Capability and Mechanism of Ammonium Adsorption from Aqueous Solution onto Various Biochars (wood, rice husk, bamboo)

In this study, adsorption of ammonium nitrogen (NH ₄ ⁺ -N) ions onto various biochars produced from biomass residues in Vietnam were studied as a function of their physicochemical properties. Three biochars, including wood biochar (WBC), rice husk biochar (RBC), and bamboo biochar (BBC), were produced under limited oxygen conditions using Top-Lid Updraft Drum technology at around 550 ^o C for 2.5 hours. Physicochemical characterization by BET surface area, Cation exchange capacity (CEC), Scanning Electron Microscopy and Fourier Transform Infrared Spectroscopy (FTIR) of the three biochars were performed in order to link their porosity and surface functional groups with their NH ₄ ⁺ -N capture capacities. The adsorption capacity was evaluated using various parameters such as adsorbent dosage, contact time, and initial adsorbate concentration. The equilibrium adsorption data were analyzed by Langmuir, Freundlich and Temkin adsorption isotherm models. The Freundlich model best described the adsorption of NH ₄ ⁺ -N onto the three biochars. The capacities of the biochar for NH ₄ ⁺ -N adsorption were in the order RBC > BBC > WBC, which correlates with their overall CEC. The Lagergren-first order and pseudo-second order models were also used to evaluate the kinetics of adsorption, and the adsorption data of NH ₄ ⁺ -N showed a good fit with the latter model. Thus, the experimental data indicates that all three boimass residue-derived biochar are suitable for maximising NH ₄ ⁺ -N adsorption from aqueous solutions, with rice husk BC being especially effective. The increased adsorption capacity of rice husk BC correlated with it having the highest CEC despite having the lowest surface area, suggesting that surface chemistry plays the greatest role in the NH ₄ ⁺ -N adsorption of all the physico-chemical parameters investigated.