Amino acids-intercalated Mg/Al layered double hydroxides as dual-electronic adsorbent for effective removal of cationic and oxyanionic metal ions

Mg/Al layered double hydroxides (Mg/Al-LDHs) directly intercalated with different amino acids (phenylalanine, tyrosine, and serine) were synthesized through a co-precipitation method, yielding Phenylalanine-LDH, Tyrosine-LDH, and Serine-LDH, respectively. The obtained LDHs were characterized with X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The results indicated that the tested amino acids were intercalated into the interlayer region of Mg/Al-LDH as a horizontal orientation and monolayer arrangement. The abundant presence of anions (CO32- and NO3-) in the interlayer region of LDHs plays an important role in the removal process of toxic metal ions. The adsorption results demonstrated that Phenylalanine-LDH exhibited the excellent maximum adsorption capacity (Q) to cations, with the sorption capacity sequence: Cu2+ (7.223 mmol/g) > Pb2+ (4.112 mmol/g) > Ni2+ (2.368 mmol/g) > Cd2+ (1.966 mmol/g), while the highest (2.901 mmol/g) was obtained by TyrosineLDH for oxyanion adsorption. The Cu2+ values of pristine Mg/Al-LDH followed the order: Ni2+ (1.366 mmol/ g) > Cu2+ (1.248 mmol/g) > Cd2+ (0.961 mmol/g) > Pb2+ (0.627 mmol/g) > MnO4- (0.509 mmol/g). Electrostatic attraction was found to be the principal mechanism that controls the adsorption of potentially toxic metals. This study proved that amino acids-intercalated LDHs are promising dual-electronic adsorbents for the simultaneous removal of cationic and oxyanionic metal ions from water media.