Facile preparation and characterization of lanthanum-loaded carboxylated multi-walled carbon nanotubes and their application for the adsorption of phosphate ions

A new carboxylated multi-walled carbon nanotubes adsorbent modified with lanthanum hydroxide (MWCNTs-COOH-La) was prepared, characterized and investigated for phosphate removal in batch experiments. The maximum adsorption capacity of the MWCNTs-COOH-La was 48.02 mg P g −1 according to Langmuir model. Structural characterizations demonstrated that the MWCNTs-COOH-La was successfully synthesized and La species was present in the form of La(OH) 3 . Batch experiments were performed under various conditions (e.g., initial concentrations, temperature, pH, co-existing ions) to investigate the removal of phosphate by MWCNTs-COOH-La. Equilibrium data agreed very well with the Langmuir model, suggesting that the adsorption feature was monolayer. The adsorption behaviors of phosphate were described better by the pseudo-second-order, indicating that the adsorption behaviors were mainly ascribed to chemic-sorption. Phosphate adsorption varied slightly at pH 3–7, but decreased significantly at higher pH values. Phosphate adsorption was slightly influenced by solution ionic strength. A high selectivity of phosphate was also observed in the presence of co-existing anions (except CO 3 2− ). The underlying mechanism for the specific adsorption of phosphate by MWCNTs-COOH-La was fully analyzed by X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy technologies. The above results revealed that the hydroxyl groups were involved in the sorption of phosphate and LaPO 4 was formed during the adsorption process. This study implied that MWCNTs-COOH-La might be an alternatively viable and promising adsorbent for removal of phosphate from aqueous solution.