An insight into the reactions occurring during the chemical activation of bone char

1 AN INSIGHT INTO THE REACTIONS OCCURRING DURING THE CHEMICAL ACTIVATION OF BONE CHAR Unai Iriarte-Velascoa, Jose L. Ayastuyb, Lorena Zudairea and Irene Sierraa* a Department of Chemical Engineering, Faculty of Pharmacy, University of the Basque Country UPV/EHU, Paseo de la Universidad, 7, 01006 Vitoria, Spain. b Department of Chemical Engineering, Faculty of Science and Technology, University of The Basque Country UPV/EHU, Barrio Sarriena, s/n, 48940 Leioa, Spain. *Corresponding author. Tel.: +34 945013290; fax: +34 6015963. Email: irene.sierr[email protected] Abstract The valorisation of animal wastes by pyrolysis has shown to be of interest, due to the versatility of the main compound of char, hydroxyapatite (HAP), which can be used in catalysis, electrochemistry, and adsorption. The utility of HAP depends to a great extent on its textural properties, which can be developed by chemical activation, through gasification reactions. In the present work thermogravimetric analysis coupled to mass spectrometry was used to gain insight into the reactions occurring during the chemical activation of pork bone char with different agents: H3PO4, H2SO4, NaOH and K2CO3. Moreover, the role of each activation reagent in the enhancement of porosity was determined. The treatment with H2SO4 resulted in a highly microporous material, suitable to be used in gaseous pollutant adsorption. Chemical activation with NaOH and K2CO3, on the contrary, led to a more equilibrated increase of microand mesoporosity, resulting in a hierarchical porous material, with an excellent potential for applications as electrode, in gas storage, catalysis and energy storage. Regarding H3PO4, it was extremely aggressive under the operating conditions used, since it removed almost all the porous structure of HAP. These results are useful to optimize the preparation method of HAP, in order to configure a material with the desired textural properties. KEYWORDS: Hydroxyapatite; Bone char; Chemical activation; Reaction mechanism; Mass spectrometry; Thermogravimetry. This is the accepted manuscript of the article that appeared in final form in Chemical Engineering Journal 251 : 217-227 (2014), which has been published in final form at https://doi.org/10.1016/j.cej.2014.04.048. © 2014 Elsevier under CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/) 2 1. Introduction As a result of Bovine Spongiform Encephalopathy crisis, the use of meat and bone meal (MBM) to feed cattle was forbidden in EU (Commission Decision 94/381/EC). Therefore, today there is a high amount of animal wastes that must be safely disposed or transformed. The combustion of MBM and its co-incineration with coal has been studied as a suitable way for energy valorisation [1, 2] and [3]. Nevertheless, the incineration of MBM gives way to a high level of emissions of dioxins and furans [4], and also NOx, due to the nitrogen content of MBM [5]. Pyrolysis constitutes one of the most reliable treatment methods for the animal bones left over. The solid fraction obtained (char) contains about 70-76 wt% calcium hydroxyapatite (HAP) Ca10(PO4)6(OH)2, 9-11 wt% carbon and 7-9 wt% CaCO3. There are other minor constituents, such as calcium sulphate (0.1-0.2 wt%) and Fe2O3 (less than 0.3 wt%) [6]. Due to its chemical structure HAP has been recently reported as a very useful material in catalysis [7] and [8], electrochemistry [9] and adsorption [10], [11], [12] and [13] processes. Spain is the world’s fourth largest pork producer, with 3.5 MTn of pigmeat produced in 2012 [14]. Consequently, the preparation of HAP-based materials from pork bone is a promising alternative, given that this process is economically feasible and environmentally friendly. The utility of HAP depends to a great extent on its textural properties. Chemical activation has been reported as a suitable method to develop a material with high porosity, which is associated with the occurrence of gasification reactions [15]. Moreover, chemical activation has several advantages over physical activation, such as: (i) higher yield; (ii) higher surface area; (iii) better development of porous structures; and (iv) the requirement of lower pyrolysis temperatures [16]. Its main drawback is the need for washing to remove the residual inorganic material. Several efforts have been made to understand the reactions that take place during the preparation of highly textured materials by chemical activation, using different carbonaceous precursors and activating agents. Senneca [1] studied product distribution during the thermal activation of MBM. Guo et al. [17] studied the chemical activation of oil-palm stone with H2SO4 and KOH. Lillo-Ródenas et al. [16, 18] investigated the 3 reactions occurring during the chemical activation of an anthracite with NaOH and KOH. Robau-Sánchez et al. [19] proposed a reaction mechanism for the activation of Quercus agrifolia char with KOH. Nevertheless, the literature concerning the preparation of porous hydroxyapatites by the chemical activation of animal wastes is scarce, and there are important gaps in the fundamentals of the process, regarding the reaction mechanisms occurring during the heat treatment. In this work thermogravimetric analysis coupled to mass spectrometry was used to gain knowledge on the chemical activation process of pork bone char with different agents: H3PO4, H2SO4, NaOH and K2CO3. The objectives of this work are: i) Identify the reactions between the constituents of bone char and the activating agents that take place during the preparation of porous hydroxyapatite. Furthermore, since carbon is one of the constituents, the results will help to better understand the fundamentals of the chemical activation process of any carbonaceous precursor. ii) Investigate the effect of the activation process on the physicochemical properties of the final product, in order to optimize the preparation method, resulting in the achievement of a HAP-based material with the desired textural properties. 2. Experimental 2.1 Production of bone char Bone char (BC) was prepared from pork chop bones collected from a local butcher’s shop. The preparation protocol was as follows: first, bones were cleaned from meat and cut into pieces of 2–5 cm. It is known that the evaporation of low molecular weight organic compounds and the decomposition of collagen occurs below 450 ºC [20]. In this way, in order to remove meat and fat, prior to chemical activation bones were precarbonized at 450 ºC, which is about half of the temperature used in the subsequent carbonization step. From now on, precarbonized sample will be referred as precursor. Precarbonization was performed using a heating rate of 10 ºC/min until the desired temperature was reached; temperature was then held constant for 1 hour. Nitrogen flow was set at 120 cm3/min, which corresponds to 8 minutes of residence time in furnace. Precarbonized samples were left to cold down in nitrogen atmosphere. The precursor was sieved and particles in the 0.25 – 0.35 mm size range were used. The precursor was divided into five parts. Four were impregnated with H3PO4 (P), H2SO4 4 (S), NaOH (N) or K2CO3 (K). The activation reagents were supplied by Panreac with laboratory grade purity, H3PO4 (86%), H2SO4 (96%), NaOH pellets (97%) and anhydrous K2CO3 (99%). The last sample was not further modified to be used as a reference (O). The impregnation ratio was established at 20 mmol of activation agent per gram of precursor (precarbonized bone char). These values are within the normal range used for the chemical activation of other low cost precursors [21], [16], [15] and [22], and ensure sufficient interaction between the activation agent and bone char matrix. For the impregnation step about 2 g of precursor were contacted with 40 cm3 of a solution containing 1 mol/L of the activating agent. Solutions were introduced in 50 mL borosilicate amber glass vials with Teflon tap and stirred at 120 rpm in a reciprocating shaker at room temperature (20 ± 2 ºC) for 24 h, to ensure the access of the activating agent to the interior of the particles. Samples were then filtered, transferred to a convection oven and dried at 80 ⁰C for 24 hours. The impregnated materials were coded according to the activating agent used: BCP, BCS, BCN, BCK and BCO. Finally, impregnated samples were pyrolysed at 800 ºC under operating conditions similar to those used during the precarbonization stage. Samples of BC were washed with distilled water until neutral pH of solution was reached, and their physicochemical properties were measured. 2.2 TG-MS study In order to investigate the reactions occurring during the activation process, thermogravimetric analysis (TG) coupled to mass spectrometry (MS) was conducted. TG analysis was performed with a Setsys evolution (Setaram) thermal analyser. About 40 mg of sample (BCP, BCS, BCN, BCK and BCO) were put into the ceramic crucible and heated under helium atmosphere from room temperature to 1000 ºC, at a heating rate of 10 ºC/min. The thermal analyser exhaust gases were analysed on-line by a mass spectrometer (MKS, Cirrus 3000). The total pressure in the analysis chamber was 10-6 Torr. The following compounds were monitored continuously: H2 (m/z = 2), CH4 (m/z = 16), H2O (m/z = 18), CN- (m/z = 26), HCN (m/z = 27), CO (m/z = 12), aldehydes (m/z = 29) and CO2 (m/z = 44). MS signals were normalized by dividing by the signal of He used as carrier. 5 2.3 Physicochemical characterization The physicochemical properties of the final samples (chemically activated at 800 ºC) were investigated. Textural properties were determined by nitrogen adsorption/desorption at 77 K, using a porosimeter (ASAP 2010, Micromeritics). Prior to the measurements samples were dried and outgassed at 200 °C under a nitrogen flow for 15 hours. BET surface area, and pore area and volume were measured. Micropore surface and volume were measured from density functional theory (DFT), while values in the mesopore and macropore ranges were determined based on the Barrett, Joyner & Halenda (BJH) method. The chemical composition and textural structure were analysed by a scanning electron microscope (JEOL JSM-7000F) equipped with energy dispersive X-ray detector (EDX). Fourier-transformed infrared (FTIR) spectra were collected using a Nicolet Protégé 460 device in the transmittance mode, in the 400-4000 cm-1 range with a resolution of 2 cm1. The KBr self-supported pellet technique was used to collect the spectra. The X-ray powder diffraction (XRD, PANalytical Xpert PRO) analyses were performed with CuKα radiation (λ = 1.5418 Å) in continuous scan mode in the 5-70º 2θ range, with a 0.02º step size. The analysis of the diffraction peaks was performed using PANalytical X´pert HighScore software. 3. Results and discussion 3.1 Chemistry of the activation process Fig. 1 depicts the evolution of mass spectrometric signals with temperature, corresponding to the main compounds released during the thermal activation of the impregnated samples of bone char. 3.1.1 Release of aldehydes, HCN and cyanides Fig. 2a and b show the evolution of the signals of HCN and cyanides, respectively. All samples, including the precursor, exhibit a well defined peak at temperatures between 300 and 600 ºC, with its maximum at around 500 ºC. It should be noticed, though, that the signal of CNis a secondary signal of HCN. If HCN was the only source of CNions, both signals should follow an invariable 6 HCN/CNratio of 1:0.17 [23]. However, the measured HCN/CNintensity ratio is around 1:0.09 for all samples, which evidences the presence of CNin pyrolysis gases. Robau-Sánchez et al. [19] reported the formation of cyanides during the activation with KOH of a carbonaceous material (Quercus agrifolia char). These authors proposed alternative mechanisms which result in the formation of cyanides in either liquid or gas form: 6 KOH (l) + 5.5 C + N2 (g) ↔ 2 KCN (l) + 2 K2CO3 + 1.5 CH4 (g) (1) 6 KOH (l) + 5.5 C + N2 (g) ↔ K2(CN)2 (g) + 2 K2CO3 + 1.5 CH4 (g) (2) Liquid cyanides might then be transformed into gaseous cyanides: 2KCN (l) ↔ K2(CN)2 (g) (3) The essential condition for cyanide formation is the presence of carbon and nitrogen. Carbon represents about 10 wt% of bone char [6], and it contains also a small amount of structural nitrogen [24]. Thus, the cyanide formation evidences that structural nitrogen took part in the activation process, probably following a reaction pathway similar to that described by Eq. (1)-(2). Regarding the formation of HCN during the pyrolysis, there could be two possibilities: (i) its direct synthesis, following a mechanism similar to that mentioned before, or (ii) its synthesis from cyanides, through the reaction of these components with H2O and CO2, H2O or acids: 2 NaCN + H2O + CO2 ↔ Na2CO3 + 2 HCN (4) KCN + H2O ↔ HCN + KOH (5) CN- + H+ ↔ HCN (6) The above-mentioned set of equations involves the reaction of OHions and consequently, it could be extended to every compound containing OHfunctionalities. In the case of BCO, there are two potential sources of OH-: (i) the P-OH functionalities of HAP and (ii) the reaction of CaCO3 constituent of bone char with water, according to Eq. (7) and (8) [25]: CO32- + H2O ↔ HCO3- + OH- (7) HCO3- ↔ CO2 + OH- (8) 7 Eq. (7), which takes place at temperatures in the 200-600 ºC range, requires a source of water. As discussed below, the dehydration of hydroxyapatite in that temperature range results in a continuous release of water (Fig. 5). The aforementioned mechanism is supported by the increased release of CNobserved for alkali treated samples (BCN and BCK) as compared to non-chemically treated sample (BCO). For BCK, the large amount of CO32ions incorporated with K2CO3 leads to the formation of OHions (Eq. (7)-(8)). It is noteworthy the opposite behaviour of both acid treated samples, suggesting that different reaction pathways occur by either H2SO4 or H3PO4 activation. The sample treated with H2SO4 shows the lowest release of CN-, slightly lower than the precursor itself. This fact could be ascribed to the partial dissolution of HAP, which leads to the formation of calcium sulphate and cation deficient HAP, Ca10-X(HPO4)X(PO4)6-X(OH)2X. [25]. Cation deficient HAP, which contains a lower amount of P-OH functionalities, would result in a lower formation of cyanides. BCP sample, on the contrary, shows the highest release of cyanides, an increase of 99% compared to BCO. This unexpected behaviour (given that this sample should have a lower amount of OHfunctionalities, due to its partial dissolution) could be related to the incongruent dissolution of HAP with H3PO4 and the formation of an amorphous structure. Fig. 9 shows the XRD patterns of the precursor, BCO, BCS and BCP samples, after being pyrolysed at 800 ºC. It is clear that the acid activation with phosphoric acid results in an important reduction of crystallinity, since only a wide band, different from those of HAP, is observed. Several authors have reported the dissolution of HAP with H3PO4. According to Krupa-Zuczek et al. [26], mild concentrations of acid would lead to the partial dissolution of HAP, resulting in a solid phase composed of HAP and CaPO3(OH)·2H2O. Other authors have reported the presence of various phosphates (CaHPO4·2H2O, Ca(H2PO4)2·H2O and Ca3(PO4)2), after acid and heat treatment [27]. The wide band observed in the XRD pattern (in the 20-30º range) could be ascribed to the coalescence of those peaks. According to this hypothesis, the aggressive treatment with H3PO4 would result in an amorphous, thermolabile structure, which would collapse during the pyrolysis step. A high amount of OHions in the solid phase could then be released, thus being available to react according to Eq. (1)-(2). 8 The release of aldehydes occurs simultaneously to that of CN- (Fig. 2c). Moreover, aldehydes follow a similar trend, given as follows: BCP >> BCK > BCN > BCO ≥ BCS. These results suggest that aldehydes are also formed through a mechanism which implies the reaction of carbon and hydroxides, which is in good agreement with their chemical formula (R-CHO). To summarize, the following aspects could be highlighted, regarding the release of aldehydes, HCN and CN-: - The proposed reaction mechanism involves the reaction of C and OHions (and structural N, in the case of HCN/CN-). - Since this mechanism implies the gasification of constituents of bone char (or incorporated species), it is expected to have an impact on the textural properties of the final material. - The alkali treatment (with either NaOH or K2CO3) has the effect of incorporating OHions, thus resulting in a higher extent of this set of reactions. Between both activation reagents, K2CO3 is more active. - Acid treated samples show an opposite behaviour. The treatment with H2SO4 leads to the partial dissolution of HAP, resulting in a slightly lower release of aldehydes and HCN/CN-. For the treatment with H3PO4, the formation of an amorphous and thermolabile structure is hypothesized. This structure would collapse during the thermal treatment, thus leaving a high amount of OHions available to react according to the proposed mechanism. 3.1.2 Release of CH4 All samples exhibit a peak of CH4 starting around 400 ºC and with its maximum near 550 ºC (Fig. 3). The release of CH4 is probably related to cyanide formation through Eq. (1)-(3), which involve the simultaneous formation of methane. This mechanism would be supported by the observed correlation between the intensity of CNand CH4 signals for each sample (Fig. 2b and 3). However, an exception occurs with the sample activated with H2SO4, being the release of CH4 higher than expected in that temperature range. This result evidences the occurrence of an additional mechanism of methane formation. It is likely that further amounts of CH4 are formed through the methanation of CO2 (Sabatier reaction, Eq. (9)) and CO (Eq. (10)): 9 CO2 + 4 H2 ↔ CH4 + 2H2O (9) CO + 3 H2 ↔ CH4 + H2O (10) Both reactions have been reported in the gasification of carbonaceous materials, such as activated carbon [28] and rice straw [29] with CO2 and steam. In this work, the intense release of CO and especially CO2 around 550 ºC (and also the peak at higher temperature, around 850 ºC) observed for BCS (Fig. 4 and Fig. 7) strongly coincide with CH4 release. These results support the occurrence of the above-mentioned methanation reactions. Furthermore, BCN and BCK also exhibit a minor peak of CH4 (Fig. 1b and c) at high temperature (800-900 ºC), which is in good agreement with the small release of CO and CO2 of these samples. It is worth highlighting the following points concerning the evolution of methane: - All samples show a peak with its maximum near 550 ºC, which could be related to the aforementioned mechanism of HCN/CNformation, that implies the simultaneous formation of methane. - The release of CH4 of the sample treated with H2SO4 (higher than expected at that temperature range, and with another peak at higher temperature) suggests the occurrence of an additional reaction mechanism, which is likely to be the methanation of CO and CO2. - Between both sets of reactions, that related to the formation of HCN/CNinvolves the gasification of constituents of bone char (or incorporated species). Methanation reactions, on the contrary, take place in gas phase and consequently, should not be related to the development of porosity. 3.1.3 Release of CO2 All samples show a low and broad band of CO2 in the 200-700 ºC range (Fig. 4). As mentioned before, in that temperature range carbonate ions present in bone char can react with water, according to Eq. (7). The resultant HCO3ions and those contained in HAP crystals are then released as CO2 and OH-, as described by Eq. (8). It is noteworthy that the treatment with sulphuric acid results in a much greater release of CO2 at mild pyrolysis conditions (200-700ºC). The reaction of H+ ions with carbonates, promoting the formation of HCO3ions (Eq. (11)) and their subsequent decomposition could explain this higher release of CO2. 16 On the other hand, water vapour reacts with carbon to produce CO (water-gas reaction): H2O + C ↔ CO + H2 (27) Ren et al. [24] studied the gasification behaviour of MBM char with steam at temperatures in the 850-1000 ºC range, and concluded that this carbonaceous material has higher gasification reactivity than several types of coal. The derivative of the measured weight loss (Fig. 8b) supports the above mechanism in which structural carbon is converted into gaseous CO. The order in weight loss (BCS >> BCK > BCN > BCP ≈ BCO) mostly coincides with CO release (BCS >> BCK > BCP > BCN > BCO). It should be noticed, though, that the weight loss of BCS should not be ascribed only to this mechanism, but also to the reaction of H2SO4 with carbon or carbonates (Eq. (12)-(13)). Gasification reactions (Eq. (26)-(27)) require a source of CO2 and H2O, respectively. In the case of acid activation (BCP, BCS), the formation of cation-deficient HAP and the reaction of carbon with sulphuric acid (Eq. (12)-(13)) are the main source of H2O and CO2 at high temperature. Consequently, it is expected that these samples give way to a higher release of CO than the precursor. Regarding alkali activation (BCN, BCK), CO2 is mainly released through the thermal decomposition of structural carbonates or those added by the activating agent. Moreover, it would also lead to an increased presence of H2O in pyrolysis gases (Eq. (7), (8) and (22)), compared to BCO. It is well known the catalytic role of alkali metals such as K, Mg, Na and Ca for both the reverse Boudouard reaction, and the gasification of carbon with steam (Eq. (26)-(27)) [24] and [41], being K the most active [41]. Several authors proposed the mechanism described by Eq. (28), in which C acts as a reducing agent, to produce CO at temperatures above 700 ºC [42]. This reaction could also be extended to the sample treated with NaOH. The required oxides (K2O and Na2O) are likely to be formed through the aforementioned thermal decomposition of carbonates (Eq. (16)-(18)). Our results are in good agreement with these observations, since the CO formation of BCN and BCK was higher and shifted to a lower temperature than that of the precursor. K2O + C ↔ 2K + CO (28) It is worth highlighting the following points concerning the evolution of CO: 17 - The first release of CO (at 300-600 ºC) is attributed to the RWGS reaction (CO is formed from CO2). This reaction takes place in gas phase, and thus, does not generate porosity. - The peak of CO above 600 ºC could not only be attributed to RWGS reaction. The additional reaction mechanism is likely to be the gasification of carbon with CO2 or H2O, favoured by the presence of alkali metals (being K the most active). These gasification reactions are expected to develop the porous structure of the material. 3.2 Textural properties of the pyrolysed materials The production yields and textural properties (surface area and pore volume) of the prepared samples of bone char are shown in Table 2. These values correspond to impregnated samples activated at 800 ºC. The production yield was maximum (21.5%) for the non-chemically treated sample (BCO). Alkali activation (samples BCN and BCK) did not further decrease the production yield, with values of 19.5 and 20.2%, respectively. On the contrary, acid activation significantly lowered the yield to 2.9 and 12.4%, by H3PO4 and H2SO4 activation, respectively. The specific surface area and pore volume vary significantly depending on the activating agent used. It is noteworthy the extremely low value of surface area and pore volume of the sample treated with H3PO4. These results, which reveal that almost all the porous structure of the precursor was removed during the chemical activation, are in good agreement with the aforementioned hypothesis. According to this hypothesis, the aggressive treatment with phosphoric acid would incongruently dissolve HAP, resulting in the formation of an amorphous and thermolabile structure. This structure would be decomposed when heating, releasing high amounts of gases (such as H2O, H2 and CH4), which would almost completely destroy the porous structure of HAP. Regarding the alkali treated samples (BCN and BCK), an increase in the total surface area is observed, compared to BCO. Furthermore, a quite homogeneous increase in the microand mesoporosity (pore volume) is observed, whereas macroporosity remains almost constant. These textural properties must be related to the extent of the reactions which generate porosity, mainly: (i) those involving the reaction of C and OHions, that generate compounds such as CNand aldehydes (Fig. 2b and c); and (ii) the gasification reactions of carbon (Eq. (26) and (27), Fig. 7), that generate CO. These reactions occur 18 in two temperature intervals: (i) 300-600 ºC: synthesis of CN-/HCN and aldehydes; and (ii) 600-1000 ºC: gasification of carbon. The release of H2 takes place in both temperature intervals (Figure 6), although it becomes more important at temperatures higher than 600 ºC. Between both sets of reactions, that implying the reaction of structural C and OHions to produce CN-/HCN and aldehydes is of greater importance, since it gives way to a higher weight loss (Fig. 8b). Table 3 shows the released amounts of aldehydes and CN- /HCN (calculated by integration), to be used as an indicator of the extent of this set of reactions. Fig. 11 displays the relationship between the released amount of CN-/HCN and the pore volume of the sample. A similar figure is obtained when the amount of aldehydes is displayed. It is observed that there is a direct relationship between the extent of the reactions (BCK > BCN > BCO) and the development of microand mesoporosity, while macroporosity remains constant. This enhancement of porosity gives way to a significant increase in surface area, measured by Stotal (59% for BCN and 74% for BCK). Concerning the set of reactions occurring at temperatures above 600 ºC, the simultaneous occurrence of the RWGS reaction (Eq. (25)) makes it difficult to measure the amount of CO or H2 effectively produced in gasification reactions. Thus, mass change in this temperature interval has been selected as an indicator of the extent of this set of reactions (Table 3). The extent of these reactions (BCK > BCN > BCO) is in good agreement with both the extent of the first set of reactions, and the development of microand mesoporosity (Table 2). Thus, it is clear that the effect of both alkali reagents is to favour the reactions that generate porosity (in the microand mesopore range) in the whole temperature range, being K2CO3 more active. This homogeneous increase of microand mesoporosity results in a hierarchical porous material, with an excellent potential for applications as electrode material, in gas storage, catalysis, and energy storage. Samples of bone char prepared by alkali treatment could be used as electrode materials for electric double-layer capacitor EDLC. According to the work of Wang et al. [43] ion-buffering reservoirs can be formed in the macropores to minimize the diffusion distances to the interior surfaces, the mesoporous walls provide low-resistant pathways for the ions through the porous particles, and the micropores strengthen the electric double layer capacitance. Consequently, the 19 combination of micro-, mesoand macroporous structures helps to maintain a good capacitive behaviour of the resultant material. Furthermore, the great increase in specific surface area allows the chemical activation with NaOH and K2CO3 to be considered as a suitable method to improve the performance of HAP as a catalytic support. Its desirable properties, i.e. structural stability, ion-exchange ability, acid-base properties and adsorption capacity, make HAP a promising material to configure heterogeneous catalysts. HAP-based catalysts have been recently employed in some chemical transformations, such as the oxidation of alcohols to aldehydes [44] or the oleic acid esterification [7]. Finally, the sample treated with H2SO4 shows a completely different trend in the development of porosity (Fig. 11), which cannot be mainly ascribed to the aforementioned reactions of carbon with OHto produce compounds such as CN-/HCN and aldehydes. According to the data of Table 2, BCS sample exhibits a remarkable increase of microporosity (135%, compared to BCO), while the macropore volume dramatically decrease (65%). Moreover, there is a trade-off between external and micropore area, since total area hardly varies. This decrease is mainly due to the reduction in macroporosity, since mesopore area remains almost constant. The development of microporosity should then be attributed to specific reactions involving H2SO4. Among the reactions which could have an impact on textural properties, two should be highlighted, according to their relative importance (measured by weight loss intensity and velocity in the TG and DTG plots, Fig. 8): the dehydration of cation-deficient HAP (Eq. (20)-(21)), and the reaction of H2SO4 with a source of carbon (Eq. (12)-(13). This selective development of microporosity results in a material suitable to be used in gaseous pollutant adsorption, to remove compounds such as SO2 [45] or VOCs [46]. 4. Conclusions When studying the development of porosity during the chemical activation of bone char, it is essential to distinguish between gasification reactions (which result in weight loss) and reactions in gas phase, which not generate porosity. Gasification processes involve the thermal decomposition or the reaction of constituents of bone char (hydroxyapatite, carbonates, carbon, nitrogen) and those compounds incorporated in the impregnation stage. 20 The mechanisms to generate porosity are: (i) the desorption of adsorbed water; (ii) those involving the reaction of carbon and OHions, which produce compounds such as CN-, HCN, aldehydes and H2; (iii) the thermal decomposition of carbonates; (iv) the gasification of carbon with CO2 or H2O; (v) the dehydration of HAP or cation-deficient hydroxyapatite; and (vi) the reaction of H2SO4 with carbon or carbonates. The mechanisms through which each activation agent enhances the development of porosity depend on the nature of the agent. Acids lead to the formation of cation deficient HAP, which is strongly dehydrated under thermal treatment. Furthermore, H2SO4 readily reacts with carbon. The main effect of alkali treatment (with NaOH or K2CO3) is the incorporation of OHions, which can then react with carbon. Other reaction pathways favoured are the thermal decomposition of carbonates and the gasification of carbon, promoted by alkali metals. The treatment with H2SO4 produces a highly microporous material, given that there is a trade-off between pores in the macropore and micropore range. Alkali treatment, on the contrary, results in a more equilibrated increase of microand mesoporosity, with no variation in the macropore range. Between both agents, the effect of K2CO3 is more pronounced than that of NaOH. Consequently, it is possible to choose the most appropriate treatment method to use, depending on the desired textural properties of the material. The selective development of microporosity allows the treatment with H2SO4 to be considered as a suitable method to configure a material to be used in gaseous pollutant adsorption, to remove compounds such as SO2 or VOCs. Alkali treatment, on the contrary, results in a hierarchical porous material, with an excellent potential for applications as electrode, in gas storage, catalysis and energy storage. Finally, under the operating conditions used in this work, the treatment with H3PO4 was extremely aggressive. Phosphoric acid is likely to incongruently dissolve HAP, forming an amorphous and thermolabile structure, which is decomposed by heating, leading to the formation of high amounts of gases, which would almost completely destroy the porous structure of HAP. 21 5. Acknowledgements The authors wish to thank to the Basque Government (UFI 11/39 (UPV/EHU)) for their economic support. 22 References 1. O. Senneca, Characterisation of meat and bone mill for coal co-firing, Fuel 87 (2008) 3262-3270. 2. J.A. Conesa, A. Fullana, R. Font, Thermal decomposition of meat and bone meal, J. Anal. Appl. Pyrolysis 70 (2003) 619-630. 3. G. Skodras, P. Grammelis, P. 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J. 245 (2014) 80-88. 32 0.0E+00 2.0E-04 4.0E-04 6.0E-04 8.0E-04 1.0E-03 0.0E+00 4.0E-05 8.0E-05 1.2E-04 1.6E-04 2.0E-04 2.4E-04 2.8E-04 0 100 200 300 400 500 600 700 800 900 1000 Ion current (A) Temperature (ºC) H2 CH4 CO2 HCN CO Aldehydes H2O CN - Fig. 1e ( e ) 33 0 100 200 300 400 500 600 700 800 900 1000 HCN ion current (a. u.) Temperature (ºC) BCO BCN BCK BCP BCS Fig. 2a (a) 34 0 100 200 300 400 500 600 700 800 900 1000 CN-ion current (a. u.) Temperature (ºC) BCO BCN BCK BCP BCS Fig. 2b (b) 35 0 100 200 300 400 500 600 700 800 900 1000 Aldehydes ion current (a. u.) Temperature (ºC) BCO BCN BCK BCP BCS Fig. 2c ( c ) 36 0 100 200 300 400 500 600 700 800 900 1000 CH 4 ion current (a. u.) Temperature (ºC) BCO BCN BCK BCP BCS Fig. 3 37 0 100 200 300 400 500 600 700 800 900 1000 CO 2 ion current (a. u.) Temperature (ºC) BCO BCN BCK BCP BCS Fig. 4 38 0 100 200 300 400 500 600 700 800 900 1000 H 2 O ion current (a. u.) Temperature (ºC) BCO BCN BCK BCP BCS Fig. 5 39 0 100 200 300 400 500 600 700 800 900 1000 H 2 ion current (a. u.) Temperature (ºC) BCO BCN BCK BCP BCS Fig. 6 40 0 100 200 300 400 500 600 700 800 900 1000 CO ion current (a. u.) Temperature (ºC) BCO BCN BCK BCP BCS Fig. 7 41 20 40 60 80 100 0 100 200 300 400 500 600 700 800 900 1000 Weight (%) Temperature (ºC) BCO BCN BCK BCP BCS Fig. 8a (a) 48 Table 1. Standard enthalpy (kJ/mol) and entropy (kJ/mol K) changes, and Gibbs free energies (kJ/mol) for the proposed reactions between the acid reagent (H2SO4 or H3PO4) and the precursor. Eq. ∆ ܪ ° ∆ ܵ ° ∆ ܩ ° ∆ ܩ (300 ºC) ∆ ܩ (600 ºC) ∆ ܩ (900 ºC) (12) -0.50 0.484 -144.8 -267.7 -386.0 -493.5 (13) -127.3 0.120 -163.2 -191.3 -214.2 -231.9 (15) 85.8 0.872 -174.1 -412.2 -669.5 -925.0 49 Table 2. Production yields and tTextural properties of prepared samples of bone char. Surface area in m2/g; pore volume in cm3/g. a DFT method b BJH method c Sum of DFT method (Dp<1.7 nm) and BJH method (Dp >1.7 nm) Sample Yield S BET S micro a S meso b S macro b S total c V micro a V meso bV macro b V total c BCO 21.5 76.2 80 32 61 172.2 0.017 0.026 0.222 0.265 BCN 19.5 106.2 151 50 73 274 0.030 0.042 0.214 0.286 BCK 20.2 110.7 166 55 80 300 0.033 0.047 0.224 0.303 BCP 2.9 3.2 n.d. 4 1 4.7 n.d. 0.002 0.004 0.006 BCS 12.4 116.8 108 32 14 153.4 0.040 0.021 0.077 0.139 50 Table 3. Released amounts of several compounds (calculated by integration), and mass loss in the 600-1000 ºC temperature range. For comparison purposes, a value of 100 has been assigned to BCO sample. Sample Aldehydes CN- + HCN Mass loss 600-1000 ºC BCO 100 100 100 BCN 122 118 113 BCK 139 133 134 BCS 94 98 508 51 HIGHLIGHTS -A highly textured material, based mainly on HAP, was prepared from pork bone char. - The reactions occurring during the chemical activation were investigated by TG-MS. - Treatment with H2SO4 resulted in a highly microporous material. - Treatment with NaOH and K2CO3 increased microand mesoporosity. - These results are useful to configure a material with the desired textural properties