Removal of Methylene Blue (MB) and Crystal Violet (CV) dyes from monocomponent and binary aqueous solutions by water hyacinth-E. Crassipes roots fixed on alginate (a low-cost adsorbent) has been investigated. The extent of adsorption was evaluated as a function of solution pH, initial dye concentration, and bead biomass loading. Kinetic sorption data were analysed by widely used models: pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models. The results showed that pseudo-second-order model better described the biosorption experimental data than the pseudo-first-order kinetic model for both dyes, whilst the Elovich model fitted the biosorption experimental data at lower dye concentrations. The intraparticle diffusion model indicated that sorption of CV and MB was characterized by rapid surface adsorption coupled with slow film diffusion process at higher initial dye concentration and at all initial bead biomass loading. The range of mean free energy values confirmed physical adsorption as the mechanism for dye removal from solution.
Increasing global production, possible toxic effects and persistence in the environment has resulted in dye pollution emerging as a significant environmental pollution issue over the past few decades [
Recently, use of organisms including bacteria, fungi, algae and plants as adsorbents has been developed as a low-cost and eco-friendly viable alternative [8,9]. In particular, biosorption, which involves dye removal from aqueous solution by passive linkage in live and dead biomasses in a mechanism that is not controlled by metabolic steps, is proving to be a popular alternative owing to its several advantages. These include simplicity of design, great efficiency for dye removal at low concentration, potential for biosorbent regeneration, high velocity of sorption and desorption, limited generation of secondary residues and more environmentally friendly life cycle of the material (e.g. easy to eliminate compared to conventional resins) [8,9]. Low-cost biomasses which have been investigated for their capacity to remove dyes from solution include peel of fruit, potato plant wastes and dried algae and ferns [10-12]. MB and CV are typical basic dyes that have been removed from aqueous solution using various adsorbents such as sawdust [
Generally, for a given biomass being studied for biosorption potential for the first time, the sorption process is initially carried out using native biomasses in the ground, fine powdered form in batch systems. Despite of the continuous operation in columns to be the preferential mode for amplifying the biosorption process to a pilot scale, the batch systems serve as pre-stage for an initial evaluation of adsorption phenomena and operational conditions before the application of the process on continuous systems. The batch systems have several advantages such as high rapid steady-state attainment, high biosorption capacity, less process cost and good particle mass transfer. However, they often suffer several drawbacks, and these include low biomass density, solid-liquid separation problems, possible biomass swelling, inability to regenerate/reuse and development of high pressure drop in the column mode [
Many studies have shown that E. crassipes can effectively remove heavy metals from aquatic systems because of the functional groups on the cell wall and their high binding affinity, fast growth and high abundance around the world [30-34]. However, of the few reports on the biosorption of dyes from contaminated waters using E. crassipes [
The concentrations of MB and CV dyes in the solutions before and after biosorption were determined by Genesys 10S (Thermo Scientific) UV/VIS Spectrophotometer. The pH of the solution was measured with a Cheker pH meter (Hanna Instruments, UK). All experiments were repeated twice with a repeatability of the results showing a variation of coefficient lower than 5%.
MB and CV stock solutions were prepared by dissolving known quantities of analytical reagent grade MB, (C16H18ClS∙H2O (x = 2.3), (C.I. 52015, M.W. 319.86)) supplied by Thomas Baker (Chemicals) Ltd., India and CV, (C25H30N3Cl (CI 42555, M.W. 407.98 g/mol)) supplied by Saarchem Ltd., SA. The two basic dyes were chosen because they have well separated absorption peaks in binary solution (663 nm for MB and 590 nm for CV). Stock solutions of 1,000 mg/L of MB and CV were prepared using de-ionised water and used to obtain standard solutions.
Sodium alginate salt purchased from Sigma Aldrich (Germany) was used as hydrocolloidal gelling material. As fixing solution, a CaCl2 (Sigma Aldrich, Germany) solution was utilized. A 2% (w/v) sodium alginate solution was prepared by dissolving 2 g of sodium alginate in 100 mL of deionised water and then agitated for 24 hours at 300 rpm to make a homogenous polymer solvent mixture. Biomass sampling and propagation has been described previously [
The optimum bead biomass loading for dye sorption was determined by contacting 0.1 g of adsorbent of biomass loadings of 2%, 4%, 6% and 8% (w/v) separately with 50 mL solution of each dye at initial concentration of 50 mg/L. The initial pH of the dye solution was set at 8 and the mixture was agitated at 150 rpm at room temperature, filtered through 0.45 mm Millipore® cellulose filters and dye remaining concentrations were determined in the filtrate.
The effect of pH on the adsorption of MB and CV in monocomponent dye solutions was studied with aqueous solutions of 50 mg/L dye concentration. The initial pH was varied in the range pH 3 - 10 using 0.1 M HNO3 and 0.1 M Na2CO3. Fifty millilitres of dye solutions were mixed with 0.1 g of 6% biomass loading, agitated at 150 rpm for 5 hr at 30˚C. After that, aliquots were filtered through 0.45 mm Millipore® cellulose filters and dye remaining concentrations were determined in the filtrate.
In order to determine the equilibrium time and sorption mechanism, adsorption tests were conducted with solutions containing 50 mg/L of MB and CV dyes in monocomponent and binary systems respectively. A fixed dose of 0.5 g of 6% biomass loading was mixed with 50 mL of each dye solution. The mixtures were agitated at 150 rpm, filtered at pre-set times up to a maximum of 200 min, and analyzed for remaining dye concentration.
For binary dye aqueous solution systems, a 1:1 dye concentration ratio was used, and the equilibrium sorption for each dye in the binary mixture was determined at the individual dye’s previously stated maximum absorption wavelength. The sorption parameters and conditions used were as described previously for monocomponent dye systems.
The reasons for studying the effect of adsorbent dosage on sorption dynamics are mainly economical: to establish the effectiveness of an adsorbent and the ability of a solute to be adsorbed with a minimum dosage.
versus bead biomass loading (%) for a fixed biomass dose of 2 g/L at initial dye concentration of 50 mg/L. As it can be seen in