Surface modification of polypropylene nonwoven with chitosan, TiO2, and Ag nanoparticles for the removal of acid dye from water

In this study, polypropylene nonwoven was coated with chitosan after being activated by oxygen/argon low-pressure plasma. The chitosan-treated sample (PP-Ch) was coated with TiO2 nanoparticles by a pad-dry method. Silver nanoparticles were in situ synthesized on the chitosan-TiO2-coated sample by a chemical reduction method. The morphology of each of the samples mentioned above was evaluated by FESEM. The efficiency of each sample in the removal of C.I. Acid Brown 248 from aqueous media was evaluated. The PP-Ch sample removed 90% of the dye at pH=3. Coating the PP-Ch sample with TiO2 and silver nanoparticles improved the dye removal efficiency under UV irradiation.


Materials and methods
PP spunpond nonwoven with a density of 20 g/m 2 was obtained from Baftineh CO., Iran, and scoured with a nonionic surfactant before use.Medium molecular weight chitosan and silver nitrate were obtained from Merck, Germany.Titanium Dioxide nanoparticles with average particle size of 15-25 nm were obtained from Tecnan, Spain.
Chitosan coating: plasma treatment was done on a piece of low-pressure equipment made by Basafan, Iran.A mixture of oxygen and argon was used as the processing gas, while the flow rate of each gas was 50 Sccm.Plasma treatment was done for 10 min with a power of 150 W. The plasma-treated samples were immediately immersed for 5 min in a solution containing 1% chitosan and 1% acetic acid.Finally, the samples were padded with the pressure of 2 bar and dried in a stenter at 80 °C for 5 min.
Silver nanoparticle coating: PP-Ch-TiO 2 samples were immersed in silver nitrate solution (200 ppm) for 10 min, and then transferred to a solution of sodium borohydride (400 ppm) for 10 min to reduce the silver ions to silver nanoparticles (PP-Ch-TiO 2 -Ag).
Dye removal studies: 5 solutions of C.I. Acid Brown 248 in distilled water (100 mg/L) were prepared at different pH values ranging from 1 to 10.To assess the dye removal efficiency of PP-Ch at each pH value, 0.1 g of the adsorbent was immersed in 25 mL of the dye solution, and the absorption of the solution was measured in different time intervals (up to 120 min) using a UV-Vis spectrophotometer (EU-220, Onlab, China) at 387 nm.The dye removal efficiency of the nanoparticle-coated samples was measured by the same procedure under UV irradiation (UVC, 400 W).
Surface morphology: field emission scanning electron microscope (Mira3, Tescan, Czech Republic) was employed to study the surface morphology of the samples after each modification stage.

Results
Figure 1 shows the FESEM images of the raw and plasma-treated samples at two magnifications.The surface of the raw fibres is smooth.It is evident that plasma treatment created some etching and increased the surface roughness due to the bombardment of the fibres with high-energy species.Furthermore, oxygen-containing groups are created on the surface of PP fibres after plasma treatment which has been approved in previous studies [2].These groups are responsible for the attachment of chitosan on the surface of PP fibres.
Coating of PP fibres with chitosan biopolymer can be seen in Figure 2a, and the presence and uniform distribution of the titanium dioxide and silver nanoparticles can be seen in Figures 2b and 2c as well.EDX images were also taken (not shown here), which confirmed that the titanium dioxide and silver nanoparticles are present on the surface and well distributed.
Figure 3 shows the percent dye removal by PP-Ch at various pH levels.It can be seen that the maximum dye removal took place at pH = 3 which is due to the protonation of the amine groups of chitosan and the electrostatic attraction between the dye anions and the positive charges on the modified fibres [15].
The change of percent dye removal by PP-Ch-TiO 2 samples containing different amounts of TiO 2 nanoparticles under UV radiation during times ranging from 15 to 60 min is shown in Figure 4.The dye removal efficiency increased by increasing the exposure time.
Figure 5 shows the dye removal efficiency of the samples after 60 min of UV irradiation.It can be seen that the optimum amount of TiO 2 nanoparticles loading was 0.5%, and the dye removal efficiency was decreased at higher TiO 2 nanoparticles loading, which may be due to the agglomeration of the nanoparticles on the surface and avoiding the accessibility of the dye molecules to the adsorbent sites.The fundamental mechanisms of TiO 2 photocatalytic activity have been discussed in many studies [13].
Figure 6 compares the chitosan-coated sample with the samples coated with optimum amounts of TiO 2 and silver nanoparticles.It can be seen that the coating with TiO 2 nanoparticles increased the dye removal efficiency under UV irradiation considerably, due to the photocatalytic activity of these nanoparticles.Adding silver nanoparticles on the surface of the PP-Ch-TiO 2 sample made a slight improvement in the dye removal efficiency.According to the literature, silver nanoparticles possess some photocatalytic activity.Furthermore, the presence of silver nanoparticles enhances the TiO 2 photocatalytic activity by trapping the electrons transferred from the conduction band of the TiO 2 semiconductor and transferring these electrons to oxygen which converts them to superoxide radicals which decompose the dye molecules.The surface plasmon resonance of silver nanoparticles extends the light absorption to the visible light region and enhances the photocatalytic activity of TiO 2 nanoparticles under visible light [13,14].

Conclusion
PP nonwoven was functionalized by plasma, chitosan, TiO 2 , and silver nanoparticles, respectively.The ability of the modified samples for the removal of an anionic dye from the water was studied.The PP-Ch sample removed about 90% of the dye at pH = 3.The application of TiO 2 and Ag nanoparticles on the surface of PP-Ch enabled the adsorbent to degrade the dye molecules under UV irradiation, and the dye removal efficiency was improved.The prepared photo-active adsorbent can be a good candidate for adsorptive and photocatalytic removal of anionic dyes from textile wastewater.

Important note
This paper has been presented at the 10th ULPAS held on 13-14 May 2022 at İstanbul Technical University, İstanbul, Turkey.

Figure 3 .
Figure 3.The effect of pH on dye removal by PP-Ch.Figure 4. Effect of time on dye removal efficiency of samples with various TiO 2 loading under UV irradiation.

Figure 4 .
Figure 3.The effect of pH on dye removal by PP-Ch.Figure 4. Effect of time on dye removal efficiency of samples with various TiO 2 loading under UV irradiation.

Figure 5 .
Figure 5.Effect of TiO 2 loading on dye removal efficiency of PP-Ch-TiO 2 samples after 60 min.