Abstract: In this study, the adsorption properties of two different marine algae (Gracilaria salicornia (red algae) and Sargassum sp. (brown algae) were investigated. Equilibrium isotherms were studied to evaluate the relative ability of the two algae to sequester Cr (VI) from aqueous solutions. The maximum biosorption capacity obtained was 45.959 mg g-1 for G. salicornia and 33.258 mg g-1 for Sargassum sp. at a solution pH of 4 and 50 mg L-1 initial chromium concentration. A significant fraction of the total Cr (VI) uptake was achieved within 60 min. Biosorbed chromium ions concentrations increased with increasing concentrations of biosorbents and increasing pH. The biosorption of Cr (VI) on G. salicornia and Sargassum sp. could best be described by the Langmuir model (R2>0.997 for Sargassum sp. and R2>0.999 for G. salicornia).
INTRODUCTION
Biosorption is an emerging and attractive technology which involves sorption of dissolved substances by a biomaterial. It is a potential technique for the removal of heavy metals from solutions and recovery of precious metals (Veglio and Beolchini, 1997). The process has gained importance due to its advantages over conventional separation techniques such as chemical precipitation, ion exchange, reverse osmosis, membrane filtration and activated carbon adsorption, which are used to remove toxic metals from waste streams. These advantages are the reusability of biomaterial, low operating cost, improved selectivity for specific metals of interest, removal of heavy metals from effluent irrespective of toxicity, short operation time and no production of secondary compounds which might be toxic (Spinti et al., 1995; Srinath et al., 2002). A number of chromium compounds have great economic importance and are used extensively in chemical, metallurgical and refractory industries. The International Agency for Research on Cancer (IARC) has determined that Cr (VI) is carcinogenic to both humans and animals (International Agency for Research on Cancer, 1997). It is well recognized that the presence of heavy metals and aromatic compounds in the environment can be detrimental to a variety of living species, including man. Hexavalent chromium (chromium (VI)) has been found together with a variety of aromatic compounds including phenol, naphthalene and trichloroethylene (TCE) at high concentrations in a number of contaminated sites. Chromium (VI) and its organic copollutants often originate from industrial sources such as leather tanning, photographic-film making, wood preservation, car manufacturing, petroleum refining and agricultural activity. Chromium (VI) and phenol containing effluents are also discharged from a wide range of industries, including dye, textile, tannery, petrochemical and many others (Patterson and Stoneham, 1985; Chirwa and Wang, 2000; Aksu and Akpinar, 2000). These industries contain Cr (III) and Cr (VI) at concentration ranging from 10 to 100 mg L-1 (Park et al., 2005). Among the biological materials investigated for heavy metal removal, the biomass of marine algae otherwise known as seaweeds has been reported to have high uptake capacities for a number of heavy metal ions (Bailey et al., 1999; Pagnelli et al., 2000). Numerous studies on metal biosorption by brown seaweeds such as Sargassum have been reported in the literature. However, the applicability of green seaweed biomass such as Ulva for metal removal has not been extensively investigated yet despite its large abundance in the world’s shorelines (Morand and Birand, 1996; Valiela et al., 1997). Over the past two decades, attention has been concentrated on identifying materials that can effectively remove heavy metals from aqueous environments. These materials are known as biosorbents and the passive binding of metals by living or dead biomass is referred to as biosorption (Schiewer and Wong, 2000). Seaweeds have been shown to be extremely efficient biosorbents with the ability to bind a variety of metals (Volesky and Holan, 1995). In particular, the potential of non-viable seaweeds in the recovery of heavy metal ions from aqueous effluents has been studied by Yun et al. (2001) and Davis et al. (2003). Seaweeds possess a high metal-binding capacity (Ramelow et al., 1992; Holan and Volesky, 1994). Metal ion uptake by biomass is believed to occur through interactions with cell wall (Wang et al., 2001). This is due to the presence of various functional groups such as carboxyl, amino, sulphate and hydroxyl groups, which can act as binding sites for metals. The main mechanisms of binding include ionic interaction sand complex formation between metal cations and ligands on the surface of the seaweeds (Yun et al., 2001). In the present study, we report the biosorption of Cr (VI) by two different marine algae. Gracilaria salicornia (red algae) and Sargassum sp. (brownalgae). The effects of contact time, concentration of algal biomass and pH were studied.
MATERIALS AND METHODS
Preparation of biomass: Gracilaria salicornia and Sargassum sp. were collected from Persian Golf coast of Iran in May 2007. After harvesting from sea, these were washed several times using deionized water to remove the sand particles and salts. They were then dried in an oven at 60°C until constant weight. Dry biomass was chopped and sieved and particles between 50-70 mesh (0.2-0.3 mm) were used for biosorption experiments.
Chromium (VI) solutions: Stock Chromium solution (500 mg L-1) was prepared by dissolving 0.1414 g of K2Cr2O7 (Merck, Darmstadt, Germany) in 100 mL of deionized water. Chromium solutions of different concentrations were prepared by adequate dilution of the stock solution with deionized water. The stock solution was standardized by titremetry.
Batch biosorption studies: All batch biosorption experiments were performed by adding 250 mg of dried biomass to 250 mL of Chromium solution in 250 mL Erlenmeyer flasks with constant Chromium concentration at 50 mg L-1. The flasks were agitated at 150 rpm for 240 min. The experiments were conducted at 25±1°C. For studying the influence of pH on the biosorption of Cr (VI), experiments were conducted at various initial metal solution pH values of 3-6. Algal mass quantity was varied from 0.5 to 4.5 g L-1 to know the optimum concentration. For equilibrium time, the initial pH was set at 4±0.2 and 1.0 g L-1 biomass was added into metal solution. Metal-free and biosorbent-free blanks were used as controls. The residual Chromium ion concentration in the solution was analyzed using atomic adsorption spectroscopy (braic). All biosorption experiments were carried out in triplicates to check the reproducibility of results.
RESULTS AND DISCUSSION
Effect of initial solution pH: Marine algae contain high content of ionizable groups (carboxyl groups from mannuronic and guluronic acids) on the cell wall polysaccharides, which suggests that the biosorption process could be affected by changes in the solution pH (Davis et al., 2000; Matheickal and Yu, 1999). The effect of pH on Chromium (VI) adsorption by the marine algae was therefore studied first. The uptake of Chromium (VI) showed a sharp increase with an increase in pH from 3.0 to 6.0 in the case of Sargassum sp. and in the case of G. salicornia (Fig. 1). Crist et al. (1992) reported that the zero-point for alga cell was at pH 3.0, above which the algal cells could develop a net negative charge. Nasseri et al. (2002) reported the removal of chromium using A. oryzae, where maximum removal was observed at pH 5 also Variation of biosorbed chromium (VI) ion concentration with time was shown (Fig. 2).
| Fig. 1: | The effect of initial pH on the uptake of chromium (VI) |
| Fig. 2: | Variation of biosorbed chromium (VI) ion concentration with time |
| Fig. 3: | Variation of biosorbed chromium (VI) ion concentration for
different biosorbent concentrations |
| Fig. 4: | Langmuir isotherm for chromium (VI) uptake by Sargassum sp. |
| Fig. 5: | Freundlich isotherm for chromium (VI) uptake by Sargassum sp. |
A significant fraction of the total Cr (VI) uptake was achieved within 60 min. The maximum biosorption capacity obtained was 45.959 mg g-1 for G. salicornia and 33.258 mg g-1 for Sargassum sp. at a solution pH of 4 and 50 mg L-1 initial chromium concentration.
Effect of algae concentration: The effect of G. salicornia or Sargassum sp. concentration on the removal of Chromium was studied using algal mass in the range of 0.5-4.5 g L-1.
| Fig. 6: | Langmuir isotherm for chromium (VI) uptake by G. salicornia |
| Fig. 7: | Freundlich isotherm for chromium (VI) uptake by G. salicornia |
Results showed an increased uptake of Cr (VI) with the increase of algae quantity. The maximum uptake was attained at about 4.5 g L-1 (Fig. 3). Karthikeyan et al. (2007) reported that an increased uptake of Cu (II) with the increase of alga quantity was shown.
Adsorption isotherms: Adsorption isotherms were obtained at an optimal pH of 4 as shown in Fig. (4-7) The solution phase concentration of Cr (VI) was varied from 10 to 100 mg L-1. The biosorption of Cr (VI) on G. salicornia and Sargassum sp. could best be described by the Langmuir model (R2>0.997 for Sargassum sp. and R2>0.999 for G. salicornia). however, the biosorption of Cr (VI) on G. salicornia could best be described by the Freundlich model (R2>0.9985). Mungasavalli et al. (2007) reported that Isotherm analysis of the biosorption pattern for chromium on Aspergillus niger pretreated biomass followed the Freundlich isotherm in the entire temperature range studied.
CONCLUSION
The results obtained showed that Sargassum sp. and G. salicornia could be used as efficient biosorbent material for the treatment of Cr (VI) ions. The uptake of Chromium (VI) showed a sharp increase with an increase in pH from 3.0 to 6.0 in the case of Sargassum sp. and in the case of G. salicornia. the strong pH dependence of Cr (VI) biosorption observed in this study could be attributed to more pronounced electrostatic attraction taking place between the biosorbents and the metal ions at higher pH. increased uptake of Cr (VI) with the increase of algae quantity could be due to the formation of biosorbent aggregates at higher biomass concentration, which in turn could reduce the effective surface area available for the biosorption. the biosorbents showed a different adsorption capacity, which might presumably be due to the presence of the different number of cell surface binding sites.
ACKNOWLEDGMENT
The authors gratefully acknowledge the support of Lorestan University of Medical Sciences.