Phosphate Uptake over the Innovative La–Fe–CNT Membrane: Structure-Activity Correlation and Mechanism Investigation

Eutrophication caused by excessive phosphorus pollution not only brings a series of environmental problems but also threatens biological safety. The adsorption method has been widely used for preventing eutrophication due to its high selectivity, environmental friendliness, easy operation, and cost-effectiveness. In this study, an easily separated magnetic lanthanum and iron-fabricated carbon nanotube (La–Fe–CNT) membrane was synthesized by a simple combined impregnation and vacuum filtration method for highly effective phosphate uptake. Characterization results show that metallic (hydr)oxide species were successfully fabricated on the CNT membrane, and phosphate was absorbed on it. The structure-activity correlation of La–Fe–CNT was quantitatively investigated by the Box–Behnken design model, and the following optimized conditions were obtained: a reaction temperature of 44 °C, synthesis time of 14.3 h, and La molar ratio of 0.53, with an adsorption capacity of 127 mg/g. La–Fe–CNT performs well over a wide pH range (142 mg/g at pH 2) with high stability (less than 2 mg/L metal leaching). Three interactions exist during the adsorption process, including electrostatic interactions, ligand exchange, and Lewis acid–base interactions. A kinetic study shows that the phosphate adsorption process is a physical-chemical process with combined intraparticle and surface film diffusion. The equilibrium phosphate adsorption capacity of La–Fe–CNT in the isotherm study is 120.2 mg/g, and the phosphate uptake process involves a complex process including both Langmuir and Freundlich adsorption. The adsorbent still retains nearly 70% of its original capacity after 5 cycles of operation, depicting its stability and sustainability for potential industrial applications.