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Phosphate biosorption characteristics of a submerged macrophyte Hydrilla verticillata
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Aquatic Botany 89 (2008) 23–26 www.elsevier.com/locate/aquabot
Phosphate biosorption characteristics of a submerged macrophyte Hydrilla verticillata Shengrui Wang a, Xiangcan Jin a,*, Haichao Zhao b, Fengchang Wu c a
Research Center of Lake Environment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China b College of Agronomy, Inner Mongolia Agriculture University, Huhhot 010018, China c State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China Received 22 January 2007; received in revised form 8 January 2008; accepted 18 January 2008 Available online 2 February 2008
Abstract Phosphate biosorption of Hydrilla verticillata was investigated and compared with its bioaccumulation characteristics. Results obtained from the biosorption isotherms and kinetics showed that maximal phosphate biosorption was 286 mg kg1, approximately equal to 6–9% of the phosphate bioaccumulation. The biosorption mainly occurred within 5 h, and was highest during the first 30 min. The initial phosphate concentration was an important factor affecting the biosorption process. Phosphate biosorption on H. verticillata was not the main phosphate removal mechanism in our experiments, but it cannot be ignored. # 2008 Elsevier B.V. All rights reserved. Keywords: Bioaccumulation; Phosphate biosorption; Water quality improvement; Aquatic plant
1. Introduction Submerged macrophytes play an important role in nutrient cycling, especially in shallow lakes (Chambers and Kalff, 1985; Qiu et al., 2001). It has been reported that water quality can be significantly improved by reintroducing submerged macrophytes (Bole and Allan, 1978; Phillips et al., 1978; Blindow et al., 1993, 2002). The mechanisms through which submerged macrophytes can act as water quality regulators have attracted much attention recently (Laura et al., 2004). It has been reported that areas with dense submerged macrophytes usually have clear water and low concentrations of nutrients and phytoplankton (Denny, 1972; Bole and Allan, 1978; Scheffer et al., 1994; Blindow et al., 2002). Several macrophyte species, e.g., Hydrilla verticillata, Myriophyllum spicatum, Phragmites australis, Eichhornia crassipes and Zostera marina were reported to have a high biological purification capability (Gersberg et al., 1984, 1986; Mann and Bavor, 1993; Romero et al., 1999; Xie et al., 2004), and to reduce the extent of sediment resuspension (James and Barko, 1990; Vermaat et al., 2000; Horppila and Nurminen, 2001).
* Corresponding author. Tel.: +86 10 84915185; fax: +86 10 84915190. E-mail address: [email protected] (X. Jin). 0304-3770/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2008.01.006
Heavy metals can be removed from the environment by biosorption on to biological materials such as algae, bacteria and aquatic plants (Patrick and Peter, 1970; Everard and Denny, 1985; Dushenkov et al., 1995; Madrid et al., 1998; Dos Santos and Lenzi, 2000; Wang and Qin, 2004). Also for some freshwater macrophytes, e.g., Potamogeton lucens, Eichhornia crassipes, Myriophyllum brasillensis, Myriophyllum spicatum, Cabomba sp. and Ceratophyllum demersum with a capacity to remove heavy metals has been demonstrated (Wang et al., 1996; Keskinkan et al., 2003). Various mechanisms have been identified for biosorption including extracellular accumulation/precipitation, cell surface biosorption/precipitation, and intracellular accumulation. The processes include metal complexation, chelation, ion exchange, adsorption and micro-precipitation (Wang et al., 1996; Veglio and Beolchini, 1997). Bioremoval by aquatic plants may involve two processes: one is the initial and reversible metal binding or biosorption, and the other is the slower and irreversible sequestration, bioaccumulation (Keskinkan et al., 2003; Ce´sar and Marco, 2004). Amines, carboxyl and thiol functional groups may be the potential binding sites. To the best of our knowledge, little studies have been carried out on the phosphate biosorption characteristics of submerged macrophytes H. verticillata, a rooted submerged species, is
24
S. Wang et al. / Aquatic Botany 89 (2008) 23–26
widely distributed in Chinese waters, it may develop a dense mat, or canopy on the water surface (Haller and Sutton, 1975), has strong adaptability, grows fast and reproduces in many ways (Langeland et al., 1992; Shearer et al., 2007). Therefore, H. verticillata is often used in lake ecological restoration in China (Wheeler and Center, 2001). The major objective of this study was to investigate the biosorption characteristics of H. verticillata on phosphate. 2. Materials and methods 2.1. Materials Plants were collected from Juma River, Beijing (398420 N, 1158340 E, 2161 m above sea levels), and placed for pretreatment in a 20 L glass tank in greenhouse at 25 2 8C with the corresponding river water for about 1 week. Apical shoots of 10 cm length were separated from the mother plant and washed with dilute HCl (0.82 mol L1) and distilled water before experimentation. Analytical grade anhydrous KH2PO4 was used as the phosphate source, and was prepared in deionized water. 2.2. Biosorption isotherms and kinetics Clusters of H. verticillata material (10.0 g wet weight, 5 shoots) were added in a series of 10 L acid washed glass tanks with 8 L of phosphate solution (anhydrous KH2PO4). The initial phosphate concentrations were 0.005, 0.02, 0.22, 0.58, 1.41 and 3.14 P mg L1, respectively. Biosorption isotherm experiments were performed at 25 2 8C in a temperaturecontrolled cabin in dark condition, with contact time of 13 h. The biomass was then removed and dried at 80 8C to constant weight to determine final dry mass and phosphate concentrations. After digestion of the dried plant material with nitric acid/ perchloric acid (5:1), total phosphorus concentrations were determined colorimetrically (Golterman et al., 1978). The solution was filtered through 0.45 mm GF/C filter membranes, and the equilibrium phosphate concentrations in the supernatant of the extraction were analyzed using the molybdenum blue method (Jin et al., 2005). The quantity of sorbed phosphate was calculated as the differences in the phosphate concentration in the solutions before and after the biosorption experiments (Wang et al., 2007). Biosorption kinetic experiments were performed similarly to the biosorption isotherm experiments. The incubation time was 13 h. At different time intervals, the solution samples were taken, filtered through 0.45 mm GF/C filter membranes and analyzed for phosphate concentrations using the molybdenum blue method (Jin et al., 2005). The quantity of the sorbed phosphate was calculated. All experiments were carried out in triplicate, and the experimental error was controlled below 6%. The results were reported as their average. During the experiments, pH in the solutions varied between 6 and 7, and no pH adjustment was made. Control experiments (with no biomass) were simultaneously carried out to ensure that biosorption was by H. verticillata biomass and not by the container.
3. Results and discussion 3.1. Biosorption isotherms A biosorption equilibrium was reached within 5 h in the studied phosphate concentration range, and the time to achieve equilibrium was related to the initial phosphate concentration (Fig. 1). After these initial experiments, further biosorption experiments were carried out. The Langmuir model is used widely in sorption studies (Ce´sar and Marco, 2004), so we used it to describe the phosphate biosorption isotherms on H. verticillata as follows: Langmuir model : Q ¼ Qmax
KC ð1 þ KCÞ
(1)
where Q is the phosphate biosorption amount (mg kg1), Qmax is the maximal phosphate biosorption amount (mg kg1), C is the biosorption equilibrium concentration (mg L1), and K is the biosorption coefficient. Linear regression was applied and model parameters were estimated. The Langmuir model described the phosphate biosorption properties of H. verticillata quite well (Qmax = 286 mg kg1, K = 17.5, r2 = 0.99, P < 0.01, n = 6). Qmax of phosphate on H. verticillata was 286 mg kg1. This result is similar to the biosorption isotherms of rice bran on Cu2+ (Wang and Qin, 2004). 3.2. Biosorption kinetics The biosorption process for porous solids is characterized by the initial amount of solute biosorption (Keskinkan et al., 2003). This can be calculated from the initial slope of the Ct/C0 vs. time curves (Ct/C0 is the ratio of the phosphate concentration at different sampling time intervals to the initial
Fig. 1. Initial phosphate biosorption experiments for Hydrilla verticillata at different initial phosphate concentrations.
S. Wang et al. / Aquatic Botany 89 (2008) 23–26
25
Table 2 The estimated n and R2 of the Q = a (C0)n Initial concentration (P mg L1)
n
R2
P
0.005 0.020 0.220 0.580 1.410 3.140
0.116 0.002 0.230 0.043 0.287 0.015 0.388 0.053 0.392 0.018 0.768 0.061
0.83 0.91 0.95 0.91 0.96 0.64
0.0030 0.0010 0.0020 0.0056 0.0004 0.0840
Table 3 Biosorption kinetics constants for phosphate on Hydrilla verticillata Concentrations (P mg L1)
0.005 0.02 0.22 0.58 1.41 3.14
Fig. 2. The variation of Ct/C0 with time.
phosphate concentration, Fig. 2). These slopes can be obtained by assuming that the relationship was linear over the experiment time (Ho et al., 1996). The estimated slopes and R2 are shown in Table 1 for different initial concentrations. Linear relationships were significant at initial concentrations from 0.005 to 1.41 mg L1 (P < 0.01, n = 8). This gave initial amounts of 0.0211, 0.0212, 0.0045, 0.0037, and 0.0022 per hour, respectively, for phosphate at different initial concentrations, and the initially sorbed phosphate decreased with increasing initial concentrations. However, the relationship was not significantly linear over the experimental period at the initial concentrations of 3.14 mg L1. This indicates that mass transfer was the main process only in the lower initial concentration range, but not a main process at higher concentrations. According to the theoretical equations for diffusion, when intraparticle diffusion was the only rate determining step, the biosorption amount is related to the square root of the initial concentration (Ho et al., 1996). This can be expressed as follows: Q ¼ aðC0 Þ
n
(2)
where Q is the phosphate biosorption amount (mg kg1), C0 is the initial phosphate concentration (mg L1), a and n are the constants. The estimated n and R2 are shown in Table 2 for different initial concentrations. The relationship was both significant and positive at the initial concentrations from 0.005 to 1.41 mg L1 (P < 0.01, n = 8). This indicates that intraparticle Table 1 Linear regression of phosphorus concentration at different sampling time intervals as a function of the initial concentration Initial concentration (P mg L1)
Slope
R2
P
0.005 0.020 0.22 0.58 1.41 3.14
0.021 0.0023 0.021 0.0035 0.005 0.0034 0.004 0.0012 0.002 0.0001 0.003 0.0001
0.96 0.98 0.84 0.88 0.78 0.42
0.0042 0.0021 0.0053 0.0064 0.0002 0.3400
Power Function model (Q = at^b) a
b
R2
p
14.87 1.873 32.88 5.256 69.00 6.781 85.03 7.389 156.36 12.543 285.07 15.437
0.12 0.034 0.23 0.056 0.29 0.075 0.39 0.031 0.39 0.053 0.77 0.058
0.79 0.91 0.95 0.91 0.95 0.73
0.0020 0.0001 0.0001 0.0003 0.0003 0.0068
diffusion was the operative mechanism and the amount determining step for phosphate biosorption on H. verticillata at lower initial concentrations. The relationship was not significantly positive at the higher initial concentrations of 3.14 mg L1. This suggests that intraparticle diffusion was not the main process for the phosphate biosorption on H. verticillata at higher initial concentrations. Biosorption kinetics data have been fitted to the Power Function model (Wang et al., 1996; Jin et al., 2005). It seems that the Power Function appears to describe the kinetics satisfactorily (Table 3). The average biosorption rates within 0– 0.5 h were the highest within 13 h for different initial phosphate concentration, ranging from 30 to 721 mg (kg h)1. As the initial concentration increased, the biosorption rate increased. 3.3. Comparison between phosphate biosorption and bioaccumulation Phosphorus concentrations of submerged macrophytes ranges from 2.6 to 5.6 g kg1 (Qiao et al., 1996; Ross et al., 1999; Wheeler and Center, 2001; Chen et al., 2002; Schulz et al., 2003). In the present study, the Qmax of phosphate on H. verticillata was 286 mg kg1,or approximately 6–9% of the phosphorus bioaccumulation by H. verticillata would be due to early sorption. Hence, the phosphate biosorption cannot be ignored as an effect for net bioaccumulation. Acknowledgements The authors wish to thank the financial support from China’s national basic research program: ‘‘Studies on the Process of Eutrophication of Lakes and the Mechanism of the Blooming of Blue Green Alga’’ (2002CB412304). We also wish to thank the staff from the Research Center of Lake Environment of the Chinese Research Academy of Environmental Sciences and the
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S. Wang et al. / Aquatic Botany 89 (2008) 23–26
anonymous reviewers for their helpful comments for improving this paper. References Blindow, I., Andersson, G., Haregy, A., 1993. Long-term pattern of alternative stable states in two shallow eutrophic lakes. Freshwater Biol. 30, 1159– 1167. Blindow, I., Hargeby, A., Andersson, G., 2002. Seasonal changes of mechanisms maintaining clear water in a shallow lake with abundant Chara vegetation. Aquat. Bot. 72, 315–334. Bole, J.B., Allan, J.R., 1978. Uptake of phosphorus from sediment by aquatic plants, Myriophyllum spicatum and Hydrilla verticillata. Water Res. 12, 353–358. Ce´sar, R.T.T., Marco, A.Z.A., 2004. Biosorption of heavy metals using rice milling by-products. Characterization and application for removal of metals from aqueous effluents. Chemosphere 54, 987–995. Chambers, P.A., Kalff, J., 1985. The influence of sediment composition and irradiance on the growth and morphology of Myriophyllum spicatum L. Aquat. Bot. 22, 253–263. Chen, Q.X., Liu, S., Wang, N., Zhiguang, S., Guo, X.S., 2002. Effect of phosphorus nutrient on physiological activity of Nymphaea tetregona Georgi. and Trapa bispinosa Roxb. Leaves. J. Nanjing Normal Univ. (Nat. Sci.) 25, 71–77 (in Chinese). Denny, P., 1972. Sites of nutrient absorption in aquatic macrophytes. J. Ecol. 60, 819–829. Dos Santos, M.C., Lenzi, E., 2000. The use of aquatic macrophytes (Eichhornia crassipes) as a biological filter in the treatment of lead contaminated effluents. Environ. Technol. 21, 615–622. Dushenkov, V., Kumar, P.B.A.N., Motto, H., Raskin, I., 1995. Rhizofiltration: the use of plants to remove heavy metals from aqueous streams. Environ. Sci. Technol. 29, 1239–1245. Everard, M., Denny, P., 1985. Flux of lead in submerged plants and its relevance to a freshwater system. Aquat. Bot. 21, 181–193. Gersberg, R.M., Elkins, B.V., Goldman, C.R., 1984. Use of artificial wetlands to remove nitrogen from wastewater. J. Water Pollut. Control Fed. 56, 152–156. Gersberg, R.M., Elkins, B.V., Lyon, S.R., Goldman, C.R., 1986. Role of aquatic plants in wastewater treatment by artificial wetlands. Water Res. 20, 363– 368. Golterman, H.L., Clymo, R.S., Ohnstad, M.A.M., 1978. Methods for Physical and Chemical Analysis of Fresh Water. Blackwell, Oxford. Haller, W.T., Sutton, D.L., 1975. Community structure and competition between Hydrilla and Vallisneria. Hyacinth Control J. 13, 48–50. Ho, Y.S., Wase, D.A.J., Forster, C.F., 1996. Kinetic studies of competitive heavy metal adsorption by sphagnum moss peat. Environ. Technol. 17, 71–77. Horppila, J., Nurminen, L., 2001. The effect of an emergent macrophyte (Typha angustifolia) on sediment resuspension in a shallow north temperate lake. Freshwater Biol. 46, 1447–1455. James, W.F., Barko, J.W., 1990. Macrophyte influences on the zonation of sediment accretion and composition in a north-temperate lake. Arch. Hydrobiol. 20, 29–42. Jin, X.C., Wang, S.R., Pang, Y., Zhao, H.Z., Zhou, X.N., 2005. The adsorption of phosphate on different trophic lake sediments. Colloid Surf. A 254, 241–248. Keskinkan, O., Goksu, M.Z.L., Yuceer, A., Basibuyuk, M., Forster, C.F., 2003. Heavy metal adsorption characteristics of a submerged aquatic plant (Myriophyllum spicatum). Process Biochem. 39, 179–183.
Langeland, K.A., Shilling, D.G., Carter, J.L., Laroche, F.B., Steward, K.K., Madiera, P.T., 1992. Chromosome morphology and number in various populations of Hydrilla verticillata (L.f.) Royle. Aquat. Bot. 42, 253–263. Laura, P., Vicente, C., Juan, J.I., Antonio, A., Joaquı´n, B., Niell, F.X., 2004. Influence of macrophytes on sediment phosphorus accumulation in a eutrophic estuary (Palmones River, Southern Spain). Aquat. Bot. 40, 1–11. Madrid, Y., Barrio-Cordoba, M.E., Camara, C., 1998. Biosorption of antimony and chromium species by Spirulina platensis and Phaseolus. Applications to bioextract antimony and chromium from natural and industrial waters. Analyst 123, 1593–1598. Mann, R.A., Bavor, H.J., 1993. Phosphorus removal in constructed wetlands using gravel and industrial waste substrata. Water Sci. Technol. 27, 107– 113. Patrick, D., Peter, W.R., 1970. Lead accumulation in Plankton blooms from Ullswater, the English Lake District. Environ. Pollut. 18, 1–9. Phillips, G.L., Eminson, D., Moss, B., 1978. A mechanism to account for macrophyte decline in progressively eutrophicated freshwaters. Aquat. Bot. 4, 2103–2126. Qiao, J.R., Chen, Y.Q., Song, F., 1996. Study on the removal rate to TP in Lake Caohai by common submerged macrophytes. Acta Scientiarum Naturalium Universitatis Pekinensis 32, 6787–6789 (in Chinese). Qiu, D.R., Wu, Z.B., Liu, B.Y., Deng, J.Q., Fu, G.P., Feng, H., 2001. The restoration of aquatic macrophytes for improving water quality in a hypertrophic shallow lake in Hubei Province. Chin. Ecol. Eng. 18, 147–156. Romero, J.A., Brix, H., Comı´n, F.A., 1999. Interactive effects of N and P on growth nutrient allocation and NH4+ up take kinetics by Phragmites australis. Aquat. Bot. 64, 369–380. Ross, M., Kuruvilla, M., Goen, H., 1999. The role of the submerged macrophyte Triglochin huegelii in domestic greywater treatment. Ecol. Eng. 12, 57–66. Scheffer, M., Van den Berg, M., Breukelaar, A., Breukers, C., Coops, H., Doef, R., Meijer, M.L., 1994. Vegetated areas with clear water in turbid shallow lakes. Aquat. Bot. 49, 193–196. Schulz, M., Rinke, K., Ko¨hler, J., 2003. A combined approach of photogrammetrical methods and field studies to determine nutrient retention by submersed macrophytes in running waters. Aquat. Bot. 76, 17–29. Shearer, J.F., Michael, J.G., Dwilette, G.M., 2007. Nutritional quality of Hydrilla verticillata (L.f.) Royle and its effects on a fungal pathogen Mycoleptodiscus terrestris (Gerd.) Ostazeski. Biol. Control 41, 175–183. Veglio, F., Beolchini, F., 1997. Removal of heavy metal ions by biosorption: a review. Hydrometallurgy 44, 301–316. Vermaat, J.E., Santamaria, L., Roos, P.J., 2000. Water flow across and sediment trapping in submerged macrophyte beds of contrasting growth form. Arch. Hydrobiol. 148, 549–562. Wang, X.S., Qin, Y., 2004. Equilibrium biosorption isotherms for of Cu2+ on rice bran. Process Biochem. 40, 1–4. Wang, T.C., Weissman, J.C., Ramesh, G., Varadarajan, R., Benemann, J.R., 1996. Parameters for removal of toxic heavy metals by water milfoil (Myriophyllum spicatum). Bull. Environ. Contam. Toxicol. 57, 779–786. Wang, S.R., Jin, X.C., Zhao, H.Z., Zhou, X.N., Wu, F.C., 2007. Effect of organic matter on sorption of dissolved organic and inorganic phosphorus in lake sediment. Colloid Surf. A 297, 154–162. Wheeler, G.S., Center, T.D., 2001. Impact of the biological control agent Hydrellia pakistanae (Diptera: Ephydridae) on the submersed aquatic weed Hydrilla verticillata (Hydrocharitaceae). Biol. Control 21, 168–181. Xie, Y.H., Wen, M.Z., Yu, D., Li, Y.K., 2004. Growth and resource allocation of water hyacinth as affected by gradually increasing nutrient concentrations. Aquat. Bot. 79, 257–266.
Aquatic Botany 89 (2008) 23–26 www.elsevier.com/locate/aquabot
Phosphate biosorption characteristics of a submerged macrophyte Hydrilla verticillata Shengrui Wang a, Xiangcan Jin a,*, Haichao Zhao b, Fengchang Wu c a
Research Center of Lake Environment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China b College of Agronomy, Inner Mongolia Agriculture University, Huhhot 010018, China c State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China Received 22 January 2007; received in revised form 8 January 2008; accepted 18 January 2008 Available online 2 February 2008
Abstract Phosphate biosorption of Hydrilla verticillata was investigated and compared with its bioaccumulation characteristics. Results obtained from the biosorption isotherms and kinetics showed that maximal phosphate biosorption was 286 mg kg1, approximately equal to 6–9% of the phosphate bioaccumulation. The biosorption mainly occurred within 5 h, and was highest during the first 30 min. The initial phosphate concentration was an important factor affecting the biosorption process. Phosphate biosorption on H. verticillata was not the main phosphate removal mechanism in our experiments, but it cannot be ignored. # 2008 Elsevier B.V. All rights reserved. Keywords: Bioaccumulation; Phosphate biosorption; Water quality improvement; Aquatic plant
1. Introduction Submerged macrophytes play an important role in nutrient cycling, especially in shallow lakes (Chambers and Kalff, 1985; Qiu et al., 2001). It has been reported that water quality can be significantly improved by reintroducing submerged macrophytes (Bole and Allan, 1978; Phillips et al., 1978; Blindow et al., 1993, 2002). The mechanisms through which submerged macrophytes can act as water quality regulators have attracted much attention recently (Laura et al., 2004). It has been reported that areas with dense submerged macrophytes usually have clear water and low concentrations of nutrients and phytoplankton (Denny, 1972; Bole and Allan, 1978; Scheffer et al., 1994; Blindow et al., 2002). Several macrophyte species, e.g., Hydrilla verticillata, Myriophyllum spicatum, Phragmites australis, Eichhornia crassipes and Zostera marina were reported to have a high biological purification capability (Gersberg et al., 1984, 1986; Mann and Bavor, 1993; Romero et al., 1999; Xie et al., 2004), and to reduce the extent of sediment resuspension (James and Barko, 1990; Vermaat et al., 2000; Horppila and Nurminen, 2001).
* Corresponding author. Tel.: +86 10 84915185; fax: +86 10 84915190. E-mail address: [email protected] (X. Jin). 0304-3770/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2008.01.006
Heavy metals can be removed from the environment by biosorption on to biological materials such as algae, bacteria and aquatic plants (Patrick and Peter, 1970; Everard and Denny, 1985; Dushenkov et al., 1995; Madrid et al., 1998; Dos Santos and Lenzi, 2000; Wang and Qin, 2004). Also for some freshwater macrophytes, e.g., Potamogeton lucens, Eichhornia crassipes, Myriophyllum brasillensis, Myriophyllum spicatum, Cabomba sp. and Ceratophyllum demersum with a capacity to remove heavy metals has been demonstrated (Wang et al., 1996; Keskinkan et al., 2003). Various mechanisms have been identified for biosorption including extracellular accumulation/precipitation, cell surface biosorption/precipitation, and intracellular accumulation. The processes include metal complexation, chelation, ion exchange, adsorption and micro-precipitation (Wang et al., 1996; Veglio and Beolchini, 1997). Bioremoval by aquatic plants may involve two processes: one is the initial and reversible metal binding or biosorption, and the other is the slower and irreversible sequestration, bioaccumulation (Keskinkan et al., 2003; Ce´sar and Marco, 2004). Amines, carboxyl and thiol functional groups may be the potential binding sites. To the best of our knowledge, little studies have been carried out on the phosphate biosorption characteristics of submerged macrophytes H. verticillata, a rooted submerged species, is
24
S. Wang et al. / Aquatic Botany 89 (2008) 23–26
widely distributed in Chinese waters, it may develop a dense mat, or canopy on the water surface (Haller and Sutton, 1975), has strong adaptability, grows fast and reproduces in many ways (Langeland et al., 1992; Shearer et al., 2007). Therefore, H. verticillata is often used in lake ecological restoration in China (Wheeler and Center, 2001). The major objective of this study was to investigate the biosorption characteristics of H. verticillata on phosphate. 2. Materials and methods 2.1. Materials Plants were collected from Juma River, Beijing (398420 N, 1158340 E, 2161 m above sea levels), and placed for pretreatment in a 20 L glass tank in greenhouse at 25 2 8C with the corresponding river water for about 1 week. Apical shoots of 10 cm length were separated from the mother plant and washed with dilute HCl (0.82 mol L1) and distilled water before experimentation. Analytical grade anhydrous KH2PO4 was used as the phosphate source, and was prepared in deionized water. 2.2. Biosorption isotherms and kinetics Clusters of H. verticillata material (10.0 g wet weight, 5 shoots) were added in a series of 10 L acid washed glass tanks with 8 L of phosphate solution (anhydrous KH2PO4). The initial phosphate concentrations were 0.005, 0.02, 0.22, 0.58, 1.41 and 3.14 P mg L1, respectively. Biosorption isotherm experiments were performed at 25 2 8C in a temperaturecontrolled cabin in dark condition, with contact time of 13 h. The biomass was then removed and dried at 80 8C to constant weight to determine final dry mass and phosphate concentrations. After digestion of the dried plant material with nitric acid/ perchloric acid (5:1), total phosphorus concentrations were determined colorimetrically (Golterman et al., 1978). The solution was filtered through 0.45 mm GF/C filter membranes, and the equilibrium phosphate concentrations in the supernatant of the extraction were analyzed using the molybdenum blue method (Jin et al., 2005). The quantity of sorbed phosphate was calculated as the differences in the phosphate concentration in the solutions before and after the biosorption experiments (Wang et al., 2007). Biosorption kinetic experiments were performed similarly to the biosorption isotherm experiments. The incubation time was 13 h. At different time intervals, the solution samples were taken, filtered through 0.45 mm GF/C filter membranes and analyzed for phosphate concentrations using the molybdenum blue method (Jin et al., 2005). The quantity of the sorbed phosphate was calculated. All experiments were carried out in triplicate, and the experimental error was controlled below 6%. The results were reported as their average. During the experiments, pH in the solutions varied between 6 and 7, and no pH adjustment was made. Control experiments (with no biomass) were simultaneously carried out to ensure that biosorption was by H. verticillata biomass and not by the container.
3. Results and discussion 3.1. Biosorption isotherms A biosorption equilibrium was reached within 5 h in the studied phosphate concentration range, and the time to achieve equilibrium was related to the initial phosphate concentration (Fig. 1). After these initial experiments, further biosorption experiments were carried out. The Langmuir model is used widely in sorption studies (Ce´sar and Marco, 2004), so we used it to describe the phosphate biosorption isotherms on H. verticillata as follows: Langmuir model : Q ¼ Qmax
KC ð1 þ KCÞ
(1)
where Q is the phosphate biosorption amount (mg kg1), Qmax is the maximal phosphate biosorption amount (mg kg1), C is the biosorption equilibrium concentration (mg L1), and K is the biosorption coefficient. Linear regression was applied and model parameters were estimated. The Langmuir model described the phosphate biosorption properties of H. verticillata quite well (Qmax = 286 mg kg1, K = 17.5, r2 = 0.99, P < 0.01, n = 6). Qmax of phosphate on H. verticillata was 286 mg kg1. This result is similar to the biosorption isotherms of rice bran on Cu2+ (Wang and Qin, 2004). 3.2. Biosorption kinetics The biosorption process for porous solids is characterized by the initial amount of solute biosorption (Keskinkan et al., 2003). This can be calculated from the initial slope of the Ct/C0 vs. time curves (Ct/C0 is the ratio of the phosphate concentration at different sampling time intervals to the initial
Fig. 1. Initial phosphate biosorption experiments for Hydrilla verticillata at different initial phosphate concentrations.
S. Wang et al. / Aquatic Botany 89 (2008) 23–26
25
Table 2 The estimated n and R2 of the Q = a (C0)n Initial concentration (P mg L1)
n
R2
P
0.005 0.020 0.220 0.580 1.410 3.140
0.116 0.002 0.230 0.043 0.287 0.015 0.388 0.053 0.392 0.018 0.768 0.061
0.83 0.91 0.95 0.91 0.96 0.64
0.0030 0.0010 0.0020 0.0056 0.0004 0.0840
Table 3 Biosorption kinetics constants for phosphate on Hydrilla verticillata Concentrations (P mg L1)
0.005 0.02 0.22 0.58 1.41 3.14
Fig. 2. The variation of Ct/C0 with time.
phosphate concentration, Fig. 2). These slopes can be obtained by assuming that the relationship was linear over the experiment time (Ho et al., 1996). The estimated slopes and R2 are shown in Table 1 for different initial concentrations. Linear relationships were significant at initial concentrations from 0.005 to 1.41 mg L1 (P < 0.01, n = 8). This gave initial amounts of 0.0211, 0.0212, 0.0045, 0.0037, and 0.0022 per hour, respectively, for phosphate at different initial concentrations, and the initially sorbed phosphate decreased with increasing initial concentrations. However, the relationship was not significantly linear over the experimental period at the initial concentrations of 3.14 mg L1. This indicates that mass transfer was the main process only in the lower initial concentration range, but not a main process at higher concentrations. According to the theoretical equations for diffusion, when intraparticle diffusion was the only rate determining step, the biosorption amount is related to the square root of the initial concentration (Ho et al., 1996). This can be expressed as follows: Q ¼ aðC0 Þ
n
(2)
where Q is the phosphate biosorption amount (mg kg1), C0 is the initial phosphate concentration (mg L1), a and n are the constants. The estimated n and R2 are shown in Table 2 for different initial concentrations. The relationship was both significant and positive at the initial concentrations from 0.005 to 1.41 mg L1 (P < 0.01, n = 8). This indicates that intraparticle Table 1 Linear regression of phosphorus concentration at different sampling time intervals as a function of the initial concentration Initial concentration (P mg L1)
Slope
R2
P
0.005 0.020 0.22 0.58 1.41 3.14
0.021 0.0023 0.021 0.0035 0.005 0.0034 0.004 0.0012 0.002 0.0001 0.003 0.0001
0.96 0.98 0.84 0.88 0.78 0.42
0.0042 0.0021 0.0053 0.0064 0.0002 0.3400
Power Function model (Q = at^b) a
b
R2
p
14.87 1.873 32.88 5.256 69.00 6.781 85.03 7.389 156.36 12.543 285.07 15.437
0.12 0.034 0.23 0.056 0.29 0.075 0.39 0.031 0.39 0.053 0.77 0.058
0.79 0.91 0.95 0.91 0.95 0.73
0.0020 0.0001 0.0001 0.0003 0.0003 0.0068
diffusion was the operative mechanism and the amount determining step for phosphate biosorption on H. verticillata at lower initial concentrations. The relationship was not significantly positive at the higher initial concentrations of 3.14 mg L1. This suggests that intraparticle diffusion was not the main process for the phosphate biosorption on H. verticillata at higher initial concentrations. Biosorption kinetics data have been fitted to the Power Function model (Wang et al., 1996; Jin et al., 2005). It seems that the Power Function appears to describe the kinetics satisfactorily (Table 3). The average biosorption rates within 0– 0.5 h were the highest within 13 h for different initial phosphate concentration, ranging from 30 to 721 mg (kg h)1. As the initial concentration increased, the biosorption rate increased. 3.3. Comparison between phosphate biosorption and bioaccumulation Phosphorus concentrations of submerged macrophytes ranges from 2.6 to 5.6 g kg1 (Qiao et al., 1996; Ross et al., 1999; Wheeler and Center, 2001; Chen et al., 2002; Schulz et al., 2003). In the present study, the Qmax of phosphate on H. verticillata was 286 mg kg1,or approximately 6–9% of the phosphorus bioaccumulation by H. verticillata would be due to early sorption. Hence, the phosphate biosorption cannot be ignored as an effect for net bioaccumulation. Acknowledgements The authors wish to thank the financial support from China’s national basic research program: ‘‘Studies on the Process of Eutrophication of Lakes and the Mechanism of the Blooming of Blue Green Alga’’ (2002CB412304). We also wish to thank the staff from the Research Center of Lake Environment of the Chinese Research Academy of Environmental Sciences and the
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