XRD analysis
Figure 1a,b displays the XRD patterns of CNSAC and Ag/CNSAC. The CNSAC's XRD pattern (Fig. 1a) exhibits a wide hump, which indicates that its structure is mostly amorphous [28]. Major peaks in the 2θ values of the Ag NPs loaded on CNSAC (Fig. 1b) are 38.08, 44.28, 64.39, and 77.37 [29]. They are allocated to the face centred cubic (FCC) metallic Ag (JCPDS No: 04-0783) lattice planes (111), (200), (220), and (311), respectively, indicating the presence of Ag NPs. The Scherrer equation was used to determine the average crystallite size of the Ag NPs loaded on the CNSAC sample [30]. Ag NPs loaded on the CNSAC sample were found to have an average crystallite size of 25 nm. These findings show that the Ag NPs were effectively loaded onto the CNSAC surface.
XPS Analysis
The XPS spectrums of Ag/CNSAC are shown in Fig. 2a-d, which reveals substantial signals of C, O, and Ag. On the other hand, high resolution XPS spectrums for Ag 3d, C 1s and O 1s of Ag/CNSAC were observed. The metallic silver is responsible for the Ag 3d 3/2 and Ag 3d 5/2 peaks, which are positioned at 367.18 eV and 374.6 eV, respectively. However, as seen in Fig. 2b, the spin-orbit splitting of the 3d doublet is 6.36 eV [31]. This B.E shows that the Ag on the CNSAC surface is metallic in character and supports the idea that Ag NPs have been effectively loaded onto the surface of the CNSAC. The C 1s (Fig. 2c) three peaks at 284.3, 285.4 and 288.1 eV represented C = C, C = O and C = C, respectively [32]. The O 1s (Fig. 2d) broad peak at 530.9 eV corresponds to the lattice oxygen of CNSAC/Ag [33]. Ag NPs will quickly become tightly adhered to the supporting CNSAC surface as a result of their interaction, which is extremely beneficial and required for the practical usage in water treatment.
Functional Group Analysis
Figure 3a, b displays the FT-IR spectra of CNSAC and Ag/CNSAC. Broad banding with a centre at around 3400 cm− 1 has been seen in all of the spectra. It is attributed to the hydroxyl group in water's O-H stretching mode (hydrogen bonded hydroxyl group). A distinct hydroxyl band is seen at a slightly higher frequency, or 3740 cm− 1, in addition to this broad and powerful one. Typically, O-H stretching results in a band at a lower frequency that behaves like it is hydrogen bound (since hydrogen bond weakens the O–H bond). As a result, the sharp band at 3740 cm− 1 may belong to a free hydroxyl group or one that is metal-bonded. The C = C vibration in the aromatics group of activated carbon is responsible for the band at 1552 cm− 1 [34]. C-O vibrations in esters, ethers, or phenol groups are responsible for the strong band seen in the area of 1300 − 900 cm− 1.
Morphological Study
Figure 4a-c shows the energy dispersive spectrum analysis (EDS) results and related SEM images of CNSAC and Ag/CNSAC. CNSAC features holes on its surface and a honeycomb-like shape [17]. More than 20% of elements other than carbon are found in the CNSAC's EDS spectra (Fig. 4b). Ag/CNSAC was clearly visible and had a sizable surface area (Fig. 4c). It has inner surface growth and large or tiny pores with many openings. EDS analysis has verified the presence of the Ag NPs loaded on CNSAC.
BET Surface Area Analysis
The porosity and surface area of the synthesized CNSAC and Ag/CNSAC are disclosed by the BET analysis. The N2 adsorption and desorption isotherm's hysteresis loop, which resembles a type IV H3 hysteresis loop with a mesopore structure [35], is shown in Fig. 5a,b. The CNSAC has a surface area of 246.2 m2/g and the loading of silver has raised that surface area to 271.5 m2/g. Similar to the CNSAC, the CNSAC's pore capacity rose from 0.281 to 0.341 cm3/g as a result of the loading of silver. The simple accommodation of dye molecules in the mesopores of both samples was validated by these results. Ag/CNSAC showed improved photocatalytic and adsorption characteristics for the removal of dye because to its greater surface area compared to CNSAC.
Photocatalytic Activity
The photocatalytic degradation rates of dye solution using CNSAC and Ag/CNSAC were estimated using Ct/C0 vs different time scales, as were the adsorption-desorption equilibrium rates of dye solution using CNSAC and Ag/CNSAC under dark conditions (Fig. 6a). The adsorption of dye molecules onto the surfaces of the CNSAC and Ag/CNSAC was shown to increase with time, and after 30 min, the majority of both samples had been saturated with MB. Approximately 1.6%, 44%, and 52% of the dye solution were subsequently adsorbed on the surfaces of MB, CNSAC, and Ag/CNSAC, respectively. Similar to this, Ag-loaded CNSAC had a photocatalytic dyes removal effectiveness of 98.33%. Finally, the CNSAC loading increased the adsorption of Ag/CNSAC from silver. Figure 6b shows the effects of contact time and % degradation of dye solution in the removal of the CNSAC and Ag/CNSAC. At 120 minutes, the removal efficiencies of MB were 66.6 and 98.3% for the two samples, respectively. In comparison to the CNSAC, the Ag loaded CNSAC exhibits a greater degradation percentage. The plot of ln (Co/Ct) with respect to irradiation duration (min) is shown in Fig. 6c, and a linear fit to the graph results in the slope of the fitted line representing the value of the rate constant (k) [36, 37]. The silver loaded CCAC have a highest value of k (0.028 min− 1) as compared to the CNSAC (0.004 min− 1) due to the high surface area. The enhanced photodegradation in Ag/CNSAC is explained by its mechanism (Fig. 6d). The Ag photocatalysts produce e−/h+ pairs with the help of sunlight irradiation. The hole may then interact with the H+ created by the H2O to create OH•. Following that, OH• will react with the MB dye molecules to cause degradation. O2 molecules in solution have the ability to absorb the electron and produce •O2− radicals. Then, •O2− reacts with the organic dye molecules in the MB to create degradation. They are the combined impacts of the MB dye's photocatalytic (Ag) and adsorption (CNSAC) degradation processes. Additionally, the CNSAC's high adsorption due to its wide surface area and the subsequent photodegradation of sunlight by the deposited Ag led to an improvement in photocatalytic performance [38–42].
Antibacterial Activity
By using the disc diffusion method, the antibacterial properties of CNSAC, Ag/CNSAC, and E. coli, V. cholera, and S. aureus were evaluated. The antibacterial activity of CNSAC, Ag/CNSAC, and the positive control (Ampicillin) against the pathogens used for analysis is shown in Fig. 7. In a similar vein, gram-negative bacteria E. coli likewise shown higher zone of inhibition (8 mm) than that of positive standard, with the maximum zone of inhibition against gram-positive S. aureus being found to be higher (11 mm) than that of the positive standard ampicillin. Gram-positive and gram-negative bacteria may have different antibacterial properties as a result of their different membrane structures. The peptidoglycan layer of gram-positive bacteria is thick, whereas the peptidoglycan layer of gram-negative bacteria is thinner but externally protected by a lipid layer [43].
The precise processes behind the inhibitory activity of Ag NPs and ACs are still not well understood. Numerous methods have been described by researchers, but several have hypothesized that NPs like silver may change the architecture of the membrane, impairing appropriate transport through it and ultimately leading to cell death. Free radicals-induced oxidative damage to the membrane is another explanation put forth [44]. By the aforementioned processes, antibacterial activity of ACs cannot be generated. But adsorption may be used to remove it. Electrostatic contact and van-der Waals forces can be used to manage the elimination of microorganisms that may contain an electrical charge [45]. In the current work, Ag NPs are primarily present in an isolated condition and on the exterior surface of CNSAC, which facilitates simple bacterial interaction with Ag NPs. As a result, the Ag NPs on the CNSAC's outside surface aid in enhancing antibacterial efficacy.