EFFECT OF PYROLYSIS TEMPERATURE ON THE TEXTURAL PROPERTIES AND THE ACTIVATION MECHANISM OF PORK BONE DERIVED HYDROXYAPATITE

U. IRIARTE-VELASCO^†, I. SIERRA^†, R. BRAVO^† and J.L. AYASTUY^‡

† Dept. of Chemical Engineering, Faculty of Pharmacy, UPV/EHU, Vitoria, Spain unai.iriarte@ehu.eus

‡ Dept. of Chemical Engineering, Fac. of Science & Technology, UPV/EHU, Leioa, Spain

Cite this arcicle as:

U. IRIARTE-VELASCO, I. SIERRA, R. BRAVO and J.L. AYASTUY (2017) “EFFECT OF PYROLYSIS TEMPERATURE ON THE TEXTURAL PROPERTIES AND THE ACTIVATION MECHANISM OF PORK BONE DERIVED HYDROXYAPATITE”, Latin American Applied Research, 47(1), pp 17-22.

Abstract-- Calcium hydroxyapatite was produced from pork bones through a three step procedure including pre-pyrolysis, chemical impregnation with K₂CO₃ and pyrolysis. The effect of pyrolysis temperature on the porosity development of the material was investigated. The specific surface area (SSA) decreases as charring temperature increases. It is observed that the chemical treatment with K₂CO₃ is inefficient at mild pyrolysis conditions, in the 550-700ºC range. On the contrary, micropore, mesopore and macropore formation is enhanced by chemical activation with K₂CO₃ at about 800ºC. The reactions involved during the chemical activation step were analysed by mass spectrometry. It was concluded that treatment with K₂CO₃ favours the release of water and CO. this phenomenon was related to SSA and pore volume development. On the contrary, activation at 900 ºC caused a remarkable reduction in SSA, especially for K₂CO₃ treated samples, due to excessive ongoing of the above reactions as deduced from the pyrolysis gases.

Keywords-- Hydroxyapatite; Bone char; Chemical activation; Physicochemical characterization; TG-MS.

I. INTRODUCTION

Owing to its desirable structural stability, ionic substitution ability and acid–base properties, the use of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) (HAp) has recently increased. HAp is a versatile material and recently it has been successfully used for the adsorption of organic pollutants (Ciobanu et al., 2014) and metals (Kim and Lee, 2014; Sicupira et al., 2014) from liquid effluents, as a support in photocatalytic systems (Chun et al., 2014) and also in electrochemical (Goodman et al., 2013) and biomedical applications (Sobczak-kupiec et al., 2013).

Many methods of synthesizing HAp have been reported, such as solid-state reaction, sol–gel, wet synthesis and hydrothermal methods (Guo et al., 2013; Salviulo et al., 2011). However, the procedures employed were relatively complicated and it has been proven difficult to obtain controlled morphology with these methods. Hard tissues in animals represent an abundant source of natural HAp (Larsen et al., 1993; Singh, 2012). If adequate methods are developed, the use of waste animal bones is a promising alternative choice for the production of HAp-based porous materials.

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 Ca₁₀(PO₄)₆(OH)₂, 9–11 wt% carbon and 7–9 wt% CaCO₃. The apatite crystals in bones can act as natural template for the formation of abundant pores. Furthermore, this process could be economically feasible as well as environmentally friendly.

Difficulties have been reported in the production of materials based on natural HAp with large specific surface area (SSA). Doostmohammadi et al. (2012) reported a surface area value of 2.2 m²/g, obtained by the charring of bovine bones in air atmosphere at 700 ºC. Hassan et al. (2008) achieved an area of 72 m²/g from the pyrolysis of camel bones in inert atmosphere at 800ºC. Murillo et al. (2011) reported some of the largest BET values, of about 127 m²/g, through the pyrolysis of chicken bones in nitrogen atmosphere at 800 ºC.

Chemical activation represents an alternative for improving the textural and surface properties of bone char. It requires lower temperature and results in higher yield than physical activation only (Lillo-Ródenas et al., 2003; Phan et al., 2006). In a previous work our research group (Iriarte-Velasco et al., 2015a; 2015b) concluded that the chemical activation with K₂CO₃ significantly enhances the textural properties of bone char. However, there are still important gaps in the fundamentals of the process, regarding the optimization of preparation conditions and the reaction mechanisms that take place. The activation temperature is one of the most important factors. Sobzak et al. (2009) investigated the textural properties of pig bone calcined in air atmosphere. Calcinations in the 650 – 900 ºC range caused variations in S_BET values in the 44 – 3 m²/g range.

In this study a porous material, mainly composed of natural HAp, has been produced by using pork bones as raw material. To the best of the authors’ knowledge, there is no study in the literature regarding the effect of the pyrolysis temperature on the production of natural HAp through the activation of pork bones with K₂CO₃. The specific objectives of the present work can be described as follows: i) investigate the effect of pyrolysis temperature on porosity development within the bone char; ii) identify the reactions that occur during the chemical activation process; and iii) determine how such reactions contribute to the evolution of porosity and specific surface area.

II. METHODS

A. Preparation of bone char

Pork chop bones were pre-pyrolysed at 450 ºC to remove meat and fat. Pre-pyrolysed bones were then contacted with a solution containing 0.05M K₂CO₃ at an impregnation ratio (r) of 1 mmol K₂CO₃/g_precursor. This impregnation ratio was selected because it results in an adequate pore development (Iriarte-Velasco et al., 2015a) and implies environmentally and economically sustainable conditions. Finally, impregnated samples were pyrolysed in nitrogen atmosphere at different temperatures (550, 700, 800 and 900 ºC). More details about the preparation procedure are given elsewhere (Iriarte-Velasco et al., 2014). Thermal treatment conditions (precharring and pyrolysis) were as follows: heating rate of 10 ºC/min until the predetermined temperature was reached, which was held constant for 1 hour, nitrogen flow of 120 mL/min, corresponding to 8 minutes of residence time in furnace. The obtained bone char was left to cold down in nitrogen atmosphere. A part of the precursor material (precharred bone) was pyrolysed without chemical treatment (BCO). K₂CO₃-treated samples were coded as BCK.

B. Characterization of bone char

A porosimeter (ASAP 2010, Micromeritics) was used to determine the textural properties, by means of nitrogen adsorption/desorption at 77 K. BET surface area, and pore area and volume were determined. Micropore surface and volume were measured from density functional theory (DFT), while values in the mesopore and macropore ranges were measured based on the Barrett, Joyner & Halenda method (BJH).

In order to investigate the reactions occurring during the activation process, thermogravimetric analysis (TG) coupled to mass spectrometry (MS) was conducted. TG analyses were performed with a Setsys Evolution (Setaram) thermal analyser. About 70 mg of impregnated samples were put into the alumina crucible and heated under helium atmosphere from room temperature up to 1000 ºC, using a heating rate of 10 ºC/min. The thermal analyser exhaust gases were analysed on-line by mass spectrometry (MKS, Cirrus 3000). The following compounds were collected continuously: H₂ (m/z=2), CH₄ (m/z=15), H₂O (m/z=18), CO (m/z=28) and HCN (m/z=27).

III. RESULTS AND DISCUSSION

A. Specific surface area and volume

Figure 1 shows the nitrogen adsorption desorption curves. All isotherms exhibited the presence of a hysteresis loop, which is a characteristic feature of the Type IV isotherm. The downward displacement of the curve with the increased calcinations temperature was indicative of the collapse of porous structure. For chemically activated samples, this effect was intensified at the highest temperatures investigated.

Figure 2 shows the total specific surface area (SSA) and pore volume (V_p) of biochars. Figure 3 exhibits the fractional surface areas. Complementary textural data are presented in Table 1. The following regions have been considered, based on the location of valleys in the pore size distribution (not shown): micropores, SSA_I (D_p<1.7 nm); small mesopores, SSA_II (1.7<D_p<5.0 nm) and the sum of large mesopores and macropores SSA_III (D_p>5.0 nm).

Figure 1. Nitrogen adsorption-desorption isotherms.

Figure 2. Influence of pyrolysis temperature on the specific surface area (SSA) and pore volume (V_p) of K₂CO₃ treated (BCK) and non-treated (BCO) bone char.

The maximum SSA (237 m²/g) was obtained for non-treated sample pyrolysed at mild temperature, 550ºC. At that temperature, there are no significant differences between treated and non-treated (BCO) samples. A significant variation in SSA is observed as temperature varies. For BCO, specific surface area continu-

Table 1. Textural properties of the prepared biochars. T, ºC; S_BET, m²/g; dp, Å; V, cm³/g.

	T	SBET	dp	VI	VII	VIII
BCO	550	142	70	0.021	0.07	0.19
	700	134	80	0.021	0.05	0.21
	800	76	107	0.017	0.03	0.22
	900	83	102	0.003	0.03	0.20
BCK	550	145	74	0.022	0.07	0.20
	700	117	89	0.017	0.04	0.23
	800	115	109	0.023	0.05	0.20
	900	75	125	n/d	0.01	0.17

Figure 3. Influence of pyrolysis temperature on the fractional surface area of non-treated (BCO) and K2CO3 (BCK) treated bone char. r=1 mmol/g.

ously decreases with charring temperature within all the temperature range investigated (550 to 900 ºC). The decrease is more pronounced at high temperatures.

Several authors (Deydier et al., 2005) have also reported a dramatic decrease in surface area of calcined meat and bone meal, from 18 to 2 m²/g, as temperature was increased from 550 to 900ºC. On the contrary, Ayllon et al. (2006) reported a dissimilar behaviour. According to their data, the specific surface area of meat and bone meal char had a local maximum at 450 ºC, and it dropped off at about 500 ºC. From 600 to 900 ⁰C, the SSA increased again, up to a maximum value of 37.7 m²/g at 900⁰C. The discrepancy in the influence of temperature could be due to either differences in source material or in the experimental set-up.

For K₂CO₃-treated sample, as activation temperature increases (in the 500-800 ºC range), the SSA of the final product is reduced, though the decrease is less pronounced, as compared to BCO. This behaviour is mainly attributed to a better preservation of the meso and macro range structure, as shown in Figure 3. In this way, at 800 ºC, the sample treated with K₂CO₃ shows larger surface area. As compared to BCO, the mesopore and macropore surface area of BCK were 78% (57 vs 32 m²/g) and 15% (70 vs 61 m²/g) larger, respectively. This suggests that the activation with K₂CO₃ requires temperatures of about 800 ºC to effectively enhance the textural properties of biochar. If the activation temperature is further raised to 900 ºC, total SSA and V_p are drastically reduced from 210 to 59 m²/g and from 0.272 to 0.179 cm³/g, respectively. This is likely due to pore widening processes which cause an almost complete suppression of surface area at dp<5 nm (Figure 3) whereas macropores still remain.

B. Thermogravimetric analysis

The results of the thermogravimetric (TG) and differential TG (dTG) analysis of non-treated and K₂CO₃-impregnated samples are shown in Figure 4. In the dTG curves one main peak (with its maximum near 470 ºC) and other small peaks (at around 100, 225, 800 and 900 ºC) are detected. The inorganic component of bone is primarily hydrated calcium phosphate, hydroxyapatite (HAp). At about 450 ºC, HAp begins to dehydroxylate to form oxyhydroxyapatite, or Ca₁₀(PO₄)₆(OH)_(2−2x)O_x, where x = vacancy.

This decomposition process is gradual and takes place over a range of temperature (Tanaka et al., 1997). The intense mass loss from around 275 °C to 600 °C is attributed to that process. Furthermore, the decomposition of collagen has also been associated to this mass loss (Etok et al., 2007). Figure 4 reveals that mass loss within this range is significantly intensified by K₂CO₃ treatment.

At above 600 ºC a broad step of continuous mass loss exists related to the thermal decomposition of inorganic constituents of bone. This peak is centred at about 800 ºC for non-treated bone char (mass loss of 3.5 %), whereas it is displaced to a higher temperature of about 900 ºC (mass loss of 5.6 %) for chemically treated sample. The activation of samples (both impregnated and non-impregnated) was performed within this range and despite the relatively low mass loss observed at high temperatures for both non-treated and K₂CO₃ treated sample, the textural properties were significantly altered. The decrease in surface area at pyrolysis temperatures above 600 ºC could be explained by an increase of HAp crystallite size (Guo et al., 2013). At high temperatures, changes in the crystalline structure have been reported, even with minimum loss of components. Dimovic et al. (2009) prepared a biochar using bovine bones. They reported a decrease of SSA, from 71 to 2 m²/g, when increasing temperature in the 600 - 1000 ºC range. The weight loss was low, about 3 %, which is very similar to the present work.

C. TG-MS analyses

In order to better understand the activation mechanism the following species were monitored in pyrolysis gases: H₂O, H₂, CO, CH₄ and HCN. The results are shown in Figure 5. Chemical treatment increases dramatically the release of H₂O and CO whereas that of H_2, CH₄ and HCN is diminished. The main release of water starts at

Figure 4. Thermogravimetric analysis of non-impregnated and chemically impregnated samples of bone char. A) TG curves; B ) dTG curves.

about 400 ºC, and could be partially ascribed to the decomposition of the surface P-OH groups of HAp by dehydration (Eq. 1), which takes place in the 400-700 ºC range (Tanaka et al., 1997). The OH^- species provided by K₂CO₃ (Iriarte-Velasco et al., 2014) could also react according to following mechanism:

. (1) (1)

The release of water observed below 200 ºC is associated to desorption of adsorbed water. The fact that the peak for the K₂CO₃ impregnated samples was lower could be related to the aforementioned dehydration capability of K₂CO₃ during the impregnation step.

Regarding the release of CO, McKee (1983) and Lillo-Ródenas et al. (2004) studied the gasification of graphite by alkali metal and found that K₂CO₃ was reduced in inert atmosphere by carbon as follows:

. (2)

Another source of CO is the RWGS reaction (Eq. 3). This reaction has been reported during the pyrolysis of carbonaceous precursors, and would take place mainly at temperatures higher than 700 ºC (Chen and Lin, 2013; Iriarte-Velasco et al., 2014; Prabowo et al., 2014).

(3)

As shown in Figure 5, the release of CO during the activation of non-impregnated sample occurs in two stages, in the 400-600 ºC and 700-1000 ºC ranges. For the chemically treated sample it takes place much more intensely and only at the lowest temperature range. The occurrence of the RWGS reaction requires the presence of both CO₂ and H₂. Nevertheless, CO₂ was not detected in pyrolysis gases. It is hypothesized, that the rapid transformation of CO₂ into CO, according to the RWGS reaction, would cause both the absence of CO₂ and the drop-off in H₂ release at around 700 ºC ( Figure 5). This hypothesis is also supported by the role of K as promoter of the RWGS reaction (Jacobs and Davis, 2010).

The release of CO₂ is well documented. Both the CaCO₃ constituent of bone char and the incorporated K₂CO₃ can undergo thermal decomposition (Eqs. 4-5) in the 600- 1000 ºC range (Lehman et al., 1998; Wang and Thomson, 1995).

(4)

(5)

The increased release of H₂O and CO suggests that K₂CO₃ can act through different mechanism; i) as dehydration agent; or ii) being reduced by carbon and releasing CO and thereby increasing the specific surface area and specific pore volume. Both processes involve gasification reactions and thus, are expected to have an impact on the textural properties of the prepared material. Also, when activation temperature reaches the boiling point of potassium 800 °C, it can diffuse into the precursor structure, causing the deformation of pore structure. These phenomena could be involved in porosity development. However, they could also cause the collapse of pores and a drastic reduction in SSA, as observed for the K₂CO₃ treated sample activated at 900 ºC (Figure 2), likely by excessive ongoing of such reactions.

CH₄ and HCN could be formed following a reaction pathway similar to that described by Robau-Sánchez et al. (2005):

(6)

The above mechanism implies the reaction of structural carbon and nitrogen with a source of OH^- ions. Figure 5D,E reveal the evolution of CH₄ and HCN which exhibit a well defined peak at temperatures between 400 and 600 ºC, with its maximum at around 500 ºC. The shape of the curves suggests a main common source, reaction Eq. (6). The observed tail in the CH₄ release curves could be ascribed to additional mechanisms of CH₄ formation, such as the methanation of CO and CO₂ (Iriarte-Velasco et al., 2014).

The release of water observed belowThe reduced release of these compounds during the charring of K₂CO₃ treated samples could be attributed to the preferential oxidation of carbon by the activating agent through Eq. (2). In this way, the consumption of much part of the carbon content in bone char would limit the occurrence of Eq. (2) and therefore would explain the limited release of cyanide and CH₄ as observed for the sample impregnated with K₂CO₃.

Figure 5. Temperature dependence of pyrolysis products for K₂CO₃ impregnated and non-impregnated bone char.

III. CONCLUSIONS

The results presented demonstrate that chemical treatment with K₂CO₃ enhances pore development more than physical activation when pyrolysis is carried out at 800 ºC, mainly in the mesopore and macropore range. At lower temperatures, in the 550-700 ºC range, the chemical treatment was inefficient. At higher temperatures, of about 900 ºC, the shrinkage in the bone char structure was more pronounced resulting in a drastic reduction in surface area and pore volume.

The investigation of pyrolysis gases by TG-MS reveals an increased release of H₂O and CO during the charring of K₂CO₃-treated bone char. The release of H₂O is related to the decomposition of HAp structure. The release of CO is partly attributed to the enhanced gasification of carbon in bone char through its reaction with K₂CO₃. The obtained results encourage further investigation into the application of TG-MS as a technique to control the textural properties of biochars produced by the pyrolysis of waste material.

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Received: January 8, 2016.

Sent to Subject Editor: April 27, 2016.

Accepted: September 29, 2016.

Recommended by Subject Editor: Orlando Alfano