Increasing detergent use, both in homes and public laundry services has produced greater volume of wastewater containing Linear Alkyl-benzene Sulfonate (LAS) in high concentration1-3 and some time the LAS concentration is found to exceed the permissible level1. Accordingly, disposal of laundry wastewater directly to the environment can create several problems. At low concentrations of LAS were toxic to certain fish, aquatic animals and aquatic plants including bacteria and algae4. It is also reported that consumption drinking water or food contaminated by LAS higher than 0.5 mg L1 can be harmful to health5. The chemical structure of LAS represented by dodecyl benzene sulfonate is displayed as Fig. 1, that is usually resistant to biodegradation1,3and thus, the toxicity and environmental persistence of these surfactants are emerging concerns. Justifying the negative effect, removal of LAS in laundry wastewater is essential.
Several treatment methods have been considered for LAS removal from water, especially the methods that can degrade the LAS effectively. The degradation methods were operated by involving OH radicals that can be created through photo-Fenton1, photo-Fenton like2, photocatalysis over TiO23,4 and ozonisation5. Photocatalytic degradation using TiO2 has recently received considerable attention for Persistent Organic Pollutants (POPs) removal due to its cost-effective technology, non-toxicity, fast oxidation rate and chemical stability4,6-7. However, the wide band gap of TiO2, that is 3.2 eV for anatase, allows it only to be excited by photons with wavelengths shorter than 385 nm or UV region that limits its application under visible light8-15. Therefore, an effort has been focused to overcome this deficiency by doping TiO2 crystal structure with either non-metal elements8,9 and metals10-15 elements.
From the metals, silver as a dopant has received intensively attention since, it can significantly decrease the band gap of TiO213-15, allowing it to be more active under visible light. In addition, Ag dopant can also effectively trap electrons that delay the recombination and it turns to improve the photocatalysis process both under UV and visible light14,15.
Ag-doped TiO2 has been frequently reported for water splitting12, degradation of organic pollutants13,15and bacterial abatement14,16. However, no reports of TiO2 doped with Ag used for LAS degradation were found in the previous papers. Under the circumstance, in this present study, Ag-doped TiO2 prepared by photodeposition method is examined for elimination of LAS in the real laundry wastewater under visible light exposure at the laboratory scale.
|Fig. 1:||Chemical structure of dodecylbenzene sulfonate
||Source: Wahyuni et al.1
To evaluate the efficiency of the visible photodegradation, several process parameters including the content of Ag-doped in TiO2, photocatalyst dosage and irradiation time were optimized in this study.
MATERIALS AND METHODS
Study area: The research was conducted in April-October, 2019, at Laboratory of Analytical and Environmental Chemistry, Department of Chemistry, Universitas Gadjah Mada, Yogyakarta, Indonesia.
Materials: TiO2, AgNO3, dodecyl-benzene sulfonate and methylene blue were purchased from E. Merck and used without any purification. Laundry wastewater taken from a laundry service in Yogyakarta, Indonesia, was used as a photodegradation subject.
Method: The research method consists of doping process, characterization of the doped photocatalyst and photoactivity of the doped photocatalyst for degradation of LAS in the laundry wastewater sample.
Doping process: Doping was performed by photodeposition of Ag(I) from AgNO3 solution over TiO2 powder under UV irradiation by following procedure: 100 mL AgNO3 solution in the beaker glass was added with 50 mg TiO2 powder to form suspense and the pH was set at 4, where the photoreduction proceeded maximally. Next, the suspense was placed in the apparatus as illustrated by Fig. 2 and it was irradiated by UV light accompanied by magnetically stirring for 24 h. The amount of Ag-doped on TiO2 was measured by Atomic Absorption Spectrophotometry (AAS) method.
A set of apparatus used for doping and LAS photodegradation processes
||Source: Wahyuni et al.16
In this step, the concentrations of Ag(I) in the solution were varied as 50×102, 100×102, 150×102, 200×102 and 2.0×102 mole L1.
Characterization: The doped photocatalysts obtained were characterized by using Shimadzu 6000X-XRD, Bruker-2000 FTIR, Variant-DRUV and TEM machines. In the FTIR analysis, the samples were pelleted with KBr and the spectra were recorded from 4000-400 cm1. The XRD patterns were recorded on XRD machine with Cu-Kα from 5-80° of diffraction angles. By using DRUV machine, the spectra were scanned at 200-800 nm of the wavelength. The samples were coated with gold for taking TEM images.
Photocatalytic degradation of LAS in the laundry wastewater: The photodegradation was conducted by batch technique in the apparatus seen in Fig. 2. The wastewater 100 mL in Erlenmeyer flask was mixed with 50 mg of TiO2-Ag, then the flask was put in the photodegradation apparatus. Next, the Erlenmeyer in the apparatus flask was irradiated with the visible lamp accompanied by magnetic stirring for a period of time. The LAS left in the wastewater was analyzed by visible spectrophotometer at the wavelength of 650 nm based on the reaction with methylene blue. The concentration of the LAS was determined by plotting its absorbance on the corresponding standard curve. The same procedure was repeated for processes with TiO2-Ag having different Ag content and various irradiation time and photocatalyst dosage.
RESULTS AND DISCUSSION
The results of the research consists of influence of the initial concentration Ag into TiO2 structure, characterization of the Ag-doped TiO2 photocatalyst by using XRD, FTIR, DRUV/Vis. and TEM methods and the photoactivity evaluation for photodegradation of LAS in the laundry wastewater with various amount of Ag-doped, irradiation time and photocatalyst dose.
Influence of Ag(I) initial concentration on the amount of Ag-doped in TiO2: The amount of Ag-doped on TiO2 prepared from various concentration of AgNO3 solution is presented as Fig. 3. The reaction of Ag-doped on TiO2 structure via photodeposition was initiated by the formation of electron (e) and hole (h+) pair (Eq. 1) during UV-light (or hv) exposure, then the electron was captured by Ag+ ion to induce reduction to form Ag0 (Eq. 2)5:
It is seen in Fig. 3, a sharp increasing amount of Ag-doped when the initial AgNO3 concentration was increased, but only slight increase was observed for the initial concentration higher than 150×103 M. For very high concentration, large amount of Ag has been doped on TiO2 structure that might cover the surface of TiO2, that inhibited the contact between TiO2 with the light. Consequently, the less electrons was released from TiO2-Ag that resulted in slower reduction of Ag(I) in the solution. Similar results were also found by previous study12,13,15,16. Based on the amount of Ag-doped, the photocatalyst prepared were coded as TiO2-Ag(46), TiO2-Ag(90), TiO2-Ag(131), TiO2-Ag(150) and TiO2-Ag(165).
XRD data: The XRD patterns of TiO2 doped with Ag is displayed as Fig. 4. Several 2θ values of 25.25°, 37.52°, 48.02°, 53.58°, 54.88°, 62.61°, 68.78°, 70.33°, 75.07° and 82.68° are observed, which were assigned to anatase TiO213,15,16. It is also observable that doping Ag caused a decrease in the XRD intensities and the decrease was larger with the increase of the amount of Ag-doped13-16. The decrease of the intensities represented the partial crystallinity destroyed, due the insertion of Ag dopant in the TiO2 lattice14,15. In addition, new peaks of Ag were not appeared, implying that Ag particles were inserted in the lattice of TiO2 and /or the small amount of Ag were formed in TiO2-Ag that were un-detectable by XRD13-16. It could be confirmed that Ag particles have been doped in the structure of TiO2.
Influence of AgNO3 initial concentration on the amount of Ag-doped in TiO2
XRD patterns of (a) TiO2, (b) TiO2-Ag (46), (c) TiO2-Ag (131) and (d) TiO2-Ag (165)
IR spectra of (a) TiO2, (b) TiO2-Ag (46), (c) TiO2-Ag (131), and (d) TiO2-Ag (165)
DRUV spectra of (a) TiO2, (b) TiO2-Ag (46), (c) TiO2-Ag (131) and (d) TiO2-Ag (165)
Band gap energy changed resulted by Ag doping
FTIR data: In the Fig. 5, it was seen that the IR spectra of all TiO2-Ag samples are similar to that of un-doped TiO2 where several peaks are observed at around 3400, 2450, 1630 and 700-500 cm1 of the wavenumbers. Some studied also reported the same IR spectra13,15,16. The peaks appearing at around 3400 and 2450 cm1 were attributed to the Ti-OH bond. In addition, the spectra also showed as harp band at ∼1630 cm1 due to the OH bending vibration of chemisorbed and/or physisorbed water molecule on the surface of the catalysts14. The strong band in the range of 700-500 cm1 appearing was attributed to stretching vibrations of Ti-O-Ti bond. The FTIR spectra of all Ag-TiO2 samples revealed a weak peak at about 1385 cm1, which was not observed for the un-doped TiO2. The intensity of this peak was seemed to increase with the increase of Ag amount doped in TiO2 . The peak at 1385 cm1 was assigned tentatively to the interaction between Ag and TiO2 particles13,15,16 implying that Ag particles have been inserted in the TiO2 structure.
DRUV data: The DRUV spectra was exhibited by Fig. 6. Attributing that Ag doping into TiO2 has shifted the absorption into longer wavelength emerging the visible area. The shift was resulted by narrowing the gap or reducing the band gap energy (Eg) as presented in Table 1. A study has also found the same finding14,15. It was implied that the narrowing was created by the insertion of Ag particles into the gap between conduction and valence bands. Further, increasing amount of Ag-doped in TiO2 gave higher effect in the declining the Eg values, but very large Ag amount caused the narrowing decreased.
TEM images of (a) TiO2, (b) TiO2-Ag (46), (c) TiO2-Ag (131) and (d) TiO2-Ag (165)
With very large Ag amount that might form larger aggregate, some Ag particles were inhibited to insert into the gap causing smaller amount of Ag could be doped.
The enhancement of light absorption in the visible region provided a possibility for improving the photocatalytic performance of TiO2 under visible light irradiation as also previously reported13-16.
TEM data: In order to investigate the surface morphology of the synthesized Ag-doped TiO2 nanoparticles, SEM studies were performed. The SEM images of TiO2 and Ag/TiO2 samples are shown in Fig. 7a-d. As can be seen, most of the TiO2 particles are spherical or square shaped with a particle size of 15-35 nm (Fig. 7a). The dark spots with diameter of 1-5 nm were seen uniformly deposited in the TiO2 structure prepared from lower Ag(I) concentration. The Ag formed in the TiO2 crystal clearly confirmed that Ag particles were inserted in the lattices. The larger dark spots and more spots on the TiO2 surface are observable with the further increasing amount of Ag-doped. Some studies reported the similar findings15,16. These data well agreed with the data from XRD and FTIR analysis was.
Effectiveness of the degradation with the condition of (1) TiO2-Ag under visible light, (2) TiO2-Ag under UV light, (3) TiO2 under visible light and (4) TiO2 under UV light
Concentration of LAS in the laundry wastewater: The concentration of LAS in the laundry wastewater was measured as 110.05 mg L1, which was beyond the permissible level in the corresponding wastewater that was 2 mg L1 as regulated by Indonesian Government1.
Influence of Ag-doping on the activity of TiO2: The results of the LAS photodgeradation under visible and UV light, whether over un-doped and doped TiO2 are illustrated in Fig. 8.
|Fig. 9:||Influence of the amount of Ag-doped in TiO2
|Fig. 10:|| Influence of the irradiation time
Influence of the photocatalyst dosage on the LAS degradation
Figure 8 assigned that the activity of the doped TiO2 or TiO2-Ag in the LAS photodegradation under visible was higher compared to that of the un-doped TiO2 photocatalyst. The improvement of the activity of the doped photocatalyst under visible light exposure was promoted by decreasing gap energy (Eg) that allowed the photocatalyst to absorb visible light, which could not occur in the un-doped TiO2 . Additionally, the doped of TiO2-Ag also showed higher activity in the UV light than the un-doped one did. In the doped photocatalyst of TiO2 -Ag, the Ag dopant was found as electron-hole separation center, where electron transfer from the TiO2 conduction band to Ag particles at the interface was thermodynamically possible, because the fermi level of TiO2 was higher than that of Ag metal11,13,16. This doping resulted in the formation of a Schottky barrier at metal semiconductor contact region and improved the photocatalytic activity11,13 of TiO2. Hence, doping of Ag atoms essentially reduced the band gap of TiO2 for the photo-excitation or red shift and simultaneously reduced the recombination rate of photogenerated electron-hole pairs.
Moreover, photodegradation over un-doped TiO2 under UV light was more effective than that of under visible light. The Eg of TiO2 was 3.2 eV that was equal to UV light allowing TiO2 to be activated by UV irradiation and so more OH radicals could be provided. In contrast, the energy of visible light was lower than the Eg of TiO2 that could not promote TiO2 in releasing electrons.
Influence of Ag amount doped in TiO2: Figure 9 illustrated that increasing amount of Ag-doped gave raise in the photodegradation efficiency and reached maximum at 131 mg g1 of Ag amount in the photocatalyst. More amount of Ag dopant should elevate the retardation of the recombination, that further enhanced the photodegradation of LAS in the wastewater. On the contrary, at the Ag content beyond its optimum value, the degradation efficiency of LAS seems to be detrimental. With further more Ag content, the Ag particles may also act as recombination centers of e and h+ that should reduce the number OH radicals available . The excess doping of metal ions might also cover the active sites on the TiO2 surface, thereby inhibiting the OH radical formation13,15,16. These conditions explained the decrease in the photodegradation. Same finding was also found by some studies12-16.
Influence of irradiation time: As seen in Fig. 10, prolong irradiation time up to 24 h could improve of the degradation, but further expansion of the irradiation time longer than 24 h, no influence on the degradation could be observed. Photodegradation than 24 h led to the photocatalyst exhausted in the OH radical formation, that was un-abled to improve the degradation15.
Influence of photocatalyst dose: Increasing dosages of the photocatalyst gives different effect on the LAS photodegradation as demonstrated by Fig. 11. The increase of the photocatalyst dosage provided more OH radicals, that could enhance the photodegradation. The larger dosage that exceeded the optimum level caused a detrimental in the photodegradation. The excessive photocatalyst made the turbidity of the mixture elevated that could inhibit the penetration light. As a consequent, the OH radical formation was prevented and the photodegradation efficiency was reduced. Other studies also found similar results13,15,16.
From this study it was concluded that, the Ag dopant was confirmed to be inserted in the TiO2 lattice that narrowed the band gap energy, allowing it to absorb visible light. It was found that doping Ag on TiO2 structure could improve its activity in the photodegradation of LAS in the laundry wastewater driven by visible light irradiation. The highest degradation (~78%) of LAS as much as 110.05 mg L1 contained in the laundry wastewater could be achieved by using TiO2-Ag with 131 mg g1 of Ag-doped during 24 h of the irradiation time and with 50 mg/100 mL of TiO2-Ag dose.
This study discovered the doping TiO2 with Ag, conducted by simpler and cheaper method that could successfully enhance TiO2 photocatalytic activity under visible light illumination for effectively removal of Linear Alkyl- benzene Sulfonate (LAS) in laundry wastewater. It can be beneficial for the developing photocatalyst material as well as a treatment method of the hazardous LAS residue in the laundry wastewater. This study will help the researchers to uncover the critical areas of wastewater treatment method that many researchers were not able to explore. Thus, a new theory on linear alkyl-benzene sulfonate photocatalytic degradation under visible light irradiation may be arrived at”.
Authors greatly thank to Universitas Gadjah Mada for the Research Project Grant through Final Project Recognition (RTA) schema with the Contract No:3193/UN.1/ DIT-LIT/LT/2019 April 11, 2019.