Bioresource Technology 189 (2015) 145–153

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Characterization of biochars and dissolved organic matter phases obtained upon hydrothermal carbonization of Elodea nuttallii J. Poerschmann a,⇑, B. Weiner a, H. Wedwitschka b, A. Zehnsdorf c, R. Koehler a, F.-D. Kopinke a a

Helmholtz Centre for Environmental Research-UFZ, Department of Environmental Engineering, Permoserstr. 15, D-04318 Leipzig, Germany Deutsches Biomasseforschungszentrum-DBFZ, Department of Biochemical Conversion, Torgauer Straße 116, D-04347 Leipzig, Germany c Helmholtz Centre for Environmental Research-UFZ, Department of Environmental Biotechnology, Permoserstr. 15, D-04318 Leipzig, Germany b

h i g h l i g h t s  Invasive Elodea nuttallii plants were subjected to hydrothermal carbonization.  Biochars contain extraordinarily high fraction of inorganic elements.  Process water is a good substrate for biogas production.  Phenols, carboxylic acids and N-functionalized proved abundant breakdown products.

a r t i c l e

i n f o

Article history: Received 27 February 2015 Received in revised form 27 March 2015 Accepted 29 March 2015 Available online 3 April 2015 Keywords: Hydrothermal carbonization Elodea nuttallii Biochar Breakdown products

a b s t r a c t The invasive aquatic plant Elodea nuttallii was subjected to hydrothermal carbonization at 200 °C and 240 °C to produce biochar. About 58% w/w of the organic carbon of the pristine plant was translocated into the solid biochar irrespectively of the operating temperature. The process water rich in dissolved organic matter proved a good substrate for biogas production. The E. nuttallii plants showed a high capability of incorporating metals into the biomass. This large inorganic fraction which was mainly transferred into the biochar (except sodium and potassium) may hamper the prospective application of biochar as soil amendment. The high ash content in biochar (40% w/w) along with its relatively low content of organic carbon (36% w/w) is associated with low higher heating values. Fatty acids were completely hydrolyzed from lipids due to hydrothermal treatment. Low molecular-weight carboxylic acids (acetic and lactic acid), phenols and phenolic acids turned out major organic breakdown products. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Hydrothermal carbonization (HTC) is an exothermic process used to convert aqueous biomass suspensions at moderate, subpyrolysis temperatures (180–250 °C) and self-generated pressure (Libra et al., 2011). In the process, two basic carbonaceous product streams are formed: biochar and process water. HTC is considered to simulate natural coalification processes due to the high similarity between elemental ratios (H/C and O/C in particular) of biochars and lignite. Over the last years, HTC has established itself as a fast, simple and carbon dioxide negative conversion technique to produce biochar from a variety of biomasses. The process water rich in dissolved organic matter (DOM) is in principle an undesirable by-product, but can be used as a raw material source for biogas ⇑ Corresponding author. Tel.: +49 341 235 1761; fax: +49 341 235 2492. E-mail address: [email protected] (J. Poerschmann). http://dx.doi.org/10.1016/j.biortech.2015.03.146 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

production (Funke et al., 2013). The potential natural raw biomass substrates, as well as agricultural and municipal waste products, are summarized in Funke et al. (2013) and Oliveira et al. (2013). The significance of utilizing ‘‘green” hydrothermal processes to produce value-added products such as biochar is especially high if waste biomasses, including those originating from aggressively spreading plants, can be used as substrates. Biochar has been used as soil amendment (Meyer et al., 2011), as sorbent (Mohan et al., 2014), and for energetic purposes (Kieseler et al., 2012). Biochars which can easily be separated from process water due to high dewatering capabilities carry a high percentage of organic carbon (OC) of the pristine biomass. Solid char formation is due to dehydration and defunctionalization processes in water followed by condensation reactions (Libra et al., 2011). The aggressively spreading submersed macrophyte E. nuttallii grows in lakes, ponds, canals, and slow-moving waters leading to interference with natural vegetations (Mooney and Cleland, 2001). Elodea waterweeds are well adapted to varying environmental

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conditions and contain allelochemicals to fight competing algae and cyanobacteria (Erhard et al., 2007). Input of nitrogen and phosphorous arising from plant decay harms the water quality. Furthermore, recreational activities on lakes dominated by E. nuttallii are hampered. In Germany, mechanical harvesting of this invasive plant has been widely applied. Potential application fields of this waste biomass include its utilization as substrate (Koyama et al., 2014) and co-substrate (Munoz Escobar et al., 2011) for biogas generation. Prospective scale up processes with (seasonally harvested) Elodea biomasses should be focused on a decentralized approach, with the HTC plant being not far away from Elodea harvest sites. Thus, extensive transportation costs (due to low dry mass, low bulk density which may be improved by chopping) and biomass decomposition can be avoided. Regarding the latter, freshly harvested biomass is known to decompose quickly, concomitantly generating a putrid odor. Preliminary and tentative results regarding HTC of E. nuttallii indicated an unexpectedly low suitability of different Elodea plants to produce biochar (Munoz Escobar et al., 2011; Pels et al., 2014). However, the mass balances of organic and inorganic elements, the capability of the process water constituents to produce biogas as performed previously (Poerschmann et al., 2014; Reza et al., 2014a), as well as the characterization of organic breakdown products need to be clarified yet. These organic breakdown products are good candidates to produce platform chemicals and value-added products (Reza et al., 2014b). This contribution is aimed at filling these gaps. Studies were performed with E. nuttallii sampled mechanically in a trench, in which the water flows slowly through the village of Kleinpoesna (20 km southeast from Leipzig). The HTC process was performed at operating temperatures of 200 °C and 240 °C for 14 h. 2. Methods 2.1. Chemicals All chemicals, solvents, and derivatization agents were purchased from Sigma–Aldrich (Munich, Germany) and used without further purification. Derivatization agents included N,O-bis (trimethylsilyl) trifluoroacetamide) (BSTFA) to form trimethylsilyl ethers (TMS), 14% w/w boron-trifluoride methanol (BF3/Meth) to produce methyl esters, and acetic anhydride to produce acetates. Isotopically labeled internal standards including phenol-d6, hydroquinone-d4, palmitic acid-d2, succinic acid-d4, and phenanthrened10 were acquired from Cambridge Isotope Lab. (Andover/MA). 2.2. Hydrothermal carbonization All mechanically harvested samples consisted of fully grown plants. Samples were taken in spring 2012. One day after sampling, an aliquot of wet plants (60 g, dry matter content 8.8% w/w) was roughly cut in small pieces for HTC experiments. Attention should be paid to remove inorganic material such as residues from the harvest attached onto the plant. Prior to HTC experiments, these inorganic residues were removed by mild washing with hot water. To perform the HTC experiments, the biomass was immersed in distilled water (120 g, containing 1.5 g/L of citric acid) and put into a glass insert, which was placed into a 250 mL volume autoclave (Fa. Roth, Karlsruhe, Germany). The autoclave was heated to 200 °C or 240 °C for 14 h holding time. The chosen experimental conditions proved straightforward regarding the completeness of the hydrothermal process (Poerschmann et al., 2014). After cooling to room temperature, the slurry was filtered in vacuum via a membrane pump. The process water was passed through a 0.45 lm pore size (cellulose) syringe filter prior to TOC and COD analysis. Then, the solid biochar was washed with distilled water followed

by drying at 105 °C overnight. HTC experiments were performed in duplicate. The DOC (dissolved organic carbon) concentration as bulk parameter was analyzed by subjecting both the filtered process water and the wash water to a total organic carbon analyzer TOC 600 (Shimadzu, Duisburg, Germany). The chemical oxygen demand (COD) was determined by a DIN-compatible micro-method with chromo-sulfuric acid (test kit LCK014, Hach Lange, Duesseldorf, Germany). The BOD was measured manometrically with a WTW BOD-analyzer OxiTop IS 6, according to the DIN-method 38 409-H51/H52. 2.3. Elemental analysis and X-ray fluorescence Elemental analysis (C, H, N, S) of biochars was performed using an automatic analyzer (CHN 932, LECO Instruments Moenchengladbach, Germany). Residual ash as well as OC content was measured on a solid carbon analyzer (C-MAT, Stroehlein Instruments, Duesseldorf, Germany). Similarly, elemental analysis and ash determination were also conducted with the process water and the pristine E. nuttallii, both of them on a dry mass basis resulting from overnight drying at 105 °C. The oxygen concentration was calculated on the basis of the difference between the ash-free dry weight of the matrix (pristine biomass, biochar 200 °C, biochar 240 °C, DOM 200 °C, DOM 240 °C) and the combined weight of the measured elements (C, H, N, S). The mass balance of inorganic analytes was determined by subjecting dried, mortar-ground homogenized samples (pristine biomass, biochar 200 °C, biochar 240 °C, aliquots of 3.0 g each) to X-ray fluorescence spectroscopy using a WDXRF-spectrometer S4 PIONEER (Bruker-AXS, Bremen, Germany), equipped with a 4 kWRh X-ray tube (75 lm Be window) and a 60 kV generator. In parallel, DOM matrices were subjected to an inductively coupled plasma mass spectrometer (ICP-MS ELAN 6100 DRC, PerkinElmer, Überlingen, Germany) after digestion with 99.5% pure HNO3 because the DOM mass yield was too low to allow X-ray fluorescence spectroscopy. Concentration data of elemental silicon measured by ICP-MS were not considered herein due to poor recoveries after the caustic digestion. 2.4. Extraction of slurries and pristine biomass The slurries were spiked with internal standards, then lyophilized. The concentration of isotopically labeled standards ranged from 500 lg g 1 dry mass of pristine biomass (DMBiomass) for succinic acid-d4 to 10 lg g 1 for phenanthrene-d10. Extraction of 200 mg aliquots of lyophilizates (including that of the pristine biomass) was performed by pressurized liquid extraction (ASE 300, ThermoFisher, Dreieich, Germany) with chloroform/methanol (1:1, v/v) at 100 °C (2 cycles, each 10 min). The combined extracts (first and second extraction) were purified by SPE using activated silica gel with acetone as solvent, gently evaporated, and finally the residue was taken up in 100 lL of chloroform. Aliquots of these extracts were subjected to silylation and methylation according to (Poerschmann et al., 2015). To determine total concentrations of fatty acids and fatty alcohols in the pristine biomass, the lyophilized E. nuttallii was subjected to a mild saponification, which was conducted with 3 M methanolic KOH solution for 0.5 h under nitrogen atmosphere, followed by release of free acids with phosphoric acid at pH = 2. Likewise, the saponified material was subjected to lyophilization. The in situ acetylation of the process water with acetic anhydride at pH  9.5 was performed as described in (Poerschmann and Trommler, 2008). 2.5. Short-chain carboxylic acids Anion chromatography was carried out on a DX500 (ThermoFisher, Dreieich, Germany) using an IonPac AS18 column

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along with the corresponding pre-column AG18 in a gradient mode with KOH as eluent (Weiss and Jensen, 2003). Samples were filtered through 0.2 lm syringe filters (PTFE; VWR international, U.S.) and diluted prior to analysis. 2.6. GC/MS analysis An Agilent GC/MS system model 5973 (Agilent, Waldbronn, Germany) equipped with electron impact ionization source was used. Separations were performed using a DB-1 capillary column (30 m  0.25 mm  0.25 lm, Agilent). In case of free short-chain carboxylic acids as target analytes, a polar thick film SolGel Wax column (30 m  0.25 mm  1.0 lm, SGE, VWR Darmstadt, Germany) was used. Splitless injection as well as a temperature programmed mode (40 °C-hold 2 min, linear ramp 6 K min 1 to 310 °C in case of the DB-1 coating and to 280 °C in case of the SolGel Wax coating) was applied. Data acquisition was in full scan mode (m/z = 25–700 amu). Quantification of organic analytes was done by means of selected ions: the three most abundant ions of a given analyte were summed up and referred to the sum of ions of the corresponding internal standard. The mean relative standard deviation (RSD) for concentrations as determined by GC/MS was below 20%, n = 3. RSD-data for saponified samples were generally higher by about 10%. If not otherwise stated, acids were quantified both as methyl esters and TMS ethers; data were averaged. 2.7. Biochemical methane potential test The biochemical methane potential test based on a German Norm for fermentation of organic materials was detailed in (Poerschmann et al., 2014). Briefly, methane production was obtained from the incubation of triplicate anaerobic batch cultures (performed under mesophilic conditions, 38 ± 3 °C for 45 days) containing both the substrate and a methanogenic inoculum (mixture of sewage sludge from a wastewater treatment plant and of digested effluent from an agricultural biogas plant) at a predetermined substrate/inoculum ratio. The biogas volume was measured daily using the Eudiometer technology (Poerschmann et al., 2014). The methane content in the biogas was determined once a week with a landfill gas sensor GA2000 (Ansyco, Karlsruhe, Germany). Once the daily biogas formation did not exceed 1% of the total accumulated biogas volume produced until that time, the experiment was finished. To monitor the inoculum performance, each test run included triplicate control cultures containing inoculum and reference microcrystalline cellulose. The methane yields were calculated according to standard conditions (273.15 K, 1.01325  105 Pa). 3. Results and discussion 3.1. HTC mass balance 3.1.1. Mass balance of organic matter Table 1 summarizes data from gravimetric and elemental analysis for the pristine biomass, the biochars and the process waters. COD and BOD10 data for the process waters are given alongside. Ratios of BOD10/COD turned out relatively low at 200 °C (0.35), but reflected good biodegradability for process water obtained from HTC at 240 °C (0.51). Further studies (not detailed here) provided strong evidence that the BOD10/COD-ratios for Elodea plants harvested at different sites in Germany proved similar despite significant differences in elemental compositions and dry weight fractions. As an example, the BOD10/COD-ratio for process water (220 °C) of Elodea plants sampled from Goitzsche lake (about 100 km north from Leipzig town) (Munoz Escobar et al., 2011) were

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0.46. As will be demonstrated below, similar conclusions can be drawn for biogas production. The combined mass yield referred to DMBiomass which was transferred into both precipitated biochar and process water amounts to 80% and 73% for 200 °C and 240 °C, respectively (see Table 1). As previously observed with brewer’s spent grain (Poerschmann et al., 2014), this yield was decreased with higher operating temperature. The mass loss of 20% at 200 °C, and 27% at 240 °C is – besides handling losses – predominantly due to the formation of gases (Oliveira et al., 2013). In an ideal scenario, the organic carbon fraction is overwhelmingly transferred into biochar with a minor DOC fraction and negligible formation of carbon dioxide. The recovery of the substrate’s OC sequestered in biochars is 58% independent of the operating temperature, and is in total 11% lower than observed with brewer’s spent grain (Poerschmann et al., 2014). Compared to a wide array of different biomasses (n = 16) subjected to HTC at 220 °C, an averaged OC-recovery of 70% (scattering between 52% and 82%) for biochars was obtained (Oliveira et al., 2013). The OC content in the biochar produced at 240 °C is higher compared to that produced at 200 °C (37.2% versus 34.5%, see Table 1). Intriguingly, the OC contents are much lower compared to biochars produced from brewer’s spent grain as substrate (72.9% at 240 °C, 67.5% at 200 °C) (Poerschmann et al., 2014). The low recovery of OC in biochars of Elodea are tied to the high ash contents, which, in turn, are associated to the Elodea plants’ outstanding capability of accumulating inorganic species (see Section 3.1.2.). As for the process water phase, the DOC mass yields amount to 23.0% (200 °C) and 20.7% (240 °C). Beyond reasonable ratios of BOD10/COD (see above), the relatively high recoveries give reason to expect a good suitability to generate biogas (see Section 3.2.). Regarding the elemental ratios, the atomic H/C ratio refers to a significant dehydrogenation (aromatization) of both biochars compared to pristine E. nuttallii, which is most pronounced for higher operating temperatures (H/C = 1.13 at 200 °C versus H/C = 0.98 at 240 °C). As expected, defunctionalization of O-containing moieties turned out higher at higher operating temperatures (O/C = 0.43 versus O/C = 0.31, Table 1). The loss of oxygen at 240 °C amounts to as high as 57.6%, which is much higher than the corresponding value at 200 °C (42.6%). The same tendency holds true for the loss of N and S. In case of process waters, decarboxylation proved of minor impact. Dehydration did not proceed at all. To better illustrate defunctionalization processes, a van-Krevelen diagram was drawn that arranges the pristine E. nuttallii along with the corresponding HTC matrices into elemental compositions of common biological molecules. (Fig. 1). The H/C and O/C atomic ratios of the biochar (240 °C) resemble those of lignite, whereas the biochar (200 °C) showed a higher O/C atomic ratio which refers to a lower degree of decarboxylation. Hence, the calorific significance of the biochar produced at 240 °C is expected to be higher. The calculation of HHV (higher heating value) data according to guidelines given by (Channiwala and Parikh, 2002) results in HHVBiochar-240 = 14.1 kJ g 1 and HHVBiochar-200 = 13.6 kJ g 1, that of the pristine biomass amounts to 12.9 kJ g 1. Both HHV data for the biochars combined with the H/C and O/C elemental ratios refer to an incomplete coalification process and insufficient energy densification, the latter expressed by the ratio HHVBiochar/HHVFeedstock. For reasons of comparison, the energy densification for brewer’s spent grain amounted to factors of 1.35 at 200 °C and 1.43 at 240 °C (Poerschmann et al., 2014). Likewise, the comparison of O/C elemental ratios of biochar and feedstock of brewer’s spent grain (Poerschmann et al., 2014) and E. nuttallii provides evidence that decarboxylation processes are of minor significance for E. nuttallii as compared to brewer’s spent grain. The ash content is fixed to about 75% in the biochar relatively independent of the operating temperature (see Table 1). Clearly,

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Table 1 Mass balances after hydrothermal treatment of Elodea nuttallii.

Elodea-Educt 200 °C-DOM

COD (g L 1)

BOD10 (g L 1)

BOD10/ COD

13.8

4.79

0.35

DMMatrix/ DMElodea (%, g/g) 100 24b

200 °C-Biochar

VStand.Biogas/mODM (mL g 1)a

456 (283)

56

Loss 240 °C-DOM

13.2

6.75

20 22

0.51

240 °C-Biochar

51

Loss

27

463 (287)

OCDM (%, g/g)

HDM (%, g/g)

NDM (%, g/g)

SDM (%, g/g)

ODM (%, g/g)

AshDM (%, g/g)

H/C

O/C

33.2 31.8 (23.0)c 34.5 (58.1) 19.0 31.3 (20.7) 37.2 (57.1) 22.2

4.32 4.51 (25.1) 3.26 (42.3) 32.6 4.37 (22.3) 3.03 (35.8) 41.9

1.96 2.68 (32.8) 1.82 (52.0) 15.2 2.55 (28.6) 1.69 (44.0) 27.4

0.92 1.75 (45.7) 0.51 (31.0) 23.3 1.56 (37.3) 0.62 (34.4) 28.2

30.6 25.3 (19.8) 20.1 (36.8) 42.6 22.7 (16.3) 15.7 (26.1) 57.6

29.2 (30.7)d 36.0 (38.5)d (29.6) 39.4 (39.6)d (75.5) 5.1 37.6 (42.8)d (28.3) 41.9 (44.3)d (73.1) 1.4

1.56 1.70

0.69 0.60

1.13

0.43

1.68

0.54

0.98

0.31

a

In parentheses: standard volume of methane (rather than standard volume of biogas VStand.Biogas) referred to ODM (dry mass of DOM). Mass yield as (non-diluted) DM-dry matter (105 °C), DMElodea = 8.8% w/w; DMElodea, DMDOM, and DMBiochar are each referred to DMBiomass (%, w/w). In parentheses: carbon recovered in the biochar, i.e. OCDOM referred to OCBiomass, OCBiochar referred to OCBiomass (%, w/w), correspondingly for HDM, NDM, SDM, ODM, AshDM each referred to the element’s content in dry Elodea. d In parenthesis: ash yield (in%) calculated on the basis of elemental concentrations given in Table 2 (see text). b

c

temperatures, the residence time proved of minor importance towards elemental composition when beyond 4 h (Gao et al., 2013).

2.0 H/C DOM 200

1.2 Lignite

Humic acids

Char 200

DOM 240 Char 240

1.6

Lipids

Elodea Fulvic acids

0.8

0.4 Condensed aromatics 0.1

0.2

0.3

0.4

0.5

0.6

0.7

O/C Fig. 1. van Krevelen diagram of Elodea feedstock and HTC products along with common biological molecules.

due to the loss of organic matter as a result of hydrothermal treatment, the ash content in the combined HTC product streams (showing a combined recovery of 100% at both temperatures) is higher as compared to the ash content in the pristine substrate. In comparison to biochars and process waters originating from HTC of a wide array of other biomasses (see Oliveira et al. (2013), Poerschmann et al. (2014) and references cited therein), the ash content in HTC product streams of E. nuttallii is extremely high (see Section 3.1.2.). Beyond insufficient decarboxylation (see above), the high ash content contributes to low HHV data. These combined findings provide evidence that setting the operating temperature has always been a compromise in HTC work (Wiedner et al., 2013): high operating temperatures are associated with higher defunctionalization, which is equivalent to the production of biochars of higher calorific value. In addition, the biochar produced from E. nuttallii at 240 °C is easier to handle: it can be ground to a fine dark brown powder, whereas that produced at 200 °C was fibrous, inhomogeneous and not easily to ground with a mortar. On the contrary, biochar mass yields are lower at high operating temperatures compared to mild HTC conditions. Moreover, high temperatures result in high total pressures due to the vapor pressure of water and an enhanced loss of carbon dioxide. This is tied to high equipment costs, especially when the process is performed in a continuous fashion. In contrast to operating

3.1.2. Mass balance of inorganics Table 2 summarizes the concentrations of individual inorganic cations along with the elements S, P and Si. Concentrations in the pristine E. nuttallii biomass as well as in the biochars were determined by X-ray fluorescence spectroscopy, those in the dried process water phases were determined by ICP-MS coupling after digestion. Bivalent calcium proved the most abundant element in the pristine biomass. It should be noted that Elodea plants were taken as a whole, thus not considering that heavy metals are not equally distributed across roots, shoots, and leaves as demonstrated in (Kaehkoenen et al., 1997). As expected (Reza et al., 2013; Poerschmann et al., 2014), calcium is almost quantitatively translocated into the biochars. The latter holds also true for phosphorous which ought to be beneficial in the prospective application of biochar as fertilizer. Predominant translocation of phosphorous into the solid biochar has also been described previously for wheat straw (Funke et al., 2013) as well as for bamboo and whey (Schneider et al., 2011). Unfortunately, potential phytotoxic elements such as copper, lead, zinc and aluminuma are also enriched in the biochar. On the contrary, monovalent potassium and sodium remain almost quantitatively in the aqueous phase. The concentration of Cd in the pristine substrate proved very low (6 lg g 1). During HTC, Cd was almost quantitatively transferred into the biochar. The combined concentrations of inorganics in biochar and in the aqueous phase provided evidence that 70% of inorganics of the pristine biomass were transferred into the solid biochar (13.94/20.06 = 0.695, see Table 2). In the face of the high ability of Elodea plants to accumulate inorganic species, they have been used in phytoremediation studies. Soils polluted with Ni and Cd were decontaminated by direct accumulation in roots and shoots (Fritioff and Greger, 2007). Similarly, Elodea plants exhibited high phosphorous storage stabilities (Thiébau, 2005) and high accumulation capacities for Hg (Regier et al., 2013). In Table 1 (‘‘AshDM” column), calculated ash yields are given (in parenthesis, italics) alongside gravimetric ash data. These yields were calculated on the basis of elemental concentrations as given in Table 2: it was understood in this framework that Ca was measured gravimetrically as CaO, K as K2O, Si as SiO2, S as SO3, P as P2O5, Mn as MnO2, Na as Na2O, Fe as Fe2O3, Al as Al2O3, and Mg as MgO. Basically, there is a good agreement between

27.7 28.7

20.4 24.9 25.5

21 40 (106) n.d. (0) 106 44 (107) n.d. (0) 107

1

Pb (lg g

47 80 (99) 4 (3) 102 97 (105) 8 (4) 109

Cu (lg g

1

)

3.2. Biogas formation from process water

(95) (6) (45) (46) (98) (6)

Data in parenthesis: recovery of the elements in% referred to Elodea dry mass. DOM-data for Si in italics: concentration calculated assuming for the residual Si concentration to be the gap to 100% (see text). a

b

(104) (6) (85) (9) (1) (92) (91) (3)

(88) (4)

0.91 0.02 3.24 87 0.03 3.82 93 Elodea substrate Biochar 200 °C DOM 200 °C P (Char + DOM)% Biochar 240 °C DOM 240 °C P (Char + DOM)%

8.96 15.2 2.41 102 17.2 2.58 104

(95)a (7)

3.20 0.05 (1) 12.1 (93) 94 0.19 (3) 14.25 (92) 95

2.81 3.26 (65) 4.09(35)b 100 3.35 (61) 5.02 (39)b 100

1.07 0.65 2.47 90 0.91 2.22 91

(34) (56)

1.02 1.73 0.16 98 1.82 0.13 94

(94) (4)

(97) (5)

0.95 1.65 0.19 102 1.61 0.15 92

(1) (86)

0.78 1.19 0.19 93 1.30 0.32 95

(87) (6)

0.39 0.62 0.22 102 0.80 0.10 110

(89) (13)

0.23 0.34 0.10 94 0.43 0.07 101

(83) (11)

740 1207 (91) 77 (3) 94 1548 (106) 13 (1) 107

156 183 (66) 146 (22) 88 316 (103) 22 (3) 106

)

1

Ni (lg g )

1

Zn (lg g Mg (%) Al (%) Fe (%) Na (%) Mn (%) P (%) S (%) Si (%) K (%) Ca (%)

Table 2 Concentrations of elements in the input Elodea biomass, in the biochars and in the process water phases (concentrations referred to dry mass of the input biomass).

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gravimetrically determined (combustion-derived) ash concentrations and concentrations calculated by using elemental concentrations of the respective matrices. In the light of a prospective application of Elodea-biochar as fuel, another shortcoming (in addition to low HHV-data) may come into effect: The high content of ash may act detrimental due to the ash’s tendency to deposit inside the HTC vessel.

)

P

(Inorg) (%)

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The utilization of water soluble organic components from HTC to produce biogas has been previously described. Cumulative findings suggest a good suitability of process waters for anaerobic digestion (Funke et al., 2013), (Oliveira et al., 2013), (Poerschmann et al., 2014). Herein, the DOC content of the E. nuttallii process water amounted to 3.05 g L 1 (200 °C) and 2.72 g L 1 (240 °C). These data proved low, but it should be noted that 60 g E. nuttallii (dry matter as low as 8.8%) were diluted with 120 g water (see Section 2.2.). Relatively high BOD10/COD-ratios of 0.35 (200 °C) and 0.51 (240 °C) (see Table 1) gave reason to expect a good suitability of both process waters to generate biogas. Indeed, as presented in Table 1, standard volumes of biogas referred to DMBiomass were measured to be as high as 456 mL g 1 (200 °C) and 463 mL g 1 (240 °C), which translates into methane yields of 283 mL g 1 (200 °C) and 287 mL g 1 (240 °C). The course of the biogas development is illustrated in the Supplemental material section (Fig. S-2). Intriguingly, biogas production proved very similar when subjecting further Elodea varieties sampled on different sites in Germany. For example, process water as obtained from HTC (220 °C) of E. nuttallii harvested from Goitzsche lake gave a biogas yield of 476 mL g 1 (methane yield 285 mL g 1) referred to organic dry mass (detailed data shown elsewhere). In comparison to biogas formation with HTC process waters arisen from brewer’s spent grain amounting to 479 mL g 1 organic dry mass (200 °C) and 654 mL g 1 (240 °C) (Poerschmann et al., 2014), the biogas/biomethane yield with E. nuttallii process waters is significantly lower. However, in contrast to the biogas development on the basis of HTC process water from wheat straw (165 mL g 1) (Funke et al., 2013) the biogas yield with process waters of E. nuttallii is higher. 3.3. Characterization of organic breakdown products in HTC slurries 3.3.1. Long-chain carboxylic acids Table 3 lists fatty acid concentrations in the HTC slurry (200 °C) along with corresponding concentrations in the pristine biomass obtained by alkaline saponification. GC/MS identification was based on methyl esters and TMS ethers. Diagnostic ions are given in the Supplementary material section (Table S-1). The fatty acid pattern of E. nuttallii is characterized by an even-over-odd predominance, with palmitic and a-linolenic acid being the most abundant surrogates (Table 3). As with other biomasses (Poerschmann et al., 2014), the overwhelming majority of fatty acids in the pristine Elodea biomass is present in bound form (acylglycerines, glycolipids, phospholipids) rather than occurring as free acids. Preliminary lipid class pre-separations using cyanopropyl bonded cartridges (Poerschmann and Carlson, 2006) provided surprising evidence that glycolipids account for the most abundant class (around 45%), while neutral and phospholipids account for around 25% each. Data in Table 3 provide evidence that 21% (7.5/35.2 = 0.21) of the total fatty acid fraction is recovered in the slurry. No intact lipids were found indicating a complete hydrolysis of lipids. The presence of nonanoic acid indicates that unsaturated surrogates (e.g. oleic acid 18:1ῳ9c) were degraded during hydrothermal treatment. Although no detailed sorption studies were performed, it is safe

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Table 3 Concentration of fatty acids (as methyl esters) in the Elodea substrate and in the HTCslurry (200 °C), data in lg g 1 DM Elodea.a

Table 4 Concentrations of short-chain carboxylic acids in the HTC slurry (200 °C), data in lg g 1 DMBiomass.a

Fatty acid (shorthand designation)

MW (Da)

CBiomass

CSlurry (200 °C)

Acids

MW (Da)

CSlurry

Nonanoic (9:0) Caprinic (10:0) Lauric (12:0) Myristic (14:0) Pentadecanoic (15:0) Palmitoleic (16:1ῳ7c) Palmitic (16:0) Heptadecanoic (17:0) a-Linolenic (18:3ῳ3,6,9c) Linoleic (18:2ῳ6,9 cc) OH-16:3b Oleic (18:1ῳ9c) Stearic (18:0) 20:5ῳ3,6,9,12,15c Arachic (20:0) 22:6 Behenic (22:0) P (Fatty acids) (mg g 1)

158 172 200 228 242 254 256 270 278 280 280 282 284 302 312 328 340

n.d. 355 535 1330 325 1710 12,550 180 10,160 4200 330 1095 1720 160 145 90 250 35.2

320 160 225 410 130 160 2890 110 1,295 430 n.d. 540 625 n.d. 37 n.d. 72 7.5

Acetic acid Formic acid Propionic acid a-M-acrylic acid i-Butyric acid n-Butyric acid 2-M-2-butenoic acid c-2-Pentenoic acid t-2-Pentenoic acid P Branched C5-acids n-Valeric acid n-Capronic acid Glycolic acid Lactic acid ß-Lactic acid Oxalic acid Acetoacetic acid 3-OH-butyric acid 2-OH-i-butyric acid 4-OH-butyric acid 2,3-Di-OH-propionic acid Levulinic acid Fumaric acid Succinic acid 2-OH-3-M-butyric acid 2-OH-valeric acid 2,4-Di-OH-butyric acid 5-Oxo-capronic acid 4-Oxo-capronic acid Glutaric acid 2-OH-3-M-valeric acid 2-OH-capronic acid 3-Oxo-1-cyclohexene-1-acid 6-Oxo-enanthic acid 2-M-glutaric acid Adipic acid 2-OH-glutaric acid Tartraic acid 7-Oxo-caprylic acid Suberic acid Azelaic acid

60 46 74 86 88 88 100 100 100 102 102 116 76 90 90 90 102 104 104 104 106 116 116 118 118 118 120 130 130 132 132 132 140 144 146 146 148 150 158 174 188

8900 (12,450)b 3650 (5830) 1780 (1920) 655 1,490 465 280 330 435 2,080 220 185 810 5650 (9210) 310 170 110 440 1,650 2,120 310 1880 470 620 1370 430 550 230 590 335 435 220 180 140 110 80 170 125 120 350 300

a Quantification was performed using palmitic-d2 as internal standard. Calibration ions: m/z = 76 amu, m/z = 88 amu, m/z = 272 amu for methyl ester, m/z = 76 amu, m/z = 119 amu, m/z = 315 amu for TMS ether (see text). b The hydroxyl-polyunsaturated fatty acid was tentatively identified by methylation followed by silylation of the free OH-group in the alkyl chain.

to assume that the major fraction of fatty acids was sorbed onto the biochar: data in (Poerschmann et al., 2014) reported for HTC-biochar originating from brewer’s spent grain that 96% of fatty acids were sorbed onto the particulate matrix (pH  4). In line with these findings, the averaged sorption rate of fatty acids on HTC-biochars based on algae was reported to be 92% (Heilmann et al., 2011). The sorption of fatty acids is regarded ambivalent in the face of a prospective application as soil amendment: sorbates may serve as substrate or may be inhibitory for bacterial activities (Wilson and Novak, 2009). The pattern of fatty alcohols in the pristine E. nuttallii was peaked at 1-tetracosanol. For analytical purposes, these alcohols were released by alkaline saponification, then subjected to silylation. The combined fatty alcohol concentration (nC18-nC28) amounted to 470 lg g 1 DMBiomass. This low content refers to a minor fraction of wax esters in the pristine biomass in comparison to glycolipids, acylglycerines, and phospholipids. The combined yield of fatty alcohols in the HTC slurry (200 °C) is lower than 50 lg g 1 DMBiomass. Similarly, the isoprenoid alcohol phytol along with its degradation product 6,10,14-trimethyl pentadecan-2-one – both of them could be detected in the pristine biomass – were completely degraded as a result of hydrothermal treatment. Total sterol concentration in the pristine E. nuttallii amounted to 530 lg g 1 DMBiomass. The pattern is characterized by high abundances of ß-sitosterol (60% of total sterol concentration) with stigmasterol, cholesterol, campesterol, and ergosterol (in this order of abundance) accounting predominantly for the remaining fraction. Ergosterol may serve as an indicator for fungal biomass colonizing Elodea plants (Parsi and Górecki, 2006). As a result of hydrothermal treatment, the total sterol concentration dropped to 28 lg g 1, i.e. the overwhelming fraction of sterols was degraded as observed with fatty alcohols. The pattern of ‘‘surviving” sterols was basically preserved, though. 3.3.2. Low molecular weight carboxylic acids Concentrations of short-chain acids including OH-acids and dicarboxylic acids in the slurry (200 °C) are given in Table 4. GC/MS identification was based on non-derivatized acids, their methyl esters and their TMS ethers. Diagnostic ions are given in the Supplementary material section (Table S-2). Corresponding

a Acetic acid to capronic acid were determined in free form by GC on SolGelWax, quantification was performed by referring to phenol-d6 as internal standard, glycolic acid to azelaic acid were determined as methyl esters as well as TMSderivatives, with succinic acid-d4 serving as internal standard (see text). b Data in parenthesis: concentrations determined by ion chromatography, quantification performed by external calibration.

concentrations measured at 240 °C are higher by 25% across the whole array of acids listed in Table 4. This finding is in line with data reported in (Hoekman et al., 2013). It should be noted that compounds listed in Table 4 proved virtually absent in acidic extracts (methanol/water = 1:10 w/w, pH = 1.5) of the pristine (lyophilized) biomass: Only minor concentrations of oxalic acid, lactic acid, aconitic acid, succinic acid and malonic acid (listed in decreasing order of abundance) could be identified. Their combined concentration was less than 150 lg g 1 DMBiomass; though. Acetic acid turned out the most abundant surrogate in the slurry. Its concentration of 12,450 lg g 1 means that 1.5% w/w of the substrate’s OC is transferred into the OC of acetic acid. The acids listed in Table 4 are overwhelmingly due to degradation of native monosaccharides along with monosaccharides originating from degradation of di-, oligo- and polysaccharides via the formation of 5-hydroxymethyl furfural or other dehydration products (see Libra et al. (2011) and references cited therein). Further sources of minor significance include proteins and lignin (Hoekman et al., 2013). Dicarboxylic acids constitute oxidation and/or hydrolysis products of unsaturated fatty acids. Acids listed in Table 4 show minor sorption capabilities onto biochar

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(Poerschmann et al., 2014). Thus, the low molecular weight organic acids remain almost exclusively in the aqueous process stream. This conclusion should be relevant in the light of producing value-added products such as levulinic acid as efficient source of biofuel on the basis of the DOM matrix. The low concentration of levulinic acid as well as the absence of levoglucosan are tied to degradation processes within the long holding time (14 h); see Section 3.3.3. Lactic acid as the second most abundant surrogate can also be considered a potential value-added chemical because it can be upgraded into the biodegradable plastic polylactic acid and can serve as precursor of acrylic acid. The third most abundant acid, formic acid, may be conceived as a value-added chemical if used as precursor of hydrogen gas. The combined concentration of acetic, formic, and lactic acid amounts to 49% of the total concentration of all identified and quantified surrogates listed in Table 4. However, the enrichment and purification of these compounds from diluted aqueous solutions has been an unresolved objective so far.

Table 5 Concentrations of O-functionalized breakdown products in the HTC slurry (200 °C), data in lg g 1 DMBiomass. Analyte class

MW (Da)

Analyte

CSlurry

Acids

112 122 136 148 150 150 166 180

2-Furoic acid Benzoic acid Benzeneacetic acid Cinnamic acid Benzenepropionic acid 3-Deoxy pentonic acid 4-OH-benzenepropionic acid 3-Deoxy hexonic acid

92 54 37 23 75 620 18 930

Phenols

94 108 110 122 124 124 138 140 152 152 154 166 196

Phenol P Cresols P Benzenediols R (C2-phenols) P (M-benzenediols) Guaiacol 4-M-guaiacol P (methoxy-benzenediols) 4-E-guaiacol Vanillin Syringol Acetoguaiacone Acetosyringone

35 22 142 17 31 97 18 26 12 25 51 65 46

Lactones

86 100 100 114

Butyrolactone 4-Valerolactone 5-Valerolactone E-Butyrolactone

43 21 14 15

Furans/furfurals

96 110 110 114 124 124 126 138

Furfural 5-M-furfural 2-Acetylfuran 3-M-furandione 2-Propionylfuran P (M-2-acetylfurans) 5-Hydroxymethyl furfural 2-M-5-propionylfuran

41 19 45 25 20 13 34 15

74 88 96 98 106 110 112 120 124 126 136 140 180

1-OH-propanone 1-OH-2-butanone P (M-2-cyclopenten-1-ones) 3-Hexen-2-one Benzaldehyde P (C -2-cyclopenten-1-ones) P 2 (M-2-OH-2-cyclopenten-1-ones) Acetophenone P (C -2-cyclopenten-1-ones) P 3 (C -2-OH-2-cyclopenten-1-ones) P 2 (OH-acetophenones) P (C -2-OH-2-cyclopenten-1-ones) P 3 Inositols

110 75 37 20 33 21 23 20 15 37 25 28 175

Alcohols/ketones

3.3.3. Oxygen-functionalized breakdown products Concentrations of oxygen-functionalized breakdown products such as phenols, phenolic acids, OH-substituted cyclopentenones, acids derived from monosaccharides and lignin are given in Table 5. GC/MS identification was based on non-derivatized analytes, TMS ethers and acetates. Diagnostic ions are given in the Supplementary material section (Table S-3). The pattern of organic intermediates listed in Table 5 is characterized by high abundances of carbohydrate and lignin-derived breakdown products. The most abundant compound class proved deoxy acids originating from monosaccharides, prominent surrogates include (erythro/threo) pairs of 3-deoxy pentonic acid and 3-deoxy-(ribo/arabino) hexonic acid. Further abundant carbohydrate-based intermediates include isomers of inositol, but no additional alcohols (glucitol, xylitol) could be found. Phenol, alkylated phenols, benzenediols, furfurals as well as cyclopentenone derivatives could be found, which may have different sources (Carrier et al., 2012). In contrast to that, guaiacol-type and syringol-type breakdown products can exclusively be assigned to lignin. Phenols listed in Table 5 may also serve as value-added products once converted into substituted benzene derivatives possessing high octane numbers. In contrast, (phytotoxic) phenols sorbed onto the biochar act in a detrimental way regarding prospective soil application of biochars. Monomeric aldohexoses and aldopentoses including glucose (arising from hydrolysis of cellulose and hemicellulose) or xylose and arabinose (from hemicellulose) could not be detected. Previous findings indicate (Reza et al., 2014a) that their concentration in HTC product streams drops sharply down with increasing operating time of hydrothermal treatment. Furfural and 5-hydroxymethyl furfural, which have been known typical HTC intermediate compounds originating from dehydration of (macromolecular) carbohydrates (Sevilla and Fuertes, 2009), could only be found in minor abundances. This finding is associated with the high holding time (14 h): most recent results (Reza et al., 2014b) demonstrate that these organic intermediates were hydrothermally degraded according to a first order kinetics, resulting in the formation of acetic, lactic and formic acid. All of them can finally degrade into carbon dioxide and water. In contrast to fatty acids, sorption of less hydrophobic analytes listed in Table 5 onto biochars is expected to be low, which is mainly due to the low specific surface area of the sorbents (Parshetti et al., 2013). The combined concentration of all analytes listed in Tables 3–6 (translated into an OC-normalized basis) proved minor in comparison to the organic carbon located in the DOM phase. As shown above, the OC of acetic acid as the most abundant breakdown product accounted for about 1.5% of the OC in the pristine biomass,

Table 6 Concentrations of N-functionalized breakdown products in HTC slurry (200 °C), data in lg g 1 DMBiomass. Analyte

MW (Da)

CSlurry

Pyrrolidine 2-Pyrrolidone Pyrrol-2-aldehyde 3-Pyridinol 2,5-Pyrrolidinedione P (C2-pyrazines) R (M-pyridinols) 2-Acetylpyrrole Acetylpyridine P (C3-pyrazines) R (M-pyridinemethanols) R (C2-pyridinols) R (M-2-acetylpyrroles) 5-Oxo-proline carboxylate R (C2-pyridinemethanols)

71 85 95 95 99 108 109 109 121 122 123 123 123 129 137

34 63 55 195 14 90 78 19 23 51 41 33 12 220 30

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which translates into about 6.5% of the DOC (23% of the biomass’ OC is transferred into DOC, see Table 1). Thus, the overwhelming majority of the DOC was macromolecular in nature as it was found with other biomasses previously (Poerschmann et al., 2014).

3.3.4. N-containing breakdown products The formation of N-containing organic intermediates such as pyridinols and pyrazines is of minor significance regarding the mass balance of nitrogen (see Table 5): the combined concentration of analytes listed in Table 5 is as low as 800 lg g 1, which translates into 1.0% nitrogen recovery (assumption: averaged mass content of nitrogen across all analytes listed in Table 5: 20%) These findings are in line with results reported for the mass balance of nitrogen for the HTC of brewer’s spent grain (Poerschmann et al., 2015). Following this estimation, the overwhelming majority of nitrogen of E. nuttallii was transferred into a macromolecular aromatic network of biochar and DOM which was reported to be bound to a polyfuran network (Baccile et al., 2011). The predominant contribution of nitrogen loss of 15.2% (see Table 1) is likely due to formation of ammonia as hydrolysis product of proteinaceous material (Wilson and Novak, 2009). In addition to ‘‘simple” N-containing breakdown products, a series of cyclic diketopiperazines was detected. Their formation as a result of hydrothermal treatment of brewer’s spent grain, which is very rich in proteins, has been detailed in (Poerschmann et al., 2015). The combined concentration of 2,5-diketopiperazines in the slurry arisen from HTC of E. nuttallii amounted to about 2,300 lg g 1 referred to DMBiomass, which translates into an additional contribution of 2.5% nitrogen recovery. Most abundant diketopiperazine surrogates included cyclo(leucine-proline) (3-(i-butyl)-hexahydropyrrolo-[1,2-a]-pyrazine-1,4-dione), cyclo (phenylalanine-proline) (hexahydropyrrolo-[1,2-a]-pyrazine-1,4dione-3-(phenylmethyl), and cyclo(leucine-phenylalanine) (2,5piperazinedione-3-benzyl-6-i-butyl). As expected (Poerschmann et al., 2015) this diketopiperazine pattern resembles that obtained on hydrothermal treatment of brewer’s spent grain. In addition to N-funtionalized organic intermediates, an array of S-functionalized surrogates such as thiazol, benzothiazol, thiophenemethanol, and acetylthiophenes could be unambiguously identified. Their formation from amino acids (cysteine, methionine) and peptides (glutathione) in biomasses rich in proteins/peptides will be detailed in a forthcoming contribution.

4. Conclusions The invasive plant E. nuttallii proved an appropriate substrate for hydrothermal conversion. The high ash content in resulting biochars which is associated with low contents of organic carbon results in biochars with low calorific values. High concentrations of (potentially phytotoxic) phenols may hamper a prospective application of these biochars as soil amendments, even if the sorption capability of biochars is low. In the face of a multitude of intermediates which may be used as a source of value-added products and platform chemicals, the potential of hydrothermal conversions in the framework of an integrated biorefinery strategy should be considered in future.

Acknowledgements We thankfully acknowledge the work of A. Raemmler (X-ray and ICP/MS analysis). The authors would also like to thank G. Weichert for technical assistance. The results are part of the research project CARBOWERT financed by the German Federal Ministry of Food, Agriculture and Consumer Protection (BMEL).

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Characterization of biochars and dissolved organic matter phases obtained upon hydrothermal carbonization of Elodea nuttallii.

The invasive aquatic plant Elodea nuttallii was subjected to hydrothermal carbonization at 200 °C and 240 °C to produce biochar. About 58% w/w of the ...
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