Sustainable heteroatom doped biochar for methylene blue adsorption with structure function insights

This study investigates the enhancement of agro-derived biochars as sustainable, low-cost adsorbents for dye-contaminated wastewater treatment. Four agricultural residues-rice straw (RS), date palm fiber (DP), sugarcane bagasse (SCB), and giant reed (Arundo donax L., GR)-were evaluated as biochar precursors. RS, the most carbon-rich, exhibited limited surface reactivity, prompting further modification via hydrothermal nitrogen (N) doping and phosphoric acid (H₃PO₄) activation. The modified biochars were characterized for surface chemistry, porosity, and morphology using elemental analysis, FTIR, SEM, and iodine number, and their methylene blue adsorption performance was systematically investigated under varying pH, adsorbent dosage, and initial dye concentration. The novelty of this work lies in the systematic comparison of hydrothermal N-doping, microwave-assisted treatment, and low-cost phosphoric acid activation on the same precursor, elucidating how complementary acidic and basic surface functionalization strategies govern adsorption performance. Adsorption was governed by electrostatic interactions, π-π stacking, hydrogen bonding, and surface complexation, influenced by the biochar's porosity, surface functional groups, and chemical composition. Hydrothermally N-doped RS (N-RS-HT) exhibited enriched nitrogen content (3.56%), increased basic site density (1.495 mmol g⁻¹), specific surface area (434.9 × 10³ m² kg⁻¹), and adsorption capacity (136.9 ± 6.9 mg g⁻¹), while RS-P showed higher acidity (8.2 mmol g⁻¹), surface area (462.8 × 10³ m² kg⁻¹), and MB uptake (145.8 ± 7.3 mg g⁻¹). Adsorption followed Langmuir isotherms, pseudo-second-order kinetics, and was spontaneous and endothermic, demonstrating complementary strategies for designing high-performance biochar for practical wastewater treatment applications.