Ci a ion: Pe sano, F.; Mali es a, C.;
Mazzo a, E. Cellulose-Based
Hyd ogels o Was ewa e T ea men :
A Focus on Me al Ions Remo al.
Polyme s 2024,16, 1292. h ps://
doi.o g/10.3390/polym16091292
Academic Edi o s: Ana-Mihaela
Ga ila, Ma iana Ioni a and Gab iela
Toade
Recei ed: 9 Ap il 2024
Re ised: 24 Ap il 2024
Accep ed: 26 Ap il 2024
Published: 5 May 2024
Copy igh : © 2024 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
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polyme s
Re iew
Cellulose-Based Hyd ogels o Was ewa e T ea men : A Focus
on Me al Ions Remo al
F ancesca Pe sano , Cosimino Mali es a and Elisabe a Mazzo a *
Labo a o y o Analy ical Chemis y, Depa men o Biological and En i onmen al Sciences and
Technologies (Di.S.Te.B.A.), Uni e si y o Salen o, ia Mon e oni, 73100 Lecce, I aly;
[email p o ec ed] (F.P.); [email p o ec ed] (C.M.)
*Co espondence: [email p o ec ed]
Abs ac : The apid wo ldwide indus ial g ow h in ecen yea s has made wa e con amina ion
by hea y me als a p oblem ha equi es an immedia e solu ion. Se e al s a egies ha e been
p oposed o he decon amina ion o was ewa e in e ms o hea y me al ions. Among hese, me hods
u ilizing adso ben ma e ials a e p e e ed due o hei cos -e ec i eness, simplici y, e ec i eness,
and scalabili y o ea ing la ge olumes o con amina ed wa e . In his con ex , hea y me al
emo al by hyd ogels based on na u ally occu ing polyme s is an a ac i e app oach o indus ial
was ewa e emedia ion as hey o e signi ican ad an ages, such as an op imal sa e y p o ile, good
biodeg adabili y, and simple and low-cos p ocedu es o hei p epa a ion. Hyd ogels ha e he
abili y o abso b signi ican olumes o wa e , allowing o he e ec i e emo al o he dissol ed
pollu an s. Fu he mo e, hey can unde go su ace chemical modi ica ions which can u he imp o e
hei abili y o e ain di e en en i onmen al pollu an s. This e iew aims o summa ize ecen
ad ances in he applica ion o hyd ogels in he ea men o hea y me al-con amina ed was ewa e ,
pa icula ly ocusing on hyd ogels based on cellulose and cellulose de i a i es. The epo ed s udies
highligh how he adso p ion p ope ies o hese ma e ials can be widely modi ied, wi h a wide
ange o adso p ion capaci y o di e en hea y me al ions a ying be ween 2.3 and 2240 mg/g. The
possibili y o de eloping new hyd ogels wi h imp o ed so p ion pe o mances is also discussed in
he e iew, wi h he aim o imp o ing hei e ec i e applica ion in eal scena ios, indica ing u u e
di ec ions in he ield.
Keywo ds: cellulose-based hyd ogels; hea y me al adso p ion; was ewa e ea men s; adso p ion
capaci y; me al ion emo al
1. In oduc ion
G owing indus ial ac i i y oge he wi h he educed a ailabili y o d inking wa e
sou ces ha e made wa e pollu ion a se ious p oblem ha equi es new e ec i e solu ions.
The indus y is esponsible o app oxima ely 20% o global wa e consump ion, con inu-
ously eleasing a ious pollu an s in o wa e sys ems, hus esul ing in i epa able damage
o ecosys ems. Con aminan s, including hea y me als, a e con inuously eleased in o
he en i onmen h ough was ewa e esul ing om human and indus ial ac i i ies [
1
].
The p esence o hea y me al ions in was ewa e ine i ably leads o hei en y in o he
su ounding en i onmen and ecological cycles, being subsequen ly abso bed by animals,
mic oo ganisms, and plan s.
Hea y me al ions, such hose o lead, coppe , a senic, cadmium, and nickel, end o
accumula e in li ing o ganisms and, due o hei oxici y, high pe meabili y, and lack o
deg adabili y, lead o a se ies o physiological diso de s, causing al e a ions in de elopmen ,
cons i u ing a se ious h ea o human heal h [
2
–
6
]. In ac , e idence accumula ed o e
he yea s links hea y me als o a ious diseases ha a lic he human popula ion, such as
cance , anemia, and hype ension, as well as damage o he ne ous sys em and kidney
Polyme s 2024,16, 1292. h ps://doi.o g/10.3390/polym16091292 h ps://www.mdpi.com/jou nal/polyme s
Polyme s 2024,16, 1292 2 o 29
ailu e. I ollows ha e ec i e emo al o hea y me al ions om was ewa e be o e hei
elease in o wa e bodies would b ing bene i s o bo h human heal h and he en i onmen
i sel [7,8].
Fu he mo e, al hough he ea h is co e ed by 70% wa e , 97% is sal y and only 3% is
a ailable o human use. The limi ed a ailabili y o wa e o human use, along wi h he
la ge olumes o con amina ed wa e esul ing om a ious indus ial ac i i ies highligh
he need o ad anced ea men s in he decon amina ion o was ewa e .
Mos con en ional s a egies used o was ewa e ea men , such as chemical p e-
cipi a ion, memb ane sepa a ion, ion exchange, and elec ochemical ea men s, p esen
limi a ions which make hei scaling up di icul , hus discou aging hei applica ion in
eal con ex s in which la ge wa e olumes ha e o be ea ed, p e e ably wi h simple and
low-cos p ocedu es based on eusable ma e ials, allowing o educ ions in he equi ed
ime and cos s [9–13].
Chemical p ecipi a ion, o example, despi e being an economical me hod, equi es
addi ional cos s o he disposal o he la ge quan i ies o sludge p oduced du ing he
pu i ica ion p ocess i sel [
14
]. Memb ane sepa a ion is a p essu e ea men me hod whose
e icacy s ongly depends on he selec ion o he memb ane, and la ge-scale applica ion is
complica ed. This p ocess is based on he p inciple o selec i e pe meabili y o memb anes,
which allows o he selec i e passage o ce ain componen s while hinde ing o p e en -
ing he passage o o he componen s. Some memb anes may no be able o selec i ely
sepa a e all ypes o hea y me als p esen in wa e . Ion exchange in ol es he use o
solid su aces ha ac as ion exchange s and a e cha ac e ized by high selec i i y owa d
ce ain ionic species, meaning ha only ce ain ypes o ions can be adso bed [
15
,
16
]. This
can be a disad an age as mul iple s eps o di e en ma e ials a e equi ed o e ec i ely
ea a mix u e o hea y me als, making he p ocess mo e complex and expensi e. Elec-
ochemical ea men s, including elec odeposi ion, elec ocoagula ion, elec o lo a ion
and elec ooxida ion, ha e he ad an age o possessing a sho p ocess ime and a simple
ope a ion, wi h he possibili y o ully au oma ion wi h no addi ional chemicals equi ed.
None heless, he side eac ion o hyd ogen gene a ion and oxygen gene a ion nega i ely
a ec ing he e iciency o he p ocess ha e o be aken in o accoun in elec odeposi ion, as
he high ene gy consump ion a ec s mainly he elec ocoagula ion p ocess, while mos o
he p ocesses su e om elec ode passi a ion, esul ing in a subsequen need o pe iodic
eplacemen [17,18].
In con as , adso p ion-based s a egies ep esen simple, low-cos , easy o implemen ,
and e ec i e me hods in he emo al o hea y me al ions, which jus i y he g owing
esea ch in e es owa d his app oach du ing ecen yea s. In he adso p ion me hod, he
ea ed aqueous ma ix is simply s i ed in he p esence o an adso ben able o bind o
con aminan s due o he p esence o su ace unc ional g oups ac ing as ac i e si es, un il
he sa u a ion o he binding si es is eached [
19
]. To achie e an e icien and con enien
adso p ion p ocess, desi ed adso ben p ope ies a e a la ge su ace a ea, nume ous ac i e
si es, biodeg adabili y, and easy assembly and a ailabili y. T adi ional adso ben s, such
as a i icial esin, ac i a ed ca bon, zeoli e, dia omaceous ea h, alumina, and silica gel,
al hough widely used in he las ew yea s, a e a ec ed by educed adso p ion capaci y
and poo egene a ion pe o mance [20,21].
In his con ex , po ous adso ben s wi h a h ee-dimensional s uc u e and la ge su ace
a ea enabling he exposu e o nume ous ac i e si es a e ecen ly eme ging, due o hei
g ea e adso p ion capaci y and educed adso p ion ime achie ed by apid di usion kine -
ics h oughou he adso ben . The e o s o inc easing he numbe o su ace unc ional
g oups and con olling he su ace a ea o imp o ing he adso p ion p ocess ha e led
o he design o ailo ed po ous ma e ials such as me al o ganic amewo ks (MOFs) o
co alen o ganic amewo ks (COFs), al hough hei applica ion in wa e ea men s ill
equi es u he s udies o inc ease hei s abili y and dec ease p oduc ion cos s, along wi h
a conduc ing c i ical e alua ion o hei oxici y.
Polyme s 2024,16, 1292 3 o 29
Among po ous adso ben ma e ials, hyd ogels occupy a key ole, combining a high
adso p ion capaci y wi h eusabili y, biodeg adabili y, and non- oxici y, educing en i on-
men al impac and p omo ing sus ainable p ac ices in wa e ea men . Hyd ogels a e
so , elas ic ma e ials made o h ee-dimensional polyme ne wo ks wi h he ex ao dina y
abili y o s o e la ge amoun s o wa e (up o hund eds o imes wi h espec o hei d y
weigh ) hanks o hei hyd ophilic polyme s uc u e, all ea u es explaining hei po en ial
and hei inc easing applica ions as adso ben s o wa e emedia ion [
22
,
23
]. Among he
wide pano ama o polyme s used o hyd ogel de elopmen , cellulose—a well-known
polysaccha ide made up o glucose uni s held oge he by ß-1,4-glucosidic bonds—occupy
a p ominen ole [
24
,
25
], being he mos abundan na u al polyme on ea h ( he main
componen o plan cell walls) wi h high biocompa ibili y, biodeg adabili y, low cos , and
con ollable p ope ies. Rela ed o his, cellulose, i s de i a i es, and nanocellulose-based
hyd ogels ha e been con enien ly exploi ed in a ple ho a o uses anging om he biomed-
ical ield (adminis a ion o he apeu ic agen s, in issue enginee ing, implan ma e ials,
and wound d essings) o ene gy and en i onmen al applica ions [
26
–
28
]. Speci ically, he
p esence o nume ous su ace unc ional g oups capable o coo dina ing me al ions in
wa e , oge he wi h he possibili y o easily ob aining an in e connec ed po ous s uc u e,
has p omo ed he applica ion o cellulose and i s de i a i es in he p oduc ion o hyd ogels
o wa e decon amina ion in e ms o hea y me als [29,30].
The p esen e iew ocuses on he main de elopmen s in he ield, p esen ing i s
he main manu ac u ing me hods o cellulose-based hyd ogels, hen desc ibing he mecha-
nisms in ol ed in hea y me al adso p ion p ocesses, and highligh ing some ecen selec ed
applica ions. P ospec i e u u e ends in he ield a e also discussed, along wi h indi-
ca ions o possible di ec ions o ake o b idge he gap be ween labo a o y esea ch and
p ac ical applica ion and ully exploi he e ec i eness o cellulose-based hyd ogels and
hei de i a i es in he emo al o me al ions om wa e .
2. Syn hesis o Cellulose- and Cellulose De i a i e-Based Hyd ogels
2.1. S uc u e o Cellulose and I s De i a i es
Cellulose hyd ogels can be de eloped om a pu e cellulose solu ion h ough physical
c oss-linking hanks o he p esence o a many hyd oxyl g oups (–OH) ha allow he
polyme ne wo k o be connec ed by hyd ogen bonding. The –OH g oups can be unc-
ionalized wi h a ious chemical modi ica ions [
31
–
39
], o en consis ing o pa ial o o al
e he i ica ion, h ough eac ions wi h o ganic species, such as e hyl and me hyl uni s [
40
].
Cellulose e he s, based on hei subs i uen s, can be classi ied as single e he s o mixed
e he s. Single cellulose e he s in ol e he subs i u ion o a single unc ional g oup in he cel-
lulose uni , while mixed e he s in ol e he subs i u ion o mo e han one unc ional g oup.
Indi idual e he s include alkyl e he s such as me hyl cellulose (MC) and hyd oxyalkyl
e he s such as hyd oxyp opyl cellulose, while mixed e he s a e mainly ep esen ed by
ca boxyme hylcellulose (CMC), hyd oxye hyl cellulose (HEC), and hyd oxyp opyl me hyl
cellulose [41,42] (Figu e 1).
Cellulose e he can be soluble in wa e depending on he chemical s uc u e o he
subs i uen as well as he ype and deg ee o subs i u ion. Typically, wa e -soluble cellulose
e he s a e cha ac e ized by a deg ee o subs i u ion be ween 0.4 and 2 [
43
]. MC is he
simples alkyl e he , soluble in a ious o ganic sol en s and in wa e depending on he
deg ee o subs i u ion. Speci ically, o a deg ee o subs i u ion be ween 1.4 and 2.0, MC
is soluble in wa e [
44
–
46
]. In e es ingly, MC has he mo-gelling p ope ies which can be
e ec i ely exploi ed in p omo ing empe a u e-con olled hyd ogel beha io [
47
]. CMC
is cha ac e ized by hyd ophilici y, s abili y in wa e , ele a ed chemical s abili y, biocom-
pa ibili y, and biodeg adabili y. Howe e , CMC is no soluble in some o ganic sol en s
such as e hanol [
48
]. HEC u ns ou o be soluble in wa e and a ious o ganic sol en s. In
addi ion, HEC can be blended wi h cellulose, allowing cellulose o be p ocessed wi hou
he use o sol en s [49].
Polyme s 2024,16, 1292 4 o 29
Polyme s 2024, 16, x FOR PEER REVIEW 4 o 31
sol en s. In addi ion, HEC can be blended wi h cellulose, allowing cellulose o be
p ocessed wi hou he use o sol en s [49].
One ascina ing ea u e o cellulose-based hyd ogels is hei biodeg adabili y, being
deg adable by ungi and bac e ia p esen in wa e , ai , and soil, con a ily o ac yla e-
based hyd ogels, ypically used a indus ial le el, which show slow deg ada ion, a e non-
biodeg adable, and a e he e o e conside ed po en ial pollu an s o wa e and soil. S o ing
cellulose in soil o 10 weeks leads o i s deg ada ion by 82–95%, depending on he ype
o soil [50,51]. When applied in decon amina ion p ocesses, such a ea u e ep esen s an
addi ional bene i allowing us o educe he en i onmen al impac o esidual ma e ials
and hus imp o ing he en i onmen al sus ainabili y o he whole p ocess.
Figu e 1. Gene al chemical s uc u e o e he cellulose. Func ional g oups (R) o he main cellulose
e he s a e epo ed.
The app oaches o p epa ing cellulose-based hyd ogels can be essen ially di ided
in o wo ypes, namely, physical and chemical c oss-linking me hods [30,52].
2.2. Physically C oss-linked Hyd ogels
Physically c oss-linked hyd ogels a e e e sible hyd ogels, assembled h ough he
in e wea ing o polyme chains by means o in a- and in e molecula hyd ogen bonding,
ionic bonding, hyd ophobic in e ac ion, and non-pe manen o ces, such as elec os a ic
and g a i a ional o ces [53,54]. They a e syn hesized wi hou he use o chemical c oss-
linking agen s, which, being oxic in mos cases, should be comple ely emo ed om he
sys em be o e use. The e o e, he esul ing hyd ogels possess inc eased biocompa ibili y
and educed o e en no oxici y [55,56].
One o he mos widely used me hods o physical c oss-linking o he p oduc ion o
cellulose-based hyd ogels wi h ele a ed abso bency is he c yogenic me hod ( epea ed
eeze– haw cycles) [57]. This p ocess leads o he o ma ion o hyd ogels due o phase
sepa a ion in he polyme solu ion a e epea ed eezing and hawing cycles. Thus, he
o ma ion o polyme c ys alli es occu s when polyme solu ions a e exposed o low
empe a u es, c ea ing insoluble polyme ne wo ks. Fu he mo e, eeze– haw cycles
allow a po ous s uc u e o be c ea ed in hyd ogels hanks o he space le by ice c ys als
Figu e 1. Gene al chemical s uc u e o e he cellulose. Func ional g oups (R) o he main cellulose
e he s a e epo ed.
One ascina ing ea u e o cellulose-based hyd ogels is hei biodeg adabili y, being
deg adable by ungi and bac e ia p esen in wa e , ai , and soil, con a ily o ac yla e-
based hyd ogels, ypically used a indus ial le el, which show slow deg ada ion, a e
non-biodeg adable, and a e he e o e conside ed po en ial pollu an s o wa e and soil.
S o ing cellulose in soil o 10 weeks leads o i s deg ada ion by 82–95%, depending on he
ype o soil [
50
,
51
]. When applied in decon amina ion p ocesses, such a ea u e ep esen s
an addi ional bene i allowing us o educe he en i onmen al impac o esidual ma e ials
and hus imp o ing he en i onmen al sus ainabili y o he whole p ocess.
The app oaches o p epa ing cellulose-based hyd ogels can be essen ially di ided
in o wo ypes, namely, physical and chemical c oss-linking me hods [30,52].
2.2. Physically C oss-Linked Hyd ogels
Physically c oss-linked hyd ogels a e e e sible hyd ogels, assembled h ough he
in e wea ing o polyme chains by means o in a- and in e molecula hyd ogen bonding,
ionic bonding, hyd ophobic in e ac ion, and non-pe manen o ces, such as elec os a ic
and g a i a ional o ces [
53
,
54
]. They a e syn hesized wi hou he use o chemical c oss-
linking agen s, which, being oxic in mos cases, should be comple ely emo ed om he
sys em be o e use. The e o e, he esul ing hyd ogels possess inc eased biocompa ibili y
and educed o e en no oxici y [55,56].
One o he mos widely used me hods o physical c oss-linking o he p oduc ion
o cellulose-based hyd ogels wi h ele a ed abso bency is he c yogenic me hod ( epea ed
eeze– haw cycles) [
57
]. This p ocess leads o he o ma ion o hyd ogels due o phase
sepa a ion in he polyme solu ion a e epea ed eezing and hawing cycles. Thus, he
o ma ion o polyme c ys alli es occu s when polyme solu ions a e exposed o low em-
pe a u es, c ea ing insoluble polyme ne wo ks. Fu he mo e, eeze– haw cycles allow
a po ous s uc u e o be c ea ed in hyd ogels hanks o he space le by ice c ys als ha
mel du ing he hawing phase. In a s udy conduc ed by Guan e al. [
58
], a hemicellulose-
based hyd ogel wi h ema kable mechanical p ope ies was de eloped h ough epea ed
eeze/ haw cycles, which led o he o ma ion o a sys em wi h a compac s uc u e cha -
ac e ized by high he mal s abili y and mechanical esis ance. The maximum comp essi e
s eng h was eached a e nine eeze/ haw cycles, highligh ing he packing o he phys-
Polyme s 2024,16, 1292 5 o 29
ically c oss-linked polyme chains induced by epea ed eeze/ haw cycles. The g oup
o Wang e al. applied he c yogenic app oach o he syn hesis o a composi e hyd ogel
consis ing o poly( inyl alcohol) (PVA) and ca boxy me hyl cellulose (CMC) (Figu e 2).
Speci ically, a mixed aqueous solu ion o PVA and CMC was ozen a
−
20
◦
C o 6 h
and hawed o 1 h a oom empe a u e, o i e eeze/ haw cycles [
57
]. F eezing he
polyme solu ions caused c ys als o o m, gene a ing insoluble hyd ogels. The physical
p ope ies and deg ee o swelling o he gels we e clea ly in luenced by he p opo ions
o PVA and CMC. Pu e PVA hyd ogels showed a swelling deg ee o 416%, while hose
wi h 1:2 PVA/CMC a io eached 1437%. The esul s indica e ha an inc ease in CMC
con en led o a educ ion in gel ac ion and an inc ease in he deg ee o swelling o
PVA/CMC hyd ogels wi h di e en CMC p opo ions. This is due o he inc easing hy-
d ophilici y o CMC and a lowe c oss-linking densi y o he hyd ogel as he amoun o
PVA dec eases. C oss-sec ion SEM images o he p epa ed hyd ogels highligh ha all
PVA/CMC hyd ogels exhibi ed a spongy s uc u e wi h high po osi y (Figu e 2).
Polyme s 2024, 16, x FOR PEER REVIEW 5 o 31
ha mel du ing he hawing phase. In a s udy conduc ed by Guan e al. [58], a
hemicellulose-based hyd ogel wi h ema kable mechanical p ope ies was de eloped
h ough epea ed eeze/ haw cycles, which led o he o ma ion o a sys em wi h a
compac s uc u e cha ac e ized by high he mal s abili y and mechanical esis ance. The
maximum comp essi e s eng h was eached a e nine eeze/ haw cycles, highligh ing
he packing o he physically c oss-linked polyme chains induced by epea ed
eeze/ haw cycles. The g oup o Wang e al. applied he c yogenic app oach o he
syn hesis o a composi e hyd ogel consis ing o poly( inyl alcohol) (PVA) and ca boxy
me hyl cellulose (CMC) (Figu e 2). Speci ically, a mixed aqueous solu ion o PVA and
CMC was ozen a −20 °C o 6 h and hawed o 1 h a oom empe a u e, o i e
eeze/ haw cycles [57]. F eezing he polyme solu ions caused c ys als o o m,
gene a ing insoluble hyd ogels. The physical p ope ies and deg ee o swelling o he gels
we e clea ly in luenced by he p opo ions o PVA and CMC. Pu e PVA hyd ogels showed
a swelling deg ee o 416%, while hose wi h 1:2 PVA/CMC a io eached 1437%. The
esul s indica e ha an inc ease in CMC con en led o a educ ion in gel ac ion and an
inc ease in he deg ee o swelling o PVA/CMC hyd ogels wi h diffe en CMC
p opo ions. This is due o he inc easing hyd ophilici y o CMC and a lowe c oss-linking
densi y o he hyd ogel as he amoun o PVA dec eases. C oss-sec ion SEM images o he
p epa ed hyd ogels highligh ha all PVA/CMC hyd ogels exhibi ed a spongy s uc u e
wi h high po osi y (Figu e 2).
Figu e 2. SEM images o he c oss-sec ion o he diffe en PVA/CMC composi e o mula ions: (A)
PVA (1:0, PVA/CMC), (B) P2C1 (2:1, PVA/CMC), (C) P1C1 (1:1, PVA/CMC), and (D) P1C2 (1:2,
PVA/CMC) (scale ba : 100 µm), om e . [57].
The main ad an ages o hyd ogels p oduced h ough physical c oss-linking o e
chemically c oss-linked ones include ease o ab ica ion, biodeg adabili y,
biocompa ibili y, educed oxici y, pos -p ocess mass modi ica ion, and emodeling. In
con as , in physical c oss-linking p ocesses, weak in e ac ions be ween polyme chains,
Figu e 2. SEM images o he c oss-sec ion o he di e en PVA/CMC composi e o mula ions:
(A) PVA (1:0, PVA/CMC), (B) P2C1 (2:1, PVA/CMC), (C) P1C1 (1:1, PVA/CMC), and (D) P1C2 (1:2,
PVA/CMC) (scale ba : 100 µm), om e . [57].
The main ad an ages o hyd ogels p oduced h ough physical c oss-linking o e
chemically c oss-linked ones include ease o ab ica ion, biodeg adabili y, biocompa ibili y,
educed oxici y, pos -p ocess mass modi ica ion, and emodeling. In con as , in physical
c oss-linking p ocesses, weak in e ac ions be ween polyme chains, such as hyd ogen
bonds o physical in e ac ions, allow he hyd ogel s uc u e o be o med mo e easily.
Fu he mo e, physical c oss-linking makes hyd ogels mo e biodeg adable, as he weak
in e ac ions wi hin he polyme ne wo k a e mo e suscep ible o biodeg ada ion compa ed
o co alen bonds [
59
]. On he o he hand, physically c oss-linked hyd ogels usually su e
Polyme s 2024,16, 1292 6 o 29
om as dissolu ion and educed s abili y due o he lack o chemical bonding wi hin he
hyd ogel ne wo k [60].
Howe e , in a ecen wo k, he g oup o Zhang e al. [
61
] p oposed he syn hesis o
a physically c oss-linked CMC-Fe
3+
/polyac ylamide dual-ne wo k hyd ogel ia a wo-
s ep p ocess. In pa icula , Fe
3+
c oss-linked CMC o ms he i s ne wo k, while he
hyd ophobic polyac ylamide o ms he second ne wo k o p ese e he in eg i y o he
esul ing hyd ogel. This sys em showed excellen mechanical p ope ies. The mechanical
p ope ies we e e alua ed by ensile and comp essi e es s. Compa ed wi h he physically
c oss-linked single CMC/polyac ylamide hyd ogel, he elas ic modulus and oughness o
he CMC-Fe3+/polyac ylamide hyd ogel inc eased by 420% and 94%, espec i ely.
2.3. Chemically C oss-Linked Hyd ogels
Chemical c oss-linking in ol es he o ma ion o co alen bonds be ween he c osslink-
ing agen s and he polyme chains which impa imp o ed s abili y and g ea mechanical
s eng h o he esul ing hyd ogels [
62
]. Chemically c oss-linked sys ems do no mel
o disin eg a e i ea ed wi h hea , and do no dissol e in wa e wi hou i s b eaking
he co alen bonds. Fo his eason, hey a e called pe pe ual sys ems [
63
]. Chemical
c oss-linking can occu h ough es e i ica ion, adical polyme iza ion, Michael addi ion,
e c. In he case o cellulose and i s de i a i es, commonly used c oss-linking agen s a e
ci ic acid, succinic acid, epichlo ohyd in, polye hylene glycol diac yla e, di inyl sul one,
and glu a aldehyde (EGDE) [30,64].
The g oup o Zhao e al. p epa ed a modi ied cellulose-based hyd ogel h ough an
inno a i e me hod o simul aneous c oss-linking and g a ing o ac ylamide and ac ylic
acid (Figu e 3) [
65
]. The syn hesis was ca ied ou by adding ammonium pe sul a e o an
aqueous solu ion o mic oc ys alline cellulose o gene a e hyd oxyl adicals unde magne ic
s i ing a 50
◦
C. Subsequen ly, a mixed solu ion o ac ylic acid (AA), ac ylamide (AM), and
N,N
′
-me hylene bisac ylamide (MBA) was slowly added, con inuing s i ing a 50
◦
C o
2 h un il comple e polyme iza ion. The physicochemical cha ac e iza ion o he hyd ogels
success ully con i med he e icien g a ing o ac yla e and ac ylamide monome s on o he
ini ial cellulosic ma e ial. Scanning elec on mic oscopy (SEM) analysis e ealed a ough
and i egula po ous s uc u e. The he mal cha ac e is ics o cellulose-based hyd ogels
we e examined h ough he mog a ime ic analysis (TGA). While he unmodi ied hyd ogel
showed a high decomposi ion a e in he empe a u e ange om 240
◦
C o 360
◦
C, o
he modi ied hyd ogel, he main decomposi ion s age was signi ican ly p olonged and he
a e was educed. The good he mal s abili y obse ed ep esen s a posi i e elemen o
p ac ical applicabili y, since i gi es hem g ea e esis ance o empe a u e a ia ions.
Ci ic acid (CA), widely used in he ood and pha maceu ical sec o s, is a highly
e icien c oss-linking agen . P esen in na u e, such as in lemon juice whe e i cons i u es
app oxima ely 5% o he composi ion, CA is p oduced on a comme cial scale h ough
he ungal e men a ion o glucose. Nume ous s udies ha e employed CA as a c oss-
linking agen in a ious cellulose-based hyd ogels as, compa ed o o he c oss-linking
agen s, CA o e s he ad an age o being inexpensi e and non- oxic [66–69]. Fo example,
Coma e al. [
66
] conduc ed c oss-linking o hyd oxyp opyl me hylcellulose wi h ci ic
acid (CA) in o de o educe he wa e sensi i i y o packaging ma e ials based on wa e -
soluble cellulose de i a i es and, abo e all, o a oid pa ial solubiliza ion o ilms in ood
p oduc s. This was achie ed by a aching he ca boxylic acid moie y o he hyd oxyp opyl
me hylcellulose polyme ia an es e i ica ion eac ion o he i s cyclic anhyd ide. Such a
eac ion exposes a new ca boxylic acid uni in he CA, which has he chemical connec i i y
necessa y o o m a new in amolecula anhyd ide moie y wi h he adjacen ca boxylic
acid uni . Subsequen ly, u he eac ion wi h cellulose e he led o c oss-linking. This
p ocess esul ed in limi ing he a ini y o he ilms wi h wa e , since he –OH g oups o
hyd oxyp opyl me hyl cellulose a e in ol ed in he es e bonds. The s udy demons a ed
ha c oss-linking o e s signi ican bene i s in educing he wa e solubili y o e he cellulose
ilms while simul aneously imp o ing mois u e ba ie p ope ies by app oxima ely 34%.
Polyme s 2024,16, 1292 7 o 29
Demi i e al. [
68
] syn hesized a biodeg adable HEC/CMC hyd ogel using CA as a c oss-
linking agen . Thei esul s highligh ed ha he concen a ion o CA and he eac ion
ime in luence he deg ee o swelling o cellulose-based hyd ogels. In pa icula , he
hyd ogels showed a high deg ee o swelling a low concen a ions o CA (1.75%, 2.75%,
and 3.75% by weigh polyme ); a swelling deg ee o 900 was achie ed in he p esence
o 3.75% CA (Figu e 4A), while CA concen a ions lowe han 1.75% esul ed in weak
c oss-linking associa ed wi h insu icien mechanical p ope ies. On he o he hand, highe
CA concen a ions (10% and 20% by weigh polyme ) p oduced hyd ogels wi h a low
deg ee o swelling (Figu e 4B), indica ing high c oss-linking. I was also obse ed ha
his ype o hyd ogel, a e swelling, had good igidi y and e ained he o iginal shape
o he ub o syn hesis. Subsequen ly, CA as a c oss-linking agen was success ully used
in ano he s udy o he syn hesis o CMC and HEC hyd ogels. In pa icula , chemical
ea men wi h CA induced –COOH unc ional g oups, wi h an inc ease in hyd ophilici y
and su ace oughness o he esul ing ma e ials [
69
], as e ealed by con ac angle and
A omic Fo ce Mic oscopy (AFM) measu emen s. The esul s indica ed a signi ican dec ease
in con ac angle, app oxima ely 50% and 67%, a e c oss-linking be ween HEC/HEC-CA
and CMC/CMC-CA hyd ogels, espec i ely. Fu he mo e, AFM p o iles e idenced ha
he addi ion o CA esul ed in an inc ease in oughness, measu ed by oo mean squa e
(RMS), wi h a e age alues o 31.82, 39.72, 9.98, and 16.56 nm o CMC, CMC-CA, HEC,
and HEC-CA, espec i ely.
Yan e al. p oduced a hyd ogel based on hyd oxyp opyl cellulose c oss-linked
wi h epichlo ohyd in and ammonia in an aqueous sodium hyd oxide solu ion [
70
]. A
empe a u e-induced phase sepa a ion (TIPS) p ocess was applied o syn hesize a mic o-
po ous c oss-linked hyd ogel which in ol es he c ea ion o a gel-like s uc u e h ough
he phase ansi ion o he polyme . Typically, he p ocess s a s wi h a solu ion con aining
hyd oxyp opyl cellulose in wa e o a compa ible sol en . Nex , he sys em is hea ed o
cooled o a speci ic empe a u e, o en abo e o below he c i ical solu ion poin o he
polyme . This empe a u e change induces phase sepa a ion, leading o he o ma ion o
a h ee-dimensional ne wo k o in e connec ed polyme chains which ap wa e o he
sol en wi hin he gela inous s uc u e.
Epichlo ohyd in as a c oss-linking agen was also used o he syn hesis o a new
supe abso ben sys em based on na i e cellulose and qua e nized cellulose (cellulose ha
has been chemically modi ied by adding qua e na y ammonium g oups o i s s uc u e) in
a NaOH/u ea aqueous solu ion o o e come he limi a ions o educed biocompa ibili y,
poo mechanical p ope ies, and a lack o an imic obial ac i i y associa ed wi h he supe -
abso ben hyd ogel no mally used in he p oduc ion o disposable diape s [
71
]. Inc easing
he densi y o he c oss-linking ne wo k using epichlo ohyd in led o an imp o emen in
he mechanical p ope ies o he hyd ogel. In addi ion, he in oduc ion o qua e nized
cellulose wi hin he hyd ogel ne wo ks no only imp o ed he swelling a io due o he
elec os a ic epulsion o he qua e na y ammonium g oups, bu also endowed he hyd o-
gel wi h an imic obial p ope ies. The p esence o an imic obial ac i i y was due o he
a ac ion o he anionic moie ies o he mic obial memb anes owa d he in e nal po es o
he polyca ionic hyd ogel, which caused i s up u e.
In his ega d, cellulose-based hyd ogels wi h in e es ing an ibac e ial p ope ies ( o
G am-posi i e bac e ia) we e de eloped in a e y ecen s udy, in which bo ax (sodium
e abo a e) was used as a c oss-linking agen [
72
]. Bo ax is commonly used as a non-
oxic ood addi i e, being cheap and soluble in wa e . Cellulose-based hyd ogels we e
syn hesized using cellulose ex ac ed om wa e hyacin h ia a dissolu ion p ocess in he
p esence o u ea and sodium hyd oxide, wi h bo ax employed o gene a e c oss-linking
be ween he –OH g oups o he cellulose molecules (Figu e 5). The use o bo ax ga e he
cellulose-based hyd ogel p oduc s ex ao dina y supe abso ben p ope ies, wi h a swelling
a io o 325% o he non-c oss-linked hyd ogels and a swelling a io o app oxima ely
900% o he c oss-linked ones.
Polyme s 2024,16, 1292 8 o 29
Polyme s 2024, 16, x FOR PEER REVIEW 7 o 31
Figu e 3. A schema ic o he syn hesis eac ion o bioadso ben s om he modi ied cellulose h ough
simul aneous c oss-linking and g a ing o bo h ac ylamide and ac ylic acid, om e . [65]. Aam:
ac ylamide; AA: ac ylic acid; MBA: N,N’-me hylene bisac ylamide.
Ci ic acid (CA), widely used in he ood and pha maceu ical sec o s, is a highly
efficien c oss-linking agen . P esen in na u e, such as in lemon juice whe e i cons i u es
app oxima ely 5% o he composi ion, CA is p oduced on a comme cial scale h ough he
ungal e men a ion o glucose. Nume ous s udies ha e employed CA as a c oss-linking
agen in a ious cellulose-based hyd ogels as, compa ed o o he c oss-linking agen s, CA
offe s he ad an age o being inexpensi e and non- oxic [66–69]. Fo example, Coma e al.
[66] conduc ed c oss-linking o hyd oxyp opyl me hylcellulose wi h ci ic acid (CA) in
o de o educe he wa e sensi i i y o packaging ma e ials based on wa e -soluble
cellulose de i a i es and, abo e all, o a oid pa ial solubiliza ion o ilms in ood
p oduc s. This was achie ed by a aching he ca boxylic acid moie y o he hyd oxyp opyl
me hylcellulose polyme ia an es e i ica ion eac ion o he i s cyclic anhyd ide. Such a
Figu e 3. A schema ic o he syn hesis eac ion o bioadso ben s om he modi ied cellulose h ough
simul aneous c oss-linking and g a ing o bo h ac ylamide and ac ylic acid, om e . [
65
]. Aam:
ac ylamide; AA: ac ylic acid; MBA: N,N′-me hylene bisac ylamide.
Polyme s 2024,16, 1292 9 o 29
Polyme s 2024, 16, x FOR PEER REVIEW 9 o 31
Figu e 4. Swelling a io e sus eac ion ime o CMC/HEC (weigh a io 3:1) hyd ogels wi h 1.75%,
2.75%, and 3.75% (w/w) o CA c oss-linke concen a ion (A). Swelling a io e sus eac ion ime o
CMC/HEC (weigh a io 3:1) hyd ogels wi h 10% and 20% (w/w) o CA c oss-linke concen a ion
(B), om e . [68].
In his ega d, cellulose-based hyd ogels wi h in e es ing an ibac e ial p ope ies ( o
G am-posi i e bac e ia) we e de eloped in a e y ecen s udy, in which bo ax (sodium
e abo a e) was used as a c oss-linking agen [72]. Bo ax is commonly used as a non- oxic
ood addi i e, being cheap and soluble in wa e . Cellulose-based hyd ogels we e
syn hesized using cellulose ex ac ed om wa e hyacin h ia a dissolu ion p ocess in he
p esence o u ea and sodium hyd oxide, wi h bo ax employed o gene a e c oss-linking
be ween he ─OH g oups o he cellulose molecules (Figu e 5). The use o bo ax ga e he
cellulose-based hyd ogel p oduc s ex ao dina y supe abso ben p ope ies, wi h a
swelling a io o 325% o he non-c oss-linked hyd ogels and a swelling a io o
app oxima ely 900% o he c oss-linked ones.
Figu e 4. Swelling a io e sus eac ion ime o CMC/HEC (weigh a io 3:1) hyd ogels wi h 1.75%,
2.75%, and 3.75% (w/w) o CA c oss-linke concen a ion (A). Swelling a io e sus eac ion ime o
CMC/HEC (weigh a io 3:1) hyd ogels wi h 10% and 20% (w/w) o CA c oss-linke concen a ion (B),
om e . [68].
Polyme s 2024, 16, x FOR PEER REVIEW 10 o 31
Figu e 5. A schema ic ep esen a ion o he c oss-linking mechanism be ween he hyd oxyl g oups
o he cellulose chains and bo ax as a c oss-linking agen , om e . [72].
3. Hyd ogels Based on Cellulose and Cellulose De i a i es o Wa e
Decon amina ion o Hea y Me als
In ecen yea s, gels based on cellulose and i s de i a i es ha e inc easingly a ac ed
esea ch in e es o hei possible applica ion as hea y me al abso ben s in was ewa e
decon amina ion p ocesses, hanks o se e al ad an ageous cha ac e is ics such as a
po ous s uc u e, nume ous unc ional g oups, and high abso p ion capaci y [73–81].
Hyd ogels, simila o ion exchange esins, can sub ac con aminan s om wa e
bodies hanks o he p esence o unc ional g oups, including ca boxyl, hyd oxyl, and
amino g oups [82–88], which can es ablish diffe en in e ac ions wi h me al ions [89]. In
addi ion, h ough he modi ica ion o he unc ional g oups, he p ope ies o he hyd ogel
can be sui ably modi ied o u he imp o e i s in e ac ion abili y wi h me al ions, o
example in oducing chela ing unc ional g oups o oppo unely cha ged moie ies able o
bind o me al ions h ough elec os a ic in e ac ion. Fu he mo e, he unique h ee-
dimensional s uc u e o he hyd ogel allows o a uni o m dis ibu ion o me al ions
h oughou he sys em, p e en ing agg ega ion and oxida ion [62]. Indeed, me al ions
uni o mly dis ibu ed wi hin he hyd ogel ma ix a e less suscep ible o in e ac ion wi h
oxygen o o he oxidizing chemical species p esen in he su ounding en i onmen ,
helping o p e en hei oxida ion. I me al ions a e oxidized du ing he emo al p ocess,
hey may lose hei abili y o be effec i ely cap u ed by adso ben s, hus educing he
o e all ea men efficiency. Also, he wa e p esen in he hyd ogel plays a ole in his
sense, no only p o iding suppo o he hyd ogel i sel , allowing i o main ain a ce ain
shape, bu also ensu ing he p esence o nano anspo channels o he ee diffusion o
me al ions and pa icipa ing in he o ma ion o hyd ogen bonds, hus cons i u ing
addi ional ac i e si es o he in e ac ion wi h he a ge ionic species [90].
In cellulose-based hyd ogels, elec os a ic a ac ion usually occu s be ween
unp o ona ed ca boxyl g oups (–COO−) and me al ca ions [79,80]. O he mechanisms,
such as complexa ion and mic op ecipi a ion, ha e been sugges ed o he abso p i e
emo al o hea y me als by using cellulose-based hyd ogels. Complexa ion can occu
be ween he lone pai o O elec ons and me al ions. Unde some ci cums ances, o
example when he empe a u e, pH, o concen a ion o o he subs ances a y, he
s abili y o he me al complex may dec ease, leading o me al ec ys alliza ion as solid
pa icles wi h he o ma ion o small solid pa icles nea he me al complexa ion si es
(mic op ecipi a ion) [81,91].
In he nex sec ions, b ie desc ip ions will be p o ided on he adso p ion kine ics
and iso he ms ha go e n he adso p ion p ocesses o hea y me al ions by hyd ogels
based on cellulose and i s de i a i es. Finally, a b oad o e iew o he applica ion o such
ma e ials as adso ben s o hea y me al ions, bo h in non-composi e and composi e
sys ems, will be p esen ed.
Figu e 5. A schema ic ep esen a ion o he c oss-linking mechanism be ween he hyd oxyl g oups o
he cellulose chains and bo ax as a c oss-linking agen , om e . [72].
Polyme s 2024,16, 1292 16 o 29
ini ial concen a ion o he me al ions in he aqueous solu ions, e ealing an adso p ion
capaci y o 90 mg/g, 69 mg/g, and 60 mg/g o Cu2+, C 6+, and As5+, espec i ely.
Hyd oxyalkyl e he CMC has also ound widesp ead use as a composi e hyd ogel
in was ewa e decon amina ion sys ems o emo ing hea y me al ions. The addi ion o
polyac ylamide (PAM) inc eased he abili y o he CMC-based hyd ogels o cap u e hea y
me als, such as Cu
2+
, Pb
2+
, and Cd
2+
, by 2.3, 3.6, and 1.5 imes compa ed o he p is ine
CMC hyd ogel, due o he p esence o –C=O and –NH2ac i e in PAM [94].
Baiya e al. p oposed he assembly o a CMC-based hyd ogel (25%) mixed wi h
PVA, employing glu a aldehyde as a c oss-linking agen o he selec i e adso p ion o
Cu
2+
ions [
109
]. Cellulose o CMC p oduc ion was ex ac ed om suga cane bagasse,
a was e p oduc de i ing om he c ushing and p essing o suga cane, wi h he aim o
ecycling was e ma e ials in an eco-sus ainable app oach. To minimize he syn hesis ime
and imp o e he eac ion e iciency, he hyd ogel was de eloped h ough a mic owa e-
assis ed i adia ion p ocess. The composi e hyd ogel e ealed an adso p ion capaci y o
Cu2+ a oom empe a u e and pH 5.0 o 2.3 mg/g.
An excellen adso ben ma e ial o he selec i e emo al o C
6+
ions was de eloped
wi h he addi ion o he polye hylenimine (PEI) polyme o CMC, cha ac e ized by a no able
quan i y o imine g oups [
110
]. The CMC/PEI composi e hyd ogel was p oduced wi h
a simple one-s ep p ocedu e, consis ing o mixing an aqueous solu ion o CMC wi h an
aqueous solu ion o PEI, o which he c oss-linking agen epichlo ohyd in was added. The
p o ona ed imine g oups (a a sligh ly acidic pH) a e able o adso b C
6+
ions h ough
elec os a ic in e ac ion, while he oxidizing hyd oxyl g oups o CMC ac by educing C
6+
ions o C
3+
. The in si u p oduced C
3+
is immobilized h ough coo dina ion wi h N and O
and hen emo ed h ough p ecipi a ion. The CMC/PEI composi e hyd ogel can be hus
egene a ed and eused, exhibi ing a C
6+
adso p ion capaci y highe han 2.0 mmol/g
e en a e ou cycles.
A composi e hyd ogel was assembled o he decon amina ion o aqueous ma ices
by emo ing cadmium and nickel based on CMC, mic oc ys alline cellulose (MCC), and
xylan, using e hylene glycol diglycidyl e he as he c oss-linking agen [
111
]. The epo ed
maximum adso p ion capaci y was 61.44 o Cd
2+
and 55.85 mg/g o Ni
2+
. The adso p ion
o me al ions can occu a he le el o he ca boxyl g oups o he CMC, o e en a he le el
o he hyd oxyl g oups o he MCC and xylan h ough ionic bonding.
Ba ch adso p ion es s o hea y me al ions om an aqueous solu ion we e ca ied
ou by he g oup o As ini e al., using a CMC-g a -poly(ac ylic acid)/mon mo illoni e
composi e hyd ogel as he adso ben [
112
]. Clay mine als such as mon mo illoni e a e in o-
duced in o he la ice s uc u e o he polyme hyd ogel in o de o imp o e i s mechanical
p ope ies. Thei inclusion is mo i a ed by hei abundance, chemical and mechanical
s abili y, and low cos , which makes hem an a ac i e cellulose immobiliza ion ma e ial.
The impac o a ious pa ame e s (such as pH and he p esence o compe ing ions) on he
adso p ion capaci y o he composi e sys em was e alua ed. The ob ained esul s con i med
ha he adso p ion e iciency o me al ions by he composi e hyd ogel is signi ican ly in lu-
enced by pH, wi h he maximum adso p ion a pH 5.0 (Figu e 7A). A his pH alue, an
adso p ion capaci y o he composi e sys ems o 146.19 mg/g and 286.67 mg/g was ob-
ained o Pb
2+
and Zn
2+
ions, espec i ely. Fou ie ans o m in a ed spec oscopy (FTIR)
spec a be o e and a e adso p ion demons a ed ha he complex o ma ion be ween
me al ions and he –COOH g oups o he hyd ogel is he main adso p ion mechanism o
hea y me al ions. Fu he mo e, compe i i e adso p ion es s e ealed ha he adso p ion
capaci y o he hyd ogel o Zn
2+
ions is highe han ha o Pb
2+
ions (Figu e 7B). Deso p-
ion s udies con i med he e ec i e me al ion deso p ion om he hyd ogel, which could
be eused wice h ough washing wi h a low pH solu ion.
Polyme s 2024,16, 1292 17 o 29
Polyme s 2024, 16, x FOR PEER REVIEW 18 o 31
Figu e 7. In luence o pH on hea y me al ion adso p ion by CMC-g a -poly(ac ylic
acid)/mon mo illoni e hyd ogel a e 6 h o ea men (A). Compe i i e adso p ion s udy a pH 5.0
o Pb
2+
and Zn
2+
om mixed solu ion wi h concen a ion o each ion ixed a 250 ppm o 500 ppm
(B), om e [112].
A no el ac ylic acid/e hylenediamine e aace ic acid (EDTA) biocha -based chi osan
(CH) composi e hyd ogel was de eloped h ough adical polyme iza ion om deg eased
was e co on [113], mainly consis ing o cellulose, hemicellulose, and lignin. The esul ing
hyd ogel was able o adso b Pb
2+
and Cu
2+
ions efficien ly, wi h an adso p ion capaci y o
1105.78 and 678.04 mg/g, espec i ely. As shown in Figu e 8, he impac o se e al ac o s
on he adso p ion capaci y o he composi e hyd ogel was e alua ed, inding ha he
eco e y o Pb
2+
and Cu
2+
ions om he adso ben was a an op imal le el e en a e i e
cycles. The composi ion o he adso ben plays a c ucial ole in he effec i eness o
adso p ion. As shown in Figu e 8A, he adso p ion pe o mance o CH/EDTA was
signi ican ly imp o ed compa ed o ha o he CH hyd ogel, due o he addi ion o EDTA
which enhances he chela ing effec o con aminan s. In pa allel, an inc ease in he amoun
o po ous co on biocha (CBC) inc eases he adso p ion capaci y o he hyd ogel. This
occu s due o he addi ion o nume ous oxygen- and ni ogen-con aining unc ional
g oups on he su ace o he CBC a e i s acid–base modi ica ion, hus p o iding mo e
ancho ing si es o pollu an s. Howe e , once he CBC con en eaches 0.03 g, he
Figu e 7. In luence o pH on hea y me al ion adso p ion by CMC-g a -poly(ac ylic acid)/mon mo i-
lloni e hyd ogel a e 6 h o ea men (A). Compe i i e adso p ion s udy a pH 5.0 o Pb
2+
and Zn
2+
om mixed solu ion wi h concen a ion o each ion ixed a 250 ppm o 500 ppm (B), om e [
112
].
A no el ac ylic acid/e hylenediamine e aace ic acid (EDTA) biocha -based chi osan
(CH) composi e hyd ogel was de eloped h ough adical polyme iza ion om deg eased
was e co on [
113
], mainly consis ing o cellulose, hemicellulose, and lignin. The esul ing
hyd ogel was able o adso b Pb
2+
and Cu
2+
ions e icien ly, wi h an adso p ion capaci y
o 1105.78 and 678.04 mg/g, espec i ely. As shown in Figu e 8, he impac o se e al
ac o s on he adso p ion capaci y o he composi e hyd ogel was e alua ed, inding ha
he eco e y o Pb
2+
and Cu
2+
ions om he adso ben was a an op imal le el e en a e
i e cycles. The composi ion o he adso ben plays a c ucial ole in he e ec i eness
o adso p ion. As shown in Figu e 8A, he adso p ion pe o mance o CH/EDTA was
signi ican ly imp o ed compa ed o ha o he CH hyd ogel, due o he addi ion o EDTA
which enhances he chela ing e ec o con aminan s. In pa allel, an inc ease in he amoun
o po ous co on biocha (CBC) inc eases he adso p ion capaci y o he hyd ogel. This
occu s due o he addi ion o nume ous oxygen- and ni ogen-con aining unc ional g oups
on he su ace o he CBC a e i s acid–base modi ica ion, hus p o iding mo e ancho ing
si es o pollu an s. Howe e , once he CBC con en eaches 0.03 g, he adso p ion capaci y
s ops inc easing signi ican ly, as a s a e o supe sa u a ion o he CBC con en is eached.
Polyme s 2024,16, 1292 18 o 29
Polyme s 2024, 16, x FOR PEER REVIEW 19 o 31
adso p ion capaci y s ops inc easing signi ican ly, as a s a e o supe sa u a ion o he CBC
con en is eached.
Figu e 8. The impac o diffe en pa ame e s on he adso p ion capaci y: he composi ion o he
adso ben gel (A), pH (B), he amoun o he adso ben (C), he ea men ime (D), and he ini ial
concen a ion o he con aminan (E), om e . [113]. CS: CH (chi osan); CBC: po ous co on biocha .
A new CH/cellulose composi e sponge adso ben sys em o he selec i e and apid
emo al o Hg2+ ions om aqueous solu ions was success ully de eloped h ough simple
c oss-linking wi h glu a aldehyde and a eeze-d ying p ocedu e [114]. The CH/cellulose
composi e sys em showed in he i s wo minu es an adso p ion o mo e han 97% o he
maximum adso p ion capaci y (equal o 495 mg/g wi h an ini ial concen a ion o 598
mg/L) in aqueous solu ions. In addi ion, he composi e sys em showed excellen
selec i i y o he adso p ion o Hg2+ ions, wi h a selec i i y coefficien 35, 277, and 461
imes highe han ha o Cd2+, Pb2+, and Cu2+ ions, espec i ely. The selec i i y coefficien
was de e mined using he sepa a ion ac o , conside ing he dis ibu ion coefficien s o
Hg2+ ions and in e e ing me al ions. XPS and FTIR s udies con i med ha he main
mechanism o Hg2+ ion adso p ion is a chela ion eac ion wi h N ligand and O-con aining
unc ional g oups. In addi ion, washing wi h 1 M ni ic acid allows he CH/cellulose
Figu e 8. The impac o di e en pa ame e s on he adso p ion capaci y: he composi ion o he
adso ben gel (A), pH (B), he amoun o he adso ben (C), he ea men ime (D), and he ini ial
concen a ion o he con aminan (E), om e . [
113
]. CS: CH (chi osan); CBC: po ous co on biocha .
A new CH/cellulose composi e sponge adso ben sys em o he selec i e and apid
emo al o Hg
2+
ions om aqueous solu ions was success ully de eloped h ough simple
c oss-linking wi h glu a aldehyde and a eeze-d ying p ocedu e [
114
]. The CH/cellulose
composi e sys em showed in he i s wo minu es an adso p ion o mo e han 97% o
he maximum adso p ion capaci y (equal o 495 mg/g wi h an ini ial concen a ion o
598 mg/L) in aqueous solu ions. In addi ion, he composi e sys em showed excellen
selec i i y o he adso p ion o Hg
2+
ions, wi h a selec i i y coe icien 35, 277, and
461 imes highe han ha o Cd
2+
, Pb
2+
, and Cu
2+
ions, espec i ely. The selec i i y
coe icien was de e mined using he sepa a ion ac o , conside ing he dis ibu ion co-
Polyme s 2024,16, 1292 19 o 29
e icien s o Hg
2+
ions and in e e ing me al ions. XPS and FTIR s udies con i med ha
he main mechanism o Hg
2+
ion adso p ion is a chela ion eac ion wi h N ligand and
O-con aining unc ional g oups. In addi ion, washing wi h 1 M ni ic acid allows he
CH/cellulose composi e sys em o be e ec i ely egene a ed, wi h sligh ly lowe mechani-
cal s eng h p ese ing an 85% adso p ion capaci y a e i e cycles.
Ano he lignocellulose-based composi e hyd ogel sys em was de eloped by dispe s-
ing TEMPO-oxidized cellulose nano ib ils (TOCNs) in an aqueous solu ion o 7% NaOH
and 12% u ea by weigh a oom empe a u e [
115
]. In he same sys em, dissol ed cellulose
was p oduced a subze o empe a u es. The TOCN dispe sion and cellulose solu ion we e
mixed a di e en empe a u es based on hei empe a u e-dependen solubili y. The
composi e hyd ogels we e ob ained by mixing a cellulose solu ion, TOCN dispe sion,
and alkaline lignin solu ion in an aqueous NaOH/u ea solu ion. The esul ing hyd ogel
showed an excellen adso p ion capaci y o Cu
2+
, equal o 540.7 mg/g (achie ed a e
45 min wi h an ini ial Cu
2+
concen a ion o 250 mg/mL) (Figu e 9). Acco ding o au ho s,
he adso p ion p ocess o Cu
2+
ions by composi e hyd ogels can be di ided in o wo dis-
inc i e phases. Ini ially, Cu
2+
ions di use on o he su ace o he composi e hyd ogels om
he su ounding solu ion. Subsequen ly, Cu
2+
ions u he bind o he ac i e adso p ion
si es (phenolic and ca boxyl g oups om TOCNs and lignin) p esen on he composi e
hyd ogels ia complexa ion. In addi ion, he 3D po ous ne wo k s uc u e, which p o ides
he maximum su ace a ea and an abundan po e s uc u e, leads o he exposu e o many
ac i e si es o he complexa ion o Cu2+ ions.
TEMPO nega i ely cha ged oxidized cellulose nano ibe s (TOCNFs) we e used o
he syn hesis o a composi e sys em h ough he sel -assembly o nanocellulose and
nanochi in [
116
]. Elec os a ic o ces guided he sel -assembly be ween TOCNFs and
he posi i ely cha ged pa ially deace yla ed chi in nano ibe s a oom empe a u e wi h-
ou he use o c oss-linking agen s. The esul ing hyb id hyd ogel was hus physically
c oss-linked h ough elec os a ic in e ac ions and H-bonding be ween TOCNFs and chi in
nano ibe s. Subsequen eeze-d ying o he hyb id hyd ogel esul ed in an ae ogel wi h a
igh ly in e connec ed po ous s uc u e and ex ao dina y adso p ion capaci y o As(III)
ion (217 mg/g) a neu al pH.
TEMPO-oxidized nanocellulose was used also o p oduce a composi e sys em wi h
sodium algina e and ca boxyme hyl chi osan, using Ca
2+
as a c oss-linking agen [
117
].
Ca boxyme hyl chi osan is a de i a i e o chi osan cha ac e ized by a high numbe o ee
–OH, –COOH, and –NH
2
g oups, hus o e ing se e al binding si es o hea y me al ions.
The gel p oduced was used in adso p ion es s o Cu
2+
and Pb
2+
om aqueous solu ions,
showing an excellen adso p ion e iciency o bo h ions, wi h an adso p ion capaci y
o 169.94 mg/g o Cu
2+
and 472.59 mg/g o Pb
2+
. In addi ion, he composi e sys em
main ained a high adso p ion e iciency e en a e i e adso p ion–deso p ion cycles.
In e es ingly, FTIR analysis be o e and a e he adso p ion o Cu
2+
and Pb
2+
con i med
he in ol emen o –OH, –COOH, –NH
2
g oups, as well as me hyl and me hylene g oups
in he adso p ion, demons a ing he key ole o di e en componen s in de e mining he
adso p ion pe o mances o he composi e hyd ogel.
A no el composi e hyd ogel based on mic oc ys alline cellulose (MCC) and CH was
ecen ly p oposed, wi h he in eg a ion o polydopamine (PDA) and polye hyleneimine
(PEI) o e ec i e adso p ion o ions om was ewa e [
118
]. Thanks o he p esence o
se e al ac i e adso p ion si es, he MCC-PDA-PEI/CH-PDA-PEI composi e sys em showed
excellen adso p ion capaci ies, equal o abou 434.8, 277.7 and 261.8 mg/g o Cu
2+
, Zn
2+
,
and Ni
2+
ions, espec i ely, in a single ion expe imen . In a mul i-ion adso p ion s udy, he
composi e hyd ogel showed sligh selec i i y in he emo al o Cu
2+
ions. Fu he mo e, he
esul s o FTIR s udies sugges ed me al ion chela ion as a possible adso p ion mechanism.
Repea ed adso p ion/deso p ion es s e ealed ha he hyd ogel main ained an adso p ion
capaci y o mo e han 40% a e ou cycles.
Polyme s 2024,16, 1292 20 o 29
Polyme s 2024, 16, x FOR PEER REVIEW 20 o 31
composi e sys em o be effec i ely egene a ed, wi h sligh ly lowe mechanical s eng h
p ese ing an 85% adso p ion capaci y a e i e cycles.
Ano he lignocellulose-based composi e hyd ogel sys em was de eloped by
dispe sing TEMPO-oxidized cellulose nano ib ils (TOCNs) in an aqueous solu ion o 7%
NaOH and 12% u ea by weigh a oom empe a u e [115]. In he same sys em, dissol ed
cellulose was p oduced a subze o empe a u es. The TOCN dispe sion and cellulose
solu ion we e mixed a diffe en empe a u es based on hei empe a u e-dependen
solubili y. The composi e hyd ogels we e ob ained by mixing a cellulose solu ion, TOCN
dispe sion, and alkaline lignin solu ion in an aqueous NaOH/u ea solu ion. The esul ing
hyd ogel showed an excellen adso p ion capaci y o Cu
2+
, equal o 540.7 mg/g (achie ed
a e 45 min wi h an ini ial Cu
2+
concen a ion o 250 mg/mL) (Figu e 9). Acco ding o
au ho s, he adso p ion p ocess o Cu
2+
ions by composi e hyd ogels can be di ided in o
wo dis inc i e phases. Ini ially, Cu
2+
ions diffuse on o he su ace o he composi e
hyd ogels om he su ounding solu ion. Subsequen ly, Cu
2+
ions u he bind o he
ac i e adso p ion si es (phenolic and ca boxyl g oups om TOCNs and lignin) p esen on
he composi e hyd ogels ia complexa ion. In addi ion, he 3D po ous ne wo k s uc u e,
which p o ides he maximum su ace a ea and an abundan po e s uc u e, leads o he
exposu e o many ac i e si es o he complexa ion o Cu
2+
ions.
Figu e 9. Syn hesis schema ic diag am and amoun o Cu
2+
adso bed on he alkaline
lignin/TOCN/dissol ed cellulose composi e hyd ogel (10/10/80) a diffe en ini ial Cu
2+
ion
concen a ions, adap ed om e . [115].
Figu e 9. Syn hesis schema ic diag am and amoun o Cu
2+
adso bed on he alkaline
lignin/TOCN/dissol ed cellulose composi e hyd ogel (10/10/80) a di e en ini ial Cu
2+
ion con-
cen a ions, adap ed om e . [115].
In a ecen s udy, MCC was used o he syn hesis o a biodeg adable MCC–gela in
adso ben o he adso p ion o Cd
2+
, Pb
2+
, and C
3+
ions om aqueous solu ions [
119
].
The p esence o se e al unc ional g oups, such as –COOH g oups, –OH g oups, and amide
g oups (–CONH
2
), could acili a e he adso p ion o hea y me al ions. The cha ac e iza ion
o he composi e hyd ogel wi h FTIR analysis, scanning elec on mic oscopy analysis (SEM),
and ene gy dispe si e X- ay analysis con i med ha he gela in was in e cala ed wi hin
he mic oc ys alline cellulose ma ix. Pe o ming ba ch adso p ion s udies, he impac o
se e al pa ame e s including pH, adso ben dosage, and ini ial me al ion concen a ion
on he emo al capaci y o hea y me al ions was e alua ed. Speci ically, i was obse ed
ha he e was a maximum adso p ion e iciency o Cd
2+
, Pb
2+
, and C
3+
o 95%, 88%, and
70%, espec i ely, a pH 6 wi h an ini ial concen a ion o each me al ion o 60 ppm and an
adso ben dosage o 1.0 g L−1.
Rod iguez e al. [
120
] de eloped ee adical c oss-linking composi e chi osan-g-
poly(ac ylic acid) hyd ogels illed wi h cellulose nanowhiske s (CNWs) and e alua ed
Polyme s 2024,16, 1292 21 o 29
he abso p ion capaci y o Pb
2+
and Cu
2+
ions om wa e . In he syn hesis o he com-
posi e hyd ogel by ee adical c oss-linking, h ee key componen s a e in ol ed such
as N,N
′
-me hylenebisac ylamide (MBA), which ac s as a c oss-linke , helping o s abi-
lize he s uc u e o he hyd ogel, po assium pe sul a e (KPS), a polyme iza ion ini ia-
o , and N,N,N
′
,N
′
- e ame hyle hylenediamine (TEMD), which ac s as an accele a o o
he polyme iza ion eac ion. Fo hese composi e sys ems, he maximum adso p ion o
Pb
2+
(818.4 mg/g) and Cu
2+
(325.5 mg/g) was ob ained wi hin he i s 30 min, a pH 4.0
and using 20 mg o he composi e ma ix wi h a CNW concen a ion o 10% (w/w). FTIR
analysis e ealed ha unc ional g oups (mainly –COOH and –OH) in composi e hyd ogels
ac as coo dina ion si es o he me al ions. Deso p ion s udies showed ha he composi e
sys em can be egene a ed ( lushing wi h a 0.1 M HCl solu ion) and eused o up o i e
cycles wi hou signi ican loss o e iciency.
Sphe e-shaped composi e sys ems ha e ecen ly been p oposed as possible adso -
ben s o he ea men o was ewa e con amina ed wi h hea y me al ions. In a ecen
wo k [
121
], a composi e adso ben based on cellulose nano ibe s and sodium algina e was
p epa ed h ough a simple c oss-linking p ocess o he e icien emo al o Pb
2+
. The
sphe es we e p oduced by mixing an aqueous solu ion o sodium algina e wi h an aqueous
solu ion o cellulose nano ibe s unde s i ing. A suspension o calcium phospha e (CaP)
was hen added and le s i ing un il a uni o m dispe sion was ob ained, a e which
a CaCl
2
solu ion was added. O e nigh s i ing esul ed in he p oduc ion o solidi ied
cellulose nano ibe /sodium algina e/CaP beads. Subsequen s i ing o he sphe es in HCl
(pH = 2.0) esul ed in he disin eg a ion o he CaP wi hin he sphe es, hus ob aining
composi e sphe es wi h a po ous s uc u e, as e ealed by scanning elec on mic oscopy.
XPS analysis con i med he in ol emen o -OH and -COOH g oups in he adso p ion
p ocess in ol ing elec os a ic a ac ion and complexa ion. Also, he e ec o empe a u e
on he adso p ion o Pb
2+
ions was s udied, e ealing an inc ease in he limi ing adso p ion
capaci y as he ea men empe a u e inc eased, wi h a limi ing adso p ion capaci y o
280.90 mg/g a 291 K, 307.69 mg/g a 297 K, and 318.47 mg/g a 303 K. Regene a ion
expe imen s wi h 1 M HCl showed ha he adso p ion capaci y o Pb
2+
on he beads
emained sa is ac o y, wi h an adso p ion abo e 80% a e i e cycles.
Mo e ecen ly, Alja e al. [
122
] p oposed composi e beads made om PVA, chi osan,
and cellulose o he emo al o Pb
2+
, Cd
2+
, Zn
2+
, and Co
2+
om wa e . The sphe es we e
ab ica ed ia a simple physical c oss-linking me hod, in which an aqueous solu ion o
PVA was mixed wi h an acidic aqueous solu ion o chi osan unde con inuous s i ing
a 70
◦
C, ollowed by cellulose powde addi ion. Subsequen d opwise addi ion o he
ob ained solu ion o a NaOH solu ion led o he o ma ion o he hyd ogel sphe es. The
mo phology and majo elemen composi ion o he composi e sphe es we e analyzed be o e
and a e hea y me al ion adso p ion ia SEM and ene gy dispe si e X- ay (EDX). SEM
images e ealed ha he hyd ogels possessed a sphe ical o sligh ly o al shape wi h a
compac su ace and a well c oss-linked s uc u e (Figu e 10). In gene al, a e adso p ion,
i was obse ed ha he su ace s uc u e o he sphe es emained as a well-c oss-linked
s uc u e, e en i he channels we e mo e compac and he po es had a signi ican ly smalle
diame e . EDX spec a (Figu e 10) clea ly showed he cha ac e is ic peaks o hea y me al
ions, con i ming hei adso p ion. Adso p ion s udies also showed he e icien emo al
o Pb
2+
, Cd
2+
, Zn
2+
, and Co
2+
ions, wi h 99, 95, 92, and 84% emo al wi hin 60 min.
A e egene a ion o he sphe es wi h 1 M HCl, he composi e hyd ogel main ained a high
emo al e iciency, which emained abo e 80% e en a e i e adso p ion–deso p ion cycles.
Table 3 epo s he capaci y adso p ion alues o he abo e p esen ed cellulose-based
hyd ogels and composi e adso ben s based on cellulose de i a i es.
Polyme s 2024,16, 1292 22 o 29
Table 3. Composi ion and adso p ion capaci y o di e en cellulose-based hyd ogels used o hea y
me al abso p ion.
Cellulose o Cellulose
De i a i e
Addi ional
Polyme
O he Componen s
o Modi ica ions Adso p ion Capaci y Re .
Non-
composi e
hyd ogels
CMC — epichlo ohyd in as
c oss-linking agen
Cu2+: 6.49 mmol/g
Pb2+: 5.15 mmol/g
Ni2+: 4.06 mmol/g
[96]
CMC — aluminum ni a e as
c oss-linking agen
Pb2+: 550 mg/g
Ni2+: 620 mg/g
Co2+: 760 mg/g
[95]
Cellulose om bamboo
ibe s — i ana e nano ubes
Cu2+: 176.4 mg/g
Pb2+: 613.5 mg/g
S 2+: 113.3 mg/g
Fe2+: 158.2 mg/g
Cd2+: 294.6 mg/g
Zn2+: 187.8 mg/g
C 3+: 97.8 mg/g
[99]
Cellulose ibe om
pineapple lea es —
modi ica ion wi h
ca boxyme hyl g oup
(CM) and
e hylenediamine e aace ic
acid (EDTA) g oup o
p oduce Cell-CM and
Cell-EDTA, espec i ely
Fo Cell-CM
Pb2+: 63.4mg/g
Cd2+: 23 mg/g
Fo Cell-EDTA
Pb2+: 41.2mg/g
Cd2+: 33.2 mg/g
[93]
HEC —
EDTA dianhyd ide
(EDTAD) as
c oss-linking agen
Pb2+: 405.37 mg/g
Cu2+: 265.47 mg/g
Cd2+: 203.36 mg/g
Zn2+: 108.36 mg/g
[49]
Ginge ibe s — ci ic acid (C6H8O7) and
hyd ochlo ic acid (HCl) Cu2+: 45.053 mg/g
[
103
]
Ju e — sodium ni i e and
ni ic acid
Pb2+:
2270 mg/g
[
104
]
Composi e
hyd ogels
CMC and
mic oc ys alline
cellulose
Xilan e hylene glycol diglycidyl
e he as a c osslinking agen
Cd2+: 61.44 mg/g
Ni2+: 55.85 mg/g
[
111
]
CMC Polyac ylic acid mon mo illoni e
Pb2+: 146.19 mg/g
Zn2+: 286.67 mg/g
[
112
]
CMC PVA — Ag+: 8.4 mg/g [57]
Deg easing co on Chi osan e hylenediamine e aace ic
acid (EDTA)
Pb2+: 1105.78 mg/g
Cu2+: 678.04 mg/g
[
113
]
Cellulose Chi osan glu a aldehyde (EGDE) as
c osslinking agen Hg2+: 495 mg/g
[
114
]
TEMPO-oxidized
nega i ely cha ged
(unidimensional)
cellulose nano ibe s
(TOCNFs)
Pa ially deace yla ed
chi in nano ibe s — As(III) ion: 217 mg/g
[
116
]
TEMPO-oxidized
nanocellulose
Ca boxyme hyl
chi osan and sodium
algina e
Ca2+ as c oss-linking agen Pb2+: 472.59 mg/g
Cu2+: 169.94 mg/g
[
117
]
Mic oc ys alline
cellulose
Chi osan,
polydopamine (PDA),
and
polye hyleneimine
(PEI)
—
Cu2+: 434.8 mg/g
Zn2+: 277.7 mg/g
Ni2+ 261.8 mg/g
[
118
]
Cellulose nanowhiske s
(CNWs)
Chi osan-g-poly
(ac ylic acid)
MBA as c oss-linking agen
and TEMED
as ca alys
Pb2+: 818.4 mg/g
Cu2+: 325.5 mg/g
[
120
]
Polyme s 2024,16, 1292 23 o 29
Polyme s 2024, 16, x FOR PEER REVIEW 23 o 31
Figu e 10. SEM images o su ace o PVA/chi osan/cellulose hyd ogel sphe es be o e and a e
adso p ion o hea y me al ions (le ). EDX spec a o hyd ogel sphe es be o e and a e adso p ion
o hea y me al ions ( igh ), om e . [122].
Table 3 epo s he capaci y adso p ion alues o he abo e p esen ed cellulose-
based hyd ogels and composi e adso ben s based on cellulose de i a i es.
Table 3. Composi ion and adso p ion capaci y o diffe en cellulose-based hyd ogels used o hea y
me al abso p ion.
Cellulose o Cellulose
De i a i e
Addi ional
Polyme
O he Componen s
o Modi ica ions Adso p ion Capaci y
Re .
Non-composi e
hyd ogels CMC ― epichlo ohyd in as
c oss-linking agen
Cu2+: 6.49 mmol/g
Pb2+: 5.15 mmol/g
Ni2+: 4.06 mmol/g
[96]
CMC ― aluminum ni a e as
c oss-linking agen
Pb2+: 550 mg/g
Ni2+: 620 mg/g
Co2+: 760 mg/g
[95]
Cellulose om bamboo
ibe s ―
i ana e nano ubes
Cu2+: 176.4 mg/g
Pb2+: 613.5 mg/g
[99]
Figu e 10. SEM images o su ace o PVA/chi osan/cellulose hyd ogel sphe es be o e and a e
adso p ion o hea y me al ions (le ). EDX spec a o hyd ogel sphe es be o e and a e adso p ion o
hea y me al ions ( igh ), om e . [122].
4. Conclusions
Polyme -based hyd ogels, due o hei abili y o abso b la ge olumes o wa e , ha e
eme ged as a p omising al e na i e ma e ial o he de elopmen o wa e decon amina ion
p ocesses. He ein, we ha e p o ided an o e iew o ecen s udies explo ing he use o
cellulose o cellulose de i a i e-based hyd ogels o was ewa e p ocessing o emo e
hea y me al con aminan s. Se e al p ope ies ha e made cellulose and i s de i a i es an
in e es ing class o ma e ials o he emedia ion o con amina ed wa e , including high
biodeg adabili y, abundance, low cos , op imal s abili y in a wide a ie y o condi ions, and
he la ge numbe o su ace unc ional g oups. Fu he mo e, since non-co alen in e ac ions,
such as hyd ogen bonds and ion pai s, a e commonly in ol ed in he emo al o hea y
me al con aminan s, cellulose-based sys ems can be po en ially eused o se e al cycles.
Polyme s 2024,16, 1292 24 o 29
Al hough hey a e he subjec o nume ous esea ch s udies and some pilo ials, hei
la ge-scale adop ion in eal wa e bodies is s ill a om ac ual implemen a ion.
This is due o he possible e ec o se e al addi ional ac o s when mo ing om
labo a o y-scale applica ions o he eal en i onmen , which can signi ican ly a ec me al
ion adso p ion p ocesses. Along wi h me al ions, eal was ewa e con ains a a ie y o con-
aminan s, including mic oplas ics, dyes, d ugs, oils, de e gen s, pes icides, and e ilize s.
The e o e, a he han me ely concen a ing on he in es iga ion o a single me al ion, u u e
adso p ion esea ch should be expanded no only o solu ions con aining di e en me al
ions o e alua ing possible c oss-in e e ence, bu abo e all, o mul ipollu an e alua ion.
O he physical/chemical cha ac e is ics o eal wa e bodies, namely, pH, empe a u e,
salini y, dissol ed oxygen, and ligh exposu e, can a ec me al ion adso p ion p ocesses due
o hei na u al a iabili y which possibly ep esen s an addi ional ac o o be conside ed.
None heless, i is a e o ind in he li e a u e he es ing o hese hyd ogels o eal wa e
samples o unde simula ed en i onmen al condi ions. The e ec i eness o hyd ogels
unde challenging en i onmen al condi ions mus he e o e be p io i ized.
En i onmen al condi ions can se e ely a ec also cellulose-based hyd ogel s abili y.
As men ioned abo e, hei biodeg adabili y ep esen s an addi ional alue o hei en i-
onmen al applica ions as i means comple e and sa e deg ada ion wi hou oxic p oduc s
being eleased. A he same ime, hei s abili y in eal wa e na u ally hos ing di e en mi-
c oo ganisms mus be conside ed and op imized o ind a easonable comp omise be ween
adso p ion p ocess kine ics and biodeg ada ion imescales. Also in his sense, hyd ogel
es s in eal en i onmen s a e manda o y.
Cos e alua ion ep esen s a key aspec o p omo ing a echnology ans e o he eal
wo ld. In he case o hyd ogels o en i onmen al applica ions, cos s a e ela ed no only o
ab ica ion p ocesses, bu also o hei egene a ion capaci y ha de e mines hei economic
iabili y. Howe e , al hough he hyd ogel egene a ion capaci y is e alua ed in a numbe
o in es iga ions, in mos cases, he adso p ion p ocess is mos ly ocused on, wi h li le o
no a en ion gi en o e ec i e hyd ogel euse. Mo e in-dep h s udies should be conduc ed
in his con ex o comp ehend he egene a ion capabili y o hyd ogels du ing a ious
cycles. Fu he mo e, he de eloped hyd ogel egene a ion p ocesses mus be sus ainable
no only in e ms o cos s bu also in e ms o en i onmen al impac : sa e pos - ea men
echniques o hea y me al deso p ion should be p e e ed o gua an eeing en i onmen
and ope a o p o ec ion du ing he whole p ocess.
Impo an ly, he p ac ical applicabili y o hyd ogels in was ewa e ea men can be
eased by hei in eg a ion wi h lowing beds, adso p ion columns, and o he de ices ha
can be easily handled by simply exposing he sys em i s ly o con amina ed wa e and
hen o egene a ion solu ions. This u he echnological s ep should be conside ed in he
design o hyd ogels as i could be e y a o able o hei la ge-scale applica ion, especially
in he case o bead-shaped hyd ogels. As discussed in he p esen ed li e a u e su ey, such
hyd ogel o ma has been success ully applied o was ewa e ea men s udies, aking
ad an age o high su ace a ea, which esul s in be e con ac wi h wa e and a la ge
adso p ion a ea. Thei packing wi hin p ope ly designed de ices allows o hei exposu e
o uni o m wa e low and makes hei handling easie , speeding up he whole p ocess.
These aspec s, i c i ically e alua ed in s udies ocused on he de elopmen o cellulose-
based adso ben s, can su ely help b idge he gap be ween labo a o y esea ch and p ac ical
applica ion, p o iding c i ical in o ma ion o he success o cellulose-based hyd ogels in
emo ing me al ions in eal en i onmen al condi ions.
Au ho Con ibu ions: Concep ualiza ion, F.P. and E.M.; me hodology, E.M.; in es iga ion, F.P.;
esou ces, C.M. and E.M.; da a cu a ion, F.P. and E.M.; w i ing-o iginal d a p epa a ion, F.P.; w i ing-
e iew and edi ing, C.M. and E.M.; supe ision, C.M. and E.M.; p ojec adminis a ion, E.M.; unding
acquisi ion, E.M. All au ho s ha e ead and ag eed o he published e sion o he manusc ip .
Funding: This wo k was pa ially unded by he Eu opean Union Ho izon Eu ope p og am unde
g an ag eemen No 101046946 (RESORB).
Polyme s 2024,16, 1292 25 o 29
Con lic s o In e es : The au ho s decla e no con lic s o in e es .
Abb e ia ions
CMC ca boxy me hyl cellulose
–COOH ca boxyl g oups
CCNF ca boxyla ed cellulose nano ibe
CM ca boxyme hyl g oup
CH chi osan
CA ci ic acid
EDTA e hylenediamine e aace ic acid g oup
EGDE glu a aldehyde
HEC hyd oxye hyl cellulose
–OH hyd oxyl g oups
MC me hylcellulose
MCC mic oc ys alline cellulose
CNF nano ib illa ed cellulose
NOCNF ni o-oxidized ca boxycellulose nano ibe s
PVA poly( inyl alcohol)
TOCNF TEMPO-oxidized nega i ely cha ged (unidimensional) cellulose nano ibe
XRD X- ay di ac ion
TEMED N,N,N′,N′- e ame hyle hylenediamine
MBA N,N′-me hylenebisac ylamide
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