Bioresource Technology 172 (2014) 212–218

Contents lists available at ScienceDirect

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

Improving abiotic reducing ability of hydrothermal biochar by low temperature oxidation under air Yunfeng Xu a, Zhenjun Lou a, Peng Yi b, Junyu Chen a, Xianlong Ma a, Yang Wang a, Mi Li a, Wen Chen a, Qiang Liu a,⇑, Jizhi Zhou a, Jia Zhang a, Guangren Qian a a b

School of Environmental and Chemical Engineering, Shanghai University, No. 99 Shangda Road, Shanghai 200444, China Shaoxing Drainage Management Co., Ltd, No. 391, ZhongXing Road (Middle), YueCheng District, Shaoxing, Zhejiang 312000, China

h i g h l i g h t s  240 °C oxidization under air improving hydrochar reducing ability towards Fe(III).  Carbonyl group supposed to be responsible for Fe

3+

transformation to Fe2+.

 Reducing ability be easily restored by further air-oxidization at 240 °C.  Oxidized hydrochar + H2O2 show potential on organic degradation.

a r t i c l e

i n f o

Article history: Received 14 July 2014 Received in revised form 2 September 2014 Accepted 4 September 2014 Available online 16 September 2014 Keywords: Hydrochar Low temperature oxidization Oxygen contained functional group Fe(III) reduction Fenton-like process

a b s t r a c t Oxidized hydrothermal biochar was prepared by hydrothermal carbonization of Spartina alterniflora biomass (240 °C for 4 h) and subsequent oxidization (240 °C for 10 min) under air. Oxidized hydrochar achieved a Fe(III) reducing capacity of 2.15 mmol/g at pH 2.0 with 120 h, which is 1.2 times higher than un-oxidized hydrochar. Low temperature oxidization increases the contents of carboxyl and carbonyl groups on hydrochar surface. It is supposed that carboxyl groups provide bonding sites for soluble Fe species and carbonyl groups are responsible for Fe3+ reduction. A Fenton-like process was established with Fe2+ replaced by oxidized hydrochar and tested for methylene blue (MB) decoloration. Oxidized hydrochar achieved a MB decolorization (200 mg/L, pH 7.0) rate of 99.21% within 3 h and demonstrates prominent prevail over H2O2 absent control test. This study reveals low temperature oxidization is an effective way to improve and restore abiotic reducing ability of hydrochar. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Biochar is the carbonaceous solid produced by thermochemical conversion of organic materials with the exclusion of oxygen and which has physiochemical properties suitable for safe and longterm storage of carbon in environment and, potentially, soil improvement (Shackley and Sohi, 2010). In recent years, biochar has attracted extensive attentions from environmental researchers due to its prominent benefit in contaminants elimination (Sun et al., 2011). Numerous studies reveal biochars obtained from slow pyrolysis of biomass and biowaste present excellent performance on the removal of heavy metals (Dong et al., 2011; Liu et al., 2013), phosphate (Yao et al., 2011) and organic pollutants (Sun et al., 2011) from aqueous phase, soil as well as sediment. The

⇑ Corresponding author. Tel.: +86 21 6613 7743; fax: +86 21 66137761. E-mail address: [email protected] (Q. Liu). http://dx.doi.org/10.1016/j.biortech.2014.09.018 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

elimination of environmental contaminants by biochar involves multiple processes, such as complexation with surface functional groups, ions exchange, chemical precipitation, cation–p bonding for heavy metals (Uchimiya et al., 2010; Harvey et al., 2011) and partition onto noncarbonized organic medium as well as adsorption on carbonized phase via p–p interaction for organic matters (Ni et al., 2011). Apart from above mentioned mechanisms, recently published works indicate abiotic reduction by biochar also contributes to the removal of heavy metal ions. For example, Hsu et al. (2009a) suggested that Cr(VI) removal by biochar proceeds with two consecutive steps: (i) the sorption of Cr(VI) and (ii) the subsequent reduction of sorbed Cr(VI) to Cr(III). Dong et al. (2011) indicated that Cr(VI) reduction is highly related to hydroxyl and carboxylate groups on biochar while Shen et al. (2012) suggested that the transform of Cr(VI) to Cr(III) are mainly regulated by phenolic moieties in biochar, which both emphasize the important roles of oxygen-containing functional groups on abiotic reduction of oxidation state heavy metal ions.

Y. Xu et al. / Bioresource Technology 172 (2014) 212–218

Hydrochar is a carbon-condensed material obtained from hydrothermal carbonization (HTC) of biomass/biowaste with deliberate purposes for fossil fuel substitution (Kang et al., 2012) or CO2 biosequestration (Sevilla et al., 2011). Compared with other carbonization methods for biochar production, HTC receives more attention due to its mild preparation condition, easy operation (such as mechanical dewatering of biosolids) and low environmental impact (Kang et al., 2012). In addition, hydrochar and their derivatives were proven to have potential in soil amendment (Du et al., 2012), contaminants sorption (for heavy metal such as uranium (Kumar et al., 2011), copper, cadmium (Regmi et al., 2012) and organic matter such as bisphenol A, 17 a-ethinyl estradiol and phenanthrene (Sun et al., 2011)) and electrochemistry (Paraknowitsch et al., 2009), which are also interested by many investigators. In contrast to biochar produced from slow pyrolysis, hydrochar possesses more abundant oxygen-containing functional groups such as carboxyl, hydroxyl, phenolic hydroxyl, quinine and lactone on its surface due to aqueous environment of hydrothermal carbonization process (Sevilla et al., 2011). Therefore, it is supposed that hydrochar can serve as an effective reducing agent for environmental remediation. However, to our best knowledge, the information regarding to the reducibility of hydrochar is scarce. In this paper, hydrochar were prepared from the biomass of Spartina alterniflora with the intention to evaluate their abiotic reducibility. Inspired by Chen et al. (2011), hydrochar samples were subjected to low-temperature oxidation under air to increase the quantities of oxygen-containing functional groups on HTC carbon surface. Hydrochar samples were characterized with elemental analysis, FT-IR and XPS and tested for its ability to reduce Fe(III) in aqueous solution under acidic pH. Fe(III) was chosen as the target of abiotic reduction process for two reasons: (i) the knowledge about reductive transform of Fe(III) to Fe(II) is well established; (ii) the information about the reducibility of hydrochar towards Fe(III) can be helpful to extend its practice application because most of contaminants degradation can be mediated or promoted by Fe redox transformation. Based on the result of Fe(III) reduction by hydrochar obtained in this experiment, a Fenton-like process was established and primarily tested for the decoloration of methylene blue (MB) with Fe being substituted by HTC carbon with the intention of developing possible application for the reducing ability of hydrochar. 2. Methods 2.1. Preparation of hydrochar samples S. alterniflora biomass was obtained from Chongming Island, Shanghai, China. The collected biomass was washed by deionized water for three times to scour off impurity such as dust and airdried for days. Raw biomass was cut into pieces, ground to pass through a 100 mesh sieve and oven-dried at 105 °C for 24 h. For hydrochar preparation, biomass powder and deionized water were sufficiently mixed (10 g to 50 ml) and placed into a 100 ml poly(tetrafluoroethylene)-lined stainless steel autoclave. After nitrogen flow (100 ml/min for 10 min) purging through the mixed suspension (to get rid of oxygen dissolved in solution), hydrothermal carbonization reaction was performed at 240 °C for 4 h. Then, autoclave was cooled down to room temperature and the obtained solid powder (hydrochar) was centrifugally separated (3000 rpm for 10 min) and alternately washed by acetone and deionized water until aqueous pH was stable. Hydrochar powder was oven dried at 105 °C for 24 h and ground to pass through a 100 mesh sieve for further use. To acquire oxidized hydrochar, a

213

25 ml crucible laden with 1 g hydrochar was oven-heated in air at 240 °C for 10 min. After cooled to room temperature, oxidized hydrochar sample was ground to pass through a 100 mesh sieve for experiment use without further treatment.

2.2. Sample characterization Elemental contents of C, H, N, and O in (oxidized) hydrochar and raw biomass were determined by an Elemental Analyzer (EA 3000 EuroVector EURO). BET surface area and pore structure of samples were determined by a Micromeritics ASAP 2020 using N2 as adsorbate at 195.74 °C. Infrared spectra were collected by a Thermo Scientific FTIR 380 spectrometer for wave numbers at 400–4000 cm1 with each sample mixed with KBr at a ratio of 1:100 (w/w). Surface morphology of samples were characterized by a scanning electron microscope (JEOL JSM6700) equipped with an energy dispersive spectroscopy detector. XPS (ESCALAB 250 Xi, USA) was used to determine elemental composition and chemical state of elements on sample surface. The contents of oxygen-containing functional groups on raw biomass and the resulting hydrochar were examined by Boehm titration, before which acidic and alkaline washing were performed for each sample to remove soluble species (Tsechansky and Graber, 2014). The structures of hydrochar and oxidized hydrochar were characterized by X-ray powder diffraction (XRD) by a D/max-2500 X-ray diffractometer with Cu-kA radiation (40 Kv, 250 Ma, k = 0.1789 nm) at a scanning speed of 4°/min and a scan range 2h of 5–80°.

2.3. Fe(III) reduction experiments All chemicals and reagents (Sinopharm Chemical Reagent Co., China) used in experiment were AR-grade without further treatment. Fe(III) solution was prepared by dissolving ferric chloride (FeCl3) in deionized water and pH values were adjusted by HCl and NaOH (0.1 M). Fe(III) reduction by hydrochar was conducted via a series of batch equilibrium and kinetics tests. Typically, 50 ml Fe(III) solution and 0.05 g hydrochar sample were mixed in a 100 ml conical flask covered with aluminum foil. After high-purity N2 purging (100 ml/min) for 10 min, each flask was airtightly sealed with stopper and agitated in a temperature-controlled shaker (25 °C) at a rate of 180 rpm. For predefined time interval, solid samples were separated from aqueous solutions by filtration and equilibrium pH was recorded by a pH meter (Delta320, Mettler Toledo). The concentrations of total iron and ferrous ions in solution were determined by phenanthroline spectrophotometry (752 N ultraviolet– visible spectrophotometer, China) and the difference between them is the concentration of Fe(III).

2.4. Fenton-like decoloration of methylene blue by hydrochar Hydrochar based Fenton-like tests for MB decoloration were also performed in batch. In general, 0.1 g hydrochar, 50 mL Methylene blue solution (pH = 7.0, 200 mg/L) and 1 mL H2O2 (30%) were mixed in a 100 mL conical flask covered with aluminum foil. The flasks were agitated in a temperature-controlled shaker (25 °C) at a rate of 180 rpm. After predefined time intervals, solid samples were immediately separated from aqueous solutions by filtration and the concentrations of MB in solution were determined by spectrophotometry (752 N ultraviolet–visible spectrophotometer, China).

Y. Xu et al. / Bioresource Technology 172 (2014) 212–218

3. Results and discussion

4.5 Fe(III) Fe(II)

4.0

3.1. Characteristics of samples

Redcuing percentage

3.5

3.2. Fe(III) reduction by hydrochar 3.2.1. Comparison between raw biomass, hydrochar and oxidized hydrochar Fig. 1 exhibits Fe(III) reducing amounts by S. alterniflora raw biomass and derived (oxidized) hydrochar at an initial Fe(III) concentration about 3.57 mmol/L and pH 2.0 within 120 h. In all tests, the concentrations of total iron in solution before and after Fe(III) reduction show negligible difference, revealing no obvious

Table 1 Elements analysis, surface area and yield of biomass, hydrochar and oxidized hydrochar.

Elemental analysisa C (%) H (%) N (%) O (%) H/C O/C

a b

Biomass

Hydrochar

47.30 9.45 1.99 41.26 2.40 0.65

75.35 7.85 1.50 15.30 1.25 0.15

Oxidized hydrochar 70.12 3.91 2.48 23.50 0.67 0.25

3.0

Fe (mmol/L)

Table 1 shows the results of element and BET analysis of S. alterniflora biomass and derived (oxidized) hydrochar. Raw biomass of S. alterniflora is rich in O content (41.26%) and has a high O/C ratio (0.65). Hydrothermal carbonization leads to the enrichment of C (75.35%) while the decrease in O (15.30%) and H (7.85%) contents of hydrochar, which makes O/C dropped dramatically (0.15). After 10 min exposure under air at 240 °C, O content in oxidized hydrochar grows back to 23.50% with O/C ratio rising to 0.25. Based on the yields of hydrochar and oxidized hydrochar, it is clear that much of C and H atoms evaporate via reacting with oxygen in air during calcinations process. From the results of BET analysis, it is obvious that heating treatment make the specific surface area of hydrochar increase significantly (up to 463.97 m2/g, about 210 times higher than un-oxidized hydrochar). As indicated by SEM image (Fig. S1a), the surface of hydrochar sample is abundant with aggregate microspheres with about 2–6 lm in diameter. The formation of these spherical nuclei is possibly attributed to the successive dehydration and polymerization reaction of polysaccharides contained in biomass under hydrothermal condition (Yao et al., 2007). After calcination at 240 °C, a part of microspheres on hydrochar surface vanished (Fig. S1b), which maybe contributes to the increase in specific surface area of oxidized samples. XRD patterns (Fig. S2) reveals no apparent crystalline structures in hydrochar and oxidized hydrochar samples except for SiO2, maybe related to aqueous preparation condition, under which most of soluble species are liable to leach out. For hydrochar, peak spacings at 0.53 and 0.40 nm can be assigned to hkl (010)t/(110)m and (110)t/(200)m crystallographic planes of the crystalline regions of cellulose respectively (Moon et al., 2011), revealing part of lignin reserved during hydrothermal carbonization. After oxidization, the emergence of turbostratic structure with hkl (002) plane indicates a disorder graphitization structure.

0.61 –

2.30 18.18

484.74 22.46

Yielda (%)

–

32.48

12.91

On a dry base. Obtained from single point at P/P0 = 0.08.

2.0 1.5 1.0 0.5 0.0

Biomass

Hydrochar

Oxidized hydrochar

Fig. 1. Fe(III) reduction by biomass, hydrochar and oxidized hydrochar.

sorption of Fe species by samples, probably due to strong competitive inhibition from protons under low pH. From Fig. 1, raw biomass shows the lowest reducing ability towards Fe(III) (0.42 mmol/g) among four samples examined. Hydrothermal carbonization leads to Fe(III) reduced by hydrochar doubles to 0.84 mmol/g. After further heating procedure, oxidized hydrochar made about 57.88% oxidized ferric being transformed within 120 h and demonstrates the highest Fe(III) reducing capacity of 2.15 mmol/g, which is almost 2.2 times as much as un-heated sample. 3.2.2. Kinetics of Fe(III) reduction Abiotic reduction kinetics of Fe(III) by raw biomass and hydrochar samples at pH 2.0 was found to be relatively fast initially (within 10 h), followed by a slower reduction process with reaction balance time over 120 h for all samples. Model fitting of experiment data with pseudo-first and second order kinetics (the results can be accessed from Fig. S3 and Table S1) show poor correlation coefficient (R2 < 0.9). Thus, the following kinetic model was established according to Park et al. (2006). Assuming the reaction between hydrochar and Fe(III) is a single electron transfer reaction:

BC þ FeðIIIÞ ! BCðoxidation stateÞ þ FeðIIÞ

ð1Þ

where BC represents the function groups responsible for Fe(III) reduction. Then, the rate of Fe(III) reduction by function groups on samples surface under constant pH can be obtained:

dC Fe =dt ¼ kC BC C Fe ðmmol=hÞ

ð2Þ

where CFe and CBC denote the concentrations of ferric ions and BC at a particular time (t), respectively and k represents the constant of reduction reaction. If defining C0,Fe and C0,BC as the initial concentrations of ferric ions and BC, the concentration of BC at t moment can be written as follow:

C BC ¼ C 0;BC  ðC 0;Fe  C Fe Þ

ð3Þ

then, Eq. (2) can be transformed to

dC Fe =dt ¼ kðC 0;BC  C 0;Fe þ C Fe ÞC Fe

ð4Þ

The integral formula of Eq. (4) is shown as follow

 ln

BET analysisb Surface area (m2/g) Average pore width (nm)

2.5

80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

Reducing percentage (%)

214

 C 0;Fe  ðC 0;BC  C 0;Fe þ C Fe Þ ¼ kðC 0;BC  C 0;Fe Þt C 0;BC C Fe

ð5Þ

If defining

y ¼ ln

  C 0;Fe  ðC 0;BC  C 0;Fe þ C Fe Þ C 0;BC C Fe

ð6Þ

215

Y. Xu et al. / Bioresource Technology 172 (2014) 212–218 100

Fe(III) Fe(II) Fe(II)/Intial Fe(III)

90 80 70

3

60 50 2 40 30 1

Fe(II)/Intial Fe(III) (%)

4

Fe (mmol/L)

Thus, at constant pH, a plot of y versus reaction time, t, should yield a straight line and the slope can be used to calculate k, the constant of reduction reaction from Eq. (5). Based on the results from Section 3.1, the values of C0,BC for raw biomass, hydrochar and oxidized hydrochar were presumably determined as 0.42, 0.84 and 2.15 mmol/g. Modeling results of Fe(III) reduction by three samples at initial pH 2.0 are shown in Fig. 2. As can be observed, the resulting lines gave close fits to experimental data (with R2 > 0.96), revealing Fe(III) reduction is first-order dependent on both the concentration of ferric ions and functional groups. From Fig. 2, k values for raw biomass, hydrochar and oxidized hydrochar are 0.03636, 0.01455, 0.02799 mmol1 h1, respectively. It should be noted that k is an average overall rate constant which assembles the roles from a varieties of functional groups responsible for Fe(III) reduction.

20 10 0

0 1.0

1.5

2.0

2.5

3.0

pH

3.2.3. pH dependent Fe(III) reduction by hydrochar The effect of initial pH on Fe(III) reduction by oxidized hydrochar was explored over a pH range of 0.5–3.0 at 25 °C. Results (Fig. 3) indicate that Fe(III) reduction by oxidized hydrochar is highly pH-dependent and the extent of Fe(III) reduction increases with pH but decreases dramatically when pH exceeds 2.0. pH dependence of Fe(III) reduction by NOM or biomass has been reported by many researchers and several explanations were proposed to interpret this phenomenon. The primary reason is the hydrolysis of Fe3+ ions at higher pH levels, which leads to the formation of hydroxy-Fe(III) species such as FeOH2+, Fe(OH)+2 or precipitated amorphous iron hydroxides (or ferrihydrite) that are more difficult to be reduced than soluble Fe3+ (Chen et al., 2003). Calculated results (Fig. S4) reveal a great proportion of Fe ions (10–67%, increases with pH) exist in the form of Fe(OH)+2 at pH 1.5–3.0. In current experiment, we noticed that a red–brown precipitation (Fe(OH)3) will be generated when solution pH is higher than 2.5. These facts demonstrate hydroxy-Fe(III) or iron hydroxides species can be formed even under strongly acidic condition, which results in low soluble Fe ions concentration as shown in Fig. 3 at pH >2.5, and thus hinders Fe(III) reduction by oxidized hydrochar. Another important factor could be an increased standard electrode potential of the Fe3+/Fe2+ couple with a decrease in pH, which renders thermodynamics of soluble Fe3+ reduction more favorable at a low pH than a high one (Bauer and Kappler, 2009). Both the reasons account for the sharp drop in Fe(III) reduction by oxidized hydrochar at pH above 2.0 in this experiment. As for the increasing Fe(III) reducing ability of oxidized hydrochar below pH 2.0, it is speculated that this should be associated with the protonation/deprotonation of functional groups on

4.5 4.0

Oxidized hydrochar Hydrochar Biomass

3.5 3.0

y

2.5 2.0 1.5

Fig. 3. Kinetic modeling of Fe(III) reduction by raw biomass, hydrochar and oxidized hydrochar.

hydrochar surface. Formation of an inner- or outer-sphere complex between the electron donor and acceptor in a redox reaction is the prerequisite for subsequent electron transfer between the redox pair. For example, Hsu et al. (2009a) reported Cr(VI) reduction by rice straw-derived carbon (RC) is favorable at low pH because oxidized state Cr is more readily bound to RC due to electrostatic attraction. From the results of Boehm titration (Table 2), oxidized hydrochar is abundant with acid sites such as carboxyl groups. High pH facilitates the deprotonation of these acidic groups and promotes the interaction between –COO with Fe(III) to form surface complexes. 3.2.4. Reducing capacity of oxidized hydrochar In order to fully assess the reducing capacity of oxidized hydrochar, 1.0 g sample was mixed with 1 L Fe(III) solution (pH = 2, 3.57 mmol/L) and agitated at a rate of 180 rpm and 25 °C. After thorough reaction (about 120 h), samples were withdrawn and re-thrown into fresh Fe(III) solution for several runs until no more Fe(II) ions were produced. The amount of Fe(III) reduction in each run is illustrated in Fig. 4. As can be seen, after 6 runs of operation, the reduction capacity of oxidized hydrochar is completely exhausted with Fe(III) reducing amount being 2.29, 1.07, 0.77, 0.50, 0.23 and 0.09 mmol/g in each run. Therefore, the total reducing capacity of oxidized hydrochar (the sum of Fe(III) reduced in 6 runs) reached 4.95 mmol/g. With the intention to test whether further oxidization is helpful for restoring the reducing ability of hydrochar, the exhausted samples were centrifugally separated (3000 rpm for 10 min), rinsed with deionized water until stable pH, oven-dried at 105 °C for 24 h and exposed under air at 240 °C for 10 min by the same way as hydrochar oxidization. The obtained materials (sample recovery ratio of 73.7% after further oxidization) were employed to repeat above test (run 8 in Fig. 4) and the result manifested this procedure make the reducing ability of hydrochar recovered by 93.90%, further confirming the effectiveness of oxidization treatment. 3.3. Mechanism presumed for Fe(III) reduction by oxidized hydrochar

1.0 0.5 0.0 0

20

40

60

80

100

t (h)

Fig. 2. Effect of pH on Fe(III) reduction by oxidized biochar.

The reduction of Fe3+ species by nature organic matters (NOM) and humic substances have been reported to be effective. These materials are known to be redox reactive for the abundance in functional groups such as sulfuryl or phenolic groups and quinoid moieties (Gardea-Torresday et al., 2000; Fimmen et al., 2007). Recently reported Cr(VI) reduction by biochar also emphasized

216

Y. Xu et al. / Bioresource Technology 172 (2014) 212–218

Table 2 The amounts of oxygen containing functional groups in biomass, hydrochar and oxidized hydrochar. Based on raw biomassa (mmol/g)

Base on sample (mmol/g)

Raw biomass Hydrochar Oxidized hydrochar a

Carboxyl

Lactone

Phenol

Carboxyl

Lactone

Phenol

0.365 0.364 1.253

0.254 0.840 0.290

0.279 0.120 0.001

0.365 0.118 0.162

0.254 0.273 0.037

0.279 0.039 –

The yields of hydrochar and oxidized hydrochar were coupled.

5

100

Fe(III) Fe(II)

90

Fe(II)/Intial Fe

4

80

3

60 50

2

40 30

1

Fe(II)/Intial Fe (%)

Fe (mmol/L)

70

20 10

0

0 0

1

2

3

4

5

6

7

8

9

Run Fig. 4. Repeated use of oxidized hydrochar for Fe(III) reduction under pH 2.0.

the importance of functional groups, such as carbonyl and carboxyl groups (Hsu et al., 2009b). This information suggests surface functional groups are responsible for the reducing ability of biomass and biochar. Therefore, biomass, hydrochar and oxidized hydrochar samples were characterized with FT-IR, Boehm titration and XPS analysis to indentify the effect of hydrothermal carbonation and low temperature oxidization on surface functional groups. 3.3.1. FT-IR The characteristic IR adsorption bands (Fig. S5) for raw biomass, hydrochar and activated hydrochar belong to hydroxyl groups (3410 cm1), C–H stretching (2925 cm1), carbonyl groups (1705 cm1), C@C stretching (1600 cm1), carboxyl groups asymmetry and symmetry biration (1400 cm1), C–H2 stretching (1255 cm1) and C–O stretching (1090 cm1 and 1050 cm1), respectively (Fanning and Vannice, 1993; Chen et al., 2011). Apparently, adsorptive intensity at 1600 cm1 increased after hydrothermal carbonization and 240 °C heating treatment. Some adsorption bands, e.g., C–H and C–H2, disappeared after 240 °C heat-treatment, indicating the loss of C atom, which is in accordance with the results of elemental analysis. 3.3.2. Boehm titration To quantify the contents of oxygen-containing functional groups of samples, Boehm titration tests were performed and the results are present in Table 2. The amounts of carboxyl, lactone and phenolic groups in raw biomass of S. alterniflora are 0.365, 0.254 and 0.279 mmol/g. Hydrothermal carbonization leads to an increase in lactone groups (0.840 mmol/g) while a decrease in phenolic groups (0.120 mmol/g) of hydrochar. As for oxidized hydrochar, much higher content of carboxyl groups (1.253 mmol/g) can be observed. On the basis of raw biomass employed, it is clear that much carboxyl groups (0.162 vs 0.118 mmol/g biomass for oxidized vs un-oxidized hydrochar) were generated via oxidization procedure, which agrees well with the results of Chen et al. (2011),

who found the amounts of carboxyl group increased with calcination temperature when glucose derived hydrochar is exposed under air. However, significant decrease of lactone (0.290 mmol/ g) and phenolic (0.001 mmol/g) groups in oxidized hydrochar is inconsistent with Chen et al. (2011)’s results, maybe related to the differences in calcination condition and feedstock. 3.3.3. XPS The XPS wide-scan spectra showed the presence of C (C1s, 295– 280 eV) and O (O1s, 540–528 eV) for all samples. C1s spectra (Fig. S6a) contains four peaks, which are corresponded to aliphatic/aromatic carbon groups (CHX/C–C/C@C, 284.6 eV, peak I), hydroxyl groups (–OH, 286.1 eV, peak II), carbonyl groups (>C@O, 287.4 eV, peak III) and carboxy groups, esters or latones (–COOR, 288.9 eV, peak IV), respectively (Yue et al., 1999). O1s spectra of samples (Fig. S6b) have four peaks with binding energies of 531.4, 532.5, 533.4 and 534.8 eV, which can be assigned as carbonyl oxygen (peak I), carbonyl oxygen in ester/anhydrides and oxygen atoms in hydroxyl groups (peak II), non-carboxyl (ether type) oxygen in ester/anhydrides (peak III) and oxygen in carboxyl groups (peak IV) (Okpalugo et al., 2005). Detailed assignment of each peak in C1s and O1s spectra can be accessed from Fig. S7. After hydrothermal carbonization, Peak I of C1S increased while Peak II decreased, which should be related to dehydration and aromatization reaction during hydrothermal carbonization process. Compared with biomass and hydrochar, it is obvious that peaks III and IV in C1S and Peaks I, III and IV in O1S of oxidized hydrochar were intensified, indicating a number of carbonyl groups, esters, latones and carboxyl groups formed during heating in air. 3.3.4. The promotion effect of oxidization on the reducing ability of hydrochar From the results of FT-IR, Boehm titration and XPS analysis, the distinct difference in the surface functional groups between oxidized and un- oxidized hydrochar lies in the contents of oxygen containing function groups, more specific, carboxyl and carbonyl groups. The effect of oxidization on the content of carboxyl groups of biochar has been proven by Chen et al. (2011) and Liu et al. (2013) and current experiment. Carbonyl groups, in particular, quinoid and semiquinone moieties are thought to be redox-active and responsible for reduction transformation of oxidation state metal ions or crystallization. Elovitz and Fish (1995) suggested the oxidation of phenols leads to the formation of quinones containing carbonyl groups. By electron paramagnetic resonance technique, Dela Cruz et al. (2012) demonstrated reactive oxygen species (ROS) such as semiquinones, cyclopentadienyls are widely present in low temperature thermal treated soils. Combined with result obtained by Boehm titration and XPS in this experiment, it is inferred that much of quinones and semiquinone groups were formed during low temperature oxidization process, which are responsible for Fe(III) reduction. Even though both Hsu et al. (2009b) and Dong et al. (2011) indicated carboxyl groups are involved in the removal of Cr(III) ions via complexation, there is still no direct evidence proving these sites are responsible for Cr(VI) reduction to Cr(III). In addition, compared

Y. Xu et al. / Bioresource Technology 172 (2014) 212–218

the amount of carboxyl groups obtained from Boehm titration and Fe(III) reducing amount in Section 3.2.1, no apparent correlation between them can be observed. According to Macrus (1993), forming an inner- or outer- sphere complex is the prerequisite of a redox reaction between electron donor and acceptor. Therefore, we still reckon carboxyl groups play positive roles in Fe(III) reduction since the deprotonated species of them provide bonding sites for Fe(III) complexation, which facilitates the contract between Fe(III) and the reducing active groups on hydrochar surface. From above, the promotion effects of oxidization treatment on Fe(III) reduction by oxidized hydrochar are supposed to lie in two aspects: (1) low temperature thermal treatment makes more carboxyl groups created on hydrochar surface, which promotes Fe(III) complexation with hydrochar samples; (2) oxidization treatment improves the contents of redox-active groups, such as quinines, and thus enhances reducing ability of hydrochar. 3.3.5. Hydrochar based Fenton-like process for MB decoloration Fenton process is a common advanced oxidation method for contaminants elimination, during which Fenton’s reagent [Fe(II) + H2O2] produces highly reactive and non-selective hydroxyl radicals (OH) capable of oxidizing and mineralizing most organic compounds. In light of excellent ability of Fe(III) reduction by hydrochar, a Fenton-like process was established and tested primarily for MB decoloration using oxidized hydrochar and H2O2 as oxidant. Fig. 5 shows MB removal in Fenton-like reaction by using biomass, hydrochar, oxidized hydrochar instead of Fe(II). Biomass/ H2O2 and hydrochar/H2O2 show close performance for MB degradation with reaction approaching equilibrium within 3 h, during which removal efficiencies of 54.6% and 59.2% were achieved, respectively. MB removal by oxidized hydrochar alone can be considered as a physical adsorption process, which required more time to attain balance and finally obtained a MB removal efficiency of 77.3% with 12 h. With the present of H2O2, MB was rapidly removed by oxidized hydrochar with a removal efficiency of 96.21% achieved within 2 h. Comparison with MB removal by oxidized hydrochar with and without H2O2, it is speculated that the former is more likely involved with MB degradation as revealed by Fang et al. (2014) who found that H2O2 can be effectively activated by biochar, which produces hydroxyl radical to degrade 2-CB. From Figs. 2 and 5, it is easy to find out MB degradation is quicker than Fe(III) reduction by oxidized hydrochar. A possible reason is that OH triggered by H2O2 is a highly active species,

217

which can react with MB instantly. Another possible factor responsible for this phenomenon maybe lies in the difference of pH employed in two tests. To avoid the precipitation of Fe species, Fe(III) reduction by oxidized hydrochar was performed at an extremely acidic condition. Under such a circumstance, the deprotonation of oxygen contained functional group is strongly inhibited. Therefore, the intrinsic rate for Fe(III) reduction by redox-active groups on oxidized hydrochar may be underestimated. Rigid pH limitation is the major drawback of Fe2+ based Fenton process. The results of this study clearly demonstrate hydrochar based Fenton system is effective for organic compounds degradation, and more importantly is adequate for higher pH condition, which extends the practical application of oxidized hydrochar and allows it be sufficient for environmental remediation in more diversified approaches, i.e. sorption and redox transformation, and in a more effective manner if the synergy of adsorption is taken into account. 4. Conclusion This experiment reveals that low temperature oxidization (240 °C) significantly improves the reducing ability of hydrochar towards Fe(III) via increasing the contents of carboxyl and carbonyl groups on surface. Fe(III) reduction by oxidized hydrochar follows pseudo-first order kinetics and shows both Fe(III) concentration and functional groups dependent. The reducing ability of hydrochar can be easily restored by further air-oxidization at 240 °C. Oxidized hydrochar based Fenton – like process achieves commendable performance for MB decoloration and presents prominent advantage over H2O2 absent control test. The results of this study are helpful for further extension of hydrochar application. Acknowledgements The authors are grateful for the financial supports from Science and Technology Commission of Shanghai Municipality (No. 12231204902) and Program for Innovative Research Team in University (No. IRT13078). 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.2014. 09.018. References

Removal efficiency (%)

100

80

60

oxidized hydrochar oxidized hydrochar+H2O2

40

hydrochar+H 2O2 biomass+H 2O2

20

0

3

6

9

12

Time (h) Fig. 5. MB removal by biomass, hydrochar and oxidized hydrochar with/without H2O2 under pH 7.0.

Bauer, I., Kappler, A., 2009. Rates and extent of reduction of Fe(III) compounds and O2 by humic substances. Environ. Sci. Technol. 43, 4902–4908. Chen, J., Gu, B.H., Royer, R.A., Burgos, W.D., 2003. The roles of natural organic matter in chemical and microbial reduction of ferric iron. Sci. Total Environ. 307, 167– 178. Chen, Z., Ma, L.J., Li, S.Q., Geng, J.X., Song, Q., Liu, J., Wang, C.L., Wang, H., Li, J., Qin, Z., 2011. Simple approach to carboxyl-rich materials through low-temperature heat treatment of hydrothermal carbon in air. Appl. Surf. Sci. 257, 8686–8691. Dela Cruz, A.L.N., Cook, R.L., Lomnicki, S.M., Dellinger, B., 2012. Effect of low temperature thermal treatment on soils contaminated with pentachlorophenol and environmentally persistent free radicals. Environ. Sci. Technol. 46, 5971– 5978. Dong, X.L., L. Ma, L.Q., Li, Y.C., 2011. Characteristics and mechanisms of hexavalent chromium removal by biochar from sugar beet tailing. J. Hazard. Mater. 190, 909–915. Du, Z.Y., Hu, B., Shi, A.M., Ma, X.C., Cheng, Y.L., Chen, P., Liu, Y.H., Lin, X.Y., Ruan, R., 2012. Cultivation of a microalga Chlorella vulgaris using recycled aqueous phase nutrients from hydrothermal carbonization process. Bioresour. Technol. 126, 354–357. Elovitz, M.S., Fish, W., 1995. Redox interactions of Cr(VI) and substituted phenols: products and mechanism. Environ. Sci. Technol. 29, 1933–1943. Fang, G.D., Gao, J., Liu, C., Dionysiou, D.D., Wang, Y., Zhou, D.M., 2014. Key role of persistent free radicals in hydrogen peroxide activation by biochar:

218

Y. Xu et al. / Bioresource Technology 172 (2014) 212–218

implications to organic contaminant degradation. Environ. Sci. Technol. 48, 1902–1910. Fanning, P.E., Vannice, M.A., 1993. A DRIFTS study of the formation of surface groups on carbon by oxidation. Carbon 31, 721–730. Fimmen, R.L., Cory, R.M., Chin, Y.P., Trouts, T.D., McKnight, D.M., 2007. Probing the oxidation–reduction properties of terrestrially and microbially derived dissolved organic matter. Geochim. Cosmochim. Acta 71, 3003–3015. Gardea-Torresday, J.L., Tiemann, K.J., Armendariz, V., Bess-Oberto, L., Chianelli, R.R., Rios, J., Parsons, J.G., Gamez, G., 2000. Characterization of Cr(VI) binding and reduction to Cr(III) by the agricultural byproducts of Avena monida (Oat) biomass. J. Hazard. Mater. 80, 175–188. Harvey, O.R., Herbert, B.E., Rhue, R.D., Kuo, L.J., 2011. Metal interactions at the biochar–water interface: energetics and structure–sorption relationships elucidated by flow adsorption microcalorimetry. Environ. Sci. Technol. 45, 5550–5556. Hsu, N.H., Wang, S.L., Liao, Y.H., Huang, S.T., Tzou, Y.M., Huang, Y.M., 2009a. Removal of hexavalent chromium from acidic aqueous solutions using rice straw-derived carbon. J. Hazard. Mater. 171, 1066–1070. Hsu, N.H., Wang, S.L., Lin, Y.C., Sheng, G.D., Lee, J.F., 2009b. Reduction of Cr(VI) by crop-residue-derived black carbon. Environ. Sci. Technol. 43, 8801–8806. Kang, S.M., Li, X.H., Fan, J., Chang, J., 2012. Characterization of hydrochars produced by hydrothermal carbonization of lignin, cellulose, D-xylose, and wood meal. Ind. Eng. Chem. Res. 51 (26), 9023–9031. Kumar, S., Loganathan, V.A., Gupta, R.B., Barnett, M.O., 2011. An Assessment of U(VI) removal from groundwater using biochar produced from hydrothermal carbonization. J. Environ. Manage. 92, 2504–2512. Liu, Z.Y., Demisie, W., Zhang, M.K., 2013. Simulated degradation of biochar and its potential environmental implications. Environ. Pollut. 179, 146–152. Moon, R.J., Martini, A., Nairn, J., Simonsen, J., Youngblood, J., 2011. Cellulose nanomaterials review: structure, properties and nanocomposites. Chem. Soc. Rev. 40, 3941–3994. Ni, J.Z., Pignatello, J.J., Xing, B.S., 2011. Adsorption of aromatic carboxylate ions to black carbon (biochar) is accompanied by proton exchange with water. Environ. Sci. Technol. 45, 9240–9248. Okpalugo, T.I.T., Papakonstantinou, P., Murphy, H., McLaughlin, J., 2005. High resolution XPS characterization of chemical functionalised MWCNTs and SWCNTs. Carbon 43, 153–161.

Paraknowitsch, J.P., Thomsa, A., Antonietti, M., 2009. Carbon colloids prepared by hydrothermal carbonization as efficient fuel for indirect carbon fuel cells. Chem. Mater. 21, 1170–1172. Park, D., Park, J.M., Yun, Y.S., 2006. Mechanisms of the removal of hexavalent chromium by biomaterials or biomaterial-based activated carbons. J. Hazard. Mater. 137, 1254–1257. Regmi, P., Moscoso, J.L.G., Kumar, S., Cao, X.Y., 2012. Removal of copper and cadmium from aqueous solution using switchgrass biochar produced via hydrothermal carbonization process. J. Environ. Manage. 109, 61–69. Sevilla, M., Macia-Agullo, J.A., Fuertes, A.B., 2011. Hydrothermal carbonization of biomass as a route for the sequestration of CO2: chemical and structural properties of the carbonized products. Biomass Bioenergy 35, 3152–3159. Shackley, S., Sohi, S., 2010. An assessment of the benefits and issues associated with the application of biochar to soil. Department for Environment, Food and Rural Affairs, UK Government, London. Shen, Y.S., Wang, S.L., Tzou, Y.M., Yan, Y.Y., Kuan, W.H., 2012. Removal of hexavalent Cr by coconut coir and derived chars – The effect of surface functionality. Bioresour. Technol. 104, 165–172. Sun, K., Ro, K., Guo, M., Novak, J., Mashayekhi, H., Xin, B.S., 2011. Sorption of bisphenol A, 17a-ethinyl estradiol and phenanthrene on thermally and hydrothermally produced biochars. Bioresour. Technol. 102, 5757–5763. Tsechansky, L., Graber, E.R., 2014. Methodological limitations to determining acidic groups at biochar surfaces via the Boehm titration. Carbon 66, 730–733. Uchimiya, M., Lima, I.M., Klasson, K.T., Chang, S.C., Wartelle, L.H., Rodgers, J.E., 2010. Immobilization of heavy metal ions (CuII, CdII, NiII, PbII) by broiler litter-derived biochars in water and soil. J. Agric. Food Chem. 58, 5538–5544. Yao, C., Shin, Y., Wang, L.Q., Windisch, C.F., Samules, W.D., Arey, B.W., Wang, C., Risen, W.M., Exarhos, G.J., 2007. Hydrothermal dehydration of aqueous fructose solutions in a closed system. J. Phys. Chem. C 111, 15141–15145. Yao, Y., Gao, B., Inyang, M., Zimmerman, A.R., Cao, X.D., Pullammanappallil, P., Yang, L.Y., 2011. Biochar derived from anaerobically digested sugar beet tailings: characterization and phosphate removal potential. Bioresour. Technol. 102, 6273–6278. Yue, Z.R., Jiang, W., Wang, L., Gardner, S.D., Pittman, C.U., 1999. Surface characterization of electrochemically oxidized carbon fibers. Carbon 37, 1785–1796.