This work studied the textural properties of three adsorbents using nitrogen adsorption–desorption at a constant low temperature (77 K). The adsorbents are (a) mesoporous silica (MS), (b) mesoporous silica modified with Fe₃O₄ (MSF), and (c) mesoporous silica modified with Fe₃O₄ and coated with deacetylated chitin (MSFDAC). The adsorption–desorption isotherm curve demonstrated that the studied adsorbents have a mesoporous structure of type IV, according to the International Union of Pure and Applied Chemistry (IUPAC) classification. The obtained results show that the Brunauer–Emmett–Teller (BET) isotherm fits the data better than the Langmuir and the Freundlich isotherms. The surface area of MS was at its maximum value as compared to MSF and MSFDAC, which were 1000.55, 168.05, and 163.63 m²/g, respectively. The analysis of particle size distribution for all adsorbent materials was in a narrow distribution, suggesting that the structures were mesoporous and well-defined. Despite slight variations in pore diameter distribution due to loading Fe₃O₄ and modification with deacetylated chitin, the MSFDAC adsorbent results in heterogeneity and unequal particle diameters. The average pore diameter increased from MS (4.6 nm) to MSF (7.32 nm) and MSFDAC (11.69 nm). Conversely, the total pore volume decreased from MS (1.15 cm³/g) to MSF (0.89 cm³/g) and MSFDAC (0.48 cm³/g). A comparative study was carried out with the previous studies, which demonstrated that the textual properties of the modified magnetic silica adsorbent biomaterial agreed with those of other studies.