Lentinus concinnus biomass was immobilized to carboxyl derivative of cellulose, carboxymethyl cellulose (CMC), in the presence of FeCl3 (0.1 mol L−1) via ionic cross-linking. The beads containing immobilized fungal biomass were incubated at 30 °C for three days to permit growth of the fungus. The free and immobilized fungal biomass were tested for adsorption of Disperse Red 60 (DR-60) from aqueous solution using bare CMC beads as a control system. The maximum adsorption of DR-60 on the free and immobilized fungal biomass was observed at pH 6.0. The adsorption of DR-60 by the free, and immobilized fungal biomass increased as the initial concentration of DR-60 in the medium increased up to 100 mg/L. The maximum adsorption capacity of the CMC beads, the free and immobilized fungal biomass (i.e. composite beads) were found to be 43.4, 65.7, and 92.6 mg g−1 dry sorbents, respectively. The equilibrium of the adsorption system was well described by Langmuir and Temkin isotherm models. Adsorption equilibrium was established in about 1.0 h. The adsorption of DR-60 on the fungal preparations followed pseudo-second-order kinetic model. It was observed that the immobilized fungal biomass has a high potential for the removal of DR-60 as a model dye from aqueous solution.

INTRODUCTION

Textile industries consume large quantities of water and chemicals, which are specified as process water. The textile process waters containing dyes have caused adverse problems for aquatic plants by reducing the light transmission and photosynthetic activity (Bayramoglu & Arica 2007; Akar et al. 2013; Adnan et al. 2015). Moreover, dyes in the water cause much bigger ecological problems and threat the many living things with toxic, mutagenic and carcinogenic effects by accumulating in the aquatic organisms (Chaudhry et al. 2014; Chen et al. 2014; Din et al. 2016; Guo et al. (2014)). Reactive, disperse, and direct dyes are the most used dyes in the textile industries. The wastewaters of the textile industries create extensive pollution problems in terms of organic toxics, metals (dyes containing metal ions), and solvents (Hu 1996; Fu & Viraraghavan 2002; Bayramoglu & Arica (2005); Asnaoui et al. 2015; Ahmady-Asbchin 2016; Attallah et al. 2016; Bayramoglu et al. (2009); Chew et al. 2016). For dyes removal from wastewaters, many physical and chemical methods such as coagulation, adsorption, chemical oxidation and ozonation have been developed. Among them, adsorption systems have been studied in the past and are still on the focus. Many microorganisms (such as bacteria, algae and fungi) have been identified for their capability of removing various dyes and metal ions from aqueous solutions (Bayramoglu et al. 2006, 2016; Couto 2009; Garza-Gonzalez et al. 2011; Kaushik et al. 2014; Demierege et al. 2015; Devi et al. 2015). In particular, fungal biomasses are used for removing of different dyes via adsorption, biodegradation, mineralization, etc. (Kaushik & Malik 2009; Marco-Urrea et al. 2010; Espinosa-Ortiz et al. 2016; Si et al. 2016). Different fungal biomasses have been tested for the removal of various dyes and metal ions from aqueous solutions, such as Trametes trogii (Bayramoglu & Arica 2016), Trichoderma asperellum (Chew et al. 2016), Phanerochaete chrysosporium (Zhao et al. 2016), Aspergillus oryzae (Zhang et al. 2015), Lentinus polychrous (Wangpradit & Chitprasert 2014), etc.

In this study, carboxymethyl cellulose (CMC) was used as a natural polymeric material for immobilization of L. concinnus biomass, since it is not toxic to fungal cells and also biodegradable. The free and immobilized fungal biomasses were tested first time for removal of Disperse Red 60 (DR-60) from aqueous solution. The removal of the model dye was studied by varying experimental conditions such as solid/liquid ratio, ionic strength, initial dye concentration, temperature, and pH of the adsorption medium. Experimental data were evaluated using Langmuir, Freundlich and Temkin isotherm models. The experimental data were also analyzed using the first-, second-order and Bangham's kinetic models.

MATERIALS AND METHODS

Materials

DR-60 was obtained from Sigma-Aldrich Chemical Co., St Louis, MO, USA. The chemical structure and some properties of the DR-60 dye are presented in Table 1. CMC sodium salt (Na-CMC; high viscosity; 1.0% in H2O at 25 °C: 700–1,550 mPa; degree of substitution: 0.60–0.95) was also obtained from Sigma Chem. Co. All other chemicals were of analytical grade and purchased from Merck AG (Darmstadt, Germany).

Table 1

The general characteristics of DR-60

Name of dyes C.I. 60756; DR-60; Kayaset RB; Elbasol Red 3B; Allilon Red 2B 
Color index number 17418-58-5 
Chemical formula C20H13NO4
 
Chemistry 1-Amino-4-hydroxy-2-phenoxy-9, 10-anthraquinine 
Molecular weight (g/mol) 331.32 
Chemical/Dye class Anthraquinine 
λmax (nm) 502 nm 
Name of dyes C.I. 60756; DR-60; Kayaset RB; Elbasol Red 3B; Allilon Red 2B 
Color index number 17418-58-5 
Chemical formula C20H13NO4
 
Chemistry 1-Amino-4-hydroxy-2-phenoxy-9, 10-anthraquinine 
Molecular weight (g/mol) 331.32 
Chemical/Dye class Anthraquinine 
λmax (nm) 502 nm 

Cultivation of Lentinus concinnus

The white rot fungus Lentinus concinnus strain MAFF 430305 was obtained from the National Institute of Agrobiological Sciences, Tsukuba, Ibaraki, Japan. Briefly, the white-rot fungus was cultivated in Sabouraud dextrose liquid medium using the shake flask method. The growth medium consisted of (g L−1 of distilled water); D-glucose (10.0); KH2PO4 (20.0); MgSO4·7H2O (0.5); NH4Cl (0.1); CaCl2·H2O (0.1); thiamine (0.001). The pH of the medium was adjusted to 4.5 before autoclaving. Once inoculated, flasks were incubated on an orbital shaker at 150 rpm for seven days at 30 °C. After incubation, the biomass was harvested from the medium by filtration and washed with distilled water.

Immobilization of L. concinnus biomass in CMC beads

L. concinnus biomass was immobilized in CMC via entrapment. The CMC solution was prepared in distilled water (2.0%, 50 mL), and then mixed with the fungal biomass (2.0 g in 50 mL growth medium). This mixture was introduced into a solution containing 0.1 mol L−1 FeCl3 through a nozzle (2.0 cm length, 1.0 mm i.d.) by means a peristaltic pump. Trivalent ferric chloride ions were used for ionic crosslinking of functional carboxyl groups of the CMC molecules. The solution was stirred to prevent aggregation of the CMC bead with each other. The fungal biomass immobilized beads (∼2 mm diameter) were cured in this solution for 15 min, and then washed with 50 mL sterile distilled water. The CMC beads with immobilized fungal biomass were then transferred to the growth medium (50 mL) in 250 mL flask, and incubated on an orbital shaker (150 rpm) at 30 °C for three days. The growth of mycelia on the CMC beads was followed during the incubation period using a microscope. After three days incubation period, the CMC beads with immobilized fungal biomass were removed from the medium by filtration and washed twice with distilled water. The immobilized fungal preparation was stored at 4 °C until use. The dry weight of the microbial growth on the CMC beads was determined by weighing (after drying in an oven at 50 °C overnight) before and after the growth period.

Adsorption studies

Effect of adsorbent dose, initial concentration of DR-60 dye, initial pH value of the solution and contact time in the adsorption experiments was investigated in a batch system. A stock solution (1,000 mg L−1) was prepared by dissolving the dye in the purified water. Adsorption experiments were carried out by mixing a certain dose of adsorbent in 50 mL of dye solution (100 mg L−1) at 25 °C and at 150 rpm for 2.0 h. The pH value of the solution was adjusted with 0.1 mol/L HCl or 0.1 mol/L NaOH solutions and a pH meter was used to obtain individual solutions with pH value ranging from 3.0 to 8.0. The effect of initial concentration of the dye on the adsorption rate and capacity was studied by varying the concentration of the dye between 10 and 200 mg L−1. The effect of temperature and ionic strength was studied at four different temperatures (i.e. 15, 25, 35 and 45 °C) and at four different NaCl concentrations (i.e. 0.25, 0.5, 0.75, and 1.0 mol L−1) at pH 6.0, respectively. All experiments were conducted in duplicates with 50 mg fungal preparations and initial concentration of the dye was 100 mg L−1 in each set experiments. The amount of adsorbed dye per unit CMC beads or fungal preparations (mg dye per g dry biomass) was obtained by using the following expression:
formula
1
where q is the amount of dye adsorbed onto the adsorbent (mg g−1); C0 and C are the concentrations of the dye (mg L−1) before and after adsorption, respectively; V is the volume of the aqueous phase (L) and M is the amount of the biomass (g). The dye concentration in the solution was analyzed using a double beam UV/vis spectrophotometer (PG Instrument Ltd, ModelT80 + ; PRC) at 502 nm for DR-60. Results given in averages were obtained from the experiments repeated three times. Calibration curve for the dye was obtained by plotting absorbance (A502) of the dye versus concentration of dye.

Elution and regeneration of the adsorbents

To determine the regeneration, and reusability of the tested adsorbents, desorption of DR-60 dye was performed using 0.01 mol L−1 HCl as desorption agent. The dye adsorbed CMC beads, free biomass or immobilized fungal biomass preparation was placed in the desorption medium and stirred at 150 rpm for 2 h at 25 °C. The final dye concentration in the aqueous phase was determined as described above. Reusability of the free and immobilized fungal preparations was studied by repeating adsorption–desorption cycles six times using the same adsorbent. After each cycle of adsorption–desorption, the adsorbent preparation was washed with 0.1 mol L−1 NaCl solution and transferred into fresh dye solution for adsorption in the succeeding cycle. The desorption ratio was calculated from the amount of dye adsorbed on the adsorbent and the final concentration of the dye in the adsorption medium.

Zeta-sizer and FTIR spectroscopy studies

The suspension of the tested adsorbents was analyzed with a Zeta-sizer (NanoZS, Malvern Instruments Ltd) in order to analyze surface charge alterations in different pH values, and the solution pH was adjusted with 0.1 mol L−1 NaOH or HCl solutions. For the ξ-potential measurement, the fungal biomass preparation (about 0.1 g) was transferred and mixed in purified water (100 mL) for 1.0 h.

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra of the fungus of L. concinnus biomass, CMC beads and fungus immobilized CMC beads were obtained by using an ATR-FTIR spectrometer (Nicolet IS 5, Thermo Electron Scientific Instruments, WI, USA).

The CMC and fungal biomass immobilized CMC beads were coated with a thin layer of gold under reduced pressure as part of sample preparation and their scanning electron micrographs were obtained using JEOL (JSM 5600) scanning electron microscope.

RESULTS AND DISCUSSION

Properties of the adsorbent systems

CMC is a water-soluble derivative of natural polymer cellulose. It is also a popular polymer compared to synthetic polymers which are mostly petroleum origin. The synthetic polymers are non-degradable and toxic making them a major cause of pollution. Carboxymethylation provides functional carboxylic groups on the cellulose backbone that can easily be cross-linked with trivalent metal ions such as ferric chloride and aluminium chloride. In this work, CMC beads were prepared by ionic cross-linking with trivalent ferric ions, considered as hard Lewis acid and be able to show strong affinity for oxygen-rich ‘Lewis bases’ such as carboxylic groups of CMC. Since Fe(III) ions are trivalent, three carboxyl groups on the CMC molecules can connect to the trivalent Fe(III) ions to form the cross-linked CMC beads. The white rot fungus L. concinnus biomass was immobilized in/on CMC beads via entrapment. In this immobilization method, covalent bound formation between support and fungal biomass was not expected. The L. concinnus biomass was embedded in a natural polymer (i.e. CMC), providing a comfortable microenvironment for the host fungal cells. Thus, the protective polymer layers could enhance biological activity of the fungal cells. Additionally, the stabilized fungal cells can be used for various biotechnological applications. The amount of immobilized L. concinnus biomass per the gram of CMC beads was 415 mg g−1.

In order to determine the functional groups responsible for DR-60 adsorption, ATR-FTIR spectroscopy was used. The ATR-FTIR spectra of CMC, L. concinnus biomass, and fungal biomass immobilized CMC beads are presented in Figure 1. In general, the ATR-FTIR spectra of all the studied adsorbents had intense peaks at a frequency level of 3,300–3,000 cm−1 representing -OH groups stretching vibration. For CMC, the strong peak at 1,582 cm−1 results from the C = O stretching band of carbonyl groups (Figure 1(a)). Fungal biomass has an intense peak compared with CMC and CMC-fungal biomass at a frequency level of 995–850 cm−1 representing C=O stretching of alcohols, esters and carboxylic acids. The peak at 1,635 and 1,539 cm−1 can be assigned for –NH bending (scissoring) and –NH2 wagging, respectively, and they are due to the presence of nitrogen containing molecules on both for the free and immobilized fungal preparations. As seen in this figure, the combination of fungal biomass with CMC resulted in an increase in the intensity of functional groups on the composite sorbent surfaces for the removal of model cationic dye from aqueous solution.
Figure 1

ATR-FTIR spectra: (a) CMC beads; (b) fungal biomass; (c) composite beads.

Figure 1

ATR-FTIR spectra: (a) CMC beads; (b) fungal biomass; (c) composite beads.

Zeta potential measurements provide information about the surface charge properties of the adsorbent. The negative or positive values of zeta potential depend on the functional groups on the sample surface, and pH value of the medium. For fungal biomass, functional groups mainly on the surface are hydroxyl, carboxyl, sulfhydryl, imidazole, phosphate, amine and amide. Some functional groups on the fungal biomass surfaces are protonated or deprotonated, corresponding to an intermediate pH, and the total surface charge density is zero at the point of zero charge. The zeta potentials as a function of solution pH (i.e. between 2.0 and 11.0) of the free and immobilized biomass of L. concinnus (i.e. composite beads) surfaces were presented in Figure 2. The zeta potential for all studied adsorbents decreased with increasing solution pH. The zero zeta potential point for free and immobilized fungal biomass was found to be at around pH 5.0 and 4.5, respectively. The negative charge density on the CMC surface significantly decreased with decreasing medium pH due to the protonation of the carboxyl groups on the CMC. The charge density in this pH range was changed from −23.8 to 5.6 mV for free fungal biomass and −29.2 and 7.3 mV for the immobilized fungal biomass. As can be seen from these results the medium pH is an important parameter to control for the adsorption of DR-60 dye. Additionally, it also determines the ionization state of the functional groups on the adsorbent, as well as the distribution of the charge density of the dye molecules.
Figure 2

Zeta potentials of free fungal biomass and composite beads at different pH values.

Figure 2

Zeta potentials of free fungal biomass and composite beads at different pH values.

The scanning electron microscope (SEM) micrographs of the CMC and fungal biomass immobilized CMC bead surfaces are presented in Figure 3(a) and 3(b), respectively. The CMC beads were spherically shaped about 2 mm diameter. The SEM micrograph of fungal biomass immobilized CMC bead surface was completely different from the bare CMC surface. There is a uniform fungal biomass distribution on the CMC surface indicating that entrapment of fungal was not localized. The uniform combination of fungal biomass with CMC is an important criterion for the removal of model dye due to the creation of large surface area on the composite adsorbent. Thus, combination of fungal biomass with CMC could also provide an additional advantage over the freely suspended fungal biomass in batch culture fungal mycelia form individually distributed spherical clumps (Ø: 2–4 mm). This tight packing of the fungal cells could also lead to diffusional restrictions and fewer adsorptive sites for target adsorbate than the fungal biomass immobilized CMC composite system (Bayramoglu et al. 2006).
Figure 3

SEM images: (a) bare CMC bead surface; (b) composite bead surface.

Figure 3

SEM images: (a) bare CMC bead surface; (b) composite bead surface.

Effect of adsorbent dosage

The removal capacity of DR-60 dye was studied by varying the adsorbent dosage between 20 and 150 mg in 50 mL at a constant dye concentration of 100 mg L−1. The effect of adsorbent dosage for the CMC, free fungal biomass, and immobilized fungal preparations are presented in Figure 4. As seen in this figure, the amount of dye adsorbed varied with initial adsorbent dosage, and increased with the increasing the sorbent dosage. This can be attributed to an increase in the availability of more adsorption sites. At adsorbent dosages > 50 mg, the incremental dye removal reached almost a constant. It was very low. That is why in the rest of the study, 50 mg adsorbent dosage in 50 L medium was used for all the studied sorbent preparations.
Figure 4

Effect of adsorbent amount on the adsorption of the dye onto CMC beads, fungal biomass, and composite beads.

Figure 4

Effect of adsorbent amount on the adsorption of the dye onto CMC beads, fungal biomass, and composite beads.

Effect of pH on adsorption of DR-60 on the adsorbents

The dye removal performances of the CMC, free and immobilized fungal biomass preparations were studied in order to evaluate their potential applications for removal of cationic organic pollutant. The pH of the medium is one of the most important parameters affecting the chemical groups on adsorbent surfaces. Because pH value of solution influences the electrical charge on both dye molecule and the binding sites of the adsorbent surface. The effect of medium pH on the adsorption capacity of the tested adsorbents was investigated between pH 3.0 and 8.0 at 25 °C, and the initial dye concentration was 100 mg L−1. At pH 6.0, a high amount of DR-60 dye was adsorbed on the studied adsorbent preparations. At this pH value, the DR-60 dye (Table 1) was positively charged by protonation of the basic amino groups, whereas the CMC, free and immobilized fungal preparations have negative charge due to the deprotonation of carboxyl groups (Figure 2). At increasing pH values, the diminution in the dye adsorption capacities might result from the change of surface charge distribution. As the pH increased, the number of positive charge on the surface of the dye molecules decreased due to the protonation of the dye molecules. Figure 5 shows the DR-60 removal performance by adsorbents. It was observed that the DR-60 adsorption by the immobilized fungal biomass was significantly higher than those of the CMC and free L. concinnus biomass. The dye removal performance of the CMC and free L. concinnus biomass was 43.4, and 65.7 mg g−1, respectively. On the other hand, this was about 92.6 mg g−1 for the immobilized fungal biomass, which is about 1.4 fold higher than that of the free counterpart. The enhancement of the observed dye removal performance for composite sorbent could be due to the increment in the adsorptive sites by combination of fungal biomass with negatively charged natural polymer (i.e. CMC). In general, the removal of dye molecules from the solution can be due to adsorption and/or degradation processes. In this work, the observed dye removal could be due to the adsorption and degradation by the free L. concinnus and composite sorbent (i.e. fungal biomass-CMC). Because of live L. concinnus, biomass was used for the free and immobilized fungal preparations, and several ligninolytic enzymes associated with this fungus for degradation of complex compound lignin such as laccase, Mn-peroxidase, and peroxidases (Kaushik & Malik 2009; Wangpradit & Chitprasert 2014).
Figure 5

Effect of pH on the dye adsorption onto CMC beads, fungal biomass, and composite beads.

Figure 5

Effect of pH on the dye adsorption onto CMC beads, fungal biomass, and composite beads.

Effect of ionic strength on DR-60 adsorption

Textile wastewater contains a remarkable number of salt ions which may influence dye adsorption processes. Therefore, the effects of salts on the dye removal performance of the CMC, free and immobilized fungal preparations were studied. For this, ionic strength of the adsorption medium was changed between 0.00 and 1.0 M by adjusting NaCl concentration. As seen in Figure 6, the amount of adsorbed dye slightly increased with an increase of NaCl concentration from 0.0 to 0.25 mol L−1. When the NaCl concentration was higher than 0.25 mol L−1, the removal percentage decreased to about 86 mg/g at 0.5 mol L−1 and to 52 mg g−1 at 1.0 mol L−1 NaCl for the immobilized fungal biomass. The lower adsorption capacity at higher ionic strength may be explained by the competition between Na+ ions for the same binding sites present on the sorbent surfaces. This behavior indicated that the ion exchange could be responsible for DR-60 removal process. Another way to explain the adverse effect of ionic strength might be to consider the change in activity coefficient of the dye, which would limit their transfer to sorbent surface. Thus, electrostatic interactions between the adsorbents and the dye molecules could be shielded by the presence of specific ionic species as reported previously (Bayramoglu et al. 2006; Sadaf et al. 2015; Tunali-Akar et al. 2016).
Figure 6

Effect of ionic strength on the dye adsorption onto CMC beads, fungal biomass, and composite beads.

Figure 6

Effect of ionic strength on the dye adsorption onto CMC beads, fungal biomass, and composite beads.

Effect of initial dye concentration and isotherm models

The initial dye concentration seems to be a very important parameter which has a pronounced impact on the adsorption process. Influences of various initial concentration of DR-60 (20–200 mg L−1) on the adsorption capacities of the CMC, free and immobilized fungal preparations are presented in Figure 7. Results showed that the removal of dye at different concentrations of 20–100 mg L−1 linearly increased, and then reached a plateau value at around 100 mg L−1. Initial dye concentration provides an important driving force to overcome all mass transfer resistance of the dye between the aqueous and solid phases. By increasing the concentration of dye from 20 to 200 mg L−1 the adsorption capacity increased from 12.8 to 65.7, and 18.2 to 92.6 mg g−1, for the free, and immobilized fungal preparations, respectively. It should be noted that at low DR-60 concentration, the adsorption sites on the adsorbents were not completely occupied, therefore the amount of adsorbed dye rapidly increased. When the initial dye concentration increased, the number of DR-60 molecule was beyond the adsorption capacity of the tested adsorbent, and then a plateau value was observed. The studied adsorbents are very negatively charged at this adsorption pH as observed in Figure 2. On the other hand, DR-60 dye is a typical cationic dye and positively charged. Thus, electrostatic attraction could greatly contribute to the adsorption of DR-60. In addition, the CMC beads' surface could provide greater and more favorable adsorptive sites for the DR-60 dye molecules. Hence, the CMC support plays as ‘helper’ for the adsorption of the tested cationic dye by enhancing the interaction between the protonated dye molecules and the fungal biomass-CMC surface. Therefore, the adsorption capacity of the fungal biomass-CMC was increased towards DR-60 dye. The composite system is promising adsorbent for the removal of dyes in terms of adsorption capacity.
Figure 7

Effect of initial dye concentration on the adsorption capacity of the adsorbent.

Figure 7

Effect of initial dye concentration on the adsorption capacity of the adsorbent.

Adsorption isotherms provide information about the capacity of the adsorbent as well as the nature of the solute-surface interaction. Therefore, three adsorption isotherm models, the Langmuir (Langmuir 1919), Freundlich (Freundlich 1906), and Temkin equations, were used to further describe the adsorption equilibrium. The calculated parameters and correlation coefficients for the adsorption of DR-60 on the CMC, free fungal and immobilized fungal biomass preparations are summarized in Table 2 for all the tested isotherm models.

Table 2

Isotherm models constants and correlation coefficients for sorption of DR-60 by the tested sorbents from aqueous solutions

Model parameters
Sorbentqmax (mg/g)b × 101 (L/mg)R2RLa
Langmuir 
 Fungal biomass 81.3 0.406 0.984 0.100 
 Bare CMC beads 64.5 0.178 0.974 0.258 
 Composite beads 96.0 3.19 0.998 0.015 
KFnR2
Freundlich 
 Fungal biomass 6.28 1.84 0.884  
 Bare CMC beads 1.76 1.41 0.891  
 Composite beads 31.8 3.62 0.739  
QTbTKTR2
Temkin 
 Fungal biomass 19.7 0.216 0.346 0.904 
 Bare CMC beads 16.2 0.015 0.145 0.943 
 Composite beads 14.9 0.166 0.812 0.808 
Model parameters
Sorbentqmax (mg/g)b × 101 (L/mg)R2RLa
Langmuir 
 Fungal biomass 81.3 0.406 0.984 0.100 
 Bare CMC beads 64.5 0.178 0.974 0.258 
 Composite beads 96.0 3.19 0.998 0.015 
KFnR2
Freundlich 
 Fungal biomass 6.28 1.84 0.884  
 Bare CMC beads 1.76 1.41 0.891  
 Composite beads 31.8 3.62 0.739  
QTbTKTR2
Temkin 
 Fungal biomass 19.7 0.216 0.346 0.904 
 Bare CMC beads 16.2 0.015 0.145 0.943 
 Composite beads 14.9 0.166 0.812 0.808 

aInitial concentrations (Co) of Disperse Red 60, 200 mg/L.

These equations can be written in the form given below to predict the adsorption capacities of the sorbents.

The Langmuir isotherm model equation:
formula
2
where qmax is the maximum adsorption capacity (mg g−1), Ce is the equilibrium DisperseRed 60 concentration in solution (mg L−1) and b is the Langmiur constant.
The Freundlich isotherm model equation:
formula
3
KF and n are the Freundlich adsorption isotherm constants characteristic of the system. KF and n are indicative of the extent of the adsorption and the degree of non-linearity between solution concentration and adsorption, respectively. Temkin isotherm is based on the assumption that heat of adsorption would decrease linearly with increase of coverage of adsorbent due to sorbate/sorbent interaction (Bayramoglu et al. 2002; Kiran & Kaushik 2008).
Linearized Temkin isotherm equation:
formula
4
where QT=RT/bT, bT is the Temkin constant related to heat of adsorption (kJ/mol), KT is the Temkin isotherm constant (L/g), R is the gas constant (8.314 J/mol K), and T is the temperature (K). The fitting plots of the Temkin model is shown in Figure 8.
Figure 8

Temkin isotherm plot of dye adsorption on DR- 60 by the tested sorbents from aqueous solutions at 25 °C.

Figure 8

Temkin isotherm plot of dye adsorption on DR- 60 by the tested sorbents from aqueous solutions at 25 °C.

The higher correlation coefficients of Langmuir model than those of the Freundlich and Temkin equations reveal that the adsorption data can be better described by the Langmuir model (Table 2). The monolayer adsorption capacity on the tested adsorbents surfaces can be calculated from Langmuir model, which is 64.5, 81.3, and 96.0 mg/g for CMC, free and immobilized fungal preparations, respectively. The calculated maximum adsorption capacities (qmax values) of the used sorbents were found to be close compared to experimental results (qexp) values for the dye. Therefore, the Langmuir isotherm model is suitable to describe the dye removal process for all the tested adsorbents. Additionally, the Langmuir constant b can quantitatively reflect the adsorption affinity of adsorbents towards to the target dye molecules. As shown in Table 2, values of b were 3.19 L/g for immobilized fungal biomass, which was much higher than that of the free fungal biomass (0.406 L/g). It indicated that the CMC increased the adsorption affinity of composite (i.e. CMC-fungal biomass) towards to the DR-60 dye.

For the Freundlich model, the magnitude of KF and n values of showed easy uptake of dye from aqueous medium with a high adsorption capacity of the free and composite sorbents. Values of n > 1 for the DR-60 dye at 25 °C indicates positive cooperativity in binding and a heterogeneous nature of adsorption. DR-60 adsorption on bare CMC beads in adsorption system followed Temkin isotherm, whereas on fungal biomass and composite beads it fitted better to Langmuir isotherm indicating change in interaction between sorbate and sorbent (Table 2). The constant bT reflects bonding energy which in turn dictates the type of interaction. In our study low values of bT (between 0.015 and 0.166 kJ/mol) indicate that interactions between the adsorbate and adsorbent are neither purely through ion-exchange nor purely through physisorption (Kiran & Kaushik 2008).

Adsorption time and kinetic models

Adsorption time-dependent experiments were studied to evaluate the adsorption kinetics. The adsorption capacities of the free and immobilized fungal biomasses for DR-60 dye increased linearly at the beginning of the adsorption process and after 60 min reached a sorption equilibrium (Figure 9). As seen in this figure, the amount of adsorbed DR-60 dye was increased linearly with time during the first 60 min. After this equilibrium period, no significant increase in the quantity of adsorbed DR-60 dye was observed with increase in contact time, thus, it was characterized as the optimum contact time. Therefore, the adsorption rate is an important parameter used to image the adsorption process. Two important physico-chemical aspects for evaluation of the adsorption process as a unit operation are the kinetics and the equilibria of adsorption. The kinetic data were analyzed using pseudo-first-order and pseudo-second-order models. The equations for these models are given as follows:
formula
5
formula
6
where qeq (mg g−1) is the experimental amount of DR-60 dye adsorbed at equilibrium, qt (mg g−1) is the amount of DR-60 dye adsorbed at time t, k1 (min−1) and k2 (g/mg min) are the equilibrium rate constants of first- and second-order sorption. The constant k2 is used to calculate the initial sorption rate ‘h’ (mg (g min)−1), at t → 0 by using h = k2qe. The application of the pseudo-second-order kinetics by plotting t/qt versus t yields the second order rate constant k2.
Figure 9

Effect of contact time on the adsorption of the dye onto adsorbents.

Figure 9

Effect of contact time on the adsorption of the dye onto adsorbents.

An addition, the value of dye concentration (C) relates to the thickness of the boundary layer. The larger C implies the greater effect of the boundary layer. Bangham's equation was used to evaluate whether the sorption is pore-diffusion controlled. Bangham's equation (Bangham & Burt 1924) is given as follows:
formula
7
where V is the volume of the solution (mL), m is the weight of sorbent (g L−1) and α (<1) and kb are constants.

The constants of pseudo-first-order, pseudo-second-order, and Bangham's equations were calculated from experimental data and presented in Table 3. As seen in this table, the correlation coefficients (R2) obtained from the pseudo-second-order model were found to be above 0.996 for all the tested sorbents, making them larger than those of the pseudo-first-order model. The results obtained from the pseudo-second-order model described the best among applied the models.

Table 3

Kinetic parameters for sorption of DR-60 by the tested sorbents from aqueous solutions

Model parameters
Biosorbentqeq,exp (mg/g)qe, cal (mg/g)k1 × 101 (min−1)R2 
First-order 
 Fungal biomass 95.7 165.8 0.86 0.964  
 Bare CMC beads 43.9 99.8 0.92 0.950  
 Composite beads 92.6 204.2 0.98 0.972  
 qeq,exp (mg/g) qe, cal (mg/g) k2 × 103 (g/mg min) h × 101 (mg/g min) R2 
Second-order 
 Fungal biomass 95.7 74.1 1.07 0.79 0.997 
 Bare CMC beads 43.9 49.7 1.62 0.80 0.996 
 Composite beads 92.6 99.0 1.46 1.45 0.999 
  α kb × 101 (L/g) R2  
Bangham's 
 Fungal biomass  0.368 0.79 0.944  
 Bare CMC beads  0.341 0.56 0.956  
 Composite beads  0.263 1.97 0.972  
Model parameters
Biosorbentqeq,exp (mg/g)qe, cal (mg/g)k1 × 101 (min−1)R2 
First-order 
 Fungal biomass 95.7 165.8 0.86 0.964  
 Bare CMC beads 43.9 99.8 0.92 0.950  
 Composite beads 92.6 204.2 0.98 0.972  
 qeq,exp (mg/g) qe, cal (mg/g) k2 × 103 (g/mg min) h × 101 (mg/g min) R2 
Second-order 
 Fungal biomass 95.7 74.1 1.07 0.79 0.997 
 Bare CMC beads 43.9 49.7 1.62 0.80 0.996 
 Composite beads 92.6 99.0 1.46 1.45 0.999 
  α kb × 101 (L/g) R2  
Bangham's 
 Fungal biomass  0.368 0.79 0.944  
 Bare CMC beads  0.341 0.56 0.956  
 Composite beads  0.263 1.97 0.972  

The calculated adsorption capacities (qeq) using the pseudo-second-order model agreed with the corresponding experimental adsorption capacities of the tested sorbents. Additionally, the composite sorbent had a high equilibrium adsorption capacity qeq, and the adsorption rates were also very fast compared to free fungal biomass. The plot was found to be linear for each tested sorbent with good correlation co-efficient (>0.9) indicating that kinetics confirmed to Bangham's equation and therefore the sorption of DR-60 onto the free fungal biomass, bare CMC and composite beads, and was pore diffusion controlled (Table 4).

Table 4

Thermodynamic parameters for sorption of DR-60 by the free, bare CMC and composite beads from aqueous solutions

SorbentT (K)qexp (mg/g)qmax (mg/g)ΔG (kJ/mol)ΔS (kJ/mol)ΔH (kJ/mol K)Ea (kJ/mol)
Fungal biomass 288 53.6 66.1 −22.2 0.20 36.4 −13.2 
298 65.7 81.3 −23.6 
308 77.5 88.2 −25.9 
318 86.3 93.7 −28.1 
Bare CMC beads 288 38.9 57.1 −20.7 0.82 3.02 −2.74 
298 43.4 64.5 −21.0 
308 44.6 65.4 −22.3 
318 46.7 67.6 −23.2 
Composite beads 288 87.1 92.6 −26.3 0.43 97.0 −5.93 
298 92.6 96.1 −28.7 
308 99.8 100.2 −35.1 
318 104.5 104.7 −39.6 
SorbentT (K)qexp (mg/g)qmax (mg/g)ΔG (kJ/mol)ΔS (kJ/mol)ΔH (kJ/mol K)Ea (kJ/mol)
Fungal biomass 288 53.6 66.1 −22.2 0.20 36.4 −13.2 
298 65.7 81.3 −23.6 
308 77.5 88.2 −25.9 
318 86.3 93.7 −28.1 
Bare CMC beads 288 38.9 57.1 −20.7 0.82 3.02 −2.74 
298 43.4 64.5 −21.0 
308 44.6 65.4 −22.3 
318 46.7 67.6 −23.2 
Composite beads 288 87.1 92.6 −26.3 0.43 97.0 −5.93 
298 92.6 96.1 −28.7 
308 99.8 100.2 −35.1 
318 104.5 104.7 −39.6 

Thermodynamic parameters

The effect of temperature on the equilibrium adsorption capacity of the free, bare CMC and composite beads for DR-60 dye was investigated in a temperature range of 15–45 °C at an initial dye concentration of 100 mg L−1. The adsorption of DR-60 dye raised slightly with increasing temperature from 298 to 318 K, indicating the process is endothermic in nature (Table 4). The reason for the increase in DR-60 dye adsorption on the fungal biomass at high temperatures could be attributed to increase in the interaction between dye molecules and functional groups on the fungal mycelia surfaces because of an increase in the energy of the system.

The following equations have been used to determine the thermodynamic parameters such as enthalpy (ΔH), Gibbs free energy (ΔG) and entropy (ΔS).
formula
8
formula
9
where R is the gas constant (8.314 J mol−1 K−1), T represents the absolute temperature (K), and Ka (b = Ka) is the association coefficient which can be calculated by using Langmuir equation (2). The values of ΔHo and ΔSo were determined from the slope and intercept of the van 't Hoff plot of ln Ka versus 1/T.

The values of the thermodynamic parameters are presented in Table 4. The negative values of ΔG suggested that the adsorption process was spontaneous, and the decreasing of ΔG with the increasing of temperature indicated that the adsorption was more favorable at high temperatures. The positive values of ΔHo confirmed the endothermic process for the adsorption of DR-60 dye, consistent with the increase of adsorption capacity as temperature increased. The positive value of ΔSo suggested that the increasing randomness between the solid/solution interface during the adsorption process.

Activation energy is an important parameter in a thermodynamic study as it determines the temperature dependence of the reaction rate. The activation energy (Ea) for the adsorption of an adsorbate onto a sorbent surface can be determined from experimental measurements of the adsorption rate constant at different temperatures according to the Arrhenius equation. The activation energy (Ea) values of the free fungal biomass, bare CMC and composite beads for DR-60 dye adsorption were found to be −13.2, −2.7 and −5.9 kJ mol−1, respectively (Table 4). In physical adsorption, the equilibrium is usually rapidly attained and easily reversible, because the energy requirements are small. These results suggest that the adsorption processes of DR-60 dye onto the studied fungal biomass, bare CMC beads and composite beads was likely to take place by physical mechanism.

Desorption studies

The use of an adsorbent in waste water treatment depends not only on the adsorptive capacity, but also on how well the adsorbent can be regenerated and reused (Bayramoglu & Arica 2011). In order to understand the detailed mechanism of dye binding, desorption studies were carried out. When 0.01 mol L−1 HCl solution was used for desorption of DR-60 dye from the free fungal biomass and composite beads, 46.1 and 24.7% desorption was obtained, respectively. In general, the electrostatic interaction between functional groups of adsorbents and cationic dye decrease at acidic pH resulting in desorption of adsorbed dye. The results in this study indicated that apart from surface binding, degradation of the surface adsorbed dye after internalization by both the free and entrapped fungal preparation also occurs.

To show the reusability, the adsorption–desorption cycle was repeated six times with the same sample of adsorbent. The adsorption capacity of the free fungal biomass and composite beads did not significantly change for tested adsorbents.

CONCLUSIONS

The removal of DR-60 dye from aqueous solutions by the free and immobilized fungal biomasses was studied in a batch system with respect to adsorbent dosage, medium pH, ionic strength, initial dye concentration, and temperature. FTIR and zeta-sizer analysis confirmed that large amount of amino, hydroxyl and carboxyl groups were responsible for adsorption of DR-60 dye on the studied biomass preparations. Among these functional groups, the carboxyl groups were essential for removal of DR-60 dye from aqueous solutions. Optimum pH value was found to be 6.0 for the removal of the dye. The removal of the dye with the studied adsorbent preparations increased with increasing temperatures. Additionally, the removal of the dye on all the tested adsorbents increased with increasing initial dye concentration. The Langmuir isotherm model was more applicable to the type of adsorption achieved by the tested adsorbents. The free and immobilized fungal biomass preparations of L. concinnus have been demonstrated that the removal performance toward to the DR-60 dye through a combination of adsorption and degradation processes due to the presence of the lignolitic enzymes associated with the fungal cells. The free and immobilized fungal biomass surfaces have negative charges as demonstrated by the zeta-sizer analyses and, therefore, the removal of cationic dye by adsorption was also favorable. The removal performance of the DR-60 dye by the composite beads was more pronounced compared to the free fungal biomass.

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