Acid-Treated Spent Coffee Bean Biochar for Nitrate and Nitrite Adsorption

Elevated concentrations of nitrate and nitrite can cause eutrophication, which degrades natural water bodies, imposing a serious threat to aquatic organisms. Past studies reported that unmodified biochars have relatively low affinity to remove nitrite from water bodies. This work aims to enhance the removal efficiency of nitrate and nitrite using acid-activated spent coffee bean biochar. The biochar was prepared from spent grounded coffee beans by carbonization at various temperatures (300–700 °C), followed by activation with hydrochloric acid. Biochar without acid activation did not show any interesting positive removal activity for both nitrate and nitrite. This is expected due to electrostatic repulsion between the negatively charged biochar surface and the nitrate and nitrite ions. Upon acid activation of the carbonized biochar, the removal efficiency improved drastically. Biochar prepared at 500 °C showed the highest removal efficiency for nitrate (85.7%) and nitrite (79.2%), after acid activation. Acid activation improved the surface functional groups (–OH) of biochar, thus increasing the electrostatic adsorption of nitrate and nitrite. The biochar showed a high affinity for adsorbing both nitrate and nitrite from aqueous solutions, which can be potentially applied in liquid effluents such as aquaculture wastewater.