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Calcium interference with continuous biosorption of zinc by Sargassum sp. (Phaeophyceae) in tubular laboratory reactors
Bioresource Technology 83 (2002) 159–163
Calcium interference with continuous biosorption of zinc by Sargassum sp. (Phaeophyceae) in tubular laboratory reactors Francisca Pess^ oa de Francßa a, Ana Paula Mora Tavares a, Antonio CarlosAugusto da Costa b,* a
b
Universidade Federal do Rio de Janeiro, Escola de Quımica, Departamento de Engenharia Bioquımica, Ilha do Fund~ ao, Rio de Janeiro, RJ 21949-900, Brazil Universidade do Estado do Rio de Janeiro, Instituto de Quımica, Departamento de Tecnologia de Processos Bioquımicos, Rua S~ ao Francisco Xavier 524, Rio de Janeiro, RJ 20550-013, Brazil Received 26 June 2001; received in revised form 10 October 2001; accepted 14 October 2001
Abstract The zinc biosorptive capacity of the brown seaweed Sargassum sp. (Phaeophyceae) was studied in the presence or absence of competing calcium ions, using a continuous system with tubular fixed-bed reactors. In order to detect the effect of calcium on zinc biosorption, a 130 mg/l zinc solution was used, and calcium was added at 50–340 mg/l. The potential zinc biosorptive capacity of the biomass was markedly influenced by the presence of ionic calcium. Zinc sorption decreased with increasing calcium concentrations, as expressed by zinc uptake rates. Calcium was effectively recovered only during the initial stages of the process, as expressed by the decrease in its uptake rates. Calcium uptake rates were also much higher than zinc uptake rates, indicating that calcium was preferentially recovered when compared to zinc. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Biosorption; Calcium; Environment; Sargassum sp.; Seaweed; Zinc
1. Introduction Biological materials provide an attractive solution for the removal of heavy metals from aqueous streams when compared to conventional chemical treatments, mainly due to their reduced cost. Classical processing of solutions containing heavy metals includes precipitation with lime, ion-exchange, activated carbon adsorption, electrodialysis and reverse osmosis. In many cases, classical strategies are not completely efficient, as heavy metal levels are not sufficiently reduced to meet the limiting concentrations established by local legislation (Vılchez et al., 1997). Brown seaweed biosorption of metallic elements has been investigated under batch conditions using Ascophyllum nodosum (de Carvalho et al., 1995) and Sargassum fluitans (Schiewer and Volesky, 1995; Leusch et al., 1996). Brown seaweeds constitute potential biomaterials for the recovery of heavy metals, mainly due to its polysaccharide content. Carboxyl groups from alginates are the main sites of
*
Corresponding author. Fax: +55-21-2587-7206. E-mail address: [email protected] (A. Carlos Augusto da Costa).
heavy metal biosorptive sorption in Sargassum sp., even though they are not the exclusive ones. In an extensive screening test that compared the biosorptive performance of different types of seaweed (brown, red and green), Sargassum sp. proved to be the best accumulator of heavy metals, due to its high biosorption capacity, low cost and low equilibrium concentrations (da Costa and de Francßa, 1996). There is a specific interest on certain types of seaweeds, such as Sargassum, Ecklonia, Ascophylum, Gracilaria and Padina, mainly due to their availability in the ocean and also due to their potential to accumulate heavy metal elements in their structures (Leusch et al., 1995). Most brown algae are found in cold seawater, being structurally complex, with some species presenting air flotation bubbles. Their cell walls consist of polysaccharides, proteins and lipids, both rich in functional groups able to interact with heavy metal ions (Remacle, 1990; Kiefer et al., 1997). Some workers have been investigating the interference of metallic ions on heavy metal biosorption in batch systems (de Carvalho et al., 1995; Figueira et al., 1997; Schiewer and Volesky, 1997). Only a few studies describe the application of continuous systems for heavy metal biosorptive sorption
0960-8524/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 1 ) 0 0 1 9 8 - 5
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F. Pess^oa de Francßa et al. / Bioresource Technology 83 (2002) 159–163
(Volesky and Prasetyo, 1994; da Costa and de Francßa, 1997), although they did not focus on the effect of competing light metal ions in the process. For the treatment of heavy metal containing effluents, the primary treatment constitutes the addition of a high amount of calcium hydroxide, in order to precipitate heavy metals as metal hydroxides. Following this stage, the effluent will contain less heavy metals, but, additionally, a high content of calcium ions. This primary treatment is less effective for the precipitation of heavy metals, thus, additional treatment must be introduced. In view of this, the principal aim of the present work was to study zinc biosorption by the brown seaweed Sargassum sp. in the presence and absence of ionic calcium, using laboratory tubular fixed-bed bioreactors.
to obtain a representative sample to conduct the experiments. After this procedure, the sample used in the experiments was representative and homogeneous in relation to the total amount of seaweed harvested initially. A detailed description of the procedure can be found in Goes et al. (1998). 2.2. Solution of metals The synthetic solutions used were prepared by dissolving ZnSO4 7H2 O and CaSO4 2H2 O in distilled water. In order to detect the effect of calcium on zinc biosorption, a 130 mg/l ionic zinc solution was used, and calcium was added at 50.0, 136.0, 258.0 and 338 mg/l. Those zinc/calcium concentrations are characteristic of zinc-producing plants. Control solutions, containing calcium only, in the same concentrations, were used.
2. Methods 2.3. Continuous biosorption system 2.1. Biomass The biomass used was the brown seaweed Sargassum sp. (Chromophyta), collected in the Northeastern coast of Brazil. Intact Sargassum sp. was harvested live from the sea, sampled, washed with distilled water and ovendried at 60 °C for 24 h for use in the experiments. From a bulk sample harvested from the sea, 1 kg of Sargassum was subsampled for use in the experiments. To ensure that homogeneous samples were used, standard sampling techniques were applied. Initially a conical heap was made with the total amount of sample harvested from the sea, after drying. This heap was then divided in four parts, to compose later into two smaller piles. A series of additional cuts was again performed, in order
Experiments were conducted in a continuous system containing three acrylic columns, arranged in series, working as fixed-bed reactors (Fig. 1). Each column was 25.0 cm high and 3.5 cm internal diameter. The system was fed with synthetic solutions, with the help of a peristaltic pump (Milan, Model 202). Solutions were pumped upwards from the bottom of the first column, being continuously transported to the additional reactors. Each of the columns was filled with 15.0 g of dried Sargassum sp., only. Solutions were pumped at 25.0 ml/min and samples collected from the top of each column every 30 minutes for analysis, through take-offs located at the left top of each reactor. pH values were monitored in the collected
Fig. 1. Diagram of the three fixed-bed bed tubular reactors.
F. Pess^oa de Francßa et al. / Bioresource Technology 83 (2002) 159–163
161
The equipment used was a Plasma Spectrometry Emission, ICP-AES (Perkin–Elmer Model 2000) for the
determination of all standard and process solutions. The emission line used to determine zinc concentrations was equal to 213.856 nm, and, for calcium determination, the emission line was 393.366 nm. The detection limit for both elements was equal to 0.1 mg/l. The frequency used was equal to 400 V (for Zn) and 600 V (for Ca). Results reported correspond to quadruplicate readings of each sample. Each sample collected from the columns (10 ml), was filtered through Millipore membranes (0.47 lm pore size), acidified with concentrated HCl solution, and submitted to ICPAES for analysis, on the same day they were collected from the reactors. ICP-AES results gave residual zinc and calcium concentrations. As initial calcium and zinc
Fig. 2. The effect of calcium on zinc uptake rates by Sargassum sp. The results correspond to zinc uptake rates for the first, second and third columns: () no calcium; (N) 50 mg/l; ( ) 136 mg/l; ( ) 258 mg/l, (r) 338 mg/l. Error bars correspond to standard mean deviations.
Fig. 3. Calcium uptake rates by Sargassum sp. during zinc biosorption. The results correspond to calcium uptake rates for the first, second and third columns: () 50 mg/l; ( ) 136 mg/l; ( ) 258 mg/l; (r) 338 mg/l. Error bars correspond to standard mean deviations.
samples (Digimed, DPMH-1). All experiments were conducted in quadruplicate. Paired t-test were conducted on all sets of experiments, considering P < 0:05 as significantly different. Confidence intervals were determined for all data, and significantly different means between paired data are reported here, along with the respective standard deviations (da Costa and de Francßa, 1998).
2.4. Analysis of metals
162
F. Pess^oa de Francßa et al. / Bioresource Technology 83 (2002) 159–163
concentrations were known it was possible to determine the amount of metals biosorbed.
3. Results The biosorptive removal of zinc from the solutions by Sargassum sp., both in the absence and presence of ionic calcium is shown in Fig. 2. Rates indicate, for outlet solutions from column 1, decreasing zinc uptake values against volume. For column 2 decreasing uptake rates were observed for two solutions containing the highest levels of calcium. For the remaining solutions differences were negligible. On the other hand, increasing zinc uptake rates were detected up to a limit, followed by an abrupt decrease, in the case of column 3. For the remaining solutions increasing rates were observed. Maximum zinc uptake rates were 3.5, 0.9 and 0.18 mM/l h, obtained from columns 1, 2 and 3, respectively. The results on calcium biosorption indicated that this ion was initially sorbed by the seaweed biomass but was subsequently released into solution (Fig. 3). Considering calcium uptake rates, there was an abrupt decrease in the curves in the initial steps of the process, followed by a smooth curve. This behavior was observed from column 1. From column 2 this decrease was not markedly observed, being more pronounced for the experiment with the solution containing the highest calcium concentration and for the experiment without calcium addition. For the remaining curves equivalent results were observed. Column 3 results indicated that only for the solutions containing 338 mg/l calcium did uptake rates decrease gradually with an increasing volume. For the rest of the solutions, differences in calcium uptake rates were less pronounced, ranging from 0 to 0.5 mM/l h. The values of pH did not change abruptly over time. pH ranges for columns 1, 2 and 3 were 5.8–6.2, 6.3–7.5, and 6.2–7.0, respectively, indicating that no marked precipitation could have occurred.
4. Discussion Measurements of outlet zinc concentrations indicated, overall, a marked decrease in the efficiency of zinc biosorption by Sargassum sp. This was observed when zinc and calcium were simultaneously present in the test solution. A lower efficiency of heavy metal sorption was observed in the first column, which can be related to the higher zinc concentration of the solution fed to the system at this point. The inlet metal solutions for columns 2 and 3 had a lower concentration since part of the metal had been already sorbed by the biomass in the previous column reactor. Based on the data in Fig. 2, it is apparent that, from column 1, the brown seaweed’s absorptive capacity for zinc was strongly influenced by the presence of calcium
(Fig. 2). In general, decreasing zinc uptake rates were observed, with significantly different results for the solution with a higher calcium concentration. These results suggest that binding sites in the structure of Sargassum sp. are probably saturated with calcium. A high concentration of zinc in the inlet solutions, led to identical zinc uptake rates, irrespective of the calcium concentration added. Maximum zinc uptake rate observed from column 1 was 3.5 mM/l h. Results from columns 2 and 3 (Fig. 2), did not strictly match the biosorption pattern observed in column 1, to some extent because part of the zinc had been removed by the first bioreactor. Results from column 2 showed a reduced zinc removal efficiency, especially for the solutions containing 338.0 and 258.0 mg/l calcium. For the additional solutions changes in zinc uptake rates were not so significant. For this second column, maximum zinc uptake rate achieved was 0.9 mM/l h. Nevertheless, a distinct behavior was observed in column 3, as much more dilute solutions were fed to this reactor – in this case, diffusion and mass transfer problems may have interfered with the continuous system. Here, for the solution containing calcium at 338.0 mg/l, zinc uptake rates increased up to a limit, then decreased rapidly. When the solution contained calcium at 258.0 mg/l increasing uptake rates of zinc were observed; however, a decreasing tendency can be observed at the final stages of the curve. This behavior seemed to repeat for the other curves, nevertheless with a different value of the maximum zinc uptake rate. These results reflect the fact that the third column presents a different kinetic behavior, due to the fact that a pre-treated solution was fed to this. Maximum zinc uptake rate, in this column, was equal to 0.18 mM/l h. Based on the results in Fig. 2 it is apparent that calcium negatively influenced the sorption of zinc by Sargassum sp. This was probably due to the competition of both elements for the same binding sites in the biomass. This was also confirmed by the higher calcium uptake rates obtained, in comparison to zinc uptake rates (Fig. 3). From columns 1, 2 and 3, maximum calcium uptake rates were 9.0, 4.5 and 0.5 mM/l h. Individually, results from column 1 (Fig. 3) indicated that calcium uptake rates markedly decreased at the initial stages of the process (up to 2 l inlet volume); afterwards this decrease in the uptake rate was not so marked. From column 2 (Fig. 3), it can be observed that calcium uptake rates decreased, less abruptly than in column 1, specially for the solution containing 338.0 mg/l calcium and the solution containing zinc only. This could be due to the effect of pre-treatment performed by the first column. For the remaining solutions, results were similar. For column 3 (Fig. 3) only for the solution containing calcium at a high concentration (338.0 mg/l), rates decreased smoothly. For the remaining solutions decreases were not substantial, with a maximum calcium uptake value of 0.5 mM/l h.
F. Pess^oa de Francßa et al. / Bioresource Technology 83 (2002) 159–163
Schiewer and Volesky (1997), examining the influence of calcium on cadmium biosorption by Sargassum sp., also found that the sorption of this heavy metal decreased as a function of increasing calcium concentration in solution. This reduction in the uptake of zinc was attributed to a high electrostatic accumulation where a high concentration of charged ions (due to the presence of calcium) competed for the same binding sites on the surface of the seaweed. Based on our data, we can infer that the biomass of Sargassum sp. did not reach saturation under the experimental conditions established. This was an indication that the seaweed still contained free binding sites for metal sorption in its structure. As for calcium biosorption, this light metal was removed from solution only during the initial steps of the process. Furthermore, it was found to be desorbed into solution, probably associated with additional calcium from constituent polysaccharides of the biomass. Such a release can be accounted for by the ionexchange properties of calcium alginates. The sorption of metals by both biological and nonbiological materials can be markedly influenced by pH. The pH values recorded during our investigation indicated that metal precipitation did not occur, because for all the combinations of zinc and calcium tested, pH values were smaller than 8.0 (Yong and Cloutier, 1993). Acknowledgements We wish to thank Conselho Nacional de Desenvolvimento Cientıfico e Tecnol ogico (CNPq) and Fundacß~ ao de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ) for financial support. References da Costa, A.C.A., de Francßa, F.P., 1996. Cadmium sorption by biosorbent seaweeds: adsorption isotherms and some process conditions. Sep. Sci. Technol. 31, 2373–2393.
163
da Costa, A.C.A., de Francßa, F.P., 1997. Biosorption of zinc, cadmium, and copper by a brown seaweed (Sargassum sp.) in a continuous fixed-bed laboratory reactor. Bioseparation 6, 335–341. da Costa, A.C.A., de Francßa, F.P., 1998. The behavior of the microalgae Tetraselmis chuii in cadmium-contaminated solutions. Aquaculture Int. 6, 57–66. Figueira, M.M., Volesky, B., Ciminelli, V., 1997. Assessment of interference in biosorption of a heavy metal. Biotechnol. Bioeng. 54, 344–350. de Carvalho, R.P., Chong, K.H., Volesky, B., 1995. Evaluation of the Cd, Cu, and Zn biosorption in two-metal systems using an algal biosorbent. Biotechnol. Prog. 11, 39–44. Kiefer, E., Sigg, L., Schosseler, P., 1997. Chemical and spectroscopic characterization of algae surfaces. Environ. Sci. Technol. 31, 759–764. Leusch, A., Holan, Z.R., Volesky, B., 1995. Biosorption of heavy metals (Cd, Cu, Ni, Pb, Zn) by chemically-reinforced biomass of marine algae. J. Chem. Technol. Biotechnol. 62, 279–288. Leusch, A., Holan, Z.R., Volesky, B., 1996. Solution and particle effects on the biosorption of heavy metals by seaweed biomass. Appl. Biochem. Biotechnol. 61, 231–248. Goes, M.A.C., Luz, A.B., Possa, M.V., 1998. Amostragem. In: Luz, A.B., Possa, M.V., Almeida, S.L. (Eds.), Tratamento de Minerios. CNPq/MCT, Rio de Janeiro, pp. 18–58. Remacle, J., 1990. The cell wall and metal binding. In: Volesky, B. (Ed.), Biosorption of Heavy Metals. CRC Press, Boca Raton, FL, pp. 83–91. Schiewer, S., Volesky, B., 1995. Modeling of the proton-metal ion exchange in biosorption. Environ. Sci. Technol. 29, 3049–3058. Schiewer, S., Volesky, B., 1997. Ionic strength and eletrostatic effects in biosorption of divalent metals ions and protons. Environ. Sci. Technol. 31, 2478–2485. Vılchez, C., Garbayo, I., Lobato, M.V., Vega, J.M., 1997. Microalgaemediated chemicals production and wastes removal. Enzyme Microbiol. Technol. 20, 562–572. Volesky, B., Prasetyo, I., 1994. Cadmium removal in a biosorption column. Biotechnol. Bioeng. 43, 1010–1015. Yong, R.N., Cloutier, R.G., 1993. Selective sequential extraction analysis of heavy-metal retention in soil. Can. Geotech. J. 30, 834–847.
Calcium interference with continuous biosorption of zinc by Sargassum sp. (Phaeophyceae) in tubular laboratory reactors Francisca Pess^ oa de Francßa a, Ana Paula Mora Tavares a, Antonio CarlosAugusto da Costa b,* a
b
Universidade Federal do Rio de Janeiro, Escola de Quımica, Departamento de Engenharia Bioquımica, Ilha do Fund~ ao, Rio de Janeiro, RJ 21949-900, Brazil Universidade do Estado do Rio de Janeiro, Instituto de Quımica, Departamento de Tecnologia de Processos Bioquımicos, Rua S~ ao Francisco Xavier 524, Rio de Janeiro, RJ 20550-013, Brazil Received 26 June 2001; received in revised form 10 October 2001; accepted 14 October 2001
Abstract The zinc biosorptive capacity of the brown seaweed Sargassum sp. (Phaeophyceae) was studied in the presence or absence of competing calcium ions, using a continuous system with tubular fixed-bed reactors. In order to detect the effect of calcium on zinc biosorption, a 130 mg/l zinc solution was used, and calcium was added at 50–340 mg/l. The potential zinc biosorptive capacity of the biomass was markedly influenced by the presence of ionic calcium. Zinc sorption decreased with increasing calcium concentrations, as expressed by zinc uptake rates. Calcium was effectively recovered only during the initial stages of the process, as expressed by the decrease in its uptake rates. Calcium uptake rates were also much higher than zinc uptake rates, indicating that calcium was preferentially recovered when compared to zinc. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Biosorption; Calcium; Environment; Sargassum sp.; Seaweed; Zinc
1. Introduction Biological materials provide an attractive solution for the removal of heavy metals from aqueous streams when compared to conventional chemical treatments, mainly due to their reduced cost. Classical processing of solutions containing heavy metals includes precipitation with lime, ion-exchange, activated carbon adsorption, electrodialysis and reverse osmosis. In many cases, classical strategies are not completely efficient, as heavy metal levels are not sufficiently reduced to meet the limiting concentrations established by local legislation (Vılchez et al., 1997). Brown seaweed biosorption of metallic elements has been investigated under batch conditions using Ascophyllum nodosum (de Carvalho et al., 1995) and Sargassum fluitans (Schiewer and Volesky, 1995; Leusch et al., 1996). Brown seaweeds constitute potential biomaterials for the recovery of heavy metals, mainly due to its polysaccharide content. Carboxyl groups from alginates are the main sites of
*
Corresponding author. Fax: +55-21-2587-7206. E-mail address: [email protected] (A. Carlos Augusto da Costa).
heavy metal biosorptive sorption in Sargassum sp., even though they are not the exclusive ones. In an extensive screening test that compared the biosorptive performance of different types of seaweed (brown, red and green), Sargassum sp. proved to be the best accumulator of heavy metals, due to its high biosorption capacity, low cost and low equilibrium concentrations (da Costa and de Francßa, 1996). There is a specific interest on certain types of seaweeds, such as Sargassum, Ecklonia, Ascophylum, Gracilaria and Padina, mainly due to their availability in the ocean and also due to their potential to accumulate heavy metal elements in their structures (Leusch et al., 1995). Most brown algae are found in cold seawater, being structurally complex, with some species presenting air flotation bubbles. Their cell walls consist of polysaccharides, proteins and lipids, both rich in functional groups able to interact with heavy metal ions (Remacle, 1990; Kiefer et al., 1997). Some workers have been investigating the interference of metallic ions on heavy metal biosorption in batch systems (de Carvalho et al., 1995; Figueira et al., 1997; Schiewer and Volesky, 1997). Only a few studies describe the application of continuous systems for heavy metal biosorptive sorption
0960-8524/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 1 ) 0 0 1 9 8 - 5
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F. Pess^oa de Francßa et al. / Bioresource Technology 83 (2002) 159–163
(Volesky and Prasetyo, 1994; da Costa and de Francßa, 1997), although they did not focus on the effect of competing light metal ions in the process. For the treatment of heavy metal containing effluents, the primary treatment constitutes the addition of a high amount of calcium hydroxide, in order to precipitate heavy metals as metal hydroxides. Following this stage, the effluent will contain less heavy metals, but, additionally, a high content of calcium ions. This primary treatment is less effective for the precipitation of heavy metals, thus, additional treatment must be introduced. In view of this, the principal aim of the present work was to study zinc biosorption by the brown seaweed Sargassum sp. in the presence and absence of ionic calcium, using laboratory tubular fixed-bed bioreactors.
to obtain a representative sample to conduct the experiments. After this procedure, the sample used in the experiments was representative and homogeneous in relation to the total amount of seaweed harvested initially. A detailed description of the procedure can be found in Goes et al. (1998). 2.2. Solution of metals The synthetic solutions used were prepared by dissolving ZnSO4 7H2 O and CaSO4 2H2 O in distilled water. In order to detect the effect of calcium on zinc biosorption, a 130 mg/l ionic zinc solution was used, and calcium was added at 50.0, 136.0, 258.0 and 338 mg/l. Those zinc/calcium concentrations are characteristic of zinc-producing plants. Control solutions, containing calcium only, in the same concentrations, were used.
2. Methods 2.3. Continuous biosorption system 2.1. Biomass The biomass used was the brown seaweed Sargassum sp. (Chromophyta), collected in the Northeastern coast of Brazil. Intact Sargassum sp. was harvested live from the sea, sampled, washed with distilled water and ovendried at 60 °C for 24 h for use in the experiments. From a bulk sample harvested from the sea, 1 kg of Sargassum was subsampled for use in the experiments. To ensure that homogeneous samples were used, standard sampling techniques were applied. Initially a conical heap was made with the total amount of sample harvested from the sea, after drying. This heap was then divided in four parts, to compose later into two smaller piles. A series of additional cuts was again performed, in order
Experiments were conducted in a continuous system containing three acrylic columns, arranged in series, working as fixed-bed reactors (Fig. 1). Each column was 25.0 cm high and 3.5 cm internal diameter. The system was fed with synthetic solutions, with the help of a peristaltic pump (Milan, Model 202). Solutions were pumped upwards from the bottom of the first column, being continuously transported to the additional reactors. Each of the columns was filled with 15.0 g of dried Sargassum sp., only. Solutions were pumped at 25.0 ml/min and samples collected from the top of each column every 30 minutes for analysis, through take-offs located at the left top of each reactor. pH values were monitored in the collected
Fig. 1. Diagram of the three fixed-bed bed tubular reactors.
F. Pess^oa de Francßa et al. / Bioresource Technology 83 (2002) 159–163
161
The equipment used was a Plasma Spectrometry Emission, ICP-AES (Perkin–Elmer Model 2000) for the
determination of all standard and process solutions. The emission line used to determine zinc concentrations was equal to 213.856 nm, and, for calcium determination, the emission line was 393.366 nm. The detection limit for both elements was equal to 0.1 mg/l. The frequency used was equal to 400 V (for Zn) and 600 V (for Ca). Results reported correspond to quadruplicate readings of each sample. Each sample collected from the columns (10 ml), was filtered through Millipore membranes (0.47 lm pore size), acidified with concentrated HCl solution, and submitted to ICPAES for analysis, on the same day they were collected from the reactors. ICP-AES results gave residual zinc and calcium concentrations. As initial calcium and zinc
Fig. 2. The effect of calcium on zinc uptake rates by Sargassum sp. The results correspond to zinc uptake rates for the first, second and third columns: () no calcium; (N) 50 mg/l; ( ) 136 mg/l; ( ) 258 mg/l, (r) 338 mg/l. Error bars correspond to standard mean deviations.
Fig. 3. Calcium uptake rates by Sargassum sp. during zinc biosorption. The results correspond to calcium uptake rates for the first, second and third columns: () 50 mg/l; ( ) 136 mg/l; ( ) 258 mg/l; (r) 338 mg/l. Error bars correspond to standard mean deviations.
samples (Digimed, DPMH-1). All experiments were conducted in quadruplicate. Paired t-test were conducted on all sets of experiments, considering P < 0:05 as significantly different. Confidence intervals were determined for all data, and significantly different means between paired data are reported here, along with the respective standard deviations (da Costa and de Francßa, 1998).
2.4. Analysis of metals
162
F. Pess^oa de Francßa et al. / Bioresource Technology 83 (2002) 159–163
concentrations were known it was possible to determine the amount of metals biosorbed.
3. Results The biosorptive removal of zinc from the solutions by Sargassum sp., both in the absence and presence of ionic calcium is shown in Fig. 2. Rates indicate, for outlet solutions from column 1, decreasing zinc uptake values against volume. For column 2 decreasing uptake rates were observed for two solutions containing the highest levels of calcium. For the remaining solutions differences were negligible. On the other hand, increasing zinc uptake rates were detected up to a limit, followed by an abrupt decrease, in the case of column 3. For the remaining solutions increasing rates were observed. Maximum zinc uptake rates were 3.5, 0.9 and 0.18 mM/l h, obtained from columns 1, 2 and 3, respectively. The results on calcium biosorption indicated that this ion was initially sorbed by the seaweed biomass but was subsequently released into solution (Fig. 3). Considering calcium uptake rates, there was an abrupt decrease in the curves in the initial steps of the process, followed by a smooth curve. This behavior was observed from column 1. From column 2 this decrease was not markedly observed, being more pronounced for the experiment with the solution containing the highest calcium concentration and for the experiment without calcium addition. For the remaining curves equivalent results were observed. Column 3 results indicated that only for the solutions containing 338 mg/l calcium did uptake rates decrease gradually with an increasing volume. For the rest of the solutions, differences in calcium uptake rates were less pronounced, ranging from 0 to 0.5 mM/l h. The values of pH did not change abruptly over time. pH ranges for columns 1, 2 and 3 were 5.8–6.2, 6.3–7.5, and 6.2–7.0, respectively, indicating that no marked precipitation could have occurred.
4. Discussion Measurements of outlet zinc concentrations indicated, overall, a marked decrease in the efficiency of zinc biosorption by Sargassum sp. This was observed when zinc and calcium were simultaneously present in the test solution. A lower efficiency of heavy metal sorption was observed in the first column, which can be related to the higher zinc concentration of the solution fed to the system at this point. The inlet metal solutions for columns 2 and 3 had a lower concentration since part of the metal had been already sorbed by the biomass in the previous column reactor. Based on the data in Fig. 2, it is apparent that, from column 1, the brown seaweed’s absorptive capacity for zinc was strongly influenced by the presence of calcium
(Fig. 2). In general, decreasing zinc uptake rates were observed, with significantly different results for the solution with a higher calcium concentration. These results suggest that binding sites in the structure of Sargassum sp. are probably saturated with calcium. A high concentration of zinc in the inlet solutions, led to identical zinc uptake rates, irrespective of the calcium concentration added. Maximum zinc uptake rate observed from column 1 was 3.5 mM/l h. Results from columns 2 and 3 (Fig. 2), did not strictly match the biosorption pattern observed in column 1, to some extent because part of the zinc had been removed by the first bioreactor. Results from column 2 showed a reduced zinc removal efficiency, especially for the solutions containing 338.0 and 258.0 mg/l calcium. For the additional solutions changes in zinc uptake rates were not so significant. For this second column, maximum zinc uptake rate achieved was 0.9 mM/l h. Nevertheless, a distinct behavior was observed in column 3, as much more dilute solutions were fed to this reactor – in this case, diffusion and mass transfer problems may have interfered with the continuous system. Here, for the solution containing calcium at 338.0 mg/l, zinc uptake rates increased up to a limit, then decreased rapidly. When the solution contained calcium at 258.0 mg/l increasing uptake rates of zinc were observed; however, a decreasing tendency can be observed at the final stages of the curve. This behavior seemed to repeat for the other curves, nevertheless with a different value of the maximum zinc uptake rate. These results reflect the fact that the third column presents a different kinetic behavior, due to the fact that a pre-treated solution was fed to this. Maximum zinc uptake rate, in this column, was equal to 0.18 mM/l h. Based on the results in Fig. 2 it is apparent that calcium negatively influenced the sorption of zinc by Sargassum sp. This was probably due to the competition of both elements for the same binding sites in the biomass. This was also confirmed by the higher calcium uptake rates obtained, in comparison to zinc uptake rates (Fig. 3). From columns 1, 2 and 3, maximum calcium uptake rates were 9.0, 4.5 and 0.5 mM/l h. Individually, results from column 1 (Fig. 3) indicated that calcium uptake rates markedly decreased at the initial stages of the process (up to 2 l inlet volume); afterwards this decrease in the uptake rate was not so marked. From column 2 (Fig. 3), it can be observed that calcium uptake rates decreased, less abruptly than in column 1, specially for the solution containing 338.0 mg/l calcium and the solution containing zinc only. This could be due to the effect of pre-treatment performed by the first column. For the remaining solutions, results were similar. For column 3 (Fig. 3) only for the solution containing calcium at a high concentration (338.0 mg/l), rates decreased smoothly. For the remaining solutions decreases were not substantial, with a maximum calcium uptake value of 0.5 mM/l h.
F. Pess^oa de Francßa et al. / Bioresource Technology 83 (2002) 159–163
Schiewer and Volesky (1997), examining the influence of calcium on cadmium biosorption by Sargassum sp., also found that the sorption of this heavy metal decreased as a function of increasing calcium concentration in solution. This reduction in the uptake of zinc was attributed to a high electrostatic accumulation where a high concentration of charged ions (due to the presence of calcium) competed for the same binding sites on the surface of the seaweed. Based on our data, we can infer that the biomass of Sargassum sp. did not reach saturation under the experimental conditions established. This was an indication that the seaweed still contained free binding sites for metal sorption in its structure. As for calcium biosorption, this light metal was removed from solution only during the initial steps of the process. Furthermore, it was found to be desorbed into solution, probably associated with additional calcium from constituent polysaccharides of the biomass. Such a release can be accounted for by the ionexchange properties of calcium alginates. The sorption of metals by both biological and nonbiological materials can be markedly influenced by pH. The pH values recorded during our investigation indicated that metal precipitation did not occur, because for all the combinations of zinc and calcium tested, pH values were smaller than 8.0 (Yong and Cloutier, 1993). Acknowledgements We wish to thank Conselho Nacional de Desenvolvimento Cientıfico e Tecnol ogico (CNPq) and Fundacß~ ao de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ) for financial support. References da Costa, A.C.A., de Francßa, F.P., 1996. Cadmium sorption by biosorbent seaweeds: adsorption isotherms and some process conditions. Sep. Sci. Technol. 31, 2373–2393.
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