Phase transition of Mg/Al-flocs to Mg/Al-layered double hydroxides during flocculation and polystyrene nanoplastics removal

Nanoplastics, a kind of emerging pollutant in natural environments, have now drawn tremendous attention worldwide. Flocculation with Mg/Al-layered double hydroxides (LDH) precursor solutions has showed great potential for removing negatively charged nanoparticles from water. In this study, the flocculation behavior and mechanism for the removal of polystyrene nanoplastics (PSNP) with Mg/Al flocs or Mg/Al LDH were systematically analyzed and investigated. During the process of flocculation, it was observed that in situ Mg/Al LDH can be gradually formed with increasing pH, in addition, PSNP were captured or attached to the surface of LDH with a turning point around pH of 5.0. In acidic solutions with pH < 5.0, the negative surface charges of PSNP were diminished mainly due to the high concentrations of hydrogen ions and the positive charges from Mg and Al ions. In a moderately alkaline solution, Mg and Al ions gradually formed crystals capturing PSNP. Electrostatic adsorption and intermolecular force are the main mechanisms via which PSNP are captured on Mg/Al flocs. Herein, PSNP removal efficiencies from water were more than 90.0%. As the problem of plastic pollution becomes more severe, in situ LDH growth flocculation can provide an efficient way for the removal of PSNP.