Tio 2 coating on alumina membranes for continuous separation of dyes

Abstract

Ceramic membranes characterised by corrosion resistance, high-temperature applicability, and reusability are especially suitable for wastewater treatment. In this research, Titania dioxide (TiO₂) was fabricated on surfaces of alumina (Al₂O₃) membranes by hydrothermal reaction and factors that influenced their morphologies were investigated. SEM images showed needle-like TiO₂ crystals filling up the pores on the membrane surfaces. We investigated the photocatalytic activity and continuous separation performance of the TiO₂ coating alumina membranes using Congo red dye as a model organic pollutant. The results showed that the dye removal ratio remained at about 98% in a 120 h continuous filtration test. Furthermore, we proposed an easy and low-cost membrane regeneration method based on the photocatalytic degradation of TiO₂ crystals. The regenerated membrane still showed excellent continuous filtration ability, indicating the good potential application in the treatment of dye effluents.

Keywords

coating membrane morphology dye photodegradation separation regeneration

Introduction

Water pollution has become a primary factor that destroys the environment and harms human health. In recent years, ceramic membranes have been used in wastewater treatment because their high mechanical strength and thermal stability are conducive to long-term usage in harsh environments [1 –6]. In addition, it is easy to recover the performance of polluted ceramic membranes through chemical cleaning, thus extending its service life in industrial applications [7,8].

So far, many ceramic membranes, such as alumina membranes, have been prepared using phase inversion combined with a high-temperature sintering method. However, due to the decomposition of polymers, voids and macropores frequently exist on the membrane surfaces and reduce the separation efficiency of membranes. To solve this problem, many researchers have tried to use monolayer or multi-layer dense coating to modify the membrane surface.

Surface modification methods for ceramic membranes include the sol–gel method [5,9 –12], low-pressure chemical vapour deposition (LPCVD) [2], atomic layer deposition (ALD) [13 –15], magnetron sputtering [16], in-situ synthesis [6], interfacial polymerisation [17], and microwave irradiation method [18]. According to the literature, coating materials, such as TiO₂, SiO₂, γ-Al₂O₃, TiO₂-doped ZrO₂ and polyamide, have been used to modify the ceramic membranes.

Manjumol et al. prepared Al₂O₃ membranes modified with multi-layer coatings of Titania and boehmite nanoparticles by a so-gel process. The obtained ultrafiltration pore-size membranes were photoactive and their multifunctional property could be utilised in water purification [9]. Mastropietro et al. fabricated an ultrathin TiO₂ layer on α-Al₂O₃ membranes, which showed high efficiency in the photodegradation of azo dye pollutants, and the self-cleaning properties rendered antifouling procedures unnecessary [5]. Chen et al. fabricated a thin SiC layer on Al₂O₃ membranes at a relatively low temperature via the LPCVD method. These membranes could be used to filter oil-in-water emulsions and showed little fouling [2]. Amirilargani et al. first synthesized two kinds of metal–organic frameworks (MOFs) materials on α-Al₂O₃ membranes, including NH₂-MIL-53(Al) and MIL-53(Al). The MOF-based membranes with unique morphology showed good adsorption performance for high concentrations of Rose Bengal dye [6]. Chen et al. demonstrated an efficient strategy to produce tubular nanofiltration membranes via ALD technology. Titanium alkoxide was deposited on ceramic membranes and then calcined into TiO₂. This thin TiO₂ separation layer provided the sieving function of nanofiltration size [15]. Hydrothermal synthesis is a facile and cost-effective method to obtain nanostructures (nanowires, nanorods, and nanoflakes), and the morphology could be easily tailored by adjusting the forming conditions [19]. However, there are few attempts to prepare TiO₂-coated alumina membrane by hydrothermal method for the application of photocatalysis combined with water filtration.

As a green technology, photocatalysis has attracted extensive attention because it can effectively eliminate toxic organic substances, bacteria, and pollutants in water [1]. Oxide semiconductor photocatalysts, such as TiO₂, ZnO, Fe₂O₃, WO₃, and SnO₂, can produce electron-hole pairs by absorbing appropriate photon energy [20 –24]. The holes and electrons participate in the oxidation process to remove organic contaminants. Among these photocatalysts, TiO₂ has the advantages of high quantum yield, low band gap energy, good chemical stability, and cost-effectiveness. Under UV irradiation, TiO₂ produces reactive oxygen species, which can effectively degrade pollutants and disinfect a variety of organisms. Recently, many researchers have focused on immobilising TiO₂ photocatalysts with ceramic membranes to improve the separation efficiency [5,9,16,25 –27]. However, it is still challenging to realise the continuous separation of wastewater and the regeneration of polluted membranes by a simple method.

In this work, TiO₂ coatings were synthesised on the surface of Al₂O₃ membrane by a hydrothermal reaction. The influences of the molar ratio of urea/Ti³⁺, TiCl₃ concentration, reaction temperature, and time on the structures of coatings were studied. The feasibility of using TiO₂ coating alumina membranes to remove Congo red dye from the water was investigated by a continuous filtration process. In addition, how to recycle separation membranes has always been an important issue in industrial applications. Although high-temperature sintering can remove pollutants in ceramic membranes, this operation is complex and energy-consuming. We propose a simple and low-cost regeneration method based on the photocatalytic degradation of TiO₂ crystals. Excellent separation performance and easy regeneration method indicate that the TiO₂ coating alumina membranes have high industrial application and development potential in the treatment of dye wastewater.

Materials and methods

The alumina membrane was prepared according to the previous report [11]. A certain amount of 15% TiCl₃ hydrochloric acid solution and urea were added to the saturated NaCl aqueous solution to obtain the hydrothermal reaction solution. Then, put the reaction solution and alumina membranes into an autoclave and heat them at different temperatures for a while. After washing and drying, the TiO₂ coating alumina membranes were obtained. The formulation and hydrothermal reaction conditions are listed in Table 1.

Table 1

The conditions of hydrothermal reaction and porosity, maximum water flux of the TiO₂ coating alumina membranes.

Samples	Urea/TiCl₃ molar ratio	TiCl₃ concentration (mol/L)	Reaction temperature (°C)	Reaction time (h)	Porosity (%)	Maximum water flux (L/m²h)
Al₂O₃	—	—	—	—	54.22	887.0
A-1	1:10	0.4	170	12	47.92	309.8
A-2	3:10	0.4	170	12	44.25	239.7
A-3	5:10	0.4	170	12	46.34	252.3
B-1	3:10	0.2	170	12	48.88	461.6
B-2	3:10	0.6	170	12	43.32	204.1
C-1	3:10	0.4	150	12	48.34	355.0
C-2	3:10	0.4	190	12	43.58	204.1
D-1	3:10	0.4	170	6	46.79	298.2
D-2	3:10	0.4	170	24	45.30	251.6

The structures of TiO₂ coating alumina membranes were characterised by Fourier transform infrared spectroscopy (FTIR, Nicolet 6700) and X-ray diffraction (XRD, Rigaku D/max 2550 V X-ray diffractometer). Field-emission scanning electron microscopy (FE-SEM, S-8010) was used to observe the morphology of TiO₂ coatings. The porosity of the membrane was tested by Archimedes method. Water flux was measured by a laboratory-made flux detector at a pressure of 0.015 MPa.

Adsorption and photocatalytic degradation tests of an unmodified alumina membrane or a TiO₂ coating membrane (Sample D-1) were performed in Congo red solution with an initial concentration of 50 mg/L. At certain intervals of ultraviolet radiation, the solution concentration was determined using a UV-2550 UV-visible spectrophotometer. The photocatalytic degradation ratio R_t was calculated using Equation (1). (1) Where C₀ and C_t represent the concentrations of solutions before and after t minutes of irradiation, respectively.

The same laboratory-made flux detector was used to measure the continuous separation performance for dyes except using Congo red solution (50 mg/L) instead of pure water. The size of the membrane was 2.6 *2.6 *0.2 (cm). UV-2550 UV-visible spectrophotometer was also used to obtain the concentration of filtrates. The separation performance of the membrane was represented by the removal ratio, which was calculated using Equation (2) (2) Where R is the removal ratio of Congo red, C₀ and C₁ are the concentrations of the initial solution and the filtrate, respectively.

Results and discussion

Effect of reaction conditions on the morphology of TiO₂ coatings

The TiO₂ coating was fabricated on the Al₂O₃ membrane surface by a hydrothermal reaction. The main factors affecting the reaction include the molar ratio of urea/TiCl₃, TiCl₃ concentration, reaction temperature, and reaction time. Figure 1 shows SEM images of the surfaces of the unmodified alumina membranes and the TiO₂ coating membranes prepared at different molar ratios of urea/TiCl₃ (as shown in Table 1).

Figure 1

SEM images of the surfaces of the unmodified membrane and the TiO₂ coating membranes prepared at different urea/TiCl₃ molar ratios (inset: identical sample at high magnification).

In Figure 1, Al₂O₃ particles can be observed on the unmodified membrane surface. Interspaces and pores appear between the alumina particles, which is attributed to the decomposition of the polymer during the sintering process. After the hydrothermal reaction, the membrane surfaces are covered with TiO₂ particles, and the number and morphology strongly depend on the molar ratio of urea/TiCl₃. When the molar ratio is 1:10, the formed TiO₂ particles are not enough to cover the entire membrane surface. As it increases to 3:10, many needle-like TiO₂ crystals have formed and tightly wrapped the alumina particles and further assembled into spherical structures, making the voids disappear and improving the membrane surface's density. However, with increasing the molar ratio of urea/TiCl₃ to 5:10, the amount of TiO₂ crystals decreases, and alumina particles and voids appear again on the membrane surface.

In the hydrothermal reaction process, an appropriate amount of urea tends to reduce the hydrolysis reaction rate and the degree of solution supersaturation by forming ligands between urea and Ti³⁺, which is conducive to inducing heterogeneous nucleation on the membrane surface [28]. At the same time, the decomposition of urea increases the pH value of the reaction solution, thus promoting the growth of TiO₂ crystals on the membrane surface [29]. When the molar ratio of urea/TiCl₃ is 1:10, urea in the reactive solution is not enough to slow down the hydrolysis reaction and promote the growth of TiO₂ crystal. When it increases to 5:10, an excess of urea suppresses the Ti³⁺ hydrolysis rate and decreases the solution's acidity, which also negatively affects the growth of TiO₂ crystals. By comparison, the molar ratio of urea/TiCl₃ around 3:10 is optimum for forming the TiO₂ crystals on the membrane surface.

The formation of the TiO₂ crystal is also affected by the TiCl₃ concentrations in the precursor solution. Figure 2 B-1 and B-2 show SEM images of the surfaces of the TiO₂ coating membranes prepared at different TiCl₃ concentrations. TiO₂ particles have formed but cannot cover the entire membrane surface when the TiCl₃ concentration is 0.2 mol/L. As the TiCl₃ concentrations increase to 0.4 mol/L (A-2) or 0.6 mol/L (B-2), the spherical structures assembled by needle-like TiO₂ crystals are deposited on the membrane surface, which makes the surface much denser.

Figure 2

SEM images of the surfaces of the TiO₂ coating membranes prepared at different TiCl₃ concentration, hydrothermal reaction time, and temperature (inset: identical sample at high magnification).

The reaction temperature and time are vital factors in adjusting the morphology of TiO₂ coatings. Figure 2 C-1 and C-2 show the membrane surfaces after hydrothermal reaction for 6 and 24 h. Some voids and cracks can be found on the membrane surface in Figure 2 C-1, indicating that the reaction time of 6 h is too short to generate enough TiO₂ crystals. When the hydrothermal reaction is prolonged to 12 h (A-2) and 24 h (C-2), the needle-like crystals form and fill the gaps on the surface. As the morphology of the membrane surface changes little, the hydrothermal reaction time is determined as 12 h based on the consideration of energy saving. Figure 2 D-1 and D-2 show SEM images of the TiO₂ coating membrane surfaces prepared at 150°C and 190°C. The TiO₂ crystals formed at 150°C are insufficient to cover the surface thoroughly. When the reaction temperature is raised to 190°C, the denser structures of TiO₂ crystals form, which is similar to that prepared at 170°C. Therefore, 170°C is a more suitable temperature for the hydrothermal reaction.

Structure and property of the TiO₂ coating alumina membrane

Figure 3 shows FTIR spectra of the unmodified membrane and the TiO₂ coating membranes obtained at different reaction times. The stretching vibration peaks of Al₂O₃ can be found at 632 and 721 cm⁻¹. The peak at 1400 cm⁻¹ and the broad peak in the range of 670∼920 cm⁻¹ are assigned to the stretching vibration of Ti-O-Ti. The bending vibration peak and wide stretching vibration peak of -OH can be found at 1640 cm⁻¹ and around 3000∼3500 cm⁻¹, which may be due to the adsorption of water molecules on the TiO₂ surface.

Figure 3

FTIR spectra of the unmodified membrane and the TiO₂ coating membranes prepared at different reaction time. (TiCl₃ Concentration: 0.4 mol/L, urea/Ti³⁺ molar ratio: 3:10, reaction temperure:170 °C).

Figure 4 is the XRD patterns of the TiO₂ coating membranes prepared at different TiCl₃ concentrations. For the sample prepared at 0.2 mol/L, the characteristic peaks of TiO₂ appear at 45.3°, 54.3°, and 62.7°, corresponding to (210), (211), and (002) of rutile, respectively. As the TiCl₃ concentration increases to 0.6 mol/L, the existence of (110), (210), (220), and (002) from typical rutile can be confirmed by the diffraction peaks at 27.4°, 45.3°, 56.5°, and 62.7°, and the peaks become sharper, indicating that more perfect rutile crystals have generated on the surface of the alumina membrane.

Figure 4

XRD patterns of the TiO₂ coating alumina membranes prepared at different TiCl₃ concentration. (Urea/Ti³⁺ molar ratio: 3:10, reaction time: 12 h, reaction temperure:170 °C).

After being modified with TiO₂ coatings, the alumina membrane surfaces become more compact, which directly affects their porosity and water flux. As shown in Table 1, the average porosity of the unmodified alumina membrane is 54.22%, while the values for all TiO₂ coating membranes decrease to a certain extent.

The water flux values of membranes are shown in Table 1 and Figure 5. At the working pressure of 0.015 MPa, the maximum water flux of the unmodified membrane is 887.0 L/(m²h). For the TiO₂ coating membranes prepared at the TiCl₃ concentration about 0.2, 0.4, and 0.6 mol/L (Samples B-1, A-2, and B-2), the maximum flux values decrease to 461.6, 239.7, and 204.1 L/(m²h), respectively. As shown in the SEM images, interspaces and pores on the member surfaces are filled by TiO₂ crystals after the hydrothermal reaction, and the dense coatings block water flow. It can also be seen from Figure 5 that the water flux value of the membrane decreases with the extension of the measurement time, which may be related to the increase in the interaction between water and membrane.

Figure 5

The variation curve of water flux with time for the unmodified membrane and the TiO₂ coating membranes prepared at different TiCl₃ concentration. (Urea/Ti³⁺ molar ratio: 3:10, reaction temperure:170°C, reaction time: 12 h).

Adsorption and photocatalytic degradation performance of the TiO₂ coating alumina membrane

Under UV irradiation, TiO₂ possesses photocatalytic degradation ability for dyes. Therefore, the TiO₂ coating alumina membranes demonstrate a potential application in eliminating Congo red dye from water. Figure 6 shows the UV spectra of the solutions after adsorption and photocatalytic degradation treatment by the unmodified membrane and the TiO₂ coating membrane (Sample A-2) for different irradiation times. The removal efficiency of the unmodified membrane is very low. After irradiation for 120 min, the absorbance of Congo red solution at 500 nm decreases from 1.2325 to 1.0503, corresponding to the removal ratio for Congo red is only 14.0%. In contrast, the removal ratio of the TiO₂ coating alumina membrane improves significantly to 94.8%. Figure 7 also shows that at the same irradiation time, the removal ratio of the TiO₂ coating membrane is much higher compared to the unmodified membrane. There is only an adsorption effect for the unmodified membrane on Congo red because alumina alone does not possess photocatalytic degradation ability. Once most of the active sites in the membrane have been occupied by dyes, the adsorption cannot further conduct. In contrast, adsorption and photocatalytic degradation are the two main driving forces for the TiO₂ coating membrane to remove dyes in the solution. The nanoscale needle-like TiO₂ crystals formed on the modified membrane surface significantly increase the contact areas between the membrane and dyes, which brings more active sites. Meanwhile, the TiO₂ coating can react with Congo red under UV irradiation, which destroys the dye chromophore and thus causes decolorisation. Therefore, based on the improved adsorption and the extra photocatalytic degradation, the TiO₂ coating alumina membrane shows a good performance for Congo red dye removal.

Figure 6

UV-visible spectra of the Congo red solution after adsorption and photocatalytic degradation treatment by (a) the unmodified membrane and (b) the TiO₂ coating membrane (Sample A-2) at different irradiation time intervals. The insets are the corresponding digital photographs of Congo red solution simultaneously when the UV-visible spectra were taken.

Figure 7

Effect of irradiation time on the removal ratios for Congo red of the unmodified alumina membrane and the TiO₂ coating membrane.

Continuous separation performance of the TiO₂ coating alumina membrane

To investigate the separation property of TiO₂ coating membranes for dyes during the continuous filtration process, we used Congo red as a model pollutant. A scheme of the filtration process is shown in Figure 8. The absorbance of the filtrate was tested and the removal ratios and cumulative filtrate volumes are listed in Table 2. The removal ratios depend on the morphology of the membrane surfaces. For the TiO₂ coating prepared at 6 h (Sample D-1), the dye removal ratio is above 97% during the first 4 h and then decreases with prolonging the filtration time. However, for the coating prepared at 12 h (Sample A-2), the dye removal ratio stays around 98% and the total filtrate volume reaches 257 ml in a 120 h continuous filtration test. The excellent separation performance is related to the high surface compactness and the nanostructure of the TiO₂ coatings. The needle-like TiO₂ crystals on the entire membrane surface of Sample A-2 significantly reduce the pore size and effectively prevent the dye molecules from penetrating. On the other hand, the TiO₂ needle-like crystals increase the contact areas with dyes, resulting in most dye molecules being adsorbed by TiO₂ during the filtration process.

Figure 8

The scheme of separation performance of the membrane module for Congo red dye during the filtration process (insert images are the modified alumina membranes before and after filtrated).

Table 2

Continuous filtration performance of the TiO₂ coating alumina membrane.

Samples	cumulative filtration time (h)	cumulative filtrate volume (ml)	dye removal ratio (%)
D-1	2	15	97.54
	4	27	97.26
	6	35	96.36
	8	42	92.00
A-2	20	73	97.97
	40	118	98.59
	60	150	98.37
	80	183	97.31
	100	218	98.23
	120	257	97.78

Regeneration of the TiO₂ coating alumina membrane

The recycling of separation membranes has always been a significant concern. For ceramic membranes, the usual regeneration method is to sinter the membrane again at high temperatures to remove the organic matter. However, high-temperature sintering consumes much energy, and the membrane needs to be removed from the module and installed after sintering, which increases the complexity. In this work, an easy and low-cost method for membrane regeneration was used to remove dyes based on the photocatalytic degradation of TiO₂ crystals. The membrane, after dye filtration, did not need to be removed from the module but was directly irradiated with UV light for 8 h.

Figure 9A shows the SEM image of the surface of the TiO₂ coating membrane (Sample A-2) after dye separation. The membrane surface is covered by many dye molecules. After photocatalytic degradation, the Congo red dye molecules disappeared and the original morphology was restored (Figure 9B). Figure 9C and D also show that the membrane surface has changed from dark red to light red after UV irradiation, indicating that most Congo red dyes have been degraded.

Figure 9

SEM images and photographs of the TiO₂ coating membrane surfaces of Sample A-2 (A and C: after dye separation, B and D: after photocatalytic degradation).

A second filtration experiment was conducted to evaluate the reusability of the membrane. The regenerative membrane maintains its excellent separation capability, and the dye removal ratio is also above 97% for the cumulative filtration time of about 160 h (Table 3). Therefore, based on the excellent chemical resistance, high-temperature resistance, and dye separation performance, the TiO₂ coating alumina membrane shows a good potential application in the treatment of dye effluents.

Table 3

Continuous filtration performance of the regenerative membrane.

cumulative filtration time (h)	cumulative filtrate volume (ml)	dye removal ratio (%)
20	32	98.02
40	66	98.29
60	98	98.05
80	123	97.96
100	144	98.02
120	165	98.14
140	189	98.21
160	217	98.08

Conclusions

In summary, alumina membranes were modified with TiO₂ layers by hydrothermal reaction for photodegradation and elimination of dyes. The molar ratio of urea/Ti³⁺, TiCl₃ concentration, reaction temperature, and reaction time are the main factors influencing the structures of the TiO₂ coatings and morphologies of the membrane surfaces. After modification, needle-like TiO₂ crystals tightly wrap alumina particles and cover the entire membrane surface, which increases the compactness of membrane surfaces and makes them possess good photocatalytic degradation and separation performances for Congo red dye. The TiO₂ coating alumina membrane shows a removal ratio of Congo red of 94.8% under UV light irradiation, which is much better than that of the unmodified membrane. Furthermore, during the continuous filtration process, the removal ratio of the TiO₂ coating membrane for Congo red solution remains at about 98% in 120 h. In addition, the contaminated membrane can directly be irradiated with ultraviolet light to degrade the organic dye molecules, and the regenerated membrane still shows a high dye removal ratio of more than 97% in the second continuous filtration. Excellent separation performance and easy regeneration method make the TiO₂ coating alumina membrane a good application prospect in the treatment of dye wastewater.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the authors.

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