Evaluation of Fermentation Waste (Corynebacterium Glutamicum) As a Biosorbent for the Treatment of Nickel(II)-Bearing Solutions

Biochemical Engineering Journal 41 (2008) 228–233

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Evaluation of fermentation waste (Corynebacterium glutamicum) as a biosorbent for the treatment of nickel(II)-bearing solutions

K. Vijayaraghavan a, Min Woo Lee b, Yeoung-Sang Yun a,∗ a Division of Environmental and Chemical Engineering, Research Institute of Industrial Technology, Chonbuk National University, Chonbuk 561-756, South Korea b Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 561-756, South Korea article info abstract

Article history: This research highlights the possibility of employing a fermentation industry waste (Corynebacterium Received 3 November 2007 glutamicum) for the removal of nickel(II) ions from aqueous solution. Furthermore, it necessitates the Received in revised form 15 February 2008 importance of detailed examinations on the possible differences in the biosorption performance, even for Accepted 28 April 2008 the same biomass, but from different origins. Two types of C. glutamicum, obtained from different industrial sources, were used in this study. With respect to nickel speciation and biosorption performance, pH Keywords: 6 was identiﬁed as an optimal condition. Of the two types of C. glutamicum used, the biomass with excess Biosorption negatively charged groups performed well in the binding of Ni2+ ions. To enhance the feasibility of using Immobilization Waste-water treatment the biomass in column mode, as well as its reuse for multiple cycles, C. glutamicum was immobilized Packed bed bioreactors in a polysulfone matrix. Both the free and immobilized biomasses performed relatively well, with max- −1 Fermentation waste imum experimental uptakes of 111.4 and 102.4 mg g , respectively. An up-ﬂow packed column loaded Nickel with immobilized biomass was employed for the removal of Ni2+ ions. The column performed well in the biosorption of nickel(II), and exhibited a delayed and favorable breakthrough curve, with Ni2+ uptake and percentage removal of 48.1 mg g−1 biomass and 60.4%, respectively. © 2008 Elsevier B.V. All rights reserved.

1. Introduction recently been published [2,4]. However, its ability to bind heavy metals is relatively unknown. The production of waste biomass from fermentation industries Even though these microbial wastes possess excellent biosorp- increases every year, with its disposal having been a central issue tion abilities, they have poor mechanical strength and little rigidity; in Korea for many years. Ocean dumping, land fill and incineration which in turn limits their application under real conditions [5]. have been the most widely used methods for the disposal of these In industrial treatment schemes, a continuous mode of opera- wastes. However, environmental issues have recently come to the tion is usually preferred, for which the sorbent should be porous forefront, which has made these disposal techniques less attractive. and sufficiently resilient to withstand the application pressures. Alternatively, fermentation wastes can be used for several applica- Therefore, alternative technologies, in particular immobilization, tions, thereby increasing their value. Several studies have reported may be adapted to enhance the mechanical strength of micro- on the use of fermentation wastes for the removal of heavy metal bial mass. Beolchini et al. [6] immobilized Sphaerotilus natans in ions, dyes and other organic pollutants [1,2]. a polysulfone matrix, and successfully reused the biosorbent for Corynebacterium glutamicum, a Gram-positive organism, 10 cycles of copper biosorption. The choice of immobilization belonging to the order of Actinomycetes, is widely used for the matrix is a key factor in the environmental application of immo- biotechnological production of amino acids. Currently, the produc- bilized biomass. The polymeric matrix determines the mechanical tion of amino acids via fermentation processes using C. glutamicum strength and chemical resistance of the final biosorbent particles amounts to 1,500,000 t of l-glutamate and 550,000 t of l-lysine per for their successive utilization during sorption–desorption cycles year [3]. Hence, the utilization of C. glutamicum waste in a further [7]. application has attracted much interest. Several reports on the Nickel, the model solute used in this study, is one of the most potential of C. glutamicum to biosorb different reactive dyes have widely used heavy metals in electroplating industries. The effluent emanating from these industries is often associates with high concentration of Ni(II) ions, which are toxic to both higher and lower ∗ Corresponding author. Tel.: +82 63 270 2308; fax: +82 63 270 2306. organisms and; thus, need to be treated prior to discharge. Thus, E-mail address: [email protected] (Y.-S. Yun). this study explored the possible utilization of fermentation waste

(C. glutamicum) for the removal of nickel(II) ions from aqueous solutions in both batch and up-ﬂow column mode of operations.

2. Materials and methods

2.1. Biosorbent and sorbate

The fermentation wastes (C. glutamicum biomass) were obtained in the form of a fine powder from two lysine fermentation industries. The biomasses obtained from Daesang Corporation (Kunsan, Korea) and BASF-Korea (Kunsan, Korea) were designated as C. glutamicum-1 and C. glutamicum-2, respectively. Both biomasses were washed with deionized water and dried subsequently at 60 ◦C for 12 h. The dried biomasses were then sieved and particles with average diameter of 125 ␮m were selected for biosorption studies. For immobilization of the biomass, a 9% (w/v) solution of poly- Fig. 1. Potentiometric titration curve of C. glutamicum (obtained from Daesang Cor- sulfone was prepared in N,N-dimethyl formamide (DMF) solution. poration, Kunsan, Korea). The curve was predicted using the proton-binding model + − After stirring the mixture for 10 h, C. glutamicum-1 (14%) was mixed (Eq. (2)). The minus and plus values represent the amounts of added H and OH , respectively. with the polysulfone slurry, with the resultant slurry dripped into deionized water, where beads were formed by a phase inversion process. The beads were then washed with deionized water, and of 20 cm. The column was then fed, using a peristaltic pump, with − placed in a water bath for 18 h to remove any residual DMF. The 100mgL 1 (pH 6) of Ni2+ solution in an up-flow mode, at a flow rate − resultant beads (1–2 mm in diameter) were then stored at 4 ◦C, and of1mLmin 1. Samples were collected at the desired time intervals designated “PIC” in this paper. using a sample collector arrangement at the exit of the column, All reagents, including NiSO4·6H2O, were of AnalaR grade and and the Ni(II) concentrations then analyzed. The operation of the purchased from Sigma–Aldrich. column was stopped when the effluent Ni concentration exceeded 97% of that at the inlet. After exhaustion of the column, desorption was carried out by pumping 0.1 M HCl upwards through the column 2.2. Potentiometric titration and FT-IR at a flow rate of 2 mL min−1. Both batch and column data modeling were performed using The titrations of the biosorbent were carried out at con- non-linear regression with the Sigma Plot (version 4.0, SPSS, USA) stant temperature (25 ◦C), with 30 mL CO stripped water and 2 software. The average percentage error between the experimental 0.3 g biomass, in 50 mL plastic bottles (high-density polyethylene). and predicted values was calculated using: Either 1 M HNO3 or NaOH was added to each biomass suspen- sion. The air-tightened bottles were then agitated at 160 rpm and N Q − Q /Q i= ( exp,i cal,i) exp,i ε (%) = 1 × allowed to equilibrate for 24 h. Thereafter, the equilibrium pH was N 100 (1) measured using an electrode (Ingold). Infrared spectra of the C. glutamicum samples were obtained where Qexp and Qcal represent the experimental and calculated using a Fourier transform infrared spectrometer (FT/IR-Nicolet metal uptake values, respectively, and N the number of measure- NEXUS-470). The sample, prepared as KBr disc, was examined ments. within the range 400–4000 cm−1 to identify the functional groups responsible for biosorption. 3. Results and discussion

2.3. Experimental procedure 3.1. Potentiometric titration

2.3.1. Batch experiments Attempts were initially made to study the functional groups Free biomass (0.1 g) or wet PIC beads (1 g) were brought into present in the two types of C. glutamicum biomass. The biomass contact with 40 mL of the desired Ni(II) concentrations in a 50-mL titration data for C. glutamicum-1 are reported in Fig. 1. These data plastic bottle (high-density polyethylene). The pH of the solution permitted the qualitative and quantitative determination of the was initially adjusted to the desired value and controlled using nature and number of binding sites present on the bacterial cell 0.1 M HCl or NaOH. The bottles were then kept in an incubated walls. To describe the titration curve, the proton-binding model rotary shaker at 160 rpm and 25 ◦C. After equilibrium had been proposed by Yun et al. [9] was used in this study. The ﬁnal form of attained, the supernatant was separated and analyzed for nickel the model equation can be expressed as concentrations [8], after appropriate dilution. M N b X b X K For the desorption experiments, nickel(II)-laden PIC beads (pre- − = i − j + W − + −1 2+ [OH ]added + + + [H ] viously exposed to 500 mg L of Ni solution at pH 6) were 1 + [H ]/Ki 1 + [H ]/Kj [H ] i= j= brought into contact with 20 mL of 0.1 M HCl for 30 min, and agi- 1 1 (2) tated on a rotary shaker at 160 rpm. The remaining procedure was − + the same as that employed in the biosorption equilibrium experi- where [OH ]added and [H ] are the concentrations of hydroxide and ments. hydrogen ions (mmol L−1) added, respectively, and X is the biomass concentration (g L−1). Eq. (2) contains two parameters per func- 2.3.2. Column experiments tional group: moles of functional group per gram of biomass (b) A glass column (1.5-cm i.d. and 25-cm height) was packed with and the equilibrium constant (K). The model assumes the presence 19.7 g (wet weight)/4.4 g (dry weight) of PIC, to yield a bed height of four binding sites. A detailed derivation of the proton-binding 230 K. Vijayaraghavan et al. / Biochemical Engineering Journal 41 (2008) 228–233

Fig. 3. The effects of solution pH on nickel(II) biosorption (initial Ni2+ concentration = 500 mg L−1; temperature = 25 ◦C). Fig. 2. FT-IR spectrum of C. glutamicum biomass (obtained from Daesang Corpora- tion, Kunsan, Korea). 3.3. pH edge

The solution pH will affect the speciation of metals and the model can be found elsewhere [2,9]. To find the number and nature activities of the functional groups in the biomass. In the present of binding sites, the model was fitted to the titration curve (Fig. 1) study, the pH edge experiments (Fig. 3) revealed that pH > 4 favored using the Marquardt–Levenberg non-linear regression algorithm nickel(II) biosorption. The negatively charged functional groups, within the Sigma Plot (version 4.0, SPSS, USA) software. Three such as carboxyl and phosphonate, can attract and bind positively negative (pKH = 2.47 ± 0.09, 4.90 ± 0.09 and 6.82 ± 0.05) and one charged nickel ions. For instance, the pKa value of the carboxylic positive (pKH = 10.35 ± 0.08) functional groups were predicted by groups was earlier identified as 4.9 (Section 3.1). Therefore, they the proton-binding model, with a high correlation coefficient of have negative charges at a pH approximately higher than 4.9, and 2+ 0.992. The first functional group (pKH = 2.47 ± 0.09) was relatively thereby, attract positively charged Ni ions. The reason for the low unknown; however, nearly 2.57 ± 0.34 mmol g−1 was present in C. Ni(II) uptake under strongly acidic conditions was due to compe- glutamicum-1. The number of carboxyl groups (pKH = 4.90 ± 0.09), tition between protons and nickel ions for occupying the binding found via the proton-binding model, was 0.41 ± 0.02 mmol g−1. sites. The nickel(II) uptake increases with increasing pH; however, The third group, whose pKH and bj values were 6.82 ± 0.05 and the control experiments revealed that nickel tended to precipitate 0.63 ± 0.02 mmol g−1, respectively, was assigned as either phos- at pH ≥ 7. Considering the speciation, within the pH range 1–6, − 2+ phonate (B-HPO4 ) or dicarboxylic groups [2]. The last functional nickel will exist only as Ni ions [14]. Above pH 6, the formation of + group (pKH = 10.35 ± 0.08) appeared to be an amine (B-NH3 ), nickel hydroxide will be initiated. Therefore, precipitation may also which generally show pKH values between 8.5 and 11, depending contribute to the nickel uptake at pH 7. This phenomenon may com- on the biomaterial [2,10]. Also, the amine groups were abundantly plicate the understanding of the actual biosorption potential of the −1 found in C. glutamicum-1 (bj = 3.85 ± 0.35 mmol g ). bacterial biomass. Therefore, pH 6 was identified as the optimum Our previous research [2] on the titration data of C. glutamicum- pH for nickel biosorption onto C. glutamicum. 2 also revealed the presence of carboxyl, phosphonate and amine On comparing the two types of C. glutamicum biomass, C. groups; however, their total contents were less than those found glutamicum-1 performed well for nickel(II) biosorption. This may be in C. glutamicum-1. The carboxyl, phosphonate and amine groups due to an excess of negatively charged groups on the cell walls of C. were present at 0.32 ± 0.01, 0.56 ± 0.02 and 0.68 ± 0.02 mmol g−1, glutamicum-1, which can attract Ni2+ ions. Hence, C. glutamicum-1 respectively [2]. was selected for further studies. Due to its poor mechanical strength and little rigidity, C. glutamicum biomass is known to cause problems during column 3.2. FT-IR analysis operations [4]. To avoid these problems, C. glutamicum-1 was immobilized in a polysulfone matrix, and subsequently employed To better understand the nature of the functional groups present for biosorption studies. The polysulfone-immobilized C. glutam- in C. glutamicum-1, the FT-IR spectrum was obtained (Fig. 2). The icum (PIC) also exhibited a similar trend to that of the free biomass, broad absorption band in the range 3600–3000 cm−1 indicated with a maximum uptake at pH ≥ 6. In comparison, PIC exhibited the existence of the amine groups and the –OH of the carboxyl slightly less Ni2+ uptake to that of the free biomass. This dimin- groups [11]. A medium strength absorption peak at 1384 cm−1 was ished uptake may be attributable to some of the binding sites not assigned to the symmetrical stretching of a carboxylic acid [12]. being easily accessible or blocked due to the immobilization pro- The phosphonate groups showed characteristic absorption peaks cess. However, this decrease will be minimal when the possibility around 1161 cm−1 (P O stretching), 1065 cm−1 (P O C stretch- of recycling of biosorbent for a number of cycles is considered. No ing) and 968 cm−1 (P OH stretching). The absorption peaks around significant Ni(II) biosorption was observed when polysulfone beads 1651 (N H bending band) and 1539 cm−1 (H N C stretching) were (without biomass) were used (Fig. 3). indicative of the existence of amine groups [13]. Thus, the FT- IR spectrum supports the presence of carboxyl, phosphonate and 3.4. Biosorption isotherms and modeling amine groups in C. glutamicum-1. Previously, Won et al. [2] confirmed the presence of carboxyl, phosphonate and amine groups in To evaluate the maximum biosorption potential of C. glutam- C. glutamicum-2. Based on these findings, the nickel(II) biosorption icum, isotherm experiments were conducted at pH 6 (Fig. 4). Typical potential of the two types of C. glutamicum was explored. L-shaped biosorption isotherms were observed for both the free K. Vijayaraghavan et al. / Biochemical Engineering Journal 41 (2008) 228–233 231

Fig. 5. Nickel(II) biosorption kinetics at pH 6 (initial Ni2+ concentration = Fig. 4. Nickel(II) biosorption isotherms at pH 6 (temperature = 25 ◦C). Curves were 500mgL−1; temperature = 25 ◦C). Curves were predicted using the pseudo-second predicted using the Toth model. order model. and immobilized forms of C. glutamicum. The ratio between the A three parameter model, viz. the Toth model, was also used Ni2+ concentration remaining in solution and that sorbed onto the to improve the fit of the biosorption isotherm data. The Toth solid decreases with increases in the Ni2+ concentration, provid- isotherm, derived from potential theory, has proven to be use- ing a concave curve with a severe plateau. As expected, a high Ni2+ ful in describing the sorption in heterogeneous systems, such as uptake was observed with the free form of C. glutamicum. phenolic compounds onto carbon. This assumes an asymmetrical Modeling of the Ni2+ isotherm data was attempted using both quasi-Gaussian energy distribution, with a widened left-hand side, the Langmuir and Toth models, which can be represented as fol- i.e., most sites have sorption energy less than the mean value [16]. lows: Further improved R2 values (>0.99) and lower percentage error val- 2+ Q bC ues (<0.4%) were observed when describing the Ni biosorption Langmuir model : Q = max f (3) isotherms using the Toth model. In the case of the free biomass, + bC 1 f −1 −1 the Qmax, bT and nT values were 119.3 mg g , 0.008 L mg and − Q b C 0.72, respectively. Whereas, for PIC, these values were 107.1 mg g 1, max T f Toth model : Q = /n n (4) −1 + b C 1 T T 0.007 L mg and 0.65, respectively. The successful application of [1 ( T f) ] the Toth model to the present data confirmed the surfaces of the −1 where Qmax is the maximum metal uptake (mg g ), b the Lang- biosorbent were heterogeneous and contained different functional −1 muir equilibrium constant (L mg ), bT the Toth model constant groups. and nT the Toth model exponent. The Langmuir model incorporates two easily interpretable constants: Qmax, which corresponds to the 3.5. Biosorption kinetics and modeling maximum achievable uptake of the system and b, which relates to the affinity between the sorbate and sorbent. The Langmuir The sorption kinetics is significant in the treatment of waste- constant “Qmax” is often used to compare the performance of biosor- water, as it provides valuable insights into the reaction pathways bents; while “b” characterizes the initial slope of the isotherm. and mechanisms of sorption reactions. Since biosorption is a Thus, for good biosorbents; in general, a high Qmax and steep ini- metabolism-independent process, it would be expected to be a tial isotherm slope (i.e., high b) are desirable [15]. The maximum very fast reaction. Experimental kinetic data for the free biomass nickel uptake values, 130.4 and 120.9 mg g−1, were found for the coincided with this expectation, with more than 90% of Ni2+ ions free biomass and PIC, respectively; whereas, b values of 0.009 and removedinthefirst3h(Fig. 5). This initial quick phase was fol- 0.008 L mg−1 were shown in the cases of free biomass and PIC, lowed by slow the attainment of equilibrium as a large number respectively. The correlation coefficient (R2) values were greater of vacant binding sites will initially be available for sorption; but than 0.99; whereas, the percentage error values were less than 1.3%. thereafter, the occupation of the remaining vacant sites will be dif- The nickel biosorption capacity observed in this study was superior ficult due to the repulsive forces between the metal ions in the to the results published in the literature (Table 1). solid and bulk phases. Compared with the free biomass, the attain-

Table 1 Comparison of the nickel biosorption capacities of the various biosorbents

Adsorbent Experimental condition Maximum uptake capacity according References to the Langmuir model (mg g−1)

Corynebacterium glutamicum M =2.5gL−1;pH6;T =25± 1 ◦C 130.4 This work PIC M =5.6gL−1;pH6;T =25± 1 ◦C 120.9 This work Sargassum wightii M =2gL−1;pH4;T =30◦C 81.2 [22] Polyporous versicolor M =0.6gL−1;pH5;T =35◦C 57.0 [23] Pseudevernia furfuracea (L.) Zopf M =2gL−1;pH4;T =35◦C 49.9 [24] Lyngbya taylorii M =2.2gL−1; pH 4.7 38.2 [25] Streptomyces rimosus M =3gL−1; pH 6.5 32.6 [26] Chlorella vulgaris M =2.2gL−1; pH 4.7 24.1 [25] C. vulgaris M =2.5gL−1;pH5;T =25◦C 13.9 [27] Thuja orientalis M =5gL−1;pH4;T =25◦C 12.4 [28] Yeast M =1gL−1; pH 6.75 11.4 [29]

M = biosorbent dosage; T = temperature. 232 K. Vijayaraghavan et al. / Biochemical Engineering Journal 41 (2008) 228–233 ment of equilibrium with PIC was time consuming, which was not altogether surprising, since when immobilized, the biomass was retained within the interior of the immobilized matrix; whereas, the binding sites of the free biomass were freely exposed to Ni2+ ions. With immobilization systems, mass transfer resistances play a significant role in deciding the rate of biosorption, but for successful immobilization systems, these mass transfer resistances should not influence the overall biomass biosorption performance. Most importantly, the metal ions should have access to all possible binding sites, even at a slower rate. The experimental biosorption kinetic data were modeled using pseudo-first and pseudo-second order kinetics, which can be represented in their non-linear forms, as follows:

Pseudo-first order model : qt = qe(1 − exp(−k1t)) (5) Fig. 6. Nickel(II) sorption and elution breakthrough curves (initial Ni2+ con- 1 −1 2+ Pseudo-second order model : qt = qe 1 − (6) centration = 100 mg L ;Ni solution pH 6; bed height = 20 cm; sorption flow − − 1 + qek2t rate=1mLmin 1; elutant = 0.1 M HCl; elutant flow rate=2mLmin 1).

−1 where qe is the amount of solute sorbed at equilibrium (mg g ); −1 With the aim of evaluating the biosorption potential of PIC dur- qt the amount of solute sorbed at time t (mg g ); k1 is the first −1 ing repeated cycles, regeneration experiments were conducted for order equilibrium rate constant (min ) and k2 the second order equilibrium rate constant (g mg−1 min−1). three cycles. The PIC beads approximately retained their first cycle −1 With the pseudo-first order model, the correlation coefficients Ni(II) uptake (85.5 mg g biomass) throughout the three cycles, were found to be above 0.98; however, the model was unable to pre- which was aided by the consistently high elution efficiencies of dict the equilibrium uptake values over the entire time period. The around 99% with the use of 0.1 M HCl as the eluent. Most impor- reason for these differences in the qe values was due to a time lag, tantly, the weight loss was insignificant (<5%) by the end of the possibly resulting from a boundary layer or an external resistance three consecutive cycles. controlling the initial stage of the sorption process [17].Inmost published cases, the pseudo-first order model did not fit well with 3.7. Biosorption in an up-flow packed column the kinetic data over the entire contact time range; therefore, generally underestimates the qe values [18,19]. Thus, the good linearity Continuous biosorption studies are of utmost importance in obtained with Lagergren plots is no guarantee that the interactions evaluating the technical feasibility of a process for real applica- will follow first order kinetics [20]. The first order equilibrium rate tions. Therefore, in this study, a PIC-loaded up-flow packed column − constants were 0.03 and 0.02 min 1 for the free biomass and PIC, was devised, and employed for the biosorption of nickel(II) ions. respectively. Fig. 6 shows the breakthrough curve generated during the biosorp- Contrary to the pseudo-first order model, the second-order tion of nickel(II) ions. The PIC column bed performed well for the − model predicted the qe values over the entire study range. The biosorption of Ni2+ ions, with the breakthrough (1 mg L 1 in the correlation coefficients were always greater than 0.99, with the pre- effluent) appearing after only 8 h of column operation. Thereafter, dicted curves showing excellent agreement with the experimental the PIC bed still performed well, resulting in a smooth breakthrough − data (Fig. 5). The equilibrium rate constant values were 4 × 10 4 curve, but finally became exhausted (99.2 mg L−1 in the effluent) − − − and 3 × 10 4 gmg 1 min 1 for the free biomass and PIC, respec- after around 32 h of column operation. The sorption zone (differ- tively. The predicted equilibrium uptake (qe) values were 97.8 and ence between the breakthrough and exhaustion times) and slope − 89.5 mg g 1 biomass for the free biomass and PIC, respectively; of the breakthrough curve (from the breakthrough to exhaustion which were in good agreement with those found experimentally. time) were 24.2 h and 5.01 mg L−1 h−1, respectively. The column uptake [4] and percentage removal were calculated as 26.1 mg g−1 −1 3.6. Desorption and regeneration of biomass beads (48.1 mg g biomass) and 60.4%, respectively. The successful design of a column sorption process requires Batch desorption experiments were aimed at evaluating the prediction of the concentration–time profile or the effluent break- possibility of recycling the biomass over three cycles. In the pH through curve [21]. Also, the maximum sorption capacity of a edge experiments (Fig. 3), under strongly acidic conditions, the H+ sorbent is required for the design. The Thomas model is able to ions were revealed to occupy negatively charged groups and; thus, fulfill this necessity, and hence was used in this study, which can very little uptake was recorded with C. glutamicum at pH 2. Thus, be expressed in non-linear form as it would be logical that the sorbed Ni2+ ions can be recovered by C k 0 = + TH Q M − C V ) (7) further reducing the pH values below 2. Therefore, 0.1 M HCl was C 1 exp F ( 0 0 eff used as an elutant, which performed very satisfactorily with both −1 the free and immobilized biomasses. Desorption efficiencies were where Q0 is the Ni(II) uptake (mg g ); kTH the Thomas model −1 −1 always greater than 99%, but considerable weight loss (approxi- rate constant (L mg h ); M the PIC weight (g) and Veff the mately 20%) was observed in the case of free biomass, due to the effluent volume (l). The Ni2+ uptake predicted by the Thomas −1 extreme conditions. Also, the separation of the biomass from the model (Q0) was 48.3 mg g biomass, which agreed well with that final elution solution was difficult; therefore, centrifugation and fil- obtained experimentally. The rate constant (kTH), which character- tration are of utmost importance. Conversely, the polysulfone beads izes the rate of solute transfer from the fluid to the solid phase, were stable under acidic conditions, with no weight loss observed. was 0.003 L mg−1 h−1. The breakthrough curve simulated by the Considering the good stability and ease of metal desorption, the PIC Thomas model, as presented in Fig. 6, coincided well with the exper- beads were employed in regeneration experiments. imental data. K. Vijayaraghavan et al. / Biochemical Engineering Journal 41 (2008) 228–233 233

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