Environ. Sci. Technol. 2001, 35, 4283-4288
Comparative Analysis of the Biosorption of Cadmium, Lead, Nickel, and Zinc by Algae S. KLIMMEK AND H.-J. STAN* Institute of Food Chemistry, Technical University Berlin, Gustav-Meyer-Allee 25, D-13355 Berlin, Germany A. WILKE, G. BUNKE, AND R. BUCHHOLZ Department of Bioprocess Engineering, Institute of Biotechnology, Technical University Berlin, Ackerstrasse 71-76, D-13355 Berlin, Germany
Thirty strains of algae were examined for their biosorption abilities in the uptake of cadmium, lead, nickel, and zinc from aqueous solution. A wide range of adsorption capacities between the different strains of algae and between the four metals can be observed. The cyanophyceae Lyngbya taylorii exhibited high uptake capacities for the four metals. The algae showed maximum capacities according to the Langmuir Adsorption Model of 1.47 mmol lead, 0.37 mmol cadmium, 0.65 mmol nickel, and 0.49 mmol zinc per gram of dry biomass. The optimum pH for L. taylorii was between pH 3 and 7 for lead, cadmium, and zinc and between pH 4 and 7 for nickel. Studies with the algae indicated a preference for the uptake of lead over cadmium, nickel, and zinc in a four metal solution. The metal binding abilities of L. taylorii could be improved by phosphorylation of the biomass. The modified biosorbent demonstrated maximum capacities of 2.52 mmol cadmium, 3.08 mmol lead, 2.79 mmol nickel, and 2.60 mmol zinc per gram of dry biomass. Investigations with phosphated L. taylorii indicated high capacities for the four metals also at low pH. The selectivity remained quite similar to the unmodified algae.
Introduction Increased knowledge about ecotoxicological effects as well as increased legal requirements for reductions in industrial emissions necessitate research and development in the area of wastewater treatment. In this context, the contamination of the environment with toxic heavy metals is a significant problem. Industrial effluents are considered to be the major sources of heavy metal contamination. Because of heavy metal accumulation in the food chain and their persistence in nature, it is necessary to remove toxic heavy metals from wastewater. Conventional technologies for the removal of heavy metals such as chemical precipitation, ion exchange, or electrochemical processes are often neither effective nor economical, especially when used for the reduction of heavy metal ions to low concentrations (1). New separation methods are required that reduce heavy metal concentrations to environmentally acceptable levels at affordable cost. Bioremoval has the potential to contribute to the achievement of this goal (2). The ability of microorganisms to remove heavy * Corresponding author phone: +49-3031472702; fax: +493031472702; e-mail:
[email protected]. 10.1021/es010063x CCC: $20.00 Published on Web 10/05/2001
2001 American Chemical Society
metals from aqueous solution has been known for some decades. The removal of the metals occurs actively only with living cells (bioaccumulation) or passively at the surface of both living and dead cells (biosorption) (3). Biosorption is a fast and reversible reaction of the heavy metals with the microorganism’s biomass (4). The use of dead biomass is of particular economic interest, because the biomaterials are used in the same way as synthetic adsorbents or ion exchangers and repeated regeneration is possible (5-8). Algae, bacteria, fungi, and yeast have proved to be potential heavy metal sorbents (8, 9). Algae, primarily marine macroalgae, play an important role in the research and development of new biosorption materials due to their high capacities, similar to commercial ion-exchange resins and their availability in nearly unlimited amounts from the ocean (10-12). Microalgae, the world’s largest group of primary producers, are under-represented in the current research field of biosorption, although they are important in the production of valuable biomaterials (1, 13-15). The microalgal byproducts, obtained e.g. from a biomaterial production, are a cheap source of biosorbents (16). In regard to their influence on the biosorption process (capacities and selectivities), it is necessary to know the binding sites for the metals at the surface of the algae. The cell wall of algae consists of a variety of polysaccharides and proteins, some of which contain anionic carboxyl, sulfate, or phosphate groups. Crist et al. (17, 18) considered carboxyl and sulfate groups as binding sites for the metals by the algae Vaucheria by displacing either an existing metal (ion exchange) or a proton (proton displacement), depending on the pH. Gardea-Torresdey et al. (19) described the importance of the carboxylate anion for the biosorption of copper(II) and aluminum(III) by five algae strains. Studies of the removal of cadmium(II) and lead(II) with the algae Saragassun fluitans (20) and copper(II), strontium(II), cadmium(II), and lead(II) with the plant Datura innoxia (21, 22) from aqueous solution have also demonstrated the great influence of anionic sites at the cell wall on the biosorption. Following these results, recent investigations (23-26) have described the increase of metal capacities of the biosorbents due to the successful introduction of additional functional groups into the biomass cell wall. The aim of this study was to investigate the biosorption of the industrially relevant heavy metals cadmium, lead, nickel, and zinc by algae. In this paper, we describe screening of 30 algae strains, most of them microalgae, for their ability to bind the four metals. Quantitative evaluation of the screening results identified the most suitable algae for an application. Further characterization of the biosorption was carried out on the most efficient species. The successful introduction of additional phosphate groups into algae cell wall is also demonstrated in this paper.
Materials and Methods Biomaterials. Algae cultures were obtained from the strain collection of algae in Go¨ttingen (SAG) and the strain collection in the Institute of Biotechnology, Technical University Berlin (ISA). The Algae were cultivated in the Institute of Biotechnology. The cultivation conditions have been described elsewhere (27). Table 1 shows the 30 algae used in this study. The fresh algae cells were harvested by centrifugation, washed with distilled water, and freeze-dried. The freezedried biomass was ground in a mortar and sieved to a particle size of maximum 250 µm. Chemicals. Analytical grades of CdCl2 (Sigma, Deisenhofen), NiCl2‚6H2O, Pb(NO3)2, and ZnCl2 (all Merck, DarmVOL. 35, NO. 21, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4283
TABLE 1. Algae Investigated during the Study taxonomic group
species
origina
Bacillariophyceae
Phaeodactylum tricornutum SAG 1090-6
Bangiophyceae
Porphyridium purpureum
SAG 112.79
Chlorophyceae
Acinastrum hantzschii Ankistodesmus densus Chlorella kessleri Chlorella species Chlorella vulgaris Dunaliella bioculata Dunaliella salina Gloeotilopsis planctonica Granulocystis verrucosa Koliella spiculiformis Raphidonema spiculiforme Tetraselmis species Anabaena cylindrica Anabena inaequealis Arthronema africanum Gloeotrichia longicauda Lyngbya taylorii Microcystis aeroginosa Microcystis species Nostoc parmeloides Phormidium species Scytonema hofmani Spirulina laxissima Spirulina maxima Spirulina platensis Synechococcus species
ISA SAG 202-1 SAG 211-11g ISA SAG 211-11b SAG 19-4 SAG 184.80 SAG 29.93 SAG 56.81 ISA ISA ISA SAG 1403-2 SAG 1403-10 ISA SAG 32.84 ISA SAG 14.85 ISA ISA ISA ISA SAG B 256.80 SAG B 84.79 SAG 257.80 ISA
Cyanophyceae
Eustigmatophyceae Eustigmatos magnus
SAG 36.89
Vaucheria dichotoma
Baltic Sea, Finland
Xanthophyceae
stadt) were used for the sorption experiments. Atomic absorption metal standards (1000 ppm), nitric acid (65%, suprapur), phosphoric acid (85%, analytical grade), and urea (analytical grade) were purchased from Merck (Darmstadt). The other chemicals were also of analytical grade. Biosorption Experiments. Biosorption experiments were carried out in batch experiments as follows. Twenty milligrams of biomass was suspended in 9 mL of metal solution (pH 5-6) and shaken continuously for 30 min. The biosorption data points in the figures and tables presented here in general were the average of duplicate experimental results, and the deviation was within 5%. Buffering was not used due to unknown effects of buffer compounds on biosorption. All experiments were conducted at room temperature. The cells were collected by centrifugation (10 min; 10 000*g). The amount of metal adsorbed by algae was calculated from the difference between the metal quantity added to the biomass and the metal content of the supernatant using the following formula
(1)
where q is the metal uptake (mmol/g), c0 and ceq are the initial and equilibrium concentrations in the solution (mmol/ l), respectively, V is the solution volume (L), and m is the mass (g) of the biosorbent used. The metal content in the supernatant was determined by graphite furnace atomic absorption spectroscopy (Zeiss AAS 4, Analytik Jena AG). Each sample was injected into the AAS instrument five times by means of an autosampler; the quantitative determination resulted in coefficients of variation of less than 3%. The pH was measured before and after the sorption experiment. The 4284
9
q ) qmaxbceq/(1 + bceq)
(2)
where b is the Langmuir constant, ratio of the adsorption/ desorption rates, i.e., related to energy of adsorption through the Arrhenius equation. For the fitting of experimental data, the Langmuir model was linearized as follows:
ceq/q ) ceq/qmax + 1/(qmaxb)
(3)
Phosphorylation of L. taylorii. The conditions were according to a method described by Meisch and Gauer, who applied it to wood and chitin (23). Algae (1.0 g) was mixed with 4.5 g of urea and 5 mL of phosphoric acid (30%). The mixture was left standing for 30 min at room temperature and then for 60 min at 70 °C in a drying oven. The phosphorylation of the biomass was carried out for 2 h at 200 °C in a muffle furnace. The phosphated (phos.) biomass was separated by centrifugation, washed with distilled water to remove the unreacted initial reagents, and treated as already described. The phosphate content of the modified biomass was determined with elemental analysis. The elemental analysis of phosphor was performed with a method according to Pu ¨ schel and Wittmann (28).
Results and Discussion
a SAG - strain collection of algae in Go ¨ ttingen; ISA - strain collection in the Institute of Biotechnology, Technical University Berlin.
q ) V(c0 - ceq)/m
desorption experiments were carried out in the same way. The metal-laden materials were generally desorbed with 0.1 N hydrochloric acid. Model To Fit Experimental Data. To determine the maximum metal sorption (qmax) to the cells, the equilibrium isotherm of metal sorption was calculated using the Langmuir sorption model
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 21, 2001
Biosorption of C. vulgaris. First investigations were carried out with C. vulgaris. The single sorption isotherms of the four metals on C. vulgaris are presented in Figure 1A. At lower equilibrium concentration, the algae adsorbed over 90% of the metals out of the solution. At higher equilibrium concentration, the sorption isotherms approached a saturation level. The experimental data in the figure were fitted using the Langmuir adsorption model. The Langmuir parameters and the correlation coefficient (r2) are summarized in Table 2. A good correlation between the experimental data and the Langmuir adsorption model is given in the isotherms for lead, cadmium, and zinc (r2 > 0.99). The sorption isotherms of the four metals by C. vulgaris illustrate maximum capacities of 0.30 mmol cadmium, 0.47 mmol lead, 0.41 mmol nickel, and 0.37 mmol zinc per gram of dry biomass, respectively. It could be observed that the affinity b (Table 2) of lead and cadmium onto algae surface was higher compared with nickel and zinc. Only a few quantitative biosorption studies with C. vulgaris have been reported in the literature, and these studies are difficult to compare. For instance, investigations of the removal of nickel by C. vulgaris described maximum capacities ranging between 0.02 mmol (29), 0.21 mmol (30), and 1 mmol (31) per gram of dry biomass. The kinetic of the biosorption of the four metals on C. vulgaris is very fast (Figure 2A). These experiments of adsorption kinetics indicated that a period of less than 30 min was sufficient to attain equilibrium for the four metals. In general, the system reached over 91% of the total biomass metal uptake within 3 min of contact. The pH of the metal solution played an important role in the biosorption of metals. The metal uptake of C. vulgaris from single metal solutions at various pH values is presented in Figure 3A. The competition of protons for the same binding sites on the algal cell wall reduces the amount of metal biosorbed at low pH, and therefore over 90% of the metals can be desorbed at pH 1, for example, with 0.1 N hydrochloric acid. There was a rapid increase in lead uptake with increasing
FIGURE 1. Sorption isotherms of Cd, Ni, Pb, and Zn by C. vulgaris (A), L. taylorii (B), and L. taylorii phos. (C). pH from 1.0 to 3.1, in cadmium uptake from pH 1.2 to 4.1, in nickel uptake from pH 1.4 to 4.7, and in zinc uptake from pH 1.6 to 4.3. The lead, cadmium, and zinc uptake reached a plateau around 4.0, nickel around 4.7. Working at over pH 6 was avoided to prevent precipitation of metal hydroxide complexes. These kinetic and pH results compare with those reported in the literature (30, 32). C. vulgaris was used as standard for the following investigations of further algae strains. Screening. The screening batch adsorption experiments were carried out with one initial concentration for each metal. The initial concentrations where the surface of C. vulgaris was saturated with the particular metal were selected for the screening. The use of saturating conditions for a screening investigation was necessary because the uptake capacities otherwise would not be comparable (2, 6, 8). The initial concentrations for the screening investigations were determined with 400 mg per L for lead and 100 mg per L for the other three metals. The results of the screening are shown in Table 3. Thirty strains of algae were examined. A wide range of adsorption capacities between the different strains of microalgae and between the four metals can be observed. The adsorption of lead shows the highest capacity for most of the strains.
S. hofmani and L. taylorii proved to be the most efficient algae under investigation. The good biosorption properties of cyanophyceae have already been described by Fehrmann and Pohl (4) and Inthorn et al. (33). The chlorophyceae A. densus and K. spiculiformis and the xanthophyceae V. dichotoma also show good biosorption properties for these metals. The lead uptake of the two best algae was determined to be over 96% of that in the solution. L. taylorii was able to remove 81% cadmium, 56% nickel, and 50% zinc out of the solution, respectively. In comparison to the best algae, D. bioculata could only bind 3% of the lead, 11% of Cadmium, 7% of nickel, and 6% of zinc in the solution, respectively. The importance of a screening investigation has been demonstrated by Volesky, Holan (8), and Veglio and Beolchini (9). Further investigations on biosorption were carried out on L. taylorii. Biosorption of L. taylorii. First of all we determined the single sorption isotherms of the four metals on the algae. The single sorption isotherms on L. taylorii are illustrated in Figure 1B. The metal uptake capabilities were evaluated by fitting the biosorption isotherms with the Langmuir adsorption model. A good correlation between the experimental data and the Langmuir adsorption model was also found in the isotherms. The Langmuir parameters and the correlation coefficient are presented in Table 2. The sorption isotherms of the four metals by L. taylorii show maximum capacities according to the Langmuir Adsorption Model of 1.47 mmol lead, 0.37 mmol cadmium, 0.65 mmol nickel, and 0.49 mmol zinc per gram of dry biomass, respectively. For L. taylorii, the lead isotherm is steeper, and the maximum capacity for lead is much higher compared to the other three metals. L. taylorii sorbed the metals tested in the decreasing order Pb > Ni > Zn > Cd. Leusch et al. (34) reported the same decreasing order for the four metals by the marine algae Ascophyllum nodosum and Saragassun fluitans. The impressive maximum lead uptake by the alga compares favorably with that determined for ion-exchange resin and the best marine macroalgae and is much higher than for natural zeolites and powdered activated carbon (2, 34, 35). The kinetics of the biosorption of the four metals on L. taylorii was again fast (Figure 2B). These adsorption kinetics experiments indicated that a period of 30 min was sufficient to attain equilibrium for the four metals. The biosorption of lead and cadmium reached over 90% of the total biomass metal uptake within 5 min of contact. The sorption of nickel and zinc was slower and reached equilibrium after 30 min. The influence of the pH for the biosorption by L. taylorii may be observed in Figure 3B. The figure illustrates that the optimum pH for L. taylorii was between pH 3 and pH 6 for lead, cadmium, and zinc and between pH 4 and 6 for nickel. Using pH values of less than 3, biosorption was reduced drastically by the protonation of the anionic ligands of the algal surfaces. The trends of pH dependence shown in Figure 3B have characteristics that are similar to those of titration curves of weak acids, with the inflection points corresponding to the pKa values of the functional group. The functional groups in the biomass, for instance carboxyl groups being weakly acidic groups, are affected by the acid-base equilibrium. A similar influence of pH on the biosorption of copper and cadmium onto a marine alga has been reported by Yu and Kaewsarn (36). Therefore the metals could be quantitatively desorbed at low pH value. As with the influence of the protons for the biosorption process, knowledge of the competition of the four metals at the binding sites on L. taylorii is also very important. The metal uptake for the four metals by L. taylorii up to initial concentration of a multimetal solution at the same molar concentration is illustrated in Table 4. Studies with L. taylorii showed a preference for the uptake of lead over cadmium, nickel, and zinc in a four metal solution VOL. 35, NO. 21, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4285
TABLE 2. Langmuir Parameters for the Biosorption Isotherms of Four Metals by Algae Cd
Ni
Pb
Zn
biomass
qmaxa
ba
r2
qmaxa
ba
r2
qmaxa
ba
r2
qmaxa
ba
r2
C. vulgaris L. taylorii L. taylorii phos.
0.30 0.37 2.52
56 13 20
0.999 0.998 0.999
0.41 0.65 2.79
4.5 1.3 6.4
0.989 0.999 0.999
0.47 1.47 3.08
38 14 57
0.999 0.998 0.999
0.37 0.49 2.60
6.0 15 135
0.993 0.995 0.999
a
All qmax and b values given in mmol/g and L/mmol.
FIGURE 2. Kinetics of metal binding by C. vulgaris (A), L. taylorii (B), and L. taylorii phos. (C). * in brackets is the initial concentration of the particular metal in g/L. at the equimolar concentration. Only lead significantly increased with an increase of the initial metal concentrations for all four metals. In the same way cadmium decreased. The metal uptake for nickel and zinc was constant over the concentration range. As can be drawn from Table 4, the competitive biosorption capacities of the biomass for cadmium, nickel, and zinc were lower than for noncompetitive conditions. The order of selectivity for competitive conditions in a four metal solution was as follows: Pb . Zn > Ni > Cd. What about the selectivity of the other three metals without lead? The results of this investigation are also presented in Table 4. It could be observed that zinc was bound prefer4286
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 21, 2001
FIGURE 3. Effect of pH on biosorption of Cd, Ni, Pb, and Zn by C. vulgaris (A), L. taylorii (B), and L. taylorii phos. (C). * in brackets is the initial concentration of the particular metal in g/L. entially over cadmium and nickel. The capacity for nickel was the lowest. The order of selectivity for competitive conditions in a three metal solution was as follows: Zn > Cd > Ni. The strong competition between lead and cadmium in the four metal solution could be confirmed with investigations in a two metal system of lead and cadmium (Table 4). These results are in agreement with those recently reported for Microcystis, Lemna, and Spirogyra (37) and Rhizopus arrhizus (38) for lead, which was also preferentially biosorbed from multimetal mixtures. Phosphorylation of L. taylorii. Several experiments were carried out to increase the capacity or to alter selectivity of
TABLE 3. Screening Results for 30 Algae Strains Pb (0.4)a
Cd (0.1)a
Ni (0.1)a
Zn (0.1)a
biomass
rem.b
qc
rem.b
qc
rem.b
qc
rem.b
qc
S. hofmani L. taylorii A. densus K. spiculiformis V. dichotoma C. kessleri M. species N. parmeloides S. maxima C. vulgaris G. longicauda R. spiculiforme A. hantzschii S. platensis P. tricornutum M. aeroginosa P. purpureum T. species G. verrucosa C. species A. cylindrica S. laxissima G. planctonica S. species P. species A. africanum E. magnus D. salina A. inaequealis D. bioculata
96 97 91 84 83 63 62 56 55 55 51 45 46 44 41 40 38 36 28 26 25 25 21 22 22 22 19 12 11 3
0.85 0.84 0.80 0.71 0.70 0.55 0.54 0.50 0.49 0.46 0.44 0.40 0.39 0.38 0.36 0.35 0.33 0.30 0.24 0.23 0.22 0.22 0.21 0.19 0.19 0.18 0.16 0.10 0.10 0.02
84 81 64 86 70 63 63 58 69 69 67 62 68 72 55 59 45 33 37 48 36 55 15 61 43 43 22 17 20 11
0.33 0.32 0.24 0.34 0.28 0.24 0.25 0.23 0.27 0.29 0.27 0.25 0.27 0.29 0.23 0.23 0.18 0.13 0.15 0.20 0.14 0.22 0.06 0.24 0.17 0.17 0.09 0.07 0.08 0.05
22 56 34 41 49 16 27 27 15 43 27 34 33 51 24 27 26 35 17 22 18 17 14 13 23 19 16 8 15 7
0.17 0.43 0.26 0.28 0.37 0.12 0.20 0.22 0.12 0.31 0.20 0.26 0.25 0.40 0.19 0.21 0.20 0.26 0.13 0.17 0.14 0.13 0.11 0.09 0.18 0.15 0.12 0.06 0.12 0.05
53 50 37 63 67 20 35 33 24 44 36 15 39 54 32 36 27 35 24 14 16 26 14 53 16 27 8 9 14 6
0.37 0.37 0.23 0.42 0.42 0.14 0.24 0.23 0.18 0.28 0.25 0.11 0.27 0.37 0.23 0.25 0.19 0.24 0.16 0.10 0.11 0.18 0.07 0.36 0.11 0.17 0.06 0.06 0.10 0.04
a In parentheses is the initial concentration of the particular metal in g/L. b Removal efficiency of the metal from the solution in %. c q is the metal uptake in mmol/g.
TABLE 4. Selectivity of Metal Binding onto L. taylorii and L. taylorii phosa Pb biomass
nMeb
L. taylorii 4
3
2
L. taylorii 4 phos.
3
c0c
rem.d
Cd
qe
rem.
0.5 99 0.22 39 1 97 0.43 14 2 88 0.78 3 3 60 0.83 n.d.f 0.5 54 1 22 2 11 0.5 100 0.23 70 1 99 0.49 21 2 97 0.92 3 3 79 1.16 n.d. 1 97 0.33 98 2 100 0.73 78 3 100 1.17 55 4 93 1.42 40 6 83 2.25 19 8 71 2.53 13 10 58 3.02 6 4 60 6 40 8 26 10 19
Ni
q 0.09 0.06 0.04 n.d. 0.10 0.09 0.09 0.15 0.09 0.03 n.d. 0.45 0.69 0.74 0.71 0.53 0.45 0.33 1.04 1.07 0.90 0.83
Zn
q
rem.
q
Σq
25 10 6 4 36 15 7
0.06 0.05 0.05 0.05 0.08 0.06 0.06
37 23 12 8 69 42 21
0.08 0.10 0.11 0.11 0.12 0.14 0.16
97 44 18 3 n.d. n.d. n.d. 33 26 16 11
0.39 0.35 0.23 0.04 n.d. n.d. n.d. 0.57 0.69 0.57 0.51
99 99 99 28 9 10 7 51 38 33 25
0.37 0.67 1.08 0.45 0.21 0.31 0.33 0.87 1.01 1.14 1.10
0.45 0.64 0.98 0.99 0.30 0.29 0.31 0.38 0.58 0.95 1.16 1.54 2.44 3.22 2.62 2.99 3.29 3.68 2.48 2.77 2.61 2.44
rem.
a The values presented are the mean values of triplicate experiments, and the standard deviation was within 7%. b Number of metals in the solution. c Initial concentration in mmol/L of the multimetallic solution at the same molar concentration. d Removal efficiency of the metal from the solution in %. e q is the metal uptake in mmol/g. f Not detectable.
the binding of the various metals by introduction of additional functional groups into the cell wall polysaccharides by L. taylorii. A phosphorylation of the hydroxyl groups on L. taylorii is the most promising method. The additional
insertion of phosphate groups into the cell wall was achieved with esterification of the hydroxyl groups of the polysaccharides using phosphoric acid in an urea melt. The yield of phosphorylated L. taylorii after the modification process was over 94% of the amount of biomass applied. The phosphate content of the alga, which was determined with elemental analysis, showed an increase from 0.6 mmol phosphor to 4.4 mmol phosphor per gram biomass. Biosorption of L. taylorii phos. The modified L. taylorii showed excellent biosorption properties (Figure 1C). The biosorption isotherms show good correlation again with the Langmuir Adsorption Model (Table 2). The sorption isotherms of the phosphorylated L. taylorii show maximum capacities of 2.52 mmol cadmium, 3.08 mmol lead, 2.79 mmol nickel, and 2.60 mmol zinc per gram of dry biomass, respectively, which are much higher than that of the natural algae. The cadmium uptake capacity was, for instance, about 7 times higher than for the natural algae. These capacities are comparable with those of the best commercial ion exchange materials (23, 35). The affinity b (Table 2) of the four metals to the surface of the modified biomass was higher compared to native L. taylorii. Only a few quantitative biosorption studies with chemical modified biomasses have been reported in the literature. Kraemer and Meisch (26) showed that the metal-binding ability of Aspergillus niger mycelial waste was improved by introduction of additional carboxyl or ethyldiamino groups into the cell wall structure. Maximum capacities for cadmium(II), cobalt(II), nickel(II), and zinc(II) were found to be in the range from 0.17 to 1.06 mmol per gram of dry biomass. Investigations of adsorption kinetics of the four metals on L. taylorii phos. showed that equilibrium was reached in a period of less than 5 min (Figure 2C). The effect of pH on biosorption of the four metals by the modified algae was different to the natural L. taylorii (Figure 3C). The biosorption capacities for cadmium and lead were observed to be constant over the pH range 4-6 and reduced only by about 28% for cadmium and about 9% for lead using metal solution at pH 1. The capacities for zinc were constant over the whole investigated pH range. It may be seen from Figure 3C that adsorption of nickel decreased with decreasing solution pH. The uptakes decreased from 2.7 mmol/g at pH 6.3 to 1.73 mmol/g for nickel at pH 1.1. However, the sorption capacities for the four metals at pH 1 were determined as being very high compared with the natural biomass. These results are very important for an application, because this new biosorbent could also be used for wastewater at low pH. Desorption of the metals, however, proves more difficult in comparison with the untreated biomass. Tests showed that the metals could be quantitatively desorbed with 3 N hydrochloric acid. Like the pH dependence for the biosorption process, the selectivity of the four metals at the binding sites on L. taylorii phos. is also very interesting. The results of metal uptake for the four metals up to initial concentration of a four metal solution at equimolar concentration are shown in Table 4. Only the lead uptake significantly increased with the increase of the initial metal concentration for all four metals similar to the results for the natural algae. The cadmium and zinc uptakes increased in the initial concentration range from 1 to 3 mmol/L and decreased in the range from 4 to 10 mmol/ L. In the initial concentration range from 1 to 3 mmol/L the binding sites of the modified algae were not saturated with lead (removal efficiency about 100%), and therefore the free binding sites could be used for cadmium and zinc. At higher initial concentration (4-10 mmol/L) these binding sites were occupied by lead, and consequently the capacities for cadmium and zinc decreased. Simultaneously, the uptake of nickel dramatically decreased in contrast to the natural algae. The order of selectivity for competitive conditions in a four VOL. 35, NO. 21, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4287
metal solution was as follows: Pb . Cd g Zn > Ni. In a three metal system without lead, it could be observed that zinc was bound selectively over cadmium and nickel similar to L. taylorii (Table 4). The capacity for nickel was the lowest. Biosorption by algae thus demonstrated itself to be an useful alternative to conventional systems for the removal of toxic metals in solution. The experiments presented underline the importance of screening because different algae species differ very much in their binding capacities for the various heavy metals. A study of binding kinetics is particularly important if more than one heavy metal should be retained from wastewater. The metal binding capacities can be influenced by chemical modification of the biomass. Phosphorylation of algae proved to be most successful in increasing the binding capacities for the less favored metals such as Cd, Ni, and Zn. L. taylorii and L. taylorii phos. have been used successfully in a technical process for the purification of wastewater, which will be described elsewhere (39).
Acknowledgments The authors gratefully acknowledge the financial support by the Deutsche Forschungsgemeinschaft (DFG), which was granted within the cooperative research program Sfb 193 “Biological treatment of industrial and commercial wastewater”. We thank Constance Richter for her excellent laboratory assistance and Mr. R. Hatton for his help in preparing this manuscript.
Literature Cited (1) Wilde, E. W.; Beneman, J. R. Biotechnol. Adv. 1993, 11, 781812. (2) Volesky, B. FEMS Microbiol. Rev. 1994, 14, 291-302. (3) Volesky, B., Ed.; Biosorption of Heavy Metals; CRC Press: Boca Raton, FL, 1990. (4) Fehrmann, C.; Pohl, P. J. Appl. Phycol. 1993, 5, 555-562. (5) Bakkaloglu, I.; Butter, T. J.; Evison, L. M.; Holland, F. S.; Hancock, I. C. Water Sci. Technol. 1998, 38, 269-277. (6) Matheickal, J. T.; Yu, Q. M. Water Sci. Technol. 1996, 34, 1-7. (7) Winter, C.; Winter, M.; Pohl, P. J. Appl. Phycol. 1994, 6, 479487. (8) Volesky, B.; Holan, Z. R. Biotechnol. Prog. 1995, 11, 235-250. (9) Veglio, F.; Beolchini, F. Hydrometallurgy 1997, 44, 301-316. (10) Garnham, G. W. In Biosorbents for metal ions; Wase, J., Forster, C., Eds.; Taylor & Francis: London, 1997; pp 11-39. (11) DeCarvalho, R. P.; Chong, K. H.; Volesky, B. Biotechnol. Prog. 1995, 11, 39-44. (12) Leusch, A.; Holan, Z. R.; Volesky, B. J. Chem. Technol. Biotechnol. 1995, 62, 279-288. (13) Wastewater Treatment with Algae; Wong, Y. S., Tam, N. F. Y., Eds.; Springer-Verlag: Berlin, 1998.
4288
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 21, 2001
(14) Matsunaga, T.; Takeyama, H.; Nakao, T.; Yamazawa, A. J. Biotechnol. 1999, 70, 33-38. (15) Bunke, G.; Go¨tz, P.; Buchholz, R. In Biotechnology; Rehm, H.-J., Reed, G. in cooperation with Pu ¨ hler, A., Stadler, P., Eds.; Volume 11a, Environmental (1) Processes I; Winter, J., Ed.; Wiley-VCH: Weinheim, 1999; pp 431-452. (16) Sandau, E.; Sandau, P.; Pulz, O.; Zimmermann, M. Acta Biotechnol. 1996, 16, 227-235. (17) Crist, R. H.; Oberholser, K.; McGarrity, J.; Crist, D. R.; Johnson, J. K.; Brittsan, J. M. Environ. Sci. Technol. 1994, 28, 496-502. (18) Crist, R. H.; Martin, J. R.; Crist, D. R. Environ. Sci. Technol. 1999, 33, 2252-2256. (19) Gardea-Torresdey, J. L.; Becker-Hapak, M. K.; Hosea, J. M.; Darnall, D. W. Environ. Sci. Technol. 1990, 24, 1372-1378. (20) Fourest, E.; Volesky, B. Environ. Sci. Technol. 1996, 30, 277282. (21) Lin, S.; Rayson, G. D. Environ. Sci. Technol. 1998, 32, 14881493. (22) Drake, L. R.; Lin, S.; Rayson, G. D. Environ. Sci. Technol. 1996, 30, 110-114. (23) Meisch, H.-U.; Gauer, J. Nachr. Chem. Technol. Lab. 1998, 46, 948-951. (24) Xie, J. Z.; Chang, H.-L.; Kilbane II, J. J. Bioresour. Technol. 1996, 57, 127-136. (25) Marshall, W. E.; Wartelle, L. H.; Boler, D. E.; Johns, M. M.; Toles, C. A. Bioresour. Technol. 1999, 69, 263-268. (26) Kraemer, M.; Meisch, H. U. Biometals 1999, 12, 241-246. (27) Wilke, A.; Bunke, G.; Go¨tz, P.; Buchholz, R. Prog. Mining Oilfield Chem. 1999, 1, 337-344. (28) Pu ¨ schel, R.; Wittmann, H. Mikrochim. Acta 1960, 352, 670-674. (29) Wong, J. P. K.; Wong, Y. S.; Tam, N. F. Y. Bioresour. Technol. 2000, 73, 133-137. (30) Lau, P. S.; Lee, H. Y.; Tsang, C. C. K.; Tam, N. F. Y., Wong, Y. S. Environ. Technol. 1999, 20, 953-961. (31) Donmez, G. C.; Aksu, Z.; Ozturk, A.; Kutsal, T. Proc. Biochem. 1999, 34, 885-892. (32) Aksu, Z. Sep. Purif. Technol. 2001, 21, 285-294. (33) Inthorn, D.; Nagase, H.; Isaji, Y.; Hirata, K.; Miyamoto, K. J. Ferment. Bioeng. 1996, 82, 580-584. (34) Leusch, A.; Holan, Z. R.; Volesky, B. Appl. Biochem. Biotechnol. 1996, 61, 231-249. (35) Ion Exchangers; Dorfner, K., Ed.; de Gruyter: Berlin, New York, 1991. (36) Yu, Q. M.; Kaewsarn, P. Sep. Sci. Technol. 1999, 34, 1595-1605. (37) Singh, S.; Pradhan, S.; Rai, L. C. Proc. Biochem. 2000, 36, 175182. (38) Sag, Y.; Kaya, A.; Kutsal, T. Separ. Sci. Technol. 2000, 35, 26012617. (39) Wilke, A.; Bunke, G.; Buchholz, R.; Klimmek, S.; Stan, H.-J. Environ. Sci. Technol. 2001, submitted for publication.
Received for review March 2, 2001. Revised manuscript received July 30, 2001. Accepted July 31, 2001. ES010063X
