Available via license: CC BY 4.0
Content may be subject to copyright.
Citation: Soran, M.-L.; Bocs
,a, M.;
Pintea, S.; Stegarescu, A.; Lung, I.;
Opri¸s, O. Commercially Biochar
Applied for Tartrazine Removal from
Aqueous Solutions. Appl. Sci. 2024,14,
53. https://doi.org/10.3390/
app14010053
Academic Editor: Victor Burlakov
Received: 10 November 2023
Revised: 13 December 2023
Accepted: 15 December 2023
Published: 20 December 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
applied
sciences
Article
Commercially Biochar Applied for Tartrazine Removal from
Aqueous Solutions
Maria-Loredana Soran 1, Mariana Bocs
,a2,3, Stelian Pintea 1, Adina Stegarescu 1, *, Ildiko Lung 1
and Ocsana Opri¸s 1
1National Institute for Research and Development of Isotopic and Molecular Technologies, 67-103 Donat,
400293 Cluj-Napoca, Romania; loredana.soran@itim-cj.ro (M.-L.S.); ildiko.lung@itim-cj.ro (I.L.);
ocsana.opris@itim-cj.ro (O.O.)
2VERNICOLOR Group, 410605 Oradea, Romania; marianabocsa@yahoo.co.uk
3Faculty of Food Engineering, Tourism and Environmental Protection, Aurel Vlaicu University,
310130 Arad, Romania
*Correspondence: adina.stegarescu@itim-cj.ro
Abstract: Biochar gained attention due to its definite physico-chemical characteristics and because it
is a cost-effective and efficient adsorbent. In this paper, commercial biochar was tested for the removal
of tartrazine from aqueous solutions. Thus, the optimum experimental conditions were determined
for several parameters (pH, temperature, initial concentration of tartrazine, biochar dose, and contact
time). The concentration of tartrazine residues was determined using UV-Vis spectrophotometry.
The best experimental results were obtained at 1 mg L
−1
concentration of tartrazine, pH 2, 30
◦
C,
18 min, and 0.9 g L
−1
adsorbent dose. The maximum removal efficiency of tartrazine obtained in
optimum conditions was 90.18%. The experimental data were analyzed by the isotherm and kinetic
models. The isotherm and kinetics of tartrazine removal on biochar follow the Langmuir isotherm
and pseudo-second-order kinetics, respectively. According to the Langmuir isotherm model, the
biochar showed a maximum adsorption capacity of 3.28 mg g
−1
. In addition, biochar demonstrated a
good reuse potential and therefore can be used for the removal of tartrazine from aqueous solutions.
Keywords: biochar; adsorption; tartrazine; isotherm; kinetic studies
1. Introduction
The fact that large amounts of synthetic dyes are released into the environment on a
daily basis by industries such as textile, paper, food product, and dyeing makes environ-
mental contamination by these dyes a pressing global concern these days [
1
]. A significant
risk is posed by their diffusion in the environment because the majority of them have
detrimental impacts on people, animals, and plants [2,3].
The azo dyes are chemical compounds that contain two hydrocarbon groups joined by
two nitrogen atoms. They account for up to 70% of all dyes used in the food, pharmaceutical,
and textile industries. Their popularity is a consequence of their higher stability with
respect to most natural food dyes and relatively low production costs. Furthermore, they
have excellent water solubility and fixation, accounting for 65% of all those used in the
industry [
4
]. Tartrazine, also referred to as Yellow 5, E102, is an azo dye that gives a
bright yellow color to the food products in which it is used (candies, soft drinks, breakfast
cereals, condiments, etc.), being one of the most widely used dyes. It is produced from
compounds extracted from coal tar, which is an asphalt derivative. Like many other
synthetic compounds, it has side effects on the human body. Among the known side effects
are allergic responses in certain individuals, especially for those suffering of asthma or
aspirin intolerance, and loss of concentration or hyperactivity for children when combined
with benzoic acid (E210). It might also be associated with blurred vision, itching, rhinitis,
fatigue, purple skin patches, etc.; however, these effects need to be confirmed by additional
Appl. Sci. 2024,14, 53. https://doi.org/10.3390/app14010053 https://www.mdpi.com/journal/applsci
Appl. Sci. 2024,14, 53 2 of 16
studies. Due to these concerns regarding its toxicity and side effects, tartrazine is already
banned in some countries. In 1959, Lockey described for the first time tartrazine-induced
urticaria [
5
]. Since then, many other studies were performed in order to elucidate the risks
associated with tartrazine consumption. While the studies of Maekawa et al. and Borzelleca
and Hallagan, from 1987 and 1988, respectively, show that there is no carcinogenic effect
from the use of tartrazine, in a study from 2002, Sasaki et al. claim that it may induce DNA
damage in mice colon [6–9].
Although the severity of the side effects of the tartrazine are still under debate, it is
clear that limiting its spread and contact with human and living cells will also prevent or
minimize the unwanted consequences that it might generate. For this reason, providing
low-cost, efficient methods for their removal from the environment could be considered as
an achievement.
Effective ways to treat polluted effluents are required since tartrazine’s chemical
treatment produces harmful chemicals and its breakdown is difficult due to the aromatic
rings on both sides of the azo group [4].
Many conventional physical and biological methods were employed for the removal
of dyes from water—among these, photodegradation, coagulation, membrane separation,
reverse osmosis electrochemical processes, and adsorption [
10
–
13
]. Most of these processes
require high operational and capital costs, which restricts their application for the removal
of contaminants from polluted waters [
14
]. Adsorption has become one of the most used
processes due to its modest costs, simple operating conditions, high flexibility, material
regeneration, and environmental friendliness [15,16].
Many adsorbents are frequently used to remove dyes from contaminated water. The
benefits of metal-oxide-based, bio-adsorbent, metal-organic framework, polymer-based,
and activated-carbon materials include ease of production, high surface area, high reactivity,
reusability, cheap cost, and high efficiency [17–23].
Recent research has used biochar and biochar-based adsorbents to remove pollutants
from aqueous media because biosorbents are desired for environmental remediation. This
can be used in both batch and continuous flow systems and it can be used in combination
with other adsorbents to enhance its adsorption capacity [
24
]. The feedstock, preparation
conditions, and modification approaches affect the properties of biochar and are responsible
for their adsorptive performances [
14
,
24
,
25
]. The characteristics of the biochar determine
the adsorption efficiency. Nonetheless, biochar is thought to be an economical and effective
method of eliminating pollutants from water [26].
Testing was performed on the ability of biochar made from different feedstocks to
remove various contaminants from aqueous solutions, including organic pollutants and
heavy metals [
27
]. Biochar and biochar-based adsorbents were discovered to be effective
in eliminating malachite green [
28
], crystal violet [
28
,
29
], Congo red [
28
], remazol black
B [
30
], reactive brilliant blue [
31
], rhodamine B [
31
,
32
], methylene blue [
29
,
33
], reactive
yellow [33], malachite green oxalate [34], safranine T [34], etc.
The AR-18 dye removal capability of N-doped biochar made from birch tree wastes
has been demonstrated to be superior [
35
]. Chen et al. prepared sorghum-straw-based
biochar that they activated with peroxydisulfate to enhance the tartrazine degradation [
36
].
To the best of our knowledge, no articles on tartrazine removal from aqueous solutions
on biochar have been found in the literature and there are only a few on modified biochar.
Thus, biochar-mediated zirconium ferrite nanocomposite [
37
] and positively charged tri-
ethylenetetramine biochar [
38
] were used for the adsorption of tartrazine. However, there
are quite a few articles regarding the adsorption of tartrazine on different adsorbents.
Activated carbon obtained from coconuts and groundnut shells at 450
◦
C and activated
with different concentration of hydrochloric acid was an efficient adsorbent for the removal
of sunset yellow and tartrazine from aqueous solutions [39].
The effectiveness of the adsorbent obtained from Moringa oleifera seeds was tested for
the removal of tartrazine from aqueous solution, obtaining a removal of 95%. The results
Appl. Sci. 2024,14, 53 3 of 16
were compared with those obtained for coconut babassu and bone-activated carbon and it
was found that the new adsorbent can be an effective alternative to activated carbon [40].
Cellulose obtained from wheat straw residues and cetyltrimethylammonium-chloride-
modified cellulose were used for the removal of Congo red and tartrazine from aqueous
solution by Villabona-Ortíz et al. [
41
]. Adsorption capacity increased from 2.31 mg g
−1
obtained for the cellulose to 18.85 mg g
−1
for the modified cellulose in the case of tartrazine
removal. In the case of the Congo red, the adsorption capacity increased very slightly from
18.5 mg g−1for cellulose to 19.92 mg g−1for the modified cellulose.
Albroomi et al. tested the activated carbon from apricot stones for tartrazine removal
in a batch and dynamic adsorption system. In batch experiments, the maximum adsorp-
tion was found to be 76 mg g
−1
at 100 mg L
−1
of tartrazine, while in fixed-bed column
experiments, as flow rate increases, the maximum adsorption capabilities decrease [42].
In this paper, commercial biochar was tested for the removal of tartrazine from aque-
ous solutions, optimizing several parameters (pH, temperature, adsorbent dose, contact
time, and initial dye concentration) that influence the adsorption process. To the best of
our knowledge, there is no available information about biochar application in tartrazine
removal from wastewater. In addition, the commercial biochar selected for the present
study is used as fertilizer in agriculture, and its interaction with pollutants is therefore
useful knowledge.
2. Materials and Methods
2.1. Materials
For this study, commercial biochar sold as horticultural soil amendment, produced
for Dr. Soil GmbH Germany, was procured from the market and used as adsorbent ma-
terial with no other synthesis steps involved. Tartrazine dye, the pollutant chosen to be
investigated in this study, was purchased from Sigma-Aldrich (Schnelldorf, Germany). As
pH-adjusting materials, HCl and NaOH solutions procured from Sigma-Aldrich (Schnell-
dorf, Germany) and VWR Chemicals (Wien, Austria), respectively, were used. Aqueous
solutions were prepared using ultrapure water lab source (Direct-Q
®
3 UV Water Purifica-
tion System, Merck, Darmstadt, Germany).
The morphological characterization of the commercial biochar was investigated
through scanning/transmission electron microscopy (STEM) using a Hitachi HD2700
microscope, operated at 200 kV and in cold field emission.
The specific surface area of biochar was determined from the Brunauer–Emmett–Teller
(BET) analysis.
The electrostatic nature of the adsorbent surface was determined with the pH of point
zero charge (pHpzc) using the so-called pH drift method [43]. The method of determining
pHpzc is according to the one described in a previous article [44].
Using the KBr pellet approach, FT-IR spectra were obtained in the 4000–400 cm
−1
spectral domain with a resolution of 4 cm
−1
on a JASCO 6100 FT-IR spectrometer. After
dispersing each sample in roughly 300 mg of anhydrous KBr, the powder was pulverized
in an agate mortar. The ground mixture was pressed into an evacuated die to produce the
pellet. The software Jasco Spectra Manager v.2 was used to gather and examine the spectra.
2.2. Adsorption Process
The adsorption process was achieved in a Berzelius beaker, under static conditions,
bringing the artificial tartrazine aqueous solution in contact with biochar and varying
different parameters: temperature, pH, tartrazine content, dose of adsorbent, and duration
of contact. At the end, to determine the tartrazine remaining unabsorbed on the biochar,
centrifugation was used for 10 min at 9000 rpm in order to separate the solution from the
adsorbent. Using the PG Instruments T80 UV-VIS spectrophotometer (Leicestershire, UK),
the absorbance at 473 nm was read to conduct the solute analysis.
Appl. Sci. 2024,14, 53 4 of 16
The following relationships were utilized to determine the effectiveness of the adsorp-
tion process and the performance of the nanocomposite as an adsorbent:
η(%)=(C0−Ct)
C0
100, (1)
qt=(C0−Ct)V
m, (2)
where
η
(%) represents the tartrazine removal degree, C
0
and C
t
(mg L
−1
) are the tartrazine
concentrations in the solution at the initial moment and at the moment t (min), respectively,
q
t
(mg g
−1
) represents the mean of the adsorption capacity, V (mL) is the volume of the
dye solution, and m (g) is the amount of biochar.
2.3. Desorption and Recycle Studies
Before desorption, the tartrazine was adsorbed on biochar, the adsorption experiment
being performed under the determined optimal conditions. EtOH was investigated for
regeneration. Thus, after adsorption on the adsorbent, 5 mL of eluent was added and at
room temperature, the mixture was agitated for 20 min at 400 rpm. At the end, the mixture
was centrifuged for 10 min at 9000 rpm and the concentration of desorbed tartrazine was
determined with UV-Vis.
Tartrazine desorption capacity (q
e,desorption
, mg g
−1
) and desorption efficiency (%)
were determined from the following relationships:
qe,desorption =Vx Cf
m(3)
D%= qe,desorption
qe,adsorption !(4)
where V is the volume of the eluent solution (L), m is the mass of the tartrazine-saturated
nanocomposite biochar, and C
f
is the tartrazine concentration in the desorption solution
(mg L−1) (g).
Four cycles of adsorption–desorption studies were performed intending to determine
the biochar regeneration capacity.
3. Results and Discussion
3.1. Biochar Characterization
3.1.1. Morphological Characterization
The TEM analysis conducted to structural characterization of the biochar showing the
presence of pores in the material (Figure 1).
The porous structure of the biochar allows the retention of water and other elements
(such as nutrients or fertilizers) that are useful for soil and for plants fertilization.
3.1.2. Surface Area
The total surface area of the biochar was calculated from nitrogen adsorption–desorption
isotherms registered at the liquid nitrogen temperature. Thus, the BET surface area was
67.2 m
2
g
−1
. Two instances of the isotherms, which show the volume of adsorbed–desorbed
nitrogen as a function of relative pressure (the ratio between the effective pressure, or p and
the saturation pressure, or p0), are shown in Figure 2.
Appl. Sci. 2024,14, 53 5 of 16
Appl.Sci.2023,13,xFORPEERREVIEW4of15
adsorbent.UsingthePGInstrumentsT80UV-VISspectrophotometer(Leicestershire,UK),
theabsorbanceat473nmwasreadtoconductthesoluteanalysis.
Thefollowingrelationshipswereutilizedtodeterminetheeffectivenessofthead-
sorptionprocessandtheperformanceofthenanocompositeasanadsorbent:
𝜂 %
100, (1)
𝑞𝐶𝐶
, (2)
whereη(%)representsthetartrazineremovaldegree,C
0
andC
t
(mgL
−1
)arethetartrazine
concentrationsinthesolutionattheinitialmomentandatthemomentt(min),respec-
tively,q
t
(mgg
−1
)representsthemeanoftheadsorptioncapacity,V(mL)isthevolumeof
thedyesolution,andm(g)istheamountofbiochar.
2.3.DesorptionandRecycleStudies
Beforedesorption,thetartrazinewasadsorbedonbiochar,theadsorptionexperi-
mentbeingperformedunderthedeterminedoptimalconditions.EtOHwasinvestigated
forregeneration.Thus,afteradsorptionontheadsorbent,5mLofeluentwasaddedand
atroomtemperature,themixturewasagitatedfor20minat400rpm.Attheend,themix-
turewascentrifugedfor10minat9000rpmandtheconcentrationofdesorbedtartrazine
wasdeterminedwithUV-Vis.
Tart razin edesorptioncapacity(q
e,desorption
,mgg
−1
)anddesorptionefficiency(%)were
determinedfromthefollowingrelationships:
𝑞,
(3)
𝐷%
,
,
(4)
whereVisthevolumeoftheeluentsolution(L),misthemassofthetartrazine-saturated
nanocompositebiochar,andC
f
isthetartrazineconcentrationinthedesorptionsolution
(mgL
−1
)(g).
Fourcyclesofadsorption–desorptionstudieswereperformedintendingtodeter-
minethebiocharregenerationcapacity.
3.ResultsandDiscussion
3.1.BiocharCharacterization
3.1.1.MorphologicalCharacterization
TheTEManalysisconductedtostructuralcharacterizationofthebiocharshowing
thepresenceofporesinthematerial(Figure1).
Figure 1. TEM images of commercial biochar.
Appl.Sci.2023,13,xFORPEERREVIEW5of15
Figure1.TEMimagesofcommercialbiochar.
Theporousstructureofthebiocharallowstheretentionofwaterandotherelements
(suchasnutrientsorfertilizers)thatareusefulforsoilandforplantsfertilization.
3.1.2.SurfaceArea
Thetotalsurfaceareaofthebiocharwascalculatedfromnitrogenadsorption–de-
sorptionisothermsregisteredattheliquidnitrogentemperature.Thus,theBETsurface
areawas67.2m
2
g
−1
.Twoinstancesoftheisotherms,whichshowthevolumeofadsorbed–
desorbednitrogenasafunctionofrelativepressure(theratiobetweentheeffectivepres-
sure,orpandthesaturationpressure,orp
0
),areshowninFigure2.
Figure2.Nitrogenadsorption–desorptionisothermsforbiochar(blue—adsorption,orange—de-
sorption).
3.1.3.FTIRAnalysis
TheFTIRspectraforthecommercialbiochar,tartrazine,andthebiocharwithad-
sorbedtartrazinearepresentedinFigure3.
Figure3.FTIRspectraofinitialbiochar,tartrazine,andbiocharwiththeadsorbeddye.
FTIRspectraofbiocharshowsanamorphousmaterialthatcontainhydroxyl(3433
cm
−1
),carbonyl(1700,1125cm
−1
),aromaticC=C(1623cm
−1
),aliphatic(2924,2857,1369
cm
−1
),andaromatic(1412,900–700cm
−1
)C−Hgroups[45].
ThevibrationalbandsintheFTIRspectrumoftartrazinecanbeassignedtovarious
functionalgroupspresentinthecompoundasfollows:3436cm
−1
(OH),2924and2854cm
−1
Figure 2. Nitrogen adsorption–desorption isotherms for biochar (blue—adsorption, orange—desorption).
3.1.3. FTIR Analysis
The FTIR spectra for the commercial biochar, tartrazine, and the biochar with adsorbed
tartrazine are presented in Figure 3.
FTIR spectra of biochar shows an amorphous material that contain hydroxyl (3433 cm
−1
),
carbonyl (1700, 1125 cm
−1
), aromatic C=C (1623 cm
−1
), aliphatic (2924, 2857, 1369 cm
−1
), and
aromatic (1412, 900–700 cm−1) C−H groups [45].
The vibrational bands in the FTIR spectrum of tartrazine can be assigned to various
functional groups present in the compound as follows: 3436 cm
−1
(OH), 2924 and 2854 cm
−1
(aliphatic C-H), 1744 sh and 1642 cm
−1
(C=O), 1599 and 1192 cm
−1
(COONa), 1562 and
1412 cm
−1
(N=N), 1347 and 1126 cm
−1
(C–H bending and C–H-aromatic ring), 1037 and
1008 cm
−1
(–S=O and C
6
H
5
–SO
2
), 768 cm
−1
(–C
6
H
5
–N=N– bonds), 694 cm
−1
(–CCO), 718
and 648 cm
−1
(the –C–C–, –C=C– and C
6
H
5
out-of-plane stretching), 567 cm
−1
(C
6
H
5
– and
R–C
6
H
4
– groups) and 528 cm
−1
(out-of-plane bending of the para-substituted aromatic
ring containing a sulphoxyl group) [46,47].
Appl. Sci. 2024,14, 53 6 of 16
Appl.Sci.2023,13,xFORPEERREVIEW5of15
Figure1.TEMimagesofcommercialbiochar.
Theporousstructureofthebiocharallowstheretentionofwaterandotherelements
(suchasnutrientsorfertilizers)thatareusefulforsoilandforplantsfertilization.
3.1.2.SurfaceArea
Thetotalsurfaceareaofthebiocharwascalculatedfromnitrogenadsorption–de-
sorptionisothermsregisteredattheliquidnitrogentemperature.Thus,theBETsurface
areawas67.2m
2
g
−1
.Twoinstancesoftheisotherms,whichshowthevolumeofadsorbed–
desorbednitrogenasafunctionofrelativepressure(theratiobetweentheeffectivepres-
sure,orpandthesaturationpressure,orp
0
),areshowninFigure2.
Figure2.Nitrogenadsorption–desorptionisothermsforbiochar(blue—adsorption,orange—de-
sorption).
3.1.3.FTIRAnalysis
TheFTIRspectraforthecommercialbiochar,tartrazine,andthebiocharwithad-
sorbedtartrazinearepresentedinFigure3.
Figure3.FTIRspectraofinitialbiochar,tartrazine,andbiocharwiththeadsorbeddye.
FTIRspectraofbiocharshowsanamorphousmaterialthatcontainhydroxyl(3433
cm
−1
),carbonyl(1700,1125cm
−1
),aromaticC=C(1623cm
−1
),aliphatic(2924,2857,1369
cm
−1
),andaromatic(1412,900–700cm
−1
)C−Hgroups[45].
ThevibrationalbandsintheFTIRspectrumoftartrazinecanbeassignedtovarious
functionalgroupspresentinthecompoundasfollows:3436cm
−1
(OH),2924and2854cm
−1
Figure 3. FTIR spectra of initial biochar, tartrazine, and biochar with the adsorbed dye.
After adsorption of tartrazine, some changes were observed on the FTIR spectrum of
the biochar, as evidence of the adsorption of the dye from the solution on the surface, such
as the following: the band of tartrazine from 1642 cm
−1
shifts and appears as a shoulder at
1634 cm
−1
on the band of biochar from 1623 cm
−1
; the band of biochar from 1369 cm
−1
shifts
to 1375 cm
−1
and appears as a shoulder, the vibrational band of biochar from 1125 cm
−1
and the band of tartrazine from 1126 cm
−1
shift and overlap at 1129 cm
−1
; the characteristic
vibrational bands of tartrazine from 1037, 718, and 648 cm
−1
are founded in the form of
a shoulder and shifted to 1046, 726, and 657 cm
−1
, respectively. The vibrational bands
of tartrazine from 718 and 648 cm
−1
shift at 726 and 657 cm
−1
, respectively, and appear
as a shoulder. The observed shifts of the vibration bands of tartrazine when the biochar
adsorbed it are probably caused by the weak intermolecular interactions, possibly of the
hydrogen bonds, that arise between the functional groups of tartrazine, namely the C=O
and –S=O groups, and the surface of biochar.
3.1.4. pH at Point of Zero Charge (pHpzc) of Biochar
The pH
pzc
of adsorbent biochar is 6.5, as can be seen from Figure 4, which suggests
that the adsorbent can be used for the adsorption of cations and anions [48].
3.2. Testing the Biochar Efficiency as Tartrazine Adsorbent from Synthetic Aqueous Solutions
In the adsorption process, the effects of some factors such as solution pH, temperature,
adsorbent dose, etc., must be taken into account. Thus, the optimal retention conditions
for the tartrazine removal from aqueous solutions, using biochar as adsorbent, were de-
termined following the influence of different parameters. For this reason, the contact time
between the biochar adsorbent and the synthetic solution, the initial pH of the aqueous
solution, the temperature of the solution during the adsorption process, the adsorbent dose
during the adsorption, and the tartrazine initial concentration were varied, and the results
are presented in the following sections.
Appl. Sci. 2024,14, 53 7 of 16
Appl.Sci.2023,13,xFORPEERREVIEW6of15
(aliphaticC-H),1744shand1642cm
−1
(C=O),1599and1192cm
−1
(COONa),1562and1412
cm
−1
(N=N),1347and1126cm
−1
(C–HbendingandC–H-aromaticring),1037and1008
cm
−1
(–S=OandC
6
H
5
–SO
2
),768cm
−1
(–C
6
H
5
–N=N–bonds),694cm
−1
(–CCO),718and648
cm
−1
(the–C–C–,–C=C–andC
6
H
5
out-of-planestretching),567cm
−1
(C
6
H
5
–andR–C
6
H
4
–
groups)and528cm
−1
(out-of-planebendingofthepara-substitutedaromaticringcontain-
ingasulphoxylgroup)[46,47].
Afteradsorptionoftartrazine,somechangeswereobservedontheFTIRspectrumof
thebiochar,asevidenceoftheadsorptionofthedyefromthesolutiononthesurface,such
asthefollowing:thebandoftartrazinefrom1642cm
−1
shiftsandappearsasashoulderat
1634cm
−1
onthebandofbiocharfrom1623cm
−1
;thebandofbiocharfrom1369cm
−1
shifts
to1375cm
−1
andappearsasashoulder,thevibrationalbandofbiocharfrom1125cm
−1
andthebandoftartrazinefrom1126cm
−1
shiftandoverlapat1129cm
−1
;thecharacteristic
vibrationalbandsoftartrazinefrom1037,718,and648cm
−1
arefoundedintheformofa
shoulderandshiftedto1046,726,and657cm
−1
,respectively.Thevibrationalbandsof
tartrazinefrom718and648cm
−1
shiftat726and657cm
−1
,respectively,andappearasa
shoulder.Theobservedshiftsofthevibrationbandsoftartrazinewhenthebiocharad-
sorbeditareprobablycausedbytheweakintermolecularinteractions,possiblyofthehy-
drogenbonds,thatarisebetweenthefunctionalgroupsoftartrazine,namelytheC=Oand
–S=Ogroups,andthesurfaceofbiochar.
3.1.4.pHatPointofZeroCharge(pH
pzc
)ofBiochar
ThepH
pzc
ofadsorbentbiocharis6.5,ascanbeseenfromFigure4,whichsuggests
thattheadsorbentcanbeusedfortheadsorptionofcationsandanions[48].
Figure4.Pointzerochargeofbiochar.
3.2.TestingtheBiocharEfficiencyasTartrazineAdsorbentfromSyntheticAqueousSolutions
Intheadsorptionprocess,theeffectsofsomefactorssuchassolutionpH,tempera-
ture,adsorbentdose,etc.,mustbetakenintoaccount.Thus,theoptimalretentioncondi-
tionsforthetartrazineremovalfromaqueoussolutions,usingbiocharasadsorbent,were
determinedfollowingtheinfluenceofdifferentparameters.Forthisreason,thecontact
timebetweenthebiocharadsorbentandthesyntheticsolution,theinitialpHoftheaque-
oussolution,thetemperatureofthesolutionduringtheadsorptionprocess,theadsorbent
doseduringtheadsorption,andthetartrazineinitialconcentrationwerevaried,andthe
resultsarepresentedinthefollowingsections.
3.2.1.TheInfluenceoftheInitialpHoftheAqueousSolutionontheAdsorptionProcess
First,5mgofbiocharwasstirredat400rpmwith5mLoftartrazineaqueoussolution
withconcentrationof5mgL
−1
,varyingthepHofthesolutionfrom2to12,forestablishing
theoptimalpHforthetartrazineremoval.Theadsorptionprocesswasperformedatroom
temperaturefor20minforeachsample.Thetwophaseswereseparatedattheendofthe
processbyfiltrationandthesolutewasanalyzed.TheresultspresentedinFigure5are
Figure 4. Point zero charge of biochar.
3.2.1. The Influence of the Initial pH of the Aqueous Solution on the Adsorption Process
First, 5 mg of biochar was stirred at 400 rpm with 5 mL of tartrazine aqueous solution
with concentration of 5 mg L
−1
, varying the pH of the solution from 2 to 12, for establishing
the optimal pH for the tartrazine removal. The adsorption process was performed at room
temperature for 20 min for each sample. The two phases were separated at the end of the
process by filtration and the solute was analyzed. The results presented in Figure 5are
showing a very low adsorption at pH values of 4 and above (only 5% or less), while for
lower pH values, there is a significant increase in removal degree up to almost 50% at pH 2.
Since pH < pH
pzc
, the adsorbent surface is positively charged and absorbs the negatively
charged dye.
The aqueous tartrazine solutions used in all subsequent tests were prepared at pH 2 in
accordance with the findings of this particular example.
3.2.2. The Influence of the Temperature of the Solution on the Adsorption Process
In order to investigate the influence of the temperature on the adsorption process, 5 mL
tartrazine solution of 5 mg L
−1
was mixed with 5 mg of biochar and stirred at 400 rpm for
20 min at temperatures ranging from 25 to 45
◦
C. The results depicted in Figure 5highlight
a clear increase in removal efficiency with a temperature of up to about 35
◦
C, followed
by a decrease in the efficiency at higher temperatures. The efficiency increases along the
mentioned temperature range from about 49% at 25
◦
C to above 88% at around 35
◦
C and
then decreases with the temperature.
According to the literature, chemisorption increases with an increase in temperature
at first as the requirement of the activation energy for the reaction is fulfilled by an increase
in temperature. Then, it decreases due to the exothermic nature of adsorption.
In our case, the pseudo-second-order kinetic model is applicable for the removal
of tartrazine on biochar, suggesting that chemisorption is the process that controls the
adsorption rate.
For economic reasons and taking into account the fact that, on a large scale, tem-
peratures higher than ambient temperature are difficult to obtain, for the subsequent
experiments, a temperature of 30 ◦C was chosen.
Appl. Sci. 2024,14, 53 8 of 16
Appl.Sci.2023,13,xFORPEERREVIEW7of15
showingaverylowadsorptionatpHvaluesof4andabove(only5%orless),whilefor
lowerpHvalues,thereisasignificantincreaseinremovaldegreeuptoalmost50%atpH
2.SincepH<pH
pzc
,theadsorbentsurfaceispositivelychargedandabsorbsthenegatively
chargeddye.
TheaqueoustartrazinesolutionsusedinallsubsequenttestswerepreparedatpH2
inaccordancewiththefindingsofthisparticularexample.
Figure5.Theinfluence
ofthepH,temperature,adsorbentdose,contacttime,andtheinitialconcen-
trationsoftartrazineadsorptionontocommercialbiochar.
3.2.2.TheInfluenceoftheTemperatureoftheSolutionontheAdsorptionProcess
Inordertoinvestigatetheinfluenceofthetemperatureontheadsorptionprocess,5
mLtartrazinesolutionof5mgL
−1
wasmixedwith5mgofbiocharandstirredat400rpm
for20minattemperaturesrangingfrom25to45°C.TheresultsdepictedinFigure5high-
lightaclearincreaseinremovalefficiencywithatemperatureofuptoabout35°C,fol-
lowedbyadecreaseintheefficiencyathighertemperatures.Theefficiencyincreasesalong
thementionedtemperaturerangefromabout49%at25°Ctoabove88%ataround35°C
andthendecreaseswiththetemperature.
Figure 5. The influence of the pH, temperature, adsorbent dose, contact time, and the initial concen-
trations of tartrazine adsorption onto commercial biochar.
3.2.3. The Influence of the Adsorbent Dose on the Adsorption Process
In order to characterize the adsorption process as a function of the adsorbent dose, the
degree of tartrazine removal was monitored while the adsorbent dose was increased from
0.2 mg L
−1
to 1.0 mg L
−1
. The measurements were conducted at 30
◦
C temperature, stirring
the mixture of 5 mL tartrazine solution of 5 mg L
−1
concentration and the corresponding
amount of adsorbent at 400 rpm for 20 min for each sample. In Figure 5, which shows the
removal efficiency with the adsorbent dose, can be observed a rather oscillatory trend in
the removal degree.
In order to obtain a good removal efficiency using the smallest possible amount of
adsorbent, further studies were carried out using 0.9 mg L−1adsorbent dose.
3.2.4. The Influence of the Tartrazine Initial Concentration on the Adsorption Process
As expected, the initial concentration of the tartrazine solution should have an effect
on the tartrazine removal degree. In order to test it, the removal efficiency was measured at
30
◦
C stirring for 20 min at 400 rpm a mixture of 5 mg biochar with 5 mL tartrazine solution
Appl. Sci. 2024,14, 53 9 of 16
of different concentrations from 1 to 20 mg L
−1
. From the results depicted in Figure 5,
it can be concluded that the maximum removal degree was obtained at low tartrazine
concentrations.
3.2.5. The Influence of the Contact Time on the Adsorption Process
In order to determine the dependence of the removal degree as a function of contact
time, 5 mL of tartrazine aqueous solution with initial concentration of 3 mg L
−1
was
brought in contact with 5 mg of biochar for time periods between 3 and 30 min, at 30
◦
C.
The tartrazine removal degree as a function of contact time between the synthetic solution
of tartrazine and the biochar is shown in Figure 5. As one can see in the mentioned figure,
the maximum removal efficiency is achieved at contact times of about 18 min, with a
removal degree of 90.18%.
3.3. Adsorption Isotherm
Adsorption isotherms are used to illustrate the relationship between the material’s
capability for removal and the concentration of the contaminated solution, as well as to
characterize the pollutant-sorbent interaction [49].
The adsorption isotherm studies were carried out by agitating 5 mg of biochar with
5 mL of tartrazine solutions at 30
◦
C for 20 min at 400 rpm. The starting concentrations of
the tartrazine solutions ranged from 1 to 20 mg L−1.
The experimental data obtained were evaluated using the linearized forms of Lang-
muir, Freundlich, and Temkin isotherm models from Table 1.
Table 1. The linearized forms of isotherm models.
Isotherm Model Linear Form Reference
Langmuir Ce
qe=1
KLqm+Ce
qm[50]
Freundlich logqe=logKF+1
nlogCe[51]
Temkin qe=BlnKT+BlnCe[52]
The most popular isotherm for physisorption data is the Langmuir isotherm [
53
], non-
ideal sorption on heterogeneous surfaces can be addressed with the Freundlich isotherm
model [
54
], and abnormal adsorbate/adsorbate communication during adsorption is taken
into consideration by the Temkin model [55].
The best model is the one in which the difference between the experimental and
theoretical estimates are the smallest. For this purpose, the coefficient of determination R
2
is used; the closer its value is to unity, the more suitable the model.
Table 2summarizes the parameters derived from the isotherms models that might
shed light on the adsorption mechanism, surface characteristics, and adsorbent affinity.
Table 2. Isotherm constants for the adsorption of tartrazine on biochar.
Isotherm Model Constants Values
Langmuir
qm[mg g−1]3.2841
KL[L g−1]17.4000
R20.9936
Freundlich
KF[L mg−1]2.6878
1/n 0.1207
R20.9451
Temkin
KT[L mg−1]1.0486
bT[J mol−1]1046.5724
R20.3315
Appl. Sci. 2024,14, 53 10 of 16
The Langmuir isotherm produced the highest correlation coefficient (R
2
) value, indi-
cating that tartrazine was adsorbed on the biochar’s monolayer surface. This indicates that
every molecule that has been adsorbed is in contact with the adsorbent’s surface layer. The
constant n was used in order to determine the kind of adsorption: if n = 1, then adsorption
is linear; if n < 1, then adsorption is chemical; and if n > 1, then adsorption is physical. In
this case, n is greater than 1, so that the adsorption of tartrazine on biochar is physical.
Furthermore, it is evident from the comparison of the three models with the experimen-
tal results (Figure 6) that the Langmuir isotherm best describes the tartrazine adsorption on
biochar. Similar results in which the Langmuir isotherm was adapted to the adsorption of
tartrazine on bentonite modified with octa-decyltrimethylammonium bromide [
56
], raw
sawdust and activated sawdust [
10
], and microcline/MWCNTs nanocomposite [
57
] were
also obtained by other authors.
Appl.Sci.2023,13,xFORPEERREVIEW10of15
Figure6.Isothermsoftartrazineadsorptiononbiochar.
Weevaluatedthetartrazineadsorptionabilityofcommercialbiocharbycomparation
withotheradsorbentreports.Table3providesasummaryofthemaximaladsorptionca-
pacityofthisandotheradsorbentsusedtoremovetartrazine.
Table3.Comparisonofseveraladsorbents’adsorptioncapacitieswithcharcoalfortartrazinere-
moval.
Adsorbentq[mgg
−1
]References
positivelychargedtriethylenetetraminebiochar85.47[38]
octadecyltrimethylammoniumbromide-modified
bentonite
43.20,145.80,
175.80,and201.00[56]
rawsawdust 0.80[10]
activatedsawdust127.00[10]
microcline37.96[57]
microcline/MWCNTs67.17[57]
chitosan/polyaniline584.00[58]
activatedredmud136.98[59]
activatedcarbonderivedfromcassavasievate20.83[60]
activatedcarbonfromZiziphusSpina-Christi160.00[61]
commercialbiochar3.28presentstudy
3.4.KineticStudiesoftheAdsorptionProcess
Inordertounderstandtherateofadsorptionandmechanism,kineticstudiesarecon-
ducted,pseudo-first-order,pseudo-second-order,andintra-particle-diffusionmodelsbe-
ingmostwidelyused.Thebest-fiedmodelisselectedconsideringthecorrelationcoeffi-
cientR
2
[62].
Theinvestigationoftartrazineadsorptionkineticsonbiocharwascarriedoutat30
°Cwithaninitialconcentrationof3mgmL
−1
broughttopH2,forvariousdurations,rang-
ingfrom3to30min.
Therateofadsorptionandpotentialadsorptionmechanismoftartrazineadsorption
onbiocharwereexaminedusinganumberofkineticsmodels.Thekineticmodelsapplied
inthisstudyandtheirlinearizedformarepresentedinTabl e 4.
Table4.Thelinearizedformsofkineticmodels.
KineticModelLinearFormReference
Pseudo-first-orderkinetic𝑙𝑜𝑔𝑞𝑞𝑙𝑜𝑔𝑞
𝑘 𝑡
2.303
[63]
Pseudo-second-orderkinetic1
𝑞
1
𝑘
𝑞
𝑡
𝑞
[64]
Intraparticlediffusion𝑞𝑘
𝑡. 𝐶[65]
Figure 6. Isotherms of tartrazine adsorption on biochar.
We evaluated the tartrazine adsorption ability of commercial biochar by comparation
with other adsorbent reports. Table 3provides a summary of the maximal adsorption
capacity of this and other adsorbents used to remove tartrazine.
Table 3. Comparison of several adsorbents’ adsorption capacities with charcoal for tartrazine removal.
Adsorbent q [mg g−1] References
positively charged triethylenetetramine biochar 85.47 [38]
octadecyltrimethylammonium
bromide-modified bentonite
43.20, 145.80,
175.80, and 201.00 [56]
raw sawdust 0.80 [10]
activated sawdust 127.00 [10]
microcline 37.96 [57]
microcline/MWCNTs 67.17 [57]
chitosan/polyaniline 584.00 [58]
activated red mud 136.98 [59]
activated carbon derived from cassava sievate 20.83 [60]
activated carbon from Ziziphus Spina-Christi 160.00 [61]
commercial biochar 3.28 present study
3.4. Kinetic Studies of the Adsorption Process
In order to understand the rate of adsorption and mechanism, kinetic studies are
conducted, pseudo-first-order, pseudo-second-order, and intra-particle-diffusion models
being most widely used. The best-fitted model is selected considering the correlation
coefficient R2[62].
Appl. Sci. 2024,14, 53 11 of 16
The investigation of tartrazine adsorption kinetics on biochar was carried out at 30
◦
C
with an initial concentration of 3 mg mL
−1
brought to pH 2, for various durations, ranging
from 3 to 30 min.
The rate of adsorption and potential adsorption mechanism of tartrazine adsorption
on biochar were examined using a number of kinetics models. The kinetic models applied
in this study and their linearized form are presented in Table 4.
Table 4. The linearized forms of kinetic models.
Kinetic Model Linear Form Reference
Pseudo-first-order kinetic log(qe−qt)=lo gqe−k1t
2.303 [63]
Pseudo-second-order kinetic 1
qe=1
k2q2
e+t
qe[64]
Intraparticle diffusion qt=kid t0.5 +C[65]
The pseudo-first-order model (Lagergren model) establishes the relationship between
the change in time and the adsorption capacity with order of one, the pseudo-second-order
kinetic model (Ho and Mckay model) shows the relationship of the adsorption capacity
and concentration with second order, and the intraparticle diffusion model (Webber and
Morris model) determines the rate-controlling factor for the adsorption process [66].
The kinetics parameters obtained for the adsorption of tartrazine on biochar are
available in Table 5.
By examining the appropriate values of R
2
, it was discovered that the removal of
tartrazine on biochar can be accomplished using the pseudo-second-order kinetic model.
This suggests that the mechanism governing the rate of its adsorption was chemisorption.
The pseudo-second-order model fit with the results obtained for adsorption of tartrazine
on biochar-mediated ZrFe
2
O
5
nanocomposites [
37
] and on positively charged triethylenete-
tramine biochar [38].
Table 5. Kinetic constants for the adsorption of tartrazine on biochar.
Kinetic Model Constants Values
Pseudo-first-order kinetic
qe[mg g−1]0.1313
k1[min−1]0.0355
R20.1867
Pseudo-second-order kinetic
qe[mg g−1]2.7579
h [mg g−1min−1]2.1245
k2[g mg−1min−1]1.0603
R20.9889
Intraparticle diffusion
kid [mg g−1min1/2]0.0630
C 2.4296
R20.1912
Additionally, Figure 7shows that the plots are divided into two linear zones and
were not linear across the whole time range. According to the literature, if the line passes
through the origin point (0, 0), the adsorption is dominated by the intraparticle diffusion,
while if not, it is a multiple adsorption process. Because the line does not pass through
the origin, this indicates that the intraparticle diffusion is not the controlling stage of the
adsorption rate of tartrazine on biochar and it is complex.
Appl. Sci. 2024,14, 53 12 of 16
Appl.Sci.2023,13,xFORPEERREVIEW11of15
Thepseudo-first-ordermodel(Lagergrenmodel)establishestherelationshipbe-
tweenthechangeintimeandtheadsorptioncapacitywithorderofone,thepseudo-sec-
ond-orderkineticmodel(HoandMckaymodel)showstherelationshipoftheadsorption
capacityandconcentrationwithsecondorder,andtheintraparticlediffusionmodel(Web-
berandMorrismodel)determinestherate-controllingfactorfortheadsorptionprocess
[66].
Thekineticsparametersobtainedfortheadsorptionoftartrazineonbiocharareavail-
ableinTab le5.
Tab l e5.Kineticconstantsfortheadsorptionoftartrazineonbiochar.
KineticModelConstantsVal ues
Pseudo-first-orderkinetic
q
e
[mgg
−1
]0.1313
k
1
[min
−1
]0.0355
R
2
0.1867
Pseudo-second-orderkinetic
q
e
[mgg
−1
]2.7579
h[mgg
−1
min
−1
]2.1245
k
2
[gmg
−1
min
−1
]1.0603
R
2
0.9889
Intraparticlediffusion
k
id
[mgg
−1
min
1/2
]0.0630
C2.4296
R
2
0.1912
ByexaminingtheappropriatevaluesofR
2
,itwasdiscoveredthattheremovaloftar-
trazineonbiocharcanbeaccomplishedusingthepseudo-second-orderkineticmodel.
Thissuggeststhatthemechanismgoverningtherateofitsadsorptionwaschemisorption.
Thepseudo-second-ordermodelfitwiththeresultsobtainedforadsorptionoftartrazine
onbiochar-mediatedZrFe
2
O
5
nanocomposites[37]andonpositivelychargedtriethylene-
tetraminebiochar[38].
Additionally,Figure7showsthattheplotsaredividedintotwolinearzonesand
werenotlinearacrossthewholetimerange.Accordingtotheliterature,ifthelinepasses
throughtheoriginpoint(0,0),theadsorptionisdominatedbytheintraparticlediffusion,
whileifnot,itisamultipleadsorptionprocess.Becausethelinedoesnotpassthroughthe
origin,thisindicatesthattheintraparticlediffusionisnotthecontrollingstageofthead-
sorptionrateoftartrazineonbiocharanditiscomplex.
Figure7.Intraparticlediffusionadsorptionkineticsoftartrazineadsorptiononbiochar.
Numerouswritershavealsoreportedontheeffectiveuseofthepseudo-second-order
modeltoreflecttheexperimentalkineticdataoftartrazineadsorptionondifferentadsor-
bents;thesereportsareshowninTabl e 6.
Figure 7. Intraparticle diffusion adsorption kinetics of tartrazine adsorption on biochar.
Numerous writers have also reported on the effective use of the pseudo-second-
order model to reflect the experimental kinetic data of tartrazine adsorption on different
adsorbents; these reports are shown in Table 6.
Table 6. Correlation of the pseudo-second-order kinetic model for tartrazine adsorption on different
adsorbents.
Adsorbent References
raw sawdust [10]
activated sawdust [10]
octadecyltrimethylammonium bromide-modified bentonite [38]
cellulose from wheat straw residues [41]
cetyltrimethylammonium chloride-modified cellulose [41]
microcline/MWCNTs nanocomposite [57]
chitosan/polyaniline [58]
activated red mud [59]
activated carbon derived from cassava sievate [60]
crosslinked chitosan-coated bentonite [67]
commercial biochar this study
3.5. Reusability of Biochar
The reusability of biochar was investigated by adsorption–desorption cycles using
EtOH as desorption eluent. The degree of dye elimination is displayed in the Figure 8after
these cycles were completed four times.
Appl.Sci.2023,13,xFORPEERREVIEW12of15
Tab l e6.Correlationofthepseudo-second-orderkineticmodelfortartrazineadsorptionondifferent
adsorbents.
AdsorbentReferences
rawsawdus
t
[10]
activatedsawdust[10]
octadecyltrimethylammoniumbromide-modifiedbentonite[38]
cellulosefromwheatstrawresidues[41]
cetyltrimethylammoniumchloride-modifiedcellulose[41]
microcline/MWCNTsnanocomposite[57]
chitosan/polyaniline[58]
activatedredmud[59]
activatedcarbonderivedfromcassavasievate[60]
crosslinkedchitosan-coatedbentonite[67]
commercialbiocharthisstudy
3.5.ReusabilityofBiochar
Thereusabilityofbiocharwasinvestigatedbyadsorption–desorptioncyclesusing
EtOHasdesorptioneluent.ThedegreeofdyeeliminationisdisplayedintheFigure8after
thesecycleswerecompletedfourtimes.
Figure8.Adsorptionoftartrazineonbiocharinfourconsecutivecycles.
Theadsorbentcanbeemployedinanumberoftreatmentcyclessince,asthefigure
shows,therewasnodiscernibledropintheamountofdyeremovedafterthefourcycles.
4.Conclusions
Thisstudyexaminestheeffectivenessofcommercialbiocharineliminatingtartrazine
fromaqueoussolutions.Theidealparametersforextractingtartrazinefromaqueousso-
lutionsweredeterminedbythisstudytobepH2,30°Cworkingtemperature,0.9g