3.1 Morphology of silk fibroin membranes
Pure SF membranes and blended SF membranes of NaCl and PEG were simply prepared by the drop-casting method. The physical characteristic (i.e., color) of pure SF membrane slightly differed from the blended membranes as presented in Fig. 2(a), (b), and (c). The blended SF membrane had high turbidity, while the membrane surface of SF/PEG was the cloudy white color. The obtained membranes are homogenous.
Figure 2(d), (e), and (f) show the morphology of the surface and cross-sectional picture of all prepared membranes in this study. As observed, the SF/NaCl has high porosity compared with the SF/PEG and SF membranes, respectively. For the SF/PEG membrane, many wrinkles were found on the membrane surface. For porosity from the natural polymer by using crystalline sodium chloride (NaCl) salt and mixed with a solution mixture main component is polyamide. Then formed by baking, NaCl crystals are dissolved with water. This crystal causes the missing portion of the sodium chloride to become porous. The sodium chloride was applied to control scaffolds with a pore size (Park et al. 2010; Yan et al. 2012). The influence of NaCl on silk fibroin solution is show in Figure 2(e) and Figure 2(h). Moreover, in contrast with Pacharawan et al. 2013, this study can improve the porosity of silk fibroin membrane using polyethylene glycol.
The FTIR spectra of SF, SF/NaCl, and SF/PEG membranes were collected to investigate the change of surface chemistry of the prepared silk membranes. As presented in Fig. 3, the characteristic structure of silk II at the adsorption peak of 1620 cm−1, were found in all membranes. This peak refers to the formation of β-sheet structure in the silk membrane.
The amount of β-sheet crystals found on SF/NaCl membrane was higher than that on SF and SF/PEG membranes. Silk fibroin is consists of β-sheet crystallites and amorphous domains, and these can building blocks (Yuan et al. 2014). Keten et al. 2010 indicated β-sheet crystallites within a few nanometers, much higher stiffness and strength must be achieved (Keten et al., 2010). Water significantly impacts the mechanical properties, making the membrane more feasible (Fu C, 2009). Meanwhile, Yuan et al. found that water molecules play a weakening forming hydrogen bonds, β-chains, and the instability of the crystallite (Cheng et al. 2014).
The hydrophilicity of the studied SF membranes was also investigated using the water contact angle method. As shown in Fig. 4, the contact angle of the SF/NaCl and SF/PEG blended membranes was decreased due to the addition of NaCl and PEG to SF matrix. Among the studied membranes, SF/NaCl blended membranes have the lowest contact angle of 22.25o followed by 57.85o for SF/PEG membrane and 72.06o for pure SF membrane. The difference in hydrophilicity in the membrane may come from the surface roughness and membrane pores (Chan and Ng, 2016; Mohammad et al. 2019). Increased surface hydrophilicity can make the membrane fouling resistant due to the water's easy diffusion through the membrane thickness. Hence, the SF/NaCl blended membrane has better chances of antifouling ability and higher water flux.
3.2 Pure water flux (flow rate)
The deionized water was treated by keeping the temperature (20ºC) and flow rate varying the pressure as 100 mbars, 200 mbars, 300bars, 400 mbars, and 500 mbars. As shown in Fig. 5, the flow rate of membrane permeation was increased from 120 mL/min to 1033.3 mL/min when pressure was increased from 100 mbars to 500 mbars. The increasing pressure during the membrane contact with water can reduce the number and strength of hydrogen bonds between β-chains, thus dramatically weakening the strength of silk fibroin (Cheng et al. 2014; Love et al. 2019; Yang et al. 2019; Kook et al. 2019).
The temperature effect of flowing rate of SF/NaCl membrane was treated by keeping the pressure (200 mbars) constant and then varying the temperature at 5ºC, 10ºC, 20ºC, 30oC, and 45ºC of initial water. The average flow rate of membrane permeation was increased from 730.0 mL/min to 1035.3 mL/min when pressure was increased from 5 oC to 45 oC. Then the average flow rate increased to 951.7 mL/min at 30 Co, but it was decreased to 1045.3 ml/min at 40 Co, as shown in Fig. 6. The water temperature between 5 – 20 oC showed flow rate stability to increase, but after 20 oC was the flow rate instability. The water molecules influence the glass transition of silk (Yazawa et al. 2016), the effect of thermal water can be changing the flow rate properties. In addition, heating solution cast fibroin can be converted to the silk II β -sheet structure commonly found in B. mori cocoon fibers (Drummy et al. 2005).
The water pH range was 7-13. The pH effect of the flowing rate of SF/NaCl membrane, the metal wastewater sample was treated by keeping the pressure (200 mbars) constant. The average flow rate of membrane permeation was increased from 180.0 mL/min to 597.3 mL/min when pH was increased from 7 to 13. Then it was decreased to 540.0 mL/min at pH 13, as shown in Fig. 7.
3.3 Heavy metal removal and efficiency.
The SF/NaCl membranes were applied to remove the three metals Cd, Pb, and Hg in deionized waters. It was the concentration 10 ppm. Various pH of metal waters of Cd, Pb, and Hg has shown the % removal in Fig. 8.
pH
Cd removal was found in the range of 10.76 – 45.36 %, as shown in Fig. 8. The highest % removal for Cd was 45.36% at pH 12. At the same time, Pb % removal was found to range from 10.80 – 61.43%. The highest % removal for Pb was 61.43% at pH 12. The highest % removal for Hg was 78.18% at pH 9. The mean concentration of Hg removal was range 70.20 – 78.18 % for pH 7-12. Comparing the % removals of Cd, Pb, and Hg found that Hg was high efficiency for metal removal by use SF/NaCl higher than Cd and Pb at pH 7-12.
Temperature
The various metal water temperature was range from 5 – 40 oC, as shown in Fig. 9. Cd removal was found in the range of 6.90 – 15.04 %. The highest % removal for Cd was 15.04% at 5 oC. At the same time, Pb % removal was found the range of 13.93 – 16.27%. The highest % removal for Pb was 16.27 % at 40 oC. The highest % removal for Hg was 72.72 % at 30 oC. The mean concentration of Hg removal was range 61.26 – 72.72 % for temperatures 5 – 40 oC. Comparing the % removals of Cd, Pb, and Hg found that Hg was high efficiency for metal removal by use SF/NaCl higher than Cd and Pb at temperature 5 – 40 oC.
Pressure
The various metal water pressure in the filtrated column was range 100 – 500 mbars as shown in Fig. 10. Cd removal was found to range from 2.90 – 16.34 %. The highest % removal for Cd was 16.34 % at pressure 500 mbars. At the same time, Pb % removal was found to the range to 2.50 – 16.13%. The highest % removal for Pb was 16.13% at pressure 200 mbars. The highest % removal for Hg was 86.87 % at pressure100 mbars. The mean concentration of Hg removal was range to 62.71 – 86.87 % for pressure 100-500 mbars. Comparing the % removals of Cd, Pb, and Hg found that Hg was high efficiency for metal removal by use SF/NaCl higher than Cd, and Pb at pressure 100-500 mbars.
Comparison of the % of metal removal is shown in Table 1. This study showed that the highest % removal of Cd, Pb, and Hg were 45.36%, 61.43%, and 86.87%, respectively. This study, apply SF/NaCl shown that % Cd removal is lower than research studies by using the MWSF (modified water-insoluble silk fibroin) and chitosan/silk fibroin films (Ramya and Sudha 2013; Gao et al., 2017). In addition, the % Pb removal is lower than the studies of Gao et al., 2017 and Zhao et al., 2020. SF/NaCl membrane is excellent for removal Hg greater than Metallic molybdenum disulfide (MoS2)/Silk nanofibril (Zhao et al., 2020).
The average membrane mass (m: unit, gram (g)) was 1.76 g, and the metal solution volume (Vs: unit, liter(L)) was range 0.12 – 1.04 liters. The adsorption capacity (mg/g) of Cd, Pb, and Hg for SF/NaCl membrane is shown in Table 2. In this study, the highest adsorption capacity of Cd was 8.50 mg/g of metal water at 5 oC, at pH 7 and, pressure 200 mbars. For Pb found that the highest adsorption capacity was 6.42 mg/g of metal water at pH 12, at 20 oC, and pressure 200 mbars. Finally. The highest adsorption capacity of Hg was 41.14 mg/g of metal water at 100 mbars, at pH 7, and at 20 oC. The pH of the metal solution found that Cd and Pb were high efficiencies of adsorption capacity at pH 12, and Hg was pH 9. In addition, the appropriate temperature of a metal solution of Cd, Pb, and Hg was at 5 oC, 40 oC, 30 oC, respectively. The best condition of absorption capacity was 8.50, 6.42, and 41.14 mg/g for Cd, Pb, and Hg, but the result was lower than the study of Cd and Pb absorption capacity of Rastogi et al. 2020.
Table 1
Comparison between the membranes in this study and others reported
Filtration Membrane
|
% Removal
|
References
|
Cd
|
Pb
|
Hg
|
SF/NaCl
|
45.36
|
61.43
|
86.87
|
This study
|
MWSF (modified water-insoluble silk fibroin)
|
65.00
|
82.00
|
-
|
Gao et al., 2017
|
Chitosan/silk fibroin
|
81.10
|
-
|
-
|
Ramya and Sudha 2013
|
Metallic molybdenum disulfide (MoS2)/Silk nanofibril
|
-
|
65.00
|
62.00
|
Zhao et al., 2020
|
Table 2
The adsorption capacity (mg/g) of Cd, Pb, and Hg for SF/NaCl membrane
Heavy metal
|
Adsorption capacity (mg/g)
: highest efficiency
|
pH
|
Temperature (oC)
|
Pressure (mbar)
|
Cd
|
4.77
(at pH 12)
|
8.50
(at 5 oC)
|
0.89
(at 500 mbars)
|
Pb
|
6.42
(at pH 12)
|
0.89
(at 40 oC)
|
5.08
(at 200 mbars)
|
Hg
|
8.07
(at pH 9)
|
4.34
(at 30 oC)
|
41.14
(at 100 mbars)
|