WO2001062990A1

WO2001062990A1 - Method and bioreactor for enzymatic metal cation reduction in solution

Info

Publication number: WO2001062990A1
Application number: PCT/GB2001/000799
Authority: WO
Inventors: Ivor Rex Harris; Lynne Elaine Macaskie; John Peter George Farr; Ping Yong; Neil Anthony Rowson
Original assignee: University of Birmingham
Current assignee: University of Birmingham
Priority date: 2000-02-23
Filing date: 2001-02-23
Publication date: 2001-08-30
Anticipated expiration: 2002-08-23
Also published as: AU2001233960A1; GB2375545A; GB0004155D0; GB0218623D0; GB2375545B

Abstract

The present invention relates to a method of removing metal cations from a metal cation-containing solution. The method comprises the steps of attaching cells having metal reductase activity to a first surface of a support in a reaction vessel, supplying hydrogen to the cells, and exposing the metal cation-containing solution to the cells during step (ii) so as to effect enzymatic reduction of the metal cations to solid metal particles. The invention also resides in a bioreactor constructed and adapted for the enzymatic reduction of metal cations in solution. The bioreactor comprises a reaction vessel (2) adapted to receive a metal cation-containing solution and a support (22) within the reaction vessel (2). The support has a first surface which is adapted to have live cells having metal reductase activity attached thereto. The bioreactor also comprises a source of hydrogen (20, 22, 24, 26) and means (22) for supplying hydrogen to the cells in use.

Description

ETHOD AND BIOREACTOR FOR ENZYMATIC METAL CATION REDUCTION IN SOLUTION

The present invention relates to an apparatus and method for enzymatically reducing metal cations in solution, particularly platinum group metal cations, to solid metal particles.

The routine use of platinum group metals (eg. palladium, platinum and rhodium) is increasing due to their widespread use in industry and in automotive catalytic converters. These metals are a diminishing resource yet their recovery and reuse is still relatively uncommon. Chemical reclamation techniques are hindered by the complex solution chemistry of these metals. Generally the metals exist as complexes in solution and precipitation techniques are not readily applicable since the metal concentration in the solution is usually low. Solution extraction techniques have been developed (eg. using 8-hydroxyquinoline or tributyl phosphate), but substantial plant development is necessary, the extraction rates are generally low and large amounts of extractant and large volumes of solvent are required. This is costly, and the solvents, which may be toxic, eventually require disposal. Electrochemical recovery of platinum group metals is feasible (their electrode potential is 0.44-0.73 V in HCI) but a large electrode surface area is required for effective deposition, and recovery of the resultant thin film of metal from the electrode is an additional factor which limits industrial application.

Biological reduction of metals is also known. In particular, it has been shown (J.R. LLoyd et. al.; Applied and Environmental Microbiology (1998) 64, 4607) that palladium metal can be precipitated from a solution containing palladium (II) in which live Desulfovibrio desulfuricans bacteria are suspended, by supplying hydrogen gas to the solution through a PTFE membrane. Various test data showed that the palladium (II) was reduced enzymatically by the bacteria.

It is an object of the present invention to provide an improved apparatus and method for enzymatically reducing metal cations in solution, particularly platinum group metal cations, to solid metal particles.

According to a first aspect of the present invention, there is provided a method of removing metal cations from a metal cation-containing solution, comprising the steps of:-

(i) attaching cells having metal reductase activity to a first surface of a support in a reaction vessel,

(ii) supplying hydrogen to the cells, and

(iii) exposing the metal cation-containing solution to the cells during step (ii) so as to effect enzymatic reduction of the metal cations to solid metal particles.

It will be appreciated that the primary purpose of said method can be to purify the solution with respect to the metal cations or to recover metals from solution. In the latter case, the method includes an additional step (iv) of removing said metal particles from the reaction vessel.

According to a second aspect of the present invention, there is provided a bioreactor constructed and adapted for the enzymatic reduction of metal cations in solution comprising:- (i) a reaction vessel adapted to receive a metal cation-containing solution,

(ii) a support within the reaction vessel having a first surface which is adapted to have cells having metal reductase activity attached thereto,

(iii) a source of hydrogen, and

(iv) means for supplying the hydrogen to the cells.

Preferably, said bioreactor is a flow-through reactor and means (eg. a pump) are provided to pass the metal cation-containing solution through the reaction vessel.

Preferably, said support is an active support. As used herein, an active support is defined as a support which effects the electrochemical injection of hydrogen or homolytic fission of hydrogen molecules into hydrogen atoms. More preferably, said active support is a palladium-based alloy (eg. Pd-Ag, Pd-Y and Pd-Ce). Most preferably, said alloy is a Pd-Ag, Pd-Y or Pd-Ce alloy containing from 20 to 25 atomic% Ag, from 8 to 10 atomic% Y or 6 atomic% Ce respectively. Particularly preferred alloys are 0.77Pd- 0.23Ag and 0.76Pd-0.24Ag.

Said hydrogen source may be a hydrogen reservoir (eg. hydrogen cylinder or metal hydride store eg. LaNi₅), in which case the hydrogen supply means may comprise means for bubbling hydrogen into the reaction vessel (eg. tubing and sparger). Preferably, however, said hydrogen source includes means for generating hydrogen, such as an electrolytic cell. Preferably, hydrogen is supplied to the first surface of the support through the support from an opposite second surface. It will be appreciated that, in the case of an active support, hydrogen will be supplied to the first surface, and hence the cells attached thereto, as nascent hydrogen atoms.

In a particularly preferred embodiment, the active support serves as the cathode of the electrolytic cell and is in the form of a tube having a closed lower end, the outer surface of the tube defining the first surface to which the cells are attached in use. In use, electrolyte solution within the tube and in contact with the second surface is separated from the metal cation- containing solution within the reaction vessel by the active support which serves as a hydrogen permeable membrane between the two solutions. Thus, it will be appreciated that the electrolyte solution can be optimised for hydrogen generation without an adverse effect on the enzymatic reduction. It will also be appreciated that in the particularly preferred embodiment, the support serves as the hydrogen supply means.

Preferably, said cells are capable of reducing at least one platinum group metal. Preferably said cells are sulphate reducing bacterial cells, more preferably D. desulfuricans cells and most preferably ATCC 29577 or NCIMB 8307 cells. Preferably said cells are resting cells.

Preferably, said metal cations are cations of one or more platinum group metals, and most preferably palladium.

An embodiment of the present invention will now be described by way of example only, with reference to the accompanying drawings in which:- Figure 1 is a schematic representation of a flow-through bioreactor in accordance with the second aspect of the present invention,

Figure 2 is a graph showing palladium (II) reduction at different flow rates through the bioreactor of Figure 1 with and without cells having metal reductase activity, and

Figures 3 and 4 are photographs at different magnifications of cells attached to a Pd-Ag alloy support.

Referring to Figure 1 , a flow-through bioreactor comprises a reaction vessel 2 (20ml flow volume) having an inlet 4 and an outlet 6, a reservoir 8 for feedstock solution and a reservoir 10 for treated solution connected by tubing 12 to the inlet 4 and outlet 6 of the reaction vessel 2 respectively, and a pump 14 directly upstream of the reaction vessel 2 for effecting and controlling flow of solution through the reaction vessel 2. In use, solution within the reaction vessel 2 is stirred by means of a magnetic stirrer unit 16 and follower 18. The bioreactor also incorporates an electrolytic cell comprising a dc power supply 20, a 0.76Pd-0.24Ag alloy cathode 22 in the form of a hollow tube (length 3.2cm, diameter 1.Ocm, approximate surface area 12cm²) closed at its bottom end and defining an electrolyte chamber 24, and a platinum anode 26 extending into the electrolyte chamber 24. The electrolyte is 1 M HN0₃.

The Pd-Ag alloy was prepared by a known method (D. Fort et. al. J. Less Comm. Met., 39, 293) and formed into the required cathode shape by casting. This alloy, like other palladium-based alloys, absorbs hydrogen gas which dissociates and diffuses as atomic hydrogen (H°) through the alloy matrix. H° is an extremely reactive species and rapidly recombines in solution to molecular hydrogen. Thus, the cathode 22 serves as a hydrogen permeable membrane between the electrolyte chamber and the solution passing through the reaction vessel.

Preparation of Cells

D. desulfuricans cells (NCIMB 8307) were cultured for 2 days under N₂ in sealed serum bottles (pre-gassed with N₂ and sterilised) in Postgates' medium C (J.R. Postgate: "The sulphate-reducing bacteria", Cambridge University Press, 1979) which contained (per litre of medium): 0.5g KH₂P0₄, 10.0g NH₄CI, 4.5g Na₂S0₄, 0.06g CaCI₂.6H20, 0.06g MgS0₄.7H₂0, 6.0g sodium lactate, 1.0g yeast extract, 0.004g FeS0₄.7H₂0 and 0.3g sodium citrate.2H₂0 adjusted to pH 7.5+0.2 with 2M NaOH. After being harvested by centrifugation under N₂, the cells were washed under N₂ three times with 20mM MOPS (morpholinopropane-sulphonic acid)-NaOH buffer (pH 7) and re-suspended in degassed N₂- sparged MOPS buffer. Prior to use, the resultant cell suspension was stored under N₂ at 4°C.

Reduction of Pd²⁺ to Pd° Example 1:

A sample of the concentrated (resting) cell suspension so prepared was diluted (=0.25mg dry weight of cells / ml of solution) anaerobically with a 2mM solution of Na₂PdCI₄ in HN03 (pH 2) (feedstock solution pre-gassed with N₂). The mixture was allowed to stand for 1 hour and poured into the reaction vessel 2 so as to partially submerge the Pd-Ag cathode 22. After overnight stirring with hydrogen generated from the electrolytic cell (3V, 20mA), Pd° was precipitated on the cells, causing them to become attached to the surface of the Pd-Ag cathode 22. Thus, the Pd-Ag cathode 22 also serves as a support for cell attachment. The level of attachment was calculated by assay of residual protein in the solution following adhesion (protein according for 50% of the dry weight of the cells) and found to be 1.5mg dry weight of cells/cm² surface area of cathode 22.

After attachment of the cells-Pd⁰ to the Pd-Ag cathode 22, H₂ was continuously generated in the electrolytic cell (3V, 20mA) and Na₂PdCI₄/HN0₃ feedstock solution was pumped from the feedstock reservoir 8 through the reaction vessel 2 (stirred) at a fixed flow rate (70- 150ml/hr) into the treated solution reservoir 10. Hydrogen permeated through the Pd-Ag cathode 22 and was supplied to the cells-Pd⁰ attached to the outer surface of the Pd-Ag cathode 22 as nascent hydrogen (although some recombination to molecular hydrogen is likely). Attachment of the cells-Pd⁰ to the Pd-Ag cathode 22 is essential in order that the cells-Pd⁰ are exposed to nascent hydrogen. As far as the inventors are aware, this represents the first use of nascent hydrogen in a reduction catalysed by bacterial cells.

Samples of the treated solution were taken after 40ml of flow at the given flow rate and analysed for Pd(ll) by differential pulse voltammetry (J.F. Lloyd et. al., supra). Example 1 was repeated using cells from the same batch. A plot of percentage palladium reduced against flow rate for the two runs (1 A and 1 B) is shown in Figure 2. Figure 3 shows the rod-like cells attached to the Pd-Ag cathode surface. At a higher magnification (Figure 4) crystal clusters of precipitated palladium associated with the cells are clearly visible, giving the cells an uneven "knobbly" appearance.

Comparative Example 1:

Example 1 was repeated in the absence of cells. A plot of percentage palladium reduced against flow rate for two independent runs (C1 A and C1 B) is shown in Figure 2. Initially, Pd particles remain attached to the membrane, but after continuous flow for 4 hours, Pd° drops from the cathode surface to the floor of the reaction vessel. The attached Pd° is difficult to remove from the cathode.

Referring to Figure 2, the effect of the presence of the adhered biomass is clearly shown. At a flow rate of 150ml/hr, Pd removal from the feedstock solution (Na₂PdCI₄) is 30% higher (about 85% vs. 55%) in the presence of adhered biomass (Example 1 ) than in the absence of biomass (Comparative Example 1 ). Greater than 75% Pd removal from solution was achieved at a flow rate of 150 ml/hr in the presence of the adhered biomass whereas a lower flow rate of 70 ml/hr was required to obtain 75% removal in its absence. As can be seen from Figure 2, the plot obtained from Example 1 has a shallower gradient than that from Comparative Example 1 , indicating that the proportional enhancement by the adhered biomass is greater at higher flow rates.

In addition it should be noted that in Example 1 , palladium removal from solution was accompanied by a build up of a black precipitate on the biomass, which on further accumulation dropped to the bottom of the reactor. Continued reduction on the biomass was maintained and after 3 days the total Pd recovered from the base of the reaction vessel 2 was 27mg/mg dry wt of biomass. Any Pd associated with the biomass can be recovered by scraping the biomass from the Pd-Ag cathode 22 (easily accomplished due to its smooth surface) followed by treatment with ultrasound and separation. In Example 1 , the amount of Pd recovered from the cathode 22 was about 5% of the amount of Pd recovered from the base of the reaction vessel 2. Energy dispersive X-ray microanalysis and powder X-ray diffraction analysis showed the precipitate to be palladium metal. Thus, the immobilisation of the cells on the substrate allows relatively easy recovery of the palladium.

It should be noted that the optimum pH for Pd reduction by NCIMB 8307 is between 4 and 7. At pH2, reduction is approximately 50% of optimum. Acidic pH was chosen for investigation because this is the usual pH for Pd- containing industrial waste (see Examples 2 and 3). Neutralisation of these very acidic solutions requires additional chemicals and the high ionic content of the solution is known to adversely affect reduction by cell suspensions to a greater degree than the benefit obtained by operating at higher pH.

Comparative Example 2:

H₂ was bubbled at a rate of 5 ml/min through a cell suspension ^"0.25mg cells/ml in HN0₃ containing 2mM Na₂PdCI₄ at pH2. Palladium removal from the feedstock in Comparative Example 2 was 3.0 ± 0.07 μmol/min/mg dry cells. At a flow rate of 150 ml/hr in Example 1 , palladium removal was about 80%. Calculating the palladium removal in Comparative Example 2 in a similar manner to Example 1 , total palladium removal over a comparable time period was 60% These results demonstrate that adherence of cells to the substrate results in a more rapid reduction than the use of a cell suspension supplied with bubbled H₂.

Example 2:

The method of Example 1 was followed using an acid leachate prepared from spent automotive catalyst powder (supplied by Sims Bird Ltd, Long Marston, UK) as the feedstock solution. The leachate was prepared by stirring the ground waste powder (particle size 0.04 - 0.7mm) with aqua regia (3:1 HCI/HN0₃) at 80°C for 2 hours, followed by filtration. The filtrate was adjusted to pH 2.5 with NaOH. The metal content of the leachate was Pt(IV) = 1.2 mM, Rh(lll) = 0.4 mM with negligible Pd(ll). The Pd content of the leachate was supplemented with a stock solution of Na₂PdCI₄ to give a final concentration of Pd(ll) = 1 .4 mM. The Cl^" and N0₃ ^" concentrations were 8.7M and 3.9M respectively.

Comparative Example 3

Comparative Example 2 was repeated using the acid leachate of Example 2. Very little ( < 15%) Pd(ll) was recovered from solution. This is thought to be due to the inability of the free cells to reduce the Pd(ll) in its chloride complexes. Surprisingly, using the process of the present invention, this effect is overcome. A comparison of percentage of Pd recovered at a flow residence time of 10-15 minutes for Example 1 (low Cl^") and Example 2 (high CL) shows that the chloride level has little or no effect on recovered Pd (83% and 87% Pd removed from solution respectively).

Example 3:

The method of Example 1 was repeated using a waste processing solution (supplied by Degussa Ltd, Germany) diluted 1 :500 with water to give a pH2.5 solution having the following composition: Pd(ll) = 2.8mM, Rh(lll) = 0.14mM, Pt(IV) = 1.9mM, N0₃ ^' < 10mM and CL < 20mM. The out-flow samples were analysed for Pt(IV) and Rh(lll) by atomic absorption spectroscopy with an AA/AE spectrophotometer 751 (Instrumentation Laboratory Inc., USA) using standard AAS techniques: air- acetylene flame, hollow cathode lamps (Pt/Rh) at bandpass 0.5nm, lamp current 10mA/5mA and wavelength 265.9nm/343.5nm for Pt(IV) and Rh(lll) respectively. The sample was analysed for Pd(ll) in the same way as described with reference to Example 1.

In addition to palladium removal (72.0% at a flow rate of 95 ml/hr), removal of Pt and Rh was also observed at the same flow rate (62.8% and 60.0% respectively). It is thought that palladium is initially precipitated on the cel ls and that this precipitated palladium provides reaction sites for the subsequent reduction of Pt and Rh. Experiments have shown that Pt and Rh are not reduced in the absence of Pd.

Claims

1 . A method of removing metal cations from a metal cation-containing solution, comprising the steps of:-

(ii) supplying hydrogen to the cells, and

2. A method as claimed in claim 1 , including an additional step (iv) of removing said metal particles from the reaction vessel.

3. A bioreactor constructed and adapted for the enzymatic reduction of metal cations in solution comprising:-

(i) a reaction vessel adapted to receive a metal cation-containing solution,

(iii) a source of hydrogen, and

(iv) means for supplying the hydrogen to the cells.

4. A bioreactor as claimed in claim 3, wherein cells having metal reductase activity are attached to said first surface of the support.

5. A bioreactor as claimed in claim 3 or 4 which is a flow-through reactor and wherein means are provided to pass the metal cation- containing solution through the reaction vessel.

6. A bioreactor as claimed in any one of claims 3 to 5, wherein said support is an active support.

7. A bioreactor as claimed in claim 6, wherein said active support is a palladium-based alloy.

8. A bioreactor as claimed in claim 7, wherein said palladium-based alloy is a Pd-Ag, Pd-Y or Pd-Ce alloy containing from 20 to 25 atomic%> Ag, from 8 to 10 atomic% Y or 6 atomic% Ce respectively.

9. A bioreactor as claimed in claim 8, wherein said palladium-based alloy is 0.77Pd-0.23Ag.

10. A bioreactor as claimed in any one of claims 3 to 9, wherein said hydrogen source includes an electrolytic cell for generating hydrogen.

1 1. A bioreactor as claimed in any one of claims 3 to 10, wherein hydrogen is supplied to the first surface of the support through the support from an opposite second surface.

12. A bioreactor as claimed in claim 6 to 1 1 , wherein the active support serves as the cathode of the electrolytic cell and is in the form of a tube having a closed lower end, the outer surface of the tube defining the first surface to which the cells are attached in use, and wherein, in use, electrolyte solution within the tube and in contact with an inner second surface of the tube is separated from the metal cation-containing solution within the reaction vessel by the active support which serves as a hydrogen permeable membrane between the two solutions.

13. A bioreactor as claimed in any one of claims 3 to 12, wherein said cells are sulphate reducing bacterial cells which are capable of reducing at least one platinum group metal.

14. A bioreactor as claimed in claim 12, wherein said cells are D. desulfuricans cells ATCC 29577 or NCIMB 8307.