Continuous Tubular Photoreactor with TiO₂ Nanoparticles Catalyst for Batik Industrial Wastewater Treatment

Abstract

Like many other textile industries, batik requires a lot of water, and its use of different dyes and other chemicals accounts for most of the water pollution (17–20%). It can reduce light penetration, which in turn may decrease photosynthetic activity, and it may also have an impact on the quantity of oxygen available for the biodegradation of aquatic microbes. Many dyes, including diazo (-N〓N-), direct, and basic dyes, are challenging to work with using standard techniques, and the byproducts may occasionally be quite hazardous. Photocatalysis with various semiconductor materials has been the focus of investigations in recent years for the wastewater treatment application. Among many semiconductors, because of its strong photocatalytic activity, outstanding stability, lack of toxicity, and affordability, TiO₂ is still the most extensively used photocatalyst. When a catalyst (TiO₂) is exposed to ultraviolet (UV) light, hydroxyl radicals (•OH) are produced in the UV/TiO₂ photocatalytic oxidation process. Organic materials therefore mineralize into CO₂, H₂O, and inorganic components. In this chapter the application of continuous tubular photoreactor with TiO₂ nanoparticles as a catalyst for Batik wastewater treatment is reviewed.

1. Introduction

Synthetic dyes are organic compounds that are widely used in various industries such as the textile industry. One form of environmental contamination that produces a lot of wastewater from the dyeing process is heavy metals and organic pollutants. If this wastewater is not adequately treated, it could pose a hazard to the ecosystem [1]. Dye wastewater tends to cause eutrophication by lowering the amount of dissolved oxygen in the water body and is difficult for nature to break down [2, 3].

Various treatment methods have been developed such as physical, biological, and chemical treatments. Physical methods such as adsorption and sedimentation have widely been used, as well as chemical methods such as coagulation and flocculation. The current method of treating wastewater has not been able to fully degrade the pollutants into less harmful molecules; instead, it only transfers the toxins to another medium. In the meantime, total breakdown can be achieved through biological treatment using microorganisms, but this requires a longer process and specific handling due to their extreme sensitivity to environmental factors [4]. Advanced oxidation process (AOP) such as photocatalysis is one of the technologies developed to degrade organic pollutants such as dyes. The process involves semiconductor material as catalyst and photon to excite an electron and holes that are responsible for the photodegradation. TiO₂ is a semiconductor that can be used as a catalyst in the photocatalysis process [5] with advantages of low price, non-toxicity, high chemical stability, and good response to light [6].

Research on photocatalysis in degrading dyes organic pollutants has been massively conducted globally with various type of dyes. The batik business in Indonesia is a sort of textile sector that has grown rapidly at the household or small- to medium-sized enterprise (SME) level, producing vast amounts of wastewater that contains dyes and is ill-equipped with appropriate wastewater treatment technologies. In order to meet the demand for wastewater treatment methods that are both sustainable and environmentally benign in order to attain zero waste in the batik industry, this chapter reports on the development of a continuous tube photoreactor using TiO₂ nanoparticles as a catalyst.

2. Batik wastewater characteristics

A historical artifact created by the Indonesian people, batik is a blend of artistic expression with modern technology. It comes in a variety of colors and patterns. Before creating a gorgeous batik fabric, the pattern is drawn with wax onto the mori cloth. Subsequently, the fabric is submerged in a dye solution. Finally, the fabric is washed multiple times to achieve an appealing hue. Massive-scale batik production generates a substantial volume and quality of wastewater during the processing period. Synthetic dyes are utilized in the production of widely appreciated batik. The wastewater that is generated conceals the risks inherent in batik fabric. Batik wastewater contains dyes, wax, paraffin, resin, and other chemical components [5, 6]. Synthetic dyes are divided into three categories: cationic, anionic, and non-ionic dyes.

Charge is the primary distinction between cationic and anionic dyes. The molecules of cationic dyes are positively charged, whereas those of anionic dyes are negatively charged. Both cationic and anionic dyes are water-soluble. Cationic dyes include Crystal violet, Methylene blue, Basic blue, and Basic red [7, 8]. Methyl orange, Remazol, Alizarin Yellow R, and Eriochrome black T (EBT) are anionic dyes [9]. Remazol dyes are categorized as reactive azo dyes because it forms an ester bond with the fiber when they bind to it [10]. The structures of various dyes can be found in Table 1. Chromophores and auxochromes are the basic components in dye molecules. The colors that are visible are the result of chromophores. Auxochromes, nevertheless, act to enhance the affinity for the fibers and facilitate the solubility of the chromophore in water [11]. Batik wastewater contains a high concentration of electrolytes, which makes it difficult for light to penetrate [10], particularly solar irradiation, a natural degrading agent. Moreover, the presence of batik dyes in aquatic environments can give rise to a multitude of adverse effects, such as respiratory issues, cancer, allergies, and ecological imbalances.

Table 1.

Chemical structure of dyes.

3. TiO₂ Photocatalysis for organic degradation

The term “photocatalysis” was used for the first time in 1911 to explain the effect of the illumination of ZnO on the bleaching of Prussian blue. The ZnO acts like a catalyst, which increases chemical reaction rate without getting consumed during the reaction process. Under ultraviolet (UV) light, Fujishima and Honda observed the phenomena of photocatalytic water splitting on a TiO₂ electrode [15], opening a new world of possibilities to produce clean hydrogen.

The term “photocatalysis” refers to the use of a catalyst to speed up a photoreaction. Among these is photosensitization, a process in which an initial absorption of radiation by a different molecular entity known as the photosensitizer causes a photochemical modification in one molecular entity [16]. Depending on the particular photoreaction, the catalyst may interact with a principal photoproduct, the substrate in either its excited or ground state, or both to speed up the reaction. The schematic of photocatalytic processes is displayed in Figure 1. After the catalyst C absorbs the light, the sensitized photoreaction can happen in two ways: (i) through energy transfer by converting the reactant of interest, A, into an activated state that can be oxidized more readily than its ground state; and (ii) via electron transfer, by acting either as an electron donor or acceptor [17].

Figure 1.

The schematic of different photocatalytic pathway, C-catalyst, A-reactant (adapted from [17]).

The advantage of photocatalysis over traditional or current technologies [18] are as follows: (1) with the ability to use renewable and pollution-free solar energy, photocatalysis offers a good substitute for the energy-intensive conventional treatment procedures (adsorption, ultrafiltration, reverse osmosis, coagulation by chemical agents, and ion exchange on synthetic adsorbent resins); (2) unlike traditional treatments that move contaminants from one phase to another, photocatalysis produces harmless products; (3) a range of dangerous substances in various wastewater streams can be destroyed via the photocatalytic method; (4) photocatalysis requires little chemical input, mild reaction conditions, and a short reaction time; and (5) it can be used with hydrogen and produces little secondary waste.

As it offers exceptional advantages, for the past 40 years, the development of photocatalysis technology has progressed rapidly in term of materials and application. Furthermore, it will become very attractive as the process can be widely used in various applications such as degradation of organic contaminants in water, removal of metallic and other inorganic pollutants, recovery of valuable metals and chemicals, water disinfections and bacterial removal, purification and disinfection of air, hydrogen generation, functional materials including self-cleaning and anti-reflective material, and new synthetic routes for chemical processes [19].

4. Photocatalysis for degradation of organic pollutant

Nowadays almost all human activity discharges a wide variety of contaminants into the environment. There are approximately 30,000 chemicals commercially available with less knowledge of their occurrence and fate in the environment. Not to mention newly emerging micro-pollutants (e.g., pharmaceuticals, antibiotics, and pathogens) that are now easily being detected in the environment. The biggest challenge is the industrial dyes and textile dyes, which have been a long-term environmental threat. During the production process, 10–20% of all dye products produced worldwide are lost in textile wastes and discharged into aquatic bodies as effluents. Organic dyes are one of the leading groups of pollutants released into wastewaters from textile and other industrial processes [18].

Advanced oxidation processes (AOPs) such as ozonolysis, ozone/H₂O₂ peroxide process, UV/ozone, UV/H₂O₂, Fe²⁺/H₂O₂ Fenton process, irradiation with electrons, vacuum ultraviolet and ultrasonic, photocatalysis, and combinations of the stated methods have been extensively explored to mitigate a great variety of pollutants present in various environmental media. Numerous studies have focused on photocatalysis using different semiconductor materials in recent years. Hydroxyl radicals (•OH) are produced during the photocatalytic oxidation process when the catalyst is exposed to light. Organic materials therefore mineralize into CO₂, H₂O, and inorganic components [20].

Photocatalytic reaction primarily depends on photon energy and the response of catalysts. Semiconducting materials are characterized by a filled valence band, a vacant conduction band, and a band gap between two bands.

If a semiconducting material acts as a catalyst, it will perform as charge sensitizers by the irradiation of light and stimulate the redox process. The fundamental steps in the process of semiconductor photocatalysis are shown in Figure 2. An electron from the valence band is excited to the conduction band when light strikes the photocatalyst surface and the energy of the incident light is equal to or greater than the photocatalyst bandgap energy. This results in an excess of negative charge in the conduction band, e−, and an electron deficiency or hole, h+, in the valence band. They can participate in redox reactions and are equivalents of oxidizing and reducing agents, respectively. Any organic species that is adsorbed on the semiconductor’s surface could undergo sequential oxidation and reduction processes with these holes and electrons to produce the required products. [21]. Photocatalysts such as TiO₂, ZnO [22], Fe₂O₃ [23], CdS [24], WO₃ [25], Cu₂O [26], and ZnS [27] have been used to effectively break down a variety of organic contaminants. TiO₂ is the most often utilized photocatalyst among those semiconductors because of its strong photocatalytic activity, high stability, non-toxicity, and affordability when employed in the wastewater treatment industry [28].

Figure 2.

The schematic of semiconductor photocatalytic mechanism.

In order to achieve greater activity and outperform TiO₂-based photocatalysts, new photocatalytic materials with unique compositions and structures are being thoroughly investigated. But there aren’t many examples of successful practical applications. From the technical and economical point of view, alternative photocatalytic materials that are as “good” as TiO₂ are hard to find. One of the most viable and practical approaches in developing better photocatalysts remains the modification of TiO₂ in various methods (e.g., impurity doping, sensitization, surface modification, integration with other nanostructured materials) [29].

5. Basic structures and properties of TiO₂ semiconductor

Since the water splitting breakthrough using semiconductor reported by Fujishima and Honda in 1972 [15], titanium dioxide (TiO₂) has been thoroughly studied as a potentially useful material in a variety of study domains, particularly those pertaining to the environment and energy [30]. TiO₂ is a transition metal oxide semiconductor with three distinct polymorph crystalline structures, namely rutile, anatase, and brookite. Each structure has distinctive parameters, which contribute to the different physical and chemical properties. The size and crystal structure of the particles determine how stable a certain phase of TiO₂ is. The most stable phase for particles larger than 35 nm is rutile. For nanoparticles smaller than 11 nm, the most stable phase is anatase. When it comes to microparticles in the 1135 nm range, brookite is most stable. Bulk rutile, anatase, and brookite have band gaps of 3.0, 3.2, and 3.14 eV, which implied its photon absorption capability in the wavelength smaller than 431, 387, and 395 nm, respectively [31]. It has been discovered that the anatase form offers the best properties for photocatalytic oxidation. When compared with rutile TiO₂, the anatase has greater photocatalytic activity. One argument that could apply is the fact that anatase shows a smaller indirect band gap than direct band gap. Indirect band gap semiconductors typically have longer charge carrier lifetimes than do direct gap semiconductors [32].

6. Photocatalytic oxidation using TiO₂

In order to initiate the inter-band transition of electrons between the lowest unoccupied molecule orbital (LUMO) and the highest occupied molecule orbital (HOMO), the photocatalytic reaction needs a minimum photon energy that is greater than TiO₂’s band gap. The photo-excited electrons are de-excited via multiple pathways following irradiation: (i) they recombine with holes on the semiconductor surfaces; (ii) they recombine with holes in the bulk volume; (iii) they are transferred to the absorbed organic/trapped or inorganic redox species (typically oxygen molecules in the aerated solution); and (iv) the holes migrate to the surfaces and receive electrons from the absorbed redox species via oxidation reactions. TiO₂ is a semiconductor with a wide bandgap (3.0–3.2 eV) that requires the excitation light wavelength range shorter than ca. 400 nm (Eg = hc/λ ≈ 1240/λ) [29].

The overall photocatalytic process including the reduction-oxidation with its surrounding component can be divided into the following steps and described further in Eqs. (1)–(12) [29]:

$$Ti{\text{O}}_2+hv\to Ti{\text{O}}_2\left({\text{e}}_{CB}{}^{-}+{\text{h}}_{VB}{}^{+}\right)$$E1

$$Ti{\text{O}}_2\left({\text{h}}_{VB}{}^{+}\right)+{\text{H}}_2\text{O}\to Ti{\text{O}}_2+{\text{H}}^{+}+OH\cdot$$E2

$$Ti{\text{O}}_2\left({\text{h}}_{VB}{}^{+}\right)+O{H}^{-}\to Ti{\text{O}}_2+OH\cdot$$E3

$$Ti{\text{O}}_2\left({\text{e}}_{CB}{}^{-}\right)+{\text{O}}_2\to Ti{\text{O}}_2+{\text{O}}_2{\cdot }^{-}$$E4

$${\text{O}}_2{\cdot }^{-}+{\text{H}}^{+}\to H{O}_2\cdot$$E5

$$H{O}_2\cdot +H{O}_2\cdot \to {\text{H}}_2{\text{O}}_2+{\text{O}}_2$$E6

$$Ti{\text{O}}_2\left({\text{e}}_{CB}{}^{-}\right)+{\text{H}}_2{\text{O}}_2\to OH\cdot +O{H}^{-}$$E7

$${\text{H}}_2{\text{O}}_2+{\text{O}}_2{\cdot }^{-}\to OH\cdot +O{H}^{-}+{\text{O}}_2$$E8

$${\text{H}}_2{\text{O}}_2+hv\to 2OH\cdot$$E9

$$\text{Organic compound}+OH•\to \text{degradation products}$$E10

$$\text{Organic compound}+Ti{O}_2\left({\text{h}}_{VB}{}^{+}\right)\to \text{oxidation products}$$E11

$$\text{Organic compound}+Ti{O}_2\left({\text{e}}_{CB}{}^{-}\right)\to \text{reduction products}$$E12

A semiconductor’s surface undergoes photocatalytic processes when photons are absorbed, which can lead to the breakdown of organic molecules or the splitting of water. Because of its ability to gather photons, it can be used to degrade organic molecules, remove contaminants from a variety of media, and oxidize or reduce materials to generate hydrogen and hydrocarbons.

7. Photoreactor for TiO₂ photocatalytic process

The performance of TiO₂ photocatalytic activity is commonly measured by the reaction rate of the degradation process. In general, the degradation of photocatalysis proceeds according to pseudo first-order reaction kinetics. Prefixing the order of a reaction with “pseudo-” has become commonplace since, in actuality, one of the reactants in catalytic systems functions as a catalyst while maintaining its concentration constant. In a similar vein, when the substrate is present in considerable excess, it completely covers the catalyst surface, and its disappearance becomes mostly concentration dependent. A pseudo zero order photocatalytic process will be seen with such saturation coverage [33]. Because the concentration of adsorption in the dark is so small, the Langmuir-Hinshelwood model is frequently employed to describe and analyze reaction kinetics.

Both organic and oxygen molecules must participate in the photocatalytic destruction of organic contaminants. Since oxygen is usually in high concentration, its concentration during photocatalysis can be thought of as constant. The photocatalytic rate, as per the law of mass-action, can be represented as [34]:

$$r=-\frac{dC\left(t\right)}{dt}=kC\left(t\right)$$E13

where k is the rate constant, and C(t) is the concentration of reactants that adsorb on the semiconductor surface at time t. Photocatalysis requires the adsorption of organic materials on the semiconductor surface. It is believed that the adsorption-desorption (A–D) equilibrium complies with a Langmuir isotherm. However, in typical photocatalytic circumstances, the organic concentration is typically quite low, therefore the Eq. (13) can be simplified to:

$$\ln \frac{C_0}{C_t}=kt$$E14

where C₀ is initial concentration of organic substances before photocatalysis, and C_t is the reactive concentration at time t during the photocatalytic reaction. Eq. (14) can be easily used to calculate the k of different photocatalytic reactions.

Numerous factors and operational parameters affect the efficiency of photocatalytic processes, particularly when it comes to organic degradation. After a thorough analysis of the key factors, it was determined that the initial reactant concentration, light wavelength, feed flow rate, irradiation time, and pH had decreasing effects on process performance, while the increase in dissolved oxygen, light intensity distribution, photocatalyst loading to some extent, and air flow rate had increasing effects [32]. The execution of TiO₂ photocatalysis technology in the wastewater treatment application requires more research and development especially in the process and kinetics aspects of the photocatalysis itself. Several types of photocatalytic reactors have been proposed and investigated for the last 20 years, which can mainly be divided into two categories according to the catalyst form: (i) TiO₂ immobilized on support materials, e.g., quartz sand, glass, glass wool matrix, ceramic or polymer membranes, noble metal etc., and (ii) TiO₂ suspended in aqueous medium [32]. Even though it has a high operating cost and necessitates additional treatment after the degradation process, the TiO₂ suspended reactor has a higher photocatalytic area to reactor volume ratio, higher mass transfer and degradation efficiency, fairly uniform catalyst distribution, and an adjustable amount of nanoparticle suspension in the reactor to deal with different compositions of treated solution. In the meantime, TiO₂ immobilized reactors do not require catalyst separation or recycling, and their degrading efficiency is lower than that of suspension mode. Additionally, they cannot have their catalyst loading adjusted to address [35].

Several different reactors were designed such as novel tube light reactor, multiple tube light reactor, fluidized bed photoreactor, fixed bed photocatalytic reactor, batch photoreactor, slurry continuous flow reactor, hexagonal annular photoreactor; spinning disk photoreactors; solar photocatalytic reactor; annular photocatalytic reactor, and tubular photocatalytic reactor, etc. Some of them are designed with the suspended type of catalyst and the other using immobilized catalyst within the reactor [32]. From the technical application point of view the immobilized TiO₂ is preferred because, following the photocatalytic process, it does not require any additional post-treatment for the recovery of catalyst particles. On the other hand, films containing immobilized TiO₂ powders had issues with the catalyst particles’ detachment. Furthermore, a portion of the porous structure is lost through the sintering process during the heating procedure that fixes the photocatalyst. They result in inefficient mass transfer by decreasing the photocatalyst’s area-to-volume ratio. As a result, there appears to be a factor of 10 difference in the efficiency of reactors with immobilized photocatalysts compared to those with dispersed TiO₂ particles (slurry) [32].

The overview of the photoreactor with the optimized parameters is summarized in Table 2. The photocatalytic activity of TiO₂ is a multifaceted function that relies on the physical characteristics of the particles. Types of dye’s component to degrade are more profound to be the driving force of the optimum degradation rather than the design of the photocatalytic reactor.

Photoreactor System	Optimal Condition	Photodegradation Performance	Ref.
Characteristics: a pilot-scale falling film photoreactor (FFR) in slurry mode has a surface area of 60x120 cm and is tilted at 35° (local latitude) Light Source: genuine solar light (35 W) Scale: 25 L	Pollutant: 10 ppm Catalyst: 100 ppm Light Intensity: 1.1 mW/cm^2 pH: 2 (2–10) Oxidating Agent: 10 mM H2O2 Stirring/Flow rate: 30 L/h	Pollutant: Methylene Blue Catalyst: Pure TiO2, and 1 ^wt.% Ag/TiO2 (by the simple chemical reduction) Efficiency: 99%	[36]
Characteristics: double-walled horizontal glass reactor (DHGR) Scale: 1–6 L Light Source: UV lamps (400 W)	Pollutant: 560 mg/L Catalyst: n.a pH: 3	Pollutant: Remazol Red RGB Catalyst: Pure TiO2 and 3 mol% Ag–TiO2 (by the modified sol–gel method) Efficiency: 94.29% (COD) and 98.44% (Color), for the immobilized catalyst	[37]
Characteristics: continuous type of reactor which is named CS4PR and built using a glass vessel Scale: 500 mL Light Source: UV Lamp (Philips TUV 36 W/G 36 T8)	The ratio of catalyst, C sulphate ion & V sample 9: 500 ppm: 500 mL/5 mg.L of RhB pH: 6.5–7.5 Stirring/Flow rate: 2–10 mL/s	Pollutant: Rhodamine B (RhB) Catalyst: TiO2/NO3- film coated ceramic Efficiency: 100%	[38]
Characteristics: homemade reactor Scale: 4 L Light Source: sunlight irradiation	Pollutant: 10 ppm Catalyst: 1 g/L	Pollutant: Methylene Blue Methyl Orange Rhodamine B Catalyst: TiO2 Degussa P25 (anatase: rutile = 3:1) Efficiency: 85%; 49% and 96%	[39]

Table 2.

Previous research on the development of photocatalytic reactor.

8. Continuous tubular photoreactor with TiO₂ nanoparticles catalyst for batik industrial wastewater treatment

8.1 Continuous tubular photoreactor prototype

A continuous tubular photoreactor prototype has been developed as shown in Figure 3. The system consists of five photoreactor tubes that are designed to allow fluid to flow either in parallel or in series depending on the ball valve that is mounted on the pipe. The box panel is a place where all the buttons to turn on or turn off system parts are stored. The frame serves as a structural support for all the equipment and is made from hollow pipe of ST 42 steel. The frame’s primary dimensions are 1200 × 365 × 1700 mm (l × w × h), and it has four roller wheels at the bottom to facilitate system mobility. The feed tank with dimensions of 600 × 300 × 300 mm (w × w × h) is made of 304 stainless steel plate with a thickness of 1 mm. It also has a glass cover on the front to make it easier to watch during the process. Polyvinyl chloride (PVC) pipes Sch 40 with a 1/2-inch diameter are used in the fluid flow system. These pipes are connected using elbow pipes at the ends and tee pipes at each intersection with the same diameter and thickness. Two primary valves are positioned at the end of the flow output and close to the pump. There are also sixteen ball valves to control the flow whether operating in series or parallel. A flowmeter tube with an 18 L/min capacity is mounted on the system’s top side.

Figure 3.

Continuous tubular photoreactor system.

The tubular photoreactor is made of 304 stainless steel, 90 cm in length and 3/4 inch in diameter with capacity 12 gallons per minute. Inside it is fitted with a UV light with a power of 40 Watt, which is shielded by a glass sleeve of 85 cm. When the main valve and pump are turned on, the solution will flow into the photoreactor. The solution flow rate is controlled with a flowmeter.

The lower the flow rate, the longer the flow will stay in the photoreactor, and the higher the flow rate, the faster the flow will exit the photoreactor. If the system is run in series, the solution will enter photoreactor 1 and photocatalytic activity will occur in the tube. Once the solution exits photoreactor 1, it proceeds to photoreactor 2 for the same procedure, and so on, until the solution reaches photoreactor 5. This system also can run in parallel. The solution will enter simultaneously into photoreactors 1, 2, 3, 4, and 5. Photocatalytic activity also occurs simultaneously in each tube photoreactor.

8.2 Continuous tubular photoreactor performance

Methylene blue solution as a representation of batik waste was run in series on a continuous tubular photoreactor with flow rate and catalyst loading variation. The initial concentration was 10 ppm, and to make sure that photocatalysis was the only factor contributing to the degradation process, the adsorption was performed for 30 minutes before the photocatalytic process began. Photocatalysis is carried out until a color change occurs in the solution, and every 30 minutes the solution is taken to analyze the remaining methylene blue concentration. The catalyst loading used is 0.25; 0.50; 0.75; 1 g/L and from that the best performance of the photodegradation was carried out by catalyst loadings of 0.75 and 1.00 g/L with almost complete degradation up to 97% (Figure 4). The degradation process was completely achieved in 30–60 minutes with the highest kinetics rate 0.059/min calculated using pseudo-first-order kinetics from the Langmuir-Hinshelwood model, which can be used to explain the reaction kinetics of methylene blue photodegradation (Eq. 14) as mentioned in previous section.

Figure 4.

The methylene blue degradation profile and kinetics constant of photocatalysis using TiO₂ nanoparticles at different catalyst loading.

The optimum catalyst loading in every system varies depending on the reactor and position of the photon source. Thus, in every system it has optimum catalyst as adding too much catalyst dose can cause dispersion, thereby reducing the effectiveness of the UV lamp from entering the solution. TiO₂ has super hydrophilic properties when irradiated by UV light [40, 41, 42]. In addition, hydrophilic substances function effectively with alcohol and water, whereas hydrophobic substances function optimally with ester and alkane substances [43].

In this study, methylene blue was dissolved into distilled water, so it can maximize the performance of TiO₂ by photo induction. Nevertheless, the TiO₂ surface can exhibit hydrophobic characteristics under certain conditions when exposed to visible or ultraviolet light; this transformation is reversible [44]. The same as Shondo et al. who conducted research on modification of the Au-TiO₂ surface with octadecyl-phosphonic acid (ODP) self-assembly monolayer produce extraordinary water repellent, they found the contact angle (CA) of the catalyst was < 5° and it became CA > 163° by means the substance changes from hydrophobic to hydrophilic during photocatalysis and it is strongly influenced by light intensity [41, 45]. In addition, the diffusion of products and reactants is influenced by the wettability of the catalyst surface. In addition to catalyst loading, different flow rates ranging from 1 to 9 L/min were used to study the degradation profile of methylene blue, which serves as a proxy for Batik dye wastewater. The result demonstrates that the fastest degradation was attained at a flow rate of 3 L/min, with a degradation kinetics constant of k = 0.01762/min (Figure 5).

Figure 5.

The methylene blue degradation profile and kinetics constant of photocatalysis using TiO₂ nanoparticles at different flow rates.

Several factors do influence photocatalysis such as light intensity, temperature, pH, pollutant concentration, and catalyst loading. Besides that, flow rate is one of the driven parameters in the photodegradation process on tubular photoreactor at a certain level. Previous experiments show that at low flow rate (less than 0.6 L/min) the TiO₂ nanoparticle tend to sediment in the lower side of the photoreactor [46]. While higher flow rate not only gives good suspension of TiO₂ nanoparticles in the methylene blue solution but also led to maximum contact as well as optimum opacity for the UV light to penetrate and activate the TiO₂ surface for photodegradation. When the number of active sites on the catalyst increases, decolorization of MB occurs. The formation of hydroxyl and superoxide as active radicals is the key point driving the degradation reaction [47]. In this system, flow rate of 1 L/min is considered a low flow rate. Sedimentation of TiO₂ nanoparticles occurs at the bottom side of the tubular photoreactor, so that the amount of TiO₂ that is active as a catalyst in photocatalysis is reduced. While the optimal TiO₂ nanoparticle for methylene blue degradation is achieved at a flow rate of 3 L/min, this also leads to the maximum contact time and optimum opacity for the UV light to penetrate and activate the TiO₂ surface for photodegradation. Given that the photodegradation rate, or k value, is lower than the rate attained by the 3 L/min, high flow rates (6–9 L/min) may have different impacts.

High flow rates have the potential to cause the suspension to turbulence, which can disrupt both the reaction and penetration of UV radiation. Additional research on the effects of flow rate (2–10 L/min) in disinfection-based photocatalysis reveals that the process is more effective at lower flow rates (2 L/min) because applying a high flow rate could cause mechanical stress and disrupt the process’s light flow and sequence [48].

8.3 Degradation of batik wastewater

The developed continuous tubular photoreactor is also used to treat batik wastewater from the batik industry in Semarang, Indonesia. The wastewater consists of Remazol Brilliant Blue, Remazol Yellow, Remazol Brilliant Violet, and Remazol Red. Like Remazol, reactive azo dyes are frequently used to color cloth fibers. Remazol yellow’s presence of peaks at 259 and 420 nm, whose wavelengths match the color of the substance’s UV–VIS spectrum, serves as evidence [49]. Then, Remazol Brilliant Violet or Remazol Brilliant Blue most likely owns the peak at 564 nm [50]. Over time, the absorbance of batik wastewater decreases. Batik wastewater’s initial absorbance values were 0.844, 0.537, and 0.203 at wavelengths of 259, 420, and 564 nm, respectively.

The result shows that dyes contained in the solution were no longer detectable within 4 hours. The Remazol Brilliant Violet or Remazol Brilliant Blue degraded within 60 min at a kinetics constant of 0.0065/min, followed by Remazol yellow degraded within 3.5 hours at 0.0037/min of kinetics constant. Smaller organic contaminants containing water and oxygen molecules are produced by this photocatalytic breakdown [51], making them safer for the environment. The most efficient batik waste degradation was observed 30 minutes after the adsorption process. This was also seen by Ariyanti et al. [48] who demonstrated that practically all RhB could be degraded at a catalyst dosage of 0.5 g/L once adsorption conditions were met for a maximum of 15 minutes (Figure 6).

Figure 6.

UV–visible spectrum and visualization of batik wastewater after photocatalysis.

In photocatalytic reactions, electrons from the valence band are excited to the conduction band, producing electron hole pairs (e−/h+) when photons with energy equal to or greater than the energy gap of semiconducting materials are absorbed [52]. When water molecules or hydroxyl ions (OH⁻) interact with photogeneration gaps (h+) in the valence band, free radicals are produced.

Conversely, excited electrons (e) in the conduction band combine with the oxygen in the solution to form superoxide radical anions (∙O₂⁻) [52]. Then, superoxide radicals react with H⁺ ions to produce hydrogen peroxide, which breaks down into OH radicals. The production of reactive oxygen species during photocatalysis can trigger oxidation and reduction, which may lead to the chromophores in the azo-reactive dye being cleaved. Based on Figure 5.5(a), the chemical structure of azo dyes consists of the backbone, auxochrome groups, chromophores groups, and solubilizing groups [53, 54]. Chromophore (N〓 N) cleavage will result in the formation of intermediate amine compounds and sulfonic acid [55]. The batik wastewater becomes decolored as a result of the chain breaking caused by the insertion of protons and electrons in the diazo N〓N bond [56]. The azo linkages (N〓N) in the molecules are broken down by photocatalysis, which is involved in the breakdown of wastewater pollution. The process starts with the azo bond cleavage (N〓N) and continues with the cleavage of single and double bonds.

Breaking bonds with the lowest energy come first for initiators, then those with the highest energy. The degradation pathways for Remazol Yellow, Remazol Brilliant Blue, Remazol Red, and Remazol Brilliant Violet are shown in Figures 5.5(b-e). At the point where the bond energy is lowest, which is C∙S (259 kJ/mol), the bond breaks. Remazol undergoes C∙S bond cleavage throughout. All things considered, Remazol Yellow is the azo dye that degrades the slowest due to the occurrence of the bonds C〓N (615 kJ/mol) and C〓O (745 kJ/mol). Aside from that, other types of Remazol have more phi (π, electron transition) compared to Remazol Yellow. The decomposition of Remazol Brilliant Blue R and Remazol Yellow FG using doped TiO₂ was also found in the research. The results show that the Remazol Brilliant Blue R decomposed more than Remazol Yellow [14].

9. Conclusions

The Implementation of TiO₂ photocatalysis as one of advanced oxidation technology (AOP) for complete removal of organic pollutants such as dyes in wastewater is very promising. Many types and designs of photocatalytic reactors are developed; among them the tubular photocatalytic reactors have more technical visibility and economic viability to be implemented for Batik wastewater industry application.

Continuous tubular photoreactor with TiO₂ nanoparticles catalyst developed for batik industrial wastewater treatment consists of five reactors arranged in parallel connected by a 1/2-inch pipe and equipped with a circulation pump, flowmeter, feed tank, panel box, and valve. This system has been tested for photocatalysis in series with nanostructured TiO₂ of 0.75–1.5 g/L catalyst loading. The degradation of methylene blue reached more than 97% and the reduction of Remazol dye in batik wastewater achieved 61%. Another processing parameter that strongly influences the performance of photoreactor is the flow rate, and the flow rate of 3 L/min is the most effective condition for the degradation of dyes in wastewater with the kinetic constant of 0.01762/min. Degradation of dyes in wastewater is a challenge and improvement still needs to be address in terms of increasing the degradation rate as well as catalyst recovery.

Acknowledgments

Authors acknowledge Universitas Diponegoro, Indonesia for the research financial support.