Arsenic and antimony removal by coconut mat-derived biochars functionalized with layered double hydroxides: Metal ion synergy, surface chemistry, and functional performance

초록

Metal-ion selection in layered double hydroxides (LDHs) can strongly affect the adsorption of oxyanion metalloids. Here, coconut mat biochar (CMB) was decorated with Mg/Fe-LDH (LDHMgFe@CMB) or Ni/Fe-LDH (LDHNiFe@CMB) and evaluated for As(V) and Sb(V) removal from water. LDHNiFe@CMB achieved markedly higher equilibrium capacities [Qe = 12.2 mg/g for As(V) and 10.8 mg/g for Sb(V)] than LDHMgFe@CMB (9.5 and 4.8 mg/g) and pristine CMB (1.4 and 1.2 mg/g). The Ni-containing composite showed sharper and more intense X-ray diffraction reflections, indicating higher crystallinity, which is consistent with enhanced layered stability expected from the smaller ionic radius of Ni2+ (69 pm) relative to Mg2+ (72 pm). Kinetic and intraparticle diffusion analyses suggest that uptake involves chemisorption with a notable diffusion contribution, and equilibrium data are well described by the Langmuir model. The main adsorption mechanism was electrostatic surface complexation and anion exchange, judging from various characterizations. Performance trends were further assessed by varying adsorbent dose, initial As(V)/Sb(V) concentration, temperature, contact time, and solution pH. Regeneration with KOH maintained high activity; after five adsorption-desorption cycles, LDHNi-Fe@CMB retained Qe values of 12.4 mg/g for As(V) and 11.3 mg/g for Sb(V). Overall, LDHNiFe@CMB is a robust and reusable adsorbent for efficient removal of As(V) and Sb(V).

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