Adsorptive Removal of Ciprofloxacin by Ferric Chloride Modified

Biochar

Huaxuan Zhao

1,2

, Zhengmei Cao

, Jin Li

1,2

, Yinhai Lang

1,2,*

College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China;

Key Laboratory of Marine Environmental Science and Ecology, Ministry of Education, Ocean University of China,

Qingdao 266100, China;

Qingdao Environmental Monitoring Central Station, Qingdao 266071, China.

Email: yhlang@ouc.edu.cn.

Key words: Ciprofloxacin, magnetic biochar, adsorption, mechanism

Abstract: A magnetic biochar (MBC) was fabricated through pyrolysis of FeCl

impregnated reed stalk powder, and

used for ciprofloxacin (CIP) adsorption from aqueous solution. The structure and properties of the MBC

were characterized by elemental analyzer, BET, SEM and FTIR. Results showed that the prepared MBC

had a large specific surface area and pore volume, which was conducive to the CIP adsorption. Adsorption

of CIP on MBC was of strong pH dependence and demonstrated a bimodal curve. Ionic strength increasing

was advantageous to the CIP adsorption. The Langmuir model and the pseudo second-order model

presented better fittings for the adsorption equilibrium and kinetic data. Adsorption thermodynamic

parameters manifested that the adsorption behavior was a spontaneous, endothermic and entropy-increasing

process. Pore-filling effect, π-π EDA interaction, electrostatic interaction, hydrogen bonding formation and

hydrophobic interaction were important mechanisms for CIP adsorption on the MBC.

1 INTRODUCTION

Ciprofloxacin (CIP) is one of the quinolone

antibiotics used to treat human and veterinary

bacterial infection because of its strong,

broad-spectrum antibacterial property and high

mobility (Wang et al., 2016). Due to its incompletely

metabolized by humans and domestic animals and

released in effluent from drug manufacturing, CIP

has become a widely monitored and frequently

detected antibiotic in surface water, wastewater of

the effluent treatment plants and hospitals (He et al.,

2015; Giger et al., 2003; Rodriguez et al., 2015).

Moreover, CIP in the environment could increase the

antibiotic resistance of the living organisms and

threaten human health (Berhane et al., 2016).

Therefore, it is imperative to eliminate CIP from the

water environments.

Adsorption method shows a promising prospect

for the elimination of quinolone antibiotics.

Magnetic biochar is an ideal adsorbent due to its

abundant raw materials, versatility and particularly

higher adsorption capacity for that the magnetization

of carbonaceous material can create additional

adsorption sites in comparison with the

non-magnetic adsorbent (Reguyal et al., 2017).

Moreover, magnetic biochar has been found to

provide simple separation after treatment using a

magnet (Thines et al., 2017). Compared with the

other common separation technologies such as

centrifugation and filtration, the magnetic separation

method is more efficient and cost-effective (Shang et

al., 2016).

These synthesized magnetic biochars have been

used for adsorption removal of multiple

contaminants, such as heavy metals, hydrophobic

organic pollutants, dyestuffs, pharmaceuticals,

entrophication anions such as nitrate and phosphate,

from aqueous solutions (Mohan et al., 2014; Bastami

and Entezari, 2012; Ma et al., 2015; Wang et al.,

2017; Usman et al., 2016; Li et al., 2016). However,

very few researches have been conducted with

respect to the synthesis of magnetic biochar using

reed biomass as raw material. Furthermore, the

studies on the CIP adsorption behaviors were not

enough.

In this task, a magnetic biochar (MBC) was

prepared by one-step synthesis through pyrolysis of

FeCl

impregnated reed stalk powder, and used for

the CIP adsorption from aqueous solution. Batch

Zhao, H., Cao, Z., Li, J. and Lang, Y.

Adsorptive Removal of Ciproﬂoxacin by Ferric Chloride Modiﬁed Biochar.

In Proceedings of the International Workshop on Environment and Geoscience (IWEG 2018), pages 56-61

ISBN: 978-989-758-342-1

adsorption experiments were employed to delve into

the adsorption behaviors and mechanisms of CIP on

MBC. The MBC served as a practical and effective

adsorbent presented great potential in wastewater

treatment.

2 MATERIALS AND METHODS

2.1 Preparation and Characteristics of

MBC

The reed stalk powder (40 g) was immersed into the

prepared FeCl

solution (40 g FeCl

ˑ6H

O in 60 mL

of ultrapure water) for 2 h and then dried under air.

The obtained biomass was pyrolyzed at a

temperature of 873.15 K for two hours in an

oxygen-limited environment. After washed and dried

overnight, the obtained magnetic biochar, denoted as

MBC, was milled and sieved to pass through a

100-mesh sieve (<0.15 mm).

The elemental compositions (C, H, S, O and N)

of the MBC were determined using an elemental

analyzer. The microscopic features were

characterized by SEM. The BET surface area and

micropore volume were measured using a

Physisorption and Chemisorption Analyzer. The

amounts of the acidic oxygen-containing groups on

the MBC surface were measured by the Boehm’s

titration method (Boehm, 1994). The surface

functional groups of the MBC were determined by

FTIR spectra in the 4000 to 400 cm

-1

spectral range.

The measurement of point of zero charge (pH

pzc

)

was on the basis of a reported paper (Bastami and

Entezari, 2012).

2.2 Adsorption Experiments

All the adsorption experiments were conducted

under the darkness condition in 150 mL Erlenmeyer

flasks with 50 mL of CIP solutions and 50 mg of

MBC samples in a reciprocating shaker at 190

r·min

-1

and 298.15 K unless stated otherwise.

Separate sets of experiments were carried out to

examine the influences of pH and ionic strength on

CIP adsorption. The pH values of CIP solutions (10

mg·L

-1

) were adjusted in the range of 4-12 with

NaOH or HCl solutions. 10 mg·L

-1

CIP solutions

containing 0 to 1.0 mol·L

-1

NaCl were prepared to

investigate the effect of ionic strength. In the

adsorption kinetic experiment, 10 mg·L

-1

CIP

solutions were used with adsorption time ranging

from 0.5 to 24 h. The initial CIP concentrations were

set from 2 to 20 mg·L

-1

at 298.15, 308.15, 318.15 K

for the adsorption isotherms study. All batch

adsorption experiments were performed with three

replicates. The CIP concentrations were analysed

with a UV-visible spectrophotometer at 272 nm.

The adsorption capacity of the MBC was

calculated by the following equation:





(







)



 (1)

Where q

(mg·g

-1

) is the amount of adsorbed CIP, C

and C

(mg·L

-1

) are the initial and equilibrium CIP

concentrations, respectively, V (L) is the solution

volume, and m (g) is the MBC weight.

3 RESULTS AND DISCUSSION

3.1 Characterizations of MBC

The calculated weight percentage of iron accounted

for 17.56% in MBC, indicating a lot of iron was

successfully loaded on the surface of MBC. MBC

had a relatively high porosity with BET surface area

of 238.3 m

ˑg

-1

and total pore volume of 0.257

ˑg

-1

. The pH

pzc

was 2.5 for MBC.

Figure 1: SEM image of the MBC.

Figure 2: FTIR analysis spectra of the MBC.

Adsorptive Removal of Ciproﬂoxacin by Ferric Chloride Modiﬁed Biochar

Details about the structures and morphologies of

the MBC were investigated using SEM analysis.

Figure 1 showed that the MBC possessed roughness

and pores within its structure. A good dispersion of

particles with different shapes has been observed on

the surface of the MBC, which clearly delineated the

presence of iron oxide particles attached and

dispersed onto the surface of MBC.

Figure 2 showed the FTIR spectra of MBC

before and after CIP adsorption. The FTIR spectra

indicated that the adsorption peak at 559 cm

-1

can be

corresponding to the Fe-O stretching vibration

(Bastami and Entezari, 2012). After adsorption, the

disappearance of this peak indicated that the Fe-O

bond can serve as adsorption sites for CIP. The peaks

of MBC at 3386, 1558, 1100 and 870 cm

-1

can be

attributed to the skeletal vibration of O-H, C=O or

C=C, and C-O bonds, respectively (Bastami and

Entezari, 2012 ; Zhou et al., 2017 ; Zhu et al., 2014 ).

The above results indicated that there were many

oxygen-contained functional groups on the MBC

surface. The Boehm’s test also indicated that many

acidic oxygen-containing groups (such as carboxyl,

lactone, phenolic hydroxyl and carbonyl) were on

the MBC surface and the total amount of these

groups was about 3.263 mmolˑg

-1

. The

oxygen-containing functional groups can provide

adsorption sites for CIP, so that CIP formed

hydrogen bonds with the groups on the MBC

surface.

3.2 Effect of pH

The influence of pH on CIP adsorption coefficient

, calculated by equation (2)] presented a bimodal

curve in Figure 3. The K

values for the adsorption

of CIP on MBC reached its first maximum at pH 6.

A further increase of pH up to 7.2 led to a decrease

in K

values. The second sharp peak was observed at

pH 11 and then the K

values decreased gradually up

to pH 12.1.













=



















(2)

CIP (pK

=6.1, pK

=8.7) can exist as cations

(CIP

), zwitterions (CIP

) and anions (CIP

−

) with the

change of solution pH values. In addition, the pH

pzc

of the MBC was at pH of 2.5. Therefore under the

experimental conditions, the MBC surface was

negatively charged. Accordingly, the first peak of the

value at pH 6 was due to the electrostatic

attraction interaction between CIP

and the

negatively charged MBC surface. When 6<pH<7.2,

the electrostatic attraction interaction decreased,

bringing about the decrease in CIP adsorption. The

second peak of the K

value at pH 11, where the final

CIP solution pH after adsorption shifted to 8.2

because of proton release from MBC surface, was

mainly attributed to the hydrophobic effect. The CIP

solubility is lower in neutral solutions than in acidic

or alkaline solutions (Jalil et al., 2015). A further

increase of pH up to 12.1, the adsorption of CIP

−

species declined on account of the electrostatic

repulsion between negatively charged MBC surface

and CIP

−

. The same phenomenon was described

when investigating sulfonamides adsorption on

acidic functionalized biochar (Ahmed et al., 2017)

Figure 3: Effect of pH on CIP adsorption on MBC.

Figure 4: Effect of ionic strength on CIP adsorption on

MBC.

3.3 Effect of Ionic Strength

As can be seen in Figure 4, the amounts of CIP

adsorbed on MBC increased by 71.4% when the

initial NaCl concentration increased from 0 to 1.0

mol·L

-1

. The salting out effect between CIP

IWEG 2018 - International Workshop on Environment and Geoscience

molecules and NaCl may result in the decrease of

CIP solubility in aqueous solution when NaCl was

added, which facilitating the diffusion of more CIP

molecules to the MBC surface and increasing the

CIP adsorption amount (Peng et al., 2016).

Figure 5: Adsorption kinetics of CIP on MBC.

Figure 6: Adsorption isotherms of CIP on MBC.

3.4 Adsorption Kinetics

As illustrated in Figure 5, the adsorption rate was

quite rapid in the first four hours and then slowed

down progressively, bringing about the equilibrium

state. There were plenty of active adsorption sites on

the MBC surface. The adsorption rate decreased

with the adsorption sites occupied (Mao et al., 2016).

The result indicated that the saturation of CIP

adsorption on MBC was obtained in approximately

24 h.

The plots of the pseudo first-order (PFO) and

pseudo second-order (PSO) kinetic models for CIP

adsorption onto the MBC were given in Figure 5.

The PSO model provided the better fitting for the

adsorption process compared with the PFO model.

The determination coefficient (R

) value of the PSO

model (0.955) was higher than that of the PFO

model (0.854). Futhermore, the value of q

e,exp

(4.11

mgˑg

-1

) was evidently closer to the predicted values

calculated theoretically by the PSO model (3.97

mgˑg

-1

), manifesting that PSO model was more

appropriate in describing the adsorption kinetic of

CIP on the MBC (Zhu et al., 2014). The results

manifested that chemisorption process was the

rate-limiting step in the elimination of CIP by MBC

(Jalil et al., 2015). This discovery was similar to the

adsorption kinetic of tetracycline on magnetic

porous biochar (Ahmed et al., 2017).

3.5 Adsorption Isotherms

In order to further investigate the adsorption

mechanisms of CIP onto the MBC, the experimental

data was fitted with Langmuir and Freundlich

models as illustrated in Figure 6. The Langmuir

models gave higher determination coefficient values

=0.974, 0.985, 0.973) to describe the adsorption

equilibrium data for CIP on MBC at 288.15, 298.15

and 308.15 K compared to Freundlich models,

indicating the monolayer coverage of CIP on the

MBC. Maximum Langmuir adsorption capacity (q

)

were 5.34, 5.41 and 6.14 mg·g

-1

for CIP adsorption

on MBC at 288.15, 298.15, 308.15 K, respectively.

For Langmuir isotherms, the values of k

increased with increasing temperature, indicating the

combining capacity for CIP on MBC increased

gradually. When R

values are between 0 and 1, the

adsorption process is favorable. In this study, the R

values were in the range of 0.058-0.105, indicating

the favorability of CIP adsorption on MBC (Reguyal

et al., 2017).

3.6 Adsorption Thermodynamics

The thermodynamic parameters (ΔG

, ΔH

, ΔS

) for

the adsorption process were calculated from the

following equations and given in Table 1.





=−ln



(3)





=



−



(4)

Where T (K) is the absolute temperature, R (8.314

Jˑmol

-1

ˑK

-1

) is the universal gas constant.

Table 1: Thermodynamic parameters of CIP adsorption on

MBC.

Temperature

(K)

(L g

-1

)

ΔG

(kJ mol

-1

)

ΔH

(kJ mol

-1

)

ΔS

(J mol

-1

288.15 0.402 -0.915

24.616 88.602

298.15 0.685 -1.801

308.15 1.070 -2.687

ΔH

was positive, suggesting that the adsorption

process was an endothermic process. The positive

Adsorptive Removal of Ciproﬂoxacin by Ferric Chloride Modiﬁed Biochar

value of ΔS

demonstrated the

randomness-increasing character of the adsorption of

CIP on MBC. The negative ΔG

indicated the

feasibility and spontaneous nature of the adsorption

of CIP onto the MBC surface. The adsorption

behavior was a spontaneous, endothermic and

entropy-increasing process. In addition, the

absolute values |ΔG

| were less than 20 kJ·mol

-1

revealing that a physical adsorption may be included

in the reaction processes (Feng et al., 2013). This

also suggested that pore-filling effect may be a

significant mechanism for the CIP adsorption on

account of the abundant pores within the MBC

structure (Deng et al., 2017).

3.7 Adsorption Mechanisms

The mechanisms of CIP adsorption on MBC may be

attributed to a combination of pore-filling effect, π-π

EDA interaction, hydrogen bonding formation,

electrostatic interaction and hydrophobic interaction.

The pore-filling effect played a significant role in

the adsorption of CIP on MBC. The rich porosity of

the MBC played a crucial role in the elimination of

CIP. Chun et al reported that the surface and pore

properties of biochars were the main factors

influencing the adsorption of hydrophobic organic

pollutants and concluded that the pore-filling effect

was a predominant mechanism (Chun et al., 2004).

Wang et al also found the pore-filling effect

promoted the norfloxacin adsorption on magnetic

biochars tremendously (Wang et al., 2017).

FTIR analysis indicated that the MBC surface

was enriched with –OH, –COOH and aromatic

groups. –OH and –COOH on the MBC surface could

form hydrogen bonding with N-containing and

F-containing groups on the CIP molecules. The

aromatic groups acting as strong electron donors can

interact with CIP molecules which had a strong

π-electron acceptor nature to form a π-π bond

according to the π-π EDA theory (Wang et al., 2017;

Ahmed et al., 2017).

Among interactions, electrostatic interaction was

also an important factor for CIP adsorption on the

MBC. It can be confirmed by the former discussion.

And the hydrophobicity significantly affected the

CIP adsorption due to the decreased solubility.

4 CONCLUSIONS

MBC with a relatively high porosity was prepared

for the elimination of CIP from aqueous solution.

Adsorption of CIP on MBC was of strong pH

dependence and presented a bimodal curve. Ionic

strength increasing was advantageous to the CIP

adsorption. The Langmuir model and the pseudo

second-order kinetic model presented better fittings

for the adsorption equilibrium and kinetic data,

respectively. The adsorption behavior was a

spontaneous, endothermic and entropy-increasing

process. The adsorption of CIP on MBC was

controlled by multiple mechanisms. MBC is a good

adsorbent for CIP adsorption.

ACKNOWLEDGMENT

This study was sponsored by the Natural Science

Fundation of Shandong Province (ZR2016EEM28).

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