Optimization and pH‐Dependent Removal of Cr(VI) from Aqueous Solutions by Fe ₃ O ₄ @SiO ₂ @Lysine Nano‐Composite: Insights from Adsorption–Reduction Mechanism

The development of effective and retrievable adsorbents for toxic hexavalent chromium (Cr(VI)) is crucial for wastewater remediation. This study presents the synthesis and application of Fe 3 O 4 @SiO 2 @Lysine magnetic nanoparticles as a high‐performance adsorbent for Cr(VI) removal. The adsorbent demonstrated exceptional dual functionality, effectively removing Cr(VI) across a broad pH range (1.0–6.0). Characterization by Fourier‐transform infrared spectroscopy confirmed that the amino functional groups on the lysine ligand are key to this performance, facilitating both the adsorption and reduction of Cr(VI) to less toxic Cr(III). A Central Composite Design (CCD) and Response Surface Methodology (RSM) were employed to optimize the process parameters, identifying the optimal conditions as follows: 0.04 g of Adsorbent dosage, 23 min of contact time, 5.38 mg·L −1 of initial Cr(VI) concentration and 2.38 of pH. Under these conditions, the adsorption isotherm followed the Langmuir model, revealing a high maximum adsorption capacity of 88.9 mg·g −1 . Kinetic studies confirmed that the process adhered to the pseudo‐second‐order model (rate constant = 0.65 g·mg −1 ·min −1 ), indicating a chemisorption‐dominated mechanism. The nanocomposite also exhibited excellent selectivity and facile magnetic separation. Furthermore, the adsorbent maintained high efficiency over five regeneration cycles, demonstrating its potential as a reusable, cost‐effective, and promising candidate for the treatment of Cr(VI)‐contaminated water.