Bacteria created uranium deposits

Surprising dominance of non-crystalline uranium compounds speaks for biogenic origin

Uranium ore from deposits in sandstone has previously been considered predominantly crystalline. But apparently not true. © USGS
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Surprise in the Underground: The richest uranium occurrences of the earth could have originated differently than thought. According to new analyzes, a large part of the uranium in these ores is not crystalline, but is bound to organic matter. These uranium compounds were probably once produced by bacteria, as the researchers report in the journal "Nature Communications". It may be necessary to rewrite the textbooks.

The element uranium is a natural component of the earth's crust - but it is found in it only in very small proportions. In some places, however, the uranium has enriched and created uranium ore deposits. Commonly, abiotic chemical reactions of the rock in an aqueous environment caused the uranium to deposit in the form of crystalline minerals.

Uranium ore on the test bench

Around half of the world's uranium comes from so-called roll-front deposits - sandstone-embedded uranium ore deposits. With them one has assumed that the uranium there is predominantly crystalline and insoluble in water - for example as uraninite (pitchblende).

But these common doctrines refute now Amrita Bhattacharyya of Colorado State University and her colleagues. For their study, they had analyzed samples from roll-front deposits in the US state of Wyoming. Among other things, they used a synchrotron radiation-based spectroscopy method and various isotope analyzes to determine the exact composition of the ore.

Organic bound instead of crystalline

It was surprising that between 58 and 89 percent of the uranium in the samples was not as crystalline uraninite as previously thought. Instead, this uranium was bound to organic matter in noncrystalline form, the researchers report. By contrast, classical uraninite crystals only accounted for between two and 29 percent of uranium. display

Uranium Mine in a Sandstone Formation in Utah. Aff Matt Affolter, wikipedia en / CC-by-sa 3.0

"Our results confirm that non-crystalline uranium is the main form of uranium in the ore of these deposits, " the researchers note. "This means a paradigm shift in our idea of ​​ore formation and ore mining." Because the new findings could mean that now the textbooks must be rewritten.

Created by bacteria

And not only that: The isotope analyzes revealed that the uranium ore must also have arisen differently than the current theory envisages. "In three out of four samples, the uranium-238 content was higher than is normally the case in the continental crust, " report Bhattacharyya and her colleagues. This speaks for a predominantly biogenic origin of these occurrences an ore formation by bacteria.

The scientists found a look at the ore specimens at the "T ter": they found various types of bacteria, including Geobacter and Pseudomonas, which are already known to reduce heavy metals such as uranium and thus gain their energy. Whether they participated in the formation of organic uranium compounds three million years ago or other similar bacterial species has not yet been resolved.

Formation of uranium ore on an abiotic and biogenic path. Bhattacharyya et al., Nature Communications / CC-by-sa 4.0

However, the current results provide further evidence that microbes are more involved in the formation of ore occurrences than previously thought. Because even for gold deposits researchers now postulate such a microbial assistance.

Increased risk of contamination

The new findings also shed new light on the possible environmental effects of uranium mining and the rehabilitation of former mining sites. Because the biogenic non-crystalline uranium can more easily pass into a water-soluble form than the crystalline uraninite, as the researchers explain.

However, this means that the risk of contamination of groundwater with uranium at such storage sites is significantly higher than previously assumed. "This has potential implications for the assessment of health threats from uranium mining, as well as for remediation of past uranium mines, " Bhattacharyya and her colleagues note. They now want to investigate further roll-front deposits in order to check whether the dominance of non-crystalline uranium compounds also applies to them. (Nature Communications, 2017; doi: 10.1038 / NCOMMS15538

(Colorado State University, 07.06.2017 - NPO)