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Article

Enhancing the Methane Yield of Salicornia spp. via Organosolv Fractionation as Part of a Halophyte Biorefinery Concept

by
Aadila Cayenne
1,
Maxwel Monção
2,
Leonidas Matsakas
2,
Ulrika Rova
2,
Paul Christakopoulos
2,
Mette H. Thomsen
3 and
Hinrich Uellendahl
1,*
1
Faculty of Mechanical and Process Engineering and Maritime Technologies, Flensburg University of Applied Sciences, Kanzleistr. 91-93, 24943 Flensburg, Germany
2
Biochemical Process Engineering, Division of Chemical Engineering, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden
3
Department of Energy Technology, Aalborg University, Niels Bohrs Vej 8, 6700 Esbjerg, Denmark
*
Author to whom correspondence should be addressed.
Energies 2024, 17(5), 1074; https://doi.org/10.3390/en17051074
Submission received: 5 January 2024 / Revised: 14 February 2024 / Accepted: 16 February 2024 / Published: 23 February 2024
(This article belongs to the Special Issue Anaerobic Digestion in the Bioeconomy)
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Abstract

:
The present research investigated the effect of organosolv pretreatment on two species of salt-tolerant Salicornia spp. biomass, Salicornia dolichostachya and Salicornia ramosissima, for increasing biomethane production through anaerobic digestion. The final biomethane yield of de-juiced green fibers of Salicornia spp. from wet fractionation increased by 23–28% after organosolv treatment. The highest methane yield of about 300 mL-CH4/gVS was found after organosolv treatment with 60% v/v ethanol solution at 200 °C for 30 min, or at 180 °C for 30 or 60 min treatment time. Furthermore, the methane production rate increased significantly, reducing the time until 95% of the final methane yield was reached from 20 days to 6–10 days for the organosolv-treated biomass. This research shows that the process of anaerobic digestion of halophyte biomass benefits from cascade processing of Salicornia fibers in a biorefinery framework by sequential wet and organosolv fractionation for full utilization of halophytic biomass.

1. Introduction

Halophytic biomass is receiving particular attention for the valorization of coastal areas and salt-affected lands, as these salt-tolerant plants can germinate under saline conditions without hindering their seeds or biomass production. These plants contain valuable ingredients that can be used for food, feed production, medical applications, and high-value-added compounds [1,2,3]. Furthermore, the residual organic matter of the halophyte biomass can be used for renewable energy production in the form of biogas via anaerobic digestion (AD). However, AD of halophytes faces primarily two challenges: the high salt content that can inhibit microorganisms involved in the AD process, and the presence of lignin in the fibrous organic matter that reduces the rate and extent of hydrolysis in the microbial degradation process [4,5]. Lignocellulosic biomass comprises cellulose and hemicellulose bound to lignin by hydrogen bonds and ester–ether bridges, thus making the biomass recalcitrant in nature [6]. Depending on the biomass type and maturity stage, lignin constitutes up to 40% of lignocellulosic biomass and provides strength to plant cell walls and a physical barrier protecting carbohydrates from microbial degradation and is therefore considered the major factor of biomass recalcitrance [6,7,8]. Consequently, the breaking of the bonds between cellulose, hemicellulose, and lignin will increase the accessibility to cellulose for microbial degradation [8,9,10,11,12].
Green halophyte biomass, cultivated from inland and coastal salt marshes, generally has a lower lignin content (2–10%) [6] than other lignocellulosic biomass like agricultural and forest residues (15–30%) [8]. This makes green halophytic biomass more amenable to AD, with a higher degradability and higher methane yields. Nevertheless, the composition of halophytic biomass with respect to the lignocellulosic fiber material may vary depending on the species, location, and growing conditions [2,5].
This study applied a green biorefinery approach by implementing a cascade of fractionation processes, thus allowing for complete valorization of green Salicornia biomass (Figure 1). As an initial step, wet fractionation of the green succulent halophytic biomass into a de-juiced green fiber and a nutrient-rich juice fraction allows for the recovery of proteins and bioactive compounds in the juice fraction, as well as reducing the salt content of the green fiber fraction for further processing and bioconversion. The botanical extracts and bioactive compounds are then extracted from the juice fraction for pharmaceutical, food additive, and cosmetic applications [3]. Subsequently, organosolv fractionation is applied for fractionation of the fiber fraction into a cellulose-rich pulp fraction and an aqueous stream of hemicellulose and lignin [13,14]. While the availability of the cellulose in the pulp is enhanced for conversion into biogas by anaerobic microorganisms [15,16,17], the dissolved lignin and hemicellulose in the aqueous stream can be used as feedstock for other processes for the production of green energy, chemicals, and materials. Previous studies have shown that organosolv treatment of agricultural and forest residues could increase the methane yield of these lignocellulose biomasses by 15–179% [18,19,20,21,22]. The present research extends the application of organosolv fractionation to increase the methane yield of halophytic biomass.
In previous studies, organosolv fractionation of de-juiced green fibers of wild Salicornia dolichostachya harvested on the west coast of Denmark and of Salicornia ramosissima cultivated under controlled greenhouse conditions in Murosa, Portugal (Riasearch) was studied to establish a benchmark for high fractionation efficiency [5,23,24]. In the present study, the methane yield of the cellulose-rich pulp fraction of S. dolichostachya and S. ramosissima generated after organosolv pretreatment was investigated to assess the effectiveness of organosolv fractionation on improving anaerobic digestibility and enhancing biomethane production of halophytic biomass in a biorefinery framework.

2. Materials and Methods

2.1. Halophyte Plant Material and Residues Used for the Biogas Potential Tests

S. dolichostachya was harvested in autumn from the Wadden Sea on the west coast of Denmark near the island of Mandø. S. ramosissima was grown in sandy soil in a greenhouse equipped with an irrigation system with saline water effluent from a recirculating aquaculture system at Riasearch, Portugal.
The collected Salicornia biomass was rinsed with freshwater and then subjected to wet fractionation, using a screw press, to obtain a green fiber and a juice fraction. As outlined in [23,24], the de-juiced green fibers were dried at 95 °C for 24 h, milled, and subsequently underwent organosolv fractionation under different experimental conditions: temperatures: 160–200 °C, treatment times: 15–60 min, solvent concentrations: 40–70% v/v ethanol (EtOH).
From the screening of different organosolv pretreatment parameters, the pulp samples showing high delignification and high performance in enzymatic saccharification trials after organosolv treatment were selected to determine the biomethane potential (BMP) and anaerobic biodegradability, as seen in Table 1.

2.2. Biomethane Potential Test Set-Up

Batch assays for determining the biomethane potential (BMP) were conducted in triplicates of 0.5 L batch vials, using the Automated Methane Potential Test System II (AMPTS II, Bioprocess Control, Lund, Sweden AB) at mesophilic temperatures (37 ± 2 °C) and a substrate-to-inoculum ratio of 0.5, in compliance with the VDI standard [25]. For the inoculum, anaerobic sludge (2.3 wt% TS, 1.35 wt% VS) was obtained from an anaerobic digester of the Wastewater Treatment Plant in Flensburg, Germany, that operates at mesophilic conditions (38–42 °C). The batch vials were agitated for 10 min every hour using a mechanical stirrer. Batches with inoculum only were used to determine the methane yield of the sludge in order to determine the yield solely of the added substrate. Microcrystalline cellulose was used as a control to verify the activity of the inoculum. The initial pH of the batch assays was 7.8 ± 0.3, which stayed stable during the AD experiments. In addition, the average methane production rate of the batch assays was computed by dividing the cumulative specific biomethane yield at the time when the yield plateaued by the corresponding fermentation duration.

2.3. Biomass Compositional Analysis

The TS, VS, and ash contents were determined for the halophyte plant biomass according to the standard methods in [26] and [27], respectively. The TS content was determined by drying at 105 °C for 24 h. The VS content was measured by subsequent incineration of the TS at 550 °C for 3 h in a muffle furnace. The cellulose, hemicellulose, and lignin composition of the de-juiced green and pretreated fibers (post organosolv) of S. dolichostachya [23] and S. ramosissima [24] were analyzed by strong acid hydrolysis according to the National Renewable Energy Laboratory protocol for the determination of structural carbohydrates and lignin in biomass [28].

2.4. Elemental Analysis

Elemental analyses of carbon (C), hydrogen (H), nitrogen (N), and oxygen (O) were performed on the de-juiced green and pretreated fibers using a CHNS-O FlashSmart elemental analyzer (Thermo Scientific, Waltham, MA, USA). Elemental analyses were used to determine the theoretical biomethane potential (TBMP), C/N ratio, and crude protein content of the biomass samples. The crude protein (CP) content was calculated by a mass factor of 6.25 based on the elemental nitrogen content, assuming that all nitrogen is bound in proteins. The sodium (Na) content of the de-juiced S. ramosissima fibers and three selected pretreated fibers from different organosolv fractionation conditions was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) at Leibniz University, Hannover, Germany.

2.5. Theoretical Biomethane Potential

The theoretical biomethane potential (TBMP) of the de-juiced green and pretreated Salicornia fibers was determined based on its elemental composition of C, H, N, and O (TBMPCHNO) and on the organic fraction composition (TBMPOFC), i.e., the cellulose, hemicellulose, lignin, and crude protein contents. The theoretical methane potential TBMPCHNO based on the elemental composition (CcHhNnOo) was calculated from Equation (1) based on the stoichiometric formula by Buswell and Mueller [29], modified by Boyle [30]. The TBMPOFC was determined using Equation (2), based on the content of carbohydrates and proteins as described elsewhere [31], assuming negligible lipid content and with the lignin being considered undegradable under anaerobic conditions. The anaerobic biodegradability (BD) was calculated based on the ratio of the biomethane potential obtained from the experimental BMP tests to that of the TBMP based on CHNO and OFC, respectively (Equation (3)).
The TBMPCHNO based on the elemental analyses implies full conversion of all organic compounds into CH4 and CO2, regardless of their biodegradability under anaerobic conditions. In contrast, the calculation of TBMPOFC based on the different organic matter fractions takes into account that lignin is not degradable under anaerobic conditions.
TBMP CHNO   ( mL - CH 4 / gVS ) = 22400   ( c 2 + h 2 + o 8 + 3 n 8 ) 12 c + h + 16 o + 14 n
c: molar ratio of C, h: molar ratio of H, o: molar ratio of O, n: molar ratio of N in biomass formula CcHhNnOo.
TBMP OFC   ( mL - CH 4 / gVS ) = 415 · % C a r b o h y d r a t e s + 496 ·   % P r o t e i n s + 1014 ·   % l i p i d s
BD CHNO / OFC   ( % ) = B M P T B M P C H N O / O F C × 100

3. Results and Discussion

3.1. Biomass Composition

The TS, VS, and ash contents of the de-juiced green fibers (DGFs) and organosolv-pretreated fibers (OPFs) of S. dolichostachya and S. ramosissima biomass are shown in Table 2. After organosolv fractionation, the ash content of the pretreated S. ramosissima fibers reduced from 12 wt% in de-juiced green fibers to 2–4 wt% TS, indicating that salts are mainly dissolved in the aqueous fraction, leaving the organosolv fractionation. This is supported by the mineral content, which shows a considerable reduction in Na content of the organosolv-pretreated S. ramosissima fibers. Accordingly, the Na content of S. ramosissima fibers pretreated at three different organosolv conditions (OPF200-30-60, OPF200-45-60, and OPF180-30-60) ranged between 3 and 6 mg/gTS, compared to the 43 mg/gTS measured in the de-juiced green fibers before pretreatment.
The results of the elemental analyses (Table 3) show a significant reduction in the nitrogen content in the pulp of S. dolichostachya after organosolv pretreatment, resulting in a much higher C/N ratio (129–140) than in the de-juiced green fibers. For S. ramosissima, with a higher nitrogen content in the de-juiced fibers, the reduction is not that pronounced, leading to a change in the C/N ratio from 20 to 36–44. Generally, the C/N ratio of organosolv-pretreated fibers exceeds the optimal C/N ratio of 10–30 for a stable AD process [32]. Consequently, using the organosolv-pretreated Salicornia fibers as a sole substrate may lead to nitrogen deficiency in the microbial community, reducing methane production [5,33]. Thus, to achieve the ideal C/N ratio for carbon-rich substrates, like organosolv-pretreated Salicornia fibers, co-digesting with nitrogen-rich substrates, such as manure, is recommended.
As depicted in Figure 2, the cellulose content was generally more than two-fold higher in the pulp after organosolv fractionation. For S. dolichostachya, the pulp contained 46–51% TS of cellulose compared to the 26% TS in the de-juiced fibers, while the cellulose content increased from 17% TS to 28–49% TS in the pretreated fibers of S. ramosissima, except for the pulp after pretreatment of S. ramosissima SramOPF160-30-60 (160 °C, 30 min, 60% EtOH). This indicates that the fractionation efficiency decreases at lower temperatures and shorter treatment durations. Thus, it is evident from the structural barrier surrounding the cellulose polymer that the lower removal of lignin resulted in the lower cellulose content. The reduction in N content after organosolv fractionation validates the decrease in crude protein content in the pretreated fibers compared to the de-juiced green fibers. Implying that proteins were dissolved in the process liquor during organosolv fractionation, the resulting fibers after pretreatment are mainly composed of carbohydrates.
Though the delignification of both Salicornia species ranged between 56 and 61% for S. dolichostachya and between 34 and 75% for S. ramosissima, the recovered cellulosic pulp for both species after organosolv fractionation had a similar lignin content to the initial biomass material. Accordingly, the delignification is calculated as the reduction in total lignin mass in the initial untreated and the recovered organosolv-pretreated fibers, as described in [23,24]. Therefore, to account for the negligible differences in lignin content between the de-juiced green and pretreated fibers (as shown in Figure 2), the recovery of the pretreated fibers after organosolv fractionation should be reflected on. The solubilization of the de-juiced Salicornia biomass was enhanced by elevating the temperature from 160 to 200 °C [23,24]. This resulted in a reduction in the yield of pretreated solids, specifically 37–41% w/w and 34–50% w/w for S. dolichostachya and S. ramosissima, respectively, accompanied by a higher proportion of cellulose [23,24]. Consequently, the pretreated solids possess a lower ratio of lignin to cellulose: 0.3 for S. dolichostachya and between 0.3 and 0.8 for S. ramosissima, as compared to 0.5 and 1.0 in the de-juiced green S. dolichostachya and S. ramosissima fibers, respectively. The same is also the case for the ratio of cellulose to hemicellulose, which increased as a result of organosolv pretreatment. This indicates that the given delignification percentage does not correlate to the higher degradability of the pretreated fibers and that it is rather the structural changes in the lignin and its weakened bonds to cellulose after organosolv fractionation that enable higher bioavailability.
Furthermore, the sum of the lignocellulosic constituents of the pretreated fibers ranged between 76 and 79% and between 68 and 76% of the TS content in S. dolichostachya and S. ramosissima, respectively. This accounts for 81–84% of the VS content (based on TS) in S. dolichostachya and 70–79% in S. ramosissima, thus confirming that other organic matter may be present in the biomass that was not analyzed. Accordingly, the content of extractives ranged between 16 and 30% TS in the pretreated S. ramosissima fibers [24]. Previous studies found low lipid contents (1.6–3.7 g/100 gTS) in three halophyte species (Salicornia europaea, Tripolium pannonicum, and Crithmum maritimum) hydroponically grown under varying salinities [2]. In another study, Salicornia bigelovii had 0.4–0.38 g/100 gTS crude lipid content in the fresh tips, while Salicornia herbacea had 0.2–0.3 g/100 gTS in the tips and shoots [34].

3.2. Experimental and Theoretical Methane Potential and Biodegradability of Salicornia spp. Fibers

The biomethane yields of the de-juiced green and pretreated fibers of S. dolichostachya after three different pretreatment conditions and of S. ramosissima after six different pretreatment conditions are shown in Figure 3 and Figure 4, respectively. Generally, the methane yield and productivity of Salicornia fibers increased considerably after organosolv pretreatment. The BMP assay course shows a more rapid degradation for all organosolv-pretreated fibers of S. dolichostachya and S. ramosissima, with a main digestion period of approximately 6–10 days in contrast to 20 days for the de-juiced green fibers. This suggests an improvement in the rate and extent of hydrolysis during anaerobic degradation of the organosolv-pretreated fibers. A 2–3-day lag phase for both the pretreated fibers and the microcrystalline cellulose reference indicates that the microbial community in the inoculum required more time to adapt and initiate the hydrolyses of cellulosic polymers.
The methane yield of S. dolichostachya fibers (Figure 3) is highest at 299 mL-CH4/gVS after organosolv pretreatment at 200 °C for 30 min with 60% v/v EtOH (SdolOPF200-30-60), and 293 mL-CH4/gVS after organosolv pretreatment at 180 °C for 60 min with 60% v/v EtOH (SdolOPF180-60-60). Compared to the methane yield of 239 mL-CH4/gVS of the de-juiced fibers before organosolv pretreatment, the methane yield increased by 25%. Increasing the pretreatment time at constant temperature (180 °C) and solvent concentration from 45 min (SdolOPF180-45-60) to 60 min (SdolOPF180-60-60) resulted in higher methane yields.
For S. ramosissima, the highest methane yield of 293 mL-CH4/gVS was obtained from the de-juiced fibers pretreated at 180 °C for 30 min with 60% v/v EtOH (SramOPF180-30-60), while the lowest methane yield of 235 mL-CH4/gVS was found after pretreatment at 180 °C for 60 min with 40% v/v EtOH (SramOPF180-60-40) (Figure 4). Increasing the pretreatment time from 30 min (SramOPF180-30-60) to 60 min (SramOPF180-60-60), at a constant temperature (180 °C) and with 60% v/v EtOH solvent solution, resulted in lower methane yields (258 mL-CH4/gVS), which was different from the correlation found for the S. dolichostachya fibers. Comparatively, increasing the pretreatment temperature from 160 °C (SramOPF160-30-60) to 180 °C (SramOPF180-30-60) and 200 °C (SramOPF200-30-60), respectively, resulted in similar methane yields. Increasing the solvent solution from 40% v/v (SramOPF180-60-40) to 60% v/v (SramOPF180-60-60) at constant treatment time (60 min) and temperature (180 °C) resulted in higher methane yields. Furthermore, the methane yields are generally correlated to the effectiveness of organosolv fractionation. While the pulp of the S. ramosissima fibers contained lower cellulose and higher hemicellulose and lignin contents after pretreatment at the lower temperature of 160 °C, the pretreated fibers contained a higher cellulose content after pretreatment at higher temperatures, notably at 200 °C (Figure 2). Comparing the results of pretreatment of the two different halophyte species, S. ramosissima fibers achieved lower methane yields than pretreated S. dolichostachya fibers. This is attributed to the generally higher cellulosic content of pretreated fibers of S. dolichostachya after organosolv fractionation, as the de-juiced fibers had a higher cellulose content compared to S. ramosissima (Figure 2).
In contrast to other studies, where an inverse relationship between lignin content and BMP yields has been observed [22], this correlation was not seen here. A possible explanation for this result may be the rather low lignin content of the green Salicornia fibers and the relatively constant wt% TS share of lignin in the pretreated fibers (12–15 wt% TS S. dolichostachya and 13–21 wt% TS S. ramosissima) after the different organosolv fractionation conditions, similar to the de-juiced material (14 wt% TS S. dolichostachya and 16 wt% TS S. ramosissima). During organosolv fractionation, lignin undergoes phase transitions between solid and liquid phases through depolymerization and repolymerization [35,36]. This results in structural changes in the lignin and the relocalization or deposition of lignin droplets on the cellulosic fiber surface [35,36,37]. Based on the findings of this study, this indicates that the fragments of lignin left on the cellulose-rich pulp surface after organosolv fractionation are presumably deposited on the pulp surface in such a way that minimally impedes microbial degradation. Therefore, the potential physical barrier that may be caused by the lignin fragments is seemingly inconsiderable, as evidenced by the improved accessibility of cellulose, resulting in generally higher biomethane yields from pretreated Salicornia fibers. Consequently, the variations in methane yields of organosolv-pretreated Salicornia fibers are based on the cleavage of the bonds between lignin, hemicellulose, and cellulose rather than on the specific lignin content.
The methane production rate of the de-juiced green and pretreated S. ramosissima fibers (Figure 5) was similar to that of the microcrystalline cellulose, probably due to the high cellulosic content of the pretreated fibers. The highest peak within 5 days of digestion indicates a large content of easily degradable organic matter. The methane production rate was highest on day four of digestion for all pretreated fibers, followed by a steady decline. The highest daily methane rate of 83 mL-CH4/(gVS·d) was observed in SramOPF180-60-60 and the lowest was 43 mL-CH4/(gVS·d) in SramOPF160-30-60. The untreated de-juiced fibers obtained an optimum daily methane rate of 42 mL-CH4/(gVS·d) on day two and this dropped until day three, with a mild rise and subsequent steady decline. However, the rate of the pretreated fibers SramOPF160-30-60, SramOPF180-60-40, and SramOPF180-60-60 declined on day 4 and peaked again on day 8 (SramOPF160-30-60) and 10 (SramOPF180-60-40 and SramOPF180-60-60). This could be attributed to the utilization of readily available sugar monomers in the first phase and other sugars after the release of cellulose and hemicellulose from the lignocellulosic matrix and subsequent hydrolysis.
Comparing the BMP yields with glucose release after enzymatic saccharification using Cellic® CTec2 [23], the highest methane yield that was achieved in SdolOPF200-30-60 (200 °C, 30 min, 60% EtOH) corresponded to the highest glucose release of 0.61 g/gbiomass within 48 h, whereas SdolOPF180-45-60 (180 °C, 45 min, 60% EtOH), with the lowest methane yield, showed the lowest release of glucose of about 0.53 g/gbiomass within 48 h. Similarly, the variations in the biomethane yields of the S. ramosissima processed fibers correlated to the different saccharification percentages reported previously [24]. S. ramosissima processed fibers showed a lower percentage of enzymatic saccharification than S. dolichostachya, having considerable variations ranging between 30 and 81% following different organosolv pretreatment conditions [24] as compared to S. dolichostachya pretreated fibers [23]. The SramOPF160-30-60 (160 °C, 30 min, 60% EtOH) fibers showed the highest cellulose hydrolysis (81%) and release of glucose during enzymatic saccharification of 0.25 g/gbiomass within 48 h and thus are expected to achieve the highest methane yield. However, SramOPF180-30-60 (180 °C, 30 min, 60% EtOH) showed a 61% enzymatic saccharification with the same release of glucose and achieved the highest methane yields. On the other hand, pretreated fiber SramOPF180-60-40 (180 °C, 60 min, 40% EtOH) showed the lowest methane yield, correlating to the lowest saccharification percent of 30% and release of glucose of 0.11 and 0.15 g/gbiomass within 24 and 48 h. Likewise, pretreated fibers SramOPF200-30-60 (200 °C, 30 min, 60% EtOH) and SramOPF200-45-60 (200 °C, 45 min, 60% EtOH) showed similar saccharification percentages and corresponding glucose releases as SramOPF180-60-40, thus showing similar methane yields.
Typically, anaerobic digesters are initiated with a blend of microbial communities that engage in syntrophic interactions to produce biomethane. The hydrolytic bacteria, in particular, release extracellular hydrolases such as cellulase, xylanase, lipase, and protease to facilitate the hydrolysis of polysaccharides, lipids, and proteins into monomers and oligomers [38]. Thus, using commercial enzymes like cellulase enzyme solution Cellic® CTec2 [23,24], which acts mainly on the hydrolysis of the cellulose polymer, may not wholly reflect the efficiency of the digestion process due to the presence of extractives and other organic matter such as proteins in the Salicornia fibers. Nevertheless, it may provide a good indication of the suitability of this pretreatment for AD due to its higher share of cellulosic content after organosolv fractionation. Correspondingly, the increase in biomethane yields and improved anaerobic degradability of the pretreated fibers correlates to the almost-complete hydrolysis (100%) of cellulose and the release of glucose during enzymatic saccharification of the S. dolichostachya pretreated fibers [23].
Table 4 presents the results of the BMP of de-juiced green S. dolichostachya and S. ramosissima fibers before and after organosolv pretreatment. The organosolv pretreatment of S. dolichostachya and S. ramosissima de-juiced green fibers generally resulted in a higher biomethane yield, under all pretreatment conditions for Salicornia fibers. This increase in methane yield was 16–25% for S. dolichostachya and 2.5–28% for S. ramosissima. The increase in methane yields can be attributed to the cleavage of the bonds in the lignocellulosic complex during organosolv fractionation, resulting in enhanced accessibility of the cellulose and hemicellulose polymers in the fibers. As seen in Table 4, the average methane production rate of pretreated S. dolichostachya increased by a maximum factor of four compared to the untreated fibers, while this increase was only a factor of two for pretreated S. ramosissima. The optimal organosolv fractionation conditions to increase the BMP of de-juiced green fibers from S. dolichostachya were 60% EtOH at 200 °C for 30 min, or 180 °C for 60 min treatment time. On the other hand, for increasing the BMP yields of fibers from S. ramosissima, the optimal organosolv pretreatment conditions were 180 °C for 30 min and 60% v/v EtOH.
In this study, theoretical methods [29,30,31] were applied to predict the biomethane yields (TBMPCHNO and TBMPOFC). The TBMPCHNO values were considerably higher than experimental BMP values (BMPEXP), whereas the TBMPOFC values were closer to the measured methane yields, as seen in Table 4. The TBMPCHNO of the pretreated S. dolichostachya and S. ramosissima fibers ranged from 441 to 435 and from 459 to 541 mL-CH4/gVS, respectively. The de-juiced green fibers’ TBMPCHNO values were estimated to be 439 mL-CH4/gVS for S. dolichostachya and 449 mL-CH4/gVS for S. ramosissima. On the other hand, the TBMPOFC estimated values of the pretreated fibers varied between 274 and 294 mL-CH4/gVS for S. dolichostachya and between 239 and 279 mL-CH4/gVS for S. ramosissima. The TBMPCHNO values being much higher is due to the fact that this calculation implies the conversion of all organic compounds, including lignin, into biomethane. Since lignin is not degraded under anaerobic conditions, the TBMPCHNO overestimates the BMP value of organic matter with a higher lignin content. Thus, the TBMPOFC derived from organic content (carbohydrates and crude proteins) can be considered a better prediction of the experimental methane potential. In addition, TBMP predictions do not take into account the fraction of degradable organic matter used for microbial growth.
Based on the BDOFC, the S. dolichostachya pretreated fibers achieved a higher anaerobic digestibility of 94–105% when compared to the 83% of the de-juiced fibers. Pretreated S. ramosissima fibers achieved a higher biodegradability of 91–125% than S. dolichostachya, while the de-juiced fibers obtained 89%, as seen in Table 4. BDOFC values of more than 100% indicate analysis inaccuracies and/or other organic content present in the biomass not considered in this study, thus underestimating the derived TBMPOFC.

3.3. Comparison to Other Studies

In general, the methane yields of organosolv-pretreated halophytic material are comparable to those of other organosolv-pretreated lignocellulosic biomass, as seen in Table 5. Treating wheat [18] and rice straw [19] under organosolv conditions of 180 °C for 60 min with 50% EtOH resulted in lignin removals of 14% and 18%, respectively. The untreated wheat and rice straw lignin contents were 18% and 17%, respectively. The pretreated wheat straw achieved a 15% increase in specific methane yield, yielding 316 mL-CH4/gVS compared to the 274 mL-CH4/gVS yield in the untreated material [18]. Organosolv-pretreated rice straw, on the other hand, achieved a specific methane yield of 332 mL-CH4/gVS, which, compared to the untreated rice straw’s yield of 235 mL-CH4/gVS, corresponded to a 42% increase [19]. In this study, similar organosolv fractionation conditions—180 °C for 60 min, with 40 and 60% EtOH (OPF180-60-40 and OPF180-60-60, respectively)—resulted in lower specific methane yields, ranging from 235 to 294 mL-CH4/gVS, compared to organosolv-pretreated wheat and rice straw.
At a lower treatment temperature of 150 °C, rice straw achieved a lower methane yield of 303 mL-CH4/gVS, correlating to a lower cellulosic content compared to pretreated rice straw at 180 °C [19]. Thus, similar to the findings in this study, at a lower temperature of 160 °C, pretreated halophytic fibers contained a lower cellulosic content. In comparison, at a lower solvent-to-substrate ratio (1:8), treating rice straw at 150 °C for 60 min with 75% EtOH and 1% H2SO4 acid catalyst achieved a delignification of 22% and a 32% higher methane yield of 153 mL-CH4/gVS than that of the untreated straw (116 mL-CH4/gVS) [22]. However, at a higher solvent-to-substrate ratio of 1:10, higher pretreatment temperatures, lower solvent solution concentration, and in the absence of a catalyst, this study achieved higher methane yields from pretreated Salicornia fibers.
Organosolv pretreatment of biomass with a higher lignin content, such as rubberwood waste (31%) [20] and sunflower stalks (27%) [21], also results in significant enhancement of the methane yields. Organosolv pretreatment, at 210 °C for 30 min with 75% EtOH, of rubberwood waste achieved a higher methane yield of 165 mL-CH4/gVS compared to 59 mL-CH4/gVS in untreated rubberwood waste [20]. A delignification of 74% relates to a 179% increase in methane yields. Using isopropanol solvent during organosolv pretreatment (180 °C, 60 min, 50% isopropanol), a methane yield of 264 mL-CH4/gVS for pretreated sunflower stalks was obtained as compared to 124 mL-CH4/gVS for untreated stalks [21]. This represents a 113% increase in methane yields following a 26% delignification of treated sunflower stalks. The use of a constant 50% isopropanol solvent under a variation of organosolv treatment temperatures (140, 160, and 180 °C) and times (30 and 60 min) resulted in a higher solid recovery of 67–74% than attained in the organosolv pretreatment of halophytic biomass [23,24]. Conversely, organosolv-pretreated cocoa bean shells obtained a lower methane yield of 219 mL-CH4/gVS compared to the 231 mL-CH4/gVS obtained from untreated material [19]. In this study, the green Salicornia fibers with a lower lignin content were more amenable to AD, thus achieving biomethane yields similar to and higher than de-juiced fibers before organosolv fractionation compared to biomass with a higher lignin content. Notably, as seen in Table 5, the increase in percentage methane yield being more than 100% for the pretreated lignified biomass makes evident the recalcitrant effect imposed by a high lignin content; thus, after pretreatment, considerably higher methane yields were achieved as opposed to untreated material. Therefore, further research should be conducted to explore the effectiveness of organosolv pretreatment on lignified halophytic biomass for enhanced biomethane production through AD.
Recent studies have investigated the efficiency of a hybrid organosolv–steam explosion pretreatment to fractionate birch [39] and spruce [40] biomass and, subsequently, its effect on biomethane potential under thermophilic conditions [41]. After hybrid pretreatment of birch biomass at 200 °C for 30 min with 70% v/v EtOH, with a related 73% delignification [39], the highest methane yield of 327 mL-CH4/gVS was achieved for pretreated birch [41]. This showed a 74% increase in methane yield compared to steam-exploded birch biomass (188 mL-CH4/gVS). However, in the hybrid pretreatment of spruce biomass, the use of an acid catalyst (1% H2SO4) at 200 °C for 30 min with 52% v/v EtOH achieved the highest delignification of 79% [40] and biomethane yield of 177 mL-CH4/gVS [41] from pretreated spruce, thus obtaining a significant increase in methane yields compared to the 11 mL-CH4/gVS achieved from steam-exploded spruce biomass.

4. Conclusions

Sequential wet and organosolv fractionation was applied to Salicornia spp. for complete valorization of halophytic biomass, as part of a green biorefinery concept. For full recovery, the AD process is implied to treat the residual fiber for biogas production. Organosolv fractionation of de-juiced green fibers of Salicornia spp. produces a cellulose-rich pulp fraction with improved anaerobic digestibility, resulting in a 23–28% increase in biomethane yields due to the removal of lignin and hemicellulose in the aqueous stream. The methane yields were highest at about 300 mL-CH4/gVS for organosolv pretreatment with 60% EtOH at 200 °C for 30 min of treatment, or at 180 °C for 60 min of treatment, for S. dolichostachya; and with 60% EtOH at 180 °C for 30 min for S. ramosissima. Furthermore, wet fractionation allows for the efficient removal of salts into the juice fraction, generating fibers with minimal salt contents, thereby avoiding any inhibitory impact on the AD process.

Author Contributions

Conceptualization, A.C., L.M. and H.U.; methodology, analysis and investigation, A.C.; resources, L.M., M.M., U.R. and P.C.; writing—original draft preparation, A.C.; writing—review and editing, A.C., L.M., H.U., M.M., M.H.T., P.C. and U.R.; visualization, A.C., L.M. and H.U.; supervision, H.U., M.H.T. and L.M.; project administration, M.H.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 862834.

Data Availability Statement

Data is contained within the article.

Acknowledgments

The authors are thankful to the Institute of Botany, Leibniz University, Hannover, Germany for collecting and providing the ICP-OES data.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Schematic diagram of the processing of Salicornia spp. in a halophyte biorefinery.
Figure 1. Schematic diagram of the processing of Salicornia spp. in a halophyte biorefinery.
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Figure 2. Lignocellulose composition, crude protein (CP) content, and delignification and hemicellulose removal percentages of de-juiced green and organosolv-pretreated S. dolichostachya and S. ramosissima fibers. CP content derived from elemental N content. a Data published in [23]; b data published in [24].
Figure 2. Lignocellulose composition, crude protein (CP) content, and delignification and hemicellulose removal percentages of de-juiced green and organosolv-pretreated S. dolichostachya and S. ramosissima fibers. CP content derived from elemental N content. a Data published in [23]; b data published in [24].
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Figure 3. Specific biomethane yield of de-juiced and organosolv-pretreated S. dolichostachya fibers.
Figure 3. Specific biomethane yield of de-juiced and organosolv-pretreated S. dolichostachya fibers.
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Figure 4. Specific biomethane yields of de-juiced and organosolv-pretreated S. ramosissima fibers.
Figure 4. Specific biomethane yields of de-juiced and organosolv-pretreated S. ramosissima fibers.
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Figure 5. Specific methane production rates of de-juiced green and organosolv-pretreated S. ramosissima fibers.
Figure 5. Specific methane production rates of de-juiced green and organosolv-pretreated S. ramosissima fibers.
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Table 1. Organosolv pretreatment conditions applied to de-juiced green S. dolichostachya and S. ramosissima fibers used for BMP trials.
Table 1. Organosolv pretreatment conditions applied to de-juiced green S. dolichostachya and S. ramosissima fibers used for BMP trials.
Sample NotationOrganosolv Treatment Conditions
TemperatureTimeSolvent Concentration
(°C)(min)(% v/v EtOH)
OPF200-30-602003060
OPF200-45-602004560
OPF180-30-601803060
OPF180-45-601804560
OPF180-60-401806040
OPF180-60-601806060
OPF160-30-601603060
Table 2. TS, VS, and ash contents of de-juiced green fibers (DGFs) and organosolv-pretreated fibers (OPF) of S. dolichostachya and S. ramosissima.
Table 2. TS, VS, and ash contents of de-juiced green fibers (DGFs) and organosolv-pretreated fibers (OPF) of S. dolichostachya and S. ramosissima.
Halophyte SamplesTSVSVSAsh
wt% FMwt% FMwt% TSwt% TS
S. dolichostachya (S. dol)
SdolDGF92.7 (0.1)88.0 (0.2)95.0 (0.2)5.0 (0.2)
SdolOPF200-30-6095.9 (0.1)89.7 (0.2)93.5 (0.2)6.5 (0.2)
SdolOPF180-45-6093.5 (0.1)87.9 (0.5)94.0 (0.6)6.1 (0.6)
SdolOPF180-60-6093.5 (0.1)87.7 (0.3)93.7 (0.2)6.3 (0.2)
S. ramosissima (S. ram)
SramDGF92.4 (0.03)81.1 (0.1)87.8 (0.1)12.2 (0.1)
SramOPF200-30-6097.6 (0.02)93.8 (0.2)96.1 (0.2)3.9 (0.2)
SramOPF200-45-6095.7 (0.1)91.9 (0.1)96.2 (0.04)3.8 (0.04)
SramOPF180-30-6097.2 (0.01)93.1 (0.3)95.8 (0.3)4.2 (0.3)
SramOPF180-60-4096.3 (0.01)94.3 (0.02)98.0 (0.02)2.0 (0.02)
SramOPF180-60-6096.2 (0.03)93.2 (0.03)96.8 (0.04)3.2 (0.04)
SramOPF160-30-6094.9 (0.02)92.6 (0.04)97.6 (0.02)2.4 (0.02)
Table 3. Elemental content, C/N ratio, and empirical formular of de-juiced green and organosolv-pretreated S. dolichostachya and S. ramosissima fibers; standard deviation in parentheses.
Table 3. Elemental content, C/N ratio, and empirical formular of de-juiced green and organosolv-pretreated S. dolichostachya and S. ramosissima fibers; standard deviation in parentheses.
Halophyte SamplesCHNOC/NEmpirical Formular
wt% TS
S. dolichostachya (S. dol)
SdolDGF43.7 (0.3)5.9 (0.1)1.3 (0.1)43.6 (0.5)33.6C39H63N1O29
SdolOPF200-30-6044.0 (0.2)6.0 (0.1)0.3 (0.01)42.7 (0.3)129.4C151H247N1O110
SdolOPF180-45-6043.2 (0.1)5.9 (0.1)0.3 (0.01)44.0 (0.4)139.5C163H265N1O124
SdolOPF180-60-6043.4 (0.2)5.9 (0.1)0.3 (0.01)43.6 (0.3)140.0C163H267N1O123
S. ramosissima (S. ram)
SramDGF41.0 (0.4)5.6 (0.1)2.1 (0.1)38.7 (0.5)19.7C23H38N1O16
SramOPF200-30-6047.9 (0.5)6.4 (0.1)1.3 (0.04)40.0 (0.6)36.3C42H68N1O27
SramOPF200-45-6049.7(0.1)6.4 (0.1)1.1 (0.01)38.4 (0.7)43.6C51H79N1O29
SramOPF180-30-6047.1 (0.5)6.3 (0.1)1.3 (0.1)40.7 (0.5)35.7C42H66N1O27
SramOPF180-60-4052.0 (0.7)6.5 (0.1)1.3 (0.1)37.7 (0.7)40.0C47H70N1O25
SramOPF180-60-6048.6 (0.7)6.3 (0.1)1.1 (0.1)40.3 (0.8)44.2C52H80N1O32
SramOPF160-30-6046.5 (0.4)6.1 (0.1)1.2 (0.1)43.3 (0.5)38.8C45H71N1O32
Table 4. Experimental and theoretical biomethane yields, average methane production rates, percent changes in methane yield, and anaerobic biodegradability of de-juiced green and organosolv-pretreated S. dolichostachya and S. ramosissima fibers; standard deviation in parentheses.
Table 4. Experimental and theoretical biomethane yields, average methane production rates, percent changes in methane yield, and anaerobic biodegradability of de-juiced green and organosolv-pretreated S. dolichostachya and S. ramosissima fibers; standard deviation in parentheses.
Halophytic SubstratesBMPEXPAverage CH4 Production RateΔCH4TBMPBD
CHNOOFCCHNOOFC
mL-CH4/gVSmL-CH4/(gVS·d)%mL-CH4/gVS%
S. dolichostachya (S. dol)
SdolDGF239.4 (10.5)9.5-4352885583
SdolOPF200-30-60299.3 (2.3)28.3+25.145928565105
SdolOPF180-45-60276.4 (3.8)25.1+15.54412946394
SdolOPF180-60-60293.6 (2.6)27.9+23.044727966105
S. ramosissima (S. ram)
SramDGF229.3 (2.8)12.1-4492585189
SramOPF200-30-60265.4 (11.4)26.5+15.74992675399
SramOPF200-45-60254.3 (4.7)25.4+10.95252794891
SramOPF180-30-60292.7 (10.9)29.3+28.048723460125
SramOPF180-60-40235.2 (3.2)23.5+2.654123243101
SramOPF180-60-60258.0 (13.9)25.7+12.750124652105
SramOPF160-30-60273.1 (3.8)26.1+19.245923560116
Table 5. Comparison of the effect of organosolv pretreatment on the biomethane yields, under mesophilic conditions, of various lignocellulosic biomass samples by different studies.
Table 5. Comparison of the effect of organosolv pretreatment on the biomethane yields, under mesophilic conditions, of various lignocellulosic biomass samples by different studies.
SubstratesOrganosolv Fractionation ConditionsSolvent-to-Substrate RatioDelignificationMethane YieldsΔCH4Reference
%mL-CH4/gVS%
S. dolichostachya200 °C, 30 min, 60% EtOH1:1056299+25This study
S. ramosissima180 °C, 30 min, 60% EtOH1:1063293+28
Wheat Straw180 °C, 60 min, 50% EtOH1:1014316+15[18]
Rice Straw18332+42[19]
Rice Straw150 °C, 60 min, 50% EtOH1:1014303+29[19]
Rice Straw150 °C, 60 min, 75% EtOH, 1% H2SO41:822153+32[22]
Sunflower Stalks180 °C, 60 min, 50% isopropanol1:1026264+113[21]
Cocoa Bean Shells180 °C, 60 min, 50% EtOH1:1012219−5[19]
Rubber Waste210 °C, 30 min, 75% EtOH1:1074165+179[20]
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MDPI and ACS Style

Cayenne, A.; Monção, M.; Matsakas, L.; Rova, U.; Christakopoulos, P.; Thomsen, M.H.; Uellendahl, H. Enhancing the Methane Yield of Salicornia spp. via Organosolv Fractionation as Part of a Halophyte Biorefinery Concept. Energies 2024, 17, 1074. https://doi.org/10.3390/en17051074

AMA Style

Cayenne A, Monção M, Matsakas L, Rova U, Christakopoulos P, Thomsen MH, Uellendahl H. Enhancing the Methane Yield of Salicornia spp. via Organosolv Fractionation as Part of a Halophyte Biorefinery Concept. Energies. 2024; 17(5):1074. https://doi.org/10.3390/en17051074

Chicago/Turabian Style

Cayenne, Aadila, Maxwel Monção, Leonidas Matsakas, Ulrika Rova, Paul Christakopoulos, Mette H. Thomsen, and Hinrich Uellendahl. 2024. "Enhancing the Methane Yield of Salicornia spp. via Organosolv Fractionation as Part of a Halophyte Biorefinery Concept" Energies 17, no. 5: 1074. https://doi.org/10.3390/en17051074

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