1. Introduction
Organic contaminations from different sources have become a major concern due to their impact on living organisms. due to the increasing environmental risk when they are discarded into water, organic pollutants especially dyes from various textile and chemical industries have received great attention, which exhibit high toxicity and potential mutagenic and carcinogenic effects.[1,2] Thus, the efficient removal of such kinds of pollutants before discharging to the water bodies is becoming a major problem. Due to the non-degradable nature and stability toward light and/or oxidizing agents, dyes complicate the selection of a suitable method for removal [3-8].
Several methods have been developed to treat the dye-containing effluents, including There are several chemical, physical and biochemical methods are used for the removal of organic dyes from effluent water [9-13], which will not ensure the complete mineralization of the organic pollutants from effluent water.
Advanced oxidation processes are perhaps the best method for removal of the toxic biodegradable pollutants [14,
15] Further, removing pathogens [16].
The degradation methods are depends the formation of reactive oxygen species like hydroxyl (OH•) and hydroperoxyl (HO2•) radicals. Metal ion dopants create the electron (or hole) traps and alter e-/h+ recombination rate, according to the following mechanism:
Mn+ + eCB- → M(n-1)+ (electron trap)
Mn+ + hVB+ →M(n+1)+ (hole trap)
Where the energy level for Mn+/M(n-l)+ lies above the valence band edge (EVB) and the energy level for Mn+/M(n+l)+ below the conduction band edge (ECB)[17]. Metal ions also may act as recombination sites for photoinduced charge carriers, so they can reduce the quantum efficiency.
Due to the interaction of their f-orbitals with different functional groups, lanthanide ions have the ability to form complexes with various Lewis bases (e.g., alcohols, amines, thiols) and also provide high surface area and forming complexes, hence can form bonds with various biopolymers. Doping of lanthanides with biopolymers provides the photocatalytic ability by increasing the adsorption capacity for organic compounds, as well as, suppresses electron-hole recombination rates during the process of photocatalytic reaction [18]. It can be assumed that doping with two dopants can show synergetic effect in improving photocatalytic activity [19].
Integrated photocatalytic composite adsorbents (IPCA) are used to degrade toxic organic and inorganic compounds in presence of UV/visible light irradiation. The compound preserves all the existing features of individual components and at the same time overcome drawbacks like rapid recombination of photogenerated electrons, low absorptivity and hindrance effect of photocatalyst. Herein, an integrated Photocatalytic Chitosan-La3+-Graphite Composite adsorbent were synthesized and characterized by various techniques. An aqueous solution of methylene blue (MB) dye was used to study the photocatalytic efficiency of the prepared IPCA. The influence of irradiation time, initial dye concentration, dosage of CS-La-GR and pH in the photodegradation process were also analyzed. The percentage of mineralization has been analyzed by measuring initial and final COD of the MB dye.
2. Materials and Methods
2.1. Materials. The textile dye Methylene Blue was obtained from M/s Sree Chemidyes, Bangalore and used to study the photdegradation without further purification. Double distilled water was used throughout the study. Chitosan (85% deacetylated) was obtained from Pelican Biotech and Chemicals Labs, Kerala (India) and the material was directly used without any further purification. The following analytical grade reagents LaCl3, 7H2O, NaOH acetic acid and glutaraldehyde were purchased from Merck chemicals. All chemicals and reagents were of analytical grade and are used without further purification.
The powder XRD measurements were performed by using X’per PRO model PANanlytical, Netherland. The diffractometer was equipped with graphite monochromatized Cu Kα radiation (λ = 1.5406 Å). FTIR spectra (4000-400 cm1 with KBr pellets) were recorded on a JASCO460 plus model, FTIR spectrometer Japan. The morphology was studied using VEGA3 TESCAN model field emission SEM instrument. The compositions of the elements in the LDH-OPAC composite were analysed using EDAX with Bruker Nano Gmbh, Germany. The thermal properties were measured using Universal V4.5A TA Instruments, USA. The absorption was measured using UV-vis spectrophotometer (Pharo 300 Merck) at 633 nm.
1.3. Photoreactor and Light Source
The photocatalytic degradation of MB dye was studied using CLGC under UV light irradiation. The irradiation was carried out using heber multi lamp photoreactor with UV lamp (254nm) which is placed parallel to the reaction vessel of 200 mL capacity. The photoreactor was also provided with a cooling fan at the back side of the chamber. The reaction vessel containing the dye solution with the photocatalyst was continuously bubbled using oxygen throughout the course of reaction for maintaining complete mixing up of the solution.
1.4. Photocatalytic Experiments
The experiments were carried out by the photocatalysts viz., CLGC under UV light irradiation at different experimental conditions. The MB dye was treated with CLGC in dark as well as in the presence of UV light irradiations separately. A batch experiment was conducted to investigate the effect of important parameters like irradiation time, catalyst dosage, pH, light intensity, co-ions, and initial concentration of dye, under UV light irradiation. After irradiation with a fixed time, a 5 ml aliquot of the reaction mixture was centrifuged, filtered and the concentration of MB dye was measured by UV-vis spectrophotometer (Pharo 300 Merck) at the wavelength of 554 nm. In the photocatalytic experiments, the extent of decolorization of dye in terms of the percentage has been calculated using the following relationship.
Decolorization % = (Co−Ct)/ Co ×100 (1)
Where Co is the initial concentration of MB dye (mg/L) and Ct is the final concentration of MB dye (mg/L). The degree of mineralization in terms of COD reduction was calculated as follows:
COD reduction = (CODInitial−CODFinal)/CODInitial×100 (2)
3. Result and Discussion
3.1. Characterisation of CS-La-GR
The FT-IR spectra of chitosan, graphite, lanthanumchloride (LaCl3•7H2O), lanthanum doped chitosan graphite composite (CS-La-GR), before and after the photocatalytic degradation were depicted in Fig. 1. FTIR spectrum of graphite shows a weak absorption peak at 674 cm-1, which correspond the bending vibration of the aromatic C-H bond[20]. The broad band at 3442 cm−1 indicates the —OH and —NH2 stretching frequencies in CS. The sharp peak at 2926 cm−1 emphasizes the -CH2 asymmetric stretching and the peak 2876 cm−1 depicted the C–H stretching vibrations indicating the presence of alkyl groups in chitosan. [21]. the bands at1667 cm-1 for carbonyl C=O stretching in amide and 1152 cm-1 for bridge-O-stretching [22]. The vibration of the La-O bond band was appeared at 624 cm−1 [23]. Chitosan -La- graphite catalyst in its FT-IR spectrum showed a slight changes in the stretching and bending frequencies of OH and NH2 groups (3432 and 1636 cm-1 ) along with minor changes in the intensities. This shift in frequencies indicates the complexation of lanthanum with NH2 and OH groups of chitosan [24]. A sharp peak at 1380 cm-1 was attributed to the bending vibration of metal-oxygen bands. A metal and metal oxide peaks were noticed in the range of 470–490 cm-1in prepared sorbents [25]
The XRD patterns of the CS-La-GC were shown in fig.2. The sharp and narrow diffraction band appeared at 2θ= 26.6 confirmed the presence pure and crystalline graphite phase in the composite [26]. the crystalline pattern of lanthanum was specified to be at 2θ = 15.9° (002), 27.4° (100), 48.6° (110), 50.7° (112), 52.4° (106) and 60.5° (107) [JCPDS card no 89-291] [27], however the peaks belonging to La are not detected in the pattern of catalyst due to complexation of lanthanum with chitosan and graphite or the relatively low percentage of lanthanum in the catalyst[19]. The peak at 45° was due to metal entrapment of polymeric matrix accentuated that the crystalline form of the composite. After photocatalytic degradation there are nomuch more changes in the catalyst surface indicating the efficiency of the [26].
The SEM image had been used to analyze the surface morphology of the CS- La-GR composite, before and after photocatalytic degradation was shown in Fig. 2 A and B respectively. The catalyst show a smooth and layered surface and it might be due to the formation of metal loaded polymeric composite. After degradation, there was a slight change in the layer structure that might be due to the uneven distribution of some of the degraded material on the surface of the catalyst.
The EDX analysis of CS- La-GR composite, before and after photocatalysis had been depicted in Fig.3A and B respectively. The main elements present in CS- La-GR composite C, N and O were recognized, which are the main constituent element present in the chitosan and graphite. The presence of band, La in EDX confirmed that La-metal was effectively doped on the polymeric matrix. In Fig.3 B the presence of peak of Sulfur in the EDX spectrum indicate that the sulfur was removed from the MB dye and the removed element was successfully adsorbed on CS- La-GR polymeric composite.
The thermal stability of CS- La-GR composite was studied by thermogravimetric analyses are shown in Fig. 5. In Fig. 5A and B, three degradation steps have been seen in CS- La-GR composite. The first degradation step observed a temperature at 100°C was owing to the loss of weakly adsorbed water molecules [28] and second of decomposition shown in the range of 250–350°C and this might be due to decomposition of pyranose ring structure present in the chitosan [29]. The third and final degradation step was noticed near 450°C and this could be ascribed to decomposition of metal complexes. This result indicates that the prepared composite was thermally stable.
The thermal properties of CS- La-GR composite had been carried out through DSC analysis within the temperature range of 25–800°C. The endothermic peaks around 80 °C, corresponded to the dehydration of CS- La-GR composite. The exothermic peaks of CS- La-GR composite were shown in the range of 292.80°C, due to the degradation of residual carbonated products present in the CS- La-GR composite. The exothermic peak at 450.58°C were attributed due to CS-GR composite and encapsulation of La3+ ions into it.
3.2. Adsorption of Dye
The MB dye had been used to study the adsorption capacity of the CS-La-GR composite at fixed contact time of 60 min and 30 mL of 100 mg/L of initial concentration of dyes and 100 mg of dosage of CS-La-GR composite were conducted in the dark and in the presence of light as well. The percentage of dye adsorption was found to be very low, when they have been treated in visible light irradiation. The discoloration of dye was found to be comparatively higher, in the presence of UV light. Further, the synchronous role of chitosan and Li3+ and graphite in the presence of UV light results in excellent photocatalytic degradation activities when compared to other sorbents.
3.3. Effect of Irradiation Time
Theextend of irradiation time in photodegradation of MB dye was studied by measuring the percentage of dye degradation at different time intervels using 100 mg of CS-La-GR composite and 30 ml of 100 mgL-1 dye solution in UV light at room temperature. The results reveal that the percentage of dye removal increases with increase of the irradiation time and reached up to 93.5% for MB dye within 60 min of irradiation. The result emphasized that the synthesized composite had very high catalytic efficiency which enables the degradation of the dyes in such short irradiation time and had huge number of active sites, for carrying out the catalytic degradation reaction.
3.4. Effect of CTC Dosage
The effect of the amount of CS-La-GR composite on the degradation of dye MB had been examined using different doses of CTC varying from 25 to 200mg. It was clear that the percentage of dye decolorisation increases with an increase of the amount of composite catalyst up to certain limit above which stagnation was perceived. Once the dye molecules were adsorbed on of CS-La-GR composite, there was no improvement was observed by further addition of the catalyst. The maximum quantity of dye degradation was reached at 100 mg of the catalyst, and hence, it had been fixed as constant for further analysis. The decrease in percentage of dye removal may be due to the non-availability of dye molecules, an increasing opacity of the suspension and an enrichment of the light reflectance, due to presence of CS-La-GR particles.
3.5. Effect of pH
The pH of the solution should have an influence in the photocatalytic degradation of the MB dye. The efficiency of the catalyst is affected by the pH of the solution. The pH of the solution was adjusted before irradiation, and it was not controlled during the course of reaction. In the case of MB dye, it is a cationic dye and the dye removal efficiency of MB from the solution was found to be very high at alkaline pH. At this pH level, the catalytic surface attains negative charge and there by attracts positively charged dye molecule. Hence, the degradation efficiency of the surface increases in the alkaline pH range due to the increase in the formation of OH radicals. The chitosan adsorbs dye molecules, which continuously supplies discolored dye molecules.
. 3.6. Mineralization of Dyes
The chemical oxygen demand (COD) analysis had been used widely as an effective method to measure the organic strength of waste water. The test allows the measurement of waste in terms of the total quantity of oxygen required for the oxidation of organic matter to CO2 and water [30]. Herein, the result of COD was used to analyse the effectiveness of the prepared photocatalysis. In this method, the MB dye solution (50 mL) and 100 mg of CS-La-GR were taken in the reactor and exposed to UV light for 30 min. Comparison of the COD values of the initial dye concentration with irradiated solution indicates that the COD values is substantially reduced. The mineralization of MB dyes was measured as the decrease in COD by photocatalysis using CS-La-GR composite and it was found that about 77% decreases from its initial COD values. The lower toxicity of the products with photodegradation clearly indicates the higher potential of CS-La-GR in the photocatalytic process for the CS-La-GR elimination of dyes from waste water
3.7. Mechanism of Dye Degradation
When CS- La-GR was dispersed in the solution with dye solution, the electrons on the surface of chitosan (CS) and graphite should ultimately transfer to the dye, and when it is irradiated by UV light, electrons (e−) in the valence band (VB) can be excited to the conduction band (CB) which results the generation of equal number of holes in the VB. Also, the photoinduced holes can be obviously confined by OH− to produce hydroxyl radical species (OH•) further, which is an exceptionally strong oxidant for the partial or complete mineralization of organic pollutants [31]. The proposed reaction mechanism responsible for the degradation of the dye using CS-La-GR depicted as follows.
CS-La-GR + hν → CS-La-GR (eCB-) + CS-La-GR (hVB+)
CS-La-GR (hVB+) + Dye → Degradation Product
CS-La-GR (hVB+) + OH- → CS-La-GR + OH•
Dye + OH• → CO2 + H2O + Salts
It should be considered that the surface modification of chitosan and graphite with La3+ would provide an effective environment and it will increase the active sites at the surface for the dye−semiconductor interaction, which in turn increases the photodegradation process. Further, chitosan and graphite itself adsorb dye molecules and it will constantly supply to La3+ for degradation. This process also contributes for increasing the efficiency of La3+ significantly.
4. Conclusions
The use of integrated photocatalytic composite adsorbents (IPCA) in Photo-oxidative degradation seems to be a promising technology that has numerous applications in environmental purification systems. Herein, a comprehensive study had been carried out on photocatalytic degradation of dye such as MB using CS- La-GR composite as photocatalytic sorbent under UV irradiation. It was observed that contact time, pH and substrate concentration were influence significantly in the photocatalytic degradation of the dye investigated. The percentage adsorption of dye on CS- La-GR composite was seemed to be much low. However, irradiation with UV light produced significant photodegradation and the percentage of dye degradation was found to be 93.5%. The mineralization of the dye was further confirmed by COD analysis and was found that 77% decrease from initial COD value, indicated the very high mineralization of the MB dye. This high photocatalytic efficiency is essentially due to the synchronous role of chitosan, graphite and Li3+ present in CS- La-GR composite, exhibited both adsorption and photodegradation activities. The kinetic study was explained in terms of the Langmuir−Hinshelwood model. These findings suggested that CS- La-GR composite was a promising catalyst for dye adsorption and degradation in aqueous solution.
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