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Bacteria used to sweeten sour gas

Published: 7 april 2017
Bacteria used to sweeten sour gas

By harnessing the power of billions of sulfur-hungry bacteria, Wetsus scientist Paweł Roman and his colleagues are developing new ways to clean “sour gas” by removing toxic substances that contribute to steel corrosion and air pollution.

When gases such as natural gas, landfill gas, and petroleum gas contain a large amount of hydrogen sulfide, thiols, and other organosulfur compounds, they are considered “sour.” Because of their adverse effects, these sulfurous compounds need to be removed from sour gas streams in a process called “sweetening.”

Roman first became interested in sulfur chemistry while working on his master’s degree at the Gdańsk University of Technology in Poland. He attended the university from 2006 to 2011, where he received a bachelor’s degree in Applied Chemistry and a master’s degree by completing a thesis on airborne research titled “Chemistry and transport of pollutants in cumulus cloud water.”

In November 2011, Roman started a PhD project on sulfur chemistry guided by the sub-department of Environmental Technology at Wageningen University. He performed his PhD project at Wetsus, graduating in May 2016. Now he works as a post-doctoral researcher at Wetsus where, among other responsibilities, he supervises a follow-up sulfur project.

Roman explained that the challenges of working with sulfur were partly what drew him toward the subject in the first place.

“During my first contact with thiols while doing my master studies, I realized that sulfur chemistry is very complex and not very popular among scientists, as it is usually associated with an unfriendly environment, i.e., sulfur compounds are often very smelly and toxic,” Roman said. “Also, measuring sulfur compounds can be problematic as many of them are unstable. I always liked challenges, so the possibility of further work with sulfur during my PhD research was very attractive to me. Actually, it was a double challenge for me, as biology is strongly involved in this project, which was quite new for me at that time.”

In his most recent work, Roman and his colleagues have focused on using bacteria to remove sulfurous compounds from sour gas. Traditionally, removal of these compounds involves physicochemical processes, which are often expensive, energy-intensive and leave relatively large leftover “tails.”

There are several advantages of using sulfur-oxidizing bacteria to break down the toxic substances into safer ones. Compared to physicochemical processes, the biocatalytic activity of microbes is efficient at very low micro-molar concentrations, low temperatures, and atmospheric pressure, which leads to lower costs. In addition, microbe-based processes do not require chemical chelating agents or produce sulfide-containing waste streams.

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                        Lab-scale sulfide-oxidizing bioreactor. Image credit: Anne Hennig.

In one recent paper published in Environmental Science & Technology, Roman and his coauthors experimentally demonstrated a full-scale biodesulfurization system that relies solely on sulfur-oxidizing bacteria to cleanse a sour gas stream of thiols, which are toxic organosulfur compounds.

The researchers found that different types of thiols (such as methanethiol, ethanethiol, and propanethiol) cause shifts in the composition of the microbial community, as the different thiols provide competitive advantages to various populations. Most importantly for industrial applications, the acclimatized populations are able to oxidize enough sulfide to elemental sulfur to allow the bioreactor to achieve stable operation, even under the continuous addition of thiols.

In the same study, the researchers also identified several negative effects of diorgano polysulfanes on the bioreactor controlling system. The results indicated the need for a new controlling strategy based on a new sensor. Roman and his colleagues have recently developed the first prototype of a sensor for robust sulfide measurement under high pH conditions. Currently, Roman is starting a company that will develop an industrial version of the sensor and bring it to market.

In a related work published in Water Research, Roman and his collaborators investigated the bacteria’s sulfur-oxidizing mechanism on a cellular level, for the first time identifying the reaction kinetics and ways in which thiols inhibit the oxidation process. This deeper understanding of the microbial-thiol interactions will lead to designing and optimizing future full-scale biodesulfurization reactors.

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Lab-scale sulfide-oxidizing bioreactor: the “milky liquid” is a solution of biologically produced colloidal sulfur particles from H2S. Image credit: Pawel Roman.

Although sour gas is Roman’s primary focus at the moment, in the past year he has also contributed to studies on microbial physiology and ecology in hypersaline environments. In two such studies, both published in Nature’s ISME Journal (the International Society for Microbial Ecology), the researchers reported on the surprising discoveries of two new groups of obligately anaerobic sulfur-reducing extremely halophilic archaea, whose existence challenges the traditional views on these prokaryotes.

The first of the two groups obtains its energy in a way that has never been observed before in the entire Archaea domain. It uses acetate as an electron donor with sulfur as an electron acceptor, forming sulfide and CO2 as the only products.

The second group of anaerobic haloarchaea uses formate or hydrogen as electron donors and elemental sulfur, thiosulfate, or dimethylsulfoxide as electron acceptors. The researchers explain that this advanced metabolic plasticity and type of respiration has never before been seen in this type of microbe, and it enables the microbes to flourish in a wide variety of saline environments. Roman discovered that, apart from the main products, both groups also release a minor fraction of thiols in the gas phase and produce polysulfides as an intermediate product.

Looking toward the future, Roman is excited about new directions in water purification and cutting-edge molecular tools.

“In the field of waste water purification, the strategy is shifting from solely purification to added values of recovered pollutants which are often valuable, such as heavy metals, biogens, granular sludge, etc.,” Roman said. “Also, Nereda technology, the EPS-producing consortia selected in the aerobic granual sludge system, is a very promising technology to replace the conventional waste water purification technology. In addition, the modern development of molecular tools makes it possible to analyze the microbial community composition of very complex consortia involved in the full-scale waste water purification systems, which provides a better opportunity to control the system.”

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Synthesis of diorgano polysulfanes later used for quantification of polysulfide anions. Image credit: Pawel Roman.
 
References:

Pawel Roman et al. “Selection and Application of Sulfide Oxidizing Microorganisms Able to Withstand Thiols in Gas Biodesulfurization Systems.” Environ. Sci. Technol., 2016, 50 (23), pp 12808–12815.

Pawel Roman et al. “Inhibition of a biological sulfide oxidation under haloalkaline conditions by thiols and diorgano polysulfanes.” Water Research.

Dimitry Y Sorokin et al. “Elemental sulfur and acetate can support life of a novel strictly anaerobic haloarchaeon.” The ISME Journal (2016) 10, 240–252.

Dimitry Y Sorokin et al. “Discovery of anaerobic lithoheterotrophic haloarchaea, ubiquitous in hypersaline habitats.” The ISME Journal. Advance online publication.

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