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| − | <br> <a href="#" class="back-to-top">Back to Top</a> | + | <h1 style="margin-top:8%; text-align:center; font-size:30px;">Conversation with B.E. Carlos Lara Valenzuela</h1> |
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| − | <li class="active"><a data-toggle="pill" href="#overview">Overview</a></li>
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| − | <li><a data-toggle="tab" href="#module1">Module 1</a></li>
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| − | <li><a data-toggle="tab" href="#module2">Module 2</a></li>
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| − | <li><a data-toggle="tab" href="#module3">Module 3</a></li>
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| − | <h4><strong>Overview</strong></h4>
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| − | <p>Monterrey is one of the leading industrialized cities in Mexico, but this growth has led to many environmental and health issues. The metropolitan area is considered as the most polluted in Latin America in terms of air quality (Clean Air Institute). One of the main reasons is the high concentration of PM10 in the atmosphere, particles so small that they are able to penetrate deep into the lungs and cause a wide variety of pulmonary and heart diseases, leading to a premature death. It is estimated that industrial activities and “primitive” e-waste recycling techniques are among the main factors for the release of these type of particles into the atmosphere [3]. Due to these alarming problematics, our main motivation for the project is to create a novel synbio system to improve the e-waste recycling technology and make it a feasible approach to reduce the pollutants, and generating a business model with the recycling of these valuable metals.</p>
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| − | <p>E-waste contains valuable metals as well as potential environmental contaminants. Effective recycling technology, that recovers the valuable materials with minimal environmental impact is quite expensive. As a result, most of the electronic waste is dumped in landfills where it is burned with other wastes generating dioxins, hydrogen chloride and other compounds that directly affect the environment. [4] </p>
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| − | <p>The main benefit of using recycled metals over their virgin materials is the significant energy saved, almost 95%![2] Additionally, e-waste represents a rich source of precious metals compared to their primary ores. For example, one ton of e-waste from PCs contains more gold than the one recovered from 17 tons of gold ore [1].</p>
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| − | <p>For these reasons we decided to make a project based on a eco-friendly bioleaching technology using two modified bacteria, <i>Acidithiobacillus ferrooxidans</i> and <i>Chromobacterium violaceum</i>, for recovering metals from the e-waste.</p>
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| − | <p><strong>References</strong></p>
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| − | <p>[1] Khaliq, A., Rhamdhani, M., Brooks, G., & Masood, S. (2014). Metal Extraction Processes for Electronic Waste and Existing Industrial Routes: A Review and Australian Perspective. Resources, 3(1), 152–179. MDPI AG. Retrieved from http://dx.doi.org/10.3390/resources3010152</p>
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| − | <p>[2] Clean Air Institute, (2012). Air Quality in Latin America: An Overview. http://www.cleanairinstitute.org/calidaddelaireamericalatina/cai-report-english.pdf</p>
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| − | <p>[3]Gangwar, C., Singh, A., Kumar, A., Chaudhry, A. K., & Tripathi, A. (2016). Appraisement of Heavy Metals in Respirable Dust (PM 10) Around E-Waste Burning and Industrial Sites of Moradabad : Accentuation on Spatial Distribution , Seasonal Variation and Potential Sources of Heavy Metal, 10(6), 14–21. http://doi.org/10.9790/2402-1006021421</p>
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| − | <p>[4]Robinson, B. H. (2009). E-waste: An assessment of global production and environmental impacts. Science of The Total Environment, 408(2), 183-191. doi:10.1016/j.scitotenv.2009.09.044</p>
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| − | <div class="container"> | + | <p style="text-indent:50px;">We approached Carlos Lara, Manager of the Mineral Processing Department of Grupo Peñoles’ Center of Investigation and Technological Research. Grupo Peñoles is the world’s biggest refined silver producer and Latin America’s leader in refined gold and lead production, so we considered this meeting of the utmost importance. We wanted to see if our project’s design was actually suited for the metal recovery from e-waste. In addition, we wanted to know if we could use our technology to improve the current methods where metals are recovered from mines and make them less damaging to the environment. </p> |
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| − | <h4><strong>Bioleaching Experiments</strong></h4>
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| − | <p style="text-indent:50px;"> Our first microorganism, A. ferrooxidans is currently used in a few mining companies for the recovery of Zn, Ni and Cu from the residual dirt and rocks of their processes. <i> A. ferrooxidans</i> can recover these metals thanks to its extracellular matrix, which has an environment that facilitates electron interchange. The enzyme tetrathionate hydrolase (TetH), which is produced inside the bacteria and then taken out to the external matrix, constantly converts tetrathionate to two molecules: thiosulfate and elemental sulfur. Tetrathionate is a key intermediate during RISC oxidation, hydrolyzed by tetrathionate hydrolase (TetH), and used as sole energy source. The overexpression of TetH in <i>A. ferrooxidans</i> leads to have a high expression of sulfates in the culture media, with more activity, more protons are released to the media and leads to a greater solubilization of metal ions. | + | |
| − | <br>The reaction that takes place is illustrated in the following figure:</p>
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| − | <p style="font-size: 10px; width:50%; margin-left:25%; text-align:justify;">Figure 1. TetH is involved in the electron transfer for A. ferrooxidans. The reaction increases the acidity of the media and releases sulfates, which chelate metals.</p>
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| − | <h4>FUR/FURBOX</h4>
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| − | <p>Although iron is the most abundant transition metal on Earth, its solubility is very low at neutral pH in aerobic environments. In such environments, Fe(II) is generally not available because it rapidly oxidizes to Fe(III), which precipitates as insoluble ferric ion complexes. (Quatrini, 2005). Thanks to the acidic conditions of<i> A. ferrooxidan</i>s Fe(II) is stable and Fe(III) is more soluble than at standard pH (7.0)</p>
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| − | <p>References
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| − | <br> Quatrini, R., Lefimil, C., Holmes, D. S., & Jedlicki, E. (2005). The ferric iron uptake regulator (Fur) from the extreme acidophile Acidithiobacillus ferrooxidans. Microbiology (Reading, England), Pt 6, 2005–2015.</p>
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| − | <br> </div> | + | <p style="text-indent:50px">Carlos Lara agreed to share his experience with us, which he has plenty of thanks to his 45-year career at Grupo Peñoles. He gave us valuable insights on the mining industry and metal recovery in Mexico, which helped us immensely and inspired us to modify our initial project. </p> |
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| − | <div class="container"> | + | <p style="text-indent:50px">Mr. Lara explained that there was an attempt here in Mexico to implement bioleaching as a way to recover metals from mines, based on previous experiences reported in mines in Chile. However, this project failed because they didn’t take into account the difference in composition of the Chilean and Mexican ores. Chilean ores are mainly composed of copper, which is perfectly suited for the metal recovery using our initial microorganism: A. ferrooxidans, but when they tried to implement this same process, they realized that it wasn’t profitable here in Mexico because Mexican ores have much higher concentrations of silver.</p> |
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| + | <p style="text-indent:50px"> This real- life experience and the fact that after collecting all the disposed electronic devices you can recover a considerable amount of gold and silver made us reconsider our project and add a new bioprocessing step with another microorganism capable of recovering silver and gold: C. violaceum. Even though during our previous research for project ideas we found that C. violaceum could be used to recover precious metals from e- waste, at first we weren' t considering to include it in our process. However, after our interview with Mr. Lara it was clear that we had to integrate C. violaceum into our process. This way we would not only be reducing pollution by recycling precious metals from an already used source, but we would also be able to propose our process as a suitable, profitable and more environmentally friendly way to recover metals from natural ores in Mexico. </ p> |
| − | <h4><strong>Aim of the module: gold & silver solubilization</strong></h4>
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| − | <p style="text-indent:50px ;"> With the acid oxidation process used in Module I, metals such as copper, zinc and nickel are solubilized by <i>A. ferrooxidans</i>, but valuable metals like gold and silver are not. However, the recovery of these last two metals is vital in making the process economically viable, due to their high market value and relative scarcity. An effective way to solubilize them is by bioleaching with a cyanide-producing organism such as <i>C. violaceum</i>.</p> | + | |
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| − | <h4><strong>Process description</strong></h4> <img style="width:100%;" class="wow zoomIn" align=center src="https://static.igem.org/mediawiki/2016/4/49/T--Tec-Monterrey--Module_2_Overall_Diagram.png">
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| − | <h4><strong>Why<i> Chromobacterium violaceum</i></strong></h4>
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| − | <br>why we chose <i>C. violaceum</i></button>
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| − | <h4 class="modal-title" id="myModalLabel">Why we chose to use C. violaceum for this process.</h4> </div>
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| − | <div class="modal-body"><i>C. violaceum</i> has several characteristics that make it ideal for our project:
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| − | <ul>
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| − | <li>It is more suited for the project compared to other bacteria that could also be used for cyanide bioleaching. (e.g.<i> Pseudomonas fluorescens</i>)
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| − | <li>It was shown by Pham & Ting (2009) that gold recovery with C. violaceum is more effective than with <i>P. fluorescens</i> as long as the e-waste has been pre-oxidized, as we have designed in Module I.</li>
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| − | <li>It has already been studied as a bacteria of biotechnological interest because of its adaptability to different stresses. Chief among these characteristics, its resistance to heavy metals and ability to grow using many different carbon sources stand out, as was described by the Brazilian National Genome Project Consortium (2003), the team that first reported sequencing its genome.</li>
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| − | <li>As it is a native cyanide producer, it is also highly resistant to it (Niven, Collins & Knowles, 1975).</li>
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| − | <li>It is relatively easy to manage in the lab (similar enough to E. coli as to allow the use of very similar lab protocols)
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| − | <li>It grows in LB media, with observed population kinetics similar to <i>E. coli.</i></li>
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| − | <li>It was transformed successfully with a pretty standard electroporation protocol (Broetto et al, 2005).</li>
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| − | <li>We were able to transform it using iGEM’s standard PSB1C3 plasmid with a calcium competency protocol for <i>E. coli</i></li>
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| − | <h4><strong>How will we improve the process?</strong></h4>
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| − | <p style="text-indent:50px;">In order to improve the efficiency of the process, we will modify C. violaceum using two basic constructs:</p>
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| − | <li>A gold/copper-sensitive cyanide producing system
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| − | <li>Though <i>C. violaceum</i> is a native cyanide producer, this basal production is relatively low and regulated by quorum sensing.
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| − | <li>In a previous study, cyanide production was successfully uncoupled from quorum sensing regulation by transformation with hydrogen cyanide synthase (hcnABC) under pARA regulation (Tay et al, 2013).</li>
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| − | <li>pGolS is a promoter that has been used previously by other iGEM teams. Specifically, Team York 2013. This promoter is induced by gold, through the binding of transcriptional activator GolS with gold, which later binds to the recognition site of the promoter allowing for higher expression of genes in the presence of gold. </li> <img class="wow zoomIn" src="https://static.igem.org/mediawiki/2016/c/c3/T--Tec-Monterrey--Module_2_golS_Diagram.png" style="width:100%;">
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| − | <li>Our construct is constituted by the pgolS promoter and the coding sequence of CueR. As the pgolS promoter has a basal transcription level, some CueR would be constantly expressed. CueR is a transcription factor that stimulates the transcription at the pgolS promoter when bound to copper or gold, as was described by the iGEM York 2013. Thus, this construct increases its own expression through positive feedback in the presence of gold or copper through the production of a copper/gold sensitive protein. Its organism of origin is <i>Salmonella typhimurium</i>. We optimized the original sequence for <i>Chromobacterium violaceum</i> and used this system along with Hydrogen Cyanide Synthase operon hcnABC in order to create a self-regulatory system that activates in the presence of gold, increasing cyanide production and enabling the solubilization of this precious metal as [Au(CN)2]–.</li>
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| − | <li>Proton Pump
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| − | <li>Hydrogen cyanide synthase generates HCN, not free CN- ions. Since free CN- ions are responsible for the reaction that solubilizes gold, we must take into account the dissociation of HCN. As HCN is a weak acid (pKa = 9.22[source?]), it requires a basic pH in order to dissociate. In the mining industry, it has been established that a pH > 9.4 is ideal (Marsden & House, 2006), as at lower pH values not enough HCN is dissociated; and at higher pH values other factors (such as salinity) become more important. [source?]</li>
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| − | <li>As shown in the modelling section, the overall gold solubilizing reaction generates H+. Thus, in order to mantain a pH = 9.5, a base has to be continously added to the reactor.</li>
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| − | <li>In order to improve the efficiency of <i>C. violaceum</i>’s activity under these alkaline conditions, we will introduce a proton pump. [explicación de por qué creemos que esto funcionaría, referenciada al paper de Suria]. Specifically, we will use the intermembrane protein Na(+)/H(+) antiporter NhaA, which is a proton pump from E. coli. It excretes one Na(+) ion in exchange for two H(+) external protons. This protein is active at alkaline pH. </li> <img class="wow zoomIn" src="http://placehold.it/350x150">
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| − | <li>Within our construct, the proton pump is regulated by a weak constitutive promoter from the Anderson collection, and the coding sequence is optimized for its expression in <i>C. violaceum</i>. We will also test it as a potential selectivity marker for <i>C. violaceum</i> under these alkaline conditions (pH=9.5).</li>
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| − | <h4><strong>Safety Considerations</strong></h4>
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| − | <h4>Post-processing:cyanide degradation module</h4>
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| − | <p style="text-indent:50px ;"> After gold is recovered in the third module, we will degrade the remaining cyanide, as it is too toxic to be released to the environment without previous treatment.</p> | + | |
| − | <p style="text-indent:50px;">We will use a different transformant of <i>C. violaceum</i> that will overexpress cyanide hydratase, a cyanide degrading enzyme. This enzyme has been found in different cyanide-degrading fungi such as <i>Aspergillus niger</i> and <i>Fusarium lateritium</i>.</p>
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| − | <p style="text-indent:50px;">Cyanide hydratase has been successfully expressed in <i>E. coli</i>, where it retains its original activity. In fact, Brown, Turner, and O'reilly (1995) reported that a modified <i>E. coli</i> reached 5.5 times the cyanide degrading activity compared to that observed in the native <i>F. lateritium</i>.</p>
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| − | <p><strong>References</strong></p>
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| − | <ol>
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| − | <li>Broetto, L., Cecagno, R., Sant'anna, F. H., Weber, S., & Schrank, I. S. (2005). Stable transformation of Chromobacterium violaceum with a broad-host-range plasmid. Appl Microbiol Biotechnol Applied Microbiology and Biotechnology, 71(4), 450-454. doi:10.1007/s00253-005-0140-5 </li>
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| − | <li>Pham, V., & Ting, Y. P. (2009). Gold Bioleaching of Electronic Waste by Cyanogenic Bacteria and its Enhancement with Bio-Oxidation. AMR Advanced Materials Research, 71-73, 661-664. doi:10.4028/www.scientific.net/amr.71-73.661</li>
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| − | <li>Niven, D. F., Collins, P. A., & Knowles, C. J. (1975). The Respiratory System of Chromobacterium violaceum Grown under Conditions of High and Low Cyanide Evolution. Journal of General Microbiology,90(2), 271-285. doi:10.1099/00221287-90-2-271</li>
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| − | <li>Tay, S. B., Natarajan, G., Rahim, M. N., Tan, H. T., Chung, M. C., Ting, Y. P., & Yew, W. S. (2013). Enhancing gold recovery from electronic waste via lixiviant metabolic engineering in Chromobacterium violaceum. Scientific Reports, 3. doi:10.1038/srep02236</li>
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| − | <li>Marsden, J. O., & House, C. I. (2006). The chemistry of gold extraction. Littleton: Society of Mining Metallurgy and Exploration.</li>
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| − | <li>Perky, R., Browner, R., Dunnei, R., & Stoitis, N. (1999). Low pH cyanidation of gold. Minerals Engineering, 12(12), 1431-1440. doi:10.1016/s0892-6875(99)00132-6</li>
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| − | <li>Brown, D. T., Turner, P. D., & O'reilly, C. (1995). Expression of the cyanide hydratase enzyme from Fusarium lateritium in Escherichia coli and identification of an essential cysteine residue. FEMS Microbiology Letters, 134(2-3), 143-146. doi:10.1111/j.1574-6968.1995.tb07928</li>
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| − | <li>Brazilian National Genome Project Consortium (2003). The complete genome sequence of Chromobacterium violaceum reveals remarkable and exploitable bacterial adaptability. Proceedings of the National Academy of Sciences, 100(20), 11660-11665. doi:10.1073/pnas.1832124100</li>
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| − | <li>Rinágelová, A., Kaplan, O., Veselá, A. B., Chmátal, M., Křenková, A., Plíhal, O., … Martínková, L. (2014). Cyanide hydratase from Aspergillus niger K10: Overproduction in Escherichia coli, purification, characterization and use in continuous cyanide degradation. Process Biochemistry, 49(3), 445–450. http://doi.org/10.1016/j.procbio.2013.12.008</li>
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| − | <h4><strong>Overview</strong></h4>
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| − | <p style="text-indent:50px;">After dissolving the metals in in modules I and II, it is necessary to precipitate them and to do this we require to reduce them to a neutral charge. Using the reduction potentials of the metallic ions in the solutions, we established a spontaneous reaction by coupling them with the oxidation of S2- from H2S into S0. To obtain this, we expressed the enzymes sulfur reductase and iron oxidase in E. coli. Sulfur reductase is used to catalyze the reduction of elemental sulfur in the presence of oxygen, with sulfite and hydrogen sulfide as products. Iron oxidase catalyzes the oxidation of Fe2+ to Fe3+, giving it a big enough reduction potential for its reduction to happen spontaneously. The electrical potential difference between the two electrodes can be used as power source to feed a small device or be stored in a battery for its later use.</p>
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| − | <p style="font-size:12px;"><strong>Figure 1. Metals recovered with Acidithiobacillus ferrooxidans.</strong> The solubilised metals are going to be then used in the galvanic cell, but they will have to reduced or oxidised first.</p>
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| − | <p style="font-size:12px;"><strong>Figure 2. Metals recovered with Chromobacterium violaceum.</strong>The solubilised metals are going to be then used in the reduction half-cell. </p>
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| − | <p style="text-indent:50px;">From module I, elemental sulfur is produced due to the metabolism of A. ferrooxidans. The composite part BBa_K2055025 uses the T7 system in order to express the enzyme sulfur reductase, which catalyzes the simultaneous oxidation and reduction of elemental sulfur in the presence of oxygen, with sulfite and hydrogen sulfide as products.</p>
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| − | <div class="row"> <img class="wow zoomIn" style="width:100%;" src="https://static.igem.org/mediawiki/2016/c/cf/T--Tec-Monterrey--Sulfur.png">
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| − | <p style="text-align:center; font-size:12px;"><strong>Figure 3. Sulfur reductase generator.</strong> The composite part is used to express the enzyme sulfur reductase. </p>
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| − | </div>
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| − | <p style="text-indent:50px;">From module I, ions of Fe+2 are produced due to the metabolism of A. ferrooxidans. The composite part BBa_K2055425 uses the T7 promoter in order to express the enzyme iron oxidase, which catalyzes the oxidation of Fe+2 to Fe+3.</p>
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| − | <div class="row"> <img class="wow zoomIn" style="width:100%;" src="https://static.igem.org/mediawiki/2016/9/91/T--Tec-Monterrey--Iron.png">
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| − | <p style="text-align:center; font-size:12px;"><strong>Figure 4. Iron oxidase generator.</strong> The composite part is used to express the enzyme iron oxidase. </p>
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| − | </div>
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| − | <p style="text-indent:50px;">In a galvanic cell, the element with the higher (or more positive) reduction potential gets reduced and the one with the lower (or more negative) reduction potential gets oxidized. From Table 1, it can be observed that copper, silver and gold be reduced to a neutral charge spontaneously when coupled with the H2S half-cell. Iron can also be reduced this way but only from Fe3+ to Fe2+. In the case of nickel and zinc, they can’t be reduced by sulfur oxidation, they needs to be recovered by using an electric current or in another cell arrangement. The energy produced when reducing copper, silver, gold and iron (Fe3+ to Fe2+) can be used to reduce the rest of them or further reduce iron. (Figure 5) </p>
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| − | <table>
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| − | <th style="width:20%;">Reaction</th>
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| − | <th style="width:20%;">Reduction Potential</th>
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| − | </tr>
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| − | <tr>
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| − | <td>a</td>
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| − | <td>a</td>
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| − | </tr>
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| − | </table>
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| − | <p style="text-align:center; font-size:12px;">Table 1. Table of standard reduction potentials. </p>
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| − | <div class="row" align=center> <img class="wow zoomIn" src="https://static.igem.org/mediawiki/2016/1/1b/T--Tec-Monterrey--celda.png">
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| − | <p style="text-align:center; font-size:12px;"><strong>Figure 5. Possible reactions occurring in the cell. </strong> In the left side, the oxidation of H2S to elemental sulfur occurs. In the right side, the reductions occur and the recovery of metals occur.</p>
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| − | </div>
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| − | <br>
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| − | <strong>References</strong>
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| − | <ol>
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| − | <li>Çolak, D., et al. (2015). Biochemical characterization of wild-type and mutant (Q9F and S21Y/V22D) iron oxidases isolated from Acidithiobacillus ferrooxidans M1. Turkish Journal of Biology, 2016(40), 166-173.</li>
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| − | <li>He, Z., et al. (2000). Cloning and heterologous expression of a sulfur oxygenase/reductase gene from the thermoacidophilic archaeon Acidianus sp S5 in Escherichia coli. FEMS Microbiology Letters , 2000(93), 217-221.</li>
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| − | <li>Smccd. (n. a.). Expanded Reduction Potential Table. Retrieved October, 2016, from http://accounts.smccd.edu/batesa/chem220/reference/Exp-Reduction.pdf</li>
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| − | <li>The UniProt Consortium. UniProt: a hub for protein information. Nucleic Acids Res. 43: D204-D212 (2015)</li>
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| − | </ol>
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| − | </div>
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| − | </div>
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| − | </div>
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