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DOI: 10.17344/acsi.2025.9145
Scientific paper
Upcycling of Plastic Waste Into Multi-Walled Carbon 
Nanotubes as Efficient Organic Dye Adsorbent
Gordana Bogoeva-Gaceva,
1,2*
 Meri Sokolovska,
3
 Viktor Stefov,
3,2
  
Metodija Najdoski
3
 and Sebastijan Kovačič
4,5*
1 
Faculty of Technology and Metallurgy, Ss. Cyril and Methodius University - Skopje, 1000 Skopje, R.N. Macedonia 
2 
Research Center for Environment and Materials, Macedonian Academy of Sciences and Arts, Skopje, R.N. Macedonia 
3 
Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Ss. Cyril and Methodius University,  
1000 Skopje, R.N. Macedonia  
4 
National Institute of Chemistry, Hajdrihova 19, 1001 Ljubljana, Slovenia
5
 Catalysis and Organic Synthesis research group, Laboratory for Organic and Polymer Chemistry,  
Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, SI-2000 Maribor, Slovenia
* Corresponding author: E-mail: gordana@tmf.ukim.edu.mk 
sebastijan.kovacic@um.si
Received: 01-08-2025
Abstract
Multi-walled CNTs with an average diameter of about 80 nm, a length of several micrometers and surface area (S
BET
) 
of 100 m
2
 g
–1
 were obtained by pyrolysis of low-density polyethylene waste. The potential of the resulting MWCNTs 
material to purify water containing organic dyes was tested with Bezaktiv Blau HE-RM (BB) and Bezaktiv Rot S-3B (BR) 
reactive dyes. 200 mg L
–1
 MWCNT material was used to follow the adsorption of 30 mg L
–1
, 40 mg L
–1
, 50 mg L
–1
 and 
60 mg L
–1
 BB and BR at pH 3 and a temperature of ~25 °C. The results have shown that this material has a high potential 
as a sorbent, and its adsorption capacity of 257 mg g
–1
 (for Bezaktiv Blau
 
HE-RM) and 213 mg g
–1
 (for Bezaktiv Rot) is 
close to some commercial MWCNTs and functionalized MWCNT-based adsorbents. The adsorption process was very 
fast, reaching 80–90% of the dye removal in 10–15 minutes, and the equilibrium time was reached in 40–60 minutes. The 
adsorption isotherm showed that the Langmuir model was more suitable than the Freundlich model for describing the 
adsorption properties of the pollutants. 
Keywords: Adsorption; anionic dyes; upcycling plastics waste; carbon nanotubes
1. Introduction
Wastewater from the textile industry affects environ-
mental conditions by generating a large amount of colored 
and toxic outlets. Among them, dyes are considered the 
main pollutants due to their stability and low biodegradabil-
ity. Reactive dyes are among widely used dyes in the textile 
industry, accounting for 29% of all other textile dyes and 
more than 50% of global consumption of dyes used for cel-
lulosic fibers coloring.
1,2
 The wastewater from the dyeing 
process discharged into a natural aquatic system not only 
reduces light transmission, but also has a negative effect on 
the photochemical activity of the water system and can 
cause mutations in aquatic organisms. It is estimated that 
10–15% of the dye is lost in the wastewater during the dye-
ing process.
3
 Dyes are removed from wastewaters by mem-
brane filtration, ion-exchange, chemical oxidation, degrada-
tion, flotation, electrochemical treatment and adsorption. 
In wastewater remediation, adsorption over highly 
porous materials has attracted a lot of attention due to its 
high efficiency, low energy consumption and simple de-
sign.
4
 Various synthetic ingredients have been effectively 
used as adsorbents to remove contaminants from water,
5–8
 
including engineered nanomaterials.
9 
The most common-
ly used adsorbent is activated carbon, which is produced 
from various waste materials and is distinguished by large 

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surface area, good adsorption capacity and the ability to 
modify/tune its external chemical functional groups and 
structural properties.
10
 Recently, the use of carbon nano-
tubes (CNTs), especially multi-walled CNTs (MWCNTs), 
for adsorption applications has gained interest.
11,12 
 Their 
large specific surface area, small pore size and hollow 
structural characteristics proven that MWCNTs have great 
potential for the adsorption of many types of organic and 
inorganic pollutants from the aqueous streams.
13–16
 In 
terms of environmental responsibility and cost efficiency, 
the production of CNT adsorbents is equally important. 
Common methods for synthesizing CNT s are chemical va-
por deposition (CVD), arc discharge and laser ablation.
17
 
Recently, however, several studies have reported on the 
conversion of plastic waste into CNTs with high applica-
tion value. One such approach to managing plastic waste is 
upcycling, a technology that uses pyrolysis to convert raw 
materials into value-added products.
18,19
In our previous study, the adsorption potential of 
MWCNTs obtained by pyrolysis of waste plastics was as-
sessed by evaluating its adsorption capacity for reactive 
textile dye, Bezaktiv Blau HE-RM.
20
 The adsorption of 
dyes by various carbonaceous materials has been reported 
in numerous studies. The dye adsorption capacities of un-
treated CNTs are quite low but can be significantly im-
proved by functionalization/modification.
9,21
 Functionali-
zation increases the number of oxygen, nitrogen or other 
groups on the surface of the CNTs and improves their dis-
persibility and thus the specific surface area.
22 
The removal 
capacity of Methylene blue by amorphous carbon was 194 
mg g
–1
, while it reaches 350 mg g
–1
 for activated carbon.
9
 
The adsorption capacity of activated carbon prepared from 
prickly pear cake activation with phosphoric acid for Me-
thyl orange dye was 336 mg g
–1
 at temperature 20 °C and 
pH ~7. This value was much higher than that of the acti-
vated CNTs (149 mg g
–1
) for the same dye. The equilibri-
um adsorption data were best described by the Freundlich 
model.
23 
The adsorption capacities of nano-Fe
3
O
4
, 
graphene oxide, CNT-chitosan and other composite nano-
materials are discussed in the work of Salamatinia.
24
 Inter-
estingly, the adsorption capacity of graphene oxide for 
Methylene blue (81.97 mg g
–1
) was much lower compared 
to CNTs with chitosan support. Depending on the surface 
area of the CNTs and the molecular structure of the cati-
onic and anionic dyes (planar or non-planar), their ad-
sorption capacity can be between 27.6 and 152 mg g
–1
. 
Using the Langmuir adsorption model, values for q
max
 of 
~100 to ~500 mg g
–1
 were found for MWCNTs doses 
250−50 mg L
–1
 for Methyl violet dye.
25 
A comparison of 
the adsorption capacities of different adsorbents for the 
anionic dyes methyl orange, Congo red, Reactive blue 4, 
Reactive red M-2BE, Direct red and Acid Blue 
26
 has 
shown that the modification of MWCNTs was a promising 
way to drastically improve the dye removal rate (from less 
than 100 mg g
–1
 to ~1000 mg g
–1
). Functionalization of 
inorganic adsorbent for anionic dyes by acidic treatment 
was applied to influence the electrostatic attraction be-
tween the clay surface and the dye molecule, resulting in 
an improvement of the adsorption capacity from 2.2 mg 
g
–1
 to 67.4 mg g
–1
.
8
 A recently published review provides 
an overview of reactive dye-containing wastewater treat-
ment methods, with hybrid technologies as a promising 
solution to achieve higher efficiency and effectiveness.
27
In addition to adsorption capacity, however, kinetics 
is also one of the fundamental characteristics for evaluat-
ing the adsorption performance of porous adsorbents. 
Since the adsorption of pollutants on porous adsorbents in 
batch experiments can be described by a series of succes-
sive processes, such as the mass transport of pollutants 
(adsorbate) in the bulk solution, the diffusion of pollutants 
through the liquid to the adsorbent surface or the adsorb-
ate diffusion within the adsorbent's porous structure, an 
understanding of the adsorption kinetics is a basic prereq-
uisite for the optimal design of adsorbents and adsorption 
processes.
28,29
 Therefore, the aim of this work is to investi-
gate both kinetic and equilibrium adsorption of two reac-
tive dyes onto MWCNTs carbonaceous material as an ad-
sorbent.
2. Experimental
2. 1. Materials
MWCNT material was obtained from low-density 
polyethylene waste by pyrolytic degradation using analyti-
cal grade cobalt(II) acetate dihydrate (Merck, Germany), 
as a catalyst in a simple one-stage procedure, as described 
in detail in our earlier publication.
30
 Carbonaceous MW-
CNT material (CM) was employed in adsorption experi-
ments without any further modification/treatment. Bezak-
tive Blau HE-RM (BB) and Bezaktive Rot S-3B (BR) 
reactive (anionic) dyes were supplied by CHT/Bezema. 
Hydrochloric acid (analytical grade, Merck, Germany), 
was used for pH dye solution adjustment. 
2. 2. Characterization of MWCNT 
Scanning electron microscopy (SEM) was performed 
on Thermo Fisher Verios 4G HP scanning electron micro-
scope. The presence of cobalt particles was additionally 
characterized using STEM-in-SEM imaging in high-angle 
annular dark-field (HAADF) and bright-field (BF) modes 
as well as an energy-dispersive X-ray (EDX) analysis.  For 
both analyses, the solid MWCNT samples were first dilut-
ed in absolute ethanol, sonicated for a few seconds to dis-
perse them and then dropped onto the carbon support 
grids and dried. Nitrogen physisorption analysis was de-
termined from the adsorption and desorption isotherms 
of N
2
 at –196 °C using a TriStar II 3020 instrument (Mi-
cromeritics Instrument, Norcross, GA, USA). The specific 
surface area of the samples was calculated by applying the 
B.E.T. theory to the nitrogen adsorption data within the 

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Bogoeva-Gaceva et al.:   Upcycling of Plastic Waste Into Multi-Walled Carbon   ...
0.06−0.30 P/P
0
 range. The MWCNT densities (ρ
P
) (an av-
erage of 10 consecutive measurements) were evaluated us-
ing a fully automated, highly precision helium pycnometry 
(Ultrapyc 5000 by Anton Paar).
2. 3. Adsorption Experiments
The dye absorbance was measured at 570 nm wave-
length for BB and 540 nm wavelength for BR, using a Cary 
50 model Varian UV-Vis Spectrophotometer. The calibra-
tion curve of BB and BR are shown in Figure 1. The aqueous 
solution of dye for adsorption experiments was prepared by 
diluting stock solution prepared from 1 g of dye powder in 
1000 mL deionized water in a volumetric flask, and then the 
solution was diluted to the desired initial dye mass concen-
tration of 30 mg L
–1
, 40 mg L
–1
, 50 mg L
–1
 and 60 mg L
–1
. An 
amount of 5−25 mg of carbonaceous adsorbent material 
(CM) was added to a 100-mL laboratory beaker with 50 mL 
of dye solution (adjusted to pH = 3), and the beaker was 
placed on a magnetic stirrer with a stirring bar and stirred 
for 30 min. The adsorption testing was done at constant 
temperature of 25 °C. The samples of the solution were tak-
en at predetermined time intervals to measure the dye con-
centration after contact with the adsorbent. The removal 
efficiency, R (%), was calculated from the final (C
f
) and ini-
tial (C
o
) mass concentrations of the dye in solution: 
The adsorption capacity (amount of dye adsorbed 
onto carbonaceous material, A, mg g
–1
) was calculated by 
the mass balance relationship:
where C
o
 is the initial dye solution mass concentration in 
mg L
–1
, C
f
 is the final dye solution mass concentration in 
mg/L
–1
, m is the mass of adsorbent in g, and V is the vol-
ume of dye solution in L.
3. Results and Discussion
3. 1. Adsorbent (MWCNTs) Properties
The pyrolysis of low-density polyethylene waste was 
carried out in a simple one-step process in the presence of 
cobalt(II) acetate dihydrate (CoAc) as a catalyst, where 
MWCNTs begin to form around the cobalt nanocatalyst 
obtained from the dissociation of CoAc. First, STEM-in-
SEM imaging was performed to investigate the morpho-
logical properties of the produced carbon material. As de-
picted in Figure 2, after heating low-density polyethylene 
waste at 700 °C for three hours, MWCNTs with an average 
diameter of about 80 nm and a length of several microme-
ters were formed, with the supported Co-nanocatalyst at 
the end of each nanotube. To confirm the presence of Co 
particles, we performed additional characterization using 
high-angle annular dark-field (HAADF) and bright-field 
(BF) modes in STEM mode. Atomic-resolution Z-contrast 
HAADF- and BF-STEM images of Co/MWCNT are 
shown in Figure 2b and 2c and revealed Co atoms at the 
end of the MWCNT. Furthermore, EDXS mapping of the 
selected cross-section of the MWCNTs confirmed that the 
nanotubes indeed contain nano-sized Co particles.
The porous properties and the associated specific 
surface area were further analyzed by nitrogen adsorp-
tion−desorption measurements. The shape of the iso-
therms is reminiscent of that of microporous materials 
following the type I isotherm, with a clear increase in N
2
 
uptake in the P/P
0
 range up to 0.1 and a well-defined hys-
teresis loop indicating the presence of a certain volume of 
mesopores. The meso-microporous structure is reflected 
in an S
BET
 of about 100 m
2
 g
−1
 (Figure 3). Next, the skeletal 
(true) density was determined by helium pycnometry and 
Figure 1. Calibration curve of Bezaktiv Blau HE-RM (a) and Bezaktiv Rot S-3B (b) reactive dyes.

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Bogoeva-Gaceva et al.:   Upcycling of Plastic Waste Into Multi-Walled Carbon   ...
found to be 2.5516 g cm
−3
, which is slightly higher than 
expected for MWCNTs (2.100 g cm
−3
), indicating that the 
MWCNTs are tipped with nanosized metallic Co as also 
shown in Figure 2.
The use of waste-derived carbonaceous materials 
holds great potential as a cost-effective adsorbent for or-
ganic dye removal, especially those with large surface are-
as. Next, the potential of the as obtained MWCNT to puri-
fy water containing reactive dyes was tested using Bezaktiv 
Blau HE-RM and Bezaktiv Rot S-3B reactive dyes. 
3. 2.  The effect of Various Parameters on Dye 
Adsorption
The adsorption process depends primarily on the 
contact area between the solid adsorbent and the dye mol-
ecule, and the dispersion of the CM is an important factor 
in the efficiency of dye removal. Under the experimental 
conditions used, the dispersibility of the MWCNTs was 
sufficient so that it was not necessary to modify its surface 
chemistry. The MWCNTs contains cobalt (originating 
from the catalyst), which can dissolve in solution under 
acidic conditions. To verify the presence of cobalt in the 
UV-spectra of the two dyes, an experiment was performed 
with cobalt-containing dye solution at pH = 3. The results 
showed that the difference in the spectra of dye solutions 
with and without cobalt was negligible. The main mecha-
nism of dye adsorption by carbon nanotubes depends on 
the nature of the dyes – anionic or cationic – and on the 
surface chemistry of the adsorbent. However, in most cas-
es, it is difficult to predict the mechanism of adsorption, 
and different possible interactions between CNTs and dyes 
Figure 2. STEM micrographs (a, d), STEM in HAADF mode (b), STEM in BF mode (c) and EDXS elemental mapping for the carbon (e) and cobalt 
(f) of the Co/MWCNT.
Figure 3. N
2
 isotherm of the Co/MWCNT (A) and the pores size 
distribution plot (B).

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Bogoeva-Gaceva et al.:   Upcycling of Plastic Waste Into Multi-Walled Carbon   ...
have been proposed.
31
 Hydrophobic, van der Waals forces, 
π-π stacking, hydrogen bonding, and electrostatic interac-
tions can act simultaneously or individually.
32 
Several 
main factors play a role in dye adsorption on CNTs, such 
as the initial mass concentration of the dye, the pH of the 
solution, the dosage of the adsorbent, and the temperature. 
Experiments by Liu
33
 have shown that the driving force of 
interaction between CNTs and dyes is the molecular struc-
ture of the dye, which is strongly influenced by the π-π 
bonds, as well as the charge of the CNTs and the dyes.
34
 In 
general, CNTs are positively charged in a wide pH range 
between 10 and 30 mV , as shown by measurements of the 
zeta potential.
35
 Therefore, CNTs are expected to be more 
favorable for the adsorption of anionic azo dyes than for 
cationic dyes, although the conformational structure of a 
dye also plays an important role.
Effect of initial pH values. The pH of the solution is 
an important factor that influences the process of dye ad-
sorption by CNTs, through controlling the strength of 
electrostatic charges in the solution or ionized dye parti-
cles. Figure 4 (a, b) shows the variation of dye adsorption 
on MWCNTs of BB and BR dyes at different pH values. As 
can be seen from Figure 4 the acidic conditions were fa-
vorable for the adsorption of both dyes on MWCNTs.  
The results are consistent with the findings of Mora-
di,
26 
who showed that a low pH of the solution was pre-
ferred for the adsorption of reactive dyes on CNTs, but al-
so on activated carbon.
36
 All further experiments were 
carried out at pH = 3.
Effect of dosage of MWCNTs. The effect of MW -
CNT dosage on dye removal is shown in Figure 4 (c, d), 
where we can observe that increasing the amount of adsor-
bent (in the range of 400−500 mg L
–1
) increases the effi-
ciency of dye removal. The percentage of dye removal from 
the solution increases from 40% (at a MWCNTs dose of 5 
mg (100 mg L
–1
)) to 98.6−99.5% (at the CM dose of 20 − 
25 mg (400−500 mg L
–1
)) for BB, and from 38% to 98−99% 
for BR (at the above mention MWCNTs doses), which is 
due to the larger MWCNT surface area and more available 
adsorbent sites.
Figure 4. Effect of solution pH on dyes removal rate: for BB (a) and for BR (b) (the contact time for both dyes was ~35 min) and effect of MWCNTs 
dosage on BB dye adsorption (c) and BR dye adsorption (d) (contact time = 30 min; pH = 3; C
0
 = 50 mg L
–1
).

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Table 1. The effect of adsorbent dose on its adsorption capacity for 
BB and BR dyes (pH = 3; t = 30 min)
                  BB                      BR 
Adsorbent 
 
Adsorption   Adsorbent
 
Adsorption   
dose /  capacity /  dose /  capacity /
mg L
–1 
mg g
–1  
mg L
–1 
mg g
–1
100 245.79  100 228.00
200 236.43  200 204.64
300 193.21  300 171.05
400 147.91  400 141.03
500 119.38  500 118.63
The effect of MWCNTs dose on adsorption capacity 
is analyzed for the initial dye concentration of 50 mg L
–1 
and the results are also shown in Table 1 for both dyes. The 
dye removal efficiency for both reactive dyes increased by 
increasing doses of MWCNTs adsorbent. The highest val-
ues of adsorption capacity determined for the dyes BB and 
BR were 246 mg g
–1
 and 228 mg g
–1
, respectively.  
Effect of initial dye concentration. The effect of the 
initial dye concentration in the solution (30−60 mg L
–1
) on 
the efficiency of its removal is shown in Figure 5.  It is obvi-
ous that the removal efficiency of the dye decreased with 
increasing dye concentration, as fewer surface-active sites 
were available at constant adsorbent dosage. In addition, 
the mass transfer resistance inhibited the adsorption of dye 
molecules from the solution phase to the surface of the 
MWCNTs. The process is faster for BB dye, and the time of 
40 to 50 minutes is sufficient to reach equilibrium dye ad-
sorption (dye removal of 99% at the lower initial BB con-
centrations of 30 mg L
–1
 and 40 mg L
–1
). The results for BB 
also show that adsorption is initially very fast, reaching 
more than 87–96% in the first 10 minutes of contact time, 
and then proceeding slowly to reaction equilibrium. The 
process for BR is also very fast in the first minutes, reaching 
over 75–88% adsorption in 10 minutes, and then the fur-
ther adsorption of the dye is slowed down (80–88% adsorp-
Figure 5. Effect of contact time on BB (A, C) and BR (B, D) adsorption by MWCNTs (pH = 3; MWCNTs dose = 200 mg L
–1
; C
o
 = 30 mg L
–1
, 40 mg 
L
–1
, 50 mg L
–1
 and 60 mg L
–1
).

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tion is reached after 60 minutes). The results show that the 
spaces available for adsorption are mainly located on the 
outer surface of the MWCNTs, and that adsorption is less 
dependent on the meso-microporous structure. This is in 
contrast to the mechanism of adsorption on activated car-
bon, which mainly depends on its porous structure, and it 
takes time for the adsorbents to diffuse through the pores.
37
 
For both reactive dyes, the maximum adsorption ca-
pacity was 148 mg g
–1
 (BB) and 132 mg g
–1
 (BR) under the 
investigated test conditions with a low adsorbent dosage 
and an initial dye concentration of 30 mg L
–1
 (Figure 6). 
The initial dye concentration has a major impact on dye 
removal efficiency as it depends on the direct relationship 
between the amount of dye and the sites available for ad-
sorption on the MWCNTs. As the initial concentration of 
the dye increases, the adsorption capacity changes as fol-
lows: 198 mg g
–1
 (C
o
 = 40 mg L
–1
), 236 mg g
–1
 (C
o
 = 50 mg 
L
–1
), 257 mg g
–1
 (C
o
 = 60 mg L
–1
) for BB; and 160 mg g
–1
 
(C
o
 = 40 mg L
–1
), 204 mg g
–1
 (C
o
 = 50 mg L
–1
), 213 mg g
–1
 
(C
o
 = 60 mg L
–1
) for BR.
3. 3. Adsorption Isotherms
For further analysis of the adsorption process, the 
Langmuir and Freundlich isotherm models were used to 
fit the experimental data. The Langmuir isotherm is ap-
plied to homogeneous adsorption under the following as-
sumptions: (a) all adsorption sites are identical; (b) each 
site retains one molecule of the given compound; and (c) 
all sites are energetically and sterically independent of the 
amount adsorbed. The Freundlich isotherm model, on the 
other hand, assumes sorption on a heterogeneous surface 
on which there are many different types of available sites 
with different free sorption energies that are not limited by 
the formation of a monolayer. Therefore, in this study, the 
adsorption isotherm models of Langmuir
38
 and Freun-
dlich
23
 are used to describe the relationship between the 
adsorbate (BB and BR dyes) and the adsorbent (MWCNT). 
The adsorption of the dye and q
e
 (mg g
–1
) at equilibrium 
was related to the dye equilibrium concentration C
e
 (mg 
L
–1
) by Eqs. (1) and (2).
Figure 6. Langmuir isotherms of BB dye (a) and BR dye (b) adsorption and Freundlich isotherms of BB dye (c), and BR dye (d) adsorption on MW-
CNT (at pH = 3, MWCNT = 200 mg L
–1
, contact time = 30 min).

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The Langmuir isotherm model:
C
e
/q
e
= 1/K
L
q
m
 + (C
e
/q
m
) (1)
C
e
 (g L
–1
) is the equilibrium concentration of the dye in the 
solution, q
e
 (g g
–1
) is the amount of dye adsorbed per unit 
mass of adsorbent, q
m
 (g g
–1
) is a constant related to the 
adsorption capacity (theoretical saturation capacity of the 
monolayer), and K
L
 (L g
–1
) is a constant related to the ad-
sorption rate. q
m
 is calculated from the slope of the straight 
line when C
e
/q
e
 is plotted against C
e
. 
The linear form of the Freundlich isotherm model is 
represented as follows:  
logq
e
 = logK
F
 + (1/n
F
)logC
e
 (2)
K
F
 (g g
–1
) (L g
–1
)
1/n
F
 is the constant related to adsorp-
tion capacity, and n
F
 is the adsorption isotherm constant. 
K
F
 and 1/n
F
 are calculated from the intercept and slope of 
the straight line of the plot logq
e
 versus logC
e
, respectively. 
The adsorption intensity n
F 
is used to describe the hetero-
geneity of the adsorption surface. Values for n
F
 between 1 
and 10 have been shown to indicate a good adsorption po-
tential of the adsorbent.
39
 The value 1/n
F
 can vary between 
0 and 1. It describes a measure of the heterogeneity of the 
surface and the adsorption intensity. 1/n
F
 = 0 means irre-
versible adsorption, 0 < 1/n
F
 < 1 is a favorable one, and 1/
n
F
 > 1 means an unfavorable type of isotherm.
40
 Figure 6a 
shows the Langmuir isotherm of the adsorption of BB on 
MWCNTs. The linear correlation coefficient ( R
2
) indicates 
the accuracy of the model and is above 0.9990. The
 
value of 
q
m
 obtained is 240.28 mg g
–1
 and the Langmuir constant, 
K
L 
= 3.91 L mg
–1
 (see Table 2). The dimensionless equilib-
rium separation parameter R
L
 can be calculated from the 
K
L
 constant and the initial concentration of the dye solu-
tion C
o
 (mg L
–1
): 
R
L
 = 1/(1 + K
L
C
o
)
If R
L
 = 0, the adsorption is irreversible. When R < 1, 
the adsorption is favorable, R
L
 = 1 suggests a linear, while 
R
L
 > 1 indicates an unfavorable adsorption of tested dye on 
the selected substrate.
39,41
 The R
L
 value of BB adsorption 
on CM was in the range far below 1 (0.0084–0.0042, for 
the initial dye concentration of C
o
 30 mg L
–1
 to 60 mg L
–1
), 
testifying that the adsorption process was favorable. The 
values of R
2
 obtained from the linear graph logq
e 
versus 
logC
e
 for the Freundlich isotherm (Figure 6c) was 0.7729 
and is far below the value obtained for the Langmuir iso-
therm model (Table 2). The calculated correlation coeffi -
cient (R
2
) for the Freundlich isotherm (Table 2) shows that 
the equilibrium data for all tested adsorbent dosages are in 
better agreement with the Langmuir isotherm than with 
the Freundlich isotherm, indicating preferential monolay-
er adsorption. This can be attributed to the size of the BB 
reactive dye molecules, which are among the smallest tex-
tile reactive dyes. As already mentioned, K
F
, n
F
, and 1/n
F
 
are constants related to the Freundlich isotherm, and the 
determined value of K
F
 is 178.08 and n
F
 = 7.21 (1/n
F
 = 
0.1386). The n
F
 value indicates the degree of nonlinearity 
between the concentration of the solution and adsorbent. 
At a value of n
F
 = 1, the adsorption is linear, while n
F 
< 1 
indicates that the adsorption process is preferably chemi-
cal. However, if the coefficient n
F
 is greater than 1, then the 
adsorption is favorable (the physical adsorption is satisfac-
tory),
42
 as is determined by Langmuir equilibrium separa-
tion parameter, R
L
.
A high linear correlation coefficient (R
2
 = 0.9998) 
was also found for the Langmuir isotherm model for the 
adsorption of BR dye on CM (Figure 6b), with the Lang-
muir constant K
L
 = 0.91 L mg
–1
 and q
max
 = 218.12 mg g
–1 
(see Table 2). The values of the Langmuir equilibrium sep-
aration parameter, R
L 
were less than 1, ranging from 0.0355 
to 0.0181 for the initial dye concentration of 30 mg L
–1
 and 
60 mg L
–1
. The values of Langmuir and Freundlich iso-
therm constants determined by the linear fit of Eqs. (1) 
and (2) are also included in Table 2. The Langmuir iso-
therm model fits better than the Freundlich isotherm (Fig-
ure 6d). The Langmuir isotherm of both dyes on the tested 
MWCNT was found to be linear over the entire concentra-
tion range. The determination coefficients R
2
 were ex-
tremely high (0.9994). These high degrees of correlation 
for the Langmuir relationship indicate that a single surface 
reaction with constant activation energy is the predomi-
nant sorption step and possibly the predominant rate-con-
trolling step.
In addition, a high K
L
 value for BB indicates its 
strong affinity to MWCNTs, compared to BR dye. Howev-
Table 2. Langmuir and Freundlich isotherm parameters determined for BB and BR dyes.
Adsorption isotherm Parametars BB BR
Langmuir q
m
/mg g
–1 
240.28 218.12
 K
L
/L g
–1 
3.91 0.91
 R
L
 (30 mg L
–1
 to 60 mg L
–1
) (0.0084; 0.0063; 0.0051; 0.0042) (0.0355; 0.0269; 0.0216; 0.0181)
 R
2 
0.9994 0.9998
Freundlich K
F 
(mg g
–1
) (mg L
–1
) 178.08 130.51
 n
F 
7.21 6.03
 R
2 
0.7729 0.9723

162
Acta Chim. Slov. 2025, 72, 154–163
Bogoeva-Gaceva et al.:   Upcycling of Plastic Waste Into Multi-Walled Carbon   ...
er, both the investigated dyes have the 1/n value of less 
than 1, i.e. 0.1386 for BB and 0.1658 for BR, indicating 
their favorable adsorption on MWCNTs. The results of our 
analysis suggest the perspective of upcycling of plastics 
waste by simple process of pyrolysis for obtaining efficient 
cost-effective adsorbent for removal of reactive dyes from 
wastewater effluents. 
4. Conclusion
In summary, we have shown that the meso-micropo-
rous MWCNTs material with a surface area of about 100 
m
2
 g
–1
, obtained in a simple process of pyrolysis of plastic 
waste, has a high potential as a sorbent for reactive dyes 
from wastewater effluents. Its adsorption capacity 
(213−257 mg g
–1
) for reactive dyes without further modi-
fication of its surface is similar to that of some commercial 
MWCNTs- and functionalized MWCNTs-based adsor-
bents. The equilibrium adsorption data for the two reac-
tive dyes analyzed, Bezaktiv Blau HE-RMand Bezaktiv Rot 
S-3B, were best described by the Langmuir model. The 
conversion of plastic waste into MWCNT material is 
therefore a practical solution for waste management and at 
the same time represents a promising contribution to the 
circular economy. 
Acknowledgements
This work was supported by the Ministry of Education, 
Science and Sport of the Republic of Slovenia and the Slove-
nian Research and Innovation Agency (Grant P2-0150).
Author contributions 
G. B.-G. and M. S. designed and performed the ex-
periments. G. B.-G., M. S., V . S., M. N. and S. K. performed 
the studies and data analysis. G. B.-G. and M. N. super-
vised the research. All authors contributed to the writing 
and editing of the manuscript.
Conflicts of interest
The authors declare no conflicts of interest.
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Povzetek
S pirolizo odpadnega polietilena smo uspešno proizvedli večstenske ogljikove nanocevke (MWCNT) s povprečnim 
premerom 80 nm, dolžino več mikrometrov in specifično površino (SBET) 100 m² g
–
¹. MWCNT smo nato testirali kot 
adsorbent za odstranjevanje organskih barvil iz vode, pri čemer smo preučevali adsorpcijo dveh reaktivnih barvil: Bez-
aktiv Blau HE-RM (BB) in Bezaktiv Rot S-3B (BR). Med testiranjem adsorpcije smo proučevali različne koncentracije 
barvil (30 mg L
–
¹, 40 mg L
–
¹, 50 mg L
–
¹ in 60 mg L
–
¹) pri različnih pH vrednostih ter uporabili različne količine MWCNT 
adsorbenta. Rezultati so pokazali, da ima ta material velik potencial kot adsorbent, saj njegova adsorpcijska kapaciteta 
znaša 257 mg g
–
¹ za Bezaktiv Blau HE-RM in 213 mg g
–
¹ za Bezaktiv Rot, kar je primerljivo z nekaterimi komercialnimi 
adsorbenti. Kinetika adsorpcije je bila izjemna, saj je bilo odstranjenih 80–90% barvila v 10 minutah, ravnotežno stanje 
pa je bilo doseženo v 40–60 minutah. Za vse proučevane kombinacije smo izračunali tudi Langmuirjevo in Freundlicho-
vo izotermo ter ugotovili, da Langmuirjev model bolje opisuje adsorpcijo MWCNT kot Freundlichov model.