Effect of basicity and sodium ions on stability of polymeric ferric sulfate as coagulants

Abstract

In this paper, the stability of polymeric ferric sulfate (PFS) prepared by direct oxidation method was studied. The relationship between Basicity and stability of PFS was investigated and the structure and morphology of the precipitates in PFS were evaluated using Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD), Thermogravimetric analysis (TG-DTG) and Field emission scanning electron microscopy (FESEM). Considering the existence of sodium ion in PFS prepared by NaClO3, the effects of Na2SO4, NaCl and NaNO3 on the stability of PFS were investigated, and the composition and formation mechanism of the precipitates were also probed. The results indicated that the Basicity has no effect on the structure and morphology of the precipitates. The precipitates in PFS prepared by NaClO3 were sodium jarosite. We also found that the Na2SO4, NaCl and NaNO3 that added into PFS significantly decreased the stability of PFS. The results also indicated that the precipitates in PFS prepared by H2O2 was an amorphous structure containing Fe3+, OH−, SO42−. However, the precipitates in PFS prepared by H2O2 added sodium salt were rhombohedra (pseudo cubic) crystals sodium jarosite, which was consisted with the precipitates in PFS prepared by NaClO3. Above all, we concluded that the existence of sodium ions was an important cause for the instability of PFS.