Adsorption of ammonium in aqueous solutions by pine sawdust and wheat straw biochars

Ammonium (NH₄⁺) is a common form of reactive nitrogen in wastewater, and its discharge to water bodies can lead to eutrophication. This study was conducted to understand NH₄⁺ adsorption mechanisms of pine sawdust and wheat straw biochars in aqueous solutions and the factors affecting NH₄⁺ removal. Biochars were produced by pyrolysing pine sawdust at 300 °C (PS300) and 550 °C (PS550) and wheat straw at 550 °C (WS550). Pseudo-second-order and Redlich-Peterson models best fitted the adsorption data. The PS300 showed the highest NH₄⁺ adsorption capacity (5.38 mg g^-1), followed by PS550 (3.37 mg g^-1) and WS550 (2.08 mg g^-1). Higher H/C and O/C ratios of PS300 (0.78 and 0.32, respectively) indicated the greater presence of functional groups on the biochar's surface as compared to PS550 (0.35 and 0.10, respectively) and WS550 (0.36 and 0.08, respectively), resulting in different NH₄⁺ adsorption through electrostatic interactions. The dominant mechanism for NH₄⁺ adsorption by the biochars was likely chemical bonding and electrostatic interaction of NH₄⁺ with the surface functional groups. Lower pyrolysis temperature resulted in a higher NH₄⁺ adsorption capacity by the pine sawdust biochar. At the same pyrolysis temperature (550 °C), the biochar made with pine sawdust as the feedstock had a higher NH₄⁺ adsorption capacity than biochar made from wheat straw. We conclude that biochars can be efficient absorbents for NH₄⁺ removal from wastewater, and the removal efficiency can be optimised by selecting different feedstocks or the pyrolysis condition for biochar production.