and has to be improved in order to make this process economically sustainable. In conclusion, this study demonstrated, for the first time, the possibility to remove and partially recover the Ru contained in industrial wastewaters by using purple nonsulfur bacteria as microbial biosorbents. At the same time, the above reported results point out many problematic issues deriving from the treatment of real wastewaters produced by the metal-plating industrial processes. Indeed, these wastewaters quite often contain a number of different chemical compounds, of both inorganic and organic nature, which are difficult to identify because their nature is confidential or covered by industrial patents, that may negatively affect the bioremoval of metals with microbial biosorbents. In any case, the use of microbial sorbents such as PNSB for the biosorption and recovery of a rare and precious metal like Ru could contribute to optimize its use, containing its production costs and reducing the environmental impact of the mining activities needed for obtaining it.

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