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.
