Production, characterization, and biogas application of magnetic hydrochar from cellulose

Bioresource Technology 186 (2015) 34–43

Contents lists available at ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Production, characterization, and biogas application of magnetic hydrochar from cellulose M. Toufiq Reza a,b,⇑, Erwin Rottler c, Rainer Tölle d, Maja Werner a, Patrice Ramm a, Jan Mumme a,e a

Leibniz Institute for Agricultural Engineering, Max-Eyth-Allee 100, Potsdam 14469, Germany Department of Chemical and Materials Engineering, University of Nevada Reno, 1664 N. Virginia St., Reno, NV 89557, USA c Institute of Earth and Environmental Science, University of Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany d Biosystems Engineering Division, Institute of Agricultural and Horticultural Sciences, Faculty of Life Sciences, Humboldt University Berlin, Berlin, Germany e UK Biochar Center, School of GeoSciences, University of Edinburgh, Crew Building, King’s Building, Edinburgh EH9 3JN, UK b

h i g h l i g h t s  Hydrothermal carbonization produces carbon-rich nano-micro size particles.  During HTC in presence of ferrites, hydrochar grows on ferrites surface.  Magnetic hydrochar has maintained its magnetic susceptibility.  Magnetic hydrochar works as support media for anaerobic biofilms.  Initial process inhibition by magnetic hydrochar needs further attention.

a r t i c l e

i n f o

Article history: Received 5 February 2015 Received in revised form 6 March 2015 Accepted 7 March 2015 Available online 14 March 2015 Keywords: Cellulose Hydrothermal carbonization Magnetic hydrochar Magnetic susceptibility Anaerobic digestion

a b s t r a c t Hydrothermal carbonization (HTC) produces carbon-rich nano-micro size particles. In this study, magnetic hydrochar (MHC) was prepared from model compound cellulose by simply adding ferrites during HTC. The effects of ferrites on HTC were evaluated by characterizing solid MHC and corresponding process liquid. Additionally, magnetic stability of MHC was tested by magnetic susceptibility method. Finally, MHC was used as support media for anaerobic films in anaerobic digestion (AD). Ash-free mass yield was around 50% less in MHC than hydrochar produced without ferrites at any certain HTC reaction condition, where organic part of MHC is mainly carbon. In fact, amorphous hydrochar was growing on the surface of inorganic ferrites. MHC maintained magnetic susceptibility regardless of reaction time at reaction temperature 250 °C. Pronounced inhibitory effects of magnetic hydrochar occurred during start-up of AD but diminished with prolong AD times. Visible biofilms were observed on the MHC by laser scanning microscope after AD. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Since the discovery of fullerenes and carbon nanotubes, carbon materials have received large interest among researchers in various fields for their excellent physical and chemical properties. Hydrothermal carbonization (HTC) is considered an effective process of producing porous carbon nano- and microspheres. To prepare functionalized carbon spheres, hydrothermal carbonization of saccharides (glucose, fructose, sucrose, etc.) and polysaccharides (e.g., cyclodextrins) are popular (Hu et al., 2010; Sun and Li, 2004). ⇑ Corresponding author at: Department of Chemical and Materials Engineering, University of Nevada Reno, 1664N. Virginia St., Reno, NV 89557, USA. Tel.: +1 (775) 784 4680; fax: +1 (775) 327 5059. E-mail address: [email protected] (M.T. Reza). http://dx.doi.org/10.1016/j.biortech.2015.03.044 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

The active residual functional groups on the surface of the carbon framework improve the hydrophobicity of micropores and the dispersion and stabilization in aqueous solution (Jiang et al., 2011). Porous carbon materials derived from HTC have been reported as base materials in various practical applications such as catalytic supports, adsorbents, lithium-ion batteries, sodium ion batteries, drug delivery, solid fuel, and gas storage media (Hu et al., 2010; Jiang et al., 2010; Reza et al., 2013a,b, 2014c). However, hydrochar, the solid product of HTC, needs to be activated or functionalized thermally or chemically prior to its material use (Kang et al., 2013). Magnetic nano- and microparticles are used in various areas of biosciences, medicine, biotechnology, environmental technology, electronics, and nondestructive testing (Safarik and Safarikova, 2009). Magnetic separation technology has been influenced more to synthesize magnetic carbon spheres. In fact, there are many

M.T. Reza et al. / Bioresource Technology 186 (2015) 34–43

literatures on synthesis and applications of magnetic biochar or magnetic carbon spheres. The common practice of magnetic pyrolysis biochar synthesis consists of several steps: (i) ferrous or ferric salts are dissolved in water and carbon sources are soaked with the salt solution at 60–80 °C for 0.5–5 h with presence of strong alkali. (ii) After soaking, the solid particles are centrifuged and dried at 80–100 °C for 2–24 h. (iii) Finally, the dried solid residue is pyrolyzed at 500–700 °C for 0.5–2 h (Atkinson et al., 2011). Understandably, the process is usually non-continuous and requires substantial time and energy. Moreover, the first stage often requires the use of harsh chemicals for functionalizing the carbon surface. Yu et al. prepared FexOy@C spheres using a low temperature process in a one-step metal impregnation, where sucrose is catalytically dehydrated in the presence of iron oxide nanoparticles (Yu et al., 2010). The lack of porosity and magnetic properties limits the application of their nanoparticles. Whereas, Atkinson et al. reported a novel iron-impregnated carbon sphere synthesis by aerosol technique. Their multi-step process requires a relatively high temperature (400 °C) (Atkinson et al., 2011). Until now, very few attempts are found on magnetic hydrochar production. For instance, FexOy@C spheres synthesized by hydrothermal treatment of glucose and ferric nitrate salt have been reported (Jia et al., 2011; Jiang et al., 2011). In most of the stated studies, hydrothermal treatment of carbohydrates was conducted at 180 °C for 9–24 h, while, a recent study reported that optimum carbonization of polysaccharides takes place above 220 °C (Diakite et al., 2013). Although biogas production through anaerobic digestion (AD) has been established for some decades, there is still need for optimization of this process in terms of process stability, higher methane yields, and inhibition problems (Ward et al., 2008). A well-known problem concerning AD is inhibition of methanogenesis by various mineral or organic compounds such as ammonia, phenols or volatile fatty acids (Chen et al., 2008). Among the range of high adsorptive materials, charcoal or biochar has been reported to mitigate inhibition by ammonia (Kumar et al., 1987; Mumme et al., 2014) and biotoxic pyrolysis products (Torri and Fabbri, 2014). The authors assumed that a higher biogas yield is related to biofilm formation on the particle surface. Methanogenic biofilms formation was also reported during AD of hydrochars (Mumme et al., 2014). Formation of such methanogenic biofilms is time consuming process, while, the common anaerobic digester is not designed to retain solid particles. Thus, combining the hydrochar with a magnetic feature could be a very promising solution for biogas process improvement. Furthermore, a recent study reported on the positive effect on process stability of AD, when using magnetic foam glass particles in the reactor (Ramm et al., 2014). The main goal of this research is to synthesize and characterize magnetic hydrochar by HTC with the presence of ferrites. Model compound cellulose was used in this study for better understanding the effects of ferrites. The magnetic susceptibility and stability of magnetic hydrochar was tested. One of the main goals of this study was to evaluate the application of magnetic hydrochar in biogas production. The hypothesis of this potential application was to use the carbon-rich surface of magnetic hydrochar for biofilm growth, while the magnetic feature can be used to prevent the biofilm from being washed out from the biogas reactor.

2. Methods 2.1. Materials The industrial microcrystalline cellulose (MCC), Avicel PH 101 (Sigma–Aldrich, Switzerland), was used as the sole feedstock in this study. Avicel PH 101 is a microcrystalline, powdery material with an average particle size of 50 lm and a bulk density of 280 kg m 3.

35

MnZn–ferrites powder (product No.: 0994) was purchased from Tridelta Weichferrite GmbH, Germany. The ore of the ferrites particle is often known as Franklinite and it is weakly paramagnetic by nature. The ferrites had the following properties (according to the manufacturer): particle size 30–80 lm, specific saturation magnetization 850 cm2 g 1, saturation flux density of 500 mT, curie temperature of 220 °C, and bulk density 1300 ± 100 kg m 3. The zeolite was obtained from Zeolith-Bentonit-Versand.de (Chemnitz, Germany), consisted mainly of natural clinoptilolite (89–95%) and had a surface area of 50–65 m2 g 1. 2.2. Synthesis of magnetic hydrochar (MHC) Dried MCC, 50 g weighed in a beaker, was transferred into a 1L Parr stirred reactor (reactor series 4520, Moline, IL, USA). Around 450 mL of deionized water (maintain 1:9 MCC, water ratio) was weighed and poured into the reactor. For magnetic hydrochar production, additional 33.4 g of ferrites was added. The content was stirred manually for 5 min to prevent a stirrer blockage. The experimental conditions were set at 250 °C at 3 K min 1 heating rate and holding time was 6 h at the desired temperature. The reaction temperature was controlled by a Parr PID (proportional– integral–differentiate) temperature controller (4848 series, Moline, IL, USA). The accuracy of the controller was ±1 °C. The pressure was not controlled, but monitored during HTC. The content was stirred continuously throughout the HTC process at 90 rpm. At the end of the reaction period, the heater was turned off and the reactor cooled down naturally. It usually takes 3–4 h to cool down from 250 to 25 °C, while pressure drops from 4–4.5 to 0.2– 0.5 MPa. The gaseous product was purged and the solid product was filtered for 20 min by a folder paper (ROTH Type 113 P filter). The process liquid was stored in a 4 °C refrigerator for further analyses. Hydrochar, the solid product, was dried overnight in a heating oven at 105 °C. The dried solid product was placed into a zip–lock bag and stored for further use. In this manuscript, hydrochar derived from cellulose without any addition of ferrites is addressed as HC and magnetic hydrochar as MHC. 2.3. Physico-chemical characterization of MHC Elemental carbon, hydrogen, nitrogen, and sulfur (CHNS) were measured using a Vario El elemental analyzer (Elementar Analysesysteme Hanau, Germany). Sulfonic acid was used in this elemental analysis as reference and two ovens were set at 1150 and 850 °C, respectively. Each sample was analyzed three times, and the oxygen content was calculated by the difference (O% = 100 (C% + H% + N% + S%)) and reported as ash-free basis. Liquid phase pH was measured directly after filtration using a WTW inoLab pH/Cond 720 m. The dry solid’s pH was measured in a 1:10 w/w solid:water solution. The solid/water mixture was shaken for 15 min before measuring the pH. The HTC process liquid TOC was measured by a TOC Analyzer 5050 A (Shimadzu Scientific Instruments, Columbia, MD, USA), as non-purgeable organic carbon (NPOC). The precision of TOC analysis was <2%. The concentrations of selected volatile organic compounds (5-HMF, 2-furfural, phenol, catechol, cresol, and resorcinol) in the process liquid were measured using a modified ICS 3000 Dionex (Thermo Scientific) with a UV detector (wavelength 280 nm) and Knaur Eurosphere II (C 18) column. A 15% acetonitrile (85% DI water) was used as mobile phase in the IC. Column temperature was set at 23 °C and flow rate was 1.0 ml min 1. Sugars (glucose, sucrose, fructose, and xylose) and organic acids (acetic, formic, and lactic acid)in the HTC process liquid were measured by Thermo Scientific Dionex Ultimate 3000 UHPLC equipped with an Eurokat H (300  8 mm) column, a RI-71 (refractive index) detector, and an Ultimate 3000 autosampler. Sulfuric acid of 0.01 N was used as

36

M.T. Reza et al. / Bioresource Technology 186 (2015) 34–43

mobile phase in the UHPLC. The flow rate was maintained at 0.8 ml min 1 and the pressure at 80 bar. The oven temperature for UHPLC was set at 35 °C during the analyses of sugars and acids in the liquid solution. For detailed physical characterization, both HC and MHC (after 60 min reaction) and the raw cellulose were analyzed by X-ray diffraction (XRD) and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. For XRD studies, the samples were ground, filled in a background-free support and brought into the XRD device. The tests were performed using a TT-3000 diffractometer in a medium Seiffert CuKa radiation (3 s step 1, 0.03° each) over the angular range of 10–80° = 2h. The data analysis was performed using the software MATCH. The Crystallography Open Database was used to obtain information on the molecular vibrations of the hydrochar. A Perkin-Elmer Spectrum 2000 ATRFTIR with mid- and far-IR capabilities was used on the raw and pretreated biomass. IR spectra of solid samples were recorded at 30 °C using ATR-FTIR. All samples were milled into fine powder for homogenization and dried at 105 °C for 24 h in an oven prior to FTIR. Around 5–10 mg of dry sample was placed in the FTIR for this analysis and pressed against the instrument’s diamond surface with its metal rod. All spectra were obtained using 200 scans for the background (air) and 64 scans for the samples, which were scanned from 500 to 4000 cm 1. The same two HC and MHC samples, which were used for XRD and ATR-FTIR, were also analyzed with scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, and nitrogen adsorption technique for pore size distribution. Powdered solid samples were dispersed on a conductive pad and introduced into the SEM equipment. The investigation was carried out using a Cambridge S200 scanning electron microscope (SEM) at an excitation voltage of 20 kV and a sample distance of 18 mm. A Si (Li) detector with SAT-window was used for EDX measurement. The measuring time per spectrum for elemental mapping was >30 min. The analysis was performed using the device’s own software. The coloring of the mappings was 1.46r, using J version of the software IMAGE. The specific surface area was determined based on nitrogen adsorption and the BET method according to ISO 9277. Barrett–Joyner–Halenda (BJH) and Dubinin–Radushkevich (DR) methods were used to determine the volume of meso- and micropores according to DIN 66134 and DIN 66135, respectively. Hitachi scanning electron microscope (SEM) S-2700 was used to investigate the influence of the process setting on HC and MHC microscopic structure. The magnetic susceptibility (a measure for the induced magnetization of a sample brought into contact with a magnetic field) per unit volume was measured using the Ferromagnetic Analyzer FMA5000 (Forgenta, Berlin, Germany). This method is based on the determination of changes in the oscillator frequency, when a probe with a volume of 5 cm3 is inserted into the alternated magnetic field of an oscillator coil. In order to calibrate susceptibility measurements, data obtained from longtime stable probes with FMA5000 were compared with a Kappa-Bridge KLY-35 and Barington MS2 B System (Klose et al., 2003). Finally, the signals were converted to susceptibility per mass unit to consider the diverse densities of the samples. 2.4. Application of magnetic hydrochar on biogas production AD was carried out at mesophilic temperatures (42 °C) based on the well-established biochemical methane potential (BMP) procedure, as described in the guideline VDI 4630 (VDI, 2006). The additives tested for their impact on biogas production were MHC (60 min, see Section 2.2) and for reference purposes plain HC (60 min), and natural zeolite. The HC was produced under the same conditions as MHC (see Section 2.2) and had a carbon content of

70.4%, an ash content of 0.6% and a pH (in H2O) of 4.84. For inoculation, the collected digestate (pH = 8.6, TS (total solids) = 1.2 %wt., VS (volatile solids) = 59.4 % of TS, NH4-N = 1.07 g kg 1, NKjel. = 1.53 g kg 1) of various laboratory scale digesters operated under mesophilic conditions at Leibniz Institute for Agricultural Engineering Potsdam (Germany) was used. Because particulate additives are usually only applied to digesters that suffer from inhibition problems, the inoculum’s concentration of NH4-N was increased to an inhibitory level of 5 g kg 1 by addition of ammonium carbonate. The fermentations were carried out in batch mode by means of 100 mL glass syringes. Each syringe was filled with 2 g of the HC, MHC, or zeolite and 30 g of inoculum. To allow statistical analysis all fermentations were performed at least in duplicates. As reference, each run included a duplicate of inoculum-only fermentation (control). Further information on the procedure can be obtained from (Mumme et al., 2014). The duration of the experiment was 158 days. On day 20 and 87, all syringes were fed with 1 g of sugar beet (Beta vulgaris) silage (pH = 3.52, TS = 21.1 %wt., VS = 83.8%). Gas production was measured (almost) daily during the first 3 weeks of the experiments and during the 3 weeks after each feeding. At other times, the measuring interval was expanded to 1–3 times per week. Methane content of the produced gas was measured at multiple times during the experiment using an Advanced Gasmitter (Sensors Europe GmbH, Erkrath, Germany). Because the methane analyzer requires 20 mL of minimum gas volume, the frequency of methane analysis varied between one or two times per week (after feedings) to one time per month. For determination of the methane production on days without measurements, equal concentrations to the next measurement day were assumed. For comparison purposes, the gas volumes were converted to standard conditions as described by (Wirth and Mumme, 2013). After the experiment, the liquid content of the two replicate syringes was merged, centrifuged and analyzed for TS, VS, C, N, H, S, and NH4N (solids phase) and NH4-N, TOC and volatile fatty acids (supernatant). After fermentation, the MHC was investigated for biofilm formation by means of a TCS SP5 (Leica, Germany) laser scanning microscope (LSM). MHC particles were placed on a petri dish (ø 58.4 mm) and separated with a magnet (ø 10 mm) from non-magnetic particles. To prevent the biofilm from dehydration, the MHC particles were kept moist by addition of tap water. By means of sample staining and subsequent LSM, two different structures of the biofilm were analyzed: extracellular polymeric substances (EPS) and cell DNA. EPS staining was conducted by Aleuria aurantia lectin labeled AlexaFluor488 (concentration: 1.6 mol AlexaFluor488 per mol protein, excitation: 450–490 nm by argon laser, emission: 510–530 nm) and DNA staining by 50 lmolar SYTO60 dye (excitation: 610–40 nm by a helium–neon laser, emission: 670–690 nm). For sample preparation, excess water was removed with a paper filter cloth. 50 lL of AlexaFluor488 dye was placed on the MHC particles and incubated for 20 min in the dark. After this, the dye was removed by paper filter and washed four times with 50 lL tap water each. After this, 50 lL of the SYTO60 dye was added and incubated again for 20 min. Subsequently, the petri dish was filled up with tap water and LSM analysis was conducted using an immersion lens.

3. Results and discussion 3.1. Chemical analyses of MHC and process liquids A series of HTC experiments was conducted with microcrystalline cellulose in presence of ferrites powder at 250 °C varying

37

Ni (mg/kg)

– 54.4 – 35.4 – 38.5 – 38.9 – 43.85 – 37.37 – 138.7 – 82 – 87.2 – 91.8 – 89.67 – 117.1 – 170.7 – 159.9 – 168 – 43.8 – 53.8 – 121.2

Cr (mg/kg) Co (mg/kg)

– 16.8 – 9.9 – 10.5 – 12.0 – 11.9 – 11.9 – 60,466 – 39,282 – 41,829 – 40,412 – 41,203 – 41,276

Zn (mg/kg) Mn (mg/kg)

– 174,734 – 120,546 – 124,567 – 123,144 – 119,200 – 117,278 – 406,370 – 368,266 – 380,854 – 206,423 – 240,823 – 328,753

Fe (mg/kg) H/C

2.02 – 0.93 1.08 0.80 0.90 0.75 0.85 0.76 0.86 0.76 0.86 7.4 – 4.9 2.0 4.7 1.8 4.4 1.8 4.6 1.8 4.6 1.8

H (%) S (%)

0.1 – 0.3 0.1 0.2 0.1 0.2 0.1 0.2 0.1 0.2 0.0 44.1 – 63.3 22.2 70.4 23.5 70.9 24.9 72.3 24.8 72.8 24.6

C (%) N (%)

0.00 – 0.01 0.01 0.08 0.01 0.00 0.01 0.11 0.01 0.00 0.01 0 100 0.9 63.6 0.6 69.5 0.6 67.7 0.7 68.2 0.3 68.6

Ash (%) pH solid

6.92 6.89 3.88 4.01 4.84 4.96 3.49 5.09 3.91 4.92 3.47 4.57 – – 38.2 21.7 40.5 22.2 44.3 19.7 42.9 24.5 44.3 24.2

Mass yield (AF%) Mass yield (%)

20 20 60 60 180 180 360 360 480 480

– –

MCC Ferrites HC MHC HC MHC HC MHC HC MHC HC MHC

– – 38.2 53.1 40.5 53.4 44.3 51.9 42.9 54.7 44.3 54.5

HTC reaction time (min) Name

Table 1 Physical properties of MHC from hydrothermal treatment of cellulose (all values expressed on dry-matter basis or stated otherwise).

HTC time from 20 to 480 min. Hydrothermal degradation of cellulose was reported for 230–280 °C and HTC times ranging of 0.5– 96 h (Diakite et al., 2013; Lu et al., 2013). However, according to previous studies, optimum temperature for HTC of cellulose is from 230 to 250 °C (Diakite et al., 2013; Lu et al., 2013). As the main goal of this study is to produce magnetic hydrochar and use it for biogas application, the higher end of optimum temperature range was chosen for this study. The higher temperature also ensures the complete carbonization at the stated reaction time. The ferrites concentration was 67% of the starting cellulose. To determine the effect of ferrites on HTC process, control runs were conducted without ferrites addition. Mass yield (the ratio of solid biochar product to the original raw biomass from which it was produced), ultimate analysis (CHNS), overall ash, and major inorganic components are reported in Table 1. For HC, the mass yield was lowest at the short HTC reaction time, but increases with time until it becomes steady, the similar trend was reported in the previous literature (Lu et al., 2013). A similar trend was also observed for pH. Cellulose, a straight link polymer, undergoes hydrolysis in subcritical water above 230 °C, followed by dehydration, condensation, re-polymerization (Reza et al., 2014b). Understandably, the mass yield is lower in short HTC reaction time due to the dominance of hydrolysis reactions. With the increase of HTC reaction time, hydrochar growth increases due to re-polymerization. In this study, the optimum mass yield was reached between 1 and 3 h at 250 °C, which is consistent with (Diakite et al., 2013). Mass yield of MHC is different than HC, mainly because of the ferrites’ high ash content. Now, the ash-free mass yield might show the impact of ferrites in HTC. Thus, ash-free mass yield is also presented in Table 1. In MHC, a lower ash-free mass yield was observed compared to the corresponding HC from Table 1. In fact, assuming that the ash results from the ferrites particles only ash-free hydrochar yield from MHC is 50% lower than hydrochar in the corresponding HC. In the presence of basic substances, production of organic acids from sugar molecules increased during HTC (Lynam et al., 2011). Ferrites might behave as a basic catalyst in MHC and shift the HTC reaction towards the production of organic acids rather than furanic intermediates (5-HMF, furfural, etc.), which are predecessors for biocrude production (Reza et al., 2014a). A decrease of pH was also observed in MHC, which confirms the increase of organic acids production during MHC. Elemental carbon content is increasing with HTC reaction time for both HC and MHC. However, the carbon content is almost three times lower in MHC than in the corresponding HC in every case. Table 2 shows chemical compounds in the HC and MHC process liquids analyzed by HPLC and GC. TOC is the highest in short reaction time for both HC and MHC, and decreases with HTC holding time. It can be noted that at any HTC condition, the MHC process liquid had a higher TOC than HC, which supports the assumed lower ash-free mass yield of MHC. For instance, the TOC in the HC process liquid after 180 min is 48% less than the MHC process liquid at similar conditions, while the mass yield of ash-free solid HC is 55% higher than the MHC after 180 min. Similar to the analyzed solid MHC, the pH of the MHC process liquid is slightly higher than the HC process liquid at any reaction condition throughout this study, though the organic acid concentration in MHC process liquid is much higher than HC process liquid. The presence of basic ferrites particles might be the reason behind this observation. It is important to notice that organic acid production during HTC is increasing rapidly with the presence of ferrites. This might be an indication of catalyzing hydrolyzed products in favor of organic acid (especially lactic acid) production rather than furan derivatives (Reza et al., 2014d). In fact, the lactic acid concentration in MHC process liquid was 2.4 times higher than that of HC after 20 min of reaction time. Now, lactic acid was found unstable under

Cu (mg/kg)

M.T. Reza et al. / Bioresource Technology 186 (2015) 34–43

38

M.T. Reza et al. / Bioresource Technology 186 (2015) 34–43

Table 2 Chemical analysis of process liquids for MHC and HC at various reaction conditions. Hydrochar

HTC reaction time (min)

pH

TOC (mg/L)

Lactic acid (mg/L)

Acetic acid (mg/L)

HMF (mg/L)

Furfural (mg/L)

Catechol (mg/L)

Phenol (mg/L)

HC MHC HC MHC HC MHC HC MHC HC MHC

20 20 60 60 180 180 360 360 480 480

2.6 3.1 2.6 3.1 2.7 3.2 2.8 3.2 2.7 3.3

16,912 18,900 10,469 17,820 7180 14,250 7415 13,085 6229 12,208

6654 15,820 3082 12,260 800 10,500 500 8062 0 8338

0 0 0 0 0 2566 0 2717 0 2772

6645 1927 690 96 0 0 1 0 1 0

1825 424 740 80 85 8 15 18 30 18

0 161 33 174 24 145 49 140 31 147

0 308 175 342 45 249 68 199 26 169

prolonged reaction time for both HC and MHC, which supports earlier findings of (Reza et al., 2014d). HMF and furfural concentrations also support the assumed catalyzing effects of ferrites on HTC. In the short reaction time (20 min), the concentrations of HMF and furfural in the MHC process liquid were 3.5 and 4.3 times lower than in the corresponding HC process liquid, respectively. As both HMF and furfural are degradable under subcritical water (Reza et al., 2014d), process liquids from both HC and MHC contain negligible amounts of HMF and furfural after 180 min. On the other hand, production of phenolic derivatives (phenol and catechol) was increased in the presence of ferrites. Catechol is stable throughout the reaction time (except for HC at 20 min) and phenol shows very slow degradation after 360 and 60 min for MHC and HC, respectively. 3.2. XRD and ATR-FTIR of HC and MHC The structure of the produced MHC materials along with raw cellulose and HC was characterized by XRD technique. The XRD patterns of the composites obtained at different conditions are shown in Fig. S1. MCC has specific peaks at 2h = 14°, 17°, 23°, and 34° (Fig. S1a). The highest intensity at 2h = 23° correspond to the crystallinity of cellulose and thereby assures the crystallinity of the used raw cellulose (Park et al., 2010). The crystallinity of cellulose is often used to describe changes in its structure after thermal or chemical treatment (Park et al., 2010). Hydrothermal destruction of cellulose usually takes place at temperatures above 220– 230 °C (Diakite et al., 2013; Sevilla and Fuertes, 2009). In this study, hydrochar is amorphous, when treated at 250 °C for 6 h. The peak for crystallinity is absent in the XRD spectra (Fig. S1b), while the wide peak was noted at 2h = 26.4°, corresponding to (0 0 2) lattice of the partially graphitized carbon (Sevilla and Fuertes, 2009). This indicates that HC and MHC were carbonized enough to produce graphitic layers (Fig. S1). So, one can point out that with the hydrothermal treatment, cellulose will lose the crystallinity and becomes more amorphous. Fig. S1c shows the XRD of MHC, where cellulose is treated in presence of ferrites at 250 °C for 6 h. The absence of peak at 2h = 23° confirms that cellulose lost its crystallinity during production of MHC. Moreover, the wide peak at 26.2° is very similar to what can be found in HC, which indicates the carbon material production of MHC is similar to HC. Now, it can also be noticed that the intensity of the peak at 26.2° is relatively small for MHC (80 counts with respect to 40), which might be caused by the presence of ferrites in MHC. The other diffraction peaks of MHC were assigned to the lattice of Franklinite and Spinel ore (a Fe–Cr–Zn– Mn based ore) with the corresponding diffraction peaks of 30.0°, 35.3°, 42.9°, 53.5°, 56.9°, 62.4°, 67.7°, and 73.4°. The first five of the peaks are reported in the literature, where magnetic hydrochar was derived from c-Fe2O3 and polysaccharides (Jiang et al., 2011). Thus, an indication of co-existing the graphite particles and ferrites together can be observed in MHC.

FTIR spectroscopy has been extensively used in biomass research, as it shows the bond energies of characteristic groups in biomass, and can indicate changes in molecular formulation resulting from various treatments (Reza et al., 2014b). Aromatic compounds have weak bonds, and therefore they stretch (vibrate) at lower wave numbers usually from 500 to 2000 cm 1. Aliphatic compounds stretch in the higher wave numbers (usually 25004000 cm 1). The FTIR spectra of raw cellulose, two different HC, and corresponding MHC were performed and the spectra are shown in Fig. S2. Raw cellulose shows strong bonds at 896, 1036, 1186, 1244, 1367, 1426, 1618, 2845, and 3300 cm 1 in the spectrum. These correspond to aromatic carbon hydrogen bond (C–H), alcohol group (C–OH), aryl–alkyl ether (C–O–C), aromatic acid (C–OH), aromatic carbon skeleton (C@C), aliphatic carbon hydrogen bond (C–H), and aliphatic hydroxyl bond (–OH), respectively (Reza et al., 2014b). With HTC, oxygen containing bonds in HC solids are deemed as volatile oxygen-rich components degrade into liquid and gaseous products (Reza et al., 2014b). Meanwhile, two new peaks at 1600 and 1700 cm 1 corresponding to C@C bonds are produced and sharpen. This might be another conformation of carbonization during HTC as previously addressed by various researchers (Diakite et al., 2013; Reza et al., 2014b). Not surprisingly, ferrites show almost no bands in FTIR, except a very small band around 1500 cm 1. In the MHC solids spectra, an evidence of both ferrites and carbon spheres produced by HTC can be found. The bands in 1600 and 1700 cm 1 are weaker and shifted compared to HC, as MHC has ferrites particles and graphitized carbon particles. As seen from the Fig. S2, the band assigned to C@C vibration in aromatics groups was shifted to lower wave numbers in MHC, possibly due to the observed formation of C–O–Fe. In addition, compared with the FTIR adsorption of c-Fe2O3 reported in the literature, two shifted peaks observed at 745 and 830 cm 1 further confirmed the formation of C–O–Fe bonds due to the interaction between ferrites and the carbon matrix. They were related to Fe–O stretching modes of the tetrahedral and octahedral sites in its spinel structure (Si et al., 2012). Thus, combined with the XRD analysis and FTIR, the formation of such carbon particles with iron phase could be noticed. 3.3. SEM–EDX investigation of MHC and HC Details about the structure and morphology of the selected samples (raw cellulose, HC at 6 h, and MHC at 6 h) were examined by SEM–EDX and the images are presented in Fig. S3. From both magnifications (500 and 1500), it can be noticed that raw cellulose has more likely the cylindrical shape, with around 200– 300 lm length and 40–80 lm diameter. With and without a presence of ferrites during HTC, both HC and MHC lost their cylindrical shape, which is consistent with other findings (Diakite et al., 2013; Park et al., 2010). In fact, no presence of cylindrical-shaped particle was observed in MHC. The cylindrical particle shown in the HC has

M.T. Reza et al. / Bioresource Technology 186 (2015) 34–43

39

(a)

(b)

(c)

Fig. 1. (a) Nitrogen adsorption–desorption isotherms of raw cellulose, HC, and MHC; (b) mesopore size distributions of raw cellulose, HC, and MHC using BJH model; and (c) micropore size distributions of raw cellulose, HC, and MHC using NLDFT model.

the dimension of around 40–80 lm length and 10–20 lm diameter. The MHC is amorphous and the particle size is probably smaller than 1 lm. The details on particle size distribution of MHC comparing raw cellulose and HC are discussed later in Section 3.4.

Fig. S3c shows the EDX analyses of raw cellulose. As expected, raw cellulose has only C and O signals (H cannot be shown in EDX). The distributions of C and O are very even in the cellulose particles. Fig. S3f shows the EDX analyses of HC. As expected, HC

40

M.T. Reza et al. / Bioresource Technology 186 (2015) 34–43

Fig. 2. (a) Impact of MHC, HC, and zeolite on methane production, (b) carbon degradation of HC and MHC, and (c and d) methane yield. Error bars show standard deviations from duplicate means.

Table 3a BET, pore volume and pore size analyses of raw cellulose, HC, and MHC.

Surface area (m2 g

1

Pore volume (cm3 g

Pore size (nm)

)

1

)

Method used

Raw cellulose

HC

MHC

MultiPoint BET BJH method cumulative adsorption surface area BJH method cumulative desorption surface area DR method cumulative adsorption surface area DR method cumulative desorption surface area t-Method external surface area t-Method micropore surface area NLDFT cumulative surface area

0.74 3.74 0.93 3.82 0.95 0.74 – 0.61

21.17 95.84 54.56 97.77 56.66 17.95 3.22 15.61

0.88 3.61 1.00 3.69 1.02 0.88 – 0.64

BJH method cumulative adsorption pore volume BJH method cumulative desorption pore volume DR method cumulative adsorption pore volume DR method cumulative desorption pore volume t-Method micropore volume HK method cumulative pore volume SF method cumulative pore volume NLDFT method cumulative pore volume

5.37  10 4.65  10 5.20  10 4.49  10 – 2.95  10 3.00  10 1.79  10

Average pore radius BJH method adsorption pore radius BJH method desorption pore radius DR method adsorption pore radius DR method desorption pore radius HK method pore radius SF method pore radius NLDFT pore radius

12.39 0.46 1.06 0.46 1.06 0.18 0.18 1.55

is dominated by carbon and oxygen signals. Weak signals might come from contaminations of dust or sand particles within the HTC reactor. The distributions of C and O are very clear in HC, however, O distribution is not as obvious as in raw cellulose, mainly because of the lack of O during the HTC. Fig. 2i shows the EDX

3 3 3 3

4 4 3

1.31  10 1.22  10 1.27  10 1.18  10 1.18  10 8.19  10 8.41  10 4.01  10 10.61 0.45 0.55 0.45 0.55 0.18 0.23 0.77

1 1 1 1 3 3 3 2

2.63  10 3 1.92  10 3 2.57  10 3 1.87  10 3 – 3.22  10 4 3.29  10 4 1.24  10 3 4.31 0.46 1.00 0.46 1.00 0.18 0.23 1.00

analyses of MHC. It can be noticed that besides C and O, other elements like Co, Ni, Fe, Mn, Zn, and Cr from ferrites are present in the MHC. One random site of the magnetic hydrochar was magnified for 2750 to illustrate carbon spheres growth on the ferrites surface

41

M.T. Reza et al. / Bioresource Technology 186 (2015) 34–43 Table 3b Magnetic susceptibility analyses of MCC, HC, and MHC.

MCC Ferrites HC MHC HC MHC HC MHC HC HC MHC

HTC time (min)

Measured frequency (mHz)

Magnetic susceptibility per unit volume (erg G 2 cm 3)

– – 20 20 60 60 180 180 360 480 480

0 185,053 2 34,874 0 37,350 5 36,279 5 3 35,851

1.2  10 1.3 1.7  10 0.2 2.8  10 0.3 3.5  10 0.3 3.5  10 2.1  10 0.3

6

5

6

5

5 5

(Fig. S4). It can be noticed that carbon-rich particles are growing on the ferrites particle surface (Fig. S4a). The stability of carbon growth depends on the roughness of the ferrites surface; otherwise, the carbon particles may be detached from the ferrites particle with severe environment. The stability of carbon particles on the ferrites of MHC is discussed in the latter Section 3.6. 3.4. Textural properties of magnetic hydrochar The N2 adsorption–desorption isotherms for raw cellulose, HC and MHC are shown in Fig. 1a. It can be seen that all of the adsorption isotherms of the composite, regardless of the activation conditions, showed type I isotherms, indicating presence of large fractions of micropores. The N2 adsorption isotherms in Fig. 1a showed that there is a significant downward shift in N2 adsorption for all samples, due to the effect of heat shrinking and oxidation and decomposition of the products. The mesopore size distribution of HC and MHC are similar (Fig. 1b). However, the proportion of microporosity (indicated by Vmic/Vt) increased from raw cellulose to HC and MHC. However, the MHC has less microporosity than HC, suggesting a preferred development of mesopores inside the MHC’s carbon phase. Raw cellulose, HC, and MHC were analyzed for BET, BJH, and DR surface area and pore volumes (Table 3a). Although the raw cellulose has a cylindrical shape, yet is has a BET surface area of 0.74 m2 g 1. HC has the maximum BET surface area of 21.2 m2 g 1, while MHC has only 0.88 m2 g 1, which is similar to raw cellulose. The BET surface area of HC is in good accordance with a 28 m2 g 1 value reported for microcrystalline cellulose treated at 230 °C for 6 h (Mumme et al., 2011). However, the point to be noted is that MHC contains 72% ash, meaning that 72% of the particle has no inner pores (as ferrites are not porous). In terms of DR micropores surface area, raw cellulose and MHC show similar microporous surface areas (3.6–3.8 m2 g 1), which is 30 times lower than that of HC (97.8 m2 g 1). Similar information can be obtained from the BJH mesoporous surface area analysis. The pore volumes (micropores, mesopores, and overall) are also calculated per gram basis, so, it is obvious that MHC with 72% ash has a lower pore volume (1.9  10 3 cm3 g 1 compared to HC with 1.2  10 1 cm3 g 1). Mesopore and micropore size distributions of raw cellulose, HC and MHC are shown in Fig. 1b and c. Obviously, the amount of mesopores increases with HTC, as confirmed by the N2 adsorption isotherms. In addition, almost all of the mesopores in the HC had diameters of less than 10 nm, which confirmed that the composite mainly contained microporous structures. From Fig. 1c, the NLDFT pore distribution for raw cellulose was found with a maximum around 1–5 nm and for HC with in even less diameter (0.75–0.9 nm). However, the MHC shows relatively larger particle size in the distribution. For the MHC, a pore size of about 10 nm micropore was dominant.

Magnetic susceptibility per unit mass (m3 kg 1)

Mass density (kg m 3)

Mass (kg)

4.8  10 9 4.8  10 4 24  10 9 2.7  10 4 9.3  10 9 3.0  10 4 0.01  10 9 2.8  10 4 0.02  10 9 0.7  10 9 2.8  10 4

0.5  103 2.7  103 0.6  103 0.9  103 0.3  103 0.9  103 0.3  103 0.9  103 0.3  103 0.3  103 0.9  103

3.4  10 3 13.4  10 3 3.4  10 3 4.5  10 3 1.5  10 3 4.3  10 3 1.5  10 3 4.5  10 3 1.4  10 3 1.4  10 3 4.3  10 3

3.5. Magnetic properties of MHC The curie temperature of ferrites is 230 °C. As the HTC reaction carried out at various reaction times at 250 °C, and hydrochar was growing on the ferrites surface, it is important to evaluate the magnetic properties of MHC. In this study, magnetic susceptibility test was applied to HC and MHC solid samples. According to the definition of magnetic susceptibility (Tung et al., 2003), the non-magnetic materials have very low mass or volume susceptibility, as can be found for MCC and HC solids in Table 3b. However, pure ferrites are considered as paramagnetic material and have a 6 and 5 order of magnitude higher volume and mass susceptibility than HC, respectively. MHC particles, as they have a very high concentration of ferrites, also show 5 order of magnitude larger volume and mass susceptibility than corresponding HC, but slightly lower than pure ferrites. Magnetic susceptibility was similar for all MHC, regardless of the HTC reaction time, which might indicate that inorganic part (ferrites) of the MHC is primarily responsible for magnetic susceptibility. Nonetheless, the MHC shows similar volume and mass susceptibility as ferrites and thus proves that HTC at 250 °C has no impact on the magnetic properties of MHC.

3.6. Biogas application of MHC The impact of methane production and carbon degradation during anaerobic digestion (AD) are tested for both MHC and HC. The AD results are presented in Fig. 2. The tested additives and the control showed distinct effects on the anaerobic digestion process (Fig. 2). Because of the inhibitory level of NH4-N, methane production of the control showed a slow but steady start (Fig. 2a). After 20 days, the control reached a mean biogas volume of 0.41 mL g 1 inoculum, which is less than the 1.07 mL g 1 produced by the uninhibited control (data not shown). Based on the start-up period, the following order of inhibition severity was found: MHC P HC > control > zeolite. Potential organic inhibitors in hydrochars (Becker et al., 2013), as well as an initial inhibition of AD by hydrochar (Mumme et al., 2014), have been reported earlier. These may be the probable causes for higher inhibition shown by MHC and HC. Also several heavy metals present in the used ferrite such as Ni, Cr and Zn are known to potentially cause inhibition in anaerobic digestion provided that they exist in a soluble, free form (Chen et al., 2008). Release and bioavailability of heavy metals from magnetic particles as well as the influence of HTC are important topics for future investigations on HTC-derived magnetic biogas additives. However, looking at the methane production at longer time, it can be assumed that additive-caused inhibition was limited to the start-up phase. The zeolite’s ability to mitigate inhibition problems is well known (Ho and Ho, 2012). As it can be assumed from

42

M.T. Reza et al. / Bioresource Technology 186 (2015) 34–43

Table 4 Impact of MHC, HC and zeolite on the fate of ammonium during anaerobic digestion and concentration of TOC in the liquid phase digestate. Additive

Initial NH4-N (mg kg

1

)

Liquid digestate NH4-N (mg kg

Control MHC HC Zeolite a

5000 5000 5000 5000

3949 3640 3233 3038

Solid digestate NH4-N (mg kg

1

)

a

% of initial NH4-N

NH4-N removal by additive (mg g

79 73 65 61

NA 4.6 10.7 13.7

1

)

TOC (mg L

1

)

1

1705 4327 4074 1373

) 4519 3911 3627 4552

Calculated by difference to control in mg NH4-N per g of additive.

the control’s accelerated methane production, the inoculum’s microflora adapted quickly to the high ammonia concentration. The degradation of HC’s carbon was quantified based on biogas production during start-up. Therefore, a carbon content of 0.5356 g L 1 was assumed and the control’s biogas volume was used to correct the inoculum’s biogas production. This showed a weak degradation of HC’s carbon and a slightly negative degradation (less carbon release than the control) of MHC carbon (Fig. 2b). It can be assumed that these observations are due to inhibition rather than recalcitrance of the hydrochars. For uninhibited digestion of hydrochar, (Mumme et al., 2014) reported a degradation of 5% of hydrochar carbon after 20 days and a maximum degradability of 10.4%. The fresh-matter based methane yield obtained by the control for the sugar beet silage was 108 mL g 1 for the first feeding and 85 mL g 1 for the second feeding (Fig. 2c and d). Rapid acceleration of methane production after each feeding shows that inhibition problems played a less important role than before. This can probably be attributed to one or several of the following effects: (i) less NH4-N in form of inhibiting free ammonia due to a pH shift by the silage’s acids and acid metabolites, (ii) adaption of the microflora, (iii) microbial uptake of NH4-N for cell growth. Volatile solid (VS) based methane yields for sugar beet silage are usually in the range of 300–400 mL g 1 (Ramm et al., 2014; Weiland, 2010). As the VS methane yields in this study (1st feeding: 609 mL g 1, 2nd feeding: 484 mL g 1) were considerably higher, an overestimation due to methane production from the inoculum can be assumed. Less methane yield for the second feeding indicate a subsidizing impact from the inoculum. Compared to the control, the first feeding showed a lower methane yield for MHC and zeolite, and a similar yield for HC. The lower yield might be attributed to biofilm growth, whereas the higher yield for HC might indicate degradation of the HC’s higher carbon content (HC: 70.4% C, MHC: 23.5% C). The methane yields from the second feeding showed less deviation and a generally lower level (77–93 mL g 1, fresh matter basis). Thus, it can be assumed that inhibitory effects and inoculum-based methane production were reduced to a minimum. A sudden increase in methane production of MHC after 20 days might be attributed to methanization of certain metabolites that are known for temporary accumulation due to unfavorable thermodynamic conditions (i.e., propionic acid). After the experiment, analysis of the digestates’ solid and liquid phase revealed a general reduction of NH4-N. This can be attributed to microbial cell growth. LSM-based analysis of the MHC’s surface texture after AD revealed that the anaerobic biofilm grows homogeneously on the whole particle, without formation of any gaps or clusters of microorganisms (Fig. S5). Additionally, all additives were observed to increase the removal of NH4-N from the liquid phase and the removed amount correlated with the surface area of the additive (Table 4). The observed range of additive-specific NH4-N removal is in good accordance with (Mumme et al., 2014). Because MHC and HC containing AD trials showed less NH4-N in both the solid and the liquid phase, the dominant sink is probably microbial growth. For zeolite, the clear accumulation of NH4-N in the solid phase indicates adsorption as a relevant

factor (Mumme et al., 2014). Comparing the liquids’ TOC values indicates that volatile organic compounds (VOCs) derived from MHC and HC were not fully degraded. This is not a surprise, as hydrochars are known to contain a huge range of different hard to degrade VOCs (Becker et al., 2013) and anaerobic degradation of HTC wastewater was reported to reduce a maximum of 50% of TOC (Wirth and Mumme, 2013). 4. Conclusions The presence of ferrites catalyzes HTC reaction towards organic acids production rather than HMF and furfural. Ferrites remain chemically unchanged during HTC. MCC degrades during HTC and amorphous carbon grows on ferrites surface. MHC has a relatively low surface area, low pore volume, and large pore size than corresponding HC. Magnetic susceptibilities of MHCs are similar to pure ferrites. AD tests revealed that MHC bears the risk of initial process inhibition, which, was reduced below detection with time. MHC also worked as support media for biofilm formation, but further tests are needed to evaluate their use as additive in AD. Acknowledgements This research is supported by funds from the Bioenergy 2021 program delegated from the German Federal Ministry of Research and Education to Project Management Juelich (PtJ). The corresponding author also acknowledges Western Sun Grant Initiatives (grant No.: C0432G-C) for financial support. The authors acknowledge Zeta Partikel Analytik for the support on SEM–EDX, XRD, and surface analyses. Additionally, the authors would like to thank Christoph Prautsch and Laureen Herklotz for their support in analytical tasks. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2015.03. 044. References Atkinson, J.D., Fortunato, M.E., Dastgheib, S.A., Rostam-Abadi, M., Rood, M.J., Suslick, K.S., 2011. Synthesis and characterization of iron-impregnated porous carbon spheres prepared by ultrasonic spray pyrolysis. Carbon 49 (2), 587–598. Becker, R., Dorgerloh, U., Helmis, M., Mumme, J., Diakite, M., Nehls, I., 2013. Hydrothermally carbonized plant materials: patterns of volatile organic compounds detected by gas chromatography. Bioresour. Technol. 130, 621– 628. Chen, Y., Cheng, J.J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: a review. Bioresour. Technol. 99 (10), 4044–4064. Diakite, M., Paul, A., Jager, C., Pielert, J., Mumme, J., 2013. Chemical and morphological changes in hydrochars derived from microcrystalline cellulose and investigated by chromatographic, spectroscopic and adsorption techniques. Bioresour. Technol. 150, 98–105. Ho, L., Ho, G., 2012. Mitigating ammonia inhibition of thermophilic anaerobic treatment of digested piggery wastewater: use of pH reduction, zeolite, biomass and humic acid. Water Res. 46 (14), 4339–4350.

M.T. Reza et al. / Bioresource Technology 186 (2015) 34–43 Hu, B., Wang, K., Wu, L.H., Yu, S.H., Antonietti, M., Titirici, M.M., 2010. Engineering carbon materials from the hydrothermal carbonization process of biomass. Adv. Mater. 22 (7), 813–828. Jia, Z.G., Peng, K.K., Li, Y.H., Zhu, R.S., 2011. Preparation and application of novel magnetically separable c-Fe2O3/activated carbon sphere adsorbent. Mater. Sci. Eng.: B Adv. Funct. Solid State Mater. 176 (11), 861–865. Jiang, J.H., Gao, Q.M., Zheng, Z.J., Xia, K.S., Hu, J., 2010. Enhanced room temperature hydrogen storage capacity of hollow nitrogen-containing carbon spheres. Int. J. Hydrogen Energy 35 (1), 210–216. Jiang, W., Zhang, X.J., Sun, Z.D., Fang, Y., Li, F.S., Chen, K., Huang, C.X., 2011. Preparation and mechanism of magnetic carbonaceous polysaccharide microspheres by low-temperature hydrothermal method. J. Magn. Magn. Mater. 323 (22), 2741–2747. Kang, S.M., Ye, J., Zhang, Y., Chang, J., 2013. Preparation of biomass hydrochar derived sulfonated catalysts and their catalytic effects for 5hydroxymethylfurfural production. RSC Adv. 3 (20), 7360–7366. Klose, S., Tolle, R., Baucker, E., Makeschin, F., 2003. Stratigraphic distribution of lignite-derived atmospheric deposits in forest soils of the Upper Lusatian region, East Germany. Water Air Soil Pollut. 142 (1–4), 3–25. Kumar, S., Jain, M.C., Chhonkar, P.K., 1987. A note on stimulation of biogas production from cattle dung by addition of charcoal. Biol. Wastes 20 (3), 209– 215. Lu, X.W., Pellechia, P.J., Flora, J.R.V., Berge, N.D., 2013. Influence of reaction time and temperature on product formation and characteristics associated with the hydrothermal carbonization of cellulose. Bioresour. Technol. 138, 180–190. Lynam, J.G., Coronella, C.J., Yan, W., Reza, M.T., Vasquez, V.R., 2011. Acetic acid and lithium chloride effects on hydrothermal carbonization of lignocellulosic biomass. Bioresour. Technol. 102 (10), 6192–6199. Mumme, J., Eckervogt, L., Pielert, J., Diakite, M., Rupp, F., Kern, J., 2011. Hydrothermal carbonization of anaerobically digested maize silage. Bioresour. Technol. 102 (19), 9255–9260. Mumme, J., Srocke, F., Heeg, K., Werner, M., 2014. Use of biochars in anaerobic digestion. Bioresour. Technol. 164, 189–197. Park, S., Baker, J.O., Himmel, M.E., Parilla, P.A., Johnson, D.K., 2010. Cellulose crystallinity index: measurement techniques and their impact on interpreting cellulase performance. Biotechnol. Biofuels 3. Ramm, P., Jost, C., Neitmann, E., Sohling, U., Menhorn, O., Weinberger, K., Mumme, J., Linke, B., 2014. Magnetic biofilm carriers – the use of novel magnetic foam glass particles in anaerobic digestion of sugar beet. J. Renewable Energy. Reza, M.T., Lynam, J.G., Uddin, M.H., Coronella, C.J., 2013. Hydrothermal carbonization: fate of inorganics. Biomass Bioenergy 49, 86–94.

43

Reza, M.T., Yan, W., Uddin, M.H., Lynam, J.G., Hoekman, S.K., Coronella, C.J., Vasquez, V.R., 2013. Reaction kinetics of hydrothermal carbonization of loblolly pine. Bioresour. Technol. 139, 161–169. Reza, M.T., Andert, J., Wirth, B., Busch, D., Pielert, J., Lynam, J.G., Mumme, J., 2014a. Hydrothermal carbonization of biomass for energy and crop production. Appl. Bioenergy 1 (1). Reza, M.T., Uddin, M.H., Lynam, J., Hoekman, S.K., Coronella, C., 2014b. Hydrothermal carbonization of loblolly pine: reaction chemistry and water balance. Biomass Conver. Biorefin. 4 (4), 311–321. Reza, M.T., Uddin, M.H., Lynam, J.G., Coronella, C.J., 2014c. Engineered pellets from dry torrefied and HTC biochar blends. Biomass Bioenergy 63, 229–238. Reza, M.T., Wirth, B., Luder, U., Werner, M., 2014d. Behavior of selected hydrolyzed and dehydrated products during hydrothermal carbonization of biomass. Bioresour. Technol. 169, 352–361. Safarik, I., Safarikova, M., 2009. Magnetic nano- and microparticles in biotechnology. Chem. Pap. 63 (5), 497–505. Sevilla, M., Fuertes, A.B., 2009. The production of carbon materials by hydrothermal carbonization of cellulose. Carbon 47 (9), 2281–2289. Si, Y., Ren, T., Li, Y., Ding, B., Yu, J.Y., 2012. Fabrication of magnetic polybenzoxazinebased carbon nanofibers with Fe3O4 inclusions with a hierarchical porous structure for water treatment. Carbon 50 (14), 5176–5185. Sun, X.M., Li, Y.D., 2004. Colloidal carbon spheres and their core/shell structures with noble-metal nanoparticles. Angew. Chem. Int. Ed. 43 (5), 597–601. Torri, C., Fabbri, D., 2014. Biochar enables anaerobic digestion of aqueous phase from intermediate pyrolysis of biomass. Bioresour. Technol. 172, 335–341. Tung, L.D., Kolesnichenko, V., Caruntu, D., Chou, N.H., O’Connor, C.J., Spinu, L., 2003. Magnetic properties of ultrafine cobalt ferrite particles. J. Appl. Phys. 93 (10), 7486–7488. VDI, 2006. Fermentation of Organic Materials – Characterisation of the Substrate, Sampling, Collection of Material Data, Fermentation Tests. Beuth Verlag, Berlin. Ward, A.J., Hobbs, P.J., Holliman, P.J., Jones, D.L., 2008. Optimisation of the anaerobic digestion of agricultural resources. Bioresour. Technol. 99 (17), 7928–7940. Weiland, P., 2010. Biogas production: current state and perspectives. Appl. Microbiol. Biotechnol. 85 (4), 849–860. Wirth, B., Mumme, J., 2013. Anaerobic digestion of waste water from hydrothermal carbonization of corn silage. Appl. Bioenergy 1, 1–10. Yu, G.B., Sun, B., Pei, Y., Xie, S.H., Yan, S.R., Qiao, M.H., Fan, K.N., Zhang, X.X., Zong, B.N., 2010. FexOy@C spheres as an excellent catalyst for Fischer–Tropsch synthesis. J. Am. Chem. Soc. 132 (3), 935–937.