Sustainable utilization of Fe₃O₄-modified activated lignite for aqueous phosphate removal and ANN modeling

Lignites are widely available and cost-effective in many countries. Sustainable methods for their utilization drive innovation, potentially advancing environmental sustainability and resource efficiency. In the present study, Fe₃O₄ (∼25.1 nm) supported on KOH-activated lignite (A−L) displayed 8 times higher phosphate removal than pristine A−L (67.6 mg/g vs. 8.5 mg/g at pH 5, 50 mg of absorbent in 25 mL of 1500 ppm [phosphate]), owing to its abundant Fe₃O₄ (10 wt% of Fe) nanoparticle content. The removal occurred within ∼2 h, following a pseudo-second-order kinetic model. Across pH levels ranging from 5.0 to 9.0, Fe₃O₄−A−L's phosphate removal occurs via both chemisorption and precipitation, as evident by kinetic, pH, and XPS analyses. The phosphate adsorption fits better with the Freundlich isotherm. The combined benefits of facile recovery, rapid phosphate uptake, straightforward regeneration, and attractive post-adsorption benefits (e.g., possibly use as a Fe, P-rich fertilizer) make magnetic Fe₃O₄−A−L a promising candidate for real-world applications. Artificial Neural Network (ANN) modeling indicates an excellent accuracy (R² = 0.99) in predicting the amount of phosphate removed by Fe₃O₄−A−L. Sensitivity analysis revealed both temperature and initial concentration as the most influencing factors. Leveraging lignite in environmentally friendly applications not only addresses immediate challenges but also aligns with sustainability goals. The study clearly articulates the potential benefits of utilizing lignite for sustainable phosphate removal and recovery, offering avenues for mitigating environmental concerns while utilizing resources efficiently.