Effects of Temperature and Activation on Biochar Chemical Properties and Their Impact on Ammonium, Nitrate, and Phosphate Sorption

There have been limited studies of how pyrolysis temperature and activation processes alter the chemical properties of biochar and how these changes influence ammonium (NH4+), nitrate (NO3−), and phosphate (PO43−) sorption. This study compared the chemical properties of biochars and activated biochars (ActBC with steam and CO2 activation) produced by slow pyrolysis at 200 (BC200), 400 (BC400), 600 (BC600), 800 (ActBC200, ActBC400, ActBC600), and 850°C (sulfachar‐S enriched biochar with steam activation). Quantitative solid‐state 13C nuclear magnetic resonance spectroscopy and elemental analysis were used to study temperature and activation on biochar chemical properties. The sorption capacity of biochars for NH4+, NO3−, and PO43− were measured by batch sorption experiments. Nuclear magnetic resonance spectroscopy data showed that BC200 contained mainly aliphatic C compounds (86% of O‐alkyl) belonging to cellulose and hemicellulose, whereas BC400 and BC600 composition was dominated by fused aromatic C structures, containing 81 and 97% aromatic C, respectively. Increasing pyrolysis temperatures decreased biochar total C but increased its cation exchange capacity, pH, and contents of total N and P, calcium, potassium, and magnesium. The BC200 released NO3− and PO43−, whereas sulfachar and ActBC200 sorbed significantly higher amounts NO3− and PO43− than BC600 by 83 and 96%, respectively, across aqueous solutions. Sulfachar and BC400 sorbed significantly greater amounts NH4+ than did the other biochars. This study shows that production temperature significantly affects biochar chemical properties and that activation increases NO3− and PO43− sorption. These results suggest that activated biochar could be useful for sorbing soil N and P, thereby reducing leaching losses. Core Ideas Pyrolysis temperature and activation alters the chemical properties of biochar. Pyrolysis temperatures >400°C increase the formation of aromatic C structures. Activation increases the sorption capacity of biochar for nitrate and phosphate. Activated biochars may have use as adsorbents for reducing nutrient‐leaching in soils.