Chemical Enginee ing Jou nal 475 (2023) 146162
A ailable online 19 Sep embe 2023
1385-8947/© 2023 The Au ho s. Published by Else ie B.V. This is an open access a icle unde he CC BY license (h p://c ea i ecommons.o g/licenses/by/4.0/).
Dual c osslinking o low-me hoxyl pec in by calcium and eu opium o he
simul aneous emo al o pha maceu icals and di alen hea y me als
Ja ie Ma ínez-Sabando
a
,
b
, F ancesco Coin
a
,
b
, Juan Ca los Raposo
c
, Ai o La a˜
naga
c
,
Jo ge H. Melillo
d
, Sil ina Ce eny
a
,
d
,
*
a
Cen o de Física de Ma e iales (CSIC, UPV/EHU)-Ma e ials Physics Cen e (MPC), Paseo Manuel de La dizabal 5, San Sebas i´
an 20018, Spain
b
Depa amen o Políme os y Ma e iales A anzados: Física, Química y Tecnología, Uni e si y o he Basque Coun y (UPV/EHU), P. Manuel La diz´
abal 3, San Sebas i´
an
20018, Spain
c
SGIke , Gene al Resea ch Se ices, Uni e si y o he Basque Coun y (UPV/EHU), (48940), Spain
d
Donos ia In e na ional Physics Cen e (DIPC), San Sebas i´
an 20018, Spain
ARTICLE INFO
Keywo ds:
Pec in
Adso p ion kine ics
Wa e ea men
C osslinking
Eme ging con aminan s
ABSTRACT
The con amina ion o wa e esou ces by hea y me als and pha maceu icals is a signi ican h ea o human
heal h and he en i onmen . Pec in is a p omising ma e ial o emo ing hea y me als om wa e . Howe e , i s
emo al e icacy o o he ypes o pollu an s is limi ed. In his s udy, we de eloped a no el app oach o enhance
he emedia ion capaci y o pec in (wi h a low deg ee o me hyla ion) by c osslinking i wi h di e en agen s:
calcium, eu opium, and hei combina ion. We pe o med scanning elec on mic oscopy, in a ed spec oscopy,
and X- ay di ac ion expe imen s o unde s and he molecula s uc u e o pec in a e gela ion wi h he h ee
agen s. Ou esul s showed ha calcium, eu opium, and hei combina ion all induce he gela ion o pec in.
Howe e , he e icula ed pec in s uc u es exhibi ed signi ican s uc u al di e ences depending on he ype o
c osslinking agen used, which a ec ed he adso p ion capaci y. Speci ically, calcium ca ions pa ially o med a
c ys alline “egg-box” s uc u e, whe eas eu opium ca ions p oduced a mo e homogeneous ne wo k wi hou
c ys alline egions. The dual-c osslinking sys em comp ising calcium and eu opium ca ions esul ed in an in-
e media e ne wo k wi h bo h c ys alline and amo phous egions. Ou indings sugges ha dual-c oss-linked
pec in is a highly e ec i e adso ben o he simul aneous emo al o bo h hea y me als and pha maceu ical
p oduc s. This no el app oach o c osslinking pec in wi h mul iple agen s has he po en ial o signi ican ly
enhance i s emedia ion capaci y, o e ing a p omising solu ion o he simul aneous emo al o mul iple pol-
lu an s om wa e .
1. In oduc ion
Chemical con amina ion o wa e bodies and wa e sho ages due o
o e exploi a ion ha e inc eased he need o e ec i e wa e ea men
and decon amina ion p ocesses. Howe e , cu en me hods o emo ing
pa hogens, o ganic impu i ies, and sal s om wa e equi e la ge
amoun s o ene gy [1,2]. Mo eo e , new con aminan s appea in i e s
because o he gene a ion o new chemicals a a high a e [3]. Hence, i is
essen ial o de elop a o dable and e icien wa e ea men me hods o
add ess he g owing demand o clean wa e while conse ing ene gy.
Con en ional wa e ea men p ocesses a e o en ene gy in ensi e,
making i necessa y o explo e new and inno a i e echnologies.
Adso p ion is a p omising echnique o decon amina ing wa e and has
been used o a long ime. Compa ed wi h o he me hods, i is ela i ely
inexpensi e and can be implemen ed in wa e ea men plan s o do-
mes ic ea men s wi hou excessi e ene gy consump ion. Adso ben s
can be en i onmen ally iendly i he ma e ials used in hei ab ica ion
a e biodeg adable and do no con ain nanopa icles. The de elopmen
and implemen a ion o ecologically sus ainable adso p ion echnologies
can help o o e come he challenges o wa e pollu ion and wa e
sca ci y in a cos -e ec i e and ene gy-e icien manne .
Pec in, a polysaccha ide p esen in he cellula walls o plan s, has
been used o emedia e wa e agains mono-, di-, and i alen hea y
me als (such as Zn
2+
, Ni
2+
, and Cd
2+
) wi h high e iciency and he
* Co esponding au ho a : Cen o de Física de Ma e iales (CSIC, UPV/EHU)-Ma e ials Physics Cen e (MPC), Paseo Manuel de La dizabal 5, San Sebas i´
an 20018,
Spain.
E-mail add ess: [email p o ec ed] (S. Ce eny).
Con en s lis s a ailable a ScienceDi ec
Chemical Enginee ing Jou nal
jou nal homepage: www.else ie .com/loca e/cej
h ps://doi.o g/10.1016/j.cej.2023.146162
Recei ed 5 June 2023; Recei ed in e ised o m 5 Augus 2023; Accep ed 18 Sep embe 2023
Chemical Enginee ing Jou nal 475 (2023) 146162
2
possibili y o eusing he adso ben [4–6]. Pec in-based adso ben s mus
be insoluble o use in wa e . C osslinking wi h calcium ions is common
o achie e his, al hough o he me al ions ha e also been used. Di alen
me al ions, such as Ca
2+
, p omo e he gela ion o pec in [7]. The
c osslinks a e es ablished be ween he ca boxyla e g oups (COO
−
, i.e.,
non-me hyl-es e i ied esidues) o neighbo ing pec in chains ia he
Ca
2+
ions, which eplace he hyd ogen bonds in nea pec in. The deg ee
o c osslinking in pec in is ela ed o i s deg ee o me hoxyla ion (D
M
),
which ep esen s he p opo ion o me hyl-es e i ied ca boxyl g oups
(COOCH
3
) in compa ison o non-es e i ied ca boxyl g oups (COOH)
wi hin he poly(galac u onic) chains a he ca bon 6 posi ion. Pec in
wi h a low D
M
demons a es a mo e e icien c osslinking beha io . A
sequence o non-me hyl-es e i ied uni s be ween he 6 [8] and 20 [9] is
necessa y o he o ma ion o hese dime s. This s uc u e is called he
“egg-box” model and was i s de eloped o algina es [10] and applied
o low-D
M
pec in [11]. La e [12,13], i was sugges ed ha Ca
2+
ions
could also es ablish mono-complexes; ha is, Ca
2+
ions can in e ac wi h
single ca boxyla e g oups. Some e iews ha e summa ized he o ma-
ion o he “egg-box” model o o he possible s uc u es in pec in [7,14].
In addi ion o Ca
2+
, pec in can also be e icula ed by ca ions such as
Zn
2+
. C osslinking wi h Zn
2+
esul s in a mo e he e ogeneous ne wo k
s uc u e because i in e ac s wi h bo h ca boxyl and hyd oxyl g oups,
whe eas Ca
2+
in e ac s only wi h ca boxyl g oups [15].
The main limi a ion o pec in-based adso ben s is ha hey a e use ul
only o he emo al o hea y me als and no o he con aminan s, such as
pha maceu icals and o he eme ging pollu an s. To add ess his, we
aimed o use Eu o c osslink pec in o enhance he emo al o pha ma-
ceu ical p oduc s. Eu is a lan hanide ha eac s wi h di e en pha ma-
ceu icals [16,17]. I has been used as a luo escen p obe o de ec
hyd ochlo o hiazide (a diu e ic d ug) [17] and e acycline (TC; an
an ibio ic) [18]. The in e ac ions be ween pha maceu icals and Eu can
also be used o o he pu poses, such as ha p esen ed he ein. I is also
impo an o no e ha he acu e oxici y o Eu was ecen ly s udied by
Des e ani e al. [19]. Eu was classi ied as a “ca ego y 4
″
ma e ial, which
means ha Eu has low oxici y when used in de icien concen a ions, as
hose employed in his wo k.
This pape p esen s he inno a i e use o Eu as a c osslinking agen
o enhance he pe o mance o pec in-based adso ben s o wa e
emedia ion. We de eloped a dual-c osslinking sys em ha u ilizes bo h
Ca
2+
and Eu
3+
ions. We i s in es iga ed he iso he ms and adso p ion
kine ics o pec in c osslinked wi h Ca
2+
and Eu
3+
sepa a ely o de e -
mine whe he Eu
3+
could be an e ec i e e icula ing agen . We hen
e alua ed a dual-c osslinking sys em (Ca
2+
+Eu
3+
) and ound ha bo h
agen s e ec i ely c osslinked pec in. Addi ionally, we assessed he
swelling p ope ies and he in e ac ion be ween Eu and pha maceu icals
using luo escence spec oscopy. We also compa ed he adso p ion ca-
paci ies o he adso ben s using h ee c osslinking sys ems (Ca
2+
, Eu
3+
,
and he dual Ca
2+
+Eu
3+
sys em) o hea y me als (zinc) and wo an i-
bio ics (TC, and Cip o loxacin (CIP)). When c osslinked wi h he dual
sys em, pec in exhibi ed excellen adso ben quali ies o he simul a-
neous emo al o hea y me als and pha maceu ical p oduc s.
2. Ma e ials and me hods
2.1. Ma e ials
Low-me hoxyl pec in (D
M
=9.9%) was sou ced om He bs ei h &
Fox. Glyce ol, calcium chlo ide, sodium hyd oxide, and eu opium
chlo ide hexahyd a e (p oduc numbe 203254) we e pu chased om
Me ck and used wi hou u he pu i ica ion. Acco ding o iscosime y
measu emen s, he molecula weigh (M
w
) o pec in was 82,000 g/mol
(see Supplemen a y Ma e ial).
2.2. P epa a ion o pec in-based ilms
A 6% w/ glyce ol and 3% w/ pec in solu ion was p epa ed in an
E lenmeye lask unde igo ous magne ic s i ing a a empe a u e o
70 ℃ un il homogeneous dissolu ion (1.5 h). The solu ion was hen
ul asonica ed a 70 ℃ o 1.5 h o homogenize he solu ion and elim-
ina e bubbles. The solu ion (10 mL) was cas ed in o a Pe i dish and
d ied in a acuum o en o 12 h. Finally, hin hyd ogels we e ob ained,
d ied in a acuum o en a 40 ℃, and s o ed un il u he use.
2.3. C osslinking me hods
Th ee c osslinking p ocedu es we e pe o med o a ilm dose o 2.5
g/L.
2.3.1. Ca
2+
c osslinking
Pec in ilms we e c osslinked using calcium chlo ide (CaCl
2
). The
d ied pec in ilms we e imme sed in a 500 mg/L Ca
2+
solu ion o 40
min.
2.3.2. Eu
3+
c osslinking
Pec in ilms we e c osslinked using eu opium chlo ide (EuCl
3
). The
d ied pec in ilms we e imme sed in a 500 mg/L Eu
3+
solu ion o 40
min.
2.3.3. Ca
2+
+Eu
3+
dual-c osslinking
The d ied pec in ilms we e c osslinked wi h Ca
2+
and Eu
3+
by
imme sing hem in a 500 mg/L Ca
2+
solu ion o 40 min ollowed by
imme sion in a Eu
3+
solu ion o 40 min.
2.3.4. Pos -c osslinking p ocessing
Fo all h ee cases, a e c osslinking, he ilms we e insed i e imes
wi h 200 mL o deionized wa e o 1 h o emo e excess c osslinking
agen . Thus, we ob ained h ee pec in ilms wi h di e en c osslinking
sys ems: P–Ca (samples c osslinked by Ca
2+
), P–Eu (samples c osslinked
by Eu
3+
), and P–Ca +Eu (samples wi h dual-c osslinking by Ca
2+
and
Eu
3+
).
To s udy he c osslinking eac ion, we pe o med iso he m and
adso p ion kine ic s udies as desc ibed below.
2.4. Cha ac e iza ion echniques
2.4.1. Fou ie - ans o m in a ed spec oscopy
Fou ie - ans o m in a ed (FT-IR) spec a we e eco ded on a Jasco
FT/IR-6300 spec ome e (Japan) om 4000 o 600 cm
−1
wi h a eso-
lu ion o 4 cm
−1
and 200 scans pe spec um in he a enua ed o al
e lec ance mode. All samples we e d ied a 40 ℃ in a acuum o en
p io o measu emen s o ensu e a cons an wa e con en in all samples.
2.4.2. Scanning elec on mic oscopy
The mo phology and composi ion o he pec in-based adso ben s
we e in es iga ed using scanning elec on mic oscopy (SEM; Quan a
250 ESEM, FEI, Ne he lands) and ene gy dispe si e X- ay (EDX) spec-
oscopy (silicon d i de ec o , EDAX). The beam ol age and cu en
we e se o 10 kV and 5 pA, espec i ely. Elec ons we e de ec ed using a
la ge- ield de ec o in low- acuum mode. The chambe was e acua ed
o 80 Pa. The samples we e moun ed on an aluminum s ub using double-
sided ape.
2.4.3. X- ay di ac ion
The d ied nea pec in and c osslinked pec in ilms we e cha ac e -
ized by powde X- ay di ac ion (XRD; Philips X’pe PRO au oma ic
di ac ome e ) using Cu K
α
adia ion (λ =1.5418 Å) wi h a seconda y
monoch oma o and PIXcel solid-s a e de ec o . The ol age and cu en
we e 40 kV and 40 mA, espec i ely. The scans we e pe o med wi h a
0.026 s ep size in he 2θ ange o 2◦–80◦a oom empe a u e. The
pe cen age c ys allini y was calcula ed as (c ys alline a ea/ o al a ea) ×
100.
J. Ma ínez-Sabando e al.
Chemical Enginee ing Jou nal 475 (2023) 146162
3
2.4.4. Swelling expe imen s
The c osslinked pec in ilms we e cu , accu a ely weighed, and
placed in ials con aining 100 mL o wa e (pH =7) unde agi a ion a
oom empe a u e. The ilms we e weigh ed a ce ain in e als, a e
d ying he su ace wa e wi h il e pape . The same sample was used o
each imme sion ime. The wa e up ake pe cen age was calcula ed as
(100 ×(M
swollen
– M
d y
))/M
d y
, whe e M
swollen
is he mass o he ilm
a e he wa e imme sion, and M
d y
s ands o he mass o he d ied ilm
be o e he wa e imme sion.
2.4.5. Induc i ely coupled plasma–a omic emission and ul a iole – isible
spec ome y
Induc i ely coupled plasma–a omic emission spec ome y
(ICP–AES; Agilen 5100) was used o measu e he equilib ium concen-
a ions o hea y me als. The pha maceu ical concen a ions we e
de e mined using ul a iole – isible (UV–Vis) spec ome y (Agilen
8453A).
2.4.6. Con ac angle
Con ac Angle me e OCA 15 Plus, Da a Physics Ins umen s GmbH,
Filde s ad , Ge many, equipped wi h a high- esolu ion CCD came a and
a high-pe o mance digi izing adap e , was used o de e mine he
we abili y o pec in ilms be o e and a e emedia ion. A 3
μ
l d ople o
dis illed H
2
O was placed on o he su ace o he c osslinked pec in.
SCA20 so wa e (Da a Physics Ins umen s GmbH, Filde s ad , Ge -
many) was used o image cap u ing and con ac angle de e mina ion.
2.4.7. Pho oluminescence
Pho oluminescence spec a we e ob ained using a Ca y Eclipse
Spec opho ome e (Agilen Technologies), wi h an exci a ion wa e-
leng h o 365 nm and sli s o 5 nm.
2.5. Ba ch adso p ion expe imen s
Adso p ion iso he m and kine ic expe imen s we e pe o med o
wo pu poses. Fi s , he c osslinking eac ions wi h he h ee agen s we e
moni o ed o de e mine he maximum adso p ion capaci ies and
app op ia e c osslinking le els o each sample. Second, he expe imen s
helped o analyze he adso p ion o hea y me als and pha maceu icals.
2.5.1. C osslinking eac ions
Adso p ion iso he ms we e ob ained using adso ben s wi h di e en
ini ial Ca
2+
o Eu
3+
concen a ions in he ange o 25–1500 mg/L. A
ixed amoun o adso ben (2.5 g/L) was used o each measu emen .
Ba ch expe imen s we e pe o med in 20 mL ubes, and he equilib ium
concen a ion was measu ed a e 24 h. Kine ic adso p ion s udies we e
pe o med a a ixed c osslinking agen concen a ion (500 mg/L) o
di e en du a ions (10 o 1440 min). C osslinking solu ions we e p e-
pa ed om CaCl
2
o EuCl
3
, and he pH was main ained a 7 by adding
0.1 M NaOH.
2.5.2. Adso p ion o hea y me als and pha maceu icals
Simila expe imen s we e pe o med o s udy he adso p ion capac-
i y o Zinc, wo an ibio ics (TC, and CIP). We used 1000 mg/L o a Zn
2+
s anda d solu ion o ICP (18562, Sigma Ald ich) and deionized wa e
o zinc dilu ions. The h ee adso ben s we e c osslinked o 40 min a a
dose o 2.5 g/L.
Adso p ion expe imen s we e pe o med in ba ches using a dose o
2.5 g/L unde agi a ion a 125 pm and (25 ±1) ◦C. Adso p ion iso-
he ms we e measu ed a a ious ini ial pollu an concen a ions. Fo
he kine ic expe imen s, we measu ed he pollu an concen a ion a
di e en imes anging om 10 o 1440 min a a ixed ini ial concen-
a ion o 150 mg/L o zinc and a 50 mg/L o he pha maceu icals.
In bo h he cases, he equilib ium concen a ion (C
eq
) and concen-
a ion a ime (C
) we e measu ed using ICP–AES; o he pha ma-
ceu icals, UV–Vis was used o measu e C
eq
and C
. The amoun o
adso bed c osslinking agen o pollu an was de e mined based on he
di e ence be ween he ini ial and inal concen a ions in he solu ion.
Aliquo s we e collec ed a ce ain in e als o measu e C
, and C
eq
was
aken as he equilib ium alue o C
. The equilib ium adso p ion ca-
paci y (q
eq
; mg/g) and adso p ion a ime (q
; mg/g) we e calcula ed as
ollows:
qeq=C0−Ceq
d(1)
q =C0−C
d(2)
whe e C
0
is ini ial concen a ion o a gi en c osslinking agen o
pollu an (mg/L) and d (g/L) is he adso ben dose. The emo al e i-
ciency (R%) was calcula ed as ollows:
R%=((C0−Ceq)
C0)×100% (3)
2.5.3. Reusabili y s udies
Fo egene a ion and eusabili y s udies, pec in ilms we e subjec ed
o i e adso p ion cycles (dose o 2.5 g/L), each ollowed by one
deso p ion. Solu ions o 10 mg/L o Zinc and 50 mg/L o Te acycline o
Cip o loxacin a pH 7 we e used o adso p ion. The inal concen a ion
was measu ed a e 2 h. Deso p ion was pe o med o 1 h using ni ic
acid 0,1 M o deso b Zinc. We used o mic and ace ic acids a 1% o TC
and CIP. Deso p ion and adso p ion we e done unde o bi al shaking a
120 pm. A washing and d ying s ep in wa e be ween adso p-
ion–deso p ion is equi ed o emo e he excess eagen s.
2.6. Adso p ion da a analysis
The adso p ion iso he ms we e i ed using he Langmui (Eq. (4) and
Redlich–Pe e son (Eq. (5) models [20].
q(Ceq) = qMKLCeq
1+KLCeq
(4)
q(Ceq) = KRPCeq
1+
α
Ceqβ(5)
whe e q is he adso p ion capaci y; q
M
is he maximum adso p ion ca-
paci y; K
L
is he Langmui cons an ; and KL
RP
, β, and
α
a e Red-
lich–Pe e son cons an s. β is a dimensionless pa ame e be ween 0 and
1, ep esen ing he he e ogenei y ac o . The close he alue o ze o, he
mo e he e ogeneous he su ace whe e he adso ba e in e ac s.
The adso p ion da a we e analyzed using a nonlinea i ing o he
empi ical pseudo- i s -o de (PFO; Eq. (6)) and pseudo-second-o de
(PSO; Eq. (7)) kine ic models [21].
PFO :q =qe(1−e−k1 )(6)
PSO :q =q2
ek2
1+qek2 (7)
whe e k
1
and k
2
a e he cha ac e is ic ime- ela ed cons an s o he PFO
and PSO models, espec i ely, indica ing he adso p ion speed a he
beginning o he adso p ion p ocess.
3. Resul s and discussion
3.1. C osslinking eac ions
3.1.1. Iso he ms
This sec ion p esen s he iso he ms (24 h) o pec in c osslinked using
Ca
2+
, Eu
3+
, and he dual- Ca
2+
+Eu
3+
c osslinking sys em. He eina e ,
hese sys ems a e deno ed as P-Ca, P-Eu, and P-Ca-Eu, espec i ely.
J. Ma ínez-Sabando e al.
Chemical Enginee ing Jou nal 475 (2023) 146162
4
Fig. 1a and b show he Ca and Eu adso p ion capaci ies o pec in a e
24 h as a unc ion o he equilib ium Ca and Eu concen a ions,
espec i ely, and Fig. 1c shows he Eu adso p ion capaci y o he ilm
c osslinked wi h Ca. The adso p ion capaci ies inc eased wi h inc easing
ini ial Ca o Eu concen a ion un il a pla eau alue was eached.
As shown in Fig. 1 and Table 1, he maximum adso p ion capaci y o
pec in o Ca was app oxima ely 21 mg/g, whe eas ha o Eu was 56
mg/g. In con as , he adso p ion capaci y o he Ca-c osslinked pec in
o Eu inc eased o 140 mg/g. Thus, he abili y o adso b Eu mo e han
doubled a e Ca-c osslinking. I can be concluded ha he dual-
c osslinking sys em p omo ed he adso p ion o Eu by he pec in ilm.
The iso he ms o pec in c osslinked wi h Ca o Eu we e i ed using
he Langmui model (Table 1). This model assumes ha adso p ion oc-
cu s a iden ical si es and ha only one molecule can be adso bed a each
loca ion [20,21]. Fo he sample e icula ed wi h he dual sys em, he
iso he m was i ed using bo h he Langmui and Redlich–Pe e son
models [22]. The Redlich–Pe e son model had a be e i (R
2
=0.99),
which implies ha his sys em does no exhibi ideal monolaye
adso p ion beha io ; he adso p ion si es can be homogeneous o he -
e ogeneous and a e no iden ical [23]. This indica es ha he s uc u es
o he samples c osslinked wi h Ca o Eu di e om hose e icula ed
wi h bo h Ca and Eu. Rega ding he pa ame e s ob ained om he
i ing, he K
L
and K
RP
alues indica e ha he ex en o he in e ac ion
be ween he adso ba e and adso ben is a o able.
3.1.2. Kine ics
We analyzed he adso p ion kine ics o pec in du ing c osslinking
using he h ee agen s. Fig. 2 shows he esul s o c osslinking wi h he
h ee agen s as a unc ion o adso p ion ime. The iso he ms show ha
he Eu up ake in he dual-c osslinking sys em was e y high compa ed
Fig. 1. Adso p ion iso he ms o pec in ilms (dose o 2.5 g/L) a pH =7 using (a) Ca, (b) Eu, and (c) Ca +Eu as he c osslinking agen s. The dashed lines in panels
(a)–(c) indica e he nonlinea i ings ob ained using he Langmui model, and he do ed–dashed line in panel (c) indica es he nonlinea i ing ob ained using he
Redlich–Pe e son model.
Table 1
Calcula ed adso p ion pa ame e s o he h ee ypes o pec in ilm using he Langmui and Redlich–Pe e son adso p ion iso he m models.
Sample Model R
2
adj
K
L
[L/mg] K
RP
[L/mg] q
Maxa
[mg/g]
P–Ca Langmui 0.99 0.050 ±0.032 — 21.2 ±0.6
P–Eu Langmui 0.99 0.32 ±0.06 — 56.2 ±0.9
P–Ca-Eu Langmui 0.96 0.65 ±0.26 — 123.4 ±5.4
P–Ca-Eu
b
Redlich–Pe e son 0.99 — 139.2 ±39.7
a
q
Max
is he maximum adso p ion capaci y.
b
Fo pec in–Ca
2+
+Eu
3+
using he Redlich–Pe e son model,
α
=1.78 ±0.64 mg/L and β =0.92 ±0.02.
Fig. 2. Adso p ion kine ics o pec in ilms (dose o 2.5 g/L) a pH =7 using (a) Ca, (b) Eu, and (c) Ca +Eu as c osslinking agen s. The dashed lines indica e he
nonlinea i ings ob ained using he PSO and PFO models.
J. Ma ínez-Sabando e al.
Chemical Enginee ing Jou nal 475 (2023) 146162
5
wi h ha in he case wi h Eu alone.
To selec an app op ia e kine ic model (PFO o PSO) and unde s and
he ion adso p ion mechanism, we used h ee c i e ia: he R
2
alue
ex ac ed om he linea i ing, Akaike c i e ion, and Bayesian c i e ion
[24]. Based on he esul s (see he comple e calcula ions in he Sup-
plemen a y Ma e ial), he c osslinking eac ions o pec in wi h Ca and
he dual sys em (Ca +Eu) we e bes i ed using he PSO model, indi-
ca ing ha he adso p ion p ocess was d i en by chemiso p ion [25]. In
con as , he c osslinking eac ion wi h Eu was bes i ed using he PFO
model, indica ing ha he adso p ion p ocess was d i en by elec os a ic
in e ac ions o physiso p ion a he han chemiso p ion [26,27]. Fig. 2
and Table 2 show he i ing esul s. Thus, we can conclude ha he
samples c osslinked wi h Ca and he dual sys em (Ca +Eu) had s onge
in e ac ions be ween he ions and pec in han hose c osslinked only
wi h Eu.
3.2. Mo phology
Fig. 3a–d shows SEM images o he un ea ed and c osslinked pec in.
Mo phological changes can be obse ed a e c osslinking wi h he h ee
agen s. P–Ca had a sphe ical g anula shape wi h a homogeneous size,
whe eas P–Eu had a mo e g anula su ace wi h a p e e en ial o ien a-
ion. In con as , P–Ca +Eu p esen ed a mo e uni o m su ace han nea
pec in.
The EDX spec a in Fig. 3 indica e ha all he c osslinked samples
con ained Ca o Eu (see elemen con en in Table S2 o SM). Nea pec in
con ained Na (Fig. 3a), a o ing he e icula ion eac ion [28]. A e
c osslinking wi h Ca (Fig. 3b) o Ca +Eu (Fig. 3d), a peak was de ec ed
a 3.70 keV, indica ing he p esence o Ca, and a e c osslinking wi h Eu
(Fig. 3c) o Ca +Eu (Fig. 3d), wo peaks ela ed o Eu we e de ec ed a
1.13 and 5.8 keV. These peaks con i med he exis ence o c osslinking in
all h ee cases. The EDX spec a did no exhibi signi ican a ia ions in
di e en sec ions o he samples, indica ing a homogeneous dispe sion
o Ca, Eu, o bo h.
3.3. S uc u al p ope ies o di e en c osslinked pec in-based adso ben s
In his sec ion, we analyze he s uc u al p ope ies o he h ee
c osslinked samples. Fig. 4a shows he in a ed spec a o he nea and
c osslinked pec in samples used o de e mine he s uc u al di e ences
be ween he h ee c osslinking agen s. The in ense band in he egion o
~3600–3000 cm
−1
co esponds o he s e ching o hyd oxyl g oups
(–OH), whe eas he bands in he egion o 3000–2850 cm
−1
ep esen
me hyl ib a ions (–CH
3
). The band a ~ 1725 cm
−1
co esponds o he
C =O s e ching o nonionic ca boxyl g oups (–COOH, –COOCH
3
),
whe eas he band a ~ 1610 cm
−1
is assigned o he asymme ic
s e ching ib a ions o ca boxyla e anions (COO
−
) [29,30].
The mos signi ican changes a e c osslinking we e obse ed in he
ca boxyla e anion band (~1600 cm
−1
). In pa icula , he band a 1591
Table 2
Calcula ed kine ic pa ame e s o he h ee ypes o pec in ilm using PFO o PSO
adso p ion models.
Sample Model R
2
adj
k
1
[L/min] k
2
[g/(mg min)] q
eq
[mg/g]
P–Ca PSO 0.99 —— 0.0064 ±0.0013 21.1 ±0.5
P–Eu PFO 0.89 0.024 ±0.007 —— 58.5 ±4.0
P–Ca +Eu PSO 0.99 —— 0.0020 ±0.0004 114.7 ±2.1
Fig. 3. Scanning elec onic mic oscopy images and EDX spec a o pec in-based ilms wi h (a) nea pec in and pec in samples c osslinked wi h (b) Ca, (c) Eu, and (d)
Ca +Eu. Fig. S3 shows images o he egions used o EDX mapping.
Fig. 4. In a ed spec a o (a) nea and c osslinked pec in using h ee di e en c osslinking agen s and (b) P–Ca and (c) P–Eu ilms c osslinked o di e en du a ions
(10, 40, and 400 min).
J. Ma ínez-Sabando e al.
Chemical Enginee ing Jou nal 475 (2023) 146162
6
cm
−1
o nea pec in shi ed o 1598 cm
−1
upon c osslinking. This shi
o highe wa enumbe s has been p e iously epo ed [28] and is
because Ca ions s ongly in e ac wi h he ionized ca boxyl g oup.
Howe e , in he case o dual c osslinking, he band shi ed sligh ly o
1590 cm
−1
, and in he case o Eu c osslinking, he shi was much mo e
e iden o 1583 cm
−1
. The e o e, unlike Ca, he Eu ions in e ac less
wi h he ionized ca boxyla e g oup, which is mos no iceable in he case
o Eu alone. We con i med his esul by analyzing he c osslinking o
pec in wi h Ca and Eu sepa a ely (no in he dual-c osslinking sys em) a
di e en du a ions (10, 40, and 400 min). Fig. 4b and c show ha , e en
in he i s ew minu es o he c osslinking eac ion, he Eu ions in e ac
less wi h he ionized ca boxyla e g oup (Fig. 4c). This sugges s ha i is
di icul o a ionalize he o ma ion o an “egg-box” s uc u e when
pec in is c osslinked wi h Eu.
Fu he e idence o he s uc u e o each c osslinking sys em can be
ound in he XRD pa e ns o he pec in ilms, as shown in Fig. 5. In
pa icula , he XRD pa e ns p o ide in o ma ion abou he amo phous
and c ys alline phases in each ma e ial [31]. Fo he nea pec in ilm, he
XRD pa e n con ained sha p c ys alline peaks a 2θ alues o 12.6◦,
22.6◦, and 33.8◦, indica ing a high deg ee o c ys allini y (see he i ing
p ocedu e in Fig. 5b). Amo phous halos we e obse ed a 2θ alues o
31◦and 41.9◦, which do no p oduce a well- esol ed XRD pa e n and
ep esen he amo phous ac ion o nea pec in. F om he mos in ense
di ac ion peak, a ypical d-spacing o 0.70 nm was ob ained o he nea
pec in ilm, which ag ees wi h p e ious epo s [32].
A e Ca c osslinking, he di ac ion peaks we e b oade han hose
o he nea pec in ilm, indica ing a lowe deg ee o c ys allini y.
Howe e , peaks a 2θ alues o 14◦, 23.7◦, and 27.0◦we e obse ed wi h
amo phous halos a 2θ alues o 30◦and 40◦. Conside ing he mos
in ense di ac ion peak, a ypical d-spacing o 0.63 nm was ob ained o
he Ca-c osslinked pec in ilm, indica ing a a ia ion in he la e al
spacing be ween pec in chains be ween he un ea ed (0.70 nm) sample
and he Ca-c osslinked one (0.63 nm). This XRD pa e n has been p e-
iously epo ed o he dime iza ion o polyme chains h ough Ca
coo dina ion, acco ding o he egg-box model [33,34].
The Ca +Eu-c osslinked sample had e en lowe c ys allini y, as he
di ac ion peaks we e e en b oade han hose o he sample e icula ed
wi h Ca. Ne e heless, we could s ill dis inguish be ween he peaks a 2θ
alues o 14.5◦and 21.0◦, wi h b oad peaks a 2θ alues o 26◦and 41◦.
Conside ing he di ac ion peak a 14.5◦, a ypical d-spacing o 0.61 nm
was ob ained. Thus, he in e laye dis ance sys ema ically dec eased o
he nea pec in, P–Ca, and P–Ca +Eu samples, while he peaks became
b oade and less in ense. Mo eo e , using he in eg a ion me hod, we
es ima ed he c ys alliza ion pe cen ages o be 34%, 21%, and 15% o
nea pec in and he Ca and Ca +Eu e icula ed samples, espec i ely.
This indica es ha a e adding Eu, he sample los hal i s c ys alline
phase, and g adual amo phiza ion o he pec in was obse ed when he
c osslinking changed om Ca o Ca +Eu.
In con as , a e c osslinking pec in wi h Eu alone, he XRD pa e n
only showed amo phous halos cen e ed a 2θ alues o 21◦and 42◦,
indica ing highly de ec i e c ys als, i any. In addi ion, he c ys alline
ac ion was educed o ze o. Thus, c osslinking wi h Eu alone induced
comple e amo phiza ion o he pec in; consequen ly, we could no ind
any signs o he “egg-box” model when he sample was c osslinked wi h
Eu.
In he absence o di alen ca ions, pu e pec in chains in e ac ia
hyd ogen bonds [9]. In he p esence o Ca ions, he pec in chains
in e ac ia nonco alen Ca b idges be ween he ca boxyl g oups o wo
di e en chains (dime iza ion [35]) and he hyd ogen bonds ha emain
in he ne wo k [36]. These esul s we e e lec ed in he FT-IR and XRD
Fig. 5. (a) Compa ison o he XRD pa e ns o pec in adso ben s using di e en c osslinking agen s. (b–d) XRD pa e ns o nea and c osslinked pec in ilms.
Expe imen al da a a e indica ed by poin s, and i ings o he expe imen al da a wi h he pseudo Voig p o iles a e indica ed by solid lines.
J. Ma ínez-Sabando e al.
Chemical Enginee ing Jou nal 475 (2023) 146162
7
esul s. When Eu a oms en e he P–Ca la ice, hey in oduce de ec s
in o he pe iodic ne wo k, as e idenced by he loss o c ys allini y and
educed in e ac ion be ween he ca boxyla e g oups. Howe e , a his
poin , we could no con i m whe he , o his sample, he e was only a
b eak o pe iodici y when Eu en e ed he ne wo k o whe he he e was
a disassembly o he “egg-box” s uc u e.
To assess his u he , we pe o med ICP–AES expe imen s o analyze
he elease o Ca ions when he sample was u he c osslinked wi h Eu.
A e Ca c osslinking, he P–Ca ilms we e washed ex ensi ely (100 mL
o wa e , 1 h each unde agi a ion, i e imes). The wa e om each
wash cycle was analyzed by ICP o de e mine whe he some o he Ca
ions we e eleased (i.e., no bound o he pec in chains). Fu he mo e,
we in es iga ed whe he , in he dual-c osslinking sys em, he e was ion
exchange be ween Ca and Eu (i.e., o de e mine i , du ing Eu c oss-
linking, some o he Ca ions we e eleased om he sample). The esul s
showed ha , du ing Eu c osslinking, almos 70% o he Ca ions we e
eleased. The e o e, Eu disassembled much o he “egg-box” s uc u e, as
mos o he Ca ions we e los . Fo example, conside ing P–Ca c osslinked
o 40 min, he Ca up ake was 2.69 ×10
−5
mol, and du ing Eu c oss-
linking, he sample eleased 1.77 ×10
−5
mol Ca. This indica ed ha he
sample los 67% o he Ca ions.
The sample c osslinked wi h only Eu los all i s c ys allini y, indi-
ca ing an amo phous sample wi h less in e ac ion be ween he ca boxyl
g oups. The es ablished c osslinks could be poin -like in e ac ions be-
ween wo o h ee chains, enabling he o ma ion o a s able 3D
ne wo k. Fig. 6 schema ically shows he expec ed s uc u es o pec in
using h ee di e en c osslinking agen s based on he SEM, XRD, and FT-
IR esul s. We expec ed an egg-box model o P–Ca, in which wo
ca boxyl g oups a e a ached o each Ca ion. Con e sely, Eu o med an
amo phous ne wo k wi h andom c osslinking poin s, as shown in
Fig. 6c. Finally, he dual-c osslinked sample wi h Eu and Ca exhibi ed
bo h c ys alline and amo phous phases, as shown in Fig. 6b.
3.4. Swelling o pec in adso ben s
The di e se s uc u es o he di e en c osslinking sys ems also
a ec ed he swelling p ope ies (Fig. 7). This p ope y is ele an in
wa e ea men because pollu an s each adso p ion si es a e di using
Fig. 6. Schema ics o h ee c osslinked sys ems: (a) P–Ca (a), (b) P–Ca +Eu, and (c) P–Eu.
Fig. 7. (a) Swelling p ope ies o pec in c osslinked wi h Ca, Eu, and Ca +Eu. (b) Con ac angle o pec in-based adso ben s.
Fig. 8. Kine ics o Zn
2+
so p ion by pec in c osslinked wi h Ca, Eu, and Ca +
Eu. Adso ben dose: 2.5 g/L, ini ial Zn
2+
concen a ion: 150 ppm. Dashed lines
ep esen he i ing o he PSO model (see Table 3).
J. Ma ínez-Sabando e al.
Chemical Enginee ing Jou nal 475 (2023) 146162
8
in o he ma ix. As swelling depends on he sample size, all he mea-
su emen s (a e age o h ee uns) we e conduc ed using samples wi h
simila dimensions ( hickness 0.154 ±0.031, 0.157 ±0.015, and 0.141
±0.024 mm o P–Ca, P–Eu, and P–Ca +Eu, espec i ely).
The P–Ca ilms eached wa e up ake equilib ium a e 2 h, whe eas
he P–Eu and P–Ca +Eu ilms eached equilib ium e y quickly (a e
15 min). The P–Ca ilm was also he mos swollen sys em, whe eas P–Eu
and P–Ca +Eu had less a ini y o wa e . This indica es ha he e we e
mo e a ailable hyd oxyl, ca boxyl, and ca boxyla e g oups in Ca-
c osslinked pec in o es ablish H-bonds wi h wa e han in he Eu and
Ca +Eu ilms.
In addi ion, o assess he hyd ophilici y beha io o he ilm su aces,
we s udied he con ac angles o he P-Ca and P-Ca-Eu ilms (see Fig. 7b).
Ini ially, bo h ilms exhibi ed simila con ac angles, app oxima ely 40◦.
This indica es compa able hyd ophobici y in hei d y s a e. I has been
epo ed ha su ace oughness a ec s he con ac angle o su aces.
Mo eo e , an inc ease in su ace oughness can lead o highe hyd o-
phobici y. In Fig. 3, we obse ed ha P-Ca exhibi ed a sphe ical g anula
shape, sugges ing ha i s mo phology migh con ibu e o i s seemingly
highe hyd ophobici y han eali y. Howe e , a e 10 min, he con ac
angle o P-Ca-Eu was highe (25.3◦) han ha o P-Ca (19.7◦). This
esul indica es ha he wa e abso p ion capaci y is highe o P-Ca
ilms, which con i ms he esul s in Fig. 7a.
3.5. Kine ics o Zn
2+
and pha maceu ical so p ion on h ee e icula ed
pec in adso ben s
This sec ion ocuses on he applica ion o he de eloped adso ben s
o wa e emedia ion. The pH o he con amina ed wa e was main-
ained a 7 in he expe imen s because his is i s na u al pH. Pec in, wi h
a low deg ee o es e i ica ion and c osslinking wi h Ca, is an excellen
adso ben o mono- and di alen hea y me als [4–6]. Howe e , o he
bes o ou knowledge, he e a e no epo s on he adso p ion o o he
eme ging con aminan s, such as pha maceu icals using pec in-based
adso ben s. I is also impo an o no e ha we ha e also analyzed he
possible Eu elease in o he wa e be o e and a e adso p ion by ICP-
AES. The esul s showed ha no Eu was eleased in o he wa e .
3.5.1. Non-simul aneous emo al o hea y me als and pha maceu icals
Fig. 8 shows he kine ic esul s ( i ed wi h he PSO model; Table 3)
o he adso p ion o Zn
2+
using he h ee adso ben s. The maximum
emo al e iciency dec eased om 75% o 47.2% and 37.9% o Ca, Ca
+Eu, and Eu, espec i ely. This was expec ed because he e we e ewe
Ca ions in he P–Ca +Eu sample han in he P–Ca sample. The P–Eu
sample was also able o adso b Zn, al hough i s emo al e iciency was
lowe han ha o he P–Ca adso ben . The eason he P–Eu sample can
adso b Zn is unclea , bu we expec i is because Zn can also c osslink
pec in and no because o a s ong in e ac ion be ween Eu and Zn.
Fu he s udies will be equi ed o con i m his hypo hesis.
Fig. 9 shows he adso p ion kine ics o he c osslinking sys ems o
wo pha maceu ical p oduc s: TC and CIP. P–Ca did no exhibi any
adso p ion capabili y o an ibio ics. Gi en he high a ini y o Eu o
bo h TC and CIP [37,38], i s emo al abili y is expec ed o be high.
Indeed, he P–Eu adso ben had good so p ion abili y o TC and CIP,
bu i is s ill highe o he P-Ca-Eu adso ben . This is because he Eu
concen a ion was highe in he P–Ca +Eu sample han in he P–Eu
sample (concen a ions o Eu we e de e mined by ICP-AES in sec ion
3.1). Table 4 shows he i ing pa ame e s co esponding o he PFO and
Table 3
PSO kine ic pa ame e s o Zn
2+
so p ion in o P–Ca, P–Eu, and P–Ca +Eu. MRE
ep esen s Maximum emo al e iciency.
Sample R
2
adj
K
2
[g/(mg min)] q
max
[mg/g] MRE[%]
P–Ca 0.96 0.0026 ±0.0007 44.6 ±1.6 75.0
P–Ca þEu 0.98 0.0032 ±0.0006 26.9 ±0.7 47.2
P–Eu 0.99 0.0008 ±0.0002 22.4 ±0.9 37.9
Fig. 9. Adso p ion kine ics o P–Eu and P–Ca +Eu o (a and b): cip o loxacin (CIP) a a dose o 2.5 g/L and 1 g/L espec i ely; (c and d): e acycline (TC) a a dose
o 0.5 g/L and 2.5 g/L, espec i ely. Ini ial adso ba e concen a ion: 50 ppm. Dashed lines ep esen s he i ing o he PFO and PSO models (see Table 4).
J. Ma ínez-Sabando e al.
Chemical Enginee ing Jou nal 475 (2023) 146162
9
PSO models o samples wi h a dose o 0.5 g/L. Rema kably, he
maximum emo al e iciency, wi h a dose o 2.5 g/L, is high, 91 and 81
o CIP and TC, espec i ely.
Finally, we also s udied he in e ac ion mechanism be ween TC and
CIP wi h P-Ca-Eu and P-Eu adso ben s. I has been p e iously s udied
ha TC can o m s able complexes wi h Eu ia he β-dike one s uc u e
p esen in TC [39]. This complexa ion shows pho oluminescence ea-
u es, and he e o e he Eu-TC in e ac ion can be de ec ed by luo es-
cence spec oscopy.
Fig. 10 shows he emission spec a gene a ed by Eu and i s complexes
wi h TC and CIP adso bed in P-Eu and P-Ca-Eu ilms (exci a ion wa e-
leng h 365 nm). Eu shows a luo escence cha ac e is ic peak a 615 nm
[40], as seen in he spec a co esponding o he ilms wi hou
an ibio ics. A e adso p ion, he signi ican enhancemen o he emis-
sion spec um a 615 nm indica es he o ma ion o he complexbe ween
Eu and β-dike one g oup on TC o CIP [41,42].
3.5.2. Compe i i e adso p ion: Simul aneous emo al o pha maceu icals
and Zn
2+
Typically, adso p ion p ocesses a e s udied using a single pollu an .
Howe e , wa e is usually pollu ed wi h se e al con aminan s [43],
which can ha e a nega i e o posi i e ole in he adso p ion p ocess. Fo
example, hea y me als can compe e wi h pha maceu icals o adso p-
ion si es [44]. In he p esen case, we expec he hea y me als o
in e ac wi h Ca ions and he pha maceu icals o in e ac wi h Eu
3+
o
COO
−
g oups.
Table 4
PFO kine ic pa ame e s o TC and CIP so p ion in o P–Eu and P–Ca +Eu ilms a a dose o 0.5 g/L.
Sample Ads
a
Model R
2
adj
k
1
[L/min)] k
2
[g/(mg min)] q
max
[mg/g] MRE
b
[%]
P–Eu CIP PFO 0.98 0.008 ±0.002 17.4 ±1.7 96
P–Eu TC PSO 0.89 0.0008 ±0.0002 14.5 ±1.5 48
P–Ca +Eu CIP PFO 0.99 0.0030 ±0.0004 71 ±3 91
P–Ca +Eu TC PSO 0.99 0.00008 ±0.00001 34.6 ±1.4 81
a
Ads: adso ba e.
b
MRE: maximum emo al e iciency (C
0
=50 mg/L).
Fig. 10. Emission spec a gene a ed by Eu and i s complexes wi h TC and CIP adso bed in (a) P-Eu and (b) P-Ca-Eu ilms (pho os show he P-Eu and Pe-Ca-Eu ilms
be o e and a e TC and CIP adso p ion unde 365 nm UV i adia ion). (c) Ca oon o he p oposed mechanism o he in e ac ion be ween TC and Eu o CIP and Eu.
Fig. 11. (a) Adso p ion iso he ms and (b) kine ics o he simul aneous emo al o Zn
2+
and TC by P–Ca +Eu. Adso ben dose: 2.5 g/L, ini ial Zn
2+
and TC con-
cen a ion: 30 ppm. Dashed lines ep esen he i ing o he Langmui model (a) and PSO model (b) (see Table 5).
J. Ma ínez-Sabando e al.
