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Analytical Letters
ISSN: 0003-2719 (Print) 1532-236X (Online) Journal homepage: https://www.tandfonline.com/loi/lanl20
Characterization of Two Bulgarian Herbs for Use
as Biosorbents for Copper(II)
Lidia Petkova Ivanova, Albena Kirilova Detcheva & Paunka Stoyanova
Vassileva
To cite this article: Lidia Petkova Ivanova, Albena Kirilova Detcheva & Paunka Stoyanova
Vassileva (2019): Characterization of Two Bulgarian Herbs for Use as Biosorbents for Copper(II),
Analytical Letters, DOI: 10.1080/00032719.2019.1587447
To link to this article: https://doi.org/10.1080/00032719.2019.1587447
Published online: 08 Mar 2019.
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Characterization of Two Bulgarian Herbs for Use as
Biosorbents for Copper(II)
Lidia Petkova Ivanova, Albena Kirilova Detcheva, and Paunka Stoyanova Vassileva
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria
ABSTRACT
Two biomaterials based on the abundant Bulgarian medicinal plants
Mentha spicata L. (denoted as MS) and Ruta graveolens L. (denoted as
RG) were investigated as environmentally friendly biosorbents for Cu(II)
removal from aqueous solutions. Grain size distribution, slurry pH, tex-
ture parameters, thermal behavior and mineralogical composition of MS
and RG were determined. Instrumental methods such as scanning elec-
tron microscopy (SEM), X-ray-diffraction (XRD), Fourier transform infra-
red (FTIR) spectroscopy, Brunauer–Emmett–Teller (BET) analysis,
differential thermal analysis (DTA), and thermogravimetric analysis (TGA)
were used for their characterization. It was demonstrated that the sur-
face morphology of both materials is rough and contains pores that
could entrap different pollutants, as well as functional groups that can
attach metal ions. In order to examine the feasibility of these materials
for use as biosorbents for heavy metals, adsorption experiments were
performed. The results for the removal of Cu(II) ions from aqueous solu-
tion reveal capabilities suggesting that both materials have potential to
be used as promising, efficient and low-cost biosorbents.
ARTICLE HISTORY
Received 10 December 2018
Accepted 22 February 2019
KEYWORDS
Biosorption;
Brunauer–Emmett–Teller
analysis; Bulgarian herbs;
copper(II) ions; differential
thermal analysis; Fourier
transform infrared
spectroscopy; scanning
electron microscopy;
thermogravimetric analysis;
X-ray-diffraction
Introduction
Many industrial processes such as mining, electroplating, dyeing, paper manufacturing
and petroleum refining produce wastewater streams containing heavy metals which are
hazardous for environment and living organisms (Suresh Kumar et al. 2015; Rugnini
et al. 2017; Moreira et al. 2019). The metals are of special concern because they are
nondegradable and therefore persistent. Numerous techniques and treatment technolo-
gies have been developed for removal of toxic elements from contaminated waters.
Traditional methods for water purification such as chemical precipitation, coagulation,
ion-exchange, and membrane processes are either too expensive or insufficiently effect-
ive to reduce the toxic element content to levels dictated by increasingly strict regula-
tions (Vassileva and Detcheva 2011).
Thus, biosorption (a term used here to describe the removal of heavy metals using a
passive binding process with nonliving organisms) is one of the most recent develop-
ments in environmental or bioresource technology (Sa
g and Kutsal 2001; Aksu 2005;
Volesky 2007; Vijayaraghavan and Yun 2008; Park, Yun, and Park 2010). Metal entrap-
ment is due to chemico-physical interactions with active groups present on the cell wall,
CONTACT Albena Kirilova Detcheva albena@svr.igic.bas.bg Institute of General and Inorganic Chemistry,
Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 11, BG-1113 Sofia, Bulgaria
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lanl.
ß2019 Taylor & Francis Group, LLC
ANALYTICAL LETTERS
https://doi.org/10.1080/00032719.2019.1587447
depending on the biosorbent nature. Their adsorption is due to the large internal sur-
face area and the availability of different functional groups which are able to attach
metal ions (Choi and Yun 2006; Lesmana et al. 2009).
According to the literature, the adsorption sites are carboxylic- and phenolic-type sur-
face OH-groups, which most probably form coordination bonds between the metal ions
and adjacent chains of OH-groups of the lignocellulosic components (Vassileva et al.
2018). The major advantages of this technology over conventional adsorption include
not only low cost, but also its high efficiency, minimization of chemical or biological
sludges, ability of biosorbent regeneration, and possibility of metal recovery following
adsorption (Volesky 2007; Park, Yun, and Park 2010). Another advantage of the bio-
sorbents is their biodegradability. In this aspect, some plant products (Chen, Chen, and
Hsu 1996;Aydın, Bulut, and Yerlikaya 2008; Oliveira et al. 2008; Iftikhar et al. 2009;
Pehlivan, Altun, and Parlayıcı2009; Zheng et al. 2009; Hossain et al. 2012a,b; Adeel
et al. 2013), have been studied for the removal of toxic metals from polluted waters.
Medical plants are of great interest because they contain numerous active phytochemi-
cals which are prone to metal binding.
Mentha spicata (commonly known as mint) belongs to the genus Mentha in the fam-
ily Labiatae (Lamiaceae) that includes other commonly grown oil yielding plants such
as basil, sage, rosemary, marjoram, lavender and thyme. There are 25–30 species within
the genus Mentha, including spearmint, peppermint, wild mint, corn mint, curled mint,
bergamot, American mint, and Korean mint, of which spearmint is the most common
(Choudhury, Kumar, and Garg 2006). It is very widely distributed in Europe, Asia,
Africa, and America and used by humans since ancient times.
Several plants of Rutaceae family are used in traditional medicine worldwide. The
most common medicinal plant of this family is Ruta graveolens L., known as rue, com-
mon rue or herb-of-grace, which is a species native to Europe. The plant is grown as an
ornamental plant and herb and is now available all over the world, though preferably
grown in a Mediterranean climate (Raghav et al. 2006). Both plants are oil yielding and
have important medicinal application and culinary use.
The adsorption possibilities of Mentha spicata L. distillation waste biomass for
removal of Cu(II) and Pb(II) from aqueous solutions have been described in the litera-
ture (Riaz et al. 2013; Ansari et al. 2016). No results concerning the biosorption on raw
Mentha spicata L. and Ruta graveolens L. have been reported to date.
The aim of the present work was to investigate two biomaterials based on the
Bulgarian commercially available medicinal plants Mentha spicata L. and Ruta graveo-
lens L. Both materials were characterized in terms of mineralogical and chemical com-
position, grain size distribution, thermal behavior, slurry pH, and texture parameters. In
order to examine the potential of these materials as adsorbents for heavy metals, Cu (II)
adsorption experiments were performed.
Experimental
Materials
The commercially available fresh leaves of Mentha spicata L. (denoted as MS) and Ruta
graveolens L. (denoted as RG) were washed several times with distilled water to remove
2 L. P. IVANOVA ET AL.
surface adhered and water-soluble particles and dried at 60 C in an electric oven for
48 h. The materials were then milled in an electric grinder. No other physical or chem-
ical treatment was performed on the materials thus obtained.
Instrumentation and methods
The grain size distribution was determined on a Dynamic Image Analyzer Camsizer
XT, X-Dry (Retsch Technology, Germany). Measurement principle-dynamic image ana-
lysis (according to ISO 13322-2) by digital image processing using a double-camera-sys-
tem (ISO 13322-2) and Quad Core PC incl. Windows 7. All measurements were
repeated and the average results were discussed.
For pH determination, sample suspensions (0.5 g of sample in 50 mL of deion-
ized water) were prepared in stoppered 100 mL glass bottles. The mixtures were
stirred for 24 h on a mechanical stirrer and after filtration, the pH values of the
aqueous solutions were measured on a pH meter model 211, Hanna instru-
ments (Germany).
X-ray diffraction (XRD) patterns were obtained on a D8 Advance System from
Bruker Inc. (Germany) using Cu Karadiation at 40 kV and 40 mA using a wavelength
of 1.5404 nm.
Moisture and ignition losses were measured by differential thermal analysis (DTA)
and thermogravimetric analysis (TGA). DTA was performed on a Setaram Labsys Evo
1600 system from room temperature up to 700 C at a heating rate of 10 C/min in
air. Alumina pans were used for the DTA studies. Sample masses from 20 to 50 mg
were used for thermal analysis as described by Kaur, Khanna, and
Aleksandrov (2017).
The Fourier transform-infrared (FTIR) spectra were measured on a Thermo Nicolet
6700 FTIR-spectrometer. Spectra were collected in the mid-infrared region (4000 to
400 cm
1
). Samples were prepared by the standard KBr pellet method.
The surface morphology of the biosorbents was observed on a scanning electron
microscope (SEM) –Tescan instrument model SEM/FIB Lyra I XMU using both sec-
ondary electrons (SE) and back scattering electrons (BSE) detectors.
SEM-micrographs of the investigated materials before and after the biosorption pro-
cess were recorded. The samples were covered with a thin layer of carbon using a sput-
ter coater (K450X Emitech) and were observed using a scanning electron microscope
(20 kV) under a vacuum of 10
2
Pa. The average particle size was evaluated by the
embedded image analyzer software (Lyra I XMU Tescan).
The porous structure of the studied materials was investigated by low-temperature
(–196 C) nitrogen adsorption using a Quantachrome Nova 1200 apparatus
(Quantachrome Instruments, USA). Before nitrogen adsorption, the samples were
degassed at 80 C for 3 h. Specific surface area was calculated on the basis of the
Brunauer–Emmett–Teller (BET) equation; the pore size distribution was calculated
according to the Barrett-Joyner-Halenda (BJH) method. The total pore volume was esti-
mated in accordance with the rule of Gurvich at a relative pressure of 0.99. The volume
of the micropores was calculated using the Dubinin-Radushkevich equation and the vol-
ume of the mesopores by the difference of V
total
–V
micro
.
ANALYTICAL LETTERS 3
Adsorption studies
Batch experiments were carried out to determine the adsorption properties of the plant
materials. Experiments were performed using stoppered 50 mL Erlenmeyer flasks con-
taining about 0.2 g of sample and 20 mL of aqueous solution of Cu
2þ
ions. The mixture
was shaken at room temperature (20 C) by an automatic device. The adsorption experi-
ments were carried out at pH 4. The initial solution pH was adjusted using 0.1 M HCl
or 0.1 M NaOH. After the experiment, the biomaterial was removed by passage through
a Millipore filter (0.2 lm). The initial and equilibrium copper concentrations were
determined by inductively coupled plasma optical emission spectrometry (ICP-OES) on
a Prodigy 7 ICP-OES spectrometer (Teledyne Leeman Labs, USA).
Deionized water and analytical grade reagents were used throughout. Working stand-
ard solutions of Cu
2þ
ions with concentrations of 50–500 mg L
1
were prepared by
stepwise dilution of a stock solution with concentration of 1 g L
1
(Merck,
Darmstadt, Germany).
All adsorption experiments were replicated and the average results were used in
data analyses.
Results and discussion
Sample characterization
Grain size distribution
Granulometric analysis showed no significant differences in the particle size distribution
for the MS and RG samples. After sample pretreatment, the resulting grain size distribu-
tion was shown to be practically monomodal with a maximum of approximately
100 lm average particle size. More than 90% of the particles were smaller than 173 lm
for MS and below 162 lm for RG (Figure 1).
XRD analysis
The XRD patterns of the samples MS and RG (Figure 2) are typical for cellulosic mate-
rials. The content of the registered crystal phase exhibit peaks identified as crystalline
forms of cellulose 1a (Mutungi et al. 2012;Ara
ujo et al. 2018). According to Ali et al.
(2016), some of the peaks are related to a less organized polysaccharide structure due to
the broad peak with a low angle at approximately 202h. Typical for both spectra is
also the presence of amorphous phase in the range between 15and 252h, which cor-
responds to amorphous phases of lignin, hemicelluloses and cellulose (Hongzhang 2014;
Yuvaraja et al. 2014).
TG and DTA analysis
The thermal behavior of the MS and RG samples are presented on Figure 3a and 3b,
respectively. The pyrolysis process is accompanied by purging of phases containing dif-
ferent hydrocarbon compounds, and finally solid carbon-containing residues are
obtained. The first region (20 to 100 C), related to the elimination of moisture and
adsorbed water, is not clearly distinguished because the samples MS and RG were ini-
tially dried at 60 C in an oven for 48 h.
4 L. P. IVANOVA ET AL.
The second region from 100 C to 200 C reveals the evaporation of compounds of
high volatility. The slope in the third stage (between 200 and 400 C) can be related to
the thermal degradation of hemicelluloses, cellulose and some lignin fractions. The final
part of the graph above 400 C can be assigned to the decomposition of the remaining
lignin (Shinde and Singarvelu 2014;Ara
ujo et al. 2018). These results are in agreement
with the XRD-analysis.
FTIR analysis
The plant materials mainly consist of cellulose, hemicellulose and lignin. They contain
many functional groups such as hydroxyl, carbonyl, carboxyl and amino groups with
characteristic chemical structures. To determine the surface functional groups respon-
sible for the adsorption properties of the studied plants, their FTIR spectra in the range
from 4000 to 400 cm
1
were recorded and shown in Figure 4.
In the IR spectrum of both materials clearly defined bands are observed which are
typical for the functional groups of the components of lignocellulosic materials
(Nakamoto 1970; Radoykova et al. 2015; Vassileva et al. 2018). The broad peak at about
Figure 2. X-ray diffraction patterns for Mentha spicata L. (MS) and Ruta graveolens L. (RG) samples.
Figure 1. Particle size distribution for Mentha spicata L. (MS) and Ruta graveolens L. (RG) samples
using dynamic image analysis. The results are reported as the mean values of two parallel
measurements.
ANALYTICAL LETTERS 5
3400 cm
1
is related to OH-groups of cellulose and lignin in the investigated materials.
The peaks at about 2920 cm
1
and 2850 cm
1
can be assigned to asymmetric and sym-
metric stretching vibrations of C-H bonds in methyl and methylene groups (Ngah and
Hanafiah 2008).
The peaks at about 1740 cm
1
are due to vibrations of C¼O bonds of carboxylic
acids or ester groups. The bands in the region of 1620–1650 cm
1
correspond to C¼O
Figure 3. Differential thermal analysis and thermogravimetric analysis curves for (a) Mentha spicata L.
(MS) and (b) Ruta graveolens L. (RG) samples from room temperature up to 700 C at a heating rate
of 10 C/min in air.
Figure 4. Fourier transform-infrared spectra (KBr-disks) of Mentha spicata L. (MS) and Ruta graveolens
L. (RG) samples.
6 L. P. IVANOVA ET AL.
stretching vibrations of aromatic carboxylic groups. The bands in the region
1510–1600 cm
1
are connected with valent oscillations of C-C bonds in the aromatic
nucleus of lignin. According to Vassileva et al. (2018), the bands at about 1420 cm
1
are related to CH
2
deformation vibrations in lignin.
A well-formed band at about 1250 cm
1
, connected with skeleton vibrations of the
aromatic rings, is observed which can also be attributed to C–O stretching of phenolic
groups. The bands about 1070 cm
1
refer to C¼O oscillations or C–O stretching of
ether and alcoholic groups, while those about 620 cm
–1
correspond to alkynes (V
azquez
et al. 2009; Iftikhar et al. 2009). The peak at about 590 cm
1
is related to the vibrational
Figure 5. Low-temperature (–196 C) nitrogen adsorption-desorption isotherms (volume of adsorbed
nitrogen versus relative pressure) and pore size distribution of (a) Mentha spicata L. (MS) and (b) Ruta
graveolens L. (RG) samples.
ANALYTICAL LETTERS 7
bending in the aromatic compounds of lignin (Nasrullah et al. 2015; Ali et al. 2016).
Among the active groups, carboxylic acid and hydroxyl groups could play the major
role in copper adsorption (Annadurai, Juang, and Lee 2003; Sheng et al. 2004; Memon
et al. 2008; Hossain et al. 2012b).
Texture parameters
The nitrogen adsorption-desorption isotherms and the pore size distribution of the
investigated plant materials are presented on Figure 5. Both isotherms belong to the
type IV with hysteresis loop which resembles the H3 type in the IUPAC classification.
This hysteresis pattern can be attributed to the crystalline agglomerates that result in a
mesoporous structure formed by the interparticle space causing the formation of sec-
ondary pores (Vassileva et al. 2016). The calculated textural parameters of MS and RG
are summarized in Table 1. The porosity diagrams are of analogs appearance which
implies that the studied materials possess pores of similar dimensions.
It is obvious that the sample MS is micro-mesoporous while the sample RG is pre-
dominantly mesoporous. The calculated average pore diameters for both samples are
16 nm (MS) and 9 nm (RG), corresponding to mesoporous structure. The sample MS
displays slightly higher values for the specific surface area and total pore volume.
The measurements of slurry pH indicated that both samples displayed equal pH val-
ues of about 7.0, which corresponds to the neutral reactions of their aqueous suspen-
sions (Table 1).
Adsorption studies
Effect of initial Cu(II) concentration
In order to evaluate the potential of the investigated plant materials as low-cost adsorb-
ents for heavy metals, adsorption experiments for copper ions were performed. Copper
was used as a model element in the present study as it is the third most used metal in
the world (Albert et al. 2018) and in high concentrations can be hazardous to environ-
ment and humans. It is classified in the second group of toxicity of metal ions and has
to be eliminated from contaminated waters (Stoyanov 1999).
The time needed to achieve adsorption equilibrium between the plant samples and
Cu(II) ions was experimentally determined to be 10 min. Nevertheless, all further
experiments were performed at a contact time of 1 h for certainty.
The initial Cu(II) concentration serves as an important driving force for overcoming
mass transfer resistance of Cu(II) between the aqueous and the solid phase.
The effect of initial Cu(II) concentration on the adsorption of copper on both studied
materials is shown on Figure 6. The amounts of adsorbed Cu(II) increased with increas-
ing initial Cu(II) concentration. This is due to the fact that higher Cu(II) concentrations
Table 1. Texture characteristics and pH of the water leachates of Mentha spicata L. (MS) and Ruta
graveolens L. (RG).
Surface
area m
2
g
–1
Total pore
volume cm
3
g
–1
Micropore
volume cm
3
g
–1
Mesopore
volume cm
3
g
–1
Average pore
diameter nm pH
Mentha spicata L. 1.0 0.005 0.002 0.003 16 7.0
Ruta graveolens L. 0.8 0.002 0 0.002 9 7.0
8 L. P. IVANOVA ET AL.
provide an increased concentration gradient, which leads to a higher probability of colli-
sion between Cu(II) ions and the active adsorption sites on the MS and RG surface,
thereby increasing the adsorption capacity (Zhang and Wang, 2015).
Sample MS displays better adsorption efficiency as compared with RS. This phenom-
enon is in agreement with the results obtained for the texture parameters of the sam-
ples. Even so, both plant materials in the present investigation could be used as effective
adsorbents for Cu(II)-ions.
SEM analysis
Scanning electron microscopy (SEM) is an extremely useful tool to identify the active
sorptive sites on the biosorbent surface. In the present study, SEM was used for
characterizing the morphology and structure of the biosorbents before and after
Cu(II) sorption and the respective micrographs are presented on Figure 7. SEM
micrographs of both initial materials indicate that the nature of the biomass is rough
and heterogeneous with many pores, allowing the sorption of various heavy metals
in different parts of the biosorbent. This points to the strong possibility for copper
to be trapped and adsorbed on the surface. The exhausted (loaded with copper) plant
materials MS and RG were examined and an X-ray mapping was also prepared
(Figure 7c,d).
There is a morphological change of the surface of both investigated plant materials
after the biosorption of Cu(II) (Figure 7). The surface of MS and RG became smoother
after biosorption of Cu(II), thus corroborating the presence of the adsorbed metal on
the surface which is in accordance with the results obtained by Oliveira et al. (2008),
Liu et al. (2012), and Hossain et al. (2012a,b). The differences observed on the SEM
images before and after Cu(II) loadings indicated that sorption took place on the sur-
face of both biosorbents. The tubular and porous cavities occupied by the adsorbed cop-
per became visible as white dots on the SEM images.
The average particle size was evaluated by an image analyzer software to be about
100 mm which confirms the results obtained from the particle size distribution.
Figure 6. Amounts of adsorbed Cu(II) ions on Mentha spicata L. (MS) and Ruta graveolens L. (RG) as a
function of initial copper concentration at room temperature (20 C), pH 4 and contact time of 1 h.
ANALYTICAL LETTERS 9
Conclusions
In the present work, the physical and chemical properties of Mentha spicata L. (MS)
and Ruta graveolens L. (RG) were studied by the use of several analytical techniques.
For both plant materials, the particle size distribution was monomodal with a max-
imum at about 100 mm and their slurry pH was neutral. XRD-curves showed the pres-
ence of one broad peak, which is characteristic for amorphous phase (lignin,
hemicelluloses and amorphous cellulose) and peaks indicating crystalline forms of cellu-
lose 1a. MS and RG showed similar TGA patterns. The first region is related to the
elimination of moisture and adsorbed water from the sample. The second region corre-
sponds to the rapid thermal decomposition of hemicelluloses, cellulose and part of
the lignin.
The FTIR spectra of MS and RG display a number of peaks indicating the complex
nature of the materials examined. It is found that both materials contain -OH, -CO,
-CH
2
and -COO
–
groups which could take part in the adsorption process and be very
effective in capturing the copper ions.
(a) (b)
(c) (d)
Figure 7. Scanning electron micrographs of the surface of (a) Mentha spicata L. (MS) and (b) Ruta
graveolens L. (RG) at 20.00kV using a magnification factor of 2.0010
3
using SE detector. The distri-
bution of copper ions on the surface is shown for (c) Mentha spicata L. (MS) and (d) Ruta graveolens
L. (RG) after adsorption.
10 L. P. IVANOVA ET AL.
The results obtained for surface area, pore volume, and morphology revealed that MS
and RG are porous biosorbents with heterogeneous structure which favors the biosorp-
tion of Cu
2þ
ions on different parts of their surfaces. Both samples are characterized
with isotherms corresponding to type IV, typical of mesoporous materials, with pore
size around 16 nm for MS and 9 nm for RG. The sample MS displays slightly higher val-
ues for the specific surface area and total pore volume.
The sample MS shows better adsorption efficiency towards Cu
2þ
compared to RS.
This phenomenon is in agreement with the results obtained for texture parameters.
Nevertheless, both plant materials characterized in the present investigation have good
potential as biosorbents for metal ions removal from aqueous solutions.
Acknowledgments
The authors thank to the program for supporting young scientists and Ph.D. students from the
Bulgarian Academy of Sciences –2017 (Project No DFNP-17-35/25.07.2017) for the finan-
cial support.
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