Sustainable Synthesis and Enhanced Photocatalytic Activity of Titanium Oxide-Clay Nanocomposite Toward Persistent Polychlorinated Biphenyl Degradation in Real Samples

This study presents the green synthesis of TiO 2 -clay nanocomposite using an eco-friendly method based on carbohydrates. Glucose and soluble starch function as the bioreductant and stabilizer agent during the reaction, respectively. The UV-vis analysis ascertained the formation of TiO 2 -clay nanocomposite presenting a distinctive absorption peak at 360 nm and an optical band gap of 4.3 eV. TEM and XRD results indicated the anatase phase of spherical TiO 2 -clay with a median crystallite size of 18.36 nm. EDX and XRF techniques revealed the presence of titanium, oxygen peaks, and TiO 2 compound (58 wt%) along with the elemental composition of clay. The BET values of specic surface area, average pore diameter, and pore volumes are 47.48 m 2 g − 1 , 17.39 nm, 0.19 cm 3 g − 1 , respectively. The benign TiO 2 -clay nanocomposite demonstrated superior photocatalytic eciency toward photodegradation of poly polychlorinated biphenyl pollutant in industrial wastewater with a removal rate of 98.32%, greater than untreated TiO 2 NPs. 2 pure anatase appropriate elemental composition with average crystallite size of 18.36 nm. The TEM, SEM results demonstrate that produced clay-incorporated TiO 2 NPs show round-like shape in which distributed on the surface of plate layers of clay with a median particle size of 50.84 nm. BET surface area of nanocomposite was estimated to be around 47.48 m 2 g − 1 . In comparison other the highly reusable TiO 2 -caly nanocomposite superior photocatalytic eciency with removal rate of 97.45% toward PCB-52 All in batch polychlorinated The Afterward, the employing three (Philips, under In all tests, were cm above a photoreactor containing the After at different time, the is sampled and the photodegradation process of PCB-52 by repeatability, three trials of of PCB-52 were carried out for each experiment. The removal as Eq.


Introduction
Aquatic pollution is a global issue that quite often occurs when hazardous substances such as microorganisms or inorganic and organic chemicals contaminate the water supplies, deteriorating freshwater quality and making it detrimental to the health of ecosystem 1 . It is reported that exceeding 80 percent of the world's untreated e uent is ultimately returned to the environment, contaminating safe water bodies including lakes, rivers, oceans, and aquifers. It is estimated that more than 400 cubic kilometers of polluted water annually are drained into the world's surface waters. Whereas water shields 75% of the world's surface, the total volume of saltwater on the earth is 97.5%, and the residual 2.5% proportion of freshwater is most unattainable sources, storing as frozen or underground water. As a result, a quite less than one percent of the earth's fresh water quantity is readily available for drinkable purposes indicating that our water sources are utterly nite 2 . Moreover, as populations rise, in many countries, the amount of water accessible to each individual is dropping in which in turn would emerge water-scarce regions in the world. Hence, pollution intensi es the problem and human activities are main source of ruined and polluted water. Water contamination leads to an algal bloom in freshwaters or marine water systems and advent waterborne pathogens such as cholera, giardia, and typhoid diseases are an account of unsafe water 3,4 . Nowadays the major sources of water pollution are classi ed as agricultural, sewage and wastewater, oil, radioactive substances pollution which are a complex of heavy metals, poly aromatic hydrocarbons and other emerging impurities 5 .
Polychlorinated biphenyls (PCBs) are a broad group of synthetic halogenated organic compounds known as chlorinated hydrocarbons. Due to chemical stability, non ammability and high boiling point, PCBs are widely utilized for various commercial and industrial applications such as heat transfer, plastics, dyes, insulating and cooling uids, pigments 6,7 , to name but few. Environmentally, disposal PCBs are notorious persistent contaminants in aquatic ecosystem and highly likely maintain tendency to accumulate in the organic entities of the environmental medium where they are ascertained 8, 9 . Although PCBs can be found in water, soil, and air, they facilely accumulate in tissue and sediment, and are highly likely detected at the highest concentrations in seafood cycles 10,11 . Therefore, the initial encounter of human to PCBs exposure in the environment was resulted from consumption of food products predominantly sh, and animal-derived foods such as meat, dairy and also to a lesser degree, vegetable crops. Studies show that discharge and exposure to a high level of inadvertent PCBs in water, may cause a variety of adverse health effects predominantly multiple types of cancer risk in humans [12][13][14][15][16] , hence; USEPA has categorized PCBs as potential human carcinogens 12 . Several procedures have been developed for PCB detection and remediation including biological, chemical, physical and thermal methods [17][18][19] . Yet, the most conventional technologies used for PCBs removal are land lling or incineration 20 . In recent years, nanotechnology as emerging eld, has demonstrated as a reliable alternative technique in effective and economical mineralization of organic contamination such PCBs and poly aromatic hydrocarbons (PAHs) 21 . In literature numerous reports have been addressed using a versatile nano-scale materials for wastewater treatment 22,23 .
Nano-sized titanium oxide (TiO 2 )-clay composite is n-type semiconductor that has hold promising potential due to photochemical stability, non-toxicity, and low cost photocatalysts for e cient treatment of domestic and industrial wastewaters 24,25 . It is found that immobilization of optical TiO 2 nanoparticles (NPs) on clay mineral would improve recovery and photocatalytic activity of TiO 2 NPs through curbing the recombination of the photogenerated hole-electron pairs 26 . Meanwhile, clay mineral due to safety, layered structure, mechanical stability, and cation exchange capacity is considered as signi cant support material comparing to other immobilizer substrates such as quartz and stainless steel 27 . A variety of fabrication recipes including dip coating, boil deposition, sol-gel, hydrothermal, impregnation, and metal organic chemical vapor deposition have been addressed for clay-modi ed titania. One can be noted that most preparation methods contain unsafe chemicals in which may induce signi cant damage to aquatic systems increment toxicity and environmental risks 28 . As result, in this attempt, we have presented a facial green synthesis for eco-friendly TiO 2 -clay nanocomposite and explored for the photodecomposition of 2,2′,5,5′-Tetrachlorobiphenyl (PCB-52) as a typical representative of PCBs congener pollutants under UV irradiation in real aquatic environment.

XRD analysis
The result of XRD pattern of green TiO 2 -clay nanocomposite was illustrated in FTIR analysis FTIR is a powerful qualitative toll in which elucidate the surface chemistry of organic-decorated nanostructures presenting ngerprint regions of main functional groups in compounds and molecules as well. As it can be seen in Fig. 2, there are an array of 8 main peaks that appeared in FTIR spectrum of TiO 2 -caly nanocomposite sample. the presence three stretching bands between 3800 and 3600 cm -1 are likely assigned to OH groups of absorbed water molecules as well as water coordinated to magnesium or surface of clay structure 32 . The fairly wide peak appread at 3419 cm -1 is attributed to O-H stretching vibration, representing hydroxyl groups of glucose and starch hydrocarbons 33 . The vibrational motion of absorbed CO 2 was observed speci cally around 2355 cm -1 on a FTIR spectrum during the sample preparation process 34

BET characterization
The textural properties including the surface area, average pore volume, and average pore size of nanopowder were obtained using BET analytical method through the adsorption of nitrogen molecules on the solid surface. In comparison to raw clay, incorporating TiO 2 NPs in plate-like clay would remarkably enhance the values for BET surface area (S BET ), pore size and pore volume as twice time as raw clay (Table 1) 37 . These experimental results impart higher capability of TiO 2 -clay nanocomposite in environmental remediation through increment the site numbers for pollutants adsorption 38 Table 1 Texture properties of pristine clay and green TiO 2 -caly nanocomposite calcinated at 500 ˚C

TEM investigation
The images results of TEM analysis illustrated in Fig. 3a-c. It is clearly indicated the distribution of TiO 2 NPs on the layer surface of montmorillonite clay. The produced green nanoscale TiO 2 -clay powder hold spherical shape with median particle size of 50.84 nm obtained via TEM-histogram of 203 particles (Fig.  3d), con rming immobilization of TiO 2 NPs on clay structure.

SEM and EDX observations
The SEM micrographs of green TiO 2 -Clay nanocomposite are depicted in Fig. 4a-c. the results indicated that montmorillonite possess a stacked plate-like sheets in which substantially decorated by round form titania particles. In addition, the roughly homogeneous distribution of TiO 2 NPs on the surface of the clay is clearly observed indicating an average particle size value of 68 nm (Fig. 4c). The elemental composition of the surface in TiO 2 -Clay nanocomposite was further examined using SEM-EDX analysis.
The results revealed the detection of the main constituent elements of clay minerals such as oxygen, silicon, magnesium, aluminum, iron, small proportion of Na, K, Ca along with titanium showing proper incorporation of TiO 2 NPs as it can be seen in Fig. 4d and  Photocatalytic activity of green TiO 2 -clay nanocomposite The photocatalytic ability of TiO 2 impregnated montmorillonite nanocomposite was explored toward the PCB-52 contaminant in real sample provided from local petrochemical wastewater. In order to compare the removal e ciency, two other types of nanosorbents including TiO 2 NPs and clay nanoparticles were also investigated under identical conditions. The degradation process of PCB-52 organic pollutant was performed in dark as well as UV light condition that optimized at pH=7, contact time 120 min, 100 mg adsorbent, the temperature of 35°C, and 35 mL of contaminant solution with a concentration of 8 mg/mL (Figs. 1-4S). As it is observed in Table 3, all applied nanoabsorbents reveal higher removal e ciency in the presence of UV light than in the dark regime. The UV-light triggered photocatalytic activity of aforementioned absorbents trend is in a sequence: TiO 2 -clay TiO 2 NPs clay NPs (Fig. 5). Discernibly, TiO 2 -clay nanocomposite with an adsorption capacity of 37.65 mg/g demonstrates superior photocatalytic degradation against of persistent organic PCB-52 pollutant with a removal rate higher than 97%. It is presumed that loading nanosized titania on the surface of montmorillonite clay afford additional active surface sites, and therefore increase the photodegradation rate of targeted substance through the production of highly reactive intermediates such as the photogenerated holes, superoxide ions, hydroxyl radical, peroxide radicals [40][41][42] . On the other hand, the presence of clay as a immobilizer signi cantly enhances the decomposition process of selected environmentally detrimental material, curbing the agglomeration, and aggregation of clay-anchored TiO 2 NPs 38,43 TiO 2 -caly nanophotocatalyst recyclability The steadiness of nanocatalyst was examined by the catalytic cycle test. At the end of each cycle, the solution was ltered and the green TiO 2 -caly was assessed in the next cycle. In this study the produced green clay-incorporated titania semiconductor was subjected to six successive run and degradation rate of polychlorinated biphenyl was calculated as seen in Fig. 6. it is found that TiO 2 -clay nanocomposite remained its remarkable photocatalytic e ciency without signi cant change in its removal percentage. The effective immobilization of nano-sized titania possibly would improve its surface area, chemical stability and photocatalytic performance during photodegradation reaction in UV region supporting the previous ndings 28 and current results (see table 3).

Conclusion
We successfully developed a facile ecofriendly approach for synthesis of green and economical TiO 2 -caly nanocomposite using carbohydrate as reducing and stabilizer agent. The biofabricated clay-supported titania were systematically studied using an array of characterization tools and its e ciency was explored toward persistent an environmental PCB-52 pollutant. XRD, EDX and XRF indicated that TiO 2caly exhibit crystal structure with pure anatase phase and possess appropriate elemental composition with an average crystallite size of 18.36 nm. The TEM, SEM results demonstrate that produced clayincorporated TiO 2 NPs show round-like shape in which distributed on the surface of plate layers of clay with a median particle size of 50.84 nm. BET surface area of nanocomposite was estimated to be around 47.48 m 2 g − 1 . In comparison with other absorbents, the highly reusable TiO 2 -caly nanocomposite con rmed superior photocatalytic e ciency with removal rate of 97.45% toward environmental PCB-52 contaminate from aqueous solution due to more active sites and ample reactive oxygen species. Owing to the global concern of hazardous chemical impact, we believe that the photocatalytic process based on biogenic approaches paves the way for a safe aquatic environment, yet, quite a lot of attempts are demanding in improving the performance of environmental photocatalysis pathways.

Raw Materials and reagents
All chemical materials were purchased from Sigma-Aldrich company. Glucose and soluble starch reagents were utilized as green template. Titanium butoxide (Ti(OBu) 4  Subsequently, the resultant is gradually impregnated with milky titanium solution and magnetically stirred for 1h to achieve a complete dissolution. In order to attain an effective reaction between the reactants, the obtained suspension was further sonicated for 120 mins. After centrifuging at 10,000 rpm for 10 min, the TiO 2 − clay sample was ltered and then wet powder heated at 120°C for 2h to remove absorbed water and other impurities to a large extent. Lastly, the product was heated to 500°C for 2h in air at a rate of 5°C/min.

Photocatalytic degradation experiment
The photocatalytic performance of the TiO 2 -clay nanocomposite against polychlorinated biphenyl was investigated at optimized conditions. All photodegradation experiments were conducted in a batch reactor. In a typical process, 100 mg of green nanocomposite was dispersed in 35 mL of 8 mg/L polychlorinated biphenyl solution. The suspension was constantly stirred in the dark for 20 min to acquire the adsorption-desorption equilibrium. Afterward, the mixture was subjected to ultraviolet light irradiation employing three 6-watt low-pressure UV lamps (Philips, λ = 365 nm) under vigorous stirring (500 rpm) at temperature 25°C. In all tests, UV lamps were maintained 10 cm above a Pyrex photoreactor containing the polluted water. After illumination at different time, the suspension is sampled and the photodegradation process of PCB-52 was monitored by UV/Vis absorption spectra at 208 nm. To ensure the repeatability, three trials of degradation of PCB-52 were carried out for each experiment. The removal percentage (%R) of PCB-52 as function of time is presented as Eq. 1: Where C o is the initial concentration of PCB-52, C t is the concentration of PCB-52 at certain reaction time t (min).

Physicochemical characterization of clay-supported TiO 2 nanocomposite
To determine the properties of green synthesized TiO 2 -clay nanocomposite, XRD (X'Pert PRO MPD, Panalytical Co., Netherlands), UV-Vis (Analytik-Jena AG, Germany), TGA (PerkinElmer, Pyris 1, USA), FTIR (BRUKER Alpha-Germany), and TEM (LEO912-AB, Omega) techniques were operated. The average crystallite size of nanocomposite was estimated using the Debye-Scherrer equation (Eq. 1), where d is the diameter of crystallite size, k is the shape factor with the value 0.9, λ is the wavelength of X-ray radiation (0.154 nm), β is the full width at half the maximum intensity (FWHM), and θ is the Bragg angle.

Declarations
Author contributions F. Buazar designed, supervised, and Writing -the original draft of this study. The synthetic experiments of nanoparticles and the relevant photocatalytic tests were accomplished by J. Chanani. Data analyses and related results of experiments were investigated with the aid of Y. Nikpour. All authors contributed to the writing and editing of the paper.

Con ict of interest
The authors declare no competing interests.