Vol.:(0123456789)
1 3
Biomass Con e sion and Bio e ine y
h ps://doi.o g/10.1007/s13399-023-04138-4
ORIGINAL ARTICLE
P epa a ion o ac i a ed ca bon ompine (Pinus adia a) sawdus
bychemical ac i a ion wi hzinc chlo ide o wood dye adso p ion
Ca a inaH.Pimen el1 · M.SoniaF ei e1 · DiegoGómez‑Díaz1 · JuliaGonzález‑Ál a ez1
Recei ed: 29 Decembe 2022 / Re ised: 14 Ma ch 2023 / Accep ed: 24 Ma ch 2023
© The Au ho (s) 2023
Abs ac
Was ewa e con aining dyes a e eleased in o wa e bodies gene a ing se ious p oblems in human heal h and ma ine li e. To
con ibu e o he solu ion o his p oblem, a no el ac i a ed ca bon was p epa ed om un ea ed pine (Pinus adia a) sawdus
by d y chemical ac i a ion wi h ZnCl2 and was used o wood dye adso p ion. The ca bon was cha ac e ized by poin o ze o
cha ge, N2 and CO2 adso p ion iso he ms, SEM-EDX, and FTIR. N2 and CO2 su ace a eas we e 471.4 and 319.5 m2 g−1,
espec i ely, wi h 91% o mic opo es. Wood dye adso p ion was s udied in unc ion o pH (2–12), adso ben dose (0.1–4 g
L−1), ime (up o 48 h o blue and ed and 12 h o black), and ini ial concen a ion (5–500 mg L−1). The equilib ium da a
o he blue and black dyes we e sa is ac o ily i ed o he F eundlich model while hose o he ed dye o he Langmui
model. Kine ic da a we e explained by he pseudo-second o de (chemiso p ion p ocess) and in apa icle di usion models.
A 5 mg L−1, a 100% emo al e iciency was achie ed a all pH o he blue dye, whe eas o he ed and black, na u al pH
(5.1) and pH = 2, espec i ely, led o he bes emo al e iciencies, 96 and 56%. Inc easing concen a ion abo e 25 mg L−1
signi ican ly educed adso p ion e iciency o blue and ed dyes. Fo he black dye inc easing he dose o 1 g L−1, he adso p-
ion e iciency eached 82% a 25 mg L−1. High emo al e iciencies we e achie ed o all dyes a 25 mg L−1 and 4 g L−1.
Keywo ds Ac i a ed ca bon· Chemical ac i a ion· Zinc chlo ide· Adso p ion· Pine sawdus · Wood dyes
1 In oduc ion
A p oblem ha has pe sis ed o e he yea s and has a ac ed
he a en ion o many esea che s is wa e pollu ion. Wi h he
huge expansion o indus ializa ion (such as ex ile, wood,
ubbe , and dye indus ies), he olume o un ea ed was e-
wa e s ha a e discha ged in o s eams and wa e bodies also
inc eased. Among he was ewa e con aminan s, pa icula ly
dyes can gene a e se ious p oblems in human heal h due
o hei oxici y, mu agenici y, and ca cinogenici y e en a
low concen a ions. Mo eo e , hey a e highly esis an o
ligh , ae obic diges ion, and oxidizing agen s due o hei
chemical s abili y [1–3]. The e o e, was ewa e ea men
is essen ial and a ious echnologies ha e been employed
o dye emo al such as elec ocoagula ion; pho oca aly ic,
oxida i e, and biochemical deg ada ion; and adso p ion.
Adso p ion has ecei ed special a en ion due o i s simplic-
i y, high e iciency, including he ea men o concen a ed
dye e luen s, low cos , and he possibili y o eusing he
adso ben h ough i s eco e y and egene a ion [4, 5].
Adso p ion on ac i a ed ca bons (AC) has been widely
used o dye emo al due o hei high po osi y, su ace a ea,
physicochemical s abili y, mechanical s eng h, and su ace
eac i i y. Howe e , due o hei high cos and egene a ion
di icul y, nume ous in es iga ions ha e ocused on hei p o-
duc ion om na u al, abundan , sa e, and low-cos p ecu -
so s [6–8]. Ac i a ed ca bons can be p epa ed by physical
(in which s eam, ni ogen, o ca bon dioxide can be used o
mode a e educ ion o he ca bonaceous ma e ) o chemi-
cal ac i a ion (a chemical dehyd a ing agen is employed o
ac i a e he biomass o cha ) by one- o wo-s ep p ocesses.
Usually, ac i a ed ca bon p epa ed by chemical ac i a ion
p esen s highe su ace a ea, smalle po e size, and has wide
use in many applica ions [9, 10]. Aga wal e al. [4] epo ed
he wo-s ep p ocess as he mos e ec i e due o he high
po osi y de eloped: (1) ca boniza ion o he ma e ial unde
an ine a mosphe e which enables he emo al o mois u e,
ola ile compounds, and o he a oma ic and hen esul ing
* Julia González-Ál a ez
[email p o ec ed]
1 Depa men o Chemical Enginee ing, School
o Enginee ing, Uni e sidade de San iago de Compos ela,
15782San iagodeCompos ela, Spain
Biomass Con e sion and Bio e ine y
1 3
in a cha wi h high ca bon con en ; and (2) ac i a ion a
ele a ed empe a u e o deg ade he cellulosic ma e ials in
he p ecu so and o a ain speci ic cha ac e is ics which
will depend on he p ocess condi ions used. Ca boniza ion
depends on se e al ac o s such as he exis ence o an ine
a mosphe e, he hea ing a e, he ca boniza ion empe a u e,
and ime [5, 10]. The esul ing cha can be ac i a ed h ough
chemical ac i a ion ha could be pe o med by imp egna ion
(we oxida ion) o by mixing he cha wi h he solid ac i a -
ing agen (d y oxida ion) [11–13]. Rega ding he ac i a ing
agen s, hey ac as dehyd a ing agen s which in luence he
decomposi ion by py olysis and inhibi he o ma ion o a .
Addi ionally, he ac i a ion agen will s ongly in luence he
p ope ies o he ac i a ed ca bon. Among he mos used
ac i a ing chemical agen s such as ZnCl2, KOH, H3PO4, and
K2CO3, which p esen an enhanced e ec on ca bon su ace
a ea and po osi y, ZnCl2 s ands ou as an excellen ac i a ing
agen [4]. The ac i a ion p ocess using ZnCl2 inc eases he
yield due o polyme iza ion and ope a es as a Lewis acid
a o ing he c ea ion o ce ain la ge- ing a oma ic com-
pounds, p oduces a empla e e ec inducing he o ma ion o
uni o m mic opo es and could be mo e selec i e [3, 11, 14,
15]. Mo eo e , he ca bon ac i a ed wi h his agen usually
p esen s ou s anding p ope ies such as la ge po e olume
wi h smalle po e size, highe su ace a ea, and abundan
eac i e g oups o con aminan up ake. Addi ionally, i has
been epo ed ha ac i a ed ca bons p epa ed using ZnCl2
as ac i a ing agen showed good esul s pa icula ly o dye
emo al [16, 17]. Ac i a ed ca bon, as gene al adso ben ,
has a long his o y o manu ac u e and applica ion, al hough
he e a e s ill a ious p oduc ion aspec s ha could be con-
side ably imp o ed [18].
Was e biomass om ag icul u al o o es byp oduc s has
a isen as he bes al e na i e since hese aw ma e ials can
be used as adso ben s wi hou ea men , o by ca boniza ion
and ac i a ion, imp o ing hei adso p ion capaci y. Besides,
unwan ed and abundan was es can be con e ed in o use ul
and aluable adso ben s [19]. The e has been demand o cos -
e ec i e p ecu so s o he p oduc ion o ac i a ed ca bon
and ecen esea ch ha e epo ed he p epa a ion o AC om
ma e ials such as g ape s alk, semi-coke, coconu shell, slash
pinewood, and S e culia ala a nu shell [5, 9, 14, 19, 20]. This
ype o ca bon-based ma e ials demons a es cha ac e is ics
such as high su ace a ea, s abili y, ange o unc ional g oups,
and a po ous s uc u e [5]. Ne e heless, depending on he
na u e o he biomass and he py olysis/ac i a ion condi ions,
biocha wi h a po ous s uc u e and di e en ca bon con en
ha ing a high su ace a ea can be p oduced [21]. Bo h chemi-
cal s uc u e and su ace chemis y depend no only on he
ype o biomass bu also on he pa s o he biomass. Cellu-
losic biomass usually consis s o cellulose, hemicelluloses, and
lignin bu he p opo ions change depending on he ma e ial
which will in luence he cha ac e is ics o ac i a ed ca bons
[22, 23]. Fu he mo e, in mos o he s udies [5, 14, 19, 20], he
aw ma e ial is washed o p e- ea ed which inc eases he use
o chemicals in he p oduc ion o ac i a ed ca bon. Di e en
was e biomass has been con e ed in o ac i a ed ca bon and
has been success ully used o emo e acid dyes om was e-
wa e s: Paulownia omen osa wood o emo e acid ed 4 [24],
apple wood o emo e acid ed 337 and acid blue 349 [25],
walnu and popla woods o emo e acid ed 18 [26], pine-cone
o emo e acid black 1 and acid blue 113 [27].
Pine ees o di e en species a e p esen in la ge amoun s
a ound he wo ld and a e among he mos comme cially alu-
able ee species o imbe o wood pulp [28]. Pine sawdus , a
byp oduc om he wood indus y, is a lignocellulosic ma e ial
con aining hemicellulose, cellulose, lignin, and some unc-
ional g oups (e.g., ca boxyl, phenolic, hyd oxyl) which makes
i a good p ecu so o p oduce ac i a ed ca bons [6]. Pine saw-
dus om Pinus adia a, an abundan species in he no hwes
o Spain, could be an excep ional choice om an economic
and en i onmen al iewpoin due o i s a ailabili y and low
cos . To he bes o ou knowledge, pine sawdus om Pinus
adia a has ne e been applied as aw ma e ial o he p oduc-
ion o ac i a ed ca bon o dye adso p ion [28, 29].
Due o he g ea conce n wi h he discha ge o was ewa e
con aining dyes in o wa e bodies ha pose a di e si y o haz-
a ds o humans and ma ine li e, he p esen s udy was ocused
on he p oduc ion and cha ac e iza ion o a no el ac i a ed ca -
bon based on un ea ed pine (Pinus adia a) sawdus (PS). The
ca bon was ac i a ed by di ec mixing wi h ZnCl2 and used o
he emo al o acid wood dyes om aqueous solu ions. Thus,
a double objec i e is achie ed, o eplace expensi e comme -
cial ac i a ed ca bons and a he same ime educe he cos s o
biomass was e managemen , in addi ion o p oposing an al e -
na i e o sol ing he challenge o dye was ewa e pollu ion.
Ba ch adso p ion expe imen s we e pe o med o e alua e he
in luence o a ious ope a ing condi ions such as pH, adso ben
dose, and ini ial dye concen a ion on he emo al e iciency.
The ca bon was cha ac e ized o i s s uc u al and su ace
p ope ies, by he poin o ze o cha ge (pHPZC), BET su ace
a ea ob ained om ni ogen (N2) and ca bon dioxide (CO2)
so p ion iso he ms, and po e olume. Also, Fou ie ans o m
in a ed spec oscopy (FTIR) and scanning elec on mic oscopy
coupled wi h ene gy dispe si e X- ay analysis (SEM-EDX)
we e pe o med be o e and a e adso p ion. Kine ic and equi-
lib ium s udies we e ca ied ou unde selec ed condi ions in
o de o analyze he adso p ion mechanism.
2 Expe imen al sec ion
2.1 Ma e ials
Pine (Pinus adia a) sawdus (PS) was p o ided by a
egional sawmill (Lugo, Spain). A e ai -d ying o
Biomass Con e sion and Bio e ine y
1 3
equilib ium mois u e con en (11.61 w %), PS was sie ed
and he ac ion be ween 0.5 and 1 mm was selec ed,
s o ed in a plas ic con aine , and used di ec ly o he AC
p oduc ion.
Acid wood dyes we e used: Blue o wood AGN-270%,
Red o wood GRA-200%, and Black Hispalan M-RN-
140%. Dye solu ions we e p epa ed by dilu ion o a s ock
aqueous solu ion o 1000 mg L−1 wi h dis illed wa e .
Na u al pH was de e mined o each dye a 5, 25, and 500
mg L−1 using a digi al pH me e (Hanna edge).
Zinc chlo ide (ZnCl2, 98%, Scha lau), sodium chlo ide
(NaCl, 99.5%, P obus), sodium hyd oxide (NaOH, 98%,
Sigma-Ald ich), and hyd ochlo ic acid (HCl, 37%, Sigma-
Ald ich) we e used.
2.2 P oduc ion o ac i a ed ca bon
To analyze he ans o ma ions o pine sawdus du ing
he he mal ea men , he mog a ime ic analysis was
pe o med om 50 o 1000°C unde N2 a mosphe e a
25 mL min−1. Acco ding o he esul s (Fig.S1), 600°C
was selec ed as ca boniza ion empe a u e since cellulose,
hemicelluloses, and lignin decomposi ion usually akes
place below his empe a u e [15, 30] and no weigh loss
occu ed a highe empe a u es.
PS was ca bonized a 600°C o 1 h unde a N2 a mos-
phe e (10 mL min−1) in a ho izon al ubula u nace
(Ca boli e, She ield) applying a amp o 5°C min−1 up o
600°C. A e wa d, i was cooled down o oom empe a-
u e. The ea e , he biocha p oduced a e ca boniza ion
was mixed by milling wi h ZnCl2 a a weigh a io o 1:4
(w/w) and ac i a ed in he u nace a 850°C o 2 h unde
a cons an N2 low a e (10 mL min−1) using a hea ing a e
o 5°C min−1 (ACZnCl2-850). A e ac i a ion, he ob ained
ma e ial was washed wi h HCl 0.1 M unde s i ing, and
la e wi h dis illed wa e unde acuum il a ion up o neu-
al pH. Finally, he washed sample was d ied o e nigh
a 105°C [12].
2.3 Ac i a ed ca bon cha ac e iza ion
The poin o ze o cha ge (pHPZC) was e alua ed acco ding
o he me hod desc ibed by Ca abinei o e al. [31]. B ie ly,
20 mg o ACZnCl2-850 was pu in con ac wi h 20 mL o a
0.01 M NaCl solu ion a 200 pm o 48 h o assu e equi-
lib ium. The pH o solu ions was adjus ed be ween 2 and
12 by adding HCl o NaOH wi h concen a ions be ween
0.5 and 2 M.
The po e olume, su ace a ea, and s uc u al p ope ies
o ACZnCl2-850 we e cha ac e ized by N2 and CO2 adso p-
ion iso he ms using an ASAP 2020 so p ion analyze
(Mic ome i ics). N2 and CO2 adso p ion iso he ms we e
eco ded a −196°C and 0°C, espec i ely. Be o e measu e-
men s, a degassing p ocedu e was ca ied ou unde acuum
a 300°C o 1 h. N2 and CO2 su ace a eas we e de e mined
using he B unaue -Emme -Telle (BET) equa ion and he
mesopo e olume and po e size dis ibu ion we e de e mined
by Ba e -Joyne -Halenda deso p ion iso he m. To al po e
olume was de e mined a a ela i e p essu e o 0.99 and his
alue was used o calcula e mic opo e olume on he basis o
mesopo e olume neglec ing he mac opo e olume ( om he
BJH equa ion) [19].
ACZnCl2-850 was analyzed by a scanning elec on mic o-
scope coupled wi h EDX (ZEISS EVO LS 15 mic oscope) o
s udy i s mo phological ea u es and o de e mine i s elemen al
composi ion be o e and a e adso p ion. Addi ionally, Fou ie
ans o m in a ed (FTIR) spec a o ACZnCl2-850, be o e and
a e adso p ion we e eco ded wi h a VARIAN FTIR 670
spec ome e o cha ac e ize he undamen al sample unc-
ional g oups.
2.4 Adso p ion kine ic expe imen s
To op imize he adso p ion p ocess, ba ch expe imen s we e
pe o med a di e en pH (2, na u al pH, 7, 9, and 12), ini-
ial dye concen a ion (5, 25, and 500 mg L−1), and adso ben
dose (0.1–4.0 g L−1) o s udy he emo al o acid blue, ed,
and black indus ial wood dyes by he ACZnCl2-850 p epa ed.
An agi a ion speed o 210 pm and a empe a u e o 25°C
we e ensu ed by a shaking wa e ba h (H20 SOW-LAUDA).
Samples we e wi hd awn a p ede ined imes and cen i uged
(Cen onic, P Selec a) o 15 min a 4000 pm. Dye concen-
a ion was de e mined by measu ing he abso bance o he
supe na an a he maximum dye wa eleng h, λmax, using a
UV/VIS spec opho ome e (V-630, Jasco), namely 602 nm
and 572 nm o blue and black dyes, espec i ely, and o he
ed one a 506 nm om pH 2 o 7, 499 nm a pH 9 excep a
500 mg L−1 which main ains a 506 nm and 483 nm a pH 12.
The so p ion e iciency was calcula ed using Eq. (1) as
he pe cen age o dye adso bed. The amoun o dye adso bed
in mg g−1 was calcula ed ollowing Eq. (2).
whe e C0 and C a e he ini ial and esidual dye concen a-
ions (mg L−1), q is he adso p ion capaci y (mg g−1), V is
he olume o he dye solu ion (L), and m is he d y mass o
adso ben used (g).
The kine ic models applied o he s udy o he adso p ion
p ocess and he in e ac ion mechanisms we e he pseudo-
i s -o de and pseudo-second-o de kine ic models and he
in apa icula di usion model.
(1)
%
Adso p ion =
(
C
0
−C
)
∕C
0
×
100
(2)
q
=
(
C
0
−C
)
V∕
m
Biomass Con e sion and Bio e ine y
1 3
The pseudo- i s -o de kine ic model is ela ed wi h he
solid capaci y and solu ion concen a ion and is applicable
only du ing he ini ial s age o he adso p ion p ocess. I is
gi en by:
whe e q and qe a e he amoun s o dye adso bed (mg g−1) a
ime (min) and a equilib ium, espec i ely, and k1 ep e-
sen s he i s -o de a e cons an (min−1) [32].
The pseudo-second-o de kine ic model can p edic he
beha io o e he whole ange o he adso p ion p ocess,
and i is based on a solid phase so p ion, usually ela ed wi h
chemiso p ion mechanism [32, 33]. The linea ized o m o
he model is:
whe e k2 (g mg−1 min−1) is he pseudo-second-o de a e
cons an .
The in apa icle di usion model dis inguishes di e en
di usion mechanisms and usually ou s eps a e desc ibed
o con ol he a e o adso p ion p ocess. These include (1)
molecula di usion om he solu ion o a ilm laye enci -
cling he adso ben ; (2) di usion occu ing om he ilm o
pa icle su ace; (3) mig a ion inside he adso ben pa icle
by “su ace di usion” o “po e di usion”; and (4) adso p-
ion up ake by many in e ac ion mechanisms such as chem-
iso p ion, physiso p ion, complexa ion, o ion exchange. The
in apa icle di usion equa ion is as ollows:
whe e C (mg g−1) is he in e cep and kid is he a e cons an
o in apa icle di usion (mg g−1 min−1/2) [4, 34].
2.5 Adso p ion equilib ium expe imen s
Equilib ium da a we e ob ained h ough ba ch expe imen s
in which 2 g L−1 o ACZnCl2-850 o black and 4 g L−1 o
ACZnCl2-850 o blue and ed dyes we e added o 25–500 mg
L−1 o dye solu ion a pH = 2 and 25°C o he equilib ium
ime (48 h) o blue and ed dyes and 12 h o black dye o
assu e s abili y.
Langmui and F eundlich iso he m models we e
employed o analyze he expe imen al da a since hey a e he
mos commonly used o he s udy o equilib ium da a [35].
Acco ding o he Langmui ’s iso he m model, he up ake o
adso ba e molecules akes place on a homogeneous su ace
wi h a limi ed numbe o adso p ion si es gi ing ise o he
o ma ion o a monolaye and no including he in e ac ion
be ween adso bed molecules. The linea ized iso he m equa-
ion is ep esen ed by Eq. (6).
(3)
log (
q
e
−q
)
=log q
e
−k
1
∕
2.303
(4)
∕q
=
(
1∕k
2
q
2
e)
+ ∕q
e
(5)
q =
k
id
1
∕
2
+C
whe e Ce (mg L−1) is he equilib ium dye concen a ion,
qe (mg g−1) is he amoun o dye a equilib ium, qm (mg
g−1), and KL (L mg−1) a e he Langmui cons an s ela ed
o adso p ion capaci y and adso p ion ene gy, espec i ely.
Dimensionless cons an sepa a ion ac o o equilib ium
pa ame e , one o he essen ial cha ac e is ics o his model,
can be exp essed by he ollowing equa ion:
whe e C0 is he ini ial dye concen a ion (mg L−1). RL indi-
ca es i he adso p ion p ocess is i e e sible, linea , o
a o able acco dingly wi h he RL alue, RL = 0, RL = 1, and
0 < RL < 1, espec i ely.
The F eundlich iso he m model assumes ha mul ilaye
adso p ion occu s in a non-uni o m way wi h di e en alues
o hea o adso p ion and wi h a iable a ini y o he e oge-
neous su ace. The linea o m o F eundlich iso he m model
is desc ibed as ollows:
whe e KF (L mg−1) indica es he adso p ion capaci y and
1/n he adso p ion in ensi y. I 1 < n < 10 indica es ha
adso p ion is a o able, i n < 1 is i e e sible and n > 10 is
un a o able [4, 5].
2.6 S a is ical analysis
All he expe imen s we e done in iplica e and he alues
a e aged. The exis ence o signi ican di e ences among he
esul s o adso p ion e iciency o ACZnCl2-850 o all dyes
was analyzed. In his ega d, he one-way analysis o a i-
ance (ANOVA) was used, ollowed by Tukey o T3 Dunne
es acco ding o he signi ican le el. All s a is ical es s
we e pe o med a a 5% signi icance le el using IBM SPSS
S a is ics 25 so wa e.
3 Resul s anddiscussion
3.1 E ec o concen a ion on hena u al pH o dye
solu ions
Na u al pH was de e mined o each dye a concen a ions
o 5, 25, and 500 mg L−1 and changes we e no iced wi h
inc easing concen a ion, bu mo e ema kably o he ed
dye (Table1).
pH is an impo an pa ame e ha could in luence he
adso p ion p ocess and he pH e ec on he adso p ion will
be e alua ed
(6)
Ce∕qe=Ce∕qm+1∕qmKL
(7)
RL
=1∕
(
1+K
L
C
0)
(8)
ln
q
e
=ln K
F
+1∕n
(
ln C
e)
Biomass Con e sion and Bio e ine y
1 3
3.2 Ac i a ed ca bon cha ac e iza ion
be o eadso p ion
The yield o ACZnCl2-850 was calcula ed a e ac i a ion and
washing and i was 19.3±1.3% (based on pine sawdus on
d y basis).
pHPZC is he pH alue a which he inal pH is equal o
he ini ial pH and ep esen s when he ne su ace cha ge
o he adso ben is ze o. Pa icula ly, he pHPZC e lec s he
acid o basic cha ac e o he adso ben su ace since he
su ace cha ge below he pHPZC is posi i e and abo e his
alue is nega i e. Thus, pHPZC is an impo an pa ame e o
unde s and he in e acial p ope ies and in e ac ions du -
ing adso p ion s udies, indica ing he abili y o he ac i a ed
ca bon o he emo al o anionic, ca ionic, o molecula
pollu an s since pHPZC indica es he p esence o acidic o
basic su ace unc ional g oups [14, 15, 36]. Figu e1 shows
he pHPZC o he ACZnCl2-850 o be 7.8.
This alue alls in he basic egion and he alue o
pHPZC > 7 demons a es he p e alence o basic g oups
o e acidic g oups. Acid wood dyes a e nega i ely cha ged
which implies ha when he pH is lowe han pHPZC, he
ACZnCl2-850 could ha e mo e a ini y wi h dyes.
BET su ace a eas we e calcula ed om N2 and CO2
adso p ion iso he ms p esen ed in Fig.2, and wi h o al and
mesopo e (2–50 nm) olumes and a e age po e diame e
calcula ed om N2 adso p ion-deso p ion iso he m o he
ACZnCl2-850 a e shown in Table2.
Ni ogen adso p ion-deso p ion iso he ms o ACZnCl2-850
show a e e sible ype I iso he m acco ding o he IUPAC
classi ica ion, sugges ing ha he ca bon is pa icula ly
mic opo ous since he signi ican up ake o N2 a lowe
Table 1 Na u al pH o blue, black, and ed dyes a di e en concen-
a ions
Na u al pH
5 mg L−1 25 mg L−1 500 mg L−1
Blue 6.0 5.1 6.6
Black 4.8 5.7 6.7
Red 5.1 6.7 9.0
Fig. 1 De e mina ion o pHPZC o ACZnCl2-850
Fig. 2 Adso p ion iso he ms o (a) ni ogen and (b) ca bon dioxide by ACZnCl2-850

Biomass Con e sion and Bio e ine y
1 3
p essu es is cha ac e is ic o he adso p ion in mic opo es
[37]. The ini ial sha p up ake could be also ela ed wi h
he p esence o small mesopo es [16]. Type I iso he ms
can be u he subdi ided in o wo ypes, I(a) and I(b), and
om he analysis o Fig.2a, i can be concluded ha , in
his case, i co esponds o a ype I(a) iso he m which is
cha ac e is ic o ma e ials wi h po e size smalle han 1 nm
[37, 38]. Addi ionally, he low amoun o ni ogen adso bed
on he su ace in he ange o pa ial p essu es used o
BET a ea de e mina ion is ela ed o he exis ence o a
low su ace a ea (Table2). P e ious s udies ha e shown
simila o highe su ace a ea alues in ac i a ed ca bons
p oduced om wood-based biomass [10]. Fu he mo e, he
su ace analysis has been comple ed by pe o ming CO2
adso p ion (Fig.2b) since he N2 adso p ion-deso p ion
iso he ms analysis can be limi ed due o mic opo e p es-
ence and low di usi i y [39]. The high CO2 adso p ion
capaci y, in compa ison wi h simila ab ica ed ca bons
[9, 40], can be a ibu ed o he ac i a ed ca bon mic opo-
ous na u e. In addi ion, he low alue o he su ace a ea
de e mined by ni ogen adso p ion in compa ison wi h he
co esponding CO2 su ace a ea indica es an impo an
p esence o ul amic opo osi y (po e diame e lowe han
0.7 nm), oge he wi h a signi ican p esence o mic opo es
(wi h po e diame e s highe han 0.7 nm) and low amoun
o mesopo es. This po e size dis ibu ion ag ees wi h he
a e age po e size o he ca bon [41–43].
The high mic opo osi y pe cen age (91%) combined wi h
he high BET su ace a ea calcula ed wi h CO2 e eals he
impo an ole o mic opo osi y on gas adso p ion. This pe -
cen age also indica es ha he chemical ac i a ion p ocess
gene a es a high-quali y ca bonaceous ma e ial in which
mic opo es a e p edominan [25].
The ype o s a ing ma e ial and ca boniza ion and
ac i a ion condi ions a e he mos impo an ac o s ha
in luence he ea u es o ac i a ed ca bon po osi y [7].
The o al su ace a ea o ACZnCl2-850 is 471.4 m2 g−1 o
which 91% co esponds o mic opo es (< 2 nm) due o
he e ec o ZnCl2 on mic opo e o ma ion ha inhibi s
he de elopmen o a and a o s he elease o ola iles
[35, 36]. Mo eo e , as seen he ac i a ed ca bon p epa ed
wi h ZnCl2 as chemical ac i a o has a well-de eloped
mic opo osi y. A p eceding s udy on he p epa a ion o
ac i a ed ca bon om Eucalyp us camaldulensis wood
showed ha he ype o ac i a ion agen and i s concen-
a ion in luence he su ace a ea and po e olume and
he BET su ace a ea and po osi y inc eased in he sub-
sequen o de : KOH > H3PO4 > ZnCl2. ZnCl2 and i s
hyd a es mainly cause he o ma ion o small and uni-
o m size mic opo es due o i s minuscule pa icle size
[10, 16]. P e ious s udies using ag icul u al was es wi h
he same ac i a ion agen epo ed simila alues o su -
ace a ea [5, 19] and mic opo osi y [14, 44].
SEM mic og aphs o ACZnCl2-850 a e p esen ed in Fig.3.
The de elopmen o po es was obse ed in he ac i a ed ca -
bon p epa ed, sugges ing a high su ace a ea and suppo ing
he in o ma ion p o ided by he su ace a ea and po osi y
analysis. As men ioned be o e, he po es on he su ace o
he ca bon we e p obably due o he ZnCl2 e apo a ion du -
ing he ac i a ion p ocess and o he ca aly ic e ec o he
ZnCl2 on he py oly ic decomposi ion [15]. Mo eo e , he
SEM image a 1000× magni ica ion (Fig.3a) shows a p omi-
nen honeycomb s uc u e o igina ed om he cell wall o
he aw sawdus [5, 12].
The elemen al composi ion o ACZnCl2-850 is shown in
Table3. High ca bon con en is obse ed ha may be due o
he decomposi ion o cellulose and hemicelluloses (be ween
200 and 400°C) and lignin (be ween 150 and 750°C) [10,
15]. Oxygen, one o he main componen s o sawdus ,
showed he opposi e beha io . The p esence o zinc and
chlo ide a oms indica es ha du ing he ac i a ion p ocess,
ZnCl2 eac ed wi h PS e ealing he ac i a ion e iciency
[14, 45].
Up o now, a ious pape s can be ound in li e a u e
on ZnCl2-ac i a ed ca bons om biomass eeds ocks, bu
appa en ly none o hem p epa ed h ough a d y ac i a ion
p ocess by milling, bu h ough imp egna ion wi h a ZnCl2
dissolu ion [7, 11]. The e o e, i can be concluded ha he
d y ac i a ion echnique is also e icien , as e , and wi h no
o ma ion o aqueous chemical esidues. EDX esul s a e
adso p ion will be discussed in Sec ion3.3.5.
To ob ain a g ea e insigh in o he unc ional g oups
on he su ace o he ma e ial be o e and a e adso p-
ion, he FTIR spec a we e collec ed. The FTIR spec a o
ACZnCl2-850 be o e and a e dye adso p ion a e shown in
Fig.4.
The occu ence o a s ong and b oad peak a ound
3400 cm−1 which is asc ibed o a s e ching ib a ion o
hyd ogen-bonded (O-H) hyd oxyl g oups om ca boxyl,
phenols, alcohols, o su ace bonded wa e is obse ed.
The adso p ion small bands be ween 2919 and 2852 cm−1
co espond o asymme ic and symme ic C-H s e ch-
ing ib a ions o alkyl side chains. The wo small peaks
a ound 2300 cm−1 could be a ibu ed o he alkyne g oup
Table 2 Su ace cha ac e iza ion o he ac i a ed ca bon p epa ed
Pa ame e s ACZnCl2-850
BET su ace a ea N2 (m2 g−1) 471.4
BET su ace a ea CO2 (m2 g−1) 319.5
To al po e olume (cm3 g−1) 0.26
Mesopo e olume (cm3 g−1) 0.03
Mic opo e olume (cm3 g−1) 0.23
A e age po e diame e (nm) 2.26
Biomass Con e sion and Bio e ine y
1 3
(C≡C). The spec al bands a 1650 and 1540 cm−1 co -
espond o he C=C a oma ic ing. Likewise, he small
band a 1380 cm−1 is associa ed o C=C s e ching a o-
ma ic ings pola ized by oxygen a oms bound nea o he
ca bon a oms [16, 37]. Finally, a b oad band a 1047 cm−1
is assigned o C-O s e ching alcohols.
3.3 Adso p ion expe imen s
3.3.1 E ec o pH
Ini ial pH plays an essen ial ole in he adso p ion p ocess,
a ec ing no only he chemis y o dye molecules bu also
Fig. 3 SEM images o ACZnCl2-850 a (a) 1000 and (b) 5000× magni-
ica ions be o e adso p ion and a e adso p ion wi h (c) blue, (d) ed,
and (e) black wood dyes (500 mg L−1; pH = 2; adso ben dose: 0.4
g L−1 o blue, 0.5 g L−1 o ed, and 1 g L−1 o black dyes; con ac
ime: 48 h o blue and ed dyes and 12 h o black dye; empe a u e:
25 °C; agi a ion speed: 210 pm) a 5000×
Table 3 Elemen al composi ion
o dyes and ACZnCl2-850 be o e
and a e adso p ion (500 mg
L−1; pH = 2; adso ben dose:
0.4 g L−1 o blue, 0.5 g L−1
o ed, and 1 g L−1 o black;
con ac ime: 48 h o blue and
ed dyes and 12 h o black dye;
empe a u e: 25 °C; agi a ion
speed: 210 pm)
w % ACZnCl2-850 Blue dye Black dye Red dye ACZnCl2-850
+blue dye
ACZnCl2-850
+black dye
ACZnCl2-850
+ ed dye
C 90.4 62.1 27.5 49.7 90.7 92.7 93.3
O 2.6 18.4 18.4 28.3 7.2 5.2 4.1
Zn 2.3 - - - 0.6 - -
S 2.1 6.7 21.3 7.1 1.8 0.1 0.2
F 1.6 - - - - - -
Cl 0.6 0.4 - 0.9 0.9 1.8 2.3
Si 0.1 - - - 0.1 0.03 0.03
Ca 0.1 - - - 0.3 0.1 0.1
N - 7.2 - 3.4 - - -
Na - 4.8 11.1 10.1 - - -
B - 0.2 - - - - -
Cu - 0.1 1.6 0.1 0.1 0.2 0.3
P - - - 0.4 - - -
C - - 20.0 - - 0.1 -
Mg - - 0.1 - - - -
Biomass Con e sion and Bio e ine y
1 3
he ca bon su ace binding si es [4, 33]. The pH e ec was
e alua ed (Fig.5) a an ini ial dye concen a ion o 5 mg L−1,
an adso ben dose o 0.5 g L−1, and a con ac ime o 48 h
o assu e equilib ium, excep o he black dye ha was no
s able o imes g ea e han 12 h as de e mined in a p e ious
wo k (accep ed o publica ion).
Figu e5 shows ha he o al emo al o he blue dye
was achie ed a all pH alues essayed. Fo he ed dye, he
highes emo al was ob ained a pH =2 (96.8%) and na u-
al pH (96.0%) bu dec eased when pH was inc eased om
7 o 12. Rega ding he black dye, he emo al pe cen ages
we e much lowe han hose o he ed and blue ones. The
g ea es elimina ion was achie ed a pH = 2 (55.6%) and
p og essi ely ell by inc easing pH om 2 o 12. Possibly,
he inc ease o dye adso p ion a pH lowe han pHPZC is
because he ac i a ed ca bon su ace is posi i ely cha ged
and becomes highly p o ona ed; hence, acid dye anion
adso p ion inc eases h ough elec os a ic a ac ion and
hyd ogen bonding [4, 5]. The e o e, acco ding o hese
esul s, he in luence o he adso ben dose was analyzed a
he mos a o able pH, na u al pH o blue and ed dyes, and
pH= 2 o he black one.
In a p e ious s udy (accep ed o publica ion), he
emo al o wood dyes by pine (Pinus adia a) sawdus was
e alua ed as unc ion o pH. I was ound ha a an ini ial
dye concen a ion o 5 mg L−1 and he op imum condi ions
(pH=2 and an adso ben dose o 6 g L−1), adso p ion capac-
i ies o 0.70, 0.75, and 0.72 mg g−1 and emo al pe cen ages
o 100, 92.4, and 99% we e ob ained o he blue, ed, and
black dyes, espec i ely. I is impo an o poin ou ha
he alues o adso p ion pe o mance depend on he condi-
ions used and i is no possible o compa e hem di ec ly.
In addi ion, he emo al o he h ee dyes by pine sawdus
biocha a he same condi ions han o ACZnCl2-850 (5 mg
L−1, pH=2, adso ben dose o 0.5 g L−1) was analyzed and
maximum adso p ion capaci ies o 2.13, 2.55, and 1.94 mg
g−1 and emo al pe cen ages o 19.7, 25.1, and 23.2% o
blue, ed, and black dyes, espec i ely, we e achie ed. These
esul s con i m he e icacy o he ac i a ion p ocess, since
he adso p ion capaci ies o PS and biocha we e lowe han
o ACZnCl2-850 (10.15, 9.36, and 5.52 mg g−1 o blue, ed,
and black, espec i ely).
3.3.2 E ec o adso ben dose
The e ec o adso ben dose on dye adso p ion was s udied
and he esul s a e p esen ed in Fig.6. Fo he blue and ed
dyes ha showed high emo al e iciencies a na u al pH
Fig. 4 FTIR spec a o ACZnCl2-850 be o e and a e adso p ion wi h
blue, ed, and black dyes (500 mg L−1; pH = 2; adso ben dose: 0.4
g L−1 o blue, 0.5 g L−1 o ed, and 1 g L−1 o black dyes; con ac
ime: 48 h o blue and ed dyes and 12 h o black dye; empe a u e:
25°C; agi a ion speed: 210 pm)
Fig. 5 E ec o pH on adso p-
ion o blue, ed, and black
wood dyes on o ACZnCl2-850
(na u al pH: 6.0, 5.1, and 4.8
o blue, ed, and black dyes,
espec i ely; dye ini ial con-
cen a ion: 5 mg L−1; adso ben
dose: 0.5 g L−1; con ac ime:
48 h o blue and ed dyes and
6 h o black dye; empe a u e:
25°C; agi a ion speed: 210
pm). Fo each dye, di e -
en le e s indica e signi ican
di e ences (p ≤ 0.05) be ween
samples
Biomass Con e sion and Bio e ine y
1 3
and 5 mg L−1, he adso ben dose was educed o 0.1 g L−1.
Fo he black dye, gi en he poo esul s a 0.5 g L−1, he
adso ben dose was inc eased up o 1.0 g L−1.
Fo he blue colo an , a 100% emo al pe cen age was
main ained by educing he dose om 0.5 o 0.4 g L−1, bu
he e was a signi ican dec ease o 31.8% when he adso ben
dose was lowe ed o 0.1 g L−1. Rega ding he ed colo an , i
was no possible o educe he adso ben dose ensu ing he
o al emo al o dye since he pe cen age dec eased o 18.5%
and 14.0% a 0.4 g L−1 and 0.1 g L−1, espec i ely. Finally,
o he black dye, an inc ease in he adso ben dose om 0.5
o 0.75 and 1.0 g L−1 inc eased he dye emo al pe cen age
om 55.6 o 78.4% and 86.5%, espec i ely. In gene al, his
beha io can be explained by he inc ease o he adso p ion
si es a ailable wi h inc easing he adso ben dose, and by a
high a io o dye molecules o emp y si es [4, 33]. Rega ding
he equilib ium adso p ion capaci y (qmax), i dec eased om
17.2 o 10.2 mg g−1 o he blue and om 9.3 o 6.8 mg g−1
o he ed wi h inc easing he adso ben dose om 0.1 o 0.5
g L−1. Fo he black dye, qmax dec eased om 5.5 o 4.2 mg
g−1 wi h inc easing he adso ben dose om 0.5 o 1 g L−1.
3.3.3 E ec o ini ial dye concen a ion
The in luence o he ini ial dye concen a ion (25 and 500
mg L−1) on dye emo al a di e en pH was s udied a he
adso ben doses selec ed om he p e ious expe imen s
(0.5 g L−1 o he blue and he ed dyes and 1 g L−1 o he
black one) and he ob ained esul s a e p esen ed in Fig.7.
Adso p ion pe cen age dec eased wi h inc easing ini ial dye
concen a ion due o he sa u a ion o he adso ben su ace
while adso p ion capaci y inc eased due o he highe con-
cen a ion g adien which ac s as a d i ing o ce o o e come
esis ances o dye mass ans e be ween he aqueous phase
and he solid phase [46].
Wi h espec o he blue wood dye, he highes adso p-
ion pe cen ages we e ob ained a pH = 2 a bo h ini ial
dye concen a ions and he maximum adso p ion capaci y
was 6.27 and 97.56 mg g−1 o 25 and 500 mg L−1, espec-
i ely. Rega ding he ed dye, he na u al pH inc eased
signi ican ly wi h inc easing he ini ial dye concen a-
ion (Table1). The e o e, he highe emo al pe cen age
occu ed a di e en pH depending on ini ial dye concen-
a ion and dec eased wi h inc easing dye concen a ion.
I was 30% a pH = 9 o 25 mg L−1 wi h an adso p ion
capaci y o 16.46 mg g−1 and 9.1% a pH = 2 o 500
mg L−1 wi h an adso p ion capaci y o 86.61 mg g−1. As
men ioned abo e, usually when he pH is less han pHPZC,
he e will be mo e H+ ions and will balance he cha ge o
anionic g oups in he adso ben su ace a o ing anionic
adso p ion due o an elec os a ic in e ac ion and hyd o-
gen bonding. A 25 mg L−1, his beha io was no e i-
ied which sugges s ha he adso p ion p ocess is p ob-
ably asc ibed o o he mechanisms [20]. In p e ious wo k
abo e men ioned, i was ound ha he pKa o hese dyes
is a ound 10 which means ha pH = 9 is abo e he pHPZC
and below he pKa, hen, he molecules a e p o ona ed and
Lewis acid-base, π-π bond, o hyd ophobic e ec s could
also occu [47]. Fo he black dye, a 25 mg L−1, he maxi-
mum emo al (82.4%) was ob ained a pH = 2 wi h an
adso p ion capaci y o 19.71 mg g−1, ollowing he same
end wi h he pH inc ease as o he low concen a ion.
Then, he ini ial concen a ion was inc eased o 500 mg
L−1 a pH = 2 and he emo al pe cen age dec eased o
13.9% wi h an adso p ion capaci y o 62.73 mg g−1. Simi-
la esul s in which he emo al pe cen age dec eased wi h
Fig. 6 E ec o adso ben
dose on adso p ion o blue,
ed, and black wood dyes on o
ACZnCl2-850 (pH: 6.0 and 5.1 o
blue and ed dyes espec i ely
and pH= 2 o black dye; dye
ini ial concen a ion: 5 mg L−1;
con ac ime: 48 h o blue and
ed dyes and 6 h o black dye;
empe a u e: 25°C; agi a ion
speed: 210 pm). Fo each dye,
di e en le e s indica e sig-
ni ican di e ences (p ≤ 0.05)
be ween samples
Biomass Con e sion and Bio e ine y
1 3
The pseudo-second-o de kine ic model was he mos
app op ia e model o desc ibe he emo al o wood dyes
by he ac i a ed ca bon p epa ed and i was demons a ed
ha in apa icle di usion was also in ol ed in he adso p-
ion mechanism. Equilib ium da a was explained by he
F eundlich model o he blue and black dyes and by he
Langmui model o he ed dye. Fu he mo e, dye adso p-
ion by he ac i a ed ca bon is con olled by se e al mech-
anisms including elec os a ic o ces, H-bonding, po e ill-
ing, and π-π in e ac ions.
The p esen wo k e ealed ha he ac i a ed ca bon
p epa ed om pine sawdus (ACZnCl2-850), an abundan
and low-cos o ganic was e, is a p omising adso ben o
he emo al o dyes om aqueous solu ions. The use o
biomass based ac i a ed ca bons ins ead o he high-cos
comme cial ones can con ibu e o he alo iza ion o low-
cos enewable was es and o en i onmen al sus ainabili y
goals, o e ing a g een app oach o he emo al o wood
dyes. Fu he s udies mus be pe o med o explo e he
e ec o coexis ing con aminan s on dye emo al.
Supplemen a y In o ma ion The online e sion con ains supplemen-
a y ma e ial a ailable a h ps:// doi. o g/ 10. 1007/ s13399- 023- 04138-4.
Au ho s’ con ibu ion Ca a ina H. Pimen el: in es iga ion, alida ion,
isualiza ion, w i ing — o iginal d a . M. Sonia F ei e: concep ualiza-
ion, me hodology, supe ision, unding acquisi ion, w i ing — e iew
and edi ing. Diego Gómez-Díaz: concep ualiza ion, me hodology,
supe ision, unding acquisi ion, w i ing — e iew and edi ing. Julia
González-Ál a ez: concep ualiza ion, me hodology, supe ision, und-
ing acquisi ion, w i ing — e iew and edi ing.
Funding Open Access unding p o ided hanks o he CRUE-CSIC
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de Galicia, g an numbe ED431B 2020/039.
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Fig. 12 P oposed adso p ion mechanism o blue, ed, and black wood dyes on ACZnCl2-850

Biomass Con e sion and Bio e ine y
1 3
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Publishe ’s no e Sp inge Na u e emains neu al wi h ega d o
ju isdic ional claims in published maps and ins i u ional a ilia ions.