Contamination of water resources by organophosphate pesticides, particularly diazinon, poses serious environmental and health risks. While chemically modified biochars have been widely studied, such treatments are costly and complex, limiting their use in resource-constrained regions. This study addresses this gap by evaluating three completely unmodified, low-cost adsorbents (natural biochar, date seed powder, and pine cone powder) for diazinon removal from aqueous solutions.

Batch adsorption experiments were conducted with adsorbent dosages of 1–5 g (initial diazinon: 15 mg/L, pH=7, 120 min, 25°C). Equilibrium data were fitted to Langmuir and Freundlich isotherms, and adsorbents were characterized by FTIR and SEM. Natural biochar showed the highest removal efficiency (approximately 65% at 5 g), while the lowest removal (nearly zero) was observed at 1 g. Specific surface areas (methylene blue method) were 0.7, 0.5, and 0.3 m²/g for biochar, pine cone, and date seed, respectively, correlating well with adsorption capacities. FTIR confirmed abundant O–H, N–H, and C=O groups on biochar, and SEM revealed its porous structure with micropores and mesopores, unlike the denser macroporous structure of lignocellulosic adsorbents.

Isotherm modeling indicated that the Freundlich model provided the best fit for date seed powder (R² = 0.96), suggesting heterogeneous multilayer adsorption. For pine cone, the Freundlich model also provided a better fit (R² = 0.78), while biochar exhibited closer agreement with the Langmuir model (R² = 0.74, b = 0.39), indicating stronger binding affinity (likely via π–π interactions and hydrogen bonding) and a tendency toward monolayer adsorption.

Overall, natural biochar, without any costly modification, can serve as an effective, economical, and environmentally sustainable adsorbent for diazinon removal from agricultural wastewater, especially in regions with limited access to advanced treatment technologies. Future work should focus on kinetics, regeneration, and pilot-scale testing with real wastewater.