Aluminum-Lanthanum Modified Coffee Husk Derived Nanobiochar for Nitrate Adsorptive Removal: Kinetics and Isothermal Studies

Adsorption technology is a promising alternative to conventional methods in wastewater treatment owing to its easiness and efficiency to remove even low-concentration nitrate and recover it. In this regard, advanced materials such as nanobiochar (derived from biomass) have shown significant promise as adsorbents. However, pure biochar often lacks sufficient effectiveness and is hard to reuse. Therefore, this research introduces an effective nitrate adsorbent based on aluminum-lanthanum (Al-La) modified nano-biochar (AL-LaCHNBC) derived from coffee husks. The adsorbent was synthesized by first digesting the coffee husk with acid, followed by a co-precipitation step. Standard characterization methods, including FTIR, XRD, and DLS, were used to evaluate the new materials. Through a series of batch adsorption experiments, its efficiency for removing nitrate was tested across various conditions, specifically examining the impact of pH, initial concentration, contact time, and adsorbent dose. The study found that an acidic environment significantly enhanced the adsorption process, achieving a maximum of 98% nitrate removal efficiency at pH 2. The optimum equilibrium nitrate adsorption capability achieved in work was 41.75 mg·g ⁻¹ under the optimal conditions of: 25 ^o C temperature, pH of 2, 55 minutes contact time, 20 mg/L of initial nitrate, and 40 mg/mL of adsorbent dose. The data strongly adhered to the Freundlich isotherm (R ²  = 0.9915), suggesting that the adsorbent surface is heterogeneous. Furthermore, the adsorption kinetics were best described by the pseudo-second-order model (R ² = 0.979), which implies that chemisorption is the primary mechanism of adsorption. The process was confirmed to be endothermic and spontaneous based on the thermodynamic analysis. The AL-LaCHNBC adsorbent proved to be recyclable using NaOH and HCl as effective eluents, and the recovered nitrate was successfully utilized as a fertilizer for growing white onion and bean seeds. Notably, when tested on real wastewater from the Akaki River, the adsorbent maintained a high maximum nitrate removal efficiency of 96.6 from 8.02 mg/L in the presence of other competing ions.