Phosphorus removal from aqueous solutions using hematite-coated expanded graphite: Isotherm modeling and thermodynamic insights

Hematite-coated expanded graphite, a newly developed nanocomposite, has demonstrated high efficiency as an adsorbent for the removal of various metal ions and inorganic contaminants from aqueous solutions. To enhance its durability and adsorption performance, hematite nanoparticles were synthesized onto expanded graphite, and the adsorption behavior for inorganic pollutants was investigated at different temperatures. The adsorption isotherms were best fitted by the Langmuir model, indicating monolayer adsorption on a homogeneous surface. The maximum adsorption capacity for phosphorus was determined to be 33.3 mg/g at 298 K. A slight decrease in adsorption capacity was observed with increasing temperature, suggesting that the process is exothermic in nature. Thermodynamic analysis revealed a Gibbs free energy change (<i>Δ</i>G) ranging from –22.4 to –22.6 kJ/mol, an enthalpy change (<i>Δ</i>H) of –89.5 kJ/mol, and an entropy change (<i>Δ</i>S) of –0.2 kJ/mol·K, further confirming the spontaneous and exothermic characteristics of the adsorption process. Combined results from isotherm and kinetic studies indicate that α-Fe<sub>2</sub>O<sub>3</sub>/EG is a promising adsorbent for the efficient removal of trace amounts of phosphorus from water.