Eco-Friendly Ammonia Adsorption from Aqueous Solutions Using Activated <i>Madhuca indica</i> Leaves Charcoal and Optimization of Parameters by RSM-BBD: A Sustainable Zero-Waste Approach

Ammonia separation from industrial effluents is a big concern for aquatic, plant, and human lives. Adsorption is best suited for separating ammonia from effluents due to its high effectiveness. In this paper, the activated <i>Madhuca indica</i> leaves charcoal (AMLC) adsorbent was synthesized and characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX) to know the characteristics of the adsorbent before and after adsorption. Further, thermal gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET) analysis were performed on the AMLC adsorbent, and a very high surface area of 964.252 m²/g was observed. The AMLC was utilized for adsorption experiments, and the adsorption parameters were optimized using response surface methodology-Box-Behnken design (RSM-BBD). The optimized parameters reported using RSM-BBD are a contact time of 123.19 min, an adsorbent dosage of 29.1 g/L, a pH of 10.33, an initial ammonia concentration of 312.28 mg/L, and a shaker speed of 82.70 rpm that gives the maximum ammonia removal efficiency of 100%. The adsorption equilibrium studies were analyzed using Langmuir, Freundlich, and Dubinin isotherms, with the Langmuir isotherm being the best fit for the data. The adsorption kinetics were modeled with pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion (IPD) models, indicating that PSO best describes the rate of kinetics. In addition, adsorption thermodynamics is also studied. The change in enthalpy (Δ<i>H</i>) was positive, suggesting that the adsorption is an endothermic process. The negative energy of entropy (Δ<i>S</i>) indicated complex formation with the AMLC adsorbent. In contrast, the positive change in Gibb's free energy showed that adsorption was not spontaneous. The present study supports UN SDG Goal 6 by providing sustainable and cost-effective solutions to water treatment. It also promotes biomass use and reduces chemical usage toward an eco-friendly water purification process.