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Removal of copper from aqueous solutions by low cost adsorbent-Kolubara lignite

Sonja Milicevica, Tamara Boljanaca, Sanja MartinovicaCorresponding Author Contact InformationE-mail The Corresponding Author, Milica Vlahovica, Vladan Milosevica, Biljana Babicb
aInstitute for Technology of Nuclear and Other Mineral Raw Materials, 86 Franchet d'Esperey Blvd., Belgrade, Serbia
bUniversity of Belgrade, Institute of Nuclear Sciences “Vinca”, Belgrade, Serbia
Received 29 July 2011; revised 28 September 2011; Accepted 2 November 2011. Available online 26 November 2011.


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.


► 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

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)
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.

Fig. 1.
FTIR spectrum of lignite and AC.
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 Csingle bondH 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 Csingle bondOsingle bondH 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.

Fig. 2.
Influence of adsorbent mass on adsorption efficiency (conditions: C0 = 50, 200, 330 mg Cu2 +/l, pH = 5, contact time 30 min).
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.

Fig. 3.
Effect of solution pH on copper adsorption using 30 g of lignite and contact time of 30 min.
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.

Fig. 4.
Effect of contact time on copper adsorption for three different masses of lignite as adsorbent: a) 30 g; b) 45 g; and c) 60 g at pH = 5;.
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.

Fig. 5.
Comparison of contact time influence on copper adsorption for lignite and activated carbon; 30 g of adsorbent, 50 mg Cu2 + /l, pH = 5.5.
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.

Fig. 6.
Adsorption of copper ions on lignite for three different initial solution concentrations.
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.

Fig. 7.
Influence of initial solution concentration and lignite mass on adsorption amount of copper.
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.

Fig. 8.
Isotherms of copper adsorption by lignite using Langmuir model.
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)View the MathML sourcewhere 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.

Fig. 9.
Comparative isotherms of copper adsorption by activated carbon and lignite.
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.

R2qm, mg/gK, l/mg

30 g0.92284.0450.038
45 g0.91913.9080.025
60 g0.97402.6250.056
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.


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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