Biomass Conversion and Biorefinery https://doi.org/10.1007/s13399-023-04478-1 REVIEW ARTICLE Adsorptive removal of siloxanes from biogas: recent advances in catalyst reusability and water content effect Seyed Alireza Vali1 · Javier Moral‑Vico1 · Xavier Font1 · Antoni Sánchez1 Received: 22 March 2023 / Revised: 29 May 2023 / Accepted: 6 June 2023 © The Author(s) 2023 Abstract The valorization of biogas as a renewable energy source faces a major obstacle regarding its purification. Siloxane is one of the impurities that cause problems such as damages to equipment of combustion engines, turbines, and boilers used for biogas conversion to heat and electricity. In this review, adsorption for siloxane removal is widely discussed, with two specific approaches: adsorbents sensitivity to water and regeneration, two essential points for industrial application. Thus, determining factors in adsorbents capacity, reusability, and water tolerance including textural properties, surface functional groups, and hydrophobicity are deeply analyzed. Studies oriented to the optimization of traditional adsorbents such as activated carbon, silica gel, and aluminosilicates as well as newly emerging adsorbents such as metal organic frameworks, graphene oxides, and waste-derived materials are studied in detail in terms of reusability and water tolerance. Although activated carbon is commercially used, its low selectivity, pore blockage due to siloxane polymerization, and unsuccessful regeneration make it disadvantageous. Silica gel, however, shows better reusability as a result of less adsorbent-adsorbate dissociation energy. In addition, aluminosilicates, despite its low adsorption capacity, proved to be more practical for real biogas due to their high hydrophobicity. Graphene oxide cost and energy efficiency in their synthesis make them more industrially appealing candidates despite their low adsorption capacity. Finally, metal organic frameworks demonstrated high selectivity, high adsorption capacity, and more efficient regeneration and therefore have more advantages and less drawbacks, although the number of published studies is still limited. Keywords Renewable energy · Biogas upgrading · Siloxane removal · Adsorption · Activated carbon · Metal organic frameworks (MOF) · Graphene oxide 1 Introduction and finally removing other impurities. Upgrading process simply refers to the separation of methane from carbon diox- Biogas valorization and utilization as a renewable source of ide to obtain high methane-enriched biogas which is the so- energy has increasingly captured the worldwide attention called biomethane [6]. However, biogas upgrading systems over the last decades towards a lesser dependence on fossil faces challenges such as the massive digestate produced that fuels [1–3]. However, its use has always had its own chal- will add more complication in terms of land use and also lenges due to the presence of undesirable substances such as the consequent greenhouse gas emissions released from the carbon dioxide, H ­ 2S, siloxanes, H ­ 2O, ­NH3, ­N2, and so forth digestate storage, transport, and manipulation. This chal- [4, 5]. Biogas treatment includes its purification and upgrad- lenge can be overcome by integrating some strategies into ing process. Purification process typically includes firstly the anaerobic digestion such as the use of biochar, which drying by dewatering, and then removing hydrogen sulfide, will lead to a carbon zero-emission to the environment. Also, the biochar produced from digestate can be easily stored, transported and exploited for soil amendment [7, 8]. In * Antoni Sánchez addition, other strategies have been investigated to facilitate antoni.sanchez@uab.cat biogas upgrading in terms of methane content enrichment 1 Composting Research Group (GICOM), Department and simultaneously removing some potential biogas impu- of Chemical, Biological, and Environmental Engineering, rities such as hydrogen sulfide [9]. Siloxane is one of the Universitat Autònoma de Barcelona, (Barcelona), biogas impurities that brings about various obstacles such as 08193 Bellaterra, Spain 13 Vol.:(0123456789) Biomass Conversion and Biorefinery corrosion or coating, as well as damages to some equipment neither will lead to a complete removal nor would be eco- such as combustion engines, turbines and boilers used in the nomically feasible. Schweigkofler and Niessner’s experi- process of valorization and conversion to heat and electricity ments on the removal of siloxane using condensation at 5 when biogas is used as a substitute for natural gas in grids or °C concludes with this statement as their results only dem- fuel cells [3, 10, 11]. Although the concentration of silox- onstrated less than 15% siloxane removal from biogas [18, ane in biogas does not exceed 0.4–0.5 mg/L [12], more and 31]. For this reason, in industrial application, this method more attempts have been made for its elimination due to its is implemented together with other techniques as adsorp- adverse effects on the equipment, which imposes high costs tion. Biological removal of siloxane is another method of maintenance and frequent equipment replacement [12, for siloxane removal, which is based on the fact that VMS 13]. There are two main types of siloxanes including poly (volatile methyl siloxanes) can be degraded by a number dimethyl siloxanes (PDM) and methyl siloxanes which are of microorganisms such as Arthrobacter, Agrobacterium, volatile (VMS). VMS is the principal siloxane that ends up Fusarium oxysporum, Methylibium sp., Pseudomonas sp., in biogas as a result of PDM hydrolysis and the volatiliza- Phyllobacterium myrsinacearum, Rhodanobacter and Xan- tion of a part of VMS available in the digester during the thomonadaceae as VMS are an appropriate carbon source for anaerobic digestion process [14–16]. VMS falls into two such microorganisms [22, 32–34]. Biotrickling filters (BTF) categories: linear and cyclic. Each category of linear and are the reactors typically used for the biological removal cyclic also has different types, which will be discussed later. of VMS. However, most of the reports demonstrated either Besides, biogas composition in terms of siloxane content low removal efficiencies or long residence times. While varies mainly because of the different digested organic sub- siloxane removal methods such as activated carbon adsorp- strates [17]. Thus, the selection of the methods for siloxane tion, phosphoric acid absorption, membrane separation, and elimination is challenging and needs to be carefully carried water scrubbing are widely used at industrial scale for VMS out according to the typology of siloxanes found in biogas. removal, biological methods have been principally examined To date, there have been various methods and technolo- at laboratory scale [35, 36]. Thus, the performance of such gies developed for the separation and removal of siloxanes biological systems needs to be enhanced in order to justify from biogas including absorption, adsorption, cryogenic their feasibility. Another technique for siloxane removal condensation, biological removal, and membrane separa- from biogas is membrane separation system that provides tion [13, 18–22]. Absorption of siloxanes can occur either high surface area and occupies low space. Membrane separa- physically or chemically. Physical absorption of biogas tion system is known to have low energy requirements and components are based on the solubility difference of the high efficiency. In addition, the membrane used for siloxane components in the solvent without any chemical reaction removal is normally of high selectivity for siloxane, and the [23–26]. Thus, in order to eliminate a specific component resultant methane purity is remarkably high, which means from biogas, the appropriate solvent needs to be selected. that not only less energy is needed but also less methane Siloxanes are hydrophobic and non-polar and therefore is lost in this system [20, 31]. However, while membrane cannot be absorbed by polar absorbents like water. Hence, technology has been well developed for C ­ O2 removal, its uti- organic solvents with long carbon chains have been tradi- lization for siloxane removal is limited. Besides, due to high tionally used for the physical absorption of siloxanes [27, investment and operational costs, the membrane technology 28]. A significant drawback regarding physical absorption for siloxane removal is still in the research phase. On the is that the siloxanes will strip from the solvent at elevated other hand, interfering compounds existing in biogas such as gas flow rates due to their high volatility. Nonetheless, when ­H2S and water vapor as well as oil vapors from engines can siloxanes go through chemical absorption, they can be con- cause significant damage to the membranes, in addition to verted to components with lower volatility, and therefore, the the problem regarding their pore blockage and fouling due to problem of their stripping from the solvent at higher gas flow the siloxane itself. Consequently, pretreatment of the biogas rates can be minimized. On the other hand, siloxanes dem- would be essential as well as the fact that membranes need onstrate extreme stability against chemical and biochemical to be renewed periodically [20, 37]. degradation, and only strong acids and bases can be useful Adsorption is more commonly used for siloxane removal for catalyzing the cleavage of Si-O bonds of siloxanes [2, due to advantages such as cost-effectiveness and easy opera- 29]. However, strong acids and bases cannot be practically tion, among others [19, 38, 39]. To date, there have been used because of the consequences in terms of large amounts several adsorbents investigated and developed for siloxane of precipitated carbonates formed in the presence of ­CO2 removal including activated carbon, silica gel, and alumi- in case of bases and potential corrosion in case of acids [2, nosilicate adsorbents as traditional ones [40–44]. However, 18, 30]. Condensation is another method that at low tem- a great effort of research has been made during the last two peratures and pressures makes the removal of some of the decades to find out more efficient adsorbents to optimize biogas impurities feasible. However, such removal technique the process and to overcome the limitations encountered 13 Biomass Conversion and Biorefinery traditional adsorbents. Particularly, activated carbon and for their synthesis [57–59]. Finally, waste-derived materi- silica gel have been two of the most commonly used adsor- als and their application in adsorption processes have also bents for siloxane separation from biogas. Activated carbon, gained interest [45, 60, 61]. Taking advantage of waste thanks to its large surface area and therefore high adsorption material and their recycling not only alleviates the envi- capacity, low cost, availability, and stability, has been used ronmental burdens of waste management and disposal and commercially [41, 42, 45]. However, some drawbacks such environmental consequences, but also eliminates the need as low selectivity for siloxane due to its affinity for meth- for producing raw materials in the framework of circular ane adsorption and its probable pore blockage due to the economy. For instance, activated carbon and carbon nano- siloxane polymerization phenomenon have been reported, a tubes were successfully synthesized from lignocellulosic process that is favored by the biogas water content [40, 43]. biomass waste, with high specific area and pore volume, Conventionally, this adsorption system is accompanied by which can be further tested for the adsorption applications a condensation process as a pretreatment to eliminate the [62, 63] . Regarding siloxane adsorption, a porous silica was water content to enhance its efficiency. Silica gel, as opposed synthesized using the residual sand from a WWTP [61], to activated carbon, demonstrated better reusability thanks a waste wood-derived biochar was utilized for the adsorp- to the weak adsorbent-adsorbate forces dominating the tion of siloxane [45], a lignocellulosic waste generated in adsorption mechanism [31, 46]. Researchers also managed food and wood industry was tested for siloxanes L2 and D4 to achieve higher adsorption efficiencies and less sensitivity adsorption [64], and a biochar modified with iron was evalu- of silica gel to water molecules in biogas by manipulating ated for L2 siloxane adsorption [65]. Therefore, adsorbents the effective factors of the adsorbent such as textural and derived from waste that have been used for siloxane removal surface properties by means of the modification of the adsor- will be also reviewed. bent synthesis and post treatment process of its preparation This review is intended to focus on the adsorptive [44, 46, 47]. Zeolites and mesoporous aluminosilicates are removal of siloxane from biogas and discuss the recent another category of adsorbents that have been used for silox- advances regarding the adsorption process including the ane separation from biogas [48, 49]. Although they did not optimization of traditional adsorbents as well as the recent show good performance in terms of adsorption capacity appearance of emerging adsorbents such as graphene oxide, compared to activated carbon, they proved to be competent metal organic frameworks (MOFs), and waste-derived mate- in terms of sensitivity to biogas water content, due to their rials. Specifically, published studies are critically reviewed high hydrophobicity. Furthermore, the manipulation of their in terms of regeneration of the adsorbents used as a factor preparation procedure by increasing aluminum dopant and that determines the adsorbents feasibility at the commercial calcination temperature had a positive effect on their pore or industrial scale. In addition to this, a specific focus on the size and therefore efficiency [46, 49]. The effect of biogas investigation of surface properties of the adsorbents such as water content on siloxane removal will be also studied in porosity, hydrophobicity, and surface functional groups is detail in this review. More recently, having unique character- presented. These parameters are the most important as far istics such as pore size and shape tunability and high surface as the adsorption capacity and reusability are concerned. area [50–52], metal organic frameworks (MOFs) have also The significance of this review is as following: the regenera- been studied as novel adsorbents for siloxane removal and tion of the adsorbents is essential as far as their industrial have demonstrated higher adsorption capacity and selectiv- application is concerned; therefore, in this review, all the ity compared to conventional adsorbents [53–55]. Addition- adsorbents are scrutinized and compared among them in ally, their regeneration is reported to be quite successful as terms of reusability considering the features of the adsor- opposed to activated carbon and silica gel, a phenomenon bents that affect and determine their reusability. These are that is attributed to the lack of polymerization of siloxane the bonding forces between the adsorbate and adsorbent onto the adsorbent surface [56]. In the most recent stud- that affect the dissociation energy of the adsorbate from the ies, graphene oxide, an emerging material for environmen- adsorbent surface, as well as issues like polymerization of tal applications such as energy storage and separation or siloxane in the adsorbents pores that leads to pore blockage water treatment, has gained attention for siloxane removal. and hampers the regeneration process. On the other hand, Reduced graphene oxide aerogels (rGOA) have been the the adsorbents are compared and evaluated in terms of their most used graphene oxides investigated for siloxane adsorp- sensitivity to water, since moisture is one of the unavoidable tion. Although the number of the studies in this regard is components of biogas, and the adsorbents that lose their few and they are not completely developed, rGOAs have adsorption capacity in its presence are indeed questionable shown high efficiency in terms of reusability in addition to with real biogas. In this sense, they will not be as practical their operation simplicity and the low amount of energy used and efficient as the adsorbents the performance of which is 13 Biomass Conversion and Biorefinery not significantly affected in the presence of water due to their removal technologies since the studies and developments on hydrophobicity. Hence, this work will help the trends for this issue have been largely reviewed [10, 69, 70]. the next studies to take into consideration to develop more industrially applicable adsorbents. 2.2 Siloxane and its effects on biogas valorization To take advantage of biogas, it must be converted to heat, 2 Biogas sources and its siloxane content steam, or electricity or used as a gas fuel substitute in vehi- cles. For these purposes, biogas needs to be used in dif- 2.1 A short review on the wide variety of biogas ferent types of equipment such as combustion engines, gas sources turbine engines, boilers, hot water systems, process heaters for combined heat and power units (CHP), and fuel cells. Biogas refers to a water-saturated gas that is generated However, as shown in Table 1, the existing impurities in as a result of a biological process by means of micro- biogas if not eliminated can cause damage to such equip- bial organisms on biodegradable organic substrates under ment, and therefore they hamper the feasibility of biogas val- anaerobic conditions. It is mainly generated during the orization process in economical and industrial applications. anaerobic digestion of biodegradable wastes such as food, One of these disruptive components in biogas is siloxanes. manure, municipal, and agricultural wastes in the anaero- Siloxanes are used as additives to cleaning agents, seal- bic digestion bioreactors and landfills [17, 66]. Developed ants, and pharmaceutical and cosmetic products as well as countries implement advanced large-scale biogas plants personal care products such as hairstyle products, lipsticks, to take advantage of biogas. Biogas is regularly applied creams, and deodorants [16, 71]. Consequently, they appear to generate heat, power, and electricity. In addition, many in wastewater and wastewater treatment plants and landfills industrial applications for its use in biogas plants as a [68]. Siloxanes can be categorized into two types: (1) PDM, substitute to natural gas are being progressed [67]. The which has higher molecular weight and low vapor pressure composition of biogas is highly influenced by the type that makes it non-volatile, as opposed to (2) methyl silox- of substrate and how the anaerobic process is designed. anes, which have lower molecular weight and are known For instance, the biogas resulting from the biodigesters as VMS [14, 15]. For example, owing to their characteris- of agricultural residues will have a composition different tics mentioned above, for siloxanes present in wastewater from that of a landfill gas recovery system or wastewater streams, a part of VMS is volatilized and leaves the liquid treatment plants (WWTPs) [3]. This will obviously affect stream, and the remaining part is adsorbed onto solid par- the biogas upgrading procedure, which is the process to ticles and solubilized in liquid medium and, consequently, obtain biomethane, removing ­CO2 and the rest of impuri- ends up in the digester. On the other hand, more than 90% ties form biogas. of PDM are not volatilized and will be mainly adsorbed on As mentioned, biogas contains 60–70% methane and the surface of solid particles and carried out with the sludge 30–40% carbon dioxide and trace amounts of impurities to the digester [14, 15]. After the anaerobic digestion pro- [11, 68]. Many studies have been conducted to eliminate cess, VMS will be released to the biogas stream due to the carbon dioxide content of biogas, towards biogas upgrading. hydrolysis of PMD as well as direct volatilization of a part This process is typically carried out through technologies of VMS that has already managed to enter the anaerobic such as physical and chemical scrubbing, pressure swing digester through adsorption onto solid particles [16]. Fig- adsorption, separation by membrane, and cryogenic separa- ure 1 graphically depicts the siloxanes fate in a wastewater tion [10]. This review will not go further in carbon dioxide treatment plant (WWTP) or in a landfill and their occurrence Table 1  Biogas main impurities and their effect on the energy recovery equipment Impurity Impact Siloxanes The irreversible decomposition reaction of siloxanes to silica and its deposition and coating the equipment Hydrogen sulfide (­ H2S) Having corrosive effect on the equipment,: hydrogen sulfide as well as its oxidation products such as ­SO2 and ­SO3 and ­H2SO4 Carbon dioxide (­ CO2) It diminishes the caloric value of biogas and have also a small corrosive effect due to carbonic acid formation and adverse effect in the alkaline fuel cells Ammonia ­(NH3) Its reaction with water leads to bases that can corrode the equipment; releases ­NOx in combustion process; it has s negative effect on fuel cells Oxygen ­(O2) Possibility of explosion Nitrogen ­(N2) Decrease the caloric value of biogas 13 Biomass Conversion and Biorefinery Fig. 1  Siloxane fate from water and wastewater treatment plants and landfills and its occurrence in biogas. VMS, volatile methyl siloxane; PDMS, polydimethyl- siloxanes; WWTP, wastewater treatment plants in biogas. Hence, siloxanes existing in biogas are mainly the paper, one of the major attempts in published studies VMS, which are either linear or cyclic. Table 2 shows the has been the manipulation of the textural properties of the main types of VMS existing in biogas [19]. adsorbents such as pore size and pore volume to enhance The irreversible decomposition reaction of siloxanes to the adsorption capacity of the tested adsorbents. However, silica has been reported to damage the existing facilities adsorbents also encounter a number of process-related obsta- since silica deposits and coats devices such as valves, spark cles. For example, a pretreatment step for biogas can be nec- plugs, and compressors, which results in shortening the essary to eliminate the impurities including water, which lifetime of the engines and an increase in the maintenance will disrupt the performance of some adsorbents. Another costs [13]. The oxidation reaction of siloxanes to form sili- issue is the heat produced due to the intrinsic exothermic con dioxide ­(SiO2) is presented in Eq. 1: nature of the adsorption process, which will add complica- tion to the adsorption process schemes. Another possible ((CH3)22SiO)n + 4nO2 → nSiO2 + 2nCO2 + 3nH2O (1) challenge regarding the development of manufacturing of Taking into account such obstacles caused by the pres- adsorbents is to keep a good balance between reducing the ence of VMS in biogas, its removal is essential for further particle size for the enhancement of intra-particle diffusion utilization of biogas as a renewable source of energy. In the kinetics and increasing the particle size to limit the pres- following section, adsorption will be discussed as a promis- sure drop [78]. Hence, more attempts need to be made to ing technology for VMS removal from biogas. alleviate the challenges and obstructions that inevitably face the adsorbents. Decontamination of biogas from siloxane by means of adsorption has been investigated to date. Since the characteristics and concentration of siloxane in biogas vary 3 Adsorptive removal of siloxane depending on the source of the waste stream [19, 31], a uni- versal adsorbent does not exist. Thus, depending on the type Adsorption can be defined as the change in concentration of siloxane, the adsorbent needs to be selected and further of a substance at the interface with the neighboring phases used. The traditional adsorbents include activated carbon, and it falls into four categories of liquid-gas, liquid-liquid, silica gel, molecular sieves, activated alumina, and alkaline solid-liquid, and solid-gas depending on the type of phases oxides [19, 31, 79]. The most recent adsorbents are reduced in contact [72]. Having excellent properties such as good graphene oxide and metal organic frameworks (MOFs), cost-efficiency, high potential of regenerability, and a spe- which proved to be effective for the removal of specific types cific design and manufacture for a specific need, adsorption of siloxanes. The numerical results of the adsorption stud- process has been extensively used in environmental pro- ies, such as the adsorption capacity of the adsorbents, the cesses such as water and wastewater treatment [73, 74] and conditions of the adsorption process, and the regeneration gas separation and purification [75, 76], among the most results, are presented in Table 3 and Table 4. Additionally, important ones. Adsorption process occurs through physical the schematic mechanism of siloxane adsorption from biogas attraction of siloxane molecule to an available site on the is displayed in Fig. 2, whereas the common regeneration external and internal solid surface of the adsorbent. What system is shown in Fig. 3. determines the accessibility of siloxane molecules to the One of the significant issues that needs to be taken into internal adsorption surface is the pore size of the adsorbent account as far as adsorption experiments are concerned is [77]. That is the reason why, as will be discussed throughout the initial concentration of the adsorbate. Typically, it is 13 Biomass Conversion and Biorefinery Table 2  Principal siloxanes in biogas: physical state and chemical structure Physical state Siloxane Formula Structure at 20 °C, 101.3 kPa Hexamethyldisiloxane (L2) Si2-O6-(CH3)6 Liquid Octamethyltrisiloxane (L3) Si3-O2-(CH3)8 Liquid Decamethylterasiloxane (L4) Si4-O3-(CH3)10 Liquid Dodecamethylpentasiloxane Si5-O4-(CH3)12 Liquid (L5) Hexamethylcyclotrisiloxane Si3-O3-(CH3)6 Solid (D3) Octamethylcyclotetrasiloxane Si4-O4-(CH3)8 Liquid (D4) Decamethylcyclopentasiloxan Si5-O5-(CH3)10 Liquid e (D5) Dodecamethylcyclohexasilox Si6-O6-(CH3)12 Liquid ane (D6) observed in adsorption studies that the adsorption capacity in real situation. Typically, the concentration of siloxane in values increase as the initial concentration of the adsorbate real biogas is not more than 0.5 mg/L [5]. Hence, the studies in the solution is higher. However, in order to have repro- that carried out their experimentation using initial siloxane ducible results in terms of adsorption capacity in the real concentrations close to these values as shown in Table 3 and large scale situation, it is highly recommended that the present more realistic results. However, in case of the studies adsorption experiments are carried out with the initial con- that used extremely high initial siloxane concentrations and centration of the adsorbate in the same range as they are achieved high adsorption capacities, the reported results are 13 Biomass Conversion and Biorefinery Table 3  Adsorbents experimental conditions and main results Adsorbent Features Removed Reaction conditions Optimum Reference siloxane adsorption Specific Total pore Average Inlet con- Tem- Gas flow capacity surface area volume pore size centration perature rate (mL/ (mg siloxane/g ­(m2/g) ­(cm3/g) (nm) (mg/L) (°C) min) adsorbent) Commercial AC 2142 1.52 - D4 1000 25 200 1732 [41] (MWV-2) 1.45 897 CuO-modified AC 666.52 0.34 1.3 D4 2 25 100 495 [80] Porous silica 734 0.45 3.2 D4 500 25 20 686 [46] Acetylated SG 369 0.86 13.8 L2 83.8 0 50 304 [47] D4 7.9 916 Methyl-functional- 1261.3 1.03 1.63 L2 83.8 0 50 346.4 [44] ized SG Polyethylene imine 538.5 0.73 4.70 L2 83.8 0 50 367 [81] regulated SG NaOH-reformed 500–600 - - D5 1.5 25 200 200 [82] silica Fe-BEA zeolite 600 - 0.75 D4 3 22 200 143 [83] BEA-38 zeolite 710 - 0.75 D4 3 22 200 135 [83] BEA-300 zeolite 620 - 0.75 D4 3 22 200 104 [83] UTC-15 zeolite 533 0.261 2.18 D4 2 25 5 104.5 [49] Amine-rGOA 167.9 0.737 3.83 L2 14.62 0 50 112.4 [57] Vitamin C-rGOA 137.9 0.88 5.08 L2 14.62 0 50 104.9 [58] β-cyclodextrin 163.5 0.68 5.93 L2 14.62 0 50 111.8 [59] modified-rGOA Citric acid- modi- 582.4 0.39 6.07 L2 38.3 0 10 188.3 [84] fied-rGOA MIL-101 MOF 2987 1.72 - D4 - 25 - 950 [56] Al-MOF fiber 293 0.1 - D4 0.45 25 Batch 8.42 [85] ZIF-71 969 - - D4 0.2 25 Batch 2.66 [54] Lignocellulosic L2 - 25 Batch 438 [62] waste-based AC 1668 0.7 - D4 512 Iron-modified 1068.43 0.58 - L2 83.8 20 50 356 [65] biochar AC activated carbon, SG silica gel, rGOA reduced graphene oxide aerogels, MOF metal organic framework, ZIF zeolitic imidazolate framework hard to be generalized to the real situation. Summarizing, in the adsorption of siloxane [86]. In addition to its high poros- the following sections, each types of adsorbents including ity and large surface area, activated carbon has shown high activated carbon, silica gel, aluminosilicates, rGOA, MOFs, adsorption capacity ranging from 100 to 1732 mg/g as well and waste-derived adsorbents will be elaborately discussed as cost efficiency, availability, and stability. However, one and compared in terms of adsorption efficiency, reusability, of the drawbacks of AC is its incapability for a selective and sensitivity to water, which are determined and affected removal of siloxane. For instance, it has demonstrated affin- by the adsorbents properties such as surface textural features ity toward the adsorption of methane, a highly negative phe- including pore size and volume and surface area, as well as nomenon. Another obstacle is its probable pore blockage due surface functional groups and hydrophobicity. to siloxane polymerization [19]. Eventually, the regeneration of the adsorbent has not been demonstrated to be efficient 3.1 Activated carbon enough, which makes the process economically weak [12]. In this regard, Gilson et al. [38] carried out the adsorption Activated carbon (AC) is an amorphous form of carbon that of L2-siloxane on a number of AC, and the results corre- has a surface area between 600 to 1600 ­m2/g. Depending on sponding to the best adsorbent confirmed a percentage of its activation or impregnation method, it can have an acidic adsorption capacity loss around 25–30% after three cycles or basic feature [19]. Having a non-polar nature, it favors of adsorption as shown in Table 4. 13 Biomass Conversion and Biorefinery Table 4  Adsorbents’ detailed Adsorbents Regeneration Reference information on the regeneration conditions and results Circumstances Cycles Adsorption capacity loss Porous silica Thermal treatment, 120 °C, 5 h 3 cycles 10% [46] Acetylated SG Thermal treatment, 110 °C, 2 h 5 cycles Negligible [47] Methyl-functionalized SG Thermal treatment, 80 °C, 3 h 4 cycles Negligible [44] Polyethylene imine regulated SG Thermal treatment, 100 °C, 2 h 15 cycles Negligible [81] NaOH-reformed silica Thermal treatment, 120 °C, 2 h 2 cycles Negligible [82] Fe-BEA zeolite Wet oxidation process 4 cycles 50% [83] Amine-rGOA Thermal treatment, 80 °C, 0.5 h 5 cycles Negligible [57] Vitamin C-rGOA Thermal treatment, 80 °C, 0.5 h 5 cycles Negligible [58] β-cyclodextrin modified-rGOA Thermal treatment, 80 °C, 0.5 h 10 cycles Negligible [59] Citric acid-modified-rGOA Thermal treatment, 100 °C, 1 h 5 cycles 5–10% [84] MIL-101 MOF Thermal treatment, 150 °C 3 cycles Negligible [56] Al-MOF fiber Thermal treatment, 200 °C, 4 h 1 cycle 30% [85] Iron-modified biochar Thermal treatment, 100 °C, 4.5 h 5 cycles Negligible [65] AC activated carbon, SG silica gel, rGOA reduced graphene oxide aerogels, MOF metal organic frame- work, ZIF zeolitic imidazolate framework Fig. 2  Schematic adsorption mechanism of siloxane from biogas Fig. 3  Schematic regenera- tion for the adsorbents used for siloxane removal 13 Biomass Conversion and Biorefinery Various factors impact the performance of siloxane the regeneration of SG was 95% effective under the same adsorption on AC, among which activated carbon surface conditions (250°C and 20 minutes) than that of AC [31]. As properties and biogas water content are the most significant revealed in this study, the characteristics of SG affecting its ones [40, 43]. For instance, it was reported that the adsorp- adsorptive efficacy are the specific surface area, pore vol- tion of siloxane suffered a tenfold decrease when the rela- ume, pore size, and hydrophobicity, which can be manipu- tive humidity increased from 50 to 70%, which was related lated to achieve better adsorption and regeneration perfor- to the competitive adsorption of water molecules into the mance. Following this strategy, Jafari et al. [46] attempted adsorbent sites [43]. For this reason, the adsorption process to regulate the textural properties of mesoporous silica by has been used together with a pretreatment process such as means of silica synthesis and post-treatment modifications refrigeration or condensation to reduce the water content and proved higher adsorption capacity (686 mg/g) for octa- and to prevent the probable saturation of AC. Besides, the methylcyclotetrasiloxane (D4) than that of commercial silica importance of adsorbent surface structure on adsorption (642 mg/g). The modified mesoporous silica also demon- efficiency can be accounted by the fact that the principal strated better regeneration stability as well as more resist- mechanism of adsorption is physical, which can be attrib- ance to humidity, a positive point due to the competitive uted to the hydrophobicity of both materials. In this sense, effect of water [43]. In another study, Liu et al. [47] acety- Yu et al. [40] investigated the effect of AC characteristics lated SG using acetic anhydrite, which showed enhancing on the adsorption capacity of D4-siloxane (octamethylcy- effects on the adsorption efficiency of SG, due to a signifi- clotetrasiloxane) and demonstrated that large micropores cant increase of micropore specific surface area, total pore and small mesopores (1.7–3 nm of diameter) were the most volume, and better hydrophobicity. Additionally, thermal favorable ones for D4 adsorption. Codony et al. [41] carried regeneration of the acetylated SG proved to be less energy out dynamic adsorption of D4 on twelve different ACs and consuming as observed in Table 4. Notably, they achieved an confirmed the significant relationship between the textural adsorption capacity of 304 mg L2/g and 916 mg D4/g at low properties of AC and the adsorption efficiency of siloxane. temperatures (Table 3). The best performance of the adsor- Additionally, they demonstrated that the humidity of biogas bent at lower temperatures was accounted for by the exo- adversely affects the adsorption capacity. However, the range thermic nature of micropores adsorption. They also demon- of adsorption capacity they achieved for these twelve acti- strated that the efficiency of methyl-functionalized SG was vated carbons were reported to be from 250 to 1732 mg/g as within 15 to 18 times higher than unmodified SG in terms shown in Table 3. Regarding the regenerability of AC, Tran of adsorption capacity of hexamethyldisilane (L2) [44]. Fur- et al. [42] investigated the role of surface chemistry on the thermore, it was also attempted to regulate the micropores regeneration ability of activated carbons and showed that the of SG using polyethylene imine, obtaining similar results ACs containing alkali metal cations are the ones favoring in siloxane removal [81]. Jung et al. [82] also proposed the the polymerization of siloxanes, which brings about the hin- surface modification of silica using NaOH for the removal drance of the regeneration of the adsorbent. Most recently, of D5. Although they obtained four times more adsorption Yang et al. [80] managed to enhance the adsorption capacity capacity than SG or unmodified silica, the regeneration of of AC up to 500 mg/g by the incorporation of copper oxide. the adsorbent was carried out only for two cycles with a Indeed, they demonstrated that the strong hydrogen bond is slight loss as shown in Table 4. responsible for the adsorption of siloxane on the CuO sur- face (C-H-Cu hydrogen bond). However, the regeneration 3.3 Aluminosilicate adsorbents study of the adsorbents was not carried out, which questions its further industrial applicability. Aluminosilicates are minerals mainly composed of alu- minum, silicon, and oxygen. Although all aluminosilicates 3.2 Silica and silica gel present the same basic chemical composition, the variety of atoms and molecular arrangements lead to different Silica gel ((SiO2).nH2O) (SG) is an amorphous porous mate- structures with different physico-chemical characteristics. rial as a result of polymerization of silicic acid using a strong Among the types of aluminosilicates, clays, zeolites, and acid like hydrochloric or sulfuric acid. The monomer of SG, mesoporous aluminosilicates stand out for their wide range silica ­(SiO2), has also been utilized for siloxane adsorption, of applications in catalysis, wastewater treatment, gas puri- but not as frequently as SG [18]. In fact, the weak adsorp- fication, and storage [48]. Among aluminosilicates, zeolites tion forces between SG and siloxane causes the desorption and mesoporous families have been used for the removal of siloxane molecules to require less dissociation energy of siloxane from biogas with the adsorption capacity rang- for regeneration, which makes SG a proper adsorbent for ing from 100 to 150 mg/g. The first work presented was by siloxane adsorption with the adsorption capacity ranging Ortega et al. [21] (2009) who investigated the adsorption from 200 to 916 mg/g. For instance, it was reported that performance of zeolite (type DAY 40) for the removal of 13 Biomass Conversion and Biorefinery siloxanes D4 and L2 and experimentally compared with AC for synthesis of silica, was reported to pass through extreme and silica gel. The results demonstrated that the adsorption pretreatments to be well prepared for the synthesis process. capacity of zeolite was quite lower than that AC, which was Contrarily, a waste wood-derived biochar was utilized with- attributed to its lower surface area (607 m ­ 2/g) compared to out any further modification for the adsorption of siloxane 2 AC (1220 ­m /g). However, zeolite, due to its high hydropho- D4 and compared with commercial AC in terms of adsorp- bicity, showed better performance in terms of less sensitivity tion capacity. The adsorption capacity of this biochar (3.5 to water as it had no capacity loss with levels of moisture mg/g) proved to be much lower than the commercial ones of 70%, a content that AC cannot tolerate [19]. Silica gel (37.5 mg/g). However, this shortcoming is supposed to also presented negative results due to its hydrophilic nature. be overcome through activation methods that improve the Nonetheless, no study of regeneration of zeolite was pre- physical properties of the adsorbent [45]. For instance, a sented. Later, Montanari et al. [87] studied NaX zeolite in lignocellulosic waste generated in food and wood industry comparison with AC and SG for the adsorption of D3 silox- was activated by different agents and then tested for silox- ane. The zeolite used was reported to show neither compa- anes L2 and D4 adsorption and compared with commer- rable adsorption performance nor any advantages compared cial AC [64]. Interestingly, it demonstrated an adsorption to the AC in terms of the regeneration process. Recently, capacity (438–512 mg/g) comparable to commercial AC. Caberra-Codony et al. [83] investigated the regeneration Notably, the temperatures at which the activation process of seven types of zeolites using a wet oxidation process of the waste-derived adsorbent was carried had a signifi- including ozonation and Fenton-like treatment. The results cant effect on the development of porosity and thus on the revealed that Fe-BEA zeolite had the highest efficiency of adsorbent adsorption performance. The range of temperature 143 mg/g as shown in Table 3. However, they only could used for the activation process was 800 to 900 °C, which achieve a complete regeneration of the zeolite Fe-BEA in the are costly. Most recently, Meng et al. [65] introduced the first cycle (Table 4); the recyclability of the adsorbent for the modification of biochar with iron and accomplished compa- successive cycles was unsuccessful due to the accumulation rable results in terms of adsorption capacity of L2 siloxane of C-containing products in the adsorbent. As pointed out from biogas (356 mg/g). Moreover, the pretreatment and before, textural properties of the adsorbents including pore activation process that was done in this study is less energy volume and size play a constructive role in the adsorbent consuming in terms of temperature (600 °C). In addition, the adsorption potential. In this respect, Jiang et al. [49], in an adsorbent remained effective when recycled 5 times with the attempt to obtain tunable textural properties, synthesized adsorption capacity practically stable as shown in Table 4. different mesoporous aluminosilicates manipulating the Anyway, more research is required to investigate more and aluminum dopant and the calcination temperatures, which different waste-derived adsorbents without energy-intensive proved to significantly affect the adsorbent pore size and treatments and yet having adsorption capacities comparable volume and, consequently, the adsorption efficiency. They to commercial or traditional adsorbents. In addition to this concluded that the volume of the mesopores and the exter- adsorption performance, regeneration of the adsorbent is nal surface area of the adsorbent improved as the aluminum essential to prove its industrial applicability. dopant ratio and the calcination temperature increased. In fact, the mesopores in this adsorbent are the space between 3.5 Emerging adsorbents the nanoparticles, and when more atoms of aluminum are added, they can hinder the aggregation of nanoparticles dur- 3.5.1 Graphene oxide ing the calcination process. Therefore, the volume of the space between nanoparticles (mesopores) will increase. Graphene oxides (GO) are monomolecular sheets com- posed of carbon, oxygen, and hydrogen. They have proved 3.4 Waste‑derived adsorbents to be a suitable candidate for diverse applications such as energy harvesting and storage, gas separation, water treat- It is worth mentioning that the utilization of waste-derived ment, catalysis, and biomedical applications [88–91]. Not- materials for different applications such as wastewater withstanding this, there has not been any reports of using treatment and gas purification such as siloxane removal has graphene oxide for siloxane removal until 2020; since then, recently appeared in literature [1, 60, 61]. few studies have been carried out on them for siloxane In this regard, Silva et al. [61] synthesized porous silica adsorption and achieved adsorption capacities ranging from using the residual sand from a WWTP. This waste-derived 104 to 190 mg/g. Recently, the adsorption of hexamethyld- adsorbent silica was used for removal of D4 and D5 silox- isiloxane (L2) by amine-reduced graphene oxide aerogels anes. However, the cost and energy intensity of the purifi- (rGOA) has been reported [57], and in comparison to GO, cation and modification process was a critical issue to be it demonstrated higher adsorption efficiency toward L2 considered. For instance, the sand waste, which was used due to a higher surface area and hydrophobicity (Table 3). 13 Biomass Conversion and Biorefinery Although the adsorption capacity of these adsorbents is However, as far as industrial application is concerned, relatively lower than that of some of the other adsorbents MOFs have shown a significant obstacle that is their poor mentioned before (Table 3), the critical point in the use of thermo-mechanical stability. To tackle such problems, such adsorbents is the simple and low energy intensive- recently, Pioquinto-García et al. [85] incorporated DUT-4 ness of their synthesis and regeneration. These authors also into polyacrylonitrile fiber (Al-MOF fiber). The results examined the usage of vitamin C as a more environmen- proved faster adsorption kinetics and the diffusion of adsorb- tally friendly reducing agent for the synthesis of rGOA and ate (D4) was roughly 3 times higher than that of DUT-4 obtained practically the same adsorption and regeneration powder. This means that MOF particles distribution on the efficiency [58]. Furthermore, in another attempt, they also fiber enhances the contact between the adsorbate and MOF took advantage of the cross linker β-cyclodextrin for its particles. Eventually, through an environmental assessment, abundant hydroxyl and amine groups for the modification the adsorbent preparation appeared to have less adverse envi- of the surface properties of rGOA. The adsorbent proved ronmental effects than those derived from the synthesis of to be consistent after 10 times of recycling while main- DUT-4 powders. Unfortunately, the Al-MOF fiber did not taining its adsorption capacity [59] (Table 4). However, show a good reusability performance, and after one regen- they achieved much higher adsorption capacity when they eration cycle, the adsorption capacity decreased by 30% attempted to modify the rGOA using citric acid, which (Table 4). In another study, Tiempos-Flores et al. [54] man- was pointed to be due to the improvement in the textural aged to enhance the hydrophobicity of zeolitic imidazolate properties (higher surface area) of the adsorbent and its framework (ZIF-71), which led to an improvement of the hydrophobicity. It is worth mentioning that this adsorbent adsorption capacity. In general, the adsorption capacities did not show as good reusability as the previous ones, but obtained with MOFs are not comparable to other typical better than some other adsorbents like AC [84]. adsorbents. ’Unfortunately, there are no systematic reports on the regeneration of MOF adsorbents, which is considered 3.5.2 Metal organic frameworks (MOFs) as a downside of MOFs as adsorbents. The authors highly recommend researchers to include the reusability studies to Metal organic frameworks (MOFs) are porous inorganic- their investigations in order to be able to establish the excel- organic hybrid constituents. Due to their particular features lence of MOFs at industrial scale for siloxane removal. such as high pore volume, high specific area, shape and pore tailorability, they have been exploited for many applications including biogas upgrading [53, 55, 92]. In this case, MOFs 4 Conclusions were mainly investigated for the separation of carbon diox- ide until recent years [53, 55]. Lately, researchers started Adsorption is a robust method for siloxane removal from the investigation of MOFs for the removal of siloxane. biogas and makes a valuable contribution to the biogas In 2013, for the first time, Mito-oka et al. [93] employed upgrading as the presence of siloxanes in biogas causes DUT-4 MOF for the D4 siloxane adsorption. They investi- damage to the equipment used for its conversion to elec- gated the adsorption of D4 in the presence of water, carbon tricity and heat. As discussed in this review, conventional dioxide, and methane in comparison with a conventional adsorbents such as activated carbon or silica gel need to be AC, and the results showed better selectivity and adsorption more investigated to be optimized in terms of adsorption capacity than that of conventional AC. This high efficiency capacity and especially reusability. However, this study did and selectivity were attributed to the strong hydrophobic- not aim to cover subjects such as comprehensive comparison ity of DUT-4 and its optimal chemical interactions with the of different methods and technologies for siloxane removal adsorbate molecules (D4). Later, chromium-based MIL-101 and elaborative discussion on studies regarding the source MOF was investigated for the adsorption of D4 by Gargiulo of biogas such as anaerobic digestion process. Furthermore, et al. [56] as shown in Tables 3 and 4. They demonstrated two main factors affecting the adsorbents regenerability high adsorption capacity of 950 mg/g and good reusabil- have been found: the adsorbate-adsorbent bonding and the ity. Contrarily to activated carbon and silica gel, adsorp- polymerization of siloxane in the pores of the adsorbent. tion of D4 on MIL-101 did not lead to a polymerization Thus, many studies are carried out to manipulate the surface of adsorbate on the adsorbent surface, which is critical for functional groups of the adsorbents towards the occurrence the successful reusability of the adsorbent. In addition, they of physical adsorption in which weak Van der Waals are achieved high adsorption capacity under ambient tempera- the bonding forces. In addition to this, water vapor typi- ture (Table 3). This is another important point, as traditional cally exists in the biogas composition, so the hydrophobic- adsorbents significantly lose their adsorption efficiency as ity degree of the adsorbents plays an essential role in the temperature increases and their best performance is limited adsorption efficiency of the siloxane. Activated carbon, to low temperatures (0 °C). due to its high surface area and availability, has been used 13 Biomass Conversion and Biorefinery commercially, but its low selectivity for siloxane and its This is important since the siloxane concentration in the real complicated regeneration due to the blockage of its pores biogas does not exceed 0.5 mg/L. Therefore, it is highly rec- because of siloxane polymerization question its utilization ommended to conduct adsorption experiments in the ranges and cost efficiency. In comparison to activated carbon, silica of siloxane concentrations close to the real biogas concentra- gel demonstrated better reusability due to the presence of tions to have more realistic results for larger and industrial weak forces between adsorbate and the silica gel surface. applications. Zeolites, despite their lower adsorption capacity compared to activated carbon and because of their high hydrophobic- Author contributions Seyed Alireza Vali: Writing, original draft; con- ity, were found more practical for siloxane adsorption from ceptualization; methodology; investigation; writing, review and editing. real biogas with high water content. GO adsorbents showed Javier Moral-Vico: Writing — review and editing. Antoni Sánchez: relatively lower adsorption capacities as well; however, the Writing, review and editing; supervision; project administration; fund- cost and energy efficiency of their synthesis and regenera- ing acquisition. Xavier Font: writing, review and editing; supervision; project administration; funding acquisition tion makes them more industrially appealing candidates. Eventually, metal organic frameworks (MOFs) proved to Funding Open Access Funding provided by Universitat Autonoma be promising for adsorption of siloxane due to their higher de Barcelona. This study was financially supported by the Spanish selectivity, high hydrophobicity, high adsorption capacity, Ministerio de Ciencia e Innovación in the call Proyectos de Tran- sición Ecológica y Transición Digital 2022. Squeezer project, ref. and more efficient regeneration owing to lack of the occur- TED2021-130407B-I00. rence of siloxane polymerization, contrarily to activated carbon, as well as maintaining high adsorption capacity at Data availability Not applicable. ambient temperature, an advantage that other adsorbents did not present. However, limited studies have been published Declarations for MOF utilization as adsorbents for siloxane removal. Ethical approval Not applicable The regeneration was also another issue that needs to be addressed in case of MOFs. Competing interests The authors declare no competing interests. Open Access This article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long 5 Future prospects and challenges as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes The concluding authors’ recommendation is that more focus were made. The images or other third party material in this article are is required to be put on the regeneration of adsorbents to included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in establish the industrialization of adsorbents for the removal the article's Creative Commons licence and your intended use is not of siloxanes. In addition, more research should be carried out permitted by statutory regulation or exceeds the permitted use, you will on the adsorbents derived from waste considering the world- need to obtain permission directly from the copyright holder. To view a wide increasing significance of circular economy. In this copy of this licence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/. respect, different potential wastes that have not been tested before for the production of adsorbents need to be evaluated. Furthermore, more attempts need to be made on decreasing the cost and energy consumption in the synthesis process of References waste-derived adsorbents as well as the optimization studies 1. 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