Facile Preparation of Cross-Linked Moringa oleifera Seed Hulls Powder/Hydroxyapatite Framework Composite for Efficient Removal of Toluidine Blue and Methyl Violet 2B from Aqueous Solution

Abstract In this study, adsorption of two cationic dyes, Toluidine Blue (TB) and Methyl violet 2B (MV 2B) from an aqueous solution was achieved by using multifunctional composite material. The formulation of the composite (MO@HA) was obtained by using Moringa oleifera seed hull powder, calcium chloride (CaCl 2 ), and ammonium hydrogenophosphate (NH 4 ) 2 HPO 4 salts. Surface morphology, functional groups, specific surface area, and surface charge of the composite were explored using Scanning electron microscopy (SEM), Fourier Transform infrared spectroscopy (FTIR), BET analysis, and point of zero charge (PZC), respectively. The composite material resulted in a structural change in the surface of the adsorbents, increased oxygen vacancies, enhancement of active sites, and a specific surface area of 735.55 m 2 g −1 . Different adsorption parameters such as pH, contact time, adsorbent dosage, and initial concentration were evaluated. The adsorption study showed that equilibrium was reached after 60 min, and the optimum adsorption pH for both dyes (TB and MV 2B) was 6. Langmuir, Freundlich, Liu, and Temkin were fitted to describe the adsorption isotherm, both TB and MV 2B had best correlation with Liu isotherm. The maximum adsorption capacity of TB and MV 2B were 341.488 and 182.453 mg g −1 , respectively. Adsorption-desorption cycling studies on the adsorbent confirmed its regeneration and reusability after 5 cycles. A possible adsorption mechanism involving electrostatic interactions, n-π bonding, and hydrogen bonding was suggested. These findings highlight a new direction in the development of efficient and sustainable adsorbent in environmental remediation, specifically in the removal of dyes from aqueous solution.