Increasing the Selectivity for Sulfur Formation in Biological Gas Desulfurization

Revealing cellular (poly)sulphide storage in electrochemically active sulphide oxidising bacteria using rotating disc electrodes

Sulphide oxidising bacteria (SOB) have the potential to be used for bioelectrochemical removal, i.e. oxidation, of sulphide from waste streams. In anaerobic conditions, SOB are able to spatially separate sulphide removal and terminal electron transfer to an electrode and act as a sulphide shuttle. However, it is not fully understood how SOB anaerobically remove sulphide and store charge equivalents, and where in this process sulphur is formed. Therefore, the redox behaviour of sulphide shuttling SOB was investigated at haloalkaline conditions using a glassy carbon rotating disc electrode (RDE) and cyclic voltammetry. Voltammograms of SOB in the absence and presence of sulphide were compared to voltammograms of abiotic sulphur species solutions. Polysulphide and sulphide showed different redox behaviour, with distinct potentials for oxidation of > -0.3 V (vs. Ag/AgCl) for polysulphide and > -0.1 V for sulphide. Comparing biotic to abiotic experiments lead to the hypothesis that SOB formed polysulphides during anaerobic sulphide removal, which stayed sorbed to the cells. With this study, further steps were taken in elucidating the mechanisms of sulphide shuttling by SOB.

Influence of oxidation-reduction potential and pH on polysulfide concentrations and chain lengths in the biological desulfurization process under haloalkaline conditions

Biological desulfurization under haloalkaline conditions has been applied worldwide to remove hydrogen sulfide (H2S) from sour gas steams. The process relies on sulfide-oxidizing bacteria (SOB) to oxidize H2S to elemental sulfur (S8), which can then be recovered and reused. Recently, a dual-reactor biological desulfurization system was implemented where an anaerobic (sulfidic) bioreactor was incorporated as an addition to a micro-oxic bioreactor, allowing for higher S8 selectivity by limiting by-product formation. The highly sulfidic bioreactor environment enabled the SOB to remove (poly)sulfides (Sx2-) in the absence of oxygen, with Sx2- speculated as a main substrate in the removal pathway, thus making it vital to understand its role in the process. The SOB are influenced by the oxidation-reduction potential (ORP) set-point of the micro-oxic bioreactor as it is used to control the product of oxidation (S8 vs. SO42-), while the uptake of Sx2- by SOB has been qualitatively linked to pH. Therefore, to quantify these effects, this work determined the concentration and speciation of Sx2- in the biological desulfurization process under various pH values and ORP set-points. The total Sx2- concentrations in the sulfidic zone increased at elevated pH (8.9) compared to low pH (< 8.0), with on average 3.3 ± 1.0 mM-S more Sx2-. Chain lengths varied, with S72- only doubling in concentration while S52- increased 9 fold, which is in contrast with observations from abiotic systems. Changes to the ORP set-point of the micro-oxic reactor did not produce substantial changes in Sx2- concentration in the sulfidic zone. This illustrates that the reduction degree of the SOB in the micro-oxic bioreactor does not enhance their ability to interact with Sx2- in the sulfidic bioreactor. This increased understanding of how both pH and ORP affect changes in Sx2- concentration and chain length can lead to improved efficiency and design of the dual-reactor biological desulfurization process.

Recent Advances on the Functionalities of Polyoxometalate-Based Ionic Liquids

Polyoxometalate (POM)-based ionic liquids (POM-ILs) are gaining increasing attention due to their diverse structures and functionalities. POMs in POM-ILs not only act as essential structural building blocks but also play a crucial role in their functional performance. With the incorporation of POMs, POM-ILs find applications in various fields such as chemical catalysis, energy science, materials science, sensors, and more. The abundant availability of POMs and other building blocks in POM-ILs, along with their versatile combination possibilities, present promising opportunities for the future. Rather than focusing solely on discovering new structures of POM-ILs, current developments in this field emphasize exploring their functions, leading to the emergence of numerous new applications. Summarizing these advancements aids in understanding the latest trends and facilitates rapid evolution. This review examines the recent five years' worth of results to analyze the new functions of POM-ILs, categorizing them based on their unique characteristics.

Harnessing the potential of the microbial sulfur cycle for environmental biotechnology

The sulfur cycle is a complex biogeochemical cycle characterized by the high variability in the oxidation states of sulfur. While sulfur is essential for life processes, certain sulfur compounds, such as hydrogen sulfide, are toxic to all life forms. Micro-organisms facilitate the sulfur cycle, playing a prominent role even in extreme environments, such as soda lakes, acid mine drainage sites, hot springs, and other harsh habitats. The activity of these micro-organisms presents unique opportunities for mitigating sulfur-based pollution and enhancing the recovery of sulfur and metals. This review highlights the application of sulfur-oxidizing and -reducing micro-organisms in environmental biotechnology through three illustrative examples. Additionally, it discusses the challenges, recent trends, and prospects associated with these applications.

Assessing the efficiency and microbial diversity of H2S-removing biotrickling filters at various pH conditions

This study aimed to investigate the operation of three parallel biotrickling filters (BTFs) in removing H2S at different pH conditions (haloalkaliphilic, neutrophilic, and acidophilic) and their associated microbial population in the biodesulfurization process. BTF columns were inoculated with enriched inoculum and experiments were performed by gradually reducing Empty Bed Retention Time (EBRT) and increasing inlet concentration in which the maximum removal efficiency and maximum elimination capacity in EBRT 60 s reached their maximum level in haloalkaline condition (91% and 179.5 g S-H2S m-3 h-1). For visualizing the attached microbial biofilms on pall rings, Scanning Electron Microscopy (SEM) was used and microbial community structure analysis by NGS showed that the most abundant phyla in haBTF, nBTF, and aBTF belong to Gammaproteobacteria, Betaproteobacteria, and Acidithiobacillia, respectively. Shannon and Simpson indexes evaluation showed a lower diversity of bacteria in the aBTF reactor than that of nBTF and haBTF and beta analysis indicated a different composition of bacteria in haBTF compared to the other two filters. These results indicated that the proper performance of BTF under haloalkaliphilic conditions is the most effective way for H2S removal from air pollutants of different industries.

Biodesulfurization of landfill biogas by a pilot-scale bioscrubber: Operational limits and microbial analysis

Biogas serves as a crucial renewable energy vector to ensure a more sustainable energy future. However, the presence of hydrogen sulfide (H2S) limits its application in various sectors, emphasizing the importance of effective H2S removal techniques for maximizing its potential. In the present study, the limits of a pilot-scale bioscrubber for biogas desulfurization was study in a real scenario. An increase in the superficial liquid velocity resulted in significant improvements in the H2S removal efficiency, increasing from 76 ± 8% (elimination capacity of 6.2 ± 0.5 gS-H2S m-3 h-1) to 97.7 ± 0.5% (elimination capacity of 8 ± 1 gS-H2S m-3 h-1) as the superficial liquid velocity increased from 50 ± 3 m h-1 to 200 ± 8 m h-1. A USL of 161.4 ± 0.5 m h-1 was able to achieve outlet H2S concentrations as low as 3 ± 1 ppmv (H2S removal efficiency of 97 ± 1%) for 7 days. High superficial liquid velocity favoured the aerobic H2S oxidation reducing the nitrate demand. The maximum EC reached throughout the operation was 50.8 ± 0.6 gS-H2S m-3 h-1 (H2S removal efficiency of 96 ± 1%) and a sulfur production of 60%. Studies in batch flocculation experiments showed sulfur removal rates up to 97.6 ± 0.9% with a cationic flocculant dose of 75 mg L-1. Microbial analysis revealed that the predominant genus with sulfo-oxidant capacity during periods of low H2S inlet load was Thioalkalispira-sulfurivermis (61-69%), while in periods of higher H2S inlet load, family Arcobacteraceae was the most prevalent (11%).

Process conditions affect microbial diversity and activity in a haloalkaline biodesulfurization system

Biodesulfurization (BD) systems that treat sour gas employ mixtures of haloalkaliphilic sulfur-oxidizing bacteria to convert sulfide to elemental sulfur. In the past years, these systems have seen major technical innovations that have led to changes in microbial community composition. Different studies have identified and discussed the microbial communities in both traditional and improved systems. However, these studies do not identify metabolically active community members and merely focus on members' presence/absence. Therefore, their results cannot confirm the activity and role of certain bacteria in the BD system. To investigate the active community members, we determined the microbial communities of six different runs of a pilot-scale BD system. 16S rRNA gene-based amplicon sequencing was performed using both DNA and RNA. A comparison of the DNA- and RNA-based sequencing results identified the active microbes in the BD system. Statistical analyses indicated that not all the existing microbes were actively involved in the system and that microbial communities continuously evolved during the operation. At the end of the run, strains affiliated with Alkalilimnicola ehrlichii and Thioalkalivibrio sulfidiphilus were confirmed as the most active key bacteria in the BD system. This study determined that microbial communities were shaped predominantly by the combination of hydraulic retention time (HRT) and sulfide concentration in the anoxic reactor and, to a lesser extent, by other operational parameters.IMPORTANCEHaloalkaliphilic sulfur-oxidizing bacteria are integral to biodesulfurization (BD) systems and are responsible for converting sulfide to sulfur. To understand the cause of conversions occurring in the BD systems, knowing which bacteria are present and active in the systems is essential. So far, only a few studies have investigated the BD system's microbial composition, but none have identified the active microbial community. Here, we reveal the metabolically active community, their succession, and their influence on product formation.

Effects of operating conditions on the in situ control of sulfur-containing odors by using a novel alternative landfill cover and its transformation mechanism

Sulfur-containing gases are main sources of landfill odors, which has become a big issue for pollution to environment and human health. Biocover is promising for treating landfill odors, with advantages of durability and environmental friendliness. In this study, charcoal sludge compost was utilized as the main effective component of a novel alternative landfill cover and the in situ control of sulfur-containing odors from municipal solid waste landfilling process was simulated under nine different operating conditions. Results showed that five sulfur-containing odors (hydrogen sulfide, H2S; methyl mercaptan, CH3SH; dimethyl sulfide, CH3SCH3; ethylmercaptan, CH3CH2SH; carbon disulfide, CS2) were monitored and removed by the biocover, with the highest removal efficiencies of 77.18% for H2S, 87.36% for CH3SH, and 92.19% for CH3SCH3 in reactor 8#, and 95.94% for CH3CH2SH and 94.44% for CS2 in reactor 3#. The orthogonal experiment showed that the factors influencing the removal efficiencies of sulfur-containing odors were ranked from high to low as follows: temperature > weight ratio > humidity content. The combination of parameters of 20% weight ratio, 25°C temperature, and 30% water content was more recommended based on the consideration of the removal efficiencies and economic benefits. The mechanisms of sulfur conversion inside biocover were analyzed. Most organic sulfur was firstly degraded to reduced sulfides or element sulfur, and then oxidized to sulfate which could be stable in the layer as the final state. In this process, sulfur-oxidizing bacteria play a great role, and the distribution of them in reactor 1#, 5#, and 8# was specifically monitored. Bradyrhizobiaceae and Rhodospirillaceae were the dominant species which can utilize sulfide as substance to produce sulfate and element sulfur, respectively. Based on the results of OUTs, the biodiversity of these sulfur-oxidizing bacteria, these microorganisms, was demonstrated to be affected by the different parameters. These results indicate that the novel alternative landfill cover modified with bamboo charcoal compost is effective in removing sulfur odors from landfills. Meanwhile, the findings have direct implications for addressing landfill odor problems through parameter adjustment.

Enhancement of bio-S0 recovery and revealing the inhibitory effect on microorganisms under high sulfide loading

Biodesulfurization is a mature technology, but obtaining biosulfur (S0) that can be easily settled naturally is still a challenge. Increasing the sulfide load is one of the known methods to obtain better settling of S0. However, the inhibitory effect of high levels of sulfide on microbes has also not been well studied. We constructed a high loading sulfide (1.55-10.86 kg S/m3/d) biological removal system. 100% sulfide removal and 0.56-2.53 kg S/m3/d S0 (7.0 ± 0.09-16.4 ± 0.25 μm) recovery were achieved at loads of 1.55-7.75 kg S/m3/d. Under the same load, S0 in the reflux sedimentation tank, which produced larger S0 particles (24.2 ± 0.73-53.8 ± 0.70 μm), increased the natural settling capacity and 45% recovery. For high level sulfide inhibitory effect, we used metagenomics and metatranscriptomics analyses. The increased sulfide load significantly inhibited the expression of flavin cytochrome c sulfide dehydrogenase subunit B (fccB) (Decreased from 615 ± 75 to 30 ± 5 TPM). At this time sulfide quinone reductase (SQR) (324 ± 185-1197 ± 51 TPM) was mainly responsible for sulfide oxidation and S0 production. When the sulfide load reached 2800 mg S/L, the SQR (730 ± 100 TPM) was also suppressed. This resulted in the accumulation of sulfide, causing suppression of carbon sequestration genes (Decreased from 3437 ± 842 to 665 ± 175 TPM). Other inhibitory effects included inhibition of microbial respiration, production of reactive oxygen species, and DNA damage. More sulfide-oxidizing bacteria (SOB) and newly identified potential SOB (99.1%) showed some activity (77.6%) upon sulfide accumulation. The main microorganisms in the sulfide accumulation environment were Thiomicrospiracea and Burkholderiaceae, whose sulfide oxidation capacity and respiration were not significantly inhibited. This study provides a new approach to enhance the natural sedimentation of S0 and describes new microbial mechanisms for the inhibitory effects of sulfide.

Polysulfide Concentration and Chain Length in the Biological Desulfurization Process: Effect of Biomass Concentration and the Sulfide Loading Rate

Removal of hydrogen sulfide (H2S) can be achieved using the sustainable biological desulfurization process, where H2S is converted to elemental sulfur using sulfide-oxidizing bacteria (SOB). A dual-bioreactor process was recently developed where an anaerobic (sulfidic) bioreactor was used between the absorber column and micro-oxic bioreactor. In the absorber column and sulfidic bioreactor, polysulfides (Sx2-) are formed due to the chemical equilibrium between H2S and sulfur (S8). Sx2- is thought to be the intermediate for SOB to produce sulfur via H2S oxidation. In this study, we quantify Sx2-, determine their chain-length distribution under high H2S loading rates, and elucidate the relationship between biomass and the observed biological removal of sulfides under anaerobic conditions. A linear relationship was observed between Sx2- concentration and H2S loading rates at a constant biomass concentration. Increasing biomass concentrations resulted in a lower measured Sx2- concentration at similar H2S loading rates in the sulfidic bioreactor. Sx2- of chain length 6 (S62-) showed a substantial decrease at higher biomass concentrations. Identifying Sx2- concentrations and their chain lengths as a function of biomass concentration and the sulfide loading rate is key in understanding and controlling sulfide uptake by the SOB. This knowledge will contribute to a better understanding of how to reach and maintain a high selectivity for S8 formation in the dual-reactor biological desulfurization process.

Synergistic between autotrophic and heterotrophic microorganisms for denitrification using bio-S as electron donor

In recent years, biological sulfur (bio-S) was employed in sulfur autotrophic denitrification (SAD) in which autotrophic Thiobacillus denitrificans and heterotrophic Stenotrophomonas maltophilia played a key role. The growth pattern of T.denitrificans and S.maltophilia exhibited a linear relationship between OD₆₀₀ and CFU when OD₆₀₀ < 0.06 and <0.1, respectively. When S.maltophilia has applied alone, the NorBC and NosZ were undetected, and denitrification was incomplete. The DsrA of S.maltophilia could produce sulfide as an alternative electron donor for T.denitrificans. Even though T.denitrificans had complete denitrification genes, its efficiency was low when used alone. The interaction of T.denitrificans and S.maltophilia reduced nitrite accumulation, leading to complete denitrification. A sufficient quantity of S.maltophilia may trigger the autotrophic denitrification activity of T.denitrificans. When the colony-forming units (CFU) ratio of S.maltophilia to T.denitrificans was reached at 2:1, the highest denitrification performance was achieved at 2.56 and 12.59 times higher than applied alone. This research provides a good understanding of the optimal microbial matching for the future application of bio-S.

Anaerobic sulphide removal by haloalkaline sulphide oxidising bacteria

Sulphide is a toxic and corrosive compound and requires removal from waste streams. Recent discoveries show that sulphide oxidising bacteria (SOB) from modern desulphurisation plants are able to spatially separate sulphide removal and oxygen reduction when exposed to intermittent anaerobic and aerobic environments. Here, SOB act as electron shuttles between electron donor and acceptor. The underlying mechanisms for electron shuttling are of yet unknown. To investigate the anaerobic sulphide removal of SOB, batch experiments and mathematical models were applied. The sulphide removal capacity decreased at increasing biomass concentrations. At 0.6 mgN/L SOB could remove up to 8 mgS/mgN in 30 min. It was found that biological activity determines sulphide removal, alongside chemical processes. Anaerobic oxidation of electron carriers was determined to only explain 0.1% of charge storage, where irreversible cleavage of long chain polysulphides could explain full sulphide storage. Different sulphide removal and intracellular storage processes are postulated.

Insights into the Superior Bioavailability of Biogenic Sulfur from the View of Its Unique Properties: The Key Role of Trace Organic Substances

Elemental sulfur (S0) is widely utilized in environmental pollution control, while its low bioavailability has become a bottleneck for S0-based biotechnologies. Biogenic sulfur (bio-S0) has been demonstrated to have superior bioavailability, while little is known about its mechanisms thus far. This study investigated the bioavailability and relevant properties of bio-S0 based on the denitrifying activity of Thiobacillus denitrificans with chemical sulfur (chem-S0) as the control. It was found that the conversion rate and removal efficiency of nitrate in the bio-S0 system were 2.23 and 2.04 times those of the chem-S0 system. Bio-S0 was not pure orthorhombic sulfur [S: 96.88 ± 0.25% (w/w)]. Trace organic substances detected on the bio-S0 surface were revealed to contribute to its hydrophilicity, resulting in better dispersibility in the aqueous liquid. In addition, the adhesion force of T. denitrificans on bio-S0 was 1.54 times that of chem-S0, endowing a higher bacterial adhesion efficiency on the sulfur particle. The weaker intermolecular binding force due to the low crystallinity of bio-S0 led to enhanced cellular uptake by attached bacteria. The mechanisms for the superior bioavailability of bio-S0 were further proposed. This study provides a comprehensive view of the superior bioavailability of bio-S0 and is beneficial to developing high-quality sulfur resources.

Adding organics to enrich mixotrophic sulfur-oxidizing bacteria under extremely acidic conditions-A novel strategy to enhance hydrogen sulfide removal

Biotreatment of high load hydrogen sulfide (H2S) can lead to rapid acidification of a bioreactor, which greatly challenges the application of bio-desulfurization technology. In this study, the bio-desulfurization performance was improved by enriching acidophilic mixotrophic sulfur-oxidizing bacteria (SOB) by adding organics under extremely acidic conditions (pH < 1.0). A biotrickling filter (BTF) for the removal of H2S was established and operated under pH < 1.0 for 420 days. In the autotrophic period, the maximum H2S elimination capacity (ECmax-H2S) was 135.8 g/m3/h with biofilm mass remaining within 11.1 g/L-BTF. The autotrophic SOB bacterium Acidithiobacillus was dominant (62.1 %). When glucose was added to the BTF system, ECmax-H2S increased by 272 % to 464.3 g/m3/h as biofilm mass increased to 22.3 g/L-BTF. The acidophilic mixotrophic SOB bacteria Mycobacterium (78.4 %) and Alicyclobacillus (20.7 %) were enriched while Acidithiobacillus was gradually eliminated (<0.1 %). Furthermore, the major sulfur metabolism pathways were identified to explore the desulfurization mechanism under extremely acidic conditions. To maintain optimal desulfurization performance and avoid biofilm overgrowth in the BTF system, biofilm mass should be maintained within 20-22 g/L-BTF. This can be achieved by adding 1.0 g/L-BTF glucose every 20 days under a load rate of H2S in 50-90 g/m3/h and a trickling liquid velocity of 1.8 m/h. Extremely acidic conditions eliminated non-aciduric microorganisms so that the addition of organics can increase the abundance of acidophilic mixotrophic SOB (>99 %), thus offering a novel strategy for enhancing H2S removal.

Removal of small elemental sulfur particles by polysulfide formation in a sulfidic reactor

For over 30 years, biological gas desulfurization under halo-alkaline conditions has been studied and optimized. This technology is currently applied in already 270 commercial installations worldwide. Sulfur particle separation, however, remains a challenge; a fraction of sulfur particles is often too small for liquid-solid separation with conventional separation technology. In this article, we report the effects of a novel sulfidic reactor, inserted in the conventional process set-up, on sulfur particle size and morphology. In the sulfidic reactor polysulfide is produced by the reaction of elemental sulfur particles and sulfide, which is again converted to elemental sulfur in a gas-lift reactor. We analyzed sulfur particles produced in continuous, long term lab-scale reactor experiments under various sulfide concentrations and sulfidic retention times. The analyses were performed with laser diffraction particle size analysis and light microscopy. These show that the smallest particles (< 1 µm) have mostly disappeared under the highest sulfide concentration (4.1 mM) and sulfidic retention time (45 min). Under these conditions also agglomeration of sulfur particles was promoted. Model calculations with thermodynamic and previously derived kinetic data on polysulfide formation confirm the experimental data on the removal of the smallest particles. Under the 'highest sulfidic pressure', the model predicts that equilibrium conditions are reached between sulfur, sulfide and polysulfide and that 100% of the sulfur particles <1 µm are dissolved by the (autocatalytic) formation of polysulfides. These experiments and modeling results demonstrate that the insertion of a novel sulfidic reactor in the conventional process set-up promotes the removal of the smallest individual sulfur particles and promotes the production of sulfur agglomerates. The novel sulfidic reactor is therefore a promising process addition with the potential to improve process operation, sulfur separation and sulfur recovery.

High rate production of concentrated sulfides from metal bearing wastewater in an expanded bed hydrogenotrophic sulfate reducing bioreactor

Metallurgical wastewaters contain high concentrations of sulfate, up to 15 g L^-1. Sulfate-reducing bioreactors are employed to treat these wastewaters, reducing sulfates to sulfides which subsequently co-precipitate metals. Sulfate loading and reduction rates are typically restricted by the total H₂S concentration. Sulfide stripping, sulfide precipitation and dilution are the main strategies employed to minimize inhibition by H₂S, but can be adversely compromised by suboptimal sulfate reduction, clogging and additional energy costs. Here, metallurgical wastewater was treated for over 250 days using two hydrogenotrophic granular activated carbon expanded bed bioreactors without additional removal of sulfides. H₂S toxicity was minimized by operating at pH 8 ± 0.15, resulting in an average sulfate removal of 7.08 ± 0.08 g L^-1, sulfide concentrations of 2.1 ± 0.2 g L^-1 and peaks up to 2.3 ± 0.2 g L^-1. A sulfate reduction rate of 20.6 ± 0.9 g L^-1 d^-1 was achieved, with maxima up to 27.2 g L^-1 d^-1, which is among the highest reported considering a literature review of 39 studies. The rates reported here are 6-8 times higher than those reported for other reactors without active sulfide removal and the only reported for expanded bed sulfate-reducing bioreactors using H₂. By increasing the influent sulfate concentration and maintaining high sulfide concentrations, sulfate reducers were promoted while fermenters and methanogens were suppressed. Industrial wastewater containing 4.4 g L^-1 sulfate, 0.036 g L^-1 nitrate and various metals (As, Fe, Tl, Zn, Ni, Sb, Co and Cd) was successfully treated with all metal(loid)s, nitrates and sulfates removed below discharge limits.

Enhanced Biodesulfurization with a Microbubble Strategy in an Airlift Bioreactor with Haloalkaliphilic Bacterium Thioalkalivibrio versutus D306

Biodesulfurization under haloalkaline conditions requires limiting oxygen and additional energy in the system to deliver high mixing quality control. This study considers biodesulfurization in an airlift bioreactor with uniform microbubbles generated by a fluidic oscillation aeration system to enhance the biological desulfurization process and its hydrodynamics. Fluidic oscillation aeration in an airlift bioreactor requires minimal energy input for microbubble generation. This aeration system produced 81.87% smaller average microbubble size than the direct aeration system in a bubble column bioreactor. The biodesulfurization phase achieved a yield of 94.94% biological sulfur, 84.91% biological sulfur selectivity, and 5.06% sulfur oxidation performance in the airlift bioreactor with the microbubble strategy. The biodesulfurization conditions of thiosulfate via Thioalkalivibrio versutus D306 are revealed in this study. The biodesulfurization conditions in the airlift bioreactor with the fluidic oscillation aeration system resulted in the complete conversion of thiosulfate with 27.64% less sulfate production and 10.34% more biological sulfur production than in the bubble column bioreactor. Therefore, pleasant hydrodynamics via an airlift bioreactor mechanism with microbubbles is favored for biodesulfurization under haloalkaline conditions.

Extremely acidic condition (pH<1.0) as a novel strategy to achieve high-efficient hydrogen sulfide removal in biotrickling filter: Biomass accumulation, sulfur oxidation pathway and microbial analysis

Extremely acidic conditions (pH < 1.0) during hydrogen sulfide (H₂S) biotreatment significantly reduce the cost of pH regulation; however, there remain challenges to its applications. The present study investigated the H₂S removal and biomass variations in biotrickling filter (BTF) under long-term highly acidic conditions. A BTF operated for 144 days at pH 0.5-1.0 achieved an H₂S elimination capacity (EC) of 109.9 g/(m³·h) (removal efficiency = 97.0%) at an empty bed retention time of 20 s, with an average biomass concentration at 20.6 g/L-BTF. The biomass concentration at neutral pH increased from 22.3 to 49.5 g/L-BTF within 28 days. In this case, elemental sulfur (S⁰) accumulated due to insufficient oxygen transfer in biofilm, which aggravated the BTF blockage problem. After long-term domestication under extremely acidic conditions, a mixotrophic acidophilic sulfur-oxidizing bacteria (SOB) Alicyclobacillus (abundance 55.4%) were enriched in the extremely acidic biofilm, while non-aciduric bacteria were eliminated, which maintained the balance of biofilm thickness. Biofilm with optimum thickness ensured oxygen transfer and H₂S oxidation, avoiding the accumulation of S⁰. The BTF performance improved due to the enrichment of active mixotrophic SOB with high abundance under extremely acidic conditions. The mixotrophic SOB is expected to be further enriched under extremely acidic conditions by adding carbohydrates to enhance H₂S removal.

Effect of sulfide on morphology and particle size of biologically produced elemental sulfur from industrial desulfurization reactors

We investigated the effect of polysulfide formation on properties of biologically produced elemental sulfur (S₈) crystals, which are produced during biological desulfurization (BD) of gas. The recent addition of an anoxic-sulfidic reactor (AnSuR) to the BD process resulted in agglomerated particles with better settleability for S₈ separation. In the AnSuR, polysulfides are formed by the reaction of bisulfide (HS^-) with S₈ and are subsequently oxidized to S₈ in a gas-lift reactor. Therefore, sulfur particles from the BD are shaped (i.e. morphology and particle size) both by formation and dissolution. We assessed the reaction of HS^- with S₈ particles in anoxic, abiotic experiments in a batch reactor using two S₈ samples from industrial BD reactors. Under these conditions, the sulfur particle surface became coarser and more porous, and in addition the smallest particles disappeared. Agglomerates initially fell apart but were reformed at a later stage. Moreover, we found different observed polysulfide formation rates for each S₈ sample, which was related to the initial morphology and size. Our findings show that particle properties can be controlled abiotically and that settleability of S₈ is increased by increasing both the HS^--S₈ ratio and retention time.

Continuous electron shuttling by sulfide oxidizing bacteria as a novel strategy to produce electric current

Sulfide oxidizing bacteria (SOB) are widely applied in industry to convert toxic H₂S into elemental sulfur. Haloalkaliphilic planktonic SOB can remove sulfide from solution under anaerobic conditions (SOB are 'charged'), and release electrons at an electrode (discharge of SOB). The effect of this electron shuttling on product formation and biomass growth is not known. Here, we study and demonstrate a continuous process in which SOB remove sulfide from solution in an anaerobic 'uptake chamber', and shuttle these electrons to the anode of an electrochemical cell, in the absence of dissolved sulfide. Two experiments over 31 and 41 days were performed. At a sulfide loading rate of 1.1 mmolS/day, electricity was produced continuously (3 A/m²) without dissolved sulfide in the anolyte. The main end product was sulfate (56% in experiment 1% and 78% in experiment 2), and 87% and 77% of the electrons in sulfide were recovered as electricity. It was found that the current density was dependent on the sulfide loading rate and not on the anode potential. Biological growth occurred, mainly at the anode as biofilm, in which the deltaproteobacterial genus Desulfurivibrio was dominating. Our results demonstrate a novel strategy to produce electricity from sulfide in an electrochemical system.

Comparative analysis of microbial communities from different full-scale haloalkaline biodesulfurization systems

Abstract

In biodesulfurization (BD) at haloalkaline and dO₂-limited conditions, sulfide-oxidizing bacteria (SOB) effectively convert sulfide into elemental sulfur that can be used in agriculture as a fertilizer and fungicide. Here we show which bacteria are present in this biotechnological process. 16S rRNA gene amplicon sequencing of biomass from ten reactors sampled in 2018 indicated the presence of 444 bacterial Amplicon Sequence Variants (ASVs). A core microbiome represented by 30 ASVs was found in all ten reactors, with Thioalkalivibrio sulfidiphilus as the most dominant species. The majority of these ASVs are phylogenetically related to bacteria previously identified in haloalkaline BD processes and in natural haloalkaline ecosystems. The source and composition of the feed gas had a great impact on the microbial community composition followed by alkalinity, sulfate, and thiosulfate concentrations. The halophilic SOB of the genus Guyparkeria (formerly known as Halothiobacillus) and heterotrophic SOB of the genus Halomonas were identified as potential indicator organisms of sulfate and thiosulfate accumulation in the BD process.

Key points

• Biodesulfurization (BD) reactors share a core microbiome

• The source and composition of the feed gas affects the microbial composition in the BD reactors

• Guyparkeria and Halomonas indicate high concentrations of sulfate and thiosulfate in the BD process

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-022-11771-y.

Advances in Oxidative Desulfurization of Fuel Oils over MOFs-Based Heterogeneous Catalysts

Catalytic oxidative desulfurization (ODS) of fuel oils is considered one of the most promising non-hydrodesulfurization technologies due to the advantages of mild reaction conditions, low cost and easy removal of aromatic sulfur compounds. Based on this reason, the preparation of highly efficient ODS catalysts has been a hot research topic in this field. Recently, metal-organic frameworks (MOFs) have attracted extensive attention due to the advantages involving abundant metal centers, high surface area, rich porosity and varied pore structures. For this, the synthesis and catalytic performance of the ODS catalysts based on MOFs materials have been widely studied. Until now, many research achievements have been obtained along this direction. In this article, we will review the advances in oxidative desulfurization of fuel oils over MOFs-based heterogeneous catalysts. The catalytic ODS performance over various types of catalysts is compared and discussed. The perspectives for future work are proposed in this field.

Electrified bioreactors: the next power-up for biometallurgical wastewater treatment

Over the past decades, biological treatment of metallurgical wastewaters has become commonplace. Passive systems require intensive land use due to their slow treatment rates, do not recover embedded resources and are poorly controllable. Active systems however require the addition of chemicals, increasing operational costs and possibly negatively affecting safety and the environment. Electrification of biological systems can reduce the use of chemicals, operational costs, surface footprint and environmental impact when compared to passive and active technologies whilst increasing the recovery of resources and the extraction of products. Electrification of low rate applications has resulted in the development of bioelectrochemical systems (BES), but electrification of high rate systems has been lagging behind due to the limited mass transfer, electron transfer and biomass density in BES. We postulate that for high rate applications, the electrification of bioreactors, for example, through the use of electrolyzers, may herald a new generation of electrified biological systems (EBS). In this review, we evaluate the latest trends in the field of biometallurgical and microbial‐electrochemical wastewater treatment and discuss the advantages and challenges of these existing treatment technologies. We advocate for future research to focus on the development of electrified bioreactors, exploring the boundaries and limitations of these systems, and their validity upon treating industrial wastewaters.

Novel Agglomeration Strategy for Elemental Sulfur Produced during Biological Gas Desulfurization

This article presents a novel crystal agglomeration strategy for elemental sulfur (S) produced during biological desulfurization (BD). A key element is the nucleophilic dissolution of S by sulfide (HS-) to polysulfides (S x 2-), which was enhanced by a sulfide-rich, anoxic reactor. This study demonstrates that with enhanced S x 2- formation, crystal agglomerates are formed with a uniform size (14.7 ± 3.1 μm). In contrast, with minimal S x 2- formation, particle size fluctuates markedly (5.6 ± 5.9 μm) due to the presence of agglomerates and single crystals. Microscopic analysis showed that the uniformly sized agglomerates had an irregular structure, whereas the loose particles and agglomerates were more defined and bipyramidal. The irregular agglomerates are explained by dissolution of S by (poly)sulfides, which likely changed the crystal surface structure and disrupted crystal growth. Furthermore, S from S x 2- appeared to form at least 5× faster than from HS- based on the average S x 2- chain length of x ≈ 5, thereby stimulating particle agglomeration. In addition, microscopy suggested that S crystal growth proceeded via amorphous S globules. Our findings imply that the crystallization product is controlled by the balance between dissolution and formation of S. This new insight has a strong potential to prevent poor S settleability in BD.

Recent advances in microbial capture of hydrogen sulfide from sour gas via sulfur-oxidizing bacteria

Biological desulfurization offers several remarkably environmental advantages of operation at ambient temperature and atmospheric pressure, no demand of toxic chemicals as well as the formation of biologically re-usable sulfur (S0), which has attracted increasing attention compared to conventionally physicochemical approaches in removing hydrogen sulfide from sour gas. However, the low biomass of SOB, the acidification of process solution, the recovery of SOB, and the selectivity of bio-S0 limit its industrial application. Therefore, more efforts should be made in the improvement of the BDS process for its industrial application via different research perspectives. This review summarized the recent research advances in the microbial capture of hydrogen sulfide from sour gas based on strain modification, absorption enhancement, and bioreactor modification. Several efficient solutions to limitations for the BDS process were proposed, which paved the way for the future development of BDS industrialization.

Microbial interaction promotes desulfurization efficiency under high pH condition

The existence of H₂S in biogas may cause equipment corrosion and considerable SO₂ emission. Commonly used biotrickling filters may cause biogas dilution or generation of explosive mixtures. Compared with biotrickling filters, two-step process such as bioscrubber filters can overcome these shortages. However, its removal efficiency was still limited due to low microbial activity under high pH condition. Here, a bioreactor filter was carried out under pH 9.0. Removal efficiency higher than 99% was achieved under sulfide loading rate reaching 4.24 kg S m^-3d^-1. Results of network and high throughput sequencing showed that Thiobacillus acted as both dominant species (accounting for 75%) and unique kinless hub in this bioreactor. Other bacteria (accounting for 25%) contributed 75% to the network, which implied the intensive interaction between Thiobacillus and others. Sulfide removal ability and pH tolerance of pure bacteria and mixed culture were considered to verify how microbial interaction influenced them. Compared with pure bacteria, mixed culture had better performance under high pH condition, which confirmed that microbial interaction promoted desulfurization efficiency under high pH condition. These results showed that intensive microbial interaction might be the key to enhance sulfide removal efficiency under high pH condition.

High-rate biological selenate reduction in a sequencing batch reactor for recovery of hexagonal selenium

Recovery of selenium (Se) from wastewater provides a solution for both securing Se supply and preventing Se pollution. Here, we developed a high-rate process for biological selenate reduction to elemental selenium. Distinctive from other studies, we aimed for a process with selenate as the main biological electron sink, with minimal formation of methane or sulfide. A sequencing batch reactor, fed with an influent containing 120 mgSe L^-1 selenate and ethanol as electron donor and carbon source, was operated for 495 days. The high rates (419 ± 17 mgSe L^-1 day^-1) were recorded between day 446 and day 495 for a hydraulic retention time of 6 h. The maximum conversion efficiency of selenate amounted to 96% with a volumetric conversion rate of 444 mgSe L^-1 day^-1, which is 6 times higher than the rates reported in the literature thus far. At the end of the experiment, a highly enriched selenate reducing biomass had developed, with a specific activity of 856 ± 26 mgSe^-1day^-1g_biomass^-1, which was nearly 1000-fold higher than that of the inoculum. No evidence was found for the formation of methane, sulfide, or volatile reduced selenium compounds like dimethyl-selenide or H₂Se, revealing a high selectivity. Ethanol was incompletely oxidized to acetate. The produced elemental selenium partially accumulated in the reactor as pure (≥80% Se of the total mixture of biomass sludge flocs and flaky aggregates, and ~100% of the specific flaky aggregates) selenium black hexagonal needles, with cluster sizes between 20 and 200 µm. The new process may serve as the basis for a high-rate technology to remove and recover pure selenium from wastewater or process streams with high selectivity.

Assembly of polyoxometalate-thiacalix[4]arene-based inorganic-organic hybrids as efficient catalytic oxidation desulfurization catalysts

Self-assembly of polyoxometalates, Ni(ii)/Ag(i) cations and tetra-[5-(mercapto)-1-methyltetrazole]-thiacalix[4]arene (L) yielded three inorganic-organic hybrids, namely, [Ni₃L₂(CH₃OH)₆(H₂O)₄][PMo₁₂O₄₀]₂·3CH₃OH·2H₂O (1), [Ni₃L₂(CH₃OH)₆(H₂O)₄][PW₁₂O₄₀]₂·3CH₃OH·2H₂O (2) and [Ag₃L(PMo₁₂O₄₀)] (3). In hybrids (1) and (2), Ni(ii) cations are linked by L ligands to produce layered frameworks, and H bonds among the [PMo₁₂O₄₀]^3-/[PW₁₂O₄₀]^3- anions and L ligands lengthen the structures to form 3D supramolecular architectures. Hybrid (3) exhibits a 3D architecture, of which Ag(i) cations not only coordinated with the N and O atoms of L ligands and [PMo₁₂O₄₀]^3- anions simultaneously, but also connected each other by Ag-Ag interactions. It is worth mentioning that 1 and 3 as recyclable catalysts show excellent heterogeneous catalytic activity in oxidation desulfurization reactions.

Effect of methanethiol on process performance, selectivity and diversity of sulfur-oxidizing bacteria in a dual bioreactor gas biodesulfurization system

This study provides important new insights on how to achieve high sulfur selectivities and stable gas biodesulfurization process operation in the presence of both methanethiol and H₂S in the feed gas. On the basis of previous research, we hypothesized that a dual bioreactor lineup (with an added anaerobic bioreactor) would favor sulfur-oxidizing bacteria (SOB) that yield a higher sulfur selectivity. Therefore, the focus of the present study was to enrich thiol-resistant SOB that can withstand methanethiol, the most prevalent and toxic thiol in sulfur-containing industrial off gases. In addition, the effect of process conditions on the SOB population dynamics was investigated. The results confirmed that thiol-resistant SOB became dominant with a concomitant increase of the sulfur selectivity from 75 mol% to 90 mol% at a loading rate of 2 mM S methanethiol day^-1. The abundant SOB in the inoculum - Thioalkalivibrio sulfidiphilus - was first outcompeted by Alkalilimnicola ehrlichii after which Thioalkalibacter halophilus eventually became the most abundant species. Furthermore, we found that the actual electron donor in our lab-scale biodesulfurization system was polysulfide, and not the primarily supplied sulfide.

Syngas as Electron Donor for Sulfate and Thiosulfate Reducing Haloalkaliphilic Microorganisms in a Gas-Lift Bioreactor

Biodesulfurization processes remove toxic and corrosive hydrogen sulfide from gas streams (e.g., natural gas, biogas, or syngas). To improve the efficiency of these processes under haloalkaline conditions, a sulfate and thiosulfate reduction step can be included. The use of H₂/CO mixtures (as in syngas) instead of pure H₂ was tested to investigate the potential cost reduction of the electron donor required. Syngas is produced in the gas-reforming process and consists mainly of H₂, carbon monoxide (CO), and carbon dioxide (CO₂). Purification of syngas to obtain pure H₂ implies higher costs because of additional post-treatment. Therefore, the use of syngas has merit in the biodesulfurization process. Initially, CO inhibited hydrogen-dependent sulfate reduction. However, after 30 days the biomass was adapted and both H₂ and CO were used as electron donors. First, formate was produced, followed by sulfate and thiosulfate reduction, and later in the reactor run acetate and methane were detected. Sulfide production rates with sulfate and thiosulfate after adaptation were comparable with previously described rates with only hydrogen. The addition of CO marginally affected the microbial community in which Tindallia sp. was dominant. Over time, acetate production increased and acetogenesis became the dominant process in the bioreactor. Around 50% of H₂/CO was converted to acetate. Acetate supported biomass growth and higher biomass concentrations were reached compared to bioreactors without CO feed. Finally, CO addition resulted in the formation of small, compact microbial aggregates. This suggests that CO or syngas can be used to stimulate aggregation in haloalkaline biodesulfurization systems.

Sulfate reduction and elemental sulfur recovery using photoelectric microbial electrolysis cell

Elemental sulfur recover from sulfate-rich environment has great significance for the sustainable development of environment and energy. This study aimed to realize simultaneous sulfate reduction and elemental sulfur recovery using a novel photoelectricity microbial electrolysis cell (PMEC) under low applied voltages. At an applied voltage of 1.2 V, the sulfate reduction rate in the PMEC reached 200 ± 2.3 mg L^-1 d^-1, and 46.3 ± 7.9% of the reduced sulfate converted to elemental sulfur. With increasing voltages from 0.8 to 1.5 V, the sulfate reduction rates enhanced from 37.8 ± 12.4 to 236 ± 18.1 mg L^-1 d^-1. The recovery efficiency of elemental sulfur from removed sulfate decreased to 35% at 1.5 V, which was attributed to the higher concentration of dissolved oxygen diffusing from the anode side. Sulfate reducing bacteria (including Desulfovibrio and Desulfomicrobium) cooperated with sulfur oxidizing bacteria (including Thiomonas and Acinetobacter) for recovering elemental sulfur that could be regulated by cathode configuration. The study provided an alternative to apply solar energy in biological sulfur recovery and reduce energy consumption of wastewater treatment.

Electron Storage in Electroactive Biofilms

Microbial electrochemical technologies (METs) are promising for sustainable applications. Recently, electron storage during intermittent operation of electroactive biofilms (EABs) has been shown to play an important role in power output and electron efficiencies. Insights into electron storage mechanisms, and the conditions under which these occur, are essential to improve microbial electrochemical conversions and to optimize biotechnological processes. Here, we discuss the two main mechanisms for electron storage in EABs: storage in the form of reduced redox active components in the electron transport chain and in the form of polymers. We review electron storage in EABs and in other microorganisms and will discuss how the mechanisms of electron storage can be influenced.

Properties of Sulfur Particles Formed in Biodesulfurization of Biogas

In the biodesulfurization (BD) process under halo-alkaline conditions, toxic hydrogen sulfide is oxidized to elemental sulfur by a mixed culture of sulfide oxidizing bacteria to clean biogas. The resulting sulfur is recovered by gravitational settling and can be used as raw material in various industries. However, if the sulfur particles do not settle, it will lead to operational difficulties. In this study, we investigated the properties of sulfur formed in five industrial BD facilities. Sulfur particles from all samples showed large differences in terms of shape, size, and settleability. Both single crystals (often bipyramidal) and aggregates thereof were observed with light and scanning electron microscopy. The small, non-settled particles account for at least 13.6% of the total number of particles and consists of small individual particles with a median of 0.3 µm. This is undesirable, because those particles cannot be removed from the BD facility by gravitational settling and lead to operational interruption. The particles with good settling properties are aggregates (5–20 µm) or large single crystals (20 µm). We provide hypotheses as to how the differences in sulfur particle properties might have occurred. These findings provide a basis for understanding the relation between sulfur particle properties and formation mechanisms.

Mechanisms of sulfur selection and sulfur secretion in a biological sulfide removal (BISURE) system

Biological desulfurization technology is a sustainable process for the sulfide removal from biogas, which has multiple advantages. In this study, a biological sulfide removal (BISURE) system was established to investigate the working performances and process mechanisms. The results showed that the sulfide removal rate was 2.30 kg-S/(m³ d), the sulfide removal efficiency was higher than 98%, the sulfur production rate was 1.76 kg-S/(m³ d), the sulfur selectivity was 75.02 ± 3.63% and the main form of products (sulfur compounds) was Rosickyite-S and S₈. The performance of BISURE system was supported by the dominant genus (abundance more than 60%) of sulfur-oxidizing bacteria (SOB) which shifted to Thiovirga at the high SLR. The sqr and dsrA genes could serve as the indicators for the pathway of two-step sulfide oxidation, i.e. "partial sulfide oxidation (PSO, sulfide → sulfur)" and "complete sulfide oxidation (CSO, sulfur → sulfate)". The sulfur selectivity was improved by enhancing PSO and inhibiting CSO with the indication of two genes. The cellular sulfur secretion was revealed, and the "outer-membrane vesicles (OMVs)-dependent" sulfur-secreting hypothesis was proposed to explain the transportation of elemental sulfur from inside to outside of SOB cells. The findings of this work provide a new perspective to understand the sulfur selection of sulfide bio-oxidation and the sulfur secretion of SOB cells so as to promote the development of biological desulfurization technology.

Effect of dimethyl disulfide on the sulfur formation and microbial community composition during the biological H2S removal from sour gas streams

Removal of organic and inorganic sulfur compounds from sour gases is required because of their toxicity and atmospheric pollution. The most common are hydrogen sulfide (H₂S) and methanethiol (MT). Under oxygen-limiting conditions about 92 mol% of sulfide is oxidized to sulfur by haloalkaliphilic sulfur-oxidizing bacteria (SOB), whilst the remainder is oxidized either biologically to sulfate or chemically to thiosulfate. MT is spontaneously oxidized to dimethyl disulfide (DMDS), which was found to inhibit the oxidation of sulfide to sulfate. Hence, we assessed the effect of DMDS on product formation in a lab-scale biodesulfurization setup. DMDS was quantified using a newly, in-house developed analytical method. Subsequently, a chemical reaction mechanism was proposed for the formation of methanethiol and dimethyl trisulfide from the reaction between sulfide and DMDS. Addition of DMDS resulted in significant inhibition of sulfate formation, leading to 96 mol% of sulfur formation. In addition, a reduction in the dominating haloalkaliphilic SOB species, Thioalkalivibrio sulfidiphilus, was observed in favor of Thioalkaibacter halophilus as a more DMDS-tolerant with the 50 % inhibition coefficient at 2.37 mM DMDS.

Enhancing the removal of H2S from biogas through refluxing of outlet gas in biological bubble-column

Biological bubble-column (BBC) is beneficial for elemental sulfur recycle from H₂S, but it's difficult to remove high concentration of H₂S in biogas efficiently due to the mass transfer limitation of H₂S from gas to liquid. In this study, a novel method with refluxing outlet gas in BBC was investigated. The results showed that gas reflux greatly enhanced the removal of high concentration of H₂S (about 5000 ppmv) from biogas. The removal efficiency of H₂S was 88.0 ± 4.1% with the reflux ratio at 1.0, which was higher than those without gas reflux (58.4 ± 1.0%), when the inlet H₂S loading was 143.1 ± 4.5 g/(m³·h). Moreover, the removal capacity of H₂S improved significantly with the increase of the reflux ratios from 1.0 to 4.0 and achieved the maximum at 271.8 ± 2.4 g/(m³·h). This might mainly be attributed to longer residence time and enhanced the mass transfer of O₂ and H₂S from gas to liquid through gas reflux.

Biologically enhanced hydrogen sulfide absorption from sour gas under haloalkaline conditions

We studied a biotechnological desulfurization process for removal of toxic hydrogen sulfide (H₂S) from sour gas. The process consists of two steps: i) Selective absorption of H₂S into a (bi)carbonate solution in the absorber column and ii) conversion of sulfide to sulfur by sulfide oxidizing bacteria (SOB) in the aerated bioreactor. In previous studies, several physico-chemical factors were assessed to explain the observed enhancement of H₂S absorption in the absorber, but a full explanation was not provided. We investigated the relation between the metabolic activity of SOB and the enhancement factor. Two continuous experiments on pilot-scale were performed to determine H₂S absorption efficiencies at different temperatures and biomass concentrations. The absorption efficiency improved at increasing temperatures, i.e. H₂S concentration in the treated gas decreased from 715 ± 265 ppmv at 25.4 °C to 69 ± 25 ppmv at 39.4 °C. The opposite trend is expected when H₂S absorption is solely determined by physico-chemical factors. Furthermore, increasing biomass concentrations to the absorber also resulted in decreased H₂S concentrations in the treated gas, from approximately 6000 ppmv without biomass to 1664 ± 126 ppmv at 44 mg N/L. From our studies it can be concluded that SOB activity enhances H₂S absorption and leads to increased H₂S removal efficiencies in biotechnological gas desulfurization.

Microbial reduction of organosulfur compounds at cathodes in bioelectrochemical systems

Organosulfur compounds, present in e.g. the pulp and paper industry, biogas and natural gas, need to be removed as they potentially affect human health and harm the environment. The treatment of organosulfur compounds is a challenge, as an economically feasible technology is lacking. In this study, we demonstrate that organosulfur compounds can be degraded to sulfide in bioelectrochemical systems (BESs). Methanethiol, ethanethiol, propanethiol and dimethyl disulfide were supplied separately to the biocathodes of BESs, which were controlled at a constant current density of 2 A/m² and 4 A/m². The decrease of methanethiol in the gas phase was correlated to the increase of dissolved sulfide in the liquid phase. A sulfur recovery, as sulfide, of 64% was found over 5 days with an addition of 0.1 ​mM methanethiol. Sulfur recoveries over 22 days with a total organosulfur compound addition of 1.85 ​mM were 18% for methanethiol and ethanethiol, 17% for propanethiol and 22% for dimethyl disulfide. No sulfide was formed in electrochemical nor biological control experiments, demonstrating that both current and microorganisms are required for the conversion of organosulfur compounds. This new application of BES for degradation of organosulfur components may unlock alternative strategies for the abatement of anthropogenic organosulfur emissions.

Development of quantitative PCR for the detection of Alkalilimnicola ehrlichii, Thioalkalivibrio sulfidiphilus and Thioalkalibacter halophilus in gas biodesulfurization processes

Chemolithoautotrophic sulfur-oxidizing bacteria (SOB) are crucial key players in biotechnological processes to remove hydrogen sulfide from sour gas streams. Several different haloalkaliphilic SOB have been detected and isolated from lab- and full-scale facilities, which all performed differently considering end product yields (sulfur and sulfate) and conversion rates. Understanding and regulating bacterial community dynamics in biodesulfurization processes will enable optimization of the process operation. We developed quantitative PCR (qPCR) assays to quantify haloalkaliphilic sulfur-oxidizing gammaproteobacterial species Alkalilimnicola ehrlichii, Thioalkalivibrio sulfidiphilus, and Thioalkalibacter halophilus that dominate bacterial communities of biodesulfurization lab- and full-scale installations at haloalkaline conditions. The specificity and PCR efficiency of novel primer sets were evaluated using pure cultures of these target species. We further validated the qPCR assays by quantification of target organisms in five globally distributed full-scale biodesulfurization installations. The qPCR assays perform a sensitive and accurate quantification of Alkalilimnicola ehrlichii, Thioalkalivibrio sulfidiphilus and Thioalkalibacter halophilus, thus providing rapid and valuable insights into process performance and SOB growth dynamics in gas biodesulfurization systems.