Phenolic compounds biosorption onto Schizophyllum commune fungus: FTIR analysis, kinetics and adsorption isotherms modeling

Chemical Engineering Journal 168 (2011) 562–571

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Phenolic compounds biosorption onto Schizophyllum commune fungus: FTIR analysis, kinetics and adsorption isotherms modeling Nadavala Siva Kumar ∗ , Kim Min Department of Safety Environmental System Engineering, Dongguk University, Gyeongju 780-714, Republic of Korea

a r t i c l e

i n f o

Article history: Received 10 October 2010 Received in revised form 5 January 2011 Accepted 5 January 2011 Keywords: Biosorption Phenolic compounds Schizophyllum commune fungus Kinetics Isotherms Modeling

a b s t r a c t The contamination of water by organic pollutants viz. phenolic compounds (phenol, 2-chlorophenol (2-CPh) and 4-chlorophenol (4-CPh)) is a worldwide environmental problem due to their highly toxic nature. The use of non-living Schizophyllum commune fungus (S. commune fungus) to remove phenol, 2CPh and 4-CPh from water under equilibrium and column flow experimental conditions was evaluated. The resulting biosorbent was characterized by BET surface area analysis, Fourier transformer infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) techniques. The effect of experimental parameters such as effect of pH, contact time, initial concentration of adsorbate and amount of biosorbent dosage was evaluated. The experimental data were fitted to various isotherm models. The maximum monolayer adsorption capacity of S. commune fungus for phenol, 2-CPh and 4-CPh was found to be 120, 178 and 244 mg/g, respectively, at 25 ± 2 ◦ C according to Langmuir model. The equilibrium time was found to be 2 h for all adsorbates to complete saturation. Kinetic studies showed the adsorption process followed pseudo second-order kinetic model. The column regeneration studies were carried out for three adsorption–desorption cycles. The eluant used for the regeneration of the adsorbent was 0.1 M NaOH. Based on the results obtained such as good uptake capacity, rapid kinetics, and its low cost, S. commune fungus appears to be a promising biosorbent material for the removal of phenolic compounds from aqueous media. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Phenolic compounds including chlorinated phenols, which are generated by petroleum and petrochemical, coal conversion, phenol producing industries, biocides, and other chemical processes, are common contaminants in wastewater [1]. The discharge of phenols containing domestic and industrial wastewater has caused the environmental contamination by phenols. Many surface waters [2,3], groundwater [4] and soils [5] have been reported to be contaminated by phenol and chlorophenols. The most important aspect is that, phenol is a colorless solid and easily miscible in water. So phenol cannot be identified in water through naked eye. Even small amount of phenol may cause severe diseases like cancer, nausea, vomiting, paralysis, smoky colored urine, etc. Stringent US Environmental Protection Agency (EPA) regulation calls for lowering phenol content in the wastewater to less than 1 mg/L [6]. In view of the high toxicity, wide prevalence and poor biodegradability of phenols, it is necessary to remove them from wastewaters before discharge into water bodies.

∗ Corresponding author. Tel.: +82 54 770 2253; fax: +82 54 770 2280. E-mail addresses: [email protected], [email protected] (N.S. Kumar). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.01.023

The toxicity of phenol, even at a trace level, attracts the environmental scientists to develop suitable technology for their removal from water. As a result of growing awareness over pollution caused by phenol release, efforts are being made to minimize their adverse effect. Currently, many treatment techniques such as biological treatment, activated carbon adsorption, solvent extraction, chemical oxidation and electrochemical methods are the most widely used methods for removing phenol and phenolic compounds from wastewaters [7]. The high cost of activated carbon has stimulated interest in the use of cheaper raw materials. Polymer-based adsorbents are widely employed for the removal of phenols, but the high cost of polymers has also stimulated interest in examining the feasibility of using cheaper adsorbents. Problems such as high cost, low efficiency, and generation of toxic by-products are the limiting factors for wide applications of some of these remediation strategies. Among these techniques, biosorption is an alternative low cost and ecofriendly technology to existing costly water treatment technologies and based on the biomaterial–pollutant interaction. Current researches in this field demonstrate that different kinds of biomaterials interact with dye molecules, heavy metals and other organic substances and they successfully remove these contaminants from aqueous media [8–15]. In the literature there are many materials reported as phenol adsorbent such as activated sludge [16], rice husk [17],

N.S. Kumar, K. Min / Chemical Engineering Journal 168 (2011) 562–571

water-insoluble cationic starch [18] and microbial cells [19]. Among these biosorbents, activated sludge is a well-known biomass used for the removal of phenolic compounds [20]. Aksu and Yener evaluated the biosorption of phenol and monochlorinated phenols on the dried activated sludge [21,22]. Kennedy et al. [23] have reported that the biosorption capacity of live anaerobic sludge was much less than that of live or dead aerobic microbes. In addition to activated sludge, some fungal mycelia and bacterial biomass have also been utilized to remove phenolic compounds through adsorption [24]. However, due to economic restraints, there is a growing interest in the preparation and use of low cost and unconventional adsorbents [25]. The use of microorganisms such as algae, bacteria and fungi with the special surface properties for the biosorption of metallic and organic pollutants from contaminated solutions has long been studied on the laboratory scale and in field studies [26–28]. Phenol, 2-chlorophenol, and 4-chlorophenol have been effectively removed using a brown algae Sargassum muticum [29]. The use of dead biomass is more advantageous than the use of live biomass because there are no toxicity concerns, no requirements of growth media or nutrients and there are easy techniques to desorb contaminants from the biomass and reuse them. Recent literature reviews reveal that the biosorption of PCP by the nonviable Aspergillus niger biomass [30,31] has not been studied so far and only recently, Mathialagan and Viraraghavan [32] conducted a factorial design analysis on the biosorption of PCP from aqueous solutions by a non-viable A. niger biomass. Fungal biomass can also take considerable quantities of organic pollutants from aqueous solutions by adsorption or a related process, even in the absence of physiological activity [33]. 2,4Dichlorophenol has been biosorbed from aqueous solutions by non-living fungal pellets of Phanerochaete chrysosporium [34,35], and bacterial strains such as Achromobacter sp. and Escherichia coli [36]. Benoit et al. [37] have studied the biosorption characteristics of (4-CP) and 2,4-DCP on fungal mycelia (living and non-living) of Emericella nidulans and Penicillium miczynskii. Fungal cell walls and their components have major role in the biosorption [38]. Daughney and Fein [39] described the biosorption of 2,4,6trichlorophenol by Bacillus subtilis. Their results showed that the rapid adsorption of the hydrophobic molecules onto inactivated fungal cell surfaces was the main phenomenon. Other workers [40,41] investigating the biosorption capacity of dead and live fungal biomass found that better removal was achieved with dead fungal biomass than with the living one. Wu and Yu [42] reported the biosorption of phenol and chlorophenols from aqueous solution by fungal mycelia. Biosorption of phenol and 2-chlorophenol by Funalia trogii pellets was studied by Bayramoglu et al. [43]. Kumar et al. have investigated the biosorption of phenolic compounds by Trametes versicolor polyporus fungus [44]. The maximum monolayer adsorption capacities of different adsorbents obtained from different sources are listed in Table 1 along with the values obtained in the present study. The focus of this research was to investigate the biosorption ability of S. commune fungus for phenolic compounds removal as a low cost and unexploited biomaterial. The S. commune fungus was characterized by scanning electron microscopy (SEM), Surface area, pore volume, and pore diameter were obtained on the basis of Brunauer, Emmet, and Teller (BET) measurements. Further this S. commune fungus was characterized before and after adsorption of phenolic compounds by Fourier transforms infrared (FTIR) spectroscopy studies. The effect of various factors, such as time of contact, biosorbent dosage, pH and initial adsorbate concentrations on this biosorption process were investigated under batch equilibrium technique. Moreover, kinetic and equilibrium models were used to fit experimental data. The column flow data were used to generate break through curves. The loaded adsorbent with pheno-

563

Table 1 Maximum adsorption capacities, Q0 (mg/g), for the adsorption of phenol, 2-CPh and 4-CPh compounds by various adsorbents. Sorbent

Bentonite Activated sludge Dried activated sludge Brown alga Sargassum muticum Funalia trogii pellets Inactive–phenol Native–phenol Native–phenol Native–2-CP Trametes versicolor polyporus Fungus Modified bentonite Bentonite & Perlite Dried Sewage sludge Activated sewage sludge Pleurotus sajor-caju fungus Phanerochaete chrysosporium fungus Schizophyllum commune fungus

Adsorbates, Q0 (mg/g) Phenol

2-CPh

4-CPh

References

1.7 86.1 –

– 102.4 281.1

– 116.3 287.2

[6] [21] [22]

4.6

79.0

251.0

[29] [43]

47.6 147 – – 50

– – 227.3 312.5 86

– – –

– – 94.0

– – –

29.46

–

–

[48]

89.3

159.4

188.9

[49]

115.7

191.5

228.8

[50]

120

178

244

Present study

112

[44]

176.6 10.63 & 5.84 –

[45] [46] [47]

lic compounds was regenerated by solvent elution method using 0.1 M NaOH as eluent. 2. Materials and methods 2.1. Chemicals Required raw materials, phenol (Junsei Chemical Co. Ltd., Tokyo, Japan), 2-CPh and 4-CPh (Samchun Chemicals, South Korea) were used without further purification. Stock solutions were prepared by dissolving 1.0 g of phenol, 2-CPh and 4-CPh individually in 1 L of double distilled water. These stock solutions were used to prepare 50, 100, 150, and 200 mg/L solutions of phenol, 2-CPh and 4-CPh. 0.1 M HCl and 0.1 M NaOH, used to adjust pH, were obtained from Samchun Chemicals, South Korea. Water used for preparation of solutions and cleaning adsorbent was generated in the laboratory via the double distillation of de-ionized water in a quartz distillation unit. 2.2. Preparation of Schizophyllum commune fungus biosorbent The S. commune fungus biomass species was collected from the Tirumala Tirupati Hills, Andhra Pradesh, India. The fungus was washed with tap water, followed by de-ionized water to remove extraneous and salts. It was then sun-dried, followed by drying in an oven at 60 ◦ C. Finally the dried fungus biomass were chopped and sieved though a 250 ␮m sieve to obtain a uniform particle size and used as the biosorbent. The dried S. commune fungus was used as biosorbent without any pretreatment for phenolic compounds adsorption. 2.3. Batch equilibrium studies Equilibrium adsorption experiments were conducted in a set of 125 mL Erlenmeyer flasks, where solutions of phenolic compounds 100 mL (phenol, 2-CPh and 4-CPh) with different initial

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concentrations (50–200 mg/L) were added in these flasks. Equal masses of 0.4 g/0.1 L of S. commune fungus were added to phenolic compounds solutions and each sample was kept in a shaking Water Bath Temperature controller of 220 rpm at 25 ± 2 ◦ C for 2 h to reach equilibrium of the solid–solution mixture. The samples were filtered through Whatman No. 50 filter paper (2.7 ␮m size particle retention) to eliminate any fine particles. Then the concentration of phenolic compounds was determined by measuring absorbance using UV–vis spectrophotometer (Shimadzu UV-1601 Spectrophotometer, Japan) at 270 nm, 274 nm and 280 nm for phenol, 2-CPh and 4-CPh, respectively. The amount of phenolic compounds adsorption at equilibrium qe (mg/g) was calculated from the following equation: qe =

(C0 − Ce )V m

(1)

where C0 and Ce (mg/L) are the liquid-phase concentrations of phenolic compounds at initial and equilibrium, respectively. V (L) is the volume of the solution, and m (g) is the mass of dry adsorbent used. The procedures of kinetic experiments were basically identical to those of equilibrium tests. The aqueous samples were taken at preset time intervals and the concentrations of phenolic compounds were similarly measured. The amount of adsorption at time t, qt (mg/g), was calculated by qt =

(C0 − Ct )V m

(2)

2.4. Column adsorption studies Column flow studies were carried out in a column made of Pyrex glass of 1 cm internal diameter and 25 cm length. The column is filled with 2 g of S. commune fungus by tapping so that the column is filled without gaps. All experiments were carried out at temperature of 25 ± 2 ◦ C. The influent solution of known concentration of aqueous solution of phenolic compounds was allowed to pass through the bed at a constant flow rate, 2.0 mL/min, in down flow manner. The complete cycle of operation of each column experiment included three steps: pH precondition of the adsorbate solution, solution flow, and adsorption of solute until column exhaustion occurs. Samples were collected at 2 h time intervals and the concentration of phenolic compounds in effluent solution was monitored spectrophotometrically after making appropriate dilutions. The effluent solution was collected as a function of time and concentration of phenolic compounds in the effluent solution was determined by measuring absorbance using Shimadzu UV-1601 Spectrophotometer. Breakthrough curves were obtained by plotting volume of the solution passed through the column vs. ratio of the column outlet concentration to the initial concentration, Ce /Ci . 2.5. Desorption studies Desorption experiments were performed for a better understanding of the adsorption processes. The regeneration of the sorbent may be crucially important for keeping the process costs down and to open the possibility of recovering the pollutant extracted from the solution. For this purpose, it is desirable to desorb the sorbed pollutants and to regenerate the material for another cycle of application. The desorption side of the process should yield the pollutants in a concentrated form and restore the material close to the original condition for effective reuse with undiminished pollutant uptake and no physical change or damage [51]. Regeneration of S. commune fungus can be succeeded by washing the phenolic compounds (phenol, 2-CPh and 4-CPh) loaded S. commune fungus with a suitable desorbing solution that must be cheap, effective, non-polluting and non-damaging to the adsorbent. Desorption of phenolic compounds was tried with a number of eluents and it

was found that the desorption occurred by sodium hydroxide easily. Desorption of phenolic compounds from loaded S. commune fungus were carried out by a solvent elution method using 0.1 M of NaOH as an eluent. The eluent was pumped in to the column at a fixed flow rate of 1.5 mL/min at temperature of 25 ± 2 ◦ C. From the start of the desorption process, effluent samples were collected at different time intervals and the concentration of the adsorbates was determined. When the concentration of the outlet solution was zero or close to zero, it was assumed that the column is regenerated. After the regeneration, the adsorbent column was washed with distilled water to remove NaOH from the column before the influent adsorbate solution was reintroduced for the subsequent adsorption–desorption cycles. The adsorption–desorption cycles were performed thrice for each phenolic solution using the same bed to check the sustainability of the bed for repeated use. Regeneration curves were obtained by plotting volume of the solution passed through the column vs. concentration of the column outlet solution.

3. Results and discussion 3.1. Characterization of biosorbent 3.1.1. Fourier transform infrared spectral analysis The FTIR spectroscopy method was used to obtain information on the nature of possible adsorbent–adsorbate interactions. The corresponding FTIR spectra of the biosorbent as recorded on a PerkinElmer 283B FTIR spectrometer over the range 4000–400 cm−1 before and after the biosorption of phenolic compounds are depicted as spectra (a)–(g) in Fig. 1. The FTIR spectrum of S. commune fungus biomass before biosorption [Fig. 1, spectrum (a)] shows a broad absorption peak at 3394 cm−1 corresponding to the overlapping of –OH and –NH peaks. A peak at 2926 cm−1 represents the C–H group, whereas the peak in the range of 1646 cm−1 is due to the presence of an amide group (protein). The peaks at 2926, 1541, 1421, 1375 and 1045 cm−1 representing C–H stretching vibrations, N–H bending (scissoring), C–H deformation of alkanes, –CH3 wagging (umbrella deformation) and C–OH stretching vibrations, respectively, were due to several functional groups. Finally, the peaks at 565 and 541 cm−1 corresponding to O–C–O scissoring and C–O bending vibrations were only observed for the fungal biomass. The similar FTIR results were reported for the biosorption of phenolic compounds on various Fungus biomasses [43,44]. In addition, it can be seen from the FTIR spectrum of the biosorbent prior to and after biosorption exhibited significant differences. This is shown in Fig. 1(b) and (c), which depict the spectra of pure phenol and of the S. commune fungus after phenol biosorption, and in Fig. 1(f) and (g) which show the spectra of pure 4-CPh and of the S. commune fungus after 4-CPh biosorption. The both phenol and 4-CP, after biosorption the O–H bond stretching frequency was shifted to a higher frequency region while the C–H stretching frequency was shifted to a lower frequency region. In contrast, Fig. 1(d) and (e), which show the situation before and after the biosorption of 2-CPh, indicate that the O–H bond stretching frequency was shifted to lower frequencies due to the presence of a Cl atom in the ortho position in the molecule. Under these circumstances, an increase will occur in the O–H bond length which, in turn, will lead to an automatic decrease in the O–H frequency, while the C–H stretching frequency is shifted to the lower frequency region. This shift and/or broadening of some of the IR spectral peaks of the biosorbent in the presence of the phenolic compounds studied provides a clear indication that the functional groups present on the biosorbent are involved in an interaction with the phenolic compounds.

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565

Fig. 1. FTIR spectra of phenolic compounds: (a) before biosorption of S. commune fungus; (b) pure phenol; (c) after phenol biosorption; (d) pure 2-chlorophenol; (e) after 2-CPh biosorption; (f) pure 4-chlorophenol; and (g) after 4-CPh biosorption.

3.1.2. Surface area analysis Surface area, pore volume and pore diameter of the S. commune fungus biosorbent were determined with BET (Brunauer, Emmett and Teller) instrument (Model No: Micromeritirics, USA). Surface area was measured by assuming that the adsorbed nitrogen forms a monolayer and possesses a molecular cross sectional area of 16.2 A˚ 2 /molecule. The isotherm plots were used to calculate the specific surface area (N2 /BET method) and average pore diameter of S. commune fungus, while micropore volume was calculated from the volume of nitrogen adsorbed at P/Po 1.4. The surface properties of S. commune fungus are summarized in Table 2.

3.1.3. Scanning electron microscopic studies Scanning electron micrographs of S. commune fungus recorded, using a software controlled digital scanning electron microscopeJEOLJSM 5410 (Eucentric Gonimeter state type) Japan, are given in Fig. 2a and b. In Fig. 2a is a general view which is composed of irregularly branched, filamentous hyphae, typical conidial chains

and well-defined rod clusters in net/mat format. At a high magnification (Fig. 2b), the chains are shown to be consisted of numerous, relatively smooth surfaced conidia and single rod of the biosorbent was focused, where an uneven surface texture along with lot of irregular surface format was observed.

Table 2 Surface properties of S. Communne fungus biomass. S. no.

Parameter

1

Surface area Single point surface area (m2 /g) BET surface area (m2 /g) Pore volume Single point adsorption total pore volume of pores (cm3 /g) Pore size ˚ Adsorption average pore diameter (A)

2 3

4

S. Communne fungus biomass 2.302 3.95 0.0041

37.18

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50

Adsorption capacity (mg/g)

45 40 35 30 25

phenol (100 mg/L) 2-chlorophenol (mg/L) 4-chlorophenol (mg/L)

20 1

2

3

4

5

6

7

8

9

10

11

pH Fig. 3. Effect of pH on the biosorption of () phenol, () 2-CPh and (䊉) 4CPh onto S. commune fungus. Experimental conditions: for phenolic compounds: initial concentrations = 100 mg/L, contact time 3 h, biosorbent dosage = 0.2 g/0.1 L, Temp = 25 ± 2 ◦ C, agitation rate 220 rpm.

Fig. 2. Scanning electron micrographs of the S. commune fungus at (a) 500× and (b) 1000× magnification.

3.2. Effect of solution pH The initial pH of adsorption medium is one of the most important parameters affecting the adsorption process. To evaluate the behaviour of the phenolic compounds biosorption process under more acid and alkaline solutions, experiments were conducted in the pH range from 2.0 to 10.0 using 0.2 g of S. commune fungus with 100 mL of 100 mg/L adsorbate solutions at 25 ± 2 ◦ C. The pH of the medium affects the solubility of phenol or chlorophenols as well as the ionization state of the functional groups on the fungal cell wall. There are several types of functional groups on the fungal cell walls to which phenolic compounds can bind and these include both hydrophobic and hydrophilic groups [such as carboxyl (–COOH), phosphate (PO4 3− ), primary and secondary amines (–NH2 , NH), thiol (–SH) and hydroxy (–OH)] [43]. The phenolic compounds considered in this study, viz. phenol, 2-CPh and 4-CPh, have pKa values of 9.9, 8.3 and 9.2 respectively; hence, they only exist as anions at high pH values [29]. All the phenolic compounds studied act as weak acids in aqueous solution, with the dissociation of hydrogen ions being strongly dependent on the pH value of the solution. These data obtained from these experiments are depicted in Fig. 3 from which it will be seen that the adsorption capacity of the S. commune fungus towards phenol, 2-CPh and 4-CPh increased as the pH value was increased from 2.0 to 5.0. As shown, the equilibrium sorption capacity of phenolic compounds increase when the pH is increased up to 5, which corresponds to the maximum adsorbed amount of phenolic com-

pounds (42 mg/g for phenol, 46 mg/g for 2-CPh and 48 mg/g for 4-CPh). Within the pH range of 5 to 7, there was no significant change. However, when pH was increased further, the sorption capacity of the S. commune fungus decreased to reach its minimum value for pH 10 (22 mg/g for phenol, 29 mg/g for 2-CPh and 34 mg/g for 4-CPh). Under these circumstances, the amino groups associated with the fungus carry positive charges that allow the fungal cell wall components to act as potential scavengers of phenolic compounds. The molecular form dominates in acidic solution, whereas the anionic form is the predominant species in alkaline media. On the other hand, at pH above 7, the solution contains predominantly phenolate anions. As well, the overall surface charge on the cells become negative as the solution pH increased (due to deprotonation of the functional groups), which probably leads to a lower electrostatic attraction between the phenolate anions and the anionic functional groups of cells surface. However, the interactive forces between phenol/chlorophenols and the biomass are rather weak in acidic solution. Similar observations have been reported previously. Thus, for example, the dried and dead fungus Pleurotus sajor-caju showed higher adsorption capacities towards the removal of chlorophenols from aquatic systems at pH 6.0 for all species [49]. The same trend was also mentioned for the case of phenol adsorption onto dried sewage sludge [47]. 3.3. Effect of biosorbent dosage In any adsorption process, the amount of adsorbent plays an important role. In order to evaluate the effect of adsorbent dose (in grams of adsorbent per 1 L of solution) on phenol, 2-CPh and 4-CPh adsorption, various amounts of S. commune fungus, in the range of 0.05 to 0.8 g, experiments were performed at pH 5.0 with 100 mL of 100 mg/L of phenolic compounds concentrations, respectively. The effect of adsorbent dose on the uptake of phenol, 2-CPh and 4-CPh on S. commune fungus was studied and is shown in Fig. 4a–c, respectively. It can be seen from the figures, that percentage removal of phenol, 2-CPh and 4-CPh increases with the increase in adsorbent dose while the adsorption capacity at equilibrium, qe (mg/g), decreases. The percentage of phenolic compounds biosorption steeply increases with the biosorbent loading up to 0.4 g/0.1 L. The increase in adsorbent rate over 0.4 g/0.1 L has not allowed any additional improvement

N.S. Kumar, K. Min / Chemical Engineering Journal 168 (2011) 562–571 100

80 70 % removal

80

% removal

60 50

60

40 40 Adsorption capacity (mg/g)

30 20

20

Adsorption capacity (mg/g)

(a)

567

10 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Amount of adsorbent dose (g) 100

100 90

90

% removal

80

80

% removal

70 70

60

60

50

50

40

Adsorption capacity (mg/g) 40

30 20

30

10

20

Adsorption capacity (mg/g)

(b)

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Amount of adsorbent dose (g) 100

% removal

90

90 80

80

% removal

70 70 60 60

50

50

Adsorption capacity (mg/g)

40 30

40

20

30

Adsorption capacity (mg/g)

(c)

100

10

20 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Amount of adsorbent dose (g) Fig. 4. Effect of biosorbent dosage level on the biosorption of (a) phenol, (b) 2-CPh, and (c) 4-CPh onto S. commune fungus [(% removal of phenol, 2-CPh, 4-CPh and biosorption capacity (mg/g)]. Experimental conditions: for phenolic compounds: initial concentrations = 100 mg/L, biosorbent dosage = 0.05–0.8 g, contact time 3 h, pH 5.0.

in adsorption. This seems to be due to the binding of almost molecules of phenolic compounds to the sorbent and the establishment of equilibrium between the molecules bounded to the sorbent and unadsorbed molecules in the solution. Thus, all our subsequent experiments were performed at an adsorbent dosage of 0.4 g/0.1 L of S. commune fungus. The adsorption capacity was found to be high at low dosages. Many factors can contribute to this adsorbent concentration effect. The most important factor is that adsorption sites remain unsaturated during the adsorption reaction. This decrease in adsorption capacity with the increase in the adsorbent dosage is mainly attributed to the non-saturation of the adsorption sites during the adsorption process [52,53]. Adsorption is maximum with 0.4 g/0.1 L of S. commune fungus and the maximum percent removal is about 95% for phenol, about 96% for 2-CPh and about 98% for 4-CPh. Therefore, the optimum

Fig. 5. Effect of contact time on phenol biosorption [Different initial phenol concentrations. () C0 = 50 mg/L, (䊉) C0 = 100 mg/L, () C0 = 150 mg/L, () C0 = 200 mg/L; pH 5.0; biosorbent dosage = 0.4 g/0.1 L; contact time 3 h, agitation rate: 220 rpm, Temp = 25 ± 2 ◦ C].

biosorbent dosage was taken as 0.4 g/0.1 L for further experiments. 3.4. Effect of agitation time Effect of shaking time on biosorption of phenolic compounds onto S. commune fungus was studied over a shaking time of 20–180 min, using 0.4 g/0.1 L of S. commune fungus, 50–200 mg/L of phenolic compounds concentrations at pH 5, 25 ± 2 ◦ C and 220 rpm shaking speed. The data are represented graphically in Figs. 5–7. The biosorption yield of phenolic compounds increased considerably until the contact time reached 2 h for all the concentrations studied (50–200 mg/L). Further increase in contact time did not enhance the biosorption, so, the optimum contact time was selected as 2 h for further experiments. Therefore this time is sufficient to attain equilibrium for the maximum removal of phenolic compounds from aqueous solutions by S. commune fungus. Adsorption rate of phenolic compounds on S. commune fungus was found to be relatively much faster than those reported for some other normal adsorbents. Thawornchaisit and Pakulanon determined that the sorption equilibrium of phenol on dried sewage sludge was reached within 20 h [47]. Adsorption of bromophenols onto carbonaceous adsorbents derived from fertilizer solid waste was performed by Bhatnagar [54] and reported an equilibrium time of about 8 h.

Fig. 6. Effect of contact time on 2-chlorophenol biosorption [Different initial 2-CPh concentrations. () C0 = 50 mg/L, (䊉) C0 = 100 mg/L, () C0 = 150 mg/L, () C0 = 200 mg/L; pH 5.0; biosorbent dosage = 0.4 g/0.1 L; contact time 3 h, agitation rate: 220 rpm, Temp = 25 ± 2 ◦ C].

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The values of k1 and R2 along with the experimental and calculated uptake capacity qe , are provided in Table 3. The pseudo-second order equation may be expressed as, 1 t t = + qt qe k2 q2e

Fig. 7. Effect of contact time on 4-chlorophenol biosorption [Different initial 4-CPh concentrations. () C0 = 50 mg/L, (䊉) C0 = 100 mg/L, () C0 = 150 mg/L, () C0 = 200 mg/L; pH 5.0; biosorbent dosage = 0.4 g/0.1 L; contact time 3 h, agitation rate: 220 rpm, Temp = 25 ± 2 ◦ C].

3.5. Batch adsorption kinetic modeling Kinetic models are used to determine the rate of adsorption process. The amount of phenolic compounds adsorbed increases with time and reaches maximum in about 2 h indicating the attainment of equilibrium condition. After this equilibrium period, the amount adsorbed did not change significantly with time. The experiments were conducted at temperature (25 ± 2 ◦ C) and with no adjustment of pH. Data on removal of phenol, 2-CPh and 4-CPh by S. commune fungus as a function of time at pH 5.0 at various initial concentrations (50–200 mg/L) are presented graphically in Figs. 5–7. In order to analyze the biosorption kinetics of phenolic compounds onto S. commune fungus, the kinetic models, Lagergren’s pseudofirst order and McKay and Ho’s pseudo-second order [55] models were applied to the experimental data. The first order rate equation of Lagergren is one of the most widely used for the sorption of a solute from liquid solution and is represented as: The Lagergren-first order kinetic model is represented as log(qe − qt ) = log qe −

 k  1 2.303

t

(3)

where qe and qt are the amount of solute adsorbed per unit weight of the adsorbent (mg/g) at equilibrium time, and at time t (min) and k1 (min−1 ) is the rate constant. The straight line plots (not shown here) of log (qe − qt ) vs. t indicate the applicability of the above equation to phenolic compounds biosorption on the biomass.

(4)

where k2 is the rate constant of second order adsorption (g mg−1 min−1 ). The slope and intercept of plots (figures not shown) of t/qt vs. t were used to calculate the second-order rate constant k2 . The validity of both kinetic models was checked through the existence of linear relationships indicated by Eqs. (3) and (4). The pseudo-second-order rate constant (k2 ), correlation coefficients constant (R2 ) along with the experimental and calculated uptake capacity (qe ) are presented in Table 3. In many cases the pseudo-first-order equation of Lagergren did not fit well to the experimental data over the entire range of contact time and was generally applicable over the initial stage of the adsorption processes. The pseudo-first-order kinetic process has been used for reversible adsorption with an equilibrium being established between liquid and solid phases, whereas, the second order kinetic model assumes that the rate limiting step may be chemisorptions [56]. In many adsorbate–adsorbent systems, where both chemical and physical adsorption occurs, the adsorption data are well correlated by the pseudo-second-order equation [57]. As it is obvious from Table 3 that R2 values for the pseudo-second-order model are much higher than those for Lagergren-first-order kinetics and their calculated qe values agreed well with the experimental qe values. It can be concluded that the pseudo-second-order kinetic model fits for the biosorption of phenolic compounds on the S. commune fungus Besides the value of R2 , the applicability of both kinetic models were further verified through normalized standard deviation q (%) defined as



q(%) = 100 ×

 [(qexp − qcal )/qexp ]2

(5)

(n − 1)

where the subscripts ‘exp’ and ‘cal’ refer to the experimental and calculated values, respectively and n is the number of data points. The lower the value of q (%) the model is a better fit for the data. The calculated rate constants for the models, their corresponding regression (R2 ) and normalized standard deviation values are listed in Table 3. The results suggested that the pseudo-second order adsorption mechanism was predominant, and that the overall rate of the phenol adsorption process appeared to be controlled by the chemisorptions process [58]. The similar phenomena have

Table 3 Biosorption rate constants of phenolic compounds on S. commune fungus. Lagergren-first order kinetic model −1

(mg/L)

qe, exp (mg/g)

k1 (min

Phenol 50 100 150 200

11.11 22.87 31.51 35.13

2-CPh 50 100 150 200 4-CPh 50 100 150 200

)

Pseudo-second order kinetic model qe, cal (mg/g)

k2 (g mg−1 min−1 )

R2

q (%)

4.24 7.73 7.04 6.85

11.26 22.12 31.74 35.46

5.02 × 10−3 3.96 × 10−3 2.61 × 10−3 2.27 × 10−3

0.996 0.997 0.999 0.998

0.004 0.026 0.001 0.002

0.994 0.996 0.992 0.994

5.14 9.53 8.16 9.00

12.97 24.69 38.16 47.46

5.08 × 10−3 4.54 × 10−3 2.54 × 10−3 2.15 × 10−3

0.997 0.998 0.997 0.998

0.043 0.017 0.009 0.006

0.991 0.998 0.999 0.994

4.77 7.45 9.59 10.66

15.36 24.93 40.32 48.30

3.90 × 10−3 3.26 × 10−3 2.78 × 10−3 2.67 × 10−3

0.999 0.997 0.994 0.998

0.009 0.040 0.009 0.024

2

qe, cal (mg/g)

R

0.015 0.013 0.016 0.016

6.53 10.15 14.78 16.74

0.981 0.992 0.998 0.997

12.45 24.90 37.42 46.73

0.018 0.015 0.018 0.017

6.80 9.52 16.04 18.69

15.42 25.97 41.12 49.87

0.014 0.015 0.014 0.013

8.68 11.79 15.65 17.29

q (%)

N.S. Kumar, K. Min / Chemical Engineering Journal 168 (2011) 562–571

569

Table 4 Isotherm parameters of Langmuir, Freundlich and D–R isotherms for biosorption of phenol, 2-CPh and 4-CPh on S. commune fungus. Adsorbates

Langmuir model

Phenol 2-CPh 4-CPh

Freundlich model

b

R

KF

n

R

˛

ˇ



R2

120 178 244

0.076 0.018 0.012

0.995 0.997 0.998

5.523 3.516 3.293

1.115 1.145 1.087

0.992 0.997 0.998

3.878 3.369 4.144

0.271 0.056 0.292

0.95 0.75 0.24

0.906 0.939 0.813

2

also been observed in the adsorption of phenol on activated carbons prepared from beet pulp [59] and plum kernels [60]. 3.6. Adsorption isotherm models The effect of concentration of phenol, 2-CPh and 4-CPh on the amount adsorbed per unit mass of adsorbent was obtained by studying the adsorption at different initial concentrations. The Langmuir isotherm is valid for monolayer adsorption on a surface containing finite number of identical sites. The amount of phenolic compounds adsorbed per unit mass of adsorbent, qe (mg/g), was correlated with the liquid phase concentration at equilibrium, Ce (mg/L), using Langmuir, Freundlich, and Redlich–Peterson adsorption isotherms. The linear form of Langmuir isotherm is given by the following equation. 1 1 1 = 0 + Ce qe Q bQ 0

(6)

where qe is the amount adsorbed (mg/g), Ce is the equilibrium concentration of the adsorbate (mg/L), and Q0 and b are the Langmuir constants related to maximum adsorption capacity and energy of adsorption, respectively. The most important characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless separation factor (RL ) which is defined as [61], RL =

1 1 + bC0

(7)

where C0 is initial concentration of phenolic compounds (mg/L) and b is the Langmuir constant (L/mg). RL values indicate whether the adsorption process is irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1), or unfavorable (RL > 1). The value of RL is less than 1 and great than 0, suggesting the favorable uptake of phenolic compounds by S. commune fungus. The Freundlich isotherm is given as, 1/n

qe = KF Ce

(8)

((mg/g) (L/mg)1/n

where KF and 1/n are indicators of the adsorption capacity and the adsorption intensity respectively. The values of KF and 1/n were calculated by plotting ln(qe ) against ln(Ce ). Among, these two models, Langmuir isotherm gives a better representation of adsorption of phenol, 2-CP and 4-CP on S. commune fungus compared to Freundlich model. Redlich and Peterson [62] proposed a three-parameter adsorption model to improve the correlation of the experimental data. The equation takes the form, qe =

˛Ce



(1 + ˇCe )

Redlich–Peterson model

Q

0

(9)

where ˛, ˇ, and  are constants for a given adsorbent–adsorbate system. The three parameters in Eq. (9) were obtained following a linear least squares fitting procedure. The values of the constants of all three models along with the regression coefficient (R2 ) are listed in Table 4. Although all the evaluated equilibrium models gave good fit to the experimental data, it can be concluded that the Langmuir is the best model describing biosorption of phenolic compounds on the S. commune fungus, as it gave the maximum R2

2

value among the considered models. Of course with three parameters and  being an adjustable parameter, the Redlich–Peterson model is expected to give the best fit among the three models. Based on the results of the study, best isotherm models fitted for phenol, 2-CPh and 4-CPh biosorption were determined in the order: Langmuir > Freundlich > Redlich–Peterson isotherm model. 3.7. Column adsorption and desorption studies The results of column flow experiments were used to obtain the breakthrough curves for adsorption of phenol, 2-CPh and 4-CPh from aqueous solutions by plotting (figures not shown) volume of elluent vs. Ce /Ci . The breakthrough capacity, which is the amount adsorbed until the effluent concentration of the adsorbate is equal to the influent solution concentration, are computed from the breakthrough curves. An examination of the curves indicates that no leakage of solute is observed up to a volume of about 80 mL of influent sorbate solution in all cases in the first cycle.To be useful in separation and removal processes, adsorbed species should be easily desorbed under suitable conditions and adsorbents should be used many times in order to decrease material costs. When the bed is exhausted or the effluent coming out of the column reaches the allowable maximum discharge level, the regeneration of the adsorption bed to recover the adsorbed material and/or to regenerate the adsorbent become quite essential. The regeneration could be accomplished by a variety of techniques such as thermal desorption, steam washing, solvent extraction etc. Each method has inherent advantages and limitations. The fixed bed columns of S. commune fungus saturated with phenol or chlorophenol is regenerated by passing 0.1 M NaOH solution as an eluent at a fixed flow rate of 1.5 mL/min. From the desorption plots (figures not shown) it is observed that the rate of desorption increases sharply reaching a maximum with 4.5 mL of 0.1 M NaOH solution and complete regeneration occurred at about 25 mL. The regenerated column is further used for the removal of phenolic compounds. The results indicate that the column gets saturated early and adsorption capacity decreases. As a result, the percent desorption also decreases from first cycle to third cycle. 4. Conclusions Biosorption is a promising alternative to replace or supplement to the currently used treatment techniques for the removal of phenolic compounds from aqueous medium. The adsorption isotherms and kinetics of phenol, 2-CPh and 4CPh were studied. The following results were obtained: 1. The maximum phenol removal was achieved at pH 5.0. The removal of phenol, 2-CPh and 4-CPh at acidic pH is high as compared to alkaline pH and this may be attributed to the fact that at higher pH it form phenolate ion, which decrease its adsorption. 2. The kinetic studies indicated that the adsorption process was extremely fast (equilibrium time is 2 h). The kinetics of phenolic compounds adsorption onto S. commune fungus followed by pseudo-second-order kinetic model. 3. When the S. commune fungus concentration was increased, the equilibrium adsorption capacity (mg/g) of S. commune fungus

570

N.S. Kumar, K. Min / Chemical Engineering Journal 168 (2011) 562–571

decreased, where as the percent removal efficiency increased. 4. The equilibrium adsorption data were fitted to Langmuir, Freundlich, and Redlich-Peterson isotherm models and the isotherm parameters were evaluated. Among the three models, the Langmuir model is capable of representing the adsorption data satisfactorily than the other two models. 5. Sodium hydroxide (0.1 M) was found to be effective in regenerating the column loaded with phenolic compounds. The results indicate that S. commune fungus show higher adsorption capacity for chlorophenols than phenol. This may be explained on the basis of difference in aqueous solubility and hydrophobic nature of phenol, 2-CPh and 4-CPh. 6. S. commune fungus is a very good alternative biosorbent for removal of phenol, 2-CPh and 4-CPh from aqueous solutions. The biosorbent was used without any chemical treatment presenting biosorption capacities of 120, 178 and 244 mg/g for phenol, 2-CPh and 4-CPh, respectively. In view of all these findings, the use of S. commune fungus as a biosorbent can be interesting at the commercial point of view because of its low cost and large availability for the removal of phenol, 2-CPh and 4-CPh compounds.

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