Zerovalent Iron Nanoparticles-Alginate Nanocomposites for Cr(VI) Removal in Water-Influence of Temperature, pH, Dissolved Oxygen, Matrix, and nZVI Surface Composition

The immobilization of zerovalent iron nanoparticles (nZVI) is a way to facilitate their use in continuous flow systems for the treatment of aqueous pollutants. In this work, two types of nZVI (powdered, NSTAR; and slurry suspended, N25) were immobilized in millimetric alginate beads (AL) by coagulation, forming nanocomposites (NCs). These NCs, N25@AL and NSTAR@AL, were structurally studied and tested for Cr(VI) removal. For both NCs types, SEM analysis showed a uniform distribution of the nanoparticles in micron-scale agglomerates, and XRD analysis revealed the preservation of alpha-Fe as the main iron phase of the immobilized nanoparticles. Additionally, Raman spectroscopy results evidenced a partial oxidation of the initially present magnetite. For both nZVI types, the Cr(VI) removal efficiency increased with temperature, decreased with pH, and did not show any significant change in anoxic or oxic conditions. On the other hand, N25@AL resulted a faster removal agent than NSTAR@AL; however, both materials had the same maximum removal capacity: 133 mg of Cr(VI) per gram of nZVI at pH 3. Cr(III) formed during the removal of Cr(VI) was retained by the alginate matrix, constituting a clear advantage against the use of free nZVI in suspension at acidic pH.

Automated summary

The immobilization of zerovalent iron nanoparticles (nZVI) in alginate beads was studied for the treatment of Cr(VI) in continuous flow systems. The immobilized nanoparticles showed a uniform distribution and preserved the alpha-Fe phase. The efficiency of Cr(VI) removal increased with temperature and decreased with pH, and there was no significant difference between anoxic and oxic conditions. The nanocomposites exhibited a high removal capacity for Cr(VI), and the formed Cr(III) was retained by the alginate matrix.