Recent advances in siloxanes removal from biogas and their efficiency: a short review

Assessing the impact of packaging materials on anoxic biotrickling filtration of siloxanes in biogas: Effectiveness of activated carbon in removal performance

Siloxanes (VMS) represent a class of organosilicon compounds known for their adverse effects on both the environment and human health. Their presence in biogas significantly hinders its economic valorisation, highlighting the need for effective treatment methods. This study investigates the performance of three different packing materials in the anoxic biofiltration of VMS (L2, L3, D4 and D5). The materials evaluated included plastic rings (BTF-1), polyurethane foam (BTF-2) and plastic rings combined with activated carbon (80:20) (BTF-3). Among them, BTF-3 exhibited superior performance, achieving maximum VMS removal efficiencies (REs) of 90%, including the complete elimination of L3 and D4, and ∼80% removal of D5, attributed to the presence of activated carbon. However, the abatement of L2 was inferior to that of other VMS (<80%), which was attributed to the activated carbon's affinity for larger molecular weights and critical diameters. In contrast, BTF-1 and BTF-2 supported maximum VMS removals of 40%. Notably, neither increasing the trickling liquid velocity from 2 to 4.5 m h⁻1 nor adding Fe-carbon nanoparticles to the solution had any impact on the BTFs' performance. Following the successful results observed in BTF-3, gas residence time was reduced from 60 to 42 min, consequently leading to an increase in the EC from 366 to 509 mg m-3 h-1 (corresponding to an RE = 87%). Despite the different performance of the BTFs, comparable bacterial communities were identified, dominated by the genera Thermomonas, Corynebacterium, Aquimonas, Thauera and Parvibaculum. The results obtained in this study highlighted the potential of activated carbon as packing material for enhancing abatement performance during biotrickling filtration and identified new bacterial genera with potential for VMS degradation.

Effect of toluene on siloxane biodegradation and microbial communities in biofilters

The removal of volatile methyl siloxanes (VMS) from landfill biogas is crucial for clean energy utilization. VMS are usually found together with aromatic compounds in landfill biogas of which toluene is the major representative. In the present study, two biofilters (BFs) packed with either woodchips and compost (WC) or perlite (PER) were used to study the (co-) removal of octamethyltrisiloxane (L3) and octamethylcyclotetrasiloxane (D4) from gas in presence and absence of toluene, used as a representative aromatic compound. The presence of low inlet toluene concentrations (315 ± 19 - 635 ± 80 mg toluene m-3) enhanced the VMS elimination capacity (EC) in both BFs by a factor of 1.8 to 12.6. The highest removal efficiencies for D4 (57.1 ± 1.1 %; EC = 0.12 ± 0.01 gD4 m-3 h-1) and L3 (52.0 ± 0.6 %; EC = 0.23 ± 0.01 gL3 m-3 h-1) were observed in the BF packed with WC. The first section of the BFs (EBRT = 9 min), where toluene was (almost) completely removed, accounted for the majority (87.7 ± 0.6 %) of the total VMS removal. Microbial analysis revealed the impact of VMS and toluene in the activated sludge, showing a clear selection for certain genera in samples influenced by VMS in the presence (X2) or absence (X1) of toluene, such as Pseudomonas (X1 = 0.91 and X2 = 12.0 %), Sphingobium (X1 = 0.09 and X2 = 4.04 %), Rhodococcus (X1 = 0.42 and X2 = 3.91 %), and Bacillus (X1 = 7.15 and X2 = 3.84 %). The significant maximum EC values obtained by the BFs (0.58 gVMS m-3 h-1) hold notable significance in a combined system framework as they could enhance the longevity of traditional physicochemical methods to remove VMS like activated carbon in diverse environmental scenarios.

Determination of emission factors from a landfill through an inverse methodology: Experimental determination of ambient air concentrations and use of numerical modelling

The application of numeric modelling for determining the impact of landfills needs for reliable emission source data. In this study, a methodology for the characterization of the emission profiles of the different sources present in landfills for emission factors determination, applying an indirect methodology, is presented. Ambient air concentrations of volatile organic compounds (VOCs), hydrogen sulphide (H2S) and ammonia (NH3) were determined in three potentially emission sources in Can Mata landfill (Hostalets de Pierola, Catalonia, Spain): dumping areas, pre-closed zone and leachate reservoir as well as in biogas, for the determination of emission factors. Multi-sorbent bed and Tenax TA tubes were used for a wide range of VOCs sampling, and analysis was conducted through TD-GC/MS. H2S and NH3 were sampled and analysed using Radiello passive samplers. The highest total VOC (TVOC) concentrations were found in dumping areas (0.7-3.5 mg m-3), followed by leachate reservoir (0.3-0.6 mg m-3) and pre-closed area (77-165 μg m-3). On the other hand, the highest H2S and NH3 concentrations were found in leachate reservoir, presenting values of 0.8-1.1 mg m-3 and 1.7-1.8 mg m-3, respectively. With the application of odour thresholds to the concentrations obtained, the most critical compounds regarding odour annoyances were determined. The highest odour units (O.U.) were found in leachate reservoir due to H2S concentrations, whereas VOCs contributed mainly to O.U. in the dumping areas. The obtained ambient air concentrations were used for the indirect determination of the emission factors through numerical modelling using a Eulerian dispersion model. The emission factors obtained for the landfill for TVOC, H2S and NH3 were in the range of 0.44-10.9 g s-1, 0.16-1.02 g s-1 and 0.23-1.82 g s-1, respectively, depending on the emission source. Reliable emission factors are crucial to obtain landfill impact maps, which are essential for the correct management of these facilities.

An Integrated Computational Approaches for Designing of Potential Piperidine based Inhibitors of Alzheimer Disease by Targeting Cholinesterase and Monoamine Oxidases Isoenzymes

The study aimed to evaluate the potential of piperidine-based 2H chromen-2-one derivatives against targeted enzymes, i.e., cholinesterase's and monoamine oxidase enzymes. The compounds were divided into three groups, i.e., 4a-m ((3,4-dimethyl-7-((1-methylpiperidin-4-yl)oxy)-2H-chromen-2-one derivatives), 5a-e (3,4-dimethyl-7-((1-methypipridin-3-yl)methoxy)-2H-chromen-2-one derivatives), and 7a-b (7-(3-(3,4-dihydroisoquinolin-2(1H)-yl)propoxy)-3,4-dimethyl-2H-chromen-2-one derivatives) with slight difference in the basic structure. The comprehensive computational investigations were conducted including density functional theories studies (DFTs), 2D-QSAR studies, molecular docking, and molecular dynamics simulations. The QSAR equation revealed that the activity of selected chromen-2-one-based piperidine derivatives is being affected by the six descriptors, i.e., Nitrogens Count, SdssCcount, SssOE-Index, T-2-2-7, ChiV6chain, and SssCH2E-Index. These descriptor values were further used for the preparation of chromen-2-one based piperidine derivatives. Based on this, 83 new derivatives were created from 7 selected parent compounds. The QSAR model predicted their IC50 values, with compound 4 k and 4kk as the most potent multi-targeted derivative. Molecular docking results exhibited these compounds as the best inhibitors; however, 4kk exhibited greater activity than the parent compounds. The results were further validated by molecular dynamic simulation studies along with the suitable physicochemical properties. These results prove to be an essential guide for the further design and development of new piperidine based chromen-2-one derivatives having better activity against neurodegenerative disorder.

Insights into the recent advances of agro-industrial waste valorization for sustainable biogas production

Recent years have seen a transition to a sustainable circular economy model that uses agro-industrial waste biomass waste to produce energy while reducing trash and greenhouse gas emissions. Biogas production from lignocellulosic biomass (LCB) is an alternative option in the hunt for clean and renewable fuels. Different approaches are employed to transform the LCB to biogas, including pretreatment, anaerobic digestion (AD), and biogas upgradation to biomethane. To maintain process stability and improve AD performance, machine learning (ML) tools are being applied in real-time monitoring, predicting, and optimizing the biogas production process. An environmental life cycle assessment approach for biogas production systems is essential to calculate greenhouse gas emissions. The current review presents a detailed overview of the utilization of agro-waste for sustainable biogas production. Different methods of waste biomass processing and valorization are discussed that contribute towards developing an efficient agro-waste to biogas-based circular economy.

Hexamethyldisiloxane Removal from Biogas Using a Fe3O4-Urea-Modified Three-Dimensional Graphene Aerogel

Volatile methyl siloxanes (VMS), which are considered to be the most troublesome impurities in current biogas-cleaning technologies, need to be removed. In this study, we fabricated a series of Fe3O4-urea-modified reduced graphene-oxide aerogels (Fe3O4-urea-rGOAs) by using industrial-grade graphene oxide as the raw material. A fixed-bed dynamic adsorption setup was built, and the adsorption properties of the Fe3O4-urea-rGOAs for hexamethyldisiloxane (L2, as a VMS model pollutant) were studied. The properties of the as-prepared samples were investigated by employing various characterization techniques (SEM, TEM, FTIR, XRD, Raman spectroscopy, and N2 adsorption/desorption techniques). The results showed that the Fe3O4-urea-rGOA-0.4 had a high specific surface area (188 m2 g-1), large porous texture (0.77 cm3 g-1), and the theoretical maximum adsorption capacity for L2 (146.5 mg g-1). The adsorption capacity considerably increased with a decrease in the bed temperature of the adsorbents, as well as with an increase in the inlet concentration of L2. More importantly, the spent Fe3O4-urea-rGOA adsorbent could be readily regenerated and showed an excellent adsorption performance. Thus, the proposed Fe3O4-urea-rGOAs are promising adsorbents for removing the VMS in biogas.

Are Si-C bonds formed in the environment and/or in technical microbiological systems?

Organosiloxanes are industrially produced worldwide in millions of tons per annum and are widely used by industry, professionals, and consumers. Some of these compounds are PBT (persistent, biaccumulative and toxic) or vPvB (very persistent and very bioaccumulative). If organosiloxanes react at all in the environment, Si-O bonds are hydrolyzed or Si-C bonds are oxidatively cleaved, to result finally in silica and carbon dioxide. In strong contrast and very unexpectedly, recently formation of new Si-CH3 bonds from siloxanes and methane by the action of microorganisms under mild ambient conditions was proposed (in landfills or digesters) and even reported (in a biotrickling filter, 30 °C). This is very surprising in view of the harsh conditions required in industrial Si-CH3 synthesis. Here, we scrutinized the pertinent papers, with the result that evidence put forward for Si-C bond formation from siloxanes and methane in technical microbiological systems is invalid, suggesting such reactions will not occur in the environment where they are even less favored by conditions. The claim of such reactions followed from erroneous calculations and misinterpretation of experimental results. We propose an alternative explanation of the experimental observations, i.e., the putative observation of such reactions was presumably due to confusion of two compounds, hexamethyldisiloxane and dimethylsilanediol, that elute at similar retention times from standard GC columns.

Are Si-C bonds cleaved by microorganisms? A critical review on biodegradation of methylsiloxanes

Methylsiloxanes, compounds that contain H₃C-Si-O subunits in their molecular structure, are emerging ubiquitous pollutants now detected in many environmental compartments. These compounds and generally Si-C bonds do not occur in living nature, but are industrially produced worldwide in millions of tons per annum and are widely used, resulting in their release to the environment. It is an open question whether or to what extent microorganisms are able to decompose these compounds. The presence of methylsiloxanes in many biogases adds to the economic relevance of this question. We here review and critically discuss, for the first time, the evidence obtained for and against degradation of methylsiloxanes by microorganisms, and in particular for microbial cleavage of Si-CH₃ bonds. As a result, no convincing demonstration of Si-C cleavage by native environmental microorganisms has been found.