Kinetic Modeling of Methylene Blue and Crystal Violet Dyes Adsorption on Alginate-Fixed Water Hyacinth in Single and Binary Systems ()
1. Introduction
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 [1]. Various treatment processes have been studied for dye removal from wastewater and these include physical separation, advanced chemical oxidation, and biological degradation [2-7]. However, these methods are expensive to operate and may produce toxic by-products. Activated carbon has been established as an effective adsorbent due to its large surface area, low density and chemical stability, but its high costs of manufacturing and regeneration make its use for large scale wastewater treatment uneconomical. These drawbacks associated with activated carbon have generated the interest for the development of its low-cost alternatives using a range of carbonaceous and other precursors.
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 [13]; pineapple leaf powder [14]; Calotropis procera [15], coniferous pinus bark powder [16], palm kernel fiber [17], and silkworm exuviae [18].
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 [19]. To improve the mechanical properties of biosorbents, immobilization, which uses techniques such as entrapment and crosslinking, has been reported [20,21]. Immobilization of biosorbent material offers advantages including the control of particle size, regeneration and reuse of the biomass, easy separation of biomass and effluent, high biomass loading and minimal clogging under continuous-flow conditions [22-26]. Common immobilization matrices used include sodium alginate, polysulphone, polyacrylamide, and polyurethane [21,27-29].
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 [29], none have reported on dye removal using immobilized biosorbent. The aim of the present study was to investigate the removal of MB and CV dyes by dried root biomass of E. crassipes immobilized on alginate. To obtain a better understanding of the biosorption mechanisms, the effect of initial dye concentration, pH and amount of bead biomass loading were studied for batch monocomponent and binary systems. The kinetic data generated were analysed using pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models.
2. Experimental
2.1. Instrument
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%.
2.2. Chemicals and Reagents
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.
2.3. Procedure
2.3.1. Preparation of Biosorbent
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 [39]. Biomass powder, (2 - 10 g) was then blended with 100 mL of 2%∙w/v sodium alginate solution to make beads with biomass loadings of 2% - 10%. Spherical beads were prepared by dropping the mixed solution into a 0.1 M calcium chloride solution. Beads 3 - 5 mm in diameter were formed by drop-wise extrusion through a pipette tip. The beads were hardened by placing them in 2% calcium chloride solution for 12 hours and washed in deionised water by agitation at 100 rpm for 30 min, discarding the solution, and then repeating the process 5 times. After removing residual water with paper towels, the beads were dried gradually at room temperature to constant weight and then used for further experiments [34].
2.4. Adsorption of MB and CV onto Alginate-Fixed E. crassipes
2.4.1 Optimum Biomass Loading
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.
2.4.2 Effect of pH
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.
2.4.3. Kinetic Study
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.
2.4.4. Binary Dye Sorption
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.
3. Results and Discussion
3.1. Optimum Biomass Loading
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. Figure 1 shows the removal efficiency (%) of MB and CV dyes
Figure 1. Effect of bead biomass loading on sorption of MB and CV onto alginate-E. crassipes beads.
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 Figure 1, the removal of both dyes is dependent upon the biomass loading in the beads. Dye removal is enhanced with increasing amount of biomass in the beads. Maximum removal efficiencies of 93 and 97% were achieved with a biomass loading of 8% for CV and MB respectively. Use of higher biomass loadings was not possible as the beads formed at biomass loadings greater than 8% were not stable in aqueous solution. It can also be seen that at all bead biomass loadings, the biosorbent removed greater quantities of MB than CV.
3.2. Effect of Initial pH
Figure 2 shows the removal efficiency of (%) of MB and CV versus initial pH of dye for an adsorbent dose of 2 g/L at initial dye concentration of 50 mg/L in monocomponent systems. As it can be seen in Figure 2, the removal of the dyes is enhanced with increasing pH. At pH 8, removal efficiencies were greater than 94% and 87% for MB and CV respectively. This is partly because the distribution of dye species in solution, i.e., the chemical speciation depends on pH of the solution. Furthermore, the net charge on the adsorbent is also dependent on solution pH. It has been shown that the functional groups responsible for sorption properties of E. crassipes include the carboxyl and hydroxyl [35]. At pH higher than isoelectric point (which is around pH 2 for E. crassipes), the carboxyl and hydroxyl groups increasingly become ionized, thus resulting in more negatively charged cell walls. As a result, MB and CV, being cationic dyes, have their removal increasingly augmented due to ionic attraction, levelling off around pH 10.