3.7.2. Methylene Blue Dye

The aqueous solution of MB dye appeared strong absorption peak at 664 nm (λmax) and reduction of MB dye with time by *C. pentandra*/Ag-NPs was monitored at this peak. The decreasing of absorbance peak at 664 nm with times reflects the decrease in the concentration of MB in the system. This peak was reduced to leucomethylene blue [47]. The reduction of MB dye by NaBH4 in the absence of *C. pentandra*/Ag-NPs as nanocatalyst is shown in Figure 17a. From the figure, it shows that a similar trend was observed as in RB dyes where there are no significant changes observed after 120 min. This proved that the reduction of MB dye by NaBH4 is not kinetically favorable [48]. However, after the addition of *C. pentandra*/Ag-NPs, the fast reduction time occurred within 20 min (99%) as shown in Figure 17b. The fastest reduction time was achieved at 20 mg/L MB using *C. pentandra*/Ag-NPs due to good distribution of Ag-NPs on the *C. pentandra* fiber surface to be contacted with the MB dye molecule and BH− <sup>4</sup> ions effectively [45]. The peak observed at 425 nm is corresponding to the peak of Ag. However, a lower intensity of the Ag peak was obtained suggesting that the release of Ag in the solution was controlled by the attachment of Ag-NPs on the *C. pentandra* fiber surface. The amount of Ag detected in all solutions was less than 5%. This result showed that the Ag-NPs loaded in *C. pentandra*/Ag-NPs can limit the release of Ag-NPs in the environment at only low concentrations of Ag. This is happened due to the Ag-NPs after loaded in *C. pentandra* fiber are strongly attached to the *C. pentandra* fiber. This result suggests that the *C. pentandra*/Ag-NPs increase the stability and avoid the agglomeration of Ag-NPs from the attachment on the *C. pentandra* fiber surface without decreasing the catalytic performance of Ag-NPs besides controlling the release of Ag-NPs into the solution.

**Figure 17.** UV–vis spectra of reduction of MB dye by (**a**) NaBH4 alone, (**b**) in the presence of *C. pentandra*/Ag-NPs and (**c**) the proposed mechanism of *C. pentandra*/Ag-NPs as nanocatalyst for the reduction of MB dye.

The proposed mechanism of catalytic reaction by *C. pentandra*/Ag-NPs on the MB dye reduction can be illustrated in Figure 17c. The catalytic reduction by *C. pentandra*/Ag-NPs followed electron transfer process from Ag-NPs catalyst to MB dye molecules. This electron transfer is depending on the small size of Ag-NPs to make contact with the dye molecules. Then, the MB dye molecules earn the electron from the catalyst surface to reduce its colorless and product.
