Adsorption mechanism of rhein-coated Fe3O4 as magnetic adsorbent based on low-field NMR.

In the present study, a magnetic adsorbent, rhein-coated magnetic Fe3O4 nanoparticle (RMNP), for Pb2+ and Mg2+ had been developed, and adsorption mechanism was studied via low-field NMR. RMNP was characterized by TEM, FTIR, and XRD. RMNP could adsorb and remove Pb2+ and Mg2+ from water and was successfully applied to remove Pb2+ and Mg2+ from wastewater, with satisfactory recovery rates and high adsorption capacities. The calculated maximum adsorption capacity for Mg2+ and Pb2+ was approximately 69.3 and 64.9 mg g-1 of RMNP, respectively, which was better than some results reported. Low-field NMR results showed that Pb2+ or Mg2+ enhanced the T2 relaxation time of RMNP, which suggested that RMNP selectively coordinated with Pb2+ or Mg2+ and led to the aggregation of RMNP, furthermore removal of Pb2+ or Mg2+ from water. The standard curves for △T2-cation concentration exhibited good line correlation. The linear ranges were from 4.2 × 10-6 to 2.0 × 10-4 mol L-1 for Pb2+ and from 5.0 × 10-6 mol L-1 to 1.0 × 10-4 mol L-1 for Mg2+, respectively. The limits of detection were 1.4 × 10-6 mol L-1 for Pb2+ and 2.1 × 10-6 mol L-1 for Mg2+, respectively. In short, low-field NMR could clearly display the interaction between RMNP and Pb2+ or Mg2+, even be used to detect Pb2+ or Mg2+ in suitable condition. Besides, this method could be expanded to study the interaction between other magnetic adsorbents and analytes.