Minerals Engineering 18 (2005) 681–689 This article is also available online at: www.elsevier.com/locate/mineng
Chromium(VI) biosorption characteristics of Neurospora crassa fungal biomass Sibel Tunali *, Ismail Kiran, Tamer Akar Department of Chemistry, University of Osmangazi, Campus of Mesßelik, 26480, Eskisßehir, Turkey Received 19 October 2004; accepted 26 November 2004
Abstract The removal of Cr(VI) from aqueous solutions by live and pretreated Neurospora crassa fungal biomass was investigated in the batch mode. The influence of solution pH, contact time and initial metal ion concentration as well as pretreatment of biomass on the biosorption efficiency were studied. All pretreatment methods were found to increase the biosorption capacity of biomass in comparison with the live biomass and biosorption capacity of acetic acid pretreated biomass was found to be 15.85 ± 0.94 mg/g biomass under optimum conditions. The adsorption constants were found from the Freundlich isotherm model at 25 °C. The biosorbent was regenerated using 10 mM NaOH solution with up to 95% recovery and reused five times in biosorption–desorption cycles successivelly. The biosorption mechanism of biosorbent was also evaluated by chemical and instrumental analysis including infrared spectroscopy, scanning electron microscopy and X-ray energy dispersion analysis. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Biotechnology; Environment; Pollution
1. Introduction Biosorption is an energy-independent binding of metals to the cell wall of organisms and utilizes viable, nonviable, immobilized and chemically pretreated biomass (Volesky, 1994). This process plays an important role in the removal of heavy metals from aqueous solutions (Martins et al., 2004). It is used not only for toxic metals removal but also for the recovery of precious metals such as gold and silver (Veglio and Beolchini, 1997). This process has gained great importance due to its potential advantages over conventional separation techniques including chemical precipitation, adsorption, membrane filtration, ion exchange and electrochemical processes, which are used to remove toxic metals from waste streams (Uslu et al., 2003; Zouboulis et al., *
Corresponding author. Tel.: +90 222 239 3750/2366; fax: +90 222 239 3578. E-mail address:
[email protected] (S. Tunali). 0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2004.11.002
2004). These advantages include the use of a low operating costs, minimization of the volume of chemical and/ or biological sludge to be disposed of and high efficiency in detoxifiying very dilute effluents (Kapoor and Viraraghavan, 1997; Kratochvil and Volesky, 1998). Chromium is a toxic metal of widespread use. Of its two most common and stable oxidation states, hexavalent chromium Cr(VI) species are known to be much more dangerous than trivalent chromium Cr(III) species. Extensive use of chromium in many industries such as electroplating, steel productions, wood preservation and leather tanning results in releasing chromium containing effluents to the environment making it a serious pollutant and a severe threat to the ecological system (Volesky, 1990; Jianlong et al., 2004). The conventional methods described above for removing chromium species from effluents are restricted because of technical or economical constrains. Therefore, recent studies have concentrated on the development of low cost processes and the use of microorganisms has received much more
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Nomenclature Ce Cf Ci EDAX FTIR Kf M
equilibrium Cr(VI) concentration (mg/l) final Cr(VI) concentration (mg/l) initial Cr(VI) concentration (mg/l) X-ray energy dispersion analysis Fourier transformer infrared spectroscopy Freundlich isotherm constant mass of biomass used (g)
attention in recent years since they carry wide range of binding sites for heavy metal ions (Goyal et al., 2003). The present work was aimed at investigating the possible use of Neurospora crassa biomass as biosorbent for the removal of Cr(VI) from aqueous solutions since it has been reported to show resistance against chromate species (Marzluf, 1970). The role played by the functional groups present in the biomass in biosorption was examined by FTIR, SEM and EDAX analysis in addition to the environmental parameters affecting the biosorption process such as pH, time and initial metal ion concentration. The equilibrium adsorption data evaluated by Freundlich isotherm model and desorption process and reusability of biosorbent were studied.
2. Materials and methods 2.1. Microorganism and growth conditions The strain used in this study was N. crassa (ATCC 12526), a pure culture, obtained from the collection of _ Plants, Drug and Scientific Research Centre (BIBAM) of Anadolu University. The cultures were routinely maintained at 4 °C on agar-potato dextrose agar slants. The pH of the liquid growth medium (Kiran et al., 2004) was adjusted to 5.5 by the addition of 1 N HNO3 before autoclaving at 121 °C for at least 20 min. Erlenmayer flasks containing the above media (100 ml) were innoculated with spore suspension (1 ml) obtained shaking sterile water (10 ml) with a mature slope of N. crassa under sterile conditions. Growth was allowed to proceed for seven days at 25 °C on a rotary shaker operating at 120 rpm. After the fungal growth, the biomass and the culture medium were separated by filteration and the resulting biomass was washed several times thoroughly with deionized water. 2.2. Chromium(VI) solutions A stock solution of Cr(VI) used in this study was prepared by dissolving an accurate quantity of K2Cr2O7 in deionized water. Other concentrations prepared from stock solution by dilution varied between 25 and
n R2 SEM V Q
Freundlich isotherm constant Regression coefficient Scanning electron microscopy volume of solution (l) amount of Cr(VI) adsorbed (mg/g biomass) at equilibrium
250 mg/l and the pH of the working solutions was adjusted to desired values with 0.1 M HCl and 0.1 M NaOH. Fresh dilutions were used for each experiments. All the chemicals used were of analytical grade. 2.3. Pretreatment of biomass An amount of live biomass (20 g wet weight) was subjected to pretreatment in an effort to study the effect of pretreatment on Cr(VI) uptake capacity of fungal biomass. The live biosorbent was autoclaved for 15 min at 121 °C and 18 psi (referred as heat inactivated biomass), boiled for 15 min in 500 ml of 0.5 N NaOH and 10% (v/v) AcOH solutions (Kapoor and Viraraghavan, 1998). Following the desired pretreatment, biomass were collected by filteration and washed with deionized water until the pH of the washing solution was close to neutral range (pH 6.9–7.1). All the pretreated biomass were then spread on petri dishes and dried in an oven at 60 °C overnight. They were powdered using a mortar and pestle and sieved to select particles 150 lm for use as abiosorbent. For the untreated control sample, living mycelium of N. crassa was directly used in the experiments. 2.4. Biosorption experiments The biosorption of Cr(VI) onto heat inactivated biomass of N. crassa from aqueous solutions was investigated in batch biosorption equilibrium experiments. Batch experiments were carried out with biosorbent samples (0.1 g) at 25 °C in Erlenmayer flasks (250 ml) on an orbital shaker operating at 125 rpm to elucidate the optimum conditions (pH, contact time and initial chromium(VI) concentration). The effect of pH on the biosorption rate of Cr(VI) on fungal biomass was investigated by equilibrating the sorption mixture containing dried biomass (0.1 g) and Cr(VI) solutions (50 ml of 100 mg/l) in the pH range 1.0–6.0. The suspensions were adjusted to the desired pH by adding HCl or NaOH at the beginning of the experiments and were not controlled further. The period of contact time was varied up to 120 min determined by using the same sorption mixture de-
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2.5. Desorption and reusability studies The reusability of biosorbent is an important parameter related to the application potential of biosorption technology (Iqbal and Edyvean, 2004). In this work heat inactivated biomass was subjected to 10 mM NaOH solution in order to determine the reusability of N. crassa biomass. To optimize the S/L ratio for desorption, the bound Cr(VI) species were eluted in 10 ml (S/L = 10), 12.5 ml (S/L = 8), 20 ml (S/L = 5) and 50 ml (S/L = 2) of the eluents. Each biosorption and desorption cycles were allowed 90 min of contact time and consecutive biosorption–desorption cycles were repeated five times using the same biosorbent (optimum S/L = 2) in solutions containing biosorbent-Cr(VI) species or biosorbent–desorbent agent for achieving sorption or desorption equilibrium. The final Cr(VI) concentration of the solutions were determined by using the procedure described above. The eluted biosorbent was washed repeatedly with deionized water to remove any residual desorbing solution and placed into metal solution for the next biosorption cycle. Desorption efficiency was calculated by using following equation. Desorption efficiency ¼
Amount of CrðVIÞ desorbed 100 Amount of CrðVIÞ adsorbed
analyzed and interpreted by FTIR spectroscopy. The spectra were recorded in a Bruker Tensor 27 Fourier transform infrared spectrometer with the samples prepared as KBr discs. The surface structure of biosorbent was analysed by scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDAX) using JEOL 560 LV SEM at 20 keV with background subtraction and a summation of 60 scans. Unloaded and chromium-laden heat inactivated N. crassa biomass samples were mounted on a stainless steel stab with a doublestick tape followed by coating with a thin layer of gold under vacuum to increase the electron conduction and to improve the quality of the micrographs.
3. Results and discussion Cr(VI) biosorption ability of N. crassa was investigated. The effect of pretreatments on biosorption capacity and desorption conditions were also studied together with the mode of metal–microbe interaction determined by FTIR, SEM and EDAX analysis. The maximum loading capacity of each biomass at the corresponding equilibrium conditions was determined using a mass balance equation expressed as in Eq. (1): Q ¼ ½ðC i C f Þ:V =M
ð1Þ
3.1. Effect of pH on chromium(VI) sorption Various factors such as initial pH, temperature and initial metal ion concentration in addition to biomass concentration effect the biosorption of metal ions on the cell wall surface of organisms. The greatest capacity of heat inactivated N. crassa biomass was obtained at pH 1.0 using Cr(VI) concentration of 100 mg/l at 25 ± 0.1 °C as can be seen in Fig. 1. The biosorption 8 7
Aadsorbed Cr(VI) (mg/g)
scribed above at optimal pH. The effect of the initial Cr(VI) concentration on the biosorption was studied at optimum pH and contact time as determined above except that the concentration of Cr(VI) in the adsorption medium was varied between 25 and 250 mg/l. Also the effect of biosorbent concentration on biosorption of Cr(VI) was investigated at optimum conditions. The samples were centrifugated at 4500 rpm for 5 min and supernatant was used to determine Cr(VI) concentration. The residual Cr(VI) concentration in the aqueous samples was determined using a spectrophotometer (Unicam UV2-100 UV/VIS) at 540 nm after complexation with 1,5-diphenyl carbazide (Greenberg et al., 1985). Data presented are the mean values from three independent experiments. Standard deviation and error bars are indicated wherever necessary. All statistical analysis was done using SPSS 9.05 for Windows where it is possible to evaluate whether the effect and the interaction among the investigated factors are significant with respect to the experimental error.
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6 5 4 3 2 1 0 0
2.6. Characterization of Cr(VI) biosorption on the basis of surface studies The chemical characteristics of heat inactivated N. crassa biomass before and after Cr(VI) sorption were
1
2
3
4
5
6
7
pH
Fig. 1. Effect of pH on Cr(VI) removal by heat inactivated biomass of N. crassa. Initial Cr(VI) concentration = 100 mg/l, the biosorbent concentration = 0.1 g/50 ml. The bars represent the standard error of the mean.
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capacity of biomass decreased with increase in pH and reached the lowest level at pH 3.0. Similar trends toward different pHÕs have been reported for a variety of microbial based biosorbents (Melo and DÕSouza, 2004; Nourbakhsh et al., 1994). At pH 2.0 the biosorption capacity did not differ significantly from pH 1.0 (p > 0.05) but the difference in biosorption capacity was found to be statistically significant at pH values between 3.0 and 6.0 comparing with pH 1.0 (p < 0.05).
12
Adsorbed Cr(VI) (mg/g)
684
8 6 4 2 0
3.2. Biosorption time of Cr(VI)
0
3.3. Effect of initial Cr(VI) and biosorbent concentrations The Cr(VI) removal capacity of heat inactivated N. crassa biomass is presented as a function of the initial concentration of Cr(VI) in the aqueous solution in Fig. 3. The experiments were carried out using Cr(VI) solutions ranging from 25 to 250 mg/l at pH 1.0 ± 0.1 at room temperature. The loading capacity increased from 1.0 to 9.15 mg/g as the initial Cr(VI) concentration was increased from 25 to 250 mg/l and the maximum uptake of Cr(VI) was determined as 9.15 ± 0.86 mg/g biomass at 250 mg/l of initial Cr(VI) concentration. The effect of biosorbent dosage in adsorption process was also studied with biosorbent materials ranging from 0.1 to 0.5 g. As depicted from Fig. 4, increasing biosorbent dosage resulted in a little increase in adsorption
8 7
50
100
150
200
250
300
Initial Cr(VI) concentration (mg/l) Fig. 3. Effect of initial metal ion concentration on Cr(VI) removal by heat inactivated biomass of N. crassa. The biosorbent concentration = 0.1 g/50 ml, pH = 1.0, contact time = 90 min. The bars represent the standard error of the mean.
11 10 Adsorbed Cr(VI) (mg/g)
Fig. 2 shows the effect of time course profiles for the adsorption of Cr(VI) by heat inactivated biomass of N. crassa. It showed that a major fraction of Cr(VI) gets sorbed onto biomass after 90 min and remained nearly constant afterwards. This suggested that biosorption process is slow and reaches saturation within 90 min. The subsequent biosorption experiments using heat inactivated biomass were performed under pH 1.0 and contact time of 90 min.
Adsorbed Cr(VI) (mg/g)
10
9 8 7 6 5 4 0
0.1
0.2
0.3
0.4
0.5
0.6
Amount of biosorbent (g) Fig. 4. Effect of biosorbent concentration on Cr(VI) removal by heat inactivated biomass of N. crassa. Initial Cr(VI) concentration = 250 mg/l, pH = 1.0, contact time = 90 min. The bars represent the standard error of the mean.
rate of Cr(VI) ions onto biomass. Cr(VI) biosorbed did not change significantly in various biosorbent amounts (P > 0.05). However, the maximum chromium loading capacity of biomass found to be 9.97 ± 1.32 g was achieved at biomass dosage of 0.3 g.
6
3.4. Analysis of adsorption isotherms
5 4 3 2 1 0 0
15
30
45
60
75
90
105
120
Time (min) Fig. 2. Effect of contact time on Cr(VI) removal by heat inactivated biomass of N. crassa. Initial Cr(VI) concentration = 100 mg/l, the biosorbent concentration = 0.1 g/50 ml. The bars represent the standard error of the mean.
The biosorptive metal uptake can be quantitatively evaluated from experimental biosorption equilibrium isotherm. The graphical expression of isotherm is a plot of the metal uptake by the per unit weight of biosorbent against the residual metal concentration in the biosorption medium. There are two widely accepted and easily linearized adsorption isotherm models, Freundlich and Langmuir models, commonly used in the literature. The Freundlich model is based on the relationship between the metal uptake capacity ‘‘Q’’ (mg/g) of biomass and the residual (equilibrium) metal ion concentration
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‘‘Ce’’ (mg/l). The general Freundlich equation is as follow: Q ¼ k C 1=n e
ð2Þ
and linearized form of this model is ð3Þ
ln Q ¼ ln k þ 1=n ln C e
where, intercept ln k (ln Kf) is a measure of adsorption capacity, and the slope 1/n is the intensity of adsorption (Volesky, 1990). It was found that the results were fitted well to the Freundlich isotherm model which yielded straight lines (Fig. 5) with the regression coefficient (R2) value of 0.989 and Freundlich isotherm constants values of 0.06 (Kf) and 1.06 (n). The magnitude of Kf and n shows both the easily uptake of heavy metal ions from aqueous solution and high adsorption capacity (Ucun et al., 2002). These values were found high enough for the biosorbent material to be used for the removal of Cr(VI) from aqueous solutions. 3.5. Effect of biomass pretreatments on Cr(VI) sorption The metal adsorbing capacity of dead cells may be greater, equivalent to, or less than that of living cells. This depends on various factors including the fungus under consideration, pretreatment methods used and type of metal ions being studied. The use of dead or pretreated cells are of choice since they offer some advantages over the corresponding live cells such as no limitations for toxicity, no requirement for growth media and nutritients, easily desorption of biosorbed metal ions and reusability of biomass as well as usability of dead biomass in traditional adsorption models (Kapoor and Viraraghavan, 1998). The biosorption capacity of native biomass of N. crassa for Cr(VI) was found to be 0.43 ± 0.22 mg/g biomass under optimum conditions (pH:1.0; contact time: 90 min; initial Cr(VI) ion concentration:250 mg/l) in this study. Pretreatment experiments applied in this study
3 2
lnqeq
1 0
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Table 1 Effect of pretreatment on Cr(VI) removal by biomass of N. crassa Pretreatment
Loading capacity (mg/g biomass)
Native biomass Heat inactivated Sodium hydroxide Acetic acid
0.43 9.15 7.40 15.85
(0.22) (0.86)* (1.24)* (0.95)*
Figures in parentheses are the standard deviation of the mean (n = 3). * The difference in Cr(VI) uptake capacity of the native and pretreated biomass was found to be statistically significant (P < 0.05).
showed that all pretreatment methods enhanced the adsorption capacities of N. crassa biomass which ranged from 7.40 ± 1.24 to 15.85 ± 0.95 mg/g biomass when compared with the native biomass (Table 1). The obtained results indicated that acetic acid pretreatment method was the most effective method increasing the biosorption capacity of the raw biomass by 15.85 ± 0.95 mg/g biomass for Cr(VI). The effect of acid pretreatments on the biosorption ability of organisms varies from high to none depending on the type of microorganisms used and the type of heavy metal ions studied. Kapoor and Viraraghavan (1998) reported that acid pretreatment decreased the biosorption capacity of A. niger whereas Huang and Huang (1996) indicated that acid pretreatment can increase the biosorption capacity of A. oryzae. They also reported that acid pretreatment caused no change in the biosorption capacity of Rhizopus oryzae. A similar observation was made in the studies on the biosorption of nickel and copper ions by Pseudomoans aeruginosa and Penicillium biomass suspended in acidic solutions (Galun et al., 1987; Sar et al., 1999). Heat and sodium hydroxide pretreatments also increased the biosorption capacity of the biomass from 0.43 ± 0.22 to 9.15 ± 0.86 and 7.40 ± 1.24 mg/g biomass, respectively. An increase observed by alkali pretreatment on biosorption uptake rate could be due to the release of certain biopolymers having high affinities towards heavy metal ions from the cell wall by autolytic enzymes released as a result of alkali pretreatment (Yan and Viraraghavan, 2000). Biosorption results of Cr(VI) reported by other researchers in the literature by various biosorbents and operating conditions are summarised in Table 2. The uptake values obtained for Cr(VI) biosorption in this study are comparable and were found to be higher than that of many corresponding biosorbents.
y = 0.9403x - 2.8501
-1
2
R = 0.989
3.6. Desorption and reuse
-2 -3 2
3
4
5
6
lnC eq Fig. 5. Freundlich adsorption isotherm of Cr(VI) on the heat inactivated biomass of N. crassa.
The regeneration of the biosorbent is one of the key factors in assessing their potential for commercial application. The desorption of adsorbed Cr(VI) was studied with heat inactivated N. crassa biomass using 10 mM NaOH solution as a desorption agent. Higher than
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Table 2 Biosorption results of Cr(VI) from the selected literature by various biosorbents and operating conditions Biosorbent material
Biosorption Operating conditions capacity (mg/g) pH T (°C) Initial metal ion Biomass References concentration (mg/l) concentration (g/l)
Chlorella vulgaris Clodophara crispata Zoogloea ramigera Rhizopus arrhizus Saccharomyces cerevisiae Pilayella littoralis Quercus ilex L. (Stem) Quercus ilex L. (Leaf) Quercus ilex L. (Root) Rhizopus nigricans Rhizopus nigricans (polyacrylamid) Neurospora crassa (AcOH pretreated)
2.98 6.20 3.40 8.40 4.30 6.55 0.07 0.08 0.09 12.70 21.22 15.85 ± 0.95a
25 25 25 25 25 25 22 ± 2 22 ± 2 22 ± 2 45 – 25
200 200 75 125 100 50 10 10 10 250 100 250
– – – – – 2.5 10 10 10 10 2 1
Nourbakhsh et al., 1994 Nourbakhsh et al., 1994 Nourbakhsh et al., 1994 Nourbakhsh et al., 1994 Nourbakhsh et al., 1994 Carrilho and Gilbert, 2000 Prasad and Freitas, 2000 Prasad and Freitas, 2000 Prasad and Freitas, 2000 Bai and Abraham, 2003 Bai and Abraham, 2003 This study
Standard deviation of the mean.
95% of the adsorbed Cr(VI) were desorbed from the biosorbent with optimum S/L = 2 (Fig. 6). The reusability of the biosorbent was tested in five consecutive adsorption–desorption cycles using the same preparation (Fig. 7). The adsorption efficiency of biomass for Cr(VI) did not change significantly and only a maximum 5% decrease was observed after 5 cycles. These results indicated that the heat inactivated N. crassa biomass offers potential to be used repeatedly in chromium(VI) adsorption studies without any detectable loss in the total adsorption capacity.
mg Cr(VI)/g biomass
a
1.0–2.0 1.0–2.0 1.0–2.0 1.0–2.0 1.0–2.0 5.5 2.7–5.0 2.6–4.8 2.6–5.4 2.0 2.0 1.0
10
Adsorption
9
Desorption
8 7 6 5 4 3 2 1 0 1
2
3
number of cycles
4
5
3.7. FTIR spectral analysis The FTIR spectra of dried and chromium(VI) loaded N. crassa biomass in the range of 400–4000 cm1were taken to obtain information on the nature of the possible cell–metal ions interactions and presented in Fig. 8.
Fig. 6. Effect of S/L ratio on Cr(VI) recovery (Cr(VI) concentration = 250 mg/l; adsorption and desorption period = 90 min; biomass dose = 0.1 g).
Fig. 7. Adsorption–desorption cycles for heat inactivated N. crassa biomass.
The FTIR spectroscopic analysis of N. crassa biomass indicated shifted broad bands at 3346, 1645 and 1408 cm1 when compared with that of chromium loaded biomass which showed the same absorption at 3409, 1658 and 1379 cm1. These groups are of the bonded –NH groups, hydroxyl groups (–OH), carboxylate anions (COO) and carbonyl groups (–CO). The differences between peaks in the FTIR spectra of dried and Cr(VI) loaded N. crassa biomass ranged from 13, 29 to 64 cm1. There was also clear intensity decrease of carboxylate ions peaks at 1645 and 1408, 1243 cm1 indicative of SO3 groups and others at 1151 and 1035 cm1 indicative of phosphate group absorption peaks (P=O and P–OH stretching). These results indicated the involvement of these functional groups in biosorption process. Finally, it should be noted that peaks in the region of lower wavenumbers (under 700 cm1) appeared as a broad singlet in comparison with that of unloaded dried biomass which contained multiply absorption peaks. This could be attributed to an interaction between Cr(VI) species and N-containing bioligands.
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Fig. 8. Infrared spectra of heat inactivated N. crassa biomass (a) unloaded and (b) Cr(VI) loaded biomass.
3.8. SEM and EDAX analysis SEM micrographs and EDAX spectra obtained before and after Cr(VI) biosorption onto heat inactivated N. crassa biomass are presented in Figs. 9–12, respec-
Fig. 9. Typical SEM micrograph of heat inactivated N. crassa.
tively. These micrographs indicated clearly the presence of new shiny bulky particles over the surface of metalloaded heat inactivated N. crassa cells. This observation
Fig. 10. Typical SEM micrograph of heat inactivated N. crassa (Cr(VI) loaded).
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Fig. 11. EDAX spectra of heat inactivated N. crassa.
Fig. 12. EDAX spectra of heat inactivated N. crassa (Cr(VI) loaded).
was confirmed by EDAX analysis which revealed Cr(VI) signals together with the presence of gold peaks in all spectra.
4. Conclusion The biosorption properties of N. crassa were studied for Cr(VI) removal from aqueous solution in the present work. The adsorption process has been shown to be affected from experimental conditions such as pH, initial metal ion concentration and contact time. The experimental data was shown to be described appropriately by the Freundlich isotherm model. Pretreatment of the raw biomass with heat, sodium hydroxide and
acetic acid enhanced the biosorption capacity of N. crassa when compared with the native biomass. Acetic acid pretreatment caused the most highest increase on biosorption ability of N. crassa among all pretreated methods. Bioaccumulation of Cr(VI) species extracellularly on the surface of N. crassa was confirmed by FTIR, SEM and EDAX analysis. Desorption and reusability studies indicated that the biosorbent could be regenerated using 10 mM NaOH solution with up to 95% recovery and reused five times in biosorption– desorption cycles successivelly. It could be concluded based on these results that acetic acid pretreated biomass of N. crassa may be used as an inexpensive, effective and easily cultivable biosorbent for the removal of Cr(VI) species from aqueous solutions.
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Acknowledgments We would like to thank Dr. Fatih DEMIRCI from Anadolu University for providing Neurospora crassa and Prof. Cengiz BAYCU and Arzu ISCAN from Osmangazi University, Eskisßehir, TURKEY for SEM and EDAX studies.
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