Abstract
A novel photocatalytic continuous system has been proposed for the treatment of tannery waste water, which has high levels of environmental pollutants. The purification process was performed by passing wastewater on a titanium dioxide (TiO2)-coated surface, which is continuously activated by irradiation of ultraviolet light. To improve the yield of the process, ferric chloride (FeCl3) was used as a coagulation agent. The organic and inorganic compounds, as well as the microorganisms in the tannery wastewater media, were degraded through a photocatalytic process. The results revealed that total dissolved solids and total suspended solids contents were significantly decreased from 8,450 and 8,990 mg·L−1 to 4,032 and 4,127 mg·L−1, respectively. Furthermore, the chemical oxygen demand content of the sample was reduced from 370 to 50 mg·L−1 after the addition of 100 mL of FeCl3 and 4 h of treatment. The same results were observed for the elimination of sulfate and chromium ions, which led to a decline in electrical conductivity. This suggests that introducing 100 mL of FeCl3 as the coagulation agent and continuous treatment with photocatalityc set-up could be considered as an effective method for the purification of tannery wastewaters.
HIGHLIGHTS
Continuous set-up fabricated for tannery wastewater treatment using photocatalytic reactions.
TiO2 photocatalyst activated by UV light for improvement of purification yield.
FeCl3, as coagulation agent, facilitated treatment by removing total dissolved solids.
Toxic ions and microorganisms were eliminated in this method.
Graphical Abstract
INTRODUCTION
Tanning is one of the oldest industries worldwide and began in ancient times by preparing leather for footwear, drums, and other musical instruments (Durai & Rajasimman 2011). Nowadays, leather tanning is a common industry particularly in Mediterranean countries, Middle East, and India (Insel et al. 2009; Mannucci et al. 2010; Natarajan et al. 2013). As the demand for leather has grown, the tanning industry developed from small places to the huge industrial districts. Moreover, the production process of the tanning progressed over the years and now consists of four main categories including: (1) hide and skin storage and beam house operations, (2) tanyard operations, (3) post-tanning operations, and (4) finishing operation (Lofrano et al. 2013). Despite the type of the production process this industry is considered as one the most hazardous industries due to high concentrations of pollutants with low biodegradability (Wang et al. 2019). The presence of the various cations and anions in tannery wastewater (TWW) leads to a high electrical conductivity and very low pH. In addition, the high concentration of solid components in the TWW in terms of total dissolved solids (TDS) and total suspended solids (TSS) restricts the implementation of many separation and purification methods. TWW also has high chemical oxygen demand (COD) and biological oxygen demand (BOD), which makes this pollutant wastewater unfavorable for different applications reuse (Al-Shaalan et al. 2019; Ali et al. 2019a, 2019b; Alothman et al. 2019; Khan et al. 2019). Furthermore, TWW could not be employed in agriculture due to the presence of hazardous ions, mainly Cr6+ and other toxic heavy metals. Numerous efforts have therefore been made to develop and improve the methods for TWW treatment and purification.
Using coagulation and flocculation agents is the most common method in purification of the TWWs. In this regards, aluminium sulfate (AlSO4), ferric chloride (FeCl3), and ferrous sulfate (FeSO4) are the most common coagulation agents which effectively reduce COD, TSS, and toxic substances such as Cr6+. Furthermore, biological treatment comprising aerobic and anaerobic processes, wetlands, and ponds is recognized as an efficient method for TWWs. It is worth mentioning that emerging treatment technologies have demonstrated significant advances in TWW purification (ALOthman 2012; Mittal et al. 2016; Ali et al. 2017, 2018; Alqadami et al. 2017, 2018; Alothman et al. 2020; Khan et al. 2020). Membrane processes, membrane bioreactors, adsorption, catalysis, and advanced oxidation process have received most attention during the last decade (Islam et al. 2018; Ahsan et al. 2019, 2020a, 2020b, 2020c, 2020d, 2021). Among these techniques, a number of studies have been carried out on the advanced oxidation processes. In this method, the production of highly reactive oxidizing agents, particularly active radical species, provides the opportunity of rapiod and unselective oxidation for a broad range of organic compounds and biological species. Irradiation of UV light onto the TWW in addition to the presence of an appropriate photocatalyst to produce reactive radicals has been investigated as effective method. The possible use of sunlight or near UV light is considered as a major advantage of this process, reducing the costs in large scale operations (Gogate & Pandit 2004). Although this method was not taken into account before 2007 (Schrank et al. 2004; Sauer et al. 2006), advances in nanotechnology has developed this method in the last decade. Degradation of TWW via dispersion of the green synthesized titanium dioxide (TiO2) nanoparticles was carried out by Goutam et al. (2018). They reported that the 2.26% of COD and 76.48% of Cr6+ from TWW was removed by treatment with green synthesized TiO2 nanoparticles, along with fabricated parabolic trough reactor. Bordes et al. reported that decoloration of TWW was successfully achieved by removing the organic components using plasma-sprayed TiO2 coatings (Bordes et al. 2015). Evaluation of flower-like zinc oxide (ZnO) microspheres performance as a photocatalyst in the removal of Cr6+ in TWW was carried out by Liu et al. (2016). According to their report, Cr6+ ions were efficiently removed by ZnO hollow microspheres, which showed good photocatalytic stability. Moreover, critical parameters of BOD, COD, and TOC in TWW were reduced to 24%, 32%, and 30%, respectively, after only 180 min of exposure to the UV lamp. Removal of the chemical compounds was explored by Zhao et al. who proposed a novel synthetic method for TiO2 nanoparticles (Zhao et al. 2017). They confirmed the applicability of the synthesized nanoparticles in TWW treatment by evaluating the organic component degradation kinetic. More recently, remarkable results have been achieved via treatment of TWW with synthesized N-doped carbon quantum dots functionalized SnS2 by Wang et al. (2019).
A review of the literature showed that availability, low cost, and high efficiency of the TiO2 nanoparticles makes it of particular interest (Zhao et al. 2017). However, the TiO2 nanoparticles dispersion in the media in batch systems is highlighted as a major challenge at an industrial-scale. To overcome this, a continuous system has been designed in the current study using a TiO2 nanoparticles-based photocatalyst coating embedded in novel experimental set-up. Moreover, TWW treatment performance has been improved through adding various amounts of FeCl3 as a coagulation agent in the pre-treatment sequence to remove solid components in the sample, which is then followed by filtration (Figure 1). Next, the filtrate entered the advanced oxidation set-up to remove the other insoluble pollutants. The pH, electrical conductivity (EC), TDS, TSS, COD, and BOD were determined to evaluate the effectivity of the proposed treatment process on the quality of the purified TWW. The concentration of hazardous ionic components (chromium and sulfate ions) was also investigated.
The proposed method for the TWW treatment; (a) pretreatment of the sample with FeCl3; (b) fabrication of the TiO2 photocatalyst coating; (c) photocatalytic set-up.
The proposed method for the TWW treatment; (a) pretreatment of the sample with FeCl3; (b) fabrication of the TiO2 photocatalyst coating; (c) photocatalytic set-up.
EXPERIMENTAL
Materials and instruments
Nitric acid (HNO3) (69%), FeCl3 anhydrate (99%) NaOH (solid pellets, 98%) were purchased from Merck, Germany. TiO2-P25 nanoparticles was supplied from Nanoshell, USA. The Fourier-transform infrared spectroscopy and attenuated total reflection infrared spectroscopy spectra were recorded using a Nicolete 800 instrument (Madison, USA) in the wavelength range of 400–4,000 cm−1. The size and distribution of nanoparticles were investigated using a scanning electron microscope (SEM) MIRA-III TESCANE (Kohoutovice, Czech Republic). The TWW sample was collected from Misha Tannery Factory (Tehran, Iran) and the characteristic are shown in Table 1.
The characteristics of the collected TWW
Parameter . | Unit . | Value . |
---|---|---|
pH | 3.99 | |
Conductivity | ms·cm−1 | 10.24 |
Total solid | mg·L−1 | 8,990 |
TDS | mg·L−1 | 8,450 |
TSS | mg·L−1 | 540 |
COD | mg·L−1 | 370 |
BOD | mg·L−1 | 350 |
Sulfate | mg·L−1 | 1,180 |
Dichromium | mg·L−1 | 7.52 |
Parameter . | Unit . | Value . |
---|---|---|
pH | 3.99 | |
Conductivity | ms·cm−1 | 10.24 |
Total solid | mg·L−1 | 8,990 |
TDS | mg·L−1 | 8,450 |
TSS | mg·L−1 | 540 |
COD | mg·L−1 | 370 |
BOD | mg·L−1 | 350 |
Sulfate | mg·L−1 | 1,180 |
Dichromium | mg·L−1 | 7.52 |
Fabrication of photocatalytic coating
The photocatalytic coating was fabricated by previously made titania powder (PMTP) thermal bonding method (Rao & Chaturvedi 2007). In this method, a suspension solution of TiO2-P25 was prepared by adding nanoparticles (1 g) to 1 L of deionized water. Then, the pH of the suspension was adjusted to 3 by dropwise addition of HNO3 solution (3 M) and the flask placed in an ultrasonic bath for 1 h at a frequency of 35 kHz. Preparation of the glass surface as the substrate for the TiO2 coating was carried out by washing with hydrogen fluoride solution (5% v/v) and rinsing with sodium hydroxide (NaOH) solution (6 M) to remove any pollutants and increment of the hydroxyl groups on the glass surface, respectively. The obtained suspension was poured on the glass and placed in an oven at 80 °C for 16 h to thoroughly dry the surface. Then the plate was placed in a furnace for 3 h at 500 °C to establish the nanoparticles on the glass surface. Finally, the fabricated plates were washed with deionized water to remove any unsettled particle from their surfaces.
Preparation of the TWW sample
The TWW sample was obtained from Misha Tannery Factory and was pretreated. To remove the solid materials from the sample, the sample was filtered three times using Whatman filter paper with pore size of 0.45 µm. Then, the collected liquid phase was treated with FeCl3 as the coagulation agent to precipitate the remaining suspended compounds in the media. The FeCl3 was added in five different concentrations: 50, 100, 150, 200, and 250 mL per L of sample. Then, the samples were stirred at room temperature for 15 min to obtain a homogenous mixture, which changed the color of the suspension from turquoise blue to black. The stirring of samples was stopped for 24 h to allow the solids to form. Lastly, the liquid was filtered prior to final treatment in the set-up.
Fabrication of photoreactor and its performance
In the current study, the TWW treatment set-up was designed to provide a continuous flow system for the wastewater over the photocatalyst surface. For this, a set-up was made by cutting Plexiglas sheets using a computerized numerical control instrument. The cut sheets were joined to each other by an acrylic-based instant adhesive to form the plotted device as depicted in Figure 1. To run the device, the TWW was poured into the container after turning the UV lamps on. Then, the TWW flow was pumped on the photocatalyst surface under UV irradiation of 365 nm from a 150 W UV light lamp. The TWW was agitated continuously by electrical agitators placed on the container floor during the treatment process (Figure 2).
Fabricated continuous water treatment device from front view (a) and bottom view (b) including the device parts: (1) pump; (2) pipe; (3) nozzles; (4) UV lamp; (5) TiO2 photocatalyst coating; (6) Plexiglas sheets (7) container; and (8) curved agitator.
Fabricated continuous water treatment device from front view (a) and bottom view (b) including the device parts: (1) pump; (2) pipe; (3) nozzles; (4) UV lamp; (5) TiO2 photocatalyst coating; (6) Plexiglas sheets (7) container; and (8) curved agitator.
Analysis methods
The pH of the samples were collected using a HANNA pH-meter. The EC was investigated by inoLab Cond 720 = . The COD and BOD of the treated TWW were evaluated using Standard Method 5220 D and Standard Method 5210 B, respectively. Standard Method 2540 C was used to estimate the total solid components, TDS, and TSS. The Cr6+ was determined using the Standard Method 3111 B Direct Air–Acetylene Flame Method. In this method, 100 mL of the sample was mixed with 1 mL 30% H2O2 before aspiration. The sulfate content was measured using the INSO – 2353 method.
RESULTS AND DISCUSSION
The structure of the prepared plates was investigated by FT-IR spectroscopy to confirm the sustainability of the TiO2 nanoparticles after the preparation process steps. This test could be helpful to distinguish any change in the nano-TiO2 structure during the hydrothermal process of attaching these particles on the glass sheet. Moreover, the FT-IR spectroscopy showed various functional groups in the TiO2 nanoparticles, which were also determined by the transmission and absorptions range. According to the FT-IR results, the stretching vibration of the hydroxyl groups was observed around 3,500 cm−1 (Figure 3). There was also an absorbing band at 1,630 and 1,383 cm−1, which could be assigned to the Ti–OH and related Ti–O bonds, respectively (León et al. 2017). Noteworthy, the most highlighted band at 690 cm−1 are considered as the characteristic peak of TiO2 which is assigned to the Ti–O stretching vibration (Figure 3).
The surface of the prepared coatings was studied using SEM. The SEM images showed that the thickness of the TiO2 coating on the glass surface was about 8.27 μm. Noteworthy, the size of the TiO2 nanoparticles increased to about 56 nm due to agglomeration during the fabrication process – about 27 nm larger than the size of the particles before use (according to the supplier information) (Figure 4).
Evaluation of the total solid (TS) components including TDS and TSS was carried out using the Standard Method 2540 C/D method to explore the efficiency in removing the TDS and TSS from the TWW. The results revealed that the blank sample contained 8,990 mL·L−1 of TS including 8,450 and 540 mL·L−1 of TDS and TSS, respectively. These values were reduced to 5,437 and 134 mL·L−1 by adding 50 mL of FeCl3 to the sample. The largest reduction was observed by adding 100 mL of FeCl3, which led to TDS and TSS reducing to 4,032 and 95 mL·L−1 (Figure 5). Noteworthy, the addition of the higher content of FeCl3 led to a slight increase in TS values compared to the concentration from adding 100 mL of FeCl3. This result was achieved after introducing extra contents of coagulation agent into the samples which led to the formation of Fe-based complexes in the aqueous media. Consequently, the TDS and TSS values increased slightly after the addition of more than 150 mL of FeCl3 and increased to 4,928 and 178.92 mL·L−1 in the sample containing 250 mL FeCl3, respectively (Figure 5).
The pH values of treated TWW samples from different contents of FeCl3.
On the other hand, conditions during the treatment process (long treatment time, UV light, low pH, and etc.) made it possible to form neutralized compounds from ionic species. Therefore, the EC changes were expected in the treated TWW samples. Investigation of the samples after termination of the treatment showed that the EC value was lower in all samples compared to the blank. Addition of 50 mL of coagulation agent into the TWW led to a reduction in the EC from 10.24 to 7.34 ms·cm−1 (Figure 7). This trend reached the minimum point in the sample containing 100 mL of FeCl3 of about 4.62 ms·cm−1 (Figure 7). It should be noted that a further decrease in EC was not observed by introducing more FeCl3 into the samples; contrarily a slight increase was observed in the EC value. The observed increase could be related to the production of the Fe3+ and Cl− ions in the aqueous media in relation to the excess content of the coagulation agent in the samples.
Considering that the biological species could be degraded by radical components, it was assumed that treating the TWW with the novel designed set-up could reduce the BOD content of the sample. This assumption was confirmed by evaluation of the BOD using the Standard Method 5210 B. Accordingly, the BOD content was reduced during the time it was in contact with the photocatalyst surface under UV light (Figure 9). It was observed that the BOD content reached to zero after 4 h in all samples except for the sample containing 50 mL of FeCl3, which is probably because of the low concentration of the coagulation agent. Nevertheless, the reducing trend of the BOD concentration in all samples revealed that the proposed method could be highlighted as an effective method in the degradation of both chemically and biologically toxic species.










The concentration of sulfate (a) and chromium (b) ions in different samples.
According to the observed results in the analysis of the TWW samples, introducing 100 mL of FeCl3 could be considered as the optimum content for coagulation agent. In this content, all contaminant parameters could be reduced to standard values or be close to the acceptable range for treated TWW. All the tests and experiments were repeated three times in the same manner using 100 mL of FeCl3 to confirm the process reliability and accuracy. The results showed that the proposed process could be presented as a novel method with high accuracy.
CONCLUSION
Continuous set-up for treatment of the TWW has been fabricated using TiO2 nanoparticles as photocatalyst agent. To overcome the disadvantages of the dispersed nanoparticles, the photocatalyst was established on the glass sheets using the PMTP method, including thermal conjugation of the TiO2 nanoparticles on the glass sheets. The TWW was first treated with FeCl3 as coagulation agent to improve the elimination yield of the pollutants. This step was followed by passing the wastewater sample on the photocatalyst sheet under UV light for various time durations. The results revealed that all the effective water quality parameters improved in the TWW sample. Evaluation of the sample pH revealed that it increased to 4–5 at the end of the process. Moreover, the EC of the samples decreased by removing the ions in purification steps from 10.24 to 4.62 ms·cm−1. Noteworthy is that the coagulation agent had an important role on the removal of the solid components. Specifically, introducing 100 mL of FeCl3 into the wastewater led to a reduction in the total solid content to 4,127 mg·L−1. Degradation of the oxygen was confirmed by COD and BOD test. According to these tests COD compounds were completely removed and the BOD content was reduced to 20 mg·L−1. Furthermore, the concentration of the and
decreased to 700 and 0.37 mg·L−1, which was in the standard range for treated TWW. Conclusively, introducing 100 mL of FeCl3 as a coagulation agent and continuous treatment in the photoreactor for 4 h was found to be the optimal procedure for treating TWW. The accuracy and reliability of the proposed procedure was confirmed by repeating the experimental tests three times.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.