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

Abstract

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 CO3 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 LaPO4 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.