Sol-gel derived bismuth-doped nickel ferrite: A promising adsorbent to tackle Cr(VI) pollution – Insights into the thermodynamics, kinetics, and isotherms

Bismuth-doped nickel ferrite nanoparticles (NiFe_2-xBi_xO₄ with x = 0.00, 0.03, 0.05) were synthesized using the sol-gel autocombustion method. The optimum bismuth doping concentration for enhanced Cr(VI) adsorption is determined to be x = 0.05. XRD analysis confirmed a mixed spinel structure for the synthesized ferrites and revealed that bismuth doping results in decreased crystallite size, increased surface area, and enhanced dislocation density. Batch adsorption studies showed improved removal efficiency with an increase in adsorbent dosage and contact time, but decreased efficiency with an increase in pH, temperature, and initial ion concentration. The introduction of Bi³⁺ ions increased the pH_pzc of NiFe₂O₄ from 6.21 to 7.18. Thermodynamic analysis showed that the adsorption process is spontaneous at lower temperatures but non-spontaneous at and above 318 K for NiFe₂O₄ and 333 K for NiFe_1.95Bi_0.05O₄. Adsorption kinetics studies reveal that the adsorption rate follows pseudo-second-order kinetics, indicating a second-order process with chemical adsorption mechanisms. Diffusion kinetic model analysis shows that Cr(VI) adsorption occurs via boundary layer diffusion. Isotherm data fit well with the Langmuir model, suggesting monolayer adsorption. Under optimized conditions (pH 1, temperature 273K, contact time 45 min), 2 g/L NiFe_2-xBi_xO4 (x = 0.00, 0.05) at 300 rpm achieved 80.698 % (NiFe₂O₄) and 92.188 % (NiFe_1.95Bi_0.05O₄) Cr(VI) removal, with adsorption capacities 40.349 and 46.094 mg/g, respectively. Regenerability studies showed desorption efficiency exceeding 98 %, allowing reuse of the adsorbents for at least 3 cycles with a negligible decrease in adsorption efficiency (<0.1 %). The study demonstrates that incorporating bismuth enhances NiFe₂O₄'s adsorption performance, offering a promising solution for Cr(VI) removal from aqueous solutions.