*2.7. Catalytic Dye Reduction Application*

The catalytic reduction of rhodamine B (RhB) and methylene blue (MB) dye by NaBH4 as a model of the reaction has followed a method of Joseph and Mathew [21] and Vidhu and Philip [22] with some modification. Prior to the experiment, a mass of 0.1 g of *C. pentandra*/Ag-NPs was added into 100 mL of dye aqueous solution (20 mg/L). Thereafter, the suspension was magnetically stirred in the dark for 6 h for MB and 1 h for RhB dye, respectively. This step is important to establish the adsorption/desorption equilibrium between dye molecules and the surface of the Ag-NPs. After achieved the equilibrium, the freshly prepared of 10 mL of 0.1 M NaBH4 solution was added to start the reaction. An amount of solution was withdrawn every 1 min for RhB dye and every 5 min for MB dye. The solution was then centrifuged and subjected to UV–vis spectrophotometer to measure the absorbance at 554 nm for RhB dye and 664 nm for MB dye. The percentage of dye reduction was calculated using Equation (5) as below:

$$\text{Dye reduction } (\%) = \frac{(\mathbb{C}\_0 - \mathbb{C}\_l)}{\mathbb{C}\_0} \times 100 \tag{5}$$

where *C*<sup>0</sup> and *Ct* are the concentration of the dye solution at a time corresponding to 0 and *t* at the characteristic wavelength, respectively [23].

The kinetics of the dye reduction reaction by *C. pentandra*/Ag-NPs was studied by following the pseudo-first-order and pseudo-second-order kinetic model. The equation of the pseudo-first-order model was described in Equation (6) [21] as below:

$$\ln\left[\mathbb{C}\_{t}\right]/\left[\mathbb{C}\_{0}\right] = kt \tag{6}$$

where, [*C*0] is the concentration of dye at time *t* = 0, [*Ct*] is the concentration at time *t, t* is reaction time and *k* is pseudo-first-order rate constant. The plot of ln[*Ct*] against *t* was plotted to get the *k* constant value.

Moreover, the equation of the pseudo-second-order model was described in Equation (7) [23] as below:

$$\frac{1}{\mathcal{C}\_t} - \frac{1}{\mathcal{C}\_0} = kt \tag{7}$$

where, *Ct* and *C*<sup>0</sup> are the concentration of the dye solution at times *t* and 0 min, respectively, *k* is the pseudo-second-order rate constant. The plot of *t*/*Ct* against *t* was plotted to get the *k* constant value.
