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Assessment of the surface chemistry of wood-derived biochars using wet chemistry, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy.

Introduction

The ability of different soil types to chemically retain nutrients is a function of the ion-exchange capacities of their inorganic and organic components. The exchange properties are influenced by both the density of reactive functional groups and the specific surface area of soil particles (Essington 2004). These properties vary depending on the type of clay minerals and hydrous metal oxides, and the presence of soil organic matter (SOM) and pyrogenic carbon (pyC), which includes charcoal and charred biomass that remain in the soil after vegetation fires (Glaser et al. 2000). PyC can be up to 75% of total organic carbon (OC) in Terra Preta soils (Glaser et al. 2000), Anthrosols with generally greater cation-exchange capacity (CEC; Lehmann 2007) than the natural highly weathered soils nearby.

Biochar is biomass thermochemically converted in an oxygen-limited environment that is intended to be used as soil conditioner to sustainably sequester C and concurrently improve soil functions without having a negative impact on the environment and human health in the short and long term (Verheijen et al. 2009). Agronomic responses to biochar application are likely to be affected by the fertility of the soil to which biochar is added, type of crop and the effect of the specific biochar used on: (1) balancing nutrient deficiencies; (2) neutralising soil acidity, common in well-drained soils where precipitation exceeds evapotranspiration; (3) improving soil physical properties (e.g. soil drainage, soil water retention); and (4) increasing nutrient retention at surface functional groups. Considerable effort is being invested in developing methodologies to characterise biochar (Jeffery et al. 2015); however, the quantification of biochar surface charge remains a challenge, because methods commonly used to characterise soils are not always applicable to biochar. Difficulties are mostly based on the fact that both the inorganic and organic fractions of biochar may contribute to CEC (Joseph et al. 2010) and that, over time: (1) the inorganic fraction will tend to dissolve if dominated by soluble salts (Yao et al. 2010); (2) acidic functional groups on charcoal surfaces will tend to increase (Cheng et al. 2008; Calvelo Pereira et al. 2014); and (3) soil particle-biochar interactions may attenuate the surface properties of biochar (Kwon and Pignatello 2005).

The amount and type of functional groups on the surface of fresh biochars depend on the type of feedstock used and the conditions of production and/or treatment. The presence of functional groups containing oxygen (O) is common, whereas the relative contribution of groups including nitrogen (N) and sulfur (S) is low, especially in biochars produced from woody materials (Joseph et al. 2010). The amount and reactivity of functional groups and delocalised electrons present in the condensed aromatic C clusters determine the apparent acidbase characteristic of the surface of charcoal (Lopez-Ramon et al. 1999). The acid-base properties of activated carbons have been studied extensively (Lopez-Ramon et al. 1999); however, the acid-base characteristics of biochar are substantially different to those of activated carbons because they are not generally formed at very high temperatures and in the presence of activating agents, such as steam or acids, as is the case for the activated carbon (Bandosz et al. 1992; Uchimiya et al. 2010).

The aim of the present study was to reconcile information obtained from different analytical techniques, namely spectroscopic measurements (Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS)) and wet chemistry (Boehm titration, potentiometric titrations, CEC), used to characterise and quantify the oxygen-containing surface functional groups present on fresh wood-derived biochar. To achieve this, we used biochar from pine, poplar and willow, produced at two different temperatures (400 and 550[degrees]C).

Materials and methods

Feedstock and biochar used

Biomass from mature pine (PI; Pin us radiata D. Don) wood chips, 1-year-old poplar (PO; Populus nigra L. 'Italica') and pruning of mature willow (WI; Salix matsudana Koidz.) was dried overnight at 65[degrees]C before their use as feedstock. The material was chipped to particle sizes ranging from 3 mm to 4 cm before pyrolysis. Biochar was produced using a self-purged gas-fired rotating drum kiln (5 L). Highest heating temperatures (HHT) selected for biochar production were 400 and 550[degrees]C. The chemical characterisation of feedstock types and biochar production has been described previously by Calvelo Pereira et al. (2011). It was found that the feedstock type, HHT and heating rate influenced the degree of biomass carbonisation and the intrinsic lability of C in biochar.

Chemical characterisation of biochar

Biochar samples were thoroughly characterised (e.g. elemental composition, pH and specific surface area) by Calvelo Pereira et al. (2011). Relevant chemical characteristics of each biochar are summarised in Table 1. Additional characterisation performed in the present study is detailed below.

Determination of CEC

The CEC of biochar was measured following a modified version of the methods developed by Matsue and Wada (1985) and Blakemore et al. (1987). Because the electrical conductivity of biochars was below 0.3 dSnT1, a specific pretreatment to remove the soluble salts was not considered (Rayment and Lyons 2011). Briefly, the method consists of equilibrating 0.2 g ground samples with 50 mL of 0.01 M S1CI2. A semimicro leaching system using a peristaltic pump with a gentle suction rate (1 mL [min.sup.-1]) was used to promote exchange. Commercial acidified sand was used as a packing element (ratio biochar : sand, 0.1 : 2, w/w), avoiding formation of channels and air-locks inside the leaching tubes during exchange. A macerated filter paper plug placed at the bottom of the column avoided any loss of material from the tubes. The equilibrium pH was measured in the Sr[Cl.sub.2] solution. The excess counter-ion solution was eliminated by a highly diluted solution of Sr[Cl.sup.2] ([approximately equal to] 0.0001 M). The retained Sr was finally replaced with 50 mL of 0.5 M HC1 (the displacement of retained Sr by [H.sup.+] in one single HC1 washing was proposed as suitable by Matsue and Wada (1985) because of the high affinity of [H.sup.+] for the negative charge sites), determining the Sr by atomic absorption spectrophotometry (AAS); the amount of Sr displaced by the acid solution was termed [CEC.sub.Sr]. A complementary determination was performed by analysing the solubilised bases (Ca, Na, Mg, and K) in 0.01 M Sr[Cl.sub.2] solution ([[summation].sub.cations]) by AAS; this fraction includes soluble + exchangeable cations.

Estimation of total surface acidity by the Boehm method

The Boehm titration method is widely cited in studies of activated carbons and other charcoal samples (Bandosz et al. 1992; Boehm 2002; Rutherford et al. 2008). Herein we added [approximately equal to] 0.125 g ground biochar samples to 40-mL centrifuge tubes. One tube each was filled by adding 25 mL standardised solution of NaHC[O.sub.3] (0.05 M), [Na.sub.2]C[0.sub.3] (0.025 M) or NaOH (0.05 M). A control without biochar for each solution was also prepared. Extractions were always performed in triplicate (n = 3). Biochar and control tubes were mechanically shaken for 16 h (overnight). The solutions were filtered through glass fibre filters and an aliquot was titrated with 0.05 M HC1. The amount of base neutralised by the biochar was determined by the difference between the solution containing sample and the control solution. Neutralisation curves were monitored using an automated titrator (pH/EP/IP Titration Workstation TIM865; Radiometer Analytical SAS, Lyon, France) in an [N.sub.2] atmosphere. The total surface acidity was calculated as the sum of the concentration of individual functional groups based on their acidic strength (Rutherford et al. 2008).

Specific analysis of functional groups on acid-washed biochar: potentiometric titration

Acid washing has been widely used for studying soil organic matter (Cooke et al. 2007). It allows a gentle acidification that saturates the surface with [H.sup.+] (Cooke et al. 2007). In the present study, 3 g ground biochar samples was suspended in 50 mL deionised water and the pH was adjusted to approximately 1 with 0.5 mL concentrated nitric acid (HN[O.sub.3]). Suspensions were shaken for 0.5 h, subsequently subjected to exhaustive dialysis (Hina et al. 2010), then dried until constant weight at 65[degrees]C and stored. The average percentage of biochar mass remaining after this chemical treatment was 86.3 [+ or -] 3.3%. These biochars are referred to as acid-washed biochar to distinguish them from the untreated biochars.

The content of acid groups on the acid-washed biochar samples was determined by suspending 0.1 g acid-washed biochar sample in 50 mL of 0.1 M KN[O.sub.3] (n = 3) as the inert electrolyte. Thereafter, biochar suspensions were potentiometrically forward titrated with 0.2 M KOH carbonate-free solution or backward titrated with 0.1 M HC1 in order to cover the pH range from 3 to 10. Because pH values of the biochar were buffered close to pH 5, only functional groups reactive in the pH range 5-10 were determined. The content of carboxylic groups in the samples was estimated empirically as the value of Q (the sample charge) versus pH charge curves at pH 8; the content of phenolic groups was estimated empirically as twice the difference between Q at pH = 8 and Q at pH = 10 (Hina et al. 2010). Q-pH curves were obtained from the potentiometric titration dataset generated for each sample. Potentiometric measures were taken continuously using an automated titrator (pH/EP/IP Titration Workstation T1M865; Radiometer Analytical SAS, Lyon, France).

Spectroscopic characterisation

In order to describe the surface chemistry of biochar samples by providing qualitative information about functional groups and at the same time evaluate any change caused by the pretreatment, both untreated and acid-washed biochar were characterised using spectroscopic analysis.

Fourier transform infrared spectroscopy

Untreated and acid-washed biochar subsamples (0.5 mg; ground <0.5 mm) were placed onto the sample chamber of an infrared spectrometer (FT/IR-4200; JASCO, Easton, MD, USA). Spectra were obtained over 256 scans with a Ge/KBr beam splitter. A resolution of 4 [cm.sup.-1] was set, covering the range from 4000 to 650 [cm.sup.-1]. The reflectance was measured and analysed using the Spectra Manager TM 11 cross-platform software (JASCO, Easton, MD, USA). The identification of representative peaks and bands was based on published data, as reported in Supplementary Material table S1 as available on journal's website. FTIR spectroscopy provides only qualitative data and thus does not allow a direct quantitative comparison with other methods.

X-Ray photoelectron spectroscopy

Surface analysis of ground (<0.5 mm) biochar samples (both untreated and acid washed) was conducted using XPS. This was performed using an XPS spectrometer (ESCALAB 250Xi; Thermo Scientific, Hemel Hempstead, Hertfordshire, UK), with mono-chromate AlK[alpha] (1486.7 eV) radiation emitted at 164 W. The spot size was 500 pm and the photoelectron takeoff angle 90[degrees]. A non-linear least-squares curve fitting with a Gaussian-Lorentzian mix functions and Shirley background subtraction was used to deconvolute the XPS spectra. Individual peaks were related with common organic functional groups or moieties based on published data (Biniak et al. 1997; Moreno-Castilla et al. 2000; Petit et al. 2009). A semiquantification of the amount of functional groups on the biochar surface was obtained by determining the total area under the corresponding curve (C 1s, O 1s, and N 1s) that corresponds to each individual group or peak defined. Based on sensitivity factors and the transmission function of the instrument, a correction was made to obtain the atomic percentage of the C, O and N.

XPS is a surface technique and analyses the outer part of the particle with a depth that depends on the electron inelastic mean free path, which, in turn, depends on porosity, material density and electron kinetic energy. The milling process performed was not aggressive (ring grinder for 10-20 s) and, indeed, the particle size obtained was around 0.5 mm, which is quite large. Furthermore, biochars have large numbers of interconnected wide pores (macropores) because of the emptying of vegetal cells during the pyrolysis process. These wide pores formed are accessible and help the development of functionalities at the surface and, what it is more important, the porosity increases the electron inelastic mean free path considerably; subsequently, depth analysed is enhanced. Therefore, the region analysed by XPS in the milled and unground samples is identical and the soft milling process has no effect on the results obtained. For these reasons, the information obtained using XPS was considered adequate to evaluate the surface chemistry of the biochars under investigation.

Evaluation of quantitative changes because of acid treatment

The atomic percentage of C, O and N obtained by XPS analysis was used to assess surface recoveries of C, O and N after acid washing; this was calculated following Dai and Johnson (1999) as follows:

% Recovery = % mass recovery x ([C.sub.i,acid-washed]/[C.sub.i.untreated])

where % mass recovery is the percentage of biochar mass remaining after chemical treatment, ([C.sub.i,acid-washed]/[C.sub.i.untreated]) is the enrichment factor (EF) of each element in the surface, [C.sub.i,acid-washed] is the surface concentration of element i in the biochar after treatment and [C.sub.i,untreated] is the surface concentration of the element i in the biochar before the treatment. For further evaluation of possible changes of the organic moieties caused by acid-washing treatment, a parameter R (Schmidt et al. 1997) was introduced to describe the changes in the surface elemental ratios (O/C, C/N, O/N) after the treatment. The factor R was defined as follows:

R = elemental ratio before treatment/elemental ratio after acid-washing treatment

Values of R [not equal to] 1.0 indicate a relative increase or decrease of some elements with respect to others.

Results

Biochar chemical characterisation and surface functional groups content

Biochar pH values were always in the basic range, with values extending from 6.9 in PI-400 to 8.8 in PO-550 (Table 1). For a specific feedstock, biochar prepared at 550[degrees]C always showed a higher pH than the corresponding biochar prepared at 400[degrees]C, the difference being around 1 unit of pH. Total C content was high, ranging from 662 g [kg.sup.-1] for WI-400 to 847 g [kg.sup.-1] for PI-550 (Table 1). Inorganic C (IC) content in WI-400, WI-550 and PO-550 ranged between 2.2 and 2.7 g [kg.sup.-1]. These samples were also those with the greatest ash content (Table 1). The content of N was small in all biochar (<17 g [kg.sup.-1]). The carbonisation process increased the concentration of N with respect to the original feedstock (data not shown).

The elemental concentrations of H and O were always greater in the biochar samples produced at 400[degrees]C than those produced at 550[degrees]C, whereas the ash content was consistently higher in the latter (Table 1). The molar ratios (0/[C.sub.org] and H/[C.sub.org]) decreased with temperature of pyrolysis, as expected (Table 1). Low temperature biochars were non-porous, with negligible surface area (< 5 [m.sup.2] [g.sup.-1]; Table 1). Biochar produced at 550[degrees]C showed much greater surface area (55-368 [m.sup.2] [g.sup.-1]; Table 1), having a narrow microporosity (<0.8 nm).

Values of the sum of soluble and exchangeable cations ([[summation].sub.cations]) ranged between 43 and 170 [mmol.sub.c](+) [kg.sup.-1] for the 400[degrees]C biochar samples and between 58 and 190 [mmol.sub.c](+) [kg.sup.-1] for the 550[degrees]C samples (Table 1). Measured [CEC.sub.Sr] (Table 1) was always lower than [[summations].sub.cations], as expected, ranging between 9 [mmol.sub.c](+) [kg.sup.-1] in PI-400 and 85 [mmol.sub.c](+) [kg.sup.-1] in WI-550. For a given feedstock, [CEC.sub.Sr] of biochar was always higher when produced at 550[degrees]C than when produced at 400[degrees]C (Table 1). However, values of total acidic surface groups, as determined by Boehm titration, were always higher in low-temperature biochar than in high-temperature biochar and ranged between 40 and 1068 mmol [kg.sup.-1] (Table I). The amount of 'moderated acid and lactone' groups ranged between 12% and 16% in 400[degrees]C biochar and between 8% and 26% in 550[degrees]C biochar (Supplementary Material table S2).

Concentrations of carboxylic groups, as estimated by potentiometric titration of acid-washed biochar samples, ranged between 20 and 70 mmol [kg.sup.-1] (Table 1); values were always higher for low-temperature biochar samples than the corresponding high-temperature samples, regardless of feedstock, in agreement with the results from the Boehm titration. Concentrations of phenolic groups were always higher than those of carboxylic groups and ranged between 94 and 349 mmol [kg.sup.-1] (Table 1). Pine biochar, regardless of temperature of production, showed a rather similar concentration of carboxylic + phenolic groups.

Spectroscopic assessment of untreated and acid-washed biochar

Fourier transform infrared spectroscopy

All FTIR spectra for untreated biochar samples (Fig. la) were characterised by the presence of common bands or shoulders as follows: (1) 3800-3300 [cm.sup.-1] region, attributed to a broad overlap of vibration of O-H in water or alcohols (Petit et al. 2009); (2) bands around 2960 and 2850 [cm.sup.-1], related with aliphatic C asymmetric and symmetric stretching (Pradhan and Sandle 1999); (3) broad band around 1590 [cm.sup.-1], attributed to the stretching vibration of aromatic rings and/or carbonyl (C=0) groups (Pradhan and Sandle 1999); (4) a band around 1440 [cm.sup.-1], associated with -COOH and CHO stretching (Chun et al. 2004); and (5) a broad band in the 830-820 [cm.sup.-1] region related to aromatic C-H vibration out of plane (Artz et al. 2008). Some differences were observed in those FTIR spectra for low-temperature biochar samples as follows: (1) PO-400 and WI-400 biochar had a specific band in the region around 1310 [cm.sup.-1], attributed to C-O stretching corresponding to syringyl rings (Wang et al. 2009); (2) WI-400 biochar showed a peak in the 1030 [cm.sup.-1] region, related to C-O vibrations in hydroxyl groups (Petit et al. 2009); and (3) those bands around 2960 and 2850 [cm.sup.-1] were absent (or very low intensity) in Pl-400 biochar (Fig. 1a). Specific features of FT1R spectra from high-temperature biochar included: (1) a band at 1470-1460 [cm.sup.-1] attributable to O-H deformation in carboxyl groups and/or C-H bending vibrations (Moreno-Castilla et al. 2000), but only in PO-550 and Wl-550; and (2) the presence of bands around 875 and 730-720 [cm.sup.-1], usually attributed to the carbonate ion (Tatzber et al. 2007). After acid washing, all FT1R spectra (Fig. 1 b) were very similar to those of the corresponding untreated samples (Fig. 1a); as expected, those peaks or bands related to carbonate (875, 730-720 [cm.sup.-1]) were absent.

X-Ray photoelectron spectroscopy

Fig. 2 shows the typical resolution spectra of the C Is region for the biochar samples analysed. The optimum fitting resolved five main peaks that were related with C functional groups or moieties: (1) aromatic C and aliphatic structures (285.0 eV: CHx, C-C, C=C); (2) C species in alcohols and phenols (286.5 eV; C-OR); (3) double-bonded C in carbonyl, quinone groups (288.0 eV; C=0); (4) carboxyl groups (289.2 eV; COOR); and (5) carbonate and/or plasmon-loss peak, [pi]-[[pi].sup.*] shake up satellite peak due to the presence of numerous [pi]-electrons (290.8 eV). Untreated biochar spectra showed a dominant aromatic or aliphatic C peak; the relative proportion of the aromatic or aliphatic peak increased with increasing pyrolysis temperature (table S3). The singly bonded O was the main component in all spectra, because C-OR groups were always higher than the double-bonded O (i.e. C=0 and COOR) groups (Fig. 2). After acid washing, the shape of the spectra was the same, but the relative distribution of each group identified was slightly different (table S3). Resolution spectra of the O Is region for the same biochar samples confirmed the main features described above and those changes induced by acid washing (Supplementary Material fig. SI). In general, after acid washing, the shape of the spectra was similar to that of untreated biochars; PI-550 and PO-550 samples showed a different peak distribution around 532.8eV (fig. SI), which was associated with C=0 moieties (Laszlo et al. 2001).

Fig. 3 shows the typical resolution spectra of the N Is region for the biochar samples analysed. Despite the very low N concentration, an optimum fitting was achieved by resolving each of the N Is spectra into two peaks; (1) pyridine-like structures (399.1 eV); and (2) quaternary-N moieties (400.9 eV). In general, the relative amount of pyridine-like moieties increased with pyrolysis temperature (Fig. 3). After acid washing, the shape of the spectra was similar .to that of the untreated samples; however, the relative distribution of the two groups changed (Fig. 3; table S3), indicating some degree of surface modification.

Acid treatment led to a slight increase in the concentration of surface O and N as measured by XPS and a change in their distribution, whereas some C was lost (Figs 2, 3; Table 2). The average [R.sub.O/C] value was 0.99 [+ or -] 0.03, suggesting that the acid washing treatment did not result in systematic changes of the surface chemical composition regarding C structures and O functional groups. However, on average [R.sub.C/N]= 1-12 [+ or -] 0.07, whereas [R.sub.0/N] = 1.10 [+ or -] 0.06 (Table 2); these relatively large values indicated that some systematic change involving N groups may occur on the biochar surface.

Discussion

Surface chemistry and ion-exchange capacity of biochar

The CEC of these fresh biochar samples was low, in the range of those reported for other wood-derived biochar (Cheng et al. 2008; Mukheijee et al. 2011), and increased with pyrolysis temperature regardless of feedstock considered, as reported previously by Singh et al. (2010). However, to date there is no standard methodology to determine the CEC of biochar (Atkinson et al. 2010) and therefore any direct comparison between estimates of oxygen-containing functional groups must be done with care. We detected an increase in CEC with pyrolysis temperature that was not consistent with the observed decrease in oxygen-containing functional groups, as determined through potentiomctric titration and the Boehm method, an observation also made by Singh et al. (2010), when studying biochars from woody and non-woody biomass. The origin of these discrepancies can be partially revealed by plotting together the data obtained from potentiometric titration and CEC (obtained at the pH of equilibrium with the Sr solution, approximately 6.44), as shown in Fig. 4. Based on Fig. 4, the charge of oxygen-containing functional groups accounted as CEC is low at that pH value; the remainder CEC is attributed to the mineral fraction (ash + carbonates), which is especially evident in high-temperature biochar (up to 87% for WI-550; Fig. 4b). Hence, a quantitative comparison of titrations and CEC was compromised by a 'masking effect' caused by the biochar's inorganic fraction. Our results indicate that the mineral fraction, even at ash contents as low as <80 g [kg.sup.-1] for the wood biochar samples used, increases CEC. In this sense, Sr may have become weakly adsorbed onto carbonates surfaces (Zachara et al. 1991) and later displaced by [H.sup.+], thus leading to an overestimation of CEC, although more research would be needed to confirm this.

The total acidity quantified in the samples under investigation by the Boehm titration is considerably larger than that obtained using other methods. Oxygen-bearing groups as lactone or ester (found in high-temperature biochar) are able to act as latent acidic groups (Ritchie and Perdue 2008), thus detected only under very alkaline conditions, as in the Boehm titration. In addition, this method accounts for chemical sites associated with other bonding phenomena (covalent bonding or ligand exchange; Mukheijee et al. 2011). Moreover, the diffusion of the small [H.sup.+] ion exchanged during the Boehm titration is not kinetically limited as could be the case of the [Sr.sup.2+] cation (hydrated radius: 412 pm; Essington 2004), thus favouring a high acidity estimation (Mukheijee et al. 2011).

Detailed chemistry of biochar's active surfaces

Aromaticity

The atomic [H/C.sub.org] ratio of the biochar used here indicates the dominance of a condensed aromatic structure for all biochars (Wang et al. 2013), especially in those produced at 550[degrees]C (Table 1). The spectroscopic survey of these samples further confirms this (Figs 1, 2). However, the fraction of aromatic signal obtained by XPS for PO-550 and WI-550 was smaller than that for PI-550 (Fig. 2; table S3). This is in agreement with the presence of non-aromatic C moieties in those two biochar samples (Calvelo Pereira et al. 2011). Moreover, data from FTIR spectra (Fig. 1a) further confirmed the presence of residual or transformed lignin and cellulose in both PI-400 and Wl-400 samples, as described by Calvelo Pereira et al. (2011).

Surface functional groups

Oxygen plays a role in determining the overall biochar surface reactivity (Spokas 2010), which can be an important driver for chemical reactions, including its degradation potential. The biochars studied herein showed a much greater proportion of single-bonded O functional groups, mainly attributed to hydroxyl or phenol moieties, than of double-bonded O moieties (Fig. 2). The amount of surface functional groups decreased with increasing pyrolysis temperature, particularly due to a decrease in the content of single-bonded O (Table 1; Fig. 2). Unfortunately, a full quantitative comparison between the data from the different analyses was not possible (Table 1 ; table S2), because the linear relationship between the amount of O-bearing groups quantified by XPS and potentiometric titration was weak (r< 0.700; P-c 0.05; data not shown). This may be explained, in part, by the presence of O either at the edges of the aromatic sheets or within ring structures as those furan-like (Knicker et al. 2008).

The amount of N-containing functional groups found in the biochar surface was small, mainly due to the relatively low amount of N in the feedstock from which biochar samples were produced. Heterocyclic polyaromatic moieties including N were found, which confirms previous results obtained from pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS; Calvelo Pereira et al. 2011). Recently, Singh et al. (2014) observed the presence of N in heterocyclic structures in fresh and aged woody biochars.

Wet chemistry: method comparison

The content of 'carboxylic functional groups' obtained using the Boehm method was low and not comparable with the corresponding values estimated by potentiometric titration (table S2; Table 1). However, the Boehm titration also suggested the presence of other oxygen-containing groups, such as lactone-like moieties. This procedure has been widely used to characterise activated carbons, but the methodology lacks standardisation, thus hampering data comparison (Goertzen et al. 2010). In addition, the methodology requires a filtering step that may cause the retention of non-structural dissolved carbon compounds (volatile matter, or labile C previously clogged in pores), thus interfering with the final titration. In contrast, direct potentiometric titration provides more consistent information on the charge distribution, including estimations of the concentration of carboxyl and phenolic groups. The use of potentiometric titrations has its own limitations, because unequivocal surface functional groups assignment is not possible; despite this, it has been used to adequately characterise very different environmental matrices (Ritchie and Perdue 2008). However, although the Boehm titration was used in untreated samples, the potentiometric titration method is specific for acid-washed samples. Through the combination of information provided by both titrations (wet chemistry) and the spectroscopic analysis (XPS), a thorough characterisation of oxygen-containing surface functional groups was achieved. However, more effort is needed to achieve an accurate quantification of such surface functional groups in biochar types obtained from other feedstock sources.

Effect of acid washing on biochar surface chemistry

As expected, the acid-washing pretreatment eliminated carbonates, as revealed by FTIR and XPS. This was further confirmed by the thermogravimetric analysis of selected acid-washed biochar samples (fig. S2). The amount of ash remaining in the sample after the acid treatment barely changed compared with that of untreated samples (data not shown) because Si oxides were not dissolved by using 0.14 M HN[O.sub.3]. Biochar surfaces were slightly affected by the acid treatment, because some C and O were released and there was a relative enrichment of N in specific functional structures (see above). The specific distribution of oxygen-containing functional groups, as described by XPS, was only altered by the acid treatment in samples PI550 and PO-550 (fig. SI). Such differences could be explained by either: (1) a specific change in the surface and/or void space of these two biochar samples; and/or (2) a poor resolution of the signal corresponding to C=0 (Laszlo et al. 2001). Despite these minor changes, acid pretreatment did not systematically modify the overall surface structure of biochar. This was further confirmed by the recovery factors obtained in this study. In addition, acid washing led to an enhancement of XPS signal from the aromatic C, leading to a better semiquantification of the aromatic fraction of biochar. Moreover, the (negative) correlation between the [H/C.sub.org] ratio and aromatic C and aliphatic structures quantified by XPS (as a percentage of signal at 285.0 eV peak) improved after acid treatment (r = -0.922) relative to that correlation for untreated biochar (r = -0.693; Table 1; table S3). Overall, the acid pretreatment chosen here is considered appropriate for an in-depth study of oxygen-containing surface functional groups in wood biochar samples with low ash content. However, the use of acid pretreatment is limited for the assessment of N-containing groups in the same biochars.

Pedological and environmental implications

The density of oxygen-containing surface functional groups of biochar affect the adsorptive properties of the material (Chun et al. 2004; Nagodavithane et al. 2014), contributing to nutrient retention and soil remediation. An increase in soil CEC with the addition of biochar has been related to an elevated charge density per unit surface, which, in turn, is related to either a high degree of oxidation, an increased hydrophilic area for cation adsorption or a combination of both (Liang et al. 2006). The combination of oxidation degree and surface area in biochars produced at 400[degrees]C rendered a higher surface functional group density compared with biochars produced at 550[degrees]C (Table 1). The amount of oxygen-containing surface functional groups will increase over time because of aging and/or weathering of biochar (Joseph et al. 2010), thus increasing CEC; more research is needed to resolve this issue.

Conclusions

We performed a thorough chemical characterisation of wood-derived biochars produced at different temperatures (400 and 550[degrees]C) and compared the measurements obtained using conventional titrations (Boehm and potentiometric methods) with common CEC measurements. The results indicated that the presence of a relatively small mineral fraction (ash content of biochar <8%) compromised a direct comparison between these procedures. The use of a simple acid-washing step was shown not to systematically alter these surfaces, and it can be used as a pretreatment when potentiometric titrations are aimed at directly assessing the charge and distribution of functional groups on the carbonaceous fraction of biochar. More research is needed to standardise these measurements, especially when considering other feedstock types (i.e. non-woody biomass). This will facilitate the use of biochar as a soil amendment beyond its C sequestration value.

http://dx.doi.org/10.1071/SR14194

Acknowledgements

The authors acknowledge the technical assistance of Dr Bin Gong (Surface Analysis Laboratory, UNSW, Sydney, NSW, Australia) for XPS analyses. Professor F. Macias and the staff of the Departamento de Edafologia y Quimica Agricola, Facultad de Biologia (Universidad de Santiago de Compostela, Spain) collaborated with FTIR and elemental characterisation. Professor F. Arce (Universidad de Santiago de Compostela, Spain) kindly reviewed and commented on a previous version of the manuscript. JAM-A acknowledges the assistance of CS1C (Spain) for their award of a Postdoctoral JAE-Doc contract.

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R. Calvelo Pereira (A,C), M. Camps Arbestain (A), M. Vazquez Sueiro (A), and J. A. Macia-Agullo (B)

(A) New Zealand Biochar Research Centre, Soil and Earth Sciences Group, Institute of Agriculture and Environment, Massey University, Private Bag 11222, Palmerston North 4442, New Zealand,

(B) Instituto Universitario de Tecnologia Quimica CSIC-UPV, Universidad Politecnica de Valencia, Av. De los Naranjos s/n, 46022, Valencia, Spain.

(C) Corresponding author. Email: R.Calvelopereira@massey.ac.nz

Table 1. Characteristics of untreated biochar from pine,
poplar and willow produced at different highest heating
temperature (HHT)

Details on elemental analysis and determination of Brunauer,
Emmet and Teller equation (BET) surface area ([S.sub.BET])
are given in Calvelo Pereira et al. (2011). IC, Inorganic C
content; [CEC.sub.Sr], amount of Sr displaced by the acid
solution; [[SIGMA].sub.cations], sum of Ca, Na, Mg, and K
displaced by 0.01 M Sr[Cl.sub.2] solution; <d.l., below
detection limit

Sample    Feedstock       HHT        pH           C
                      ([degrees]C)         (g [kg.sup.-1])

PI-400    Pine            400        6.9         767
PO-400    Poplar                     7.2         755
WI-400    Willow                     7.5         662
PI-550    Pine            550        7.9         847
PO-550    Poplar                     8.8         758
WI-550    Willow                     8.6         791

Sample           N                 H                 O
          (g [kg.sup.-1])   (g [kg.sup.-1])   (g [kg.sup.-1])

PI-400           6                46                145
PO-400          10                42                152
WI-400          15                35                231
PI-550           6                35                71
PO-550          11                36                130
WI-550          17                35                82

Sample        Ash (A)             IC
          (g [kg.sup.-1])   (g [kg.sup.-1])

PI-400          37               <d.l.
PO-400          40               <d.l.
WI-400          57                2.7
PI-550          41               <d.l.
PO-550          65                2.2
WI-550          75                2.3

Sample                 Atomic ratio          [S.sub.BET]
                                             ([m.sup.2]
                                             [g.sup.-1])
          O/[C.sub.org]   H/[C.sub.org]

PI-400        0.14            0.71                1
PO-400        0.15            0.66                3
WI-400        0.26            0.63                3
PI-550        0.06            0.50               368
PO-550        0.13            0.56               55
WI-550        0.08            0.52               149

Sample    [[SIGMA].sub.cations]         CECsr
            ([mmol.sub.c](+)]     ([mmol.sub.c](+)]
              [kg.sup.-1])          [kg.sup.-1])

PI-400             43                     9
PO-400             150                   29
WI-400             170                   32
PI-550             58                    10
PO-550             116                   46
WI-550             190                   85

Sample                Groups (mmol [kg.sup.-1])

          Boehm: total         Potentiometric
            acidity            titrations

                         -COOH    Phenolic

PI-400        544          40        94
PO-400        1068         40       187
WI-400        970          70       349
PI-550         40          31       101
PO-550        201          20       111
WI-550        322          48       218

(A) At 900[degrees]C.

Table 2. Enrichment of C, O and N (as analysed by X-ray
pliotoclcctron spectroscopy (XPS)) and changes in chemical
bulk composition of biochar from pine, poplar and willow
produced at contrasted different highest heating temperature
(HHT) after acid-washing pretreatment with nitric acid EF,
enrichment factor, calculated as (content of element in
[biochar.sub.acid-washed])-(content of element in
[biochar.sub.untreated]); % Rec, percentage mass recovery x
EF; R, calculated as (elemental [ratio.sub.untreated])-(elemental
[ratio.sub.acid-washed])

Sample    Feedstock       HHT              C               C
                      ([degrees]C)
                                      EF     % Rec    EF     % Rec

PI-400    Pine            400        1.00    87.0    1.00    86.7
PO-400    Poplar                     1.00    85.9    1.01    87.3
WI-400    Willow                     1.00    85.0    0.99    84.3
PI-550    Pine            550        0.99    91.6    1.05    96.8
PO-550    Poplar                     0.99    81.7    1.04    85.6
WI-550    Willow                     1.00    85.2    0.98    83.5

Sample          N         [R.sub.O/C   [R.sub.C/N   [R.sub.O/N

           EF     % Rec

PI-400    1.16    100.8      1.00         1.16         1.16
PO-400    1.17    101.0      0.98         1.18         1.16
WI-400    1.01    85.5       1.01         1.01         1.01
PI-550    1.16    107.1      0.95         1.17         1.11
PO-550    1.10    90.2       0.95         1.10         1.05
WI-550    1.08    92.3       1.02         1.08         1.11
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Author:Pereira, R. Calvelo; Arbestain, M. Camps; Sueiro, M. Vazquez; Macia-Agullo, A.
Publication:Soil Research
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Date:Oct 1, 2015
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