Synthesis, characterisation and electrochemical behaviour of calcium carbonate nanoparticles derived from automobile oxy-acetylene gas welding waste

In actualizing the management of this toxic waste, resource recovery was designed to convert the Ca(OH)₂ rich waste (generated by automobile welding process) to calcium carbonate nanoparticles (CaCO₃NPs). The synthesized nanoparticles were characterized using XRF, SEM-EDX, XRD, FTIR, XPS, TGA/DTA, BET, DFT and EIS. Results from XRF and SEM-EDX analyses showed calcium as the dominant element, with CaO and Ca contents of 98.59 wt% and 84.30 wt%, respectively. However, based on XRD, XPS, XRF and SEM-EDX results, minor impurities, including Si Al, Fe, and Mg, were also detected at concentrations less than 0.2% respectively.. SEM micrographs also showed rough, irregular, partially agglomerated surfaces and features that reflect heterogeneous particle sizes. XRD patterns identified a dominant calcite phase (~ 81%) and minor vaterite (< 19%), with crystallite sizes ranging from 12.0 to 21.6 nm (average 16.28 nm) while the FTIR spectrum showed characteristic carbonate vibrations at 1507.71, 1149.88, and 877.79 cm⁻¹, that futher confirms high purity. Thermal analysis of the nanoparticles indicated initial weight loss of less than 3% below 200 °C due to adsorbed moisture, and complete decomposition between 600 and 850 °C, with an activation energy of 178 kJ/mol and estimated purity of 97.75%. BET and DFT analyses revealed high surface areas (134 m²/g, BET; 36.6 m²/g, DFT), mesoporous structure, average pore diameter of 2.65 nm, pore volume of 0.16678 m³/g, and surface area-to-volume ratio of 819 m⁻¹. Electrochemical impedance spectroscopy in 0.5 M H₂SO₄ showed two relaxation processes in Nyquist plots. The first relaxation represented a high-frequency grain boundary processes having R_ct = 4.0 × 10⁶ Ω, C = 3.98 × 10⁻⁹ F, τ = 0.016 s, θ_min = 43°, n = 0.38, Q = 1.66 × 10⁻⁸ S sⁿ, and C_dl = 204 μF, indicating dispersive capacitive behaviour and significant interfacial polarization. However, the low-frequency bulk charge transfer processes had R_ct = 2.4 × 10⁷ Ω, C = 2.10 × 10⁻¹³ F, τ = 5.03 × 10⁻⁶ s, θ_min = 73°, n = 0.81, Q = 1.36 × 10⁻¹² S sⁿ, and C_dl = 34.3 μF, indicating high resistance and dielectric behaviour. Frequency-dependent capacitance analysis further confirmed strong double-layer formation at low frequencies and intrinsic insulating characteristics at high frequencies. The combination of nanoscale dimensions, high surface area, mesoporosity, thermal stability, wide bandgap (6.01 eV), and distinctive electrochemical behaviour indicated that the CaCO₃NPs has potentials for dielectric fillers, corrosion-resistant coatings, adsorbents, and inert supports in electrochemical applications.