An eco-friendly approach to soil remediation: evaluating clay–biochar composite for used motor oil removal through kinetic, isotherm, and thermodynamic models

This study investigates the efficiency of the Clay–Biochar Composite (CBC) in the remediation of soil contaminated by used motor oil (UMO). The CBC was synthesized through the carbonization of acid-activated bentonite (AC) [derived from natural bentonite clay (NC)], which was combined with Loquat Seed Biochar (LSB) [derived from Raw Loquat Seed (RLS)]. Comprehensive characterization for NC, AC, RLS, LSB, and CBC to assess their physicochemical properties and potential for UMO adsorption was conducted via Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray (EDX), Fourier Transform Infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) analysis techniques. Additionally, the effects of adsorption parameters such as (UMO) concentration, adsorption time, and adsorbent dosage on removal efficiency were studied. At an optimal dose of 0.3 g and 50 °C, with a contact duration of 300 minutes, the CBC removed 99.89% of oil, reaching a maximum adsorption capacity of 41.62 mg g⁻¹. The results indicated that the Freundlich isotherm (R² = 0.995) and second-order kinetic models (R² = 0.996) more effectively fitted to describe UMO adsorption, suggesting that multilayer adsorption was involved. Also, thermodynamic analysis showed that motor oil adsorption is spontaneous and endothermic. The ΔG° values ranged from −16.05 to −17.69 kJ mol⁻¹, with a positive ΔH° of 19.23 J mol⁻¹ and a ΔS° of 54.84 J mol⁻¹. K indicates increased randomness at the solid–liquid interface. Real-world experiments, which involved examining actual contaminated soil samples, validated the practical applicability of the CBC for UMO remediation. The CBC established the possibility of reuse, achieving 90% oil desorption. These results showed that the CBC significantly enhanced adsorption affinity, highlighting its potential as a cost-effective and eco-friendly adsorbent for oil pollution remediation.