Removal of organic dye compounds in water and wastewater samples based on covalent organic frameworks -titanium dioxide before analysis by UV-VIS spectroscopy

Received 24 Nov 2020 Revised form 5 Feb 2021 Accepted 25 Feb 2021 Available online 30 Mar 2021 *Corresponding Author: Mehdi Ranjbar Email: Mehdi.Ranjbar@kmu.ac.ir https://doi.org/10.24200/amecj.v4.i01.131 ------------------------


Introduction
Nanomaterials are particles that are in the size range between 1-100 nm. The importance of nanomaterials in terms of strength is the presence of active sites and their low density. Nanomaterials have a wide range of applications in optical data storage, sensors, light and durable building materials, and wastewater treatment. The gained importance of the conventional heating methods (CHM) is due to the microwaves interact with the reactants at the molecular level. By CHM, the electromagnetic energy is transferred and converted to heat by rapid kinetics through the motion of the molecules [1][2][3]. Today, the conservation of water resources has been increasingly considered by various international organizations such as WHO, FDA and EPA. By increasing of population growth as a result of over-exploitation of limited water resources and on the other hand, water pollution due to various biological, agricultural and industrial activities caused to water crisis in future years. Methylene organic compounds such as methyl red, methyl orange and methyl blue are the photoactive phenothiazine dyes [4][5][6]. Paints are one of the main and most important pollutants that are used in various industries to dye related products. Therefore, a significant amount of pollution caused by pigments is produced due to their extensive release into the effluent. The presence of these dyes in water is inappropriate even at very low concentrations and causes widespread environmental pollution by pharmaceutical industries [7]. Recently, many methods based on nanostructure materials was used for removal organic compounds. Hence, a simple and rapid microwave assisted combustion technique was used for synthesis of CdO nanospheres for removal pollution in waters [6]. Microwave assisted reverse microemulsion process as an easy, low cost and fast method can be used for synthesis of nano emulsions [8]. Photocatalytic degradation of compounds using nanoparticles with ultraviolet light is one of the advanced oxidation methods that is expanding in recent decades [9,10]. Photocatalyst as a catalyst activated in the presence of light is which absorbs light to produce a chemical reaction in the environment [11,12]. When the UV rays reach to a surface covered with a photocatalyst the electroncavity can react with molecules on the surface of the particles [13]. Bio photocatalytic materials [14] as a kind of photocatalyst was used in water purification based on degradation and adsorption process [15], during the adsorption process, solute molecules are removed from the solution and adsorbed by the adsorbent. Most of the molecules are adsorbed on the surface of the adsorbent pores and small extent on the outer surface of the particles. The adsorption transfer from the solution to the adsorbent continues until the concentration of the solvent remaining in the solution is in equilibrium with the concentration of the solvent adsorbed by the adsorbent. When equilibrium is established, the adsorption transfer stops. Adsorption equilibrium is established in the dynamic sense when the rate of adsorption of the adsorbed component on the surface is equal to its rate of absorption [16], self-cleaning glasses and organic molecules degradation [17]. Hydrothermal synthesis method has many advantage such as energy storage [18], simplicity [19], low cost [20], better nuclear control [21], pollution-free (because reaction takes place indoors) [22], better diffusivity [23], high reaction speed and better shape control [24]. In recent years, many nanocomposites used for degradation or adsorption of organic dyes [25][26][27]. The unique structure of the non-ionic surfactant which their unique structure enables them to encapsulate water-soluble and water-repellent materials [28,29] and organic building constituent units [30]. One of the new techniques in removing emerging pollutants is the use of environmentally friendly modifiers and reducing the bioavailability of pollutants in the environment [31]. In recent years, bio-charcoal has been gradually approbated which as black gold can be used to solve the problem of sustainable development of agriculture and other aspects by the academic community [32], [33], [34], [35]. In this study, the COFs synthesized and used as a novel adsorbent for removal dyes based on SDA-SPE procedure in water samples after semi-degradation of organic dye by titanium dioxide (TiO 2 ) nanoparticales in presence of UV radiation. The recovery and absorption capacity of dyes with COFs/TiO 2 were calculated before determined by UV-VIS spectrometer.

Instrumental
Varian UV-VIS spectrophotometer was used for this study (Cary 50, USA). UV-VIS spectrophotometer included dual beam, the monochromator, the wavelength ranges between 190-1100 nm, the spectral bandwidth (1.5 nm), the dual Si diode detectors, the quartz over coated optics based on scan rates of 24000 nm min -1 and computer operating system. The Power Consumption of UV-VIS spectrophotometer has supply of 100 -240 volts AC and frequency 50 -60 Hz. The condition of UV-VIS spectrophotometers was shown in Table 1. The unique optical design enables to measure dye samples, also, the large or odd-shaped samples to be measured. The highly focused beam also provides superior coupling to fiber optics caused to use the UV-Vis in different matrixes. Powder X-ray diffraction (XRD) was prepared by a PRO X-ray diffractometer. Scanning electron microscopy images were obtained using gun design using a point-source cathode of tungsten (SEM, Philips XL 30 FEG). The transmission electron microscope was used for preparation particle size of COFs (TEM, Philips EM 300).

Synthesis of COFs
Covalent organic frameworks (COFs) has two/ three dimensional structures(2D,3D) which was generated by reactions between organic precursors. The covalent bonds depended on porous and crystalline form. The COFs is an improvements of organic material based on coordination chemistry. We synthesis COFs was done based on the Yaghi method and COFs framework scaffolds were prepared by the boronate linkages using solvothermal synthetic methods [36][37]. In fact, the synthesis of COFs was obtained by condensation reactions of C 6 H 4 [B(OH) 2 ] 2 with C 18 H 6 (OH) 6 and finally the carbon structure of C 9 H 4 BO 2 (COF 5 ) produced. Moreover, the two nozzles electrospinning was used to fabricate the scaffolds. The electrospinning experimental setup was a nano model (Tehran, Iran) with two nozzles. The voltage applied at the tip of the needle was 18 kV. The mass flow rates were 0.5 ml h -1 , and distance between the tip of the needle and the collector was maintained at 15 cm. The speed of the rotary collector was 400 rpm and scanning distance was 10 cm. Experimental conditions for the preparation of self-assembled carbon nanostructures was shown in Table 2.

Removal procedure
For each experiment, a dye solution with a concentration of 5 mg per 1000 mL (5 mg L -1 ) was prepared and 0.5 g of COFs as catalyst was added to it in presence of TIO 2 /UV. Then the pH of the solution was adjusted with buffer solution between 3-9 and irradiated with UV radiation. Then, the all samples, was stirred with a magnetic stirrer for 15 min. By SDA-SPE procedure, the amount of dye Recovery (R%) = (C I -C F )/C I ×100 (EQ.1)

Structural Analysis
By full width of the half maximum (FWHM) based on the Debay-Scherer equation, the particle size of COFs nanocomposites was obtained approximately 50-100 nm. Figure 1 shows the X-ray diffraction analysis (XRD) of self-assembled carbon nanostructures in different conditions. The results show that by increasing of temperature based on solvothermal method from 180•C to 220•C, the crystallized amount of synthesized carbon nanostructures (COFs) gradually increases. The sharper peaks without any noisy peaks was seen. Interestingly, the position (2θ) of the peaks have not changed, and only the intensity of the peaks increased, which indicates an increase in the degree of purity in products.

Measurement size (DLS)
By increasing of temperature for COFs nanocomposites with solvothermal method, the sintering process and crystallization of COFs was increased and caused to smaller size of nano structures. DLS analysis was displayed in Figure 2.

Morphological and pore characteristics
Microscopic morphology and particle-size distribution (PSD) of the products were visualized by scanning electron microscopy (SEM) images. The

Photocatalytic semi-degradation/adsorption and mechanism
The Figure 4 demonstrate the photocatalytic semidegradation of methylene red, methylene blue and methylene orange organic dyes using COFs/TiO 2 modified with biochar in time cycles (0 min-40 min). The results show that the percent degradation /adsorption of MO is more than 99 % which can be depended on the low steric hindrance of MO structure. Also amount the percent degradation/ adsorption of MR is a little more than MB which can also be related to low steric hindrance of MR structure ratio to MB. By reducing of time contact of photocatalyst with organic dyes, the extraction recovery of dyes decreased up to 40-54 % by SDA-SPE procedure. TiO 2 a highly efficient photocatalyst has a small band gap energy and relatively positive valance band edge under UV-light irradiation. TiO 2 caused to photogenerated electron-hole pairs and the transfer of the charge carriers. The charge carriers transfered to the surface of the TiO 2 and react with O 2 , H 2 O, or OH groups and generated radicals. The decomposition of Dyes was initiated by the attack of •OH on the methyl group of the benzene ring of dye, leading to make a new compound. On the other hand, the presence of COFs can enhance the adsorption ability of the Dye via π-π stacking interaction between the π-electrons of the benzene rings in dye molecules and π-electron rich region of COFs nanostructure. The effect of pH on the photocatalytic degradation/ adsorption of MB, MR and MO organic dyes were revealed in Figure 5. As results the pH don't effected on extraction dyes on COFs/TiO2 based on UV radiation by SDA-SPE procedure. By procedure, π-π stacking interaction between the π-electrons of the benzene rings in dye molecules and π-electron rich region of COFs nanostructure caused to efficient adsorption after semi degradation process.
Removal of dye organic compounds by COFs Aida Bahadori et al

Kinetic study
Adsorbents in nano sizes of COFs have been shown to have better adsorption performance compared to other materials due to their high specific surface area, small size, and lack of internal penetration resistance. The most widespread models of surface adsorption isotherms for discontinuous systems are the Langmuir and Freundlich models. By the proposed procedure the R 2 for COFs/TiO 2 was achieved about 0.9988 and 0.9575 for the Langmuir and Freundlich models. So, the Langmuir model selected and used for this work. Most nanosorbents follow the Langmuir adsorption isotherm model, which can be evidence of the homogeneity of their surfaces and the fragmentation of the adsorption process. Quasi-first-order and quasi-second-order synthetic models were well used to describe the dye adsorption process on the surface of nanosorbents. Large amounts of adsorption rate constants in both models can indicate rapid adsorption of dye molecules. After desorption, the nanosorbent can be used for several cycles (23 N).

Validation of method
The validation of SDA-SPE method based on COFs and COFs/TiO 2 in present of UV irradiation is important to evaluate the correct statistical results for extraction dye from waters. In SDA-SPE method, 5 mg L -1 of dyes was made and used for extraction dyes from water samples and validated by spiking of standard solution to real samples (Table 3).

Conclusions
In this study, COFs and COFs-TiO 2 as a novel sorbent based on UV radiation was used for dye removal/extraction from water samples by SDA-SPE procedure. Based on results, the applied, fast, sensitive method based on COFs/TiO 2 /UV was demonstrated. The semi degradation/adsorption The results showed us, the activity and reaction of COFs/TiO 2 /UV were more than COFs, primary carbon structure and TiO 2 in optimized conditions. Therefore, the fast, simple and efficient method based on SDA-SPE was used for removal of dye from waters by nanotechnology.

Acknowledgments
Authors are grateful to council of Chemistry and Pharmaceutical Sciences and Cosmetic Products Research Center, Kerman University of Medical Sciences, Kerman, Iran with grant number 98000651.