Water-soluble oxidized starch in complexation of Fe(III), Cu(II), Ni(II) and Zn(II) ions

The structure of the bromate-oxidized wheat starch (OS) contains partly opened glucose units with carbonyl and carboxyl groups at C2-, C3- or C6-positions. OS with a variable degree of oxidation (DO) was studied in alkaline conditions as a water-soluble complexing agent for Fe(III), Cu(II), Ni(II) and Zn(II) ions, which are common in various wastewaters. Complexation was studied by inductively coupled plasma-optical emission spectrometry (ICP-OES) in a single metal ion or multi-metal ion solutions. The DO affected the efficiency of the complexation with metal ions. OS with the high DO (carboxyl and carbonyl DO of 0.72 and 0.23, respectively) complexed and held Fe(III) or Zn(II) ions in a soluble form effectively in 0.5 mM single ion alkaline solution with the molar ratio of 0.65:1 of oxidized starch-to-metal ion (OS-to-M). The OS-to-M molar ratio of 1.3:1 was required to form a soluble complex with Cu(II) or Ni(II) ions. These complexes were thermally stable at the temperature range of 20-60 °C. OS with the low DO (carboxyl and carbonyl DO 0.47 and 0.17, respectively) complexed Zn(II) ions highly, Cu(II) and Ni(II) ions poorly and Fe(III) ions only partly. In the multi-metal ion solution of OS the solubility of these metal ions improved with the increasing DO of starch, which followed the same tendency as was observed in the single metal ion systems. The increased molar ratio of OS-to-M improved the complexation and solubility of the metal ions in all multi-metal ion series. As the soluble multi-metal ion complexes were reanalyzed after 7 days, all solutions had kept the high complexation and solubility of metal ions (ca. 90%). Complexation by OS did not show a selective binding of the ions in the multi-metal ion solution. It was concluded that the flexible, opened ring structure units of OS prevented the selective binding to metal ions but made the complexes highly stable. Titrimetric studies of OS-Fe(III) complexation showed that each anhydroglucose unit of OS had more than one coordination site and as the content of OS increased, the free sites coordinated to Fe(III) ions and formed cross-linked starch structures.