Phenolic compounds are among the most hazardous organic pollutants commonly detected in industrial wastewater due to their toxicity, persistence, and resistance to biodegradation. In this study, phosphoric acid-activated biochar derived from date kernels (DK) was synthesized as a sustainable and low-cost adsorbent for phenol removal from aqueous solutions and real refinery wastewater. The activation process significantly enhanced the physicochemical properties of the biochar, increasing the BET surface area from 2.11 to 1087 m²/g and the pore volume from 0.0031 to 0.818 cm³/g. The prepared adsorbent was characterized using FTIR, SEM, BET, and XRD analyses. Batch adsorption experiments were conducted under different operational conditions, including pH (3–11), contact time (30–150 min), adsorbent dosage (0.4–1.2 g), initial phenol concentration (50–250 mg/L), and agitation speed (100–250 rpm). The optimum adsorption conditions were achieved at pH 5, contact time of 120 min, adsorbent dosage of 0.8 g, initial phenol concentration of 50 mg/L, and agitation speed of 200 rpm, resulting in a phenol removal efficiency exceeding 99%. Adsorption equilibrium data were best described by the Langmuir isotherm model (R² = 0.9943), indicating predominantly monolayer adsorption, while kinetic analysis showed the best fit with the pseudo-second-order model (R² = 0.999). The Dubinin–Radushkevich model suggested that physical adsorption interactions also contributed to the overall adsorption mechanism. Application of the developed adsorbent to real refinery wastewater reduced phenol concentration from 10.65 to 1.11 mg/L with a removal efficiency of 89.58% and the adsorption capacity is 62 mg/g. The findings demonstrate that phosphoric acid-activated date kernel biochar is a promising, eco-friendly adsorbent for efficient phenol removal from wastewater.