Investigation of the key mechanisms and optimum conditions of high-effective phosphate removal by bimetallic La-Fe-CNT film

Decreasing phosphate concentration in wastewater before discharge is crucial for preventing eutrophication. In this study, a novel La-Fe-CNT film was developed and its mechanism study for highly efficient phosphate adsorption was carried out. The morphology, thermal, structure and element analysis suggested that La and Fe hydroxide species were successfully fabricated on the CNT film and phosphate could be successfully adsorbed. The adsorption condition optimization of the La-Fe-CNT film was carried out by response surface methodology (RSM) with a Box–Behnken design (BBD). A low pH and low temperature were beneficial for the adsorption process, while the phosphate concentration showed little impact on phosphate uptake. It was indicated that pH = 4, temperature = 35.6 °C and phosphate concentration = 500 mg/L were the optimum adsorption conditions for La-Fe-CNT, with the optimized adsorption capacity of 137.8 mg/g, which was in good agreement with the actual value (140.3 mg/g). Thermodynamic analysis revealed that the qm of La-Fe-CNT film calculated from Langmuir model decreased from 164.9 mg/g to 121.7 mg/g from 30 to 80 °C, and the adsorption occurred through a spontaneous-exothermic-multimechanism process. This physicochemical process was controlled by both intraparticle and surface film diffusion. Adsorption experiments in presence of different ionic strengths and co-existing anions further confirmed the occurrence of inner-sphere and outer-sphere complexation during phosphate uptake. Overall, the La-Fe-CNT film showed ideal phosphate removal performance with high selectivity, indicating its potential for actual wastewater purification.