Methyl Siloxanes Research Articles

Methyl siloxanes in soils from a large silicone-manufacturing site, China: concentrations, distributions and potential human exposure.

Methyl siloxanes are widely found in the environment, but little is known about the distributions of these chemicals in soils especially in areas where they are manufactured. We determined the concentrations of four cyclic (D3-D6) and 13 linear methyl siloxanes (L4-L16) in the soils from a siloxane-manufacturing site in China; the total concentrations of these 17 siloxanes (TSi) in the soils were 17.1-3,191 (median, 134) ng/g. We did not find extremely high concentrations of siloxanes in the soils. The median concentrations of total cyclic siloxanes (TCSi) were approximately sevenfold higher than those of total linear congeners. Hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane contributed a median of 59.7% and 20.3% of the TSi concentrations, respectively. Higher concentrations of soil TCSi were found in the silicone-manufacturing area relative to the other study areas. Source analysis indicated that industrial activities contributed substantially to soil siloxanes, in addition to the contribution of the siloxane emissions from specific consumer products. We calculated that the median values of daily TSi intakes through soil ingestion were 0.021 and 0.138ng/kg-body weight/day for adults and children, respectively, under high exposure scenarios. Although our estimated daily intakes of the chemicals from soils were low, more research is required to improve our understanding of the health risks posed to humans exposed to siloxanes through other pathways.

Assessment bioaccumulation factor (BAF) of methyl siloxanes in crucian carp (Carassius auratus) around a siloxane production factory

Methyl siloxanes are identified as emerging persistent toxic compounds and the ecological environment risks of these compounds have been caused of great concern worldwide. In this study, the concentrations of methyl siloxanes were reported in dissolved water and crucian carp around a methyl siloxane production factory located in Liaoning Province, Northeast China. D4, D5, D6, D7, L4, L5 and L6 were detectable both in dissolved water and crucian carp. The total concentrations of 7 methyl siloxanes (Σ7MS) were 14 ± 6.3 ng/L in dissolved water and 43 ± 22 ng/g ww in crucian carp, respectively. D5 has the highest concentration both in dissolved water (5.5 ± 3.5 ng/L) and crucian carp (17 ± 11 ng/g ww). Based on the monitoring values, bioaccumulation factor (BAF) of these compounds were calculated. Significant bioaccumulation potential was observed for D4 (BAF = 5900 ± 3500 L/kg) based on the bioaccumulation criteria suggested by USEPA and EU (BAF > 5000 L/kg). To our understanding, this is the first report of BAF values of methyl siloxane in field study, which will provide important support for further assessment of bioaccumulation of these compounds.

Modeling cyclic volatile methylsiloxanes removal efficiency from wastewater by ZnO-coated aluminum anode using artificial neural networks

Usage of cyclic volatile methyl siloxanes (cVMSs) in the industrial process is unavoidable due to their superior properties; however, it is hazardous to human health. Photocatalytic zinc oxide coated aluminum anode is used to degrade the cVMSs in wastewater. In this work, we investigated the relationship among degradation process parameters such as current density (4–20 mA/cm2), initial pH (5–9), plate distance (8–24 cm), UV intensity (0–120 W), and reaction time (30–100 min) vis-a-vis cVMSs removal efficiency by using data-driven artificial neural networks(ANN) model. The ANN model was trained using a backpropagation algorithm with the sigmoid activation function between input, hidden, and the output layers. Two hidden layers with eight neurons in each layer presented the minimum average training error (0.24) and higher (0.99) correlation coefficient values (both Pearson’s r and Adj. R2) as compared with the conventional regression model. The effect and relationship between the parameters and cVMSs removal efficiency were analyzed by single, two variable sensitivity analysis, qualitative and quantitative estimation.

Influence of Sampling Methods and Storage Condition on Volatile Methyl Siloxanes Quantification in Biogas

Influence of Sampling Methods and Storage Condition on Volatile Methyl Siloxanes Quantification in Biogas

Methyl siloxanes in road dust from a large silicone manufacturing site in China: implications of human exposure.

Methyl siloxanes are becoming increasingly prevalent in the environment because of their extensive use in various consumer products. Little is known about the distribution of these chemicals around factories or their presence in road dust. We conducted a survey of four cyclic (D3-D6) and 13 linear (L4-L16) siloxanes in road dusts around a manufacturing site and found a total siloxane concentration range of 47.3-3.16 × 103ng/g (median 372). The predominant siloxanes in the road dust samples were D3, D4, D5, and D6, with median contributions of 65.3%, 17.9%, 6.36%, and 2.16% of the total siloxane concentrations, respectively. Our reported concentrations were comparable to those found in household dusts in previous studies. No high concentrations of siloxanes were observed in the road dust samples from the manufacturing site, which suggests that siloxanes in outdoor dusts are likely affected by many factors. Under a high exposure scenario, the daily intakes of total siloxanes via road dust ingestion at the 95th percentile were 2.13 and 0.313ng/kg-bw/day for children and adults, respectively. Although the estimated exposure to siloxanes in outdoor dust for humans was low, more studies on the distributions and risks of siloxanes released from manufacturing sites are needed.

Air pollution caused by phthalates and cyclic siloxanes in Hanoi, Vietnam: Levels, distribution characteristics, and implications for inhalation exposure

Contamination status and distribution characteristics of ten phthalic acid esters (PAEs) and three cyclic volatile methyl siloxanes (CSs) were determined in the air (gas and particle) samples collected from indoor and outdoor spaces of several chemistry laboratories, offices, and homes from urban area of Hanoi, the capital city of Vietnam. Air concentrations of Σ10PAEs (median 688; range 142–2390 ng m−3) and Σ3CSs (171; not detected–1100 ng m−3) in the indoor air samples were significantly higher than those measured in the outdoor ones (Σ10PAEs: 161; 34.1–515 ng m−3 and Σ3CSs: 43.2; not detected–258 ng m−3), partly suggesting the predominance of indoor emission sources of these substances. There were significant positive correlations in total air concentrations of phthalates and siloxanes between the indoor and outdoor air samples. The most predominant phthalates were diethyl-, di-n-butyl-, diisobutyl-, and di(2-ethylhexyl) phthalate. For siloxanes, D5 and D6 were more abundant than D4 in most samples. Except for di(2-ethylhexyl)- and di-n-octyl phthalate in some locations, almost all the compounds were likely associated with gas phase than particle phase. Daily intake doses of airborne phthalates and siloxanes, and non-cancer and cancer risks of selected phthalates were estimated for different exposure groups such as adults, children, and university subjects (e.g., laboratory staff and students), indicating relatively low levels of risk.

Distribution characteristics of methylsiloxanes in atmospheric environment of Saitama, Japan: Diurnal and seasonal variations and emission source apportionment

The large production volume of methylsiloxanes (MSs), combined with their high mobility/volatility and persistence, is a matter of concern from the atmospheric pollution perspective. Therefore, we evaluated of the concentrations and emission sources of MSs, including 7 cyclic methylsiloxanes (D3–D9; CMSs, the number refers to the number of Si–O bonds) and 13 linear methylsiloxanes (L3–L15; LMSs) in ambient air collected from Saitama, Japan. This is a first study regarding the evaluation of 20 methylsiloxanes in the Japanese atmosphere. We improved the air sampling methodology by determination the stability of D5 during a 7-d air sampling and arbitrary sample storage period using polystyrene-divinyl benzene copolymer sorbent (Sep-Pak plus PS-2). We analyzed air samples for MSs seasonally collected from nine locations in Saitama, including urban, suburban, rural, and mountainous areas. The mean CMS and LMS concentrations were 358 ng m−3 and 13.4 ng m−3, respectively. The D5 concentrations were distributed widely, with high concentrations in urban/suburban populous areas and dispersed at low concentrations in surrounding areas (north and mountainous areas). We analyzed 7-d air samples collected every week over a year and found apparent seasonal and periodic trends in the CMS concentrations. In the diurnal sampling campaign, we observed periodic fluctuations in ambient CMSs, with an inverse relationship with the atmospheric boundary layer development during the day. Backward trajectories and the prevailing wind direction during the sampling period indicated that the specific profiles of D4 observed in fall/winter weeks and north of Saitama could be ascribed to northwestward air-mass advection. We employed a novel approach in estimating CMSs emission sources and source apportionment by using non-negative matrix factorization (NMF). The concentration matrix was divided successfully into two factors (emission sources) namely, personal care and household products and industrial activities.

Methylsiloxanes in plasma from potentially exposed populations and an assessment of the associated inhalation exposure risk

Methylsiloxanes (MSs) are ubiquitous in indoor air and pose an important health risk. Thus, assessments of indoor inhalation exposure by measuring MSs levels in plasma are needed. In this study, we measured plasma MSs concentrations and evaluated daily indoor inhalation exposure in potentially exposed populations, including residents of industrial areas, university campus, and residential areas, all located in southwestern China. The concentrations of MSs in indoor air (gas-phase and PM2.5) collected from factory housing and from girls’ dormitories on university campus were approximately one to three orders of magnitude higher than in parallel samples from other areas. The consequences of MSs exposure were investigated by measuring MSs levels in the plasma samples of the exposed populations. Relatively high levels of cyclic MSs (CMSs: D4–D6) were found in the plasma of the co-resident family members of factory workers and in female college students living in campus dormitories. The highest levels of CMSs (D4–D6) and linear MSs (L5–L16), 2.3 × 102 and 2.0 × 102 ng/mL, respectively, were detected in the very young (0–3 years old) co-resident children of factory workers. The average daily dose via inhalation (ADDinh) in different groups showed that the ADDinh values of all MSs (D4–D6, L5–L16) were one to two orders of magnitude higher in the co-resident family members of factory workers and in female college students than in other groups, indicating that both populations should be considered as potentially highly exposed to MSs. A further assessment showed that inhalation exposure is the main source of CMSs (D4–D6) in plasma for people exposed to high indoor air levels of these compounds. Although the health risk assessment showed that the health risk from inhalation exposure to D4 and D5 was acceptable for all of the studied groups based on the current chronic reference dose (cRfD), the maximum ADDinh,CMSs value in 0- to 3-year-old children was only 7.9-fold below the cRfD. Because the toxicity of other MSs is unknown, the potential health risk of MSs to very young children via inhalation exposure should be further analysed.

Spatial variations of linear and cyclic volatile methyl siloxanes in a river basin and their air-water exchange

Linear and cyclic volatile methylsiloxanes (VMS) are Si and O containing organic compounds that are increasingly subject to scientific studies in recent years. They are used as solvents and coating materials in personal care products, cleaning materials and industrial applications. In this study, it was aimed to investigate the spatial variations of VMS levels in river water and ambient air in Kucuk Menderes Basin and their air-water exchange patterns. Grab river water samples and passive ambient air samples collected from 10 sites throughout Kucuk Menderes Basin located in Izmir region in Turkey, and they were analyzed for VMS. The concentrations of ΣVMS in river water and ambient air ranged from 48.4 to 148 ng L−1 and from 41.7 to 433 ng m−3, respectively. Among the VMS compounds, the predominant compounds were found to be D5 and D3 in ambient air and river water, respectively. In general, concentrations of all VMS congeners increased towards downstream of the river for both water and ambient air samples and there were substantial fluctuations between the sampling points. These fluctuations may be due to wastewater discharges at some sites (increase), followed by loss by volatilization along the river (decrease). VMS concentrations measured in air and river water samples were used to estimate the net air-water exchange fluxes. In all cases, fluxes were negative, ranging between −159 (L4) and −11100 (D3) ng m−2 day, indicating volatilization from water is a source for atmospheric VMS.

Can low-temperature point discharge Be used as atomic emission source for sensitive determination of cyclic volatile methylsiloxanes?

Despite of increased interest in the application of miniature microplasma atomic spectrometry for environmental analytical chemistry, the amenable element detection range is limited to some metal elements and carbon due to it low power consumption. In this work, the generation of silicon atomic emission (251.6 nm and 288.2 nm) from the organosiloxanes was found possible in a low-temperature, low-power, and compact point discharge. Consequence, a tiny point discharge silicon optical emission spectrometer (μPD-OES) was exploited, and used as a novel GC detector for the determination of various cyclic volatile methyl siloxanes (cVMSs). Under the optimized conditions, the developed system provided limits of detection (LODs) of 0.2 mg L−1, 0.04 mg L−1, 0.03 mg L−1 and 0.02 mg L−1 of Si for hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane and dodecamethylcyclohexasiloxane, respectively. Meanwhile, relative standard deviations (RSDs) of better than 2.3% were obtained. In contrast to gas chromatography mass spectrometer, GC-μPD-OES significantly simplifies the experimental setup with low power consumption and a miniature configuration. As far as we know, this work reports for the first time that silicon atomic emission can be generated in such low temperature microplasma. The accuracy of this system was validated by determining cVMSs in five daily-used shampoo samples collected from retail store, providing satisfactory recoveries (84%–114%) and excellent agreement with values determined by GC-MS at the 95% confidence level.

Cyclic volatile methylsiloxanes (cVMSs) in the air of the wastewater treatment plants in Dalian, China - Levels, emissions, and trends

Passive air samplers comprising sorbent-Tenax-TA thermal desorption tubes were applied for monitoring cyclic volatile methyl siloxanes (cVMSs) in the air above the aeration tanks of eight wastewater treatment plants (WWTPs, including five “open” and three “close” plants) in Dalian, China. The aim was to investigate the inputs of cVMSs from WWTPs to the air throughout a year (June 2016–July 2017). The ∑cVMS concentrations were averaged at 3.14 ± 6.84 μg·m−³ with D5 as the dominant compound (contributing to 63.8% of the ∑cVMS concentration). The annual average concentrations of D4, D5, and D6 in the air of the “close” WWTPs were 5.33 ± 4.63, 13.4 ± 14.4, and 1.05 ± 1.47 μg·m−³, and 1.31 ± 1.29, 1.57 ± 2.01, and 0.301 ± 0.340 μg·m−³ in the air from the “open” WWTPs, respectively. For both the close and open aeration tanks, the concentration of ∑cVMSs was the highest in summer and the lowest in winter, showing a significant correlation with ambient temperature (linear regression; p < 0.01). A simplified Gaussian dispersion model and a single chamber model were introduced to estimate annual emissions (kg·yr−1) of cVMSs from “open” and “close” WWTPs, ranging from 86.9 to 165 kg yr−1 and from 203 to 278 kg yr−1, respectively. Examining the relationship between the per capita emissions of cVMSs and average property prices, our results indicated that a greater amount of personal care products were used/discharged by people with relatively higher socioeconomic status.

Atmospheric Chemistry of Volatile Methyl Siloxanes: Kinetics and Products of Oxidation by OH Radicals and Cl Atoms.

Volatile methyl siloxanes (VMS) are ubiquitous anthropogenic pollutants that have recently come under scrutiny for their potential toxicity and environmental persistence. In this work, we determined the rate constants for oxidation by OH radicals and Cl atoms at 297 ± 3 K and atmospheric pressure in Boulder, CO (∼860 mbar) of hexamethyldisiloxane (L2), octamethyltrisiloxane (L3), decamethyltetrasiloxane (L4), dodecamethylpentasiloxane (L5), hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5). Measured rate constants with OH radicals were (1.20 ± 0.09) × 10-12, (1.7 ± 0.1) × 10-12, (2.5 ± 0.2) × 10-12, (3.4 ± 0.5) × 10-12, (0.86 ± 0.09) × 10-12, (1.3 ± 0.1) × 10-12, and (2.1 ± 0.1) × 10-12 cm3 molec-1 s-1, for L2, L3, L4, L5, D3, D4, and D5, respectively. The rate constants for reactions with Cl atoms with the same compounds were (1.44 ± 0.05) × 10-10, (1.85 ± 0.05) × 10-10, (2.2 ± 0.1) × 10-10, (2.9 ± 0.1) × 10-10, (0.56 ± 0.05) × 10-10, (1.16 ± 0.08) × 10-10, and (1.8 ± 0.1) × 10-10 cm3 molec-1 s-1, respectively. Substituent factors of F(-Si(CH3)2OR) and F(-SiCH3(OR)2) are proposed for use in AOPWIN, a common model for OH radical rate constant estimations. Cl atoms can remove percentage levels of VMS globally with potentially increased importance in urban areas.

Assessment of Gas–Particle Phase Distribution and Source Apportionment of Atmospheric Phthalate Esters and Volatile Methyl Siloxanes in Vietnamese Micro-Environments

In this study, the gas–particle distribution characteristics and emission sources of phthalate esters (PAEs) and volatile methyl siloxanes (VMSs) were evaluated for indoor air samples collected from different micro-environments such as homes, offices, kindergartens, hair salons, laboratories, and cars in 4 cities and provinces of Hanoi, Bac Ninh, Thai Binh, and Tuyen Quang, northern Vietnam. In general, total concentrations of PAEs and VMSs were higher in gas phase as compared to particle phase; however, phase distribution profiles of individual compounds were strongly related to their structures and physicochemical properties. For examples, low-molecular-weight compounds such as dimethyl phthalate, diethyl phthalate, D3, D4, L4, and L5 were more abundant in gas phase, while heavier compounds like di(2-ethylhexyl) phthalate and L8 were preferentially associated with particle phase. Assessment of PAE emission sources is relatively difficult because they have been applied in different consumer products and materials. Significant correlation between cyclic VMSs (e.g., D4, D5, and D6) was observed, suggesting their applications in cosmetics and personal care products.&#x0D; Keywords: Phthalate esters, volatile methyl siloxanes, indoor air, phase distribution, source apportionment.&#x0D;

Indoor levels of volatile organic compounds at Florentine museum environments in Italy.

Indoor Air Quality monitoring in cultural institutions is of particular concern to protect these places and the cultural heritage content. An indoor monitoring campaign was performed in three museums in Florence (Italy) to determine the occurrence and levels of volatile organic compounds (VOCs). VOCs of interest included BTEX (benzene, toluene, ethylbenzene, xylenes), terpenes, aldehydes, organic acids, and cyclic volatile methyl siloxanes (cVMS). The most abundant VOCs in all samples analyzed were BTEX, which were strictly related to the traffic source, followed by siloxanes and terpenes. Among BTEX, toluene was always the most abundant followed by xylenes, ethylbenzene, and benzene. cVMS in exhibition rooms with the presence of visitors showed higher values compared to samples collected when the museums were closed. Terpenes showed not only the influence of vegetation-biogenic sources surrounding a museum but could also be related to the wood used for the construction of showcases and furniture and the use of cleaning products. Data obtained also showed the presence of organic acids and aldehydes whose source can be traced back to exhibits themselves and wood-based furniture. Assessing the levels of organic acids in museums is important because, over time, it can cause deterioration of the artifacts.

A review on biogas purification through hydrogen sulphide removal

Biogas has been recognized as a clean and renewable form of energy that is produced from biodegradable organic materials via an anaerobic digestion. In fact, biogas has been well expected to substitute current conventional sources of energy. The main composition of biogas includes methane (CH4) and carbon dioxide (CO2), which is accompanied by different contaminants in varied quantities such as ammonia (NH3), water vapour (H2O), hydrogen sulphide (H2S), methyl siloxanes, nitrogen (N2), oxygen (O2), halogenated volatile organic compounds (VOCs), carbon monoxide (CO) and hydrocarbons. Biogas has been widely utilized as either engine fuel or starting material for chemicals, hydrogen and/or synthesis gas productions. For certain applications, a certain purity degree of biogas is required. Technically, the presence of trace components in biogas has been known to negatively affect engine performance. Therefore, a removal of contaminants, particularly H2S and CO2, is expected to significantly improve biogas quality, thus enhancing its performance in various applications. Besides, the removal is performed to meet standardized gas specifications for its usage as vehicle fuel or being injected into a natural gas grid. In practices, different methods for biogas cleaning and upgrading have been recognized, in which differ in terms of functions, efficiency, and required quality of input gas. This study, therefore, aims at reviewing various H2S removal-based biogas cleaning techniques during and after digestion process.

Comparative assessment of two biotrickling filters for siloxanes removal: Effect of the addition of an organic phase

Biogas produced at wastewater treatment plants and landfills contains trace levels of volatile methyl siloxanes (VMS) that are responsible for abrasion, corrosion and erosion of equipment during biogas storage and combustion. This research comparatively evaluated the removal of the most common VMS (L2, L3, D4, and D5) under aerobic conditions in a conventional biotrickling filter (BTF) and a two-phase partitioning BTF (TP-BTF) with silicone oil (at 30%) as organic phase. The TP-BTF showed a superior performance compared to the conventional BTF, increasing the total VMS removal from <30% in the BTF up to ∼70% in the TP-BTF. The highest REs in the TP-BTF were recorded for D4 and D5, reaching values of 80–90%, corresponding to ECs between 0.12 and 0.17 g m−3.h−1. Slightly lower values were obtained for L3 (70–80%), and the lowest performance was recorded for L2 (20–60%) due to the high vapor pressure of this siloxane and therefore its lower affinity by the organic phase. Surprisingly, despite the different inocula used, a similar microbial community was found by the end of operation of both BTFs, with KMBC-112, Reynarella and Chitinophaga as the dominant genera.

Investigation of volatile methyl siloxanes in biogas and the ambient environment in a landfill

Landfill biogas is a potential alternative for fossil fuel, but the containing impurities, volatile methyl siloxanes (simplified as siloxanes), often cause serious problems in gas turbines when applied to generate electricity. In this research, a collecting and analyzing method based on solvent adsorption and purge and trap-gas chromatography-mass spectrometry was established to determine the siloxanes in biogas from a landfill in Jinan, China, and adjacent ambient samples, such as soil, air, and leachate of the landfill. The results showed that, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) accounted for 63% of total siloxanes; and without considering D4 and D5, the order of detected siloxanes in concentration was found relating to Gibbs free energies of molecules, namely that higher abundant siloxane (except for D4 and D5) usually had lower Gibbs free energy. Additionally, the mass ratio between D4 and octamethyltrisiloxane (L3) in the biogas varied with different garbage age in landfills, possibly revealing the breaking-down of larger siloxane molecules with time. The samples, which were collected from environmental samples adjacent to the landfill, such as soil, water, and air, presented much higher siloxane level than urban or rural area away from landfills. The current H2S scrubber of the landfill biogas could decrease the total siloxanes from 10.7 to 5.75 mg/m3 due to Fe2O3 and a refrigerant drier in a purification system and cyclic siloxanes were more easily removed than linear ones.

Tube-type passive sampling of cyclic volatile methyl siloxanes (cVMSs) and benzene series simultaneously in indoor air: uptake rate determination and field application.

Cyclic volatile methyl siloxanes (cVMSs) and benzene series compounds have attracted increasing attention because they are associated with various adverse health effects. In this study, we developed and validated a novel passive air sampling method (PAS), using Tenax-TA as the sorbent, to measure the concentrations of cVMSs (D4, D5, and D6) and benzene series compounds simultaneously in indoor air. The uptake rates (R), which were calibrated side-by-side by an active air sampling method (Tenax-TA-AAS), ranged from 0.10 to 0.56 mL min-1 depending on chemicals, and the R of the TD tube with Tenax-TA was controlled by the sampler material-side and not the air-side. The passive sampling efficiency (PSE) of styrene and m,p-xylene was close to 1. Based on Pearson correlation analysis, a negative correlation was found between the molecular weight of compounds and their uptake rates. Furthermore, the calibrated Tenax-TA-PAS method was used to quantify cVMSs and benzene series compounds in a living room and an office environment in the urban area of Dalian, China. The concentrations of toluene were the highest in both the living room (10.4 μg m-3) and office (7.02 μg m-3) among the target compounds.

Toxicity identification and evaluation of palm oil mill effluent and its effects on the planktonic crustacean Daphnia magna

Palm oil mill effluent (POME) contains complex and highly biodegradable organic matters so discharging it without appropriate treatment may lead to environmental problems. POME final discharge quality is normally determined based on conventional chemical detection such as by biological oxygen demand (BOD) and chemical oxygen demand (COD). The novelty of the present study is that the toxicity effects of the POME final discharge samples were evaluated based on whole effluent toxicity (WET) and toxicity identification evaluation (TIE) tests using Daphnia magna. The toxicity unit (TU) values were recorded to be in the range from TU = 1.1–11 obtained from WET, and the TIE manipulation tests suggested that a substantial amount of toxic compounds was contained in the POME final discharge. Phenol, 2,6-bis (1,1-dimethylethyl)- and heavy metals such as Cu and Zn were detected in all the effluents and were recognized as being the main toxicants in the POME final discharge. GC/MS analyses also successfully identified cyclic volatile methyl siloxanes; cyclotetrasiloxane, octamethyl- (D4), cyclopentasiloxane, decamethyl- (D5), cyclohexasiloxane, dodecamethyl- (D6). D4 was detected at 0.0148–0.0357 mg/L, which could be potentially toxic. The palm oil industry used only water in the form of steam to process the fruits, and the presence of these compounds might be derived from the detergents and grease used in palm oil mill cleaning and maintenance operations. An appropriate treatment process is thus required to eliminate these toxicants from the POME final discharge. It is recommended that two approaches, chemical-based monitoring as well as biological toxicity-based monitoring, should be utilized for achieving an acceptable quality of POME final discharge in the future.

Biogas purification through membrane bioreactors: Experimental study on siloxane separation and biodegradation

Sewage biogas valorization to different energy applications is hampered by the presence of volatile methyl siloxanes. Despite the high operating costs, adsorption onto activated carbon is the most implemented technology for siloxane removal from biogas. In order to purify biogas sustainably, the current work explores the diffusion of siloxanes (octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane) together with other biogas impurities (limonene, toluene and hexane) through polydimethylsiloxane membranes. Abiotic tests revealed transport efficiencies above 75% towards a clean air stream for most compounds, although the transport of the most hydrophobic pollutants was challenged when water was circulated through the shell side of the membrane. Moreover, the performance of a hollow-fiber membrane bioreactor, inoculated with anerobic active sludge, was evaluated towards biogas purification in anoxic conditions. Toluene and limonene were successfully degraded, hexane’s removal efficiency was positively correlated with the residence time, and siloxanes removal was achieved up to 21%. CO2 was detected in the outlet gas as the mineralization product as well as some byproducts from the degradation of limonene and siloxanes. The presence of 1% of O2 in the gas, as a strategy to substitute NO3−, efficiently supported high removal for volatile organic compounds and moderate for siloxanes, which would ultimately reduce the operating costs of the technology.