Efficient Adsorptive Removal of Methyl Orange Over Engineered Pure NiO and Doped NiO:M Nanoparticles (M= Cr3+, Mn2+, Co2+, Cu2+ or Zn2+) Synthesized via a Simple Route

Pure and doped nickel oxide (NiO) nanopowders with crystallite sizes in the range of 4.3–9.9 nm were synthesized via thermal decomposition of oxalate precursors at 400 °C in air, incorporating 10 mol% of transition metal dopants (Cr³⁺, Mn²⁺, Co²⁺, Cu²⁺, and Zn²⁺). The materials exhibited a high specific surface area of 179.3 m² g⁻¹, contributing to their enhanced adsorption performance toward methyl orange (MO). The maximum adsorption capacity reached 35.79 mg g⁻¹, with removal efficiencies up to 99% for Cu²⁺-, Co²⁺-, and Mn²⁺-doped NiO under optimized conditions. The adsorption process was found to be spontaneous and endothermic, with an enthalpy change (ΔH°) of 76.79 kJ mol⁻¹, indicating a chemisorption-dominated mechanism. Kinetic analysis showed excellent agreement with the pseudo-second-order model (R² ≈ 0.99–1.00), while equilibrium data were best described by the Freundlich isotherm, confirming heterogeneous surface adsorption. The enhanced performance is attributed to nanoscale crystallite size and dopant-induced lattice defects, which increase active surface sites. UV–Vis spectroscopy was used to monitor adsorption, while XRD, EDS, SEM/TEM, and BET analyses confirmed the structural, compositional, morphological, and surface properties of the synthesized materials. These findings demonstrate that doped NiO nanoparticles are highly efficient and promising adsorbents for dye-contaminated water remediation. • Pure and metal-doped NiO nanoparticles were synthesized via a simple route. • Ultra-small NiO crystallites (4.3–9.9 nm) were successfully obtained. • Metal doping significantly enhanced methyl orange adsorption efficiency. • Adsorption followed the Freundlich isotherm and pseudo-second-order kinetics. • Thermodynamics revealed spontaneous, endothermic chemisorption behavior.