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 <i>via</i> 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 (<i>R</i>² = 0.995) and second-order kinetic models (<i>R</i>² = 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 Δ<i>G</i>° values ranged from -16.05 to -17.69 kJ mol^-1, with a positive Δ<i>H</i>° of 19.23 J mol^-1 and a Δ<i>S</i>° 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.