A study on the impact of the reaction mechanism of the thermochemical activation of bone char (by pyrolysis and carbonization)

Jou nal o Analy ical and Applied Py olysis 171 (2023) 105973 A ailable online 10 Ap il 2023 0165-2370/© 2023 The Au ho (s). Published by Else ie B.V. This is an open access a icle unde he CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by- nc-nd/4.0/). A s udy on he impac o he eac ion mechanism o he he mochemical ac i a ion o bone cha (by py olysis and ca boniza ion) I ene Sie a a , * , Jos´ e L. Ayas uy b , Miguel A. Gu i´ e ez-O iz b , Unai I ia e-Velasco a a Depa men o Chemical Enginee ing, Facul y o Pha macy, Uni e si y o he Basque Coun y UPV/EHU, Paseo de la Uni e sidad 7, 01006 Vi o ia-Gas eiz, Spain b Depa men o Chemical Enginee ing, Facul y o Science and Technology, Uni e si y o he Basque Coun y UPV/EHU, Ba io Sa iena s/n, 48940 Leioa, Spain ARTICLE INFO Keywo ds: Bioapa i e Bone cha Ac i a ion mechanism Mass spec ome y H 2 SO 4 K 2 CO 3 ABSTRACT Po ous ma e ials based on bioapa i e we e p epa ed h ough he he mochemical ac i a ion o po k bone cha , using di e en eagen s (H 2 SO 4 and K 2 CO 3 ), hea ing a mosphe e (ine and oxidizing) and ac i a ion empe a- u e. The mog a ime ic analysis coupled o mass spec ome y was used o p opose a de ailed ac i a ion mechanism. Rega ding he e ec o each eagen on he ac i a ion, whe eas ha o H 2 SO 4 is a ibu ed o speci ic eac ions, he e ec o K 2 CO 3 is asc ibed o (i) he ole o K + as a ca alys o eac ions such as he gasi ica ion o ca bon and (ii) he inco po a ion o ca bona es, in ol ed in se e al eac ions. The bes ex u al p ope ies a e ob ained in an oxidizing a mosphe e a 350 ºC. The bene icial e ec o he combus ion eac ions a an in e - media e empe a u e esul s in a cos -e ec i e p epa a ion p o ocol. H 2 SO 4 leads o a selec i e de elopmen o mic opo osi y. The oxidizing a mosphe e is in ol ed in he gene a ion o OH - unc ional g oups. These esul s demons a e he easibili y o con igu ing ac i a ion p o ocols o ailo he physicochemical p ope ies o biapa i e-based ma e ials, o use in speci ic ields. 1. In oduc ion The e is a g owing in e es in ma e ials ha all in o he calcium phospha e spec um. Among hem, hyd oxyapa i e (HAp), [Ca 10 (PO 4 ) 6 (OH) 2 ] is one o he mos p omising. HAp, a mine al o m o calcium phospha e, is a e sa ile ma e ial due o i s s uc u al s abili y, acid–base p ope ies and ionic subs i u ion abili y. Hyd oxyapa i e can be chemically syn hesized using a ious p ocedu es such as p ecipi a- ion, hyd olysis, sol–gel app oach, hyd o he mal, emulsion, d y me hods and mechanochemical syn hesis [1]. In he con igu a ion o ma e ials based on HAp, he use o na u al bio- esou ces such as was e animal bones is a p omising choice among he possible op ions, since bioapa i e is he main ino ganic componen o bones. The global slaugh e indus y p oduces billions o kg o animal bone esidues e e y yea . The use o mea and bone meal (MBM) o eed ca le was o bidden in EU (Commission Decision 94/381/EC), as a esul o he bo ine spongi o m encephalopa hy c isis. Consequen ly, he e is a high amoun o animal was es ha mus be sa ely disposed o ans- o med. The de elopmen o an adequa e echnology o he con e sion o was e ma e ial in o alue-added p oduc s will help o e come his g owing challenge. The he mochemical con e sion o was e animal bones can be used o p oduce po ous ma e ials. Thus, one o he ad- an ages o his s a egy is he educ ion o he high amoun o animal was es o be sa ely disposed, wi h he co esponding en i onmen al bene i . The solid p oduced con ains mainly biological apa i e, a ami- ica ion o hyd oxyapa i e. Ma e ials based on hyd oxyapa i e ha e been used in se e al ields such as he emo al o pollu an s om liquid and gaseous s eams [2–5], in ca aly ic sys ems [6], in elec ochemis y [6,7] and in biomedical applica ions [8,9]. Consequen ly, he p oduc ion o po ous ma e ials om was e animal bones, an abundan and a ailable low-cos ma e ial, could be economically easible as well as en i onmen ally iendly. The po en ial applica ions o hese ma e ials a e closely ela ed o hei physicochemical p ope ies such as speci ic su ace a ea, po osi y, Ca/P mola a io, phase composi ion and pa icle size [10]. Fo example, a nanoc ys alline HAp wi h high su ace a ea and small pa icle size can o e an adequa e so p ion capaci y and cell compa ibili y o be applied as adso ben and bioma e ial [11]. Thus, i adequa e ea men me hods a e de eloped, he use o animal bones could ep esen a p omising al e na i e o he p oduc ion o bioapa i e-based po ous ma e ials wi h ailo ed p ope ies. The mal ea men cons i u es one o he mos eliable ea men * Co esponding au ho . E-mail add ess: [email p o ec ed] (I. Sie a). Con en s lis s a ailable a ScienceDi ec Jou nal o Analy ical and Applied Py olysis jou nal homepage: www.else ie .com/loca e/jaap h ps://doi.o g/10.1016/j.jaap.2023.105973 Recei ed 9 Feb ua y 2023; Recei ed in e ised o m 29 Ma ch 2023; Accep ed 9 Ap il 2023 Jou nal o Analy ical and Applied Py olysis 171 (2023) 105973 2 me hods o he esidual animal bones. The solid ac ion ob ained, con aining mainly biological apa i e, is e e ed o as bone cha . The physicochemical p ope ies o his ma e ial can be enhanced by chem- ical ac i a ion. The chemical ac i a ion o biomass p ecu so s has se e al ad an ages o e physical ac i a ion: (i) highe yield, (ii) lowe ac i a ion empe a u es equi ed, (iii) less ime equi ed in he ac i a- ion p ocess, (i ) possibili y o inco po a ing sui able unc ional g oups, ( ) be e de elopmen o he po ous s uc u e, and ( i) highe su ace a ea [12]. The mos commonly used eagen s o chemical ac i a ion include ino ganic sal s (e.g. K 2 CO 3 ), alkali hyd oxides (KOH, NaOH), as well as ino ganic acids (such as H 2 SO 4 and H 3 PO 4 ) [13,14]. Al hough he li e a u e conce ning he p epa a ion o po ous ma e- ials h ough he chemical ac i a ion o bone cha is sca ce, he e a e se e al s udies ha demons a e he sui abili y o his p epa a ion me hod. Fo example, in a p e ious wo k [15] we de e mined ha he acid ea men o bone cha wi h ei he H 2 SO 4 o H 3 PO 4 inc eased he BET su ace a ea by abou 80%, compa ed o he sample p epa ed by physical ac i a ion only. Fu he mo e, he ea men wi h H 2 SO 4 led o a d ama ic inc ease in mic opo osi y, up o 263%. The alkali ea men wi h K 2 CO 3 and NaOH also p o ed o be e ec i e [16], wi h an inc ease in S BET o 45% and 39%, espec i ely, compa ed o he sample ob ained by physical ac i a ion. The knowledge o he mechanism in ol ed in he ac i a ion o bone cha and i s impac on he p ope ies o he ma e ial may be conside - ably bene icial o con igu e a ma e ial wi h he desi ed physicochemical p ope ies. Se e al e o s ha e been made o unde s and he eac ions ha ake place du ing he p epa a ion o po ous ma e ials by chemical ac i a ion, using di e en ca bonaceous p ecu so s and ac i a ing agen s. Guo and Lua [17] s udied he chemical ac i a ion o oil-palm s one wi h H 2 SO 4 and KOH. Lillo-Rodenas e al. [18,19] in es iga ed he eac ions occu ing du ing he chemical ac i a ion o an an h aci e wi h NaOH and KOH. Robau-S´ anchez e al. [20] p oposed a eac ion mechanism o he ac i a ion o Que cus ag i olia cha wi h KOH. Recen ly, we s udied he eac ion mechanism ha akes place du ing he ac i a ion o sewage sludge wi h NaOH and K 2 CO 3 [21]. The use o animal bones o p epa e po ous ma e ials equi es he de elopmen o speci ic eac ion mechanisms, gi en i s pa icula composi ion, wi h na u al apa i e being he main componen . In his ega d, he li e a u e conce ning he eac ion mechanism ha ake place du ing he he mal ac i a ion o p ecu so s de i ed om animal bones is sca ce. Senneca [22] epo ed he p oduc s eleased du ing he he mal ea men o mea and bone meal. Al hough hey used syn he ic HAp, he s udy o Yasukawa e al. [23] is also o in e es . These au ho s s udied he he mal decomposi ion mechanism o HAp wi h TG-MS, ocused on he elease o H 2 O and CO 2 . To he bes o he au ho s’ knowledge, ou p e ious s udies a e he only s udies ega ding he in es iga ion o he ac i a ion mechanism ha akes place du ing he chemical ac i a ion o bone cha [15,16,24]. These s udies a e ocused on he chemical ac i a ion o bone cha wi h di e en eagen s (NaOH, KOH, K 2 CO 3 , H 2 SO 4 , H 3 PO 4 ) in an ine a mosphe e. The aim o his s udy is o ex end he a o emen ioned in es iga ion, in o de o pe o m a comp ehensi e and de ailed s udy o he ac i a ion mechanism o bo h py olysis and ca boniza ion. Fo his pu pose, he mog a ime ic analysis coupled o mass spec ome y was used o in es iga e he gases eleased du ing he chemical ac i a ion o po k bone cha wi h H 2 SO 4 and K 2 CO 3 . These eagen s we e chosen based on hei di e en na u e (an acid and an alkali sal ), as well as on he esul s o a p e ious in es iga ion [2], which de e mined ha bo h a ou he de elopmen o po osi y and e ec i ely modi y he po e size dis ibu ion. Mo eo e , he s udy add esses he s uc u al ea angemen s and modi ica ions in he chemical composi ion o he ma e ials (measu ed by ni ogen adso p ion-deso p ion, XRD, FTIR, SEM, elemen al analysis and EDX) in e ms o : (i) he hea ing a mosphe e (ine o oxidizing), (ii) he highes ea men empe a u e, and (iii) he chemical eagen , H 2 SO 4 and K 2 CO 3 . The esul s will be help ul o con igu e ac i a ion p o ocols o bioapa i e-based ma e ials wi h ailo ed physicochemical p ope ies o use in speci ic ields. 2. Ma e ials and me hods 2.1. P oduc ion o bone cha Bone cha (BC) was p epa ed om po k chop bones collec ed om a local bu che ’s shop. The p epa a ion p o ocol was as ollows: i s , bones we e cleaned om mea and cu in o pieces o 2–5 cm. In o de o emo e mea and a , p io o chemical ac i a ion bones we e p eca - bonized in ai low (120 cm 3 min −1 , co esponding o 8 min o esidence ime in u nace) a 500 ºC (hea ing a e o 10 ºC min −1 , hold 1 h), using a qua z ube u nace. He ea e , he p eca bonized sample will be e e ed as p ecu so . The p ecu so was sie ed and pa icles in he 0.09 – 0.25 mm size ange we e selec ed. The p ecu so was di ided in o h ee pa s. Two we e imp egna ed wi h ei he H 2 SO 4 (S) o K 2 CO 3 (K). The las sample was no u he modi ied o be used as a e e ence (O). Fo he imp egna ion s ep, abou 2 g o he p ecu so we e placed in o con ac wi h 40 cm 3 o a solu ion con aining he ac i a ing agen , using a a io o 0.2 mmol H2SO4 g p ecu so −1 and 5 mmol K2CO3 g p ecu so −1 , based on ou p e i- ous expe ience [15,16]. The solu ions we e s i ed a oom empe a u e (20 ±2 ºC) o 24 h, o ensu e he access o he ac i a ing agen o he in e io o he pa icles. The samples we e hen il e ed, ans e ed o a con ec ion o en and d ied a 80 ⁰C o 24 h. The imp egna ed samples we e hea ed using ope a ing condi ions simila o hose used du ing he p eca boniza ion s ep. The highes ea men empe a u e (HTT) alues we e 600 and 800 ºC o ine –ni ogen– a mosphe e, and 350 and 550 ºC o oxidizing –ai – a mo- sphe e. These alues we e selec ed based on he esul s o he TG-MS analysis o imp egna ed samples, as shown below. The p epa ed sam- ples we e coded acco ding o he ac i a ing agen , he HTT alue and he a mosphe e: ni ogen (N) and ai (A). Fo example, BCKA-550 e e s o he sample o bone cha p epa ed by imp egna ion wi h K 2 CO 3 and ac i a ed in ai a 550 ºC. 2.2. TG-MS s udy The mog a ime ic analysis (TG) coupled o mass spec ome y (MS) was conduc ed wi h wo aims: (i) in es iga e he eac ions occu ing du ing he he mal ea men o imp egna ed samples, and (ii) selec he HTT. TG analysis was pe o med using a Se sys e olu ion (Se a am) he mal analyse . Abou 40 mg o imp egna ed samples we e pu in o he ce amic c ucible and hea ed in an ine (helium) o oxidizing (ai ) a mosphe e om oom empe a u e o 900 ºC, a a hea ing a e o 5 ºC min −1 . The exhaus gases we e analysed on-line by a mass spec ome e (MKS, Ci us LM99). The o al p essu e in he analysis chambe was 10 −6 o (1 o =133.32 Pa). The ollowing compounds we e moni- o ed con inuously: H 2 (m/z =2), H 2 O (m/z =18), CO (m/z =12) and CO 2 (m/z =44). MS signals we e no malized by di iding by sample mass. 2.3. Physicochemical cha ac e iza ion The ex u al p ope ies we e de e mined by ni ogen adso p ion/ deso p ion a 77 K, using a po osime e (ASAP 2010, Mic ome i ics). P io o he measu emen s samples we e d ied and ou gassed a 200 ◦C in a ni ogen low o 15 h. BET su ace a ea, po e a ea and po e olume we e measu ed. The mic opo e su ace and olume we e ob ained by - plo me hod, whe eas alues in he mesopo e and mac opo e anges we e de e mined based on he Ba e , Joyne & Halenda (BJH) me hod. The c ys alline s uc u e was analysed by XRD using a Philips PW1710 di ac ome e . X- ay powde di ac ion pa e ns we e collec ed by using a Philips X′pe PRO au oma ic di ac ome e ope - a ing a 40 kV and 40 mA, in he a- he a con igu a ion, seconda y I. Sie a e al. Jou nal o Analy ical and Applied Py olysis 171 (2023) 105973 3 monoch oma o wi h Cu-K α adia ion (λ =1.5418 Å; 1 Å =10 −10 m) and a PIXcel solid s a e de ec o (ac i e leng h in 2θ =3.347º). The Rie eld me hod was used o mic os uc u e analysis (p o ile e ine- men ) and es ima ion o he uni cell pa ame e s. The su ace p ope ies o he ma e ials we e analysed by a scanning elec on mic oscope (FEG-SEM Hi achi S-4800) and he quan i a i e analysis o he su ace was pe o med by an EDX de ec o (Ox o d In- s umen X-Max). Fou ie - ans o med in a ed (FTIR) spec a we e collec ed using a Nicole P o ´ eg´ e 460 de ice in he ansmi ance mode, o e he 400–4000 cm −1 ange wi h a esolu ion o 2 cm −1 . The KB sel - suppo ed pelle echnique was used o collec he spec a. The con en s o ca bon, hyd ogen, and ni ogen we e measu ed using a CHNS-O Eu o EA3000 elemen al analyse (Eu oVec o , I aly). 3. Resul s and discussion 3.1. Chemis y o he ac i a ion p ocess Fig. 1 shows he esul s o he he mog a ime ic analysis o he imp egna ed p ecu so samples in bo h ine (helium) and oxidizing (ai ) a mosphe es. The la e leads o a highe mass loss (17 w % s. 11–12 w %). Mo eo e , he mass loss occu s a di e en empe a u e in e als depending on he a mosphe e. O e he 50–130 ºC ange, mass loss is mainly asc ibed o he elease o adso bed wa e , being a ound 1.4% and 1.9 w %, o ine and ai a mosphe e, espec i ely. O e he 130–330 ºC ange, mass losses o a ound 1.7 and 2.2 w %, espec i ely, ake place, pa ly asc ibed o he deso p ion o la ice wa e . Such alues e lec he ela i ely impo an amoun o he adso bed and la ice wa e con ained in he p ecu so . Be ween 330 and 550 ºC, a ound 11 w % o mass loss akes place in ai a mosphe e, being much lowe (<2%) in helium a mosphe e. In his case, he mass loss occu s mo e p og essi ely o e he whole empe a u e ange (130 – 900 ºC), wi h an inc ease in mass loss a e abo e 600 ºC. In he oxida i e a mosphe e, se e al p o- nounced peaks can be iden i ied in he de i a i e TG p o ile. These esul s e eal ha di e en eac ions and p ocesses ake place du ing he he mal ac i a ion o chemically imp egna ed samples. The mal analysis was coupled o mass spec ome y in o de o iden i y hose eac ions. In o de o p opose a eac ion mechanism o he ac i a ion p ocess, he composi ion o he p ecu so (p eca bonized bones) mus be conside ed. XRD esul s (3.2.4. sec ion) con i m ha he p ecu so con ains mainly bioapa i e. Biological apa i es di e chemically om s oichiome ic HAp in ha hey con ain addi ional elemen s subs i u ed in o he HAp la ice. Na u al apa i e can be exp essed wi h he gene al o mula (Ca,M) 10 (PO 4 ,Y) 6 (OH,Z) 2 , whe e M ep esen s mono alen (Na + , K + ) o di alen (Mg 2+ , Zn 2+ , S 2+ ) ca ions ha can subs i u e Ca 2+ in he la ice, Y ep esen s anions such as CO 3 2− o HPO 4 2- ha can sub- s i u e PO 4 3− , and Z can be mono alen (F − , Cl − ) o di alen (CO 3 2− ) anions ha subs i u e OH − [11]. The ca bon con en in he p ecu so is no ewo hy (9.2 w %, Table 1). I is expec ed ha he p eca boniza ion o bones a 500 ºC esul s in he incomple e emo al o o ganic ma e [25] and, mo eo e , ca bon can be ound in ca bona e ions in bioapa i e (de ec ed by FTIR, 3.2.3 sec ion). Biological apa i es in bone mine als and consequen ly, bone cha , con ain signi ican amoun s o ca bona e ions (Cheung e al. [26] epo ed a con en o CO 3 2- o 7–9 w % in bone cha ). As p e iously discussed, bioapa i e can hos CO 3 2- ions in wo possible anionic si es in he apa i e la ice: (i) he OH - posi ion, o ming ype A ca bona ed apa i e and (ii) he PO 4 3- loca ion, o ming ype B ca bona ed apa i e. Mo eo e , he simul aneous loca ion o ca bona es a bo h posi ions leads o a ype AB ca bona ed apa i e [11]. 3.1.1. Samples ac i a ed in ine a mosphe e Fig. 2 depic s he e olu ion o mass spec ome ic signals wi h em- pe a u e, co esponding o he main compounds eleased du ing he he mal ac i a ion o he p ecu so . Fig. 2a co esponds o he p ecu - so wi hou imp egna ion, ac i a ed in he ine (helium) a mosphe e. The mass loss up o 300 ºC may be mainly a ibu ed o he elease o wa e . The signal o wa e shows he ollowing ea u es: (i) a main deso p ion peak, asc ibed o he elease o adso bed wa e , (ii) a shoulde nea 250 ºC, a ibu ed o he loss o la ice wa e [27] and (iii) a con inuous elease in he 300–900 ºC ange, assigned o he dehy- d oxyla ion o P-OH g oups o bioapa i e, which akes place mainly in he 300–700 ºC ange [28]: 2 P−OH ↔ P−O−P +H 2 O (g) (1) Fig. 1. The mog a ime ic analysis o he imp egna ed p ecu so . a) TG cu es ob ained in ine (He) and oxidizing (ai ) a mosphe e; b) DTG cu es ob ained in ine (He) a mosphe e; c) DTG cu es ob ained in oxidizing (ai ) a mosphe e. I. Sie a e al. Jou nal o Analy ical and Applied Py olysis 171 (2023) 105973 4 The elease o CO 2 begins a 200 ºC and p og essi ely inc eases in he 200–700 ºC ange. The loss o CO 2 may be pa ially a ibu ed o he eac ion o CO 3 2- cons i uen o bone cha wi h wa e (Eqs. (2) and (3)) [23]. CO 3 2- +H 2 O ↔ HCO 3 - +OH - (2) HCO 3 - ↔ CO 2 +OH - (3) Fu he mo e, he he mal decomposi ion o ca bona es abo e 600 ºC (Eq. (4) is well documen ed [29,30]. The esul ing oxide ions may un- de go a subsequen hyd a ion (Eq. (5)) [31]: CO 3 2- ↔ CO 2 +O 2- (4) O 2- +H 2 O ↔ 2 OH - (5) O in e es is ha he mass loss a high empe a u e (abo e 600 ºC, Fig. 1) coincides wi h he elease o H 2 and CO. The elease o H 2 , s a ing a 500 ºC, could be explained by he eac ion o OH - ions wi h he ca bon cons i uen o he p ecu so , as p oposed o o he ca bo- naceous p ecu so s [18,19,24]: 6 OH - +2 C ↔ 3 H 2 +2 CO 3 2- (6) The e a e wo sou ces o OH - ions: (i) he P-OH unc ionali ies o bioapa i e, and (ii) he OH - ions o med om ca bona es (Eqs. (2)–(5)). The elease o CO, ha akes place a empe a u es abo e 600 ºC, is likely due o he gasi ica ion o he ca bon cons i uen o bone cha . Two eac ions could be in ol ed: (i) he e e se Boudoua d eac ion, in which CO 2 eac s wi h ca bon (Eq. (7)) [32], and (ii) wa e -gas eac ion, in which wa e eac s wi h ca bon (Eq. (8)) [33]: CO 2 +C ↔ 2 CO (7) H 2 O +C ↔ CO +H 2 (8) Fu he mo e, he eac ion mechanism desc ibed by Eqs. (9)-(10) should no be disca ded, in which C ac s as a educing agen in he p oduc ion o CO [34]. The equi ed oxide is likely o be o med h ough he a o emen ioned he mal decomposi ion o CO 3 2- (Eq. 4). O 2- +C ↔ CO (9) CO 3 2- +2 C ↔ 3CO (10) The shape o he signal o CO sugges s he exis ence o o e lapping p ocesses. Mo eo e , he shoulde in he H 2 e olu ion p o ile, a a ound 850 ºC, would suppo he occu ence o Eq. (8). The ea men wi h K 2 CO 3 inc eases he in ense weigh loss abo e 600 ºC, which akes place a lowe empe a u e (Fig. 1b). As occu ed o he p is ine p ecu so , a signi ican signal o wa e is obse ed (Fig. 2c). The a enua ion o he igh shoulde and he diminished elease o wa e in he 330–550 ºC ange indica e ha he in ensi y o he s eps o wa e deso p ion and dehyd oxyla ion o P-OH g oups is lessened. The signal o CO 2 , a ibu ed o he eac ion o ca bona es wi h wa e (Eqs. (2) and (3)) and o he he mal decomposi ion o ca bona es (Eq. (4)), ollows a simila p o ile o ha in he p is ine p ecu so . In his case, apa om he CO 3 2- cons i uen o bone cha , he inco po a ed K 2 CO 3 ep esen s an addi ional sou ce o ca bona es, which a e inco po a ed by ion exchange. As p e iously explained, he mass loss a high empe a u e (abo e 600 ºC) is associa ed wi h eac ions ha esul in he elease o H 2 and CO. The ea men wi h K 2 CO 3 has an impac on he ex en o hose eac ions, as con i med by TG (Fig. 1). Rega ding he elease o CO (mainly a ibu ed o he gasi ica ion o ca bon cons i uen o bone cha ), he ca aly ic ole o alkali me als (such as K, Mg, Na and Ca) is well known, o bo h he e e se Boudoua d eac ion and he gasi ica- ion o ca bon wi h s eam (Eqs. (7)–(8)) [33,35], wi h K (inco po a ed by ion exchange) being he mos ac i e [35]. Fu he mo e, he inco - po a ion o ca bona e ions ep esen s an addi ional sou ce o ca bon. O in e es is ha he shape o he DTG cu e (Fig. 1b) a high empe a u e (abo e 600 ºC) is in conco dance wi h he CO p o ile (Fig. 2), sugges ing ha he eac ions ha esul in he elease o CO (mainly ca bon gasi- ica ion eac ions, Eqs. (7)–(8)) con ibu e o mass loss o a g ea e ex en han hose in ol ing he elease o H 2 . Mo eo e , he imp egna- ion wi h K 2 CO 3 (an addi ional sou ce o ca bon) may inc ease he elease o H 2 , h ough he eac ion o OH - ions wi h ca bon (Eq. (6)). Indeed, i is expec ed ha he amoun o OH - ions will inc ease due o he eac ion o he ca bona es (Eqs. (2)–(5)). As a consequence o he inco po a ion o K + ions (as e idenced by he da a o Table 1), he occu ence o he e e se wa e gas shi e- ac ion (RWGS) (Eq. (11)), epo ed in he py olysis o ca bonaceous p ecu so s [36,37], is also expec ed o ake place. The ole o K as p o- mo e o he RWGS eac ion is well es ablished [38,39]. CO 2 +H 2 ↔ CO +H 2 O (11) The ea men wi h H 2 SO 4 educes he mass loss (i.e. 12%, Fig. 1a). The p incipal di e ence occu s in he high empe a u e ange, whe e mass loss is he lowes and akes place a highe empe a u es (Fig. 1b). Con a ily, as obse ed in he DTG plo , he mass loss in he 400–500 ºC ange inc eases sligh ly, as compa ed o he non-imp egna ed and he sample imp egna ed wi h K 2 CO 3 . I is hypo hesized ha he enhanced mass loss a medium empe a u e is ela ed o he elease o CO 2 . The eac ion o H + ions o he acid wi h ca bona es would p omo e he o ma ion o HCO 3 - ions (Eq. (12)), which would hen be decomposed o CO 2 (Eq. (3)). CO 3 2- +H + ↔ HCO 3 - (12) As p e iously men ioned, he mass loss abo e 600 ºC is mainly ela ed o he e olu ion o H 2 and CO (Fig. 2e). The elease o H 2 is mos ly a ibu ed o Eq. (6), in which he e is a eac ion wi h OH - ions and C. The lowe a ailabili y o OH - ions o C would esul in a lowe ex en o his eac ion. Mo eo e , he p e ious ea men wi h H 2 SO 4 could lead o speci ic eac ions in ol ing ei he ca bon o CO 3 2- cons i uen s o bone cha (Eqs. (13–14)) [24]: 2 H 2 SO 4 +C ↔ CO 2 +2 SO 2 +2 H 2 O (13) Table 1 Chemical composi ion o he p epa ed ma e ials (w %). H, C and N de e mined by elemen al analysis. The es o he elemen s measu ed by EDX. Ca/P gi en as mola a io. Sample H C N Ca P O K Na Mg Cl S Ca/P P ecu so 0.81 9.21 1.67 30.6 13.8 41.1 0.81 0.85 0.49 0.32 0.00 1.71 BCKA-350 1.39 4.64 1.19 31.0 13.8 44.2 1.45 0.61 0.54 0.15 0.00 1.74 BCKA-550 0.82 0.76 0.20 32.1 14.7 44.9 1.71 0.67 0.55 0.21 0.00 1.68 BCKN-600 1.04 0.31 <0.1 34.4 16.2 41.1 1.42 0.63 0.6 0.21 0.00 1.64 BCKN-800 0.62 0.11 <0.1 34.0 15.2 43.4 1.05 0.76 0.61 n/d 0.00 1.73 BCSA-350 1.04 6.70 1.49 29.1 13.4 42.9 n/d 0.5 0.39 0.15 0.07 1.68 BCSA-550 1.01 0.47 0.25 34.8 16.1 41.6 0.31 0.62 0.44 0.2 0.08 1.67 BCSN-600 0.60 0.37 0.13 34.4 15.7 41.4 0.45 0.75 0.46 0.21 0.16 1.70 BCSN-800 0.76 <0.1 <0.1 35.4 15.8 42.5 0.15 0.63 0.61 0.14 0.00 1.73 I. Sie a e al. Jou nal o Analy ical and Applied Py olysis 171 (2023) 105973 5 Fig. 2. Mass spec ome y da a o he compounds eleased du ing he hea ea men . a) non-imp egna ed p ecu so in ine (He) a mosphe e; b) non-imp egna ed p ecu so in oxidizing (ai ) a mosphe e; c) p ecu so imp egna ed wi h K 2 CO 3 in ine a mosphe e; d) p ecu so imp egna ed wi h K 2 CO 3 in oxidizing a mosphe e; e) p ecu so imp egna ed wi h H 2 SO 4 in ine a mosphe e; ) p ecu so imp egna ed wi h H 2 SO 4 in oxidizing a mosphe e. I. Sie a e al. Jou nal o Analy ical and Applied Py olysis 171 (2023) 105973 6 H 2 SO 4 +CO 3 2- ↔ SO 4 2- +CO 2 +H 2 O (14) Eqs. (13) and (14) would esul in a lowe amoun o C and OH - ( o med om ca bona es h ough Eqs. (2)–(5)). The ac ha high amoun s o CO 2 o H 2 O a e no eleased (acco ding o he DTG, Fig. 1b) could be explained by he occu ence o bo h eac ions du ing he imp egna ion s ep, be o e he he mal ac i a ion. Conce ning CO ( o med h ough Eqs. (7)-(10), he lowe a ailabili y o ca bon as a consequence o Eqs. (13) and (14) would also explain i s lowe elease. Fu he mo e, he ea men wi h H 2 SO 4 could also esul in he pa ial dissolu ion o bioapa i e ( ega ded as a ami ica ion o HAp) and he o ma ion o calcium sul a e and calcium de icien bioapa i e (Ca- de icien HAp is exp essed wi h he o mula Ca 10−X (HPO 4 ) X (PO 4 ) 6−X (OH) 2−X ) [23]. Ne e heless, he Ca/P a ios shown in Table 1 a e close o he s oichiome ic alue o 1.67 o HAp, and do no show a dec ease in BCS se ies, compa ed o he p ecu so and he BCK se ies. Howe e , he inco po a ion o sul a e ions in o bio- apa i e should also be aken in o accoun . Indeed, i has been epo ed ha SO 4 2- can subs i u e HPO 4 2- ion (bo h ions ha ing he same alence and e y simila ionic adius) [40]. The esul s o Table 1 e eal a small amoun o S in BCS samples, whe eas i is absen in he p ecu so and BCK samples. This p ocess, ha esul s in an inc ease o he Ca/P a io, may o se he dec ease in he a io p oduced in he gene a ion o calcium-de icien bioapa i e. Among he p oposed eac ions o imp egna ed and non- imp egna ed samples, he RWGS eac ion akes place in gas phase and hus, does no con ibu e o gene a ing po osi y. In con as , he eac ion mechanisms ha in ol e he gasi ica ion o cons i uen s o bone cha o inco po a ed species, a e expec ed o ha e an impac on he ex u al p ope ies o he ma e ials. 3.1.2. Samples ac i a ed in oxidizing a mosphe e Acco ding o he DTG plo (Fig. 1c), he weigh loss in he oxidizing en i onmen akes place in h ee s ages: low empe a u e (below 300 ºC), in e media e empe a u e (300–550 ºC) and high empe a u e (abo e 550 ºC). Among hese in e als, he highes weigh loss akes place ac oss he in e media e empe a u e ange (300–550 ºC). Fig. 2b displays he main p oduc s eleased du ing he he mal ac i a ion o he p ecu so . The i s s age o mass loss is mainly ela ed o he elease o wa e . As occu ed wi h samples ac i a ed in ine a - mosphe e, he signal o wa e shows a p incipal peak, a ibu ed o he deso p ion o adso bed wa e , ollowed by a con inuous elease, asc ibed o he elimina ion o la ice wa e , as well as he P-OH g oups o apa i e by dehyd oxyla ion (Eq. (1)). The mos ema kable di e ence wi h espec o he ine a mosphe e occu s in he medium empe a u e ange, in which combus ion eac ions ake place p e e en ially o o he eac ions: C +O 2 ↔ CO 2 (15) 2H +1 2O2↔ H2O(16) Two o e lapping peaks can be dis inguished in he CO 2 p o ile. I is hypo hesized ha hese peaks a e due o ei he he p e e en ial com- bus ion o ca bon loca ed nea me als such as Ca, K, Mg, Na (Table 1) ha ac as ca alys s o he combus ion o ca bonaceous species (peak a low empe a u e) [41,42] o o he he e ogeneous na u e o ca bona- ceous species ( o example, ca bon p esen in he s uc u e o bioapa i e as ca bona es, and ca bon p esen in he emaining o ganic ma e ). Addi ionally, he e is a small peak o CO 2 wi h i s maximum nea 650 ºC, asc ibed o he he mal decomposi ion o ca bona es (Eq. (4)). The elease o hyd ogen and CO, ha akes place in he in e media e em- pe a u e ange, is almos negligible. Fig. 2d shows he e olu ion o he main signals du ing he ac i a ion in ai a mosphe e o he p ecu so imp egna ed wi h K 2 CO 3 . The elease o wa e and CO 2 ollow a simila p o ile, compa ed o he non- imp egna ed p ecu so . Rega ding he signal o CO 2 , he wo o e - lapping peaks appea mo e clea ly de ined in his case and he low empe a u e peak is sligh ly shi ed o a lowe empe a u e, which could be ela ed o he a o emen ioned ca aly ic ac i i y o he inco po a ed K me al (da a in Table 1 clea ly show ha ion subs i u ion akes place). The peak o CO 2 a high empe a u e, asc ibed o he he mal decom- posi ion o ca bona es, is also be e de ined, owing o he amoun o ca bona es inco po a ed du ing he imp egna ion. The p incipal di e ences a e obse ed in he elease o CO and H 2 , almos absen in he non-imp egna ed p ecu so . Th ee possible pa h- ways could be p oposed o he signal o CO, ha exhibi s a simila shape o ha o CO 2 : (i) he incomple e combus ion o ca bon; (ii) he e e se wa e gas shi eac ion (RWGS) (Eq.(11)), ha p oduces CO and H 2 O, p omo ed by he inco po a ed K; (iii) he gasi ica ion o ca bon h ough he e e se Boudoua d eac ion (Eq. (7)) and h ough he wa e -gas eac ion (Eq. (8)), ca alyzed by K, as p e iously discussed. Conce ning he signal o H 2 , a clea peak is obse ed a a ound 450 ºC, which is de ini ely much lowe han he H 2 de ec ion empe a u e in ine a mosphe e (>600 ºC). Apa om he wa e -gas eac ion (Eq. (8)), he a o emen ioned eac ion o OH - ions wi h ca bon (Eq. (6)) could be esponsible o he elease o H 2 . The imp egna ion wi h K 2 CO 3 may ha e an impac in bo h eac ions: (i) he wa e -gas eac ion is ca alysed by K, and (ii) he inco po a ion o CO 3 2- in he posi ion o phospha e would inc ease he amoun o bo h ca bon and OH - ions, he la e o med h ough he eac ion o ca bona es (Eqs. (2)–(5)). Finally, he e olu ion o he main p oduc s eleased du ing he ac i a ion o he sample ea ed wi h H 2 SO 4 is shown in Fig. 2 . The e ec o he acid ea men is e idenced by he al e a ion o he eleased compounds in bo h he medium and high empe a u e ange. As p e i- ously no ed, he main mass loss e en , which akes place in he in e - media e empe a u e ange, is shi ed o highe empe a u es by a ound 25 ºC, in compa ison wi h non-imp egna ed sample (Fig. 1). The signal o CO 2 , mainly asc ibed o he combus ion o ca bon, sugges s he ex- is ence o wo o e lapping peaks, bu less de ined han in he case o he sample imp egna ed wi h K 2 CO 3 . This beha iou could be pa ly a ibu ed o he lowe amoun o me als ha ca alyse he combus ion, such as K (Table 1). Mo eo e , he emo al o pa o he ca bonaceous ma e (bo h ca bona es and he emaining o ganic ma e ) would e- sul s in less de ined peaks o CO 2 . The signal o CO shows he same shape as ha o CO 2 . Mo eo e , he absence o he CO 2 peak a a ound 650 ºC suppo s he pa ial emo al o ca bona es by he acid (Eq. (14)). Acco ding o he FTIR esul s, dis- cussed below (3.2.3. sec ion, Table S1), all samples ac i a ed wi h H 2 SO 4 show he cha ac e is ic peaks o ca bona e, excep o he sample ac i a ed a he highes empe a u e (BCSN-800). Thus, ca bona es we e no comple ely emo ed du ing he imp egna ion s ep, be o e he he mal ea men . The pa ial elimina ion o ca bona es du ing he imp egna ion would explain he absence o he peak o CO 2 nea 650 ºC. Indeed, he emaining ca bona es would be comple ely emo ed by combus ion, and he e would be no ca bona e a ailable o emo al by he mal decomposi ion. The emo al o he emaining o ganic ma e and ca bona es by he acid (Eqs. (13)-(14)) would also lead o a educed amoun o he H 2 eleased a in e media e empe a u e, o med om he eac ion o OH - and C (Eq. (6)). In ac , in addi ion o he lowe amoun o C a ailable, he e would be less OH - ions a ailable, as a consequence o : (i) he emo al o ca bona es, sou ce o OH - h ough Eqs. (2)–(5); and (ii) he pa ial dissolu ion o apa i e, esul ing in he o ma ion o a ca ion de icien s uc u e, which con ains a lowe amoun o P-OH unc ionali ies. Excep o he RWGS eac ion, all he eac ion mechanisms a e ex- pec ed o de elop po osi y, since hey imply he gasi ica ion o cons i - uen s o bone cha o inco po a ed species. I. Sie a e al. Jou nal o Analy ical and Applied Py olysis 171 (2023) 105973 7 3.2. Cha ac e iza ion o bone cha s The impac o he a o emen ioned eac ion mechanisms on he ex u al and chemical p ope ies o he ma e ials was in es iga ed. Two le els o HTT we e s udied o each hea ing a mosphe e, es ablished on he basis o he TG-MS esul s (Fig. 1). The ollowing HTT alues we e selec ed: 350 and 550 ºC o he oxidizing a mosphe e, and 600 and 800 ºC o he ine a mosphe e. The p epa ed ma e ials we e ho oughly cha ac e ised by elemen al analysis, N 2 adso p ion-deso p ion, XRD, SEM, EDX and FTIR analyses. 3.2.1. Chemical composi ion Elemen al and EDX analyses show he p esence o abundan Ca, P, C and O, oge he wi h o he elemen s such as K, Na, Mg and Cl (Table 1), e lec ing he lexibili y o na u al apa i es ega ding he p esence o di e en ions. Fu he mo e, as explained p e iously, he chemical ea men esul s in he inco po a ion o se e al species. The samples ea ed wi h K 2 CO 3 show an inc ease in he amoun o K, inco po a ed by ion exchange wi h Ca. I is also ema kable ha he samples p epa ed by ea men wi h H 2 SO 4 possess a small amoun o S (absen in bo h he p ecu so and samples ea ed wi h K 2 CO 3 ), e lec ing he inco po a ion o SO 4 2- (subs i u ing HPO 4 2- ). 3.2.2. Tex u al p ope ies and po e size dis ibu ion The SEM images o he p ecu so and he samples ea ed in di e en a mosphe e and empe a u e a e p esen ed in Fig. S1 (Supplemen a y ma e ial). Pa icles wi h la and compac su aces in he p ecu so a e con e ed in o coa se pa icles a e he ea men . Fo he samples ea ed in ai a mosphe e, he su ace ex u e becomes oughe and mo e po ous a e hea ing in he 350–550 ºC ange. Rega ding he he mal ea men in ine a mosphe e, empe a u es in he 600–800 ºC ange esul in he agmen a ion o compac su aces. I is obse ed ha he bioapa i e c ys als a e ema kably de eloped when inc easing he HTT. Table 2 summa izes he ex u al p ope ies o bone cha samples and Fig. 3 displays he po e size dis ibu ion (PSD). As obse ed, he ex u al p ope ies a y signi ican ly depending on he ea men applied (ac i- a ing agen , a mosphe e and empe a u e). These ex u al p ope ies should be ela ed o he ex en o he a o emen ioned eac ions and p ocesses ha gene a e po osi y h ough he gasi ica ion o cons i uen s o he p ecu so and species inco po a ed in he imp egna ion s age. Mainly: (i) he he mal decomposi ion and ola iliza ion o he emaining o ganic ma e ; (ii) he deso p ion o adso bed and la ice wa e ; (iii) he dehyd oxyla ion o apa i e (Eq. (1)); (i ) he eac ion o ca bon wi h OH - ions o p oduce H 2 (Eq. (6)); ( ) he p ocesses in ol ing ca bona es ( ans o ma ion in o OH - ions and he mal decomposi ion) (Eqs. (2)–(5)); ( i) he gasi ica ion o ca bon wi h ei he CO 2 o H 2 O (Eqs. (7)–(8)); ( ii) speci ic eac ions in ol ing he acid, such as he o ma ion o HCO 3 - (Eq. (12)) and i s subsequen decomposi ion (Eq. (3)) and he eac ion o H 2 SO 4 wi h ca bon o ca bona es (Eqs. (13)-(14)); ( iii) he combus ion o C and H (Eqs. (15)-(16)). When he ac i a ion is pe o med in ni ogen a mosphe e, he highes ac i a ion empe a u e (800 ºC) esul s in a e y agg essi e ea men ha leads o an almos comple e des uc ion o he po ous s uc u e. The ac i a ion a lowe empe a u e (600 ºC) has a di e en e ec depending on he ac i a ing agen . The ea men wi h K 2 CO 3 p oduces a ma e ial wi h a e y low S BET (17.0 m 2 g −1 ) and po osi y, whe eas he ma e ial ac i a ed wi h H 2 SO 4 be e p ese es he po ous s uc u e, wi h a S mic o simila o ha o he p ecu so . I s mos dis inc i e cha ac e is ic is he deple ion o mesopo osi y (Fig. 3), along wi h an inc ease o mac opo osi y, esul ing in a highe a e age po e size (344 Å s. 154 Å o he p ecu so ). On he o he hand, he la ges alues o speci ic su ace a ea a e ob ained in oxidizing a mosphe e a HTT=350 ºC (66.7% o inc ease o K 2 CO 3 and 78.1% o H 2 SO 4 , compa ed o he p ecu so ). As shown in Fig. 3, bo h ac i a ing agen s esul in a signi ican inc ease o small mesopo es (d p <200 Å), along wi h a dec ease o po es la ge han ha size. These esul s sugges ha he combus ion eac ions ha ake place a in e media e empe a u e (Eqs. (15)-(16)) ha e a bene icial e ec on he ex u al p ope ies. In he case o he ea men wi h H 2 SO 4 , he e is also a huge inc ease in S mic o (84.2%), whe eas he imp egna ion wi h K 2 CO 3 esul s in a dec ease (30.9%). The selec i e de elopmen o mic opo osi y h ough he imp egna ion wi h H 2 SO 4 has been p e i- ously epo ed o o he ca bonaceous p ecu so s based on animal bones [24]. This enhancemen o mic opo osi y may be a ibu ed o he a o emen ioned speci ic eac ions in ol ing he acid (Eqs. (12)-(14)). Mo eo e , since hese eac ions esul in a lowe amoun o ca bon, ca bona es and OH - ions, a limi a ion in he ex en o he eac ions Table 2 Tex u al p ope ies o p epa ed samples o bone cha and weigh loss du ing he he mal ea men o imp egna ed samples. Sample S BET (m 2 g −1 ) S mic oa (m 2 g −1 ) S ex a (m 2 g −1 ) V pb (cm 3 g −1 ) Po e size b (nm) HAP c ys al size (nm) Mu le weigh loss (w %) P ecu so 63.4 10.0 53.4 0.3106 15.4 14.4 - BCKA-350 105.7 6.9 98.8 0.3636 11.3 20.2 6.6 BCKA-550 38.5 8.9 29.6 0.2743 28.7 24.5 15.7 BCKN-600 17.0 4.3 12.6 0.0483 13.8 33.7 17.2 BCKN-800 1.2 1.2 0.0 0.0035 56.2 93.7 19.8 BCSA-350 112.9 18.4 94.5 0.3436 11.0 14.6 3.2 BCSA-550 56.6 11.3 45.3 0.3709 24.8 18.8 15.4 BCSN-600 42.0 9.5 32.5 0.3617 34.4 15.2 16.6 BCSN-800 6.2 2.4 3.7 0.0163 22.1 99.2 19.2 a -plo me hod b BJH me hod Fig. 3. Po e size dis ibu ion o he p epa ed ma e ials. I. Sie a e al. Jou nal o Analy ical and Applied Py olysis 171 (2023) 105973 8 in ol ing hose species could be expec ed, hus limi ing he opening o he gene a ed po es. When he ac i a ion is pe o med in ai a mosphe e a highe em- pe a u e (HTT o 550 ºC), he e is a conside able emo al o he meso- po ous s uc u e below 100 Å (Fig. 3). This beha iou is accompanied wi h an inc ease o la ge mesopo es (d p >~ 250–300 Å) and mac o- po es, esul ing in a highe a e age po e size. The g ea e ex en o he combus ion eac ions could explain hese esul s, since he highe amoun o gases eleased could esul in he opening o po es. The in- c ease o mic opo osi y wi h inc easing HTT is no ewo hy in he sample imp egna ed wi h K 2 CO 3 (8.9 m 2 g −1 o S mic o a 550 ºC s. 6.9 m 2 g −1 a 350 ºC). Con a ily, in he H 2 SO 4 ea ed sample S mic o dec eases wi h inc easing HTT om 350 ºC o 550 ºC (18.4 s. 11.3 m 2 g −1 ). Ne e - heless, in he la e sample, he po osi y in he mic opo e ange is compa able o ha o he p ecu so (10.0 m 2 g −1 ). The e ec o he a mosphe e is no ewo hy when BCKA-550 and BCSA-550 a e compa ed o BCKN-600 and BCSN-600, espec i ely. The samples ac i a ed in ai a mosphe e possess much be e ex u al p ope ies ac oss he whole po e ange, sugges ing he bene icial e ec o he combus ion eac ions o ca bon and hyd ogen ha ake place p e e en ially o he o he se o eac ions. As p e iously discussed, his posi i e e ec o he a mosphe e can be maximized by pe o ming he ac i a ion a lowe empe a u e (350 ºC). 3.2.3. FTIR analysis Fig. 4 displays he FTIR spec a o he p ecu so and he p epa ed samples o bone cha . The spec a show he cha ac e is ic peaks o phospha es (phospha e s e ching bands a 1040 and 960 cm −1 , he bending ib a ions o hese unc ional g oups a 602 cm −1 and he an isymme ic s e ching mode o phospha e a 1090 cm −1 [43]. The weak peak a 870 cm −1 , could be a ibu ed o ei he he P-O s e ching ib a ion [44] o he bending mode o ca bona e g oups [45]. Ca bona es a e obse ed a 1410 and 1460 cm −1 (C-O s e ching mode), excep o BCSN-800. Table S1 shows he a io be ween he a ea o he peaks o ca bona es and hose o phospha es, in o de o e lec he ela i e amoun o ca bona es o each sample. As expec ed, BCK samples possess a highe amoun o ca bona es han bo h he p ecu so and BCS samples, due o he inco po a ed ca bona es. Mo eo e , he eac ion o H 2 SO 4 wi h ca bona es (Eqs. (12) and (14)) would also explain he lowe amoun o BCS samples. In bo h se ies (BCK and BCS), he amoun o ca bona es dec eases when empe a u e is inc eased, as a consequence o : (i) he he mal decomposi ion o ca bona es (Eq. (4)), ha akes place abo e 600 ºC; and (ii) he eac ion o CO 3 2- wi h wa e (Eqs. (2)–(3)), in he 200–700 ºC empe a u e ange. The same dec easing end when empe a u e is inc eased is obse ed o he band nea 1630 cm −1 , p e iously asc ibed o ca bona e ions [46]. Fu he mo e, he cha ac- e is ic bands o o ganic ma e a e also obse ed o all samples (bands a 2850 and 2920 cm −1 ), co esponding o he symme ic and asym- me ic CH 2 s e ching bands o o ganic ma e [47]. The b oad band a abou 3400 cm −1 is commonly asc ibed o –OH unc ional g oups, and could be pa ially a ibu ed o adso bed wa e . As expec ed, he e is a dec ease in he in ensi y o his band when empe a u e is inc eased, in line wi h he abo e-men ioned eac ions ha in ol e OH - , such as: (i) he elimina ion o he P-OH g oups o HAP by dehyd oxyla ion, Eq. (1); (ii) he eac ion o OH - ions wi h ca bon o p oduce H 2 , Eq. (6). Mo eo e , o in e es is ha he oxidan a mosphe e esul s in a highe peak ( o example, A OH- /A phospha e is 0.75 o BCSA- 550 and 0.57 o BCSN-600), sugges ing ha OH - unc ional g oups a e gene a ed om he eac ion o oxygen wi h he hyd ogen p esen (e. g. in he emaining o ganic ma e ). 3.2.4. XRD analysis The XRD pa e ns o he p epa ed solids (Fig. 5) show di ac ion peaks consis en wi h he apa i e s uc u e a 26.0◦, 28.3◦, 29.2, 32.0◦, 32.4◦, 33.1◦, 34.3◦, 40.0◦, 46.9◦and 49.7◦. The e ined la ice pa ame- e s (in Å, shown in Table 3) o he p ecu so (a =9.40809, c =6.8784) we e smalle han hose o pu e hyd oxyapa i e (a =9.4180, c=6.884, ICDD 00–009–0432), e lec ing he di e en chemical composi ion o biological apa i es as a consequence o he subs i u ion o di e en ions in he la ice. No o he c ys alline phases we e obse ed in he analysed samples, excep o BCSN-800, in which some aces o whi locki e (Ca 18 Mg 2 H 2 (PO 4 ) 14 , pd 70–2064) a e obse ed. The c ys allini y o he ma e ials inc eases wi h empe a u e, in conco dance wi h he li e a- u e. Ooi e al. [48] epo ed ha he he mal ea men abo e 600 ºC success ully elimina es he o ganic ma e om bones, hus leading o an inc ease o he c ys allini y. The XRD pa e ns show a highe deg ee o c ys allini y o he samples imp egna ed wi h K 2 CO 3 , he e ec being no ewo hy o he samples ac i a ed a 600 ºC in ine a mosphe e (BCKN-600 s. BCSN-600). This lowe c ys allini y o BCS samples could be due o he ac ha SO 4 2- ions we e inco po a ed in o he bioapa i e la ice o some ex en (as shown in Table 1) by ion exchange wi h HPO 4 - , in good conco dance wi h he esul s o Toyama e al. [40]. The K 2 CO 3 - ea ed se ies ollows he expec ed end whe e he apa i e c ys al size inc eases wi h empe a u e (Table 2) [49]. Howe e , o he H 2 SO 4 ea ed samples, apa i e c ys alli es o simila size (15–19 nm) a e de eloped in he HTT =350–600 ºC ange. Fo bo h Fig. 4. FTIR spec a o he p ecu so and samples o bone cha . a) samples p epa ed by imp egna ion wi h K 2 CO 3 ; b) samples p epa ed by imp egna ion wi h H 2 SO 4 . I. Sie a e al. Jou nal o Analy ical and Applied Py olysis 171 (2023) 105973 9 chemical agen s, a he highes alue o 800 ºC, he c ys alli e size in- c eases signi ican ly. In gene al e ms, a simila HTT, la ge apa i e c ys alli es a e o med in K 2 CO 3 ea ed samples. The la ice pa ame e s a and c we e calcula ed using he Rie eld me hod (Table 3). La ice expansion is obse ed upon chemical ac i a- ion, which sugges s a possible inco po a ion o species in o he apa i e amewo k, along wi h o he eac ions o p ocesses ha can al e he s uc u e o bioapa i e. The expansion o he c ys al la ice is di e en , depending on he ac i a ing agen used. Rega ding he ac i a ion wi h K 2 CO 3 , he expansion o la ice pa ame e c is highe ( wo o h ee imes) han ha o pa ame e a, wi h he excep ion o BCKN-800 ( o which a lowe and mo e equilib a ed inc ease o a and c pa ame e s is obse ed). Among he p ocesses ha esul in an expansion o he c ys al la ice, he ionic exchange o he inco po a ed K + and CO 3 2- ions may play an impo an ole. Indeed, he ionic adii o K + and CO 3 2- a e la ge han hose o Ca 2+ , and OH - , espec i ely. I is ema kable ha he expansion in bo h a and c la ice dimensions gene ally dec eases wi h empe a u e. This e ec would be a esul o p ocesses ha cause he con ac ion o he c ys al la ice, such as he dehyd oxyla ion o bioapa i e (Eq. (1)) and he ans o ma ion o ca bona es in o OH - ions (Eqs. (2)–(5)). Mo eo e , i has been epo ed ha he loss o la ice wa e causes a con ac ion in he a-la ice dimension [50]. By con as , he H 2 SO 4 ea men causes a mo e ma ked expansion in he a axis (wi h he excep ion o BCSA-350, wi h a balanced inc ease in bo h pa ame e s), sugges ing ha he ac i a ion wi h H 2 SO 4 al e s he c ys alline s uc u e h ough a di e en mechanism. Rega ding he ion exchange, as explained in 3.1.1 sec ion, SO 4 2- may be inco po a ed in o he bioapa i e s uc u e, subs i u ing HPO 4 - (as epo ed by Toyama e al. [40]). Ne e heless, since he ionic adii o bo h SO 4 2- and HPO 4 - a e e y simila , his ion exchange would no esul in he expansion o he c ys al la ice. Among he speci ic eac ions p omo ed by he acid ha could esul in he expansion o he la ice, he inco po a ion o H + ions in o he apa i e s uc u e h ough he o ma ion o HCO 3 - ions should be highligh ed (Eq. (12)). 4. Conclusions The ac i a ion mechanism p oposed o he p ecu so in ine a - mosphe e (py olysis) includes: (i) he he mal decomposi ion and ola iliza ion o o ganic ma e ; (ii) he deso p ion o adso bed and la ice wa e ; (iii) he dehyd oxyla ion o apa i e; (i ) he eac ion o ca bon wi h OH - ions o p oduce H 2 ; ( ) he p ocesses in ol ing ca - bona es ( ans o ma ion in o OH - ions and he mal decomposi ion); ( i) he gasi ica ion o ca bon wi h ei he CO 2 o H 2 O and ( ii) he e e se wa e gas shi eac ion. When he oxidizing a mosphe e is used (ca boniza ion), he combus ion o bo h C and H - which akes place p e e en ially - mus be added o he eac ion scheme. The e ec o K 2 CO 3 on he ac i a ion is p obably a consequence o he inco po a ion o bo h K + and CO 3 2- ions in o he bioapa i e la ice by ion exchange. K + is a ca alys o he ollowing eac ions: (i) he com- bus ion o C and H; (ii) he gasi ica ion o ca bon wi h ei he CO 2 o H 2 O and (iii) he RWGS eac ion. Mo eo e , he inc ease o he amoun o CO 3 2- esul s in a highe ex en o he eac ions in ol ing ca bona es and/ o OH - ions ( o med om ca bona es). Rega ding H 2 SO 4 , i is hypo he- sized ha i s e ec is mainly a consequence o speci ic eac ions: (i) he eac ion o H 2 SO 4 wi h ca bon o ca bona es and (ii) he o ma ion o HCO 3 - and i s subsequen decomposi ion. Since hese eac ions esul in a lowe amoun o ca bon, ca bona es and OH - ions, a limi a ion in he ex en o he a o emen ioned eac ions in ol ing hose species could be expec ed. The bes ex u al p ope ies a e ob ained in oxidizing a mosphe e a 350 ºC, e lec ing he bene icial e ec o he combus ion eac ions a in e media e empe a u e. The economic bene i o using a mode a e empe a u e should be highligh ed. Be ween bo h eagen s, H 2 SO 4 p o ides be e esul s in S BET and, u he mo e, leads o a selec i e de elopmen o mic opo osi y. This enhancemen in mic opo osi y may be asc ibed o ei he he occu ence o he a o emen ioned eac ion in ol ing he acid and/o o he limi a ion in he ex en o o he e- ac ions – hus limi ing he opening o he gene a ed po es. The FTIR esul s sugges ha he oxidizing a mosphe e is in ol ed in he gene - a ion o OH - unc ional g oups. The XRD esul s e eal an al e a ion in he s uc u e o bioapa i e: K 2 CO 3 leads o a mo e ma ked expansion in he c axis, whe eas o H 2 SO 4 he expansion is highe in he a axis. These esul s demons a e he easibili y o con igu ing he mo- chemical ac i a ion p o ocols (choosing he mos sui able ac i a ing agen , a mosphe e o ea men empe a u e) o ailo he physico- chemical p ope ies o bioapa i e-based ma e ials, o use in speci ic ields. CRediT au ho ship con ibu ion s a emen I. Sie a: Fo mal analysis, W i ing – o iginal d a . Jos´ e L. Ayas uy: Concep ualiza ion, Funding acquisi ion, In es iga ion. Miguel A. Gu i´ e ez-O iz: Funding acquisi ion, Supe ision. U. I ia e-Velasco: Concep ualiza ion, Funding acquisi ion, In es iga ion, W i ing – e iew & edi ing. Fig. 5. XRD pa e ns o he p ecu so and samples o bone cha . a) p ecu so ; b) samples p epa ed by imp egna ion wi h K 2 CO 3 ; c) samples p epa ed by imp egna ion wi h H 2 SO 4 . Table 3 La ice pa ame e s ob ained by Rie eld e inemen . La ice pa ame e s a, b c Del a a (%) Del a c (%) Cell olume, Å 3 P ecu so 9.40809 6.8784 527.25 BCKA-350 9.4171 6.8991 0.10 0.30 529.852 BCKA-550 9.41439 6.8882 0.07 0.14 528.717 BCKN-600 9.41148 6.8844 0.04 0.09 528.094 BCKN-800 9.41336 6.8819 0.06 0.05 528.115 BCSA-350 9.43117 6.8968 0.25 0.27 531.264 BCSA-550 9.42198 6.8845 0.15 0.09 529.282 BCSN-600 9.42118 6.8831 0.14 0.07 529.082 BCSN-800 9.42663 6.8834 0.20 0.07 529.719 I. Sie a e al.