*2.3. Photocatalytic Test*

A stock solution of 1000 ppm was prepared from Levafix Dark Blue, and a further dilution was carried out when needed to obtain 50, 100, 150, and 200 ppm. Initially, 100 mL of the 100 ppm of Levafix Dark Blue dye-containing aqueous solution was poured into a glass container to subject it to the photocatalytic test. Then, a certain amount of Fuller's earth augmented with hydrogen peroxide (30% *w*/*v*) supplied by Sigma-Aldrich (Burlington, MA, USA) was poured into a container as the source of the Fenton photocatalyst and placed in a photochemical reactor. The mixture was magnetically stirred and kept under UV illumination after the pH was adjusted, if desired (using AD1030, Adwa instrument, Szeged, Hungary), over the range of 3.0 to 8.0. The pH of the dye aqueous solution was adjusted to the desired values by using diluted H2SO4 (1:9) and/or 1M NaOH solutions (Sigma-Aldrich). All chemicals were used as received from the supplier without further treatment or purification.

In order to validate the effect of the Levafix Dark Blue dye concentration on the extent of photocatalytic oxidation, the polluted water with the reagents was subjected to the photocatalytic system. A UV lamp (15 W, 230 V/50 Hz, with a 253.7 nm wavelength) was used to emit UV light during the reaction. The lamp was covered with a silica tube jacket for lamp protection, still allowing the UV to penetrate the dye-containing solution. The sleeved UV lamp was located inside a glass vessel containing the wastewater solution to well induce and accelerate the photocatalytic reaction. The photo-reactor had a 250 mL volume, and the reactor was fully exposed to UV light.

In regular time intervals (every 10 min), the samples were subjected to analysis after filtration (0.45 μm) to remove the remaining excess catalyst using a UV–visible spectrophotometer (Unico UV-2100, Franksville, WI, USA). The results of the analysis were recorded, and the data are presented as the dye percentage removal. The experimental setup is summarized in Figure 1.

**Figure 1.** Schematic representation of the catalyst preparation and treatment steps.

### *2.4. Characterization Study*

The morphologies of the attained Fuller's earth sample were explored and imaged using a field-emission scanning electron microscope (FE-SEM) (FE-SEM, Quanta FEG 250, Technical Cell, Kolkata, India). The typically used magnifications were ×8000 and ×60,000. Furthermore, this instrument was supplemented by energy-dispersive X-ray spectroscopy (EDX) in order to assess the content of principal oxides in the Fuller's earth. The oxides were examined via the energy-dispersive spectrum. Moreover, a Fourier transform infrared FTIR spectrum was obtained using the Jasco FT/IR-4100 model type (Jasco Inc., Mary's Court Easton, MD, USA).
