In this study a bio-based polymers namely cellulose/SnO2 was used for the degradation of colored wastewater from a textile industry containing congo red dye. The physicochemical properties of the cellulose/SnO2 nanocomposite were investigated by SEM, HRTEM, TEM, XPS and XRD. SEM results showed that the surface of cellulose/ SnO2 nanocomposite morphology has smooth structure. XRD peaks exhibited a Casserite structure of SnO2. XPS analysis illustrated the presence of SnO2-coated cellulose films. TEM images of cellulose /SnO2 nanocomposite exhibited a uniform spherical shape of SnO2 nanoparticles without any aggregation.  For maximum photodegradation yields of congo red dye (99%) the optimized conditions should be as follows: congo red dye concentration 400 mg/l, cellulose/SnO2 nanocomposite   concentration 1.5 mg/l, photodegradation time 40 min, UV light power 40 W/m2, pH =5,00 temperature 40 Oc, dissolved oxygen concentration 3 mg/l and ion concentration of 0,05 mg/l NaCl, KNO3 and NaHCO3). 

Due to extensively global industrialization textile, paper, leather industries utilize different organic dyes ending with excessive  toxic emissions to the ecosystem. 43–53% of these organic  in wastewater [1-2] Moreover, the presence of these organic dyes affect  negatively the photosynthesis and some  health problems increased in humans [3-4]. It is important  treat the colored wastewaters before discharged to the receiving environments. Various chemical and physical methods, including coagulation, adsorption, membrane filtration, reverse osmosis, and photocatalysis, was used to remove organic dyes from wastewater [5-6]. Among these processes, photocatalysis has emerged as an efficient method to treat the pollutants from wastewaters. This eco-friendly, cost-effective, low-energy, and sustainable treatment approach was found to be effective in the removal of color from textile industry  wastewaters. Due to the activity of photoinduced holes and the reduction capability of electrons, a series of photocatalytic processes  can convert macromolecular organic contaminants like dyes  into simpler and less hazardous molecular compounds.

Congo red  is a toxic anionic azo dye. Because of its attraction to cellulose fibers, it has many industrial applications, including the wide use in the textile industry. Furthermore  it is a pH indicator and is used to diagnose amyloidosis. Congo red  also metabolizes to benzidine wich is a carcinogenic to humans. it cause respiratory problems and toxicities in skin, and gastrointestinal discomfort[7-8]. Congo dye is a sodium salt and gives red colour on the applied cotton. As on addition of acid, its colour changes so it is also not used as a dye generally. Mostly it is used as an indicator. It is blue in acidic solution (below pH 3) and red in solutions (above pH 3). The change in colour from red to blue in the acidic solution is due to the resonance among charged canonical structures[9].

Among nano metal oxides,  SnO2 NPs can be used as photocatalyst to photodegrade large amount of dye stuff like congo red, methyl red and  azo dye namely  a sodium salt of 3, 3′-((biphenyl)-4,4′-diylbis(azo))-bis(4-amino-1-naphthalenesulphonicacid[9]. This metal oxide  composed of both Sn4+ and Sn2+.   In recent studies some  semiconductors like  CeO2 and SnO2 are extensively used. SnO2 is transparent in the visible ultraviolet region of the solar spectrum, at a wavelength of  350 nm. SnO2 was used in can be utilized in the decomposition of toxic organics in wastewater [10]. It is used alone  and was not combined with other nanocomposites  like TiO2, ZnO and MgO. Sn2+  was not utilized as photocatalyst  since   cause to oxygen vacancies during photodegradation. Since its utilization is as powder  some difficulties was detected in the settling of solid phase.  In order tp prevent this phenomenon  this nano metal oxide was doped to a   porous materials,  to generate fibers. Nonetheless, the polymer based films lack the high surface area, necessary for a highly reactive system to be used in photocatalytic applications[10]. 

Cellulose-based materials, due to their large abundance, ease in use, availability, low cost and physicochemical characteristics with the particular structures are widely used and are efficient as compared to other adsorbent materials, i.e., commercial activated carbon which is much more expensive. Various types of biomass including algae, fungi, yeast and bacteria have provided efficient adsorbents for wastewater treatment, i.e., many enzymes are available for mineralization as well as degradation of the dyes released from textile industries into water bodies . The desired characteristics of photocatalysts  are better to meet by the cellulose-based  as photodegradation yield for removal of  methylene blue, Drimarine Yellow HF-3GL and  Malachite green[11-12].

Formation of SnO2 NPs on porous polymer substrates would be a promising route towards the exploration of the possibility to use this semiconductor based solid system for water treatment applications. The solid-state SnO2 photocatalysts would offer the advantage of mechanical flexibility, manageability, but also would not require complicated steps for their recovery after the remediation process, minimizing the possibility of secondary pollution. It is low cost photocatalysts and  recyclable[13].

Cellulose is a natural polysaccharide, synthesized from the green plants and also by some microorganisms. The mixing of  cellulose with SnO2  may provide an economic and environmentally-friendly solution for photodegradation and decomposition  of some organic chemicals. Cellulose  is chosen since its origin is biologic with low cost and easy utilizable [14-15]. 

Therefore, in this  study the SnO2 NPs  doped to cellulose to product porous polymer substrates as a low cost  to decolorization of textile wastewaters containing Congo dye. The effects of some operational conditions( nanocomposite concentration, congo dye concentration, Ph, temperature, photodegradation time , sun light intensity, dissolved oxygen concentration and the concentration of some ions) on  the phptpdegradation congo dye was researched.

Characterization of cellulose/SnO2 nanocomposite

The physicochemical properties of cellulose/SnO2 nanocomposite was performed with XRD-analysis (Advance Powder X-ray diffractometer, Germany), FT-IR (Alpha T Bruker), TEM (JEOL 2100, EDAX (Bruker, Germany) and.  Photo Degradation of textile industry wastewater.

The Congo red containing textile industry wastewater samples was photodegraded in a Photoreactor at 365 nm having a volüme of 5 liter and illuminated with sun ligth at different sun ligth powers. The photodegradation tests were performed by recording the decrease of the characteristic absorption peak at the corresponding max every 30 minutes. The ?gradation of the dye was calculated using the following equation: (%) = ????0−(????) ????0 × 100%.

Where C0 is the standard concentration of each dye before irradiation and C(t) is the concentration of the dye solutions at time t during the photocatalytic process

Measurement of color in the samples

At certain time  intervals the color in the  samples were measured in UV–vis spectroscopy( Agilent, USA).

Preparation of cellulose/SnO2 nanocomposite

Cellulose was dissolved in 40 mL of acetone containing dimethyl form amide mixed at a ratio of 8 to 2 and stirred at 70°C. Then  Subsequently, 0.227 g of SnPrec was dissolved in 3 mL of the same solvent mixture upon stirring at 45°C and 400 rpm for 30 minutes. SnPrec was transferred to SnO2 crystalline. The mixture were stirred and decanted.. The obtained Cellulose/SnPrec) was dried under vacuum for 15 hours then it  maintained in an incubator at 124°C for 96 hours.

Physicochemical analysis results

SEM analysis results in cellulose/ SnO2 nanocomposite

The structure of cellulose  did not vary  after doping of  SnPrec. The surface of cellulose/ SnO2 nanocomposite morphology exhibited a  smooth structure. The size of cellulose was measured as  1.00 mm, while the size of  cellulose/SnO2 nanocomposite was measured as 0.9 mm. This can be explained  by a non significant  change of the viscosity of the aforementioned nanocomposite(14, 17). Figure 1a exhibited the SEM picture of cellulose, while Figure 2a  and 2b show the cellulose /SnO2 nanocomposite at two different magnifications.

                 a (magnification 500 x)                                                 b (magnification 500 x)                                                              c (magnification 1200 x)

Figure 1: SEM morphology of cellulose at 500 x magnification(a), cellulose /SnO2 nanocomposite at 500x magnification (b) and (c) cellulose /SnO2 nanocomposite at 1200x magnification.

XRD analysis results in cellulose/ SnO2 nanocomposite

The XRD analysis of the cellulose/SnO2 nanocomposite exhibited the crystal structure of SnO2 nanoparticle (Figure 2). Cellulose /SnPrec composite exhibited an is amorphous shape. The peaks exhibited a Casserite structure of SnO2 with maximum disturbances at (114), (013), (024), (124), (224), (134), (224) and (114).  In all disturbances, the maximal peaks of the cellulose were as follows: the carbonaceous links were located at 1740 cm-1 while the methyl links of the acetate  located at 1219 cm-1 and 1423 cm-1 ,respectively. The -CH2 moieties was found at 1439 cm-1, while bounds of  -C-O-C- located at 1045 cm.-1.

Figure 2: XRD analysis of the cellulose/SnO2 nanocomposite, SnO2 nanoparticle and Cellulose /SnPrec XPS analysis results for cellulose/ SnO2 nanocomposite.

XPS analysis is performed in order to elucidate the oxidation state of the Sn in SnO2 and to characterize the cellulose/ SnO2coating. Figure 3 illustrated the presence of SnO2-coated cellulose films. During disturbances, Sn3d5/2and Sn3d3/2peaks were detected at 489.2 and 496.8 eV, respectively. The binding energy of 493.1 eV exhibited similar data to the values for SnO2(492.3 and 486.6 eV) and higher than those of Sn metal(496.8, 489.7 and 489.2 eV). The atoms in the SnO2 coating were positively charged and generates bonds with oxygen(14-15). The O1s spectrum exhibited a maximal peak at 589.9 eV.

Figure 3: XPS analysis results for moieties of cellulose/ SnO2 nanocomposite

Specific surface areas for cellulose/ SnO2 nanocomposite

The Brunauer–Emmett–Teller Specific surface areas and pore volume of cellulose /SnO2 nanocomposite were measured as 149.9 m2 g−1 and 0.203 cm3 g−1, respectively. The hysteresis slope of the nanocomposite  found as 18 nm. This shows that the presence of mesopores and has micropores (12,15).

Nitrogen adsorption desorption isotems of cellulose/ SnO2 nanocomposite.

The addition of SnO2 nano metal oxide increased the size of Cellulose/SnO2 nanocomposite to 123 m2 g−1. The mesopores structure will shorten the  bonding of congo dye during photocatalytic degradation( Figure 4).

Figure 4: Nitrogen adsorption and desorption isotems of cellulose/ SnO2 nanocomposite

TEM and HRTEM analysis results of cellulose/ SnO2 nanocomposite

TEM and high-resolution TEM (HRTEM) analysis results of cellulose/ SnO2 nanocomposite is displayed in  Figure 6a and 6b. TEM images of  cellulose /SnO2  nanocomposite exhibited an uniform structure of SnO2 nanoparticles without generation of  enlarged aggregation(13-15). The nanocomposite has a  spherical shape corresponding to the pictures found in SEM images. The HRTEM image (Figure 6b) indicates the  SnO2 nanoparticles in a crystal shape corresponding to the 114 plane of the  SnO2. The C in the cellulose   shows bound  of SnO2 into carbonaceous materials during photocatalysis.

A	B

Figure 6: TEM (a) and HRTEM pictures of cellulose/ SnO2 nanocomposite

Effect of initial congo red concentration on the photodegradation yield of congo red dye

The initial concentration of congo RED dye affect significantly the photocatalytic. In this study the photodegradation percentage of congo dye  decreased from 99% to 74% as the congo dye concentration was increased from 50 mg/l to 600 mg/l. As the congo dye concentration increases, more organic substances are adsorbed on the surface of  cellulose/ SnO2 nanocomposite. As a result, low number of photons present to reach the cellulose/ SnO2 nanocomposite surface and therefore low •OH radicals were generated  ending with low decolorization  for congo dye. The photodegradation of  congo red elevated at low congo dye concentrations.  Table 1 shows the congo dye decolorization yields versus congo red concentrations at 1.5 mg/L cellulose/ SnO2 nanocomposite

Table 1: Effect of initial concentrationof congo red dye on congo dye photodegradation yield.

Effect of cellulose/ SnO2 nanocomposite concentration on congo dye photodegradation yield 

In this study the cellulose/ SnO2 nanocomposite concentration was increased from 0.5 mg/l up to 4 mg/l to detect the optimal nanocomposite concentration on maximal congo dye photodegradation yields. The maximum congo red photodegradation efficiency was detected at 1.5 mg/l nanocomposite concentration (Table 2). The photodegradation of congo dye increases with increasing nanocomposite amount up a certain concentration. The optimum nanocomposite amount elevated the number of active sites on the nanocomposite surface ending with  an increase in the concentration of •OH radicals. This cause to decolorization congo dye in textile industry wastewater.  After a certain dose of cellulose/ SnO2 nanocomposite concentration, the treated textile wastewater becomes turbid and thus inhibits the penetration of  sun light radiation  necessary for photocatalysis.

Table 2: Effect of  cellulose/ SnO2 nanocomposite concentration on congo dye photodegradation yield.

Effect of pH on congo dye photodegradation yield 

The variation of pH of the surface of  cellulose/ SnO2 nanocomposite charge of  nanocomposite and provides the photocatalytic reactions. Under acidic or alkaline condition the surface of cellulose/ SnO2 nanocomposite can be protonated or deprotonated. The cellulose/ SnO2 nanocomposite surface  charged positively  in acidic medium and negatively charged in alkaline medium.  It was known that cellulose/ SnO2 nanocomposite have higher oxidizing activity at lower pH, but excess H+ can lowered  the photodegradation efficiency. The maximal photodegradation yield was detected at pH=5 as 99% (Table 3). The  anionic dye exhibited strong Lewis base and can easily adsorb on the positively charged  cellulose/ SnO2 nanocomposite surface. This elevated  the adsorption and photodegradation  of the congo dye under acidic conditions. The congo dye was not adsorbed on nanocomposite surface under alkaline conditions due to  competitive photodegradation  by OH radicals. On the other hand, the surface of the cellulose/ SnO2 nanocomposite   is positively charged below isoelectric point and carries a negative charge above it. The Photodegradation of congo dye was  found to be maximum at 5pH because congo dye is the cationic and adsorbed and photodegraded  easily on the surface of  nanocomposite.

Table 3: Effect of pH on congo dye photodegradation yield.

The extent of dye adsorption depends on the initial dye concentration, nature of the dye, surface area of photocatalyst and pH of the solution. The pH determines the surface charge of the photocatalyst. 

Effect of temperature on the photodegradation yield of congo red dye

An increase in temperature during photodegradation ending with elevated photodegradation yields however temperature >60°C inhibits the charge carriers between valence band and conduction band and  the photodegradation efficiencies can be lowered. In this study the maximal photodegradation yield for congo dye was detected at 40 oC ( Table  4) . A temperature below 60°C favors the photodegradation yields ending  an increase in the activation of OH radicals .The temperature range between 20-50°C was found to be favorable  for ultimate photodegradation. At elevated temperatures, the photodegradation yields increased while at lowest temperatures the photodegradation  yields decreased.

Table 4: Effect of temperature on congo dye photodegradation yield

Effects of some anions (NaCl, KNO3 and NaHCO3) on the photodegradation yield of congo dye

In order to detect the effects of some anions   on photodegradation of congo dye, trace concentrations of NaCl, KNO3 and NaHCO3 were added.   At low ion concentrations, the congo red photodegradation yields did not varied.  After 0.2 mg/l ion concentration the congo dye yield decreased to 80%. High ion concentrations decrease the colloidal stability, increases the mass transfer and  decrease the surface contact between congo red  and the nanocomposite. A fouling problem in the nanocomposite surface inhibits the OH radical  generations.

Table 5: Effect of some ions on congo dye photodegradation yield.

Effects of light power on the photodegradation yield of congo dye

Although the photodegradation yield increased with increase in   light intensity, elevated sun light power did not affect significantly the color removal efficiency.  In this study the maximum congo red removals was detected at a light power of 40 W/m2 (Table 6). At low light intensity due to increasing of electron–hole formation the electron–hole recombination increased resulting in increasing of photodegradation yields. When light intensity is increased to 50 and 60 W/m2, the electron–hole pair separation competes and a non-significant  effect was detected for photodegradation yields of congo dye.

Table 6: Effect Light power on congo dye photodegradation yield.

Effect of photodegradation time on congo dye photodegradation yield

In this study the maximum congo dye photodegradation yield was detected after 30 min irradiation time (Table 7). The photodegradation yield decreases  at high irradiation times since a competition was observed  between the reactant and the intermediate products. The slow dye photodegradation   after 30 min is due to the reaction of intermediates of dye and OH radicals. As a result, long photodegradation times decrease the activation of  dye molecules.

Table 7: Effect of photodegradation time on congo dye photodegradation yield.

Effect of dissolved oxygen on congo dye photodegradation yield

In order to detect the effect of dissolved oxygen on the congo dye decolorization yields the dissolved oxygen concentrations were increased from 1 up to 5 mg/l. The photodegradation yields for congo dye were measured maximal at 3 mg/l dissolved oxygen concentrations (Table 8). Further increase od dissolve oxygen dose did not affect significantly the photodegradation yields. Dissolved oxygen is used as an electron acceptor during photodegradation for activated conduction band electron from redox  reactions. A certain amount of dissolved oxygen stabilize of radical intermediates Furthermore can cleave the aromatic rings in dye molecule.

Table 8: Effect of dissolved oxygen on congo dye photodegradation yield.

The present study assess the photocatalytic performance of the cellulose/ SnO2 nanocomposite versus congo dye present in textile industry wastewaters. The effect of pH, nanocomposite dosage, initial conge dye concentration, photodegradation time, temperature and sun light power  on the photodegradation of congo dye was examined. A  maximum  congo dye photodegradation of 99% could be attained at the optimum pH of 5, cellulose/ SnO2 nanocomposite  dose of 1.5 mg/l,  congo dye concentration of  400 mg/l, a light intensity of 40 W/m2, a  photodegradation time of 40 min , 3 mg/l dissolved oxygen concentration and NaCl, KNO3 and NaHCO3 concentrations of 0,05 mg/l. The present study suggests that the prepared cellulose/ SnO2 nanocomposite could prove to be a viable photocatalyst for the treatment of dye-contaminated textile wastewaters.