Exploiting nickel–organic-frameworks in the temperature-controlled synthesis of carbonized nickel nanoparticles for magnetic purification of acidic water

Abstract

This work explores the effect of carbonization temperature (600 °C vs. 1000 °C) on the microstructure, magnetic properties, and adsorption performance of nickel organic framework (NiOF)-derived nanoparticles (NPs) for the remediation of acidic water using Cr(VI) and methylene blue (MB) as a typical heavy metal and organic dye, respectively. Carbonizing NiOF at 600 °C resulted in the formation of Ni NPs with both face-centered cubic (FCC) and hexagonal-close-packed (HCP) crystal structures, with subsequent encapsulation in multilayered graphitic carbon that serves as a protective barrier against Ni NP oxidation. However, carbonization of NiOF at 1000 °C induced the formation of FCC Ni NPs exclusively, accompanied by a substantial increase in saturation magnetization. After ethanol purification, these NPs showed ferromagnetic character that allows for rapid and efficient magnetic recovery. Further purification through double water washing markedly improves adsorption kinetics, achieving equilibrium for Cr(VI) and MB within 5–12 minutes. This enhancement is attributed to greater accessibility of active sites and increased surface functional group density. Adsorption kinetics are adequately described by both pseudo-first-order and pseudo-second-order models; however, kinetic fitting alone is insufficient to elucidate the underlying mechanism. Instead, mechanistic interpretation is supported by independent evidence, including pH-dependent ζ-potential measurements, pollutant speciation, and textural and microstructural characterization. These results indicate that, at pH 3, Cr(VI) uptake is primarily governed by electrostatic attraction, whereas MB adsorption is dominated by π–π stacking and dispersion interactions on more graphitic surfaces. These findings underscore the carbonization temperature as a critical parameter for tailoring the structural, magnetic, and adsorption properties of NiOF-derived NPs, enabling the design of high-performance, magnetically recoverable adsorbents for water purification.