Haroun Sebei

Doan Pham
email: doan.phamminh@mines-albi.fr

Nathalie Lyczko

Patrick Sharrock

Ange Nzihou

Hydroxyapatite-based sorbents: elaboration, characterization and application for the removal of catechol from the aqueous phase

Keywords: Hydroxyapatite, catechol, sorption, wastewater treatment, mechanism Cedex 09, France

Hydroxyapatite (HAP) is highly considered as good sorbent for the removal of metals from the aqueous phase. However, soluble metals co-exist with organic pollutants in wastewaters. But little work has been devoted to investigate the reactivity of HAP for the removal of organic compounds. The main objective of this work is to study the reactivity of HAP-based sorbents for the removal of catechol as a model organic pollutant from an aqueous solution. Thus, HAP sorbents were firstly synthesized using calcium carbonate and potassium dihydrogen phosphate under moderate conditions (25-80°C, atmospheric pressure). A zinc-doped HAP was also used as sorbent, which was obtained from the contact of HAP with an aqueous solution of zinc nitrate. All the sorbents were characterized by different standard physico-chemical techniques. The sorption of catechol was carried out in a batch reactor under stirring at room temperature and pressure. Zinc-doped HAP sorbent was found to be more reactive than non-doped HAP sorbents for the fixation of catechol. The highest sorption capacity was of 15 mg of C per gram of zincdoped HAP sorbent. The results obtained suggest the reaction scheme of HAP sorbents with metals and organic pollutants when HAP sorbents were used for the treatment of complex wastewaters.

Introduction

The sector of wastewater treatment continues to evolve with new challenges and targets. Nowadays, different configurations exist concerning the treatment of various wastewaters. For urbanized zones, domestic wastewater is usually canalized and commonly treated by a wastewater treatment plant (WWTP) using conventional biological treatment. For a given industrial zone, wastewater from various industrial activities is usually collected and (pre)treated by specific processes. In the case of a mixed zone wherein industrial activities coexist or are in the proximity of an urbanized zone, conventional WWTP receives both domestic and industrial wastewater. In this case, the pretreatment of non-biodegradable organic compounds and toxic heavy metals is crucial for biological digestion. As an example, the WWTP of Graulhet city (Tarn Department, France) receives domestic wastewater as well as wastewater from various industries, particularly from tanneries of the related region. The removal of heavy metal allows accelerating the biological treatment yield. The aeration time is reduced leading to operational energy saving, taking into account the fact that aeration corresponds to about 50% of the operational cost of a given WWTP.

Heavy metal removal can be done by various techniques: precipitation, adsorption, complexation and cationic exchange [START_REF] Srivastava | A review on progress of heavy metal removal using adsorbents of microbial and plant origin[END_REF][START_REF] Ahmady-Asbchin | Natural seaweed waste as sorbent for heavy metal removal from solution[END_REF]. For the adsorption process, numerous research works have been devoted to the investigation of calcium hydroxyapatite-based sorbents (HAP). HAP, with the general chemical formula Ca 10 (PO 4 ) 6 (OH) 2 , has an exceptional capacity for the fixation of different heavy metals such as Pb 2+ , Zn 2+ , Cd 2+ , Ni 2+ , Cu 2+ , etc. [START_REF] Pham Minh | Apatitic calcium phosphates: synthesis, characterization and reactivity in the removal of lead(II) from aqueous solution[END_REF][START_REF] Mobasherpour | Comparative of the removal of Pb 2+ , Cd 2+ and Ni 2+ by nano crystallite hydroxyapatite from aqueous solutions: adsorption isotherm study[END_REF][START_REF] Skwarek | Natural seaweed waste as sorbent for heavy metal removal from solution[END_REF][START_REF] Batton | Trapping heavy metals by using calcium hydroxyapatite and dielectrophoresis[END_REF]. Our previous work showed that the sorption capacity of HAP-based sorbents for Pb 2+ fixation could reach up to 1226 mg g -1 , which is much higher than the sorption capacity of conventional activated carbon [START_REF] Pham Minh | Calcium phosphate based materials starting from calcium carbonate and orthophosphoric acid for the removal of lead(II) from an aqueous solution[END_REF]. Similar results were obtained for Zn 2+ fixation [START_REF] Sebei | Sorption behavior of Zn(II) ions on synthetic apatitic calciumphosphates[END_REF]. The results obtained from our research group allowed the first industrial scale-up by our industrial partner, for the treatment of specific wastewaters containing high concentration of heavy metal (nickel and chromium).

If the fixation of heavy metals by HAP-based sorbents is highly studied, the removal of organic pollutants using HAP-based materials is relatively recent. Till date, most of studies focused on the fixation of phenol as the model organic pollutant compound [START_REF] Alzaydien | Equilibrium, kinetic and thermodynamic studies on the adsorption of phenol onto activated phosphate rock[END_REF][START_REF] Bahdod | Adsorption of phenol from an aqueous solution by selected apatite adsorbents: kinetic process and impact of the surface properties[END_REF][START_REF] Bouyarmane | Pyridine and phenol removal using natural and synthetic apatites as low cost sorbents: influence of porosity and surface interactions[END_REF][START_REF] Lin | Study the adsorption of phenol from aqueous solution on hydroxyapatite nanopowders[END_REF]. But we can find some works reported on the removal of other organic molecules such as light carboxylic acids, amino-acids and dyes [START_REF] Bengtsson | Solubility and surface complexation of apatites[END_REF][START_REF] Vega | Adsorption of fumaric and maleic acids onto hydroxyapatite: a thermodynamic study[END_REF][START_REF] Wei | Interaction between low molecular weight organic acids and hydroxyapatite with different degrees of crystallinity[END_REF][START_REF] Mahmud | Adsorption of direct yellow 27 from water by poorly crystalline hydroxyapatite prepared via precipitation method[END_REF]. Thus, the removal of phenol by synthetic and natural HAP has been investigated. The initial concentration of phenol had an important effect by providing a driving force, enough for surpassing the limit of mass transfer of phenol from the liquid medium to sorbent surface [START_REF] Alzaydien | Equilibrium, kinetic and thermodynamic studies on the adsorption of phenol onto activated phosphate rock[END_REF][START_REF] Lin | Study the adsorption of phenol from aqueous solution on hydroxyapatite nanopowders[END_REF]. For the phenol concentration ranging from 2 to 400 mg L -1 , the sorption capacity reached around 4-10 mg g -1 [START_REF] Bahdod | Adsorption of phenol from an aqueous solution by selected apatite adsorbents: kinetic process and impact of the surface properties[END_REF][START_REF] Bouyarmane | Pyridine and phenol removal using natural and synthetic apatites as low cost sorbents: influence of porosity and surface interactions[END_REF][START_REF] Lin | Study the adsorption of phenol from aqueous solution on hydroxyapatite nanopowders[END_REF]. Specific surface area (by BET method, S BET ) of HAP-based sorbent was reported as a crucial factor for the sorption capacity of phenol removal. Bahdod et al. [START_REF] Bahdod | Adsorption of phenol from an aqueous solution by selected apatite adsorbents: kinetic process and impact of the surface properties[END_REF] investigated the reactivity of three natural HAP-based sorbents having S BET of 35, 55 and 235 m 2 g -1 . The increase of S BET led to the increase of the sorption capacity from 4 to 8 mg g -1 . However, this was not linear, which was later explained by the fact that the microporosity of HAP of high S BET did not allow the penetration of phenol molecules. However, this microporosity was not further detailed [START_REF] Bahdod | Adsorption of phenol from an aqueous solution by selected apatite adsorbents: kinetic process and impact of the surface properties[END_REF]. The effect of pH on the sorption of phenol was also investigated in a large range of 2-12 [START_REF] Alzaydien | Equilibrium, kinetic and thermodynamic studies on the adsorption of phenol onto activated phosphate rock[END_REF][START_REF] Bouyarmane | Pyridine and phenol removal using natural and synthetic apatites as low cost sorbents: influence of porosity and surface interactions[END_REF][START_REF] Lin | Study the adsorption of phenol from aqueous solution on hydroxyapatite nanopowders[END_REF]. The results obtained were different from each other. Lin et al. [START_REF] Lin | Study the adsorption of phenol from aqueous solution on hydroxyapatite nanopowders[END_REF] showed that the pH of the solution after sorption experiment was close to that before sorption experiment, and did not depend on the initial pH. They found that the sorption capacity decreased from 2 to 8.2 then increased above 8.2. The maximum sorption capacity was obtained at the pH of 2. Similar results were observed by Bouyarmane et al. [START_REF] Bouyarmane | Pyridine and phenol removal using natural and synthetic apatites as low cost sorbents: influence of porosity and surface interactions[END_REF] On the other hand, Alzaydien and Manasreh [START_REF] Alzaydien | Equilibrium, kinetic and thermodynamic studies on the adsorption of phenol onto activated phosphate rock[END_REF] found the maximum performance at the pH of 8. They explained the decrease of the sorption capacity at high pH (>9.9) by electrostatic repulsions between negative charges of sorbent particles and phenolate groups. At pH lower than 8, wherein the phenol was not dissociated, the authors attributed the phenol fixation by 'dispersive interactions'.

Another aromatic model compound investigated was catechol (or 1,2-dihydroxybenzene, C 6 H 4 (OH) 2 ). Chirdon et al. [START_REF] Chirdon | Adsorption of catechol and comparative solutes on hydroxyapatite[END_REF] studied the adhesion of catechol and other organic compounds on the surface of a synthetic HAP. The adhesion work of these organic compounds on the HAP surface was thus determined. With two adjacent hydroxyl groups, catechol had the highest adhesion energy, around 90 mJ m -2 . Molecules containing alcohol, amine and carboxyl groups had lower adhesion energy (around 60-70 mJ m -2 ). The sorption capacity of HAP sorbent for catechol was also determined by adsorption isotherm, and reached 35 µmmol g -1 . This last was higher than that obtained with the alcohol, amine and carboxyl compounds investigated [START_REF] Chirdon | Adsorption of catechol and comparative solutes on hydroxyapatite[END_REF].

About the sorption of light carboxylic acids using HAP-based sorbents, Wei et al. [START_REF] Wei | Interaction between low molecular weight organic acids and hydroxyapatite with different degrees of crystallinity[END_REF] investigated the reactivity of both amorphous and crystalline HAP for the removal of oxalic acid (HOOC-COOH), citric acid ((HOOC-CH 2 ) 2 C(OH)(COOH)) and malic acid (HOOC-CH 2 -CH(OH)(COOH)). The fixation of oxalic acid (0.5 mmol g -1 ) was more important than that of citric and malic acids (<0.2 mmol g -1 ). This was explained by high polarity of oxalic acid which favors the fixation of this acid by the polar surface of HAP sorbents. In addition, oxalate ions could form an insoluble complex with calcium cation (Ca 2+ ) generated from the acid attack on HAP surface particles. This favors also the fixation of oxalic acid compared to two others. Bengtsson [START_REF] Bengtsson | Solubility and surface complexation of apatites[END_REF] studied the interaction of malonic acid (HOOC-CH 2 -COOH), citric acid and mellitic acid (C 6 -(COOH) 6 ) with synthetic HAP sorbents. They attributed the fixation of these acids by the formation of outer sphere complex at the water-HAP particle interfaces as well as the cooperative electrostatic interactions. Thus, the sorption was favored at pH lower than 8. Above pH of 8, the retention of these acids strongly decreased [START_REF] Bengtsson | Solubility and surface complexation of apatites[END_REF]. Vega and Colinas [START_REF] Vega | Adsorption of fumaric and maleic acids onto hydroxyapatite: a thermodynamic study[END_REF] investigated the removal of isomeric fumaric and maleic acids (HOOC-CH 2 =CH 2 -COOH) using a synthetic HAP. They demonstrated that these acids were practically fixed on the surface of HAP particles in carboxylate form. The highest performances were observed at pH of 4.1 for fumaric acid (pK a of 3 and 4.2) and 5.2 for maleic acid (pK a of 1.8 and 6.1) [START_REF] Vega | Adsorption of fumaric and maleic acids onto hydroxyapatite: a thermodynamic study[END_REF]. We note that these light carboxylic acids are generally biodegradable and can be digested by the conventional biological process. On the other hand, (poly)phenolic compounds such as phenol and catechol are more bio-recalcitrant and usually need a pretreatment before the biological digestion [START_REF] Pham Minh | Catalytic wet air oxidation of olive oil mill effluents: 4. treatment and detoxification of real effluents[END_REF].

The reactivity of a synthetic HAP for the simultaneous removal of metals and organic compounds from a real wastewater withdrawn from the WWTP of Graulhet city (France) was recently reported by our research team [START_REF] Lyczko | Calcium phosphate sorbent for environmental application[END_REF]. Using a semi-continuous pilot reactor wherein the real wastewater continuously passed through a batch of HAP sorbent, the experiment was kept for a long reaction time of 30 days. The sorption performance was monitored for various elements (Pb, Ni, Sb, Se, Zn, As, Cd, Co, Cr, Cu, Mn, Fe, Mo, Al) and for the total organic carbon (TOC) value during the experiment. For the fixation of metals, the performance reached above 60% for most of the investigated metals. High performance of TOC fixation was also observed with the weight loss up to 35% obtained by TG-DSC analysis of the sorbent recovered after 30 days of use. This result demonstrated that HAP removed simultaneously metals and organic pollutants from the aqueous phase. This is particularly interesting for the case of the WWTP which receive both domestic and industrial wastewaters. HAP allows reducing most of the metals present in this mixed wastewater, thus favoring the ulterior biological treatment step. However, as presented above, there is little work that investigates the fixation of organic pollutants by HAP-based sorbents, in particular phenolic compounds other than phenol. And the results obtained were sometimes contradictory. So this task merits further investigations.

This work aimed to study the removal of catechol (1,2dihydroxybenzene, C 6 H 4 (OH) 2 ) from an aqueous solution by different synthetic HAP-based sorbents. The main objective was to investigate the interaction of HAP-based matrix with a phenolic model compound. The results of this work will contribute to the understanding of the performance of the synthesized HAP-based sorbents for the treatment of a real effluent, as mentioned above [START_REF] Lyczko | Calcium phosphate sorbent for environmental application[END_REF].

Experimental

Synthesis of HAP-based sorbents

Calcium carbonate powder (CaCO 3 , 98%, Fisher Scientific) and potassium dihydrogen orthophosphate (KH 2 PO 4 , 99%, Fisher Scientific) were used for the synthesis of HAP sorbents. The synthesis was carried out in a 1 L glass reactor thermostated at 25°C or 80°C during the synthesis. Firstly, an aqueous solution of KH 2 PO 4 (1.2 mol L -1 ) was prepared. Then, calcite powder was progressively added into the solution of KH 2 PO 4 . The final theoretical molar ratio of Ca to P was 1.67, which is characteristic for stoichiometric HAP. The mixture was stirred for 48 h with the rotation rate of 350 rpm with a helical propeller. After 48 h, the suspension was filtered on a 0.45-µm hydrophilic filter to separate liquid and solid phases. The solid was washed several times with permuted water, dried overnight at 105°C before further analyses and utilization. The liquid phase was acidified with nitric acid to avoid all secondary precipitation from calcium cations and orthophosphate species present in the liquid phase. The solids obtained at 25°C and 80°C were thereafter called HAP_25 and HAP_80 which were then used as sorbents for the removal of catechol.

Another sorbent used for the removal of catechol was derived from HAP_25 after its utilization for Zn 2+ removal. Our previous study was done on the fixation of Zn 2+ (initial concentration of 1500 mg L -1 ) by HAP_25 [START_REF] Sebei | Sorption behavior of Zn(II) ions on synthetic apatitic calciumphosphates[END_REF]. We have recovered this solid, called hereafter HAP_25_Zn, for use in the removal of catechol. The composition of three sorbents used in this work is given in Table 1.

Sorbents' characterization and analysis

The pH of zero charge point (pH ZCP ) was experimentally determined according to the previous description [START_REF] Faria | Adsorption of anionic and cationic dyes on activated carbons with different surface chemistries[END_REF][START_REF] Smiciklas | The point of zero charge and sorption of cadmium (ii) and strontium (II) ions on synthetic hydroxyapatite[END_REF]. Elemental analysis was done with the inductive coupled plasma atomic emission spectroscopy (ICP-AES, HORIBA Jobin Yvon Ultima 2 apparatus).

The specific surface area was determined using the BET method (S BET ) with a MICROMERITICS ASAP 2010 apparatus. The true density was obtained from a helium pycnometry (MICROMERITICS Accupyc 1330). Thermogravimetric analysis coupled with differential scanning calorimetry (TG-DSC) was performed with an SDT Q600 (TA instruments). For each analysis, around 30 mg of sample was used under N 2 flow rate of 100 NmL min -1 . Fourier transformed infra-red spectroscopy (FTIR) was carried out with a SHIMADZU -8400S spectrophotometer scanning between 4000 and 500 cm -1 . The particle size distribution of the sorbents was determined with a MALVERN Laser Mastersizer Hydro 2000 in liquid medium. X-ray diffraction (XRD) was conducted using a Philips X'Pert diffractometer with copper anti-cathode. The identification of crystalline phases was carried out by comparing the XRD patterns with JCPDS standards.

Sorption experiments

The removal of catechol was carried out in a 350-mL glass batch reactor at room temperature and atmospheric pressure. For each experiment, 300 mL of an aqueous solution of catechol were added to the reactor under stirring (300 rpm). The initial concentration of catechol was fixed in the range of 100-1000 mg L -1 , which corresponds to the TOC concentration of 65-653 mg L -1 . The sorption was started by adding a weighed amount of a sorbent into the reactor. All the experiments were carried out under room temperature and atmospheric pressure. The reaction time was fixed for 2 h. This duration was chosen to limit the effect of the auto-oxidation of catechol by dissolved oxygen in water. After 2 h of reaction, the concentration of catechol was determined by high performance liquid chromatography (HPLC, Dionex system, PRP-X 300 column of 250 × 4.1 mm, UV detector). TOC measurements were carried out with a Shimadzu 5050 TOC analyzer. It allows analyzing the TOC of both the liquid and the solid phases. The conversion of catechol and liquid TOC was calculated according to the following during the fixation of Zn 2+ [START_REF] Sebei | Sorption behavior of Zn(II) ions on synthetic apatitic calciumphosphates[END_REF]. Some trace amounts of potassium was still observed in all the sorbents. For the zinc-doped sorbent (HAP_25_Zn), calcite was always present in this sorbent (JCPDS standard N°01-072-1937). The presence of CAP or HAP was still confirmed with the main peak at 32°. On the other hand, OCP disappeared, which suggests the participation of this compound to the fixation of Zn 2+ [START_REF] Sebei | Sorption behavior of Zn(II) ions on synthetic apatitic calciumphosphates[END_REF]. Despite the high content of Zn in this sorbent (11.8 wt.%), as shown by ICP-AES analysis (Table 1), any crystalline phase of zinc was observed for this sorbent. Did zinc exist under amorphous forms? Further analysis is needed to identify the chemical structure of zinc-containing compounds.

XRD characterization

For the sorbent obtained at 80°C (HAP_80), different apatitic calcium phosphates were present: HAP, CAP, OCP. Compared to the sorbent synthesized at 25°C, HAP_80 had a higher crystallinity with fine XRD peaks and good resolutions. Finally, crystalline calcite was still present, because the synthesis reaction of CaCO 3 with KH 2 PO 4 was not finished after 48 h (95%).

TG-DSC analysis

TG analysis of the initial sorbent is presented in Figure 2. DTG signals allowed highlighting the thermal behavior of the sorbents. The weight losses below 600°C could be attributed to the dehydration of surface water, brushite (CaHPO 4 •2H 2 O), monetite (CaHPO 4 ) or hopeite (Zn 3 (PO 4 ) 2 •4H 2 O) [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF][START_REF] Rootare | Vapor phase adsorption of water on hydroxyapatite[END_REF]. The presence of brushite as intermediate of the HAP synthesis was already confirmed by XRD. This compound thermally decomposed around 130-200°C to form monetite. This compound decomposed near 430°C [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF]. On the other hand, the hydydration of hopeite is usually complex which depends on its crystallinity as well as its dimorphic α and β forms [START_REF] Herschke | Polymer controlled mineralization of zinc phosphate hydrates and applications in corrosion protection, catalysis and biomedicine[END_REF]. In general, the dehydration of this compound can take place around 120-190°C then 280-370°C [START_REF] Herschke | Polymer controlled mineralization of zinc phosphate hydrates and applications in corrosion protection, catalysis and biomedicine[END_REF].

All three sorbents had weight losses around 600 and 800°C. These correspond to the decomposition of the residual calcium carbonate (close to 600°C) and CAP (close to 800°C). The decarbonation of CAP has been previously discussed [START_REF] Pham Minh | Synthesis of calcium hydroxyapatite from calcium carbonate and different orthophosphate sources: a comparative study[END_REF][START_REF] Pham Minh | Novel one-step synthesis and characterization of bone-like carbonated apatite from calcium carbonate, calcium hydroxide and orthophosphoric acid as economical starting materials[END_REF][START_REF] Pham Minh | Carbonated hydroxyapatite starting from calcite and different orthophosphates under moderate hydrothermal conditions: synthesis and surface reactivity in simulated body fluid[END_REF].

FTIR analysis

Figure 3 shows FTIR spectra of the initial sorbents. For HAP_25 and HAP_80, the characteristic bands of apatitic phosphate groups (PO 3- 4 ) were observed around 1030 cm -1 , and at 600 and 560 cm -1 [START_REF] Fowler | Infrared studies of apatites. I. vibrational assignements for calcium, strontium, and barium hydroxyapatites utilizing isotopic substitution[END_REF]. The large peak around 1030 cm -1 can be explained by the low crystallinity of the sorbents. The large vibration band at 3600-3000 cm -1 was due to molecular water. The very week vibration band at 630 cm -1 could be attributed to hydroxyl groups (OH -) of the apatitic structure. The peaks at 1400, 872 and 712 cm -1 correspond to the

Particle size distribution

Figure 4 presents the particle volume distribution of the sorbents as a function of the particle size. In all cases, different particle populations were observed. Fine particles around 1 µm were present at low volume distribution. The accumulated volume of particles smaller than 2 µm did not exceed 5%. HAP_25 contained the second population which extended from 2 to around 85 µm. The peak of this second population seemed to be under bimodal mode. Compared to HAP_25, HAP_80 had in addition the third population in the range of 85-450 µm. The presence of large calcium phosphate particles in these sorbents might be due to the agglomeration phenomenon, as suggested by SEM analysis (Figure 5). For HAP_25_Zn sorbent, particle volume distribution was similar to that of HAP_25. However, the bimodal mode of the second population was clear and the particle size of this sorbent extended up to around 165 µm. Table 2 summarizes the value of d 90 of all the sorbents.

3.1.6. Specific surface area (S BET ), true density, point of zero charge (pH ZPC ) and solubility product (K s ) Table 2 summarizes different physico-chemical properties of the sorbents used in this work. HAP_25 had high specific surface area of 145 m 2 g -1 , explained by the presence of mesopores in this sorbent [START_REF] Sebei | Sorption behavior of Zn(II) ions on synthetic apatitic calciumphosphates[END_REF]. HAP_80 had smaller S BET compared to HAP_25, which was probably due to the higher synthesis temperature. Consequently, HAP_80 was slightly denser than HAP_25. Both these sorbents had similar pH ZPC , but HAP_80 was much less soluble in water than HAP_25. This could certainly be due to the higher crystallinity of HAP_80 compared to HAP_25, as evidenced above by XRD characterization. For the HAP_25_Zn sorbent, the fixation of Zn 2+ (11.8 wt.%) led to a decrease of S BET , pH ZPC and K s , as well as a slight increase of the true density compared to the HAP_25 sorbent. The large difference of the solubility product between HAP_25 and HAP_25_Zn explained the high affinity of apatitic sorbent for Zn 2+ fixation [START_REF] Sebei | Sorption behavior of Zn(II) ions on synthetic apatitic calciumphosphates[END_REF].

Results on catechol sorption

Evolution of the concentration of catechol and TOC in the aqueous solution

The first experiments were carried out with an aqueous solution containing 1000 mg L -1 of catechol. Figure 6 presents the fraction of catechol and TOC that remained in the solution after 2 h of contact with 5 g L -1 of sorbent. At this high concentration of catechol (1000 mg L -1 ), the conversion of catechol reached from 6.2% to 13.2% after 2 h of contact with the sorbents. Catechol could be adsorbed on the surface of the sorbents and/or chemically degraded into intermediates by oxidation processes. TOC measurement of the liquid phase showed that in all cases, liquid TOC conversion was smaller than catechol conversion. So, a fraction of catechol was transformed into intermediates which were soluble in the aqueous phase. Both the nondoped sorbents (HAP_80 and HAP_25) showed low conversion of liquid TOC which ranged from 0.7 to 1.2%. The HAP_25_Zn sorbent showed high reactivity with 5.6% of liquid TOC conversion, which must be probably due to the catalytic property and/or the high affinity of Zn species for the degradation and fixation of organic compounds.

Figure 7 shows the results obtained with HAP_25 and HAP_25_Zn for the removal of catechol at a low concentration of 100 mg L -1 . The decrease of the initial concentration of catechol led to higher liquid TOC and catechol conversions, compared to the results of Figure 6. High conversions of catechol and liquid TOC were observed with HAP_25 and HAP_25_Zn. On the other hand, HAP_80 showed very low reactivity. This will be explained in the next sections.

Evolution of pH in the liquid phase during the sorption

Figure 8 shows the evolution of pH as a function of contact time during the sorption of catechol. At the concentration range of catechol of 100-1000 mg L -1 , the initial pH was around 5.2-5.3. In all cases, the pH increased quickly above 7.5 within the first minutes of contact with the sorbents. Then, the pH slowly decreased with the contact time, but remained higher than 7.5 in all cases. This stagnant pH might be explained by the buffer effect of the presence of residual calcium carbonate and/ or calcium phosphate compounds. For both the concentrations of catechol, the pH order was: HAP_25 > HAP_80 > HAP_25_Zn. A previous study showed that the increase of pH is favorable for the degradation of catechol [START_REF] Lebedev | Effect of group ii metal cations on catecholate oxidation[END_REF]. But the most reactive sorbent was HAP_25_Zn, which led to the lowest pH in the solution. This must be due to the effect of Zn on the degradation/fixation of catechol compared to non-doped sorbents.

Role of soluble metals (Ca 2+ , Zn 2+ )

In aqueous solution, calcium phosphates are sensitive to pH and can be leached during the test. The concentration of soluble zinc and calcium after 2 h of test was determined and presented in Figure 9. Among the two non-doped sorbents, HAP_25 had a higher solubility than that of HAP_80. In fact, HAP_25 had a lower crystallinity compared to HAP_80 (Figure 1). The higher solubility of HAP_25 might be also explained by the presence of intermediates such as brushite, monetite and calcite, which are generally more soluble than apatitic components. For HAP_25_Zn, both soluble calcium and zinc were observed but the concentration of calcium was high than that of zinc. From the results in Figure 9, we could determine the distribution of zinc and calcium in liquid and solid phases at 2 h of test (Table 3). The results in Figure 9 and Table 3 show that it is necessary to investigate the influence of the soluble metals on the degradation of catechol. Supplementary experiments have been carried out without solid sorbent at constant pH. In order to keep the pH unchanged, a buffer solution was prepared using tris (hydroxymethyl)aminomethane (TRIS, C 4 H 11 NO 3 ) of 0.2 M. The pH of this solution was adjusted to 8.5 by using 7.8 M HNO 3 solution [START_REF] Feenstra | Light scattering studies on solutions containing calcium phosphates[END_REF]. This buffer solution was then used for preparing 100 mg L -1 catechol solution for sorption tests. These were performed with and without soluble zinc or calcium addition (nitrate salts). The amount of zinc nitrate or calcium nitrate was calculated to get 0.9 mmol L -1 in the final solution of the sorption test. This metal concentration was much higher than the soluble zinc and calcium determined in Figure 9 in order to clarify the effect of soluble metals. Figure 10 presents the results obtained.

Catechol decomposed itself in the aqueous solution. The conversion of 3.37% was observed after 2 h of reaction at pH 8.5, without catalyst. The addition of soluble calcium or zinc accelerated this decomposition. Zinc was found to be more efficient with 15.9% of catechol conversion compared to calcium with 7.5 of catechol conversion after 2 h of contact. The presence of soluble zinc or calcium accelerated only the degradation of catechol, but it did not affect the mineralization. The value of liquid TOC was practically unchanged with or without soluble metal addition (Figure 10). In fact, the mineralization of organic compounds into CO 2 and H 2 O in the aqueous phase needs generally high temperature and pressure in the absence or in the presence of a homogeneous metal catalyst [START_REF] Luck | Wet air oxidation: past, present and future[END_REF][START_REF] Zapico | Influence of operation conditions on the copper-catalysed homogeneous wet oxidation of phenol: development of a kinetic model[END_REF].

The catalytic effect of divalent metallic cations such as Zn 2+ , Ca 2+ , Mg 2+ , Ba 2+ , Sr 2+ on the decomposition of catechol has been previously demonstration by Lebedev et al. [START_REF] Lebedev | Effect of group ii metal cations on catecholate oxidation[END_REF] Thus, a metal-catechol complex was formed which favors the dissociation (deprotonation) of hydroxyl groups. This additional deprotonation favors the transfer and the propagation of free radicals in the reaction medium, which are responsible for the initiation degradation of catechol [START_REF] Lebedev | Effect of group ii metal cations on catecholate oxidation[END_REF]. In another study, Bhatt and Shah also demonstrated the formation of metal-catechol complex for various divalent metals including Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ , Pb 2+ [START_REF] Bhatt | Sorption studies of heavy metal ions by salicylic acid-formaldehyde-catechol terpolymeric resin: isotherm, kinetic and thermodynamics[END_REF].

Carbon balance

Sorption results in Figures 6 and7 show that the liquid TOC conversion was not negligible using HAP-based sorbents. This liquid TOC conversion could be attributed to the fixation of organic species on the surface of the sorbent, and/or the mineralization of organic compounds. In order to determinate the nature of the liquid TOC conversion, the solids recovered after sorption tests were analyzed by solid TOC. Table 4 shows the TOC distribution in solid and liquid phases.

In all cases, the carbon balance could be buckled. The sum of TOC in liquid and solid phases was close to the initial quantity of TOC introduced into the reactor. This evidences that any mineralization took place at room temperature and pressure under the presence of HAPbased sorbents. The addition of zinc on the surface of HAP strongly improved the sorption capacity. The fixation of organic compounds on the surface of a solid sorbent is generally attributed to two coordination modes. The first one calls for the formation of inner sphere complex, wherein organic ligand replaces water or hydroxyl groups of the liquid-solid interface to form metal-ligand bonds. The second one corresponds to the formation of the outer sphere complex, wherein organic ligand is fixed around the liquid-solid interface by hydrogen bonds [START_REF] Mcbride | Chemisorption of catechol on gibbsite, boehmite, and noncrystalline alumina surfaces[END_REF][START_REF] Nordin | Adsorption of o-phthalate at the water-boehmite (γ-AlOOH) interface: evidence for two coordination modes[END_REF]. In our case, the improvement of the sorption reactivity by zinc addition suggests that the formation of metal-ligand bonds must be favored, as illustrated in Figure 11. At the pH range of 7.5-8.5 as observed in our experiments (Figure 8), catechol exists mainly under the non-deprotonation (C 6 H 4 (OH) 2 ) or semi-deprotonation (C 6 H 4 (OH)(O -)) forms. At this pH range, the surface of the sorbents used in this work must be negatively charged, taking into account the value of pH ZPC in Table 2. The nondeprotonation of catechol was more favorable with HAP_25_Zn than with HAP_25 or HAP_80, according to the pH evolution in Figure 8. This form of catechol has higher complexation with negative solid surface than the semi-deprotonation form which explains high reactivity of HAP_25_Zn. In addition, according to IUPAC Stability Constants Database, zinc has higher metal-ligand complexation constant with catechol than that of calcium, which allows also to explain the favorable effect of zinc addition on the fixation of this catechol.

These results help us to understand that, when HAPbased sorbent is used for the treatment of real wastewater containing simultaneously soluble metals and organic pollutants [START_REF] Lyczko | Calcium phosphate sorbent for environmental application[END_REF], it is possible that soluble metals are firstly fixed on the surface of the sorbent, followed by the fixation of organic compounds by ligand-metal complexation. Further characterizations need to be done with used sorbents to confirm this. Table 4. TOC distribution in liquid and solid phases after 2 h of test: initial concentration of catechol of 100 mg L -1 , sorbent concentration of 5 g L -1 (sorption results of Figure 7). From the results in Table 4, the sorption capacity of the sorbent could be calculated. The highest value was obtained with HAP_25_Zn, which reached 15 mg of C per gram of sorbent. This result was very positive since activated carbon, which is specifically developed for the removal of organic compounds, was found to have the sorption capacity of 92 mg of C per gram of sorbent [START_REF] Kumar | Adsorption of resorcinol and catechol on granular activated carbon: equilibrium and kinetics[END_REF].

Conclusions

HAP-based sorbents have been synthesized and investigated in the removal of catechol from an aqueous solution. Three sorbents have been evaluated: HAP_25 (synthesized at room temperature), HAP_80 (synthesized at 80°C) and HAP_25_Zn (solid recovered after the fixation of Zn 2+ by HAP_25). The most reactive sorbent was HAP_25_Zn, followed by HAP_25, then HAP_80. The pH of the sorption medium was found to be a crucial factor for the fixation of catechol. In the pH range observed for the sorption experiments, the surface of all the sorbents was negatively charged, which favored the complexation with the non-deprotonation (C 6 H 4 (OH) 2 ) or semi-deprotonation (C 6 H 4 (OH)(O -)) forms of catechol. Soluble calcium (Ca 2+ ) and zinc (Zn 2+ ) contributed also to the degradation of catechol. In all cases, any mineralization was observed, as confirmed by the carbon balance analysis.

Figure 1

 1 shows the XRD patterns of the initial sorbents used in this work. For the sorbent synthesized at 25°C (HAP_25), different crystalline phases were observed, including the residual calcite (JCPDS standard N°01-072-1937), HAP (JCPDS standard N°00-001-1008) with the main peak at 32°, octacalcium bis(hydrogenphosphate) tetrakis(phosphate) pentahydrate (OCP, Ca 8 (HPO 4 ) 2 (PO 4 ) 4 •5H 2 O, JCPDS standard N°01-047-0260) and carbonated hydroxyapatites (CAP). The presence of OCP was further confirmed by its main peak at 4.7°. The presence of CAP was thereafter confirmed by TG-DSC analysis thereafter.

Figure 1 .

 1 Figure 1. XRD patterns of the initial sorbents used in this work; (*) calcite; (•) HAP; (▪) OCP; (►) CAP.

Figure 3 .

 3 Figure 3. FTIR spectra of the initial sorbents used in this work.

Figure 2 .

 2 Figure 2. TG-DSC analysis of the initial sorbents used in this work; (a) weight loss, (b) derivative weight loss (DTG).

Figure 4 .

 4 Figure 4. Volume distribution as a function of particle size of the initial sorbents used in this work; particle size in base 10-logarithmic scale.

Figure 5 .

 5 Figure 5. Example of SEM image of HAP_80 sorbent; large particles were formed by agglomeration of small particles.

Figure 7 .

 7 Figure 7. Conversion of catechol and TOC after 2 h of test: initial concentration of catechol of 100 mg L -1 , sorbent concentration of 5 g L -1 .

Figure 6 .

 6 Figure 6. Conversion of catechol and TOC after 2 h of test: initial concentration of catechol of 1000 mg L -1 , sorbent concentration of 5 g L -1 .

Figure 8 .

 8 Figure 8. pH evolution during catechol removal with the initial concentration of catechol of 1000 mg L -1 (a) and 100 mg L -1 (b); inserts pH within three first min.

Figure 9 .Figure 10 .

 910 Figure 9. Concentration of soluble zinc and calcium in the aqueous phase after 2 h of test with the initial concentration of catechol of 100 mg L -1 and sorbent concentration of 5 g L -1 .

Sorbent 8 Figure 11 .

 811 Figure 11. Illustration of catechol-metal bond; metal: Zn or Ca.

Table 1 .

 1 Chemical composition of the sorbents used in this work. Firstly, the composition of all the sorbents was determined by ICP-AES (Table1). For the sorbent synthesized at 25°C, the conversion of CaCO 3 and KH 2 PO 4 into apatitic calcium phosphate reached 70%[START_REF] Sebei | Sorption behavior of Zn(II) ions on synthetic apatitic calciumphosphates[END_REF]. The bulk molar ratio of Ca to P of this sorbent was 2.38. At the synthesis temperature of 80°C, the conversion of CaCO 3 and KH 2 PO 4 reached 95%. The bulk molar ratio of Ca to P of this sorbent was 2.05. For the zinc-containing sorbent, the molar ratio of Ca to P decreased to 1.88 because of the partial replacement of Ca 2+ by Zn 2+

	Sorbent	Ca, mmol g -1	P, mmol g -1	Ca/P	Zn, mg g -1
	HAP_25	8.9	3.7	2.38	0
	HAP_80	9.4	4.6	2.05	0
	HAP_25_Zn	7.1	3.8	1.88

Table 2 .

 2 Physico-chemical properties of the initial sorbents used in this work.

	Sorbent	S BET , m 2 g -1	True density, g cm -1	pH ZPC	K s	d 90 , µm
	HAP_25 HAP_80 HAP_25_Zn	145 66 123	2.61 2.79 2.84	9.5 9.5 8.9	1.8*10 -94 6.8*10 -98 5.4*10 -99	95 235 85

Acknowledgements

We gratefully acknowledge our industrial partner for the financial support. We also thank the colleagues at the RAPSODEE center, Mines Albi, for their technical help.

Disclosure statement

No potential conflict of interest was reported by the authors.


Doan Pham
email: doan.phamminh@mines-albi.fr

Ngoc Dung Tran

Ange Nzihou

Patrick Sharrock

Novel one-step synthesis and characterization of bone-like carbonated apatite from calcium carbonate, calcium hydroxide and orthophosphoric acid as economical starting materials

Introduction

Calcium hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 , Ca-HA) is found to be the main mineral phase of human hard tissues [1]. However, pure Ca-HA never occurs in any biological system but co-exists with its derivatives [2]. These are formed by partial replacement of calcium (Ca 2+ ), orthophosphate (PO 4

3À

) or hydroxide (OH À ) groups by other species such as Mg 2+ , Na + , K + , F À , Cl À , and CO 3 2À [3,4].

Among them, carbonated apatite (CAP) is the closest biomimetic solid resembling the minerals in calcified tissues [4][5][6]. The amount of carbonate present in human bone mineral reaches typically 4-8 wt% [3,7]. Recent studies showed that substituted Ca-HA, such as CAP, were more effective than pure Ca-HA for skeletal implants [7][8][9]. This leads to a growing interest in the development of such substituted materials. Many hydroxyapatite types have been synthesized, characterized and examined for use as implantable bone-compatible biomaterials [10][11][12]. In synthesis procedures for pure Ca-HA, atmospheric carbon dioxide can be incorporated in the precipitated solids which lead to phosphate substitution with carbonate ions. This yields Ca-HA with low carbonate content and non-stoichiometric Ca/P ratios [13,14]. Only the more closely stoichiometric Ca-HA withstands the high temperature related to ceramic sintering or plasma-spraying, without decomposition into lime or tricalcium phosphate (TCP). CAP containing higher carbonate amounts can be synthesized by multi-step synthesis procedures including the precipitation of soluble calcium salts (Ca 2+ ) with PO 4 3À in the presence of CO 3 2À (liquid route) [15] or the heating up to more than 1000 8C of a calcium phosphate under carbon dioxide flux (thermal route) [16]. These methods have disadvantages of added manipulations for the elimination of waste counter ions or heating step under controlled carbon dioxide. CAP starting from calcium carbonate as the source of both calcium cations (Ca 2+ ) and carbonate anions (CO 3

2À

) and potassium dihydrogen orthophosphate (KH 2 PO 4 ) under atmospheric pressure was briefly mentioned by Lomo [17]. This route has recently been investigated under higher pressures of hydrothermal conditions [15,18,19]. Despite the severe reaction conditions used (high temperature, long reaction time and water rinsing), the conversion of calcium carbonate was not complete [19]. So, it is of current interest to find simple synthesis procedures that lead to CAP with controlled carbonate contents.

In this study, the synthesis of CAP using calcium carbonate (calcite), calcium hydroxide and orthophosphoric acid was investigated under moderate conditions. Orthophosphoric acid was chosen because of its strongest acidity among the available orthophosphate sources and the absence of alkali cations in the final CAP product. There is growing interest in the development of substituted calcium hydroxyapatite (Ca-HA) for biomedical applications. Carbonated apatite (CAP) appears as an important substituted Ca-HA because of its better biocompatibility compared to pure Ca-HA. This paper reports a novel pressure-mediated onestep synthesis of CAP starting from orthophosphoric acid, calcite and calcium hydroxide as available and high-purity starting materials. Under moderate synthesis conditions (80 8C and <13 bar), CAP with different carbonate contents could be obtained. The ratio of calcite/calcium hydroxide mixture played a crucial role for both the advancement of reaction and the carbonate content inserted in CAP's structure. At 80 8C, the total decomposition of calcite required a calcite/calcium hydroxide mixture containing at least a half of calcite. CAPs containing 2.25-4.17 wt% of carbonate inserted in its structure were obtained as a function of the composition of calcite/calcium hydroxide mixture. The results open a simple but effective way for the synthesis of high quality biomimetic CAP.

Various molar ratios of calcium carbonate to calcium hydroxide were used in order to control the amount of carbonate inserted in the apatitic structure. The results showed that CAP with different carbonate contents could be obtained in a one-step synthesis at 80 8C and pressures below 13 atm, without a washing step.

Materials and methods

Calcite powder (CaCO 3 , 98 wt%) from Fisher Scientific, calcium hydroxide powder (Ca(OH) 2 , 98 wt%) from Acros Organics and orthophosphoric acid (H 3 PO 4 , 85 wt% in water) from Merck were used as received. CAP synthesis was carried out in a 250 mL stainless steel reactor (Top Industrial) which was equipped with an electrical heating jacket and a magnetic stirrer. For each reaction, calcite or mixture of calcite/calcium hydroxide containing 100 mmol of calcium and 45 mL of water were fed into the reactor. After closing, 66.7 mmol of orthophosphoric acid were quickly injected into the reactor. This led to the starting molar ratio of calcium to phosphorus of 1.67 (Table 1). During the reaction, the stirring rate was set at 800 rpm and the reactor was thermostated at 80 8C for 48 h. This reaction temperature was chosen because supplementary experiments showed that the temperatures lower than 80 8C were not sufficient for a good decomposition of calcite particles. After 48 h of reaction, the reactor was freely cooled down to room temperature. Solid products were separated from liquid phase by filtration using 0.45 mm filter paper. Then they were dried at 50 8C for 48 h before further characterizations.

Both solid and liquid phases obtained from the filtration step were analyzed using different analysis and characterization techniques. Elemental analysis was carried out using inductively coupled plasma atomic emission spectroscopy (ICP-AES, HORIBA Jobin Yvon Ultima 2 apparatus). Thermogravimetry (TG) was performed in a TA Instruments SDTQ600 analyzer with a heating rate of 5 8C min À1 under air flux (100 mL min À1 ). Thermo-mechanical analysis (TMA) was carried out in a SETARAM SetSys 16/18 analyzer with 5 g constant load on the powder sample. X-ray diffraction (XRD) data of the solids collected using a Phillips Panalytical X'pert Pro MPD diffractometer with a Cu Ka (1.543 A ˚) radiation source. Fourier transform infrared (FTIR) spectroscopy was performed using a Shimadzu 8400S spectrometer. Particle size distribution was determined by laser scattering in a Mastersizer 2000 (Malvern Instruments Ltd., Malvern, UK) in the particle size range of 0.020-2000 mm. Scanning electron microscopy (SEM) was carried out on a Philips XL30 ESEM apparatus (FEI Company) which was coupled with an energy-dispersive X-ray spectroscopy (EDX analysis).

Results

Elemental analysis

The liquid separated from the filtration of the reaction mixture was acidified with concentrated nitric acid to avoid all further re-precipitation. Solid products were mineralized using concentrated nitric acid. The resulting homogeneous solutions were analyzed by ICP-AES technique and results are presented in Table 2. In all cases, the contents of soluble calcium and phosphorus in the liquid phase were lower than 1.4% of the initial quantities of calcium and phosphorus introduced in the reactor. So, the precipitation of orthophosphate species was quite complete and most of the calcium existed in solid phases after 48 h of reaction. Because of the absence of any counter ions in the liquid phase, no washing step was required for the purification of the final solid products. This brings a significant advantage of the present synthesis process in comparison with the synthesis using soluble calcium salts and/or alkali orthophosphates, since the washing step is usually arduous when small particles are formed.

The molar ratios of the bulk solids were higher than that of the stoichiometric Ca-HA (1.67). This result is discussed in more details in the section of FTIR analysis.

Decomposition of calcium carbonate

The reaction of calcite with orthophosphoric acid led to the formation of carbon dioxide which can exist in both gas and soluble states. Pressure in the reactor increased with the formation of carbon dioxide in gas phase. Taking into account the volume of gas phase in the reactor, the partial pressure of water vapor at 80 8C and using the ideal gas law, the quantity of carbon dioxide in gas phase could be calculated from the final pressure at 48 h of reaction. Fig. 1 indicates the relative advancement of the reaction via the calculated amounts of carbon dioxide. The decomposition of calcite reached at least 43, 89, 83 and 88% in the syntheses using 25, 50, 75 and 100% of calcite as calcium source, respectively,

, mmol

Fig. 1. Quantity of carbon dioxide in gas phase (CO gas 2 ) calculated from the final reaction pressure as a function of the molar percentage of calcite in the initial mixture of calcite and calcium hydroxide. which does not consider the amount of carbon dioxide dissolved in the liquid phase.

XRD characterization

In order to identify crystalline phases of the solid products, XRD characterization was performed and the results are presented in Fig. 2. For the solid product starting from 100% calcium carbonate (CAP100/0), all diffraction peaks could be attributed to calcium hydroxyapatite (Ca-HA). CAP might be also present but they seemed to be relatively low in comparison with that of Ca-HA. No evidence of calcium carbonate was observed, indicating that the decomposition of the initial calcite must be quite complete for this synthesis.

Both CAP75/25 and CAP50/50 products, which were formed from the mixture of calcite and calcium hydroxide, had similar XRD patterns compared to that of CAP100/0. Most of their peaks could be also attributed to Ca-HA. Trace amounts of octocalcium phosphate (OCP, Ca 8 (HPO 4 ) 2 (PO 4 ) 4 Á5H 2 O) could be also found. In fact, the final pH of the reaction mixture was around 7-8. This pH is favorable for the formation of OCP because in this pH range, the main soluble phosphate species are H 2 PO 4 À and HPO 4 2À [26]. This result was supported by previous work on the synthesis of Ca-HA under similar conditions, at atmospheric pressure [26]. As observed previously for CAP100/0, calcite was almost absent in these two solid products, which was in agreement with the results in Fig. 1.

On the other hand, the results changed notably for CAP25/75. Calcite, Ca-HA and OCP were found to be the main crystalline phases present in this sample. A small amount of crystalline calcium hydroxide was found, as illustrated by the low intensity of its main diffraction peak at 39.068. Calcium hydroxide has higher solubility and basicity than calcite. Thus, orthophosphoric acid introduced in the mixture of calcite and calcium hydroxide preferentially consumes the calcium hydroxide. When a high amount of calcium hydroxide is used such as for the synthesis of CAP25/75, the pH of reaction mixture remains at high value, which disfavors the decomposition of calcite.

In all cases, XRD results revealed the presence of CAP at levels much lower than those of Ca-HA. The identification of CAP by XRD is delicate because in most cases, the main diffraction peaks of CAP are close to those of Ca-HA and would require Rietveld refinement for increased resolution. Other characterization techniques were used to highlight the formation of CAP.

IR characterization

IR spectra of the solid products are presented in Fig. 3. In the wavenumber range from 4000 to 1700 cm À1 (not presented), there was only a very broad weak peak around 3500 cm À1 , which is attributed to the stretching of molecular water. The vibrations of orthophosphate groups are characterized by the absorption bands in the wavelength ranges of 650-550 cm À1 and 1300-910 cm À1 [20]. According to XRD results, calcite was only notably present in CAP25/75 as confirmed by its net absorption band at 711 cm À1 . This is one principal band of calcite and is not superimposed with bands of orthophosphates groups or carbonates groups present in apatitic structure [21].

The presence of carbonate groups inserted in the apatitic structure to form CAP was clearly revealed by IR spectra. The replacement of PO 4 3À groups present in Ca-HA's structure by CO 3 2À groups led to the formation of B-type CAP, which is characterized by the bi-modal peak at 1450/1415 cm À1 and the peak at 870 cm À1 [22,23]. Thus, B-type CAP was present in all solid products but its content was higher in CAP100/0, CAP75/25 and CAP50/50 than that in CAP25/75. The higher intensity at 1415 cm À1 compared to that at 1450 cm À1 of CAP25/75 was due to the principal peak at 1389 cm À1 of calcite that remained in this product. The formation of B-type CAP explained the molar ratio of Ca/P in Table 1, which was slightly higher than that of the stoichiometric Ca-HA. On the other hand, A-type CAP is formed when OH À groups of Ca-HA's structure are replaced by CO 3 2À groups, which have the characteristic bands at 1545 and 880 cm À1 [22,23]. So, A-type CAP was only formed in CAP100/0, CAP75/25 and CAP50/50, and was practically not present in CAP25/75.

Thermal analysis (TG)

Fig. 4 shows the results of the thermal analysis of the products in the wide range of temperatures from 25 to 1400 8C. A weight loss smaller than 1 wt% took place below 100 8C which corresponds to the removal of surface moisture. The next weight loss of 1-2 wt%, which occurred continuously from 100 8C to about 340-380 8C, could be attributed to the removal of lattice molecular water [24,25]. The weight loss in the temperature range of 380-500 8C was attributed to the condensation of HPO 4 2À groups, present in nonstoichiometric apatite such as OCP, to form calcium pyrophosphate (Ca 2 P 2 O 7 ), which was observed for all products. Remaining calcite decomposed at 610-670 8C [26]. DTG signal of both the condensation of HPO 4 2À groups and the decomposition of calcite increased with the increase of calcium hydroxide content in the initial reactant This showed the unfavorable effect of calcium hydroxide content for the decomposition of calcite, as explained in XRD section. According to XRD results, unreacted calcium hydroxide remained only notably in CAP25/75. Its thermal decomposition is characterized by two separated peaks [27]. The first weight loss starting at 325 8C was partially superimposed with the condensation of HPO 4 2À groups, and the second weight loss starting at 600 8C was practically superimposed with the thermal decomposition of calcite, highlighted by its DTG curve. The decarbonation of CAP took place in the temperature range of 670-1250 8C. According to IR results, A-type CAP was not present in CAP25/75. Thus, we deduce that the thermal decarbonation of A-type CAP and B-type CAP took place 870-1250 8C and 670-870 8C, respectively. Finally, the last weight loss at 1250-1340 8C was attributed to the dehydration of OH À groups present in Ca-HA's structure [28]. From TG curves in Fig. 4, the content of different types of carbonate present in the solid products could be calculated (Table 3). As observed by XRD characterization, the increase of calcium hydroxide content in the initial mixture of calcium source led to the increase of remaining calcite content. In all cases, the carbonate contents of B-type CAP were close to each other. On the other hand, the carbonate content of A-type CAP decreased with the increase of calcium hydroxide content in the initial mixture of calcium source, which was in good agreement with IR results. The total amount of carbonate obtained in CAP100/0 and CAP75/25 approximated the amount of carbonate present in the mineral phase of bone, dentin and enamel, which is in the range of 4-8% [3,7,22]. It is evident that the composition of the initial mixture of calcium source is an efficient parameter to control the carbonate content for the one-step synthesis of CAP, since the conversion of both initial calcium and orthophosphate sources into desired solid products was practically total in the synthesis of CAP100/0, CAP75/ 25, CAP50/50. Taking into account the quasi-complete decomposition of calcite, the high selectivity in apatitic compounds and the weight of the solid products recovered after filtration and drying steps, the yield of the synthesis procedure in desired CAP powder could be calculated which reached at least 97% for CAP100/0, CAP75/25 and CAP50/50.

SEM, particle size distribution

The results of SEM observation are illustrated in Figs. 5 and6. In the left-hand side of Fig. 5 (100 mm scale), particles of sizes ranging from several nm to hundreds of mm could be observed for CAP100/ 0, CAP75/25 and CAP50/50. The images on the right-hand side of Fig. 5 (10 mm scale) revealed that the larger particles consisted of the agglomeration of smaller ones.

The agglomerates of these solids had a flat-needle-like morphology. This was previously observed for apatitic calcium phosphates synthesized under hydrothermal conditions [19,29] or at atmospheric pressure [30]. The formation of these particles could be attributed to the growth of CAP particles along the c-axis direction, where CO 3 2À groups replaced OH À groups [31,32]. For CAP50/50, in addition to the presence of particles of flat-needle-like morphology, particles of sheet structure could be also observed, which were attributed to apparent morphology of OCP, which contains HPO 4

2À

groups [33].

SEM images of CAP25/75 are shown in Fig. 6. At 100 mm scale corresponding to a magnification of 200 times (Fig. 6(A)), particle sizes varied also in a large range as observed in Fig. 5 for other solids. At higher magnification, calcium phosphate particles of sheet-like structure were clearly observed, which was attributed to the presence at large amount of OCP (Fig. 6(B)). Remaining calcite particles were also frequently present, as confirmed by EDX analysis (Fig. 6(C)). We observed also that the remaining calcite particles were usually covered by calcium phosphate layers. This explained why the FTIR signal of calcite was not significant despite its high content in CAP25/75 (Fig. 3).

To better quantify the results of SEM observations, particle size distribution was examined and Fig. 7 shows the volume distribution as a function of particle size. The initial calcite powder had a Gaussian distribution in the particle size ranging from 6 to 70 mm. On the other hand, the initial calcium hydroxide had different categories of particle sizes, which varied from 0.3 mm to about 240 mm.

The volume distribution of all four solid products can be classified into two categories. The first one consisted of fine particles ranging from 0.35 to about 1.5 mm. The volume occupied by this category was not significant and did not exceed 1.4% of the total volume of the solids. The second one included particles larger than 1.5 mm, which stretched up to 310, 400, 450 and 710 mm for CAP100/0, CAP75/25, CAP50/50 and CAP25/75, respectively, indicating different levels of agglomeration of particles for each product. The higher the calcium hydroxide content was, the higher the pH of calcite/calcium hydroxide mixture was, and the more the acid-base reaction was violent leading to stronger agglomeration. In all cases, these results confirmed SEM observations of Figs. 5 and 6.

Thermo-mechanical analysis

Used as biometerials, apatitic calcium phosphate based products can be stabilized at high temperature in order to avoid all further evolution of materials. This section was dedicated to the study of the thermo-mechanical behavior of CAP100/0 as the selected product.

Thermal shrinkage is defined as (L À L 0 )/L 0 or DL/L 0 , where L 0 is the initial length of sample, and L is the length of sample measured at temperature T or time t. Fig. 8(A) shows the non-isothermal analysis of the CAP100/0 sample in the temperature range of 30-1000 8C. The temperature of 880 8C was found to be the critical point for the thermal shrinkage of CAP100/0 where the shrinkage accelerated. Only a slight thermal effect on the shrinkage was observed below 880 8C which was smaller than 0.02%. On the other hand, it became significant above 880 8C and reached À0.46% when the temperature rose to 1000 8C (Fig. 8(A)). during the heating time to reach the desired temperature. No significant thermal effect was observed at 600 and 800 8C for an isothermal time of 300 min, confirming that CAP100/0 was thermo-mechanically stable at these temperatures. As expected, the isothermal shrinkage took place at 1000 8C and reached À3.85% after 300 min.

The thermal shrinkage can be attributed to the sintering phenomenon wherein denser mass is formed and chemical reaction, i.e. the decarbonation or dehydration. Fig. 9 shows SEM images of CAP100/0 sample after TMA. The isothermal time of 300 min at 600 and 800 8C had no significant effect on the surface morphology of CAP particles, compared to the solid before thermal treatment (Fig. 5 for the CAP100/0 sample). At 1000 8C, particles of flat-needle-like morphology became only partially rounded. Probably, this morphology distortion of CAP particles corresponded to the first step of sintering wherein the formation of the contact areas between adjacent particles started [START_REF] Bailliez | Transmittance (a. u.) Wavenumber (cm -1 ) 105[END_REF].

TG analysis showed that the heating of CAP solids led to different chemical reactions. We are interested in the thermal stability of carbonate anions present in apatitic structure of CAP. As shown in Fig. 10 for IR spectra of CAP100/0 sample before and after TMA, the decarbonation of both A-type (bands at 1545 and 880 cm À1 ) and B-type (bands at 1450, 1415 and 870 cm À1 ) CAP was only partial after the plateau time of 300 min at different temperatures. This demonstrates the possibility to stabilize CAP using thermal treatment while conserving its carbonate content.

Conclusions

For the first time, carbonated apatite (CAP) of high purity was successfully synthesized by a one-step synthesis process using orthophosphoric acid, and a mixture of calcite and calcium hydroxide as simple starting reactants under moderate conditions (80 8C, <13 bar). Carbon dioxide was formed as the only byproduct and no washing step was required for the purification of final products. Both A-type and B-type CAP were formed under the synthesis conditions used.

The total content of carbonate inserted in apatitic structure could be controlled by varying the calcite content in the initial mixture of calcium source. We obtained CAP containing 2.25-4.17 wt% of carbonate with the initial mixture of calcium source containing 50-100% calcite. For a complete decomposition of calcite, an initial mixture of calcite and calcium hydroxide containing at least 50% of calcite was mandatory.

The CAP100/0 sample prepared from 100% calcite and containing 4.17% carbonate, was found to be thermo-mechanically stable up to 880 8C. This offers a good possibility for the synthesis of CAP for biomedical applications.

Fig. 2 .Fig. 3 .

 23 Fig. 2. XRD patterns of the solid products; Ca-HA (JCPDS standard No 01-072-1243); (&) calcite (JCPDS standard No 00-047-1743); (^) OCP (JCPDS standard N0 00-026-1056); (") calcium hydroxide (JCPDS standard No 00-050-0008).

Fig. 4 .

 4 Fig. 4. TG (A) and DTG (B) curves of the solid products analyzed in the temperature range of 25-1400 8C under air atmosphere.

Fig. 5 .

 5 Fig. 5. SEM images of CAP100/0, CAP75/25 and CAP50/50.

Fig. 8 (

 8 B) illustrates the isothermal shrinkage at different temperatures before and after the critical point. The initial value of DL/L 0 of each curve corresponded to the shrinkage of the sample

Fig. 6 . 2 Fig. 7 .

 627 Fig. 6. SEM images of CAP25/75.

Fig. 8 .

 8 Fig.8. TMA of CAP100/0; (A) non-isothermal analysis with the heating rate of 10 8C min À1 ; (B) isothermal analysis.

Fig. 9 .

 9 Fig. 9. SEM images of CAP100/0 after sintering at different temperatures.

Fig. 10 .

 10 Fig.10. IR spectra of CAP100/0 before (105 and after (600, 800 and 1000 8C) TMA.

Table 1

 1 Composition of the starting reactant mixtures and designation of solid products; other condition: synthesis temperature of 80 8C; stirring rate of 800 rpm; initial water of 45 g.

	Calcium source		Phosphate	Solid product
			source	designation
	CaCO 3	Ca(OH) 2	H 3 PO 4 ,
	powder, mmol	powder, mmol	mmol
	100	0	66.7	CAP100/0
	75	25	66.7	CAP75/25
	50	50	66.7	CAP50/50
	25	75	66.7	CAP25/75

Table 2

 2 Results of elemental analysis using ICP-AES technique; Ca Liq and P Liq : amount of calcium and phosphorus remained in the liquid phase; Ca/P: molar ratio of bulk solid products.

	Solid	Ca Liq (%)	P Liq (%)	Ca/P
	CAP100/0	1.4	1.1	1.88
	CAP75/25	0.9	1.2	1.84
	CAP50/50	1.4	1.2	1.82
	CAP25/75	1.3	1.2	1.74

Table 3

 3 Carbonate contents in the solid products determined by TG analysis.

	Product	CO 3 2À (wt%)
		CaCO 3	B-type CAP	A-type CAP	CAP
		(610-670 8C)	(670-780 8C)	(780-1250 8C)	(670-1250 8C)
	CAP100/0	0.30	0.63	3.5	4.17
	CAP75/25	0.75	0.69	2.73	3.42
	CAP50/50	1.20	0.63	1.62	2.25
	CAP25/75	<4.7 a	0.54	0	0.54

a Superimposition of TG signals of CaCO 3 decomposition and Ca(OH) 2 dehydration, giving an approximate value.

Acknowledgments

The authors acknowledge gratefully colleagues at RAPSODEE center, Nathalie Lyczko, Christine Rolland and Philippe Accart for different characterization measurements.


Augustina Ephraim
email: aug.ephraim@gmail.com

Doan Pham

Damien Lebonnois

Carlos Peregrina

Patrick Sharrock

Ange Nzihou

Co-pyrolysis of wood and plastics

Keywords:

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The rapid growth in world population over the last century combined with industrial development and urbanization has resulted in two major issues: the sharp increase in waste production and demand for energy. In Europe, the current generation rates of wood and plastic waste are 70.5 and 26.2 Mt/yr which are accompanied by low recycling rates of 31% and 17% respectively [START_REF] Jrc-Ipts | Inorganic chloride species (b) Organic chloride species Fig. 6. Theoretical chloride species yield from the pyrolysis of 30 wt% PVC/PW between 150 °C and 750[END_REF]. Hence, research into opportunities for converting wood and plastic waste to energy is strongly needed. Pyrolysis is a promising technique for recovering energy inherent in combustible solid waste which involves the thermal decomposition of the waste in an inert atmosphere to yield three primary products: gas (H 2 , CO, CO 2 , CH 4 and light hydrocarbons), liquid hydrocarbons (tars and oils) and char (carbon material).

In recent years, co-pyrolysis of woody biomass and plastic waste has gained considerable research interest mainly due to the positive contribution of plastics to the product yield and the calorific value of the oil produced [START_REF] Pinto | Production of liquid hydrocarbons from rice crop wastes mixtures by co-pyrolysis and co-hydropyrolysis[END_REF][START_REF] Abnisa | A review on co-pyrolysis of biomass: an optional technique to obtain a high-grade pyrolysis oil[END_REF][START_REF] Cornelissen | Flash co-pyrolysis of biomass with polyhydroxybutyrate: Part 1. Influence on bio-oil yield, water content, heating value and the production of chemicals[END_REF][START_REF] Cornelissen | Flash co-pyrolysis of biomass with polylactic acid. Part 1: Influence on bio-oil yield and heating value[END_REF][START_REF] Liu | Preparation of liquid chemical feedstocks by co-pyrolysis of electronic waste and biomass without formation of polybrominated dibenzo-p-dioxins[END_REF]. According to Johannes et al. [START_REF] Johannes | Synergy in co-pyrolysis of oil shale and pine sawdust in autoclaves[END_REF], the interactions or synergy between biomass and plastics depends on various factors including the type and contact of the fuel components, pyrolysis duration, temperature and heating rate, and catalysts. Hence, it is not surprising that conflicting reports sometimes arise in literature concerning the differences between the actual and predicted yields of the pyrolysis products, where the predicted yields are calculated as linearly proportional to the contributions of the pure components. For example, Grieco and Baldi [START_REF] Grieco | Pyrolysis of polyethylene mixed with paper and wood: interaction effects on tar, char and gas yields[END_REF] observed insignificant interactions between low density polyethylene (LDPE) and beech wood during co-pyrolysis in a capsule at heating rates of 0.1 and 1 °C/s. However, Yang et al. [START_REF] Yang | Fast co-pyrolysis of low density polyethylene and biomass residue for oil production[END_REF] observed a significant synegistic effect on oil yield when LDPE and cedar wood were co-pyrolyzed at 600 °C in a dropdown tube reactor at rapid heating rates.

Research works on the co-pyrolysis of biomass and plastic waste have principally focused on the influence of temperature, heating rate and reaction time on product yield and quality [START_REF] Papuga | Temperature and time influence on the waste plastics pyrolysis in the fixed bed reactor[END_REF][START_REF] Sun | Study on the pyrolytic behavior of wood-plastic composites using Py-GC/MS[END_REF][START_REF] Paradela | Study of the co-pyrolysis of biomass and plastic wastes[END_REF][START_REF] Sharypov | Co-pyrolysis of wood biomass and synthetic polymer mixtures. Part I: influence of experimental conditions on the evolution of solids, liquids and gases[END_REF]. Nonetheless, some authors, such as Oyedun et al. [START_REF] Oyedun | Thermogravimetric analysis of the pyrolysis characteristics and kinetics of plastics and biomass blends[END_REF] have shown that the type and plastic content in the sample also play an important role. Unfortunately, the few studies on the influence of plastic content on product yield are narrowly-focused on polyolefins (polyethylene (PE) or polypropylene (PP)) because this plastic type generates high oil yields and is the most abundant in waste streams [START_REF] Anal | Bio-oil production via co-pyrolysis of almond shell as biomass and high density polyethylene[END_REF][START_REF] Grieco | Pyrolysis of polyethylene mixed with paper and wood: interaction effects on tar, char and gas yields[END_REF][START_REF] Caglar | Isothermal co-pyrolysis of hazelnut shell and ultra-high molecular weight polyethylene: the effect of temperature and composition on the amount of pyrolysis products[END_REF]. Consequently, the operating conditions chosen for these studies aim at maximising oil yields for liquid fuel production and involve low temperatures (400-500 °C), and short gas residence times (< 15 min). Furthermore, with respect to research works on the influence of plastic content in biomass on gas composition and heating value, Grieco et al. [START_REF] Grieco | Pyrolysis of polyethylene mixed with paper and wood: interaction effects on tar, char and gas yields[END_REF] and Paradela et al. [START_REF] Paradela | Study of the co-pyrolysis of biomass and plastic wastes[END_REF] have observed significant interactions. However, Paradela et al. [START_REF] Paradela | Study of the co-pyrolysis of biomass and plastic wastes[END_REF] conducted their experiments with a fixed plastic waste mixture of 56 wt % PE, 17 wt% PS and 27 wt% PP and Grieco et al. [START_REF] Grieco | Pyrolysis of polyethylene mixed with paper and wood: interaction effects on tar, char and gas yields[END_REF] performed theirs using only PE as the plastic type. Hence, there remains a knowledge gap in literature about the contribution of non-polyolefins such as PS and PVC on the gas composition and heating value derived from co-pyrolysis with biomass. Thus, in this work, we will investigate the influence of PS and PVC content in wood on the co-pyrolysis product yields, gas specie yields and heating value, under conditions that favour gas production: high temperature, low heating rate and long gas residence time.

Experimental methods

Wood and plastic samples

The materials used for the tests were poplar wood (PW), PVC and PS, whose characterstics are displayed in Tables 1 and2. The particle size of each pure material was less than 1 mm. Several wood/plastic mixtures for each plastic type were prepared having different plastic contents: 0-5-10-30-50-100 wt% plastic.

Pyrolysis reactor and procedure

Fig. 1 gives a schematic representation of the experimental setup. For each test, 10 g of sample was placed in a quartz crucible. The sample was heated in a furnace to 750 °C at a heating rate of 20 °C/min under nitrogen atmosphere (33 mL/min) and was maintained at this temperature for 30 min before being cooled to room temperature. These operating conditions allowed a reaction time of 66 min which is sufficient for secondary cracking of volatiles to occur in order to form more gas and secondary char. At the reactor outlet, the volatile oil was condensed in an impinger bottle filled with cotton wool and cooled to 0 °C. Thus, in this case, the term 'oil' represents a mixture of bio-tar, oil, wax and small amounts of water produced from the pyrolysis process. For experiments with PVC, the oil condenser was replaced with two impinger bottles containing aqueous solutions of KOH to trap HCl gas. Next, the total amount of the non-condensable gas produced from pyrolysis was collected in a Tedlar bag. Its composition was analyzed by a micro-gas chromatograph (Micro-GC, A3000 Agilent). The quantification of HCl concentration in the aqueous phase was conducted using X-ray fluorescence (XRF, PANanalytical).

The char yield was calculated by dividing the weight loss of the sample and crucible before and after pyrolysis by the total initial sample mass. The gas yield y gas , was calculated based on the following equations:

= V Q t x gas N N 2 2
(1)

∑ = y ρ x V m ( ) gas i n i i gas s (2)
where V gas is the total gas volume, Q N2 and x N2 are the volumetric flowrate and volume fraction of the nitrogen carrier gas respectively, t is the reaction time, ρ i and x i are the density and volume fraction of each gas specie i, measured at normal temperature and pressure, and m s is the initial sample mass. To estimate the oil yield, the difference between the yields of char and gas was calculated.

The gas lower heating value LHV gas in units of MJ/Nm 3 was esti- mated based on the lower heating values of each gas specie i (MJ/kg), using the following equation:

∑ = LHV LHV ρ x gas i n i i i (3)
To evaluate the synergy between poplar wood and the plastic samples, the theoretical product yields were calculated according to the following formula:

= + y w y w y calc 1 1 2 2 (4)
where y 1 and y 2 are the pyrolysis yields of pure wood and plastic; w 1 and w 2 are the mass proportions of wood and plastic in the sample respec- tively. Therefore, the interaction or synergy parameter y ∆ , is the dif- ference between the experimental and calculated product yields from co-pyrolysis.

= - y y y ∆ exp calc
(5)

Results

Effect on char yield

Fig. 2 and Table 3 present the yields of char, oil and gas with respect to the different plastic contents in the samples. It can be seen in Table 3 that the char yield from pure PW pyrolysis is 20.6 wt% whereas half as much char is produced from pure PVC (10.5 wt%) and virtually no char is produced from pure PS. Williams et al. [START_REF] Williams | The pyrolysis of individual plastics and a plastic mixture in a fixed bed reactor[END_REF] have obtained similar values for char yield from the pyrolysis of these pure plastics.

Regarding the co-pyrolysis of PW and PS, a small positive synergy can be observed in Fig. with a maximum difference of approximately 2.5 wt% between the experimental and theoretical char yield. This indicates that the presence of PS promotes secondary reactions of volatiles, such as condensation and repolymerisation, to form secondary char.

For co-pyrolysis with PVC, a strong positive synergy can be seen in Fig. 2a, where the maximum synergy of 8 wt% occurs at a PVC content of 30 wt%. Cepeliogullar and Putun [START_REF] Aullar | Products characterization study of a slow pyrolysis of biomass-plastic mixtures in a fixed-bed reactor[END_REF] have also obtained significantly higher char yields compared with the theoretical yield for a mixture of 1:1 weight ratio of PVC and biomass (hazelnut shell, sunflower residues and E. rigida). An explanation for this phenomenon could be drawn from the work of Matsuzawa et al. [START_REF] Matsuzawa | Acceleration of cellulose co-pyrolysis with polymer[END_REF][START_REF] Matsuzawa | Evaluation of char fuel ratio in municipal pyrolysis waste[END_REF] who suggest that HCl produced from PVC pyrolysis promotes the dehydration and cross-linking of cellulose in biomass, which in turn promotes the charring of cellulose.

Effect on oil and gas yields

As shown in Table 3, the oil and gas yield of pure PW is 63.7 wt% and 15.7 wt%. Pure PS generates a high oil yield of 99.5 wt%, due to its relatively high volatile and low ash content (Table 1). Thus, the oil yield from PS/PW co-pyrolysis increases with PS content, which follows a similar trend reported in literature for co-pyrolysis with other biomass types at temperatures of roughly 400 °C [START_REF] Abnisa | A review on co-pyrolysis of biomass: an optional technique to obtain a high-grade pyrolysis oil[END_REF][START_REF] Abnisa | Pyrolysis of mixtures of palm shell and polystyrene: an optional method to produce a high-grade of pyrolysis oil[END_REF][START_REF] Abnisa | Co-pyrolysis of palm shell and polystyrene waste mixtures to synthesis liquid fuel[END_REF][START_REF] Brebu | Co-pyrolysis of pine cone with synthetic polymers[END_REF]. However, in Fig. 2b, a negative synergy can be observed for PS/PW co-pyrolysis, which has not yet been demonstrated in literature. Oyedun et al. [START_REF] Oyedun | Thermogravimetric analysis of the pyrolysis characteristics and kinetics of plastics and biomass blends[END_REF] have found from thermogravimetric analysis, that lower temperature and energy are required to pyrolyze PS/biomass blends than are required for the pyrolysis of other mixtures such as HDPE/biomass. It is therefore possible, in our opinion, that the oil formed at lower temperatures in the presence of PS has more time to undergo secondary cracking reactions to produce more gaseous products, and thereby decrease the overall oil yield. This suggestion is supported by the observation, in Fig. 2c, that the gas yield increases relative to the predicted yield for PS contents below 50 wt%.

Pure PVC has a relatively low oil yield (30.3 wt%) and high gas yield (59.2 wt%) compared to PW and PS. By observing Fig. 2b, the copyrolysis of PVC and PW has a positive synergistic effect on oil yield, with a maximum synergy of 11 wt% at 50 wt% PVC content. A corresponding strongly negative synergy in gas yield can be seen in Fig. 2c, where the maximum synergy of 18 wt% occurs at 30 wt% PVC content. Zhou et al. [START_REF] Zhou | Effect of interactions of PVC and biomass components on the formation of polycyclic aromatic hydrocarbons (PAH) during fast co-pyrolysis[END_REF] have recently observed a significant increase in oil yield for mixtures of PVC with three biomass componentshemicellulose, cellulose and ligninduring fast co-pyrolysis at 800 °C in a fixed bed reactor. They propose that the biomass and/or bio-char can act as a catalyst to promote chain scission of PVC, resulting in the increased production of chlorinated oil compounds. This theory will be examined in Section 3.5.

Effect on gas specie yields

Table 4 displays the yield of light gases derived from the co-pyrolysis of the different plastic and wood samples, and Table 5 shows the yield of the heavier hydrocarbon gases produced, which we will conveniently refer to as C H

x y . In Fig. 3, the variation of the various gas specie yields as a function of plastic content is presented, which reveals non-linear trends and therefore, the existence of interactions between PW and plastics.

Concerning PS, the yields of the gas species generally decrease as PS content in PW increases because oil production is favoured by this plastic (Fig. 2b). However, significant positive synergies can be observed for H 2 , CH 4 , CO and CO 2 , and to a lesser degree for C H

x y compounds. This implies, that secondary reactions occur between PS and the wood volatile products to promote the formation of lighter gases.

In Fig. 3, we can observe that PVC contents below 50 wt% in PW results in negative synergies for CH 4 , C H

x y , CO and CO 2 yields. This indicates that interactions between PVC and PW favours the production of oxygenated hydrocarbons in the oil and char products. Nonetheless, a positive synergy for H 2 yield can be seen, which in our opinion, may be due to enhanced condensation and cyclization reactions of polyene species produced from the dechlorination of PVC. These cyclization reactions of polyene take place via both radical and molecular mechanisms to form H 2 , polycyclic aromatic hydrocarbons (PAH) and char [START_REF] Marongiu | Thermal degradation of poly (vinyl chloride)[END_REF]. Our proposed mechanism for enhanced H 2 production is supported by the recent works of Çepelioullar et al. [START_REF] Aullar | Products characterization study of a slow pyrolysis of biomass-plastic mixtures in a fixed-bed reactor[END_REF][START_REF] Aepelioullar | A pyrolysis study for the thermal and kinetic characteristics of an agricultural waste with two different plastic wastes[END_REF] who obtained an increased amount of PAH from the co-pyrolysis of PVC and biomass mixtures.

It is interesting to observe in Fig. 3b that although HCl yield increases as a function of PVC content, a strong negative synergy can be observed, whereby HCl gas yield was found to decrease by 53.6 wt% when PVC content was decreased from 100 to 30 wt.%. This result is consistent with a recent TG-FTIR analysis by Zhou et al. [START_REF] Zhou | Interactions of three municipal solid waste components during co-pyrolysis[END_REF] who report that, compared to pure PVC pyrolysis, the maximum HCl peak strongly weakens when PVC is co-pyrolysed with orange peel and tissue paper in a 1:1 weight ratio mixture. Furthermore, Kuramochi et al. [START_REF] Kuramochi | HCl emission during co-pyrolysis of demolition wood with a small amount of PVC film and the effect of wood constituents on HCl emission reduction[END_REF] have reported a significant reduction in HCl emission when demolition wood is co-pyrolyzed with 1 wt% PVC film. However, the mechanism for HCl-biomass interaction is not well-defined in literature [START_REF] Yu | Thermal degradation of PVC: a review[END_REF]. Kuramochi et al. [START_REF] Kuramochi | HCl emission during co-pyrolysis of demolition wood with a small amount of PVC film and the effect of wood constituents on HCl emission reduction[END_REF] have pyrolyzed mixtures of PVC with three constituents of biomass (cellulose, hemicellulose and lignin) separately. Their results show that cellulose has an insignificant effect on HCl emission, whereas hemicellulose, and lignin to a lesser extent, reduce HCl emissions by fixing most of the Cl molecules in the char product. A contradictory result has been presented by Zhou et al. [START_REF] Zhou | Thermogravimetric characteristics of typical municipal solid waste fractions during co-pyrolysis[END_REF] who have found that cellulose can also decrease HCl emission to a similar degree as hemicellulose and lignin. To further add to the confusion, another recent work by the same author suggests that the Cl molecules are fixed in the liquid product to form chlorinated oil compounds [START_REF] Zhou | Effect of interactions of PVC and biomass components on the formation of polycyclic aromatic hydrocarbons (PAH) during fast co-pyrolysis[END_REF]. It is therefore evident that more investigations are needed to better understand the mechanisms that govern the interactions between HCl and biomass during co-pyrolysis with PVC. This point will be further addressed in Section 3.5.

Effect on gas heating value

Table 6 and Fig. 4 present the evolution of the gas lower heating value (LHV) with plastic content. Factors that can increase the LHV of a gas include high concentrations of hydrogen and hydrocarbons. Fig. 4 shows that at conditions of normal temperature (20 °C) and atmospheric pressure, the LHV of gas derived from pure PW pyrolysis is about 19 MJ/Nm 3 , while higher LHV values are observed for pure PS and PVC in the order of 23 and 26 MJ/Nm 3 respectively. However, the gas LHV is observed to not vary significantly with plastic content below 50 wt%, because the hydrogen and carbon content of these plastics preferentially distribute to the oil product of PS, and the char and HCl gas products of PVC. PVC content in the sample mixture with PW. The Cl content in the char and condensed gas products were analysed using X-ray fluorescence (XRF, PANanalytical) and the Cl content in oil was calculated by difference. The corresponding yields of Cl in the pyrolysis products were then determined on the basis of the initial mass of Cl in the PVC/PW samples. It can be observed in Fig. 5 that products from pure PW pyrolysis contain insignificant amounts of Cl whereas for pure PVC, 96.9 wt % of Cl is released in the gas phase, thus leaving only 2.9 and 0.04 wt% Cl in the oil and char phase respectively. Compared to the products of pure PVC pyrolysis, the Cl yield decreases in the gas phase by 89.9 wt% when the PVC content is lowered from 100 wt% to 30 wt%, and the corresponding Cl yields in the oil and char products increase by 76.2 and 13.8 wt% respectively. Furthermore, even at a low PVC content of 5 wt%, the Cl distribution in oil is significantly higher than in the char and gas phases. Hence, it is clear that the Cl molecules from co-pyrolysis of PVC/PW are preferentially trapped in the oil product. Our result therefore supports the aforementioned hypothesis presented by Zhou et al. [START_REF] Zhou | Effect of interactions of PVC and biomass components on the formation of polycyclic aromatic hydrocarbons (PAH) during fast co-pyrolysis[END_REF] that the Cl molecules are fixed in the oil product.

Effect of PVC content on chlorine distribution

A further investigation was made to predict the distribution of chloride species in the char, gas and oil products, using thermodynamic equilibrium calculations based on the Gibbs free energy minimization approach. These calculations were performed using FactSage V.6.3. and a database developed by Said et al. [START_REF] Said | Behavior of heavy metals during gasification of phytoextraction plants: thermochemical modelling[END_REF]. The chosen input data for the calculations are:

• Feedstock: 10 g of 30 wt% PVC/PW, with its composition of organic and inorganic elements displayed in Table • Reactor temperature range: 150-750 °C.

• Reactor pressure: 1 atm Figs. 6a and b display the equilibrium mass percentages of inorganic and organic chloride species with respect to the total mass of chloride species and as a function of temperature, respectively. Fig. 6a shows that above 200 °C, 95 wt% of chloride species is released into the gas phase as HCl. In addition, Cl is mainly trapped in the inorganic solid phase as CaCl 2 (4 wt%). It is interesting to observe in Fig. 6b that organic chloride species are formed in trace amounts, the most abundant of which is dimethyl phosphinic chloride (C 2 H 6 ClOP), which forms a pinkish-white crystal at room temperature.

By comparing the predicted yields of chloride species from the thermodynamic equilibrium calculations, with those obtained experimentally from the pyrolysis of 30 wt% PVC/PW (Fig. 5), we can first conclude that chlorinated hydrocarbons are only in trace amounts in the condensed oil phase (a mixture of bio-tar, oil, wax and water), which contradicts the proposal by Zhou et al. [START_REF] Zhou | Effect of interactions of PVC and biomass components on the formation of polycyclic aromatic hydrocarbons (PAH) during fast co-pyrolysis[END_REF]. Secondly, we propose that the high chlorine content observed in the oil phase is mainly due to HCl gas dissolved in the water fraction of the oil upon condensation. This statement is supported by our observation in Fig. 5 that HCl yield in the gas phase has a similar but opposite trend to the chloride species yield in the oil phase with respect to the initial PVC content in the sample. Future work should be done on the analysis of organic chlorinated compounds in the oil phase, using for example GC-MSMS technique.

Conclusion

Experimental work on the co-pyrolysis of poplar wood (PW) and the plastics, PS and PVC, has been presented in this paper in which different synergistic effects of plastic type and content on product yield, gas specie yield and gas heating value were studied. Regarding char yield, PVC was found to provide a relatively large positive synergy with a maximum value of 8 wt% at 30 wt% PVC content, whereas PS only showed a slightly positive synergy (2.5 wt% maximum). The addition of PS generally increased oil yield and lowered gas yield relative to the pyrolysis of pure PW. However, for oil production, a small negative synergy was seen for PS, although the reverse effect on gas yield was observed. Interestingly, the presence of PVC had a significant positive synergy on oil yield with a maximum value of 11 wt% at 50 wt% PVC content. This was linked to a strong negative synergy in gas (mainly HCl) production, where a maximum synergy of 18 wt% was observed at 30 wt% PVC content.

With respect to the influence of plastic type and ratio on gas specie yield, the addition of PS resulted in positive synergies in the formation of H 2 , CH 4 , CO and CO 2 , although insignificant interactions were seen for C H

x y compounds. With regard to PVC, an increase in H 2 yield was observed accompanied by a positive synergistic effect. Furthermore, HCl gas yield was found to decrease by 53.6 wt% when PVC content was decreased from 100 to 30 wt%. A further analysis of the distribution of Cl molecules in the char, gas and oil products using thermodynamic equilibrium calculations, revealed that Cl was largely fixed in the condensed oil phase mainly as dissolved HCl gas in the water fraction, although some chlorine was also trapped in the solid residue in the form of metal chloride salts. Concerning the gas lower heating value (LHV), the addition of PS and PVC to the sample had an insignificant synergistic effect below 50 wt% plastic content.

Our work consolidates the understanding of the effect of synergy between wood, PVC and PS, on product yield, gas specie yield and gas heating value under operating conditions that favor gas production. Moreover, by comparing the results of our work with literature, we have identified the need for a better understanding of the interactions involving HCl during co-pyrolysis of biomass and PVC. Our future work will therefore focus on modelling the kinetics of interactions between poplar wood and PVC during co-pyrolysis.

Fig. 1 .

 1 Fig. 1. Experimental setup of pyrolysis equipments.

Fig. 2 .

 2 Fig. 2. Influence of plastic type and content on product yield.

Fig. 5

 5 Fig.5presents the distribution of Cl molecules in the char, gas and condensed oil (bio-tar, oil, wax and water) products as a function of the

Fig. 3 .

 3 Fig. 3. Influence of plastic type and content on gas composition.

Fig. 4 .

 4 Fig. 4. Influence of plastic type and content on gas heating value.

Fig. 5 .

 5 Fig. 5. Influence of PVC content on distribution of chlorine in char, gas and oil products.

Table 1

 1 Proximate analysis (as-received basis) of wood and plastic samples.

		Moisture (%) Ash (%) Volatile matter	Fixed carbon	LHV (MJ/
				(%)	(%)		kg)
	PW	8.0	2.0	85.1	4.0		17.1
	PS	0.0	0.0	99.6	0.4		39.8
	PVC 0.0	0.0	95.8	4.2		19.3
	Table 2
	Ultimate analysis (dry ash-free basis) of wood and plastic samples.
		C (%)	H (%)	O (%)	N (%)	S (%)	Cl (%)
	PW	49.9	6.4	42.7	1.0	0.0	0.01
	PS	92.4	7.8	0.0	0.0	0.0	0.0
	PVC	38.7	4.8	0.0	0.0	0.0	56.5

Table 3

 3 Product yield from the pyrolysis of PW, PVC, PS and their mixtures

			PVC			PS
	Plastic	Char	Gas	Oil *	Char	Gas	Oil*
	Content (wt%)	(wt%)	(wt%)	(wt%)	(wt%)	(wt%)	(wt%)
	0 (100 wt% PW)	20.6	15.7	63.7	20.6	15.7	63.7
	5	26.1	13.5	60.5	20.9	16.1	63.0
	10	25.7	11.0	63.3	19.9	16.0	64.2
	30	25.4	10.3	64.2	16.3	15.1	68.6
	50	20.2	21.5	58.2	13.1	8.30	78.6
	100	10.5	59.2	30.3	0.00	0.50	99.5
	* Calculated by difference and represents a mixture of bio-tar, oil, wax and
	water.

Table 4

 4 Light gas yields from the pyrolysis of PW, PVC, PS and their mixtures.

	PVC

Table 5

 5 Hydrocarbon gas (C Hx y ) yields from the pyrolysis of PW, PVC, PS and their mixtures.

			PVC			PS
	Plastic	C 2 H 4	C 2 H 6	C 3 H 8	C 2 H 4	C 2 H 6	C 3 H 8
	Content (wt%)	(wt%)	(wt%)	(wt%)	(wt%)	(wt%)	(wt%)
	0 (100 wt% PW)	0.26	0.20	0.19	0.26	0.20	0.19
	5	0.19	0.12	0.13	0.38	0.18	0.22
	10	0.13	0.12	0.11	0.34	0.18	0.24
	30	0.18	0.17	0.13	0.34	0.15	0.15
	50	0.23	0.29	0.17	0.28	0.10	0.09
	100	0.69	0.45	0.24	0.09	0.01	0.00

Table 6

 6 Gas lower heating value (MJ/Nm3) from the pyrolysis of PW, PVC, PS and their mixtures.

		PVC	PS
	0 (100 wt% PW)	18.9	18.9
	5	18.2	19.0
	10	17.9	19.0
	30	18.2	18.3
	50	18.5	19.0
	100	25.6	22.8

Table 7

 7 Elementary composition of 30 wt% PVC/PW.

	30 wt% PVC/PW