Volatile Siloxanes Emissions: Impact and Sustainable Abatement Perspectives

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.

Agri-food waste biosorbents for volatile organic compounds removal from air and industrial gases - A review

Approximately 1.3 billion metric tons of agricultural and food waste is produced annually, highlighting the need for appropriate processing and management strategies. This paper provides an exhaustive overview of the utilization of agri-food waste as a biosorbents for the elimination of volatile organic compounds (VOCs) from gaseous streams. The review paper underscores the critical role of waste management in the context of a circular economy, wherein waste is not viewed as a final product, but rather as a valuable resource for innovative processes. This perspective is consistent with the principles of resource efficiency and sustainability. Various types of waste have been described as effective biosorbents, and methods for biosorbents preparation have been discussed, including thermal treatment, surface activation, and doping with nitrogen, phosphorus, and sulfur atoms. This review further investigates the applications of these biosorbents in adsorbing VOCs from gaseous streams and elucidates the primary mechanisms governing the adsorption process. Additionally, this study sheds light on methods of biosorbents regeneration, which is a key aspect of practical applications. The paper concludes with a critical commentary and discussion of future perspectives in this field, emphasizing the need for more research and innovation in waste management to fully realize the potential of a circular economy. This review serves as a valuable resource for researchers and practitioners interested in the potential use of agri-food waste biosorbents for VOCs removal, marking a significant first step toward considering these aspects together.

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.

Physiological impact of personal care product constituents on non-target aquatic organisms

Personal care products (PCPs) are products used in cleaning, beautification, grooming, and personal hygiene. The rise in diversity, usage, and availability of PCPs has resulted in their higher accumulation in the environment. Thus, these constitute an emerging category of environmental contaminants due to the potential of its constituents (chemical and non-chemical) to induce various physiological effects even at lower concentrations (ng/L). For analyzing the impact of the PCPs constituents on the non-target organism about 300 article including research articles, review articles and guidelines were studied from 2000 to 2023. This review aims to firstly discuss the fate and accumulation of PCPs in the aquatic environment and organisms; secondly provides overview of environmental risks that are linked to PCPs; thirdly review the trends, current status of regulations and risks associated with PCPs and finally discuss the knowledge gaps and future perspectives for future research. The article discusses important constituents of PCPs such as antimicrobials, cleansing agents and disinfectants, fragrances, insect repellent, moisturizers, plasticizers, preservatives, surfactants, UV filters, and UV stabilizers. Each of them has been found to display certain toxic impact on the aquatic organisms especially the plasticizers and UV filters. These continuously and persistently release biologically active and inactive components which interferes with the physiological system of the non-target organism such as fish, corals, shrimps, bivalves, algae, etc. With a rise in the number of toxicity reports, concerns are being raised over the potential impacts of these contaminant on aquatic organism and humans. The rate of adoption of nanotechnology in PCPs is greater than the evaluation of the safety risk associated with the nano-additives. Hence, this review article presents the current state of knowledge on PCPs in aquatic ecosystems.

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.

Removal of Hexamethyldisiloxane via a Novel Hydrophobic (3-Aminopropyl)Trimethoxysilane-Modified Activated Porous Carbon

Volatile methyl siloxanes (VMS) must be removed because the formation of silica in the combustion process seriously affects the resource utilization of biogas. Herein, a series of APTMS ((3-aminopropyl)trimethoxysilane)-modified activated porous carbon (APC) adsorbents (named APTMS@APC) were prepared for VMS efficient removal. The as-prepared adsorbents were characterized using SEM, FTIR, Raman, X-ray diffraction analyses, and N2 adsorption/desorption. The results showed that the surface modification with APTMS enhanced the hydrophobicity of APC with the water contact angle increasing from 74.3° (hydrophilic) to 127.1° (hydrophobic), and meanwhile improved its texture properties with the SBET increasing from 981 to 1274 m2 g-1. The maximum breakthrough adsorption capacity of APTMS@APC for hexamethyldisiloxane (L2, model pollutant) was 360.1 mg g-1. Effects of an inlet L2 concentration (31.04-83.82 mg L-1) and a bed temperature (0-50 °C) on the removal of L2 were investigated. Meanwhile, after five adsorption-desorption cycles, the APTMS@APC demonstrated a superior cycling performance. This indicated that the hydrophobic APTMS@APC has a great significance to remove VMS.

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.

VOCs absorption from gas streams using deep eutectic solvents - A review

Volatile organic compounds (VOCs) are one of the most severe atmospheric pollutants. They are mainly emitted into the atmosphere from anthropogenic sources such as automobile exhaust, incomplete fuel combustion, and various industrial processes. VOCs not only cause hazards to human health or the environment but also adversely affect industrial installation components due to their specific properties, i.e., corrosive and reactivity. Therefore, much attention is being paid to developing new methods for capturing VOCs from gaseous streams, i.e., air, process streams, waste streams, or gaseous fuels. Among the available technologies, absorption based on deep eutectic solvents (DES) is widely studied as a green alternative to other commercial processes. This literature review presents a critical summary of the achievements in capturing individual VOCs using DES. The types of used DES and their physicochemical properties affecting absorption efficiency, available methods for evaluating the effectiveness of new technologies, and the possibility of regeneration of DES are described. In addition, critical comments on the new gas purification methods and future perspectives are included.

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.

Anoxic desulphurisation of biogas from full-scale anaerobic digesters in suspended biomass bioreactors valorising previously nitrified digestate centrate

Nitrification of centrate from anaerobic sewage sludge digestion presents a major opportunity as an electron acceptor in anoxic biogas biodesulphurisation. Nitritation and nitrification inhibition by free ammonia was detected at laboratory scale, but was avoided during the scale-up operation in a 4 m³ reactor treating ammonium loads up to 19 gN m^-3 h^-1. This nitrate-rich stream was fed to two pilot-scale suspended biomass bioreactors (SBBs) treating real biogas for 220 days. After an adaptation period of 21 days, nitrate and alkalinity concentrations in the liquid medium below 10 mgN L^-1 and 100 mgCaCO₃ L^-1 were found to limit hydrogen sulphide (H₂S) oxidation. Once controlled, 95% of the H₂S was removed in SBB1 and 90% in SBB2, at a gas residence time (GRT) of 5.9 min, treating average values of 321 ± 205 ppmv and 457 ± 205 ppmv, respectively. Outlet H₂S concentrations of 16 ± 24 ppmv in SBB1 and 46 ± 39 ppmv in SBB2 were obtained, which are below the requirements of biogas combustion heat and power engines. Unlike H₂S, siloxanes were not removed with these GRTs. The results demonstrate the feasibility of the combined process for H₂S treatment, potential valorisation of precipitated elemental sulphur and a reduction in the reagents currently used to control H₂S.

Syngas biomethanation: Current state and future perspectives

In regions highly dependent on fossil fuels imports, biomethane represents a promising biofuel for the transition to a bio-based circular economy. While biomethane is typically produced via anaerobic digestion and upgrading, biomethanation of the synthesis gas (syngas) derived from the gasification of recalcitrant solid waste has emerged as a promising alternative. This work presents a comprehensive and in-depth analysis of the state-of-the-art and most recent advances in the field, compiling the potential of this technology along with the bottlenecks requiring further research. The key design and operational parameters governing syngas production and biomethanation (e.g. organic feedstock, gasifier design, microbiology, bioreactor configuration, etc.) are critically analysed.

Considerations on the Analysis of E-900 Food Additive: An NMR Perspective

Food additives are in widespread use in the food industry to extend the shelf life of food, improve its organoleptic characteristics or facilitate industrial processing. Their use is not without controversy, which makes regulation and control crucial for food safety and public health. Among food additives, silicone-based antifoaming agents (polysiloxanes or E900) are difficult to analyze and quantify due to their polymeric nature. Currently, there is no official method of quantifying this additive in foods. In this context, nuclear magnetic resonance (NMR) is a quantitative method for speciation analysis of silicon compounds almost without known interferents. In this work, we describe the evolution of the regulation of the E900 additive, discuss different analytic methods quantifying polydimethylsiloxanes (PDMS), and propose a new method based on NMR suitable for analyzing the content of E900 in the form of PDMS in various types of food from dietary oils to marmalades and jellies, among others. The proposed method consists of a previous quantitative concentration of PDMS by liquid-liquid extraction and the monitoring of the quantification using a bis(trimethylsilyl)benzene (BTMSB) standard to control the variability, ranging within 2-7%, depending on the food. This simple, direct, and reproducible procedure for aqueous and lipidic foods may help to monitor and fill a gap in regulatory legislation regarding the E900 additive.