1
AN INSIGHT INTO THE REACTIONS OCCURRING DURING
THE CHEMICAL ACTIVATION OF BONE CHAR
Unai I ia e-Velascoa, Jose L. Ayas uyb, Lo ena Zudai ea and I ene Sie aa*
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, Spain.
b Depa men o Chemical Enginee ing, Facul y o Science and Technology, Uni e si y
o The Basque Coun y UPV/EHU, Ba io Sa iena, s/n, 48940 Leioa, Spain.
*Co esponding au ho . Tel.: +34 945013290; ax: +34 6015963. Email:
i ene.sie [email p o ec ed]
Abs ac
The alo isa ion o animal was es by py olysis has shown o be o in e es , due o he
e sa ili y o he main compound o cha , hyd oxyapa i e (HAP), which can be used in
ca alysis, elec ochemis y, and adso p ion. The u ili y o HAP depends o a g ea ex en
on i s ex u al p ope ies, which can be de eloped by chemical ac i a ion, h ough
gasi ica ion eac ions. In he p esen wo k he mog a ime ic analysis coupled o mass
spec ome y was used o gain insigh in o he eac ions occu ing du ing he chemical
ac i a ion o po k bone cha wi h di e en agen s: H3PO4, H2SO4, NaOH and K2CO3.
Mo eo e , he ole o each ac i a ion eagen in he enhancemen o po osi y was
de e mined. The ea men wi h H2SO4 esul ed in a highly mic opo ous ma e ial,
sui able o be used in gaseous pollu an adso p ion. Chemical ac i a ion wi h NaOH and
K2CO3, on he con a y, led o a mo e equilib a ed inc ease o mic o- and mesopo osi y,
esul ing in a hie a chical po ous ma e ial, wi h an excellen po en ial o applica ions as
elec ode, in gas s o age, ca alysis and ene gy s o age. Rega ding H3PO4, i was
ex emely agg essi e unde he ope a ing condi ions used, since i emo ed almos all
he po ous s uc u e o HAP. These esul s a e use ul o op imize he p epa a ion
me hod o HAP, in o de o con igu e a ma e ial wi h he desi ed ex u al p ope ies.
KEYWORDS: Hyd oxyapa i e; Bone cha ; Chemical ac i a ion; Reac ion mechanism;
Mass spec ome y; The mog a ime y.
This is he accep ed manusc ip o he a icle ha appea ed in inal o m in Chemical Enginee ing Jou nal 251 : 217-227 (2014), which has
been published in inal o m a h ps://doi.o g/10.1016/j.cej.2014.04.048. © 2014 Else ie unde CC BY-NC-ND license (h p://
c ea i ecommons.o g/licenses/by-nc-nd/4.0/)
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1. In oduc ion
As a esul o Bo ine Spongi o m Encephalopa hy c isis, he use o mea and bone meal
(MBM) o eed ca le was o bidden in EU (Commission Decision 94/381/EC).
The e o e, oday he e is a high amoun o animal was es ha mus be sa ely disposed o
ans o med. The combus ion o MBM and i s co-incine a ion wi h coal has been
s udied as a sui able way o ene gy alo isa ion [1, 2] and [3]. Ne e heless, he
incine a ion o MBM gi es way o a high le el o emissions o dioxins and u ans [4],
and also NOx, due o he ni ogen con en o MBM [5].
Py olysis cons i u es one o he mos eliable ea men me hods o he animal bones
le o e . The solid ac ion ob ained (cha ) con ains abou 70-76 w % calcium
hyd oxyapa i e (HAP) Ca10(PO4)6(OH)2, 9-11 w % ca bon and 7-9 w % CaCO3. The e
a e o he mino cons i uen s, such as calcium sulpha e (0.1-0.2 w %) and Fe2O3 (less
han 0.3 w %) [6]. Due o i s chemical s uc u e HAP has been ecen ly epo ed as a
e y use ul ma e ial in ca alysis [7] and [8], elec ochemis y [9] and adso p ion [10],
[11], [12] and [13] p ocesses.
Spain is he wo ld’s ou h la ges po k p oduce , wi h 3.5 MTn o pigmea p oduced in
2012 [14]. Consequen ly, he p epa a ion o HAP-based ma e ials om po k bone is a
p omising al e na i e, gi en ha his p ocess is economically easible and
en i onmen ally iendly.
The u ili y o HAP depends o a g ea ex en on i s ex u al p ope ies. Chemical
ac i a ion has been epo ed as a sui able me hod o de elop a ma e ial wi h high
po osi y, which is associa ed wi h he occu ence o gasi ica ion eac ions [15].
Mo eo e , chemical ac i a ion has se e al ad an ages o e physical ac i a ion, such as:
(i) highe yield; (ii) highe su ace a ea; (iii) be e de elopmen o po ous s uc u es;
and (i ) he equi emen o lowe py olysis empe a u es [16]. I s main d awback is he
need o washing o emo e he esidual ino ganic ma e ial.
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 highly ex u ed 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. Senneca [1] s udied p oduc dis ibu ion
du ing he he mal ac i a ion o MBM. Guo e al. [17] s udied he chemical ac i a ion
o oil-palm s one wi h H2SO4 and KOH. Lillo-Ródenas e al. [16, 18] in es iga ed he
3
eac ions occu ing du ing he chemical ac i a ion o an an h aci e wi h NaOH and
KOH. Robau-Sánchez e al. [19] p oposed a eac ion mechanism o he ac i a ion o
Que cus ag i olia cha wi h KOH. Ne e heless, he li e a u e conce ning he
p epa a ion o po ous hyd oxyapa i es by he chemical ac i a ion o animal was es is
sca ce, and he e a e impo an gaps in he undamen als o he p ocess, ega ding he
eac ion mechanisms occu ing du ing he hea ea men .
In his wo k he mog a ime ic analysis coupled o mass spec ome y was used o gain
knowledge on he chemical ac i a ion p ocess o po k bone cha wi h di e en agen s:
H3PO4, H2SO4, NaOH and K2CO3. The objec i es o his wo k a e: i) Iden i y he
eac ions be ween he cons i uen s o bone cha and he ac i a ing agen s ha ake place
du ing he p epa a ion o po ous hyd oxyapa i e. Fu he mo e, since ca bon is one o he
cons i uen s, he esul s will help o be e unde s and he undamen als o he chemical
ac i a ion p ocess o any ca bonaceous p ecu so . ii) In es iga e he e ec o he
ac i a ion p ocess on he physicochemical p ope ies o he inal p oduc , in o de o
op imize he p epa a ion me hod, esul ing in he achie emen o a HAP-based ma e ial
wi h he desi ed ex u al p ope ies.
2. Expe imen al
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. I is known ha he e apo a ion o low molecula weigh
o ganic compounds and he decomposi ion o collagen occu s below 450 ºC [20]. In his
way, in o de o emo e mea and a , p io o chemical ac i a ion bones we e
p eca bonized a 450 ºC, which is abou hal o he empe a u e used in he subsequen
ca boniza ion s ep. F om now on, p eca bonized sample will be e e ed as p ecu so .
P eca boniza ion was pe o med using a hea ing a e o 10 ºC/min un il he desi ed
empe a u e was eached; empe a u e was hen held cons an o 1 hou . Ni ogen low
was se a 120 cm3/min, which co esponds o 8 minu es o esidence ime in u nace.
P eca bonized samples we e le o cold down in ni ogen a mosphe e.
The p ecu so was sie ed and pa icles in he 0.25 – 0.35 mm size ange we e used. The
p ecu so was di ided in o i e pa s. Fou we e imp egna ed wi h H3PO4 (P), H2SO4
4
(S), NaOH (N) o K2CO3 (K). The ac i a ion eagen s we e supplied by Pan eac wi h
labo a o y g ade pu i y, H3PO4 (86%), H2SO4 (96%), NaOH pelle s (97%) and
anhyd ous K2CO3 (99%). The las sample was no u he modi ied o be used as a
e e ence (O).
The imp egna ion a io was es ablished a 20 mmol o ac i a ion agen pe g am o
p ecu so (p eca bonized bone cha ). These alues a e wi hin he no mal ange used o
he chemical ac i a ion o o he low cos p ecu so s [21], [16], [15] and [22], and ensu e
su icien in e ac ion be ween he ac i a ion agen and bone cha ma ix. Fo he
imp egna ion s ep abou 2 g o p ecu so we e con ac ed wi h 40 cm3 o a solu ion
con aining 1 mol/L o he ac i a ing agen . Solu ions we e in oduced in 50 mL
bo osilica e ambe glass ials wi h Te lon ap and s i ed a 120 pm in a ecip oca ing
shake 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. 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 hou s. The imp egna ed ma e ials we e coded
acco ding o he ac i a ing agen used: BCP, BCS, BCN, BCK and BCO.
Finally, imp egna ed samples we e py olysed a 800 ºC unde ope a ing condi ions
simila o hose used du ing he p eca boniza ion s age. Samples o BC we e washed
wi h dis illed wa e un il neu al pH o solu ion was eached, and hei physicochemical
p ope ies we e measu ed.
2.2 TG-MS s udy
In o de o in es iga e he eac ions occu ing du ing he ac i a ion p ocess,
he mog a ime ic analysis (TG) coupled o mass spec ome y (MS) was conduc ed.
TG analysis was pe o med wi h a Se sys e olu ion (Se a am) he mal analyse . Abou
40 mg o sample (BCP, BCS, BCN, BCK and BCO) we e pu in o he ce amic c ucible
and hea ed unde helium a mosphe e om oom empe a u e o 1000 ºC, a a hea ing
a e o 10 ºC/min. The he mal analyse exhaus gases we e analysed on-line by a mass
spec ome e (MKS, Ci us 3000). The o al p essu e in he analysis chambe was 10-6
To . The ollowing compounds we e moni o ed con inuously: H2 (m/z = 2), CH4 (m/z
= 16), H2O (m/z = 18), CN- (m/z = 26), HCN (m/z = 27), CO (m/z = 12), aldehydes
(m/z = 29) and CO2 (m/z = 44). MS signals we e no malized by di iding by he signal
o He used as ca ie .
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2.3 Physicochemical cha ac e iza ion
The physicochemical p ope ies o he inal samples (chemically ac i a ed a 800 ºC)
we e in es iga ed. Tex 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 unde a ni ogen low
o 15 hou s. BET su ace a ea, and po e a ea and olume we e measu ed. Mic opo e
su ace and olume we e measu ed om densi y unc ional heo y (DFT), while 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 chemical composi ion and ex u al s uc u e we e analysed by a scanning elec on
mic oscope (JEOL JSM-7000F) equipped wi h ene gy dispe si e X- ay de ec o (EDX).
Fou ie - ans o med in a ed (FTIR) spec a we e collec ed using a Nicole P o égé 460
de ice in he ansmi ance mode, in 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 X- ay
powde di ac ion (XRD, PANaly ical Xpe PRO) analyses we e pe o med wi h
CuKα adia ion (λ = 1.5418 Å) in con inuous scan mode in he 5-70º 2θ ange, wi h a
0.02º s ep size. The analysis o he di ac ion peaks was pe o med using PANaly ical
X´pe HighSco e so wa e.
3. Resul s and discussion
3.1 Chemis y o he ac i a ion p ocess
Fig. 1 depic s he e olu ion o mass spec ome ic signals wi h empe a u e,
co esponding o he main compounds eleased du ing he he mal ac i a ion o he
imp egna ed samples o bone cha .
3.1.1 Release o aldehydes, HCN and cyanides
Fig. 2a and b show he e olu ion o he signals o HCN and cyanides, espec i ely. All
samples, including he p ecu so , exhibi a well de ined peak a empe a u es be ween
300 and 600 ºC, wi h i s maximum a a ound 500 ºC.
I should be no iced, hough, ha he signal o CN- is a seconda y signal o HCN. I
HCN was he only sou ce o CN- ions, bo h signals should ollow an in a iable
6
HCN/CN- a io o 1:0.17 [23]. Howe e , he measu ed HCN/CN- in ensi y a io is
a ound 1:0.09 o all samples, which e idences he p esence o CN- in py olysis gases.
Robau-Sánchez e al. [19] epo ed he o ma ion o cyanides du ing he ac i a ion wi h
KOH o a ca bonaceous ma e ial (Que cus ag i olia cha ). These au ho s p oposed
al e na i e mechanisms which esul in he o ma ion o cyanides in ei he liquid o gas
o m:
6 KOH (l) + 5.5 C + N2 (g) ↔ 2 KCN (l) + 2 K2CO3 + 1.5 CH4 (g) (1)
6 KOH (l) + 5.5 C + N2 (g) ↔ K2(CN)2 (g) + 2 K2CO3 + 1.5 CH4 (g) (2)
Liquid cyanides migh hen be ans o med in o gaseous cyanides:
2KCN (l) ↔ K2(CN)2 (g) (3)
The essen ial condi ion o cyanide o ma ion is he p esence o ca bon and ni ogen.
Ca bon ep esen s abou 10 w % o bone cha [6], and i con ains also a small amoun o
s uc u al ni ogen [24]. Thus, he cyanide o ma ion e idences ha s uc u al ni ogen
ook pa in he ac i a ion p ocess, p obably ollowing a eac ion pa hway simila o ha
desc ibed by Eq. (1)-(2).
Rega ding he o ma ion o HCN du ing he py olysis, he e could be wo possibili ies:
(i) i s di ec syn hesis, ollowing a mechanism simila o ha men ioned be o e, o (ii)
i s syn hesis om cyanides, h ough he eac ion o hese componen s wi h H2O and
CO2, H2O o acids:
2 NaCN + H2O + CO2 ↔ Na2CO3 + 2 HCN (4)
KCN + H2O ↔ HCN + KOH (5)
CN- + H+ ↔ HCN (6)
The abo e-men ioned se o equa ions in ol es he eac ion o OH- ions and
consequen ly, i could be ex ended o e e y compound con aining OH- unc ionali ies.
In he case o BCO, he e a e wo po en ial sou ces o OH-: (i) he P-OH unc ionali ies
o HAP and (ii) he eac ion o CaCO3 cons i uen o bone cha wi h wa e , acco ding o
Eq. (7) and (8) [25]:
CO32- + H2O ↔ HCO3- + OH- (7)
HCO3- ↔ CO2 + OH- (8)
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Eq. (7), which akes place a empe a u es in he 200-600 ºC ange, equi es a sou ce o
wa e . As discussed below, he dehyd a ion o hyd oxyapa i e in ha empe a u e ange
esul s in a con inuous elease o wa e (Fig. 5).
The a o emen ioned mechanism is suppo ed by he inc eased elease o CN- obse ed
o alkali ea ed samples (BCN and BCK) as compa ed o non-chemically ea ed
sample (BCO). Fo BCK, he la ge amoun o CO32- ions inco po a ed wi h K2CO3 leads
o he o ma ion o OH- ions (Eq. (7)-(8)).
I is no ewo hy he opposi e beha iou o bo h acid ea ed samples, sugges ing ha
di e en eac ion pa hways occu by ei he H2SO4 o H3PO4 ac i a ion. The sample
ea ed wi h H2SO4 shows he lowes elease o CN-, sligh ly lowe han he p ecu so
i sel . This ac could be asc ibed o he pa ial dissolu ion o HAP, which leads o he
o ma ion o calcium sulpha e and ca ion de icien HAP, Ca10-X(HPO4)X(PO4)6-X(OH)2-
X. [25]. Ca ion de icien HAP, which con ains a lowe amoun o P-OH unc ionali ies,
would esul in a lowe o ma ion o cyanides.
BCP sample, on he con a y, shows he highes elease o cyanides, an inc ease o 99%
compa ed o BCO. This unexpec ed beha iou (gi en ha his sample should ha e a
lowe amoun o OH- unc ionali ies, due o i s pa ial dissolu ion) could be ela ed o
he incong uen dissolu ion o HAP wi h H3PO4 and he o ma ion o an amo phous
s uc u e. Fig. 9 shows he XRD pa e ns o he p ecu so , BCO, BCS and BCP samples,
a e being py olysed a 800 ºC. I is clea ha he acid ac i a ion wi h phospho ic acid
esul s in an impo an educ ion o c ys allini y, since only a wide band, di e en om
hose o HAP, is obse ed. Se e al au ho s ha e epo ed he dissolu ion o HAP wi h
H3PO4. Acco ding o K upa-Zuczek e al. [26], mild concen a ions o acid would lead
o he pa ial dissolu ion o HAP, esul ing in a solid phase composed o HAP and
CaPO3(OH)·2H2O. O he au ho s ha e epo ed he p esence o a ious phospha es
(CaHPO4·2H2O, Ca(H2PO4)2·H2O and Ca3(PO4)2), a e acid and hea ea men [27].
The wide band obse ed in he XRD pa e n (in he 20-30º ange) could be asc ibed o
he coalescence o hose peaks.
Acco ding o his hypo hesis, he agg essi e ea men wi h H3PO4 would esul in an
amo phous, he molabile s uc u e, which would collapse du ing he py olysis s ep. A
high amoun o OH- ions in he solid phase could hen be eleased, hus being a ailable
o eac acco ding o Eq. (1)-(2).
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The elease o aldehydes occu s simul aneously o ha o CN- (Fig. 2c). Mo eo e ,
aldehydes ollow a simila end, gi en as ollows: BCP >> BCK > BCN > BCO ≥
BCS. These esul s sugges ha aldehydes a e also o med h ough a mechanism which
implies he eac ion o ca bon and hyd oxides, which is in good ag eemen wi h hei
chemical o mula (R-CHO).
To summa ize, he ollowing aspec s could be highligh ed, ega ding he elease o
aldehydes, HCN and CN-:
- The p oposed eac ion mechanism in ol es he eac ion o C and OH- ions (and
s uc u al N, in he case o HCN/CN-).
- Since his mechanism implies he gasi ica ion o cons i uen s o bone cha (o
inco po a ed species), i is expec ed o ha e an impac on he ex u al p ope ies o he
inal ma e ial.
- The alkali ea men (wi h ei he NaOH o K2CO3) has he e ec o inco po a ing OH-
ions, hus esul ing in a highe ex en o his se o eac ions. Be ween bo h ac i a ion
eagen s, K2CO3 is mo e ac i e.
- Acid ea ed samples show an opposi e beha iou . The ea men wi h H2SO4 leads o
he pa ial dissolu ion o HAP, esul ing in a sligh ly lowe elease o aldehydes and
HCN/CN-. Fo he ea men wi h H3PO4, he o ma ion o an amo phous and
he molabile s uc u e is hypo hesized. This s uc u e would collapse du ing he he mal
ea men , hus lea ing a high amoun o OH- ions a ailable o eac acco ding o he
p oposed mechanism.
3.1.2 Release o CH4
All samples exhibi a peak o CH4 s a ing a ound 400 ºC and wi h i s maximum nea
550 ºC (Fig. 3). The elease o CH4 is p obably ela ed o cyanide o ma ion h ough Eq.
(1)-(3), which in ol e he simul aneous o ma ion o me hane. This mechanism would
be suppo ed by he obse ed co ela ion be ween he in ensi y o CN- and CH4 signals
o each sample (Fig. 2b and 3). Howe e , an excep ion occu s wi h he sample
ac i a ed wi h H2SO4, being he elease o CH4 highe han expec ed in ha empe a u e
ange. This esul e idences he occu ence o an addi ional mechanism o me hane
o ma ion. I is likely ha u he amoun s o CH4 a e o med h ough he me hana ion
o CO2 (Saba ie eac ion, Eq. (9)) and CO (Eq. (10)):
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CO2 + 4 H2 ↔ CH4 + 2H2O (9)
CO + 3 H2 ↔ CH4 + H2O (10)
Bo h eac ions ha e been epo ed in he gasi ica ion o ca bonaceous ma e ials, such as
ac i a ed ca bon [28] and ice s aw [29] wi h CO2 and s eam. In his wo k, he in ense
elease o CO and especially CO2 a ound 550 ºC (and also he peak a highe
empe a u e, a ound 850 ºC) obse ed o BCS (Fig. 4 and Fig. 7) s ongly coincide
wi h CH4 elease. These esul s suppo he occu ence o he abo e-men ioned
me hana ion eac ions. Fu he mo e, BCN and BCK also exhibi a mino peak o CH4
(Fig. 1b and c) a high empe a u e (800-900 ºC), which is in good ag eemen wi h he
small elease o CO and CO2 o hese samples.
I is wo h highligh ing he ollowing poin s conce ning he e olu ion o me hane:
- All samples show a peak wi h i s maximum nea 550 ºC, which could be ela ed o he
a o emen ioned mechanism o HCN/CN- o ma ion, ha implies he simul aneous
o ma ion o me hane.
- The elease o CH4 o he sample ea ed wi h H2SO4 (highe han expec ed a ha
empe a u e ange, and wi h ano he peak a highe empe a u e) sugges s he
occu ence o an addi ional eac ion mechanism, which is likely o be he me hana ion
o CO and CO2.
- Be ween bo h se s o eac ions, ha ela ed o he o ma ion o HCN/CN- in ol es he
gasi ica ion o cons i uen s o bone cha (o inco po a ed species). Me hana ion
eac ions, on he con a y, ake place in gas phase and consequen ly, should no be
ela ed o he de elopmen o po osi y.
3.1.3 Release o CO2
All samples show a low and b oad band o CO2 in he 200-700 ºC ange (Fig. 4). As
men ioned be o e, in ha empe a u e ange ca bona e ions p esen in bone cha can
eac wi h wa e , acco ding o Eq. (7). The esul an HCO3- ions and hose con ained in
HAP c ys als a e hen eleased as CO2 and OH-, as desc ibed by Eq. (8). I is
no ewo hy ha he ea men wi h sulphu ic acid esul s in a much g ea e elease o
CO2 a mild py olysis condi ions (200-700ºC). The eac ion o H+ ions wi h ca bona es,
p omo ing he o ma ion o HCO3- ions (Eq. (11)) and hei subsequen decomposi ion
could explain his highe elease o CO2.
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On he o he hand, wa e apou eac s wi h ca bon o p oduce CO (wa e -gas eac ion):
H2O + C ↔ CO + H2 (27)
Ren e al. [24] s udied he gasi ica ion beha iou o MBM cha wi h s eam a
empe a u es in he 850-1000 ºC ange, and concluded ha his ca bonaceous ma e ial
has highe gasi ica ion eac i i y han se e al ypes o coal.
The de i a i e o he measu ed weigh loss (Fig. 8b) suppo s he abo e mechanism in
which s uc u al ca bon is con e ed in o gaseous CO. The o de in weigh loss (BCS
>> BCK > BCN > BCP ≈ BCO) mos ly coincides wi h CO elease (BCS >> BCK >
BCP > BCN > BCO). I should be no iced, hough, ha he weigh loss o BCS should
no be asc ibed only o his mechanism, bu also o he eac ion o H2SO4 wi h ca bon o
ca bona es (Eq. (12)-(13)).
Gasi ica ion eac ions (Eq. (26)-(27)) equi e a sou ce o CO2 and H2O, espec i ely. In
he case o acid ac i a ion (BCP, BCS), he o ma ion o ca ion-de icien HAP and he
eac ion o ca bon wi h sulphu ic acid (Eq. (12)-(13)) a e he main sou ce o H2O and
CO2 a high empe a u e. Consequen ly, i is expec ed ha hese samples gi e way o a
highe elease o CO han he p ecu so .
Rega ding alkali ac i a ion (BCN, BCK), CO2 is mainly eleased h ough he he mal
decomposi ion o s uc u al ca bona es o hose added by he ac i a ing agen .
Mo eo e , i would also lead o an inc eased p esence o H2O in py olysis gases (Eq.
(7), (8) and (22)), compa ed o BCO. I is well known he ca aly ic ole o alkali me als
such as K, Mg, Na and Ca o bo h he e e se Boudoua d eac ion, and he gasi ica ion
o ca bon wi h s eam (Eq. (26)-(27)) [24] and [41], being K he mos ac i e [41].
Se e al au ho s p oposed he mechanism desc ibed by Eq. (28), in which C ac s as a
educing agen , o p oduce CO a empe a u es abo e 700 ºC [42]. This eac ion could
also be ex ended o he sample ea ed wi h NaOH. The equi ed oxides (K2O and
Na2O) a e likely o be o med h ough he a o emen ioned he mal decomposi ion o
ca bona es (Eq. (16)-(18)). Ou esul s a e in good ag eemen wi h hese obse a ions,
since he CO o ma ion o BCN and BCK was highe and shi ed o a lowe empe a u e
han ha o he p ecu so .
K2O + C ↔ 2K + CO (28)
I is wo h highligh ing he ollowing poin s conce ning he e olu ion o CO:
17
- The i s elease o CO (a 300-600 ºC) is a ibu ed o he RWGS eac ion (CO is
o med om CO2). This eac ion akes place in gas phase, and hus, does no gene a e
po osi y.
- The peak o CO abo e 600 ºC could no only be a ibu ed o RWGS eac ion. The
addi ional eac ion mechanism is likely o be he gasi ica ion o ca bon wi h CO2 o
H2O, a ou ed by he p esence o alkali me als (being K he mos ac i e). These
gasi ica ion eac ions a e expec ed o de elop he po ous s uc u e o he ma e ial.
3.2 Tex u al p ope ies o he py olysed ma e ials
The p oduc ion yields and ex u al p ope ies (su ace a ea and po e olume) o he
p epa ed samples o bone cha a e shown in Table 2. These alues co espond o
imp egna ed samples ac i a ed a 800 ºC. The p oduc ion yield was maximum (21.5%)
o he non-chemically ea ed sample (BCO). Alkali ac i a ion (samples BCN and
BCK) did no u he dec ease he p oduc ion yield, wi h alues o 19.5 and 20.2%,
espec i ely. On he con a y, acid ac i a ion signi ican ly lowe ed he yield o 2.9 and
12.4%, by H3PO4 and H2SO4 ac i a ion, espec i ely.
The speci ic su ace a ea and po e olume a y signi ican ly depending on he
ac i a ing agen used. I is no ewo hy he ex emely low alue o su ace a ea and po e
olume o he sample ea ed wi h H3PO4. These esul s, which e eal ha almos all he
po ous s uc u e o he p ecu so was emo ed du ing he chemical ac i a ion, a e in
good ag eemen wi h he a o emen ioned hypo hesis. Acco ding o his hypo hesis, he
agg essi e ea men wi h phospho ic acid would incong uen ly dissol e HAP, esul ing
in he o ma ion o an amo phous and he molabile s uc u e. This s uc u e would be
decomposed when hea ing, eleasing high amoun s o gases (such as H2O, H2 and CH4),
which would almos comple ely des oy he po ous s uc u e o HAP.
Rega ding he alkali ea ed samples (BCN and BCK), an inc ease in he o al su ace
a ea is obse ed, compa ed o BCO. Fu he mo e, a qui e homogeneous inc ease in he
mic o- and mesopo osi y (po e olume) is obse ed, whe eas mac opo osi y emains
almos cons an . These ex u al p ope ies mus be ela ed o he ex en o he eac ions
which gene a e po osi y, mainly: (i) hose in ol ing he eac ion o C and OH- ions, ha
gene a e compounds such as CN- and aldehydes (Fig. 2b and c); and (ii) he gasi ica ion
eac ions o ca bon (Eq. (26) and (27), Fig. 7), ha gene a e CO. These eac ions occu
18
in wo empe a u e in e als: (i) 300-600 ºC: syn hesis o CN-/HCN and aldehydes; and
(ii) 600-1000 ºC: gasi ica ion o ca bon. The elease o H2 akes place in bo h
empe a u e in e als (Figu e 6), al hough i becomes mo e impo an a empe a u es
highe han 600 ºC.
Be ween bo h se s o eac ions, ha implying he eac ion o s uc u al C and OH- ions
o p oduce CN-/HCN and aldehydes is o g ea e impo ance, since i gi es way o a
highe weigh loss (Fig. 8b). Table 3 shows he eleased amoun s o aldehydes and CN-
/HCN (calcula ed by in eg a ion), o be used as an indica o o he ex en o his se o
eac ions. Fig. 11 displays he ela ionship be ween he eleased amoun o CN-/HCN
and he po e olume o he sample. A simila igu e is ob ained when he amoun o
aldehydes is displayed. I is obse ed ha he e is a di ec ela ionship be ween he
ex en o he eac ions (BCK > BCN > BCO) and he de elopmen o mic o- and
mesopo osi y, while mac opo osi y emains cons an . This enhancemen o po osi y
gi es way o a signi ican inc ease in su ace a ea, measu ed by S o al (59% o BCN and
74% o BCK).
Conce ning he se o eac ions occu ing a empe a u es abo e 600 ºC, he
simul aneous occu ence o he RWGS eac ion (Eq. (25)) makes i di icul o measu e
he amoun o CO o H2 e ec i ely p oduced in gasi ica ion eac ions. Thus, mass
change in his empe a u e in e al has been selec ed as an indica o o he ex en o his
se o eac ions (Table 3). The ex en o hese eac ions (BCK > BCN > BCO) is in good
ag eemen wi h bo h he ex en o he i s se o eac ions, and he de elopmen o
mic o- and mesopo osi y (Table 2). Thus, i is clea ha he e ec o bo h alkali
eagen s is o a ou he eac ions ha gene a e po osi y (in he mic o- and mesopo e
ange) in he whole empe a u e ange, being K2CO3 mo e ac i e.
This homogeneous inc ease o mic o- and mesopo osi y esul s in a hie a chical po ous
ma e ial, wi h an excellen po en ial o applica ions as elec ode ma e ial, in gas
s o age, ca alysis, and ene gy s o age. Samples o bone cha p epa ed by alkali
ea men could be used as elec ode ma e ials o elec ic double-laye capaci o EDLC.
Acco ding o he wo k o Wang e al. [43] ion-bu e ing ese oi s can be o med in he
mac opo es o minimize he di usion dis ances o he in e io su aces, he mesopo ous
walls p o ide low- esis an pa hways o he ions h ough he po ous pa icles, and he
mic opo es s eng hen he elec ic double laye capaci ance. Consequen ly, he
19
combina ion o mic o-, meso- and mac opo ous s uc u es helps o main ain a good
capaci i e beha iou o he esul an ma e ial.
Fu he mo e, he g ea inc ease in speci ic su ace a ea allows he chemical ac i a ion
wi h NaOH and K2CO3 o be conside ed as a sui able me hod o imp o e he
pe o mance o HAP as a ca aly ic suppo . I s desi able p ope ies, i.e. s uc u al
s abili y, ion-exchange abili y, acid-base p ope ies and adso p ion capaci y, make HAP
a p omising ma e ial o con igu e he e ogeneous ca alys s. HAP-based ca alys s ha e
been ecen ly employed in some chemical ans o ma ions, such as he oxida ion o
alcohols o aldehydes [44] o he oleic acid es e i ica ion [7].
Finally, he sample ea ed wi h H2SO4 shows a comple ely di e en end in he
de elopmen o po osi y (Fig. 11), which canno be mainly asc ibed o he
a o emen ioned eac ions o ca bon wi h OH- o p oduce compounds such as CN-/HCN
and aldehydes. Acco ding o he da a o Table 2, BCS sample exhibi s a ema kable
inc ease o mic opo osi y (135%, compa ed o BCO), while he mac opo e olume
d ama ically dec ease (65%). Mo eo e , he e is a ade-o be ween ex e nal and
mic opo e a ea, since o al a ea ha dly a ies. This dec ease is mainly due o he
educ ion in mac opo osi y, since mesopo e a ea emains almos cons an .
The de elopmen o mic opo osi y should hen be a ibu ed o speci ic eac ions
in ol ing H2SO4. Among he eac ions which could ha e an impac on ex u al
p ope ies, wo should be highligh ed, acco ding o hei ela i e impo ance (measu ed
by weigh loss in ensi y and eloci y in he TG and DTG plo s, Fig. 8): he dehyd a ion
o ca ion-de icien HAP (Eq. (20)-(21)), and he eac ion o H2SO4 wi h a sou ce o
ca bon (Eq. (12)-(13). This selec i e de elopmen o mic opo osi y esul s in a ma e ial
sui able o be used in gaseous pollu an adso p ion, o emo e compounds such as SO2
[45] o VOCs [46].
4. Conclusions
When s udying he de elopmen o po osi y du ing he chemical ac i a ion o bone
cha , i is essen ial o dis inguish be ween gasi ica ion eac ions (which esul in weigh
loss) and eac ions in gas phase, which no gene a e po osi y. Gasi ica ion p ocesses
in ol e he he mal decomposi ion o he eac ion o cons i uen s o bone cha
(hyd oxyapa i e, ca bona es, ca bon, ni ogen) and hose compounds inco po a ed in he
imp egna ion s age.
20
The mechanisms o gene a e po osi y a e: (i) he deso p ion o adso bed wa e ; (ii) hose
in ol ing he eac ion o ca bon and OH- ions, which p oduce compounds such as CN-,
HCN, aldehydes and H2; (iii) he he mal decomposi ion o ca bona es; (i ) he
gasi ica ion o ca bon wi h CO2 o H2O; ( ) he dehyd a ion o HAP o ca ion-de icien
hyd oxyapa i e; and ( i) he eac ion o H2SO4 wi h ca bon o ca bona es.
The mechanisms h ough which each ac i a ion agen enhances he de elopmen o
po osi y depend on he na u e o he agen . Acids lead o he o ma ion o ca ion
de icien HAP, which is s ongly dehyd a ed unde he mal ea men . Fu he mo e,
H2SO4 eadily eac s wi h ca bon. The main e ec o alkali ea men (wi h NaOH o
K2CO3) is he inco po a ion o OH- ions, which can hen eac wi h ca bon. O he
eac ion pa hways a ou ed a e he he mal decomposi ion o ca bona es and he
gasi ica ion o ca bon, p omo ed by alkali me als.
The ea men wi h H2SO4 p oduces a highly mic opo ous ma e ial, gi en ha he e is a
ade-o be ween po es in he mac opo e and mic opo e ange. Alkali ea men , on he
con a y, esul s in a mo e equilib a ed inc ease o mic o- and mesopo osi y, wi h no
a ia ion in he mac opo e ange. Be ween bo h agen s, he e ec o K2CO3 is mo e
p onounced han ha o NaOH.
Consequen ly, i is possible o choose he mos app op ia e ea men me hod o use,
depending on he desi ed ex u al p ope ies o he ma e ial. The selec i e de elopmen
o mic opo osi y allows he ea men wi h H2SO4 o be conside ed as a sui able me hod
o con igu e a ma e ial o be used in gaseous pollu an adso p ion, o emo e
compounds such as SO2 o VOCs. Alkali ea men , on he con a y, esul s in a
hie a chical po ous ma e ial, wi h an excellen po en ial o applica ions as elec ode, in
gas s o age, ca alysis and ene gy s o age.
Finally, unde he ope a ing condi ions used in his wo k, he ea men wi h H3PO4 was
ex emely agg essi e. Phospho ic acid is likely o incong uen ly dissol e HAP, o ming
an amo phous and he molabile s uc u e, which is decomposed by hea ing, leading o
he o ma ion o high amoun s o gases, which would almos comple ely des oy he
po ous s uc u e o HAP.
21
5. Acknowledgemen s
The au ho s wish o hank o he Basque Go e nmen (UFI 11/39 (UPV/EHU)) o hei
economic suppo .
22
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32
0.0E+00
2.0E-04
4.0E-04
6.0E-04
8.0E-04
1.0E-03
0.0E+00
4.0E-05
8.0E-05
1.2E-04
1.6E-04
2.0E-04
2.4E-04
2.8E-04
0 100 200 300 400 500 600 700 800 900 1000
Ion cu en (A)
Tempe a u e (ºC)
H2
CH4
CO2
HCN
CO
Aldehydes
H2O
CN
-
Fig. 1e
(
e
)
33
0 100 200 300 400 500 600 700 800 900 1000
HCN ion cu en (a. u.)
Tempe a u e (ºC)
BCO
BCN
BCK
BCP
BCS
Fig. 2a
(a)
34
0 100 200 300 400 500 600 700 800 900 1000
CN-ion cu en (a. u.)
Tempe a u e (ºC)
BCO
BCN
BCK
BCP
BCS
Fig. 2b
(b)
35
0 100 200 300 400 500 600 700 800 900 1000
Aldehydes ion cu en (a. u.)
Tempe a u e (ºC)
BCO
BCN
BCK
BCP
BCS
Fig. 2c
(
c
)
36
0 100 200 300 400 500 600 700 800 900 1000
CH
4
ion cu en (a. u.)
Tempe a u e (ºC)
BCO
BCN
BCK
BCP
BCS
Fig. 3
37
0 100 200 300 400 500 600 700 800 900 1000
CO
2
ion cu en (a. u.)
Tempe a u e (ºC)
BCO
BCN
BCK
BCP
BCS
Fig. 4
38
0 100 200 300 400 500 600 700 800 900 1000
H
2
O ion cu en (a. u.)
Tempe a u e (ºC)
BCO
BCN
BCK
BCP
BCS
Fig. 5
39
0 100 200 300 400 500 600 700 800 900 1000
H
2
ion cu en (a. u.)
Tempe a u e (ºC)
BCO
BCN
BCK
BCP
BCS
Fig. 6
40
0 100 200 300 400 500 600 700 800 900 1000
CO ion cu en (a. u.)
Tempe a u e (ºC)
BCO
BCN
BCK
BCP
BCS
Fig. 7
41
20
40
60
80
100
0 100 200 300 400 500 600 700 800 900 1000
Weigh (%)
Tempe a u e (ºC)
BCO
BCN
BCK
BCP
BCS
Fig. 8a
(a)
48
Table 1. S anda d en halpy (kJ/mol) and en opy (kJ/mol K) changes, and Gibbs ee
ene gies (kJ/mol) o he p oposed eac ions be ween he acid eagen (H2SO4 o H3PO4)
and he p ecu so .
Eq.
∆
ܪ
°
∆
ܵ
°
∆
ܩ
°
∆
ܩ
(300 ºC)
∆
ܩ
(600 ºC)
∆
ܩ
(900 ºC)
(12) -0.50 0.484 -144.8 -267.7 -386.0 -493.5
(13) -127.3
0.120 -163.2 -191.3 -214.2 -231.9
(15) 85.8 0.872 -174.1 -412.2 -669.5 -925.0
49
Table 2. P oduc ion yields and Tex u al p ope ies o p epa ed samples o bone cha .
Su ace a ea in m2/g; po e olume in cm3/g.
a DFT me hod
b BJH me hod
c Sum o DFT me hod (Dp<1.7 nm) and BJH me hod (Dp >1.7 nm)
Sample
Yield S
BET
S
mic o
a S
meso
b S
mac o
b S
o al
c V
mic o
a V
meso
bV
mac o
b V
o al
c
BCO 21.5 76.2 80 32 61 172.2 0.017 0.026 0.222 0.265
BCN 19.5 106.2 151 50 73 274 0.030 0.042 0.214 0.286
BCK 20.2 110.7 166 55 80 300 0.033 0.047 0.224 0.303
BCP 2.9 3.2 n.d. 4 1 4.7 n.d. 0.002 0.004 0.006
BCS 12.4 116.8 108 32 14 153.4 0.040 0.021 0.077 0.139
50
Table 3. Released amoun s o se e al compounds (calcula ed by in eg a ion), and mass
loss in he 600-1000 ºC empe a u e ange. Fo compa ison pu poses, a alue o 100 has
been assigned o BCO sample.
Sample
Aldehydes CN- + HCN Mass loss
600-1000 ºC
BCO 100 100 100
BCN 122 118 113
BCK 139 133 134
BCS 94 98 508
51
HIGHLIGHTS
-A highly ex u ed ma e ial, based mainly on HAP, was p epa ed om po k bone cha .
- The eac ions occu ing du ing he chemical ac i a ion we e in es iga ed by TG-MS.
- T ea men wi h H2SO4 esul ed in a highly mic opo ous ma e ial.
- T ea men wi h NaOH and K2CO3 inc eased mic o- and mesopo osi y.
- These esul s a e use ul o con igu e a ma e ial wi h he desi ed ex u al p ope ies
