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Single- and Dual-Component Biosorption of Reactive Black 5 and Reactive Orange 16 onto Polysulfone-Immobilized Esterified Corynebacterium glutamicum K. Vijayaraghavan,* Sung Wook Won, and Yeoung-Sang Yun* DiVision of EnVironmental and Chemical Engineering, Research Institute of Industrial Technology, Chonbuk National UniVersity, Chonju 561-756, South Korea
The ability of polysulfone-immobilized esterified Corynebacterium glutamicum (PIEC) to biosorb two reactive dyes [Reactive Black 5 (RB5) and Reactive Orange 16 (RO16)] from single- and dual-dye solutions has been investigated. Single-dye biosorption isotherm experiments revealed that PIEC performed well in the biosorption of RO16, with a higher experimental uptake of 248.1 mg/g, compared to 174.1 mg/g for RB5. Conversely, the uptake of RO16 was suppressed almost 2.5 times in the presence of RB5, whereas PIEC maintained similar RB5 uptakes in both single- and dual-dye systems. Several factors might be responsible for this effect, the most important being the number of sulfonate groups and the size of each reactive dye. An attempt to achieve the continuous biosorption of reactive dyes from single- and dual-dye solutions using an upflow packed column was successful, with PIEC being regenerated and reused for three cycles. 1. Introduction Reactive dyes, one of the prominent and most widely used types of azo dyes, are typically azo-based chromophores combined with different reactive groups. They are extensively used in many textile-based industries because of their favorable characteristics, such as bright color, water-fastness, and simple application.1 However, up to 50% of reactive dyes are lost through hydrolysis during the dyeing process, and therefore, a large quantity of the dyes appears in wastewater.2 These dyestuffs are designed to resist biodegradation and are barely removed from effluents using conventional wastewater treatments, such as activated sludge.3 In recent years, biosorption has been described by several investigators as an efficient and economical method for the remediation of reactive-dye-bearing wastewaters. Some efficient biosorbents reported in the literature for reactive-dye biosorption include Corynebacterium glutamicum,4 Escherichia coli,5 Pseudomonas luteola,5 and Rhizopus arrhizus.6 C. glutamicum, which was used in the present study, is a gram-positive organism and widely used for the biotechnological production of amino acids. Our previous examinations7 using C. glutamicum for the biosorption of reactive dyes revealed that strong acidic conditions are required to obtain maximum uptake. This is also the case for other microorganisms for the biosorption of reactive dyes, with the results from several publications coinciding on this aspect.4,8 This might be because of nature of the functional groups responsible for the biosorption, and suitable chemical modification might solve this practical problem. Even though microbial biomass exhibits excellent dye biosorption abilities, it has poor mechanical strength and little rigidity, which, in turn, limits its application under real conditions. Immobilization is a possible practical method for the successful reuse of this biosorbent over multiple cycles. The choice of immobilization matrix is a key factor in the environmental application of immobilized biomass, which determines * To whom correspondence should be addressed. Tel.: +82 63 270 2308 (Y.-S.Y.), +82 63 270 2308 (K.V.). Fax: +82 63 270 2306 (Y.-S.Y.), +82 63 270 2306 (K.V.). E-mail:
[email protected] (Y.-S.Y.),
[email protected] (K.V.).
the mechanical strength and chemical resistance of the final biosorbent particles.9 Although considerable information has been accumulated on the biosorption of single-component dye systems, many industries discharge effluents that contain several dye components. In these cases, biosorption becomes competitive, where one solute competes with another to occupy the binding sites. Multicomponent dye adsorption has been the subject of a few studies,10,11 but the mechanism and effects of competition remain to be fully understood. The evaluation and predication of multicomponent sorption equilibria are tedious and are still among the most challenging of the problems in the adsorption field. Therefore, the objective of this work was to study dual-dye biosorption in batch and column modes of operation. To achieve this goal, polysulfone-immobilized esterified Corynebacterium glutamicum (PIEC) was used as the biosorbent for Reactive Black 5 and Reactive Orange 16 in both single- and dual-dye systems. 2. Materials and Methods 2.1. Biosorbent. The fermentation wastes (C. glutamicum biomass) were obtained in the form of a dried powder from a lysine fermentation industry (BASF-Korea, Kunsan, Korea). The biomass was ground and sieved to obtain particle sizes within the range 0.1-0.25 mm. For esterification, 2.5 g of biomass was mixed with 150 mL of anhydrous methanol and 1.5 mL of concentrated HCl in a rotary shaker at 125 rpm and 25 °C for 6 h.12 The suspension was then filtered and washed sequentially with deionized water, 0.2 M sodium carbonate, and finally deionized water. The biomass was dried in an oven at 60 °C for 12 h and subsequently used. The general esterification scheme is as follows H+
RCOOH + CH3OH 98 RCOOCH3 + H2O
(1)
A 9% (w/v) solution of polysulfone was prepared in N,Ndimethylformamide (DMF) solution. After the mixture had been stirred for 10 h, the esterified biomass (14%) was mixed with the polysulfone slurry. The slurry was dripped into deionized water, where beads formed by phase inversion. The beads were
10.1021/ie071537p CCC: $40.75 © 2008 American Chemical Society Published on Web 04/04/2008
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Figure 1. Chemical structures of (a) Reactive Black 5 and (b) Reactive Orange 16. Table 1. Characteristics of Reactive Black 5 and Reactive Orange 16
color index number molecular weight chemical formula purity (%) λmax (nm) calibration constant at 597 nm calibration constant at 494 nm
Reactive Black 5
Reactive Orange 16
20505 991.82 C26H21N5Na4O19S6 55 597 0.0197 0.0080
17757 617.54 C20H17N3Na2O11S3 50 494 0.0001 0.0174
then washed with deionized water and placed in a water bath for 18 h to remove all residual DMF. The resultant polysulfoneimmobilized esterified C. glutamicum (PIEC) beads (1-2 mm diameter) were then stored at 4 °C. 2.2. Dyes and Analysis. Reactive Black 5 (RB5) and Reactive Orange 16 (RO16) were purchased from Sigma-Aldrich Korea Ltd. (Yongin, Korea); their chemical structures are presented in Figure 1, and their general characteristics are listed in Table 1. Dye concentrations were analyzed using a spectrophotometer (UV-2450, Shimadzu, Kyoto, Japan). For single-dye solutions, the dye concentrations were determined by measuring the optical densities at the corresponding maximum wavelengths λmax. In the case of binary mixtures, the dye concentrations were determined as previously described.13 For a two-solute system, components A and B were measured at λ1 and λ2, respectively, to give optical densities of d1 and d2
CA )
CB )
KB2d1 - KB1d2 KA1KB2 - KA2KB1 KA1d2 - KA2d1 KA1KB2 - KA2KB1
(2)
2.3. Batch Experimental Procedure. Batch biosorption experiments were conducted in a 50-mL plastic bottle (highdensity polyethylene), using 1 g (wet weight) of PIEC and 40 mL of dye solution. For dual-dye solutions, the dye concentration ratio was always maintained at 1:1 (in weight basis). The pH of the solution was initially adjusted and controlled using either 0.1 M HCl or 0.1 M NaOH. The dye-biosorbent mixture was then agitated in an incubated shaker at 160 rpm. After 16 h of contact, the supernatant was separated, and the dye concentrations were analyzed after appropriate dilution. In the present study, the uptake capacity of PIEC is reported in terms of the amount of dye biosorbed per gram of dry beads. However, for the sake of better understanding, the amount of biomass per gram of dry beads was calculated on the basis of the mass balance, which was determined as 0.56 g of biomass per gram of dry beads. The dye-loaded PIEC, which was previously exposed to 500 mg/L of each dye solution at pH 4, was separated from the solution by filtration. The biosorbent was then brought into contact with a known volume of 0.1 M NaOH for 2 h, on a rotary shaker at 160 rpm. The remaining procedure was the same as that employed in the biosorption equilibrium experiments. 2.4. Modeling of Batch Experimental Data. The singlecomponent biosorption isotherms were characterized using the Langmuir and Freundlich models, which can be expressed in their nonlinear forms as
Langmuir model Q)
where KA1, KB1, KA2, and KB2 are the calibration constants for components A and B at the two wavelengths λ1 and λ2, respectively (Table 1).
(4)
where Cf is the final dye concentration (mg/L).
Freundlich model Q ) KFCf1/n
(5)
where Qmax is the maximum dye uptake (mg/g), b is the Langmuir equilibrium constant (L/mg), KF is the Freundlich constant [(mg/g)(L/mg)1/n], and n the Freundlich constant. The multicomponent biosorption isotherms were assessed using an extended Langmuir equation with a constant interaction factor,14 which can be represented for binary mixtures as
Q1 ) Q2 )
Qmax1b1(C1/η1) 1 + b1(C1/η1) + b2(C2/η2) Qmax2b2(C2/η2) 1 + b1(C1/η1) + b2(C2/η2)
(6)
(7)
where Qmax1, Qmax2, b1, and b2 are the single-component Langmuir parameters for the first (RB5) and second (RO16) dyes and η1 and η2 are the corresponding interaction factors. The calculation of the interaction factor is based on minimization of the error function10
100
n
∑
n - p i)1 (3)
QmaxbCf 1 + bCf
[
]
(Qe,meas - Qe,calc)2 Qe,meas
(8)
i
where n and p are the numbers of data points and parameters, and Qe,meas and Qe,calc represents measured and calculated dye uptake values (mg/g), respectively. The Sheindrof-Rebhun-Sheintuch (SRS) equation15 was also employed in the present study; for binary mixtures, it can be expressed as16
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(Q)ij ) KFiCfi[Cfi + θijCfj][(1/ni)-1]
(9)
where (Q)ij is the amount of solute i adsorbed per unit weight of PIEC in the presence of solute j, KFi is the single-component Freundlich constant for solute i, ni is the Freundlich exponent for solute i, and θij is the competitive coefficient. All model parameters were evaluated using nonlinear regression employing the SigmaPlot (version 4.0, SPSS, Chicago, IL) software. 2.5. Column Experimental Procedure. A glass column with an inner diameter of 1 cm and a height of 12 cm was employed for the column experiments. The column was initially packed with 4.8 g (wet weight) of immobilized beads, and operated at a flow rate of 0.5 mL/min using a peristaltic pump. The initial concentrations were fixed at 100 and 200 mg/L for the singleand dual-dye systems, respectively (with 100 mg of RB5/L and 100 mg of RO16/L in the latter case). Effluent samples were collected at the exit of the column at different time intervals and then analyzed for their dye concentrations. Regeneration experiments were conducted by employing 0.1 M NaOH as the elutant at 1 mL/min. After elution, deionized water was pumped through the column to wash the bed until the pH of the wash water stabilized near 7.0. The regenerated bed was reused for the next cycle. 2.6. Analysis of Column Experimental Data. The breakthrough (tb) and exhaustion times (te) represent the times at which outlet dye concentration reached 1 and 95 mg/L, respectively. The column dye uptake was calculated as previously described.17 The other column parameters were calculated as follows
total dye removal removal (%) )
mad × 100 C0Fte
(10)
desorption efficiency E (%) )
md × 100 mad
(11)
where C0 is the initial dye concentration (mg/L), F is the flow rate (L/h), mad is the mass of dye adsorbed (mg), and md the mass of dye desorbed (mg). 3. Results and Discussion 3.1. Effect of pH and Dye Biosorption Mechanism. In single-dye systems, PIEC performed well in the biosorption of both RO16 and RB5 in the pH range of 1-4 (Figure 2), with approximately 94% decolorization at pH 4. A decreased dye uptake was observed with further increases in the solution pH. It should be noted that the uptake values presented in Figure 2 are on a weight basis (mg/g), but on a mole basis (mmol/g), PIEC biosorbed more RO16 than RB5. The cell walls of grampositive bacteria mainly consist of a peptidoglycan layer connected by amino acid bridges.18 Embedded in the grampositive cell wall are polyalcohols, known as teichoic acids, which give an overall negative charge to the bacterial cell wall because of the presence of phosphodiester bonds between teichoic acid monomers.19 In our previous studies, we confirmed that C. glutamicum is mainly composed of carboxyl, amine, and phosphonate groups. Under acidic conditions, because of protonation of the functional groups, the biomass has a net positive charge.4 Conversely, in solutions, the reactive dyes RB5 and RO16 exist as ROSO3- and Na+ ions. Therefore, the colored
Figure 2. Effects of pH on the uptakes of RB5 and RO16 by PIEC in single- and dual-dye systems [temperature ) 25 ( 1 °C, agitation speed ) 160 rpm, C0 (single-dye) ) 500 mg/L, C0 (dual-dye) ) 1000 mg/L (500 mg of RB5/L + 500 mg of RO16/L)].
negatively charged dye ions exhibit an electrostatic attraction toward the positively charged cell surface. In particular, the amino groups present in C. glutamicum are mainly responsible for the biosorption of reactive dyes, with the hydrogen ions acting as bridging ligands between the bacterial cell wall and the dye molecules.7 It is a well-known fact that reactive dyes are designed to form irreversible covalent bonds with the amino acid residues of cellulosic fibers. Our previous study also identified the number of amine groups in C. glutamicum to be 0.68 mmol/g.4 Also, it should be noted that the pKa values of amine groups lie at 9.1. This means that they are completely protonated at pH values of less than approximately 7.4 However, the presence of other functional groups, such as a negatively charged carboxyl groups, gives rise to a repulsion against the negatively charged dye anions. The pKa value of carboxylic groups usually lies within the range of 3.5-4.5.4,20 Therefore, carboxyl groups are negatively charged at pH values higher than approximately 4 and electrostatically hinder the access of reactive-dye anions to the bacterial biomass. These are the reasons why highly acidic conditions (pH < 2) are required for the optimum biosorption of reactive dyes, although such conditions are highly unfavorable in reality. However, in this study, the C. glutamicum biomass was esterified, and hence, constant optimum reactive-dye biosorption capacities were observed up to pH 4 (Figure 2). In the case of dual-dye systems, PIEC almost maintained its original RB5 biosorption capacity in the presence of RO16, exhibiting a slight decrease in the uptake of RB5 only at pH g 5. In contrast, the uptake of RO16 was affected at all pH values examined. At pH 4, PIEC exhibited uptakes of 83.5 mg of RB5/g (0.084 mmol of RB5/g) and 69.4 mg of RO16/g (0.112 mmol of RO16/g). Adsorption is a process in which several parameters can be involved, related to either the sorbent characteristics (specific surface area, pore size distribution, etc.) or the sorbate structural features (molecular weight, shape, molar volume, flexibility, branching, etc.).21 In several instances, adsorption seems to be correlated with the molecular size of the dye: a tendency might be that the smaller the dye, the faster and more extensively it will be sorbed.11,21 However, it should be noted that, when the pores are large enough to admit RO16 but too small to admit RB5, pore blockage might be the dominant competition mechanism. However, single-component biosorption data (Figure 2) revealed that almost complete RB5 and RO16 decolorization was observed. Therefore, direct site competition might be the predominant competition mechanism,22 with both dyes competing to occupy the same amine group. Both RB5 and RO16 are based on vinyl sulfone and have
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Figure 3. Biosorption isotherms at pH 4 for single- and dual-dye systems (temperature ) 25 ( 1 °C, agitation speed ) 160 rpm). Single-component isotherm curves (solid lines) were predicted using the Freundlich model, and dual-component isotherm curves (dotted lines) were predicted using the SRS equation.
moderate reactivities. As shown in Figure 1, RB5 has two sulfate and two sulfonic acid groups compared to one of each in RO16. The sulfonate groups are related to the biosorption of reactive dyes, and each associates with one amine group of PIEC. This accounts for the utilization of two amine groups by RB5, but only one in the case of RO16.11,23 Therefore, in singlecomponent systems, the smaller size and single sulfonic acid group of RO16 appear to be more accessible than the two groups of RB5, as was also shown by our experimental results (Figure 2). However, in binary systems, the additional utilization of the amine groups by RB5 seems to affect the biosorption of RO16. 3.2. Biosorption Isotherm and Modeling. PIEC exhibited steeper isotherms for RO16 than for RB5 (Figure 3). In general, the steepness of the isotherm indicates the degree of affinity of the sorbate toward the sorbent.9,24 Owing to the smaller size of RO16, its affinity would be expected to be higher than that of the bulky RB5. At pH 4, the highest experimental uptakes were 174.1 mg of RB5/g (0.176 mmol of RB5/g) and 248.1 mg of RO16/g (0.402 mmol of RO16/g). A reverse trend was observed in the dual-component biosorption isotherm, where PIEC accommodated more RB5 than RO16 ions (Figure 3). In the presence of RO16, PIEC biosorbed almost identical, but slightly lower, amounts of RB5 ions as in the pure RB5 system. Conversely, the uptake of RO16 was severely affected in the presence of RB5 compared to its biosorption in the pure state. To be precise, the highest experimental uptakes observed in the dual-dye systems were 154.5 mg/g (0.156 mmol/g) and 99.3 mg/g (0.161 mmol/g) for RB5 and RO16, respectively. For the purpose of comparison, on a mole basis, the single-component uptake of RO16 was approximately 2.3 times higher than that of RB5. This result roughly coincides with our prediction that two amino groups were associated with the biosorption of RB5. In addition, the total molar uptake (of both RB5 and RO16) in the binary systems was 0.317 mmol/g, and this was even less than the uptake of RO16 in its pure state. This result also supports the conclusion that RB5 competes well and overutilizes the amino groups, which results in decreased RO16 uptake. Also, interactions between the two reactive dyes would be significant, affecting the two components to different extents. This explanation is supported by the fact that RO16 biosorption seems to be affected even at low RB5 concentrations. In other words, at low dye concentrations, adequate binding sites are available to accommodate both dye anions, but the uptake of RO16 still
seems to be affected. This clearly indicates that the repulsion caused by RB5 inhibits RO16 from associating with amine groups. Application of the Langmuir model to describe the biosorption isotherms resulted in an underprediction of maximum dye uptake values. The Qmax values of RB5 and RO16 were determined to be 241.6 and 169.3 mg/g, respectively. The values of the constant b, which represents the affinity between the sorbent and sorbate, were 0.018 and 0.019 L/mg, respectively, for RB5 and RO16. This result indicates that the two dyes have almost equal affinities for the amine groups of C. glutamicum. Therefore, the competition between the two dyes in binary systems might be due to the repulsion between dye anions and the stoichiometric relationship between the dye anions and available amine groups. The correlation coefficients and percent error values were within the ranges 0.964-0.986 and 2-10%, respectively. The Freundlich model described the isotherm data with high correlation (0.971-0.992) and low percent error values (1-7%) compared to the Langmuir model. The binding capacity constant KF values were determined to be 43.7 and 41.0 (mg/g)(L/mg)1/n for RB5 and RO16, respectively, whereas the Freundlich affinity constants were actually 0.19 for RB5 and 0.25 for RO16. For dual-component isotherm modeling, an extended Langmuir equation with a constant interaction factor was used. The traditional extended Langmuir equation assumes no interaction between solutes, which is not valid in real conditions.10 To incorporate sorbate-sorbate interactions and competition, an interaction factor (η) was introduced into the extended Langmuir equation.14 In the present study, η was assumed to be constant and specific for each dye in the dual-component system; values of 0.02 and 0.27 were determined for RB5 and RO16, respectively. The model was able to reasonably describe the RB5 biosorption isotherm in the dual-dye system. However, it was found to be completely inadequate for the RO16 isotherm, as a very low correlation coefficient (0.36) and high error value (81%) were obtained. The reason for this discrepancy might be that the competition and interactions between the dye solutes were significant and the saturation capacities of the two dyes in the single-solute systems were also different.10 It should be noted that single-component Langmuir isotherm model constants were used to model the multicomponent biosorption data.10 Also, the interaction factor, η, does not have a strong theoretical foundation; indeed, several investigators have employed the model with only limited success.10,14 In a subsequent attempt at dual-component modeling, we used the SRS equation. This is a multicomponent Freundlich-type equation that includes a competitive coefficient (θij) and is based on the assumption that an exponential distribution of adsorption energies is available for each solute. Interestingly, the model was able to describe RB5 and RO16 dual-component isotherms, with both acceptable correlation coefficients and percent error values, within the ranges of 0.838-0.947 and 3.8-12.6%, respectively. As was already shown in our experiments, the effect of RB5 competition on RO16 biosorption was significant, and this result was confirmed by the SRS equation yielding a competitive coefficient of 3.75 compared to 0.02 for RO16 over RB5. 3.3. Biosorption Kinetics and Modeling. Analysis of singledye biosorption kinetic data (Figure 4) revealed that the lowermolecular-weight RO16 biosorbed, and reached equilibrium, faster than the bulky RB5. In binary systems, the uptake of RO16 was lower than that of RB5. Comparison of the singleand dual-dye data shows that PIEC retained RB5 capacity in
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Figure 4. Biosorption kinetics of single- and dual-dye systems [pH ) 4, temperature ) 25 ( 1 °C, agitation speed ) 160 rpm, C0 (single-dye) ) 500 mg/L, C0 (dual-dye) ) 1000 mg/L (500 mg of RB5/L + 500 mg of RO16/L)].
both cases whereas RO16 uptake was severely affected in the presence of RB5. Also, it is worth noting that the attainment of RO16 equilibrium was delayed in the presence of RB5, directly indicating the difficulty of RO16 in accessing the amine groups of PIEC. In an attempt to visualize the influence of mass-transfer resistance on dye uptake, the kinetic data were analyzed using the equation proposed by Weber and Morris25
qt ) kit1/2
(12)
where q+ is the uptake at any time (mg/g) and ki is the intraparticle diffusion constant (mg/g‚min0.5). According to the Weber-Morris model, a plot of qt versus t1/2 should be linear if intraparticle diffusion is involved in the sorption process, and if this line passes through the origin, then intraparticle diffusion is the rate-controlling step. However, plots of the Weber-Morris model for both single-component RB5 and RO16 data showed multilinearity (figures not presented). This implies that the process involves more than one kinetic stage (or sorption rate).26 For instance, the biosorption of RB5 onto PIEC exhibited three stages, including a first linear portion up to t1/2 < 5.5, a second portion for 5.5 < t1/2 < 13.4, and a third portion for 13.4 < t1/2. The first can be attributed to the sorption of RB5 over the surface of biomass and hence is the fastest sorption stage. The second, ascribed to intraparticle diffusion, is relatively slow. The third stage can be regarded as reflecting diffusion through smaller pores, which is followed by the establishment of equilibrium. Thus, intraparticle diffusion is clearly not the fully operative mechanism, as the slope of the plot does not pass through the origin. The calculated values of the intraparticle diffusion constants (ki) were 4.2 and 5.0 mg/g‚min0.5 for the biosorptions of RB5 and RO16, respectively. Thus, the smaller RO16 molecules produced a higher diffusion coefficient, resulting in a higher biosorption capacity (on a mole basis). In the case of dual-dye systems, the rate of dye uptake depends not only on the diffusion mechanisms but also on the extent of competition between the dye solutes. Each dye will repel the other and compete to occupy the same binding sites. These factors usually further delay attainment of equilibrium, as was also observed in our experiments (Figure 4). Therefore, it is not appropriate to describe the dual-component kinetics data using the Weber-Morris model, and this was not attempted in this study. 3.4. Desorption and Regeneration. Biosorption experiments revealed that reactive dyes can be effectively sorbed under acidic
Figure 5. Breakthrough curves during single- and dual-dye biosorption in an upflow PIEC loaded column [bed height ) 10 cm, flow rate ) 0.5 mL/ min, inlet solution pH ) 4, C0 (single-dye) ) 100 mg/L, C0 (dual-dye) ) 200 mg/L (100 mg of RB5/L + 100 mg of RO16/L)].
conditions, and therefore, desorption would be expected under alkaline conditions. Our previous study also confirmed the necessity of an alkaline environment for the effective elution of reactive dyes, which was provided by 0.1 M NaOH.7 The volume of NaOH was fixed at 20 and 25 mL for the singleand dual-dye systems, respectively. The use of two different elutant volumes was due to the fact that the total numbers of moles of dye present on the biosorbent were different in the two cases. The elutant performed well in both systems, with desorption efficiencies of >99.4% in all cases. The reason for these superior desorption efficiencies might be that, under strongly basic (high pH) conditions, the number of negatively charged sites increases. These negatively charged sites on the sorbent surface favor desorption of dye anions as a result of electrostatic repulsion. In the next step, PIEC was employed in three sorptiondesorption cycles. PIEC maintained consistent reactive-dye biosorption capacities, and 0.1 M NaOH provided full desorption efficiencies in all three cycles examined. In single-dye systems, the uptakes of RB5 and RO16 at the end of third cycle were 86.4 and 83.2 mg/g, respectively, with a decrease of only 1.4% compared to those obtained in the first cycle. Even though competition exists in dual-dye systems, PIEC maintained consistent RB5 and RO16 uptakes of 82.6 and 64.2 mg/g, respectively, in all three cycles examined. In all cases and all cycles, the desorption efficiencies were greater than 98.2%. These results imply that the binding sites of C. glutamicum are active enough to accommodate reactive dyes over multiple cycles. Polysulfone immobilization helped the biosorbent to perform well during the regeneration experiments and is known to be stable in both acidic and alkaline environments. Also, the weight loss was insignificant (
