**2.5 Methylene blue removal testing**

The setup of MB removal testing was prepared as in the previous study [15]. 10 mg/L of MB solution was prepared in a 200 mL beaker. 0.8 g of the composite photocatalyst was added into a beaker. The suspension was first stirred without a UV lamp using a magnetic stirrer for 30 min for adsorption equilibrium. After taking the sample for adsorption, the UV lamp and the stirrer were turned on for photocatalytic activity testing. The experiment was conducted for 150 min and for each 10 min interval, the aliquot samples were taken and centrifuged at 6000 rpm [16] for 5 minutes to measure the concentration of MB with a UV–Vis spectrophotometer at 665 nm as the maximum absorbance [17]. The samples were analyzed by a UV–Vis spectrophotometer.

The calibration curve is a method for determining the sample concentration that must be completed before the photocatalytic suspension test. The absorbance value of each sample from each experiment run was determined using a UV–Vis spectrophotometer. The sample absorption value can be translated into concentration using the linear equation obtained from the calibration curve (ppm). After normalizing the measurement, the removal of MB can be computed using the formula:

$$\text{MB}\,\,\text{removed}\,\%=\frac{\left(\text{C}\_{o}-\text{C}\_{t}\right)}{\text{C}\_{o}}\times\text{100\%}\tag{1}$$

where R is the removal efficiency of MB in percentage, Co (mg/L) as initiate al concentration of dye solution, and Ct (mg/L) as the concentration of dye solution at equilibrium [18].
