Our process
Our Process is applicable on many ways
Our process can begin with many different sources. As platinum accumulate in many places, its recovery can begin from a lot of places and substrates. In fact the first step of our project would change if the basic matter change. Moreover, solutions of phytoremediation provide a lot of substrates and by-products our project could start from.
Indeed our process could start from ashes
The Source (step1)
Sewage Sludge
(Incineration)
Ashes
Bioleaching
The Bioleaching process allow a far better recovery of platinum. Indeed, the drop of pH is required for the metals solubilization. This step could be realized with chemicals as chlorhydric acid in the actual industry. But in order to achieve a greener process as possible, we decided to lower the pH with biological ways, as the leaching accomplished by Thiobacillus. This method is widely use in mines, e.g. for copper mines in South America, so this will be a reliable step already tested in industrial conditions.
This method rely on the ability of the bacteria Thiobacilus to acidify its medium until a pH of 1. The bacterium solution is applied on the ashes and liquid part dropping from it constitute the leachate i.e. a very acidified solutions containing solubilized metals particles mostly in ionic form.
This step wouldn't be obligatory in our process, but it could improve our recovery yield while still using a environmentally friendly approach. Moreover, after leachning, the remained ashes will present a decreased concentration in metals. If this concentrations are lowered enough, the ashes are now ready to be spreaded on fields. Of courses before it, steps of alcanization and destruction of potentially Thiobacillus cells should be performed. If concentrations are still too high for spreading, this step may be repeated.
This step consist in spreading a Thiobacillus solution on the ashes then recollecting the produced leachate. This step could be also realised not with Thiobacillus but with chemicals solutions.
Siderophore mediated recovery
Once the platinum is leached, we need to recover it. If leaching (by biological or chemical methods) is clearly recommended to improves solubilization of metals, it does not improves the concentrations. As we worked with synthetic biology, we decided to use what it is commonly employed by cells to catch metals, i.e. siderophore. Siderophores are well known to catch iron most of all but some of them have an affinity with others metals as platinum. So in our process, we planned to work with such a siderophore, called Desferrioxamine B. This one is already employed to recover platinum in mines, and have shown high capacity to recover platinum from ores [1].
Recovery yields with DFHOB can reach 75% of the total platinum if both of these conditions are fully respected: a lowering pH level leaching step should be performed, as well as a alcanization of medium (until a pH range between 8 and 9) just before addition of siderophore. The last step devoted to rise up the pH level will be realized using a standard buffer, e.g. a Tris buffer.
So where is the innovation in this step? Firstly, in our case, DFHOB won't be applied on the same materials where is commonly used, in our cases not ores but a leachate of ashes. Basically the main difference will be the metal concentration.
Secondly, DFHOB is usually synthesized chemically, we'll rather produce it in high amounts with bacteria. Indeed, operon of the Desferrioxamine B biosynthesis from Streptomyces coelicolor will be cloned into a E. coli bacteria strains in order to produce it, hence lowering the costs of required basic matter as production by bacteria needs especially an appropriate medium and good growth conditions.
DFHOB is a derivative of diamines moelcule and therefore its Biosynthesis start with a amino acid, lysine. Lysine is quite expansive, and as we are aware about the cost of our process we decided to use a cheap source of lysine the corn steep liquor. Such a lysine source is already in use in industry since it's cheap, amino acid provided, produced in industrial amounts and well known as a excellent source of nitrogen in growth media. So in this step we hope we could produce DFHOB in high quantities with a affordable cost.
Following previous uses of DFHOB, 75% of platinum can be recovered with 3mM DFHOB solution on 100g at 5ppm platinum concentrated ore. That allow us to estimate that in order to reach a 75% yield (max yield obtained) we 'll need to add approximately 3mg of DFHOB per µg of platinum (see Raw calculations).
For instance....; CALCULS POUR 1G COMBIEN PRODUIT UNE ECOLI DE DFHOB,,???
To sum up, this step consist in adding an appropriated quantity of Siderophore producer E. coli to the leachate.
Siderophore producer lysis
Our engineered E. coli, i.e. our siderophore producer will produce a lot of DFHOB molecules in its cytoplasm after induction. But theory tell us that all produced molecules may not go out of the cell hence the membranous export system of E. coli is not adapted to the DFHOB. To resolve this we decided to realize the lyse of bacteria in order to release the totality of siderophore molecules in the media.
A wide range of industrial lysis technique are available to perform this step. ( MENTON GAULIN ??)
This step consist in lyse the bacterial cells.
Siderophore recoverer addition
Given that the siderophore have been released on the media, the majority of platinum (most of all in a ionic form) is now strongly bounded with it. It has to be noted that DFHOB specificity is not perfect, so many others metals should from complex with it, especially iron.
In this step we aim to take back the complex siderophore-platinum from the media. As Streptomyces coelicolor is a DFHOB producer organism, it possess the appropriate system to import it. That why a certain amount of S. coelicolor will be added in the growth media during an optimized time of incubation.
We can estimate the amount of bacteria we have to add and also the time of incubation knowing the input capacity of bacteria.
For instance......
In this step, a certain quantity of S. coelicolor will be added to the media during determined duration.
At this step, S. coelicolor cells have imported the vast majority of Siderophore-platinum complexes. The aim of this step is to remove the bacteria from the solution and hence remove platinum with it. Several industrial techniques are relevant to do that as centrifugation or ultrafiltration. At the end of both of these techniques, the cells will be separated from the ashes.
This step allow two important things :
-The sludge ashes contained in the leachate will be rid of the metals especially since the siderophore would have caught a lot of others metals.
This allow the spreading in the fields of the sludge ashes, thus answering the great issue of the sewage sludge problematic.
If the metals concentration are not lowered enough to permit spreading, a additional removing metal process should be performed (step 4 to 7).
-It move platinum from the whole solution space to the cells cytoplasm space, thus increasing its concentration by reduction of the volume.
Concentration increase is hard to estimate since the volume of the leachate (step 3) is not precisely known.
instance with estimation
This step consist in removing the cells either by centrifugation or by filtration. At this step, all remained sludge ashes get out of the process.
Siderophore recoverer destruction
Once the siderophore has fulfilled its function we need to get rid of it. Indeed the bound between platinum and it is very strong and it would be irrelevant to try to destroy it. That's why we decided to destroy the siderophore itself by a thermal degradation that is to say a incineration. This step will destroy the main componenent of the cells, will remove the water and will destroy most of molecules, including our siderphore. As in the first step the burning will reduce the volume of the our materials (from cells to ashes) therefore enhancing its platinum and others metals concentration by a X20 factor.
This step present another advantage: in the next step, the most wanted form of platinum is a ionic one and high heat is well known to produce rather ionic particles. Thus the next step will be performed in best conditions.
In this step cells filtrates or pellets are incinerated, the rest of the process will be performed with the produced ashes.
Glossary
Bioleaching : in simple words, leaching is a metal extraction technique which rely on solubilization of metals ores (ore must be soluble and impurities must be insoluble) in a aqueous solution, by using strong acid solutions. The leachate is the solution containing the solubilized metals of interest. Bioleaching in the same process but involves uses of living organism.
DFHOB: Desferioxamine B, is a molecule produced by (among others) Streptomyces pilosus [2] able to catch metals (as platinum) with a very high affinity.
Corn steep liquor: a by-product of an industrial process (wet-milling) applied on corn kernels. As the kernels are steeped in water solution, the process produce an amino acid, vitamins and minerals enriched solution in high volumes.
Raw calculations
- ↑ Bau and al., 2015 http://dx.doi.org.gate1.inist.fr/10.1016/j.hydromet.2015.01.002
- ↑ Müller, Matzanke and Raymond., 1984 https://www.scopus.com/record/display.uri?eid=2-s2.0-0021137021&origin=inward&txGid=0
