Evaluation of different isotherm models, kinetic, thermodynamic, and copper biosorption efficiency of Lobaria pulmonaria (L.) Hoffm.

Abstract

The biosorption characteristics of Cu(II) ions from aqueous solution using Lobaria pulmonaria (L.) Hoffm. biomass were investigated. The biosorption efficiency of Cu(II) onto biomass was significantly influenced by the operating parameters. The maximum biosorption efficiency of L. pulmonaria was 65.3% at 10 mg/L initial metal concentration for 5 g/L lichen biomass dosage. The biosorption of Cu(II) ions onto biomass fits the Langmuir isotherm model and the pseudo-second-order kinetic model well. The thermodynamic parameters indicate the feasibility and exothermic and spontaneous nature of the biosorption. The effective desorption achieved with HCl was 96%. Information on the nature of possible interactions between the functional groups of the L. pulmonaria biomass and Cu(II) ions was obtained via Fourier transform infrared (FTIR) spectroscopy. The results indicated that the carboxyl (–COOH) and hydroxyl (–OH) groups of the biomass were mainly involved in the biosorption of Cu(II) onto L. pulmonaria biomass. The L. pulmonaria is a promising biosorbent for Cu(II) ions because of its availability, low cost, and high metal biosorption and desorption capacities.

Implications: Lobaria pulmonaria is a promising biosorbent for Cu(II) ions because of its availability, low cost, and high metal biosorption and desorption capacities. To the best of our knowledge, this is the first paper on the biosorption Cu by L. pulmonaria.

Introduction

The removal and recovery of heavy metals from wastewater are important for environmental protection and human health. Several methods have been used to remove heavy metals in high concentrations from aqueous solutions, including precipitation, phytoextraction, ultrafiltration, reverse osmosis, electrodialysis ion exchange, activated carbon sorption, and membrane technology.

Heavy metal usually comes from compounds that can be toxic, carcinogenic, or mutagenic, even in very low concentrations (Ruiz-Manriquez et al., 1997). Conventional methods of removing metals from wastewaters are generally expensive and have many limitations (Volesky, 1990). Alternative methods of metal removal and recovery based on biological materials have been considered (Raras, 1995).

Biosorption of heavy metals from aqueous solution can be considered as an alternative technology in industrial wastewater treatment. Adsorbent metarials (biosorbents) derived from suitable biomass can be used for the effective removal and recovery of heavy metal ions from wastewater streams. The main advantage of biosorption technology is its effectiveness in reducing the concentration of inexpensive biosorbent materials (Feng and Aldrich, 2004).

The uptake of heavy metal ions by bacteria, fungi, lichens, etc., is classified into three categories: cell surface binding, intercellular accumulation, and extracellular accumulation (Rich and Cherry, 1987). In the biosorption process, a physicochemical reaction occurs between metal species and cell components of live and dead cell material (Javed et al., 2007). Bioaccumulation is the preferred term when living organisms are used. The uptake of metal ions by dead cells also can be defined as biosorption. Several investigators have reported the potential use of living and dead microbial biomass or plants to adsorb heavy metal ions from aqueous solutions (Crist et al., 1981; Rich and Cherry, 1987). The nonliving biomass of bacteria (Crist et al., 1981), fungi (Jeon et al., 2001; Bulut and Tez, 2007), yeast (Yan and Viraraghavan, 2003; Prasher et al., 2004), algae (Göksungur et al., 2005; Han et al., 2005), lichens (Ekmekyapar et al., 2006; Uluözlü et al., 2008), and plant material (Akcin et al., 2001; Hanif et al., 2007) has been reported as a method of effective and economical removal of a variety of toxic heavy metals from wastewater.

Lichens are dual organisms, an association between an alga and a fungus (Antonelli et al., 1998; Lin et al., 2003). Lichens show remarkable differences with respect to their sensitivity to heavy metals. Some species are highly tolerant to high concentrations of transition metals, including Cu (Purvis and Halls, 1996), Fe (Hauck et al., 2007), and Mn (Hauck and Paul, 2005). Cu- and Fe-tolerant lichens include hyperaccumulors inhabiting metal-rich rock and slag (Lange and Ziegler, 1963; Pawlik-Skowrońska et al., 2006).

Heavy metals have become one of the most important environmental pollutants in the world. Among different organic and inorganic pollutants, heavy metal ions are serious poisons capable of being assimilated and stored in the tissues of living organisms, causing noticeable health problems (Gupta and Ali, 2004). Unlike organic pollutants, heavy metals are nonbiodegradable and therefore their removal is extremely important in terms of health of living specimens. In 1978, the U.S. Environmental Protection Agency prepared a list of organic and inorganic pollutants that are found in wastewater and constitute serious health hazards. The 13 metals found in this list are antimony, arsenic, beryllium, cadmium, chromium, copper, lead, mercury, nickel, selenium, silver, thallium, and zinc. Growing attention is being given to the potential health and environmental hazards presented by heavy metals (Sari et al., 2010).

Biosorption is one of the most suitable technologies in recent years. Biosorption has a potential marketing advantage over other traditional wastewater treatment technologies because it is cheaper and environmentally friendly. The main advantages of this technique are the reusability of biomaterial, low operating cost, improved selectivity for specific metals of interest, removal of heavy metals from effluent irrespective of toxicity, short operation time, and no production of secondary compounds that might be toxic (Gupta and Rastogi 2006, 2008, Gupta and Bhattacharyya, 2006).

Alternative methods of metal removal and recovery based on biological materials have been considered (Raras, 1995). Recently, the removal of metal ions by low-cost and eco-friendly biosorption processes utilizing lichen biomass has been explored (Ekmekyapar et al., 2006; Uluözlü et al., 2008, 2010; Bingöl et al., 2009; Tüzen et al., 2009). Lichen thallus is particularly favored because lichens do not have root systems, nor do they have waxy cuticles, and thus they are strongly dependent on wet and dry deposition for their mineral nutrients (Bermudez et al., 2009), especially epiphytic species. They have a high surface–volume ratio and ion exchange properties; unlike many other plants, they lack variability in morphology throughout the growing season and they have no cuticle. Lichens are widely used as biomonitors, demonstrating an exceptional capability to accumulate trace metals (Nash, 1989; Bargagli, 1998; Conti and Cecchetti, 2001; Aslan et al., 2004, 2006, 2010, 2011, Cansaran-Duman, 2011; Cansaran-Duman et al., 2009, 2011, 2012).

In this research, the adsorbsion properties of lichen biomass Lobaria pulmonaria (L.) Hoffm. for copper (II) were investigated using a batch adsorption method. The effect of initial metal ion concentration, initial pH, biosorbent concentration, stirring speed, and contact time on biosorption efficiency were studied. To the best of our knowledge, this is the first paper on the biosorbence of Cu by L. pulmonaria. This material was chosen as a biosorbent in this study because it is natural and easily available and thus is a low-cost biomass for dissolved metal ions.

Materials and Methods

Biosorbent preparation

The lichen biomass (Lobaria pulmonaria (L.) Hoffm.) was used as a biosorbent for the biosorption of copper (II) ions. Specimens of L. pulmonaria were collected from the Yenice Forest. The study area is located between 44°62′18″ N and 45°73′56″ E in the western part of the Black Sea region and belongs to the Yenice district in the province of Karabük (44°62′18″ N, and 45°73′56″, Anatolia, Turkey, collection and determination of lichen species was made by D. Cansaran-Duman), approximately 400 m above sea level. Lichen samples were collected in July 2005 and all samples were stored at the University of Ankara Herbarium.

Preparation of stock solution

The stock solution of Cu(II) (1,073 g/L) was prepared by dissolving a weighed quantity of CuCl₂.2H₂O in deionized water. The required concentration was prepared from the stock solution by dilution.

Fourier transform infrared spectroscopy

The surface functional groups on the biosorbent material were identified using Fourier transform infrared (FTIR) spectroscopy. About 1 mg L. pulmonaria was ground with about 100 mg of finely divided spectroscopic-grade KBr powder. The mixture was placed in a die and pressed under high pressure. The obtained disks were placed in the FTIR spectrometer and FTIR spectra were recorded in the region of 400–4000 cm⁻¹.

Biosorption studies

Dried lichen was added to different concentrations (5–100 mg/L) of a 50-mL copper (II) solution. The pH of each solution was adjusted to the required value (2.0, 3.0, 4.0, 5.0, 6.0) using 0.1 N HCl and ammonia solutions. The biosorbent concentrations and stirring speeds varied from 1 to 50 g/L and 50 to 250 rpm, respectively. The mixture was stirred in a shaker at a constant speed for 60 min at 20°C. The samples were then collected at certain time intervals, filtered to remove the suspended biomass, and analyzed for residual copper (II). The Cu(II) concentration in the supernatant solution was determined using a flame atomic absorption spectrophotometer (GBC Avanta Ver. 2.02, Scientific Equipment, USA) at 324 nm. All experiments were performed in a batch stirred system.

Desorption studies

L. pulmonaria (L.) Hoffm.was added to a 10-mg/L Cu(II) solution shaken for 60 min at 150 rpm. The system was filtered via blue band filter paper and the Cu(II)-loaded L. pulmonaria was washed with fresh distilled water. Keeping the solid-to-liquid ratio at 5 g/L, a new mixture was prepared and shaken on a rotary shaker for 15 min at 150 rpm. Different desorbents including HCl (0.1 M), HNO₃ (0.1 M), EDTA (0.5 M), NaCl (0.1 M), and H₂O were used in the desorption studies. The reaction mixture was then filtered and the supernatant was used to determine the Cu(II) concentrations after desorption.

The same procedure was applied to the blank solution. The desorbed Cu(II) ions from L. pulmonaria were determined using flame atomic absorption spectrophotomety at 324 nm.

Results and Discussion

FTIR analysis

Information on the nature of possible interactions between the functional groups of the L. pulmonaria (L.) Hoffm. biomass and Cu(II) ions was obtained via FTIR spectroscopy. The FTIR spectra of dried unloaded biomass and Cu(II)-loaded biomass were obtained (Figures 1a and 1b). The broad and strong bands at 3308–3343cm⁻¹ were due to bound hydroxyl (–OH) or amine (-NH) groups. The peaks at 1644–1645 cm⁻¹ were attributed to the stretching vibration of the carboxyl group (-C=O). The bands observed at 1038 cm⁻¹ were assigned to CO stretching of alcohols and carboxylic acids. Peaks of the C–H group were observed at 2929 cm⁻¹. The stretching vibration of the OH group shifted from 3308 to 3343 cm⁻¹ for Cu(II)-loaded biomass. These results revealed that the chemical interactions between the metal ions and the hydroxyl groups occur on the biomass surface. The peaks at 1644 and 1428 cm⁻¹ were attributed to the stretching vibration of the carboxyl group (-C=O). The carboxyl peaks at 1644 and 1428 cm⁻¹ were shifted to 1645 and 1448 cm⁻¹ for the Cu(II)-loaded biomass. The results indicated that the carboxyl (–COOH) and hydroxyl (–OH) groups of the biomass were mainly involved in the biosorption of Cu(II) onto L. pulmonaria biomass. The peaks observed at 2929 cm⁻¹ did not change after biosorption of Cu(II), showing that the C-H group did not participate in the biosorption of Cu(II). Similar FTIR results were reported for Cu(II) biosorption onto Botrytis cinerea fungal biomass (Akar and Tunali, 2005).

Figure 1. FTIR spectra of Lobaria pulmonaria biomass.

Effect of pH on Cu(II) biosorption

The acidity of the solution is one of the important factors affecting the biosorption of metals. The effect of pH on the biosorption of Cu(II) ions onto L. pulmonaria (L.) Hoffm. biomass was studied by lichen biomass with Cu(II) solution at various pH values ranging from 2.0 to 6.0. Cu(II) removal sharply increased from 14.08% at pH 2.0 to 61.76% at pH 5.0 (Figure 2a). The change in biosorption levels with pH can be explained by the availability of charged groups at the biosorbent surface, which is necessary for the biosorption of metals to proceed.

Figure 2. Effect of pH on biosorption efficiency (a) and initial Cu(II) concentration on Cu(II) uptake (q_e = mg Cu (II)/g biomass) and biosorption efficiency (b). C_o  = 10 mg/L, T = 20°C, m = 5 g/L, stirring speed = 150 rpm, contact time (t) = 60 min.

The cell wall of L. pulmonaria becomes positively charged at lower pH values, due to an increase in hydrogen ion concentration responsible for the reduction in biosorption capacity. In contrast, at higher pH values the biosorption capacity of Cu(II) is high due to the attraction between the biomass and metal, because the cell wall surface is more negatively charged. A further increase in pH resulted in precipitation of insoluble copper hydroxides.

The competition between metal ions and hydrogen ions for the active sites on the biosorbent surface is directly related to the acidity of the solution (Lodeiro et al., 2006). Metal binding onto biomass during biosorption involves complex mechanisms, such as ion exchange, chelation, adsorption by physical forces, and ion entrapment in inter- and intrafibrillar capillaries and spaces of the cell structural network of a biosorbent (Chojnacka et al., 2005; Sari and Tuzen, 2009). The FTIR spectroscopic analysis showed that the L. pulmonaria has carboxyl, hydroxyl, and amine functional groups, which tend to participate in metal ion bindings depending on the pH value of the solution.

Effect of initial Cu(II) concentration on biosorption

Biosorption experiments were carried out at different initial copper (II) concentrations ranging from 5 to 100 mg/L. The effect of initial metal concentration on the biosorption capacity of lichen biomass is presented in Figure 2b. The biosorption efficiency decreased with the increasing initial concentration of metal ions. The amount of Cu(II) adsorbed on the biomass (mg Cu(II)/g biomass) increased with an increase in the initial concentration of Cu(II). These results can be explained by the increase in the number of ions competing for the available binding sites on the biomass at higher concentrations.

Effect of biosorbent dosage on biosorption

Copper (II) biosorption on lichen biomass was studied at various biosorbent concentrations (1 to 50 g/L). The experimental results are presented in Figure 3a. The percentage removal of Cu(II) increased with increasing adsorption surface area but a further increase in biosorbent concentration did not change the removal efficiency. Partial aggregation occurred at higher biomass dosage, giving rise to a loss of available surface area for metal ion uptake (Uluözlü et al., 2008). Copper (II) removal efficiency was at its maximum at 5 g/L of biosorbent.

Figure 3. Effect of biosorbent concentration (a) and stirring speed (b). C_o  = 10 mg/L, pH 5.0, T = 20°C, stirring speed = 150 rpm, t = 60 min.

Effect of stirring speed on biosorption

Biosorption studies were carried out with a shaker at pH 5.0 and 10 mg/L initial Cu(II) ion concentration. The stirring speed varied from 50 to 250 rpm. The maximum Cu(II) removal efficiency was obtained at 150 rpm. The increase in stirring speed from 50 to 150 rpm resulted in an increase in Cu(II) ion removal efficiency due to an increase in mass transfer rate with an increase in stirring speed. The decrease in the boundry layer thickness with increased stirring speeds resulted in a reduction of the surface film resistance (Benefield et al., 1982). This led to quicker and easier adsorption of metal ions onto the biosorbent surface. A further increase in stirring speed did not show an increase in the biosorption efficiency (Figure 3b).

Effect of biosorption time and temperature on biosorption

The biosorption time is an important parameter for the practical application of biosorbents. A mixture of the metal solution and dried lichen was continuously stirred in a shaker at 150 rpm for 52 hr and the effect of biosorption time on biosorption was determined. The metal uptake by dried lichen was fast and equilibrium was reached within 60 min. Biosorption continued with a small increase in removal of metal ions after the equilibrium time, as shown in Figure 4a. The increase in residual concentration of Cu ions in the solution with time suggests that physical adsorption is effective in the bonding mechanism of Cu(II) ions on lichen biomass. Adsorption is generally called physical adsorption and occurs as a result of van der Waals forces, which are usually predominant at low temperatures, characterized by a relatively low energy of adsorption, and the adsorbed molecules are not attached to a specific site at the surface. Thus, the experimental period was determined as 60 min.

Figure 4. Effect of contact time (a) and temperature (b) on biosorption. Initial metal concentration (C_o ) = 10 mg/L, pH 5.0, temperature (T) = 20°C, biosorbent concentration (m) = 5 g/L, stirring speed = 150 rpm.

The removal efficiency of Cu(II) from aqueous solution is also affected by the temperature of the medium. The temperature dependency of Cu(II) biosorption was determined at varying temperatures (20–50°C). The biosorption efficiency of Cu(II) decreased from 78.73 to 76.8% as the temperature increased from 20 to 50°C over 60 min (Figure 4b). The results indicated the exothermic nature of Cu(II) biosorption onto L. pulmonaria(L.) Hoffm. biomass. The decrease in biosorption of Cu(II) with increasing temperature may be due to an increase in the desorption of Cu(II) from the interface to the solution (Ekmekyapar et al., 2006; Sari et al., 2007; Ucun et al., 2009). The optimum temperature was selected as 20°C for further biosorption experiments.

Biosorption isotherm models

Adsorption isotherms that describe the nature of the interaction between sorbate and biosorbent are important in optimizing the utilization of biosorbents. Thus, analysis of equilibrium data is useful for practical design and operation of adsorption systems. Two equilibrium models, namely, the Langmuir and Freundlich isotherm models, were used to examine the biosorption mechanism and surface properties of the biomass (Freundlich, 1906; Langmuir, 1918).

The Langmuir isotherm, which indicates a reduction of the available specific homogeneous interaction sites within the sorbent with increasing metal ion concentration, is a well-known model for the biosorption of a solute from an aqueous solution. The monolayer type of adsorption is essentially described by the Langmuir model, represented by the following equation:

(1)

where q _m is the maximum Cu(II) uptake of dried L. pulmonaria (L.) Hoffm. to form a complete monolayer on the surface at high metal concentration. C_e (mg/g) is the metal concentration at equilibrium and K _L is the Langmuir equilibrium constant with the free energy of the reaction (L/mg or mM⁻¹). q_e is the biosorption capacity at equilibrium (mg/g). The linearized form of eq 1 is

(2)

A plot of 1/q_e against 1/C_e gives a straight line with a slope of 1/q_m (mg/g) K _L (L/mg) and an intercept of 1/q_m . The dimensionless biosorption intensity (R _L) is defined by

(3)

where C_o is the initial concentration in the solution (mg/L). For favorable adsorption, 0 < R _L < 1, and R _L >1, R _L = 1, and R _L = 0 describe unfavorable, linear, and irreversible adsorption, respectively (Gupta et al., 2006).

The Freundlich model, which assumes a heterogeneous adsorption surface and active sites with different energies, is given below:

(4)

where n represents the Freundlich constant (dimensionless) related to the adsorption intensity, which varies with the heterogeneity of the material. K_f is the Freundlich adsorption constant related to the adsorption capacity. The linearized form of eq 4 is given by eq 5:

(5)

Copper (II) adsorption isotherms for L. pulmonaria are presented in Figure 5. Adsorption isotherm constants for Cu(II) ion biosorption are indicated in Table 1.

Figure 5. Langmuir isotherm (a) and Freundlich isotherm (b) plots for the biosorption of Cu(II) onto Lobaria pulmonaria biomass. Biomass dosage = 5 g/L, contact time = 60 min, pH 5, stirring speed = 150 rpm, temperature = 20°C.

Table 1. Adsorption isotherm constants for biosorption of Cu(II) onto Lobaria pulmonaria biomass

Langmuir parameters			Freundlich parameters
qm (mg/g)	K L	R ^2	1/n	Kf	R ^2
12.12	0.0341	0.9993	0.66	0.6	0.9917

The Langmuir isotherm fits the experimental data very well because it presents higher linear regression coefficients (R ) with a value of 0.9993. The obtained q_m value was higher than the experimental q_e value. The biosorption of Cu(II) ions onto L. pulmonaria, which is regarded as monolayer biosorption, takes place at the active sites on the surface of the biomass. The binding energy on the whole surface of the L. pulmonaria was uniform. The R _L value was calculated as 0.745, which falls between 0 and 1, suggesting favorable adsorption. The values of K_f and 1/n were found to be 0.6 and 0.66, respectively, for Cu(II) biosorption. The 1/n values were between 0 and 1, indicating that the biosorption of Cu(II) onto L. pulmonaria was favorable under the experimental conditions. The R ² value was found to be 0.9898, indicating that the Freundlich model did not adequately describe the relationship between the amount of Cu(II) adsorbed by the biomass and its equilibrium concentration in the solution. Thus, the Langmuir adsorption isotherm showed a better fit to adsorption data with respect to the Freundlich isotherm. The highest biosorption capacity was found to be 12.12 mg/g. A comparison of copper biosorption capacities for various adsorbents was reported in the literature (Aksu and Isoğlu, 2005; Zhu et al., 2009). The direct comparison of adsorbent capacity was due to the varying experimental conditions. The biosorption capacity of L. pulmonaria is actually comparable to many other biomass products. The study indicates that the tested L. pulmonaria is a potential biosorbent for the removal of Cu(II) ions from aqueous solutions.

Kinetics of biosorption

Adsorption kinetics explain the solute uptake rate, which in turn controls the equilibrium time (Grimm et al., 2008). Biosorption experiments were carryied out at four temperatures, which depended on the amount captured with time. Lagergren's pseudo-first-order and pseudo-second-order models were applied to the experimental data in order to clarify the biosorption kinetics of Cu(II) ions onto L. pulmonaria (L.) Hoffm. biomass (Lagergren, 1898; Ho and McKay, 2000). The linearized form of the pseudo-first-order rate equation given by Lagergren is as follows:

(6)

where q and q_e are the solid-phase copper ion concentrations at any time (t) and at equilibrium (mg/g), respectively, and k ₁ is the pseudo-first-order adsorption rate constant. The values of k ₁ can be determined from the slope of the linear plot of ln(q_e  − q) vs. t.

The pseudo-second-order kinetic model can be presented by the linear equation:

(7)

where k ₂ (g/mg min) is the rate constant of the second-order equation, q_t (mg/g) is the amount of biosorption time t (min), and q_e is the amount of biosorption equilibrium (mg/g).

The rate constant k ₁ was calculated from the slope by plotting ln (q_e  − q_t ) versus t (Table 2). The correlation coefficients obtained using the Lagergren model were relatively low. Moreover, the experimental q_e values did not corroborate the calculated data. This suggests that the adsorption of Cu(II) onto L. pulmonaria cannot be considered as a pseudo-first-order kinetic.

Table 2. Kinetic parameters obtained from pseudo-first-order and pseudo-second-order models for biosorption of Cu(II) onto Lobaria pulmonaria biomass at different temperatures

	Pseudo-first-order			Pseudo-second-order
Temperature (°C)	k 1 (1/min)	qe (mg/g)	R ^2	k 2 (g/mg min)	qe (mg/g)	R ^2
20	7.5 × 10^−3	0.18	0.9595	2.16	2.13	0.9992
30	8.9 × 10^−3	0.24	0.9114	2.57	2.44	0.9856
40	6.9 × 10^−3	0.11	0.9454	1.60	2.09	0.9997
50	5.9 × 10^−3	0.08	0.9695	1.42	2.03	0.9989

The pseudo-second-order model was tested in order to understand the adsorption kinetics of Cu(II) onto L. pulmonaria biomass. Pseudo-second-order kinetic plots at different temperatures for biosorption of Cu (II) onto L. pulmonaria biomass are presented in Figure 6a. The rate at which the adsorption sites were covered was proportional to the square of the number of unoccupied sites, and the number of occupied sites was proportional to the metal ion adsorbed.

Figure 6. Pseudo-second-order kinetic plots at different temperatures (a) and plot of ln K _D vs. 1/T for the estimation of thermodynamic parameters (b) for biosorption of Cu(II) onto Lobaria pulmonaria.

The results show that the obtained correlation factors for the pseudo-first-order model were not very good, whereas the results obtained for the pseudo-second-order kinetic model were. The correlation factors of the pseudo-second-order kinetics model in the range of 0.9856 and 0.9997 show that the pseudo-second-order model accurately represents the experimental behavior. The pseudo-second-order model has been successfully applied to the description of biosorption of different metal ions by different bioadsorbents (S.S. Gupta and Bhattacharyya, 2006; Sari et al., 2007; Sari and Tuzen, 2009a, 2009b). The biosorption of Cu(II) ions onto L. pulmonaria biomass first the pseudo-second-order kinetic model well.

Biosorption thermodynamics

The thermodynamic parameters including the change in free energy (ΔG ^ˆ), enthalpy (ΔH ^ˆ), and entropy (ΔS ^ˆ) were calculated to describe thermodynamic behavior of Cu(II) ion biosorption onto L. pulmonaria (L.) Hoffm. biomass. The thermodynamic parameters were calculated using the following equations:

(8)

where R is the universal gas constant (8.314 J/(mol K)), T is the temperature (K), and K _D (q_e /C_e ) are the distribution coefficients (Aravindhan et al., 2007). The enthalpy (ΔH ^ˆ) and entropy (ΔS ^ˆ) parameters were estimated as follows:

(9)

The enthalpy (ΔH ^ˆ) and entropy (ΔS ^ˆ) can be calculated from the slope and intercept of the plot of ln K _D vs. 1/T, respectively.

The thermodynamic data obtained from biosorption of Cu(II) ions onto L. pulmonaria biomass are plotted in Figure 6b. The ΔG ^ˆ values were calculated as −5.042, −4.332, −3.2, and −2,658 kJ/mol for biosorption of Cu(II) ions onto L. pulmonaria biomass at 20, 30, 40, and 50°C, respectively. The negative values of ΔG indicated that the biosorption of Cu(II) ions onto L. pulmonaria biomass was spontaneous and feasible. The decrease in the ΔG values with a temperature increase showed a decrease in feasibility of biosorption at higher temperatures. The change in the enthalpy and entropy values are presented in Table 3. ΔH ^ˆ was found to be −29.03 J/mol for the biosorption of Cu(II) ions onto L. pulmonaria biomass. The negative ΔH ^ˆ value indicates the exothermic nature of the biosorption processes at 20–50°C. ΔS ^ˆ was found to be −81.80 J/mol K for the biosorption of Cu(II) ions onto L. pulmonaria biomass. The negative ΔS ^ˆ value is an indication of a decrease in the randomness at the solid–solution interface during the biosorption process.

Table 3. Thermodynamic parameters for the biosorption of Cu(II) onto Lobaria pulmonaria biomass

Metal	ΔH ^ˆ (J/mol)	ΔS ^ˆ (J/mol K)
Copper	−29.03	−81.80

Desorption

HCl, HNO₃ EDTA, NaCl, and H₂O were used to performed the desorption studies for Cu(II)-loaded L. pulmonaria (L.) Hoffm. As can be seen from the desorption results, the highest recovery of Cu(II) was determined with HCl and HNO₃ and EDTA after 15 min at room temperature (Figure 7). Desorption of 96, 84, 64, 12, and 5% was obtained with HCl, HNO₃ EDTA, NaCl, and H₂O, respectively. Desorption using distilled water (H₂O) and NaCl was almost negligible. Effective desorption was obtained with HCl.

Figure 7. Effect of various agents on desorption of Cu(II) from Lobaria pulmonaria (desorption time 15 min).

Conclusion

This study focused on the biosorption of Cu(II) onto L. pulmonaria (L.) Hoffm. biomass from aqueous solution. The maximum biosorption efficiency of biomass was 65.3% at 10 mg/L metal concentration for 5 g/L lichen dosage. The biosorption of Cu(II) ions onto biomass fit the Langmuir isotherm model and the pseudo-second-order kinetic model well. The thermodynamic parameters indicate the feasibility and exothermic and spontaneous nature of the biosorption. Effective desorption of 96% was achieved with HCl. L. pulmonaria is a promising biosorbent for Cu(II) ions with high metal biosorption and desorption capacities, availability, and low cost.