Ind. Eng. Chem. Res. 2003, 42, 4881-4887
4881
Copper Biosorption onto Rhizopus Oligosporus: pH-Edge Tests and Related Kinetic and Equilibrium Modeling F. Beolchini,† F. Pagnanelli,‡ A. P. Reverberi,§ and F. Veglio` *,† Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita` degli Studi di L’Aquila, 67040 Monteluco di Roio, L’Aquila, Italy, Dipartimento di Chimica, Facolta` di SMFN, Universita` degli Studi “La Sapienza”, Piazzale A. Moro 5, 00185 Rome, Italy, and Dipartimento di Ingegneria Chimica e di Processo “G. B. Bonino”, Universita` degli Studi di Genova, via Opera Pia 5, 16145 (Albaro) Genova, Italy
In this paper copper biosorption on a specially propagated biomass of Rhizopus oligosporus has been studied considering kinetic and equilibrium aspects. A rough biomass characterization performed by titration tests suggested that over pH 3-4 all biomass active sites seem available for metal adsorption. A maximum copper-specific uptake of about 140 mg/g was observed at pH 5. Kinetic tests evidenced that the process is very fast: in fact, biosorption equilibrium is reached within the first 20 min. Furtermore, kinetic data were successfully modeled by second-order models. Equilibrium data were obtained by pH-edge tests, and copper uptake was modeled considering the pH and equilibrium concentration in solution as independent variables: different empirical and semiempirical models have been proposed to model biosorption equilibrium and successfully fitted to experimental data. 1. Introduction Biosorption of heavy metals from aqueous solution can be considered as an alternative technology in industrial wastewater treatments to remove heavy metals, as demonstrated by several researchers, because it is possible to use cheap adsorption materials that can be competitive with respect to conventional technologies.1,2 Some papers that try to summarize the results reported on biosorption of heavy metals are available in the wide literature1-4 considering single-metal5,6 and multimetal systems.5,7,8 Biosorption of heavy metals can be explained by considering different kinds of chemical and physical interactions among the functional groups present on the cell wall and the heavy metals in solution.4 The active sites responsible for the heavy-metal uptake on the cell wall are very different according to the biosorbent nature: carboxylic, phosphate, sulfate, ammino, amidic, and hydroxylic groups are the most commonly found.9,10 Qualitative or semiquantitative tests to determine the characteristics of these adsorption sites can be realized by titration tests of the selected biomass.11,12 The analysis of titration curves outputs the influence of pH on the deprotonation of the functional groups on the cell wall:6 for this reason pH is one of the most influential environmental factors.13-15 The aim of this work was to study the effect of several process parameters on the kinetics and equilibrium of copper biosorption by Rhizopus oligosporus. R. oligosporus is a fast-growing filamentous fungus, usually isolated from fermented food. R. oligosporus may be a suitable specially propagated biomass that can be obtained as waste biomass during the purification * To whom correspondence should be addressed. E-mail:
[email protected]. † Universita` degli Studi di L’Aquila. ‡ Universita` degli Studi “La Sapienza”. § Universita` degli Studi di Genova.
process of starch wastes. In particular, this biomass has been considered as animal feed and to recover R-glucoamylase from the fermentation broth.16 In the present paper, the influence of the cultivation procedure has been considered and the pH effect was monitored during kinetic and equilibrium tests: these last experiments were carried out either by the pH-edge procedure17 or by a standard procedure. In both cases, suitable kinetic and equilibrium models have been utilized to fit experimental data: some empirical and semiempirical models18,19 are here considered to be used in the data fitting of heavy-metal equilibrium biosorption, in which pH has to be taken into consideration. This approach permits one to analyze equilibrium data by avoiding the pH control during a selected trial. Therefore, these tests were carried out not only to verify the potential application of R. oligosporus in purification processes but also to propose a methodological approach to the study of equilibrium biosorption data in which adsorption isotherms have to be obtained by also considering the pH effect. A similar approach was successfully applied on biosorption studies utilizing Sphaerotilus natans, calcium alginate, and olive-mill residues.17,19 2. Materials and Methods Biomass Preparation and Its Characterization. R. oligosporus has been supplied by CRAB (Consorzio per le Ricerche Applicate alla Biotecnologia, Avezzano, Italy). It is cultivated in agar plate using solid media.16 The microorganism is inoculated in submerged culture using shaken flasks (volume 100 mL) for biomass production. Three cultural media have been used during the biomass cultivation: (a) medium no. 1 (named NEW) containing glucose, 40 g/L; yeast extract, 10 g/L; KH2PO4, 1 g/L; MgSO4‚7H2O, 0.5 g/L; NaNO3, 1 g/L; (b) medium no. 2 (named MEM) containing malt extract, 20 g/L; (c) medium no. 3 containing starch, 10 g/L; yeast extract, 5 g/L; polypeptone, 5 g/L; K2HPO4, 0.2 g/L; MgSO4‚7H2O, 0.2 g/L.16 In all cases the cultural media have been sterilized in an autoclave at 120 °C for 20 min.
10.1021/ie020829h CCC: $25.00 © 2003 American Chemical Society Published on Web 09/09/2003
4882 Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003
Several shaken flasks (about 60) were inoculated in sterile conditions for each cultural media and then placed in a shaker (New Brunswick model G25) at 35 °C and 150 rpm. The biomass was recovered after 2 and 7 days of cultivation period, filtered under pressure (3 bar) in a Millipore filter (0.45 µm), washed three times with distilled water, dried in an oven at 80 °C for 2 h, and frozen at -10 °C for the next lyophilization process. No further treatments were performed for the fungus inactivation. Finally, the biomass produced was lyophilized (temperature -53 °C, vacuum pressure 76 mTorr) and stored.20 Biomass lyophilization allows more reliable results to be obtained than using fresh cells, avoiding contamination, and not changing biosorption performances during the biosorption tests. A selected amount of lyophilized mold (1 g) was suspended in 100 mL of an acid solution maintained at pH 2 by 1 M HCl. Then, this suspension was potentiometrically titrated by adding step by step 0.1 M NaOH: after each addition, an equilibration time of 1 h was considered before reading the pH values. These tests were carried out to roughly characterize the active sites present on the cell wall responsible for biosorption.6 Kinetic Tests. Kinetic tests were carried out in shaken flasks under the following experimental conditions: A selected amount of lyophilized biomass (0.1 g) was placed in a shaken flask with a known volume of distilled water and rehydrated for 1 h. Then, a selected volume of a copper solution (prepared by dissolving CuSO4 in distilled water, 1 g/L) was added to the shaken flask while maintaining the liquid total volume at 100 mL: 35, 70, and 140 mg/L were the initial copper concentrations selected for the kinetic tests. Two levels of pH were selected in these tests (pH 4 and 5). This parameter was continuously monitored and controlled by adding 0.1 N NaOH or HCl. The shaken flasks were then placed in a shaker at 250 min-1 at room temperature (25 °C), and heavy-metal uptake was quantified by monitoring the copper concentration in the solution over time. Samples of 1.5 mL were taken after 5, 10, 20, 30, 45, and 60 min, and the copper concentration was determined by an atomic absorption spectrophotometer (Varian Spectra 2000). Obviously, the volume reduction due to sampling was taken into account when calculating the copper uptake. In these tests the effect of cultural media (nos. 1 and 2), cultivation age (2 and 7 days), initial copper concentration (35, 70, and 140 mg/L), and pH (4 and 5) were taken into consideration. Equilibrium Tests. Equilibrium biosorption tests were realized with two experimental procedures: sorption tests at constant pH (noted in the following as the standard method, STD) and pH-edge tests. In both cases a selected amount of lyophilized biomass (0.1 g) was placed in a shaken flask with a known volume of distilled water: the biomass was rehydrated for 1 h. A selected volume of a copper solution (prepared by dissolving CuSO4 in distilled water, 1 g/L) was added to the shaken flask while maintaining the liquid total volume at 100 mL. The shaken flasks were then placed in a shaker at 250 min-1 at room temperature (25 °C), and heavy-metal uptake was monitored by sampling the solution and measuring the copper concentration by an atomic absorption spectrophotometer (Varian Spectra 2000): the heavy-metal uptake and the concentration in the solid phase q (mg of Cu2+/g of biomass) were estimated by material balance.6 In the first series of adsorption tests (STD), the pH was continuously moni-
tored and controlled by adding 0.1 M HCl or NaOH until the equilibrium conditions were reached (after 1 h): each isotherm (at constant pH) was obtained by increasing the initial copper concentration in different shaken flasks (initial copper concentration: 10, 50, 72, 144, 204, 272, 366, 488, and 680 mg/L). The second series of tests (pH-edge tests) were carried out as STD tests but the pH was changed from low to high values (from about pH 3 to 5, by adding 0.1 or 1 M NaOH) in each shaken flask and vice versa (from pH 5 to 3, by adding 0.1 N or 1 M HCl): in this manner, the total amount of copper is constant for each test and the copper in solution is monitored after each pH change has been induced. In both cases, particular care was paid to have negligible dilution by alkali or acid solutions during the pH control. After each pH change, an equilibration time of 60 min was used before the collection of the liquid sample in the pH-edge tests obtained by increasing the pH values (from 3 to 5); on the other hand, sampling times of 30 and 45 min were selected during the pHedge tests carried out by decreasing the pH (from 5 to 3). The q values were calculated also by considering the copper collected during the sampling procedure in order to avoid a propagation of this systematic error. The copper concentration in the liquid phase was determined by an atomic absorption spectrophotometer (Varian Spectra 2000). All samples were diluted with HNO3 at pH 2 and stored at 4 °C before the analysis. Most of the biosorption tests were replicated twice, and the coefficient of variation values ranged from 2 to 5%. 3. Results Media Cultivation and Biomass Characterization. Preliminary biosorption tests were performed to select the cultural medium that produces a biomass able to remove the largest amount of copper. It is known in the literature that microbial cultivation procedures may influence the biosorption capacity of a selected microorganism. For this reason, three kinds of biomasses were prepared using the cultural media no. 1-3 after 2 days of incubation in shaken flasks. Then, these three lyophilized biomasses were tested in biosorption tests by adding a selected amount of copper. The tested experimental conditions are reported in the following: biomass concentration 1 g/L; initial copper concentration 90 mg/L; pH 4.5; equilibrium time 1 h. From the analysis of the experimental results (not reported here), the media no. 1 and 2 were the most suitable cultural media: copper uptakes of 60%, 51%, and 19% were obtained for the media no. 1-3, respectively, in the investigated experimental conditions. Media no. 1 and 2 are named NEW (no. 1) and MEM (no. 2) in the following, and they then have been considered for the subsequent biosorption tests. In particular, R. oligosporus was prepared with these two cultural media by collecting the biomass after 2 and 7 days of incubation in shaken flasks and carrying out the entire experimental procedure to lyophilize the microorganism. The four samples of biomass (MEM after 2 and 7 days of incubation and NEW obtained for the same incubation periods) were roughly characterized by titration tests.6,11 The analysis titration curves (not shown here) highlight the absence of a single inflection point in the titration curve, as a confirmation of the etherogeneity of biomass functional groups. Many researchers have used this procedure to determine possible functional
Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003 4883 Table 1. Results of the Data Fitting Using Equation 1: qeq (mg/g, a) and K (g/mg/min, b) as a Function of Operating Conditions medium days pH MEM
2 7
NEW
2 7
MEM Figure 1. Kinetic results obtained with MEM biomass after 2 days of incubation (pH 4). Points represent experimental data, while lines have been calculated by eq 1.
groups responsible for the sorption phenomena.1,6,11 Fungus species’ cellular membrane is rich in chitine and chitosan,21 and several researchers3,5 have already demonstrated a fungi biosorption capacity with respect to several heavy metals. The related functional groups present on the cell membrane may be in a protonated form for low pH values and in an anionic form for large pH values; consequently, cation adsorption strictly depends on the pH, either decreasing or improving according to pH values. Therefore, a comparison between the pK values reported in the literature and the observed experimental results gives an indication of pH conditions favorable for Cu(II) sorption: in particular, the sites responsible for copper uptake increase with decreasing H+ concentration, and over pH 3-4, we can hypothesize that all of the functional groups are available for copper biosorption. The analysis of these titration curves suggests that about 1-3 mequiv of heavy metal/g of biomass could be adsorbed on the cell membrane. This value can be estimated by the difference between the titration points at pH 7 and 3.5. In any case, the results reported above permitted one to establish the experimental conditions for next kinetic and equilibrium tests. Kinetic Tests. Kinetic tests were carried out in order to test the effect of the different biomass cultivation procedures (media MEM and NEW, after 2 and 7 days of incubation period each) on the copper kinetic uptake, for different initial copper concentrations, C0 (35, 70, and 140 mg/L). Figure 1 shows the experimental results for the biomass MEM obtained after 2 days of incubation as an example. Copper-specific uptake versus time profiles suggest that equilibrium is reached within the first 20 min. The experimental data were fitted by using the following second-order kinetic model:
dq ) K(qeq - q)2 dt
(1)
The regression analysis was performed using the integrated and linearized forms of eq 1 and minimizing the following objective function by Solver (Microsoft Excel): n
Φ)
∑ i)1
{ [ ( qi - qeq -
1
qeq
) ]} -1
- Kt
2
(2)
2 7
NEW
2 7
4.00 5.00 4.00 5.00 4.00 5.00 4.00 5.00 4.00 5.00 4.00 5.00 4.00 5.00 4.00 5.00
copper initial concn (mg/L) 35
70
(a) qeq (mg/g) 9.7 ( 0.4 17 ( 1 15.4 ( 0.5 29.1 ( 0.9 18 ( 1 15 ( 1 13.2 ( 0.8 26 ( 1 11.7 ( 0.4 23.2 ( 0.6 18.8 ( 0.2 32.6 ( 0.8 15 ( 1 25 ( 2 20.0 ( 0.4 41 ( 1 (b) K (g/mg/min) 0.013 ( 0.003 0.018 ( 0.008 0.025 ( 0.006 0.03 ( 0.01 0.02 ( 0.01 0.010 ( 0.004 0.023 ( 0.009 0.017 ( 0.007 0.018 ( 0.003 0.024 ( 0.006 0.048 ( 0.003 0.05 ( 0.02 0.010 ( 0.003 0.02 ( 0.01 0.023 ( 0.009 0.012 ( 0.003
140 37 ( 1 56 ( 1 27 ( 2 50 ( 2 44 ( 2 64 ( 1 48 ( 2 66 ( 1 0.018 ( 0.007 0.05 ( 0.03 0.01 ( 0.01 0.02 ( 0.01 0.02 ( 0.02 0.03 ( 0.01 0.03 ( 0.02 0.03 ( 0.01
In this way, parameters qeq and K were found for each experimental condition (Table 1). The statistical analysis was performed as described in previous papers.18,19 Results reported in Table 1 and Figure 1 suggest that the empirical model is suitable for describing these kinetic data, as also reported by other workers.22 Furthermore, an analysis of the variance (ANOVA23) was performed and aimed at the individualization of the investigated operating conditions’ effects on parameters qeq and K. The analysis evidenced the obvious relationship between parameter qeq and the initial copper concentration (significance 100%) and the positive effect of pH on the biosorption process (significance 99%), as expected from titration tests. On the contrary, parameter K did not seem to be influenced by any of the investigated factors: neither the kinds of cultural media, the biomass age, nor the pH, under the investigated operating conditions. This might be due to a lack of data points in the ascending part of the uptake curves: more samples should have been taken within the first 5 min. Equilibrium Tests. The subsequent experimental runs were focused on the study of the equilibrium conditions. Considering that the two biomasses MEM and NEW had behavior similar to that of sorption performances, just R. oligosporus obtained with MEM media was used in equilibrium tests. According to the results obtained in the first characterization tests, the pH was monitored in the range 3-5; the largest pH values were not considered in order to avoid copper precipitation phenomena. Two procedures, standard equilibrium tests (STD) and pH-edge tests, were carried out to verify if similar experimental results are obtained in both cases. In fact, whereas under STD conditions the biomass contacts the solution containing the heavy metal only one time, under pH-edge conditions the same amount of biomass reaches different equilibrium points, which are generated by the continuous change of pH by the addition of alkali or acid solutions. So, it is necessary to verify if some hysteresis phenomena take place during biosorption. The experimental results obtained under pH-edge conditions are reported in Table 2. The equilibrium tests
4884 Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003 Table 2. Experimental Results Obtained in Equilibrium Adsorption pH-Edge Tests: Biomass Concentration 1 g/L; Temperature 25 °C; pH from 3 to 5 pH 3.00
pH 3.50
pH 3.75
pH 4.00
Ceq qeq Ceq qeq Ceq qeq Ceq qeq (mg/L) (mg/g) (mg/L) (mg/g) (mg/L) (mg/g) (mg/L) (mg/g) 5.20 34.62 61.40 124.20 182.00 242.00 346.00 470.00 654.00
4.80 15.38 10.60 19.80 22.00 30.00 20.00 18.00 26.00
pH 4.25
4.41 30.91 58.30 118.90 172.00 229.00 329.00 450.00 644.00
5.58 19.05 13.67 25.05 31.90 42.87 36.83 37.80 35.90
pH 4.50
4.20 29.47 52.40 113.10 169.00 223.00 311.00 435.00 624.00
5.79 20.46 19.45 30.73 34.84 48.75 54.47 52.50 55.50
pH 4.75
3.83 27.00 50.00 109.10 160.00 216.00 295.00 421.00 614.00
6.15 22.86 21.78 34.61 43.57 55.54 69.99 66.08 65.20
pH 5.00
Ceq qeq Ceq qeq Ceq qeq Ceq qeq (mg/L) (mg/g) (mg/L) (mg/g) (mg/L) (mg/g) (mg/L) (mg/g) 3.72 25.83 47.40 103.30 152.00 210.00 271.00 408.00 608.00
6.25 23.98 24.28 40.18 51.25 61.30 93.03 78.56 70.96
3.50 24.70 47.00 101.10 148.00 199.00 286.00 390.00 596.00
6.46 25.06 24.66 42.27 55.05 71.75 78.78 95.66 82.36
3.30 6.65 3.17 6.77 24.20 25.53 23.20 26.46 44.90 26.63 44.50 27.00 93.10 49.79 82.40 59.74 145.00 57.87 131.00 70.89 191.00 79.27 179.00 90.43 264.00 99.46 245.00 117.13 375.00 109.76 360.00 123.71 582.00 95.52 564.00 112.26
Figure 3. Copper equilibrium concentration as a function of pH during pH-edge tests: results have been obtained by increasing the pH from 3 to 5 (empty symbols) and decreasing the pH from 5 to 3 (full symbols) after 30 and 45 min of equilibration time.
Similar results were observed in previous works reported elsewhere using calcium alginate as the biosorbent.17 2. No significant differences are observed during the adsorption (pH from 3 to 5) and desorption tests (pH from 5 to 3): moreover, after 60 min of equilibration time, the two curves (adsorption and desorption lines) are the same, taking into account the experimental error (about 5%; see Figure 3). The experimental results obtained under adsorption pH-edge conditions (from pH 3 to 5) were utilized to model the equilibrium of copper biosorption onto R. oligosporus. The analysis of these data was performed with a classical approach by using Langmuir, Freundlich, and Redlich-Peterson models6 and using alternative models that include the pH factor as an independent variable. Similar comparisons were performed elsewhere using different biosorbents.17,19 The Langmuir, Freundlich, and Redlich-Peterson models are respectively reported in the following:
Figure 2. Adsorption equilibrium isotherm (pH 4, biomass concentration 1 g/L): comparison between STD and pH-edge tests.
under STD conditions were carried out at pH 3-5, and the obtained results are similar to the ones obtained under pH-edge conditions. All the experimental data are not reported here, and Figure 2 shows an example of this comparison. In any case, all of the experiments were carried out with the same biomass concentration (1 g/L). Moreover, for each initial copper concentration, the pHedge tests were realized by increasing the pH from 3 to 5 and vice versa. In the tests realized by decreasing the pH (from 5 to 3), two samples were collected after 30 and 45 min of equilibration time to check if adsorption and desorption give the same final equilibrium results (data not reported here): also in this case, these experiments were carried out to check the eventual presence of hysteresis phenomena during the adsorption-desoprtion process. An example of these results obtained under pH-edge conditions is reported in Figure 3. From the analysis of the results in Figures 2 and 3, the following points come out: 1. No significant differences can be observed between STD and pH-edge equilibrium tests (see Figure 2).
qmaxCeq Ks + Ceq
(3)
qeq ) KFCeq1/n
(4)
qeq )
qeq )
KR-PCeq 1 + aCeqb
(5)
The estimated parameters of these models have been evaluated by regression analysis: Table 3 reports the results of this analysis. Besides these usual equilibrium models well described and reported in the literature (see all of the references), further empirical and semiempirical models were used to fit equilibrium data by also considering the pH factor as an independent variable. These are summarized in the following:17,19
q)
Ceq R1pH + R2 R3 + pH (R4pH + R5) + Ceq
q)
Ceq R1eR2pH R1 R4 + Ceq 1 - (1 - eR2pH) R3
(6)
(7a)
Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003 4885 Table 3. Parameters of the Most Utilized Equilibrium Models Described in the Literature for Adsorption Isotherms at Constant pH Langmuir
Freundlich
Redlich-Peterson
pH
Ks (mg/L)
qmax (mg/g)
KFa
1/n
KR-P (1/g)
a [(mg/L)1/b]
b
3.00 3.50 3.75 4.00 4.25 4.50 4.75 5.00
34 ( 1 46 ( 2 85.4 ( 0.9 86 ( 3 74 ( 2 96 ( 4 103 ( 2 108 ( 1
20 ( 20 40 ( 20 63 ( 10 80 ( 20 85 ( 30 101 ( 20 120 ( 30 106 ( 30
6(3 6(3 5(2 6(2 7(4 7(3 7(3 8(4
0.2 ( 0.1 0.3 ( 0.1 0.38 ( 0.06 0.39 ( 0.08 0.4 ( 0.1 0.42 ( 0.08 0.43 ( 0.09 0.4 ( 0.1
0.6 ( 0.6 0.4 ( 0.1 0.8 ( 0.4 0.5 ( 0.1 0.43 ( 0.07 0.49 ( 0.07 0.53 ( 0.06 0.68 ( 0.06
0.02 ( 0.04 0.0008 ( 0.0009 0.02 ( 0.03 0.0007 ( 0.0008 0.000006 ( 0.000007 0.00006 ( 0.00005 0.000008 ( 0.000007 0.000010 ( 0.000008
1.0 ( 0.1 1.4 ( 0.1 0.9 ( 0.1 1.3 ( 0.1 2.0 ( 0.1 1.7 ( 0.1 1.9 ( 0.1 1.91 ( 0.08
a
mg/g (mg/L)-1.
q)
R1eR2pH Ceq R1 (R4pH + R5) + Ceq 1 - (1 - eR2pH) R3 R1
q)
-pH
10 R2
1+ q)
R1 -pH
10 1+ R2
(
Ceq R3 + Ceq Ceq
)
10-pH R3 1 + + Ceq R4
(7b)
(8a)
(8b)
These equations have been successfully applied to describe the equilibrium properties of S. natans6 and calcium alginate17 during copper(II) biosorption at different pH values. All of these equations have been built considering the Langmuir model (eq 3) as a reference model, introducing the pH effect in the two parameters qmax and Ks as shown:
qeq )
qmax(pH) Ceq Ks(pH) + Ceq
Figure 4. pH dependence of Langmuir model parameters (eq 3).
(9)
Equation 9 represents a general form of eqs 6-8. In fact, at constant pH all of the equations degenerate in the classical Langmuir model. Figure 4 shows the Langmuir model’s parameters versus pH profiles.6,17,19 The empirical model reported in eq 6 was named model 1; in eqs 7a,b we have reported two different versions of the logistic equation coupled with the Langmuir model,19 both named models 2a and 2b; eqs 8a and 8b have been originated from noncompetitive biosorption models6,17,19 with some empirical changes introduced considering the obtained experimental data (named models 3a and 3b, respectively). In particular, models 3a and 3b have been derived assuming that the cell wall is mainly characterized by one kind of active site and neglecting the ion charge; further implementations have been empirically carried out on parameter Ks of the original Langmuir model, considering its pH dependence. Many details about the modeling can be found elsewhere.17,19 The experimental data (q vs Ceq) obtained at different equilibrium pH values have been fitted by a nonlinear regression method to evaluate the adjustable parameters of each model (Ri; i ) 1, p). Figure 5 shows example results obtained in the case of model 3a, and Table 4 reports the estimated values of the models’ constants together with other statistical parameters23,24 that also permit one to evaluate the model goodness.18,25
Figure 5. Equilibrium isotherms of copper biosorption by R. oligosporus. Points represent experimental data, while lines have been calculated by model 3a (eq 8a).
The performances of the selected models were also compared by using an F test (not reported here).17-19 This statistical tool permits one to evaluate if there is a difference in the precision of the investigated tested models. Considering the results of the F tests that compare Sres2 with respect to the lowest model residual variance (not reported here) and the scatter diagrams (see Figure 6 as an example), it was possible to conclude that the best model is 3a because it has a low Sres2 value, it was derived from a physical model of the cell wall, and it permits one to fit all of the experimental data with the lowest number of parameters (p ) 3). Moreover, this model seems to be quite general and adequate because similar results were obtained using Arthrobacter sp.,18 S. natans,6,19 and calcium alginate17 as adsorbent materials.
4886 Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003 Table 4. Parameters of the Selected Models Obtaind by Regression Analysis and Related Statistical Analysis (See Text for Details) R1 R2 R3 R4 R5 Sres2 R2
model 1
model 2a
model 2b
model 3a
model 3b
60 ( 20 -170 ( 60 130 ( 2 -390 ( 10 -2.9 ( 0.2 54.9 0.98
1.3 ( 0.8 1.1 ( 0.2 240 ( 70 125 ( 10
2(1 1.0 ( 0.3 280 ( 150 60 ( 90 -70 ( 300 55.1 0,98
150 ( 8 1 × 10-4 ( 2 × 10-5 120 ( 20
150 ( 10 1 × 10-4 ( 2 × 10-5 75 ( 10 114000 ( 4000
76.9 1.04
101.2 1.04
56.5 0.99
Table 5. Comparison between This Work and Other Results Found in the Literature operating conditions sorbent material
Figure 6. Scatter diagram obtained in the case of model 3a.
R. oligosporus 25.4 R. oligosporus 76.2 R. oligosporus 142.9 S. natans 57.2 S. natans 38.1 S. natans 9.5 S. natans 2.6 Arthrobacter sp. 44.8 Chlorella vulgaris 36.5 Rhizophus arrhizus 35.2 Zoogloea ramigera 35.9 Streptomyces noursei 11.4 Ascophillum nodosum 61.9
4. Conclusions In this paper the experimental results of copper biosorption on R. oligosporus are presented. In particular, the analysis of experimental data led to the following results: (i) Two cultivation media have been individuated, which is optimal for R. oligosporus copper biosorption; no significant difference was observed between biomasses cultivated for 2 and 7 days. (ii) A rough characterization of R. oligosporus by protonation suggested that over pH 3-4 all biomass active sites seem available for metal adsorption. (iii) Kinetic tests demonstrated that biosorption equilibrium is reached within the first 20 min. Moreover, a second-order model has been successfully fitted to copper-specific uptake versus time profiles, and the effect of the main operating conditions on the model parameters has been evidenced. (iv) Concerning equilibrium tests under different pH conditions, no significant difference was observed between the STD and pH-edge tests. This implies that the pH-edge standard procedure can be applied in equilibrium studies with several practical advantages (saving biosorbent material and laboratory time). Furthermore, data fitting of equilibrium biosorption data has been performed, also including the pH as an independent variable of the proposed models. In this manner it is possible to carry out biosorption tests by avoiding pH control during the building of an adsorption isotherm, and it is necessary just to monitor the equilibrium pH during the biosorption runs, as obtained in pH-edge tests.17 This last aspect presents one of the main aims of the work, that is, to propose a methodological approach to the study of equilibrium biosorption data in which adsorption isotherms have to be obtained by also considering the pH effect. In fact, an equilibrium model is a basis in the development of a mathematical model for the simulation of biosorption processes in engineer-
qmax (mg/g) pH 3 4 5 6 5 4 3 6 4 4 4 5.5 4.5
T (°C)
biomass (g/L)
25 25 25 25 25 25 25 25 25 25 25 30 25
1 1 1 1 1 1 1 2 2 1 1 0.4 na
ref this work this work this work 6 6 6 6 23 38 5 39 40 Chong and Volesky, 1995
ing processes, such as fixed-bed columns26 and membrane processes.27 Furthermore, these tests evidenced the potential application of R. oligosporus in the practical operation of purification processes. In fact, a comparison between this work and other results in the literature (Table 5) shows that R. oligosporus gives very good copper sorption performances with respect to other biosorbents, under the investigated experimental conditions. A maximum copper-specific uptake of 142.9 mg/g was estimated at pH 5, while other typical values in the literature are lower. Obviously, an industrial application of biosorption with the specially propagated R. oligosporus would be too expensive and not convenient, also considering these high sorption abilities. In fact, for industrial application, R. oligosporus might be obtained as a waste biomass, during the purification process of starch wastewaters.28 Further work is in progress aimed at the study of R. oligosporus biosorption properties in multimetal systems. Acknowledgment The authors thank Mrs. Lia Mosca and Mr. Marcello Centofanti for their precious and professional contributions during the execution of the chemical analysis and Dr. Patrizio Di Lillo for his helpful and precious cooperation in the experimental tests and data analysis. List of Symbols a ) Redlich-Peterson parameter (Lb/mgb) b ) Redlich-Peterson parameter KR-P ) Redlich-Peterson parameter (L/g) K ) kinetic constant (g/mg/min) KF ) Freundlich constant (mg1-1/n/g/L1/n) Ceq ) heavy-metal equilibrium concentration (mg/L) Ks ) Langmuir constant (mg/L) n ) Frendlich constant p ) number of model parameters
Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003 4887 q ) heavy-metal uptake (mg/g) qmax ) maximum heavy-metal uptake (mg/g) qeq ) equilibrium heavy-metal uptake (mg/g) Sres2 ) residual variance (mg2/g2) t ) time (min) R1 ) model parameter (eqs 6-8; mg/g) R2 ) model parameter (model 1, eq 6; mg/g) R2 ) model parameter (eqs 7 and 8) R3 ) model parameter (model 1, eq 6) R3 ) model parameter (model 2, eqs 7a and 7b; mg/g) R3 ) model parameter (model 3, eqs 8a and 8b; mg/L) R4 ) model parameter (models 1 and 2, eqs 6 and 7; mg/L) R4 ) model parameter (model 3b, eq 8b) R5 ) model parameter (model 1, eq 6; mg/L) Φ ) objective function (mg2/g2)
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Received for review October 21, 2002 Revised manuscript received July 14, 2003 Accepted July 27, 2003 IE020829H
