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

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