Removal and recovery of lead fixed-bed biosorption with immobilized bacterial biomass

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Pergamon

WadeL Tech. Vol. 38.No. 4-5 . pp. 171-178,1998. IAWQ C 1998 Publishedby ElsevierScienceLtd, Printedin GreatBritain. All rightsreserved 0273-1223/98 $19'00+ 0'00

PII: S0273-1223(98)OO526-5

REMOVAL AND RECOVERY OF LEAD FIXED-BED BIOSORPTION WITH IMMOBILIZED BACTERIAL BIOMASS Jo-Shu Chang, Jeng-Charn Huang, Chia-Cheng Chang and Tsueng-Jeng Tam Department01ChemicalEngineering. Feng Chia University. 100 Wen-Hwa Road.

Taichung, Taiwan, ROC

ABSTRACT Fixed-bed columns packed with immobilized biomass of Pseudomonas aeruginosa PU21 were utilized to remove lead(Pb) from the contaminated water. Effects of the immobilization method, bed length, flow rate, and the particle size on the performance of Pb removal by the biosorption columns were systematically investigated. Calcium alginate-immobilized cellswere found to hold better Pb capacity than polyacrylamide (PAA)-entrapped cells. Typical saturation capacity of calcium alginate (CA)-immobilized cells was 280 mg Pb/g, and 31 mg Pb/g for PAA-immobilized cells. Results of fixed-bed biosorption showed that the breakthrough time (Ib) appeared to increase withthe bed length, but decreased withthe flowrate. The typical overall adsorption efficiency (Q) was within 50-600/.. and did not appreciably fluctuate withchanges in the operation conditions or the particle size. The initial rate of adsorption was facilitated nearly 40% as the size of immobilized cells was reduced from 3.5 mm to 2 mm, whereas the particle size did not affect the equilibrium adsorption of the immobilized biomass. The length of unused bed (LUB) remained constant with different bed length, while it slightly increased with the raising of the Pb loading rate. The metal-laden column was regenerated by elution of HCI solution (pH 2.0). For up to four adsorption/desorption (AID) cycles, the metal recovery efficiency of eachcyclewasover 98%, andthe recovery ratio was 8:I and 27: J for PAA and CA-immobilized cells. respectively. The regenerated beds were able to restoreover 66% of their original adsorption capacity afterfoursuccessive AID cycles.~ J998 Published by Elsevier Science Ltd. Allrights reserved

KEYWORDS Immobilized cells. biosorption, alginate, polyacrylamide, fixed-bed reactor. lead. Pseudomonas aeruginosa

INTRODUCTION Biomass of a variety of microorganisms, including bacteria. fungi, and algae, is capable of adsorbing or accumulating metal ions due to metal-attracting compositions on their cell walls or via metabolismdependent intracellular metal uptake mechanisms (Gadd, 1988), The use of biomass as a potential metal adsorbent has caughtgreat attention, and much work has been devoted to the field of so called "biosorption" for the past decades (Volesky and Holan, 1995). However, effortsto develop practical biosorption processes havebeenrelatively lacking. Although freely suspended biomass mayhavebettercontactwith the adsorbates during the adsorption, the biomass suspension is normally not the practical form for the direct use in biosorption processes. The biomass is often immobilized to enhance its stability, mechanical strength, reusability, and the easeof handling, and thus cell immobilization techniques have beenincorporated into the development of biosorption processes (Volesky and Holan, 1995). Biosorption with immobilized cells is frequently designed as a fixed-bed reactor. inside which desired types of immobilized biomass are packed (Macaskie, 1990; Volesky and Prasetyo, 1994). However, thus far little work has been contributed to 171

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determine the effects of the process characteristics (type of immobilization. bed length. flow rate, particle size,etc.) on the performance of the fixed-bed biosorber. Our previous work (Chang and Hong, 1994; Chang et al., 1997) has shown that biomass of Pseudomonas aeruginosa PU21 can effectively adsorbheavy metals, including mercury, lead.copper, and cadmium. In this study, cells of P. aeruginosa PU21 were immobilized at different biomass loading by two cell-entrapment approaches. The resulting immobilized cells were examined for their ability to adsorb Pb in up-flow fixedbedcolumns. The efficiency of removal and recovery of Pb from the adsorption columns wasevaluated with different operation conditions. The characteristic parameters of the fixed-bed process. such as breakthrough time (16), length of unused bed (LUB), and total adsorption efficiency (Q) were identified to provide informative data for the design of large-scale biosorption processes. MATERIALS ANDMETHODS Bacterial strain and cultivation

Pseudomonas aeruginosa PU21, an auxotrophic derivative of PAOI. was isolated from clinical sewage by Jacoby (1986). The strain harbors a 142.5 Kb plasmid Rip64 which encodes for the narrow-spectrum mercury resistance (Jacoby, 1986). The strain was cultivated aerobically at 37°C in Luria-Bertani (LB) broth (Difco), which was amended with 25 mg Hg2+JL to avoid contamination from other microorganisms. Mass production of the biomass was achieved with a 5 L fennentor (Eyela Jar Fennentor MDF) equipped with devices that measure and control temperature, dissolved oxygen level(DO), pH, and agitation speed. Cell immobilization methods

Preparation of biomass: Cells of P. aeruginosa PU21 were harvested by centrifugation (15.000 g, 8 min) from early-stationary cultures with cell density of approximately 1-2 gIL. After being rinsed twice with deionized, reverse osmotically treated water, the cells were prepared at designated concentrations with phosphate-buffered saline(PBS)to be readyfor cell immobilization operations. Immobilization with calcium alginate (CA) : A cell solution of 15-20 gIL was mixed with sodium alginate (Sigma) at a 2% weight to volume (w/v) ratio. The mixture was introduced into a syringe, and was then pressured to drop into 0.1 M CaCh solution to form particles of approximately 2 mm in diameter. The particles weresuspended in the CaCh solution for at least 12 h to enhance their mechanical stability. Immobilization with polyacrylamide (PAA): Detailed procedures were described elsewhere (Nakajima and Skaguchi, 1993). In general. 4.5 ml of the prepared cell solution (15-20 gIL) was rapidly mixed with a solution containing 0.68g of acrylarnide monomer, 0.034g of N,N'-methylene-bisacrylarnide, 0.34 ml of potassium persulphate (5%, w/v), and 0.3 ml of 3-(dimethylamino)propionitrile (2.5%, w/v) on a shallow plate. All the chemical reagents described here were obtained from Sigma. After completion of polymerization (about I h), the resulting gel-like slice was cut into 2.5 mm cubes, which were rinsed with deionized waterbefore beingutilizedin the adsorption operations. Measurement of Lead

The lead compound used in this study was Pb(N03n (Riedel-de Haen, Inc.) Total Pb concentration in the solution wasmeasured with a Polarized Zeeman Atomic Absorption Spectrometer (Hitachi Model-l-6100). Adsorption Equilibrium Experiments

Immobilized cells of P. aeruginosa PU21 were suspended in solutions amended with Pb2+ in the concentration range of 0-2000mgIL. In general, each adsorption batch contained 1.4 gramsof immobilized cells per liter of Pb solution. The adsorption solutions were gently agitated at 3~C, and pH values of the

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solutions were initially adjusted to 5.0. As the adsorption reached equilibrium (5 to 10 h), samples were takenfrom eachbatch. andthe metal concentration in the supernatants wasmeasured. Adsorption of Pb by immobilized cells with different biomass loading

The immobilized cells wereprepared with similar procedures described in previous sections, except that the cell concentrations were adjusted to obtain different weight ratios between the biomass and the support matrix. Batch adsorption experiments were then performed with the immobilized cells to investigate the effectof biomass loadings. The initial Pb concentration was 1000 mgIL for CA-immobilized cells. and 300 mg/L for PAA-immobilized cells. The pH was 5.0 initially. The concentration of the solute (Pb) was measured as the adsorption reached equilibrium. Operation of Fixed-bed Biosorption

The immobilized cells were stacked intoglasscolumns (2.2em in diameter) withthe bed length in the range of 5 to 21 em, The Ph-bearing solution (10 mg/L) was continuously pumped upward into the column to avoidchannel effects. The void space for the beds with PAA-immobilized cells (2.5 mm cubes) was 0.543, and for beds packed with 2, 3. 3.5 mm particles of CA-immobilized cells were 0.618, 0.591, and 0.468, respectively. The Pb loading rate was ranged from 300 to 1200 mllh. Samples were collected from the effluent to measure for residual Pb concentrations. As the bed wassaturated, the Pb loading was terminated, and the bed waselutedwith HCI solution (pH 2.0)to recover the loaded Pb ions.The acid regeneration was operated at a flowrate identical to that for metal feeding. The regenerated bed was washed thoroughly with deionized waterbefore beingusedfor thenextadsorption run. The characteristics, as well as effectiveness of the fixed-bed biosorption, were evaluated with the breakthrough time (Ib), total adsorption efficiency (Q), and length of unused bed (LUB). All those parameters were determined from the breakthrough curves. The tb is defined as the time spanduringwhich the effluent concentration of Pb was under 10% of Pb concentration in the feed. The Q valueis the ratio of total amount of Pb adsorbed at the time of saturation versus total amount of Pb flow in for the same time interval. The LUB value, frequently usedfor scale-up estimation, represents the fraction of the bed which is not utilized at the breakthrough time. By Ruthven's (1984) definition, LUB=L(l_/~), where L denotes total bed length; tb denotes breakthrough time, and t· denotes the time when the effluent Pb concentration is 50% of its entering concentration. For more efficient mass-transfer (or lessmass-transfer resistance) the Ib is closerto I., resulting in a smaller LUB. The LUB depends on the mass-transfer rate, the flow rate, and the shapeof the equilibrium curve, but is assumed to be independent of total bed length (Ruthven, 1984). Therefore, a large scale unit can be designed according to the LUB value obtained from small-scale laboratory tests for the samesuperficial velocity and particle size. RESULTS ANDDISCUSSION Adsorption isotherms of Pb on the immobilized bleserbents

Theadsorption isotherms ofPb on P. aeruginosa PU21 cells immobilized withPAA (biomass loading" 7.4 wt%) and CA (biomass loading=38 wr'1o) were demonstrated in Fig. 1 and Fig. 2, respectively, Adsorption equilibria for cell-free PAA and CA matrices were also determined as the blank control (Fig. I and 2). Langmuir and Freundlich models wereutilized to describe the experimental data, and the estimated kinetic constants were listed in Table I. It can be clearly observed from Fig. I and 2 that CA-immobilized cells exhibited much higher Pb adsorption capacity (280 mg/g) than PAA-immobilized cells (31 mglg) did. Langmuir isotherm had good prediction for the adsorption equilibrium of Pb on both types of immobilized cells, while the deviation between Freundlich model estimation and the experimental data was relatively higher (Table I). Table 1 also shows that the dissociation constant (Ktt) was over 10-fold lower for CAimmobilized cells than for PAA-immobilized cells, suggesting that CA-immobilized cells had betteraffinity to Ph than the PAA-immobilized cellsdid. It wasalso found that the cell-free CA matrix showed strong Ph

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adsorption ability(Table 1), and its saturation capacity for Pb (350 mglg)was slightly higher than that of the immobilized cells (Fig. 2). In contrast, Pb adsorption by the PAA matrix was very limited (Fig. 1). These results seemto show that the alginate matrix may be an effective metal biosorbent. In fact, there have been examples in the literature demonstrating that alginate and other biopolymers wereappliedfor the removal of heavy metals (Janget al, 1991).

3Or-----------, Freundlich model 25

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Langmuir model PAA- immobilized cell

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PM-matrix

400

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CA-matrix CA-immobilized cell Freundlich model Langmuir model

6

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.

o ....

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o

200

400

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100

1000

500

Equilibrium concentration (mg PbIL)

1000

1500

2000

Equilibrium concentration (mg PbIL)

Fig.1 Adsorption isotherms of Pb on polyacrylamide (PAA)-immobilized cells, and on the PAA matrix

Fig.2 Adsorption isotherms of Pb on calcium alginate (CA)-immobilized cells, and on the CA matrix

Table1 Kinetic parameters of adsorption isotherms estimated by Langmuir and Freundlich Models Langmuir model Typeof Adsorbent calcium alginate matrix calcium alginate immobilized cell polyacrylamide matrix polyacrylamide immobilized cell

KtJ

qmaJI

6.7 20.7

350 280

243

31

Freundlich model

Sumof square residuals 2977 3735 50

k

lin

17.5

Sumof square residuals 0.1298 8160 0.1837 9012

2.26

0.3482

143.5

22

llll

Langmuir model: q=qmIXCe/{KtrI"Ce); Freundlich model: q=kCe ; where, q: adsorption capacity for Ph qmax: maximal Pb adsorption capacity, Ce: equilibrium concentration ofPb, Kc dissociation constant Effed of biomass loadings on biosorption As shownin Fig. 3, there is a general trend that increases in the weightratio of biomass in the immobilized cells facilitated Pb adsorption efficiency. As the biomass loading of CA-immobilized cells was increased from 0 to 44 wt%, the adsorption efficiency increased only 10% (from 84 to 94%), which is much less significant than a six-foldenhancement observed for PAA-immobilized cells when the biomass loading was increased from 0 to 7%. These results seem to be closely related to the Pb adsorption capacity of the two kinds of immobilization matrices. The improvement of adsorption efficiency by increasing biomass loading on CA-immobilized cells was greatly restricted by a high adsorption capacity of cell-free CA matrix, which already contributed to 84% of Pb adsorption efficiency, when the initial Pb concentration was as high as 1000 mgIL. On the other hand, since PAA matrix can barely adsorb Pb (Fig. 1), there is much room for improvement of the adsorption efficiency by raising the biomass loading on PAA-immobilized cells, in which the biomass contributed to the majority ofPb adsorption capacity.

Fixed-bed biosorption

17S

Fixed-bed biosorption with polyacrylamide-immobilized cells

Figure 4 demonstrates the typical breakthrough curve of Pb with a fixed-bed bioreactor containing PAAimmobilized cells. In the fixed-bed operation, the bed length (L) was 16em, flow rate (v) was 300 mlIh, and the biomassloading was 7.4 wflo. The fixed-bed adsorption was also performed at the same flow rate for 11em and 21-cm beds to investigate the effect of the bed-length on the breakthrough time (tb) an4 total adsorption efficiency (Q). The results were presented in Table 2, which shows that as the bed len~ was increased from 11 em to 21 em, the lb was extendedcorrespondinglyfrom 2.4 h to 4.3 h, whereasthe Qvalue did not vary appreciably. When the 16-cm fixed-bed was fully saturated, the loaded Pb was desorbed by flushing with HCIsolution(pH=2). The desorption profile, presented in Fig. 4, exhibiteda sharp increase of Pb concentration at the beginning of acid elution. with the maximum concentration peaked at 190 mg/L, which is 19-fold of feeding Pb concentration in the adsorption operation. Above 98% of loaded Pb was recovered after being eluted with HCI solutionfor only 1.5 h (equivalent to a flow-in volume of 450 ml), in contrast to about 12-hfeeding of Ph-bearing solutionof 10 mg PblLto fullysaturatethe column. That is, the acid-elution resulted in a recovery ratio (a ratio between volume flow in and recovery volume) of 8:1. The adsorption/desorption (AID) cycle was repeated three times to justify the reusability of the column. As indicated in Table 3. the lb decreased slightly as the number of cycle increased. The decrease in lb after repeated uses of the column was most likely attributed to the HCI washing operations. Acid treatmentmay either alter the conformation of Ph-adsorption sites on the immobilized cells, or may destroy part of metalbinding functional groups. Besides, a portion of biomass in the column may be washed out after repeated AID operations. Nevertheless. the acid treatment has been a popular approach to regenerate metal-laden biomass, and was found to cause minor effects on the regenerated biosorbents (Mattuschka and Straube, 1993). The Q values of the three AID cycles remained in the range of around 50-61%, and the regenerated biomass retained 80% and 71 % of its original Pb adsorption capacity for the second and third cycle, respectively.

3O,.-----------r M

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2

3

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. '; ~ "D Biema.. loading for Polylayllmide celli (wI%) o

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Fig.3 Effects of biomass loading on Pb adsorption efficiency for CA- and PAA·immobilized cells Table 2 The effects of bed-length on Pb adsorption characteristics for PAA-immobilized cells bed length(cm) 11 2.4 16 3.5 21 4.3

ts(h)

Q(%)

8.13 11.S6 16.98

47.4 58.6 50.9

Fig.4 Adsorption and desorptionprofiles of PAAimmobilized cells in the fixed-bed operation (bed length=16cm) Table 3 The effects of AID cycles on Pb adsorption characteristics for PAA-immobilized cells tb (h) ts(h) Q (%) Regeneration efficiency (%) cycle1 3.47 10.90 61. 100 cycle2 1.25 14.24 so 80 cycle3 1.11 IS.83 so 71

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Fixed-bed biosorption with calcium alginate-immobilized cells

The effect of bed-length: Fixed-bed columns with the bed-length of 5, 10, and IS em were operated at a constantflow rate of 450 mVL. The biomass loading of the packedCA-immobilized biosorbent was 38 wr'1o, and the particle size was 2.0 mm. The resulting breakthrough curves of Pb were illustrated in Fig. 5, which demonstrate a similar trend of concentration profiles for the three bed-lengths, except that the curve tended to shift to the right as the bed length was increased. Increasing the bed length from 5 em to 15 em led to considerable prolongation of ts from 30 h to 190 h (Table 4). Also indicated in Table 4, the total adsorption efficiency (Q) was 77% and 79%. respectively, for the bed-length of 10 and 15 em, whereas the 5-cm bed had a lower Q value (51 %), which may be due to a relatively small amount of adsorbents in a shorter bed. The LUB evaluated from the curves in Fig. 5 was found to be independent of the bed length, and the LUB value was around 3.0 em (Table 4). This indicates that the elongation of bed length did not alter the mass transferpatterns of Pb to the packed adsorbent. Inspection of Fig. 4 and 5 shows that with similaroperation conditions, the beds with CA-immobilized cells exhibited phenomenally longer tb and larger Pb removal capacity than those of PAA columns, which is consistent with what was observedfrom the batch adsorption results(Fig. 1-3)

\.2 1------------.~

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Table 4 The effect of bed-length on Pb adsorption characteristics for CA-immobilized cells

Scm

to

5 10 15

0.1

, 110

100

1110

Lo 200

2110

:lOO

3IlO

Adsorption time (h) i f '

10

IS

20

25

30

35

40

Desorption time(hI

Fig.5 Adsorption and desorption profiles of CAimmobilized cells in the fixed-bed operations

30 100 190

135 185 290

Q(%)

LUB(cm)

51 79 77

3.2 3.0 3.1

Table 5 The effect of flow rate on Pb adsorption characteristics for CA-immobilized cells flow rate(ml/h) tb(h) ts(h) Q(%) LUB(cm) 390 450 900 1200

58 30 11 2

142 135 75 47

60 51 56 53

2.1 3.2 3.7 4.6

The effect offlow rate: The 5-cm-deep beds containing CA-immobilized cells (biomass loading=38 wr'1o, particle size=2.0 mm) were operated at four flow rates (v), 390, 450, ~OO, and 1200 mllh to investigate the effect of metal feeding rate. As indicated in Table 5, increase in the flow rate resulted in a decrease in the breakthrough time (tb). When v was raised to 1200 ml/h, the tb nearly vanished, suggesting that Ph may be overloaded as v exceeded 1200ml/h. Table 5 also shows that the LUB value increased with flow rate, while Q valueswere essentially unchanged. It seems reasonable to observe an increase of LUB when the flow rate is turned faster, since increasing the flow rate makes a shorter retention time, which may cause a negative effecton the mass transfer efficiency of the adsorbate. The effect of particle size: With 5-cm beds operated at 450 ml/h, the breakthrough curves of Pb for different particle sizes were presented in Fig. 6, which demonstrates that the pore diffusion rate could dominate the Pb adsorption process during the early stage of the operation. Therefore. a faster Ph removal was observed for the smaller-size particles in the first 70 h (Fig. 6). The three breakthrough curves intersected at around 70 h, after which the diffusion limitationwas less apparent, and the adsorption on large particles becamemore significant, while the small particlesreached equilibrium earlier (Fig. 6). As indicated in Fig. 7, where the cumulative Pb adsorption was plotted as a function of time, the saturationcapacity for the three particle sizes was similar at 221, 250, and 228 mg/g for the particle size of 2, 3, and 3.5 mm,

Fixed-bed biosorption

177

respectively. This resultsuggests that changes in the particlesize from 2 to 3.5 rom did not significantly alter the adsorption equilibrium of the adsorbent. The initial adsorption rate for different particle sizes were evaluated from initial slopes of the cumulative adsorption curves (Fig. 7). As expected. the resulting initial adsorption rate was in the order of2 mm > 3 nun> 3.5 mm, and the value was 3.32, 3.05, and 2.30 mg/gIh. respectively. 1.2 , - - - - - - - - - - - - ,

:lCOr--------_-,

0.8

0.4 - - O"'2mm - - O"3mm - - - O:z3.5mm 0.0 ""--.---,-.,--,---,-,--...,.--1

o

~

~

00 00

100~01~100

Time(h)

Fig.6 Breakthrough curves for Pb on CAimmobilized cells with different particle sizes

o ~-'--'-"--.---,-.,--,--l o ~ ~ ~ ~ ~ m ~ ~ Time (h)

Fig.7 Cumulative adsorption profiles for Pb by CA-immobilized cells withdifferent particlesizes

Ph recovery and fixed-bed regeneration: A fully loaded 5-cm-deep bed with CA·immobilized cells was regenerated by elution with HCI (pH 2.0). The trend of the desorption profile (Fig. 5) was quite similar to that for the PAA column (Fig. 4), except that for the CA columnthe peak concentration was higher (nearly 340 mgIL), and the elutiontime for a complete recovery was longer(about5 h). The corresponding recovery ratio was 27:I, in contrast to a recovery ratio of 8:I obtained from the PAA column. The CA column was repeatedly operated four times, and the tb gradually decreased as the numberof AID cycle increased (Table 6). However. the LUB and Q values remained nearly constant (Table 6), excluding a smaller LUB in the secondcycle. The resultsshown in Table 6 suggest that the mass transfer pattern was essentially maintained despiteoffour NO cycles. and the decrease of total Pb adsorption capacity was lessthan 34 %. Table6 The effect of AID cycles on Pb adsorption characteristics for CA-immobilized cells tb(h) t,(h) Q(%) LUB(cm) Regeneration efficiency(%) 3.2 100 cycle 1 30 135 50 81 2.3 cycle 2 21 87 57. 73 3.0 75 60 cycle3 IS 66 3.5 cycle4 11 69 60

Table 7 Comparison of Pb adsorption characteristics for CA matrix and CA-immobilized cells Adsorbent tb(h) ts(h) Q(%)LUB(cm) CAmatrix 20 CA-immobilized 30 cells

123 53 140 51

3.6 3.2

Comparison of Columns containing cell-free alginate matrix and alginate-immobilized cells: Biosorption columns packed with cell-free calcium alginate particles (2.0 nun in diameter) and with calcium alginate. immobilized cells (also around 2.0 mm in diameter) were compared for their Pb adsorption capacity in the treatment of 10 mg Pb/L solution. Table 7 summarizes the adsorption characteristics of the two types of biosorbers. The cell-bearing bed exhibited longer tb and ts than those obtained from the cell-free CA bed, whereas the Q and LUB values of the two columns were quite similar. Although the results show that addition of biomass of P. aeruginosa PU21 made only a slight enhancement in the adsorption capacity, the cost of the biomass is significantly lower than that of the alginate matrix. More importantly, the biomass appeared to demonstrate predominant selectivity for Pb over Cu and Cd (Changand Chen, 1998), while CA

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matrix showed poor specificity for the heavy metals (Chang and Huang, in press). It may be critical to incorporate the biomass withthe immobilized biosorbent whenthe selectivity becomes the majorconcern. CONCLUSIONS Batchadsorption resultsshowed that the equilibriwn capacity of the CA-entrapped biomass of P. aeruginosa PU21 was 280 mg Pb/g, and was 350 mg Pb/g for the cell-free CA matrix. Both materials demonstrated an effective adsorption ability for Pb. In contrast. the PAAmatrix can barely adsorb Pb, and PAA-immobilized cells exhibited a capacity of 31 mg Pb/g, much smallerthan that for the CA-immobilized biomass. Increases in the biomass loading led to higher adsorption capacity of CA- and PAA-immobilized biomass, while the effectof biomass loadings was moresignificant for PAA-immobilized cells. The'column studiesshowed that increases in the bed length and decreases in the flowrate all causedthe extension of tb. The effectof particle size (2-3.5 mm) primarily occurred during the first 70 h of operation, with the initial adsorption rate decreased in the order of2 mm>3 mm>3.5 mm, whereas the equilibriwn adsorption capacity was similarfor the threeparticle sizesexamined. In contrast to tb, the LUB and Q values wereless sensitive to the changes of the operation and bed conditions. Fullyloaded beds wereregenerated by acid washing, resulting in more than 98% recovery of Pb with a recovery ratio of 27:1 for CA-immobilized cells, and 8:1 for PAA-immobilized cells. The columns were operated for up to four AID cycles without considerable changes in the mass transfer patterns of the bed. whereas the total adsorption capacity and tb declined slightly after repeated uses of the column. It can be concluded that CA-immobilized biomass of P. aeruginosa PU21 is an excellent metal biosorbent, and has a promising potential for practical applications to the removal of heavy metals from industrial effluents. However, PAA-immobilized cells were much less effective in all the respects evaluated. ACKNOWLEDGEMENTS Theauthors gratefully acknowledge the financial support fromNational Science Council of the Republic of ChinaunderGrantNo. NSC-84-2214-E-035-Q04. REFERENCES Chang J. S. and Chen C. C. (1998) Quantitative analysis and equilibriwn models of selective biosorption in multi-metal systems usinga bacterial biosorbent. Separ. Sci. Technol. 33(5),611-632. Chang J. S. and Huang J. C., Selective adsorption and recovery of heavy metals by immobilized bacterial biomass in sequential fixed-bed columns. Biotechnol. Progr., in press. Chang 1. S., Law R. and Chang C. C. (1997) Biosorption of lead, copper, and cadmium by biomass of Pseudomonas aeruginosa PU21. Wat. Res. 31(1),1651·1658. Chang J. S. and HongJ. (1994)Biosorption of mercury by the inactivated cells of Pseudomonas aeruginosa PU21 (Rip64). Biotechnol. Bioengr. 44,999-1006. Gadd G. M. (1988) Accwnulation of metals by microorganisms and algae. In: Biotechnology, H. J. Rehm and G. Reed (eds), vol. 6b, VCHPublishers, Weinheim, Germany, pp. 401-430 Jacoby G A (1986) Resistance plasmids in Pseudomonas aeruginosa. In: The Bacteria, I. C. Gunsalus, J. R. Sokatch and L. N. Ornston(eds),vol X, Chapter 17,Academic Press,Orlando, FL,pp. 497-514. Jang L. K., Lopez S. L., Eastman S. L. and Pryfogle P. (1991)Recovery of copperand cobaltby biopolymer gels. Biotechnol, Bioengr. 37,266-273. Mattusehka B. and Straube G. (1993)Biosorption of metalsby a waste biomass. J. Chem. Tech. Biotechnol. 58:57·63. Nakajima A. and Sakaguchi T. (1993) Accwnulation of uraniwn by basidiomycetes. Appl. Microbiol. Biotechnol. 38, 574-578. Ruthven D. M. (1984)Principles ofadsorption andadsorption process. Wiley, New York. Volesky B. and HolanZ. R. (1995)Biosorption of heavymetals. Biotechnol. Progr. 11,235-250. Volesky B. and Prasetyo I. (1994) Cd removal in a biosorption column. Biotechnol. Bioeng. 43, 1010-10I5.