Removal of copper from aqueous solutions by low cost adsorbent-Kolubara lignite
Sonja Milicevica, Tamara Boljanaca, Sanja Martinovica, 
, 
, Milica Vlahovica, Vladan Milosevica, Biljana Babicb
, , Milica Vlahovica, Vladan Milosevica, Biljana Babicb
Received 29 July 2011; revised 28 September 2011; Accepted 2 November 2011. Available online 26 November 2011.
Abstract
Serbian lignite from “Kolubara” deposit was used as a low cost adsorbent for removal of copper ions (Cu2 +)
from aqueous solutions. Lignite was subjected to the elementary and
technical analysis as well as BET and FTIR analysis due to complete
characterization. Basic comparison between lignite and activated carbon
was also done. As a method, batch adsorption procedure was applied.
Adsorption efficiency was studied as a function of the initial metal
concentration, pH of the solution, contact time, and amount of the
adsorbent. Optimum removal of copper ions was achieved at pH values of
5.0. About 90% of copper cations were removed in 5 min of contact time
from the solution with the lowest copper concentration (50 mg Cu2 +/l)
regardless adsorbent amount, while the same effect of adsorption was
achieved in 60 min in case of solutions with higher concentrations of
copper. It was concluded that the effect of adsorbent amount on
adsorption kinetics is evident but not crucial. It was proved that the
experimental results of copper adsorption fit well to a Langmuirian type
isotherm which was used to describe monitored adsorption phenomena. The
calculated adsorption capacities of lignite for copper adsorption
decrease with increasing adsorbent amount. The study proved that tested
lignite is very efficient adsorbent material, especially in case of low
copper concentration in aqueous solution where the usual methods are
either economically unrewarding or technically complicated. This
behavior can be explained by FTIR spectrum despite a small specific
surface area of lignite. Namely, many bands (peaks) are attributed to
the functional groups that they are involved in chemisorption and ionic
exchange, basic mechanisms of copper adsorption.
Highlights
►
Serbian lignite was used as adsorbent for copper removal from aqueous
solutions. ► Influence of concentration, pH, contact time and adsorbent
amount was studied. ► Tested lignite is very efficient adsorbent
material. ► Copper adsorption results fit well to Langmuirian type
isotherm.
Keywords: Adsorption; Lignite; Copper; Kinetics
Article Outline
- 1. Introduction
- 2. Experimental
- 2.1. Adsorbent
- 2.2. Surface area
- 2.3. Fourier transform infrared analysis
- 2.4. Adsorption tests
- 3. Results and discussion
- 4. Conclusion
- Acknowledgments
- References
1. Introduction
Copper
is known as one of the most common toxic and hazardous metals which is
often used in electrical, mining, and electronic industries, iron, steel
and non-ferrous production, electroplating, metal finishing, printing
and photographic procedures. Copper, as well as the other heavy metals,
is released into the environment in a number of different ways and it
finds the way to get into the water-streams and thus make environmental
contamination that presents threat to humans, animals, and plants. This
can cause serious and complex problem [1], [2], [3] and [4].
Concentrations
of copper and other heavy metals from the wastewater and water-streams
have to be reduced in order to satisfy rigid legislative standards. They
can be removed by various technologies, most often expensive or
inefficient and technically complicated especially because of limited
low residual concentrations required by the EPA (Environmental
Protection Agency) [2], [3] and [4].
The conventional techniques for heavy metals removing from aqueous
solutions include oxidation, reduction, chemical precipitation,
filtration, ion exchange, adsorption, membrane techniques, electrolytic
or liquid extraction, reverse osmosis, biological process [2] and [5]. Each of these methods is used only in special cases since it has some limitations in practice [6].
Namely, the major disadvantage of almost all mentioned methods is
production of new hazardous waste, mostly solid, at the end of the
treatment [1].
Nowadays, many researches have been involved in development of new
inexpensive materials and methods for the treatment of wastewater
containing heavy metals, for example natural adsorbents such are
zeolites, wood, lignite, metal oxides, fly ash, coal, and waste biomass [7], [8] and [9].
Predominant mechanism in this process is ion exchange, but also there
is surface adsorption, chemisorption, complexation and
adsorption-complexation [2], [3], [4] and [10].
Carbon based materials are very common for the removal of color and
organic matter from wastewater. Activated carbon is a preferred
adsorbent, but its application is often restricted due to its high cost [11].
On the other side, in spite of relatively small adsorption capacity of
lower quality coals, compared with expensive synthetic materials used
for ionic exchange, lignite is considered as a very attractive material
for metals removal since it is widely available and inexpensive [2].
It should be emphasized that removing heavy metals from the wastewaters
present in relatively low concentration is rather difficult [6].
Recently, the use of lignite in wastewater treatment has become more
and more attractive since it can be good substitution for synthetic and
expensive activated carbon. Lignite possesses all the necessary
characteristics that make it a very efficient material for the removal
of copper and other heavy metals from wastewater [2], [6], [11] and [12].
Lignite
is the youngest type of coal, member of the solid fuels and one of the
abundant natural resorces. Lignite has a high content of exchangable
functional groups that make it suitable and efficient for the removal of
heavy metals from wastewater. Lignites have high cation exchange
capacity forming with metal ions complexes. Namely, lignites have high
content of oxygen fixed in carboxile (− COOH), alcocholic (− OH) and
carbonyl (= C = O) groups representing active centers of the ion
exchange. Owing to these properties, lignites take part in ion exchange
mechanism and in wastewater treatment as a medium for heavy metals
removing. Lignites are usually amorphous and fibrous or woody in
texture. Their structure consists of water-filled pores and capillaries
and exhibits high moisture contents (30–70%) [6], [11], [13] and [14].
The
primary aim of this study was to determine adsorption parameters of low
cost Kolubara lignite during the removal of copper from the synthetic
aqueous solutions. The idea was to perform experiments with lignite in
the form suitable for usage in thermal power plant for the reason of
comparison with the activated carbon. It was found that this lignite is
an excellent adsorbent for copper, especially in case of aqueous
solution with small copper concentration.
2. Experimental
2.1. Adsorbent
Lignite
from Kolubara deposit, field B, used as a fuel in power plant Nikola
Tesla, was applied in experiments as an adsorbent material. First, it
was dried at 45 °C for 24 h, and then grounded and sieved. Fractions
from 1 to 2 mm were used for the adsorption experiment, while fine
fractions, under 1 mm, were used for elementary analyses. Heating value
was measured using an automatic calorimeter. Activated carbon (Merk,
Germany), most common adsorbent in wastewater treatment, was used for
comparation with lignite as adsorbent agent.
2.2. Surface area
Adsorption and dersorption isotherms of N2
were measured on lignite and activated carbon samples, at − 196 °C,
using the gravimetric McBain method. The specific surface area (SBET), pore size distribution was estimated by applying BJH method [15] to desorption branch of isotherms and mesopore surface and micropore volume were estimated by using the high resolution αs plot method [16], [17] and [18]. Micropore surface (Smic) was calculated by substracting Smeso from SBET.
2.3. Fourier transform infrared analysis
Functional
groups in organic samples (lignite and activated carbon) were examined
by using the FTIR method of analysis. The IR measurements were carried
out by a Fourier Transform Infra Red (FT-IR) spectrophotometer based on
changes in dipole moment resulting from bond vibration upon absorption
of IR radiation. FTIR-ATR (Fourier transform infrared attenuated total
reflection spectroscopy) spectroscopic analyses were carried out at room
temperature using a Nicolet 380 spectrophotometer in the spectral range
of 4000 to 400 cm− 1, with a resolution of 4 cm− 1. The datasets were averaged over 64 scans.
2.4. Adsorption tests
The
kinetics of copper adsorption on lignite was conducted by batch
technique at ambient temperature in aqueous solutions under continuous
stirring conditions.
The
procedure was as follows: weighted amount of lignite was placed into a
glass vessel with cover. Prepared copper solution was added and then
agitated. In order to quantify adsorption efficiency (percent of
adsorbed metal), suspension was filtered and residual copper ion
concentration in the filtrate was determined by Perkin Elmer Atomic
Adsorption Spectroscopy (AAS) type AAnalyst 300.
Synthetic aqueous solution of copper was prepared by dissolving of appropriate amount of CuSO4·5H2O
salt in deionised water. Volume of the solutions was constant (250 ml),
as well as stirring conditions. Effects of three different
concentrations of the initial solution (50, 200, and 330 mg Cu2 +/l)
on adsorption were investigated. Influence of the initial solution's pH
on the adsorption efficiency was observed during the experiment. pH was
measured by pH meter and kept in the range of 2–6 by using diluted 0.1 M
HNO3 or 0.1 M NaOH solution. Also, the effect of three
different amounts of air dried lignite (30, 45, and 60 g) on adsorption
was followed during the experiment. All experiments were monitored
depending on contact time up to one hour.
The
lignite saturated with copper was treated with deionized water. The
analysis of the obtained solution proved literature assessment that
there was no leaching from the adsorbent [2].
3. Results and discussion
3.1. Characterisation of adsorbent
Elementary
and technical analysis as well as specific surface area of lignite from
Kolubara deposit, conducted according to the standard procedure, is
given in Table 1.
Table 1. Basic characteristics of the lignite Kolubara deposite, field B.
| Content (%) | Heating value (kJ/kg) | BET (m2/g) | |||||
|---|---|---|---|---|---|---|---|
| C | H | N + O | S | H2O | Ash | ||
| 65.75 | 5.79 | 27.76 | 1.13 | 46.30 | 11.14 | 9499 | 1 |
According to the IUPAC classification [19], nitrogen adsorption–desorption isotherm for lignite, as the amount of N2
adsorbed as function of relative pressure at − 196 °C, is of type I
which is associated with nonporous and macroporous materials. Specific
surface area calculated by BET equation (SBET) is 1 m2/g.
On the other side, isotherm for activated carbon is of type IV and with
a hysteresis loop which is associated with mesoporeous materials. The
shape of hysteresis loop is of type H4 which indicates a narrow slit
pores and large amount of micropores [20]. Specific surface area calculated by BET equation (SBET) is 758 m2/g.
Pore
size distribution of activated carbon shows that the sample is mostly
microporous with certain amount of mesopores and the pores radius below 7
nm. Actually, activated carbon consists of micropore amount (Smic) = 683 m2/g and mesopore amount (Smeso) = 75 m2/g.
FTIR spectra of analyzed lignite and activated carbon are shown in Fig. 1. Bands were identify by comparison to the literature [[21], [22], [23], [24], [25], [26], [27] and [28]].
These two spectra differ significantly in the peaks and it is obvious
that lignite has much more functional groups than activated carbon.
Six peak areas observed in diagram of Fig. 1 are: hydorxyl group region (3100–3700 cm− 1), aliphatic stretching region (2931–2855 cm− 1), aromatic carbon (peaks at 1618 and 1606 cm− 1), aliphatic bending region (1509–1371 cm− 1), cellulose and lignin region (1300–1000 cm− 1), and the aromatic out-of-plane region 900–700 cm− 1) were measured. Additionally, intense vibrations at 3698 cm− 1, 3620 cm− 1, 531 cm− 1 and 469 cm− 1 are attributed to clay and silicate minerals. The small peaks in the rand of 3698 cm-1 and 3700 cm-1 can be assigned to the crystal water which exists in clay minerals of the matrix lignite samples [29].
Lignite
spectra show typical infrared characteristics of the organic compound,
coal, including aliphatic C-H stretching bands at 2924 cm− 1 and 2856 cm− 1, C = C or C = O aromatic ring stretching vibrations at 1610 cm− 1 and at 1506 cm− 1, as well as aliphatic C
H stretching bands at 1455 cm− 1, 1370 cm− 1.
H stretching bands at 1455 cm− 1, 1370 cm− 1.
The broad band at ~ 3406 cm-1
is attributed to –OH stretching vibrations of hydrogen bonded hydroxyl
groups of absorbed water either of clay minerals or phenol groups.
The bands at ~ 2931 cm− 1 and ~ 2855 cm− 1 are attributed to aliphatic CH vibration of − CH3 and − CH2 stretching vibrations, respectively.
The strong band at ~ 1606 cm− 1 is attributed either to C = O or C = C aromatic ring stretching vibrations.
The band at ~ 1505 cm− 1 is due to C = O stretching vibrations.
The band at ~ 1454 cm− 1 is attributed to symmetric aliphatic C-H vibrations of methylene (CH2) and methoxyl (OCH3).
The band at ~ 1370 cm− 1 is due to symmetric aliphatic C-H bending vibrations of methyl groups (OCH3).
The band at ~ 1265 cm− 1 is attributed to C-O stretching vibrations.
The peak at ~ 1033 cm− 1 is due to COH bonds in cellulose as well as to C-O stretching vibrations of aliphatic ethers (R-O-R`) and alcohols (R-OH).
OH bonds in cellulose as well as to C-O stretching vibrations of aliphatic ethers (R-O-R`) and alcohols (R-OH).
Adsorption
is a process of mass transfer of adsorbate in solution to the adsorbent
surface driven by physical and/or chemical forces. For adsorbate, its
adsorption capacity and mechanisms are closely associated with the
adsorbent surface characteristics. Based on that, it can be concluded
that driving mechanism of lignite adsorption is based on chemisorption
since there are many functional groups involved in ion exchange. On the
other side, the adsorption properties of activated carbon are govern by
physisorption since it has high values of specific surface area as well
as micro and meso porosity. It can be explained by diffusion and
transport processes within meso- and micropores. Since lignite is
classified as nonporous and macroporous materials and specific surface
area is small, it can be assumed that all identified functional groups
involve in ion exchange during the adsorption are placed at the surface
of lignite. Namely, when express in terms of per unit surface area,
lignite seems to give a surprisingly good adsorption capacity compared
with the activated carbon.
3.2. Adsorption kinetics
3.2.1. Effect of the adsorbent amount on copper removal
Effect of the adsorbent amount on adsorption efficiency is shown in Fig. 2.
Increasing amount of lignite from 30 to 60 g for the same concentration
of copper in solution leads to reduction of the adsorbed metal amount
per mass unit of lignite. This is particularly obvious in the solution
with higher copper concentration while this influence weaks in the
solutions with the lowest copper concentration. It suggests that
experiments should be directed to the small amounts of lignite in order
to achieve better efficiency and to determine capacity of the adsorbent.
On the other hand, tested masses of lignite under optimal conditions do
not substantially affect the adsorption efficiency regardless the high
concentration of the initial solution.
3.2.2. Effect of pH solution on copper removal
Carboxyl
and hydroxyl groups are the main exchangeable functional groups that
take part in the adsorption of metal ions onto lignite derived
adsorbents. With increasing of pH solution, these functionalities
dissociate, i.e., become deprotonated and negatively charged. During the
adsorption, H+ and other exchangeable cations (e.g. Na+, Ca2 +, and Mg2 +) are substituted with metal cations and released from the adsorbent to the solution [11].
Generally,
very important parameter that should be controled during the adsorption
process is pH of the initial aqueous solution. Lignite mass of 30 g and
contact time of 30 min used in this part of study were fixed.
The
metal cations in the aqueous solution convert to different hydrolysis
products. At low to high pH values, copper ions exist as Cu2 +, Cu(OH)+, and as neutral compound Cu(OH)2. The dominant species of copper in the pH range from 3 to 5 are Cu2 + and Cu(OH)+ ions, while the copper occurs as insoluble Cu(OH)2(s) above pH 6.3. Experiments were performed with the pH values in the range of 2–6, since Cu(OH)2
started to precipitate above pH of 6. Increase of pH from 2 to 5 leads
to the rise of adsorption efficiency from 20 to 94%, respectively [4], [5] and [6].
Fig. 3 shows influence of pH value on sorption efficiency of lignite.
Based on the results presented in Fig. 3,
it is obvious that the percentage of adsorbed copper ions suddenly
increases with rising of pH reaching the highest value at pH of 4–5. It
can be explained by observation that the increase of pH value induces
replacement of hydrogen ions from the surface of the lignite with the
copper ions resulting in improvement of the adsorption effeciency
extent.
Hydrogen
ions induce metal complexation because they have great affinity for
many complexing and ion exchange sites. At very low pH (< 2.0)
functional groups (hydroxyl, carboxyl, phenol, methoxyl, etc.) of the
coals are protonated. Equilibrium reaction of metal adsorption can be
considered as follows [6]:(1) Coal − COOH = Coal − COO- + H(aq)+(2) Cu(aq)+ 2 + 2 Coal − COO- = (Coal − COO)2Cu(3) Cu(aq)+ 2 + 2 OH- = Cu(OH)2
Due to high concentration of H+ ions for the pH lower than 2, equilibrium of the Reaction (1)
will be shifted to the left side according the equilibrium law. Since
sites of ion exchange on the lignite are mainly protonated, less
available groups for ion exchange become available. As expected, the
efficiency generally increases with increasing pH, while the effect of
pH is indistincitve or even reverse. The increase of the adsorption
effeciency is the most explicit for pH values between 2 and 4, probably
reflecting progressive deprotonation of carboxylic groups. Namely, in
mentioned pH range, the carboxyl groups (− COOH) from the lignite can
lose H+ and be appreciably deprotonated. That will shift the Reaction (2)
to the right, while the increase of the solution pH increases copper
ion removal. In this pH range, process of ion exchange is the major
mehanism for the removal of copper ions from the aqueous solution. As
already mentioned, hydrolysis Reaction (3) happened at pH ≥ 6 and copper hydroxide precipitation was occurred [6], [30] and [31]. It is obvious that optimum pH for Cu2 +
adsorption by lignite is 5.0, so the adsorption experiments regarding
influence of the adsorbent amount, initial concnentration and contact
time were performed with pH value of 5.
3.2.3. Effect of contact time on the copper removal
In
order to achieve the equilibrium state with maximally reduced
adsorption time, tests were carried out with greater amounts of coal
thus providing large number of available active adsorption sites on free
adsorbent surface.
Namely,
possible use of small amounts of coal that can provide good adsorption
efficiency but for more reasonable time was a goal of this testing. The
efficiency of copper adsorption from aqueous solutions with three
different initial concentrations (50, 200, and 330 mg Cu2 +/l) on three different amounts of lignite (30, 45, and 60 g) was observed depending on contact time up to 60 min, as shown in Fig. 4.
Based on the change in metal concentration in the aqueous solution
before and after achieving equilibrium adsorption, the adsorption
efficiency was calculated.
Copper removal from the aqueous solutions in the first 5 min has a significant practical value.
In
all cases, the majority of copper ions was removed at the beginning of
the adsorption proceess, during the first 5 min of contact time.
Besides, it is obvious that the adsorption occurred in 3 stages. The
first stage lasted for five minutes. In this period, there was decrease
in Cu2 + ions concentration of 65–92% depending on the solution concentration and the adsorbent amount. The adsorption of Cu2 + in this stage happened so quickly. During the second stage within the next 15 min removing of Cu2 +
was more than 90% for all solution concentrations and adsorbent amounts
while the adsorption happened much slowly. It is very important to
determine the equilibrium time for each type of lignite used as an
adsorbent material. Equilibrium time, that is the contact time
characterized by unchanging Cu2 + concentration in the
solution, was achieved after 30 min for all used concentrations of
solutions and amounts of adsorbent; this period is denoted as the third
stage of the adsorption.
High
adsorption rate at the beginning of the adsorption process is due to
the numerous readily available active adsorbing sites on the adsorbent
surface; that is the large uncovered surface area of lignite which was
provided by high amount of lignite while the copper ions can interact
easily [6].
Additionally, the driving force for the adsorption is the difference
between concentration of copper in the solution and solid/liquid
interface which has the highest value at the beginning of the process,
resulting in fast adsorption. Lower slopes of the curves confirm that
the second stage was a bit lower due to lower diffusion velocity of
copper ions within the pores of the lignite structure. It can be
observed that the best adsorption efficiency (> 90%) was achieved in
the case of initial solution with the lowest copper ions concentration
(50 mg Cu2 +/l) for all adsorbent amounts. In addition, the
smallest difference in adsorption efficiency of three initial solutions
concentration was observed by using 60 g of lignite.
Sorption
efficiency of lignite and activated carbon in case of low initial
concentration depending on contact time is presented in Fig. 5.
It is obvious that activated carbon shows better adsorption efficiency
than lignite for longer contact time, but these differences are
insignificant. Namely, it should be emphasized that used lignite is low
cost raw material and plentiful in Serbia while activated carbon is more
expensive due to pretreatment.
3.2.4. Effect of initial concentration on copper removal and adsorption isotherm
Effect of the initial solution concentration (50, 200, and 330 mg Cu2 +/l) on the copper removal was observed and the obtained results are shown in Fig. 6.
Different amounts of the adsorbent were used (30, 45, and 60 g). It is
obvious that for all lignite amounts, the adsorption of Cu2 + and, therefore, adsorption efficiency decreases with the increase of the initial solution concentration.
It
was found that the equilibrium time was around 30 min, so the results
of the initial solution concentration influence on copper removal are
shown for that period. Effect of the initial solution concentration on
copper adsorption was investigated for the following conditions: V = 250
ml, pH = 5, t = 20 °C.
For all used initial solutions, the amount of adsorbed Cu2 +
ions decreases with the increase of the concentration. This is
especially emphasized in case of the adsorbent mass of 30 g, where the
adsorption efficiency decreases from 94% to 86% for the initial
concentrations of 50 mg/l and 330 mg/l, respectively. Based on the
results presented in Fig. 6,
it is obvious that lignite is an effective adsorbent material for
copper removal from the aqueous solutions, especially in case of low
solution concentration; the adsorption efficiency is 94–97% from the
initial solution with concentration of 50 mg Cu2 +/l.
In
solutions with low concentration (regardless the adsorbent mass), the
ratio of surface active sites (funcional groups) on lignite to the total
copper ions in solution is high and hence all metal ions may interact
with the adsorbent and be removed from the solution. Since the driving
force that presents concentration gradient is stronger in case of high
concentrations, adsorbed amount of Cu2 + per unit of absorbent mass will be higher, Fig. 7.
It
is obvious that the influence of lignite mass on adsorption efficiency
is visible but not crucial particularly in the cases of low
concentrations. The influence of adsorbent mass increases with
concentration rising; this is evident in case of initial solution
concentration of 330 mg Cu2 +/l.
Isotherm of copper adsorption by lignite is shown in Fig. 8. Initial experimental conditions were: lignite amount of 30 g, pH = 5.0 and contact time of 30 min.
Adsorption
isotherm used for describing results of copper adsorption on lignite as
an adsorbent is presented using the equation of Langmuir, Fig. 6. Results for copper adsorption fit excellent to a Langmuirian type isotherm expressed by the following equation [2] and [4]:(1) 
where q is the amount of metal ion absorbed per unit mass of lignite (mg/g), Ce is the equilibrium copper concentration (mg/l), qmax is the maximum adsorption capacity (mg/g), b is constant related to adsorption intensity (l/mg) [2] and [4].
where q is the amount of metal ion absorbed per unit mass of lignite (mg/g), Ce is the equilibrium copper concentration (mg/l), qmax is the maximum adsorption capacity (mg/g), b is constant related to adsorption intensity (l/mg) [2] and [4].
The
Lagnmuir model is probably the best-known and most widely applied
adsorption isotherm, since it shows good agreement with a wide variety
of experimental data. It should be emphasized that Langmuir isotherm can
be applied to the adsorption on completely homogenous surface with
negligible interaction between adsorbed molecules. Regardless the basic
assumption that this model can not be applied for heterogeneous
adsorbent surface, it was quite successful in predicting the
experimental saturation capacity of the applied adsorbent [6].
Comparative adsorption isotherms of activated carbon and lignite are shown in Fig. 9. It is evident that lignite shows better adsorption properties than activated carbon.
As seen in Fig. 1.,
FTIR spectra, lignite has more specific adsorption bands than activated
carbon. On the other side, specific surface area of activated carbon is
higher as well as number of micro and meso pores. It can be concluded
that influence of oxygen functional groups is dominant for explaining
adsorption behavior of lignite. Also, results of FTIR functionality
analysis suggests that chemical adsorption plays important role for high
adsorption efficiency of lignite.
Experimental
data of the present work were excellent fitted to the Langmuir equation
since the regression analysis gave high correlation coefficients R2 > 0.9, as shown in Table 2.
The maximum adsorption capacities calculated by the Langmuir equation
were 4.045, 3.908 and 2.625 mg/g for lignite masses of 30, 45 and 60 g,
respectively (Table 2).
Table 2. Langmuir parameters for copper adsorption on lignite.
| R2 | qm, mg/g | K, l/mg | |
|---|---|---|---|
| Lignite | |||
| 30 g | 0.9228 | 4.045 | 0.038 |
| 45 g | 0.9191 | 3.908 | 0.025 |
| 60 g | 0.9740 | 2.625 | 0.056 |
| AC | 0.9497 | 3.229 | 0.026 |
The shape of all the isotherms is of ”L1” type according to Giles classifications for isotherms [4] and [32] which indicated that the curves do not reach any plateau (the adsorbent does not show clearly a limited adsorption capacity) [4] and [33].
„L“ or Langmuir isotherm type is usually associated with ionic
substrates, like metal cations, adsorption with weak competition from
the solvent molecules [32] and [34].
The
obtained adsorption capacities are not in accordance with those that
are usually reported in the literature because of high used masses of
the adsorbent. These results indicate that the saturation of lignite by
copper ions was not achieved regardless the initial concentration of
solution. It is obvious that the increasing amounts of lignite from 30 g
to 60 g for the same initial solution concentration lead to decrease of
the adsorption capacity value. Numerous free, available, and active
sites on the adsorbent surface prove the assumption that much smaller
lignite amount than 30 g can give good results in means of removing
copper from the aqueous solution.
4. Conclusion
Because
of plentiful amounts of lignite in Serbia and rationalization of the
adsorption process, the idea of this research was to avoid pre-treatment
and use adsorbent in its raw form suitable for application in thermal
power plant.
This research shown that lignite from Kolubara deposit is highly effective, inexpensive and naturally available adsorbent for Cu2 +
removal from aqueous solutions because of the environmental protection.
In order to support explanation of lignite adsorption efficiency, FTIR
analysis and determination of specific surface area were done. Also, all
results were compared with the activated carbon because it is the most
common adsorbent in wastewater treatment.
The effects of pH, initial solution concentration, the adsorbent amount, and contact time on adsorption process of Cu2 + on lignite were followed.
In
a metal-lignite system, interaction process happened in the interval of
pH = 2,0–5.0. The maximum adsorption is achieved if the pH solution is
around 5.0. It was found that the rate of adsorption is very high at the
beginning of the process in case of low copper concentration in the
initial solution. In the first 5 min, it reaches 90% due to higher
amount of the adsorbent than it is usual. Sufficient contact time for
both adsorbents is 30 min, since they achieve equilibrium for the
mentioned time. Optimal parameters presented in this paper were: pH =
5.0, C0 = 50 mg Cu2 +/l, τ = 30 min, the adsorbent
amount of 30 g. The calculated adsorption capacities are not in
accordance with literaturely available values because high adsorbent
masses were used. As the surface area of the lignite was significantly
lower than that of the activated carbons, it can be assumed that the
adsorption capacity of the lignite was augmented by chemisorption.
Isothermal
tests show that the adsorption data agrees well with Langmuir isotherm
model. The obtained maximum adsorption capacities were 4.045, 3.908 and
2.625 mg/g for lignite masses of 30, 45 and 60 g, respectively.
It
can be concluded that in order to achieve better efficiency and
economy, further investigation should be directed to the usage of much
smaller amounts of lignite that can provide complete saturation by
copper ions to the maximum utilization of adsorbent in wastewater
treatment.. Most important advantages of lignite as a potential
industrial absorbent compared to activated carbon are that no
pre-treatment is required, its low cost, high adsorption capacity, and
plentiful resources.
Acknowledgments
This research has been financed by the Ministry of Science and Technological Development of Republic of Serbia as a part of the project TR 33007. The authors would like to express their gratitude for this support.
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