INTRODUCTION:

Textile industrial effluent has one of the major accounts among all the industrial toxic discharge and it is estimated that approximately 10-15% of the dye lost into waste water during manufacturing and processing of dyes from textile industries¹. The amount of dye releases depends on the type of dye used and it varies from 2-50% loss due to the dye-fiber interaction². Textile synthetic dyes are highly stable organic molecules and their complex aromatic structure makes them highly recalcitrant to degradation, thus remain in the water bodies for longer period³. The content of dye (10-200 mg/L) in textile waste water is highly visible and affects aquatic ecosystem, as they undergo various chemical and biological disturbances. Beyond its color, they affect living systems by transforming into toxic, mutagenic and carcinogenic amines⁴.

In addition, the dye wastewater has high TOC, BOD, COD and extreme pH, high salt content⁵. Therefore, the dye-based industries are major concerned about the treatment of waste water before discharging the dyes into the environment. Among the synthetic dyes, congo red (sodium 3, 3’- ([1,1'-biphenyl]-4,4'-diyl) bis (4-aminonaphthalene-1-sulfonate)) is an azo dye mostly used in textile industries, as it has strong non-covalent affinity to the cellulose fibers. Azo dyes have characteristic one or more azo bonds (-N=N-). These dyes can be decolorized by breaking the azo bond (s) through reduction reaction, but the reaction products, like aromatic amines cannot be fully degraded. They are mutagenic and carcinogenic to humans and animals and toxic to aquatic living system, food-chain⁶.

Various physical, chemical, and biological decolorization procedures such as membrane separation, microbial degradation, photocatalysis, electrochemical oxidation etc. are adopted in order to remove the colorants from the waste water⁷. These methods are limited due to its disadvantages like feasibility, generation of secondary pollutants, treatment cost, and efficiency of dye removal. Adsorption method gives satisfactory results because it allows a complete decolorization of dyes, reuse and reactivation⁸. In this process, a soluble material (dye) from the solvent (wastewater) is adsorbed on the porous solid surface (adsorbent) by means of physical interactions or chemical bonds⁹. Natural molecules and wastes can be utilized as cost-effective, eco-friendly decolorizing adsorbents. Calcium hydroxyapatite (HAp), Ca₁₀(PO₄)₆(OH)₂, is a stable inorganic material, widely used for water purification systems, chromatographic techniques, biomedical applications, such as artificial bone, teeth etc.^10,11. HAp has high adsorption efficiency due to its smaller size with larger surface area. Fish scales are rich sources of mainly HAp along with some other biomolecules like collagen, vitamins, fatty acids etc.¹². Approximately 4% fish scale of 50% of total fish waste is produced by the fish processing industry¹³. Thus extraction of HAp from the fish waste considered as economically as well as environmentally friendly technique. In view of this waste as abundant source, the aim of the present work was the extraction of HAp from the fish scale waste and used for decolorization of congo red (CR) on this biomaterial by adsorption process. The effect of various factors on the adsorption like the contact time, adsorbent dosage, pH the initial concentration of CR was also analyzed.

MATERIALS AND METHODS:

Chemicals:

Congo red, the toxic dye used in this study was obtained from Hi-Media, India. Absorbance maxima (λ_max) for the dye was obtained using UV-Vis Spectrophotometer (UV 8500 II Techomb) and found to be 620 nm. All the chemicals of analytical grade were purchased from Hi-Media, India and used without further purification.

Collection and pretreatment of materials:

The raw fish scales of Labeorohita was freshly collected from a local market of Guntur, India. The scales was soaked and rinsed for several times in distilled water to remove contaminants. Then the fish scales were dried at room temperature for overnight and kept in air tight polythene bag.

Preparation of hydroxyapatite from fish scales:

The initial step of extraction of hydroxyapatiteis demineralization which involves soaking of scales in 4% HCl at 27°C for 36 h, followed by washing for several times with distilled water, then the demineralized scales were deproteinized in 5% NaOH at 90°C and incubated for 24h. After the incubation time, the residues are collected and rinsed to neutrality and dried at 60°C in an oven. The dried scales were calcined in muffle furnace in temperature range between 800°C-1000°C at a heating rate of 4.5°C/min in 5h. The obtained hydroxyapatite powder was cooled and used for studies.

Analytical studies of hydroxyapatite:

The synthesized hydroxyapatite from fish scales were characterized by using different analytical techniques. The structural analysis of hydroxyapatite was recorded using SEM (FEI Quanta 200 SEM) with secondary electron detectors at an accelerating voltage of 20 kV. To confirm the crystalline nature of hydroxyapatite, powder XRD experiment was carried out by X’Pert Pro A Analytical diffractometer with Cu Kα radiation in the 2θ range operated at a voltage of 45 kV and a current of 40 mA. The surface chemistry of hydroxyapatite was analyzed by FTIR experiment. The HAp powder was mixed with potassium bromide, and dried overnight at 60°C. The resulting pellet was used to determine the surface functional groups by IR spectroscopy (Jasco-FTIR 4100typeA, Japan). The result was recorded between 4000 and 400 cm⁻¹ at a resolution of 4 cm⁻¹ in transmittance mode.

Decolorization experiment:

The decolorization of dye was performed in batch mode. A stock solution of congo red (1000 mg/L) was prepared with distilled water. The adsorption studies carried out with accurate amount of HAp powder with aqueous dye solution in 250 mL Erlenmeyer flask. The mixture was agitated at 200 rpm at 27 °C. After the respective incubation period, solution was centrifuged and final concentration of dye was measured as absorbance at 620 nm using UV-vis spectrophotometer (UV 8500 II Techomb). The influence of different factors such as initial dye concentration (10-100mg/L), contact time (0-360 min), adsorbent dosage (1-10g/L), pH (2-10) on decolorization of dye was evaluated. Finally percentage of decolorization was calculated using equation as follows¹⁴:

×100

RESULTS AND DISCUSSION:

Characterization of HAp by microscopic studies

Fig 1 represents the SEM image of HAppowder extracted from the biowaste fish scale. Low magnified micrograph depicts that the spherical shaped nano-sized particles along with micro-scale particles form aggregation. The different range of particle size depends on the calcination temperature. The size of the particles increased with increase of calcin temperature, because at higher temperature HAp molecules move faster and leads to collide with each other^15,16. The result of this experiment was confirmed by the presence of intense peaks in the XRD profile.

Fig 1: Scanning electron micrographs of hydroxyapatite particles

XRD pattern reveals the crystallinity and the structural information of the HAp powder, shown in Fig 2. According to XRD spectrum, HAp extracted from the fish scale is crystalline in nature and has a sharp peak at 2θ value of 31.9 ° along with small peaks, which can be indexed (211) Bragg’s reflections plane, characteristics to pure hydroxyapatite¹⁷. The high intense peak and corresponding peaks in XRD diffractograms of the HAp powder confirms the complete crystallization of the HAp at high calcination temperature. These results matches well with the standards in JCPDS file no. 090432. The absence of additional peaks prove the success of calcination process, that means the increase in calcin temperature continued until to the point where the calcite can completely transform into HAp, because it decrease the impurities¹⁸.

Fig 2: Representative XRD pattern of HAp particles

FTIR spectroscopic analysis was carried out to identify the possible functional group present in the biopolymer hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), extracted from the fish scale waste in the range of 4000-400 cm^-1. Fig 3 shows the all characteristic IR band of HAp, such as peak for phosphate, hydroxyl and carbonate groups. The strong absorption band at 3396and 1616 cm^-1represents stretching and bending vibration of O–H bond of water in the crystal lattice^13,19. The spectra centered in the range of 900-1100 cm^-1 corresponds to asymmetric stretching vibration mode for PO₄^3- group. A very strong band positioned at 1024 cm^-1due to the phosphate group absorbance intensity. The spectrum of IR at 560 and 603 cm^-1were referred as the strong n4 symmetric P-O stretching vibration of a phosphate group²⁰. The most common absorption band found in HAp at 1415 cm^-1, can be attributed to asymmetric stretching mode of CO₃^2- (carbonate group)²¹. It is clear that the obtained results are in accordance with the XRD profile since all the functional groups contained in the crystalline HAp molecule are identified by FTIR spectroscopy.

Fig 3: FTIR absorption spectrum of hydroxyapatite

Influence of different parameters on decolorization of dye using hydroxyapatite:

Hydroxyapatite is an easily available, cost-effective adsorbing material that may be used commercially to control the dye water pollution. The functional properties of the adsorbent can be improved by studying the effects of various parameters on the decolorization process.

Effect of contact time on decolorization:

The effect of contact time on adsorption of CR was studied at different contact time (0-360 min) with initial dye concentration 50 mg/L, 1 g/L adsorbent mass at pH 3. Fig 4 depicts that in the initial stages, increases in dye decolorization with increase time, and reaches saturation at 180 min, further no significant change was found in dye adsorption with prolonging time. The rapid adsorption at the beginning steps may be due to availability of large number of active adsorption site on the adsorbent surface and attain the equilibrium with maximum decolorization of 88%. But after the saturation point, the adsorption of dye on the remaining unoccupied site may be difficult because of repulsive forces of adsorbate ions between liquid and solid phase²². The decolorization percentage was almost constant after the equilibrium contact time. The optimum contact time of CR was used for further decolorization study.

Fig 4: Effect of contact time on decolorization

Effect of pH on adsorption of congo red:

pH is an important parameter affecting the adsorption of dyes because change in pH in the bulk phase can lead to ionization of the functional groups on the adsorbent or adsorbate molecules. The variation of pH on adsorption capacity of hydroxyapatite was represented in Fig 5. The dye adsorption experiment was carried out with varying pH (2-10) and constant dye concentration and adsorbent dose for 180 min. The percentage dye decolorization was higher in lower pH, while adsorption percentage decreased with increase in pH. The maximum decolorization was found at pH 4 of 93% and decreased steeply above the pH 8. The results suggest that at acidic pH, the negatively charged HAp become protonated and thus positively charged which can interact with negatively functional group (SO₃^ˉ) of congo red by attractive electrostatic forces, causing increase in dye adsorption. But at alkaline pH, decolorization reduced dye due to repulsive forces between negatively charged HAp and SO₃^ˉ group of dye molecules or presence of excessive OH^ˉionscompeting with an ionic group of CR for the same adsorption site. Thus highly pH-dependent adsorption study reveals that the acidic pH favors the decolorization of CR; similar result was also reported by Reddy et al.²³.

Fig 5: Effect of pH on dye decolorization

Effect of adsorbent amount on decolorization of dye:

The influence of adsorbent dose on adsorption of CR was represented in Fig 6. The decolorization of CR was evaluated at different amount of HAp ranging from 1 to 10 g/L at pH 4 for 180 min of contact time. The initial CR concentration of the solution was fixed at 50 mg/L for all the decolorization tests. The adsorption of dye increased with increasing HAp amount up to 2 g/L and reached to optimum decolorization of 95%. For a constant adsorbate concentration, the increase in adsorbent dose provides numerous vacant adsorption sites on larger surface area that favors CR adsorption²⁴. In addition, further increase in adsorbent dose did not show significant decolorization of CR. This may be due to the unavailability of the active site on HAp surface. Thus the obtained optimum adsorbent dose was used for further study.

Fig 6: Percentage of dye decolorization removal on varying amount of adsorbent dose

Effect of initial dye concentration on decolorization:

Percentage of decolorization of CR using HAp was investigated for the dye concentrations of 10-100 mg/L at pH 4 for 180 min. Fig 7a shows significant increases in decolorization of dye with increase in initial dye concentration. The maximum dye adsorption of 98% was observed at 60 mg/L of initial concentration of dye solution. This pattern of decolorization depicts monolayer adsorption of dye molecules on HAp surface. The inset of Fig 7b shows the decolorization of congo red using fish scale hydroxyapatite at optimum condition. Above the optimum dye concentration, the adsorbent has very lesser number of vacant dye-binding site, thus there is no major changesin decolorization percentage at higher dye concentration.

Fig 7:(a) Effect of initial concentration of dye on decolorization, (b) Inset image shows congo red decolorization using hydroxyapatite

CONCLUSION:

A simple, eco-friendly method was adopted to decolorize the dye by using a renewable and low-cost adsorbent. Hydroxyapatite was extracted from the fish scale waste and used to decolorize congo red dye from the aqueous solution. The adsorption properties of the adsorbent were verified in terms of particle size, functional groups on the surface, surface area by different analytical techniques. The physical and chemical properties of hydroxyapatite prove the purity of the material that showed excellent dye decolorization at optimum condition. Thus hydroxyapatite can be employed as an effective adsorbent for the treatment of dye-polluted waste water.

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Received on 21.02.2019 Modified on 10.03.2019

Research J. Pharm. and Tech. 2019; 12(6):2917- 2921.

DOI: 10.5958/0974-360X.2019.00491.8