Ammonium Adsorption from Aqueous Solutions Using MnFe2O4-Functionalized Coffee Husk Biochar Nanocomposites: An Evaluation of HNO3 and NaOH Activation

Abstract

This research investigated the development of MnFe₂O₄-based composite adsorbents (MFO@CFA and MFO@CFB) by activating coffee husk-derived biochar with HNO₃ and NaOH, followed by synthesis through coprecipitation and hydrothermal techniques. The structural and chemical properties of the nanocomposites were characterized by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. Batch adsorption experiments were used to assess ammonium removal efficiency across a range of conditions: solution pH (3–8), contact time (5–120 min), adsorbent dosage (0.2–3 g L^-1), and initial ammonium concentration (10–60 mg L^-1). The findings indicated that compared with MFO@CFB, which was synthesized from NaOH-activated biochar, MFO@ CFA, derived from HNO₃-activated biochar, presented enhanced surface func-tionality and a superior adsorption capacity. Langmuir isotherm analysis revealed maximum adsorption capacities of 55.25 mg g^-1 for MFO@CFA and 28.73 mg g^-1 for MFO@CFB, highlighting the potential of MFO@CFA as an effective adsor-bent for ammonium remediation in wastewater treatment. Reusability tests over five adsorption–desorption cycles revealed that MFO@CFB and MFO@CFA retained more than 50% of their initial NH₄⁺ adsorption capacity after regene-ration, which was supported by alkaline desorption and stable MnFe₂O₄-enabled magnetic separation.