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Biosorption of cadmium by algal biomass: Adsorption and desorption characteristics
~
Pergamon
Wal. ScL Tuh. Vol. 35. No.7. pp. 115-122, 1997. C 1997 IA wQ. Pubhshed by Elsevier Science LId Printed in Oreat Britain.
PH: S0273-1223(97)OO121-2
0273-1223197 $17·00 + (H)O
BIOSORPTION OF CADMIUM BY ALGAL BIOMASS: ADSORPTION AND DESORPTION CHARACTERISTICS K. H. Chu, M. A. Hashim, S. M. Phang and V. B. Samuel Institute of Advanced Studies. University of Malaya. 50603 Kl«lla Lumpur. Malaysia
ABSTRACT The adsorption and desol]ltion characteristics of a biosol]ltion process comprising the biomass of the marine alga Sargassum baccularia. cadmium ions and desorbing agents hydlochloric acid and ethylenediaminetetraacetic acid (EOTA) were investigated using a batch reactor system. Both desorbents were effective in stripping adsorbed cadlnium from the biomass. It was found that HCI at pH 2 could desorb 80% of the cadmium initially loaded onto the biomass. Almost complete recovery of cadmium was achieved by a 3.24 mM EOTA solution. The reusability of the biomass was tested in five consecutive adsol]ltion• desol]ltion cycles. The quantity of cadmium desorbcd over the five cycles with either HO or EOTA as desorbent corresponded well to the quantity loaded. indicating that complete desol]ltion was readily achieved. However. the cadmium uptake capacity of the biomass deteriorated with repeated use of HCI or EOTA. HO was found to have reduced cadmium uptake by 56% while the reduction for EDTA was nearly 40% over the five adsorption-desorption cycles. EDTA thus emerged as a slightly better desorbing agent compared with HCI. After completion of the five cycles it was found that 30% of the original biomass weight had been lost with HCI as the desorbent. EOTA exhibited desol]ltion behaviour similar to that of HCI by causing a biomass loss of 16%. The loss of biomass indicates that some dissolution of biomass components containing cadlnium binding sites apparently occurred. reducing the cadmium uptake capacity of the biomass in multiple cycles of adsol]ltion-desol]ltion. C 19971AWQ. Published by Elsevier Science Ltd
KEYWORDS Adsorption; algae; biosorption; cadmium; desorption; heavy metals; seaweed; wastewater.
INTRODUCTION The ability of some algae to remove toxic metal ions from aqueous solutions has been known for some time (Maeda and Sakaguchi, 1990). Recently. this phenomenon has attracted the attention of researchers due to possible commercial exploitation in the rapidly growing field of industrial wastewater treatment. This treatment technology. commonly known as biosorption. often employs microbial biomass to treat industrial wastewater containing heavy metals generated by many industrial activities such as electroplating. metal finishing or printed circuit board manufacturing. In general. the biosorption process relies on reversible binding of metal ions to the surface of nonliving microbial biomass because dead organisms are not affected by toxic waste. Although the effectiveness of biosorption for heavy metal removal has been demonstrated in numerous studies (Volesky. 1990). commercial acceptance of a treatment process based on biosorption will largely 115
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K. H. CHU et al.
depend on its ability to compete with existing technologies on a cost basis. It has been reported that the cost of biomass production plays a major role in determining the overall cost of a biosorption process (Kuyucak, 1990). Since the cost of producing biomass specifically for metal removal through cultivation is generally high, various researchers have proposed the use of spent microbial biomass produced by the fermentation industry. For example, the waste biomass of fungi generated by industry has been employed to remove heavy metals from aqueous solutions (Fourest et al., 1994). Another source of low-cost biomass comes from the abundant biological materials found in nature such as marine macroalgae, or seaweeds as they are more generally known. Volesky (1994) revealed the impressive binding capacities of seaweeds for a wide range of heavy metals. To design and optimise a biosorption process for industrial applications, it is important to elucidate the adsorption as well as desorption behaviour of the biomass so that it can be reused in mUltiple cycles, increasing substantially the cost-effectiveness of the process. In addition, it is equally important to select an efficient and cost-effective desorbing agent to strip adsorbed metals from the biomass. The metal adsorption behaviour of various biomass has been widely reported (Volesky, 1990). However, relatively few studies have addressed in detail the issues of biomass reusability and selection of appropriate desorbents. This work was conducted to examine the metal adsorption and desorption characteristics of the marine alga Sargassum baccularia. In particular, the ability of two different desorbents to strip adsorbed cadmium, a toxic metal commonly found in industrial wastewater, from the biomass of S. baccularia was investigated. MATERIALS AND METHODS Fresh samples of the brown alga Sargassum baccularia were collected from a beach situated on the west coast of Peninsular Malaysia. These samples were washed thoroughly with distilled deionised water to remove adhering particles and dried to constant weight. The dried samples were then ground and sieved to a size range of 500-710 J.Lm. A stock solution of cadmium was prepared by dissolving cadmium nitrate salt in distilled deionised water. All working cadmium solutions were prepared by diluting the stock solution. The biomass of S. baccularia was first loaded with cadmium by conducting batch experiments. The initial cadmium concentration and solution pH were fixed at 0.44 mM (50 ppm) and 5.0, respectively. 0.1 g of the seaweed biomass was added to each flask containing a cadmium solution of 0.05 I, giving a biomass concentration of 2 gil. The flasks were incubated for five hours at 25°C and 150 rpm. These experimental conditions have been found to be optimal for the uptake of cadmium by the biomass (Samuel, 1996). Kinetic studies have shown that a contact time of approximately 30 minutes was sufficient for the adsorption process to attain eqUilibrium (Samuel, 1996). After eqUilibration, the slurry was filtered and the residual cadmium concentration determined with an inductively coupled plasma spectrometer. The amount of cadmium bound to the biomass was calculated from a mass balance. The amount of cadmium adsorbed by the biomass in a series of experiments under identical conditions was fairly constant; about 0.02 mmol in most cases. To standardise the initial conditions of the desorption experiments, only biomass with a cadmium loading of exactly 0.02 mmol was selected for further investigation. The cadmium-laden biomass was filtered and washed repeatedly with deionised water to remove any residual cadmium solution. After washing, excess water was removed from the biomass. The biomass was then placed in a flask containing 0.02 I of a desorbing agent. Two different desorbents were used to strip the adsorbed cadmium: hydrochloric acid and disodium salt of ethylenediaminetetraacetic acid (EDTA). The flasks were incubated at 25°C and 150 rpm for 24 hours. Kinetic studies showed that a contact time of approximately 90 minutes was sufficient for the desorption process to reach equilibrium (Samuel, 1996). Following each desorption of the biomass with either HCI or EDTA, the slurry was filtered and the cadmium concentration of the filtrate determined. The amount of desorbed cadmium was calculated from a mass balance. The ability of hydrochloric acid to strip the adsorbed cadmium was examined in the 2-6 pH range while that of EDTA was tested in the 0.2-3.24 mM concentration range. All desorption experiments were performed in triplicates.
Biosorplion of cadmium by algal biomass
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The above adsorption and desorption procedures were repeated for five cycles using the same biomass to assess the effect of desorption on the ability of the biomass to take up cadmium again. Following each desorption of the biomass with either HCI or EDTA, the biomass was filtered, washed with distilled deionised water, dried to a constant weight and reloaded with cadmium. RESULTS AND DISCUSSION Uptake of metal ions by nonviable algal biomass is mainly due to adsorption or ion exchange on the surface of the biomass. The surface of marine algae consists of polysaccharides and proteins which offer a host of functional groups capable of binding to metal ions. Brown algae such as Sargassum baccularia used in this study contain high concentrations of sulphated polysaccharides. It has been postulated that the function of these polysaccharides, which are absent in land plants, is to enable marine algae to selectively adsorb trace metal ions such as potassium and calcium in a saline medium through ion exchange (Percival and McDowell. 1967).
In addition to sulphated polysaccharides. the surface of brown algae contains alginic acid. a copolymer composed of L-guluronic and D-mannuronic acids, which have anionic carboxylate sites at neutral pH. Alginic acid is an important component of the brown algae. constituting 14 to 40% of the dry weight of these seaweeds (Percival and McDowell, 1967). The cation exchange properties of alginic acid have been extensively studied. For example. Haug and Smidsrod (1967) showed that the presence of the divalent metals calcium, magnesium and strontium in brown algae was largely due to ion exchange between seawater and alginate found in their cell wall. A more recent study (Kuyucak and Volesky. 1989a) revealed that alginate in the cell wall of the brown alga Ascophyllum nodosum plays an important role in cobalt uptake. These observations imply that metal uptake by nonliving marine algae is essentially an easily reversible physicochemical process since metal ions are deposited on the surface of the marine algae. In this study the ability of a mineral acid (HCI) and a chelating agent (EDTA) to reverse cadmium uptake by the brown alga S. baccularia was investigated using a batch reactor system. Adsorption experiments were first conducted to load 0.02 mmol of cadmium onto 0.1 g of biomass. resulting in an equilibrium uptake of 0.2 mmol cadmium/g biomass. It should be noted that under these experimental conditions the maximum capacity of the biomass for cadmium could easily reach 0.7 mmollg (Chu et aL, 1996). Hydrochloric acid has been employed by Holan et al. (1993) and Aldor et al. (1995) to strip adsorbed cadmium from algal biomass. In this study the suitability of HCI to process cadmium-laden algal biomass was tested in the 2-6 pH range. Figure 1 shows the desorption efficiency of HCI as a function of pH. Desorption efficiency is defined as percent extraction of the cadmium initially loaded onto the biomass. It is clear that HCl was able to desorb approximately 80% of the adsorbed cadmium at pH 2. In the 3-6 pH range desorption of the adsorbed cadmium was negligible. The ability of HCl at pH 2 to strip most of the adsorbed cadmium may be partly attributed to ion exchange. It has been postulated that the high concentration of hydrogen ions at low pH values is responsible for the displacement of adsorbed metals via the ion exchange mechanism (Crist et al.• 1992, 1994). In addition to ion exchange. desorption of the adsorbed cadmium by HCI may be attributed to the acid's ability to dissolve certain groups of polysaccharides found on the surface of the biomass. The dissolution of these polysaccharides with cadmium binding sites would release cadmium back into the solution. Similar desorption studies using the chelating agent ethylenediaminetetraacetic acid (EDTA) as the desorbent were also conducted. The desorption effectiveness of EDTA was examined in the 0.2-3.24 mM concentration range. The pH of these EDTA solutions varied between 4.5 and 5.0. The pH was not standardised because the previous study with HCl indicates that the effect of pH on desorption was negligible at pH > 4.
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Figure 2 shows that the desorption efficiency of EDTA increased with increasing EDTA concentration. The desorption efficiency at the lowest EDTA concentration examined was 21 % while at the highest EDTA concentration of 3.24 mM almost complete desorption of the adsorbed cadmium was achieved.
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Based on the above results, five consecutive cycles of adsorption-desorption were conducted under conditions of maximum desorption, Figures 3 and 4 show the results obtained using HCI at pH 2 and EDTA at 3.24 mM as the desorbents. respectively. The open bars depict the amount of cadmium adsorbed while the solid bars represent the amount of cadmium desorbed in each cycle. In the case of HCl. Fig. 3 clearly shows a gradual decrease in cadmium uptake with increasing cycle. After a sequence of five cycles. the cadmium uptake capacity of the biomass had been reduced to approximately 44% of the original cadmium uptake capacity of the virgin biomass. A small fraction of the cadmium initially loaded in the first cycle was not desorbed. However, in the second cycle the amount of cadmium desorbed was higher than the amount of cadmium adsorbed. The excess cadmium was most probably the residual cadmium that was not desorbed in the first cycle. This could also explain the sharp reduction in cadmium uptake in the second cycle. Cadmium taken up in the first cycle which was not desorbed resulted
Biosorption of cadmium by algal biomass
119
in a ~~crease in the number of bind!ng sites. available for cadmium uptake in the second cycle. In the remammg three cycles, the desorptIon efficIency was close to 100%, indicating that the desorption efficiency of HCl increased with increasing cycle.
Figure 3. Five cycles of cadmium adsorption-desorption with hydrochloric acid as the desorbing agent
The gradual reduction of cadmium uptake in five cycles of adsorption-desorption indicates that the binding sites on the surface of the biomass were either destroyed or morphologically altered by HC!. It is known that concentrated HCI can rupture the structure of alginate chains and destroy the hydrogen bonding capacity of the alginate by hydrolysis (Percival and McDowell, 1967). Consequently, repeated exposure of the biomass to a strong acidic environment could lead to a reduction of metal binding sites. This hypothesis is supported by the observation that there is a gradual biomass weight loss over the five cycles, as shown in Fig. S. The amount of biomass used at the beginning of each cycle with HCI is depicted as open bars in Fig. S. The initial amount of biomass employed in the first cycle was 0.1 g. This amount had been reduced to 0.07 g by the time the biomass reached the fifth cycle, resulting in a biomass loss of 30%. Since great care had been taken to minimise the loss of biomass due to handling over the course of the five cycles, the observed loss of biomass could be a result of the dissolution of alginate by HCI. Kuyucak and Volesky (1989b) reported that a biomass weight loss of 42% also occurred over three cycles of adsorption-
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K. H. CHU el al.
Results of the biomass recycling experiments with EDTA are shown in Fig. 4. EDT A exhibited desorption behaviour similar to that of He!. There is a gradual decrease in cadmium uptake with increasing cycle. However, cadmium uptake in the fifth cycle was approximately 62% of that observed in the first cycle. This is higher than the 44% obtained with He!. The desorption efficiency in all five cycles was very high; almost complete recovery of cadmium was observed in each cycle.
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The reduction in cadmium uptake was possibly due to cell wall damage as a result of repeated exposure of the biomass to EDTA solutions. As EDT A is a strong chelating agent, the configuration of the binding sites on the biomass surface may have been altered following extraction of adsorbed cadmium by EDT A to form highly stable complexes in solution. However, biomass weight loss was also observed in this case. The solid bars in Fig. 5 show that the amount of biomass had been reduced from 0.1 g in the fust cycle to 0.084 g in the fifth cycle, a reduction of 16%. This reduction is much smaller than that observed with He!. The loss of biomass may have contributed to the reduction in cadmium uptake although it is not known whether EDTA has the ability to dissolve cell wall components of the biomass such as alginate. Table 1. Total amounts of cadmium adsorbed and desorbed over five cycles of adsorption-desorption with EDTA and Hel as the desorbents Desorbent
cadmium adsorbed (mmol)
cadmium desorbed (mmol)
EDTA
0.083
0.081
Hel
0.067
0.065
The results shown in Figures 3 and 4 suggest that both Hel and EDTA were effective in stripping almost all of the adsorbed cadmium from the biomass in each cycle except for the fust cycle with Hel where a small fraction of the adsorbed cadmium remained in the biomass. It should however be noted that the residual cadmium was subsequently removed in the second cycle. Table 1 shows the sum of cadmium adsorbed and desorbed over the entire five cycles. It is clear that cadmium uptake by S. baccularia is easily reversible with no accumulation of irreversibly bound cadmium on the biomass. Unfortunately, exposure of the biomass to both desorbents resulted in a decrease in cadmium uptake with increasing cycle. Table 1 shows that the total amount of cadmium taken up by the biomass with EDTA as the desorbent is about 1.24 times higher than that adsorbed by the biomass when Hel is used as the desorbent. It can therefore be concluded that EDTA is a more benign desorbent than He!. However, selection of EDTA as a desorbent also depends on other
Biosorplion of cadmium by algal biomass
121
equally important factors such as its cost and potential problems associated with further processing of the extracted cadmium which forms highly stable complexes with EDTA. It is interesting to compare the results reported here with similar studies conducted using marine algae and HCI or EDT A. Kuyucak and Volesky (l989b) conducted three cycles of batch adsorption-desorption using H2S04 at pH 5.2 and HCI at pH 2.7 to desorb cobalt from the native biomass of the brown alga Ascophyllum nodosum. They reported that cobalt uptake and biomass weight decreased by 37 and 42%. respectively. These observations are in good agreement with the results reported here. A subsequent study by the same laboratory using chemically modified biomass of the same alga showed that desorption of cadmium with 0.1-0.5 M HCI resulted in no changes of the biomass cadmium uptake capacity through five adsorption-desorption cycles (Holan et al.• 1993). A third study indicates that another chemically modified algal biomass of Sargassum fluitans showed no decrease in cadmium uptake upon three cycles of adsorption-desorption with 0.1 M HCI (Aldor et al.. 1995). They claimed that chemical modification appears to increase the resistance of the algal biomass to HCI during the desorption step. An interesting point to note is that different desorption times were employed in these studies. For example. Kuyucak and Volesky (1989b) conducted their desorption experiments "overnight" while the duration of our desorption step per cycle was 24 hours. These relatively long desorption times could have allowed HCI to do enough damage to the native biomass. resulting in progressive deterioration of its metal uptake capacity in multiple cycles. By contrast. in the study by Holan et al. (1993) batch desorption was carried out until equilibrium was reached or up to 12 hours while Aldor et al. (1995) employed a desorption duration of 2 hours. These relatively short desorption times could have prevented HCI from inflicting severe damage on the chemically modified biomass. aIlowing it to retain much of its original cadmium uptake capacity in multiple cycles. It is thus not clear at this time whether the superior performance of the chemically modified biomass over native biomass in multiple adsorption-desorption cycles is a result of biomass reinforcement by chemical treatment or a result of short desorption time.
If the metal uptake capacity of a biomass in a batch reactor system with a given desorbent is a function of the desorption time. then the adsorption-desorption cycles. whose number is always stated in this type of studies. wiII not be a suitable performance indicator. In such cases. the desorption time per cycle should always be specified in order to give a more accurate comparison of different biomass. The effect of desorption time on the metal uptake capacity of the algal biomass in multiple cycles is currently being investigated in our laboratory and the results will be reported in a future communication. CONCLUSIONS Two desorbents. HCI and EDTA. were found to be effective in stripping adsorbed cadmium from the biomass of the marine alga S. baccularia over five consecutive cycles of adsorption-desorption. HCI at pH 2 and EDTA at 3.24 mM successfully desorbed most of the adsorbed cadmium. However, both desorbents damaged the binding sites enough to seriously impair cadmium uptake in subsequent cycles. HCI was found to have reduced cadmium uptake over the five cycles by 56%; for the five cycles with EDTA. the reduction was nearly 40%. Biomass weight loss was observed in both cases. indicating that the reduction in cadmium uptake could be a result of the dissolution of metal-binding alginate from the biomass ceIl wall by the two desorbents. It can therefore be concluded that neither HCI nor EDTA seem to be attractive as a desorbent although both possess excellent desorption efficiency. However. it may be possible to minimise the reduction in cadmium uptake in multiple cycles by optimising the desorption duration in each cycle. The influence of desorption time on the metal uptake capacity of biomass in multiple cycles is not known at this time. REFERENCES Aldor. I.• Fouresl. E. and Volesky. B. (1995). Desorplion of cadmium from algal biosorbenl. Canadian J. Chem. Eng .• 73. ~16522. MT 35:1·[
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Chu, K. H., Hashim, M. A., Phang, S. M. and Samuel, V. B. (1996). Biosorption of heavy metals by tropical marine algae. Proc. 7th Congress of Asian Pacific Confederation of Chemical Engineers, vol. 2, pp 527-531. Crist, R H .. Oberholser, K.• McGarrity, J .• Crist, D. R, Johnson, J. K. and Brittsan J. M. (1992). Interaction of metals and protons with algae. 3. Marine algae with emphasis on lead and aluminum. Environ. Sci. Techno!., 26, 496-502. Crist, D. R., Crist, R. H., Martin, J. R and Watson, J. R (1994). Ion exchange systems in proton-metal reaction with algal cell walls. FEMS Microbiol. Rev., 14, 309-314. Fourest, E., Canal, C. and Roux, J.-c. (1994). Improvement of heavy metal biosorption by mycelial dead biomasses (Rhizopus arrhizus. Mucor mlehei and Penicillium chrysogenum): pH control and cationic activation. FEMS Microbiol. Rev., 14, 325-332. Haug, A. and Smidsrod, O. (1967). Strontium. calcium and magnesium in brown algae. Nature, 215, 1167- 1168. Holan, Z. R, Volesky, B. and Prasetyo, I. (1993). Biosorption of cadmium by biomass of marine algae. Biotechnol. Bioeng., 41, 819-82'. Kuyucak, N. (1990). Feasibility of biosorbents application. In: Biosorption of Heavy Metals, B. Volesky (Ed.), CRC Press. Boca Raton, pp 371-378. Kuyucak. N. and Volesky. B. (l989a). The mechanism of cobalt biosorption. Biotechnol. Bioeng.• 33. 823- 831. Kuyucak, N. and Volesky. B. (l989b). Desorption of cobalt-laden algal biosorbent. Biotechnol. Bioeng., 33, 815-822. Maeda, S. and Sakaguchi, T. (1990). Accumulation and detoxification of toxic metal elements by algae. In: Introduction to Applied Phycology. I. Akatsuka (Ed.). SPB Academic Publishing bv. The Hague. pp. 109-136. Percival. E. and McDowell, R. H. (1967). Chemistry and Enzymology of Marine Algal Polysaccharides. Academic Press. London. Samuel. V. B. (1996). Biosorption of cadmium and copper by the biomass of Sargassum baccularia (Phaeophyta). M. Biotech. Thesis. University of Malaya, Kuala Lumpur. Malaysia. Volesky, B. (Ed.) (1990). Biosorption of Heavy Metals. CRe Press, Boca Raton. Volesky, B. (1994). Advances in biosorption of metals: selection of bIOmass types. FEMS Microbiol. Rev., 14, 291-302.
Pergamon
Wal. ScL Tuh. Vol. 35. No.7. pp. 115-122, 1997. C 1997 IA wQ. Pubhshed by Elsevier Science LId Printed in Oreat Britain.
PH: S0273-1223(97)OO121-2
0273-1223197 $17·00 + (H)O
BIOSORPTION OF CADMIUM BY ALGAL BIOMASS: ADSORPTION AND DESORPTION CHARACTERISTICS K. H. Chu, M. A. Hashim, S. M. Phang and V. B. Samuel Institute of Advanced Studies. University of Malaya. 50603 Kl«lla Lumpur. Malaysia
ABSTRACT The adsorption and desol]ltion characteristics of a biosol]ltion process comprising the biomass of the marine alga Sargassum baccularia. cadmium ions and desorbing agents hydlochloric acid and ethylenediaminetetraacetic acid (EOTA) were investigated using a batch reactor system. Both desorbents were effective in stripping adsorbed cadlnium from the biomass. It was found that HCI at pH 2 could desorb 80% of the cadmium initially loaded onto the biomass. Almost complete recovery of cadmium was achieved by a 3.24 mM EOTA solution. The reusability of the biomass was tested in five consecutive adsol]ltion• desol]ltion cycles. The quantity of cadmium desorbcd over the five cycles with either HO or EOTA as desorbent corresponded well to the quantity loaded. indicating that complete desol]ltion was readily achieved. However. the cadmium uptake capacity of the biomass deteriorated with repeated use of HCI or EOTA. HO was found to have reduced cadmium uptake by 56% while the reduction for EDTA was nearly 40% over the five adsorption-desorption cycles. EDTA thus emerged as a slightly better desorbing agent compared with HCI. After completion of the five cycles it was found that 30% of the original biomass weight had been lost with HCI as the desorbent. EOTA exhibited desol]ltion behaviour similar to that of HCI by causing a biomass loss of 16%. The loss of biomass indicates that some dissolution of biomass components containing cadlnium binding sites apparently occurred. reducing the cadmium uptake capacity of the biomass in multiple cycles of adsol]ltion-desol]ltion. C 19971AWQ. Published by Elsevier Science Ltd
KEYWORDS Adsorption; algae; biosorption; cadmium; desorption; heavy metals; seaweed; wastewater.
INTRODUCTION The ability of some algae to remove toxic metal ions from aqueous solutions has been known for some time (Maeda and Sakaguchi, 1990). Recently. this phenomenon has attracted the attention of researchers due to possible commercial exploitation in the rapidly growing field of industrial wastewater treatment. This treatment technology. commonly known as biosorption. often employs microbial biomass to treat industrial wastewater containing heavy metals generated by many industrial activities such as electroplating. metal finishing or printed circuit board manufacturing. In general. the biosorption process relies on reversible binding of metal ions to the surface of nonliving microbial biomass because dead organisms are not affected by toxic waste. Although the effectiveness of biosorption for heavy metal removal has been demonstrated in numerous studies (Volesky. 1990). commercial acceptance of a treatment process based on biosorption will largely 115
116
K. H. CHU et al.
depend on its ability to compete with existing technologies on a cost basis. It has been reported that the cost of biomass production plays a major role in determining the overall cost of a biosorption process (Kuyucak, 1990). Since the cost of producing biomass specifically for metal removal through cultivation is generally high, various researchers have proposed the use of spent microbial biomass produced by the fermentation industry. For example, the waste biomass of fungi generated by industry has been employed to remove heavy metals from aqueous solutions (Fourest et al., 1994). Another source of low-cost biomass comes from the abundant biological materials found in nature such as marine macroalgae, or seaweeds as they are more generally known. Volesky (1994) revealed the impressive binding capacities of seaweeds for a wide range of heavy metals. To design and optimise a biosorption process for industrial applications, it is important to elucidate the adsorption as well as desorption behaviour of the biomass so that it can be reused in mUltiple cycles, increasing substantially the cost-effectiveness of the process. In addition, it is equally important to select an efficient and cost-effective desorbing agent to strip adsorbed metals from the biomass. The metal adsorption behaviour of various biomass has been widely reported (Volesky, 1990). However, relatively few studies have addressed in detail the issues of biomass reusability and selection of appropriate desorbents. This work was conducted to examine the metal adsorption and desorption characteristics of the marine alga Sargassum baccularia. In particular, the ability of two different desorbents to strip adsorbed cadmium, a toxic metal commonly found in industrial wastewater, from the biomass of S. baccularia was investigated. MATERIALS AND METHODS Fresh samples of the brown alga Sargassum baccularia were collected from a beach situated on the west coast of Peninsular Malaysia. These samples were washed thoroughly with distilled deionised water to remove adhering particles and dried to constant weight. The dried samples were then ground and sieved to a size range of 500-710 J.Lm. A stock solution of cadmium was prepared by dissolving cadmium nitrate salt in distilled deionised water. All working cadmium solutions were prepared by diluting the stock solution. The biomass of S. baccularia was first loaded with cadmium by conducting batch experiments. The initial cadmium concentration and solution pH were fixed at 0.44 mM (50 ppm) and 5.0, respectively. 0.1 g of the seaweed biomass was added to each flask containing a cadmium solution of 0.05 I, giving a biomass concentration of 2 gil. The flasks were incubated for five hours at 25°C and 150 rpm. These experimental conditions have been found to be optimal for the uptake of cadmium by the biomass (Samuel, 1996). Kinetic studies have shown that a contact time of approximately 30 minutes was sufficient for the adsorption process to attain eqUilibrium (Samuel, 1996). After eqUilibration, the slurry was filtered and the residual cadmium concentration determined with an inductively coupled plasma spectrometer. The amount of cadmium bound to the biomass was calculated from a mass balance. The amount of cadmium adsorbed by the biomass in a series of experiments under identical conditions was fairly constant; about 0.02 mmol in most cases. To standardise the initial conditions of the desorption experiments, only biomass with a cadmium loading of exactly 0.02 mmol was selected for further investigation. The cadmium-laden biomass was filtered and washed repeatedly with deionised water to remove any residual cadmium solution. After washing, excess water was removed from the biomass. The biomass was then placed in a flask containing 0.02 I of a desorbing agent. Two different desorbents were used to strip the adsorbed cadmium: hydrochloric acid and disodium salt of ethylenediaminetetraacetic acid (EDTA). The flasks were incubated at 25°C and 150 rpm for 24 hours. Kinetic studies showed that a contact time of approximately 90 minutes was sufficient for the desorption process to reach equilibrium (Samuel, 1996). Following each desorption of the biomass with either HCI or EDTA, the slurry was filtered and the cadmium concentration of the filtrate determined. The amount of desorbed cadmium was calculated from a mass balance. The ability of hydrochloric acid to strip the adsorbed cadmium was examined in the 2-6 pH range while that of EDTA was tested in the 0.2-3.24 mM concentration range. All desorption experiments were performed in triplicates.
Biosorplion of cadmium by algal biomass
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The above adsorption and desorption procedures were repeated for five cycles using the same biomass to assess the effect of desorption on the ability of the biomass to take up cadmium again. Following each desorption of the biomass with either HCI or EDTA, the biomass was filtered, washed with distilled deionised water, dried to a constant weight and reloaded with cadmium. RESULTS AND DISCUSSION Uptake of metal ions by nonviable algal biomass is mainly due to adsorption or ion exchange on the surface of the biomass. The surface of marine algae consists of polysaccharides and proteins which offer a host of functional groups capable of binding to metal ions. Brown algae such as Sargassum baccularia used in this study contain high concentrations of sulphated polysaccharides. It has been postulated that the function of these polysaccharides, which are absent in land plants, is to enable marine algae to selectively adsorb trace metal ions such as potassium and calcium in a saline medium through ion exchange (Percival and McDowell. 1967).
In addition to sulphated polysaccharides. the surface of brown algae contains alginic acid. a copolymer composed of L-guluronic and D-mannuronic acids, which have anionic carboxylate sites at neutral pH. Alginic acid is an important component of the brown algae. constituting 14 to 40% of the dry weight of these seaweeds (Percival and McDowell, 1967). The cation exchange properties of alginic acid have been extensively studied. For example. Haug and Smidsrod (1967) showed that the presence of the divalent metals calcium, magnesium and strontium in brown algae was largely due to ion exchange between seawater and alginate found in their cell wall. A more recent study (Kuyucak and Volesky. 1989a) revealed that alginate in the cell wall of the brown alga Ascophyllum nodosum plays an important role in cobalt uptake. These observations imply that metal uptake by nonliving marine algae is essentially an easily reversible physicochemical process since metal ions are deposited on the surface of the marine algae. In this study the ability of a mineral acid (HCI) and a chelating agent (EDTA) to reverse cadmium uptake by the brown alga S. baccularia was investigated using a batch reactor system. Adsorption experiments were first conducted to load 0.02 mmol of cadmium onto 0.1 g of biomass. resulting in an equilibrium uptake of 0.2 mmol cadmium/g biomass. It should be noted that under these experimental conditions the maximum capacity of the biomass for cadmium could easily reach 0.7 mmollg (Chu et aL, 1996). Hydrochloric acid has been employed by Holan et al. (1993) and Aldor et al. (1995) to strip adsorbed cadmium from algal biomass. In this study the suitability of HCI to process cadmium-laden algal biomass was tested in the 2-6 pH range. Figure 1 shows the desorption efficiency of HCI as a function of pH. Desorption efficiency is defined as percent extraction of the cadmium initially loaded onto the biomass. It is clear that HCl was able to desorb approximately 80% of the adsorbed cadmium at pH 2. In the 3-6 pH range desorption of the adsorbed cadmium was negligible. The ability of HCl at pH 2 to strip most of the adsorbed cadmium may be partly attributed to ion exchange. It has been postulated that the high concentration of hydrogen ions at low pH values is responsible for the displacement of adsorbed metals via the ion exchange mechanism (Crist et al.• 1992, 1994). In addition to ion exchange. desorption of the adsorbed cadmium by HCI may be attributed to the acid's ability to dissolve certain groups of polysaccharides found on the surface of the biomass. The dissolution of these polysaccharides with cadmium binding sites would release cadmium back into the solution. Similar desorption studies using the chelating agent ethylenediaminetetraacetic acid (EDTA) as the desorbent were also conducted. The desorption effectiveness of EDTA was examined in the 0.2-3.24 mM concentration range. The pH of these EDTA solutions varied between 4.5 and 5.0. The pH was not standardised because the previous study with HCl indicates that the effect of pH on desorption was negligible at pH > 4.
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Figure 2 shows that the desorption efficiency of EDTA increased with increasing EDTA concentration. The desorption efficiency at the lowest EDTA concentration examined was 21 % while at the highest EDTA concentration of 3.24 mM almost complete desorption of the adsorbed cadmium was achieved.
100
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Figure 2. Cadmium desorption by EDTA. Desorption efficiency is expressed as percent extraction of cadmium initially loaded on the biomass.
Based on the above results, five consecutive cycles of adsorption-desorption were conducted under conditions of maximum desorption, Figures 3 and 4 show the results obtained using HCI at pH 2 and EDTA at 3.24 mM as the desorbents. respectively. The open bars depict the amount of cadmium adsorbed while the solid bars represent the amount of cadmium desorbed in each cycle. In the case of HCl. Fig. 3 clearly shows a gradual decrease in cadmium uptake with increasing cycle. After a sequence of five cycles. the cadmium uptake capacity of the biomass had been reduced to approximately 44% of the original cadmium uptake capacity of the virgin biomass. A small fraction of the cadmium initially loaded in the first cycle was not desorbed. However, in the second cycle the amount of cadmium desorbed was higher than the amount of cadmium adsorbed. The excess cadmium was most probably the residual cadmium that was not desorbed in the first cycle. This could also explain the sharp reduction in cadmium uptake in the second cycle. Cadmium taken up in the first cycle which was not desorbed resulted
Biosorption of cadmium by algal biomass
119
in a ~~crease in the number of bind!ng sites. available for cadmium uptake in the second cycle. In the remammg three cycles, the desorptIon efficIency was close to 100%, indicating that the desorption efficiency of HCl increased with increasing cycle.
Figure 3. Five cycles of cadmium adsorption-desorption with hydrochloric acid as the desorbing agent
The gradual reduction of cadmium uptake in five cycles of adsorption-desorption indicates that the binding sites on the surface of the biomass were either destroyed or morphologically altered by HC!. It is known that concentrated HCI can rupture the structure of alginate chains and destroy the hydrogen bonding capacity of the alginate by hydrolysis (Percival and McDowell, 1967). Consequently, repeated exposure of the biomass to a strong acidic environment could lead to a reduction of metal binding sites. This hypothesis is supported by the observation that there is a gradual biomass weight loss over the five cycles, as shown in Fig. S. The amount of biomass used at the beginning of each cycle with HCI is depicted as open bars in Fig. S. The initial amount of biomass employed in the first cycle was 0.1 g. This amount had been reduced to 0.07 g by the time the biomass reached the fifth cycle, resulting in a biomass loss of 30%. Since great care had been taken to minimise the loss of biomass due to handling over the course of the five cycles, the observed loss of biomass could be a result of the dissolution of alginate by HCI. Kuyucak and Volesky (1989b) reported that a biomass weight loss of 42% also occurred over three cycles of adsorption-
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K. H. CHU el al.
Results of the biomass recycling experiments with EDTA are shown in Fig. 4. EDT A exhibited desorption behaviour similar to that of He!. There is a gradual decrease in cadmium uptake with increasing cycle. However, cadmium uptake in the fifth cycle was approximately 62% of that observed in the first cycle. This is higher than the 44% obtained with He!. The desorption efficiency in all five cycles was very high; almost complete recovery of cadmium was observed in each cycle.
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The reduction in cadmium uptake was possibly due to cell wall damage as a result of repeated exposure of the biomass to EDTA solutions. As EDT A is a strong chelating agent, the configuration of the binding sites on the biomass surface may have been altered following extraction of adsorbed cadmium by EDT A to form highly stable complexes in solution. However, biomass weight loss was also observed in this case. The solid bars in Fig. 5 show that the amount of biomass had been reduced from 0.1 g in the fust cycle to 0.084 g in the fifth cycle, a reduction of 16%. This reduction is much smaller than that observed with He!. The loss of biomass may have contributed to the reduction in cadmium uptake although it is not known whether EDTA has the ability to dissolve cell wall components of the biomass such as alginate. Table 1. Total amounts of cadmium adsorbed and desorbed over five cycles of adsorption-desorption with EDTA and Hel as the desorbents Desorbent
cadmium adsorbed (mmol)
cadmium desorbed (mmol)
EDTA
0.083
0.081
Hel
0.067
0.065
The results shown in Figures 3 and 4 suggest that both Hel and EDTA were effective in stripping almost all of the adsorbed cadmium from the biomass in each cycle except for the fust cycle with Hel where a small fraction of the adsorbed cadmium remained in the biomass. It should however be noted that the residual cadmium was subsequently removed in the second cycle. Table 1 shows the sum of cadmium adsorbed and desorbed over the entire five cycles. It is clear that cadmium uptake by S. baccularia is easily reversible with no accumulation of irreversibly bound cadmium on the biomass. Unfortunately, exposure of the biomass to both desorbents resulted in a decrease in cadmium uptake with increasing cycle. Table 1 shows that the total amount of cadmium taken up by the biomass with EDTA as the desorbent is about 1.24 times higher than that adsorbed by the biomass when Hel is used as the desorbent. It can therefore be concluded that EDTA is a more benign desorbent than He!. However, selection of EDTA as a desorbent also depends on other
Biosorplion of cadmium by algal biomass
121
equally important factors such as its cost and potential problems associated with further processing of the extracted cadmium which forms highly stable complexes with EDTA. It is interesting to compare the results reported here with similar studies conducted using marine algae and HCI or EDT A. Kuyucak and Volesky (l989b) conducted three cycles of batch adsorption-desorption using H2S04 at pH 5.2 and HCI at pH 2.7 to desorb cobalt from the native biomass of the brown alga Ascophyllum nodosum. They reported that cobalt uptake and biomass weight decreased by 37 and 42%. respectively. These observations are in good agreement with the results reported here. A subsequent study by the same laboratory using chemically modified biomass of the same alga showed that desorption of cadmium with 0.1-0.5 M HCI resulted in no changes of the biomass cadmium uptake capacity through five adsorption-desorption cycles (Holan et al.• 1993). A third study indicates that another chemically modified algal biomass of Sargassum fluitans showed no decrease in cadmium uptake upon three cycles of adsorption-desorption with 0.1 M HCI (Aldor et al.. 1995). They claimed that chemical modification appears to increase the resistance of the algal biomass to HCI during the desorption step. An interesting point to note is that different desorption times were employed in these studies. For example. Kuyucak and Volesky (1989b) conducted their desorption experiments "overnight" while the duration of our desorption step per cycle was 24 hours. These relatively long desorption times could have allowed HCI to do enough damage to the native biomass. resulting in progressive deterioration of its metal uptake capacity in multiple cycles. By contrast. in the study by Holan et al. (1993) batch desorption was carried out until equilibrium was reached or up to 12 hours while Aldor et al. (1995) employed a desorption duration of 2 hours. These relatively short desorption times could have prevented HCI from inflicting severe damage on the chemically modified biomass. aIlowing it to retain much of its original cadmium uptake capacity in multiple cycles. It is thus not clear at this time whether the superior performance of the chemically modified biomass over native biomass in multiple adsorption-desorption cycles is a result of biomass reinforcement by chemical treatment or a result of short desorption time.
If the metal uptake capacity of a biomass in a batch reactor system with a given desorbent is a function of the desorption time. then the adsorption-desorption cycles. whose number is always stated in this type of studies. wiII not be a suitable performance indicator. In such cases. the desorption time per cycle should always be specified in order to give a more accurate comparison of different biomass. The effect of desorption time on the metal uptake capacity of the algal biomass in multiple cycles is currently being investigated in our laboratory and the results will be reported in a future communication. CONCLUSIONS Two desorbents. HCI and EDTA. were found to be effective in stripping adsorbed cadmium from the biomass of the marine alga S. baccularia over five consecutive cycles of adsorption-desorption. HCI at pH 2 and EDTA at 3.24 mM successfully desorbed most of the adsorbed cadmium. However, both desorbents damaged the binding sites enough to seriously impair cadmium uptake in subsequent cycles. HCI was found to have reduced cadmium uptake over the five cycles by 56%; for the five cycles with EDTA. the reduction was nearly 40%. Biomass weight loss was observed in both cases. indicating that the reduction in cadmium uptake could be a result of the dissolution of metal-binding alginate from the biomass ceIl wall by the two desorbents. It can therefore be concluded that neither HCI nor EDTA seem to be attractive as a desorbent although both possess excellent desorption efficiency. However. it may be possible to minimise the reduction in cadmium uptake in multiple cycles by optimising the desorption duration in each cycle. The influence of desorption time on the metal uptake capacity of biomass in multiple cycles is not known at this time. REFERENCES Aldor. I.• Fouresl. E. and Volesky. B. (1995). Desorplion of cadmium from algal biosorbenl. Canadian J. Chem. Eng .• 73. ~16522. MT 35:1·[
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Chu, K. H., Hashim, M. A., Phang, S. M. and Samuel, V. B. (1996). Biosorption of heavy metals by tropical marine algae. Proc. 7th Congress of Asian Pacific Confederation of Chemical Engineers, vol. 2, pp 527-531. Crist, R H .. Oberholser, K.• McGarrity, J .• Crist, D. R, Johnson, J. K. and Brittsan J. M. (1992). Interaction of metals and protons with algae. 3. Marine algae with emphasis on lead and aluminum. Environ. Sci. Techno!., 26, 496-502. Crist, D. R., Crist, R. H., Martin, J. R and Watson, J. R (1994). Ion exchange systems in proton-metal reaction with algal cell walls. FEMS Microbiol. Rev., 14, 309-314. Fourest, E., Canal, C. and Roux, J.-c. (1994). Improvement of heavy metal biosorption by mycelial dead biomasses (Rhizopus arrhizus. Mucor mlehei and Penicillium chrysogenum): pH control and cationic activation. FEMS Microbiol. Rev., 14, 325-332. Haug, A. and Smidsrod, O. (1967). Strontium. calcium and magnesium in brown algae. Nature, 215, 1167- 1168. Holan, Z. R, Volesky, B. and Prasetyo, I. (1993). Biosorption of cadmium by biomass of marine algae. Biotechnol. Bioeng., 41, 819-82'. Kuyucak, N. (1990). Feasibility of biosorbents application. In: Biosorption of Heavy Metals, B. Volesky (Ed.), CRC Press. Boca Raton, pp 371-378. Kuyucak. N. and Volesky. B. (l989a). The mechanism of cobalt biosorption. Biotechnol. Bioeng.• 33. 823- 831. Kuyucak, N. and Volesky. B. (l989b). Desorption of cobalt-laden algal biosorbent. Biotechnol. Bioeng., 33, 815-822. Maeda, S. and Sakaguchi, T. (1990). Accumulation and detoxification of toxic metal elements by algae. In: Introduction to Applied Phycology. I. Akatsuka (Ed.). SPB Academic Publishing bv. The Hague. pp. 109-136. Percival. E. and McDowell, R. H. (1967). Chemistry and Enzymology of Marine Algal Polysaccharides. Academic Press. London. Samuel. V. B. (1996). Biosorption of cadmium and copper by the biomass of Sargassum baccularia (Phaeophyta). M. Biotech. Thesis. University of Malaya, Kuala Lumpur. Malaysia. Volesky, B. (Ed.) (1990). Biosorption of Heavy Metals. CRe Press, Boca Raton. Volesky, B. (1994). Advances in biosorption of metals: selection of bIOmass types. FEMS Microbiol. Rev., 14, 291-302.