Integrating chemical modification pathways and machine learning for optimization of nitrate removal by rapeseed (Brassica napus L.) biochar

Nitrate contamination from agricultural and industrial activities poses significant risks to human health and aquatic ecosystems. Biochar, particularly when chemically modified, has emerged as a sustainable and effective adsorbent; however, the influence of modification sequence and operational conditions on nitrate removal is not fully understood. In this study, rapeseed (Brassica napus L.)-derived biochars were modified with aluminum through three distinct pathways: pre-pyrolysis (MP), post-pyrolysis (PM), and post-pyrolysis with re-pyrolysis (PMP). Comprehensive characterization using BET, SEM, EDS, XRD, and FTIR showed that PM biochar exhibited the highest surface area, uniform mesoporous structure, stable aluminum content, and abundant oxygen-containing functional groups, resulting in superior nitrate adsorption. Batch experiments demonstrated that adsorption efficiency is strongly affected by operational parameters, including initial nitrate concentration, contact time, adsorbent dose, solution pH, and the presence of competing anions, with CO₃²⁻ and SO₄²⁻ having the strongest inhibitory effects. Regeneration tests indicated that PM biochar retained ~ 76% of its initial adsorption capacity after five adsorption-desorption cycles, confirming its reusability. Nonlinear machine learning models, including Random Forest (RF), Support Vector Regression (SVR), and Linear Regression (LR), were applied to predict nitrate removal, with RF achieving the highest predictive accuracy (R² = 0.892, RMSE = 0.078), demonstrating robustness and generalization. This work highlights the critical role of modification sequence in tailoring biochar structure and functionality and integrates AI-based modeling to provide a data-driven framework for designing high-performance biochars for sustainable nitrate removal from water.