The sustainable synthesis of photocatalysts has attracted significant research interest because of their role in environmental and energy applications. This work aimed to improve a previously developed green synthesis route for ferrous oxalate dihydrate (FOD) by reducing the processing time while maintaining material performance and sustainability. For this purpose, FOD was hydrothermally synthesized from naturally occurring Ecuadorian black sands and an oxalic acid solution under subcritical conditions. After the high-pressure reaction, the resulting suspensions were filtered and subjected to either conventional oven drying or microwave-assisted drying as a faster alternative. The synthesized materials were characterized for crystallinity, morphology, and particle size using Raman spectroscopy, scanning electron microscopy (SEM), and dynamic light scattering (DLS). The analysis revealed the formation of a well-defined crystalline phase, with most particles ranging between 1 and 2 μm in size. SEM images showed compact particles with relatively uniform morphology. The FOD materials were tested for the degradation of phenol in water under visible light, reaching phenol removal efficiencies above 90%, with variations in performance associated with the drying method. Notably, the microwave-dried sample exhibited catalytic behavior comparable to that of conventionally dried materials, suggesting a viable strategy for reducing processing time. These results demonstrate the potential of mineral-rich waste materials as precursors for photocatalysts and highlight the promise of rapid, scalable synthesis techniques. The study underscores the importance of optimizing drying methods and reaction parameters for future scale-up, with further research needed to understand how processing conditions influence surface properties and catalytic performance.