Removal of ammonium from water by a KOH-treated bentonite biochar composite

In this study, a novel adsorbent—KOH-treated bentonite biochar composite (BRK) derived from natural bentonite and rice husk—was successfully synthesized to remove ammonium (NH₄⁺) from water. Adsorbent preparation involved pyrolysis at 400 °C followed by activation of biochar with KOH to produce BRK. Various techniques were applied to characterize the investigated adsorbent, including Fourier-transform infrared spectroscopy (FTIR), N₂ adsorption analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM) integrated with energy-dispersive X-ray (EDX) spectroscopy. Batch experiments were conducted for elucidating the factors influencing the adsorption process, including pH, contact time, temperature, initial ammonium concentration, and the presence of co-existing cations in the solution. The results showed that the pH of the solution strongly affected BRK’s adsorption capacity for NH₄⁺ ions. Co-existing cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺) significantly reduced the removal efficiency of NH₄⁺ ions. The Langmuir adsorption capacity of BRK for NH₄⁺ followed the order: 22.51 mg/g (10 °C) > 20.57 mg/g (30 °C) > 16.22 mg/g (50 °C). The negative standard enthalpy change (∆H°) obtained in thermodynamic study suggested that the adsorption process of NH₄⁺ was exothermic. The kinetic experiments demonstrated that adsorption equilibrium was achieved after 30 min of contact. Ion exchange was found to be the main adsorption mechanism for removing NH₄⁺ by BRK. This study proved that BRK is a low-cost and sustainable adsorbent derived from natural bentonite and rice husk and is advantageous for removing NH₄⁺ from water.