Hydrothermal synthesis temperature induces sponge-like loose silica structure: A potential support for Fe₂O₃-based adsorbent in treating As(V)-contaminated water

Low cost Fe₂O₃-based sorbents with an exceptional selectivity toward the targeted As(V) pollutant have gained extensive attention in water treatment. However, their structural features often influence removal performance. In this respect, we present herein a rational design of silica-supported Fe₂O₃ sorbents with an enhanced morphological structure based on a simple temperature-induced process. Low-hydrothermal temperature synthesis (60 and 100 °C) provided a large silica-cluster size with a close packed structure (S-60 and S-100), contributing to an increase in mass transport resistance. Fe₂O₃/S-60 with 6.2-nm pore width silica achieved a maximum As(V) uptake capacity (q_m) of only 3.5 mg g⁻¹. Supporting Fe₂O₃ on S-100 with an approximately two-fold increase in the pore size (13 nm) did not lead to any evident enhancement in q_e (3.7 mg g⁻¹). However, expanding the pore window up to 22.6 nm (S-140) and 39.5 nm (S-180), along with changing from close-packed to sponge-like loose structures induced by high-temperature synthesis (140 °C and 180 °C), resulted in substantial increases in q_m. Fe₂O₃/S-140 had 1.7 and 1.6 times higher q_m (5.9 mg g⁻¹) than Fe₂O₃/S-100 and Fe₂O₃/S-60, respectively. The highest q_m (7.4 mg g⁻¹) was achieved for Fe₂O₃/S-180, which was attributed to its relatively small-sized silica cluster and the largest cavities that facilitated easier access by As(V) to adsorbing sites.