Diesel exhaust soot (DES), a hazardous by-product of incomplete diesel combustion, was valorized into activated nanocarbon (ANC) through phosphoric acid activation for phenol removal from water. The synthesized ANC was characterized by FTIR, XRD, FE-SEM, TEM, EDX, and AFM. The results revealed predominantly spherical nanoparticles with an average size below 30 nm, a turbostratic carbon structure, and oxygen-containing surface functional groups favorable for adsorption. The adsorption performance was evaluated as a function of adsorbent dosage, contact time, initial phenol concentration, and temperature. The optimum adsorbent dosage and equilibrium time were 0.1 g and 90 min, respectively, while the highest removal efficiency was obtained at an initial phenol concentration of 10 mg L⁻¹. The equilibrium data were best described by the Langmuir model (R² = 0.999), with a maximum adsorption capacity of 23.98 mg g⁻¹, indicating predominantly monolayer adsorption. Thermodynamic analysis confirmed that phenol adsorption was spontaneous (ΔG = −22.42 to −25.79 kJ mol⁻¹) and endothermic (ΔH = +11.43 kJ mol⁻¹), with a positive entropy change (ΔS° = +113.47 J mol⁻¹ K⁻¹). These findings demonstrate the potential of DES as a low-cost precursor for functional nanocarbon adsorbents and provide a sustainable approach for converting combustion waste into a value-added material for water purification.