Structure-performance relationship of biochars derived from agricultural wastes for nitrate adsorption: The role of surface charge and porosity

Nitrate pollution in water poses significant environmental and health risks, such as methemoglobinemia and eutrophication, necessitating the development of sustainable and cost-effective remediation technologies. This study investigates the potential of biochar derived from abundant agricultural wastes—coconut shell (CSB), corn cob (CCB), and mahogany sawdust (MSB)—as adsorbents for nitrate removal, aiming to establish a correlation between feedstock-dependent physicochemical properties and adsorption performance. Biochars were produced via slow pyrolysis at 280 °C–400 °C and subsequently characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, BET surface area analysis, point of zero charge (pHzc), and iodine number determination. Batch adsorption experiments were conducted using a 50 mg/L nitrate solution to evaluate the effects of contact time, adsorbent dosage, and pH on nitrate uptake. CCB emerged as the most effective adsorbent, exhibiting the highest nitrate adsorption capacity of 4.373 mg/g at an optimal pH of 5. This superior performance is attributed to the unique combination of high microporosity, where the iodine number (450.92 mg/g) proved to be a more representative indicator of accessible surface area than BET analysis due to the kinetic limitations of N 2 gas in ultramicropores, and a favorable alkaline pHzc of 8.0. Mechanistic analysis confirmed that the protonation of oxygen-containing functional groups at pH levels below pHzc facilitated strong electrostatic attraction toward nitrate anions. Furthermore, adsorption isotherm analysis revealed that nitrate uptake on CCB followed the Freundlich model, indicating multilayer adsorption on a heterogeneous surface, whereas CSB was better described by the Langmuir model. These findings establish a critical structure-performance relationship, underscoring the potential of corn cob as a superior feedstock for engineering effective biochar adsorbents for anionic water pollutants. • Corn cob biochar exhibited superior nitrate adsorption capacity. • Electrostatic attraction at pH < pHzc was the dominant removal mechanism. • Iodine number predicted adsorption capacity better than BET surface area. • Adsorption on corn cob biochar followed the Freundlich multilayer model. • Feedstock selection critically governed biochar physicochemical properties.