Efficient Separation of Heavy Metals by Magnetic Nanostructured Beads

ino ganics
A icle
E icien Sepa a ion o Hea y Me als by Magne ic
Nanos uc u ed Beads
Lisand a de Cas o Al es , Susana Yáñez-Vila * , Yolanda Piñei o-Redondo and JoséRi as
Applied Physic Depa men , NANOMAG Labo a o y, Resea ch Technological Ins i u e, Uni e sidade de San iago
de Compos ela (USC), 15782 San iago de Compos ela, Spain; lisand ac is ina.decas [email protected] (L.d.C.A.);
yolanda. [email protected] (Y.P.-R.); jose. i [email protected] (J.R.)
*Co espondence: [email p o ec ed]
Recei ed: 9 Ap il 2020; Accep ed: 24 June 2020; Published: 26 June 2020


Abs ac :
This s udy epo s he abili y o magne ic algina e ac i a ed ca bon (MAAC) beads o
emo e Cd(II), Hg(II), and Ni(II) om wa e in a mono-me al and e na y sys em. The adso p ion
capaci y o he MAAC beads was highes in he mono-me al sys em. The emo al e iciency o such
me al ions alls in he ange o 20–80% and i ollowed he o de Cd(II) >Ni(II) >Hg(II). The model
ha bes i ed in he e na y sys em was he F eundlich iso he m, while in he mono-sys em i was
he Langmui iso he m. The maximum Cd(II), Hg(II), and Ni(II) adso p ion capaci ies calcula ed
om he F eundlich iso he m in he mono-me al sys em we e 7.09, 5.08, and 4.82 (mg/g) (mg/L)
1/n
,
espec i ely. Lowe adso p ion capaci y was obse ed in he e na y sys em due o he compe i ion o
me al ions o a ailable adso p ion si es. Deso p ion and eusabili y expe imen s demons a ed he
MAAC beads could be used o a leas i e consecu i e adso p ion/deso p ion cycles. These indings
sugges he p ac ical use o he MAAC beads as e icien adso ben o he emo al o hea y me als
om was ewa e .
Keywo ds:
hea y me als; magne i e nanopa icles; adso p ion; nanocomposi e; hyb id;
mul i-me al; wa e
1. In oduc ion
Wa e pollu ion by hea y me als has become a se ious p oblem due o he ad e se e ec s on
ecosys ems and human heal h. Mo e speci ically, cadmium (Cd), me cu y (Hg), lead (Pb), o nickel
(Ni) a e known o be highly ca cinogenic and mu agenic a low concen a ions, and may p oduce
acu e oxici y o e en dead in li ing o ganisms, when p esen sligh ly abo e hei allowed limi s [
1
–
3
].
Al hough di e se echnologies ha e been de eloped o he emo al o hea y me als om wa e
sou ces [
4
], he e is s ill an u gen need o acile cleaning p ocedu es ha ensu e high e iciency in he
low concen a ion anges. Chemical p ecipi a ion, ion p ecipi a ion, ion exchange, and adso p ion a e
some o he mos used echniques due o hei po en ial o scaling up. Among hem, he use o na u al
biopolyme s, such as algina e, aga ose, chi in and pec in [
5
–
7
] in me al bioso p ion om was ewa e s
has gained much a en ion in ecen yea s. Algina e is a polysaccha ide de i ed om b own algae and
in he majo i y o he s udies i has been used in he o m o calcium algina e beads, due o i s p ac ical
handling [
8
]. Ne e heless, he sepa a ion o he loaded bioma e ial om he medium is o en a
p oblem. To o e come his p oblem, magne i e nanopa icles (Fe
3
O
4
-NPs) a e being inco po a ed on o
he bioso ben ma ix [
9
–
11
] gi ing he possibili y o magne ically manipula e and sepa a e he hyb id
ma e ials om he wa e ma ix. Magne ic algina e beads a e a e y a ac i e ma e ial wi h mul iple
p ope ies such as high speci ic su ace a ea, apid eco e y, cos -e ec i eness, and chemical e sa ili y,
o which hey a e amenable o be combined wi h ma e ials o inc ease hei a ini y o pollu an s.
Humic acid [
12
], Cyanex 302 [
12
] and mic oalgae [
7
] we e inco po a ed in o algina e beads o hei
Ino ganics 2020,8, 40; doi:10.3390/ino ganics8060040 www.mdpi.com/jou nal/ino ganics
Ino ganics 2020,8, 40 2 o 12
a ini y o me al ions. Di e en au ho s ha e shown he abili y o se e al algina e beads composi ion
o up ake me al ions om aqueous solu ions. I was obse ed ha magne ic algina e beads con aining
silica coa ed wi h i on ca bide nanopa icles enhanced mo e he adso p ion o coppe ions han algina e
beads alone [
13
]. In addi ion, i was s udied ha magne ic nanopa icles unc ionalized wi h ci a e
ions p esen an enhanced adso p ion o Pb(II) me al ions om solu ion [
14
]. In one o ou p e ious
wo ks [
15
], magne ic algina e beads ailo ed wi h comme cial ac i a ed ca bon e ealed a high capaci y
o cadmium ions up ake. A 35% emo al pe cen age o cadmium ions was achie ed o e 1 h wi h
less han 15 mg o adso ben used. These nanos uc u ed beads e ealed o be a g ea assessmen o
indus ial use, o hei high adso p ion su ace a ea, easy handling, and magne ic sepa a ion om
any aqueous media. Fo hese easons, he same nanos uc u ed beads we e used in he p esen s udy
and es ed unde mo e ealis ic condi ions by s udying hei adso p ion capaci y when exposed o a
mix u e o hea y me als. Wi h his aim, di e en adso p ion es s we e pe o med on mono-me al
and e na y sys ems, comp ising Cd(II), Hg(II) and Ni(II) me als ions, which a e commonly ound in
was e wa e om indus y and mining e luen s. To gain insigh s in he adso p ion mechanism o he
ele an aspec s (e.g., me al-adso ben mechanisms, me al dis ibu ion on beads su ace and in e nal
s uc u e, me al deso p ion, euse, po osi y, among o he s) we e s udied in de ail.
2. Resul s and Discussion
2.1. Mo phology o Magne ic Beads
The mo phology o he MAAC beads and comme cial ac i a ed ca bon was examined using
scanning elec on mic oscopy (SEM) and ansmission elec on mic oscopy (TEM). In Figu e 1a,
SEM mic og aph shows ha comme cial ac i a ed ca bon p esen s a diso de ed laye -like s uc u e,
which was u he s udied wi h TEM, e eals a wide a ia ion in he laye size ange be ween 40
µ
m o
320
µ
m (Figu e 1b). SEM mic og aph (Figu e 1c) o a ep esen a i e MAAC bead shows a sphe ical
mo phology wi h a po ous and laye -like s uc u e on he su ace inhe i ed om he p ecu so
comme cial ac i a ed ca bon. The in e nal s uc u e, as can be obse ed in Figu e 1d, is a combina ion
o ac i a ed ca bon laye s wi hin he in e connec ed po ous ne wo k o algina e in MAAC beads.
Ino ganics 2020, 8, x 2 o 13
we e inco po a ed in o algina e beads o hei a ini y o me al ions. Di e en au ho s ha e shown
he abili y o se e al algina e beads composi ion o up ake me al ions om aqueous solu ions. I was
obse ed ha magne ic algina e beads con aining silica coa ed wi h i on ca bide nanopa icles
enhanced mo e he adso p ion o coppe ions han algina e beads alone [13]. In addi ion, i was
s udied ha magne ic nanopa icles unc ionalized wi h ci a e ions p esen an enhanced adso p ion
o Pb (II) me al ions om solu ion [14]. In one o ou p e ious wo ks [15], magne ic algina e beads
ailo ed wi h comme cial ac i a ed ca bon e ealed a high capaci y o cadmium ions up ake. A 35%
emo al pe cen age o cadmium ions was achie ed o e 1 h wi h less han 15 mg o adso ben used.
These nanos uc u ed beads e ealed o be a g ea assessmen o indus ial use, o hei high
adso p ion su ace a ea, easy handling, and magne ic sepa a ion om any aqueous media. Fo hese
easons, he same nanos uc u ed beads we e used in he p esen s udy and es ed unde mo e
ealis ic condi ions by s udying hei adso p ion capaci y when exposed o a mix u e o hea y me als.
Wi h his aim, di e en adso p ion es s we e pe o med on mono-me al and e na y sys ems,
comp ising Cd (II), Hg (II) and Ni (II) me als ions, which a e commonly ound in was e wa e om
indus y and mining e luen s. To gain insigh s in he adso p ion mechanism o he ele an aspec s
(e.g., me al-adso ben mechanisms, me al dis ibu ion on beads su ace and in e nal s uc u e, me al
deso p ion, euse, po osi y, among o he s) we e s udied in de ail.
2. Resul s and Discussion
2.1. Mo phology o Magne ic Beads
The mo phology o he MAAC beads and comme cial ac i a ed ca bon was examined using
scanning elec on mic oscopy (SEM) and ansmission elec on mic oscopy (TEM). In Figu e 1a, SEM
mic og aph shows ha comme cial ac i a ed ca bon p esen s a diso de ed laye -like s uc u e, which
was u he s udied wi h TEM, e eals a wide a ia ion in he laye size ange be ween 40 µm o 320
µm (Figu e 1b). SEM mic og aph (Figu e 1c) o a ep esen a i e MAAC bead shows a sphe ical
mo phology wi h a po ous and laye -like s uc u e on he su ace inhe i ed om he p ecu so
comme cial ac i a ed ca bon. The in e nal s uc u e, as can be obse ed in Figu e 1d, is a combina ion
o ac i a ed ca bon laye s wi hin he in e connec ed po ous ne wo k o algina e in MAAC beads.
Figu e 1. Comme cial ac i a ed ca bon SEM (a) and TEM (b) mic og aph. SEM image o he MAAC
bead su ace (c) and in e nal s uc u e (d).
2.2. S uc u al and Tex u al Cha ac e iza ion
The X- ay di ac ion pa e n o he MAAC beads in Figu e 2a shows he p esence o sha p
di ac ion peaks loca ed a 2θ = 30.1, 35.5, 43.2, 53.5, 57.1, 62.7°. This is he cha ac e is ic di ac ion
pa e n a ising om he e lec ion o planes (022), (113), (004), (224), (115) and (044) co esponding o
c ys alline Fe3O4-NPs embedded in he po ous beads. The MAAC bead’s su ace was also analyzed
by Fou ie ans o m in a ed (FT-IR)spec oscopy. As shown in Figu e 2b, he peaks a 3228 cm−1
and 1076 cm−1 a e ela ed o he -OH and -C-O s e ching ib a ion bands o algina e, espec i ely,
Figu e 1.
Comme cial ac i a ed ca bon SEM (
a
) and TEM (
b
) mic og aph. SEM image o he MAAC
bead su ace (c) and in e nal s uc u e (d).
2.2. S uc u al and Tex u al Cha ac e iza ion
The X- ay di ac ion pa e n o he MAAC beads in Figu e 2a shows he p esence o sha p
di ac ion peaks loca ed a 2
θ
=30.1, 35.5, 43.2, 53.5, 57.1, 62.7
◦
. This is he cha ac e is ic di ac ion
pa e n a ising om he e lec ion o planes (022), (113), (004), (224), (115) and (044) co esponding o
c ys alline Fe
3
O
4
-NPs embedded in he po ous beads. The MAAC bead’s su ace was also analyzed
by Fou ie ans o m in a ed (FT-IR)spec oscopy. As shown in Figu e 2b, he peaks a 3228 cm
−1
and
Ino ganics 2020,8, 40 3 o 12
1076 cm
−1
a e ela ed o he –OH and –C–O s e ching ib a ion bands o algina e, espec i ely, while
peaks a 1585 and 1286 cm−1a e a ibu ed o he asymme ic and symme ic s e ching ib a ions o
he ca boxyl g oups o algina e, espec i ely [
16
]. Finally, he band a 558 cm
−1
is due o collec i e
ib a ions o he magne i e la ice [
17
], which con i ms he success ul inco po a ion o Fe
3
O
4
-NPs in o
he algina e ma ix.
Ino ganics 2020, 8, x 3 o 13
while peaks a 1585 and 1286 cm
−1
a e a ibu ed o he asymme ic and symme ic s e ching
ib a ions o he ca boxyl g oups o algina e, espec i ely [16]. Finally, he band a 558 cm
−1
is due o
collec i e ib a ions o he magne i e la ice [17], which con i ms he success ul inco po a ion o
Fe
3
O
4
-NPs in o he algina e ma ix.
(a) (b)
Figu e 2. (a) X- ay di ac ion and in a ed spec a and (b) IR spec a o MAAC beads.
The mean pa icle size o he Fe
3
O
4
nanopa icles employed o he syn hesis o he MAAC beads
we e calcula ed om he X- ay di ac ion (XRD) pa e n (Figu e S1) acco ding o he linewid h o
he (113) plane e ac ion peak using Sche e equa ion:
L = Kλ/βcosθ
(1)
whe e L is he mean size o he o de ed (c ys alline) domains, λ is he X- ay wa eleng h, β is he
wid h o he XRD peak a hal heigh , K is a shape ac o , abou 0.9 o magne i e and θ is he B agg
angle The pa icle diame e calcula ed om he X- ay di ac og am, using he Sche e equa ion was
9.58 nm. By TEM he image (Figu e S1) we can obse e nanopa icles wi h a size o 20 nm. The
di e ence be ween TEM and XRD can be a ibu ed o he ac ha “c ys alli e size” is no
synonymous wi h “pa icle size”, while XRD is sensi i e o he c ys alli e size inside he pa icles.
2.3. Speci ic Su ace A e and Po e-Dis ibu ion
The ex u al p ope ies o comme cial ac i a ed ca bon (comme cial AC) and MAAC beads we e
analyzed wi h B unaue –Emme –Telle (BET) po osime y and shown in Table 1. The su ace a ea
o comme cial AC (849.32 m
2
/g) is wi hin he heo ic ange o ac i a ed ca bons (500 o 3.000 m
2
/g)
[8]. The su ace a ea o he MAAC beads is 107.13 m
2
/g which is wo o de s o magni ude highe han
he epo ed 6.25 m
2
/g o calcium algina e beads [18], and can be asc ibed o he p esence o ac i a ed
ca bon. The po e olume o he MAAC beads (0.075 cm
3
/g) was smalle han he one o comme cial
AC (0.28 cm
3
/g). In addi ion, he po e diame e size o he MAAC beads (1.24 nm) is sligh ly smalle
han he comme cial AC (1.26 nm). This could be a ibu ed o he deposi ion o he comme cial AC
on he su ace o he MAAC beads (bo h su ace and in e nally), leading o a comple e illing o he
smalle po es.
Table 1. Tex u al pa ame e s o Comme cial AC and MAAC beads.
Samples BET Su ace A ea (S
BET
)
1
(m
2
/
g) Po e Volume (cm
3
/
g) Po e diame e
2
(nm)
Comme cial AC 849.32 0.28 1.26
MAAC 107.13 0.075 1.24
1
S
BET
is he BET su ace a ea e alua ed a a ela i e p essu e (p/p0) o 0.99.
2
Po e diame e calcula ed
using he Ba e -Joyne -Halenda (BJH) me hod.
Figu e 2. (a) X- ay di ac ion and in a ed spec a and (b) IR spec a o MAAC beads.
The mean pa icle size o he Fe
3
O
4
nanopa icles employed o he syn hesis o he MAAC beads
we e calcula ed om he X- ay di ac ion (XRD) pa e n (Figu e S1) acco ding o he linewid h o he
(113) plane e ac ion peak using Sche e equa ion:
L=Kλ/βcosθ(1)
whe e L is he mean size o he o de ed (c ys alline) domains,
λ
is he X- ay wa eleng h,
β
is he wid h
o he XRD peak a hal heigh , K is a shape ac o , abou 0.9 o magne i e and
θ
is he B agg angle The
pa icle diame e calcula ed om he X- ay di ac og am, using he Sche e equa ion was 9.58 nm.
By TEM he image (Figu e S1) we can obse e nanopa icles wi h a size o 20 nm. The di e ence
be ween TEM and XRD can be a ibu ed o he ac ha “c ys alli e size” is no synonymous wi h
“pa icle size”, while XRD is sensi i e o he c ys alli e size inside he pa icles.
2.3. Speci ic Su ace A e and Po e-Dis ibu ion
The ex u al p ope ies o comme cial ac i a ed ca bon (comme cial AC) and MAAC beads we e
analyzed wi h B unaue –Emme –Telle (BET) po osime y and shown in Table 1. The su ace a ea o
comme cial AC (849.32 m
2
/g) is wi hin he heo ic ange o ac i a ed ca bons (500 o 3.000 m
2
/g) [
8
].
The su ace a ea o he MAAC beads is 107.13 m
2
/g which is wo o de s o magni ude highe han he
epo ed 6.25 m
2
/g o calcium algina e beads [
18
], and can be asc ibed o he p esence o ac i a ed
ca bon. The po e olume o he MAAC beads (0.075 cm
3
/g) was smalle han he one o comme cial
AC (0.28 cm
3
/g). In addi ion, he po e diame e size o he MAAC beads (1.24 nm) is sligh ly smalle
han he comme cial AC (1.26 nm). This could be a ibu ed o he deposi ion o he comme cial AC
on he su ace o he MAAC beads (bo h su ace and in e nally), leading o a comple e illing o he
smalle po es.
Ino ganics 2020,8, 40 4 o 12
Table 1. Tex u al pa ame e s o Comme cial AC and MAAC beads.
Samples BET Su ace A ea (SBET)1(m2/g) Po e Volume (cm3/g) Po e Diame e 2(nm)
Comme cial AC 849.32 0.28 1.26
MAAC 107.13 0.075 1.24
1
S
BET
is he BET su ace a ea e alua ed a a ela i e p essu e (p/p0) o 0.99.
2
Po e diame e calcula ed using he
Ba e –Joyne –Halenda (BJH) me hod.
2.4. Magne ic P ope ies o MAAC Beads
Figu e 3a shows he a ia ion o magne iza ion, M, as a unc ion o empe a u e o MAAC beads
in he ange 5 o 350 K in an ex e nal magne ic ield o 100 Oe eco ded in ze o- ield cooling (ZFC)
and ield cooling (FC). F om he cu es i is clea ly obse ed he supe imposi ion o he ZFC and FC
cu es ake place a 275 K. The supe imposi ion o ZFC and FC cu es is one o he cha ac e is ic
ea u es o a supe pa amagne ic sys em. The magne ic con en on he MAAC beads was calcula ed by
he mog a ime ic analyses (TGA), which was equal o 23%. Figu e 3b illus a es he magne iza ion
cu es o ba e Fe
3
O
4
-NPs and o he MAAC nanocomposi e beads. The sa u a ion o magne iza ion
(Ms) o he syn hesized Fe
3
O
4
-NPs (69.23 emu/g) was highe han he obse ed o he MAAC beads
(48.62 emu/g). This may be a ibu ed o he coa ing e ec o algina e apping he Fe
3
O
4
-NPs in he
gel ma ix. Howe e , he MAAC beads ha e supe pa amagne ic beha io and a e easily sepa a ed
om solu ion wi h he help o an ex e nal magne ic o ce.
Ino ganics 2020, 8, x 4 o 13
2.4. Magne ic P ope ies o MAAC Beads
Figu e 3a shows he a ia ion o magne iza ion, M, as a unc ion o empe a u e o MAAC beads
in he ange 5 o 350 K in an ex e nal magne ic ield o 100 Oe eco ded in ze o- ield cooling (ZFC)
and ield cooling (FC). F om he cu es i is clea ly obse ed he supe imposi ion o he ZFC and FC
cu es ake place a 275 K. The supe imposi ion o ZFC and FC cu es is one o he cha ac e is ic
ea u es o a supe pa amagne ic sys em. The magne ic con en on he MAAC beads was calcula ed
by he mog a ime ic analyses (TGA), which was equal o 23%. Figu e 3b illus a es he
magne iza ion cu es o ba e Fe
3
O
4
-NPs and o he MAAC nanocomposi e beads. The sa u a ion o
magne iza ion (Ms) o he syn hesized Fe
3
O
4
-NPs (69.23 emu/g) was highe han he obse ed o
he MAAC beads (48.62 emu/g). This may be a ibu ed o he coa ing e ec o algina e apping he
Fe
3
O
4
-NPs in he gel ma ix. Howe e , he MAAC beads ha e supe pa amagne ic beha io and a e
easily sepa a ed om solu ion wi h he help o an ex e nal magne ic o ce.
(a) (b)
Figu e 3. ZFC and FC cu e eco ded a 100 Oe (a) and magne iza ion cu e o MAAC bead and
Fe
3
O
4
-NPs a 25 °C (b).
3. Adso p ion S udy
3.1. Deso p ion and Reusabili y
The eusabili y o he MAAC bead was s udy by epe i i e adso p ion and deso p ion cycles o
Cd (II) me al ions using 0.01 M HCl solu ion, as he deso p ion solu ion. The q (mg/g) deso p ion o
cadmium was calcula ed di ec ly om he amoun o cadmium adso bed and amoun o cadmium
deso bed using Equa ion (2):
𝑞(𝑑𝑒𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛)=(𝑀 −𝑀
)𝑉
𝑀 (2)
whe e M
adso b
and M
deso b
a e he adso bed and deso bed amoun o me al ions (mg/g), espec i ely; V
(L) is he olume o deso p ion solu ion and M (g) he mass o he MAAC beads used. The deso p ion
pe cen age was calcula ed using he ollowing Equa ion (3):
% 𝐷𝑒𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 = 𝑀
𝑀
× 100 (3)
The deso p ion o cadmium om MAAC bead showed a ai deso p ion pe cen age o e he i e
cycles, as can be obse ed in Figu e 4. Howe e , he adso p ion capaci y dec eased wi h inc easing
egene a ion cycle numbe , excep o on cycle numbe wo. The cadmium me al ions ha e en e ed
inside o he beads s uc u e a e he i s cycle (Figu e S2) esul ing in he maximum adso p ion
capaci y on cycle wo. Con inuous adso p ion esul ed on in e nal po e sa u a ion and consequen ly,
on he dec eased o adso p ion capaci y and inc ease amoun o deso bed cadmium me al ions ound
Figu e 3.
ZFC and FC cu e eco ded a 100 Oe (
a
) and magne iza ion cu e o MAAC bead and
Fe3O4-NPs a 25 ◦C (b).
3. Adso p ion S udy
3.1. Deso p ion and Reusabili y
The eusabili y o he MAAC bead was s udy by epe i i e adso p ion and deso p ion cycles o
Cd(II) me al ions using 0.01 M HCl solu ion, as he deso p ion solu ion. The q (mg/g) deso p ion o
cadmium was calcula ed di ec ly om he amoun o cadmium adso bed and amoun o cadmium
deso bed using Equa ion (2):
q(deso p ion)=(Madso b −Mdeso b)V
M(2)
Ino ganics 2020,8, 40 5 o 12
whe e M
adso b
and M
deso b
a e he adso bed and deso bed amoun o me al ions (mg/g), espec i ely;
V(L) is he olume o deso p ion solu ion and M(g) he mass o he MAAC beads used. The deso p ion
pe cen age was calcula ed using he ollowing Equa ion (3):
%Deso p ion =Mdeso b
Madso b
×100 (3)
The deso p ion o cadmium om MAAC bead showed a ai deso p ion pe cen age o e he i e
cycles, as can be obse ed in Figu e 4. Howe e , he adso p ion capaci y dec eased wi h inc easing
egene a ion cycle numbe , excep o on cycle numbe wo. The cadmium me al ions ha e en e ed
inside o he beads s uc u e a e he i s cycle (Figu e S2) esul ing in he maximum adso p ion
capaci y on cycle wo. Con inuous adso p ion esul ed on in e nal po e sa u a ion and consequen ly,
on he dec eased o adso p ion capaci y and inc ease amoun o deso bed cadmium me al ions ound
in solu ion. Fu he mo e, he esul s sugges a educ ion o he me al ions om he ma ix in he
i s ou h cycles and mo e han 50% o me al was eco e ed. A e he i h cycle, he deso p ion
pe cen age dec eased o 20%, since cadmium me al ions emained inside o he beads s uc u e
a e he hi d deso p ion cycle. Thus, i can be said ha hese beads ha e he po en ial o be
euse up o ou cycles unde he chosen condi ions. Fo u he s udies, ac o s, such as ime and
deso ben concen a ion mus be conside ed o maximum deso p ion capaci y o mo e han 5 cycles.
In p e iously s udies [
19
], hyd ochlo ic acid p o ed o be a good deso ben wi hin 2 h o epe i i e
adso p ion and deso p ion cycles.
Ino ganics 2020, 8, x 5 o 13
in solu ion. Fu he mo e, he esul s sugges a educ ion o he me al ions om he ma ix in he i s
ou h cycles and mo e han 50% o me al was eco e ed. A e he i h cycle, he deso p ion
pe cen age dec eased o 20%, since cadmium me al ions emained inside o he beads s uc u e a e
he hi d deso p ion cycle. Thus, i can be said ha hese beads ha e he po en ial o be euse up o
ou cycles unde he chosen condi ions. Fo u he s udies, ac o s, such as ime and deso ben
concen a ion mus be conside ed o maximum deso p ion capaci y o mo e han 5 cycles. In
p e iously s udies [19], hyd ochlo ic acid p o ed o be a good deso ben wi hin 2 h o epe i i e
adso p ion and deso p ion cycles.
Figu e 4. Adso p ion and Deso p ion cycles o cadmium om MAAC beads (mean ± SE o 15
eplica es).
3.2. E ec o pH
The in luence o pH on Cd (II), Hg (II) and Ni (II) adso p ion capaci y by he MAAC beads was
s udied a pH ange o (2.0 o 9.0). In Figu e 5, he adso bed me al ions pe adso ben mass q (mg/g)
a e p esen ed o all es ed pH alues. As can be seen, he adso p ion p ocess o Cd (II) and Ni (II),
was cons an be ween he pH alues o 4.5 o 7.0, ollowed by an inc ease a highe pH alues. On
con a y, Hg (II) adso p ion depends in a non-p edic able way on he pH, a aining a maximum
adso p ion capaci y a pH 4.5, ollowed by a s eep dec ease a pH 6.5 and a la ge inc emen up o pH
9.0. The highe concen a ion o hyd olyzed ions such as H
+
allow an enhanced binding o Hg (II)
me al ions o he sodium algina e su ace [19]. Besides his binding mechanism being mo e enhanced
a pH 2.0 o Hg (II) ions, a pH 4.5 all me al ions e ealed o ha e a cons an and high adso p ion
capaci y a his pH, being he selec ed o he expe imen s a he mono and e na y sys em. The
inc easing adso p ion a highe pH alues o all me al ions may be a ibu ed o he o ma ion o
hyd oxyl ions [20].
Figu e 5. E ec o ini ial pH on Cd (II), Hg (II) and Ni (II) adso p ion on o MAAC beads (mean
s anda d de ia ion ± 0.3).
Figu e 4.
Adso p ionandDeso p ioncycleso cadmium omMAACbeads(mean
±
SEo 15 eplica es).
3.2. E ec o pH
The in luence o pH on Cd(II), Hg(II) and Ni(II) adso p ion capaci y by he MAAC beads was s udied
a pH ange o (2.0 o 9.0). In Figu e 5, he adso bed me al ions pe adso ben mass q (mg/g) a e p esen ed
o all es ed pH alues. As can be seen, he adso p ion p ocess o Cd(II) and Ni(II), was cons an
be ween he pH alues o 4.5 o 7.0, ollowed by an inc ease a highe pH alues. On con a y, Hg(II)
adso p ion depends in a non-p edic able way on he pH, a aining a maximum adso p ion capaci y a pH
4.5, ollowed by a s eep dec ease a pH 6.5 and a la ge inc emen up o pH 9.0. The highe concen a ion
o hyd olyzed ions such as H
+
allow an enhanced binding o Hg(II) me al ions o he sodium algina e
su ace [
19
]. Besides his binding mechanism being mo e enhanced a pH 2.0 o Hg(II) ions, a pH 4.5 all
me al ions e ealed o ha e a cons an and high adso p ion capaci y a his pH, being he selec ed o he
expe imen s a he mono and e na y sys em. The inc easing adso p ion a highe pH alues o all me al
ions may be a ibu ed o he o ma ion o hyd oxyl ions [20].
A highe pH alues, he dec eased adso p ion capaci y by Cd(II) coincides wi h he dec easing
concen a ion o Cd(II) and he p ecipi a ion o Cd(OH)
2
in o solu ion. The Cd(OH)
2
ionic species a e
adso bed by he MAAC beads occupying he a ailable si es and p e en ing he u he adso p ion o
Cd(II) ions [
21
]. Fo nickel an inc easing end a highe pH alues was obse ed. This cha ac e is ic
end is a ibu ed o he speci ic adso p ion o ca ionic hyd oxo-complexes, which is he pH ange

Ino ganics 2020,8, 40 6 o 12
mo e a o able o he o ma ion o hese species [
22
]. The hyd olysis o me cu y, on he o he hand,
begins a e y low pH alues (pH <4.5) wi h he o ma ion o Hg(OH)
+
and a pH alues (pH >4.5)
he adso p ion capaci y suddenly inc eases because o he o ma ion o me cu y neu al species
Hg(OH)2[23].
Ino ganics 2020, 8, x 5 o 13
in solu ion. Fu he mo e, he esul s sugges a educ ion o he me al ions om he ma ix in he i s
ou h cycles and mo e han 50% o me al was eco e ed. A e he i h cycle, he deso p ion
pe cen age dec eased o 20%, since cadmium me al ions emained inside o he beads s uc u e a e
he hi d deso p ion cycle. Thus, i can be said ha hese beads ha e he po en ial o be euse up o
ou cycles unde he chosen condi ions. Fo u he s udies, ac o s, such as ime and deso ben
concen a ion mus be conside ed o maximum deso p ion capaci y o mo e han 5 cycles. In
p e iously s udies [19], hyd ochlo ic acid p o ed o be a good deso ben wi hin 2 h o epe i i e
adso p ion and deso p ion cycles.
Figu e 4. Adso p ion and Deso p ion cycles o cadmium om MAAC beads (mean ± SE o 15
eplica es).
3.2. E ec o pH
The in luence o pH on Cd (II), Hg (II) and Ni (II) adso p ion capaci y by he MAAC beads was
s udied a pH ange o (2.0 o 9.0). In Figu e 5, he adso bed me al ions pe adso ben mass q (mg/g)
a e p esen ed o all es ed pH alues. As can be seen, he adso p ion p ocess o Cd (II) and Ni (II),
was cons an be ween he pH alues o 4.5 o 7.0, ollowed by an inc ease a highe pH alues. On
con a y, Hg (II) adso p ion depends in a non-p edic able way on he pH, a aining a maximum
adso p ion capaci y a pH 4.5, ollowed by a s eep dec ease a pH 6.5 and a la ge inc emen up o pH
9.0. The highe concen a ion o hyd olyzed ions such as H
+
allow an enhanced binding o Hg (II)
me al ions o he sodium algina e su ace [19]. Besides his binding mechanism being mo e enhanced
a pH 2.0 o Hg (II) ions, a pH 4.5 all me al ions e ealed o ha e a cons an and high adso p ion
capaci y a his pH, being he selec ed o he expe imen s a he mono and e na y sys em. The
inc easing adso p ion a highe pH alues o all me al ions may be a ibu ed o he o ma ion o
hyd oxyl ions [20].
Figu e 5. E ec o ini ial pH on Cd (II), Hg (II) and Ni (II) adso p ion on o MAAC beads (mean
s anda d de ia ion ± 0.3).
Figu e 5.
E ec o ini ial pH on Cd(II), Hg(II) and Ni(II) adso p ion on o MAAC beads (mean s anda d
de ia ion ±0.3).
3.3. Adso p ion o Mono and Te na y Sys ems
Ba ch adso p ion expe imen s we e pe o med using a de ined amoun o MAAC beads in mono
and e na y sys ems con aining Cd(II), Ni(II) and Hg(II) me al ions. The e ec o he ini ial me al ion
concen a ion on adso p ion was s udied o solu ions p epa ed wi h a se o concen a ions 10 o
250 mg/L, and applying p e iously op imized condi ions (magne ic agi a ion a 300 pm, using 14 mg
o adso ben du ing 6 h) a ixed pH =4.5. The equilib ium adso p ion capaci y (
qe
) was calcula ed
acco ding o he ollowing equa ion [24]:
qe=(C0−Ce)V
M(4)
The emo al e iciency (R%) o Cd(II), Hg(II) and Ni(II) in he mono-componen sys em was
calcula ed using he ollowing equa ion:
R%=(C0−Ce)
C0
×100 (5)
whe e
qe
is he equilib ium adso p ion capaci y (mg/g);
C0
and
Ce
a e he ini ial and equilib ium
concen a ion (mg/L) o me al ions, espec i ely; Vis he olume o wo king solu ion (L) and Mis he
weigh (g) o adso ben used. Al e na i ely, q, can be exp essed in e ms o mola i y, q(mol/g), o gain
insigh s in o he numbe o moles (a oms) ha adso b on he cleaning beads (Figu e S3).
In Figu e 6, he adso p ion capaci y, q(mg/g), o Cd(II), Ni(II) and Hg(II) by he MAAC beads, o
he mono and e na y me al adso p ion ba ch es s a e p esen ed e sus he ini ial concen a ion in
he ba ch solu ion o each me al. F om all es ed me als unde he cu en expe imen al condi ions,
i is e iden ha Cd(II) ions a e p e e en ially adso bed in bo h es s, being in he mono-me al case
(Figu e 6a) he adso p ion capaci y o Cd(II) ions wice he adso p ion obse ed o Ni(II) and Hg(II).
In addi ion, in he mono-me al es , he adso p ion o cadmium and nickel ions inc eases uni o mly
indica ing ongoing adso p ion p ocess on o he MAAC beads, on con a y o me cu y, which shows a
dec eased adso p ion a he ini ial concen a ion o 150 mg/L. In he e na y me al sys em (Figu e 6b),
he adso p ion capaci y is gene ally smalle han in he mono-me al case, indica ing a compe i ion
be ween he me al ions in he e na y sys em o a ailable binding si es on he MAAC beads [
25
], wi h a
s iking excep ion a ini ial me al concen a ion o C=150 mg/L, whe e all me al ions a e adso bed
Ino ganics 2020,8, 40 7 o 12
wi h mo e e icacy. Mo eo e , he simila i ies o he adso p ion capaci y cu e o all ions in he e na y
sys em wi h he me cu y adso p ion ea u e in he mono-me al sys em sugges s ha me cu y has an
impo an ole modula ing he adso p ion mechanism, which will dese e u u e s udies.
The emo al e iciency dec eases when he ini ial me al ions concen a ion is inc eased (Figu e 7),
a end ha was al eady obse ed in a p e ious s udy [
15
]. This beha io is mainly asc ibed o he
sa u a ion o he a ailable binding si es du ing he adso p ion p ocess, leading o a educ ion o he
adso p ion capaci y.
Ino ganics 2020, 8, x 6 o 13
A highe pH alues, he dec eased adso p ion capaci y by Cd (II) coincides wi h he dec easing
concen a ion o Cd (II) and he p ecipi a ion o Cd(OH)2 in o solu ion. The Cd(OH)2 ionic species a e
adso bed by he MAAC beads occupying he a ailable si es and p e en ing he u he adso p ion o
Cd (II) ions [21]. Fo nickel an inc easing end a highe pH alues was obse ed. This cha ac e is ic
end is a ibu ed o he speci ic adso p ion o ca ionic hyd oxo-complexes, which is he pH ange
mo e a o able o he o ma ion o hese species [22]. The hyd olysis o me cu y, on he o he hand,
begins a e y low pH alues (pH < 4.5) wi h he o ma ion o Hg(OH)+ and a pH alues (pH > 4.5)
he adso p ion capaci y suddenly inc eases because o he o ma ion o me cu y neu al species
Hg(OH)2 [23].
3.3. Adso p ion o Mono and Te na y Sys ems
Ba ch adso p ion expe imen s we e pe o med using a de ined amoun o MAAC beads in mono
and e na y sys ems con aining Cd (II), Ni (II) and Hg (II) me al ions. The e ec o he ini ial me al
ion concen a ion on adso p ion was s udied o solu ions p epa ed wi h a se o concen a ions 10 o
250 mg/L, and applying p e iously op imized condi ions (magne ic agi a ion a 300 pm, using 14
mg o adso ben du ing 6 h) a ixed pH = 4.5. The equilib ium adso p ion capaci y (𝑞) was calcula ed
acco ding o he ollowing equa ion [24]:
𝑞=(𝐶−𝐶
)𝑉
M (4)
The emo al e iciency (R%) o Cd (II), Hg (II) and Ni (II) in he mono-componen sys em was
calcula ed using he ollowing equa ion:
𝑅% = (𝐶−𝐶
)
𝐶
× 100 (5)
whe e 𝑞 is he equilib ium adso p ion capaci y (mg/g); 𝐶 and 𝐶 a e he ini ial and equilib ium
concen a ion (mg/L) o me al ions, espec i ely; V is he olume o wo king solu ion (L) and M is he
weigh (g) o adso ben used. Al e na i ely, q, can be exp essed in e ms o mola i y, q (mol/g), o
gain insigh s in o he numbe o moles (a oms) ha adso b on he cleaning beads (Figu e S3).
In Figu e 6, he adso p ion capaci y, q (mg/g), o Cd (II), Ni (II) and Hg (II) by he MAAC beads,
o he mono and e na y me al adso p ion ba ch es s a e p esen ed e sus he ini ial concen a ion
in he ba ch solu ion o each me al. F om all es ed me als unde he cu en expe imen al condi ions,
i is e iden ha Cd (II) ions a e p e e en ially adso bed in bo h es s, being in he mono-me al case
(Figu e 6a) he adso p ion capaci y o Cd (II) ions wice he adso p ion obse ed o Ni (II) and Hg
(II).
(a) (b)
Figu e 6. E ec o ini ial me al concen a ion on he adso p ion capaci y o MAAC beads o (a) mono-
me al and (mean SD ± 1.45) (b) e na y expe imen a pH 4.5 (mean SD ± 1.27).
Figu e 6.
E ec o ini ial me al concen a ion on he adso p ion capaci y o MAAC beads o
(a) mono-me al and (mean SD ±1.45) (b) e na y expe imen a pH 4.5 (mean SD ±1.27).
Ino ganics 2020, 8, x 7 o 13
In addi ion, in he mono-me al es , he adso p ion o cadmium and nickel ions inc eases
uni o mly indica ing ongoing adso p ion p ocess on o he MAAC beads, on con a y o me cu y,
which shows a dec eased adso p ion a he ini ial concen a ion o 150 mg/L. In he e na y me al
sys em (Figu e 6b), he adso p ion capaci y is gene ally smalle han in he mono-me al case,
indica ing a compe i ion be ween he me al ions in he e na y sys em o a ailable binding si es on
he MAAC beads [25], wi h a s iking excep ion a ini ial me al concen a ion o C = 150 mg/L, whe e
all me al ions a e adso bed wi h mo e e icacy. Mo eo e , he simila i ies o he adso p ion capaci y
cu e o all ions in he e na y sys em wi h he me cu y adso p ion ea u e in he mono-me al sys em
sugges s ha me cu y has an impo an ole modula ing he adso p ion mechanism, which will
dese e u u e s udies.
The emo al e iciency dec eases when he ini ial me al ions concen a ion is inc eased (Figu e
7), a end ha was al eady obse ed in a p e ious s udy [15]. This beha io is mainly asc ibed o he
sa u a ion o he a ailable binding si es du ing he adso p ion p ocess, leading o a educ ion o he
adso p ion capaci y.
Figu e 7. Remo al e iciency o mono-me al and e na y expe imen s a pH 4.5.
3.4. Compe i i e Adso p ion E alua ion
The in e ac i e e ec o Cd (II), Hg (II) and Ni (II) was in es iga ed in he e na y sys em. Fo
ha , an e alua ion a io was in oduced o assess he ype o adso p ion compe i ion be ween each
me al ion in he sys em and combined [26]. The e alua ion a io is exp essed by he ollowing
Equa ion (6):
𝐸=𝑄
󰆒
𝑄
(6)
whe e 𝐸, is he e alua ion a io; 𝑄
󰆒 (mg/g) is he amoun o me al ions adso bed in a e na y sys em
and 𝑄 (mg/g) is he amoun o me al ions adso bed in he mono-me al sys em. I he e alua ion
a io, E > 1, he p esence o o he me al ions ha e enhanced he adso p ion o o he me al ions in
solu ion (syne gism e ec ); when E = 1, his means ha he p esence o o he me al ions would no
in luence he adso p ion o ano he me al ion; and when E < 1, he p esence o ano he me al ion
would supp ess he adso p ion o one ano he (an agonism e ec ). The e alua ion a ios o
indi idual Cd (II), Hg (II) and Ni (II) in he e na y sys em and he whole mix u e a e lis ed in Table
2.
Figu e 7. Remo al e iciency o mono-me al and e na y expe imen s a pH 4.5.
3.4. Compe i i e Adso p ion E alua ion
The in e ac i e e ec o Cd(II), Hg(II) and Ni(II) was in es iga ed in he e na y sys em. Fo ha ,
an e alua ion a io was in oduced o assess he ype o adso p ion compe i ion be ween each me al
ion in he sys em and combined [
26
]. The e alua ion a io is exp essed by he ollowing Equa ion (6):
E=Q0
e
Qe(6)
whe e
E
, is he e alua ion a io;
Q0
e
(mg/g) is he amoun o me al ions adso bed in a e na y sys em
and
Qe
(mg/g) is he amoun o me al ions adso bed in he mono-me al sys em. I he e alua ion a io,
E>1, he p esence o o he me al ions ha e enhanced he adso p ion o o he me al ions in solu ion
(syne gism e ec ); when E=1, his means ha he p esence o o he me al ions would no in luence he
Ino ganics 2020,8, 40 8 o 12
adso p ion o ano he me al ion; and when E<1, he p esence o ano he me al ion would supp ess
he adso p ion o one ano he (an agonism e ec ). The e alua ion a ios o indi idual Cd(II), Hg(II)
and Ni(II) in he e na y sys em and he whole mix u e a e lis ed in Table 2.
Table 2.
Indi idual and sum o he e alua ion a ios o Cd(II), Hg(II) and Ni(II) in he e na y sys em
o MAAC beads.
Ini ial Concen a ion (mg/L) 10 70 150 200 250
Adso ben Me al Ions E alua ion Ra ios
MAAC
Cd(II) 1.08 0.58 1.10 0.35 0.56
Hg(II) 0.14 0.66 1.46 0.44 1.36
Ni(II) 1.05 0.63 1.38 0.20 0.26
Cd(II) +Hg(II) +Ni(II) 0.68 0.71 1.48 0.32 0.48
Cadmium and nickel in bo h mono and e na y sys ems we e mos e icien ly adso bed han
me cu y (Figu e 6a,b). On con a y, he s udy o he e alua ion a ios in e ms o compe i i e scena io
e eals ha me cu y ions a e he leas supp essed o all h ee me al ions. The esul s compiled in
Table 2, e eal an an agonis ic e ec (E<1) almos in all concen a ions in he indi idual me als and on
he whole mix u e o he e na y sys em, excep a C=150 (mg/L), whe e E=1.48 (E>1), indica ing an
a e age syne gis ic beha io be ween he me al ions. Fo high alues o ini ial me al concen a ions o
C=200 (mg/L) and C=250 (mg/L), he deg ee o supp ession is la ge o nickel and cadmium ions,
while me cu y adso p ion is a o ed by a syne gis ic e ec . Mo eo e , o C
≥
70 (mg/L) me cu y is he
leas supp essed me al and bene i s om an enhancemen e ec , in pa due o he inc ease elec os a ic
epulsion among he ca ions ha would limi he adso p ion o he me al ion [27].
3.5. Adso p ion Iso he ms
To de e mine he adso p ion capaci y o he MAAC beads in a mono-me al and e na y sys em,
adso p ion s udies wi h ini ial concen a ions anging om C=10 (mg/L), o C=250 (mg/L), we e
ca ied ou . The adso p ion equilib ium was s udied i ing he expe imen al da a o he linea
equa ions o Langmui and F eundlich iso he m models.
Langmui iso he m model [28]:
Ce
qe
=1
KLqm
+Ce
qm
(7)
F eundlich iso he m model [29]:
logqe=logKF+1
nlogCe(8)
whe e
qe
(mg/g) is he amoun o me al ions adso bed;
Ce
(mg/L) is he adso ba e concen a ion in
solu ion, bo h a equilib ium; K
L
(L/mg) is he Langmui adso p ion cons an ; and
qm
(mg/g) is he
maximum adso p ion capaci y o monolaye o ma ion on he adso ben . The alue
KF
can be de ined
as he adso p ion o dis ibu ion coe icien and ep esen s he quan i y o me al ions adso bed on o
he beads. The alue o 1
/n
indica es su ace he e ogenei y, which becomes mo e he e ogeneous as
i s alue ge s close o ze o. A undamen al cha ac e is ic o he Langmui iso he m is o p edic he
a ini y be ween so ba e and so ben using a dimensionless cons an , known as sepa a ion ac o
RL
,
which can be ep esen ed as:
RL=1
1+KLC0(9)
whe e
C0
(mg/L) is he adso ba e ini ial concen a ion. The alue o
RL
s ands be ween 0 and 1
o a o able adso p ion, while
RL
>1 ep esen s un a o able adso p ion,
RL
=1 ep esen s linea
adso p ion and RL=0 o i e e sible adso p ion p ocesses [28].
Ino ganics 2020,8, 40 9 o 12
Table 3lis s he pa ame e s o Langmui and F eundlich iso he ms models compu ed om he
expe imen al es s using MAAC beads o mono-me al and e na y sys ems adso p ion.
Table 3. Langmui and F eundlich iso he m pa ame e s.
Adso p ion Sys em Me al
Ions
Langmui Pa ame e s F eundlich Pa ame e s
qm
(mg/g)
KL
(L/mg) RLR2n1/nKF
(mg1−(1/n)L1/ng−1)R2
Cd(II) Cd(II) 59.17 0.048 0.103 0.981 2.52 0.397 7.09 0.983
Hg(II) Hg(II) 25.00 0.040 0.118 0.922 3.94 0.254 5.08 0.865
Ni(II) Ni(II) 37.04 0.031 0.144 0.945 2.79 0.358 4.82 0.995
Cd(II) +Hg(II) +
Ni(II)
Cd(II) 56.18 0.029 0.339 0.758 2.49 0.401 6.11 0.889
Hg(II) 181.82 0.007 0.164 0.681 1.50 0.668 3.35 0.901
Ni(II) 172.71 0.001 0.456 0.561 1.14 0.878 0.35 0.953
The models o Langmui and F eundlich equa ions in gene al desc ibed he da a well, al hough
he mono-me al sys em seems o be be e desc ibed by Langmui while he e na y sys em by he
F eundlich iso he m, as sugges ed by he co ela ion coe icien alues R
2
. The Langmui cons an , K
L
alues we e highe o he mono-me al sys em, e ealing a highe adso p ion capaci y. Con a y o
he e na y sys em, whe e he p esence o o he me al ions in he sys em dec eased he adso p ion
capaci y due o he compe i ion o a ailable adso p ion si es on o he MAAC beads. The R
L
pa ame e
alues s and below 1, indica ing a o able and weakly e e sible adso p ion o s udied me al ions
on o MAAC beads. The alues o nde e mined wi h he F eundlich equa ion we e gene ally highe
han 1.0, indica ing he e ogeneous adso p ion p ocess o all me al ions on o he beads. Among all
me als, Cd(II) was he mos highly adso bed wi h a F eundlich
KF
cons an o 7.09 and 6.11 o bo h
mono-me al and e na y sys em, espec i ely. In ou s udy, he adso p ion canno be simply ela ed
o he physicochemical p ope ies o me al ca ions, since cadmium wi h highe a omic weigh and
ionic adius han nickel was mo e in ensely adso bed [
30
]. Thus, adso p ion in he e na y sys em
was be e desc ibed by he F eundlich adso p ion iso he m, e ealing he e ogeneous su ace wi h
di e en a ini y si es on MAAC beads.
4. Ma e ials and Me hods
4.1. Syn hesis o MAAC Beads
Thep ocedu eo heMAACbeadssyn hesisisdesc ibedinou p e iouss udy[
15
]. B ie ly,Fe
3
O
4
-NPs
we e syn hesize by e e se cop ecipi a ion me hod [
16
]. In he syn hesis, 15 mL o 1.0 M FeCl
3·
6H
2
O
(Al a Aesa , Mad id, Spain) and 0.5 M FeSO
4·
7H
2
O (Sigma, S . Louis, MO, USA) we e mixed and added
d opwise in o a 3.5 M NH
4
OH solu ion o 20 mL a 60
◦
C. The eac ion p oceeded o 30 min unde
mechanical agi a ion. The magne ic nanopa icles we e hen washed and e-dispe sed in dis illed wa e .
Beads we e p epa ed in c oss-linking solu ion using calcium chlo ide solu ions as he c oss-linking
agen . Nex , 2.0 g o sodium algina e (Sigma, S . Louis, MO, USA) was subsequen ly added o he
p e iously p epa ed Fe
3
O
4
-NPs solu ion (35 mL). A e ob aining a homogeneous solu ion, 3.0 g
o comme cial ac i a ed ca bon was added and he solu ion was mechanically agi a ed o 4 h.
The ob ained suspension was added d opwise in o a p e iously p epa ed ba h o 0.13 M CaCl
2
(Sigma,
S . Louis, MO, USA) and 450
µ
L Tween 20 (Fluka, S einheim, Ge many) unde con inuous magne ic
speed o 450 pm using a New E a NE-300 sy inge pump (Biogen, Mad id, Spain). Beads we e ins an ly
o med and we e le in he ba h a ound 30 min o ha dening. A e wa ds, he beads we e collec ed
wi h a magne and cleaned wi h dis illed wa e . Finally, he magne ic beads we e d ied a 60 ◦C.
4.2. E ec o pH
The e ec o pH on adso p ion capaci y o Cd(II), Hg(II) and Ni(II) me al ions was conduc ed
indi idually by mixing (14 mg) o adso ben wi h 20 mL o 10 mg/L
−1
me al ions concen a ion.
The adso p ion capaci y was s udied a pH alues o (2.0, 3.0, 4.5, 5.0, 6.5, 7.0, 9.0) unde magne ic