Synthesis of Fe-zeolites from Amazonian kaolinite for methylene blue removal: adsorption and photocatalytic activity

Zeolites exhibit high adsorption capacities for organic substrates and catalytic activity owing to their unique structural features, such as uniform pore sizes and Lewis acidity. However, their inherent UV-vis transparency (<i>λ</i> > 240 nm) limits their efficacy as photocatalysts. This limitation can be addressed by incorporating transition metals into their framework or encapsulating semiconductors within their porous structures. In this study, Fe-zeolites were synthesized through alkaline fusion of natural Amazonian kaolinite, followed by hydrothermal treatment with varying Fe³⁺ loadings from Fe(NO₃)₃. In the absence of Fe(NO₃)₃, an LTA zeolite was obtained under these experimental conditions. However, the introduction of Fe(NO₃)₃ led to the formation of a cancrinite (CAN) zeolite, suggesting that nitrate anions acted as a structure-directing agent. XRD and Mössbauer spectroscopy confirmed the presence of Fe-doped cancrinite and dispersed maghemite within FeZEO-3 and FeZEO-5. Methylene blue adsorption onto Fe-zeolites was best described using the Langmuir isotherm model and followed pseudo-second-order kinetics. An increase in iron content from 0 to 5% (w/w) led to enhanced photoactivity, as evidenced by a decrease in band gap energy from 3.21 eV to 2.93 eV and finally to 2.53 eV. This improved photoactivity was further supported by the observation of hydroxylated methylene blue intermediates during the photocatalysis process as the FeZEO-5 material removed up to 90% of the dye.