## 3 Results and Discussion
### Photocatalytic Removal Efficiency  
Fe\({}_{3}\)O\({}_{4}\) and BiOI/Fe\({}_{3}\)O\({}_{4}\) were synthesized using the solvothermal method following the reported procedure. The synthesized magnetite is shown in Figure 9a, and the BiOI/Fe\({}_{3}\)O\({}_{4}\) photocatalyst in Figure 9b. Figure 8: VSM magnetization curve for Fe\({}_{3}\)O\({}_{4}\) and BF-5 (BiOI/Fe\({}_{3}\)O\({}_{4}\)). Figure 7: FTIR spectra (**a**) BiOI; (**b**) Fe\({}_{3}\)O\({}_{4}\); and (**c**) BF-5. There are several synthesis routes to manufacture this class of materials. However, the solvothermal synthesis that uses ethylene glycol as a solvent and equimolar amounts of Bismuth nitrate pentahydrate (Bi(NO\({}_{3}\))\({}_{3}\).5H\({}_{2}\)O) and of KI has been used with positive results in obtaining BiOI/Fe\({}_{3}\)O\({}_{4}\) photocatalysts with high photocatalytic activity and magnetic properties [20, 64]. It is important to consider the molar ratio between Bi and Fe in the manufacture of the BiOI/Fe\({}_{3}\)O\({}_{4}\) photocatalyst, as an optimal ratio favors the growth of BiOI sheets on the surface of the magnetic core, forming a core-layer type structure [18]. This structure prevents iron oxide (magnetic) corrosion in the aqueous environment and improves the material's photocatalytic activity [18, 20, 22]. In addition, the ratio between Fe\({}_{3}\)O\({}_{4}\) and BiOI influences the recombination process of photogenerated charge carriers, as the recombination rate of electrons and holes is reduced, to some extent, by the addition of Fe\({}_{3}\)O\({}_{4}\)[18]. This means that the smaller the amount of magnetite, the higher the recombination rate will be (Figure 10). The research conducted by [PERSON] et al. [22], [PERSON] et al. [21], and [PERSON] et al. [18] indicate that a Bi/Fe ratio greater than 5:1 would reduce its photocatalytic activity due to an increased recombination of the electron-hole pairs. Considering this background, four stoichiometric ratios between Bi/Fe: 2:1 (BF-2), 3:1 (BF-3), 5:1 (BF-5), and 7:1 (BF-7) were evaluated in this study (Figure 11), the results of which indicated that the most favorable stoichiometric ratio was the 5:1 ratio (Figure 10). Control experiments were also carried out to evaluate the photocatalytic activities of the pure compounds of BiOI and Fe\({}_{3}\)O\({}_{4}\), as the degradation of caffeic acid without a catalyst in suspension was also measured under simulated visible light radiation using 25W LED  
Figure 10: Percentage of degradation after 180 min of the model pollutant under different Bi/Fe ratios. Figure 9: (**a**) Fe\({}_{3}\)O\({}_{4}\) and (**b**) BiOI/Fe\({}_{3}\)O\({}_{4}\) synthesized by the solvothermal method. lamps. This compares the photocatalytic performance of the BiOI/Fe\({}_{3}\)O\({}_{4}\) heterostructures synthesized after 180 min of reaction. The synthesized compounds BF-2, BF-3, BF-5, and BF-7 showed photocatalytic efficiencies in caffeic acid degradation of 59.1%, 65.0%, 77.2%, and 71.7%, respectively. An analysis of variance (ANOVA) was performed with a 95% confidence level to determine the significance of the differences in these performances. The \(p\)-value obtained (0.018) indicates statistically significant differences between the efficiencies of the different compounds, which suggests that variations in the composition and structure of photocatalysts significantly influence their ability to degrade phenolic compounds. This is consistent with the research conducted by [PERSON] et al. [18], whose results indicate that the ratio between Bi and Fe influences the recombination process of photogenerated charge carriers as the recombination rate is reduced, to some extent, by the addition of magnetite. It was determined that the reaction kinetics for the degradation of the model contaminant using BF-5 are pseudo-first-order; other authors have obtained the same results [18; 19; 21].