Reducing the use of nanotitanium dioxide by switching from single photocatalysis to combined photocatalysis-cavitation in dye elimination

Accepted Manuscript Title: Reducing the use of nanotitanium dioxide by switching from single photocatalysis to combined photocatalysis-cavitation in dye elimination Authors: Pooya Arbab, Bita Ayati, MohammadReza Ansari PII: DOI: Reference:

S0957-5820(18)30639-6 https://doi.org/10.1016/j.psep.2018.10.012 PSEP 1542

To appear in:

Process Safety and Environment Protection

Received date: Revised date: Accepted date:

7-8-2018 16-10-2018 16-10-2018

Please cite this article as: Arbab P, Ayati B, Ansari M, Reducing the use of nanotitanium dioxide by switching from single photocatalysis to combined photocatalysiscavitation in dye elimination, Process Safety and Environmental Protection (2018), https://doi.org/10.1016/j.psep.2018.10.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Reducing the use of nanotitanium dioxide by switching from single photocatalysis to combined photocatalysis-cavitation in dye elimination Pooya Arbab1, Bita Ayati2*, MohammadReza Ansari3

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1- Ph.D. Candidate of Environmental Civil Eng., Faculty of Civil and Environmental Eng., Tarbiat Modares University, e-mail: [email protected] Declarations of interest: none 2- Associate Prof., Environmental Civil Eng. Dept., Faculty of Civil and Environmental Eng., Tarbiat Modares University, e-mail: [email protected] (*Corresponding Author) P.O.Box: 14115-397, Tel: +98 8288 3328, Fax: +98 8288 4914 Declarations of interest: none 3- Associate Prof., Energy Conversion Dept., Faculty of Mechanical Eng., Tarbiat Modares University, e-mail: [email protected] Declarations of interest: none

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Graphical abstract

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Highlights

The elimination of Reactive Black 5 (RB5) was investigated through the photocatalytic process alone and a combination of photocatalysis and new form of cavitation (hydrodynamic cavitation) using nanotitanium dioxide as a photocatalyst.

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Hydrodynamic cavitation (HC) was generated by an orifice plate with a 7 mm hole diameter at the inlet pressure of 4 bars.

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The effect of hydrodynamic cavitation on photocatalysis was assessed in terms of reducing the use of nanomaterials.

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The cost (energy and nanophotocatalyst) of single photocatalysis and combined photocatalysis-cavitation was obtained for equal efficiency (the single process cost seven times more than the combined process).

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A comparison was performed with various other textile wastewater treatment methods.

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Abstract The present study was conducted to investigate the elimination of Reactive Black 5 (RB5) through the photocatalytic process alone and a combination of photocatalysis and cavitation using nanotitanium dioxide as a photocatalyst. To use a new form of cavitation, hydrodynamic cavitation (HC) was generated by an orifice plate with a 7 mm hole diameter at the inlet pressure of 4 bars. First, the photocatalytic process was optimized by changing the parameters of pH, nanotitanium dioxide concentration, irradiation power and dye concentration; then, by adding cavitation and producing combined photocatalysis-cavitation and with the same optimal conditions obtained in the previous stage, the amount of nanophotocatalyst was adjusted, and the amount of nanotitanium dioxide required to yield an efficiency equal to that of the single process was calculated. Finally, by measuring the amount of current consumed in these processes, the cost of nanotitanium dioxide and the electrical energy consumption, the total cost of both the single and the combined processes was estimated in the same efficiency. The results showed that using cavitation significantly reduces the amount of nanomaterials used from 100 to 8.4 mg/L and the total cost of the process to one-seventh, which suggests that cavitation is a very promising process.

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Keywords Reactive Black 5; Photocatalytic process; Hydrodynamic cavitation; Nanotitanium dioxide; Orifice plate; Cost

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1. Introduction

Organic dyes are highly non-degradable compounds that cause genetic problems, toxicity and carcinogenicity for humans, and their direct release into the environment adversely affects the natural photosynthesis cycle (Çalışkan et al. 2017; Franco et al. 2017; Gupta et al. 2016). Conventional methods of treatment, such as physical, chemical and biological processes, have failed in the decomposition and mineralization of organic materials and have merely transferred them to another phase that requires further separation and thus imposes additional operational costs (Bokhale et al. 2014; Joseph et al. 2011; Mosleh et al. 2016). Most of dyes are used in the textile industry and almost 20% of global water pollution is caused by these industries (Gul et al. 2016). The removal of organic dyes from wastewater has turned into a major research concern (Ferreira et al. 2016; Machado et al. 2016; Pavan et al. 2014; Yola et al. 2014). Among these dyes, Reactive Black 5 (RB5) is one of

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the most commonly used dyes for dyeing cotton, nylon, etc. in the textile industry. RB5 has a high stability against traditional treatment like that other synthetic dyes (El Bouraie and El Din 2016). Advanced oxidation processes (AOPs) are greatly emphasized nowadays for the full mineralization of organic materials (Luo et al. 2018a; Luo et al. 2017; Luo et al. 2018b; Thejaswini et al. 2017). These methods can effectively decompose dye molecules into water and carbon dioxide. Among them, heterogeneous photocatalysis using semiconductor metal oxide as the photocatalyst has become a more developed approach in recent years that can be effectively used in dye decomposition (Thejaswini et al. 2017). In the photocatalytic process, when the photocatalyst is irradiated, photon excitation causes the transfer of an electron from the valence band to the conduction band, thereby creating charge carriers (e- and h+) (Bamba et al. 2017; Liang et al. 2017; Mosleh and Rahimi 2017). By surfacing, these carriers can initiate redox reactions. Organic compounds adsorbed onto the photocatalyst surface react with reactive radicals or intermediate species 

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(such as  OH , h+, O 2 , HO2 , H2O2 and O2). It is currently accepted that pollutant decomposition takes place in the photocatalytic process in two ways, including the direct reaction of the pollutant molecules with the charge carrier pair, and the reaction of molecules with oxygen species or radicals emerged from the adsorbed oxygen and water reaction with the charge carrier pair (Bamba et al. 2017). Studies have referred to various semiconductors such as TiO 2, ZnO, Fe2O3, CdS and ZnS. Of this list, TiO2 has been widely used in heterogeneous photocatalytic systems due to well-known properties such as high chemical stability, good optical activity and better efficiency (Thejaswini et al. 2017). Photocatalysis with TiO2 is a promising method for the decomposition of organic pollutants (Wankhade Atul et al. 2013; Yang et al. 2008; Zhu et al. 2013). This process generates reactive radical species (mainly hydroxyl radicals) that are involved in the decomposition and mineralization of organic materials in wastewater (Wang et al. 2017). Nevertheless, photocatalytic oxidation is affected by the limitations in mass transfer and the gradual inactivation caused by the accumulation of pollutants and peripheral products on the catalyst surface, which can be a huge barrier to the larger-scale use of this technology in actual sewage treatment (Wang et al. 2017). To solve this problem, a number of researchers have tried adding cavitation to photocatalytic reaction systems for improving the optical efficiency of TiO2 (Eren 2012; Gogate 2008; Wang et al. 2011). Cavitation is a phenomenon characterized by the formation, development and explosion of vapor bubbles in a liquid medium. Ultrasonic and hydrodynamic methods are two simple approaches for producing cavitation (Wang et al. 2017). The ultrasonic technology has widely been studied for use in the decomposition of organic pollutants in water (Eren 2012; Merouani et al. 2010; Wei et al. 2017; Xiao et al. 2013; Xiao et al. 2014a; Xiao et al. 2014b). Ultrasonic irradiation, however, is not economically feasible in industrial activities due to the inefficient largescale distribution of cavitation activity and the inefficiency of electrical energy transmission to liquids (Kalumuck and Chahine 2000). Only recently, researchers have begun research on a new way of producing cavities called hydrodynamic cavitation and few studies have been carried out on this method (either singly or in combination) to decompose aqueous pollutants (Çalışkan et al. 2017; Rajoriya et al. 2017; Sayyaadi 2015; Wang et al. 2017). Hydrodynamic cavitation is an alternative technique that can overcome the problems of ultrasonic cavitation (Wang et al. 2017) and be used as an advanced oxidation method for the removal of chemicals from wastewater (Çalışkan et al. 2017). When liquid passes through constrictions such as orifice plate or venturi, the pressure at the constriction falls below the liquid vapor pressure, and the rapid passage of liquid creates a number of bubbles that subsequently collapse as pressure improves downstream. The collapse of bubbles during cavitation produces localized transient hot spots at a high pressure and temperature, which leads to the splitting of the volatile molecules of the pollutant and water and thereby the production of free radicals. In addition, the collapse of bubbles produces high-speed micro-jets and severe shockwaves, which may be a factor for surface cleansing or erosion, the fragmentation of solid particles and the increased rate of mass transfer (Wang et al. 2017). Hydrodynamic cavitation is a green technology for the decomposition or even mineralization of water pollutants that can be performed in normal conditions without the help of chemicals or external catalysts (Schmid 2010). The combination of this technology has been studied with other advanced oxidation processes such as H 2O2 (Joshi and Gogate 2012; Raut-Jadhav et al. 2013a), Fenton (Joshi and Gogate 2012; Pradhan and Gogate 2010) and ozonation (Wu et al. 2011). Nevertheless, there are very few reports on the combination of hydrodynamic cavitation and photocatalysis (Bagal and Gogate 2014; Wang et al. 2011). Table 1 presents a number of these studies.

In this study, first, the process of photocatalysis was optimized for the removal of RB5; then, the effect of hydrodynamic cavitation on photocatalysis was assessed in terms of reducing the use of nanomaterials and the 3

total cost. The reduction in nanophotocatalyst consumption was then measured and compared to the amount of consumption obtained with photocatalysis alone, and a cost analysis was carried out that took account of the costs of energy and nanophotocatalyst used. Finally, a comparison was performed with various other textile wastewater treatment methods. 2. Materials and Methods 2.1. Materials

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The reactive black 5 textile dye was procured from Nasaj Sabet Company (Iran) and used with no further purification. Table 2 and Fig.1 present the chemical properties and structure of this dye. The nanocatalyst used in the present study was the commercial Degussa P25 powder, which contains nanoparticles of titanium dioxide (TiO2) with anatase and rutile crystal structures at a 70:30 ratio. To achieve the required pH, the acid H 2SO4 and the base NaOH were procured from Merck Company (Germany). Deionized water was used in the preparation of all the solutions.

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2.2. System specifications

2.3. Devices

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Figures 2 and 3 present the actual and schematic plan of the system used. This closed-circuit setup consisted of a supply tank, a 1.6 KW pump, pressure gauges and valves. The tank used was made of stainless steel with a thermal jacket for temperature control. The capacity of the reactor was 8.25 liters. The end of the supply tank was connected to the pump suction part, and the pump outlet was divided into two branches, including a bypass flow and a cavitating device. In this study, cavitation was generated by an orifice plate with 7 mm hole diameter at the inlet pressure of 4 bars. The bypass branch was used to control current flow in the branch with the cavitating device, and control valves 1, 2 and 3 were located at the pump inlet, prior to the cavitating device and in the bypass branch. Pressure gauges 1 and 2 were installed before and after the cavitating device to control the inlet and downstream pressures. Also, for ultraviolet irradiation, four 15 W UV-C lamps (Philips Company) inside two quartz tubes installed next to the tank and the lamp illumination control circuit were used.

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A Hach spectrophotometer (DR-4000 model) for analyzing the RB5 solution using calibration at the maximum wavelength of 598 nm, a Metrohm pH meter (691 model) for measuring the samples’ pH, a Kern scale (PLS360-3 model) with a precision of 0.001 grams for weighing, a centrifuge (Sigma101 model) for the separation of the photocatalyst nanoparticles and a Hioki ammeter (3280-10 model) for recording the consumed current were used. A 1.6 KW pump (Pentax, Italy) and pressure gauges (Victory) were used in the pilot.

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2.4. Methods

In each test, 8.25 liters of the solution was prepared by adding RB5 dye, and was then poured into the tank after adjusting its pH through the addition of H2SO4 or NaOH and the required amount of nanoTiO2. After adjusting the lamps and turning on the pump, the inlet pressure was reached to a specific value through the bypass control valve (0 bar for photocatalysisalone and 4 bars for combined photocatalysis-cavitation). The solution temperature was kept constant at 30±2 °C using a thermal jacket. Each test lasted 120 minutes, with 20-minute sampling intervals. The decolorization rate was found using Eq. (1): ED(%) 

(Ci  Ct ) 100 Ci

(1)

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Where ED indicates the extent of decolorization in percentage, and Ci and Ct indicate the initial and instant concentration of RB5 during the process. In this study, photocatalysis was assessed first, and various parameters, including pH, photocatalyst concentration, irradiation power and dye concentration were tested as in Table 3 and the optimal values were obtained. Next, to assess the effect of cavitation on the amount of nanomaterials used and the total cost, hydrodynamic cavitation was used in combination with photocatalysis through an orifice plate with 7 mm hole diameter at the inlet pressure of 4 bars (based on the optimization performed in the previous study) .The tests were performed using the single factorial method (in each stage, one parameter was variable and the rest were kept constant). All the tests were repeated at least twice, and errors of the reported values were in the 5% range.

3. Results and Discussion 3.1. The photocatalytic system

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3.1.1. Determining the optimum pH

Fig. 4 shows the decolorization rate for different pH values after the completion of the test (120 minutes). As shown, efficiency increases as pH reduces and tends toward acidity, but decreases as pH increases and tends toward basicity. The highest efficiency (60.16%) was found at pH=3 and the lowest (45.99%) at pH=11. Hence, pH=3 was chosen as the optimum value of pH.

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This occurrence was due to the dependence of TiO2 surface charge on the environment pH. The point of zero charge (PZC) for TiO2 was between 5 and 7, and negative or positive surface charges were obtained at pH values higher or lower than this range, in respective order. Therefore, the increased efficiency of the process in acidic pH is due to the attraction between anionic dye RB5 and positively charged TiO 2, and the reduced efficiency in basic pH is caused by the repulsion between anionic dye and negatively charged TiO2 (Chong et al. 2015). In addition, the rate of recombination of  OH radicals is lower in acidic conditions, and consequently more radicals are available for RB5 oxidation (Gore et al. 2014). 3.1.2. Determining the optimum concentration of nanophotocatalyst

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Fig. 5 presents the decolorization rate at different nanophotocatalyst concentrations. Increasing the nanophotocatalyst concentration from 50 to 100 mg/L was associated with an increase in decolorization efficiency from 44.38% to 55.97%, but a further increase to 200 mg/L in the nanophotocatalyst concentration reduced the process efficiency to 53.21%. Therefore, 100 mg/L was taken as the optimum nanophotocatalyst concentration.

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The increased rate of decolorization is due to the increase in the amount of nanophotocatalyst surface available for the greater adsorption of dye molecules and the higher absorption of photon energy, which ultimately lead to the further formation of reactive radicals and dye decomposition (Thejaswini et al. 2017; Velmurugan et al. 2014). The reduced decolorization rate at higher photocatalyst concentration is due to the accumulation of particles and the over opacity of the medium, which leads to the dispersion and reduced penetration of light and subsequently a limited photocatalysis (Bamba et al. 2017; Thejaswini et al. 2017). 3.1.3. Determining the optimal irradiation intensity

Fig. 6 presents the decolorization rate over time at different levels of irradiation intensity (various watts of the radiation source). The decolorization rate increased from 21.95% to 36.72% and 55.97% as the source power increased from 15 to 30 and 60 W. Thus, 60 W was chosen as the optimal irradiation intensity. The efficiency of the dye photolysis improves with increasing the radiation intensity (Bendjabeur et al. 2018). In addition, at a high light intensity, the effective number of photons available for the photocatalyst 5

excitation and the production of an electron-cavity pair often increases, which leads to the further formation of reactive radical species and a subsequent increase in the decomposition rate (Liang et al. 2017; Thejaswini et al. 2016). 3.1.4. Determining the optimum dye concentration

Fig. 7 presents the decolorization rate at various initial concentrations of pollutants. The tests performed showed that the process efficiency decreases with an increase in the initial concentration of the dye. The increase in the initial concentration from 30 to 50 and 100 mg/L reduced the decolorization rate from 60.16% to 47.15% and 25.60%. Thus, the 30 mg/L concentration was chosen as the optimum value.

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The limited number of places on the surface of nanophotocatalyst particles can be a controlling factor in the photocatalytic decomposition process (Khaksar et al. 2017). With an increase in the concentration of pollutants, the photocatalyst surface absorbs a larger number of pollutant molecules and the need for oxidants (radicals) thus increases. Since the oxidant production rate is constant, the number of free radicals available for photocatalytic decomposition at higher concentrations of pollutants will be insufficient. Therefore, increasing the concentration of pollutants entails a reduced process efficiency (Bahnemann et al. 2007). The highest efficiency of photocatalysis (60.16%) was thus obtained at the following optimal conditions: Dye concentration=30 mg/L, nanophotocatalyst concentration=100 mg/L, pH=3 and irradiation power=60 W. 3.2. The equalization of combined photocatalysis-cavitation

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Next, cavitation was used in conjunction with photocatalysis, and tests were run with photocatalyst amounts from 100 to 0 mg/L, and the other parameters were allocated the same optimum values obtained in the previous stage. The results are shown in Fig. 8.

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As shown, the addition of cavitation to photocatalysis was associated with a significant increase in decolorization. For example, at a photocatalyst concentration of 100 mg/L, efficiency increased by 23% (from 60.16% in the single process to 83.17% in the combined process), and even in the case that did not use nanotitanium dioxide (i.e. combined photolysis-cavitation), efficiency was 52%, which is a significant rate. Given the decline in efficiency with a reduction in nanotitanium dioxide, the amount of nanophotocatalyst that yielded the same efficiency as the single process (60.16%) in the combination process was calculated as 8.4 mg/L. This occurrence can be attributed to the ability of hydrodynamic cavitation to continually prevent the accumulation of photocatalyst particles and refresh surfaces due to the shockwave activity generated by cavitation, which results in an increase in the effective catalyst surface and optical activity through the combination process (Chen and Smirniotis 2002; Gogate and Patil 2015; Wang et al. 2017). In addition, the radical species produced by hydrodynamic cavitation can also contribute to the photocatalytic reaction (Arrojo et al. 2007; Stock et al. 2000). A larger number of free radicals are produced in combined photocatalysis-hydrodynamic cavitation

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compared to single photocatalysis. In addition to the  OH radicals produced in photocatalytic reactions, hydrodynamic cavitation also provides strong oxidizing conditions by cavitation. The formation, development and subsequent collapse of cavities release high amounts of energy through cavitation that lead to the production of hydroxyl radicals. Moreover, a hydrodynamic cavitation reaction produces hydrogen peroxide, which splits into hydroxyl radicals under ultraviolet irradiation (Chakinala et al. 2009). It has also been argued that the

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oxidation of dye molecules is initiated by free radicals (  OH ), and thermal pyrolysis then occurs in the cavitation cavities (Behnajady et al. 2008; Saharan et al. 2011). 3.3. The cost of single and combined processes and a review of other cost estimation studies

Given the equivalent amount of TiO2 found in the combined process in the previous stage, and given the amount of electrical current used in both processes (measured through the ammeter), the cost of each of the two processes was calculated (Table 4). If the cost of one Kilowatt-Hour electricity consumed in Iran is assumed to be A, then the cost of one gram of nanotitanium dioxide powder in Iran will be 60 A (A=320 IRR in 2017-18).

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Therefore, the cost of single photocatalysis was obtained as 51.51 A and the cost of combined photocatalysis-cavitation as 7.08 A for equal efficiencies (the single process cost 7.3 times more than the combined process). Given the cost breakdown listed in the table, it is evident that the reduction in the cost of nanomaterials consumed is much more significant than the increase in the cost of energy consumed due to the use of cavitation, which reduces the total cost. The increase in the energy consumed is due to the inlet pressure of 4 bars exerted on the orifice plate in the combined process compared to the 0 bar pressure exerted in the single process, and the significant reduction in the amount of nanomaterial used is due to the concurrent addition of cavitation, which leads to the production of the same efficiency by using a less amount of photocatalyst. To date, researchers have mostly focused on qualitative interpretations and related scientific techniques, and there has been no quantitative cost analysis for pollutant control in textile industries for decision making purposes. The cost analysis of wastewater treatment is therefore an essential strategy. The methods used, the region and the type of industry in question are key factors affecting the cost of water pollution control (Rodrigues et al. 2014). There have been few cost analysis studies in this field so far (Holkar et al. 2016). Table 5 shows the cost estimates obtained in previous studies and the present research. Nevertheless, this table is merely an overview, since treatment conditions and pollutants are different. Evidently, the cost reported in the present study is higher than that reported in the other studies.

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Future studies need to focus on the cost analysis of more processes in textile wastewater treatment, such as advanced oxidation and combined and biological processes (Holkar et al. 2016). To lower the operational costs of combined approaches in the decomposition of organic materials in the textile industry, it is important to minimize the percentage of mineralization so as to reduce the energy and chemical consumption (Rodrigues et al. 2014; Vergili et al. 2012). For cost optimization, it is important to identify the most expensive parts of the processes first, and to then minimize or replace them with more effective strategies; for example, the reuse of photocatalyst in photocatalysis or the use of hydrogen peroxide present in bleaching wastewater. The cost of all these potential combinations (combined processes) and the toxicity of the intermediate materials produced through the treatments should be compared with each other. The combination with the lowest costs and toxicity should then be used for the treatment of dyes in textile wastewaters (Holkar et al. 2016).

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4. Conclusion

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Hydrodynamic cavitation is a modern and promising approach for dealing with resistant industrial pollutants. The benefits of this process include an easier large scale use due to the utilization of the reactive radicals produced and the decomposition of pollutants through them. Both photocatalytic and cavitation processes contain the same reactive radical production mechanism, and using hydrodynamic cavitation in combination with photocatalysis therefore increases the efficiency. Considering the substantial costs of producing and procuring nanophotocatalysts such as nanotitanium dioxide, this approach (combined photocatalysis and cavitation) leads to less consumption of nanomaterial and lower operational costs and is therefore cost-effective. At optimal conditions, photocatalysis yielded a decolorization rate of 60.16%. With the addition of cavitation in the form of combined photocatalysis-cavitation, the decolorization efficiency increased to 83.17%, and the nanophotocatalyst used in the combined process for yielding the same efficiency as the single process (60.16%) decreased from 100 to 8.4 mg/L. Switching from the single photocatalytic process to combined photocatalysis-cavitation reduced the operational costs to one-seventh of the initial costs. Given the lack of cost analysis studies, this issue should be further considered in wastewater treatment. The cost of all the potential combinations of different processes and the toxicity of the intermediate materials produced from the treatments should be assessed and used as a criterion for comparison and choosing the optimal system.

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cavitation with various advanced oxidation processes (AOPs) Journal of Environmental Chemical Engineering 1:850-857 Rodrigues CS, Madeira LM, Boaventura RA (2014) Synthetic textile dyeing wastewater treatment by integration of advanced oxidation and biological processes–performance analysis with costs reduction Journal of Environmental Chemical Engineering 2:1027-1039 Saharan VK, Badve MP, Pandit AB (2011) Degradation of Reactive Red 120 dye using hydrodynamic cavitation Chemical Engineering Journal 178:100-107 Sayyaadi H (2015) Enhanced cavitation–oxidation process of non-VOC aqueous solution using hydrodynamic cavitation reactor Chemical Engineering Journal 272:79-91 Schmid A (2010) MTBE degradation by hydrodynamic induced cavitation Water Science and Technology 61:2591-2594 Solmaz SKA, Birgül A, Üstün GE, Yonar T (2006) Colour and COD removal from textile effluent by coagulation and advanced oxidation processes Coloration Technology 122:102-109 Stock NL, Peller J, Vinodgopal K, Kamat PV (2000) Combinative sonolysis and photocatalysis for textile dye degradation Environmental science & technology 34:1747-1750 Thejaswini T, Prabhakaran D, Maheswari MA (2016) Soft synthesis of potassium co-doped Al–ZnO nanocomposites: a comprehensive study on their visible-light driven photocatalytic activity on dye degradation Journal of materials science 51:8187-8208 Thejaswini T, Prabhakaran D, Maheswari MA (2017) Ultrasound assisted synthesis of nano-rod embedded petal designed α-Bi2O3-ZnO nanoparticles and their ultra-responsive visible light induced photocatalytic properties Journal of Photochemistry and Photobiology A: Chemistry 335:217-229 Üstün GE, Solmaz SKA, Birgül A (2007) Regeneration of industrial district wastewater using a combination of Fenton process and ion exchange—A case study Resources, Conservation and Recycling 52:425-440 Vandevivere PC, Bianchi R, Verstraete W (1998) Treatment and reuse of wastewater from the textile wet‐processing industry: Review of emerging technologies Journal of Chemical Technology and Biotechnology 72:289-302 Velmurugan R, Krishnakumar B, Swaminathan M (2014) Synthesis of Pd co-doped nano-TiO2–SO42– and its synergetic effect on the solar photodegradation of Reactive Red 120 dye Materials Science in Semiconductor Processing 25:163-172 Vergili I, Kaya Y, Sen U, Gönder ZB, Aydiner C (2012) Techno-economic analysis of textile dye bath wastewater treatment by integrated membrane processes under the zero liquid discharge approach Resources, Conservation and Recycling 58:25-35 Wang X, Jia J, Wang Y (2011) Degradation of CI Reactive Red 2 through photocatalysis coupled with water jet cavitation Journal of hazardous materials 185:315-321 Wang X, Jia J, Wang Y (2017) Combination of photocatalysis with hydrodynamic cavitation for degradation of tetracycline Chemical Engineering Journal 315:274-282 Wankhade Atul V, Gaikwad G, Dhonde M, Khaty N, Thakare S (2013) Removal of organic pollutant from water by heterogenous photocatalysis: a review Res J Chem Environ 17:84e94 Wei Z, Villamena FA, Weavers LK (2017) Kinetics and mechanism of ultrasonic activation of persulfate: an in situ EPR spin trapping study Environmental Science & Technology 51:34103417 Wu Z, Franke M, Ondruschka B, Zhang Y, Ren Y, Braeutigam P, Wang W (2011) Enhanced effect of suction-cavitation on the ozonation of phenol Journal of hazardous materials 190:375-380 Xiao R, Diaz-Rivera D, He Z, Weavers LK (2013) Using pulsed wave ultrasound to evaluate the suitability of hydroxyl radical scavengers in sonochemical systems Ultrasonics sonochemistry 20:990-996 Xiao R, He Z, Diaz-Rivera D, Pee GY, Weavers LK (2014a) Sonochemical degradation of ciprofloxacin and ibuprofen in the presence of matrix organic compounds Ultrasonics sonochemistry 21:428-435 Xiao R, Wei Z, Chen D, Weavers LK (2014b) Kinetics and mechanism of sonochemical degradation of pharmaceuticals in municipal wastewater Environmental science & technology 48:96759683

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Yang L, Liya EY, Ray MB (2008) Degradation of paracetamol in aqueous solutions by TiO2 photocatalysis Water research 42:3480-3488 Yola ML, Eren T, Atar N, Wang S (2014) Adsorptive and photocatalytic removal of reactive dyes by silver nanoparticle-colemanite ore waste Chemical Engineering Journal 242:333-340 Zhu X-D, Wang Y-J, Sun R-J, Zhou D-M (2013) Photocatalytic degradation of tetracycline in aqueous solution by nanosized TiO2 Chemosphere 92:925-932

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Fig. 2- Image of experimental setup

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Fig. 1- RB5 chemical structure

Fig. 3- Schematic representation of the experimental setup (V1, V2, V3: Control Valves, V4: Sample or Waste Valve - G1, G2: Pressure Gauges)

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Fig. 4- Extent of dye removal for process at different levels of pH after 120 minutes (dye concentration=30 mg/L, TiO2 concentration=100 mg/L, power of irradiation=60 W)

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Fig. 5- Extent of dye removal for process at different levels of TiO2 concentration after 120 minutes (dye concentration=30 mg/L, power of irradiation=60 W, pH=8)

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Fig. 6- Extent of dye removal for process at different levels of irradiation power after 120 minutes (dye concentration=30 mg/L, TiO2 concentration=100 mg/L, pH=8)

Fig. 7- Extent of dye removal for process at different levels of initial concentration after 120 minutes (TiO2 concentration=100 mg/L, power of irradiation=60 W, pH=3) 13

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Fig. 8- Extent of dye removal for process at different levels of TiO2 concentration after 120 minutes (dye concentration=30 mg/L, power of irradiation=60 W, pH=3)

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Table 1- Recent researches on the combination of hydrodynamic cavitation and photocatalytic process

Subject of study

Conditions

Result

Reference

Degradation of Reactive Red 180 (RR180)

Concentration of RR180 = 100 mg/L ZnO dosage = 1000 mg/ L Reaction time = 90 min

Degradation efficiency was 95%

(Çalışkan et al. 2017)

Degradation of tetracycline

Concentration of tetracycline = 30 mg/L TiO2 dosage = 100 mg/ L Reaction time = 90 min

Degradation efficiency 78.2%

was

Degradation of C.I. Reactive Red 2 (RR2)

Concentration of RR2 = 80 mg/L TiO2 dosage = 100 mg/ L Reaction time = 90 min

Degradation efficiency 60.5%

was

Degradation of diclofenac sodium

Concentration of diclofenac sodium = 20 mg/L TiO2 dosage = 200 mg/ L Reaction time = 120 min

Degradation efficiency 79.4%

was

Degradation of imidacloprid

Concentration of imidacloprid = 25 mg/L Nb2O5 dosage = 200 mg/ L Reaction time = 120 min

Name

Reactive Black 5 (diazo) Remazol Black B

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Synonyms

C26H21N5Na4O19S6

Molecular weight

991.82 g. mol -1

C.I. number

C.I. 20505

C.B. number

CB0419426

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Molecular formula

Application class

Cotton

Chemical class

Azo

CAS registry number

17095-24-8

Natural pH

8

λmax

598 nm

Water solubility

82 g. L -1

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Degradation efficiency was 55.18 %

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Table 2- Characteristics of RB5

)Wang et al. 2017(

)Bagal and Gogate 2014(

)Raut-Jadhav et al. 2013b(

Table 3- Parameters investigated in the photocatalysis process Variable parameter And other constant conditions

Levels

pH

3, 4, 6, 8, 9, 11 30 100 60 50, 100, 200

Dye concentration (mg/L) Power of irradiation (W) pH

30 60 8

Power of irradiation (W)

15, 30, 60

Dye concentration (mg/L) Photocatalyst concentration (mg/L) pH

30 100 8

Dye concentration (mg/L)

30, 50, 100

Photocatalyst concentration (mg/L) Power of irradiation (W) pH

100 60 3

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Photocatalyst concentration (mg/L)

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Dye concentration (mg/L) Photocatalyst concentration (mg/L) Power of irradiation (W)

Table 4- Calculation the cost of two processes based on nanomaterial and energy consumption

Process

I (A)

E (KWh)

TiO2

Cost

Pump

Lamps

Pump

Lamps

Total

mg/L

gr

E

TiO2

Total

Photocatalysis

4.3

0.27

1.89

0.12

2.01

100

0.825

2.01 A

49.50 A

51.51 A

Cavitation + Photocatalysis

6.4

0.27

2.82

0.12

2.94

8.4

0.069

2.94 A

4.14 A

7.08 A

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I: Current consumption E: Energy consumption

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Table 5- Cost of textile wastewater treatment techniques

Process for treatment of textile wastewater

Treatment cost ($/m3) (the initial investment, sludge disposal cost and labor cost are excluded)

Fenton process followed by activated sludge

0.4 USD per m3

Fenton oxidation

0.59 USD per m3

Fe3+/H2O2

0.57 USD per m3

Reference

Complete decolorization

(Vandevivere et al. 1998)

95%

(Solmaz et al. 2006)

71%

(Solmaz et al. 2006)

4.94 USD per m3

97%

(Solmaz et al. 2006)

5.02 USD per m3

99%

(Solmaz et al. 2006)

3.5 USD per m3 (cost of sludge disposal 1.5 USD per m3)

98%

(Üstün et al. 2007)

Electrochemical process

0.66 USD per m3

98%

(Mook et al. 2017)

Hydrodynamic Cavitation + Photocatalysis

6.2 USD per m3

60%

This article

Ozonation

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Ozonation and H2O2 (peroxone)

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Color removal

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Fenton process followed by coagulation (polyaluminium chloride) followed by ion exchange process

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