A New Generation of Porous Polymer Materials from Polystyrene Waste: Synthesis and Adsorption of Nitrate Anions in Aqueous Media

A simple approach was developed to efficiently graft diethylene triamine onto polystyrene waste using succinic anhydride as a tracer to remove nitrate anions from aqueous solutions. Infrared spectroscopic data showed characteristic signs at 3395 cm⁻¹ and 1695 cm⁻¹ corresponding to N-H and C=O (ester and amide), confirming the grafting of DETA onto PS. The zeta potential study showed that the PS-Succ-DETA adsorbent had a pH_iep of 8.2, and its charge was positive when the pH was lower than the pH_iep. Parameters affecting nitrate adsorption, such as dosage, initial concentration, pH, and contact time, were studied. The adsorption data corresponded well to the Langmuir isotherm with an R² correlation coefficient of 0.998, and the adsorption capacity was found to be 195.65 mg/g. The adsorption kinetics of NO₃⁻ ions by PS-Succ-DETA corresponded perfectly to the PS-II model, with an R² coefficient of 0.999. The negative value of ΔG (−10.02 kJ/mol), ΔH (−18.76 kJ/mol), and ΔS (−28.83 J/K/mol) indicates that NO₃- adsorption is spontaneous exothermic and suggests a decrease in randomness at the solid-liquid interface during the adsorption. The mechanism of adsorption of nitrate ions onto PS-Succ-DETA occurs via electrostatic interactions and hydrogen bonds between the NO₃⁻ ions and the -NH₂ and NH functions of PS-Succ-DETA.