Nitric Acid Post-Treatment of Physically Activated Hydrochars: Contrasting Effects on Methylene Blue and Phenol Adsorption

ABSTRACT: Hydrochar is a low-cost carbon precursor for water treatment, but its adsorption performance depends on both texture and surface chemistry. This work evaluates whether a nitric acid (HNO₃) post-treatment can direct physically activated hydrochar adsorbents toward a target contaminant class. An industrially produced hydrochar was activated at 800 °C for 2 h under N₂, CO₂, or steam, then treated with 70% HNO₃. Adsorption was evaluated using methylene blue as a representative cationic dye and phenol as a representative aromatic micropollutant. The post-treatment altered surface chemistry rather than texture: total acidic groups increased by 86 to 123% and basic groups decreased by 56 to 66%, lowering the point of zero charge to 4.5–5.3, while Brunauer-Emmett-Teller (BET) surface areas were largely preserved (largest reduction of 14%, most changes below 5%). These changes had opposing consequences for the two adsorbates: methylene blue capacity increased by up to 135%, reaching 264 mg·g⁻¹ for the post-treated steam-activated hydrochar, whereas phenol capacity decreased by up to 58% from a maximum of 167 mg·g⁻¹ for the same steam-activated char before post-treatment. The HNO₃ post-treatment is therefore selective rather than universally beneficial as it valorizes hydrochar adsorbents for cationic contaminants while reducing their affinity for neutral aromatics.