Bimetallic Zinc-Iron-Modified Sugarcane Bagasse Biochar for Simultaneous Adsorption of Arsenic and Oxytetracycline from Wastewater

Abstract

Arsenic (As), a highly toxic and carcinogenic heavy metal, poses significant risks to soil and water quality, while oxytetracycline (OTC), a widely used antibiotic, contributes to environmental pollution due to excessive human usage. Addressing the coexistence of multiple pollutants in the environment, this study investigates the simultaneous adsorption of As(III) and OTC using a novel bimetallic zinc-iron-modified biochar (1Zn-1Fe-1SBC). The developed adsorbent demonstrates enhanced recovery, improved adsorption efficiency, and cost-effective operation. Characterization results revealed a high carbon-to-hydrogen ratio (C/H) and a specific surface area of 1137 m2 g−1 for 1Zn-1Fe-1SBC. Isotherm modeling indicated maximum adsorption capacities of 34.7 mg g−1 for As(III) and 172.4 mg g−1 for OTC. Thermodynamic analysis confirmed that the adsorption processes for both pollutants were spontaneous (ΔG < 0), endothermic (ΔH > 0), and driven by chemical adsorption (ΔH > 80 kJ mol−1), with increased system disorder (ΔS > 0). The adsorption mechanisms involved multiple interactions, including pore filling, hydrogen bonding, electrostatic attraction, complexation, and π-π interactions. These findings underscore the potential of 1Zn-1Fe-1SBC as a promising adsorbent for the remediation of wastewater containing coexisting pollutants.

Full text

Academic Editor: Dipendu Saha
Received: 31 December 2024
Revised: 22 January 2025
Accepted: 24 January 2025
Published: 27 January 2025
Citation: Nguyen, N.-T.; Lin, A.-B.;
Chang, C.-T.; Hong, G.-B. Bimetallic
Zinc-Iron-Modified Sugarcane Bagasse
Biochar for Simultaneous Adsorption
of Arsenic and Oxytetracycline from
Wastewater. Molecules 2025,30, 572.
https://doi.org/10.3390/
molecules30030572
Copyright: © 2025 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/).
Article
Bimetallic Zinc-Iron-Modified Sugarcane Bagasse Biochar for
Simultaneous Adsorption of Arsenic and Oxytetracycline
from Wastewater
Nhat-Thien Nguyen 1, An-Bang Lin 2, Chang-Tang Chang 2, * and Gui-Bing Hong 1, *
1Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No. 1,
Sec. 3, Zhongxiao E. Rd., Taipei City 106, Taiwan; nguyennhatthien333@gmail.com
2Department of Environmental Engineering, National Ilan University, Yilan City 26047, Taiwan;
ampang1419@gmail.com
*Correspondence: ctchang@niu.edu.tw (C.-T.C.); lukehong@ntut.edu.tw (G.-B.H.);
Tel.: +886-2-2771-2171 (G.-B.H.)
Abstract: Arsenic (As), a highly toxic and carcinogenic heavy metal, poses significant risks
to soil and water quality, while oxytetracycline (OTC), a widely used antibiotic, contributes
to environmental pollution due to excessive human usage. Addressing the coexistence of
multiple pollutants in the environment, this study investigates the simultaneous adsorption
of As(III) and OTC using a novel bimetallic zinc-iron-modified biochar (1Zn-1Fe-1SBC).
The developed adsorbent demonstrates enhanced recovery, improved adsorption efficiency,
and cost-effective operation. Characterization results revealed a high carbon-to-hydrogen
ratio (C/H) and a specific surface area of 1137 m
2
g
−1
for 1Zn-1Fe-1SBC. Isotherm modeling
indicated maximum adsorption capacities of 34.7 mg g
−1
for As(III) and 172.4 mg g
−1
for
OTC. Thermodynamic analysis confirmed that the adsorption processes for both pollutants
were spontaneous (
∆
G < 0), endothermic (
∆
H > 0), and driven by chemical adsorption
(
∆
H > 80 kJ mol
−1
), with increased system disorder (
∆
S > 0). The adsorption mechanisms
involved multiple interactions, including pore filling, hydrogen bonding, electrostatic
attraction, complexation, and
π
-
π
interactions. These findings underscore the potential
of 1Zn-1Fe-1SBC as a promising adsorbent for the remediation of wastewater containing
coexisting pollutants.
Keywords: sugar cane bagasse; biochar; adsorption; arsenic; oxytetracycline
1. Introduction
Arsenic (As) is a highly toxic and carcinogenic heavy metal that poses a significant
threat to global soil and water quality. Its contamination has been widely reported in
river systems across various countries, including the United States, China, Turkey, and
Vietnam. The presence of arsenic in water is associated with severe environmental and
health risks, leading to diseases such as black foot disease, diabetes, hyperkeratosis, cancer,
hypertension, and neuropathy. Regulatory agencies have established stringent standards
to mitigate these risks; for instance, the Ministry of Environment of the Executive Yuan has
set arsenic limits in groundwater at 0.05 mg L
−1
and 0.005 mg L
−1
for general and drinking
water protection zones, respectively. Despite these efforts, arsenic pollution continues
to be a pressing issue worldwide [
1
]. Arsenic primarily exists in aqueous solutions as
arsenite (As(III)) and arsenate (As(V)) [
2
]. Generally, inorganic arsenic compounds exhibit
higher toxicity compared to their organic counterparts. Among the inorganic species,
As(III) is more toxic than As(V), as it binds with greater affinity to vicinal sulfhydryl
Molecules 2025,30, 572 https://doi.org/10.3390/molecules30030572

Molecules 2025,30, 572 2 of 25
groups, interacting with various proteins and inhibiting their function. Furthermore, As(III)
demonstrates greater stability than As(V), attributed to its electronic configuration [3,4].
Oxytetracycline (OTC), a widely used antibiotic in human medicine and animal
husbandry, has become another critical environmental pollutant. While antibiotics are
invaluable in combating bacterial infections and improving life expectancy, their extensive
use and improper disposal have resulted in significant contamination. Incomplete metabolic
degradation of antibiotics in humans (70–90%) exacerbates this issue, as unmetabolized
residues are released into the environment. According to the World Health Organization,
antibiotic pollution poses one of the greatest threats to global health, food security, and
ecological stability, with adverse impacts on plants, animals, and ecosystems [5].
Pharmaceutical and personal care products (PPCPs) have been linked to abnormalities
in human and microbial systems, as reported in several studies [
6
–
8
]. These compounds,
which contaminate rivers and other aquatic ecosystems, have become a significant focus
of research due to their adverse effects on aquatic organisms. Efforts to eliminate PPCPs
from the environment have been extensively studied [
9
,
10
]. PPCPs have been detected
in effluents [
11
], surface waters such as rivers [
12
], and within sewage treatment systems.
Among the wide range of pharmaceuticals and over-the-counter products, antimicrobials
are widely used in both human medicine and animal husbandry [
13
]. Their antimicrobial
properties make them resistant to degradation within biological systems [
14
], necessi-
tating the development of advanced adsorption techniques for the effective removal of
oxytetracycline (OTC).
A variety of technologies have been developed to address heavy metal and antibiotic
contamination in water, including chemical precipitation [
15
], biological treatment [
16
],
ion exchange [
17
], filtration [
18
], flotation [
19
], redox reactions [
20
], electrochemistry [
21
],
adsorption [
22
], and membrane filtration [
23
]. Among these, adsorption has emerged as a
promising method due to its effectiveness in removing and recovering pollutants. However,
traditional adsorbents often suffer from high costs, limited recovery efficiency, and technical
challenges, hindering their widespread adoption. Sugarcane bagasse (SCB) biochar, a
carbon-rich material derived from agricultural waste, represents a sustainable and cost-
effective alternative for environmental remediation. However, unmodified biochar typically
exhibits low adsorption efficiency, necessitating chemical or structural modifications to
enhance its pollutant removal capabilities. SCB biochar has shown promise as an adsorbent
for heavy metal removal from soil and water, attributed to its microporous structure
and large specific surface area [
24
,
25
]. Its surface is enriched with functional groups
such as carboxyl (-COOH), hydroxyl (-OH), and amino (-NH
2
), facilitating adsorption
via mechanisms like electrostatic attraction, cation exchange, surface complexation, and
electron donation [
26
]. A noteworthy example is the development of a zinc-iron bimetallic
system for water purification applications. Chen et al. [
27
] prepared modified walnut shell
biochar (ZF@WBC) through zinc-iron bimetallic oxide modification at 600
◦
C under oxygen-
limited conditions. Characterization revealed that ZF@WBC exhibited a significantly
enhanced surface area (376 m
2
g
−1
) and total pore volume (0.205 cm
3
g
−1
) compared to
unmodified walnut shell biochar. Adsorption experiments demonstrated that ZF@WBC
achieved a maximum adsorption capacity of 104.26 mg g
−1
for Pb(II), which is 2.57 times
higher than that of unmodified biochar, highlighting its improved performance [27].
This study focuses on the development of a chemically modified biochar derived from
SCB, an abundant agricultural byproduct. By incorporating zinc chloride (ZnCl
2
) and
ferric chloride (FeCl
3
) during the modification process, the biochar’s adsorption properties,
including surface area, pore structure, and functional groups, are significantly improved.
This modified biochar not only demonstrates enhanced adsorption performance for As(III)
and OTC but also offers advantages such as reduced operational costs, material recyclability,

Molecules 2025,30, 572 3 of 25
and reusability. The primary objectives of this research are to develop a cost-effective
biochar capable of simultaneously removing As(III) and OTC, optimize the composition of
zinc and iron in the modification process, establish optimal adsorption parameters, and
elucidate the underlying mechanisms driving pollutant removal. This work aims to provide
an innovative and sustainable solution for addressing the challenges of multi-pollutant
contamination in water systems.
2. Results and Discussion
2.1. Characterization Analysis
2.1.1. SEM Analysis Results
Scanning electron microscopy (SEM) was employed to investigate the surface mor-
phology and properties of SCB, SBC, 1Zn-1Fe-1SBC, and 1Zn-1Fe-1SBC-As-OTC (after
adsorption of As and OTC onto 1Zn-1Fe-1SBC). The SEM images are presented
in Figure 1
.
The analysis revealed that the biochar materials, including SCB, exhibited pores of varying
sizes and uneven, multilayered surface structures. Figure 1b shows that SBC possesses
a relatively well-developed pore structure compared to SCB (Figure 1a), primarily due
to the decomposition of organic matter on the SCB surface under high-temperature py-
rolysis, which resulted in the formation of a stable pore network on the SBC surface. In
Figure 1c, the surface of 1Zn-1Fe-1SBC modified with ZnCl
2
and FeCl
3
is shown to ex-
hibit a significantly enhanced pore structure and particle formation, attributed to ZnCl
2
impregnation. Consistent with findings by Lin et al. [
28
], the SCB pores underwent sub-
stantial corrosion following FeCl
3
modification, leading to an increased specific surface
area and the deposition of metal residues on the material’s surface. The modifications
using ZnCl
2
and FeCl
3
enhanced the material’s specific surface area and pore volume. BET
and EDS analyses further confirmed this enhancement, showing an increase in specific
surface area from 16.8 m
2
g
−1
to 1136.8 m
2
g
−1
and pore volume from 0.02 cm
3
g
−1
to
0.55 cm
3
g
−1
as detailed in Table 1. Additionally, EDS mapping revealed the uniform
distribution of Zn and Fe elements across the material’s surface, as illustrated in
Figure S1
in the
Supplementary Materials
. These modifications using ZnCl
2
and FeCl
3
effectively
improved the material’s adsorption properties by significantly enhancing its surface area
and pore volume.
Molecules 2025, 30, x FOR PEER REVIEW 4 of 25
Figure 1. SEM analysis results: (a) SCB, (b) SBC, (c) 1Zn-1Fe-1SBC, and (d) 1Zn-1Fe-1SBC-As-OTC.
Table 1. Specific surface area and pore structure of various biochars.
Materials Surface Area
(m
2
g
−1
)
Average Pore Volume
(cm
3
g
−1
)
Pore Size
(nm)
SBC 16.8 0.02 4.65
1Zn-1SBC 1314.5 0.64 1.92
1Fe-1SBC 97.3 0.31 3.28
1Zn-1Fe-1SBC 1136.8 0.55 1.98
1Zn-1Fe-1SBC-As-OTC 945.7 0.43 3.05
1Zn-1Fe-1SBC-5 cycles 989.5 0.45 2.31
Figure 1d illustrates the surface morphology of the 1Zn-1Fe-1SBC-As-OTC material,
which shows numerous particles deposited on the surface, forming deposition-induced
pores. These particles correspond to adsorbed As(III) and OTC, leading to a slight reduc-
tion in specific surface area (from 1136.8 m
2
g
−1
to 945.7 m
2
g
−1
) and pore volume (from 0.55
cm
3
g
−1
to 0.43 cm
3
g
−1
), as confirmed by BET (Table 1) and EDS analyses. Additionally,
EDS mapping identified the presence of As on the material’s surface (as shown in Figure
S2), aligning with the findings reported by Liu et al. [29]. Overall, the 1Zn-1Fe-1SBC ma-
terial developed in this study successfully adsorbed As(III) and OTC, demonstrating its
potential for effective pollutant removal.
2.1.2. EDS Analysis Results
Energy-dispersive X-ray spectroscopy (EDS) was employed to analyze the composi-
tion and distribution of chemical elements on the surfaces of SCB, SBC, 1Zn-1Fe-1SBC,
and 1Zn-1Fe-1SBC-As-OTC materials. The results, presented in Table 2, reveal the pri-
mary elemental compositions of the materials. For SCB and SBC, the main elements de-
tected were C, O, and Si, while the dominant elements for 1Zn-1Fe-1SBC and 1Zn-1Fe-
1SBC-As-OTC included C, O, Si, Zn, Fe, and As. As shown in Figure S3, the distribution
of C and O on the surface of SCB was uniform, aributed to the abundance of oxygen-
containing functional groups and surface-bound water molecules [30]. The presence of Si
on the SCB surface likely results from the interaction between sugarcane straw and soil,
incorporating nutrients and minerals from the soil into the sugarcane stalks [31]. Figure
Figure 1. SEM analysis results: (a) SCB, (b) SBC, (c) 1Zn-1Fe-1SBC, and (d) 1Zn-1Fe-1SBC-As-OTC.

Molecules 2025,30, 572 4 of 25
Table 1. Specific surface area and pore structure of various biochars.
Materials Surface Area
(m2g−1)
Average Pore Volume
(cm3g−1)
Pore Size
(nm)
SBC 16.8 0.02 4.65
1Zn-1SBC 1314.5 0.64 1.92
1Fe-1SBC 97.3 0.31 3.28
1Zn-1Fe-1SBC 1136.8 0.55 1.98
1Zn-1Fe-1SBC-As-OTC 945.7 0.43 3.05
1Zn-1Fe-1SBC-5 cycles 989.5 0.45 2.31
Figure 1d illustrates the surface morphology of the 1Zn-1Fe-1SBC-As-OTC material,
which shows numerous particles deposited on the surface, forming deposition-induced
pores. These particles correspond to adsorbed As(III) and OTC, leading to a slight reduc-
tion in specific surface area (from 1136.8 m
2
g
−1
to 945.7 m
2
g
−1
) and pore volume (from
0.55 cm3g−1
to 0.43 cm
3
g
−1
), as confirmed by BET (Table 1) and EDS analyses. Addition-
ally, EDS mapping identified the presence of As on the material’s surface (as shown in
Figure S2), aligning with the findings reported by Liu et al. [
29
]. Overall, the 1Zn-1Fe-1SBC
material developed in this study successfully adsorbed As(III) and OTC, demonstrating its
potential for effective pollutant removal.
2.1.2. EDS Analysis Results
Energy-dispersive X-ray spectroscopy (EDS) was employed to analyze the composition
and distribution of chemical elements on the surfaces of SCB, SBC, 1Zn-1Fe-1SBC, and
1Zn-1Fe-1SBC-As-OTC materials. The results, presented in Table 2, reveal the primary
elemental compositions of the materials. For SCB and SBC, the main elements detected
were C, O, and Si, while the dominant elements for 1Zn-1Fe-1SBC and 1Zn-1Fe-1SBC-As-
OTC included C, O, Si, Zn, Fe, and As. As shown in Figure S3, the distribution of C and
O on the surface of SCB was uniform, attributed to the abundance of oxygen-containing
functional groups and surface-bound water molecules [
30
]. The presence of Si on the SCB
surface likely results from the interaction between sugarcane straw and soil, incorporating
nutrients and minerals from the soil into the sugarcane stalks [
31
]. Figure S4 illustrates
that the O content in SBC decreases significantly, while the levels of C and Si increase,
as confirmed by the numerical elemental compositions provided in Table 2. This change
is attributed to the substantial reduction in organic matter during the high-temperature
pyrolysis of SCB, leading to an increase in the relative proportions of inorganic C and Si.
As confirmed in Figure S1, the EDS analysis demonstrates the successful incorporation
of Zn and Fe onto the surface of 1Zn-1Fe-1SBC following modification with ZnCl
2
and
FeCl
3
. These elements are observed to be widely and uniformly distributed across the
material’s surface. Notably, no detectable Cl was found after retesting the EDS, further
confirming the successful dechlorination of ZnCl
2
and FeCl
3
during the modification
process. This evidence supports the conclusion of “complete dechlorination” and aligns
with the data presented in Table 2. Figure S2 illustrates that after the adsorption of As(III)
and OTC by 1Zn-1Fe-1SBC, the presence of As and OTC on the material’s surface is
evident. Additionally, the EDS results confirm the presence of N, indicating the successful
adsorption of OTC on the material surface. The O content increases significantly during the
adsorption process, which can be attributed to the specific interaction mechanisms between
As(III) and the biochar (SBC) surface. Specifically, the metal oxyhydroxide nanoparticles
present on the SBC surface facilitate the replacement of OH-ligands in As(III) molecules,
forming mono- and bidentate complexes. These complexes enable As(III) to adhere to the
surface, contributing to the observed increase in O content. These findings underscore the

Molecules 2025,30, 572 5 of 25
material’s efficacy in capturing target pollutants through surface adsorption and validate
the compositional integrity and adsorption mechanisms discussed in this study.
Table 2. EDS elemental analysis results.
SCB SBC 1Zn-1Fe-1SBC 1Zn-1Fe-1SBC-As-OTC
W (%) A (%) W (%) A (%) W (%) A (%) W (%) A (%)
C (K) 55.84 62.78 86.75 89.97 80.37 86.57 65.08 78.30
O (K) 44.04 37.14 12.38 9.74 15.17 12.26 19.01 17.17
Si (K) 0.12 0.08 0.87 0.39 0.26 0.12 1.52 0.78
Zn (L) - - - - 0.09 0.02 0.45 0.10
Fe (K) - - - - 4.11 1.03 13.86 3.63
As (L) - - - - - - 0.08 0.02
N (K) - - - - - - 0.40 0.01
Total 100 100 100 100 100 100 100 100
2.1.3. FTIR Analysis Results
Fourier-transform infrared (FTIR) spectroscopy was employed to investigate the func-
tional groups present in the materials (SCB, SBC, 1Zn-1Fe-1SBC, and 1Zn-1Fe-1SBC-As)
within the range of 650–4000 cm
−1
. The results, presented in Figure 2, reveal that SCB,
due to its complex organic and inorganic composition, exhibits a variety of functional
groups and carbohydrate-related features. The characteristic peaks observed in the range
of 3000–3600 cm
−1
correspond to the stretching vibrations of -OH and -NH
2
groups, which
are associated with water molecules and amine groups, respectively. The peaks observed
at 2849 and 2930 cm
−1
are attributed to the C-H stretching vibrations in the -CH and
-CH
2
groups. All SBC samples exhibit C=O vibrations at 1760 cm
−1
, which is consistent
with previous studies on biochar and its derivatives [
32
,
33
]. The stretching vibration of
the aromatic C=C ring corresponds to the band observed at 1500–1600 cm
−1
. The peak
band at 1470–1500 cm
−1
corresponds to bending vibrations that may include contributions
from C–H, CH
2
, and CH
3
groups, as distinguishing these vibrations in this range can
be challenging without additional analysis [
34
]. Vibrations of C–C bonds are observed
in the 1000–1100 cm
−1
range. However, in the spectra of (bio)polymers, this range typi-
cally overlaps with contributions from C–O and C–H bonds, which complicates precise
assignments [
35
]. Notably, after adsorption of As(III) onto 1Zn-1Fe-1SBC, a new band
appears at 852 cm
−1
, corresponding to As-OH bending vibrations. This observation aligns
with findings reported by Chen et al. [
36
], Rago et al. [
37
], Coates et al. [
38
], and Park
et al. [
39
]. For SBC, 1Zn-1SBC, 1Fe-1SBC, and 1Zn-1Fe-1SBC, the bending vibrations of
-OH and -NH
2
groups were significantly reduced, leaving prominent peaks for C=O and
C-O groups. This is attributed to the decomposition of water molecules and amine groups
during high-temperature pyrolysis. According to Minaei et al. [
40
], the modification and
pyrolysis of biomass (e.g., sludge) result in a simplified FTIR spectrum, with distinct C=O
and C-O bending vibrations observed at 1560 and 1016 cm
−1
, respectively. Consistent
with the findings of Park et al. [
39
] and Cen et al. [
41
], the bending vibration of As-OH
appears in the 800–900 cm
−1
range after As adsorption. In this study, the high-temperature
pyrolysis of SCB led to substantial sintering of organic matter in SBC, 1Zn-1SBC, 1Fe-1SBC,
and 1Zn-1Fe-1SBC, resulting in the disappearance of most carbon-containing functional
groups. However, the adsorption of As(III) by 1Zn-1Fe-1SBC introduced an As-OH bending
vibration band at 852 cm−1, indicative of successful adsorption.

Molecules 2025,30, 572 6 of 25
Molecules 2025, 30, x FOR PEER REVIEW 6 of 25
1000–1100 cm−1 range. However, in the spectra of (bio)polymers, this range typically over-
laps with contributions from C–O and C–H bonds, which complicates precise assignments
[35]. Notably, after adsorption of As(III) onto 1Zn-1Fe-1SBC, a new band appears at 852
cm−1, corresponding to As-OH bending vibrations. This observation aligns with findings
reported by Chen et al. [36], Rago et al. [37], Coates et al. [38], and Park et al. [39]. For SBC,
1Zn-1SBC, 1Fe-1SBC, and 1Zn-1Fe-1SBC, the bending vibrations of -OH and -NH2 groups
were significantly reduced, leaving prominent peaks for C=O and C-O groups. This is at-
tributed to the decomposition of water molecules and amine groups during high-temper-
ature pyrolysis. According to Minaei et al. [40], the modification and pyrolysis of biomass
(e.g., sludge) result in a simplified FTIR spectrum, with distinct C=O and C-O bending
vibrations observed at 1560 and 1016 cm−1, respectively. Consistent with the findings of
Park et al. [39] and Cen et al. [41], the bending vibration of As-OH appears in the 800–900
cm−1 range after As adsorption. In this study, the high-temperature pyrolysis of SCB led
to substantial sintering of organic maer in SBC, 1Zn-1SBC, 1Fe-1SBC, and 1Zn-1Fe-1SBC,
resulting in the disappearance of most carbon-containing functional groups. However, the
adsorption of As(III) by 1Zn-1Fe-1SBC introduced an As-OH bending vibration band at
852 cm−1, indicative of successful adsorption.
Figure 2. FTIR analysis of various biochars.
2.1.4. TGA Analysis Results
Thermogravimetric analysis (TGA) was employed to evaluate the thermal stability
and compositional characteristics of the materials SCB, SBC, 1Zn-1SBC, 1Fe-1SBC, and
1Zn-1Fe-1SBC. The materials were subjected to a temperature range of 25–800 °C under a
nitrogen atmosphere. The weight loss profiles are illustrated in Figure 3. The initial weight
loss, aributed to water evaporation and the volatilization of organic compounds, was
approximately 6.0%, 5.9%, 5.2%, 5.8%, and 29% for SCB, SBC, 1Zn-1SBC, 1Fe-1SBC, and
1Zn-1Fe-1SBC, respectively [42]. The second stage of weight loss occurred in the temper-
ature range of 200–400 °C, associated with the thermal decomposition of lignin, hemicel-
lulose, and cellulose. During this stage, SCB showed a weight loss of approximately 73.1%,
while SBC, 1Zn-1SBC, 1Fe-1SBC, and 1Zn-1Fe-1SBC exhibited minimal weight losses of
0.5%, 0.4%, 0.3%, and 0.4%, respectively. The third stage of weight loss, observed between
400–800 °C, was primarily due to the decomposition of residual organic maer. The
weight losses during this phase were 18.1%, 22.6%, 19.4%, 7.8%, and 18.4% for SCB, SBC,
Figure 2. FTIR analysis of various biochars.
2.1.4. TGA Analysis Results
Thermogravimetric analysis (TGA) was employed to evaluate the thermal stability
and compositional characteristics of the materials SCB, SBC, 1Zn-1SBC, 1Fe-1SBC, and
1Zn-1Fe-1SBC. The materials were subjected to a temperature range of 25–800
◦
C under
a nitrogen atmosphere. The weight loss profiles are illustrated in Figure 3. The initial
weight loss, attributed to water evaporation and the volatilization of organic compounds,
was approximately 6.0%, 5.9%, 5.2%, 5.8%, and 29% for SCB, SBC, 1Zn-1SBC, 1Fe-1SBC,
and 1Zn-1Fe-1SBC, respectively [
42
]. The second stage of weight loss occurred in the
temperature range of 200–400
◦
C, associated with the thermal decomposition of lignin,
hemicellulose, and cellulose. During this stage, SCB showed a weight loss of approximately
73.1%, while SBC, 1Zn-1SBC, 1Fe-1SBC, and 1Zn-1Fe-1SBC exhibited minimal weight
losses of 0.5%, 0.4%, 0.3%, and 0.4%, respectively. The third stage of weight loss, observed
between 400–800
◦
C, was primarily due to the decomposition of residual organic matter.
The weight losses during this phase were 18.1%, 22.6%, 19.4%, 7.8%, and 18.4% for SCB,
SBC, 1Zn-1SBC, 1Fe-1SBC, and 1Zn-1Fe-1SBC, respectively. The reduced organic content
in Zn- and Fe-modified biochars (1Zn-1SBC, 1Fe-1SBC, and 1Zn-1Fe-1SBC) contributed to
their lower overall weight losses. Moreover, the presence of Zn and Fe, which are inorganic
elements with high melting points, enhanced the thermal stability of the materials at
elevated temperatures.
These observations are consistent with previous studies. According to Marx et al. [
43
],
Stylianou et al. [
44
], and Kaikiti et al. [
45
], weight loss between 30–200
◦
C is predomi-
nantly associated with water evaporation and organic volatilization. The rapid weight
loss between 200–1000
◦
C is attributed to the high-temperature decomposition of lignin
(250–900
◦
C), cellulose (300–400
◦
C), and hemicellulose (220–315
◦
C). Hu et al. [
46
] reported
similar trends in thermogravimetric behavior for biochars and metal-doped biochars. For
example, undoped biochar (BC) exhibited a weight loss of 88% between 300–600
◦
C, while
Zn-Fe-modified biochar (ZF-BC) and tri-metallic spinel biochar (MZF-BC) showed reduced
weight losses of 62% and 50.7%, respectively. This reduction is attributed to the lower
organic matter content in metal-doped biochars. Consistently, the Zn- and Fe-modified
biochar 1Zn-1Fe-1SBC in this study demonstrated superior thermal stability compared to
other biochars, particularly at high temperatures.

Molecules 2025,30, 572 7 of 25
Molecules 2025, 30, x FOR PEER REVIEW 7 of 25
1Zn-1SBC, 1Fe-1SBC, and 1Zn-1Fe-1SBC, respectively. The reduced organic content in Zn-
and Fe-modified biochars (1Zn-1SBC, 1Fe-1SBC, and 1Zn-1Fe-1SBC) contributed to their
lower overall weight losses. Moreover, the presence of Zn and Fe, which are inorganic
elements with high melting points, enhanced the thermal stability of the materials at ele-
vated temperatures.
These observations are consistent with previous studies. According to Marx et al.
[43], Stylianou et al. [44], and Kaikiti et al. [45], weight loss between 30–200 °C is predom-
inantly associated with water evaporation and organic volatilization. The rapid weight
loss between 200–1000 °C is aributed to the high-temperature decomposition of lignin
(250–900 °C), cellulose (300–400 °C), and hemicellulose (220–315 °C). Hu et al. [46] re-
ported similar trends in thermogravimetric behavior for biochars and metal-doped bio-
chars. For example, undoped biochar (BC) exhibited a weight loss of 88% between 300–
600 °C, while Zn-Fe-modified biochar (ZF-BC) and tri-metallic spinel biochar (MZF-BC)
showed reduced weight losses of 62% and 50.7%, respectively. This reduction is aributed
to the lower organic maer content in metal-doped biochars. Consistently, the Zn- and
Fe-modified biochar 1Zn-1Fe-1SBC in this study demonstrated superior thermal stability
compared to other biochars, particularly at high temperatures.
Figure 3. TGA analysis of various biochars.
2.1.5. BET Analysis Results
The pore characteristics, specific surface area, and pore size distribution of SBC, 1Zn-
1SBC, 1Fe-1SBC, 1Zn-1Fe-1SBC, and 1Zn-1Fe-1SBC-As-OTC were analyzed using
Brunauer–Emmett–Teller (BET) isothermal adsorption/desorption methods. The results,
presented in Figures 4 and 5 and summarized in Table 1, provide insights into the influence
of material modifications and adsorption processes on these parameters. The specific sur-
face areas of SBC, 1Zn-1SBC, 1Fe-1SBC, 1Zn-1Fe-1SBC, and 1Zn-1Fe-1SBC-As-OTC were
16.8, 1314.5, 97.3, 1136.8, and 945.7 m
2
g
−1
, respectively. Correspondingly, their pore volumes
were 0.02, 0.64, 0.31, 0.55, and 0.43 cm
3
g
−1
, while their average pore sizes were 4.65, 1.92,
3.28, 1.98, and 3.05 nm. These results demonstrate that modifications with ZnCl
2
and FeCl
3
significantly increased both the specific surface area and pore volume compared to unmod-
ified SBC. The enhancement in these parameters is primarily attributed to the etching effects
of ZnCl
2
and FeCl
3
, which created new cavities and pores on the material surface. For in-
stance, the specific surface area of SBC increased from 16.8 to 1136.8 m
2
g
−1
, and the pore
volume increased from 0.02 to 0.55 cm
3
g
−1
after modification. However, after the adsorption
of As(III) and OTC onto 1Zn-1Fe-1SBC, both specific surface area and pore volume de-
creased significantly, with the specific surface area reducing from 1136.8 to 945.7 m
2
g
−1
and
the pore volume reducing from 0.55 to 0.43 cm
3
g
−1
. This reduction is attributed to pore
Figure 3. TGA analysis of various biochars.
2.1.5. BET Analysis Results
The pore characteristics, specific surface area, and pore size distribution of SBC,
1Zn-1SBC, 1Fe-1SBC, 1Zn-1Fe-1SBC, and 1Zn-1Fe-1SBC-As-OTC were analyzed using
Brunauer–Emmett–Teller (BET) isothermal adsorption/desorption methods. The results,
presented in Figures 4and 5and summarized in Table 1, provide insights into the influence
of material modifications and adsorption processes on these parameters. The specific
surface areas of SBC, 1Zn-1SBC, 1Fe-1SBC, 1Zn-1Fe-1SBC, and 1Zn-1Fe-1SBC-As-OTC
were 16.8, 1314.5, 97.3, 1136.8, and 945.7 m
2
g
−1
, respectively. Correspondingly, their pore
volumes were 0.02, 0.64, 0.31, 0.55, and 0.43 cm
3
g
−1
, while their average pore sizes were
4.65, 1.92, 3.28, 1.98, and 3.05 nm. These results demonstrate that modifications with ZnCl
2
and FeCl
3
significantly increased both the specific surface area and pore volume compared
to unmodified SBC. The enhancement in these parameters is primarily attributed to the
etching effects of ZnCl
2
and FeCl
3
, which created new cavities and pores on the material
surface. For instance, the specific surface area of SBC increased from 16.8 to 1136.8 m
2
g
−1
,
and the pore volume increased from 0.02 to 0.55 cm
3
g
−1
after modification. However,
after the adsorption of As(III) and OTC onto 1Zn-1Fe-1SBC, both specific surface area and
pore volume decreased significantly, with the specific surface area reducing from 1136.8 to
945.7 m
2
g
−1
and the pore volume reducing from 0.55 to 0.43 cm
3
g
−1
. This reduction is
attributed to pore blockage caused by the adsorption of As(III) and OTC. Furthermore, after
five adsorption cycles (1Zn-1Fe-1SBC5 cycles), the specific surface area and pore volume of
1Zn-1Fe-1SBC decreased slightly, from 1136.8 to 989.5 m
2
g
−1
and from 0.55 to
0.45 cm3g−1
,
respectively, indicating that the pore structure remained intact despite repeated usage.
These findings are consistent with results reported by Li et al. [
47
], where biochar
modified with ZnSO
4
and FeCl
3
exhibited a significant increase in specific surface area
and pore volume compared to unmodified biochar. For example, the specific surface
area of Mikania micrantha Kunth biochar increased from 4.2 m
2
g
−1
to 54.1 m
2
g
−1
after
modification, with a corresponding increase in pore volume from 0.02 to 0.14 cm
3
g
−1
.
However, after the adsorption of OTC, these parameters decreased, suggesting effective
adsorption and pore occupation. In this study, the modification of SBC with ZnCl
2
and
FeCl
3
significantly enhanced the specific surface area and pore volume, making it a highly
effective material for the adsorption of As(III) and OTC. The observed decrease in these
parameters post-adsorption confirms the successful loading of these contaminants onto the
modified material.

Molecules 2025,30, 572 8 of 25
Molecules 2025, 30, x FOR PEER REVIEW 8 of 25
blockage caused by the adsorption of As(III) and OTC. Furthermore, after five adsorption
cycles (1Zn-1Fe-1SBC5 cycles), the specific surface area and pore volume of 1Zn-1Fe-1SBC
decreased slightly, from 1136.8 to 989.5 m2 g−1 and from 0.55 to 0.45 cm3 g−1, respectively,
indicating that the pore structure remained intact despite repeated usage.
These findings are consistent with results reported by Li et al. [47], where biochar mod-
ified with ZnSO₄ and FeCl3 exhibited a significant increase in specific surface area and pore
volume compared to unmodified biochar. For example, the specific surface area of Mikania
micrantha Kunth biochar increased from 4.2 m2 g−1 to 54.1 m2 g−1 after modification, with a
corresponding increase in pore volume from 0.02 to 0.14 cm3 g−1. However, after the adsorp-
tion of OTC, these parameters decreased, suggesting effective adsorption and pore occupa-
tion. In this study, the modification of SBC with ZnCl2 and FeCl3 significantly enhanced the
specific surface area and pore volume, making it a highly effective material for the adsorp-
tion of As(III) and OTC. The observed decrease in these parameters post-adsorption con-
firms the successful loading of these contaminants onto the modified material.
Figure 4. The N2 adsorption–desorption isotherm of biochars.
Figure 5. The pore size distribution paerns of biochars.
Figure 4. The N2adsorption–desorption isotherm of biochars.
Molecules 2025, 30, x FOR PEER REVIEW 8 of 25
blockage caused by the adsorption of As(III) and OTC. Furthermore, after five adsorption
cycles (1Zn-1Fe-1SBC5 cycles), the specific surface area and pore volume of 1Zn-1Fe-1SBC
decreased slightly, from 1136.8 to 989.5 m2 g−1 and from 0.55 to 0.45 cm3 g−1, respectively,
indicating that the pore structure remained intact despite repeated usage.
These findings are consistent with results reported by Li et al. [47], where biochar mod-
ified with ZnSO₄ and FeCl3 exhibited a significant increase in specific surface area and pore
volume compared to unmodified biochar. For example, the specific surface area of Mikania
micrantha Kunth biochar increased from 4.2 m2 g−1 to 54.1 m2 g−1 after modification, with a
corresponding increase in pore volume from 0.02 to 0.14 cm3 g−1. However, after the adsorp-
tion of OTC, these parameters decreased, suggesting effective adsorption and pore occupa-
tion. In this study, the modification of SBC with ZnCl2 and FeCl3 significantly enhanced the
specific surface area and pore volume, making it a highly effective material for the adsorp-
tion of As(III) and OTC. The observed decrease in these parameters post-adsorption con-
firms the successful loading of these contaminants onto the modified material.
Figure 4. The N2 adsorption–desorption isotherm of biochars.
Figure 5. The pore size distribution paerns of biochars.
Figure 5. The pore size distribution patterns of biochars.
2.1.6. XPS Analysis Results
To investigate the composition, content, and molecular structure characteristics of the
carbon (C) element on the surfaces of 1Zn-1Fe-1SBC and 1Zn-1Fe-1SBC-As-OTC, X-ray
photoelectron spectroscopy (XPS) analysis was conducted on the C1s electron spectra.
As shown in Figure 6, the binding energies of C-C/C=C, C-O, C=O, and
π
-
π
* shakeup
satellite interactions were detected at 284.8, 285.1, 285.8, and ~6.8 eV, respectively [
48
].
From Figure 6a, the binding energies corresponding to C-C/C=C, C-O, C=O, and
π
-
π
*
shakeup satellite features were observed on the 1Zn-1Fe-1SBC surface, while Figure 6b
reveals that these binding energies were also present on the 1Zn-1Fe-1SBC-As-OTC surface,
with respective area ratios of 17.3%, 27.0%, 31.8%, and 23.9%. The increased
π
-
π
* shakeup
satellite area (23.9%) in 1Zn-1Fe-1SBC-As-OTC compared to 1Zn-1Fe-1SBC suggests en-
hanced
π
-
π
electron interactions between the biochar (acceptor) and the adsorbed As(III)
and OTC molecules (donors). This phenomenon is consistent with the findings of Zhang
et al. [
49
], who reported an increase in the
π
-
π
* shakeup satellite area from 8.90% to 9.32%
in Sycamore Flocs Biochar after the adsorption of OTC-HCl, attributed to the interaction of
π-πelectron donor-acceptor systems.

Molecules 2025,30, 572 9 of 25
Molecules 2025, 30, x FOR PEER REVIEW 9 of 25
2.1.6. XPS Analysis Results
To investigate the composition, content, and molecular structure characteristics of
the carbon (C) element on the surfaces of 1Zn-1Fe-1SBC and 1Zn-1Fe-1SBC-As-OTC, X-
ray photoelectron spectroscopy (XPS) analysis was conducted on the C1s electron spectra.
As shown in Figure 6, the binding energies of C-C/C=C, C-O, C=O, and π-π* shakeup
satellite interactions were detected at 284.8, 285.1, 285.8, and ~6.8 eV, respectively [48].
From Figure 6a, the binding energies corresponding to C-C/C=C, C-O, C=O, and π-π*
shakeup satellite features were observed on the 1Zn-1Fe-1SBC surface, while Figure 6b
reveals that these binding energies were also present on the 1Zn-1Fe-1SBC-As-OTC sur-
face, with respective area ratios of 17.3%, 27.0%, 31.8%, and 23.9%. The increased π-π*
shakeup satellite area (23.9%) in 1Zn-1Fe-1SBC-As-OTC compared to 1Zn-1Fe-1SBC sug-
gests enhanced π-π electron interactions between the biochar (acceptor) and the adsorbed
As(III) and OTC molecules (donors). This phenomenon is consistent with the findings of
Zhang et al. [49], who reported an increase in the π-π* shakeup satellite area from 8.90%
to 9.32% in Sycamore Flocs Biochar after the adsorption of OTC-HCl, aributed to the
interaction of π-π electron donor-acceptor systems.
Figure 6. Electron spectra of C1s from XPS: (a) 1Zn-1Fe-SBC and (b) 1Zn-1Fe-1SBC-As-OTC.
To further analyze the oxygen (O) element on the surfaces of 1Zn-1Fe-1SBC and 1Zn-
1Fe-1SBC-As-OTC, XPS O1s spectra were examined. Figure 7 indicates the presence of Fe-
O, C-O, and C=O binding energies on the biochar surface, located at 529.5, 530.9, and 532.6
eV, respectively. From Figure 7a, the binding energy areas for Fe-O, C-O, and C=O on the
1Zn-1Fe-1SBC surface were 29.1%, 47.8%, and 23.1%, respectively. In contrast, Figure 7b
shows that the respective binding energy areas on the 1Zn-1Fe-1SBC-As-OTC surface
were reduced to 21.7%, 39.3%, and 39.0%. The C-O and C=O binding energy variations
observed in Figure 7b can indeed be aributed to the sorbed OTC, as OTC contains its
own C-C, C=C, C-O, and C=O functional groups. These functional groups may contribute
to the changes in the C-O and C=O regions. However, we believe that the reduction in the
Fe-O binding energy area is primarily due to interactions between the Fe sites and the
sorbed OTC molecules. These interactions likely cause a shift in the electron distribution,
resulting in changes in the Fe-O contribution, which indicates that the adsorption of OTC
has influenced the surface structure of the material. The results of Yoon et al. [50] strongly
suggested adsorption of As occurred via a specific chemical reaction between As and Fe–
O functional groups on magnetite. The overall results suggest the use of FeCl
3
is a feasible
practical approach to control the intrinsic pH of biochar and impart additional function-
ality that enables effective treatment of As.
Figure 6. Electron spectra of C1s from XPS: (a) 1Zn-1Fe-SBC and (b) 1Zn-1Fe-1SBC-As-OTC.
To further analyze the oxygen (O) element on the surfaces of 1Zn-1Fe-1SBC and 1Zn-
1Fe-1SBC-As-OTC, XPS O1s spectra were examined. Figure 7indicates the presence of
Fe-O, C-O, and C=O binding energies on the biochar surface, located at 529.5, 530.9, and
532.6 eV, respectively. From Figure 7a, the binding energy areas for Fe-O, C-O, and C=O on
the 1Zn-1Fe-1SBC surface were 29.1%, 47.8%, and 23.1%, respectively. In contrast, Figure 7b
shows that the respective binding energy areas on the 1Zn-1Fe-1SBC-As-OTC surface
were reduced to 21.7%, 39.3%, and 39.0%. The C-O and C=O binding energy variations
observed in Figure 7b can indeed be attributed to the sorbed OTC, as OTC contains its
own C-C, C=C, C-O, and C=O functional groups. These functional groups may contribute
to the changes in the C-O and C=O regions. However, we believe that the reduction in
the Fe-O binding energy area is primarily due to interactions between the Fe sites and the
sorbed OTC molecules. These interactions likely cause a shift in the electron distribution,
resulting in changes in the Fe-O contribution, which indicates that the adsorption of OTC
has influenced the surface structure of the material. The results of Yoon et al. [
50
] strongly
suggested adsorption of As occurred via a specific chemical reaction between As and Fe–O
functional groups on magnetite. The overall results suggest the use of FeCl
3
is a feasible
practical approach to control the intrinsic pH of biochar and impart additional functionality
that enables effective treatment of As.
Molecules 2025, 30, x FOR PEER REVIEW 10 of 25
Figure 7. O1s electron spectra of XPS: (a) 1Zn-1Fe-SBC and (b) 1Zn-1Fe-1SBC-As-OTC.
To explore the presence of arsenic (As) on the surface of 1Zn-1Fe-1SBC-As-OTC, the
As3d spectra were analyzed via XPS. As shown in Figure 8, the binding energy of As(III)
was identified at 45.0 eV, with an area ratio of 100%, confirming the successful adsorption
of As(III) onto the surface of 1Zn-1Fe-1SBC. This observation is consistent with the findings
of Zama et al. [51], who reported the appearance of As(III) binding energy at 45.1 eV on the
surface of Aspen Wood Biochar after the adsorption of As(III). These results collectively
demonstrate the successful adsorption of As(III) and OTC onto the surface of the modified
biochar, elucidating the molecular interactions responsible for these adsorption processes.
Figure 8. As3d electron spectra of XPS from 1Zn-1Fe-SBC-As-OTC.
2.2. Results of Simultaneous Adsorption of As(III) and OTC
2.2.1. Comparison of Different Biochars
The adsorption efficiencies of four biochars—SBC, 1Zn-1SBC, 1Fe-1SBC, and 1Zn-
1Fe-1SBC—towards As(III) were 9%, 49%, 43%, and 91%, respectively, corresponding to
adsorption capacities of 0.06, 2.46, 2.15, and 4.55 mg g
−1
, as illustrated in Figure 9. The
adsorption capacities observed in this study are higher than those reported by Lin et al.
[52] for metal-biochar (3.6 mg g
−1
). Similarly, the adsorption efficiencies of these biochars
for OTC were 10%, 84%, 40%, and 92%, respectively, with adsorption capacities of 0.25,
16.8, 7.98, and 18.4 mg g
−1
. The results indicate that 1Zn-1Fe-1SBC exhibited the highest
adsorption efficiencies for both As(III) and OTC. This superior performance can be at-
tributed to the synergistic effects of ZnCl
2
and FeCl
3
during the modification process,
which enhanced the specific surface area, pore volume, and functional group availability
on the biochar surface. These structural and chemical improvements significantly contrib-
uted to the adsorption of both As(III) and OTC.
Figure 7. O1s electron spectra of XPS: (a) 1Zn-1Fe-SBC and (b) 1Zn-1Fe-1SBC-As-OTC.
To explore the presence of arsenic (As) on the surface of 1Zn-1Fe-1SBC-As-OTC, the
As3d spectra were analyzed via XPS. As shown in Figure 8, the binding energy of As(III)
was identified at 45.0 eV, with an area ratio of 100%, confirming the successful adsorption
of As(III) onto the surface of 1Zn-1Fe-1SBC. This observation is consistent with the findings
of Zama et al. [
51
], who reported the appearance of As(III) binding energy at 45.1 eV on the

Molecules 2025,30, 572 10 of 25
surface of Aspen Wood Biochar after the adsorption of As(III). These results collectively
demonstrate the successful adsorption of As(III) and OTC onto the surface of the modified
biochar, elucidating the molecular interactions responsible for these adsorption processes.
Molecules 2025, 30, x FOR PEER REVIEW 10 of 25
Figure 7. O1s electron spectra of XPS: (a) 1Zn-1Fe-SBC and (b) 1Zn-1Fe-1SBC-As-OTC.
To explore the presence of arsenic (As) on the surface of 1Zn-1Fe-1SBC-As-OTC, the
As3d spectra were analyzed via XPS. As shown in Figure 8, the binding energy of As(III)
was identified at 45.0 eV, with an area ratio of 100%, confirming the successful adsorption
of As(III) onto the surface of 1Zn-1Fe-1SBC. This observation is consistent with the findings
of Zama et al. [51], who reported the appearance of As(III) binding energy at 45.1 eV on the
surface of Aspen Wood Biochar after the adsorption of As(III). These results collectively
demonstrate the successful adsorption of As(III) and OTC onto the surface of the modified
biochar, elucidating the molecular interactions responsible for these adsorption processes.
Figure 8. As3d electron spectra of XPS from 1Zn-1Fe-SBC-As-OTC.
2.2. Results of Simultaneous Adsorption of As(III) and OTC
2.2.1. Comparison of Different Biochars
The adsorption efficiencies of four biochars—SBC, 1Zn-1SBC, 1Fe-1SBC, and 1Zn-
1Fe-1SBC—towards As(III) were 9%, 49%, 43%, and 91%, respectively, corresponding to
adsorption capacities of 0.06, 2.46, 2.15, and 4.55 mg g
−1
, as illustrated in Figure 9. The
adsorption capacities observed in this study are higher than those reported by Lin et al.
[52] for metal-biochar (3.6 mg g
−1
). Similarly, the adsorption efficiencies of these biochars
for OTC were 10%, 84%, 40%, and 92%, respectively, with adsorption capacities of 0.25,
16.8, 7.98, and 18.4 mg g
−1
. The results indicate that 1Zn-1Fe-1SBC exhibited the highest
adsorption efficiencies for both As(III) and OTC. This superior performance can be at-
tributed to the synergistic effects of ZnCl
2
and FeCl
3
during the modification process,
which enhanced the specific surface area, pore volume, and functional group availability
on the biochar surface. These structural and chemical improvements significantly contrib-
uted to the adsorption of both As(III) and OTC.
Figure 8. As3d electron spectra of XPS from 1Zn-1Fe-SBC-As-OTC.
2.2. Results of Simultaneous Adsorption of As(III) and OTC
2.2.1. Comparison of Different Biochars
The adsorption efficiencies of four biochars—SBC, 1Zn-1SBC, 1Fe-1SBC, and
1Zn-1Fe-1SBC—towards As(III) were 9%, 49%, 43%, and 91%, respectively, corresponding
to adsorption capacities of 0.06, 2.46, 2.15, and 4.55 mg g
−1
, as illustrated in Figure 9. The
adsorption capacities observed in this study are higher than those reported by
Lin et al. [52]
for metal-biochar (3.6 mg g
−1
). Similarly, the adsorption efficiencies of these biochars for
OTC were 10%, 84%, 40%, and 92%, respectively, with adsorption capacities of 0.25, 16.8,
7.98, and 18.4 mg g
−1
. The results indicate that 1Zn-1Fe-1SBC exhibited the highest adsorp-
tion efficiencies for both As(III) and OTC. This superior performance can be attributed to the
synergistic effects of ZnCl
2
and FeCl
3
during the modification process, which enhanced the
specific surface area, pore volume, and functional group availability on the biochar surface.
These structural and chemical improvements significantly contributed to the adsorption of
both As(III) and OTC.
Molecules 2025, 30, x FOR PEER REVIEW 11 of 25
Figure 9. Comparison of biochar absorption performance: (a) As(III) and (b) OTC. (C0 of As(III) = 1
ppm, C0 of OTC = 4 ppm, dose = 0.2 g/L, T = 25 °C, pH = 5).
2.2.2. Effect of Fe and Zn Content
The adsorption efficiencies of biochars modified with varying ratios of ZnCl2 and
FeCl3—4Zn-1Fe-1SBC, 2Zn-1Fe-1SBC, 1Zn-1Fe-1SBC, and 1Zn-2Fe-1SBC—for As(III)
were 61%, 72%, 91%, and 80%, respectively, with corresponding adsorption capacities of
3.05, 3.60, 4.55, and 4.00 mg g−1. Similarly, the adsorption efficiencies for OTC were 66%,
70%, 92%, and 80%, respectively, with adsorption capacities of 13.2, 14.1, 18.4, and 16.0
mg g−1, as illustrated in Figure 10. These results demonstrate that increasing the propor-
tion of metal modifiers initially enhances the adsorption efficiency of biochar for both
As(III) and OTC. However, when the metal content exceeds an optimal level, the adsorp-
tion efficiency declines. This reduction is primarily aributed to excessive metal content,
which can block pores on the biochar surface, thereby reducing the available adsorption
sites. Comparable findings were reported by Li et al. [53], who investigated the effect of
ZnCl2 content on the adsorption efficiency of chromium using domestic sewage sludge
carbon (DSSC). The adsorption capacities for DSSC mixed with 33%, 50%, 60%, and 67%
ZnCl2 were 66.1, 68.8, 66.3, and 55.9 mg g−1, respectively. The study revealed that while an
optimal ZnCl2 ratio enhances porosity and adsorption capacity, excessive ZnCl2 leads to
pore blockage, reducing adsorption efficiency. Similarly, the optimal adsorption efficiency
for As(III) and OTC in this study was achieved with an appropriate Zn-to-Fe ratio in the
biochar. Both insufficient and excessive metal content adversely impacted adsorption per-
formance. This underscores the importance of balancing the metal modifier ratios to max-
imize the adsorption capabilities of the biochar.
Figure 10. Comparison of biochar absorption performance with non-identical Zn and Fe content: (a)
As(III) and (b) OTC. (C0 of As(III) = 1 ppm, C0 of OTC = 4 ppm, dose = 0.2 g L−1, T = 25 °C, pH = 5).
Figure 9. Comparison of biochar absorption performance: (a) As(III) and (b) OTC. (C0of As(III) = 1
ppm, C0of OTC = 4 ppm, dose = 0.2 g/L, T = 25 ◦C, pH = 5).
2.2.2. Effect of Fe and Zn Content
The adsorption efficiencies of biochars modified with varying ratios of ZnCl
2
and
FeCl
3
—4Zn-1Fe-1SBC, 2Zn-1Fe-1SBC, 1Zn-1Fe-1SBC, and 1Zn-2Fe-1SBC—for As(III) were
61%, 72%, 91%, and 80%, respectively, with corresponding adsorption capacities of 3.05,
3.60, 4.55, and 4.00 mg g
−1
. Similarly, the adsorption efficiencies for OTC were 66%, 70%,
92%, and 80%, respectively, with adsorption capacities of 13.2, 14.1, 18.4, and 16.0 mg g
−1
,

Molecules 2025,30, 572 11 of 25
as illustrated in Figure 10. These results demonstrate that increasing the proportion of
metal modifiers initially enhances the adsorption efficiency of biochar for both As(III) and
OTC. However, when the metal content exceeds an optimal level, the adsorption efficiency
declines. This reduction is primarily attributed to excessive metal content, which can block
pores on the biochar surface, thereby reducing the available adsorption sites. Comparable
findings were reported by Li et al. [
53
], who investigated the effect of ZnCl
2
content on
the adsorption efficiency of chromium using domestic sewage sludge carbon (DSSC). The
adsorption capacities for DSSC mixed with 33%, 50%, 60%, and 67% ZnCl
2
were 66.1,
68.8, 66.3, and 55.9 mg g
−1
, respectively. The study revealed that while an optimal ZnCl
2
ratio enhances porosity and adsorption capacity, excessive ZnCl
2
leads to pore blockage,
reducing adsorption efficiency. Similarly, the optimal adsorption efficiency for As(III)
and OTC in this study was achieved with an appropriate Zn-to-Fe ratio in the biochar.
Both insufficient and excessive metal content adversely impacted adsorption performance.
This underscores the importance of balancing the metal modifier ratios to maximize the
adsorption capabilities of the biochar.
Molecules 2025, 30, x FOR PEER REVIEW 11 of 25
Figure 9. Comparison of biochar absorption performance: (a) As(III) and (b) OTC. (C0 of As(III) = 1
ppm, C0 of OTC = 4 ppm, dose = 0.2 g/L, T = 25 °C, pH = 5).
2.2.2. Effect of Fe and Zn Content
The adsorption efficiencies of biochars modified with varying ratios of ZnCl2 and
FeCl3—4Zn-1Fe-1SBC, 2Zn-1Fe-1SBC, 1Zn-1Fe-1SBC, and 1Zn-2Fe-1SBC—for As(III)
were 61%, 72%, 91%, and 80%, respectively, with corresponding adsorption capacities of
3.05, 3.60, 4.55, and 4.00 mg g−1. Similarly, the adsorption efficiencies for OTC were 66%,
70%, 92%, and 80%, respectively, with adsorption capacities of 13.2, 14.1, 18.4, and 16.0
mg g−1, as illustrated in Figure 10. These results demonstrate that increasing the propor-
tion of metal modifiers initially enhances the adsorption efficiency of biochar for both
As(III) and OTC. However, when the metal content exceeds an optimal level, the adsorp-
tion efficiency declines. This reduction is primarily aributed to excessive metal content,
which can block pores on the biochar surface, thereby reducing the available adsorption
sites. Comparable findings were reported by Li et al. [53], who investigated the effect of
ZnCl2 content on the adsorption efficiency of chromium using domestic sewage sludge
carbon (DSSC). The adsorption capacities for DSSC mixed with 33%, 50%, 60%, and 67%
ZnCl2 were 66.1, 68.8, 66.3, and 55.9 mg g−1, respectively. The study revealed that while an
optimal ZnCl2 ratio enhances porosity and adsorption capacity, excessive ZnCl2 leads to
pore blockage, reducing adsorption efficiency. Similarly, the optimal adsorption efficiency
for As(III) and OTC in this study was achieved with an appropriate Zn-to-Fe ratio in the
biochar. Both insufficient and excessive metal content adversely impacted adsorption per-
formance. This underscores the importance of balancing the metal modifier ratios to max-
imize the adsorption capabilities of the biochar.
Figure 10. Comparison of biochar absorption performance with non-identical Zn and Fe content: (a)
As(III) and (b) OTC. (C0 of As(III) = 1 ppm, C0 of OTC = 4 ppm, dose = 0.2 g L−1, T = 25 °C, pH = 5).
Figure 10. Comparison of biochar absorption performance with non-identical Zn and Fe content:
(a) As(III) and (b) OTC. (C
0
of As(III) = 1 ppm, C
0
of OTC = 4 ppm, dose = 0.2 g L
−1
, T = 25
◦
C,
pH = 5).
2.2.3. Effect of pH
This study investigates the impact of SBC, 1Zn-1SBC, 1Fe-1SBC, and 1Zn-1Fe-1SBC
materials on the adsorption capacity for As(III) and OTC at different pH values (3, 5, 7, 9,
and 11). The initial concentrations of As(III) and OTC were 1 ppm and 4 ppm, respectively.
The material dosage was 0.2 g L
−1
, the temperature was maintained at 25
◦
C, and the
contact time was set to 120 min to evaluate the effect of pH on the adsorption of As(III)
and OTC.
As shown in Figure S5, 1Zn-1Fe-1SBC exhibited the highest adsorption efficiency
for As(III), with removal rates of 89%, 91%, 86%, 82%, and 76% at pH values of 3, 5, 7,
9, and 11, respectively. The corresponding adsorption capacities were 4.43, 4.56, 4.32,
4.08, and 3.77 mg g
−1
. The isoelectric points of SBC, 1Zn-1SBC, 1Fe-1SBC, and 1Zn-1Fe-
1SBC were found to be at pH 3.9, 5.2, 6.5, and 5.9, respectively (Figure S6). When the
pH is lower than the isoelectric point of the biochar, the materials carry a positive charge,
while at pH values above the isoelectric point, the materials are negatively charged. The
experimental results indicate that the adsorption of As(III) is most effective when the pH
is between 3 and 5. As the pH increases from 5 to 11, the adsorption capacity for As(III)
decreases. This decrease is primarily due to the pH’s influence on the biochar, which
not only affects the adsorption reaction but also alters the chemical form of As(III) in the
solution. As(III) has three dissociation constants: pK
a1
, pK
a2
, and pK
a3
, with values of

Molecules 2025,30, 572 12 of 25
9.2, 12.1, and 13.4, respectively. Between pH 3.22 and 7.46, As(III) predominantly exists as
neutral ions (H
2
AsO
3
), and between pH 7.46 and 14.0, it exists mainly in anionic forms
(H2AsO3−, HAsO32−, and AsO33−).
Similarly, Figure S7 shows that 1Zn-1Fe-1SBC exhibits the best adsorption efficiency
for OTC, with removal rates of 84%, 92%, 84%, 79%, and 72% at pH values of 3, 5, 7, 9, and
11, respectively. The corresponding adsorption capacities were 16.9, 18.4, 16.8, 15.8, and
14.4 mg g
−1
. The experimental results demonstrate that the adsorption capacity for OTC
increases as the pH value rises from 3 to 5, but decreases as the pH increases further from
5 to 11. OTC has three dissociation constants: pK
a1
, pK
a2
, and pK
a3
, with values of 3.22,
7.46, and 8.94, respectively. Between pH 3.22 and 7.46, OTC primarily exists in its cationic
(OTC
+
) and neutral (OTC
±
) forms. Between pH 7.46 and 8.94, OTC is mainly in its anionic
form (OTC
−
), and between pH 8.94 and 14.0, it exists predominantly in the forms OTC
−
and OTC2−.
2.3. Isothermal Adsorption Model Correlation Results
The adsorption behavior of 1Zn-1Fe-1SBC for As(III) and OTC was evaluated using
the Langmuir, Freundlich, and Temkin isothermal adsorption models, which are among the
most widely applied models for adsorption studies. The fitting results are summarized in
Table 3and Figure S8. The correlation coefficients (R
2
) for the Langmuir model fitting of 1Zn-
1Fe-1SBC adsorption of As(III) and OTC were both 0.999, indicating excellent agreement.
For the Freundlich model, the R
2
values were 0.996 and 0.997, respectively, while the
Temkin model yielded R
2
values of 0.929 and 0.926 for As(III) and OTC, respectively.
These results suggest that the Langmuir model provides the best fit for describing the
adsorption behavior of 1Zn-1Fe-1SBC, as its correlation coefficients are superior to those of
the Freundlich and Temkin models. Based on the Langmuir model analysis, the maximum
adsorption capacities for 1Zn-1Fe-1SBC were determined to be 34.72 mg g
−1
for As(III)
and 172.41 mg g
−1
for OTC. According to the Freundlich model analysis, the n-values for
As(III) and OTC adsorption were both greater than one, indicating that the adsorption
process is favorable. The Temkin model analysis showed that higher K
T
values correspond
to lower adsorption capacities, with the K
T
values indicating a stronger adsorption effect
of 1Zn-1Fe-1SBC for OTC compared to As(III).
Table 3. Parameters for the analysis of isothermal adsorption model of biochar.
Model
Langmuir Freundlich Temkin
KL
(L mg−1)
Qm
(mg g−1)R2KF
(L g−1)nR2KT
(kJ mol−1)
AT
(m
3
mole
−1
)
R2
1Zn-1Fe-
1SBC-As 0.15 34.72 0.999 4.22 1.20 0.996 610.06 4.08 0.929
1Zn-1Fe-
1SBC-OTC 0.03 172.41 0.999 5.26 1.16 0.997 141.92 0.99 0.926
For comparison, Xia et al. [
54
] reported a maximum adsorption capacity of
27.67 mg g−1
for ZnCl
2
-activated biochar, as determined by the Langmuir model. Furthermore,
Zhao et al. [55]
emphasized that the n-value from the Freundlich model is a critical in-
dicator of adsorption efficiency, with n > 1 signifying effective adsorption of pollutants,
such as antibiotics, by biochar. In conclusion, the 1Zn-1Fe-1SBC synthesized in this study
exhibits superior adsorption capacities for As(III) and OTC, outperforming previously
reported materials and demonstrating its potential as an effective adsorbent.

Molecules 2025,30, 572 13 of 25
2.4. Adsorption Kinetic Model Correlation Results
The adsorption kinetics of 1Zn-1Fe-1SBC for As(III) and OTC were analyzed using
the pseudo-first-order and pseudo-second-order kinetic models, which are widely applied
to study adsorption processes. The fitting results are presented in Table 4. The squared
correlation coefficients (R
2
) for the pseudo-first-order model for the adsorption of As(III)
and OTC ranged from 0.845 to 0.994 and 0.894 to 0.992, respectively. For the pseudo-second-
order model, the R
2
values ranged from 0.968 to 0.989 for As(III) and from 0.970 to 0.994
for OTC. These results indicate that both kinetic models are suitable for describing the
adsorption behavior of 1Zn-1Fe-1SBC, as the R
2
values for both models are generally greater
than 0.9. The findings are consistent with prior research. Norberto et al. [
56
] analyzed the
adsorption kinetics of As(III) and As(V) using Canola Straw Biochar (CSB). Their results
showed that the R
2
value for the pseudo-first-order model was 0.962 for As(III) and 0.983
for As(V), while the R
2
value for the pseudo-second-order model was 0.997 for As(III)
and 0.958 for As(V). These findings suggest that both physical and chemical adsorption
processes are involved in the adsorption of As(III) and As(V) by CSB, as both models
provided good fits. Similarly, the results of this study demonstrate that the adsorption
kinetics of As(III) and OTC onto 1Zn-1Fe-1SBC can be effectively described by both the
pseudo-first-order and pseudo-second-order models, indicating the occurrence of both
physical and chemical adsorption mechanisms.
Table 4. Parameters for analysis of biochar kinetic model.
Model Pseudo-First-Order Pseudo-Second-Order
k1qeR2k2qeR2
1Zn-1Fe-
1SBC-AS
1 ppm 0.125 1.151 0.994 0.317 1.276 0.980
2 ppm 0.083 2.154 0.986 0.135 2.423 0.968
4 ppm 0.061 3.626 0.948 0.090 4.114 0.976
8 ppm 0.051 5.577 0.886 0.078 6.325 0.986
16 ppm 0.043 9.442 0.845 0.054 10.132 0.989
1Zn-1Fe-
1SBC-OTC
1 ppm 0.210 4.65 0.992 0.154 5.13 0.994
2 ppm 0.142 8.75 0.973 0.059 10.1 0.990
4 ppm 0.078 13.1 0.936 0.033 17.5 0.989
8 ppm 0.054 23.3 0.894 0.019 27.5 0.987
16 ppm 0.067 45.2 0.973 0.007 50.5 0.970
The adsorption process of As(III) and OTC onto 1Zn-1Fe-1SBC was also analyzed us-
ing the intra-particle diffusion model to investigate the internal diffusion mechanisms. The
initial concentrations of As(III) and OTC were set at 1.0 ppm and 4.0 ppm, respectively, with
a material dosage of 0.2 g L
−1
. The experiments were conducted at 25
◦
C, with the pH ad-
justed to 5, and the contact time ranging from 0 to 120 min. The adsorption reaction curves
for both As(III) and OTC were divided into three distinct stages, representing different diffu-
sion mechanisms. The first stage corresponds to volumetric diffusion (K
1
), the second stage
to thin-film diffusion (K
2
), and the third stage to intra-particle diffusion (K
3
). The regression
results, as summarized in Table 5, indicate that the K
1
values for As(III) and OTC adsorp-
tion were 0.922 mg/g
·
min
0.5
and 3.012 mg/g
·
min
0.5
, respectively. The K
2
values were
0.411 mg/g
·
min
0.5
and 1.390 mg/g
·
min
0.5
, while the K
3
values were
0.116 mg/g·min0.5
and 0.246 mg/g
·
min
0.5
, respectively. The analysis shows that the
K1values
in the first stage
are significantly higher than the K
2
and K
3
values, indicating that volumetric diffusion is
the dominant adsorption mechanism. Thin-film diffusion and intra-particle diffusion are
secondary processes that contribute to the overall adsorption mechanism. Similar findings
were reported by Feng et al. [
57
], who studied the adsorption of As(III) using iron-modified

Molecules 2025,30, 572 14 of 25
biochar (Fe-BC300 and Fe-BC500). Their adsorption reaction curves were also divided into
three stages. In their study, the reaction rate constants for the first stage (K
1
, 0–60 min)
were 4.79 mg/g
·
min
0.5
and 3.12 mg/g
·
min
0.5
, respectively, which were notably higher
than those of the subsequent stages. This indicates that the primary adsorption process
was the diffusion of As(III) from the liquid phase to the material’s surface. The second
stage (K
2
, 120–360 min) demonstrated reaction rate constants of
0.83 mg/g·min0.5
and
0.93 mg/g·min0.5
, signifying the diffusion of As(III) from the surface to the inner layers
of the material. In the third stage (K
3
, 960–1440 min), the reaction rate constants were
0.32 mg/g·min0.5
and 0.68 mg/g
·
min
0.5
, respectively, indicating diffusion into the pores of
the material. As the adsorption capacity increased, diffusion resistance also rose, leading to
a decrease in the adsorption rate until equilibrium was achieved. The results of this study
suggest that the adsorption of As(III) and OTC onto 1Zn-1Fe-1SBC is primarily governed
by volumetric diffusion, with thin-film diffusion playing a secondary role.
Table 5. Parameters of biochar particle diffusion analysis.
Model First Stage Second Stage Third Stage
K1
(mg/g min0.5)R2K2
(mg/g min0.5)R2K3
(mg/g min0.5)R2
1Zn-1Fe-
1SBC-AS 0.922 0.997 0.411 0.993 0.116 0.997
1Zn-1Fe-
1SBC-OTC 3.012 0.998 1.390 0.999 0.246 0.996
2.5. Thermodynamic Analysis Results
The thermodynamic parameters, including Gibbs free energy (
∆
G), enthalpy (
∆
H),
and entropy (
∆
S), for the adsorption of As(III) and OTC onto 1Zn-1Fe-1SBC were evaluated
at varying temperatures. Experiments were conducted at 15, 25, 35, and 45
◦
C (288, 298, 308,
and 318 K), respectively. The initial concentrations of As(III) and OTC were fixed at
1 ppm
and 4 ppm, respectively, with a material dosage of 0.2 g L
−1
. The pH was maintained at
5, and the contact time was set to 120 min to facilitate the determination of
∆
G,
∆
H, and
∆
S. The thermodynamic analysis results are summarized in Table 6. The negative values of
∆
G for both As(III) and OTC adsorption (as shown in Table 5) indicate that the process is
spontaneous. Furthermore, the magnitude of
∆
G decreases significantly with increasing
temperature, suggesting that higher temperatures enhance the adsorption reaction, thereby
facilitating the diffusion of As(III) and OTC to the material’s surface. Notably, the
∆
G values
for OTC adsorption are smaller than those for As(III) adsorption, demonstrating that 1Zn-
1Fe-1SBC exhibits a stronger adsorption affinity for OTC. The positive
∆
H values indicate
that the adsorption of As(III) and OTC is endothermic in nature. For adsorption processes
where
∆
H is less than 80 kJ mol
−1
, the mechanism is considered physical adsorption,
whereas
∆
H values exceeding 80 kJ mol
−1
suggest chemical adsorption. In this study,
the
∆
H values confirm that the adsorption of As(III) and OTC by 1Zn-1Fe-1SBC involves
chemical adsorption [
58
]. Additionally, the positive
∆
S values imply an increase in system
disorder during adsorption. This indicates an increased probability of collision between the
adsorbent and As(III) or OTC molecules. Collectively, the
∆
G,
∆
H, and
∆
S values reveal
that the adsorption reaction is thermodynamically favorable and becomes more efficient
with rising temperature.

Molecules 2025,30, 572 15 of 25
Table 6. Parameters of thermodynamic models for the adsorption of As(III) and OTC by
various biochars.
Material Temperature
(K)
∆G
(kJ mol−1)
∆H
(kJ mol−1)
∆S
(J mol−1K−1)
1Zn-1Fe-
1SBC-As
288 −7.026
82.60 309.61
298 −9.167
308 −12.26
318 −16.40
1Zn-1Fe-
1SBC-OTC
288 −10.49
84.45 329.60
298 −13.47
308 −17.64
318 −20.07
2.6. Arrhenius Model Analysis Results
The reaction rate constants (k
1
) for the adsorption of As(III) and OTC by 1Zn-1Fe-1SBC
at 15, 25, 35, and 45
◦
C (288, 298, 308, and 318 K) were analyzed using the Arrhenius model
to evaluate the activation energy (E
a
) of the adsorption process at different temperatures.
The results of the analysis are presented in Table 7. The calculated E
a
values for the ad-
sorption of As(III) and OTC by 1Zn-1Fe-1SBC were 21.82 and 26.07 kJ mol
−1
, respectively.
The pre-exponential factors (A) were determined to be 32.9 and 754.3 s
−1
, respectively. Ac-
cording to the Arrhenius model simulation, the E
a
value greater than 21 kJ mol
−1
confirms
that the adsorption process is governed by a chemical reaction mechanism. Furthermore,
the higher E
a
value for OTC adsorption compared to As(III) adsorption indicates that the
adsorption of OTC involves a higher activation energy, suggesting a more energy-intensive
reaction for OTC adsorption on 1Zn-1Fe-1SBC [
59
]. Jin et al. [
60
] reported similar findings
using nanosized Fe/Ni-functionalized ZIF-8 (ZIF-8-Fe/Ni) for the adsorption of OTC. Their
study, employing the Arrhenius model, found an E
a
value of 22.9 kJ mol
−1
with an R
2
of
0.993. This aligns with the criterion that an E
a
value exceeding 21 kJ mol
−1
characterizes the
adsorption process as a chemical reaction. These results further corroborate the chemical
nature of the adsorption mechanism for 1Zn-1Fe-1SBC.
Table 7. Arrhenius model analysis of biochar.
Material
Temperature
(K) k1Ea
(kJ mol−1)
A
(s−1)R2
1Zn-1Fe-
1SBC-As
288 0.020
21.82 32.92 0.954
298 0.024
308 0.029
318 0.041
1Zn-1Fe-
1SBC-OTC
288 0.021
26.07 754.3 0.985
298 0.027
308 0.042
318 0.052
2.7. Adsorption Mechanism of 1Zn-1Fe-1SBC for As(III) and OTC
The adsorption mechanisms of 1Zn-1Fe-1SBC for As(III) and OTC were investigated
in this study. A comprehensive understanding of the adsorption pathways was achieved
through a combination of literature references and experimental data. Figure 11 illustrates
the five primary reaction pathways involved in the adsorption process.

Molecules 2025,30, 572 16 of 25
Molecules 2025, 30, x FOR PEER REVIEW 16 of 25
1Zn-1Fe-
1SBC-OTC
288 0.021
26.07 754.3 0.985
298 0.027
308 0.042
318 0.052
2.7. Adsorption Mechanism of 1Zn-1Fe-1SBC for As(III) and OTC
The adsorption mechanisms of 1Zn-1Fe-1SBC for As(III) and OTC were investigated
in this study. A comprehensive understanding of the adsorption pathways was achieved
through a combination of literature references and experimental data. Figure 11 illustrates
the five primary reaction pathways involved in the adsorption process.
Figure 11. Adsorption mechanism of As(III) and OTC on 1Zn-1Fe-1SBC.
2.7.1. Pore Blocking and Surface Effects
Energy-dispersive X-ray spectroscopy (EDS) revealed the presence of As on the sur-
face of 1Zn-1Fe-1SBC following adsorption, while X-ray photoelectron spectroscopy (XPS)
confirmed the appearance of As(III) binding energy peaks. Brunauer–Emme–Teller
(BET) analysis indicated that the specific surface area and pore volume of 1Zn-1Fe-1SBC
decreased significantly after adsorption, from 1136.8 m
2
g
−1
and 0.55 cm
3
g
−1
to 945.7 m
2
g
−1
and 0.43 cm
3
g
−1
, respectively. This reduction demonstrates that the adsorption of As(III)
and OTC caused pore blockage. Intra-particle diffusion model analysis further confirmed
that As(III) and OTC ions were successfully adsorbed into the material’s pores during the
adsorption process.
2.7.2. Formation of Hydrogen Bonds
The functional groups present in biochar, particularly those containing hydrogen at-
oms, facilitated the formation of hydrogen bonds with As(III) and OTC ions in aqueous
solutions, enhancing the stability of the adsorption process [28]. Fourier-transform infra-
red (FTIR) spectroscopy showed an absorption band at 852 cm
−1
, corresponding to the
bending vibration of As-OH after adsorption [39]. Furthermore, the high adsorption effi-
ciency observed at pH 5 can be aributed to surface protonation, which promotes hydro-
gen bond formation between functional groups on 1Zn-1Fe-1SBC and the adsorbed ions
[47]. These hydrogen bonds provide additional aractive forces, stabilizing the adsorption
of arsenic ions.
Figure 11. Adsorption mechanism of As(III) and OTC on 1Zn-1Fe-1SBC.
2.7.1. Pore Blocking and Surface Effects
Energy-dispersive X-ray spectroscopy (EDS) revealed the presence of As on the surface
of 1Zn-1Fe-1SBC following adsorption, while X-ray photoelectron spectroscopy (XPS)
confirmed the appearance of As(III) binding energy peaks. Brunauer–Emmett–Teller (BET)
analysis indicated that the specific surface area and pore volume of 1Zn-1Fe-1SBC decreased
significantly after adsorption, from 1136.8 m
2
g
−1
and 0.55 cm
3
g
−1
to 945.7 m
2
g
−1
and
0.43 cm3g−1, respectively. This reduction demonstrates that the adsorption of As(III) and
OTC caused pore blockage. Intra-particle diffusion model analysis further confirmed that
As(III) and OTC ions were successfully adsorbed into the material’s pores during the
adsorption process.
2.7.2. Formation of Hydrogen Bonds
The functional groups present in biochar, particularly those containing hydrogen
atoms, facilitated the formation of hydrogen bonds with As(III) and OTC ions in aqueous
solutions, enhancing the stability of the adsorption process [
28
]. Fourier-transform infrared
(FTIR) spectroscopy showed an absorption band at 852 cm
−1
, corresponding to the bending
vibration of As-OH after adsorption [
39
]. Furthermore, the high adsorption efficiency
observed at pH 5 can be attributed to surface protonation, which promotes hydrogen
bond formation between functional groups on 1Zn-1Fe-1SBC and the adsorbed ions [
47
].
These hydrogen bonds provide additional attractive forces, stabilizing the adsorption of
arsenic ions.
2.7.3. Electrostatic Interactions
FTIR analysis revealed that 1Zn-1Fe-1SBC possessed functional groups with varying
charge states, enabling electrostatic interactions with As(III) and OTC ions in solution [
28
].
The adsorption behavior was strongly influenced by the pH of the solution, which de-
termined the charge states of both the adsorbate and adsorbent. The isoelectric point of
1Zn-1Fe-1SBC was found to be pH 5.9. Under acidic conditions, As(III) primarily exists as
H
3
AsO
3
and H
2
AsO
3−
, reducing electrostatic repulsion. OTC exists as OTC
+
and OTC
±
,
where the latter is less repelled by the material. These electrostatic interactions played a
critical role in the effective adsorption of As(III) and OTC ions at varying pH levels [61].
2.7.4. Surface Complexation
FTIR spectra indicated the presence of As-OH bending vibrations at 852 cm
−1
after
adsorption, while XPS analysis demonstrated the binding of As to the surface of 1Zn-1Fe-

Molecules 2025,30, 572 17 of 25
1SBC. The functional groups and metal elements (C-O, C=O, Fe, and Zn) on the material
contributed to the formation of surface complexes with As(III) and OTC ions, further
enhancing the adsorption process [62].
2.7.5. π-πInteractions
FTIR and XPS analyses identified functional groups such as -OH, C=C, and C=O
on the surface of 1Zn-1Fe-1SBC. These groups acted as electron donors, facilitating
π
-
π
interactions that enhanced the adsorption of As(III) and OTC. XPS data for C1s electron
spectroscopy showed a decrease in functional groups (C-C and C=C) and an increase in
π
-
π
interactions from 23.1% to 23.9% after adsorption, providing further evidence of the role of
π
-
π
interactions in the adsorption process [
63
]. In conclusion, the adsorption of As(III) and
OTC by 1Zn-1Fe-1SBC involves multiple mechanisms, including pore blocking, hydrogen
bond formation, electrostatic interactions, surface complexation, and
π
-
π
interactions.
These synergistic effects contribute to the material’s high adsorption efficiency.
3. Materials and Methods
3.1. Preparation of Biochar
The sugarcane bagasse (SCB) was initially washed three times with ultrapure water to
remove impurities and then dried at 105
◦
C for 24 h under ambient conditions. The dried
material was pulverized using a crusher to obtain powdered pretreated SCB. The SCB was
then placed in a quartz tube, and pyrolysis was performed in a tube furnace. The heating
rate was set at 5
◦
C min
−1
, and the pyrolysis temperature was maintained at 550
◦
C for 2 h
under an oxygen-free environment using nitrogen gas (N
2
). After the furnace cooled, the
pyrolyzed product was removed and treated with 3 M HCl for 30 min at approximately
90
◦
C to eliminate residual organic matter. The biochar was then thoroughly washed with
ultrapure water until a neutral pH (7) was achieved. The product was dried in an oven at
105
◦
C for 12 h and subsequently sieved through a 100-mesh sieve to obtain carbonized
biochar (SBC).
3.2. Preparation of Modified Biochar
3.2.1. Monometallic Modified Biochar
Monometallic modified biochar was synthesized by incorporating ZnCl
2
(Shimakyu
Co., Ltd., Osaka, Japan, 98%) or FeCl
3
(Shimakyu Co., Ltd., Japan, 45%) into SBC. Specifi-
cally, 10 g of SBC and varying amounts of ZnCl
2
or FeCl
3
(2.5, 5.0, 10.0, and 20.0 g) were
mixed in a beaker containing 150 mL of ultrapure water. The mixture was stirred at 85 ◦C
for 2 h using a magnetic stirrer and then dried in an oven at 105
◦
C for 24 h under ambient
atmosphere to remove excess water. The dried mixture was placed in a quartz tube and
subjected to pyrolysis under nitrogen flow at a heating rate of 5
◦
C min
−1
, maintaining a
temperature of 550
◦
C for 2 h. The resulting product was washed with 3 M HCl for 30 min
at approximately 90
◦
C to remove residual impurities, followed by thorough washing with
ultrapure water until a neutral pH was achieved. The final product was dried in an oven
at 105 ◦C for 12 h and sieved through a 100-mesh sieve to obtain ZnCl2or FeCl3modified
biochar (designated as XZn-YSBC or XFe-YSBC, where X/Y represents the ratio of Zn or Fe
to SBC content: 1/4, 1/2, 1/1, or 2/1).
3.2.2. Bimetallic Modified Biochar
Bimetallic modified biochar was prepared by mixing SBC with both ZnCl
2
and FeCl
3
.
A mixture containing 10 g of SBC, 10 g of ZnCl
2
, and varying amounts of FeCl
3
(2.5, 5.0,
10.0, and 20.0 g) was stirred in a beaker with 150 mL of ultrapure water at 85
◦
C for 2 h.
The mixture was then dried in an oven at 105
◦
C for 24 h under an ambient atmosphere

Molecules 2025,30, 572 18 of 25
to remove water and activate the SBC. The activated mixture was subjected to pyrolysis
in a quartz tube under nitrogen flow, with the heating rate and temperature parameters
identical to those for monometallic biochar. The pyrolyzed product was washed with
3 M HCl
, heated to approximately 90
◦
C, and rinsed with ultrapure water until a neutral
pH was achieved. The final product was dried at 105
◦
C for 12 h and sieved through a
100-mesh sieve to obtain bimetallic modified biochar (designated as XZn-YFe-1SBC, where
X/Y represents the Zn to Fe ratio: 4/1, 2/1, 1/1, or 1/2).
3.3. Material Characterization
The synthesized biochar materials were characterized using a range of analytical
techniques to assess their physical and chemical properties. Fourier transform infrared
spectroscopy (FTIR, NiCoLET- iS10 model, Perkin Elmer Inc., Waltham, MA, USA) was
employed to identify functional groups, with the following operating conditions: number
of scans (50), resolution (4 cm
−1
), spectral range (4000 to 500 cm
−1
), and acquisition time
(2.47 s). The samples were prepared by the standard KBr pellets method. Scanning electron
microscopy (SEM, Hitachi High-Tech Corporation, Tokyo, Japan) coupled with energy-
dispersive X-ray spectroscopy (EDS, S-4700 model, Hitachi High-Tech Science Corporation,
Tokyo, Japan) provided insights into surface morphology and elemental composition.
X-ray photoelectron spectroscopy (XPS, Nexsa G2 model, Thermo Fisher Scientific Inc.,
Waltham, MA, USA) was utilized to examine the chemical states and crystal structures, with
calibration performed by the ISO 15472 method under an excitation wavelength of
290 nm
and a scanned area of 5 mm. Nitrogen adsorption/desorption isotherm analysis (BET,
ASAP 2020 model, Micromeritics Inc., Norcross, GA, USA) was conducted to determine
specific surface area and pore structure. Thermogravimetric analysis (TGA, ZCT-1 model,
TA Instruments Inc., New Castle, DE, USA) and elemental analysis (EA, MICRO cube
model, Elementar Analysensysteme GmbH Inc., Langenselbold, Germany) were performed
to evaluate the thermal stability and elemental composition of the biochar materials.
3.4. Material Performance Valuation
The adsorption performance of the biochar materials was assessed through batch
experiments targeting As(III) and oxytetracycline (OTC) in aqueous solutions. The con-
centration of As(III) was measured using inductively coupled plasma mass spectrometry
(ICP-MS, 4100 MP-AES model, Thermo Fisher Scientific Inc., SA, USA). The instrument has
a detection limit of 10 ppb. To ensure data reliability, all experiments were conducted in
triplicate, yielding a standard deviation of 20 ppb. Calibration with standard solutions was
performed regularly to maintain accuracy.
For OTC, the adsorption capacity was evaluated using a UV-Vis spectrophotometer (U-
3900 model, Hitachi High-Tech Science Corporation, Tokyo, Japan). A standard
1 mg L−1
OTC solution was prepared, and the maximum absorption wavelength (272 nm) was
determined through multiple wavelength scans. The concentration changes in OTC after
adsorption by modified biochar were calculated based on the absorbance values, providing
insights into the material’s performance in removing OTC from aqueous solutions.
3.5. Isothermal Adsorption Model
3.5.1. Langmuir Isotherm Model
The Langmuir isotherm model is widely used to describe the adsorption of solute
species onto solid adsorbents. It assumes monolayer adsorption on a surface with a finite
number of identical sites. The theoretical derivation leads to an equation that establishes a

Molecules 2025,30, 572 19 of 25
relationship between the surface’s active sites undergoing adsorption and the equilibrium
concentration. The Langmuir model is expressed as [64]:
Q=ab ×Ce
1+aCe(1)
Taking the reciprocal of both sides results in:
1
Q=1
b+1
ab ×Ce(2)
Here, C
e
is the equilibrium concentration of the adsorbate in solution, and Qis the
amount of adsorbate adsorbed per unit mass of adsorbent. The constants aand brepresent
the adsorption energy and capacity, respectively. By plotting 1/Qagainst 1/C
e
, the values
of aand bcan be determined from the slope and intercept.
3.5.2. Freundlich Isotherm Model
The Freundlich isotherm is an empirical model describing the relationship between the
concentration of the adsorbate on the adsorbent surface and its concentration in solution. It
is expressed as [65]:
Q=KCe
1
n(3)
Taking the natural logarithm of both sides yields:
ln Q=ln K+1
n(ln Ce)(4)
Here, Kand nare the Freundlich constants related to adsorption capacity and intensity,
respectively. By plotting lnQagainst C
e
,K, and ncan be obtained from the intercept and
slope, respectively.
3.5.3. Temkin Isotherm Model
The Temkin isotherm model accounts for adsorbent-adsorbate interactions by assum-
ing that the heat of adsorption decreases linearly with coverage rather than logarithmically.
It is derived based on a uniform distribution of binding energies. The model is represented
by the equations [66]:
qT=qe=RT
bln(ATCe)(5)
qe=RT
bln AT+RT
bln Ce(6)
In these equations, q
T
represents the adsorption capacity predicted by the Temkin
model (mg g
−1
), q
e
is the equilibrium adsorption capacity (mg g
−1
), R is the universal
gas constant (8.314 J mol
−1
K
−1
), T is the absolute temperature (K), b is the Temkin
constant, A
T
is the Temkin equilibrium binding constant (L g
−1
), and C
e
is the equilibrium
concentration (ppm).
3.6. Adsorption Kinetics Models
3.6.1. Pseudo-First-Order Kinetics
The pseudo-first-order kinetic model assumes that the adsorption rate is proportional
to the concentration of unadsorbed species. The equation is given as:
dqt
dt =k1(qe−qt)(7)

Molecules 2025,30, 572 20 of 25
Here, q
t
is the amount adsorbed at time t(mg g
−1
), k
1
is the pseudo-first-order rate
constant (min
−1
), and q
e
is the adsorption capacity at equilibrium. Integrating this equation
with the initial condition qt= 0 at t= 0 yields [67]:
log(qe−qt) = log qe−k1
2.303t(8)
3.6.2. Pseudo-Second-Order Kinetics
The pseudo-second-order kinetic model assumes that the adsorption rate depends on
the square of the concentration of unadsorbed species. It is described as:
dqt
dt =k2(qe−qt)2(9)
Integrating this equation with the condition qt= 0 at t= 0 gives [68]:
t
qt
=1
k2qe2+1
qe
t(10)
Here, k2is the pseudo-second-order rate constant (g mg−1min−1).
3.6.3. Intra-Particle Diffusion Model
The intra-particle diffusion model evaluates the rate of adsorption based on the square
root of time. It is expressed as:
qt=kit1/2 +C(11)
Here, k
i
(mg g
−1
min
−1/2
) is the rate constant for intra-particle diffusion, and Cis a
constant related to boundary layer thickness [69,70].
3.7. Thermodynamic Model
Thermodynamics primarily utilizes the relationships between Gibbs free energy (
∆
G),
entropy (
∆
S), and enthalpy (
∆
H) to analyze the reaction mechanisms of pollutants in
systems undergoing temperature changes [
71
]. The adsorption process is classified as
spontaneous when
∆
G is less than 0 kJ mol
−1
and as non-spontaneous when
∆
G equals
0 kJ mol−1
. Conversely, if
∆
G is greater than 0 kJ mol
−1
, the adsorption process is again con-
sidered spontaneous. The
∆
H value further elucidates the nature of the adsorption process.
When
∆
H is less than 0 kJ mol
−1
, the process is exothermic, indicating that adsorption is
more effective at lower temperatures. In contrast, a
∆
H greater than
0 kJ mol−1
signifies an
endothermic process, where the adsorption capacity decreases as the temperature increases.
Additionally, the magnitude of
∆
H provides insight into the type of adsorption: values
between 0 and 80 kJ mol
−1
indicate physical adsorption, while values between 80 and
400 kJ mol−1
suggest chemical adsorption [
72
,
73
]. The thermodynamic equations used are
as follows:
Kd=qe/Ce(12)
∆G=−RT ×ln(Kd)(13)
ln(Kd) = ∆S
R+−∆H
RT (14)
where, K
d
is the reaction equilibrium constant (L mg
−1
), C
e
is the concentration of
adsorption reaction equilibrium (mg L
−1
), q
e
is the adsorption capacity at adsorption
equilibrium (mg g−1).

Molecules 2025,30, 572 21 of 25
3.8. Arrhenius Model
The Arrhenius equation relates the reaction rate constant (k) to temperature, providing
insights into activation energy (Ea) as follows:
k=A×e−Ea
RT (15)
Taking the natural logarithm yields:
ln(k) = ln(A)−Ea
RT(16)
Here, A is the pre-exponential factor, and Eais the activation energy (kJ mol−1) [74].
4. Conclusions
The SEM-EDS analysis revealed that the modification of biochar with ZnCl
2
and
FeCl
3
significantly enhanced pore formation and facilitated the loading of Zn and Fe
particles onto the surface of 1Zn-1Fe-1SBC. Following the adsorption of As(III) by 1Zn-
1Fe-1SBC, the presence of As elements was confirmed on the material’s surface. TGA
analysis demonstrated that 1Zn-1Fe-1SBC exhibited high thermal stability at elevated
temperatures. BET analysis indicated that the specific surface area and pore volume of
the biochar increased markedly after ZnCl
2
and FeCl
3
modification, from 16.8 m
2
g
−1
to 1136.8 m
2
g
−1
and from 0.02 cm
3
g
−1
to 0.55 cm
3
g
−1
, respectively. However, both
the specific surface area and pore volume decreased significantly to 945.7 m
2
g
−1
and
0.43 cm3g−1
, respectively, after the adsorption of As(III) and OTC by 1Zn-1Fe-1SBC. FTIR
analysis identified an As-OH bending vibration at 852 cm
−1
following the adsorption of
As(III), confirming the interaction between As(III) and the material. XPS analysis revealed
that the ratios of oxygen-containing functional groups, such as C-O and C=O, as well
as
π
-
π
interactions, were significantly higher in the modified biochar compared to the
unmodified biochar. This enhancement in functional groups and
π
-
π
electron interactions
facilitated the effective adsorption of As(III) and OTC by 1Zn-1Fe-1SBC. These findings
underscore the effectiveness of ZnCl
2
and FeCl
3
modifications in improving the adsorption
performance and thermal stability of biochar, making 1Zn-1Fe-1SBC a promising material
for environmental remediation applications.
Supplementary Materials: The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/molecules30030572/s1, Figure S1: SEM and EDS mapping of
1Zn-1Fe-1SBC. (a) SEM image of 1Zn-1Fe-1SBC, (b) C, (c) O, (d) Si, (e) Fe, and (f) Zn.; Figure S2: SEM
and EDS mapping of 1Zn-1Fe-1SBC-As-OTC. (a) SEM image of 1Zn-1Fe-1SBC-As-OTC, (b) C, (c) O,
(d) Si, (e) Fe, (f) Zn, (g) As, and (h) N.; Figure S3. SEM and EDS mapping of SCB. (a) SEM image
of SCB, (b) C, (c) O, and (d) Si.; Figure S4. SEM and EDS mapping of SBC. (a) SEM image of SBC,
(b) C, (c) O, and (d) Si.; Figure S5. Effect of different pH values on As(III) adsorption capacity and
efficiency. (a) SBC, (b) 1Zn-1SBC, (c)1Fe-1SBC, and (d) 1Zn-1Fe-SBC.; Figure S6. Effect of pH on the
isoelectric potential of materials.; Figure S7. Effect of different pH values on OTC adsorption capacity
and efficiency. (a) SBC (b) 1Zn-1SBC (c)1Fe-1SBC (d) 1Zn-1Fe-SBC.; Figure S8. Isothermal adsorption
mode analysis of As(III) adsorption results. (a) As(III), and (b) OTC.
Author Contributions: Conceptualization and methodology, C.-T.C. and G.-B.H.; investigation and
formal analysis, N.-T.N. and A.-B.L.; resources, C.-T.C. and G.-B.H.; data curation, N.-T.N. and
A.-B.L.; writing—original draft preparation, C.-T.C., N.-T.N., A.-B.L. and G.-B.H.; writing—G.-B.H.
All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.

Molecules 2025,30, 572 22 of 25
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available in this article.
Acknowledgments: The authors thank the anonymous reviewers for their comments.
Conflicts of Interest: The authors declare no conflicts of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
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