Researchers from the University of Houston, with colleagues at the University of São Paolo in Brazil, have demonstrated how copper-resistant bacterium from a copper mine in Brazil convert CuSO₄ (copper sulfate) ions into zero-valent Cu (metallic copper). An open-access paper on their research is published in Science Advances.

Gracioso et al.

The idea of having bacteria in mines is not new, but the unanswered question was: what are they doing in the mines? By putting the bacteria inside an electronic microscope, we were able to figure out the physics and analyze it. We found out the bacteria were isolating single atom copper. In terms of chemistry, this is extremely difficult to derive. Typically, harsh chemicals are used in order to produce single atoms of any element. This bacterium is creating it naturally that is very impressive.

—Francisco C. Robles Hernandez, professor at the UH College of Technology and co-corresponding author

Mining copper often leads to toxic exposures and challenges on drawing out substantial volume for commercial use. Approximately one billion tons of copper are estimated in global reserves, according to the Copper Development Association Inc., with roughly 12.5 million metric tons per year mined. This aggregates to roughly 65 years of remaining reserves.

Part of the supply challenge comes from limited available copper in high concentration in the earth’s crust, but the other challenge is the exposure to sulfur dioxide and nitrogen dioxide in the copper smelting and production process to concentrate the metal into useful quantities.

The novelty of this discovery is that microbes in the environment can easily transform copper sulfate into zero valent single atom copper. This is a breakthrough because the current synthetic process of single atom zerovalent copper is typically not clean, it is labor intensive and expensive.

The microbes utilize a unique biological pathway with an array of proteins that can extract copper (II) (Cu²⁺) and convert it into single-atom zero-valent copper (Cu⁰). The aim of the microbes is to create a less toxic environment for themselves by converting the ionic copper into single-atom copper, but at the same time they make something that is beneficial for us too.

—Debora Rodrigues, Ezekiel Cullen Professor of Engineering at the University of Houston Cullen College of Engineering, co-corresponding author

Proposed mechanism of monoatomic copper synthesis assisted by the ferritin protein. The protein contains an iron core that transfers the electrons to Cu²⁺ ions bound to the nucleation sites on the exterior of the ferritin protein shell to produce Cu⁰. To restore the hole, citrate is oxidized, and the electron is able to continue to flow to reduce additional Cu²⁺ ions present inside the bacterium. Gracioso et al.

With a focus in electronic microscopy, Robles examined samples from Rodrigues’ findings in Brazilian copper mines and he determined the single atom nature of the copper. Rodrigues’ and Ellen Aquino Perpetuo’s (professor at the University of Sao Paulo) groups further identified the bacterial process for converting copper sulfate to elemental copper—a rare find.

Research results demonstrate this new conversion process as an alternative to produce single atoms of metallic coper is safer, and more efficient versus current methods (i.e. chemical vapor deposition, sputtering and femtosecond laser ablation).

We have only worked with one bacterium, but that may not be the only one out there that performs a similar function. The next step for this particular research is harvesting the copper from these cells and using it for practical applications.

—Debora Rodrigues

Resources

• Louise Hase Gracioso, Janire Peña-Bahamonde, Bruno Karolski, Bruna Bacaro Borrego, Elen Aquino Perpetuo, Claudio Augusto Oller Do Nascimento, Hiroki Hashiguchi, Maria Aparecida Juliano, Francisco C. Robles Hernandez, Debora Frigi Rodrigues (2021) “Copper mining bacteria: Converting toxic copper ions into a stable single-atom copper” Science Advances doi: 10.1126/sciadv.abd9210